The Precipitation of Gold Using Zinc Sheets

 

By practicing the precipitation of gold from old cyanide solutions using zinc sheets no gold will remain in the solution standing above the precipitate after being carefully treated by this method, and the gases produced is not a serious issue as they are in the process for extracting the gold by a mineral acid. Metallic zinc in the form of sheets is more appropriate to use than either turnings, clippings, granulated into flakes, filings or any other form of finely divided zinc. Also the sheets can be easily hung in the solutions and readily withdrawn when all the gold was precipitated or at any other stage of the operation. Basically, it is not necessary when a little of acid is used that the zinc has clean surfaces, for if coated with ordinary oxide, it will work quite satisfactorily.

It is important to indicate that when the zinc is immersed in the solution its oxide is attacked and dissolved, and the zinc turns a blackish color, and if any gold attached itself to the surface at first, which is unlikely, it is spongy and non-adherent and is quickly removed by the non-interrupted action of the sulphuric acid on the zinc. If the zinc sheet surface was scraped quite clean and placed in the gold cyanide solution without the acid, the gold would deposit on the surface in a thin layer by chemical or electrolytic action, but only in a thin layer, and the moment it became covered all over, all action would immediately cease and the operation then prove a failure. Sulphuric acid gives a simple remedy for this precipitation process by creating to the zinc a free surface for the continual production and escape of the hydrogen gas which is produced from the reactions caused within the solution, which is an advantage.

Without adding the acid, a zinc hydroxide may be formed on the zinc sheet and this compound interferes with the action of the process also by putting a stop to the precipitation. Excessive addition of sulphuric acid must be avoided due to it causes a rapid action on the zinc sheet; a very little and added occasionally as required is much the best form of treatment. When all the gold was precipitated, the clear solution may be siphoned off and discarded, as it will contain extremely minor contents of gold. The precipitation product is then ready to be transferred to the drying furnace and there heated until all the moisture was evaporated. No washing is required to be done by this process, and will not be necessary to filter the remaining solution away from the precipitate, as that will vanish under the heat of the furnace. The dried product is next reduced to a fine powder, mixed with a good reducing flux and melted in a crucible, when a button of mixed metals will be obtained without loss occurring, such as is often experienced in washing precipitates.

The crucibles in which these products are melted must be of the kind known as round fluxing pots. Other crucibles such blacklead and plumbago does gives a good results due to the action of the fluxes on the material of which they are fabricated. These special crucibles when being used are sometimes placed within plumbago pots as precaution against losses due to cracks, which seldom happens.