Note: Descriptions are shown in the official language in which they were submitted.
C9Q~il6
C-6645 This invention relates to the recovery of lime from
calcium hypochlorite solutions.
A variety of processes and modifications are known
in the art for manufacturing calcium hypochlorite.
U.S. Patent No. 1,713,669, issued to Rob rt B.
MacMullin et al on May 21, 1929, discloses a process in
which an aqueous slurry of lime is chlorinated, solid
impurities are removed, and caustic and chlorine are
added to produce calcium hypochlorite and alkali chloride.
The concentration of the calcium hypochlorite-alkali
chloride mixture is kept dilute so that the alkali
chloride remains in solution while the calcium hypochlorite
dihydrate is precipitated. After separation of the solid
calcium hypochlorite dihydrate, the filtrate may be
treated with lime to form basic calcium hypochlorite or
used as bleach liquor.
U~S~ Patent No. 1,718,284, issued to Anthony George
et al on June 25, 1929, describes a process in which lime
and caustic soda are added to an eutectic solution of
sodium chloride and calcium hypochlorite, the mixture i5
chlorinated and sodium chloride is quickly precipitated
and separated from an unstable solution of calcium hypo-
chlorite. Calcium hypochlorite dihydrate is then precip
itated. In an alternate-embodiment, sodium hypochlorite and
lime are chlorinatPd in an eutectic solution of sodium
chloride and calcium hypochlorite with the ~odiu~ chloride
and calcium hypochlorite dihydrate being precipitated and
separated sequentially as de~cribed above.
--2--
.
C-6645 U.S. Patent No. 1,718,285, issued to Anthony George
on June 25, 1929, discloses a process for chlorinating an
aqueous slurry of lime in the presence of a solution of
. sodium hypochlorite to form a slurry of calcium hypochlorite .
dihydrate. The slurry is evaporated to dryness to recover
. calcium hypochlorite, or alternatively the calcium hypo- .:
chlorite is precipitated from a dilute NaCl solution, then
separated and dri~d.
A process for calcium hypochlorite dihydrate manufacture
is disclosed in U.S. Patent No. 3,572,989, issued to Seiji
Tatara et al on Ma.rch 30, 1971, in which concentrated sodium
hydroxide is added to an aqueou~ solution saturated with .
calcium hypochlorite and sodium chloride, and in a first
step, the mixture is chlorinated and sodium chloride
crystals are removed. In a second step, slaked lime and
water are added to -the reaction mixture and the mixture
is chlorinated. The resulting calcium hypochlorite
dihydrate crystals are separated, and the mother liquor is
recycled to the first step.
In all of the above processes the separation of calcium
hypochlorite from sodium chloride takes place by either
a) precipitating sodium chloride from a metastable
solution of calcium hypochlorite where the separation
must be carried out quickly to be effective, or
b) precipitating calcium hypochlorite dihydrate from
sodium chloride in solutions which must be very
dilute to hold the ~odium chloride in solutionO
In both a) and b) above, there is a loss of product by
co-precipitation or incomplete precipit:ation and in b)
above there is the necessity to treat or dispose of large
volumes of a dilute sodium chloride solution containing
calcium hypochlorite values.
The process described in U.S. Patent No. 3,895,099,
issued July 15, 1975, describes another process for
preparing calcium hypochlorite wherein the filtrate, after
removal of calcium hypochlorite dihydrate crystals is
reacted with an alkali metal hydroxide to form a lime
slurry, the lime is separated and recycled to the calcium
hypochlorite reactor. Although this technique permits the
recovery of calcium values which are normally lost in the
previously defined calcium hypochlorite processes, one of
the problems encountered is the difficulty in separating
the line particles from the resulting lime slurry.
There is a need in the art at -the present time to
improve the recovery of calcium values in calci~m hypochlorite
processes and to improve the separation rate of lime from lime
slurry formed in an effort to recover calcium values.
It is a primary object of this invention to provide
an improved process for separating lime from lime slurry.
Another object of this invention is to provide an
improved process for preparing calcium hypochlorite from
lim~, alkali metal hydroxide and chlorine.
A further object of this invention is to provide:an
improved process for preparing calcium hypochlorite wherein
.
~ ~t~ ~ ~ 6
-6645 the proportion of impurities in waste streams is substantially
reduced.
These and other objects of the invention are
accomplished in a process for pxecipitating lime in
a reaction of an alkali metal hydroxide with an aqueous
solution of calcium hypochlorite to form a lime slurry,
and separating the lime from the resulting slurry, utilizing
an improvement which comprises maintaining a p~I in the
range from about 11 to about 13O5 during said reaction,
whereby hemi-basic calcium hypochlorite er~stals form
in said lime slurry and aid in the separation of the lime
from the slurry. In addition, washing of the lime particles
to remove impurities is easily 0ffected because of the
improved contact between wash liquor and lime particles
and ease of separation of wash liquor from the lime
particles that is effected by the presence of hemi-basic
calcium hypachlorite crystals in the lime particles~
More in detail, any a~ueous solution containing
calcium hypochlorite may be processed in accordance with
the process of this invention. Filtrates processed in
calcium hypochlorite processes af~er separation of the
calcium hypochlorite product, are particularly useful
as a starting material in the process of this invention.
Generally, these filtrates are aqueous solutions containing
soluble calcium hypochlorite values as well as reaction
by-products such as sodium chloride and sodium hypochlorite ,
or calcium chloride. A typical filtrate obtained i~ com-
~5--
~L~7~
C-6645 mercial calcium hypochlorite processes will have the
following analyses:
Components Pe:rcent ~y Wei~ht
~road Range Narrow Range
Calcium hypochlorite 5-12 9.5-10.5
Sodium hypochlorite 0-3.0 0-0.5
Sodium chloride 16-20 17-18
Calcium chloride 0-2 0-0.5
Water 68-75 69-73
While the above analysis is a typical analysis of a starting
calcium hypochlorite solution, it will be recognized by
those slcilled in the art that the composition of the calcium
hypochlorite filtrate will depend upon the type of process
employed in the preparation of calcium hypochlorite. For
example, in some commercial processes, sodium chloride is
separated as a solid before calcium hypochlorite is separated,
and the resulting filtrate will contain substantially less
sodium chloride. In other processes, no sodium components
are employed and the main impurity is calcium chloride
rather than sodium chloride. In the calcium systems, a
typical filtrate may have the following analysis.
Com~onents Percent b~ ~eight
Broad Range Narrow Range
Calcium hypochlorite 1-10 1-8
Calcium chloride 15-40 18-35
Calcium hydroxide b .1 `1. o o . 4-o . 6
Water S0-85 60-80
., ,.: .:
6--
. - . - .
31 ~)7~6
C-6645 The bene~its of applicant's improved process are also
applicable to such filtrates.
Any alkaline compound capable of precipitating calcium
values as lime fxom the aqueous solution of calcium hypo-
chlorite may be employed in this process. Typical examples
of alkaline compounds are alkali metal hydroxides such as
sodium hydroxide, potassium hydroxide, lithium hydroxide
and mixtures thereof. However, other suitable alkaline
hydroxides include barium hydroxide, cesium hydroxide,
rubidium hydroxide and strontium hydroxide.
Sufficient alkaline hydroxide is added to the aqueous
solution of calcium hypochlorite to adjust the pH to a range
from about 11 to about 13.5, and preferably from about 12
to about 12.5. When sufficient alkaline hydroxide is added
to adjust the pH of the calcium hypochlorite solution within
the above ranges, a large portion of soluble calcium values
is precipitated as lime. This may be accomplished batchwise
or continuously, the latter technique being preferred. In
addition, a portion of the lime and calcium hypochlorite
values is co-precipitated as hemi-basic calcium hypochlorite
crystals having the empirical formula:
Ca(OCl)2 1/2Ca(OH)~.
These hemi-basic crystals are substantially larger than the
lime particles and range from about 2 to about 30 in microns
in diameter~ In contrast, the lime particles have a size
of less than about 2, and generally~ less than about 1 micron
in diameter. The crystalline structure and particle size of
the hemi-basic calcium hypochlorite crystals aid in the
.
~7~6
C-6645 separation of the lime particles from the resulting slurry.
Filtration rates have been increased by a factor of from
about 10 to 20 when compared wlth the filtration rate of
lime slurries which do not contain the hemi-basic calcium
hypochlorite crystals in accordance with the process of
this invention. When the alkaline hydroxide is added to
the aqueous calcium hypochlorite solution in a propoxtion
sufficient to adjust the pH in excess of about 14, which
is about 4 to about 5 percent excess alkali, the excess alkaline
hydroxide decomposes the hemi-basic crystals and the
filtration rate of the lime slurry decreases drastically.
If the proportion of alkaline compound added to the calcium
hypochlorite solution is not enough to form a pH above
about 11.0, decreased precipitation of the calcium values
as lime occurs and a substantial content of calcium remains
in the filtrate, resulting in substantial calcium values
entering the sodium hypochlorite system.
In one embodiment of the invention, the alkaline
hydroxide, generally as a 50 percent by weiyht aqueous solu-
tion of sodium hydroxide, is added to a dilution tank in
which a recycle stream of precipitated paste filtrate is
used to dilute the sodium hydroxide to about 10 to 20
percent NaOH. This diluted sodium hdyroxide over~lows into
a reactor where it reacts with the incoming paste filtrate
containing calcium hypochlorite. With more concentrated NaOH
streams, the reaction is difficult to control and the
filterability- varies widely. If water were used to dilute
~; the caustic to the 10 to 20 percent NaOH, too much water
,
~ ~.
--8--
. ' ~
~a~7~
C-6645 would be added to the process requiring more evaporation in
the chlorinator evaporator. For examp:Le, 50 percent sodium
hydroxide has only 1 pound of water per pound of NaOH
as compared to 9 pounds of water per pound of NaOH at
10 percent caustic.
The reaction between the alkaline compound and the
calcium hypochlorite solution is carried out at a temperature
from about 10 to about 50 and preferably from about
27-32C. The reaction is generally carried out in a
continuous reactor at atmospheric pressure, but slightly
elevated or reduced pressure may be used, if desired.
The residence time in the reaction zone or a suitable
storage zone prior to recovering or separating the mixture
of lime and hemi-basic calcium hypochlorite from the reaction
mass generally ranges from about 20 to about 180 minutes and
preferably from about 25 to about 45 minutes. Extended
storage periods of the reaction mass, particularly at high
caustic concentrations, will adversely affect the filtration
rate.
After the reaction is completed a suitable separation
technique such as filtrati.on, centrifuging and the like is
employed to separate the lime product and co-precipitated
hemi-basic calcium hypochlorite crystals from the resulting
slurryl When vacuum filters are employed, filtration rates
from about ~0 to about 40 gallons per hour per square
foot o filter surface are generally obtained. Washing of
the filter cake to remove adhering liquid containing im-
purities is easily effected because o the improved fil-
tration rate imparted by the hemi-basic calcium hypochlorite
particles.
_g_ .
The washed filter cake is generally comprised of the
following components.
Components Percent by Weight
Broad Range Narrow Range
: Calcium hypochlorite 0.5-8 1-3
Calcium hydroxide (Total25-40 30-34
Sodium Hypochlorite 0.1-2.5 0.4-0.8
Sodium chloride 0.2-2.0 0.5-1.0
Water 50-75 60-70 :.
Calcium hypochlorite -
hemi-basic 0.5-10.0 1-4
Free Lime 25-35 28-34
It can be seen from the above analysis that, the weight
.ratio of lime to the crystalline hemi-basic calcium
hypochlorite in the filter cake generally ranges from about
2.5:1 to about 50:1 and preferably from about 7:1 to about ,~
30:1.
The liquid recovered after separation of the lime from
-. the slurry generally contains the following components in
the following proportions.
: .:
.. ' .:
: ', '
--10-- ..
:. ...
,
.. . . ~ . .
:. .. . .
C-6645 Components Percent by Weight
Broad Ran~eNarrow Range
Calcium hypochlorite 0~2~0 0-0.1
Calcium hydroxide 0.01-1.0 0.02-0.0S
Sodium hypochlorite 8.50 12.09.5-10.0
Sodium chloride 15O0-18 16.5-17.0
Water 70-77 72-75
NaOH 0-1.0 0.1-0.4
pH 11-13.512.1-12.5
The wet lime cake, particularly if produced in a contin-
uous calcium hypochlorite process, can be recycled to the
mixing zone and then the slurry chlorinator. Because of the
washing, there is substantially no recycle of sodium chloride
and other impurities in the lime, thereby reducing the size
of recycle streams and improving the yield of calcium
hypochlorite based upon initial limP fed to the reaction.
The liquid recovered after separation of the lime can be
further processed to recover salt values by evaporation,
caustic addition, and chlorination. It may also be used
in the preparation of a bleach solution.
....
~7~
As indicated above, the technique for recovering
lime from aqueous solutions of calcium hypochlorite in
accordance with this invention can be applied to virtually
any known process for preparing calcium hypochlorite where
calcium values are lost af-ter separation of the calcium
hypochlorite dihydrate crystals. The novel process of this
invention in particularly suitable for use in the process for
preparing calcium hypochlorite described in U.S. Patent No.
3,895,099, issued July 15, 1975.
Figure 1 is a flow sheet of a preferred embo~iment of
the present invention showing the separation of calcium
hypochlorite and precipitation of lime from the paste liquor.
More in detail, in the process of Figure 1, lime (both
fresh lime and recycle lime of the type described more fully
below), water and sodium hypochlorite are admixed in mixing
`~ zone 1 to form a mixing zone slurry~ Mixing zone 1 is a
mixing tank or other suitable container with agitation means
for blending the various components fed thereto. There may
be some degree of reaction between the various components
fed to mixing zone 1, but an important function of mixing
zone 1 is to admix the components, and therefore it is referred
to as a "mixing zone", even though some "reaction" may occur in
some embodiments of the inventionO
The resulting mixing zone slurry is conveyed to slurry
chlorinator 2 and reacted with chlorine. Slurry chlorinator
2 is any suitable chlorination apparatus provided with
'
, ~ . : . ~,
1~7~(1336
C-6645 agitation means for maximum contact ~etween chlorine and
slurry. It is preferred to employ as slurry chlorinator
2 an evaporator chlorinator which utilize~ the chlorination
technique described in U.S. Patent No. 3,241,912, issued
to Bernard H. Nicolaisen on March 22, 1966. Tempera~ure
within slurry chlorinator 2 is main~ai.ned within the range
from about 0 to about 50 and preferably from about 20 to
about 32C.
During chlorination of the slurry in slurry chlori-
nator ~, lime reacts with chlorine to form calcium hypo-
chlorite in accordance with Equation (1):
tl) Ca(OH)2 ~ ~12 ~ 1/2 Ca(ClO)2 + H2O ~ 1/2 CaC12
Some sodium hydroxide may be present in the mixing
zone slurry as a result of feeding sodium hydroxide ~not
shown) to mixing ~one 1, or it may be present in the recycle
lime fed to a mixing zone 1. Any sodium hydroxide :
present in the sluxry chlorinator 2 ic reacted with
chlorine to form sodium hypochlorite in accordance with
; Equation (2):
(2) 2NaOH ~ C12 ~ NaClO ~- NaCl + H2O
Sodium hypochlorite present in slurry chlorinator
2 reacts with calcium chloride to form sodium chloride and
additional calcium hypochlorite in accordance with Equation
(3):
: (3) NaClO ~ 1/2 CaC12 ~ 1/2 Ca(ClO)2 ~ NaCl
:~ -13-
.. . . . .
C-6645 The primary products of slurry chlorinator 2 are
calcium hypochlorite, sodium chloride and water. At
start-up of the process, it is preferred, but not necessary
to fill slurry chlorinator 2 with a sluxry or "paste" of
calcium hypochlorite solids suspended in an aqueous solution
of sodium chloride and calcium hypochlorite, with an excess
alkali concentration in the slurry being less than about 1.0
and pre~erably less than about 0.50 percent by weight. The
rate of feed of mixing zone slurry and chlorine to slurry
chlorinator 2 and rate of evaporation of water, if any, are
adjusted to maintain the concentxation of unreacted alkali
during the reaction below about 1.0 percent. Continuous
chlorination of the slurry in this manner causes the formation
of coarse calcium hypochlorite dihydrate crystals which are
: much more easily separated fxom paste liquor in cake separator
3, than are calcium hypochlorite dihydrate crystals prepared
in a conventional triple salt process or a batch type process.
A portion of the resulting "paste" comprised of solid
! calcium hypochlorite dihydrate and a "paste" liquor, which
is predominately an aqueous solution of sodium chloride and
calcium hypochlorite is continuously withdrawn from slurry
chlorinator 2 and conveyed to cake separator 3.
Cake separator 3 is a filter, centrifuge, or other
suitable solid-liquid separating apparatus capable of
separating a moist cake of calcium hypochlorite dihydrate
crystals from the aqueous solution o sodium chloride and
calcium hypochlorite.
-14-
~L~)7~ 6
C 6645 Moist cake from cake separator 3 generally contains
from about 40 to about 6C percent by weight of calcium
hypochlorite dihydrate, from about 2 to about 15 percent
by weight of sodi~n chloride, and from about 38 to about
52 percent by weight of water. Moist cake is generally
conveyed to dryer 4 where it is heated to remove most of
the water. Dryer 4 is any suitable drying unit or unit~
capable of reducing the moisture content of the oalcium
hypochlorite caXe to the desired level without causing
excess decomposition of the calcium hypochlorite particles.
Generally the water content of the calcium hypochlorite
is reduced in dryer 4 to below about 10 percent by weight
and preferably from about 0.5 to about 7.5 percent by
weight. The calcium hypochlorite content of the dried
calcium hypochlorite generally ranges from about 65 to
about 85, and preferably from about 70 to about 80 percent
by weight. The remainder of the dried calcium hypochlorite
is predominately sodium chloride. The dried product is
then placed in suitable containers, with or without prior
size classification or other processing such as pelletizing,
prior to use in water treatment or other utility.
"Paste liquox'l (or "paste filtrate" when cake
separator 3 is a filter) is an aqueous sodium chloride
solution from cake separator 3 which also contains soluble
calcium hypochlorite. This paste liquor is conveyed to
caustic reactor 5, which is any suitable mixing tank
reactor provided wi~h agitation means, where it is reacted
with an aqueous solution of a soluble metal hydroxide,
preferably sodium hydroxide, to form a lime slurryO
~15-
~7~3~i
C-6645 The aqueous solution of sodium hydroxide is prepared
by admixing sodium hydroxide or other alkaline compounds
in dilution tank 6 with a portion of the lime slurry from
caustic reactor 5. The lime slurry is conveyed from
caustic reactor 5 to dilution tank 6 by means of caustic
reactor pump 7. SUfficient lime slurry is recycled to the
; dilution tank to form an alkaline aqueous slurry in which
the proportion of sodium hydroxide or other soluble metal
hydroxide is in the range from about 10 -to ahout 20 percent
by weight.
; The recycle flow of lime slurry is preferably maintained
uniform, and as the reaction requires more or less alkali
to maintain the desired pH in reactor 5, the control system
adds more or less caustic to the dilution tank resulting in
a variation of the NaOH concentration in the dilution system.
The slurry in the caustic reactor 5 is maintained at a
pH of about 11.0 to about 13.5~ and preferably from about
12 to 12.5 by incxeasing or decreasing the caustic flow
to dilution tank 6.
A portion of the lime slurry is conveyed to lime
separator 8 such as a filter, where the solid lime particles
and the resulting hemi-basic calcium hypochlorite crystals
are separated from the aqueous mother liquor, which pre-
dominates in sodium chloride.
The reaction between the alkaline compound such as
sodium hydroxide and the calcium hypochlorite in the paste
liquor proceeds in accordance with Equations (4) and (5).
. . , , , : .
, . . . . , ,: ... .
1~'7~
C-6645 (4) 2NaOH + Ca(ClO)2 -- ~ Ca(OH)2 ~ 2NaClO
(5) 1/2 Ca(OH)2 ~ Ca(OCl)2 ) Ca(OCl)2 1/2 Ca (~)2
The crystals of lime and hemi-basic calcium hypo-
chlorite crystals in the cake formed i:n lime separator
8 are easily washed with water to remove excessive salt
from the lime cake and the washed cake is recycled to
mixing zone 1.
Mother liquor, the aqueous solution of sodium
chloride and sodium hypochlorite recovered in lime
separator 8, may be utilized as bleach liquor. This
mother liquor contains essentially no calcium hypochlorite
values. Therefore, a much larger percentage of lime fed
into the system is recovered as large crystals Qf calcium
hypochlorite dihydrate rather than as an impurity in
an effiuent stream or sold as a less expensive bleach
liquor. However, mother liquor from lime separator 8 is
preferably recycled to ~he proces~ as described more fully
below.
In a second embodiment of the invention an aqueous
solution of sodium chloride and sodium hypochlorite from
lime separator 8 i9 conveyed to an evaporator-chlorinator
~not shown~ where the aqueous solution is concentrated
by evaporation and is reacted with sodium hydroxide and
chlorine to form a slurry of solid sodium chlorlde in an
aqueous solution of sodium hypochlorite.
-17
8~
C-6645 This evaporation and reaction is carried out in any
suitable evaporation apparatus and chlorinator-reactor
provided with agitation means. Simultaneous evaporation
and chlorination may be carried out in an evaporative
chlorinator using the chlorination technique described in
U.S. Patent No. 3,241,912, issued to Bernard H. Nicolaisen
on March 22, 1966.
: In a preferred embodiment of the invention, the
mother liquor is first concentrated.by evaporation with
steam, in an evaporator (not shown) and the resulting con-
centrated mother liquor is then reacted with chlorine and
sodium hydroxide in the evaporator-chlorinator, utilizing
the heat of chlorination to complete evaporation of the
mother liquor and effect precipitation of salt in the
mother liquor.
Evaporation prior to reaction in the evaporator-
chlorinator is generally more economical since the rate
of evaporation of water is more rapid from the dilute
mo~her liquor prior to reacting and therefore smaller,
less expensive evaporatoxs may be used than are required
~or the more concentrated slurry of salt and mother liquorO .~:
In another embodiment o~ the invention~ no chlorine .
or sodium hydroxide are added to the evapora~or-chlorinator
and only evaporation takes place in the evaporator-
chlorinator to effect precipitation of salt in the mother
liquor. The degree of evaporation will depend upon the .
initial mother liquor concentration,
18-
D86
C-6645 When chlorination is used, the temperature during
chlorination is generally maintained within the range
from about 0 to a~out 50 and preferably from about 20 to
about 32C.
In this embodiment, sufficient chlorine and sodium
hydroxide are added to and sufficient water is removed from
the mother liquor fed to the evaporator-chlorinator to
maintain in the solution portion of the resulting slurry a
sodium hypochlorite concentration within the range from
about 15 to about 40 and preferably from about 25 to about
35 percent by weight. In addition, the soluble soclium
chloride concentration in the solution portion of the slurry
from evaporator-chlorinator is maintained from about 4 to
about 14 and preferably from about 4.8 to about 7.7 percent
be weight. The solid sodium chloride concentration in
'' the slurry from the evaporator~chlorinatox ranges from a~out
15 to about 35, and preerably rom about 18 to about 28
percent by weight. The resulting slurry i9 conveyed to a
, salt separator (not shown) which is a suitable filker,
centrifuge or similar solid-liquid separating apparatus.
In the salt separator, relatively pure sodium chloride
crystals are separated from the aqueous sodium hypochlorite
solution. These crystals may be used in the preparation
of br,ine which is used as a feed material for electrolytic
cells used in the preparation of chlorine and sodium
hydroxide.
~19-
C-66~5 The aqueous solution of sodium h~pochlorite which is
also separated from the salt separator~ is recycled to
mixing zone 1 for further admixing with fresh lime and
recylced lime.
This embodiment of the invention not only results in
easily filtexable lime, large crystals of hemi-basic
calcium hypochlorite and more efficient utilization of calcium
values as calcium hypochlorite, as in the embodiment of
Figure 1, but also produces relatively pure sodium chloride
in solid form, which has utility in the preparation of
brine feed for electrolytic cells. There are no impure
aqueous solutions of sodium chloride, calcium hypochlorite
or the like which need to be disposed of in the latter
embodiment thereby avoiding a seriou~ pollution pro~lem.
A third em~odiment of the invention includes the
second embodiment above as well as a technique for purifying
at least a portion of the fresh lime fed to the p~ocess
prio~ to admixing with sodium hypochlorite in mixing zone
1 and recycling at least a portion of the paste liquor to
mixing zone 1.
-20-
1Ci7q;~
C-6645 In the third embodiment, an aqueous slurry of fresh
lime is chlorinated in lime chlorinator (not shown) to form
an aqueous solution of calcium hypochlorite and calcium
chloxide in accordance with Equation (1). Impurities in
the fresh lime include insoluble impurities such as silica,
aluminum salts, iron salts, magnesium ~alts, magnesia,
unburned limèstone (calcium carbonate and magnesium
caxbonate) and other compounds in ~race quantities. The~e
impurities present in the fresh lime remain insoluble in
the aqueous solution formed in the lime chlorinator and
form a slurry with the.aqueous solution of calcium hypo-
chlorite and ~alcium chloride. This slurry is conveyed
to an impurity separator (not shown), which is a suitable
solid-liquid separator such as a filter, centrifuge or the
like, where solid impurities are separatedO Solid impurities
~rom the impurity separator are generally disposed of as
solid waste, land fill or the like. The aqueous solution
of calcium hypochlorite and calcium chloride from the
impurity separator is conveyed to mixing zone 1. As
indicatPd above, the primary function of mixing zone 1 is
to ef~ect admixing of the components fed thereto. However,
in the third embodiment~ the calcium chloride component of
the aqueous solution from the impurity separator is reacted
in mixing zone 1 with sodium hypochlorite to ~orm an aqueous
: .
.
~70~)8~i
C-6645 solution of calcium h~pochlorite and sodium chloride in
accordance with Equation t3~.
In the third emhodiment, at least a portion of the
fresh lLme is purified in the lime chls)rinator and impurity
separator prior to feeding to mixing zone 1, and a portion
of the fresh lime fed to mixing zone 1 may be untreated.
The ratio of fresh lime in each of these feed streams
depends upon the initial purity and activity of the fresh
lime fed to the process as well as specifications for
" 10 impurities required for the calcium hypochlorite productO
Thus, if the fresh lime i5 relatively pure and the ~tandards
for purity of the calcium hypochlorite are not high, then
little or no fresh lime has to be purified prior to
feeding to mixing zone 1. However, if the lime is relatively
impure, then a large fraction or all of the ~resh lime is ~ :
purified in the lime chlorinator and impurity separator
prior to feeding to mixing zone 1.
In all of the above defined embodiments, in order to
maintain within the mixing zone slurry the concentration
of lime (both fresh lime, if added, and recycled lime)
within the desired range, and the concentration of sodium
hypochlorite within the desired range, additional sodium
hydroxide may be added to mixing zone 1. In addition, in
order to improve control of the chlorination and heat
transfer in slurry chlorinator 2 a portion of the paste
liquor from cake separator 3 may be recycled to mixing
zone 1.
22-
,
C-6645 The primary raw materials for the pro~e~s of this
invention are lime, sodium hydroxide, chlorine cmd water.
Fresh lime is added to the process at mixing zone 1
and~or the lime chlorinator. One of the advantages of a
preferred embodiment of this invention is that relatively
impure lime may be utilized to prepare a relatively pure
calcium hypochlorite product. For example, lime having an
active lime content as low as 85 percent by weight or less
may be added to the lime chlorinator in accordance with
the third embodiment of this invention, and procluce a
relatively pure calcium hypochlorite product. Generally
the active lime content of the lime fed to the lime
chlorinator and/or mixing zone 1 ranges from about 85 to
about 100 percent, and preferably from about 90 to about
97 p rcent by weight of active lime. Lime impurities of
the type described above may range from about n to about
15 percent and generally from about 3 to about 10 percent
by weight of the lime.
Typical illustrative specifications for a preferred ~ -
fresh lime feed and for an acceptable lime feed are as
follows:
Component Preferred Acceptable
C~(OH)2 min.% g5.0 85
CaCO3 max.~ 1.0 3.0
MyO max.% 0.5 3~5
SiO~ max.~ 0.5 2.5
Fe2O3~A12O3 max.~ 0.5 1.5
CaSO4 max.~ 0.5 1.5
-2~-
. . , . . :
v~
C-6645 Generally from 0 to less than about 1~2 of the lime feed
; meeting the above-illustrative preferxed specifications
does not need to be processed in accordance with the lime
purification embodiment of the third embodiment. However,
when processing lime feed meeting the above illustrative
acceptable specifications a major portion up to all of the
feed may have to be processed in accordance with the lime
purification step of the third embodiment. The average
particle size of fresh lime added to the process generally
is substantially all -325 mesh (wet screen analy~is) but
particles up to about ~~00 mesh may be employed if desired.
When the impurity content of lime in the lime feed is
greater than about S percent by weight, it may be desirable
to add the lime chlorinator a carbonating agent such as
carbon dioxide or sodium carbonate to enhance precipita-
tion and removal of the impurities in solid form from
impurity separator 10.
As indicated above, fresh lime is fed to the system
either through the lime chlorinator or mixing zone 1, or
a combination of both. It is preferred to feed-from
about 25 to about 100 percent of the fresh lime into the
lim~ chlorinator and any balance of fresh lime being fed
into mixing zone 1. When all of the fre~h lime is fed to
the lime chlorinator then substantially all of the lime
fed to mixing zone 1 is in the form of recycle lime
slurry, the concentrated slurry of precipitated lime.
Fresh lime is fed to the lime chlorinator as an aqueous
slurry containing from about 10 to about 30 percent, and
-24-
C~6645 preferably ~rom about 15 to about 20 percent by weight of
active lime. Into mixing zone 1, lime may be added dry or
as a slurry up to about 50 percent lime by weight,
Lime added to mixing zone 1 as recycle lime rom lime
separator 8 is substantially pure lime~ having an active
lime content of from about 85 to about 100 percent by
weight. The solid content of the recycle lime slurry
generally ranges from about 20 to about 50 and preferably
from about 25 to about 35 percent by weight of solids,
which are predominately lime, and containing some hemi-
basic cryskals of calcium hypochlorite.
Sodium hydroxide is added to dilution tank 6, the
evaporator chlorinator and if desired, to mixing zone 1,
as a concentrated aqueous solution, generally ranging
from about 40 to about 60 percent by weight of sodium
hydroxide. However, sodium hydroxide may be added in
anhydrous form to dilution tank 6 and the evaporator-
chlorinator, and if desired, to mixing zone 1.
In addition, sodium hypochlorite added to mixing
~one 1 in Figure 1 may be prep~red by chlorinating an
a~ueous solution of sodium hydroxide, in a suitable
agitated chlorinator reactor (not shown). The sodium
hydroxide concentration in the aqueous solution used to
make sodium hypochlorite for feed to mixing zone 1 of
Figure 1 ranges from about 20 to about 75 and preferably
fromabout 35 to about 5S percent by weight. In the second
and third embodiments, sodium hypochlorite solution is added
to the mixing zone as a recycle stream from the ~alt
separator. Concen~ration of this recycle stream by
~2~-
. .
~7~
.
C-6645 evapoxation in the evaporator-chlorinator is generally
sufficient to maintain the desired sodium hypochlorite
concentration in mixing zone l. However, if desired,
any necessary additional sodium hydroxide may be added
dire¢tly to mixing zone 1 or additional sodium hypo-
chlorite may be prepared by chlorinating an aqueous solution
of sodium hydroxide and then addad to mixing zone 1 as
described above with respect to Figure 1, or may be added
to mother liquor prior to or simultaneous with chlorination
in the evaporator-chlorinator.
The overall reaction for the process of this invention
may be illustrated by Equation ~6):
~6) Ca(OH)2 + 2C12 + 2NaOH ~ Ca~ClO)2 ~ 2NaC1 ~ 2H2O
Thus, the stoichiometric proportion of fresh sodium hydroxide
fed to the process is equivalent to two moles of sodium
hydroxide per mole of active lime present in the fresh lime
fed to the process. As indicated in the embodiment of
Figure 1, sodium hydroxide is fed to caustic reactor 5 and
may be used to form sodium hypochlorite fed to mixing zone
1. In the second embodiment, sodium hydroxide is added to
caustic reactor S and the evaporator-chlorinator (not shown).
In the third embodiment, sodium hydroxide is added to caustic
reactor 5, and to either the evaporator-chlorinator or
mixing zone 1, or both. The relative ratio of proportions
of sodium hydroxide added to the di~ferent units of the
di~ferent embodimen~s may be varied over a wide range.
Generally, above about 20 and preferably ~rom about 22 to
`
-26-
.
0~8~
C-6645 about 35 percent o~ the stoichiometric proportion of sodium
hydroxide required in Equation (6~ is added to caustic
reactor 5 to precipitate lime and hemi~basic calcium hypo-
chlorite crystals from the paste liquor. The balance of
the stoichiometric proportion of sodium hydroxide is added
in the embodiment of Figure 1 as sodium hypochlorite to
mixing zone 1. The balance of sodium hydroxide is added
to the evaporator-chlorinator in the second embodiment.
The balance o sodium hydroxide is added either entirely
to evaporator-chlorinator in the thixd embodiment, or
up to about 70 percent of the balance of the stoichio-
metric proportion may be added to mixing zone 1.
In the embodiment of Figure 1 and the second embodi-
ment where lime ~both fresh and recycled) and sodiwm
hypochlorite solution are added to mixing zone 1, the
resulting mixing zone slurry has a lime concentration
ranging from about 1 to about 25 and preferably from about
2 to about 20 weight percent and a sodlum hypochlorite
concentration ranging from about 1.5 to about 30, and
: 20 preferably from about 2 to about 26 percent by weight. In
the third embodiment, when all or part of the fresh lime
is purified in the lime chlorinator and impurity separator
to form an aqueous solution of calcium hypochlorite and
calcium chloride which is fed to mixing zone 1~ the resulting
mixing zone slurry has a lime concentration and sodium
hypochlorite concentration within the above ranges as well
as a concentration of calcium hypochlorite ranging from
a~out 0 to about 30, and preferably from about 5 to about
26 percent by weight. All of the calcium chloride fed to
-27-
.
~C~7~
C-6645 mixing zone 1 re~cts with sodium hypochlorite to form
calcium hypochlorite and sodium chloride in acco.rdance
with Equation (3).
The ultimate water content in the mixing zone slurry
is controlled by adjusting the water content of the various
feed streams to mixing zone 1. For example, the water
content of the aqueous solution of calcium hypochlorite
and calcium chloride (when lime purification of the third
embodiment is used), the water content of any lime slurry
added, ~either fxesh or recycled lime) the water content
of the sodium hypochlorite (either fresh sodium hypo-
chlorite or recycled), and.if desired, the quantity of re-
cycled paste liquor from cake separator 3 are controlled
to obtain a mixing zone slurry of the desired concentration
range described above.
.As indicated above, slurry-chlorinator 2 at start-up,
is preferably filled, but not necessarily filled, with a
slurry of calcium hypochlorite solids suspended in an aqueous
solution of sodium chloride. The excess lime or other
alkali in the slurxy is maintained below about 1.0 and
preferably less than about 0.5 percent by weight of the
sluxry. The feed xate of mixing zone slurry and chlorine to
mixing slurry chlorinator 2 and the withdrawal rate of the
resulting calcium hypochlorite paste are adjusted to achieve
substantially complete chlorination of the calcium hydroxide
values fed to slurry chlorinator 2 in mixing zone slurry,
while maintaining the free lime or alkali conce.ntration
in slurry chlorinator 2 preferably below about 1.0 percent
by weight of the slurry.
-28-
~7(~
C-6645 Chlorine is added to slurry chlorinator 2 as well as
the evaporator-chlorinator and lime chlorinator in either
gaseous or liquid form. The chlorination reactions are
carried out preferably continuously in an evaporator type
chlorinator of the type described above.
Paske from slurry chlorinator 2 is predominately a
slurry of calcium hypochlorite dihydrate in an aqueous
solution of sodium chloride and calcillm hypochlorite~ The
paste contains calcium hypochlorite dihydrate crystals in
the concentration of from about 10 to about 35 and preferably
from about 15 to about 30 percent by weight. These crystals
are predominately rectangular platelets which are several
microns in thickness, and have substantially equal sides
ranging from about 50 to about 300 microns in length with
the major portion haviny sides ranginy from about 100
microns to about 250 microns in length. Generally, less
than about 5 percent of the crystals are "twin crystals"
which entrain paste liquor, which are difficult to separate
from the paste liquor, ancl which are difficult to dry.
Since more than about 95 percent of the calcium hypochlorite
dihydrate crystals obtained by the process of this invention
may be large platelets or cohesive agglomerates, there is
a minimal amount of paste liquor entrained in the crystals
during the separation in cake separator 3, even when filtered
on a drum filter. The crystals are easier to separate from
the paste liquor in cake separator 3 and are easier to dry
in dryar 4 than crystals produced by conventional calcium
hypochlorite techniques. In prior art techniques, more
-29-
~7~
C-6645 expensive high speed titanium centri~uges are necessary to
obtain crystals of equivalent purity.
Moist cake from cake separator 3 contains from about
40 to about 60 percent by weight of Ca(OCl)2 ~ 2H2O, from
about 2 to a~out lS percent by weight of NaCl, and from
about 40 to 50 percent by weight of water. This moist
cake may be used ~irectly in the treatment of water systems
such as swimming pools and the like, but is generally dried
and stored prior to use. The moist cake is dried by known
means, for example, using a spray dryer, turbodryer or
vacuum dryer where the appropriate temperature ranges are
employed to reduce the water content to the desired level.
In the process of the present invention, the cake is dried,
for example in a turbodryer with hot air while maintaining
the product temperature in the range from about 35 to about
110C, and preferably from about 40 to about 95C to give
a product having a calcium hypochlorite content from about
65 to about 85, a water content below about 10 percent by
weight and the bulk of the remainder being sodium chloride.
Pas~e liquor from cake separator 3 generally has a
sodium chloride concentration ranging from about 15 to
; about 22 percent, and preferably from about 17 to about 20
percent by weight, a calcium hypochlorite concentration
ranging ~rom about 7 to about 15 percent and preexably
~rom about 8 to about 12 percent by weight, and a water
content ranging from about 60 to about 75 percent and
preferably ~rom about 68 to about 73 pexcent by weight.
-30-
3 ~7~
C-6645 As indicated in the third embodiment, a portion of
the paste liquor may be recycled to mixing zone l, if
desired, to improve control of the chl.orination and heat
transfer in slurry chlorinator 2. Generally, from 0 to
about 40 and preferably from about 0 t:o about 10 percent
by weight of the pasteliquor is recycl.ed to mixing zone
l, the balance being conveyed to caust.ic reactor 5.
As discussed above, a concentrated alkali is diluted
to about 10 to 20 percent in dilution tank 6 with lime
slurry from caustic reactor 5, and paste liquor is then
reacted with this alkaline slurry in caustic reactor 5 to
precipitate lime and a small proportion of hemi-basic
calcium hypochlorite crystals. The resulting lime slurry
is conveyed to lime separator 8. The lime slurry is
separated in lime separator 8 to form a wet lime cake and a . .
mother liquor containing from about 7 to about 20 and
preferably from about 8 to about 15 percent by weight of
sodium hypochlorit~ and from about 15 to about 22 and prefer-
ably from about 17 to about 20 percent by weight of sodium
chloride. This mother liquor may be sold as bleach liquor
but is preferably conveyed to the evaporator-chlorinator
as described above, wherein it is reacted with chlorine
and sodium hydorxide and evaporated to effect precipitation
of sodium chloride. Sodium chloride is separated from the
resulting solution in the salt sepaxator and may be used
to prepare a brine feed for electrolytic cells. The
resulting aqueous solution from the salt separator contains
from about 15 to about 40 and preferably from about 18 to
about 35 pexcent by weight of sodium hypochlorite and from
. .
-31-
- .
C-6645 about 4 to about 14 and preferably from about 4 to about 8
percent by weight of sodium chloride.
Recovered lime cake, after washing, from lime
separator 8 is recycled as recycled lime to mixing zone 1.
Recycled lime contains from about 25 to about 50 and pre-
ferably from about 35 to about 45 perc:ent by weight o
~olid lime having a purity of from about 85 perc~nt to
about 100 percent by weight of active limet The lower
part of this purity range is obtained when higher propor-
tions of calcium hypochlorite-hemi-basic crystals are
co-precipitated. However, more concentrated lime slurries
or cakes may be recovered from lime separator 8, if desired.
The process of the present invention is preferably
carried out on a continuous basls which permits higher
rates of chlorination and thus increased rates of production.
Continuous chlorination also produces calcium hypochlorite
dihydrate crystals which are more easily separated by the
solid-liquid method of separation employed and which are
easier to dry.
The reaction conditions during the chlorination steps
have been described above. Ganerally, the evaporation
steps are carried out at temperatures ranging ~rom about
18 to about 45C and at pressures ranging from about 15
to about 35 mm Hg. All of the other steps of the process
are carried out at ambient pressure and temperature
conditions.
-32-
~0~ 86`
C 6645 Various operating problems occurring during ~atch
chlorination are aliminated, Lime for this process does
not need to he the high purity lime needed for most con-
ventional commercial processes and therefore lime from
virtually any source can ~e employed.
The following examples are presented to illustrate
the invention more fully. All parts and percentages are
by weight unless otherwise specified.
. . .
-33-
C-6645 EXAMPLE 1
Using the process illustrated in Figure 1, 5146 parts
per hour of a solution containing 27.97 percent NaOCl were
admixed in mixing zone 1, an agitated vessel, with 4058 parts
per hour of an aqueous lime slurry containing 30.2 percent
active lime,. The slurry was prepared from 1270 parts per
hour of raw lime containing g6.5 percent Ca(O~)2 (the balance
being silica, iron oxide, aluminum oxide and the like~, and
369 parts per hour of recycled precipitated lime from lime
separator 8 as a 35 percent lime cake, The mixture was
blended in the mixing zone to form a slurry, which was
transferred to slurry chlorinator 2 where 1561 parts per hour
of liquid chlorine were added while the slurry was thoroughly
agitated and the temperature maintained at about 30C. The
calcium hypochlorite paste which formed in slurry chlori-
nator 2 contained calcium hypochlorite dihydrate crystals
in an aqueous NaCl solution. This paste was withdrawn from
the slurry chlorinator at the rate of about 11381 parts per
hour. The paste contained 26.05 percent Ca(OCl)2, 15.16
percent NaCl and 56.67 p0rcent ~2 The remainder was
chlorate (0.5~ percent) and inert ingredients (1.08 percent).
The feed rate of mixing zone slurry and chlorine and
the withdrawal rate of paste from slurry chlorinator 2 were
adjusted to maintain concentration of free alkali in slurry
; chlorinator 2 of less than about 0.5 percent.
The resulting paste was conveyed to cake separator 3,
which was a filter, where the paste was separated into a
moist cake and a paste liquor. The crystals of the calcium
hypochlorite dihydrate in the moist cake were flat platelets
-34
-, . . - . ' : ~
~0~ 6
C-6645 of a few microns in thickness and sides which ranged fro~
about 50 to about 300 microns in length with about 70 to
about 90 percent of the crystals being larger khan 100
microns in length.
Moist cake was removed from the filter at a rate of
4762 parts per hour. The moist cake, which contained 46.26
percent Ca(OCl)2, 9.43 percent NaCl and 42.~2 percent
H~O, the remainder being chlorates (0.34 percent) and inert
ingredients (0.67 percent~, was transferred to a dryer
and dried with hot air while maintaining the product tempera-
ture inthe range of from 45 to 90~C. A dried calcium
hypochlorite product was recovered from the dryer at the rate
of 2451 parts per hour, containing about 74.7 percent Ca(OCl)2,
18.6 percent NaCl and having a water content of less than
1 percent, the balance being calcium chloride ~0.5), calcium
chlorate (0.9), calcium hydroxide (2.1), and calci~m car-
bonate (2.2). From the filter, 6369 parts per hour of paste
liquor containing about 10.15 percent Ca(OCl)2, 19.68
percent NaCl and 67.57 percent H2O were recovered and trans-
ferred to a caustic reactor 5. In caustic reactor 5, paste
liquor was reacted with 2800 parts per hour of an a~ueous
alkaline lime slurry. This alkaline lime slurry was pre-
pared by recycling 1943 parts per hour of the lime slurry
ormed in caustic reactor 5 to dilution tank 6, where it
was admixed with 857 parts per hour of a 50 percent aqueous
sodium hydroxide solution to form an alkaline lime slurry
in which the solution component of the slurry cont:ained
about 15.3 percent by weight of sodium hydroxide. Suffi-
cient alkaline lime slurry from dilution tank 6 was fed to
.~:
-35-
7~6
C-6645 caustic reactor 5 with the paste liquo:r to maintain a pH
of 12.07 in the solution component of the lime slurry
and caustic reactor 5.
Lime slurry from caustic reactor S was conveyed at
the rate of 7264 parts per hour to a f.ilter, which served
as lime separator 8, where the slurry was separated into a
mother liquor and a lime cake containing some h~mi-basic
calcium hypochlorite crystals. The lime cake was washed
with water and recycled to mixing zone 1. Analyses of the
mother liquor, the unwashed lime cake and the washed lime
cake were as follows:
Perc0nt by Wei~ht
Mother
U~3~b ~ Washed Lime Cake Liquor
ca(OCl)2 3.8~I 4.23 0
Ca(OH)~ 30.41 32.85 0.07
NaOCl 6.96 0.16 9.5
NaCl 11.44 1.25 16.67
H2O 47.35 61.50 73.63
NaOH 0.13
pH 12.07
When 300 ccs. o the lime slurry produced in caustic
reactor 5 was filtered on a fritted ylass filter under a
vacuu~ of 23 inches o mercury, 2 minutes and 26 seconds
were required to complete the filtration, which was equivalent
to a filtration of about 0.487 gallons per hour per square
foot.
~36-
.~ ~ .. .. .
C-6645 For purposes of comparison the procedure of this
example was repeated except that the pH of caustic reactor
was maintained above about 14, which has an excess caustic
concentration of 1.63 percent NaOH, which preverlted the
formation of hemi-basic calcium hypochlorite crystals in the
lime slurry. Filtering 300 ~cs. of the resulting lime
slurry in the same kype of fritted glass filter required
39 minutes and 10 seconds to complete the filtration, which
was equivalent to a filtration rate of about 0.030 gallons
per hour per square foot.
: The moist lime cake from the filter 8 was recycled
. .
to mixing 20ne 1.
Clarified mother liquor was recovered from lime
separator 8 at the rate of about 6649 parts per hour, and
stored for use as a liquid bleach.
~37~
