STUDY ON SELECTIVE PRECIPITATION OF PLATINUM AND BASE METALS IN LIQUID-LIQUID AND GAS-LIQUID CHLORIDE SYSTEMS: FOCUS ON CONCEPTUAL PROCESS DESIGN

JOHN SIAME (john.siame@cbu.ac.zm)
FACULTY OF ENGINEERING AND THE BUILT ENVIRONMENT, Department of Chemical, Metallurgical and Materials Engineering, TSHWANE UNIVERSITY OF TECHNOLOGY
March, 2013
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John Siame is a Zambian Citizen aged 44. He completed his high school education in
1988 at Mpelembe Secondary School in Kitwe, Zambia and was later enrolled at Moscow Institute of Chemical Technology for a master’s degree in Chemical Engineering. He received his Master’s degree in 1997 following a research dissertation titled “Design and Production of Injection-molded Products from Polyolefin with a Production Capacity of 360 ton/year”.
Mr. Siame joined the University of Zambia as a part-time Lecture/Tutor in 1998 and two years later was appointed Lecturer at the Copperbelt University, Kitwe, Zambia within the Department of Chemical Engineering where he is currently based.
Mr. Siame enrolled as M Tech-Chemical Engineering student at Tshwane University of Technology (TUT) in 2006 and was awarded his master’s degree in September 2008 following a research dissertation entitled “Physical Beneficiation of PGM Tailings by Gravity Separation and Elutriation Techniques”. Between 2006 and 2008, Mr. Siame presented a few chemical engineering courses to undergraduate students in the Department of Chemical, Metallurgical and Materials Engineering.
Mr. Siame registered as doctorate student in the Chemical Engineering programme at TUT in 2008. He has published papers in peer reviewed journals and conference proceedings at National and International levels. His final doctoral Thesis is titled:
“Study of Selective Precipitation of Platinum and Base Metals in Liquid-Liquid and Gas-Liquid Chloride Systems: Focus on Conceptual Process Design”.

Mr. Siame investigated and validated the concepts of selective precipitation of platinum from industrial effluents contaminated by excess base metal ions using Sulphur-bearing liquids and gas systems. The concept of using a sulphur-bearing gas system to sequester metals such as platinum from industrial effluents is both economically and environmentally attractive. In fact, the sulphur dioxide-mediated precipitation concept is unique in that it provides the opportunity to capture sulphur gas from the smelter flue gases in order to precipitate valuable metals like platinum from effluents in a semi-pure sulphide crystalline state making downstream processing at the refinery or smelter both profitable and energy efficient. Overall, this new technology could reduce the impact of toxic sulphur dioxide gas on the environment significantly.
This doctorate thesis has laid a foundation for future technologies to be developed around selective recovery of valuable platinum group metals from chloride solutions containing many base metal impurities simply by using an in-expensive resource such sulphur dioxide or hydrogen sulphide from the smelter flue gases. Platinum reacts preferentially and very fast with sulphur atoms in solutions.
This research project was carried out under the supervision of Prof Henry Kasaini who is currently the Director of Science and Technology at Rare Element Resources Corporation in Denver, USA.
 

Abstract

This study provides experimental data and new perspectives on selective precipitation of platinum group metals (PGMs) in the presence of base metals while at the same time reviewing the mass transfer characteristics and models associated with metal sulphides precipitation in liquid-liquid and gas-liquid systems. A contrast was made between the complex purification methods for PGM solutions in industry and the simplicity of reactive precipitation. The concept of using dissolved sulphur atoms from either gaseous or liquid phase to separate platinum ions from base metal ions (iron, cobalt, chromium and nickel) instantaneously formed the main hallmark of this study. Mass transfer coefficients in liquid phase and diffusivity of sulphur atoms at the gas-liquid interface were described according to classical molecular transport models. A typical capex and opex of a conceptual platinum precipitation circuit has been highlighted on the basis of laboratory data and not detailed industrial pilot test work. It was recommended that laboratory data should be scaled up prior to future pilot tests taking into account all the constraints of bench scale experiments and lack of detailed metal crystallization data. This study did not focus on generating experimental data towards modelling of colloids and crystal formation due to lack of specialized equipments. However, precipitates were collected at the end of a fixed interval and then subjected to SEM, EDS and XRD analyses to confirm the presence of metal species and morphology. The composition of precipitates (PtS, FeS and CoS) was determined on the basis of EDS analyses. In terms of mass distribution of species in the precipitate (obtained in single component chloride solutions), the mass fraction of platinum was 26.54 %wt and that of amorphous sulphur was 66.88 %wt. This means that the mass fraction of amorphous sulphur was higher than PtS by a factor of ~2.5. In the case of multi-component chloride solutions, the metal sulphides in the precipitates were determined as follows: 21.09 %wt PtS, 2.16 %wt FeS, 1.36 %wt CoS and 0.63 %wt Cr2S3. Again, the mass of sulphur relative to platinum was found to be higher by a factor of ~2.5. The implied separation factors between Pt and base metals, as observed from the quality of the precipitate, are significantly high. This means that this product can be refined directly without further purification.

In the platinum industry, production of platinum group metals (PGMs) is carried out in numerous stages starting with the recovery of a sulphide concentrate (100 – 400 g/T, PGMs) by flotation and then obtaining a matte (400 – 6000 g/T, PGMs) at the Smelter Plant (appendix A, B and C). The matte contains several base metal oxides as impurities such as Fe-oxide, Co-oxides, Cr-oxide, Ni-oxide, Se-oxides, and Bi-oxides. The atomized matte pellets are subjected to H2SO4 acid digestion, distillation and precipitation processes to remove all based metals. PGM elements are unstable in sulphate media and therefore remain in the leach residue. Subsequently, the PGM-rich residues are solubilized in chloride media and are subjected to a complex separation and purification process using ion exchange resins. Therefore, the overall process of separating and purifying PGMs in industry is both chemically complex and energy intensive. The performance and cost of resins impact strongly on the efficiency and profitability of the PGM industry in South Africa.

In this study, the objective was to investigate and validate the concept of selective precipitation of platinum from chloride media using sulphur-bearing liquids or gases. The concept of using sulphur atoms from the gas phase seemed economically and environmentally attractive. Platinum ions react readily with sulphur atoms to form stable platinum sulphide precipitates (PtS) in acidic media. The source of sulphur atoms could be a liquid or gas phase. In this case, sodium thiosulphate solution (Na2S2O3) or sulphur dioxide gas (SO2) was employed as precursors. All the metal ion species were prepared in chloride media before the experimental work.

From batch tests in L-L system selective precipitation of platinum (Pt) from base metals (Fe, Co and Cr)was achieved at small dosages of Na2S2O3(0.05 – 1.0 M) and short time intervals ( 3-5 minutes ). On the basis of precipitation kinetics, data from liquid-liquid batch tests were interpreted in terms of Pt selectivity and recovery to precipitates, order of reaction and reaction constants. A differential method of data analysis was used to obtain an nth-order rate equation. The nth-order kinetics model makes it possible to determine the order of a reaction (n) and rate constant (kR). Thereafter, precipitation studies were extended to gas-liquid system using a continuously stirred tank reactor (CSTR). Mass transfer, recovery and selectivities of platinum were quantified in the G-L systems using mass balance and liquid film diffusion model. The model describes absorption of gas molecules through a gas-liquid interface accompanied by a precipitative reaction in the liquid phase. Moderate liquid temperatures (25-50 oC) and SO2 gas pressure (0.5-2.0 bars) were applied. Metal concentrations were maintained equal (100 mg/L) in a solution with high acid strength (1–4 M, HCl). Resistance to mass transfer in the gas phase was discounted because the gas phase was homogeneous.

The main achievements in this study were the following:
 Platinum was selectively precipitated from dilute acidic chloride solutions containing base metals (Fe, Co and Cr). The separation factors associated with batch tests in L-L system and continuously tests in G-L system were found to be of the following order {βPt/Fe (112) >βPt/Cr (9) >βPt/Co (5)} and {βPt/Fe (28.5) >βPt/Cr (2.8) >βPt/Co (2.5)} respectively. The mass of sulphur required for precipitating 1g of platinum in L-L precipitation system and G-L precipitation system was estimated to be 3.2 g and 1.024 g respectively.
 High solubility of sulphur (65g/g of chloride solution) was achieved at moderate temperature (25 °C) and pressure (2.0 bars).
 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 mass transfer coefficients, using Two Film Model were successfully tested and fit the experimental data.
 Mass balance of metals and sulphur were successfully performed and a conceptual process was proposed to separate and purify platinum.
 A method to recycle sulphur between the hydrometallurgical and smelter plants has been proposed.

Therefore, this study was concluded with recommendations to industry to perform a pilot absorption/precipitation study involving a matte leach solution to assess the benefits of using sulphur dioxide gas as precipitant in reducing the cost of metal precipitation in solution and reducing environmental pollution.