Gold refining

What happens to the gold and silver after it leaves Waihi?

Doré bullion from the Martha and Correnso mines in Waihi is delivered to the refinery at the Western Australian Mint. Bars are checked, weighed, and their description recorded on a computer and in a manual register.

The bars are then melted in a 300kw furnace and transferred to a pouring pot from which samples are taken for assay.

The molten material is cast into ingots which are air cooled, cleaned, labelled, weighed and compared to the receival weight to ensure that all the precious metal is accounted for.

Assay samples are processed in a dual stream independent of the remainder of the precious metals.

The assay values are applied against the nett weight of the total deposit and the result of the calculated gold and silver values is credited to the Waihi Metal Account. Transfers can be made to bullion dealers as financial transactions.

The refining procedure is then completed by placing the ingots (anodes) in vats containing a nitric acid solution and applying an electric charge. The ingot dissolves and the silver content is plated out on a cathode plate. The resulting silver crystal is removed from the cell, washed, dried, and melted into the desired shape as pure silver.

The gold content is captured in a ‘mud bag’ around the anode. It is washed and dried, then melted and refined in a chlorination process where chlorine gas is introduced into the melting crucible. Gold is separated from the remaining base metals and is poured into granules or bars for commercial sale.

References and acknowledgements
Material on this page has been prepared from information kindly supplied by the Western Australian Mint.