DISSOLUTION PHENOMENA OF GOLD WITH COMBINATION OF MANGANESE DIOXIDE AND HYDROCHLORIC ACID

摘要

Common lixiviation methods for precious metals, such as aqua regia (used mainly for industrial purposes), iodine/iodide, bleaching powder-hydrochloric acid/sodium chloride and iron(Ⅲ)-sulfuric acid/thiourea, are based on combinations of complexing agents and oxidation agents. As the leaching solutions used in these processes are highly poisonous, the resulting waste liquid requires careful handling. Here, we propose and examine the usefulness of a new lixiviation process for precious metals relying on a combination of manganese dioxide and hydrochloric acid in an airtight container. In the case of aqua regia, Pd and Au are leached rapidly while most of the Ag and Pt are not leached. In iodine/iodide lixiviation, Au and Pd are leached efficiently while, again, most of the Ag and Pt are not leached. Bleaching powder-hydrochloric acid/sodium chloride lixiviation results in fast dissolution of Au and Pd without leaching most of the Ag and Pt. In the case of iron(Ⅲ)-sulfuric acid/thiourea lixiviation, Au and Ag are leached rapidly, while Pd and Pt are not leached at all. These phenomena are explained by the complexing and oxidizing abilities of the lixiviation agents for each of these precious metals. In this study, we propose the leaching of Au with a combination of manganese dioxide and dilute hydrochloric acid in an airtight container. The dissolution of Au by this method results in 90% leaching or more after 3h with 0.5mol/dm~3 HCl. This method is shown to be useful for the leaching of precious metals from industrial wastes such as automobile catalysts and printed circuit boards.