This invention relates to a method and an apparatus for continuously processing material suspensions comprising particles of different size, for example lignocellulose- -containing fibrous material.

By separating coarse particles from a material suspension by means of a screen device, the suspension can be div¬ ided into an accept portion and a reject portion. These portions can thereafter be processed separately in diff¬ erent ways. It cannot be avoided, however, that a subst¬ antial amount of small particles follow along with the reject. For solving this problem, additional screening steps for the reject are required for recovering these small particles. For being able to recover also the rej¬ ect, special equipment is required.

According to the present invention, the material suspens¬ ion is separated by a screen device into an accept port¬ ion and a reject portion, and the reject portion is pass¬ ed directly through a refiner for reducing the size of the particles. The rej.ect is thereafter returned to the screen device for repeated separation. This combination yields the unique effect that the material suspension supplied is converted to 100% accept, i.e. a suspension without overdi ension particles. Separated coarse part¬ icles passing through the refiner and returned to the screen device, thus, circulate in one or several sequenc¬ es until the dimension of the particles permit their passage through the screen device.

The refiner is provided with two refiner members, which are rotatable relative to each other and define a div¬ erging gap. Owing to the fact that the gap is designed with an inlet of a smaller diameter and an outlet of a greater diameter, a vacuum is created at the inlet. As a result thereof, the suspension (reject) enriched on

greater particles on the inlet side of the screen device is sucked into and passed through the gap. The fluidiz- ation arising at the entrance into the gap can contrib¬ ute to rendering the refining more efficient. The flow can be adjusted by special means, for example valves and means for adjusting the gap.

According to the invention, different types of screen devices can be utilized, for example rotary or station¬ ary screening cylinders. Other types of screens (vibrat¬ ing, pulsating ones, etc,) can also be imagined.

The characterizing features of the invention become app¬ arent from the attached claims.

The invention is described in greater detail in the foll¬ owing, with reference to the accompanying drawings, which illustrate two embodiments of the invention.

The embodiments shown comprise a refiner consisting of a rotary conic refiner member 10, the refining surface of which is provided with grooves 12 and bars 14, the long¬ itudinal direction of which substantially coincide with the generatrices of the conic refiner surface.

The rotary refiner member 10 co-operates with a station¬ ary refiner member 20, the surface of which is designed similar to the surface of the rotary refiner member 10. Between the refiner members 10,20 a gap 22 is formed,which can be adjusted according to the particle size desired at the discharge zone 24 of the refiner by moving the rot¬ ary refiner member 10 in axial direction. The said refiner member is supported on a shaft 11, which is mounted in an axially movable bearing housing 13 and driven via an axially adjustable coupling 14 by a motor, the shaft 15 of which is connected to said coupling. The axially mov¬ able bearing housing 13 is adjusted to and retained in desired position by means of a piston 16, which is mount¬ ed on the bearing housing and enclosed in a cylinder housing 17, which is rigidly secured in the stationary casing 30 of the refiner.

The distance between the rotary and the stationary refin¬ er member 10,20 is adjusted in known manner, see for ex¬ ample SE-PS 214 707 _. by means of a hydraulic servo valve 40, which is actuated by a sensing arm 44 mounted on the axially movable bearing housing, which arm carries an adjusting screw 46 through a threaded connection 45, and by which the bearing housing can be adjusted to desired position. The adjusted position is thereafter maintained constant automatically, in that the sensing arm 44 act¬ uates the servo valve 40, which adjusts and controls the hydraulic pressure in the cylinder chambers 18 and 19.

The embodiment shown in Fig. 1 comprises a screen device of the kind disclosed in SE-PS 7802653-1. The screen dev¬ ice consists of a stationary screening cylinder 50, which is rigidly connected in the pressurized casing 30. The material suspension is intended to be supplied through an inlet 64 via an adjusting valve 65. The screening cyl¬ inder 50 is provided with slits or perforations of the size permitting the passage for accepted particle size, but which prevents the passage of greater particles. The screening cylinder 50 is concentrically enclosed by a rot¬ ary outer cylinder 60, which is provided with relatively great feed openings, through which the material suspens¬ ion supplied at overpressure and intensely agitated can pass without particle separation. The material suspension supplied is moved by the supply pressure toward the part¬ icle separating screening cylinder 50, where particles with overdimensions are prevented from passing through the apertures of the screening cylinder. Particles of oversize accumulated on the screening cylinder 50 are removed from the cylinder surface by means of a series of radial tubular members 66, which are attached on the rotary cylinder 60 and upon rotation of the cylinder gen¬ erate a vacuum, by which the fibre particles deposited are removed from the screening cylinder and passed to the space 63 outside the rotary cylinder 60. In said space 63 the suspension is enriched with oversize part-

icles, which were not permitted to pass through the screening cylinder 50. By means of the axial flow of the unprocessed suspension supplied in the direction to the space between the rotary cylinder 60 and an enclosing cylinder 67, the suspension enriched on great particles is moved in axial direction toward the gap 22. The accept is conducted away from the interior of the screening cylinder 50 via an outlet 68 with a valve 69.

The embodiment shown in Fig. 2 comprises a refiner of the same kind as shown in Fig. 1 in combination with a diff¬ erent type of screen - derøice. This screen .device is of the kind disclosed in SE-PS 354 677 and consists here of a stationary screening cylinder 70, which is rigidly conn¬ ected in the pressurized casing 30. An inlet 71 with ad¬ justing valve 72 is provided for the material suspension. The screening cylinder 70 is provided with perforations of a size permitting the passage of accepted particle size. The screening cylinder 70 is located spaced from the casing 30 so as to define an accept chamber 73 on the outside, from which the accept can be conducted away through an outlet 74 with a valve 75. Inside of the screening cylinder 70 and concentrically therewith a rot¬ ary pulsation cylinder 7 is located, which is provided with surface deformations 77 in the form of depressions and/or projections. The rotation of the pulsation cylind¬ er 76, preferably together with the rotary refiner device 10, generates a flow along the screening cylinder 70 which is favourable for particle separation, and at the same time the perforations are kept clean. The particles unable to pass through the perforations of the screening cylinder 70 continue past the screening surface and are sucked into the gap 22 as described above.

At both embodiments an. intense fluidization of the susp¬ ension is effected by the rotary members, the rotary cyl¬ inder 60 and, respectively, the pulsation cylinder 76, which are attached on the rotary refiner member 10. This

implies that the reject enters the gap 22 in fluidized state,and thereby the efficiency of the refining- is im¬ proved. The strong fluidization further renders it poss¬ ible that the refining is carried out at a concentration at optimum for refining.

At both embodiments a return conduit 80 is connected to the discharge zone 24 of the refiner This conduit 80 is coupled to the inlet side of the refiner devices 50,70. A control valve 81 is provided in the conduit 80 for controlling the recycling. The apparatus according to the invention, thus, acts as a screening or separation means without external reject flow. The invention renders possible substantial savings in respect of both build¬ ing and machine equipment for the recovery and process¬ ing of the rejected portion of the material suspension. The apparatus according to the invention, thus, elivers 100 accept, i.e. accepted particle size. The processed fibre suspension is discharged through the outlet 68 and, respectively, 74 as 100% accept. The outgoing flow is adjusted by valves 69 and, respectively, 75 by means of a control device 84, which is connected to a pressure sensing device 85, by means of which a const¬ ant hydraulic pressure is maintained in the casing 30.

The difference in pressure required for the recirculat- ion through the conduit 80 is generated by means of the refiner as described above. The circulation flow can be controlled by means of the valve 81. The adjustment to be made, of course, depends on the quality of the susp¬ ension supplied and on the quality desired of the out¬ going suspension.

The invention, of course, is not restricted to the emb¬ odiments described above, but can be varied within the scope of the invention idea.