Thesis (PhD)--Stellenbosch University, 2012. / 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.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/71705 |
| Date | 12 1900 |
| Creators | Dorfling, Christie |
| Contributors | Bradshaw, S. M., Akdogan, G., Stellenbosch University. Faculty of Engineering. Dept. of Process Engineering. |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | English |
| Type | Thesis |
| Format | 214 p. |
| Rights | Stellenbosch University, Stellenbosch University |
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