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Mechanisms and kinetics of atmospheric sphalerite oxidative and non-oxidative leaching

Henning, Adriaan Johannes (2018-03)

Thesis (MEng)--Stellenbosch University, 2018.

Thesis

ENGLISH SUMMARY: A kinetic study of the non-oxidative and oxidative leaching of sphalerite concentrates, under elevated temperatures (75 – 95 °C) and atmospheric pressure is presented in this dissertation. Sphalerite, a zinc sulphide ore, is commonly associated with impurities and other sulphides (i.e. chalcopyrite, galena, pyrite etc.). The mineralogical nature of sphalerite concentrates is complex and the chemistry of iron-containing reactive systems is generally poorly understood, especially under aggressive hydrometallurgical conditions. The aim of this work was the development of an engineering model capable of describing the rate and extent of sphalerite leaching in non-ferric and ferric containing systems. The mathematical framework presented in this thesis consists of various objectives, each addressing thermodynamic and kinetic aspects of the primary leach process. Comprehensive literature investigations are presented which constitutes the mechanisms and rate models, supplemented by phenomenological data obtained from batch experimentation. The different objectives are each covered in a chapter of this dissertation, and include the following: i) a solution thermodynamic framework, ii) intrinsic oxidation mechanisms and rate expressions and iii) quantification and validation of the intrinsic rate expression. Thermodynamic considerations provided a rigorous framework for the interpretation of the solution chemistry, with the explicit recognition of the important solution species. Speciation measurements from various literature sources were utilised to construct the Pitzer model for the various subsystems of the ZnSO4 – Fe2(SO4)3 – FeSO4 – H2SO4 – H2O system. The model gave accurate speciation trends up to concentrations of 1.5 M ZnSO4, 1.5 M FeSO4, 1.5 M Fe2(SO4)3 and 2 M H2SO4. The model distinguishes between inner- and outer-sphere complexes, which was achieved through the inclusion of Raman spectroscopic stability constants. Contact ion pair (CIP) formations was predicted by the Pitzer model and shown results with suitable accuracy for the application in modelling the ionic aqueous solution relevant to this metallurgical kinetic study. A detailed investigation into the electrochemical and mineralogical nature of natural sphalerite gave insights to the leaching mechanism. Iron impurity was found to be integral to sphalerites dissolution mechanism, with the electron exchange at the mineral surface limiting reaction rate. Polarization of sphalerite particle surface by the electrolytic solution caused surface states (barriers) that limits the rate of movement of charge carriers (i.e. electrons). A mechanism was proposed based on the assumption that the first electron or proton transfer step are the rate-limiting step of the non-oxidative and oxidative leaching mechanism. The resulting electrochemical half reactions from the mechanism was used to define the activation polarisation relationships, the Butler-Volmer equations. Through application of the mixed potential theory of metallic corrosion, rate expression for the non-oxidative and oxidative leaching of sphalerite were derived. Experimental batch data obtained from Dr JDT Steyl (1996) were used to quantify and validate the rate parameters of the derived rate expressions. The shrinking core model was applied within a batch reactor model to predict the leaching extents of sphalerite under various initial conditions. The rate parameter regression followed a two-fold strategy whereby the model was first linearized and regressed using a linear regression technique, and obtaining preliminary kinetic constants at average solution compositions. The second strategy consisted of a detailed differential batch reactor model including the solution speciation model and concentrate characteristics, which was used to quantify the intrinsic rate parameters using a non-linear regression technique. A sphalerite leaching mechanism and intrinsic reaction rate model was proposed in this study and the model was quantified using phenomenological batch data. The model was found to be able to predict the leaching rate of sphalerite.

AFRIKAANSE OPSOMMING: ‘n Kinetiese studie van die nie-oksidatiewe en oksidatiewe loging van sfalerietkonsentraat, onderhewig aan verhewe temperatuur (75 – 95 °C) en atmosferiese druk, word in hierde proefskrif voorgelê. Sfaleriet, ʼn sinksulfied erts, word tipies geassosieer met onsuiwerhede en ander sulfiede (m.a.w. chalkopiriet, galeniet, piriet, ens.). Die mineralogiese aard van sfalerietkonsentraat is kompleks, en die chemie van ysterbevattende reaktiewe stelsels word in die algemeen nie goed verstaan nie, veral nie onder aggressiewe hidrometallurgiese toestande nie. Die doel van hierdie werk is die ontwikkeling van ʼn ingenieursmodel wat in staat is om die snelheid en omvang van sfalerietloging in nie-femiese en femiese stelsels te beskryf. Die wiskundige raamwerk wat in hierdie proefskrip voorgelê word, bestaan uit verskeie doelwitte wat termodinamiese en kinetiese aspekte van die primêre logingsproses aanspreek. Die verskillende doelwitte word elk in ʼn hoofstuk van hierdie proefskrif aangespreek, en sluit die volgende in: i) ʼn oplossing termodinamiese raamwerk, ii) intrinsieke oksidasiemeganismes en snelheiduitdrukkings en iii) kwantifisering en validering van die intrinsieke snelheiduitdrukking. Termodinamiese oorwegings verskaf ‘n gestrenge raamwerk vir die interpretasie van die oplossingchemie, met die uitdruklike erkenning van die belangrike oplossingspesies. Spesiasie-metings van verskeie beskikbare literatuurbronne is gebruik om die Pitzermodel vir die verskeie substelsels van die ZnSO4 – Fe2(SO4)3 – FeSO4 – H2SO4 – H2O stelsel saam te stel. Die model verskaf akkurate spesiasietendense vir konsentrasies tot en met 1.5 M ZnSO4, 1.5 M FeSO4, 1.5 M Fe2(SO4)3 en 2 M H2SO4. Die model onderskei tussen binne- en buitesfeerkomplekse, wat moontlik gemaak is deur die insluiting van Raman spektroskopiese stabiliteitkonstantes. Kontakioonpaar(KIP)-formasies is voorspel deur die Pitzermodel en toon resultate met genoegsame akkuraatheid vir toepassing in die modellering van die ionise wateroplossing relevant tot hierdie metallurgiese kinetiese studie. ʼn Gedetailleerde ondersoek rakende die elektrochemiese en mineralogiese geaardheid van natuurlike sfaleriet het insigte verskaf oor die logingmeganisme. Ysteronsuiwerhede is gevind om integraal te wees tot die sfalerietontbindingmeganisme, met die elektronuitruil by die mineraaloppervlak wat dien as die beperkende reaksiesnelheid. Polarisering van die sfalerietpartikeloppervlak deur die elektrolietoplossing veroorsaak oppervlaktoestande (versperrings) wat die bewegingsnelheid van ladingdraers (m.a.w. elektrone) beperk. ʼn Meganisme is voorgestel gebaseer op die aanname dat die eerste elektron- of protonoordragstop dien as die snelheid-beperkende stap van die nie-oksidatiewe en oksidatiewe logingmeganisme. Die resultante elektrochemiese halfreaksies van hierdie meganisme is gebruik om die aktiveringpolariseringverwantskappe te beskryf deur middel van die Butler-Volmer vergelykings. Snelheiduitdrukkings vir die nie-oksidatiewe en oksidatiewe loging van sfaleriet is afgelei deur middel van die toepassing van gemengde-potensiaalteorie van metaalkorrosie. Eksperimentele enkelladingdata verkry vanaf Dr JDT Steyl (1996) is gebruik om die snelheidparameters van die afgeleide snelheiduitdrukkings te kwantifiseer en te valideer. Die krimpende kernmodel is toegepas binne die enkelladingreaktormodel om die logingomvang van sfaleriet onder verskeie begintoestande te voorspel. Die snelheidparameterregressie het ʼn tweedelige strategie gevolg, waar die model eerstens gelineariseer en geregresseer is deur middel van ʼn lineêre regressietegniek, wat die voorlopige kinetiese konstantes vir gemiddelde oplossingsamestelling gelewer het. Die tweede strategie het bestaan uit ʼn gedetailleerde differensiaal enkelladingreaktormodel (insluitende die oplossingspesiasiemodel en konsentraateienskappe) wat gebruik is om die intrinsieke snelheidparameters met ʼn nie-lineêre regressietegniek te kwantifiseer. ʼn Sfalerietlogingmeganisme en intrinsieke reaksiesnelheidmodel is in hierdie werk voorgestel en die model is gekwantifiseer deur middel van fenomenologiese enkelladingdata. Dit is gevind dat die model die logingsnelheid van sfaleriet kan voorspel.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/103692
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