A method and a device for sampling material from a moving belt conveyor

Abstract: The invention provides a sampler for removing material such as ore from a mining operation, from a moving belt conveyor. The sampler comprises a frame (19) having sweeping means (38, 44) located between a pair of spaced blades (24) and behind their cutting edges. A drive means (26, 28, 30, 31) is arranged to pass the frame periodically across the moving belt in order to confine the material between the blades in a sampling zone which extends obliquely across the belt. Material confined in the sampling zone is displaced from the belt while the blades traverse the belt. The material displaced from the belt is conveniently reduced to a homogeneous state for assaying.

Description: BACKGROUND OF THE INVENTION This invention relates to a method and a device for sampling material from a moving belt conveyor. The material to be sampled may be the run-of-mine product of mines such as metal mines in which the mineral to be recovered is unevenly distributed in the ore removed from the mining area. The run-of-mine product may be considered to be the product reaching the first stage of ore processing after leaving the mining area. Typically, in an underground gold or platinum mine, the run-of-mine product arrives at the surface through a shaft, having been blasted and screened underground. The ore in such mines contains mineral values which are not consistent from one working area of the mine to the others. There is also inconsistency in the distribution of the mineral in ore taken from the same working area. In underground gold mines, for instance, the ore may be concentrated in relatively narrow reefs or seams surrounded by barren country rock. Some country rock is always removed in such cases together with the metal-bearing reef or seam, and its presence necessarily affects the grade of the ore processed. It is highly desirable to sample the run-of-mine product for its mineral values in order to establish the efficiency of the extraction process. Because of the large tonnages typically handled in metal mines, and the disruption to production which would occur by periodically stopping a moving conveyor to remove a representative sample, run-of-mine sampling has not successfully been carried out on metal mines in the past, as far as the applicant is aware. Fully representative sampling of bulk products is possible where the product is homogeneous. For instance, in the processing of food products such as maize and flour, it is possible to extract a satisfactory sample at almost any point. The sample in such cases need not be large: only sufficient product need be removed to satisfy the needs of the sampling procedure itself. Typically in such instances a sample can be scooped off a moving or stationary conveyor such as a belt conveyor to provide a fully representative sample of the product at that point in the plant and at that particular time. The scoop acts as a displacement means which is periodically passed across the belt to displace material across the belt and off it. The material rests in this apparatus on the floor of the scoop. It has been appreciated that in order to conduct proper sampling of the run-of-mine product on metal mines, it is necessary to take a portion of the entire stream of material passing through a particular point at a particular time. The run-of-mine product in such mines is often transported, at least at some stage, on belt conveyors. Hitherto, to conduct run-of-mine sampling, the conveyor has been stopped, a former placed over a portion of the belt, and the entire body of ore contained within the former has been removed. The removal has, at least in some instances, taken place by laborious manual methods. These procedures have been slow and somewhat erratic since the consistency of operation of the person voiding the former cannot readily be sustained. Since a substantial proportion of the metal is often found in fines which settle at the bottom of a troughed belt conveyor, any significant part of the fines left behind can destroy the accuracy of the sampling result. The problem is rendered more complicated by the fact that the ores are often highly abrasive so that there is a good deal of wear on any but the most robust of equipment. The problem resolves itself largely into one of repeatability. It is necessary to be able to undertake a consistently repeatable procedure and one which does not disturb, to any real extent, the continuity of flow of the ore into the processing plant. If the conveyor from which the sample is to be taken is a conventional belt conveyor the problem is compounded by the fact that belt geometry varies from point to point along the length of the conveyor. The belt, generally of rubber, is supported on a series of rollers, arranged in sets along the conveyor frame. Each set is arranged in a trough configuration. Between each pair of adjacent roller sets the belt tends to trough in a longitudinal direction in addition to the troughing imposed transversely by the rollers, so that a mechanism which merely travels in a straight line across the belt to eject a portion of the ore on it would not provide a sufficiently complete body of material to be adequate for proper sampling. The material contained in the trough, both longitudinal and transverse, would not be removed, and its presence would distort the results of the assaying procedure carried out on the sample extracted. Prior sampling apparatus known to the Inventors has generally suffered from one or more deficiencies, making the accuracy of obtained samples doubtful. U.S. Patent 4,790,196 (Gould), for example, illustrates a "snowplowing effect" which arises when a sampler comprising a pair of spaced plates passes across a belt conveyor in perpendicular relationship to its direction of motion. This has the result of disturbing the material being sampled between the plates, while displacing outwardly adjacent material from the belt conveyor. Gould's patent essentially addresses one aspect of this problem by maximising the effective opening between the moving plates as they traverse the width of the belt. More particularly the leading edges of the plates are spaced from each other along a line extending in transverse relationship to the belt conveyor. During each sampling cycle the plates are displaced either in perpendicular relationship, or at an oblique angle in relation to the direction of motion of the belt conveyor, thereby minimising disturbances of any material lying between the plates. It is evident from the illustrations in Gould's patent, however, that the paths traced by the leading edges of the plates are not necessarily followed by the remaining portions of the plates as they traverse the belt conveyor. This has the unavoidable result of causing at least some spillage of material lying along the outer edges of the plates. In a particularly preferred embodiment of Gould's invention a pair of plates is mounted on a movable carriage running on rails extending on either side of a belt conveyor. By matching the speed of the carriage with that of the belt conveyor material may be sampled from a band extending across the width of the belt in perpendicular relationship to its direction of motion. This sampling arrangement minimises the disturbance of any material lying on either side of each plate. It is evident from the contents of Gould's patent, however, that such a sampling arrangement could extend over 6 metres (18 feet), which is considered too elaborate to warrant the likely sampling costs. Moreover Gould's patent does not address the difficulties in sampling material from belt conveyors which are troughed both in the transverse and the longitudinal directions. European patent application EP 373 281 (Long) seeks to offer a solution to this shortcoming by providing a curved plate which is locatable below a chosen belt conveyor in the region from which a sample is to be drawn. The sampler disclosed in Long's patent includes a pair of spaced plates arranged in perpendicular relationship to the direction of motion of the belt conveyor. The described sampling device is intended primarily for taking samples of coal of which the largest expected lump has a diameter of approximately 50mm (2 inches). The spacing between the plates is accordingly comparatively small, being about 150mm (6 inches). By choosing a comparatively high rate of displacement, in the order of 4.6 m per second (180 inches per second), a relative brief traverse time of less than 0.25 seconds is obtainable. This sampling arrangement is said to be acceptable for coal sampling. It appears unsuitable, however, for sampling ore in which valuable minerals are randomly distributed among pieces of low-value country rock having sizes in excess of 50mm. The width of the plates, and the spacing between them, for a similar sampling arrangement would accordingly have to be substantially larger, thereby re-introducing the "snowplowing effect" already described in Gould's US Patent 4,790,1096. It is not however necessary that every particle of the body of material removed from the stream of the ore should be collected. Provided the sampling procedure is entirely consistent and the extent of removal of the ore from the conveyor is understood, together with the overall distribution of metal in the ore, a satisfactory sampling result can be achieved by importing the necessary bias into the assay results. With these and related circumstances in mind, it is an object of the invention to provide a method and apparatus for sampling material from a moving belt conveyor, such as the run-of-mine products of metal mines and similar mines, so that a successful and consistent results can be achieved. SUMMARY OF THE INVENTION According to the present invention, a method of sampling material is provided from a moving belt conveyor which includes a conveyor belt which is troughed in transverse relationship to its longitudinal direction of motion, the method of sampling including the steps of periodically passing a pair of spaced plates secured in oblique relationship to a rotatable shaft arranged above the belt in parallel relationship to its direction of motion so that the plates pass across the belt to confine material in a sampling zone extending obliquely across the belt when the shaft is rotated about its longitudinal axis, and displacing the material in this sampling zone off the belt while the plates traverse the belt, characterised in that the belt is longitudinally supported in a region beneath the sampling zone in order to keep the portion of the belt from which material is being sampled substantially straight in the direction of motion of the belt, while maintaining its transverse troughing. The material is preferably displaced from the belt by sweeping the space located between the plates and behind their leading edges. Various forms of sweeping action, singly or in combination, may be used. Material removed from The belt will generally be reduced to a homogeneous state, a portion of the homogeneous product then being assayed. A sampler for removing material from a moving belt conveyor which includes a conveyor belt which is troughed in transverse relationship to its longitudinal direction of motion, the sampler including a frame which includes a pair of spaced plates having leading edges, the plates being secured in oblique relationship to a rotatable shaft arranged above the belt in parallel relationship to its direction of motion, the plates being adapted to confine material between each other in a sampling zone which extends obliquely across the belt, is characterised in that a support means is provided for longitudinally supporting the belt below the sampling zone for keeping the belt substantially straight in its direction of motion, while maintaining its transverse troughing. The guard extending adjacent to, and in overlapping relationship with an edge of the belt may be provided. The sweeping means may comprise or include a web which extends between the plates for sweeping the surface of the belt. Alternatively or in addition the sweeping means may include a set of bristles arranged between the plates. In order to assist the sweeping action the sampler may further include means for agitating material in the sampling zone by means of a continuous stream of air routed through a passage in the shaft to which the plates are secured. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a simplified side elevation view of a portion of a belt conveyor for run-of-mine product at a metal mine equipped with a sampler of the invention; Figure 2 is a plan view of the belt conveyor of figure 1, without its roof structure; Figure 3 is a vertical section view along the line III - III of figure 1. Figure 4 is a perspective view of a frame and associated components of the sampler of figures 1 - 3; Figure 5 is a semi-schematic cross-sectional view through the belt seen in preceding figures, with certain other components and features indicated; Figure 6 is a plan view of a portion of the belt of the preceding figures, showing the zone swept by the sampler of the invention. DESCRIPTION OF A PREFERRED EMBODIMENT In the drawings, a mine belt conveyor 10 comprises a base 12 of a conventional type supporting a series of sets of horizontal rollers 14 and inclined rollers 15 on which a rubber conveyor belt 16 is supported. The belt 16 is inclined to travel upward in the direction of the arrow A and the belt carries run-of-mine ore arriving from underground up a mine shaft towards a processing plant (not illustrated). A sampler of the invention is located below a roof structure 20 supported by a base 18 and covering a zone of the belt 16. Supported above the belt between pedestal bearings 21 is a sampler frame 19 comprising a tubular shaft 22 on which are fixed two plates 24 of paddle shape, spaced apart and parallel to each other. The axis of the shaft 22 is inclined at an angle of approximately 40° or more to the planes of the plates 24. This angle is dictated by several factors such as the linear speed of the belt 16 and the rotational speed of the shaft 22, as appears more fully from the further description of the invention. Drive means comprising an electrical motor 26 operating through V belts enclosed in a housing 28 transmits motion to a gearbox 30, which is connected via a coupling 31 to the shaft 22. As appears from figure 1 the shaft 22 is arranged parallel to the direction of movement of the belt 16. The shaft 22 has at its lower end a fixed inlet cap 32 for receiving compressed air through a hose 34 that is connected to a source 36 of compressed air and directs this compressed air through a seal into the interior of the shaft 22. The detailed construction of the sampler frame 19 is best seen in figure 4. It includes, between the plates 24, a rectangular web 38 located in a plane to which the axis of the shaft 22 is parallel. Each plate 24 has a leading edge 40 which, on rotation of the shaft 22, describes a locus 41 (figure 6) extending obliquely across the belt, with the web 38 between and behind the leading edges of the plates 24. The loci 41 define between them a sampling zone 50 extending obliquely across the belt 16, as illustrated in figure 6. The web 38, which extends to the radially outer edges of the plates 24, sweeps the surface of the belt 16 and displaces ore situated in the sampling zone 50 of the belt on rotation of the shaft 22, depositing this material in a chute 42 (figure 1) located on one side of the sampler frame 18. Adjacent to the web 38 is a series of rod-like supports on which bristles 44 are fixed. Between each line of the bristles 44 is a rigid tube 46 which is perforated along its length. The holes in the tubes 46 are downwardly directed so that compressed air which they emit impinges on the belt 16 and agitates material on it. The bristles 44 enhance the sweeping action of the sampler which is further enhanced by the draughts of air provided through the tube 46. The sampler is arranged, through a conventional timer mechanism schematically shown at 48 in figure 3, to cause the shaft 22 to undergo periodically a complete revolution at predetermined time intervals. These intervals will be determined by several factors, including the nature and quantity of material on the belt and the speed of movement of the belt. A typical belt speed is one metre per second and a typical frequency of sampling is once every 15 minutes. The angle at which the plates 24 are arranged in relation to the axis of the shaft 22 is important. More particularly the loci 41 on the belt 16 described by the leading edges 40 should be followed as nearly as possible by the radial edges of the plates 24. It is important to arrange the sampler frame 19 so that, in relation to the belt 16, it traverses the belt 16 at an oblique angle as shown in figure 6. The speed of the motor 26 is so co-ordinated with the motion of the belt 16 that, when the shaft 22 rotates, the ore on the belt is confined between the plates 24 and does not tend to accumulate against the aft plate 24, nor does the sampler tend to overtake material on the belt 16. The material on the zone 50 of the belt is cleanly swept by the sampler into the chute 42, without disturbing to any significant extent the ore to either side of that zone 50. It has been found that the combined action of the plates 24, web 38, bristles 44, and compressed air tubes 46 is to sweep the belt 16 virtually entirely of the material in the zone 50 traversed by the sampler as the belt proceeds up the conveyor 10. The belt 16 is conventionally troughed in a transverse direction, as shown in figures 3 and 5, in view of its support on the inclined sets of rollers 15. The belt has a natural tendency to trough in the longitudinal direction too between each set of rollers 14 spaced along the length of the base 12. The longitudinal trough is unacceptable for the purposes of the invention, and for this reason a curved plate 52 (figure 5) is fixed to the conveyor base 18 in the zone of the sampler. The plate 52 bears against the lower surface of the belt 16 and keeps it longitudinally supported while permitting the required transverse troughing in the sampling zone. In figure 5 it will also be seen that, at the edge 54 of the belt 16 first approached by the plates 24 of the sampler frame 19 as the shaft 22 rotates, a guard 56 is provided. This consists of a curved strip of steel, fixed to the plate 52, so that it extends over the edge 54 of the belt and prevents this edge from lifting. (Such lifting could of course lead to obstruction of the plates 24 and web 38 as they first cross the edge 54 of the belt; potential damage to the belt 16 and sampler frame 19 is accordingly avoided.) The locus of the lower edge of the plates 24 of the sampler is shown in figure 5 by the line 41, and the direction of rotation of the sampler frame by the arrow "B". In practice, for a conventional belt of 1050 mm width, it has been found adequate to provide a sampler frame 19 in which the distance between the plates 24 (measured along the axis of the shaft 22) is 600 mm. Steel plate of approximately 12 mm thickness is used for the plates 24 and web 38. The compressed air reaching the agitator tubes 46 arrives through tubes 62, 64 connected to the interior of the shaft 22 and jointed by a union 66. A feature of the invention is that the sampler removes from the belt 16 substantially all the ore in the zone 50. Naturally total removal of every particle in this zone 50 is not possible, but a consistently high degree of removal has been found possible by the method and apparatus of the invention. To the extent that any material regularly remains on the belt 16 in the zone 50, a bias is present which can be statistically catered for when calculating the results of assaying. Material entering the chute 42 on each occasion of sampling is collected and crushed to a suitable consistency. This mixes the material so that it becomes a product in which the metal values are homogeneously distributed through the sample. A portion of the sample which is statistically adequate for the purpose is then removed from the sample and assayed by conventional techniques to determine its mineral content. As noted above, the raw assay result can be adjusted to take account of any bias known to exist in the system, for instance to represent the known value of material consistently left behind on the belt 16 in the zone 50. A sampler as described above has the necessary robustness to be reliable over a long period. This is essential for accurate sampling. Various alternative constructions are of course possible. For instance, blades of a different form from the plates 24 might be provided to lead the sampler frame into and through the material on the belt and confine this material within the sampling zone. In some embodiments the axis of the shaft 22 may not be parallel to the direction of movement of the belt 16. In this case the angle of obliquity of the pates 24 to the shaft 22 will differ from that in the embodiment illustrated, and may even be zero degrees.

Claims: A method of sampling material from a moving belt conveyor (10) which includes a conveyor belt (16) which is troughed in transverse relationship to its longitudinal direction of motion (A), the method of sampling including the steps of periodically passing a pair of spaced plates (24) secured in oblique relationship to a rotatable shaft (22) arranged above the belt (16) in parallel relationship to its direction of motion (A) so that the plates (24) pass across the belt to confine material in a sampling zone (50) extending obliquely across the belt when the shaft (22) is rotated about its longitudinal axis, and displacing the material in this sampling zone (50) off the belt (16) while the plates traverse the belt, characterised in that the belt (16) is longitudinally supported in a region beneath the sampling zone (50) in order to keep the portion of the belt (16) from which material is being sampled substantially straight in the direction of motion (A) of the belt (16), while maintaining its transverse troughing. A sampler for removing material from a moving belt conveyor (10) which includes a conveyor belt (16) which is troughed in transverse relationship to its longitudinal direction of motion (A), the sampler including a frame (19) which includes a pair of spaced plates (24) having leading edges. (40), the plates being secured in oblique relationship to a rotatable shaft (22) arranged above the belt (16) in parallel relationship to its direction of motion (A), the plates (24) being adapted to confine material between each other in a sampling zone (50) which extends obliquely across the belt, characterised in that a support means (52) is provided for longitudinally supporting the belt (16) below the sampling zone (50) for keeping the belt (16) substantially straight in its direction of motion (A), while maintaining its transverse troughing. A method of sampling material according to claim 1 characterised in that the plates (24) are secured to a tubular shaft (22). A method of sampling material according to claim 3, characterised by the step of directing air from the interior of the shaft (22) through perforated tubes (46) into the sampling zone (50) to agitate the material being displaced off the belt (16). A sampler according to claim 2, characterised in that a guard (56) extending adjacent to a lateral edge (54) of the belt (16) is connected to the support means (52) to prevent the plates (24) from lifting the edge (54) and damaging the belt (16), when first approaching the edge (54) of the belt (16). A sampler according to claim 2 or claim 5, characterised by agitating means (46) which receives air from the interior of the shaft (22) for directing air into the sampling zone (50) during use to agitate the material being displaced off the belt (16).

Inventors: ENGLISH RALPH HENRY, VAN DER MERWE EUGENE, ENGLISH, RALPH HENRY, VAN DER MERWE, EUGENE
Applicants: JOHANNESBURG CONS INVEST, JOHANNESBURG CONSOLIDATED INVESTMENT COMPANY LIMITED
Application year: 1990
Main classification: G01N1
IPC codes: G01N1