Oxidative Extraction of Hardwood and Softwood Kraft Pulps with Sodium Carbonate-Sodium Hydroxide Mixtures or Oxidized White Liquor and Multi-Stage Bleaching, 1996 Pulping Conference Proceedings

V.R. Parthasarathy

Jaakko Poyry

Fluor Daniel

Mill and synthetically prepared alkalis were evaluated against sodium hydroxide for EO or EOP extraction of HW and SW pulps. The alkalis included sodium carbonate, mixtures of sodium carbonate and sodium hydroxide, kraft and soda white liquors, soda green liquor, and oxidized white liquors (mill and synthetic liquors). The rate of decomposition of hydrogen peroxide was used to screen them for their suitability to substitute caustic in OP, EP and EOP delignfication of hardwood and softwood kraft pulps. Decomposition of hydrogen peroxide was negligible in synthetically prepared alkalis except in kraft white liquor. It was significant in mill soda liquors but rapid in mill kraft liquor. In mill kraft liquor, sodium led the decomposition of hydrogen peroxide. In mill soda liquors, the transition metals had initiated the decomposition. Therefore, both mill and synthetic kraft liquors are unsuitable for use in EO and EOP extraction of pulps. Magnesium salt was more efficient than Na5 -DTPA in curtailing the decomposition of hydrogen peroxide in sulfide free liquors such as soda (SWL) and oxidized white liquor (OWL). Bleaching of sodium carbonate extracted pulps resulted in 88-87 %ISO brightness, if chlorine dioxide dosage at the final D-stages is increased by 30%. The pulp had a lower viscosity than sodium hydroxide extracted and bleached pulp. The EO and EOP extraction of kraft pulp with sodium carbonate established that it cannot be used alone and should be extended with sodium hydroxide. Mixtures of sodium hydroxide and sodium carbonate (50/50 or 60/40) and synthetically prepared OWL was found to be as good as NaOH for the EO and EOP extraction of pulps.