fig1 of the drawings shows a preferred embodiment of apparatus 10 . screen 10 consists of an inner perforated casing 12 , preferably of cylindrical configuration , extending generally upwardly , preferably in slightly inclined relation to the vertical . the casing 12 has an inlet 14 adjacent its lower end , and adjacent its upper end a rejects outlet 16 feeding a trough 18 . an outer main chamber 20 extends about the casing 12 in spaced relationship thereto and in communication therewith through the perforations 22 , which constitute accepts outlet means from the casing 12 into the main chamber . the perforations in the casing may be of a variety of geometric shapes although round holes have been found to perform satisfactorily . the important consideration is that the perforations be large enough to allow the fibers to pass through with the liquid . in most instances this requires that the perforations have a maximum dimension of no less than 2 millimeters and usually a maximum dimension of no more than 6 millimeters . of course , where all or most of the contaminants are very large , the size of the openings can be increased accordingly . where round holes are used they will have a diameter no less than approximately 2 millimeters , and perforations of square , triangular or other shape will be sized similarly . slots may also be used and their maximum dimension , their length , will be greater than 2 millimeters . the width of the slots is preferably no greater than approximately 1 millimeter . a screw 24 consisting of a hollow shaft 26 and the screw flights 28 , is rotatably mounted adjacent its upper and lower ends , as indicated at 30 and 32 , and driven by a motor 34 through suitable gearing 36 . the shaft 26 is provided with perforations 38 at a point thereon above the normal liquid level in the casing 12 , indicated by the line 40 . each of the openings 38 is provided with a deflector 42 , as best seen in fig2 of the drawings , although for purposes of clarity some of the deflectors have been omitted from fig4 . additionally , the outer edge of the screw flights 28 , or a portion thereof , may be provided with a brush , as indicated at 44 , to wipe continuously the inner surface of the casing 12 . again , only a portion of the brush is shown in fig1 although it will be apparent that the brush may extend the length of flights 28 , or a portion thereof . in operation , the mixture of liquid , free fibers and other solids is received by the screen 10 through its inlet 14 . as the screw rotates within the casing 12 , the free fibers and liquid flow outwardly through the perforations 22 into the outer main chamber , while the other coarse solids are carried upwardly by the screw 24 to the rejects outlet 16 and chute 18 . the liquid and free fibers collected in the main chamber are removed therefrom through the accepts outlet 46 positioned adjacent the lower end of the main chamber . to wash any free fibers that may have collected on the other solids as they are conveyed upwardly out of the system by the screw 24 , shower water may be supplied to the hollow shaft 26 through the pipe 48 , the shower water passing through perforations 38 and being deflected downwardly by deflectors 42 toward the lower end of the perforated casing 12 . as a result , the useful fiber recovery rate of the apparatus is increased by reducing the amount of valuable paper fibers carried out with the rejected solids through the outlet 16 . it will be seen therefore , that in operating the screen 10 , the feed into inlet 14 consists of a mixture of liquid and fibers having a substantial amount of solid rejects . the consistency of the accepts from the inner casing will usually be less than 3 % by weight of fibers , and for many operations , about 1 / 2 % by weight of fibers . this can be attained by controlling the consistency of the feed into the casing 12 , by the amount of shower liquid ejected through the perforations 38 , or both . the material passing through the perforations 22 constitutes the accepts of the process and the material carried upwardly by the flights 28 constitutes the rejects . this is in contrast to the usual screw thickening operation wherein the objective is to carry all suspended solids upwardly by means of the screw flights while only liquid drains out through perforations in the casing surrounding the screw . it will also be noted that with the present invention the accepts passing through the perforations 22 consist of liquid and the bulk of the paper fiber fed into the apparatus and from which the rejects have been separated . in conventional screw thickening operations the bulk of the paper fiber is removed by the screw flights and ejected at the top . understandably , in a conventional thickener the amount of fibers passing through the perforations with the liquid is kept as low as possible . to do this the perforations in the casing in which the screw revolves are kept as small as possible . as noted above , the minimum size of these openings is limited by manufacturing considerations and the desired throughput for the apparatus . except insofar as limited in this regard , however , the openings are kept as small as possible . in contrast in the screen 10 , the perforations 22 are kept as large as possible so that as much fiber as possible will pass through the perforations 22 . thus , if the perforations are round holes , the diameter of the perforations 22 will be generally in excess of 2 millimeters . as a practical matter in order to avoid an excess of rejects from escaping with the accepts , the perforations will generally not be in excess of 6 millimeters , although it will be appreciated that when working with fibers containing relatively large contaminants the openings may even be increased above 6 millimeters . conversely , smaller holes may be required for the rejection of very fine contaminants . in any case , it will be seen that the basic principles of operation of apparatus in accordance with the present invention are exactly the opposite of those governing the design and operation of conventional screw thickening apparatus . fig3 of the drawings shows schematically a basic system in accordance with the present invention . thus , a supply 50 of pulp slurry containing contaminants of some type is conveyed to a screen 10 with rejected material being ejected from the top of the screen while accepted materials , paper fibers and liquid , are directed to further treatment or storage apparatus 52 . in a specific application of the invention as seen in fig4 the supply of liquid , paper fibers and contaminants may come from pulping apparatus , such as a pulping tub 60 having a rotor 62 rotatably mounted in the bottom thereof and fed with materials high in paper fiber content , such as waste paper or refuse containing a high proportion of fibrous materials . the tub , which may be of the type shown in u . s . pat . no . 3 , 595 , 488 , may be fed by a conveyor 64 while liquid , such as water , is fed into the tub from any convenient source , as indicated at 66 . from the tub 60 , a pump 68 conveys the pulped material , which will consist of liquid , free fibrous material , undefibered fibrous material and other solids , to a cleaner such as a cyclone cleaner 70 where , typically , heavy inorganic solids are removed from the system . thereafter , the resulting slurry is conveyed to a screening apparatus 72 which delivers liquid and accepted fiber to a stock holding chest 74 while rejecting the remaining material to a deflaker 76 for further comminution thereof . from the deflaker 76 , the material is conveyed to a screen 10 in accordance with the present invention , where liquid and free fibers are separated from undefibered materials and other solid contaminants as described above . the latter are rejected by the screen , while the accepted material is returned to the system , either upstream or downstream of the cyclone 70 as desired . in the system described above , paper making fibers are extracted from waste paper or other wastes containing a high proportion of fibrous material , such as refuse . it will be apparent , however , that the present system may be utilized for screening paper making fibers to remove many other types of contaminants . an example of another system is the screening of fibers following a digesting operation , where the contaminants are essentially fibers but in undefibered form , such as knots . with reference to fig5 of the drawings , a system of this type is shown , including a digester 80 provided with wood chips from a conveyor 82 and with cooking liquor through the conduit 84 . after cooking at elevated temperatures and pressures for a predetermined amount of time , the contents of the digester 80 are expelled from the digester to the blow tank 86 . the resulting mixture of spent cooking liquors , free fibers and undefibered fibrous materials , such as knots , is diluted and then directed to the inlet 14 of the screen 10 . as in the system described above in connection with fig4 of the drawings , the material passes through the inlet 14 into the casing 12 , from which the spent cooking liquor and free fibers are expressed through the perforations in the casing into the outer main chamber 20 . the contaminants , such as knots , are conveyed upwardly by the screw flights 28 to be ejected throgh the rejects outlet 16 for disposal or further treatment . weak liquor or other diluting liquid may be pumped into the hollow shaft 26 to spray the knots being rejected from the system and remove free fibers clinging to the knots to improve the yield of the system . the accepted material , that is , the spent liquor and free fibers , are then directed to further processing . in applications of this type , the consistency of the accepts will be in the range of 3 / 4 % to 2 % by weight of fiber . additionally since the size of the rejects will be fairly large , the maximum dimension of the openings in the casing can be increased up to approximately twenty millimeters . while specific systems in accordance with the present invention are described above , it will be apparent that screen 10 is capable of general application to paper fiber treatment systems for screening paper pulp , regardless of how the pulp is prepared originally . systems in accordance with the present invention therefore may be characterized broadly as using a screw type apparatus for screening a supply of contaminant containing paper stock to separate the contaminants from the paper fibers and liquid of suspension . while the methods and forms of apparatus herein described constitute preferred embodiments of the invention , it is to be understood that the invention is not limited to these precise methods and forms of apparatus , and that changes may be made therein without departing from the scope of the invention .