When dealing with acid mine drainage (AMD), the common accepted practice is to develop a predictive model using methodologies such as acid-base accounting (ABA). These models are used to evaluate the neutralization potential (NP) and evaluate acid potential (AP) of geological materials that comprise mine waste and tailings; however, creating accurate predictive models for rocks of low neutralizing and low acid producing potentials is difficult. Through modeling a series of synthesized alkalic igneous rock compositions, static acid-base accounting (ABA) testing methods were conducted. These rock compositions were designed over a range in feldspar compositions: quartz syenite (Or > Ab); quartz monzonite (Or = Ab); albite-bearing quartz monzodiorite (Or < Ab); and labradorite-bearing quartz monzodiorite (Or < Lab). By means of ABA testing, the experiments demonstrated that with minimal change in sulphide content (increasing from 0.1% to 0.5%) the neutralization potential of the rock can be overcome.
In addition to ABA testing, static water-rock reaction experiments were conducted; in which the synthesized rocks were reacted at a 1:50 ratio with sulphuric acid solutions with varying pH values (2.0, 4.0, and 6.0) for a 4 week and 8 week period. The intent of each solution was to simulate a different AMD scenario: pH 2.0 to be an analogue of highly acid waters; pH 4.0 to be weakly acid and natural waters mixture; and pH 6.0 to be more representative of natural waters. The low to near neutral waters (pH 4.0 and pH 6.0) showed that feldspar composition has little to no effect in changing the buffering capacity, as the pH condition for initial pH 4.0 and 6.0 waters were neutralized to a pH in the range of 8.58 to 9.44. For pH 2.0 waters, the pH of the sulphuric acid solution were moderately buffered by the rock compositions to varying degrees; however, did not fully neutralize the waters (final pH ranged from 2.69 to 4.04). Feldspar composition in this situation is important in controlling the extent of buffering achieved. The largest buffering capacity was from the albite-bearing quartz monzodiorite, followed by labradorite-bearing quartz monzodiorite, and with the least amount by buffered by the quartz syenite.