Coal Preparation » Dewatering
The preferred filtration device for flotation concentrates in most Australian plants is currently the horizontal vacuum belt filter. This machine has many advantages such as high reliability, low maintenance, ease of control, high solids capture and able to accommodate rapid changes in feed solids and particle size distribution. But, the capital cost is high (over $1M), not only for the unit itself but also for the extra building area required to house it as a result of its large foot print.
The coal industry currently dewaters spiral product with fine coal scroll centrifuges. These units use a wedge wire screen, with a nominal aperture of 0.4 to 0.5 mm and operate at approximately 100 g's. These units are relatively inexpensive compared to vacuum filters and occupy a comparatively very small footprint within the washery, less than 4-5 m2. This project addresses the question of whether the scroll centrifuge could be adapted to produce low product moisture with flotation concentrate and what changes would need to be made to achieve this result. The main changes to the scroll centrifuge envisaged a new design of basket to improve capture of the finest (- 0.1mm) particles and also optimisation of the centrifugal force to ensure adequate dewatering of the final product. The inability to capture ultrafine particles when dewatering spiral product can potentially be a positive asset, allowing the clays to report to the centrate and thus removed to improve product quality.
The project was conducted in three stages; laboratory studies, pilot testing and full scale trials.
LMPE has recently developed a high g force centrifuge, termed the 'Wallaby', which was developed for an entirely different market. It was apparent that it could be suitable for this project, even though the basic design is significantly different to the FC1200. The screen used in this test work is a chrome plated laser cut screen, which had a nominal slot aperture of 0.1 mm. Each slot was approximately 5 mm long.
There were two aspects to the test program:
· Firstly, assess the centrifuge as a dewatering unit processing flotation concentrate (Dewatering Characteristics).
· The second aspect was to assess the classifying ability of centrifuge/laser cut screen unit, and how that impacts on the specific energy of the product coal; (Classifying Characteristics).
· There is a significant decrease in product moisture as g force for the centrifuge is increased. Forces of about 350g's are required for effective dewatering and there seems to be no real benefit in operating above this g force.
· There is a small increase in product moisture as feed rate increases and a decrease in product moisture as solids content of the feed increases.
· There is a significant reduction in product recovery as the g force increases. This is also seen with decreasing solids content of the feed.
· The feed ash value has no influence on the value of the product moisture
· Both the final product moisture and the solids recovery have a dependence on the centrifuge g force, however the relationship for product moisture is much larger than that for solids recovery.
· The solids content had a different effect on product moisture in the full scale machine compared to the pilot scale machine. Increasing the solids content of the feed slurry actually decreased the moisture content. The engineering difference in the full scale machine was that it did not have an internal scroll which fixed the volume available for the bed.
· The 'Wallaby' produced a lower product moisture compared to the vacuum drum filter, when the centrifuge was operated at a g force greater than 350 and a feed solids content of 35% or greater with a capacity of about 20 m3/hr.
· The slots in the screen may allow the passage of any thin plate-shaped particles through the screen, reducing the performance of the classifying unit. Changing the shape of the slots to round holes, maintaining the same open area, should reduce this bypassing and consequently improve classifier performance.
· In all cases the calorific value for the product was greater than the feed and the centrate is lower than the feed. During centrifuging, the centrate carried with it the higher ash (lower calorific) ultra fine particles, resulting in a product with higher calorific value.
· Analysis suggests that for coal with particle size less than between 0.03 and 0.04 mm, its value with respect to its thermal energy becomes negative.