US3633743A

US3633743A - Process and apparatus for classifying fibres

Info

Publication number: US3633743A
Application number: US821850A
Authority: US
Inventors: Ronald William Gooding; Noel James Parratt
Original assignee: National Research Development Corp UK
Current assignee: National Research Development Corp UK; National Research Development Corp of India
Priority date: 1968-05-07
Filing date: 1969-05-05
Publication date: 1972-01-11
Anticipated expiration: 1989-01-11
Also published as: FR2009860A1; DE1923230C3; DE1923230A1; DE1923230B2; NL6906953A; CH490898A; GB1265803A

Abstract

A process and apparatus for classifying fibers for length wherein the fibers are dispersed in a liquid which is tangentially flowed onto a screen, which has a plurality of holes of a size in accordance with the length of fibers desired to be retained on the screen. The screen is in the form of a closed loop which is continuously circulating past the dispersion supply position at a rate sufficient to ensure that only a thin layer of fibers is applied to the screen. After passing the supply position the screen is inverted and a stream of fluid is applied to remove the fibers retained thereon.

Description

United States Patent Continuation-impart of application Ser. No. 610,661, Jan. 20, 1967, now Patent No. 3,490,585. This application May 5, 1969, Ser. No. 821,850

The portion of the term of the patent subsequent to Jan. 20, 1987, has been disclaimed.

[54] PROCESS AND APPARATUS FOR CLASSIFYING FlBRES 36 Claims, 7 Drawing Figs.

[52] U.S. Cl 209/250, 209/254, 209/270, 209/303, 209/380 [51] Int. Cl B07b 1/22 [50] Field of Search 209/380, 403, 254, 295, 273, 274, 270, 303, 250; 210/403 [56] References Cited UNITED STATES PATENTS 1,882,662 10/1932 Haug 209/270 3,490,585 1/1970 Gooding et a1. 209/5 138,823 5/1873 Strawn 209/380 X 158,626 1/1875 Choat 209/380 X 2,653,521 9/1953 Ahlfors 210/403 X 2,748,951 6/1956 Dubach.... 209/254 X 2,751,079 6/1956 Ahlmann.. 209/295 2,845,848 8/1958 Bowen 209/273 X 2,910,142 12/1959 Fontein 209/274 3,024,91 l 3/1962 .Ianson 209/268 3,145,164 8/1964 Jonkman 209/270 X 3,174,622 3/1965 Lamort 209/273 3,276,854 10/1966 Mathewson 209/380 X 3,278,039 10/1966 Nilsson 209/270 X 3,394,809 7/1968 Hunter 209/273 FORElGN PATENTS 664 2/1880 Great Britain 209/380 Primary Examiner- Frank W. Lutter Attorney-Cushman, Darby & Cushman ABSTRACT: A process and apparatus for classifying fibers for length wherein the fibers are dispersed in a liquid which is tangentially flowed onto a screen, which has a plurality of holes of a size in accordance with the length of fibers desired to be retained on the screen. The screen is in the form of a closed loop which is continuously circulating past the dispersion supply position at a rate sufficient to ensure that only a thin layer of fibers is applied to the screen. After passing the supply position the screen is inverted and a stream of fluid is applied to remove the fibers retained thereon.

PATENIED JIIIII I I972 SHEET 2 [1F 5 M LOG FIBRE LENGTH.

FIG. 3.

PATENTEU JAN. 1 1972 WEIGHT IN A /5 LENGTH INTERVAL.

' SHEET 3 OF 5 KA M LOG FIBRE LENGTH.

FIG. 4.

PATENTED JAN] 1 1972 SHEET 0F 5 PATENTED JAN] 1 I972 3.633.743

' SHEET 5 BF 5 PROCESS AND APPARATUS FOR CLASSIFYlNG-FEBRES This application is a continuation-in-part of our application, Ser. No. 610,661 filed Jan. th, 1967, now US. Pat. No. 3,490,585.

The present invention relates to methods and apparatus for the length classification of fibers, that is the separation of fibers of widely different lengths into fractions having lengths within specified ranges. Hitherto, little commercial effort has been devoted to the length classification of fibers partly because there has been no obvious industrial need for, and advantage in a successful process. Traditional fiber-handling industries such as the paper industry have found it sufficient that the fiber feed is freed from macroscopic impurities such as dirt, undispersed fiber clumps, etc., which might otherwise spoil 'the appearance of the finished product. lndeed it is probable that the better quality of a paper product would be obtained from a random distribution of fibers by length.

Potentially sizeable new industry is now developing, however, based on the manufacture of composite materials in which a matrix of metal or synthetic resin is reinforced by fibers of strong materials such as the single crystal fibers of ceramic materials known as whiskers. In order to manufacture strong composite materials it is vital that the reinforcing fibers have closely controlled physical characteristics so that they may be densely packed into the composite matrix and exert their maximum reinforcing effect. One of the necessary physical characteristics of the fibers which must be so controlled is the fiber length and it is the aim of the present invention to achieve length classification processes which yield classified fiber fractions eminently suited to the reinforcement of composite materials. We are unaware of any previously described processes which achieve this aim and indeed are unaware even that the requirement has been previously noted.

Classification of materials by size is commonly performed by filter screens consisting of a mesh or grid whose interstices will retain material greater than a chosen size and thereby classify it from lesser sized material. Such processes can normally only be applied to materials which are substantially symmetrical, e.g., gravel, since unsymmetrical materials such as fibers which have considerably different values for length and thickness, often of the order of 100:1, present a different cross section to a filter screen according to their angle of attack. Thus, a fiber which was 50 millimeters long and 0.025 millimeter thick might readily pass end-on through a screen having appertures 0.05 millimeter wide, whereas a fiber only 0.1 millimeter long might well be retained by the screen if it approached the screen in a sideways manner.

We have now discovered that filter screens surprisingly may be used successfully to classify fibers by length if the fibers are given a major component of velocity in a direction parallel to the surface of the screen at their point of application. In this way a component of force tending to drive fibers end on through the screen is drastically reduced or eliminated and the vast majority of the fibers deposit on the screen by a sideways approach so that the screen effectively allows through fibers having lengths less than a chosen value. Conveniently, the appropriate velocity component is imparted to the fibers to be classified by flowing a liquid dispersion of the fibers tangentially (or more properly secanthally) onto the inner surface a curved screen which is propelled passed the dispersion supply position.

A second major problem is achieving classification of fibers by length is a problem common to most filtering operations. This is the buildup of retained material on the filter screen which prevents the passage of undersized material which it is desired should be passed by the filter screen. Many methods of reducing this disadvantage have been advocated, typical suggestions being to provide an oversize screen which will clog" with retained material until a clogged screen having approximately the correct dimensions is obtained for a useful filtering period; to provide a scraper to frequently remove retained material from the screen and allow free passage to the undersized material; or to wash or allow to fall free from the screen,

the retained material at frequent intervals. We overcome the problem by applying the fibers to the screen in a layer which is so thin that the undersized fibers are not hindered from passing through the screen. We combine the use of the thin layer with means for removing the retained fibers which impinge on the retained fibers which a velocity component normal to the plane of the screen sufficient to lift the retained fibers away from the screen.

According to one embodiment of the present invention a process, normally a continuous process, for classifying fibers for length comprises dispersing the fibers in a liquid to form a fiber dispersion, flowing the dispersion at a dispersion supply position onto a screen in the form of a closed loop having a plurality of holes of similar size which is predetermined in accordance with the minimum length of fibers which is desired to be retained on the screen, propelling the screen past the dispersion supply position at a rate which is at least sufficient to ensure that no more than a thin layer of fibers is applied to the screen and fibers shorter than those desired to be retained are filtered through the screen, the dispersion being flowed on to the screen with a velocity component in the direction of movement of the screen and only a small velocity component in a direction perpendicular to the screen with the result that the fiber dispersion flows steadily onto the screen and the fibers of at least the desired length can settle and be retained by the screen while shorter fibers are filtered through the screen, inverting the screen after it has passed the dispersion supply position so that the retained fibers are on the underside of the screen, and flushing liquid through the screen to remove the retained fibers from the screen.

In a preferred form of the process, the fiber dispersion if filtered through a cylindrical screen which is rotating at an even rate about a horizontal axis, the dispersion supply position being situated inside the cylinder above the lower part of its course, and the fibers retained on the screen are removed therefrom by a liquid flushed down through the screen from above the higher part of its course.

According to a feature of the invention the dispersion is arranged to flow onto the screen in a direction substantially tangential to the direction of motion of the screen, and preferably at a similar speed. Turbulent flow and disturbances, which would tend to drive through the screen more fibers longer than mesh size, are thereby minimized, as is the possibility of damage to brittle fibers. This may be achieved by flowing the dispersion over a lip extending the width of the screen and which is parallel to and close to the screen. In a preferred form the lip is made of flexible material and its end just touches the screen. If long enough this would fulfill its purpose despite any eccentricity of the screen.

By severely limiting the thickness of the layer of fibers which is allowed to be formed on the screen in accordance with the invention, the classification of fibers can be carried out effectively and the filtration of comparatively shorter fibers through the screen is not significantly impeded. The thin layer allowed to form should normally be no more than a very few, i.e., less than five monolayers thick and preferably not more than 1% monolayers thick where a monolayer is regarded as having a thickness equal to the average diameter of the great majority of the fibers. Preferred embodiment of the invention operate with a layer thickness of between and percent of a monolayer.

The concentration of the fibers in the liquid is decided having regard to two conditions. The fibers must be dispersed so that they do not interfere. However, the efficiency of the process is reduced with increase of liquid not only because there is a limit to the rate at which liquid can be applied to the screen but because the liquid tends to wash all fibers through the screen. Themaximum permissible concentration varies inversely with the average length of the fibers.

it will be appreciated that for a particular apparatus the rate of application of fibers must be related to the rate of movement of the screen. These rates will normally be as high as possible consistent with effective operation of the apparatus and in practice it has been found that there is normally a limiting rate of movement of the screen above which the screen does not filter the fibers as required and difficulty is experienced in removing the fibers from the screen. The permitted rate of application of fibers at this maximum screen rate can be readily determined as in practice if it is exceeded the dispersion snowballs on the screen.

The dispersion may contain a wetting agent to aid the free dispersion of the fibers. The wetting agent should be of a type which does not froth or foam, as foam generated when the dispersion is applied'may tend to carry fibers intended to pass through the screen.

While some types of fibers exhibit no greater variation in their diameter between fibers, others, such as asbestos fibers, may vary by a factor of or more. As the tensile properties of the composite are generally better for higher aspect ratio fibers, those which are coarse may be undesirable. These latter types of fibers are preferably classified for diameter before being classified for length. The diameter classification may be carried out using a hydrocyclone or an elutriation column.

Good separation of fibers into two length categories can be achieved by using the above-described length classification process, but the efficiency of the process can be increased further by providing a second screen in form of a closed loop around the first screen and having holes the same size as those of the first screen and which is propelled with it. In the case of the cylindrical screen, the second screen is conveniently concentric with the first. A high proportion of the fibers in the longer category will be retained by the first screen, and most of the remainder by the second.

The process for classifying fibers for length in accordance with the invention may be used with particular advantage in the length classification of fibers which are prone to mechanical damage or distortion, as the dispersion of the fibers in a liquid causes them to be cushioned against shocks.

A preferred form of apparatus for carrying out the process of the invention comprises one or more cylindrical screens of a similarly sized mesh, the screens being disposed concentrically about a common horizontal axis and adapted to be driven at a controlled rate of rotation. The dispersion is flowed onto the screen from a dispersion supply weir situated above the lower part of the inner surface of the inside screen so that the dispersion may be flowed onto the whole operative width of the screens at a controlled even rate. Means are provided which can flush liquid across the whole operative width of the screen or each screen from above its higher part, with receptacles for the liquid and the fibers thereby washed from the screens provided below.

The classification process as described separates fibers into two groups in accordance with their length with respect to the size of the holes in the screen. For wire mesh screens substantially all fibers longer than about twice the mesh size are retained while substantially all fibers less than about half a mesh size filter through the screen.

In order to separate fibers into more than two length groups, either the fibers filtered through, or those removed from, the screen or screens may be applied to further screens having holes of a different size. The order in which such a multistage process is carried out is decided having regard to the proportions of fibers of each length category contained in the bulk. When the short fibers predominate, as is usual, the dispersion is first flowed onto a screen having small holes, and the fibers removed from the screen are dispersed again and passed to screens of relatively larger size holes. A multistage process is conveniently carried out in apparatus having a common drive for the different sets of screens.

In order that the invention may be more clearly understood a classification apparatus and its method of use will now be described, by way of example, with reference to the accompanying drawings, of which:

FIG. I is a diagrammatic side elevation of the apparatus,

FIG. 2 is a diagrammatic view on the section II-II FIG. 1,

FIG. 3 is a graph of the separation achieved with a single unit of two similar screens, and

FIG. 4 is a graph showing the separation achieved in a multistage process, compared with the initial length distribution of the fibers.

As shown in FIGS. 1 and 2 the apparatus has a cylindrical inner screen I and a concentric cylindrical outer screen 2 with a common horizontal axis. The screens 1 and 2 are made of metal gauze of similar mesh. A dispersion supply weir 3 is situated inside the inner screen so that it can flow dispersion onto the whole width of the screen I. It has a flexible lip 4 made of neoprene rubber, whose free edge touches the screen, which acts to even out the flow of the dispersion and to direct it to approach the screen as nearly tangentially as possible. Situated below the outer screen 2 is a filtrate tray 5. Liquid flushing means in the form of a spray pipe 6, for spraying liquid on to the outer screen 2, is situated above the highest part of the outer screen 2. A receptacle for the liquid in the form of a gutter 7 is provided inside the outer screen 2 immediately below the spray pipe 6. Above the inner screen 1 there is an inner screen spray pipe 8, and a gutter 9 is held inside the screen 1 just below the pipe 8. The two

gutters

8 and 9 feed into a common products pipe 10. The screens 1 and 2 are attached to a backplate 11 having a drive connection to an electric motor 12.

The apparatus is used to classify fibers into lengths above and below a given length in the following manner. A dispersion of the fibers in a liquid containing a nonfoaming wetting agent such as a polyalcohol so that the fibers are freely dispersed, is fed into the weir 3 whence it flows evenly at a controlled rate via the lip 4 onto the whole operative width of the screen in a direction substantially tangential to the screen. The screens 1 and 2 are set rotating by the motor 12 in the direction indicated in FIG. 1. The rotation rate of the screens is such that not more than approximately a monolayer of fibers is applied to the inner screen 1. In a preferred mode of operation, the dispersion of fibers is flowed onto the screen with a velocity substantially the same as the velocity of the screen. The filtrate from the screen 1 drops to the outer screen 2. Most of the longer fibers will have been retained on the inner screen 1, and substantially the remainder of these will be retained on the inner surface of the screen 2. Most of the shorter fibers pass through both screens and are caught in the drain 4. The monolayer of long fibers remains on the screens as they revolve until they pass under their

respective sprays

6 and 8 when the fibers are washed into the

gutters

7 and 9 and pass into the products pipe 10.

The fibers from the products pipe 10 may be redispersed and passed to another similar apparatus having screens of a larger mesh than the one described for further classification.

One practical apparatus, used for classifying silicon nitride whiskers whose diameters are about l-2 microns and whose lengths vary from 10 microns to about 1 mm., has

screens

1 and 2, 6 inches deep and 1% feet and 2 feet in diameter respectively. The fines, or shortest whiskers are taken out by screens having a 400-mesh gauze, or apertures of 0.036 inch square. The whiskers are dispersed in water, about 0.5 gram per liter, the water containing a small amount of a wetting agent, and the whisker bulk having between about 10 and 10 fibers per gram. The screens are rotated at 10 rpm. The dispersion is arranged to flow over the weir at a rate of about 1 liter per minute.

The whiskers retained on the screens and subsequently removed were redispersed and treated in a second similar apparatus but whose screens were ZOO-mesh size (apertures 0.065 inch). The whiskers retained on these screens were treated in a third apparatus whose screens were IOO-mesh size (apertures 0.143 inch).

The separation of fibers achieved by passing them through two similar screens in a single stage of the process is illustrated in FIG. 3. FIG. 3 is a graph showing the weight-percentage of each length of fibers passed through the screens. The length scale is logarithmic, and the mesh size (M) is indicated on the graph, where M is equal to the reciprocal of the mesh number. M, measured in inches, is thus the length of the aperture plus one wire diameter, The graph shows that all fibers less than about 0.4 M are passed through the screen, while all fibers greater than about 2 M are retained, the amount retained being proportioned to the fiber length between about 0.4 M and 2 M.

The separation of fibers achieved in a multistage process is shown in FIG. 4 which is a graph of the initial length distribution in a bulk of fibers (curve A) and of the length distribution of fibers retained on one screen (M1) but passed through the next stage (M2) (Curve i3) plotted on scales of the weightpercentage of fibers in each length category, of 2 mm., and the logarithm of the fiber length. The aperture size M2 is twice that M1. The curve B shows that about 40 to 50 weight percent of the fibers collected in this stage of the process are of length between the two aperture sizes.

Whiskers such as silicon nitride whiskers are capable of reinforcing materials to a degree dependent upon their aspect ratios. For example whiskers on aspect ratio of the order of 100 are suitable for enhancing the modulus of elasticity of plastics materials. If these plastics materials were to be used at stresses above those at which they are elastic the whiskers are insufficiently long considering the maximum specific shear stress obtainable between the each whisker and its matrix, and cannot reinforce the matrix for strength. Table 1 below shows the reinforcing power of silicon nitride whiskers of different aspect ratios:

TABLE I Order of Aspect Ratio Reinforcing Power of Whiskers l0 fillers, or elastic modulus alone in metals and resins.

The process of the invention is capable of separating a bulk of fibers into difierent length categories with a high degree of distinction between each category. The fibers can be used in a matrix with predictable results, so that a reinforced matrix can be manufactured to specification.

Another practical apparatus, used for classifying asbestos fibers, has three sets of screens 1 and 2 of overall dimensions similar to those of the first practical example, but the mesh sizes are 30 mesh, mesh, and 54; inch mesh. Asbestos fibers obtained in a desired diameter range by passage through a hydrocyclone system, are dispersed in water in a concentration of about 3 grams per liter of water containing a small amount of poly(oxyethylene) oxypropylene(pluronic) and fed at about 3 liters per minute onto the first screen set which rotates at about 10 rpm.

The concentration of 0.5 gram per liter for the 100-mesh screen is a maximum for the whiskers, whose average length is comparatively long, (0.5 mm.). The concentration has to be even less for whiskers retained on the screen and treated subsequently, as the average length is higher, providing another reason for carrying out a multistage process in the order described, furthermore a small-mesh screen can pass a considerably heavier amount of short whiskers per unit time per unit screen area than a large-mesh screen of the same unit area can pass longer whiskers in the same unit time.

It was found that in the two examples described above, a greater speed of the inner screen than about 50 feet/minute. caused splashing and carryover of liquid, while a faster feed rate or a much higher whisker concentration reduces the efficiency of separation, as fibers build up on the screen and shorter fibers are retained.

The embodiment of the invention hereinbefore described are normally extremely effective and achieve good separation of fibers by length. However, the throughput of apparatus in which the processes are carried out is generally low since only a very thin layer of fiber dispersion can be fed onto the rotating screen in order that the passage of shorter fibers through the screen is not unduly obstructed; and the rate of rotation of the drum is relatively slow so that substantially all of the shorter fibers and the dispersion medium may pass through the screen under the influence of gravity before reaching a position at which the fiber retained on the rotating screen are washed from an upper part of the screen and collected. The present invention also aims to provide an improvement in or modification of the processes and apparatus aforementioned in this application in which extremely high throughput rates of fiber may be achieved without significant reduction in the efficiency of the classification of the fibers by length.

According to the present invention, a process for classifying fibers for length comprises dispersing the fibers in a liquid to form a fiber dispersion, feeding the dispersion at a dispersion supply position onto a screen which is in the form of a closed loop having a plurality of holes of similar size which is predetermined in accordance with the minimum length of fibers which is desired to be retained on the screen, propelling the screen past the dispersion supply position at a rate, in accordance with the rate of application of the fibers, which is at least sufficient to ensure that no more than a thin layer of fibers may be applied to the screen and propelling the screen with an angular velocity sufficient to ensure that the major part of the fibers and dispersion medium passed through screen are so passed by the centrifugal acceleration resulting from this angular velocity, and removing the retained fibers from the screen with a jet of gas directed at a suitable angle to the screen at a subsequent part of its course.

In a preferred form of process, the fiber dispersion is passed onto a cylindrical screen which is rotating at an even rate about a horizontal axis, the dispersion supply position being situated inside the cylinder above the lower part of its course and the jet of gas being directed at an upper part of the screen. The rate of rotation of the screen is chosen to give the angular velocity required to exert the desired centrifugal acceleration on the dispersion. The rate of rotation is dependent upon factors such as the concentration of the dispersion and the radius of the cylindrical screen, but will usually lie in the range 300l,000 revolutions per minute (r.p.m.) In general, the higher the rate of rotation the higher will be the throughput of the apparatus over a given period. The volume of dispersion which may be passed on to the screen will normally increase with increments in rate of rotation of the screen and is limited in general only by the requirement that the major bulk of the shorter fibers and the dispersion medium must have passed through the screen before the screen reaches a position where the retained fibers are removed by the jet of gas.

The centrifugal acceleration resulting from the screen rotation will, of course, depend upon the diameter of the cylindrical screen, but normally will be well above 3 g (where 3 represents the acceleration due to gravity) and generally in the range lO-lOO g. Although centrifugal accelerations above this range may be used, there is generally no advantage because difficulties in construction of the apparatus become prohibitive at higher accelerations with insufficient improvement in throughput to justify the effort involved.

We have found that the fibers retained by the screen can be removed readily only by a high-speed jet of gas directed at the screen, normally from the side of the screen remote from the retained fibers. Attempts to achieve removal by a water jet as hitherto described in this specification are not successful as the jet is broken into a fine spray and/or deflected by contact with the rapidly moving screen and there is therefore no force transmitted through the screen to the retained fibers which is sufficient to remove them from the screen. The jet of gas is conveniently an air jet delivered through a slit orifice extending in a radial plane across the whole width of the screen. The preferred angle that the gas jet strikes the screen will be determined to some extent by the rate of rotation of the screen and by the force exerted by the gas jet. Clearly, the jet must apply a component of force to the retained fibers through the screen in a direction perpendicular to the direction of travel of the screen which is sufficient to part the fibers from the screen. It is found that the resultant path of the fibers on separation from the screen initially should be at an angle of at least 25 and preferably 3035 to the plane of fiber travel immediately before removal from the screen, i.e., to the tangential velocity of the screen at the point to which the gas jet is applied, in order to prevent the removed fiber recontacting the screen.

The path followed by the fibers removed by the gas jet is the resultant of the velocity component transmitted by the screen and the velocity component due to the gas jet and, in accordance with a feature of the invention, a collecting chamber is provided which has a receiving orifice to intercept the removed fibers and of a suitable cross section to receive substantially all of the fibers. To further assist smooth collection of the fibers, the chamber rearward of the orifice follows the resultant path of the removed fibers and, additionally, air may be continuously withdrawn into the orifice and through the collecting chamber to assist in drawing fibers into the chamber.

The minimum permissible velocity of the gas jet may be readily determined by simple experiment as it is normally the velocity just sufficient to remove entrained dispersion medium from the screen. Typically, for a screen moving at 30 feet/second, an effective gas jet directed through the screen radially at right angles to the direction of screen movement requires a speed of about 200 feet/second and should exceed at least I50 feet/second. The narrower dimension of the slit, the slit thickness, must be adequate for the gas jet to apply an acceleration to the retained fibers sufficient to remove them from the screen. The minimum useful slit thickness is generally about 0.25 inch. Although the slit thickness can be much greater than 0.25 inch there is no useful purpose to be served by this since it merely necessitates accelerating an increased volume of gas to achieve substantially the same effect.

The dispersion is preferably arranged to flow onto the screen in a direction substantially tangential to the direction of motion of the screen and with a velocity comparable to that of the screen. This tangential flow is normally achieved by passing the dispersion through a slit orifice shaped to provide an even flow of dispersion over the whole width of the screen. Turbulent flow and disturbances which would tend to drive through the screen more fibers longer than mesh size, are thereby minimized, as is the possibility of damage to brittle fibers.

By severely limiting the thickness of the layer of fibers which is allowed to be formed on the screen in accordance with the invention, the classification of fibers can be carried out effectively and the filtration of comparatively shorter fibers through the screen is not significantly impeded. The thin layer allowed to form should normally be no more than a very few, i.e., less than monolayers thick, where a monolayer is regarded as having a thickness equal to the average diameter of the great majority of the fibers. For the highest efficiency the thickness should not exceed about 1% monolayers. The desired thickness of the fiber layer is dependent upon the concentration of the fibers in the fiber dispersion, the rate of feed of the dispersion on to the screen and the evenness of distribution of the dispersion over the whole screen. A satisfactory fiber layer thickness may be readily achieved by altering one or more of these variables.

The concentration of the fibers in the liquid is decided having regard to two conditions. The fibers must be dispersed sufficiently not to interfere with each other. However, the efficiency of the process is reduced with increase of liquid because large volumes of liquid tend to wash all the fibers, large and small, through the screen. Conversely sufficient liquid must be present to ensure that the smaller fibers are not retained. For a given rate of rotation and liquid feed rate, the maximum permissible concentration varies inversely with the average length of the fibers. A screen of a given mesh size is found to entrain a quantity of liquid which substantially constant over a wide range of angular velocities of the screen and as an approximate guide not less than about l0 times this quantity of liquid should be applied to achieve adequate classification of fibers by length. This quantity of liquid may all be applied to the screen as the dispersion medium of the fiber dispersion, or alternatively some of the required quantity may be applied as dispersion medium and the remainder at a washing position between the position at which the dispersion is fed onto the screen and the position at which retained fiber is removed by the gas jet.

While some types of fiber exhibit no great variation in their diameter between fibers, others, such as asbestos fibers, may vary by a factor of 10 or more. As the tensile properties of the composite are generally better for higher aspect ratio fibers, those which are coarse may be undesirable. These latter types of fibers are preferably classified for diameter before being classified for length. The diameter classification may be carried out using a hydrocyclone network or an elutriation column.

In order that this embodiment of the invention may be more clearly understood classification apparatus and its method of use will now be described, by way of example, with reference to the accompanying drawings in which:

FIG. 5 is a diagrammatic part-sectioned side elevation of one form of classification apparatus;

FIG. 6 is a diagrammatic view onthe section lI-II in FIG. 5 and incorporating views on AA and CC and a section on BB in FIG. 5;

and FIG. 7 is a diagrammatic view similar to FIG. 6 of a classification apparatus having a recycling provision.

As shown in FIGS. 5 and 6 the apparatus has a cylindrical screen and a concentric outer casing 2 with a common horizontal shaft 3. The screen 1 is made of metal gauze. A dispersion supply weir 4 is situated inside the inner screen so that it can flow dispersion through slot 5 onto the whole width of the screen 1, and acts to even out the flow of the dispersion and to direct it to approach the screen as nearly tangentially as possible. Situated below the outer casing 2 is a fines outlet 6. Air jet flushing means in the form of a nozzle 7 is situated above an upper part of the outer screen 2. The screen 1 is driven by an electric motor (not shown) attached to the shaft 3.

The apparatus is used to classify fibers into lengths above and below a given length in the following manner. A dispersion of the fibers in a liquid so that the fibers are freely dispersed, is fed into the weir whence it flows evenly at a controlled rate via the slot orifice 5 onto the whole operative width of the screen in a direction substantially tangential to the screen. The screen 1 is set rotating in the direction indicated in FIG. 5. The feed rate of dispersion on to the screens is such that not more than approximately a monolayer of fibers is applied to the inner screen 1. The filtrate from the screen 1 consisting of finer fibers and dispersion medium is centrifuged to the outer casing 2 while most of the longer fibers are retained on the screen 1. The monolayer of longer fibers remains on the screen as it revolves until it passes under air jet 7 when the fibers are blown into the collecting chamber 8 and sucked away through pipe 9. The separated fiber fractions may be redispersed and passed to another similar apparatus having a screen of a different mesh from the one described for further classification.

One practical apparatus, used for classifying asbestos fibers whose diameters are about 10-100 microns and whose lengths vary from 01. mm. to mm., has a screen 9 inches deep and 2 feet in diameter. The screen has a 30-mesh wire gauze (i.e., apertures of about 0.57 mm.).

The asbestos fibers, as a 0.1 percent by weight suspension in water are fed at a rate of pounds asbestos per hour on to the screen which is rotated at 300 r.p.m. The finer fibers and dispersion medium, water collect in the outer casing 2 and the fibers retained on the screen are removed by an air blast of 200 feet per second from jet 7 and collected in chamber 8. It is found that the asbestos is divided into two fractions of approximately equal weight, the fibers retained on the screen predominantly having lengths between 1 mm. and 10 mm. and the fibers passing through the screen having lengths up to 1 Referring now to FIG. 7, a modified form of classification apparatus has provision for recycling the fibers passed through by an initial screening process so that a high proportion of any longer fibers passing through the screen initially are removed from the recycled material. Recycling is conveniently achieved by axially dividing the screen 1 into two annular portions by means of a peripheral separator 17, initially feeding the dispersion on to one portion of the screen via weir l3, recycling the centrifuged liquid and fines through pipe 15 with the aid of a pump (not shown) and feeding the recycled material on to the other portion of the divided screen via weir 14. The longer fibers retained on each portion of the screen are removed by a slit-shaped gas jet 7 into receiver 8 which may be divided by an upstanding wall 12 into a separate collecting chamber for each portion of the divided screen. The fibers so collected are removed, preferably with suction, through pipes 10 and 11 and may if desired be united to give a consolidated fraction of longer fibers. In general the long fibers derived from the recycled material and removed through pipe 11 will predominantly have lengths at the shorter end of the retained fiber range. An annular baffle 18 is provided on the inside of the casing 2 which cooperates with the separator 17 to keep the discharge from each portion of the screen separate. Since the proportion of longer fibers is less in the recycled material than in the initial feed of unclassifed fiber dispersion, the width of the screen portion required for the recycled material is considerably less than that for the initial feed.

Another mode of operation of the divided screen apparatus of FIG. 7 is one which provides the maximum resolution of fiber length, as distinct from the maximum retention of long fibers. This uses a feedback circuit, as follows. The suspension is fed on at weir 13 as before, but the long retained fibers, removed at 10, are returned to be fed on at weir 14, where further separation removes traces of short fiber, and the doubly screened long fiber is the extracted at 11. The traces of short fiber, which inevitably contain a few long fibers, are recovered through 18 and recombined with fresh feed dispersion being fed on at weir 13. The finer output is removed directly through pipe 13 for this mode of operation and no longer returned.

It will be appreciated that this modified form of classification apparatus is not necessarily limited to a screen divided into two portions and may consist of a multiply divided screen. If each portion of screen is constructed of different mesh sizes it may also be possible to achieve a multiple fractionation of the fibers by length, although the difficulties in constructing a suitable apparatus will be readily apparent.

We claim:

1. A method of classifying fibers by length which comprises (a) forming a liquid dispersion of the fiber, (b) feeding the dispersion at a controlled rate and concentration onto a screen which is in the form of a closed loop moving continuously in one direction and having a plurality of holes of a similar size, said size being predetermined in accordance with the minimum length of fibers which is desired to be retained on said screen, said concentration and rate being such that no more than five monolayers of fibers are applied to said screen and that the fibers flow onto the screen with the major component of their velocity in the direction of movement of the screen, (c) removing retained longer fibers from the screen after the bulk of the shorter fibers have passed through the screen by impinging upon said retained fibers a gas jet having a velocity component normal to the plane of the screen sufficient to lift the retained fibers away from the screen and separately collecting said shorter fibers and said longer fibers, and (d) propelling the screen at an angular velocity sufficient to ensure that the major part of the shorter fibers and dispersion medium are passed through the screen by the centrifugal acceleration resulting from this angular velocity.

2. A process according to claim 1 in which the closed loop is a cylindrical screen which rotates at an even rate about a horizontal axis in which the dispersion supply position is situated inside the cylindrical screen above the lower part of the course of the screen, and in which the jet of gas is directed to an upper part of the screen.

3. A process according to claim 2 and in which the rate of rotation of the screen is between 300 and 1,000 revolutions per minute.

4. A process according to claim 2 and in which the centrifugal acceleration resulting from the rotation of the cylindrical screen is between 10 and times the acceleration due to gravity.

5. A process according to claim 2 in which the jet of gas is an air jet delivered through a slit orifice extending in a radial plane across the whole width of the screen.

6. A process according to claim 5 wherein the slit thickness is about 0.25 inch.

7. A process according to claim 1 and in which the jet of gas applies to fibers retained on the screen a component of force through the screen sufficient to ensure that the fibers thereby removed from the screen have a resultant path at an angle of at least 25 to the plane of fiber travel immediately before removal from the screen.

8. A process according to claim 7 and in which the said angle is between 30 and 35.

9. A process according to claim 1 and in which a collecting chamber is provided which has a receiving orifice to intercept the removed fibers of a cross section suitable to receive substantially all of the fibers removed by the jet of gas.

10. A process according to claim 9 and in which the collecting chamber rearward of the orifice is shaped to follow the resultant path of fibers removed from the screen by the jet of gas.

1 l. A process according to claim 10 and in which air is continuously withdrawn into the orifice and through the collecting chamber to assist the smooth collection of the said fibers.

12. A process according to claim 1 and in which the jet of gas is directed onto the screen at right angles to the direction of screen movement with a speed of at least feet/second.

13. A process according to claim 1 wherein the dispersion is flowed onto the screen with a velocity substantially the same as the velocity of the screen.

14. A process according to claim 13 wherein the dispersion is supplied through a slit-shaped orifice extending across the width of the screen.

15. A process according to claim 1 and in which the thickness of the layer of fibers formed on the screen is limited to not more than about 1% monolayers.

16. A process according to claim 1 and in which the quantity of liquid applied to the screen to achieve fiber classification is approximately 10 times that amount entrained by the screen.

17. A process according to claim 16 in which the said quantity of liquid is supplied wholly as the liquid phase of the fiber dispersion.

18. A process according to claim 16 and in which the said quantity of liquid is applied to the screen partly as the liquid phase of the fiber dispersion and partly at a washing position between the position at which the dispersion is fed onto the screen and the position at which retained fiber is removed by the jet of gas.

19. A process according to claim 1 and in which a dispersion of the separated short fibers is subsequently recycled through the screen to further improve their classification by length.

20. A process according to claim 1 and in which a dispersion of separated long fibers is subsequently recycled through the screen to further improve their classification by length.

21. Apparatus for classifying fibers by length which comprises a screen in the form of a closed loop having a plurality of holes of a similar size which is predetermined in accordance with the minimum length of the fibers desired to be retained on the screen, dispersion supply means mounted above the screen for flowing a dispersion of fibers in a liquid at a controlled rate onto the screen at a dispersion supply position, gas jet supply means for removing retained fibers from the screen, means for collecting fibers passed through the screen, means forcollecting fibers removed from the screen by the gas jet, and driving means operably linked with the screen for propelling the screen past the dispersion supply position with an angular velocity sufficient to ensure that the major part of the fibers and dispersion medium to be passed through the screen are so passed by the centrifugal acceleration resulting from this angular velocity.

22. Apparatus according to claim 21 in which the closed loop is a cylindrical screen which rotates to an even rate about a horizontal axis in which the dispersion supply position is situated inside the cylindrical screen above the lower part of the course of the screen, and in which the jet of gas is directed at an upper part of the screen.

23. Apparatus according to claim 22 and in which the rate of rotation of the screen is between 300 and 1,000 revolutions per minute.

24. Apparatus according to claim 22 and in which the centrifugal acceleration resulting from the rotation of the cylindrical screen is between and 100 times the acceleration due to gravity.

25. Apparatus according to claim 22 in which the jet of gas is an air jet delivered through a slit orifice extending in a radial plane across the whole width of the screen.

26. Apparatus according to claim 25 wherein the slit thickness is about 0.25 inch."

27. Apparatus according to claim 22 and in which the jet of gas applies to fibers retained on the screen a component of force through the screen sufficient to ensure that the fibers thereby removed from the screen have a resultant path at an angle of at least 25 to the plane of fiber travel immediately before removal from the screen.

28. Apparatus according to claim 27 and in which the said angle is between 30 and 35.

29. Apparatus according to claim 22 and in which a collecting chamber is provided which has a receiving orifice to intercept the removed fiber of a cross section suitable to receive substantially all of the fibers removed by the jet of gas.

30. Apparatus according to claim 29 and in which the collecting chamber rearward of the orifice is shaped to follow the resultant path of fibers removed from the screen by the jet of gas.

31. Apparatus according to claim 29 and in which air is continuously withdrawn into the orifice and through the collecting chamber to assist the smooth collection of the said fibers.

32. Apparatus according to claim 22 and in which the jet of gas is directed onto the screen at right angles to the direction of screen movement with a speed of at least feet/second.

33. Apparatus according to claim 22 and in which the cylindrical screen is internally divided into a first and a second cylindrical section by a circumferential separating wall.

34. Apparatus for classifying fibers by length, which comprises (a) a screen in the form of a closed loop having a plurality of holes of a similar size which is predetermined in accordance with the minimum length of the fibers to be retained on the screen, said screen being cylindrical and rotatable about a horizontal axis and being internally divided into a first and second section by a circumferential separating wall, b) first and second dispersion supply means being positioned inside the cylindrical screen sections near the lowest part of the course of the screen, (c) a gas jet supply means for removing retained fibers from the screen, said gas jet being directed at the upper part of each of said cylindrical screen sections, (d) first and second collecting means for collecting fibers passed through said first and second screen sections respectively (e) means for collecting fibers removed from the screen by the gas jet and (f) driving means operably linked with the screen for propelling the screen past the dispersion supply position with an angular velocity sufficient to ensure that the major part of the fibers and dispersion medium to be passed through the screen are so passed by the centrifugal acceleration resulting from this angular velocity.

35. Apparatus according to claim 34 and in which there is provided means to feed fibers passing through the first section into the dispersion supply means of the second section to optimize the separation of longer fibers.

36. Apparatus according to claim 34 and in which there is provided means to feed fibers retained by the first section into the dispersion supply means of the second section to optimize the classification of fibers by length.

Claims

4. A process according to claim 2 and in which the centrifugal acceleration resulting from the rotation of the cylindrical screen is between 10 and 100 times the acceleration due to gravity.

7. A process according to claim 1 and in which the jet of gas applies to fibers retained on the screen a component of force through the screen sufficient to ensure that the fibers thereby removed from the screen have a resultant path at an angle of at least 25* to the plane of fiber travel immediately before removal from the screen.

8. A process according to claim 7 and in which the said angle is between 30* and 35* .

11. A process according to claim 10 and in which air is continuously withdrawn into the orifice and through the collecting chamber to assist the smooth collection of the said fibers.

12. A process according to claim 1 and in which the jet of gas is directed onto the screen at right angles to the direction of screen movement with a speed of at least 150 feet/second.

15. A process according to claim 1 and in which the thickness of the layer of fibers formed on the screen is limited to not more than about 1 1/2 monolayers.

21. Apparatus for classifying fibers by length which comprises a screen in the form of a closed loop having a plurality of holes of a similar size which is predetermined in accordance with the minimum length of the fibers desired to be retained on the screen, dispersion supply means mounted above the screen for flowing a dispersion of fibers in a liquid at a controlled rate onto the screen at a dispersion supply position, gas jet supply means for removing retained fibers from the screen, means for collecting fibers passed through the screen, means for collecting fibers removed from the screen by the gas jet, and driving means operably linked with the screen for propelling the screen past the dispersion supply position with an angular velocity sufficient to ensure that the major part of the fibers and dispersion medium to be passed through the screen are so passed by the centrifugal acceleration resulting from this angular velocity.

24. Apparatus according to claim 22 and in which the centrifugal acceleration resulting from the rotation of the cylindrical screen is between 10 and 100 times the acceleration due to gravity.

26. Apparatus according to claim 25 wherein the slit thickness is about 0.25 inch.

27. Apparatus according to claim 22 and in which the jet of gas applies to fibers retained on the screen a component of force through the screen sufficient to ensure that the fibers thereby removed from the screen have a resultant path at an angle of at least 25* to the plane of fiber travel immediately before removal from the screen.

28. Apparatus according to claim 27 and in which the said angle is between 30* and 35*.

32. Apparatus according to claim 22 and in which the jet of gas is directed onto the screen at right angles to the direction of screen movement with a speed of at least 150 feet/second.

34. Apparatus for classifying fibers by length, which comprises (a) a screen in the form of a closed loop having a plurality of holes of a similar size which is predetermined in accordance with the minimum length of the fibers to be retained on the screen, said screen being cylindrical and rotatable about a horizontal axis and being internally divided into a first and second section by a circumferential separating wall, (b) first and second dispersion supply means being positioned inside the cylindrical screen sections near the lowest part of the course of the screen, (c) a gas jet supply means for removing retained fibers from the screen, said gas jet being directed at the upper part of each of said cylindrical screen sections, (d) first and second collecting means for collecting fibers passed through said first and second screen sections respectively (e) means for collecting fibers removed from the screen by the gas jet and (f) driving means operably linked with the screen for propelling the screen past the dispersion supply position with an angular velocity sufficient to ensure that the major part of the fibers and dispersion medium to be passed through the screen are so passed by the centrifugal acceleration resulting from this angular velocity.