Bleaching Applications

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.

pH measurements in the D and E stages have historically been among the most difficult applications in the pulp mill. 

The high pulp consistency of the process stream and harsh bleaching chemicals degrade the performance of most general purpose sensors. Those sensors tend to require regular cleaning and may not be able to withstand the strongly oxidizing environment.

The effective operation of a chlorine dioxide generator requires continuous coordination of multiple reagent addition rates and steam heating rates. 

Any deviation from design operating conditions can lead to side reactions (waste of reagents), poor product strength, and potentially dangerous scenarios. This is a complex task for even the most skilled operators, who must constantly manage the addition of multiple reagents and steam heating rates under dynamic reaction conditions.