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.

The proposed magnesium-barium-oxide process consists of metal removal with Mg(OH) 2 , magnesium and sulphate removal with Ba(OH) 2 and calcium removal with CO 2 . The raw materials, Mg(OH) 2 and Ba(OH) 2 are recovered from the BaSO 4 and Mg(OH) 2 sludges that are produced. Laboratory studies showed that metals are removed to low levels. This includes iron(II), the dominant metal ion in mine water, which is first oxidised to iron(III), whereafter it precipitates as Fe(OH) 3 resulting in residual levels of Fe(II) in the mine water of less than 20 mg/ℓ. Sulphate is also removed to less than 25 mg/ℓ. The final sulphate concentration is a function of the amount of Ba(OH) 2 dosed, as the amount of sulphate removed is stoichiometrically equivalent to the Ba(OH) 2 dosage. During CO 2 -dosing, CaCO 3 is precipitated to the saturation level of CaCO 3 .

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The removal of sulphate and metals from mine water was assessed using the integrated barium carbonate process and the co-precipitation of barium sulphate with calcium carbonate. The rate of sulphate removal was influenced by the BaCO 3 concentration and the cation associated with sulphate, and increased with increasing BaCO 3 -concentration. BaCO 3 can only be used for removal of sulphate that is associated with calcium, as calcium is needed to remove the added carbonate associated with the barium cation. Sulphate removal was only marginally influenced by alkalinity. Sulphide can be stripped with CO 2 from a BaS-solution. The (CO 2 dosed/sulphide removed) molar ratio was close to unity for the first 50% of sulphide in solution. The stripped H 2 Sgas can be absorbed into zinc acetate. BaSO 4 and CaCO 3 can be converted simultaneously to BaS and CaO, respectively at an optimum temperature of 1050˚C. The CaCO 3 /BaSO 4 molar ratio has little influence on the yield of BaS. The running cost of the barium carbonate process for the removal of 2 g/L of sulphate totalled ZAR1.28/m³ (US$1.00 = ZAR7.0, Feb 2007), the capital redemption cost was R1.08/m³, and the value of the products (water and sulphur) totalled R2.76/m³.

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