WO1994008220A1

WO1994008220A1 - Device for measuring the flow properties of particulate materials

Info

Publication number: WO1994008220A1
Application number: PCT/NO1993/000135
Authority: WO
Inventors: Gisle G. Enstad; Gisle F. Knutsen; Lars P. Maltby
Original assignee: Powder Science And Technology Research A/S
Priority date: 1992-10-02
Filing date: 1993-09-09
Publication date: 1994-04-14
Also published as: NO923867L; NO179652B; NO179652C; NO923867D0; AU5158393A

Abstract

It is disclosed a device for measuring flow properties in particulate materials after uniaxial consolidation, said device comprising a frame (1, 2, 3, 4) wherein there is mounted a movable piston (14) which may be moved through a sample chamber in a removable matrix (15) to compact a sample of a particulate material (21). The device comprises further a measurement apparatus to register the movement of the movable piston (14) as well as other measurement instruments (20) for registering the power being applied to the sample of the particulate material.

Description

Device for measuring the flow properties of particulate materials.

Background for the invention: The present invention concerns a device for measuring the flow-properties in particulate materials. The device according to the invention is especially suited to measure axial tension and deformation and optionally also radial tension in the particulate material to determine its folw- properties during uniaxial consolidation.

Fon industry using, handling or producing particulate materials such as cement, sand, granulates etc., the flow properties of the particulate materials are often of imperative importance. Many industries dealing with such industrial branches have a need for controlling the flow properties of the particulate materials, and for this end it has up till now been used different types of complicated equipment being hampered by lacks such as depandence of the operator, imprecision and measurement data with varying reliability.

It is thus known from US patent 4.616.508 and GB patent 2.158.951 as well as EP patent 0.042.598 devices for measuring the flow propertis in particulate materials, but these are complicated devices being based on measurements of pressure across partitions or membranes, and may be hampered with sources for errors associated with inhomogenous packing of the particulate material, or these devices may be very time-consuming and complicated to operate.

The need for a new, simple and quick apparatus for measuring flow properties with little operator dependence is thus present. Such a type of a device must be fast enough for it being present as a natural part of a quality control for the particulate material and it must also be based on the correct physical principles. ith "correct physical principles" is meant the powder is examined under conditions

SUBSTITUTE SHEET resembling closely those being present in the actual production or treatment process. The apparatus for the examination must also be sensitive for small variations in the fluidity and flow properties of the particulate material.

General principle for the invention:

The basis for the present invention is a cylindrical sample of the particulate material being examined (see fig. la nd fig. lb showing a description of the flow properties at a) consolidation under axial main tension σ*-_ and b) determi¬ nation of power f_c) . The sample is subjected to an axial consolidation tension, σ^ causing a deformation _ __ . A cylindrical matrix enveloping the sample makes it impossible for the sample to expand in a radial direction so that the radial deformation, σ₃, equals zero. The axial tension will cause a radial tension acting against the cylinder wall, and this will be lower than the axisal consolidation tension, σ-*_ (σ**L > σ₃) . This makes the sample as a total to be com- pacted. This type of consolidation where the compacting is performed in one direction without deformation in other directions, is called an uniaxial consolidation. When the sample has been consolidated at the chosen value for σ**_ during a predetermined time, the axial tension is according to the invention reduced to zero.

For the sample to be packed homogenously during the consolidation, it is important to minimalize the friction between the cylinder wall and the particulate material. In the device according to the invention this may as an example be performed by using an elastical rubber membrane and a thin layer of oil, even if other aiding equipment also may be used, e.g. a plastic membrane or similar device. By using this configuration the wall friction may be reduced to a minimum. Special embodiments to achieve this will be given in the detailed description of the invention.

When examining the strength of the sample of the particulate

SUBSTITUTE SHEET material, the cylindrical matrix is removed so that the sample has the opportunity to expand in radial direction. Then the axial tension is again increased until the sample breaks apart at a value f_c. The flow properties are characteized by showing the breakdown force, f_c, as a function of the consolidation tension, σ__ . An example of this is shown in fig. 2 representing a breakdown-function showing the power, f_c, as a function of the consolidation tension σ*_j_, wherein the breaking strength of a particulate material is shown at four different consolidation levels.

Concerning evaluation/presentation of measurement data obtained by the above mentioned consolidation and process according to the invention, these may be represented in several forms which the person skilled in the art may evaluate from case to case. As examples of such a represen¬ tation the following may be mentioned:

1. If a company is to use the measurement data obtained by the process and apparatus according to the invention for e.g. quality control of a particulate product, it will be natural for the person to set a consolidation level (σ-*_) to which the samples are to be compacted. (Choice of such a level may be evaluated by the person skilled in the art.) By examining the material at a constant σ-^-value, there may after a few tests be determined the normal value for the f_c- value. In a standard quality control one will accordingly be able to determine a certain value for f_c which should not be exceeded. If there is performed tests below this value no problems will arise when handling the powder, but if one on the other hand lies above this value there may arise problems e.g. by the powder material then being able to plug exits in inter alia silos etc. by the powder then being able to form self-supporting material bridges or "stoppers".

2. If several types of powders are to be determined according to the invention it will be natural to separate

SUBSTITUTE SHEET the materials into different kohesivity-classes. For this purpose the ratio may be used. In similarity with the example above there is used a constant value for σ . A natural cathegorysation will then be as follows:

°J/^fc ^{< 2}'° Extremely cohesive

2,0 < σ-_]_/f_c < 5,0 Very cohesive

5,0 < σ-_j/fc < 10,0 Cohesive

10,0 < ^i/f_c Little cohesive

3. A third way to evaluate the measurement results found according to the invention, is to perform tests at four consolidation levels (four values for σ-_j . By measuring the consolidation strength it will be possible to find a function describing the consolidation strength as a function of the degree of consolidation at an uniaxial compression corresponding to the well-known flow function. An example of such an exhibition is given in the performance example below.

For the matrix to be easily removable from the sample after consolidation it is given a slightly conical shape. The above given general disclosure is given for a cylindrical sample, but also other shapes such as e.g. mainly cubical or pyramidal may be used without departing from the idea behind the present invention.

Detailed description of an embodiment of the invention: Figure 3 and 4 show an embodiment of the device according to the invention. The device is carried by a frame comprising top 1, bottom 3 and back plates 2 as well as side plates 4. On the top plate 1 there is mounted a wheel 5 by using two axial bearings 6 so that it may be freely rotated. The wheel 5 is in its turn mounted in a trapeezegrooved nut 7 making it possible to elevate and lower a grooved rod 8, said rod 8 being secured to the uppermost of two ball-leads 9,10. Alternatively there may be mounted a motor with a ball screw to move the ball lead 9. The elevatable and

SUBSTITUTE SHEET lowerable grooved rod 8 is connected to a piston rod 11 via the upper lead 9 and via two bearings 12. By using a long pretrating screw 13 a piston 14 is secured to the end of the piston rod 11. The piston 14 may be moved through a matrix 15 having a penetrating and mainly cylindrical bore. The piston 14 may be secured the matrix 15 by using a bail 16 and be libertated from the piston rod 11 by using the penetrating screw 13. When the wheel 5 is rotated back the opposite way, the uppermost ball lead 9 will be elevated and the piston rod 11 and the piston 14 will be separated.

The matrix 15, e.g. being made of transparent PMMA, is in its turn secured to the lower part of the two ball leads 10. The two ball leads 9,10 may be moved vertically along two poles 17 being secured to the back-plate 2. By placing the two ball leads 9,10 on identical poles 17, it will, as long as the mounting is correctly performed, be ensured that there is continuous parallellity between the center of the opening in the matrix 15 and the piston rod 11 with the piston 14. The matrix 15 is placed in a cut out groove in the securing plate on the lower ball lead 10, and may thus not be moved relatively to this ball lead 10.

The matrix may be disassembled from the lower ball lead 10 by using a quick-coupling. Such a coupling may e.g. comprise two bails 26 being clamped over two penetrating bolts 25 in the matrix 15. As mentioned above it is also possible to disassemble the piston 14 from the piston rod 11 and secure this to the matrix 15 by using the bail 16. By truning the wheel 5 back it will be possible to elevate the uppermost ball lead 9 (without piston) . This makes it possible to remove the matrix 15 (with piston) so that the matrix 15 may be filled with the relevant particulate material which is to be examined.

The matrix 15 rests on an end cup 18 which in its turn stands on a platform 19 on which there is located a weighing cell 20. This weighing cell 20 makes it possible to

SUBSTITUTE SHEET register the strength of the force being applied on the powder sample 21 in the test chamber in the matrix 15. In the end cup 18 it will also be possible to register the strain acting in the sample 21 by using a pressure cell (pressure transducer) . Radial tensions may be registered by using stretch pads (tension pads) being mounted on the matrix wall according to a special mounting pattern, optionally by using a pressure transducer mounted directly in the matrix wall.

A detailed figure of the matrix 15 with piston 14 is shown in fig. 5. The matrix 15 comprises e.g. a square block with a penetrating bore. The bore is in this embodiment approximately cylindrical with a weakly incresing diameter towards the bottom. Such a mainly cylindrical bore may have conicity with e.g. 0,5° inclination of the walls, but also other angles may be possible, and with a mainly cylindrical bore the inclination of the walls may lie within the interval 0-5°. As mentioned previously it is not necessary to form the bore mainly as a cylinder, even if this shape is preferred on account of its simplicity, but may also have other geometical shapes, and the inclination of the walls in the test chamber need not be mainly vertical, even if this is preferred. What is necessary, however, is that the above mentioned general states are fulfilled so that the flow properties of the sample 21 of a particulate material may be examined at the specified conditions. The indicated conicity of the walls in the test chamber is present so that the sample more easily may slip off the matrix wall when the matrix 15 is elevated up and past the sample 21 of particu¬ late material.

As mentioned above it is important to obtain minimal friction between the sample 21 of the particulate material and the walls in the matrix 15. If the friction in this area is too large the sample 21 may not be packed ho o- genously within the matrix walls. To avoid this friction it is stretched a membrane 23 from the periphery of the piston

SUBSTITUTE SHEET 14 to the bottom of the matrix 15. Between the membrane 23 and the matrix wall 15 it will preferably be a thin layer of oil or some other kind of lubricant. The inner wall in the matrix 15 is preferably highly polished. When the piston 14 is moved vertically downwards in the matrix 15, the stretch will thus decrease in the membrane 23 and this will follow the deformation to which the sample is subjected. This will contribute to eliminate the friction between the powder and the membrane wall. Towards the bottom of the matrix 15 the membrane 23 is stretched to the lower edge of the matrix as shown in fig. 5. There will not be any relative movement between the membrane 23 and the lower edge of the matrix 15. This is avoided by clamping the membrane along the periphery of the bottom of the matrix by using a securing ring 27.

Oil being present between the membrane and the matrix wall 15, may of course have a tendency to collect towards the bottom of this space. To avoid a too great collection of oil there is present a number, preferably three, small radial bores 28 through the matrix wall 15 in the lowermost part of the matrix 15. Excessive oil may thus escape through these bores 28 and this oil may e.g. be returned through an airing aperture 24 to the top of the matrix 15. When the piston 14 is pressed down into the matrix 15, it is also possible that there are formed air pockets between the membrane and the matrix wall. This air will have a possibility to migrate through the same radial bores 28. Then the air will have a possibility to eascape through an airing hole 24 in the matrix 15. When the sample of the particulate materiale is consolidated under the tension σ**_, air will be expelled from the sample. This air may be led out from the test volume by using a filter 22.

Example of a use of the device according to the invention: To examine the flow properties of a particulate material

(standard powder CRM - 116, certified by CBR (the Community Bureau of Reference) [Akers, R.J. : The Certification of a Limestone Powder for Jenike Shear Testing - CRM 116,

SUBSTITUTE SHEET Loughborough University of Technology, CBR/163/90] com¬ prising limestone powder which is conventionally used for testing apparatuses measuring flow properties of particulate materials, it was used an apparatus as shown in the present figures 3-7 and which is generally described above.

The apparatus according to the invention being used in the disclosed test example, has a total height of 820 mm, a width of 340 mm and a depth of 330 mm. To measure the deformation of the sample for then to be able to calculate the bulk density, there is used a standard micrometer, e.g. Mitutoyo Digimatic 572-300. A deformation measurement device of the type Lucas/Schaevitz L.V.D.T. (Linear Variable Differential Transformer) may also be used. To measure the consolidation tension in the sample and later the strength, there may be used two different methods (in parallell or individually) . One form for registering is a so-called weighing platform (corresponding to a standard weight) . In one embodiment of the measuring device according to the invention this is of the fabrication TEDEA (model no. 1040) . The other form for registering axial tension is done by using pressure measuring device (pressure transducer) in the bottom of the end cup, and this may be a stadard pressure cell or pressure transducer with a diameter equal to the area but not exceeding the smallest diameter of the sample material. Further it is important that the measuring device is not deforemd significantly as a consequence of the acting tension. It is also preferred that the measuring exactness of such a measuring device is relatively high, and that a reasonable deviation of the measuring data for equal measurements should be ± 0,05 kPa.

In the test being examplified above, the initial sample voume is 126 cm³. The sample quantity may of course be larger than this examplified volume, and the volume given as an example is not of critical importance, but should for the examplified test be above 130 cm³.

SUBSTITUTE SHEET The test was initiated by removing the matrix 15 with the seured piston 14 from the rest of the apparatus as explained earlier. The matrix 15 is filled with the particulate material bu turning this upside down and filling the material form the underside. After the matrix 15 has been closed with the end cup 18, the matrix is then mounted back into the measuring apparatus according to the invention by using the clamping bail 26 so that the matrix 15 is standing securely in the lower ball lead 10. The wheel 5 is rotated until the piston rod 11 comes into contact with the piston 14, and the piston is then secured by using the penetrating screw 13 in the piston rod 11. Then the bail 16 securing the the piston 14 to the matrix 15 is removed. Each part of the testing apparatus according to this embodiment of the invention will in this situation have a position as shown in fig. fig. 6.

After this position has been reached, the wheel 5 is then rotated slowly so that the piston 14 is moved slowly downwards and the sample 21 is consolidated. This operation is continued until there has bee achieved the vanted value for the axial tension σ± . The deformation to which the sample is subjected may be read from an e.g. securely mounted micrometer so that the bulk density may be deter- mined. Such a micrometer may e.g. be located with a movable point on the upermost ball lead 9 and with a stationary reference point or reading scale on the back plate 2 of the device so that the relative movement of the piston 14 relative to a constant zero point may be registered.

When the powder sample has been subjected to the axial tension for a predetermined time, the wheel 5 is turned back so that the sample is unloaded from the consolidation tension σ*_j_. How much the wheel must be rotated depends on the elastic properties of the powder sample and on the consollidation level. Then the lower ball lead 10 is moved upwards so that the matrix 15 is clear of the piston 14 and the sample is left standing freely on the end cup 18 without

SUBSTITUTE SHEET any form of radial support. This position of the measuring apparatus according to the invention is shown in fig. 7. The bore in the matrix 15 has, as mentioned previously, a certain conicity and is polished on the inside. Further the membrane 23 is secured to the lower edge of the matrix 15 so that the sample 21 may be liberated from the matrix 15 and the membrane 23 without influencing the sample's 21 properties. The time during which the sample is subjected to a load below the consolidation tension σ-_ is not of decisive importance but should be the same for measurements being performed in a series. It may be advantageous to use a standardized time of e.g. 2 minutes to finish consoli¬ dating the sample, but also longer or shorter periods of time are possible. Such time intervals will be obvious to the person skilled in the art and may additionally be dependent on the type of material which is to be examined. In the present example it is used 2 minutes as consolidation time.

By using a longer period of time it will, depending on the powder, be found that it is achieved a larger strength. This fenomenon is called time consolidation and is caused by several factors, inter alia the building of liquid or solid matter bridges between the particles in the sample. If it accordingly is not interesting to examine this time consolidation, one should not use too long a time between the end of the consolidation and the strength testing. With a "waiting" time of 2 minutes it should be possible to minimalize such a time consolidation.

By again turning the wheel 5 the sample may againg be put under a load. Thereby the sample is deformed by increasing the axial tension pressure until there is obtained a break in the sample 21, and a maximum tension value, f_c, is read from the weighing platform 20. f_c is specific for the strength of the sample after consolidation with the previously applied axial tension α-^.

SUBSTITUTE SHEET To determine the consolidation strength it will, in the case where the sample 21 has a certain and sufficient cohesivity, be possible to register the maximum value for f_c without the sample breaking totally apart, and the weight of the sample may be determined after the test has been performed.

Alternatively the weight of the sample may be determined before the test is started.

It will thus be possible to make corrections for the weight of the sample in the measurements of σ and f_c. It may, as mentioned earlier, e.g. be used a weighing platform or a pressure measuring device (pressure transducer) which will be calibrated before the test is started, it be the consolidation step or the strength measurement step. Since both the pressure cell and the weighing platform are located below the sample, there will in practice have to be added a fraction of the weight of the sample as a correction. This is the case for both the σ-^ and the f_c values.

This addition may in practice be determined in two ways. The simplest way is to add half the weight of the sample (i.e. the amount of this weight in tension at the relevant cross sectional area of the sample (d = 35,4 mm in the present example) ) .

There exists, however, an alternate method for deterining this addition. Since the piston may be moved very accu¬ rately there is a possibility to determine the strength value f_c before the actual sample breaks apart. When the sample breaks apart there will be estabished so-called breaking planes in the sample. By moving the piston very slowly downwards there may be observed where this breaking plane starts. If it is observed that this break starts in the lower part of the sample, there is added the whole weight of the sample in the calculations. If the break starts in the upper part, nothing will be added. Such considerations will, however, be easily made by the person skilled in the art.

SUBSTITUTE SHEET In fig. 8 there is shown results from the test described above with the indicated standard material and with the process and the device according to the invention. Below there is in table 1 given corresponding measurement values for σ*-_ and f_c.

Table 1:

Measurement point. σ-L 6,80 kPa 11,67 kPa 19,59 kPa 31,57 kPa f_c 2.70 kPa 3.82 kPa 5,15 kPa 6.69 kPa

It is, however, important to notice that the process and the device according to the invention may be used for all types of particulate and cohesive materials, and it is in reality found no lower limit for the particle size in such materials which are to be tested. It is, however, found an upper limit for the particle size wich is suitable in the material which is to be examined, but such an upper limit will for a large part depend on what kind of material which is to be examined. It is thus obvious that the properties of the material will have to be considered since there obviously is a difference between e.g. cement and sand, but such a consideration may easily be performed by the person skilled in the art without extensive experimentation. The limit¬ ation for the upper suitable particle size lies generally in the cohesivity of the material of the paricle sample and such a cohesivity limit varies widely from material to material as indicated above.

As an example of the particle size in a material which may be used for measuring the flow properties according to the invention there may be mentioned the limestone powder being used as a standardization powder and which is indicated above. This powder comprises particles whereof 90% is below 6 um. Further 50% of the particles are below 4 urn and 10% are below 2,2 μm in diameter. It will further be

SUBSTITUTE SHEET remarked that this is a powder with a very narrow particle size distribution and other particle size distributions as well as other types of material may of course be measured with the process and the device according to the invention. (Another example of a material which may be measured is cement having a paricle size wherein 90% is below 20 urn, 50% is below 11 um and 10% is below 2 μτa. in diameter.)

Alternate fields of use for the device according to the invention:

The device according to the invention as well as the use thereof has above been disclosed in connection with a special embodiment for the examination of the flow proper¬ ties of particulate materials. The disclosed principle and the divice making use thereof gives a quick indication of the properties of materials and provides an opportunity to distinguish beween relatively small variations in the behaviour of the materials. The device and the measuring method will thus be excellently suited for quality control of particulate materials and the device may also have a wide field of use in industry handling and producing particulte materials. The device is, as previously mentioned, excellently suited for measuring time consolidation effects.

The presently disclosed principle and the device for measuring friction-free deformation of material samples may, however, also be used in other connections where it is important to achieve a homogenous bulk density throughout the complete material sample. As examples thereof it may be mentioned packing of chromatograph colou ns and tabletation of different kinds of products such as medications etc.

Furthermore the mesuring device according to the invention is suited for determining powder-mechanical properties such as tension relaxations and creep properties, said properties being to a certain degree related to the elastic and elasto- plastic properties of the powder material. Additionally it is furthermore possible to examine the effect of the loading

SUBSTITUTE SHEET speed on the strength of the powder sample. All these properties are important to determine to understand the behaviour of the particulate materials during production and handling.

SUBSTITUTE SHEET

Claims

C l a i m s

1. Device for dermining the flow properties of particulate materials, c h a r a c t e r i z e d i n that it comprises a frame

(1,2,3,4) wherein there is mounted a movable piston (14) which may be passed through a sample chamber in a removable matrix (15) to compact and test under pressure a sample of a particulate material (21) , said device further comprising a measuring device to register the deformation of the sample

(21) as well as other mesuring devices (20) to register the tensions being applied to the sample of the particulate material.

2. Device according to claim 1, c h a r a c t e r i z e d i n that the sample chamber in the matrix (15) has smoothly polished walls and also preferably includes a lubricnat-coated inner membrane (23) secured to the lower edge of the piston (14) and the lower edge of the matrix (15) .

3. Device according to claim 1 or 2, c h a r a c t e r i z e d i n that the sample chamber in the matrix (15) has a mainly cylindrical shape with a lower diameter larger than the upper diameter and where teh walls has in inclination angle, preferably of 0,5°.

4. Device according to claim 1 - 3, c h a r a c t e r i z e d i n that the device for moving the piston (14) e.g. comprises a wheel (5) connected for influencing the piston (14) via a piston rod (11) running i lead devices (9,10,17) and being influenced by the wheel (5) via a treaded rod (8) .

5. Device according to claim 1 - 4, c h a r a c t e r i z e d i n that the lower ball lead (10) together with the matrix (15) may be elevated along the axes (17) after consolidation of the sample (21) so that the

SUBSTITUTE SHEET sample (21) may be subjected to further testing without the surrounding matrix (15) .

6. Device according to claim 1 - 5, c h a r a c t e r i z e d i n that the removable matrix

(15) is secured by using removable clamping bails (26) being mounted to a lower ball lead (10) for the piston rod (11) via a fast coupling.

SUBSTITUTE SHEET AMENDED CLAIMS

[received by the International Bureau on 7 March 1994 (07.03.94); original claims 1-6 replaced by amended claims 1-6; new claims 7,8 added; (2 pages)]

1. Device for determining the flow properties of powders, comprising a frame (1,2,3,4) wherein there is mounted a movable piston ( 14 ) which may be pressed through a sample chamber in a removable die (15) to compact and test under pressure a sample of a powder ( 21 ) , as well as other mea¬ suring devices (20) to register the tensions being applied to the powder sample, c h a r a c t e r i z e d i n that the sample chamber in the die (15) has smoothly polished walls, which are downwards inclined forming a cross section which is narr¬ owing upwards in the die and expanding downwards in the die.

2. Device according to claim 1, c h a r a c t e r i z e d i n that the sample chamber has a mainly cylindrical shape.

3. Device according to claim 1 or 2, c h a r a c t e r i z e d i n that the walls of the sample chamber has an inclination of at least 0,1°.

4. Device according to any of the previous claims, c h a r a c t e r i z e d i n that the sample chamber in the die (15) is provided with a lubricant-coated inner membrane (23) .

5. Device according to claim 4, c h a r a c t e r i z e d i n that the membrane (23) is stretched from the periphery of a piston (14) in the die (15) to the bottom edge of the die (15).

6. Device according to any of the previous claims, c h a r a c t e r i z e d i n that the die ( 15 ) is arranged to a lower ball lead (10), which may be elevated along shafts (17) after consolidation of the sample (21) in the die (15)

7. Device according to any of the previous claims, c h a r a c t e r i z e d i n that the die (15) is removably secured through clamping bails (26) being moun¬ ted to the lower ball lead (10) for a piston rod (11) via a quick-coupling.

8. The use of a device according to any of the claims 1 7 for testing powders for finding the time-consolidation properties of the relevant powder.