Observations on increasing dust counts in coal mines
Dust Buster News

In recent years coal mines have experienced increasing dust counts after a period of stability when dust counts were achieved between 2 and 5 mg/m³. The mines have asked CDC to come to their rescue and the following observations are intended to help in this matter.

The CDC wet fan scrubbers represented a big step forward over the old dry fan units and whilst they followed well proven and well tried techniques, modern technology was used in the design of the key components to evolve a very efficient, light-weight scrubber with low power and water requirements.

The efficiency of the in-line wet scrubber is between 95% and 98% depending on the particle size analysis. Efficiencies on sub micronic dusts will be quite low perhaps of the order of 20% to 30% and for courser dusts the efficiency will be up close to 100%. This means that for every 100 kg of coal dust that enters the scrubber, between 2 and 5 kg of dust will be expelled from the unit. It therefore follows that any increase in production rates will increase the amount of dust ingested by the unit and correspondingly the amount of dust expelled from the unit will rise by the same proportion.

There has been an increasing tendency to mount the scrubber in the cat frame and close to the cutter head. Whilst this increases dust collection efficiency, there is also a tendency to collect not only the air-borne dust but small lumps of coal which get thrown into the scrubber. These small lumps are pulverised by the impeller blade, thereby adding considerably to the dust load (wear on the impeller is also considerably increased resulting in very much reduced impeller life).

We have found a number of instances of the incorrect size scrubber being used and this can be for a number of reasons. The DME requirement is for an air volume of .4 m³/sec for every m² of coal face. However, smaller units are fitted for one or other of the following reasons:-

In some mines a figure lower than the .4 m³ per m² of coal face has been used but there is no justification for this.

With increasing production rates and increasing dust loads, the screens on the scrubber are being loaded at a much higher rate, whereas with low dust loads the combination of the dust removal in the scrubber section and the spray nozzle system on the screens was sufficient to keep the screens clean with very little additional maintenance, this is no longer the case and screens should be checked on a daily basis and either cleaned or changed if necessary. Blocked screens mean reduced air flow and poor dust collection.

The wet fan scrubber works in conjunction with a number of other parameters to achieve effective dust suppression. The other factors that are involved are:-

a) Adequate and effective ventilation system.
b) Good incoming air and water quality.
c) Good pick condition maintained.
d) The correct cutting sequence followed.
e) The scrubber is properly maintained.
f) The spray fan system is properly installed and properly maintained.
g) The operation of the air movers over the conveyor.

The current range of CDC in-line wet fan scrubbers have been developed over the years into a small, compact, highly efficient system and the dust collection efficiency has been optimised and is unlikely to have significant improvements to its collection rate.

The correct sizing of the unit for the coal face being mined is paramount and the position of the scrubber on the CM needs careful consideration to ensure that only air-borne dust is captured.

Optimisation of all the other factors affecting dust suppression is also important and probably plays a more significant role than is fully appreciated. Unless these factors are also fully taken into consideration, a scrubber operating at even 100% efficiency could still result in high dust counts.