Bleach Plant Scale Control Best Practices to Minimize Barium Sulfate and Calcium Oxalate Scale, Down Time and Cost

摘要

Chlorine dioxide used as a delignification agent in the first stage (DO) of a bleach plant is a common practice in North America. Compared to a conventional chlorination stage, using chlorine dioxide at the DO stage requires a relatively high pH (2.5 - 4.5) to achieve maximum delignification and bleaching efficiency. These operating conditions often result in an increased risk of developing either barium sulphate and/or calcium oxalate scale, depending on the operating pH range. This can lead to significant production losses, extra maintenance costs, high bleaching chemical costs and quality issues. Through process modification, many mills are able to reduce or eliminate calcium oxalate scale formation by running the DO stage at a relatively low pH. These same mills incur higher costs though as a result of higher acid and caustic costs. For mills with higher barium levels, lowering the pH in the first chlorine dioxide (D0) stage will also increase the risk of barium sulphate scale, particularly if the mill uses spent acid from chlorine dioxide generation. For mills having limited water supply or using water with high hardness, calcium oxalate issues can be even more problematic when those mills operate at the higher end of the pH range (3.5 - 4.5). This paper will discuss how several mills have improved their bleach operation efficiency, reduced down time and decreased maintenance costs with a scale control program that manages both barium sulphate and calcium oxalate scale. The program provides additional benefit by allowing the D0 stage to operate at a slightly higher pH, in resulting in incremental of caustic and acid savings.