A Method for Iron Determination During Copper Electrometallurgy and Its Application to the Calculation of Current Efficiency
With the objective of developing statistical models that describe iron kinetics under typical copper electrolytes as well as its effects on current efficiency and influence in cathodic and anodic behavior, a detailed electrochemical analysis is done in this work as a contribution to implement hydrometallurgical processes in the refining or winning process to produce high purity copper. This thesis evaluates the oxidation-reduction potential (ORP) of the Fe3+/Fe2+ couple in the H2SO4-Fe2(SO4)3-FeSO4-H2O and H2SO4-CuSO4-Fe2(SO4)3-FeSO4-H2O systems, with addition of different copper concentrations and temperatures up to 70℃, typically employed in the industry. This study further validates and expands a thermodynamic expression developed by Yue et al. to predict the redox potential of the H2SO4-CuSO4-Fe2(SO4)3-FeSO4-H2O system. This expression establishes a mathematical relationship between temperature, Fe3+/Fe2+ ratio and ORP, and therefore provides an alternative way of the determination of ferric and ferrous concentration in the electrolyte based on the measurements of ORP and T. Furthermore, a model developed by Khouraibchia and Moats (2009,2010) [1, 2] to calculate current efficiency based on current density and concentrations of copper and ferric is employed in this work in an attempt to evaluate the current efficiency-ferric/ferrous concentration relationship. Various sets of open circuit potential (OCP) tests were conducted to study the thermodynamic tendency of Fe3+/Fe2+ nominal ratio with temperature, copper concentration, acid concentration and iron concentration. Potentiometric determination of the ferrous iron was conducted to study the presence of iron ions and determine species in solution. All electrochemical assays were carried out by preparing a synthetic solution utilizing a standard three-electrode cell to simulate a small-scale electrowinning cell and also, by expanding parameters, an electrorefining cell. Redox potential of the Fe3+/Fe2+ couple was measured via a potentiostat/galvanostat VersaSTAT 3F and temperature was controlled with a PolySci circulating bath. Potentiometric titrations where carried out with an accumet ab200 pH/conductivity meter and a 50 ml burette for volumetric measurements. Measured ORP results are in well agreement with the predicted ORP from developed equation at all nominal Fe3+/Fe2+ ratios and temperatures (25 to 70℃), with no more than ±3 mV difference. This confirm the validation of the model developed by Yue et al. by reliable prediction of measured redox potential based on 2 variables; nominal Fe3+/Fe2+ ratio and temperature. The measured free Fe2+ concentration by potentiometry showed an average percent error of 2.53% when compared to initial values and no observable relationship with temperature variation. This further validates the expression developed by Yue et al., qualitatively and quantitatively. These findings expand the applicability of the equation to predict the redox potential in the H2SO4-Fe2(SO4)3-FeSO4-H2O and H2SO4-CuSO4-Fe2(SO4)3-FeSO4-H2O systems and provide an alternative through this equation to avoid complicated speciation calculations and volumetric methods for ferric and ferrous determination. The applicability of the equation in the current efficiency loss caused by iron in copper electrolytes was investigated using Eq. (2.12) for current efficiency. Results of the presumable current efficiencies obtained by using equation (2.12) showed several inconsistencies among CE values from the two industrial solutions (ER and EW) and consequently equation (2.7) couldn’t be employed to establish a relation to the current efficiency factor. This is due to the lack of a complete set of data from each solution such as real ferric/ferrous ratio and ORP measurements. Further industrial data will be investigated to support this method to apply Eq. (2.7) in iron determination and for the direct calculation of current efficiency.
Cruz, Daniel Pedro, "A Method for Iron Determination During Copper Electrometallurgy and Its Application to the Calculation of Current Efficiency" (2019). ETD Collection for University of Texas, El Paso. AAI27737895.