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:—

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.

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.