Mining and Milling

After mining, the uranium must be concentrated before further operations are carried out. This is done in the mills, which are located near the mines. Here the uranium ore content is increased from a few tenths of a percent (in the ore) to 85-95% (in a semirefined concentrate known as yellowcake ), while eliminating other elements that are present (the tailings ). 

The normal choice ( 80% of all U.S. ores) for chemical leaching is acid leaching with sulfuric acid (due to its low cost and great availability). For the ores that have high carbonate content, an alkaline leach with Na2CC 3 is used to prevent high acid concentration. 

The problem in acid leaching is that only U(VI) dissolves in H2S04. Any U(IV) present must be oxidized to U(VI) prior to leaching. The chemical steps can be summarized as 

The final product thus appears in three chemical forms, U02+, [U02(S04)2]2-, and [U02(S04)3]4 . 

In alkaline leaching of high carbonate materials, one takes advantage of the unique solubility of [U02(C03)3]4-. One begins with finely divided material and must also deal with the oxidation of any U(IV) that is present. The basic reaction (pun intended) is 

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The exploration, evaluation, and development of zinc and lead ore bodies in North and Central America are discussed in Ref. 12. A survey of world zinc production in Ref. 13 gives all operating mines and mills, and their methods, production, and chemical analysis of the products zinc smelters are included. 

Hazards of Production. In most zinc mines, zinc is present as the sulfide and coexists with other minerals, especiaHy lead, copper, and cadmium. Therefore, the escape of zinc from mines and mills is accompanied by these other often more toxic materials. Mining and concentrating, usuaHy by flotations, does not present any unusual hazards to personnel. Atmospheric poHution is of Httle consequence at mine sites, but considerable effort is required to flocculate and settle fine ore particles, which would find their way into receiving waters. 

Canada. Canada has one sizeable mine and at least five more possibly commercial deposits, one or two of which might be brought onstream in the next few years. The new Lac-des-Iles mine and mill of Stratmin Inc. has, for practical purposes, displaced Asbury Graphite s older and smaller mine and mill at Notre Dame du Laus. Stratmin states that the capacity of the operation is 25,000 tons per year as concentrate and this might be substantially increased later. 

The presence of radiation in the workplace - which is an inevitable consequence of the radioactivity of uranium - requires that additional safety precautions be taken over and above those observed in other similar workplaces. There are generally three sources from which radiation exposure may occur (i) radiation emitted from uranium ore in-situ and/or during handling (ii) airborne radiation resulting from the decay of radon gas released from the ore and uranium dust and (iii) contamination by ore dust or concentrate. Radiation levels around uranium mining and milling facilities are quite low - for the most part only a few times the natural background levels - and they decrease rapidly as the distance from... 

Dust created by mining and milling activities blown by the wind from ore stockpiles is a potential source of environmental contamination. The watering of stockpiles or erection of enclosures is resorted to when necessary in order to prevent the spread of dust. Where ventilation systems collect significant amounts of uranium dust in mills, scrubbers are used to remove the dust from the air before it is discharged to the environment. 

There are three principal types of wastes that arise in mining and milling operations (i) mine wastes (ii) milling wastes and (iii) waste water. Each type of waste is subjected to an appropriate management strategy. 

Waste water from a number of sources in mining and milling operations, which cannot be re-used, must be treated before discharge to the environment. Standards have been established for the maximum permissible concentrations of specific contaminants in the discharged water. 

The zinc industry employs 2200 workers at mines and mills and 1400 at primary smelters.14 The four primary zinc smelters in the U.S. are located in Illinois, Oklahoma, Tennessee, and Pennsylvania. There are ten secondary zinc recovery plants in the U.S.17... 

Near Mo mine and mill, Colorado, irrigated with Mo-contaminated effluent from uranium mill 61 (49-72) 4... 

McConnell, M.A., V.M.S. Ramanujam, N.W. Alcock, G.J. Gabehart, and W.W. Au. 1998. Distribution of uranium-238 in environmental samples from a residential area impacted by mining and milling activities. Environ. Toxicol. Client. 17 841-850. 

The U.S. Bureau of Mines-Salt Lake Research Center is conducting research to develop new biohydrometallurgical techniques to decontaminate mining and milling wastes containing heavy metals and toxic chemicals. Bacterial techniques are being developed to remove cyanide and selenium from waste water. 

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