Coal Preparation » Process Control
Separation density (RD50) and efficiency (Ep) are two important performance parameters of dense medium cyclones (DMCs). These two parameters are not directly measured but calculated from particle density partition curves. Currently partition curves for DMCs are obtained from washability analysis or density tracer test. However, these analysis cannot be performed routinely due to relatively high cost and time constraints. Therefore, the RD50 and Ep are not routinely determined, let alone monitored on-line. Consequently, undesirable RD50 and/or Ep changes caused by unusual operating conditions and faults are not detected in a timely manner. Moreover, parallel DMC modules could be operating under different separation densities and efficiencies. The yield loss from operating in this way is very costly and variation in product quality will also be high.
In this project, an on-line measurement system for the on-line determination of dense medium cyclone separation density (RD50) and efficiency (Ep) from measured feed flowrate, the ratio of medium to coal, medium density in feed, overflow and underflow streams has been developed by integrating and modifying the electrical impedance spectroscopy based technique developed in previous ACARP projects. Software for the measurements and data communication was also developed. An inclined channel flow guide in the front of the EIS electrode assembly has been developed to release air bubbles, which introduce significant noise in the measured EIS spectrum.
Plant-based trials of the on-line measurement system have been successfully carried out at a Bowen Basin plant. It has been found that the on-line measurement system can monitor the DMC feed flowrate, the ratio of medium to coal, the medium density in feed, overflow and underflow streams. The medium density measured by EIS has a similar accuracy as that from a Marcy gauge. Although the EIS technique has a lower precision than the Nucleonic gauge, the EIS technique is more reliable to measure the medium density in situations where the types of fine contaminants change frequently.
Mathematical models for estimating RD50 and Ep from the on-line measurements of process parameters have been developed. In order to identify the most suitable model, three different types of models have been investigated: the modified Suspension-Partition Model [Hu et al., 2001], empirical correlations and the numerical tracer test model. Density tracer test and float-sink data from plant trials and other sources have been used to validate the model predictions. Due to lack of approaches to estimating model parameters and the requirement of intensive computation of CFD model, the numerical tracer test model was not successful. Empirical models can estimate RD50 and Ep with an accuracy level depending on the operating conditions.
A Suspension-Partition Model has been modified for predicting RD50 and Ep from measured process parameters. All model parameters can be estimated from the measured process parameters. The model permits the partition curves of any particle size fractions to be generated. This model is particularly suitable for on-line application due to its simple computation. From this model, equations has been developed for the determinations of the medium splitting interface and the particle separation boundary. A simple one-parameter equation for the calculation of RD50 has been derived from this simplified theoretical model as:
Algorithms and procedures for predicting Ep have been developed. The predictions of RD50 and/or Ep from the Suspension-Partition Model agree very well with those obtained from 56 cases carried out in this project or from the literature.
The average error of RD50 predictions using the above equation for 56 cases carried out in this project or from the literature is less than 0.035 RD. Considering an error of about 0.02 RD in the medium density measurement and significant errors in the determinations of RD50 and Ep from float-sink analysis, the predictions from the modified suspension-partition model should be regarded as practical and useful.
The measurement of medium density by EIS does not suffer from the bias problem as that encountered in Nucleonic gauge. The precision of EIS technique in the plant-based trials is lower than that from Nucleonic gauge. But in a stable upward pipe flow, the precision of EIS technique can be as high as 0.005. Due to the acceptable accuracy, the EIS technique and the model for RD50 prediction will be very useful for monitoring and control of the differences in the feed medium density and cut points in parallel loops.
The modified Suspension-Partition Model is also useful for the predictions DMC surging and vortex overloading. It has been found from a limited number of cases that the value of the difference between the medium splitting interface and the particle separation boundary is a reliable criterion for surging occurrence or vortex-finder overloading. Therefore, further work on the application of the model to the predictions of DMC surging and vortex-finder overloading are highly recommended. Further investigations are also required to extend the application of the model to operation limits of DMCs and non-conventional DMCs. One of the potential improvements in the suspension-partition model is to include the effect of the ratio of medium to coal.
The EIS concept has been shown to be able to indicate the density of the DMC medium and the ratio of medium to coal at the pilot plant and plant scales. It needs to be recognised that these outcomes were achieved for a limited time with experienced researchers in attendance. This indicates that the concept has the potential to be an important monitoring device in this economic significant aspect of coal preparation.
Interest is being expressed in a commercially available device by both industry and equipment manufacturers/suppliers. The next step is the development of appropriate EIS devices for longer term trials and the conduct of these trials. The emphasis would be 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.