Causticising.
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:—
Na2CO3
Sodium
Carbonate.
+
CaO
Lime.
+
H2O
Water.
=
2 NaOH
Caustic
Soda.
+
CaCO3.
Calcium
Carbonate.
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:—
| Calcium carbonate | 40·02 |
|---|---|
| Calcium hydrate | 5·13 |
| Silica | 4·01 |
| Sodium hydrate | 2·13 |
| Oxide of iron and alumina | 0·30 |
| Water | 48·10 |
| Other constituents | 0·31 |
| 100·00 |
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.
{193}
CHAPTER XIII. PAPER TESTING.
There are two points of view from which a paper may be tested: first, of physical or mechanical properties; secondly, of material composition. We shall consider the subject according to this division.
(I.) Quantitative measurements of such properties as resistances to breaking and tearing strains are seldom made by English paper-makers. In Germany, on the other hand, the matter has been very thoroughly investigated in connection with the work of the Königl. Techn. Versuchsanstalt, Berlin, and through the agency and influence of Prof. Sell, and C. Hofmann, a department has been organised exclusively for the work of paper testing. The results of the tests are becoming widely recognised by practical men and the trade in that country, as affording a true index of the quality of a paper. It is therefore of importance to give an outline of the methods employed.
The determination of the strain or weight which a paper is capable of supporting is a very obvious measure of the strength of the paper. Observations of the limiting strain or breaking weight are sometimes made by paper-makers, but the apparatus and method employed are usually crude. The simplest means consist in clamping the paper—a strip of standard length and breadth, arbitrarily chosen—at one end, the clamp being firmly held in a fixed support, and to the other attaching by means of a similar clamp, an ordinary scale pan, the whole arrangement hanging vertically. Into the pan, weights are added in due succession until fracture of the strip is determined. It is scarcely necessary to point {194} out that the errors of experiment with such a method are very great: indeed it has been found that even with the refined apparatus about to be described the errors are not inconsiderable. However, by exhaustive investigation, according to the well-known “law of errors,” these have been quantified, and a careful operator can therefore obtain results which are trustworthy. The apparatus in question is the Hartig-Reusch machine.[14] It is shown in sectional elevation and plan in Figs. 75 and 76.
[14] A complete description of this machine is given in ‘Civil Engineer,’ 1879.
The principle will be readily grasped by inspection of the diagrammatic representation of its essential parts—Fig. 77. The strip of paper is held horizontally by the clamps a and b, a being held by the fixed support A, b by the movable carriage B. B is connected by means of a swivel with the spiral spring F, and this again is similarly connected with the screw, which is made to rotate by the wheel D. By turning D, therefore, the spiral may be extended, and a corresponding strain communicated through B and b to the paper. The paper undergoes a certain elongation under the strain, and the carriage B moves from right to left in consequence. The rotation of the screw is continued, and the extension of the spiral proceeds until the paper is fractured. At this point it is required to determine, (1) the elongation of the spiral which is the measure of the breaking strain, and (2) the distance through which the carriage has moved, i.e. the elongation of the paper. Both these effects are communicated to the pencil G, the latter directly, since the pencil-holder is in rigid connection with B, the former through the rod I, from which, by a special arrangement, the horizontal is converted into a vertical motion of the pencil. This, therefore, traces a curve, of which the ordinates represent the strains, and the abscissæ the elongations of the paper produced by the strain.
The scale shown in Fig. 76 indicates the exact position of the clamp A. {195}
| FIG. 77. | FIG. 76. |
{196}
The values for the spiral spring—i.e. extension for a given load—having been determined by previous observations in a special apparatus, the curve obtained is at once a measure and a permanent record of these cardinal factors, breaking strain and elasticity. As with all other such instruments, the recording apparatus introduces certain errors, which, however, by careful investigation and modification in accordance with the results, have been reduced to a minimum. Nevertheless, the director, Dr. Martens, has recently adopted a simpler instrument, altogether similar in principle, but based upon a direct reading of the two movements, in which of course these errors do not appear. For the student, however, the recording instrument is the more instructive, and we have given it preference for description here, more especially as no difference in essential parts is involved.
Those who wish to pursue the matter into the most interesting details of the investigations made upon the subject, are referred to the papers published by the Institute for 1885.[15]
[15] Mittheilungen a. d. Königl. Techn. Versuchsanst, Berlin.
In testing the strength of papers by this or similar machines, it is important to observe the hygrometric state of the atmosphere at the time the trials are made, as this has been found to exert a considerable influence on the results, a paper being weaker the moister the atmosphere.
The results of the tests are expressed in the following terms:—The elongation is given directly in percentage of the original length. This is uniformly taken at 180 mm., a length arrived at after laborious investigation, as minimising the errors of experiment; in other words, as giving mean value with the minimum of variation. For the breaking strain an ingenious expression has been arrived at, viz. the length of the paper which suspended vertically, with one end hanging freely, the other fixed, would determine fracture at the fixed end. As the breaking strain would vary with the thickness, the numbers obtained in {197} units of force or weight for strips of constant breadth, would need correction in order to admit of strict comparison with one another. By substituting an expression in terms of the paper itself—since a paper of greater thickness, and requiring therefore a proportionately greater force to fracture it, weighs more per unit of area, and in the same proportion—all the numbers for breaking strains are strictly comparative one with the other. In the same way also the question of width may be disregarded.
A further mechanical test, forming a part of the scheme of investigation, is the resistance of the paper to rubbing. This test is an altogether empirical one, as the following brief description will show:—A piece of the paper, about 6 inches square, crumpled by successive folding in two directions at right angles, is grasped by the thumb and forefinger of each hand, at a distance of 3–4 inches apart. It is then rubbed upon itself across the thumbs a given number of times (seven is the number chosen) and held up to the light. If no holes are visible, the rubbing is repeated. The number of times necessary to repeat the rubbing until holes appear is the measure of the resistance. A sufficient uniformity in the results of this test has been observed to make it the basis of a classification of papers, in regard to their resistance to such disintegration; they are divided into the following eight groups, beginning with the lowest:—
- 0. Extremely weak.
- 1. Very weak.
- 2. Weak.
- 3. Medium.
- 4. Moderately strong.
- 5. Strong.
- 6. Very strong.
- 7. Extremely strong.
The classification of papers on the results of these tests cannot be more lucidly given than in the following scheme, under which the results are officially recorded:—
| Class. | 1 | 2 | 3 | 4 | 5 | 6 |
|---|---|---|---|---|---|---|
| a. Mean breaking length (metres) not less than | 6000 | 5000 | 4000 | 3000 | 2000 | 1000 |
| b. Mean elongation (per cent.) at fracture not less than | 4·5 | 4 | 3 | 2·5 | 2 | 1·5 |
| c. Resistance to rubbing | 6 | 6 | 5 | 4 | 3 | 1 |
{198}
This classification is based on the results of some hundreds of observations. It is interesting to note the differences observed in the numbers for a and b according to the direction for the test in which the paper is cut, i.e. in the direction in which it was run on the paper machine, or at right angles (see Chap. XI. p. [171]). The mean ratio for the breaking lengths (strains) may be taken as 1 : 1·6, i.e. the paper is about 40 per cent.[16] weaker across the web; the elongation under strain on the other hand is about double.
[16] In the statement of results the mean of the numbers obtained in the two directions is given.
It is also of interest to note the influence of the glazing process (p. [167]) upon the quality of the paper as determined by these tests.
First, we must notice the effect of the treatment upon the substance of the paper itself. The mean reduction of thickness is 23 per cent. On the other hand, the reduction of weight, calculated per unit of surface (square metre), is 6·7 per cent., whence we may infer an increase of surface, flattening out, in the process. These quantities, but more particularly the latter, will doubtless vary with the various methods of glazing and with the materials of which the paper is composed.
The breaking length (strain) shows a mean increase of about 8 per cent.; the elongation under strain, on the other hand, a diminution of 6 per cent.
For an interesting discussion of the question of the relative strengths of machine and hand-made paper see ‘Paper,’ by Richard Parkinson.
FIG. 78.
The thickness of a paper may be determined by means of an ordinary micrometer, such as is shown in Fig. 78. The paper is placed in the jaws of the instrument, and the screw {199} advanced until it touches the paper. The thickness is then read off on the scale. Other forms of apparatus are sold for the same purpose. In making a determination of the thickness of a paper it is necessary to take the mean of a series of observations at different points of the sheet, as the thickness may vary somewhat.
Determination of Composition of Papers.
(II.) The analysis of a paper naturally divides itself into two parts:—(a) The determination of the nature of the fibrous material of which it is composed; and (b) the identification of such adventitious substances as size and filling material.
(
a
A fragment of the paper is soaked for some time in glycerine, and is then carefully teased out with a pair of needles, and the fragments laid on a glass slip with a drop of glycerine. A cover-glass is then laid on and lightly pressed down so as to spread the fibres in a thin layer.
The microscopical features of the different fibres have been already described, and it is only necessary now to summarise the chief characteristics of the more important materials.