Low cost and efficient recovery of noble metals from urban mines (e.g., used electronic equipments) is important for securing scarce resources. We are developing a noble metal recovery technology utilizing electroless displacement deposition on silicon (Si)¹. In this method, noble metals can be recovered by adding Si powder in metal salt solutions containing hydrofluoric acid (HF)^2,3. For safety concerns, we are considering the use of highly basic solution instead of toxic HF solution. We have reported the effects of solution pH on the gold recovery from a basic solution⁴. In the practical applications, gold is recovered from leaching solutions. In this study, we examined the gold recovery from a basic solution simulating a thiosulfate-based leaching solution.

0.20 M (M: mol dm^-3) Si powder (Kojundo Chemical Laboratory, 99.99%) with a particle diameter of 45 μm or less was added to a simple basic solution of 1 mM hydrogen tetrachloroaurate (III) solution pH-adjusted with sodium hydroxide, or a simulated leaching solution containing 1 mM sodium gold (I) sulfite, 1 M ammonium thiosulfate, 1 M ammonium hydroxide, and 0.5 mM copper (II) sulfate, which solution pH was at 9.3. The solution was stirred at 200 rpm using a magnetic stirrer and kept at 298 K of temperature. After filtering the solution, the concentrations of the metal in the filtrate were measured by an ICP-AES (Hitachi, SPS7800) and the recovery was determined from the change in concentration before and after the recovery process.

Figure 1 shows the recovery of gold from the simple basic solution (Fig. 1a) and the simulated leaching solution (Fig. 1b). The recovering rate form the simple basic solution was increased with solution pH. The long time treatment at the solution pH of 8 for 24h increased the recovery to 100%. Using bare Si powder, which was pretreated with a HF solution for dissolving an oxide layer on Si surface, shortened the treatment time to 45 min for getting 100% of recovery from the solution at the pH of 8. These results indicate that Si oxide layer inhibits gold recovery from the basic solution without fluoride species. The recovery of gold from the simulated leaching solution increased with treatment time and reached 98% or more at the treatment time of 120 min (Fig. 1b). The recovery of copper, which was added as the oxidation aid, was less than 10%. This suggests the possibility that gold is selectively recovered from the leaching solution including less noble metal than gold.

ACKNOWLEDGEMENT

The present work was partly supported by JSPS KAKENHI (JP26289276).

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