Alexander Watt

TREATMENT OF ESPARTO.

Preliminary Treatment.—Picking.—Willowing Esparto.—Boiling Esparto.—Sinclair's Esparto Boiler.—Roeckner's Boiler.—Mallary's Process.—Carbonell's Process.—Washing Boiled Esparto.—Young's Process.—Bleaching the Esparto.

Preliminary Treatment: Picking.—Esparto is imported in bales or trusses, tightly compressed by hydraulic presses, and bound with twisted bands of the same material, much in the same manner as hay, except that which comes from Tripoli, which is bound with iron bands. The bands being cut, the loosened material is then spread out upon tables, partly covered with iron, or galvanised-iron, netting, to allow earthy matter or sand to pass through to a receptacle beneath. Here it is carefully picked by women and girls, who remove all roots, other kinds of grass, weeds, and heather. The material thus cleansed from impurities is transferred to the boiling-room. This careful preliminary treatment has been found necessary, since pieces of root and other vegetable matters which may be present are liable to resist the action of the bleaching liquor to a greater extent than the grass itself, and therefore produce specks, or "sheave" as they are termed at the mill, in the manufactured paper.

At some mills, however, as at the Horton Kirby Mills of Messrs. Spalding and Hodge, at South Darenth, for example, the cleaning of esparto is admirably effected by means of a willow, or esparto-cleaner, constructed by Messrs. Masson, Scott, and Bertram, which entirely supersedes the system of hand-picking. Having recently visited the mill referred to, we were enabled, through the courtesy of Mr. Sydney Spalding, to witness the action of this willow, which appeared to perform its functions with perfect uniformity, and to clean the grass most effectually. The rationale of the operation of willowing esparto may be thus described:—

Willowing Esparto.—A bale of the grass is unbound at a short distance from the machine, and the grass, which is in the form of small bundles or sheaves, tied with bands of the same material, is thrown by a woman on to a table or platform placed by the side of the willow, and a second woman, standing near the hopper of the machine, takes the bundles, a few at a time, and drops them into the hopper. The machine being in motion, in a few moments the grass, freed from its bands and dirty matters, appears in a perfectly loose condition at the wider end of the drum, and passes upward along a travelling-table to a room above, in the floor of which are the man-holes of a series of esparto boilers. During the passage of the loosened fibre, women standing on steps or platforms at the sides of the travelling-table are enabled to examine the material, and to remove any objectionable matters that may be present. Beneath the drum of the machine is a pipe, through which the dust and dirty matters are drawn away by means of a fan.

Boiling Esparto.—In the boiling-room at the mill referred to is a series of vertical stationary boilers, each about twenty feet high, and capable of holding about three tons of grass. The man-holes of these boilers pass through the floor of a room above, being nearly level with it, into which the cleaned esparto is conveyed, as described, by the travelling-table of the willowing machine. In this room is a series of compartments in which the willowed esparto is stored until required for boiling, when it is fed into the boilers by means of two-pronged forks provided for the purpose. The boiler being partially charged with caustic ley at 14° Twad., the esparto is introduced, and steam also, by which the esparto becomes softened, and thus a larger quantity of the fibre can be charged into the vessel. When the full charge of ley and esparto have been introduced the head of the boiler is securely fixed by means of its bolts, and steam then turned on until a pressure of about 20 lbs. to the square inch has been reached, which pressure is kept up for about three hours, when the steam is shut off and the blow-off tap opened. When the steam is blown off, the spent liquor is run off, and hot water then run into the boiler, steam again turned on, and the boiling kept up for about twenty minutes to half an hour, at the end of which time the steam is shut off and the blow-pipe opened. As soon as the steam has blown off, the washing water is run off by the bottom pipe, and the grass allowed to drain as thoroughly as possible. A door at the lower end of the boiler is then opened, and the grass emptied into trucks and conveyed to the washing-engines.

Sinclair's Esparto Boiler.—Another form of boiler, known as Sinclair's boiler, of the vertical cylindrical type, is shown in Figs. 15 and 16. It is constructed on what is termed the "vomiting" principle, but without the central vomiting-pipes generally used, and is fitted with one or more vomiting-pipes close to the side, two diametrically opposite pipes being used by preference. Steam jet pipes, with upwardly-directed nozzles, are fitted into the vomiting-pipes at points a little above the bends, between the vertical and horizontal parts. The liquid or ley thrown up the vomiting-pipes by the action of the steam is delivered from the upper ends of the pipes over a diaphragm or plate fixed near the top of the boiler, and the liquid is retained at a certain depth on the diaphragm by a number of small tubes fixed in it, and the liquid becomes well heated by the steam before overflowing down the tubes, which tubes also serve to distribute it uniformly over the fibrous materials in the boiler. A casing is formed at the bottom of the boiler, and in some cases extended more or less up the sides, and is supplied with steam, which should be superheated, or of high pressure. With this arrangement the heat in the boiler is maintained without the excessive condensation of steam and consequent dilution and weakening of the liquors which occurs in ordinary boilers. Figs. 15 and 16 are horizontal and vertical sections of one form of this boiler. The boiler is made with a vertical cylindrical shell, 1; with a flat top, 2; and flat bottom, 3; and there is an inner or second bottom, 4; the space between it and the bottom, 3, being for steam to assist in heating the contents of the boiler. At a little distance above the inner bottom, 4, there is the usual perforated horizontal diaphragm, 5, down through which the liquid or ley drains from the fibre. Two diametrically opposite vertical vomiting-pipes, 6, are formed by the attachment of curved plates to the cylindrical shell, 1, and these vomiting-pipes, 6, have their upper ends above a horizontal diaphragm, 7, attached by stays to the boiler top, 2. This diaphragm is perforated, and short tubes, 8, are fixed in the perforations so as to project upwards, by which arrangement the liquid, rising up the vomiting-pipes, 6, lies on the diaphragm to the depth of the tubes, 8, and overflows down through them all equally, so as to be uniformly distributed over the materials in the boiler. Steam jet nozzles, 9, are fitted in the lower parts of the vomiting-pipes, being supplied with steam by pipes, 10, from one of which a branch, 11, supplies steam to the double bottom, 3, 4. The steam jets cause the liquid to be drawn from under the perforated diaphragm, 5, and thrown up the pipes, 6, whereby a constant circulation of the liquid through the fibre is maintained. The liquors are drawn off by the pipe, 15. In another form of boiler Mr. Sinclair employs vomit-pipes formed of thin steel plates riveted to opposite sides of the boiler, and the liquid which drains through the perforated double bottom is forced upward through the vomit-pipes to the perforated plates above, through which it distributes over the material in fine jets. The boiler is capable of holding from 2 to 3 tons of esparto, and under a pressure of from 40 to 50 lbs. the boiling occupies about two hours.

Roeckner's Boiler.—This boiler, of which an illustration of two in series is given in Fig. 17, has been extensively adopted by paper manufacturers. It will be noticed that the vomit-pipe A is placed outside the boiler, and the steam enters at the cock B, forcing the liquor up the vomit-pipe A and distributing it over the esparto. A pipe C is used for heating the liquor by means of waste steam at the commencement of the operation. The grass is fed into the boiler at the opening D. At E E are gauges for showing the height of the liquor in the boiler, F F F are pipes for the supply of steam, strong ley, and water, and the door G is for the discharge of the boiled grass. Each boiler is capable of holding 3 tons of esparto, and the boiling is completed in about two and a half hours, at a pressure of from 35 to 40 lbs. per square inch. It is said that the boiler effects a saving both in time and the amount of soda used.

Mallary's Process.—By this process the inventor says that he obtains the fibre in greater length, and gets rid of the gummy and resinous matters in a more economical way than by the present system. The materials used form a species of soap, with which and with the addition of water, the esparto is boiled. To carry out his process, he places in a boiler a suitable quantity of water, to which caustic soda, or a ley of the required strength to suit the nature of the fibre, is added; magnesite, or carbonate of magnesia, in the proportion of about 2 per cent. of the fibrous material, or a solution of sulphate of magnesia, is then added and mixed with the ley. He next adds "an improved saponaceous compound" to produce the required result, and when the boiling is completed, the stuff is treated as ordinary stock, to be applied for paper-making or other uses. The proportions are as follows:—2 gallons of petroleum or its products, 1 gallon of mustard oil, 10 to 15 lbs. of caustic soda, and 1 per cent. of boracic acid. These are placed in a copper and heated for 1 to 2 hours, until properly saponified. From 3 to 6 gallons of the "saponaceous compound" are added to the ley and magnesite, previously placed in the boiler with the fibre, and the boiling is kept up for the usual length of time, when the fibre will be found "beautifully soft, and the greater portion of the gum, silica, and resinous matters removed, or so softened as to be no hindrance to the perfect separation of the fibres, whilst the strength, silkiness, and softness are preserved in all their natural integrity." Considering that caustic soda ley "of the required strength" forms an essential part of this process, we should imagine that the auxiliaries mentioned would scarcely be necessary.

Carbonell's Process.—In this process, devised by M. Carbonell, of Paris, 200 lbs. of raw esparto are placed in a wooden vat furnished with a perforated steam-pipe, 20 lbs. of soda and 30 lbs. of quicklime being mixed with it: the vat is then supplied with cold water until the esparto is completely covered. Steam is then turned on, and the materials boiled for 4 hours. The spent liquor is then drained off, and the esparto submitted to hydraulic pressure. It is afterwards washed and broken in a rag engine, and in about 15 minutes is reduced to half-stuff. 20 lbs. of chloride of lime dissolved in water are then introduced, and the cylinder kept in motion as usual. In another vessel, lined with lead, 1¼ lb. of sulphuric acid is dissolved in 3 lbs. of water, and this gradually added to the pulp, which immediately assumes a reddish colour; but in the course of about three quarters of an hour it becomes perfectly white, when the pulp is ready for the paper-maker.

In the boiling of esparto, several important points have to be considered. The kind of esparto to be treated is the first consideration, since this grass differs materially in character in the different countries from which it is imported. Spanish esparto is considered the best for paper-making, as it is stronger in fibre and yields a whiter pulp than other varieties. Of the African espartos there are several varieties, which are known respectively as Oran, Tripoli, Sfax, Gabes, and Susa. Of these, the first-named (Algerian esparto) is held in highest estimation amongst paper-makers, since it more closely resembles Spanish esparto than the other varieties, though not so hard and stiff as the latter. These grasses usually have a length of about 10 to 12 inches. Tripoli esparto has an entirely different growth, being sometimes as long as 2½ or 3 feet, and proportionately stouter, and is also softer than Oran esparto, which is not so hard as the Spanish variety. Tripoli esparto does not yield a strong paper by itself, but in conjunction with Oran esparto gives more favourable results. Sfax and Gabes espartos have a closer resemblance to Oran than Tripoli, but are not so strong as Oran, being green and spongy, and not so dry as the latter variety. Susa esparto of good quality is said to equal Oran, but not to yield so high a percentage of fibre.

The next important consideration is to determine the percentage of caustic alkali which should be used per hundredweight of the particular variety of esparto to be treated, and we cannot do better than give the following proportions recommended by Mr. Dunbar.

Mr. Dunbar says that the above figures "insure a first-class boil, with the steam pressure of 25 lbs. and not exceeding 30 lbs., but are liable to alteration according to circumstances—such as the form of boilers, quality of the water for boiling purposes, and steam facilities, which ought at all times to be steady and uniform to get the absolute regularity required."

Respecting the strength of caustic ley used for boiling esparto, as indicated by Twaddell's hydrometer, this appears to range from 7° to 15°, some preferring to boil with stronger liquors than others. The time occupied in boiling also varies at different mills, and depends greatly upon the character of the boiler used. We are informed that a Sinclair boiler will turn out, on an average, three boils in twenty-four hours, including filling, boiling, discharging, &c., the boiling occupying about four hours for each batch of grass.

The boiling being completed, the liquor is run off into tanks, to be afterwards treated for the recovery of the soda, and the esparto is then subjected to a second boiling with water only for about 20 minutes. The liquor from the second boiling is sometimes thrown away, even when the soda from the first liquor is recovered; but a more economical method is to use this liquor, in lieu of water, strengthened with soda for a first boiling; or to mix it with the first liquors and evaporate the whole together. The second boiling being finished, the steam is turned off, and water then run in and steam again turned on for a short time, and the water then run off and the esparto allowed to drain thoroughly. The boiled grass is then discharged into trucks which convey it to the washing engines.

The liquor resulting from the boiling of esparto, which is of a dark brown colour, contains nearly all the soda originally used, but it also contains silicious, resinous, and other vegetable matters which it has dissolved out of the grass, the silica taking the form of silicate of soda. The esparto liquor, which was formerly allowed to run to waste, polluting our rivers to a serious extent, is now treated by several ingenious methods for the recovery of the soda with considerable advantage alike to the manufacturer and the public. The process consists essentially in boiling down the liquor to dryness, and incinerating the residue. During the process of incineration the carbonaceous matter extracted from the grass is converted into carbonic acid, which, combining with the soda, reconverts it into carbonate of soda, which is afterwards causticised with lime in the usual way, and the caustic soda thus obtained is again used in the boiling of esparto. Although one or other of the "recovery" processes is adopted at a good many of our paper-mills, the recovery of the soda is by no means universal as yet, but the time will doubtless soon arrive when the economical advantages of the process will be fully recognised. Indeed, we know it to be the fact that some manufacturers are watching, with keen interest, the progress of some of the newer systems of soda recovery, with the full intention eventually of adopting one or other of them.

Washing Boiled Esparto.—This operation is usually performed in engines similar to those used in washing rags, but in some mills the boiled grass is washed in a series of tanks, so arranged that water flows in at one end of the series, thence passing in succession through each batch of grass in the other tanks, and finally issues at the farthest end of the series as a very concentrated liquor. By this arrangement there is great economy of water, while at the same time no loss of fibre occurs. The concentrated washing liquors thus obtained may be evaporated, and the alkali recovered, which would be an undoubted saving, since these liquors obtained in the ordinary way by washing in the boilers are generally run off as waste. The engines used for washing esparto and converting it into half-stuff are generally of large size, and capable of treating a ton of boiled esparto. In this engine, however, there is no bed-plate, as the action of the roll alone is sufficient to reduce the boiled and softened esparto to half-stuff. A drum-washer is also furnished to the engine, which carries off the dirty washing water, while an equivalent proportion of clean water is kept constantly running into the engine from an elbowed pipe at its end. In charging the washing-engine, it is first about three parts filled with water, when the washing cylinder is lowered, and the esparto is then put in, care being taken not to introduce more of the material than will work freely under the action of the roll; if the mass be too stiff, portions of the material may be imperfectly washed. While the washing is in progress, the workman, armed with a wooden paddle, constantly stirs the esparto, clearing it away from the sides of the engine, so that none of the material may escape a perfect washing. At the bottom of the engine is a "sand-trap," covered with perforated zinc, through which any sand or other solid particles which may be present escape. When the washing is complete, the fresh water supply is shut off, and the drum-washer allowed to run until enough water has been removed to make room for the bleaching liquor.

Young's Process.—By this process the boiled and strained esparto is passed through elastic covered rollers, so adjusted as to split up and squeeze out the dissolved matters or liquid from the fibres, thus leaving them clean and open for the access of the bleaching liquor.

Bleaching the Esparto.—It is usual to bleach esparto in the washing engine, for which purpose a tank of bleaching liquor of the required strength (about 6° T. for Spanish) is placed close to the engine, which is provided with a pipe leading to the engine and another pipe proceeding from the tank in which the bleaching liquor is stored. The supply tank is furnished inside with a gauge, divided into inches—each inch representing so many gallons of liquor—by means of which the workman is enabled to regulate the quantity of bleaching liquor he is instructed by the manager or foreman to introduce into the engine. About half an hour after the bleach has become well incorporated with the fibre, sulphuric acid in the proportion of six ounces of the acid (which must be well diluted with water) to each hundredweight of the fibre. The dilute acid should be added gradually, and the proportions given must not be exceeded. The bleaching being completed, the half-stuff is next treated in a machine termed the presse-pÂte, which not only cleanses the material from sand and dirt, but also separates all knots and other imperfections from the fibre in a most effectual and economical manner. Indeed, we were much struck with the excellent working of this machine at Messrs. Spalding and Hodge's mill, at South Darenth, and the remarkably fine quality of the finished pulp obtained through its agency. The presse-pÂte was formerly used in the preparation of pulp from straw, but its advantages in the treatment of esparto are now fully recognised. The apparatus and method of working it may be thus briefly described:—

The machine is on the principle of the wet end of a paper machine, and consists of several stone chests for holding the bleached half-stuff, in which are fitted agitators to keep the stuff in suitable condition. From these chests the stuff is pumped into a mixing box, and from thence over a series of sand traps made of wood, and with slips of wood fixed in the bottom, in which any sand present is retained. The stuff then passes into a series of strainers, which, while allowing the clean fibre to pass through, retain all impurities, such as knots, &c., and the clean stuff is allowed to flow on to the wire-cloth in such a quantity as to form a thick web of pulp. A greater portion of the water escapes through the wire-cloth, but a further portion is removed by the passage of the pulp across two vacuum boxes, connected with four powerful vacuum pumps, which renders the half-stuff sufficiently dry to handle; but to render it still more so, it now passes between couch rolls, and is either run into webs, or, as is sometimes the case, it is discharged into boxes, the web of pulp thus treated being about an inch in thickness.