Challenges for tomorrow

God stopped creating land and fossil fuels a long time ago.  Therefore, it is significantly important to harness the sun and sea for the creation of wealth.  India is endowed with inexhaustible resources of seawater and the art of salt making is as old as mankind.  The Salt Satyagraha – or Salt March to Dandi – led by Mahatma Gandhi was an important milestone in India’s freedom struggle from Britain.
 
Salt affects our life and our needs more than anything else.  In industry, salts are important raw materials for heavy chemical industries that require high quantities of salt with fewer impurities, like calcium magnesium.
 
Continuous and vigorous efforts are needed to maintain the growth potential by way of tapping new and latent resources.  Although a lot has been achieved on the salt front, some areas are still untapped.  Marine chemicals recovery is one such area that is calling for concentrated efforts and input to tap into this inexhaustible resource from sea.


Salt production

During the process of salt manufacture by solar evaporation, gypsum is precipitated first, before the salt begins to separate.  Approximately 30 - 50 kg of gypsum is produced for every ton of salt produced.  Except for small quantities of gypsum being supplied to cement plants, there is no organic outlet for this marine gypsum.
 
The major consumption of salt is for edible purpose and industrial use.  The alkali industry soda ash and caustic soda are the principal industrial users.

The marine chemical industry of today is generally limited to the manufacture of salt from seawater by solar evaporators.  About 47 m³ of seawater is required to produce one-ton salt and 43 tons of water has to be evaporated by solar energy, which equivalent to about 5.5 tons of coal for every ton of salt produced.
 
Bittern

In theory, for every ton of salt harvested, about 1.5 m³ of bittern is produced. Taking into consideration percolation and other losses, one cubic meter is available for further processing.  Chemicals in the bittern corresponding to 10 million tons of salt produced:

 
Amongst the other chemicals present in seawater, presently few salt manufacturers recover only bromine from bittern.  Two kg of bromine can be recovered per ton of salt produced.

Bittern is a source rich with chemicals, but unfortunately not much attention seems to have been given to this resource.  The non-recovery of these useful chemicals can be attributed to:

a)      Bittern is available in a scattered form in different salt works.  Large salt works are basically owned by the alkali industry and smaller salt works are scattered and do not have sufficient bittern to consider further processing.  The essential prerequisite of any marine chemicals complex is the collection and storage of bittern with minimum seepage loss.

b)      Solar evaporation of bittern is much slower compared to the brine before the salt precipitation stage.  In addition, unlike the precipitation of quality salt, no single salt is precipitated.  During the evaporation there are over-lapping solids phases, and therefore the crops obtained are mixtures of different salts that need further processing.

c)      Solar evaporation is also a limiting factor that depends on the climatic conditions.  For example, TATA Chemicals Limited’s, Mithapur (India), evaporation beyond 320 Be is a rare condition.  The duration of going beyond  320 Be is limited to a few days a  week.  During this peak period of evaporation, the crops obtained are not sufficient to run the plant continuously throughout the year.

d)      The magnesium and potassium salt present in bittern are known to form double salt and also the hydrates.  Due to the diurnal temperature variations, sometimes the composition of the solid crop obtained tends to differ.

e)      Most of the salt manufacturers are on the Saurashtra coast and it is not possible to reach even 360 Be for more than two months in a year.                    

With 360 Be, the Potash-rich fractions begin to separate out from above and it is clear that a bittern-base complex cannot be planned, so solar evaporation has to be resorted to.  As the temperature of the system rises, the equilibrium points of solid/liquid phase of the multi-component bittern system change and some of the solids phases of the solar evaporations area completely disappear or merge into other phases.


Further possibilities

TATA chem is India's first major scientific venture toward the recovery of chemicals from seawater.  Engineers and scientists are looking toward the future of marine chemicals, including:
 
1)      Gypsum and solar salt yields improvement
 
The pilot level experiment can be conducted to use the clear effluent of soda ash in salt works to improve the yields of solar salt and gypsum.
 
           MgSO₄  +  CaCl₂  --->   CaSO₄  +  MgCl₂
 
2)      Potassium chloride and potassium sulfate production and further by electrolysis a potassium hydroxide
3)      Production of magnesium compounds eg. Magnesium hydroxide and Carbonates etc.

The integrated chemical complex from sea water and bitterns is shown here:




Conclusion

India's solar salt production is likely to exceed 15 million tons, which creates a potential for 30,000 tons of Bromine 350,000 tons of KCL and 1 million tons of Magnesium oxide (MgO).
 
Besides the above chemicals, there will also be potential for about 0.6 million tons of precipitated Gypsum and 1.3 million tons of vacuum-evaporated salt per year.  All these process will produce about 1,000 million gallons of good water (condensate) per year.  Large integrated chemical complexes having captive power generation facilities will need substantial investments to be phased out during the next decades.  Thus, leading chemical manufacturers, R&D centers and the scientists and engineers of India have very important and challenging tasks ahead for the recovery of marine chemicals, which will create scientific and economic fields this century.

It is imperative that an all out effort is made to exploit the entire potential so as to ensure availability of potash for industry as well as the formation of the balance of fertilizer so vital to agriculture growth.


By: P.S. Dwivedi, Assistant Manager of Technical Services, TATA Chemicals
psdwivedi@tatachemicals.com