Prevention of the generation of acid drainage waters in a heap consisting of uranium-bearing mining wastes

Different methods to prevent and/or inhibit this generation were tested and one of the most efficient of them is present in this paper.

Data about the chemical composition of the mining wastes and their essential geotechnical parameters are shown in Tables 1 and 2, respectively. Pyrite was the main ore mineral. Uranium was present mainly as uraninite, nasturan, torbernite. Chalcopyrite was the main copper-bearing mineral but some secondary copper sulphides such as covellite and chalcocite were also present, together with some copper oxide minerals. Zinc was present mainly as sphalerite, and lead mainly as galena. Arsenopyrite was the main arsenic-bearing mineral. Quartz was the main mineral of the host rock.

Table 2.

The elemental analysis of the heap effluents was done by atomic adsorption spectrometry and induced coupled plasma spectrometry. Sulphate was determined photometrically. The radioactivity of the samples was measured, using the solid residues remaining after their evaporation, by means of a low background gamma-spectrometer ORTEC (HpGe-detector with a high distinguishing ability).

The main geotechnical characteristics of the heap, such as permeability and wet bulk density, were measured in situ using sand-core method (U.S. Environmental Protection Agency, 1991). True density measurements were carried out in the laboratory using undisturbed core samples. Such samples were also used for determination of their acid generation and net neutralization potentials using static acid-base accounting tests.

The isolation, identification and enumeration of the microorganisms were carried out by methods described elsewhere (Karavaiko et al., 1988; Johnson and Hallberg, 2003; Escobar et al., 2008).

The inhibition of the generation of acid drainage waters was achieved by means of construction of an about 40 – 45 cm cover on the mining wastes. A mixture of a slightly leached cinnamonic forest soil (which is typical for this region), with some solid biodegradable organic substrates (plant compost, animal manure, straw) and crushed limestone, was used to form this heap cover with a slightly alkaline pH (about 7.5 – 8).
The main component of the heap cover, i.e. the slightly leached cinnamonic forest soil, had pH of 6.8 (in H₂O) and contained 1.90 % humus, 78.1 % SiO₂, 2.35 % Fe₂O₃, 0.10 % total nitrogen, 2.21 % K₂O, 0.15 % P₂O₅ as the essential components. The soil contained viable microflora which practically was the main source of microorganisms in the constructed cover on the mining wastes. The fertility of the soil cover was enhanced by adding compost to the soil at amounts of 10 – 20 % on dry weight.

The preparation of the compost, i.e. the procedure of composting, was carried out with a mixture consisting of plant (leaf) biomass, animal manure, cinnamonic forest soil and a biofertilzier obtained by processing of activated sludge from the system for municipal wastewater treatment (Nicolova et al., 2012). The components were mixed in a ratio of 7:1:1:1 on dry weight.

The batches of compost produced in this way had pH of about 6.8 – 7.1 and contained 50 – 55 % dry substances, with 40 – 45 % ash content and 55 – 60 % organic content ( with 40 – 45 % organic carbon and nitrogen content of 2.0 – 2.5 % at a ratio of C:N of about 20 (all these contents are on dry weight). The moisture of the different batches of compost was about 45 – 50 %.

The heap cover contained some non-ferrous metals (Cu, Zn, Pb, Ni, Co) but each of them in concentrations lower than 50 mg/kg on dry weight)

The mesophilic chemolithotrophic bacteria At. ferrooxidans and L. ferrooxidans were the main species participating in these oxidation processes but some moderate thermophiles (mainly Sulfobacillus thermosufidooxidans) were also involved, although in lower extent (Table 3).

Table 5.

The construction of the soil cover deeply changed the environmental conditions in the mining wastes located below this cover. The former heap of mining wastes changed into a mainly anaerobic ecosystem, with a neutral pH, i.e. a system unfavourable for the growth and activity of the acidophilic, mainly aerobic, chemolithotrophic bacteria. The former typical mineral ecosystem gradually was enriched in organic compounds transported from the soil cover by the drainage waters. This new anaerobic system containing biodegradable organic substrates at almost neutral pH was inhabited mainly by different sulphate-reducing bacteria. The metabolism of these bacteria was connected with the process of the microbial dissimilatory sulphate reduction, which generated hydrogen sulphide (H₂S) as an essential product. The hydrogen sulphide precipitated efficiently as the relevant sulphide each heavy metal penetrated into this anaerobic zone by the drainage waters from the top soil cover. The soluble hexavalent uranium was reduced and precipitated as the insoluble tetravalent form.

Escobar, B., Bustos, K., Morales, G. and Salazar, O., 2008. Rapid and specific detection of Acidithiobacillus ferrooxidans and Leptospirillum ferrooxidans by PCR, Hydrometallurgy, 92, 102 – 106.

Johnson, D.B. and Hallberg, K.B., 2003. The microbiology of acidic mine waters, Res. Microbiol., 154, 446 - 473.

Karavaiko, G.I., Rossi, G., Agate, A.D., Groudev, S.N. and Avakyan, Z.A. (Eds.), 1988. Biogeotechnology of Metals, Mannual, Center for International Projects GKNT, Moscow.

Nicolova, M.N., Spasova, I.I., Georgiev, P.S. and Groudev, S.N., 2012. Bacterial leaching of activated sludge for recovery of non-ferrous metals and fertilizer for agriculture, Annual of the University of Mining and Geology, Sofia, part II, 162 – 165.

U.S. Environmental Protection Agency, 1991. Description and Sampling of Contaminated Soils – A Field Guide (EPA/625/12 – 91/002 Technology Transfer), Centre for Environmental Research Information, U,S. EPA, Cincinati, Ohio.