WO2015040274A1

WO2015040274A1 - Method and device for measuring a settled bed of solids in a multiphase system

Info

Publication number: WO2015040274A1
Application number: PCT/FI2014/050705
Authority: WO
Inventors: Marko Latva-Kokko; Jari KOURUNEN
Original assignee: Outotec Finland Oy
Current assignee: Outotec Finland Oy
Priority date: 2013-09-19
Filing date: 2014-09-16
Publication date: 2015-03-26
Anticipated expiration: 2016-03-19
Also published as: FI20135940L; FI20135940A7

Abstract

In minerals processing and hydrometallurgical industries,settled solids tend to form a solid layer at the bottom of mechanically stirred process vessels, which can lead to severe malfunction or even total shutdown of the process unit. The present invention introduces a method and a system for the real-time measuring of a settled bed of solids(5)in a container(1) comprising a multiphase system(2, 3)using at least one probe (6) comprising together a plurality of electrodes (7).

Description

METHOD AND DEVICE FOR MEASURING A SETTLED BED OF SOLIDS IN A MULTIPHASE SYSTEM

FIELD OF THE INVENTION

The present invention is applied in minerals processing and hydrometallurgical industries. The in^¬ vention relates to the monitoring of the height of a settled bed of solids in e.g. mechanically stirred process vessels, such as flotation cells and storage tanks .

BACKGROUND OF THE INVENTION

In minerals processing and hydrometallurgical industries, settled solids tend to form a solid layer at the bottom of mechanically stirred process vessels, such as flotation cells, stirred tank reactors and storage tanks. This accumulation can lead to severe malfunction or even total shutdown of the process unit. Therefore, real-time measurement or monitoring of the height of the settled bed of solids gives an opportunity to follow the situation in real time and make corrective action like increasing agitation or decreasing pulp density in time. Thus, said measure- ment is of a crucial importance.

There are some level measurement equipment that can measure the interface between a liquid and a settled bed of solids. One method is to use a vibrat^¬ ing probe extending through the wall of the tank to detect the desired interface. Another type of inter^¬ face level measurement equipment uses the reflection of sonar pulses to detect the level of solids on the bottom of the vessel.

An apparatus arranged to determine an inter- face between two components, such as water-oil or wa^¬ ter-sediment, is disclosed in W02010001089. Said appa- ratus comprises a plurality of electrodes configured to measure a potential difference between the measure^¬ ment electrode and the reference electrode, which measured potential difference is indicative of the in- terface between the components.

All the above techniques suffer from limita^¬ tions and they are not reliable in a multiphase envi^¬ ronment, such as three phase system (gas-liquid- solid) , where multiple interfaces are also present (gas-liquid, liquid-slurry, slurry-settled solids) . Contamination of the measurement equipment is often a problem in the existing techniques. In addition, with methods that utilize the reflection of sonar pulses, calibration in the multiphase environment is an issue due to fluctuation in the amounts of liquid, solid particles and gas bubbles.

SUMMARY

The present invention introduces a method for the real-time measuring of a settled bed of solids in a container comprising a multiphase system using at least one probe comprising together a plurality of electrodes capable of being in contact with the medi^¬ um, and the method comprises the steps of injecting currents or voltages through at least two electrodes; measuring voltages or currents, respectively, through the at least two electrodes; calculating the electri^¬ cal conductivity distribution in the multiphase system the basis of the measurement results and the known in- jected currents, which calculation comprises recon^¬ struction of a vertical conductivity profile as a function of location in the multiphase system; and concluding the height of the settled bed of solids on the basis of the electrical conductivity distribution and electrode location information.

In an embodiment of the invention, the geome^¬ try of the used at least one probe is linear and the plurality of electrodes are assembled in the same ver^¬ tical plane at a predetermined distance from each oth^¬ er .

In an embodiment of the invention, the method further comprises the step of monitoring contamination of the electrodes by measuring contact impedances be^¬ tween each electrode and the multiphase system.

In an embodiment of the invention, the method further comprises the step of estimating voltage loss- es due to contamination on electrode surfaces during calculation of the electrical conductivity distribu^¬ tion.

In an embodiment of the invention, at least one probe is submerged to the bottom of the container.

In an embodiment of the invention, at least one probe is in contact with the solid settled on the bottom of the container.

In an embodiment of the invention, the con^¬ tainer comprises at least one of the following: a flo- tation cell, a stirred tank reactor, a storage tank, a filter feed tank, a conditioning tank, a thickener.

In an embodiment of the invention, the multi^¬ phase system comprises a three phase system of a gas, a liquid and a solid.

In an embodiment of the invention, the method further comprises mixing the multiphase system in the container; submerging the at least one probe to the bottom of the container to be in contact with the sol^¬ id settled on the bottom of the container; monitoring contamination of the electrodes by measuring contact impedances between each electrode and the multiphase system; calculating the electrical conductivity dis^¬ tribution in the multiphase system on the basis of the measurement results, the contact impedances and the known injected currents.

In an embodiment of the invention, at least one probe is submerged so that it is in contact with both the solid settled on the bottom of the container and with the slurry phase above the solid.

In an embodiment of the invention, compensat^¬ ing, during calculation of the electrical conductivity distribution, inaccuracy of the measurements due to contamination of the electrodes that are in contact with the solid by estimating voltage losses over the measured contact impedances.

Another aspect of the invention comprises a system for the real-time measuring of a settled bed of solids in a container comprising a multiphase system. The system comprises a probe arrangement of at least one probe comprising together a plurality of elec^¬ trodes capable of being in contact with the multiphase system; a current source configured to inject currents or voltages through at least two electrodes; measuring means configured to measure voltages or currents, re^¬ spectively, through the at least two electrodes; a processor configured to control the measurements; the processor being further configured to calculate the electrical conductivity distribution in the multiphase system on the basis of the measurement results and the known injected currents, which calculation comprises reconstruction of a vertical conductivity profile as a function of location in the multiphase system, conclude, on the basis of the electrical conductivity distribution and electrode location information, the height of the settled bed of solids.

In an embodiment of the invention, the geome- try of the at least one probe is linear and the plu^¬ rality of electrodes are assembled in the same verti^¬ cal plane at a predetermined distance from each other.

In an embodiment of the invention, the meas^¬ uring means are configured to monitor contamination of the electrodes by measuring contact impedances between each electrode and the medium to be analyzed. In an embodiment of the invention, the meas^¬ uring means are configured to estimate voltage losses due to contamination on electrode surfaces during cal^¬ culation of the electrical conductivity distribution.

In an embodiment of the invention, a mixing mechanism is arranged to mix the multiphase system in the container; at least one probe of the probe ar^¬ rangement is submerged to the bottom of the container to be in contact with the solid settled on the bottom of the container; the measuring means are further configured to monitor contamination of the electrodes by measuring contact impedances between each electrode and the medium to be analyzed; the processor is fur^¬ ther configured to calculate the electrical conductiv- ity distribution in the multiphase system on the basis of the measurement results, the measured contact im^¬ pedances and the known injected currents.

In an embodiment of the invention, the at least one submerged probe is arranged to be in contact with both the solid settled on the bottom of the con^¬ tainer and with the slurry phase above the solid.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a further understanding of the invention and constitute a part of this specification, illus^¬ trate embodiments of the invention and together with the description help to explain the principles of the invention. In the drawings:

Figure 1 illustrates a flotation sell com^¬ prising a probe arrangement according to an example of the invention. Figures 2a and 2b illustrate curves depicting measured conductivity height profiles at different ro^¬ tation speeds of the tank, where the distance d (in cm) from the bottom of the tank is shown as a function of conductivity σ (in mS/cm) .

DETAILED DESCRIPTION

Reference will now be made in detail to the embodiments of the present invention, examples of which are illustrated in the accompanying drawings.

The present invention introduces techniques based on a computational electrical resistance tomog^¬ raphy approach which is applied to be used with a probe arrangement for measuring a settled bed of sol- ids in real time in a container comprising a multiphase system.

In this approach, metal electrodes can be at^¬ tached on a surface of a probe, through which sinusoi^¬ dal currents are injected to the target substance and resulting voltages are measured through at least two electrodes. Alternatively, voltages can be supplied between any two of the electrodes, and the resulting currents may be measured through the electrodes. The electronics in the system hardware handles the injec- tion, the measurements and the analysis performed based on the measurement results.

The probe arrangement may comprise one or more separate probes. The probe (s) is submerged to the bottom of a container comprising a multiphase system, such as a flotation cell, a stirred tank reactor, or a storage tank, for measuring a settled bed of solids in a continuous real time measurement that updates the desired value (height of the settled solids bed) with at least a few seconds' interval. The probe arrange- ment according to the invention is capable of detect^¬ ing the interface between e.g. a settled bed of solids and mixed slurry that contains fluctuating amounts of liquid, solid particles and gas bubbles.

A model based computational approach is uti^¬ lized to analyze the measured data. This means that such an approach takes into account for instance the geometry of the probe, the geometry of the container, as well as possible contamination of the electrodes. Through mathematical analysis of the model, the loca^¬ tion of the different interfaces such as the slurry- settled solid interface can be detected, based on which the height of the settled bed of the solid on the bottom of the container can be determined.

In the method according to the present inven^¬ tion an injection signal, which can be either injected voltage or current, is applied to the electrodes. An estimate of the electrical conductivity of the target substance as a function of location is calculated based on the measured voltages and the known injected currents. Measured data is used to determine the actu- al situation, i.e. the conductivity distribution which caused the observations - this approach is known as an inverse problem. The calculation is based on a mathe^¬ matical model determining the relation between the injected currents, the electrical conductivity distribu- tion of the target substance and the voltages of the electrodes. Said technique is called electrical imped^¬ ance tomography (EIT) , and is applied in e.g. medical imaging or soil characterization. One advantage of EIT is that it is based on a mathematical model which also takes into account the electrode impedances; the model can be also modified to take into account the voltage losses due to contamination on electrode surfaces.

In an exemplary arrangement of the invention, the probe comprises 16 electrodes in operation. Said probe was submerged to the bottom of the container, which was a flotation cell with a fluid volume of 501. Said flotation cell was filled with an authentic slur- ry sample that contained 33w% of solids. The probe was submerged so that the lowest electrode was at a 7cm distance from the bottom of the container. The rota^¬ tion speed of the mixing mechanism was alternated and height of the settled solids bed was measured with and without air feed.

It shall be noted, however, that another num^¬ ber of electrodes is also applicable, but at least two electrodes are always needed for supplying and measur- ing voltages or currents between the electrodes. As already mentioned, the probe arrangement may comprise one or more separate probes. The geometry of a probe can be linear, i.e. said probe can be formulated as a straight piece of probe. The electrodes can be placed or arranged in the same vertical plane at a predeter^¬ mined distance from each other. However, in every arrangement each electrode shall be insulated from the remainder of the electrodes in order for the current or voltage measurement to be possible.

More precisely, the electrodes can also be connected to the surface of a straight piece of body in such a way that electrodes remain electrically in^¬ sulated from each other. The probe itself can be con^¬ nected or fixed to any material, e.g. the walls of the container or flotation cell, in a way that the elec^¬ trode's contact with the surrounding medium or substance can easily be achieved. Also the alignment or angle, in which the straight probe is submerged in the container, can be selected. However, in order to re- construct a vertical conductivity profile as a func^¬ tion of location in the multiphase system, electrodes shall not be located on the same horizontal plane. Therefore, the probe shall not be submerged in the container in such a way that all the electrodes are in the same horizontal plane.

In one embodiment of the invention, the ef^¬ fect of contamination or dirtying of at least one electrode in the probe arrangement is taken into ac^¬ count. As widely known, the contamination around the electrode (s) leads into a non-ideal connection between the metallic electrode and target substance, which further causes additional electric resistance. The non-ideal connection can be seen as an additional voltage drop and it can be expressed by a quantity called contact impedance. The voltage (or current) measured through a pair of electrodes is generally a function of the injected current (or voltage), the conductivity distribution in the path of the electrical current and the contact impedances between the electrodes and the surrounding medium to be measured. The contact impedances may be used to compensate for the dirtying of the electrodes by inserting them to the calculation model as additional voltage loss pa^¬ rameters .

The obtained height of the settled bed of a solid in the container can be used when determining if said accumulation of a solid is significant enough and can lead to malfunction of the container. If the height of the solid layer formed at bottom of a con^¬ tainer reaches predetermined high levels, it will be possible to make corrective action like increasing ag- itation or decreasing pulp density in time.

Figure 1 illustrates a measurement arrange^¬ ment in a container 1, such as a flotation tank or cell, a stirred tank reactor, a storage tank, a filter feed tank, a conditioning tank, a thickener, etc. Ma- terial can be fed into and away from the container and it comprises solid particles suspended in a liquid phase 2 with or without gas 3 (slurry or gassed slur^¬ ry) . The container comprises a mixing mechanism 4 necessary e.g. for the flotation process. A separate vol- ume(s) of solid 5 is settled on the bottom of the con^¬ tainer. There can also be a transitional layer between the slurry phase and liquid phase or the liquid phase and gas phase (not shown) .

A probe arrangement comprising in this case a single probe 6 is submerged or lowered into the con- tainer 1 and fixed in its measurement position, pref^¬ erably in such a manner that the lower end of the probe 6 is in contact with the bottom of the container 1. In practice the probe is submerged so that it is in contact with both the settled solid 5 at the bottom of the container and with the slurry phase that may con^¬ tain also gas bubbles.

The probe arrangement comprises a set of electrodes 7. Here, 16 electrodes are used. The two lowest electrodes on the probe are in contact with the settled solid 5 and the rest of the electrodes are in contact with the slurry phase 2. Naturally, also an^¬ other configuration is possible depending on the size of the probe, the number of the electrodes and the height of the settled solid. In the illustrated ar- rangement the probe is aligned in a vertical plane, i.e. it is submerged perpendicularly to the bottom of the container. However, the alignment or angle of the probe in relation to the bottom of the container may vary according to the desired measurement, as long as not all the electrodes are in the same horizontal plane .

The controller 8 determines the vertical co^¬ ordinates of the probe and its electrodes in relation to the container geometry. Said controller also takes care of the current or voltage supply and all the voltage or current measurements between different pairs of electrodes 7. A computer (i.e. a processor) 9 or a similar device performs all the required calcula^¬ tions and data storing. All the measurement, analysis and calculation steps may be executed through a com^¬ puter program implemented in the controller 8, computer 9 or through an external server (not shown) located remotely in the network. The process control means, such as providing a signal to change a parameter val^¬ ue, e.g. an input rate of the medium to be fed into the process, increasing agitation or decreasing pulp density in time, can also be implemented through the controller 8 or computer 9. It can be noted that the controller 8 may comprise a motor directing the probe arrangement and being aware of the orientation and lo^¬ cation of the probe (s) all the time, while the comput- er 9 controls the motor and the overall process, e.g. flotation process.

Figures 2a and 2b illustrate curves depict^¬ ing measured conductivity height profiles at different rotation speeds of the tank, where the distance d (in cm) from the bottom of the tank is shown as a function of conductivity σ (in mS/cm) . In all the measurements the probe was submerged to the bottom of the container so that the lower end of the probe was in contact with the bottom of the container and the lowest electrode was at a 7cm distance from the bottom. In Figure 2a the rotation speed of the mixing mechanism (i.e. rotor) was 192rpm, in Figure 2b it was 230rpm. The meas^¬ urements were performed implementing the method ac^¬ cording to the present invention, and on the basis of the measurement results the electrical conductivity distribution was determined in this multiphase system. Measurement results were obtained with and without air feed to the container.

By observing the conductivity σ behavior one can clearly see that there are three different phases in the container, each phase having a different conductivity. The height of the solid settled on the bot^¬ tom of the container can be estimated from the dis^¬ tance d value on the y-axis that correspond to the first rapid change of conductivity σ on the x-axis. From the curves one can determine that the height of the solid, settled on the bottom of the tank at the point where the probe was located, was approximately between 7 and 8 cm in Fig. 2a and between 6 and 7 cm in Fig. 2b. Visual inspections confirmed that in these cases the height of the settled bed of solids was 8.0cm and 7.0cm, respectively.

There are numerous advantages of the present invention compared to the prior art measurement meth^¬ ods and arrangements. It shall be emphasized that the measurement arrangement according to the present in- vention, where at least one probe is submerged to the bottom of the container and is in contact (i.e. at least one of the electrodes is in contact) with the solid that is settled on the bottom of the container, enables a faster and more reliable calculation, be- cause the currents can be injected directly to the settled solid. Studies performed by the applicant showed that arrangements where probes comprising elec^¬ trodes are located in the container above the settled solid, so that none of the electrodes is in contact with the solid, do not provide accurate results. In the method according to the present invention, the in^¬ accuracy of the measurements caused by fouling or con^¬ tamination of the electrodes that are in contact with the solid is solved by measuring contact impedances between each electrode and the multiphase system and estimating voltage losses due to contamination on electrode surfaces during calculation of the electrical conductivity distribution.

The method according to the present invention is particularly suitable for calculating the electrical conductivity distribution in a multiphase system and concluding the height of the settled bed of sol^¬ ids, because settled solids do not contain a notable amount of gas - the electrical conductivity of the solid is significantly lower than that of a slurry or gas-containing slurry phase. Due to the large contrast (i.e. difference) in conductivity between settled sol- ids and other phases, the mathematical models provide reliable results when calculating the electrical con^¬ ductivity distribution. In addition, the modeling and calculation is fast compared to e.g. the determining of the electrical conductivity distribution of a two- phase system comprising gas and liquid phases or froth and slurry.

The known methods that are based on a poten^¬ tial difference measurement between the measurement electrode and the reference electrode are not directly applicable to flotation cells or stirred tank reac^¬ tors, because in said cells and reactors the bottom part of the tank contains a multitude of solid phase particles. These particles act as insulators and may contaminate the electrodes in a short period of time.

The other known solutions for determining a conductivity profile of a target are not directly ap^¬ plicable to flotation cells or stirred tank reactors, because in said cells and reactors the solid phase particles, which act as insulators, are mixed in the whole volume of the cell or reactor. Therefore, in flotation cells or stirred tank reactors, it is in practice very hard to find a pure liquid phase without any impurities. Further, gas bubbles that are mixed in the whole volume of the cell or reactor also act as an insulator and thus make it very difficult to detect an interphase between e.g. solid and liquid phases.

The requirements for successful determination of the height of a settled bed of solids are: the liq- uid phase (if any) present in the container shall be electrically conductive and the electrical conductivi^¬ ty of said liquid phase and settled solid(s) shall not be the same.

It is obvious to a person skilled in the art that with the advancement of technology, the basic idea of the invention may be implemented in various ways. The invention and its embodiments are thus not limited to the examples described above; instead, they may vary within the scope of the claims.

Claims

1. A method for the real-time measuring of a settled bed of solids in a container comprising a mul^¬ tiphase system using at least one probe comprising to^¬ gether a plurality of electrodes capable of being in contact with the medium, the method comprising:

- injecting currents or voltages through at least two electrodes;

- measuring voltages or currents, respective^¬ ly, through the at least two electrodes;

- calculating the electrical conductivity distribution in the multiphase system on the basis of the measurement results and the known injected currents, which calcula^¬ tion comprises reconstruction of a vertical conductivity profile as a function of loca^¬ tion in the multiphase system;

- concluding, on the basis of the electrical conductivity distribution and electrode lo^¬ cation information, the height of the set^¬ tled bed of solids.

2. The method according to claim 1, wherein the geometry of the used at least one probe is linear and the plurality of electrodes are assembled in the same vertical plane at a predetermined distance from each other.

3. The method according to any of the claims 1-2, further comprising:

- monitoring contamination of the electrodes by measuring contact impedances between each electrode and the multiphase system.

4. The method according to any of the claims 1-3, further comprising:

- estimating voltage losses due to contamina^¬ tion on electrode surfaces during calcula^¬ tion of the electrical conductivity distri^¬ bution .

5. The method according to any of the claims 1-4, wherein:

- at least one probe is submerged to the bot^¬ tom of the container.

6. The method according to any of the claims 1-5, wherein:

- at least one probe is in contact with the solid settled on the bottom of the contain^¬ er .

7. The method according to any of the claims 1-6, wherein the container comprises at least one of the following: a flotation cell, a stirred tank reactor, a storage tank, a filter feed tank, a conditioning tank, a thickener.

8. The method according to any of the claims 1-7, wherein the multiphase system comprises a three phase system of a gas, a liquid and a solid.

9. The method according to claim 1 or 2 , comprising :

- mixing the multiphase system in the container;

- submerging the at least one probe to the bottom of the container to be in contact with the solid settled on the bottom of the container;

- monitoring contamination of the electrodes by measuring contact impedances between each electrode and the multiphase system;

- calculating the electrical conductivity distribution in the multiphase system on the basis of the measurement results, the contact impedances and the known injected currents .

10. The method according to claim 9, wherein at least one probe is submerged so that it is in con^¬ tact with both the solid settled on the bottom of the container and with the slurry phase above the solid.

11. The method according to any of the claims 9-11, further comprising compensating, during calculation of the electrical conductivity distribu^¬ tion, inaccuracy of the measurements due to contamina^¬ tion of the electrodes that are in contact with the solid by estimating voltage losses over the measured contact impedances.

12. A system for the real-time measuring of a settled bed of solids (5) in a container (1) com^¬ prising a multiphase system (2, 3), said system comprising

- a probe arrangement of at least one probe (6) comprising together a plurality of electrodes (7) capable of being in contact with the multiphase system;

- a current source (8) configured to inject currents or voltages through at least two electrodes ;

- measuring means (8) configured to measure voltages or currents, respectively, through the at least two electrodes;

- a processor (9) configured to control the measurements; the processor (9) being fur^¬ ther configured to:

- calculate the electrical conductivity dis^¬ tribution in the multiphase system on the basis of the measurement results and the known injected currents, which calculation comprises reconstruction of a vertical con^¬ ductivity profile as a function of location in the multiphase system,

- conclude, on the basis of the electrical conductivity distribution and electrode lo^¬ cation information, the height of the set^¬ tled bed of solids.

13. The system according to claim 9, wherein the geometry of the at least one probe (6) is linear and the plurality of electrodes (7) are assembled in the same vertical plane at a predetermined distance from each other.

14. The system according to any of the claims 9-10, wherein the measuring means (8) are fur^¬ ther configured to:

- monitor contamination of the electrodes by measuring contact impedances between each electrode and the medium to be analyzed.

15. The system according to any of the claims 9-11, wherein the measuring means (8) are fur^¬ ther configured to:

- estimate voltage losses due to contamina^¬ tion on electrode surfaces during calcula- tion of the electrical conductivity distri^¬ bution .

16. The system according to any of the claims 9-12, wherein at least one probe (6) is sub^¬ merged to the bottom of the container (1) .

17. The system according to any of the claims 9-13, wherein at least one probe (6) is in con^¬ tact with the solid (5) settled on the bottom of the container ( 1 ) .

18. The system according to claim 10 or 11, wherein:

- a mixing mechanism (4) is arranged to mix the multiphase system in the container (1);

- at least one probe of the probe arrangement is submerged to the bottom of the container to be in contact with the solid settled on the bottom of the container (l);the measuring means (8) are further configured to monitor contamination of the electrodes by measuring contact impedances between each electrode (7) and the medium to be ana^¬ lyzed; - the processor (9) is further configured to calculate the electrical conductivity dis^¬ tribution in the multiphase system on the basis of the measurement results, the meas^¬ ured contact impedances and the known in^¬ jected currents.

19. The system according to claim 10 or 11, wherein the at least one submerged probe is arranged to be in contact with both the solid settled on the bottom of the container and with the slurry phase above the solid.