Laboratory Pilot Test of Chromium (III) Isolation from Acid Extract of Tannery Sludge

High concentrations of Al, Fe, Ca, Mg, and Zn in the tanning agent are detrimental to the properties of tanned leather. To recycle the Cr(III) from the tannery sludge to the tanning agent, these five metals must be removed from the acid extract obtained by sulfuric-acid treatment of tannery sludge. A new approach has been developed to recycle the Cr(III) and to remove other metals. Summarily, this pilot test includes a simple six steps process for the extraction of metals from tannery sludge and the following separation of Cr(III) from interfering elements (Al, Fe, Ca, Mg, Zn) in the acid extract (or filtrate). In Step 1, metals were extracted from tannery sludge with sulfuric acid at about pH 1–2. A filtration was then performed to separate the residual solids and the liquid. The second step involves the dilution of filtrate and the precipitation of Al and Fe at pH 4.8. The third step involves the precipitation of Cr(III) at pH 7.0 for the separation of Ca and Mg from Cr(III). As a result, a precipitate containing less interfering metals was obtained in the third step. In order to recover more Cr(III), the precipitate obtained in Step 2 was dissolved once again with sulfuric acid in the fourth step. The fifth and sixth steps were similar to the second and third steps respectively, except that the pH in the sixth step was adjusted to 8.3 not 7.0. After the sixth step, a precipitate containing less interfering metals was also obtained. A 44% of Cr recovery yield was obtained in the final product. The Cr concentration in this product was 32.5% and the ratio of interfering metals to Cr concentration was inferior or equal to 0.02.