Electrochemical recovery of sodium and sulfur species from spent caustic streams

Today, sulfide from raw fuel gasses is removed mostly via absorption in a sodium hydroxide stream in a wet scrubber. This creates a spent caustic stream (SCS), which is treated via various approaches. However, these treatments have some disadvantages such as a high operating cost, a high chemical usage or low robustness. Moreover, these treatments often convert sulfide to sulfate, so no recovery of other products from sulfide oxidation can be achieved. In this thesis, the possibility of recovery of sodium hydroxide and sulfur compounds from a SCS was investigated. A two-compartment electrochemical cell was used. An iridium tantalum mixed metal oxide coated titanium anode was placed in the anode compartment to oxidize sulfide to its sulfur oxidation products, such as elemental sulfur, sulfate, thiosulfate, sulfite and polysulfides. A cation exchange membrane was used in order to selectively transfer sodium ions to the cathode compartment in an attempt to create a concentrated sodium hydroxide stream with the hydroxide electrochemically produced at the stainless steel mesh cathode. An artificial SCS (4 wt% sodium hydroxide, 1 wt% sulfide-S) and deionized water were continuously fed to the anode and cathode compartment respectively. In a first part, a stable sulfide removal (68 ± 1 %) and cell voltage (2.74 ± 0.10 V) were observed during a two and a half months operation at 100 A m-2 and 46.9 ± 2.3 g S L-1 d-1. Sodium hydroxide was efficiently recovered in the catholyte (4.96 ± 0.36 wt%) with a coulombic efficiency of 96 ± 2 %. In a next step, the effect of an increasing sulfide loading rate was investigated. Higher sulfide loading rates (i.e. higher anolyte flowrate) resulted in lower sulfide removal efficiencies, but coulombic efficiencies for sulfide increased. Increasing the sulfide loading rate also resulted in less sulfate production. The effect of current density was investigated in a third set of experiments. Increasing the current density increased the sulfate and thiosulfate concentration and decreased polysulfides and elemental sulfur in the anode effluent. 200 A m-2 current density and sulfide loading rate of 40.2 g ± 3.9 S L-1 d-1 were found to be limiting for the system, as the cell voltage increased due to a built up of elemental sulfur on the anode. The highest sulfide removal efficiency (86 ± 3 %) was recorded at 150 A m-2 and 42.9 ± 5.2 g S L-1 d-1 . At 50 A m2and 45.3 ± 5.8 g S L-1 d-1, a high removal efficiency (73 ± 1 %) combined with high polysulfide and elemental sulfur production (2.7 ± 1.4 g S L-1) were observed. These operational conditions are recommended to harvest high value calcium polysulfide that can be used as a fungicide or for remediation of heavy metals polluted soils. Further research on the performance and stability of the electrochemical system with real SCS will help in determining the viability of the process as an alternative for treating SCS.