Patent GB-646971-A (GB)
Title: Continuous precipitation of cellulose derivatives
IPC: C08B3/28
Filed: 1948-04-27
Abstract: <PICT:0646971/IV (a)/1> Organic derivatives of cellulose are continuously precipitated from solution in a solvent which is either miscible with water in all proportions, or which contains a substantial proportion of one or more constituents miscible with water in all proportions, by passing the solution into a precipitating chamber, passing an aqueous liquid into the precipitating chamber, vigorously stirring and agitating the solution and aqueous liquid to effect intimate mixing of the two and precipitation of the cellulose derivative, milling and grinding the precipitate of cellulose derivative in the precipitating chamber as it is formed, and discharging the milled and ground precipitate from the precipitation chamber.  The invention is more particularly applicable to the precipitation of organic esters of cellulose from solutions in which they have been formed.  The precipitating chamber may comprise a hammer mill of either the fixed or the swing hammer type.  Alternatively various forms of crusher may be used.  As shown, cellulose acetate dope from an acetylator or hydrolyser (not shown) is introduced in controlled amounts into a precipitating and grinding chamber 6 by means of a valved pipe provided at its end with a distributor (not shown) so that the dope passes therefrom into the precipitating chamber in the form of a ribbon.  Water or a dilute acid is likewise introduced into the chamber 6 in controlled amounts and the aqueous liquid and acid dope are thoroughly mixed by revolving swing hammers 9-12 pivotally mounted on a disc 7 attached to a rotating shaft 8.  The clearance between the hammers and the periphery of the grinding chamber is adjusted to grind the precipitated particles to a predetermined size.  The shaft 8 may carry more than one disc and set of hammers and a corrugated crushing-plate may be installed in the upper part of the chamber 6. The ground precipitated particles are washed by the liquid continuously entering the chamber 6 through perforations 18 into a chamber 19 from which they are discharged for further treatment. The spent acid contains about 25 per cent of acetic acid. Cellulose propionate, acetate butyrate, or acetate propionate may be similarly precipitated from their respective acid dopes.  The precipitation may follow a hydrolysis treatment or the completion of the acetylation reaction.  The invention is also applicable to the treatment of previously manufactured organic esters of cellulose which may be dissolved in a suitable solvent and then precipitated.  The invention may be applied to acid dopes containing about 9-35 per cent by weight of an organic ester of cellulose, but it is preferred to use a dope containing about 20-30 per cent of ester in the case of cellulose acetate and about 15-20 per cent of ester in the case of cellulose propionate.  The proportion of water or dilute acid introduced into the precipitating chamber should usually be sufficient to reduce the acidity below about 40 per cent in the case of cellulose acetate dope and below about 50 per cent in the case of cellulose propionate. Specification 337,366, [Class 2 (ii)], is referred to.