Материал: Апсе ENVIRONMENTAL PROTECTION 2014

for 63 years, the expensive uranium resources can prolong this time interval on 28 years, i.e. total cheap and expensive uranium resources will be able to meet the demands of the world nuclear power for natural uranium during 91 years.

Nearly 85% of the reasonably assured and inferred uranium resources are placed in seven countries: in America (Canada, Brazil), Africa (SAR), Eurasia (Kazakhstan, China, Russia) and in Australia. The same situation takes place with uranium production. Annual rate of natural uranium production (40-44 thousand tons) does not meet the

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demands of the world nuclear power (59 thousand tons). The deficit of natural uranium is covered by the previously mined uranium ores.

Some data on total capacities of national nuclear energy systems in 2011 and on NPP shares in gross electricity generation are presented in Table 1.3.

As is seen, the countries possessing main deposits of uranium ores are the main producers of natural uranium too but they are not obligatory the possessors of the well-developed nuclear power system. For example, all African countries and Australia have no NPP in operation at all. Quite the contrary, Asian countries Japan and South Korea have no any available resources of natural uranium, both these countries are the main uranium importers in the world, but there are the welldeveloped nuclear power systems in these states.

Of 312 GWe produced by the countries with the well-developed nuclear power industry, about 113 GWe are generated by North American countries, 123 GWe – by European countries and 76 G We – by Asian countries. The remaining 61 GWe are generated by the countries, nonmembers of the top-ten list.

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The following four methods are mainly used for recovery of natural uranium:

1.Underground extraction from uranium mines.

2.Uranium extraction from open-cast mines.

3.Underground leaching, or in-situ leaching of uranium deposits.

4.Uranium extraction from seawater.

When uranium-containing minerals are already recovered from the

Earth’s crust with application of the first two methods, uranium ore undergoes the hydro-metallurgical (HM) treatment. The hydrometallurgical technologies are based on good solubility of the uraniumbearing minerals by acidic and alkaline solutions.

Natural uranium can be recovered from uranium ores by means of the following consecutive procedures:

1.Crashing and physical concentration of uranium ore by removal of the barren (dead) rocks.

2.Leaching (dissolution) of uranium ore in acidic or carbonate solutions.

3.Selective separation of uranium from the solutions or pulps by technologies of sorption, extraction and chemical precipitation.

4.Production of dry uranium concentrate ( 95% U3O8).

5.Production of pure (refined) uranium compounds with application of the affinage technologies.

The following methods can be used to concentrate uranium ore by separation of the uranium-bearing rocks from the barren rocks:

1.Radiometrical separation. The radiometrical method is based on the higher radioactivity of the uranium-bearing rocks. Uranium ore is milled into pieces with typical sizes about 20-30 cm. The ore pieces are examined by monitoring the natural gamma-radioactivity of each ore piece and removing the barren pieces. This method is able to remove up to 50% of the barren rocks.

2.Gravitational separation. The gravitational method is based on different densities of the uranium-bearing minerals (6, 5-10,5 g/cm3) and the barren rocks (2,5-2,7 g/cm3). Uranium ore is milled into pieces with typical sizes about 1 mm, and the ore pieces are put into a waterfilled vessel. The heavier pieces sink onto bottom and can be next collected. The gravitational method is often combined with a floatation separation.

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3. Floatation separation. The floatation method is based on different densities and different abilities to be moistened by water of the ura- nium-bearing minerals and the barren rocks. Uranium ore is milled into pieces with typical sizes about 0,1 mm, and the ore pieces are put into a water-filled vessel. Air flow is pumped from the bottom. The lighter pieces of the barren rocks are sticking to the air bubbles and going to the water surface while the heavier pieces of the uranium-bearing minerals gradually sink onto the vessel bottom where they can be then collected. The separation process can be quickened by introducing some floatation reagents into the water-filled vessel to change purposefully natural ability of the uranium-bearing minerals to be moistened by water.

The next step in the HM-treatment is a leaching (removal by dissolving) of uranium compounds from uranium ore. Depending on chemical composition of uranium ore, one of two leaching materials can be used, namely acidic or carbonate solutions.

The acid leaching is a more widely used technology. Sulphuric acid H2SO4, nitric acid HNO3 and hydrochloric acid HCl may be used as a leaching reagent. The carbonate leaching is applied at large content of impurities which can actively interact with acidic solutions. Soda NaHCO3, sodium bi-carbonate Na2CO3 and ammonium carbonate (NH4)2CO3 may be also used as a leaching reagent.

After uranium compounds were leached from uranium ore, these uranium compounds can be selectively derived from liquid acidic or carbonate solutions by using the following three methods:

1.Sorption on organic ion-exchange resins.

2.Extraction by organic liquid (extractant).

3.Chemical precipitation from solutions.

Uranium can be sorbed from pulps and clarified solutions. Extrac-

tion and chemical precipitation of uranium compounds can be performed from the well-clarified solutions only. Uranium-bearing solutions can be clarified by:

1.Settling of solid particles in large vessels.

2.Filtration of the solutions obtained after removal of solid particles by settling through thick layers of sand, silica gel and activated charcoal.

The sorption method is based on the selective ability of some organic ion-exchange resins to sorb primarily uranium compounds on

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their surface. Small spherical granules of an ion-exchange resin are mixed with uranium-bearing solution, and the granules sorb selectively uranium compounds. Since ion-exchange resins are lighter than liquid solutions, the granules can be easily collected and removed for further de-sorption of uranium compounds from their surface. The uranium washing off the granules is named as a de-sorption, or elution process with an eluate as a final product. Neutral or alkaline soda solutions are widely used as eluents.

Other method is also used to derive uranium compounds from acidic or carbonate aqueous solutions. This is a method of uranium extraction by organic substances. From viewpoint of a general chemistry, the extraction process is based on a solvation reaction that can unite molecules of quite different materials into a single stable compound (solvate). The simplest example of the solvation reaction is a hydration of salts that leads to formation of stable hydrates with the following chemical formula “salt·nH 2O”. The uranium extraction process is based on the property of some organic dissolvents (extractants), immiscible with water, to form complex chemical compounds with uranium salts. The extraction process must be followed by the re-extraction process, i.e. dissolution of uranium-bearing solvates by excess quantity of a neutral dissolvent and, thus, formation of a highly concentrated uranium solution.

When the clarified acidic or carbonate solution contacts with organic extractant, uranium is distributed between aqueous and organic phases. The most uranium quantity goes into organic phase. Then, these phases are separated, and uranium re-extracted from the organic phase. Light water or low-concentrated nitric acid HNO3 can be used as reextractants. Several consecutive applications of the extraction-re- extraction process can derive up to 99,7% U contained in the mined natural uranium ore.

One else method of uranium derivation from acidic or carbonate solutions is a chemical precipitation. The precipitation process can deal with the clarified acidic or carbonate solutions produced by leaching of uranium ore and with large volumes of low-concentrated uraniumbearing solutions produced by the de-sorption or re-extraction processes.

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