METHOD FOR REDUCING HEXAVALENT CHROMIUM IN OXIDIC SOLIDS

Abstract

Process for reducing hexavalent chromium in oxidic solids, which comprises the steps: a) heating of the oxidic solid containing Cr(VI) to a temperature of from 600 to 1400 C. in an atmosphere containing less than 0.1% by volume of an oxidizing gas and b) cooling of the reaction product obtained after step a) to a temperature below 100 C. in an atmosphere containing less than 0.1% by volume of an oxidizing gas,
characterized in that no reducing agent is added to the oxidic solid or to the atmosphere in step a) and b) in the process.

Claims

1. A process for reducing hexavalent chromium in oxidic solids, the process comprising: a) heating an oxidic solid containing Cr(VI) to a temperature of 600 to 1400 C. in an atmosphere containing less than 0.1% by volume of an oxidizing gas to produce a reaction product; and b) cooling the reaction product to a temperature below 100 C. in an atmosphere containing less than 0.1% by volume of an oxidizing gas to produce a resultant product: wherein no reducing agent is added to the oxidic solid or to the atmosphere in steps a) and b) in the process.

2. The process according to claim 1, wherein the oxidic solid a chrome ore residue.

3. The process according to claim 1, wherein the oxidic solid comprises up to 80,000 ppm of Cr(VI).

4. The process according to claim 1, wherein the Cr(VI) in the oxidic solid is present as sodium monochromate (Na.sub.2CrO.sub.4).

5. The process according to claim 1, wherein the oxidic solid comprises: chromium(III) oxide (Cr.sub.2O.sub.3): from 7 to 13% by weight, preferably from 7.5 to 12.5% by weight aluminium oxide (Al.sub.2O.sub.3): from 10 to 30% by weight, preferably from 18 to 24% by weight iron(III) oxide (Fe.sub.2O.sub.3): from 42 to 50% by weight, preferably from 42 to 48% by weight magnesium oxide (MgO): from 9 to 18% by weight, preferably from 10to 17% by weight calcium oxide (CaO): <10% by weight, preferably <5% by weight silicon oxide (SiO.sub.2): from 0 to 3% by weight, preferably from 1to 3% by weight vanadium oxide (V.sub.2O.sub.5): <1% by weight, preferably <0.5% by weight sodium oxide (Na.sub.2O): from 0 to 5% by weight, preferably from 2 to 5% by weight; and sodium monochromate (Na.sub.2CrO.sub.4); from 0.3 to 4.7% by weight.

6. The process according to claim 1, wherein at least 90% of the particles of the oxidic solid are smaller than 500 m, particularly preferably smaller than 300 m.

7. The process according to claim 1, wherein the atmosphere in step a) is selected from the group consisting of an inert gas atmosphere and a vacuum.

8. The process according to claim 7, wherein the atmosphere in step a) is an inert gas atmosphere and comprises at least 90% by volume of one or more gases selected from the noble gas.

9. The process according to claim 7, wherein the atmosphere in step a) is an atmosphere having a pressure of less than 800 mbar.

10. The process according to claim 1, wherein the oxidic solid in step a) is heated to a temperature of 850 C. to 1200 C.

11. The process according to claim 1, wherein the heating in step a) is carried out in a continuously operating or discontinuously operating reactor.

12. The process according to claim 11, wherein the reactor is a reactor which is indirectly heated by means of gas or electricity.

13. The process according to claim 11, wherein the reactor is an indirectly electrically heated horizontal tube furnace,

14. The process according to claim 1, wherein the reaction product is cooled to a temperature below 40 C.

15. The process according to claim 1, wherein the heating in step a) and the cooling in step b) are done in the same reactor, wherein, after producing the reaction product in the reactor in step a), indirect heating of the reactor is switched off, the reaction product is left in the reactor for step b), and the reactor is subsequently cooled by introducing inert gas into the reactor.

16. The process according to claim 1, wherein the oxidic solid comprises 1000 to 15,000 ppm of Cr(VI), and the resultant product has a a Cr(VI) content of <1000 ppm.

17. The process according to claim 16, wherein: the atmosphere in step a) contains less than 0.1% by volume of an oxidizing gas, and is at least one of an inert gas atmosphere and a vacuum; the oxidic solid in step a) is heated to a temperature of 850 C. to 1200 C.; the oxidic solid is a particulate chrome ore residue wherein at least 90% of the particles of the oxidic solid are smaller than 300 m; the Cr(VI) in the chrome ore residue is present as sodium monochromate (Na.sub.2CrO.sub.4); the chrome ore residue comprises 0.3 to 4.7% by weight sodium monochromate (Na.sub.2CrO.sub.4); and the atmosphere in step b) is an inert gas atmosphere, wherein inert gas is introduced in step b) to cool the reaction product to a temperature below 40 C.,

18. The process according to claim 17, wherein: steps a) and b) are performed consecutively in the same reactor; and the atmosphere in step a) contains less than 0.01% by volume of oxygen, and is at least one of an inert gas atmosphere and a vacuum, wherein: the inert gas atmosphere comprises at least 95% by volume of one or more gases selected from helium, argon, nitrogen and carbon dioxide; and the vacuum atmosphere is at a pressure of less than 650 mbar.

19. The process according to claim 16, wherein: the oxidic solid in step a) is heated to a temperature of 950 C. to 1150 C.; the reaction product in step b) is cooled to a temperature below 30 C.; the reactor is configured for continuous operation and is an indirectly, electrically heated, rotary tube furnace, oriented horizontally and having two-zones; the inert gas atmosphere comprises at least 99.9% by volume of one or more of nitrogen and carbon dioxide; the vacuum atmosphere is at a pressure of less than 450 mbar; the chrome ore residue is a residue obtained in the oxidative alkaline digestion of chrome ores for the production of sodium monochromate, and additionally comprises: 7.5 to 12.5% by weight chromium(III) oxide (Cr.sub.2O.sub.3); 18 to 24% by weight aluminium oxide (Al.sub.2O.sub.3); 42 to 48% by weight iron(III) oxide (Fe.sub.2O.sub.3); 10 to 17% by weight magnesium oxide (MgO); <5% by weight calcium oxide (CaO); 1 to 3% by weight silicon oxide (SiO.sub.2); <0.5% by weight vanadium oxide (V.sub.2O.sub.5); and 2 to 5% by weight sodium oxide (Na.sub.2O).

20. A process for reducing hexavalent chromium in chrome ore residues, wherein the chrome ore residues contain hexavalent chromium Cr(VI) in the form of sodium monochromate (Na.sub.2CrO.sub.4), the process comprising: a) heating the chrome ore residue to a temperature of 850 C. to 1200 C. in an atmosphere containing less than 0.1% by volume of an oxidizing gas to produce a reaction product; and b) cooling the reaction product to a temperature below 40 C. in an atmosphere containing less than 0.1% by volume of an oxidizing gas to provide a product having a Cr(VI) content of <1000 ppm, wherein steps a) and b) are performed consecutively in the same reactor, and no reducing agent is added to the ore residue or to the atmosphere in steps a) and b).

Description

EXAMPLES

Determination of the Cr(VI) Content

[0093] Description of the test methods used:

[0094] Modified Alkaline Digestion Process

[0095] The determination of the Cr(VI) content of the oxidic solids used as starting materials and also the reaction products obtained was carried out by a method based on the alkaline digestion process described in USEPA SW-846 Method 3060A.

[0096] When the oxidic solid contains more than 2% by weight of water, it is dried to constant weight at 120 C. and then weighed out. However, in contrast to the process described in USEPA SW-846 Method 3060A, not from 2.4 g to 2.6 g of the sample to be examined are digested, but instead from 9.9 g to 10.1 g (balance accuracy 0.0001 g) of the oxidic solid are transferred quantitatively into a reaction flask having a protective gas connection. 50 ml of the alkaline digestion solution (produced by dissolving 20.0 g of NaOH (0.5 M) and 29.7 g of Na.sub.2CO.sub.3 (0.28 M) in 1.001 of demineralized water), 2 ml of a Mg(NO.sub.3).sub.2 solution (prepared by dissolving 60.0 g of Mg(NO.sub.3).sub.2*6 H.sub.2O in 1001 of demineralized water) and 0.5 ml of a buffer solution having pH=7 are then added. The suspension is heated to boiling in a nitrogen atmosphere while stirring and heated under reflux for one hour, After one hour, the suspension is cooled to room temperature while stirring. The mixture is subsequently filtered in air and the filter cake is intensively washed with demineralized water. The mother liquor and washings obtained during filtration and washing are combined in a 500 ml standard flask, made up to the mark with demineralized water and analysed for Cr(VI) as described below. In contrast to the process described in USEPA SW-846 Method 3060A, a significantly larger amount of sample is thus used, but the alkaline extract is finally made up to 500 ml instead of 250 ml in the standard flask. Nevertheless, twice the Cr(VI) concentration in the standard solution used for the Cr(VI) determination by UV/VIS spectroscopy results from the above-described method, compared to the process described in USEPA SW-846 Method 3060A.

[0097] UV/Vis Spectroscopy for Determining the Chromium(VI) Content

[0098] A small amount of the clear solution is taken off from the standard flask containing the alkaline extract obtained from the alkaline digestion process and brought to a pH of 7 by means of dilute hydrochloric acid. This generally gives a precipitate of aluminium and silicon hydroxides, which is centrifuged off. The clear centrifugate obtained is filtered through a 0.45 m syringe filter and its Cr(VI) content after setting of the pH is determined as 1,5-diphenylcarbazide complex by means of UV/Vis spectroscopy as described in USEPA Method 218.7. The measured Cr(VI) concentration is, if it can be quantified, back-calculated taking into account the dilution brought about by the setting of the pH with the dilute hydrochloric acid to the mass of the oxidic solid content originally used.

[0099] The determination of the Cr(VI) content was carried out at a wavelength of 539 nm on an automated UV/Vis spectrometer, model Metrohm 844 UV/VIS Compact IC. In this instrument, the monochromate is firstly separated off from other anions by means of an anion-exchange column before being reacted with 1,5-diphenylcarbazide in an after-column reactor and determined spectrophotometrically. In the case of the instrument used, the Cc(VI) determination limit is 0.0128 mg/l of Cr(VI). Taking into account 10 g of dried oxidic solid used for the above-described alkaline digestion process, this gives a determination limit of 0.64 mg of Cr(VI) per kg of oxidic solid, corresponding to 640 ppb of Cr(VI),

Examples 1-7

[0100] The invention is described in more detail by the following examples without the invention being intended to be restricted thereby.

[0101] For the following examples, chrome ore residue from the industrial process for producing sodium monochromate from chromite via an oxidative alkaline digestion with sodium carbonate (known as no lime process, CaO content <5% by weight) was used. The chrome ore residue obtained in the form of a moist filter cake in the process for producing sodium monochromate after solid-liquid separation was merely dried but not sieved or milled.

[0102] General Procedure

[0103] Dried chrome ore residue whose Cr(VI) content had been determined by the above-described modified alkaline digestion process was heated in crucibles in an indirectly electrically heated horizontal tube furnace. The tube diameter was 70 mm and the total length was 1500mm, of which about 500 mm were heated. The crucibles were located in the heated region during the experiments.

[0104] The dried chrome ore residue was introduced into the cold furnace, both ends were closed in a gastight manner and nitrogen (>99.9990% by volume of nitrogen) was introduced from one end and at the opposite end was conveyed via an outlet opening and an immersed tube into the exhaust air. The furnace was heated to the desired target temperature under these conditions under a nitrogen gas stream (about 2 l/min at a furnace volume of about 6 1 l), maintained at this temperature for the desired time and then cooled again.

[0105] After cooling, the black reduced chrome ore residue reaction product was taken out and worked up by the above-described alkaline digestion process and the Cr(VI) content of the alkaline extract was determined by means of UV/Vis spectroscopy.

Example 1

[0106] Dried chrome ore residue (Cr.sub.2O.sub.3: 8.7%, Al.sub.2O.sub.3:21.8%, Fe.sub.2O.sub.3: 46.5%, V.sub.2O.sub.5:0.04%, SiO.sub.2: 1.3%, MgO: 13.3%, CaO: 0.04%, Na.sub.2O: 2,9%, all figures in % by weight) having a content of 1409 ppm of Cr(VI) (corresponds to 0.43% by weight of Na.sub.2CrO.sub.4) was introduced into silicon carbide crucibles and heated at 900 C. under a nitrogen atmosphere as described in the general procedure for two hours and subsequently cooled to below 100 C. in a nitrogen atmosphere. The Cr(VI) content of the reaction product was found to be 31 ppm.

Example 2

[0107] Dried chrome ore residue (Cr.sub.2O.sub.3: 9.2%, Al.sub.2O.sub.3: 19.8%, Fe.sub.2O.sub.5: 44.5%, V.sub.2O.sub.5: 0.04%, SiO.sub.2: 1.4%, MgO: 14.6%, CaO: 0.04%, Na.sub.2O: 2.5%, all figures in % by weight) having a content of 1474 ppm of Cr(VI) (corresponds to 0.45% by weight of Na.sub.2CrO.sub.4) was introduced into silicon carbide crucibles and heated at 900 C. under a nitrogen atmosphere as described in the general procedure for two hours and subsequently cooled to below 100 C. in a nitrogen atmosphere. The Cr(VI) content of the reaction product was found to be 40 ppm.

Example 3

[0108] Dried chrome ore residue (Cr.sub.2O.sub.3: 9.2%, Al.sub.2O.sub.3: 19.8%, Fe.sub.2O.sub.3: 44.5%, V.sub.2O.sub.5: 0.04%, SiO.sub.2:1.4%, MgO 14.6%, CaO: 0.04%, Na.sub.2O: 2.5%, all figures in % by weight) having a content of 1474 ppm of Cr(VI) (corresponds to 0.45% by weight of Na.sub.2CrO.sub.4) was introduced into silicon carbide crucibles and heated at 1100 C. under a nitrogen atmosphere as described in the general procedure for four hours and subsequently cooled to below 100 C. in a nitrogen atmosphere. The Cr(VI) content of the reaction product was found to be <0.64 ppm.

Example 4

[0109] Dried chrome ore residue (Cr.sub.2O.sub.3: 8.9%, Al.sub.2O.sub.3: 22.3%, Fe.sub.2O.sub.3: 43.8%, V.sub.2O.sub.5: 0.04%, SiO.sub.2: 1.5%, MgO: 14.3%; CaO: 0.04%, Na.sub.2O: 3.5%, all figures in % by weight) having a content of 7995 ppm of Cr(VI) (corresponds to 2.49% by weight of Na.sub.2CrO.sub.4) was introduced into silicon carbide crucibles and heated at 1100 C. under a nitrogen atmosphere as described in the general procedure for four hours and subsequently cooled to below 100 C. in a nitrogen atmosphere. The Cr(VI) content of the reaction product was found to be <0.64 ppm.

Example 5

[0110] Dried chrome ore residue (Cr.sub.2O.sub.3: 8.9%, Al.sub.2O.sub.3: 22.3%, Fe.sub.2O.sub.3: 43.8%, V.sub.2O.sub.5: 0.04%, SiO.sub.2: 1.5%, MgO: 14.3%, CaO: 0.04%, Na.sub.2O: 3.5%, all figures % by weight) having a content of 7995 ppm of Cr(VI) (corresponds to 2.49% by weight of Na.sub.2CrO.sub.4) was introduced into silicon carbide crucibles and heated at 1000 C. under a nitrogen atmosphere as described in the general procedure for four hours and subsequently cooled to below 1000 C. in a nitrogen atmosphere. The Cr(VI) content of the reaction product was found to be 27 ppm.

Example 6

[0111] Dried chrome ore residue (Cr.sub.2O.sub.3: 8.9%, Al.sub.2O.sub.3: 22.3%, Fe.sub.2O.sub.3: 43.8%, V.sub.2O.sub.5: 0.04%, SiO.sub.2: 1.5%, MgO: 14.3%, CaO: 0.04%, Na.sub.2O: 3.5%, all figures in % by weight) having a content of 7995 ppm of Cr(VI) (corresponds to 2.49% by weight of Na.sub.2CrO.sub.4) was introduced into silicon carbide crucibles and heated at 1100 C. under a nitrogen atmosphere as described in the general procedure for two hours and subsequently cooled to below 100 C. in a nitrogen atmosphere. The Cr(VI) content of the reaction product was found to be <0.64 ppm.

Example 7

[0112] Dried chrome ore residue (Cr.sub.2O.sub.3: 8.3%, Al.sub.2O.sub.3: 22.01%, Fe.sub.2O.sub.3: 45.7%, V.sub.2O.sub.3: 0.07%, SiO.sub.2: 1.4%, MgO: 10.4%, CuO: 0.06%, Na.sub.2O: 3.4%, all figures in % by weight) having a content of 5714 ppm of Cr(VI) (corresponds to 1.8% by weight of Na.sub.2CrO.sub.4) was introduced into silicon carbide crucibles and heated at 1000 C. in a vacuum of 400 mbar as described above departing from the general procedure for two hours and subsequently cooled to below 100 C. under the same vacuum. The Cr(VI) content of the reaction product was found to be <0.64 ppm.