Influence of Electrolyte on an Energy-Saving Copper Recycling Process Using Ammoniacal Alkaline Solutions

Abstract

In order to make the energy-saving copper recycling process more economical by reducing the power consumption during copper electrowinning, the influence of the bath composition on the saturated concentration of cuprous ion, the electric conductivity, the cathodic current efficiency and the power consumption were studied in an ammonia-ammonium chloride-cuprous/cupric ion solution. The saturated cuprous ion concentration increased with the ammonia and ammonium chloride concentrations. The electric conductivity of the solution increased in proportion to the ammonium chloride concentration, because ammonium chloride acts as a supporting electrolyte. When the ammonia concentration increased from 3 to 10 kmolÁm À3 or when the cuprous ion concentration increased from 0.5 to 2 kmolÁm À3 , the cathodic current efficiency decreased by 5-10% at a current density higher than 500 AÁm À2 . This results from an increase in the cupric ion concentration formed by the unavoidable oxidation and the disproportionation of cuprous ions. The low current efficiency, however, had little influence on the power consumption compared to the influence of the electric conductivity that was the dominant factor determining the power consumption. Actually, when the concentration of ammonium chloride increased from 2 to 4 kmolÁm À3 , because of its function of increasing the electric conductivity of the solution, the power consumption was reduced by half. Based on this study, a suitable bath composition for the energy-saving copper recycling process was determined to be 3-10 kmolÁm À3 ammonia, 4-5 kmolÁm À3 ammonium chloride and 1-2 kmolÁm À3 cuprous ions.

Hydrometallurgical process for the recycling of copper using anodic oxidation of cuprous ammine complexes and flow-through electrolysis

“…The calculated power consumption requirements were 460 and 770 kWh t −1 at the current densities of 250 and 500 A m −2 , respectively. These were much lower than that in the conventional copper electrowinning process, which requires 2000-2200 kWh t −1 at the current density of 200-300 A m −2 [11], in spite of the longer interpolar distance of 10 cm, and were similar to those reported in our previous study which were calculated based on the electrode potentials and conductivity of the electrolyte, 480 and 930 kWh t −1 at the current densities of 200 and 500 A m −2 , respectively [6]. Although it was difficult to discuss the details of the power consumption due to the difficulty of measuring the anode potential in the present cell design, our previous study showed that the voltage required for the electrode process was quite low, 0.34 V at the current density of 500 A m −2 for example, including the theoretical decomposition voltage of 0.20 V. Thus, this low power consumption is attributable to the low theoretical decomposition voltage of the present electrolysis and the low over potentials on the anode and cathode, as well as the fact that copper is electrowon from the monovalent state.…”

“…7. Although the solution was treated with copper particles to reduce the Cu(II), the feed solution and catholyte contained Cu(II) on the order of 0.1 or 0.01 kmol m −3 due to the unavoidable oxidation and/or disproportionation of Cu(I) [6]. As shown in this figure, the Cu(II) concentrations gradually increased or decreased with time and then reached nearly constant values.…”

“…In order to take advantage of this, however, both the low overpotential and high current efficiency are desired for both the cathode and anode reactions. On the one hand, we and some other researchers have reported a high current efficiency and low overpotential as for copper electrodeposition from Cu(I) in ammoniacal solutions [4][5][6]12,13]. On the other hand, a literature search showed that little attention has been given to the anodic oxidation of Cu(I).…”

“…Simultaneously, the residual Cu(I) is oxidized to Cu(II) on the anode. Like other similar attempts using monovalent copper ions [9,10], this process is advantageous in terms of power consumption requirement during the electrowinning stage, which is expected to be 2-4 times lower [5,6] than that of the conventional copper electrowinning from a sulfuric acid and cupric sulfate solution [11]. Such a low power consumption requirement is attributed to the low theoretical decomposition voltage as well as the fact that copper is electrowon from the monovalent state, i.e., Cu(I) [3].…”

“…Although copper could be recovered from these wastes in a pyrometallurgical process [1,2], a hydrometallurgical process has certain advantages such that the capital cost is generally lower than a pyrometallurgical process and that it could be economically operated even on a small scale. Our research group has been studying a hydrometallurgical copper recycling process for PCBs [3][4][5][6][7][8]. In this process, copper in the waste is dissolved as cuprous ammine complexes (Cu(I)) using a solution containing ammonia, an ammonium salt, and cupric ammine complexes (Cu(II)) [3], and metallic copper is then electrowon from the Cu(I) [4][5][6] after removing impurities from the solution [7].…”