Factors Contributing to Calcium Oxalate Scale at the Simpson Tacoma Kraft Mill, 2001 Pulping Conference Proceedings

D. W. Reid, M. L. Hinck--The Simpson Tacoma bleached kraft pulp mill experienced severe calcium oxalate scaling in the D0 stage washer and bleach plant sewer heat exchanger. After removing the calcium oxalate, the mill implemented two process changes to contend with the calcium oxalate: pH control in the D0 stage and a partial re-opening of the counter- current washing strategy. A study was undertaken to determine the relative impact of washing strategy and D0 stage pH on calcium oxalate formation. Extensive testing was performed at the mill and used to tune a computer model of the bleach plant. Various operating strategies were simulated and the data were compared to published kraft bleaching filtrate solubility data. It was determined that strict countercurrent washing did not cause the calcium oxalate scale outbreak. D0 stage pH was found to have minimal impact on calcium oxalate precipitation in the washer but was shown to be a factor in the sewer heat exchanger where temperatures are lower. These results allowed the mill to run strict countercurrent washing with more confidence and save 8.6 m 3 /ADMT (150 l/min) of hot water in the bleach plant. The mill has also been able to shorten the list of potential causes of the scale outbreak and believes that good pH control is the key to sustainable operation.