Chromium removal and recovery from tannery wastes. Laboratory investigation and field experience on a 10 m3/d demonstration plant

After extensive laboratory investigation of an ion exchange based process for selective removal, separation and recovery of Cr(III), AI(III) and Fe(III) from tannery wastes (spent chrome baths and leather washing waters), a 10 m3/d mobile pilot plant was assembled to demonstrate technical reliability and economic feasibility of the process. The IERECHROM (Ion Exchange REmoval of CHROMium) process is based on a weak electrolyte carboxyl resin, able to remove the metals from the liquid effluent followed by selective separation and recovery during a regeneration step. The resin is regenerated with alkaline hydrogen peroxide brines (0.15 M H2O2, 1M NaCl, O.1M NaOH, pH ≈ 11) through an internal oxidation of chromic species to chromate, whereas aluminium is co-eluted after hydrolysis as aluminate ion. Ferric species are not released by the resin in these conditions and are easily regenerated by subsequent acidic elution (1M H2SO4). Aluminate is thus separated from chromate ion in the alkaline spent regeneration eluate by pH adjustment to 8.5 and precipitation of aluminum hydroxide. In this paper the basic principles of the process are reported and the promising data obtained with a 10 m3/d mobile demonslration plant running at a tannery site in the Naples area.