Manufacture of calcium hypochlorite

A process for making calcium hypochlorite from caustic soda calcium hydroxide and chlorine by feeding to a stirred reactor an aqueous solution of the total required amount of caustic soda and an aqueous suspension of up to 80% by weight of the total required amount of calcium hydroxide, passing chlorine through the mixture until a redox potential of 650 to 800 mV is reached and while continuing chlorination adding the remaining of the calcium hydroxide at such rate with respect to chlorine that redox potential gradually increases to between 850 and 950 mV. This process prevents thickening and caking which are a recognized problem with conventional batch processes.

This invention relates to improvements in the batchwise manufacture of 
calcium hypochlorite. 
Calcium hypochlorite is widely used for water treatment and in other 
sanitizing applications and for this purpose, the material may contain 
minor amounts of impurities such as calcium chloride and sodium chloride. 
For commercial acceptance, calcium hypochlorite should contain at least 
65% available chlorine less than 2% water and have a certain particle size 
distribution. One process which can be used to manufacture a product 
meeting this specification is the interraction of calcium hydroxide, 
caustic soda and chlorine according to the following equation: 
EQU 3Ca(OH).sub.2 +2NaOH+4Cl.sub.2 .fwdarw.2Ca(OCl).sub.2 +2NaCl+CaCl.sub.2 
+4H.sub.2 O 
A wide range of process exists for the production of calcium hypochlorite. 
Batchwise processes were first developed and rapidly abandoned because 
they faced a critical thickening/caking problem which limits their use. To 
minimize the viscosity problem which made it most difficult to stir and 
chlorinate the lime slurry, it was necessary to add large amounts of 
dilution water. However such addition of extra dilution water resulted in 
substantial loss in process yield. 
Through further development, continuous operation within some specific 
conditions permitted to avoid this viscosity problem and obtain higher 
yield through optimization of the amount of process water used. Based on 
this, numerous continuous processes were developed industrially. However, 
they operate in more or less narrow conditions gap which makes their 
control more critical. This leads to less flexible processes which are 
more influenced by fluctuations of the operating conditions. 
Furthermore, the continuous processes, by their nature, require more 
complex equipment than the batch process and thus necessitate higher 
capital investment. This makes such processes much less economically 
attractive for small production requirements especially when one considers 
that equipment used in this technology is made out of titanium. 
Typically, the conventional batch process is carried out by passing 
chlorine gas into a slurry of lime and caustic soda in water. When the 
reaction is completed, the product is separated from the mother liquor and 
dried. Conveniently, the sodium hydroxide may be supplied as a 
concentrated solution as normally obtained from caustic soda-chlorine 
manufacturing operations. In any case, the sodium hydroxide should be 
dissolved in water prior to chlorination. The high grade lime is added as 
a slurry to the sodium hydroxide solution and chlorine is passed through 
the agitated sodium hydroxide/calcium hydroxide slurry at appropriate rate 
while the temperature is maintained in the range of 5.degree. 
C.-40.degree. C. 
After reaction, the calcium hypochlorite is recovered and separated from 
the mother liquor in the batch by any suitable conventional solids-liquid 
separation technique. The final stages of the process are drying and 
"forming" of the product to meet the needs of particular consumers. 
Various processing alternatives are well known. 
In those developed batch processes, all caustic soda and lime required for 
the reaction is added into the reactor before starting the chlorination. 
During chlorination, the viscosity of the reacting slurry varies, reaching 
a maximum when about 30% of total chlorine has been added. This maximum 
viscosity peak always corresponds to 650 to 700 mV redox potential 
measurement. At this point, the viscosity can be 100 times or more higher 
than normal and the reacting slurry thickens severely. Sometimes hard 
caking of the whole reactor batch can be observed. 
The reason for this thickening and caking to occur is not entirely known 
and the ever present possibility of such a situation developing always has 
been an objection for major industrial development of the batch process. 
It has now been found that the batch process operating conditions may be 
modified to avoid such a viscosity problem, 
Accordingly, in the known process of preparing a slurry of calcium 
hypochlorite by reacting chlorine with an aqueous slurry of calcium 
hydroxide and sodium hydroxide, the present invention provides the 
improvement consisting of: 
(a) loading a stirred reactor with an aqueous solution of the total 
required amount of sodium hydroxide, and from 10% to 80% by weight of the 
total required amount of calcium hydroxide as an aqueous suspension. 
(b) passing chlorine through this suspension until the reaction mixture has 
reached a redox potential in the range of 650 to 800 mV; 
(c) while continuing chlorination, continuously adding the remaining of the 
calcium hydroxide at such a rate with respect to chlorine that the redox 
potential gradually increases to a value from 850 to 950 mV; and 
(d) collecting the resulting slurry of calcium hypochlorite, while 
maintaining the temperature of the reaction mixture throughout the 
reaction in the range from 10.degree. C. to 30.degree. C. 
The rates of addition of the reagents are not narrowly critical. The 
temperature of the reaction may be partially controlled by the rate of 
addition of the reactants. The reactants may be cooled by any convenient 
method. Preferably temperature is controlled by means of cooling coils. 
The product from the process comprises an aqueous slurry of calcium 
hypochlorite and the latter may be separated from the mother liquor and 
dried by any convenient method known in the art. Before solids-liquids 
separation and further processing by any of said convenient methods, the 
calcium hypochlorite slurry can advantageously be cooled down for maximum 
crystallization of soluble hypochlorite. 
The molar ratio of calcium hydroxide to caustic soda to chlorine is 
approximately that required by the equation shown above i.e. 3:2:4. The 
amount of caustic soda is not critical and can be varied as long as the 
total amount of chlorine is adjusted to meet the stoichiometric demand. In 
the first step of the process, calcium hydroxide is fed as a suspension in 
the aqueous solution of the caustic soda. In the third step the calcium 
hydroxide is fed as a suspension in sufficient water to produce a reactor 
slurry which is adequately stirrable and pumpable. The total of water used 
in the process is not critical in that it does not substantially affect 
the occurring chemical reactions but it has importance with regard to 
process yield and product quality. Whereas, for instance, a process using 
a certain amount of water would yield a product having 85% available 
chlorine, the same process using 50% less water will result in a product 
having only 55% available chlorine and an increase of 40% in yield. 
Because of this phenomenon which is well known to those versed in the art, 
care must be exercised in formulating the charge of reactants to select 
the amount of water which will meet the required specifications for yield 
and product quality. 
The proper amount of calcium hydroxide (lime) to be added in the first step 
of the process can vary from 10% to 80% by weight of the total amount 
required in the process and is apparently function of the physical 
properties of the particular lime used as a starting material. From the 
following examples, it will be realized that the amount of lime which has 
to be fed into the first step in order to obtain optimum results varies 
widely from one particular lime to another. While it is stated above that 
this may depend upon the physical properties of the particular lime used 
as starting material, the reasons why this is so are not known. Therefore 
the proper amount of lime to be added in the first step must be determined 
experimentally. 
Compared with the conventional batch process, the present process offers 
several important advantages. For instance it improves control of the 
viscosity of the reaction mixture. Indeed whereas, in the conventional 
batch process the viscosity of the reaction mixture increases markedly as 
the reaction proceeds, the present two step lime addition technique allows 
the reaction mixture to remain fluid at anytime. 
The above advantage coupled with control of the rate of addition of lime in 
the third step of the process allow formation of crystals of good setting 
rate easily filtered and containing relatively low moisture (about 30%). 
The process is highly reproducible and can be computer controlled through 
measurement of the oxidoreduction potential of the reaction mixture. 
In addition the process has a high flexibility. A wide range of product 
quality can be achieved by varying the total amount of water and caustic 
soda used. Such variation has no significant effect on the reaction 
mixture viscosity during chlorination.

The invention is illustrated by, but by no means limited to the following 
examples. 
EXAMPLE I 
Calcium hypochlorite was prepared in nine batches using lime A (SOBRENICK 
lime). Batches 1 and 2 were carried out by the conventional batch process 
whereas batches 3 to 9 were made according to the process of the present 
invention. In all of the batches, the reaction temperature was maintained 
at 15.degree. C. to 25.degree. C. and the total amount of lime used was 
the same. Total amount of water and/or total amount of caustic soda could 
be varied from one batch to the other. In batches 3 to 9 carried out by 
the process of the invention, two thirds of the total lime was added with 
the total caustic soda in the first step of the process whereas one third 
was added in the third step as a 30% water slurry. The results obtained 
with the nine batches were as follows: 
TABLE I 
______________________________________ 
Total Total % Available 
Batch Total NaOH Lime % Yield 
Cl.sub.2 in dried 
No. H.sub.2 O kg 
kg kg on Cl.sub.2 
product 
______________________________________ 
1 209.5 24.2 71.3 0 0 
(caking; product lost 
viscosity &gt; 500 units) 
56.3 84.9 
2 251.9 24.2 71.3 (hard caking but batch 
saved) 
viscosity &gt; 20 units 
3 209.5 24.2 71.3 68.4 84.4 
4 189.0 24.2 71.3 72.3 83.0 
5 178.0 24.2 71.3 80.2 78.0 
6 168.0 24.2 71.3 81.1 69.1 
7 225.0 24.2 71.3 65.5 * 
8 225.0 26.5 71.3 70.1 * 
9 225.0 30.1 71.3 72.0 * 
______________________________________ 
*not calculated 
In all of batches 3 to 9 the viscosity was around 5 units. 
EXAMPLE 2 
Calcium hypochlorite was prepared in four batches using lime B (Steel 
Brothers lime) at a reaction temperature of 15.degree. C. to 25.degree. C. 
All batches were done using a total of 215 kg H.sub.2 O, 23.4 kg NaOH and 
65.6 kg lime (97% Ca(OH).sub.2). 
To start the reaction, all the caustic soda with required amounts of water 
and lime were loaded into the reactor. For batches 2 to 4, part of the 
lime and enough water to get a 30% lime suspension were saved for a second 
lime addition. The results were as follows: 
TABLE II 
______________________________________ 
% of % 
Total Lime used in Available 
Batch 1st 2nd % Yield on 
Cl2 in Settling 
No. Addition Addition CL.sub.2 
Product Test 
______________________________________ 
1 100 0 0 0 -- 
caking; batch lost 
2 66 33 severe 900 
caking 
3 30 70 81.3 71.7 540 
4 20 80 77.1 63.5 495 
______________________________________ 
Settling Test: 
Total volume of thickened slurry after 2 hours settling as compared to an 
initial slurry sample of 1000 cc.