GB813625A - Improvements in or relating to the hardening of cellulose acetate precipitates - Google Patents

Abstract

<PICT:0813625/IV (a)/1> <PICT:0813625/IV (a)/2> <PICT:0813625/IV (a)/3> A process for hardening cellulose acetate immediately after continuously precipitating it from solution comprises forming a continuous stream of a slurry of the freshly precipitated acetate in the precipitating medium, continously dividing this stream into successive separate batches, and then passing these separate batches continuously and successively through a hardening zone while maintaining each batch substantially separate from the succeeding batch. This process may suitably be carried out in a cylindrical vessel mounted for rotation about its horizontal longitudinal axis and having an inlet at one end, an outlet at the other, and a helical vane having a number of convolutions of uniform pitch mounted in the vessel and adapted on rotation to divide material entering and passing through the vessel to the outlet at a constant rate into separate equal batches. As shown, a hollow cylindrical tank 11 carries a circumferential ring gear 14 driven by gear 21 which causes the tank to rotate. Within the tank is provided a continuous helical vane 22 which makes a number of uniform convolutions along the length of the tank and the outer edge of which is welded to the wall of the tank throughout its length. The central part of the vane is open, the inner edge 24 of the vane also forming a helix of uniform diameter and pitch along the length of the vane. At the discharge end of the tank is provided a second helical vane 32, the external and internal diameters of which are the same as those of the first vane 22, though this second vane makes only one complete convolution and occupies a position midway between adjacent convolutions of the first vane 22. In operation, the freshly precipitated cellulose acetate suspension in dilute acetic acid is fed continuously into the tank by means of pipe 29. As the tank rotates the material is divided up into separate batches by vane 22, and each batch moves towards the far end of the tank. Before reaching the outlet end of the tank, which is frusto-conical in shape, each batch is divided again in two by the action of vane 32, after which one complete rotation serves to discharge each divided batch through outlet opening 38, the diameter of which should be greater than the diameter of the inner edge of helical vane 22. The helical devices ensure that each batch passes through the hardening tank 11 with comparatively little variation in the time of passage. Specifications 783,504 and 813,626, [Group XXX], are referred to.