NO179652B - Device for determining flow properties of powdered materials - Google Patents

Description

Background for the invention:

The present invention relates to a device for determining the flow properties of powdery materials. In particular, the device according to the invention is suitable for measuring axial stress and deformation, and possibly also radial stress in the powdery material to determine its flow properties during uniaxial consolidation.

For industry that uses, processes or produces particulate materials such as cement, sand, granules, etc., the flow properties of the particulate materials are often of decisive importance. Many companies within such branches of industry need to be able to control the flow properties of the particulate materials, and for this, various types of complicated equipment have been used until now, which are fraught with shortcomings such as operator dependence, inaccuracy and measurement values with varying reliability.

It is thus known from US patent no. 4,616,508 a device for measuring the flow properties of particulate materials. However, with such a device, a relatively large pressure is used to compact the examined material than is the case with the present invention. The disadvantage of this previously known device is that with this it is not possible to examine the flow properties of particulate materials where the material is compacted to a very small degree, as such material would then fall apart before the measurement with this prior technique. This problem is avoided with the device according to the present invention.

Likewise, GB patent 2,158,951 describes a device for measuring the flow properties of particulate materials, but here it is particularly a question of pressure measurement over partitions or membranes, and this may be subject to sources of error that are associated with inhomogeneous packing of the particulate material and the device is here also time-consuming and complicated to use.

In EP patent no. 0,042,598, a device for measuring flow properties is also described, but here the device is electrically driven and is also subject to many of the above-mentioned error sources.

The need for a new, simple, fast and less operator-dependent apparatus for measuring flow properties is therefore present. Such a form of apparatus must be fast enough that it can be included as a natural part of a quality control of the particulate material and must also be based on the correct physical principles. By correct physical principles is meant that the powder is examined under conditions which are approximately the same as those present in the real production or treatment process. The examination apparatus must also be sensitive to small variations in the fluidity and flow properties of the particulate material.

This is achieved by means of a device of the type mentioned at the outset, and whose characteristics appear from the characteristics of claim 1.

General principle of the invention:

The starting point for the present invention is a cylindrical sample of the particulate material to be measured (see fig. 1). The sample is subjected to an axial consolidation stress, a1#, which causes a deformation e1. A cylindrical matrix enclosing the sample makes it impossible for the sample to expand in the radial direction, so that the radial deformation, e3, is equal to zero. The axial stress will cause a radial stress acting against the cylinder wall, and this will be lower than the axial consolidation stress, ax ( a1 > a3) . This means that the sample is overall compacted. This type of consolidation, where compaction occurs in one direction, without deformation in other directions, is called uniaxial consolidation. When the sample has been consolidated at the chosen value for ox over a predetermined time, the axial stress according to the invention is reduced to zero.

In order for the sample to be packed homogeneously during consolidation, it is important to minimize the friction between the cylinder wall and the particulate material. In the device according to the invention, this is done, for example, by means of an elastic rubber membrane and a thin layer of oil, although other aids can also be used, for example a plastic membrane or the like. With the help of this design, wall friction can be reduced to a minimum. Particular embodiments for achieving this will be given in the detailed description of the invention.

When examining the strength of the sample of the particulate material, the cylindrical matrix is removed, so that the sample has the opportunity to expand in the radial direction. The axial stress is then increased again until the sample collapses at a stress value fc. The flow properties are characterized by showing the breakdown strength, fc, as a function of the consolidation stress, ax. An example of this is shown in fig. 2, where the degradation strength of a particulate material is shown at four different levels of consolidation.

When it comes to the assessment/presentation of measurement data obtained by the above-mentioned consolidation with the device according to the invention, these can be represented in several forms which the person skilled in the art will be able to assess from case to case. As examples of such representation, the following can be mentioned: 1. If a company is to use the measurement data obtained with the apparatus according to the present invention for, for example, quality control of a particular product, it will be natural for the person concerned to decide on a consor- suffering level (ax) to which the samples are compacted. (Choosing such a level can be assessed by the person skilled in the art.) By examining the material at a constant ax value, the normal level for the fc value can be determined after a few trials. With standard quality control, one will therefore be able to find a specific value for fc that should not be exceeded. If investigations are carried out below this value, there will be no problems with the handling of the powder, but on the other hand if it is above this value, problems may arise, for example in that the powder material will then be able to block outlet openings such as in silos and the like by that in the powder will be able to form self-supporting material bridges or "plugs". 2. If several types of powders are to be examined with the device according to the invention, it will be natural to separate the materials into different cohesiveness classes. For this purpose, the ratio ox/ tc can be used. Similar to the example above, a constant ^-value is used. A natural distinction would then be as follows:

o1/ fc < 2.0 Extremely cohesive

2.0 < oJtc < 5.0 Very cohesive 5.0 < (Ti/fc < 10.0 Cohesive

10.0 < ajtc Little cohesive

3. A third way of assessing the measurement results found with the device according to the invention is to carry out investigations at four levels of consolidation (four values for ax). By measuring the consolidation strength, it will be possible to produce a function that describes the consolidation strength as a function of degree of consolidation in uniaxial compression, corresponding to the well-known flow function. An example of such a presentation is given in the design example below.

In order for the matrix to be easily removed from the sample after consolidation, it is given a slightly conical design. The above-mentioned general description is given for a cylindrical sample, but also other designs, such as for example approximately cubic or pyramidal, can be used without deviating from the idea behind the present invention.

Detailed description of an embodiment of the invention: Figures 3 and 4 show the design of an embodiment of the device according to the invention. The device is supported by a frame which includes top 1, bottom 3 and back plates 2, as well as two side plates 4. A steering wheel 5 is mounted on the top plate 1 using two axial bearings 6 so that it can be turned freely. The steering wheel 5 is again mounted in a trapezoidal threaded nut 7 which makes it possible to raise and lower a threaded rod 8, which rod 8 is attached to the top of two ball guides 9,10. Alternatively, a motor with a ball screw can be mounted to move the ball guide 9. The raising and lowering threaded rod 8 is connected to a piston rod 11 via the upper guide 9 and via two brackets 12. By means of a long continuous screw 13, a piston 14 attached to the end of the piston rod 11. The piston 14 can be passed through a die 15 which has a continuous, essentially cylindrical bore. The piston 14 can be attached to the matrix 15 by means of a hoop 16 and released from the piston rod 11 by means of the through screw 13. When the steering wheel 5 is turned back in the opposite direction, the upper ball guide 9 will be raised and the piston rod 11 and the piston 14 will separate.

The matrix 15, which is for example made of transparent PMMA, is in turn attached to the lower of the two ball guides 10. The two ball guides 9,10 can be moved vertically along two axes 17 which are attached to the back plate 2. to put the two ball guides 9,10 on identical shafts 17, as long as the assembly is correctly carried out, one will make sure that there is always parallelism 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 recessed groove in the fixing plate on the bottom ball guide 10, and thus cannot be moved in relation to this ball guide 10.

The matrix can be dismantled from the lower ball guide 10 by means of a quick coupling. Such a connection can, for example, comprise two hoops 26 which span over two through bolts 25 in the matrix 15. As previously mentioned, it is also possible to dismantle the piston 14 from the piston rod 11 and attach this to the matrix 15 using the hoop 16. By turning steering wheel 5 back, it will be possible to raise the upper ball guide 9 (without piston). This makes it possible to remove the matrix 15 (with stamp) so that the matrix 15 can be filled with the relevant particulate material to be examined.

The matrix 15 rests on an end cup 18, which in turn stands on a plateau 19 where a weighing cell 20 is mounted. This weighing cell 20 makes it possible to record the force acting on the powder sample 21 in the sample chamber in the matrix 15. In the end cup 18, also be possible to record the voltage acting in the sample 21 by means of a pressure cell. Radial stresses will be able to be recorded using strain gauges mounted on the matrix wall according to a special device setup, possibly using a pressure cell mounted directly in the matrix wall.

A detailed sketch of the matrix 15 with stamp 14 is shown in fig. 5. The matrix 15 comprises, for example, a square block with a through bore. In this embodiment, the bore is approximately cylindrical with a slightly increasing diameter towards the bottom. Such an essentially cylindrical bore can have a conicity of, for example, 0.5° inclination on the walls, but other angles can also be possible, and with an approximately cylindrical bore, the inclination of the walls could lie within the interval 0-5°. As mentioned earlier, it is also not necessary to design the bore approximately in the form of a cylinder, although this design is preferred due to its simplicity, but can also have other geometric shapes, nor does the slope of the walls in the test chamber need to be approximate vertical, although this is also preferred. What is necessary, however, is that the above-mentioned general conditions are fulfilled, so that the flow properties of the sample 21 of particulate material can be examined under the stated conditions. The stated conicity of the walls of the sample chamber exists so that the sample will more easily release the matrix wall when the matrix 15 is to be raised up and past the sample 21 of particulate material.

As mentioned above, it is important to have minimal friction between the sample 21 of the particulate material and the walls of the matrix 15. If the friction in this area is too high, the sample 21 will not be able to be packed homogeneously within the matrix walls. To avoid this friction, a membrane 23 has been stretched from the periphery of the piston 14 to the bottom of the matrix 15. Between the membrane 23 and the matrix wall 15 there will preferably be a thin layer of oil or other lubricant. The inner wall of the matrix 15 is preferably highly polished. When the piston 14 is moved vertically downwards in the matrix 15, the tension will thus decrease in the membrane 23 and this will follow the deformation to which the sample 21 is subjected. This will help to eliminate friction between powder and membrane wall. Towards the bottom of the matrix 15, the membrane 23 is stretched to the bottom edge of the matrix as shown in fig. 5. There will be no relative movement between the membrane 23 and the bottom edge of the matrix 15. This is avoided by clamping the membrane along the periphery of the matrix bottom by means of a retaining ring 27.

Oil that lies between the membrane 23 and the matrix wall. 15, will naturally tend to accumulate towards the bottom of this space. In order not to get a large accumulation of oil, there are preferably a number, for example three, small radial bores (28) through the matrix wall 15 in the lower part of the matrix 15. Excess oil will thus be able to escape through these bores (28) and this oil can, for example, be returned through an air hole 24 to the top of the matrix 15. When the piston 14 is pressed down into the matrix 15, there will also be opportunities for air pockets to form between the membrane and the matrix wall. This air will have an opportunity to migrate through the same radial bores (28). The air will then have the opportunity to escape through an air hole 24 in the matrix 15. When the sample of the particulate material is consolidated under the consolidation stress a1#, air will be forced out of the sample. This air can be led out of the sample volume by means of a filter 22 .

Example of application of the device according to the invention: To examine the flow properties of a particulate material (standardization powder CRM - 116, certified by BCR "the Community Bureau of Reference") [Akers, R.J.: The Certification of a Limestone Powder for Jenike Shear Testing - CRM 116, Loughbobough University of Technology, BCR/163/90] consisting of limestone powder which is conventionally used to examine apparatus that measure flow properties of particulate materials, an apparatus was used as shown in the present figures 3- 7 and which is generally described above.

The apparatus according to the invention which is used in the test example described has a total height of 820 mm, a width of 340 mm and a depth of 330 mm. To measure deformation of the sample, a standard micrometer is used, for example Mitutoyo Digimatic 572-300. A deformation gauge of the type Lucas/Schaevitz L.V.D.T. (Linear Variable Differential Transformer) can also be used. To measure the consolidation stress ax on the sample and later the strength, two different methods can be used (parallel or individually). One form of registration is a so-called weighing platform (corresponding to a standard scale). In one embodiment of the measuring device according to the invention, this is made by TEDEA (mode 1 no. 1040). The other form of axial stress recording is done by means of a pressure gauge at the bottom of the end cup, and it can be a standard pressure cell or pressure transducer with a diameter of the measurement area that does not exceed the smallest diameter of the test piece. Furthermore, it is important that the measuring device is deformed little as a result of the affecting voltage. It is also preferred that the measurement accuracy of such a measuring device is relatively high, and a reasonable deviation of the measurement data for identical measurements should be a maximum of ± 0.05 kPa.

In the exemplary experiment described below, the initial sample volume is 126 cm<3>. The sample quantity can of course be larger than this exemplary volume, and the volume given as an example is not of decisive importance, but should for the exemplary experiment be over 130cm<3>.

The investigation was initiated by removing the die 15 with the fixed piston 14 from the rest of the apparatus as explained above. The matrix 15 is filled with the particulate material by turning it upside down and filling in the material from the underside. After the matrix 15 has been closed with the end cup 18, the matrix 15 is then fitted back into the measuring apparatus according to the invention with the help of the clamping rings 26 so that the matrix 15 remains steady in the lowermost ball guide 10. The steering wheel 5 is turned until the piston rod 11 comes into contact with the piston 14, and the piston is then screwed on using the through screw 13 in the piston rod 11. Then the bracket 16 which attaches the piston 14 to the matrix 15 is pulled out. The individual parts in the test apparatus according to this embodiment of the invention will in this situation have a location as shown in fig. 6.

After this position has been achieved, the steering wheel 5 is then turned slowly so that the piston 14 is moved slowly downwards and the sample 21 is consolidated. This operation is continued until a desired value for the axial tension a1 is achieved. The deformation to which the sample is subjected can be read on, for example, a fixed micrometer. Such a micrometer can, for example, be placed with a movable point on the top ball guide 9, and with a fixed reference point or reading scale on the device's back plate 2 so that the relative movement of the piston 14 in relation to a constant zero point can be recorded.

When the powder sample 21 has been under the axial stress for a predetermined time, the steering wheel 5 is turned back so that the sample is relieved of the consolidation stress a1. How much the steering wheel 5 has to be turned back depends on the elastic properties of the powder sample 21 and the level of consolidation. The lower ball guide 10 is then pulled up so that the matrix 15 is guided over the piston 14, and the sample 21 is left free in the end cup 18 without any form of radial support. This position of the measuring device according to the invention is shown in fig. 7. The bore in the matrix 15 has, as previously mentioned, a certain taper, and is polished inside. Furthermore, the membrane 23 is attached to the lower edge of the matrix 15 so that the sample 21 can be released from the matrix 15 and the membrane 23 without affecting the properties of the sample 21. The time that the sample is loaded under the consolidation stress cr1# is not of decisive importance, but should be the same for measurements made in series. It can be advantageous to use a standardized time of, for example, 2 minutes to fully consolidate the sample, but shorter and longer times are also possible. Such times will be known or obvious to the person skilled in the art, and may also depend on the type of material to be examined. In the present example, 2 minutes have been used as consolidation time.

By using a longer time, it will be found, depending on the powder, that a higher strength is achieved. This phenomenon is called time consolidation, and is due to several factors, including the building of liquid or solid bridges between the particles in the sample. If it is not interesting to examine this time consolidation, an excessively long time between the end of consolidation and strength testing should therefore not be used. With a "waiting" time of 2 minutes it should be possible to minimize such time consolidation.

By turning the steering wheel 5 again, the sample 21 can be loaded again. In this way, the sample is deformed by increasing the axial stress until fracture occurs in the sample 21, and a maximum stress value, fc, is read from the weighing platform 20. fc denotes the strength of the sample after consolidation with the previously applied axial stress ax.

In order to calculate the consolidation strength, in the event that the sample 21 has a certain and sufficient cohesiveness, it will be possible to record the maximum value of fc without the sample completely collapsing, and the weight of the sample can be determined after the test has been carried out. Alternatively, the weight of the sample can be determined before starting the experiment.

It will thus be possible to correct for the weight of the sample in the measurements of ax and fc. As mentioned, for example, a weighing platform or a pressure gauge can be used which will be set to zero before the experiment is started, be it the consolidation stage or the strength measurement stage. As both the pressure cell and the weighing platform are placed under the sample, a part of the sample weight will have to be added to the calculations in practice for correction. This applies to both the ax and fc values.

In practice, this supplement can be determined in two ways. The simplest way is to add half the sample weight (that is, how large this weight becomes in tension at the relevant cross-sectional area of the sample (d = 35.4 mm in the present

Example)).

However, there is also another method for determining this supplement. As the piston can be moved very precisely, there is an opportunity to determine the strength value fc before the sample itself breaks down. When the sample breaks down, so-called fracture planes will occur in the sample. By moving the piston very slowly downwards, it will be possible to observe where this fracture plane starts. If this fracture is observed to start at the bottom of the specimen, the entire weight of the specimen is added to the calculations. If the break starts at the top, nothing will be added. However, such assessments can easily be carried out by the expert.

Fig. 8 shows results from the experiment described above with the aforementioned standard material and with the device according to the invention. Table 1 below shows the corresponding measured values for ax and fc.

However, it is important to note that the device according to the invention can be used for all types of particulate and cohesive materials, and there is in reality no lower limit for the particle size in such materials to be examined. There is, however, a certain upper limit for the particle size that is suitable in the material to be examined, but such an upper limit will largely depend on which material is to be examined. It is thus obvious that the properties of the materials will have to be assessed, as there is obviously a difference between, for example, cement and sand, but such an assessment can easily be made by the person skilled in the art without extensive experimentation. The limit for the upper suitable particle size generally lies in the cohesiveness of the material of the particle sample, and such a cohesiveness limit varies greatly from material to material, as mentioned above.

As an example of a particle size in a material that can be used when measuring flow properties according to the invention, the limestone powder that is used as standardization powder and that is mentioned above can be mentioned. This powder consists of particles of which 90% are below 6 µm. Furthermore, 50% of the particles are below 4 µm and 10% are below 2.2 µm in diameter. It will further be noted that this is a powder with a very narrow particle size distribution, and other particle size distributions as well as other material types can of course be measured with the device according to the invention. Another example that can be measured is cement which has a particle size where 90% is below 20 /xm, 50% is below 11 /xm and 10% is below 2 /xm in diameter.

Alternative areas of application for the device according to the invention: The device according to the invention and its use have been described above in connection with a special embodiment for investigating the flow properties of particulate materials. The described principle as well as the device that uses this, gives a quick indication of the material's properties, and gives opportunities to distinguish between relatively small variations in the material's behaviour. The device and measuring method will therefore be well suited for quality control of particulate materials, and the device will also be able to have wide application within industry that processes and produces particulate materials. The device is, as already mentioned, suitable for measuring time consolidation effects.

Claims (7)

1. Device for determining the flow properties of powdery materials, comprising a frame (1,2,3,4) on which is mounted a movable piston (14) which can be passed through a sample chamber in a removable matrix (15) to press together and examine a powder sample (21) under pressure load, as well as other measuring devices (20) to record the stresses applied to the powder sample, characterized in that the sample chamber in the matrix (15) has smooth polished walls that slope downwards and form a cross-section that tapers upwards in the matrix and expanding downwards in the matrix. 2. Device according to claim 1, characterized in that the sample chamber preferably has a cylindrical shape. 3. Device according to claim 1 or 2, characterized in that the walls of the test chamber have an inclination of at least 0.5°. 4. Device according to any one of the preceding claims, characterized in that the sample chamber in the matrix is equipped with a lubricant-coated inner membrane (23). 5. Device according to any of the preceding claims, characterized in that the matrix (15) is placed on a lower ball guide (10), which can be raised along shafts (17) after consolidation of a sample (21) in the matrix (15). 6. Device according to any one of the preceding claims, characterized in that the matrix (15) is removably attached by clamps (26), which are mounted on a lower ball guide (10) for a piston rod (11) via a quick coupling. 7. Use of the device according to any one of claims 1-6 for examining powders to find time consolidation properties of the relevant powder.