Effects of recycling peroxide liquor on brightness of mechanical pulp

The high cost of bleaching softwood mechanical pulps to 80% brightness with peroxide can be partly offset by recycling the unreacted peroxide in the effluent. However, the situation is complicated by the presence of dissolved organic material, aged silicate, and calcium extracted from the pulp. This study examines the effects of these materials on brightening efficiency. Dissolved organic material did not consume peroxide in subsequent cycles. Aged silicate stabilized peroxide as efficiently as fresh silicate. Interaction of silicate with calcium and magnesium caused silicon residue to precipitate on the fibers, with deposition increasing with the number of cycles. Calcium silicate adhered strongly to stainless steel, while magnesium silicate did not.