zontal vacuum belt filter dries fine coal slurry which is typically 20% solids to a cake of 20-40% moisture. On-line analysis on the belt filter gives the
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82 Paper C1 Horizontal Vacuum Belt Filter Control Using On-line Moisture Analysis at Gregory Coal Minepaper C1 Horizontal Vacuum Belt Filter Control Using On-line Moisture Analysis at Gregory Coal MineG France, A von Muralt Callidan Instruments Abstract The paper summarises the recent ACARP project C14061 where a microwave moisture analyser was ˛tted across a horizontal vacuum belt ˛lter at the Gregory coal preparation plant. The hori -zontal vacuum belt ˛lter dries ˛ne coal slurry which is typically 20% solids to a cake of 20-40% moisture. On-line analysis on the belt ˛lter gives the opportunity for feed back control and minimal delay time. The purpose of this project was to gain a better understanding of belt ˛lter control by examin -ing the moisture on-line. The integration of a moisture result into the control loop may provide a cake of more consistent moisture as the drying time can be adjusted real-time to maintain a desired outlet moisture range. After determining the accuracy of the analyser was 1.0% to 2% at 2 standard deviations (depending on the range) it was possible to monitor moisture online. It was found that the moisture analyser proved very useful to optimise process parameters such as ˝occulant addi -tion and belt speed for a desired ˛lter cake moisture. Introduction Horizontal vacuum belt ˛lters are a common dewatering technique for ˛ne coal applications in Australian coal mines. Through the use of these ˛lters, material as coarse as spiral product or as ˛ne as froth ˝otation concentrates can be dewatered e˚ectively. Horizontal vacuum belt ˛lters work by applying vacuum below a ˛lter media. As the name implies the ˛lter is horizontal in the zone where the vacuum is applied. The ˛lter media is therefore ˝at and, for continuous separation to occur, the media must also be moved continu -ously. Generally the horizontal vacuum belt ˛lter is designed to allow about one-third of the conveyor length for drainage of free moisture and the remainder for drawing air through the drying zone. Whilst there are a number of controls to adjust the performance of horizontal vacuum belt ˛lters such as belt speed, cake depth, ˝occulant addition, and vacuum control, to optimise the process requires extensive laboratory sampling to observe a parameter™s e˚ect
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8311th Australian Coal Preparation Conference and Exhibition Proceedings 2007 on the output moisture. Figure 1 below displays a cross sectional view of a typical horizontal vacuum belt ˛lter, the slurry is deposited on top of the cloth on the left side, and dropping o˚ the right side of the belt as dewatered product. Figure 1 Cross-section of a Horizontal Vacuum Belt Filter This project examines the installation of an on-line instrument to measure moisture in real-time. If proven successful, this will allow operators and process engineers to accurately adjust process parameters to meet the desired product moisture. It would also eliminate the costly need to manually sample the ˛lter belts, which in many cases are performed only every two hours. In the early stages of this project Callidan Instruments undertook to develop a microwave based moisture analyser which would predict the moisture content of the ˛ne coal bed across the conveyor belt. The analyser was designed to independently determine the percentage moisture at three di˚erent locations across the conveyor belt. For each location a microwave transmitter was located below the conveyor and an opposing microwave receiver located above the conveyor and ˛lter cake product. One of the major design concerns for this applica -tion was to develop the correct microwave operating parameters to cater for approximately 20- 40 mm of ˛lter cake which could vary in moisture from 20% to 50%. Project Objectives The objectives of this project were to: Ł Design, develop and supply an on-line microwave moisture analyser with 3 measurement heads across a horizontal vacuum belt ˛lter, and outputting three moisture results simultaneously. Ł Install commission and calibrate the analyser at the Gregory CHPP. Ł Evaluate and determine the accuracy of the moisture result. Ł Provide a report on the accuracy and e˚ectiveness of the installed instrumentation and the resultant improved e˚ect upon belt ˛lter control. Ł Identify control strategies for belt ˛lter control using an on-line moisture analyser.
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84Paper C1 Horizontal Vacuum Belt Filter Control Using On-line Moisture Analysis at Gregory Coal MineProject Stages and Outcomes Stage 1 Œ Design and Development of an On-line Moisture Analyser for a Horizontal Vacuum Belt Filter The ˛rst objective of this project was the design and development of a microwave moisture analyser which would be capable of accurately predicting the moisture content at three inde -pendent points across the 5m span of the ˛lter cloth. Whilst there are a number of techniques available for the determination of moisture in material such as ˛ne coal, it was decided to use a microwave transmission technique. This technique passes a microwave signal from below the conveyor fabric, through the ˛lter cake bed and is detected by the microwave receiver. By using statistical analysis tools (chie˝y linear regression) on both the measured microwave attenuation, phase shift, and the ˛lter cake depth an algorithm was determined which could predict the total moisture content of the ˛lter cake. The analyser needed to be designed such that three independent readings of moisture could be obtained from the analyser, hence three pair of sensors where equally placed across the conveyor. The analyser then needed to accept the ˛lter cake depth signal from the already installed ultrasonic sensor. Typically ultrasonic depth sensors would be provided for each microwave sensor, however due to ˛nancial and complexity constrictions, the single ultrasonic was used and the thickness across the belt was assumed to be uniform. In addition to the sensor heads and measurement techniques of the analyser, the analyser needed to incorporate an integral processor, user display, and industry standard analog, digital and serial outputs. Over a period of approx 2 months the analyser design speci˛cation was established, the necessary components ordered and the analyser assembled and tested with Callidan Instru -ments R&D facility in Mackay. Figure 2 and Figure 3 below shows the analyser being manufac -tured in Mackay and the sensor head. Figure 2 Manufacture of Analyser
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8511th Australian Coal Preparation Conference and Exhibition Proceedings 2007 Figure 3 Sensor Head Stage 2 Œ Installation and Commissioning The second objective of this project was the installation, commissioning and calibration of the analyser. The analyser was installed at Gregory in June 2005. The installation of the analyser needed to occur during a routine shutdown period of the Gregory coal handling and prepara -tion plant. The necessary safety requirements for both the mechanical and electrical installa -tion were considered. Mechanical and electrical installations were then conducted safely and without any di˜culties. The unit consists of the conveyor frame which supports the 3 pairs of transmitters/sensors. Located within 5m is the control cabinet which is designed to operate from a standard 110v or 240v power supply. The analyser automatically begins reporting moisture when the ˛lter cake reaches a pre-deter -mined minimum depth which was found to be 10mm. The results of the analyser can be supplied to the control room via analog, serial, or Ethernet, enabling moisture trends to be displayed for the bene˛t of the operators. The analyser can supply an instantaneous (1 second) or a con˛gurable rolling average moisture result at each sensor location or alternatively an average of the 3 results. The microwave transmitters located beneath the cake emit RF energy at less than 5mW or +3dBm. For comparison a mobile phone emits between 50mW and 800mW of RF energy. Figure 4 and Figure 5 show the analyser location prior to the analyser being installed and after the installation.
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86Paper C1 Horizontal Vacuum Belt Filter Control Using On-line Moisture Analysis at Gregory Coal MineFigure 4 Discharge End of Conveyor Prior to Installation Figure 5 Installation of Sensor Heads During Shutdown Period Stage 3 Œ Moisture Analyser Calibration The analyser was calibrated by sampling ˛lter cake directly from the discharge conveyor. A sample of approx 5kg was collected next to each sensor over a period of approx 10 seconds and samples were collected 10 minutes apart. The collected samples were analysed for total moisture content. In total 60 samples were taken for the purpose of calibration, 20 samples per sensor. The analyser moisture result was also averaged over the same 10 second period. It is important to understand the analyser carries out the on-line determination of moisture by measuring the microwave response through the ˛lter cake, however for this to occur indepen -dently from the depth of ˛lter cake the depth signal must be used. For this particular ˛lter belt it was assumed that measuring the cake depth at one location (i.e. the only existing ultrasonic sensor in line with sensor 1) would be su˜cient for all three measurements as ultrasonics could not be provided for each sensor. Figure 6 below highlights the calibration performance of the ˛rst sensor.
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87 11th Australian Coal Preparation Conference and Exhibition Proceedings 2007 Figure 6 Sensor 1 Sampling Data The results from Sensor 1 indicated that the analyser could operate to an accuracy of better than 2.7% (2 std dev) moisture over a range of 25% Π45%. Sensors 2 & 3 did not achieve this level of accuracy and are included below. However for all data sets the F statistic for the data is greater than the critical value indicating that there is a relationship between analyser and lab results, and that this relationship did not occur by chance. For the F statistic calculation, the probability of erroneously concluding a relationship or alpha value was set at 0.05. Figure 7 Sensor 2 Sampling Data
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89 11th Australian Coal Preparation Conference and Exhibition Proceedings 2007 Figure 9 Trends of Moisture Analyser and Laboratories When applying the 3 data sets to the Grubbs Estimator program it can be determined that the analyser error is approx 1.0% moisture at a 95% con˛dence level whilst the two sampling tech -niques yield a slightly greater error (approx 1.1% and 1.2%). Figure 10 below shows the resul -tant Grubbs calculated instrument and laboratory techniques. Figure 10 Grubbs Estimator Reporting of Instrument and Laboratory Errors Stage 5 Œ Investigation into Control Mechanisms and their Optimisation The horizontal vacuum belt ˛lter used for this trial unfortunately had some serious maintenance concerns. The ˛lter belt was scheduled for replacement in late September as the current belt was fouling and partially fiblindedfl. In addition the spray bar nozzles at both the front end of the belt ˛lter (prior to slurry addition) and mid way along the belt ˛lter were partially inoperative.
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90 Paper C1 Horizontal Vacuum Belt Filter Control Using On-line Moisture Analysis at Gregory Coal MineThe result of this poorly performing belt ˛lter was that a large variation in moisture was experi -enced across the width of the belt ˛lter. Typically sensor 1 & 2 experienced extremely high moisture variations and variable depth of product. Sensor 3 experienced much more stable cake depths which resulted in a much more consistent moisture reading. Figure 11 below highlights the vastly di˚ering moisture readings experienced over a 2 hour period. Figure 11 Two Hour Trend of Moistures and Cake Depth What can be observed from this data is the clear relationship the cake depth has with moisture content. Considering that the depth measurement only occurs near Sensor 1, it is interesting to observe the relationship between the two below in Figure 12. Figure 12 Analyser Moisture vs Filter Cake Depth
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91 11th Australian Coal Preparation Conference and Exhibition Proceedings 2007 Whilst depth of ˛lter cake is only a part of the complexity of controlling moisture it can be observed from Figure 12 that typical performance plots for moisture control can be now easily obtained using the calibrated and veri˛ed moisture analyser where previously this exercise required extensive sampling and analysis. After the bed depth trial above, the ˛lter cloth was replaced and no more data was collected for 2-3 months. With the installation of the new ˛lter cloth the ˛lter cake pro˛les appeared much more consistent and of much lower moisture content. Figure 13 displays an example of the e˚ect of belt speed on the on-line moisture result. Typi -cally the plant operators run the belt speed at 0%, however during this test the belt speed was incrementally adjusted up to 50%. A speed of 0% simply represents the minimum belt speed of 30mm/s whilst 100% represents the belts maximum belt speed of 200mm/s or 60t/h. It can be observed that as the belt speed approaches 35% of its maximum speed the moisture content begins to rise rapidly. Whilst this graph clearly displays a relationship between belt speed and moisture it should be understood that the belt speed is not the only contributor to the change in ˛lter cake mois -ture. It was observed during the test that as the belt speed increases the bed depth decreases, hence two e˚ects are contributing together during the this test. Figure 13 Analyser Moisture vs Belt Speed The e˚ect of ˝occulant on on-line moisture was tested (Figure 14). Plant operators typically operate the belt ˛lter with an addition of 100% ˝occulant, meaning the ˝occulant pump is operating at 100% of its capacity. Figure 14 indicates that ˝occulant addition could be reduced to 80% before any appreciable change in moisture was observed on that occasion.
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92 Paper C1 Horizontal Vacuum Belt Filter Control Using On-line Moisture Analysis at Gregory Coal MineFigure 14 Analyser Moisture vs Flocculant Addition Typically operators run belt ˛lters for good discharge and adjust the firightfl bed thickness to achieve this. There is currently no easy way of knowing what the best approach for moisture control should be. Traditional practices usually involve a manual sample being taken directly from the belt discharge every two hours. Not only is this sample indicative of perhaps a snap -shot of 2-3 seconds of belt ˛lter operation the analysis results can take up to 4 hours to be received by the operator. By this time the operation of the belt ˛lter can and usually does change considerably. This project demonstrates the value of an instrument measuring belt ˛lter moisture online so that e˚ective control strategies can be implemented. It is the author™s belief that the accuracy achieved from the analyser would clearly add bene˛t to strategies for belt ˛lter control. Having an instrument reliably provide direct on-line information to an accuracy of better than 1.0% moisture would be a clear advantage over previous labour intensive sampling practices. The control strategies for optimum belt ˛lter control will vary considerable depending upon the objectives of each operation. Not all plants may require the lowest moisture possible as some operations will consider throughput and ˝occulant usage as equally important param -eters. There may even be the case of wanting the product to be of higher moisture content to meet client speci˛cations. Regardless of plant objectives, there is little debate that the use of an accurate and reliable on-line moisture result will enable plant operators to tune the belt ˛lter for their own objectives. The ˛ndings of this project suggest there is opportunity for this type of analyser in almost every belt ˛lter application world wide. The analyser could be used in industry for more e˚ec -tive means to monitor product quality and e˜cient use of ˝occulant.
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