WO1988005532A1

WO1988005532A1 - Methods and apparatus for monitoring the flocculation state of particles in a suspension

Info

Publication number: WO1988005532A1
Application number: PCT/GB1988/000043
Authority: WO
Inventors: Terence Wilfred Webb; Peter John Molloy; Gerald Henry Meeten
Original assignee: ECC International Ltd
Current assignee: Imerys Minerals Ltd
Priority date: 1987-01-22
Filing date: 1988-01-22
Publication date: 1988-07-28
Anticipated expiration: 1989-07-22
Also published as: GB2202051B; GB8801457D0; GB2202051A; GB8701368D0

Abstract

Methods and apparatus for monitoring the flocculation state of particles in a suspension in which a field is applied to a region of the suspension and an electrical property of the suspension is monitored in both the presence and absence of the field. The relative change in the value of the property in the presence of the field with respect to its value in the absence of the field is used to monitor the level of flocculation. The field may be an electric, shear, magnetic, acoustic or gravitational field. Although various electrical properties may be monitored several examples of apparatus are disclosed which monitor the conductance of a region. In one embodiment the field is a shear field resulting from the flow of the suspension induced by a pump (65) through a tube (62) between carbon electrodes (60, 61). A conductivity measuring arrangement (67) measures the conductance of the path through the suspension between the electrodes (60, 61) in both the flow and no flow states and a processor (68) controls the pump (65) and records the conductance measured.

Description

METHODS AND APPARATUS FOR MONITORING THE FLOCCULATION STATE OF PARTICLES IN A SUSPENSION

The present invention relates to methods and apparatus for monitoring the flocculation state of particles in a suspension.

In various industrial processes particles of material are in suspension in a liquid and for correct and efficient processing need to be in a de-flocculated state. For example, in processing kaolin or naturally occurring calcium carbonate to render them into a fine particulate form for use in paper pigmentation the kaolin or ground calcium carbonate particles are in a water suspension in the early stages of processing and are in various states of locculation. To produce de- flocculated suspensions for further processing de- flocculant is added to produce, ideally, 100% de- flocculation. In the case of kaolin the addition of excess de-flocculant is more expensive than using the optimum amount but, except with extreme amounts of de- flocculant, does not cause re-flocculation. In the case of, for example, calcium carbonate the amount of de-flocculant that is used is more critical since the addition of de-flocculant in excess of that required for 100% de-flocculation will cause re-flocculation and the smaller the particle sizes that are involved, the more critical becomes the amount of de-flocculation that is used, i.e. the band of acceptable de-flocculant quantities that will de-flocculate without causing re- flocculation narrows. Once the suspensions are de-flocculated, then filtering processes can be used to extract particles of desired fineness. If floes still exist in the suspension then clearly even if fine particles are contained in the floes they cannot be extracted by filtering through standard mesh sizes. The obtaining of 100% de-flocculated suspensions is therefore of critical importance in many processes. In the case of the processing of kaolin 100% deflocculation can obviously be achieved simply by adding an excess of de- flocculant. However, this is expensive and significantly increases the processing costs. In the case of the processing of calcium carbonate particles in suspension the cost of the de-flocculant is even more significant as a proportion of the processing cost and in addition the correct level of de-flocculant is essential to ensure 100% deflocculation for satisfactory further processing of the suspension. In all cases it is obviously desirable to have an on-line facility for measuring the de-flocculant state to enable accurate control of the addition of de- flocculant.

Most current methods of monitoring the state of flocculation of a -suspension are based on optical detection techniques. These techniques require optical components such as photodetectors and lasers, which are expensive and complex. It is necessary to provide a window to enable radiation to be passed to the sample to be observed, and this window can become dirty or otherwise inoperative. There are limits on the size of cell which can be used with optical detection techniques, since the techniques impose limitations on the construction of the cell, for example it is necessary to use thin sections.

A device manufactured by Water Instrumentation Division monitors and controls the dosage of chemical to permit more efficient water clarification and sludge dewatering. The device monitors and indicates the proper feed rate of flocculant/coagulant chemicals by measuring an electric current generated when particles in water are temporarily immobilised and liquid is forced to flow past the particles. The device operates using a reciprocating piston which creates an alternating current by stripping counter ions from the immobilised particles. Apart from the complexity arising from the moving piston, this device is only suitable for monitoring the usage of chemical in a suspension with a very low concentration, about 1 or 2%.

The present invention seeks to provide a method or apparatus for determining the state of deflocculation, which avoids the problems mentioned above. According to one aspect of the present invention, there is provided a method of monitoring characteristics of particles in a suspension, by applying a field to a region of suspension to cause the particles to adopt an ordered orientation and monitoring any change in the value of an electrical property of said region in the presence of the field as a result of the ordered orientation, relative to its value in the absence of the field.

According to another aspect of the present invention there is provided a method of monitoring changes in the state of flocculation of particles in a suspension by applying a field to a region of suspension to cause alignment of deflocculated particles in the region and monitoring changes in the relative values of an electrical property of the suspension in the presence and absence of the field.

Thus, according to this aspect of the invention, the change in electrical property arising from the change in alignment of the particles is monitored to indicate the state of flocculation.

The electrical property may be a voltage, or an electrical property, which may be derived therefrom such as current, resistance, conductivity or permittivity. The value of the electrical property varies depending on the freedom of movement of the particles. Thus, in a fully flocculated suspension, an ordering force will have little, if any, detectable effect on the flocculation and only a small or zero change in the electrical property will be observed. In a fully defloccula ed state, all particles will be free to move, to generate the maximum change in the electrical property. The applied field may be shear, magnetic, acoustic, gravitational or centrifugal.

As examples, a magnetic filed can be created by electromagnets, an acoustic filed by an ultrasonic transducer and a shear field by creating a velocity gradient in the suspension.

The field can be applied periodically as a pulse with the electrical property being monitored during and on the removal of the pulse to establish the change in the property. The magnitude of the change varies with the state of flocculation and this variation may be monitored whilst adding deflocculant to the suspension to establish when deflocculation has been achieved. The applied field may be any of the afore-mentioned, or it may be electrical.

The state of flocculation of the suspension can be monitored by observing the highest value of the electrical property immediately the field is removed or time may be allowed for the suspension to reach a new equilibrium state. The size of particles in the suspension may be monitored by monitoring the change with time of the electrical property following removal of the pulse. The present invention also provides apparatus for monitoring characteristics of particles in a suspension, which particles, when deflocculated, are such that they can move to adopt an ordered orientation in an applied field, the apparatus comprising means for applying a field to a region of the suspension, and means for monitoring changes in the relative values of an electrical property of said region in the presence and absence of said field.

One embodiment of the invention uses a conductivity cell comprising carbon electrodes at opposite ends of a region of suspension, means for applying a magnetic field to the region or a portion thereof and means for measuring the change in conductivity between the carbon electrodes.

A further embodiment uses carbon electrodes each with an aperture therein and with the apertures aligned with and at opposite ends of a length of tube, means for feeding suspension into the aperture of one electrode to pass through the tube and through and out of the aperture in the other electrode, and means for measuring the conductivity of the suspension between the electrodes both during non-turbulent flow of the suspension-through"the-tube and- in the absence of flow.

A yet further embodiment comprises two spaced co- axially arranged toroids, means for producing a flow of suspension through the central apertures of both toroids, means for feeding a current to one of the toroids and means for measuring the current induced in the other toroid to obtain an indication of the conductance of the suspension. For a better understanding of the present invention, and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which:-

Figures 1a to 1b illustrate respectively a circuit diagram and electrode layout of one form of system for applying a pulsed electrical field to a suspension, and in which the monitored electrical property is DC voltage;

Figures 2a to 2d are graphs illustrating the effect of flocculation state on electrical response; Figures 3a to 3d are graphs illustrating the effect of particle size on the electrical response; Figure 4 illustrates a system using an applied acoustic field, and in which the electrical property monitored is voltage; Figure 5 shows diagrammatically a system for measuring the conductance of a suspension in the presence of a magnetic field;

Figure 6 shows diagrammatically a system for measuring conductance of a flowing suspension passing through two toroidal coils;

Figure 7 show diagrammatically a further system for measuring the conductance of a flowing suspension; Figure 8 shows a conductivity/time response curve for the system^' of Figure 7; Figure 9 is a graph of results obtained with the system of Figure 7; and

Figure 10 shows diagrammatically a kaolinite processing arrangement employing a system corresponding to that of Figure 7. Figures 1a and 1b show a system using a pulsed electrical field, and in which the electrical property is a DC voltage.

The suspension 1 is held by a container 2. Electrodes 3 and 4 are immersed in the suspension 1 and are connected to respective normally closed switches 5,6. A DC voltage supply 7 is connected between the switches 5,6, and a peak hold voltmeter 9 is connected across the electrodes 3, 4 by way of a normally open switch 8. A 20ms relay 10 is connected across the supply 7.

Operation of the system is as follows: Switches 5 and 6 are closed to apply an electric field to the suspension, thus causing any free particles in the suspension to adopt an ordered orientation. Subsequently, relay 10 operates to open switches 5 and 6 and to close switch 8 to connect the electrodes 3,4 to the voltmeter 9. The maximum voltage obtained is recorded and is representative of the number of free particles is the suspension. In order to measure the maximum voltage, it should be measured as soon as possible after removal of the applied electrical field.

Thus the field is applied as a voltage pluse of about 10-500V per mm of sample gap, for example with a duration of a few milliseconds (to avoid overheating of the sample). It is possible to obtain information concerning the state of flocculation of the suspension by monitoring the maximum voltage on removal of the pulse. The differences in magnitude of this voltage can be seen from Figures 2b to 2d. Figure 2a illustrates the switching pulse for the aligning field, and Figures 2b and 2d illustrate the electrical response curves, referred to the switching pulse, for flocculated, partially deflocculated and fully deflocculated suspensions respectively. The greater the state of deflocculation, the larger the voltage. Monitoring the change in voltage between presence and absence of the aligning field thus provides a means of monitoring the level of flocculation of a suspension which can be used to monitor and control the addition of de-flocculant to the suspension to achieve complete or substantially complete deflocculation.

In addition to information on the state of flocculation or deflocculation of the suspension as discussed above, it is possible by analysing the electrical response curve to obtain information concerning particle size in a deflocculated suspension. Figure 3a shows the switching pulse for the aligning field, and Figures 3b to 3d show the electrical response curves for small particles, medium size particles and large particles respectively. As can be seen, the relaxation portion of the response curve for large particles is more extended than that for small particles. This is due to their relative inertia, inter-particle forces and Brownian motion which creates random orientation on the removal of the ordering force or field.

Figure 4 shows a system using an applied acoustic field, and in which the electrical property is voltage. In this system, an ultrasonic generator 15 actuates a transducer 14 to set up the acoustic field in the container 2 to cause the free particles in the suspension 1 to align. A voltmeter 13 is connected to the electrodes 3 and 4 to provide a reading related to the number of free particles. Figure 5 shows apparatus for use in a method of determining the flocculation state of a suspension in which the conductance of a path through the suspension is used as the electric characteristic to determine the state of flocculation. This apparatus comprises two tanks 30 and 31 for the suspension 33, the tanks being coupled together by a tube 32. Carbon electrodes 34 and 35 extend into the suspension in tanks 30 and 31 respectively. Conductors 36 and 37 connected to electrodes 34 and 35 respectively are connected to a conductance measuring circuit arrangement (not shown) ♦ The conductance measuring circuit arrangement may be of any suitable form.

Positioned on opposite sides of the tube 32 are the poles 38 and 39 of a magnetic system such that lines of magnetic force extend transversely across the tube. The poles are of an electro-magnetic arrangement which enables ready establishment or removal of the field. The arrangement of Figure 6 is for use in assessing the level of flocculation in a suspension of particles which can be orientated using a magnetic field; such a suspension is SPS kaolinite. It is known that the predominant magnetic field orientation of SPS particles is such that their platelet normals prefer to lie 90° to the direction of the field. This assumes that the particles are free discreet particles. However, particles that are flocculated are unable to be orientated by the magnetic field. The magnetic field orientation for kaolinite is thought to be due to magnetic impurities in or on the particles. Tests using the apparatus of Figure 6 have shown that the conductance of the path between the electrodes through the tube 32 is dependant upon the state of flocculation of the suspension and thus the measurement of this conductance relative to the conductance in the absence of a magnetic field can be used to give a measure of the flocculation state.

Whilst the usefulness of this apparatus and method of assessing the state of flocculation is not dependant upon the theory now to be advanced, it is believed that the reason for the change in the conductivity is the preference of the kaolinite particles to align such that their platelet normals lie at 90° to the direction of the magnetic field, thereby increasing the tortuosity of the current paths for current flowing at 90° to the direction of the magnetic field. Hence the conductivity of the suspension in the tube decreases upon activation of the magnet if the suspension contains discreet particles. No change in tortuosity of the current path occurs if the particles are all flocculated particles since they will not be aligned by the magnetic field.

Figure 6 shows a further embodiment employing elongational flow or shear flow of the suspension and using such flow to orient the particles of the suspension. Measurement of the conductance is again the method used for assessing the level of flocculation of the suspension. The apparatus uses an electrode-free method of measuring conductance and has a tank 40 for a suspension 41 with an inlet conduit 47 and an outlet conduit 49 for providing a flow of suspension into and out of the tank. A toroid measuring arrangement 42 (for example, a Wayne-Kerr conductivity loop cell C321 ) is positioned in the suspension and comprises two toroids 43 and 44 co-axially arranged and spaced apart by a fixed distance. The electrical connections to the toroids 43 and 44 are referenced 45 and 46 respectively. The end 48 of conduit 47 passes co- axially through toroid 43 and projects into the space between this toroid and toroid 44. The end portion 48 and 47 is surrounded by a hollow frusto-conical cover 50 with rectangular apertures 51 therein. The frusto- conical cover 50, which may be made of polystyrene, holds the end of the pipe 48 centrally in the toroid device and the approximately rectangular windows in the walls of the cover provide a current-path linking the two toroids 43 and 44. As shown, the toroid device is totally immersed in the suspension and the flow of the suspension is produced by a peristaltic pump (not shown) at a rate of 3-6.5 mis^-'' .

In this method of measuring the flocculation state of the suspension, a current is fed to toroid 43 through conductors 45 and the magnetic field produced thereby induces a current, in the suspension flowing through the toroid 43, having a magnitude which depends upon the conductance of the suspension. This current then produces an induced voltage in the toroid 43 , the magnitude of which in turn corresponds to the magnitude of the current in the suspension. The voltage produced in the toroid 44 is fed via the cable 46 to a suitable measuring arrangement (not shown). A measure of the relative conductance (flowing/non-flowing suspension) for different degrees of flocculation are thereby obtainable. Using the arrangement of Figure 6 definite changes in relative conductivity of the flowing/non-flowing suspension for changes in the flocculation level of the suspension, were detected for a kaolin suspension, i.e. for a suspension having platelet particles.

Figure 7 shows diagrammatically a further embodiment of the invention and in particular a laboratory arrangement which has provided satisfactory results for assessing de-flocculation levels of kaolin in suspension. This arrangement comprises two carbon electrodes 60 and 61 each having a thickness of approximately 1 cm with a bore of 7 mm2 cross section. A tube 62 of corresponding bore, of plastics material, couples in liquid-tight fashion the two apertures in the electrode 60 and 61. A suspension input tube 63 is connected in liquid-tight fashion to the opposite end of the bore in electrode 60 and an output tube 64 is connected in liquid-tight fashion to the other end of the bore in electrode 61. The output pipe 64 passes around a peristaltic pump 65 and its output end empties into a tank 66.

A conductivity metering arrangement 67 is electrically connected to the electrodes 60 and 61 and additionally via a lead 70 (shown by a dotted line) to a processor unit 68. The processor unit 68 is also connected to the peristaltic pump 65 to control its operation by a further lead 71 , which is also shown as a dotted line. A printer 69 is coupled to the processor unit 68 to print out results of the processor.

In operation a kaolin suspension is pumped from a source (not shown) by operation of the peristaltic pump 65 and drawn via input tube 63 through the bore in electrode 60, the pipe 62, the bore 60 in the electrode 61 , and the output pipe 64 into the storage tank 66 .

The suspension flows in a non-turbulent manner through the tube 62 and is in contact with the inner surfaces of the bores in the carbon electrodes 60 and 61.

Using this method successive measurements of conductivity were made on a kaolin pit wash suspension for different dosages of de-flocculant. Measurements were made at each de-flocculant dosage both with the suspension flowing and with the suspension stationary, i.e. with the pump on and off under the control of microprocessor 68. The conductivity measurements from the conductivity meter were processed in the microprocessor 68 and fed to the printer 69 to print out the different conductivity measurements.

Figure 9 shows a typical response curve for the output for the conductivity meter and shows the conductivity measured during periods of flow 0 to A and C upwards, as well as the conductivity measurement from cessation of flow at point A to commencement of flow at point C. As can be seen, during the periods of flow there is an alternating component of conductivity but the average or base conductivity level is significantly higher than the conductivity at no flow. After cessation of flow (at A) the conductivity falls, reaching a stable low level after approximately 30 seconds. Figure 10 shows a graph of the change in conductivity of the suspension between the flow and no- flow conditions-represented as a percentage of the conductivity with no flow - against level of deflocculant added to the suspension. Clearly the conductivity ordinate in Figure 9 starts at a conductivity level significantly above 0 since as can be seen from Figure 10 the percentage change in conductivity between flow and no flow, ranges from approximately 1 1/2 % to 6 % for different de- flocculant dosages.

As can been seen from Figure 10 the percentage change in conductivity rises steeply at first when the suspension still contains flocculated particles and levels out once the suspension is fully de-flocculated. At this point further addition of flocculant does not result in a change in the on/off conductance percentage.

Figure 8 shows a deflocculation processing arrangement for use in a kaolin production plant. In this arrangement parts which correspond to the apparatus of Figure 7 have been given the same reference numerals. All the components 60 to 65 are the same as or similar to those of Figure 7, although the tube 62 is indicated as being partially coiled. This is optional and does not affect the non-turbulent flow of the suspension through the tube. The processor is referenced 68^ and corresponds to a combination of the units 67 and 68 .of Figure 7, being provided with means for both measuring and processing the conductivity between the electrodes 60 and 61 via leads represented by dotted lines 72 and 73. Output of the microprocessor 68^ are fed to a display unit 81 and also via a lead shown by dotted lines 74 to a control valve 76 in a line from a de-flocculant supply 78, the latter output being a valve control output. A source of clay (kaolin) in suspension is indicated by the reference 77 and this is fed to a mixer 75 and from this mixer to an output 79. Deflocculant can also be fed to the clay suspension feeding into the mixer, via the control valve 76 and a pipe 82. The processing arrangement is required to add sufficient de-flocculant to the suspension to substantially fully deflocculate the clay in suspension and to feed this fully deflocculated suspension to the output 79. The conductivity measuring arrangement is used in a closed control loop with the microprocessor

68¹ to control the flow of deflocculant to the mixer 75 to achieve and maintain this level of deflocculation. To achieve this, on starting up of the plant the microprocessor controls the addition of deflocculant in stages to the clay pitwash and the mixed pitwash and 5 deflocculant is sampled under the control of the microcomputer 68¹ via tube 63 using the peristaltic pump 65. Measurements are made of the conductivity with flow and no flow on each sample and deflocculant added by control of the valve 76 by the microprocessor

10 until the percentage change in conductivity levels off as shown in Figure 10. In carrying out this process the microcomputer stores the results and hence the percentage change in conductivity against deflocculant curve for the pitwash is stored in the memory of the

15 computer. The level of deflocculant added to the suspension is then set by the computer to a value which

„ , is just below the Level representing maximum change in conductivity at a level say of approximately 2% of this maximum conductivity percentage change.

20 Every 30 minutes the de-flocculant level setting is verified by both slightly increasing and slightly decreasing the level of deflocculant added under the control of the processor and taking conductivity flow/non-flow measurements for both steps. If the

25 percentage change in conductivity measured with slight increase and slight decrease of the deflocculant level correspondingly shows a slight increase and decrease respectively, then it means that the operating deflocculant setting is still positioned on the very

30 low gradient proportion of the curve just below the maximum percentage change in conductivity level. If no change in conductivity percentage occurs with change in deflocculant level it means that the level of deflocculant is too high and the deflocculant is to be

35 reduced until the operating point is again on the shallow gradient proportion of the curve. If on decrease of the de-flocculant there is a sharp drop in the percentage conductivity measurement for flow/no- flow then the level of de-flocculant is too low since it means that the operating point has fallen to a steeper part of the curve of Figure 10 and it is necessary to increase the operating deflocculant dosage level.

The apparatus shown in Figure 8 provides a simple and very effective way of achieving deflocculant dosage control in an on-line production process.

Carbon electrodes have proved to be resistant to a build up of a clay coating which would detrimentally affect the conductivity measurement. Stainless steel electrodes were tried but suffered from such a build up of a clay coating from the suspension. The carbon electrodes in the laboratory arrangement remained substantially free from coating after 20 days of continuous use.

Adding deflocculant to a kaolin suspension significantly changes the base non-flow conductivity of the suspension and it is for this reason that it is the change in conductivity of the flowing suspension relative to the non-flowing suspension which is important. Simple monitoring of solely the flowing conductivity would not be satisfactory by comparison.

Claims

1. A method of monitoring characteristics of particles in a suspension, by applying a field to a region of suspension to cause the particles to adopt an ordered orientation and monitoring any change in the value of an electrical property of said region in the presence of the field as a result of the ordered orientation relative to its value in the absence of the field.

2. A method of monitoring changes in the state of flocculation of particles in a suspension by applying a field to a region of suspension to cause alignment of deflocculated particles in the region and monitoring changes in the relative values of an electrical property of the suspension in the presence and absence of the field.

3.. A method according to claim-2 wherein the field is applied periodically as de-flocculant is added to the suspension to monitor changes in the flocculation state.

4. A method as claimed in any of claims 1 to 3, in which the applied field is electrical.

5. A method as claimed in any of claims 1 to 3, in which the applied field is a shear field.

6. A method as claimed in any of claims 1 to 3, in which the applied field is magnetic.

7. A method as claimed in any of claims 1 to 3, in which the applied field is acoustic.

8. A method as claimed in any of claims 1 to 3, in which the applied field is gravitational.

9. A method as claimed in claim 1 , or in any of the claims 4 to 8 when appendant thereto, in which said field is applied as a pulse and the change with time of the electrical property is monitored in response to removal of the pulse.

10. A method according to any of claims 1 to 9 wherein the said electrical property is the conductivity of said region of the suspension.

11. A method according to claim 10 appended to claim 5 wherein said shear field is produced by causing a flow of the suspension in said region.

12. A method according to claim 10 wherein the conductivity is monitored using a current flowing in the suspension and wherein a magnetic field is applied to the suspension transverse to the direction of the current.

13. Apparatus for carrying out the method of claim 12 comprising two containers for the suspension linked by a tube; a magnetic system for producing a magnetic field transverse to and passing through the tube; two electrodes each to extend into a suspension in a respective one of the containers; and monitoring means connected to the- electrodes-for"monitoring i_he- conductivity of the path therebetween.

14. Apparatus for carrying out the method of claim 13 for monitoring the state of flocculation of platelet shaped particles, such as kaolin, in a suspension, comprising a container; two spaced toroids mounted co-axially in the container; a first pipe passing co-axially into the aperture in one toroid a second pipe positioned on the side of the second toroid opposite to said one toroid; means for providing a flow of suspension between the pipes through the container; means for feeding a current to one of the toroids; and means for monitoring voltages induced in the other toroid to monitor the conductance of the suspension flowing between the toroids.

15. Apparatus for carrying out the method of claim- 11 comprising two carbon electrodes, each having an aperture therein; a tube coupling the apertures in the two electrodes; feeding means for feeding said suspension into one aperture to pass non-turbulently through the tube and out of the other aperture, the suspension in the tube forming said region of suspension; and means for measuring the conductivity of the suspension between the electrodes during flow and with no flow.

16. A processing arrangement for deflocculating particles in a suspension comprising apparatus according to claim 16; a deflocculent supply; a mixing arrangement for mixing deflocculant with said suspension and connected to said deflocculant supply; a control valve in the connection between said deflocculant supply and mixing arrangement; means for supplying mixed suspension from said mixing arrangement to said feeding means; and means for adjusting the control valve to add sufficient deflocculant to said mixture to keep at a substantially maximum value the change in conductivity measured between non-flow and flow of the suspension in said tube.