<\/span>Stage 1: Metals Removal and Pretreatment<\/span><\/h3>\nThis is where the chemistry gets settled. The standard sequence is:<\/p>\n
\n- Chromium reduction<\/strong> \u2014 drop pH to 2\u20133, add a reducing agent, hold for reaction time.<\/li>\n
- Cyanide oxidation<\/strong> \u2014 alkaline chlorination or another oxidation route, if cyanide is present.<\/li>\n
- Neutralization and precipitation<\/strong> \u2014 raise pH to the range where each metal hydroxide has minimum solubility. That range is different for each metal, which is why a single setpoint rarely gets you everything.<\/li>\n
- \u041a\u043e\u0430\u0433\u0443\u043b\u044f\u0446\u0438\u044f \u0438 \u0444\u043b\u043e\u043a\u0443\u043b\u044f\u0446\u0438\u044f<\/strong> \u2014 polymer dosing to build settleable floc.<\/li>\n
- Clarification or dissolved air flotation<\/strong> \u2014 solids separation.<\/li>\n
- Filtration<\/strong> \u2014 multimedia or ultrafiltration to protect the membranes downstream.<\/li>\n<\/ol>\n
For chelated or complexed metals, hydroxide precipitation alone will not work. I use sulfide precipitation or a specialized heavy metal precipitant in those cases, and I always run jar tests before committing to a dosing strategy.<\/p>\n
The sludge from this stage goes to a screw press sludge dewatering<\/a> unit. In my experience, a screw press handles metal hydroxide sludge well, produces a cake in the 20\u201330% dry solids range depending on the feed, and needs far less operator attention than a plate-and-frame press.<\/p>\n![\"Chemical](\"https:\/\/memvatop.com\/blogimg\/wastewater_14.webp\")
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