Treatment of ferric sulfate waste solutions for the production of ammonium ferric sulfate dodecahydrate

Abstract

The process of precipitation of ammonium ferric sulfate dodecahydrate (NH₄Fe(SO₄)₂·12H₂O) from waste solutions, obtained during autoclave oxidation of pyrite concentrate has been studied. A special feature of these solutions is the high concentration of Fe(III) ions (>60 g L^–1) and sulfuric acid (> 61 g L^–1). Based on comprehensive laboratory tests, the study determined the optimal conditions for the precipitation process of ammonium ferric sulfate dodecahydrate (AFS) by salting out with ammonium sulfate: reagent excess (100% over stoichiometric, temperature 276 K, time 1 h). The process should be conducted under continuous slow stirring which would not allow forming of large crystals that are difficult to remove from the reactor. The test work confirmed that high quality crystals can be produced by prior oxidation of Fe(II) to Fe(III) ions using hydrogen peroxide and copper removal from the solution.