Describes a method of producing a magnesium sulfate and hydrous ammonia double salt or Boussingaultite ((NH4)2SO4.MgSO4.6H2O), using as a source of magnesium a liquid effluent rich in magnesium sulfate originally from hydrometallurgical processes for the production of metals such as nickel, copper, rare earths. According to the invention, the process route for the production of Boussingaultite with physical properties suitable for use in fertilizer mixtures involves the steps of precipitating the Boussingaultite double salt, filtration and thermal drying.