Uranium ore processing

Uranium ore is mined in several ways: by open pit, underground or by leaching uranium from low-grade ores  Uranium ore typically contains 0.1 to 0.25 percent of actual uranium oxides. So extensive measures must be employed to extract the metal from its ore. Uranium ore is crushed and rendered into a fine powder and then leached with either an acid or alkali. The leachate is then subjected to one of several sequences of precipitation, solvent extraction, and ion exchange. The resulting mixture, called yellowcake, contains at least 75 percent uranium oxides. Yellowcake is then generally further refined using nitric acid to create a solution of uranyl nitrate. Additional solvent extraction procedures finish the process.

Commercial-grade uranium can be produced through the reduction of uranium halides with alkali or alkaline earth metals. Uranium metal can also be made through electrolysis of KUF5 or UF4, dissolved in a molten calcium chloride (CaCl2) and sodium chloride (NaCl). Very pure uranium can be produced through the thermal decomposition of uranium halides on a hot filament. 

Cascades of gas centrifuges are used to enrich uranium ore to concentrate its fissionable isotopes. Enrichment of uranium ore through isotope separation to concentrate the fissionable uranium-235 is needed for use in nuclear power plants and nuclear weapons. A majority of neutrons released by a fissioning atom of uranium-235 must impact other uranium-235 atoms to sustain the nuclear chain reaction needed for these applications. The concentration and amount of uranium-235 needed to achieve this is called a 'critical mass.'

To be considered 'enriched' the uranium-235 fraction has to be increased to significantly greater than its concentration in naturally-occurring uranium. Enriched uranium typically has a uranium-235 concentration of between 3 and 5 percent.

The gas centrifuge process, where gaseous uranium hexafluoride (UF6) is separated by weight using high-speed centrifuges, has become the cheapest and leading enrichment process (lighter UF6 concentrates in the center of the centrifuge).   

The most common forms of uranium oxide are triuranium octaoxide (U3O8) and the aforementioned UO2.  Both oxide forms are solids that have low solubility in water and are relatively stable over a wide range of environmental conditions

Triuranium octaoxide is (depending on conditions) the most stable compound of uranium and is the form most commonly found in nature. Uranium dioxide is the form in which uranium is most commonly used as a nuclear reactor fuel. At ambient temperatures, UO2 will gradually convert to U3O8. Because of their stability, uranium oxides are generally considered the preferred chemical form for storage or disposal

At room temperatures, UF6 has a high vapor pressure, making it useful in the gaseous diffusion process to separate highly valuable uranium-235 from the far more common uranium-238 isotope. This compound can be prepared from uranium dioxide and uranium hydride by the following process:

UO2 + 4HF + heat (500 °C) → UF4 + 2H2O

UF4 + F2 + heat (350°) → UF6

The resulting UF6 white solid is highly reactive (by fluorination), easily sublimes (emitting a nearly perfect gas vapor), and is the most volatile compound of uranium known to exist.   

The main use of uranium in the civilian sector is to fuel commercial nuclear power plants; by the time it is completely fissioned, one kilogram of uranium can theoretically produce about 20 trillion joules of energy (20 × 1012 joules); as much electricity as 1500 metric ton of coal.

The south Basin Uranium deposits 

In the south of the Basin, the Tummalapalle belt with low-grade strata-bound carbonate uranium mineralisation is 160 km long, and appears increasingly prospective – AMD reports 37,000 tU in 15 km of it and over 100,000 tU overall, extending down dip to 1000 metres. Some secondary mineralisation is reported in the Srisailam sub-basin.

In August 2007 the government approved a new US$ 270 million underground mine and mill at Tummalapalle near Pulivendula in Kadapa district of Andhra Pradesh, 300 km south of Hyderabad. Its resources have been revised upwards by AMD to 71,690 tU (March 2014) and its cost to Rs 19 billion ($430 million), and to the end of 2012 expenditure was Rs 11 billion ($202 million). First commercial production was in June 2012, using an innovative pressurised alkaline leaching process (this being the first time alkaline leaching is used in India). Production is expected to reach 220 tU/yr as sodium diuranate, and in 2013 mill capacity was being doubled at a cost of Rs 8 billion ($147 million). An expansion of or from the Tummalapalle project is the Kanampalle U project, with 38,000 tU reserves. Further southern mineralisation near Tummalapalle are Motuntulapalle, Muthanapalle, and Rachakuntapalle 

In Karnataka, to the west of north Cuddapah Basin, UCIL is planning a small underground uranium mine in the Bhima basin at Gogi in Gulbarga area from 2014, after undertaking a feasibility study, and getting central government approval in mid-2011, state approval in November 2011 and explicit state support in June 2012. A portable mill is planned for Diggi or Saidpur nearby, using conventional alkaline leaching. Total cost is about $135 million. Resources are 4250 tU at 0.1% (seen as relatively high-grade) including 2600 tU reserves, sufficient for 15 years mine life, at 127 tU/yr, from fracture/fault-controlled uranium mineralisation. UCIL plans also to utilise the uranium deposits in the Bhima belt from Sedam in Gulbarga to Muddebihal in Bijapur.

India's Uranium mines and mills

State, district	Mine	Mill	Operating from	tU per year
Jharkhand	Jaduguda	Jaduguda	1967 (mine) 1968 (mill)	200 total from mill
	Bhatin	Jaduguda	1967
	Narwapahar	Jaduguda	1995
	Bagjata	Jaduguda	2008
Jharkhand, East Singhbum dist.	Turamdih	Turamdih	2003 (u/g mine) 2008 (mill)	190 total from mill
	Banduhurang	Turamdih	2007 (open pit)
	Mohuldih	Turamdih	2012
Andhra Pradesh, Kadapa/YSR district	Tummalapalle	Tummalapalle	2012 2015 (mill)	220 increasing to 330
Andhra Pradesh, Kadapa/YSR district	Tummalapalle	Kanampalle?	2017?
Telengana, Nalgonda district	Lambapur-Peddagattu	Seripally/Mallapuram	2024? (open pit + 3 u/g)	130
Karnataka, Yadgir (Gulbarga) district	Gogi	Diggi/Saidapur	2020? (underground)	130
Meghalaya, West Khasi Hills district	Kylleng-Pyndeng-Sohiong-Mawthabah (KPM), (Domiasiat), Wakhyn	Mawthabah	2022? (open pit)	340

Acid Leaching

Acid leaching has the advantage of being more effective with difficult ores, requiring lower temperatures and leaching times compared to alkaline solutions. It also requires less pretreatment than alkaline leaching, most notably because the particle size from the grinding process does not need to be as small.  Acid leaching is sometimes also referred to as heap leaching because the leaching process can be performed on large "heaps" of uranium ore that have been collected from mines. The chemistry of the leaching process revolves around oxidation of the uranium compounds, which is typically achieved using manganese dioxide (MnO2), sodium chlorate (NaClO3), and Fe(II) salts.   Sulfuric acid is typically used due to the solubility of uranyl sulfate complexes.  The reaction is typically performed at slightly elevated temperatures (~60C) and can often release H2, H2S, and CO2 gases during the process.  The uranium, which typically begins in the tetravalent state, goes through a series of reactions, eventually leading to the formation of the desired complex, [UO2(SO4)3]4-. While the solubility of this complex makes sulfuric acid a desirable leaching agent, nitric and hydrochloric acid can also be used, but are typically not due to their higher cost and corrosiveness.

Alkaline Leaching

While both acidic and alkaline leaching agents are used, alkaline leaching has some significant advantages. Alkaline solutions tend to be more selective to uranium minerals, which means the solution will contain fewer impurities. Consequently, the uranium oxide (commonly called "yellow cake") can be directly precipitated without purification. Furthermore, the solutions are less corrosive and can be recycled without the annoyance of increasing impurity concentrations.  The alkaline leaching process relies on the formation of highly soluble uranyl tricarbonate, UO2(CO3)34-. As in the case of acid leaching, oxidizers are used to maintain the presence of the hexavalent U6+ cation. This can be achieved by simply introducing oxygen into the solution by bubbling air into the solution.  The leaching agents used are sodium bicarbonate and sodium carbonate. This prevents formation of uranyl hydroxide compounds. Due to the slower reactivity of the alkaline solutions, increased pressures and temperatures are sometimes used to accelerate the process.