System and apparatus for the measurement of particle size distribution

Abstract

An apparatus and system for automatically determining the particle size distribution of a sample, such as an ore concentrate. The apparatus includes a vertically movable frame, actuator, and at least one load cell for measuring the weight of sieves in a stack before and after sample processing. The sieves are configured to be lifted and separated for individual weighing. A controller determines the particle size distribution based on the weight difference and known density. The sieves are adapted for automated handling and include fluid apertures and a closable discharge opening.

Description

SYSTEM AND APPARATUS FOR THE MEASUREMENT OF PARTICLE SIZEDISTRIBUTIONBACKGROUND OF THE INVENTION

[0001] This invention relates generally to the measuring of the particle size distribution of various materials and more particularly, to a system for measuring the particle size distribution of ores in mineral processing.

[0002] In mineral processing, particle size measurement is used to keep track of the comminution quality, to liberate concentrate, and to reduce energy and material losses in the plant. Laser diffraction, dynamic light scattering, and optical and image analysis techniques are preferred as these can take rapid and accurate particle size measurements. These devices are suitable for measuring particles in the range of 0.2 to 500 pm. However, these devices are expensive to purchase and to maintain and therefore increase the CAPEX and OPEX of the process.

[0003] To overcome the high cost, a sieve analysis technique is often used. The sieve analysis technique determines the material’s particle size distribution by separating fine particles from coarse particles in a sample.

[0004] Wet and dry sieving are two forms of sieve analysis techniques that can be performed. Wet sieves are appropriate for particle sizes ranging from 20 pm to 3mm, while dry sieves are suitable for particle sizes ranging from 30 pm to 125mm. The material to be analysed is vibrated through a sequence of sieves with successively decreasing mesh sizes using a single motion or a combination of horizontal, vertical, and circular motions to ensure complete dispersion of the particles. The particles will orient themselves and land between the screens with aperturesbigger and smaller than their equivalent spherical diameter. A cumulative mass distribution of material is then obtained by adding the mass fractions on the screens, from the smallest to the greatest mesh size. The particle mass fraction is measured and weighed to generate a cumulative particle size distribution. A generated particle size distribution curve gives information about the dominating particle size range and the material can then be classified as fines, silt, sand and coarse material.

[0005] Although cost effective, the sieving technique is labour intensive as it requires repeated stacking and unstacking of the sieve screens and the manual weighing thereof. This is also time consuming and therefore increases the response time of any changes which may be required in a milling circuit. Additionally, for quality control purposes, consistent and repeatable results are imperative. This is difficult to achieve in a manually operated system.

[0006] In mineral processing, a typical sample of ore or concentrate to be analysed is in a slurry phase and a wet sieving technique is required. A problem encountered with this technique is the inability to measure sub-sieve particles and the difficulty in cleaning the mesh screens after each test cycle.

[0007] The invention aims, at least partly, to address the aforementioned issues.SUMMARY OF INVENTION

[0008] The invention provides an apparatus for use in automatically measuring the particle size distribution of a material in a sample, wherein the apparatus includes a support structure, a frame which is engaged with the support structure, and which is vertically moveable relative to the support structure, an actuator means for causing movement of the frame relative to thesupport structure, and at least one load cell engaged with the frame for measuring weight applied thereto.

[0009] The sample may be in the form of an ore concentrate, such as a slurry. However, the invention is not limited to this phase or form.

[0010] The frame may include a body and a plurality of lifter members mounted to the body at spaced apart locations on the body.

[0011] The actuator means may be a stepper motor which drives a linear actuator on the support structure to cause the vertical movement of the frame.

[0012] The apparatus may be operably engaged with a programmable logic controller.

[0013] In a first form of the invention, each lifter member is in the form of a fork which includes a pair of generally horizontal tines. A plurality of load cells is provided at spaced apart locations on the body of the frame to measure respective weights applied thereto. The plurality of load cells may be mounted to the body of the frame or to the fork members. Preferably each load cell is engaged with a respective fork member.

[0014] In use of the apparatus, the tines of a fork member are engaged with a respective sieve in a stack of sieves. The sieves are stacked in accordance with decreasing mesh size. Actuation of the actuator means causes the frame to move vertically upwardly, thereby lifting the fork members and sieves. The fork members are spaced apart by a distance greater than the height of each sieve. This ensures that the sieves may be separated by the respective fork members to measure the weight of each sieve.

[0015] In this way, each sieve exerts a weight on a respective fork member which is recorded by the associated load cell as a first weight for a particular sieve.

[0016] After measuring the first weight, a sample of material to be measured e.g. a sample of an ore concentrate in a slurry phase is placed on an upper sieve in the stack of sieves, and a sieve shaker agitates the stack. The agitation causes the sieved portions of the sample to move through the succession of sieves such that the sample is split into a succession of groups or fractions of the sample wherein the groups, moving from an uppermost group to a lowermost group, contain particles with sizes in progressively smaller size ranges. Thus, due to the agitation, the particles land between the sieves with mesh apertures bigger, and smaller, than their equivalent spherical diameter.

[0017] The apparatus may then be used to measure a second weight of the sieves, including the particles in each group retained on each sieve, in the same manner as set out above. Each second weight is stored in the controller.

[0018] The particle size distribution of the material in the sample may then be calculated by the controller using the difference between the second and first weights of the sieves to provide a measure of the mass of material on each sieve (the mass of the respective group of particles) and the known density for the material in the sample. A particle size distribution curve may then be generated for the sample.

[0019] According to a second form of the invention, the body of the frame includes first and second opposed sides and a third side extending between said first and second opposed sides. Corresponding lifter members are provided on an inner surface of the first and second opposed sides along a length thereof. A load cell is engaged with the third side at an upper end of thebody to measure the weight applied to the body. The load cell may be mounted to the support structure by means of a load cell shelf.

[0020] In use of the apparatus according to the second form of the invention, the corresponding lifter members on the first and second opposed sides of the body are engageable with a respective sieve in a stack of sieves. The sieves are stacked in accordance with decreasing mesh size. The body exerts a weight which is measured by the load cell. Actuation of the actuator means causes the frame to move vertically upwardly by a first distance, thereby lifting the body until respective corresponding lifter members are engaged with a first sieve in the stack of sieves, lifting the sieve out of engagement with the stack of sieves. The weight exerted by the body is then recorded by the load cell. The difference between the weight of the body and the weight of the body and first sieve provides a measure of the weight of the first sieve. The frame is then caused to move vertically upwardly by a second distance which is greater than the first distance. This causes corresponding lifter members to engage with the first sieve and a second sieve in the stack of sieves. The weight exerted by the body on the load cell is then measured. From these measurements, the weight of the second sieve in the stack of sieves can be calculated. This process is repeated, wherein the frame is caused to move vertically upwardly in increasing increments of distance until all of the sieves in the stack have been measured. In this way, a first weight for each sieve is calculated and stored in the controller.

[0021] After measuring the first weight, a sample of material to be measured is placed on an upper sieve in the stack of sieves, and a sieve shaker agitates the stack. The agitation causes the sieved portions of the sample to move through the succession of sieves such that the sample is split into a succession of groups or fractions of the sample wherein the groups, moving from an uppermost group to a lowermost group, contain particles with sizes inprogressively smaller size ranges. Thus, due to the agitation, the particles land between the sieves with mesh apertures bigger, and smaller, than their equivalent spherical diameter.

[0022] The apparatus may then be used to measure a second weight of the sieves, including the particles in each group retained on each sieve, in the same manner as set out above. Each second weight is stored in the controller.

[0023] The particle size distribution of the material in the sample may then be calculated by the controller as set out above.

[0024] The invention provides an automated on-line system for measuring the particle size distribution of a material, including: a) means for providing a sample of material to be measured; b) a plurality of sieves wherein each sieve has a mesh screen of a predetermined mesh size and wherein the sieves are stacked vertically in order of decreasing mesh size; c) sieve shaking means to agitate the sieves whereby the agitation causes sieved portions of the sample to move vertically downwardly through the succession of sieves such that the sample is split into a succession of groups or fractions of the sample wherein the groups, moving from an uppermost group and a lowermost group, contain particles with sizes in progressively smaller size ranges. d) an apparatus for measuring the weight of each sieve before the addition of the sample to provide a first weight of the sieve and for measuring the weight of the groups of particles on the sieves to provide a second weight wherein the difference between the weights provides a measure of the mass of the particles in, and of, the groups on the respective sieves; ande) a controller which, using the difference between the second weight and the first weight obtained from each sieve and the known density of the sample to be measured, calculates the size distribution of the particles in each sieve and generates a particle size distribution curve for the sample.

[0025] The system may include means to measure the density of the material in the sample which includes a vessel having an internal volume for receiving the sample to be measured, the vessel including an automatic discharge valve, an optional level sensor to determine the volume of the sample in the vessel and a load cell for measuring the weight of the vessel prior to the sample being added to the vessel to determine a tare weight of the vessel and once the sample has been added to the vessel to provide the weight of the sample.

[0026] The measured volume and respective weights of the vessel and the sample may be recorded in corresponding tags in the controller to calculate a density value of the material.

[0027] The vessel may be a conical cylinder.

[0028] Each sieve may include a frame having a base and first and second opposed sides and third and fourth opposed sides engaged with the base. The base may define an opening which is engageable with a mesh screen. The mesh screen may be formed from a nylon material for ease of cleaning. The mesh screens may have apertures sizes in the range of 30pm to 850 pm.

[0029] The frame may include respective flanges on the first and second opposed sides thereof.

[0030] One or more apertures may be provided in the third side of each frame to provide an opening through which water may be sprayed using a suitable nozzle. An opening is provided in the fourth side of the frame and a retractable closure may be provided to close the opening,according to requirement. The opening may be closed using the closure when the stack of sieves is agitated. The closure may be moved into a retracted position to allow the material in the sample to be washed from the mesh screen.

[0031] The controller may be any suitable programmable logic controller.

[0032] The apparatus may be of the kind described above and may include a support structure, a frame which is engaged with the support structure, and which is vertically moveable relative to the support structure, an actuator for causing vertical movement of the frame relative to the support structure, and at least one load cell to measure weight applied thereto.

[0033] The frame may include a body and a plurality of lifter members mounted to the body.

[0034] A first weight of each sieve before the sample of material is added may be measured by the apparatus in the manners described above. Once the sample has been added to the stack of sieves and the stack is agitated for a period of 3 to 5 minutes, the apparatus measures the second weight of each sieve including the mass of the respective group of particles retained on the screen in the manner described above. The first weight and the second weight of each sieve is recorded in the controller.

[0035] Once the second weight has been recorded, the sieves may be cleaned using one or more water nozzles in communication with the one or more apertures in the third side of the sieve frame. The material may be washed from the screen and may pass through the opening in the fourth side of the frame.

[0036] Using the calculation of the density value for the material in the sample and the mass of material on each sieve i.e. , the mass of the respective groups of the particulate material, thecontroller calculates the percentage of particles passing through each sieve and a particle size distribution curve for the material in the sample is then generated.

[0037] The system is preferably carried out continuously. Each cycle may take approximately 10 minutes.DESCRIPTION OF THE DRAWINGS

[0038] The invention is further described by way of examples with reference to the accompanying drawings in which:Figure 1 is perspective view of an apparatus according to one form of the invention;Figure 2 is a side view of the apparatus in Figure 1 and a stack of sieves engaged with the apparatus according to the invention;Figure 3 is a side view of an apparatus according to a second form of the invention;Figure 4 is perspective view of an apparatus according to a second form of the invention and a stack of sieves engaged with the apparatus according to the invention;Figure 5 is a perspective view of a sieve for use with the apparatus according to the invention; andFigure 6 is a block diagram of a system according to the invention.DESCRIPTION OF PREFERRED EMBODIMENT

[0039] Figure 1 is a perspective view of an apparatus 10 according to one form of the invention. The apparatus includes a support structure 12, and a frame 14 which is secured to the structure 12 via an intermediate block and which is vertically moveable relative to, thesupport structure 12. The frame 14 includes a body in the form of vertical support 16 and a plurality of lifter members (20) in the form of fork members 20A, 20B, 20C, 20D, 20E at spaced apart locations on the support 16. Each fork member 20 has a pair of generally horizontal tines 22A, 22B. A respective load cell 18 is associated with each fork member 20. The load cells 18 are used to measure respective weights applied to each fork member 20A, 20B, 20C... 20E.

[0040] An actuator means 24, in the form of a stepper motor and a linear actuator on the support structure 12, is operable to cause vertical movement of the frame 14. The stepper motor drives the mechanism on the linear actuator and causes the frame 14 to move vertically relative to the support structure 12. The mechanism of the linear actuator is well known in the art and will not be described further herein.

[0041] In use, the apparatus 10 engages with a stack of sieves 26 - see Figure 5. The stack comprises a plurality of vertically stacked sieves 28. As shown in Figure 5, each sieve 28 includes a frame 30 which includes a base 32 with opposed first and second sides 34 and 36 and opposed third and fourth sides 38 and 40. The base 32 defines an opening 32A for receiving a mesh screen 42 of a predetermined mesh size. Preferably the mesh size is in the range of 30 pm to 850 pm. The first and second opposed sides 34 and 36 terminate in respective flanges 44 and 46.

[0042] An aperture 50 is provided in the third side 38 of the frame 30 to provide access to one or more water nozzles (not shown). An opening 48 is provided in the fourth side 40 of the frame 30.

[0043] The sieves 28 are stacked in order of decreasing mesh size with the sieve having the largest mesh size at the top of the stack 26 and the sieve having the smallest mesh size at the bottom of the stack 26.

[0044] The apparatus 10 is used to obtain a first weight for each sieve 28, before a sample of material is added to the stack 26. Actuation of the stepper motor 24 causes actuation of the linear actuator 12 and frame 14 to move vertically upwardly. The spacing h between the fork members 20A, 20B, 20C...20E is greater than the height hi of each sieve 28. In this way, as the frame 14 moves vertically upwardly, the tines 22A and 22B of each fork member 20A, 20B, 20C...20E engage with respective flanges 44 and 46 on the first and second opposed sides 34 and 36 of the sieves 28 and each sieve 28 may be lifted separately. Once the frame 14 has been moved a desired distance and the sieves 20 have been lifted, each load cell is able to measure the weight of each respective sieve. The frame 14 is then caused to move vertically downwardly to return to its original starting position. The first weight for each sieve is stored in a tag in a controller (not shown), operatively engaged with the apparatus 10.

[0045] Once the first weight has been measured for each sieve 28, a sample of material is directed to a top of the stack 26 i.e., onto the sieve with the largest mesh apertures. The stack is agitated using a suitable sieve shaking means (not shown) thereby causing the material to move vertically downwardly through the stack 26 of sieves 28. The particles of the sample land between the mesh screens 42 of the sieves 28 with apertures bigger and smaller than their equivalent spherical diameter. In this way the sample is divided into groups or formations with the particles in each group being progressively smaller in size than in the upper preceding groups. Once the particles have settled, the apparatus 10 is used to obtain a second weight for each sieve 28 in the manner described above. The second weight of each sieve 28 is stored in a tag in the controller.

[0046] The difference between the first and second weights for each sieve 28, provides a measure of the mass of particles retained in the groups or fractions of particles on the respective sieves 28. Using the calculated mass for each sieve of a predetermined mesh size and theknown density of the material in the sample, the controller determines the percentage (by mass) of particles passing through each sieve and a particle size distribution curve can be provided for the material in the sample.

[0047] Figure 3 is a side view of an apparatus 10A according to a second embodiment of the invention. In this embodiment, a single load cell is used to measure the respective weights of the sieves in a stack of sieves. The apparatus 10A is in many respects the same as the apparatus 10 and therefore like features will include like reference numerals. The apparatus 10A includes a support structure 12A, and a frame 14A which is secured to the structure 12A via a shelf, and which is vertically moveable relative to, the support structure 12A. The frame 14A includes a body 16A with first and second opposed sides 52 and 54 with a third side 56 extending between the first and second opposed sides 52 and 54. A plurality of lifter members 20A, 20B, 20C...20F are provided on inner surfaces 58 and 60 of the respective opposed sides 52 and 54.

[0048] A load cell 18A is engaged with the third side 56 at an upper end 60 of the body to measure the weight applied to the body. The load cell 18A is mounted to the support structure by means of a load cell shelf 62.

[0049] An actuator means 24A, in the form of a stepper motor and a linear actuator on the support structure 12A, is operable to cause vertical movement of the frame 14A. The stepper motor drives the mechanism on the linear actuator and causes the frame 14A to move vertically relative to the support structure 12A.

[0050] In use of the apparatus 10A, the corresponding lifter members 20A and 20B, 20C and 20D, 20E and 20F on the first and second opposed sides 52, 54 of the body 16A are engaged with a respective sieve 28A in a stack of sieves 26A. The sieves are of the kind illustrated inFigure 5. The sieves 28C are stacked in accordance with decreasing mesh size. The body 16A exerts a weight which is measured by the load cell 18A. Actuation of the actuator means 24A causes the frame 14A to move vertically upwardly by a first distance d, thereby lifting the body 16A until respective corresponding lifter members 20A and 20B are engaged with a first sieve 28A in the stack of sieves 26, lifting the sieve out of engagement with the stack of sieves 26. The weight exerted by the body 16A is then in then recorded by the load cell 18A. The difference between the weight of the body 16A and the weight of the body 16A and first sieve provides a measure of the weight of the first sieve. The frame is then caused to move vertically upwardly by a second distance d2 which is greater than the distance d1. This causes corresponding lifter members 20C, 20D to engage with t a second sieve 28A in the stack of sieves 26A. The weight exerted by the body 16A on the load cell 18A is then measured. From these measurements, the weight of the second sieve 28A in the stack of sieves 26A can be calculated. This process is repeated, wherein the frame is caused to move vertically upwardly in increasing increments of distance (i.e. wherein d1 < d2 < d3) until all the sieves 28A in the stack 26A have been measured. In this way, a first weight for each sieve 28A is calculated.

[0051] The frame 14A is then caused to move vertically downwardly to return to its original starting position. The first weight for each sieve is stored in a tag in a controller (not shown), operatively engaged with the apparatus 10A.

[0052] Once the first weight has been measured for each sieve 28A, a sample of material is directed to a top of the stack 26A i.e., onto the sieve with the largest mesh apertures. The stack is agitated using a suitable sieve shaking means (not shown) thereby causing the material to move vertically downwardly through the stack 26A of sieves 28A. The particles of the sample land between the mesh screens 42A of the sieves 28A with apertures bigger and smaller than their equivalent spherical diameter. In this way the sample is divided into groups or formationswith the particles in each group being progressively smaller in size than in the upper preceding groups. Once the particles have settled, the apparatus 10A is used to obtain a second weight for each sieve 28A in the manner described above. The second weight of each sieve 28A is stored in a tag in the controller.

[0053] The particle size distribution of the material in the sample may then be calculated by the controller as set out above.

[0054] Figure 5 is a block diagram of an automated on-line system 100 for measuring the particle size distribution of a material using the apparatus of the invention. The system 100 is operated using a suitable programmable logic controller 102.

[0055] The system 100 includes means 104 for providing a slurry sample 106 having a mass in the range of (100g to 2 kg) of a material e.g., ore or concentrate to be measured. The means can be of any suitable form e.g., an automatic riffle sampler of the kind known in the art, and the invention is not limited in this respect. The use of a riffle sampler ensures that the sample of material selected for measurement is representative of a bulk sample of the material, ensuring more accurate results.

[0056] The density of the material in the sample 106 is then measured (108) using a vessel with an automatic discharge valve, an ultrasonic level sensor to determine the volume of the sample in the vessel, and a load cell for measuring the weight of the vessel. The measured volume 110 and respective weights 112 of the vessel and the sample may be recorded in corresponding tags in the controller 102 to calculate a density value of the material.

[0057] The sample 106 is then caused to move to a stack of sieves 114 wherein the sieves are stacked in a vertically descending order of decreasing mesh size. The sieves are of the kind illustrated in Figure 3.

[0058] The weight of each sieve is then measured (116) using an apparatus 10 according to the invention in the manner described above. A first weight 118 of each sieve (i.e. , the weight before the sample is added) is measured and recorded in the controller 102. Agitation of the stack of sieves 108 for a period of 3 to 5 minutes causes the sample to move vertically downwardly and, due to the sieving action, the sample is divided into fractions wherein the particles in one fraction have a size range which is larger than the size range of the particles in an adjacent lower fraction. The particles of the sample land between the mesh screens of the sieves in the stack 108 with apertures bigger and smaller than their equivalent spherical diameter. Once the particles have settled, the apparatus 10 is used to obtain a second weight 120 for each sieve in the manner described above. The second weight 120 of each sieve is stored in a tag in the controller 102.

[0059] Once the second weight 120 has been recorded, water nozzles (not shown) are aligned with the aperture in the fourth side of the sieve frame and water is sprayed to clean 122 the material from the mesh screens. The material is washed from the sieves and passes through the opening in the third side of the sieve frame.

[0060] Using the first weight 118 and second weight 120 for each sieve, the controller 102 calculates the mass of particles in each fraction retained on the respective sieves. This mass, together with the measurement of the density 108 of the material in the sample 106 is used to calculate the percentage of particles passing through each sieve and a particle size distribution curve can be generated for the sample.

[0061] The apparatus 10 and system 100 provide a cost-effective solution to measure the particle size distribution of the material in a sample. The use of the apparatus 10 embodies an automated sieving technique which allows a larger sample of material to be measured. This improves the accuracy of the results and allows a wide range of particle sizes to be measured. Furthermore, each cycle completed by the system takes approximately 10 minutes, which is a vast improvement on the prior art manual techniques thereby ensuring a quick response time for any adjustments which may need to be made in a mineral processing plant and improving the OPEX of the process.

Claims

CLAIMS1. An apparatus (10) for use in automatically measuring the particle size distribution of a material in a sample, wherein the apparatus (10) includes a support structure (12), a frame (14) which is engaged with the support structure (12), and which is vertically moveable relative to the support structure (12), an actuator means (24) for causing movement of the frame (14) relative to the support structure (12), and at least one load cell (18) engaged with the frame (14) for measuring weight applied thereto.

2. An apparatus (10) according to claim 1 , wherein the frame (14) comprises a body (16) and a plurality of lifter members (20) mounted to the body (16) at spaced apart locations.

3. An apparatus (10) according to claim 2, wherein each lifter member (20) comprises a fork having a pair of generally horizontal tines (20A, 20B) configured to engage a respective sieve (28) in a stack of sieves (26).

4. An apparatus (10) according to claim 3, comprising a plurality of load cells (18), each load cell (20) being associated with a respective fork member (20) for measuring the weight of a respective sieve (28) engaged by the fork member (20).

5. An apparatus (10) according to any one of claims 1 to 4, wherein the actuator means (24) comprises a stepper motor configured to drive a linear actuator mounted on the support structure (12).

6. An apparatus (10) according to any one of claims 1 to 5, wherein the frame (14) is operably engaged with a programmable logic controller.

7. An apparatus (10) according to claim 4, wherein the sieves (28) are stacked (26) vertically in order of decreasing mesh size and are configured to be agitated to separate the particles of the sample into fractions based on particle size.

8. An apparatus (10) according to claim 7, which is configured to measure a first weight of each sieve (28) prior to addition of the sample, and a second weight after particle separation, wherein the difference between the first and second weights represents the mass of particles retained on each sieve (28).

9. An apparatus (10A) according to any one of claims 1 to 3, wherein the frame (14A) includes a body (16A) with first and second opposed sides (52, 54) and a third side (56) extending between the first and second sides (52, 54), wherein corresponding lifter members (20A, 20B, 20C) are mounted on inner surfaces (58, 60) of the first and second sides (52, 54), and a load cell (18A) is engaged with the third side (56) to measure the total weight applied to the frame (14A).

10. An apparatus (10A) according to claim 9, wherein actuation of the actuator means (24A) incrementally raises the frame (14A) to successively engage lifter members (20A, 20B, 20C) with respective sieves (28A) in a stack of sieves (26A), such that the load cell (18A) records increasing cumulative weights allowing for calculation of individual sieve (28A) weights.

11. An apparatus (10, 10A) according to claim 10, wherein each sieve (28, 28A) includes a frame (30) having a base (32) and first (34), second (36), third (38), and fourth (40) sides, the base (32) defining an opening (32A) configured to receive a mesh screen (42), and the mesh screen (42) having apertures in the range of 30 pm to 850 pm.

12. An apparatus (10, 10A) according to claim 11 , wherein the first and second sides (34, 36) of the sieve frame (30) include respective flanges (44, 46) configured to be engaged by the lifter members (20).

13. An apparatus (10, 10A) according to any one of claims 11 or 12, wherein the mesh screen (42) is formed from nylon.

14. An apparatus (10, 10A) according to any one of claims 11 to 13, wherein the third side (38) of the sieve (28) includes at least one aperture (50) configured to receive fluid from a cleaning nozzle.

15. An apparatus (10, 10A) according to any one of claims 11 to 14, wherein the fourth side (40) of the sieve (28) includes a closable opening (48) with a retractable closure, the closure being closed during agitation of the sieves (28) and opened during washing of the sieves (48).

16. An apparatus (10, 10A) according to any one of claims 1 to 15 wherein the sample is in the form of an ore concentrate.

17. An automated on-line system (100) for measuring the particle size distribution of a material, including:(a) means (104) for providing a sample of material (106) to be measured;(b) a plurality of sieves (114), each sieve having a mesh screen of a predetermined mesh size, the sieves being stacked vertically in order of decreasing mesh size;(c) sieve shaking means configured to agitate the sieves such that the agitation causes sieved portions of the sample to move vertically downward through the succession of sieves, thereby splitting the sample into a succession of groups or fractions containing particles with progressively smaller size ranges from an uppermost group to a lowermost group;(d) an apparatus (10, 10A) according to any one of claims 1 to 15 configured to measure (116) the weight of each sieve (114) before addition of the sample to provide a first weight (118) of the sieve and to measure the weight of the groups of particles on the sieves to provide a second weight (120), wherein the difference between the second (120) and first (118) weights provides a measure of the mass of the particles in the respective sieves; and(e) a controller (102) configured to calculate, based on the difference between the second (120) and first (118) weights for each sieve (114) and a known density of the sample, the size distribution of the particles in each sieve and to generate a particle size distribution curve for the sample.

18. A system (100) according to claim 17, further comprising a density measuring unit (108) including a vessel for receiving the sample, the vessel being provided with an automatic discharge valve, a load cell for measuring the weight (112) of the vessel and sample, and optionally a level sensor for measuring sample volume (110).

19. A system (100) according to claim 18, wherein the vessel is a conical cylinder.

20. A system (100) according to any one of claims 17 to 19, configured to perform continuous measurement cycles of approximately 10 minutes each.

21. A system (100) according to any one of claims 17 to 20 wherein each sieve (114) is a sieve according to any one of claims 11 to 15.

22. A system (100) according to any one of claims 17 to 21 wherein the sample of material(106) is an ore concentrate such as a slurry.

23. A method of automatically measuring particle size distribution of a material sample, comprising: a) measuring a first weight of each of a plurality of stacked sieves using the apparatus of any one of claims 1 to 16; b) placing a sample of material onto an upper sieve; c) agitating the stack to cause particle separation; d) measuring a second weight of each sieve including retained particles; e) determining the mass of particles retained on each sieve from the difference between second and first weights; and f) calculating and outputting a particle size distribution curve for the sample.

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