A kraft pulp production line at Nippon Paper Iwakuni Mill, # 5 KP, was rebuilt in 1994 and has been operating well. The new line, originally designed as1, 200 daily metric tons for bleached hardwood pulp, has three bleaching stages, C-E⁄H-D, each with a Diffuser Washer to intend energy saving. Since the start-up we have focused on improving the washing efficiency and reducing water consumption.
Although the water consumption rate had been improved with increasing the production, we had difficulty in the bleaching efficiency and productivity due to scale trouble on the Diffuser Washer screens. A chemical washing to those screens was effective in removing calcium scale. Furthermore we found a method to prevent the scale by dosing caustic soda to washings on the secondary stage of the Diffuser Washer. (Pat. ) These approaches have led to improvement of bleaching efficiency and expansion of production ; 1450 mtpd in 1999, 1550 mtpd in 2002. As the result of increase in productivity, the water consumption rate in bleaching process has been significantly improved.