Electrolytic Bleaching.

The electrolysing vat consists of a rectangular vessel of slate or other suitable material. The negative electrode may be constructed of zinc; for the positive it is necessary to employ platinum.

The electrolysed solution has been found to possess very remarkable properties, which have considerable bearing upon the economy of the process. If a solution be taken of equal {116} oxidising efficiency with one of calcium hypochlorite, as indicated by the arsenious acid test (see p. [206]), it is found that the former possesses greater bleaching efficiency than the latter in the proportion of 5 to 3. Moreover, the bleaching is much more rapid, and the loss of weight which the substances undergo is less, for equal degrees of whiteness obtained.

It has been shown that by the electrolytic method the bleaching of paper pulp can be effected at nearly one-half the cost of bleaching powder. The process is now being industrially carried out. Those interested in the subject are referred to a paper by the authors in the ‘Journal of the Society of Chemical Industry’ for April 1887.

{117}

CHAPTER VIII. BEATING.

The bleached pulp is now ready for the final treatment. If it were attempted to make paper from the pulp in the state in which it leaves the “potchers” or “steeping” chests, it would be found to be wanting in evenness of texture and uniformity of surface. This result can only be secured by an elaborate process of comminution or disintegration. This is in effect the result produced by the action of the “beaters.”

They resemble in general appearance the breaking engines previously described; the roll, however, carries more knives, and it is usually let down much nearer to the bed-plate. In the case of fibrous substances, whose ultimate fibres are relatively short (see table, p. [39]), it is only necessary to split up the filaments into their constituent fibres: esparto, straw, and wood are of this class. In the case of straw, the disintegration is for the most part accomplished in the boiling and bleaching processes, and therefore but little work devolves upon the beater. Esparto and wood require a certain amount of beating, but this should be regulated to the drawing asunder of the individual fibres, any cutting of the fibres being carefully avoided. This is accomplished by adjusting the distance of the roll from the bed-plate, so that by the friction of the fibres upon themselves, when passing over the plate, a kind of rubbing or “brushing” is produced. If a carefully-made paper of esparto or wood be examined by the microscope, it will be found that the majority of the fibres preserve the pointed or slightly rounded ends {118} characteristic of bast cells. On the other hand, it is obvious that cotton, whose ultimate fibres have a length of 20 to 40 mm., and flax (25–30 mm.), with other similar fibres, will require to be broken up into short fragments in order to develop to the fullest the felting property of the pulp. Not only are the fibres reduced to the most favourable dimensions, but in cotton and linen a further contributory advantage accrues; for on account of the internal structure of the ultimate fibres they tend to split up at the point of rupture into a number of fibrillæ, which, in the case of cotton, take the form of a network; and in case of linen, are seen as a bundle of distinct fibres parallel to and continuous with the fibre. This gives to ends of the fragments a ragged contour, which has considerable influence on the felting power of the pulp, and therefore on the strength of the paper into which it is made.

With these fibres, therefore, the “cutting” as distinguished from the “breaking” action should be avoided as much as possible, otherwise the effect described above will not be produced, and the fibres will show instead a clean cut. The appearance shown by cotton and linen pulp, when thoroughly “beaten” and ready to be made into paper, is given in the frontispiece, which is taken from the author’s micro-photographs.

The “half-stuff” is furnished to the “beater,” or “beating” engine as it is sometimes called, previously partially filled with water. The furnishing is done in successive portions, the first being allowed to mix thoroughly with the water before another lot is added. This is continued until the mass is so thick that it will only just turn round in the beater under the action of the “roll.” Owing to the construction of the beater it frequently happens that a portion of the pulp lodges in the corners, from which the beaterman removes it by means of a wooden paddle, which also serves to push forward the pulp to the roll in case the motion is inclined to be sluggish. The proportion of water to pulp should not be too high, otherwise the beating is not so effective; at the same time, {119} if the mass is allowed to get too thick, imperfect circulation results.

The operation of “beating” occupies a considerable time, and consumes a large amount of power. Cotton and linen rags naturally take longer—in some cases as much as ten hours is given to the operation.

Esparto, on the other hand, can be sufficiently disintegrated in from two to four hours.

Wood pulp requires very gentle beating; it is therefore necessary to prolong the time to about six hours.

These differences in the duration and method of beating should be borne in mind when pulps of different natures are mixed together in the same beater, as is frequently the case. It is better, with very dissimilar fibres, to “beat” each separately, and only to mix them in the stuff-chests. This, however, is open to the objection that the pulps may be insufficiently mixed.

The length of fibre to which pulps should be reduced depends to some extent upon the kind of paper to which it is to be applied. The authors have examined a number of papers by well-known makers, and find the dimensions in millimetres of various pulps to be as follows:—

Fibre.	Maximum.	Minimum.	Mean.
Cotton	1·32	0·23	0·75
Linen	1·20	0·20	0·76
Esparto	1·40	0·40	1·00
Straw	1·50	0·50	0·88
Wood	2·60	1·00	2·00

Wood and straw pulp, when imported in the form of dry sheets, may, before being beaten, be conveniently disintegrated and thoroughly mixed with water by means of the edge-runner described under “Broke Paper,” p. [104], Fig. 28.

In making the finer kinds of paper, the roller bars or knives, instead of being made of steel are made of bronze, thus any contamination with oxide of iron is avoided. This is especially liable to take place in case of steel knives when the beater has been allowed to stand for some time. {120}

The inside of the heater itself is often lined with lead, a material which is not liable to oxidize, and which can very readily be cleaned.

When a beater has been running for some time the knives of the roll and the bed-plate become worn and so far reduced that they must be taken out and re-cut. The bed-plate is removed, firmly fixed in the bed of a planing machine, and the edges trimmed by means of a chisel, so as to cut the knives at the proper angle.

The roll and bed-plate are shown in section in Fig. 32. Fig. 33 is a plan of a bed-plate, and Fig. 34 illustrates the manner in which the knives are fixed.

FIG. 32.

It will be seen that the knives in the bed-plate are placed so that they do not lie parallel with those of the roll. This arrangement imitates to some extent the action of a pair of scissors. Occasionally the knives are slightly bent, so as to form a very obtuse angle. Bed-plates so fitted are called {121} “knee-plates.” They are largely used in America, but not much in this country.

FIG. 33.

FIG. 34.

To obviate the necessity of removing the roll, a small machine has been devised whereby the knives can be cut in situ. This machine, which can be firmly fixed between the mid-feather and the side of the beater, consists of a small steam-engine which actuates a movable cutter, which is made to pass to and fro horizontally along the edge of each knife in succession. The engine is supplied with steam by a piece of strong flexible rubber tubing.

The ordinary form of beater contains only one roll, though some have been constructed containing two or even four rolls. In America, beaters of a totally different construction are much in vogue. The most important of these are the Jordan and Kingsland beaters, so called from the names of the inventors. The former consists essentially of a roll in the shape of a truncated cone, fitted with knives in the usual way, revolving in an iron box of corresponding shape, furnished with knives placed in the direction of its length, but at slightly different angles. The half-stuff enters the beater at the narrow end through a box provided with an {122} arrangement for regulating the flow, and is discharged by two or more openings in the cover at the wider end.

The Kingsland engine consists of a circular chamber, the sides of which are covered with knives, and between which a circular plate, also furnished with knives, revolves. The pulp enters through a pipe in the centre of one of the sides of the chamber, and flows out through an opening in the opposite side.

Another form of beater is that invented by S. L. Gould. The only essential difference between it and the Kingsland beater is that, instead of having a plate which revolves vertically against two stationary ones, the plate, which is placed horizontally, is furnished with knives on one side only, and revolves upon but one fixed plate, much in the same way as a pair of millstones.

The half-stuff supplied to these forms of beater is generally disintegrated to a greater extent in the breaker than is the case with those of the ordinary construction, as it is necessary to make it flow easily through them, and this could not be done if the fibres were kept long. A single passage of the pulp through the beater is sufficient to beat it up ready for making into paper. The chief advantages claimed for them is that they are more economical, both of time and power; also that the pulp is more regularly beaten.

Certain new forms of beaters have lately been introduced in this country, and are rapidly gaining in favour, chiefly on account of the saving of driving power, and the space occupied, compared with the amount of “stuff” they are capable of beating. Fig. 35 shows the construction of Forbes’ patent engine, as manufactured by Messrs. G. & W. Bertram, of Edinburgh. This beater is divided into three channels. The rolls, one of which is shown uncovered, are placed in the outer channels, while in the centre one the mixing and circulating wheel a is placed, the peculiar construction of which enables it to throw the pulp alternately into the two outer channels, from whence, after passing under the rolls again, it enters the central channel at the other end of the beater. {123}

In Umpherston’s patent engine great economy of space is effected by causing the pulp to travel over and under the backfall (a, Fig. 36). Another advantage is that the stuff circulates freely with less water than in the ordinary forms, thus increasing its output. It is also claimed that the pulp is beaten with less power, and as it is less liable to lodge in corners it is more evenly beaten.

FIG. 35.

The construction of the engine will be readily understood by reference to Fig. 36. The direction which the stuff takes is indicated by the arrows.

Cooke and Hibbert’s beater, as manufactured by Messrs. Masson, Scott and Bertram, resembles, to a certain extent, Gould’s engine, previously described; it differs from it, however, in that the discs are placed vertically, instead of horizontally. Moreover, the general construction of the beater more nearly resembles those in ordinary use. Fig. 37, which shows it in plan and in section at the line D E, illustrates its construction.

FIG. 36.

FIG. 37.

The shaded portions A represent the stationary disc. It is furnished for about two-thirds of its circumference with steel bars or knives, placed tangentially. B represents the {125} revolving disc on which similar knives are placed radially round the whole circumference. The direction that the pulp takes is indicated by the arrows. After passing between the plates it is thrown violently into the trough at G, the result being that it is very thoroughly mixed. The distance between the plates can be regulated by means of the gearing H. The engine is driven by the pulley C fixed on the shaft F. It is claimed that by means of this form of beater a great saving in time and power is effected, and that moreover the pulp is obtained in a more even condition.

FIG. 38.

FIG. 39.

The quality of the water used to furnish the engine is a matter of very great importance, especially in the manufacture of high-class papers. Above all it should be free from suspended matter, and from dissolved iron; the former finds its way directly into the paper, and the latter is liable to become precipitated in the pulp as oxide, thus injuriously affecting its colour. Careful settling and filtration are sufficient to eliminate insoluble matter; soluble impurities are more difficult of removal; therefore the water should, if possible, be free from them. In most mills settling ponds are provided, for the purpose of removing suspended matter, and in addition it is usual to employ woollen filter-bags {126} fastened to the nozzle of the pipe that supplies the beaters with water.

For methods of purifying water see p. [211].

The beaters are generally driven from a separate engine, and are connected with it by a system of spur-wheels, pinions, and belts. Messrs. G. and W. Bertram have lately introduced a system of direct driving of beaters, whereby a great saving in power is effected. It is illustrated in plan and elevation in Figs. 38 and 39. The crank-shaft of the engine is coupled direct on to the main driving-shaft. Large pulleys are keyed on to this shaft, from which the power is taken directly on to the pulleys connected with the rolls.

{127}

CHAPTER IX. LOADING, SIZING, COLOURING, ETC.

The bleached half-stuff as it leaves the steeping chests usually contains an excess of bleaching liquor, which can be removed in two ways, viz. by washing or by decomposition with an “antichlor.” The first method has the advantage of not only removing the bleach, but of also eliminating the chloride of calcium, partly existing ready formed, and also that resulting from the decomposition of the calcium hypochlorite originally present in the bleach. On the other hand, this method takes some time, and consumes a large amount of water, which in some mills is a matter of considerable importance. For this purpose, many beaters are provided with one or more drum washers (see Fig. 36). An additional objection to this method lies in the fact that a certain quantity of fibre passes through the meshes of the wire-cloth covering the washers, and is thus lost.

The more usual plan is to remove the bleach by decomposing it with an “antichlor.” The substance generally employed for this purpose is sodium hyposulphite, or thiosulphate as it is now called, which, in presence of calcium hypochlorite, is oxidized to sodium sulphate, the latter being reduced to calcium chloride. Double decomposition then takes place between these salts, with the formation of calcium sulphate and sodium chloride. The reactions which take place may be expressed by the following equation:—

2 (Ca(ClO)₂)

Calcium
hypochlorite.

Na₂S₂O₃

Sodium
thiosulphate.

H₂O

Water.

2 CaSO₄

Calcium
sulphate.

2 HCl

Hydro-
chloric
acid.

2 NaCl.

Sodium
chloride.

{128}

The above decomposition does not accurately represent the action of bleach upon sodium thiosulphate. If the solutions employed are very dilute, the decomposition may take place in another direction, viz.:—

Ca(ClO)₂

Calcium
hypochlorite.

4 Na₂S₂O₃

Sodium
thiosulphate.

H₂O

Water.

2 Na₂S₄O₆

Sodium
tetrathionate.

2 NaCl

Sodium
chloride.

2 NaOH

Caustic
soda.

CaO.

Lime.

At the particular degree of dilution which occurs in a beater, the bleach is decomposed almost entirely according to the first of the equations; from which, on calculating, it will be seen that 158 parts of sodium thiosulphate are equivalent to 286 parts of calcium hypochlorite. As commercial sodium thiosulphate contains 36·3 per cent. of water, and bleaching powder 70 per cent. of calcium hypochlorite, on the basis of 35 per cent. available chlorine it follows that 248 parts of the former are required to neutralize 409 parts of the latter.

Within the last few years other forms of “antichlor” have been introduced, such, for example, as the various sulphites. The most important of these is sodium sulphite, which has been manufactured by a patent process at a cheap rate, by Gaskell, Deacon, & Co., Widnes. Their product contained as much as 75 per cent. of Na₂SO₃, ordinary crystallized sodium sulphite containing only 50 per cent.

Sulphites are converted by the action of bleach into sulphates, thus:—

Ca(OCl)₂

Calcium
hypochlorite.

2 Na₂SO₃

Sodium
sulphite.

CaSO₄

Calcium
sulphate.

2 Na₂SO₄

Sodium
sulphate.

2 NaCl

Sodium
chloride.

From this equation, it will be seen that 252 parts of sodium sulphite will neutralize 143 parts of calcium hypochlorite, or 204·3 parts of bleaching powder. Assuming that crystallized sodium sulphite contains 50 per cent. Na₂SO₃, the same amount of bleach would require 504 parts. As Messrs. Gaskell, Deacon & Co.’s sulphite contains 75 per cent. Na₂SO₃, only 336 parts are needed. Comparing these numbers with {129} those given above for sodium hyposulphite, it will be seen that 204·5 parts of bleach require for neutralisation 129 parts of sodium thiosulphate and 504 parts of crystallised sodium sulphite, or 336 parts of the stronger product.

Sodium sulphite is preferred to sodium thiosulphate by some paper makers, notwithstanding the fact that even in its most concentrated form, nearly three times as much is required to produce a certain result. It is said that when it is used the wire cloth of the machine is preserved for a longer time than if sodium thiosulphate is employed. This may be due to the fact that with the latter a certain amount of free acid is always formed, which of course would act injuriously on the wire; whereas, when sodium sulphite is used, the products of decomposition are neutral salts without any action upon metals. (See the above equations.)

A very cheap “antichlor” may be prepared by boiling together lime and sulphur. One hundred and sixty-eight parts of lime, made into a milk with water, are heated to boiling in an iron vessel. Three hundred and eighty-four parts of flour of sulphur or ground sulphur are then added in small quantities at a time, and the boiling continued until the whole is dissolved. The liquid which is now of a deep yellow colour, is allowed to settle and cool and is then ready for use. It contains a mixture of calcium thiosulphate and calcium pentasulphide, the latter compound giving to it its deep yellow colour. The following equation represents the action which takes place between the lime and the sulphur:—

3 CaO

Lime.

12 S

Sulphur.

CaS₂O₃

Calcium
thiosulphate.

2 CaS₅

Calcium
pentasulphide.

The decomposition which takes place when the mixture is acted upon by calcium hypochlorite may be represented as follows:—

2 (CaS₂O₃

Calcium
thio-
sulphate.

2 CaS₅)

Calcium
penta-
sulphide.

9 (Ca(OCl)₂)

Calcium
hypochlorite.

6 CaSO₄

Calcium
sulphate.

18 S

Sulphur.

9 CaCl₂

Calcium
chloride.

{130}

From this it is seen that a large quantity of free sulphur is formed, which is precipitated as a fine yellowish powder in the fibre. This not only affects the colour of the pulp, but is objectionable on account of its liability, in contact with the hot cylinders of the paper machine, or by the action of moisture over a long period of time to become oxidised to sulphuric acid, which undoubtedly, even if present only in very small quantities, has an injurious effect upon the finished paper. (See hydracellulose.) This property of sulphur and its action upon cellulose may be shown by the following experiment. Take a piece of water-leaf paper, rub into it a mixture of flour of sulphur and water. If the paper thus treated be rapidly dried, it will be found to be weakened or even slightly charred in those portions which have been in contact with the sulphur. (See also Chem. Soc. Journ., p. 249, 1879.)

Whichever variety of “antichlor” is used an excess should be carefully avoided, as all act more or less upon the size and colouring matter added to the pulp subsequently. The proper method is to run in small quantities of a solution of the antichlor at a time, testing the pulps, after allowing a few minutes for complete admixture after each addition. The test is made by immersing a piece of iodide of potassium and starch paper, which will be turned blue so long as any calcium hypochlorite is present. These papers are made as follows:—

Three grms. of starch are ground up with a small quantity of water and poured into 700 cc. of boiling water, and to the solution 1 grm. of iodide of potassium and 0·5 grm. carbonate of soda are added. Sheets of white paper, water-leaf in preference, are now soaked in the solution and dried. In presence of calcium hypochlorite, iodine is liberated from the potassium iodide, and acting upon the starch forms with it a characteristic blue colour.