CA1187036A - Chromate removal from concentrated chlorate solution by chemical precipitation - Google Patents

Abstract

ABSTRACT _ A process for removing and recovering precipitated solid particles of divalent or trivalent chromium compounds from an aqueous alkali metal chlorate solution containing them which comprises passing the aqueous solution through a first centrifuge to obtain an overflow solution and an under-flow solution, filtering the overflow solution and adding the filter cake therefrom to the underflow solution to form a slurry, diluting the slurry with more water and passing through a second centrifuge to form a second overflow aqueous solution and a second underflow aqueous solution. The second overflow aqueous stream is then mixed with an alkali metal halide to form a brine and is fed to an electrolytic cell.

Description

6~q~4 CHRO~TE REMOVAL FROM CONCENTRATED CHLOR~TE
SOLUTION BY CHEMICAL PRECIPITATION
This invention relates to a process ~or removiny hexavalent chromium from impure aqueous alkali metal chlorate solutions. More particularly, it relates to the removal of alkali metal dichromates from concen-trated aqueous solutions of alkali metal chlor~tes ~y reacting said solutions with inorganic sulfur containing comp~unds~ Most particularly, the present invention xelates to the xemoval of sodium dichromate from concentrated aqueous sodium chlorate solutions without significantly altering the ratio of sodium chlorate to sodium chloride in the strong oxidizing solution.
In the producti.on of chlorine dioxide for use in the bleaching of chemical celluosic pulps, an aqueous sol.ution of an alkaii meta]. chlorate and an alkali metal chloride, particularly sodium chlorate and sodium chloride is reacted with a rnineral acid r particularly sulfuric acid~ In such a process, the aqueous solution ~ contains sodium chlorate and sodium chloride in a selected ratio which is conducive to optimum production of chlorine dioxide.
The aqueous solution of the sodium chlorate and sodium chloride is conventionally produced by the electrolysis of brine in electrolytic cells, usually bipolar electrolytic cells, the extent of the electroly-sis suitably being controlled tc produce an eflluent from the cell in which the sodium chlorate and sodium chloride have essentially the selected ratio for optimum production of chlorine dioxide in the chlorine dio~ide 3~

generator. The selected ratio of sodium chlorate to sodium chloride is in the ran~e from about 1:1 to about - 20:1 and preferably in the range from about 2:1 to about 15:1. However, in the electrolysis of the ~odium chloricle in the bipolar electrolytic cells to form sodium chlorate, it is conventional to add chromates to the electrolyte in the cell because the chromates sign-ficantly improve the current ef~iciency of the cells in the oonversion of the sodium chloride to sodium chlorate. Chromate ion occurs in signi~icant amounts in the cell effluent~ I-t is desirable to remove chromate ion from the cell effluen~ before it is processed in the chlorine dioxide generator. Also, chromium is a valuable material; hence, it is desirable lS to recover it for reprocessing into chro~ate for sub-sequent re-use in the electrolytic cells. Fuxther, if the chromate passes into the chlorine dioxide generator, it may be present in the waste effluent from the chlorine dioxide generator, and eventually reappear in the outside environment as a toxic pollutant.
U.S. Patent ~o~ 3,961,029 issued to Yoshio Senoo on June 1, 1976 discloses that chromate ion can be re-covered from an aqueous waste liquor by employing in a first step a reducing agent such as sodium hydrogen sulfite at an acidic pH and precipitating the chromium as chromium hydroxide at an alkaline pH. ~.S. Patent No. 3,981,965 issued to Alan B. Gancy et al on September 21, 19?6 dis~loses a method of treating solid waste material containing minor amount~ of water soluble chromium compounds with sodium dithionite. However, there is no teaching of ~he applicability of either o~
i these aforementioned processes to a highly concentratedaqueous alkali metal chlorate solution, a stron~ oxidizing 501ution .

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U.S. Patent No. 3,8~3,769 issued to Harold deVere Partridge, et al on October 22, 1974 teaches a process whereby alkali metal chlorate solutions containing hexavalent chromium are treated with at least about a 3-molar proportion of a water-soluble su~fide such as sodium sulfide, sodium hisulfide, potassium bisulfide or hydrogen sulfide. Preferably an excess of the sulfide is added to the solution. Then the solution is acidified to a pH below about 5, whereby the trivalent chromium is precipitated as an insoluble product, and removed from the solution. Partridge et a~ discloses- the treatment in this way of chlorate solutions having about 350 grams per liter of NaClO3, but do not ~isclose the treatment of higher concentrations of sodium chlorate.
A further discussion of the above-noted patent and other chemical methods of precipitating Cr(OH)3 is found in U.S. Patent No. 4,086,150 issued to Bruno Rindl et al on April 25, 1978.
The above-mentioned prior art processes for the removal of hexavalent chromium from aqueous solutions containing hexavale~t chromium have serious drawbacks in that H2S gas may be generated and undesirable sulfur compounds such as elemental sulfur may be ~ormed in the sodium chlorate solution~
In spite of these and other methods, there is i a long felt need at the present time for a process to I remove hexavalent chromium from an aqueous solution of alkali metal chlorate by the selective reduction of hexavalent chromium, without significant reduction of 30 - alkali metal chlorate, at a pH value which results in the separation of chromium compounds as for example an insoluble trivalenf or divalent chromium hydroxide, which can easily be removed from the aqueous alkali metal chlorate solution~

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Ob~ects It is a primary object of this invention to remove hexavalent chromium from concentrated aqueous solu-tions of alkali metal chlorates.
It is another object of this invention to remove hexa-valent chromium from aqueous solutions of concentrated alkali metal chlorates by the selective reduction of hexavalent chromium, without reduction of alkali metal chlorate or production of hydrogen sulfide.
It is yet another object of this invention to remove hexavalent chromium from concentrated aqueous solutions of alkali metal chlorate solution without the introduction of significant amounts of impuri-ties resulting from the reducing agent employed or reaction products thereof.
It is still another object of this invention to remove hexavalent chromium from aqueous solutions of concentrated alkali metal chlorates without significantly altering the ratio of alkali metal chlorate to alkali metal chloride in the so:Lutions.
It is an additional object of this invention to recover and reuse precipitated chromium hydroxide.
It is a further object of this invention to remove alkali metal hypohalite from aqueous solutions of concentrated alkali metal chlorates.

2~ It is a still further object of this invention to remove heavy metals from aqueous solutions of concentrated alkali metal chlorates.

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-4a-Brief Description of the Invention .
The foregoinq objects o the in~en-- tion are accomplished in a process for removing hexavalent chromium from an impure aqueous alkali metal chlorate solution containing hex~valent chromium as an impurity which comprises:
(a) adjusting the pH of the impure aqueous solution to a first intermediate pH in the range from a~out 9 to about 13;
~; (b) xeacting an inorganic sulfur compound selected from the group consisting of alkali metal bisulfite, alkali metal sulfite, alkali metal thiosulfa~e, alkali ; metal thiosulfite, alkali metal metabisulfite, alkaline earth metal bisulfite, alkaline earth metal thiosulfate, alkaline earth metal ; metabisulfite, alkali metal dithionite, alkaline earth met~l dithionite~ ferric ; sulfite, ferric thiosulfate, ammonIum sulfite, ammonium bisulfite, ammonium thiosulfatP, and mixtures thereof with the impure aqueous solution to form an aqueous slurry o~ solid particle~ of trivalent and 2ivalent chromium ~compounds in an aqueous solution;
(c) adjusting the pH of the aqueous slurry of alkali metal chlorate to a second intermediate pH in the range from about 2 to about 4;
(d) further adjusting the pH of the aqueous slurry to a final pH in the range from about 6 to about 8; and (e) separating the solid particles of divalent and trivalent chromium compounds from the aqueous solution.
The FIGURE shows a schematic view of a process for removal and recovery of chromium precipitates.

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D~tailed Description of th Invention Any aqueous solution of alkali metal chlorate which contains reducible hexavalent ~hromium may be treated according to the process of this invention.
S Typical solutions are aqueous alkali metal chloxate solutions, aqueous alkaline earth metal ch~orate solutions, and mixtures thereof.
Typical alkali metal chlorates include sodium chlorate, potassi~ chlorate, lithium chlorate, rubidium chlorate, cesium chlorate and mixtures thereof.
Typical alkaline earth metal chlorates include beryllium chlorate, magnesium chlorate, calcium chlorate, strontium chlorate, barium chlorate, radium chlorate and mixtures thereof.
To simplify the description, the invention will be described in terms of an aqueous sodium ch~orate-solu-tion although any other alkali metal chlorate, alkaline earth metal chlorate or mixtures thereof may be substituted with equivalent results. For example, one may substitute potassium chlorate, lithium chlorate, barium chlorate, magnesium chlorate, calcium chlorate, mixtures thereof and the like.
The electrolytic production of sodium chlorate has been effected in electrochemical cells typically provided with graphite, steel, or titanium cathodes~
To minimize corrosion of steel cathode an~ to improve the overall chlorate efficiency of the cell possibly by decxeasing hypohalite reduction at the cathode, hexavalent chromium compounds have been conventionally added to the cell liquor. The hexavalent chromium compound is usually added to the cell liquor in the form o~ sodium dichromate in an amount sufficient to provide a hexavalent chromium concentration in the range from about 0.1 to about 20.0 and preferably from about 0.2 to about lO.Ograms sodium dichromate per liter of solution .

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Hexavalent chromium may be added to sodium chl~rate cell liquor in any suitable form. For example, water soluble hexavalent chromium may be supplied as Na2CrO4 (sodium chromate), Na2Cr207 (sodium dichromate), CrO3 (chromic acid), potassium dichromate, potassium chromate, mixtures thereof and the likeO ~hroughout this specification and claims, when reference is made to hexavalent chxomium, it is to be understood that any source of soluble hexavalent chromium may be substituted with equivalent results.

In Encyclopedia of Chemical Technology, by Kir}.-Othmer, Third Edition, Volume 5, pages 633-645, a variety of electrochemical cells are described for producing sodium chlorate.
. 5 A typical analysis of an aqueous sodium chlorate liquor from a typical sodi~n chlorate cell plant is as ~ollows:

.. COJnpOnent Typical ~1in /Max. Min./Max.
. Analysis AnalysisAnalysis (grams PreferredBroa~
~ per liter) (grams(grams i per liter)per liter) NaClO3 600 250 - 675100 - 750 lS NaCl 150 100 - 30020 - 400 NaOCl 1 0.2 - 40.1 - 5 Na2SO4 5 1 - 100.1 - 30 Hexavalent Chxomium 2 0.2 - 100.1 - 20 Greater or les~er concentrations of ~he components shown above may be present in aqueous sodium ohlorate l;quor.
In addition, other minor components may also be present.
Additionally, as indicated in the Kirk-Othmer : article cited above, the presence of alkali metal hypo-halites such as sodium hypochlorite, potassium hypo-chlorite, mixtures thereof and the like is not a strict requirement of this invention but such alkali metal hypohalites are expected components in a typical electrochemical cell plant pxoducing alkali metal chlorates such as sodium chlorate.
The term "impure.aqueous sodium chlorate liguor"
is.employed ~hroughout the claims and description to denote any a~ali metal or alkaline eart~ metal chlorate liquor containing reducible hexavalent chromium.
The pH o~ typical impure aqueous sodium chlorate ; liquor is in the range from about 2 to ~bout 12 ana preferably from about 3 to about 10.

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In the process of this inventlon ! when the pH of an im-pure aqueouC sodium chlorate solution containing hexavalent chromium is less than about 9, sufficient inorganic base is added to adjust the initial pH to a first intermediate pH
which is within the range from about 9 to about 13 and prefe-rably from about 10 to about 12.
Typical inorgani~ bases include aqueous solutions of sodium hydroxide, potassium hydroxide, mixtures thereof and the like. The particular inorganic base chosen is one that preferably corresponds to the particular alkali metal chlo-rate solution undergoing chromate removal. For example, when treating a sodium chlorate solution, then it is preferred to employ sodium hydroxide as the inorganic base.
The inoryanic base is typically an aqueous solution of the inorganic base having a concentratiOn in the range from about 5 to about 75 and preferably from about 10 to about 60 perccnt inorganic base by weight.
Sufficient water soluble inorganic sulfur containing compound is added to the impure aqueOus sodium chlorate solu-tLon to provide an amount of inoryanic sulfur containing com pound relative to the hexavalent chromium in excess of the molar ratio. For example~ the molar ratio of Na2S2O4 to Na2Cr2O7 is in the range from about 2.2:1 to about 8-1 and preferably from about 2.4:1 to about 4:1. The molar ratio of Na2SO3 to Na2Cr2O7 is in the range from about 7.6:1 to about 14:1 and preferably from about 8:1 to about 12:1. The 2S2O5 to Na2Cr2O7 is in the range from ab 3~1 to about 10:1 and preferably from about 2.7:1 to about 5.1.
The inorganic sulfur containing compound also reacts with any alkali metal hypohalite present such as alkali metal hypochlorite to remove alkali metal hypochlorite by chemical reaction.

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A preferred method of removing alkali metal hypo-halite present in impure sodium chlorate solution is to chemically decompose the hypohalite before prac.icing the process of this invention.
The inorganic sulfur containing compound also reacts with any heavy metals present~such as mercury, to pre~
cipitate heavy metals by chemical reaction, for example mercury sulfide.

Typical water soluble inorganic sulfur containing compounds which may be employed in the process of this invention include (i) alkali metal bisulfite such as sodium bisulfite, potassium bisulfite, lithium bisulfite and mixtures thereof; (ii) alkaline earth metal bisulfite such as calcium bisulfite, magnesium bi.sulfite and mixtures thereof; (iii) alkali metal metabisnlfite such as sodium metabisulfite, po~assium metabisulfite, lithium metabisulfite, and mixtures thereof; (iv) alkali metal sulfite such as sodium sulfite, potassium sulfite, lithium sulfi~e and mixtures thereof; (v) alkaline earth metal sulfi~e such as calcium dihydrogen sulfite, magnesium dihy~rogen sulfite; (vi) alkali metal thiosulfate such as sodium thiosulfate, potassium thiosulfate, lithium thiosulfate, and mixtures thereof; (vii) alkaline earth metal thiosulfate such as calcium thiosulfate,. maynesium thiosulfate, and mixtures thereof; 5viii) alkali metal di.thionite,such as sodium dithionite., potassium dithionite, lithium dithionite, and mixtures thereof;

(i~ alkaline earth metal ~i~hionitesuch as ~alcium dithionite, magnesium dithionite,and mixtures thereof; and ~x ~ ferric sulfite, ferric thiosulate, and mixtures thereof;
and (xi) ammonium sulfite, ammonium hvdrogen s~lfite, ammonium thiosulfate, and mixtures there~; (xii) alkaline earth metal-metabisulfite such 25 calciu~
me~abisulf~e,-~gnesium-~etabi~ul~ite, an~ mixtures thereof;

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Without being bound by theory, it is believed that an inorganic sulfur compound such as sodium thiosulfate reacts with sodium dichromate and water to form divalent chromium hydroxide and sodium sulfate ~ccording to equation (1):

( 1) 2S23 + Na2Cr27 + 2H2o 2Cr(OH)2 + 2Na2SO4.

Similarly, it is bel.ieved that an inorganic sulfur containing compound such as sodium sulfite reacts with sodium dichromate, water, a~d hydrochloric ac.id to form chromium hydroxide, sodium sulfate, and sodium chloride according to equation (2):

(2) 2S03 +.Na2Cr2o7 + H20 + 2HC

2Cr(OH)2 ~ 4Na2SO4 + 2~aCl.

Similcrly, it is believed that sodium dithionite reacts with sodiurn dichromate and water to form chromium hydro~ide and sodium sulfate accor~ing to equation (3):

(3) Na2cr2o7 + Na2S2~4 + 3H2 2Cr(OH)3 ~ 2Na2SO4~

Similarlv, it is believed that sodium bisulfite reacts ith sodium dichromate, water, and hydrochloric acia to form chromium hydroxide, scdium sulfate, and sodium chloride according to eauation (4):

(4) 3Na2SO3 + Na2Cr2O7 + 2H2 2Cr(OH)3 + 3~a2SO~ + 2 ~aCl.

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~ dditional inorganic su.lfur containina compound must be added to the impure aqueous sodium chlorate solution to provide a molar ratio of additional inorganic s~-lfur containing compound to alkali metal hypohalite in the aqueous solution in the range from about 0.4:1 to a~out 2:1 and preferably from about 0.5:1 to about 0.6:1.
Yet additiv!aL ;norganic sulfur containing compounds must be added to the impure aqueous sodium chlorate solution to provide a molar ratio of additional inorganîc sulfur containing compound to heavy metals such as mercury in.the aqueous solution in the range from about 1:1 to about 10:1 and preferably to about 2:1 to about 8:1.

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The pH of the impu.re aqueous sodium chlorate solution is maintained in the aforemention~a range for a time period in the range from about 5 to about 60 and preferably from about 10 to about 30 minutes while the S solution is vigorously stirred. Any suitable agitation means may be employed; however, air agitation is preferred.
Without being bound by theory, it is believed that a chromium chloride complex hydrate CrC13-X~2O is 10 , formed at the high alkaline pH which is not fully reduced.
It has been found in these reactions that the formation of and subsequent removal of the solid trivalent chromium compound Cr(OH)3 and the solia divalent chromium compound Cr(OH)2 are remarkably enhanced when the pEI of the impure solution~ after inorganic sulfur containing compound has been added to the impure aqueous sodium chlorate solutionr is pH
adju5ted in the range from about 2 to.about 4 and preferably from about 2.5 to about 3~5.
It is further believed that lowering o~ the pH to a strong acid region effects a more complete reduction of the CrC13 XH2O complex hydrate to soluble Cr(OH)3.
When the first intermediate pH of the impure aqueous sodium chlorate solution is greater than about 9, sufficient inorganic acid is added to ad~ust ~he first intermediate pH to a second intermediate p~
within the range from about 2 to about 4 an~ preferably from about 2.5 to about 3Ø
. Any i.norganic acid may be employed in the process of this invention which is capable of controlling the pH
of the impure aqueous sodium chlorate solution in the-range from about 2 to about 4 and which does not act as a reducing agent for the alkali metal chlorate and hexavalent chromium present in the solution~

Typical inorganic acids include acids such as hydrochloric acid, sulfuric acid, phosphoric acid, nitric acid, orthc-phosphoric acid, pyrophosphoric aci~, mixtures thereof and the like.
S The inorganic acid is typically an aqueous solution of the inorganic acid having a concentration in the range from about 1 to about 15 and preferably from about

5 to about 10 percent inorganic acid by weight.
The solution is maintained at a second intermediate pH in the aforedescribed range for a time period in the range from about 5 to about 60 and preferably fxom about 10 to about 30 minutes while the solution is vigorously stirred. Any suitable agitation means may he employed;
however, air agitation i5 preferred.
Thereafter, the pH of the solution is adjusted to a final pH value in the range rrom about 6 to about 8 and preferably from about 6.5 to about 7.5 by the addi~
tion of a sufficient amount of the previously described inorganic bases. As the pH of the solution tends to ~0 become more basic, the Cr(OH~3 becomes less soluble in solution.
The temperature of the impure aqueous sodium chlorate solution during reaction, precipitativn and p~
adjustment steps is in the range from about 20~ to about 100C and preferably from about 25 to about 80C.
The reaction is generally carried out at atmospheric pressure but super~ or subatmospheric pressure may be employed, if desired Either a batch or a continuous operation can be used, although the batch system is preferable.
The aforementioned process steps of reaction and pH
adjustment result in the formation of a slurry of solia particles of Cr(OH)3 or Cr(OH)2 (from about 1 to abou~
5 percent ~y weight), hexavalent chromium (less than about 5 ppm), in an aqueous solution also containing sodi~m chlorate and sodium chloride in a weight ratio essentially the same as the weight ratio in the impure aqueous sodium chlorate solution~

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The solid particles of trivalen~ chromium compound, CrlOH)3,and divalent chromium compound, CrtOH)2, are separated from the sodium chlorate solution by any ; suitable solid-liauid separation techniquer such as by settling, filtering, and the like.
E30wever, a preferred method of removal and recovery of chromium precipitates such as Cr(OH)3 ana Cr(OH~2 by centrifuging from aqueous sodium chlorate solutions is shown in the accompanying FIGURE.
In the FIGVRE, aqueous solution of soaium c~lorate as formed in the process of this invention is con~eyed by process line 1 to a first centrifuge 2 to produce an overflow stream 3 and an underflow stream 4~
First centrifuge 2 is preferably a solids ejecting disc type centrifuge such as a centrifuge marketed by the DeLaval Company known as a BRPX-213 solids ejec~ing centrifu~e disc type.
The Cr(OH)3 solids content in overflow stream 3 IS
in the xange from about 0. ol to about 0005 percent by weight. After exiting from first centrifuge 2, the overflow stream 3 is conveyed to a final ~ilter 6 to produce a clarified stream 7 and a.filter cake (not shown). A preferable final filter 6 is a polishing filter containing filter cartridge comprised o polyethylene or polyvinyl chloride having about a 5-micron filter size.An automatic pr~ssure activate~
filter is preferred as final filter 6 although any suitable manual filter could be employed.
Clarified stream 7 contains less than about 0.001 percent by weight solid Cr(OH)3 and Cr(OH~2 an~ is conveyed to chlorate product storage (not shown) for sale or is otherwise utili7ed.
The clarified liquid in stream 7 is an aqueous solution of sodium chlorate and sodium chloride which is subs~antially free of chromium ions, Cr(OH)2 ana Cr(OH)3. The clarified solution is useful as feed for chlorine dioxide senerators since the w~ight ra~io of alkali metal chlorate, such as sodium chlorate, to alkali metal chloride, such as sodi.um chloride, remains substantially the same throughout the process of this invention.
Filter cake (CrlOH)3 solids) not shown is peri.odically removed from final filter 6 as required by backwashing final filter 6 with clari.fied stream 7 to produce backwash stream 8. Backwash stream 8 and underflow stream 4 from centrifuge 2 containing in the range from about 15 to about 30 percent solids are conveyed to and combined together i.n dilution tank 9 to form a slurry.
Water or brine is added as needed to the slurry in dilution tank 9 to maintain the specific gravity of the resulti.ng slurry in the range from about 1.0 to about 1.2. Speciflc gravity reduction is done to i.mprove the solids separation from the liquid.
Effluent stream 10 from dilution tank 9 is conveyed to a second centrifuge 11 to produce an overflow stream 12 and an underflow stream 13.
An addikional final filter (not shown) may be employed on overflow stream 12 to further clarify overflow stream 12. Solids collected therein could be conveyed to dilution tank 9.
Overflow stream 12 contains in the range from about 0.001 ~o about 0.05 percent solids and valuable chlorate and is thereafter conveyed to salt saturator 14 where an alkali metal halide or salt such as sodium chloride, potassiùm chloride, or mi.~tures thereof is added as required to saturate ove.rflow stream 12. Salt saturated overflow stream 17 thereby produced i.s thereafter conveyed to a sodium chlorate plant feed storage (not shown) for use in electrolyti.cally preparing sodium chlorate or as otherwise utilized. The : particular alkali metal halide or salt added to Qverflow stream 12 typically corresponds to the particular chlorate liquor employed herein. For example, sodium chloride 3S salt or sodium chloride brine is added to overflow stream 12 in salt saturator l~ when sodium chlorate is employed in the process of this invention.

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Underflow stream 13 from second centrifuge 11 contains s~lids of Cr(OH)3 and Cr(OH)2 in the range from about 80 to about 90 percent by weight and is typically drummed for resale as Cr(OH)3 or Cr(OH~2.
The solid particles of trivalent chromium compound or divalent chromium compound in underflow stream 13 may be water-washed, if desired, and then oxidized to the hexavalent state for use in electrolyte s~di~n chloxate cells or otherwise disposed of.
Centrifuge 11 is preferably a basket type centrifuge although any other suitable soli~-liquid separator capable o. precipate removal can be employed.
Those of skill in the art will recognize that a plurality of centrifuges may be employed as first centrifuge 2 and second centrifuge 11. Such pluralities of ~entrifuges may be arranged in series or par~llel flow concepts.
Aqueous solutions of sodium chlorate containing chrom.ium hydroxide may be prepared by the method previously descr;.bed .in this invention or by any other method described in t~e prior art. For examplel other method~ of precipitating chromium h~droxide from solutions contai.nir,~ hexavalent chromium are disclosed in U..S. Patent No. 3,961,029, supra; ~.S. Patent No.
3,901,805 issued to ~onald Stewart on August 26, 1975;
U~S. .Patent No. 3,981,965, supra; U.SO Patent No.
4,086,150, supra; ~.S. Patent ~o. 3,493,328 issued to Garmt J. ~ieuwenhuis on Fe~ruary 3, 1970; and U.S~
Patent llo. 3,896,209 issued to Louis Bvron Fourmier on Jul~ 22, 1975.
The process of this invention recovers valuable chromium compounds in a nearly pure state for subseque~t reuse as described above~

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The following exalnples are presented to define the invention more fully without any intention of limiting it there~y. A11 parts and percentages are by weight unless indi.cated otherwise.

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Example 1 In this example, hexavalent chromium was substantially removed from a concentrated aqueous SOaium chlorate solution by employing sodium bisulfite.
About 500 milliliters of a conc~ntrated aqueous solution of sodium chlorate containing about 637 grams sodium chlorate per liter~
about 43 grams sodium chloride per liter, and about 0.5 grams sodium dichromate (Na2Cr207 2H~0) was prepared. The initial pH of the aqueous sodium chlorate solution was about 7~ The weight ratio of sodium chlorate to sodium chloride was calculated to about 14.8:1. The initial temperature of the aqueous sodium chlorate solution was maintained at about 50C in this example~
Sufficient dilute (about 20 percent strength) aqeuous sodium hydroxide solution was added to about 500 milliliters of the sodium chlorate solution to increase the initial pH to a first int~rmediate pH oE about 11~7.
About six milliliters of about a 40 weight percent aqueous solution o sodium bisulfite (about 2.50 gxams sodium bisulfite) was added to the 500-milliliter portion of the aqueous sodium chlorate solution.

The color of the resultant solutiol~ changed from bright yellow to blue-green. The resultant solution was vigorously stirred for about 15 minutes.
A sufficient ~nount of about a five percent a~ueous solution of hydrochloric acid was slowly admixed with the a~ueous sodium chlorate solution to decrease the p~
to a second intermediate pH of about 3. As the pH of the solution slowly decreased from about 11 to about 9 and then to about 3, the resultant solution changed color from blue-grePn to dark blue-green and the intensity of the dark blue-green increased.

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.v - 17 -The solution was vigorously stirred for about another 15 minutes.
A sufficient amount o~ about 20 percent NaO~ was admixed with the aqueous sodium chlorate solution to increase the pH to a final pH of about 7.2. A dark blue-green precipitate formed at a pH of about 5.
The sol.ution was then filtered through a fine millipore glass fiber filter of about 0.5 micron size using an aspirator forming a filter ca]~e of solid dark blue-green particles of an insoluble chromium compound believed to be Cr(OII)3 and an essentially colorless filtrate.
The odor of H2S was absent during the entire example.
The concentration of hexavalent chromium in the essentially colorless ~iltrate was determined to be less than about 0.01 part chromium per million by weight by atomic absorption method. This analysis indicated nearly complete hexavalent chromi.um removal from the concentrated aqueous solution of sod.i.um chlorate. Analysis of the fil-trat~ indicated essentially no sodium chlorate reacted. The fi.nal sodium chlorate to sodium chloride weight ratio was calculated to be about 13.6:1.

Example 2 In this example, hexavalent chromium was sub-stantially removed from a concentrated aqueous sodium chlorate solution by employing sodium sulfite.
About 500 milliliters of a concentrated aqueous solution of sodium chlorate containing about 640 grams sodium chlorate per liter, about 45 grams sodium chloride per liter, and about 0.5 grams sodium dichromate (Na2Cr2O7 2H2O) was prepared. The initial pH of the aqueous sodium chlorate solution was about 7. The weight ratio of sodium chlorate to sodium chlori~e was calculated to be about 14.2:1. The initial temperature of the aqueous sodium chlorate solution was maintained at about 50C in this example.

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Sufficient dilute (about 20 percent strength) aqueous sodium hydroxide solution was adde~ to about 500 milliliters of -the sodium chlorate solu-tion to ad~ust the initial pH to a first intermediate pH
of about 11.~.
About 12 milliliters of about a 9 weight percent aqueous solution of sodium sulfite (about 1.10 grams sodium sulfite) was added to about a ~00 milliliter portion of the aqueous sodium chlorate solution.

The color of the resultant solution changed from bright yellow to green and then to ~lue-green. The resultant solution was stirred for about 15 minutes.
A sufficient amount o a five percent aqueous solution of hydrochloric acid was slowly a~nixed with the aqueous sodium chlorate solution to decrease the first intermediate pH to a second intermediate pH of about 2.5. As the pH of the solution slo-lty decreased rom about 11 to abo~t 9 and then to about 2.5, the xesultant solution cn~nged color from blue-yreen to dark blue-green. The solution was stirred for about another 15 minutes~
A sufficient amount of about 20 percent NaOH was admixed with the aqueous sodium chlorate solution to increase tlle second inte~nediate pH to a final pH of about 7.1. A brown/yello~ precipitate forned at a pH
of about 5.5.
The solution was then filtered throu~h a fine millipore glass fiber filter o about 0.5 ~icron siz~
using an aspirator forming a brown/yellow filter cake of solid particles of an insoluble chromium compound believed to be Cr(O~)~ an~ a colorless filtrate.
The odor of H2S was--absent during the entire example .
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The concentration of hexavalent chromium in the essentially colorless filtrate was determined to be less than about 0.01 part chromium per million ~y weight by atomic absorption. This analysis indicated nearly complete hexavalent chromium removal from the concen-trated aqueous solution of sodium chlorate. Analysis of the filtrate indicated essentially no sodium chlorate reacted. The final sodium chlorate to sodium chloride weight ratio was calculated to be about 13.2:1.

Example 3 In this example, hexavalent chromium was substantially removed from a concentrated aqueous sodium ; chlorate solution by employing sodium thiosulfate.
Suficient dilute (about 20 percent strength) 15 aqueous sodium hydroxide solution was added to about a 500-milliliter portion of the sodium chlorate solution prepared in Example 2 to adjust the initial pH
to a first intermediate pH of about~ 6, About 5 millilitersof about 50 weight percent 20 a~ueous solution of sodium thiosulfate (about 2.50 grams sodium thiosulfate) was added to about a 500-milliliter a~ueous sodium chlorate solution as described in Example 2.

, The color of the resultant solution changed from bright yellow to green and then to blue-green. The resultant solution was stirred for about 15 minutes.
A sufficient amount of five percent aqueous solution of hydrochloric acid was slowly aamixed with the aqueous sodium chlorate solution to decrease the first intermediate pH to a second intermediate pH of about 2.7. As the pH of the solution slowly decreased from about 11 to about 9 and then to about ~.7, the resultant solution changed color from blue-green to a murky brown blue-green color.

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A sufficient amount of about 20 percent NaOH was admixed with the aqueous sodium chlorate solution to increase the second intermediate p~ to a final pH of about 7.1. A brown/yellow precipitate ormed at a pH
of about 5.5.
The solution was then filtered through a fine millipore glass fiber filter of about 0.5 micron size using an aspirator forming a brown/yellow filter cake of solid particles of an insoluble ch~omium compound believed to be Cr(O~2 and a cc,lorless filtrate~-The concentration of hexavalent chromium in the colorless filtrate~ was determined to be less than abou~
0-5 part chromium per mi.llion by weight by atomic absorption. This analysis indicated nearly complete hexavalent chromium removal from the concentrated aqueous solution of the sodium chlorate. Analysis of the filtrate indicated essentially no sodi~n chlorate reacted. The final sodium chlorate to sodium chloride weight ratio was calculated to be about 13.2:1.
~0 The odor of H2S was absent during the entire example.

. Example 4 In this example, hexavalent chromium was substantially removed from a concentrated aqueous sodium chlorate solution by employing sodium dithionite.
A small amount of dilute (about 20 percent strength) aqueous sodium hydroxide solution was added to about a 500-milliliter portion of the sodium chlorate solution prepared in Example 1 -to adjust the initial pH of about 6.9 to a first intermeaiate pH
of about 7.

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J ~ About 1.10 grams of solid sodium dithionite was added to tlle 500-millili~er portion of the aqueous sodium chlorate solution. The color of the resultant solution changed from briyht yellow to green and then to blue green. The resultant solution was stirred for about 10 minutes.
A sufficient amount of a five percent aqueous solution o~ hydrochloric acid was admixed with the a~ueous sodium chlorate solution to decrease the first intermediate pH to a second intermediate pH of about Z.7. The resultant solution changed color from blue-green to dark blue-green as the pH of the solution slowly decreased from about 7 to about 2 . 7. The solu-tion ~as stirred for another 15 minutes.
A sufficient amount of about 20 percent NaOH was admixed with the aqueous sodium chlorate solution to increase the second,intermediate pH to a final pH of about 6 ~ 9 . A blue precipitate formed at a pH ~f about 5.
The solution was then filtered through a ~ine millipore glass fiber filter of about 0.5 micron size using an aspirator forming a filter cake o~ blue soli~
particles of insoluble trivalent chromium compound and a co~orless filtrate.
The concentration of hexavalent chromium in the colorless filtrate was determined to be less than about 0.01 part chromium pex million by atomic absorption.
This analysis indicated nearly completP hexavalent chromium removal from the concentrated a~ueous solution of impure aqueous sodium chlorate. Analysis of the filtrate indicated essentially no sodium chlorate reacted. The final sodium chlorate-to sodium chloride weight ratio was calculated to be about 14.Z:l - The odor of H2S was absent during this example.

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Claims (2)

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

1. A process for removing and recovering precipitated solid particles of divalent and trivalent chromium compounds from an aqueous alkali metal chlorate solution containing them which comprises:
(a) passing said aqueous solution containing said solid particles through a first centrifuge to form:
(i) an overflow aqueous solution having a solid particle concentration in the range from about 0.02 to about 0.05 percent, and (ii) an underflow aqueous solution having a solid particle concentration in the range from about 10 to about 30 percent;
(b) filtering said overflow aqueous stream to form a product filtrate and a filter cake;
(c) combining said filter cake with said underflow aqueous stream of said first centrifuge to form a slurry;
(d) admixing water with said slurry to form a slurry of reduced specific gravity;
(e) passing said slurry of reduced specific gravity through a second centrifuge to form:
(i) a second overflow aqueous solution having a solid particle concentration in the range from about .01 to about .05 percent, and (ii) a second underflow aqueous solution having a solid particle concentration of said di-valent and trivalent chromium compounds in the range from about 80 to about 90 percent;

(f) admixing said second overflow aqueous stream with an alkali metal halide to form a brine;
(g) feeding said brine to an electrolytic cell; and (h) recovering said second underflow aqueous solution containing said divalent and trivalent chromium compounds .

2. The process of claim 1 wherein said particles of divalent and trivalent chromium compounds recovered as said second underflow aqueous solution are oxidized to the hexa-valent state and admixed with said brine in said electroly-tic cell.