| Summary: | Thesis (PhD)--Stellenbosch University, 2015. === ENGLISH ABSTRACT: Recent developments in the atmospheric treatment of low-grade nickel laterite ores at
Anglo American plc has culminated in the conceptual iron-focused laterite (ARFe) process.
In addition to the recovery of nickel and cobalt from laterite ore, this process
uniquely aims to recover iron as a saleable by-product. The reduction of soluble iron(III)
(Fe(III)) by sulfur dioxide gas (SO2) is central to the ARFe concept and represents a complex,
multiphase system involving simultaneous gas-liquid mass transfer, thermodynamic
speciation and chemical reaction. The chemistry of iron-containing systems is generally
poorly understood and accurately predicting their behaviour is challenging, especially
under aggressive hydrometallurgical conditions.
The primary objective of this work is the development of an engineering model capable
of describing the rate and extent of ferric reduction with SO2 under conditions
typical of the ARFe process. Thermodynamic considerations provide a rigorous framework
for the interpretation of chemical reactions, however little experimental data are
openly available for the associated solution species in acidic iron sulfate systems.
A key contribution of this work, and critical for the development of the overall model,
is the direct measurement of speciation in iron sulfate solutions. Raman and UV-vis
spectroscopy were utilised to make direct speciation measurements in the various subsystems
of the Fe2(SO4)3-FeSO4-H2SO4-H2O system that were previously unavailable in
the open literature. The FeSO+4 and Fe(SO4)–
2 species were explicitly identified and measurements
were supported and rationalised by static computational quantum mechanical
calculations and ultimately permit the calibration of a robust, ion-interaction solution model with the explicit recognition of the important solution species up to 1.6 mol/kg
Fe2(SO4)3, 0.8 mol/kg H2SO4 over 25 – 90 C.
Batch and continuous Fe(III) reduction kinetics were measured and the effects of initial
Fe2(SO4)3 and H2SO4 concentrations, temperature and in-situ neutralisation quantified.
The retardation effect of sulfuric acid was observed to be the most significant
factor influencing the initial reaction rate and the achievable extent of reduction at fixed
residence time, which varied between about 20 and 80 % after 180 minutes of reaction.
A reaction mechanism that is limited by the slow ligand-to-metal electron transfer
in the FeIIISO+3 solution species’ decomposition is proposed and spectroscopic measurements
and computational quantum mechanical calculations are used to support this
mechanism. A kinetic model, comprising a system of differential mass-balance equations,
is incorporated into the thermodynamic framework. This reaction model permits
the prediction of kinetic profiles over the full range of experimental conditions and can
be incorporated into more elaborate simulation models of the ARFe circuit.
The specific original contributions of this work are
• The direct measurement of aqueous speciation in the Fe2(SO4)3-H2SO4-H2O system
by Raman and UV-vis spectroscopy
• The development of a modelling framework to characterise speciation, activity coefficients
and solubility in the mixed Fe2(SO4)3-FeSO4-H2SO4-H2O system.
• The measurement of Fe(III) reduction kinetics using SO2 in concentrated sulfate
solutions as a function of initial composition and temperature.
• The development of a solution reaction model of Fe(III) reduction with SO2 that
accurately predicts the solution speciation and reaction rate with time as a function
of composition and temperature.
Lastly, the vast complexity of industrial systems will nearly always result in a lack
of specific experimental data that are required for the development of phenomenological
models. This work emphasises the crucial role that engineering studies hold in the generation
of such data to derive maximum practical value for industrial process development
and optimisation. === AFRIKAANSE OPSOMMING: Onlangse ontwikkelinge in die atmosferiese behandeling van lae-graad nikkel lateriet erts
by Anglo American plc het gelei tot die konseptuele yster gefokus lateriet (ARFe) proses.
Bykommend tot die herwinning van nikkel en kobalt uit laterite erts is hierdie proses
uniek en daarop gemik om yster te herwin as ’n verkoopbare by-produk. Die vermindering
van oplosbare yster(III) (Fe(III)) met swaeldioksied (SO2) is sentraal tot die ARFe
konsep en verteenwoordig ’n komplekse, multifase stelsel wat gelyktydige gas-vloeistof
massa-oordrag, termodinamiese spesiasie en chemiese reaksie behels. Die oplossingschemie
van ysterstelsels word, oor die algemeen, swak verstaan en om hul gedrag akuraat
te voorspel is ’n uitdaging, veral onder aggressiewe hidrometallurgiese kondisies.
Die primêre doel van hierdie werk is die ontwikkeling van ’n ingenieursmodel wat
die tempo en omvang van yster(III) vermindering met SO2 onder tipiese ARFe proses
toestande beskryf. Termodinamiese oorwegings stel ’n streng raamwerk voor vir die interpretasie
van chemiese reaksies, alhoewel daar egter min eksperimentele data openlik
beskikbaar is vir die gepaardgaande oplossing spesies in suur yster(III) sulfaat stelsels.
’n Belangrike bydrae van hierdie werk, en van kritieke belang vir die ontwikkeling van
die algehele model, is die direkte meting van spesiasie in yster(III) sulfaat oplossings.
Raman en UV-vis spektroskopie is gebruik om direkte spesiasie metings te maak in die
verskillende subsisteme van die Fe2(SO4)3-FeSO4-H2SO4-H2O stelsel wat voorheen nie
in die oop literatuur beskikbaar was nie. Die FeSO+4 en Fe(SO4)–
2 spesies is ekplisiet geïdentifiseer, terwyl die metings ondersteun en gerasionaliseer is deur statiese kwantummeganiese
berekeninge wat uiteindelik die kalibrasie van ’n robuuste, ioon-interaksie
model tot gevolg hê wat ook die belangrike oplossingspesies duidelik beklemtoon tot en
met 1.6 mol/kg Fe2(SO4)3, 0.8 mol/kg H2SO4 en tussen 25 – 90°C.
Enkellading en kontinue yster(III) verminderingskinetika is gemeet en die gevolge
van die aanvanklike Fe2(SO4)3 en H2SO4 konsentrasies, temperatuur en in-situ neutralisasie
is gekwantifiseer. Die waargeneemde vertragingseffek van swaelsuur is die mees
beduidende faktor wat die aanvanklike reaksietempo en die haalbare reaksie omvangsvermindering
na ’n vaste residensietyd van 180 minute bepaal, wat wissel tussen ongeveer
20 en 80%.
’n Reaksiemeganisme word voorgestel wat beperk word deur die stadige ligand-totmetaal
elektronoordrag in ontbinding van die Fe(III)SO+3 oplossing-spesies en wat verder
deur spektroskopiese metings en kwantummeganiese berekenings ondersteun word. A
kinetiese model, wat bestaan uit ’n stelsel van gedifferensieerde massa-balans vergelykings,
is in die termodinamiese raamwerk geïnkorporeer. Hierdie reaksie-model laat die
voorspelling van kinetiese profiele toe oor die volle omvang van die eksperimentele toestande
en kan in meer uitgebreide simulasie modelle van die ARFe proces geinkorporeer word.
Die spesifieke en oorspronklike bydraes van hierdie werk is
• Die direkte meting van die spesiasie in die Fe2(SO4)3-H2SO4-H2O stelsel deur
Raman en UV-vis spektroskopie
• Die ontwikkeling van ’n modelraamwerk om spesiasie, aktiwiteitskoëffisiënte en
oplosbaarheid in die gemengde Fe2(SO4)3-FeSO4-H2SO4-H2O stelsel te karakteriseer.
• Die meting van yster(III) vermideringskinetieka deur SO2 in gekonsentreerde sulfate
oplossings te gebruik as ’n funksie van die aanvanklike samestelling en temperatuur.
• Die ontwikkeling van ’n oplossingsreaksie-model van yster(III) vermindering met
SO2 wat die oplossing-spesiasie en reaksietempo met die tyd as ’n funksie van samestelling
en temperatuur akkuraat voorspel.
Laastens, die oorgrote kompleksiteit van industriële stelsels sal byna altyd lei tot ’n
gebrek van spesifieke eksperimentele data wat nodig is vir die ontwikkeling van fenomenologiese
modelle. Hierdie werk beklemtoon die belangrike rol wat ingenieursstudies
speel in die generasie van data wat sodanig tot maksimum praktiese waarde vir industriële
prosesontwikkeling en optimalisering lei.
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