Effect of Prolonged Beating.

The importance of a knowledge of the precise effects produced by the beating of pulp cannot be emphasised too much, and any contributions to the subject along the lines of special research will be welcomed by all students of cellulose.

Fig. 46.—Cotton Pulp beaten 8 hours.

Some experiments were conducted by the writer in 1906 with cotton rags, in order to determine the results obtained by beating the pulp for a prolonged period under exact and specific conditions.

Fig. 47.—Cotton Pulp beaten 37 hours.

The cotton rags, of good quality, were boiled with caustic soda in the usual way for six or seven hours, at a pressure of 15 to 20 lbs., washed and partially broken down in the rag breaker, and finally bleached, made into half-stuff, and then transferred to a Hollander beating engine.

The particular conditions specified for the beating operation were that the beaterman should manipulate the pulp according to his usual routine for the manufacture of the paper which he was accustomed to make from these rags. In this case the routine process meant beating for eight hours, by which time the pulp was ready for the paper machine. In the ordinary course the pulp would be discharged into the stuff chest, and converted into a strong, thin, bank paper.

During the prolonged beating the pulp became very soft and “greasy,” and when made up into sheets the paper as it dried exhibited remarkable differences in shrinkage, the dry sheets obtained from pulp beaten thirty-seven hours being much smaller than those obtained from pulp beaten only four or six hours. The actual shrinkage is shown in the following table:—

Hours. Area of Sheet.
Sq. mm.		 Loss of Area.
Sq. mm.		 Relative Areas.
Deckle 100		 Shrinkage
per cent.
026,384·0—100·0—
426,076·0308·098·91·1
625,520·1863·996·73·3
825,160·01,224·095·44·6
1024,794·81,589·293·96·1
1324,467·41,916·692·87·2
1524,215·22,168·891·88·2
1724,024·02,360·090·99·1
1923,616·22,767·889·610·4
2123,616·02,768·089·610·4
2323,535·72,848·389·310·7
2523,329·93,054·188·511·5
2722,920·53,463·586·913·1
2922,831·23,552·886·513·5
3122,492·93,891·185·314·7
3321,917·24,466·883·116·9
3521,226·15,157·980·519·5
3720,778·85,605·278·821·2

If these results are plotted in the form of a curve the relation between the period of beating and the shrinkage in area is clearly shown. For the first twenty hours the shrinkage is proportional to the period of beating, after which the curve assumes an irregular shape, showing a tendency for shrinkage to proceed at a faster rate.

Weight and Substance of the Paper.—The shrinkage of the paper after prolonged beating indicates a closer and denser sheet, so that for papers of equal thickness the weight per unit area was much greater in the case of the pulp beaten for the full period. The results obtained are very interesting, and the following summary for a few of the readings obtained will serve to show the alteration effected.

Hours.Weight of
20,000 sq. mm.
Grams.		Thickness of
Sheet.
mm.		Grams per
sq. metre.		Lbs. per ream
480 sheets,
20" × 30".
Class A
8-10 hrs.		1·875·18393·7538·23
Class B
19-21 hrs.		2·043·189102·1541·65
Class C
33-35 hrs.		2·203·189110·1544·93

Sizing and Glazing Effects.—The behaviour of the waterleaf paper after sizing and glazing gave some interesting results. In the first place, the effect of the altered density of the paper is strikingly shown by the amount of the size absorbed. Certain selected sheets were passed through a solution of ordinary gelatine in the usual way, and subsequently dried. The amount of gelatine absorbed differs in a remarkable degree, as shown in table.

Tensile Strength of the Paper.—It is interesting to note that the tensile strength of the waterleaf papers appears to remain fairly constant throughout the whole period of beating. But this uniformity is greatly altered by the operations of sizing and glazing.

Percentage of Air-dry Gelatine absorbed by the Waterleaf Sheets.

Hours. Percentage of Size absorbed. Mean.
1st Trial. 2nd Trial. 3rd Trial.
85·56·06·25·9
105·46·86·56·2
193·85·04·54·4
214·83·94·64·4
332·71·72·42·3
352·41·91·72·0

These results are rather remarkable. The prolonged beating does not seem to have affected the tensile strength of the waterleaf, and the practical loss of strength which actually occurs in the more completely finished paper does not manifest itself until after the sizing process. The importance of the gelatine as a factor in the ultimate strength is thus clearly and strikingly demonstrated.

Tests for Strength on Original Waterleaf Paper.

Hours. Mean result of Readings.
Lbs.		 Mean Strength of the Paper.
Lbs.
8a 14·112·1
b 10·1
10a 15·413·2
b 10·9
19a 16·514·0
b 11·4
21a 15·214·0
b 12·8
33a 13·412·4
b 11·4
35a 14·513·6
b 12·7

Tests for Strength on Papers, Sized only.

Hours. Mean result of Readings.
Lbs. 		Mean Strength of the Paper.
Lbs.
8a 22·720·0
b 17·3
10a 28·523·2
b 18·0
19a 22·521·0
b 19·5
21a 26·021·7
b 17·5
33a 15·015·0
b 15·0
35a 14·215·3
b 16·5

Tests for Strength on Paper Sized and Glazed.

Hours. Mean result of Readings.
Lbs.		 Mean Strength of the Paper.
Lbs.
8a 25·823·6
b 21·4
10a 28·423·6
b 18·9
19a 27·022·9
b 18·9
21a 24·922·7
b 20·6
33a 16·115·2
b 14·4
35a 17·516·2
b 15·0

It may also be noticed that the strength of the finished paper after twenty hours' beating, as in class B, is equal to that of the paper after nine hours' beating, as in class A. This is curious, especially in view of the fact that the percentage of gelatine in the papers of class B. is only 4·4 per cent. as against 6·0 per cent. in class A.

The relation of the percentage of gelatine to the period of beating thus becomes a matter of interest, and well worth investigation. The figures are suggestive of further experimental research along definite lines.

Fig. 48.—Plan and Sectional Elevation of a “Hollander.”

Developments in Beating Engines.—Since the introduction of the Hollander beating engine, about A.D. 1670, other types of beater almost too numerous to mention have been devised to supersede it, but the fact remains that the principle of the original Hollander and its general design are still adhered to in the engines used by paper-makers for high-class work.

The alterations and improvements which have taken place during the last fifty years relate chiefly to the modifications naturally arising from the introduction of fibres not requiring such drastic treatment as rags.

The machines now in use for reducing half-stuff to beaten pulp ready for the paper machine may be classified as follows:—

(1) Beaters of the Hollander type, in which the circulation of the pulp in the engine and the actual beating process are both effected by the beater roll.

(2) Beaters of the circulator type, in which the movement of the pulp is maintained by a special contrivance, and the beater roll used only for beating.

(3) Beaters of the stone roll type in which the roll and bedplate are either or both composed of stone, granite, or similar non-metallic substance.

(4) Refiners, containing conical shaped beater rolls working in a conical shell fitted with stationary knives.

Fig. 49.—Beating Engine with Four Beater Rolls.

The Hollander.—This beating engine in its simplest form consists of an oval shaped trough, divided into two channels by a “midfeather,” which does not, however, reach completely from one end to the other.

In one of the channels the bed of the trough slopes up slightly to the place where the “bedplate” is fixed. The bedplate consists of a number of stout metal bars or knives firmly fastened into an iron frame, which lies across this channel. The beater roll, a heavy cast-iron roll provided with projecting knives or blades arranged in clumps of three around the circumference, and supported on bearings at each side of the engine, revolves above the bedplate with the knives adjusted to any required distance from it, the raising or lowering of the beater roll for this purpose being effected by the use of adjustable bearings.

The bed of the trough behind the beater roll rises sharply up from the bedplate and then falls away suddenly, as shown in the diagram, forming the “backfall.”

When the engine is in operation the mixture of water and pulp is drawn between the knives and circulated round the trough. The material is disintegrated into fibres of the required condition, discharged over the backfall, and kept in a state of continual circulation, and the beating maintained until the stuff has been sufficiently treated.

The dimensions of the engine vary according to the capacity, which is usually expressed in terms of the amount of dry pulp the beater will hold, and the following figures may be taken as giving the average sizes:—

— 2 cwt. Engine. 5 cwt. Engine.
Length11 ft. 0 in.16 ft. 0 in.
Width5 ft. 6 in.8 ft. 0 in.
Depth (average) 2 ft. 3 in.2 ft. 9 in.
Diameter of roll3 ft. 6 in.3 ft. 6 in.

Sundry modifications in the form and arrangement of the beater have been tried from time to time. In 1869 Granville patented the substitution of a second beater roll in place of the stationary bedplate for the purpose of hastening the operation. Repeated attempts have been made to construct a beating engine with two or more rolls, but it is evident that such a device could hardly succeed, since it would be impossible to ensure proper adjustment of the rolls, and in that case one roll might be doing all the work.

The first machine of this type was patented in 1872 by Salt. Similar beaters were devised by Forbes in 1880, Macfarlane in 1886, Pickles in 1894, who proposed to use three rolls, and Partington in 1901. Hoffman describes a beating engine which was working in America containing four rolls, as shown in the diagram.

The Umpherston.—A notable modification of the Hollander, having an arrangement by which the two channels of the engines are placed under one another, and one which is largely used for fibres, is the Umpherston. Several engines differing in detail, but embodying the same principle, have been built in imitation of this one.

Fig. 50.—Umpherston Beater.

Bedplates of large working surface were first tried in England by Cooke and Hibbert, in 1878, but in practice it has been found that no serious deviations from the narrow type of plate are of much value. As a matter of fact it is held by some paper-makers that one or two knives would be sufficient if they could be relied on to keep true and in proper adjustment.

The Circulating Type of Beater.—The addition of some device for keeping the pulp in circulation apart from the action of the roll has received considerable attention. The early experiments in this direction with the Hollander led ultimately to the construction of the engine of the circulator type mentioned in class 2.

Fig. 51.—Section of Umpherston Beating Engine.

Thus, in 1872, Nugent patented a special paddle to be used in the Hollander, by which the pulp in the trough of the beater was impelled towards the roll. Many other plans were tried for this purpose, and details can be seen in the List of Patents (see page [192]).

The introduction of the beaters with special means of circulating the pulp was found to be of the greatest service in the treatment of stuff like esparto and wood pulp, since these materials did not require the drastic measures necessary with rag pulp. In 1890 several engines of this class were being adopted, amongst which may be mentioned Hemmer's, Reed's and Taylor's. The pulp discharged from the beater roll was drawn through an independent pipe or channel by means of an Archimedean screw, or a centrifugal pump.

Stone Beater Rolls.—The substitution of stone for metal in the roll and bedplate of the engine brings about some remarkable changes in the nature of the beaten stuff. The fibre is submitted to the action of rough surfaces rather than that due to the contact of sharp edges, with the result that the disintegration is much more rapid, and produces a “wet” working pulp suitable for imitation parchments and similar papers. The latest materials used for this purpose are basalt lava stone in Germany, and carborundum in America.

Fig. 52.—Nugent's Beating Engine with Paddles for Circulating the Pulp.

Care is necessary in the manipulation of these beaters to prevent fracture of the stone parts. In the Wagg Jordan engine this danger is materially reduced by the construction of the working parts.

Refiners.—In these engines the beater roll is a conical shaped drum carrying the knives, which revolve inside a conical shell completely lined with fixed knives. The fibres are thus cut up to the desired length, but before discharge from the engine they pass between two circular discs, one stationary and the other revolving in a vertical position. The effect of the discs is to tear or bruise the fibres rather than to cut them.

The refiner is best employed to clear or brush out the mass of pulp after a certain amount of preliminary treatment in the beater, as the refiner cannot produce the effects obtained by actual beating as in the Hollander.

Fig. 53.—A “Tower” Beating Engine with Centrifugal Pump for Circulating Pulp.

Power Consumption.—The long treatment required to thoroughly pulp a strong material demands a great amount of power. Engines differ considerably in their power consumption, and comparisons are frequently made in terms of the power required to beat a given weight of pulp. But this is not always a true criterion of efficient work. Some types of beater are suitable for producing certain results, and the mere substitution of a beater consuming less power is worse than useless unless it can be shown that the same effects are being obtained. The efficiency of the Hollander for the beating of rag pulp, in spite of the high power consumption, is a case in point.

Fig. 54.—Working Parts of a Modern Refining Engine.

With this factor properly considered, the power required for beating becomes an interesting study. Many detailed experiments have been published from time to time, the most recent being those described by Beadle.