Coal Preparation » Dewatering
Experimental work at Coppabella by Sedgman suggested that a moisture reduction could be obtained via a reduction in the basket speed with a commensurate increase in basket life. This finding raised the issue of our lack of understanding of :
- The importance of the method of placing the coarse coal feed into the centrifuge,
- Bringing this feed coal up to the rotational speed of the basket, and
- The movement of the bed of coal over the basket.
The objective of the project was to improve the understanding of dewatering performance and wear life of a basket centrifuge via optimal presentation of the coarse coal to the basket, its subsequent movement over the screen and the impact of the centrifugal force being applied.
The project required the collaboration of staff from CSIRO, Sedgman Pty Ltd and Ludowici Mineral Processing Equipment Pty Ltd.
The system under investigation can be described as :
- Coal, with a size range of -50+0.5mm, falling from a drain and rinse screen into a pipe connected to a centrifuge feed chute.
- The coal then moves through the chute at high velocity (possibly up to 8 m/sec).
- It impacts the distribution plate at the back of the centrifuge at high speed where it should be redirected to the basket wall.
- At this point the coal forms a bed on the screen.
- The angle of the basket should be less than the angle of friction, and
- the particles are moved over the basket assisted by axial vibration which effectively causes some fluidisation of the particle bed.
The important control factors are :
- There is sufficient centrifugal acceleration (Zg) to obtain an acceptable dewatering outcome.
- The particles are subjected to that force for sufficient time for the film drainage to take place.
The work program was split between two aspects :
- Controlling and modifying the movement of coal through the centrifuge. This meant examining and controlling the delivery of coal on to the basket via the chute, and the movement of coal across the basket before discharge.
- Observing and measuring the movement of material (coal) through the chute and centrifuge and the moisture content of the coal.
The work program was further divided into a laboratory and full scale programs.
One of the main aspects of the laboratory program was the development of a novel chute design to improve the presentation of the coal to the basket. The best design at this scale was the use of a channel rather than a rounded pipe section, curved at the end to aid in the redirection of the feed and tilting the channel end to aid in the distribution of the feed across the chute.
For the plant investigation, a basket with full length accelerator bars was constructed to accelerate the coal up to the basket speed in as short a time as possible and to help minimise bed slippage along the full length of the basket.
The following conclusions were made from the results :
- The moisture contents were higher than expected due to the centrifuge being lightly loaded. This is at odds with conventional thinking which suggests that a lightly loaded centrifuge should perform best due to unhindered dewatering of thin beds. Analysis of particle movement across the basket suggests that the particles have a low kinetic friction factor which means that they are fluidised and exit the basket quickly, reducing residence time in the basket. Optimisation of the centrifuge for lower feed rates may improve the dewatering performance of the centrifuge, optimisation may include lowering the vibration amplitude or adding a discharge lip to the basket, both will increase the residence time of the coal in the centrifuge.
- As the centrifuge speed is increased, the increased centrifugal acceleration (Zg) appears to be balanced by the decreased residence time and reduced bed depth for the normal chute with minimal impact on the moisture content. Although the increased force acting on the water surrounding the particle drives it off faster, unfortunately, there is insufficient time to effectively remove it from the particle.
- The residence time does decrease with increasing centrifugal acceleration (Zg) for baskets fitted with either the normal or long accelerator bars. Interestingly, the basket fitted with long accelerator bars had lower residence times and slightly higher moisture compared to the conventional basket, at least for the larger particle sizes.
- The slightly higher product moisture for the basket with long accelerator bars is related to the reduced residence time. Optimisation by reducing the amplitude of the basket vibration may also reduce this moisture.
- The curved chute resulted in higher moisture values than the normal chute; it is believed that the new method of entry results in a more fluidised bed with a lower kinetic friction factor, thus reducing the residence time.
- The curved chute had similar moistures for both the normal basket and the basket fitted with long accelerator bars except for the higher Zg on the normal basket. The high moisture value is linked with the rapid movement of the coal over the basket surface.
- The main difference between the Coppabella investigation and this one at Morevale was the level of loading in the centrifuge and the significantly different bed depths.
- It is suspected that bed depth increases the kinetic friction factor and the balance between the impacts of centrifugal acceleration (Zg)