In situ calcite formation in limestone-saturated water leaching of acid rock waste

The result of leaching of a 75% acid rock/25% limestone column with limestone-saturated solution has shown that the pH of the effluent recovered from 2.5, after apparent loss of acid neutralizing capacity after 4 years with water leaching, to pH 7 in less than 3 years. Bulk assay results, XRD and SEM/EDS analyses of samples from the column at 384 weeks (pH 3.6) and 522 weeks (pH 6.9) during this recovery have suggested that this is due to formation in situ of fine calcite. Calcite, initially blended to the column material at 25 wt.% was not found in the XRD of the 384 week sample but is clearly found in the 522 week XRD. This increased calcite content appears to be derived from the limestone-saturated water as finely divided solid precipitated in the drying cycles in the column. This result is confirmed by assessment of the 522 week sample as non-acid forming. Loss of some reactive aluminosilicate minerals, formation of secondary, precipitated, surface-attached gypsum and loss of fine secondary jarosite occurs across this pH range but fine, surface-attached jarosite is still found in the 522 week sample implying relatively slow dissolution kinetics. In comparison with the 384 week sample, armouring of highly reacted pyrite particles by surface layers of iron oxyhydroxides and aluminosilicates has become more extensive at 522 weeks after return of the pH to neutral values. This is consistent with results from Freeport field samples from limestone blended test pads where pyrite armouring was also substantially increased at higher pH. The results suggest that it may be possible to effectively maintain neutral pH and passivate pyrite, reducing oxidation rates by more than an order of magnitude, using limestone-saturated solution dump feed rather than bulk limestone blending or covers.