Coal Sizing and Classification

Vibrating screens in coal classification by size

Classification by size is one of the fundamental operations of coal preparation. Screens can classify a broad range of sizes and are used for various applications throughout the coal preparation plant, including raw coal pre-treatment and dry fines extraction, pre-sizing of feeds to separation processes, recovery and dewatering of products after separation processes and sizing of washed coal to meet market requirements. The main screen types currently in use are static screens and vibrating screens.

The most common application of a static screen is the sieve bend, constructed as an arc or bend, with the sieve surfaces offering very steep to progressively lower angles to the flow of material. The most common application of sieve bends is to remove large volumes of water prior to material passing to a vibrating dewatering screen.

Vibrating screens for coal sizing and classification

Vibrating screens are widely used to size and dewater coal in the range 200-0.25mm. A wide range of screen sizes and designs are available to meet the specific requirements of each application. One of the most significant developments in screening technology in recent years has been the introduction of new materials for screen decks to reduce wear, increase screening performance and reduce noise at coal preparation plant.

Vibrating screens become less effective as particle size decreases and accurate size classification of fine and ultrafine coal remains a problem. These sizes tend to be classified using the principle of differential settling velocity.

Hydrocylones in coal preparation

In the past, a range of classifiers operating on this principle have been applied, including hydrocyclones, spiral classifiers and settling towers. Settling towers are still in common use for the classification of process water for jig-washing plant.

However, a major feature in recent years has been the increasing use of hydrocyclones in coal preparation. Hydrocylones are used for many applications including control of fine solids recirculating in process water, classification of fine from ultrafine coal prior to further cleaning and pre-concentration of fine coal before dewatering.

Over the years, there has been considerable R&D into means of improving the accuracy of separation in hydrocyclones and, currently, a number of high-efficiency units are available. However, the hydrocyclone has the inherent problem that some ultrafines (or slimes) always report to the coarse product. Efforts to minimise this misplacement of slimes have included optimisation of hydrocyclone design to minimise water flow, and hence the entrainment of slimes, to the underflow and injection of clean water close to the cyclone underflow spigot. A recent approach of interest has been the use of multiple-stage hydrocyclone circuits to increase overall accuracy of separation. In Australia, a three-stage hydrocyclone system has been tested recently and found to produce a much sharper separation, both improving the recovery of coarse coal and minimising the misplacement of slimes.

However, despite the improved performance, multiple-stage hydrocyclone systems are relatively expensive to install and operate and their use will be limited to applications where the additional cost can be justified.