Humidity Cell Research Articles

A Comparison Between Predictive Tests Results and Natural Weathering in the Figueira Mine, in Southern Brazils

Many studies on acid mine drainage (AMD) use predictive methods, such as humidity cells (HC), to study this problem, as it is often not possible to predict acid generation from field tests. However, some comparisons of HC testing with reality have proven to be unrealistic. In this study, we compared the results of the HC test with historical data from the Figueira mine, in the State of Parana (southern Brazil). To carry out the work,we collected a fresh coal tailings sample from the Figueira Mine, and compared monthly measurements of pH and electrical conductivity (EC), and concentrations of Fe, Al, Mn, and Zn, over a 5 year period (from 2013 to 2017) with the predictive test results performed following protocol B of the ASTM D5744-13 (2018) standard. The elements were analyzed by ICP-OES. The pH and EC of the HC test were lower than the field data. The HC test underestimated the concentrations of Fe and Mn, while Al and Zn were overestimated. We believe that the overestimations were due to the aluminosilicate buffering capacity and cation exchange capacity of the soil.

Nitrate release from waste rock dumps in the Elk Valley, British Columbia, Canada

The origin, distribution and leaching of nitrate (NO3−) from coal waste rock dumps in the Elk Valley, British Columbia, Canada were defined using chemical and NO3− isotope analyses (δ15N- and δ18O-NO3−) of solids samples of pre- and post-blast waste rock and from thick (up to 180m) unsaturated waste rock dump profiles constructed between 1982 and 2012 as well as water samples collected from a rock drain located at the base of one dump and effluent from humidity cell (HC) and leach pad (LP) tests on waste rock. δ15N- and δ18O-NO3− values and NO3− concentrations of waste rock and rock drain waters confirmed the source of NO3− in the waste rock to be explosives and that limited to no denitrification occurs in the dump. The average mass of N released during blasting was estimated to be about 3–6% of the N in the explosives. NO3− concentrations in the fresh-blast waste rock and recently placed waste rock used for the HC and LP experiments were highly variable, ranging from below detection to 241mg/kg. The mean and median concentrations of these samples ranged from 10–30mg/kg. In this range of concentrations, the initial aqueous concentration of fresh-blasted waste rock could range from approximately 200–600mg NO3−-N/L. Flushing of NO3− from the HCs, LPs and a deep field profile was simulated using a scale dependent leaching efficiency (f) where f ranged from 5–15% for HCs, to 35–80% for the LPs, to 80–90% for the field profile. Our findings show aqueous phase NO3− from blasting residuals is present at highly variable initial concentrations in waste rock and the majority of this NO3− (>75%) should be flushed by recharging water during displacement of the first stored water volume.

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

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.