Alexander Watt

TREATMENT OF VARIOUS FIBRES.

Treatment of Straw.—Bentley and Jackson's Boiler.—Boiling the Straw.—Bertrams' Edge-runner.—M. A. C. Mellier's Process.—Manilla, Jute, etc.—Waste Paper.—Boiling Waste Paper.—Ryan's Process for Treating Waste Paper.

Treatment of Straw.—As a paper-making material, the employment of straw is of very early date, a patent for producing paper from straw having been taken out by Matthias Koops as far back as 1801. The material, however, was used in its unbleached state, and formed a very ugly paper. White paper was not obtained from straw until 1841, but no really practical method of treating this material was devised until about ten years later, in France, when MM. Coupier and Mellier introduced a process which, with subsequent modifications, has been extensively adopted. A great advance in the manufacture of paper from straw has since been effected by the introduction of various boilers, specially constructed for boiling the material at high pressures, and for keeping the alkaline liquors freely circulated amongst the fibre during the progress of the boiling. These boilers are of different forms—being either cylindrical or spherical—and are preferably of the revolving type, which causes the caustic ley employed in the boiling to become uniformly mixed with the fibre. Sometimes the vomiting boilers described elsewhere are used by paper-makers in preference to those referred to.

Bentley and Jackson's Boiler.—This boiler, a representation of which is shown in Fig. 18, is 7 feet in diameter, 18 feet long on the cylindrical surface, with hemispherical ends of Martin-Siemens steel plate 7/16 inch thick in the shell, and ½ inch thick in the ends. It is double riveted in the longitudinal seams, has two manholes 3 × 2, forged out of solid steel plate. Inside are two perforated lifting plates or shelves, each 1 foot wide, ¼ inch thick, the full length of the shell, and secured to the ends by strong angle-irons; it is supported on two turned cast-iron trunnions. These boilers are tested by hydraulic pressure to 120 lbs. per square inch.

The varieties of straw generally used for paper-making in this country are wheat and oats, though rye and barley straws are also used, but in a lesser degree. The treatment of straw differs greatly at different mills, some makers using strong liquors and boiling at a lower pressure, while others prefer to use less caustic soda and boil at a higher pressure. There can be little doubt, however, that the high temperatures resulting from boiling at very high steam pressure must deteriorate the fibre considerably, causing subsequent loss of fibre in the processes of washing and bleaching.

Boiling the Straw.—The straw is first cut into short lengths of one or two inches by means of a chaff-cutter, or by a machine similar to a rag-cutter, and the cut material is then driven by an air-blast through a wooden tube into a chamber having coarse wire-gauze sides: a second chamber surrounds this, in which the dust from the straw collects as it passes through the wire gauze. The winnowed straw, freed from dust and dirt, is then conveyed in sacks to the boilers. In charging the boilers, a certain quantity of ley is first introduced, and steam also, and the cut straw then added, which soon becomes softened, and sinks to the bottom of the boiler, when further quantities of the material are added, until the full charge has been given. The requisite proportion of ley and water is then run in and the head of the boiler secured in its place. Steam is now turned on, until a pressure of 20 to 40 lbs., or even more, has been reached, when the boiling is kept up for 3½ to 8 hours, according to the pressure used and the strength of the alkaline liquor, which varies from 9° to 16° Tw. From 10 to 20 lbs. of caustic soda per cwt. of straw are generally required to boil the material thoroughly. The boiling being complete, steam is turned off, and when the boiler has somewhat cooled, the material, which is in the form of a pulp, is discharged by the pipes beneath into a large tank or strainer, the bottom of which is fitted with a series of plates having long narrow openings or slits, through which the liquor drains. The pulp is then washed with water, and again allowed to drain thoroughly, after which it is dug out and transferred to the potcher to be again washed and bleached. At some mills the straw is boiled whole and not subjected to any preliminary cutting In such cases the boiled straw, not being so fully pulped as when cut into short lengths, is emptied from the boiler through the manholes used for charging the material into the boiler.

Bertrams' Edge-runner.—For the purpose of crushing the knots of the straw, and other hard particles derived from weeds, etc., a machine termed the "koller-gang" or "edge-runner" is sometimes employed. This machine, which is manufactured by Bertrams, Limited, and of which an illustration is given in Fig. 19, consists of two large millstones, made from hard red granite, the surfaces of which are sometimes grooved with V-shaped equidistant grooves. These stones are worked by a horizontal spindle, and are caused to revolve very rapidly in an iron basin, in which the washed pulp is placed, and by this means the knots and harder portions of the fibre not fully acted upon by the caustic alkali, become so reduced as to be more readily accessible to the action of the bleach, and thus a very superior straw pulp is produced. In using this machine in the way indicated, the washed pulp is mixed in a chest provided with agitators, with water, is then pumped into a second chest above it, from whence it flows into the basin shown in the engraving, while the stones are revolving.

M. A. C. Mellier's Process.—By this method the straw is first cut into small lengths as usual; it is then steeped for a few hours in hot water, and afterwards placed by preference in a jacketed boiler, the object being to heat the materials without weakening the ley by the direct introduction of steam into the body of the material. The boiler is to be heated to a pressure of 70 lbs. to the square inch, or to a temperature of about 310° F., by which means, it is said, a considerable saving of alkali is effected, as also time and fuel, as compared with the ordinary practice of boiling. The alkaline ley which M. Mellier prefers to use is from 2° to 3° B., or of the specific gravity of from 1·013 to 1·020, and in the proportion of about 70 gallons of such solution to each cwt. of straw. The boiler should revolve very slowly, making about 1 or 2 revolutions per minute. The boiling occupies about 3 hours, at the pressure named, when the steam is turned off and cold water passed through the jacket of the boiler, which assists in cooling the pulp, the water thus used being afterwards employed in washing the pulp. The pulp is then thoroughly washed until the last water runs off quite clear, when it is next steeped for about an hour in hot water acidulated with sulphuric acid, in the proportion of about 2 per cent. of the weight of the fibre. The pulp is then washed with cold water, when it is ready for bleaching in the usual way.

Manilla, Jute, etc.—Previous to boiling these fibres it is usual to cut them into short pieces by a machine such as is used for cutting straw, after which they are cleaned in a willowing and dusting machine. The boiling is then conducted in the same way as for esparto. Manilla fibre is not so much used in this country as in the United States, where its employment forms an important feature in the manufacture of certain kinds of paper. Some idea of the extent to which it is used by the paper-makers of America may be gleaned from the following statement of Mr. Wyatt:—"Another large and important branch of the American paper trade are the mills running on news and Manilla paper. Many of these mills turn out a vast quantity of paper, running up to two hundred tons per week, besides making their own ground wood pulp. The American news is composed mainly of ground wood pulp, with an admixture of about 15 to 25 per cent. of sulphite wood or jute fibre, and not much loading, and the machines are run at high speed. What is termed Manilla paper is very largely used in the States, and much more so than with us for common writings, envelopes, and wrapping papers. The paper is composed of Manilla, jute fibre, old papers, etc., and is highly finished at the machine. I was told of one mill belonging to a large company running altogether six mills on news and Manilla, turning out, with one 96-inch machine and beater capacity of 1,800 lbs., and one Jordan, 10 to 12 tons of 2,000 lbs., of Manilla paper per day at an average speed of 200 feet per minute."

Jute is seldom reduced to the condition of a fine white pulp since the treatment necessary to obtain that condition would result in a weak fibre; it is usual, therefore, to only partially reduce the material, when a strong fibre is obtained, which, lacking in whiteness, is used for coarse papers. This also applies to Adamsonia, or Baobab, another description of bast obtained from the West Coast of Africa. These fibres are chiefly used for papers which require strength rather than whiteness of colour, such as wrapping papers, &c.

"Broke" paper is a term applied to paper which has been imperfectly formed on the paper machine or damaged while passing over the drying cylinders. Imperfect sheets when they are not sold as retree, and clean waste paper, also come under this designation and are re-converted into pulp after undergoing the treatment described below.

Waste Paper.—In treating waste paper for conversion into pulp for paper-making, it is doubtless advisable to separate, as far as can be done economically, papers which have been written upon with common ink, as old letters, documents, &c., from printed papers, since the latter require a more severe treatment than the former. While simple boiling in water containing a little soda-ash will discharge ordinary writing ink, printer's ink can only be extracted by using rather strong solutions of soda-ash or caustic soda; and even with this treatment it can only be rendered serviceable for an inferior paper, owing to the grey colour of the resulting pulp, due to the carbon of the printer's ink, upon which the alkali has no solvent effect.

Boiling Waste Paper.—This is sometimes effected in iron vats, about 8 feet deep and 8 feet in diameter at the bottom, and about 6 inches wider at the top. At the bottom of each vat is a false bottom, closely perforated with small holes. Steam is introduced by a pipe below the false bottom, which passes through the perforations and thus becomes uniformly distributed to all parts of the vat. To facilitate the emptying of the vats, the false bottoms have connected to them three or four iron rods, to the tops of which iron chains are hooked, and by this means the false bottom, carrying the mass of boiled paper can be raised by a steam hoisting engine or crane and deposited where desired. When the boiling is commenced, the vat should first be about one-fourth filled with a solution of soda-ash, and the steam then turned on. When the liquor boils, the papers having been previously dusted, are introduced gradually, and well distributed through the liquor; if they are thrown into the vat in large quantities at a time, and especially if they are in a compact state, the portions in contact may not be reached by the liquor, and an imperfect boiling will be the result. To ensure a uniform distribution of the boiling liquor over the surface of the material, an iron pipe extends from the centre of the false bottom to nearly the top of the vat, and this pipe is covered with a hood, which causes the soda liquor to be evenly spread over the whole mass. The vats are either cased with wood or coated with asbestos to prevent the escape of heat, and the vessel is covered with a flat iron cover, which is generally in two halves. The steam enters the tubs at the side, below the false bottom, and the exhausted liquor is drawn off through a valve connected to the bottom of the vat. In some mills the liquor is not drawn off after each boiling, but the boiled paper is hoisted from the vat as before described, and the liquor strengthened by the addition of from 10 to 20 lbs. of soda-ash for each 100 lbs. of the paper to be next boiled. Paper that is thickly coated with printing ink requires an extra dose of soda-ash. The boiling is continued for twelve to twenty-four hours according to the nature and condition of the waste paper under treatment.

Waste papers are frequently boiled, after dusting, in revolving boilers, in a solution of soda-ash or caustic soda, but it not unfrequently happens that some portions of the material become so agglomerated or half pulped during the boiling that the alkali fails to reach all the ink, and as this cannot be removed by the after processes of washing and breaking, it remains in the body of the pulp and necessarily forms a constituent part of the paper to be produced from it. The mass, when discharged from the boiler and drained is then conveyed to the washing-engine, in which it becomes broken and freed from alkali and so much of the ink as may have been dissolved or loosened, and it is afterwards treated in the beater and mixed with varying portions of other paper stock, according to the quality of paper to be produced. In some mills the boiled waste paper is disintegrated after boiling, by means of the edge-runner (Fig. 19).

Ryan's Process for Treating Waste Paper.—The following process for treating waste paper so as to produce a "first-class clean paper" therefrom, was patented by Mr. J. T. Ryan, of Ohio. The waste paper is first passed through a duster in the usual way, all thick old books being previously torn apart to separate the leaves. The papers are then boiled in a hot alkaline liquor without pulping them, whereby the alkali acts on the surfaces of the papers, and dissolves off, carrying away all the ink into the liquor. The papers, which are still in sheet form, are then drained as free as convenient from the alkaline liquor, and are next washed in the washing-engine, which leaves the material perfectly clean. It is then pulped in the beating-engine; and it is claimed that it can be formed into first-class paper without the addition of any new or expensive paper stock. The details of the process are thus given by the patentee: "Into a bucking-keir put a soda-ash solution having a density of 5° B., at 160° F., put in the stock, and shower for eight hours at a temperature of 160° F., without pulping the paper, then lift and drain, and cleanse well in the washing-engine; then pulp and form into paper. As the draining will always be imperfect, each charge removed will carry away some of the soda-ash solution, and leave the remainder of impaired strength. After each drainage add water to make up for loss in quantity of the solution, and add enough soda-ash solution at a density of 13° B., to bring all the liquor up to 5° B. at 160° F. In about eighteen working days the liquor will have accumulated considerable ink and other matter. Then blow one half of the liquor, and restore the quantity for proper working. None of the soda-ash solution is wasted, except such as falls to drain and what is blown out as last mentioned." In carrying out this process every care must be taken to guard against pulping before the alkali is washed out.