Methods for estimation of long-term non-carbonate neutralisation of acid rock drainage

In the long-term phase of an acid rock drainage (ARD) evolution profile, after any short-term neutralisation capacity provided by carbonate minerals is exhausted, the net acid release is a product of a declining acid generation rate (AGR) and a slower, long-term acid neutralisation rate mainly provided by gangue silicate minerals. At some point, the AGR and the non-carbonate acid neutralisation rate (ANRnc) will be similar. Matching of the AGR and ANRnc near 10 mg H2SO4/kg/week is demonstrated in data from 10-year columns. This long-term neutralisation is not measured at present in any accepted assessment tests. Methods to estimate ANRnc, based on silicate mineralogy and solution assays from long-term column leach tests, are compared. Good agreement is demonstrated between rates measured from the solution assay data and those calculated from mineralogy using kinetic databases. More rigorous analysis of the leachate chemistry of selected long-term leach tests also suggests possible cover design criteria based on the maximum AGR that will maintain a pH > 4 in leachate from ARD materials. The data show a distinct break at an AGR of 3 mg H2SO4/kg/week, below which no leachate pH is less than 4. The results indicate that an AGR of 10 t H2SO4/ha/year is conservative and a suitable cover design target for ARD control that would be matched by ANRnc.