|Title of Invention||
"AN IMPROVED FOR THE RECOVERY OF SODIUM CHLORIDE CONTAING LOW CA++ IMPURITY FROM SEA BRINE"
|Abstract||An improved process for the recovery of sodium chloride containing low Ca++ impurity from sea brine which comprises introducing sea brine into series of concentrating ponds, conventionally called reservoir & condenser till brine density reaches 25.5° Be" adding to the said brine to a polysaccharide in concentration in the range of 50-150 ppm to obtain the brine strach mixture, evaporating the said brine starch mixture in a crystallizer pond to get crystals of sodium chloride containing Ca++ impurity in the range of 0.07-0.1 percent.|
|Full Text||This invention relates to an improved process for
the .recovery of Sodium Chloride containing low ,Ca++
impurity from sea brine
The present invention particularly relates to the preparation of high purity. Common salt from sea brine having low calcium sulphate using polysaccharide as an additive during crystallization process. ' Crystallization of common salt-sodium chloride from sea water by solar evaporation is a well known and long established industrial process. In the process for recovery of salt: from sea brine, the sea water is subjected to solar evaporation. Degree Baume is an alternative unit of expressing specific gravity of a liquid or homogeneous solution. The relation of degree baume with specific gravity is explicitly given by following equation for solutions; denser than water.
During the progressive evaporation, the brine
o eventually obtains 25.6 Be',.at which stage, it is fed
o to salt crystallizers. The brine of 25.6 Be' is al-
o lowed to evaporate by solar radiation upto 2 9 Be'
density. During this operation as the residual calcium
and sulphate ions start forming very fine micelle due to
decrease in solubility of calcium sulphate and remains
suspended in the initial stage. As the brine is also saturated with sodium chloride it starts crystallizing,, on calcium sulphate nuclei.'. This results in the occluded impurities of calcium sulphate in salt. Calcium sulphate crystallizing at later stage of evaporation .remains adhered on the surface "of salt which remains as surface impurities. The surface impurities can be reduced by washing operation while occluded impurities remain as it is evenafter washing. (Kreb-Swiss, Chem. Age of India Vol.39, Dec.1988) Salt washing processes with wet hammering or wet grinding result in removal of occluded calcium sulphate to greater extent which involves many unit operations such as : crushing, agitating, centrifuging, etc. Because of these reasons the washed salt becomes expensive, and there is inevitable loss of salt ranging from 10-15%, depending on' the contacting1 process employed for the purpose. Attempts have been made by many workers to reduce calcium sulphate by adding minute quantity of straight chain aliphatic carboxylic acid-saturated or unsaturated-such as oleic, stearic palmitic, myristic, margaric and:linoleic, individually or in admixture. (P.F. Joseph' and R.T. John, Brit. Pat. No. 1284944 (1972) .
The formation of calc.um sulphate crystals in crystal lattice of sodium chloride calcium will have a small overall.positive charge. The carboxylic group of
the fatty acid additive on the other hand will have a small negative charge and there will be a tendency for the carboxylic group of the acid molecule to attach itself to the calcium sulphate crystal, with the hydrophobic hydrocarbon chain of the acid molecule forming a sort of protective covering against other crystals in the system.
In a similar manner carbohydrates having polyelec-trolytic properties such as water soluble starch, carboxy methyl cellulose (CMC) etc. are known as complex forming -chemicals and are used in industries. The crystallographic studies of carbohydrate-calcium complexes reveal that carbohydrates can chelate calcium ions in several ways. Uncharged carbohydrates commonly use pairs of adjacent hydroxyl groups to chelate calcium ions. Carbohydrates that contain carboxylate groups can also chelate calcium ion by use of a single car-boxylate-oxygen atom combined with hydroxyl or ring oxygen atoms.;: In calcium-carbohydrate complexes, Ca-O distances range between 2.3 and 2.6 Ao, and only sevenfold or eightfold coordination polyhedra are found. "Metal Ions Biological Systems" edt Helmut Sigel, Vol.17 (1984) Marul Dekker Inc., New York, Basel.
With the; advent of membrane cell technology for preparation of alkali and chlorine, the emphasis on
requirement , of high purity salt has increased. In fact, the membrane cell should receive brine in cell compartment bearing metalic ion impurity reduced to ppb level. Most of methods are being employed including chemical treatment followed by ion exchange purification of brine to desired level of purity. In the course of brine preparation, if the raw salt is used having minimum impurity, chemical dosing and subsequent load on ionexchanger is reduced to great extent resulting in improved brine treatment economy. Further, Ca++ is difficult to get rid off by simple washing method due to lower solubility of calcium compound and occlusion of .Ca.+ + impurity during salt crystallization. Against this Mg++ impurities are easy to remove on simple washing.
In view of the above facts, the research work was carried out to minimise the Ca++ impurity in salt during crystallization in situ.
The main object of the present invention is to provide a process for the preparation of high purity common salt HNaCl) from sea brine having low calcium sulphate content using polysaccharide as an additive during crystallization process.
Crystallization of sodium chloride from brine by solar evaporation is a well established industrial
process. The sea brine is fed to a large compartment,
called reservoir and then led to compartments called
condenser and finally to crystallizers on progressive
evaporation. ;> The sea brine has normally a density of
o ' '
3-3.5 Be' when taken to reservoir and concentrated upto
o 8-10 Be1 therein. Most of the carbonate and iron
compounds are separated there. The brine is taken to
condenser where it is concentrated upto 25.5 Be In
the condenser, most of CaSO separates out. The brine
is now saturated with NaCl and fed to crystallizers,
v;here sodium chloride crystallizes till brine attains
density 29 Be'. The mother liquor at 29 Be' conventionally called bittern is discharged for recovery of bromine, potash and magnesium chemicals.
A common objectionable impurity in the crystalline salt obtained by evaporation of brine is calcium sulphate which crystallizes out from the brine alongwith sodium chloride. It is present as an impurity in the sodium chloride produced by the evaporation, both, as an inclusion within the sodium chloride, crystal, and as an external admixture with the crystal. The external portion of Ga++ impurity could be removed to some extent by washing depending,on its solubility but the occluded crystals of calcium sulphate cannot be removed by any such simple method.
In accordance with the present invention, there is
provided a method of crystallizing sodium chloride out
o of saturated: brine of about 25.5 Be', obtained on solar
o evaporation of sea brine of 3- 3.5 Be'. The said brine
contains calcium sulphate impurity. The brine is
treated with;'additive slurry of required concentration
before introducing the said brine into the crystalliz-
er. The sodium chloride obtained on evaporation of
the treated *brine is containing unusually low calcium
The addit!ive of this invention is a polysaccharide
starch such .as made out of maize, potato, tapioca or
any other natural product containing starch. The
polysaccharide can be added to saturated brine
(25.5 Be1) by making its slurry; in water in adequate
concentration and proportion prior to the introduction
of the said brine, to the salt crystallizers. The
effective minimum quantity of the additive can be
determined by the analysis of the final product salt.
However, there will be no additional advantage in going
beyond a certain optimum quantity, which depends on
effectiveness, economics etc.
Accordingly the prese: invention provides an improved process for the recovery of sodium chloride containing low Ca++ impurity from sea' brine which comprises introducing sea brine into series of
concentrating ponds, conventionally called reservoir
o & condenser till brine density , reaches 25.5 Be'
adding to ;the said brine to a polysaccharide in
concentration in the range of 50-150 ppm to obtain the
brine strach mixture, evaporating the said brine
starch mixture in a crystallizer pond to get crystals
of sodium chloride containing Ca++ impurity in the:
range of 0.07-0.1 percent.
The starch granule is composed of linear and branched starch molecules associated by hydrogen bonding either directly or through water-hydrate bridges to form radially; oriented micelles or crystalline areas of various degrees of order. An interconnected three dimensional micellar lattice is formed by the participation of segments of individual molecules in several micellar areas. The overall strength of the micellar network (which is itself dependent on the degree of association and the molecular arrangement) controls the behaviour of.; starch in water. For activating the starch molecules thermal gelatinization is one of the simple method. If an aqueous suspension of starch is heated the granules change in appearance when critical temperature is reached. This gelatinous active form of starch is employed in the present work.
It will be understood that this explanation while
plausible, does not fully explain the phenomena occur
ring and that in any case, the present invention is not
predicated .on any particular theory explaining these
It is indicated that where the product collected is crystalline sodium chloride the salt crystals obtained by the process of this invention have a much reduced occluded calcium content namely generally 0.07 to 0.1 percent.
The following examples will further illustrate the practice of *this invention but it will be understood that the invention is not limited to the particular embodiments disclosed.
5 L of 251.8 Be1 saturated sea brine was taken. 0.5
g of good quality of starch made from tapioca was
taken in 5 ml hot water stirred well and made reactive
by adding the paste to boiling water and cooking it
till gel is formed.' It was:mixed with sea brine and
was allowed to solar evaporate. At 2 9oBe' salt fraction
was separated and allowed to drain off adherent mother
liquor for 48;hours. Salt was analysed for calcium to
o 0.04 percent; A blank experiment with 25.8 Be' sea
brine was carried out in identical condition except
hat of adding the chemical. Salt sample of this blank experiment was.analysed to 0.14 percent of calcium.
A precrystallizer was chosen having the dimension
18m x 10m x 0.3 0m for starch treatment experiment. The
o precrystallizer was filled with 25.3 Be' sea brine
which contained 0.36 Ca++ and 13.5 Mg++ gram per litre alongwith other marine salts in"their standard proportion. The depth of the brine was kept 22.5 centimeters .
The uniform paste of 4.0 kg soluble starch was prepared in 10 L of water and was added slowly into 110• L of hot water. The temperature was maintained for about 2 0 minutes to convert the whole mass into gel form. This reactive starch solution was evenly sprayed in the saturated sea brine and mixed with wooden rakers. The processed brine was filled in the adjacent crystallizer of the same size. Such five doses of the starch treatment were given in series to the same crystallizer. After this treatment heaps of salt were prepared and; three samples collected from different beaps were analysed which showed 0.068, 0.071 and 0.072 percent of calcium as Ca++. Blank experiment in the identical condition showed 0.16 percent of calcium
Preparation of reactive starch : 25 Kg of soluble starch made from tapioca was taken and thick paste was prepaired in about 60 L of water. This starch paste was slowly added into 24 0 L boiling water and allowed to cook for about half an hour when it was completely converted into gel form.
Solar salt crystallizer chosen for experimentation
was having the size of 38.5m x 3Sm x 0.30m. The bottom
of the crystallizer was made impervious. The crystal-
o lizer was filled with 25.5 Be' density which normally
contains calcium as Ca++ between 0.03 to 0.04 gram/100 ml. The depth of the brine was measured to 16.7 cms. To this brine in crystallizer., the reactive starch gel as mentioned above was sprayed through out the crystallizer uniformly. The intimate mixing was achieved by raking manually the brine with wooden rakers. The
crystallizer was then expo'sed for solar evaporation
o till the brine attained 29 Be' density. Again the same
quantity (16.7cms) 25.5 Be' was filled and dose of same
quantity of reactive starch, gel was employed without
removing salt bed of prr .s charge. Thus three
consequetive charges of starch were carried out. After
attaining 29 Be' density of the brine after third
charge mother liquor was drained off. Crystallizer
were charged with starch treated 25.5 Be' brine before heaping the" salt in crystallizer. Salt heaps of the crystallized,, salt were prepared. The samples collected from each heap were analysed. Average salt sample contained calcium content between 0.06 to 0.07 percent. Blank experiment in the identical condition showed 0.15 percent of calcium as Ca++.
The field' experiments were conducted in the precrystallizer having the size 60m x 50m x 0.30m.
Preparation of reactive starch : 50 Kg of the soluble starch was mixed with 100 L of water to make uniform slurry. The slurry was then slowly fed into 400 litres of nearly boiling water and maintained the temperature till all starch was converted into reactive gel formation-:
Concentrated sea brine of 25.5 to 26 Be density
was filled in a precrystallizer pan of 60m x 50m x
0.3 0m size. The depth of brine was kept 16.0 cms. The
reactive slurry was evenly distributed in the brine by
vanual labour. Raking was accomplished using wooden
rakers manually. On the next day the treated brine was
transferred through channels into the salt crystalliz-
ers After three days same quantity of fresh brine of
o 25.5 Be' was filled in the precrystallizer and dose of
reactive starch gel made from 50 Kg starch was applied
and the next day brine was transferred into the same
salt crystallizer. Thus three consecutive doses of
starch was given and brine was allowed to concentrate
o " o till it reached 29 Be'. This 29 Be' brine was then
drained off and salt heaps were prepared after filling
o freshly starch gel treated 25.5 Be' sea brine. Twelve
heaps were prepared and samples from each one were analysed for calcium content. The range of calcium in salt as Ca++ was between 0.03 to 0.06 percent giving the average value of 0.042 percent. The similar salt sample from the blank experiment contained 0.145 percent calcium as Ca++.
We Claim :
1. An improved process for the recovery of sodium
chloride containing low Ca++ impurity from sea brine
which comprises introducing sea brine into series
of concentrating ponds, conventionally called
reservoir & condenser till brine density reaches
o 25.5 Be' adding to the said brine to a polysaccharide in concentration in the range of 50-150 ppm to olitain" the brine strach mixture, evaporating the said brine starch' mixture in a crystallizer pond to get crystals of sodium chloride containing Ca++ impurity in the range of 0.07-0.1 percent.
2. An improved process as claimed in Claim 1 wherein
the polysaccharide used is starch which is preferably
obtained from natural sources maize, potato, tapioca.
3. An improved process for the recovery of sodium
chloride containing low Ca++ impurity from sea brine
substantially as herein described with reference to
|Indian Patent Application Number||315/DEL/1995|
|PG Journal Number||3/2004|
|Date of Filing||24-Feb-1995|
|Name of Patentee||COUNCIL OF SCIENTIFIC AND INDUSTRIAL RESEARCH|
|Applicant Address||RAFI MARG, NEW DELHI-110001, INDIA.|
|PCT International Classification Number||C01D 3/04|
|PCT International Application Number||N/A|
|PCT International Filing date|