Recovery and the quality of Gypsum from Natural and Artificial Pans of Various Salt-Ponds of Kanyakumari and Tuticorin Districts, Tamilnadu, India.
A. Kumaresan*, C. Vaithyanathan
Department of Chemistry, S.T. Hindu College, Nagercoil – 629002, Kanyakumari District, Tamilnadu, India.
*Corresponding Author E-mail: naga.susa83@gmail.com
ABSTRACT:
The story of the salt is the story of mankind. Salt has played a vital role in the development of man’s activities, trade, politics, culture and world’s economy from pre-historic time. The most abundant source of common salt (NaCl) is sea water, in view of that 71% of earth surface is covered with sea water, which contains 2.5% common salt. Next to sea water the concentration of sodium chloride is high in subsoil brine and backwater brine. The production of common salt is mainly by solar evaporation of sea brine, subsoil and backwater brines. Thus the technique of solar salt production involves fractional crystallization of the salts in different brines. While the process for solar evaporation of brines is same around the world, but the product quality vary considerably. During the production of salt, the valuable by-product of gypsum (CaSO4.2H2O) is also obtained. The sea water contains about 3.6% CaSO4, which is in-exhaustible and can be renewable. The recovery of gypsum from different brines is substantial and its comparable with the production of commercially available mineral gypsum in market. The most influential parameters potentially affecting the crystallization process of gypsum were water, salinity, temperature, type and amount of dissolved organic compounds, PH and organic waste products from microorganisms. The quality of gypsum from natural and artificial pans in the salt ponds of Kanyakumari and Tuticorin districts were taken in to consideration.
KEYWORDS: Gypsum, Reservoir, Condenser, Crystallizer, Bittern.
Sea water contains more than 60 elements including gold and uranium. The predominant ions are chloride, sodium, magnesium, calcium, potassium, sulphate and bromine. Around 30, 000 acres of land in Tamilnadu are being utilized in the manufacture of salt and also various by-products like gypsum, calcium oxide etc. Gypsum and salt production depend on the factors like absorption of solar energy, air humidity, temperature and wind velocity. Gypsum is a hydrated calcium sulphate (CaSO4.2H2O) with a composition of 79% CaSO4 and 21% water (pure gypsum).
The quality of gypsum is determined from the % of CaSO4 and also % of combined water.
Study Area:
The Kanyakumari district is in the southern tip of India. Here the salt pans are located in Puthalam and Swamithoppu. In Puthalam, subsoil brine is used and in Swamithoppu, backwater brine is utilized.
Tuticorin district is located on southeast of Tamilnadu. The salt pan at Tuticorin uses sea brine for salt production.
Chemistry of salt pans:
The evaporation process of brines is conveniently divided in to four distinct phases. Reservoir is the first phase and its density ranges from 3 – 13 degree Baume (Be), when iron, magnesium and calcium precipitate as their carbonates. Carbonates of iron and magnesium crystallizes completely by 13°Be, but CaCO3 crystallizes till 90% and the remaining 10% precipitates at 15°Be ie, in the next stage. The second phase (Condenser) extending from 13°– 25.4° Be which centers around gypsum. 85% of the gypsum is precipitated in this phase. The crystallizer (Third phase) extends between 25.4 to 30°. Be in which sodium chloride precipitates out in this phase. At the end of this phase, when the brine density is around 30° Be, the liquor is called bittern because of its characteristic bitter taste.
MATERIALS AND METHODS:
Ten litres of samples were collected separately from three different sources for the study of artificial evaporation. The samples were exposed to sunlight in plastic buckets. The reduction in volume and the increasing brine density were monitored daily. The density of the brine was measured by Baume hydrometer. The rate of artificial evaporation was presented in table 1.
During the course of evaporation, gypsum was crystallized between 16° Be to 25° Be. The formed crystals were collected, dried and weighed separately and the quantity was presented in table II.
The collected gypsum from various brines through artificial evaporation was correlated with the gypsum from natural brines and also with commercially available gypsum samples availed from the market through chemical analysis. The percentage of CaSO4.n H20 (Hydrated calcium sulphate), the percentage of combined water and the percentage of free water were determined by standard procedures. The percentage of combined water analysis is used to determine the percent of chemically combined water in gypsum which inturn is used to calculate the purity of gypsum.. The free water analysis determines the amount of free water contained in the sample as opposed to chemically combined water. The percentage of hydrated calcium sulphate, combined water and free water was presented in table III.
RESULTS AND DISCUSSION:
Evaporation proceeded with a gradual decrease in the volume of the brines exposed to sunlight. In our study 10 litres of sea brine was reduced to just 220 ml at 30 0 Be. Similarly, 10 litres of the subsoil brine was reduced to 360 ml and the same quantity of backwater brine was reduced to 340 ml at 30° Be.
Table I : Rate of artificial evaporation
|
Sea Brine |
Subsoil Brine |
Backwater Brine |
|||
|
Volume (litres) |
Brine Density (°Be) |
Volume (litres) |
Brine Density (°Be) |
Volume (litres) |
Brine Density (°Be) |
|
10.0 |
5.0 |
10.0 |
5.0 |
10.0 |
5.5 |
|
9.070 |
5.5 |
9.100 |
5.5 |
9.120 |
6.5 |
|
7.970 |
6.0 |
8.050 |
6.0 |
8.070 |
7.0 |
|
6.970 |
6.5 |
7.090 |
7.0 |
7.095 |
8.0 |
|
5.810 |
8.0 |
6.026 |
8.0 |
6.044 |
9.25 |
|
4.560 |
10.0 |
4.900 |
9.75 |
4.930 |
11.5 |
|
3.590 |
12.25 |
4.060 |
11.5 |
4.080 |
13.5 |
|
3.085 |
14.0 |
3.630 |
13.5 |
3.280 |
16.0 |
|
2.630 |
16.5 |
3.220 |
14.5 |
2.985 |
18.5 |
|
2.410 |
17.5 |
2.740 |
16.0 |
2.510 |
20.0 |
|
2.120 |
19.0 |
2.405 |
18.0 |
2.305 |
21.5 |
|
1.925 |
20.5 |
1.890 |
21.5 |
2.025 |
22.5 |
|
1.715 |
21.5 |
1.580 |
24.0 |
1.890 |
24.5 |
|
1.230 |
23.5 |
0.978 |
26.5 |
1.730 |
26.0 |
|
1.040 |
24.5 |
0.645 |
28.5 |
0.945 |
27.5 |
|
0.835 |
26.5 |
0.360 |
30.0 |
0.972 |
29.5 |
|
0.725 |
27.5 |
|
|
0.340 |
30.0 |
|
0.580 |
28.5 |
|
|
|
|
|
0.220 |
30.0 |
|
|
|
|
Table II : Yield of gypsum and Sodium chloride from artificial pans.
|
Source |
Volume of sample (litres) |
Weight of gypsum (gms) |
Weight of NaCl (gms) |
|
Sea brine |
10 |
18.51 |
450.04 |
|
Subsoil brine |
10 |
16.20 |
401.73 |
|
Backwater brine |
10 |
15.16 |
420.66 |
Table III: Analysis of Gypsum:
|
Natural Evaporation |
Artificial Evaporation |
Samples availed from market |
||||||
|
Parameters |
Sea brine |
Subsoil brine |
Backwater brine |
Sea brine |
Subsoil brine |
Backwater brine |
Low grade gypsum |
High grade gypsum |
|
% of CaSO4.nH2O |
91.632 |
86.107 |
82.028 |
94.265 |
89.931 |
87.611 |
83.527 |
96.53 |
|
% of Combined water |
16.1 |
15.3 |
13.15 |
14.2 |
13.4 |
12.3 |
11.64 |
14.36 |
|
% of CaSO4 (% of CaSO4.n H2O - % of combined water) |
75.532 |
70.807 |
68.878 |
80.065 |
76.531 |
75.311 |
71.887 |
82.17 |
|
% of free water |
1.01 |
1.03 |
3.04 |
1.01 |
1.01 |
3.00 |
0.85 |
0.37 |
In the beginning, the rate of evaporation of sea brine was higher than subsoil and backwater brine. But as the brine density increased, the rate of evaporation was maximum in subsoil brine, moderate in backwater brine and lowest in sea brine, because of the fact that the concentration of different ions in sea brine was found to be maximum when compared to the other two different brines.
% of Hydrated calcium sulphate (CaSO4.n H2O) :
In natural evaporation, marine gypsum (seabrine) contains 91.632 % of hydrated calcium sulphate and subsoil gypsum has 86.107 % and backwater gypsum contains 82.028 %. But in artificial evaporation sea brine gypsum has 94.265 %, While that of the subsoil sample contains 89.931 % and backwater gypsum has 87.611 % of hydrated calcium sulphate. commercially available low grade gypsum in market has 83.527 % and high grade gypsum is 96.53 %. The reason for the high value of % of hydrated calcium sulphate in the case of artificial pans may be due to high temperature, low wind velocity, less surface area , and less humidity etc.
% of combined water:
The combined water content in marine gypsum from natural evaporation is 16.1 %, subsoil is 15.3 % and 13.15 % in backwater. In artificial evaporation, sea brine gypsum contains 14.2% of combined water, 13.4 % in subsoil gypsum, and 12.3% in backwater gypsum. This indicated that the moisture content was found to be higher in natural pans, which give the higher percentage of combined water than artificial pans. In market low grade gypsum, it was 11.64% and high grade gypsum contains 14.36% of combined water. It showed that higher the % of CaSO4 in gypsum the higher % of water combined.
% of Calcium sulphate:
In natural evaporation, sea brine gypsum has 75.532 %, subsoil gypsum has 70.807% and 68.878 % was in backwater gypsum. In artificial evaporation 80.065 % of calcium sulphate was in sea brine gypsum, 76.531 % in subsoil gypsum and 75.311 % in backwater gypsum. This marginal higher percentage of CaSO4 in sea brine gypsum (natural and artificial) indicated that the brine had comparatively more amount of calcium sulphate than subsoil and backwater.
In market low grade gypsum, it was 71.887 % and high grade gypsum it was 82.17 %. As sea brine gypsum has more % of calcium sulphate than subsoil and back water gypsum samples, these can be applied for the manufacture of surgical plaster, cement manufacture etc.
% of free water:
The free water content in sea brine, subsoil and backwater brine gypsum were almost similar in the samples of both natural and artificial pans. The low grade gypsum in market had 0.85% and high grade gypsum had 0.37% free water in the samples.
CONCLUSION:
The data suggests that the quality of gypsum from sea brine (marine gypsum) in natural and artificial pans were of remarkable high quality than subsoil and backwater gypsum. The percentage of calcium sulphate in sea brine is almost the same to that of market gypsum. It was very clear from our finding that sea brine gypsum is of better quality when compared to subsoil and backwater samples of gypsum
ACKNOWLEDGEMENT:
The authors are thankful to the Principal and Management of S.T. Hindu College Nagercoil, TN, India.
REFERENCES:
1. R. Prakash, Training course in salt Technology, Salt Department, Govt. of. India, 12, 4-22 August, 1981
2. Indian Minerals Year Book 2013 (Part-III; Mineral Reviews), Ministry of Mines, Govt. of India.
3. The Chemistry and Technology of Gypsum, ASTM STP 861 ASTM, 1984, PP 22 +_47
4. Gypsum Fact sheet.docx. Incitec pivot Ltd.
5. Ullmann's Encyclopedia of Industrial chemistry, 1995, Vol A25, Sulphur, p.563
6. M.L. John, Mineral Resources of the Sea, Elsevier Publications, Amsterdam, Holland, 242, 1964.`
Received on 10.04.2016 Modified on 23.04.2016
Accepted on 17.05.2016 © AJRC All right reserved
Asian J. Research Chem. 9(4): April, 2016; Page 185-187
DOI: 10.5958/0974-4150.2016.00030.4