CHROMIUM PRECIPITATION FROM TANNING SPENT LIQUORS USING INDUSTRIAL ALKALINE RESIDUES - A COMPARATIVE-STUDY

Chromium precipitation from spent tanning liquors using industrial alk aline tailings is described and removal efficiencies are compared with those obtained with traditional chemicals as NaOH, CaO and MgO: (i) u sing steelmaking dusts at dosages up to 70 g l(-1) residual Cr remains greater than or equal to 3.7 mg l(-1). Moreover Cr and mainly Pb are partially leached by the spent tanning liquor, therefore limiting the use of this industrial residue as Cr precipitant; (ii) a dosage of 80 g l(-1) (dry basis) of chemical sludge from a water treatment plant re sults in a removal efficiency of 99.97% Cr and residual Cr less than o r equal to 2 mg l(-1) under experimental conditions that include stirr ing at 100 rpm for 1 h and settling for 23 h. Sedimentation time may b e reduced to 2 h if stirring is extended to 2 h. Resulting sludge volu me is about 400 ml l(-1). However, during Cr precipitation, Al is leac hed from the added product up to about 40 mg Al per litre of supernata nt; (iii) precipitation using acetylene production sludge only require s a dosage of 16 g l(-1) (dry basis) to remove 99.96% Cr after stirrin g for 1 h and settling for 2 h. The soluble Cr concentration in the cl arified effluent is less than or equal to 2 mg l(-1) and the sludge vo lume about 500 ml l(-1). This Cr level can also be achieved at a dosag e of 14 g l(-1), provided the stirring time is increased to 3 h; (iv) these results are comparable with those using either CaO or MgO at sim ilar dosages (14 and 18 mg l(-1), respectively); and, (v) NaOH at dosa ges between 6.4 and 14 g l(-1) proved to be not sufficiently effective for Cr precipitation. Although removal efficiencies up to 99.9% are a chieved, residual Cr is always above 8.7 mg l(-1). Additionally, the r esulting sludge is not very dense, thus leading to high sludge volume production (about 800 ml l(-1) spent liquor). (C) 1998 Elsevier Scienc e Ltd. All rights reserved.