Characterisation and dynamic modelling of the behaviour of platinum group metals in high pressure sulphuric acid/oxygen leaching systems

Dorfling, Christie (2012-12)

Thesis (PhD)--Stellenbosch University, 2012.

Thesis

ENGLISH ABSTRACT: Sulphuric acid/oxygen pressure leaching is typically employed on Base Metal Refineries (BMRs) to selectively dissolve base metals from platinum group metal (PGM) bearing nickelcopper matte. Optimal operation of this processing step requires an understanding of the system chemistry and the effects of process variables on base metal and PGM leaching behaviour. This project aimed to aid in the development of an improved understanding of the high pressure leaching system. The effects of temperature, pressure, acid concentration, and solid to liquid ratio on the leaching behaviour were determined experimentally using a two litre autoclave. For conditions comparable to that typically used at the Western Platinum Ltd. BMR, changes in the acid concentration had the largest effect on the copper leaching behaviour. Increasing the initial acid concentration from 140 g H2SO4/ℓ to 165 g H2SO4/ℓ resulted in the copper dissolution decreasing from 88.7% to 75.3% on average for the experiments performed at different temperatures (116°C, 130°C) and pressures (7 bar, 9 bar), and with different solids contents (80 g/ℓ, 130 g/ℓ). In the case of the other precious metals (OPMs), temperature was determined to be the process variable with the largest effect on the leaching kinetics. The average percentage rhodium dissolution achieved after seven hours of leaching at different conditions (pressure, acid concentration, and solids content were varied) increased from 58.3% at 116°C to 83.6% at 130°C. Similar effects were observed for ruthenium (96.2% dissolution at 130°C; 79.4% dissolution at 116°C) and iridium (81.8% dissolution at 130°C; 46.9% dissolution at 116°C). The rate of copper leaching was found to be limited by the rate of oxygen transfer from the gaseous phase to the liquid phase, while the remainder of the reactions were chemical reaction limited. The extent of OPM leaching was found to be dependent on the rate and extent of copper leaching. A set of 21 chemical reactions was proposed to describe the leaching behaviour, and the shape factors and reaction rate constants were determined by the method of least squares to minimise the error between the predicted concentrations and the experimental data. Apart from direct base metal leaching reactions, six cationic exchange reactions contribute to the leaching of copper sulphides and nickel sulphides by precipitation of OPM oxides. Three leaching reactions for each of the OPMs (one for sulphide phases, one for metallic phases, and one for oxide phases) resulted in satisfactory modelling of the system behaviour. Activation energies of -26.2 kJ/mol and -5.9 kJ/mol were calculated for the digenite acid leaching reaction and the covellite direct oxidation reaction, respectively, which confirmed that the rates of these reactions were mass transfer limited. The activation energies for the remainder of the base metal leaching reactions exceeded 30 kJ/mol. The activation energies of the reactions accounting for rhodium sulphide leaching, rhodium leaching, and rhodium oxide leaching, were calculated to be 64.2 kJ/mol, 138.5 kJ/mol, and 116.2 kJ/mol, respectively. Similar activation energies were calculated for the respective Ru and Ir leaching reactions. The rate of OPM sulphide leaching was typically an order of magnitude and three orders of magnitude larger than the rate of OPM leaching reactions and OPM oxide leaching reactions, respectively. The autoclave at the Western Platinum Ltd. BMR was modelled assuming a monosized distribution of the feed and approximating the autoclave as four ideal continuously stirred tank reactors. The steady state solution employed the sequential modular approach in MATLAB, while the dynamic simulation involved solving a set of 217 differential equations derived from mass and energy balances simultaneously in MATLAB. The model was used successfully to evaluate the effects that changes in the leaching temperature, leaching pressure, acid feed rate, and solids feed rate have on the extent of base metal and OPM leaching in the autoclave. The optimum operating conditions depend on the flow rates and compositions of the feed streams. A feed stream containing 10.3 wt% solids (825 kg solids/h) and an acid addition rate of 28.6 kg/h were considered as typical operating conditions for model-based analysis. More than 95% copper dissolution and no OPM dissolution were predicted when performing the pressure leaching at a pressure of 8 bar and a temperature of approximately 123°C. Decreasing the pressure resulted in lower copper dissolution when OPM leaching started to occur. Increasing the temperature resulted in reduced copper leaching, while decreasing the temperature resulted in a longer OPM leaching period and hence higher OPM dissolution. Model-based analysis furthermore showed that the relative amounts and relative leaching rates of digenite and covellite significantly influence the percentage copper dissolution achieved when noticeable OPM leaching start to occur.

AFRIKAANSE OPSOMMING: Swawelsuur/suurstof hoë druk loging word tipies op Basis Metaal Raffinaderye (BMRe) gebruik om basis metale selektief op te los vanuit platinum groep metaal (PGM) bevattende nikkel-koper mat. Optimale bedryf van hierdie prosesstap vereis ʼn begrip van die sisteem se chemie en die effekte wat proses veranderlikes op die logingsgedrag van basis metale en PGMe het. Hierdie projek het ten doel gehad om ʼn beter begrip van die hoë druk loging sisteem te ontwikkel. Die effekte van temperatuur, druk, suur konsentrasie, en vastestof tot vloeistof verhouding op die logingsgedrag is eksperimenteel met behulp van ʼn twee liter outoklaaf bepaal. Vir toestande vergelykbaar met dié wat tipies by die Western Platinum Bpk. BMR gebruik word, het veranderinge in die suurkonsentrasie die grootste effek op die logingsgedrag van koper gehad. Verhoging van die aanvanklike suurkonsentrasie van 140 g H2SO4/ℓ na 165 g H2SO4/ℓ het tot gevolg gehad dat die gemiddelde koper oplossing afgeneem het van 88.7% na 75.3% vir die eksperimente wat by verskillende temperature (116°C, 130°C) en drukke (7 bar, 9 bar), en met verskillende vastestof inhoud (80 g/ℓ, 130 g/ℓ), uitgevoer is. In die geval van die ander edelmetale (AEM) is bevind dat die temperatuur die prosesveranderlike met die grootste effek op die logingskinetika is. Die gemiddelde persentasie rodium oplossing wat na sewe ure se loging by verskillende toestande (druk, suurkonsentrasie, en vastestof inhoud is varieer) behaal is, het toegeneem van 58.3% by 116°C na 83.6% by 130°C. Soortgelyke effekte is waargeneem vir rutenium (96.2% oplossing by 130°C; 79.4% oplossing by 116°C) en iridium (81.8% oplossing by 130°C; 46.9% oplossing by 116°C). Dit is bevind dat die tempo van koper loging beperk is deur die tempo van suurstof oordrag vanaf die gas na die vloeistoffase, terwyl chemiese reaksies beperkend was vir die res van die reaksies. Die mate van AEM loging was afhanklik van die tempo en mate van koper loging. ʼn Stel van 21 reaksies is voorgestel om die logingsgedrag te beskryf, en die vorm faktore en reaksie tempo konstantes is bepaal deur middel van die metode van kleinste kwadrate om die fout tussen die voorspelde konsentrasies en die eksperimentele data te minimeer. Afgesien van die direkte basis metaal logingsreaksies het ses kationiese uitruilingsreaksies bygedra tot die loging van kopersulfiede en nikkelsulfiede deur presipitasie van AEM oksiede. Drie logingsreaksies vir elk van die AEMe (een vir die sulfied fase, een vir die metaal fase, en een vir die oksied fase) het bevredigende modellering van die sisteem se gedrag tot gevolg gehad. Aktiveringsenergieë van -26.2 kJ/mol en -5.9 kJ/mol is bereken vir die Cu1.8S suur logingsreaksie en die CuS direkte oksidasie reaksie, onderskeidelik, wat bevestig het dat die tempo’s van hierdie reaksies deur massa oordrag beperk is. Die aktiveringsenergieë vir die res van die basis metaal logingsreaksies het 30 kJ/mol oorskry. Die aktiveringsenergieë vir die reaksies wat die rodiumsulfied loging, rodium loging, en rodiumoksied loging beskryf is as 64.2 kJ/mol, 138.5 kJ/mol, en 116.2 kJ/mol, onderskeidelik, bereken. Soortgelyke aktiveringsenergieë is bereken vir die onderskeie Ru en Ir logingsreaksies. Die tempo van AEM sulfied loging was tipies ʼn ordegrootte en drie ordegroottes groter as die tempo van AEM logingsreaksie en AEM oksied logingsreaksies, onderskeidelik. Die outoklaaf by die Western Platinum Bpk. BMR is gemodelleer deur ʼn enkelgrootte verspreiding vir die voer te aanvaar en die outoklaaf as vier ideale kontinu geroerde tenk reaktore te benader. Die oplossing vir gestadige toestande het die sekwensiële modulêre benadering toegepas in MATLAB, terwyl die dinamiese simulasie die gelyktydige oplos van 217 differensiale vergelykings, wat vanaf massa- en energiebalanse afgelei is, in MATLAB behels het. Die model is suksesvol gebruik om die effekte wat veranderinge in die logingstemperatuur, logingsdruk, suur voertempo, en vastestof voertempo op die mate van basis metaal en AEM loging in die outoklaaf het, te bepaal. Die optimale bedryfstoestande is afhanklik van die vloeitempo’s en samestellings van die voerstrome. ʼn Voerstroom wat 10.3 massa% vastestof (825 kg vastestof per uur) bevat en ʼn suur voertempo van 28.6 kg/h is as tipiese bedryfstoestande beskou vir model-gebaseerde analises. Meer as 95% koper oplossing sonder enige AEM oplossing is voorspel wanneer die loging by ʼn druk van 8 bar en ʼn temperatuur van ongeveer 123°C uitgevoer word. ʼn Vermindering van die druk het tot gevolg gehad dat laer koper loging behaal is toe AEM loging begin plaasvind het. ʼn Verhoging in die temperatuur het laer koper loging tot gevolg gehad, terwyl laer temperature ʼn langer AEM logingsperiode en gevolglik hoër AEM loging tot gevolg het. Model-gebaseerde analises het verder getoon dat die relatiewe hoeveelhede en relatiewe tempo’s van loging van Cu1.8S en CuS ʼn beduidende invloed het op die persentasie koper wat geloog is wanneer beduidende AEM loging begin plaasvind.

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