| A new technology for extracting and recovering platinum group metals (PGMs) has been investigated and established by pressure cyanidation from two typical resources, flotation concentrates of Yunnan Jinbaoshan low-grade proterozoic Pt-Pd sulfide ores and spent auto-catalysts.Since the low-grade Pt-Pd flotation concentrates are not appropriate to be treated by traditional pyro-metallurgical processes, an innovated all-in-hydrometallurgical flowsheet was suggested in present paper, including pressure acid leaching, pressure cyanide leaching and zinc cementation. The laboratory small-scale, 600g scale, and 5kg scale batch experiments were carried out. The new technology broke through the limitation of treating sulfide ores containing PGMs only by pyro-metallurgical processes. The developed new method has higher percent metal extraction, shorter deposing-procedure, lower pollution and better operational atmosphere for the treatment of the low-grade proterozoic platinum ores in China.In laboratory small-scale experiments, effects of different technical conditions on recovery of Cu and Ni for pressure H2SO4 leaching pre-treatment, and on percent cyanide extraction of Pt and Pd for pressure cyanide leaching processes were investigated respectively. The optimal operation conditions for pressure H2SO4 leaching process are concentrate granularity 98% -74 μm, added H2SO420wt%, temperature 200℃, time 8h, oxygen pressure 2.0MPa and L:S=4:1. Percent Cu, Ni and Co recovery reached is more than 99%. For pressure cyanide leaching processes, the optimal operation parameters are 2.5wt% of NaCN, 160℃, 1h, Pair 2.0MPa and L:S=4:1. The results for 600g-batch unit experiments showed that the percent Pt and Pd extractions for two-step pressure cyanide leaching are more than 98%.The process development unit for 5kg batch in 50 L autoclave also reproduced the same results as small-scale experiments. The research indicated that the overall extraction of Pt and Pd from flotation concentrates to cementation enrichments were 90%94% and 99% respectively. The content of Pt and Pd in cementation enrichments was 56.0% 59.3%, which was 6000 times higher than that in flotation concentrates. The content of precious metals, including Rh, Ir, Au and Ag, were enriched to 70%90%. Cu, Ni and Co leaching recovery also reached more than 99%. The metal balance of Pt, Pd, Cu and Ni in process development unit was carried out. The new technology was also evaluated from the behavior of S, Fe, MgO, SiO2 and precious metals by comparing with traditional pyro-metallurgical processes.For spent automobile catalysts, present investigation indicated that the pretreatment of catalysts was the key process to obtain higher cyanide extraction of PGMs. A newflow-sheet involving pressure NaOH pre-leaching, pressure cyanide leaching and zinc cementation of precious metals was suggested. The laboratory small-scale, 600g scale and 5kg scale batch experiments were carried out. The experimental results were better than those reported by the cyanidation technology of U.S.B.M. The new technology made a breakthrough in recovery of PGMs from spent auto-catalysts, presented various advantages over current oxidation leaching process, and has better-industrialized applications.In laboratory small-scale experiments, effects of different technical conditions on cyanide leaching recovery of Pt, Pd and Rh were investigated for both pressure NaOH leaching pretreatment and pressure cyanide leaching processes. The optimal operation conditions for pressure NaOH leaching process were spent auto-catalysts granularity > 90% -74 u m, added NaOH 10wt%, temperature 160°C, time 2h, oxygen pressure 2.0MPa, and L:S=4:1. For pressure cyanide leaching processes, the optimal operation parameters were 5wt% of NaCN, 160°C, lh, Pair 2.0MPa and L:S=4:1.The results of magnified process for 600g-batch unit showed that the recovery of PGMs could reproduce the results of laboratory small-scale experiments. Under optimal operation conditions, percent Pt, Pd and Rh extraction calculated on the basis of slags reached 97%98%, 98%99% and 92%95% respectively. The technology also held true for end-slags by current ambient temperature oxidation acid-leaching processes. For the end- slags, more than 90% of PGMs could still be recovered using the new technology, but it is almost impossible by using the acid-leaching method for further extraction of PGMs.The process development unit for 5kg batch in 50 L autoclave indicated that, the recoveries of Pt, Pd and Rh after two-steps of pressure cyanide leaching could reach 95%—96%, 97%—98% and 90%92% for recovery of PGMs from spent auto-catalysts containing Pt+Pd+Rh^lOOOg/t—2000g/t respectively, and the recoveries of Pt, Pd and Rh after only one-step pressure cyanidation could also reach 87%89%, 88% 93% and 83%89% for acid-leaching end-slags containing Pt+Pd+Rh?=80g/t120g/t respectively.Based on above systemic technical research, the mineralogy of leaching processes was investigated, influence of pressure H2SO4 pre-leaching and pressure NaOH pre-leaching on latter cyanide leaching process were discussed, thermodynamics and kinetics of pressure cyanide leaching were also analyzed, and some new explanations from the atomic level for reaction micro-mechanism were proposed.For low-grade Pt-Pd flotation concentrates, the investigation on changes of mineralography of Pt, Pd, Cu and Ni minerals in pressure hydrometallurgy processesindicated that, after pressure acid pre-leaching process, Cu and Ni sulfides disappeared, which were oxidized as sulfates and leached into solutions, Fe remained as Fe2O3 and FeO(OH) in slags, while precious metal minerals could be exposed out and concentrated into slags. Therefore, precious metals could be separated from Cu, Ni and Co in the process. Because the wrappage of sulfides was uncovered at 200 °C and in the presence of excessive O2, precious metal mineral grains were exposed in solutions. Due to high oxidation potential of slurry and existence of Cl" in water, Pd and Pt were possiblely dissolved a little into the solution, but they could be easily recovered from H2SO4 medium by Cu cementation at room temperature. The thermodynamic research showed that, when increasing reaction temperature, the metallic dissolved region in E-pH diagram of Me-S- H2O transferred towards higher acidity and lower oxidation potential. Under higher slurry oxidation potential or lower acidity, FeS was easy to form Fe2O3, but Cu and Ni sulfide ores could be oxided completely as sulfates. For promoting the process, the reaction temperature should be higher than 160°C, and higher than 175°C is better. The kinetics research denoted that, the oxidation of base metal sulfide ores followed the order of Fe > Ni, Co > Cu, and the reaction rate depends on temperature, O2 pressure, initial H2SO4 concentration and slurry stirring speed. The reaction rate was dually controlled by chemical reaction and diffusion in the initial stage of pressure acid leaching, but was mainly by chemical reaction in the latter stage.For spent automobile catalysts, the pressure NaOH leaching reaction was the same as that of aluminum mineral by Baier method. The mineralogical investigation indicated that the AI2O3 coat and part of AI2O3 and SiC>2 carrier material of auto-catalysts could be dissolved in leaching process, so wrappage of carrier for PGMs was uncovered. However, when the leaching process proceeded in higher NaOH concentration at higher temperature for longer time, the undissolved Na2O-Al2O3-2SiO2-2H2O would be formed, which made PGMs re wrapped and contacted hardly with NaCN. The thermodynamic analysis based on Al2O3-Na2O-H2O solubility-graph showed that, to unpack the wrappage of carrier materials, the leaching reaction should be carried out within the unsaturated region. When reaction temperature was elevated, the unsaturated region was enlarged, which is in favor of leaching, but the initial NaOH concentration should not be too high under chosen reaction temperature. The kinetics research denoted that, the efficiency of unpacking the wrappage of carrier minerals was related to different controlling conditions for pressure NaOH leaching reaction. The reaction rate was dually controlled by chemical reaction and diffusion process. During the initial stage of leaching, the reaction rate was mainly controlled by dissolution of carrier minerals, but when above-mentioned undissolved resultant was formed, the diffusion process became to the reactionrate-controlling step. The formation of Na2OAl2O3xSiO2nH2O in NaOH leaching slags decreased the reaction rate, increased the consume of NaOH and the possibility of re-wrapped PGMs.For the pressure cyanide leaching process of PGMs, thermodynamics analysis indicated the ion species of Pt, Pd and Rh existed stably in cyanide solutions. Three kinds of possible chemical reactions were proposed. The high-temperature E-pH diagram of ￡ Pt-CN-H2O was discussed, the cyanidation thermodynamics data were calculated, and formula of (T) versus pHr was also deduced. The kinetics research on the difference in behaviors of PGMs in cyanide leaching process was carried out. The effects of reaction temperature, NaCN concentration, and O2 pressure on reaction rate were discussed, and the leaching reaction rate controlling-step was analyzed. Based on the results of cyanide leaching kinetics, behavior of Pt, Pd and Rh in pressure cyanide leaching process and reaction micro-mechanism were explained at atomic level. The result showed that the leaching-order of Pd > Pt > Rh could be described by their surface-state chemical reaction activity, and was the synergetic influence of metal-bond strength and atomic-state stability. The fact that the high-temperature stability of Pt(CN)42" is higher than that of Pd(CN)42" could be explained from the heavy platinum-group complexes having higher thermodynamic stability and kinetic inertia than light platinum-group complexes with same structure. On the other hand, the higher stability of Rh(CN)63" than that of Pd(CN)42" could be explained from the chemical reaction activity of the PGM cyanide complex ions having different geometrical structure.
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