Calcium Chloride Recovery in Soda Ash Production by Solvay's Process

Table of Contents

Contents

Acknowledgement

List of Tables

List of Figures

List of Acronyms

Abstract

1.Introduction
1.2 Objectives
1.2.1 General Objective
1.2.2 Specific Objectives

2. Literature Review
2.1Calcium Chloride
2.2 Properties of Calcium Chloride
2.3 Calcium Chloride Solutions
2.4 Production Methods of Calcium Chloride
2.5 Economic Aspects
2.6 Grades and Specifications
2.7 Environmental Concerns
2.8 Health and Safety Factors
2.9 Uses of Calcium Chloride
2.9.1Deicing
2.9.2 Roadbed Stabilization/Dust Control
2.9.3 Oilfield Uses
2.9.4 Accelerator in Ready-Mix Concrete
2.9.5 Food
2.10 Central Salt and Chemical Research Institute
2.10.1 Solvay Process
2.10.2 Merferberg Process
2.11 Calcium Chloride Purification
2.11.1 Background of the Invention

3. Calcium Chloride Recovery Processes
3.1 Evaporative Crystallization Process
3.1.1 Byproduct Calcium Chloride
3.1.2Chemical Reactions
3.2 Evaporation of Distiller Waste Setting Ponds
3.2.1 Raw Materials
3.2.2 Raw Waste Loads
3.2.3 Process Description
3.3 Ion Exchange Process for Purification of Calcium Chloride
3.3.1 Advantages
3.3.2 How Does the Separation Work
3.3.2 Technical Considerations

4. Process Selection and Detailed Description
4.1 Crystallize Evaporation Process in Detail
4.1.1 Crystallize Evaporation Process
4.1.2 Sedimentation process
4.1.2 Flotation
4.2 Multiple Effect Evaporators
4.2.1 Feeding of Multiple Effect Evaporators
4.2.2 Advantages of Multiple Effect Evaporators
4.2.3 Rising Film Tubular Evaporator
4.2.4 pH adjustment
4.3 Filtration and Drying of Calcium Chloride
4.3.1Rotary Louvre Dryers

5. Major Engineering Problems
5.1 Choking in Dryer
5.2 pH adjustment
5.3 Conversion of Liquid Calcium Chloride to the flakes of calcium chloride Special features of cooling drum flaker:
5.5 Product Cooling Systems
5.6 Storage and Handling of flake Calcium Chloride
5.7 Packaged Product Storage

6. Cost Estimation
6.1 Raw Materials
6.2 Utilities
6.3 Equipment Costs
6.4 Total Initial Investment Cost
6.5 Financial Evaluation
6.5.1Profitability
6.5.2 Ratios
6.5.3 Break-even Analysis
6.5.4 Payback Period
6.5.5 Internal Rate of Return

7. Result Analysis
7.1 Evaporative Crystallization Process
7.2 Evaporation of Distiller Waste Setting Ponds
7.3 Ion Exchange Process for Purification of Calcium Chloride

8. Conclusion

References

Appendices
Appendix A: Properties of Calcium Chloride Hydrates (Shearer, W.L., 1978)
Appendix B: Calcium Chloride Specifications (U.S. Environmental Protection Agency, 2006)
Appendix C: Market and uses for calcium chloride (Donald, E. Garrett, 1996)
Appendix D: Parameters for the process of CaCl2 recovery from the soda ash (Department of Scientific and Industrial Research, 1995)

List of Tables

Table 2.1 Calcium chloride statistics

Table 2.2 United States statistics

Table 2.3 United States imports for consumption of crude calcium chloride

Table 2.4 Sieve Analysis for calcium chloride commercial grades, Mass%, Passing

Table 2.5 Calcium chloride use in United States

Table 3.1 Amount of raw waste loads

Table 4.1 Steam consumption and running costs of evaporators

Table 6.1 Raw material requirement and cost

Table 6.2 Utilities consumption and cost

Table 6.3 Machinery cost

Table 6.4 Total investment cost

Table 6.5 Total annual production cost

List of Figures

Figure 1.1 Calcium chloride

Figure 1.2 The phase relationship among calcium chloride, its hydrates, & a saturated solution

Figure 3.1 Solvay Soda Ash process flow diagram

Figure 3.2 Calcium chloride recovery process

Figure 3.3 Ion-exchange process using membrane

Figure 3.4 Ion-exchange

Figure 4.1 Crystallize evaporation process for recovery of calcium chloride

Figure 4.2 Solution for recovery of calcium chloride

Figure 4.3 Continuous sedimentation plant

Figure 4.4 Multiple effect evaporator-forward feed

Figure 4.5 Rising film tubular

Figure 4.6 Rotary Louvre dryer

List of Acronyms

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Abstract

There is a lot of wastage of calcium chloride going on daily and lot of valuable resources are being wasted. If this continues and any solution is not provided then this problem can prove to be very vital for the industry. Based on this study we look forward to design a major project based on recovery of calcium chloride, which will be fruitful to the industry to solve this setback and hence will be profit making concept. The focus on recovery of calcium chloride and minimizing the problem can lead to significant cost savings and industrial growth. Our solution provides the methods for recovery of calcium chloride from which evaporative crystallization process is very economical and efficient process. The final product will be in flake form (calcium chloride dehydrate) having 78 - 80% purity. It can be further dried to get calcium chloride anhydrous having purity level of 94%.The main advantage of this system is physical form of product (non dusty) and the cost of operation is far less as compared to conventional drying systems. The present demand for the calcium chloride is estimated at 53 tons per annum. The demand is expected to reach at 137.5 tones by the year 2018. The calcium chloride recovery from Solvay’s process solution succeeds in addressing all of these requirements where previous efforts have failed.

Key Words: Calcium chloride recovery, flake calcium chloride, Evaporative crystallization, Solvay’s process

1. Introduction

Wastage of calcium chloride in lumps can affect all aspects of a business. Soda ash production companies are making the waste of CaCl2 as a by-product which they kept at any place, no disposal, and no removal. Since last 85 years Soda Ash Production Company is doing the same thing. So many acres of soil are non useful due to this type of wastage it may also cause soil pollution (Solvey S.A., unpublished data).

We also many proceed to remove the hardness of water by CaCl2.Calcium chloride is mainly used as agent for gas drying, for the preparation of cooling brines, as drilling agent, as de-icing agent and as anti-frosting agent for concrete, as binding agent for de-dusting, and also for the preparation of solutions as heating media. The product is typically available in form of flakes and granules with 78% CaCl2 and 95% CaCl2; for food and pharmaceutical applications in form of crystallized and over calcined material with 78% CaCl2 ( Chemical Market Reporter ,2002).The main production is originated from the Solvay process as by-product to soda ash as flake material. The concentration of the calcium chloride liquor from this soda ash process is effected in evaporative crystallizers to avoid incrustations with gypsum and sodium chloride. Minor capacities are generated from the liquid effluents of the flue-gas desulfurization plants behind power stations and waste incineration units by evaporation crystallization. The plants here are a combination of a heavy metals precipitation and an evaporative crystallization with gypsum seeding. The current business environment is more competitive than ever. Any company that possesses even a slight advantage whether with collaborative applications or customer relationship management tools can dominate a market. Now more than ever, companies are looking for a competitive edge (Solvey S.A., unpublished data). The solution succeeds in purifying all of these requirements where previous efforts have failed.

1.2 Objectives

1.2.1 General Objective

The general objective of this research is calcium chloride recovery in soda ash production by Solvay’s process.

1.2.2 Specific Objectives

The specific objectives are:

- To design a major project based on recovery of calcium chloride (by product).
- To minimize the cost of calcium chloride recovery.
- To identify efficient and economical process methods of calcium chloride recovery.

2. Literature Review

2.1 Calcium Chloride

Calcium chloride (CaCl2) is a white, crystalline salt that is very soluble in water. In its anhydrous form it is 36.11% calcium and 63.89% chlorine. It forms mono-, di-, tetra-, and hexahydrates. Calcium chloride is found in small quantities, along with other salts, in seawater and in many mineral springs. It also occurs as a constituent of some natural mineral deposits. Natural brines account for 70 -75% of the United States CaCl2 production (Kemp et al.1985).

Calcium chloride was discovered in the 15th century but received little attention or study until the latter part of the 18th century. All of the early work was done with laboratory prepared samples, since it was not produced on a commercial scale until after the ammonia-soda process for manufacture of soda ash was in operation. It was actually considered a waste product until its uses were discovered (Shearer, W.L., 1978).

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Fig.2.1 Calcium chloride (Shearer, W.L., 1978)

2.2 Properties of Calcium Chloride

The properties of calcium chloride and its hydrates are summarized in appendix A. Accurate data are now available for the heats of fusion of the hexahydrate, the incongruent fusion of the tetrahydrate, and the molar heat capacities of the hexahydrate, tetrahydrate, and dihydrate. These data are important when considering the calcium chloride hydrates as thermal storage media. A reevaluation and extension of the phase relationships of the calcium chloride hydrates has led to new values for the heats of infinite dilution for the dihydrate, monohydrate, 0.33-hydrate, and pure calcium chloride (Shearer, W.L., 1978).

2.3 Calcium Chloride Solutions

Because of its high solubility in water, calcium chloride is used to obtain solutions having relatively high densities. For example, densities as high as 1430 kg/m3 are achieved at 208oC and 1570 kg/m3 at 808oC. The oil- and gas-drilling industries frequently exploit these high densities when completing or reworking wells. Density, or specific gravity, can also be used to determine the molal concentration, c, of calcium chloride in water.

c = 30.8 -129.6 d + 180.8 d2-106.8 d3+ 24.89 d4

where, c is in units of moles of calcium chloride per kg of water and d is the specific gravity of solution relative to water at 258oC.

The densities of calcium chloride solution at various wt% CaCl2 values and different temperatures have been listed. Densities and apparent molar volumes of aqueous calcium chloride solutions at temperatures from 323 K (508oC) to 600 K (3278oC) and at pressures up to 40 MPa (395 atm) have also been reported. Viscosity is an important property of calcium chloride solutions in terms of engineering design and in application of such solutions to flow through porous media. Data and equations for estimating viscosities of calcium chloride solutions over the temperature range of 20-5080C are available (O’neil et al., 2001).

Numerous studies on the thermodynamics of calcium chloride solutions were published in the 1980s. Many of these were oriented toward verifying and expanding the Pitzer equations for determination of activity coefficients and other parameters in electrolyte solutions of high ionic strength. A review article covering much of this work is available. Application of Pitzer equations to the modeling of brine density as a function of composition, temperature, and pressure has been successfully carried out (O’neil et al., 2001).

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Fig.2.2.The phase relationship among calcium chloride, its hydrates, and a saturated solution (O’neil et al., 2001).

2.4 Production Methods of Calcium Chloride

Calcium chloride is produced in commercial amounts using many different procedures refining of natural brines, reaction of calcium hydroxide with ammonium chloride in Solvay soda ash production, and reaction of hydrochloric acid with calcium carbonate. The first two processes account for over 90% of the total calcium chloride production. In the United States, the primary route for making calcium chloride is by the evaporation of underground brines. An integrated process is used to extract various brine components. Calcium chloride is derived from the brine left over after processing magnesium chloride into magnesium hydroxide. This brine is ca. 25% CaCl2. A 32-45% solution is produced after being processed through a double- or triple-effect evaporator. Unwanted alkali chlorides are precipitated and removed. The brine is then further evaporated, attaining a 78-94% calcium chloride concentration (Chemical Profiles in Chemical Marketing Reporter, 1999).

Production involves removal of other chlorides (primarily magnesium) by precipitation and filtration followed by concentration of the calcium chloride solution, either for ultimate sale or for evaporation to dry product. Commercial dry products vary by the amount of water removed and by the nature of the drying equipment used. Production and capacity figures for the United States are indicated in Table 2.1(Chemical profiles in chemical marketing reporter1999).

Table 2.1 Calcium chloride statistics (Chemical profiles in chemical marketing reporter, 1999)

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Mixed chloride brines not yet passing specifications containing calcium chloride, magnesium chloride, sodium chloride, and minor quantities of other salts are also produced in the United States in considerable quantities from natural brines and are marketed mainly in the form of brines of various concentrations. The Solvay Process Company introduced the ammonia soda process, which originated in Belgium in 1861, to the United States by 1881. Through a series of purchases and sales, this company now operates under the name of General Chemical with United States headquarters in Parsippany, NJ, and Canadian headquarters in Mississauga, Ontario. About 95% of synthetic calcium chloride produced is recovered from this process. The sole producer of calcium chloride in the United States by that route closed operation in the 1890s and consolidated production with Canada (Miller et al.,1990).

In 1896, brine was concentrated further to produce a solid containing 73-75% actual calcium chloride. In 1900, a granular product was produced, which greatly facilitated handling and dissolving. Later the granular product was replaced by flaked material containing 77-80% actual calcium chloride that, through high-temperature drying, acquires a superficial anhydrous coating, thus preventing caking. The ammonia soda process involves the reaction of sodium chloride (ordinary salt) with calcium carbonate (limestone) using ammonia as a catalyst to form sodium carbonate (soda ash) and calcium chloride. The process was originally designed to produce soda ash, producing calcium chloride as a waste product (Miller et al.,1990).

However, the importance of calcium chloride has grown such that calcium chloride is now considered a co-product rather than a by-product. Additional commercial material is available by action of hydrochloric acid on limestone. Typically the hydrochloric acid is a by-product of some other commercial process and the conversion to calcium chloride is motivated by waste avoidance

CaCO3 + 2 HCl à CaCl2 + H2O

Significant quantities of calcium chloride are produced in the United States, Canada, Mexico, Germany, Belgium, Sweden, Finland, Norway, and Japan. In 1989 there were 10 producers of calcium chloride in the United States. In 1990 this decreased to nine, Table 2. The Dow Chemical Company and Wilkinson Corporation recover calcium chloride from brines in Mason and Lapeer Counties, MI. Calcium chloride pellets, flake, and liquid were produced by Dow’s Ludington plant. Wilkinson markets calcium chloride solutions. National Chloride Company of America, Cargill’s Leslie Salt Company, and Hills Brothers Chemical Company also produces calcium chloride from dry-lake brine wells in San Bernadino County, CA. Hills Brothers also produced calcium chloride from an operation near Cadiz Lake, CA, and marketed calcium chloride that resulted as a by-product of magnesium production in Rowly, UT, produced by Magnesium Corporation of America (Miller et al.,1990).

Calcium chloride was synthesized by Tetra Chemicals at a plant near Lake Charles, LA., and from its liquid plant at Norco, LA. Calcium chloride was recovered as a by-product of the reaction of hydrochloric acid and limestone and marketed by Allied Signal (now Honeywell) Incorporated. Occidental Chemical Corporation also manufactured calcium chloride from this process in Tacoma, WI. Solution production is centered on Michigan (brines), California and Utah (brines), and Louisiana (by-product acid). Michigan is the leading state in natural calcium chloride production with California second (Miller et al.,1990).

2.5 Economic Aspects

Calcium chloride consumption is very dependent on the weather. The deicing, dust control, and road stabilization markets are, thus, effected by these conditions. In 1990 and 1991 the winter was mild, thus hurting the deicing market. However, in 1996, production delays, a harsh winter in 1995, and early signs of another one for 1996 created some snugness in the dry calcium chloride market (Chemical Profiles in Chemical Marketing Reporter, 1999).

As for liquid calcium chloride, which is used largely in dust control and oil well completion, a rainy summer in 1996 reduced demand for calcium chloride as a dust-control product. Aside from yearly changes in precipitation, these markets remain fairly stable. However, oil-drilling activity has declined, slowing the expected increase in the growth of this market. Use as a concrete set accelerator should see an increase as the construction industry continues to soar. About 12% of calcium chloride goes into cement manufacture and concrete accelerating. Industry sources believe that the use of calcium chloride as a growth-enhancing macronutrient may be a future market in the agricultural sector (Chemical Profiles in Chemical Marketing Reporter, 1999).

There is currently an excess of capacity in the calcium chloride industry, which is only expected to become more acute as additional synthetic or byproduct capacity increases. Calcium chloride production is used as a solution to the oversupply of hydrochloric acid. As processes convert from using caustic soda to using hydrated lime for propylene oxide production, an additional 225,000 tons of calcium chloride by-product has the potential for being generated. With capacity outweighing demand, new niche markets are being developed for using the product: mining, water treatment, fertilizer, pulp and paper, agriculture, and food-grade applications.

Table 2.2 United States statistics (Miller et al.,1990)

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Table 2.2 gives a summary of the calcium chloride statistics for production, value, exports, imports for consumption, and consumption. Exports for 1990 and 1991 totaled 23,300 and 30,568 metric tons, respectively. Of this, 16,463 (70%) and 25,006 (81%) metric tons were exported to Canada. Statistics for the United States exports by country are given in Table 3. Calcium chloride canvassing discontinued beginning 1993. However, export statistics were gathered for 1999: 66,197 metric tons were exported with a value of $18,319,470. Of this total, 52.5% ($9,920,969) was to Canada, 2% ($511,167) was to Mexico, 4% ($714,340) was to Trinidad, 4.78% ($631,990) was to Venezuela, 6% ($731,333) was to Italy, 4.5% ($466,411) was to the Netherlands, and 14.9% ($2,420,665) was to United Arab Emirates. The rest was in small amounts experted to other countries (Miller et al.,1990).

Estimated imports of calcium chloride increased more than ten-fold between 1984 and 1988, from 10,000 to 139,700 metric tons on a 100 wt% basis. The United States imports most of its calcium chloride from Canada (1990, 109,880 metric tons; 1991, 92,838 metric tons). The location of production facilities close to the United States/Canada border make this a particularly inviting country to export from because of calcium chloride’s use as a deicing material. The other countries the United States imports calcium chloride from are Mexico (1989, 17,800 metric tons), the former Federal Republic of Germany (1989, 6,900 metric tons), and Sweden (1989, 4,800 metric tons). Table 2.3 lists the United States imports for consumption of calcium by country for 1990 and 1991. As stated previously, calcium chloride canvassing discontinued beginning 1993.However, import statistics were gathered for 1999: 219,249 metric tons were imported with a valve of $26,810,352. Of this total, 78% ($14,347, 984) was from Canada, 16% ($7,160,676) was from Mexico, and 4.7% ($2,342, 994) was from Finland, with the rest being in small amounts from other countries (Miller et al.,1990).

Table 2.3United States imports for consumption of crude calcium chloride (Miller et al.,1990)

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2.6 Grades and Specifications

Most solution calcium chloride is sold as 38 or 45 wt% concentration; however, different uses require concentrations ranging from 28 to 45 wt%. The principal uses (deicing and dust control) do not require high-purity calcium chloride. However, it must be free of chemicals harmful to the environment. Producers ship the most concentrated form, and the distributors make final adjustments in concentration (Camfored Information Services,1999).

The majority of dry calcium chloride comes in one of two forms: flake or pellet. Lesser amounts are sold as mini pellets, powders, or briquettes. For a product containing 90.5% calcium chloride, the American Society of Testing Materials (ASTM) and the American Association of State Highway and Transportation Officials (AASHTO) has set up standards for calcium chloride content (assay), total alkali chlorides (less than 8.0% as NaCl), total magnesium (less than 0.5% as magnesium chloride), and other impurities (less than 1.0% after accounting for sodium, calcium, potassium, and magnesium chlorides, water, and calcium hydroxide). There are three grades of commercial calcium chloride: Grade 1, 77 wt% CaCl2 minimum; Grade 2, 90 wt% CaCl2; and Grade 3, 94 wt% CaCl2. Adjusted standards exist for all grades. Calcium chloride Manufactured in the United States routinely meets these standards. Table2. 6 summarized sieve analysis for key commercial grades. Calcium chloride meeting the Food Chemical Codex (FCC) specifications is used as a food additive. The specifications for this grade of anhydrous calcium chloride are as follows: assay, not less than 93.0%; arsenic (As), less than 3ppm; fluoride, less than 0.004%; heavy metals ( Pb), less than 0.002%; lead, less than 10ppm; magnesium and alkali salts, less than 5%; acid-insoluble material, less than 0.02%; and no particles of sample greater than 2 mm in any dimension (Camfored Information Services,1994).

2.7 Environmental Concerns

Calcium chloride is not considered to be harmful to the environment. Calcium is essential for all organisms. At concentrations above 1000 ppm, calcium chloride has been found to retard plant growth and can damage plant foliage. These effects are most likely caused by excess chloride ion as calcium is a nutrient for plants. In testing United States water supplies, high chloride concentrations are rarely found, even in areas of high salt usage for ice and dust control ( U.S. Environmental Protection Agency,2004).

Table 2.4 Sieve analysis f or commercial grades CaCl2, and mass % passing (Technology in the Indian Soda Ash Industry, 1995).

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Calcium chloride is found in the marine environment. Many organisms and aquatic species are tolerant of the concentrations of calcium and chloride ions in seawater (400ppm calcium, 18,900ppm chloride ions). Toxicity arises when possible toxic doses of calcium chloride from spills, surface runoff, or underground percolation get into typically freshwater streams or aquifers. Various agencies have guidelines for calcium and chloride in potable water. The European Economic Community (EEC) is the only agency to have a minimum specification for calcium in softened water (U.S. Environmental Protection Agency, 2006)

The ability of plants to take up calcium chloride (ion selectivity) and the Toxicity of calcium in plants and soils varies widely. Studies of herbaceous crop Species, where water defect is not a constraint, point to low levels of chloride ion as being responsible for inhibiting growth. However, deicing salts can be toxic to roadside vegetation. The use of both calcium chloride and sodium chloride as deicing salts and the effects on various grasses, shrubs, and trees has been studied. As calcium chloride use with sodium chloride is more effective at deicing roads, thus less is used, the overall chloride ion content is lower than with rock salt alone. From studies in Europe, calcium chloride in blends of deicing salts can have beneficial effects on the regulation of sodium, and of potassium over sodium, in spruce trees. Recommendations for calcium chloride tolerant species are available. Concentrations of 10,000-20,000ppm in water have been shown to be hazardous to animals and fish. The effects vary widely, ranging from reduced growth rate and impaired reproduction to death. Both calcium chloride (35% solution) and oil-field brine received the lowest toxicity ratings in a study, indicating the environmental advantages of using these products (U.S. Environmental Protection Agency, 2006)

2.8 Health and Safety Factors

In general, calcium chloride is not considered to be toxic. Because calcium chloride is hygroscopic, common safety precautions should be used: wearing gloves, long-sleeved clothing, shoes, and safety glasses. Contact with skin may cause mild irritation on dry skin. Strong solutions or solid in contact with moist skin may cause severe irritation and possibly burns. Calcium chloride can irritate and burn eyes from the heat of hydrolysis and chloride irritation. Inhalation may irritate the lungs, nose, and throat with symptoms of coughing and shortness of breath. Ingestion may cause irritation to the mucous membrane due to the heat of hydrolysis. Large amounts can cause gastrointestinal upset, vomiting, and abdominal pain. Dry bulk calcium chloride can be stored in construction-grade bins. Care should be taken to minimize moisture. It should be kept in a tightly closed container, stored in a cool, dry, ventilated area (U.S. Environmental Protection Agency, 2006).

2.9 Uses of Calcium Chloride

Calcium chloride, manufactured for over 100 years, has been used for a variety of purposes. The primary CaCl2 markets have not changed since the 1950s. A breakdown of the United States consumption by percent is given in table 2.5. All markets and uses are summarized in Table 8. Significant markets in the United States are for deicing during winter, roadbed stabilization, and as a dust palliative during the summer. Use as an accelerator in the ready-mix concrete industry is sizable, but there is still concern about chlorine usage because of possible corrosion of steel in highways and buildings. Calcium chloride is also used in oil- and gas-well drilling (Shearer et al., 1978).

Table 2.5 CaCl2 use in the United States (Shearer et al., 1978)

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2.9.1 Deicing

The largest market for calcium chloride is for deicing roads, sidewalks, and parking lots (30%). It is more effective than rock salt at lower temperatures. Calcium chloride melts ice at temperatures as low as -518C (-60 8F). Because it liberates heat upon exposure to moisture, ice melts quickly after application. Anhydrous calcium chloride, 94-97 wt% calcium chloride pellets, and 77-80 wt% calcium chloride flakes are used for highway deicing and in institutional and consumer markets. Under normal conditions, when temperature drops below -98C, untreated road salt loses its ability to generate quickly the heat necessary for melting snow and ice. Calcium chloride solutions (28-32 wt %) are used with rock salt or abrasives such as sand or cinders before spreading on highways to enhance their effectiveness. The result is more efficient utilization of road salt and safer roads. Instead of watching the road salt bounce off the roads during colder days, it will be actively melting the ice and snow. Solutions of 42-45 wt% concentration are also used to pretreat stockpiles of these materials. Calcium chloride is the deicer of choice for use at temperatures less than -6.78oC (Shearer et al., 1978).

2.9.2 Roadbed Stabilization/Dust Control

One of the earliest uses of calcium chloride was for dust control and roadbed stabilization of unpaved gravel roads. Dust control accounts for ca. 25% of calcium chloride production. Because calcium chloride is hygroscopic and deliquescent, it absorbs moisture from the atmosphere and forms a solution, binding the dust particles and keeping the surface damp. Calcium chloride in dry and solution forms are used both typically and mixed with the aggregate. If aggregate is mixed with dry calcium chloride or a calcium chloride solution and then compacted, the presence of calcium chloride draws in moisture to bind the fine particles in the aggregate matrix. This process leads to well compacted, maximum density gravel road. Due to its low vapor pressure, calcium chloride is slow to evaporate; thus, this dust-free condition is retained over a long period of time (Shearer et al., 1978).

2.9.3 Oilfield Uses

Calcium chloride has two uses in the oil field: as a primary ingredient in completion fluids and as the brine phase in an invert emulsion oil mud. An excellent review of oil-well drilling fluids is available (Shearer et al., 1978).

2.9.4 Accelerator in Ready-Mix Concrete

Calcium chloride has been used in concrete since 1885 and finds application mainly in cold weather, when it allows the strength gain to approach that of concrete cured under normal curing temperatures. In normal conditions, calcium chloride is used to speed up the setting and hardening process for earlier finishing or mold turnaround. Effects of calcium chloride on concrete properties are also widely studied and quantified. Aside from affecting setting time, calcium chloride has a minor effect on fresh concrete properties. It has been observed that addition of CaCl2 slightly increases the workability, reduces the amount of water required to produce a given slump, and reduces bleeding. Using calcium chloride significantly reduces initial and final setting times of concrete. The total effect of adding calcium chloride depends on dosage, type of cement used, and temperature of the mix. Addition of as little as 1-2% calcium chloride accelerates the set time of concrete, giving it a high early strength development. It is not antifreeze, but by using it during cold weather, it can offset problems associated with lower temperatures. Reviews of the concerns and possible remedies of calcium chloride corrosion problems in concrete are available. There is no consensus on what the safe levels of calcium chloride in concrete are (Shearer et al., 1978).

2.9.5 Food

Calcium chloride is used in the food industry to increase firmness of fruits and vegetables, such as tomatoes, cucumbers, and jalapenos, and prevent spoilage during processing. Food-grade calcium chloride is used in cheese making to aid in rennet coagulation and to replace calcium lost in pasteurization. It also is used in the brewing industry both to control the mineral salt characteristics of the water and as a basic component of certain beers (Shearer et al., 1978).

2.10 Central Salt and Chemical Research Institute

Bhavnagar-based Central Salt Marine and Chemical Research Institute (CSMCRI) has standardized and internationally patented a novel process of converting the discharge emanating from soda ash and salt-making units into value-added products (VAP). A novel cost-effective process that helps derive three times more gypsum from the distellar waste, emanating from the soda ash and salt-making units, using the Solvay process for production, has been standardised and granted US patent,” Dr Pushpito Ghosh, Director of CSMCRI, a laboratory of Council for Scientific and Industrial Research (CSIR) ( Kiefer et al., 2002).

2.10.1 Solvay Process

The distiller waste (calcium chloride), which is a by-product of landmark Solvay process, is currently being discharged directly into the sea by the industry. But once this waste is added with brine to forcibly precipitate out gypsum, the common salt derived is much purer compared to the traditional salt, currently drawn by the industry using conventional methods, Dr Ghosh claimed. Salt itself is required for soda ash manufacturing, which makes the invention especially more significant. If adopted by the industry, the process shall help reduce the discharge of distiller waste into the sea, besides reducing expenses involved in the purification of salt for manufacturing of soda ash, chloralkalies or even for edible applications, he claimed. In addition, one could realize the value from the additional amount of gypsum obtained, Dr Ghosh said. Once the salt is produced, the leftover sulphate-free bitten would also be ideal for recovery of low sodium salt, he said, adding that separate international patents have been granted to the institute for this. In a more recent development, the gypsum, derived from using this process, can further be value-added. Even though, it is on a very small scale currently, efforts are underway to scale it up, he said. It can be converted into ammonium sulphate which is a fertilizer, Dr Ghosh said ( Kiefer et al., 2002).

2.10.2 Merferberg Process

The gypsum can be value-added, using the merferberg process to obtain ammonium sulphate fertilizer and calcium carbonate. Interestingly, calcium carbonate, too, is required in the Solvay process to manufacture soda ash, and this can help reduce quarrying of lime stone, a natural resource, Dr Ghosh claimed. CSMCRI is in dialogue with the Gujarat Pollution Control Board (GPCB) on how this novel technology can be put to use in the state, which is a major manufacturing hub for soda ash and salt. The major players into soda ash or salt manufacturing in the state are Gujarat Heavy Chemicals Ltd, Tata Chemicals, Nirma and Gujarat Alkalies and Chemicals Ltd (Kiefer et al., 2002).

2.11 Calcium Chloride Purification

This invention relates in general to a process for removing fluoride by ion exchange. Specifically, this invention relates to a process for manufacturing low fluoride calcium chloride, or removing soluble fluoride from calcium chloride using a naturally occurring mineral to purify the calcium chloride. More specially, this invention relates to a process for purifying calcium chloride by removing soluble fluoride using hydroxyapatite (Kostick et al., 2006).

2.11.1 Background of the Invention

Calcium chloride is used in different applications, some of which require "food-grade" calcium chloride that contains low concentrations of fluorides and other contaminants. For example, calcium chloride is used in bisphenol-A plants to break the hydrochloric acid/water azeotrope in hydrochloric acid recovery columns. In this particular application, fluoride ions will concentrate and convert to hydrogen fluoride in the HC1 recovery column. Hydrogen fluoride, known to dissolve glass, creates pin holes in the recovery column, disrupting the recovery process and creating leakage problems. Food grade calcium chloride is also used in actual food applications, which naturally require high quality materials (Kostick et al., 2006).

The fluoride concentration in "food-grade" calcium chloride is typically less than 10 ppm. However, this grade of calcium chloride is often difficult to obtain and is therefore
expensive. It would thus be desirable to remove the fluoride ions from the calcium chloride solution prior to its use in applications requiring low-fluoride, or "food grade" quality
calcium chloride. Many present methods for removing fluoride ions from process and wastewater streams are inadequate or cost prohibitive for obtaining the desired fluoride-
free calcium chloride solution because they are inapplicable when calcium and chloride concentrations are high. Calcium fluoride used as a seed for creating enhanced calcium fluoride particles in order to remove soluble fluoride from the wastewater streams (Kust et al., 1995)

The use of adsorbents to remove fluoride ions in solution has also been effective under certain conditions. For example, European Patent number EP0191893 discloses contacting a solution containing fluorine compounds with various hydrated rare earth oxide adsorbents (Nomura et al.) .Similarly; International Publication number WO 98/10851 teaches the removal of fluoride ions in solution by passing the solution through an ion exchange resin to produce an ultrapure hydrofluoric acid. However, these methods do not solve the problem of removing fluoride ions from solutions containing high calcium and chloride ion concentrations, thereby generating a purified calcium chloride stream for use in later processing. These methods also do not produce a calcium chloride solution with as little as less than 1 ppm of fluoride. It would also be advantageous to have an easy, cost effective method of manufacturing low fluoride calcium chloride (Nomura et al., 2003).

Briefly, the invention relates to a method for removing fluoride from aqueous solution. More specifically, the invention relates to the removal of soluble fluoride from a calcium chloride solution to produce purified calcium chloride with extremely low concentrations of fluoride in the range of 0 to 10 ppm. In one embodiment of the present invention, fluoride is 5 removed from calcium chloride solution by causing ion-exchange between the solution and an ion-containing material. According to one aspect of this invention, the ion containing material is a natural material that is mixed with the calcium chloride in the form of slurry. Ion exchange occurs whereby the fluoride ions in solution are substituted for the hydroxide ions in the material. The natural material is hydroxyapatite, or calcium phosphate/calcium hydroxide composite. The contact between the fluoride ions and the slurry causes ion exchange between the solution and slurry, causing adsorption of the fluoride ions. Chloride ions are too big to exchange for hydroxide ions in the hydroxyapatite matrix, therefore the chloride ions stay in solution. The solubility of fluoridated hydroxyapatite is extremely small. The solubility product, Ksp of fluor hydroxyapatite, is Ca10 (PO4)6(F2OH) 2 is 3.16X10"60. As a result, the fluoride ions remain in the hydroxyapatite matrix and do not re-enter solution during the purification process. The resulting purified calcium chloride solution is altered out or removed from the slurry according to the designated application or end use for the product. The resulting product has a fluoride concentration of as low as less than 1 ppm, and has broad uses in applications requiring ultra low fluoride concentration calcium chloride. According to another embodiment of the present invention, calcium chloride with ultra low fluoride concentrations is manufactured by mixing lime or calcium carbonate with aqueous hydrochloric acid and calcium tri-phosphate (http://www.ncl.ac.uk/dental/oralbiol/oralenv/tutori- als/calciumphosphate.htm, 2003).

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