Lake Magadi facility
Tata Chemicals Magadi produces both soda ash and common salt at the Lake Magadi facility in Kenya.
||Soda ash manufacturing
Soda ash manufacturing
At Tata Chemicals Magadi's manufacturing facility, soda ash is obtained by washing and calcining trona, a naturally occurring form of sodium sesquicarbonate, extracted from Lake Magadi in Kenya (see photo gallery).
The alkali deposits at Lake Magadi exist as solid trona crystals with interstitial saturated alkaline liquor. The sodium sesquicarbonate crystals are crushed, slurried and pumped to the washery plant or dewatering tower by use of dredgers.
At the washery plant, washing and blending is carried out to achieve required specifications. As a final stage in the washery, the interstitial liquor is displaced by processed water to drop the sodium chloride level associated with the liquor from 11.5 per cent to around 0.33 per cent.
The crushed refined soda (CRS) is then conveyed to the calciner plant. Here it is fed into two large rotary kilns where the main chemical reaction takes place. The CRS is loaded in the rotary kiln and a continuous flow of hot gases is blown through the kiln across the reaction bed to chemically decompose sodium sesquicarbonate into soda ash, carbon dioxide and water vapour. This material is conveyed to the grinding and screening plant where the physical grading of the product takes place before the product is conveyed to the storage silos.
At the silos the product is bagged in 50kg bags for sale to local customers or moved to shipping terminal in Mombasa, Kenya by specially designed hopper wagons. At Mombasa the product is bagged into 50kg or 1tonne bags depending on the requirements of the customer. Soda ash can also be shipped in bulk form.
At the Lake Magadi salt pans, common salt (sodium chloride or NaCl) is extracted from the lake waters by solar evaporation. The climate of the area (low rainfall and high evaporation rates) makes this process very suitable; Magadi has a net evaporation rate of over 400mm per annum.
The Magadi Lake brines are normally saturated with a dissolved solids content of approximately 32 per cent W/W. Of this, the sodium chloride content varies from 30 - 34 per cent, the rest being almost exclusively sodium carbonate compounds with some sodium fluoride in small quantities.
The brines are pumped to pre concentration ponds where solid deposition takes place. Because of the higher concentration of sodium carbonate compounds in the brines, sodium carbonate monohydrate is deposited predominantly in the pre concentration ponds. When the brine contains equal amounts of sodium chloride and sodium carbonate, it is transferred to the salt making ponds.
With more evaporation, a mixture of soda and common salt is deposited as solids in the salt making ponds. Because of the daily temperature differences and the solubility characteristics of sodium carbonate, the final stages of evaporation result in a superficial layer of sodium chloride rarely more than 1cm thick. This 1cm thick layer is manually harvested and transported to the factory for processing into various grades of sodium chloride for different salt markets.