EP0335922A4 - Method and apparatus for friction sorting of particulate materials. - Google Patents

Abstract

Processes by which a mixture of two or more discrete particulate materials, each of the materials having a different sliding coefficient of friction, can be separated using methods which take advantage of velocity differences generated by application of a force to the mixture over a surface as a function of its sliding coefficient of friction. In a first embodiment, the mixture is fed by a supply means (116, 118, 120) to the surface of a rotating disc (122), whereby the materials will be induced to move by centrifugal action. A second embodiment employs a surface (230) inclined to the horizontal and vibrated, causing materials of different coefficients of friction traverse the surface (230) along separate paths. A third disclosed embodiment uses an inclined slide (316) to provide separate momentum effected paths to the materials.

Description

METHOD AND APPARATUS FOR FRICTION SORTING OF PARTICULATE MATERIALS

This application is a Continuation-In-Part of United States Patent Application Serial No. 07/097,877, filed September 17, 1987.

FIELD OF THE INVENTION This invention relates to novel methods and apparatus for the separation or beneficiation of a mixture of two or more discrete particulate materials, e.g., a mixture of two or more granular or rocklike mineral materials, each of which materials has a different sliding coefficient of friction. More particularly, this invention relates to sorting of dissimilar materials by methods which take advantage of their differences in sliding coefficient of friction. This invention further relates to improvements in the art of separation of mixtures of unlike material masses, e.g., mineral mixtures, wherein the separability of the constituents of these mixture results primarily from differences between their respective sliding coefficients of friction rather than from their shape or degree of sphericity.

BACKGROUND OF THE INVENTION

Ores and other minerals when mined usually contain various impurities, i.e., the desired mineral species usually occurs in admixture with other minerals. Thus, the desired mineral or ore usually must be separated from the rest of the material as mined.

Talc, for example, occurs in nature in rock formations in which it is typically associated with other minerals, such as dolomite, chlorite, quartz, pyrite, magnesite, calcite, feldspar, mica, or mixtures thereof. For ease of description, as used in this application, "dolomite" shall be taken to mean dolomite and/or the aforementioned other minerals with which talc shall be admixed or otherwise associated in nature. A run-of-mine ore is generally composed, for the most part, of rocks of predominately one mineral species, e.g., talc rocks are admixed with dolomite rocks or the like. A very small percentage of conglomerate rocks containing varying mixtures of mineral species, such as talc combined with dolomite in the same rock, may be present as well.

Talc is commonly separated from other minerals, for example, dolomite, by manual sorting or flotation processes. Manual or hand sorting relies upon visual differences between the mineral species such as color variations, degree of granulation, size of the material lumps and the like which are perceptible to the persons doing the sorting. Manual sorting is, obviously labor intensive. It can also give rise to disabling injuries, including carpal tunnel syndrome. Flotation processes are capital intensive, chiefly due to the very expensiveequipment necessary to carry them out. Furthermore, vast quantities of water are needed, water that is not available in many mining regions such as ones located in Montana and Australia.

Numerous attempts have been made to develop automated sorting processes for sorting mineral species. Among these are optical sorting, which relies upon optical sensorperceptible visual differences in light reflection from the surfaces of the ores or minerals to be separated, sinkfloat processes, which rely upon specific gravity differences in the materials being separated, and electrostatic separation methods based either on electrophoresis or dielectrophoresis, which relies upon the differences in conductivity or shape of the mixture's components. None of these automated sorting processes, however, have been completely successful in that they can be affected by color variations between the particles, shape variations, specific gravity differences and mineral size variations, to name just a few factors. A number of automated sorting processes based on particle shape have been developed. For example, the separation of coal from its associated rock is shown in U.S. Patents Nos. 1,030,042, issued June 18, 1912 to Wilmot et al. and 1,190,926, issued July 11, 1916 to Lotozky. Coal suitable for separation by these patented processes is granular in form and generally of a more or less spherical configuration. The associated rock, which includes slate, must on the other hand be present in the form of more or less flat shaped pieces.

In the Wilmot et al. process a mixture of coal and its associated rock is placed in a chute having a rotating disc halfway down its length. The coal, being generally spherical in shape, rolls down the chute and comes to rest in a bin at the bottom of the chute. The associated, generally flat rock slides down the chute until it reaches the rotating disc, where it comes to rest and is carried away from the chute by the disc.

In the Lotozky process a chute is not used. Instead, a mixture of coal and rock is fed onto the surface of an inclined rotating disc in a direction opposite to that in which the disc is moving. The coal, which is again generally spherical in shape, continues to roll down the disc in its original direction. The flat rock comes to rest on the disc and is carried away.

These separation processes rely upon the shape of the particles to be separated, in particular the extent to which the particles being separated are or are not spherical, thus utilizing both rolling and sliding coefficients of friction in the sorting process rather than differences between the sliding coefficients of friction of the two types of particles being separated. Furthermore, Wilmot et al.'s and Lotozky's rotating discs are used solely to physically carry slate or other flat rocks out of the slate/coal stream, not to impart centrifugal acceleration to separate coal from the other materials present. Other automated sorting methods and apparatus based solely on particle shape include, for example, those disclosed in U.S. Patent No. 4 ,059,189, issued November 22, 1977 to John. The separation of particles with identical composition but different shape is again based on the degree of sphericity of the particles being separated as demonstrated by their differences in rolling and sliding coefficients of friction. Yet another automated apparatus for separating particles based upon their degree of sphericity is shown in U.S. Patent No. 3,485,360, issued December 23, 1969 to Deinken et al. The Deinken et al. apparatus consists of a rotating disc to which a mixture containing generally spherical and irregularly shaped, generally nonspherical particles of identical composition is fed. The spherical particles roll off the disc, while irregularly shaped particles are forcibly removed from the surface of the disc. Another automated separation process, that disclosed in U.S. Patent No. 1,744,967, issued January 28, 1930 to Johnson, requires the application of an electrostatic field. The Johnson process operates on a frictional difference obtained chiefly by increasing gravitational force by applying an electrostatic force to take advantage of the fact that flat particles create a stronger electrostatic ξield than spherical particles. Automated sorting processes have also been developed to separate particles by differences in their adhesive properties; see U.S. Patent No. 3,508,645, issued April 28, 1970 to Conrad. In the Conrad process sticky particles, such as chicken meat, are made to adhere to a moving surface by static bonding while non-sticky particles, such as associated chicken bones, slide off the moving surface.

Other separation processes have been used to separate material mixtures by gravity concentration using the density differences between the mixture components. These processes may be carried out on ore concentrating tables, a form of a vibratory table. None of the above-mentioned automated sorting processes separate different mineral species by taking advantage of differences in sliding coefficients of friction exhibited by the mineral species being separated. It has now been discovered that the constituents of mixtures of discrete particulate materials, e.g., a mixture of two or more granular or rocklike mineral materials of dissimilar chemical constitution but similar physical configuration, can be separated one from another by a novel sorting technique that utilizes differences/ in the sliding coefficients of friction of the materials being separated, thus obviating the need to form such materials into different shapes to effect separation thereof.

It is, therefore, an object of this invention to provide methods and apparatus for separating different materials, including but not limited to different minerals, having different sliding coefficients of friction by taking advantage of such sliding coefficient of friction differences. It is also an object of this invention to provide methods and apparatus for the separation of talc from associated minerals and rocks utilizing the differences in the sliding coefficients of friction exhibited by talc and such associated mineral species to produce high-grade talc products and upgraded talc mixtures.

These and other objects, as well as the nature, scope and utilization of the invention will become readily apparent to those skilled in the art from the following description, the drawings and the appended claims.

SUMMARY OF THE INVENTION

This invention is based on the discovery that any mixture of two or more discrete particulate materials, each having significant differences from the others present in the mixture in their sliding coefficients of friction, can be sorted utilizing such frictional differences. Such particulate mixtures are separated by contacting them with a surface upon which the individual components of the mixture exhibit sliding coefficient of friction differences, and separation is achieved by differences in the movement of the individual components over the surface resulting from the differences in their sliding coefficients of friction. The surface upon which the materials exhibit differences in sliding coefficient of friction may be part of an apparatus in which either accelerative of decelerative forces are applied to the materials to cause differences in sliding movement of the components of the material mixture over the surface. Such apparatus may be of any configuration which effectuates such separations, including but not limited to apparatus containing slides, rotating discs, centrifuges, rotating cylinders, vibrating tables and the like.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a top plan view schematic representation of a rotating disc apparatus embodying this invention which uses a feed conveyor in conjunction with a feed chute and doctor blade to introduce the material mixture being separated to the surface of the disc on which separation takes place.

Figure 1a is a partial plan view schematicrepresentation of a modified embodiment of rotating disc apparatus embodying the invention.

Figure 2 is a side view schematic representation of the rotating disc apparatus depicted in Figure 1.

Figure 3 is a top plan view schematic representation of a vibratory table apparatus embodying this invention wherein the material mixture being separated is introduced by means of a screw conveyor to the surface of the vibratory table on which separation is effected.

Figure 4 is a side view schematic representation of a vibratory table apparatus depicted in Figure 3 showing the tilt of the vibratory table. Figure 5 is a front view schematic representation of the vibratory table apparatus depicted in Figure 3 showing the slope of the vibratory table.

Figure 6 is a side schematic representation of a velocity sorting slide apparatus embodying this invention which uses a feed chute together with a reciprocating push bar as the feed mechanism to introduce the material mixture to be separated to the surface of the slide on which separation is effected. Figure 7 is a top plan view schematic Representation of the velocity sorting slide apparatus depicted in Figure 6.

DETAILED DESCRIPTION OF THE INVENTION Among the mixtures of discrete particulate materials which can be separated in accordance with this invention by using sliding coefficient of friction differences between these materials are ores and minerals in the form of granular or rocklike masses from run-of-mine ores, the desired mineral or ore being separated in such cases from rocks made up in whole or part of' other materials. Naturally occurring mineral mixtures, e.g., combinations of any of talc, dolomite, chlorite, quartz, pyrite, magnesite, calcite, feldspar, mica, such as talc and dolomite, talc and chlorite, chlorite and dolomite, chlorite and quartz, and the like, are particularly suitable for separation in this fashion. Talc, a hydroxylated magnesium silicate which can be represented by the formula Mg₁₂Si₁₆O₄₀(OH)₈, occurs in nature, as indicated above, in rock formations associated with various mineral species. The most common of these minerals are dolomite, a species of limestone which can be represented by the formula CaMg(CO₃)₂; chlorite, a monoclinic silicate of any of aluminum, ferrous iron and magnesium; quartz, represented by the formula SiO₂; pyrite, represented by the formula FeS₂; magnesite, represented by the formula MgCO₃; calcite, a species of limestone which can be represented by the formula CaCO₃; feldspar, a silicate of aluminum with the metals of potassium, sodium, and calcium; and mica, a phyllosilicate mineral which can be represented by the general formula (K,Na,Ca) (Mg,Fe,Li,Al)₂-₃(Al,Si)₄O₁₀(OH,F)₂. A run-of-mine ore from a talc mine fed to any separation process, including a process in accordance with this invention, typically is in the form of mixtures of rocks ranging in size from fines to larger particles up to about 20 inches in characteristic size. For the most part, each rock in such mixture is made up of predominately one mineral species. A very small percentage of conglomerate rocks may be present which contain mixtures of mineral species. The run-of-mine ore may also contain sand, other rocklike particles and gangue. Such mineral mixtures are separated in accordance with this invention by taking advantage of the differences between their individual components' sliding coefficients of friction. Coefficient of friction, in broad terms, is a measure of the resistance of an object to movement over a surface expressed in one of three forms: static, dynamic or rolling coefficient of friction. Static coefficient of friction, the largest of these three frictional forces, is a measure of the force required to initiate movement of the object over the surface calculated by taking the tangent of the angle of incline required to initiate movement over the surface. Dynamic coefficient of friction, although smaller than static coefficient of friction, does not differ appreciably from static coefficient of friction at low velocities, and is a measure of the force necessary to maintain the object in a sliding motion over the surface. Dynamic coefficienxt of friction is calculated by taking the tangent of the angle of incline required to maintain a constant velocity for the object moving over the surface. An object's static or dynamic coefficient of friction is referred to as its sliding coefficient of friction, and is a measure of the object's resistance to sliding. Rolling coefficient of friction is substantially smaller than sliding coefficient of friction for any given object, whether the sliding coefficient of friction is expressed either as the static or dynamic coefficient, and is a measure of the force needed to maintain an object in a rolling motion across a surface. Rolling coefficient of friction is dependent upon the degree of sphericity of the object and may be calculated by taking the tangent of the angle of incline needed to maintain the object rolling over a surface at a constant velocity.

Coefficient of friction, however expressed, i.e., as static, dynamic, sliding or rolling coefficient of friction, when multiplied by an object's force normal to a surface, gives the force necessary to move the object along the surface at a constant velocity or, in the case of static coefficient of friction, to initiate such movement.

A material's sliding coefficient of friction is unique not only to the material, per se, but also to each surface with which the material comes in contact, and will be affected by such variables as surface hardness, the smoothness of the surface finish, the degree to which the surface is amorphous in character, the material's grain size, and whatever coatings, such as fluids, dust or other contaminants, are found on the surface or associated with the material contacting the surface. Thus, a mixture of discrete particulate materials having similar physical configurations but dissimilar chemical compositions can be separated using the process and apparatus of this invention based on the differences they exhibit in sliding coefficient of friction on any particular surface, unaffected by rock size or rock geometry.

Material mixture separation by sliding coefficient of friction differences in accordance with this invention may be achieved by means of any of velocity difference sorting, slide-retain sorting or differential braking sorting, depending on how the material mixture being separated is made to move across a surface as a function of the sliding coefficients of friction of the mixture's components.

In velocity difference sorting one of the materials being separated will be made to slide at a significantly faster rate over the surface than the other material(s). When the ratio of acceleration of a material mixture parallel to normal on a surface along which the material mixture is moving is greater than the sliding coefficients of friction of the components of the material mixture, the comoonents will move along the surface each at a velocity inversely proportional to its sliding coefficient of friction. Thus, the lower the sliding coefficient of friction against the surface the faster the component will move. In slide-retain sorting one material in the mixture being separated is made to move along the surface while the other(s) remain stationary. The ratio of acceleration parallel to normal on a surface, which is proportional to the sliding coefficients of friction of the materials, is such that the component having the lower sliding* coefficient of friction will move along the surface while the component(s) having the higher sliding coefficient of friction will remain stationary on the surface.

Differential braking sorting, a variation of velocity difference sorting, relies on one material slowing down faster than the other material (s) when the components of a mixture being separated are introduced to a surface at the same initial velocity. The rate of deceleration of a material moving parallel to normal on a surface is directly proportional to its sliding coefficient of friction. Thus, materials having higher sliding coefficients of friction slow down more than materials having lower sliding coefficients of friction.

An. apparatus used to effectuate material separations in accordance with this invention will comprise means to supply the materials to be separated to a surface upon which the materials to be separated exhibit sufficient differences in their sliding coefficients of friction associated with means to apply a force to cause movement of such materials over the surface so that these frictional differences can be displayed. Such applied forces can be accelerative or decelerative in nature, and may include gravitational and centrifugal forces.

One type of apparatus which can be used in practicing the present invention can be termed generally a rotating disc sorting apparatus, such as depicted in Figures 1 and 2, shown in the top plan and side views, respectively. When utilizing such an apparatus, a multicomponent particulate mixture, e. g. , a mineral mixture such as mixtures of talc and dolomite rocks, is fed by means of a feed system onto a separation surface. In the feed system, the multicomponent mixture is fed from a hopper 110 onto a vibrating feeder 112. The vibrating feeder 112 contains a screen 114, which permits fines and other small extraneous particulate materials to be removed from the system as the multicomponent mixture is fed through the vibrating feeder 112. The screen 114 may consist of a sheet metal plate with punch holes of approximately 1.5 inches in diameter. The screened material mixture is fed from the vibrating feed 112 to a feed conveyor 118 by means of a slide 116. The slide 116 may have any angle of inclination, and preferably will have a V-shaped trough to accelerate and place the particles or rocks being separated in a single file on a feed conveyor 118. For example, the slide 116 may have a thirty degree V-shaped trough, inclined at an angle of 35 degrees from the horizontal. This provides adequate alignment and spacing of the mixture particles for the disc unit 122 to function properly. The material mixture moves along the feed conveyor 118 to a feed slide 120, which is used to transfer the particulate mixture from the feed conveyor 118 to the surface of the rotating disc 122. For example, the feed slide 120 may be three feet in length and have an angle of inclination of 16 degrees. The feed conveyor 118 is operated at any suitable speed, preferably at a velocity that enables the material mixture as it exits the feed slide 120 onto the disc 122. to have a velocity substantially equal to the tangential velocity of the disc 122 at that point, i.e., the feed point of the rotating disc 122. The velocity of the feed conveyor 118 generally will be greater than the tangential velocity of the disc 122 at the feed point since the accelerative energy of the material mixture tends to dissipate as the material mixture slides down the feed slide 120, hits the doctor blade 124, and changes direction on .the surface of the disc 122. For example, when the disc 122 has a diameter of six feet, a rotational speed of thirty rpm, and a feed point at the 18 inch radius of the disc 122, then the feed conveyor 118 is operated at a velocity of seven feet per second, which results in the material mixture having a velocity of 4.7 feet per second as it exits the feed slide 120. onto the surface of the disc 122, which is the approximate tangential velocity of the disc 122 at the

18 inch radius feed point. At low feed rates the feed slide 120 may also comprise a separation surface embodied by the present invention and as such may- apply accelerative or decelerative forces to the material mixture thus further enhancing the separation of the material mixture. At higher feed rates, the feed slide 120 ceases to function as a separation surface due to the interaction of the material mixture. In such interactions the material component with the lower sliding coefficient of friction shoves the material component(s) having higher coefficients of friction down the feed slide 120 resulting in the material mixture components having essentially the same velocity as they come into contact with the surface of the rotating disc 122. For example, at a feed rate of about forty tons per hour of a mixture of talc and dolomite rocks the velocities are approximately equal for the talc and dolomite when they contact the disc 122. The surface of the feed slide 120 may be the same or different from that of the surface of the rotating disc 122. A feed doctor blade 124 is used to place, with variable to no spacing, the particles or rocks making up the particulate mixture in a single file on the surface of the disc 122 so as to prevent any portion of this material from either being pushed or trapped by the remainder of the mixture. The feed doctor blade 124 may have any configuration that assists in the placement of the material mixture on the surface of the disc 122 without causing bouncing or rolling the material mixture. Preferably the feed doctor blade 124 is curved, wherein the degree of curvature may be altered to change the feed point. The feed point will vary depending upon such factors as the composition of the particulate mixture, the size of the particles or rocks being separated and their sliding coefficient of friction differences, and the rotational speed, diameter, surface material and profile of the disc 122. The diameter of the disc 122 may be of any size sufficient to effect separation and is preferably from about two feet to about thirty feet in diameter. The disc 122 is rotated in a counterclockwise direction about an axis 126 using any conventional means such as an electric motor (not shown). The rotational speed of the disc 122 will be such that the two or more different species which- make up the particulate mixture develop velocity differences, based on their respective sliding coefficients of friction, that will depend on which of the three embodiments which can be utilized when practicing this invention - - velocity difference sorting, slide retain sorting or differential braking sorting - - is being practiced at any particular time. For a given size disc, as the characteristic size of the particles in the mixture decreases the rotational speed of the disc will be increased to overcome interference with the disc surface's frictional characteristics by fines and the like. And for a given particulate mixture composition and throughput, as the size of the disc 122 is increased its rotational speed will be decreased. The rotational speed of the disc 122 may be any speed, however, due to physical limitations on fabrication and use the rotation speed of the disc 122 preferably will be from about 5 rpm to about 2000 rpm.

The component of the particulate mixture having the lower (or lowest, in the case of three or more components) sliding coefficient of friction on the surface of the disc 122 slides on the surface towards the perimeter of the disc 122 and off the edge of the disc 122, where it is deposited in a bin 130 from which it can be removed by means such as a conveyor 132. The component having a higher sliding coefficient of friction on the disc surface remains stationary, or moves at a slower speed towards the perimeter of the disc 122 at a location on the perimeter different from that to which materials having the lower sliding coefficient of friction move. Any material remaining on the surface of the disc 122 after a single revolution is removed forcibly from the surface by means such as a reject doctor blade 140 or any other suitable means such as a scraper, air jet, vacuum or the like, into a reject bin 142. The contents of the bin 142 can be removed by means of a conveyor 144. Additional bin(s) and removal means may be placed at locations on the perimeter between the bins 130 and 142, such as bin 146 and conveyor 148. The proportion of material components recovered in any additional bin will depend upon its location between the bins 130 and 142, with a percentage of material with the lower sliding coefficient of friction increasing as its distance from the bin 130 decreases. Material recovered from additional bin(s) may be subjected to a further sorting, either by recycling back to the feed hopper 110 or by feeding to another separation or sorting process. The surface of the disc 122 can be wetted, by means of a water spray 150, if desired, to impart different frictional surface qualities. A cleaning means 158 may be employed to remove materials other than the discrete particulate materials being separated, e.g., sand and chips, from the surface of the disc 122. The cleaning means 158 may consist of a rotating nylon brush unit or any other suitable cleaning means that effectively removes extraneous material from the surface of the disc 122 including, but not limited to, a rotary brush unit, a fluid jet, vacuum means, a scraper, or a combination thereof.

The surface profile of the disc 122 can be varied depending upon the amount of gravitational force to be used in the separation process. Any suitable profile may be used including, but not limited to, disc surfaces having a radial cross section that is flat, convex, concave or in the form of a shallow cone. In the case of a flat disc, the minimum acceleration required to slide at least one of the materials being separated will be equal to its sliding coefficient of friction on the disc surface since the force of such material normal to the plane of the disc surface is equal to this material's weight. Preferably the disc 122 will have a concave, generally friesto-conical profile as shown in Figure 2. Such profiles have a higher capacity, and thus permit the separation of larger volumes of materials during a given time period. A concave profile also helps to prevent bouncing and rolling of the materials being separated.

In a preferred embodiment, the surface profile of the disc 122 is selected so that a constant ratio of acceleration or deceleration of the components of the material mixture to be separated is maintained over the surface of the disc 122.

Figures 3, 4 and 5, respectively, depict the top plan, side and front views, respectively, of another type of apparatus in the form of a vibratory table which can be used in practicing the present invention. A multicomponent particulate mixture is fed from a hopper 210 to a screw conveyor 212 which then feeds the material mixture onto the surface 230 of the vibratory table 225. The vibratory table 225 is subjected to vibration in the form of cyclical accelerative force imparted to the vibratory table 225 and an angle to the normal and in a direction indicated by the arrow 226 in Figure 3. This angle may vary but preferably is about thirty degrees from the horizontal. This cyclical accelerative force results in the particles being conveyed on the surface 230 of the vibratory table 225 by means of a series of actions which may be termed pitches and catches, a pitch being the action during which the particles are being thrown forward while the accelerative force is applied, and a catch being the particles landing on the surface as a result of cessation of the accelerative force. The path of the particles is determined by the stroke amplitude and frequency of the vibration imparted to the vibratory table 225. Increases in either the stroke amplitude or thrust force throws the particles forward a greater difference. An increase in the stroke frequency increases the number of such throws during a given time period.

During operation of the vibratory table 225, the multicomponent particulate mixture is subjected to two forces: a cyclical accelerative force in the feed direction and a gravitational force. As a result of the cyclical accelerative force, i.e., the vibration imparted to the vibratory table 225, the particles are thrown forward. When the particulate mixture contacts the surface 230, the component having the higher sliding coefficient of friction remains substantially stationary on the surface 230. The component having the lower sliding coefficient of friction contacts the surface 230 and slides. The slide direction is dependent upon the gravitational forces exerted on the particles, determined by the slope 260 and the tilt 270 of the surface 230.

During each stroke the particles are thrown forward with an accelerative force of from about three times the force of gravity (3 g's) to about 25 g's. The distance that the particles are thrown forward is dependent upon the sliding coefficients of friction of the particles. The particles having the higher coefficient of friction are thrown further forward than the particles having the lower sliding coefficient(s) of friction. The stroke amplitude is decreased as the thrust force is increased, for example, when the thrust force is increased from about 8 g's to about 25 g's the stroke amplitude is decreased from about 1/2 inch to about 1/64 inch. The stroke frequency will depend upon the particle size, the degree of slope 260 and the tilt 270 of the vibratory table 225 are adjusted so that the component of the particulate mixture having the higher sliding coefficient of friction does not slide on the surface 230, while the component with the lower sliding coefficient of friction will slide, thus enabling separation of the particle mixture when the direction of feed is uphill. The tilt 270 of the surface 230 of the vibratory table 225 is depicted in Figure 4 and varies from about zero degrees to about 45 degrees from the horizontal. The tilt 270 is. used to help spread the particle mixture across the surface 230 of the vibratory table 225 to differentiate the velocities of the particles being separated. The slope 260 of the vibratory table 225 is depicted in Figure 5 and also varies from about zero degrees to about 45 degrees from the horizontal. As the degree of slope 260 of the surface 230 of the vibratory table 225 is increased, the forward motion of the particle mixture is inhibited. This inhibition of forward particle motion enables a shorter surface to be used. The stroke amplitude and frequency are adjusted to match the tilt 270 of the vibratory table 225 and the average particle size of the particles being separated. For example, a mixture of 5/8 inch size talc and dolomite particles can be separated on a wetted aluminum oxide surfaced vibratory table with a slope of ten degrees and a tilt of five degrees with a stroke amplitude of 3/8 inch and frequency of 500 cycles per minute. The particulate mixture component having the higher sliding coefficient of friction is conveyed uphill on the surface 230 by means of a cyclical accelerative force in the direction of feed and is discharged into a bin 240. The contents of the bin 240 may be emptied by any suitable means such as a conveyor 242. The component of the particulate mixture having the lower (or lowest in the case of three or more components) sliding coefficient of friction on the surface of vibratory table 225 slides on the surface 230 in a direction opposite to the direction of feed, i.e., downhill, and is discharged into a bin 244, which may be emptied by means of a conveyor 246. This apparatus is particularly well suited for the separation of material having a characteristic size of six inches or less, preferably a characteristic size of one inch or less. To impart different frictional characteristics to the surface 230, a water spray 250 may be used to wet the surface 230. Cleaning means are not needed since the surface 230 is an essentially self-cleaning surface. Yet another type of apparatus which can be used in practicing this invention, one utilizing gravitational forces, is represented by the velocity sorting slide apparatus shown in Figures 6 and 7. The mixture to be separated is fed to the separation surface 316 by means of a feed system, consisting of a feed chute 310 which feeds the material mixture onto a platform 312 where it is moved onto the separation surface 316 by means of a reciprocating push bar 314. The separation surface 316 is inclined at angle of inclination 318 and is surfaced with material on which the mixture components of the particulate mixturebeing separated exhibit differences in their sliding coefficients of friction.

Any feed system may be used, preferably one operating in a cyclical manner, such as the reciprocating feed bar 314 illustrated in Figures 6 and 7. Cyclical operation helps to allow suffxicient time for the material mixture to separate without particle collision and interaction that interferes with the development of velocity differences dependent upon the sliding frictional difference between the material mixture components. A feed conveyor system may also be used provided sufficient spacing is allowed on the conveyor between the individual particles or discrete masses making up the material mixture. For a mixture of talc and dolomite rocks, the minimum spacing between rocks on such feed conveyor should be held to about 18 inches.

The velocity sorting slide apparatus depicted in Figures 6 and 7 has a surface 316 with a slope 318 that is variably adjustable in response to factors which include the surface material, the components making up the particulate material mixture being separated, the relative amounts of each component, the length of the slope, the initial velocity, if any, imparted to the mixture being separated, the sliding coefficient of friction differences, the sliding coefficients of friction, and the like. This angle will vary depending upon the velocities of the rocks, and can readily be determined by simple repositioning for any particular apparatus configuration and for any particular mixture of materials.

A velocity sorting slide apparatus may have a profile ranging from a flat to a curved surface, and, if curved, the surface can be either concave or convex. The angle of inclination can vary along the length of the slide. In a preferred embodiment, the slide has a, reduced angle of inclination at the bottom of the slide just before the end of the slide. This reduced angle of inclination imparts a braking action to the particulate material mixture which helps to further accentuate the sliding frictional differences between its components.

The surface of the slide 316 may optionally be wetted by means of a water spray 330. A cleaning means (not illustrated) may also be employed to remove materials other than the material mixture being separated, e.g., sand, fines and chips, from the surface 1 of the slide 316. Such cleaning means may include, for example, a brush, a fluid jet, vacuum means, a scraper and the like.

The separated materials leave the surface of the slide 316 in a parabolic trajectory, the path of which is inversely proportional to the sliding coefficient of friction of the materials separated. Thus, a material with a higher sliding coefficient of friction will exhibit a lower velocity and a smaller trajectory, and will be deposited in a bin 320, while the material with a lower sliding coefficient of friction and consequently a higher velocity and larger trajectory will be deposited in a bin 322. An adjustable divider gate in the form of a pivotally mounted baffle 324 is placed between the bins 320 and 322, enabling adjustment of the apparatus for differing feeds, surface, surface conditions, velocities, apparatus lengths, angles of inclination and the like. The material collected in each of the bins 320 and 322 can be separately removed by any suitable means such as the conveyors 326 and 328. The separation may be further enhanced by an increase in the vertical distance between the slide 316 and the collection bins 320 and 322.

This apparatus may be used for velocity difference sorting and differential braking sorting, depending upon the initial velocity imparted to the material being separated. A larger velocity, such as that needed for the differential braking process, may be imparted by means of a feed system such as that shown in. Figures 1 and 2.

Furthermore, this apparatus may easily be adapted for use in slide retain sorting by the addition of any suitable material sorting means, such as a scraper, suction or an air jet, to remove the material remaining on the separation surface.

The apparatus depicted in Figures 1 through 7 may be used to separate a material mixture by any of the three embodiments which can be utilized when practicing this invention - - velocity difference sorting, slide-retain sorting and differential braking sorting - - or a combination thereof. The specific sorting process used will depend upon, among other factors, the material mixture composition, the relative amounts of each component of the mixture, the characteristic size of the mixture, the range of characteristic sizes present in the mixture, the separation surface, and the initial velocity, if any, imparted to the material mixture before introduction to the separation surface.

Preferred surface materials are ones that accentuate -the differences between sliding coefficients of friction of the materials to be separated. However, any surface is acceptable so long as the materials to be separated in fact have a difference in their sliding coefficients of friction on such surface. The smaller the frictional differences are, the more difficult separation becomes, until the point is reached at which even improved equipment design will not prevent incomplete or poor separation.

The higher the sliding coefficient of friction for the surface, the greater the probability that a portion of, or all of, the material will be induced to roll or bounce, a condition to be avoided when practicing this invention. Rolling or bouncing materials will not contact the surface for a sufficient period of time to develop the sliding coefficient of friction differences essential to the sorting process of the present invention. Among the separation surface materials which can be used in practicing this invention on which materials being separated exhibit significant differences in their sliding coefficients of friction are ceramics, e.g., abrasion resistant tiles and bricks, metals such a stainless steel, and high density abrasion-resistant plastics such as high molecular weight polyethylene. The separation surface material will preferably resist abrasion by the particulate materials being separated, since abrasion of the surface can adversely affect the separation process. Accordingly, the surface will preferably be of an equal or greater degree of hardnessx as the hardest of the components of the particulate mixture undergoing separation.

The separation surface may be composed of more than one material. For example, a slide having a reduced angle of inclination at its lower end may be surfaced with a different material over each of its differently angled portions to help further accentuate the sliding coefficient of friction differences exhibited by the materials being separated as they pass successively over such surfaces. When more than one material is used, the joints between the materials will preferably be made flush or approximately so, since differences in surface heights may adversely affect the movement of all or part of the material mixture over the surface.

A particularly preferred surface for the separation of mixtures of talc and dolomite rocks is an aluminum-oxide ceramic surface composed of a very fine grain 85% alumina product with a Moh hardness of 9.3 or greater, such as Cerasurf alumina brick (Coors Ceramic Company, Golden, Colorado), and especially preferred are such surfaces which have been wetted with water. The separation surface itself is an essentially smooth, unbroken surface free of any substantial dips or protrusions that may affect the movement of the material mixture to be separated across the surface. When more than one surface section is used, either of the same or different materials, the surface sections are preferably adjusted by any conventional means, e.g_*, sanding, so that the sections are approximately flush and even. For example, when abrasion resistant bricks, e.g., Cerasurf alumina bricks, are used, the bricks are aligned and grout placed therebetween to give a generally smooth surface. Any edges that protrude are sanded, shaved or filed so as to make the surface level or approximately so.

Figure la illustrates an alternative embodiment of rotating disc 122 that is particularly suitable for utility in the arrangement of Figure 1 in an application subject to conditions of extreme wear and/or damage. The rotating disc 122 is similar in all respects to the disc 122 of the Figure l embodiment, except that processed steel plates form an annulus 123 in the disc in the region most prone to wear and/or damage. It has been found that, by forming the plates of the annulus 123 of mild steel whose exposed sliding surfaces have been stress hardened by sand blasting or shot peening, the resultant surface produces sliding coefficients of friction with talc and with dolomite that are substantially the same as the sliding coefficients of friction of these materials on the Cerasurf alumina brick referred to in connection with the Figure 1 embodiment. Of interest is the fact that mere exposure of unprocessed mild steel plates to impacting by dolomite and talc ore rocks is capable, in time, of producing a surface condition not substantially unlike that produced by shot peening. The time required under standard operating conditions is from about 24 to about 48 hours of equipment operation.

In practice, most preferable results are obtained when the sliding coefficient of friction of dolomite is about twice that of talc on a given sliding surface. Acceptable, but less preferable, results can be obtained, however, as long as there is an appreciable difference between the sliding coefficient of friction of the materials to be separated on the sliding surface. In the separation of talc from dolomite, most preferable results are obtained with a sliding coefficient of friction for dolomite of about 0.53 and about 0.25 for talc. Acceptable results are obtained with sliding coefficients of friction of dolomite in the range of about 0.45 to 0.55 and of talc in the range of about 0.20 to 0.25.. Lower ratios of the respective sliding coefficients can nonetheless be made operable by adjusting the operating conditions of the equipment, e.g., operating the disc at a lower rotational speed and/or with reduced material feed rates.

Wetting of the surface by water or other fluids can accentuate frictional differences. And while in some cases wetting may have no discernible effect on the sliding coefficient of friction of one component, it can significantly reduce the sliding coefficient of friction of another component. Furthermore, for some materials the use of a fluid assists in the development of velocity differences for both components, apparently caused by the fluid's lubrication effect. The components of the particulate mixtures separated in accordance with this invention are preferably in the form of rocks, rather than fines, which can have a broad range of characteristic sizes, and particularly sizes such that the ratio of the smallest to the largest size is about 1 to 6. As the size range is narrowed, separation efficiency and capacity are increased. Preferably, sand and fines associated with the material mixture are removed prior to introduction of the mixture to the separation surface. Such removal means may include screening or washing of the material mixture to be separated.

For a talc/dolomite mixture the rocks being separated preferably will have a characteristic size ranging from about two inches to about twelve inches, and more preferably will be screened to smaller size ranges such as mixtures in which rocks of from about two to about six inches predominate and mixtures in which rocks of from about six to twelve inches predominate.

Example 1 A slide 34 inches long with a 33-1/2 degree angle of inclination from the horizontal and a 4.2 foot vertical drop was used. The slide was surfaced with Cerasurf, a fine grain abrasion-resistant 85% alumina brick, Moh harness of 9.3, manufactured by Coors Ceramic Company, Golden, Colorado. A vertical divider was placed 24 inches in horizontal distance from the slide. A mixture of rocks, ranging from two to twelve inches in characteristic size, 61.7% of which were talc, 30.5% dolomite and 7.8% talc-dolomite conglomerate was hand fed one rock at a time to the slide. The contents of two boxes in which the rocks were collected were as follows:

Talc Box Dolomite Box

63.125 lbs. talc 0 lbs. talc

6.125 lbs. conglomerate 1.875 lbs. conglomerate 0 lbs. dolomite 31.25 lbs. dolomite Example 2 A flat rotating disc 18 feet in diameter rotating at 14.0 rpm is used. The surface is wetted Cerasurf abrasion-resistant brick. A talc-dolomite mixture having rocks ranging in size from two inches to twelve inches is fed to the disc at a point six feet from the center. A slide two feet long placed at an angle of 40 degrees from the horizontal is used to introduce the talc and dolomite rock mixture to the surface of the rotating disc. The talc rocks slide off the disc, while the dolomite rocks are retained and removed by a reject doctor blade.

Example 3 A mixture of talc rocks and dolomite rocks was hand fed by placing one rock at a time onto a slide four feet long, surfaced with Cerasurf abrasion-resistant brick and wet with water, placed at an angle of inclination of 23 degrees from the horizontal. The talc/dolomite rock mixture remained stationary on the surface and would not slide.

Examples 4 - 9

The slide employed in Example 3 was adjusted to an angle of inclination of thirty degrees from the horizontal.

A talc/dolomite rock mixture having a characteristic size of about two to about 12 inches, the composition of which is indicated in Table 1, was then manually fed to the slide surface. The results of size separations carried on these rock mixtures are set forth in Table I.

TABLE I Feed fibs) Recovery Box Example Talc Dolomite Talc Dolomite % Recovery % Talc

4 69 202 67 25 97 73

5 69 202 67 0 97 100

6 69 20 26 41 93 99

7 29 16 12 90 100 100

8 26 108 25 2 96 93

9 26 103 25 6 96 81

Example 10

A mixture of 53 pounds of talc and 125 pounds of dolomite in the form of rocks having a characteristic size of about two to about ten inches was hand fed to a three foot long slide having an angle of inclination of 34 degrees. The slide was surfaced with Silicon Carbide manufactured by Coors Ceramic Company, Golden, Colorado, as was kept wetted with water. Fifty-two pounds of talc rocks and 16 pounds of dolomite rocks were collected in the talc recovery box, a talc recovery of 98% with a purity of 77%. Many dolomite rocks rolled on the sμrface rather than sliding, thus accounting for the number of dolomite rocks found in the talc recovery box. Example 11 A mixture of 29 pounds of talc rock and 59 pounds of dolomite rock having a characteristic size of six to 12 inches was fed to a slide consisting of an upper section three feet long inclined at an angle of 25 degrees in which the first two feet were surfaced with 304 stainless steel, the remaining slide surface was Cerasurf alumina brick. The lower section of the slide, one foot long and having an angle of inclination of 19 degrees was also surfaced with Cerasurf alumina brick. The slide surface was kept wetted with water. Twenty-nine pounds of talc rock and no dolomite rock were collected in the talc recovery box, yielding 100% talc recovery with a purity of 100%. Example 12

A compound slide was used to separate a manually fed mixture of 42 pounds of talc rock and 84 pounds of dolomite rock having a characteristic size of two to six inches. The upper section of the slide was the same as that in

Example 11 and was inclined at an angle of 30 degrees. The lower slide section was two feet long, inclined at an angle of 26 degrees, and surfaced with Cerasurf. The slide surface was kept wetted with water. In the talc box, there was collected 42 pounds of talc rock and one pound of dolomite rock giving 100% recovery of the talc with a product of 90% purity.

Example 13 A compound slide was used to separate a manually fed mixture of 120 pounds of talc rock and 116 pounds of dolomite rock having a characteristic size of two to 12 inches. The slide consisted of an upper section three feet long surfaced with 304 stainless steel and inclined at an angle of 31 degrees, and a lower section two feet long surfaced with Cerasurf and inclined 13 degrees. The slide surface was kept wetted with water. The rock recovered in the talc box consisted of 120 pounds of talc and four pounds of dolomite, yielding a recovery of 100% of the talc at a purity of 97%.

Example 14

A mixture of talc and dolomite rocks ranging from two to 12 inches in characteristic size was fed to a compound slide by means of a conveyor belt operating at 462 feet per minute. The rock mixture left the conveyor belt at a parabolic trajectory and contacted the slide at the point where the slide angle of 16 degrees was tangent to. the trajectory. The compound slide used consisted of an upper section of 5.5 feet Cerasurf surface inclined 16 degrees and a lower section of one foot Cerasurf surface inclined six degrees. The slide surface was kept wetted with water. From the talc box 102 pounds of talc rock and seven pounds of dolomite rock were recovered to give a talc recovery of 90% at a degree of purity of 94%.

Example 15

A mixture of 101 pounds of talc rocks and 224 pounds of dolomite rocks, having a characteristic size of about two to 12 inches, was hand fed to a conveyor belt operating at 7.5 feet per second. The rock mixture was fed from the conveyor belt to a six foot diameter disc of the type depicted schematically in Figure 1 rotating at 28 rpm. The outermost 12 inch section of the disc was upwardly inclined at an angle of 14.6 degrees. The disc surface was Cerasurf kept wetted with water. The talc box collected 100 pounds of talc and 10 pounds of dolomite for a 99% recovery with a product having a 91% purity. The reject box contained 214 pounds of dolomite and one pound of talc in the form of a single talc rock with one calcite side. The dolomite rocks in the talc box had either rolled or had been shoved off the disc by talc rocks.

Examples 16 to 25 A talc and dolomite roqk mixture was fed to the six foot diameter disc sorter schematically represented in Figure 1. The rock mixture was fed from a hopper to a vibratory feeder, which screened out fines up to 1.5 inches in characteristic size, and then onto a conveyor belt. A V-trough slide was used to then transfer the mixture to the rotating disc. The rotating disc had a Cerasurf surface that was kept wetted with water. The results are shown in Table II. TABLE II

Percent Talc feed total disc belt rate feed speed speed ore

Example (TPH) (lb) feed product recovered (rpm) (fps) size

16 27 728 16 75 94 29.6 7.1 2"-12"

17 28 757 24 86 88 28.4 7.1 2"-12"

18 41 752 24 89 82 28.4 7.1 2"-12"

19 41 740 23 85 76 28.4 7.1 2"-12"

20 45 1715 56 96 90 28.2 7.1 2"-12"

21 70 798 40 94 97 28.4 7.0 4"-12"

22 60 612 45 83 96 29.3 8.0 2"-4"

23 53 741 36 92 78 28.3 8.0 2"-4"

24 28 727 36 85 78 28.2 8.0 2"-4"

25 26 1724 35 90 94 28.0 7.0 2"-12"

Examples 26 - 30 A mixture of talc rocks and dolomite rocks, having the characteristic size and composition as indicated in Table III, was hand fed to a vibratory table of the type depicted schematically in Figure 3. The vibratory feeder had a slope of 10 degrees, a tilt of five degrees, a stroke amplitude of 3/16 inch and a stroke frequency of approximately 500 to 800 cycles per minute. The slide surface was kept wetted with water. The results are shown in Table III.

TABLE III

Feed Product

Size Talc Dolomite % %

Example (inches) (lb.) (lb.) Talc Recovery

26 3/4 4.0 4.0 98 99

27 3/4 4.0 6.0 99 93

28 3/4 3.0 10.0 94 100

29 1/2 4.0 4.0 93 95

30 1/2 4.0 6.0 88 93

Examples 31 - 33

A mixture of talc and dolomite rocks as hand fed to the vibratory table of Examples 26-30 in which the degree of tilt changed to three degrees. The feed composition, particle size and the results are shown in Table IV.

TABLE IV

Feed Product

Size Talc Dolomite % %

Example (inches) (lb.) (lb.) Talc Recovery

31 5/16 9.0 9.0 99 96

32 1/4 7.2 10.8 100 99

33 1/4 5.4 12.6 100 100 The above discussion of this invention is directed primarily to preferred embodiments and practices thereof. It will be readily apparent to those skilled in the art that further changes and modifications in the actual implementation of the concepts described herein can easily be made without departing from the spirit and scope of the invention as defined by the following claims.

Claims

1. A method of separating on the basis of material composition a mixture of two or more discrete particulate materials of disparate composition which comprises the steps of: selecting a surface material that manifests a different sliding coefficient of friction between the respective materials of said mixture; placing said mixture on said surface in a condition for sliding; applying a force tending toward creating relative sliding movement between said mixture and said surface; and separating said discrete materials of said mixture on the basis of the velocity differences therebetween.

2. The method of claim 1 including the step of inhibiting rolling movement of said particulate materials on said surface.

3. The method of claim 1 wherein said mixture comprises granular or rocklike discrete particulate materials.

4-. The method of claim 3 wherein said mixture comprises talc and dolomite rocks.

5. The method of claim 4 wherein the ratio of sliding coefficients of friction of the respective mixture components on said surface is about 2 to 1.

6. The method of claim 4 wherein the ratio of sliding coefficients of friction of the respective mixture components on said surface is in the range of about 0.45 to 0.55 for dolomite and of about 0.20 to 0.25 for talc.

7. The method of claim 6 wherein the respective materials in said mixture are accelerated on said surface.

8. The method of claim 6 wherein the respective materials in said mixture are decelerated on said surface.

9. The method of claim 6 wherein one of the materials in said mixture remains substantially stationary on said surface.

10. The method according to claim 7 including the step of rotating said surface whereby said materials will be induced to move by centrifugal action.

11. The method according to claim 10 including the step of placing said mixture on said surface at a velocity substantially equal to the tangential velocity of said surface at the point of disposition of said material.

12. The method according to claim 1 including the steps of inclining said surface with respect to the horizontal and imparting vibratory motion thereto whereby said particles of different sliding coefficient of friction traverse said surface along separate paths.

13. The method according to claim 1 including the steps of inducing movement of said particles with respect to said surface by gravity whereby said particles having different sliding coefficient of friction traverse separate momentum-effected paths, and collecting the respective particles in spatially separated collectors.

14. The method according to claim 1 including the step of varying the effective distance between said collectors in response to varying operating conditions.

15. Apparatus for the separation of a mixture of two or more discrete particulate materials of disparate composition, comprising: a separation surface formed of a material to produce distinct sliding coefficients of friction between the respective particulate materials; means for feeding said mixture materials to said separation surface for sliding movement with respect thereto; means for imparting sliding movement to said mixture materials with respect to said surface whereby the respective of said materials will traverse distinct velocity-determined paths; and means for collecting the respective of said mixture elements at the end of said distinct paths.

16. Apparatus according to claim 15 in which said particulate materials are talc and dolomite and said surface comprises a material effective to create a sliding coefficient of friction ratio between said particulate materials of about 2 to 1.

17. Apparatus according to claim 16 in which said surface material produces a sliding coefficient of friction for dolomite in the range of about 0.45 to about 0.55 and for talc in the range of about 0.20 to 0.25.

18. Apparatus according to claim 16 in which said surface material comprises an aluminum oxide ceramic.

19. Apparatus according to claim 16 in which said surface material comprises a metal having a work hardened surface.

20. Apparatus according to claim 19 in which said surface material comprises a mild steel having a work hardened surface produced by shot peening.

21. Apparatus according to claim 16 in which said sliding surface comprises a composite surface structure including a wear resistant surface formed of a work hardened metal adjacent a less wear resistant ceramic material.

22. Apparatus according to claim 15 including means for wetting said sliding surface with water for varying said sliding coefficients of friction.

23. Apparatus according to claim 15 in which said sliding surface is positioned in an oblique disposition, and including means for imparting vibration to said surface whereby said particulate material traverses said surface along separate paths.

24. Apparatus according to claim 23 in which said oblique disposition of said sliding surface in cooperation with said vibrating surface imparts a cyclical accelerative force and a gravitational force to said particulate material for directing said respective materials along separate paths.

25. Apparatus according to claim 24 in which said oblique disposition is defined by a slope and tilt of said plate effective upon each application of cyclical accelerative force to said surface in the feed direction, to effect a corresponding movement of said particle elements and said gravitational force intermediate said cyclical accelerative force applications is operative to effect sliding movement of only the element of said mixture having the lower sliding coefficient of friction.

26. Apparatus according to claim 25 in which said slope and said tilt of said surface are each of a magnitude of up to about 45 degrees.

27. Apparatus according to claim 15 in which said sliding surface is a rotatable disc, means for rotating said disc at a predetermined rate of angular velocity and means for supplying said mixture material to said disc for sliding movement with respect thereto.

28. Apparatus according to claim 27 in which said material supply means includes means to supply said mixture material to said disc at a linear velocity substantially equal to the tangential velocity of said disc at the point of supply of said mixture to said disc.

29. Apparatus according to claim 28 in which said material supply means includes a movable conveyor for imparting a predetermined linear velocity to said mixture material, and means for transferring said moving mixture material from said conveyor to said disc.

30. Apparatus according to claim 29 in which said mixture transfer means is a feed slide interposed between said conveyor and said disc.

31. Apparatus according to claim 30 in which said feed slide is downwardly inclined between said conveyor and said disc.

32. Apparatus according to claim 28 including, means for supplying said mixture in substantial in-line disposition to said disc.

33. Apparatus according to claim 32 in which said mixture supply means includes a substantially V-shaped trough for feeding said mixture material to said conveyor.

34. Apparatus according to claim 27 in which said sliding surface comprises a smooth ceramic surface.

35. Apparatus according to claim 34 in which said ceramic is aluminum oxide ceramic.

36. Apparatus according to claim 34 in which said sliding surface includes a wear resistant portion formed of a metal having a work hardened surface.

37. Apparatus according to claim 34 in which said sliding surface includes a wear resistant surface adjacent said ceramic surface for receiving material supplied to said disc, said wear resistent surface being formed of steel having a work hardened surface.

38. Apparatus according to claim 27 in which said rotating disc is a generally concave disc having substantially conically shaped surface adjacent its outer periphery.