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

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 g l−1 residual Cr remains ≥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 results in a removal efficiency of 99.97% Cr and residual Cr≤2 mg l−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 l−1. However, during Cr precipitation, Al is leached from the added product up to about 40 mg Al per litre of supernatant; (iii) precipitation using acetylene production sludge only requires a dosage of 16 g l−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 l−1 and the sludge volume about 500 ml l−1. This Cr level can also be achieved at a dosage 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 similar dosages (14 and 18 mg l−1, respectively); and, (v) NaOH at dosages 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 achieved, residual Cr is always above 8.7 mg l−1. Additionally, the resulting sludge is not very dense, thus leading to high sludge volume production (about 800 ml l−1 spent liquor).