INTRODUCTION:

The production of common salt is one of the most antiquated and generally dispersed industries in the world. The method of solar evaporation of brine is same around the world, but the fabricating strategy and product quality shift impressively.¹Evaporation is the chief operation included within the concentration of weak brine to a stage at which it is saturated with the salts dissolved in it resulting in the separation.² India is the third biggest salt producing nation within the world after China and USA.

The amount of 5960.98 tons are traded to other nations and 110.05 lakh tons are expended by Indian industries and the remaining being utilized for human consumption.³ Tamilnadu is the second largest producer of solar salt in India following to Gujarat. In Tuticorin district constitutes 75% of the total salt generation of Tamilnadu and 30% that of India. The Kanyakumari district lying within the southernmost portion of India, which has produces a considerable amount of salt.

Chemistry of solar salt manufacture:

The evaporation process of solar salt is appropriately classified into four distinct phases. The first phase (reservoir) is from 3-13°Be, when most of the carbonates precipitate as their salts of iron, magnesium and calcium. The second phase (condenser) extending from 13°- 25.4°Be, Centres around gypsum and it crystallizes as needle shaped crystals of CaSO₄.2H₂O from 13°Be to 16°Be and thereafter the anhydrous calcium sulphate. The third phase (crystallizer) extends between 25.4 and 30°Be and sodium chloride predominately precipitates out in this phase. At higher levels of salinity, the crystallization slows down considerably and is complete only with the completion of evaporation.⁴ At the end of this phase, when the concentration of brine reaches above 30°Be, the liquor is called bittern (fourth phase), because of its characteristic bitter taste.⁵

STUDY AREA:

Kanyakumari District (Latitude 8.0883°N and Longitude 77.5385°E), Tamilnadu, India.

Puthalam Situated 3 km away from Manakudy sea coast and is 13 Km away from Cape Comorin in the western region. The total area of the salt- pan is 138.68 ha. This salt-pan normally uses subsoil brine for salt manufacture.

MATERIAL AND METHODS:

Brine samples were collected monthly for a period of one year from different stages viz, source, reservoir, condenser, crystallizer and bittern from the subsoil salt-pan of puthalm in Kanyakumari district. The collected samples were subjected to chemical analysis of chloride, sulphate, calcium, magnesium, sodium and potassium by standard procedure.⁶ Sodium and potassium were estimated flame photometrically, calcium and magnesium ion concentrations were determined by complexometric titrations. Similarly, chloride ion by Argentometry titration and sulphate by precipitation method. For the determination of Ca, Mg, Cl and SO₄, the brine samples were diluted 10 times i.e, 25ml of the brine samples were diluted to 250ml with distilled water and kept as stock solution. The rainfall data were obtained from the meteorological department which is correlated with chemical parameters.

RESULT AND DISCUSSION:

Percentage of Chloride:

The percentage of chloride at various stages in the salt-pans were maximum in the months of January, February and March and also in between the monsoon periods. The minimum value of chloride was recorded in October due to the dilution of brine by rainfall. The percentage of chloride gradually increased from source to bittern. In spite of sodium chloride being isolated in the crystallizer phase, it was interesting to note that there was no reduction in the chloride content at the bittern stage as there is good content of chlorides of magnesium and potassium salts at this final stage. The average value was 3.116% for the sources and the reservoir stage had an average value of 5.266%. The average value of the percentage of chloride for the condenser, crystalizer and bitterns are 10.075%, 15.437% and 27.542%. In all the stages, the percentage of chloride was maximum in bittern. (Table-1)

Percentage of sulphate:

The gradual increase in the percentage of sulphate from source to bittern was observed. This was in accordance with earlier works regarding the production of sulphur from brines.⁷ Though calcium sulphate is removed in the primary stage i.e, before the condenser level, the continuing increase is due to the remaining gypsum and salts like sulphates of magnesium and potassium which got separated only after 30° Be. The values were maximum in January, February, March and also between the monsoon periods of June and August. But as northeast monsoon became active, the values went on decreasing from September to October and minimum values were observed in October.⁸ The percentage of sulphate was maximum in bittern (2.684%) and minimum in source ie, 0.360%. (Table-1)

Percentage of Calcium:

The percentage of calcium was almost similar at various stages of this salt-pan. It was found that the average value of calcium increased from source to reservoir and the maximum amount of calcium sulphate got separated before the condenser stage and that the values went on decreasing from reservoir to bittern.

It was clearly evident that subsoil brine had lower concentration of calcium initially but once calcium sulphate got separated, the average value of calcium was almost similar for the remaining stages. (Table-1)

Percentage of Magnesium:

The percentage of magnesium gradually increased from source to the bittern stage. This was due to the fact that, during the course of evaporation, magnesium chloride and magnesium sulphate stay in the solution until the brine reaches 30° Be, i.e, the bittern stage.⁹

The average percentage of magnesium at the source, reservoir, condenser, crystallizer and bittern has 0.242%, 0.600%, 0.991%, 1.772% and 3.005%. (Table-2)

Percentage of Sodium:

As the percentage of sodium increased from source to crystallizer, the same value is decreased in the bittern stage due to maximum sodium chloride getting separated at 29.5° Be i.e., in crystallizer stage. Since maximum sodium chloride was separated out before reaching the density 30°Be, the value was minimum at bittern stage. The average ranged between 0.710% and 1.235%. (Table-2)

Percentage of Potassium:

Regarding potassium, the values gradually increased from source to bittern because the potassium salt was separated only after the 30°Be. The average value of potassium at source was 0.029%, reservoir, condenser, crystallizer and bittern has 0.037%, 0.045%, 0.057% and 0.072%. (Table-2)

Rainfall:

The southwest and the northeast monsoons being active in southern district of Kanyakumari district, they bring heavy rain from the periods of May to July and from September to December respectively. This salt-pan received the maximum rainfall almost throughout the year, ie, average is 83.3mm, particularly heavier during monsoon periods. During the month of May maximum rainfall at 260.5mm and the rainfall was minimum in pre-monsoon periods of January, February and March and so the salt manufacture was active during these months. (Table-3)

Table-1: Percentage of Chloride, Sulphate and Calcium

Ions	stages	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec	mean
Cl	1	3.5	3.5	3.4	3.2	3.0	3.1	3.1	3.2	3.0	2.6	2.8	3.0	3.11
	2	6.1	5.8	5.4	5.3	4.8	5.3	5.5	5.6	5.1	4.3	4.7	5.3	5.26
	3	10.8	10.7	10.1	9.9	9.3	10.4	10.3	10.5	9.9	9.3	10	9.7	10.0
	4	16.4	15.8	15.4	14.9	X	15.1	15.7	15.3	X	X	X	14.9	15.4
	5	28.3	27.9	27.6	27.2	X	27.3	27.9	X	X	X	X	26.6	27.5
SO₄	1	0.43	0.41	0.40	0.38	0.35	0.37	0.35	0.38	0.37	0.29	0.30	0.32	0.36
	2	1.20	1.13	0.97	0.86	0.71	0.91	0.85	0.93	0.88	0.58	0.64	0.84	0.87
	3	1.74	1.67	1.33	1.22	1.09	1.24	1.06	1.23	1.18	0.74	0.88	1.09	1.20
	4	2.66	2.59	2.28	2.44	X	2.17	2.02	1.91	X	X	X	1.68	2.21
	5	3.08	3.04	2.97	2.48	X	2.69	2.47	X	X	X	X	2.06	2.68
Ca	1	0.05	0.05	0.04	0.03	0.03	0.039	0.04	0.04	0.03	0.02	0.02	0.03	0.03
	2	0.13	0.12	0.12	0.08	0.07	0.083	0.09	0.10	0.05	0.04	0.04	0.08	0.08
	3	0.08	0.08	0.07	0.06	0.05	0.067	0.08	0.09	0.04	0.01	0.03	0.07	0.06
	4	0.06	0.06	0.06	0.05	X	0.059	0.06	0.07	X	X	X	0.05	0.06
	5	0.05	0.04	0.04	0.04	X	0.053	0.05	X	X	X	X	0.03	0.04

Stages: 1.Source 2.Reservoir 3.Condenser 4.Crystallizer 5.Bittern

X: Samples not available due to rain

Table-2: Percentage of Magnesium, Sodium and Potassium

Ions	stages	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec	mean
Mg	1	0.32	0.26	0.28	0.23	0.22	0.25	0.23	0.25	0.24	0.19	0.21	0.23	0.24
	2	0.77	0.63	0.71	0.57	0.54	0.68	0.61	0.62	0.50	0.42	0.51	0.65	0.60
	3	1.44	1.37	1.41	0.97	0.73	0.96	0.84	0.82	0.81	0.67	0.93	0.95	0.99
	4	2.04	1.95	2.01	1.74	X	1.54	1.51	1.72	X	X	X	1.67	1.77
	5	3.36	3.14	3.21	3.13	X	2.97	2.44	X	X	X	X	2.79	3.00
Na	1	0.89	0.79	0.73	0.66	0.64	0.72	0.69	0.74	0.68	0.61	0.63	0.74	0.71
	2	1.03	0.92	0.81	0.80	0.80	0.93	0.88	0.93	0.83	0.72	0.77	0.92	0.86
	3	1.28	1.08	0.99	0.96	0.91	1.01	0.94	1.09	0.99	0.82	0.92	1.01	1.00
	4	1.52	1.31	1.19	1.04	X	1.21	1.07	1.32	X	X	X	1.22	1.23
	5	0.84	0.81	0.77	0.72	X	0.75	0.69	X	X	X	X	0.76	0.76
K	1	0.032	0.032	0.030	0.029	0.029	0.031	0.030	0.030	0.028	0.027	0.027	0.028	0.02
	2	0.044	0.040	0.038	0.039	0.036	0.040	0.037	0.036	0.037	0.033	0.035	0.038	0.03
	3	0.052	0.049	0.047	0.044	0.043	0.050	0.048	0.041	0.043	0.039	0.041	0.044	0.04
	4	0.062	0.061	0.058	0.059	X	0.056	0.055	0.047	X	X	X	0.059	0.05
	5	0.080	0.079	0.071	0.070	X	0.068	0.069	X	X	X	X	0.068	0.07

Stages: 1.Source 2.Reservoir 3.Condenser 4.Crystallizer 5.Bittern

X: Samples not available due to rain

Table-3: Total Rainfall (mm)

Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec	mean
6.2	8.7	6.9	26.2	260.5	29.3	29.3	63.6	73.7	233.1	214.1	47.5	83.3

CONCLUSION:

The chemical parameters of brine at various stages were found and that all the parameters are maximum in the pre-monsoon period of January to March and they were minimum in the monsoon period. From the above values, it was clear that the amount of chemicals present at various stages of the different salt- pans were found to be different. The above information gives the idea to the salt-makers, academician and researchers for the management of salt-pans.

REFERENCES:

1. United Nations Industrial Development Organisation, Procedure of Expert group meeting, Rome 25-29, 1968.

2. Singaravelu S., Training Course in Salt Technology, Salt Department, Government of India, 37, 4-22 August 1981.

3. Annual Report, Salt Commission of India, Salt Department, Ministry of Commerce and Industry, Department of Industrial policy and Promotion, Government of India, 2014-2015.

4. Sorgeloos P and Tackaert W., Proceedings of the International Symposium on Biotechnology of Salt Pond, Tang Gu, PR China, 13-21 September 1990.

5. Krumbein W.E., Philip G., Tager A.G., Waheb S.A and Wale A.M., Evaporitic Sedimentation and Microbial mats in a Salina System (Port Fouad, Egypt), International Journal of Salt lake Research, 4, 95-116 .1995.

6. Vogel A.I., A Text book of Quantitative Inorganic Analysis, ELBS, London 1978.

7. Hutchington G.F., A Treatise in Limnology, John Wiley and Sons, New york, 124 .1975.

8. Jensen J.H and Nylander A.F, Magnesium chloride from naturally occurring brines and evaporates, Mining Engineering, 16(11), 64-66 .1964. Hadzeriga P, Trans.AMIE, 229, 169 .1965.

Received on 29.08.2021 Modified on 12.09.2021

Asian J. Research Chem. 2021; 14(6):421-424.

DOI: 10.52711/0974-4150.2021.00072