The sub-soil brine was analysed in order to establish its density and ionic composition. The chemical analysis is provided in Table 1 and Table 2.
Table 2
Probable composition of salts dissolved Didwana Lake brine
Na2SO4 (%w/v)
|
MgCl2 (%w/v)
|
KCl (%w/v)
|
NaCl (%w/v)
|
Na2CO3 (%w/v)
|
NaHCO3 (%w/v)
|
5.17
|
0.0014
|
0.07
|
14.38
|
0.32
|
0.84
|
It has been observed that lake brine has higher sulphate content as compared to the sea brine at similar density. Table 1 shows, that 1 L of lake brine contains 34 g of SO42− at 18.2oBé [oBé =145×[1-(1/SG)] whereas sea brine analysis shows 13.5 g sulphate at a similar density, with a probable concentration of nearly 5 (w/v) % of Na2SO4 along with 14 (w/v) % of NaCl (Table 2). Lake brine has also very low content of Mg2+ and Ca2+ as compared to the seawater. The common salt has been crystalized from this brine via solar evaporation method. The composition of salt crystallized is given in Table 3. As can be seen from Table 3, the salt is highly contaminated with Na2SO4 (23.46 % w/w) which is very difficult to wash, and not suitable either for edible or industrial application.
Table 3
Chemical analysis of salt obtained Didwana Lake brine
CaSO4
(%w/w)
|
Na2SO4
(%w/w)
|
MgCl2
(%w/w)
|
NaCl
(%w/w)
|
KCl
(%w/w)
|
Na2CO3
(%w/w)
|
NaHCO3
(%w/w)
|
0.03
|
23.46
|
0.09
|
74.05
|
0.05
|
2.08
|
0.24
|
The dolomite rocks of have CaO and MgO in the range of 26-44% and 16-22% respectively. Therefore, MDP generated while processing such rocks was reacted with HCl to generate water-soluble inorganic calcium or magnesium based salts solutions and insolubles. In a typical experiment, 321 g of marble powder was dissolved 535 ml HCl (11 N) and kept overnight for preparation of slurry/solution. Insolubles residue (59 g) was filtered and salt slurry (542 ml) thus obtained was analysed for chemical composition (Table 4, Table 5).
Table 4 Chemical analysis of salt slurry obtained by mixing of MDP and HCl
Ca2+(%w/v)
|
Mg2+ (%w/v)
|
SO42− (%w/v)
|
10.39
|
6.16
|
0.099
|
Table 5 Probable composition of salts in the slurry obtained by mixing of MDP and HCl
CaCl2 (%w/v)
|
MgCl2 (%w/v)
|
CaSO4 (%w/v)
|
acid insoluble (%w/v)
|
28.72
|
24.09
|
0.14
|
18.38
|
Therefore, the filtered slurry (225 ml) was used as economic additives in lake brine (1500 ml brine) to make a stoichiometric proportion of Ca2+ to SO42−. Composition of the resulting brine is given in Table 6.
Table 6 Ionic composition of lake brine after addition of slurry obtained by mixing of MDP and HCl
oBé
|
Ca2+ (%w/v)
|
Mg2+ (%w/v)
|
SO42− (%w/v)
|
Cl− (%w/v)
|
K+ (%w/v)
|
18.8
|
0.19
|
0.78
|
0.69
|
9.61
|
0.03
|
Resulting solution was subjected to solar evaporation to crystallize CaSO4·2H2O and improve quality and yield of NaCl. CaSO4·2H2O fraction was crystallized between 18.8 to 24.0oBé, and analysed for chemical composition and purity.
Table 7 Ionic and probable composition of CaSO4·2H2O fraction collected between 18.8 - 24.0oBé
Ionic Composition
|
oBé
|
Ca2+
(%w/w)
|
Mg2+
(%w/w)
|
SO42−
(%w/w)
|
Cl−
(%w/w)
|
K+
(% w/w)
|
18.8 - 24.0
|
19.80
|
0.61
|
48.86
|
2.42
|
Nil
|
Probable Composition
|
oBé
|
CaSO4.2H2O
(%w/w)
|
MgSO4 (%w/w)
|
MgCl2
(%w/w)
|
KCl
(%w/w)
|
NaCl
(%w/w)
|
18.8 - 24.0
|
85.14
|
1.68
|
1.06
|
Nil
|
2.69
|
As can be seen from Table 7, CaSO4·2H2O with a purity of higher than 85 (%w/w) could be directly precipitated and purity was improved up to 96 (%w/w) by washing the soluble impurities with a weak brine. Purity of gypsum crystals were examined using P-XRD and FTIR (Figure 1left, right). The XRD pattern of precipitated CaSO4·2H2O (Figure 1left) showed prominent peaks with d-spacing at 7.55, 4.26, 3.79, and 3.06 Å which are comparable with corresponding d-values of 7.52, 4.25, 3.76, and 3.05Å reported in literature (Follner et al., 2002). The vibrational spectral characteristics of the precipitated gypsum crystals are shown in Figure (1right). The absorption bands at 602, 669 and 1117 cm−1 correspond to the different modes of SO42−. Presence of H2O in gypsum is confirmed from the band around 1630 cm−1 and the broad band around 3400 cm−1 (Prasad et al., 2005).
Morphology of crystals was examined by FE-SEM (Figure 2). SEM images have shown that crystals have grown from soft spongy dumble or spherical shaped to tabular pseudo-hexagonal and rigid. EDX spectra shows some minor impurities of sodium chloride which can be easily washed away by simple washing
After CaSO4·2H2O recovery, brine was further concentrated up to 29oBé and pure NaCl was collected between 24 to 29 oBé. Analysis of NaCl (ionic and probable composition) crystallized between 24 to 29oBé is provided in Table 8 and 9.
Table 8 Ionic composition of NaCl fractions collected between 24.0-29.0oBé
oBe’
|
Ca2+
% (w/w)
|
Mg2+
% (w/w)
|
SO42−
% (w/w)
|
Cl−
% (w/w)
|
K+
% (w/w)
|
24.0 - 25.0
|
0.25
|
0.17
|
0.66
|
58.88
|
0.03
|
25.0 - 26.0
|
0.46
|
0.29
|
1.18
|
57.17
|
0.01
|
26.0 - 27.0
|
0.33
|
0.51
|
1.08
|
57.86
|
0.01
|
27.0 - 28.0
|
0.06
|
0.57
|
0.49
|
59.67
|
0.04
|
28.0 - 29.0
|
0.12
|
0.67
|
2.84
|
58.31
|
0.04
|
Table 9 Probable composition of NaCl fractions collected between 24.0-29.0oBé
oBé
|
CaSO4 (%w/w)
|
MgSO4 (% w/w)
|
MgCl2 (%w/w)
|
KCl (%w/w)
|
NaCl (%w/w)
|
24.0 - 25.0
|
0.87
|
0.08
|
0.60
|
0.05
|
98.39
|
25.0 - 26.0
|
1.62
|
0.12
|
1.09
|
0.02
|
97.15
|
26.0 - 27.0
|
1.19
|
0.36
|
1.81
|
0.03
|
96.61
|
27. 0 - 28.0
|
0.20
|
0.44
|
1.92
|
0.07
|
97.37
|
28.0 - 29.0
|
0.41
|
3.20
|
0.13
|
0.07
|
96.19
|
The purity of as such collected NaCl was always > 96 %w/w meeting specifications of edible grade salt, which could be further improved to > 98.5 %w/w by simple washing with water and meeting industrial grade salt specifications. Purity of washed salt has been ascertained from P-XRD pattern and FTIR spectra of unwashed and washed samples (Fig. 3 left, right).
Morphology of crystallized NaCl indicated growth of crystals from hollow to dense cubic structures (Figure 4). EDX spectra shows a very high purity of the crystallized salt. After separation of NaCl at 29oBé filtrate was recovered and analysed for Mg2+ content (Table 10).
Table 10 Ionic and probable composition of filtrate at 29.0oBé
Ionic Composition
|
oBé
|
Ca2+
(%w/v)
|
Mg2+
(%w/v)
|
SO42-
(%w/v)
|
Cl-
(%w/v)
|
K+
(% w/v)
|
29.0
|
0.03
|
5.57
|
2.30
|
20.99
|
0.23
|
Probable Composition
|
oBé
|
CaSO4
(%w/v)
|
MgSO4
(%w/v)
|
MgCl2
(%w/v)
|
KCl
(%w/v)
|
NaCl
(%w/v)
|
29.0
|
0.11
|
2.78
|
19.59
|
0.43
|
10.22
|
Filtrate contained approximately 5.6 (%w/v) of Mg2+, which is quite suitable to recover magnesium salts economically. From magnesium rich solutions, different types of magnesium carbonate hydrates can be synthesized by carefully adjusting the reaction temperature and pH value of the initial reaction solution in the precipitation process. Magnesium carbonate normally crystallizes in the di-, tri-, or pentahydrates (MgCO3·2H2O, MgCO3·3H2O or MgCO3·5H2O) which are colourless crystals having triclinic and monoclinic structures. Therefore, we used magnesium rich filtrate to crystallize magnesium carbonate hydrate. MgCO3·6H2O was prepared using a known quantity of Na2CO3 and was separated from sodium chloride solution. In a typical experiment, clear Na2CO3 solution was added to the dilute bittern in stoichiometric ratios of magnesium to carbonate. The precipitated slurry of hydrated MgCO3 was stirred and then allowed to stand. Slurry was put under filtration and washed with fresh water to make it free from chloride and sulphate. Wet cake was then dispersed in fresh water such that the slurry concentration is reduced. This dispersion was heated to 50–60oC and maintained at this temperature for 30 minutes to transform into dried basic magnesium carbonate. The complete integrated scheme of crystallization of gypsum, sodium chloride and magnesium carbonate is shown in Figure 5.
During investigations it has been found that a rare form of magnesium carbonate hexahydrate (MgCO3·6H2O) has been crystallized (Rincke et al., 2020). MgCO3·6H2O formed was analysed for chemical composition (Table 11), and characterized for its bulk density, moisture content, purity and morphology.
Table 11 Ionic composition of precipitated light basic MgCO3·6H2O
Sample details
|
Mg
(% w/w)
|
Ca
(% w/w)
|
Cl
(% w/w)
|
MgO
(% w/w)
|
CaO
(% w/w)
|
KCl
(% w/w)
|
MgCO3 unwashed
|
17.84
|
0.98
|
15.48
|
29.61
|
1.37
|
0.04
|
MgCO3
Washed
|
26.18
|
0.06
|
0.26
|
43.46
|
0.08
|
0.03
|
Based on the above analysis purity of washed MgCO3.6H2O was found to be > 98.5 % w/w. Bulk density of the washed sample was found to be 130.50 and 217.50 g/l for loose and pack respectively, meeting the criteria of light basic MgCO3 suitable for industrial applications.
The product was initially characterized using TGA in order to assess water of hydration (Figure 6). TGA profiles indicated small weight loss up to 110oC indicating loss of moisture and the process of decomposition starts at around 100oC and completes around 500oC. The thermograms indicated that the weight loss occurred in different stages, about 60 wt%. In first stage, up to 160oC around 14.9 wt% loss occurs, between 160 to 280oC approx. 12.4 wt% and between 280 to 500oC approx. 33.6 wt% loss occurs. After a calculation from the weight loss in different temperature stages of the TGA curve, the material obtained have a simple formula of MgCO3·6H2O. The purity of product was further characterized using PXRD.Figure 5 (left) shows the PXRD pattern of product phase which matches with the magnesium carbonate hexahydrate (MgCO3·6H2O) reported (Rincke et al., 2020) and complements the results obtained from TGA profiles. PXRD pattern of product phase MgCO3·6H2O in comparison with the reference data for MgCO3·3H2O (Nesquehonite, 98-016-9540) is provided in Figure 7 (right).
Figure 8 shows the IR spectra of crystallized MgCO3·6H2O. IR spectra is very similar to those of MgCO3·3H2O which is confirmed by the presence of ~850 cm-1 (υ2 mode), 1117 cm-1 (υ1 mode), 1485 and 1426 cm-1 (υ3 mode) CO32- adsorption bands (Farmer, 1974; White, 1971; Zhang et al., 2006). The CO32- asymmetric stretching vibrations are observed as a strong band split in two at 1485 and 1426 cm-1 cm–1 (υ3 mode) (Sawada et al., 1978). In addition, a faint band around 1645 cm-1 corresponding to the O-H bending mode of water. Broad bands in the range of 3650-3000 cm-1 corresponds to O-H of water molecules in the compound. These bands have much difference between them, which is likely originating from a different number of water of crystallization. A sharp band around 3650 cm-1 is assigned to the free O-H vibration (Zhang et al., 2006).
Morphology of crystals depends upon several conditions viz. composition of reaction mixture, reaction temperature, carbonation time, pH of the solution etc. Figure 9 provides a set of typical SEM images of a dried sample under the investigated conditions. Here, fine nano-sized thick plates like particles are produced which arrange into small rod like structures. Elemental analysis and EDX spectra shows a very high purity of the crystallized magnesium carbonate (Figure 9).
Techno economic analysis
Based on the common salt, magnesium carbonate and gypsum production production rates and chemical input requirements, techno economic feasibility of production of 1 ton/day capacity plant is evaluated (Table 12). The economic viability of salts produced has been assessed based on the following economic indicators: Plant establishment costs: capital expenditure (CAPEX); Operating expenditure (OPEX); Cost recovery from struvite production (revenue). Equipment sizes based on the mass production and cost estimation of the process equipment has been estimated as provided in Table 13. The viability calculations have been done based on pilot scale experiments.
Table 12 Basic engineering design of common salt, magnesium carbonate and gypsum production
Capacity of the Plant (TPA) – Common Salt
|
3540
|
Capacity of the Plant (TPA) - Magnesium Carbonate
|
300
|
Capacity of the Plant (TPA) - Gypsum
|
1166
|
Basis
|
2 shifts/day,
300 Working days per annum
|
Design Basis
|
10% Design Margin
|
Capital Cost (Rs. in lakhs)
|
110.95
|
Annual Sales Revenue (Rs. in lakhs)
|
194.36
|
Annual Profit Before Tax
|
28.83
|
Pay Back Period (Years)
|
3.83
|
Return on Capital Investment
|
22.19%
|
Break Even Point
|
39.36%
|
Assumptions:
|
|
Economic life of plant
|
10 Years
|
Construction Period
|
6 months
|
Capacity Utilisation
|
75% - 1st Year; 85% - 2nd Year;
100% - 3rd Year onwards
|
Table 13 Total capital investment (TCI)
Sr.No.
|
Description
|
Amount (Rs. in lakhs)
|
1
|
Civil Works
|
12.88
|
2
|
Plant & Equipment
|
86.74
|
3
|
Furniture & Fixtures
|
2.00
|
4
|
Laboratory & QC Equipment
|
5.00
|
|
Grand Total
|
110.95
|
Raw materials cost as estimated according to actual consumption from pilot experimental results and prevailing costs, other components of operating costs are presented in Table 14. The rates of raw material and utilities mentioned below are prevailing rates in market. Standard norms have been taken for depreciation, maintenance and repair cost which are applicable for chemical plant.
Table 14 Raw material and utilities consumption and production cost
Description
|
Rate, Rs. per unit
|
Consumption per MT of MgCO3
|
Total cost per annum
(Rs. In lakhs)
|
Raw Material
|
|
|
|
Hydrochloric Acid (M3)
|
1500
|
10
|
44.46
|
Marble dust (MT)
|
1000
|
6
|
17.79
|
Sub Soil Brine (M3)
|
10
|
65
|
1.96
|
Na2CO3 (MT)
|
25000
|
1
|
79.01
|
Total
|
|
|
143.23
|
Utilities (in Plant only)
|
|
|
|
Electricity (KWH)
|
7.5
|
132.97
|
2.99
|
Water
|
10
|
22
|
0.65
|
Total
|
|
|
3.64
|
Salaries & Wages
|
|
|
9.18
|
Depreciation
|
5% of Plant & Equipment, 10% of civil work
|
5.62
|
Maintenance & Repair
|
2.5% of Plant & Equipment, 1.5% of civil work
|
2.36
|
Packing Cost
|
|
1.50
|
Grand Total
|
|
165.53
|
Sales revenue
|
Rate (Rs. per unit)
|
Unit
|
|
Salt (Sodium Chloride)
|
500
|
3540 (TPA)
|
17.70
|
Magnesium Carbonate
|
55000
|
300 (TPA)
|
165.00
|
Gypsum
|
1000
|
1166 (TPA)
|
11.66
|
Total (Rs. in lakhs)
|
|
|
194.36
|
Profit before tax (Rs. In lakhs per annum)
|
28.83
|
Manpower requirements with their monthly and annual remunerations for running of plant are given below in Table 15.
Table 15 Manpower requirement
Description
|
No.
|
Monthly Salary (Rs.)
|
Months
|
Annual Salary (Rs.)
|
Supervisor
|
1
|
15000
|
12
|
180000
|
Plant Operator
|
1
|
12500
|
12
|
150000
|
Unskilled worker
|
3
|
10000
|
12
|
360000
|
|
|
Total
|
|
690000
|
Fringe Benefit @33%
|
|
|
|
227700
|
Total
|
917700
|
Return on capital investment costing has been done with the assumptions that the economic life of equipment is ten years, two-shift per day basis and no bank borrowings for capital expenditures. The calculations are provided in Table 16.
Table 16 Return on capital investment
Profit before tax over 10 years’ period (Rs. in lakhs)
|
246.22
|
Average Annual Operating Surplus (Rs. in lakhs)
|
24.62
|
Capital Investment (Rs. in lakhs)
|
110.95
|
Return on Capital Investment
|
22.19%
|
Break even analysis has been done with 100% capacity utilization which is normally achieved during third year of operation. It has been assumed that capacity utilization during first and second year will be 75% and 80% respectively. The calculations are given in Table 17.
Table 17 Break even analysis (Based on 3rd year of operation L.E. 100% capacity utilization)
Items
|
Cost break up
|
Fixed
|
Variable
|
Raw Materials & Utilities
|
0.00
|
146.87
|
Packing materials
|
0.00
|
1.50
|
Salaries & Wages
|
10.12
|
0.00
|
Maintenance & Repairs
|
2.36
|
0.00
|
Depreciation
|
5.62
|
0.00
|
Total
|
18.10
|
148.37
|
Annual Sales Revenue (Rs. in lakhs)
|
=
|
194.36
|
Break Even Point
|
=
|
18.10 x 100
|
|
|
194.36
|
-
|
148.37
|
|
=
|
39.36%
|
|
|
Payback period was calculated based on six years of operation using net surplus and cumulative net surplus amounts and is provided in Table 18.
Table 18 Payback period
Year of operation
|
Net Surplus (Rs. in lakhs)
(i.e. Profit + Depreciation)
|
Cumulative Net Surplus
(Rs. in lakhs)
|
1
|
22.95
|
22.95
|
2
|
27.09
|
50.05
|
3
|
33.51
|
83.56
|
4
|
33.01
|
116.57
|
5
|
32.47
|
149.04
|
6
|
31.92
|
180.96
|
Capital Investment (Rs. Lakhs) =
|
110.95
|
|
|
|
|
|
|
110.95
|
-
|
116.57
|
Payback period =
|
4
|
+
|
|
|
|
|
|
|
149.04
|
-
|
116.57
|
=
|
3.83
|
Years
|
|
|
|
Techno-economic analysis indicates that production of common salt, magnesium carbonate and gypsum will generate enough revenue to recover the cost of production and make profit with a payback period of approximately 4 years. Recovery of salts in current process is thus concluded to be technically feasible and the economically affordable.