CA1172827A - Process for manufacture of calcium hypochlorite - Google Patents

Abstract

PROCESS FOR MANUFACTURE OF CALCIUM HYPOCHLORITE

Abstract Of The Disclosure A novel process for the production of granular calcium hypochlorite compositions is provided which comprises reacting neutral calcium hypochlorite with an alkaline solution comprised of an alkali metal hypochlorite and an alkali metal hydroxide. The reaction produces a slurry from which a wet cake comprised of neutral calcium hypochlorite and a residual amount of hemibasic calcium hypochlorite is recovered and dried. During the drying process in which granular calcium hypochlorite compositions are obtained, there is a substantial reduction in the loss of available chlorine from the product.

Description

~t7~7 PROCESS FOR MANUFACTURE OF CALCIUM HYPOCHLORITE
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This invention relates to the manufacture of calcium hypochlorite. Calcium hypochlorite is S a co~nercial bleaching and sanitizing agent used particularly in the disinfection of swl~ning pool waters.
~nong the processes employed commercially - for the production of calcium hypochlorite are several which produce neutral calcium hypochlorite, Ca(OCl)2, in the anhydrous form, or as a hydrated product containiny at least 4 percent of water. These widely employed processes have ef~iciently produced calcium hypochlorite compositions o~ hiyh quality.
As energy costs have rapidly increased, ways lS have been sought to lower energy requirements ~or producing calcium hypochlorite, for example, by improving the drying of the product so that the loss of available chlorine during the drying stage is reduced.
In the past, methods for improving neutral ; 20 calcium hypochlorite processes have included those which ` attempted to eliminate impurities such as calcium chloride from the product.
; U.S. Patent No. 1,713,650, issued May 21, 1929, i~ to A. Georye and R. B. MacMullin, neutral calcium hypo-chlorite is treated with a sodi~n hypochlorite solution in an amount approximately equivalent to the calcium chloride present to reduce the calcium chloride concentration by converting it to neutral calcium hypochlorite.
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In a similar process, as described in U.S.
: Patent No. 1,713,654, issued May 21, 1929, to J. A. Guyer, the calcium chloride content in the neutral calci~n hypochlorite is reduced by forming lime upon reaction with sodium hydroxide.
Alkali metal salts such a.s sodium hypochlorite, sodium hydroxide, sodium carbonate, or trisodium phos-phate are used to reduce the calcium chloride concen-txation in a process for producing calci'um hypochlorite by reacting lime with hypochlorous acid. ~he process, ,as described in U~S. Patent No. 3,134,641, issued May 26, lg64, to R. D. Gleichert, is carried out while maintaining the pH of 10 to 10.5 in the reaction mixture.
While it is desirable, as shown by the above ' 15 processes, to minimize the concentration of calcium chloride or other undesirable impurities, their elimi-nation alone does not result in reduced energy requirements.
Thus there is a need at the present time for an improved process for the produc~ion of calcium hypochlorite having lower energy requirements for drying.
' It is an object o the present invention to provide a process ~or khe production of ~alcium hypo~
chlorite,having a reduced loss of available chlorine duriny its dryincJ.
Another object of the present invention is to provide a process for the production of calcium hypochlorite having improved filterability o~ the slurry form.
A further object of the present invention is to provid,e a process for the production of calcium hypochlorite having lower energy requirements.

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These and other objects of the invention will be apparent from the following detailed description o~
the invention.
The novel process for the production of ` 5 granular calcium hypochlorite compositions comprises:
a) reacting neutral calcium hypo-chlorite with an alkaline solution comprised of an alkali metal hypo-- chlorite and an alkali metal hydroxide to produce a slurry comprised of neutral calcium hypochlorite and a residual amount of hemibasic calcium hypochlorite, and lS b) recovering a wet cake of the neutral calcium hypochlorite and the residual amount of hemibasic calcium hypo-chlorite and drying -the wet cake to produce granular calci~m hypochlorite compositions.
More in detail, the novel process o the present invention employs as one reactant a slurxy of neutral calci.-um hypochlorite, u~ually in the dihydrate form. The qlurry is substant:ially fre~ o~ components Z5 which provide active residual alkalinity. Active residual alkalinity as used in the present specification is defined as the presence of Ca(OH)2, hemibasic calcium hypochlorite, or dibasic calcium hypochlorite in the reactants or products. Neutral calcium hypochlorite slurries may be prepared by any of several processes including, for example, the triple salt process, the '~ reaction of hypochlorous acid with a lime slurry, and the chlorination of dibasic calcium hypochlorite. In a : preferred embodiment, the neutral calcium hypochlorite slurry is produced by the triple salt process. The triple : salt process reacts ~ chlorinated lime slurry comprised of ,~ calcium hypochlori.te and calcium chloride with a slurry of triple salt crys~als having the formula Ca(OCl)2 -NaOCl NaCl 12H20. The lime slurry may be prepared, :

~4-for example, by mixing lime having an active lime content of from about 85 to about 100, and preferably from about 90 to about 97 percent by weight with water to produce an a~ueous lime slurry containing from about l0 to about 50, and preferably from about 15 to about 45 percent by weight of active lime. Active lirae is de~ined by the amount of Ca~OH)2 in the lime.
Typical illustrative specifications or an acceptable lime and for a pre~erred lime are ~s follows:
Component Acceptable Preferred Ca(OH)2 min.% 95.0 98 CaCo3 max.~ 1.0 0.8 : MgO max.% 0.8 0.5 SiO2 max.~ 0.5 0~Z
Fe2O3 ~ A1~03 max.%0.3 0.1 CaSO4 max.~ 0-3 0 05 The average particle size of lime employed in the process generally is substantially all 325 me~h (wet screen analysis) but particles up to about -200 mesh may be employed if desired. ~rhe lime slurry is fed to a reactor to which i~ al~o added chlorine in ei~her ga~eou~
or liquid ~orm. The reactor may be any suitable chlorination apparatus provided with agitation means for maximum contact between chlorine and the lime slurry.
; A particularly suitable reactor is an evaporator-chlorinator of the type used in the chlorination process described in U.S. Patent No. 3,241,912, issued to B. H. Nicolaisen on March 22, 1966. During the chlorina-tion, the temperature within the reactor is maintained within the range from about 0 to about 30C. and prefer-; ably from about 20 to about 25C. Chlorination of the lime slurry forms calcium hypochlorite and calcium chlorid~ in accordance with Equation (1).
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(1) Ca(OH)2 + Cl2 ~ 1/2 Ca(ClO)2 ~ H2O ~ l/2 CaC12 Chlorination of the lime slurry is continued until substantially all of the active lime, Ca(OH)2 is reacted. The chlorinated lime slurry pxoduced is comprised of calcium hypochlorite and calcium chloride.
The slurry is alkaline having a total residual alkalinity which includes inacti~e residual alXalinity provided by lime impurities such as calcium carbonate and magnesia and an active residual alkalinity as defined above.
The active residual alkalinity in the slurry is less than ~2percent by weight.
The chlorinated lime slurry produced is reacted : with a slurry of triple salt cryscals, Ca(OCl)2 NaOCl -NaCl 12H20. Triple salt crystals are produced by lS the chlorination of a slurry of hydrated lime and a sodium hypochlorite solution where the reaction mixture is cooled to a temperature below about -18C. The crystals are separated from the reaction mixture, ~or example, by filtering. The crystals recovered are essentially neutral and have an active alkalinlty content of less than 0.05percent by weight. The triple salt cry~tals are admixed with the chlorinated lime slurry in amount~ which provide su~flcient concen-krations o~ sodium hypochlorite to react with the calcium ~ 25 chloride present in the chlorinated lime slurry and ,; reduce its concentration to less than about 0.5 percent by weight. So~i~D~-hypochlorite present in the triple salt crystals reac~s with calcium chloride to produce a neutral calci~m ~ypochlorite slurry comprised o~ said ` - 30 calcium hypochlorite dihydrate, CatOCl)2 2H2O and ~'; a solution containing sodium chloxide.

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~7~?J7 ; This neutral slurry is admixed with an alkaline solution comprised of an alkali metal hypochlorite and an alkali metal hydroxide. Any suitable alkali metal hypochlorite may be used, for example, sodium hypo-- 5 chlorite, potassium hypochloxite, or lithium hypochlorite, wi~h sodium hypochlorite being preferred. Similarly, alkali metal hydroxides which may be employed include sodium hydroxide, potassium hydroxide, or lithium hydroxide with sodium hydroxide beiny preferred. In order to simplify the disclosure, the alkaline solution of the invention will be described hereafter in terms of sodium hypochlorite and sodium hydroxide. To prevent undesired dilution of the reaction mixture, the concentration of the sodiwm hypochlorite is maintained in the range of from about 20 to about 32 and preferably from about 22 to about 30 percent by weight. The amount of sodium hydroxide present in the alkaline solution is sufficient to produce a residual amount of hemibasic calcium hypochlorite by reaction of the alkaline solution with neutral calcium hypochlorite in the slurry~
Suitable amounts include from about 2 to about ?J0, and preferably from about 4 to about 20 percent b~ weight of sodium hydroxide. While the sodium hydroxid~ may be added to the hypochlorite solution a~ a solid, it is preferred to use a concentrated aqueous solution containing, for example, 50 percent by weight of NaOH.
The reaction is carried out at temperatures in the range of from about 22 to about 30C., preferably at from about 25 to about 28C.
The reaction mixture should be agitated in ; a manner which thoroughly admixes the alkaline solution with the slu\rry. This results in the production of small, fine crystals of hemibasic calcium hypochlorlte which are thoroughly dispersed throughout the reac-tion mixture.
The reaction between the sodium hypochlorite solution containing sodium hydroxide and the calcium hypochlorite in the neutral slurry proceeds in accordance wi-th Equations (2) and ~3).

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_ 7_ 2NaOH ~ Ca~Cl0)2 -~ Ca(OH)~ ~ 2NaC10 ~2) 1/2Ca(OH)2 ~ Ca(OCl)2 ~ Ga(~Gl)2 1/2ca(0H)2 (3) After the reaction is completed, the slurry comprised of neutral calcium hypochlorite dihydrate, sodium chloride, and a residual amount of hemi~aqic calcium hypochlorite is subjected to a solid-liquid separation ~echnique such as filtering or centrifuging to recover a moist cake of neutral calcium hypochlorite, hemibasic calcium hypochlorite, and sodium chloride.
Where the moist cake~ is separated by filtering, the hemibasic calcium hypochlorite crystals presente~
increase the rate of filtration of the slurry over that of slurries produced in conventional processes for producing neutral calcium hypochlorite. The residual amount of hemibasic calcium hypochlorite present in the moist cake is in the range of ~rom about 1 to about 4, and preferably from about 1.5 to about 2.5 percent by weighk. The moist cake is conveyed to a dr~er which is any suitable apparatus capahle of reducing the moisture ; 20 content of the calcium hypochlorite cake to the desired level without causiny excessive decomposition of the calcium hypochlorite particles. During ~i:Lteriny, a iltrate i9 xecovered which comprises a sodium chloride ; soiution containing hypochlorite value~. This filtrate ; 25 may be recycled, ~or example, for use in the production o triple salt crystals.
By insreasing the filtration rate, the process of the present invention provides increased productivity and permits a greater amount of calcium hypochlorite cake to he dried per unit of time resulting in a lower energy cost per unit of product.

During the drying process, the calcium hypo-chlorite cake undergoes substantially less decompo~ition than neutral calcium hypochlorite cakes produced, for example, by conventional processes, resultins in a higher available chlorine content in the dried product.
After drying, a calcium hypochlorite composi-tion is obtained comprised of from about 60 to about 70 percent by weight of neutral calcium hypochlorite, from about 1o to about 20 , and preferably from about 13 to about 16 percent by weight of hemibasic calcium hypochlorite to provide a total calcium hypochlorite concentration in the dry product of from about 65 to about 85, and preferably from about 70 to about 80 percent by weight. The water content is in the range of from about 0.5 to about 10 and preferably from about 5.0 to about 7.5 percent by weight. The remainder of the dry calcium hypochlorite composition is predominantly sodium chloride. The product is then placed in suitable containers, with or without prior size classification or other processing such as pelletizincJ, prior to use, for example, in the sanltation of water supplies.
The novel process o~ the invention may be - conducted ~ a continuous proc~ss by proclucirly the neutral hypochlorite slurry while simultaneously introducing controlled amounts of the alkaline solution ! of the alkali metal hypochlorite and alkali metal hydroxide. When operated as a batch process, for example, a chlorinated lime slurry is reacted in a first stage with a triple salt slurry to produce a slurry of neutral calcium hypochlorite dihydrate. This neutral calcium hypochlorite slurry is then reacted in a second stage to produce the calcium hypochlorite slurry comprised of neutral calcium hypochlorite, a residual amount of hemibasic calcium hypochlorite, and sodium chloride.
The separation and drying of the calcium hypochlorite cake i5 carried out as described above.

~'7~ 7 In a preferred embodiment, the slurry of triple salt crystals is admixed with the chlorinated lime slurry in amounts which provide insufficient concentra~
- tions of sodium hypochlorite to completely react with the calcium chloride present in the chlorinated lime slurry. Sodium hypochlorite present in the trlple ; salt crystals reacts with calcium chloride to produce `; neutral calcium hypochlorite. Unreacted calcium chlorideconcentrations are in the range of from about 2 ~ 10 about 5 and preferably from about 2.5 to about 3.5 : percent by weight. The remaining calcium chloride concentration in the neutral calcium hypochlorite '; slurry is reacted with the sodium hypochlorite in the alkaline solution to produce ad~itional neutral calcium hypochlorite and reduce the concentration of CaC12 to . less than about 0.5 percent by weight. The sodium hydroxide present in the alkaline solution reacts with neutral calcium hypochlorite present to produce the residual amoun~`bf`hemibasic calcium hypochlorite as . 20 described above.
. The fo~lowing examples are presented to illustrate the invention more fully. ~11 parts and ` percentages are by weight unless otherwise speci~ied.

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. EX~PLE 1 A solution of sodium hypochlorite (24 g) was mixed with 9.0 g of a 50 percent solution of sodium hydroxide to produce an al~aline sodium hypochlorite . 5 solution containlng 22;19 percent NaOCl and 13.64 percent NaOH. The alkaline solution (33 g) was blénded in a reactor equipped with a variable speed agitator with a neutral calcium hypochlorite slurry (490 g) contai~ing less than 0.3 percent of calcium chloride, to ~orm a ~inal calcium hypochlorite slurry which contained 21.48 percent Ca(OCl)2 and 0.25 percent NaOCl and had an . active residual alkalinity of 8.27 percent in the form o~
hemibasic calcium hypochlorite. This paste was filtered :; to yield a calcium hypochlorite filter cake having a moisture content of 45.69 percent and an available : chlorine content of 38.79 percent. The cake was dried . in a.vacuum oven at 100 mm Hg absolute pressure and 65C. to produce a hydrated granular calcium hypochlorite product containing 6.6 percent water and 6~.48 percent available chlqrine (wet basi~), indicating a 1058 on drying o~ 6.34 percent o~ the available chlorine (dry basi.s).

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`~! EXAMPLE 2 Triple salt crystals (500 g), having the formula Ca(OCl)2 NaOCl NaCl 12H20 in which the NaOCl concentration was 9.12 percent, were blended with ~ a chlorinated lime slurry haviny a calcium chloride ;. concentration of 15.23 percent. A first calcium ~ hypochlorite slurry was produced containing 28.29 percent `- Ca(OCl)~ 2H2O and 3.21 percent CaC12. In a separate reactor, a solution of sodium hypochlorite (197 g) .. 10 was mixed with 2.73 g of a 50 percent solution of sodium hydroxid~ to produce an alkaline sodium hypo-chloritP solution containing 29.61 percent NaOCl and

2.29 percent NaOH. The alkaline solution (90 g) was ., blended with the first calcium hypochlorite slurry to ., 15 produce a slurry paste containing 28.42 perceIlt Ca(OC1~2 . and 0.56 percent CaC12 and having an alkalinity of 2.87 percent. in the form of hemibasic calcium hypochlorite, Ca(OCl)2 1/2Ca(0~)2. The final paste was filtered to yield a calcium hypochlorite filter cake having a moisture content of 45.16 percent and an available chlorine content of 43.25 percent. The cake wa~ dried in a vacuum oven at 100 mm Hg absolute pressure and 65C. to produce a granular product contain.ing 8:2 percent water and 69.57 percent available chlorine (wet basis), indicath~g a loss on drying o~ 3.91 percent of nvailable chlorine (dry basis).

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_OMPARATIVE EX~MPLE A
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. Triple salt crystals (500 g) of the formula Ca(OCl)2 NaOCl NaCl 12H20 containing 10.53 percent NaOCl were mixed with 333 g of a chlorinated lime slurry :; 5 containing 13.04 percent CaC12 to produce a calcium .. hypochIorite paste containing 22.98 percent Ca(OCl)2 and O.22 p~rcent CaC12. The calcium hypochlorite paste was filtered to p.rovide a filter cake having a moisture content of 44.93 percen~ and an available chlorine content 42~71 percent. The filter cake was dried in a vacuum oven at 100 mm Hg absolute pxessure and : 65C.. to produce a granular product containing 67.~9 percent of available chlorine (dry basis) indicating a loss on drying of 12.46 percent of available chlorine :~ 15 (dry basis).
Using the novel process of Examples l and 2 resulted in a reduction of 49% and 69% respectively in the loss of available chlorine during drying when compared with the prior art process o~ Comparati~e : 20 Example A.

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~:~ A solution of sodium hypochlorite (187 g) was mixed with 12.9 g of a 50 percent solution of sodium hydroxide to produce an alkaline sodium hypochlorite ' 5 solution containing 30.38 percent NaOC1 and 4.66 ~, percent NaOH. The alkaline solu~ion (122 g) was blended in a reactor having a vaxiabl~ speed agitator with a , chlorinated lime slurry (449 g) having a calcium chloride concentration of 15.72 percent, and triple salt crystals (500 g~, having the ~ormula Ca(OCl)2 NaOCl NaCl 12~20, in which the NaOCl concentration was 10.93 percent. A
;v calcium hypochlorite paste was produced which contained 26.53 percent Ca(OCl)2 and 0.40 percent CaC12 and had `~ an alkalinity of 2~34 percent in the form of hemibasic calcium hypochlorite. The ~inal paste was filtered to yield a calcium hypochlorite ~ilter cake having a moisture content of 51.87 percent and an available chlorine of 37~78 percent. The cake was dried in a vacuum oven at 100 mm Hg absolute pressure and 65C. to produce a granular product containing 73.22 percent available chlorine (dry basis) indicating a loss on drying Oe 6.72 percent o~ available chlorine (dry basis).

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Claims (15)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A process for the production of granular calcium hypochlorite compositions which comprises:
a) reacting neutral calcium hypochlorite with an alkaline solution comprised of an alkali metal hypochlorite and an alkali metal hydroxide, the concentration of said alkali metal hypochlorite being from about 20 to about 32 percent by weight and the concentration of said alkali metal hydroxide being from about 2 to about 20 percent by weight, to produce a slurry comprised of neutral calcium hypochlorite and a residual amount of hemibasic calcium hypochlorite, and b) recovering a wet cake of said neutral calcium hypochlorite and said residual amount of hemibasic calcium hypochlorite and drying said wet cake to produce granular calcium hypochlorite compositions, comprised of said neutral calcium hypochlorite and from about 10 to about 20 percent by weight of hemibasic calcium hypochlorite.

2. The process of claim 1 in which said alkali metal hypochlorite is selected from the group consisting of sodium hypochlorite, potassium hypochlorite, and lithium hypochlorite

3. The process of claim 2 in which said alkali metal hydroxide in said alkaline solution is selected from the group consisting of sodium hydroxide, potassium hydroxide, and lithium hydroxide.

4. The calcium hypochlorite composition produced by the process of claim 3.

5. A process for the production of granular calcium hypochlorite compositions which comprises:

a) reacting a chlorinated lime slurry comprised of calcium hypochlorite and calcium chloride with a slurry of triple salt crystals comprised of Ca(OCl)2 ? NaOCl ?
NaCl ? 12H2O, and an alkaline solution comprised of an alkali metal hypochlorite and from about 2 to about 20 percent by weight of an alkali metal hydroxide, to pro-duce a calcium hypochlorite slurry comprised of neutral calcium hypochlorite, sodium chloride, and a residual amount of hemibasic calcium hypochlorite, and b) recovering a wet cake of said neutral calcium hypochlorite, said sodium chloride, and said residual amount of hemibasic calcium hypochlorite and drying said wet cake to produce granular calcium hypochlorite com-positions comprised of said neutral calcium hypochlorite, said sodium chloride and from about 10 to about 20 percent by weight of said hemibasic calcium hypochlorite.

6. The calcium hypochlorite compositions produced by the process of claim 5.

7. A process for the production of granular calcium hypochlorite compositions which comprises:
a) chlorinating an aqueous slurry of lime to pro-duce a chlorinated lime slurry comprised of calcium hypo-chlorite and calcium chloride, b) reacting said chlorinated lime slurry with crys-tals of a triple salt comprised of Ca(OCl)2 ? NaOCl ? NaCl ? 12H2O to produce a first calcium hypochlorite slurry com-prised of neutral calcium hypochlorite, sodium chloride and calcium chloride, said calcium chloride being present in an amount from about 2 to about 5 percent by weight, c) reacting said first calcium hypochlorite slurry with an alkaline solution comprised of sodium hypochlorite containing from about 2 to about 20 percent by weight of sodium hydroxide to form a second calcium hypochlorite slurry comprised of neutral calcium hypochlorite, sodium chloride, and a residual amount of hemibasic calcium hypo-chlorite, and d) recovering a wet cake of said neutral calcium hypo-chlorite, said sodium chloride and said residual amount of hemibasic calcium hypochlorite and drying said wet cake to produce granular calcium hypochlorite compositions comprised of said neutral calcium hypochlorite and from about 10 to 20 percent by weight of said hemibasic calcium hypochlorite.

8. The calcium hypochlorite compositions produced by the process of claim 7.

9. The process of Claim 1, 5 or 7 in which said granular hypochlorite compositions have a water content in the range of from about 5.0 to about 7.5 percent by weight.

10. The process of Claim 5 or 7 in which said chlorinated lime slurry has an active residual alkalinity content of less than 0.2 percent by weight.

11. The process of Claim 1 in which said alkali metal hypochlorite is sodium hypochlorite and said alkali metal hydroxide is sodium hydroxide.

12. The process of Claim 5 in which said alkali metal hypochlorite is sodium hypochlorite and said alkali metal hydroxide is sodium hydroxide.

13. The calcium hypochlorite compositions produced by the process of claim 11.

14. The calcium hypochlorite compositions produced by the process of claim 12.

15. A calcium hypochlorite composition comprised of neutral calcium hypochlorite and from about 10 to about 20 percent by weight of hemibasic calcium hypochlorite and having a water content of from about 5.0 to about 7.5 percent by weight.