As we have already pointed out, caustic soda and lime are the only alkalis employed by the paper-maker for boiling fibres; the special cases in which carbonate of soda is used have been mentioned in their proper place.

The preparation of milk of lime requires little or no de­scrip­tion, as the operation is a simple and tolerably familiar one. Before using, it should be passed through a fine wire sieve, to keep back sand, coal, and similar impurities which the lime invariably contains.

Caustic soda is met with in commerce in four forms, differing from each other in alkaline strength and colour, and of course in price. The lowest quality is what is known as caustic “bottoms”: it consists of that portion remaining at the bottom of the caustic-pot after the clear fused caustic soda has been ladled out, and it contains a considerable quantity of ferric oxide and other insoluble impurities. It is of a dark reddish brown colour, and contains 50–60 per cent. of alkali (Na₂O). Its use in paper-making cannot be recommended, except for the preparation of the very lowest grade of pulp. The solution should be allowed to remain at rest, in order that the insoluble matter may subside.

The next in quality is known as cream caustic, so called from its slightly brown colour. It is usually sold containing 60 per cent. of alkali, in the form of a tolerably friable mass, having a crystalline structure. It is a very suitable form of alkali for the paper-maker.

The next in order is what is called 60 per cent. white. It {178} consists of a hard white mass, requiring considerable force to break it. Though whiter in appearance than cream caustic, it is in reality less pure, as it contains a considerable quantity of salt. It is made by continuing the evaporation of the caustic liquor to a further point than is the case with cream caustic, the result being that the whole of the water is driven off. A small quantity of nitre is then added to oxidise the sulphides and other compounds which impart the colour to cream caustic. Common salt is then added to reduce its strength to 60 per cent., in order to satisfy the whim of the consumer, who insists upon an article of a given definite strength. On this account it is somewhat inferior to cream caustic, and is, moreover, more expensive.

The highest quality of all is white 70 per cent. This resembles white 60 per cent. in appearance, but is much purer, and more expensive.

A very pure form of caustic soda has within the last few years been introduced by the Greenbank Alkali Company. It consists of small fragments, more or less crystalline, of almost pure sodium hydrate. It can be packed in wooden casks, and it dissolves very readily in water. It contains 76 per cent. of alkali. It is, however, much too costly for boiling purposes; it might, however, be used for the preparation of rosin size.

The following analyses of different forms of caustic soda and their relative prices will possibly be of interest:—

Their relative prices per ton f.o.b Liverpool, in December 1886, were:—70 per cent. white, 8l. 5s.; 60 per cent. white, 7l. 5s.; 60 per cent. cream, 7l.

All the above-mentioned forms of caustic soda occur in the form of more or less hard masses, and are contained in thin wrought-iron drums, which are filled with the caustic in a fused state. The soda is removed by breaking the drum with a chisel and hammer. The lumps of caustic may be put direct into the boiler, though it is better to dissolve them previously in water and allow any insoluble impurities to settle to the bottom of the solution.

Instead of buying caustic soda direct, it is the custom of some paper-makers to prepare it for themselves from carbonate of soda, which can now be obtained in various forms, some of great purity.

It occurs as caustic ash, containing about 40 per cent. of alkali as carbonate, and about 10 per cent. as caustic soda; soda-ash containing about 52 per cent. total alkali, nearly all as carbonate; and refined soda-ash containing 57–58 per cent. of alkali as carbonate. Some forms of carbonate of soda, such, for example, as those made by the ammonia process, are of even greater purity. The method of converting sodium carbonate into caustic soda will be described subsequently.

Unless the arrangements for washing and draining the lime-mud are very perfect, it is probably cheaper for the paper-maker to buy his caustic direct from the maker.

The apparatus for ac­com­plish­ing the evap­o­ra­tion varies with almost every mill. In some, it is very primitive and {181} crude, consisting perhaps of only a furnace for incinerating the residue, and over it a pan containing the liquor, the latter being heated and evaporated by the heat from the furnace. It is obvious that, with such an arrangement, a large quantity of heat must be wasted. To economise as much as possible of this waste heat, various plans have been suggested. That of Roeckner, of Newcastle, appears to be to a great extent efficacious. It consists practically of a series of shallow trays B (Fig. 67) placed in a brick chamber, alternated so as to allow the heated air from the furnace below to play upon the surface of each in succession, on its way to the chimney, with which the whole system is in connection. Above the chamber containing these trays, is a large tank C, containing a store of the liquor to be evaporated, placed there so as still further to economise the heat, and from which the liquor runs on to the trays. The furnace A is of the ordinary re­ver­ber­a­tory kind; below it, and connected with it by a kind of damper, is a large chamber J, where the calcined residue from the furnace is put to cool, thus preventing any nuisance from the smell of the burning mass. The chamber is provided with a pipe L, through which the vapours pass into the furnace. Several pipes E from the furnace pass through the {182} tank, to assist in warming the liquor. The residue, when cold, is drawn through doors from the chamber below the furnace. Roeckner has devised an apparatus (Fig. 68), consisting of a small chamber containing a series of pipes A, through which a stream of cold water constantly runs, in connection with the flue from his evaporator, for the purpose of condensing volatile bodies, and thus preventing, to a certain extent, contamination of the surrounding air.

A very economical form of evaporator is that invented by Porion, a French distiller, and named after him. It is shown in sectional elevation and plan in Figs. 69 and 70. It is largely used on the Continent, and also in England and Scotland. It consists of a large chamber k, the floor of which is slightly inclined from the chimney shaft, and through which the waste heat from the furnace a passes.

The liquor to be evaporated is run in at the end nearest the chimney from the tank placed above the chamber c. A number of cast-iron fanners i, dip into the liquor and revolve rapidly, usually at the rate of about 300 revolutions per {183} minute, producing and filling the chamber with a very fine spray, thus presenting a very large evaporating surface.

Between the furnace and the evaporator are placed the chambers c and f. In c a number of brick walls d are so placed that the flames from the furnace are intercepted and broken up. The object of this is to give time for all the products of combustion to be thoroughly burnt up, which would not be the case without the “smell-consumer,” as these chambers are called. This part is an addition to the original evaporator, and was devised by Messrs. Menzies and Davis. The liquor after having been concentrated in the chamber k runs into a trough placed alongside the doors h and flows into one or other of the furnace beds b where it is still further concentrated, and the residue ignited by the flames from the fires a. The draught can be regulated by the damper g, and also by one placed near the shaft j. The doors e, in the smell-consuming chamber, are for the purpose of cleaning out. The fanners i are worked by a small steam engine, not shown in the drawing. Under properly regulated conditions very excellent results can be obtained with this evaporator. The temperature of the gases near the chimney should not be higher than about 85°. By running the fanners at a very high speed the temperature of the gases may be even further reduced, thus showing the completeness of the evaporation.

This form of evaporator is open to the objection that the whole of the sulphur in the coal employed for the furnaces, finds its way into the recovered soda. It combines with the alkali to form sulphite of soda, part of which is decomposed in the furnace with formation of sodium sulphate, sulphide, and other sulphur compounds. The same objection, of course, applies, though perhaps in a less degree, to all systems of evaporation in which the flame is in contact with the liquor to be evaporated.

The Porion evaporator can be erected at very small cost, and costs but little for maintenance. It is capable of producing ³?/?₄ ton of recovered soda per ton of coal with liquors {184} of the usual strength. It has proved itself to be perhaps the most economical evaporator existing.

Some time ago there was erected in Lancashire an evaporator invented and patented by Mr. Alfred Chapman. It is shown in Figs. 71, 72, 73, and 74. The evaporation is effected at a low temperature in three vacuum pans E, and with the unusual result that the concentrated liquor gelatinises after leaving the third vacuum-pan, instead of taking the ordinary form of the concentrated products of other evaporators. It is said that this apparatus gives an excellent product, with great economy of labour and water, and with no drainage of foul liquor from the buildings. Observations extending over three months have proved that it evaporates 22 lb. of water from the liquor per lb. of coal used under the boiler. It is however very costly to erect.

The waste liquor is discharged into the tank A, whence it is pumped by the donkey-engine B, through the feed-heater C, {185} into the boiler D, which receives heat from the incinerating furnace H, and, in case of need, from an auxiliary furnace shown on the plan, under the feed-liquor-heater. The steam produced in D is taken to the first vacuum-pan at E, and having heated its contents, the products of evaporation pass over into the tubes of the second pan; this, in its turn, gives up its products of evaporation to the third, whence they go to the condenser of the vacuum-engine F. Thus the heat from the furnace H is used for incinerating the concentrated liquor {186} on its bed, for beating the feed-liquor in the feed-heater pipes, and for making steam out of the liquor itself in the boiler; this steam finally drives the donkey-pump and vacuum-engine, and causes the evaporation in the three vacuum-pans E. One advantage of this evaporator is the fact that the liquor is evaporated out of contact with the furnace gases.

Whatever be the method of evaporating or concentrating the liquor, the final treatment in the furnaces is much the same in every case. The furnaces shown in Figs. 69 and 70 may be taken to represent the ordinary form. The concentrated liquor is run on to either of the beds b, where the last portions of water are driven off by the heat from the fireplaces a, and the residual mass is ignited until all the organic matter contained in the liquors is carbonised and the soda is converted into carbonate of soda. This takes place in about 4 hours, according to the degree of concentration of the liquor as it is run into the furnaces. The running in of liquor should be done with great care, as explosions sometimes occur through the sudden liberation of steam on the liquor coming in contact with the hot beds. The charge should be allowed to remain in the furnace until it is thoroughly carbonised and all volatile matters have been driven off, otherwise a nuisance may be caused when the still burning mass is exposed to the air. Roeckner’s evaporator is provided with a special chamber into which the charge is drawn (J, Fig. 67).

The composition of the recovered soda varies with the nature of the liquors from which it has been obtained, and, as has been already pointed out, with the form of evaporator employed. It consists essentially of carbonate of soda, together with a certain amount of silicate of soda, if derived from liquors in which straw or esparto have been boiled, chloride of sodium, sulphate of soda, sulphite of soda, sulphide of sodium, and other sulphur compounds, the rest being made up of carbon and insoluble impurities. The amount of soda varies from 35 to 45 per cent. (Na₂O). The following analysis {187} will give some idea of the composition of Recovered Soda:—

The whole of the soda present as sulphur compounds is not lost, as a large proportion of it is present as sodium sulphite, most of which is converted into caustic soda by the causticising process.

A certain amount of soda is carried forward, partly mechanically and partly volatilised, to the flue leading to the chimney. This accumulates, and may be from time to time removed in the form of fine dust. It contains, besides carbonate of soda, much sulphate and chloride. In two different samples examined by the authors, the amounts of soda (Na₂O) present were 25·0 and 27·1 per cent.

This operation is known as “causticising,” and consists in heating a solution of the soda with lime. The decomposition which takes place is shown in the following equation:—

The recovered soda should be dissolved in separate vessels. Perhaps the best form of apparatus is a series of lixiviating tanks such as are used for dissolving the alkali in black ash. {188} By this means a nearly perfect exhaustion of the mass can be effected with a minimum of labour. Special tanks are sometimes made for the purpose, provided with mechanical stirrers.

It is essential in dissolving the recovered soda that a high temperature should be employed, as otherwise a portion of the soda present as silicate of soda will be lost, as it is only with difficulty soluble, and requires rather prolonged heating with water. Whatever the form of apparatus employed it should be so arranged that, after running off the strong liquor, the insoluble residue may be further treated with water. In the case of the vats mentioned above, this process is made continuous, pure water being run in at one end, and strong liquor flowing from the other. If other forms are used, the liquor after settling, may be run off by means of a pipe passing through the bottom or side of the vessel, and near the bottom, and consisting of two parts, one long, and one short. The short part is stationary, and is connected to the longer part by means of a movable knee joint, allowing it to be deflected. The liquor having settled sufficiently, the movable limb is lowered beneath the surface of the liquor which is then allowed to flow through. As the surface of the liquor falls, the pipe is gradually lowered. In this way the clear solution can be run off without disturbing the residue at the bottom. The open end of the pipe is usually covered with coarse wire gauze, to keep back insoluble impurities. With properly calcined recovered soda, the solution should be bright and almost colourless. If at all brown in colour, and if it has an empyreumatic odour, it indicates imperfect calcination. The residue in the dissolving tanks consists chiefly of carbonaceous matter, together with some soda, insoluble matter, &c.

The liquor is now ready to be causticised. This should be done in a separate vessel, although it is the practice in many mills to perform this operation in the same vessel in which the solution of the soda has been conducted. A good form of causticiser can be made from an old egg-shaped boiler, by cutting it in two along its length. {189}

It should be provided with two or more vertical steam pipes, connected at the bottom of the boiler with a horizontal pipe perforated with numerous holes. The vertical steam pipes should be furnished with injectors, whereby air is drawn in, and forced with the steam through the holes in the horizontal pipe. The stream of air serves the double purpose of thoroughly agitating the liquor and of oxidising any sodium sulphide in the recovered soda. The liquor before causticising should be reduced in strength to about 20–25 degrees Twaddle, which may be done with the washings of the residue from the recovered soda, or from the washings obtained subsequently from the lime-mud. This strength should never be exceeded, otherwise imperfect conversion into caustic soda is the result. This is due to the fact that concentrated solutions of caustic soda react upon calcium carbonate, forming sodium carbonate, and calcium hydrate, the reaction being the reverse of that indicated in the above equation. If the liquors are very strong in carbonate of soda, and comparatively free from sulphate, they should not be causticised at much over 20° Twaddle, if they contain much sulphate, and therefore less carbonate, the higher strength can with safety be adopted.

The causticising vessel should be provided with a stout iron cage or basket, into which the lime can be put. This should be securely fastened to the vessel, and should dip into the liquid.

The liquor having been properly diluted, is now heated by means of the steam pipes, and the lime put into its cage. It should be put in in lumps. As the liquor reaches the boiling point, the reaction will proceed rapidly, and the lime will gradually disappear; fresh lumps should be added if necessary. If the liquor is sufficiently heated the causticising will be complete in from two to three hours. The liquor should be tested from time to time; this is usually done by a workman. He withdraws a sample of the liquor, and after allowing the calcium carbonate to subside, pours off a portion of the clear liquid into a glass vessel. He {190} then adds an excess of either sulphuric or hydrochloric acid. If any effervescence takes place, due to the evolution of carbonic acid gas, he knows that the operation of causticising is incomplete; the heating must therefore be continued. It is difficult, without an undue expenditure of time and steam, to convert the whole of the soda into caustic: it should however be so perfect, that on testing only a very slight effervescence occurs. It is quite easy to convert as much as 95 per cent. of the soda, or even more. The actual amount converted can only be ascertained by a careful analysis of the liquor.

The amount of lime used is generally somewhat in excess of the theoretical quantity; 106 parts of sodium carbonate (Na₂CO₃) require 56 parts of lime (CaO): it is necessary, however, to add about 60 parts. A very good plan is to conduct two or even three causticisings in the same vessel without cleaning out or removing the calcium carbonate, using in the first operation a large excess of lime. The causticising being completed, the calcium carbonate and excess of lime are allowed to settle down, and the clear liquor run off by an arrangement such as that already described in the dissolving process. Fresh solution is then run in and the whole mass heated for some time, until the excess of lime is converted into carbonate. Fresh lime is then added if necessary until the conversion of the carbonate of soda is complete. The liquor is then allowed to settle, and is run off as before: this operation may again be repeated.

The residual calcium carbonate, or “lime-mud” as it is called in alkali works, is then washed once or twice by running in water, boiling up, allowing to settle, and running off the clear liquor. If these liquors are too weak for use in boiling fibres, they may be used for diluting fresh recovered soda liquor before causticising, or for dissolving the soda.

Some arrangement should be provided for removing as much as possible of the liquor from the lime-mud before throwing it away or otherwise disposing of it. This is best done by throwing it on a filter made of layers of stones, {191} ashes and sand, and covered at the top with perforated iron plates. The filter is connected with a vacuum pump. In this way very perfect draining is ac­com­plished, and the mud forms a hard mass on the surface of the filter, from which it can be easily removed with spades. In this form it contains only 50–60 per cent. of water. If properly washed it should contain in this state only about 2 per cent. of alkali (Na₂O). By careful manipulation, even this amount can be reduced.

The importance of thoroughly washing the mud can hardly be too much insisted upon. Where proper means are not employed for draining, the washing should be made more perfect. The lime-mud consists chiefly of carbonate of lime, together with silicate, free lime, &c. The following analysis is of a mud obtained by causticising recovered soda derived from the liquors in which esparto and straw had been boiled:—

As already pointed out the liquors contain a certain amount of soda, as sodium sulphide and other sulphur compounds. The presence of the former, if in large quantities, is objectionable, as it is liable to discolour fibres boiled in liquors containing it. It is therefore best to remove it. This can be conveniently done by blowing air into the liquors during the process of causticising: this has the effect of oxidising it to sulphate of soda, in which form it is harmless.

The air can of course be blown into the liquor by means of a pump; the most economical way is to connect with the steam pipe an injector constructed on the principle of the injectors used for feeding boilers and for other purposes. By this means a strong current of air is drawn in, and {192} being forced with the stream to the bottom of the liquor, passes through it in a number of fine streams.

If the amount of sulphide present be very high it may be necessary to prolong the oxidising operation beyond the time necessary for complete causticising.

In many paper-mills the causticising is conducted in circular vessels furnished with mechanical agitators. These are more expensive than the simple form described above, and they possess no special advantages. The use of causticisers in which neither mechanical agitation nor agitation by means of air is provided for is exceedingly wasteful of labour, time, steam, and soda. The lime-mud settles at the bottom as a hard mass, very difficult to manipulate.