Measuring container and method for determining the particle size distribution of a sediment

Abstract

The invention relates to a measuring container applicable for testing the particle size distribution of samples, more specifically samples containing particles or sediments, specifically for determining said particle size distribution, and to a method for determining the particle size distribution of said samples. The technical solution according to the invention is essentially characterised in that by artificially accelerating the sedimentation / settlement of the particles of the liquid sample (hereinafter: suspension) containing the particles being tested, the known sedimentation measurement based on pressure measurement can be accelerated, and the pressure difference to be measured can be increased.

Description

[0001] MEASURING CONTAINER AND METHOD FOR DETERMINING THE PARTICLE SIZE

[0002] DISTRIBUTION OF A SEDIMENT

[0003] Technical Field

[0004] The present invention relates to a measuring container applicable for testing the particle size distribution of samples, more specifically samples containing particles or sediments, specifically for determining said particle size distribution, and to a method for determining the particle size distribution of said samples. The invention can be particularly preferably applied for testing suspension soil samples.

[0005] Background Art

[0006] Knowledge of the particle size distribution (or in an alternative term, the particle distribution) is essential in many fields. It is indispensable to know the particle distribution for example for adjusting the physical properties of ceramics made from powder-like starting materials applying ceramics sintering technology (e.g. the pore size or the distribution thereof in the sintered finished product), or for soil tests e.g. for estimating the water retention capacity or nutrient storage capacity of soils, or even for classifying the soil being tested. Particle distribution is a parameter that is equally important in cosmetics industry, in chemical industry (catalyst production), and in construction industry (i.e. , the production of diverse types of concrete).

[0007] A variety of measurement techniques exist nowadays for laboratory measurements of particle distribution, involving the application of dedicated measurement devices, namely apparatus based on X-ray absorption or laser diffraction. Although these dedicated devices allow rapid and automated sample analysis due to their measurement principles, their use is excessively costly. In addition to that, these techniques are intended for determining the real mean / average physical dimensions of the particles rather than for measuring the nominal particle diameter interpreted according to the conventional definition.

[0008] Testing processes based on the sedimentation principle are widely applied in laboratory geotechnical tests of various soil samples. Such a process is described in the Hungarian standard MSZ EN ISO 17892-4 for the determination of the particle size distribution curve of soil particles by measuring the settlement properties of the soil particles in the test fluid applying the well-known physical law of friction, known as Stokes’ law, considering the particles as spherical objects. A significant disadvantage of the process according to the standard is that - depending on the soil type - it may require 2-48 hours to complete, which implies that the soil samples can only be analysed and classified very slowly. Accordingly, when a large number of soil samples have to be tested, obtaining the results requires a significantly long time, even a few weeks. In addition to that, the process according to the standard is difficult to automate, i.e. , it essentially requires constant monitoring by a human.

[0009] A known solution for measuring particle distribution is the sedimentation method based on pressure measurement, which involves measuring the absolute pressure change (i.e., the change of the pressure value measured relative to the external atmospheric pressure) during the settlement of a liquid containing particles in an expediently vertical measuring tube at a single location (that is, at a single depth) chosen along the length of the tube, followed by deriving particle distribution and the particle distribution curve from the result obtained this way based on Stokes’s law using an appropriate mathematical model. The change of density occurring during the measurement along the measurement tube due to particle sedimentation, and consequently the measurable pressure change will be a very small value. This implies that applying this technique in practice necessitates the measurement of minor differences between relatively high pressure values, which is inaccurate and unfavourable, i.e., the measurement results will have significant errors. Due to the applied mathematical model, this may significantly affect the accuracy of determining particle distribution.

[0010] In addition to that, the sedimentation technique based on pressure measurement allows measuring only a single sample at a time, so it does not allow the simultaneous testing of multiple samples, i.e., testing the samples under identical external measurement conditions; which samples may for example be “parallel” samples made of identical material (e.g. soil, ceramic powder, pigment particles, etc.), or samples taken from various materials.

[0011] The prior art includes the use of geotechnical centrifuges and their application for soil sample analysis. Such a geotechnical centrifuge is disclosed in e.g. the European publication EP-0,744,611 A1 (non-exhaustive reference), which among others provides an example for attaching specific measurement devices at a desired angle of inclination to one or more arms of the centrifuge. Disclosure of the Invention

[0012] The objective of the present invention is to eliminate or at least alleviate the disadvantages of the above-described technical solutions.

[0013] Our other objective is to provide a sedimentation testing method for determining particle distribution based on pressure measurement, i.e., a measuring container and a method, which provide a particle distribution measurement of improved accuracy, with shorter measurement time, improved repeatability, and higher reliability.

[0014] A further objective is to provide a sedimentation testing method for determining particle distribution based on pressure measurement that allows the simultaneous measurement of a plurality of samples while increasing the reliability of the measurement.

[0015] Our research has led to the recognition that by artificially accelerating the sedimentation / settlement of the particles of the liquid sample (hereinafter: suspension) containing the particles being tested, the known sedimentation measurement based on pressure measurement can be accelerated, and the pressure difference to be measured can be increased.

[0016] To this end, the tests are performed applying a special measuring container, in which measuring container the gravitational acceleration driving the sedimentation / settlement of the particles in the vertical position of the measuring container is augmented. The gravitational acceleration is augmented by rotating the measuring container steadily along a circular path. This is achieved by attaching the measuring container to one of the arms of a centrifuge, for example one known from medical or geotechnical science, and by operating the centrifuge, subjecting the measuring container to an apparent acceleration having the same effect as gravitational acceleration. The centrifugal acceleration of the measuring container is “sensed” by the particles of the suspension located in the measuring container as a force accelerating their sedimentation / settlement. The sedimentation measurement for determining particle distribution based on pressure measurement is thus performed in an augmented acceleration field, replacing the effect of the gravitational field in the mathematical model applied for processing the measurement results with the effect of the field corresponding to centrifugal acceleration.

[0017] Without going into mathematical details, it can be maintained that due to the increased acceleration, the sedimentation / settlement of the particles becomes more rapid (expectedly nearly linearly with centrifugal acceleration), because fluid flow around the particles remains nearly laminar even in the case of larger-sized particles and increased settlement rate, that is, the time required for performing the measurement of particle distribution will shorten proportionally depending on the applied centrifugal acceleration, e.g. to 1 / 1 Oth-1 / 100th of the original time at acceleration values of 10-100g. Therefore, a measurement originally requiring 24 hours will expectedly require a time between 15 and 150 minutes when performed according to the invention.

[0018] Due to the elevated value of apparent acceleration occurring in the measuring container when it is rotated, the measured pressure difference also increases with respect to the value measurable at an acceleration of 1 g ( i. e. , gravitational acceleration occurring in an upright, stationary position of the measuring container), which improves the accuracy of the measurement.

[0019] Further, such a measuring container is applied as a measuring container that in a preferred embodiment is constituted by two vessels attached together in a mutually parallel position, the vessels being opened up to each other at one or more identical points of their length, the so-called bleeding points. The one or more opened-up portions are configured to allow pressure propagation between the vessels but prevent material flow therebetween, with a respective pressure differential sensor being introduced in each opened-up portion such that the pressure measuring port on one side of the sensor is connected to one of the vessels, the pressure measuring port on the other side thereof being connected to the other vessel. The applied pressure differential sensor is expediently a small-sized (optionally, miniaturised) high-resolution pressure differential sensor. The open ends of the two vessels have features configured to be connectible to a common pressure source. The vessels constituting the measuring container preferably have a cylindrical configuration, but they may be of any shape.

[0020] The sedimentation method for determining particle distribution based on pressure measurement according to the invention is performed by filling one of the vessels with the suspension to be tested (sample to be tested), and filling the other vessel with the liquid applied for preparing a suspension containing the particles, in other words, the suspending solution (reference liquid). Thereafter, the vessels are sealed by connecting the open ends 30 of the two filled-up vessels to the common pressure source. After this preparation step, the measuring container is attached to one of the arms of the centrifuge together with the common pressure source, and then, by setting the arm of the centrifuge to rotate at a given rotational speed, an increased apparent acceleration acting on the prepared sample and the reference liquid is induced. Thereafter, if necessary, after stirring the sample, the measurement is started which comprises measuring and recording over time a difference between the pressure differentials occurring in each of the vessels with respective pressure differential sensors introduced between corresponding bleeding points of the vessels. If necessary, as the measurement progresses, apparent acceleration may be further increased by increasing the rotational speed of the arm of the centrifuge. Based on the time-pressure difference relationship, the particle distribution curve, particularly its “lower” section, can be calculated from the recorded measurement data based on the modified Stokes’ law and taking into account the varying apparent acceleration. The duration of the measurement is a fraction of the time taken by conventional sedimentation tests.

[0021] Because during the measurement the two vessels are connected in an airtight manner to the common pressure source, in the method according to the invention the result of the pressure measurement performed on the vessel containing the reference liquid is not the external atmospheric pressure, but the pressure of the suspending liquid not containing suspended particles (the decisive amount of the part of the absolute pressure measured at any depth above atmospheric pressure results from that), so the pressure difference to be measured arises only from the weight of the particles floating in the suspension. This pressure difference is further increased by the application of centrifugal acceleration, making it possible to measure it more easily and more accurately (because low pressure differential changes can be measured more accurately with sensors having measurement ranges optimised for the range in question).

[0022] Therefore, by applying the method according to the invention it is not necessary to perform measurements of the increased total fluid pressure. As set forth above, it is sufficient instead to measure the difference between the pressure levels of the reference liquid and the suspending liquid carrying the particles at an identical “height” (i.e., due to the presence of centrifugal acceleration, at a distance r from the geometrical axis of the rotating centrifuge). Therefore, instead of measuring small differences between large pressure values it is required to measure small pressure differences, which can be performed with a pressure meter having a measurement range corresponding to the pressure difference to be measured. Thereby, the signal- to-nose ratio and the accuracy of measurements is improved.

[0023] Furthermore, if the vessels of the measuring container are configured to have a plurality of corresponding bleeding points, with a respective pressure differential sensor being preferably introduced, as set forth above, between the corresponding point of each pair, then the density function of the particle distribution can be determined in the radially variable acceleration field on the basis of measurement data obtained by measuring the temporal variation of the pressure distribution in the fluid column containing the particles undergoing sedimentation / settlement, whereby more accurate results can be obtained.

[0024] Furthermore, such a measuring container is applied as a measuring container that - in another preferred embodiment - is constituted by at least three vessels attached together in a mutually parallel position, with at least one of the vessels being filled with the reference liquid, the other vessels being filled with liquids containing either identical or at least partially different samples to be measured, before being introduced into the augmented apparent-acceleration field 15. The vessel receiving the reference liquid is opened up to each of the other vessels individually, pair by pair at one or more points of their lengths, i.e. , at the bleeding point(s). The one or more opened-up portions between the vessel receiving reference liquid and one of the vessels receiving samples is configured to allow pressure propagation between the vessels but prevent material flow therebetween, and with a respective pressure differential sensor being introduced in each opened-up portion such that the pressure measuring port on one side of the sensor is connected to the vessel receiving reference liquid, and the pressure measuring port on the other side thereof is connected to the sample vessel. The applied pressure differential sensor is expediently a small-sized (optionally, miniaturised) high-resolution pressure differential sensor. The open ends of the vessel receiving the reference liquid and the vessels receiving the samples have features configured to be connectible to a common pressure source. The vessels constituting this embodiment of the measuring container preferably have a cylindrical configuration, but they may have any shape.

[0025] In line with the above, the sedimentation testing method for determining particle distribution based on pressure measurement according to the invention, i.e., the measuring container and measurement method, as well as the preferred embodiments of the measuring container and the preferred variants of the method have been provided by providing the measuring container and the method according to the appended claims.

[0026] Brief Description of the Drawings

[0027] In the following, the invention will be described in detail referring to the accompanying drawing, where

[0028] Fig. 1 is a schematic longitudinal sectional view of the two-vessel embodiment of the measuring container applied for the testing method according to the invention,

[0029] Fig. 2A is a schematic longitudinal sectional view of the multi-vessel embodiment of the measuring container applied for the testing method according to the invention, and

[0030] Fig. 2B is a schematic top plan view depicting the multi-vessel measuring container illustrated in Fig. 2A.

[0031] Modes for Carrying out the Invention

[0032] Fig. 1 depicts schematically the two-vessel embodiment of the measuring container applied for the testing method according to the invention. The measuring container in question includes a vessel 1 adapted for receiving a suspension, and a vessel 2 adapted for receiving a clean suspending liquid, in other words, a reference liquid that is free from particles of the sample to be tested. The vessels 1 , 2 are vessels that are closed at one of their ends and open at their other end. The vessels 1 , 2 are preferably secured together in mutually parallel positions, with the inside space thereof being interconnected at corresponding bleeding points (bores) located at one or more distances from one of their ends by means of interconnecting passages 4 (conduits). Pressure differential sensors 7-i (or differential pressure cells) are introduced in the passages 4 (i denotes the number of mutually independent sensors in the separate passages 4 connecting the vessels 1 , 2) such that the measurement port at one side of each sensor is connected to a given bleeding point of one vessel 1 , while the measurement port at the other side is connected to a corresponding bleeding point of the other vessel 2. The pressure differential sensors 7-i thus allow for determining the difference between the pressure levels prevailing inside the vessels 1 , 2 at the given height level of the two vessels 1 , 2. The passages 4 between the bleeding points of the vessels 1 , 2 and the measurement ports on the corresponding side of the pressure differential sensors 7-i are filled with such an incompressible liquid that may be different from the suspending liquid and can be allowed to be present at the measurement ports of both sides of the sensor; in light of the sample to be measured selecting this liquid is obvious to the skilled person.

[0033] Structural components 3 adapted for preventing material flow / mixing of the materials but allowing pressure propagation are arranged in the passages 4 at the bleeding points of the vessels 1 , 2. The structural components 3 can be constituted by open-pore filtering stones placed at appropriate locations of the passages 4, or some kind of membrane, expediently made of a thin material.

[0034] The pressure differential sensors 7-i are high-accuracy pressure differential sensors. The pressure differential sensors 7-i expediently have such accuracy that a linear density change of 0.0001 g / cm of the liquid can be measured at a depth of 5 cm with an acceleration of 10 g. Furthermore, the pressure differential sensors 7-i are pressure sensors with measurement ports of which the inside is wettable at both sides by the liquid that fills the passage 4, with no material flow being produced between the two sides of the sensors.

[0035] After filling the vessels 1 , 2 with suspension and with clean suspending liquid, respectively, the open ends of the vessels 1 , 2 are connected with an airtight seal to a common pressure source 8 in a manner allowing pressure propagation.

[0036] The measuring container further comprises accelerometers 5-1 , 5-2, preferably MEMS or other type of accelerometer, one of which is arranged at each end of the vessels 1 , 2. Based on the acceleration-time functions measured in the course of performing the tests utilising the accelerometers 5-1 , 5-2, and on the pressure-time functions measured utilising pressure sensors, the mean density of the liquid located above the given measurement point can be calculated by model fitting known to the skilled person. The basis of the model - like the basis of conventional hydrometric analysis - is Stokes’ law, but due to the variable acceleration field during the test the movement of the particles and the expected pressure levels at given depths must be calculated applying dedicated functions.

[0037] Furthermore, if necessary, the measurement according to the invention can be performed repeatedly, optionally applying different acceleration values; the “in flight” stirring of the samples in the vessel 1 is provided by a mechanical or hydraulic apparatus 6 (i.e. apparatus for circulating the suspension), but the samples can also be stirred applying an arrangement that turns the vessel 1 upside down by reversing the ends thereof located proximate the rotation axis and facing outward. Figs. 2A and 2B, respectively, depict in a schematic longitudinal sectional and a top plan view, an exemplary multi-vessel (preferably at least 3-vessel) embodiment of the measuring container applicable with the testing method according to the invention. Like reference numerals applied in Figs. 2A-2B and in Fig. 1 refer to like structural components / subassemblies. In the arrangement illustrated schematically in Figs. 2A and 2B the suspension containing the sample particles is received in vessels 1-1 , 1 -2, ..., 1-j, where j denotes the total number of such vessels. The vessel 2 receiving the reference liquid is connected to each of the vessels 1 forming “pairs”, as described above in relation to the arrangement depicted in Fig. 1 . In the passages 4 between the vessel 2 and the other vessels 1 -1 , 1-2, ... , 1-j pressure differential sensors 7-j-(1 -i) are arranged in a manner described above, where i denotes the combined number of bleeding points.

[0038] The centrifuge applied for creating an apparent acceleration field, i.e., a centrifugal acceleration that is identical to the effects of gravitational acceleration from the aspect of the sedimentation / settlement of the particles, has a configuration that is similar to the configuration of medical diagnostic centrifuges or the centrifuges widely used in the field of geotechnics, while in respect of its geometrical dimensions it is between the dimensions of the centrifuges applied in the two mentioned fields.

[0039] Specifically, the centrifuge is expediently such a centrifuge that is able to provide a centrifugal acceleration of approximately 300g at the end of the measuring containers connected to one or more arms thereof, (g is the gravitational acceleration at the location where the test according to the invention is performed). The load-bearing capacity of the centrifuge is expediently 20 kg per arm, more preferably 15 kg per arm, the working radius of the centrifuge preferably being 0.4-0.6 m. At the same time, however, modifying these parameters of, and / or, if necessary, resizing the centrifuge to be applied for performing the method in order to adapt it to the measurement device to be applied, or vice versa, adapting the geometric parameters of the measurement device to the centrifuge at hand to achieve the required centrifugal acceleration is a trivial routine task for the person skilled in the art.

[0040] The measuring container is provided with a mechanism allowing its connection to one of the horizontal arms of the applied centrifuge. The mechanism is preferably a swinging bucket attached e.g. via an articulated joint to the given arm of the centrifuge, which allows that when the arm of the centrifuge rotates at a given rotational speed co, the bucket, and together with it, the geometrical longitudinal axis of the given measuring container (in particular, for example, the vessel receiving reference liquid) can incline at an arbitrary angle with respect to the vertical. To preserve the balanced rotation of the centrifuge, a number of measuring containers corresponding to the number of arms of the centrifuge are attached to the centrifuge, preferably in a symmetrical arrangement (i.e. , one measurement vessel to each arm). Accordingly, for the simultaneous measurement of particle distribution usually 1 to 10 measuring containers, more preferably 2 to 8, still more preferably 2 to 6 and particularly preferably 4 to 6 measuring containers are applied.

[0041] If the centrifuge is configured such that measuring containers with an identical arrangement (i.e. comprising the same number of vessels 1 ) can be attached to its oppositely located arms, then during operation it may be necessary to move a minimal balancing weight in order to provide the continuous balancing of the centrifuge made necessary by centre-of-gravity shifts caused by particles settling at different rates. To control the balancing system, the phase of the oscillations caused by eccentricity and the position of the arms must be measured with appropriate accelerometers, and, based on these data, the balance weight must be moved to a position that minimises the vibrations. As the measurement progresses, the required position of the balancing weight may change continuously.

Claims

Claims1. Measuring container for determining the particle size distribution of a sediment, characterised in that it consists of at least two vessels (1 , 2) that are open at one side and are secured together parallel to each other, said vessels (1 , 2) being opened up to each other at at least one point along their length - the bleeding point(s) - in a manner that allows pressure propagation but prevents material flow, a pressure differential sensor (7-i) is disposed in the opened-up portion, of which the pressure measuring port on one side being connected to one of the vessels (1 , 2) and the pressure measurement port on the other side being connected to the other vessel (1 , 2), and the open end of the vessels (1 , 2) is configured to be connectible to a common pressure source (8).2 The measuring container according to claim 1 , characterised in that it consists of three or more vessels (1 , 2) secured together parallel to each other, with one of the vessels (2) being opened up to each of the other vessels (1 ) individually, pair by pair at one or more points of their lengths, i.e. , at the bleeding point(s), in a manner that allows pressure propagation but prevents material flow.

3. The measuring container according to claims 1 or 2, characterised in that the vessels (1 , 2) constituting the measuring container are cylindrical.4 The measuring container according to any of claims 1 -3, characterised in that the vessels (1 , 2) constituting the measuring container are opened up to each other at a plurality of bleeding points along their length.

5. The measuring container according to any of claims 1 -4, characterised in that the bleeding points are connected by passages (4), for example conduits, that are filled with incompressible liquid.

6. The measuring container according to any of claims 1 -5, characterised in that structural components (3) adapted to allow pressure propagation but prevent material flow are arranged in the passages, said structural components being filtering stones or membranes.

7. The measuring container according to any of claims 1 -6, characterised in that the pressure differential sensor (7-i) is a small-sized, high-resolution pressure differential sensor (7-i).

8. The measuring container according to any of claims 1 -7, characterised in that a respective accelerometer (5-1 , 5-2) is arranged at each end of each vessel.

9. Method for determining the particle size distribution of a sediment, characterised by filling one of the vessels (2) of the measuring container according to any of claims 1 -8 with a liquid utilised for producing a suspension containing the particles (reference liquid), and filling the other vessels (1 ) with the suspension to be tested, and then, by connecting the open ends of the vessels (1 , 2) to a common pressure source (8), sealing the vessels (1 , 2) in an airtight manner, subsequently, attaching the measuring container to one of the arms of the centrifuge together with the common pressure source, then, by setting the arm of the centrifuge to rotate at a given rotational speed, generating the increased apparent acceleration acting on the suspension and the reference liquid, and starting the measurement, if necessary after stirring the suspension to be tested, wherein a differential between the pressure changes occurring individually in each of the vessels (1 , 2) filled with the two different materials is measured and recorded over time with respective pressure differential sensors (8) introduced between the corresponding bleeding points of the vessels ( 1, 2).

10. The method according to claim 9, characterised in that as the measurement progresses, apparent acceleration is further increased by increasing the rotational speed of the arm of the centrifuge.

11. The method according to claims 9 or 10, characterised in that in the case of more than two vessels (1 , 2), one of the vessels (2) is filled with a liquid utilised for producing a suspension containing the particles (reference liquid), while at least a subset of the other vessels (1 ) is filled with various suspensions to be tested.

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