Experimental study of acidity-consuming processes in mining waste rock: some influences of mineralogy and particle size

Weathering reactions producing and consuming acid in fresh waste rack sampl es from the Aitik Cu mine in northern Sweden have been investigated. Batch- scale (0.15 kg) acid titrations with waste rock of different particle sizes were operated for 5 months. The pH was adjusted to a nearly constant level , similar to that in effluents from waste rock dumps at the site (pH near 3 .5). The reactions were followed by analysing for all major dissolved eleme nts (K, Na, Mg, Ca, Si, Al, SO4, Cu, Zn, Fe) in aliquots of solution from t he reaction vessels. In addition, the solids were physically and chemically characterised in terms of mineralogy, chemical composition, particle size distribution, surface area and porosity. The results show that the alkalini ty production is initially dominated by a rapid dissolution of small amount s of calcite and rapidly exchangeable base cations on silicate surfaces. St eady-state dissolution of primary silicate minerals also generates alkalini ty. The total alkalinity is nearly balanced by input of acid from the stead y-state oxidation of sulphides, such that the pH 3.1-3.4 can be maintained without external input of acid or base. There is a large difference in weat hering rates between fine materials and larger waste rock particles (diamet ers (d) > 0.25 mm) for both sulphides and silicates. As a result particles with d smaller than 0.25 mm contribute to approximately 80% of the sulphide and silicate dissolution. Calcite dissolution can initially maintain a neu tral pH but with time becomes limited by intra-particle diffusion. Calcite within particles larger than 5-10 mm reacts too slowly to neutralise the ac id produced from sulphides. (C) 1998 Elsevier Science Ltd. All rights reser ved.