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Sulfur dioxide SO2 is a gas used for precipitating gold from gold chloride solutions. It is very specific for gold in chloride solution and it can precipitate a very pure product even from a dirty solution. The main contaminant is drag down when small amounts of the original solution gets locked in with the gold crystals and traces of platinum group metals. SO2 is so good at precipitating gold from solution that after precipitation no gold can be detected with stannous chloride, even from as dilute solutions as one gram per liter.
In a weakly acidic solution with copper chloride, SO2 can precipitate CuCl salt. When there is a large excess of hydrochloric acid in the solution the CuCl will not precipitate as it is soluble. Any precipitated CuCl can be washed away with a bit of hydrochloric acid in the following wash cycles.
In nitrate solutions it can form a complex salt and precipitate palladium.
Sulfur dioxide is mainly used by professional refineries as it takes some special equipment to get started. For the hobby refiner it is easier to use SMB in an acidic gold chloride solution where SO2 is created in the solution when it reacts with the hydrochloric acid.
Sulfur dioxide is a toxic gas with a very strong smell and can give acute respiratory problems when inhaled, especially for asthmatic individuals.
Use it only in a fume hood or outside with the wind coming from the side.
Chemical reaction with gold chloride
3 SO2 + 2 AuCl3 + 4 H2O = 3 H2SO4 + 6 HCl + 2 Au