Bleaching is a whitening process that is used in the paper industry to produce paper with high brightness. Chemicals – such as chlorine gas (C), sodium hypochlorite (H), oxygen (O), hydrogen peroxide (X), ozone (Z) and chlorine dioxide (D) – are used in various combinations to produce pulp with the desired properties. Recent limitations on the use of chlorine (and hypochlorite) have resulted in more extensive use of chlorine dioxide as an alternate-bleaching chemical. Chlorine dioxide bleaching is referred to as elemental chlorine free (ECF) and is the process used in over 50% of US mills.
The process
The chlorine dioxide is mixed with the pulp and contacted in each D (chlorine dioxide) tower for about 1 hour per stage. The primary purpose of bleaching is to maximise removal of the lignin-binding agents in the pulp without degradation of carbohydrates, which weakens the final product (paper). The pH is maintained at relatively low levels (around 4) to foster rapid chemical reaction. Higher pH levels require more chlorine dioxide for equivalent bleaching action.
Following bleaching in each D tower, the reacted lignin is removed by dissolving the lignin under high pH conditions (typically over 11). Lower pH levels will not dissolve all of the lignin. This caustic extraction (E) stage is preceded by a washing step to reclaim the chlorine dioxide and to minimise carry over of the acidic chemicals. The amount of caustic needed is proportional to the amount of bleach added in the D stage.
Bleach plant sequences vary widely from mill to mill; however, the D-E-D-E stage sequence is quite common. At least two bleaching stages are generally required to produce the specified brightness, with additional stages for more specialised uses. Consistency (% solids) is typically maintained at a high level (up to 10%) to minimise water use. Temperature is controlled around 60°C and doses of 5% chlorine dioxide are common.
The measurements
pH is measured in the D tower to control the bleaching reaction conditions. pH is also measured in the E stages to control addition of the caustic used for extraction. ORP can be used in the washer between the D and E stages to verify that rinsing of the chlorine dioxide has been completed. Attempts to use ORP to control chlorine dioxide dosage in the D tower have been mostly unsuccessful due to the nonlinear character of ORP.
The instruments
pH measurements in the D tower 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 cleaning regularly and frequently, and they may not be able to withstand the strongly oxidising environment.
The TupH Model 398R pH and ORP sensors are well suited for this kind of application. The wide area junction provides resistance to coating, while the patented helical pathway (prevents process intrusion into the inner reference chamber. Construction materials of Tefzel and Kalrez are used to withstand the strongly oxidising environment.
The Models 54 pH/ORP and 3081 pH/ORP Fisher-Rosemount analysers are ideal instruments for monitoring and controlling pH and ORP in pulp and paper mills. All configuration and calibration can be conducted remotely using the Hart protocol and the Fisher-Rosemount AMS PC interface. The combination of the Model 398R with either the AC-powered Model 54 or the DC-powered Model 3081 provides ease of installation, ease of operation and the lowest maintenance possible.
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