How does sodium metabisulfite differ from sodium bisulfite?
Chemically they are equivalent in use, because metabisulfite hydrolyses to bisulfite on dissolution. The difference is physical. Metabisulfite is a solid carrying far more available sulphur dioxide per tonne, with much longer shelf life in sealed packaging; bisulfite solution is around 60 percent water and oxidises slowly in the drum.
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When is the solid preferable to the solution?
Where freight distance is significant, where storage life matters, or where dosing strength needs to be varied. Shipping a solution means shipping water, and a drum of bisulfite loses strength over months. Against that, the solid must be dissolved before dosing, which needs make-up equipment, and handling the powder releases irritant sulphur dioxide.
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Why is sodium metabisulfite used in gold processing?
As a reducing agent and for cyanide destruction. In the SO2 and air process it is used to oxidise free and weakly complexed cyanide in tailings to cyanate, which is far less toxic, allowing tailings to be discharged within consent. It is also used to precipitate gold from solution in some refining routes.
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What are the handling precautions?
It releases sulphur dioxide, which is an irritant to eyes and respiratory tract, particularly when the powder is handled or when it contacts acid. Adequate ventilation and respiratory protection are required for powder handling, and it must be stored away from acids and oxidising agents in a cool, dry, sealed condition.
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