Abstract
Using a modified Sobek method to determine the neutralizing potential (NP) of waste rock, we have come across several instances where the NP greatly exceeds the neutralising capacity determined using carbonate content. The common factor in these cases is the presence of ultramafic intrusions, both with and without serpentine alteration.
We examined the measurement of NP for a number of ultramafic minerals including olivine, forsterite and lizardite using a modified Sobek method and found the measured NP to be very sensitive to both the amount of acid added and temperature. While the determination of NP for many silicate minerals is relatively insensitive to the test conditions, generally providing limited NP, and most carbonate minerals are quantitatively dissolved, the amounts of these ultramafic minerals which dissolved during NP testing proved to be very dependent on the test conditions.
In contrast to the modified Sobek method, the acid buffering characteristic curve (ABCC) method provided much reduced NP values, demonstrating limited NP for the ultramafic minerals measured under the ABCC test conditions of significantly lower acid addition and ambient temperature. When the ABCC method was applied to mine wastes containing ultramafic mineralisation, the measured NP gave much closer agreement with carbonate NP, demonstrating that ultramafics are likely to provide only limited NP, which may be greatly over-estimated if a modified Sobek method is employed to determine NP.