CA1290135C - Calcium hypochlorite process - Google Patents
Calcium hypochlorite processInfo
- Publication number
- CA1290135C CA1290135C CA000523544A CA523544A CA1290135C CA 1290135 C CA1290135 C CA 1290135C CA 000523544 A CA000523544 A CA 000523544A CA 523544 A CA523544 A CA 523544A CA 1290135 C CA1290135 C CA 1290135C
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- Prior art keywords
- mother liquor
- calcium
- calcium hypochlorite
- dibasic
- hydroxide
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
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Abstract
ABSTRACT
A process for the production of calcium hyopchlorite from calcium hydroxide and chlorine in the presence of sodium hydroxide and dibasic calcium hypochlorite wherein the dibasic calcium hypochlorite is prepared from the calcium hypochlorite containing primary mother liquor and recycled evaporator mother liquor, and calcium hydroxide.
The process provides improved quality calcium hypochlorite products and sodium chloride by-product. It also avoids the separation of co-crystallized calcium hypochlorite and sodium chloride and the precipitation and separation of calcium hydroxide as part of the recovery process.
A process for the production of calcium hyopchlorite from calcium hydroxide and chlorine in the presence of sodium hydroxide and dibasic calcium hypochlorite wherein the dibasic calcium hypochlorite is prepared from the calcium hypochlorite containing primary mother liquor and recycled evaporator mother liquor, and calcium hydroxide.
The process provides improved quality calcium hypochlorite products and sodium chloride by-product. It also avoids the separation of co-crystallized calcium hypochlorite and sodium chloride and the precipitation and separation of calcium hydroxide as part of the recovery process.
Description
~90~3~;
Calcium hypochlorite is used primarily in the solid form as a bleaching and sanitizing agent, particularly for swimming pool chlorination. The typical commerci~al product contains 65-75~ calcium hypochlorite with the balance being : 5 mainly sodium chloride, small quantities of cal:cium hydroxide and moisture.
Calcium hypochlorite is produced industrially by reacting an aqueous mixture of calcium hydroxide and ~odium hydroxlde with chlorine accordlng to the overall equatl~on Ca(OH32 + 2NaOH + 2C12 ~ t Ca(OCl)2 + 2NaCl + 2H20 ~: :
: The crystalline calcium hypochlorite produced is separated from the aqueous phase as a wet cake and contains : ~ . ; : : a~slgnificant quantity of the aqueous phase, or mother liquor, which ~onsists mainly of calcium hypochlorite and sodium chloride in solution. When the crystalline calcium hypochlorite cake is dried thi.s qodium chloride remains in the product and, typically, constitutes between 15 and 20%
of the final product by weight.
Thé sodium chloride content in the solid final product does not constitute all the sodium chloride produced in the primary reaction and the sodium chloride remaining in : solution in the mother~liquor must be removed from the ~: ~ process either by disposition of the mother liquor or by :
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.. ~, ~, , ~ , "
~29~135 C-I-L 714 .
; some other ~eans. However, disposal o~ the mother liguor is not economically attractive as the mother liquor contains significant quantities of calcium hypochlorite in solution.
Two of the raw materials for the calcium hypochlorite process, sodium hydroxide and chlorine, are produced industrially by electrolyzing an aqueous solution of sodium chloride to produce sodium hydroxide, chlorine and hydrogen, viz:-2NaCl -~ 2H2O J 2NaOH ~ C12 + H2 Since sodium chloride is inevitably a by-product of - the calcium hypochlorite process it would be advantageous if this sodium chloride by-product were suitable for sodium hydroxide and chlorine production. However, in this respect, it is important that the sodium chloride contain minimal quantities of calcium ion as calcium ion is deleterious to the operation of the electrolytic cells used for sodium hydroxide and chlorine production and, thus, must be reduced to low concentrations in the feed brin~ to these cells.
Sodium hydroxide is produced industrially in electrolytic cells of the mercury, diaphragm or m~mbrane type~. Mercury cells typically produce sodium hydroxide at approximately 50% strength directly. Diaphragm cells produce a low strength sodium hydroxide mixed with sodium chloride and this mixture is evaporated to ~ive a 50~
caustic product and to remove most of the sodium chloride.
~embrane cells produce sodium hydroxide at up to 35%
concentration. As sodium hydroxide is a raw material for the production of calcium hypochlorite it follows that, using commercial strength caustic, water will also be introduced along with the sodium hydroxide. This quantity of water, plus that produced during the chemical reaction , ~ , ' ,, , ;~ ' , ' :
. . ,, : , : , ~;~9~35 to manufacture calcium hypochlorite exceeds the water that leaves the process with the product calcium hypochlorite (typically 3-10% by weight o~ the final product). This water imbalance could be managed by the use of highly concentrated sodium hydroxide (greater than 50~ strength~
to reduce the water entering the process or by evaporation of the excess water from within the process. However, high concentration sodium hydroxide is produced by the evaporation of commercial strength solution under vacuum and, thus, hi~h concentration sodium hydroxide is a more expensive feed material than concentrations commercially available.
It is known that calcium hypochlorite may be partially recovered from mother liquor containing calcium hypochlorite, sodium chloride and water by the addition of calcium hydroxide. The addition of calcium hydroxide - causes the precipitation of dibasic calcium hypochlorite, according to the equation.
Ca(OCl)~ + 2Ca(OH)2 ~ Ca(OCl)2O2Ca~OH)2 The dibasic calcium hypochlorite so produced may be separated by conventional separation techniques and recycled to a slurry chlorinator. In the slurry chlorinator the dibasic compound is chlorinated together with sodium hydroxide to form calcium hypochlorite, sodium chloride and water. All or part o~ the calcium hydroxide required for the overall reaction may be added to the slurry chlorinator. The total quantity of calcium hydroxide required for the process is in excess of that required for the production of dibasic calcium hypochlorite, and therefore, if all the calcium hydroxide required for the overall reaction is added to the dibasic calcium hypochlorite reactor, the dibasic calcium hypochlorite after separation from the mother liquor will ~ ' , .
.
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.
129~35 contain excess calcium hydroxide.
The mother liquor remaining after separation of the dibasic calcium hypochlorite contains residual calcium hypochlorite and sodium chloride in solution and thus a S complete recovery of the total quantity of calcium hypochlorite produced is not obtained.
The patent literature describes many processes for the production of calcium hypochlorite of which the following are relevant to the present invention.
U.S. Patent No. 1,718,284 describes a process in which calcium hydroxide and sodium hydroxide are mixed with a eutectic solution of calcium hypochlorite and sodium chloride and then chlorinated. Sodium chloride cystallizes from the metastable solution of calcium hypochlorite and is carefully separated. Crystallization of calcium hypochlorite from the remaining liquor is then initiated.
U.S. Patent No. 3,572,989 describes a process in which recycled mother liquor containing calcium hypochlorite and sodium chloride is mixed with sodium hydroxide of high concentration and chlorinated. Sodium chloride crystals are precipitated and removed. The remaining liquor is mixed with calcium hydroxide and chlorinated to produce crystalline calcium hypochlorite which is separated. The mother liquor is recycled U.S. Patent No. 3,767,775 decr;bes a process in which sodium hydroxide, calcium hydroxide, water and a mother liquor saturated with calcium hypochlorite and sodium chloride from a later stage in the process are chlorinated.
Calcium hypochlorite and sodium chloride are co-crystaI-lized, separated from each other and the mother liquor recovered and recycled to the chlorinator.
U.S. Patent No. 3,~95,099 describes a process for the manufacture of calcium h~pochlorite in which the mother liquor containing dissolved calcium h~pochlorite and sodium chloride is reacted with sodium hydroxide to produce a ~ . .
~29~l35 slurry of calcium hydroxide in sodium hypochlorite and sodium chloride. The calcium hydroxide is separated and recycled to the primary reactor.
V.S. Patent No. 3,950,499 describes a process for the manufacture of calcium hypochlorite in which mother liquor containing calcium hypochlorite and sodium chloride is mixed with sodium hydroxide and chlorinated. Sodium chloride crystals are produced and are separated from the aqueous liquor. Calcium hydroxide and sodium hydroxide are added to this aqueous liquor and the mixture is chlorinated. A slurry containing sodium chloride and calcium hypochlorite crystals is produced. The crystals of calcium hypochlorite and sodium chloride are separated from each other and the mother liquor recycled.
U.S. Patent No. 3,954,948 describes a process for the manufacture of calcium hypochlorite in which calcium hydroxide and sodium hypochlorite are chlorinated to form a solution containing crystals of calcium hypochlorite. The calcium hypochlorite crystals are separated and the solution reacted with sodium hydroxide to produce a calcium hydroxide slurry. The calcium hydro~ide is separated and recycled. The remaining solution may be used as bleach liquor or processed to produce solid sodium chloride and a sodium hypochlorite solution which may be recycled.
U.S. Patent No. 4,196,184 describes an improved version of the above process in which the calcium hydroxide slurry is produced under closely controlled conditions which result in the formation of some hemi-basic calcium hypochlorite which improves the separation characteristics of the calcium hydroxide.
U.S. Patent No. 4,258,024 describes a process in which calcium hydroxide is added to a recycled mother liquor containing calcium hypochlorite. Crystalline dibasic calcium hypochlorite is produced and separated from the mother liquor. The dibasic hypochlorite is mixed with high .
. .
: . .
. . . .. .
129~35 C-I-L 714 strength sodium hydroxide and chlorinated to produce a slurry containing calcium hypochlorite and sodium chloride crystals. The crystals are separated from each other and the mother liquor recycled.
U.S. Patent Nos. 4,328,200 and 49390,512 describe a process in which sodium chloride and calcium hypochlorite are co-crystallized and the crystals then separated. A
recycled aqueous solution of sodium chloride and calcium hypochlorite is mixed with sodium hydroxide and calcium hydroxide chlorinated in the presence of a seed bed of sodium chloride and calcium hypochlorite crystals. The two types of crystal are then separated in a classification zone. The crystals are separated romm the mother liquor which is then recycled.
~rom a review of the above prior art and from a review of patents not described hereinabove which relates only to improvements in specific process steps, it is clear that processes in which sodium chloride and calcium hypochlorite crystals are co-crystallized from a~solution are inherently difficult to operate. Many techniques are described to enhance the separation efficiency, particularly with regard to the crystallizing conditions necessary to grow crystals o~ different sizes which may be physically separated. It is also clear that there will be some contamination of the calcium hypochlorite crystals with sodium chloride crystals and vice versa. Use of wash water to improve the quality of the products is not practical as these processes operate under a tight water balance, maintained by using high strength caustic soda, to obtain the required product yield.
It is/ thus, an object of this invention to provide an improved industrial process ~or the production of calcium hypochlorite in which the calcium hypochlorite produced is effectively all recovered as product and the excess co-produced sodium chloride lS recovered in crystalline , ,:' .
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129~5 C-I-L 714 form containing calcium impurities in concentrations similar to those typically present in sodium chloride used for chlorine and sodium hydroxide production.
It is a further object of the present invention to s provide a commercial process of improved flexibility in the production of calcium hypochlorite.
It is a further object to provide a process using a single step chlorination.
It is a further object to provide a process which avoids the separation of co-crystallized calcium hypochlorite and sodium chloride and avoids the precipitation and separation of calcium hydroxide as part of the recovery process.
It is a further object to provide a process utilizing the common commodity 50~ sodium hydroxide as a raw material without further evaporation, or to use sodium hydroxide at lower than 50% concentration.
It is a further object to provide a process in which effectively all of the calcium and hypochlorite values produced in the overall reaction are accounted for in the desired solid calcium hypochlorite product.
It is a further object to provide a process which does not produce a solution containing calcium and hypochlorite values either as a by-product stream of lesser value, or as a waste stream of zero or negative value due to disposal costs.
It is a further object to provide a process which is free of objectionable effluents, wherein the only outputs from the process are the desired solid calcium hypochlorite product, solid sodium chloride, and condensed water vapour.
~ccordingly, the invention provides (a) reacting aqueous sodium hydroxide, calcium hydroxide and dibasic calcium hypochlorite in a slurry chlorinator with chlorine to produce crystalline calcium hypochlorite and a primary mother liquor .
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~90135 containing calcium hypochlorite;
(b) separating said crystalline calcium hypochlorite from said primary mother liquor;
(c) adding calcium hydroxide and an evaporator mother liquor, as hereinafter defined, to said primary mother liquor to effect production of crystalline dibasic calcium hypochlorite and a dibasic mother liquor;
(d) separating said crystalline dibasic calcium hypochlorite from said dibasic mother liquor;
(e) feeding said crystalline dibasic calcium hypochlorite to said slurry chlorinator;
(f) evaporating said dibasic mother liquor to produce crystallized sodium chloride and said evaporator mother liquor;
(g) separating said crystaIlized sodium chloride from said evaporator mother liquor; and (h) feeding said evaporator mother liquor to said primary :~ :
mother liquor as hereinbefore de~fined in step (c).
The calcium hydroxide of use in:the slurry:chlorinator may be added directly as such or, preferably, added with the dibasic calcium hypochlorite as a solid mixture of dibasic calcium hypochlorite - calcium hydroxide produced : by .the addition of calcium hydroxide in excess to produce said solid mixture of the formula: ~
. .
: Ca(OC12~2.2 Ca(OH)2 / Ca(OH)2 ~ :
The resultant dibasic mother liquor is subjected to 30 evaporation as hereinbefore defined. .
Accordingly, in a preferred feature the invention ; provides a process as hereinbefore defined wherein said ¢alcium hydroxide is added in step (c) in stoichiometric excess of said calcium hypochIorite in said primary mother : 35 liquor to effect production of a solid mixture of :
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~90135 C-I L 714 crystalline dibasic calcium hypochlorite-calcium hydroxide and dibasic mother liquor; and further comprising separating said solid mixture from said dibasic mother liquor and feeding said solid mixture to said slurry S chlorinator and evaporating said dibasic mother liquor as hereinbefore defined.
In an alternative preferred feature, the invention provides a process as hereinbefore defined wherein said calcium hydroxide is added in step ~c) in stoichiometric excess of calcium hypochlorite present in the primary mother liquor and the evaporator mother liquor to effect production of said solid mixture of crystalline dibasic calcium hypochlorite - calcium hydroxide.
It is well known that hypochlorite solutions are subject to loss of active chlorine values, the rate of loss being dependent on hypochlorite concentration, alkalinity, temperature and the presence of catalysts such as cobalt and nickel. The loss of active chlorine values can be by decomposition to chloride and oxygen or by chemical conversion ~o chlorate. The rate of loss of active chlorine values can be minimized ~y selecting conditions of high pH and low temperature in the absence of catalysts.
It is a preferred feature of the invention that the evaporation of the dibasic mother liquor step takes place at low hypochlorite concentration and at a pH of 10.5 A low temperature is maintained by carrying out the evaporation under vacuum. The combination of low temperature and short evaporator residence time leads to an insignificant loss o~ active chlorine in the evaporation step. It is clear that the selection of said appropriate evaporation conditions resides in the ~kill of the art.
By the ~traight forward design adjustment of increasin~ the water evaporation in the evaporator the use of any required amount of water for washing crystal streams within the process is permitted. This water is then ', :
--~ . ., ~ . . , . - . . . :
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. ' , ', ": ' ", .. , , .. , ~29~35 C-I-L 714 evaporated, condensed and recycled to the washing stages.
In this way sodium chloride crystals of high purity can be produced. In contrast, processes using high strength caustic and not employing evaporation cannot accommodate the addition of wash water to the system and thus can co-produce only relatively impure product sodium chIoride.
The separation steps may be carried out using conventional separation equipment such as settlers, filters, centrifuges and screens. The preparation of crystals of a specific crystal size to permit the separations is not necessary. The typical complex process steps of the art re~uired to simultaneously co-crystallize sodium chloride and calcium hypochlorite and subsequently separate the two crystal types are thus avoided in the present process.
The process can use sodium hydroxide at all commercial streng~hs including the low strength (30 to 35%) sodium hydroxide typically produced by membrane cells. The additional water introduced with the lower strength caustic may be evaporated in the evaporator and pure condensate recycled to a chloralkali plant.
It will be appreciated by those knowledgeable in the -art that the process of the invention is adaptable and suited to batch, semi-continuous or continuous operations and is, thus, suitable for a wide ~ange of plant capacities.
It can further bé seen that the instant process required only a single chlorination step. This si~plifies the process control instrumentation and reduces capital cost.
In order that the invention may be better understood a preferred embodiment will now be described, by way of example only, with reference to the accompanying figure which shows a simplified block diagram of a preferred 35 process according to the in~ention.
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~290~5 C-I-L 714 Commercial strength sodium hydroxide, typically, 30-50% by weight NaOH in water is fed as stream 10 to a slurry chlorinator 11 wherein it is combined with a mixture of dibasic calcium hypochlorite-calcium hydroxide fed as stream 12 from a dibasic separator 13 as hereinafter described. Chlorine is passed as stream 14 into slurry chlorinator 11 until essentially all of the calcium hydroxide present is converted to calcium hypochlorite.
The chlorination process is exothermic and takes place at a controlled temperature in the range of 15 - 30C with cooling.
The slurry after chlorination has the general composition:
Typical Range Ca(OCl)2 22 % 20-24 NaCl 16 % 15-17 %
Ca(OH)2 0-3% 0.1-0.6%
Miscellaneous0.2% 0.0-0.5%
H2O 61.5~ 60-65 After chlorination, the slurry is fed as stream 15 to a separator 16 wherein calcium hypochlorite as product is ` removed as stream 17 and the resultant neutral primary mother liqu~r is transferred as skream 18 to a dibasic crystallizer 19. Separator 16 may be any suitable -equipment for separating the crystalline calcium hypochlorite from the mother liquor, such as a filter or centrifuge. The type of equipment and the method of its operation determines the composition of the calclum hypochlorite stream 17 but this stream typically contains:
Ca(OCl)2 40-60%
NaCl 4-15%
H2O 30-50%
- .
.
,~ , . ..
~9~35 --1~--In a further stage of the industrial process, not shown in the Figure, the calcium hypochlorite of stream 17 is dried, under carefully controlled conditions to avoid or minimize product decomposition, to give a final "dry"
product containing less than 10% by weight of water, typically 5 to 8~.
In a variation on the basic process part of the neutral primary mother liquor stream 18 may be recycled to slurry chlorinator 11 to control the desired slurry strength in the slurry chlorinator 11. The slurry strength in slurry chlorinator 11 will vary depending upon the concéntration of the sodium hydroxide used as feed material. The recycle of neutral primary mother liquor i~
advantageous if the chlorination is performed batchwise.
To dibasic crystallizer 19 is also added calcium hydroxide, as stream 20, and a recycled evaporator mother liquor, as stream 21, as hereinafter described. In dibasic crystallizer 19, most of the calcium hypochlorite of the primary mother liquor is crystallized as dibasic caIcium ;
hypochlorite, Ca(OCl)2.2Ca(OH)2 by the calcium hydroxide added. The calcium hydroxide may be added directly to dibasic crystallizer 19 or, to improve the operability by minimizing the formation of lumps of calcium hydroxide, may be mixed with a recycled dibasic mother liquor (as hereinafter described) in a mixer 22 prior to addition to the dibasic crystallizer as stream 23. The~quantity of calcium hydroxide~introduced into the dibasic crystallizer may be any quantity up to the total quantity required ~or the overall reaction However, it is preferred that the quantity added iq sufficient to maintain at least a molar ratio of 2 parts calcium hydroxide to one part calcium hypochlorite. If the total ~uantity of calcium hydroxide i9 not added to the dibasic crystallizer l9 the balance of the lime may be added to slurry chlorinator 11. Although not advantageous, the source of calcium hydroxide may be ' .: ' ,. :
. .
129~13~ C-I-L 714 from calcium chloride and a base. Dibasic crystallizer 19 is typically an agitated mixing vessel. The temperature in the dibasic crystallizer may range from aprroximately 15 -80C but is preferably in the range of 40 - 50C which 5 provides a crystal slurry of good separation characteristics.
The crystal slurry from dibasic crystallizer 19 is fed as stream 24 to dibasic separator 13 and separated into solids stream 12 consisting of the mixture of dibasic calcium hypochlorite and calcium hydroxide and dibasic mother liquor stream 25. The dibasic separator may be any suitable separation device such as a filter or centrifuge.
The composition of the dibasic calcium hypochlorite/
calcium hydroxide mixture after separation, stream 12, will vary depending on the equipment selected and depending on the use of an optional water wash but will typically analyze as shown below:
Typical ~E~
Ca(OCl)2 21% 16-30%
Ca(OH)2 30% 20-40%
NaCl 11~ 1-15%
H2O 38% 30-50% ~ -Dibasic mother liquor, stream 25, contains approximately 3~ dissolved calcium hypochlorite and 20-23%
dissolved sodium chloride. The solution is alkaline, approximately pH 11. Optionally part of the liquor may be recycled to prepare a calcium hydroxide slurry as previously described. The balance of the liquor stream is evaporated in an evaporator crystallizer 26 to give a solutlon containing approximately 8 to 10% calcium hypochlorite in solution. During the evaporation process sodium chloride is crystallized from the solution. To minimize decomposition of hypochlorite the evaporation .
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~Z90135 C-I-L 714 process is performed under vacuum so that the operating temperature of the evaporator is in the range of 30-60C.
The water vapour evaporated is condensed. Part of this water may be recycled for process use, if required, with the balance (representing the difference between the water introduced into the overall process with the sodium hydroxide and that removed in the dry product) discharged as condensate, as stream 27.
A salt crystal slurry, stream 28, is withdrawn from evaporator crystallizer 26 and separated into a salt stream 29 and a concentrated recycled dibasic mother liquor stream 21 as hereinbefore described in a salt separator 30. Salt separator 30 may be any suitable separation device such as a filter or centrifuge. Salt is removed from the process as stream 29. The quality of the salt by-product may be impoved by washing the salt with water or brine to dispIace residual debasic mother liquor on the crystals, thereby reducing the calcium content of the salt stream 29.
Typical analyses are:
Unwashed Washed NaCl balance balance Ca 0.3-0.85% 0.1-0.25%
~32 2-5% 2-5%
The concentrated dibasic mother liquor stream 25 is of similar composition to that of neutral primary mother liquor stream 18 and may be used in place o~ this primary mother liquor in, for example, slurry chlorinator 11, if advantageous.
Thus, it can be readily seen that the process provides almost complete recovery o~ the calcium and hypochlorite values in the process streams, thus maximizing the process yield and minimiæing raw material requirements.
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Calcium hypochlorite is used primarily in the solid form as a bleaching and sanitizing agent, particularly for swimming pool chlorination. The typical commerci~al product contains 65-75~ calcium hypochlorite with the balance being : 5 mainly sodium chloride, small quantities of cal:cium hydroxide and moisture.
Calcium hypochlorite is produced industrially by reacting an aqueous mixture of calcium hydroxide and ~odium hydroxlde with chlorine accordlng to the overall equatl~on Ca(OH32 + 2NaOH + 2C12 ~ t Ca(OCl)2 + 2NaCl + 2H20 ~: :
: The crystalline calcium hypochlorite produced is separated from the aqueous phase as a wet cake and contains : ~ . ; : : a~slgnificant quantity of the aqueous phase, or mother liquor, which ~onsists mainly of calcium hypochlorite and sodium chloride in solution. When the crystalline calcium hypochlorite cake is dried thi.s qodium chloride remains in the product and, typically, constitutes between 15 and 20%
of the final product by weight.
Thé sodium chloride content in the solid final product does not constitute all the sodium chloride produced in the primary reaction and the sodium chloride remaining in : solution in the mother~liquor must be removed from the ~: ~ process either by disposition of the mother liquor or by :
:: ::: `
.. ~, ~, , ~ , "
~29~135 C-I-L 714 .
; some other ~eans. However, disposal o~ the mother liguor is not economically attractive as the mother liquor contains significant quantities of calcium hypochlorite in solution.
Two of the raw materials for the calcium hypochlorite process, sodium hydroxide and chlorine, are produced industrially by electrolyzing an aqueous solution of sodium chloride to produce sodium hydroxide, chlorine and hydrogen, viz:-2NaCl -~ 2H2O J 2NaOH ~ C12 + H2 Since sodium chloride is inevitably a by-product of - the calcium hypochlorite process it would be advantageous if this sodium chloride by-product were suitable for sodium hydroxide and chlorine production. However, in this respect, it is important that the sodium chloride contain minimal quantities of calcium ion as calcium ion is deleterious to the operation of the electrolytic cells used for sodium hydroxide and chlorine production and, thus, must be reduced to low concentrations in the feed brin~ to these cells.
Sodium hydroxide is produced industrially in electrolytic cells of the mercury, diaphragm or m~mbrane type~. Mercury cells typically produce sodium hydroxide at approximately 50% strength directly. Diaphragm cells produce a low strength sodium hydroxide mixed with sodium chloride and this mixture is evaporated to ~ive a 50~
caustic product and to remove most of the sodium chloride.
~embrane cells produce sodium hydroxide at up to 35%
concentration. As sodium hydroxide is a raw material for the production of calcium hypochlorite it follows that, using commercial strength caustic, water will also be introduced along with the sodium hydroxide. This quantity of water, plus that produced during the chemical reaction , ~ , ' ,, , ;~ ' , ' :
. . ,, : , : , ~;~9~35 to manufacture calcium hypochlorite exceeds the water that leaves the process with the product calcium hypochlorite (typically 3-10% by weight o~ the final product). This water imbalance could be managed by the use of highly concentrated sodium hydroxide (greater than 50~ strength~
to reduce the water entering the process or by evaporation of the excess water from within the process. However, high concentration sodium hydroxide is produced by the evaporation of commercial strength solution under vacuum and, thus, hi~h concentration sodium hydroxide is a more expensive feed material than concentrations commercially available.
It is known that calcium hypochlorite may be partially recovered from mother liquor containing calcium hypochlorite, sodium chloride and water by the addition of calcium hydroxide. The addition of calcium hydroxide - causes the precipitation of dibasic calcium hypochlorite, according to the equation.
Ca(OCl)~ + 2Ca(OH)2 ~ Ca(OCl)2O2Ca~OH)2 The dibasic calcium hypochlorite so produced may be separated by conventional separation techniques and recycled to a slurry chlorinator. In the slurry chlorinator the dibasic compound is chlorinated together with sodium hydroxide to form calcium hypochlorite, sodium chloride and water. All or part o~ the calcium hydroxide required for the overall reaction may be added to the slurry chlorinator. The total quantity of calcium hydroxide required for the process is in excess of that required for the production of dibasic calcium hypochlorite, and therefore, if all the calcium hydroxide required for the overall reaction is added to the dibasic calcium hypochlorite reactor, the dibasic calcium hypochlorite after separation from the mother liquor will ~ ' , .
.
' -.
.
129~35 contain excess calcium hydroxide.
The mother liquor remaining after separation of the dibasic calcium hypochlorite contains residual calcium hypochlorite and sodium chloride in solution and thus a S complete recovery of the total quantity of calcium hypochlorite produced is not obtained.
The patent literature describes many processes for the production of calcium hypochlorite of which the following are relevant to the present invention.
U.S. Patent No. 1,718,284 describes a process in which calcium hydroxide and sodium hydroxide are mixed with a eutectic solution of calcium hypochlorite and sodium chloride and then chlorinated. Sodium chloride cystallizes from the metastable solution of calcium hypochlorite and is carefully separated. Crystallization of calcium hypochlorite from the remaining liquor is then initiated.
U.S. Patent No. 3,572,989 describes a process in which recycled mother liquor containing calcium hypochlorite and sodium chloride is mixed with sodium hydroxide of high concentration and chlorinated. Sodium chloride crystals are precipitated and removed. The remaining liquor is mixed with calcium hydroxide and chlorinated to produce crystalline calcium hypochlorite which is separated. The mother liquor is recycled U.S. Patent No. 3,767,775 decr;bes a process in which sodium hydroxide, calcium hydroxide, water and a mother liquor saturated with calcium hypochlorite and sodium chloride from a later stage in the process are chlorinated.
Calcium hypochlorite and sodium chloride are co-crystaI-lized, separated from each other and the mother liquor recovered and recycled to the chlorinator.
U.S. Patent No. 3,~95,099 describes a process for the manufacture of calcium h~pochlorite in which the mother liquor containing dissolved calcium h~pochlorite and sodium chloride is reacted with sodium hydroxide to produce a ~ . .
~29~l35 slurry of calcium hydroxide in sodium hypochlorite and sodium chloride. The calcium hydroxide is separated and recycled to the primary reactor.
V.S. Patent No. 3,950,499 describes a process for the manufacture of calcium hypochlorite in which mother liquor containing calcium hypochlorite and sodium chloride is mixed with sodium hydroxide and chlorinated. Sodium chloride crystals are produced and are separated from the aqueous liquor. Calcium hydroxide and sodium hydroxide are added to this aqueous liquor and the mixture is chlorinated. A slurry containing sodium chloride and calcium hypochlorite crystals is produced. The crystals of calcium hypochlorite and sodium chloride are separated from each other and the mother liquor recycled.
U.S. Patent No. 3,954,948 describes a process for the manufacture of calcium hypochlorite in which calcium hydroxide and sodium hypochlorite are chlorinated to form a solution containing crystals of calcium hypochlorite. The calcium hypochlorite crystals are separated and the solution reacted with sodium hydroxide to produce a calcium hydroxide slurry. The calcium hydro~ide is separated and recycled. The remaining solution may be used as bleach liquor or processed to produce solid sodium chloride and a sodium hypochlorite solution which may be recycled.
U.S. Patent No. 4,196,184 describes an improved version of the above process in which the calcium hydroxide slurry is produced under closely controlled conditions which result in the formation of some hemi-basic calcium hypochlorite which improves the separation characteristics of the calcium hydroxide.
U.S. Patent No. 4,258,024 describes a process in which calcium hydroxide is added to a recycled mother liquor containing calcium hypochlorite. Crystalline dibasic calcium hypochlorite is produced and separated from the mother liquor. The dibasic hypochlorite is mixed with high .
. .
: . .
. . . .. .
129~35 C-I-L 714 strength sodium hydroxide and chlorinated to produce a slurry containing calcium hypochlorite and sodium chloride crystals. The crystals are separated from each other and the mother liquor recycled.
U.S. Patent Nos. 4,328,200 and 49390,512 describe a process in which sodium chloride and calcium hypochlorite are co-crystallized and the crystals then separated. A
recycled aqueous solution of sodium chloride and calcium hypochlorite is mixed with sodium hydroxide and calcium hydroxide chlorinated in the presence of a seed bed of sodium chloride and calcium hypochlorite crystals. The two types of crystal are then separated in a classification zone. The crystals are separated romm the mother liquor which is then recycled.
~rom a review of the above prior art and from a review of patents not described hereinabove which relates only to improvements in specific process steps, it is clear that processes in which sodium chloride and calcium hypochlorite crystals are co-crystallized from a~solution are inherently difficult to operate. Many techniques are described to enhance the separation efficiency, particularly with regard to the crystallizing conditions necessary to grow crystals o~ different sizes which may be physically separated. It is also clear that there will be some contamination of the calcium hypochlorite crystals with sodium chloride crystals and vice versa. Use of wash water to improve the quality of the products is not practical as these processes operate under a tight water balance, maintained by using high strength caustic soda, to obtain the required product yield.
It is/ thus, an object of this invention to provide an improved industrial process ~or the production of calcium hypochlorite in which the calcium hypochlorite produced is effectively all recovered as product and the excess co-produced sodium chloride lS recovered in crystalline , ,:' .
:
.~
:
, :,: ,- , ` ' ` . : :
129~5 C-I-L 714 form containing calcium impurities in concentrations similar to those typically present in sodium chloride used for chlorine and sodium hydroxide production.
It is a further object of the present invention to s provide a commercial process of improved flexibility in the production of calcium hypochlorite.
It is a further object to provide a process using a single step chlorination.
It is a further object to provide a process which avoids the separation of co-crystallized calcium hypochlorite and sodium chloride and avoids the precipitation and separation of calcium hydroxide as part of the recovery process.
It is a further object to provide a process utilizing the common commodity 50~ sodium hydroxide as a raw material without further evaporation, or to use sodium hydroxide at lower than 50% concentration.
It is a further object to provide a process in which effectively all of the calcium and hypochlorite values produced in the overall reaction are accounted for in the desired solid calcium hypochlorite product.
It is a further object to provide a process which does not produce a solution containing calcium and hypochlorite values either as a by-product stream of lesser value, or as a waste stream of zero or negative value due to disposal costs.
It is a further object to provide a process which is free of objectionable effluents, wherein the only outputs from the process are the desired solid calcium hypochlorite product, solid sodium chloride, and condensed water vapour.
~ccordingly, the invention provides (a) reacting aqueous sodium hydroxide, calcium hydroxide and dibasic calcium hypochlorite in a slurry chlorinator with chlorine to produce crystalline calcium hypochlorite and a primary mother liquor .
.. .
, ', . '' i' . :, .
,, : ~ :
~90135 containing calcium hypochlorite;
(b) separating said crystalline calcium hypochlorite from said primary mother liquor;
(c) adding calcium hydroxide and an evaporator mother liquor, as hereinafter defined, to said primary mother liquor to effect production of crystalline dibasic calcium hypochlorite and a dibasic mother liquor;
(d) separating said crystalline dibasic calcium hypochlorite from said dibasic mother liquor;
(e) feeding said crystalline dibasic calcium hypochlorite to said slurry chlorinator;
(f) evaporating said dibasic mother liquor to produce crystallized sodium chloride and said evaporator mother liquor;
(g) separating said crystaIlized sodium chloride from said evaporator mother liquor; and (h) feeding said evaporator mother liquor to said primary :~ :
mother liquor as hereinbefore de~fined in step (c).
The calcium hydroxide of use in:the slurry:chlorinator may be added directly as such or, preferably, added with the dibasic calcium hypochlorite as a solid mixture of dibasic calcium hypochlorite - calcium hydroxide produced : by .the addition of calcium hydroxide in excess to produce said solid mixture of the formula: ~
. .
: Ca(OC12~2.2 Ca(OH)2 / Ca(OH)2 ~ :
The resultant dibasic mother liquor is subjected to 30 evaporation as hereinbefore defined. .
Accordingly, in a preferred feature the invention ; provides a process as hereinbefore defined wherein said ¢alcium hydroxide is added in step (c) in stoichiometric excess of said calcium hypochIorite in said primary mother : 35 liquor to effect production of a solid mixture of :
, .. . ..
: .. ,: . :
.. ~, ,, ~. .. ..
~90135 C-I L 714 crystalline dibasic calcium hypochlorite-calcium hydroxide and dibasic mother liquor; and further comprising separating said solid mixture from said dibasic mother liquor and feeding said solid mixture to said slurry S chlorinator and evaporating said dibasic mother liquor as hereinbefore defined.
In an alternative preferred feature, the invention provides a process as hereinbefore defined wherein said calcium hydroxide is added in step ~c) in stoichiometric excess of calcium hypochlorite present in the primary mother liquor and the evaporator mother liquor to effect production of said solid mixture of crystalline dibasic calcium hypochlorite - calcium hydroxide.
It is well known that hypochlorite solutions are subject to loss of active chlorine values, the rate of loss being dependent on hypochlorite concentration, alkalinity, temperature and the presence of catalysts such as cobalt and nickel. The loss of active chlorine values can be by decomposition to chloride and oxygen or by chemical conversion ~o chlorate. The rate of loss of active chlorine values can be minimized ~y selecting conditions of high pH and low temperature in the absence of catalysts.
It is a preferred feature of the invention that the evaporation of the dibasic mother liquor step takes place at low hypochlorite concentration and at a pH of 10.5 A low temperature is maintained by carrying out the evaporation under vacuum. The combination of low temperature and short evaporator residence time leads to an insignificant loss o~ active chlorine in the evaporation step. It is clear that the selection of said appropriate evaporation conditions resides in the ~kill of the art.
By the ~traight forward design adjustment of increasin~ the water evaporation in the evaporator the use of any required amount of water for washing crystal streams within the process is permitted. This water is then ', :
--~ . ., ~ . . , . - . . . :
- . , , , ., .. ,: .
. ' , ', ": ' ", .. , , .. , ~29~35 C-I-L 714 evaporated, condensed and recycled to the washing stages.
In this way sodium chloride crystals of high purity can be produced. In contrast, processes using high strength caustic and not employing evaporation cannot accommodate the addition of wash water to the system and thus can co-produce only relatively impure product sodium chIoride.
The separation steps may be carried out using conventional separation equipment such as settlers, filters, centrifuges and screens. The preparation of crystals of a specific crystal size to permit the separations is not necessary. The typical complex process steps of the art re~uired to simultaneously co-crystallize sodium chloride and calcium hypochlorite and subsequently separate the two crystal types are thus avoided in the present process.
The process can use sodium hydroxide at all commercial streng~hs including the low strength (30 to 35%) sodium hydroxide typically produced by membrane cells. The additional water introduced with the lower strength caustic may be evaporated in the evaporator and pure condensate recycled to a chloralkali plant.
It will be appreciated by those knowledgeable in the -art that the process of the invention is adaptable and suited to batch, semi-continuous or continuous operations and is, thus, suitable for a wide ~ange of plant capacities.
It can further bé seen that the instant process required only a single chlorination step. This si~plifies the process control instrumentation and reduces capital cost.
In order that the invention may be better understood a preferred embodiment will now be described, by way of example only, with reference to the accompanying figure which shows a simplified block diagram of a preferred 35 process according to the in~ention.
~:
:
' " ' ' : ' ,~ ' ': `
.
.
~290~5 C-I-L 714 Commercial strength sodium hydroxide, typically, 30-50% by weight NaOH in water is fed as stream 10 to a slurry chlorinator 11 wherein it is combined with a mixture of dibasic calcium hypochlorite-calcium hydroxide fed as stream 12 from a dibasic separator 13 as hereinafter described. Chlorine is passed as stream 14 into slurry chlorinator 11 until essentially all of the calcium hydroxide present is converted to calcium hypochlorite.
The chlorination process is exothermic and takes place at a controlled temperature in the range of 15 - 30C with cooling.
The slurry after chlorination has the general composition:
Typical Range Ca(OCl)2 22 % 20-24 NaCl 16 % 15-17 %
Ca(OH)2 0-3% 0.1-0.6%
Miscellaneous0.2% 0.0-0.5%
H2O 61.5~ 60-65 After chlorination, the slurry is fed as stream 15 to a separator 16 wherein calcium hypochlorite as product is ` removed as stream 17 and the resultant neutral primary mother liqu~r is transferred as skream 18 to a dibasic crystallizer 19. Separator 16 may be any suitable -equipment for separating the crystalline calcium hypochlorite from the mother liquor, such as a filter or centrifuge. The type of equipment and the method of its operation determines the composition of the calclum hypochlorite stream 17 but this stream typically contains:
Ca(OCl)2 40-60%
NaCl 4-15%
H2O 30-50%
- .
.
,~ , . ..
~9~35 --1~--In a further stage of the industrial process, not shown in the Figure, the calcium hypochlorite of stream 17 is dried, under carefully controlled conditions to avoid or minimize product decomposition, to give a final "dry"
product containing less than 10% by weight of water, typically 5 to 8~.
In a variation on the basic process part of the neutral primary mother liquor stream 18 may be recycled to slurry chlorinator 11 to control the desired slurry strength in the slurry chlorinator 11. The slurry strength in slurry chlorinator 11 will vary depending upon the concéntration of the sodium hydroxide used as feed material. The recycle of neutral primary mother liquor i~
advantageous if the chlorination is performed batchwise.
To dibasic crystallizer 19 is also added calcium hydroxide, as stream 20, and a recycled evaporator mother liquor, as stream 21, as hereinafter described. In dibasic crystallizer 19, most of the calcium hypochlorite of the primary mother liquor is crystallized as dibasic caIcium ;
hypochlorite, Ca(OCl)2.2Ca(OH)2 by the calcium hydroxide added. The calcium hydroxide may be added directly to dibasic crystallizer 19 or, to improve the operability by minimizing the formation of lumps of calcium hydroxide, may be mixed with a recycled dibasic mother liquor (as hereinafter described) in a mixer 22 prior to addition to the dibasic crystallizer as stream 23. The~quantity of calcium hydroxide~introduced into the dibasic crystallizer may be any quantity up to the total quantity required ~or the overall reaction However, it is preferred that the quantity added iq sufficient to maintain at least a molar ratio of 2 parts calcium hydroxide to one part calcium hypochlorite. If the total ~uantity of calcium hydroxide i9 not added to the dibasic crystallizer l9 the balance of the lime may be added to slurry chlorinator 11. Although not advantageous, the source of calcium hydroxide may be ' .: ' ,. :
. .
129~13~ C-I-L 714 from calcium chloride and a base. Dibasic crystallizer 19 is typically an agitated mixing vessel. The temperature in the dibasic crystallizer may range from aprroximately 15 -80C but is preferably in the range of 40 - 50C which 5 provides a crystal slurry of good separation characteristics.
The crystal slurry from dibasic crystallizer 19 is fed as stream 24 to dibasic separator 13 and separated into solids stream 12 consisting of the mixture of dibasic calcium hypochlorite and calcium hydroxide and dibasic mother liquor stream 25. The dibasic separator may be any suitable separation device such as a filter or centrifuge.
The composition of the dibasic calcium hypochlorite/
calcium hydroxide mixture after separation, stream 12, will vary depending on the equipment selected and depending on the use of an optional water wash but will typically analyze as shown below:
Typical ~E~
Ca(OCl)2 21% 16-30%
Ca(OH)2 30% 20-40%
NaCl 11~ 1-15%
H2O 38% 30-50% ~ -Dibasic mother liquor, stream 25, contains approximately 3~ dissolved calcium hypochlorite and 20-23%
dissolved sodium chloride. The solution is alkaline, approximately pH 11. Optionally part of the liquor may be recycled to prepare a calcium hydroxide slurry as previously described. The balance of the liquor stream is evaporated in an evaporator crystallizer 26 to give a solutlon containing approximately 8 to 10% calcium hypochlorite in solution. During the evaporation process sodium chloride is crystallized from the solution. To minimize decomposition of hypochlorite the evaporation .
:
.
' ~' ' ' ' ': ' ' ';' ', '; , ' ~ " ''.', ' :. ' `
~Z90135 C-I-L 714 process is performed under vacuum so that the operating temperature of the evaporator is in the range of 30-60C.
The water vapour evaporated is condensed. Part of this water may be recycled for process use, if required, with the balance (representing the difference between the water introduced into the overall process with the sodium hydroxide and that removed in the dry product) discharged as condensate, as stream 27.
A salt crystal slurry, stream 28, is withdrawn from evaporator crystallizer 26 and separated into a salt stream 29 and a concentrated recycled dibasic mother liquor stream 21 as hereinbefore described in a salt separator 30. Salt separator 30 may be any suitable separation device such as a filter or centrifuge. Salt is removed from the process as stream 29. The quality of the salt by-product may be impoved by washing the salt with water or brine to dispIace residual debasic mother liquor on the crystals, thereby reducing the calcium content of the salt stream 29.
Typical analyses are:
Unwashed Washed NaCl balance balance Ca 0.3-0.85% 0.1-0.25%
~32 2-5% 2-5%
The concentrated dibasic mother liquor stream 25 is of similar composition to that of neutral primary mother liquor stream 18 and may be used in place o~ this primary mother liquor in, for example, slurry chlorinator 11, if advantageous.
Thus, it can be readily seen that the process provides almost complete recovery o~ the calcium and hypochlorite values in the process streams, thus maximizing the process yield and minimiæing raw material requirements.
"
. ' ' ' ' :
.. :.,
Claims (3)
1. A process for the production of calcium hypochlorite comprising the steps of:
(a) reacting aqueous sodium hydroxide, calcium hydroxide and dibasic calcium hypochlorite in a slurry chlorinator with chlorine to produce crystalline calcium hypochlorite and a primary mother liquor containing calcium hypochlorite;
(b) separating said crystalline calcium hypochlorite from said primary mother liquor;
(c) adding calcium hydroxide and an evaporator mother liquor, as hereinafter defined, to said primary mother liquor to effect production of crystalline dibasic calcium hypochlorite and a dibasic mother liquor;
(d) separating said crystalline dibasic calcium hypochlorite from said dibasic mother liquor;
(e) feeding said crystalline dibasic calcium hypochlorite to said slurry chlorinator;
(f) evaporating said dibasic mother liquor to produce crystallized sodium chloride and said evaporator mother liquor;
(g) separating said crystalized sodium chloride from said evaporator mother liquor; and (h) feeding said evaporator mother liquor to said primary mother liquor as hereinbefore defined in step (c).
(a) reacting aqueous sodium hydroxide, calcium hydroxide and dibasic calcium hypochlorite in a slurry chlorinator with chlorine to produce crystalline calcium hypochlorite and a primary mother liquor containing calcium hypochlorite;
(b) separating said crystalline calcium hypochlorite from said primary mother liquor;
(c) adding calcium hydroxide and an evaporator mother liquor, as hereinafter defined, to said primary mother liquor to effect production of crystalline dibasic calcium hypochlorite and a dibasic mother liquor;
(d) separating said crystalline dibasic calcium hypochlorite from said dibasic mother liquor;
(e) feeding said crystalline dibasic calcium hypochlorite to said slurry chlorinator;
(f) evaporating said dibasic mother liquor to produce crystallized sodium chloride and said evaporator mother liquor;
(g) separating said crystalized sodium chloride from said evaporator mother liquor; and (h) feeding said evaporator mother liquor to said primary mother liquor as hereinbefore defined in step (c).
2. A process as claimed in Claim 1 wherein said calcium hydroxide is added in step (c) in stoichiometric excess of said calcium hypochlorite in said primary mother liquor to effect production of a solid mixture of crystalline dibasic calcium hypochlorite-calcium hydroxide and dibasic mother liquor; and further comprising separating said solid mixture from said dibasic mother liquor and feeding said solid mixture to said slurry chlorinator and evaporating said dibasic mother liquor as hereinbefore defined.
3. A process as claimed in Claim 2 wherein said calcium hydroxide is added in step (c) in a molar ratio of two parts calcium hydroxide to one part calcium hypochlorite.
Priority Applications (4)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000523544A CA1290135C (en) | 1986-11-21 | 1986-11-21 | Calcium hypochlorite process |
| NZ22242387A NZ222423A (en) | 1986-11-21 | 1987-11-03 | Production of calcium hypochlorite |
| AU80899/87A AU593404B2 (en) | 1986-11-21 | 1987-11-06 | Calcium hypochlorite process |
| ZA878382A ZA878382B (en) | 1986-11-21 | 1987-11-09 | Calcium hypochlorite process |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000523544A CA1290135C (en) | 1986-11-21 | 1986-11-21 | Calcium hypochlorite process |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1290135C true CA1290135C (en) | 1991-10-08 |
Family
ID=4134399
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000523544A Expired - Lifetime CA1290135C (en) | 1986-11-21 | 1986-11-21 | Calcium hypochlorite process |
Country Status (4)
| Country | Link |
|---|---|
| AU (1) | AU593404B2 (en) |
| CA (1) | CA1290135C (en) |
| NZ (1) | NZ222423A (en) |
| ZA (1) | ZA878382B (en) |
-
1986
- 1986-11-21 CA CA000523544A patent/CA1290135C/en not_active Expired - Lifetime
-
1987
- 1987-11-03 NZ NZ22242387A patent/NZ222423A/en unknown
- 1987-11-06 AU AU80899/87A patent/AU593404B2/en not_active Ceased
- 1987-11-09 ZA ZA878382A patent/ZA878382B/en unknown
Also Published As
| Publication number | Publication date |
|---|---|
| AU593404B2 (en) | 1990-02-08 |
| AU8089987A (en) | 1988-05-26 |
| NZ222423A (en) | 1989-03-29 |
| ZA878382B (en) | 1988-05-03 |
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