Selective Precipitation of Pt and Base Metals in Liquid-Liquid Chloride Systems

Transcription

1 Selective Precipitation of Pt and Base Metals in Liquid-Liquid Chloride Systems J. Siame, H. Kasaini Abstract The concept of selective precipitation of Pt and base metals in Liquid-Liquid (L-L) chloride media using S-bearing liquids was investigated in a stirred batch reactor. Pt ions react readily with S-atoms to form stable PtS precipitates in acidic media. The source of S-atoms was sodium thiosulphate solution (Na 2 S 2 O 3 ). All the metal ion species were prepared in chloride media before the experimental work. The selective precipitation of Pt from base metals was achieved at small dosages of Na 2 S 2 O 3 ( M) and short time intervals (3-5 minutes). Experimental data were interpreted in terms of Pt selectivity and recovery to precipitates, order of reaction and reaction constants. Differential method of data analysis was used to obtain an nth-order rate equation. The order of reaction for Pt was approximately unit (n = 1.3 and k R = 0.004). Pt was selectively precipitated from dilute acidic chloride solutions containing base metals. A solid precipitate (8.22 g) was successfully obtained at an initial Pt concentration of 100 mg/l in solution and HCl concentration of 4 M. The recovery of Pt was 99.5%. The separation factors associated with batch tests in L-L chloride system were found to be of the following order { β (Pt/Fe) = 112 > β (Pt/Co) = 9 > β (Pt/Cr) = 5 }. The mass of Sulphur required for precipitating 1g of Pt in L-L precipitation system was found to be 3.2 g. Keywords Platinum, selective precipitation, separation factor, stirred batch reactor, thiosulphate. O I. INTRODUCTION NE of the principle causes of water pollution is the industrial discharge of wastes and by-products. The pollution of surface and ground water by heavy metal species represents a significant environmental threat. The mining and mineral processing industries generate large quantities of metal polluted wastewaters, while acid mine drainage (AMD) is one of the most widespread forms of pollution in the world [1, 2]. Therefore, the global concerns about the impact of wastewater effluents from the mines on the environment have led to the mineral processing industry leaders to adopt new strategies of recycling or capturing mine waste. Several treatment techniques have been used to remove heavy metals from wastewater. The common practice of recovering platinum group metals (PGMs) and base metals from industrial solutions is through hydroxide precipitation and ion exchange processes using imported resins. In this regard, there is a lot of data on precipitation of metal J. Siame is with the Copperbelt University, School of Mines and Mineral Sciences, Department of Chemical Engineering, P.O. Box 21692, Kitwe, Zambia. (Phone: ; Fax: ; E- mail:jsiamem@yahoo.co.uk). H. Kasaini is with Rare Element Resources Corporation, Denver, USA ( hydroxides in liquid-liquid systems. In the platinum industry, resins are widely used for separating and purifying individual elements in the PGM group (Pt, Pd, Ru, Rh, Ir and Os). Hydroxide precipitation and ion exchange processes are driven by the cost of reagents such as quicklime, soda ash and resins. In case of South Africa, imported resins constitute a huge proportion of the operational budgets in metallurgical plants. Apart from ion exchange processes (in which the resins are used), solvent extraction (SX) is an alternative process for purifying base metals and PGMs. However, efficient performance of SX plants requires stringent measures such as low dissolved silica, low concentration of metals with high oxidation numbers (Fe 3+, Cr 3+, etc.), and usage of selective extractants which are equally very expensive. Any attempts to replace ion exchange units and SX mixer-settlers with a costeffective and selective precipitation process generate huge interest from industry. Sulphide precipitation of metal ions is a viable alternative process because of the possible high degree of metal ions removal over a broad range of HCl concentration. Moreover, it has several advantages over hydroxide precipitation, such as the low solubility and high stability of metal sulphides. Most of the metals can, therefore, be precipitated as sulphides. In addition, metal sulphide sludge has better dewatering characteristics than hydroxides. Precipitation also has wide applications in hydrometallurgy [3-6], water treatment [7-11] and the production of sulphide films for semiconductors devices [12] and cathode ray tubes [13]. Solubility product values of several metal sulphides as well as solubility product expressions with respect to activities are shown in Table 1. However, the solid products produced by precipitation is often amorphous and it solubility depends on the method of precipitation, particle size and age [14]. A comparison of solubilities of metal hydroxides and metal sulphides is given in literature [15]. The most important metal sulphide reactions are: H 2 S, HS H + Me 2+ + S 2 MS (solid) (1) where Me 2+ is any bivalent metal ion. With sulphide precipitation a residual concentration of less than 0.1 mg/l can be achieved at 4 < ph < 12. Currently, two methods, which differ in the technique of delivering sulphide ions, are available [16] namely, the soluble sulphide method and sparingly soluble sulphide method. Table I quantifies the sparingly soluble nature of several metal sulphide compounds. 88

2 TABLE I SOLUBILITY PRODUCTS (K sp ) OF SOME METAL SULPHIDES AT 25 C [15]. Compound Formula K sp Log K sp Chromium (III) Sulphide Cr 2 S Cobalt (II) Sulphide CoS Iron(II) Sulphide FeS Platinum(II) Sulphide PtS A. Previous Work Precipitation of PGMs with solutions containing sulphur atoms has been reported in literature by several researchers. Selective precipitation of PGMs from mixed chloride solutions using ammonium chloride solution (NH 4 Cl) has been reported by Habashi [17] and Schreier, [18] and a longer contact time and high temperatures were found to be having a negative influence on precipitation process [18]. Increased contact time results in co-precipitation of impurities and thus reduces purity and at higher temperatures recovery is compromised because the solubility of PGMs increases. It is known that more or less all PGM-chloride complexes are susceptible to hydrolysis, especially at higher temperatures, and this can have influence on the completeness of the reaction and the separation effect [18]. The use of sodium formate to precipitate precious metals from acidic media has also been reported as an alternative to zinc reduction process, and the process is ph dependent and it is also not easy to precipitate iridium [19]. Kasaini et al. [20] performed some tests which showed that when sulphur atoms are immobilized on the Activated Carbon (AC) surface, the affinity properties towards Pd anions are developed on the surface of carbons in solution. The concepts above support the preposition that the presence of sulphur atoms in chloride solution will induce a selective reaction with PGMs and base metals depending on the reactivity and charges of the ions in solution. Unfortunately, not much is known about the exact reaction rate of S 2- with the metal ions, although some precipitation models are available in literature. The reaction rate at which S 2- and metal ions react to form solid metal sulphide is determined by two processes: firstly, the formation of new metal sulphide nuclei and, secondly, the growth of existing precipitates. These precipitation models state that the rate of formation of new precipitate particles is a highly non linear function of the concentration of the reacting components [21], [22]. Furthermore, the rate of growth of existing particles seems to be the result of a number of process steps (like diffusion of reacting components to the precipitate surface and the rate at which the ions are incorporated in the crystal), all of which can be rate limiting. The aim of the present work is to study the kinetics of Pt sulphide precipitation in liquid-liquid chloride systems containing excess base metal ions in an attempt to understand the basic mechanism that control the process. A search of the literature reveals that only a few kinetics studies have previously been performed on the aqueous phase reactions which produce sulphides deposits. B. Kinetics Study and Process Modelling The kinetic study was conducted in order to establish the parameters which affect metal (iron, cobalt and chromium) precipitation in liquid-liquid and gas-liquid chloride systems. The scope of the study was predominantly concerned with the precipitation rates of platinum as this was the main target metal in the mixed chloride systems. Furthermore, the precipitation rate constants (k R ) and separation coefficients β (Pt/Fe) or β (Pt/Co) or β (Pt/Cr) of all the metals were quantified for liquid-liquid systems. C. The nth-order reaction model In liquid-liquid (L-L) precipitation systems, the rate of reaction in the solution phase where the metal ions react with sulphur ions can be evaluated by using the nth-order reaction model: r A = dc A dt = k RC A n (2) where dc A dt is the rate of disappearance of species A in solution (L/mol.s), C A is the concentration of species A (mol/l) at time t (seconds), k R is the rate constant and n is the order of reaction. The nth-order of reaction makes it possible to determine the order of a reaction and rate constant. A differential method of data analysis may be applied to determine the rate equation by plotting concentration (C A ) versus time (t), [23]: Taking logarithms of both sides of Eq. 3 gives log( dc A /dt) = log k R + n log C A (3) A plot of log(dc A dt) versus log (C A ) will yield a straight line. The slope and intercept of the best line gives (n) and (log k R ) respectively. Therefore, the rate equation is: r A = dc A dt = k R D. Separation coefficients L n 1 mol n 1 s C A n, mol L s Separation coefficient (β) is the ratio of rate constants values for the respective metals in solution. It is a dimensionless value. In this research study, the separation coefficients are evaluated as follows: β Pt/Fe = k R(Pt), β k Pt/Co = k R(Pt), R(Fe) k R β = k R(Pt) (Co) Pt/Cr k R (Cr) (5) A high value of β indicates the extent to which a metal is precipitated. II. MATERIALS AND METHODS Reagents used throughout the study were pure analytical standard grade solutions of platinum (H 2 PtCl 6 ), iron (FeCl 2 ), cobalt (CoCl 2 )R and chromium (CrCl 3 ) of 1000 mg/l each; 32% analytical grade HCl acid solution. Commercially (4) 89

3 available hydrated sodium thiosulphate (Na 2 S 2 O 3 5H 2 O) crystals were purchased from Merck Chemical (Pty) Ltd in South Africa. The standard solutions were prepared in equal concentrations (100 mg/l) using distilled water. Distilled water was produced in the laboratory at Tshwane University of Technology, South Africa. Sodium thiosulphate (Na 2 S 2 O 3 ) solutions of different concentrations (0.05, 0.1 and 1.0 M) were prepared by dissolving a known quantity (mass) of sodium thiosulphate pellets in small amounts of distilled water placed in a 1000 ml volumetric flask. The mixture was stirred until the crystals completely dissolved in distilled water and then more distilled water was added to make the solution reach the flask s etched mark of 1000 ml. Sodium thiosulphate solution was used as a precipitant for metal in all the L-L precipitation tests. Different concentrations of hydrochloric acid solutions (1.0, 3.0 and 4.0 M) were prepared by diluting HCl acid solution (10 M, 32% HCl) with known quantities of distilled water. A. Analytical Techniques A.1 Solution samples Barren solutions which remained after precipitation were analyzed for platinum, iron, cobalt and chromium by means of an Inductively Coupled Plasma Optical Emission Spectrophotometer (ICP OES) instrument (Shimadzu model ICPE 9000) at Tshwane University of Technology (South Africa). The raw data for the analyzed samples is given in appendix G. A.2. Solid samples The solid samples (precipitates) were analyzed using a scanning electron microscope (SEM) instrument at Nuclear Energy Corporation of South Africa (NECSA). A.3 Experimental set-up Initial liquid-liquid (L-L) batch tests were carried out in 1 L glass vessel equipped with a perforated lid. The reactor vessel consisted of the following: electrical stirrer, heating element (hot plate), thermometer and ph meter. The reactor vessel is shown in Fig. 1 and the operation conditions of the reactor vessel are listed in Table II. A.4 Experimental Procedure Bench scale L-L batch tests runs were conducted using a 1 L glass vessel. A mixed chloride solution was prepared containing PtCl 6 2 anions and base metal cations (Fe 2+, Co 2+ and Cr 3+ ) in equal concentration (100 mg/l) and high acidic strength (1 M HCl). Na 2 S 2 O 3 solution was used as a precipitant for metal ions. Fig. 1. Schematic diagram of experimental setup for liquidliquid precipitation under ambient conditions: stirred vessel reactor (1); electric stirrer (2); ph meter (3). TABLE II EXPERIMENTAL CONDITIONS: L-L PRECIPITATION. Temperature 25 ºC Liquid volume 1 x 10-3 m 3 Stirrer speed 2-16 s -1 Initial conc. of Na 2 S 2 O M Feed-precipitant ratio 1:1 Contact time 40 min Metal ions conc. (Pt, Fe, Co, Cr) 100 mgl -1 HCl conc. 1-4 M The initial concentration of Na 2 S 2 O 3 solution was varied in the range M. The molar ratio of the feed solution to the precipitant solution was kept constant at 1:1 while contact time was averaged 24 hrs. For each test, samples were taken from the reaction vessel at the same point using a 10 ml syringe for the time intervals of 0, 1, 3, 5, 7, 10, 15, 20, 25, and 40 minutes. The samples were filtered immediately to prevent further precipitation and the clear filtrates were analyzed for solution metals by means of Inductively Coupled Plasma-Optical Emission Spectrometry (Shimadzu model ICPE , ICP-OES). Filtration of suspensions may not be effective to remove all colloidal micro and nanoparticles and in fact some unreacted thiosulphate molecules might remain in the filtrate sample. It is difficult to stop a reaction from occurring in a homogeneous filtrate sample containing residual metal ions and thiosulphates molecules. Therefore, it was found necessary to standardize both the time required for filtration and time lag before ICP analyses. The data obtained was used to establish the average or estimated reaction kinetics. In the case of sampling the precipitates, sufficient time was allowed to allow for extensive nucleation, crystal growth and agglomeration of particles to take place. Therefore, the suspension and precipitates were filtered from solutions after 24 hours; the composite precipitate from the batch glass vessel was filtered and dried at room temperature for 24 hrs. The induction time for precipitate formation was determined visually and recorded. Solid precipitate samples were analysed by XRD, SEM and EDS to determine the level of impurities and morphology. The results were analysed mainly to determine the amount of sulphur (from Na 2 S 2 O 3 ) required precipitating metals, Pt selectivity and recovery of precipitates, order of reaction and reaction constants. 90

4 III. RESULTS AND DISCUSSION The selective precipitation characteristics of platinum and base metals were investigated in a batch liquid-liquid (L-L) using sodium thiosulphate. According to speciation chemistry, platinum may exist as chloro-complex anions with or without water ligands. Sodium thiosulphates is the potential precipitant for transitional metals. The results obtained in the present work are discussed. A.1 Effect of sodium thiosulphate concentration Fig. 2, illustrate the change in metal concentrations of [PtCl 6 ] 2-, Fe 2+, Co 2+ and Cr 3+ in chloride concentration of 4 M, HCl. The concentration of sulphur containing molecules in the liquid precipitant depends on the initial concentrations of thiosulphate reagent (Na 2 S 2 O 3 ), which was varied in the range (0.05 M - 1 M). The molar ratio of the metals (Pt, Fe, Co and Cr) to sulphide ion concentrations was maintained at 1:1. In Fig. 2, metal ion concentrations decreased with contact time at different rates. The rate of Pt precipitation was found to be faster and preferential at low sodium thiosulphate concentration (0.05 M). In fact, Pt metal ions were precipitated out within short contact time (<10 min). Except for Cr, all metals precipitated out of solution after prolonged contact time. According to literature, chromium does not form the sulphide precipitates easily at standard temperature and pressure and normally requires an additional reduction step before precipitation. The reduction step is easy, but produces a significant quantity of sludge [24]. Therefore, the poor precipitation of Cr was attributed to a much higher solubility product of Cr 2 S 3 [15] than the other metal sulphides (Pt, Fe and Co). With regard to rapid precipitation of Pt metal ions, this result was attributed to a much lower solubility product as compared to other metal cations of iron and cobalt (PtS: K sp = ; FeS: K sp = ; and CoS: K sp = ). The precipitation reaction of PtS is indeed due to ionic bonding, so are the rest of the metal sulphides forming. The Pt metal ions are electrophilic that is why the lone electron pair on water molecules and the negative chloride ions form complexes with Pt metal ions. Similarly, the other metal cations of iron, cobalt and chromium form complexes with the chloride ions before reacting with sulphide ions to precipitate as metal sulphides. With regard to stability, the Pt metal ions require lots of chloride ions. This implied that at low Cl - concentration, water ligands easily bond with Pt metal ions. Therefore, in the presence of excess negatively charged sulphide ions, water ligands as well as chloride ions may be replaced chemically within the chloro-complex structure in order to crystallize the platinum ions. By contrast, cations (Fe 2+, Co 2+ and Cr 3+ ) form stable structures with chloride ions and therefore chemical reduction with sulphide ions is a slow process. It is therefore possible to separate Pt metal ions from base metal ions before disposal of heavy metal sulphide precipitates. In previous studies, the applications of sulphides (sodium hydrogen sulphide, NaHS) to precipitate PGMs and gold from a mixed sulphate/chloride solution have been reported [25]. However, the selectivities of PGMs were low and unsatisfactory. In the present study, the ability of sodium thiosulphate having two sulphur atoms to react with Pt ions selectively was high as shown in Fig. 2. According to Fig. 2 selectivity of Pt over base metals reached infinity within a short contact time (<10 min). The amount of platinum recovered from all chloride solutions averaged above 99.5% in short contact time (<10 min) as shown in Fig. 3. The first and second order kinetics did not fit the precipitation data in Fig. 2. Hence the expressions for the n-th order of reaction, as described by combining Eq. 2 and Eq. 3 was used to evaluate the order (n) and rate constant (k R ) of precipitation as shown by Fig. 4. The order of reaction for platinum was found to be approximately unity (n = 1.3 and k R =0.004). The result implied that the rate of reaction between sulphide ions and Pt chloro-complex anions was affected by the sulphide ion concentrations and proceeded rapidly at low Na 2 S 2 O 3 concentration. Fig. 2. A plot of metal concentration vs. contact time. Precipitation kinetics of Pt and base metals from a mixed solution in L-L batch test at ambient conditions, stirring speed = 500 rpm; molar feed ratio metal ions: sulphide ions = 1:1; [HCl] = 4 M; [Na 2 S 2 O 3 ] initial = 0.05 M; [Me n+ ] feed = 100 ppm; contact time = 40 min. Fig. 3. A plot of percentage recovery vs. [Me n+ ] o. Effect of [Me n+ ] o on recovery (%) of Pt and base metals from a mixed solution in liquid-liquid batch test after 24 h, stirring speed = 500 rpm; molar feed ratio metal ions: sulphide ions = 1:1; [HCl] = 4 M; [Na 2 S 2 O 3 ] initial = 0.05 M. 91

5 also studied. According to Fig. 6, the final mass of Pt in the precipitate (reactor vessel discharge solution) increased significantly with increase in HCl acid strength up to 8.4 g (Pt recovery = 99.5%) at ambient conditions. This result was attributed to the enhanced activities of sulphur and platinum ions in acidic chloride solutions. Fig. 4. A plot of log( dc Pt dt) vs. log C Pt. Determination of (n) and (k R ) values of Pt using differential method of data analysis at ambient conditions, stirring speed = 500 rpm; molar feed ratio platinum: sulphide ions = 1:1; [HCl] = 4 M; [Na 2 S 2 O 3 ] initial = 0.05 M; [Pt] feed = 100 ppm; contact time = 40 min. A.2 Effect of hydrochloric acid concentration According to the speciation diagrams of Pt in chloride solutions [26-28], the most abundant Pt species are chlorocomplex anions (HCl conc. >0.01 M). Metal speciation of platinum in the bulk solution is dependent on chloride ion concentration. For solution with ph values ranging from , platinum exists as [PtCl 3 ] - (15%), PtCl 2 (65%), and PtCl + (20%) complexes. Platinum (IV) in chloride solutions can form different kinds of chloride complexes depending on hydrochloric acid concentration as shown in Table III. Therefore, in this investigation platinum (IV) forms only one kind of complexes [PtCl 6 ] 2 in concentration range of 1-4 M hydrochloric acid [29]. Fig. 5. A plot of Pt concentration vs. contact time. Precipitation kinetics of Pt at different acid strength (1, 3 and 4 M HCl) in liquid-liquid precipitation batch test at ambient conditions, stirring speed = 500 rpm; molar feed ratio platinum: sulphide ions = 1:1; [Na 2 S 2 O 3 ] initial = 0.05 M; [Pt] feed = 100 ppm; contact time = 40 min. TABLE III DIFFERENT KINDS OF CHLORIDE COMPLEXES [29]. Acid concentration Complex form M HCl [PtCl 6 ] M HCl 20% [Pt(OH)Cl 5 ] 2-, 80% [PtCl 6 ] M HCl [Pt(OH)Cl 5 ] 2-, [Pt(OH) 2 Cl 4 ] 2- ph 7-13 [Pt(OH) 5 Cl] 2- ph > 13 [Pt(OH) 6 ] 2- Fig. 5 shows the change of Pt concentration in the solution while precipitation was occurring. At high acid strength (4 M, HCl) or under excess proton concentration (H + ), the rate at which Pt ions disappeared from solution was comparably faster than the base metal ions. In retrospect, this result means that rapid precipitation of Pt occurred in chloride solutions at high acid strength in the first three (3) minutes. This implies that the substitution kinetics is influenced by the hydrogen ion concentration in solution which is a favourable condition for the precipitation of platinum sulphide. For comparison purposes, the effect of proton concentration on the final mass of Pt precipitate (PtS + S) was Fig. 6. A plot of mass of precipitate (PtS + S) vs. contact time. Effect of HCl concentration (1. 3 and 4 M HCl) on the mass of precipitate. Stirring speed = 500 rpm; molar feed ratio platinum: sulphide ions = 1:1; [Na 2 S 2 O 3 ] initial = 0.05 M; [Pt] feed = 100 ppm; contact time = 24 h at ambient conditions. A.3 Determination of separation coefficients The rate constants (k R ) values for each metal and precipitant (Na 2 S 2 O 3 ) were determined from the intercept of the straight lines by a linear regression method (Eq. 3 and Fig. 4). The separation coefficient (β) is evaluated as the ratio of k R values for respective metals (Eq. 5). The separation coefficient associated with platinum, iron, cobalt and chromium in L-L precipitation (chloride systems) at equilibrium increased in the order β (Pt/Co) < β (Pt/Cr) < β (Pt/Fe) β Pt/Co, indicating a good separation between platinum and base metals for the 92

6 liquid-liquid precipitation process. If the contact time increases, then separation factor is compromised. A.4 Effect of sodium thiosulphate addition x 10-6 m x10-6 m x 10-6 m3 Platinum may exist as chloro-complex anions with or without water ligands. The most labile ions are those without water ligands in the molecular structure at high chloride concentration. In this study, the effects of Na 2 S 2 O 3 (0.05, 0.1 and 1.0 M) and HCl (1, 3 and 4 M) concentration on metal precipitation were investigated. The initial concentration of each metal (Pt and base metals ions) in the chloride solution was kept constant at 100 mg/l. All the absorption tests were carried out at high HCl acid strength ( M) to ensure that dissolved sulphur species and metal ions were maintained at high activity levels. This implies that negatively charged species of platinum ions were free of water ligands. Therefore, dissolved sulphur ions (S 2- ) reacts with platinum by replacing the chloride ions in the electrophilic chloro-complex structure and subsequently crystallizing Pt and base metals as a sulphide aggregates. The effect of Na 2 S 2 O 3 addition was studied by varying the volume (dosage) from 3 x 10-6 m 3 to 13 x 10-6 m 3. Fig. 7 represents the change in metal concentration of Pt from a mixed chloride solution containing equal amounts (100 ppm) of Fe, Co and Cr ions at various Na 2 S 2 O 3 volumes (dosages). The individual concentration profiles of metal impurities are not shown in Fig. 7. The rate of metal precipitation is indirectly derived from the change in metal concentrations in the solution with time. It was not possible to filter the solids from the solution during the kinetics study, however, the final mass of precipitation solids was only determined when there was no further change in metal concentrations or when the metal concentrations were significantly small. It is clear that faster precipitation kinetics for Pt from mixed chloride solution containing Fe, Co and Cr were achieved at very big dosages (< 12.3 x 10-6 m 3 ) of Na 2 S 2 O 3. At high dosage of Na 2 S 2 O 3, about 99.5% of the Pt was recovered within short contact period (<5 min), while it took more than 10 min with 7.6 x 10-6 and 3.12 x 10-6 m 3 to get close to the same Pt precipitation kinetics. 1V. CONCLUSIONS The precipitation of Pt and base metals (Fe, Co and Cr) by L-L reactions were conducted over a wide range of solutions concentrations. The effect of concentrations (Na 2 S 2 O 3 ), residence time and acidic condition on the precipitation kinetics were investigated. The rate of Pt precipitation was found to be faster than the precipitation of the base metals at low Na 2 S 2 O 3 concentration (0.05 M). Pt recovery from batch L-L precipitation in chloride systems using Na 2 S 2 O 3 solution averaged above 99.5% in a short contact time (10 min) within the HCl concentration range of 1 4 M. Precipitation was confirmed by the evidence of Pt in the precipitate according to the XRD analysis. The first and second order kinetics did not fit the precipitation data. The order of reactions (n) and rate constants (k R ) were evaluated using the n-th order kinetics model. Pt conc. (mg/l) Time (min) Fig. 7. A plot of Pt concentration vs. contact time. The effect of Na 2 S 2 O 3 dosage on Pt and base metals precipitation process. Conditions: stirring speed = 500 rpm; molar feed ratio metal ions: sulphide ions = 1:1; T = 25 C; [HCl] = 4 M; [Na 2 S 2 O 3 ] initial = 0.05 M; [Me n+ ] feed = 100 ppm; contact time = 40 min. The order of reaction for platinum was found to be approximately unit (n = 1.3 and k R = 0.004). This would imply the first order kinetics. Two types of precipitation profile were observed at different acidic strength; namely a rapid precipitation stage of Pt in the first 3 min followed by a slow precipitation stage. The rate of Pt precipitation was found to be faster at high HCl concentration (4 M). The amount of Pt precipitate in the batch reactor discharge solution increased significantly with an increase in HCl concentration (Pt recovery 99.5%) at ambient conditions. The average selectivities of platinum to base metals at an average concentration of 100 mg/l of Pt and base metals were found to be in following order β (Pt/Fe) = 112 > β (Pt/Co) = 9 > β (Pt/Cr) = 5. The fastest Pt and base metals precipitation kinetics were achieved at a volume (dosage) of 12.3 x 10-6 m 3 of Na 2 S 2 O 3 solution. ACKNOWLEDGMENT The authors are grateful to Tshwane University of Technology, Department of Science & Technology (Innovation and Technology Fund), Pretoria, South Africa and the Copperbelt University, Kitwe, Zambia for financial support. REFERENCES [1] B. Gazea, K. Adam and A. Kontpoulos, A review of passive systems for the treatment of acid mine drainage, Mineral Engineering, vol. 1, pp , [2] D.B. Johnson and K.B. Hallberg, Acid mine drainage remediation options a review, Science of the Total Environment, vol. 338 (1 2), pp. 3 14, [3] C.S. Simon, Hydrogen sulphide as a hydrometallurgical reagent, In: M.E. Wordsworth and F.T. Davies (Editors), Unit Process in Hydrometallurgy. Gordon and Breach, New York, pp , [4] T. Tuominen and P.O. Groenquist, Hydrogen sulphide as precipitation reagent in hydrometallurgy, Erzmetall, vol. 22, pp ,

7 [5] T.K. Roy, Preparing nickel and cobalt concentrates, Ind. Eng. Chem., vol. 53, pp , [6] M. Harris, D.M. Meyer and K. Auerswald, The production of electrolytic manganese in South Africa, J. Southern African Institute of Mining and Metallurgy, vol. 77 (7), pp , [7] B. M. Kim, Treatment of metal containing wastewater with calcium sulphide, AIChE Symp. Ser., vol. 77(209), pp , [8] A. Van den Steen, J. M. Pollni, B. Kalala and T. Shungen, Development of cobalt sulphite solution purification by sulphide precipitation, In Proc. Symp. Extractive Metallurgy of Nickel and Cobalt (Phoenix, Ariz., Jan ), Metallurgical Soc., Pennsylvania, [9] C. H. Bijsterveld, The precipitation of cobalt and manganese sulphides from manganese sulphate-ammonium sulphate electrolyte, M. Sc. Diss., Witwatersrand, Johannesburg, [10] F. S. Waitman and A. M. Roberson, Precipitation of copper from an acid mine water. US Bur. Mines Rep. Investigations 3746, [11] D. Bhattacharya, A. B. Jumawan, G. Sun, C. Sund-Hagelberg and K. Schwitzgebel, Precipitation of heavy metal with sodium sulphite. AIChE Symp. Ser, vol. 77, pp , [12] G. A. Kiraev and A. A. Uritskaya, Kinetics for the chemical deposition of cadmium sulphide thin film, Izv. Akad. Nauk USSR, Neorg. Mater. vol. 2 (9), pp , [13] C.C. Nesbitt, J. L. Hendrix and J. H. Nelson, Use of thiourea for precipitation of heavy metals in metallurgical operation effluents, I. M. M., London, [14] T. Moeller and R. O Corner, Ions in Aqueous Systems. McGraw-Hill, New York, [15] J. Snyder, (Jr.), Handbook of Chemistry and Physics, 71 st Ed., CRC Press, Boston, [16] R. Williams, P. N. Yocom and F.S. Stofko, Preparation and properties of spherical zinc sulphide particles, J. Colloid Interf. Sci., vol. 106, pp , [17] F. Habashi, Principles of Extractive Metallurgy, Vol. 2 Gordon and Breach, Science Publishers, Inc. New York, pp. 203, [18] G. Schreier, and C. Edtmaier, Separation of Ir, Pd, and Rh from secondary Pt scrap by precipitation and calcinations, Hydrometallurgy, vol. 68, pp , [19] H. G. Julsing, and R. I. McCrindle, The use of sodium formate for the recovery of precious metals from acidic base metal effluents, Journal of Chem. Technol. & Biotechnology., vol. 76(4), pp (6), [20] H. Kasaini, R. C. Everson, and O. S. L. Bruinsma, Selective Adsorption of Platinum from Mixed solutions Containing Base Metals Using Chemically Modified Activated Carbons, Separation Science and Technology, vol. 40, pp , [21] A. E. Nielsen, Kinetics of nucleation, Pergamon Press Ltd. [22] O. Söhnel, and J. Garside, Precipitation, basic principles and industrial Applications, Butterworth Heinemann Ltd., Oxford. [23] O. Levenspiel, Chemical Reaction Engineering, 3rd ed., John Wiley & Sons, Inc., New York, 1999, pp [24] J. R. Aldrich, A Better Heavy Metal of Waste Treatment Method. Metal Finishing, vol. 82, pp , [25] D. Dreisinger, W. Murray, D. Hunter, K. Baxter, J. Ferron, and C. Fleming, The Application of the PLATSOL Process to Copper- Nickel-Cobalt-PGE/PGM Concentrates from PolyMet Mining's NorthMet Deposit, Presented at ALTA, Perth, WA. Unpublished, [26] M. COX, Liquid-Liquid Extraction in Hydrometallurgy, In: THORNTON, J. D. (Ed.). Science and Practice of Liquid-Liquid Extraction. Clarendon Press: Oxford, vol. 2, pp. 57, [27] K. Kondo, H. Nishio, and F. NAKASHIO, Kinetics of Solvent Extraction of Palladium and Didodecylmonothiophosphoric Acid, Journal of Chemical Engineering, vol. 22, pp , [28] H. Kasaini, M. Goto, and S. Furusaki, Selective Separation of Pd(II), Rh(III) and Ru(III) Ions from a Mixed Chloride Solution Using Activated Carbon Pellets, Separation Science and Technology, vol. 35(9), pp , [29] S. I. Ginzburg, N. A. Ezerskaia, I. W. Prokofiewa, N. W. Fedorenko, W. I. Szlenskaia, and N. K. BELSKI, Analytical Chemistry of the Platinum Group Metals, Nauka, Moscow,