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:
Tons
NaCl
1,500,000
MgCl2
1,200,000
MgSO4
700,000
KCl
238,000
Bromine
20,000
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.
