Mechanical Wood Pulp.
The wood is first cut up into blocks, the size of which is determined by the width of the stones used for grinding; any knots present are cut out with an axe. The stones are made of sandstone, and are covered over three quadrants with an iron casing, the remaining quadrant being exposed. The surfaces of the stones are made rough by the pressure of a steel roll studded with points, and which is pressed against it while revolving. In addition to this, channels about 1⁄4 in. deep are cut into the stone at distances of 2–3 in. They are made in two sets, crossing each other in the centre of the stone, and serve to carry off the pulp to the sides of the stone, in addition to giving increased grinding-surface. The pressure of the blocks of wood against the stones is steadily maintained by screws worked by suitable gearing; this is necessary in order to obtain a pulp of uniform character. A stream of water is kept constantly playing on the stone; by this means, the pulp as fast as it is formed can be conveniently carried away. It is first passed {106} through a rake, which retains small pieces of wood that have escaped grinding. The stream of pulp then passes through the sorters, the object of which is to keep back such portions of the wood as have not been sufficiently disintegrated. These consist of cylinders about 3 ft. long and 2 ft. in diameter, covered with a coarse wire-cloth. The fibres that are retained by this wire fall into the refiners, which consist of a couple of horizontal cylinders of sandstone, the upper one only of which revolves. Here they are further {107} disintegrated, and are again passed through the wire-cloth; this is repeated until all the fibres have passed through. The pulp, after passing through the first sorter, may be conducted through a series of gradually increasing fineness, and, by this means, be separated into different qualities. Though pulp so prepared cannot compete with chemically-prepared stuff, as the fibres are extremely short, and have comparatively little felting-power, it may be used with advantage as a sort of filling-material.
FIG. 28.
Various modifications of the foregoing process have from time to time been proposed; among others, that of softening the wood by previous soaking in water, or steaming, seems to be valuable, as by so doing, it is highly probable that a longer fibre could be obtained, the soft wood being more readily torn away by the stones. Some inventors have proposed to replace the sandstone by an artificial stone containing a large quantity of emery.
An improved method of preparing mechanical wood pulp, lately patented by Mr. A. L. Thune, of Christiania, has been communicated to us by Mr. Carl Christensen.
| FIG. 29. | FIG. 30. |
The apparatus employed is shown in Figs. 28, 29, and 30. Fig. 28 illustrates an arrangement of grinding apparatus fixed direct on to a turbine. The stone is fastened on to the shaft S worked by the turbine T. The wood in the form of small blocks is kept in contact with the stone by a number of hydraulic presses P. {108}
A somewhat similar arrangement, but placed horizontally, is shown in front and side elevation in Figs. 29 and 30. The same letters correspond.
FIG. 31.
The ground and sorted pulp is made into thick sheets by means of the board machine shown in Fig. 31. The pulp mixed with water passes down the shoot D into the vat B in which the cylinder K revolves. This cylinder is covered with wire-cloth, and as it revolves it takes with it a certain quantity of pulp in the form of a continuous sheet. This sheet is taken on to the endless travelling felt F by the small couch roll E. When it reaches the rolls C it is wound round the upper one, from which it is removed when a sufficient thickness is obtained. Obtained in this form the pulp is readily transported.
The woods commonly employed are white pine and aspen. The latter yields a pulp of a better colour, but of inferior strength than the former.
Paper containing mechanical wood pulp is very liable to become discoloured by the action of air and light, the ligno-celluloses being much more readily acted upon than the celluloses isolated from them. Such fibre is, moreover, devoid of much felting power; it has, in fact, little to recommend it but its comparative cheapness. It is nevertheless used in large quantities, some cheap papers being made entirely from it. {109}
| Specific Gravity. | Degrees Twaddle.[*] | Per cent. Na2O. | Equivalent Percentage of 60 per cent. Caustic Soda. |
|---|---|---|---|
| 1·0040 | 0·80 | 0·302 | 0·503 |
| 1·0081 | 1·62 | 0·601 | 1·001 |
| 1·0163 | 3·26 | 1·209 | 2·015 |
| 1·0246 | 4·92 | 1·813 | 3·021 |
| 1·0330 | 6·60 | 2·418 | 4·030 |
| 1·0414 | 8·28 | 3·022 | 5·037 |
| 1·0500 | 10·00 | 3·626 | 6·043 |
| 1·0587 | 11·74 | 4·231 | 7·051 |
| 1·0675 | 13·50 | 4·835 | 8·059 |
| 1·0764 | 15·28 | 5·440 | 9·067 |
| 1·0855 | 17·10 | 6·044 | 10·073 |
| 1·0948 | 18·96 | 6·648 | 11·080 |
| 1·1042 | 20·84 | 7·253 | 12·090 |
| 1·1137 | 22·74 | 7·857 | 13·095 |
| 1·1233 | 24·66 | 8·462 | 14·103 |
| 1·1330 | 26·60 | 9·066 | 15·110 |
| 1·1428 | 28·56 | 9·670 | 16·117 |
| 1·1528 | 30·56 | 10·275 | 17·125 |
| 1·1630 | 32·60 | 10·879 | 18·131 |
| 1·1734 | 34·68 | 11·484 | 19·140 |
| 1·1841 | 36·82 | 12·088 | 20·147 |
| 1·1948 | 38·96 | 12·692 | 21·153 |
| 1·2058 | 41·16 | 13·297 | 22·161 |
| 1·2178 | 43·56 | 13·901 | 23·170 |
| 1·2280 | 45·60 | 14·506 | 24·177 |
| 1·2392 | 47·84 | 15·110 | 25·170 |
NOTE.—It must be borne in mind that the above numbers refer only to solutions of pure caustic soda. With liquors containing sodium chloride, sulphate, &c., the specific gravity will give an erroneous view of the amount of alkali present, as these salts of course raise the gravity. For example, a liquor prepared by causticising a solution of recovered soda has a specific gravity of 1·05 (10° Tw.). According to the above table this corresponds to 6·043 per cent. of 60 per cent. caustic. Tested by means of standard acid it showed 4·520 per cent. Too much reliance should therefore not be placed upon determinations of specific gravity, but in important cases the actual amount of alkali should be determined by titration with standard acid.
[*] To convert degrees Twaddle into specific gravity, multiply by 5, add 1000 and divide by 1000: thus 7° Tw.,
{110}
CHAPTER VII. BLEACHING.
The methods of bleaching being essentially the same for all pulps, it will only be necessary to describe one in detail.
The process depends upon the use of a solution of calcium hypochlorite, prepared by dissolving bleaching powder in water. This solution is best prepared in a vessel provided with stirrers or agitators, whereby a thorough mixture of the bleaching powder and water is obtained. Bleaching powder always contains a certain amount of free lime and calcium carbonate, which remain undissolved in the water. This should be allowed to settle to the bottom of the vessel, and the clear liquor run off. The residue may then be again treated with water, or with weak liquor obtained by washing the residue. If the washings are too weak to be conveniently used for bleaching, they may be stored in a separate tank and used for dissolving fresh powder. In this way a strong stock solution can be readily prepared, the powder at the same time being thoroughly exhausted. The washing should be repeated as frequently as possible, after which the residue may be allowed to drain. This operation is best conducted on a filter bed provided with an air-pump similar to that described in Chapter XII. The residue is obtained in this way as a hard cake, containing about 60 per cent. of water. If the above operations have been properly conducted, it should not contain more than about 0·25 per cent. of chlorine.
The original powder should contain from 33·0 to 35·0 per cent. of “available chlorine,” that is, chlorine which is effective in the bleaching process. {111}
Owing to the ease with which it decomposes it should be kept cool and not exposed to the air.
If possible, one or more large store tanks should be provided for the strong bleach liquor, so that time may be given for the complete separation of the insoluble portions. The liquor can be drawn off with a “siphon” pipe without disturbing the residue.
It is of the utmost importance that only clear liquor be used, as a milky solution of calcium hypochlorite is much more sluggish in its action than one which is perfectly clear and bright.
A convenient strength for the stock solution is 6° Twaddle: this corresponds to about half a pound of bleaching powder to the gallon.
The method of testing the powder and its solution will be described in Chapter XIV.
The washed and broken pulp is placed in the “potcher” together with the necessary quantity of bleaching liquor and as much water as is required to produce complete circulation of the mass. In many mills the breaker itself answers the purpose of a potcher also. The quantity of water should be kept as low as possible, as it is found that by the use of strong solutions less bleaching powder is required than with weak solutions; moreover, less time is required to produce the effect. On the other hand, with very strong solutions, the pulp is liable to be injuriously affected.
Straw and esparto pulps are sometimes bleached in large potchers made of brick covered with cement. The circulation of the pulp is produced by the action of paddles made of wood or, preferably, of iron.
Many “potchers” are provided with steam pipes, whereby the mass of pulp and liquor is heated. This should be done with great care, so as to avoid superheating in any part, as this is certain to cause destruction of the fibre. The better plan, where it can be adopted, is to uniformly heat the pulp before running in the bleach.
In certain cases the action of the calcium hypochlorite is {112} assisted and accelerated by the addition of either sulphuric or hydrochloric acids; these combine with the lime and liberate hypochlorous acid, which has a more rapid action than its calcium salt. If the acid be added in too large quantity, or of too great a strength, it sometimes happens that instead of hypochlorous acid, chlorine gas is given off, part of which escapes into the air, thus causing loss. Moreover, the chlorine is liable to enter into combination with the fibre substance forming the yellow chlorination products described on p. [18].
This liability is greater in the case of highly lignified fibres, such as wood or jute: in these cases, therefore, the use of acids should be avoided. When used the acid should be largely diluted with water and added gradually. The best plan is to allow the action of the bleach to continue for some time, only adding the acid when it is nearly exhausted. In this way risk of damage to the pulp is avoided.
The diluted acid should be conveyed by a leaden pipe passing down to the floor of the potcher, and perforated at its lower extremity.
Other means of accelerating the action of the bleaching powder have been suggested, such, for example, as the use of bicarbonate of soda, which by double decomposition forms with the bleaching powder carbonate of lime and free hypochlorous acid. It is, however, much too expensive an agent.
The amount of bleaching powder necessary to produce a good white colour differs materially with the fibre to be bleached, and of course with the degree of resolution of the fibre substance in the preliminary treatment with caustic soda.
The following numbers may be taken to be approximations of the amount necessary to bleach well-boiled pulps:—
| Per cent. Powder calculated on Unboiled Fibre. | |
|---|---|
| Esparto | 10 to 15 |
| Straw | 7 to 10 |
| Wood | 15 to 20 |
{113}
A well-boiled pulp should contain from 90 to 95 per cent. of cellulose.
Very excellent results are sometimes obtained by substituting sodium hypochlorite for the calcium compound. This is done by decomposing a solution of bleaching powder with an equivalent quantity of either sodium sulphate or sodium carbonate. The precipitate of calcium sulphate or carbonate is allowed to settle and the clear liquor run off. Such a solution is more sluggish in its action than one of calcium hypochlorite, but it is more easily controlled, and is less liable to injure any material treated with it.
The time necessary to produce a perfect bleach depends on the nature and condition of the pulp, on the degree of concentration of the liquor, and on the temperature at which the operation is conducted. Some paper-makers prefer to extend the bleaching over a considerable period of time, but this involves the employment of a larger system of “potchers,” which in many instances is an objection. If this method is adopted, especially in the case of straw or esparto, the action of the paddles or roll should be stopped, as otherwise the fibres are liable to felt together in the form of small grains, which sometimes escape the action of the beating roll, and occasionally find their way into the paper, causing an unevenness of surface. By giving longer time to the bleaching there is less risk of damage to the fibre, and consequently a greater yield of bleached pulp.
The action of the bleaching solution should, as far as possible, be confined to the non-cellulose portions of the fibre. A certain action on the cellulose itself invariably takes place, but it should be reduced to a minimum. The action of bleach upon cellulose has been indicated in Chapter I., p. [10]. For further information on this subject the reader is referred to the ‘Journal of the Society of Chemical Industry,’ 1884, April 29 and May 29.
A very good method of bleaching consists in a preliminary treatment in the ordinary way in the potcher, followed by a prolonged steeping in tanks. {114}
In the case of pulps which are very difficult to bleach, the action can be greatly assisted by washing out the products of the bleaching action, treating the pulp for a short time with a weak alkaline solution, again washing, and then repeating the bleaching process. In this way very refractory pulps can easily be bleached. Even an intermediate washing with water is useful.
In most cases, where a fairly good colour has been obtained by the use of bleaching powder alone, the colour is greatly improved by a souring or treatment with weak hydrochloric acid, or, better still, with solution of sulphurous acid.
Or the bleaching may be conducted in two or more operations with intermediate treatments with acid.
All these modifications, however, are more or less troublesome, and should only be resorted to when the ordinary method fails.
After bleaching, a certain quantity of calcium hypochlorite always remains in the pulp. It may be removed in the potcher itself, or when the pulp reaches the beater. In any case, the methods adopted are the same. (See p. [127].)
In some mills it is the practice to free the bleached pulp from the products of the decomposition of the bleach by pressing the pulp in hydraulic presses, or by draining in vats provided with perforated bottoms. Or the pulp may be made to pass through a pulp-saver (see Fig. 52, p. [155].)
Chlorine gas as a bleaching agent has been almost entirely superseded by the more manageable calcium hypochlorite. Its employment is open to the serious objection indicated before, viz. the liability to form difficultly removable chlorine compounds. Its use as a disintegrating agent has already been alluded to (see p. [102]).
A process has lately been invented (English patent, No. 11,333, 1884), by Prof. Lunge, which involves the use of acetic acid. The quantity required is, however, very small, as during the process of bleaching it becomes regenerated. The free lime in the bleaching powder solution should first be {115} nearly neutralised with a cheaper acid, such as hydrochloric or sulphuric, followed then by the addition of the acetic acid. This process, we are informed, gives excellent results with high-class material, such as the best cotton and linen rags; it is, however, not to be recommended for such materials as straw or esparto.
For the bleaching of rags the process invented by Thompson (English patent, No. 595, 1883) has proved very effective. The method consists in saturating the material with a weak solution of bleaching powder, and then exposing them to the action of carbonic acid gas. In this way the bleaching action is made very rapid and effective.