Investigating the influence of electrolyte composition on electrodeposition in copper electrowinning

Chibwe, Chalwe (2020-03)

Thesis (MEng)--Stellenbosch University, 2020.

Thesis

ENGLISH ABSTRACT: The major concern in copper electrowinning is to deposit smooth, dense, pure copper at high current efficiency and low energy consumption. Electrolyte physicochemical propertiesnamely density, diffusion coefficients and conductivity affect the mass transfer and energy consumption in the cell. These properties are dependent on electrolyte composition. Control of growth and structure of the deposit determines the deposit morphology/smoothness and is strongly dependent on the current distribution over the cathode surface. The study investigated the influence of electrolyte composition on copper deposition via consideration of electrolyte physicochemical properties and current distribution in the cell. The electrolyte components were copperions, sulphuric acid, iron ionsand polyacrylamide(PAM)additive. The effect of other factors such as cell/electrode alignment on current distribution cannot be ignored, but were beyond the scopeof study, therefore were kept constant.The research approach was divided into two stages: establishing the relationship of electrolyte composition to physicochemical properties and modelling a copper electrodeposition process to predict current distribution at the cathode surface. A 5 factor, 2 and 3 level design of experiment was performed to determine the effect of copper (35 and 45 g/l), sulphuric acid (160 and 180 g/l), iron (1, 3 and 6 g/l), PAM additive (2, 5 and 10 mg/l) and temperature (45 and 55°C) on electrolyte density, conductivity and diffusion coefficient in synthetic copper electrowinning electrolytes. Density and conductivity were measured using a pycnometer and conductivity meter respectively. Diffusion coefficients were determined from the limiting current using linear sweep voltammetry.COMSOL Multiphysics, a finite element software was used to generate an electrowinning model for predicting current distribution at the cathode surface. Experiments were conducted for model validation. The current density was determined from deposit thickness by applying Faraday’s law. The results showed that increase in copper, acid, and iron concentration increased density but decreased iffusion coefficient. Conductivity improved with addition of acid but reduced with addition of metallic elements (copper, iron). The polyacrylamide additive had no effect on the properties. It was suggested that the addition of high atomic weight (copper, iron) elements increased density whilst impeding the movement of ions in the electrolyte whereas hydrogen ions improved electrolyte conductivity. Mathematical correlations for each property as a function of electrolyte composition were developed and were supported by previous studies.The copper electrowinning model predicted outputs such as species concentration and current distribution. Model and experimental current distribution compared well with each other. High current densities were observed near the cathode top and bottom with relatively uniform distribution at the cathode centre. This was attributed to the mass transfer phenomena, which facilitated less resistive path of ions in these regions. The model under-predicted the current density magnitude likely due to model limitations. The influence of electrolyte composition on current distribution profile was minimal, the effect was primarily on the magnitude of current density. Experimental and modelled current density both slightly increased with increase in copper concentration whereas variation in acid concentration caused a slight increase only in experimental current density, the modelled current density values remained the same.

AFRIKAANSE OPSOMMING: Die groot kommer in koperelektroherwinning is om gladde, digte, suiwer koper by hoë stroom effektiwiteit en lae energiegebruik te deponeer. Elektroliet fisikochemiese eienskappe, genaamd digtheid, diffusie koëffisiënte en geleidingsvermoë, affekteer die massa-oordrag en energie gebruik in die sel. Hierdie eienskappe is afhanklik van elektrolietkomposisie. Beheer oor groei en struktuur van die deposito bepaal die deposito morfologie/gladheid en is grootliks afhanklik van die stroomdistribusie oor die katode-oppervlak.Die studie het die invloed van elektrolietkomposisie op koperdeponering ondersoek via oorweging van elektroliet fisikochemiese eienskappe en stroomdistribusie in die sel. Die elektrolietkomponente was koperione, swaelsuur, ysterione en poliakrielamied-(PAM) bymiddel. Die effek van ander faktore soos sel-/elektrodebelyning op stroomdistribusie kan nie geïgnoreer word nie, maar is buite die bestek van die studie, en is daarom konstant gehou. Die navorsingsbenadering is verdeel in twee stadiums: die bepaling van die verhouding tussen elektrolietkomposisie en fisikochemiese eienskappe, en modellering van ’n koperelektrodeponeringsproses om stroomdistribusie by die katode-oppervlak te voorspel. ’n 5-faktor, 2-en 3-vlak ontwerp van eksperiment is uitgevoer om die effek van koper (35 en 45 g/l), swaelsuur (160 en 180 g/l), yster (1, 3 en 6 g/l) PAM-bymiddel (2,5 en 10 mg/l) en temperatuur (45 en 55 °C) op elektrolietdigtheid, geleidingsvermoë en diffusiekoëffisiënt in sintetiese koper elektroherwinning elektroliete te bepaal. Digtheid en geleidingsvermoë is gemeet deur ’n piknometer en geleidingsvermoëmeter onderskeidelik te gebruik. Diffusiekoëffisiënte is bepaal uit die beperkte stroom deur liniêre stryk voltammetrie te gebruik. COMSOL Multiphysics, ’n eindige element sagteware is gebruik om ’n elektroherwinningmodel te genereer om stroomdistribusie by die katode-oppervlak te voorspel. Eksperimente is uitgevoer vir modelvalidasie. Die stroomdigtheid is bepaal uit depositodigtheid deur Faraday se wet toe te pas.Die resultate het gewys dat verhoging in koper-, suur-en ysterkonsentrasie digtheid verhoog het, maar die diffusiekoëffisiënt verlaag het. Geleidingsvermoë het verbeter met byvoeging van suur maar verlaag met byvoeging van metaalelemente (koper, yster). Die PAM-bymiddel het geen effek op die eienskappe gehad nie. Dis voorgestel dat die byvoeging van hoë atomiese gewig-(koper, yster) elemente digtheid verhoog het terwyl die beweging van ione in die elektroliet belemmer is, waar waterstofione elektrolietgeleidingsvermoë verbeter het. Wiskundige korrelasies vir elke eienskap as ’n funksie van elektrolietkomposisie isontwikkel en ondersteun deur vorige studies.Die koperelektroherwinningmodel het uitsette soos spesiekonsentrasie en stroomdistribusie voorspel. Model en eksperimentele stroomdistribusie het goed vergelyk met mekaar. Hoë stroomdigthede is waargeneem naby die katode se bokant en onderkant met relatiewe uniforme distribusie by die katode se vimiddel. Dis toegeskryf aan die massa-oordragfenomeen, wat ʼn laer weerstandpad van ione in hierdie streke gefasiliteer het. Die model het die stroomdigtheidgrootte ondervoorspel, waarskynlik as gevolg van modelbeperkinge.Die invloed van elektrolietkomposisie op stroomdistribusieprofiel was minimaal, die effek was primêr op die grootte van stroomdigtheid. Eksperimentele en gemodelleerde stroomdigtheid het beide effens verhoog met verhoging in koperkonsentrasie, waar variasie in suurkonsentrasie ’n effense verhoging slegs in eksperimentele stroomdigtheid veroorsaak het, die gemodelleerde stroomdigtheidwaardes het dieselfde gebly.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/108313
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