The Indian salt industry remains largely labour-intensive.In the marine salt units, seawater, which has 3 to 4 degrees baume concentration, is let in by tidal action through gates into salt ponds called reservoirs. Here it attains a concentration level of 6-8 degrees baume. The solution then moves by gravity into an enclosure called a condensor where the concentration level increases to 15-20 degrees baume. Finally it moves into another enclosure called a harvesting pond or crystalliser. Here the concentration reaches 26-29 degrees baume which is the level required for salt to crystallise.
It may be mentioned that between 3.5 to 8 degrees baume, iron salts and other organic material settle down at the bottom of the reservoirs while in the condensors, calcium and magnesium precipitate as carbonate and sulphate salts between 8 to 18 degrees baume. For removing the residual impurities the solution is washed at 26 degrees baume.
Harvesting
Normally harvesting salt takes 25 to 30 days in summer and 60 to 70 days in winter. With four to five harvests in a year, the standard production rate is 20 kg per square metre or 80 tonnes per acre annually. Harvesting is usually done manually in India and salt is stored in the open. However, in the case of refineries like Tata Chemicals and Saurashtra Chemicals, most of the harvesting is done mechanically with minimal requirement of labour. In a few days, a crust of 10-15 mm envelops the stockpile. Exposure to monsoons thickens the crust further and pickaxes have to be used to break the stockpiles when the crust thickens to 75 mm or more. Though the rains wash away the salt, they improve its quality. But washing losses are considerable and pegged at about 10-15 per cent of production. Curing generally takes up to a year.
In the case of inland salt manufacture, salt is produced by solar evaporation of inland brine available from wells about 30 to 50 feet deep. These have a density of 15 to 18 degrees baume which is concentrated by circulating the solution in pans of 130 x 250 ft dimension till it achieves a density of 24 degrees baume. This is when salt crystals begin to form. Thereafter, the salt is collected manually with pickaxes.
The investment for inland salt manufacture is much less than that for marine salt works since the infrastructure required is smaller. Inland salt manufacturers require a minimum investment of Rs 2 lakh to create infrastructure for salt production. Marine salt works require an initial investment of at least Rs 50 lakh as permanent salt beds have to be constructed. But inland salt manufacturers reap only one annual crop of around 500 tonnes salt per well compared to one crop reaped every two to three months by the marine salt manufacturers.