E. J. Bevan

We have already, in Chapter V., discussed in a general way the principles upon which the treatment of the various fibres should be based; we now proceed to consider each fibre in detail, giving at the same time such information as is necessary regarding the preliminary treatment of the various fibres and describing the forms of apparatus in which these operations are conducted. We shall consider the fibres in the order of their simplicity of treatment.

Rags

(Linen and Cotton).—The treatment necessary for rags differs largely with the quality and, of course, with the kind of paper for which they are intended. These different qualities are known in the trade by different names and marks, such for example as the following:—New linen pieces, new cotton pieces, superfines, dark fines, grey linen, sailcloth, seconds, thirds, &c., &c.

The two former consist of the cuttings produced in the manufacture of various garments, &c.; not having been worn, they are usually free from dirt, and are, in fact, if bleached, nearly pure cellulose, containing only the starch and other sizing material which has been added in the process of finishing the goods. They may, of course, contain considerable quantities of china clay or other loading material. In purchasing rags, therefore, regard should be had to the probable presence of these bodies. Such rags require only a very slight treatment; in fact, for certain classes of paper they might be used without any preparation. If, as is sometimes the case, the rags are unbleached, a rather more severe boiling is necessary. {80}

FIG. 16.

The greater part of the rags used in paper-making, however, consist of the residual portions of garments, household linen, &c., which vary in quality from clean, almost unused portions, to the very foulest sorts; the latter require a very drastic treatment. The first thing to be done with the rags is to “sort” and cut them into convenient pieces. This is usually done by women, who stand at tables furnished with broad knives firmly fixed into them, with the backs towards the worker, and inclined at a slight angle. Near to the women are placed a number of boxes, corresponding with the number of qualities of rags, lined at the bottom with coarse wire gauze, into which the different sorts are put. The distinctions made are purely arbitrary, but as a general rule the rags are sorted with special reference to their colour and the material of which they are composed. The coloured rags may be allowed to accumulate, and then made into a coloured paper. During the process of cutting, all hard substances, {81} such as buttons, pieces of iron, &c., are carefully removed. The rags are cut into pieces of from two to five inches square. In some mills machines are used for cutting.

Fig. 16 shows the construction of a machine that may be used either for rags or rope. The material is passed into the machine along the table A, where it passes between the stationary knife C and the knives B, fixed in the revolving drum D. The cut rags fall into a receptacle placed underneath the drum.

Notwithstanding the extra expense of cutting by hand, it is nevertheless preferred by many, especially for the finer grades of paper. One reason for this is to be found in the fact that more perfect sorting and removal of impurities can be effected. It is moreover said that less waste of fibre occurs in the subsequent operations.

FIG. 17.

The next process which the rags undergo is that of removing all loose extraneous matter. This may be done in a machine such as is shown in Fig. 17. The rags are fed continuously by the endless travelling platform A into the willow. This consists of two wrought-iron drums, BB, furnished with wrought-iron teeth C, which when the drums revolve pass rapidly near to the stationary teeth fixed into the cast-iron framework of the willow. The sides are covered in with cast-iron doors, and the top is covered over {82} with sheet iron. Underneath the drums is a grating for the escape of dust.

The rags, having been thoroughly beaten by the teeth of the drums, pass into the duster D, consisting of a kind of hollow cylinder E, made of strong iron bars securely fastened to the circular ends F, the bars being covered with wire cloth or perforated zinc. It is made to revolve almost horizontally, a slight dip being given to it in order that the rags may be carried forward to the lower end. The bars or skeleton of the cylinder are furnished with a number of teeth securely bolted on. The whole is enclosed in a strong wooden box in which it revolves, and which serves to collect the dust passing through the wire cloth. The cylinder E is driven by the gearing G. This combined willow and duster may be used for dusting and cleaning not only rags, but almost any other kind of fibre. The willow and duster sometimes form two separate machines.

The cleaned and dusted rags are now ready to be boiled, although it is the practice in some mills to give them a preliminary washing with water.

The boiling may be conducted either in spherical or cylindrical boilers, or in the ordinary vomiting boilers described under Esparto. (See p. 92.) In Fig. 18 is shown, part in section and part in elevation, a spherical rag-boiler, as manufactured by Messrs. G. & W. Bertram.

This boiler, of from 8 to 9 feet diameter, is supported by means of the hollow journals A on the standards B, and is made to revolve by means of the gearing C.

Steam enters by the pipe D, which is fitted with a safety-valve E, and vacuum valve F. The steam on entering the boiler is distributed by means of the “baffle-plate” G. Lye enters by the pipe H. The boiler is fitted with two doors I, wherewith to fill and empty; the waste lye is run off by the cock J. L is a small blow-through cock. The false bottom K prevents the rags from choking up the cock and also serves to drain off the waste liquor.

FIG. 18.

The alkali employed may be either caustic lime, caustic {83} soda, sodium carbonate, or a mixture of the latter and lime, which is of course equivalent to using caustic soda. The proportion of alkali depends upon so many considerations that it is quite impossible to give exact information on this point. In the case of caustic soda, a quantity equal to from 5–10 per cent. on the rags may be taken to be a fair average. If lime be used it should be slaked with water, made into a thin milk and carefully filtered through fine wire cloth to keep back the particles of sand, coal, &c., which lime is always liable to contain. From 5–10 per cent. may be used. The amount of lime actually dissolved in the water is relatively small (1·3 grms. per litre); the portion in solution however, rapidly combines with the grease, dirt, and colouring matter of the rags and forms with them insoluble compounds, a fresh portion of lime being at the same time dissolved. This formation of insoluble compounds constitutes an important objection to the use of lime, as they are liable {84} to remain to some extent fixed in the rags and are with great difficulty removed by washing. For this reason, therefore, the more soluble alkali is to be preferred. Moreover, the lime sometimes tends to exert a hardening effect upon the cellulose. Notwithstanding these objections, lime is used by some paper-makers in preference to soda. In making choice of the chemical for boiling, much depends on the quality of the rags and the nature of the paper for which they are intended, so that no definite rules can be given.

The time of boiling varies from 2 to 6 hours, according to the quality of rags, the chemical employed, and the pressure. The use of very high pressures should be avoided as far as possible, as there is a danger, owing to the correspondingly high temperature, of fixing the dirt and colouring matters instead of dissolving them.

The quantity of water should be kept as low as may be, in order to have as strong a solution of alkali as possible; this effects a saving both in the time of boiling, and in the alkali. This is of great importance where it is necessary to evaporate the whole of the waste liquors. It should also be remembered that a certain amount of water is always formed by condensation in all boilers in which live steam is used. On the other hand, if too little water be added, the rags are liable to become “burned” and the fibre therefore weakened. During the operation the boiler should be made to revolve slowly, in order to produce thorough circulation of the liquor.

The boiling being completed, the pressure is allowed to fall, either by cooling or by blowing off from a cock usually provided for that purpose, and the liquor allowed to collect at the bottom of the boiler. It is then run off by the cock J (Fig. 18), and the rags drained as much as possible. Water is then run in to give the rags a preliminary washing. If time permits the steam may be turned in and the operation assisted. After again draining, the rags are withdrawn from the boiler into any suitable receptacle. A convenient form is that of a rectangular iron box on wheels, which can be readily transferred from one part of the mill to another. {85}

The next process is that of washing. This is usually performed in a washer or breaker, the construction of which is shown in Fig. 19.

FIG. 19.

It consists essentially of a rectangular vessel with rounded ends, in the centre of which, but not extending the whole {86} way, is a partition B, known as the “mid-feather.” The roll A, which is furnished with a number of steel knives G, and driven from the wheel H, revolves in one of the compartments formed by the mid-feather. In this compartment the floor is inclined in such a way as to bring the pulp well under the roll, as shown by the dotted line D. Immediately under the roll is what is called the “bed-plate,” the end of which is seen at I, extending up to the mid-feather, and fitted with knives similar to those in the roll A. The arrangement of the knives, both in the bed-plate and the roll, is similar to that given in Figs. 33 and 34. The distance between the roll and the bed-plate can be varied at will by means of the handle E, which is so arranged as to raise both ends of the roll simultaneously. In those breakers of an older pattern, one end only of the roll was raised, and thus the knives became worn unequally.

After passing between the roll and the bed-plate, the pulp flows down the “back-fall” D', and finds its way round to the other side of the mid-feather. On the inclined part of the floor, and immediately in front of the bed-plate I, a small depression is made, covered with an iron grating, for the purpose of catching buttons, small pieces of stone, and other heavy substances that may have found their way into the rags. There is generally a similar grating with rather finer openings on the other side of the mid-feather. The engine is constructed of iron, generally made in one casting.

The dirty water from the rags is removed by the “drum-washer” C. It is divided into compartments by the partitions shown by the dotted line c. The centre of the drum is formed of a conical tube, the narrow end of which is towards the mid-feather. The ends of the drum are generally made of mahogany, as this is found to stand the action of alkali better than any other wood. The periphery is covered with fine copper or brass wire-cloth, laid on to a backing of a much coarser material. An improved form of backing has been introduced lately, which is much more durable than wire. It is formed of brass cut into the form shown in {87} Fig. 20. The drum can be raised or lowered by the small wheel F, and it is driven by a belt on the shaft that bears the roll.

FIG. 20.

The wash-water passes through the wire cloth into the compartments formed by the partitions c, and finding its way down to the narrow end of the inner conical tube, flows out through the side of the drum into a trough which is placed across the washer to receive it. Or it may, as shown in the drawing, be conducted through the mid-feather itself, which is made hollow at this part for the purpose.

Another form of drum-washer, called the siphon-washer, is sometimes used. Its construction will be understood by reference to Figs. 21 and 22.

The drum is simply a hollow cylinder of wire cloth, the ends of which are formed of wood. Inside the cylinder is the siphon tube A, into which the water passing through the wire cloth flows. The continuation of the siphon tube B (Fig. 22) is made of flexible tubing. The action of the siphon is commenced by filling it with water through the cock C (Fig. 22). The water then flows in the direction indicated by the arrows. The tube A is fixed, and passes through a hollow journal fitted on to the end of the drum. {88} The other end is connected with the rod E, on which the driving pulley D is placed.

FIG. 21.

FIG. 22.

The water passing through the wire cloth on the drum carries with it a certain amount of fibre which, unless special precautions are taken (see “Save-all,” p. 154) is lost. The amount is considerable in the case of weak rags, and care should therefore be taken that the washing is not prolonged more than is consistent with proper cleansing, and that the pulp should not be too much broken up at this stage.

The washed and broken pulp goes by the name of “half-stuff.” {89}

The following is the usual plan of treating rags in the washer:—It is first half-filled with water, and the rags from the boiler put in gradually until nearly filled. Water is now allowed to flow in at the opposite end to where the drum-washer is placed. This, by the action of the roll, mixes thoroughly with the pulp and extracts all the soluble matter, and also carries with it fine insoluble impurities.

The action of the knives in the roll on the rags passing between it and the knives in the bed-plate serves to break them up and thoroughly disintegrate them. The dirty water then passes away through the drum-washer, the stream of pure water being regulated so as to keep the level constant. This is continued until only pure water passes away. The supply is then stopped, the washer still being kept in action. As the level falls the drum is lowered by means of the handle F. When sufficiently drained the pulp is discharged through a valve in the bottom of the engine. It is now ready to be bleached. This may be done in separate engines called “potchers,” somewhat resembling the breaker or washer already described, or it may be done in the breaker itself. The process of bleaching will be described in Chapter VII.

Occasionally, the bleaching process is conducted in the “beater” itself, but this is not to be recommended.

Esparto.

—This fibre, on account of its high percentage of non-cellulose constituents, requires a large amount of soda to resolve it; on the other hand, being of the nature of a pecto-cellulose, the process of disintegration may be conducted at a low pressure; in fact, it is the practice at some mills to boil in open vessels, in which case, however, a larger amount of soda is required. As has been already stated, it may be taken as a general rule for all fibres, that within certain limits the higher the pressure employed the less soda is required. The quantity necessary also varies with the district from which the grass comes. Caustic soda is the chemical invariably employed; lime, on account of its forming insoluble compounds with the {90} non-cellulose portion of the grass, is inapplicable. Not only does the amount of soda depend upon the pressure, but it also depends to a considerable extent on the form of boiler employed. The use of rotary boilers is objectionable, as the esparto tends to collect together into compact masses, which are with difficulty penetrated by the liquor. It is therefore the almost universal practice to employ stationary boilers.

The first treatment that the esparto undergoes is that of “picking.” The bands of the bales in which esparto is packed, generally by hydraulic pressure, are cut, and the grass is spread out on tables by women, who carefully remove such impurities as weeds, root-ends, &c., which from their nature are with great difficulty boiled and bleached, and which if not removed would be liable to appear in the finished paper as dark-coloured specks, technically known as “sheave.” This treatment is called “dry-picking,” in contradistinction to a subsequent process, known as “wet-picking.” A portion of the table is covered with coarse wire gauze, through which when the grass is spread over it loose particles of sand, dirt, &c., escape. The cleaning of esparto can be much better ac­com­plished by means of machinery. Fig. 23 is an elevation of a machine manufactured for the purpose by Messrs. Masson, Scott and Bertram. The grass, which may be in the form of sheaves, as taken from the bales, is put in by the hopper A. It passes thence to a conical {91} drum made of steel bars placed very close together, driven rapidly by the shaft B. It is provided with five rows of teeth. Fixed to the sides of the willow there is also a row of stationary teeth. The grass is thoroughly broken up and dusted by the action of the teeth, and is transferred to the wide end of the willow at C, where it is carried forward on the travelling tables D. During its passage along the tables it can be examined and picked by women standing on the platforms E. The dust and dirt passing through the steel bars of the drum are drawn away by a fan through the pipe F.

This machine is also adapted so as to carry the cleaned grass to the boiler-house. It is taken forward by means of travelling rakes over the tops of a series of boilers, any one of which can be furnished by simply opening a door corresponding in position with the lid of the boiler.

Boiling.—The ordinary form of boiler is shown in Fig. 24. It is known as a vomiting boiler. The grass is put in by the door E, which is hinged, and is counterbalanced by the weights L. It is securely fastened down by the screws F. Steam enters by the inner pipe A, which dips a little below the perforated false bottom B. Surrounding the steam-pipe is a wider pipe C, open at the top, which is made slightly trumpet-shaped; also open at the bottom, where it ends in a kind of shoulder, on which the false bottom rests. The lower part has two or more openings G cut away, through which the liquor can freely pass.

In order to get as large a charge into the boiler as possible, steam is turned on while furnishing, and a quantity of caustic soda lye is also run in, which has the effect of softening the grass and making it more compact. This is continued until the boiler is well filled.

The action of the boiler is as follows:—The steam passing through the pipe A heats the liquor that has drained from the grass through the perforations in the false bottom, and forcing it up the wide pipe C, causes it to strike against the dome or bonnet D, and distribute itself again over the grass. {92} This is technically known as “vomiting.” In this way a constant circulation of liquor is kept up.

The boiler is emptied by the door H and the liquor is run off by the cock I. The boilers are usually supplied with a safety valve E, and also with a pipe for letting off the steam when the boiling is finished. The boiling generally takes from four to six hours. The quantity of soda necessary depends upon the nature of the grass, Spanish requiring less than African. {93}

Routledge, to whom the introduction of esparto as a paper-making material is due, gives 10 per cent. as the proper quantity.

The pressures employed vary from 5 lb. to 45 lb. There has been a growing tendency of late to employ the higher pressures for reasons already stated.

Of late years great improvements in the form of the boiler have been introduced. Fig. 25 is an illustration of Roeckner’s Patent Boiler. The vomit pipe is outside the boiler. Steam entering by the cock D, forces the liquor up the vertical vomit pipe and distributes it over the grass. The pipe K is used for heating the liquor at the commencement of the operation by means of waste steam. F F are gauges for indicating the height of the liquor. The grass is put in by the opening G. The pipes A B and C are for the supply of {94} steam, strong lye and water. The boiled grass is discharged by the circular door E.

The boiler holds three tons of grass and the boiling is completed in about 2¹?/?₂ hours, the pressure being about 35–40 lb. per square inch. It is claimed that by its use a saving both of time and soda is effected.

Sinclair’s Patent Boiler is shown in Fig. 26. The vomit pipes, of which there are two, are made of thin steel plates {95} riveted to opposite sides of the boiler. The liquor drains through the perforated false bottom, and is then forced up the vomit pipes above the perforated plates through which it is distributed over the grass in a number of fine jets. The boiler is charged by the door e and emptied by j. The small cock a is used as a blow-through cock; the opening b is used for blowing-off steam when emptying the boiler; c for letting in caustic soda lye; d for water. Steam enters by the small branch of the T pipe at the bottom of the boiler, the other, g being used for running off the waste liquor. A boiler holds from two to three tons of grass, and a boiling is completed in about two hours at a pressure of 40–50 lb. per square inch.

The two forms of boiler above described possess the obvious advantage of having the whole or nearly the whole of the interior available for holding grass; the boilers can therefore be made to hold more, and the boiling is moreover much more evenly ac­com­plished.

Washing.—The boiling being completed, the steam is allowed to escape and the liquor to collect at the bottom of the boiler where it is run away by the pipes placed at the bottom for the purpose. Water is then run in and steam turned on for a short time; this is also run off, and the grass drained as completely as possible. The boiled grass is then emptied into trucks and taken away to the washing engines. These resemble those already described under rags.

During the process of washing, a certain amount of the shorter fibres find their way through the meshes of the wire-cloth. In addition to this a large proportion of the cellular tissue surrounding the fibro-vascular bundles (see Fig. 12, p. 57) is carried away. If the wash-water be examined under the microscope, it will be seen to consist largely of this cellular matter. Though this entails a certain loss of cellulose, its removal is in other respects advantageous, as it is possessed of hardly any “felting” properties, and it is moreover exceedingly difficult to bleach.

The amount of fibre actually obtained in practice is but little below that contained in the grass. A certain loss is {96} inevitable, but this probably does not exceed 1 or 2 per cent.

The percentage of cellulose in esparto is given on p. 57. The following numbers obtained by the authors are somewhat higher. They are calculated on the absolutely dry fibre; those mentioned are on the air-dry samples:—

There is no doubt that considerable differences occur even between different specimens of the same kind of grass.

It is the practice in some mills to wash the grass in a series of tanks connected together in the same way as the lixiviating tanks of an alkali works. They are so arranged that pure water flows in at one end, passes through fresh lots of grass in succession, and issues at the further end highly charged with the soluble products of the grass. By such an arrangement the grass can be washed without any loss of fibre, and with a minimum quantity of water. This latter feature is of great importance in mills where it is necessary to evaporate the whole of the waste liquors from the esparto, as they are then obtained in a very concentrated form. Even with such an arrangement it is advisable to give the grass a short final treatment in the washing engine.

The washing having been completed, and the esparto having been broken up into “half-stuff,” it is now ready to be bleached (see Chap. VII.).

The presse-pÂte system, originally adopted for the treatment of straw, has of late years been extensively applied to esparto.

The presse-pÂte consists of the wet end of a paper machine, and is furnished with sand-tables and strainers. The pulp is allowed to flow on to the wire cloth, so as to form a thick web of pulp. The bulk of the water passes away through the wire cloth; a further quantity is removed by the {97} vacuum-boxes and couch-rolls. The pulp, containing from 50 to 60 per cent. of water, is wound round an iron rod until a sufficiently large roll is formed.

The advantages of the presse-pÂte system are the possibility of the removal of dirt and unboiled portions by means of the sand-tables and strainers, and the very complete washing and removal of the products of the action of bleaching powder.

It also enables manufacturers to dispense with the somewhat costly methods of dry and wet picking.

The presse-pÂte system can also be applied to the unbleached pulp.

It may be interesting at this point to say something about the substances which are removed from the esparto by the caustic soda. On referring to p. 57 it will be seen that the original grass contains nearly half its weight of extractive matter, the removal of which has to be effected. Only a small proportion of this is extractive matter in the ordinary sense of the term, i.e. that can be extracted by the usual solvents; the remainder is intimately combined with the cellulose. The action of the caustic soda is to resolve these bodies, the cellulose remaining behind, and the other constituents being dissolved as resinous bodies by the soda. A certain proportion of the mineral constituents, notably the silica, dissolves in the lye, the latter forming silicate of soda.

On neutralizing the liquor with an acid, the bulk of the dissolved constituents is thrown down as a dark brown resinous mass. If this crude product be purified, it is found to consist of a definite body having the formula C₂₁H₂₄O₈. By the action of chlorine on this resin a bright yellow chlorine compound is formed, resembling the compound from jute described on p. 18. If the resin be treated with concentrated nitric acid, a bright yellow body is formed, which forms definite compounds with bases, and which has the property of dyeing animal fibres a bright orange colour. In addition to this body, a yellowish-white wax can be isolated. It sometimes happens that this wax is but imperfectly dissolved in the caustic lye; if the waste liquor be allowed {98} to stand, the wax is occasionally found to collect in small quantities on the surface.

Various attempts have been made from time to time to remove from the waste liquors the soluble matter derived from the esparto by the addition of lime. This throws it down as a very voluminous precipitate, exceeding difficult of filtration. The filtrate contains only caustic soda. The difficulties of removal of the precipitate, however, are almost insurmountable. The usual method of disposing of the liquor is to evaporate it to dryness and ignite it, as will be subsequently described under the head of “Soda Recovery,” Chapter XII.

The kinds of straw in general use are wheat, oat, rye, and barley; the two first constituting the bulk of the raw material, at least in this country. Some idea of the composition of straw may be gathered from the analyses given on p. 59.

It will be seen that the amount of cellulose is quite as high as in esparto, but for the reasons above given and from the fact that a large proportion of the cellulose consists of cellular tissue (see Figs. 12 and 13), which is easily attacked by soda and readily passes through the meshes of the {99} drum-washers and the wire-cloth of the presse-pÂte and paper machine, the yield obtained in practice falls considerably below that from esparto.

In addition to the numbers already quoted on p. 59, the following results of analyses made from time to time by the authors, may be interesting. The percentages are calculated on the absolutely dry material.

The yield of pulp is greatly influenced by the conditions under which it is obtained; high pressures and temperatures exerting considerable influence on the result. This is probably the case with straw more than any other fibre, on account of the physical and chemical nature of the cellulose. It is doubtful whether much more than 35 per cent. is actually obtained in practice.

Straw is usually boiled in cylindrical rotary boilers. Some paper-makers, however, prefer to use one or other of the different forms of vomiting boilers already described.

The use of a rotary boiler is open to certain objections. In the first place, it is doubtful if they are as economical of soda as the vomiting boilers, especially the more recent patterns; and, secondly, the rotation of the boiler so disintegrates the pulp, that a certain proportion of the cellulose, especially those fibres which are short, is liable to be lost during the subsequent treatment. It seems probable that the best results would be obtained if the rotation of the boiler were reduced to a minimum, in fact, sufficient only to produce thorough circulation of the liquor.

The proportion of soda necessary to boil straw thoroughly is, for reasons already stated, greater than is the case with esparto. The amount varies from 10 to 20 per cent. on the raw material. {100}

The different processes to which straw is subjected vary greatly. The following may be taken as a general indication of the methods employed. The straw is usually cut into short pieces of about 1–2 in. in length by means of an ordinary chaff-cutter. The cut straw is carried by means of a blast of air along a wooden tube or shaft into a chamber, the sides of which are made of coarse iron gauze. This chamber is itself inclosed in another chamber, in which the dust and dirt accompanying the straw collect. The clean straw is then placed in sacks and conveyed to the boiler-house.

Owing to the bulky nature of straw, it is difficult to get a large charge into a boiler at one operation; it is therefore usual, after having filled it as completely as possible, to run in a portion of the lye required, and to turn on the steam for a few minutes. This has the effect of so far softening the straw as to make it lie closely at the bottom of the boiler and to allow of a further quantity being put in. This having been ac­com­plished, the remainder of the lye required is run in, together with the requisite quantity of water, and the steam turned on. The pressure may vary from 10 to 50, or even 80 lbs. per sq. in., and the time from 4–8 hours.

When the boiler has sufficiently cooled, the charge is run out by a cock in the bottom. Owing to the rotary action of the boiler the straw is in the state of fine pulp, having been almost completely disintegrated; so fine, in fact, that it flows readily through a 3-in. pipe. It is run into large tanks with perforated tile bottoms, where the excess of liquor is allowed to drain away and the pulp washed by the addition of water. It is then dug out and taken to be bleached. Before this is done, however, it may be necessary to give it a further washing. This may be done in the potcher itself. Instead of washing the straw in tanks, it may be washed in an ordinary washing engine, such as has been already described. Owing to the finely divided state of the pulp, however, this method, unless the meshes of the wire-cloth covering the drum washer are very fine, entails {101} a considerable loss of fibre. It is more suitable in cases in which the straw has been boiled in stationary boilers, and in which therefore it is less disintegrated. When stationary boilers are employed, it is not necessary to cut the straw very fine; in fact, it is sometimes put into the boiler whole. The cutting has this advantage, that it loosens the adhering dirt. In cases where the action of the soda has not been carried far, or when the straw has been put into the boiler whole, the pulp will not be disintegrated to the same extent, and it would not flow through a narrow pipe. It is necessary therefore to discharge the boiler through the doors used for filling.

A novel form of washer especially adapted for straw, whereby the washing is effected with the minimum quantity of water, has been used on the Continent to a considerable extent, and to some extent in this country. The pulp is caused to pass along a series of revolving cones covered with wire-cloth, through which the liquor escapes. As it reaches the end of each cone, the pulp is emptied into a small tank containing water from the cones further on in the series. It is carried forward by means of hollow bent arms connected with the inside of the next cone, which then discharges it at the other end, to be again carried forward.

As already stated, the presse-pÂte system is largely adopted for straw pulp; it has been described under Esparto. It may sometimes be employed with advantage before bleaching, though it is generally used after.

A method of treating straw is sometimes adopted which produces a pulp of higher quality than is obtainable by the ordinary method. The washed pulp is mixed in a chest provided with stirrers, with a large quantity of water, and is then pumped into another chest placed at a higher level, from which it flows between a pair of hard granite millstones, the surfaces of which are cut into radial grooves. The stones are fixed horizontally, and are made to revolve at a very rapid rate. During the passage of the pulp through the stones, the knots, weeds, and other hard portions of the {102} straw which may have resisted the action of the alkali are reduced to a fine state of division, and are thus more readily acted upon in the subsequent operation of bleaching. The degree of fineness to which the pulp is reduced can be determined by regulating its flow and the distance between the stones. Pulp produced in this way is of a very even character, and is comparatively free from unbleached particles.

Within the last few years a process of treating straw has been introduced which, on account of the high quality of the pulp produced, has attracted considerable attention from paper-makers. It consists in exposing pulp obtained in the usual way by the action of caustic soda to the action of chlorine gas. The method was patented in England on March 3rd, 1880 (No. 938), by Friedrich Carl Glaser. The amount of soda in the preliminary boiling is reduced much below that necessary for perfect pulping, the result being that the cellulose is much less liable to be destroyed, and thus a greater yield is obtained. The pulp is thoroughly washed and partially freed from moisture in a centrifugal machine until only about 70 per cent. remains. The partially dried pulp is then exposed in leaden or stone chambers for some hours in an atmosphere of chlorine, produced by the action of hydrochloric acid upon manganese dioxide. By the action of the chlorine, those portions of the straw which have been but imperfectly acted upon by the caustic soda in the boiling process are completely disintegrated, or, rather, are rendered more susceptible to the action of the bleaching liquor employed subsequently. The consequence is that when bleaching liquor is added, the whole of the straw is rapidly reduced to the state of pure cellulose, and the finished product is remarkably free from anything like unbleached portions. Owing to the fact that the action of the caustic is minimised, a considerable proportion of the more easily degraded cellulose survives, and the yield of pulp is thereby increased. The process is, however, necessarily expensive. The action of the chlorine will be readily understood by a reference to p. 19. {103}

Owing to the fact that a considerable proportion of the pulp obtained from straw consists of cellular tissue (see Fig. 13, p. 58), which, although cellulose, is devoid of fibrous structure, paper made from straw only is found to be much weaker than that made from fibres such as esparto. On the other hand, being a cheap material, and one capable of yielding a very white pulp with a moderate quantity of bleach, it is held in considerable repute by many paper-makers, especially as a material for mixing with esparto. It is sometimes mixed with cotton and linen rags, even in the finest qualities of paper.

A cheaper method, and one which is largely adopted in mills where only a coarse dark-coloured paper is required, consists in boiling the jute or other fibre in lime. Such a treatment is, however, not so effective as one in which caustic soda is used, and the fibre produced is harder and coarser in every way. The yield of pulp, however, owing to the fact that the fibre is imperfectly resolved, is greater. In many cases, from questions of economy, lime may be advantageously employed.

Jute can only be employed successfully when it is imperfectly resolved, that is to say, when it is not reduced to the state of pure cellulose. In such a state it forms a comparatively weak fibre, owing to the fact that it is resolved into its individual fibres. If the filaments are not completely resolved, then a strong fibre is the result, but in this case, the pulp cannot be obtained in a pure white state. Owing to this fact, jute has had up to the present only a limited consumption, except for the manufacture of coarse papers. {104} Jute and Adansonia are largely used for papers where strength is of more importance than appearance, such, for example, as paper for telegram forms, strong wrapping paper, &c.

When an alkaline treatment is adopted, the alkali employed is, at least in this country, caustic soda. In Germany the sulphide of sodium processes have been very largely adopted. The alkali method is, however, being superseded by the more perfect and more economical method of treatment with sulphurous acid or acid sulphites.

The wood is first cut up into blocks, the size of which is determined by the width of the stones used for grinding; any knots present are cut out with an axe. The stones are made of sandstone, and are covered over three quadrants with an iron casing, the remaining quadrant being exposed. The surfaces of the stones are made rough by the pressure of a steel roll studded with points, and which is pressed against it while revolving. In addition to this, channels about ¹?/?₄ in. deep are cut into the stone at distances of 2–3 in. They are made in two sets, crossing each other in the centre of the stone, and serve to carry off the pulp to the sides of the stone, in addition to giving increased grinding-surface. The pressure of the blocks of wood against the stones is steadily maintained by screws worked by suitable gearing; this is necessary in order to obtain a pulp of uniform character. A stream of water is kept constantly playing on the stone; by this means, the pulp as fast as it is formed can be conveniently carried away. It is first passed {106} through a rake, which retains small pieces of wood that have escaped grinding. The stream of pulp then passes through the sorters, the object of which is to keep back such portions of the wood as have not been sufficiently disintegrated. These consist of cylinders about 3 ft. long and 2 ft. in diameter, covered with a coarse wire-cloth. The fibres that are retained by this wire fall into the refiners, which consist of a couple of horizontal cylinders of sandstone, the upper one only of which revolves. Here they are further {107} disintegrated, and are again passed through the wire-cloth; this is repeated until all the fibres have passed through. The pulp, after passing through the first sorter, may be conducted through a series of gradually increasing fineness, and, by this means, be separated into different qualities. Though pulp so prepared cannot compete with chemically-prepared stuff, as the fibres are extremely short, and have comparatively little felting-power, it may be used with advantage as a sort of filling-material.

Various modifications of the foregoing process have from time to time been proposed; among others, that of softening the wood by previous soaking in water, or steaming, seems to be valuable, as by so doing, it is highly probable that a longer fibre could be obtained, the soft wood being more readily torn away by the stones. Some inventors have proposed to replace the sandstone by an artificial stone containing a large quantity of emery.

An improved method of preparing mechanical wood pulp, lately patented by Mr. A. L. Thune, of Christiania, has been communicated to us by Mr. Carl Christensen.