Note: Descriptions are shown in the official language in which they were submitted.
1 ~ 63~
BACKGI?OI]MD OF TilE INVENTION
-~ The present invention relates generally to a
process for removiny chromium values from aqueous
solutions of alkali metal halates, and more par-
ticularly, to an improved process for recovexing
chromium compounds from alkali metal chlorate~
containing cell liquors thereby reducing the poten-
tial hazard of environmental contamination.
Electrolytic cells used in the manufacture of
chlorine and sodium hydroxide from brine solutions
are well known. In such cells, chlorine is pro-
duced at the anode and sodium hydroxide is produced
at the cathode. Because chlorine and sodium
hydroxide react chemically to produce sodium
hypochlorite, in chlorine cells, membranes or
diaphragms, or other suitable separating means are
interposed between the electrodes to prevent such
reactions. In chlorate-type cells, however, the
chlorine produced at the anode is absorbed by the
electrolyte and subsequently hydrolyzed to yield
hypochlorous acid. The hypochlorous acid then
equilibrates, HOCl~ H++C10 , which yields sodium
hypochlorite when reacted in the presence of the
products of the cathode, e. g. . . hydroxyl ions.
~5 The hypochlorous acid then reacts with the sodium
hypochlorite to yield sodium chlorate.
'~
1;~63~19
In order to enhance the buffer action in the
cell and to minimize the reduction of hypochlorites
and chlorates by hydrogen produced at the cathode
it has been customary to add to the cell electrolyte
frcm about 0.0~5 to about 10 grams per liter (gpl) or
more, alkali metal chromates/dichromates, chromic
acid or other soluble hexavalent chrome-containing
compound. Consequently, from their addition to the
cell, chromium (VI) compounds reappear as impurities
in chlorate cell products imparting an objectionab]e
yellow-like color to chlorate solutions and crystals.
Furthermore, in some instances, the presence of
chromium (VI) compounds in alkali metal chlorate
solutions used, for example, in chlorine dioxide
generators impedes salt cake crystal growth, which is
desirable for most effective separation of unreacted
sulfate to take place in the generator. Under such
circumstances there is always the danger that chromium
in a +6 oxidation state will be passed into plant
effluent streams from industrial processes and reappear
in the environment as potentially toxic pollutants.
Of no less importance too, is the fact that chrome
compounds are valuable commodities, and it would be a
highly desirable economic benefit if they could be
recovered and recycled back to the cell for further use
in connectlon with the production of sodium chlorate.
Heretofore, various methods have been proposed
for the removal of hexavalent chromium compounds. For
. example, U.S. 3,427,236 discloses a process ~or remov-
.~ 30 ing chromium from alkali metal chlorates using barium
3 ~ ~ ~
.
precipitation. Barium has been used success~ully
in the removal of chrome, but as in the case of
chrome, barium is also a toxic materia] and a
potential health hazard~ In addition, processes
involving barium precipitation are costly, requir-
ing excess amounts of barium salts to make commer-
cially acceptable water-white chlorate products.
Other in1uential factors which make barium precipi-
tation non-economic is the fact that barium recovery
processes in current use are also too costly to
be practical. Other considerations in the use of
barium chloride are the requirements for a separate
step for hypochlorite "kill" by the addition of
reagents, such as urea, possible post precipitation
of barium in the filtrate, and low pH which can cause
chlorate product to decompose. U.S. ~,086,150
utilizes an "iron-mud" composition which is prepared
by first reacting ferrous salts with sodium hydroxide
which is then added to the cell liquor to precipitate
trivalent chromium compounds. According to this method,
the production of iron-mud requires large excesses of
ferrous salts ranging from 3 to 5 times the stoichio-
metric amount needed to reduce the chromate present
in the solution. Under such circumstances, the large
excesses of ferrous salts present create significant
problems due to the formation of gelatinous-like
precipitate which has a tendency to clog filtration
systems severely slowing the rate of filtration and
recovery o purified chlorate solutions. This process
is also conducted at high pH values, in the range of
10 to 12, which subjects the ferrous hydroxide to
possible oxidation by air~ U.~. 4,086,150 provides no
means for recovering chro~ium val~les.
I ~63~19
U.S. 3,616,344 suggests the addition of
soluble iron (II) salts to salt-free solutions
of sodium chlorate rather than chlorate cell
liquors. The salt-free solutions of the '344
patent are used in the electrochemical machining
of chromium-containing metals. This patent pro-
vides no method for further treatment and ~ecovexy
of the chrome va]ues after the solutions have
become contaminated after being used in machining
chrome-containing metals. Still other earlier
processes have been suggested for removing chromium
impurities from cell liquor by precipitation using
sodium sulfide, e.g. . . .U.S. 3,843,769. But,
discoloration and turbidity of the treated solution,
the Eormation of elemental sulphur, hydrogen sulfide
odors and possible evolution of chlorine dioxide
through inadvertent addition of excess acid have
been associated with such processes.
Accordingly, it has now been dlscovered that
chromium (VI) compounds present as impurities in
alkali metal halate-containing liquors produced in
electrolytic cells may be recovered by a virtually
~uantitative reaction through the reduction of
hexavalent chromium compounds producing a virtually
colorless, chrome-free or substantially chrome-free
solution of alkali metal halate/halide. By avoidance
of pH extremes, according to the disclosed process
possible decomposition of sodium chlorate or oxidation
of iron (II~ compounds is reduced. A precipitate of
hydrous ferric-chrcmic cxide formed during the first step of the
;
--5--
process is oxidized selectively whereby only the tri-
valent chromium compounds are solubilized leaving an
innocuous chrome-free or substantially chrome-free
disposable iron oxide. A solution of alkali metal
chromate resulting may be recycled to the electrolytic
cell for further use in halate production.
~ ccordingly, the invention seeks to provide
an improved process for making alkali metal halate-
containing products which are virtually free of all
chromium compounds~
The invention also seeks to provide a pro- -
cess whereby chromium compounds used in the electro-
chemical production of alkali metal halates are recovered
~ and made available for further use in halate production
! reducing or eliminating the potential risk of environ-
mental contamination from chrome.
Still further the invention seeks to provide
a highly efficient and economic process for the
virtual quantitative removal of chrome impurities from
alkali metal halates utilizing readily available, low
cost re~ctant~.
i
1 1~3~19
~5a-
In accordance with the invention there is
provided a process for making substantially chrome-
free alkali-metal halates which comprises: a) forrning
a slurry by reaeting a chromium (VI) contaminated
. halate-eontaining eell liquor with a redueing agent
seleeted from the group consisting of: i) iron (II)
compounds, ii) hydrazine or hydrazine salts, and iii)
produets made from the reaetion of i3 and ii), b)
, separating solids from the slurry of a) and reeovering
an alkali metal halate solution substantially devoid of
chromium eompounds.
Broadly, the invention relates to a proeess for
the eleetrochemical production of alkali metal halates
r
~;
whereby chromiurn compounds present as impurities
are removed therefrom to provlde chrome-free
halate-containing products. One embodiment of
the process comprises the steps of (a) treating
cell liquor made in a halate-type electrolytic
cell with a stoichiometric amount of an iron (II)
compound(s) to form a hydrous ferric-chromic
oxide precipitate, the pH of the treated liquor
being adjusted continuously in the range of 5.5
to 9.5, the cell li~uGr being treated comprising
at least 2S0 gpl alkali metal halate, at least
75 gpl alkali metal halide and at least 0~005 gpl
of chromium (VI) compound(s); (b) separating the~
hydrous ferric-chromic oxide precipitate from the
cell liquor ko produce an alkali metal halate-
containi.ng solution substantially free of chromium
compound(s); (c) removing the chromium compound(s)
from the precipitate by reacting with an oxidizing
agent whereby the chromium compound(s) is solubilized,
and (d) separating the residual hydrous ferric oxide
precipitate from the solubilized chromium compound(s),
the solubilized chromium compound(s) being suitable
for recycling back to the electrolytic cell for
further use in the production of alkali metal
halates. The residual precipitate of hydrous ferric
oxide being in suitable condition for disposal afker
further washing.
Although the present process involves treating
all alkali metal halate cell liquors having chrome
impurities, such as potassium chlorate, sodium
chlorate, and the like, the description herein will
be directed more particularly to the manufacture of sodium
chlorate. H~ever, it is to be understood that in describing ~le
63~19
production of sodium chlorate, other alkali metal
chlorates are also applicable and are to be included.
The invention will be readily understood by the
reference to the following descriptions of embodiments
S taken in conjunction with the drawing which provides
a schematic representation of the chemical process.
The ~roduction of sodium chlorate is carried out
by the electrolysis of ~rine in a cell 1 equipped with
anode 5 and cathode 7 in spaced relationship with each
other. Cell 1 illustrated in thè drawing containing
no diaphra~m or membrane disposed between anode 5
and cathode 7 is but one example o any number of
electrolytic cell designs which may be used specifically
for the production of sodium chlorate. The liquor 3
produced in such a cell will typically have a low pH
usually in the range of about 3 to 6, and will comprise
from about 250 to about 750 gpl sodium chlorate, from
about 75 to about 200 gpl sodium chloride, from 0 to 6
gpl sodium hypochlorite and a dichromate salt, such as
potassium or sodium dichrornate as an impurity, usually
in an amount ranging from about 0.005 to 10 gpl. The
liquor is usually treated to eliminate substantially
all residual hypochlorite by means known in the art,
such as by the addition o~ urea, catalytic decay ~sing
salts of cobalt, copper, platinum, and nickel, or by
simply retaining the liquor in a holding tank where
the remaining hypochlorite is converted to chlorate.
However, according to the disclosed invention, the lron
(II) compounds employed as a reactant are capable of
performing a dual function, namely to reduce the
chromium (VI) compounds and also decompose any remain-
ing hypochlor~ite in the cell liquor~ ~ence, the need
63~9
for an independent hypochlorite "kill" step is no
longer a requirement in carrying out the immediate
invention. Nevertheless, if desired, it is still
possible to use a pretreatment step for the elimina-
tion of any residual hypochlorite in the cell liquor.
Treater-reactor 17 equipped with ayitator 19
is charged with sodium chlorate-containing cell
liquor containing chromium (VI) salt impuri~ies.
The pH is adjusted and maintained at 5.5 to 9.5, and
more preferably, in a range of about 6 to about 9 by
the addition of sodium or potassium hydroxide or
alternatively, by the addition of chlor-alkali cell
caustic catholyte liquor or other alkali or alkaline
materials, e. g. . . .sodium carbonate, lime or
mixtures thexeof. The pH of the cell liquor in
reactor 17 is regulated by pH control 13 equipped
with pH electrode 15 positioned in the interior of
the reactor. The pH of the reaction mixture is there-
by constantly monitored. Should, for example, the pH
of the reaction mixture fall below the prescribed
level, control 13 actuates control valve 11 feeding
a sufficient amount of sodium hydroxide to the mix-
ture which will restore the pH to the desired level.
A precipitate of hydrous ferric-chromic oxide
16 is produced in reactor 17 preferably by the addi-
tion of about a stoichiometric amount of iron (II)
compound(s) 9 per mole of hexavalent chromium present
in the liquor. However, in practice the quantity of
iron (II) actually employed may range in an amount
somewhat under or over 3 moles of iron (II) per mole
of chromium (VI), depending on the level of purity
desired or grade of sodium chlorate being produced.
1 :1 83~L :1. 9
~9.
Generally, this will range from a~out 2.5 to a~out
3.5 moles of divalent iron per mole of chromium lVI).
In those instances where the cell liquor contains
some residual sodium hypochlorite which was not
eliminated by a prior hypo-kill procedure, additional
or excess amounts of divalent iron should be employed.
Normally, the extra divalent iron added will be an
amount sufficient to reduce the sodium hypochlori~e,
or, in other words, from about 1.5 to about 2.5 moles
of additional iron (II) per mole of hypochlorite.
The iron (II) compound(s) 9 is added to the~`cell
li~uor in reactor 17 simultaneously or subsequently
to the addition of the caustic soda while the mixture
is agitated. This will minimize the corroslon of
steel agitators, etc., in the reaction vessel and the
risk of forming chlorine and/or chlorine dioxide under
acid conditions resulting from the introduction of
acidic divalent iron salts. The reaction is conducted
at temperatures which may range from about 20 to 95 C.,
~0 although temperatures ranging from about 45 to 75 C.,
are preferred. Virtually any readily available divalent
iron compound may be used according to the process, and
includes,for example, ferrous sulfa-te, ferrous sulfite,
ferrous chloride, ferrous nitrate, and mixtures thereof.
The hydrous ferric-chromic oxide precipitate
contains essentially all of the chromium originally
in the cell liquor, provided the amount of divalent
iron added is at least 3 moles per mole of hexavalent
chromium. The solids in the mixture are separated by
filter 21 which may also be a centrifuge, not shown.
Alternatively, separation may be accomplished by a
combination of bo-th centrifugation and filtration
1 3 ~ 3 ~
-~o
utilizing techniques known in the art. To aid in
separation and reduce viscosity of the mixture,
brine 24 may be added to the mixture. The recovered
filtrate 23 is a colorless sodium chlorate-chloride
solution which is free or substan-tially free of
chromium compounds. Chrome-free sodium chlorate
crystals may be prepared from solution 23 by -treat-
ing in chlorate crystallizer 31 of any conventional
design. Alternatively, solution 23 may be processed
by selective crystallization of sodium chlorate from
an aqueous solution containing sodium chloride, w~ich
comprises introduciny sodium hydroxide into said
solution in an amount sufficient to depress the solubil-
ity of the sodium chlorate in cooling the solution from
lS an initial temperature of from 80 to 100 C. to a final
temperature from ahout 25 to 40 C. The solubility of
the sodium chlorate is yreatly reduced whereas the
solubility of the sodium chloride is not appreciably
affected. Details of the process are disclosed in U. S.
Patent No. 3,690,845 which is incorporated-by-reference
herein.
Chrome-free sodium chlorate-containing solutions
_ may also be prepared from filtrate 23, such as R~2
solution commonly used in the production of chlorine-
dioxide. R-2 solutions are those having both sodium
chlorate and sodium chloride wherein the chloride to
chlorate mole ratio is typically about 1.00 to about
1.09. In those instances where brine 24 is added to the
reaction mixture duriny filtration the need for adding
make-up brine to the final solution to bring solution
29 up to R-2 specification may be eliminated.
~ :1 6 ~ '3
- ~:11-
Although the preeipitate 25 comprising
Fe203/Cr203.n H20 may be discarded, aceording to the
present invention, it is preferred to reeover the
ehromium values in a form whieh permits their
further use in the production of sodium chlorate.
In this regard, it was diseovered that the trivalent
chromium of precipitate 2S can be selectively treated
by slurrying the solids 39 with an oxidizing agent
33 in a second reaction vessel 35. With the aid of
mixer 37 the chromium in the precipitate is selectively
eonverted to a soluble form by oxidation to a ~6 state,
e.g~ . . .sodium chromate, leaving an unreacted solid
comprising hydrous ferric oxide. This water insoluble
precipitate ean be made into a dis~osable solid by
further treatment, i. e. . .washing to reeover
residual ehromium (VI) compounds, or alternatively,
eonverted back to recyclable iron (IIl salts, as dis-
elosed hereinbelow.
Oxidizing agents 33 suitable for use according to
the present invention, may inelude a wide range of
materials, but pre~erably include o~idiæing agents
readily available in electrochemical plants. For
example, solutions of bleach containing soidum hypo~
chlorite, usually 5~ in water made by bubbling chlorine
gas into sodium hydroxide, are most satisfactory.
Alternatively, solutions of bleach made from chlor-
alkali eell scrubber liquor o~ hypochlorite-eontaining
chlorate cell liquor may be employed. Generally, a
sufficient amount of oxidizer is employed to convert
all the trivalent chromium to solubilized hexavalent
chromium, althou~h excess amounts may be used without
~ ~ 6 ~
-12-
., ~
adverse results. More particularly, at least a
stoichiometric amount of oxidizer is needed, and
in the case of sodium hypochlorite, at least 1.5
moles of hypochlorite per mole of trivalent chro~e
is added ~o the rea~tion mixture. Other oxidizers,
such as solutions of hydrogen peroxide, preferrably
30~ by weight, may be employed. Gaseous oxidizing
agents like oxygen, air and ozone may also be use~ul.
The reaction is carried out under ambient conditions,
most satisfactorily at between 20 and 75 C. The
mixture of precipitate and oxidizer 39 is se~arated
in vessel 41 which may be either a filter or a centrl-
fuge or a combination of both. Filtrate 43 which
comprises sodium dichromate plus some sodium chloride
and possibly sodium chlorate, and any unreacted
oxidizer, e. g. . . . sodium hypochlorite is suitable
for recycling back to chlorate cell 1 via loop ~7,
or alternatively, to brine dissolver of conventional
design (not shown). Because residue ~S comprises
mainly insoluble hydrous ~erric oxide in the form of
a gelatinous materlal it has a tendency to adsorb
chrome values from the mixture. The hydrous ferric
oxide, however, can be made into a substantially
chrome-fxee solid suitable for disposal by washing
with water and/or brine solutions to recover the soluble
chromium values (not illustrated). The washing solu-
tions containing the recovered chromium values may be
returned to cell 1 via loop 47. Residue 51 may also
be converted back to reusable iron (II) compounds by
reducing it, for example, with sulfur dioxide employing
methods known in the art. The ferrous salts may then
be recycled as new reducing agents via line 53.
:.
--]3-
The volume of precipitate~l solids occurriny from
the reduction of hexavalent chromium compounds in
chlorate cell liquor with divalent iron may in some
instances be appreciable. Under such circumstances,
it may be desirable to reduce the quantity of solids
thereby minimizing both material handling requirements
and time needed for separation, e.g. filtxation of the
slurry and ultimate recovery of chrome-~ree sodium
chlorate. Accordingly, the present invention contem-
plates as an alternative embodiment the use of hydrazineor salts of hydrazine replacing at least part of the
divalent iron requirements previously described. Although
up to 100% of the divalent iron requirements may be re-
placed with hydrazine eliminating all iron (II) from the
reaction, up to about 85%, and more preferably, from
about 25 to 75% of the total iron (II) requirements may
be replaced with hydrazine or their salts. By utilizing
the product made from the reaction of hydrazine and di-
valent iron the hydrazine component provides supplementary
reducing power such that the quantity of iron (II) salts
needed to remove substantially all chromium (VI) in the
cell liquor is lowered. It was discovered that because
hydrazine forms no residual precipitate of its own in
chlorate cell liquor having chrome impurities a reduction
in the quantity of iror- (II) salts in the reaction mixture
will result in a lowering of the solids loading factor in
the separation step. As a further advantage, the reacticn product of
one mole of hydrazlne and two moles of divalent iron, which may form
a chemical c~llplex at pH below 8, balances the iron in the
rcaction product such that when reacting with chromate-containing
cell liquor a nearly neutral pll is achieved, thereby
eliminating the need for either acid or base to adjust the
pH.
J 3 63~ ~ ~
As previously disclosed, in the extreme hydrazine
may be utilized as the sole reducing agent in remov-
ing chrome impurities from chlorate cell liquor.
When used alone, hydrazine is supplemented wlth an
acid, such as hydrochloric acid, to control the pH at
about 5 to 7 which will effectively reduce the
chromium (VT) compound(s) in the impure chlorate cell
liquor to chromium (III). Under such conditions, a
slurry of iron-free hyc'rcus chromic oxide is formed
which is separated from the cell liquor by methods
disclosed hereinabove. Chromium (VI) may in turn be
recovered by treating the hydrous chromic oxide with
an oxidizing agent, e.g. hypochlorite solution, alkaline
hydrogen peroxide, ozone, etc. In the absence o~ iron
in the reaction, the solution containing solublized
chromium (VI) compound(s) preferably after simple polish-
ing filtration, may be returned to the cell. However,
the product made from the reaction of divalent iron and
hydrazine or salts of hydrazine ls the preferred mode
for this alternative embodiment. The reaction product
can be made in-situ by the addition of both
the hydrazine and the divalent iron to the cell liquor,
or alternatively~ by preforming the product which
is then added to the chrome-containing cell liquor.
~5 Up to 85~, and more preferably, up to 75~ of
the divalent iron required to reduce the chromium (VI)
to chromium (III) may be substituted with either
hydrazineAox salts of hydrazine. Calts of hydrazine
include both inorganic and organic types, such as the
monohydrochloride, the hydrobromide, hydrosulfate, and
the like. Organic salts and derivatives of hydrazine,
such as hydrazine oxalate, mono and dimethyl hydrazine,
semicarbazide, although useful) are less attractive
alternatives to hydrazine and inorganic sa]ts,
since the introduction of carbonaceo~ls materials in
3~19
-~5-
chlorate solutions is preferabl~ avoided, especially
those which may be returned to chlorate cells or used
in chlorine dioxide generators. The divalent iron
is replaced with hydrazine at a rate ranging from
about 1 to 2 parts by weight of anhydrous hydrazine
per 7 parts of ferrous iron replaced
The following specific examples demonstrate the
process of the instant invention, however, it is to
- be understood that these examples are for illustrative
purposes only, and do not purport to be wholly
definitive as to conditions and scope.
EXAMPL~ I
A 250 ml. sample of "hypo-killed" cell liquor from
a sodium chlorate electrolyzer was placed in a 500 ml.
beaker, agitated gently and heated to 74 C. The cellliquor
contained 334 gpl sodium chlorate, 206 gpl sodium chloride
and 476 ppm Cr as Na2Cr207 and Na2CrO4. Simultaneously,
a solution of 3 grams of ferrous sulfate heptahydrate
in 20 ml. of water and a solution of 20~ sodium hy-
droxide in water were added slowly over a period ofS minutes. The feed rates were adjusted to maintain a
pH of 8 + 1 during the entire addition period and the
mixture was agitated for an additional 10 minutes.
100 ml. of the slurry was filtered through two layers
of #1 Whatman filter paper. The filtrate contained less
than 1 ppm chromium. The filter cake was washed 3 times
with ]00 ml. portions of saturated brine. The solids
were mixed with 10 ml. of 5% sodium hypochlorite solution,
a~itated for 20 minutes and transferred to a filter hav-
in~ two layers of #1 Whatman filter paper and ~ashed withthree 100 ml. portions of saturated brine. The combined
filtrates ancl solids were analyzecl for chromium and iron
both bv atomic absorbtion. Results showed that all of
:`
:
1 :363~.~g
-16-
the iron is recovered as a res~due and at least 81%
of the chromium remained in the filtrate for recycle
to the chlorate cell.
EXAMPLE II
A 4 liter beaker is filled with a solution com-
prising 33~ gpl sodium chlorate, 206 gpl sodium
chloride and 476 ppm of Cr as Na2Cr207O The solution
has a temperature of 60 C. and an initial pH of 4.5.
While agitating by means of a magnetic stirrer bar
at the bottom of the beaker 48 grams of ferrous
sulfate heptahydrate (25%) in brine (290 gpl sodium
chloride) and sodium hydroxide (20~ in water) are
simultaneously added to the sodium chlorate containing
solution. During the addition of the ferrous sulfate
and sodium hydroxide the p~ is held at 8 + 1. A brown
slurx~ is formed containing about 0.5% solids.
A 30 ml. portion of the slurry is filtered
through a Millipore brand vacuum filter apparatus hav-
- ing a ~l Whatman filter paper covered with 0.2 grams
of Dicalite Speedplus diatomaceous earth having a cross
sectional area of 0.01 Et. . The colorless filtrate
contains 313 gpl sodium chlorate, 203 gpl sodium chloride
and less t~lan l ppm NaCr207.2H20.
The filter cake containing diatomaceous earth and
hydrous ferric-chromic oxide, water and residual amounts
of sodium chlorate and sodium chloride is placed in lO0
ml. beaker equipped with a stirrer. lO ml. of 5~ sodium
hypochlorite solution also containing about 4% sodium
chloride is added for 30 minutes at room temperature~
The resulting slurry is filtered agaln through dia-
tomaceous earth and washed four times with lO ml. water
.,
~ ~ 6 ~
on the filter. The resulting yellow filtrate con-
taining 85~ of the chromium originally present in
the chlorate solution is treated with sodium chloride
rendering it suitable for returning to the electrolytic
cells for further manufacture of sodium chlorate. The
washed filter cake containing mainly hydrous ferric
oxide is dissolved in HCl and sulfur dioxide bubbled
in to form ferrous salts for return to the ferric-
chromic oxide precipitation step.
EXAMPLE III
A slurry was prepared by reacting 1 gram of ferrous
sulfate heptahydrate and 10 ml. of water with 1 gram of
a 10% aqueous hydrazine solution. This slurried reaction
mixture was added to 250 ml. of a sodium chlorate-con-~ning
cell liquor having 206 gpl sodium chloride, 334 gpl sodium
chlorate, and 476 ppm chromium as sodium chromate. The
temperature of the solution was 85 C. After l hour
of mixing a 30 ml. sample of the slurry was filtered in
a Millipore apparatus, as described in Example II. The
30 ml. sample required 103 seconds for filtration to
; be completed, approximately one-third the tirne required
for filtering the equivalent slurry prepared frorn
ferrous sulfate exclusively. The filtrate contained
less than l ppm chromium.
The residue from the precipitation step was subse-
quently treated with l ml. of 5% sodium hypochlorite
solution to produce a slurry which upon filtration and
washiny yielded a solution containing 72% of the chromium
originally in the residue.
3~ 1 9
..
EXAMPLE IV
A 250 ml. sample of sodium chlorate cell liquor
containing 206 gpl sodium chloride, 334 gpl o sodium
chlorate, and 476 ppm chromium at 70 C. was treated
with 2 grams of a 10% hydrazine solution. The pH of
the reaction mixture was held at about 6 by the
simultaneous addition of 6 N hydrochloric acid. A
slurry of Cr203.nH20 was formed. After l hour the
slurry was filtered and allo~ed to cool to room
temperature. The solution was polish filtered to re-
move light turbidityO The filtrate contained lessthan 3 ppm chromium~ A 30 mlO sample of the slurry
required a filtration time of 50.6 seconds. This
represents one-sixth of the time required to filter
a 30 ml. slurry sample prepared from a reaction of
ferrous sulfate with cell liquor. The residue from
the 30 ml. filtration was treated with 5 ml. of 5%
sodium hypochlorite, washed S times with lO ml.
portions of water and one 50 ml. portion. The filtrate
and washings contained nearly 100~ of the chromium
originally contained in the residue.
While the invention has been described in con-
junction with specific examples thereof, this is
illustrative only. Accordingly, many alternatives, modi-
fications, and variations will be apparent to those
skilled in the art in light of the foregoing description
and it is therefore intended to embrace all such alterna-
tives, modifications and variations as to fall within
the spirit and broad scope of the appended claims.
