Spent caustic streams (SCS) are produced during alkaline scrubbing of sulfide containing sour gases.
Conventional methods mainly involve considerable chemical dosing or energy expenditures entailing
high cost but limited benefits. Here we propose an electrochemical treatment approach involving anodic
sulfide oxidation preferentially to sulfur coupled to cathodic caustic recovery using a two-compartment
electrochemical system. Batch experiments showed sulfide removal efficiencies of 84 ± 4% with
concomitant 57 ± 4% efficient caustic production in the catholyte at a final concentration of 6.4 ± 0.1 wt%
NaOH (1.6 M) at an applied current density of 100 A m2. Subsequent long-term continuous experiments
showed that stable cell voltages (i.e. 2.7 ± 0.1 V) as well as constant sulfide removal efficiencies of 67 ± 5%
at a loading rate of 47 g(S) L1 h1 were achieved over a period of 77 days. Caustic was produced at
industrially relevant strengths for scrubbing (i.e. 5.1 ± 0.9 wt% NaOH) at current efficiencies of 96 ± 2%.
Current density between 0 and 200 A m2 and sulfide loading rates of 50e200 g(S) L1 d1 were tested.
The higher the current density the more oxidized the sulfur species produced and the higher the sulfide
oxidation. On the contrary, high loading rate resulted in a reduction of sulfide oxidation efficiency. The
results obtained in this study together with engineering calculations show that the proposed process
could represent a cost-effective approach for sodium and sulfur recovery from SCS.
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