Since decopperized anode slime generated by copper electrolysis contains large amounts of precious metals such as gold, platinum, silver, selenium, palladium, and tellurium, these precious metals are recovered from the decopperized anode slime. For example, decopperized anode slime is subjected to oxidation roasting to convert selenium into selenium dioxide and vaporize the selenium dioxide, and the selenium dioxide is cooled and condensed to recover the selenium. In addition, the remaining matter of the oxidation roasting is melted and impurities therein are removed to produce a raw silver plate. The raw silver plate is set as an anode to electrolytically recover silver. The electrolytic slime after recovering silver by electrolysis is further subjected to parting (leaching using nitric acid) to recover platinum and palladium from the liquid. A raw gold plate is produced from the dissolution residues after the parting and is set as an anode to recover gold by electrolysis. Such a processing method has been known.
As another method of recovering precious metals from decopperized anode slime, a processing method (Hoffmann process) including recovering silver chloride by carrying out chlorination leaching of decopperized anode slime, extracting gold with a solvent from the liquid after the leaching, recovering selenium by reduction and distillation from the liquid after the extraction, and recovering gold and platinum from the liquid after the distillation has been known.
In the chlorination leaching step, the decopperized anode slime is repulped with a hydrochloric acid and an oxidant is added thereto to perform oxidation leaching. Due to this chlorination leaching, silver is converted into silver chloride, and gold, platinum, and palladium are also dissolved in the hydrochloric acid liquid. A hydrogen peroxide or a chlorine gas is used as the oxidant (Non-Patent Document 1).
Regarding the chlorination leaching, a processing method has been known in which copper removal electrolytic slime is made into a slurry with water, and in oxidation leaching by injection of a chlorine gas, the concentration of chloride in the leaching solution is controlled to a certain amount or less to improve a leaching rate of gold, silver, and the like (Patent Document 1).