Coal Preparation » General
The current high demand for coal for the export market has raised the issue of the limitations to plant performance and production as the amount of run of mine coal feed to the plant is increased. While there are some obvious limitations such as the amount which the feed conveyor to the plant can physically carry, there are likely to be some very important but currently non quantified impacts on the metallurgical efficiency of the various unit operations within the plant.
The aim of this project was to identify the real limits to capacity for the major unit operations in coal preparation plants. The specific aim of this project therefore was to focus on the DMC circuit (desliming screens, drain and rinse screens and the DMC).
The dense medium cyclone circuit consisting of the dense medium cyclone, desliming and drain & rinse screens provided the initial focus, and the impact of marked changes in solids feed tonnages on the DMC performance was considered. Plant solids feed tonnages were increased by approximately 10%, 20% and 30% above normal capacity and samples were collected of the DMC feed (desliming screen product), DMC overflow (product drain and rinse screen) and the DMC underflow (rejects or middlings drain and rinse screens). Samples of the underflow from the drain and rinse screens (medium) and the feed medium were taken at the same time as the screen products were taken.
It was expected that a simple increase of the solids feed rate to the DMC circuit above the nominal capacity level would result in measureable changes in the efficiency of the DMC and/or associated screening prior to and after the cyclone.
The two investigations conducted in the project have provided evidence that both DMC circuits considered were not stressed during the experiments with regard to metallurgical performance, but there are important findings :
- In both cases, there were plant operational issues which were emerging and would have had detrimental implications with larger solids feed rates. In the first plant, the ability to provide sufficient power to maintain the process was being compromised, while in the latter case, the physical loadings on the screens were of concern.
- The particular sets of design factors (inlet, spigot, vortex finder diameters) and control factors (feed pressure, feed medium density, raw coal washability) led to hydrodynamic fluid flows within the DMCs at both plants which were found to be stable with respect to the changes in solids flow rate which were applied in this investigation.
- The fluid mechanic factors, feed, overflow and underflow medium densities and the related amount of medium in the overflow stream, provide an indication of the hydrodynamic environment in the cyclone. But it appears, within the level of accuracy with which these factors can be measured, there was no detectable change in the environment.
- The level of accuracy with which important variables can be measured in investigations of large DMC in plants is not high and any conclusions drawn from this type of work must take this issue into account.
This finding does not imply that a DMC can not be overloaded, but it means that the design and control conditions found at these two plants did not allow it to become evident.
While the performance characteristics of the DMC in the second investigation was within conventional expectations, the outcome from the first investigation indicates a situation where low density tails and RD50 values for the fine particles can occur in conditions other than those observed for crowding at the vortex finder. This situation can occur where there is, insufficient pressure to provide a effective separation force. This is an important finding since low density tails of order 10% would have significant commercial ramifications depending on the amount of clean fine coal in the DMC feed. For 10% of the DMC feed in this zone, a loss of 1% of the feed would result; at 2 mtpa of feed to the DMC circuit and a value of say $50 per tonne, this would equate to $1 mpa loss.
The second investigation provides some initial guidelines for operating DMC at high solids loading. There needs to be sufficient capacity for the solids being fed to the cyclone to exit at the appropriate portal without encountering overcrowding. The pressure (flowrate) should be such that there is sufficient medium available to carry the solids through the exit ports.
The results from this work will allow a more refined understanding of the interaction between the factors controlling the performance of a DMC. It is recommended that this is used to define further experiments to further crystallise this understanding.
Further investigation/analysis is required on the impact of medium:volumetric solids ratios on the DMC performance. At what point does the hydrodynamic environment change?
The level of accuracy with which DMC investigations in plants can be carried out is not clearly defined and there needs to be some effort to quantify this situation. This has implications for plant management and on-line monitoring as well as the level of understanding of the behaviour of these important separation devices and the potential they can achieve.
The effect of higher the capacity on downstream processes such as thickeners was outside the scope of this investigation. These effects need to be considered when increasing the tonnage to the DMC circuits.
The screening efficiency was determined by measuring the amount of minus 0.5mm material in the screen oversize flow stream at the Bowen Basin plant and is shown in Table 8. The values recorded in this Table along with the amount of magnetite present suggest that the low head screens in this plant did not have any real change in their efficiency as the tonnages to the screen was increased.
Misplaced fines in these circuits was low and therefore the actually increases and decreases as shown in Table 8 did not have any measurable effect on the efficiency of the circuit. However the effect of fines is evident in the size by density partition curves produced for the Bowen Basin (Figures 14 and 15) and the difference between the tracer data and full float sink analysis for the Hunter Valley plant (Figure 12). The effect of fines as shown by the float sink analysis for the minus 4mm material in the Bowen Basin DMC was to increase the D50 as expected. In the Hunter Valley plant the fines had the effect of decreasing the D50 and increasing the amount of low density material reporting to rejects, this was due to the low operating pressure and small vortex finder diameter.