Factors contributing to calcium oxalate scale at the Simpson Tacoma Kraft mill.

Application: Mills with calcium oxalate scale can use the approach described in this paper to help solve their scale problem.

Simpson Tacoma Kraft's bleached kraft mill in Tacoma, Washington, USA, had a severe outbreak of calcium oxalate (Ca[C.sub.2][O.sub.4]) scale deposits on the DO stage bleach plant washer. At the peak of the problem, the bleach plant had to be shut down as often as once every three days to hydroblast scale from the washer.

The scale outbreak was stopped by reducing DO stage pH to 2.5 and partially opening the washing system by switching from Eop filtrate to hot water on the first wash stage of the DO washer. There was a need to better understand the cause of the scale outbreak in order to minimize higher acid, water, and energy costs associated with the solution.

A computer model was constructed based on extensive mill testing and used to predict calcium and oxalate concentrations for different washing strategies. We plotted data from the model on solubility curves specific to bleach plant filtrates. That indicated that the mill could safely return to the closed washing strategy it had formerly used. This resulted in lower costs due to reduced hot water usage (8.6 [m.sup.3]/air dried metric tons) and the associated energy required to produce it.

The data also confirmed the mill's experience that DO stage pH is a critical factor for controlling calcium oxalate scale in the bleach plant heat exchanger (in which about 8800 kW is recovered by using DO filtrate to heat fresh water).

Additional samples were taken after the mill returned to its original washing strategy (with Eop filtrate on the first stage of the DO washer) and the model's predictions were found to be very close to the measured values.

Reid is senior process engineer, Eka Chemicals, Marietta, Georgia, USA; Hinck is fiberline manager, Simpson Tacoma Kraft, Tacoma, Washington, USA. Email Reid at [e-mail] or Hinck at [e-mail]