US1978962A - Washing of nitrocellulose and the like - Google Patents

Description

Patented Oct. 30, 1934 UNITED STATES PATENT OFFICE Milton 0. Schur, Berlin, N. H., assignor to Brown Company, Berlin, N. 11., a corporation of Maine No Drawing. Application May 14, 1930, Serial No. 452,522

3 Claims.

This invention in its broadest aspects relates to the recovery of liquid from a liquid bath in which porous sheet material is submerged, its object being to make possible a practically complete displacement of the liquid from within the pores of the material. Briefly stated, the process of the present invention comprehends the removal of the given liquid from the bath in which the sheets are submerged, While supplying thereto another liquid capable of forming a distinctlayer in association with the bath. This invention more specifically relates to the manufacture of nitrocelluloses, its general object being to make possible important advantages and economies in this line of manufacture and at the same time to produce a high grade product. In accordancewith the process of the present invention, sheets of interfelted cellulose fiber are steeped in a bath of suitable nitrating acid, and after nitration has been 'eiiected to the desired degree, the acid bath is progressively withdrawn at the bottom of the bath while it is being displaced by Water which is delivered gently onto the surface of the acid bath, preferably at the same rate as the rate of withdrawal of the acid. I have found that such a process is advantageous, in that the rate of withdrawal or draining of acid from the bath may be comparatively rapid, with little mixing of the water and acid at the interface formed between the acid and water layers, as the draining is continued until the interface between the water and acid layers has disappeared, so that substantially only water is present in the bath along with the sheets. It is further advantageous, in that the cellulose is used in a condition in which it lends itself to easy handling, storing, and shipment, both before and after nitration. Other advantages of the process will appear as this description proceeds.

The displacement of the nitrating acid by water has been applied in connection with the nitration of cotton linters, but, in such case, unless the rate of displacement is exceedingly slow, there is substantial mixing of acid and water across the liquid interface; and, although theacid may be almost entirely displaced from the interstices of the mass of nitrocellulose, the last portions of acid drained ofi may be so diluted with water as to make their reclamation for reuse in nitration difficult and expensive. 'The proponents of this socalled displacement method as applied to cotton linters have recommended a rate of displacement of about two inches in depth of nitrating vessel per hour (see British Patent No. 8,278, to the Thompsons, dated April 9, 1903). This means that if the vessel has any substantial depth, say,

24 inches, twelve hours may be consumed in the draining operation. In investigating this method,

I have found that in the preparation of nitrocellulose from cotton linters there is serious weakening of the last, say, 15% of the acid drained from the nitrocellulose, even when the rate of displacement is only two inches per hour. To be sure, practically all of the acid is displaced from the nitrocellulose, but the weakening of the last portion of the acid, say the last 15%, makes the reclamation of this portion for reuse in nitration hardly worth While. When the rate of displacement is increased, the weakening effect becomes more marked; and, when the rate is increased to as high as 30 inches per hour, there is so much mixing and dilution of the acid with water at the liquid interface that pronounced heating ensues and the yield of nitrocellulose is seriously decreased. These troubles with the displacement method as applied to cotton linters probably more than offset the benefits made possible thereby and account for its not being generally adopted by the industry.

On the other hand, when sheets of cellulose are nitrated, the displacement of acid by water may be practised to very much greater advantage, for not only is the acid displaced completely by Water floating on it, but the displacement is effected with comparatively little mixing at the interface, even though the rate of draining is as high as 30 inches per hour. Such a rapid rate of draining is important, for two reasons, as with slow draining:

(l) The nitrocellulose at the bottom of the nitrating vessel is exposed to the action of the nitrating acid much longer than the upper layers, with the result that the nitrocellulose charge may be of non-uniform character;

(2) The apparatus and the nitrating acid are tied up unduly, so that more equipment and acid are necessary for a given production capacity.

The difierence in results of practising the displacement process with sheets of interfelted cellulose fiber as against cellulose in bulk or shreaded condition is probably due to the fact that with the sheets, particularly when placed in the bath vertically and carefully separated from one another, the liquids have comparatively little tendency to mix, probably by reason of the smooth surfaces of the sheets, the straight line flow through the capillary pores in the sheets afiorded by the interfelted relationship of the fibers and the comparatively narrow clearances between the sheets; whereas, with cellulose in bulk, because of the lumpy character of the charge, unless the interface is lowered exceedingly slowly, the draining is more or less irregular and turbulent from lump to lump and mixing is apt to ensue. Regardless of what may be the actual causes underlying the surprising and important results that I realize by the practice of my invention, the fact remains that the surfaces of the sheets, the pores therein, and their other characteristics comport with a maintenance of a substantially undisturbed condition at the interfacebetween the nitrating acid and the water,so much so thata' substantially unmixed condition of these liquids 7 can be maintained at a rate of the movement of the liquid interface as high as about 30 inches per hour, as already described. Aside from the advantages and economies accruing from the actual nitration operation, the feature of using cellulose'in sheet form is of utmost commercial significance, particularly when the cellulose employed as a raw material is wood pulp which has been refined to high alpha cellulose content and possesses other qualities'rendering it suitable as a base for nitrocellulose manufacture. 'In such case, the pulp manufacturer is equipped to run theprocessed pulp in the form of an aqueous suspension over .so-called pulp driers, which are built as large units capable of handling at low expense as high as 200,01" more, tons of dry fiber per day. These sheets, which'are known as drier sheets, may be of a thickness of about .040 inches and otherwise be similar to those being used currently as a raw materialfor the production of artificial silk by the viscose process. When used as a raw material for nitration in the process of the present invention, the sheets may be supported vertically' in closely spaced, parallel relation, as in 3 frames; or they may be suspended as from glass rods or clamps. They should be lowered suffi ciently'in the nitrating vessel to permit the bath of nitrating acid to cover them completely, their spaced relation thus permitting uniform access of acid to allthe surfaces of the sheets.

, The process of the present invention is applicable tocellulose suitable for nitration of various origins, including wood pulp and cotton fiber. While, as hereinbefore noted, it is advantageous to use comparatively thick sheets of unbeaten washing, stabilizing, and dehydrating operations.

This-means easy handling and a compact and convenient form in which to store and ship the nitrocellulose. It further means that not only may thenitrating equipment be of the simplest kind, but further that the other equipment of the nitrating'pl'ant may be of the same character, and

that labor costs may be relatively low.

The principles of the'present invention are applicable when it is desired to displace a given a liquid bath in which the sheets are submerged with mother liquid iloatable on the given liquid. For instance, after the acid bath in which the sheets are suspended has been displaced by water to form a water bath, and thesheets have preferably been stabilized in the usual manner, i. e., by

boiling in slightly acidulated water, and washed, the water bath may be displaced by alcohol to effect a dehydration of the sheets.

I claim:

1. A process of uniformly nitrating cellulose and rapidly removing the mixed nitrating acid therefrom, which comprises maintaining integral porous sheets of interfelted cellulose fiber in closely spaced vertical relation in a bath of the mixed nitrating acid until nitration has been effected; progressively draining the acid from the bath while delivering water onto the surface of the bath to form a floating water layer, and continuing such draining until the interface between the water and acid layers has disappeared and substantially only water is present in the bath along with the sheets, the surfaces of such sheets, the pores therein, and their other characteristics comporting with a maintenance of a substantially undisturbed condition at the interface betweensuch liquids and a substantially unmixed condition of the liquids at a rate of move ment of the liquid interface up to about 30 inches per hour, whereby the rapid drainage of said mixed nitrating acid may be effectedso asto maintain the uniformity of. nitration of said sheets. 7 i

2. A process of uniformly nitrating cellulose and rapidly removing the mixed nitrating acid therefrom, which comprises maintaining integral porous sheets of interfelted cellulose fiber ina bath of the mixed nitrating acid until nitration has been effected, said sheetsbeing in closely spaced relationship and defining channels between the sheets down through which channels the mixed nitrating acid can have substantially unobstructeddrainage, progressively draining the acid from the bath while delivering water onto the surface of the bath to form a floating water layer, and continuing such draining until the in,- terface between the water and. acid layers has disappeared and substantially only water is present in the bath along with the sheets, the surfaces of such sheets, the pores therein, and their other characteristics comporting with a maintenance of a substantially undisturbedcondition at the interface between such liquids and a substantially un-. mixed condition of the liquids at a rate of movement of the liquid interface up to about 30 inches per vhour, whereby the rapid drainage of said mixed nitrating acid may be effected so as to maintain the uniformity of nitration ofsaid sheets. I

3. A process of uniformly nitrating cellulose,

rapidly displacing the mixed nitrating acid with ing such draining until the interface between the Water and acid layers has disappeared and substantially only water is present in the bath along with the sheets, progressively draining water from the bath while delivering alcohol onto the surface of the bath to form afloatin'g alcohol layer and continuing such'draining until the interface between the alcohol and water layers has'disap peared and substantially only alcohol is present IIO in the bath, the surfaces of such sheets, the pores therein, and their other characteristics comporting with a maintenance of a substantially undisturbed condition at the interface between such liquids and a substantially unmixed condition of V the liquids at a rate of movement of the liquid interface up to about 30inches per hour, whereby