From ore to yellow cake

The rod mill and the ball mill (huge circular machines) where the natural ore, already reduced from 300 mm size to 25 mm size, is further reduced to 1 mm and then 74 micron sizes.-The rod mill and the ball mill (huge circular machines) where the natural ore, already reduced from 300 mm size to 25 mm size, is further reduced to 1 mm and then 74 micron sizes.

The ore extracted from a mine is processed to yield yellow cake, or uranium in a compound form, in a processing plant. The ore is first crushed to a uniform particle size, and the fine ore is then subjected to chemical leaching. The extracted yellow cake is transported to the Nuclear Fuel Complex in Hyderabad, where it is fabricated into fuel bundles for use in Pressurised Heavy Water Reactors.

Conventionally, processing plants/ mills, such as those in Jaduguda and Turamdih in Jharkhand, use the acid leaching technique. But the high lime content of the ore at Tummalapalle meant that acid leaching could not be done. Scientists at the Bhabha Atomic Research Centre (BARC), Trombay, then developed the alkali leaching technique for this ore.

The ore is crushed in three stages: from 300 mm size to 150 mm and then to 25 mm. The fine ore is sent to a rod mill, where it is further reduced to 1 mm size. Crushing is a dry process, but the grinding in the mill uses recycled water from the downstream stages.

In hydrocyclones [devices used to separate particles in a liquid suspension], the ore gets separated into coarse and finer fractions. The coarse fraction goes to the ball mill for further grinding, says S. Siddique, General Manager (Technical Services) at the Tummalapalle plant.

The fine ore has a diameter of 74 micron (a micron is one-thousandth of a millimetre), which is the desired size for further processing. The ore is in the form of a finely ground greyish slurry when it reaches the leaching section, which is the heart of the plant.

The slurry is dewatered on horizontal belt filters and the resultant cake travels in the form of broad sheets to a mixer where it is re-pulped with recycled uranium-bearing liquor and fed into two gigantic autoclaves. Here, the uranium is converted to liquid state in seven hours. The outer face of the autoclave is lined with glass wool and encased in aluminium sheets.

The dissolved-uranium-bearing slurry is washed thoroughly on the belt filters to separate all the uranium-bearing liquid from the waste. These filtering devices are akin to tea strainers, said S.K. Malhotra, the spokesperson of the Department of Atomic Energy.

In the autoclaves, we maintain a temperature of 130 {+0} Celsius and a pressure of seven kg per square cm. We add sodium carbonate, sodium bicarbonate and oxygen into the autoclave. After seven hours, the slurry is sent for filtration. The filtration results in two products: cake and liquor. This cake, after thorough washing, is despatched to the tailing pond for containment and the liquor is precipitated with sodium hydroxide, or caustic soda. What is available after precipitation is sodium diuranate, that is, yellow cake. The fine product is dried, packed and sent to the NFC in Hyderabad for fabrication into fuel bundles.

T.S. Subramanian