Coal Preparation » Fine Coal
At present, there is no single device for the recovery of coal from feed materials less than 2 mm in diameter. Fine particles are usually separated in two stages. Gravity separation is used for particles in the range 300 µm to 2 mm, followed by froth flotation for the fraction below 300 µm. It would be desirable for these two stages to be carried out in a single step, with the aim of improving recoveries, improving the dewatering of product and tailings, reducing manpower costs, and simplifying plant flowsheets and operational procedures. A new type of froth flotation machine, has been developed for this purpose. Known as the NovaCell, it combines a high-shear reactor for capturing ultrafine particles, with a fluidised bed for recovery of coarse coal particles.
In this report, the results of an investigation into the use of a NovaCell, operating on feed particles in the range 0 to 2 mm, from an operating plant are presented. In continuous operation, high combustibles recoveries were achieved between 92 to 97%, averaging 94.2%. Yields were 83.5 to 88.7%, average 86.3%; and the product ashes were 10.5 to 14.8%, average 12.4%.
The NovaCell is unique in delivering two product streams - a normal froth product, and a product that arises from the formation of particle-bubble clusters in the flotation cell. The cluster product flows out of the top of the cell and on to a sieve bend to recover the coarser particles. In the tests, the fraction recovered in the froth discharge was generally larger than that in clusters. Another unique feature is the delivery of two tailings streams from the cell. The underflow tails consists of coarse silica particles that have settled to the bottom of the cell. The overflow tails are fine silica particles that have been elutriated out of the bed.
As a replacement for the two-stage separation technologies currently in use for fine coal, the NovaCell appears to be a success. Advantages over existing two-stage plants include:
· Improved yields and combustible recoveries;
· Improved product and tails dewatering. Results show product and tails d80 particle sizes are 1390 µm and 1222 µm respectively, pointing to relatively easy dewatering;
· With coarser particle sizes, flocculant consumption will be reduced;
· No need to deslime the feed. The NovaCell gives high combustibles recoveries in the < 75 µm fraction, and good ash rejection;
· Reduced capital costs; spirals or TBS eliminated;
· Reduced operating costs, reduced labour requirements;
· Easier troubleshooting/expansion/optimisation studies due to simplified flowsheet and modular design;
· Reduced water use - the Cell can accept feeds with much higher percent solids than traditional flotation cells;
· Forgiving to changes in feed or operational variables. Floatable particles that drop out of the froth are recovered in the screen discharge if the froth is overloaded;
· Relatively easier maintenance and low wear and tear (no moving parts); and
· Improved bubble-particle collision rates (fast flotation therefore increased throughput for a given cell volume). Typical residence time 1 to 3 mins.