This invention relates to the manufacture of calcium hypochlorite. More particularly, this invention relates to an improved process for the manufacture of calcium hypochlorite using hypochlorous acid as a reactant.

It has long been a desire to produce calcium hypochlorite by the reaction of lime with a hypochlorous acid solution to eliminate or minimize the concentration of chloride ions in solutions or slurries of calcium hypochlorite and thus avoid the formation of effluents containing both hypochlorite and chloride ions.

In one approach, an aqueous hypochlorite solution was prepared in or extracted with an organic solvent such as ethyl alcohol, carbon tetrachloride, or methyl ethyl ketone. Suitable examples of this approach are found in U.S. Patent No. 1,481,039, issued January 15, 1924 to M. C. Taylor et al; and U.S. Patent No. 3,578,393, issued May 11, 1971 to J. A. ojtowicz et al. The use of organic solvents in producing hypochlorous acid solutions not only increases process costs but the presence of organic materials as impurities in the calcium hypochlorite product is

undesirable as calcium hypochlorite is decomposed by a large variety of organic materials.

U.S. Patent No. 2,429,531, issued October 21, 1947 by E. C. Soule et al teach a process for producing calcium hypochlorite by the neutralization or reaction of hypochlorous acid with a basic calcium hypochlorite such as dibasic calcium hypochlorite and/or hemibasic hypochlorite. Soule et al found that the direct reaction of lime with hypochlorous acid produced dilute concentrations of calcium hypochlorite in low yields. The reasons for this is the formation of considerable quantities of chlorate. The formation of the chlorate is attributed to the oxidation of hypochlorite ion.

Additionally, the use of aqueous solutions of hypochlorous acid in the preparation of calcium hypochlorite is taught, for example, in U.S. Patent Nos. 3,134,641, issued May 26, 1964 by R. D. Gleichert; 4,146,578, issued March 27, 1979 by J. P. Brennan et al; 4,147,761, issued April 3, 1979 by J. A. Wojtowicz et al; and 4,416,864, issued November 22, 1983 by J. A. Wojtowicz. However, in these processes the hypochlorous acid solutions employed were not used as the sole chlorinating agent and the use of an additional chlorinating agent such as chlorine or an alkali metal hypochlorite was required.

Processes employing hypochlorous acid and these additional chlorinating agents produce a calcium hypochlorite product containing substantial amounts of chloride ions. In addition, to recover the product, a separation step is requirec" which removes the solid calcium hypochlorite product containing chloride ions from a solution containing dissolved chloride ions and hypochlorite ions. The volume of effluent containing chloride and hypochlorite ions is greater than that

which can be recycled to the process. Thus, the excess effluent requires an additional application or if this is not available, it must be treated to reduce or remove the hypochlorite ions present prior to disposal. Further, the hypochlorite ion concentrations are relatively low and if they are to be recovered, the solutions require concentration.

To remove chloride ions from a calcium hypochlorite product Murakami et al, in U.S Patent No.4,355,014, issued October 19, 1982 propose to wash the product with large volumes of water. This method, however, is not commercially feasible as it reduces the product yield by dissolving calcium hypochlorite while generating large volumes of solutions containing low concentrations of dissolved chloride and hypochlorite ions.

Thus, prior art processes for producing calcium hypochlorite by the reaction of lime with hypochlorous acid suffer from several disadvantages including the production of a calcium hypochlorite product containing substantial amounts of chloride ions; the requirement of a separation step of the product from an effluent solution containing dissolved chloride and hypochlorite ions, and the use or disposal of excess volumes of effluents containing chloride and hypochlorite ions.

Now a novel process for producing calcium hypochlorite has been discovered that employs concentrated solutions of hypochlorous acid to produce a calcium hypochlorite product in the absence of supplementary chlorinating agents such as chlorine or an alkali metal hypochlorite. The process is free of effluents containing mixtures of chloride ions and hypochlorite ions which require disposal. In addition, the calcium hypochlorite product is of a high assay and

high purity and does not contain chloride ions formed by the use of chlorine or an alkali metal hypochlorite as chlorinating agents. Further, the process requires reduced amounts of fresh water and produces calcium hypochlorite slurries having higher concentrations of solids which can be dried at reduced energy requirements and thus reduced energy costs.

These and other advantages are accomplished in a process for producing calcium hypochlorite which consists essentially of reacting lime or a lime slurry containing greater than 30% by weight of Ca(OH)_ with a chlorinating agent consisting of a hypochlorous acid solution containing at least about 35 percent by weight of HOC1 to produce a slurry of calcium hypochlorite dihydrate crystals in a solution of calcium hypochlorite, the slurry containing at least 30% of Ca(0Cl) ₂ and being substantially free of alkali metal and chloride ions.

More in detail, the novel process of the present invention employs as one reactant a concentrated hypochlorous acid solution containing at least 35 percent by weight of HOC1. The method of producing these high purity concentrated HOC1 solutions is that in which a gaseous mixture, having high concentrations of hypochlorous acid vapors and chlorine monoxide g:^ and controlled amounts of water vapor is produced. The gaseous mixture can be produced by the process described by J. P. Brennan et al in U.S. Patent No. 4,147,761. The gaseous mixture having high concentrations of hypochlorous acid vapors and chlorine monoxide gas and controlled amounts of water vapor may also contain varying amounts of chlorine gas. To remove any solid particles, such as alkali metal chloride particles,

which may be entrained, the gaseous mixture may be passed through a separation means. To produce the concentrated solutions of hypochlorous, the gaseous mixture is converted to hypochlorous acid, for example, by condensing the gaseous mixture at low temperatures such as those in the range of from about -5°C to about +20°C and preferably from about 0°C to about +10°C. Condensation of the gaseous mixture is operated at autogenous pressures for the temperatures employed. The concentrated solutions of hypochlorous acid contain from about 35 percent to about 75 percent of H0C1, preferably from about 40 to about 60 percent, more preferably from about 42 to about 55, and most preferably from about 45 to about 50 percent by weight of HOC1. The hypochlorous acid solutions are substantially free of ionic impurities such as chloride ions, and alkali metal ions and have low concentrations of dissolved chlorine. For example, concentrations of the chloride ion are less than about 50 parts per million; the alkali metal ion concentration is less than about 50 parts per million. The dissolved chlorine concentration in the hypochlorous acid solution is less than about 3 percent, and preferably less than about 1 percent by weight. In the process of the present invention, the concentrated hypochlorous acid is initially reacted with lime.

The lime employed can be any suitable lime having an active lime content of from about 85 to about 99, and preferably from about 90 to about 98 percent, where active lime is defined as the weight percent of Ca(OH) ₂ in the lime. The lime employed usually contains impurities such as iron compounds, silica, aluminum salts, magnesium salts, manganese, unburned

limestone (calcium carbonate and magnesium carbonate) and other compounds in trace quantities. These impurities represent from about 1 to about 15, and preferably from about 2 to about 8 percent by weight of the lime. More preferred are limes having low concentrations of heavy metal compounds such as those of iron and manganese.

Lime slurries employed in the novel process of the invention contain greater than 30 percent by weight of Ca(OH) ₂ . For example, the lime slurries contain from about 32 to about 45%, preferably, from about 33 to about 44% and, more preferably from about 35 to about 43 percent by weight of Ca(OH) ₂ -

Reacting these thick slurries of lime substantially eliminates the formation of liquid effluents requiring reuse or disposal. In the process of the invention, the concentrated lime slurries are added to and admixed with the concentrated solutions of hypochlorous acid. The reaction is carried out using vigorous agitation to assure the production of a uniform or homogeneous slurry of neutral calcium hypochlorite dihydrate. As the process is preferably operated continuously, control of the reaction can be accomplished, for example, by adding the concentrated lime slurry to the hypochlorous acid solution at a rate which maintains the desired pH of the reaction mixture. Suitably the pH is maintained in the range of from about 10 to about 10.8, and preferably at from about 10.2 to about 10.5. The reaction between the concentrated hypochlorous acid solution and the concentrated lime slurry is carried out at a temperature in the range of from about 15° to about 30°C, and preferably from about 20° to about 25°C. As the hypochlorous acid solution is maintained at low temperatures, for example, below

about 10°C, its use provides a portion of the cooling required for this exothermic reaction.

A slurry of neutral calcium hypochlorite dihydrate crystals is produced in the reaction which is expressed by the equation:

2HOC1 + Ca(OH) ₂ > Ca(OCl> ₂ .2H ₂ 0 (1).

The product is a dense slurry of calcium hypochlorite dihydrate solids suspended in an aqueous solution of calcium hypochlorite. The slurry, containing at least 30%, and preferably from about 35 to about 50 percent by weight of Ca(OCl) ₂ , is dried without a solid-liquid separation step has a suspended solid content, from about 10 to about 30 percent, and preferably from about 15 to about 30 percent by weight. The slurry is substantially free of alkali metal ions and contains less than about 4 percent by weight of calcium salts such as lime, calcium chloride, and calcium chlorate.

In a preferred embodiment, a portion of the slurry of calcium hypochlorite dihydrate produced is used to "wet" the lime and form the lime slurry. When mixed with lime, the slurry of calcium hypochlorite dihydrate reacts to produce crystals of dibasic calcium hypochlorite in a reaction represented by the following equation:

2Ca(OH) ₂ + Ca(OCl) ₂ .2H ₂ 0 > Ca(OCl) ₂ .2Ca(OH) ₂ (2)

+ 2H ₂ 0

Reacting a portion of the slurry of calcium hypochlorite dihydrate with the lime significantly reduces the requirements for fresh water to be used in

the process. The slurry of dibasic calcium hypochlorite is then reacted with the concentrated hypochlorous acid solution to produce a slurry of neutral calcium hypochlorite dihydrate crystals. This reaction is represented by the equation:

Ca(OCl) ₂ .2Ca(OH) ₂ + 4HOC1 > 3Ca(OCl) ₂ + 4H ₂ 0 (3)

While the process of the present invention may be conducted batchwise, it is preferably operated with the hypochlorous acid solution and a slurry of lime, dibasic calcium hypochlorite or mixtures thereof, being charged continuously to the reactor. The rate of addition of reactants provides the reaction mixture with a slurry having a suspended solids concentration in the range of at least about 10 percent, preferably of about 10 to about 30 percent and more preferably about 15 to about 30 percent by weight.

A slurry of neutral calcium dihydrate crystals is continuously recovered from the reaction mixture. The slurry is substantially free of alkali metal ions, and has very low concentrations of chloride ions, i.e. less than about 3 percent, and preferably less than about 2 percent by weight.

In an alternate embodiment, a mother liquor may be recovered from the slurry of calcium hypochlorite by separation of the calcium hypochlorite dihydrate crystals. The mother liquor is a concentrated solution containing at least 15 percent, and normally from about 20 to about 25 percent by weight of Ca(OCl) ₂ - This concentrated calcium hypochlorite solution having very low concentrations of impurities may be used or sold as a bleach solution or used in the preparation of the

lime slurry.

The slurry may be used directly in the treatment of water systems such as swimming pools and the like, but is generally dried and stored prior to use. The slurry is dried by known means, for example, using a spray dryer, turbodryer or vacuum dryer where the appropriate temperature ranges are employed to reduce the water content to the desired level. A preferred method of drying employs a fluidized spray drier having a fluidized bed of granular particles into which off-sized product is fed after being agglomerated to adjust particle size.

The dried calcium hypochlorite product is then placed in suitable containers, with or without prior size classification or other processing such as pelletizing, prior to use in water treatment or any other application. During the drying of the product or in a size classification operation, fine particles of the dried calcium hypochlorite are recovered. In one embodiment of the process, these fine particles of dried calcium hypochlorite are admixed with a lime solution. This permits the recovery of the calcium hypochlorite values without forming an effluent or requiring other means of disposal of the fine particles. The novel process of the present invention employing highly concentrated solutions of hypochlorous acid and thick lime slurries produces calcium hypochlorite in a reaction mixture which does not require the use of supplementary chlorinating agents such as chlorine or an alkali metal hypochlorite and the product is therefore substantially free of chloride ions. The process does not produce effluents containing chloride and hypochlorite ions which require separation, treatment and disposal. The process can

thus be operated as a "closed loop".

The preferred embodiment of the continuous process of the invention is operated without solid-liquid separating equipment, such as vacuum filters, which significantly reduces both the capital costs and the maintenance costs.

The hydrated calcium hypochlorite product which is produced by the process of the present invention contains at least 70 percent, for example, from about 75 to about 95 percent, and preferably from about 80 to about 95 percent by weight of Ca(OCl) ₂ . The hydrated calcium hypochlorite has a water content in the range of 4 to about 20, and preferably from about 7 to about 20 percent by weight. Hydrated calcium hypochlorite products produced by commercial processes presently in operation have had to lower the concentrations of Ca(OCl) ₂ in the product to permit the water contents required for improved safety and handling properties. The process of the present invention produces hydrated calcium hypochlorite having the water content required for improved safety properties and yet having very high assays of Ca(OCl) ₂ .

Surprisingly, the high purity calcium hypochlorite product produced by the process of the present invention is substantially free of alkali metal chlorides and contains less than about 4 percent, preferably less than about 3 percent, and more preferably less than about 2.5 percent by weight of calcium chloride.

To further illustrate the present invention, the following example(s) are presented without any intention of being limited thereby. All parts and percentages are by weight unless otherwise indicated.

EXAMPLE 1

To a hypochlorinator reactor was continuously charged a slurry of dibasic calcium hypochlorite crystals having a total solids content of 30.7 percent at a rate of 535 parts per hour. Simultaneously, at a rate of 113 parts per hour was added an aqueous hypochlorous acid solution having a concentration of HOC1 of 50 percent by weight. The reaction mixture was agitated and maintained at a temperature of about 30°C. A slurry of calcium hypochlorite dihydrate was produced which was conveyed to a filter which separated a cake of calcium hypochlorite having a Ca(OCl)_ concentration of 37.9 percent by weight and a water content of 56.9 percent from a calcium hypochlorite mother liquor containing 20.9 percent by weight of Ca(OCl) _? . The calcium hypochlorite dihydrate cake was dried by a convector heating dryer to a product containing 85.1 percent by weight of Ca(OCl) ₂ , a water content of 4.7 percent by weight and a calcium chloride concentration of 1.4 percent by weight. Alkali metal ions could not be detected in the product.

The mother liquor was recycled to a crystallizer reactor and admixed with additional lime to produce the dibasic calcium hypochlorite slurry charged to the hypochlorinator reactor.

EXAMPLE 2

A gaseous mixture containing an average concentration of 23.7 parts by weight of chlorine monoxide, 65.5 parts by weight of Cl ₂ , and 1.8 parts by weight of water vapor was continuously passed through a cyclone separator to remove any entrained

solid particles of alkali metal chloride. The solid-free gaseous mixture at a temperature of 85-90°C was passed through a vertical shell and tube heat exchanger maintained at a temperature of about 0°C and a pressure of about 3-4 torr gauge to condense a portion of the chlorine monoxide and substantially all of the water vapor to produce an aqueous hypochlorous acid solution containing 40 to 55 percent by weight of HOC1. The hypochlorous acid solution had a pH of about 1 and the dissolved chlorine concentration was determined to be about 1 percent by weight. The concentrated hypochlorous acid solution was continuously added to a hypochlorinator reactor at a rate of 113 parts per hour. Also added to the reactor was a slurry of dibasic calcium hypochlorite at a rate of 373 parts per hour. The reaction mixture was agitated and maintained at a temperature of about 30°C. A slurry of calcium hypochlorite dihydrate was produced which was conveyed to a filter which separated a cake of calcium hypochlorite having a Ca(OCl) ₂ concentration of 42.7 percent by weight and a water content of 53.8 percent from a calcium hypochlorite mother liquor containing 20.8 percent by weight of Ca(0Cl) ₂ . The calcium hypochlorite dihydrate cake was dried by a convector heating dryer to a product containing 84.1 percent by weight of Ca(OCl) ₂ , a water content of 7.8 percent by weight and a calcium chloride concentration of 1.2 percent by weight. Alkali metal ions could not be detected in the product. The mother liquor was recycled to a crystallizer reactor and admixed with additional lime to produce the dibasic calcium hypochlorite slurry charged to the hypochlorinator reactor.

EXAMPLE 3

A slurry of dibasic calcium hypochlorite crystals was continuously fed to a hypochlorinator reactor at a rate controlled by pH. Simultaneous addition of 5 hypochlorous acid solution, containing 47% by wt. of HOC1, at approximately 0.5 gallons/min. (gp ) produced a paste slurry containing about 38% Ca(OCl) ₂ , 0.3% total alkalinity as Ca(OH) ₂ , 1.3% chloride salts of calcium and sodium, 0.5% Ca(C10_) ₂ and 60% water

10 (by difference) . Temperatures of the hypochlorinator reactor were maintained between 20°C and 25°C by circulating chilled brine through cooling coils immersed in the reactor. Vigorous agitation was maintained to assure rapid dispersion of the

15 hypochlorous acid and dibasic slurry into the paste slurry. A portion of this paste was spray dried to produce product, the analysis averaging 81.6% Ca(OCl) ₂ , 5.8% total alkalinity as Ca(OH) ₂ , 1.8% chloride salts of calcium and sodium, 2.5%

20 Ca(C10 ) ₂ , and 8.5% H ₂ 0. The other portion of the paste was recycled to a dibasic calcium hypochlorite crystallizer at a rate of about 0.3 to 0.4 gpm.

The dibasic crystallizer system consisted of a

25.crystallizer reactor, a lime slurry makeup tank, a hypochlorinator reactor feed tank, and a hydroclone. The dibasic slurry produced in the crystallizer was fed to the hydroclone at a rate of about 4 gpm. The thicker underflow dropped into the hypochlorinator

30 reactor feed tank. The thin hydroclone overflow fed the lime slurry makeup tank where lime was added at a rate of about 100 lbs/hr.

A small amount of water was also added to the lime slurry makeup tank for overall water balance control to maintain 60% water in the paste slurry. This lime slurry fed the dibasic crystallizer reactor along with the recycled paste slurry to produce a dibasic calcium hypochlorite slurry with a slight excess of free lime.

The temperature in the dibasic crystallizer reactor was maintained between 30°C and 35°C by recirculating the dibasic slurry through a shell and tube heat exchanger using low pressure steam. The stream analyses in the dibasic crystallizer system were.

* Total alkalinity

EXAMPLE 4

A hypochlorinator was operated continuously by feeding a concentrated solution of HOCl (47-50%) at a rate of about 7 gallons per minute. Also fed to the hypochlorinator was a concentrated slurry of hydrated lime (40-42% Ca(OH) ₂ ) at a controlled rate such that the pH of the paste slurry was maintained at about 10.2. The reactants were mixed using a high shear mixer (Lightning™ mixer, Mixing Equipment Co., Rochester N.Y.). The paste slurry level in the

hypochlorinator was controlled by continuously drawing off a portion of the paste slurry to a storage vessel.

The analysis of the paste slurry averaged 35% Ca(OCl) ₂ , 0.2% Ca(OH) ₂ , 0.5% CaCl ₂ , 0.5% Ca(C10 ₃ ) ₂ , and 63.8% H ₂ 0. From the storage vessel the paste slurry was fed directly to a fluidized bed spray dryer and dried to produce a calcium hypochlorite product containing containing 78-80% (Ca(OCl) ₂ and 11-14% H ₂ 0 (by difference).

EXAMPLE 5

The hypochlorinator of Example 4 was operated continuously by feeding a concentrated solution of HOCl (47-50%) at a rate of about 7.5 gallons per minute. Also fed to the hypochlorinator was a dilute chlorinated lime solution at a rate of about 1 gallon per minute (6.5% Ca(0Cl) ₂ , 2% Ca(OH) ₂ , 25CaCl ₂ ,

0.2%CaClO_ and the balance water). A concentrated slurry of hydrated lime (38-41% Ca(OH) ₂ ) was continuously added at a controlled rate such that the pH of the paste slurry was maintained at about 10.2. The reactants were mixed using a high shear mixer (Lightning™ mixer, Mixing Equipment Co., Rochester N.Y.). The paste slurry level in the hypochlorinator was controlled by continuously drawing off a portion of the paste slurry to a storage vessel. The analysis of the paste slurry averaged 33.5% (Ca(OCl) ₂ , 0.3% Ca(OH) , 0.8% CaCl ₂ , 0.4% Ca(C10 ₃ ) ₂ and 65% H ₂ 0. From the storage vessel the paste slurry was fed directly to a fluidized bed spray dryer and dried to produce a calcium hypochlorite product containing 78-80% (Ca(0Cl) ₂ and 11-14% H ₂ 0 (by difference). During operation of the dryer, fine particles of

calcium hypochlorite recovered from the exhaust were fed to a scrubber containing a dilute solution of lime and a chlorinated lime solution produced. This chlorinated lime solution was recycled to the hypochlorinator.