Electrochemical treatment of industrial sulfidic spent caustic streams for sulfide removal and caustic recovery.

Alkaline spent caustic streams (SCS) produced in the petrochemical and chemical manufacturing industry, contain high concentrations of reactive sulfide (HS-) and caustic soda (NaOH). Common treatment methods entail high operational costs while not recovering the possible resources that SCS contain. Here we studied the electrochemical treatment of SCS from a chemical manufacturing industry in an electrolysis cell, aiming at anodic HS- removal and cathodic NaOH, devoid of sulfide, recovery. Using a synthetic SCS we first evaluated the HS- oxidation product distribution over time, as well as the HS- removal and the NaOH recovery, as a function of current density. In a second step, we investigated the operational aspects of such treatment for the industrial SCS, under 300 A m-2 fixed current density. In an electrolysis cell receiving 205 ± 60 g S L-1 d-1 HS- over 20 days of continuous operation, HS- was removed with a 38.0 ± 7.7 % removal and ∼80 % coulombic efficiency, with a concomitant recovery of a ∼12 wt.% NaOH solution. The low cell voltage obtained (1.75 ± 0.12 V), resulted in low energy requirements of 3.7 ± 0.6 kW h kg-1 S and 6.3 ± 0.4 kW h kg-1 NaOH and suggests techno-economic viability of this process.