Metallurgy Interview Question:

How much gold concentration should be in the organic phase (DBC) before reduction by oxalic acid solution? Is it effective on purity of gold in reduction step? What is the ratio of DBC to oxalic acid solution in this step?

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It really does not matter the concentration of gold in the DBC, it will respond well to very small concentrations or it will generally load up at about 25 grams of gold in a liter of DBC. This does not present a problem, as the DBC is completely re-useable, although I generally figure on a 3% to 4% loss per cycle in handling, etc. Therefore, if your original acid had 50 grams of gold in it, you would do two extractions into DBC using a liter of the extracting solvent. You should continue to extract from the pregnant gold solution until you are sure no gold is left in it. If the pH of the acid solution is not too high a simple stannous chloride test will tell you if there is still gold in it, and that is sensitive to the ppb level.
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