IMPROVEMENTS IN A PROCESS AND APPARATUS FOR FORMING PARTICULATE LAYERS

Abstract: A process for forming a layer of particulate material, comprising the steps of forming a foamed dispersion of said particulate material, supplying said foamed dispersion to a predetermined zone of a plane through which a foraminous element is moving, applying a vacuum to said plane across said zone on the other side of said foraminous element, so as to drain foam therethrough and form a particulate layer thereon, removing from said drained foam any free air drawn through or past the foraminous element during drainage, and continuously recycling the resulting foam for re-use in forming the foamed particulate dispersion

Description: This invention relates to improvements in a process and apparatus for forming fibrous or particulate layers, from foamed dispersions. UK Patent Specification No. 1397378 discloses a process and apparatus for making fibrous webs from a foamed dispersion of fibres. The process disclosed involves the generation of a foamed fibrous dispersion, the draining of the dispersion on a foraminous material to form a web without substantial breakdown of the foam, and continuously recycling the drained foam for re-use in forming new fibrous dispersion. The character of the foam used in the foregoing system needs to be relatively stable, so as to both achieve an even distribution of fibres on the foraminous material and facilitate recycling of the foam. Typically, such foams have an air content in excess of 65 %, a viscosity of at least 22 seconds measured by Ford Cup No. 4 according to British Standard No. 1733, and in which the air is contained in very small bubbles of in the region of 0.2 millimetres in diameter. The foregoing patent specification proposes the use of a conventional paper machine Fourdrinier wire and headbox in order to lay down the foamed dispersion. This configuration was, however, susbsequently discovered to have certain disadvantages for use with foam. Thus the vacuum applied through the wet vacuum boxed to drain down the foam for recycling also drew in free air, that is, air uncombined as bubbles in the foam. This free air had the effect of destabilizing the foam in the hydraulic conditions existing in the recycling circuit and produced variations in the character of the recycled foam. As a result, the inability to control the induction of free air imposed a limit on the level of vacuum which could be applied, and thus on the speed and production capacity of the system. UK Patent Specification 1 427 272 also discloses a process and apparatus for making fibrous webs from a foamed dispersion of fibres and again uses a wire and a wet suction box to remove the foam. Once again the disadvantage is that the vacuum boxes also draw in air and this has the effect of de-stabilising the foam, the air and foam are passed to a separator but variations in the character of the recycled foam causes difficulty in pumping. British Patent Specification 1 129 757 also suggests that liquid and froth collected from the suction boxes can be separated from the air in a cyclone-type separator of known kind and the air is stated to be exhausted by a wet vacuum pump, the separated liquid and froth being returned to source. The same difficulties arise with this construction as with the others referred to above. The invention therefore provides a process for forming a layer of particulate material, comprising the steps of forming a foamed dispersion of said particulate material, supplying said foamed dispersion to a predetermined zone of a plane through which a foraminous element is moving, applying a vacuum to said plane across said zone on the other side of said foraminous element by vacuum means, so as to drain foam through said element and form a particulate layer thereon, characterised by removing from said drained foam any free air drawn with said foam through or past the foraminous element said free air being removed from within or at a point closely adjacent to said vacuum means so as to substantially inhibit destabilisation of the foam by said free air prior to entry into means for recycling the foam for re-use in forming further foamed particulate dispersion. In this specification, the term "particulate material" is to be taken as including any particles which are capable of being disperse- in a foamed dispersion and specifically as including fibres, for example of wood pulp, glass, metal or, carbon or other suitable fibres. In another aspect, the invention provides apparatus for forming a layer of particulate material from a foamed dispersion of such material which comprises a movable foraminous element, a headbox having an extended slice disposed above said foraminous element to define a layer forming zone, means for generating a foamed fibrous dispersion and supplying it to said headbox, wet vacuum box means disposed below said foraminous element co-extensively with said layer forming zone and slice, for draining foam through said foraminous element to form a particulate layer characterised by means for removing from said foam any free air drawn with said foam through or past the foraminous element, said means acting to remove said free air from within or at a point closely adjacent to said vacuum box means so as to substantially inhibit destabilisation of the foam by said free air prior to entry into means for continuously recycling the resulting foam to said foam generating means. The means for removing the free air from the drained foam may comprise a secondary vacuum line connected to the vacuum box means at a position above the level of the foam therein. Alternatively, the outlet duct from the vacuum box means may include an in-line centrifugal separator through which the free air is removed by vacuum before the foam is recycled. The invention will now be further described with reference to the accompanying drawings in which :- Fig. 1 is a sectional side elevation of part of an apparatus for laying down a particulate or fibrous layer on a foraminous element from a foamed dispersion; Fig. 2 is a view on the line II-II of Fig. 1, showing in addition one embodiment of an improvement according to the present invention; and, Fig. 3 is a view on the line II-II of Fig. 1, showing in addition a second embodiment of an improvement according to the invention. Referring to the drawings, an endless foraminous belt 1, for example a Fourdrinier wire passes over a breast roll 2 and across a series of wet vacuum boxes 3 which are evacuated through ducts 4 by a positive displacement pump 5. A headbox 6 is located above the Fourdrinier wire 1 and has a slice opening 7 co-extensive with the vacuum boxes 3. In use, foamed fibrous dispersion 8 is fed to the headbox 6 through an inlet duct 9. The dispersion passes through the slice 7 and is drained through the Fourdrinier wire 1 to deposit a fibrous web 10 on the moving Fourdrinier wire. Drainage of the foam through the wire is assisted by the evacuation of the wet vacuum boxes 3 caused by operation of the positive displacement pump 5. As best seen in Fig. 2, in order ot faciliate movement of the Fourdrnier wire 1, gaps 11 must be left between the wire and the upper and lower edges of the wet vacuum boxes and slice opening. The partial vacuum generated in the wet vacuum boxes by the positive displacement pump 5 therefore invitably draws in free air through the gaps 11. The presence of this free air in the system has the effect of destabilizing the drained foam 12 being recycled by the pump 5 to a foamed fibrous dispersion generator of known kind (not shown) which is connected to the inlet duct 9 of the headbox. Also as shown in Fig. 2, the present invention provides for the secondary application of vacuum by a conventional vacuum pump, such as a liquid ring vacuum pump, through the line 13 and a control valve 14 which may be automatic from a point above the level of the drained foam 12 in the wet vacuum boxes 3. Turning now to Fig. 3, this shows an arrangement similar to that of Fig. 2 but omitting the vacuum line 13 and control valve 14. The remaining features of this embodiment are identified by the same reference numerals as in Fig. 2 and will not be further described. In this embodiment, a centrifugal separator 15 is included in the outlet duct 4 from the vacuum box 3. The centrifugal separator has connected thereto a vacuum line 16 and an automatic control valve 17. The centrifugal separator serves to separate the free air from the foam and discharge it from the system through the vacuum line 16 and valve 17. By arranging for secondary vacuum applied through the line 13 to remove the free air inducted through the gaps 11, destabilization of the drained foam 12 is prevented. As a result, existent characteristics are maintained in the foam being recycled, the need for make up raw materials is minimised and the rate of production of the web 10 is substantially increased. The secondary vacuum can also be controlled to increase the pressure differential across the Fourdrinier wire, and thereby increase the weight per unit area of the web 10 being laid down.

Claims: A process for forming a layer of particulate material (10), comprising the steps of forming a foamed dispersion (8) of said particulate material, supplying said foamed dispersion (8) to a predetermined zone of a plane through which a foraminous element (1) is moving, applying a vacuum to said plane across said zone on the other side of said foraminous element (1) by vacuum means (3), so as to drain foam through said element (1) and form a particulate layer (10) thereon, characterised by removing from said drained foam any free air drawn with said foam through or past the foraminous element (1) said free air being removed from within or at a point (15) closely adjacent to said vacuum means (3) so as to substantially inhibit destabilisation of the foam by said free air prior to entry into means (4, 5) for recycling the foam for re-use in forming further foamed particulate dispersion. A process as claimed in claim 1 characterised in that the particulate material includes fibres: A process as claimed in claim 2 characterised in that the fibres are of wood pulp, glass, metal, or carbon. Apparatus for forming a layer of particulate material (10) from a foamed dispersion (8) of such material comprising a movable foraminous element (1), a headbox (6) having an extended slice (7) disposed above said foraminous element (1) to define a layer forming zone, means for generating a foamed fibrous dispersion and supplying it to said headbox (6), wet vacuum box means (3) disposed below said foraminous element (1) co-extensively with said layer forming zone and slice (7), for draining foam through said foraminous element (1) to form a particulate layer (10) characterised by means (13, 15, 16) for removing from said foam any free air drawn with said foam through or past the foraminous element (1), said means (13, 15, 16) acting to remove said free air from within or at a point closely adjacent to said vacuum box means (3) so as to substantially inhibit destabilisation of the foam by said free air prior to entry into means (5) for continuously recycling the resulting foam to said foam generating means. Apparatus as claimed in claim 4 characterised in that the means for removing the free air from the drained foam comprises a secondary vacuum line (13) connected to the vacuum box means (3) at a position above the level of the foam therein. Apparatus as claimed in claim 4 characterised in that an outlet duct from the vacuum box means (3) includes an inline centrifugal separator (15) through which the free air is removed by vacuum before the foam is recycled.

Inventors: WALLACE, PETER LIAM, WILLIS, ANTHONY JOHN
Applicants: THE WIGGINS TEAPE GROUP LIMITED
Application year: 1988
Main classification: D21F11
IPC codes: D21F11, D21F1