Practical Data calculated from Experiments.

Recovery of Spent Liquor.—As it is possible to recover 75 to 80 per cent. of the soda originally used in digesting the esparto, the washing of the boiled grass is conducted on scientific principles in order to ensure a maximum recovery of soda at a minimum cost.

The recovery is effected by evaporating down the black liquor, together with the washing waters, to a thick syrupy mass, which can be burnt. The organic and resinous constituents of the esparto which have been dissolved out by the caustic soda, forming the soluble soda compounds, ignite readily, and during combustion the organic soda compounds are converted more or less completely into crude carbonate of soda.

It is obvious, then, that the cost of recovery depends mainly on the quantity of weak washing water which has to be evaporated. Consequently methods are devised by means of which the grass is thoroughly washed with as little water as possible, and some of the methods are very ingenious.

The spent liquors and washing waters are evaporated to a small bulk in a vacuum multiple effect apparatus, and the thick liquid mass obtained by evaporation is burnt either in a rotary furnace or on an ordinary hearth. Every precaution is taken to effect this operation with a minimum quantity of coal. The burning off of this mass results in the formation of a black substance which is taken away from the furnace and allowed to char or slowly burn until the impure white soda ash, or carbonate of soda, is obtained.

Two systems of recovery are in general use, which deserve a brief notice:—

Direct Evaporation.—The liquors may be evaporated to a small bulk ready for incineration by treatment in long shallow pans or furnaces, the heat necessary for the process being obtained mainly from the combustion of the thick concentrated liquor. The most familiar type of this form of apparatus is the Porion evaporator.

Evaporation Table.

Showing the volume of liquor obtained by evaporating 1,000 gallons of weak black lye of density d to a higher density D.

Example:—1,000 gallons of weak liquor at a density of 7° Twaddell are reduced to a volume of 200 gallons having a density of 35° Twaddell, or to a volume of 140 gallons with a density of 50° Twaddell, by evaporation.

Preparation of Caustic Soda.—The crude soda ash recovered from previous boiling operations is dissolved in large lixiviating tanks and extracted with hot water. The clear solution obtained after all impurities have been allowed to settle is pumped up into the causticising tanks, where it is converted into caustic soda, the loss due to the amount of soda not recovered being made up by the addition of ordinary soda ash. The causticising pans are large circular iron vessels usually 9 feet diameter and 8 or 9 feet deep, into which a known volume of the recovered carbonate of soda solution is placed.

A weighed quantity of ordinary quicklime is then put into a perforated iron cage which is fixed inside the causticising pan at such a level that the whole of the lime is immersed in the solution. The liquor is kept in constant circulation by means of an agitator and heated to boiling point, with the result that the chemical reaction sets in, the carbonate of soda being converted into caustic soda and the lime being thrown out as chalk. When the operation is completed, the steam is turned off and the chalk allowed to settle. The clear liquor is carefully strained off and pumped up into store tanks from which the required quantities are drawn off into the digesters as circumstances demand.

Washing.—The grass which has been partially washed in the digester is dug out by the workmen and discharged through a manhole fitted on one side of the digester near the bottom. It is then conveyed in any convenient manner to the breaking engine, in which the grass is more completely washed. This important machine has already been described on page 53. The floor of the vessel slopes slightly upward towards the front of the roll and falls suddenly behind the roll, in order to promote a circulation of the contents of the engine round and round the vessel.

A definite weight of boiled grass is thrown into the engine together with a large quantity of fresh water. The circulation of the roll draws the mixture of pulp and water between the knives, breaking it up and at the same time discharging it behind the beater roll, and producing a continuous circulation of the mixture in the two sections of the vessels.

The dirty water is continuously removed from the vessel by means of a “drum-washer.” This is a large hollow drum, the outer surface of which consists of a fine wire cloth, the interior of the washer being fitted with specially curved scoops. The drum-washer is lowered until it is half immersed in the mixture of pulp and water, and as it rotates the dirty water finds its way through the wire cloth, being caught up by the internal scoops and discharged through a pipe to a drain outside the breaking engine. At the same time fresh water is run into the vessel at one end, and the continuous washing of the pulp thus effected.

Bleaching.—The clean boiled grass is bleached by means of a solution of chloride of lime.

There are several methods used for this purpose, each of which has special advantages of its own, though this is largely a question of local conditions:—

(A) The pulp can be bleached in the washing engine directly the grass has been sufficiently cleaned. In this case the flow of fresh water is stopped and as much water as possible removed by means of the drum-washer. The drum-washer is then raised out of the pulp and a known volume of bleaching powder solution corresponding to a definite weight of dry powder is added to the contents of the breaking engine. The amount used depends on the quantity of dry grass in the breaking engine, the usual proportion being 8 to 10 per cent. on the calculated air-dry weight of raw grass. As the stuff circulates round the engine the colour gradually changes from dark yellow to white.

The process is sometimes hastened by blowing a small quantity of steam into the mixture and thereby raising its temperature. Considerable care must be exercised in using heat, because pulp bleached quickly by this means is liable to lose colour at the later stages of manufacture.

When the pulp has been bleached to the required extent, the drum-washer is again lowered into contact with the bleached pulp, and the latter is thoroughly washed so as to be quite free from traces of bleach and other soluble impurities.

(B) Esparto is often bleached in a “Tower” bleaching engine which consists of a tall cylindrical vessel of 9 feet diameter, and 15 or 16 feet deep, at the bottom of which is fixed a small centrifugal pump.

The boiled grass together with sufficient water and clear bleaching powder solution is placed in the engine; the centrifugal pump draws the mixture from the bottom of the vessel and discharges it, by means of a large external pipe, direct into the top of the vessel, where, as it falls, it comes into contact with a circular baffle-plate, which distributes the pulp evenly over the surface of the mixture in the vessel. A continuous and rapid circulation is thus maintained, and the process is said to be very effective. The bleached pulp is subsequently washed free from any traces of bleach.

(C) Esparto is frequently bleached by the “steeping” process. In this case the pulp is washed in the breaking engine, mixed with the required quantity of bleach, and at once discharged through the outlet pipes of the engine into large brick tanks, where the bleach is allowed to act quietly upon the boiled grass. This method produces a pulp of good colour and is economical.

Whichever process of bleaching is adopted, it is necessary to remove all the by-products formed during the process, as these soluble by-products if left in the mixture produce a lowering of colour.

The presence of small traces of bleaching powder solution can be detected by the use of starch and potassium iodide test papers. If a handful of the pulp after bleaching, when squeezed out, does not turn the test paper violet or blue, then the absence of any free bleach is taken for granted. The slightest trace of bleach will turn such test papers blue or violet according to the amount present. This is the test usually applied by the men in charge of the bleaching operations.

Making Sheets of Esparto Pulp.—For convenience in handling, it is usual to work up the washed and bleached pulp into the form of moist sheets. This is effected on a machine known as a “presse-pâte,” an apparatus which closely resembles the wet end of a paper machine. It consists of a set of flat strainers or screens, a horizontal wire similar to the paper machine wire, provided with deckles, the usual couch rolls, and press rolls.

The production of esparto pulp by bacteriological fermentation is an idea of later date. According to the inventor, the grass is crushed mechanically by means of rollers and then immersed in sea water inoculated with special bacillus obtained from esparto, and gradually resolved into cellulose and soluble by-products by fermentation which is complete in about eleven days. The commercial value of this idea has not yet been demonstrated.

Esparto Pulp: Microscopical Features.

The pulp of esparto when examined under the microscope is easily recognised, first by the characteristic appearance of the long slender cylindrical-shaped fibres, and secondly by the numerous cells always present. These cells consist of cuticular vessels with serrated edges, and also of small pear-shaped seed hairs, the shape of which is a ready means of identifying esparto. An examination of the transverse section of the raw material indicates the source of these pear-shaped vessels.

Test for Esparto in Papers.—Paper containing esparto fibre may be tested by means of a weak solution of aniline sulphate. The suspected paper is gently heated in the test reagent, and if esparto is present the paper turns a rose-red or pink colour, the depth of colour being a measure of the amount of esparto. Most of the modern book papers are prepared from chemical wood pulp and esparto mixed in varying proportions, and while this test can be used as a means of detecting a small or a large proportion of esparto, a microscopical examination is required for a more accurate estimation.

The proportions used by the paper-maker depend upon the weighing out of the wood pulp and esparto more or less accurately, while the microscopical test is based upon the relative proportions as represented by the volume of fibres of each class on the glass slip placed under the microscope. Since the wood pulp consists of a number of broad flat ribbon-like fibres, and the esparto of small cylindrical fibres, considerable practice is necessary in making a proper analysis of the two constituents in paper.

The use of straw for the manufacture of paper was first brought prominently into notice about the year 1800 by Matthias Koops, who published a book printed on paper made from straw, but it was not until 1860 that this material was used in any large quantity.

The production of a white paper pulp from straw is carried out in a manner similar to that used in the case of esparto fibre, viz., by digestion with caustic soda under pressure and subsequent bleaching. As the straw contains considerable quantities of siliceous matter, the chemical treatment necessary to reduce the material to paper pulp is more severe, a stronger solution of caustic soda being used, and the process of digestion being carried out at a higher temperature.

For the best quality of straw cellulose, the material is cut up into small pieces by machines which resemble an ordinary chaff-cutter, and the knots taken out by a separating machine. In most cases, however, the whole straw is simply cut up into small lengths of about one to two inches long, and placed at once in the digester. When the straw is contaminated with foreign weeds, sand, husks, and similar substances, as is usually the case, it is carefully hand-picked by girls, who remove these impurities, which tend to produce particles of unbleached matter in the finished pulp. The expense of this preliminary cleaning process is more than compensated for by the enhanced value of the bleached straw pulp.

Digesting.—The cut straw is boiled in rotary cylindrical or spherical vessels, stationary upright boilers of the vomiting type being seldom employed because the circulation of the caustic soda liquor does not take place freely with straw packed in the latter.

As the material is very bulky, some of the liquor is first put into the boiler and the steam admitted while the straw is being thrown in. By this means the straw is softened and reduced in bulk, so that a larger quantity can be added before the digester is quite full. The full amount of caustic soda is then made up by further additions of liquor, and the contents of the digester heated by high-pressure steam for four to six hours.

The conditions of treatment are shown by the following trial:—

The caustic soda was added in the form of a liquor, having a volume of 2,012 gallons and a specific gravity of 1·055.

Washing.—The boiled straw is discharged into large tanks placed below the digester and washed with hot water, the smallest possible quantity being used consistent with complete washing in order to prevent the accumulation of large volumes of weak lye. The spent liquor and washing waters are drained off into store tanks and evaporated in a multiple effect apparatus by the same process as that used for esparto pulp. The last washings are usually run away because the percentage of soda in them is too small to pay for the cost of recovery.

The final washing of the straw pulp is completed by the use of a breaking engine or potcher. As straw pulp contains a large proportion of cellular matter which cannot be regarded as true fibres, there is always a danger of considerable loss in yield if the use of the breaking engine is extensively adopted, because the short cells escape through the meshes of the drum-washer. The washing is most economically effected in the tanks if a good yield of pulp is required.

Separating out Knots.—The broken pulp from the breaking engines is diluted with large quantities of water and pumped over sand traps in order to remove knots and weeds which have resisted the action of the caustic soda. These traps consist of long shallow trays, perhaps sixty to eighty yards long, one yard wide, and nine inches deep, containing boards which stretch from side to side, sloping at an angle, and nailed to the bottom of the trays. The dilute pulp flows through the trays, leaving the heavy particles, knots, and foreign matter behind the sloping boards, and finally passes over the strainers, which retain any large coarse pieces still remaining.

Making Sheets of Pulp.—The mixture from the strainers contains a large excess of water which has to be removed before the pulp can be bleached. For this purpose a wet press machine (see page 103) or a presse-pâte (see page 85) is employed, and the wet sheets of pulp are then ready for bleaching.

Bleaching.—The process by which the pulp is bleached is exactly similar to that used for treating esparto.

From 1870 to 1890 large quantities of straw were used for the manufacture of newspaper in conjunction with esparto and wood pulp, but the price of the material was gradually advanced so that it could not be used with advantage, especially as the production of wood pulp gave a material which was much cheaper, and which could be utilised at once without chemical treatment.

In the manufacture of newspaper the tendency during recent years has been to make the paper mill operations as mechanical as possible and to dispense with the preliminary operations which are essential for the manufacture of half-stuff, the chemical processes being left in the hands of the pulp manufacturers.

The manufacture of straw cellulose is now practically confined to Germany, but small quantities of the bleached straw cellulose are imported because the pulp imparts certain qualities to paper which improve it, notably in making cheap printing papers harder and more opaque.

Microscopical Features of Straw.

The paper pulp obtained from straw consists of a mixture of short fibres together with a large proportion of oval-shaped cells. The fibres are short and somewhat resemble esparto, but the presence of the smaller cells is a sure indication of the straw pulp. The fibres themselves closely resemble the fibres of esparto, but as a rule the latter are long slender fibres, while the straw fibre is very often bent and twisted or slightly kinked.

The only method of distinguishing between straw and esparto is by examination with the microscope. There is no chemical reagent known which will produce a colour reaction on a paper containing straw that will serve to distinguish it from a paper containing esparto. If such papers are gently heated in a weak solution of aniline sulphate a pink colour is slowly developed, the intensity of which is to some extent a measure of the amount of straw or esparto present.

Straw and esparto are usually described in text-books under one heading, partly because the fibres possess strong resemblances in physical and chemical constitution, and partly because the methods of manufacture are identical. At the same time the qualities of the two pulps are so different that they cannot be used indiscriminately, the one for the other. Straw cellulose cannot be utilised in the place of esparto, particularly for light bulky papers. Hence in magazine and book papers containing a fibre which gives a pink coloration with aniline sulphate it is fairly safe to assume that esparto pulp is present.

CHAPTER V

WOOD PULP AND WOOD PULP PAPERS

The Manufacture of Mechanical Wood Pulp.

Wood is converted into pulp suitable for the manufacture of paper by methods which produce two distinct varieties. The first is mechanical wood pulp, so called because it is made by a purely mechanical process. The second is termed chemical wood pulp from the fact that the material is submitted to chemical treatment.

Ground Wood and Cellulose.—The two varieties of pulp are sometimes distinguished by the use of the terms ground wood and cellulose. In the former case the description implies a product consisting of pulp obtained by grinding wood into a fibrous condition, while in the second the word suggests a purified chemical product freed from the resinous and non-fibrous constituents found in wood. This is, in fact, the essential difference, for mechanical wood pulp consists of fibres which have been torn away from wood by means of a grindstone; it differs but slightly in chemical composition from the original raw material and contains most of the complex substances natural to wood. Chemical wood pulp, on the other hand, consists of fibre isolated from wood in such a manner that the complex non-fibrous substances are more or less entirely removed. The difference between these two pulps is shown in the following approximate analysis of spruce wood, and of the pulp derived from it. The composition of the mechanical pulp is practically identical with that of the wood itself.

Composition of Spruce Wood, and of Chemical Wood Pulp (Spruce).

The use of mechanical wood pulp is generally confined to the manufacture of news, common printings and packing papers, cardboards, and boxboards. It possesses very little strength, quickly discolours when exposed to light and air, and gradually loses its fibrous character. The chemical wood pulp is a strong fibre, from which high-class papers can be manufactured, the colour and strength of which leave little to be desired.

Species of Wood.—The woods most commonly used for the manufacture of wood pulp belong to the order Coniferæ, or cone-bearing trees. In Europe the spruce and silver fir are the chief species, while in America spruce, balsam, pine, and fir are employed. The harder woods, such as hemlock, beech, larch and others, are not converted into pulp by the mechanical process.

Timber Operations.—The trees are cut down in the early part of winter by gangs of men specially trained to the work. The organisation of a lumber camp when the operations are of an extensive character is very complete and carefully arranged, every detail being attended to in order to get out the wood as cheaply and expeditiously as possible. The branches and small tops are removed from the trees when they are fallen, and the trunks cut into logs of 12, 14, or 16 feet in length, and afterwards piled up on the banks of the nearest river, or on the ice, ready for the breaking up of the winter.

As soon as the ice breaks up and the rivers become navigable the logs are floated down to their destination, in some cases hundreds of miles from the scene of operations. Where rivers are not available the timber is brought out by horses or bullocks, or by means of a light railway.

Log Cutting.—As the timber arrives at the mill it is carefully measured, both as to its diameter and length, in order that a record may be kept of the quantity used. Some of the logs are piled up in the storeyard for use in the winter, and the remainder converted into pulp day by day. The logs are first cut into short pieces about 2 feet long by means of a powerful circular saw, the arrangements for this work being devised so as to keep down the cost of labour as much as possible. All waste pieces are thrown aside to be utilised as fuel.

Barking.—The bark on the logs is removed in one or two ways. Much of it is knocked off during the transfer from the forest to the mill, but even then the wood requires to be cleaned. In Norway and Sweden the wood is treated in a tumbler or a barker, while in America and Canada the use of the tumbler is practically unknown.

The barker consists of a heavy iron disc fitted with knives, usually three in number, which project from the surface of the disc about half or three-quarters of an inch. The barker rotates in a vertical position, and the short pieces of wood are brought one by one into contact with the disc in such a manner that the bark is shaved off by the knives. The machine is provided with conveniences for pressing the wood against the disc and for turning the logs as they are barked.

The tumbler system is quite different. In this case the short pieces are thrown into a large circular drum with hot water, and the bark taken off by the friction of the pieces as the drum rotates. The loss of material is of course less in this process, but the wood is not cleaned quite so effectively.

Grinding.—The main feature of the grinding process is the attrition of the wood when held against the surface of a rapidly revolving grindstone, the fibres as they are rubbed off being instantly carried away from the stone by a current of water. A complete description of the machines used and the modifications of the process practised by manufacturers is impossible in this book, but the following points will be sufficient.

The machine consists of a large grindstone about 54 inches in diameter, and 27 inches thick. It rotates in a vertical or in a horizontal position at a high speed. The stone revolves inside a casing which is provided with a number of pockets, so called, into which the pieces of wood are thrown at regular intervals, as fast as the wood is ground by the friction of the stone.

A continual stream of water playing upon the surface of the stone washes away the pulp into a tank or pit below the machine.

The quality of the pulp may be varied by the conditions under which it is made. By limiting the proportion of water so that the wood remains in contact with the stone for a longer time the temperature of the mass in the pockets rises. Such hot ground pulp, as it is termed, is tough and strong.

When the fibres are washed away from the stone as fast as they are produced the temperature does not rise, and cold ground pulp is made, which is not characterised by the somewhat leathery feel of the pulp made at the higher temperature.

The surface of the stone plays an important part also. If the stone is smooth the wood is rubbed away slowly, but if the surface has been roughened and grooved by means of a special tool the fibres are torn away quickly. In the first case the pulp comes from the stone in a finely-ground state and in a uniform condition, while in the second the pulp is coarse and chippy.

The output of the machine is, however, much increased by the use of sharp stones and by the application of considerable pressure to the blocks of wood.

Screening.—The mixture of water and pulp leaving the grinder falls into a tank below the stone, all large chips being retained by means of a perforated plate. The finer pulp, still too coarse for use, is then pumped to the screens, which serve to remove all chippy and coarse fibres and produce a uniform material. The shaking sieve consists of a shallow tray, the bottom of which is a brass plate or series of plates perforated with small holes or slits. The pulp flows on to the tray, which is kept in a state of violent agitation, the fine pulp passing through the holes and the coarser pieces working down to the lower edge of the tray into a trough which carries them away. The flat screen is somewhat different in construction, but the principle of separation is the same. It consists of brass perforated plates forming the bottom of a shallow cast-iron tray, continually agitated by means of cams fixed to the under surface of the trays.

The centrifugal screen is a cage made of finely perforated brass sheeting which revolves at a very high rate of speed inside a circular cast-iron vessel. The pulp flows into the interior of the cage, the fine fibres being forced through the screen by the centrifugal action of the machine, and the coarse material is retained.

Wet Pressing.—The pulp leaving the screens is mixed with such a large quantity of water that it is necessary to concentrate it. This is effected by means of the wet press machine (Fig. 41). The pulp and water are pumped into a wooden box in which revolves a large hollow drum, the surface of this drum consisting of a fine wire cloth of about 60 or 70 mesh. The drum is not entirely immersed in the mixture, so that as it rotates the pulp forms a skin or thin sheet on the surface, and the water passes away through the wire into the interior of the hollow drum. The drum carries the thin sheet out of the box and above the level of the mixture until it comes into contact with an endless blanket or felt, which is pressed against that part of the drum not immersed in the liquid.

By this means the thin sheet is transferred to the felt and carried between squeezing rolls to the finishing rolls. The felt, carrying on its upper surface the thin sheet of pulp, passes between two rolls, usually 16 to 20 inches in diameter, the upper being made of wood and the lower one of cast iron. The pulp adheres to the upper drum and the felt passes round the lower drum back to the box containing the mixture of pulp and water; the thin sheet is continuously wound on the upper roll until a certain thickness is reached.

When this occurs the attendant removes the thick sheet by a dexterous movement of a sharp stick across the face of the roll. The wet pulp at this stage consists of 30 per cent. air-dry pulp and 70 per cent. of water.

Hydraulic Pressing.—The sheets taken from the wet press machine are folded into a convenient shape and piled up, coarse pieces of sacking being placed between the sheets. At stated intervals the piles are submitted to pressure in hydraulic presses in order to remove further quantities of water, which slowly drains away through the sacking. In this way a mass of pulp in the form of thick folded sheets containing 50 per cent. of dry wood pulp is produced.

The pieces of sacking are taken out and the sheets put up in bales of any required weight, usually 2 cwt. or 4 cwt.

The Manufacture of Chemical Wood Pulp.

Most vegetable fibres are converted into pulp by alkaline processes, that is by digesting the raw material with caustic soda and similar alkaline substances. Wood may be treated in two ways, one of which is the ordinary soda process, and the other an acid treatment requiring the use of sulphurous acid.

Preparation of the Wood.—The logs of wood are cut up and barked exactly as in the case of mechanical pulp. The short two-foot pieces are then cut up into small flakes about one inch square and half an inch thick by means of a machine known as a chipper. This is similar in construction to a barker, consisting of a heavy iron disc rotating at a high speed inside a stout cover. The disc revolves in a vertical position, and three projecting knives slice up the logs into flakes. For this purpose the disc is provided with three slots which radiate from the centre towards the circumference for about 12 inches. The knives can be adjusted so that they stand up through the slots and above the surface of the disc to any required distance.

In order to ensure uniformity in the size of the chips, the practice is frequently adopted of sifting the wood leaving the chipper. The sieve is a large skeleton drum, the outer surface of which is made of a coarse wire cloth capable of passing all pieces of the size mentioned. Larger chips and pieces are retained in the drum as it revolves in a horizontal position and only fall out on reaching the extreme end of the machine.

The Digesters.—The object of boiling the wood under pressure with chemicals is to dissociate the valuable fibrous portion of the plant from the resinous and non-fibrous portion. In this process the wood loses half its weight, the yield of pulp being about 50 per cent., and the remainder is dissolved out by the chemical solution. The conditions of treatment are extremely varied in character, the quality of the pulp produced varying in proportion.

The digesters are either spherical, cylindrical, or egg-shaped, being constructed to revolve at a slow rate of speed, or fixed permanently in an upright position. Spherical boilers are usually 9 or 10 feet in diameter, the cylindrical digesters being 40 or 50 feet high and 12 or 15 feet diameter, the larger ones being capable of taking 20 tons of wood for each operation.