Uranium Production From ore to yellow cake

Uranium Corporation of India's mining and processing facilities in Bihar's Singhbhum district present a range of capabilities in the specialized field

Natural uranium converted into pellets is the vital fuel for eight nuclear reactors in the country, called Pressurized Heavy Water Reactors (PHWRs), two each at Rawatbhatta in Rajasthan, Kalpakkam in Tamil Nadu, Narora in Uttar Pradesh and Kakrapar in Gujarat. Heavy water is both the moderator and the coolant in these reactors.

The mining and milling (that is, processing) of uranium ore, the first steps leading to nuclear power generation, are undertaken in India by the public sector Uranium Corporation of India Limited (UCIL), which comes under the administrative control of the Department of Atomic Energy (DAE). UCIL itself was founded in 1967 with its headquarters at Jaduguda in east Singhbhum district of Bihar, where the country's first underground uranium mine was established the same year. UCIL manages the mining of uranium ore and its processing into uranium oxide concentrates or yellow cake. Yellow cake is sintered and made into fuel pellets at the Nuclear Fuel Complex, Hyderabad.

Uranium is found in nature in the form of minerals. There are more than 100 such minerals present in almost all rocks. The German scientist Martin Heinrich Klaproth isolated uranium in 1789 in a sample of pitchblende. (It was only in 1896 that A.H. Bacquerel discovered that uranium underwent radioactive decay. The discovery of nuclear fission by Otto Hahn and F. Strassmann in 1939 brought uranium into the limelight).

The Atomic Minerals Division of the DAE is responsible for uranium prospecting; UCIL mines and process the ore. Commercially exploitable levels of ore deposits were found at Jaduguda in 1950 and at nearby Bhatin and Narwapahar later in the 160 km-long and 2-km wide Singbhum thrust belt.

Picturesque Jaduguda is 230 km from Calcutta and 24 km from Jamshedpur. It is hemmed in by the Chhotanagpur hills. The valley is awash with paddy fields. The Subarnarekha flows nearby. People belonging to the Santhal, Munda and Ho tribes live in the area.

Although the ore is mined at all the three underground mines, all milling is carried out at Jaduguda. Here the mine and the mill are next to each other.

As you step into the "cage" in the shaft and the switch is thrown, it is dark all around. The lift speeds down. In a couple of minutes, you step out - 605 metres below the surface of the earth, in an area that is well lit and ventilated. Now you are in the heart of the Jaduguda mine.

The tunnels go in different directions. Occasionally, miners with their machinery appear. A conveyor belt transports lumps of uranium ore into measuring bin which, in turn, empties the ore into a skip. The skip, a huge bucket, is hoisted and the ore is emptied on the surface.

The circular, concrete-lined main mine shaft has a diameter of 5 metres and it sunk to a depth of 640 m. It was sunk in two stages- the first stage went to a depth of 315 m and the second stage from 315m. Later, an auxiliary third shaft to a further depth of 350 m was sunk. The three shafts together to a depth of 990 m.

J.L. Bhasin, who until his death on August 2 of a heart attack, was the Chairman and Managing Director of UCIL, was in charge of the design of the three shafts. (Bhasin had briefed a group of visiting journalists on the activities of UCIL, just last month.)

On top of the shaft and on the surface is a 41-m concrete tower that houses machinery. Two multi-rope friction winders control the skip and the cage. The skip can haul 5 tonnes of ore at a time from a loading station situated 605 m below the surface. The cage has two decks to transport men and materials.

As you go down the shaft, there is an operating level every 65m, which is connected to the ore drivers. At every alternate level there is a pumping station.

One sub-station with high tension lines of 6,600 volts could be spotted at a depth of 434 m. Elsewhere, in a rock-cut "temple", is a control room with automatic panels.

S.C. Bhowmik, Mines Superintendent, explained how the uranium ore bodies are priced out. "Earlier, we used the hand-drill jack hammer. Now we use electrically operated jumbo drills."

The ore extracted from different levels is collected in a central crushing station, 580 m below the ground. The crushing is done in the mine itself. A nearby washing station, the rocks are sprayed with water. Then a big jaw crusher breaks the ore lumps to a size of about 200 mm The crushed ore is stored in ore pockets below the crushing station, at a depth of 605 m. Here a conveyor belt loads the crusted ore into a measuring bin which, in turn, loads the ore into the skip. The ore is finally hoisted to the surface in the skip. This ore is transported to the mill. About 750 tonnes of ore a day is extracted at Jaduguda.

Bhasin stressed the importance given to the safety of the workers. He said: "We lay greater emphasis on providing adequate ventilation in all the three mines. The most effective way of controlling radon and airborne radioactive dust from the mine air is proper ventilation." At Jaduguda, the main shaft is used to supply fresh air. There are also two exhaust fans, located on the surface of the eastern and western ends of the mine, which get rid of impure air.

Located 4 km west of Jaduguda is the much smaller Bhatin mine. Entry into this mine, which goes to a depth of 135 m is by; means of an adit and two winzes. Bhatin's uranium deposit is the western extension of the Jaduguda ore body. This mine has been producing ore since 1986-87 and production now is around 150 tonnes a day. This ore is transported to the Jaduguda mill.

The Narwapahar mine is 12 km away from Jaduguda. It is one of the most modern mines in the country. Bhasin said state-of-the-art technology involving "trackless mining with decline" and "ramps in stopes" was utilized here for ore extraction from depth of upto 140 m.

The Narwapahar mines also an underground one, but the preferred entry here is down a ramp rather than through the shaft. This is the only underground mine in India where such dual-type access is possible. At a production point, a jumbo drill bores into the rock that contains uranium ore. Trucks transport the ore outside.

The Narwapahar mine was set up at a cost of Rs.216 crores. The 5-m diameter shaft here is sunk to a depth of 355 m. The working level is upto 275 m. The mine now produces about 650 tonnes of ore a day; it is planned to increase production to about 1,000 tonnes.

According to R.P. Sengupta, General Manager of the Narwapahar mine, trackless mining equipment such as jumbo drill, low profile dump trucks and passenger carriers were introduced for the first time in the country here.

D. Acharya, mine manager, said the jumbo drill used drill bits made of tungsten carbide. As these drill bits cut into the rock, rock particles flew out.

The mill at Jaduguda is a modern complex with a massive chemical house. It can process 1,000 tonnes of ore a day. Here there is also a sulphuric acid manufacturing unit and a plant to recover byproducts.

According to K.K. Beri, Director (Technical), UCIL, the processing is done in three stages: crushing, grinding and leaching. The ore is crushed and ground and uranium leached into solution by sulphuric acid. The solution is filtered, concentrated and purified, and uranium is precipitated in the form of magnesium diuranate or yellow cake. Crushing is done in stages in rollers with 4-inch diameter iron rods and steel balls. The rods and the steel balls impact on the ore, smashing it. The smashed ore is sieved into 4-inch, 2.5-inch, and one-inch particles. The 4-inch and 2.5 inch particles are again crushed, to produce one-inch particles, which are mixed with water (to prevent dust formation) and ground.

The grinding process a slurry which is 40 percent solid; it is thickened into 60 per cent solid state. This is filtered to 80 per cent solid state. "This ore pulp is dissolved in sulphuric acid in a process that lasts about 12 hours at a temperature of 40 ^o to 50 ^o C. Thus, the uranium gets oxidized and dissolved. The left-out solids are gangue minerals - waste products. The uranium bearing solution is filtered and purified by the ion exchange process of clarification. Subsequently, the uranium is separated as magnesium diurante after a reaction with magnesia slurry.

A.R.Ray, General Manager (Electrical), UCIL, said that at a certain percentage of the hydrogen value, the uranium gets precipitated. The yellow cake thus formed is sent to the NFC to be sintered into uranium bundles. Once uranium is extracted, magnetite is recovered. Magnetite has applications in the coal industry.

All the process after leaching takes place in the chemical house. Double filtration is also done in the chemical house and its main purpose is to take out the uranium solution.