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Abstract. Viscosity and density are highly important physicochemical properties of copper electrolyte since they affect the purity of cathode copper and energy consumption [1,2] affecting the mass and heat transfer conditions in the cell [3]. Increasing viscosity and density decreases the rate in which the anode slime falls to the bottom of the cell [4,5] and lowers the diffusion coefficient of cupric ion (DCu2+) [6]. Decreasing the falling rate of anode slime increases movement of the slime to other directions than downward [4,5]. If the anode slime ends up on the cathode, the impurities could entrap into coating [4]. Due to that the aim is to keep the viscosity and density sufficiently low [4]. According to the studies of Price and Davenport [3], Subbaiah and Das [2], Devochkin et al. [7] as well as Jarjoura et al. [8] increasing the concentration of copper, nickel and sulfuric acid increases both viscosity and density, while temperature decreases these properties. In addition, Price and Davenport [1] researched the effect of impurities arsenic and iron as well as Subbaiah and Das [2] the effect of iron, manganese and cobalt. All of these researchers found that those impurities increased both viscosity and density. The density and kinematic viscosity of copper electrorefining electrolytes have not been extensively researched under electrorefining conditions. The kinematic viscosity is also an important factor in the equation defining DCu2+ using a rotating disc electrode technique [6]. The errors in the viscosity values cause significant error to DCu2+. Thus, this work introduces mathematical models for the density and kinematic viscosity. The kinematic viscosity of the test electrolytes was measured with a Ubbelohde capillary viscometer from Schott-Geräte GmbH and the density with a glass tube oscillator DMA 40 Digital Density Meter from Anton Paar K. G. The temperature (50, 60, 70 °C) and electrolyte composition were used as variables. The composition variables investigated were copper (40, 50, 60 g/dm 3 ), nickel (0, 10, 20 g/dm 3 ) and sulfuric acid (130, 145, 160 g/dm 3 ) in all models, and additionally the effect of arsenic acid for the viscosity was studied (0, 15, 30 g/dm 3 ). The electrolytes used in these tests were prepared from CuSO4•5H2O (99-100 %), NiSO4•7H2O (99-100 %), H2SO4 (95-97 %) and arsenic acid (containing As 151700 mg/dm 3 , Bi 6.2 mg/dm 3 , Se 0.07 mg/dm 3 , Te 18.6 mg/dm 3 , Ag 0.2 mg/dm 3 , Cu 4794 mg/dm 3 , Ni 1688 mg/dm 3 , Pb 28.62 mg/dm 3 and Sb 3954 mg/dm 3 ). The results were normalized using known water values for viscosity as well as water and air values for the density. Based on these results the models for density (ρ, g/cm 3 ) and kinematic viscosity (ν, mm 2 /s) were designed, refined and analyzed using modeling and design tool MODDE, Equation 1 for density and 2 for kinematic viscosity: a Corresponding author: taina.kalliomaki@aalto.fi , 010 (2016)

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Abstract:The physico-chemical properties of the copper electrolyte significantly affect the energy consumption of the electrorefining process and the quality of the cathode product. Favorable conditions for electrorefining processes are typically achieved by keeping both the electrolyte conductivity and diffusion coefficient of Cu(II) high, while ensuring low electrolyte viscosity. In this work the conductivity of the copper electrorefining electrolyte was investigated as a function of temperature (50-70˝C) and concentrations of copper (Cu(II), 40-60 g/L), nickel (Ni(II), 0-20 g/L), arsenic (As(III), 0-30 g/L) and sulfuric acid (160-220 g/L). In total 165 different combinations of these factors were studied. The results were treated using factorial analysis, and as a result, four electrolyte conductivity models were built up. Models were constructed both with and without arsenic as the presence of As(III) appeared to cause non-linearity in some factor effects and thus impacted the conductivity in more complex ways than previously detailed in literature. In all models the combined effect of factors was shown to be minor when compared to the effect of single factors. Conductivity was shown to increase when copper, nickel and arsenic concentrations were decreased and increase with increased temperature and acidity. Moreover, the arsenic concentration was shown to decrease the level of conductivity more than previously suggested in the literature.

Diffusion and convection are the main modes of mass transport that occur during copper electrorefining. Diffusion determines the rate of copper transfer across the diffusion layer, which in turn, affects the dissolution of the anode and deposition on the cathode. The diffusion coefficient of cupric ion (DCu(II)) is a typical property that can be defined from the limiting current density (jlim) values. In this work, the limiting current densities were measured for 24 different synthetic copper electrolytes over a temperature range of 50-70 °C using a rotating disc electrode (RDE). From this data, a model for jlim and the corresponding models for DCu(II) were constructed using Levich (Model L), Koutecký-Levich (Model K) and mixed-control Newman equations (Model M). The models for jlim and DCu(II) were designed, refined and analyzed using the modeling and design tool MODDE, with the temperature, copper, nickel, arsenic and sulfuric acid concentrations as variables. Results from this research show for the first time that an increase in arsenic concentration has a reciprocal effect on the DCu(II) under copper electrorefining conditions. Furthermore, the models were validated with 11 industrial electrorefining electrolytes with known compositions. Model L (DCu(II) based on Levich equation) was shown to provide the highest correlation with the industrial solutions when compared to the other models (Model K and M) considered and previously published diffusion coefficient models. Overall, this work provides an explanation for the previously observed data variances in the literature, investigates for the first time the combined effect of parameters on DCu(II) value in industrial copper electrolysis and clarifies the effect of arsenic on the DCu(II) of copper electrorefining electrolytes.