CA1163419A - Process for removing chromium compounds from alkali metal halates - Google Patents

Abstract

ABSTRACT

Alkali metal halates, such as sodium chlorate-containing liquors when produced in electrolytic cells may contain hexavalent chromium compounds which are objectionable impurities when permitted to remain in such products. These impure cell liquors adjusted to a pH of 5.5 to 9.5 are treated with divalent iron salts, such as ferrous sulfate or products formed from the reaction of divalent iron salts and hydrazine to reduce the soluble chromium (VI) compounds present to a hydrous ferric-chromic oxide precipitate. Separation of the pre-cipitate leaves a colorless chlorate-containing solution which is virtually free of chrome impurities. The chrome values of the precipitate are recovered as soluble chromium (VI) compounds by reacting with bleach or other oxidizing agents leav-ing an unreacted, disposable sol ? of nearly chrome-free hydrous ferric oxide. The recovered chromium compounds are available for recycling back to the electrolytic cell.

Description

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 ;

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 ~

., ~
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 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.

Claims

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

Claim 1 A process for making substantially chrome-free alkali metal halates which comprises:
(a) forming a slurry by reacting a chromium (VI) contaminated halate-containing cell liquor with a product made from the reaction of an iron (II) compound(s) and hydrazine or salts thereof, said slurry comprising solids of hydrous ferric-chromic oxide, and (b) recovering an alkali metal halate solution substantially devoid of chromium compounds by separating the solids from the slurry of step (a).

Claim 2 The process of claim 1 wherein the iron (II) com-pound is a salt selected from the group consisting of ferrous sulfate, ferrous sulfite, ferrous chloride, ferrous nitrate and mixtures thereof.

Claim 3 The process of claim 2 wherein the solids of step (b) are reacted further with an oxidizing agent to produce a chromium (VI)-containing slurry having solids of hydrous ferric oxide, said solids being separated from the slurry to provide a solution of chromium (VI) compound(s) suitable for further use in the electrolytic production of alkali metal halates.

Claim 4 The process of claim 2 wherein the iron (II) com-pound(s) and hydrazine are added independently to the cell liquor.

Claim 5 A process for making substantially chrome-free alkali metal halate which comprises:
(a) forming a slurry by reacting a chromium (VI) contaminated halate-containing cell liquor with about 2.5 to 3.5 moles of an iron (II) compound(s) per mole of chromium (VI) compound(s) present in said liquor while maintaining a pH in a range from 5.5 to 9.5, said cell liquor comprising at least 250 gpl alkali metal halate, at least 75 gpl alkali metal halide and at least 0.005 gpl of chromium (VI) compound(s), (b) recovering an alkali metal halate-containing solution substantially devoid of chromium compound(s) by separating the solids from the slurry of step (a), (c) recovering the chromium compound(s) from the solids of step (b) by reacting with an oxidizing agent whereby the chromium com-pound(s) is solubilized , and separating the residual iron-containing precipitate from the solubilized chromium compound(s), said solubilized chromium compound(s) being suit-able for recycling for further use in the electrolytic production of alkali metal halates.

Claim 6 The process of claim 5 wherein about a stoichio-metric amount of iron (II) compound(s) is added to the cell liquor in step (a).

Claim 7 The process of claim 5 wherein up to 85% of the iron (II) compound(s) are replaced with hydrazine or salts thereof.

Claim 8 The process of claim 7 wherein products formed from the reaction of iron (II) compound(s) and the hydrazine are preformed before reacting with the cell liquor.

Claim 9 The process of claim 5 wherein the pH of the cell liquor is adjusted to a range of about 6 to 9 during the addition of the iron (II) compound(s), said adjust-ment being made by the addition of an alkali metal hydroxide or chlor-alkali cell catholyte liquor.

Claim 10 The process of claim 9 wherein the iron (II) com-pound(s) of step (a) is an inorganic salt selected from the group consisting of ferrous chloride, ferrous sulfate, ferrous sulfite, ferrous nitrate, and mixtures thereof.

Claim 11 The process of claims 5 additional or 6 whereby additional iron (II) compound(s) is added in an amount sufficient to re-duce any alkali metal hypochlorite in the cell liquor.

Claim 12 The process of claim 10 wherein the pH of the cell liquor is about 6.5 to 8.5 and the iron (II) compound is ferrous sulfate.

13. The process of claim 5, wherein the cell liquor being treated comprises sodium chlorate, sodium chloride and the chromium (VI) compound(s) is selected from the group of salts consisting of alkali metal chromate, alkali metal dichromate and mixtures thereof.

14. The process of claim 13, wherein the oxidizing agent of step (c) is a member selected from the group consisting of alkali metal hypochlorite, chlorate cell liquor, peroxide, ozone, air and oxygen.

15. The process of claim 14, wherein the oxidizing agent is sodium hypochlorite.

16. The process of claim 14, wherein the sub-stantially chrome-free halate solution of step (b) comprises a mixture of sodium chlorate and chloride with a chloride to chlorate mole ratio of about 1.00 .
to about 1.09.

17. The process of claim 14, wherein the halate solution of step (b) is a sodium chlorate solution and includes a step of converting said sodium chlorate solution to substantially chrome-free sodium chlorate crystals.

18. The process of claim 5, wherein the solubilized chromium compound(s) have an oxidation state of +6 and are recycled back to a chlorate cell.

Claim -19-A process of making sodium chlorate with reduced hazard of environmental contamination from chrome which comprises:
(a) adjusting the pH of an alkali metal halate-containing electrolytic cell liquor to about 6.5 to 8.5 said liquor being contaminated with alkali metal chromate and/or dichromate, (b) forming a slurry by reacting the liquor with about 2.5 to 3.5 moles of an iron (II) reduc-ing agent per mole of chromium (VI) in the cell liquor, said slurry having solids com-prising hydrous ferric-chromic oxide, (c) recovering an alkali metal halate-containing solution substantially free of chrome com-pounds by separating the solids from the slurry, and (d) converting the hydrous chromic oxide to a soluble form which is suitable for reuse in a halate-type electrolytic cell by reacting the solids with an oxidizing agent.

Claim -20-The process of claim 19, including the step of recycling the oxidized chromium compound back to chlorate-type electrolytic cell.

Claim 21 The process of claim 19, including an additional amount of iron (II) reducing agent sufficient to eliminate any sodium hypochlorite in the cell liquor.

22. The process of claim 19, wherein from about 25 to about 75% of the iron (II) reducing agent is replaced with hydrazine or salts thereof.

23. The process of claim 21, wherein the additional amount of iron (II) reducing agent is from about 1.5 to 2.5 moles per mole of sodium hypo-chlorite present in the cell liquor.

24. The process of claim 1, wherein the iron (II) reducing agent is ferrous sulfate and the oxidiz-ing agent is bleach.

25. The process of claim 19, wherein the chrome-free halate-containing solution comprises both sodium chlorate and sodium chloride in a chloride to chlorate mole ratio of about 1.00 to about 1.09.

26. The process of claim 19, wherein the halate-containing solution of step (c) is made into sodium chlorate crystals.

27. A process for making substantially chrome-free alkali-metal halates which comprises:
a) forming a slurry by reacting a chromium (VI) contaminated halate-containing cell liquor with a reducing agent selected from the group consisting of:

i) iron (II) compounds, and ii) reaction products of iron (II) compounds and hydrazine or hydrazine salts, b) separating solids from the slurry of a) and recovering an alkali metal halate solution substantially devoid of chromium compounds.