Modified Abrasion pH and NAGpH Testing of Minerals

The original abrasion pH test of Stevens and Carron (1948) is based on grinding and wetting of minerals and subsequent pH measurements of the minerals’ paste and suspensions. Such pH values allow insights into the behaviour of individual minerals upon fluid:mineral reactions in surface environments including mine sites and waste repositories. This study proposes a modified abrasion pH method which includes the use of 0.01 M CaCl₂, an operationally defined grain size (<0.075 mm), and high precision pH measurements. Several mineral specimens (n = 20) were obtained from commercial suppliers for the modified abrasion pH testwork, with the acquired specimens having a range of purities and resultant pH values. The modified abrasion pH testwork demonstrates that small admixtures to monomineralic samples caused significant pH changes. Only pure monomineralic samples provide a true indication of the modified abrasion pH of that mineral type. The NAGpH method aims to oxidize sulfide minerals but the reliability of this method in waste classification can be compromised depending on the sample mineralogy. This study demonstrates that NAGpH measurements of some carbonate minerals led to very alkaline pH values that are possibly due to the formation of Ca and Mg hydroxides. Such reactions do not occur in waste environments and therefore, NAGpH measurements of carbonate-rich wastes may overestimate their acid buffering capacity.