Testing a Standard Test Method for Laboratory Weathering on Sulphide Containing Mine Tailings

A Standard Test Method (ASTM D 5744), for laboratory weathering using a humidity cell procedure, was conducted on two sulfide containing tailings samples from a metallurgical site in Ontario (Canada). To test the reliability of this method, results were compared with those of previous studies on the same samples. The mineralogy of the samples differs only by the addition of a small amount of natrojarosite in one of the samples. The sulfide oxidation rate and release rates of elements during weathering were monitored. Both processes are influenced strongly by the capacity of the material to generate acidity, which was enhanced by the presence of natrojarosite. Acid-neutralization processes occurring during the laboratory test were affected by reaction kinetics. In the laboratory tests, the carbonate minerals more readily neutralized acid compared to field observations, where a more prolonged solid-solution interaction allowed chemical equilibration to be reached. As a consequence, the capacity of this standard test to predict weathering reactions is limited by its inability to reproduce the chronology of full scale reactions. Introduction Sulfidic wastes from ore processing, including waste rock or mill tailings, are typically deposited in piles or impoundments nearby the mine workings. In these piles sulfide minerals are exposed to air and undergo oxidation reactions. This oxidation may lead to generation of acid effluents and the release of Fe, SO4 and potentially toxic elements into surface and ground waters, causing environmental damage. The goal of the scientific community is to determine with sufficient certainty how sulfidic wastes from a particular site will behave, in particular the rate at which the material oxidizes under a range of climatic conditions and the ability of site materials to neutralize acid. Different approaches to this topic include detailed field investigations and/or laboratory studies. The advantage in conducting laboratory experiments is the lower cost and the more timely acquisition of data, but also the possibility to conduct experiments prior to disposal of mine wastes. The presupposition is that laboratory-scale measurements of oxidation rates of sulfidic mine wastes are sufficiently similar to field measurements so that the laboratory rates can be used with confidence in predictive models of full scale waste piles (Bennett et al., 2000). In this study, a series of humidity test cell experiments was conducted under controlled laboratory conditions using fresh unoxidized tailings from a mine-waste disposal facility at the Kidd Creek metallurgical site near Timmins, Ontario. Results were correlated with those obtained from previous experiments conducted on the same material, which examined acid neutralization relations in isolation from acid-producing reactions (Jurjovec et al., 2002, 2003). In contrast, the humidity test cell experiments evaluate weathering when both acid generation and acid neutralization reactions are allowed to occur simultaneously. The experimental results were used to estimate parameters controlling the oxidation rates of sulfide materials, and to predict mechanisms controlling the mobility of metals in the pore water. Material Solid materials used in this study are: a) one sample (labeled KC) of fresh unoxidized tailings collected at the concentrator at the Kidd Creek metallurgical site in Ontario, Canada; b) one sample (labeled KCJ) of the same tailings mixed with 3% wt. of natrojarosite, a waste derived from the adjacent Zn refinery. Since 1985 the natrojarosite has been co-disposed with mine tailings and deposited in a single impoundment. The 3% of natrojarosite added to the KCJ sample reproduces the proportion co-disposed in the field. Jurjovec et al. (2003), who previously conducted experiments on these samples, verified that the presence of natrojarosite diminishes the ability of tailings to buffer acidic solutions. The mineralogy of the tailings was determined by Jambor et al. (1993) to be as follows: 15 wt. % sulfides (mainly pyrite [FeS2], secondarily pyrrhotite [Fe1-xS], chalcopyrite [CuFeS2], and sphalerite [(Zn,Fe)S]), 8 wt. % carbonates (siderite [FeCO3], ankerite-dolomite [Ca(Fe,Mg)(CO3)2], calcite [CaCO3]) and Fe-oxides [Fe2O3], 49 wt.% quartz [SiO2], 24 wt.% chlorite [(Mg,Fe)5Al(Si3Al)O10(OH)8], and the balance of other silicates and aluminum-silicates. IMWA Symposium 2007: Water in Mining Environments, R. Cidu & F. Frau (Eds), 27th 31st May 2007, Cagliari, Italy