DIGESTERS, LINE SULPHITE FIBER. Sulphite fiber, or pure wood cellulose, su persedes rag stock in paper-making. The wood in (-hips or disks is boiled in great digesters with a solution of bisulphite of lime, and the main engineering problem lies in the construc tion of a suitable, economical, and lasting digester.
Tito following notes on digesters are Condensed from a valuable paper on Lime Sulphite Fiber 31anufacture in the' United States, by Major 0. E. Michaelis, U. S. A. (see Scientific American Supplement, No. 7:12, 1890): Exteriorly all the arc of metal. all of open hearth steel or iron plate, except the Schenk, which is of so-called deoxidized bronze. All are approximately cylindrical. except the Partington, which is spherical. The cylinders are upright in the Ritter-Kellner and Sehunk processes: in the Mit seherlich and Graham they are horizontal. The digesters are fixed, with the exception of the Partington and Graham, which revolve, the Graham about its longer axis. Considered merely as a vessel strong enough to stand a given pressure, the only available substance of which the digester can be made, looking from an economical standpoint. is iron or steel. The majority of the digesters are made of rolled iron plates; the Detroit, of open-hearth steel. There is no reason why our gun-iron, with a tensile strength approximating 40.000 lbs.. should not lie available for digest ers. They could he turned out, in sections ready for assembling; the advantages of such a substitution for the complicated rivat-work shell are evident. At remote inland points the large digesters must lie assembled in situ. and boiler-makers must now be transported for the purpose. A properly handled wrench would sulliee to set up the sectional east-iron construc tion. A 14 X 40 ft. east-iron digester tuts been designed, with a factor of safety of II, which will cost less than the riveted apparatus, to say nothing of the facility with which it can be transported and the ease with which it can lie assembled by unskilled labor. We come now in Ihw instil' of I he digester. Owing to the well-known affinity of the hisulphite solution for iron, all digesters made of this 'natal lutist be lined with it resistant, fluid-tight material, as a proteetioal against the solvent tetion of the "arid" mixtnre. The Schenk digester, 0 tun met al construction of deoxidized bronze, is assumed to be sufficiently resistant to the solution without protecting lining. The Graham, p'sytingIom and Ritter-I:1)11m- digesters are all kali-lined, the Mitscherlieh lire-brick lilted. The brinks used are of special form, made of Gerund refraetory clay the same as used in the mann lact tire of the Nassau Seltzer jugs.
1.4 Itys.—The vital point in these sulphite lies in the ability of the digester to resist the erosive ;lethl of the livid solution and its gaseous prodnets. Lead lots for centuries been used as a lining material in the manufacture of sniphori, Co I hat its application to the present sulphite, fiber proeesses lay tienm at hand. It is used in the Graham, lrytmglos, and Nitter-Kellner digesters. In speakine.: of vulph le process the Encyclo pedia Brilauttirtt uses the following language: `• The pulp or tiber produced by all these processes is of excellent quality, and can be prepared at a cost greatly lower than the soda process. The strength of the fiber is maintained unimpaired even after bleaching, mid white paper made solely from such fiber is in respect superior to that manufactured solely from pulp prepared by boiling with caustic soda. Dr. itsclierlich's process has been exten sively adopted in Germany, and there seems little doubt that these processes will in time sup plant the use of soda in the case of wood. The great objection to them all is that, as they all depend on the use of bisnlphite, which, being an acid salt, can not be worked in an iron boiler, the boiler must be lined with lead, and great difficulty has been encountered in keeping the lead lining of the boiler in repair." The primary, indispensable condition in protecting iron sulphite boilers with lead is that the lining must be continuous—that is, liquid-tight. Now, lead has a linear coefficient of ex pansion much more than double that of iron: in these processes it is subject to a change of temperature of at least 240° F. (300*--60°), and the unavoidable resulting flow of the metal can not he compensated for by permitting sections to expand and to contract freely upon each other, for that would require open joints, a violation of our primary condition. The lead lining must in some way be attached to the iron shell, for otherwise it would soon collapse, or go to pieces in some other way. Only three practical was offer themselves for the attach ment of the lead lining to the iron. It may he bolted on at proper points; it may be, to borrow a plumber's phrase, "tacked on" aL appropriate places, or it may be completely sol dered on. The first two methods permit, as is evident, under variations of temperature, changes in the superficial area of the lining; the latter method forcibly resists this, and limits the flow of the lead during the life of the solder union to molecular expansion only.