Traditionally acid mine water is neutralised with lime (Ca(OH)2) Limestone (CaCO3) can be used as an alternative to lime as it is much cheaper. The aim of this study was to investigate the practicality of using cheaper limestone (CaCO3) as an alternative. Beaker and semi-continuous studies were carried out to determine the quality of the water after lime treatment to various pH-values, the influence of CaCO3 dosage, particle size and aeration time on neutralisation rate and the effect of gypsum, magnesium, iron(II) and iron (III).
The rate of neutralisation is fast when stoichiometric dosages of lime are applied. If sufficient crystallisation is allowed, partial removal of sulphate is achieved, as well as complete removal of heavy metals, depending on the pH of the treated water. With limestone the rate of CaCO3 neutralisation depends on the dosage of CaCO3 and particle size. Aeration marginally accelerated the rate of neutralisation as a result of CO2 stripping. Partial sulphate removal is achieved as a result of CaSO4 crystallisation while magnesium, if present, co-precipitates with the CaSO4. Iron(III) and aluminium(III) are effectively removed but the rate of neutralisation is dramatically retarded by dissolved iron(II). Iron(III) has no influence on the rate of acid water neutralisation. During semi-continuous fluidised bed studies, CaCO3 in the limestone was completely utilised when the particle size was greater than 0.150 mm. The capital costs for lime and limestone neutralisation in a f luidised-bed reactor are similar, but the chemical cost in case of limestone neutralisation amounts to only 29% of that of lime.