11

C—3a.

of grooves reduces the screenings and disintegrates the wood more nearly to individual fibres. A stone grooved spirally to the axis produces, in general, a shorter-fibred pulp and less screenings than the straight cut. The third most important factor affecting quality is the pressure with which the wood is pressed on to the stone. For a given species of wood, quality of stone, and the character of its surface, a shorterfibred pulp and less screenings will be produced by a lower pressure. The temperature of grinding, which, is controlled by the amount of water used, is not as important as the foregoing factors. Pulp ground hot is freer and contains more long fibres. Practically it, is only necessary to use enough water to prevent burning of the wood and to maintain the desired consistency of pulp in the grinder-pit. Factors affecting quality and production are interrelated, and sometimes one must be sacrificed to the other. Production is increased by increasing the sharpness of the stone-surface, the peripheral speed, and the pressure. While an increase in the last two also requires an increase in the power-input, the power-consumption per ton will in general be lowered because of the increased production. Laboratory Semi-commercial Grinding Tests. Selection of Woods. In carrying out the experiments reported here, the effort was directed entirely toward producing a newsprint grade of pulp. However, departing from conventional practice, considerable attention was devoted to the use of hardwood as a possible source of mechanical pulp. After due consideration two species were chosen for study. Insignis pine was selected from the softwoods, as being most suitable from the standpoint of colour, relative freedom from pitch, and quantity available as pulpwood. Tawa, the hardwood, was likewise selected on the basis of its excellent colour and the fact that it has at present little value for lumber. Of the other species, rimu, although also a softwood of low pitch content, was not considered, because of its dark colour, while the same objection, together with high resin contents, eliminated the other species from the picture. The Grinder Equipment and Methods of Tests. The experimental wood-pulp grinder (Plate 3), while much smaller than a commercial grinder, is very similar in design in all respects. It was made by the Bayley Manufacturing Co., Milwaukee, Wisconsin. It has two pockets, the principal dimensions of which are as follows : Cylinder, 7-25 in. ; cylinder area, 41-25 sq. in. ; pocket dimensions, 8 in. by 11 in. ; pocket area, 88 sq. in. ; hydraulic pressure foot within pocket, 7-5 in. by 9-5 in. ; hydraulic-pressure foot within pocket area, 71 -3 sip in. The Lombard pulp-stone is 25 in. in diameter, with 11 in. face. The grinder is driven by a. Westinghouse 75 horse-power variable-speed motor, belted with alO in. leather belt. A. Bristol recording tachometer and a Westinghouse indicating watt-hour meter respectively indicate the speed and the power-consumption. The grinding pressure is controlled by a reducing-valve in the water-line to the cylinders. The pressure of the woods against the stone varies within certain limits for any given pressure on the cylinder. This is due to the constantly changing area of content, which in turn is partly influenced by the size and shape of the wood. In these experiments it is estimated that the average area of wood in contact with the stone was 74 sq. in. For insignis pine it was probably greater than this, as most of the billets were large and nearly covered the entire width of the pocket. The tawa was small round wood, and the entire width of the pocket was not always covered. The average area of contact for the tawa was undoubtedly less than 74 sq. in. The water-showers are used to cool the stone and wash off the pulp. One of these is located on the top of the stone between the pockets ; the other is low down at the back of the grinder. The lower one was found the more effective in keeping the stone cool and the pulp in the pit uniformly of the desired consistency. The upper one was used occasionally when excessive heat developed from such causes as the grinding of a knot or a wedged sliver. Only one pocket was used at a time, mainly because of the limited power available. As one pocket was grinding while the other was being filled, a uniform load was being applied throughout a run, and reliable measurements of power-consumption were easily obtained. . The peripheral speed of the stone was 3,000 ft. per minute for all experiments. Tests used to determine Quality of Pulf. As the wood was being ground, the pulp was run over a wet machine, where an average sample was taken. The sample was used for the determination of the moisture content and freeness; also for a microscopic slide from which photomicrographs were taken. The moisture analysis was used in determining the yield of pulp. The freeness was measured with a Green's slowness-tester, which was of the type recently standardized by the Forest Products Laboratory of Canada, and was obtained from the Star Brass Manufacturing Co., Boston. The pulp from the wet machine was slushed in a beater and made into a dry sheet of waterleaf pulp by running it over the Fourdrinier machines without calendering. Samples of the pulp were cut from the sheet for the determination of the weight per ream, thickness, bursting and tearing strengths, colour, and dirt-particles. The strength-testing instruments are described in Appendix XL The colour was measured by means of the Ives tintphotometer (Appendix IV). The dirt-particles were counted in 5 sq. in. of the pulp-sheet with the aid of a reading-glass.