Factors Affecting Mechanical Pulp Brightening When Peroxide Liquor is Recycled, 1992 Pulping Conference Proceedings

High peroxide concentration is required to bleach softwood mechanical pulps to brightnesses in the range of 80% Elrepho. The economics of the process dictate that recycling of the residual peroxide be considered. However, together with the residual peroxide, the recycled effluent will contain dissolved organic material, "aged" silicate and, in most cases. calcium extracted from the pulp.

In an investigation of the effects of these variables on brightening efficiency, bleaching experiments were conducted for 4 - 6 cycles with 60% of the effluent being recycled. Dissolved organic material did not consume peroxide in subsequent cycles. The "aged" silicate was as efficient as fresh silicate in stabilizing peroxide. Compounds formed by silicate inter-acting with calcium and magnesium caused precipitation of silicon residue on the fibers. This deposition increased as the cycle number increased. Results showed that calcium silicate adhered strongly to stainless steel while magnesium silicate did not.