Chrome Reduction / Cyanide Destruction Control System

Regulatorily load-bearing setpoints that belong in gated synthesized content rather than an open spec table. Chrome reduction: lower pH to ~2.5 then add a reducing agent (sulfur dioxide, sodium sulfite or sodium metabisulfite); optimum pH 2.0-2.5 because the reaction slows significantly by pH 3.0 and is very slow at pH 4.0; minimum detention 15 minutes; ORP setpoint -300mV (-400mV max) generally adequate at pH 2.0; explicit vendor warning that lowering pH further than needed risks releasing hazardous sulfur dioxide gas. Cyanide destruction batch: raise pH to 11.5 with NaOH, add chlorine until ORP reaches +450mV, hold pH 11.5, then neutralize with acid plus a small amount of chlorine to convert cyanate to carbonate — with acid injection manually locked out to prevent pH dropping below 11. Two-step: stage 1 pH must be adjusted rapidly to avoid releasing extremely toxic cyanogen chloride gas, target oxidation below 0.1 ppm; stage 2 lowers pH with sulfuric acid, and modern systems often use up to a 4-hour detention time. Named cyanide-waste-generating processes: plating of copper, brass, gold and zinc; conversion coatings; chemical milling; barrel finishing; burnishing; heat treating; electrochemical machining. Materials to confirm: Not a material cell — process-control chemistry; wetted materials come from the electrode and valve cells above. Specific chemicals include hexavalent chromium (Cr6+), trivalent chromium (Cr3+), sulfur dioxide, sodium sulfite, sodium metabisulfite, cyanide, cyanate, cyanogen chloride (hazard product), sodium hydroxide, sulfuric acid, chlorine.

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