Corrosion and wear of plates in the production of refiner mechanical pulp

Rideout, David Walter George (1984) Corrosion and wear of plates in the production of refiner mechanical pulp. Masters thesis, Memorial University of Newfoundland.

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Abstract

Disk refining has become an increasingly important process for the manufacture of mechanical pulp for the production of newsprint and other "groundwood" speciality papers. The history of the development of chip refiners beginning with their use in pulp property modifications in the early 1900's and their use for semichemical and mechanical pulping processes at present is briefly renewed. -- Disk refining, as a pulping process, has in spite of its promise to replace chemical pulp not been without its technical difficulties; the major ones being the high energy inputs required as compared to stone groundwood, variations in quality and inadequate plate life. Each of the variables associated with these difficulties is reviewed in an extensive literature review. -- Since refiner plate wear and refiner plate corrosion both involve loss of plate material and are critical factors in plate life, the review of literature was extended to cover these variables. Although literature on wear and corrosion of refiner plates is extensive, little was found directly pertaining to refiner plate life. Because of this deficiency of information in both these important areas, experimental work was conducted on both the wear and corrosion of Ni-hard refiner plates. -- The wear of refiner plates run on a Sprout Waldron 42-1B refiner at Price (Nfld.) Pulp and Paper Company Limited, Grand Falls, Newfoundland, was monitored using a systems analytical approach coupled with standard v metallurgical techniques. The refiner plates after use were found to have worn in sharply defined annular zones at randomly defined radii. In each zone the wear was asymmetrical, with one of the opposing plates wearing more than the other. The location of the wear zone and the disk experiencing wear (rotor or stator) were found to be random. -- At first, it was believed that this unusual wear pattern was the result of plate clashing and/or damage by foreign material. Metallurgical and electrochemical studies of the plates within the framework of systems analysis clearly showed that this was not the case, and that the metal removal patterns observed were indicative of that produced by a process not dissimilar to electrochemical machining. -- The corrosion characteristics of Ni-Hard plates were analyzed by potentiostatic polarization methods using an intact refiner plate as the working electrode. A potential was impressed upon this electrode causing it to corrode. The resultant current density as a function of applied potential was plotted giving a polarization curve for Ni-Hard. The procedure followed was based upon ASTM Standard Method G5, modified such that an intact refiner plate could be used. No attempt was made to duplicate the temperature and consistency conditions in the refining zone of a commercial refiner because of the experimental difficulties involved and the fact that these conditions are still not well understood. In spite of this limitation, the results gave some interesting insights into the effect that additives have in extending refiner plate life. Results showed that corrosion rates were lower when dissolved oxygen was high, bringing into question the belief that the role played by Na₂SO₃ in enhancing refiner plate life is via a mechanism of oxygen scavenging. -- The potential for extending refiner plate life by corrosion control techniques is discussed. These include: -- 1.. control of O₂ content, -- 2. alloying of plates, -- 3. anodic protection, -- 4. cathodic protection.

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