WASTES FROM MILLING OPERATIONS

The waste products from the conventional milling process are known as tailings (not to be confused with uranium tails, the depleted product from the uranium enrichment process). The tailing slurry is considered the most significant waste from the milling process. This waste stream is a mixture of leached solid ore and waste solutions from the grinding, leaching, uranium purification, precipitation and washing circuits of the mill. Because uranium makes up only a small part of the ore, the tailings are essentially of the same volume as ore fed to mill.

Tailings are pumped from the plant as slurry and returned underground, back to the mined-out pit or, in some cases, deposited in specially engineered tailings dams. The tailings also contain any heavy metals originally present in the ore. Therefore, if provision is not made to completely contain the material, the tailings may be a long term source of these substances which may enter the groundwater below the impoundment [17, 18].

Liquid waste from milling operations and runoff from the mine stockpiles are collected in secure retention ponds. These can be lined with clay as uranium in solution can pass through sand but adheres to negatively charged clay particles. Heavy metals and other contaminants may be isolated and recovered and the liquid portion is either recirculated back to the mill or naturally evaporated away. For example, the McLean Lake operation uses barium chloride, lime and ferric sulphate to precipitate arsenic and radium, neutralize acidic waste and prepare the tailings for disposal in the tailings management facility [19]. For more details on tailings management, the reader is referred to the case studies highlighted in the IAEA [20].

The impacts on the environment from uranium mining and milling residues are not all related to the radionuclide content alone. The nature of the processes used in the milling may results in increased availability of a wide range of heavy metals in the tailings, or the residues of process reagents such as sulphate, ammonia, chloride, pyrite, kerosene and sulphuric acid may have the potential to cause adverse environmental impact [21].

The presence of non-radiological contaminants can exacerbate the availability of the radionuclides to the environment. In this regard, the potential for uranium mining and milling residues to cause environmental harm is little different from that of other forms of mining, and the resultant impacts may be quite similar. Indeed, it is not adequate to consider the radiological risk only. The other effects may include [22]:

a) The chemical toxicity of the radionuclides, including uranium;

b) The chemical toxicity of heavy metals and metallic compounds;

c) The chemical toxicity of non-metallic minerals and compounds in the ore or introduced during processing (e.g. sulphuric acid, kerosene);

d) Acidity, resulting from sulphidic (ore) minerals or acid introduced during milling;

e) Increased turbidity in surface waters;

f) Increased salinity.

The types of non-radiological contaminants that may cause harm are dependent on the mineralization in the ore body, the gangue mineralogy, the overburden mineralogy and the processing technique used in the mill. Elevated acidity plays a major role in increasing the mobility of heavy metals in aqueous solution, including uranium, as well as copper, arsenic, cadmium and other metals. The transport of chemical residues from mill tailings into the environment through aquatic or atmospheric dispersion needs to be controlled and kept to the absolute minimum achievable using best practicable technology and ‘as low as reasonably achievable’ principle (ALARA). Other processes that might act on uranium mill tailings to produce hazardous conditions are climate, biological processes and chemico-mineralogical changes. Furthermore, many of the contaminants, especially the heavy metals, retain the same levels of toxicity permanently, unlike the radionuclides which gradually decay and become less dangerous with time [21].

8.1. Atmospheric releases

The processing of uranium ore in concentrating mills generates wastes and effluents that are both radioactive and nonradioactive. Solid, liquid and gaseous effluents are released to the environment. The atmospheric releases from uranium mining are, for the most part, similar to releases from conventional mines. They are, in addition to typical releases, radon and radon progeny, and radioactively contaminated dusts [17].

8.2. Airborne chemical contaminants

Airborne chemical (nonradioactive) contaminants released to the environment during uranium milling operations include fuel combustion products (oxides of carbon, nitrogen and sulphur) from process steam boilers, and power generation, sulphuric acid fumes in small concentrations from the leach tanks vent systems, and vaporised organic reagents, mostly kerosene, from the solvent extraction ventilation system. In addition, where sulphuric acid is produced on site, sulphur dioxide is exhausted to the atmosphere if no desulphurisation equipment is installed [17].

8.3. Radioactive airborne effluents

Radioactive airborne effluents from milling include dust and radon gas released into the air from ore stockpiles, crushing and grinding of ore, drying and packing of yellowcake, and from the tailings retention system. The amount of dust produced in the processing operations is reduced by ventilation extract scrubbers and/or filters. Short lived radon progeny, resulting from the decay of radon, are a major source of radiation exposure for uranium mine workers, particularly in underground mines. Ventilation is used in underground mines to remove radon and thereby limit the exposure to its progeny. However, the expelling of the radon and its progeny from underground mines results in dispersal of these radionuclides into the environment. At in situ leach (ISL) mines radon gas is dissolved in the uranium bearing solution

that is pumped from the ore body. This radon may be released if the solution is exposed to the environment in tanks or ponds [17].

The tailings contain nearly all of the naturally occurring radioactive progeny from the decay of uranium, notably thorium–230 and radium–226. The presence of radium–226 provides a long term source of radon. As radium decays, radon gas is formed. During the life of the mine, the tailings are generally covered by water to reduce surface radioactivity and radon gas emission.

On completion of the mining project, it is normal for the tailings to be covered with at about two metres of clay and topsoil. This reduces the surface radiation to levels normal for the region and allows vegetation to cover the area.

REFERENCES

[1] INTERNATIONAL ATOMIC ENERGY AGENCY, Uranium Extraction Technology. Tech. Rep. Series No. 359, IAEA, Vienna (1993).

[2] SCHNEIDER E., CARLSEN, B., TAVRIDES, E., VAN DER HOEVEN, C., PHATHANAPIROM, U., A top–down assessment of energy, water and land use in uranium mining, milling, and refining, Energy Economics 40 (2013) 911–926.

[3] TAYLOR G., FARRINGTON, V., WOODS, P., RING, R., MOLLOY, R., Review of Environmental Impacts of the Acid In Situ Leach Uranium Mining Process. CSIRO Land and Water Client Report (2004).

[4] KHAN Y., SHAH, S. S., SIDDIQ, M., Selection of Lixiviant System for the Alkaline in Situ Leaching of Uranium from an Arkosic Type of Sandstone and Measuring the Dissolution Behaviour of some Metals and Non–Metals, J. Chem. Soc. Pak. 34 4 uranium mining technology, IAEA–TECDOC–1239, IAEA, Vienna (2001).

[7] TAYLOR, A., “Innovations and Trends in Uranium Ore Treatment”, Uranium Sessions at ALTA 2012 (Proc. Conf., Perth, Australia) ALTA Metallurgical Services, Melbourne (2012).

[8] POOL, T.C., “Technology and the uranium industry”, The Uranium Production Cycle and the Environment (Proc. Symp. Vienna, 2000), IAEA–CSP–10/P, IAEA, Vienna (2002) 261–270.

[9] INTERNATIONAL ATOMIC ENERGY AGENCY, Establishment of Uranium Mining and Processing Operations in the Context of Sustainable Development, Nuclear Energy Series No. NF–T1.1, IAEA, Vienna (2009).

[10] DUNN, G., TEO, Y. Y., “The Critical Role of Gangue Element Chemistry in Heap and Agitated Leaching of Uranium Ores”, (Proc. of Uranium Sessions at ALTA 2012, Perth, Australia) ALTA Metallurgical Servies, Melbourne (2012).

[11] VENTER, R., BOYLETT, M, “The evaluation of various oxidants used in acid leaching of uranium”, Hydrometallurgy Conference 2009, The Southern African Institute of Mining and Metallurgy (2009).

[12] EDWARDS C.R., OLIVER, A.J., Uranium processing: A review of current methods and technology, J. Minerals, Metals & Materials Society (TMS) 52 9 (2000) 12–20.

[13] LÖFSTRÖM–ENGDAHL, E., ANEHEIM, E., EKBERG, C., FOREMAN, M., SKARNEMARK, G., “Diluent effects in solvent extraction”, Actinide Recycling by Separation and Transmutation (ACSEPT) (Proc. First Int. Workshop, Lisbon, Portugal, 2010) (2010).

[14] INTERNATIONAL ATOMIC ENERGY AGENCY, Nuclear Technology Review 2012, IAEA, Vienna (2012).

[15] DOUGLAS BELLE, W., “The History of Uranium recovery from Phosphoric acid”, Fertilizer Industry: Fertilizer Development and Environment Protection (Proc. Conf., Jeddah, Saudi Arabia) (2008).

[16] SCHNELL, H. An overview of uranium production from unconventional resources, AREVA NC/BU Mines (2009).

[17] ORGANISATION FOR ECONOMIC CO–OPERATION AND DEVELOPMENT – NUCLEAR ENERGY AGENCY, INTERNATIONAL ATOMIC ENERGY AGENCY, Environmental Activities in Uranium Mining and Milling, OECD, Paris (1999).

[18] INTERNATIONAL ATOMIC ENERGY AGENCY, Management of Radioactive Waste from the Mining and milling of Ores. Safety Standards Series No. WS–G–1.2, IAEA, Vienna (2002).

[19] CLIFTON, A.W., BARSI, R.G., MISFELDT, G.A., “Decommissioning: a critical component of the design for uranium tailings management facilities”, The Uranium Production Cycle and the Environment (Proc. Symp. Vienna, 2000), IAEA–CSP–10/P, IAEA, Vienna (2002) 313–324.

[20] INTERNATIONAL ATOMIC ENERGY AGENCY, Classification of Radioactive Waste. General Safety Guide No. GSG–1, IAEA, Vienna (2009).

[21] WAGGITT P. A review of worldwide practices for disposal of uranium mill tailings, Supervising Scientist for the Alligator Rivers Region Tech. Mem. 48, Australian Publishing Service, Canberra (1994).

[22] INTERNATIONAL ATOMIC ENERGY AGENCY, Environmental Contamination from Uranium Production Facilities and their Remediation, (Proc. International Workshop Lisbon, 2004), STI/PUB/1829, IAEA, Vienna (2005).

EXTRACTION OF HAZARDOUS CONSTITUENTS FROM TAILINGS RESULTING FROM PROCESSING OF HIGH GRADE URANIUM ORE³¹

H.G. JUNG³², J. HEIDUK, F. SCHEUERMANN NUKEM Technologies Engineering Services GmbH Alzenau, Germany

Abstract

Conventional production of uranium across the world creates millions of tonnes of tailings annually, which are typically placed in above ground tailings impoundments. However, particularly tailings resulting from leaching of high grade uranium ores have the potential to cause serious environmental impacts. In spite of significant improvements which have been made in recent years to operational and short term safety of those tailings impoundments, their provision of reliable long term safety is still not satisfying. By advancement of the processing technology for uranium ore a solution is achieved to comprehensively eliminate risks which can derive from those tailings. Removal of radionuclides other than uranium as well as toxic metals can be achieved, if also these are intentionally extracted. The overall objective is to prevent the generation of future uranium production legacies with associated high follow–up costs.

1. BACKGROUND

The surficial disposal of tailings in engineered impoundments has been considered to be generally a reasonable solution. By relatively moderate effort a disposal opportunity has been provided, which apparently has been meeting operational requirements.

Conventional above ground tailings impoundments, however, can also comprise disadvantages.

If properly engineered, such facilities are able to cope with challenges like dam failure or seepage generation during operation and in the short term (e.g. up to some centuries). However, it is potentially a fundamental disadvantage of conventional tailings impoundments that long term safety and integrity, which from a radiological point of view can be necessary up to several tens of thousands of years, may be provided insufficiently.

Provided that maintenance and monitoring measures are ensured, conventional tailings impoundments can provide integrity maximum for a timespan of up to 1000 years [1]. Their integrity duration can be even much shorter in case of high georisks (like earthquakes, floods etc.). Furthermore, in many countries institutional control is assumed to be reliable not longer than 300 years [2].

Consequently, the disposal of tailings resulting from conventional processing (e.g. of high grade uranium ore in above ground tailings impoundments) appears to be a temporary rather than a true long term solution to their containment.

2. JUSTIFICATION

Of particular concern for the long term safety of applicable tailings impoundments is that long-lived daughter nuclides of uranium are conventionally disposed of with tailings. The contained radioactivity is predominantly caused by radium (mainly Ra–226) and its daughter nuclides like

31 Paper first presented at the Technical Meeting of the UMREG in Bad Schlema, Germany, AugustSeptember 2015.

32 Corresponding author, hagen.jung@ymail.com