The manufacture of paper is a highly technical industry, which requires a practical knowledge of mechanical engineering, as well as an intimate acquaintance with the many important chemical problems connected with the art.

The following brief description of the various chemicals used in the manufacture of paper is divided into certain classes, based upon the order of the operations through which the raw material passes before its final conversion into paper:—

(1) The alkaline processes used for treating raw fibre: soda ash; caustic soda; lime; recovered ash.

(2) The conversion of wood into sulphite pulp: sulphur; limestone.

(3) The operation of bleaching: bleaching powder; antichlors; acids.

(4) The sizing and loading of paper: casein; gelatine; rosin size; alum; starch; silicate of soda; pigments and soluble dyes; mordants.

Mineral substances for loading: clay, blanc fixe, etc.

Carbonate of Soda.—This substance, also known under the trade names of alkali and soda ash, is used in the paper mill for the manufacture of caustic soda. It is purchased by the paper-maker from the chemical works, and used together with the recovered ash (see page 78) for the production of caustic soda solution, which is required in the treatment of raw fibres.

It is also used for the preparation of rosin size (see “Rosin Size”) and in softening hard waters for steam-raising purposes.

Sodium Carbonate Table.

Showing percentage by weight and pounds per 100 gallons in solutions of various densities.

Analysis.—The value of soda ash, carbonate of soda, and recovered ash depends on the amount of available alkali (Na₂O) present.

A weighed quantity (15·5 grammes conveniently) is dissolved in a measured volume of distilled water (500 c.c.), and titrated with standard normal hydrochloric acid, methyl orange indicator being used.

Caustic Soda.—Raw vegetable fibres may be reduced to the condition of paper pulp by treatment with caustic soda. In practice this process is largely resorted to for the manufacture of pulp from esparto, straw, and wood, the spent caustic soda being recovered and used again.

The paper-maker prepares the caustic required for digesting the raw material from recovered ash and carbonate of soda.

A convenient volume of clear liquor obtained by lixiviating the recovered ash is boiled with lime in suitable causticising pans, the reaction being represented as follows:—

According to this equation, 100 lbs. of soda ash require 53 lbs. of quicklime, but a slight excess is generally added, 58 or 60 lbs. being the usual amount actually employed. Several precautions should be observed in the process of causticising.

(1) The liquor from the recovered soda should be bright and clear, indicating complete incineration of the ash.

(2) The liquor is best causticised at a density between 1·050 and 1·100 (10-20, Twaddell). With stronger solutions the reaction is complicated and the yield of caustic soda reduced. Lunge has shown that if the density of the solution is 1·025 the proportion of soda causticised is 99·5 per cent., whereas at a density of 1·150 it is only 94·5 per cent. In the latter case the caustic soda formed acts upon the chalk produced and is reconverted into carbonate.

(3) The large quantities of chalk residue resulting from the reaction must be thoroughly and carefully washed. The economy of the whole process depends in no small measure upon this seemingly small detail.

Caustic Soda Tables.

Showing quantity of liquor obtained from 1 cwt. of caustic soda and the amount of caustic soda in 100 gallons of liquor (adapted from Lunge and others).

Dilution Table for Strong Liquors.

Showing number of gallons of water required to reduce the density of 100 gallons of liquor from a higher density, D, to a lower density, d. (See page 163).

Lime and Limestone.—Carbonate of soda and recovered ash are converted into caustic soda by means of lime. About sixty parts of lime are necessary for the conversion of 100 parts of carbonate of soda. Large quantities of insoluble carbonate of lime are produced in this operation, and great care is necessary to prevent a loss of caustic soda which occurs if the residue is not thoroughly washed. In some cases the residual chalk is drained by vacuum filters in order to remove all traces of soluble alkali. Processes have been devised for calcining the residue so as to convert the carbonate into caustic lime to be used over again, but no economical and practical method has yet been found. The treatment of the residual chalk with sulphuric acid for the production of calcium sulphate appears feasible, but the substance obtained is very impure, and therefore has little commercial value.

Limestone is required in considerable quantity for the preparation of sulphite of lime for the manufacture of wood pulp.

Recovered Ash.—The black liquor obtained during the process of the boiling of straw, esparto, and other paper-making fibres contains a large proportion of non-fibrous organic constituents derived from the fibres, the quantity of which may be gauged from the fact that these fibres generally lose 50 per cent. of their weight when being boiled. The black liquor on evaporation yields a thick resinous mass, which is converted into carbonate of soda when burnt.

Advantage is taken of this fact to carry out a process of incineration on a large scale, so that heat derived from the burning off of the resinous mass is utilised for evaporation of weaker liquors. The ash is drawn from special furnaces, put aside, and allowed to char quietly, so that the carbonaceous matter is more or less completely burnt away. The ash in this form contains about 40 per cent. of soda, its composition being determined by the nature of the fibre which has been treated. In the case of straw, the amount of silicate is considerable, as shown by the following typical analysis:—

At the present time there is no process in general use for the recovery of the liquors used in the treatment of wood by the sulphite process. Many schemes have been proposed, the most promising of which is that of Drewsen.

Sulphur and Sulphites.—The pale yellow brittle substance known as sulphur is too familiar to require any detailed description. It unites with oxygen in various proportions, and these in contact with water form the various sulphur acids known to commerce. Sulphur burned with a limited quantity of air forms sulphurous acid gas, and this substance is the chief product of oxidation, which by further treatment can be converted into sulphites.

In the manufacture of the sulphur compounds required in the preparation of wood pulp, the furnace for burning the sulphur consists of a flat-bottomed cast iron retort which is very shallow, and provided with a curved top, to which a pipe is fixed, so that the sulphurous acid may be conveyed away from the furnace. In the most recent form of sulphur oven a small conical-shaped revolving furnace is employed, which produces a satisfactory gas of constant composition very economically.

Bisulphite of Lime.—This compound is obtained when the sulphurous acid gas is brought into contact with moistened limestone. In the manufacture of bisulphite of lime on a large scale the sulphurous acid gas is drawn or pumped up tall circular towers filled with blocks of limestone, kept moistened by a carefully regulated stream of water flowing from the top of the tower.

In another system known as the acid tank process, the gas is forced into large circular vats containing milk of lime.

In either case a solution is prepared containing bisulphite of lime, together with a certain proportion of free sulphurous acid, the object of the pulp manufacturer being to obtain a solution containing as large a proportion of free sulphurous acid as possible. The composition of a solution will vary on this account, and the following may be quoted as being an example of such a liquor:—

For experimental purposes the bisulphite of lime solution may be prepared by passing sulphurous acid gas into a mixture of water and sulphite of lime. The latter compound is insoluble in water, but gradually dissolves when the gas is absorbed. A known weight of sulphite of lime is added to a measured volume of water, and the sulphurous acid gas discharged into the mixture from a siphon of compressed sulphurous acid. The amount of gas absorbed is determined by weighing the siphon before and after use, the loss of weight representing the gas discharged.

The following figures may be quoted as an example:—

The composition of the solution prepared is—

Analysis.—The examination of sulphite liquors for free and combined sulphurous acid is made by means of standard iodine solution and normal caustic soda solution.

A known volume of the sulphite liquor is first titrated with standard iodine solution, the number of cubic centimetres required being a measure of the total sulphurous acid.

Each cubic centimetre standard iodine solution = ·0032 grammes SO₂. The titrated liquor is then treated with standard caustic soda in quantity sufficient to exactly neutralise the acid. The volume of caustic soda solution used minus the number of cubic centimetres of iodine first added is a measure of the free sulphurous acid.

Bleaching Powder.—This substance is prepared on a large scale by allowing chlorine gas to act upon dry slaked lime. The lime absorbs nearly one-half its weight of chlorine and forms a dry white powder, having a very pungent odour. The best bleaching powder contains about 37 per cent. of what is termed “available chlorine.” The substance, on being treated with water, gives a greenish-coloured solution known as bleach liquor, and when raw paper-making material, after having been digested with caustic soda, is treated with this solution, it is gradually bleached to a white colour. The composition of the powder may be represented approximately as follows:—

Since the amount of bleach used for wood pulps varies from 8 per cent. to 25 per cent. of powder on the dry wood pulp, the cost of bleaching in some cases is considerable. The economy of the process depends in some measure upon the care exercised in the purchase of bleaching powder of standard quality, the storage of same in a dark, cool place, and the efficient treatment or exhaustion of the powder when the bleach liquor is prepared.

The powder is usually agitated for about an hour with water sufficient to produce a liquor of 13°-15° Twaddell. The undissolved powder is allowed to settle and the clear solution siphoned off, after which the sediment is washed once or twice to remove all the soluble matter completely.

Bleach Liquor Table.

Showing for bleaching powder solutions of known density the quantity of powder necessary to produce 100 gallons of liquor and the number of gallons obtained from 1 cwt. of powder (adapted from Lunge and Beichofen).

The best method for extracting powder is to agitate the material with water for a short period, and to stop the mixing process directly the maximum density has been obtained, which usually takes place in 15 minutes. Prolonged agitating prevents the powder from settling readily.

The maximum quantities of liquor which can be obtained from bleaching powder are shown on page 162. The following table is useful as showing the amount of water required for diluting strong liquors, the figures being applicable to any solution independent of the nature of the dissolved substance.

Dilution Table for Weak Liquors.

Showing number of gallons of water required to reduce the density of 100 gallons of liquor from a higher density, D, to a lower density, d. (See page 157.)

Antichlors.—The residues of chlorine which may be left in pulp after bleaching are frequently neutralised by the use of substances termed antichlors, which react with the calcium hypochlorite, converting it into chlorides.

The sodium hyposulphite is the most frequently used antichlor, the reaction between this and hypochlorite resulting in the formation of calcium sulphate and sodium chloride; 100 lbs. of commercial bleaching powder will require 30 lbs. of crystallised sodium hyposulphite.

The sulphites of soda and lime also act as antichlors, reducing the hypochlorite of calcium into sulphate of lime or soda. The chief advantage of the use of sulphites is to be found in the fact that the substances obtained by the reaction are neutral.

The best practice in bleaching is to avoid the necessity for using any forms of antichlors by careful regulation of the bleaching process. It has already been suggested in previous references to bleaching that the desired results are obtained when the pulp and bleach are left in contact with one another in tanks or drainers until the bleach is completely exhausted, the residual salts in solution being removed by thorough washing.

Gelatine.—For animal-sized or tub-sized papers gelatine is used. It can be prepared by the paper-maker from hide clippings, sheep skins, bone, etc., or can be purchased ready made.

Beadle gives the following interesting details as to the amount of gelatine which can be obtained from wet hide pieces:—

Weight of Wet Hide Pieces, 2,128 lbs.

Percentage of gelatine on weight of wet skins = 13·69.

A similar trial on the same class of wet hide pieces gave a yield of 13·23 per cent.

Two trials, of a somewhat different class of wet hide pieces, gave respectively 13·11 and 12·8 per cent.

The temperature of the draught water should be approximately as follows:—

In the final draught it is often necessary to use live steam at the finish, but this should be avoided if possible.

The water contained in wet hide pieces varies from 77 to 90 per cent. in the different pieces, but in the bulk the average may be taken at 85 per cent.

Casein.—Casein is the nitrogenous principle of milk, and belongs to the class of proteids which are definite compounds of oxygen, hydrogen, carbon, and nitrogen, forming the basis of the most important constituents of all animal fibres, albumen, casein, and gluten. A very pure form of casein is cheese made from skimmed milk. Casein belongs to that class of albumens which are soluble in water, e.g., egg albumen, blood albumen or serum, and lactalbumen, or milk albumen; these are mostly precipitated from solution by saturation with sodium chloride (common salt) or magnesium sulphate; but they are all coagulated by heat.

By the action of rennet on milk the proteid or albumen principle is converted into a curd (casein). This curd, when freed from fats, is insoluble in water, but is soluble in dilute acids, or alkalies, or alkaline carbonates, from which substances, however, it is reprecipitated by acidulation. Instead of the above method, casein may be precipitated from milk by saturation with sulphate of magnesia, and washing the precipitate with a solution of that salt until the washings contain no albumen, and then redissolving the prepared casein by adding water. The salt still adhering to the precipitate enables it to dissolve. On a large scale the casein is usually prepared by treating the milk with acid.

Casein is readily dissolved by alkalies and alkaline carbonates, borax, boracic acid solution, caustic soda, and bicarbonate of soda.

Starch.—This substance is used in many classes of paper for improving the surface and finish. It is added to the pulp in the beating engine in the dry form as powder, or in the form of starch paste, produced by boiling the starch in water.

The viscosity of the starch paste is somewhat increased by the addition of a small quantity of alkali, but due care must be exercised in boiling, which should only be carried out sufficiently to cause the starch granules to burst, as any excessive boiling causes the starch paste to lose some of its viscosity.

The presence of starch in paper is detected by the blue coloration produced when the paper is dipped into a weak solution of iodine. The determination of the exact percentage of starch in a paper is a matter of some difficulty.

Silicate of Soda.—The precipitation of gelatinous silica upon the pulp in the beating engine is generally regarded as favourable to the production of a sheet of paper having what is known as a harder finish. The precipitation is effected by adding a solution of silicate of soda to the beating engine, with the subsequent addition of sufficient sulphate of alumina to react with the silicate of soda.

Analysis of Commercial Alums.

(Griffin and Little.)

Table showing Value of Solutions of Aluminium Sulphate.

Alum.—Alum is one of the most important substances required in the manufacture of paper, its chief function relating to the sizing of paper. Various forms are utilised for this purpose, the purest being sulphate of alumina, required for high grade papers, and the cheaper form known as alum cake, for news and common printing.

The alum is manufactured on a large scale by heating china clay or bauxite with sulphuric acid. This reaction gives sulphate of alumina together with silica. If the mass is heated to dryness, it is sold under the name of alum cake. If the mass is extracted with hot water and the insoluble silica filtered off, the solution can be evaporated down for the production of sulphate of alumina, which is sold in the form of large cakes or in the form of crystals.

By careful selection of raw material a sulphate of alumina can be prepared almost entirely free from iron. The presence of the latter is undesirable, since on exposure to air the sulphate of iron produced during the manufacture of the alum is slowly oxidised and turns brown. Ultimately this affects the colour of the finished paper.

Alum is added to solutions of animal size or gelatine in order to thicken the solution and render it more viscous. It also acts as a preservative, and is used for regulating the absorption of the gelatine by the paper, the penetration effects being materially varied by the extent to which the alum is utilised.

In the process of engine sizing, a term applied to the application of rosin size on account of the fact that the process is completed in the beating engine, alum plays an important part. The mere addition of the prepared rosin soap to the mixture of pulp and water in the beating engine does not size the paper, but the alum precipitates the rosin from its solution, producing a complex mixture said to consist of resinate of alumina and free rosin particles, and subsequently the heat of the paper machine drying cylinders renders the paper more or less impermeable to moisture.

The appearance and tone of paper, more particularly of coloured papers, are brightened by the use of an excess of alum over and above that necessary to precipitate the rosin soap.

Rosin Size.—This substance is used chiefly for the sizing of news and cheap printing papers, and is also employed together with gelatine for the commoner writing papers. It is prepared by boiling rosin with carbonate of soda under various conditions.

Rosin, sometimes called colophony, is obtained from the sap of certain firs and pine trees. This on distillation yields spirits of turpentine, leaving behind as a residue the mixture of substances to which is given the name rosin. It behaves as an acid, and therefore will combine with certain alkaline oxides, producing soluble resinates.

The nature of the rosin soap used in the paper mill varies according to the conditions under which the size is prepared. If a large proportion of rosin is used, then the size obtained consists of a mixture of resinate of soda together with free rosin dissolved in the solution. If the proportion of rosin is small compared with the amount of carbonate of soda, the composition of the final mixture is quite different. The difference in treatment results in the formation of—

(A) Neutral Size, prepared by boiling a known weight of rosin with sufficient alkali to combine with it and form a neutral resinate of soda. Theoretically this may be obtained by using 630 parts of rosin to 100 parts of soda ash. It is doubtful how far the reaction is completed so as to produce an exactly neutral solution containing only resinate of soda.

(B) Acid Size.—When the proportion of rosin is largely increased the soda becomes converted into the alkaline resinate, and the excess of rosin is gradually dissolved in the resinate formed.

The practical operations necessary for the preparation of the size are comparatively simple. In the case of size containing relatively small percentages of free rosin, the boiling is conducted in open vessels, but for the manufacture of rosin size containing large proportions of free rosin boiling under pressure in closed vessels must be resorted to.

With the open pan process a steam jacketed pan is used, and the required quantity of alkali, dissolved in water, is placed therein and heated to boiling point. The rosin well powdered is added in small quantities from time to time, this being effected cautiously in order that the carbonic acid gas set free during the process may readily escape. The rosin is generally completely saponified after four or five hours' boiling. It is then passed through strainers into store tanks, from which it is drawn into the beating engines as required.

In the case of rosin boiled under pressure a cylindrical vessel provided with a manhole at the top is used. The correct amounts of alkali and water are put into the digester, and also the rosin in a powdered form, the digester being fitted with a perforated plate placed about two feet above the bottom of the vessel in order to prevent the rosin forming into a hard mass at the bottom of the digester.

It is possible in this way to manufacture a thick size containing 30 or 40 per cent. of free rosin and a comparatively small proportion of water. Many paper mill firms prefer to purchase such size ready made.

The most recent modification of the ordinary rosin size is a compound prepared by treating rosin with silicate of soda. This alkali dissolves rosin readily, and the soap obtained when suitably diluted with water decomposes in the beating engine on the addition of aluminium sulphate, with the precipitation of a gelatinous silica which assists in hardening the paper.

Bacon has patented a process in which powdered rosin is melted down with dry crystalline silicate of soda. The resultant product is ground to a fine powder, which is then ready for use. It dissolves easily in water, and when decomposed with the proper proportion of alum gives a gelatinous viscous mass said to have excellent sizing properties.

The advantages of a dry powdered rosin size readily soluble in water are obvious.

Loading.—The term “loading” is applied to the various substances which are employed for the purpose, as it is commonly supposed, of making paper heavy. But china clay and similar materials are not added simply in order to give weight to the paper, since they serve to produce opacity and to improve the surface of papers which could not be satisfactorily made unless such materials were used.

Examination of Paper for Loading.—If a piece of paper is crumpled up, placed in a small crucible, and then ignited until all the carbonaceous matter has been burnt off, a residue is left in the crucible which may be white or coloured. This is usually termed the ash of the paper. The amount of ash present is determined by taking a weighed quantity of paper and weighing the residue obtained. Special appliances can be obtained for making rapid determinations of the ash in paper, but for occasional analyses they are not required.

China Clay.—This is the best known and most commonly used loading. The purest form of this material is kaolin, a natural substance formed by the gradual decomposition of felspathic rocks arising from exposure to the long-continued action of air and water. The clay occurs in great abundance in Dorset, Cornwall, and Devon, the southern counties in England, where the most famous deposits are found.

The natural mineral is levigated with water, and the mixture allowed to flow through a series of settling ponds, so that the clay gradually settles in the form of a fine deposit. The clay is dried and packed in bags. Its value is controlled largely by the purity of its colour and its freedom from grit and sand. It is essentially a silicate of alumina, having the approximate composition—

The specific gravity of the dry substance is 2·50.

It is utilised as a loading in all kinds of paper, and forms also the main ingredient in the coating found on ordinary art and chromo papers.

Ash containing China Clay.—In news, cheap printings, and common art papers the ash almost invariably contains china clay. This substance is insoluble in dilute acids, but is acted upon by concentrated sulphuric acid when digested for some time. A simple test for the presence of china clay in ash is the blue coloration which is obtained when the ash after being ignited is gradually heated with a few drops of solution of cobalt nitrate. China clay can be decomposed by fusion with carbonate of soda in a crucible. By this means silicate of alumina is decomposed, and the alumina goes into solution, the silica remaining as an insoluble residue. The filtered solution is boiled with an excess of ammonia which gives a gelatinous precipitate of aluminium hydrate.

Sulphate of Lime.—This compound is valued chiefly for its brilliancy of colour, being used in high-class papers. It is slightly soluble in water, to the extent of about 23 lbs. in 1,000 gallons, and this fact must be taken into account when the material is added to the pulp in the beating engine.

It occurs naturally in a variety of forms, such as gypsum, alabaster, selenite, the first of which when finely powdered is sold to the paper-maker as gypsum, powdered plaster, and under other fancy names.

It can be prepared artificially by adding sulphuric acid to solutions of calcium salts; and the precipitated product so obtained is sold as terra alba, pearl hardening, satinite, mineral white, etc.

The tests for sulphate of lime in paper ash are based upon the following reactions:—

Calcium sulphate is soluble in dilute hydrochloric acid. The addition of a few drops of barium chloride to the solution produces a dense heavy precipitate, indicating the sulphate. A small quantity of ammonium oxalate solution added to another portion of the dissolved calcium salt previously neutralised with ammonia produces a precipitate and indicates calcium.

A microscopic test of paper for the presence of sulphate of lime is based upon the slight solubility of the salt in water. The paper is boiled with some distilled water. The water is evaporated to a small bulk and transferred to a glass slip, and the gradual formation of characteristic sulphate of lime crystals can be seen by means of the microscope as the water cools down.

French Chalk.—This material is prepared by grinding talc into a fine powder, and possesses a good colour and a somewhat soapy feel. It is a silicate of magnesia, having the approximate composition—

Other silicates of magnesia used for paper-making are agalite and asbestine, the latter being a finely ground asbestos.

The composition of asbestos is approximately—