GB2076146A - Method and Apparatus for Sorting - Google Patents

Abstract

The invention concerns a method of sorting comprising irradiating a plurality of objects (2), such as pieces of ore, with microwave energy at 6 and sorting the irradiated objects (2) in accordance with their temperature or with a property functionally related thereto; e.g. infra-red radiation at 11. A time lag of 10-60 seconds may occur between irradiation & sorting. <IMAGE>

Description

SPECIFICATION Method and Apparatus for Sorting This invention concerns a method and an apparatus for sorting objects such as pieces of ore or pieces of coal and shale. Although, however, the present invention is primarily concerned with the sorting of minerals, the invention is applicable to the sorting of vegetables and other foodstuffs, and to the sorting of industrial objects.

Optical sorting has been established for many years in the food, mineral and general materials field as a means of separating discrete particles where a measurable optical difference occurs between the various particles in the "visible" part of the electro-magnetic spectrum.

Modern techniques have gradually extended the range over which such optical differences can be measured to embrace the shorter wavelengths (approaching the near ultra-violet and the longer wavelengths (near the infra-red). Sorting machines based on such modern techniques generally uniformly expose the particles to a steady "light" source of the appropriate wavelength range and then, by the use of suitable lens systems and detectors, measure the reflection, refraction or absorption of the chosen wavelengths. Scanning systems are sometimes employed to give increased resolution.

A more recent development in sorting machines relies on the fluorescent properties of some materials, that is the phenomenon of the absorption of electro-magnetic waves at one frequency and the re-radiation of the waves at a lower frequency. Notable examples of the use of this phenomenon are: 1. The irradiation of minerals such as scheelite with short wave ultra-violet light to produce visible fluorescence.

2. The "X ray" radiation of diamond bearing ores to cause the diamonds to fluoresce at visible wavelengths.

In both cases photo-sensors are employed to detect the visible wavelength re-radiated.

There still exist, however, a number of sorting problems which do not respond to any of the previously mentioned techniques and in some cases there may be amenable to "hand-sorting".

At first sight it might be thought that any "handsorting" process could be automated. One must remember, however, that a human operator can respond to a much greater range of input information than automatic apparatus devised up to the present time, e.g. the human operator can respond to shape, size, quality of surface (rough, smooth, crystalline) and to very small colour and optical reflectance differences.

One Example of this is in the separation of coal from shale where, although hand sorting is sometimes employed, to date no optical method of separation has been successful, in particular because the material is often dusty or wet.

Although methods exist for mechanical separation (e.g. by heavy media separation) it would seem to be an advantage if a dry process could be developed.

In British Patent No. 1,365,107 there is disclosed a sorting apparatus in which crocidolite containing rocks are heated by high intensity fuel oil burners and the resulting infra-red emission of the rocks, which is dependent on their crocidolite content, is detected by infra-red sensors. In this apparatus, however, the sensors have to be closely adjacent to the heating point since the heating of the rocks is only superficial and is thus very temporary. Moreover, unsatisfactory results are obtained if the rocks are wet since this prevents penetration of the infra-red heating rays from the oil burners.

According, therefore, to the present invention, there is provided a method of sorting comprising irradiating a plurality of objects with microwave energy, and sorting the irradiated objects in accordance with their temperature or with a property functionally related thereto.

Preferably, the irradiated objects are sorted in accordance with their infra-red radiation.

The term "microwave", as used in this specification is intended to cover all radio frequencies of at least 300 megacycles per second. The preferred range is 900 to 23,000 megacycles per second since the United States Federal Communications Commission has assigned the frequencies of 915, 2,450, 5,800 and 22,125 megacycles per second for industrial, scientific and medical uses.

Of these frequencies, the one most commonly used at the present time for microwave heating is 2,450 megacycles per second, which corresponds two a wavelength of 4.8" (12.19 cms). The frequency would be highly suitable for the present invention, although in some cases it could b advantageous to use higher frequencies e.g.

where the objects to be irradiated have a dimension smaller than 4.8" (12.19 cms).

In contrast to the superficial heating effected in the apparatus of British Patent No. 1,365,107, the microwave energy employed in the case of the present invention penetrates the objects and causes internal heating thereof. Moreover, the microwave energy acts selectively, the energy being absorbed more readily by materials which have a high dielectric loss, and the presence of surface water does not materially affect the heating.

For example, if particles of shale and coal are exposed to microwave energy at a frequency of 2,450 megacycles per second, the shale rapidly increases in temperature whilst the coal remains more or less cool. If these particles are exposed to microwave energy for a period of 10-60 seconds, temperature increases of greater than 2 degrees centigrade are achieved on the shale.

Microwave energy is radiant in source.

However, as its effects are to cause a rise in temperature of some bodies, this temperature rise can be measured electro-magnetically.

If the object is considered as a block body radiator, it will behave according to the Stefan Boltzmann law for the transfer of heat from a black body. If it is assumed that for practical purposes the initial temperature of the objects would be between OOC and 5O0C, then the "black body" radiation would lie between 8 and 10 ym.

It is well-known that the heating effect of microwave energy in food is caused by the rise in temperature of the water molecules as they are "jostled" by the microwave energy. In the case of coal/shale, however, the phenomenon is probably a dielectric heating effect, the shale having a greater dielectric loss than the coal and thus absorbing more energy. It is noticeable that after the initial heating effect, which is internal there is a redistribution of the heat through the body. This would be an advantage in some sorting applications and would also enable the sensing of infra-red radiation to be less critical.

Exp.riments with native rock which have molybdenum disulphide inclusions normally too small and too "near" in reflectance to the native rock to be sorted by normal optical methods, show that the molybdenum inclusion rapidly heats up and then redistributes the heat gradually over a larger area enabiing such rocks to be sorted from those which do not have such inclusions.

The objects are preferably irradiated with microwave energy for a period of 10-60 seconds.

There is preferably a time lag e.g. in the range of 10 to 60 seconds, between the irradiation of the objects with microwave energy and the sorting of the irradiated objects. Such a time lag allows the internal heat in the objects to be generally distributed therethrough, thus making the infra-red sensing less affected by the surface area of the objects. This enables an examination to be made of the rocks or other objects for quite small inclusions therein which may not be apparent from a study of the surfaces of the objects.

The objects are preferably sorted while in free flight.

The invention also comprises sorting apparatus comprising means for irradiating a plurality of objects with microwave energy, and means for sorting the irradiated objects in accordance with their temperature or with a property functionally related thereto.

Preferabiy, the apparatus comprises a microwave heating section, first transport means for continuously passing objects through the microwave heating section, second transport means for continuously passing the irradiated objects from the microwave heating system to a sorting zone, sensor means for sensing the temperature or related property of the irradiated objects passing through the sorting zone, and separator means, controlled by the sensor means, for separating the irradiated objects into those which have and those which do not have a predetermined temperature or related property.

The sensor means are preferably infra-red sensor means arranged to sense the infra-red radiation of the irradiated objects.

The second transport means may comprise an horizontal endless belt from whose downstream end the irradiated objects fall in free flight.

The sensor means preferably examine the irradiated objects in free flight.

The separator means may comprise pneumatic ejection means for directing compressed air onto irradiated objects which either have or do not have the said predetermined temperature or related property.

The invention is illustrated, merely by way oft example, in the accompanying drawing which is a diagrammatic sketch of a sorting machine according to the present invention.

Referring to the drawing, a sorting machine comprises a hopper 1 adapted to contain objects to be sorted which may be constituted by particles 2 of coal and shale. The particles 2 are fed onto a tray 3 which is vibrated by an electromagnetic or other vibrator 4 so as to pass onto the upper rim of an horizontal endless belt 5.

The endless belt 5 serves to pass the particles 2 continuously through a microwave heating oven 6 where the particles 2 are heated from above and from below. The endless belt 5 is a slow moving belt so that the particles 2 are in the microwave heating oven 6, and are irradiated with microwave energy therein, for a period of 10-60 seconds, preferably 30 to 160 seconds.

The microwave heating oven may employ 5 to 10 Kilowatts of microwave energy input produced by microwave generators known per se which produce coherent wavelengths in narrow bands of the sTectrum, e.g. 2,450 megacycles per second, the depth of penetration of such energy being a function of frequency. Such heating is unaffected by the external temperature and the heat transfer coefficient of the particles 2.

The irradiated particles 2 leaving the downstream end of the endless belt 5 slide over a chute 7 and fall onto the upper rim of an horizontal endless belt 8 by means of which they are continuously passed to a sorting zone 10.

There is a time lag, e.g. of 10 to 60 seconds, between the irradiation of the particles 2 in the microwave heating oven 6 and the sorting of the irradiated particles in the sorting zone 1 0. As pointed out above, this time lag allows the internal heat in the irradiated particles 2 to be generally distributed therethrough. It also enables the microwave heating oven 6 to be isolated as far as possible from personnel involved in operating the sorting machine.

The particles 2 fall in free flight from the downstream end of the endless belt 8. The latter may be a broad non-channelised belt so that the particles fall therefrom as a random stream of particles, the stream being many particles wide.

Alternatively, the endless belt 8 may be channelised (by means not shown) so that the particles 2 fall therefrom in a plurality of predetermined single files. The endless belt 8 may move at a speed of 100" (254 cms) per second.

While the particles 2 are in free flight, they are viewed by one or more infra-red sensors 11 which may be provided with a thin layer of lithium tantalate so as to be responsive to the infra-red radiation of the particles 2 passing through the sorting zone 10 and thus to a property functionally related to their temperature. A single scanned sensor 11 could, if desired, be used, or alternatively a bank of parallel sensors 11 could be used. the sensors 11, in addition to sensing the infra-red radiation, may also, if desired, sense the area and position of the particles 2.

A comparator 12 compares signals from the sensor or sensors 11 with a datum so that, when a particle 2 is detected whose temperature is above (or below) a predetermined value, a signal is sent to one of the ejectors of a bank of pneumatic ejectors 13.

Thus if the particles are coal and shale, and a particular particle has a temperature higher than a predetermined value, this indicates that the particle consists mainly or wholly of shale. such a particle is then deflected by a puff of compressed air from the respective ejector 14 into a "reject" chute 14, while particles of coal, which will be at a lower temperature, wilt pass undeflected in free flight into an "accept" chute 1 5.

Although the invention has been described above in connection with the separation of shale from coal, the invention may be used in many other fields, e.g. for sorting seeds which have viable embryos from those which do not, since the heating effect on a viable embryo may be less than that on a non-viable embryo. Similarly, a vegetable or other product with a hollow heart would not absorb as much microwave energy as one which did not have this fault and could therefore be separated therefrom.

Claims (14)

Claims

1. A method of sorting comprising a plurality of objects with microwave energy, and sorting the irradiated objects in accordance with their temperature or with a property functionally related thereto.

2. A method as claimed in claim 1 in which the irradiated objects are sorted in accordance with their infra-red radiation.

3. A method as claimed in claim 1 or 2 in which the objects are irradiated with microwave energy for a period of 10-60 seconds.

4. A method as claimed in any preceding claim in which there is a time lag between the irradiation of the objects with microwave energy and the sorting of the irradiated objects.

5. A method as claimed in claim 4 in which the said time lag is in the range of 10 to 60 seconds.

6. A method as claimed in any preceding claim in which the objects are sorted while in free flight.

7. A method as claimed in any preceding claim in which the microwave energy is at a frequency in the range of 900 to 23,000 megacycles per second.

8. A method as claimed in any preceding claim in which the said objects are particles of coal and shale.

9. A method of sorting substantially as hereinbefore described.

10. sorting apparatus comprising means for irradiating a plurality of objects with microwave energy, and means for sorting the irradiated objects in accordance with their temperature or with a property functionally related thereto.

11. Apparatus as claimed in claim 10 comprising a micro-wave heating section, first transport means for continuously passing objects through the microwave heating section, second transport means for continuously passing the irradiated objects from the microwave heating system to a sorting zone, sensor means for sensing the temperature or related property of the irradiated objects passing through the sorting zone, and separator means, controlled by the sensor means, for separating irradiated objects into those which have and those which do not have a predetermined temperature or related property.

12. Apparatus as claimed in claim 11 in which the sensor means are infra-red sensor means arranged to sense the infra-red radiation of the irradiated objects.

13. Apparatus as claimed in claim 11 or 12 in which the second transport means comprises an horizontal endless belt from whose downstream end the irradiated objects fall in free flight.

14. Apparatus as claimed in claim 13 in which the sensor means examine the irradiated objects in free flight.

1 5. Apparatus as claimed in any of claims 11-14 in which the separator means comprise pneumatic objection means for directing compressed air onto irradiated objects which either have or do not have the said predetermined temperature or related property.

1 6. Sorting apparatus substantially as hereinbefore described with reference to and as shown in the accompanying drawings.