WO1997014950A1

WO1997014950A1 - Method and apparatus for sizing particulate material

Info

Publication number: WO1997014950A1
Application number: PCT/AU1996/000650
Authority: WO
Inventors: Ian Bruce Browne; Kenneth John William Lieber; Lip-The Young
Original assignee: Scientific Industrial Automation Pty. Limited
Priority date: 1995-10-16
Filing date: 1996-10-16
Publication date: 1997-04-24
Also published as: AUPN599495A0

Abstract

A method and apparatus for particle size analysis of particulate material. The method and apparatus utilise the blob analysis of an image of a moving substantially monolayer sample of the particulate material. The blob analysis is performed to obtain information about at least the sizes of the particulate material in the image of the monolayer. The monolayer is preferably established by allowing the particles to fall under the action of gravity. In an embodiment of the invention, the particulate material falls through the field of view (11) of a camera (10) which generates output signals indicative of an image of the monolayer. The blob analysis is performed by a processing system (14) connected with a camera (10) to provide an analysis of the size of the particles.

Description

"Method & apparatus for sizing particulate material"

Technical Field

This invention concerns size measurement. More particularly, it concerns non-contact sizing of particulate material such as material of the type obtained in mineral mining operations, using "blob analysis" techniques. However, the invention is not limited in its application to the measurement of the size of lumps of rock, coal, ores and other mined materials.

Background Art

The sizing of materials such as coal, ore, quarried rock and minerals, and the output of crushers, has always been an important factor in the mining and mineral processing industries. Currently, the most commonly used method of measuring the size of such materials involves discharging a sample of the material into the top of a multi-level sizing station or "sieve". The sample falls through progressively finer screens, with the larger lumps of rock, coal or the like being retained first, the smaller lumps falling further before being caught by a screen, and the finest particles descending to the lowest level of the "sieve".

The main disadvantages of this method of sizing are:

(a) the analysis of the size distribution of the sample can be specified only on the basis of the hole sizes of the screens used;

(b) the sizing is a slow process; (c) only a small quantity of the ore, coal or the like is sampled by this method, so that in many instances the estimated size distribution does not represent the true size distribution of the material due to poor statistics or segregation; (d) the cost of building a sizing station is high (millions of dollars) and the maintenance and operation of a sizing station is labour intensive and thus expensive; and (e) significant errors can occur due to breakage of the screens and build-up of material over the screens which can prevent smaller particles falling through that screen.

Nevertheless, despite the fact that it has been known for many years that sizing stations and their operation have these defects, no effective alternative method of sizing has been proposed.

Disclosure of the Present Invention

It is an object of the present invention to provide a new method and apparatus for sizing particulate materials which, when applied to sizing of rocks and lumps of coal, ores and the like, overcomes, or substantially diminishes the adverse effect of, the disadvantageous features of the current sizing method and equipment, noted above.

To achieve this objective, the present invention utilises a technique known as "blob analysis" (which is also termed, by some people, "connectivity analysis" or "connected components analysis"). Blob analysis has been used in industrial vision systems that have been developed for object recognition, defect classification and image enhancing in the manufacturing, automotive and electronics industries.

In the measurement of particulate materials in accordance with the present invention, the blob analysis is carried out on an image of a moving new monolayer of the particulate material. Preferably the particulate materials are back-lit, so that the projected area of the particle is observed by the camera. In such an application, blob analysis techniques can provide, for each image that is produced, information about the projected areas of the particles, their minimum radii, maximum radii, orientations, perimeters, holes, centroid, and other shape information.

Thus, according to the present invention, there is provided a method of particle size analysis of a sample of a particulate material, said method comprising the steps of:

(a) establishing a moving substantially monolayer sample of the particulate material and an image of a selected portion of the monolayer; and (b) performing blob analysis of the image to obtain information about at least the sizes of the particulate material in the selected portion of the monolayer.

The present invention also provides apparatus for obtaining a size distribution of a sample of particulate material, comprising:

(a) means to establish a moving substantially monolayer sample of the particulate material;

(b) means to generate an image of a selected portion of the monolayer; and (c) means to perform blob analysis of the image and to produce therefrom information about the size distribution of particles in the selected portion of the monolayer.

Preferably, the moving substantially monolayer sample is established allowing particles to fall under the action of gravity. In one form of the invention, this is achieved by running the particles off the end of a conveyor belt moving at a speed determined to give a desired separation of the particles. In another form of the invention, the sample is directed onto a vibrating feeder to at least partially form a new monolayer of the particles which are subsequently allowed to fall from an end of the feeder. Preferably, the particles falling from the end of the vibrating feeder are directed onto a second vibrating feeder running at a higher throughput rate to increase the distance between the particles.

The image is preferably generated by a camera having a field of view through which the monolayer passes. The camera preferably provides an output signal indicative of the image within the field of view and the blob analysis is perfoπned on the output signal.

Preferably, the particulate material in the field of view is back-lit. Preferably, the camera is either a linear array (or linescan) camera or an area array camera. With a linescan camera, continuous illumination of the particles is required; if an area array camera without an electronic shutter or external mechanical shutter is used, the illumination of the particulate material must be strobed. When applied to mineral or rock sizing, the sample of particulate material may conveniently be a continuous sample diverted from a main conveyor, to which the sample is returned following a determination of its size distribution.

The method and apparatus for particle size analysis according to this invention provides a number of advantages over prior art systems including:

(i) size distribution analysis based on user selected criteria and not on available physical screen size; (ii) increased speed of analysis, allowing on-line sizing of materials;

(iii) an ability to sample a greater percentage of raw material and thus improve the accuracy of analysis; (iv) lesser installation operation and maintenance costs compared to sizing stations; (v) greater accuracy of analysis compared to screening techniques due to degration of sample material being avoided;

(vi) an ability to provide information about other particle parameters such as area and perimeter; (vii) improved precision due to better statistics resulting from a high percentage of stream sampling; and (viii) extra sizing units can be placed at other sites and information transmitted via electrical or fibre optic cables to the processing system without the need to transport sample to the processor or to build a new multi-level screen sizing system.

An embodiment of the present invention will now be described, by way of example only. In the following description, reference will be made to the accompanying drawings.

Brief Description of the Drawings

Figure 1 is a partly schematic, perspective sketch showing the application of this invention to the analysis of mined coal or the like; and Figure 2 is a schematic sketch similar to Figure 1 showing some modifications to the invention.

Best Modes for Carrying Out the Invention

The arrangement shown in Figure 1 includes a camera 10 having an imaged region (field of view) 11, which is back-lit by a diffused back-light source 12. As noted above, the camera 10 is a type that has the image seen converted into electrical data signals, which are output through a cable 13 which is connected to a processing system 14.

Two kinds of camera may be used as the camera 10, namely a linear array (or linescan) camera or an area array camera. Although the hardware and software usually associated with processing systems generally supports area array cameras, some systems are able to process the output of linear array cameras.

The lighting configuration used depends upon the camera that has been selected. If the camera 10 is a linear array, linescan camera or shuttered camera, the light source 12 is a continuous source. If the camera 10 is an area array camera, the light source 12 is strobed. In either case, the light source is placed behind a diffuser panel, thus providing back-lighting for objects traversing the field of view 11 of the camera 10. To form a back-light surface having an even illumination, a complete optical fibre illumination system is required. If the camera 10 is a linear array camera, the optical fibre 19 which illuminates the diffuser panel can be a light pipe which provides a light slice or flat beam of light. If an area array camera is used, the evenness of the back-light is more important, so that a matrix of optical fibres can provide the illumination of the back-lit panel or if a shuttered camera is used a continuous light source such as a set of fluorescent tubes using high frequency ballasts can be used.

The processing system 14 may be any one of a number of commercially available suitably programmed systems. Suitable simple processing systems, comprising a microcomputer with add-on processing boards, are manufactured by companies such as (this list is not exhaustive) Imaging Technology, Matrox, Datacube and Data Translation. Faster processing systems are marketed by companies such as Allen-Bradley, Adept Technology, Cognax, IRI and AISI. These faster systems contain specific hardware and software to perform the blob analysis. Their response speed can be improved further by designing custom or semi-custom software and hardware to perform the necessary image processing. Such equipment with enhanced performance may be able to threshold and perform connectivity analysis (blob analysis) in a pipeline.

A sample of the material to be sized is brought to the vicinity of the camera by a conveyor 15. The sample may be of coal, ore, rock, the output of a crusher, or any suitable material. Normally, it will be a sample diverted from a main conveyor (not shown) that is transporting the material to a required destination (a stock pile or a loading point, for example).

The end ofthe conveyor 15 is positioned above the imaged region 11 of the camera 10, so that material on the conveyor 15 falls, essentially as a monolayer, through the imaged region 11. As the material on the conveyor falls, the individual lumps 16 of coal, ore or rock (or the like) singulate. That is, they effectively become separated from each other. They are back-lit by the diffused light source 12 and imaged by the camera 10. The camera image is processed to produce size data which is used to establish the size distribution of the sample.

The falling lumps 16 are caught by a suitably positioned hopper 17, about a metre below the imaged region 11, and are transported away from the analysis region by a lower conveyor 18. Typically, the lower conveyor will return the sample to the main conveyor from which it was diverted.

Figure 2 shows an arrangement similar to Figure 1 with some modifications. For ease of understanding, the same reference numerals have been used to identify corresponding features. In the arrangement shown in Figure 2, a main conveyor 19 is shown carrying particulate material. A primary sampling device 20 is provided to divert a selected amount of the particulate material from the main conveyor 19 into a hopper 21. The material is withdrawn from hopper 21 by two vibrating feeders 22 and 23. These vibrating feeders are of substantially known type with an adjustable vibration rate and/or vibration amplitude and/or inclination to provide for a varying rate of throughput or speed. The first feeder 22 determines the feed rate and the second feeder 23 distributes the material so that it discharges in a thin monolayer. As in the case of the arrangement shown in Figure 1, a image region or measurement window 11 is provided and has back-lighting by a light box 12. In the Figure 2 arrangement, the light box 12 and camera 10 are surrounded by an environmental enclosure 24 to protect the arrangement from environmental dust. The environmental housing is protected from ingress of dust by either positive pressure or by suction. Additionally, a pneumatic wiper (not shown) is provided to clean the surface of the back light 12 and an electric wiper (not shown) is provided to clean the camera window. In the arrangement of Figure 2, the connection between the camera unit and the processing system 14 is by way of an optical fibre link. This allows communication over distance of up to 2kms and overcomes difficulties associated with electrical interference.

As in the case with the arrangement shown in Figure 1, the particulate material is discharged to a lower conveyor 18 which returns the analysed material to the main conveyor.

The sequential vibrating feeders 22, 23 of the Figure 2 arrangement give an improved reliability of separation and a better distribution of the particles in the field of view of camera 10. The software associated with processing system 14, is able to detect overlapping particles and reject them for the purposes of the measurement of size distribution. This is achieved by finding a centroid of each blob and then measuring the maximum and minimum radii. The ratio of the maximum and minimum radii is determined and the blob is identified as overlapping particles if the ratio is too large for the material being measured.

The weight percent particle distribution can be calculated using the processed image information and a variety of algorithms. The camera 10 used can be a CCD (charge coupled device) camera using an electronic shutter controlled by a computer to freeze the image and transfer it to a frame buffer associated with the processing system 14. In the alternative, a stroboscopic backlight can be used as described in relation to the Figure 1 arrangement. Where a CCD camera with an electronic shutter is used, the fluorescent backlight must have a high frequency ballast in order to provide a consistent light level image.

The software associated with the operation of the system can perform checks to determine whether the backlight and/or camera lens glass has dust on the relevant surfaces. There are two preset levels of degregated performance determined by the system. The lower level gives a warning signal and the second higher level an alarm signal. These signals can be used to activate the pneumatic and electric wipers (not shown) described above. The following tables illustrate some laboratory trial data obtain comparing the sizing system of the present invention to screen sizing. The materials used for the process were Sinter and Coke of the kind typically used in a blast furnace.

Table 1 - SINTER

FRACTION NOMINAL % DEVIATION CUMULATIVE (VISION - SCREEN)

+ 63 mm 1 ± 0.5

+ 40 mm 6 ± 0.5

+ 30 mm 15 ± 1.0

+ 20 mm 30 ± 1.0

+ 10 mm 60 ± 1.5

+ 8 mm 80 ± 0.8

+ 5 mm 95 ± 1.5

+ 3 mm 99 ± 0.5 Table 2 - COKE

FRACTION NOMINAL % DEVIATION CUMULATIVE (VISION - SCREEN)

+ 100 mm - -

+ 80 mm 5 ± 1.0

+ 50 mm 50 ± 0.5

+ 40 mm 85 ± 2.0

+ 30 mm 99 ± 1.0

Table 3 - REPEATABILITY - Stabilised Coke Sample (Run 8 Times)

FRACTION CUMULATIVE % STANDARD DEVIATION

+ 63 mm 1.47 0.69

+ 50 mm 8.07 0.62

+ 40 mm 22.62 0.95

+ 30 mm 60.37 0.67

+ 20 mm 93.59 0.26

It will be apparent that the illustrated arrangements can perform an accurate size analysis and present realistic size distribution data in respect of a meaningful sample of the mined material or crusher output. The analysis is performed quickly. The equipment required to perform the sampling and analysis is at least an order of magnitude less expensive to install than a conventional sizing station. The system used for the analysis is essentially automated and has little labour associated with its running and maintenance.

It will also be apparent that although exemplary embodiments of the present invention have been illustrated and described in this specification, variations to and modifications of that embodiment can be made without departing from the present inventive concept.

Claims

1. A method of particle size analysis of a sample of a particulate material, said method comprising the steps of:

(a) establishing a moving substantially monolayer sample of the particulate material and generating an image of a selected portion of the monolayer; and

(b) performing blob analysis of the image to obtain information about at least the sizes of the particulate material in the selected portion of the monolayer.

2. A method as claimed in claim 1, wherein said moving substantially monolayer sample is established by the step of allowing particles to fall under the action of gravity.

3. A method as claimed in claim 2, wherein said moving substantially monolayer sample is established by the step of running said particles off the end of a conveyor belt moving at a speed determined to give a desired separation of the particles.

4. A method as claimed in claim 2, wherein said moving substantially monolayer sample is established by the steps of directing a sample onto a vibrating feeder to at least partially create a new monolayer of said sample and subsequently allowing the particles to fall from an end of said feeder to separate the particles.

5. A method as claimed in claim 4, wherein said moving substantially monolayer sample is established by the additional step of directing the particles falling from an end of said vibrating feeder onto a second vibrating feeder running at a higher throughput rate to improve the monolayer of said sample.

6. A method as claimed in any one of claims 1 to 5, wherein said image is generated by a camera having a field of view through which said monolayer passes.

7. A method as claimed in claim 6, wherein said camera provides an output signal indicative of the image within said field of view and said blob analysis is performed on said output signal.

8. A method as claimed in claim 6 or claim 7, wherein said camera produces an image by the use of a strobe lighting system.

9. A method as claimed in claim 6 or claim 7, wherein said camera produces an image by use of a continuous lighting system.

10. A method as claimed in claim 8 or claim 9, wherein said lighting system employs back lighting of the field of view of said camera.

11. A method as claimed in claim 7 or claim 9, wherein said camera includes an electronic shutter which is used to produce said image.

12. A method as claimed in any one of claims 6 to 11, wherein said camera is an area array camera.

13. A method as claimed in claim 6 or claim 7, wherein said camera is a linear array camera.

14. A method as claimed in any one of claims 1 to 13, wherein said monolayer sample is a continuous sample diverted from a main conveyor of said particulate material.

15. A method as claimed in any one of claims 1 to 14, wherein overlapping or touching particles are detected and rejected.

16. A method as claimed in claim 15, wherein overlapping or touching particles are detected by determining the centroid of each particle and the ratio of the maximum to minimum radii of each particle from the centroid and identifying a particle as an overlapping or touching particle where the ratio is larger than that for the particulate material being measured.

17. An apparatus for obtaining a size distribution of a sample of particulate material, comprising:

(b) means to generate an image of a selected portion of the monolayer; and

(c) means to perform blob analysis of the image and to produce therefrom information about the size distribution of particles in the selected portion of the monolayer.

18. An apparatus as claimed in claim 15, wherein said moving substantially monolayer sample is established by allowing particles to fall under the action of gravity.

19. An apparatus as claimed in claim 18, wherein said moving substantially monolayer sample is established by running said particles off the end of a conveyor belt moving at a speed determined to give a desired separation of the particles.

20. An apparatus as claimed in claim 18, wherein said moving substantially monolayer sample is established by directing a sample onto a vibrating feeder to at least partially create a new monolayer of said sample and subsequently allowing the particles to fall from an end of said feeder to separate the particles.

21. An apparatus as claimed in claim 20, wherein said moving substantially monolayer sa ple is established by directing the particles falling from an end of said vibrating feeder onto a second vibrating feeder running at a higher throughput rate to improve the monolayer of said sample.

22. An apparatus as claimed in any one of claims 17 to 21, wherein said image is generated by a camera having a field of view through which said monolayer passes.

23. An apparatus as claimed in claim 22, wherein said camera provides an output signal indicative of the image within said field of view and said blob analysis is performed on said output signal.

24. An apparatus as claimed in claim 22 or claim 23, wherein said camera produces an image by the use of a strobe lighting system.

25. An apparatus as claimed in claim 22 or claim 23, wherein said camera produces an image by use of a continuous lighting system.

26. An apparatus as claimed in claim 24 or claim 25, wherein said lighting system employs back lighting of the field of view of said camera.

27. An apparatus as claimed in claim 23 or 25, wherein said camera includes an electronic shutter which is used to produce said image.

28. An apparatus as claimed in any one of claims 22 to 27, wherein said camera is an area array camera.

29. An apparatus as claimed in claim 22 or claim 23, wherein said camera is a linear array camera.

30. An apparatus as claimed in any one of claims 17 to 29, wherein said monolayer sample is a sample diverted from a main conveyor of said particulate material.

31. An apparatus as claimed in any one of claims 17 to 30, wherein overlapping or touching particles are detected and rejected.

32. A method as claimed in claim 31, wherein overlapping or touching particles are detected by determining the centroid of each particle and the ratio of the maximum to minimum radii of each particle from the centroid and identifying a particle as an overlapping or touching particle where the ratio is larger than that for the particulate material being measured.