Process for recycling chromium from waste water containing hexavalent chromium

Abstract

A process for recovering chromium from hexavalent chromium-containing wastewater comprises the following steps: (1) extracting hexavalent chromium in wastewater to an organic phase by using an extracting agent, and separating hexavalent chromium from a water phase, so as to acquire a hexavalent chromium-loaded organic phase; (2) reducing the hexavalent chromium-loaded organic phase by using an aqueous solution of an organic reducing agent, reducing hexavalent chromium into trivalent chromium, reversely extracting trivalent chromium into the water phase, and separating the organic phase from the water phase to acquire a solution of the trivalent chromium and a renewable organic phase, wherein the organic reducing agent is one or a mixture of alcohols, aldehydes and carboxylic acids having the carbon atom number ranging 1 to 3; and (3) performing solvent evaporation on the solution of trivalent chromium, catalyzing, and recovering the trivalent chromium.

Claims

1. A process for recycling chromium from waste water containing hexavalent chromium, comprising the following steps: (1) adjusting the pH value of the waste water containing chromium to 1.0-2.0 by using 10-50 wt % of sulfuric acid, then extracting hexavalent chromium contained in waste water to a trialkylamine-kerosene solution containing 30 wt % of trialkylamine as an extraction agent and kerosene as a solvent, and separating the organic phase from an aqueous phase to obtain the organic phase loaded with the hexavalent chromium, wherein the flow velocity ratio of aqueous phase to organic phase in extraction is (10-25):1; (2) reducing the hexavalent chromium in organic phase to trivalent chromium by using an aqueous solution of an organic reducing agent, performing reverse extraction to feed the trivalent chromium into the aqueous phase, and performing separation of aqueous phase and organic phase to obtain a solution containing the trivalent chromium and a regenerated organic phase, wherein the organic reducing agent is ethanol, the aqueous solution contains 50 wt % of ethanol, and the flow velocity ratio of aqueous phase to organic phase in reverse extraction is (1:1)-(1:5); and (3) performing solvent evaporation and then crystallization on the solution containing the trivalent chromium to recycle the trivalent chromium.

2. The process according to claim 1, wherein the regenerated organic phase in the step (2) is fed back to the step (1) to be used as the extraction agent for extraction of the hexavalent chromium in the waste water.

3. The process according to claim 2, wherein the reverse extraction in the step (2) is a two-stage counter current reverse extraction.

4. The process according to claim 1, wherein the evaporated solvent in the step (3) is recycled and is used for reducing the organic phase loaded with the hexavalent chromium in the step (2).

5. The process according to claim 1, wherein the extraction in the step (1) is a three-stage counter current extraction.

6. The process according to claim 1, wherein the reverse extraction in the step (2) is a two-stage counter current reverse extraction.

Description

DETAILED DESCRIPTION OF THE EMBODIMENTS

(1) In the process provided by the embodiment, the waste water containing chromium is the one in a waste water collection pool of a chromic salt plant, and the content of the hexavalent chromium is 53 mg/L. The process comprises the following steps: preparing a trialkylamin-kerosene solution containing 30 wt % of the extraction agent by taking trialkylamin as an extraction agent and kerosene as a solvent; preparing dilute sulfuric acid containing 10 wt % of sulfuric acid from concentrated sulfuric acid, and adjusting the pH value of the waste water containing chromium to 1.0-2.0; adding the prepared 30 wt % trialkylamin-kerosene solution to the waste water containing chromium to extract the hexavalent chromium, wherein the flow velocity ratio of aqueous phase to oil phase is 15:1 (i.e., the waste water containing chromium is concentrated by 15 times) in extraction; performing three-stage counter current extraction, providing a sampling opening at the extraction raffinate outlet, and performing online sampling analysis by using a spectrophotometer according to the analysis method under Chinese national standards GB7467-1987, wherein the concentration of effluent chromium (Cr.sup.6+) is less than 0.1 mg/L; performing reduction and reverse extraction on the obtained organic phase loaded with the hexavalent chromium using aqueous solution containing 50 wt % of ethanol which is prepared by industrial ethanol, wherein the flow velocity ratio of aqueous phase to oil phase is 1:5 in reverse extraction, wherein the reverse extraction is two-stage counter current reverse extraction; performing post-treatment on the trivalent chromium in the aqueous phase, evaporating and recycling excessive ethanol, concentrating and crystallizing the aqueous solution to obtain chromium sulfate product, and feeding back the regenerated organic phase as the extraction agent for repeated use.