CA2041005A1 - Process for the production of sodium dichromate - Google Patents
Process for the production of sodium dichromateInfo
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- CA2041005A1 CA2041005A1 CA002041005A CA2041005A CA2041005A1 CA 2041005 A1 CA2041005 A1 CA 2041005A1 CA 002041005 A CA002041005 A CA 002041005A CA 2041005 A CA2041005 A CA 2041005A CA 2041005 A1 CA2041005 A1 CA 2041005A1
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- sodium
- solution
- dichromate
- monochromate
- ions
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- C—CHEMISTRY; METALLURGY
- C01—INORGANIC CHEMISTRY
- C01G—COMPOUNDS CONTAINING METALS NOT COVERED BY SUBCLASSES C01D OR C01F
- C01G37/00—Compounds of chromium
- C01G37/14—Chromates; Bichromates
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Abstract
ABSTRACT OF THE DISCLOSURE
The invention relates to a process for the production of sodium dichromate and sodium dichromate solu-tions by oxidative roasting of chrome ores under alkaline conditions, leaching of the furnace clinker obtained with water or an aqueous chromate-containing solution, adjust-ment of the pH to from 7 to 9.5, removal of the insoluble constituents by filtration, a sodium monochromate solution being obtained, conversion of the monochromate ions of this solution into dichromate ions by acidification and crystal-lization of sodium dichromate by concentration of this solution, characterized in that the acidification is carried out with carbon dioxide under pressure with removal of sodium hydrogen carbonate, the remaining solution is then very largely freed from sodium monochromate by cooling to a temperature below 10°C and filtration, any monochro-mate ions still present in the remaining solution are con-verted into dichromate ions by addition of an acid and the sodium monochromate filtered off is added to the sodium monochromate solution before conversion with carbon dioxide into a sodium dichromate solution.
Le A 27 685
The invention relates to a process for the production of sodium dichromate and sodium dichromate solu-tions by oxidative roasting of chrome ores under alkaline conditions, leaching of the furnace clinker obtained with water or an aqueous chromate-containing solution, adjust-ment of the pH to from 7 to 9.5, removal of the insoluble constituents by filtration, a sodium monochromate solution being obtained, conversion of the monochromate ions of this solution into dichromate ions by acidification and crystal-lization of sodium dichromate by concentration of this solution, characterized in that the acidification is carried out with carbon dioxide under pressure with removal of sodium hydrogen carbonate, the remaining solution is then very largely freed from sodium monochromate by cooling to a temperature below 10°C and filtration, any monochro-mate ions still present in the remaining solution are con-verted into dichromate ions by addition of an acid and the sodium monochromate filtered off is added to the sodium monochromate solution before conversion with carbon dioxide into a sodium dichromate solution.
Le A 27 685
Description
2 0 ~
A PROCESS FOR THE PRODUCTION OF SODIUM DICHROMATE
_ _ BACKGROVND OF THE INVENTION
~his invention relates to a process for the production of sodium dichromate and sodium dichromate solutions.
According to Winnacker-Kuchler, Chemische Technologie, Vol. 2, 4th Edition, 1982, pp. 657 - 664. sodium dichromate is produced by roasting of-chrome ore with soda ash and/or sodium hydroxide and atmospheric oxygen under alkaline conditions.
A sodium monochromate solution containing from 300 to SOO g/l Na2CO4 is obtained by leaching the furnace clinker leaving the -roasting furnace with water or-a chromate containing aqueous solution, adjusting the pH value of the mash from 7 to 9.5 and removing the insoluble constituents by filtration. The pH value is generally adjusted with sulfuric acid and/or with sodium dichromate solution. To produce sodium dichromate, the monochromate ions of the solution are converted into dichromate ions either by acidification with sulfuric acid, by acidification with carbon dioxide under pressure or by electrolytic acidifi-cation. Where sulfuric acid is used for acidification, sodium sulfate is formed and precipitates when the solution is concentrated to approximately 70% by weight Na2Cr207 2H20.
The chromate-containing sodium sulfate obtained has to be worked up.
For acidification with carbon dioxide, the sodium monochromate solution is adjusted to a concentration of from 750 to 1,000 g/l Na2CrO~ snd is saturated with carbonic acid in autoclaves by introduction of carbon dioxide under a pressure of from 0.5 to 1.5 MPa (5 to 15 bar). 75 to 9S~ conversion of monochro-mate ions into dichromate ions is obtained with precipita-tion of sodium hydrogen carbonate. The remaining conver-sion to 100% can be obtained by introduction of carbon dioxide in another stage after concentration to 1,300 g/l Le A 27 685 Na2CrO~ (already for~ed Na2CrOr-2H20 expressed in Na2CrO~
equivalents). On an industrial scale, however, this final conversion step is very complicated. The final conversion step may also be carried out with sulfuric acid or by electrolysis.
As mentioned above, chromate-containing sodium sulfate is formed during the reaction with sulfuric acid a~ has to be worked up.
Electrolytic acidification is carried out by introduc-tion of the sodium monochromate solution or rather thesodium monochromate soluti~ converted to a level of from 75 to 95% into sodium dicbromate into the anode compartments of electrolysis oells equipped with cation excbanger membranes.- During the electrolysis process, sodium ions migrate through the membrane into the cathode compartments of the cells filled with water or witb an aqueous solution.
The sodium ions form an aqueous solution containing sodium hydroxide witb the hydroxide ions formed at the cathode w~th s~ta~x~s evDlution of h~x~n. In tbe ~x~e ooqp~ireot,~he chromate ions are converted into dichromate ions by the hydrogen ions formed at the anode with simultaneous evolu-tion of oxygen. This process is also difficult to carry out on an industrial scale.
To produce sodium dichromate crystals, the sodium di-chromate solutions obtainable in the processes mentionedabove are concentrated by evaporation of water. The sodium dichromate which crystallizes is separated off and dried.
Now, the present invention adopts a different, ~imple approach to the production of sodium dichromate solutions ~nd sodium dichro at- crystal~ which does not have any of the disadvantages of the proc-~es described above.
SUMMARY OF ~HE INVENTION
The present invention relates to a process for the production of sodium dichromate and sodium dichromate ~olu-tions by ro~sting o~ chro~e ores under alkalineconditions, leaching of the furnace clinker obtained with Le A 27 685 2 ~ o ~
water or an ~queous chromate-containing solution, adjust-aent of the pH to from 7 to 9.5, removal of the insoluble constituents by filtration, a sodium monochromate solution being obtained, conversion of the monochromate ions of this solution into dichromate ions by acidification and crystal-lization of sodium dichromate by concentration of this solution, characterized in that the acidification is carried out with carbon dioxide under pressure with removal of sodium hydrogen carbonate, the remaining solution is then very largely freed from sodium monochromate by cooling to a temperature below lO C and filtration, any monochro-mate ions still present in the remaining solution are con-verted into dichromate ions by addition of an acid and the sodium monochromate filtered off is added to the sodium monochromate solution before conversion with carbon dioxide into a sodium dichromate solution.
The solution remaining after acidification with carbon dioxide under pressure and removal of sodium hydrogen car-bonate is preferably cooled to temperatures of from O to -25-C and, more preferably, to temperatures of from -10 to -20-C. It is of advantage to concentrate the sodium monoch-romate solution to contents of from 500 to 1,000 g/l Na2CrO~
: before acidification and to carry out the acidification with carbon dioxide under pressure to a degree of acidific-ation of the monochromate ions into dichromate ions of from 40 to 95t. Although degrees of acidification of less than 40% are possible, they do lead to very large recirculation volumes which ~a~e the process difficult to carry out on an industrial ~cAle.
. DESCRIPTION OF THE PREFERRED EMBODIMENTS
In one particularly preferred embodiment of the pro-ce~s according to the invention, the sodium monochromate solution is concentrated before acidification to contents of from 750 to 1,000 g/l Na2CrO~ and the acidification with carbon dioxide under pressure is carried out to a degree of acidification of the monochromate ions into dichromate ions Le A 27 685 3 2 ~
of from 80 to 95%.
Solutions in which up to 99% of the chromate ions of the solution are present as dichromate ions are obtained by acidification of sodium monochromate solutions with carbon dioxide under pressure and cooling of the solution af~er removal of sodium hydrogen carbonate to temperatures below lO-C and filtration of precipitated sodium monochromate.
Solutions such as these, which only contain minimal resi- -dues of sodium monochromate,-can be used in numerous pro-duction processes, such as the electrolytic production of chromic acid, the production of chrome tanning materials and in the oxidation of organic compounds.
For the production of sodium dichromate solutions, in which all the chromate ions are present as dichromate ions, and for the production of sodium dichromate crystals, the remaining monochromate ions of the solution may be con-verted into dichromate ions either with an acid or by elec-trolysis. The monochromate ions still present after cool-ing and filtration are preferably converted by addition of chromic acid and/or a solution containing chromic acid.
Suitable chromic acid solutions are obtained, for example, in the electrolytic production of chromic acid.
The process according to the invention is described in the following with reference to Fig. l. The variant of the process according to the invention illustrated in Fig. l represents a particularly advantageous embodiment.
Chrome ore is digested by oxidative roasting with ~X~ ash and atm~ rlc o~n at frcm 1,000 to 1,100C under alkaline conditions in the presence of a ~Tier mater~ in a rotary klln (1). The furnace cllnker formed is then leached with water or dilute chromate solution and adjusted with sodium dichromate solution to a pH value of from 7 to 9.5 (2). Soluble alkali compounds of iron, aluminium and silicon are thus converted into insoluble and readily filterable hydroxides or oxide hydrates which are Le A 27 685 4 ~q~
removed together with the insoluble constituents of the furnace clinker (3). The sodium monochromate solution formed, which contains from 300 to 500 g/l Na2CrO4, is then freed from dissolved vanadate by addition of calcium oxide at pH values of from lO to 13, as described in EP-A-47 799.
The solution, which now contains calcium, is freed from most of the alkaline earth metal ions and other polyvalent cations by precipitation as carbonates through the addition of, or n situ formation of, sodium carbonate (4). Preci-pitation is preferably carried out at temperatures of from 50 to lOO-C, at pH values of from 8 to 12 and with an approximately 2 to lO-fold molar carbonate excess, based on the quantity of alkaline earth metal ions. A sodium monochromate solution freed from polyvalent cations to a total content of less than 5 mg/l is obtained after filtra-tion. The content of polyvalent cations in this solution can be even further reduced by means of suitable selective cation exchangers. The sodium monochromate solution is then concentrated to contents of from 750 to l,000 g/l of Na2CrO4 (5) by evaporation in one or more stages.
In the now concentrated solution, a pH value below 6.5 is established by introduction of carbon dioxide in one or more stages to a final pressure of from 0.5 to i.s MPa (5 -15 bar) at a final temperature not exceeding 50-C, an 80 to 95% conversion of the sodium monochromate into sodium dichromate being obtained in this way with precipitation of . sodium hydrogen carbonate (6). Carbon dioxide may be in-troduced in gaseous or liquid form.
The sodium hydrogen carbonate is removed from the suspension formed under continuing carbon dioxide pressure or, after expansion, the sodium hydrogen carbonate is quickly removed before its back-reaction with the sodium dichromate.
The sodium h~drogen carbonate removed is calcined at from 200 to 300-C, optionally with addition of sodium .
Le A 27 685 5 i hydroxide, to form soda ~sh which is ~ed to the roasting of the chrome ore (1).
After removal of a sidestream for pH adjustment of the leached furnace clinker, the resulting sodium monochromate/
sodium dichro~ate solution separated from the sodium hydro-gen carbonate is then cooled to -10 to -20-C in one or more stirrer-equipped vessels and, after a residence time of 5 to 60 minutes, is freed from the sodium monochromate accu-mulating by filtration (7). Sodium dichromate solutions in which 97 to 99% of the chromate is present as dichromate are obtained. The sodium monochromate filtered off is then concentrated by evaporation (5), optionally after addition of water, before the pH adjustment (2).
~he sodium monochromate filtered off, optionally after washing and drying, may also be used as such, for example for the production of corrosion inhibitors.
To avoid the accumulation of impurities, particularly sulfate ions, it may be advisable to deliver part of the sodium monochromate filtered off to the final acidification stage (8). It has proved to be of particular advantage in this connection to use low-sulfur fuels, preferably natural gas, in the roastin~ of the chrome ore (1).
The sodium monochromate/sodium dichromate solution may be coo}ed by standard methods of the type described, for example, in "UlImanns Encyklopadie der Technischen Chemie", 4th Edition, Vol. 3, pages 186 to 218, Verlag Chemie, Wein-. heim, 1973. If liquid carbon dioxide is used for acidifi-cation, it is advisable to cool the solution either direct-ly or indirectly through a heat exchanger using a suitable heat exchange medium by evaporation of carbon dioxide.
Part of the solution obtained during cooling (7) is used for the electrolytic production of chromic acid (11) or another application. The other part is converted with mother liguor containing chromic acid from the chromic acid filtration stage (13), with solid chromic acid or with .
Le A 27 685 6 . --,~
Q ~ ~
chromic acid solution into a sodium dichromate solution in which all the chromate ions are present as dichromate ions (8). This solution is then used either as such or after - concentration by evaporation to from 60 to 70% by weight Na2Cr20~ H20 t9) For the production of sodium dichromate crystals, the solution is concentrated to approximately 1,650 g/l Na2Cr207 2H20 with precipitation of sodium dichromate which is removed from the solution by filtration (10).
In the electrolytic production of chromic acid, the sodium dichromate/sodium monochromate solution is converted into a solution containing chromic acid by multistage electrolysis at from 50 to 90-C in two-compartment electro-lysis cells with cation-selective membranes are partitions (11~. The electrolysis is carried out by introduction of the solution mentioned into the anode compartment of the first stage. After partial conversion of the dichromate into chromic acid, the solution then flows into the second stage, in which it undergoes further partial conversion into chromic acid, and so on to the last stage in which a degree of conversion of the dichromate into chromic acid of from 55 to 70% is achieved, corresponding to a molar ratio of sodium ions to chromic acid of from 0.45 : 0.55 to 0.30 : 0.70. Although there may be any number of stages, 6- to 15-stage electrolysis is preferred.
The solution containing chromic acid formed during - electrolysis and residual sodium dichromate is concentrated by evaporation to a water content of approximately 12 to 22% by weight water at temperatures of from 55 to llO-C, most of the chromic acid crystallizing out (12). The suspension formed is then separated by centrifugation at from 50 to llO-C into a solid consisting essentially of crystalline chromic acid and a liquid phase hereinafter referred to as the mother liguor (13).
The mother liquor obtained, optionally after dilution Le A 27 685 7 2 ~
with water, is returned to the electrolysis process at a suitable point, i.e. at a stage in which the dichromate conversion is similar. To avoid a heavy accumulation of impurities in the system, part of the mother liquor is removed and used in the final acidification (8). The crystalline chromic acid is freed from adhering mother liquor by single or repeated washing with from 10 to 50% by weight, based on the weight of the solid, of saturated or substantially saturated chromic acid solution and by centrifugation after each wash. The washed pure chromic acid crystals may then be used for the intended purpose either as such or after drying.
T~e process according to the invention is illustrated by the following Examples.
EXAMPLES
GENERAL COMMENTS
Sodium dichromate/sodium monochromate solutions prepared by dissolution of sodium dichromate and sodium monochromate were used in Examples 1 to 5. The composi-tions of the solutions corresponded to those in which theycan be present in the industrial production of sodium di-chromate after acidification with carbon dioxide under pressure. A partly acidified sodium dichromate solution obtained in an industrial process after acidification with carbon dioxide was used in Example 6. The soiutions were cooled in an electrically operated laboratory cryostat.
In Examples 1 to 5, q~antities of ~0 ml were cooled to the temperatures shown, kept at those temperatures for 60 . minutes and then filtered through a cooled glass frit.
,. .
Exam~lel A solution having the following composition was cooled to temperatures down to -25-C and worked up as described.
ComDosition of the solution:
Total chromate content of the solution (in Na2CrO~ eguivalents): 60.1% by weight - 900 g/l Le A 27 685 8 Na2CrO7 2H2o 46.7 % by weight Na2Cro4 9.3 % by weight Degree of acidification~: 84.5 %
) The so-called degree of acidification indicates the percentage of the total chromate content, expressed as Na2CrO~ equivalents, which is present as dichromate.
The results of the test are shown in Table 1.
Table 1 Temper- Concentration Concentration Degree of ature Na2Cr207 2~20 Na2CrO4 acidifi-cation 15 ll-C Beginning of Na2CrO4 crystallization -5-C 53.6% by weight 3.3% by weight 94.7%
-lO-C 55.2% by weight 2.0% by weight 96.8%
-l5-C 56.2% by weight 1.1% by weight 98.3%
-20-C 58.6% by weight 1.0% by weight 98.3%
-25-C 59.0% by weight 1.1% by weight 98.3%
Exam~le 2 A solution having the following composition was cooled to temperatures down to -25-C and worked up as described.
Com~osition of the solution:
Total chromate content of the solution (in Na2CrO4 equivalents): 59.3% by weight ~ 874 g/l Na2CrO?-2H20: 48.9 % by weight Na2CrO4: 6.1 % by weight Degree of acidification: 89.7 %
The results of the test are shown in Table 2.
Le A 27 685 9 , Table 2 Temper- Concentration Concentration Degree of ature Na2Cr207-2H2o Na2CrO4 acidifi-cation 7-C Beginning of Na2CrO4 crystallization -5-C 53~5% by weight 2.5% by weight 95.9%
-lO-C 53.9% by weight 1.7% by weight 97.3%
-l5-C 55.2% by weight 1.1% by weight 98.2%
10-20 C 54.4% by weight 1.1% by weight 98.2%
-25-C 57.1% by weight 0.7% by weight 98.9%
~xam~le 3 A solution having the following composition was cooled to -20 and -2S-C and worked up as described.
Com~osition of the solution:
Total chromate content of the solution (in Na2CrO4 equivalents): 62.7% by weight - 892 g/l Na2CrO7 2H20: 48.7 % by weight Na2Cro4 9.8 % by weight Degree of acidification: 84.4 %
The results are shown in Table 3.
Table 3 Temper- Concentration Concentration Degree of ature Na2Cr2O7 2H2o Na2CrO~ acidifi-cation !
lO-C Beginning of Na2CrO~ crystallization -20-C 54.0S by weight 1.2~ by weight 98.0%
-25-C 57.1% by weight 0.6~ by weight 99.1%
~xam~le 4 A solution having the following composition was cooled to -15, -20 and -25-C and worked ~p as described.
.
Le A 27 685 10 2~ J I
Com~osition of the solution:
Total chromate content of the solutisn (in Na2CrO4 equivalents): 54.0% by weight - 765 g/l Na2CrO7-2H2O 44.4 % by weight Na2Cro4 5.7 S by weight Degree of acidification: 89.4 %
The results of the test are shown in Table 4.
Table 4 Temper- Concentration Concentration Degree of ature Na2Cr20~ 2H20 Na2CrO4 acidifi-cation O-C~ Beginning of Na2CrO4 crystallization -15-C 48.5% by weight 1.9% by weight 96.6%
-20-C 47.5% by weight 1.2% by weight 97.7~
-25-C 55.2% by weight 1.1% by weight 98.2%
ExamDle 5 A solution having a total chromate content of 42.7% by weight - 563 g/l (expressed in Na2CrO4 equivalents) and a degree of acidification of 49.1% was cooled to -12-C and worked up as described. The results are shown in Table 5.
Table 5 Concentration Concentration Degree of Na2cr2or 2H20 Na2CrO4 acidifi-cation Solution before cooling 19.3% by weight 21.7% by weight 49.1%
. 35 Solution after cool-ing to -12-C
and filtra-tion 33.2% by weight 4.7% by weight 88.5%
,;
The crystallization of Na2CrO4 began at lO C. The solution solidified on cooling to below 13-C.
Example 6 Quantities of 200 ml of a 93% acidified technical sodium dichromate solution having the following composition were cooled to 0, -10, -15 and -20-C, stirred for 1 hour and filtered through a cooled glass frit.
Composition of the solution:
Total chromate content of the solution (in Na2CrO4 equivalents): 63.9% by weight - 990 g/l Na2CrO7 2H2 54.7 % by weight Na2CrO4: 4.5 % by weight The results of the test are shown in Table 6.
Table 6 Temper- Concentration Concentration Degree of ature Na2Cr207-2H20 Na2CrO4 acidifi-cation O C 57.0% by weight 2.6% by weight 96.0%
-5 C 57.7% by weight 2.0% by weight 96.9%
-lO-C 59.4% by weight 1.3% by weight 98.0%
30-15-C 59.8% by weight 1.0% by weight 98.5%
-20-C 60.2% by weight 0.7% by weight 98.9%
It is understood that the specification and examples are illustrative but not limitative of the present invention and that other embodiments within the spirit and scope of the invention will suggest themselves to those skilled in the art.
Le A 27 685 12
A PROCESS FOR THE PRODUCTION OF SODIUM DICHROMATE
_ _ BACKGROVND OF THE INVENTION
~his invention relates to a process for the production of sodium dichromate and sodium dichromate solutions.
According to Winnacker-Kuchler, Chemische Technologie, Vol. 2, 4th Edition, 1982, pp. 657 - 664. sodium dichromate is produced by roasting of-chrome ore with soda ash and/or sodium hydroxide and atmospheric oxygen under alkaline conditions.
A sodium monochromate solution containing from 300 to SOO g/l Na2CO4 is obtained by leaching the furnace clinker leaving the -roasting furnace with water or-a chromate containing aqueous solution, adjusting the pH value of the mash from 7 to 9.5 and removing the insoluble constituents by filtration. The pH value is generally adjusted with sulfuric acid and/or with sodium dichromate solution. To produce sodium dichromate, the monochromate ions of the solution are converted into dichromate ions either by acidification with sulfuric acid, by acidification with carbon dioxide under pressure or by electrolytic acidifi-cation. Where sulfuric acid is used for acidification, sodium sulfate is formed and precipitates when the solution is concentrated to approximately 70% by weight Na2Cr207 2H20.
The chromate-containing sodium sulfate obtained has to be worked up.
For acidification with carbon dioxide, the sodium monochromate solution is adjusted to a concentration of from 750 to 1,000 g/l Na2CrO~ snd is saturated with carbonic acid in autoclaves by introduction of carbon dioxide under a pressure of from 0.5 to 1.5 MPa (5 to 15 bar). 75 to 9S~ conversion of monochro-mate ions into dichromate ions is obtained with precipita-tion of sodium hydrogen carbonate. The remaining conver-sion to 100% can be obtained by introduction of carbon dioxide in another stage after concentration to 1,300 g/l Le A 27 685 Na2CrO~ (already for~ed Na2CrOr-2H20 expressed in Na2CrO~
equivalents). On an industrial scale, however, this final conversion step is very complicated. The final conversion step may also be carried out with sulfuric acid or by electrolysis.
As mentioned above, chromate-containing sodium sulfate is formed during the reaction with sulfuric acid a~ has to be worked up.
Electrolytic acidification is carried out by introduc-tion of the sodium monochromate solution or rather thesodium monochromate soluti~ converted to a level of from 75 to 95% into sodium dicbromate into the anode compartments of electrolysis oells equipped with cation excbanger membranes.- During the electrolysis process, sodium ions migrate through the membrane into the cathode compartments of the cells filled with water or witb an aqueous solution.
The sodium ions form an aqueous solution containing sodium hydroxide witb the hydroxide ions formed at the cathode w~th s~ta~x~s evDlution of h~x~n. In tbe ~x~e ooqp~ireot,~he chromate ions are converted into dichromate ions by the hydrogen ions formed at the anode with simultaneous evolu-tion of oxygen. This process is also difficult to carry out on an industrial scale.
To produce sodium dichromate crystals, the sodium di-chromate solutions obtainable in the processes mentionedabove are concentrated by evaporation of water. The sodium dichromate which crystallizes is separated off and dried.
Now, the present invention adopts a different, ~imple approach to the production of sodium dichromate solutions ~nd sodium dichro at- crystal~ which does not have any of the disadvantages of the proc-~es described above.
SUMMARY OF ~HE INVENTION
The present invention relates to a process for the production of sodium dichromate and sodium dichromate ~olu-tions by ro~sting o~ chro~e ores under alkalineconditions, leaching of the furnace clinker obtained with Le A 27 685 2 ~ o ~
water or an ~queous chromate-containing solution, adjust-aent of the pH to from 7 to 9.5, removal of the insoluble constituents by filtration, a sodium monochromate solution being obtained, conversion of the monochromate ions of this solution into dichromate ions by acidification and crystal-lization of sodium dichromate by concentration of this solution, characterized in that the acidification is carried out with carbon dioxide under pressure with removal of sodium hydrogen carbonate, the remaining solution is then very largely freed from sodium monochromate by cooling to a temperature below lO C and filtration, any monochro-mate ions still present in the remaining solution are con-verted into dichromate ions by addition of an acid and the sodium monochromate filtered off is added to the sodium monochromate solution before conversion with carbon dioxide into a sodium dichromate solution.
The solution remaining after acidification with carbon dioxide under pressure and removal of sodium hydrogen car-bonate is preferably cooled to temperatures of from O to -25-C and, more preferably, to temperatures of from -10 to -20-C. It is of advantage to concentrate the sodium monoch-romate solution to contents of from 500 to 1,000 g/l Na2CrO~
: before acidification and to carry out the acidification with carbon dioxide under pressure to a degree of acidific-ation of the monochromate ions into dichromate ions of from 40 to 95t. Although degrees of acidification of less than 40% are possible, they do lead to very large recirculation volumes which ~a~e the process difficult to carry out on an industrial ~cAle.
. DESCRIPTION OF THE PREFERRED EMBODIMENTS
In one particularly preferred embodiment of the pro-ce~s according to the invention, the sodium monochromate solution is concentrated before acidification to contents of from 750 to 1,000 g/l Na2CrO~ and the acidification with carbon dioxide under pressure is carried out to a degree of acidification of the monochromate ions into dichromate ions Le A 27 685 3 2 ~
of from 80 to 95%.
Solutions in which up to 99% of the chromate ions of the solution are present as dichromate ions are obtained by acidification of sodium monochromate solutions with carbon dioxide under pressure and cooling of the solution af~er removal of sodium hydrogen carbonate to temperatures below lO-C and filtration of precipitated sodium monochromate.
Solutions such as these, which only contain minimal resi- -dues of sodium monochromate,-can be used in numerous pro-duction processes, such as the electrolytic production of chromic acid, the production of chrome tanning materials and in the oxidation of organic compounds.
For the production of sodium dichromate solutions, in which all the chromate ions are present as dichromate ions, and for the production of sodium dichromate crystals, the remaining monochromate ions of the solution may be con-verted into dichromate ions either with an acid or by elec-trolysis. The monochromate ions still present after cool-ing and filtration are preferably converted by addition of chromic acid and/or a solution containing chromic acid.
Suitable chromic acid solutions are obtained, for example, in the electrolytic production of chromic acid.
The process according to the invention is described in the following with reference to Fig. l. The variant of the process according to the invention illustrated in Fig. l represents a particularly advantageous embodiment.
Chrome ore is digested by oxidative roasting with ~X~ ash and atm~ rlc o~n at frcm 1,000 to 1,100C under alkaline conditions in the presence of a ~Tier mater~ in a rotary klln (1). The furnace cllnker formed is then leached with water or dilute chromate solution and adjusted with sodium dichromate solution to a pH value of from 7 to 9.5 (2). Soluble alkali compounds of iron, aluminium and silicon are thus converted into insoluble and readily filterable hydroxides or oxide hydrates which are Le A 27 685 4 ~q~
removed together with the insoluble constituents of the furnace clinker (3). The sodium monochromate solution formed, which contains from 300 to 500 g/l Na2CrO4, is then freed from dissolved vanadate by addition of calcium oxide at pH values of from lO to 13, as described in EP-A-47 799.
The solution, which now contains calcium, is freed from most of the alkaline earth metal ions and other polyvalent cations by precipitation as carbonates through the addition of, or n situ formation of, sodium carbonate (4). Preci-pitation is preferably carried out at temperatures of from 50 to lOO-C, at pH values of from 8 to 12 and with an approximately 2 to lO-fold molar carbonate excess, based on the quantity of alkaline earth metal ions. A sodium monochromate solution freed from polyvalent cations to a total content of less than 5 mg/l is obtained after filtra-tion. The content of polyvalent cations in this solution can be even further reduced by means of suitable selective cation exchangers. The sodium monochromate solution is then concentrated to contents of from 750 to l,000 g/l of Na2CrO4 (5) by evaporation in one or more stages.
In the now concentrated solution, a pH value below 6.5 is established by introduction of carbon dioxide in one or more stages to a final pressure of from 0.5 to i.s MPa (5 -15 bar) at a final temperature not exceeding 50-C, an 80 to 95% conversion of the sodium monochromate into sodium dichromate being obtained in this way with precipitation of . sodium hydrogen carbonate (6). Carbon dioxide may be in-troduced in gaseous or liquid form.
The sodium hydrogen carbonate is removed from the suspension formed under continuing carbon dioxide pressure or, after expansion, the sodium hydrogen carbonate is quickly removed before its back-reaction with the sodium dichromate.
The sodium h~drogen carbonate removed is calcined at from 200 to 300-C, optionally with addition of sodium .
Le A 27 685 5 i hydroxide, to form soda ~sh which is ~ed to the roasting of the chrome ore (1).
After removal of a sidestream for pH adjustment of the leached furnace clinker, the resulting sodium monochromate/
sodium dichro~ate solution separated from the sodium hydro-gen carbonate is then cooled to -10 to -20-C in one or more stirrer-equipped vessels and, after a residence time of 5 to 60 minutes, is freed from the sodium monochromate accu-mulating by filtration (7). Sodium dichromate solutions in which 97 to 99% of the chromate is present as dichromate are obtained. The sodium monochromate filtered off is then concentrated by evaporation (5), optionally after addition of water, before the pH adjustment (2).
~he sodium monochromate filtered off, optionally after washing and drying, may also be used as such, for example for the production of corrosion inhibitors.
To avoid the accumulation of impurities, particularly sulfate ions, it may be advisable to deliver part of the sodium monochromate filtered off to the final acidification stage (8). It has proved to be of particular advantage in this connection to use low-sulfur fuels, preferably natural gas, in the roastin~ of the chrome ore (1).
The sodium monochromate/sodium dichromate solution may be coo}ed by standard methods of the type described, for example, in "UlImanns Encyklopadie der Technischen Chemie", 4th Edition, Vol. 3, pages 186 to 218, Verlag Chemie, Wein-. heim, 1973. If liquid carbon dioxide is used for acidifi-cation, it is advisable to cool the solution either direct-ly or indirectly through a heat exchanger using a suitable heat exchange medium by evaporation of carbon dioxide.
Part of the solution obtained during cooling (7) is used for the electrolytic production of chromic acid (11) or another application. The other part is converted with mother liguor containing chromic acid from the chromic acid filtration stage (13), with solid chromic acid or with .
Le A 27 685 6 . --,~
Q ~ ~
chromic acid solution into a sodium dichromate solution in which all the chromate ions are present as dichromate ions (8). This solution is then used either as such or after - concentration by evaporation to from 60 to 70% by weight Na2Cr20~ H20 t9) For the production of sodium dichromate crystals, the solution is concentrated to approximately 1,650 g/l Na2Cr207 2H20 with precipitation of sodium dichromate which is removed from the solution by filtration (10).
In the electrolytic production of chromic acid, the sodium dichromate/sodium monochromate solution is converted into a solution containing chromic acid by multistage electrolysis at from 50 to 90-C in two-compartment electro-lysis cells with cation-selective membranes are partitions (11~. The electrolysis is carried out by introduction of the solution mentioned into the anode compartment of the first stage. After partial conversion of the dichromate into chromic acid, the solution then flows into the second stage, in which it undergoes further partial conversion into chromic acid, and so on to the last stage in which a degree of conversion of the dichromate into chromic acid of from 55 to 70% is achieved, corresponding to a molar ratio of sodium ions to chromic acid of from 0.45 : 0.55 to 0.30 : 0.70. Although there may be any number of stages, 6- to 15-stage electrolysis is preferred.
The solution containing chromic acid formed during - electrolysis and residual sodium dichromate is concentrated by evaporation to a water content of approximately 12 to 22% by weight water at temperatures of from 55 to llO-C, most of the chromic acid crystallizing out (12). The suspension formed is then separated by centrifugation at from 50 to llO-C into a solid consisting essentially of crystalline chromic acid and a liquid phase hereinafter referred to as the mother liguor (13).
The mother liquor obtained, optionally after dilution Le A 27 685 7 2 ~
with water, is returned to the electrolysis process at a suitable point, i.e. at a stage in which the dichromate conversion is similar. To avoid a heavy accumulation of impurities in the system, part of the mother liquor is removed and used in the final acidification (8). The crystalline chromic acid is freed from adhering mother liquor by single or repeated washing with from 10 to 50% by weight, based on the weight of the solid, of saturated or substantially saturated chromic acid solution and by centrifugation after each wash. The washed pure chromic acid crystals may then be used for the intended purpose either as such or after drying.
T~e process according to the invention is illustrated by the following Examples.
EXAMPLES
GENERAL COMMENTS
Sodium dichromate/sodium monochromate solutions prepared by dissolution of sodium dichromate and sodium monochromate were used in Examples 1 to 5. The composi-tions of the solutions corresponded to those in which theycan be present in the industrial production of sodium di-chromate after acidification with carbon dioxide under pressure. A partly acidified sodium dichromate solution obtained in an industrial process after acidification with carbon dioxide was used in Example 6. The soiutions were cooled in an electrically operated laboratory cryostat.
In Examples 1 to 5, q~antities of ~0 ml were cooled to the temperatures shown, kept at those temperatures for 60 . minutes and then filtered through a cooled glass frit.
,. .
Exam~lel A solution having the following composition was cooled to temperatures down to -25-C and worked up as described.
ComDosition of the solution:
Total chromate content of the solution (in Na2CrO~ eguivalents): 60.1% by weight - 900 g/l Le A 27 685 8 Na2CrO7 2H2o 46.7 % by weight Na2Cro4 9.3 % by weight Degree of acidification~: 84.5 %
) The so-called degree of acidification indicates the percentage of the total chromate content, expressed as Na2CrO~ equivalents, which is present as dichromate.
The results of the test are shown in Table 1.
Table 1 Temper- Concentration Concentration Degree of ature Na2Cr207 2~20 Na2CrO4 acidifi-cation 15 ll-C Beginning of Na2CrO4 crystallization -5-C 53.6% by weight 3.3% by weight 94.7%
-lO-C 55.2% by weight 2.0% by weight 96.8%
-l5-C 56.2% by weight 1.1% by weight 98.3%
-20-C 58.6% by weight 1.0% by weight 98.3%
-25-C 59.0% by weight 1.1% by weight 98.3%
Exam~le 2 A solution having the following composition was cooled to temperatures down to -25-C and worked up as described.
Com~osition of the solution:
Total chromate content of the solution (in Na2CrO4 equivalents): 59.3% by weight ~ 874 g/l Na2CrO?-2H20: 48.9 % by weight Na2CrO4: 6.1 % by weight Degree of acidification: 89.7 %
The results of the test are shown in Table 2.
Le A 27 685 9 , Table 2 Temper- Concentration Concentration Degree of ature Na2Cr207-2H2o Na2CrO4 acidifi-cation 7-C Beginning of Na2CrO4 crystallization -5-C 53~5% by weight 2.5% by weight 95.9%
-lO-C 53.9% by weight 1.7% by weight 97.3%
-l5-C 55.2% by weight 1.1% by weight 98.2%
10-20 C 54.4% by weight 1.1% by weight 98.2%
-25-C 57.1% by weight 0.7% by weight 98.9%
~xam~le 3 A solution having the following composition was cooled to -20 and -2S-C and worked up as described.
Com~osition of the solution:
Total chromate content of the solution (in Na2CrO4 equivalents): 62.7% by weight - 892 g/l Na2CrO7 2H20: 48.7 % by weight Na2Cro4 9.8 % by weight Degree of acidification: 84.4 %
The results are shown in Table 3.
Table 3 Temper- Concentration Concentration Degree of ature Na2Cr2O7 2H2o Na2CrO~ acidifi-cation !
lO-C Beginning of Na2CrO~ crystallization -20-C 54.0S by weight 1.2~ by weight 98.0%
-25-C 57.1% by weight 0.6~ by weight 99.1%
~xam~le 4 A solution having the following composition was cooled to -15, -20 and -25-C and worked ~p as described.
.
Le A 27 685 10 2~ J I
Com~osition of the solution:
Total chromate content of the solutisn (in Na2CrO4 equivalents): 54.0% by weight - 765 g/l Na2CrO7-2H2O 44.4 % by weight Na2Cro4 5.7 S by weight Degree of acidification: 89.4 %
The results of the test are shown in Table 4.
Table 4 Temper- Concentration Concentration Degree of ature Na2Cr20~ 2H20 Na2CrO4 acidifi-cation O-C~ Beginning of Na2CrO4 crystallization -15-C 48.5% by weight 1.9% by weight 96.6%
-20-C 47.5% by weight 1.2% by weight 97.7~
-25-C 55.2% by weight 1.1% by weight 98.2%
ExamDle 5 A solution having a total chromate content of 42.7% by weight - 563 g/l (expressed in Na2CrO4 equivalents) and a degree of acidification of 49.1% was cooled to -12-C and worked up as described. The results are shown in Table 5.
Table 5 Concentration Concentration Degree of Na2cr2or 2H20 Na2CrO4 acidifi-cation Solution before cooling 19.3% by weight 21.7% by weight 49.1%
. 35 Solution after cool-ing to -12-C
and filtra-tion 33.2% by weight 4.7% by weight 88.5%
,;
The crystallization of Na2CrO4 began at lO C. The solution solidified on cooling to below 13-C.
Example 6 Quantities of 200 ml of a 93% acidified technical sodium dichromate solution having the following composition were cooled to 0, -10, -15 and -20-C, stirred for 1 hour and filtered through a cooled glass frit.
Composition of the solution:
Total chromate content of the solution (in Na2CrO4 equivalents): 63.9% by weight - 990 g/l Na2CrO7 2H2 54.7 % by weight Na2CrO4: 4.5 % by weight The results of the test are shown in Table 6.
Table 6 Temper- Concentration Concentration Degree of ature Na2Cr207-2H20 Na2CrO4 acidifi-cation O C 57.0% by weight 2.6% by weight 96.0%
-5 C 57.7% by weight 2.0% by weight 96.9%
-lO-C 59.4% by weight 1.3% by weight 98.0%
30-15-C 59.8% by weight 1.0% by weight 98.5%
-20-C 60.2% by weight 0.7% by weight 98.9%
It is understood that the specification and examples are illustrative but not limitative of the present invention and that other embodiments within the spirit and scope of the invention will suggest themselves to those skilled in the art.
Le A 27 685 12
Claims (7)
1. A process for the production of sodium dichromate and sodium dichromate solutions by oxidative roasting of chrome ores under alkaline conditions, leaching of the furnace clinker obtained with water or an aqueous chromate-containing solution, adjustment of the pH to from 7 to 9.5, removal of the insoluble constituents by filtration, a sodium monochromate solution being obtained, conversion of the monochromate ions of this solution into dichromate ions by acidification and crystallization of sodium dichromate by concentration of this solution, characterized in that the acidification is carried out with carbon dioxide under pressure with removal of sodium hydrogen carbonate, the remaining solution is then very largely freed from sodium monochromate by cooling to a temperature below 10°C and filtration, any monochromate ions still present in the remaining solution are converted into dichromate ions by addition of an acid and the sodium monochromate filtered off is added to the sodium monochromate solution before conversion with carbon dioxide into a sodium dichromate solution.
2. A process according to claim 1, wherein the cooling is carried out to temperatures of from 0 to -25°C.
3. A process according to claim 1, wherein the cooling is carried out to temperatures of from -10 to -20°C.
4. A process according to claim 1, wherein the sodium monochromate solution is concentrated before acidification to a concentration of from 500 to 1,000 g/l Na2CrO4.
5. A process according to claim 1, wherein the acidification with carbon dioxide under pressure is Le A 27 685 13 carried out to a degree of acidification of the mono-chromate ions into dichromate ions of from 40 to 95%.
6. A process according to claim 1, wherein the sodium monochromate solution is concentrated before acidification to a concentration of from 750 to 1,000 g/l Na2CrO4 and the acidification with carbon dioxide under pressure is carried out to a degree of acidification of the monochromate ions to dichromate ions of from 80 to 95%.
7. A process according to claim 1, wherein the mono-chromate ions present after cooling and filtration are converted by addition of a component comprising chromic acid.
Le A 27 685 14
Le A 27 685 14
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| DE4013335A DE4013335A1 (en) | 1990-04-26 | 1990-04-26 | METHOD FOR PRODUCING SODIUM SEALANT |
| DEP4013335.4 | 1990-04-26 |
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| Publication Number | Publication Date |
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| CA2041005A1 true CA2041005A1 (en) | 1991-10-27 |
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| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA002041005A Abandoned CA2041005A1 (en) | 1990-04-26 | 1991-04-23 | Process for the production of sodium dichromate |
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| EP (1) | EP0453913B1 (en) |
| JP (1) | JP3262127B2 (en) |
| BR (1) | BR9101664A (en) |
| CA (1) | CA2041005A1 (en) |
| DE (2) | DE4013335A1 (en) |
| ES (1) | ES2063394T3 (en) |
| PL (1) | PL165627B1 (en) |
| RO (1) | RO110459B1 (en) |
| RU (1) | RU2008262C1 (en) |
| TR (1) | TR25954A (en) |
| ZA (1) | ZA913113B (en) |
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|---|---|---|---|---|
| DE10217206A1 (en) * | 2002-04-18 | 2003-11-06 | Bayer Ag | Process for recycling vanadium from chrome ore as vanadium V-oxide |
| CN102649585B (en) * | 2011-08-22 | 2014-05-14 | 四川省银河化学股份有限公司 | Preparation method of sodium bichromate |
Family Cites Families (5)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| DE379410C (en) * | 1921-04-14 | 1923-08-22 | Griesheim Elektron Chem Fab | Process for the production of sodium dichromate from sodium chromate using carbonic acid |
| FR1223186A (en) * | 1958-05-05 | 1960-06-15 | Diamond Alkali Co | Process for the purification of chromate and dichromate liquors |
| RO64038A2 (en) * | 1972-12-01 | 1978-07-15 | Inst De Chime | PROCESS FOR CONVERTING SODIUM CHROMATE TO SODIUM BICHROMATE |
| US3933972A (en) * | 1974-02-11 | 1976-01-20 | Products Chimiques Ugine Kuhlmann | Process for preparing pure sodium bichromate |
| DE3837266A1 (en) * | 1988-11-03 | 1990-05-10 | Bayer Ag | PROCESS FOR PREPARING SODIUM DICHROMATE |
-
1990
- 1990-04-26 DE DE4013335A patent/DE4013335A1/en not_active Withdrawn
-
1991
- 1991-04-13 ES ES91105917T patent/ES2063394T3/en not_active Expired - Lifetime
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- 1991-04-13 EP EP91105917A patent/EP0453913B1/en not_active Expired - Lifetime
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- 1991-04-22 RO RO147405A patent/RO110459B1/en unknown
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| ZA913113B (en) | 1992-02-26 |
| BR9101664A (en) | 1991-12-10 |
| PL290011A1 (en) | 1992-01-13 |
| DE4013335A1 (en) | 1991-10-31 |
| EP0453913A2 (en) | 1991-10-30 |
| RO110459B1 (en) | 1996-01-30 |
| DE59103520D1 (en) | 1994-12-22 |
| ES2063394T3 (en) | 1995-01-01 |
| JPH04228432A (en) | 1992-08-18 |
| TR25954A (en) | 1993-11-01 |
| EP0453913A3 (en) | 1992-05-20 |
| PL165627B1 (en) | 1995-01-31 |
| JP3262127B2 (en) | 2002-03-04 |
| RU2008262C1 (en) | 1994-02-28 |
| EP0453913B1 (en) | 1994-11-17 |
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