Chromium precipitation from tanning spent liquors using industrial alkaline residues: A comparative study

Abstract

Chromium precipitation from spent tanning liquors using industrial alkaline tailings is described and removal efficiencies are compared with those obtained with traditional chemicals as NaOH, CaO and MgO: (i) using steelmaking dusts at dosages up to 70 gl-1 residual Cr remains 3.7 mgl- 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 gl-1 (dry basis) of chemical sludge from a water treatment plant results in a removal efficiency of 99.97% Cr and residual Cr2 mgl-1 under experimental conditions that include stirring at 100 rpm for 1 h and settling for 23 h. Sedimentation time may be reduced to 2 h if stirring is extended to 2 h. Resulting sludge volume is about 400 ml 1- 1. However, during Cr precipitation, A1 is leached from the added product up to about 40 mg A1 per litre of supernatant; (iii) precipitation using acetylene production sludge only requires a dosage of 16 gl-1 (dry basis) to remove 99.96% Cr after stirring for 1 h and settling for 2 h. The soluble Cr concentration in the clarified effluent is 2 mg 1-1 and the sludge volume about 500 ml 1-1. This Cr level can also be achieved at a dosage of 14 g 1-1, provided the stirring time is increased to 3 h; (iv) these results are comparable with those using either CaO or MgO at similar dosages (14 and 18 mg l-1, respectively); and, (v) NaOH at dosages between 6.4 and 14 gl-1 proved to be not sufficiently effective for Cr precipitation. Although removal efficiencies up to 99.9% are achieved, residual Cr is always above 8.7 mg 1-1. Additionally, the resulting sludge is not very dense, thus leading to high sludge volume production (about 800 ml 1-1 spent liquor).