Valves used in this application must be designed for high corrosion and accurate control.
After digestion, end products that are required to be whitened are bleached using chlorine dioxide which preserves pulp strength and provides stable brightness. Modern mills typically have pulp stock enter the oxygen delignification stage before the medium consistency pulp heads to the conventional bleaching sequence. These can vary from four to six separate stages depending on the end-user’s requirements.
A standard mill would utilize a DEopDED sequence, or an alternating sequence of chlorine dioxide (D), alkaline extraction stages (E), oxygen (O), and peroxide (P) brightening stage. This alternating sequence of chlorine dioxide and alkaline helps to break down the increasingly smaller amounts of residual lignin. Valves used in this application are subjected to high corrosion due to the chlorine dioxide material, and accurate control is necessary to avoid overspending on the high cost chemicals.