US3843769A

US3843769A - Process for removing hexavalent chromium from chlorate containing liquors

Info

Publication number: US3843769A
Application number: US00405963A
Authority: US
Inventors: H Partridge; J Hildyard
Original assignee: Hooker Chemical Corp
Current assignee: Occidental Chemical Corp
Priority date: 1972-01-20
Filing date: 1973-10-12
Publication date: 1974-10-22
Anticipated expiration: 1991-10-22

Abstract

A method has been devised for removing hexavalent chromium values from concentrated aqueous alkali metal chlorate solutions by treating said solutions with a soluble sulfide, neutralizing the treated mixture and thereafter separating an insoluble trivalent chromium product from the solution. Hypochlorites, when present, may be simultaneously removed from the chlorate solution by reaction with excess sulfide reagent.

Description

United States Patent [191 Partridge et al.

[ PROCESS FOR REMOVING HEXAVALENT CHROMIUM FROM CHLORATE CONTAINING LIQUORS [75] lnventors: Harold deVere Partridge, Wilson;

Joseph M. I-Iildyard, Youngstown,

both of N.Y.

[73] Assignee: Hooker Chemical Corporation,

Niagra Falls, NY.

[22] Filed: Oct. 12, 1973 21 Appl. No.: 405,963

Related U.S. Application Data [63] Continuation-in-part of Ser. No. 219,375, Jan. 20,

l972, abandoned.

[52] U.S. Cl 423/475, 423/472, 423/607 [51] Int. Cl. ..C01b 11/00, C01b 11/14 [58] Field of Search 423/475, 472, 607, 184,

[ Oct. 22, 1974 [56] References Cited UNITED STATES PATENTS 2,784,057 3/l957 Chisnall 423/179 3,002,815 l0/l96l Heinze 423/607 3,6l6,344 l0/l97l Peterson et al. 423/607 X Primary Examiner-Edward Stern Attorney, Agent, or F irm-Peter F. Casella, Richard P. Mueller [57] ABSTRACT 11 Claims, No Drawings I PROCESS FOR REMOVING HEXAVALENT CHROMIUM FROM CHLORATE CONTAINING LIQUORS BACKGROUND OF INVENTION The present invention relates to a method for removing hexavalent chromium values from concentrated aqueous solutions of alkali metal chlorates. More particularly it relates to the simultaneous removal of alkali metal dichromates and alkali metal hypochlorites from concentrated aqueous solutions of alkali metal chlorates by treating said solutions with water soluble sulfides.

It is known to prepare alkali metal chlorates, specifically sodium chlorate, by the electrolysis of brine wherein the alkali metal hydroxide formed at the cathode reacts with the chlorine formed at the anode. The cathode which is generally fabricated from iron or similar corrodible metal may, it is known, be protected from oxidation by the inclusion in the electrolyte of substances, such as an alkali metal dichromate, specifically sodium dichromate. The dichromates being highly colored salts are objectionable impurities when permitted to remain in the chlorate product and it is generally the practice to treat the chlorate solution with a soluble metal salt, such as a soluble barium salt, to remove the dichromate impurity as an insoluble barium salt. Such prior art processes have serious objections. Soluble barium salts are not only relatively expensive but also combine with any sulfate salts present producing difficultly filterable precipitates. In addition, excess barium salts must be precipitated, e.g., with sodium carbonate, and removed by filtration before the cell liquor can be processed further. Moreover, barium salts present pollution problems in the disposal of wastes from processes utilizing alkali metal chlorates.

OBJECTS OF THE INVENTION Itis thus a principal object of this invention to devise an economical and efficient method for removing hexavalent chromium values from concentrated aqueous solutions of alkali metal chlorates.

Another object is to devise a process for the simultaneous removal of hexavalent chromium and alkali metal hypochlorite from alkali metal chlorate solutions.

Other objects will be obvious from the following description of the present invention.

SUMMARY OF THE INVENTION In accordance with the present invention, it has now been found that hexavalent chromium values can be substantially completely removed from concentrated aqueous solutions of alkali metal chlorates by treating said solutions contaminated with a hexavalent chromium compound with an amount of a soluble sulfide sufficient to reduce the hexavalent chromium to the trivalent stage, neutralizing the alkaline solution to a pH of below about 5, and thereafter separating the resultant insoluble trivalent chromium product from the chlorate solution. It has been found also that any alkali metal hypochlorite present in the aqueous alkali metal chlorate solution may be removed simultaneously by the addition of a sufficient excess of the soluble sulfide.

Although the exact course of the reaction is not known, it is probable that the reaction proceeds according to the following equations, using sodium dichromate and sodium sulfide as the reactants in acid solution:

NQ-gCTzO- 3 3 H2504 H2O 2 Cr(OH) 38 3 Na SO 2 NaOH Similarly it may be postulated that the reaction between sodium hypochlorite and sodium sulfide proceeds as follows:

4 NaClO Na S Na S0, 4 NaCl NaClO Na S H O S NaCl 2 NaOH or combining equations (2) and (3) 5 NaClO 2 Na S H O Na SO 5 NaCl 2 NaOH S (4) Accordingly, although the precise stoichiometry of the reactions occurring in the pr oces s of the present invention is not known, it is believed that at least three mol proportions of the soluble sulfide, e.g., sodium sulfide, are necessary to convert one mol proportion of the hexavalent chromium compound, e.g., sodium dichromate, to the trivalent stage and further at least about 0.4 molar proportion of the soluble sulfide is required to kill each mol proportion of alkali metal hypochlorite, e.g., sodium hypochlorite, when present in the aqueous alkali metal chlorate composition. Preferably, in carrying outthe process of this invention an ex cess, and especially at least a 100 percent excess, of the amount of soluble sulfide stoichiometrically required to react with the hexavalent chromium values present in the chlorate liquor is used. Thereby, on acidification all of the hexavalent chromium is reduced to an insoluble trivalent chromium product and any hypochlorite present is removed or killed.

DETAILED DESCRIPTION OF THE INVENTION In accordance with a preferred mode of carrying out this process of this invention, a concentrated aqueous solution of sodium chlorate, such as is obtained by the electrolysis of brine and which contains about 200 to 400 grams per liter (gpl) of sodium chlorate about 100 to 200 gpl of sodium chloride and about 0.2 to about l.5 gpl of sodium dichromate (Na Cr O '2I-I O) is heated to and maintained at from about 40 to about C. To this alkaline solution about 0.5 to about 0.7 gpl of sodium sulfide, equivalent to from about 10 to about 20 mols of sodium sulfide per mol of sodium dichromate Na Cr O -2H O), is added. The resultant alkaline yellow solution, the pH of which is above about 8 and may be as high as 11 or more, is neutralized to a pH within the range of about 4 to about 5 by the addition of sulfuric acid. The solution becomes colorless and a precipitate, which is probably a hydrated chromic oxide, appears and is removed by filtration, preferably after adjusting the pH of the slurry to above and rate solution. Some or all of the excess sulfide, over that required to react with the hexavalent chromium and hypochlorite is removed during the process when the mass is rendered acid.

present in the solution can be used. for example. Such large apparent excesses of sulfide are preferred in order to not only reduce the chromium values but also to simultaneously remove any hypochlorite from the chloespecially to about pH of 7, with an alkaline agent such 5 The temperature of the reaction is not critical. Conas caustic soda. The clarified solution contains less than Vehiehtty, the Process is carried out at about 400 to about 2 ppm f chromium and is f f sodium about 80 C, and preferably at about 65 C. The latter pochgorim temperature is that at which the concentrated alkaline The reaction between alkali metal chlorates and al- Chlorate o u is discharged from y electrolysis kali metal sulfides proceeds very slowly if at all in alka- Cells and 1t ls m eohyehteht o treat the sotuttoh at line media. Since concentrated aqueous solutions ofsothis potht to remove the ehtomtum Vatuesp di hl are lk li L h a H f at least tures above and below the preferred range can be used about 8, and especially in the presence of strongly alkaalthough the reactions proceed at a slower rate below line alkali metal sulfides it is obvious that the reduction about C, and above about 800 the energy of the hexavalent chromium values proceeds the more pended in heating the solution is wasted. slowly, if at all, in the strongly alkaline reaction media Following formation of the chromic hydroxide preof this process. It is therefore, highly desirable to renoipitate When the solution is acidified to 3 P below der the reaction mixture acid to expedite the reaction. about e slurry may be filtered. It has been found This can be observed by the decolorization of the mixthat at this point, the sludge iS not readily filterable. ture on addition of acid, However, lk li t l h1 Addition of filter aids improves the filtration at this rates decompose with the formation of chlorine dioxide g but it is preferred to neutralize the acid ur y to in the presence of acids and sodium chloride, especially 3 P of about 5 or higher, with an alkaline agent such at pHs below 4, and at temperatures above ambient. as caustic soda or the like y this Procedure not y Accordingly it is preferred to adjust the of the reaciS the decomposition Of sodium chlorate minimized but i mass to a H f' b 5 Once h color f h l also the separation of the insoluble chromic hydroxide i h b di h d L aft completion f h sludge is facilitated. Separation of the insoluble matereduction of the hexavalent chromium values and aprial can be accomplished a known muhhehes, for I pearance of the hydrated chromic oxide precipitate. ample, y filtrattoh, eehtrlfugatloh and the This final adjustment is preferably carried out to a pH The fottowithg examples will Illustrate the Process of of above 5 and especially to a pH of about 7 prior to the Present thyehtioh- Parts h Percentages are y filtration of the insoluble chromic hydroxide. weight and temperatures are glyeh in degrees eehti' Any water soluble sulfide can be used in the process grade, unless otherwise pe of the resent invention. Thus for exam le, sodium sulfide, p tassium sulfide, sodium bisulfide potassium bi- EXAMPLE I sulfide, or the free acid, hydrogen sulfide can be used. A concentrated aqueous solution of sodium chlorate Hydrogen sulfide when added to the alkaline chlorate containing 350 grams per liter (gpl") of sodium chloliquor would be converted to sodium sulfide or bisulrate, 140 gpl sodium chloride and 0.l95 gpl sodium difide. "White Liquor," as used in Kraft pulp manufacchromate (Na Cr- O '2H O) was heated to and mainturing plants, and which contains approximately 70 gpl tained at 65. Sufficient of an alkaline aqueous solution NaOH and 30 gpl Na S can be used as the source of the 40 of sodium sulfide (White Liquor containing about 70 soluble sulfide also. Alkali metal polysulfides, e.g. percent NaOH and 30 percent Na S) was added to in- Na S can also be used. troduce 0.54 gpl sodium sulfide into the solution. The Sodium sulfide and sodium bisulfide because of their resulting yellow colored solution had a pH of l 1. This general availability and low cost are generally presolution was rendered acid, by the addition of sulfuric ferred. acid to a pH between 4.0 and 4.5. The solution was agi- The amount of soluble sulfide used should be at least tated as it became colorless and a precipitate appeared. that amount which will reduce the hexavalent chro- Thereafterthe acidic mixture was neutralized to a pH mium values to the trivalent state. Based on the stoichiof about 7 and filtered. The trivalent chromium ion ometry postulated above, it is believed that at least 3 content of the clarified solution was determined by mol proportions of sulfide are required to reduce one atomic absorption. mol proportion of hexavalent chromium to the trivalent This experiment was repeated using sufficient stage. Preferably an excess is used, and accordingly White Liquor to introduce 0.63 gpl sodium sulfide about 6 mols of sulfide and especially from about 10 to into the mass. 20 mol proportions of sulfide per mol proportion of The data obtained in these experiments are set out in hexavalent chromium compound present is used. the following Table l.

TABLET Initial Conc. Na,S Mol Ratio Cr Na,Cr,O,.2H,O Added Na,S/Na,Cr,O .2H O in solution 0.l95 gpl 0.54 gpl 10.56 l.9 ppm 0.i95 0.63 gpl 12.32 0.3 ppm About 10 or l2 to about 13 mol proportions of sulfide EXAMPLE ll per mol proportion of hexavalent chromium compound The procedure of Example I was repeated using a concentrated sodium chlorate solution containing 360 gpl NaClO 2l6 gpl NaCl and 1.4 gpl Na Cr O '2H O. The results obtained are set out in the following Table II.

To a synthetic chlorate cell liquor containing about 350 gpl NaClO to about 150 gpl NaCl and 0.15 gpl ble sulfide per molar proportion of hexavalent chromium compound present,

2. neutralizing the resultant solution to a pH of about Na Cr O '2H O, at about 65 degrees sufficient aqueous 4 F about thereby Preclpltatmg "Willem sodium bisulfide containing 0.5 gpl sodium bisulfide ase all lhsehlble product, and saying 71.5 percent in solid state was added to intro- Separatmg F lhsbhlble mvaieht Chromium P duce 0.357 gpl NaHS into the solution. The resulting from Sald Solutionyeuow solution having a of about 9 was rendg ed 2- The process Of Claim 1 wherein Said metal acid by the addition of sulfuric acid to a pH of 4.2. The Chlorate is sodium Chlorate. mixture was then divided into three equal portions and 1 The Process of Claim 2 wherein the Said Solution th portions w neutralized 1 H 5, or 6, or 7 with is treated with an amount of soluble sulfide which is at sodium hydroxide and then filtered. The clarified soluleast sufficient to reduce the hexavalent chromium valtions were assayed by atomic absorption for trivalent ues to the trivalent stage and also to react with any alchromium. Table III is a record of the results obtained. kali metal hypochlorite present.

TABLE Ill Initial Conc. NaHS Mol Ratio Cr* *in Na,Cr,0,.2H,O added NaHS/Na,Cr,O,.2H,O Filtered Solution 0.15 gpl 0.357 gpl 12.67 0.1 at pH 5 0.15 gpl 0.357 gpl l2.67 0.1 at pH 6 0.15 gpl 0.357 gpl [2.67 0.1 at pH 7 EXAMPLE IV 4. The process of claim 2 wherein the said solution A concentrated sodium chlorate solution containing treated at least i 6 m0] proportions of about 350 gpl Sodium Chlorate, about 140 gpl Sodium ble sulfide per mol proportlon of hexavalent chromium chloride, 3.5 x 10' moles (0.26075 gpl) sodium hypo- Impound W 2 501mm; Chlorhe and 2 X 0- mole 0 2 gpl) sodium 5. The process of claim 4 wherein the said solution dichromate (Na Cr 0;'2H O) was treated with 7.0 X treated Wlth from about 10 to about P 10- moles 0392 gpl) Sodium bi lfid at about 5. t1ons of soluble sulfide per mol proportlon ot hexava- The pH of the solution was adjusted to 4.7 by the addilent Chromium compound Present in said solutiontion of aqueous sulfuric acid. This amount of sodium The PmeeSS of Claim 5 wherein the Said Solution bisulfide represents about 100 percent excess over the is treated with from about 10 to about 13 P p stoichiometric equivalent amount of NaHS required to tiOhS of Sodium Sulfide P m0] Proportion of hexava' reduce the hexavalent chromium to trivalent chrolent chromium compound present in said solution. mium. The solution was agitated at about 65 for about 7. The process of claim 2 in which the said solution 30 minutes and then filtered. The filtrate contained less is treated with sodium bisulfide. than 0.1 ppm of hexavalent chromium. 8. The process of claim 5 in which the said solution This invention has been described in the above speciis treated with about 12 to about 13 mol proportions of fication and the best modes for carrying it out illussodium bisulfide per mol proportion of hexavalent trated in the above examples. As will be obvious to chromium compound present in said solution. those skilled in this art many variations in the details set The P s Of Claim 5 wherein the Said SOIUUOH, t h i can b d i h departing f h after treatment with the soluble sulfide, and prior to cope or i i f h invention separating the insoluble trivalent chromium product is wh t i l i d i adjusted to a pH between above about 5 and about 7. l. A process for removing hexavalent chromium values from alkaline aqueous solutions of alkali metal The Process of Claim 1 wherein the Process is chlorates which Comprises the Steps f r1ed out at a temperature of from about 40 to about I. treating a concentrated aqueous solution of an 'alkali meml Chlorate having a pH above about 8 with ll The process of claim 10 wherem the process 1s at least about 3 molar proportions of a water soluearned out at i i

Claims

1. A PROCESS FOR REMOVING HEXAVALENT CHROMIUM VALUES FROM ALKALINE AQUEOUS SOLUTIONS OF ALKALI METAL CHLORATES WHICH COMPRISES THE STEPS OF

1. TREATING A CONCENTRATED AQUEOUS SOLUTION OF AN ALKALI METAL CHLORATE HAVING A PH ABOVE 8 WITH AT LEAST ABOUT 3 MOLAR PROPORTIONS OF A WATER SOLUBLE SULFIDE PER MOLAR PROPORTION OF HEXAVALENT CHROMIUM COMPOUND PRESENT,

2. NEUTRALIZING THE RESULTANT SOLUTIOON TO A PH OF ABOUT 4 TO ABOUT 5 THEREBY PRECIPITATING TRIVALENT CHROMIUM AS AN INSOLUBLE PRODUCT, AND

2. neutralizing the resultant solution to a pH of about 4 to about 5 thereby precipitating trivalent chromium as an insoluble product, and

2. The process of claim 1 wherein said alkali metal chlorate is sodium chlorate.

3. The process of claim 2 wherein the said solution is treated with an amount of soluble sulfide which is at least sufficient to reduce the hexavalent chromium values to the trivalent stage and also to react with any alkali metal hypochlorite present.

3. separating the insoluble trivalent chromium product from said solution.

3. SEPARATING THE INSOLUBLE TRIVALENT CHROMIUM PRODUCT FROM SAID SOLUTION.

4. The process of claim 2 wherein the said solution is treated with at least about 6 mol proportions of soluble sulfide per mol proportion of hexavalent chromium compound present in said solution.

5. The process of claim 4 wherein the said solution is treated with from about 10 to about 20 mol proportions of soluble sulfide per mol proportion of hexavalent chromium compound present in said solution.

6. The process of claim 5 wherein the said solution is treated with from about 10 to about 13 mol proportions of sodium sulfide per mol proportion of hexavalent chromium compound present in said solution.

7. The process of claim 2 in which the said solution is treated with sodium bisulfide.

8. The process of claim 5 in which the said solution is treated with about 12 to about 13 mol proportions of sodium bisulfide per mol proportion of hexavalent chromium compound present in said solution.

9. The process of claim 5 wherein the said solution, after treatment with the soluble sulfide, and prior to separating the insoluble trivalent chromium product is adjusted to a pH between above about 5 and about 7.

10. The process of claim 1 wherein the process is carried out at a temperature of from about 40* to about 80* C.

11. The process of claim 10 wherein the process is carried out at about 65* C.