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Hydrolysis of metallic ions in mineral processing circuits and its effect on flotation Laskowski, J. (Janusz), 1936-; Castro Flores, Sergio H. Apr 30, 2017

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COM	2017	Conference	of	Metallurgists,		Vancouver,	August	27-30,	2017		     Hydrolysis of metallic ions in mineral processing circuits and its effect on flotation  J. S. Laskowskia* and S. Castrob  aN. B. Keevil Institute of Mining Engineering University of  British Columbia Vancouver, B.C., Canada            *Correponding author: jsl@apsc.ubc.ca bWater Research Centre for Agriculture and Mining (CRHIAM),  University of Concepcion, Concepcion, Chile   ABSTRACT            Hydrolysis of metallic ions depends on pH and the products of  hydrolysis either activate flotation, can lead to inadvertent activation of gangue particles, but can also depress flotation of valuable components. One of the most recently studied topics is the use of seawater in flotation. Seawater is a concentrated NaCl solution (about 0,6 mole/L) that also contains considerable amount of Mg2+ and Ca2+ ions. The hydrolysis products of Mg2+ strongly depress flotation of molybdenite over the pH ranges over which they form in the process water. This effect can be eliminated by removal of hydrolysing ions from the process water or by carrying out the flotation at the pH over which such a hydrolysis can be avoided.  The former can be exemplified by prior precipitation of the harmful hydrolysing ions as, for example, in the University of Concepcion Patent. The latter can be exemplified by the use of metabisulfite to depress pyrite in the flotation of Cu-Mo sulfide ores in seawater, the process that is carried out at much lower pH which  avoids formation of Mg2+ hydrolysis products.      Keywords  Activation, Adsorption,  Depression, Flotation, Ion  hydrolysis, Metallic ions, Seawater                   COM	2017	Conference	of	Metallurgists,		Vancouver,	August	27-30,	2017		1. Introduction            Metallic ions, depending on pH, hydrolyze in aqueous environment. In general, unhydrolyzed species, Me2+, predominate in acidic solutions, while MeOH+, Me(OH)2 and Me(OH)3- appear when pH is made more alkaline.             Metallic ions can appear in flotation pulps not only when added as flotation activators (e.g. Cu2+ in the flotation of sphalerite or as pyrrhotite’s  activator in the flotation of platinum group minerals in South Africa), they may also be present as a result of the processes in which different mineral particles suspended in water participate. The solubility of sulfides is so low that these minerals might be assumed inert. However, all the sulfides are reactive with oxygen, with the oxidation products being appreciably soluble in water. Therefore, metallic ions commonly appear in the pulps containing sulfide minerals. Perhaps the most studied have been copper sulfides, and it is well established that copper ions appear in the pulps resulting from grinding copper ores (Lascelles & Finch, 2002).            For our understanding of the hydrolysis phenomena and the effect of ion hydrolysis on adsorption of these ions onto mineral particles the most important have been publications coming from two teams: Professor Tom Healy’s and Professor Maurice Fuerstenau’s.             Experimental adsorption isotherm for Co2+ ions on silica is shown in Figure 1 (James & Healy, 1972a). The figure also shows hydrolysis products of Co2+ as a function of pH for Co2+ concentration of 1.2x10-4 M. As it is seen, over the acidic pH range over which Co2+ ions predominate in solution the adsorption is negligible. However, once the pH of CoOH+ formation is exceeded the adsorption  of Co onto silica dramatically increases.                               Figure 1. Experimental adsorption isotherm for Co2+ at 1.2 x 10-4 M on silica at 25 oC. Hydrolysis products of  Co2+ are also shown as the percentage of each aquo comlex as a function of pH (from James & Healy,1972a; with permission of Elsevier).            The importance of these findings for understanding of flotation phenomena was clearly demonstrated by M.C. Fuerstenau and Palmer (1976).  Figure 2 shows that quartz (the mineral that is negatively charged in almost whole pH range) as expected does not float with anionic collector. This mineral can, however, be activated by metallic ions. It is activated over the pH range over which the tested metallic ions hydrolyse, form hydroxy-complexes and adsorb onto minerals. The adsorbing hydrolyzed species depending on the flotation process can either activate or depress this process. What is important here is the adsorption of metallic ion hydrolysis products and the fact that this adsorption takes place over the pH range over which such hydrolysis products appear in solution. Since the ion hydrolysis phenomena are well known it is possible to manipulate the flotation process in such a way that it either leads to activation or depression.   COM	2017	Conference	of	Metallurgists,		Vancouver,	August	27-30,	2017		                      pH0 2 4 6 8 10 12 14Flotation recovery,  %020406080100Col 1 vs Col 2 Col 3 vs Col 4 Col 5 vs Col 6 Col 7 vs Col 8 Col 9 vs Col 10 Col 11 vs Col 12 Fe3+ Al3+ Pb2+ Mn2+ Mg2+ Co2+pH of hydroxy complex formationFeOH2+     AlOH2+      PbOH+      MnOH+     MgOH+     CoOH+	   Figure 2.  The pH ranges of quartz flotation with sodium alkyl sulfonate (10-4 M) in the presence of different metallic ions (10-4M) (from Fuerstenau & Palmer, 1976; with permission of the Society for Mining, Metallurgy and Exploration).             The process water recycled in plant closed water circuits constitutes a very concentrated electrolyte solution. Perhaps the best example is Mt. Keith plant in Western Australia. To mimic this plant process water the synthetic process water prepared with the use of: 48.2 g/L of NaCl, 24.4. g/L of MgSO4.7H2O, 2.1 g/L of KCl, 1,25 g/L of NaCl and 0.29 g/L of NaHCO3 is often used (Merve-Genc et al., 2012). Limited resources of fresh water in many parts of the world (as, for example, in the Atacama desert in Chile) make the use of seawater the only sustainable solution. Sea water is a concentrated solution of NaCl (about 0.6 M) with high content of Mg2+ (1.3 g/L) of Ca2+ (0.4 g/L) and also sulfate ions (2.7 g/L). The ions present in such process waters – as it will be shown -  may heavily affect the flotation processes. To complicate it further, in some flotation processes, metallic ions are added to the pulp to activate flotation of some particular minerals.   2. Cu2+ ions as activators in flotation            As it is known sphalerite does not float without activation and copper sulfate is commonly applied to activate it. In our studies on activation of sphalerite by Cu2+ we used the electrokinetic measurements (Laskowski at al.,1997) following the interpretation introduced by James and Healy (1972b). Figure 4 shows schematically the general zeta potential – pH curve in the presence and absence of hydrolysable ions. The curve reveals three charge point reversals (CR.1, CR.2 and CR.3). The position of CR.1 coincides with the i.e.p. of SiO2 (solid particles used in the experiment). The high pH charge reversal point (CR.3) results from coating of SiO2 surface by metal hydroxide; CR.3 is the i.e.p. of this metal hydroxide. The CR.2 point is at the pH of surface precipitation of metal 3ydroxyl-complexes induced by pH.   COM	2017	Conference	of	Metallurgists,		Vancouver,	August	27-30,	2017		                     Figure 3. Effect of pH and conditioning time on the zeta potential of sphalerite in the presence of copper ions. A, in H2O; B, in 10-3 M CuSO4, 5 min; C, in 10-3 M CuSO4, 15 hrs; D, zeta potential of chalcocite (from Laskowski et al., 1997; with permission of Elsevier).                          Figure 4. Schematic illustration of the general electrophoretic mobility behavior of colloidal systems  in  the presence and absence of hydrolysable ions (from James & Healy, 1972b; with permission of Elsevier).            Following the James and Healy’s interpretation, Figure 3 shows that copper hydroxy-complexes accumulate on the sphalerite surface over the pH range over which Cu2+ ions hydrolyze. These tests also show that since copper hydroxide is not stable on the ZnS surface it is converted into copper sulfide COM	2017	Conference	of	Metallurgists,		Vancouver,	August	27-30,	2017		(activation) with time.              Exceptional activity of Cu2+ hydroxy-complexes confirms Figure 5 (Iskra & Laskowski, 1969). It shows the effect of pH on the flotation of quartz made hydrophobic by methylation (by reaction with trimethylchlorosilane) when frother is the only reagent used. As this Figure demonstrates, at low copper ion concentration the flotation is depressed over a narrow pH range from 6.5 to 7.5. The agreement of these flotation tests with the distribution diagram for Cu2+ ions (Fig. 6) is excellent. The pH range over which the flotation of  hydrophobic methylated quartz is depressed coincides perfectly with the pH range over which Cu2+ hydroxy-complexes predominate in solution.                           Figure 5.  Flotation of methylated quartz at 20 mg/L concentration of α –terpineol without  Cu2+ (curve A), at 1.5x10-4 M Cu2+ (curve B), and at 1.5x10-3 M Cu2+ (curve C) (from Iskra & Laskowski, 1969; with permission of Taylor and Francis).                                                                                  Figure 6. Logarithmic concentration diagram for 1x10-4 Cu2+ (from Fuerstenau & Palmer, 1976; with permission of the Society for Mining, Metallurgy and Exploration).            The hydroxy-complexes may be responsible for either depression (Figure 5) or activation (Figures 2 and  7). As Figure 7 shows the methylated hydrophobic quartz, which was depressed by Cu2+  ion hydrolysis products, over exactly the same pH range may be activated. The activation takes place when the adsorbed hydrolysis products COM	2017	Conference	of	Metallurgists,		Vancouver,	August	27-30,	2017		become adsorption centers for the added collector. This happens when to the depressed methylated quartz sodium oleate collector is added. Fuerstenau et al (1988) used talc to show how adsorption of hydrophilic hydroxy-complexes  depress flotation of hydrophobic by nature talc.                                    Figure 7. Flotation of methylated quartz as a function of pH in 1.5 x 10-5 M solution of Cu2+ at  α-terpienol concentration of 20 mg/L (curve o), and at sodium oleate concentration of 1.5 x 10-5 (curve ●)             In South Africa in the flotation of sulfide ores containing platinum group elements copper ions are utilized to activate and enhance recovery of pyrrhotite. This can, however, cause inadvertent activation  of gangue. Figure 8    0 4 8 12 16 20Time, min020406080100Pyroxene recovery. %pH = 9pH = 4No reagents                                         Figure 8. Effect of pH on flotation of pyroxene with SIBX at 5x10-5 M Cu2+  (from Mailula et al., 2003; with permission of CIM ). COM	2017	Conference	of	Metallurgists,		Vancouver,	August	27-30,	2017		shows the effect of pH on flotation of pyroxene with iso-butyl xanthate at 5 x 10-5M Cu2+ (Mailula et al., 2003). As this figure indicates while at pH 4 the activation is negligible, it is very substantial at pH of 9. This effect can be reduced when addition of copper salts follows  addition of xanthate.     3. Flotation in seawater       In our recent paper (Laskowski & Castro, 2015)  the high electrolyte process water systems were classified into:  A, High electrolyte concentration systems which do not require adjustment of pH (A1) and which require pH changes (A2); B, The system in which collector precipitates with ions present in the pulp; C, Saturated NaCl-KCl brines as in the flotation of potash ores.            Salt flotation process (Klassen & Mokrousov, 1963), the process in which inherently hydrophobic minerals are floated in electrolyte solutions without any other  reagents, is the best example of A1. The flotation of Cu-Mo sulfide ore in seawater, the process in which pH is raised using lime to depress pyrite, exemplifies A2. The flotation of phosphate ores in seawater with the use fatty acid collector exemplifies case B. In this case Ca2+ and Mg2+ ions must be complexed and removed from the system to avoid precipitation of the collector (Yousef et al., 2003; Nanthakumar et al., 2009). 6-7 molar NaCl-KCl saturated brines constitute a special case and require a separate discussion (Laskowski, 2013).            The main problems in flotation of most sulfide ores result from the presence of pyrite. Common way to depress its flotation is by the use of lime and highly alkaline pH. In the case of Cu-Mo ores these problems result from the fact that this widely used flotation technology was developed to control flotation of pyrite and not to optimise flotation of molybdenite. Lime and high pH depresses not only pyrite but it is also responsible for poor recovery of molybdenite (Castro, 2012; Castro et al . 2014).           The main thesis of this paper is that the depressing effect of the hydrolysis products of metallic  ions can be avoided either by removing these ions or the products of their hydrolysis from the flotation system, or by carrying out the process at the pH range over which these metallic ions do not hydrolyze. The research needed to develop the technology of flotation in which seawater is used instead of fresh water can be broadly split into two streams: water desalination projects aimed at reduction  of seawater salinity, and physico-chemical studies aimed at better understanding of what really causes molybdenite depression. While water desalination by reverse osmosis is now seriously considered by many companies, the University of Concepcion Patent (Castro, 2010) shows that removal of all ions from seawater is not necessary since only some of them cause depression of molybdenite. This latter technology will be discussed further in this paper.      Previous publications showed that magnesium hydroxy-complexes and magnesium hydroxide  accumulatying on the surface of molybdenite particles are responsible for molybdenite depression (Castro 2012, Castro et al., 2014; Nagaraj et al., 2016). Also, depressing effect of kaolinite on chalcopyrite flotation in seawater  is        COM	2017	Conference	of	Metallurgists,		Vancouver,	August	27-30,	2017		  pH4 6 8 10 12Induction time, ms010203040Molybdenite recovery,  %020406080Induction time, MgCl2 1x10-3 MMicroflotation ,MgCl2 1x10-3 MBoundary Mg(OH)2 precipitation Figure 9. Induction time and molybdenite recovery as a function of pH in 1x10-3 M MgCl2 solutions (from Castro at al., 2014) is the most severe in the pH range over which Mg2+ hydrolyze (Uribe et al., 2017a). In our more recent tests the induction  time technique was used to study the effect of Mg2+ ions on wettability of molybdenite particles, and these tests, as shown in Figure 9, entirely confirms  the previous conclusions (Castro et al., 2014).  For simplicity only the results obtained in 10-3 M MgCl2 solution are shown here, the results obtained  in seawater and in the 0.6 M NaCl solution containing 1,300 ppm Mg2+ ions totally agree with what is shown in Figure 9. All this clearly demonstrate that the Mg2+ ions hydrolysis products are responsible for the observed phenomena.   If coating of the molybdenite surface by precipitating magnesium species is responsible for depression (as in the case of slime coating) then dispersion of these coagulating particles should be able to restore molybdenite   Figure 10. Recovery of molybdenite as a function of pH at  various concentrations of SHMP (from Rebolledo et al., 2017). COM	2017	Conference	of	Metallurgists,		Vancouver,	August	27-30,	2017		flotation. This was tested (Rebolledo et al., 2017). Sodium hexamethaphosphate (SHMP) was used as a dispersant since Lu et al. 2011) found that SHMP very actively interacts with Mg2+ ions. Figure 10 shows that the molybdenite flotation depressed over the pH range from  9.5 to 10.5 can be restored when SHMP is utilized.  In the same series of the tests also the effect of pH and added SHMP on pyrite flotation was studied. It was found  that depression of pyrite started only when pH of 10 was exceeded, that is only when Ca2+ hydrolysis products started appearing  in the solution.   The results which were selected to illustrate the main thesis of this paper all agree that the key factor  in all these results is pH: the flotation results are not affected by pH when it is far away from those in which Mg2+ ions hydrolize. When the process is carried out at the pH at which Mg2+ hydrolyze these hydrolysing ions must be removed from  the system. In the University of Concepcion Patent (Castro, 2010)  this is achieved by treating  seawater with  lime, precipitation of magnesium hydroxides and its separation from the treated process water by decantation/settling. When pyrite depressant that works in a slightly acidic environment, that is far from the dangerous pH range, is used the flotation of molybdenite is improved (Uribe et al., 2017b). Figure 11 shows pH ranges of hydrolysis for  Mg2+ ions.                                   Figure 11. Logarithmic concentration diagram for 5 x 10-4 M Mg2+.      4. Conclusions  1. Metallic ions hydrolyze in water in a given pH range. 2. The hydrolysis products are exceptionally surface active and can either activate or depress floatability of given particles. 3. The Mg2+ hydrolysis products tend to accumulate on the surface of molybdenite particles and depress their flotation.  4. Seawater  is a concentrated solution of NaCl (about 0.6 M) that also contains Mg 2+ ions (1,300 mg/L) and Ca2+ ions (400 mg/L). The presence of Mg2+ ions in seawater is the main reason for depression of molybdenite flotation when pH is raised with lime to depress pyrite. COM	2017	Conference	of	Metallurgists,		Vancouver,	August	27-30,	2017		5. Depression of molybdenite in seawater can be reduced by either elimination of such hydrolyzing ions from the process water (desalination, prior precipitation of Mg2+ ions from seawater) or by carrying out the flotation  at much lower pH using the pyrite depressant that works under acidic conditions.     References  Castro, S., 2010. Proceso para pre-tratar agua de mar y otras aguas salinas para su utilización en procesos     industriales. University of Concepcion. Chilean Patent, No 00475/INAPI (May 12th, 2010).  Castro, S., 2012. Challenges in flotation of  Cu-Mo sulfide ores in sea water. In J. Drelich (Ed.), Water  in Mineral   Processing, SME, pp. 29-40.  Castro, S., Uribe, L. & Laskowski J.S., 2014. Depression of inherently hydrophobic minerals by hydrolyzable   metal cations: molybdenite depression in seawater. Proc. 27th Int. Mineral Processing Congress, Santiago,   pp. 207-217.  Fuerstenau, M. C. & Palmer, B. R., 1976. Anionic flotation of oxides and silicates. In M.C. Fuersteanu (Ed),   Flotation – A.M. Gaudin Memorial Volume, Vol. 1, pp. 148-196.   Fuerstenau, M.C., Lopez-Valdivieso, A. & Fuerstenau, D.W., 1988. Interfacial phenomena involved in  	 talc	flotation and depression. Int. J. Mineral Processing, 23, 161-170.  Iskra, J. & Laskowski J.S., 1969. Copper ions in the flotation process: effect of CuSO4 on flotation of methylated  quartz. Trans. IMM, Sec. C., 78, 113-115.   James, R.O. & Healy, T.W., 1972a. Adsorption of hydrolysable metal ions at the oxide-water interface, Part I,   J. Coll. Interf .Sci., 40, 42-52.  James, R.O. & Healy, T.W., 1972b. Adsorption of hydrolysable metal ions at the oxide-water interface, Part II,   J. Coll. Interf. Sci., 40, 53-64.  Klassen, V.I. & Mokrousov, V.A., 1963. An Introduction to the Theory of Flotation, Butterworths, London, pp.   338-142.  Lascelles, D. & Finch, J. A., 2002. Quantifying accidental activation. Parts I & II. Minerals Engineering, 15, 567-  576.   Laskowski, J.S., Liu, Q. & Zhan, Y., 1997. Sphalerite activation: flotation and electrokinetic studies, Minerals   Engineering, 10, 787-802.  Laskowski, J.S., 2013. From amine molecules adsorption to amine precipitate transport by bubbles: a  potash           ore flotation mechanism,  Minerals Engineering, 45, 170-179.  Laskowski, J. S. & Castro, S., 2015. Flotation in concentrated electrolyte solutions, Int. J. Mineral Processing, 144,   50-55.  Merve-Genc, A., Kilickaplan, I. & Laskowski, J,.S., 2012. Effect of pulp rheology on flotation of nickel sulphide   ore with fibrous gangue particles, Canadian Metallurgical. Quarterly, 51, 368-375.   Mailula, T. D., Bradshaw, D. J., Harris, P. J. & Laskowski, J. S. (2003). Copper ions in flotation of  sulfide ores. In:   C.O. Gomez and C.A. Barahona, (Eds.). Proc. Copper 2003 Int. Conference, Vol. 3, pp. 243-256. COM	2017	Conference	of	Metallurgists,		Vancouver,	August	27-30,	2017		 Nagaraj, D. R., Farinato, R. & Tercero, N., 2016. Probing the interface and interphase region of molybdenite edge   and face in ore flotation pulps: effect of Mg2+ ions and their hydrolysis products. Proc. 28th  Int. Mineral   Pocessing Congress, Quebec City, 2016.  Rebolledo, E., Laskowski, J.S., Gutierrez, L. & Castro, S., 2017. Use of dispersants in flotation of molybdenite in   seawater, Minerals Engineering, 100, 71-74.  Uribe, L., Gutierrez, L., Laskowski, J.S. & Castro, S., 2017a. Role of calcium and magnesium cations in the   interactions between kaolinite and chalcopyrite in seawater, Physicochemical Problems of Mineral   Processing, 53, 737-749,   Uribe, L., Gutierrez, L., Castro, S. & Laskowski, J.S., 2017b. Effect of redox conditions and copper  ions on the   depression of pyrite by sodium metabisulfite and sodium sulfite in seawater, CIM Journal, submitted.   .   


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