Uranium (atomic number 92) takes up about 4 ppm (parts per million) of the earth crust. Uranium is more abundant than the common elements such as silver, mercury, cadmium, and so on. The amount of economically recoverable uranium in the world has been estimated to be about 5.5 million tons (2007 estimate). Uranium mined from the earth crust is known as “natural ura­nium” that contains two main isotopes U235 (0.71%) and U238 (99.28%) along with a very minor presence (0.006%) of U234. Uranium in general is found in

a variety of minerals, such as pitchblende (U3O8), Uraninite (UO2) followed by carnotite (K2O ■ 2UO3 ■ V2O5 ■ nH2O). High-grade uranium ores are mined in Kazakhstan, Canada, Australia, Namibia, South Africa, Niger, the Rocky Moun­tain region of the United States, and many other nations. Kazakhstan leads the world production of uranium (27%) followed by Canada (20%) and Australia (20%), as per 2010 estimate. However, it can be noted that a sizable portion of uranium is also produced by reprocessing spent nuclear reactor fuels. Note that existing laws presently forbid reprocessing of spent nuclear fuels in the United States. So, almost all commercial uranium fuels in the United States need to be made following extraction from the ores. On the other hand, France uses reprocessing of spent nuclear fuels as one of the major methods for meeting its fuel needs.

7.2.1.1 Extraction of Uranium

Over several decades, many uranium extraction methods have been pursued. Here, only some of the very common extraction techniques are discussed. Almost all ura­nium minerals are present in the ore with a variety of gangue (impurity) materials. Hence, it is essential to separate the gangue minerals from the mineral with a metallic value. The metal extraction process thus contains mineral beneficiation or ore dressing techniques as the first steps that increase the metal value of the ore by removing the gangue materials. The uranium extraction is generally achieved by using chemical methods, as entirely physical beneficiation methods are not effec­tive enough to liberate ore minerals of value. For recovery of the most metal con­tent, it becomes necessary to pursue leaching and precipitation reactions. One of the most common processes is to leach finely ground uranium ores by dilute acids. Sulfuric acid (H2SO4), nitric acid (HNO3), and hydrochloric acid (HCl) can be used for leaching. However, the latter two acids adversely affect the process economics. Also, they present corrosion problems for the process equipment. However, if the solvent extraction process is to be used after leaching, nitric acid must be used. Under certain conditions, sodium carbonate (Na2CO3) is also used as suitable leaching agent.

Leaching is nothing but a dissolution reaction in which uranium forms a soluble compound that remains dissolved in the solution. It is known that uranium can go into solution only when it is in a hexavalent state. This is a common requirement for both acid and alkali leaching. Thus, if uranium is present in the tetravalent state, it needs to be oxidized to the higher oxidation state. This is obtained by the presence of trivalent iron or pentavalent vanadium, which happens to coexist with uranium in most uranium ores. The reactions for acid and alkali leaching are given below:

2U3O8 + 6H2SO4 + (O2) = 6UO2SO4 + 6H2O (7.3)

2U3O8 + 18Na2CO3 + 6H2O + (O2) = 6Na4UO2(CO3)3 + 12NaOH (7.4)

Acid leaching generally leads to greater recovery of uranium than carbonate leaching. However, there are some limitations of acid leaching. This cannot be used for ores that contain magnesium and/or calcium carbonates as these

compounds tend to react with acid leachants wasting an excessive amount of acid. Furthermore, because of corrosion problems, the equipment and procedure used in acid leaching are far more expensive. Conversely, corrosion problems are not so severe (thus reducing the process costs) in alkali leaching. It also allows reagent recovery. However, alkali leaching is not suitable for leaching ores with contents of high gypsum or sulfide or refractory constituent.

After leaching, uranium is to be recovered from the leach solution. This can be done by following one of the methods — chemical precipitation, ion exchange, and solvent extraction. Here, we discuss the ion exchange method. This method is used for recovering uranium after acid leaching by sulfuric acid. The method is based on the principle that uranyl sulfate ions can be selectively removed by allowing them to be adsorbed by an ion exchange resin, and subsequently the adsorbed ions could be separated from the loaded resin with a solution concentrated with nitrate or chlo­ride ions. It can be noted that the ion exchange process is also applicable to alkali leaching process. From the processes discussed above, eventually a yellow powder containing 80-85% of UO2 can be obtained. Nitric acid is used as a dissolving agent for this powder subsequent filtration. This reaction produces uranyl nitrate that is extracted with the help of ether. Then water is used to wash the filtrate to produce an aqueous solution from which pure ammonium diuranate [(NH4)2U2O7] is pre­cipitated by passing ammonia gas. Following the separation of diuranate, it is dried and reduced to uranium dioxide in hydrogen at 650 °C. In another process (Dryway process), ammonium diuranate can be converted to uranium trioxide (UO3) by heating it to remove ammonia and steam. Uranium trioxide is then treated with hydrogen at ~600 °C to produce uranium dioxide and then this dioxide is reacted with anhydrous hydrogen fluoride to obtain the uranium tetrafluoride (UF4) that can then be reduced with either calcium or magnesium to produce metallic ura­nium. Some of the reactions involved in the Dryway process are given below:

UO3 + H2 ! UO2 + H2O (7.5)

UO2 + 4HF! UF4 + 2H2O (7.6)

UF4 + 2Mg! U + 2MgF2 (7.7a)

UF4 + 2Ca! U + 2CaF2 (7.7b)

For enrichment of uranium (i. e., to increase the amount of fissile atom U235 den­sity), there are specific methods like membrane separation, centrifuging, and so on, using UF6. Depending on the condition of uranium (unenriched, slightly enriched, or highly enriched), specific details of the process may change to avoid any possibility of obtaining critical mass for fission chain reaction.