Improving Organic Removal in Brown Stock Washing Process, 2017PEERS

Pulp mills around the globe are often asked to place their focus on two divergent goals: (1) to maximize production rates, and (2) to improving both the overall efficiency of the mill and on reducing variable costs of manufacturing,  Increasing the production rates of the pulp mill places significant stress on the ability of the brown stock washing system to remove both organic and inorganic components from the pulp to keep the cost of bleaching chemicals at reasonable levels and to minimize the amount of soda makeup that the mill must purchase.  Simultaneous actions to reduce fresh water consumption to meet ever-tightening environmental regulations and contribute to lower costs have exacerbated this stress level on the washing systems as well.

For many years, mills have attempted to utilize silicone-based wash aids to increase the drainage on the brown stock washers to try to maximize overall efficiency of the washing process in order to improve soda losses, O2 Delignification efficiency and reduce bleaching costs.  Silicone-based products provided more drainage than the previous oil-based products, and continuous improvements in the silicone resins used in the manufacture of these products has kept improving their performance levels.  However, there are limitations to the amount of drainage that these products can provide at acceptable feed rates.  In addition, over-use of the product or incorrect application of the defoamer has been responsible for deposition throughout the pulping process, as well as on the pulp dryer or paper machine and the amount of siloxane present in the final pulp/paper had to be monitored routinely to prevent issues at the end customer.  Since improved drainage is not a guarantee of organic removal, additional methods must be utilized to maximize the benefits in both the O2 and bleaching sequences. 

This paper will briefly review the chemical reactions which occur in the O2 Delignification process, and how they are affected by various process conditions which change the organic content of the pulp entering the process.  It will then review a process-based approach to optimizing the cooking and washing areas of the mill, and review a new and unique chemical approach to optimize the brown stock washing process. Proper use of this combined process- and chemical-based approach has provided significant savings to multiple international mills, allowing them to maximize production and improve pulp quality while minimizing overall operating costs simultaneously, and a case study of this methodology will be provided. 

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