Chlorine Dioxide is introduced to the pulp in the D
0 tower where it reacts for approximately 1 hour. This fi rst stage chemical reaction removes most of the lignin binding agents in the pulp without destroying the carbohydrates, which would weaken the fi nal product. pH is maintained at a fairly low value (approx 2 - 4 pH) to insure a rapid chemical reaction. A higher pH level would consume more ClO
2 for a given degree of bleaching. Temperatures can vary between 45 to 85°C (113 to 185°F).
pH Measurement Challenges
pH measurement in the bleaching process stages have proven to be particularly challenging. The high pulp densities (up to 15%) and the harsh chemistries are beyond the capabilities of most pH sensors. The typical pH sensor tends to require cleaning regularly and frequently, and few sensor reference cells are able to withstand the strongly oxidizing environment. Often pH sensors are not installed directly into the process but
mounted downstream of extraction sample valves. This creates lag time between pH measurement and what is actually occurring in the process.