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Mechanisms for the improved flotation of ultrafine pentlandite and its separation from lizardite in saline water
Minerals Engineering 36–38 (2012) 284–290
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Mechanisms for the improved flotation of ultrafine pentlandite and its separation from lizardite in saline water Yongjun Peng a,⇑, Dee Bradshaw b a b
School of Chemical Engineering, University of Queensland, St. Lucia, Brisbane, QLD 4072, Australia Julius Kruttschnitt Mineral Research Centre, University of Queensland, Isles Road, Indooroopilly, Brisbane, QLD 4068, Australia
a r t i c l e
i n f o
Article history: Available online 19 June 2012 Keywords: Ultrafine flotation Pentlandite Lizardite Saline water Particle interactions
a b s t r a c t In this work, the flotation of ultrafine pentlandite and its separation from lizardite in de-ionized water, and bore water of high ionic strength were studied. In de-ionized water, the flotation separation was poor due to low pentlandite recovery and high lizardite entrainment. However, bore water increased pentlandite flotation recovery while reducing lizardite entrainment. The possible mechanisms responsible for the improved flotation separation in bore water were investigated by electrokinetic studies, Scanning Electronic Microscopy (SEM) analysis and settling tests. It was found that the reduction of electrical double layer forces between particles in bore water might mitigate the coating of lizardite particles on pentlandite surfaces resulting in the improved pentlandite flotation. The reduction of electrical double layer forces might also induce the aggregation of lizardite particles and therefore enhance lizardite rejection. This study provides a new direction to address slime coating and high gangue entrainment in ultrafine mineral flotation by electrolytes. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction The flotation of pentlandite minerals in Western Australia has historically been difficult due to low nickel grade and a large amount of serpentine minerals in deposits. At the largest nickel deposit, Mt. Keith Mine, nickel grade of the feed is about 0.6%, while MgO grade is about 38% equivalent to about 90% serpentine minerals. While lizardite is the major serpentine mineral, antigorite and chrysotile, the fibrous serpentine minerals, also occur in some minor ore bodies. To increase pentlandite flotation and its separation from serpentine, split flotation has been implemented at major pentlandite flotation plants. The grinding discharge is pumped to cyclone classification from which underflows and overflows pass to flotation streams dealing with different sizes. Nickel recovery in the ultrafine flotation stream ( 8 lm) is usually less than 70% when a normal lizardite ore is treated, which has been attributed to the low overall nickel recovery (Senior and Thomas, 2005). When a fibrous ore is treated, effective flotation in the ultrafine stream is not possible. Another characteristics of these flotation plants is the use of hyper saline water as only bore water is available. Since flotation mechanisms in saline water are not clear and most flotation plants in the world use fresh water, there has been debate over whether fresh water should be introduced to the pentlandite flotation plants to increase nickel recovery and grade. ⇑ Corresponding author. Tel.: +61 7 3365 7156; fax: +61 7 3365 3888. E-mail address: [email protected] (Y. Peng). 0892-6875/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.mineng.2012.05.015
However, previous studies based on the flotation of ultrafine fractions of a normal ore obtained from the Mt. Keith Mine indicated that pentlandite flotation was very poor in de-ionized water with high lizardite gangue entrainment, whilst bore water improved pentlandite flotation and reduced lizardite entrainment significantly (Peng and Seaman, 2011). The objective of this current research is to identify possible mechanisms responsible for the improved ultrafine pentlandite flotation and the reduced ultrafine lizardite entrainment in saline water. The separation of pentlandite from serpentine by froth flotation has been studied in fresh water by a great number of researchers. Edwards et al. (1980) investigated the effect of lizardite and chrysotile on the flotation of pentlandite. They found that pentlandite flotation was depressed in the presence of lizardite or chrysotile. By using electrophoresis and Scanning Electron Microscopy (SEM) measurements, they identified the slime coating of serpentine minerals on the pentlandite surface which was attributed to the depressed pentlandite flotation. This mechanism was also identified by Bremmell et al. (2005) using flotation, zeta potential measurements, SEM and Atomic Force Microscope (AFM) analysis on a mineral system consisting of pentlandite and lizardite. The slime coating results from the electrostatic attraction between negatively charged pentlandite and positively charged lizardite and chrysotile in neutral and weakly alkaline flotation conditions. Edwards et al. (1980), Bremmell et al. (2005) and Pietrobon et al. (1997) used negatively charged chemical additives including carboxy methyl cellulose (CMC) to modify the charge of serpentine
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minerals and therefore improved pentlandite flotation. Edwards et al. (1980) demonstrated the mitigated chrysotile coating on the pentlandite surface in the presence of CMC by SEM measurements, while Bremmell et al. (2005) demonstrated a reduced attraction between lizardite and pentlandite in CMC solutions. Mukund et al. (2010) studied the effect of serpentine fibers on pulp rheology and the flotation of copper sulfide minerals. They observed the alteration of pulp rheology and flotation performance simultaneously by serpentine fibers. An arrested system was observed when the content of serpentine fibers in the ore reached a high value dramatically reducing the copper flotation recovery. This change was proposed to be due to the strong network formation of the fibrous serpentine minerals that locked copper minerals and prevented bubble dispersion and/or collector–mineral interaction. The mechanism proposed in the literature to be responsible for the negative effect of serpentine minerals on pentlandite or copper mineral flotation may explain the poor flotation behavior of pentlandite in de-ionized water in previous studies (Peng and Seaman, 2011), but the mechanisms responsible for the improved pentlandite flotation and lizardite rejection in saline water are not clear. In fact, the beneficial effect of saline water on coal flotation has also been observed and a number of studies have been conducted to investigate the mechanisms. In general, the improved coal flotation in saline water compared to fresh water has been attributed to the increased bubble–particle attachment. Yoon (1982) and Paulson and Pugh (1996) proposed that reduced bubble sizes and increased population in electrolyte solutions increased the encounter efficiency of bubble–particle attachment. Fuerstenau et al. (1983) and Yoon and Sabey (1989) attributed the increased bubble–particle attachment to the reduction of zeta potential of both bubbles and particles resulting from the compression of electrical double-layer in the presence of electrolytes. Another mechanism proposed is that the inorganic electrolytes destabilized the hydrated layers surrounding coal particles and reduced their surface hydration therefore enhancing the bubble–particle attachment (Klassen and Mokrousov, 1963). Nanobubbles or nanopancakes were also observed to form on coal surfaces in electrolyte solutions facilitating bubble–particle attachment (Mishchuk, 2005; Zhang and Ducker, 2007). Studies in coal flotation in saline water have not taken into account the behavior of gangue minerals. It is unlikely that the increased bubble–particle attachment is a major contributor to the improved flotation of pentlandite against lizardite in saline water in previous studies since it cannot explain the decreased serpentine entrainment. The increased bubble–particle attachment in saline water should increase the collection of serpentine minerals as well. Furthermore, in pentlandite flotation, serpentine minerals play an important role due to the surface coating (Edwards et al., 1980; Bremmell et al., 2005) or the changed pulp rheology (Mukund et al., 2010). The improved pentlandite flotation in saline water may be associated with the modified behavior of lizardite minerals. The entrainment of fine gangue minerals through water films between air bubbles is a well-known problem in flotation (Trahar and Warren, 1976; Warren, 1985). The entrainment of gangue minerals is significant at a particle size smaller than 30 lm and the smaller the particle size, the higher the entrainment (Trahar and Warren, 1976; Warren, 1985). As a result, the size of particles
plays an important role in mechanical entrainment of particles in mineral flotation. To reduce gangue entrainment in flotation, a high depth of froth has been used to allow the drainage of gangue minerals from froth (Neethling and Cilliers, 2009). However, in the flotation of ultrafine nickel streams in Western Australia, very shallow froth is generated. Similar shallow froth was generated in flotation using both saline water and de-ionized water in the laboratory in the previous study (Peng and Seaman, 2011). It is unlikely that bore water produced a higher depth of froth resulting in the reduced lizardite entrainment. It has been demonstrated that mechanical entrainment of fine gangue particles in flotation may be reduced by inorganic coagulants or organic flocculants. Liu et al. (2006), Cao and Liu (2006) and Gong et al. (2010) show a relationship between the reduced entrainment of various fine gangue minerals and their aggregated sizes indicated from sedimentation tests. Tao et al. (2007) and Xu et al. (2012) demonstrated the reduced entrainment of clay minerals in the flotation of oil sand, coal and potash through the aggregation of clay minerals by polymers. The reduced lizardite entrainment in flotation in saline water may be associated with the aggregation of particles. 2. Experimental 2.1. Materials and reagents Pentlandite (NiFeS2), and lizardite (Mg3Si2O5(OH)4) pure minerals were obtained from Ward’s Natural Science Establishment (US). Lizardite is the major serpentine mineral in pentlandite deposits in Western Australia, in particular, Mt. Keith Mine which was targeted in the previous study (Peng and Seaman, 2011). These samples were crushed through a roll crusher and then screened to collect the +0.6–3.2 mm particle size fraction. The processed samples were then sealed in polyethylene bags purged with nitrogen gases as mill feeds. Sodium ethyl xanthate and H407 (a type of polyglycol ether), industrial grade, were used as the collector and frother, respectively. They have been used in pentlandite flotation plants in Western Australia and a number of studies indicate that they are the best combination in the flotation. Other chemicals used in this research were AR grade. Bore water was obtained from a pentlandite flotation plant in Western Australia. The chemical analysis of the bore water is shown in Table 1. 2.2. Grinding and flotation Mineral grinding and flotation were conducted at room temperature, 25 °C. Pentlandite (100 g), lizardite (100 g) or their mixture (100 g altogether) was combined with 0.15 dm3 of bore water or de-ionized water and ground with 10 kg of stainless steel rods in a laboratory stainless steel mill such that 90% of the particles present were less than 8 lm in diameter. After grinding the pulp was transferred to a 1.5 dm3 flotation cell and then conditioned with collector (100 g/t) and frother (10 g/t). The solid percentage in flotation was about 6.5%. A similar solid percentage has been used in the flotation of the ultrafine stream in the nickel flotation plants in Western Australia. In flotation, five concentrates were collected after cumulative times of 1, 2, 4, 8 and 16 min at an air flow rate of 2.5 dm3 min 1. The flotation froth was scraped every 10 s. Both de-ionized and saline water were tested in this study. The chosen water type was utilized in all stages of grinding and flotation.
Table 1 Composition of the bore water (mg/L). Na
K
Ca
Mg
SiO2
Cd
Cr
Cu
Ni
Cl
SO4
20,000
940
400
5100
4.9
0.002
0.001
<0.005
0.033
32,000
23,000
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When bore water was used, the pH during grinding and flotation was constant, about 9.0, due to the buffer effect of the water. When de-ionized water was used, NaOH solutions were used to maintain pH at 9.0 during grinding and flotation. The grinding and flotation procedure is similar in this study where single minerals are used and in the previous study where a real nickel ore was used. 2.3. Techniques 2.3.1. Scanning electron microscopy Samples before the flotation of pentlandite–lizardite mixtures were taken, washed with de-ionized or saline water and then analyzed by a CAMSCAN CS44FE scanning electron microscope (SEM) fitted with a Camscan Energy Dispersive X-ray spectrometer (EDXS). During analysis, a focused electron beam was rastered across the sample surface. The secondary or backscattered electrons produced were used to map the surface topography and compositional contrast. EDXS was used to provide a means of elemental identification on the surface of interest.
35
Cumulative MgO recovery (%)
286
Bore water1 Bore water2 De-ionised water1 De-ionised water2
30 25 20 15 10 5 0 0
20
40
60
80
Cumulative Ni recovery (%) Fig. 1. Repeat flotation tests on the mixture of 10% pentlandite and 90% lizardite in de-ionized water and bore water.
Cumulative Ni recovery (%)
90
2.3.2. Zeta potential measurement Particle electrophoretic mobility was measured with a Rank Brothers Microelectrophoresis Mark II apparatus. Single pentlandite or lizardite mineral particles were ground in a ceramic mill to about 8 lm. Particles were conditioned in a reaction vessel with de-ionized water mixed with saline water at pH 9.0 for 30 min. Then pH was adjusted by the addition of NaOH solutions. 10 mobility measurements at each of the two stationary planes were performed at each pH value. The average mobility was converted to the zeta potential using the Smoluchowski equation.
75 60 45 30
Bore water: De-ionised water: 0% lizardite 0% lizardite 10% lizardite 10% lizardite 50% lizardite 50% lizardite 90% lizardite 90% lizardite
15 0 0
2.3.3. Settling tests The particle slurry was conditioned in the flotation cell and then transferred to a 1.5 dm3 graduated cylinder. The cylinder was then stoppered and inverted 10 times to further mix the slurry, and then set still in the upright position. The bed volume was monitored as a function of time. The settling rate is an indication of pulp stability or the extent of mineral aggregation. The quicker the bed settles, the lower the pulp stability and the stronger the mineral aggregation. 3. Results 3.1. Flotation A mixture consisting of 10% pentlandite and 90% lizardite with the Ni grade being about 3.2% was tested first. Fig. 1 shows repeat flotation tests of this mixture in bore water and de-ionized water. As can be seen, in de-ionized water, Ni recovery was low but MgO recovery was high. At the completion of 16 min of flotation, they were 39% and 31%, respectively. In bore water, Ni recovery was increased to 73% while MgO recovery was decreased to 21%. As a result, bore water with high ionic strength increased pentlandite flotation and its selectivity against lizardite significantly. The flotation of pentlandite–lizardite mixtures in this research and the flotation of ultrafine fractions of the pentlandite ore in the previous study show exactly the same patterns in de-ionized water and bore water. In the previous study where ultrafine fractions of a real nickel ore with about 0.6% Ni grade was used, Ni and MgO recoveries were 38% and 24% in de-ionized water, respectively, whilst 69% and 17%, respectively, in saline water, at the completion of 32 min of flotation (Peng and Seaman, 2011). The effect of lizardite proportions on pentlandite flotation and lizardite entrainment in de-ionized water and bore water was studied. The results are shown in Figs. 2 and 3. Fig. 2 shows Ni
4
8
12
16
20
Flotation time (min) Fig. 2. Effect of the lizardite proportion on pentlandite flotation in de-ionized water and bore water.
recovery as a function of flotation time. In de-ionized water, additions of lizardite all depressed pentlandite flotation. 10% lizardite (the lowest lizardite proportion examined in this research) in the mixture significantly depressed pentlandite flotation. This is in good agreement with observations by other researchers (Edwards et al., 1980; Bremmell et al., 2005). It is interesting to find that the lizardite proportion had little effect on pentlandite flotation in bore water. Similar Ni recovery was obtained from the flotation of mixtures with 10%, 50% and 90% lizardite. It was only slightly lower than that obtained from the flotation of 100% pentlandite. It is evident that bore water mitigate the deleterious effect of lizardite on pentlandite flotation. Fig. 3 shows MgO recovery as a function of flotation time. MgO recovery was much lower in bore water than in de-ionized water. This is consistent with results indicated in Fig. 1. Fig. 3 also shows that the lizardite proportion had little effect on MgO recovery. The recovery of lizardite in both de-ionized water and bore water should be due to mechanical entrainment. The particle size after grinding in all tests in Fig. 3 was the same, less than 8 lm, resulting in the same lizardite entrainment in bore water or de-ionized water. Figs. 2 and 3 indicate that water type is a critical factor in the control of pentlandite flotation and lizardite entrainment. Therefore, flotation tests were conducted in different water by mixing de-ionized water with bore water to change the salinity. Fig. 4 shows MgO recovery as a function of Ni recovery from the flotation of 10% pentlandite mixed with 90% lizardite in 100% de-ionized water, 1% bore water, 5% bore water and 100% bore water. It can be seen that compared with 100% de-ionized water, an addition
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Cumulative MgO recovery (%)
40
10% lizardite 50% lizardite 90% lizardite 100% lizardite
30
10% lizardite 50% lizardite 90% lizardite 100% lizardite
of 1% bore water increased Ni recovery and decreased MgO recovery significantly. An addition of 5% bore water further increased Ni recovery and decreased MgO recovery with the MgO recovery-Ni recovery curve close to that with 100% bore water. Apparently, with the proportion of bore water, the adverse effect of lizardite on pentlandite flotation and lizardite entrainment was reduced at the same time. 5% bore water may provide the critical salinity above which no significant improvement was observed. Flotation tests above strongly suggest that the negative effect of lizardite on pentlandite flotation can be mitigated in bore water and high salinity of the water is the contributor. Studies published in the literature indicate that the negative effect of serpentine minerals on pentlandite flotation may be associated with the increased pulp viscosity and slime coating in fresh water. It is unlikely that pulp viscosity plays a role in this study due to the low percentage solids (6.5%) used in flotation. Rheology measurements indicated that the viscosity of the flotation pulp was similar to water regardless of the proportion of lizardite. It is therefore hypothesized that bore water may mitigate lizardite coating on pentlandite surfaces resulting in the improved flotation. This mechanism is investigated in the following sections by SEM analysis and zeta potential measurement to probe the interaction between lizardite and pentlandite particles. While SEM analysis directly examines the coating of lizardite on pentlandite surfaces, zeta potential measurement provides information on the electrostatic interaction between pentlandite and lizardite particles which is attributed to the coating. In addition, a number of researchers correlated the reduced fine gangue entrainment with the faster settling of gangue particles as a result of the enlarged size with the addition of inorganic coagulants or organic flocculants (Liu et al., 2006; Cao and Liu, 2006; Gong et al., 2010; Tao et al., 2007; Xu et al., 2012). In the following sections, settling tests are also conducted to correlate lizardite entrainment in flotation with possible mineral aggregation in bore water.
De-ionised water
20
Bore water 10
0 0
4
8
12
16
20
Flotation time (min) Fig. 3. Effect of the lizardite proportion on lizardite rejection in de-ionized water and bore water.
Cumulative MgO recovery (%)
35
100 de-ionised water
30
1% bore water
25
5% bore water 100% bore water
20 15 10 5 0 0
20
40
60
80
Cumulative Ni recovery (%) Fig. 4. Effect of the bore water proportion on pentlandite flotation and lizardite rejection: 10% pentlandite mixed with 90% lizardite.
0
1
2
3
4
5
6
7
8
9
10
11
12
13
Fig. 5. SEM image (top) and EDX (bottom) of pentlandite surfaces attached with lizardite particles in de-ionized water: 10% pentlandite mixed with 90% lizardite.
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Fig. 6. SEM image (top) and EDX (bottom) of pentlandite surfaces attached with lizardite particles in bore water: 10% pentlandite mixed with 90% lizardite.
3.2. SEM analysis
3.3. Zeta potential measurements Zeta potentials on pentlandite and lizardite in de-ionized water with a background electrolyte solution of 10 3 M KNO3 were measured. Zeta potential measurements in bore water were conducted without a further addition of KNO3. The results are indicated in Fig. 7. It can be seen that in de-ionized water corresponding to the zero proportion of bore water, pentlandite was strongly negatively charged with a zeta potential about 20 mV, while lizardite
Zeta potential (mV)
Figs. 5 and 6 show SEM images together with EDX of flotation feed samples in de-ionized water and saline water, respectively. The secondary electron images (SEI) clearly differentiated pentlandite (A) from lizardite (B). The backscattered electron images (SBI) shows that fine lizardite particles were attached on pentlandite surfaces differently in de-ionized water and bore water. In de-ionized water, fine lizardite particles were evenly distributed on almost all the pentlandite surface in black dots. In bore water, the top pentlandite surface was quite clean although some lizardite particles were distributed sporadically. From the SBI image in Fig. 6, it seems that the fine lizardite particles did aggregate in bore water and loosely rest on the edges of pentlandite particles. The EDX analysis from pentlandite particles located are also shown in the figures. In addition to the S, Fe and Ni signals attributed to pentlandite, distinct signals were also detected from O, Si and Mg showing attached particles or layers attributed to lizardite particles. A comparison of the EDX results in Figs. 5 and 6 clearly indicates that much more serpentine particles were attached on pentlandite surfaces in de-ionized water than in bore water due to the greater concentrations of Mg, Si and O. SEM analysis clearly shows that lizardite coating on pentlandite surfaces was mitigated and lizardite particles were aggregated in bore water. These are further examined in the following sections.
30 20 10 0 -10 Pentlandite Lizardite
-20 -30 0
5
10
15
20
Proportion of bore water (%) Fig. 7. Zeta potential of pentlandite and lizardite particles as a function of the proportion of bore water at pH 9.0.
was strongly positively charged with a zeta potential about 28 mV. This is consistent with the observations by Edwards et al. (1980) and Bremmell et al. (2005). However, with the addition of 1% bore water, the magnitude of the zeta potential was reduced significantly. With 5% bore water, the zeta potentials reached a minimum, close to zero. Apparently, the electrolytes in bore water compressed electrical double layers of particles and 5% bore water completely minimized the electrostatic interaction between pentlandite and lizardite. The zeta potential measurement in Fig. 7 is consistent with pentlandite flotation in Fig. 4. In de-ionized water, the strong electrostatic interaction between pentlandite and lizardite corresponds to the low pentlandite flotation. With the addition of bore water, the electrostatic interaction between pentlandite and lizardite is reduced, corresponding to the mitigation of lizardite coating on pentlandite surfaces in the SEM analysis (Fig. 6) and the improved pentlandite flotation. Above 5% bore water, pentlandite flotation
Y. Peng, D. Bradshaw / Minerals Engineering 36–38 (2012) 284–290
1500
Bed volume (mL)
1200 900 600
1% bore water 5% bore water
300
100% bore water 100 de-ionised water
0 0
10
20
30
40
50
60
Settling time (min) Fig. 8. Bed volume of lizardite suspensions as a function of settling time in deionized and bore water.
changes only slightly and so does the zeta potential of pentlandite and lizardite particles. 3.4. Settling tests Fig. 8 shows the settling rate of lizardite suspensions in 100% de-ionized water, 1% bore water mixed with de-ionized water, 5% bore water mixed with de-ionized water and 100% bore water. Obviously, the settling rate of lizardite suspensions varied in different water. In 100% de-ionized water, the suspension was very stable as indicated by the slow settling rate. Additions of bore water caused the suspension unstable with an increased settling rate. Again, the settling rates with 5% bore water and 100% bore water were similar. Zeta potential measurement, settling tests and lizardite entrainment in flotation are well correlated. The electrolytes in bore water which compress the electrical double layers on lizardite particles, as shown by the reduced magnitude of zeta potentials, promote lizardite aggregation. In fact, mineral aggregation in saline water due to the reduction of electrical double layers has been widely observed (Rattanakawin and Hogg, 2001; Liang et al., 2007). The aggregated lizardite particles may be responsible for the decreased gangue entrainment in flotation. 4. Discussion The flotation of ultrafine pentlandite and its separation from lizardite are contrasting in de-ionized water and bore water. In de-ionized water, low Ni recovery was obtained with high MgO recovery, resulting in the poor separation. Bore water not only increased Ni recovery but also decreased MgO recovery subsequently improving pentlandite flotation against lizardite. The improved pentlandite flotation and the reduced lizardite entrainment in bore water should be associated with the changed inter-particle interaction caused by electrolytes, which is discussed below. Based on the DLVO theory, when two particles are brought into contact they are subjected to van der Waals and electrical double layer forces (Liang et al., 2007). Van der Waals forces are due to the interaction between two dipoles that are either permanent or induced. They are weak, attractive and insensitive to variations in electrolyte strength and pH. Electrical double layer forces are repulsive between two like-charged particles, without which all dispersed particles may aggregate together and precipitate out of solutions as a solid cake. In this study, lizardite and pentlandite are oppositely charged at pH about 9.0 and an electrostatic attraction occurs between them. However, electrical double layer forces may be manipulated by electrolytes. It is well known that
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electrolytes compress electrical double layers and therefore reduce the surface charges on particles (Rattanakawin and Hogg, 2001; Liang et al., 2007). The compression of electrical double layers not only reduces the electrical double layer repulsive forces between like-charged particles, but also the attractive forces between oppositely charged particles. In de-ionized water, the electrostatic attraction between serpentine and lizardite is strong. It is therefore anticipated that lizardite particles can coat pentlandite surfaces and then depress pentlandite flotation. It is unlikely that rheology play a dominant role in pentlandite flotation in this study. In this study, the effect of pulp viscosity caused by ultrafine lizardite on the flotation was minimized by using low percentage solids in flotation, which is consistent with the practice in the nickel flotation plants in Western Australia and other flotation plants where ultrafine particles are treated. In de-ionized water lizardite particles are well dispersed as well due to electrical double layer repulsive forces and therefore, the high entrainment of ultrafine lizardite particles in de-ionized water is expected. In contrast, in bore water, both the electrostatic interaction between pentlandite and lizardite, and electrical double layer repulsive forces among lizardite particles are minimized due to the compression of electrical double layer by electrolytes. As a result, the coating of lizardite particles on pentlandite surfaces is mitigated while lizardite particles aggregate together. This may result in the improved pentlandite flotation and reduced lizardite gangue entrainment in bore water. In coal flotation using saline water, the reduced bubble sizes (Yoon, 1982; Paulson and Pugh, 1996), the reduced zeta potential of both bubbles and coal particles (Fuerstenau et al., 1983; Yoon and Sabey, 1989), the reduced hydration on coal surfaces (Klassen and Mokrousov, 1963) and the formation of nanobubbles or nanopancakes on coal surfaces (Mishchuk, 2005; Zhang and Ducker, 2007) in electrolyte solutions were attributed to the increased bubble–coal attachment and therefore coal flotation. It is not likely that these mechanisms play a dominant role in increasing pentlandite flotation in bore water in this study because they do not take into account the behavior of the gangue mineral which is intimately linked with the low flotation of pentlandite in de-ionized water. However, these mechanisms may facilitate pentlandite flotation when lizardite coating on pentlandite surfaces is mitigated by bore water. It is observed that the entrapment of pentlandite particles in lizardite aggregates is insignificant in flotation using bore water. This is consistent with the observation from other researchers using inorganic coagulants or organic flocculants to aggregate various fine gangue minerals and then reduce their entrainment without affecting the recovery of valuable minerals (Liu et al., 2006; Cao and Liu, 2006; Gong et al., 2010; Tao et al., 2007; Xu et al., 2012). Although lizardite particles have a strong tendency to aggregate in bore water, it is unlikely that large aggregates can form in flotation with air purging and agitation. Sanding did not occur in flotation using bore water either. It is not clear the size of the aggregated lizardite particles in turbulent and quiescent zones in flotation which is under the investigation presently. This study also indicates that 5% bore water is sufficient to improve the flotation. A further increase in the proportion of bore water only slightly improves the flotation. Zeta potential measurement and settling tests suggest that 5% bore water completely compresses the electrical double layers and reduce the electrical double layer forces. The reduction of electrostatic interaction in saline water is a well-known phenomenon. This study explores this phenomenon in ultrafine particle flotation and its effect on slime coating and high gangue entrainment, two of the major barriers preventing efficient mineral separation (Cao and Liu, 2006; Liu et al., 2006; Duarte and Grano, 2007; Rao et al., 1988). Electrolytes which
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mitigate slime coating and minimize fine gangue entrainment provide a new way to improve fine and ultrafine particle flotation. 5. Conclusions The current study indicates the importance of mineral interactions in the flotation of ultrafine pentlandite and its separation from lizardite. In de-ionized water, the coating of serpentine particles on pentlandite surfaces by an electrostatic attraction depresses pentlandite flotation. The dispersed fine lizardite particles also cause high gangue entrainment in flotation. Electrolytes in bore water may mitigate the coating of serpentine particles on pentlandite surfaces and induce lizardite particles to aggregate together by reducing electrical double layer forces, resulting in the improved pentlandite flotation and the decreased lizardite entrainment. Acknowledgment The authors gratefully acknowledge the New Start Grant awarded to the first author by the University of Queensland. References Bremmell, K.E., Fornasiero, D., Ralston, J., 2005. Pentlandite–lizardite interactions and implications for their separation by flotation. Colloids Surf. A: Physicochem. Eng. Aspects 252, 207–212. Cao, M., Liu, Q., 2006. Re-examining the functions of zinc sulfate as a selective depressant in differential flotation. J. Colloid Interface Sci. 301, 523–531. Duarte, A.C.P., Grano, S.R., 2007. Mechanism for the recovery of silicate gangue minerals in the flotation of ultrafine sphalerite. Miner. Eng. 20, 766–775. Edwards, G.R., Kipkie, W.B., Agar, G.E., 1980. The effect of slime coatings of the serpentine minerals, chrysotile and lizardite on pentlandite flotation. Int. J. Miner. Process. 7, 33–42. Fuerstenau, D.W., Rosenbaum, J.M., Laskowski, J.S., 1983. Effect of surface functional groups on the flotation of coal. Colloids Surf. 8, 153–174. Gong, J., Peng, Y., Yeung, A., Liu, Q., 2010. Reducing quartz gangue entrainment in sulphide ore flotation by high molecular weight polyethylene oxide. Int. J. Miner. Process. 97, 44–51.
Klassen, V.I., Mokrousov, V.A., 1963. An Introduction to the Theory of Flotation. Butterworths, London. Liang, Y., Hilal, N., Langston, P., Starov, V., 2007. Interaction forces between colloidal particles in liquid: theory and experiment. Adv. Colloid Interface Sci. 135, 151– 166. Liu, Q., Wannas, D., Peng, Y., 2006. Exploiting the dual functions of polymer depressants in fine particle flotation. Int. J. Miner. Process. 80, 244–254. Mishchuk, N., 2005. The role of hydrophobicity and dissolved gases in nonequilibrium surface phenomena. Colloids Surf., A 267, 139–152. Mukund, V., Nagaraj, D.R., Partha P., Somasundaran P. 2010. Effect of altered silicates in flotation performance: role of changes in pulp rheology. In: Pawlik, M. (Ed.). The 8th UBC-McGill_university of Alberta Symposium Proceedings of the 49th Annual Conference of Metallurgists of CIM, Vancouver, BC, Canada. Neethling, S.J., Cilliers, J.J., 2009. The entrainment factor in froth flotation: model for particle size and other operating parameter effects. Int. J. Miner. Process. 93, 141–148. Paulson, O., Pugh, R.J., 1996. Flotation of inherently hydrophobic particles in aqueous solutions of inorganic electrolytes. Langmuir 12, 4808–4813. Peng, Y., Seaman, D., 2011. The flotation of slime-fine fractions of Mt. Keith pentlandite ore in de-ionised and saline water. Miner. Eng. 24, 479–481. Pietrobon, M.C., Grano, S.R., Sobieraj, S., Ralston, J., 1997. Recovery mechanisms for pentlandite and MgO-bearing gangue minerals in nickel ores from Western Australia. Miner. Eng. 10, 775–786. Rao, S.R., Espinosa-Gomez, R., Finch, J.A., Biss, R., 1988. Effects of water chemistry on the flotation of pyrochlore and silicate minerals. Miner. Eng. 1, 189–202. Rattanakawin, C., Hogg, H., 2001. Aggregate size distributions in flocculation. Colloids Surf. A: Physicochem. Eng. Aspects 177, 87–98. Senior, G.D., Thomas, S.A., 2005. Development and implementation of a new flowsheet for the flotation of a low grade nickel ore. Int. J. Miner. Process. 78, 49–61. Tao, D., Zhou, X.H., Zhao, C., Fan, M.M., Chen, G.L., Aron, M., Wright, J., 2007. Coal and potash flotation enhancement using a clay binder. Can. Metall. Q. 46, 243–250. Trahar, W.J., Warren, L.J., 1976. The floatability of very fine particles, a review. Int. J. Miner. Process. 3, 103–131. Warren, L.J., 1985. Determination of the contributions of true flotation and entrainment in batch flotation tests. Int. J. Miner. Process. 14, 33–44. Xu G., Tao, D., Dopico, P., Johnson, S., Hines, J., Kennedy, D. 2012. Clay binder enhanced extraction of bitumen from Canadian oil sand. In: SME Annual Meeting, Seattle, WA, USA. Yoon, R.H., 1982. Flotation of coal using micro-bubbles and inorganic salts. Min. Congr. J. 68, 76–80. Yoon, R.H., Sabey, J.B., 1989. Coal flotation in inorganic salt solutions. In: Botsaris, G.D., Glazman, Y.M. (Eds.), Interfacial Phenomena in Coal Technology. Marcel Dekker, New York. Zhang, X.H., Ducker, W., 2007. Formation of interfacial nanodroplets through changes in solvent quality. Langmuir 23, 12478–12480.
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Minerals Engineering journal homepage: www.elsevier.com/locate/mineng
Mechanisms for the improved flotation of ultrafine pentlandite and its separation from lizardite in saline water Yongjun Peng a,⇑, Dee Bradshaw b a b
School of Chemical Engineering, University of Queensland, St. Lucia, Brisbane, QLD 4072, Australia Julius Kruttschnitt Mineral Research Centre, University of Queensland, Isles Road, Indooroopilly, Brisbane, QLD 4068, Australia
a r t i c l e
i n f o
Article history: Available online 19 June 2012 Keywords: Ultrafine flotation Pentlandite Lizardite Saline water Particle interactions
a b s t r a c t In this work, the flotation of ultrafine pentlandite and its separation from lizardite in de-ionized water, and bore water of high ionic strength were studied. In de-ionized water, the flotation separation was poor due to low pentlandite recovery and high lizardite entrainment. However, bore water increased pentlandite flotation recovery while reducing lizardite entrainment. The possible mechanisms responsible for the improved flotation separation in bore water were investigated by electrokinetic studies, Scanning Electronic Microscopy (SEM) analysis and settling tests. It was found that the reduction of electrical double layer forces between particles in bore water might mitigate the coating of lizardite particles on pentlandite surfaces resulting in the improved pentlandite flotation. The reduction of electrical double layer forces might also induce the aggregation of lizardite particles and therefore enhance lizardite rejection. This study provides a new direction to address slime coating and high gangue entrainment in ultrafine mineral flotation by electrolytes. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction The flotation of pentlandite minerals in Western Australia has historically been difficult due to low nickel grade and a large amount of serpentine minerals in deposits. At the largest nickel deposit, Mt. Keith Mine, nickel grade of the feed is about 0.6%, while MgO grade is about 38% equivalent to about 90% serpentine minerals. While lizardite is the major serpentine mineral, antigorite and chrysotile, the fibrous serpentine minerals, also occur in some minor ore bodies. To increase pentlandite flotation and its separation from serpentine, split flotation has been implemented at major pentlandite flotation plants. The grinding discharge is pumped to cyclone classification from which underflows and overflows pass to flotation streams dealing with different sizes. Nickel recovery in the ultrafine flotation stream ( 8 lm) is usually less than 70% when a normal lizardite ore is treated, which has been attributed to the low overall nickel recovery (Senior and Thomas, 2005). When a fibrous ore is treated, effective flotation in the ultrafine stream is not possible. Another characteristics of these flotation plants is the use of hyper saline water as only bore water is available. Since flotation mechanisms in saline water are not clear and most flotation plants in the world use fresh water, there has been debate over whether fresh water should be introduced to the pentlandite flotation plants to increase nickel recovery and grade. ⇑ Corresponding author. Tel.: +61 7 3365 7156; fax: +61 7 3365 3888. E-mail address: [email protected] (Y. Peng). 0892-6875/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.mineng.2012.05.015
However, previous studies based on the flotation of ultrafine fractions of a normal ore obtained from the Mt. Keith Mine indicated that pentlandite flotation was very poor in de-ionized water with high lizardite gangue entrainment, whilst bore water improved pentlandite flotation and reduced lizardite entrainment significantly (Peng and Seaman, 2011). The objective of this current research is to identify possible mechanisms responsible for the improved ultrafine pentlandite flotation and the reduced ultrafine lizardite entrainment in saline water. The separation of pentlandite from serpentine by froth flotation has been studied in fresh water by a great number of researchers. Edwards et al. (1980) investigated the effect of lizardite and chrysotile on the flotation of pentlandite. They found that pentlandite flotation was depressed in the presence of lizardite or chrysotile. By using electrophoresis and Scanning Electron Microscopy (SEM) measurements, they identified the slime coating of serpentine minerals on the pentlandite surface which was attributed to the depressed pentlandite flotation. This mechanism was also identified by Bremmell et al. (2005) using flotation, zeta potential measurements, SEM and Atomic Force Microscope (AFM) analysis on a mineral system consisting of pentlandite and lizardite. The slime coating results from the electrostatic attraction between negatively charged pentlandite and positively charged lizardite and chrysotile in neutral and weakly alkaline flotation conditions. Edwards et al. (1980), Bremmell et al. (2005) and Pietrobon et al. (1997) used negatively charged chemical additives including carboxy methyl cellulose (CMC) to modify the charge of serpentine
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minerals and therefore improved pentlandite flotation. Edwards et al. (1980) demonstrated the mitigated chrysotile coating on the pentlandite surface in the presence of CMC by SEM measurements, while Bremmell et al. (2005) demonstrated a reduced attraction between lizardite and pentlandite in CMC solutions. Mukund et al. (2010) studied the effect of serpentine fibers on pulp rheology and the flotation of copper sulfide minerals. They observed the alteration of pulp rheology and flotation performance simultaneously by serpentine fibers. An arrested system was observed when the content of serpentine fibers in the ore reached a high value dramatically reducing the copper flotation recovery. This change was proposed to be due to the strong network formation of the fibrous serpentine minerals that locked copper minerals and prevented bubble dispersion and/or collector–mineral interaction. The mechanism proposed in the literature to be responsible for the negative effect of serpentine minerals on pentlandite or copper mineral flotation may explain the poor flotation behavior of pentlandite in de-ionized water in previous studies (Peng and Seaman, 2011), but the mechanisms responsible for the improved pentlandite flotation and lizardite rejection in saline water are not clear. In fact, the beneficial effect of saline water on coal flotation has also been observed and a number of studies have been conducted to investigate the mechanisms. In general, the improved coal flotation in saline water compared to fresh water has been attributed to the increased bubble–particle attachment. Yoon (1982) and Paulson and Pugh (1996) proposed that reduced bubble sizes and increased population in electrolyte solutions increased the encounter efficiency of bubble–particle attachment. Fuerstenau et al. (1983) and Yoon and Sabey (1989) attributed the increased bubble–particle attachment to the reduction of zeta potential of both bubbles and particles resulting from the compression of electrical double-layer in the presence of electrolytes. Another mechanism proposed is that the inorganic electrolytes destabilized the hydrated layers surrounding coal particles and reduced their surface hydration therefore enhancing the bubble–particle attachment (Klassen and Mokrousov, 1963). Nanobubbles or nanopancakes were also observed to form on coal surfaces in electrolyte solutions facilitating bubble–particle attachment (Mishchuk, 2005; Zhang and Ducker, 2007). Studies in coal flotation in saline water have not taken into account the behavior of gangue minerals. It is unlikely that the increased bubble–particle attachment is a major contributor to the improved flotation of pentlandite against lizardite in saline water in previous studies since it cannot explain the decreased serpentine entrainment. The increased bubble–particle attachment in saline water should increase the collection of serpentine minerals as well. Furthermore, in pentlandite flotation, serpentine minerals play an important role due to the surface coating (Edwards et al., 1980; Bremmell et al., 2005) or the changed pulp rheology (Mukund et al., 2010). The improved pentlandite flotation in saline water may be associated with the modified behavior of lizardite minerals. The entrainment of fine gangue minerals through water films between air bubbles is a well-known problem in flotation (Trahar and Warren, 1976; Warren, 1985). The entrainment of gangue minerals is significant at a particle size smaller than 30 lm and the smaller the particle size, the higher the entrainment (Trahar and Warren, 1976; Warren, 1985). As a result, the size of particles
plays an important role in mechanical entrainment of particles in mineral flotation. To reduce gangue entrainment in flotation, a high depth of froth has been used to allow the drainage of gangue minerals from froth (Neethling and Cilliers, 2009). However, in the flotation of ultrafine nickel streams in Western Australia, very shallow froth is generated. Similar shallow froth was generated in flotation using both saline water and de-ionized water in the laboratory in the previous study (Peng and Seaman, 2011). It is unlikely that bore water produced a higher depth of froth resulting in the reduced lizardite entrainment. It has been demonstrated that mechanical entrainment of fine gangue particles in flotation may be reduced by inorganic coagulants or organic flocculants. Liu et al. (2006), Cao and Liu (2006) and Gong et al. (2010) show a relationship between the reduced entrainment of various fine gangue minerals and their aggregated sizes indicated from sedimentation tests. Tao et al. (2007) and Xu et al. (2012) demonstrated the reduced entrainment of clay minerals in the flotation of oil sand, coal and potash through the aggregation of clay minerals by polymers. The reduced lizardite entrainment in flotation in saline water may be associated with the aggregation of particles. 2. Experimental 2.1. Materials and reagents Pentlandite (NiFeS2), and lizardite (Mg3Si2O5(OH)4) pure minerals were obtained from Ward’s Natural Science Establishment (US). Lizardite is the major serpentine mineral in pentlandite deposits in Western Australia, in particular, Mt. Keith Mine which was targeted in the previous study (Peng and Seaman, 2011). These samples were crushed through a roll crusher and then screened to collect the +0.6–3.2 mm particle size fraction. The processed samples were then sealed in polyethylene bags purged with nitrogen gases as mill feeds. Sodium ethyl xanthate and H407 (a type of polyglycol ether), industrial grade, were used as the collector and frother, respectively. They have been used in pentlandite flotation plants in Western Australia and a number of studies indicate that they are the best combination in the flotation. Other chemicals used in this research were AR grade. Bore water was obtained from a pentlandite flotation plant in Western Australia. The chemical analysis of the bore water is shown in Table 1. 2.2. Grinding and flotation Mineral grinding and flotation were conducted at room temperature, 25 °C. Pentlandite (100 g), lizardite (100 g) or their mixture (100 g altogether) was combined with 0.15 dm3 of bore water or de-ionized water and ground with 10 kg of stainless steel rods in a laboratory stainless steel mill such that 90% of the particles present were less than 8 lm in diameter. After grinding the pulp was transferred to a 1.5 dm3 flotation cell and then conditioned with collector (100 g/t) and frother (10 g/t). The solid percentage in flotation was about 6.5%. A similar solid percentage has been used in the flotation of the ultrafine stream in the nickel flotation plants in Western Australia. In flotation, five concentrates were collected after cumulative times of 1, 2, 4, 8 and 16 min at an air flow rate of 2.5 dm3 min 1. The flotation froth was scraped every 10 s. Both de-ionized and saline water were tested in this study. The chosen water type was utilized in all stages of grinding and flotation.
Table 1 Composition of the bore water (mg/L). Na
K
Ca
Mg
SiO2
Cd
Cr
Cu
Ni
Cl
SO4
20,000
940
400
5100
4.9
0.002
0.001
<0.005
0.033
32,000
23,000
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When bore water was used, the pH during grinding and flotation was constant, about 9.0, due to the buffer effect of the water. When de-ionized water was used, NaOH solutions were used to maintain pH at 9.0 during grinding and flotation. The grinding and flotation procedure is similar in this study where single minerals are used and in the previous study where a real nickel ore was used. 2.3. Techniques 2.3.1. Scanning electron microscopy Samples before the flotation of pentlandite–lizardite mixtures were taken, washed with de-ionized or saline water and then analyzed by a CAMSCAN CS44FE scanning electron microscope (SEM) fitted with a Camscan Energy Dispersive X-ray spectrometer (EDXS). During analysis, a focused electron beam was rastered across the sample surface. The secondary or backscattered electrons produced were used to map the surface topography and compositional contrast. EDXS was used to provide a means of elemental identification on the surface of interest.
35
Cumulative MgO recovery (%)
286
Bore water1 Bore water2 De-ionised water1 De-ionised water2
30 25 20 15 10 5 0 0
20
40
60
80
Cumulative Ni recovery (%) Fig. 1. Repeat flotation tests on the mixture of 10% pentlandite and 90% lizardite in de-ionized water and bore water.
Cumulative Ni recovery (%)
90
2.3.2. Zeta potential measurement Particle electrophoretic mobility was measured with a Rank Brothers Microelectrophoresis Mark II apparatus. Single pentlandite or lizardite mineral particles were ground in a ceramic mill to about 8 lm. Particles were conditioned in a reaction vessel with de-ionized water mixed with saline water at pH 9.0 for 30 min. Then pH was adjusted by the addition of NaOH solutions. 10 mobility measurements at each of the two stationary planes were performed at each pH value. The average mobility was converted to the zeta potential using the Smoluchowski equation.
75 60 45 30
Bore water: De-ionised water: 0% lizardite 0% lizardite 10% lizardite 10% lizardite 50% lizardite 50% lizardite 90% lizardite 90% lizardite
15 0 0
2.3.3. Settling tests The particle slurry was conditioned in the flotation cell and then transferred to a 1.5 dm3 graduated cylinder. The cylinder was then stoppered and inverted 10 times to further mix the slurry, and then set still in the upright position. The bed volume was monitored as a function of time. The settling rate is an indication of pulp stability or the extent of mineral aggregation. The quicker the bed settles, the lower the pulp stability and the stronger the mineral aggregation. 3. Results 3.1. Flotation A mixture consisting of 10% pentlandite and 90% lizardite with the Ni grade being about 3.2% was tested first. Fig. 1 shows repeat flotation tests of this mixture in bore water and de-ionized water. As can be seen, in de-ionized water, Ni recovery was low but MgO recovery was high. At the completion of 16 min of flotation, they were 39% and 31%, respectively. In bore water, Ni recovery was increased to 73% while MgO recovery was decreased to 21%. As a result, bore water with high ionic strength increased pentlandite flotation and its selectivity against lizardite significantly. The flotation of pentlandite–lizardite mixtures in this research and the flotation of ultrafine fractions of the pentlandite ore in the previous study show exactly the same patterns in de-ionized water and bore water. In the previous study where ultrafine fractions of a real nickel ore with about 0.6% Ni grade was used, Ni and MgO recoveries were 38% and 24% in de-ionized water, respectively, whilst 69% and 17%, respectively, in saline water, at the completion of 32 min of flotation (Peng and Seaman, 2011). The effect of lizardite proportions on pentlandite flotation and lizardite entrainment in de-ionized water and bore water was studied. The results are shown in Figs. 2 and 3. Fig. 2 shows Ni
4
8
12
16
20
Flotation time (min) Fig. 2. Effect of the lizardite proportion on pentlandite flotation in de-ionized water and bore water.
recovery as a function of flotation time. In de-ionized water, additions of lizardite all depressed pentlandite flotation. 10% lizardite (the lowest lizardite proportion examined in this research) in the mixture significantly depressed pentlandite flotation. This is in good agreement with observations by other researchers (Edwards et al., 1980; Bremmell et al., 2005). It is interesting to find that the lizardite proportion had little effect on pentlandite flotation in bore water. Similar Ni recovery was obtained from the flotation of mixtures with 10%, 50% and 90% lizardite. It was only slightly lower than that obtained from the flotation of 100% pentlandite. It is evident that bore water mitigate the deleterious effect of lizardite on pentlandite flotation. Fig. 3 shows MgO recovery as a function of flotation time. MgO recovery was much lower in bore water than in de-ionized water. This is consistent with results indicated in Fig. 1. Fig. 3 also shows that the lizardite proportion had little effect on MgO recovery. The recovery of lizardite in both de-ionized water and bore water should be due to mechanical entrainment. The particle size after grinding in all tests in Fig. 3 was the same, less than 8 lm, resulting in the same lizardite entrainment in bore water or de-ionized water. Figs. 2 and 3 indicate that water type is a critical factor in the control of pentlandite flotation and lizardite entrainment. Therefore, flotation tests were conducted in different water by mixing de-ionized water with bore water to change the salinity. Fig. 4 shows MgO recovery as a function of Ni recovery from the flotation of 10% pentlandite mixed with 90% lizardite in 100% de-ionized water, 1% bore water, 5% bore water and 100% bore water. It can be seen that compared with 100% de-ionized water, an addition
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Cumulative MgO recovery (%)
40
10% lizardite 50% lizardite 90% lizardite 100% lizardite
30
10% lizardite 50% lizardite 90% lizardite 100% lizardite
of 1% bore water increased Ni recovery and decreased MgO recovery significantly. An addition of 5% bore water further increased Ni recovery and decreased MgO recovery with the MgO recovery-Ni recovery curve close to that with 100% bore water. Apparently, with the proportion of bore water, the adverse effect of lizardite on pentlandite flotation and lizardite entrainment was reduced at the same time. 5% bore water may provide the critical salinity above which no significant improvement was observed. Flotation tests above strongly suggest that the negative effect of lizardite on pentlandite flotation can be mitigated in bore water and high salinity of the water is the contributor. Studies published in the literature indicate that the negative effect of serpentine minerals on pentlandite flotation may be associated with the increased pulp viscosity and slime coating in fresh water. It is unlikely that pulp viscosity plays a role in this study due to the low percentage solids (6.5%) used in flotation. Rheology measurements indicated that the viscosity of the flotation pulp was similar to water regardless of the proportion of lizardite. It is therefore hypothesized that bore water may mitigate lizardite coating on pentlandite surfaces resulting in the improved flotation. This mechanism is investigated in the following sections by SEM analysis and zeta potential measurement to probe the interaction between lizardite and pentlandite particles. While SEM analysis directly examines the coating of lizardite on pentlandite surfaces, zeta potential measurement provides information on the electrostatic interaction between pentlandite and lizardite particles which is attributed to the coating. In addition, a number of researchers correlated the reduced fine gangue entrainment with the faster settling of gangue particles as a result of the enlarged size with the addition of inorganic coagulants or organic flocculants (Liu et al., 2006; Cao and Liu, 2006; Gong et al., 2010; Tao et al., 2007; Xu et al., 2012). In the following sections, settling tests are also conducted to correlate lizardite entrainment in flotation with possible mineral aggregation in bore water.
De-ionised water
20
Bore water 10
0 0
4
8
12
16
20
Flotation time (min) Fig. 3. Effect of the lizardite proportion on lizardite rejection in de-ionized water and bore water.
Cumulative MgO recovery (%)
35
100 de-ionised water
30
1% bore water
25
5% bore water 100% bore water
20 15 10 5 0 0
20
40
60
80
Cumulative Ni recovery (%) Fig. 4. Effect of the bore water proportion on pentlandite flotation and lizardite rejection: 10% pentlandite mixed with 90% lizardite.
0
1
2
3
4
5
6
7
8
9
10
11
12
13
Fig. 5. SEM image (top) and EDX (bottom) of pentlandite surfaces attached with lizardite particles in de-ionized water: 10% pentlandite mixed with 90% lizardite.
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Fig. 6. SEM image (top) and EDX (bottom) of pentlandite surfaces attached with lizardite particles in bore water: 10% pentlandite mixed with 90% lizardite.
3.2. SEM analysis
3.3. Zeta potential measurements Zeta potentials on pentlandite and lizardite in de-ionized water with a background electrolyte solution of 10 3 M KNO3 were measured. Zeta potential measurements in bore water were conducted without a further addition of KNO3. The results are indicated in Fig. 7. It can be seen that in de-ionized water corresponding to the zero proportion of bore water, pentlandite was strongly negatively charged with a zeta potential about 20 mV, while lizardite
Zeta potential (mV)
Figs. 5 and 6 show SEM images together with EDX of flotation feed samples in de-ionized water and saline water, respectively. The secondary electron images (SEI) clearly differentiated pentlandite (A) from lizardite (B). The backscattered electron images (SBI) shows that fine lizardite particles were attached on pentlandite surfaces differently in de-ionized water and bore water. In de-ionized water, fine lizardite particles were evenly distributed on almost all the pentlandite surface in black dots. In bore water, the top pentlandite surface was quite clean although some lizardite particles were distributed sporadically. From the SBI image in Fig. 6, it seems that the fine lizardite particles did aggregate in bore water and loosely rest on the edges of pentlandite particles. The EDX analysis from pentlandite particles located are also shown in the figures. In addition to the S, Fe and Ni signals attributed to pentlandite, distinct signals were also detected from O, Si and Mg showing attached particles or layers attributed to lizardite particles. A comparison of the EDX results in Figs. 5 and 6 clearly indicates that much more serpentine particles were attached on pentlandite surfaces in de-ionized water than in bore water due to the greater concentrations of Mg, Si and O. SEM analysis clearly shows that lizardite coating on pentlandite surfaces was mitigated and lizardite particles were aggregated in bore water. These are further examined in the following sections.
30 20 10 0 -10 Pentlandite Lizardite
-20 -30 0
5
10
15
20
Proportion of bore water (%) Fig. 7. Zeta potential of pentlandite and lizardite particles as a function of the proportion of bore water at pH 9.0.
was strongly positively charged with a zeta potential about 28 mV. This is consistent with the observations by Edwards et al. (1980) and Bremmell et al. (2005). However, with the addition of 1% bore water, the magnitude of the zeta potential was reduced significantly. With 5% bore water, the zeta potentials reached a minimum, close to zero. Apparently, the electrolytes in bore water compressed electrical double layers of particles and 5% bore water completely minimized the electrostatic interaction between pentlandite and lizardite. The zeta potential measurement in Fig. 7 is consistent with pentlandite flotation in Fig. 4. In de-ionized water, the strong electrostatic interaction between pentlandite and lizardite corresponds to the low pentlandite flotation. With the addition of bore water, the electrostatic interaction between pentlandite and lizardite is reduced, corresponding to the mitigation of lizardite coating on pentlandite surfaces in the SEM analysis (Fig. 6) and the improved pentlandite flotation. Above 5% bore water, pentlandite flotation
Y. Peng, D. Bradshaw / Minerals Engineering 36–38 (2012) 284–290
1500
Bed volume (mL)
1200 900 600
1% bore water 5% bore water
300
100% bore water 100 de-ionised water
0 0
10
20
30
40
50
60
Settling time (min) Fig. 8. Bed volume of lizardite suspensions as a function of settling time in deionized and bore water.
changes only slightly and so does the zeta potential of pentlandite and lizardite particles. 3.4. Settling tests Fig. 8 shows the settling rate of lizardite suspensions in 100% de-ionized water, 1% bore water mixed with de-ionized water, 5% bore water mixed with de-ionized water and 100% bore water. Obviously, the settling rate of lizardite suspensions varied in different water. In 100% de-ionized water, the suspension was very stable as indicated by the slow settling rate. Additions of bore water caused the suspension unstable with an increased settling rate. Again, the settling rates with 5% bore water and 100% bore water were similar. Zeta potential measurement, settling tests and lizardite entrainment in flotation are well correlated. The electrolytes in bore water which compress the electrical double layers on lizardite particles, as shown by the reduced magnitude of zeta potentials, promote lizardite aggregation. In fact, mineral aggregation in saline water due to the reduction of electrical double layers has been widely observed (Rattanakawin and Hogg, 2001; Liang et al., 2007). The aggregated lizardite particles may be responsible for the decreased gangue entrainment in flotation. 4. Discussion The flotation of ultrafine pentlandite and its separation from lizardite are contrasting in de-ionized water and bore water. In de-ionized water, low Ni recovery was obtained with high MgO recovery, resulting in the poor separation. Bore water not only increased Ni recovery but also decreased MgO recovery subsequently improving pentlandite flotation against lizardite. The improved pentlandite flotation and the reduced lizardite entrainment in bore water should be associated with the changed inter-particle interaction caused by electrolytes, which is discussed below. Based on the DLVO theory, when two particles are brought into contact they are subjected to van der Waals and electrical double layer forces (Liang et al., 2007). Van der Waals forces are due to the interaction between two dipoles that are either permanent or induced. They are weak, attractive and insensitive to variations in electrolyte strength and pH. Electrical double layer forces are repulsive between two like-charged particles, without which all dispersed particles may aggregate together and precipitate out of solutions as a solid cake. In this study, lizardite and pentlandite are oppositely charged at pH about 9.0 and an electrostatic attraction occurs between them. However, electrical double layer forces may be manipulated by electrolytes. It is well known that
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electrolytes compress electrical double layers and therefore reduce the surface charges on particles (Rattanakawin and Hogg, 2001; Liang et al., 2007). The compression of electrical double layers not only reduces the electrical double layer repulsive forces between like-charged particles, but also the attractive forces between oppositely charged particles. In de-ionized water, the electrostatic attraction between serpentine and lizardite is strong. It is therefore anticipated that lizardite particles can coat pentlandite surfaces and then depress pentlandite flotation. It is unlikely that rheology play a dominant role in pentlandite flotation in this study. In this study, the effect of pulp viscosity caused by ultrafine lizardite on the flotation was minimized by using low percentage solids in flotation, which is consistent with the practice in the nickel flotation plants in Western Australia and other flotation plants where ultrafine particles are treated. In de-ionized water lizardite particles are well dispersed as well due to electrical double layer repulsive forces and therefore, the high entrainment of ultrafine lizardite particles in de-ionized water is expected. In contrast, in bore water, both the electrostatic interaction between pentlandite and lizardite, and electrical double layer repulsive forces among lizardite particles are minimized due to the compression of electrical double layer by electrolytes. As a result, the coating of lizardite particles on pentlandite surfaces is mitigated while lizardite particles aggregate together. This may result in the improved pentlandite flotation and reduced lizardite gangue entrainment in bore water. In coal flotation using saline water, the reduced bubble sizes (Yoon, 1982; Paulson and Pugh, 1996), the reduced zeta potential of both bubbles and coal particles (Fuerstenau et al., 1983; Yoon and Sabey, 1989), the reduced hydration on coal surfaces (Klassen and Mokrousov, 1963) and the formation of nanobubbles or nanopancakes on coal surfaces (Mishchuk, 2005; Zhang and Ducker, 2007) in electrolyte solutions were attributed to the increased bubble–coal attachment and therefore coal flotation. It is not likely that these mechanisms play a dominant role in increasing pentlandite flotation in bore water in this study because they do not take into account the behavior of the gangue mineral which is intimately linked with the low flotation of pentlandite in de-ionized water. However, these mechanisms may facilitate pentlandite flotation when lizardite coating on pentlandite surfaces is mitigated by bore water. It is observed that the entrapment of pentlandite particles in lizardite aggregates is insignificant in flotation using bore water. This is consistent with the observation from other researchers using inorganic coagulants or organic flocculants to aggregate various fine gangue minerals and then reduce their entrainment without affecting the recovery of valuable minerals (Liu et al., 2006; Cao and Liu, 2006; Gong et al., 2010; Tao et al., 2007; Xu et al., 2012). Although lizardite particles have a strong tendency to aggregate in bore water, it is unlikely that large aggregates can form in flotation with air purging and agitation. Sanding did not occur in flotation using bore water either. It is not clear the size of the aggregated lizardite particles in turbulent and quiescent zones in flotation which is under the investigation presently. This study also indicates that 5% bore water is sufficient to improve the flotation. A further increase in the proportion of bore water only slightly improves the flotation. Zeta potential measurement and settling tests suggest that 5% bore water completely compresses the electrical double layers and reduce the electrical double layer forces. The reduction of electrostatic interaction in saline water is a well-known phenomenon. This study explores this phenomenon in ultrafine particle flotation and its effect on slime coating and high gangue entrainment, two of the major barriers preventing efficient mineral separation (Cao and Liu, 2006; Liu et al., 2006; Duarte and Grano, 2007; Rao et al., 1988). Electrolytes which
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mitigate slime coating and minimize fine gangue entrainment provide a new way to improve fine and ultrafine particle flotation. 5. Conclusions The current study indicates the importance of mineral interactions in the flotation of ultrafine pentlandite and its separation from lizardite. In de-ionized water, the coating of serpentine particles on pentlandite surfaces by an electrostatic attraction depresses pentlandite flotation. The dispersed fine lizardite particles also cause high gangue entrainment in flotation. Electrolytes in bore water may mitigate the coating of serpentine particles on pentlandite surfaces and induce lizardite particles to aggregate together by reducing electrical double layer forces, resulting in the improved pentlandite flotation and the decreased lizardite entrainment. Acknowledgment The authors gratefully acknowledge the New Start Grant awarded to the first author by the University of Queensland. References Bremmell, K.E., Fornasiero, D., Ralston, J., 2005. Pentlandite–lizardite interactions and implications for their separation by flotation. Colloids Surf. A: Physicochem. Eng. Aspects 252, 207–212. Cao, M., Liu, Q., 2006. Re-examining the functions of zinc sulfate as a selective depressant in differential flotation. J. Colloid Interface Sci. 301, 523–531. Duarte, A.C.P., Grano, S.R., 2007. Mechanism for the recovery of silicate gangue minerals in the flotation of ultrafine sphalerite. Miner. Eng. 20, 766–775. Edwards, G.R., Kipkie, W.B., Agar, G.E., 1980. The effect of slime coatings of the serpentine minerals, chrysotile and lizardite on pentlandite flotation. Int. J. Miner. Process. 7, 33–42. Fuerstenau, D.W., Rosenbaum, J.M., Laskowski, J.S., 1983. Effect of surface functional groups on the flotation of coal. Colloids Surf. 8, 153–174. Gong, J., Peng, Y., Yeung, A., Liu, Q., 2010. Reducing quartz gangue entrainment in sulphide ore flotation by high molecular weight polyethylene oxide. Int. J. Miner. Process. 97, 44–51.
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