Detoxification treatment method for extracting and recycling chromium from hexavalent chromium-containing residues

Abstract

A method for extracting and recycling chromium from hexavalent chromium-containing residues includes the following steps: 1) adding water to the hexavalent chromium-containing residues and mixing uniformly; 2) adding sodium sulfate, sodium chlorate and sulfuric acid to a solution obtained in step 1) and stirring sufficiently to obtain a mixed liquid; 3) treating the mixed liquid by a hydrothermal method or direct heating; 4) after the heating treatment, naturally cooling a solid-liquid mixture to room temperature for holding; 5) separating solid residues and a chromium-containing supernatant, and washing filtered residues with water and then drying; 6) precipitating the supernatant and the water used for washing the filtered residues with a precipitant CaCl2, then centrifugally washing, dewatering and drying the precipitates; and 7) recycling a chromium-containing solution for returning to a work section, or for a treatment of recycling chromium.

Claims

1. A detoxification treatment method for extracting and recycling chromium from hexavalent chromium-containing residues, a main component of the residues is particles of calcium sulfate dihydrate which adsorb sodium chromate, characterized in that, the detoxification treatment method comprises steps as follows: 1) adding water to the hexavalent chromium-containing residues and mixing uniformly, forming a solution; 2) adding mineralizers to the solution obtained in step 1) and stirring sufficiently to obtain a mixed liquid; and the mineralizers are sodium sulfate, sodium chlorate and sulfuric acid; 3) treating the mixed liquid obtained in step 2) by a hydrothermal method or direct heating; 4) naturally cooling a solid-liquid mixture obtained in step 3) to room temperature for holding, thereby growing solid calcium sulfate crystals in the residues; 5) separating solid residues and a chromium-containing supernatant, and washing the solid residues with water and then drying; 6) precipitating the supernatant and the water used for washing the solid residues with a precipitant CaCl.sub.2, then centrifugally washing, dewatering and drying the precipitates; and 7) recycling a chromium-containing solution obtained in step 6) for a treatment of recycling chromium; the treatment of recycling chromium has specific steps as follows: adding a reductant to the chromium-containing solution, with hexavalent chromium in the solution being reduced to trivalent chromium, then adding a precipitant NaOH to generate precipitate of Cr(OH).sub.3, and finally calcining the precipitate of Cr(OH).sub.3 into Cr.sub.2O.sub.3; and the reductant is sodium sulfide or sodium bisulfite.

2. The detoxification treatment method for extracting and recycling chromium from hexavalent chromium-containing residues according to claim 1, wherein a w/w solid-to-liquid ratio after water is added to the hexavalent chromium-containing residues in step 1) is 1:0.5-1:10.

3. The detoxification treatment method for extracting and recycling chromium from hexavalent chromium-containing residues according to claim 1, wherein sodium sulfate in step 2) in the mixed liquid has a concentration of 0.1 mol/L-1 mol/L.

4. The detoxification treatment method for extracting and recycling chromium from hexavalent chromium-containing residues according to claim 1, wherein sodium chlorate in step 2) in the mixed liquid has a concentration of 0.1 mol/L-1 mol/L.

5. The detoxification treatment method for extracting and recycling chromium from hexavalent chromium-containing residues according to claim 1, wherein the mixed liquid in step 2) has a pH value of 1-5.

6. The detoxification treatment method for extracting and recycling chromium from hexavalent chromium-containing residues according to claim 1, wherein a temperature of the hydrothermal method or direct heating in step 3) is controlled between 50 C. and 200 C.

7. The detoxification treatment method for extracting and recycling chromium from hexavalent chromium-containing residues according to claim 1, wherein a time for treating the mixed liquid in step 3) is 2 to 10 hours.

8. The detoxification treatment method for extracting and recycling chromium from hexavalent chromium-containing residues according to claim 1, wherein a time for naturally cooling the solid-liquid mixture in step 4) is 0.5 to 48 hours.

Description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

(1) The present invention is further described in connection with embodiments, but the implementations of the present invention are not limited by these.

(2) Embodiment 1

(3) 1) 0.5 ton of hexavalent chromium-containing residues were added to a hydrothermal kettle, then water was added until a solid-to-liquid ratio (w/w) was 1:1, with sufficient stirring, and sodium sulfate, sodium chlorate and sulfuric acid were added, and a concentration of sodium sulfate was adjusted to 1 mol/L and a concentration of sodium chlorate was adjusted to 1 mol/L in the system. A pH value was 1. A temperature was set at 50 C. and a time for heat preservation was 2 hours.

(4) 2) 2 hours later, heating was stopped and the system was cooled naturally to room temperature and held for 24 hours,

(5) 3) After the holding, a supernatant containing hexavalent chromium was poured out. Residues which had been subjected to the hydrothermal treatment were subjected to centrifugal washing and dewatering. Filtered residues were dried to obtain white solids or white and yellowish solids.

(6) 4) A suitable amount of CaCl.sub.2 was added to the poured supernatant and the water used for washing the filtered residues, to remove excess SO.sub.4.sup.2 from the solution, followed by stirring until white precipitate generated. The precipitate was subjected to centrifugal washing, dewatering and drying.

(7) 5) The chromium-containing solution after the precipitate treatment was reused in the production or collected in a waste water treatment station to perform restore, recycle and purification treatments. Reductants (such as sodium sulfide and sodium bisulfite) were added to reduce the dissolved hexavalent chromium into trivalent chromium, and a precipitant NaOH was added to generate precipitates of Cr(OH).sub.3. The treated water could reach the national standard off drainage. During the whole process, water could be recycled in the system.

(8) 6) Final products were residues mainly containing calcium sulfate and chromic mud mainly containing Cr(OH).sub.3, respectively. The residues of calcium sulfate could be used as raw materials in industries such as rubber, plastics, fertilizer, paint, textile and papermaking. The chromic mud could be calcined to Cr.sub.2O.sub.3, to realize recycling of chromium.

(9) A concentration of the leached hexavalent chromium from the original chromium-containing residues in this embodiment was 273 mg/L, and a concentration of the leached hexavalent chromium from the treated filtered residues was 1.93 mg/L.

(10) Embodiment 2

(11) 1 ton of hexavalent chromium-containing residues were added to the hydrothermal kettle, then water was added until the solid-to-liquid ratio (w/w) was 1:0.5, with sufficient stirring, and sodium sulfate, sodium chlorate and sulfuric acid were added. The concentration of sodium sulfate in the system was adjusted to 0.1 the concentration of sodium chlorate was 1 mol/L and the pH value was 3, with sufficient stirring, and the hydrothermal kettle was turned off. The temperature was set at 150 C. and the time for heat preservation was 6 hours. After the heating was stopped, the system was held for 48 hours. Other process was the same as that in the Embodiment 1.

(12) The concentration of the leached hexavalent chromium from the, original chromium-containing residues in this embodiment was 265 mg/L, and the concentration of the leached hexavalent chromium from the treated filtered residues was 1.52 mg/L.

(13) Embodiment 3

(14) 1 ton of hexavalent chromium-containing residues were added to the hydrothermal kettle, then water was added until the solid-to-liquid ratio (w/w) was 1:10, with sufficient stirring, and sodium sulfate, sodium chlorate and sulfuric acid were added. The concentration of sodium sulfate in the system was adjusted to 0.5 mol/L, the concentration of sodium chlorate was 0.5 mol/L and the pH value was 2, with sufficient stirring, and the hydrothermal kettle was turned off. The temperature was set at 200 C. and the time for heat preservation was 10 hours. After the heating was stopped, the system was held for 0.5 hour. Other process was the same as that in the Embodiment 1.

(15) The concentration of the leached hexavalent chromium from the original chromium-containing residues in this embodiment was 286 mg/L, and the concentration of the leached hexavalent chromium from the treated filtered residues was 0.84 mg/L.

(16) Embodiment 4

(17) 1.5 tons of hexavalent chromium-containing residues were added to the hydrothermal kettle, then water was added until the solid-to-liquid ratio (w/w) was 1:5, with sufficient stirring, and sodium sulfate, sodium chlorate and sulfuric acid were added. The concentration of sodium, sulfate in the system was adjusted to 0.5 mol/L, the concentration of sodium chlorate was 0.1 mol/L and the pH value was 2, with sufficient stirring, and the hydrothermal kettle was turned off. The temperature was set at 180 C. and the time for heat preservation was 3 hours. After the heating was stopped, the system was held for 12 hours. Other process was the same as that in the Embodiment 1.

(18) The concentration of the leached hexavalent chromium from the original chromium-containing residues in this embodiment was 278 mg/L, and the concentration of the leached hexavalent chromium from the treated filtered residues was 0.57 mg/L.

(19) Embodiment 5

(20) 2 tons of hexavalent chromium-containing residues were added to the hydrothermal kettle, then water was added until the solid-to-liquid ratio (w/w) was 1:1, with sufficient stirring, and sodium sulfate, sodium chlorate and sulfuric acid were added. The concentration of sodium sulfate in the system was adjusted to 1 mol/L, the concentration of sodium chlorate was 0.1 mol/L and the pH value was 5, with sufficient stirring, and the hydrothermal kettle was turned off. The temperature was set at 200 C. and the time for heat preservation was 6 hours. After the heating was stopped, the system was held for 6 hours. Other process was the same as that in the Embodiment 1.

(21) The concentration of the leached hexavalent chromium from the original chromium-containing residues in this embodiment was 252 mm/L, and the concentration of the leached hexavalent chromium from the treated filtered residues was 0.34 mg/L.

(22) Similarly, technical solutions given according to the protection scope defined by the claims and the description may also give a plurality of embodiments which belong to the protection scope of the present invention.