CHAPTER V.

Chapter 95,651 wordsPublic domain

PROCESSES FOR ISOLATING CELLULOSE FROM PLANT SUBSTANCES.

We are now in a position to discuss generally the various methods by which the paper-maker obtains cellulose from the different raw materials. The special treatments necessary for each of the more important of these will be given more fully subsequently. These methods, though they vary considerably with the different materials to be treated, may be roughly divided into two groups—the alkaline and the acid processes. The former, which we will call _a_, comprises all processes in which caustic soda, carbonate of soda and lime, or mixtures of these are employed; the latter, _b_, the various processes which have been introduced of late years, involving the use of sulphurous acid, either alone or in combination with a base, such as lime or magnesia.

The processes in group _a_ may be applied to the treatment of every known fibrous vegetable material; those in group _b_ are at the present time applied exclusively to the preparation of pulp from wood.

Group _a_ may be conveniently subdivided according to the nature of the materials operated upon.

It is obvious that those materials, such as bleached cotton and linen threads and rags, which have already undergone treatment by the textile manufacturer, and are therefore more or less already in the state of pure cellulose, require but little chemical treatment at the hands of the paper-maker, whose attention is therefore chiefly directed to the removal of such adventitious matters as size, grease, &c. This can generally be effected by the employment of weak solutions of caustic soda, or even of lime at a low pressure. {63}

We now have to consider the treatment of those materials which consist of the compound celluloses.

All methods for the isolation of cellulose from the compound celluloses depend upon a hydrolytic resolution of their constituents, i.e. a splitting up by combination with water into cellulose on the one hand, and a series of soluble derivatives on the other. This, as will be shown, may to a certain extent be accomplished by the action of water itself at a high temperature. As, however, the products of such an action are acid bodies, which, if allowed to remain in contact with the cellulose, would injuriously affect it, and would induce the production of complicated bodies, the removal of which from the cellulose would become increasingly difficult, it is necessary to have present a body such as caustic soda, which by combining with these acid bodies, removes them as such from the sphere of action.

In certain raw materials, such as straw and esparto, we have, in addition to the compound celluloses which form, so to speak, the groundwork, a certain proportion of fatty and resinous bodies, whose removal is brought about by the action of the caustic soda converting them into soaps.

The compound celluloses may be divided, as we have seen, into three classes—pecto-celluloses, ligno-celluloses, and adipo-celluloses. The last-named being present in only a few paper-making materials, and then only in very small proportion, is without any practical interest. We are concerned therefore with the two former only.

(1) _Pecto-celluloses._—From what we have seen of the nature of the pecto-celluloses, it follows that for their resolution a tolerably simple treatment with soda will be sufficient.

The chief members of this group in which the paper-maker is interested, are esparto and straw. The former being but very slightly lignified, its resolution can be effected at a very low pressure; in fact it is even possible by using tolerably strong solutions to do it by boiling in open vessels.

Straw, on the other hand, though it possesses certain features in common with esparto, is more lignified, and in {64} consequence, necessitates a much more energetic action to complete its resolution. It is therefore usual to boil it at somewhat high pressures (40–70 lb.), and with larger quantities of soda than are demanded in the case of esparto. It is, moreover, essential, so to regulate the treatment, that even the most resistent portions of the straw, such, for example, as the knots, shall be completely resolved, otherwise unbleached portions are liable to find their way into the finished paper. By increasing the quantity of soda, it is possible to boil under reduced pressure, and _vice versâ_; this may be taken as a general principle applicable in all other cases.

We now come to the second class of celluloses, that is to say, the ligno-celluloses. Of these, perhaps the most important are jute, manilla, and wood.

For their _complete_ resolution, it is imperative to employ very strong solutions at very high temperatures and pressures; thus in the case of wood it is necessary to boil at temperatures corresponding to a pressure of 100–120 lb. per square inch.

The use of strong caustic soda solutions at high temperatures is attended with very serious objections, apart from the question of cost. Among these may be mentioned the destructive action of the soda upon the cellulose itself, involving a considerable loss of pulp; the de-hydration, condensation, and oxidation of the non-cellulose portion of the fibre (which, as we have seen (p. 25) is of the nature of an aldehyde, and therefore peculiarly susceptible of such changes), and the formation of dark coloured products which remain intimately combined with the residual cellulose; the inconvenience of working at very high temperatures and pressures.

With a view to removing the difficulties attending the use of soda, various processes have from time to time been proposed, in which the hydrolytic agent is a solution of sulphurous acid in water, either alone or in combination with a base. These processes are comprised under group B. {65} Several of them have lately acquired great commercial and industrial importance, and a clear understanding of them is therefore essential to the student of paper-making.

Though applicable to lignified fibres generally, they have only been applied up to the present time to wood, and it is in this connection, therefore, that we propose to consider them.

In order to bring out more clearly the fundamental differences underlying the two classes of processes, it will be well to consider, somewhat in detail, these processes, which, although they have now a merely historical interest, will assist the student in thoroughly grasping the rationale of the modern and improved methods.

Water Process.—Wood is to a certain extent resolved by treatment with water at a high temperature, the degree of disintegration being, within certain limits, proportional to the temperature at which it is digested. The volatile products of the resolution are chiefly acids of the acetic series, furfurol and terpenes. The non-volatile soluble products are for the most part acids. This process was studied by Fry, who, in 1866, made it the subject of a patent. He found that under the most favourable conditions of treatment, the yield of brown pulp amounted to 70 per cent. of the wood employed. The objections to the process were sufficiently obvious: the pulp produced was of low quality, and the soluble products acted powerfully on the iron digesters employed. The chemical causes of its failure lie in the prominence of the conditions which determine re-combinations of the products of resolution with one another, viz. oxidation, high temperature, and the presence of acids. It is worthy of note that in one experiment the amount of acids produced from 100 parts wood were equivalent to 3·4 parts of soda (Na_{2}O). This was by far the most serious defect in the process, and it eventually caused its abandonment in favour of the older method of treatment with caustic alkali.[9]

[Footnote 9: See Journ. Soc. of Arts., February 1883, p. 223.]

Alkali Processes.—The first to develop the alkali process {66} were Watt and Burgess, in America (1853–1857), and Houghton in this country. To follow up the several modifications introduced is unnecessary; they differ from one another in the economical balance which they strike between temperature and quantity of chemicals: thus Watt and Burgess employed a lye of 12° Baumé, and digested at 60 lb. pressure (equal to 152° C.), whereas Houghton used a lye of 4° B., at 100–180 lb. One great defect of these processes was the gradual neutralisation of the alkali, necessitating as a compensating condition the employment of a very high temperature towards the end of the operation. To overcome this objection, Ungerer devised a process which aimed at securing a properly graduated action of the lye. This process, being based on an eminently rational view of the action of the alkali, is worthy of a more extended notice, especially as the principle on which it is founded is capable of wide application. A system of nine vertical digesters, connected together in series by means of pipes and cocks, so that the liquid contents can be transferred from one to the other in succession, is the main feature of the method employed. In connection with this system there is a double service of pipes, by which the hot soda lye at 90 lb. pressure, and cold water, may be respectively introduced. The operation is so conducted that in five of the digesters wood is being treated at a high temperature, in three the pulp is being washed, while the remaining one is being charged with fresh wood.

The whole round of operations may be described thus, numbering the digesters according to their position in the series:—Nos. 1, 2, and 3, Washing the finished pulp; 4, Final treatment with lye at the extreme pressure of 90 lb.; 5, Digesting at lower pressure with lye previously employed in 4; 6, Treatment at still lower pressure with lye which has already done duty in 4 and 5; and so on to 8, where the liquor, having passed through the series, is employed at 15–20 lb. pressure on a fresh charge of wood. The washings are conducted in such a way that the water which is introduced into 1 displaces the solution in each digester without {67} the last being allowed to escape. The introduction of the fresh liquor at 4 is similarly accomplished, 1, 2, and 3 being cut off by closing the cocks which control the ingress and egress of liquid.

The pulp produced by this method is of a sufficiently pale colour to be capable of application for many purposes without bleaching. It is converted into a perfectly white pulp by treatment in the ordinary way with from 3 to 4 per cent. of bleaching powder, calculated in the dry pulp.

As far as it goes the method is practically perfect, and it appears to have given sound results, at least as regards the main product. The principal disadvantages were the complications attending such an elaborate system of working which would doubtless make it costly.

We have next to consider a class of processes which are based primarily upon the action of an alkali, but which introduce the simultaneous action of a soluble sulphide, generally that of sodium. As far back as 1855 Jullion patented in this country a process of this nature, which, however, does not appear to have been applied to wood. The results obtained with such materials as straw and esparto appear to have been very unsatisfactory, and to have led to the early abandonment of the method.

Recently the increased attention which has been given to wood as a raw material has led to a revival of these sulphide processes for its treatment. We believe that at least two processes of this class are being worked on the continent, viz. those of Dahl and Blitz.

That of the former consists in heating wood, at pressures varying from 60–120 lb. per square inch, with a solution of the following composition:—

Sodium sulphate 37 Sodium carbonate 8 Sodium hydrate 24 Sodium sulphide 28 Various compounds 3 ─── 100

{68}

This solution is obtained only after the process has been some time at work, and it then remains tolerably constant.

The wood, to commence with, is boiled with a solution containing a large quantity of sodium sulphate, with a small quantity of sodium hydrate, obtained by boiling sodium sulphate with lime. The liquor from this operation is then evaporated and calcined, the result being a fused mass which, when dissolved in water, yields a liquor containing the above-mentioned ingredients. The loss of chemicals is made up by the addition of fresh quantities of sodium sulphate.

From experiments which the authors have made with this process, it appears, that while it is perfectly true that pulp can be obtained, it is necessary to employ either very strong solution or very high pressures. This is equivalent to saying that either the hydrolytic action is weak, or the tendencies to reversal of the hydrolysis are strong, and indeed both are to be predicted from the nature of the solution. One great objection to the process is that during the boiling a quantity of very foully-smelling compounds are produced.

The process of Blitz consists in heating wood to from 50–60 lb. pressure, with a solution of sodium sulphide, containing an infinitesimal proportion of vanadate of ammonia.

It is difficult to see what influence the presence of the costly vanadate of ammonia can have on the result. Actual experiment on this point has failed to convince us of its efficacy.

Although in these sulphide processes a strongly reducing agent is present, its effect in preventing oxidation is more than outweighed by the weakening of the hydrolytic action of the soda, both directly by its combination with sulphur, and indirectly by the complication which it induces by combination with the soluble bye-products. Further, as regards the final product, they offer no advantage over the caustic alkali processes. There is, however, a considerable advantage in point of economy, which should lead to their adoption in districts where the production of malodorous gases is permitted. {69}

Acid Processes.—We now come to consider the processes which are based upon the hydrolytic action of acids as a primary condition. We have seen that in the water process this condition obtains as a secondary result, and that while it aids in resolving the wood, it also determines the limit of the action, in consequence of its promoting secondary combination amongst the soluble products. The use of powerful oxidising acids, such as nitric and nitro-hydrochloric, induces an action which is sufficiently drastic to cause a complete resolution of the non-cellulose constituents of the wood with the formation of soluble products. The use of these acids has been made the subject of numerous patents.

The process of Barre and Blondel consisted in digesting the wood in the cold for twenty-four hours in 50 per cent. nitric acid, by which it was converted into a soft fibrous mass. This was boiled for some hours in water, afterwards in a solution of sodium carbonate, and finally bleached in the ordinary way. Many other patents have been taken out in this direction, but none have met with practical success, owing doubtless, to the high cost of nitric acid, and the difficulty of recovery. A process has lately been patented by Young and Pettigrew (No. 14,988, 1884) in which they claim the use of either nitric or nitrous acids; the acid fumes which are evolved being condensed and reconverted into nitric acid. The process has not yet, however, been tried on a practical scale.

The resolution of wood by the hydrolytic action of non-oxidising acids, is for reasons already stated, very imperfect. An important feature, however, of this action is the production of glucose as a bye-product, and a process having the double object of producing this, together with a brown pulp, was patented by Bachet and Machard in 1864, and worked for some time in Switzerland. The process consisted in boiling the wood for twelve hours in dilute hydrochloric acid; the solution, which contained glucose to the amount of 20 per cent. of the weight of the wood, was drawn off, neutralised with chalk, and fermented by the addition of {70} yeast. Alcohol was separated from the fermented liquor in the usual way. The residual pulp, after being ground with stones, was made into coarse packing paper. The process failed commercially.

The next processes in the order of the development of our subject, but not chronologically, are those involving the use of reducing acids, i.e. those which combine with their hydrolytic function a de-oxidising or anti-oxidising action. Of these the only one which has been practically investigated is sulphurous acid. A full account of the earlier discoveries in regard to the use of this reagent, for the special purpose under consideration, will be found in the _Papier Zeitung_, 1884, No. 51, p. 1927; this should be consulted by those who are interested in the history of a subject which has now grown to be of the first importance.

The first patented process claiming the use of sulphurous acid, in the form of its aqueous solution, for the purpose of disintegrating wood, was that of Tilghmann (1866). So far as is known, however, the results obtained by the inventor with the acid alone were such as to lead him to abandon it for solutions containing a certain proportion of base. The reasons, doubtless, were the greater facility of preparation and simplicity of application of such solutions.

More recently Professor Pictet, of Geneva, on the basis of the experience acquired by him in the preparation and employment of sulphurous acid in his patent freezing processes, has revived and perfected a process which in other hands had proved impracticable.

The method and apparatus employed will be found fully described in the author’s specification. The process consists in digesting the wood in closed vessels with an aqueous solution of the acid (10–12 per cent. SO_{2}) at 176°–194° F. (80°–90° C.), and under the corresponding pressure (60–75 lb. per square inch). A complete disintegration of pine wood is effected in about twenty-four hours’ treatment. The yield of pulp is, we are informed, from 40–50 per cent. of the wood (_Pinus sylvestris_); it is also stated to be readily bleached {71} with a moderate amount of bleaching powder, and to yield a pulp of the highest quality. Further, the recovery of the sulphurous acid, for which special apparatus is provided, is practically complete (95–98 per cent.). In view of the probable utilisation of the bye-products, it is interesting to observe that they are obtained in a form in which they can very readily be treated.

The process being as yet undeveloped on a large scale, we cannot do more than recommend it to the consideration of paper-makers. At the same time we would draw attention to the features of interest which it possesses in regard to the general aspect of our subject. The resolution which it effects is obviously one of simple hydrolysis, and it affords an indirect proof of the chemical complications which attend all other processes for disintegrating wood which employ temperature much above the boiling-point of water. The soluble bye-products are obtained in the simplest form, and may be isolated by mere evaporation. As regards the chemical features of the pulp, they will presumably be different from those of any other, whether on the side of advantage or not it remains to decide. Provided the process fulfils the condition of economical working, we have no hesitation in predicting for it a considerable development.

The difficulties attending the employment of a gaseous acid have doubtless operated in deterring many, in addition to the originator, from a complete investigation of its action, and have led them to adopt in preference one of its several compounds with bases, the employment of which defines a group of processes to which we now desire to call attention. There is a variety of processes agreeing in the essential feature of employing a bisulphite as the disintegrating reagent, and differing from one another only in the less essential details of method.

The general principle of these processes will be readily understood from what has gone before. The chief agency is the hydrolytic action of sulphurous acid, aided by the conditions of high temperature and pressure: and the subsidiary {72} agencies are—(1) The prevention of oxidation; (2) The removal from the sphere of action of the soluble products of resolution in combination with the sulphite as a true double compound. This mode of union is well known to be peculiarly characteristic of all aldehydes, to which class we have shown that the non-cellulose constituents of wood belong. The combination is expressed by the general equation—

M.CHO + SO_{3}HNa = M.CH(OH/SO_{3})Na.

Aldehyde. Sodium Double compound. bi-sulphite.

(3) The removal of a portion of the constituents in combination with the base, i.e. with expulsion of sulphurous acid.

The verification of these views has been afforded in the course of an investigation of the bye-products from the Ekman process, undertaken by the authors for Messrs. Thomson, Bonar & Co., the proprietors of the patent.

As the various bisulphite processes differ chiefly in their working details, the essential feature of the employment of a solution of sulphurous acid, combined with a certain amount of base, being common to them all, it will not be necessary to do more than examine one or two typical cases. We shall select those of Ekman and Partington, the latter being based upon Francke’s patent, with certain modifications and additions.

(1) The Chemicals employed.—In both cases sulphur is used as the source of the sulphurous acid. The Ekman process starts from magnesite, a native carbonate of magnesia. This is converted by calcination into magnesia, and in this state is introduced into lead-lined towers, through which is passed sulphurous acid gas, obtained by burning sulphur in suitable furnaces, while a stream of water trickles down from the top. The supply of the gas being properly adjusted, a continuous production of a solution of magnesium bisulphite of uniform strength is obtained. To secure this uniformity, constant attention is necessary, and further, the loss of sulphurous acid by oxidation to sulphuric acid, and by diffusion, is, in consequence of its distribution over so large a surface, {73} and with free exposure to the air, necessarily very large. It is unnecessary, however, to point out that the preparation of the magnesium bisulphite is capable of very considerable modification; in fact, we understand that numerous improvements in this direction have already been introduced by the proprietors of the process.

Partington’s method, in addition to the advantage of starting from lime, the cheapest of all basic substances, is free from the disadvantages referred to as attending the use of a tower. It consists in passing the sulphurous acid gas through a series of three vessels connected together with pipes. Into these vessels a charge of milk of lime is introduced, and the two first being closed air-tight, the gas is passed in. The third remains open to the air, and from it escapes a continuous stream of nitrogen, resulting from the removal of the oxygen by the burning sulphur, from the stream of air which is blown through the sulphur retort at a pressure of 5 lb. to the square inch. The expenditure for chemicals is under 1_l._ per ton, whereas, by the Ekman process, it has been variously estimated at from 2_l._ to 3_l._ 10_s._ per ton. Provided therefore, that as good pulp can be obtained by Partington as by Ekman, the former offers considerable advantages both from point of view of simplicity and economy. Doubtless the cost of chemicals could be somewhat reduced in both systems by the adoption of a cheaper form of sulphur, such as pyrites.

Preparation of the Wood.—Although the acid sulphite processes can, by suitable modifications, be made available for the preparation of cellulose from almost all kinds of wood, yet it is necessary, in order to insure uniformity of result, not only to take care that the wood is carefully selected, but that it is freed from bark and roots, which, if allowed to remain, would be imperfectly pulped, and would be exceedingly difficult to bleach. The most suitable kind of wood is _Pinus sylvestris_.

After the bark has been carefully removed the logs are sawn into boards of ordinary sizes. This treatment {74} exposes the knots, which are then removed either by boring or cutting out with an axe. The pieces are then passed through a machine, which cuts them or rather breaks them into fragments about an inch long. In order that the acid sulphite may as quickly as possible enter into the wood, these fragments must be crushed and opened out by passing through heavy rollers: moreover, the wood is cut _across_ the grain, by which means it is more or less split up into laminæ. A machine for this purpose has been patented by Fry, Ransome, and Wilkie (No. 11,389, 1884).

The Boiling Process.—Ekman uses a lead-lined, jacketed cylindrical boiler, suspended on trunnions, so that it can be turned upside down for discharging. The pressure in the outer jacket is 70 lb. per square inch, while inside the boiler it is 90 lb., the difference being due to the tension of the sulphurous acid gas. The duration of boiling is from ten to twelve hours. Partington, on the other hand, boils with live steam, and therefore is enabled to use a boiler of simpler construction. His boilers are spherical, lead-lined and rotary. The pressure employed does not exceed 60 lb. per square inch, the time of boiling being about sixteen hours.

Various other sulphite processes have been from time to time devised, which differ but little either in principle or in working detail from those described, and therefore a brief mention of them will suffice. Mitscherlich employs a bisulphite of lime in somewhat weaker solution than that employed by Francke or Partington, and in order to produce the same result he is obliged to extend the time of boiling to from 40 to 60 hours.

Graham, after treating the wood with a mono-sulphite, injects into the boiler either a solution of a bisulphite or gaseous or liquid sulphurous acid.

Flodgvist uses a solution prepared by dissolving bones in sulphurous acid solution.

The cellulosic products of the various processes present certain slight differences, but so far as has been investigated, not such as to constitute a distinct advantage for one or {75} another. One feature of these processes, to which but little attention has hitherto been paid, is that of the utilisation of their bye-products. These bodies are, as already indicated, obtained in a condition not materially different from that of the parent substance, the non-cellulose constituents of the wood. One important property of these bodies, and one capable of considerable application, has been made the basis of a patent by the authors. When a solution of gelatin is added to the liquors in which wood has been treated by any of the bisulphite or sulphurous acid processes, a body is precipitated in the form of a caoutchouc-like mass which can be dissolved in neutral sodium sulphite solution, or in weakly alkaline solutions, and can be reprecipitated on addition of alum to the solution. The substance consists of a compound of the gelatin with the non-cellulose constituents resembling, but not identical with, as has been supposed, the precipitate obtained by adding tannic acid to a solution of gelatin. The solution of this substance in sodium sulphite can be applied to many of the purposes to which gelatin itself is ordinarily applied.

A serious objection to all processes involving the use of sulphurous acid or acid sulphite, and one which has to some extent deterred paper-makers from adopting them, is the necessity of using a boiler lined with lead. In addition to the fact that such boilers are heavy and costly, they are objectionable on account of the difficulty and expense of keeping them in thorough repair. This difficulty arises from the fact that, owing to the different co-efficients of expansion of lead and iron, the lead lining originally placed in contact with the iron or steel shell of the boiler, becoming detached on heating, swells out in various places, which are thus rendered weaker and liable to fracture. This is especially the case if, in the course of lining the boiler, any air has been allowed to remain between the two linings. Various expedients have been proposed to obviate these defects but with only partial success. Graham (Eng. Pat. No. 5367, 1883) coats the plate of his boiler by pouring molten lead upon the perfectly clean surface which has been {76} previously prepared with zinc chloride. Sometimes minute holes are drilled in the iron plate to allow the escape of air.

Notwithstanding all the attempts that have been made, the use of a lead-lined boiler is a serious objection, and any process which will fulfil the conditions on which the bi-sulphite processes depend, and which at the same time admits of the use of the ordinary iron boilers, is an obvious desideratum. Such a process is that patented by one of the authors (Cross, No. 4984, 1880), claiming the use of solutions of the neutral sulphites with or without the further addition of an alkali. This process is very suitable for the treatment of fibres such as straw, jute, &c., but not economically applicable to the preparation of wood pulp on account of the cost of chemicals. The preservative effect of the sulphite is shown by the largely increased yield of pulp.

The chief advantages of the various bi-sulphite processes for the preparation of wood pulp over the old soda processes are, (1) the increased yield of fibre; (2) the preservation of its original strength, and (3), the economy in chemicals.

It is well known that cellulose, even in its most stable forms, is attacked more or less by hot solutions of the caustic alkalis, and is more or less profoundly modified according to the strength of the solution and the temperature at which it is employed; while on the other hand the solutions of the bisulphites affect it but little. Thus, the authors have exposed jute cellulose, which is of the same nature as that obtained from wood, to the action of magnesium bisulphite under the conditions which obtain in the Ekman process, without its undergoing sensible loss of weight, and further, they have determined the amount of cellulose in freshly cut grasses, in which it is but imperfectly elaborated, and therefore very susceptible of modification, both before and after treatment by the process, and in each case the percentage obtained was the same.

There is therefore no doubt that in the bisulphite processes, even the structurally delicate forms of cellulose are conserved. As before indicated, the influence of this upon the yield of cellulose is of course very marked; the yield from white pine, {77} by the soda process, being approximately 33 per cent., whereas by the Ekman process it is from 45–50 per cent. It is important to remember that this increased yield of cellulose cannot fail to affect the quality, and therefore the paper-making properties of the pulp. Papers made entirely by the bisulphite processes have a strongly marked individuality, being distinguished by their hardness and transparency. In regard to the bleaching of the pulp, it will be found that some practical experience is required for the control of the operation, so as to produce uniform results.

Although the pulp produced by these processes is of a very light-grey colour, so light indeed that they may be used for certain low classes of white paper without further treatment, yet to obtain a pure white colour almost as much bleaching powder is required as in the case of soda pulps, although the latter are much darker in colour. The chief points to be observed are (1) the complete removal of the sulphites from the pulp before the addition of bleaching liquor, which would otherwise be ineffectively consumed; (2) the avoidance, in washing, of waters containing iron, which would combine with certain constituents of the pulp to form dark-coloured bodies.

The following Table, compiled from Dr. Hugo Müller’s ‘Pflanzenfaser,’ is of interest, as indicating the probable yield of pulp from different woods, compared with the amounts obtained in actual practice by the Watt and Burgess (soda) process:—

----------+--------+----------+--------+---------+------------+------------ | | Water | | Lignin, | | Pulp—Watt | Water. | Extract. | Resin. | &c. | Cellulose. |and Burgess | | | | | | Process. +--------+----------+--------+---------+------------+------------ Birch | 12·48 | 2·65 | 1·14 | 28·21 | 55·52 | 40·00 Pine | 12·87 | 4·05 | 1·63 | 28·18 | 53·27 | 34·70 Oak | 13·12 | 12·20 | 0·91 | 34·30 | 39·47 | 20·60 Chestnut | 12·03 | 5·41 | 1·10 | 28·82 | 52·64 | 25·10 ----------+--------+----------+--------+---------+------------+------------

The following Table, which gives a concise account of the various typical processes that have been employed for the preparation of wood pulp, will be readily understood in connection with the theoretical considerations previously discussed:— {78}

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