Refuse Sorting And Transparency Sorting

Abstract

A method and apparatus for sorting refuse into its components wherein the refuse is comminuted into a mass of particles, and a fraction containing a mixture of glass particles, which are at least partially transparent, and substantially opaque particles are sorted from the remainder of the refuse. The particles in the separated fraction are passed seriatim through a transparency sorter which senses the degree of transparency of the particles and sorts the particles in accordance with the sensed transparency values. The transparent sorted glass particles can also be sorted according to color during the transparency and color sorting process. The transparency sorter preferably comprises a strip light source with a plurality of photosensitive means, such as photodiodes, aligned in a row with the strip light source. The photosensitive means scan across the light source as the particles fall past the light source. Sorting is effected when a certain minimum number or percentage of the photosensitive means sense darkness as the particles pass the photosensitive means.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to sorting of particles according to their transparent properties. In one of its aspects, the invention relates to sorting of a refuse stream to remove substantially opaque particles from a glass particulate fraction. In another of its aspects, the invention relates to sorting substantially opaque particles from glass particles which are substantially transparent.

2. State of the Prior Art

In U.S. Pat. No. 3,650,396 to Robert M. Gillespie and Hugh R. Rhys, there is disclosed and claimed a method and apparatus for sorting a refuse stream into various components. A fraction containing substantially only glass particles is sorted from the remainder of the refuse particles and the glass containing fraction is photometrically sorted according to color. It has been found that a small amount of stones, ceramic crock-ery, smaller bits of metals, wood, hard rubber, bone, clinker and other such non-glass materials are included in the glass fraction. These undesirable non-glass particles sometimes report with the flint or colorless glass obtained when flint glass is sorted from colored glass. These non-glass particles must be further sorted from the glass particles to yield a satisfactory result.

Apparatus have been devised to sort translucent particles according to their translucent value. For example, Fraenkel in U.S. Pat. 3,197,647 discloses and claims an appartus for sorting translucent particles according to their translucent properties by passing a polarized light through the particles and employing a detector opposite the source of the light with a filter crossed with respect to the direction of polarization of the light. However, insofar as presently aware, no apparatus has been devised for sorting particles according to the transparent properties, or the ability of the particles to pass light energy in undisturbed form.

SUMMARY OF THE INVENTION

According to the invention, there is provided a method and apparatus for removing stone, ceramic crockery, and other such opaque particles from a glass concentrate which has been sorted from a comminuted mass of refuse. The particles of the glass concentrate, including the opaque particles, are fed seriatim through a photometric sensing zone wherein each of the particles is photometrically measured for the transparent qualities thereof. The particles are thereafter sorted in singular fashion in accordance with the sensed transparent qualities thereof. The opaque particles are thereby removed from the glass concentrate, leaving the concentrate free from opaque particles.

Desirably, the photometric sensing zone includes a strip light source and a plurality of scanning photosensitive means, such as photodiodes. The photosensitive means are aligned in a row with the strip light source and scan across the strip light source as the particles pass between the light source and the photosensitive means. Each of the photosensitive means generates a signal representative of the intensity of light viewed thereby during the scanning operation. When the light intensity for a predetermined number of photosensitive means drops below a predetermined value, sorting of the particles causing the drop in the light value is accordingly effected.

In one embodiment, the particles are projected in a free-fall trajectory through the photometric sensing zone. During the free-fall trajectory of the particles, the particles are sensed not only for their transparency value but also for their metal content and for color of the transparent glass particles. The particles are thereafter sorted in accordance with their transparent value, the metal content thereof, and according to the color of the particles.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described with reference to the accompanying drawings in which:

FIG. 1 is a schematic representation of a sorting system and method according to the invention;

FIG. 2 is a side elevational view in section of a transparency sorter according to the invention;

FIG. 3 if a partial sectional view seen along lines 3--3 of FIG. 2;

FIG. 4 is a schematic representation of the projection of shadows onto a row of light sensors by an opaque particle;

FIG. 5 is a schematic representation of the projection of light energy through a transparent glass particle and onto the row of light sensors;

FIG. 6 is a schematic representation of an electrical circuit used in connection with the transparency sorter;

FIG. 7 is a schematic representation of wave forms of the electrical circuit; and

FIG. 8 is a schematic representation of an alternate embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the drawings, and to FIG. 1 in particular, there is shown a system for sorting refuse into its various components and having as one end product a glass fraction comprising substantially only flint glass and a glass fraction comprising substantially only colored glass.

Domestic trash or other suitable refuse is fed to a pulverizer unit 12 which pulverizes or comminutes the feed producing a fiber and pulp fraction and a concentrated solids fraction. A suitable pulverizer is a Black Clawson Hydrapulper manufactured by the Black Clawson Company of Middletown, Ohio. The solids fraction is passed to a cyclone separator 14 which removes a water and fiber fraction from an upper portion and a more concentrated solids fraction from a bottom portion.

The dewatered solids fraction is passed by a suitable conveyor to a washer and distributor 16 where water is added thereto. The watered fraction is distributed onto a screening and washing machine 18 having a hopper 20 with an outlet 22 at the bottom thereof and a screen 24 across the top. The screen 24 has vibratory motion imparted thereto to move the solids fraction along the top of the screen 24 where it is subjected to a further water wash from a pair of nozzles 26 and 28 positioned above the screen. A suitable screen has 1/4 inch openings to permit particles of 1/4 inch or less to pass therethrough into hopper 20. These particles or fines are removed through the outlet 22. Those particles larger than 1/4 inch then move to the lower end of the screening and washing machine 18 onto a second screen 34. Vibratory motion is imparted to this screen also. This screen 34 preferably has openings of 3/4 of an inch to permit particles smaller than 3/4 of an inch to pass therethrough. The larger particles move to the bottom end of screen 34 where they are removed and passed through a recycle line 36 back to the pulverizer 12. The recycle line can be a pneumatic conveyor or any suitable means for returning the oversized particles back to the pulverizer. For example, the oversize particles can even be carted back to the pulverizer in a truck. The particles collected in the hopper 30 will have a size predominately between 1/4 and 3/4 inch. A suitable screening and washing device is manufactured by Derrick Manufacturing Co. of Buffalo, N.Y.

The particles are removed from the bottom of the hopper 30 through an outlet 32 and passed to a fluidized bed conveyor and dryer 38. As the particles are passed through this fluidized bed dryer 38, air is directed upwardly therethrough to remove water therefrom. The air is heated and supplied through a line 40.

The pulp and fibrous fraction from the pulverizer 12 can be passed to a fluid bed reactor 43 after suitable water removal steps (not shown) and incinerated in the reactor 43. Excess heat in the form of hot air, suitably scrubbed clean of noxious elements, may be bled from the fluid bed reactor 43, diluted with cold air if required, and admitted to the dryer either directly or through a heat exchanger 44 through line 42. Alternatively, the dryer may have its own provision for heat generation. Suitable machines are manufactured by the Door Oliver Co. of Stamford, Connecticut (reactor), and the Jeffrey Manufacturing Co. of Columbus, Ohio (dryer). The dryer may include a cooling stage if required. Alternatively, the hot gasses may be used to convey the material directly through a pneumatic conveyor such as that manufactured by the Meyer Machine Co. of San Antonio, Texas.

The dried particles are removed from an upper end of the dryer 38 and passed to a magnetic separator 46. Any suitable magnetic separator can be used. An example of such a separator is a Rotating Drum Type Magnetic Separator, manufactured by the Dings Magnetic Co. of Milwaukee, Wisconsin.

A magnetic fraction is separated from a nonmagnetic fraction in the separtor 46. The nonmagnetic fraction is passed to a first pneumatic separator 48. The separator 48 has an inlet 50, a light fraction outlet 52, and a heavy fraction outlet 54. Air is supplied to a bottom portion of the separator and passed upwardly therethrough to entrain particles of lighter specific gravity. The particles of lower specific gravity are removed through outlet 52 and passed to the inlet 58 of a second pneumatic separator 56. This separator 56 has outlet 62 for a heavy fraction and an outlet 60 for a light fraction. Air is passed upwardly through the separator 56 to separate particles of lower specific gravity from particles of higher specific gravity. The lower specific gravity particles are removed through outlet 60. The flow of air through separators 48 and 56 is regulated to entrain those particles whose specific gravity is lighter than glass and to permit glass particles to settle to the bottom of the separator for removal. This lighter fraction contains predominately light weight metals such as aluminum, stones, ceramics, bones, wood, rubber and plastics. This lighter fraction can be passed to an aluminum concentrating apparatus where an aluminum rich fraction can be recovered for reuse. The separtion is achieved by regulation of the upward air velocity to split the feed according to the different settling velocities of its particulate components. This differential is a function of article shape as well as specific gravity. The split can be made in free or hindered settling conditions. A suitable air separator is manufactured by Sortex Co. of North America, Inc., Lowell, Michigan.

The heavier fractions separated from separators 48 and 56 comprises essentially glass and some residual stones and ceramic, and conceivably some balled up aluminum. These heavy glass fractions are combined and passed to a hopper dispenser 64 which dispenses the particles one at a time to a transparency sorting apparatus 66 wherein the particles are separated according to their ability to pass light. The opaque particles, such as the stones, crockery, and other ceramics, metals, wood, hard rubber, bone, clinker, etc., are separated out and the resulting glass fraction is passed to a pair of hopper dispensers 68 which dispense the particles one at a time to a photometric sorting apparatus 70. The glass particles are separated according to color in the photometric sorting apparatus 70 producing a colored cullet fraction and a flint colored fraction. The flint colored fraction, which is worth considerably more than the colored cullet fraction, can be remelted and used for making clear bottles or other types of clear glassware. The colored fraction can be either further separated by other photometric sensing means (not shown) into different colors, or can be used as a melt for brown bottles.

A more detailed discussion of the color sorting apparatus is disclosed in U.S. Pat. No. 3,650,396 which is incorporated herein by reference. Also disclosed in said U.S. Pat. No. 3,650,396 is another method of sorting refuse to obtain the glass fraction which contains a minor amount of opaque particles. The system disclosed above for obtaining the glass fraction with the opaque particles is only illustrative and other systems can be employed.

Referring now to FIGS. 2 and 3, the transparency sorting apparatus 66 comprises a housing 72, open at both top and bottom to allow particles 84 to pass freely therethrough. The particles are fed seriatim from a suitable feeding device (not shown) to the transparency sorting apparatus 66. Such feeding devices are well known in the art of photoelectric sorting. An example of a suitable feeding device is disclosed in Fraenkel U.S. Pat. No. 3 197 647.

A strip light source 74, such as a fluorescent tube, is mounted in a darkened box 75 within the housing 72 in back of a mask 76 having a slot opening 77. A lens 78 is supported across the housing from the light source 74 to focus the light passing through the slot 77 onto a plurality of photosensors 80. An air ejector 82 is mounted beneath the housing 72 to deflect the trajectory of the particles 84 under certain conditions to be described hereinafter. The photosensors 80 preferably comprise an array of photodiodes adapted to sense the intensity of the light from the light source. The photodiodes 80 are separate sensing units which are programmed to scan across the slot 77 and report the intensity received at the photodiodes in a sequential manner. Such a programmed array of photodiodes are well known as, for example, the type sold by Reticon Corp. of Mountainview, California. The array of diodes gives a high resolution of light intensity across the slot 77. Thus, the photodiodes are suitably adapted to resolve the transparent characteristics of particles passing between the diodes and the slot 77.

The number of photodiodes in the array will depend on the resolution desired and the width of the slot 77. The size of the particles 84 will determine the width of the slot 77. If for example, as desired, the particles are in the range of 1/4 to 3/4 of an inch, the slot width will be about one and one-half inches long. For such a slot length, a diode array of about 64 units would be suitable. For purposes of simplicity, only a few of such diodes have been schematically represented in the drawings.

In lieu of the fluorescent light source 74 and mask 76, there can be provided a collimated strip of light from a remote "cold" high intensity light source transmitted through a fiber-optic light guide. Fiber-optic light guides are well known, for example the type sold by American Optical Corp.

Reference is now made to FIG. 4 wherein there is shown schematically the effect of an opaque particle passing between the light source and the diode array. An opaque particle 84 passing between the light source 74 and the array of diodes 80 would provide a particular light pattern on the diodes. A shadow will be cast by the opaque particle 84 onto an area 88 of the diode array. On either side of the shadow area 88 there will be portion 86 of full light and portions 90 between the shadow area 88 and the light portions 86. Thus, in any given scan by the diode array when a particle 84 is passing through the viewing area, certain of the diodes will report full light, certain of the diodes will report partial light, and other diodes will report darkness.

The effect of a glass particle passing in front of the photosensors 80 is illustrated in FIg. 5 to which reference is now made. The glass particle 92 may have a facial area 94 and side edges 96 and 98 which are disposed at an acute angle to the facial area 94. These side edges 96 and 98 may appear translucent or even opaque to the diodes due to reflection of the light from internal surfaces thereof. Thus, as the array of diodes 80 scans across the face of the glass particle 92, the diodes in areas 100 and 108 will sense full light whereas the diodes in areas 102, 104 and 106 will sense somewhat attenuated light depending on the color of the glass particle 92. Possibly the diodes in areas 102 and 106 will sense a strongly attenuated light, even dark, whereas the diodes in area 104 will sense only a slightly attenuated light.

The purpose of the transparency sorter is to sort stones and ceramic particles such as crockery and the like from colored and flint glass particles. Therefore, the sorting apparatus must distinguish between the opaque particles 84 and the glass particles 92, notwithstanding that a dark or opaque signal may be obtained from one or more diodes when scanning a glass particle. An electrical system for distinguishing between opaque and transparent particles is illustrated in FIG. 6 to which reference is now made.

The photodiodes 80 are connected to amplifiers 110 which amplify the signals. The amplified signals are passed through comparators 112 which are set at a predetermined level. The output from any given comparator will be off when the light sensed by a corresponding photodiode is above a predetermined level of intensity. Thus, when there is nothing passing in front of the photodiode, the output from the corresponding comparator will be off. When an opaque particle passes in front of a particular photodiode, the level of intensity sensed by the photodiode will drop to a point at which the comparator is turned on. The output from the comparators 112 are applied to an OR gate 114 and to a ratio computer 116. If any of the comparators are turned on, the OR gate will apply a signal to a pulse width monostable circuit 120 through a gate 118. The monostable circuit 120 applies a signal for a predetermined length of time to an ejector drive 122. The duration of the signal from the monostable circuit 120 is a predetermined length sufficient to eject the particle as it passes in front of the ejector. The monostable circuit 120 also has a built-in delay circuit to delay the signal to the ejector 122 a sufficient time to permit the particle to fall from the position aligned with the photosensors 80 to a position aligned with the ejector 82.

The ratio computer 116 determines the ratio of comparators which have been turned on to those whch are off. In other words, the ratio computer determines the ratio of dark to light areas on the array of photosensors 80. If the radio is sufficiently large, the ratio computer 116 will apply a signal to the gate 118 to permit the OR gate 114 to apply the signal to the monostable circuit 120. Gate 118 is controlled by the ratio computer 116. Thus, when no signal is applied to gate 118 from the ratio computer 116, it remains closed and blocks the signal from the OR gate 114. Thus, if the ratio of on to off comparators is small, the gate 118 will remain closed. If the ratio is above a predetermined level, the gate 118 will be open to permit operation of the monostable circuit 120.

A counter circuit can be used in lieu of a ratio computer 116. The counter circuit (not shown) would count the number of comparators turned on during any given scan. If the number is below a predetermined number, the gate 118 would remain closed. If the number of on comparators is above a predetermined number, the gate 118 would be turned on.

The ratio set on computer 116 would vary depending on the nature of the feed and the size of the product. The ratio, as well as the predetermined number for the counter can be determined empirically for each feed size.

The output from the gate 118 under two separate conditions are illustrated in FIG. 7. The output will be substantially straight line 118a in the event that a glass particle passes in front of the photosensors 80. On the other hand, in the event that an opaque particle passes in front of the photosensors 80, the output will show a sqaure wave pattern 118b. The wave pattern for the output for the monostable circuit 120 would appear as a straight line (not shown) when a transparent glass particle passes in front of the photosensors 80. The pattern 120a illustrates the form of the wave otput from the monostable circuit 120 when an opaque particle falls through the sensing zone in front of the photosensors 80. As illustrated in FIG. 7, a delay occurs between the time the particle enters the viewing zone and the time at which the output signal results from the monostable circuit 120.

Briefly, the operation of the transparency sorting apparatus 66 is as follows: As the particles pass through the housing 72, the transparency of each particle is sensed by the array of photodiodes. In the event that the particle is opaque, as would be the case of a stone or ceramic crockery, a certain percentage of photodiodes would be dark as the particle passes through the housing 72. The dark photodiodes will thereafter turn on corresponding comparators 112 which will turn on OR gate 114. The ratio being sufficiently high, the gate 118 will be turned on by the ratio computer 116 to cause a signal to be sent by the monostable circuit 120 to the ejector drive 122. The ejector will deflect the trajectory of the particle and it will be collected in a separate bin or on a separate conveyor.

When a glass particle passes through the housing 72, very few, if any, of the photosensors 80 will detect an attenuation of light energy. In any case, the number of photosensors detecting a substantial attenuation of light energy will be sufficiently small so that the ratio of dark to light photosensors will be below the predetermined level set in the ratio computer 116. For example, the ratio computer can be set at a 10-20 percent ratio. In any case, the gate 118 will remain off and the particle will be permitted to continue its trajectory into a collector (not shown).

In the manner described above, transparent or highly translucent particles are separated from opaque particles.

Reference is now made to FIg. 8 for a description of a second embodiment of the invention. In this embodiment, like numerals have been used to designate like parts. The second embodiment illustrates schematically a device for simultaneously sorting metals, ceramics, and other opaque particles from glass particles and, in addition, for sorting the glass according to color.

Particles of flint glass 92a, colored glass 92b, opaque particles 84 and non-magnetic metal particles 134 are fed seriatim on a conveyor belt 130 which is conventionally trained around a wheel 132. The particles are projected off the end of the belt 130 and pass through a transparency detector of the type described above, the transparency detector including a light source 74, a mask 76 which projects light at an array of photosensors 80 behind a lens 78. The opaqueness of the particles is detected by the array of photosensors 80 in a manner described above. The outputs from the photosensors are applied to a control circuit 136 which supplies a control signal through a delay 138 to an ejector drive 122 of an ejector 82. The particles also pass through a VHF search coil 140 which detects the metal particles 143, and applies a signal to a control circuit 142. In the event that the particles are metallic, a signal is sent through a delay 144 to ejector drive 122 of ejector 82. The particles also pass through a color sensor 146 having a plurality of photodetectors 148 which sense the color of the particles in a well known manner. The outputs from the photosensors 148 are applied to a control circuit 150 which, in accordance with a predetermined color, will send a signal through a delay circuit 152 to an ejector drive 156 for ejector 154. The ejectors 154 and 82 are positioned at an angle of about 90.degree. with respect to each other so that each deflects a particle in a different direction. The transparency detector and the color sensor are pitched at a sufficient angle to each other so that light from one system does not affect the other.

The photodetectors 148 are set to detect the color of the particles passing through the color sensor 146 and to actuate the ejector 154 responsive thereto. Thus, if a particle is a piece of colored glass, it will be ejected to the left as viewed in FIG. 8. On the other hand, if the particle is a metal or opaque ceramic particle, it will be ejected in a direction perpendicular to the plane of the drawing and 90.degree. from the direction of deflection of the colored glass particles. Still further, if the particles are flint glass, the trajectory thereof will be unaffected. Separate collectors (not shown) are provided to collect the three separate fractions of particles thus sorted.

Reasonable variation and modification are possible within the scope of the foregoing disclosure, the drawings, and the appended claims without departing from the spirit of the invention.

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. In a method of sorting refuse into its components wherein said refuse is comminuted into a mass of particles, a fraction containing substantially a major portion of glass particles which are at least partially transparent and a minor portion of opaque particles are sorted from the remainder of the refuse, the improvement which comprises:

feeding said glass and opaque particles in said fraction seriatim through a photometric sensing zone and therein photoelectrically sensing the degree of transparency of each of said particles; and

sorting said particles in accordance with the transparent value of each of said particles thus sensed,

whereby said opaque particles are sorted from said glass particles.

2. A method of sorting refuse according to claim 1 and further comprising the step of sensing a property of said particles related to color, and thereafter sorting said glass particles in accordance with the sensed color-related property of said particles, whereby said glass particles are sorted according to color.

3. A method of sorting refuse according to claim 2 wherein said particles are projected in a free-fall trajectory through said photometric sensing zone and said transparency sorting and said color sorting take place during said free-fall trajectory.

4. A method of sorting refuse according to claim 3 and further comprising the step of sensing a property of said particles during said free-fall trajectory related to the metal content thereof, and sorting said particles in accordance with the value of the said metal-related property thus sensed.

5. In a system for sorting refuse into its components, said system having means to comminute said refuse into a mass of particulate particles, and means to sort a fraction containing substantially only a major portion of glass particles which are at least partially transparent and a minor portion of opaque particles from the remainder of said refuse, the improvement which comprises:

means for photometrically sensing the property of said particles related to the transparency thereof;

means for feeding said glass and opaque particles seriatim to said transparency sensing means; and

means for sorting said particles in accordance with the transparency related property sensed, whereby said opaque particles are sorted from said glass particles.

6. A system for sorting refuse according to claim 5 wherein said photometric sensing means includes a strip light source and a plurality of photosensitive means spaced from said light source in a row and aligned with said light source, said particles being fed between said light source and said row of photosensitive means.

7. A system for sorting refuse according to claim 6 wherein each of said photosensitive means generates a signal representative of the value of the light detected thereby, and said sorting means includes means coupled to said photosensitive means for sorting said particles when a predetermined number of said photosensitive means have an output signal below a predetermined value.

8. A system for sorting refuse according to claim 7 wherein said photosensitive means are photosensitive diodes.

9. A system for sorting refuse according to claim 8 and further comprising means to effect scanning of said diodes across said light source.

10. A system for sorting refuse according to claim 5 wherein said feeding means includes means to feed said particles in a free-fall trajectory through said photometric sensing means and said sorting means includes means to alter the trajectory path of said particles.

11. A system for sorting refuse according to claim 10 and further comprising means for sorting said glass particles according to color.

12. A system for sorting refuse according to claim 11 wherein said color sorting means includes means to detect a property of said glass particles related to color during its free-fall trajectory thereof, and means for altering said free-fall trajectory in accordance with the value of the property thus detected.

13. A method of sorting opaque particles from a mixture of said opaque particles and glass particles wherein said glass particles are at least partially transparent, said method comprising the steps of:

passing said particles in said concentrate seriatim through a transparency sensing zone and therein sensing the transparent quality of said particles; and

sorting said particles according to the value of transparency thus sensed.

14. A method of sorting opaque particles according to claim 13 wherein said sensing zone includes a strip light source and a plurality of photosensitive sensing elements aligned in a row with said strip light source, and further comprising the step of scanning said photosensitive elements across said strip light source as said particles pass between said strip light source and said photosensitive elements.

15. A method according to claim 14 and further comprising the step of calculating the number of photosensitive elements which detect a light value below a predetermined value and sorting said particles when said calculated number exceeds a predetermined minimum.

16. An apparatus for sorting opaque particles from a mixture of said particles and glass particles which are at least partially transparent, said apparatus comprising:

transparency sensing means for sensing the degree of transparency of particles;

means for feeding said mixture of particles seriatim to said transparency sensing means wherein the transparent qualities of each of said particles is serially sensed; and

means for sorting said particles in accordance with the degree of transparency sensed by said transparency sensing means.

17. An apparatus for sorting opaque particles according to claim 16 wherein said transparency sensing means includes a strip light source and a plurality of photosensitive means in a row and aligned with said strip light source, and said feeding means passes said particles between said strip light source and said row of photosensitive means.

18. An apparatus for sorting opaque particles according to claim 17 and further comprising means to effect scanning of said photosensitive means across said strip light source.

19. An apparatus for sorting opaque particles according to claim 18 wherein each of said photosensitive means generates a signal representative of the intensity of light sensed thereby; and means coupled to said photosensitive means for calculating the number of said photosensitive means which sense a light intensity below a predetermined value, and means for effecting sorting of said particles when said calculated number exceeds a predetermined value.

20. An apparatus for sorting opaque particles according to claim 19 wherein said calculating means calculates the ratio of the number of said photosensitive means which sense a light intensity below a predetermined value to the number of said photosensitive means which sense a light intensity above said predetermined value.

21. An apparatus for sorting opaque particles according to claim 16 and further comprising means for sensing a property of said particles related to the color thereof and means to effect sorting of said articles according to the value of color-related property thus detected.

22. An apparatus for sorting opaque particles according to claim 21 and further comprising means for sensing a property of said particles related to the metal content thereof, and means for sorting said particles according to the value of the metal-related property thus sensed.

23. An apparatus for sorting opaque particles according to claim 22 wherein said transparency sensing means, said color sensing means, and said metal sensing means are aligned with respect to each other for passage therethrough of said particles in a single free-fall trajectory; and said feeding means projects said particles in a free-fall trajectory through each of said transparency sensing means, said color sensing means, and said metal sensing means.