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molybdenum flotation circuit
Int. J. Miner. Process. 93 (2009) 256–266
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International Journal of Mineral Processing j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / i j m i n p r o
A study of mechanisms affecting molybdenite recovery in a bulk copper/ molybdenum flotation circuit M. Zanin ⁎, I. Ametov, S. Grano, L. Zhou, W. Skinner Ian Wark Research Institute, University of South Australia (The ARC Special Research Centre for Particle and Material Interfaces), Mawson Lakes Campus, Adelaide, South Australia 5095, Australia
a r t i c l e
i n f o
Article history: Received 23 April 2009 Received in revised form 2 October 2009 Accepted 3 October 2009 Keywords: Froth flotation Molybdenite Sulphide ores Ore mineralogy
a b s t r a c t Molybdenite flotation in the bulk copper/molybdenum flotation circuit at Kennecott Utah Copper was studied by means of a combination of plant metallurgical surveys, laboratory flotation tests, mineralogical analysis (QEM-Scan), surface analysis (ToF-SIMS) and contact angle measurements. It was demonstrated that molybdenite recovery is influenced by flotation feed solids percent and by the mineralogy of the host rock. Molybdenite recovery was consistently higher at reduced flotation feed solids percent. Furthermore, the recovery of molybdenite was significantly lower from flotation feeds with high limestone skarn ore content. The major factors affecting the flotation recovery of molybdenite from both porphyry and skarn copper ores are discussed. It is suggested that the lower flotation recovery of molybdenite compared to the copper sulphide is determined by several factors, including particle morphology, inherent hydrophobicity and possible formation of slime coatings in the presence of gangue minerals typical of skarn ores. Implications on plant performance are discussed, and solutions to restore molybdenite recovery presented. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Characteristically, in porphyry copper flotation plants, molybdenum exhibits lower recovery than copper, in spite of the apparent natural hydrophobicity of molybdenite (Kelebek, 1988). Furthermore, molybdenum recovery displays high variability. While copper recovery is usually between 80% and 90%, molybdenum recovery may range between 25% and 85% (Crozier, 1979). Among the copper sulphide minerals, chalcopyrite usually has higher flotation rate and recovery, but also chalcocite, bornite, digenite and covellite can be recovered to values higher than 80% if the relevant electrochemical conditions are maintained in the slurry to minimise surface oxidation (Orwe et al., 1998). Molybdenite recovery, on the contrary, may vary significantly from operation to operation, and also within different ore bodies in the same operation. Copper and molybdenum recovery data for a one year period of a typical porphyry copper flotation plant are reported in Fig. 1, showing high variability of molybdenum recovery with time. Extensive research has been carried out in an attempt to link the flotation response of molybdenite to the mineral crystal structure, textural features and lithology. The degree of crystallisation is one of the factors that have been reported to affect molybdenite flotation (Hernlund, 1961; Shirley, 1981; Podobnik and Shirley, 1982). Wellcrystallised molybdenite is considered fast floating, while the almost amorphous variety is either slow floating or non-floating (Hernlund, ⁎ Corresponding author. Tel.: +61 8 8302 3263; fax: +61 8 8302 3683. E-mail address: [email protected] (M. Zanin). 0301-7516/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.minpro.2009.10.001
1961). The occurrence of large molybdenite crystals in vein controlled mineralisation as opposed to finely disseminated molybdenite has also been reported (Sutulov, 1975; Podobnik and Shirley, 1982), but no clear connection with flotation response has been established. Triffett and Bradshaw (2008) demonstrated that molybdenite particles with high aspect ratio (major axis over minor axis) have higher probability of reporting to the concentrate, and that coarse particles with high perimeter to area ratio tend to report to the tailings. It could be noted that aspect ratio itself may not be the critical factor, but it may be a proxy for hydrophobicity, as discussed further below. Ametov et al. (2008) conducted a series of surveys at different porphyry copper flotation plants. In the operations investigated, the recovery of molybdenite in the rougher/scavengers of the bulk Cu/Mo flotation circuit was consistently lower than the recovery of copper sulphide, the difference ranging from about 2% to 12% (Ametov et al., 2008). Furthermore, a reduction in the flotation feed solids percent (expressed as solids percent in the slurry by weight) resulted in an increase in molybdenite recovery, while copper flotation was almost unaffected (Ametov et al., 2008). The different behaviour of copper minerals and molybdenite with respect to feed solids percent was explained in terms of flotation hydrodynamics (Ametov et al., 2008). 1.1. Mechanisms affecting molybdenite recovery The low and highly variable flotation recovery of molybdenum may be a result of several factors, all related to the properties of the molybdenite (MoS2) mineral. Molybdenite crystal structure consists of hexagonal layers of molybdenum atoms between two layers of
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the particles formed are characterised by strongly hydrophobic and inert faces and hydrophilic and reactive edges, generated by the breakage of the covalent bonds. These peculiarities determine the flotation behaviour of molybdenite, which is a combination of (a) particle morphology (shape and size) in relation to hydrodynamics (Ametov et al., 2008), (b) particle inherent hydrophobicity, an important factor controlled to a degree by the face/edge ratio (Chander and Fuerstenau, 1972; Hoover, 1980), (c) particle–particle interactions between molybdenite and gangue minerals (Raghavan and Hsu, 1984), and (d) particle recovery across the froth phase (Dippenaar, 1982; Zanin et al., 2008). The possible effect of each contributing factor is discussed further below, and it is a purpose of this paper to probe the relative importance of some of these mechanisms. Fig. 1. Historical recovery data for a typical porphyry copper plant. Bulk copper/ molybdenum flotation circuit.
1.2. Hydrodynamic effects (a) In flotation, the rate at which particles are removed from the slurry by air bubbles can be represented by (Newell and Grano, 2007):
sulphur atoms (Fig. 2). Strong covalent bonds act within S–Mo–S layers, but only weak van der Waals forces between adjacent S–S sheets (Lince and Frantz, 2000). This strong anisotropy causes preferential cleavage of the molybdenite crystal along the adjacent S–S sheets. As a result, during grinding, platelet shaped fragments, exfoliating from larger particles, are generally produced. Furthermore,
dNp = kNp = −Zpb × Ecoll dt
ð1Þ
in which Np is the number of particles in the slurry at time t, k the flotation rate constant, Zpb the collision frequency, and Ecoll the collection efficiency. The collection efficiency Ecoll can be described as a product of the collision (Ec), attachment (Ea) and stability (Es) efficiencies: Ecoll = Ec × Ea × Es
ð2Þ
Hydrodynamic conditions have direct influence on both collision frequency and collection efficiency. Important hydrodynamic parameters are bubble diameter, bubble velocity and turbulent energy dissipation (Newell and Grano, 2007). Ametov et al. (2008) argued that, molybdenite particles, due to their peculiar shape factor, could be more sensitive to hydrodynamic effects than copper mineral particles. In an agitated slurry, platelet shaped molybdenite particles may align along streamlines of the suspending liquid and, therefore, have lower probability of collision with bubbles. In effect, this hypothesis purports that the hydrodynamic diameter is the minimum dimension of the platelets. Increasing turbulence would increase collision frequency and efficiency, and therefore increase the rate of particle collection. This could be achieved either by increasing the impeller rotational speed or by reducing the feed solids percent. Damping of local energy dissipation occurs in suspensions containing higher volume percent of solids, as higher volume percent of solids produces higher slurry viscosity, particularly in the case of interacting particles (Schubert, 1999). Reducing the feed solids percent reduces the slurry viscosity and increases turbulence, having in turn a positive effect on particlebubble collision efficiency (Ec). 1.3. Inherent hydrophobicity (b)
Fig. 2. Crystal structure of molybdenite (from Lince and Frantz, 2000).
Due to the cleavage mechanisms of molybdenite, very thin particles are potentially produced in grinding. The overall degree of hydrophobicity of molybdenite particles depends on the relative surface exposure of faces (hydrophobic) and edges (hydrophilic). In flotation, molybdenite particles with higher exposure of edges will have lower probability of attachment to air bubbles. On the contrary, particles with a high face/edge ratio will have higher probability of being recovered, in agreement with the findings of Triffett and Bradshaw (2008). The overall degree of hydrophobicity of molybdenite particles can also be reduced by the adsorption of metal ions in solution (Raghavan and Hsu, 1984). Molybdenite has negative zeta
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potential across a wide pH range (Wie and Fuerstenau, 1974). However, adsorption of positively charged ions at the edges may reduce the magnitude, or even reverse the sign of the zeta potential. Calcium ions, in particular, have been shown to adsorb in the intermediate to high pH range (Healy, 1984). Adsorbed Ca2+ ions on the particle edges may reduce the contact angle and flotability of molybdenite particles. Furthermore, significant deformation of particles may occur in tumbling mills (Hoover, 1980), and it is common to find bent, distorted, or striated molybdenite particles in the flotation feed (Triffett and Bradshaw, 2008). Under these conditions, higher exposure of edges may occur, due to the breakage of the covalent bonds, and the spatial distribution and orientation of the hydrophilic edges may determine the probability of attachment of particles to air bubbles on collision. Oily collector is typically added in molybdenite flotation to enhance the mineral's hydrophobicity. The presence of exposed hydrophilic edges on the bent and distorted particles may also prevent spreading of oil droplets on hydrophobic surfaces, thus reducing the effect of the collector. 1.4. Inter-particle interactions with gangue minerals (c) The high reactivity of molybdenite edges may also determine particle–particle interactions with other minerals in the slurry, in the form of slime coatings. Raghavan and Hsu (1984) showed that, in the presence of Ca2+ ions, the addition of silica to a system of molybdenite particles causes a sharp decrease in molybdenite flotation recovery. It is possible that the calcium ions act as a ‘bridge’, favouring adhesion between negatively charged molybdenite and silica particles. This mechanism may be relevant in the flotation of an ore with specific gangue mineralisation and dissolution of ions. Inter-particle interactions between some of the gangue minerals and molybdenite may cause slime coatings on the latter, reducing its flotation recovery. This effect could be significant at high pH and high alkalinity, due to the bridging effect of the adsorbed Ca2+ ions. This possibility is explored in this paper. 1.5. Froth phase recovery (d) Inherent hydrophobicity and shape may also play a role in the transportation of molybdenite particles across the froth phase. Dippenaar (1982) and Hemmings (1981) found a correlation between size and hydrophobicity of the suspended particles and destabilisation of the froth phase by film rupture. For a given particle size, the greater the contact angle, the greater is the destructive compressive stress induced on the thin film (Hemmings, 1981). This implies that intermediate to coarse particles having contact angle greater than 90° are effective film breakers in froth flotation (Dippenaar, 1982). Flat and elongated molybdenite particles may fall into this category, and have, for this reason, lower recovery across the froth phase (Zanin et al., 2009), also called froth recovery (Savassi et al., 1997). In a separate investigation (Zanin et al., 2008), it was shown that froth recovery in the roughers of a porphyry copper flotation plant decreases significantly down the bank (from 60% in the first cells to 20% in the last cells), and that froth recovery of molybdenite is generally lower than froth recovery of the copper sulphide. Stability of the froth phase is therefore a factor which should be taken into consideration at plant scale, particularly in the rougher/scavengers, where major losses of coarse molybdenite occur. In this paper, however, the focus was on identifying factors which affect molybdenite recovery in the collection zone, while froth phase issues have been addressed elsewhere (Zanin et al., 2008). In the current study, investigation was carried out at Kennecott Utah Copperton Concentrator. Plant surveys have been undertaken on flotation feeds having different mineralogy, and ancillary laboratory flotation tests have been carried out plant slurries. Tests on controlled
Table 1 Average head grade of quartzite and limestone skarn ore samples.
Quartzite Skarn
Cu [%]
Fe [%]
Mo [ppm]
S [%]
SiO2 [%]
Al2O3 [%]
CaO [%]
K2O [%]
MgO [%]
0.50 0.45
1.9 9.6
600 21
0.4 1.5
63 44
13.8 1.7
2.0 24
7.0 0.3
5.0 2.7
flotation feeds (synthetic mineral mixtures and blends of different ore types) have been carried out at the Ian Wark Research Institute.
2. Materials and methods 2.1. Ores The Kennecott Utah Copperton concentrator treats porphyry copper ore, in which molybdenite occurs as a minor phase. Typically, Mo concentrations range from 0.02% to 0.06%. Distinct zones of mineralisation and alteration are mined at Kennecott, such that four main ore types have been described (Triffett and Bradshaw, 2008), including a quartzite ore showing high molybdenum grade and recovery and a more difficult to float, low grade, limestone skarn ore. In the current study, plant surveys have been undertaken with the plant processing blends of the four main ore types in different proportions, and laboratory flotation tests have been performed both on plant slurries and controlled blends of quartzite and skarn ores, prepared at laboratory scale. Average head grades of the quartzite and skarn ore samples are reported in Table 1, and the mineralogical composition (modal distribution by QEM-Scan) in Table 2. The two ores present distinct gangue minerals: large amounts of quartz and feldspar in the quartzite ore sample, and actinolite, andradite, talc and calcite abundant in the limestone skarn ore sample (shown in Table 2 as the Skarn ore). Single mineral molybdenite samples from WILLYAMA-GEO Discoveries were used to produce coarse molybdenite particles which were used for contact angle measurement by the sessile drop method.
2.2. Reagents The reagents used in this study, both in the plant and laboratory tests, were: oily collector for molybdenite (generic diesel oil was used in the laboratory tests), at a typical addition rate of 22 g/t, dicresyldithiophosphate (S-8989) collector for copper minerals, at a typical addition rate of 20 g/t, and methylisobutyl-carbinol (MIBC) as frother (15 to 35 g/t). The pH was adjusted by adding lime, with the exception of some diagnostic tests in which KOH was used, as outlined in 2.3.4. In the laboratory flotation tests carried out at the Ian Wark Research Institute, synthetic process water was used, prepared by dissolving
Table 2 Mineralogical analysis of quartzite and limestone skarn ore samples. Mineral
Formula
Modal [%] Quartzite
Skarn
Quartz K_Feldspar Plagioclase Montmorillonite Biotite Actinolite Andradite (Al) Talc Calcite
SiO2 KAlSi3O8 NaxCa(1 − x) (Al,Si)AlSi2O8 (Na,Ca)(Al,Mg)6(Si4O10)3(OH)6 − nH2O K(Fe,Mg)3AlSi3O10(F,OH)2 Ca2(Mg, Fe)5Si8O22(OH)2 Ca3Al2(SiO4)3 Mg3Si4O10(OH)2 CaCO3
24.3 38. 7 8.1 4. 7 15.0 0.1 0.14 0.01 1.0
10.9 1.0 0.02 0.04 0.06 8.2 58.7 0.08 4.9
Modal mineral distribution for the major gangue minerals determined by QEM-Scan.
M. Zanin et al. / Int. J. Miner. Process. 93 (2009) 256–266 Table 3 Composition of synthetic process water compared with a sample of process water from the KUCC copper circuit. Ion
Na+ Ca++ K+ Mg++ Cl− SO−− 4 HCO− 3 TDS pH Cond. [μ S/cm]
Synthetic H2O
Process H2O
[ppm]
[ppm]
1308 798 87 129 1818 2653 189 6980 7.6 6700
1310 798 87 129 1940 2590 160 7010 7.2 8000
salts in demineralised water to reproduce the composition of Kennecott process water (Table 3). 2.3. Methods 2.3.1. Plant surveys The Copperton concentrator consists of grinding circuits (SAG milling followed by ball milling) and two flotation circuits: a bulk copper flotation circuit, where copper/molybdenum concentrate is produced, and a molybdenite flotation plant, in which molybdenite in the bulk concentrate is separated from the copper minerals. At the time of investigation, the copper circuit consisted of 5 parallel rows of rougher/scavenger cells. The rougher concentrate was treated in rougher cleaners (2 parallel rows of cells) without regrinding. The scavenger concentrate reported to regrinding (ball milling) along with the rougher cleaner tailing. The regrind circuit product was upgraded in three scavenger cleaner stages, the final concentrate from which, combined with the rougher cleaner concentrate, produced the bulk copper/molybdenum concentrate. The target d80 of the flotation feed was 200 μm, and the pH in the rougher/scavengers was controlled between 9.5 and 10. A bulk concentrate assaying about 25–30% Cu and 2–4% Mo was produced, depending on the ore blend processed. Since most of the molybdenite losses occurred in the rougher/scavengers of the copper circuit, this part of the circuit became the focus of investigations. Four plant surveys were undertaken, in which timed lip samples and in-pulp samples were collected from each cell down one of the rougher/scavenger rows. In each survey, four sampling rounds were performed, over a period of 90 min, in which multiple samples were collected from the rougher/scavenger tailings to reduce experimental error. Prior to sampling, plant stability was ensured from the control room, and plant data (throughput, feed solids percent, air flows, and lip levels) were recorded. All metallurgical samples were initially assayed unsized. Samples from selected surveys were sized by wet/ dry sieving and assayed on a size-by-size basis. Data reconciliation and mass balance were performed using the software Bilmat 9.3™. Surveys 1 and 2 were performed with the plant processing a blend with high quartzite ore content, while surveys 3 and 4 were carried out with the plant processing a blend with high limestone skarn ore content. Surveys 1 and 4 were performed at lower solids percent in the flotation feed, while surveys 2 and 3 were carried out at higher solids percent (Table 4). The percent solids, which under standard operating conditions ranged between 30% and 35%, was adjusted by varying the water flowrate to the rougher flotation distribution box. The influence of feed solids percent on the recovery of molybdenite from the two different ore blends was studied. During survey 1, additional samples of slurry from the concentrate of cells 1 and 11 were collected for surface analysis by ToF-SIMS. Samples were placed in plastic vials, purged with nitrogen to remove oxygen and frozen in liquid nitrogen. The samples were stored frozen
259
in a cryogenic container until ToF-SIMS analysis was carried out. Since the focus of the study was recovery of coarse molybdenite, the ToFSIMS analysis was carried out on the +150 μm particles, which were isolated by wet sieving. The surface chemistry of the fast (concentrate from cell 1) and slow (concentrate from cell 11) floating particles was investigated, in order to correlate possible differences in surface composition to flotation response. It was not possible to analyse molybdenite in the flotation tailings, which ideally contain the nonfloating molybdenite, due to the extremely low molybdenum grade, i.e. a statistically significant number of molybdenite particles could not be analysed. No samples of slurry for ToF-SIMS analysis were collected during surveys 3–4. Therefore, it was not possible to compare the surface composition of molybdenite in surveys with different feed ore blends and gangue mineralogy. This important aspect of the study was, however, conducted on samples generated at laboratory scale, as described further below. 2.3.2. Laboratory flotation tests on plant slurries In parallel with the surveys, laboratory flotation tests were undertaken on conditioned rougher feed slurries collected in the plant. The tests were conducted using a 5 l Agitair flotation machine with forced air supply. The samples were taken from the flotation feed box where all reagents except frother were added. An impeller speed of 1000 rpm and air flowrate of 5 l/min were used in the tests. The slurry (conditioned rougher feed) was floated without further collector addition, and at the same pH (9.5–10) as the plant. Concentrates were collected after 1, 3, 6 and 10 min. All the flotation products were assayed on an unsized and size-by-size basis. Unsized and size-by-size recoveries were calculated. 2.3.3. Laboratory flotation tests on reconstructed feeds In diagnostic tests carried out at the Ian Wark Research Institute, samples of quartzite and limestone skarn ores provided by Kennecott were used to reproduce plant slurries. Three different controlled feed blends were prepared: 100% quartzite, 75:25 quartzite/skarn and 50:50 quartzite/ skarn ores. The ore blends were crushed using laboratory jaw and cone crushers, homogenised and ground in a laboratory Galigher tumbling mill, using stainless steel rods as grinding media. Grinding was calibrated to a target d80 = 200 μm, by varying the grind time for each feed type. Synthetic process water (Table 3) was used to simulate plant conditions. The same pH and reagents scheme were used as in the tests carried out at Kennecott. Flotation tests were carried out as described in 2.3.2. Similarly to the plant surveys, after each laboratory flotation test the coarse particles (+150 μm) from the first concentrate and the last concentrate were collected by wet screening and analysed by ToFSIMS. Surface analysis was conducted for the concentrates collected in flotation tests on both 100% quartzite ore and 50:50 blend of quartzite and skarn ores, to draw a link between molybdenite surface composition, gangue mineralogy and flotation response . 2.3.4. Laboratory flotation tests on coarse molybdenite particles The effect of calcium and magnesium ions in solution and gangue minerals on the recovery of molybdenite was studied separately. Coarse molybdenite particles (+150 μm) were obtained by dry screening of a molybdenite concentrate from Kennecott (final product Table 4 Feed composition and operating conditions during the plant surveys. Ore blend type
Cu
Mo
Survey
Solids
[%]
[%]
#
[%]
High quartzite
0.3
0.06
High skarn
0.5
0.06
1 2 3 4
27 35 35 27
pH
9.8 10.0 10.1 10.1
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of the molybdenum plant). Particles were washed in Ensolv (n-propyl bromide N 93%) and ethanol to clean the surface from any residual collector. Samples of quartzite and skarn ore were floated to remove copper minerals and molybdenite, and the flotation tailings were collected. The coarse molybdenite particles were then blended into the tailings of quartzite and skarn ores, to produce synthetic ore containing 0.15% MoS2 by weight. Samples were floated, with the collectors and frother added according to the conditioning scheme described in 2.2. Low conductivity water, produced by reverse osmosis, two stages of ion exchange and two stages of activated carbon prior to final filtration, was used in these experiments. The water pH was adjusted to 10 by adding KOH, and the concentration of ions was varied by adding Ca(NO3)2 and Mg(NO3)2. 2.4. Characterisation techniques 2.4.1. Contact angle of molybdenite particles Large molybdenite crystals were obtained from WILLYAMA-GEO Discoveries. Molybdenite crystals were cut and cleaved using a scalpel, to expose fresh crystal edges and faces. The contact angle was measured at different pH values and Ca2+ concentrations in the ranges typically observed in the plant (8–11 for pH and 0–10− 2 M for Ca2+ ions). After conditioning, the water advancing and receding contact angle of face and edge of individual molybdenite particles were measured by the sessile drop method. 2.4.2. ToF-SIMS TOF-SIMS spectra were obtained using a PHI TRIFT II System equipped with a gallium liquid metal ion gun (LMIG) in pulsed mode. For insulating samples the surface charge is compensated using a pulsed electron flood gun. The mineral samples were mounted on to indium foil. The primary beam current employed in the present study was 600 pA (DC measurement). In static mode, the analysis is confined to the top two monolayers. An excitation voltage of 25 kV was used in un-bunched mode to give a spatial resolution of better than 0.5 μm and pulse length adjusted to give a mass resolution (m/ Δm) of ~ 4000. Imaging of the sample involved mapping for positive and negative ions for surface regions of mineral particles of interest, i.e. MoS2. Several tens of particles in each processing stream are analysed and the data statistically presented in terms of normalised (to total ion yield) intensity of signals and 95% confidence intervals. In this mode, statistical differences in surface chemistry between the same minerals in different streams or under different pulp conditions may be discerned. In the present case, we are investigating surface chemistry differences between floating and non(slow)-floating MoS2 particles. 3. Results 3.1. Plant Studies and Laboratory Tests on Plant Slurries The grade/recovery relationship for copper and molybdenum for the four plant surveys and the laboratory flotation tests carried out in parallel with the plant surveys are reported in Fig. 3. The ultimate recovery (combined rougher/scavenger flotation) for copper and molybdenum is also reported in Table 5. In the plant surveys, molybdenum recovery ranged from 75% to 92%, while copper recovery from 85% to 92%. The recovery of molybdenum was consistently lower than copper recovery, the difference increasing at higher feed solids percent and in the presence of limestone skarn ore. For both copper and molybdenum, the lowest recovery was achieved at high solids percent and in the presence of limestone skarn ore (survey 3). Compared to copper, molybdenum recovery was more sensitive to the feed solids percent, being significantly enhanced when solids percent was reduced from 35% to 27%. Both copper and molybdenum recoveries were lower in the presence of limestone
skarn ore in the feed blend, but molybdenum recovery decreased more than copper recovery. With a reduction in the feed solids percent, however, it was possible to partially restore molybdenum recovery to values characteristic of quartzite ore (from 75% to 85%). The laboratory tests (Fig. 3c and d) showed generally higher recoveries for both copper and molybdenum compared to the plant surveys (Fig. 3a and b), but with a similar trend with respect to feed composition and solids percent. Copper recovery was marginally affected by changes in feed solids percent in the absence of limestone skarn ore, showing some difference (90% recovery at 35% solids versus 93% recovery at 27% solids) when limestone skarn ore was blended to flotation feed. Molybdenum recovery, on the contrary, was always higher at low solids percent. Differences in recovery became significant (93% versus 85%) in the presence of limestone skarn ore. Size-by-size analysis of the flotation products was undertaken for surveys 3 and 4 (feed blend with high skarn ore), in which a significant increase in molybdenum recovery was obtained by reducing the feed solids percent. Results (Fig. 4a) revealed increased molybdenum recovery across all size ranges, but particularly for the coarse size fractions (+150 μm). The effect on copper was much less pronounced. In the laboratory scale tests, the recovery of molybdenum was higher than in the plant across all size ranges. Furthermore, a reduction in the feed solids percent produced a significant effect on coarse (+150 μm) molybdenum bearing particles, for which recovery increased from 60% to 80% when the feed solids percent was reduced from 35% to 27%. Two additional flotation tests were undertaken at laboratory scale, at 30% and 45% solids in the rougher flotation feed. These concentrations were achieved by manipulating samples of plant feed collected during survey 3 (35% solids), the former by diluting with process water, the latter by filtering and re-suspending the solids in process water. The trend (Fig. 4d) confirmed what was observed in the previous tests, showing high sensitivity of the coarse molybdenite particles to feed solids percent. The feed solids percent is an important driver to molybdenite recovery overall, and particularly in coarse particle size fractions. In Fig. 4, bar charts reporting the distribution of copper and molybdenum in the feed are superimposed on the recovery curves. The area of each bar in the charts is proportional to the relative mass of mineral in the specified particle size range. Considering that, at the standard plant grind, the +150 μm size fraction contains about 20% of the total molybdenite in the feed, the recovery of this size fraction is critical to the overall molybdenite recovery. Surface analysis results on the slurry samples collected during survey 1 (in which the flotation feed consisted mainly of quartzite type ore, and no limestone skarn was present) are reported in Fig. 5. No significant difference in surface composition between fast (concentrate from Cell 1) and slow (concentrate from Cell 11) floating molybdenites was found in this particular case of the quartzite ore type. It could therefore be concluded that the smaller differences in molybdenite recovery noted between plant and laboratory scale (Fig. 3) may be ascribed to changes in hydrodynamics. However, this conclusion will depend on the feed type, as discussed further below. 3.2. Laboratory flotation tests on reconstructed feeds The flotation recoveries of copper and molybdenum in laboratory scale tests on blends of quartzite ore and skarn ore are reported in Fig. 6. A sharp decrease in molybdenite recovery was observed at increasing concentration of limestone skarn ore in the feed. The ultimate molybdenite recovery decreased from 92% in the absence of skarn ore to 85% with 25% skarn ore and 56% with 50% skarn ore in the flotation feed. Copper flotation was affected to a much lesser extent. This is in agreement with the plant surveys and tests carried out on plant slurries. It should be noted that, since the limestone skarn ore
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Fig. 3. Grade/recovery relationship for copper and molybdenum in the plant surveys (a and b) and in the laboratory tests on plant feed (c and d). The ultimate recovery at the completion of flotation is also indicated (dashed lines).
contains very little molybdenum (Table 1), it is the molybdenite particles in the quartzite ore which are depressed in the presence of skarn ore. The ToF-SIMS spectra for the fast floating molybdenite (Con 1) and slow floating molybdenite (Con 4) collected in the tests with 0% and 50% skarn ore in the flotation feed are reported in Fig. 7. Samples of fast (Con 1) and slow (Con 4) floating molybdenite in the presence of quartzite type ore only, showed very small differences in surface chemical composition. The different species appear in the same proportion on the surface of the two samples, within statistical error (Fig. 7). This is in agreement with previous
observations at plant scale, which also showed no surface chemical differences on molybdenite. In the presence of 50% skarn ore, on the contrary, samples of fast and slow floating molybdenites showed differences in surface chemistry (Fig. 7). In the fast floating sample (Con 1) there are much more exposed molybdenum and sulphur compared to the slow floating sample (Con 4). Furthermore, Ca, K, Fe, as well as O and OH groups, were more abundant on the latter sample. Significantly higher Mg on the surface of molybdenite in the presence of skarn ore was also noted in both fast and slow floating fractions (Fig. 7a). 3.3. Laboratory flotation tests on coarse molybdenite particles
Table 5 Ultimate recovery of copper and molybdenum in plant (after rougher/scavenger flotation) and laboratory. Ore blend type
Survey
Solids
Plant recovery
Lab recovery
#
[%]
Cu [%]
Mo [%]
Cu [%]
Mo [%]
High quartzite
1 2 3 4
27 35 35 27
92 92 85 88
92 88 75 85
95 94 90 93
95 92 85 93
High skarn
The effect of calcium and magnesium ions in solution on coarse molybdenite (+150 μm) recovery from quartzite and limestone skarn ores is presented in Fig. 8. In the case of quartzite ore, the cumulative recovery of molybdenite was high, approaching 90% after 8 min of flotation. The addition of calcium and magnesium ions to the pulp did not have an effect on molybdenite flotation. In contrast, molybdenite recovery from limestone skarn ore was considerably lower (60%) even in the absence of calcium and magnesium ions, and decreased further to 30% when Ca2+ and Mg2+ ions were added. Apparently, there is a
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Fig. 4. Size-by-size recovery of copper and molybdenum in plant surveys 3 and 4 (after rougher/scavenger flotation) (a and b) and in the laboratory flotation tests on plant feed (c and d). At laboratory scale, additional feed solids percent values (30% and 45%) were obtained by manipulating plant feed samples. The distribution of copper and molybdenum in the feed is also reported. Feed blend contained high limestone skarn ore.
synergistic effect between the presence in solution of Ca2+ and Mg2+ ions and gangue minerals of the skarn ore. The effect of pH and calcium ions on the contact angle of molybdenite was also investigated (Fig. 9). The tests showed that faces and edges of molybdenite particles have considerably different contact angles. Face contact angle was high (about 100°) and independent of solution pH and calcium concentration. In contrast, contact angle on the edges was low (maximum 45°), and decreased with increased pH and calcium ion concentration ([Ca2+] N 4 × 10− 3 M). 4. Discussion From an industrial perspective, the most important finding from the plant studies was the consistent increase in molybdenite recovery in rougher/scavenger flotation at reduced feed solids percent. This is in agreement with previous observations for different flotation plants (Ametov et al., 2008). The coarse (+ 150 μm) size fractions were affected the most by changes in feed solids percent, and therefore fluctuations in the overall recovery of molybdenite in the plant are determined to a large extent by the flotation response of the coarse
particles. Particle size and shape, morphology and surface composition are all contributing factors. Insufficient molybdenite liberation in the coarse size fractions was also considered in the first instance as a possible cause for the low flotation recovery. However, QEM-Scan analysis of the scavenger tailings (Triffett, private communication) showed that molybdenite in the + 150 μm size fraction mainly consists of liberated particles. This confirms that other mechanisms, as outlined in Section 1.1, play significant roles. The hydrodynamic behaviour of the flat molybdenite particles in a turbulent environment may explain the increase in molybdenum recovery observed at reduced solids percent (Fig. 3). Higher local turbulent energy dissipation may increase the collision efficiency for the flat and elongated particles, as hypothesised in Section 1.2. The fact that molybdenite recovery in laboratory flotation tests was higher than in the plant (Fig. 4) may also be due hydrodynamic effects. Compared to the plant cells, laboratory batch flotation cells have higher local turbulent energy dissipation (Newell and Grano, 2007), and therefore higher collision efficiency, Ec, and, according to Eq. (1), higher flotation rate. An interesting feature is that it is the coarse and liberated molybdenite that is mainly affected, the recovery of which is
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Fig. 5. Positive (a) and negative (b) ToF-SIMS normalised intensities for + 150 μm molybdenite particles. Average with 95% confidence, N N 23. Fast floating (Cell 1) and slow floating (Cell 11) molybdenite from plant survey 1 (no skarn ore in the feed).
Fig. 6. Copper and molybdenum recovery in laboratory flotation tests on reconstructed feeds (blends of quartzite and skarn ores in different ratios). Tests at 35% solids (synthetic process water used); Agitair 5 l cell; impeller speed 1000 rpm; air 7 l/min. 0% skarn ore = 100% quartzite ore.
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Fig. 7. Positive (a) and negative (b) ToF-SIMS normalised intensities for on + 150 μm molybdenite particles. Average with 95% confidence, N N 23. Fast floating (Con 1) and slow floating (Con 4) coarse particles (N 150 μm) in tests with 100% quartzite ore and 50:50 blend of quartzite and skarn ores.
presumably limited by stability efficiency, Es (Pyke et al., 2003). The high surface/bulk ratio of the coarse molybdenite particles may however be beneficial for the stability of particle–bubble aggregates, because of a higher surface force/inertial force ratio. Surface forces favour attachment of particles to bubbles, while inertial forces promote detachment. A high surface/bulk ratio results in high stability efficiency, even in high turbulent conditions such as in laboratory flotation and at reduced solids percent. Another outcome of the plant studies was that the effect of a reduction in feed solids percent on molybdenite recovery was high for some feed blends (high limestone skarn) while it was much lower for others (high quartzite). This was observed at plant scale (Fig. 3b), in laboratory flotation tests on plant slurries (Fig. 3d), and in tests on reconstructed feed blends (Fig. 6). Therefore, it is likely that factors other than hydrodynamics and collision efficiency, such as surface modification and particle–particle interactions also play a role. ToFSIMS analysis indicated the presence of elements associated with hydrophilic components on the slow floating coarse molybdenite particles in the presence of skarn ore (Fig. 7). This could be either from the deposition of fine slimes or adsorption of dissolved ions. In any case, the effect is strongly ore type dependent. In the absence of skarn ore, no difference in surface composition between fast and slow molybdenite was apparent at plant (Fig. 5) and laboratory (Fig. 7)
scales, suggesting that the different flotation recoveries of molybdenite at high and low solids percent may be ascribed to hydrodynamic effects, possibly arising from the intrinsic shape of the particles, rather than from the surface cleaning of slime coatings. Therefore, the hypothesis of slime coatings may hold for some feed types and be less important for others. It is surmised that at low pulp density there is less likelihood of inter-particle interactions causing slime coatings. A reduction in slime coatings on the surface of molybdenite particles results in increased contact angle. Even a small increase in contact angle could cause previously non-floatable coarse particles to become floatable and the ultimate recovery is therefore increased. in Kennecott process The high concentration of Ca2+ and SO2− 4 water (Table 3) may also suggest deposition of calcite and gypsum from solution due to oversaturation. This could also be a contributing factor, but it is not sufficient to justify the different degrees of molybdenite depression observed in the presence of quartzite and skarn ores. Diagnostic flotation tests on coarse molybdenite particles in the presence of limestone skarn ore and different levels of calcium and magnesium ions in solution (Fig. 8) corroborate the hypothesis that interactions between molybdenite and some gangue minerals in the skarn ore lead to molybdenite depression. The effect could be induced by metal ions adsorbed at the molybdenite edges, as also suggested by contact angle measurements carried out separately on
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5. Conclusions
Fig. 8. Recovery of coarse molybdenite particles (+ 150 μm) in flotation tests on synthetic feeds. Molybdenite particles from the final concentrate, after surface cleaning, were blended with ground quartzite and skarn ore (0.15% MoS2 by weight). Tests were in demineralised water, at pH 10, in the presence and absence of Ca2+ and Mg2+ ions.
faces and edges (Fig. 9). The edges of molybdenite particles are particularly reactive, and adsorption of Ca2+ and/or Mg2+ may have the combined effect of reducing the overall hydrophobicity of molybdenite particles and “bridging” for particle–particle interactions with the negatively charged gangue minerals. This is also in agreement with the findings of Raghavan and Hsu (1984) on molybdenite/silica mineral systems. Slime coatings are possibly causing molybdenite depression in the presence of skarn ore. Minerals which are much more abundant in the limestone skarn ore are: actinolite, andradite, talc and calcite (Table 2). These minerals are potentially responsible for the high Mg, Ca and Fe observed on the surface of molybdenite when floated in the presence of limestone skarn ore (Fig. 7). This conclusion would be consistent with the fact that the presence of the above mentioned minerals in the flotation feed has been correlated to periods of low molybdenite recovery (Triffett et al., 2008).
The experimental results confirm that the flotation of molybdenite from porphyry copper ores is more sensitive to the operating environment than that of the copper sulphide minerals. The recovery of molybdenum in the rougher/scavengers of the bulk Cu/Mo flotation circuit at Kennecott Utah Copper is highly variable, and always lower than the recovery of copper. Particularly low molybdenum recovery was observed in the presence of limestone skarn ore. As a general rule, it was found that operating the rougher/scavenger flotation rows at a lower feed solids percent (27% against 35%) ensures higher and more stable molybdenite recovery. Several factors have been identified which could affect molybdenite flotation, all of them related to the peculiar properties of molybdenite. Preferential cleavage along the weakly bound S–S layers may impart to the molybdenite particles singular hydrodynamic behaviour (low collision efficiency due to the flat and elongated particle shape), and anisotropy in hydrophobicity (hydrophobic faces and hydrophilic, highly reactive, edges). It has been shown that edges have much lower contact angle compared to faces, in particular at high pH (pH N 10) and high alkaline conditions ([Ca2+] N 10− 2 M), as in typical flotation environment. When limestone skarn ore was blended to the feed in laboratory flotation tests, molybdenite recovery decreased significantly, similarly to what was observed in the plant. Higher concentrations of Ca, Fe, Mg and K were measured on the slow floating molybdenite particles compared to the fast floating, which have been correlated to the presence of some gangue minerals typical of the skarn ore. A ‘bridging’ effect of calcium and magnesium adsorbed on the surface of molybdenite and some fine gangue particles in the slurry is possible and may lead to the formation of slime coatings, thus reducing the flotation recovery of molybdenite. In terms of strategies to increase molybdenite recovery at plant scale, operating the rougher/scavengers at low solid percent was a solution which gave immediate benefits. This benefit may be a result of a combination of hydrodynamic effects (higher local turbulent energy dissipation and collision efficiency), rheology and reduced formation of slime coatings. It also appears that preventing slimes from adsorbing on molybdenite particles during flotation may benefit molybdenite recovery. This is the subject for future investigation. Acknowledgements The authors would like to acknowledge the financial support of the sponsors of the AMIRA P260E project and the Australian Research Council (ARC). Support from the Management at Kennecott Utah Copper and site personnel is also gratefully acknowledged. References
Fig. 9. Contact angle (advancing) of coarse molybdenite particles (+150 μm) measured in proximity of face and edges, after conditioning at different pH values and Ca2+ concentrations. Contact angle measured by means of the sessile drop method.
Ametov, I., Grano, S.R., Zanin, M., Gredelj, S., Magnuson, R., Bolles, T., Triffett, B., 2008. In: Zuo, W.D., Yao, S.C., Liang, W.F., Cheng, Z.L., Long, H. (Eds.), Copper and molybdenite recovery in plant and batch laboratory cells in porphyry copper rougher flotation: Proceedings of XXIV International Mineral Processing Congress, Beijing, China 24–28 September 2008, volume 1, pp. 1129–1137. Chander, S., Fuerstenau, D.W., 1972. On the natural flotability of molybdenite. Trans. Am. Inst. Min. Metall. Engrs. 252, 62–69. Crozier, R.D., 1979. Flotation reagent practice in primary and by-product molybdenite recovery. Min. Mag. 140, 174–178. Dippenaar, A., 1982. The destabilisation of froth by solids. I. The mechanism of film rupture. Int. J. Miner. Process. 9, 1–14. Healy, T.W., 1984. Pulp chemistry, surface chemistry, and flotation. Symp. Ser. — Australas. Inst. Min. Metall. 40, 43–56. Hemmings, C.E., 1981. On the significance of flotation froth liquid lamella thickness. Trans. Inst. Min. Metall. Sect. C 90, C96–C102. Hernlund, R.W., 1961. Extraction of molybdenite from copper flotation products. Q. J. Colo. Sch. Min. 56, 177–196. Hoover, M.R., 1980. Water chemistry effects in the flotation of sulphide ores — a review and discussion for molybdenite. In: Jones, M.J. (Ed.), Complex Sulphide Ores. IMM, London, pp. 100–112.
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Kelebek, S., 1988. Critical surface tension of wetting and of floatability of molybdenite and sulphur. J. Colloid Interface Sci. 124, 504–514. Lince, J.R., Frantz, P., 2000. Anisotropic oxidation of MoS2 crystallites studied by angleresolved X-ray photoelectron spectroscopy. Tribol. Lett. 9, 211–218. Newell, R., Grano, S., 2007. Hydrodynamics and scale-up in rushton turbine flotation cells: Part 1 — cell hydrodynamics. Int. J. Miner. Process. 81, 224–236. Orwe, D., Grano, S.R., Lauder, D.W., 1998. Increasing fine copper recovery at the Ok Tedi concentrator, Papua New Guinea. Miner. Eng. 11, 171–187. Podobnik, D.M., Shirley, J.F., 1982. Molybdenite recovery at Cuajone. Min. Eng. 1473–1477 October. Pyke, B., Fornasiero, D., Ralston, J., 2003. Bubble particle heterocoagulation under turbulent conditions. J. Colloid Interface Sci. 265, 141–151. Raghavan, S., Hsu, L.L., 1984. Factors affecting the flotation recovery of molybdenite from porphyry copper ore. Int. J. Miner. Process. 12, 145–162. Savassi, O.N., Alexander, D.J., Johnson, N.W., Manlapig, E.V., Franzidis, J.-P., 1997. Measurement of froth recovery of attached particles in industrial cells. In: Lauder, D. (Ed.), Proceedings of the Sixth Mill Operators Conference, Aust. Inst. Min. Metall. Publ., Melbourne, pp. 149–155.
Schubert, H., 1999. On the turbulence-controlled microprocesses in flotation machines. Int. J. Miner. Process. 56, 257–276. Shirley, J.F., 1981. Byproduct molybdenite plant design. Can. Min. J. 1981, 27–28 March. Sutulov, A., 1975. Copper Porphyries. Miller Freeman Publications, San Francisco. Triffett, B., Bradshaw, D., 2008. The role of morphology and host rock lithology on the flotation behaviour of molybdenite at Kennecott Utah Copper. AusIMM Publication Series, 9th International Congress for Applied Mineralogy, ICAM 2008 — Proceedings, 2008, pp. 465–473. Triffett, B., Veloo, C., Adair, B.J.I., Bradshaw, D., 2008. An investigation on the factors affecting the recovery of molybdenite in the Kennecott Utah Copper bulk flotation circuit. Miner. Eng. 21, 832–840. Wie, J.M., Fuerstenau, D.W., 1974. Effect of dextrin on surface properties and the flotation of molybdenite. Int. J. Miner. Process. 1, 17–32. Zanin, M., Gredelj, S., Grano, S.R., 2008. Factors affecting froth stability in mineral flotation and implications on minerals recovery: a case study. In: Kuyvenhoven, R., Gomez, C., Casali, A. (Eds.), Proceedings of Procemin 2008, Santiago (Chile), pp. 197–206. Zanin, M., Wightman, E., Grano, S.R., Franzidis, J.-P., 2009. Quantifying contributions to froth stability in porphyry copper plants. Int. J. Miner. Process. 91, 19–27.
Contents lists available at ScienceDirect
International Journal of Mineral Processing j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / i j m i n p r o
A study of mechanisms affecting molybdenite recovery in a bulk copper/ molybdenum flotation circuit M. Zanin ⁎, I. Ametov, S. Grano, L. Zhou, W. Skinner Ian Wark Research Institute, University of South Australia (The ARC Special Research Centre for Particle and Material Interfaces), Mawson Lakes Campus, Adelaide, South Australia 5095, Australia
a r t i c l e
i n f o
Article history: Received 23 April 2009 Received in revised form 2 October 2009 Accepted 3 October 2009 Keywords: Froth flotation Molybdenite Sulphide ores Ore mineralogy
a b s t r a c t Molybdenite flotation in the bulk copper/molybdenum flotation circuit at Kennecott Utah Copper was studied by means of a combination of plant metallurgical surveys, laboratory flotation tests, mineralogical analysis (QEM-Scan), surface analysis (ToF-SIMS) and contact angle measurements. It was demonstrated that molybdenite recovery is influenced by flotation feed solids percent and by the mineralogy of the host rock. Molybdenite recovery was consistently higher at reduced flotation feed solids percent. Furthermore, the recovery of molybdenite was significantly lower from flotation feeds with high limestone skarn ore content. The major factors affecting the flotation recovery of molybdenite from both porphyry and skarn copper ores are discussed. It is suggested that the lower flotation recovery of molybdenite compared to the copper sulphide is determined by several factors, including particle morphology, inherent hydrophobicity and possible formation of slime coatings in the presence of gangue minerals typical of skarn ores. Implications on plant performance are discussed, and solutions to restore molybdenite recovery presented. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Characteristically, in porphyry copper flotation plants, molybdenum exhibits lower recovery than copper, in spite of the apparent natural hydrophobicity of molybdenite (Kelebek, 1988). Furthermore, molybdenum recovery displays high variability. While copper recovery is usually between 80% and 90%, molybdenum recovery may range between 25% and 85% (Crozier, 1979). Among the copper sulphide minerals, chalcopyrite usually has higher flotation rate and recovery, but also chalcocite, bornite, digenite and covellite can be recovered to values higher than 80% if the relevant electrochemical conditions are maintained in the slurry to minimise surface oxidation (Orwe et al., 1998). Molybdenite recovery, on the contrary, may vary significantly from operation to operation, and also within different ore bodies in the same operation. Copper and molybdenum recovery data for a one year period of a typical porphyry copper flotation plant are reported in Fig. 1, showing high variability of molybdenum recovery with time. Extensive research has been carried out in an attempt to link the flotation response of molybdenite to the mineral crystal structure, textural features and lithology. The degree of crystallisation is one of the factors that have been reported to affect molybdenite flotation (Hernlund, 1961; Shirley, 1981; Podobnik and Shirley, 1982). Wellcrystallised molybdenite is considered fast floating, while the almost amorphous variety is either slow floating or non-floating (Hernlund, ⁎ Corresponding author. Tel.: +61 8 8302 3263; fax: +61 8 8302 3683. E-mail address: [email protected] (M. Zanin). 0301-7516/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.minpro.2009.10.001
1961). The occurrence of large molybdenite crystals in vein controlled mineralisation as opposed to finely disseminated molybdenite has also been reported (Sutulov, 1975; Podobnik and Shirley, 1982), but no clear connection with flotation response has been established. Triffett and Bradshaw (2008) demonstrated that molybdenite particles with high aspect ratio (major axis over minor axis) have higher probability of reporting to the concentrate, and that coarse particles with high perimeter to area ratio tend to report to the tailings. It could be noted that aspect ratio itself may not be the critical factor, but it may be a proxy for hydrophobicity, as discussed further below. Ametov et al. (2008) conducted a series of surveys at different porphyry copper flotation plants. In the operations investigated, the recovery of molybdenite in the rougher/scavengers of the bulk Cu/Mo flotation circuit was consistently lower than the recovery of copper sulphide, the difference ranging from about 2% to 12% (Ametov et al., 2008). Furthermore, a reduction in the flotation feed solids percent (expressed as solids percent in the slurry by weight) resulted in an increase in molybdenite recovery, while copper flotation was almost unaffected (Ametov et al., 2008). The different behaviour of copper minerals and molybdenite with respect to feed solids percent was explained in terms of flotation hydrodynamics (Ametov et al., 2008). 1.1. Mechanisms affecting molybdenite recovery The low and highly variable flotation recovery of molybdenum may be a result of several factors, all related to the properties of the molybdenite (MoS2) mineral. Molybdenite crystal structure consists of hexagonal layers of molybdenum atoms between two layers of
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the particles formed are characterised by strongly hydrophobic and inert faces and hydrophilic and reactive edges, generated by the breakage of the covalent bonds. These peculiarities determine the flotation behaviour of molybdenite, which is a combination of (a) particle morphology (shape and size) in relation to hydrodynamics (Ametov et al., 2008), (b) particle inherent hydrophobicity, an important factor controlled to a degree by the face/edge ratio (Chander and Fuerstenau, 1972; Hoover, 1980), (c) particle–particle interactions between molybdenite and gangue minerals (Raghavan and Hsu, 1984), and (d) particle recovery across the froth phase (Dippenaar, 1982; Zanin et al., 2008). The possible effect of each contributing factor is discussed further below, and it is a purpose of this paper to probe the relative importance of some of these mechanisms. Fig. 1. Historical recovery data for a typical porphyry copper plant. Bulk copper/ molybdenum flotation circuit.
1.2. Hydrodynamic effects (a) In flotation, the rate at which particles are removed from the slurry by air bubbles can be represented by (Newell and Grano, 2007):
sulphur atoms (Fig. 2). Strong covalent bonds act within S–Mo–S layers, but only weak van der Waals forces between adjacent S–S sheets (Lince and Frantz, 2000). This strong anisotropy causes preferential cleavage of the molybdenite crystal along the adjacent S–S sheets. As a result, during grinding, platelet shaped fragments, exfoliating from larger particles, are generally produced. Furthermore,
dNp = kNp = −Zpb × Ecoll dt
ð1Þ
in which Np is the number of particles in the slurry at time t, k the flotation rate constant, Zpb the collision frequency, and Ecoll the collection efficiency. The collection efficiency Ecoll can be described as a product of the collision (Ec), attachment (Ea) and stability (Es) efficiencies: Ecoll = Ec × Ea × Es
ð2Þ
Hydrodynamic conditions have direct influence on both collision frequency and collection efficiency. Important hydrodynamic parameters are bubble diameter, bubble velocity and turbulent energy dissipation (Newell and Grano, 2007). Ametov et al. (2008) argued that, molybdenite particles, due to their peculiar shape factor, could be more sensitive to hydrodynamic effects than copper mineral particles. In an agitated slurry, platelet shaped molybdenite particles may align along streamlines of the suspending liquid and, therefore, have lower probability of collision with bubbles. In effect, this hypothesis purports that the hydrodynamic diameter is the minimum dimension of the platelets. Increasing turbulence would increase collision frequency and efficiency, and therefore increase the rate of particle collection. This could be achieved either by increasing the impeller rotational speed or by reducing the feed solids percent. Damping of local energy dissipation occurs in suspensions containing higher volume percent of solids, as higher volume percent of solids produces higher slurry viscosity, particularly in the case of interacting particles (Schubert, 1999). Reducing the feed solids percent reduces the slurry viscosity and increases turbulence, having in turn a positive effect on particlebubble collision efficiency (Ec). 1.3. Inherent hydrophobicity (b)
Fig. 2. Crystal structure of molybdenite (from Lince and Frantz, 2000).
Due to the cleavage mechanisms of molybdenite, very thin particles are potentially produced in grinding. The overall degree of hydrophobicity of molybdenite particles depends on the relative surface exposure of faces (hydrophobic) and edges (hydrophilic). In flotation, molybdenite particles with higher exposure of edges will have lower probability of attachment to air bubbles. On the contrary, particles with a high face/edge ratio will have higher probability of being recovered, in agreement with the findings of Triffett and Bradshaw (2008). The overall degree of hydrophobicity of molybdenite particles can also be reduced by the adsorption of metal ions in solution (Raghavan and Hsu, 1984). Molybdenite has negative zeta
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potential across a wide pH range (Wie and Fuerstenau, 1974). However, adsorption of positively charged ions at the edges may reduce the magnitude, or even reverse the sign of the zeta potential. Calcium ions, in particular, have been shown to adsorb in the intermediate to high pH range (Healy, 1984). Adsorbed Ca2+ ions on the particle edges may reduce the contact angle and flotability of molybdenite particles. Furthermore, significant deformation of particles may occur in tumbling mills (Hoover, 1980), and it is common to find bent, distorted, or striated molybdenite particles in the flotation feed (Triffett and Bradshaw, 2008). Under these conditions, higher exposure of edges may occur, due to the breakage of the covalent bonds, and the spatial distribution and orientation of the hydrophilic edges may determine the probability of attachment of particles to air bubbles on collision. Oily collector is typically added in molybdenite flotation to enhance the mineral's hydrophobicity. The presence of exposed hydrophilic edges on the bent and distorted particles may also prevent spreading of oil droplets on hydrophobic surfaces, thus reducing the effect of the collector. 1.4. Inter-particle interactions with gangue minerals (c) The high reactivity of molybdenite edges may also determine particle–particle interactions with other minerals in the slurry, in the form of slime coatings. Raghavan and Hsu (1984) showed that, in the presence of Ca2+ ions, the addition of silica to a system of molybdenite particles causes a sharp decrease in molybdenite flotation recovery. It is possible that the calcium ions act as a ‘bridge’, favouring adhesion between negatively charged molybdenite and silica particles. This mechanism may be relevant in the flotation of an ore with specific gangue mineralisation and dissolution of ions. Inter-particle interactions between some of the gangue minerals and molybdenite may cause slime coatings on the latter, reducing its flotation recovery. This effect could be significant at high pH and high alkalinity, due to the bridging effect of the adsorbed Ca2+ ions. This possibility is explored in this paper. 1.5. Froth phase recovery (d) Inherent hydrophobicity and shape may also play a role in the transportation of molybdenite particles across the froth phase. Dippenaar (1982) and Hemmings (1981) found a correlation between size and hydrophobicity of the suspended particles and destabilisation of the froth phase by film rupture. For a given particle size, the greater the contact angle, the greater is the destructive compressive stress induced on the thin film (Hemmings, 1981). This implies that intermediate to coarse particles having contact angle greater than 90° are effective film breakers in froth flotation (Dippenaar, 1982). Flat and elongated molybdenite particles may fall into this category, and have, for this reason, lower recovery across the froth phase (Zanin et al., 2009), also called froth recovery (Savassi et al., 1997). In a separate investigation (Zanin et al., 2008), it was shown that froth recovery in the roughers of a porphyry copper flotation plant decreases significantly down the bank (from 60% in the first cells to 20% in the last cells), and that froth recovery of molybdenite is generally lower than froth recovery of the copper sulphide. Stability of the froth phase is therefore a factor which should be taken into consideration at plant scale, particularly in the rougher/scavengers, where major losses of coarse molybdenite occur. In this paper, however, the focus was on identifying factors which affect molybdenite recovery in the collection zone, while froth phase issues have been addressed elsewhere (Zanin et al., 2008). In the current study, investigation was carried out at Kennecott Utah Copperton Concentrator. Plant surveys have been undertaken on flotation feeds having different mineralogy, and ancillary laboratory flotation tests have been carried out plant slurries. Tests on controlled
Table 1 Average head grade of quartzite and limestone skarn ore samples.
Quartzite Skarn
Cu [%]
Fe [%]
Mo [ppm]
S [%]
SiO2 [%]
Al2O3 [%]
CaO [%]
K2O [%]
MgO [%]
0.50 0.45
1.9 9.6
600 21
0.4 1.5
63 44
13.8 1.7
2.0 24
7.0 0.3
5.0 2.7
flotation feeds (synthetic mineral mixtures and blends of different ore types) have been carried out at the Ian Wark Research Institute.
2. Materials and methods 2.1. Ores The Kennecott Utah Copperton concentrator treats porphyry copper ore, in which molybdenite occurs as a minor phase. Typically, Mo concentrations range from 0.02% to 0.06%. Distinct zones of mineralisation and alteration are mined at Kennecott, such that four main ore types have been described (Triffett and Bradshaw, 2008), including a quartzite ore showing high molybdenum grade and recovery and a more difficult to float, low grade, limestone skarn ore. In the current study, plant surveys have been undertaken with the plant processing blends of the four main ore types in different proportions, and laboratory flotation tests have been performed both on plant slurries and controlled blends of quartzite and skarn ores, prepared at laboratory scale. Average head grades of the quartzite and skarn ore samples are reported in Table 1, and the mineralogical composition (modal distribution by QEM-Scan) in Table 2. The two ores present distinct gangue minerals: large amounts of quartz and feldspar in the quartzite ore sample, and actinolite, andradite, talc and calcite abundant in the limestone skarn ore sample (shown in Table 2 as the Skarn ore). Single mineral molybdenite samples from WILLYAMA-GEO Discoveries were used to produce coarse molybdenite particles which were used for contact angle measurement by the sessile drop method.
2.2. Reagents The reagents used in this study, both in the plant and laboratory tests, were: oily collector for molybdenite (generic diesel oil was used in the laboratory tests), at a typical addition rate of 22 g/t, dicresyldithiophosphate (S-8989) collector for copper minerals, at a typical addition rate of 20 g/t, and methylisobutyl-carbinol (MIBC) as frother (15 to 35 g/t). The pH was adjusted by adding lime, with the exception of some diagnostic tests in which KOH was used, as outlined in 2.3.4. In the laboratory flotation tests carried out at the Ian Wark Research Institute, synthetic process water was used, prepared by dissolving
Table 2 Mineralogical analysis of quartzite and limestone skarn ore samples. Mineral
Formula
Modal [%] Quartzite
Skarn
Quartz K_Feldspar Plagioclase Montmorillonite Biotite Actinolite Andradite (Al) Talc Calcite
SiO2 KAlSi3O8 NaxCa(1 − x) (Al,Si)AlSi2O8 (Na,Ca)(Al,Mg)6(Si4O10)3(OH)6 − nH2O K(Fe,Mg)3AlSi3O10(F,OH)2 Ca2(Mg, Fe)5Si8O22(OH)2 Ca3Al2(SiO4)3 Mg3Si4O10(OH)2 CaCO3
24.3 38. 7 8.1 4. 7 15.0 0.1 0.14 0.01 1.0
10.9 1.0 0.02 0.04 0.06 8.2 58.7 0.08 4.9
Modal mineral distribution for the major gangue minerals determined by QEM-Scan.
M. Zanin et al. / Int. J. Miner. Process. 93 (2009) 256–266 Table 3 Composition of synthetic process water compared with a sample of process water from the KUCC copper circuit. Ion
Na+ Ca++ K+ Mg++ Cl− SO−− 4 HCO− 3 TDS pH Cond. [μ S/cm]
Synthetic H2O
Process H2O
[ppm]
[ppm]
1308 798 87 129 1818 2653 189 6980 7.6 6700
1310 798 87 129 1940 2590 160 7010 7.2 8000
salts in demineralised water to reproduce the composition of Kennecott process water (Table 3). 2.3. Methods 2.3.1. Plant surveys The Copperton concentrator consists of grinding circuits (SAG milling followed by ball milling) and two flotation circuits: a bulk copper flotation circuit, where copper/molybdenum concentrate is produced, and a molybdenite flotation plant, in which molybdenite in the bulk concentrate is separated from the copper minerals. At the time of investigation, the copper circuit consisted of 5 parallel rows of rougher/scavenger cells. The rougher concentrate was treated in rougher cleaners (2 parallel rows of cells) without regrinding. The scavenger concentrate reported to regrinding (ball milling) along with the rougher cleaner tailing. The regrind circuit product was upgraded in three scavenger cleaner stages, the final concentrate from which, combined with the rougher cleaner concentrate, produced the bulk copper/molybdenum concentrate. The target d80 of the flotation feed was 200 μm, and the pH in the rougher/scavengers was controlled between 9.5 and 10. A bulk concentrate assaying about 25–30% Cu and 2–4% Mo was produced, depending on the ore blend processed. Since most of the molybdenite losses occurred in the rougher/scavengers of the copper circuit, this part of the circuit became the focus of investigations. Four plant surveys were undertaken, in which timed lip samples and in-pulp samples were collected from each cell down one of the rougher/scavenger rows. In each survey, four sampling rounds were performed, over a period of 90 min, in which multiple samples were collected from the rougher/scavenger tailings to reduce experimental error. Prior to sampling, plant stability was ensured from the control room, and plant data (throughput, feed solids percent, air flows, and lip levels) were recorded. All metallurgical samples were initially assayed unsized. Samples from selected surveys were sized by wet/ dry sieving and assayed on a size-by-size basis. Data reconciliation and mass balance were performed using the software Bilmat 9.3™. Surveys 1 and 2 were performed with the plant processing a blend with high quartzite ore content, while surveys 3 and 4 were carried out with the plant processing a blend with high limestone skarn ore content. Surveys 1 and 4 were performed at lower solids percent in the flotation feed, while surveys 2 and 3 were carried out at higher solids percent (Table 4). The percent solids, which under standard operating conditions ranged between 30% and 35%, was adjusted by varying the water flowrate to the rougher flotation distribution box. The influence of feed solids percent on the recovery of molybdenite from the two different ore blends was studied. During survey 1, additional samples of slurry from the concentrate of cells 1 and 11 were collected for surface analysis by ToF-SIMS. Samples were placed in plastic vials, purged with nitrogen to remove oxygen and frozen in liquid nitrogen. The samples were stored frozen
259
in a cryogenic container until ToF-SIMS analysis was carried out. Since the focus of the study was recovery of coarse molybdenite, the ToFSIMS analysis was carried out on the +150 μm particles, which were isolated by wet sieving. The surface chemistry of the fast (concentrate from cell 1) and slow (concentrate from cell 11) floating particles was investigated, in order to correlate possible differences in surface composition to flotation response. It was not possible to analyse molybdenite in the flotation tailings, which ideally contain the nonfloating molybdenite, due to the extremely low molybdenum grade, i.e. a statistically significant number of molybdenite particles could not be analysed. No samples of slurry for ToF-SIMS analysis were collected during surveys 3–4. Therefore, it was not possible to compare the surface composition of molybdenite in surveys with different feed ore blends and gangue mineralogy. This important aspect of the study was, however, conducted on samples generated at laboratory scale, as described further below. 2.3.2. Laboratory flotation tests on plant slurries In parallel with the surveys, laboratory flotation tests were undertaken on conditioned rougher feed slurries collected in the plant. The tests were conducted using a 5 l Agitair flotation machine with forced air supply. The samples were taken from the flotation feed box where all reagents except frother were added. An impeller speed of 1000 rpm and air flowrate of 5 l/min were used in the tests. The slurry (conditioned rougher feed) was floated without further collector addition, and at the same pH (9.5–10) as the plant. Concentrates were collected after 1, 3, 6 and 10 min. All the flotation products were assayed on an unsized and size-by-size basis. Unsized and size-by-size recoveries were calculated. 2.3.3. Laboratory flotation tests on reconstructed feeds In diagnostic tests carried out at the Ian Wark Research Institute, samples of quartzite and limestone skarn ores provided by Kennecott were used to reproduce plant slurries. Three different controlled feed blends were prepared: 100% quartzite, 75:25 quartzite/skarn and 50:50 quartzite/ skarn ores. The ore blends were crushed using laboratory jaw and cone crushers, homogenised and ground in a laboratory Galigher tumbling mill, using stainless steel rods as grinding media. Grinding was calibrated to a target d80 = 200 μm, by varying the grind time for each feed type. Synthetic process water (Table 3) was used to simulate plant conditions. The same pH and reagents scheme were used as in the tests carried out at Kennecott. Flotation tests were carried out as described in 2.3.2. Similarly to the plant surveys, after each laboratory flotation test the coarse particles (+150 μm) from the first concentrate and the last concentrate were collected by wet screening and analysed by ToFSIMS. Surface analysis was conducted for the concentrates collected in flotation tests on both 100% quartzite ore and 50:50 blend of quartzite and skarn ores, to draw a link between molybdenite surface composition, gangue mineralogy and flotation response . 2.3.4. Laboratory flotation tests on coarse molybdenite particles The effect of calcium and magnesium ions in solution and gangue minerals on the recovery of molybdenite was studied separately. Coarse molybdenite particles (+150 μm) were obtained by dry screening of a molybdenite concentrate from Kennecott (final product Table 4 Feed composition and operating conditions during the plant surveys. Ore blend type
Cu
Mo
Survey
Solids
[%]
[%]
#
[%]
High quartzite
0.3
0.06
High skarn
0.5
0.06
1 2 3 4
27 35 35 27
pH
9.8 10.0 10.1 10.1
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of the molybdenum plant). Particles were washed in Ensolv (n-propyl bromide N 93%) and ethanol to clean the surface from any residual collector. Samples of quartzite and skarn ore were floated to remove copper minerals and molybdenite, and the flotation tailings were collected. The coarse molybdenite particles were then blended into the tailings of quartzite and skarn ores, to produce synthetic ore containing 0.15% MoS2 by weight. Samples were floated, with the collectors and frother added according to the conditioning scheme described in 2.2. Low conductivity water, produced by reverse osmosis, two stages of ion exchange and two stages of activated carbon prior to final filtration, was used in these experiments. The water pH was adjusted to 10 by adding KOH, and the concentration of ions was varied by adding Ca(NO3)2 and Mg(NO3)2. 2.4. Characterisation techniques 2.4.1. Contact angle of molybdenite particles Large molybdenite crystals were obtained from WILLYAMA-GEO Discoveries. Molybdenite crystals were cut and cleaved using a scalpel, to expose fresh crystal edges and faces. The contact angle was measured at different pH values and Ca2+ concentrations in the ranges typically observed in the plant (8–11 for pH and 0–10− 2 M for Ca2+ ions). After conditioning, the water advancing and receding contact angle of face and edge of individual molybdenite particles were measured by the sessile drop method. 2.4.2. ToF-SIMS TOF-SIMS spectra were obtained using a PHI TRIFT II System equipped with a gallium liquid metal ion gun (LMIG) in pulsed mode. For insulating samples the surface charge is compensated using a pulsed electron flood gun. The mineral samples were mounted on to indium foil. The primary beam current employed in the present study was 600 pA (DC measurement). In static mode, the analysis is confined to the top two monolayers. An excitation voltage of 25 kV was used in un-bunched mode to give a spatial resolution of better than 0.5 μm and pulse length adjusted to give a mass resolution (m/ Δm) of ~ 4000. Imaging of the sample involved mapping for positive and negative ions for surface regions of mineral particles of interest, i.e. MoS2. Several tens of particles in each processing stream are analysed and the data statistically presented in terms of normalised (to total ion yield) intensity of signals and 95% confidence intervals. In this mode, statistical differences in surface chemistry between the same minerals in different streams or under different pulp conditions may be discerned. In the present case, we are investigating surface chemistry differences between floating and non(slow)-floating MoS2 particles. 3. Results 3.1. Plant Studies and Laboratory Tests on Plant Slurries The grade/recovery relationship for copper and molybdenum for the four plant surveys and the laboratory flotation tests carried out in parallel with the plant surveys are reported in Fig. 3. The ultimate recovery (combined rougher/scavenger flotation) for copper and molybdenum is also reported in Table 5. In the plant surveys, molybdenum recovery ranged from 75% to 92%, while copper recovery from 85% to 92%. The recovery of molybdenum was consistently lower than copper recovery, the difference increasing at higher feed solids percent and in the presence of limestone skarn ore. For both copper and molybdenum, the lowest recovery was achieved at high solids percent and in the presence of limestone skarn ore (survey 3). Compared to copper, molybdenum recovery was more sensitive to the feed solids percent, being significantly enhanced when solids percent was reduced from 35% to 27%. Both copper and molybdenum recoveries were lower in the presence of limestone
skarn ore in the feed blend, but molybdenum recovery decreased more than copper recovery. With a reduction in the feed solids percent, however, it was possible to partially restore molybdenum recovery to values characteristic of quartzite ore (from 75% to 85%). The laboratory tests (Fig. 3c and d) showed generally higher recoveries for both copper and molybdenum compared to the plant surveys (Fig. 3a and b), but with a similar trend with respect to feed composition and solids percent. Copper recovery was marginally affected by changes in feed solids percent in the absence of limestone skarn ore, showing some difference (90% recovery at 35% solids versus 93% recovery at 27% solids) when limestone skarn ore was blended to flotation feed. Molybdenum recovery, on the contrary, was always higher at low solids percent. Differences in recovery became significant (93% versus 85%) in the presence of limestone skarn ore. Size-by-size analysis of the flotation products was undertaken for surveys 3 and 4 (feed blend with high skarn ore), in which a significant increase in molybdenum recovery was obtained by reducing the feed solids percent. Results (Fig. 4a) revealed increased molybdenum recovery across all size ranges, but particularly for the coarse size fractions (+150 μm). The effect on copper was much less pronounced. In the laboratory scale tests, the recovery of molybdenum was higher than in the plant across all size ranges. Furthermore, a reduction in the feed solids percent produced a significant effect on coarse (+150 μm) molybdenum bearing particles, for which recovery increased from 60% to 80% when the feed solids percent was reduced from 35% to 27%. Two additional flotation tests were undertaken at laboratory scale, at 30% and 45% solids in the rougher flotation feed. These concentrations were achieved by manipulating samples of plant feed collected during survey 3 (35% solids), the former by diluting with process water, the latter by filtering and re-suspending the solids in process water. The trend (Fig. 4d) confirmed what was observed in the previous tests, showing high sensitivity of the coarse molybdenite particles to feed solids percent. The feed solids percent is an important driver to molybdenite recovery overall, and particularly in coarse particle size fractions. In Fig. 4, bar charts reporting the distribution of copper and molybdenum in the feed are superimposed on the recovery curves. The area of each bar in the charts is proportional to the relative mass of mineral in the specified particle size range. Considering that, at the standard plant grind, the +150 μm size fraction contains about 20% of the total molybdenite in the feed, the recovery of this size fraction is critical to the overall molybdenite recovery. Surface analysis results on the slurry samples collected during survey 1 (in which the flotation feed consisted mainly of quartzite type ore, and no limestone skarn was present) are reported in Fig. 5. No significant difference in surface composition between fast (concentrate from Cell 1) and slow (concentrate from Cell 11) floating molybdenites was found in this particular case of the quartzite ore type. It could therefore be concluded that the smaller differences in molybdenite recovery noted between plant and laboratory scale (Fig. 3) may be ascribed to changes in hydrodynamics. However, this conclusion will depend on the feed type, as discussed further below. 3.2. Laboratory flotation tests on reconstructed feeds The flotation recoveries of copper and molybdenum in laboratory scale tests on blends of quartzite ore and skarn ore are reported in Fig. 6. A sharp decrease in molybdenite recovery was observed at increasing concentration of limestone skarn ore in the feed. The ultimate molybdenite recovery decreased from 92% in the absence of skarn ore to 85% with 25% skarn ore and 56% with 50% skarn ore in the flotation feed. Copper flotation was affected to a much lesser extent. This is in agreement with the plant surveys and tests carried out on plant slurries. It should be noted that, since the limestone skarn ore
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Fig. 3. Grade/recovery relationship for copper and molybdenum in the plant surveys (a and b) and in the laboratory tests on plant feed (c and d). The ultimate recovery at the completion of flotation is also indicated (dashed lines).
contains very little molybdenum (Table 1), it is the molybdenite particles in the quartzite ore which are depressed in the presence of skarn ore. The ToF-SIMS spectra for the fast floating molybdenite (Con 1) and slow floating molybdenite (Con 4) collected in the tests with 0% and 50% skarn ore in the flotation feed are reported in Fig. 7. Samples of fast (Con 1) and slow (Con 4) floating molybdenite in the presence of quartzite type ore only, showed very small differences in surface chemical composition. The different species appear in the same proportion on the surface of the two samples, within statistical error (Fig. 7). This is in agreement with previous
observations at plant scale, which also showed no surface chemical differences on molybdenite. In the presence of 50% skarn ore, on the contrary, samples of fast and slow floating molybdenites showed differences in surface chemistry (Fig. 7). In the fast floating sample (Con 1) there are much more exposed molybdenum and sulphur compared to the slow floating sample (Con 4). Furthermore, Ca, K, Fe, as well as O and OH groups, were more abundant on the latter sample. Significantly higher Mg on the surface of molybdenite in the presence of skarn ore was also noted in both fast and slow floating fractions (Fig. 7a). 3.3. Laboratory flotation tests on coarse molybdenite particles
Table 5 Ultimate recovery of copper and molybdenum in plant (after rougher/scavenger flotation) and laboratory. Ore blend type
Survey
Solids
Plant recovery
Lab recovery
#
[%]
Cu [%]
Mo [%]
Cu [%]
Mo [%]
High quartzite
1 2 3 4
27 35 35 27
92 92 85 88
92 88 75 85
95 94 90 93
95 92 85 93
High skarn
The effect of calcium and magnesium ions in solution on coarse molybdenite (+150 μm) recovery from quartzite and limestone skarn ores is presented in Fig. 8. In the case of quartzite ore, the cumulative recovery of molybdenite was high, approaching 90% after 8 min of flotation. The addition of calcium and magnesium ions to the pulp did not have an effect on molybdenite flotation. In contrast, molybdenite recovery from limestone skarn ore was considerably lower (60%) even in the absence of calcium and magnesium ions, and decreased further to 30% when Ca2+ and Mg2+ ions were added. Apparently, there is a
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Fig. 4. Size-by-size recovery of copper and molybdenum in plant surveys 3 and 4 (after rougher/scavenger flotation) (a and b) and in the laboratory flotation tests on plant feed (c and d). At laboratory scale, additional feed solids percent values (30% and 45%) were obtained by manipulating plant feed samples. The distribution of copper and molybdenum in the feed is also reported. Feed blend contained high limestone skarn ore.
synergistic effect between the presence in solution of Ca2+ and Mg2+ ions and gangue minerals of the skarn ore. The effect of pH and calcium ions on the contact angle of molybdenite was also investigated (Fig. 9). The tests showed that faces and edges of molybdenite particles have considerably different contact angles. Face contact angle was high (about 100°) and independent of solution pH and calcium concentration. In contrast, contact angle on the edges was low (maximum 45°), and decreased with increased pH and calcium ion concentration ([Ca2+] N 4 × 10− 3 M). 4. Discussion From an industrial perspective, the most important finding from the plant studies was the consistent increase in molybdenite recovery in rougher/scavenger flotation at reduced feed solids percent. This is in agreement with previous observations for different flotation plants (Ametov et al., 2008). The coarse (+ 150 μm) size fractions were affected the most by changes in feed solids percent, and therefore fluctuations in the overall recovery of molybdenite in the plant are determined to a large extent by the flotation response of the coarse
particles. Particle size and shape, morphology and surface composition are all contributing factors. Insufficient molybdenite liberation in the coarse size fractions was also considered in the first instance as a possible cause for the low flotation recovery. However, QEM-Scan analysis of the scavenger tailings (Triffett, private communication) showed that molybdenite in the + 150 μm size fraction mainly consists of liberated particles. This confirms that other mechanisms, as outlined in Section 1.1, play significant roles. The hydrodynamic behaviour of the flat molybdenite particles in a turbulent environment may explain the increase in molybdenum recovery observed at reduced solids percent (Fig. 3). Higher local turbulent energy dissipation may increase the collision efficiency for the flat and elongated particles, as hypothesised in Section 1.2. The fact that molybdenite recovery in laboratory flotation tests was higher than in the plant (Fig. 4) may also be due hydrodynamic effects. Compared to the plant cells, laboratory batch flotation cells have higher local turbulent energy dissipation (Newell and Grano, 2007), and therefore higher collision efficiency, Ec, and, according to Eq. (1), higher flotation rate. An interesting feature is that it is the coarse and liberated molybdenite that is mainly affected, the recovery of which is
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263
Fig. 5. Positive (a) and negative (b) ToF-SIMS normalised intensities for + 150 μm molybdenite particles. Average with 95% confidence, N N 23. Fast floating (Cell 1) and slow floating (Cell 11) molybdenite from plant survey 1 (no skarn ore in the feed).
Fig. 6. Copper and molybdenum recovery in laboratory flotation tests on reconstructed feeds (blends of quartzite and skarn ores in different ratios). Tests at 35% solids (synthetic process water used); Agitair 5 l cell; impeller speed 1000 rpm; air 7 l/min. 0% skarn ore = 100% quartzite ore.
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Fig. 7. Positive (a) and negative (b) ToF-SIMS normalised intensities for on + 150 μm molybdenite particles. Average with 95% confidence, N N 23. Fast floating (Con 1) and slow floating (Con 4) coarse particles (N 150 μm) in tests with 100% quartzite ore and 50:50 blend of quartzite and skarn ores.
presumably limited by stability efficiency, Es (Pyke et al., 2003). The high surface/bulk ratio of the coarse molybdenite particles may however be beneficial for the stability of particle–bubble aggregates, because of a higher surface force/inertial force ratio. Surface forces favour attachment of particles to bubbles, while inertial forces promote detachment. A high surface/bulk ratio results in high stability efficiency, even in high turbulent conditions such as in laboratory flotation and at reduced solids percent. Another outcome of the plant studies was that the effect of a reduction in feed solids percent on molybdenite recovery was high for some feed blends (high limestone skarn) while it was much lower for others (high quartzite). This was observed at plant scale (Fig. 3b), in laboratory flotation tests on plant slurries (Fig. 3d), and in tests on reconstructed feed blends (Fig. 6). Therefore, it is likely that factors other than hydrodynamics and collision efficiency, such as surface modification and particle–particle interactions also play a role. ToFSIMS analysis indicated the presence of elements associated with hydrophilic components on the slow floating coarse molybdenite particles in the presence of skarn ore (Fig. 7). This could be either from the deposition of fine slimes or adsorption of dissolved ions. In any case, the effect is strongly ore type dependent. In the absence of skarn ore, no difference in surface composition between fast and slow molybdenite was apparent at plant (Fig. 5) and laboratory (Fig. 7)
scales, suggesting that the different flotation recoveries of molybdenite at high and low solids percent may be ascribed to hydrodynamic effects, possibly arising from the intrinsic shape of the particles, rather than from the surface cleaning of slime coatings. Therefore, the hypothesis of slime coatings may hold for some feed types and be less important for others. It is surmised that at low pulp density there is less likelihood of inter-particle interactions causing slime coatings. A reduction in slime coatings on the surface of molybdenite particles results in increased contact angle. Even a small increase in contact angle could cause previously non-floatable coarse particles to become floatable and the ultimate recovery is therefore increased. in Kennecott process The high concentration of Ca2+ and SO2− 4 water (Table 3) may also suggest deposition of calcite and gypsum from solution due to oversaturation. This could also be a contributing factor, but it is not sufficient to justify the different degrees of molybdenite depression observed in the presence of quartzite and skarn ores. Diagnostic flotation tests on coarse molybdenite particles in the presence of limestone skarn ore and different levels of calcium and magnesium ions in solution (Fig. 8) corroborate the hypothesis that interactions between molybdenite and some gangue minerals in the skarn ore lead to molybdenite depression. The effect could be induced by metal ions adsorbed at the molybdenite edges, as also suggested by contact angle measurements carried out separately on
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5. Conclusions
Fig. 8. Recovery of coarse molybdenite particles (+ 150 μm) in flotation tests on synthetic feeds. Molybdenite particles from the final concentrate, after surface cleaning, were blended with ground quartzite and skarn ore (0.15% MoS2 by weight). Tests were in demineralised water, at pH 10, in the presence and absence of Ca2+ and Mg2+ ions.
faces and edges (Fig. 9). The edges of molybdenite particles are particularly reactive, and adsorption of Ca2+ and/or Mg2+ may have the combined effect of reducing the overall hydrophobicity of molybdenite particles and “bridging” for particle–particle interactions with the negatively charged gangue minerals. This is also in agreement with the findings of Raghavan and Hsu (1984) on molybdenite/silica mineral systems. Slime coatings are possibly causing molybdenite depression in the presence of skarn ore. Minerals which are much more abundant in the limestone skarn ore are: actinolite, andradite, talc and calcite (Table 2). These minerals are potentially responsible for the high Mg, Ca and Fe observed on the surface of molybdenite when floated in the presence of limestone skarn ore (Fig. 7). This conclusion would be consistent with the fact that the presence of the above mentioned minerals in the flotation feed has been correlated to periods of low molybdenite recovery (Triffett et al., 2008).
The experimental results confirm that the flotation of molybdenite from porphyry copper ores is more sensitive to the operating environment than that of the copper sulphide minerals. The recovery of molybdenum in the rougher/scavengers of the bulk Cu/Mo flotation circuit at Kennecott Utah Copper is highly variable, and always lower than the recovery of copper. Particularly low molybdenum recovery was observed in the presence of limestone skarn ore. As a general rule, it was found that operating the rougher/scavenger flotation rows at a lower feed solids percent (27% against 35%) ensures higher and more stable molybdenite recovery. Several factors have been identified which could affect molybdenite flotation, all of them related to the peculiar properties of molybdenite. Preferential cleavage along the weakly bound S–S layers may impart to the molybdenite particles singular hydrodynamic behaviour (low collision efficiency due to the flat and elongated particle shape), and anisotropy in hydrophobicity (hydrophobic faces and hydrophilic, highly reactive, edges). It has been shown that edges have much lower contact angle compared to faces, in particular at high pH (pH N 10) and high alkaline conditions ([Ca2+] N 10− 2 M), as in typical flotation environment. When limestone skarn ore was blended to the feed in laboratory flotation tests, molybdenite recovery decreased significantly, similarly to what was observed in the plant. Higher concentrations of Ca, Fe, Mg and K were measured on the slow floating molybdenite particles compared to the fast floating, which have been correlated to the presence of some gangue minerals typical of the skarn ore. A ‘bridging’ effect of calcium and magnesium adsorbed on the surface of molybdenite and some fine gangue particles in the slurry is possible and may lead to the formation of slime coatings, thus reducing the flotation recovery of molybdenite. In terms of strategies to increase molybdenite recovery at plant scale, operating the rougher/scavengers at low solid percent was a solution which gave immediate benefits. This benefit may be a result of a combination of hydrodynamic effects (higher local turbulent energy dissipation and collision efficiency), rheology and reduced formation of slime coatings. It also appears that preventing slimes from adsorbing on molybdenite particles during flotation may benefit molybdenite recovery. This is the subject for future investigation. Acknowledgements The authors would like to acknowledge the financial support of the sponsors of the AMIRA P260E project and the Australian Research Council (ARC). Support from the Management at Kennecott Utah Copper and site personnel is also gratefully acknowledged. References
Fig. 9. Contact angle (advancing) of coarse molybdenite particles (+150 μm) measured in proximity of face and edges, after conditioning at different pH values and Ca2+ concentrations. Contact angle measured by means of the sessile drop method.
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