CA2616564A1 - Method of and control system for controlling electrostatic separator - Google Patents
Method of and control system for controlling electrostatic separator Download PDFInfo
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- CA2616564A1 CA2616564A1 CA002616564A CA2616564A CA2616564A1 CA 2616564 A1 CA2616564 A1 CA 2616564A1 CA 002616564 A CA002616564 A CA 002616564A CA 2616564 A CA2616564 A CA 2616564A CA 2616564 A1 CA2616564 A1 CA 2616564A1
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- separator
- grade
- control variable
- set point
- product stream
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B03—SEPARATION OF SOLID MATERIALS USING LIQUIDS OR USING PNEUMATIC TABLES OR JIGS; MAGNETIC OR ELECTROSTATIC SEPARATION OF SOLID MATERIALS FROM SOLID MATERIALS OR FLUIDS; SEPARATION BY HIGH-VOLTAGE ELECTRIC FIELDS
- B03C—MAGNETIC OR ELECTROSTATIC SEPARATION OF SOLID MATERIALS FROM SOLID MATERIALS OR FLUIDS; SEPARATION BY HIGH-VOLTAGE ELECTRIC FIELDS
- B03C3/00—Separating dispersed particles from gases or vapour, e.g. air, by electrostatic effect
- B03C3/34—Constructional details or accessories or operation thereof
- B03C3/66—Applications of electricity supply techniques
- B03C3/68—Control systems therefor
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B03—SEPARATION OF SOLID MATERIALS USING LIQUIDS OR USING PNEUMATIC TABLES OR JIGS; MAGNETIC OR ELECTROSTATIC SEPARATION OF SOLID MATERIALS FROM SOLID MATERIALS OR FLUIDS; SEPARATION BY HIGH-VOLTAGE ELECTRIC FIELDS
- B03C—MAGNETIC OR ELECTROSTATIC SEPARATION OF SOLID MATERIALS FROM SOLID MATERIALS OR FLUIDS; SEPARATION BY HIGH-VOLTAGE ELECTRIC FIELDS
- B03C3/00—Separating dispersed particles from gases or vapour, e.g. air, by electrostatic effect
- B03C3/02—Plant or installations having external electricity supply
- B03C3/04—Plant or installations having external electricity supply dry type
- B03C3/14—Plant or installations having external electricity supply dry type characterised by the additional use of mechanical effects, e.g. gravity
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- Automation & Control Theory (AREA)
- Electrostatic Separation (AREA)
Abstract
A method and control system are provided for controlling an electrostatic separator 22. The separator is controlled by selecting a set point value for the grade of one of its product streams, spectroscopically measuring the grade of that product stream and using a simple controller to adjust one of the control variables of the separator 22 automatically to maintain the grade of the one product stream at the set point value. A second control variable is manually adjusted and the automatic control is allowed to adjust the first control variable to maintain the output grade at the set point value. The yield is monitored manually and the second control variable is adjusted manually to optimise the yield, while maintaining the set point grade value.
Description
2 PCT/ZA2006/000090 METHOD OF AND CONTROL SYSTEM FOR CONTROLLING ELECTROSTATIC
SEPARATOR
FIELD OF THE INVENTION
THIS INVENTION relates a method of and a control system for controlling an electrostatic separator.
BACKGROUND TO THE INVENTION
Electrostatic separators have been in use in the mineral sands industry for over fifty years and in the recycling industry for many years.
Figure 1 of the accompanying drawings diagrammatically illustrates an electrostatic separator which includes a metal roll 10 which is electrically earthed. A
brush 12, which maybe electrically conductive and electrically earthed serves to sweep non-conductive particles off the roll. The roll as illustrated in Figure 1, rotates clockwise.
A high voltage wire 14 extends along the roll close to its outer surface and there is a coronal discharge between the wire 14 and the roll 10. Mineral sand, or other solid particles containing the materials to be separated, is fed, as shown by arrow A, as a thin stream onto the roll. As the particles pass the wire 14 they are charged. The so-called conductor particles lose their charge almost immediately as it leaks away to the earthed drum 10 and are thrown off the roll as indicated by the arrow B. The non-conductor particles retain their charge for longer and are "pinned"
to the drum. They either fall off later than the conductor particles, as indicated by arrow C, or are swept off by the brush 12.
Adjustable splitter plates 16 and 18 keep the conductor and non-conductor particles apart. There is inevitably, between the separated conductor and non-conductor particles, an area where what are called "middlings" fall.
Middlings are a mixture of conductor and non-conductor particles. The arrows D
and E indicate the path of the middlings.
An earthed static electrode 20 may be used, charged to the same electric polarity as the corona wire 14 assists in discharging the conductor particies, attracting them by electric induction and thus freeing them from the roll 10.
A conventional electrostatic separator has a number of controls which the operator uses to obtain maximum yield and acceptable mineral grade.
Specifically the speed of the roll 10, the voltage on the wire 14, the voltage on static electrode 20, the positions of the corona wire 14 and the electrode 20, the positions of the splitter plates 16 and 18, air humidity, mineral particle temperature and mineral feed rate can usually be adjusted.
Analysis of the conductor and non-conductor particles can be used to determine whether the grade of the products emerging from the separator is acceptable. The difficulty is that a proper.analysis may take many hours. The
SEPARATOR
FIELD OF THE INVENTION
THIS INVENTION relates a method of and a control system for controlling an electrostatic separator.
BACKGROUND TO THE INVENTION
Electrostatic separators have been in use in the mineral sands industry for over fifty years and in the recycling industry for many years.
Figure 1 of the accompanying drawings diagrammatically illustrates an electrostatic separator which includes a metal roll 10 which is electrically earthed. A
brush 12, which maybe electrically conductive and electrically earthed serves to sweep non-conductive particles off the roll. The roll as illustrated in Figure 1, rotates clockwise.
A high voltage wire 14 extends along the roll close to its outer surface and there is a coronal discharge between the wire 14 and the roll 10. Mineral sand, or other solid particles containing the materials to be separated, is fed, as shown by arrow A, as a thin stream onto the roll. As the particles pass the wire 14 they are charged. The so-called conductor particles lose their charge almost immediately as it leaks away to the earthed drum 10 and are thrown off the roll as indicated by the arrow B. The non-conductor particles retain their charge for longer and are "pinned"
to the drum. They either fall off later than the conductor particles, as indicated by arrow C, or are swept off by the brush 12.
Adjustable splitter plates 16 and 18 keep the conductor and non-conductor particles apart. There is inevitably, between the separated conductor and non-conductor particles, an area where what are called "middlings" fall.
Middlings are a mixture of conductor and non-conductor particles. The arrows D
and E indicate the path of the middlings.
An earthed static electrode 20 may be used, charged to the same electric polarity as the corona wire 14 assists in discharging the conductor particies, attracting them by electric induction and thus freeing them from the roll 10.
A conventional electrostatic separator has a number of controls which the operator uses to obtain maximum yield and acceptable mineral grade.
Specifically the speed of the roll 10, the voltage on the wire 14, the voltage on static electrode 20, the positions of the corona wire 14 and the electrode 20, the positions of the splitter plates 16 and 18, air humidity, mineral particle temperature and mineral feed rate can usually be adjusted.
Analysis of the conductor and non-conductor particles can be used to determine whether the grade of the products emerging from the separator is acceptable. The difficulty is that a proper.analysis may take many hours. The
3 results, when given to the operator, are not current and in fact are so far out of date as to have little value. A quick but error-prone "snap-shot" analysis can be obtained within thirty or sixty minutes. The difficulty with this is that the sample taken at any specific instant may not be a true reflection of the average yield and purity being obtained. The complexity of manually adjusting all the important controls of an electrostatic separator to achieve firstly a stable grade and secondly a maximum yield, is mostly too much for a human operator. As a result of this, it is the skill and experience of the operator that plays the predominate roll in determining the efficiency of the separation process.
An ideal solution to the manual problem described above would be an automatic multiple-input, multi-output modern control system that measures both grade and yield, and adjusts some or all of the separator controls automatically to achieve a stable grade at a pre-determined set point and to achieve a maximum yield at the same time, as is well known in the art of advanced automatic control, e.g. Model Predictive Control. However such an advanced automatic control system has practical drawbacks in a plant environment, where the standard instrumentation skill levels are more focussed on single-input, single output control systems for instance standard PID (Proportional-Integration-Derivative) control systems and therefore any maintenance may be expensive. Another practical drawback is that the technical complexity of such a solution, specifically the implementation of combinations of safe operating ranges for all the separator controls, can make the commissioning of such a system expensive as well.
An ideal solution to the manual problem described above would be an automatic multiple-input, multi-output modern control system that measures both grade and yield, and adjusts some or all of the separator controls automatically to achieve a stable grade at a pre-determined set point and to achieve a maximum yield at the same time, as is well known in the art of advanced automatic control, e.g. Model Predictive Control. However such an advanced automatic control system has practical drawbacks in a plant environment, where the standard instrumentation skill levels are more focussed on single-input, single output control systems for instance standard PID (Proportional-Integration-Derivative) control systems and therefore any maintenance may be expensive. Another practical drawback is that the technical complexity of such a solution, specifically the implementation of combinations of safe operating ranges for all the separator controls, can make the commissioning of such a system expensive as well.
4 One object of the present invention is to provide a method of controlling an electrostatic separator in a semi-automatic manner thereby to improve the efficiency of the separation process, but in a low cost manner, and technically simple enough to be maintained in a plant environment.
Another object of the present invention is to provide a control system for an eiectrostatic separator which improves the efficiency of the separator process, but a low cost system, that is technically simple enough to be maintained in a plant environment.
BRIEF DESCRIPTION OF THE INVENTION
According to one aspect of the present invention there is provided a method of controlling an electrostatic separator, said method comprising selecting a set point value for the grade of one of the product streams emerging from the separator, measuring the grade of said one product stream, using the result of the measurement to adjust a first of the control variabies of the separator automatically by application of a simple, industry-standard controller to maintain the grade of said one product stream generally at the set point value, manually adjusting a second control variable of the separator, allowing said automatic adjustment of said first control variable in response to the measurement result to take place, to maintain said output stream grade generally at the set point value, manually monitoring the yield and manually adjusting the second control variable or another control variable to optimise the yield, while continually and automatically maintaining said output stream grade generally at the set point value, with the simple control system.
The grade of said one product stream may be measured optically, e.g.
it may be measured spectroscopically or by measuring X-ray fluorescence.
The term "grade" means the percentage of said one stream which is comprised of the material which is preferably required to be in said one stream.
Thus a grade of 90% means that 90% by weight of the material in the product stream is of the material which is required in that product stream.
"Yield", also termed "recovery",, is the ratio between the amount of the required or preferable material in said one stream and the amount of the preferable material in the feed stream, expressed as a percentage.
The set point is preferably an optimum value of the grade of the product stream.
The first control variable is preferably the voltage of the corona discharge wire or it can be the rate of rotation of the roll, or any one of the other control variables of the separator. Further, the second control variable can be any one of the control variables of the separator, such as the voltage of the corona discharge wire, rotation of the roll, voltage of a static electrode, position of the corona discharge wire, position of the static electrode, splitter plate position, material feed rate, material temperature and/or air humidity, provided that the second control variable is not the same as the first control variable.
According to another aspect of the present invention there is provided a control system for an electrostatic separator, said control system comprising means for determining the yield of at least one product stream emerging from the separator and manually operable means for adjusting at least one manually operable control variable of the separator, wherein said control system further includes means for analysing the grade of said one product stream emerging from the separator, means for automatically controlling one of the control variables of the separator using a simple, industry-standard controller, in response to an output signal from the analysing means to adjust the grade of said one product stream to a set point value by application of a simple controller The means for analysing the grade of said one product stream may be an optical meter such as a spectrometer or a meter for measuring X-ray fluorescence.
BRIEF DESCRIPTION OF THE DRAWINGS
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 non-limiting example, to the accompanying drawings in which:
Figure 2 is a schematic representation of a test rig;
Figure 3 shows the correlation of input parameters with product grade, in the test rig of Figure 2;
Figure 4 shows the impact on the test rig of Figure 2, of variation of the separation roll drive speed;
Figure 5 shows the impact on the test rig of Figure 2, of variation of the HT
voltage;
Figure 6 is a schematic representation of the basic control concept for the test rig of Figure 2;
Figure 7 is a table of coupling factors for the major control parameters;
Figure 8 is a schematic of the control model for the test rig of Figure 2;
Figure 9 shows the effect on the test rig of Figure 2, of roll motor speed change;
Figure 10 shows the effect on the test rig of Figure 2, of changed roll speed and reiative humidity during an early morning start-up;
Figure 11 shows the effect on the test fig of Figure 2, of a change in temperature and relative humidity;
Figure 12 shows the effect on the test fig of Figure 2, of a change in humidity and set point; and Figure 13 shows changes in parameters while recovery in the test rig of Figure 2 is optimised.
DETAILED DESCRIPTION OF THE DRAWINGS
The invention is described by way of non-limiting example with reference to its application to electrostatic separators used for separating particulate minerals. However, the invention is useful in all the applications of electrostatic separators of which the applicant is aware, e.g. the removal of metals from a feed-stream of plastic for recycling. The invention also finds application in the processing of other conductor and non-conductors, for instance different types of recycled materials.
Referring to Figure 2, a pilot-scale test installation was constructed at the facilities of the Department of Process Engineering at the University of Stellenbosch to enable continuous tests to be carried out in a controlled environment. A laboratory electrostatic roll separator 22, similar to the electrostatic separator shown in Figure 1, was set up to process sand in the form of particulate ore samples in a closed loop. The feed chute 24 was fitted with electrical heating elements by which the feed temperature could be adjusted. The speed of both feed and main roller drive motors could be controlled as well as the setting of the voltage of the high voltage corona wire 14. The position of the corona wire 14, static electrode 20 and splitter flaps or plates 16,18 on the discharge were all manually adjustable. Mass flow of the feed was measured by calibration of a roll feeder 26.
It is to be understood that, while the invention is described with reference to a research pilot plant setup, it is primarily intended for application to electrostatic separators in an industrial production environment.
The splitter plates 16,18 were set to produce two exit product streams only, i.e. a "conductor" stream that dropped off the front of the separation roller 10 and a "non-conductor" stream that was discharged at the earthed brush 12 at the rear of the separation roller. Setting the splitter plates 16,18 in this way avoided discharging particles in a middlings stream and mass balance analysis to the test installation was simplified. Mass-flow of the conductor stream was measured in-line with a calibrated slot discharge mass flow meter 28.
The product, i.e. the non-conductor stream, was continuously spectroscopically analysed with a Blue Cube MQi in-line mineral quantifier 30 and the product mass flow was calculated by the difference between the feed mass flow and the conductor stream mass flow. Both discharge streams were combined and re-circulated in an endless loop by the combination of a vibration conveyor 32 and a bucket elevator 34.
The test installation was provided with instrumentation and facilities to achieve automatic control and logging of the ore feed mass flow, ore feed temperature, separation roll speed and the voltage of the corona wire. In addition, the product mass flow, product grade and relative humidity (inside the separator casing) were logged and were used as input to the control program. The actual values of all these settings were logged at one second intervals. The other settings were adjusted and logged manually.
A large sample of typical heavy minerals dry mill feed, with known composition, was continuously circulated to determine the characteristics of the installation and to prove the control system. The installation was used to separate non conductive Zircon containing particles from the sample.
Initially, the installation was set up and run continuously, with the corona wire 14 and static electrode 20 (HT plate) switched off, to homogenise the ore sample. Thereafter the corona wire 14 and static electrode 20 were switched on and separation started. Under manual control, parameters were adjusted until stable operation was achieved. The parameters were then adjusted up and down to determine relative sensitivity. A multi-input, multi-output dynamic model of the electrostatic separator was extracted by the application of standard system identification techniques for automatic control and is shown in Figure 3. The initial tests showed that the separator roll speed and corona wire voltage (HT
voltage) setting had the most significant impacts on the product grade and recovery. A
series of tests were then conducted to quantify the sensitivity and practical use of these two parameters.
The impact of variation of the separation roll drive speed is shown in Figure 4. The roll drive motor speed was increased from 50 Hz, in 5 Hz steps, to 70 Hz. As more "in-between" particles were thrown off the roll, the non-conductors product stream mass flow decreased and the product grade improved inversely in steps from 88% to 98% in Zircon content. With the known feed grade of 81 %, the calculated recovery, or yield, reduced from 93% to 86%.
Figure 5 shows the effect of the HT voltage being decreased from 21 kV, stepwise to 14 kV. With the lowered HT voltage applied, less of the in-between or middlings particles pinned to the roll 10 and the product mass flow reduced from 63 g/s down to 46 g/s but product grade improved inversely from 88% to 98%
Zircon content. However, as to be expected, recovery of Zircon dropped from 97% to 80%.
The tests to determine the effects on the installation of varying separation roll drive speed and HT voltage, were conducted within 20 minutes and provided, quantified and consistent results.
Figure 6 shows a schematic of the basic control concept envisaged for controlling the separator. The initial tests were used to determine the time constants for the control system for the installation and the sensitivity of the roll separator speed to the most important adjustable parameters. The experimentally determined coupling factors for the major control parameters are shown in the table of Figure 7.
In these experiments, it was observed that plant reaction time to changes in high voltage and roll speed was about 2 seconds. The flow rate through the MQi sensor 30 was estimated at 4 seconds and the MQi data integration time was set at 9 seconds. The total lag of the MQi output after changes in control parameters thus added up to 15 seconds. To allow for potential system noise and to ensure stability, the overall time delay was set to 25 seconds for modelling purposes. The time constant of the scale, at 10 seconds, was comfortably within this margin.
The control model finally used, is shown in Figure 8. The model was designed to maintain automatically, a constant grade of output product (in this case the non-conductor stream) as the set point and to allow a human operator to maximise product mass flow automatically, implying maximised recovery, while maintaining the desired grade.
The data logging and control program were implemented on a laptop computer. All interfacing was done on the basis of industry standard 4-20 mA
loop or 0-10 V analogue signals and A/D converters or RS232 data link. The test rig was instrumented to log and display the following parameters at a rate of 1 Hz.
Output was calibrated in the stated units:
Roll speed Drive Motor supply frequency: Hz Feed rate Drive Motor supply frequency: Hz High voltage setting kV
Ore temperature c Relative humidity %
Product flow rate gls Product grade % Zircon The parameters of the control model were conservatively set as a simple proportional control with a time constant of 200 seconds. An integral function was used to ensure return to the set point. No derivative function was necessary. To test the control function, automatic control was directed at the single input parameter of High Voltage (corona wire voltage or HT), directed at maintaining product grade at a set point. Other parameters were then adjusted manually for creating disturbances.
Figure 9 shows the effect of a sharp roll motor speed change from 65Hz to 45Hz. All other input parameters were held constant. The product grade dropped sharply from the stable condition of 92% to 88% Zircon. At the same time the product mass flow increased. The automatic control action, reducing the HT
voltage, then corrected the grade back to the set point over a period of 300 seconds.
Mass flow adapted to the new operating conditions. The new setting ensured that on-grade production was maintained, although at a lower recovery. It is therefore clear that the operator has direct and simple control over recovery, while the control system continuously keeps the grade constant. The rather slow control action ensured that no unstable situation arose. The small kink in the product flow graph around 1250 s was due to occasional product hang-ups in the discharge chutes.
Figure 10 shows the results of an early morning start-up of the separator, with high relative humidity. The feed heater thermostat controlled the feed temperature within a band of 6 C. The controller quickly stabilised the grade at the 92% set point and maintained grade, by adjusting the HT, as the relative humidity dropped. The operation was stable within 10 minutes after start-up. A step reduction in roll speed disrupted the operation but the controller adjusted the HT
setting back to a new balanced setting and maintained product grade. A small ripple is visible in Figure 10, due to the cyclical variation in product temperature.
Figure 11 shows how the controller handled a change in ore temperature. The relative humidity, measured within the machine housing, changed inversely with the ore temperature. The controller could keep the product grade close to the set point by adjusting the HT Voltage. Product mass flow varied due to the changing conditions.
Figure 12 shows how the settings parameters changed in response to a change in humidity and set point. The left hand side of the graph shows the response due a change in relative humidity. The ripple, due to the cyclic ore temperature regulation, is also visible. When the set point was reduced from 93% to 87% in one step, the voltage settings followed in small steps until the new set point was reached in about 2 minutes. Non-conductor mass flow increased accordingly.
Figure 13 demonstrate the final control model. From the initial working point, product grade was controlled at 92% Zircon content. To improve recovery, the roll speed was increased. The product mass flow showed a small drop and the grade increased above the set point. As the control system increased the voltage to bring the grade back to the set point, product mass flow increased and recovery improved. Recovery was calculated and presented in real time to the operator.
The same pattern repeated itself until the roll speed reached 70Hz, when recovery dropped slightly although grade was still maintained. From the initial setting, the optimum for roll speed and HT was reached with a production gain of more than 10%, while maintaining grade. In this case, the operator was thus able to find a set of separator settings leading to better recovery, at the same product grade, in an easy, practical way.
The invention described allows automatic control of electrostatic separators and rapid optimisation of machine settings, with the advantages of continuously maintaining product output grade, optimum recovery while maintaining the grade, improved production without the burden of regular manual tuning, improved plant yield by optimised flow circuits, higher throughput by less recycling of off-grade product, and allowing multi-parameter plant optimisation.
Another object of the present invention is to provide a control system for an eiectrostatic separator which improves the efficiency of the separator process, but a low cost system, that is technically simple enough to be maintained in a plant environment.
BRIEF DESCRIPTION OF THE INVENTION
According to one aspect of the present invention there is provided a method of controlling an electrostatic separator, said method comprising selecting a set point value for the grade of one of the product streams emerging from the separator, measuring the grade of said one product stream, using the result of the measurement to adjust a first of the control variabies of the separator automatically by application of a simple, industry-standard controller to maintain the grade of said one product stream generally at the set point value, manually adjusting a second control variable of the separator, allowing said automatic adjustment of said first control variable in response to the measurement result to take place, to maintain said output stream grade generally at the set point value, manually monitoring the yield and manually adjusting the second control variable or another control variable to optimise the yield, while continually and automatically maintaining said output stream grade generally at the set point value, with the simple control system.
The grade of said one product stream may be measured optically, e.g.
it may be measured spectroscopically or by measuring X-ray fluorescence.
The term "grade" means the percentage of said one stream which is comprised of the material which is preferably required to be in said one stream.
Thus a grade of 90% means that 90% by weight of the material in the product stream is of the material which is required in that product stream.
"Yield", also termed "recovery",, is the ratio between the amount of the required or preferable material in said one stream and the amount of the preferable material in the feed stream, expressed as a percentage.
The set point is preferably an optimum value of the grade of the product stream.
The first control variable is preferably the voltage of the corona discharge wire or it can be the rate of rotation of the roll, or any one of the other control variables of the separator. Further, the second control variable can be any one of the control variables of the separator, such as the voltage of the corona discharge wire, rotation of the roll, voltage of a static electrode, position of the corona discharge wire, position of the static electrode, splitter plate position, material feed rate, material temperature and/or air humidity, provided that the second control variable is not the same as the first control variable.
According to another aspect of the present invention there is provided a control system for an electrostatic separator, said control system comprising means for determining the yield of at least one product stream emerging from the separator and manually operable means for adjusting at least one manually operable control variable of the separator, wherein said control system further includes means for analysing the grade of said one product stream emerging from the separator, means for automatically controlling one of the control variables of the separator using a simple, industry-standard controller, in response to an output signal from the analysing means to adjust the grade of said one product stream to a set point value by application of a simple controller The means for analysing the grade of said one product stream may be an optical meter such as a spectrometer or a meter for measuring X-ray fluorescence.
BRIEF DESCRIPTION OF THE DRAWINGS
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 non-limiting example, to the accompanying drawings in which:
Figure 2 is a schematic representation of a test rig;
Figure 3 shows the correlation of input parameters with product grade, in the test rig of Figure 2;
Figure 4 shows the impact on the test rig of Figure 2, of variation of the separation roll drive speed;
Figure 5 shows the impact on the test rig of Figure 2, of variation of the HT
voltage;
Figure 6 is a schematic representation of the basic control concept for the test rig of Figure 2;
Figure 7 is a table of coupling factors for the major control parameters;
Figure 8 is a schematic of the control model for the test rig of Figure 2;
Figure 9 shows the effect on the test rig of Figure 2, of roll motor speed change;
Figure 10 shows the effect on the test rig of Figure 2, of changed roll speed and reiative humidity during an early morning start-up;
Figure 11 shows the effect on the test fig of Figure 2, of a change in temperature and relative humidity;
Figure 12 shows the effect on the test fig of Figure 2, of a change in humidity and set point; and Figure 13 shows changes in parameters while recovery in the test rig of Figure 2 is optimised.
DETAILED DESCRIPTION OF THE DRAWINGS
The invention is described by way of non-limiting example with reference to its application to electrostatic separators used for separating particulate minerals. However, the invention is useful in all the applications of electrostatic separators of which the applicant is aware, e.g. the removal of metals from a feed-stream of plastic for recycling. The invention also finds application in the processing of other conductor and non-conductors, for instance different types of recycled materials.
Referring to Figure 2, a pilot-scale test installation was constructed at the facilities of the Department of Process Engineering at the University of Stellenbosch to enable continuous tests to be carried out in a controlled environment. A laboratory electrostatic roll separator 22, similar to the electrostatic separator shown in Figure 1, was set up to process sand in the form of particulate ore samples in a closed loop. The feed chute 24 was fitted with electrical heating elements by which the feed temperature could be adjusted. The speed of both feed and main roller drive motors could be controlled as well as the setting of the voltage of the high voltage corona wire 14. The position of the corona wire 14, static electrode 20 and splitter flaps or plates 16,18 on the discharge were all manually adjustable. Mass flow of the feed was measured by calibration of a roll feeder 26.
It is to be understood that, while the invention is described with reference to a research pilot plant setup, it is primarily intended for application to electrostatic separators in an industrial production environment.
The splitter plates 16,18 were set to produce two exit product streams only, i.e. a "conductor" stream that dropped off the front of the separation roller 10 and a "non-conductor" stream that was discharged at the earthed brush 12 at the rear of the separation roller. Setting the splitter plates 16,18 in this way avoided discharging particles in a middlings stream and mass balance analysis to the test installation was simplified. Mass-flow of the conductor stream was measured in-line with a calibrated slot discharge mass flow meter 28.
The product, i.e. the non-conductor stream, was continuously spectroscopically analysed with a Blue Cube MQi in-line mineral quantifier 30 and the product mass flow was calculated by the difference between the feed mass flow and the conductor stream mass flow. Both discharge streams were combined and re-circulated in an endless loop by the combination of a vibration conveyor 32 and a bucket elevator 34.
The test installation was provided with instrumentation and facilities to achieve automatic control and logging of the ore feed mass flow, ore feed temperature, separation roll speed and the voltage of the corona wire. In addition, the product mass flow, product grade and relative humidity (inside the separator casing) were logged and were used as input to the control program. The actual values of all these settings were logged at one second intervals. The other settings were adjusted and logged manually.
A large sample of typical heavy minerals dry mill feed, with known composition, was continuously circulated to determine the characteristics of the installation and to prove the control system. The installation was used to separate non conductive Zircon containing particles from the sample.
Initially, the installation was set up and run continuously, with the corona wire 14 and static electrode 20 (HT plate) switched off, to homogenise the ore sample. Thereafter the corona wire 14 and static electrode 20 were switched on and separation started. Under manual control, parameters were adjusted until stable operation was achieved. The parameters were then adjusted up and down to determine relative sensitivity. A multi-input, multi-output dynamic model of the electrostatic separator was extracted by the application of standard system identification techniques for automatic control and is shown in Figure 3. The initial tests showed that the separator roll speed and corona wire voltage (HT
voltage) setting had the most significant impacts on the product grade and recovery. A
series of tests were then conducted to quantify the sensitivity and practical use of these two parameters.
The impact of variation of the separation roll drive speed is shown in Figure 4. The roll drive motor speed was increased from 50 Hz, in 5 Hz steps, to 70 Hz. As more "in-between" particles were thrown off the roll, the non-conductors product stream mass flow decreased and the product grade improved inversely in steps from 88% to 98% in Zircon content. With the known feed grade of 81 %, the calculated recovery, or yield, reduced from 93% to 86%.
Figure 5 shows the effect of the HT voltage being decreased from 21 kV, stepwise to 14 kV. With the lowered HT voltage applied, less of the in-between or middlings particles pinned to the roll 10 and the product mass flow reduced from 63 g/s down to 46 g/s but product grade improved inversely from 88% to 98%
Zircon content. However, as to be expected, recovery of Zircon dropped from 97% to 80%.
The tests to determine the effects on the installation of varying separation roll drive speed and HT voltage, were conducted within 20 minutes and provided, quantified and consistent results.
Figure 6 shows a schematic of the basic control concept envisaged for controlling the separator. The initial tests were used to determine the time constants for the control system for the installation and the sensitivity of the roll separator speed to the most important adjustable parameters. The experimentally determined coupling factors for the major control parameters are shown in the table of Figure 7.
In these experiments, it was observed that plant reaction time to changes in high voltage and roll speed was about 2 seconds. The flow rate through the MQi sensor 30 was estimated at 4 seconds and the MQi data integration time was set at 9 seconds. The total lag of the MQi output after changes in control parameters thus added up to 15 seconds. To allow for potential system noise and to ensure stability, the overall time delay was set to 25 seconds for modelling purposes. The time constant of the scale, at 10 seconds, was comfortably within this margin.
The control model finally used, is shown in Figure 8. The model was designed to maintain automatically, a constant grade of output product (in this case the non-conductor stream) as the set point and to allow a human operator to maximise product mass flow automatically, implying maximised recovery, while maintaining the desired grade.
The data logging and control program were implemented on a laptop computer. All interfacing was done on the basis of industry standard 4-20 mA
loop or 0-10 V analogue signals and A/D converters or RS232 data link. The test rig was instrumented to log and display the following parameters at a rate of 1 Hz.
Output was calibrated in the stated units:
Roll speed Drive Motor supply frequency: Hz Feed rate Drive Motor supply frequency: Hz High voltage setting kV
Ore temperature c Relative humidity %
Product flow rate gls Product grade % Zircon The parameters of the control model were conservatively set as a simple proportional control with a time constant of 200 seconds. An integral function was used to ensure return to the set point. No derivative function was necessary. To test the control function, automatic control was directed at the single input parameter of High Voltage (corona wire voltage or HT), directed at maintaining product grade at a set point. Other parameters were then adjusted manually for creating disturbances.
Figure 9 shows the effect of a sharp roll motor speed change from 65Hz to 45Hz. All other input parameters were held constant. The product grade dropped sharply from the stable condition of 92% to 88% Zircon. At the same time the product mass flow increased. The automatic control action, reducing the HT
voltage, then corrected the grade back to the set point over a period of 300 seconds.
Mass flow adapted to the new operating conditions. The new setting ensured that on-grade production was maintained, although at a lower recovery. It is therefore clear that the operator has direct and simple control over recovery, while the control system continuously keeps the grade constant. The rather slow control action ensured that no unstable situation arose. The small kink in the product flow graph around 1250 s was due to occasional product hang-ups in the discharge chutes.
Figure 10 shows the results of an early morning start-up of the separator, with high relative humidity. The feed heater thermostat controlled the feed temperature within a band of 6 C. The controller quickly stabilised the grade at the 92% set point and maintained grade, by adjusting the HT, as the relative humidity dropped. The operation was stable within 10 minutes after start-up. A step reduction in roll speed disrupted the operation but the controller adjusted the HT
setting back to a new balanced setting and maintained product grade. A small ripple is visible in Figure 10, due to the cyclical variation in product temperature.
Figure 11 shows how the controller handled a change in ore temperature. The relative humidity, measured within the machine housing, changed inversely with the ore temperature. The controller could keep the product grade close to the set point by adjusting the HT Voltage. Product mass flow varied due to the changing conditions.
Figure 12 shows how the settings parameters changed in response to a change in humidity and set point. The left hand side of the graph shows the response due a change in relative humidity. The ripple, due to the cyclic ore temperature regulation, is also visible. When the set point was reduced from 93% to 87% in one step, the voltage settings followed in small steps until the new set point was reached in about 2 minutes. Non-conductor mass flow increased accordingly.
Figure 13 demonstrate the final control model. From the initial working point, product grade was controlled at 92% Zircon content. To improve recovery, the roll speed was increased. The product mass flow showed a small drop and the grade increased above the set point. As the control system increased the voltage to bring the grade back to the set point, product mass flow increased and recovery improved. Recovery was calculated and presented in real time to the operator.
The same pattern repeated itself until the roll speed reached 70Hz, when recovery dropped slightly although grade was still maintained. From the initial setting, the optimum for roll speed and HT was reached with a production gain of more than 10%, while maintaining grade. In this case, the operator was thus able to find a set of separator settings leading to better recovery, at the same product grade, in an easy, practical way.
The invention described allows automatic control of electrostatic separators and rapid optimisation of machine settings, with the advantages of continuously maintaining product output grade, optimum recovery while maintaining the grade, improved production without the burden of regular manual tuning, improved plant yield by optimised flow circuits, higher throughput by less recycling of off-grade product, and allowing multi-parameter plant optimisation.
Claims (16)
1. A method of controlling a roll-type electrostatic separator 22, said method comprising selecting a set point value for the grade of one of the product streams emerging from the separator, measuring the grade of said one product stream, using the result of the measurement to adjust a first of the control variables of the separator 22 automatically by application of a controller to maintain the grade of said one product stream generally at the set point value, characterised by manually adjusting a second control variable of the separator 22, allowing said automatic adjustment of said first control variable in response to the measurement result to take place, to maintain said output, stream grade generally at the set point value, manually monitoring the yield and manually adjusting the second control variable to optimise the yield, while continually maintaining said output stream grade generally at the set point value.
2. A method as claimed in claim 1, characterised in that the grade of said one product stream is measured optically.
3. A method as claimed in claim 2, characterised in that the grade of said one product stream is measured spectroscopically.
4. A method as claimed in claim 2, characterised in that the grade of said one product stream is measured by X-ray fluorescence measurement.
5. A method as claimed in any one of the preceding claims, characterised in that the set point is an optimum value of the grade of the product stream.
6. A method as claimed in any one of the preceding claims, characterised in that said second control variable is the voltage of a corona discharge wire 14 of the separator 22.
7. A method as claimed in any one of claims 1 to 5, characterised in that said second control variable is the rate of rotation of a roll 10 of the separator 22.
8. A method as claimed in any one of claims 1 to 5, characterised in that said second control variable is the voltage of a static electrode 20 of the separator 22.
9. A method as claimed in any one of claims 1 to 5, characterised in that said second control variable is the position of a corona discharge wire 14 of the separator 22.
10. A method as claimed in any one of claims 1 to 5, characterised in that said second control variable is the position of a static electrode 20 of the separator 22.
11. A method as claimed in any one of claims 1 to 5, characterised in that said second control variable is the position of a splitter plate 16, 18 of the separator 22.
12. A method as claimed in any one of claims 1 to 5, characterised in that said second control variable is the feed rate of material to the separator 22.
13. A control system for a roll-type electrostatic separator 22, said control system comprising means 26,28 for determining the yield of at least one product stream emerging from the separator 22 and manually operable means for adjusting at least one manually operable control variable of the separator 22, characterised in that said control system further includes means 30 for analysing the grade of said one product stream emerging from the separator 22, means for automatically controlling one of the control variables of the separator 22 in response to an output signal from the analysing means 30 to adjust the grade of said one product stream to a set point value by application of a simple controller.
14. A system as claimed in claim 13, characterised in that the analysing means 30 is an optical meter.
15. A system as claimed in claim 14, characterised in that the analysing means 30 is a spectrometer.
16. A system as claimed in claim 14, characterised in that the analysing means 30 is a meter for X-ray fluorescence.
Applications Claiming Priority (3)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| ZA200505946 | 2005-07-25 | ||
| ZA2005/05946 | 2005-07-25 | ||
| PCT/ZA2006/000090 WO2007014402A1 (en) | 2005-07-25 | 2006-07-24 | Method of and control system for controlling electrostatic separator |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA2616564A1 true CA2616564A1 (en) | 2007-02-01 |
Family
ID=37205218
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA002616564A Abandoned CA2616564A1 (en) | 2005-07-25 | 2006-07-24 | Method of and control system for controlling electrostatic separator |
Country Status (4)
| Country | Link |
|---|---|
| AU (1) | AU2006272493B2 (en) |
| CA (1) | CA2616564A1 (en) |
| WO (1) | WO2007014402A1 (en) |
| ZA (1) | ZA200801761B (en) |
Families Citing this family (4)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US8552326B2 (en) * | 2010-09-03 | 2013-10-08 | Separation Technologies Llc | Electrostatic separation control system |
| US9393573B2 (en) | 2014-04-24 | 2016-07-19 | Separation Technologies Llc | Continuous belt for belt-type separator devices |
| US9764332B2 (en) | 2015-02-13 | 2017-09-19 | Separation Technologies Llc | Edge air nozzles for belt-type separator devices |
| WO2020070241A1 (en) | 2018-10-04 | 2020-04-09 | Provargo A/S | Electronic beverage dosing and pouring spout |
Family Cites Families (9)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| GB1372844A (en) * | 1971-09-14 | 1974-11-06 | Westinghouse Brake & Signal | Indicator arrangements for electrostatic precipitators energised by rectified alternating current |
| US4811197A (en) * | 1987-09-02 | 1989-03-07 | Environmental Elements Corp. | Electrostatic dust collector system |
| US5068811A (en) * | 1990-07-27 | 1991-11-26 | Bha Group, Inc. | Electrical control system for electrostatic precipitator |
| IT1247069B (en) * | 1991-01-25 | 1994-12-12 | Giannantonio Nardotto | ELECTROSTATIC PURIFIER |
| IT1245165B (en) * | 1991-02-15 | 1994-09-13 | Ente Naz Energia Elettrica | SYSTEM TO CONTROL AND REGULATE VOLTAGE PULSE POWER SUPPLIES FOR ELECTROSTATIC PRECIPITATORS |
| US6066194A (en) * | 1998-04-17 | 2000-05-23 | American Standard Inc. | Electronic room air cleaner with variable speed motor |
| CA2501778A1 (en) * | 2002-10-30 | 2004-05-21 | The Procter & Gamble Company | Dynamic electrostatic aerosol collection apparatus for collecting and sampling airborne particulate matter |
| GB0300688D0 (en) * | 2003-01-13 | 2003-02-12 | Gallaher Ltd | Contaminant removal device and method |
| US7025806B2 (en) * | 2003-11-25 | 2006-04-11 | Stri{dot over (o)}nAir, Inc. | Electrically enhanced air filtration with improved efficacy |
-
2006
- 2006-07-24 WO PCT/ZA2006/000090 patent/WO2007014402A1/en active Application Filing
- 2006-07-24 AU AU2006272493A patent/AU2006272493B2/en active Active
- 2006-07-24 CA CA002616564A patent/CA2616564A1/en not_active Abandoned
-
2008
- 2008-02-22 ZA ZA200801761A patent/ZA200801761B/en unknown
Also Published As
| Publication number | Publication date |
|---|---|
| ZA200801761B (en) | 2009-06-24 |
| AU2006272493A1 (en) | 2007-02-01 |
| AU2006272493B2 (en) | 2009-10-22 |
| WO2007014402A1 (en) | 2007-02-01 |
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Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| EEER | Examination request | ||
| FZDE | Discontinued |
Effective date: 20150616 |