Patent GB-672645-A (GB)
Title: Method and apparatus for producing homogeneous viscose
IPC: D01F2/08
Filed: 1948-04-21
Abstract: A mass comprising a slurry of particulate cellulose xanthate in an alkaline liquid which is a solvent for the xanthate is converted into a homogeneous viscose by passing the mass through one or more hoppers each provided at the bottom with a pair of rotating nip rolls for propelling portions of the mass adjacent to the rolls in circulating paths while in the hopper or hoppers, thereby effecting mixing of the mass, and squeezing the portion in the nip to disperse the cellulose xanthate in the liquid and to form the squeezed portion into a homogeneous viscose film, which is withdrawn from the mass in the hopper by the rolls, each of the nip rolls having a scraper or doctor blade adapted to be moved into contact with the roll surface to remove the film therefrom. The solvent action of the alkaline liquid on the xanthate may be physical or chemical and the alkaline liquid may be a deflocculating and a dispersing agent as well as a solvent for the xanthate particles.  The homogeneous viscose product is free from minute suspended residual gel-like bodies which have resisted complete solution and which may be detected under the microscope by a described method involving the addition of an aqueous suspension of titanium dioxide pigment to the viscose.  In an example, a slurry of cellulose xanthate crumbs and dilute aqueous sodium hydroxide solution is fed into the uppermost hopper 3 (Figs. 1 and 2) of the mixing machine from a primary mixer (not shown).  The hoppers 3 are supported by cross-members 4 secured to the frame 2 and each of the hoppers except the lowermost is provided with a pair of smooth, closely-spaced rollers 6 and 7. Each of the hoppers except the lowermost is provided with end plates 8 which extend to the nip of the rollers to prevent overflow of the material.  All of the rollers 6 and 7 have upon their shafts equal spur gears 9 and 10 <PICT:0672645/IV (a)/1> <PICT:0672645/IV (a)/2> <PICT:0672645/IV (a)/3> <PICT:0672645/IV (a)/4> (Figs. 2 and 4).  Each of the rollers 6 is individually driven (by means not shown) through the gears 12 carried on the roller shaft, and in turn drives a roller 7 through the intermeshing of the spur gears 9 and 10 (Fig. 4).  The roller shafts fit into spring - loaded bearing blocks 14 and 15 mounted on the angle irons 16 (Fig. 1), bearing blocks 14 being fixed and bearing blocks 15 being slideable on the angle irons in response to rotation of spring-loaded screws 17 rotatably secured to block 15 and provided with hand wheels 18 for rotating them and thereby adjusting the gap between the rollers 6 and 7.  Curved doctor blades 19 and 20 fit closely against the bottoms of rollers 6 and 7 and scrape material from the surfaces of the rollers, the blades having projections 21 and 22 which assist in directing the material scraped off one pair of rollers into the next succeeding hopper in the series.  The doctor blades are pivotally secured at 23 and 24 to supporting blocks 25 and 26 and are moved towards and away from the rollers by rotation of the threaded rods 29 and 30 by means of the hand wheels 33 and 34.  The rollers 6 and 7 of each pair of rollers in the series are spaced apart a distance which decreases progressively from the uppermost to the lowermost pair.  Since it is preferred to rotate all of the pairs of rollers at equal circumferential speed, the rollers and their associated hoppers increase progressively in length from the uppermost to the lowermost pair in such a manner that when all the rollers are rotating in the feeding direction, the rate of feed of the material is constant throughout the machine.  Alternatively, the rollers may all be of the same length, but rotated at circumferential speeds which increase progressively from the uppermost to the lowermost pair in the series. Since the rollers 6 and 7 of the uppermost hopper 3, like all the other pairs of rollers, are rotated in the feeding direction, as shown by the arrows, the slurry in this hopper is continuously fed into the gap between the rollers as a film on their surfaces, squeezed between the rollers, scraped off the lower surfaces of the rollers by the doctor blades 19 and 20 and drops into the next succeeding hopper 3, from which it is eventually withdrawn as a film on the surfaces of the rollers 6 and 7 at the bottom of that hopper. This sequence is repeated throughout the succeeding hopper units and the viscose thus produced is continuously withdrawn from the lowermost hopper through pipe 5 and directed to a ripening tank. The portion of the mass which accumulates temporarily in each hopper is continuously homogenized as a result of the rotation of the rollers in contact with the bottom of the mass and the fresh charge of material entering each hopper is well mixed with material already accumulated in the hopper before it is withdrawn from the hopper by the rotating rollers in the form of a film. Each hopper may be provided with conventional automatic control devices to prevent it from becoming empty or overflowing.  Such devices may comprise liquid level detectors, one for the maximum and the other for the minimum depth of material desired in any hopper.  The maximum depth detector may be arranged to control the flow of material to the hopper either by reversing the direction of rotation of the rolls in the unit above or by lifting the scrapers from the rolls in the unit above to allow the film on the rolls to be carried back into the mass of material in the hopper above.  The minimum depth detector may produce the same effects in the hopper unit in which the detector is fitted.  In another example, a slurry of cellulose xanthate particles in dilute aqueous sodium hydroxide solution is delivered to the uppermost hopper 39 (Fig. 3) of a mixing machine at a rate of 6 lbs. 12 oz. of cellulose xanthate and 16 lbs. of sodium hydroxide solution at 1 1/2 minute intervals.  At the commencement of operations the rollers 6 and 7 below the uppermost hopper 39 are rotated in the direction of the arrows, so that the slurry is held temporarily in hopper 39 and therein subjected to gentle kneading and rolling.  After 1 1/2 minutes the direction of rotation of the rollers 6 and 7 associated with hopper 39 is reversed and the contents of hopper 39 are discharged as a film into the second hopper, the rolls of which are rotating in the non-discharging direction.  The slurry is now mixed for 1 1/2 minutes in the second hopper before, upon reversal of the directions of rotation of all the rollers of the machine, the material in the second hopper is discharged into the third, in which the material is again allowed to accumulate and so on throughout the six hoppers of the machine.  Two machines 45 and 46 in side-by-side relation may be alternately charged with slurry from the feed-pipe 47 provided with a two-way valve 48.  The rollers of the machine 45 are always rotated in directions opposite to the directions of rotation of the corresponding rollers of the machine 46.  In this manner, viscose is produced continuously and delivered to a common reservoir through the pipe 55.  The structural details of the machines shown in Fig. 3 are similar to those shown in Figs. 1, 2 and 4, except that the gears 9, 10 and 12 mounted on the roller shafts drive alternate pairs of rollers 6 and 7 in opposite directions.  An automatic timing device may be provided for periodically and simultaneously reversing the direction of rotation of all of the rollers.  The diameters of all the rollers may be 8 inches, and their speed 200 r.p.m.  They may be spaced apart a distance decreasing progressively from 0.004 inch for the uppermost pair to 0.001 inch for the lowermost pair and may progressively increase in length from 16 inches for each roller of the uppermost pair to 64 inches for each roller of the lowermost pair, the lengths of the hoppers progressively increasing correspondingly to that of the rollers.  It is stated that no excessive heat is developed by undue mechanical agitation of the material, that artificial cooling of the material is thus rendered unnecessary and that the viscose is comparatively free from gas bubbles.