ASTM normalised humidity cell kinetic test: protocol improvements for optimal sulphide tailings reactivity.

The humidity cell (HC) kinetic method is widely used to assess acid-generating potential and is the only method approved by the American Society for Testing and Materials (ASTM). A protocol modification of the HC involves keeping the sample permanently at an optimal degree of saturation, between 40 and 60%, corresponding to maximal sulphide reactivity. Two mine tailings with different acid-generating potentials were submitted to both modified and standard ASTM HC protocols. The influence of the thickness and the porosity of the sample into the HC were also investigated. The results showed that, for a given sample, the oxidation rate under the modified ASTM protocol was higher than the standard ASTM protocol, which underestimated tailings reactivity due to excessive drying of the sample. When the porosity of the sample decreased, and its thickness increased, the difference between the standard and the modified HC protocol results decreased. These differences were more significant when the sample was more acid-generating. It is concluded that the best protocol for concentrator tailings to allow maximum reactivity and more conservative AMD prediction is to install a compacted 1 kg sample with 50% saturation (which should be maintained at a saturation of 40-60%) and a porosity representative of that expected in the field.
The humidity cell (HC) kinetic method is widely used to assess acid-generating potential and is the only method approved by the American Society for Testing and Materials (ASTM). A protocol modification of the HC involves keeping the sample permanently at an optimal degree of saturation, between 40 and 60%, corresponding to maximal sulphide reactivity. Two mine tailings with different acid-generating potentials were submitted to both modified and standard ASTM HC protocols. The influence of the thickness and the porosity of the sample into the HC were also investigated. The results showed that, for a given sample, the oxidation rate under the modified ASTM protocol was higher than the standard ASTM protocol, which underestimated tailings reactivity due to excessive drying of the sample. When the porosity of the sample decreased, and its thickness increased, the difference between the standard and the modified HC protocol results decreased. These differences were more significant when the sample was more acid-generating. It is concluded that the best protocol for concentrator tailings to allow maximum reactivity and more conservative AMD prediction is to install a compacted 1 kg sample with 50% saturation (which should be maintained at a saturation of 40-60%) and a porosity representative of that expected in the field.