Sulphate precipitation from mine water as ettringite, Ca6Al2(SO4)3(OH)12ยท26H2O, is generally indicated to be implemented as a second treatment step, following the heavy metal precipitation or as a third step, after gypsum seeded crystallization. The whole process may be seen as too complicated by the operators, which always prefer simpler flow sheets, with fewer steps and with lower investment cost, but this treatment process
sequence is justified by sludge management depending on type and by the economic reasons related to the aluminium required input. However, for some mine water
sources, a single step precipitation process can be considered, specifically for sources
with sulphate concentration below 2000 mg/L, out of which those containing more than
50 mg/L of aluminium, are of special interest. Single step metal and sulphate ions
precipitation can have the benefit of using the aluminium already present in the mine
water for the ettringite formation. This was experimentally tested for a Romanian mine
water source having typically 350 to 500 mg/L iron, 80 to 100 mg/L aluminium and
sulphate below 2000 mg/L. Results for continuous flow reaction system and batch
laboratory tests were similar, leading to the conclusion that aluminium from the mine
water is responsible for a relevant proportion of the precipitated sulphate. Of certain
interest is the settling behaviour of the composite sludge, consisting of metal hydroxy
compounds and ettringite.