March 2016
|
World Coal
|
69
CHEMICAL
ENHANCEMENT
Mert Kerem Eraydin and John C. Roach, Nalco Chemical Co., USA,
highlight the opportunities to improve coal dewatering operations with
chemical dewatering aids.
T
oday, coal producers must operate in difficult and dynamic
circumstances, adjusting quickly to keep pace with both market
fluctuations and strict environmental regulations. As a result,
every bit of improvement in terms of recovery and product
quality may have a significant impact on the viability of any operation.
Briefly, coal processing includes classification, cleaning and dewatering
with unique techniques for coarse and fine size fractions. Clean coal
products from both coarse and fine circuits are then blended in order to
meet strict quality requirements specified in the sales contract.
In general, coarse coal can be dewatered using screens and centrifuges,
with relatively high efficiency and low cost. As such, the coarse fraction is
generally not the primary focus of dewatering efforts. However, fine size
fractions require more intensive processing techniques to achieve contract
specifications and are often associated with fluctuating efficiency and
increased cost. For fines, froth flotation is widely used as the cleaning
method for producing a high-quality product, but the clean coal product
may contain over 80% moisture. The excessive water must be discarded
and the surface moisture must be decreased to a point where the fine clean
coal can be blended with the coarse fraction.
Most dewatering equipment applies a mechanical force to discard the
surface moisture; centrifuges and vacuum filters are commonly used. In
addition, filter presses, which require even more mechanical force, have
also gained popularity for ultra-fines. In many cases, further thermal
drying is necessary to achieve required moisture limits.
Each dewatering technique, whether for coarse or fine coal, presents
its own advantages and limitations, such as unsatisfactory dewatering
kinetics or the loss of ultra-fines to waste streams. In order to realise
these limitations, the authors believe that chemical dewatering aids may
