Treatment of solid and liquid effluents from tan-yards for removing and recovering the chromium in the effluents

Abstract

[EN] The present invention relates to the integral treatment of chromium containing wastes from tan-yards according to the diagram of figure (1). There is provided one or two solid (1) and/or liquid (2) effluents which contain chromium. The solid effluent is subjected to a treatment (4) with perborate (3) producing a chromium-free residue (5) which is treated by conventional methods (for example hydrolysis in order to produce gums or chelagen) and waters which contain the chromium present as Cr(VI). Said Cr(VI) is reduced to Cr(III) by reduction in acid medium at (6). In a reactor (7) the solution containing Cr(III) is treated continuously with an alkaline solution (8), to give waters with less than 1 ppm of Cr (9) and a pulp (10) containing Cr(OH)¿3?.3H¿2?O from which is obtained, by acidification with sulphuric acid, a concentrated solution of chromium sulphate reusable in the same tan-yard. The present invention provides for the precipitation of Cr(III) contained in solutions, by using magnesium hydroxide (or a mixture with NaOH); the chromium-containing solution is slowly and continuously added to another solution, maintained at a temperature between 60 and 63 °C, to which is also added the neutralizing agent. Thereby, the precipitate obtained has particular characteristics of solubility and settling which cannot be obtained in a precipitate produced with other neutralizers. In the solid wastes, the oxidation is performed by stirring these chopped wastes with an aqueous solution which contains a perborate in the proportion of at least 1.5 mol of BO¿3??-¿ per at-g of Cr contained in the residues (for example 2.35 g of NaBO¿3? per g of Cr), during a period of time longer than 10 minutes, the recommended amount of perborate to be used, if sodic salt is used, being from 3.3 to 3.7 g of NaBO¿3? per g of Cr contained in the residue to be treated and the recommended treatment time being 45 minutes. The perborate may be substituted by a mixture of oxygenated water, H¿2?O¿2?, and a borate (for example borax) in a solution whose pH must be comprised between 7.5 and 11.5, preferably between 8.5 and 9.5.