Disc refiner

Rowell and Rowell (1989) acetylated Scandinavian spruce Picea abies) wood chips, then subsequently reduced these to fibres in a laboratory disc refiner, fibre production did not result in loss of acetyl content, but it was found that new water sorption sites were produced as a consequence of the refining process. In addition, these workers modified a variety of lignocellulosic materials and found that all of the materials studied exhibited the same reduction in EMC at comparable WPGs. 

Wood chips are preheated in steam before passage through disc refiners. Heating is meant for softening the lignin portion of wood and to promote fiber separation. This pulp is stronger than that produced by the ground-wood process. 

Wood chips are processed in mild chemical liquor and subjected to mechanical refining using disc refiners. Semichemical pulping liquors have variable composition ranging from sodium hydroxide alone, alkaline sulfite (sodium sulfite - - sodium carbonate), mixtures of sodium hydroxide and sodium carbonate, to Kraft green or white liquors [3]. Sodium sulfite/ sodium carbonate liquor is most commonly used and the pulp product obtained thereafter is referred to as neutral sulfite semichemical (NSSC) pulp. 

This process involves a mild chemical treatment of wood chips in sodium hydroxide or sodium bisulfite before or during steaming. Chemically treated chips are passed through mechanical disc refiners. 

Chemimechanical pulps usually are produced by soaking the chips in solutions of NaOH or Na2S03, and then refining in disc refiners to produce a groundwood-type pulp. Chemical consumption is very low, and yields are usually 85-95 percent. 

In order to enhance the bonding capability of the fibers, it is necessary to mechanically beat or refine them in equipment such as beaters, jordans, or disc refiners. This treatment of the pulp slurry at about 3-6 percent consistency is done by passing the pulp between the two rotating surfaces of the refiner. These surfaces contain metal bars and operate at very close clearances. As the fibers pass between the bars, they are made more flexible, and a larger surface for bonding is developed by the mechanical action. 

Figure 12.11. Pressurised disc refiner for MDF fibre (courtesy Sunds Defibrator, Sweden).

Figure 12.12. (a) Refiner plate, (b) Schematic of the pressurised disc refiner unit. 

Pulverization can reduce the size as well as the crystallinity of cellulosic materials and increase the surface area and bulk density. It is also possible to separate part of the hgnin from carbohydrates which makes it easier for microorganisms to digest cellulose. Various equipment, such as a compression mill, a bead mill, an extruder, a roll mill and disc refiners, etc., can be used for pulverization. Unfortunately these methods tend to be very expensive and too energy intensive. For sohd-state fermentation, if the particles are too fine, the oxygen mass transfer will become a big problem therefore, hghtly crushed or just ground raw material will suffice. 

Idling load or no-load power is defined as the power drawn by the refiner motor when the refiner plates are widest apart and the refiner motor was running at normal operating rpm with stock flowing through the refiner. It can be measured or calculated. For a double-disc refiner, the idling load can be calculated from Equation (13.3) (Dernier and Silver , 1995) ... 

The C-factor itself is a function of filling geometry, rotation speed, consistency, fibre length and fibre coarseness. For a disc refiner, the C factor is given by Equation (13.12). 

Retrofitting of the existing double-disc refiner used to refine unbleached long-fibre Pinus radiata kraft pulp to make flat sack kraft paper by an HC refining system followed by LC refining enabled the mill to manufacture premium-grade semi-extensible sack kraft papers... 

The paper machine in this exercise manufactures two-ply kraft liner board grade packaging paper using unbleached long-fibre kraft pulps. The refining operation of the stock approach system of the top ply comprised two 42 double-disc refiners... 

Lumiainen, J.L. (1995). Comparison of the mode of operations between conical and disc refiners. The 1st CALAR Symposium PTS IZP in Heidenau, Germany, December 1st, 1995. 

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