چكيده به لاتين
In the present research, we investigated continuous Ni electrowinning from low-grade sulfate solutions in a lab-scale diaphragm cell. A series of preliminary experiments were conducted to assess the functionality of the cell. An appropriate electrolyte composition was found to be 20 g/L boric acid, 20 g/L sodium sulfate, 0.1 g/L thiourea, and pH range of 4.2-4.7. We considered a two-stage statistical design of experiments strategy for the main experiments. In the first stage, a four factor two-level full factorial design was conducted in order to determine the effects of temperature, specific flow rate, current density, nickel concentration and their mutual interactions on the current efficiency, specific energy consumption, pH variation of the anolyte, and the quality of the cathode product. In the second stage, a central composite design response surface methodology was adopted to optimize the process. The results indicated that (1) increasing the specific flow rate increases the current efficiency but decreases pH variation of the anolyte. Also, it provides a better controlling of pH at lower concentrations, and it has no significant effect on the cell voltage and the quality of cathode; (2) increasing the current density rises the cell voltage and pH variation, but it causes a decline in the quality of cathode; (3) by increasing the nickel concentration in the electrolyte, the current efficiency significantly increased and the quality of cathode improved while voltage and pH variation decreased. Finally, a multiple optimization approach based on the desirability function suggested the optimum condition as the specific flow rate of 4.8 h−1, current density of 2.5 A/dm2, and nickel concentration of 30 g/L. A verification test under the optimum condition leads to a current efficiency of 92.78%, a cell voltage of 2.91 V, and a dense, adhering, and uniform nickel electrodeposit. Moreover, long-run experiments with 5 and 10 hours duration confirmed the steadiness of the system with satisfactory results. Finally, we performed electrowinning experiment on two real nickel sulfate solutions with Ni concentration of 31.4 and 25.9 g/L after acid leaching of different nickel catalysts and iron removal. The respective current efficiencies of 90.89% and 87.70%, and the cell voltages of 2.9 and 3.2 V approved the suitabity of the adopted system for actual purposes.
Keywords: Nickel electrowinning, diaphragm cell, nickel catalyst, two-level full factorial design, central composite design.
