Thesis (MScEng)--Stellenbosch University, 2011. / ENGLISH ABSTRACT: In a first stage atmospheric leach at the Lonmin Marikana base metals refinery,
nickel-copper-iron-sulphur Peirce Smith converter matte is leached in recycled
electrolyte from the electrowinning section. The electrolyte contains sulphuric acid,
copper and nickel sulphates, and a small amount of iron sulphate. The converter
matte contains mostly nickel, copper and sulphur (typically 48 %, 28 % and 23 %,
respectively), but also minor amounts (<5 %) iron and cobalt. The matte also
contains platinum group elements (PGEs) and other precious metals totalling 0.2 –
0.7 % (platinum, palladium, iridium, rhodium, ruthenium, osmium and some gold).
The predominant mineral phases are heazlewoodite, chalcocite and a nickel-copper
alloy phase, as well as some entrained slag and spinel minerals. The purpose of the
first stage leach is to extract nickel, while simultaneously precipitating copper and
PGEs contained in the recycled electrolyte. Nickel, cobalt and iron are leached by
acid and oxygen. Copper is precipitated by a redox reaction in which copper ions
oxidise nickel from the matte. The purpose of this study was to determine the effects
of key variables on the performance of the first stage leach (specifically on the
removal of PGEs and copper from solution and the overall extraction of nickel) and
to improve fundamental understanding of these effects.
Batch leaching tests were carried out to investigate the effects of the following
factors: availability of oxygen, initial acid concentration, initial copper concentration,
iron endpoint (iron content of the matte), solids/liquid ratio and stirring rate. Liquid
samples were analysed with Atomic Absorption Spectroscopy (AA) to determine
leaching kinetics. Characterisation of solid samples from leach tests by quantitative
X-Ray diffraction (XRD) and scanning electron microscopy with an energy dispersive
system (SEM-EDS) helped to improve understanding of the leaching mechanism.
The oxidative leaching mechanism entails an initial period in which the alloy phase is
leached by acid and oxygen, while copper reacts with the nickel-copper-alloy and
heazlewoodite phases (which react galvanically with each other) to form a chalcocite
precipitate. In a second reaction period, heazlewoodite was transformed to millerite
by acid leaching and the particle structure became more porous. The rate of copper precipitation and nickel extraction were faster during the second reaction period than
the first reaction period. Some copper leaching occurred once the leachable nickel
(60 – 70 %) had been dissolved, provided that the solution was strongly acidic (pH < 2).
The non-oxidative leaching mechanism entails a galvanic interaction, between the
nickel-copper-alloy and heazlewoodite phases, in which nickel is leached from both
phases and copper is precipitated as chalcocite. Leaching by acid was negligible in
most non-oxidative tests. An initial fast period of copper precipitation was followed
by a second slower period. The decrease in reaction rate can probably be linked to
the decreasing availability of the nickel-copper-alloy phase. During non-oxidative
leaching, the particle structure remained mostly intact. Copper precipitation kinetics
under non-oxidative conditions was found to be slower than under oxidative
conditions. The faster copper precipitation kinetics under oxidative conditions is
most likely caused by an increase in porosity and reaction area as nickel is leached
from the matte by acid and oxygen.
The initial acid concentration, solids/liquid ratio and Fe-endpoint were the most
important factors determining reaction kinetics under oxidative conditions. Low initial
acid concentrations (37 g/L) and a high solids/liquid ratio improved the extent of
copper precipitation. Nickel extraction was enhanced by low solids/liquid ratios and
high initial acid concentrations (74 g/L). Nickel extraction was significantly less
(56 % less in one instance) when leaching high iron mattes (5.7 % Fe) rather than
low iron mattes (< 1 % Fe). Copper precipitation was initially faster when leaching a
high iron matte, but slower nickel leaching from high iron mattes led to an excess of
available acid, which resulted in copper being leached. The results suggest that high
iron mattes will lead to poor copper and PGE precipitation in the first stage leach and
also to lower nickel extractions. Consequently, Peirce Smith converting at the plant
must be carefully controlled to avoid high iron mattes.
Under non-oxidative conditions, the solids/liquid ratio and Fe-endpoint were the most
important factors. The rate of copper precipitation was faster when a high iron matte
was leached, so that a higher percentage copper was precipitated and more nickel
was extracted from the matte. / AFRIKAANSE OPSOMMING: As ‘n eerste stap in die Lonmin Marikana basis-metale veredelingsaanleg word
nikkel-koper-yster-swawel Peirce-Smith-converter-mat geloog in elektroliet wat
hersirkuleer word vanaf die aanleg se koper-elektroplaterings-afdeling. Die loging
word by atmosferiese druk uitgevoer. Die elektroliet bevat swawelsuur, koper- en
nikkel-sulfate en ‘n klein hoeveelheid ystersulfaat. Die mat bevat hoofsaaklik nikkel,
koper en swawel (tipies 48 %, 28 % en 23 %), maar ook klein hoeveelhede (< 5 %)
yster en kobalt. Verder maak Platinum Groep Elemente (PGE’s) en ander
waardevolle metale (platinum, palladium, iridium, rhodium, ruthenium, osmium en
goud) 0.2 % tot 0.7 % van die massa van die mat uit. In terme van minerale bestaan
die materiaal hoofsaaklik uit heazlewoodite, chalcocite en ‘n nikkel-koper allooi fase,
asook slak en spinel minerale, wat tydens Peirce-Smith-converting weens
meesleuring in die mat rapporteer. Die doel van die eerste stadium loog is om nikkel
op te los, terwyl koper en PGE’s wat in die elektroliet voorkom presipiteer moet word.
Nikkel, kobalt en koper word geloog in reaksies met suurstof en swawelsuur. Koper
word presipiteer deur middel van ‘n redoks reaksie waarin koper-ione nikkel in die
mat oksideer. Die doel van hierdie studie was om die effekte van
sleutelveranderlikes op die proses te bepaal (spesifiek hoe nikkel-loging en koper
presipitasie affekteer word) en om fundamentele begrip van die veranderlikes en hul
effekte te verkry.
Lot loogtoetse is uitgevoer op ‘n laboratorium-skaal en die effekte van die volgende
faktore is ondersoek: beskibaarheid van suurstof, begin suurkonsentrasie, yster
eindpunt (die ysterinhoud van die mat), vastestof/vloeistof verhouding en die
roertempo. Vloeistof monsters geneem tydens loogtoetse is geanaliseer met behulp
van Atoom Absorpsie Spektroskopie (AA) om kinetika te bepaal. Vastestof monsters
is ook geneem tydens loogtoetse en kwantitatiewe X-straal diffraksie (XRD), asook
skanderings-elektron-mikroskopie met ‘n energie dispersie sisteem (SEM-EDS) is
gebruik om die materiaal te karakteriseer en die logingsmeganisme te verduidelik.
Die oksidatiewe logingsmeganisme behels ‘n aanvanklike periode waartydens die
allooi fase geloog word deur suur en suurstof, terwyl koper presipiteer om chalcocite te vorm as gevolg van ‘n reaksie waarin galvanise interaksie tussen die nikkel-koperallooi
en heazlewoodite fases ‘n belangrike rol speel. In ‘n tweede reaksie periode is
heazlewoodite geloog deur suur om millerite te vorm. Tydens hierdie tweede fase
het die partikel struktuur meer porieus geword. Die tempo van koper presipitasie en
nikkel loging was vinniger tydens die tweede reaksie periode as tydens die eerste.
Koper is geloog indien die oplossing baie suur was (pH < 2) en die loogbare nikkel
(60 – 70 %) reeds opgelos het.
Die nie-oksidatiewe logingsmeganisme behels galvaniese interaksie tussen die
nikkel-koper-allooi en heazlewoodite fases, wat lei tot koper presipitasie as
chalcocite. Loging deur swawelsuur was onbeduidend. ‘n Aanvanklike vinnige
periode van koper presipitasie tydens nie-oksidatiewe toetse is gevolg deur ‘n
tweede stadiger periode. Die afname in reaksietempo kan waarskynlik verklaar word
deur die afnemende beskikbaarheid van die nikkel-koper-allooi fase. Tydens nieoksidatiewe
loging het die partikel struktuur redelik onveranderd gebly. Koper
presipitasie kinetika in nie-oksidatiewe toetse was stadiger as in oksidatiewe toetse.
Die belangrikste faktore wat kinetika in oksidatiewe toetse beïnvloed het was die
suurkonsentrasie, vastestof/vloeistof verhouding en die yster-eindpunt. Lae beginsuurkonsentrasies
(37 g/L) en ‘n hoë vastestof/vloeistof verhouding het gelei daartoe
dat meer koper uit die elektroliet herwin is. Nikkel ekstraksie was hoër indien die
vastestof/vloeistof verhouding laag was en die begin suurkonsentrasie hoog (74 g/L).
Nikkel ekstraksie was beduidend laer (56 % laer in een geval) wanneer hoë-yster
mat (5.7 % Fe) geloog is, eerder as lae yster mat (< 1 % Fe). Wanneer ‘n hoë yster
mat geloog is, was koper presipitasie aanvanklik vinniger, maar weens stadige
nikkel-ekstraksie-tempos was ‘n oormaat van suur beskikbaar sodat koper uiteindelik
geloog is. PGE presipitasie is ook nadelig beïnvloed wanneer koper geloog is en
veral tydens toetse met hoë yster mat.
Die mees belangrike faktore wat nie-oksidatiewe loging beïnvloed het was die
vastestof/vloeistof verhouding en die yster-eindpunt. Die tempo van koper
presipitasie was vinniger in toetse met ‘n hoë yster mat, sodat ‘n hoër persentasie
koper presipiteer het en meer nikkel opgelos het wanneer ‘n hoë yster mat geloog is.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/18069 |
| Date | 12 1900 |
| Creators | Van Schalkwyk, R. F. |
| Contributors | Akdogan, G., Stellenbosch University. Faculty of Engineering. Dept. of Process Engineering. |
| Publisher | Stellenbosch : Stellenbosch University |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | English |
| Type | Thesis |
| Format | 228 p. : ill. |
| Rights | Stellenbosch University |
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