On the Issue of Extracting Gold from Spent Concentrated Electrolytes

P. E. Bogdanov, L. D. Livkina, N. V. Pelisko

(Presented by the scientific seminar of the Department of Inorganic Chemistry)

Owing to their high chemical stability and decorative properties, gold coatings have found broad application in jewelry, instrument making, and the watch industry. Depending on the nature and importance of the coating being obtained, gilding electrolytes are worked until certain concentrations of gold in the electrolyte are reached.

Several methods are known for extracting gold from spent electrolytes. For the regeneration of gold from electrolytes containing not less than 0.5 g/l Au, the cementation method with zinc dust is mainly recommended [1]. Other methods of precipitating gold, used in quantitative analysis, are often employed [2]. In a number of cases, extraction of gold from spent cyanide and ferrocyanide electrolytes becomes burdensome for production because of the long duration of the processes, the high cost of the methods, and also the need to observe special occupational-safety conditions for workers.

Weiner and Dassoian [3] indicate that aluminum can be used to extract gold from spent electrolytes. Unfortunately, the authors do not provide experimental data.

In this connection, in the proposed work aluminum was tested for extracting gold from electrolytes with concentrations up to 13 g/l Au. The possibility of extracting gold from cyanide and ferrocyanide alkaline electrolytes was investigated. It was shown that extraction of gold by aluminum from the electrolyte is significantly accelerated in the presence of a NaOH solution. In Fig. 1 this dependence is given for cyanide and ferrocyanide electrolytes at alkali concentrations KOH and NaOH—40 g/l. As can be seen from this figure, the rate of precipitation of gold by aluminum, when a NaOH solution is introduced into the electrolyte, increases approximately threefold in comparison with a KOH solution. The dependence of the completeness of gold extraction on the concentration of NaOH solution introduced into the electrolyte is shown in Fig. 2. Under the conditions studied (initial concentration of Au in the electrolyte 12.55 g/l, \(t = 25^\circ\mathrm{C}\), \(\tau = 18\) h), the degree of gold extraction reaches 98.8% at a NaOH concentration of 40 g/l. The use of NaOH solutions of higher concentrations increases the solubility of aluminum in the electrolyte.

When a NaOH solution is introduced into the electrolyte, cyanurate is observed to precipitate; as gold is extracted by aluminum, it dissolves and passes into solution. Raising the temperature increases the rate at which cyanurate passes into solution.

As experiments have shown, cementation of gold by aluminum from cyanideileswi

Fig. 1. Dependence of the completeness of gold extraction (in % wt.) with aluminum on time. Curves: 1—ferrocyanide electrolyte with addition of NaOH, 2—cyanide electrolyte with addition of NaOH, 3—ferrocyanide electrolyte with addition of KOH, 4—cyanide electrolyte with addition of KOH. Model solutions were taken with a gold content of 10 g/l.

Fig. 2. Dependence of the completeness of gold extraction (in %) with aluminum on the amount of NaOH solution introduced into the electrolyte.

electrolytes proceed analogously to ferrocyanide electrolytes. In contrast to ferrocyanide electrolytes, when gold is cemented from cyanide electrolytes, in the initial period the process is somewhat slowed. This phenomenon can be explained by the fact that in a ferrocyanide electrolyte, when gold is displaced by aluminum, the complex \([Au(CN)_4]\) is less stable than the complex \([Au(CN)_2]\) in the case of cyanide electrolytes. In other words, trivalent gold is reduced faster than monovalent gold.

The influence of the size of the contact surface of aluminum with the electrolyte on the completeness of gold extraction was also studied. The experiments showed that when the contact surface is varied in a ratio of 1:10:1000 (with the same weighed portion of aluminum), the completeness of gold extraction remains practically constant. It should only be noted that when finely cut aluminum is used, the rate of its dissolution in the alkaline solution increases.

Simultaneously with the process of extracting gold from electrolytes containing impurities, other metals are also reduced, for example, Cu, Fe, Ni. These side processes require additional consumption of aluminum. The minimum consumption of aluminum is 0.2–0.5 g per 1 g of gold present in the electrolyte. Gold losses during displacement by aluminum do not exceed 0.1% (wt.), and during precipitation with zinc ...

~3%. Thus, the process of precipitating gold with aluminum from spent iron-hydrogen sulfide and cyanide electrolytes can be carried out over 24–48 hours (for Au concentrations in the electrolyte from 1 to 20 g/L), instead of 240–300 hours when zinc is used as the precipitant.

For extracting gold from spent electrolytes with a low metal concentration (below 0.5 g/L), it is advisable to use ion-exchange resins. The authors of this article used the anion exchangers EDE-10P, AV-17, and AN-2F. The rate at which the electrolyte was passed through (with an Au concentration of up to 0.2 g/L) was 250 mL/hour at an electrolyte pH of 8–9. The best results were obtained with the AV-17 anion exchanger (the completeness of gold extraction was 95%).

REFERENCES

1. M. I. Berdnikov. Extraction of Gold and Silver from Precious-Metal Wastes. ONTI, 1936.

2. V. F. Gillebrand et al. Practical Manual of Inorganic Analysis. Goskhimizdat, 1957.

3. Ya. V. Vyainer, M. A. Dasoyan. Technology of Electrochemical Coatings. Mashgiz, 1962.