Methods for removing hexavalent chromium
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The method for removing hexavalent chromium involves using iron filings or ferrous salts to reduce the hexavalent chromium in chromic acid to trivalent chromium, followed by the use of alkalis or lime to produce less toxic chromium hydroxide, after which the precipitate is filtered out for centralized treatment. Below are the steps for treating chromium-containing wastewater in our factory: 1. The chromium-containing wastewater is sent to the reduction reaction tank, and hydrochloric acid is added; 2. The pH value is adjusted to between 2.5 and 3.5. Maintain for more than 30 minutes. 3 Add ferrous sulfate or iron filings to reduce hexavalent chromium to trivalent chromium. 4 Transfer to the neutralization tank, add sodium hydroxide to adjust the pH value between 8 and 9, so that the chromium ions are completely precipitated. 5 Add a flocculant, then transfer to the sedimentation tank and subsequently to the air flotation water purifier. 6 Pass through the air flotation water purifier, then through a membrane-covered sand filter, followed by an activated carbon filter, before being discharged into the release tank.
The poster’s approach is correct; in applications, waste sulfuric acid is generally used instead of hydrochloric acid, as the working conditions for hydrochloric acid are too harsh due to the need for stirring as well； The two pH ranges can be adjusted further; there’s no need to use so much acid or base ； Activated carbon helps in the absorption of trace chromium.
This post explains it well; it’s exactly what I wanted to know.
Can’t we skip the step of using hydrochloric acid? Can’t it be reduced by ferrous sulfate in a neutral environment? The book states that under alkaline conditions, Cr(OH)3 is formed directly. This saves hydrochloric acid and reduces the amount of alkali that needs to be added as well.