Generation of sodium oxide and discharge of carbon by the electrolysis of multi-component molten salt systems: A recycle process for kraft pulping chemicals

The electrochemistry for a novel electrolytic recycle process for the recycling of used wood pulping chemicals, mixtures of sodium carbonate, sodium sulfide and sodium sulfate, into mixtures containing sodium oxide (a pre-cursor to sodium hydroxide) and sodium sulfide in the molten state (820 to 860°C) has been investigated and feasibility confirmed. In the mixtures, the reaction limits at the potential window are sulfate reduction to sulfide and oxide and carbonate oxidation to carbon dioxide and oxygen. Oxide and sulfide oxidation occur at potentials less positive than carbonate oxidation, therefore separation of the anolyte and catholyte are necessary. A sulfur and sodium balance is maintained on the melt during electrolysis experiments indicating sulfate oxidation to sulfur trioxide and oxygen does not occur. Literature, thermodynamic calculations of standard potentials of electrochemical reactions, cyclic voltammetry, gas analysis during electrolysis experiments and post-electrolysis analysis of dissolved inorganics confirm the electrolytic generation of sodium oxide and sulfide while simultaneously cleaning the molten salts by electrolytic production of carbon-containing gas.