Under the surface of the dry and saline sands of Sua Pan, a naturally occurring, carbonate-rich resource, brine, is found. By drilling wells into the underground aquifers below the pan surface, brine is pumped to solar evaporation ponds from where the extraction process begins.
Wells are sunk into the pan surface to extract the alka-saline brine. The total area of the wellfield is approximately 380 km2 with the centre of the wellfield being 15 km from the plant production site.
Well brine is pumped into shallow ponds operating with a brine depth of about 20 cm; provide large surface area (over 22 km2) for evaporation to occur, using the heat of the sun, thereby concentrating the brine and precipitating out salt.
Two coal fired 85 t/h, 65 bar pressure, boilers produce the process steam requirements and to generate 20 MW of power to satisfy the electrical requirements of the plant.
Carbon dioxide is extracted from the boiler flue gas and used to carbonate the brine. Concentrated brine from the solar ponds is pumped to a series of large, agitated, fibreglass lined steel tanks. In these vessels, carbon dioxide reacts with brine to convert the sodium carbonate to sodium bicarbonate which is less soluble and therefore easier to separate from the liquid.
Bicarbonate slurry from carbonators is cooled in a series of 3 heat exchangers – crystalliser units to allow the sodium bicarbonate to crystallise out.
The slurry leaving the crystallisers is fed to a thickener which raises the solids concentration to 25%. The thickened slurry is fed onto two large horizontal belt filters where the bicarbonate crystals are washed to remove impurities.
The washed sodium bicarbonate cake is fed to two large rotary kilns in which it is indirectly heated by steam, causing the sodium bicarbonate to be converted into sodium carbonate or light soda ash with the liberation of carbon dioxide and water vapour. The carbon dioxide is compressed and recycled to carbonation.
Light soda ash is compacted in the “nip” between two large horizontally orientated rollers to produce soda ash flakes, which are milled and screened to produce dense soda ash.
The brine remaining after removal of the carbonates is called bitterns. The bitterns are evaporated in large solar ponds until dry, but the crystallised salt produced is not harvested and so it builds up. The height of the bitterns ponds is limited to ensure no visual effects on the area. These ponds are constructed on the pan surface so that any seepage does not contaminate underground freshwater sources.
Salt deposited at the solar ponds crystallisers is harvested and hauled to the salt plant where it is washed counter currently with heated bitterns. The washed salt is screened into fine and coarse salt. Coarse salt is dewatered on the coarse salt centrifuge and stored at washed salt stockpile area. Fines are rid of greater than 0.2 mm particles at the fine screen before dewatered on the fine salt centrifuge. Dewatered fine salt is dried in the fluidised bed dryer and passed to the fluidised bed cooler for temperature reduction. The dried fine salt is fed to the milling plant where it is screened into product (0.2 mm – 1.0 mm) and oversize. The oversize is fed to the hammer mill and roll mill parallel to each other. The milled salt is then separated into oversize (recycled), product and super fines.
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