Use of dispersants in flotation of molybdenite in seawater

Minerals Engineering 100 (2017) 71–74

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Use of dispersants in flotation of molybdenite in seawater Erick Rebolledo a, Janusz S. Laskowski b,⇑, Leopoldo Gutierrez c, Sergio Castro a a

Water Research Centre for Agriculture and Mining (CRHIAM), University of Concepcion, Chile N.B. Keevil Institute of Mining Engineering, University of British Columbia, Canada c Department of Metallurgical Engineering, University of Concepcion, Chile b

a r t i c l e

i n f o

Article history: Received 9 August 2016 Revised 29 September 2016 Accepted 2 October 2016

Keywords: Molybdenite Molybdenite flotation Molybdenite depression Slime-coating Seawater Sodium hexametaphosphate

a b s t r a c t Coating of valuable mineral particles by hydrophilic slimes is known to depress flotation of such particles. This is referred to as a slime-coating. Such a slime coating takes place in the flotation of molybdenite in seawater when pH of the pulp is raised to depress pyrite and Mg(OH)+ hydroxy-complexes and precipitating magnesium hydroxide accumulate on the molybdenite surface. The dispersant tested in this paper, sodium hexametaphosphate, is shown to be able to restore molybdenite flotation in the pH range in which it is depressed by magnesium species. Addition of hexametaphosphate was found not to affect floatability of pyrite. Ó 2016 Elsevier Ltd. All rights reserved.

2.1. Samples and reagents

molybdenum concentrate from one of the Chilean coppermolybdenum concentrators. The final sample obtained after the 3 cleaning stages was washed with sodium hydrosulfide (NaSH), and diethyl-ether in order to remove residual organic reagents (e.g. collectors present in the sample). The chemical analyses indicate that the molybdenite sample had 59% Mo. Particle size analyses were determined using a Sympatec’s HELOS-Rodos laser diffraction system. Fig. 1 shows the particle size distribution of the molybdenite sample utilized in the study which indicates a top particle size of 60 lm. The pyrite sample was prepared from mineralized rock specimens, which were first ground using a ceramic mortar and pestle to obtain a sample in the size range of 212 + 75 lm. The ground material was further purified by using a magnetic separator to remove magnetic minerals occluded in the pyrite specimens. XRD analysis indicates that the pyrite content in the sample was 98% with minor content of molybdenite, quartz and iron oxides. Methyl isobutyl carbinol (MIBC) was used as frother, sodium hexametaphosphate (SHMP) dispersant was obtained from Sigma Aldrich (97% purity), and purified potassium amyl xanthate was used in the tests with pyrite. pH was adjusted using lime in all the tests.

Sample of molybdenite used in this work was obtained after several cleaning stages of collectorless flotation of a commercial

2.2. Procedures

1. Introduction Limited resources of fresh water in many countries forces mining industries in these countries to use seawater in mineral processing operations. Over the last five years it has been demonstrated (Castro, 2010, 2012; Castro et al., 2012, 2014) that molybdenite flotation in seawater in the flotation of Cu-Mo sulfide ores is depressed whenever pH is raised to control flotation of pyrite. Depression of molybdenite flotation under such conditions is caused by precipitating magnesium hydroxy-complexes and hydroxide. Since this depression by ‘‘slime coating” is caused by precipitating/coagulating hydroxides on the surface of molybdenite, it is logical to expect that the use of dispersants that remove precipitating/coagulating hydroxides from the molybdenite surface should diminish depression. In this paper sodium hexametaphosphate dispersant is utilized in the tests with molybdenite to evaluate this idea. 2. Materials and methods

⇑ Corresponding author. E-mail address: [email protected] (J.S. Laskowski). http://dx.doi.org/10.1016/j.mineng.2016.10.004 0892-6875/Ó 2016 Elsevier Ltd. All rights reserved.

The floatability of molybdenite over the pH range from 6 to 11 was tested using 1 g samples of molybdenite and 150 mL

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Fig. 1. Particle size distribution of molybdenite sample.

Partridge-Smith micro-flotation cell at a nitrogen flowrate of 17 mL/min. The flotation tests were carried out following two procedures: (i) In the first procedure, molybdenite was initially conditioned in seawater during 2 min. Then, pH was adjusted to the required value, and dispersant (varying concentrations), diesel (100 ppm), and MIBC (10 ppm) were added and conditioned for additional 3 min. Finally, the gas valve was opened to start the process of flotation which was carried out over 2 min, scraping the froth every 10 s. The pulp level in the micro-flotation cell was kept constant by adding a background solution prepared at the same composition and pH of the original solution. Concentrates and tailings were dried and the recovery of molybdenite was calculated as the mass of concentrate divided by the total mass of concentrate and tailings. (ii) In the second procedure, the pH of seawater was firstly adjusted, the solution was conditioned for 2 min to reach the pH required value, then molybdenite was added and conditioned for additional 2 min. At this moment, dispersant, diesel and MIBC were added and conditioned for additional 3 min. Then, the micro-flotation tests were carried out as in the first procedure. (iii) After addition of SHMP a slight reduction of pH by 0.1–0.2 units was recorded and it was readjusted to the required level. The floatability of pyrite in seawater was also tested over the pH range from 6 to 11, with and without SHMP. These experiments were conducted using 1 g samples of pyrite in a 150 mL Partridge-Smith micro-flotation cell, at a nitrogen flowrate of 17 mL/min. In these experiments, pyrite was initially conditioned in seawater during 2 min. Then, the pH of the suspension was adjusted to the required value and kept under mixing for 2 min, after which potassium amyl xanthate (25 ppm) and MIBC (10 ppm) were added and conditioned for additional 3 min. Finally, the gas valve of the micro-flotation cell was opened and pyrite was floated for 2 min. All the micro-flotation tests were performed in duplicates, and some of them even in triplicates. Reproducibility of the experi-

Fig. 2. Effect of pH on standard flotation tests with molybdenite using 100 ppm of Diesel oil and 10 ppm of MIBC in fresh water and seawater. In these tests molybdenite was first conditioned in seawater before pH was adjusted and flotation agents were added.

Fig. 3. Effect of concentration of sodium hexametaphosphate at various pH values on standard flotation tests with molybdenite in which molybdenite was first conditioned in seawater before pH was adjusted and flotation agents were added.

ments was good with an average standard deviation of 1.3 percentage points. The largest recorded standard deviation was 1.7 percent points. 3. Results As seen from Fig. 2, in the tests carried out in seawater in alkaline solutions (pH 9–11), the flotation of molybdenite is strongly depressed as it was reported in our earlier publications (Castro et al., 2012, 2014).

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Fig. 4. Recovery of molybdenite as a function of pH at various concentrations of SHMP (replotted from Fig. 3). In these tests molybdenite was first conditioned in seawater before pH of sweater was adjusted and followed by addition of flotation agents.

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Fig. 5. Effect of concentration of sodium hexametaphosphate at various pH values on standard flotation tests with molybdenite in which pH of seawater was first adjusted and then molybdenite was added following by flotation agents.

The results shown in Fig. 3 are replotted versus pH in Fig. 4. The results shown in Fig. 5 are replotted in Fig. 6 versus pH. Lime is used to raise pH in the flotation of Cu-Mo sulfide ores to depress pyrite. It was thus important to test how SHMP affects pyrite flotation, and pyrite depression in particular. This is shown in Fig. 7. As this figure demonstrates pyrite flotation depressed in the alkaline pH range is not restored by addition of SHMP.

4. Discussion As classical study on the effect of slime coating (clays) on wettability of coal surface revealed (Jawett et al., 1956), the use of dispersants can efficiently disperse/remove the coating from the coal surface restoring coal’s inherent hydrophobicity. In this paper we are dealing with similar situation. Molybdenite is also inherently hydrophobic. Its floatability is depressed over the pH range over which magnesium hydroxy-complexes and hydroxide precipitate. Similarly to the case studied by Jawett et al. (1956) the flotation (depressed over the pH range 9.5–10.5) can be restored with the use of hexametaphosphate, one of the very effective dispersing agents. This result is interesting from both the fundamental and practical engineering points of view. On one hand it confirms the depression mechanism postulated by us a few years ago (Castro et al., 2012, 2014), and on the other it seems to open additional alternative to prevent unwanted molybdenite depression in the flotation of Cu-Mo sulfide ores in seawater. As it is known, because of the content of pyrite in Cu-Mo ores, the flotation of these ores is carried out in alkaline solutions adjusted with the use of lime. Over the pH range needed to depress pyrite, Mg(OH)+ complex and magnesium hydroxide precipitate and depress molybdenite flotation. Practical solution may be the pre-treatment with lime of the seawater used in the flotation, the process patented by the University of Concepcion (Castro, 2010). Another option is to float at lower pH (pH 7–9) and depress pyrite not with lime but with some other reagents (e.g. sodium metabisulfite, Gorain et al., 2016). These preliminary results indicate that flotation with the use of dispersants such as sodium hexametaphosphate (SHMP) may restore depressed molybdenite

Fig. 6. Recovery of molybdenite as a function of pH at various concentrations of SHMP (replotted from Fig. 5).

floatability in seawater and thus this suggests that the traditional process in which lime is utilized to depress pyrite might still be possible. As Figs. 4 and 6 demonstrate, the tests carried out following two different procedures gave slightly different results. However, in both cases the molybdenite floatability in the alkaline pH range was restored with the use of SHMP and this important effect is the main focus of this paper. Comparison of Figs. 4 and 6 with Fig. 7 reveals that molybdenite flotation is depressed over the pH range from 9.5 to 10.5, whereas pyrite flotation is affected only when pH is higher than 10.5. These differences lead to the conclusion that while magnesium

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electrostatic interactions constitute a dominant factor in depression of molybdenite by precipitating magnesium species but this mechanism is beyond the scope of this paper. It is also to be pointed out that SHMP has the ability of forming soluble complexes with many multivalent cations, and can be used as very efficient water softener sequestering calcium and magnesium ions. According to Lu et al. (2011) it can dissolve some magnesium ions from the surface of serpentine. In line with this is removal of the magnesium hydroxy-complexes and hydroxides from the surface of depressed molybdenite. It is interesting that SHMP does not affect flotation of pyrite (Fig. 7). This may also result from the fact that while inherently hydrophobic molybdenite is floated with the use of water-insoluble hydrocarbon oils, all other sulfides, including pyrite, require the use of xanthates or xanthate-like collectors. The difference is that while the former involves only weak physical interactions between oily droplets and hydrophobic molybdenite surface, the later involves chemical/electrochemical interaction between the collector molecules and sulfide surface. Displacement of the collector from the mineral surface by precipitating hydroxide may then be much easier in the case of molybdenite.

Fig. 7. Effect of concentration of sodium hexametaphosphate at various pH values on standard flotation tests with pyrite in seawater. Lime was used to adjust pH, and potassium amyl xanthate (25 ppm) and MIBC (10 ppm) as collector and frother, respectively.

5. Conclusions 1. It was confirmed that the flotation of molybdenite in seawater is depressed over the pH range from 9.5 to 10.5, that is over the pH range over which magnesium hydroxy-complexes and magnesium hydroxide precipitate. 2. The use of sodium hexametaphosphate dispersant restores molybdenite flotation over this alkaline pH range. This opens an additional alternative to prevent the unwanted molybdenite depression in the flotation of Cu-Mo sulfide ores in seawater. 3. Pyrite flotation is depressed when pH is higher than 10.5 and the floatability of pyrite over this pH range is not restored by SHMP.

Acknowledgements The authors wish to thank CRHIAM for financing this work through CONICYT/FONDAP-15130015 project. Leopoldo Gutierrez wants to thank the support of Conicyt/Fondecyt Initiation Project No. 11140184. References Fig. 8. Calculated concentration of magnesium species in solution with magnesium concentration of 5  10 4 mol/L (Yu-lin et al., 2011).

hydroxy-complexes and hydroxide are responsible for depression of molybdenite, only precipitating calcium species depress flotation of pyrite. Various pieces of evidence gathered over many years (Fuerstenau, 1958) indicate that particle-particle interactions responsible for the slime coating are dominated by electrostatic forces. The induction time measurements carried out with molybdenite in seawater, discussed in one of our papers (Castro et al., 2014), revealed maximum molybdenite depression around pH 10. As Fig. 8 shows, Mg(OH)+ complex concentration is the highest around pH of 9.5; also Mg(OH)2 starts precipitating around pH 9.5–10. Li and Somasundaran (1991) measured zeta potential of bubbles in solution of magnesium salts and found that these bubbles acquire positive charge around pH of 10. Schott (1981) reported the iso-electric point for magnesium hydroxide to be situated around pH of 10.8. All this seems to corroborate the idea that

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