In the refining of precious metals, a mixture of hydrochloric and nitric acid is conventionally used.  With  such a mixture, there is the possibility that NOx gases are formed.  More and more, such fumes cannot be released freely into the environment.

An alternative method is to replace the oxidizing power of nitric acid by  hydrogen peroxide thereby eliminating the formation of NOx fumes.  The reaction is rapid at low temperatures and the solution can be filtered to remove insoluble materials, silver being precipitated as a chloride.  The gold is recovered from solution using a reducing agent such as sulphur dioxide or sulphite leaving impurities such as copper in solution.  The gold produced is reported to be + 99.99 % purity.