Fehér Béla (szerk.): Az ásványok vonzásában, Tanulmányok a 60 éves Szakáll Sándor tiszteletére (Miskolc, 2014)
Móricz Ferenc - Mádai Ferenc - Walder Ingar F.: Szulfidos bányászati meddőkben lezajló piritoxidáció időbeni változása
180 F. Móricz, F. Mádai & I. F. Wälder The Itos mine is a polymetallic vein deposit in Bolivia, had been mined for silver and tin until 1990, leaving behind much and huge tailings and mine waste heaps. Quite often the pyrite content of the waste reaches or exceeds 10%. Serious ARD effects take place in the mine waste heaps. These processes can be well characterized with the pH 1 and 2 of the seepage water, which forms serious alteration in the waste itself and the neighbouring rocks. In three consecutive years the pyrite oxidation rate was investigated on the same 7 samples by column test. 56 months pauses were left between the column test periods, which allow' mimicking the alternation of wet and dry periods, typical for the place. Thus the results give much more information, than the oxidation rate in the individual column test periods, showing the changes by time. This applied method gave good result to characterize the behaviour of the waste in long-term. The column test was complemented with mineralogical analyses, such as electron probe microanalysis. The mineralogical and column test analyses show, that the changes of the pyrite oxidation rate spilt the samples into three different groups, one where the oxidation rate decreases, second where it increases with time and the third where oxidation rate is maximal and stays stable for several years. 1. Introduction In the past two decades, the effect of Acid Rock Drainage (ARD) has become the leading environmental problem in metal mining. Weathering starts to degrade sulphidic type ore, which is unstable on the surface environment, resulting low pH and mobilizes heavy metal contamination. The release of ARD to surface- and groundwater deteriorates the water quality and may cause depletion of alkalinity, acidification, bioaccumulation of metals, accumulation of metal in sediments, effects on habitats, elimination of sensitive species and unstable ecosystems. Following the “nature pays principle”, in the Itos mine, a polymetallic vein deposit was mined for silver and tin until 1990, giving one third of the world’s silver production in the 19th century (Chance, 1948a, b), leaving behind a great number of huge tailings and mine waste heaps. Quite often the pyrite content of the waste reaches or exceeds 10%. Oxidizing sulphides produce serious acid rock drainage affects in Oruro, the mining town next to the mine waste heaps. These processes can be characterized by the pH 1 of the seepage water, which mobilizes heavy metals from the waste itself or the neighbouring rocks (Wälder etal., 2010). In three consecutive years the pyrite oxidation rate was investigated on the same seven samples by column test, which was complemented with mineralogical analyses (EPMA, XRPD). Five-to-seven-month pauses were left between the column test periods, which allow mimicking the alternation of wet and dry periods, typical of the climate of Oruro. Thus the results give much more information than the oxidation rate for an individual column test period, showing the changes by time. The column test shows that samples can be split by the changes of the pyrite oxidation rate into three different stages: where the oxidation rate shows increasing (marked type “A”) or decreasing tendencies (marked type “C”) or where the oxidation is maximal, so stable in the different years (marked type “B”). 2. Theoretical part and the samples As it was mentioned in the previous part, the problem is caused by the oxidizing iron sulphides - mainly pyrite - when it reacts with water and oxygen at surface conditions as shown in equation (1) (Seal & Hammarstrom, 2003):
