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
Temporal changes of pyrite oxidation in sulphidic mine wastes 181 FeS2 + H,0 + (7/2)02 -► 2(S04)2- + Fe2+ + 2H+ (1) For investigating the oxidation rate of the samples, a special combination of humidity cell and column test methods was used. In the column test, experiments can be used to determine the kinetic behaviour of wastes when exposed to atmospheric weathering (subaerial storage), or stored under water cover (sub-aqueous storage). The advantage of the humidity cell test is that, primary reaction products are flushed out by rinsing water (Lapakko, 2003). The main concept for the experiments was that using the column test, continuous airflow was pumped through the samples to maintain the maximal rate of pyrite oxidation. The through flowing air is more than the pyrite could use for its oxidization. In this way the oxidation rate is controllable and maintained as maximal during the column test, so the process works like a “worst case scenario”. In cyclic time periods the samples were washed through and the seepage water was analysed for basic chemical parameters, anion and cation content. Using equation (1), the pyrite oxidation rate could be determined in four different ways, such as 1) from the oxygen consumption, 2) the sulphate or 3) iron content and 4) from the H+, which defines the pH. In this paper, pyrite oxidation rate was determined by inverse calculation from pH, which could be easily and accurately measured, thus the H+ content in the seepage could be determined correctly. Based on equation (1), it was shown that the correlation ratio between the pyrite and H+ is 1:2, because each mole of pyrite results in 2 moles of acid. In the column test 1000 cm3 distilled water was used for flushing, thus theoretically total oxidation of 59.99 mg pyrite is needed for 1 dm3 of 1 mmol/1 H+ content seepage. The masses of the samples were premeasured, so for better comparison the results were calculated back to mass unit of oxidized pyrite for each kilogram of samples. For pyrite oxidation rate we use equation (2), to be determined from the measured pH of the rinse. where Rpyox is the pyrite oxidation rate (in unit of g pyrite / week / kg of sample), pymw is the molecular weight of pyrite (119.98 g/mol), Vt is the volume (in dm3) of the rinse, Nd is time (in days) between rinsing the samples and Ms is the mass (in kg) of the sample. The aim in the column test is to reach a steady state in the oxidizing system, which could mean stability in sulphate release rate, pH or other seepage water parameters. Previous studies and practice showed that in the case of fresh pyritic samples a year (easily even more time) is needed in the column to reach a steady state, while in the case of already oxidized samples this time is shorter, covering only some months (approximately 2-4 months). Seven samples were collected by Ingar Wälder in 2007 and 2008 in Itos mining district, Bolivia from the surface vadose-zone from approximately 10-30 cm depth of the waste heaps side walls, on which the column tests were completed. The samples represent different geotechnical and mineralogical scenarios, which are shown in Table I (Móricz et al., 2010). From the point of geotechnical parameters and pyrite content, the sample Bol 1 R, \pyox (2)
