aqua regia gold refining

for low cost separation of gold, silver, platinum and palladium from base metals in metallic high grade alloys, concentrates and from

heterogeneous scrap

blue arrows or lines identify liquids, brown solids; the weight of lines and arrows is an indication forrelative solid or liquid mass flow rate in question on a logarithmic scale.

Feed The main focus of the aqua regia process and its many derivations is the refining of metallic goldscrap as well as complex heterogeneous high grade metal-ash scrap. Typical feed is mine gold,dental scrap, jewellery scrap, selected electronic scrap fractions.

Inquartation

For the hydrometallurgical separation of gold and silver it is necessary to dilute the metallic feedto an extent that all of the silver in the resulting alloy will be soluble by nitic acid whereas gold isleft in the residue. Depending upon the process metallurgy the residue appears as a sponge or aneasily filtering and handable packed bed. Different metals may be used for the inquartation basedon the process strategy and environmental concern of the refiner.

The noxious fumes challenge The appearance of noxious fumes during the processes of dissolution and gold reduction is a majorimpact of the aqua regia gold refining system and may cause large investments in NOx scrubberswhich may even exceed the investment into the rest of the process by far if the process has notbeen designed carefully. But the problem of NOx emissions during dissolution may be reduced to avery low extent by means an appropriate reactor design whereas the emission of NOx from thereduction of excessive nitric acid during gold reduction may be very much limited by chemicalpretreatment.

Platinum and palladium coprecipitation If platinum and palladium is present in the feed they are separated commonly by precipitation fromthe combined filtrates/decantates of AgCl precipitation and and gold reduction. (This is not thebest but the simplest process). But precipitation of platinum and palladium from aqua regia liquorsis not quantitative ! Many pgm refiners have developed their own inhouse processes to recoverthe pgm traces values - more or less acceptable in terms of cost. The optimum process is aplatinum palladium trace coprecipitation by a selective organic ligand avoiding salination and/orcontamination of the spent liquor. (The precipitate is recycled to incineration and further pgmrefining). HEV offers this process and the chemical.

Refined gold The primary gold product of the aqua regia separation process above is gold sponge (purity>99.95% Au). If fine gold (purity >99.99% Au) is requested from the local market gold sponge ismolten to anodes and electrolyzed. Below the typical "cauliflower" shape precipitation of 99.99%gold onto the surface of the cathode in an electrowinning cell (Wohlwill type) is shown (optimalprocess conditions). Due to the highly porous surface of the material the cathodes obtained needadditional treatment to remove spent electrolyte and incorporated anode sludge.

Direct electrolysis of gold If the gold content of a metallic scrap is high and the impurity level of silver, platinum, palladiumand base metals is low it is reasonable to directly electrolyse gold instead of aqua regiapretreatment followed by electrolysis.

HEV · D-63571 Gelnhausen · Rhönstraße 106 · phone 0049-6051-968928 · fax -968929 email : HEV@halwachs.de · internet : www.halwachs.de