Waste jarosite and alunite will be ineffective sulfur and potassium fertilisers

Abstract

The processing of ilmenite to synthetic rutile produces an acidic sulfate- and iron-rich solution from which the mineral jarosite [KFe ₃ (OH)₆ (SO ₄ ) ₂ ] can be precipitated. Jarosite and isostructural alunite [KAl ₃ (OH)₆ (SO ₄ ) ₂ ], which may also be precipitated from processing waste, have the potential to be effective slow-release fertilisers on sandy soils. A glasshouse experiment was conducted on a very sandy soil in which the effectiveness of these two compounds as sulfur and potassium fertilisers was compared with that of control fertilisers (gypsum and KCl). Neither compound proved to be an effective fertiliser. Equilibrium solubilities of sulfate (63 mM, 173 mM) and potassium (60 mM, 70 mM) for jarosite and alunite, respectively, were very low, which is consistent with the relative ineffectiveness of these compounds as fertilisers.