Oxalic acid pretreatment for mechanical pulping greatly improves paper strength while maintaining scattering power and reducing shives and triglycerides

In this paper we introduce a new technology based on a mild chemical pretreatment process prior to mechanical pulping. Chips are treated with a dilute solution of oxalic acid (OA) for only 10 minute at 130°C, in a typical example. The properties of the pulp produced by this OA process are quite different from those obtained via conventional chemical pretreatments, providing a much more valuable pulp, and at higher yields. Refining results obtained at the Andritz Pilot Plant in Springfield, Ohio show an outstanding combination of strength improvements, energy savings, and shive and triglycerides reduction, with only a modest loss in brightness. Under suitable conditions the light scattering coefficient of the oxalic-treated treated chips was fully recovered without losing the above benefits. Maintaining light scattering coefficient with this treatment is of great interest, particularly to those mills producing lightweight coated paper (LWC). Refining results are presented here, along with an evaluation of the process economics for a LWC example case. Based on strength improvements (kraft replacement in the final furnish) and energy savings, the production cost savings are substantial. For the LWC example, at least $9 million/year net savings before license fees can be realized in an operation producing 372 t/d of OA mechanical pulp for 600 t/d of blended LWC furnish. Based on this strong economic potential, several mechanical pulp mills have expressed interest in conducting in-mill evaluations. Planning for mill trials is currently underway. INTRODUCTION Presently about 25% of the world's wood pulp is produced by mechanical pulping. This volume is expected to increase as raw materials become more difficult to obtain: With twice the yield of chemical pulping processes, mechanical pulping is a practical way to extend these resources. Mechanical pulping is electrical energy-intensive (a mill producing 300-ton pulp per day spends about $10 million in electricity cost per year) and yields paper with less strength compared to paper produced from chemical pulping processes. These disadvantages limit the use of mechanical pulps in many grades of paper. In many cases, chemical (kraft) pulp is blended with mechanical pulp to impart strength to the paper produced. However, chemical pulp is expensive and produces large amounts of both air and water pollutants in its processing. The new OA process modifies the internal bonding in the wood fiber composite structure, producing chips that behave significantly differently during the subsequent refining operation of mechanical pulping. The resulting pulp has substantially better fiber properties, giving much higher strength while at the same time providing energy savings in the refining process. Development History The OA process had its origins with the biopulping research [1-2] conducted at the University of Wisconsin Biotechnology Center and the USDA Forest Service Forest Products Laboratory (FPL), Madison, WI. That research clearly demonstrated the removal of calcium from wood chip cell walls during two-week fungal pretreatment. Addition of calcium (up to 2000 ppm on a dry weight basis) to the wood chips at the time of fungal inoculation completely inhibited biopulping efficacy (Akhtar et al., unpublished). Observed deposition of calcium oxalate on the surface and inside of the wood chips, and the lower pH of the fungus-treated chips, were suggestive that oxalic acid produced by the fungus could be involved in biopulping. Several follow-up experiments were conducted by Akhtar et al. wherein pretreatment of southern yellow pine chips with a dilute solution of oxalic acid prior to mechanical pulping resulted in substantial energy savings and fiber strength improvements. Process development work was conducted in the University of Wisconsin Department of Chemical Engineering, in collaboration with BioPulping International (BPI), Madison, WI, and with the assistance of FPL. A PCT application on the new technology was filed by the Wisconsin Alumni Research Foundation (WARF). Process development and optimization has continued to date, with process commercialization by OxaTech, Inc., a subsidiary of BPI. Figure 1. Representative OA process scheme and relationship to TMP line. OA Process Overview The OA process is incorporated into the mechanical pulping line in a manner similar to that of traditional chemical pretreatments. Figure 1 depicts the general components needed for the process and shows the relation to the existing components of a TMP line. Feed chips are intercepted after washing and screening. The process typically involves presteaming, impregnation, a reaction residence-time vessel, and deimpregnation. Yield loss is approximately 3−3.5% in the case of southern pine. Results of TMP refining testing at Andritz Southern yellow pine chips were treated with dilute OA solution for 10 minutes at 130°C. The treated chips were subsequently TMP-refined at Andritz to a series of freeness levels. Untreated chips from the same supply batch were refined analogously for comparison purposes. Energy requirements were determined and the pulp properties were analyzed. Table I presents the results for three-pass refining to 40 CSF. Table I. TMP refining results at 40 freeness. Untreated OA Process Treated