ASTM Normalized Humidity Cell Kinetic Test: Protocol Improvements for Optimal Sulfide Tailings Reactivity

Abstract

The humidity cell (HC) kinetic method is widely used to assess acid-generating potential and is the only method normalized by the American Society for Testing and Materials (ASTM). HC test results can be influenced by complete drying of the sample or by a long water saturation step during weekly cycles, which significantly reduces sulfide oxidation rates, leading to erroneous interpretations. This paper investigates a protocol modification of the HC, which involves keeping the sample permanently at an optimal degree of saturation, between 40 and 60 %, corresponding to maximal sulfide reactivity, as demonstrated in the literature. For this purpose, 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 obtained clearly show 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. On the other hand, 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.