THE EFFECT OF METAL-CATIONS ON THE KINETICS OF LIMESTONE NEUTRALIZATION OF ACID WATERS

Limestone (CaCO3), is a lower cost alternative to lime (CaO) for the n eutralisation of acid water, but the limestone neutralisation reaction is impaired by iron(II), iron(III) and aluminium in solution. This pa per describes the kinetics of limestone neutralisation in the presence of these metals. The reaction rate is affected by the type of metal c ation, by the concentration of the cation and by pH. At pH levels belo w 2.0 the limestone dissolution reaction rate decreases sharply with i ncreasing pH. In the pH range 4.0 to 5.5 the reaction rate decreases l inearly with increasing pH. The pH range 2.0 to 4.0 is a transition ra nge, from the non-linear to linear dissolution rate characteristics. M etal concentrations below 80 mg l(-1): At pH levels less than 4, iron( II) had the strongest suppressing effect followed by aluminium, while the presence of iron(III) increased the reaction rate. In the pH range 4.0 to 5.5 aluminium had the strongest suppressing effect followed by iron(III) and iron(II). Metal concentrations above 80 mg l(-1): Iron( II) and aluminium suppress the reaction rate at all pH levels. At pH l evels less than 4 iron(II) had the strongest suppressing effect, follo wed by aluminium. In the pH range 4.0 to 5.5 aluminium had the stronge st suppressing effect followed by iron(II). With iron(III) the rate is suppressed at pH levels below 2, however the rate is speeded up in th e pH range 25 to 3.5. At higher pH levels, the iron(III) concentration is limited to less than 80 mg l(-1) because of precipitation of iron( III) at pH levels higher than 2.5. The extent to which the overall neu tralisation reaction proceeds was modelled to assist in reactor design . The overall reaction is impaired most by aluminium, followed by iron (II) and iron(III).