Process for preparing chromium(III) oxide

Abstract

The present invention relates to a process for preparing chromium(III) oxide by reaction of alkali metal chromate with gaseous ammonia, subsequent hydrolysis, isolation of the hydrolysis product and calcination.

Claims

1. A process for preparing chromium(III) oxide, the process comprising: a) reacting an alkali metal chromate with gaseous ammonia at a temperature of 200 to 600 C., thereby forming a reaction product, b) hydrolysing the reaction product, wherein the hydrolysing comprises one of: reducing the pH of water used for the hydrolysing before hydrolysing, or reducing the pH of a resultant alkaline mother liquor during or after hydrolysing, to a pH value of from 4 to 11, by adding an acid to the water or resultant mother liquor, whereby a hydrolysis product is formed, c) isolating the hydrolysis product formed in step b), and d) calcinating the hydrolysis product of step c) at a temperature of 700 to 1400 C. to produce a calcined chromium(III) oxide product.

2. The process according to claim 1, wherein reacting the alkali metal chromate with gaseous ammonia comprises reacting the alkali metal chromate with gaseous ammonia in an indirectly heated reactor.

3. The process according to claim 1, wherein the alkali metal chromate is sodium dichromate, sodium dichromate dihydrate or potassium dichromate.

4. The process according to claim 1, wherein the acid is an inorganic acid or an organic acid.

5. The process according to claim 1, further comprising: suspending the calcined product obtained from step d) in water, and subsequently drying the calcined product.

6. The process according to claim 1, further comprising: suspending the calcined product obtained from step d) in water, washing the calcined product with water one or more times, subsequently drying the calcined product, and thereafter milling the calcined product.

7. The process according to claim 1, further comprising adding before calcinating, in an amount of 0.01% by weight to 3.0% by weight, based on a product used for the calcinating, a compound selected from the group consisting of alkali metal halides, ammonium halides, alkaline earth metal halides, fluorides, chlorides, bromides, iodides of sodium or potassium or ammonium, alkali metal hydroxides, sodium hydroxide, potassium hydroxide, and chromic acid.

8. The process according to claim 1, wherein hydrolyzing the reaction product comprises reducing the pH to a pH value of 5 to 10.

9. The process according to claim 1, wherein calcining the hydrolysis product comprises calcinating the hydrolysis product at a temperature of 800 to 1300 C.

10. The process according to claim 1, wherein reacting the alkali metal chromate with gaseous ammonia comprises reacting the alkali metal chromate with gaseous ammonia in a rotary tube furnace or in a fluidized bed.

11. The process according to claim 1, wherein the acid is carbon dioxide.

12. The process according to claim 1, further comprising adding before calcinating, in an amount of 0.02% by weight to 1.0% by weight, based on a product used for the calcinating, a compound selected from the group consisting of alkali metal halides, ammonium halides, alkaline earth metal halides, fluorides, chlorides, bromides, iodides of sodium or potassium or ammonium, alkali metal hydroxides, sodium hydroxide, potassium hydroxide, and chromic acid.

13. The process according to claim 1, wherein: the alkali metal chromate has a particle size of less than 1000 m, and the temperature for reacting the alkali metal chromate with the gaseous ammonia is 200 to 500 C.

14. The process according to claim 1, wherein: the alkali metal chromate has a particle size of less than 300 m, the reaction of the alkali metal chromate with the gaseous ammonia is started at a temperature of 200 to 300 C. and is increased continuously or step wise, and the process further comprises removing samples of the reaction product during the reaction of the alkali metal chromate with the gaseous ammonia, forming a suspension of the sampled product in water, checking the pH of the suspension, and ending the reaction of the alkali metal chromate with the gaseous ammonia when the pH of a resulting suspension of the reaction product reaches at least 11.

15. The process according to claim 14, wherein the reaction is ended when the pH of a resulting suspension of the reaction product reaches at least 13.

16. The process according to claim 14, wherein: the alkali metal chromate is sodium dichromate, sodium dichromate dihydrate or potassium dichromate, the temperature for reacting the alkali metal chromate with the gaseous ammonia is 200 to 500 C., the acid is carbon dioxide, after isolating the hydrolysis product, the hydrolysis product is further washed with water mixed with an acid to reduce the pH, and after calcining, the method further comprises: suspending the calcined product in water, washing the calcined product with water one or more times, subsequently drying the calcined product, and thereafter milling the calcined product.

Description

EXAMPLES

Example 1

(1) 60 g of anhydrous, comminuted sodium dichromate Na.sub.2Cr.sub.2O.sub.7 were introduced into a glass container provided with gas inlet and outlet with a glass frit. The bottom was completely covered and the temperature sensor dipped into the loose material. The glass container was introduced into a regulated furnace. The glass container was heated under nitrogen to a wall temperature of 300 C. At an internal temperature of 220 C., the stream of nitrogen was replaced by a moderate stream of ammonia which flowed through the product from the bottom. After 50 minutes, the internal temperature rose within a few minutes from 220 C. to 371 C. as a result of the exothermic reaction. After 60 minutes, the wall temperature was increased to 400 C. over 10 minutes to complete the reaction. After a further 6.5 hours, the ammonia was again displaced by nitrogen and the apparatus was cooled to room temperature. 50.3 g of reaction product were obtained.

(2) The coarsely comminuted reaction product was slurried in 200 ml of water and hydrolyzed. The strongly alkaline suspension was brought to a pH of 6 by means of acetic acid, subsequently filtered on a suction filter and the filter cake was dried at 120 C. without further washing. It was then ignited at 1250 C. for 2 hours. The ignited chromium(III) oxide was again suspended in water, washed with water and subsequently dried at 120 C.

(3) The chromium(III) oxide obtained in this way had an Na content, calculated as Na metal, of 210 ppm. The yield of chromium(III) oxide was 92% based on the chromium present in the starting material Na.sub.2Cr.sub.2O.sub.7.

Example 2

(4) The reaction of 60 g of comminuted sodium dichromate Na.sub.2Cr.sub.2O.sub.7 with gaseous ammonia was carried out as described in Example 1.

(5) The comminuted reaction product was slurried in 200 ml of water and hydrolyzed, giving a suspension having a pH of 14. Carbon dioxide was then passed under atmospheric pressure into the suspension until a pH of 9.4 was obtained and could no longer be reduced further. The suspension was then briefly heated to 95 C. and filtered on a suction filter. The filter cake was resuspended in 200 ml of water and the pH of the suspension was set to 7.7 by introduction of carbon dioxide under atmospheric pressure. A further lowering of the pH was not possible. The suspension was then once again briefly heated to 95 C. and filtered on a suction filter. The filter cake was resuspended in 200 ml of water and the pH of the suspension was set to 6.8 by introduction of carbon dioxide under atmospheric pressure. Further lowering of the pH was not possible. The suspension was then briefly heated to 95 C. and filtered on a suction filter. The filter cake was dried at 120 C. It was subsequently ignited at 1250 C. for 2 hours. The ignited chromium(III) oxide was again suspended in water, washed with water and finally dried at 120 C.

(6) The chromium(III) oxide obtained in this way had an Na content, calculated as Na metal, of 130 ppm. The Cr content indicated a purity of 98.9%. The yield of chromium(III) oxide was 91% based on the chromium present in the starting material Na.sub.2Cr.sub.2O.sub.7.

Example 3

(7) The reaction of 60 g of comminuted sodium dichromate Na.sub.2Cr.sub.2O.sub.7 with gaseous ammonia was carried out as described in Example 1.

(8) The reaction product which had been comminuted to a particle size of less than 600 m was slurried in 200 ml of water and hydrolyzed. 2.0 g of a 0.1% strength aqueous solution of a flocculation auxiliary (copolymer of acrylamide and sodium acrylate) were then added. Carbon dioxide was then passed in under atmospheric pressure at room temperature while stirring until a pH of 9.9 had been established. A further lowering of the pH was not possible. The suspension was then briefly boiled and filtered on a suction filter. The filter cake was resuspended in 200 ml of water, 2.0 g of the abovementioned 0.1% strength aqueous solution of the flocculation auxiliary were added and the pH of the suspension was set to 7.4 by introduction of carbon dioxide under atmospheric pressure. The suspension was then again briefly boiled and filtered on a suction filter. The filter cake was resuspended in 200 ml of water, 2.0 g of the abovementioned 0.1% strength aqueous solution of the flocculation auxiliary was added and the pH of the suspension was set to 6.5 by introduction of carbon dioxide under atmospheric pressure. The suspension was then again briefly boiled and filtered on a suction filter. The filter cake was dried at 120 C. It was subsequently ignited at 1250 C. for 2 hours. The ignited chromium(III) oxide was again suspended in water, washed with water and finally dried at 120 C.

(9) The chromium(III) oxide obtained in this way had an Na content, calculated as Na metal, of 310 ppm. The Cr content indicates a purity of 99.7%. The yield of chromium(III) oxide was 96% based on the chromium present in the starting material Na.sub.2Cr.sub.2O.sub.7.

Example 4

(10) Comminuted sodium chromite NaCrO.sub.2 and comminuted sodium dichromate Na.sub.2Cr.sub.2O.sub.7 are mixed in a molar Cr(III):Cr(VI) ratio of 1:1 and heated to 350 C. under an inert gas atmosphere. After one hour, the temperature is increased at 3 C./min to 450 C. and maintained at 450 C. for 30 minutes. The reaction product obtained is dark green. According to an X-ray powder diffraction pattern, it consists of chromium(III) oxide and sodium monochromate Na.sub.2CrO.sub.4:
2NaCrO.sub.2+Na.sub.2Cr.sub.2O.sub.7.fwdarw.2Na.sub.2CrO.sub.4+Cr.sub.2O.sub.3(10)

(11) The reaction product was reacted at 500 C. with gaseous ammonia which was used as a mixture containing 13.6% by volume of ammonia and an inert gas, with the reduction commencing from about 350 C. A weight loss of 10.65% occurs during the reduction, which is in good agreement with the expected reaction to form sodium chromite:
2Na.sub.2CrO.sub.4+Cr.sub.2O.sub.3+2NH.sub.3.fwdarw.4NaCrO.sub.2+3H.sub.2O+N.sub.2(11)

(12) The sodium chromite NaCrO.sub.2 obtained after the reaction with ammonia according to equation (11) can be worked up as described in Examples 1 to 3.

Comparative Example 1 (without Lowering of the pH During the Hydrolysis)

(13) The reaction of 60 g of comminuted sodium dichromate Na.sub.2Cr.sub.2O.sub.7 with gaseous ammonia was carried out as described in Example 1.

(14) The comminuted reaction product was slurried in 100 ml of water and hydrolyzed, giving an alkaline suspension. This was then filtered on a suction filter and the filter cake was resuspended in 100 ml of water for 1 hour and again filtered on a suction filter. The filter cake was dried at 120 C. and subsequently ignited at 1250 C. for 2 hours. The ignited chromium(III) oxide was again suspended in water, washed with water and finally dried at 120 C.

(15) The chromium(III) oxide obtained in this way had an Na content, calculated as Na metal, of 540 ppm. The yield of chromium(III) oxide was 65%, based on the chromium present in the starting material Na.sub.2Cr.sub.2O.sub.7.

Comparative Example 2 (in Accordance with CN-101475217 without Lowering of the pH During the Hydrolysis)

(16) Sodium dichromate Na.sub.2Cr.sub.2O.sub.7 was, as described in CN-101475217, reacted with gaseous ammonia at 350 C. for 1 hour. The reaction product obtained was cooled and washed three times at 80 C. with the indicated amount of water. After filtration, the solid had a moisture content of 29% and was mixed with 2% by weight of an equimolar mixture of P.sub.2O.sub.3, Al.sub.2O.sub.3, ZnO, Sb.sub.2O.sub.3, BaO and TiO.sub.2 and ignited at 1100 C. for 1 hour. The product was subsequently again washed, dried and comminuted a number of times. The yield of chromium(III) oxide was 44.6%, based on the chromium present in the starting material Na.sub.2Cr.sub.2O.sub.7.

Comparative Example 3 (Based on CN-101475217 Without Lowering of the pH During the Hydrolysis)

(17) Sodium dichromate Na.sub.2Cr.sub.2O.sub.7 was reacted with gaseous ammonia as described in Example 1. The reaction product obtained was cooled and then, in accordance with CN-101475217 worked up further and reacted as described in Comparative Example 2. The yield of chromium(III) oxide was 65%, based on the chromium present in the starting material Na.sub.2Cr.sub.2O.sub.7.