WO2007110733A1

WO2007110733A1 - A process for the recovery of superior quality and super white industrial grade salt from subsoil/sea brines

Info

Publication number: WO2007110733A1
Application number: PCT/IB2007/000736
Authority: WO
Inventors: Indrajit Mukhopadhyay; Pushpito Kumar Ghosh; Abdulhamid Usmanbhai Hamidini; Puthoor Mohandas Vadakke
Original assignee: Council Of Scientific And Industrial Research
Priority date: 2006-03-28
Filing date: 2007-03-23
Publication date: 2007-10-04

Abstract

The present invention provides a process for the production of superior quality industrial grade salt with improved whiteness / brightness. The process comprises dosing of alum to the sea / sub-soil brine in the density range of 16-24 °Be' at a predetermined optimum level. The floes formed are allowed to settle under gravity and the clarified brine is subjected to either solar or forced evaporation for the crystallization of salt. The salt crystals so produced are defect free with a whiteness index of 88-89 and meeting all the specifications prescribed for industrial grade I salt. The process is simple and can be applied for any type of brine irrespective of its source and can be adopted by any salt works irrespective of its size and geographic location. The salt is devoid of any trace element impurities and can be fortified with iodine by known methods.

Description

A PROCESS FOR THE RECOVERY OF SUPERIOR QUALITY AND SUPER WHITE INDUSTRIAL GRADE SALT FROM SUBSOIL/ SEA

BRINES Field of the invention

The present invention relates to a process for recovery of superior quality and super white industrial grade salt in the field from subsoil / sea brines in a cost-effective manner. The process is applicable to a wide range of brine compositions and can be adopted in any salt works irrespective of the source of brine and location.

Background and prior art references

The world salt production has crossed one hundred million tons per annum. About 60% of the salt produced is used for industrial applications, chlor-alkali industries being the major consumers and 40% salt goes for human consumption. Superior quality industrial grade salt is preferred by chlor-alkali industries as the use of such salt considerably reduces the brine pre-treatment cost. Moreover, super-white refined salt is highly consumer attractive and preferred for edible purposes.

Majority of the salt produced in the world does not comply with the industrial specifications due to higher level of impurities like Ca, Mg, SO₄ and presence of undesirable trace elements. Reference may be made to the research article "Rain Washing of Common Salt Heaps" by M. P. Bhatt et al. appeared in Salt Research and Industry 10 (2), 1974, pi 3 wherein it is reported that sea salt after rain washing contains about 0.21 % Ca, 0.06 % Mg and 0.60 % SO₄. In the article "Washing of Strip Mined Rock and Solar Salt at Leslie Salt Corporation US " (Symposium on SaIt-I, Vol.1, the Northern Ohio Geological society Incorporation, Cleveland (1961 ), p 449- 464), A Woodhill has reported that Ca, Mg and SO₄ impurities in solar salt can be reduced by mechanical washing. The main disadvantage of the method is that there is a 15 - 20 % loss of salt and requires high capital investment. Moreover, the maximum level of reduction of Ca is 70 %. In the article "Manufacture of Solar Salt by Series Feeding System" by R. B. Bhatt et al.(Salt Research and Industry, 11, 1979, p 9) it has been reported that solar salt with less impurities of Ca can be produced from sea water by series feeding method wherein the brine is subjected to evaporation over narrow density ranges for fractional crystallization of salt and the salt is harvested in two stages i.e. between 25.5 - 27° Be' and 27 - 29° Be'. The drawback of this process is that the yield of pure salt is reduced and the salt harvested at the second stage is contaminated more with Mg and SO₄ impurities, which can only be removed by resorting to mechanical washing. In their patent application (Indian Patent Application No. 315DEL95) entitled "A Process for the Preparation of Sodium Chloride Containing Low Ca Impurity from Sea Brine in Solar Salt Works" by M. H. Vyas et al. it is claimed that Ca can be reduced up to 70% in the harvested salt through treatment of concentrated brine with polyelectrolytes like activated starch solution. The drawbacks of the process are that it is not applicable to subsoil brines due to higher Mg to Ca ratio and also difficult to implement on a large scale commercial production because of large requirements of starch solution.

Moreover, Mg and SO₄ impurities cannot be reduced by the above method. In their patent application (....) entitled " An Improved Process for the Removal of Ca ions from the Brine by Marine Cyano Bacteria" by S. Misra et al. it has been claimed that low Ca salt can be produced from sea / subsoil brine by mopping up Ca in the brine through certain types of marine cyano bacteria. The drawback of this process is that it is not readily amenable to scale up and Mg and SO₄ impurities do not get reduced in salt. In their patent application (US Patent No. 6, 776,972 dated 17/08/2004, PCTNo. 03/035550 dt. 01/05/2003) entitled "A Process for Recovery of Common Salt and Marine Chemicals from Brine in Integrated Manner" by R. N. Vora et al. it is claimed that common salt and marine chemicals of high purity can be recovered in an integrated manner by desulphating brine of low density, prior to the crystallization of Salt with insitu generated CaCl₂ or with distiller waste liquor containing CaCl₂. The process works excellently well in solar salt works where soda ash manufacture and salt production are carried out in the same premises or there is availability of distiller waste liquor in the near by vicinity. Otherwise, production of salt on stand-alone basis becomes rather costly.

It is well known that the suspended matter is removed from surface water, effluents, wastewater, liquid waste and water from various other sources by sedimentation technique. To improve the conditions for sedimentation coagulating agents such as iron salts or aluminium salts have long been employed. Alum (A1₂(SO₄)3.18H₂O) being a very cheap source of aluminium sulfate, is widely used as a coagulating agent for the above purpose (K. Dentel and J. M. Gosset, J. Am. Water Works Assoc. Apr. 1988, p 187 - 188). Sulphate ion alum appear to act as a catalyst in the formation of polynuclear complexes and their linkage to form a solid lattice (A. C. Vermeulen et al. J. Colloid Interface Sci. 57, pi 15 (1976)). However, the application of alum for the clarification of brine with an aim to produce good quality salt has not been reported till date.

Objects of the invention

The main object of the present invention is to provide a process for the recovery of superior quality salt from brines obviating the drawbacks as detailed above. Another object of the present invention is to improve the quality of salt to meet industrial grade I specification in a cost effective manner through a simple process.

Yet another object of the present invention is to produce salt with improved whiteness/brightness

Yet another object of the present invention is to produce defect free salt to prevent the contamination of impurities, which are otherwise entrapped in salt crystal.

Yet another object of the present invention is to use cheaper flocculating agents to clarify brine in order to eliminate the presence of the trace elements, which otherwise makes the salt unfit for chlor-alkali manufacture.

Yet another object of the present invention is to settle the suspended CaSO₄ impurities along with the floes formed by the flocculating agents before the brine is charged in the crystallizers for salt crystallization to reduce the Ca and SO₄ impurities in salt.

Yet another object of the present invention is to reduce the magnesium impurities in salt through pretreatment of brine with suitable flocculating agents. Yet another object of the present invention is to see that the process is simple and applicable to any type of brine and any salt works irrespective of its location.

Still another object of the present invention is to make the process suitable for salt production in solar salt fields as well as through forced evaporation as the case may be. Yet another object of the present invention is to ensure that the process is cost effective and can be adopted easily even by marginal salt producers. Summary of the present invention

Accordingly, this invention relates to a process for the recovery of superior quality and defect free industrial grade salt with improved whiteness/brightness from sea / subsoil brine comprising the steps of: dosing of alum to dilute sub- soil/ sea brine in the density range of 16-24° Be'at a concentration level of 30-40ppm of alum in a solar evaporation pan or inflow brine, allowing the floe so formed to settle under gravity, evaporating the supernatant clarified brine in an evaporation pans / reservoir pans to a concentration of about 29⁰Be', followed by centrifugation, washing with diluted brine and drying to obtain the crystallized salt.

In another embodiment of the invention, superior quality industrial grade salt obtained has the specification: Ca = 0.03 %, Mg = 0.04 %, SO₄ = 0.12 - 0.15 % by weight with

>99.6 % NaCl.

A process as claimed in claim 1, wherein whiteness / brightness index of the salt obtained is in therange of 88-90.

A process as claimed in claim 1, wherein the salt of the above specification is obtained through forced evaporation.

A process as claimed in claim 1, wherein the alum used for brine clarification is of commercial grade or is from cheaply available source. A process as claimed in claim 1, wherein no extra means are required for applying alum dosage to the brine.

A process as claimed in claim 1, wherein the floe formed is settled under gravity with out employing any mechanical means.

A process as claimed in claim 1, wherein the process is applicable to any type of brine irrespective of its source.

A process as claimed in claim 1, wherein the process can be adopted in any salt works, large, medium or small, irrespective of its geographic location.

A process as claimed in claim 1, wherein the heap washing of salt is carried out with dilute brine available in the nearby vicinity. A process as claimed in claim 1, wherein the loss of salt during washing is minimum and the leach brine is reused for salt production. A process as claimed in claim 1, wherein defect free salt crystals obtained has no entrapped impurities.

A process as claimed in claim 1, wherein the impurities of Ca, Mg and SO₄ are reduced to bare minimum in salt. A process as claimed in claim 1, wherein the salt obtained is totally free from all the trace element impurities.

A process as claimed in claim 1, wherein the thermodynamic properties of the brine system is unaffected due to the optimized dosage level of alum.

A process as claimed in claim 1, wherein the salt obtained with the required quantity of iodate as per known methods to produce edible iodised salt.

The present invention relates to the recovery of superior quality of industrial grade salt with improved whiteness/brightness from subsoil/sea brine in a cost effective manner. The process involves dosing of brine of appropriate density with alum solution at an optimized concentration in a specially designed evaporation pan. The coagulants along with the suspended particles form floes, which are allowed to settle under gravity. The clarified brine is then taken through gravity to another pan and subjected to solar evaporation facilitating the crystallization of pure gypsum. On attaining proper density the supernatant brine is charged in the crystallizers for salt crystallization. Alternately the clarified brine is subjected to forced evaporation in an open pan evaporator or multiple effect evaporators for salt crystallization. The bittern left after the recovery of salt is discharged from the crystallizers on attaining appropriate density of the concentrated brine.

In an embodiment of the present invention, superior quality industrial grade salt can be produced in a cost effective manner from sea / sub-soil brine of 3-24 ⁰Be' density.

In another embodiment of the present invention, the recovery of super grade salt from sea/sub-soil brines can be most efficiently achieved through the dosing of alum in a predetermined optimum concentration.

In yet another embodiment of the present invention, the dosing of alum is given to sea/sub-soil brine in the density range of 16-24°Be'.

In yet another embodiment of the present invention, the dosing of alum can be conveniently given in solar salt pans or in the brine inflow channels. In yet another embodiment of the present invention, the floes formed by alum dosage are settled on its own under gravity.

In yet another embodiment of the present invention, the entire process of dosing alum, subsequent settling of floes and evaporation of clarified brine for salt recovery can be carried out in the filed in large solar pans.

In yet another embodiment of the present invention, the evaporation of clarified brine can be carried out following the forced evaporation methodology with out affecting the yield or purity of end product.

In yet another embodiment of the present invention, the process can be suitable to any type of brine irrespective of its source.

In yet another embodiment of the present invention, the process can be applied to any salt works irrespective of its size and geographic location.

In yet another embodiment of the present invention, the process is made so simple and cost effective that even unskilled marginal salt producers can easily adopt it. In still another embodiment of the present invention, the salt crystals can be made defect free with minimum chance for contamination of impurities.

Detailed description of the invention

Subsoil or sea brine, as the case may be, of 16-24°Be' density is dosed with alum solution at an optimized concentration of 40 ppm. Alum solution is prepared by dissolving a calculated quantity in dilute sea or subsoil brine in the density range of 4-

14°Be'. The dosing of alum solution is facilitated either by spraying it over the brine in a reservoir or by its continuous mixing, in the channel, with the in-flowing brine to a reservoir. In the former method, the optimum requirement of alum to be dosed is calculated based on the volume of brine in the reservoir. The volume of brine is measured as per the known procedure usually followed in a solar salt works. In the latter case, the optimum quantity of alum to be dosed is arrived at based on the flow rate of brine in the reservoir.

In the purification of brine by coagulation and settling certain fundamental factors are involved such as nature of coagulant, size of aggregates, apparent density, completeness of reactions etc. These factors are controlled by the quantity of the coagulating agents, pH control, electrolyte concentration etc. When alum is added to water/brine it goes into solution with formation of aluminium ion (Al³⁺) and sulfate ion (SO₄)^2":

Al₂(S(M)₃ <→ 2Al³⁺ + 3(SO₄)^2"

There are also present in the solution H⁺ and OH^" ions from the slight ionization of water:

H₂O <→ H⁺ + OH^'

Since aluminium hydroxide is a weak base, there will be a tendency for the formation of unionized hydrate:

Al³⁺ + 3 OH^" <→ Al(OH)₃

Since the solubility of this hydrate is extremely low, it will tend to come out of the solution in the form of a colloidal precipitate:

Al(OH)₃ <→ Al(OH)₃ Dissolved Solid

The law of mass action thus far governs the reactions, so that as hydroxide ion is removed, the concentration of hydrogen ion builds up to a point where aluminium hydroxide will no longer be precipitated. The alteration of pH, therefore, has an important effect on the precipitation. The dosage of alum is thus adjusted in such a way that the pH of the resultant brine does not cross the optimum level as within a pH range of 5.5 - 7.0 the precipitate will not be pure hydroxide and here the colloidal nature of the precipitate comes into profound significance. Al(OH)₃ acts as a positive sol and is coagulated by negative ions, i.e. sulfate released from the alum. Since the feed brine already contains excess sulfate ions there is no dearth of negative ions and the dose of alum can be restricted to minimum possible requirement. The suspended particles in the brine are adsorbed on the colloidal gel and form a floe. The floe, being a colloidal gel and having an enormous surface, exhibits greater adsorbing powers, which are responsible for the removal of suspended matter, color, trace elements and organic substances. The pH range over which aluminium sulfate is more effective is 5.3 - 8.7.

The alum treated brine is left undisturbed for a minimum period of 12 hours to facilitate the settling of floe through gravity. The clarified brine is carried to a second evaporating pan and subjected to solar evaporation in a usual manner. Within the density range of 15 - 25⁰Be' gypsum crystallizes out and is removed from the system. The concentrated brine on attaining 25°Be' is charged in the crystallizers for salt crystallization. The bittern left after the recovery of salt is discharged at an appropriate density (29⁰Be') and the salt is harvested. Alternately, the concentrated and clarified brine of 25°Be' is subjected to forced evaporation for the recovery of salt. When the dosing of alum is carried out in the in-flow brine in a channel, the floe gets well settled in the intermediate channels and pans by the time it attains the right density in the final pond from where the brine is pumped for forced evaporation.

The following examples are given by the way of illustration and therefore should not be construed to limit the scope of the present invention.

Example 1 In this example 1 L (L = litre) of sea brine of density 24° Be' and having the chemical composition: Mg²⁺= 11.50 g/L (g = gram); Ca²⁺= 0.54 g/L; SO₄ ^2" = 17.4 g/L and Na⁺ = 96.1 g/L (243 g/L as NaCl); was dosed with alum solution in a 2 L glass beaker so as to reach an alum concentration of 30 - 40 ppm in the brine. The post-treated brine was left undisturbed for 12 - 16 hours to facilitate the settling of the floe produced due to the dosing of alum. The supernatant brine was decanted out and evaporated up to a concentration of 29 ⁰Be'. The salt crystallized during the course of evaporation was centrifuged, washed with dilute brine and dried. The salt was characterized for its physical properties and chemical composition. The whiteness index of the salt samples (for uniform mesh size) was measured following the standard method using MgO as a primary reference with a whiteness index of 99. The salt so produced had the following specifications: Ca = 0.030%, Mg = 0.040 %, SO₄ = 0.12 % by weight with > 99.6 % NaCl where as the salt produced in the same procedure without any chemical treatment of sea brine contains about 0.20-0.25 % Ca, 0.20 - 0.30 % Mg and 0.50 - 0.60 % sulphate with 98 - 98.5 % NaCl. The whiteness index of salt produced from the treated brine was measured as 87-88 as against 78-80 for the normal salt produced from untreated sea brine. Example-2

In this example 1 L (L = litre) of sub-soil brine of density 23 ⁰Be' and having the chemical composition: Mg²⁺ = 12.02 g/L (g = gram); Ca²⁺ = 1.0 g/L; SO₄ ^2" = 8.0 g/L and Na⁺ = 92.4 g/L (235 g/L as NaCl); was dosed with alum solution in a 2 L glass beaker so as to reach an alum concentration of 30 - 40 ppm in the brine. The post- treated brine was left undisturbed for 12 - 16 hours to facilitate the settling of the floe produced due to the dosing of alum. Recovery of salt from the supernatant brine was carried out as mentioned in Example 1. The salt produced from sub-soil brine had the following specifications: Ca = 0.030%, Mg = 0.040 %, SO₄ = 0.15 % by weight with > 99.6 % NaCl where as the salt produced in the same procedure without any chemical treatment of sub-soil brine contains about 0.25-0.45 % Ca, 0.20 - 0.35 % Mg and 0.60 - 0.80 % sulfate with 98 - 98.5 % NaCl. The whiteness index of salt produced from the treated brine was measured as 87 as against 78-79 for the normal salt produced from untreated sub-soil brine. Example 3

In this example the experiment was conducted in the field using sub-soil brine and utilizing solar energy for evaporation of brine in open pans. Subsoil brine of 23 ⁰Be' density having the chemical composition as: Mg²⁺ = 12.02 g/L (g = gram); Ca²⁺ = 1.0 g/L; SO₄ ^2' = 8.0 g/L and Na⁺= 92.4 g/L (235 g/L as NaCl) was filled in an open pan of size 130 m X 38 m up to a depth of 0.3 m. The volume of brine was measured as 1482 m³. About 40 kg of commercial alum was dissolved in 400 L of dilute sub-soil brine and the alum solution so prepared was dosed into the above brine by spraying in order to achieve nearly homogeneous mixing. The alum treated brine was left undisturbed in the pan for 12 - 16 hours enabling the floes to settle completely under gravity. The supernatant clear brine was carried to an adjacent leveled pan through specially designed channel and allowed to evaporate till 25 ⁰Be'. The concentrated brine was then fed to a third evaporating pan and allowed to evaporate using solar energy till a density of 29 ⁰Be' was reached. The salt crystallized in the pan was harvested and heap washed with dilute sub-soil brine following the normal washing process and the bittern was discharged for further down stream processing for the recovery of valuable chemicals. The heap washed salt analyzed on an average Ca = 0.02 - 0.03 %, Mg = 0.04 - 0.06 % and SO₄ = 0.10 - 0.15 % by weight with >99.6 % NaCl meeting all the requirements specified for industrial grade I salt. The heap washed salt produced from sub-soil brine with out any chemical treatment is found to contain on an average 0.25 - 0.45 % Ca, 0.10 - 0.30 % Mg and 0.70 - 1.0 % SO₄ with about 98 % NaCl.

Example 4

In this example the experiment was conducted in a large commercial salt works using sea brine for salt production. Concentrated sea brine of 24 ⁰Be' density having the chemical composition as: Mg²⁺ = 11.50 g/L (g = gram); Ca²⁺ = 0.54 g/L; SO₄ ^2" = 17.4 g/L and Na⁺ = 96.1 g/L (243 g/L as NaCl) entering a reservoir pond through a channel was dosed with a calculated quantity of alum at the entry point such that the brine is mixed with alum to a concentration level of 30 - 40 ppm. The requirement of alum was arrived at based on the in-flow rate of brine in the reservoir. The floe so formed got settled under gravity by the time the brine reached the storage pond from where it was pumped to the evaporators for salt crystallization through forced evaporation. About 7500 m³ of brine was treated with alum in 24 hours duration and about 1500 ton of salt was produced. The salt was found to have the following specifications: Ca = 0.03 %, Mg = 0.05 %, SO₄ = 0.16 % by weight with > 99.6 % NaCl (on dry basis). The whiteness index of the salt was measured as 88-89 as against 84 - 85 for the salt obtained with out any chemical treatment of brine.

The main advantages of the present invention are:

(1) Superior quality industrial grade salt is produced in the filed in a cost effective manner.

(2) The salt produced through the above mentioned process shows improved physical characteristics such as whiteness/brightness and improved free flow ability.

(3) The floes formed during the process settle on its own under gravity and is not carried away along with the brine. On the other hand the solid bed formed by the floe in the pans helps in eliminating the percolation losses of the brine.

(4) The salt is free from any trace element impurities, which are otherwise difficult to be eliminated when salt is produced through the normal route.

(5) No mechanical washing is required to upgrade the salt, which is a normal practice in all the salt works to achieve the desired salt specification. (6) The process works equally well with any type brine irrespective of its source and composition.

(7) The salt crystals produced are defect free and there is no possibility of its contamination with any foreign elements. (8) The process can be implemented in any salt works irrespective of site and geographic location.

(9) The process is simple and can be adopted even by marginal and traditional salt producers in their salt works.

(10) The low and optimum dosage of alum ensures the production of salt free from aluminium contamination.

(11) The low dosage of alum does not change the thermodynamic equilibrium of the brine system and the normal process of salt crystallization is not affected in any way.

Claims

We Claim :

1. A process for the recovery of superior quality and defect free industrial grade salt with improved whiteness/brightness from sea / subsoil brine, the said process comprising the steps of: a) dosing of alum to dilute sub- soil/ sea brine in the density range of 16-24⁰Be' , at a concentration level of 30-40ppm of alum in a solar evaporation pan or inflow brine, b) allowing the fioc so formed to settle under gravity and c) evaporating the supernatant clarified brine in an evaporation pans/reservoir pans to a concentration of about 29⁰Be', followed by centrifugation, washing with diluted brine and drying to obtain the crystallized salt.

2. A process as claimed in claim 1, wherein superior quality industrial grade salt obtained has the following specifications: Ca = 0.03 %, Mg = 0.04 %, SO₄ = 0.12 - 0.15 % by weight with >99.6 % NaCl.

3. A process as claimed in claim 1, wherein whiteness / brightness index of the salt obtained is in the range of 88-90.

4. A process as claimed in claim 1, wherein the salt of the above specification is obtained through forced evaporation.

5. A process as claimed in claim 1, wherein the alum used for brine clarification is of commercial grade or from a cheaply available source.

6. A process as claimed in claim 1, wherein no extra means are required for applying alum dosage to the brine.

7. A process as claimed in claim 1, wherein the floe formed is settled under gravity with out employing any mechanical means.

8. A process as claimed in claim 1, wherein the process is applicable to any type of brine irrespective of its source.

9. A process as claimed in claim 1, wherein the process can be adopted in any salt works, large, medium or small, irrespective of its geographic location.

10. A process as claimed in claim 1, wherein the heap washing of salt is carried out with dilute brine available in the nearby vicinity.

11. A process as claimed in claim 1, wherein the loss of salt during washing is minimum and the leach brine is reused for salt production.

12. A process as claimed in claim 1, wherein defect free salt crystals obtained has no entrapped impurities.

13. A process as claimed in claim 1, wherein the impurities of Ca, Mg and SO₄ are reduced to bare minimum in salt.

14. A process as claimed in claim 1, wherein the salt obtained is totally free from all the trace element impurities.

15. A process as claimed in claim 1, wherein the thermodynamic properties of the brine system is unaffected due to the optimized dosage level of alum.

16. A process as claimed in claim 1, wherein the salt obtained is fortified with the required quantity of iodate as per known methods to produce edible iodised salt.

17. A process for the recovery of superior quality and defect free industrial grade salt with improved whiteness/brightness from sea / subsoil brine, substantially as herein described with reference to the examples.