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 for relative solid or liquid mass flow rate in question on a logarithmic scale.
The main focus of the aqua regia process and its many derivations is the refining of metallic gold scrap as well as complex heterogeneous high grade metal-ash scrap. Typical feed is mine gold, dental scrap, jewellery scrap, selected electronic scrap fractions.
For the hydrometallurgical separation of gold and silver it is necessary to dilute the metallic feed to an extent that all of the silver in the resulting alloy will be soluble by nitic acid whereas gold is left in the residue. Depending upon the process metallurgy the residue appears as a sponge or an easily filtering and handable packed bed. Different metals may be used for the inquartation based on 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 major impact of the aqua regia gold refining system and may cause large investments in NOx scrubbers which may even exceed the investment into the rest of the process by far if the process has not been designed carefully. But the problem of NOx emissions during dissolution may be reduced to a very low extent by means an appropriate reactor design whereas the emission of NOx from the reduction of excessive nitric acid during gold reduction may be very much limited by chemical pretreatment.
Platinum and palladium coprecipitation
If platinum and palladium is present in the feed they are separated commonly by precipitation from the combined filtrates/decantates of AgCl precipitation and and gold reduction. (This is not the best but the simplest process). But precipitation of platinum and palladium from aqua regia liquors is not quantitative ! Many pgm refiners have developed their own inhouse processes to recover the pgm traces values - more or less acceptable in terms of cost. The optimum process is a platinum palladium trace coprecipitation by a selective organic ligand avoiding salination and/or contamination of the spent liquor. (The precipitate is recycled to incineration and further pgm refining).
HEV offers this process and the chemical.
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 is molten 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 (optimal process conditions). Due to the highly porous surface of the material the cathodes obtained need additional 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, palladium and base metals is low it is reasonable to directly electrolyse gold instead of aqua regia pretreatment followed by electrolysis.
· D-63571 Gelnhausen · Rhönstraße 106 ·
phone 0049-6051-968928 · fax -968929
email : HEV@halwachs.de · internet : www.halwachs.de