Dissolving method

#The present invention relates to a method and a device for preparing a starch solution; a solution prepared in accordance with this method; and the use of such a solution for making paper.

#In papermaking, cationic starch is widely used as a retention agent and strengthening agent. In recent years, cationic starch has to a large extent been employed together with anionic, inorganic materials, such as bentonite and various silica-based sols, to improve the retention and/or drainage in papermaking, see, for instance, EP Patent 41,056 and PCT Applications WO 86/00100 and WO 89/12661.

#It has recently become possible to prepare high-cationized starch, i.e. starch with a degree of substitution exceeding 0.07. For instance EP Patent Specifications 233,336 and 303,039 disclose how this can be done by dry cationization, the latter disclosing the use of sodium aluminate as catalyst.

#An embodiment of the invention will be described in more detail below, reference being had to the accompanying drawings, in which

#FIG. 1 is a basic outline of a dissolution device;

#FIG. 2 is a lateral section, and FIG. 3 is an axial section of a preferred beater; and

#FIG. 1 shows a silo 1 for starch powder, which is connected to a screw conveyor 2 ending above a funnel 3. The upper part of the funnel 3 has a tangential inlet 4 for water, while the lower part merges with a water conduit 14 from a levelling tank 7 which has a float 8 for controlling the water level. The water conduit 14 extends, via a beater 5 and a pump 6, preferably an eccentric screw pump, to a jet boiler 10 which is fed with water vapor from the vapor conduit 15. The outlet 11 of the jet boiler 10 merges with a conduit 12 for excess water to form an outlet conduit 13. If the beater 5 is a rotating displacement pump, the pump 6 can be dispensed with.

#FIGS. 2, 3 and 4 illustrate a suitable beater 5. An axial inlet 21 communicates with a substantially cylindrical housing 20 whose inner surface area is formed with longitudinal grooves 27 and transverse grooves 28, and has a tangential outlet 27. In the housing is arranged an inclined and substantially flat rotor disc 24 whose central portion is fixed to a driving shaft 25 connected to a motor. The rotor disc 24 extends substantially along the entire axial length of the housing and is substantially elliptical in the plane, such that the distance between the periphery of the disc 24 and the surface area of the housing 20 is essentially constant when the disc rotates. Further, the periphery of the rotor disc 24 has teeth 29 which fit in the transverse grooves 28 of the housing 20, but which do not come into contact with the housing wall. The outlet 22 is formed with a segment 26 having holes or slits to prevent the passage of too-large lumps. FIG. 4 shows the essentially elliptical extension of the toothed rotor disc 24 in the plane.

#In a method for dissolving starch powder according to the invention, the powder is fed from the silo 1 through the screw conveyor 2 to the funnel 3, where it is mixed with water supplied tangentially through the inlet 4, whereby a vortex is formed. The mixture is then further diluted with water from the levelling tank 7. Preferably, about 15-60% of the water is received from the funnel, the remainder coming from the levelling tank. Then, the mixture is conducted through the inlet 21 to the beater 5, where it is subjected to strong shear forces in the housing 20 with the rotating disc 24, whereby any agglomerates are broken up and the particles are wetted before leaving the beater through the outlet 22. The beater thus contributes to the pumping action, but not as much as the pump 6 forwarding the mixture to the jet boiler 10 into which it is injected together with water vapor from the vapor conduit 15. The pressure in the jet boiler 10 is sufficiently high to prevent boiling at the working temperature which suitably is from about 60.degree. to about 130.degree. C. At the outlet 11, all the starch has been dissolved, and the solution is mixed with excess water from the conduit 12 before the positive pressure, if any, is released, thus preventing the solution from boiling.