Evolution of methyl mercaptan in its-assisted kraft pulping process

The pulp industry, one of the oldest and largest chemical industries globally, plays a critical role in biomass utilization. Increasing kraft pulp yield has long perplexed researchers and engineers. Given that methyl mercaptan (MeSH) can be self-produced during the kraft pulping process, revisiting MeSH-assisted yield enhancement technology offers great attraction for pulp industry. MeSH’s strong nucleophilicity enables its consumption through both demethylation and non-demethylation reactions. Investigating the MeSH’s evolution at typical pulping conditions is crucial for achieving higher recovery rate of MeSH. This study found that higher pulping temperatures, increased alkali charge, and larger Na₂S ratios improve the recyclable residual MeSH content in black liquors, increasing it from 17 to 36%. Under moderate pulping conditions, the residual MeSH content was approximately 30% of the original MeSH. Dimethyl sulfide (DMS), a valuable demethylation product of lignin or hemicellulose by MeSH, was found in black liquors at levels up to 15% of the original MeSH. Approximately half of MeSH participated in non-demethylation reactions and could not be recycled. X-ray photoelectron spectroscopy (XPS) analysis of separated kraft lignins suggested the generation pathway and chemical structure of new methylsulfur (MeS)-containing lignin. For economic balance and profitability, DMS recycling is proposed as cost compensation, or by adopting partial MeSH-assisted kraft pulping in batch digesting systems.