Dissection of the NAG pH test: tracking efficacy through examining reaction products

In this study we sought to better understand the mechanics of the NAG pH test and determine the influence each variable has in controlling the resulting NAG pH, a value which is commonly used by the mining industry for waste classification. Three bulk samples (20 kg) representative of waste types A (alkaline), B (weakly-NAF) and D (PAF) were obtained from the Savage River mine, Tasmania. Variables tested included initial reaction time, heating temperature, heating length, post reaction cooling time and hydrogen peroxide (H₂O₂) strength (n=126). Our observations showed that for this suite, a multi-addition NAG pH test facilitated efficient sulphide oxidation for low sulphidesulphur samples (i.e., < 0.3 wt. %) with an initial reaction time of at least 480 minutes given per 15% H₂O₂ addition, heating to 80 to 90 °C for 2.5 hours, and then cooling of the reaction solution overnight with the NAG pH reading taken the following morning. This study demonstrated that using the recommended standard conditions given in Smart et al. (2002) is not appropriate for general use. Rather a suite of representative waste samples should initially be selected and the optimal conditions to facilitate sulphide oxidation determined on a site-per-site basis.