Coal Preparation » Fine Coal
The overall objective of this project was to develop an effective and inexpensive on-line monitoring system for the improved monitoring and control of fine coal flotation processes.
STAGE ONE REPORT
The specific aims of stage 1 are to investigate how the electrical impedance spectrum (EIS) of the froth phase in the flotation process respond to the changes of operating conditions and to develop approaches for applying information from EIS measurements in the on-line monitoring of flotation performance under different conditions in a pilot-plant scale generic coal flotation column.
The key to effective process monitoring and optimising control of fine coal flotation processes is accurate and reliable measurements of process parameters. Whereas some parameters in fine coal flotation circuits can be measured continuously using relatively simple and cheap sensors, the effective on-line monitoring and optimal control of fine coal flotation circuits are still far from being achieved. This is because of the strong inertia of the flotation process, a still inadequate knowledge of suitable variables for the on-line monitoring of the process efficiency and the lack of appropriate on-line measurement instrumentation. Off-line determination of key performance parameters not only involves time-consuming process stream sampling and costly laboratory analyses, but also suffers from excessive time delays that limit the effectiveness of analysis of results as on-line feedback signals for closed loop control systems.
In this project, an innovative on-line monitoring system for the flotation process has been developed based on the electrical impedance spectroscopy technique. The electrical impedance spectrum of the froth phase has been found to be sensitive to the froth structure and characteristics. The froth structure variation or fluctuation caused by bubble coalescence and rupture can be sensed by the variation or fluctuation of electrical impedance in the froth phase. A new approach, in which a Stability Index (SI) has been defined, has been developed to quantify the stability of the froth structure. A froth phase having a SI value higher than 400 has been found to be unstable and the corresponding operating conditions will be unfavourable in terms of the separation efficiency of the flotation process. The most important finding from this project is that the EIS pattern of the froth phase is closely associated with the solids and water content of the froth phase. The EIS of the froth phase having low values of the real impedance component are favourable patterns, and the corresponding operating conditions are also favourable for the flotation process. Due to this relationship, the pattern of the EIS of the froth phase can provide useful information regarding the flotation performance.
An improved measurement technique for solids concentration of slurry mixtures has been developed based on the intelligent data analysis of electrical impedance spectra measured at different amplitudes of excitation signal. It has been demonstrated that this improved EIS based technique is capable of measuring solids contents in slurries down to 2% by volume.
By combining EIS measurements with flowrates measured by a cross-correlation flowmeter (developed in ACARP project C10046), the monitoring system can be used in the following ways:
- Online determination of product yield.
- Control of reagent addition
- Flotation performance monitoring
A data analysis procedure has been developed for the flotation performance evaluation and process diagnosis. In this procedure, a comprehensive analysis of data on the froth stability, the EIS pattern of the froth phase, flotation yield, solids contents in process streams, froth depth, reagent dosages and other information is carried out to evaluate the flotation performance and to identify abnormal or unfavourable operating conditions, such as:
- Sub-optimal dosage of frother or collector
- Inappropriate feed solids flowrate causing froth layer overload or underload
- Inappropriate pulp level or froth depth
- Inappropriate air flowrate
The benefit to coal producers is an improvement of on-line monitoring which will result in increased flotation plant operating efficiency, optimised reagent usage, reduced need for human intervention and the timely detection of problems and faults.
STAGE 2 REPORT
Due to the lack of appropriate approaches and methods, the adjustments to coal flotation operating conditions for the better control of the process are often made by the plant operator on the basis of visual observation (colour and mobility of froth, colour of tails, proportion of coarse material in the tails) and depends to a large extent on the experience and ability of the operator. This can lead to problems of poor flotation performance and reagent overdosing. Whereas some parameters in fine coal flotation circuits can be measured continuously using relatively simple and cheap sensors, the effective on-line monitoring and optimal control of fine coal flotation circuits are still far from being achieved. This is because of a still inadequate knowledge of suitable variables for the on-line monitoring of the process efficiency, the lack of appropriate on-line measurement instrumentation and the strong inertia of the flotation process.
The overall objective of this project is, therefore, to develop an effective and inexpensive on-line monitoring system for the improved operation of fine coal flotation processes. The specific objective is to carry out plant-based trials of the EIS technique developed in ACARP projects C10047-Stage 1, C9045 and C10046.
Plant-based trials of the EIS technique for monitoring flotation processes have been carried out at a Bowen Basin coal preparation plant. It has been demonstrated in the plant-based trials that the change of flotation performance caused by variations in operating conditions can be monitored by the measurement of EIS in the froth phase. The value of Zr0 (i.e. the lowest value of the real impedance component on the EIS spectrum) and the stability ratio (an index based statistic analysis of impedance fluctuations in the froth phase for quantifying the froth stability) of the froth phase can be correlated with the flotation performance. The EIS of the froth phase can sensitively respond to the change of flotation performance. Under a suitable froth stability (i.e. the stability ratio is between 12 to 14%), the less the Zr0 value, the better the yield. For cases with unstable froth structure (i.e. the stability ratio is higher than 14%), the flotation performance is always poor. In the optimizing control of flotation processes, the value of Zr0 can be used as the objective function to be minimised by adjusting operation conditions with the constraints of the stability ratio in the range of 12 to 14%.
In the preparation for the plant-based trials, developments on electronics and sensor probes were also carried out. Due to the lack of a commercially available spectrometer suitable for EIS field applications, one of the major preparation steps for the plant trials was to design and construct an EIS spectrometer. After a long development period, a robust research prototype with the required accuracy has been developed, but the measurement speed of the spectrometer is yet to be improved to catch the dynamic changes in the froth phase of fine coal flotation processes. A sensor assembly, which integrates the sensor probe for the cross-correlation flowmeter, the EIS electrode for solids content and slurry sampler, was developed and tested through the existing slurry sampling ports in the plant-based trials. The use of the existing sampling ports for installing the sensor assembly is a cost-effective approach for both the current plant-based trials and the future commercialization of the technique.
Investigations of the effect of EIS electrode configurations and the sensor position indicated that the EIS in the froth phase should be measured preferably using the two electrode configuration and at the outer overflow weir. The characteristics of the froth phase exiting from the outer weir were found to be significantly different from that from the inner weir. It was also found that more than six turnover volumes were required to reach a new steady state after adjusting operating conditions. The non-uniformity in the exited forth phase and the slow cell dynamics were believed to be associated with the uneven distribution of hydrodynamic conditions in the cell. Therefore, the EIS technique could be also a diagnostic tool for detecting hydrodynamic problems in flotation cells.
The froth stability can be quantitatively monitored by measuring the impedance at 10kHz as a function of time for a given period of time. The ratio of standard deviation to the mean value of the impedance variation in 100 seconds was defined as the Stability Ratio (SR) to quantify the froth stability. As the SR value is statistically significant, a small change in the SR value could correspond to significant changes in the measured impedance fluctuations. It has been found that a froth phase with a SR in the range of 12 to 14% is suitable. A froth phase with a SR higher than 14% will become unstable, leading to poor flotation performance, while a SR value lower than 12% could give rise to an excessively stable froth phase which would lead to a high ash value materials in the concentrate.
Feed flowrate was successfully monitored by the cross-correlation flowmeter. The flow velocity in the tailings pipe was able to be measured but the flowrate could not be estimated due to the undefined cross-section area of the flow path. The solids content can be measured using the sensor assembly and a suitable impedance meter.
The following work is recommended towards the application of the EIS technique in the monitoring and control of flotation processes:
- Interest is being expressed in commercially available devices by both industry and equipment manufacturers/suppliers. The next step is to improve the measurement speed of the CSIRO EIS spectrometer for longer term trials and the conduct of these trials. The emphasis would be on the ability to provide information consistently on a longer term basis in the physically robust environment encountered in a coal preparation plant. The mechanism of supporting and conducting this next step is currently being investigated.
- Intelligent data analysis approaches need to be developed to evaluate flotation performance and identify causes for poor separation efficiency based on the on-line EIS measurement. The development of corresponding control strategies is also required to optimize the operation of flotation processes.
- The EIS in the froth phase could be used as a diagnostic tool for evaluating new designs of flotation cells and modifications to existing cells and for detecting hydrodynamic problems associated with poor flotation performance.
- Significant efforts have been made to develop a quantitative approach for linking the flotation process performance with the EIS measurement (This was not a task component in this ACARP project). Due to the lack of sufficient time, no significant progress has been made. However, the researchers still believe that the link is not only possible but also necessary. It is therefore recommended that the work in this regard should be continued.