Method for reducing hexavalent chromium in oxidic solids

Abstract

Process for reducing hexavalent chromium in oxidic solids, which comprises the steps: a) mixing of the oxidic solid containing Cr(VI) with a carbon-containing compound which is liquid in the range from 20 to 100° C., b) treatment of the mixture obtained after a) in an indirectly heated reactor at a temperature of from 700° C. to 1100° C., particularly preferably at a temperature of from 800° C. to 1000° C., under a protective atmosphere, c) cooling of the reaction product obtained after b) to at least 300° C., preferably at least 150° C., under a protective atmosphere.

Claims

1. A process for reducing hexavalent chromium in oxidic solids, the process comprising: a) mixing an oxidic solid containing Cr(VI) with a carbon-containing compound to form a mixture, wherein the carbon-containing compound is a compound that is liquid at temperatures from 20° C. to 100° C., b) treating the mixture obtained after a) in an indirectly heated reactor at a temperature of 700° C. to 1100° C. under a protective atmosphere to form a reaction product, and c) cooling of the reaction product obtained after b) to a temperature of 300° C. or less under a protective atmosphere to form a reduced process product.

2. The process according to claim 1, wherein the oxidic solid contains a proportion of less than 15% by weight of calcium oxide.

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

4. The process according to claim 1, wherein the carbon-containing compound comprises a polyhydroxy compound.

5. The process according to claim 1, wherein, instead of vaporizing, the carbon-containing compound decomposes at temperatures of ≧150° C.

6. The process according to claim 1, wherein the carbon-containing compound comprises glycerol, polyethylene glycol having a molar mass of from 380 to 420, or a mixture of the two in a mass ratio of from 4:1 to 15:1, based on the mass of Cr(VI).

7. The process according to claim 1, wherein carbon of the carbon-containing compound is used in a molar ratio to Cr(VI) of from 5:1 to 35:1.

8. The process according to claim 1, wherein: the carbon-containing compound comprises glycerol, polyethylene glycol having a molar mass of from 380 to 420, or a mixture of the two; and carbon of the carbon-containing compound is used in a molar ratio to Cr(VI) of from 9:1 to 23:1.

9. The process according to claim 1, further comprising conducting step a) in a continuously operating mixing apparatus.

10. The process according to claim 1, further comprising conducting step b) in a continuously operating indirectly heated reactor.

11. The process according to claim 1, further comprising conducting step c) in a continuously operating cooling apparatus.

12. The process according to claim 1, wherein the oxidic solid has a water content.

13. The process according to claim 1, further comprising in step a), adding auxiliaries which improve the processability of the mixture produced, wherein the auxiliaries are selected from the group consisting of silicas, silicates, aluminates, and aluminosilicates.

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

15. The process according to claim 3, wherein the chrome ore residue contains up to 10,000 ppm of Cr(VI).

16. The process according to claim 3, wherein the chrome ore residue comprises a mixture of chrome ore residue and other residue, with the mixture containing up to 20 000 ppm of Cr(VI).

17. The process according to claim 1, wherein the process product has a Cr(VI) content of <640 ppb.

Description

EXAMPLES

(1) 1. Determination of the Cr(VI) Content

(2) Description of the Test Methods Used:

(3) Modified Alkaline Digestion Process I:

(4) 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.

(5) 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 100 l 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 1.00 l of demineralized water) and 0.5 ml of a buffer solution having pH=7 are then added. The suspension is blanketed with nitrogen protective gas, heated to boiling while stirring and heated under reflux for one hour, with continuous blanketing with nitrogen protective gas being carried out. After one hour, the suspension is cooled to room temperature while stirring, with the blanketing with nitrogen being maintained. 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 procedure 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 procedure described in USEPA SW-846 method 3060A.

(6) Modified Alkaline Digestion Process II:

(7) The reduced oxidic solids obtained by the process of the invention contain only very small amounts of Cr(VI) which are so small that they cannot be quantified in the alkaline extract obtained by the above-described process. In order to attain a further increase in the sensitivity of the determination of Cr(VI) in the reduced oxidic solid, the reduced oxidic solid samples as obtained from the process of the invention were additionally subjected to a modified alkaline digestion process II. Here, from 29.9 g to 30.1 g (balance accuracy 0.0001 g) of the oxidic solid were used and extracted as described above.

(8) The mother liquor and washings obtained after filtration and washing are combined in a 250 ml standard flask and made up to the mark with demineralised water. This modified alkaline digestion process II thus gives an alkaline extract which, compared to the process described in USEPA SW-846 method 3060A, has twelve times the Cr(VI) concentration. The analysis for chromium(VI) is carried out as described below.

(9) UV/Vis Spectroscopy for Determining Chromium(VI) Content:

(10) A small amount of the clear solution is taken off from the standard flask containing the alkaline extract obtained from the modified alkaline digestion process I or II 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 μm 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 adjustment of the pH with the dilate hydrochloric add to the basis of the mass of the oxidic solid originally used.

(11) 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 Cr(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 I gives a determination limit of 0.64 mg of Cr(VI) per kg of oxidic solid, corresponding to 640 ppb of Cr(VI). Taking into account 30 g of dried oxidic solid used for the above-described modified alkaline digestion process I gives a determination limit of 0.107 mg of Cr(VI) per kg of oxidic solid, corresponding to 107 ppb of Cr(VI).

Examples 1-6

(12) The invention is explained in more detail by the following examples without the invention being intended to be restricted thereby.

(13) 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. The composition of the dried chrome ore residue can be seen from the following Table 1.

(14) General Procedure

(15) Dried chrome ore residue whose Cr(VI) content had been determined by the above-described modified alkaline digestion method 1 was mixed with a carbon containing compound which is liquid in the range from 20 to 100° C. (glycerol or PEG having a molar mass of from 330 to 420) in order to obtain wetted chrome ore residue particles. This mixture was introduced into an electrically indirectly heated rotary tube furnace. The furnace tube had a total length of 1.5 m, of which 1.1 m were heated. The tube diameter was 0.3 m and the furnace was operated at a particular speed of rotation. A particular reaction temperature and a protective atmosphere of carbon dioxide prevailed in the furnace. The product obtained was cooled to at least 150° C. under a carbon dioxide atmosphere.

(16) After cooling, the black reduced chrome are residue reaction product was taken out, sieved through a 300 μm sieve and about 10 g of the fines were worked up by the above-described alkaline digestion process I and the Cr(VI) content of the alkaline extract was determined by means of UV/Vis spectroscopy. It was below the determination limit of 640 ppb. The Cr(VI) content was then still below the determination limit even when the reduced chrome ore residue reaction product was worked up by the above-described modified alkaline digestion process II. The Cr(VI) content of the reduced chrome ore residue reaction product was thus below 107 ppb of Cr(VI). The reduced chrome ore residue obtained thus no longer contained Cr(VI) which could be detected in this way.

(17) Example 6 was carried out according to the above-described general procedure, with the modification that a dried Cr(VI)-containing calcium vanadate filter cake formed in the separation of vanadate from a sodium monochromate solution was added to the dried chrome ore residue (3.5% by weight based on the dried chrome ore residue, vanadium content 13.80% by weight, Cr(VI) content 6.46% by weight) and the mixture was mixed as described above with a carbon-containing compound which is liquid in the range from 20 to 100° C.

(18) Table 1 shows the reaction parameters of six reaction examples carried out in the manner described above. In addition, the composition of the dried chrome ore residue used in the individual examples is indicated (calculated as metal oxides, scaled to 100%). Polyethylene glycol having a molar mass of from 380 to 420 is designated as PEG-400 in Table 1.

(19) TABLE-US-00001 TABLE 1 Example 1 2 3 4 5 6 Carbon-containing compound which is liquid PEG-400, Glycerol, PEG-400, Glycerol, PEG-400, PEG-400, in the range from 20 to 100° C. and its 1.0 1.5 1.5 1.0 0.75 1.5 proportion based on the mass of dried chrome ore residue [% by weight] Mass ratio of carbon-containing compound 8.9:1.0 13.3:1.0 9.4:1.0 10.3:1.0 8.8:1.0 4.2:1.0 which is liquid in the range from 20 to 100° C.: Cr(VI) Molar ratio of carbon (of the carbon- 20.73 22.51 22.02 17.37 20.65 9.85 containing compound which is liquid in the range from 20 to 100° C.):Cr(VI) Input rate into the reactor [kg/h] 17.6 13.8 11.2 25.7 10.3 20.0 Rotational speed [rpm] 3 3 2 3 2 3 Furnace temperature [° C.] 900 900 800 900 900 900 Cr(VI) content [ppm] 1129 1129 1594 975 850 3563 Cr.sub.2O.sub.3 content [% by weight] 9.55 9.55 8.17 9.29 9.54 9.72 Al.sub.2O.sub.3 content [% by weight] 22.83 22.83 22.88 23.04 23.00 23.07 Fe.sub.2O.sub.3 content [% by weight] 44.23 44.23 45.37 44.32 45.73 46.32 MgO [% by weight] 14.85 14.85 14.63 14.53 15.21 14.82 CaO content [% by weight] 0.65 0.65 0.76 0.71 0.08 0.07 SiO.sub.2 content [% by weight] 2.58 2.58 2.45 2.43 1.65 1.49 Na.sub.2O content [% by weight] 4.20 4.20 4.66 4.60 3.72 3.43 Others [% by weight] 1.11 1.11 1.08 1.08 1.07 1.08
2. Determination or Abrasion and Water Absorption

(20) Paving stones were produced from a mixture of cement and sand (ratio 1:8) and water without addition of the reduced oxidic solid (Example 1, reference specimens). Under identical conditions, paving stones in which 20% (Example 2) or 30% (Example 3) of the cement had been replaced by the reduced oxidic solid obtained after step c) of the process of the invention were produced. After curing for 28 days, the abrasion and the water absorption were determined on the paving stones obtained—in accordance with the method described in SANS 1058:2012 (South African National Standard 1058:2012, Edition 2.1 “Concrete paving blocks”). However, as a modification of SANS 1058:2012, the two properties were determined only on three specimens.

(21) It can be seen that both abrasion and water absorption is significantly lower in the case of the paving stones in which 20% of the cement had been replaced by the reduced oxidic solid obtained after step c) of the process of the invention (Example 2) than in the case of the paving stones produced without the reduced oxidic solid (Example 1). The paving stones in which 30% of the cement had been replaced by the reduced oxidic solid (Example 3) have even better values. The paving stones of Example 3 fulfil the requirements of SANS 1058:2012 in respect of abrasion and water absorption.

(22) TABLE-US-00002 Abrasion per paving Water absorption per Example stone in g paving stone in % 1 (Reference specimens) 27.5 7.1 35.2 7.4 25.2 7.5 2 16.0 6.8 19.6 7.4 17.7 7.1 3 7.0 5.0 5.7 5.9 8.6 5.7