METHOD FOR PROCESSING SLUDGE RESULTING FROM SODIUM CHROMATE PRODUCTION Russian patent published in 1997 - IPC

FIELD: processing of sludge resulting from oxidizing decomposition of chrome-containing ores for recovering sodium chromate and magnesium compounds, such as magnesium carbonate or magnesium oxide; environmental control. SUBSTANCE: proposed method comprises following steps of: decomposing magnesium and chrome (YI) compounds contained in sludge by treating latter with aqueous suspension of sulfuric acid at liquid-to-solids weight ratio of 3-8 in suspension, at temperature 70-100 C, at final pH value of reaction medium equal to 4-7, said pH value being maintained by adding acid at flow rate 2.5-7 l H₂SO₄/kg MgOsludgexh. Solution of magnesium sulfate and chromic acid thus-formed is separated from processed sludge, i.e. solid phase which, after finally reducing residual chrome (IY) to chrome (III), is subjected to heat treatment at temperature 150-180 C. Purification of magnesium sulfate solutions from chrome and calcium compounds is combined with chrome (YI) reduction step and with chrome (III) hydroxide precipitation step by successively treating these compounds with sulphur-containing reducing agent and sodium carbonate at pH 4.8-7.8 and temperature 40-90 C. Magnesium carbonate or hydroxycarbonate are precipitated from purified magnesium sulfate solution, followed by subjecting them to heat treatment to obtain high-quality magnesium oxide. EFFECT: method offers multiple advantages, viz, simplified process, fewer items of equipment required, sludge can be both decontaminated and processed to environmentally safe and useful products. 3 cl, 2 tbl