GB2181243A - Method and apparatus for measuring or monitoring density or rheological properties of liquids or slurries - Google Patents

Abstract

The method and apparatus involves the monitoring of the driving impedance of an ultrasonic transducer (6), which is encapsulated suitably (7) and immersed in the liquid body and comparing (generally electronically) instantaneous values with empirically derived data for the same type of liquid body. The apparatus may embody a temperature compensating arrangement (8) to counteract any effects of a fluctuating temperature of the liquid body. The method and apparatus is particularly suitable for use in monitoring the density of slurries of ore in an aqueous medium. <IMAGE>

Description

SPECIFICATION Method and apparatus for measuring or monitoring density or rheological properties of liquids or slurries This invention relates to the measuring or monitoring of density or rheological properties (that is to say viscosity, elasticity or plasticity) of liquids, slurries, dispersions or emulsions at any particular area in a body of such liquid or slurry simply by the use of a fixed or movable probe or other detector member. More particularly, but not exclusively, the invention is concerned with the monitoring or measurement of densities ofslurries generally formed in the extraction o r or treatment of finely sub-divided, mined ores during their treatment to recover required values therein.The invention is also concerned with the measurement of a ratio between the quantity of two components in an emulsion provided the rheological properties of each compo nent are sufficiently different to enable a meaningful, ratio dependantsignal to be obtained.

In accordance with one aspectofthis invention there is provided a method of detecting or monitoring a variable physical property being at least one ofthe densityora rheological property ofa liquid body selected from homogeneous liquid bodies and heterogeneous liquid bodies such as slurries and emulsions or of detecting deviations from a required value of such physical property, the method comprising the monitoring ordetecting of the driving impedance our a variable related thereto of an ultrasonic transducer located ata required position in such liquid body and relating the value of such driving impedance orvariable related thereto to a required value thereof orthe physical property itself orto a deviation from such required value.

Fu rther featu res of the invention provide for the ultrasonic transducer to be a piezoelectric ceramictransducer (PZT); for the ultrasonictransducerto be driven at a predetermined frequency in which casethe impedance of the transducer is monitored or detected; for the values detected to be compensated fortemperature; and for the detected values to be related to empirically determined data forthe particular liquid orslurry concerned.

It is notfully understood howthe driving impedance of an ultrasonictransmitter is affected by the density or rheological properties, in particular, density of a slurry or liquid but experimentation conducted to date indicates that each different slurry or liquid will behave differently and will effect different variations in the driving impedance of any particular ultrasonic transducer. Accordingly it is necessaryto calibrate any particular ultrasonictransducerfor use in a particulartype of liquid body.

As the invention is envisaged to be particularly useful in the monitoring or detection of slurries encountered in the mining industry, it is in this direction that at leastthevast majority of research has taken place to date.

With such slurries, and with a preferred form of ultrasonic transducer being a piezoelectric ceramictransducer, a test probe has been tested on frequencies ranging from 0,2MHz to 3MHzandthere is a drop-off sensitivity as frequency is increased. The optimum frequency will depend upon the application. The tests on slurry have been limited to the 1 0 > m to 3OOim particle size range. However, this does not suggest that the principle may not operate satisfactorily outside of this range.

The invention also provides apparatus for carrying out the above defined method and comprising a detector member in the form of an ultrasonictransducer encapsulated and carried on a suitable mounting thereforand a driving and detector circuitfor connection to thetransducerto drive same and detectthe driving impedance thereof or a variable related to such driving impedance.

Further features of this aspect of the invention provideforthe detector member two be carried at one end of an elongate probe,forthe driving circuitto be an active bridge circuit wherein an output voltage from the transducer isto be detected and related to the density orrheological property of a liquid or slurry; forthe detected driving voltage to be inputted to a computer system with empirically obtained information aboutthe relevant slurry or liquid to achieve density measurements; and forthe detector memberto embodytempera- ture sensing means connected to a compensatorto compensate fortemperature variations ofthe liquid or slurry.

In orderthatthe invention may be more fully understood, one embodimentthereofwill now be described with reference to the accompanying drawings.

in the drawings: Figure lisa schematic illustration of a sensorassemblyto be mounted atthe end of a probe; Figure2 illustrates the operative end of a probe embodying the sensor assembly illustrated in Figure 1; Figure3 is a block diagram ofthe driving and measurement system; Figure4is a circuit diagram ofthe active bridge; Figure 5 is an equivalent circuit of a piezoelectric transducer; Figure 6 is a graph of actual density against detected density for a test system after correction for temperature fluctuations; Figure 7 is a graph of a measured solvent concentration against actual solvent concentration; and Figure 8shows the impedance curves generated to determine the operating frequency for a particular PZT.

A piezoelectric transducer can be characterised by a simple equivalent circuit as shown in Figure 5. The components in a series arm 41 represent the notional piezoelectric properties of the ceramic transducer, the component L being the resonating mass ofthe ceramic, and Cthe elastic compliance. The componentR represents mechanical losses. CO is the capacitance between the electrodes on the surface of the ceramic, and R0 is a resistance representing the dielectric losses of the transducer. R1 is the acoustic load that varies with changes in the surrounding medium - it is this parameterthat results inthechanges in the medium of interest.

As illustrated in Figures 1 and 2 the detector unit, generally indicated by numeral 1, is housed,for use, in a robust housing 2 carried on a galvanised iron pipe arrangement3with a window 4 providing access tothe detectorunitthrough the wall of the robust housing.

The detector unit itself, as shown in Figure 1, comprises an electronic 4-pin connector 5 carrying at one end of 20 mm piezoelectric ceramic transducer disc 6 (in this case a lead zirconite titanate transducer) encapsulated in a suitable epoxy resin 7. It is to be mentioned thatthe thickness of such encapsulating resin should not betoo great as this adversely affects the sensitivity of the transducer to density changes.

Thetype of encapsulation will depend upon the application, as the encapsulation must maintain its properties in whatever environment it is used.

The method of arriving atthe correctthickness of encapsulation is to encapsulate thetransducerto an equal thickness on either side and then to remove layers of the material until the greatest sensitivity to changes in the medium is attained.

Also encapsulated within the epoxy resin is a temperature-sensitive transducer8 forenabling temperature compensation to be implemented as will be described below.

A driving circuitforthe detector unit is controlled by a stable frequency sourceS, the output of which is fed via a driving circuit 10tothe measurement and output circuit which is illustrated in block diagram form in Figure3 and which provides an output current (orvoltage) VO. The whole circuit thus comprises frequency generator9 providing a stabilized outputfrequencyofa desired value and which supplies a driver 10 and thence an active bridge circuit 11 into which is connected the transducer 6. The output from the bridge circuit is fed to a rectifier 12thence a zeroing circuit 13, a low pass filter 14 and finallyto a converter 15.

Where required the output is temperature compensated by a compatible commercial temperature converter 1 which is connected to the temperature dependant transducer 8. The outputs from both the converter 15 and temperature converter 16 are fed to a computer 18.

The method of calibrating the instrumentwasthe same whether used for mining -slurry density measure ments orfor emulsion ratio measurements, and was asfollows: The probe was immersed in the liquid body of interest, having a known density or ratio; the medium was then heated over the temperature range at which the instrument was to operate. Values of V0 and Vtwere logged during heating and this procedure was repeated using slurries of different densities oremulsions of different ratios.

Non-linear regression was applied to the resulting data and an expression derived relating density or emulsion ratio to the two variables V0 and VT. The resulting expression tooktheform: F=a1 +a2VO+a3VT+a4VOVT+aSVT where: Fis densityorratio VO istheoutputvoltage proportionalto driving impedance VTisthe output voltage proportional to temperature an are fitted coefficients.

The results obtained, after calibration are shown in Figures 6 and 7wheretesting ofthe instrumentwas carried outon a numberofslurries of different densities and temperatures (Figure 6) and emulsions of different ratios and temperatures (Figure7). The probe was immersed in the medium and the resulting values of V0 and VTwere substituted in the aforementioned expression and eitherthe density or the ratio calculated.

The output frequency range of the circuit was from 500 kHz to 590 kHz in 10 kHz increments.

Figure 8 shows the impedance curves that were generated to determine the operating frequency for the PZT.

The two graphs of impedance vs. frequency of the driver are in respect of two slurries - one of 1,8 gm/cc,and one of 1,1 gm/cc, these densities being the extremes of the range of interest.

The probe was immersed in the lighter slurry and the impedance measured at 300 frequencies in the range 0,4MHz to MHz, using an impedance analyser. (A signal generator and RMSvoltmetercould also be used).

These values ofimpedancewere plotted and this produced the curve marked '1,1 g/cc'. This procedure was repeated using the heavier slurry, and the resulting curve is shown, marked '1,8 g/cc'.

There was a range of frequencies atwhich a reasonable difference in impedance occurred. The operating frequencywas chosen to fall within this range at a point as near as possibletothat of the maximum change in impedance.

Forthe purpose of circuit description it is assumed that the instrument is being used to measure slurry density in the range 1,1 to 1,8 gm/cc.

The active bridge (see Figure 9) was set to provide the largest change in voltage at the output, when the probe was immersed in slurries at either extremeof the range of interest.

Having done this, the probe was immersed in the slurry of lowest density: a d.c. voltage was present atthe output of the rectifier and this voltage was applied to the zeroing circuit and the circuit was adjusted to provide an output current of4mAfrom the converter.

The probe was then immersed in a slurry of highest density and was adjusted to give an output currentfrom the converter of 20mA.

Using the above described device, and with the required calibration, and empirical relationships between the final voltage outputs, a computer system simply compares its programmed information as to voltage and density with the instantaneous voltage signal in order to provide a density measurement. Figures 6 and 7 show the performance curves ofthe instrument when used in slurry (Figure 6) and emulsion (Figure 7).

It will be understood that the invention provides for an extremely useful control instrument in that density can be measured at specific points in a container or pachuca and can thus be employed for checking on mixing efficiencies as well as forth simple control of densities ofslurriesforexample, in carbon-in-pulp gold extraction processes.

Clearly, for each system to which the invention isto be applied, a set of data must be obtained initiallyto enablethe comparison of output voltage to be achieved and related to density.

It is envisaged that othervariables dependant on the impedance of an ultrasonic transducer could be employed.

It is envisaged that the invention will be highly useful in process control.

Claims (17)

1. A method of detecting or monitoring avariable physical property being at least one ofthe densityora rheological property of a liquid body selected from homogeneous liquid bodies and heterogeneous liquid bodies such as slurries and emulsions or of detecting deviations from a required value of such physical property, the method comprising the monitoring or detecting of the driving impedance orvariable related thereto ofan ultrasonic transducer located at a required position in such liquid body and relating thevalueof such driving impedance or variable related thereto to a requ ired va I ue thereof or the physical property itself or to a deviation from such required value.

2. A method as claimed in claim 1 in which the ultrasonic transducer is a piezoelectricceramictransducer.

3. A method as claimed in either of claims 1 or2 in which the ultrasonic transducer is driven ata predetermined frequency and the impedance of the transducer is monitored ordetected.

4. A method as claimed in any one of claims 1 to 3 in which the detected values are compensated for temperature variations of the liquid body.

5. A method as claimed in anyone ofthe preceding claims in which the detected values are related to empirically determined values or data forthe same type of liquid body.

6. A method as claimed in any one of the preceding claims in which the liquid body is a slurry.

7. A method as claimed in claim 6 in which the solid particles have a size range of from 10m to300m.

8. A method as claimed in any one of the preceding claims in which the physical property being monitored or detected is the density ofthe liquid body.

9. A method as claimed in any one ofthe preceding claims in which the frequency of operation ofthe ultrasonic transducer is between 0,2MHz and 3MHz.

10. A method of detecting or monitoring a physical property of a liquid body substantially as herein described with reference to the accompanying drawings.

11. Apparatus for carrying out a method as claimed in any one of the preceding claims and comprising a detector member in the form of an ultrasonic transducer encapsulated in a suitable material and carried on a mounting therefor.

12. Apparatus as claimed in claim 11 and further including a driving and detectorcircuitforconnectionto thetransducerto drive same and detect the driving i m pedance thereof or a variable related to such driving impedance.

13. Apparatus as claimed in claim 12 in which the driving circuit is an active bridge circuit wherein an output voltage from the transducer is detectable for relating to the relevant physical property of a liquid body.

14. Apparatus as claimed in eitherofclaims 12 or 13 in which the apparatus includes a computersystem adapted to receive an outputforthe detectorcircuitandto compare same to empirically derived values.

15. Apparatus as claimed in any one of claims 11 to 14 in which the detector member is carried at one end of an elongate probe.

16. Apparatus as claimed in any one of claims 11 to 15 in which temperature sensing means are provided in the detector member and the apparatus is adapted to compensate fortemperature variations of a liquid body.

17. Apparatus substantially as herein described with reference to the accompanying drawings.