Small Scale Gold Refining by Zinc precipitation & Gold cathodes

    Gold mining operations treat their ore by cyanidation and precious metals are recovered from the pregnant solution by Zinc Dust (Merryl Crowe process) or activated carbon and electrowinning. Big operations treat zinc precipitates or cathodes by smelting in order to obtain a gold-silver product (Bullion). However, small operations don’t treat their final products because they have economical and technical restrictions. Therefore, here is a process on how to extract, refining your own gold.

    Considering the aspects mentioned above, the objective of this paper is show the possibilities of employing a refining process without investing a large amount of money like the refining processes that are used everyday in many jewelry shops.

    TREATMENT OF ZINC PRECIPITATES.

    A zinc precipitate can have a chemical composition like this: 12% gold, 5% silver, 12% CaO, 20% Cu, 11% Zn, 5% Zinc.  The process involves the following stages:

    • Treatment with nitric acid.
    • Silver precipitation.
    • Gold dissolution with aqua regia.
    • Gold precipitation.

    Treatment with nitric acid: Zinc precipitated is leached with hot nitric acid so that can be dissolved silver and common metals. Then, the slurry is filtered and the residue is washed with hot water. The result depends mainly of nitric acid dosage and temperature. The next table shows some possible results.

    Silver precipitation: The obtained silver nitrate solution is treated with sodium chloride in order to precipitate silver as silver chloride. The reaction is shown below.

    NaCL + AgNO3 = AgCl + NaNO3

    The amount of silver in the solution from acid leaching can be calculated by simple calculation. Now, it is necessary know the addition of sodium chloride. For this must be considered the following rule of thumb: each gram of silver in solution requires 0.55 grams NaCl.

    The silver chloride precipitated is filtered and washed with distilled water, after, must be transferred to a beaker for its treatment with zinc dust which has been previously acidified lightly with sulphuric acid. The metallic product obtained is filtered and washed with hot distilled water. The final product is dried and finally smelted in order to get metallic silver.

    Gold dissolution with aqua regia. The precipitated obtained in Step (a) is leached with hot aqua regia (85 – 90 oC). The amount of aqua regia to be employed must be in excess (at least 100 times) to the required theoretically. The reaction is shown below.

    Au + 3HNO3 + 4HCl = HAuCl4 + 3NO2 + 3H2O

    After attacking with aqua regia will be added urea which decreases the residual HNO.

    6 HNO3 + 5CO(NH2)2 = 8N2 + 5CO2 + 13H2O

    The slurry is filtered and the residue is washed with hot distilled water until eliminate a yellowish coloration in the filtered solution.

    Gold precipitation. Metallic gold is precipitated from the solution obtained in step (c) by adding sodium bisulphite. In theory, each gram of gold needs 1.89 grams of sodium bisulphite. However, must be added more, approximately 1.5 more times.

    2HAuCl4 + 2NaHSO3 = 2Au + 4HCl + Na2SO4 + SO2

    The gold precipitated is filtered and washed with hot distilled water and an ammonium solution. Finally is dried and smelted. The smelting process can be done with borax, silica and saltpeter for one hour to 1100 oC.

    There are alternative reagents to sodium bisulphite such as oxalic acid, sulphurous, acid and sulphur dioxide.  The cost of treating zinc precipitated can be in the range 8 to 12 dollars/Kg zinc precipitated.

    With this treatment gold recovery is higher than 98% and the impurities are almost imperceptible because the obtained gold is practically pure, 995 to 999 fine.

    TREATMENT OF GOLD CATHODES.

    Steel wool cathodes need a previous treatment with hot hydrochloric acid in order to dissolve all the metallic iron. The residue obtained is filtered and washed with distilled water. Then, the residue will be treated with aqua regia as was described in the previous steps for zinc precipitates.

    Flowsheet for each treatment is shown in Figure 1 and 2.

    Batch

    HNO3

    (mol/l)

    Extraction (%)

    Ag

    Pb

    Zn

    Cu

    1

    2

    42

    80

    98

    94

    2

    5

    55

    84

    99

    98