Reduction of Dust in Return Roadways of Longwall Faces

Underground » Health and Safety

Published: November 96Project Number: C3082

Get ReportAuthor: B Barker, David Humphreys | SIMTARS

The concentration of methane is relatively easily measured on a coal face, and therefore can be controlled to prevent the development of dangerous conditions, but it is much more difficult to measure airborne dust concentrations.

As a result, very little is known about the concentration of total airborne dust in the vicinity of a coal face, nor with regard to the total amount of dust produced on a face and its fallout distribution in the return ventilation system.

Coal dust can contribute to an underground explosion in two different ways. The more usual way is that a smaller explosion of methane takes place initially which causes a blast of air to travel along the mine roadways lifting coal dust from the floor, ribs and roof. The flame of the initial methane explosion ignites the suspended coal dust particles which burn very rapidly producing large volumes of hot and toxic gases.

Further coal dust is lifted into the air ahead of the burning coal dust cloud, and the coal dust explosion will continue until there is no longer a supply of coal dust. Clearly the total amount of coal dust in the mine roadways is an important contributing factor in the risk associated with the coal dust explosions.

Coal dust may also increase the risks of an underground explosion by forming, with methane, a hybrid atmosphere which is explosive, but in which neither the coal dust or methane concentrations separately constitute an explosion hazard. This phenomenon is not well understood and is difficult to investigate.

Research continues to determine the nature of hybrid explosive mixtures of coal and methane, but very little is known about the concentrations of dust that occur in coal mine and which might contribute to the formation of an explosive hybrid mixture.

Objective & Method

The intention of this research project was to investigate the concentrations and rates of production of total airborne dust in and around a producing coal face.

To do this a number of different methods of sampling coal mine air to determine the total airborne dust concentration were utilised or developed for use underground.

Most significant of these was a modified cyclone dust sampling probe based on the BCURA stack sampling cyclone, which was tested and successfully used for this purpose. Reproducible and accurate results were obtained with this instrument within the range of dust concentrations expected to occur on a longwall face.

The collection efficiency was found to be about 85-90% so long as care was taken to maintain isokinetic sampling and the inlet air velocity was above 2.3 metres/second. The drawbacks encountered were the reliance on a compressed air supply to operate the cyclone which severely hampered the use of the instrument, and the long sampling period of up to 3 minutes which limited the sensitivity to short term peaks.

A filter paper dust probe was also designed in an attempt to overcome the problem associated with the cyclone dust probe's reliance on compressed air. Measurements were also taken with a Hund respirable dust meter as a cross reference against respirable dust concentrations. Due to safety considerations in moving around an operating longwall face, almost all measurements were taken in the tailgate return roadway.


The average total dust concentrations were all extremely low, with all the mine sites having averages below 1 gram/m³. The peak dust concentration observed was 5.8 grams/m³, which was recorded on only one occasion. Significant levels of over 1 gram/m³ were observed when the longwall shearer was at the tailgate end of the face near to the sampling position.

Because of the difficulties of operating the sampling equipment on an operating longwall face, it was not possible to conduct intensive sampling close to the shearer. It is probable that higher dust concentrations exist nearer the shearer than measured here. It is also probable that higher dust concentrations could occur over short periods of time which could not be detected with the available equipment due to the extended sampling period of 1 to 3 minutes.

None of the samples obtained in this project were of a concentration to suggest that they represent a directly explosive mixture of coal dust in air throughout the actions of coal cutting and breakage alone. However, more intensive sampling closer to the main areas of coal breakage would probably show much higher dust concentrations and these warrant further investigation.

Particle size analysis of the samples obtained showed that the average was about 25-50µm varying from mine to mine. This is considerably lower than 200µm which is accepted as the upper limit for stone dust.

There is concern that differential settling rates between coal and stone dust in return airways could result in high concentrations of coal dust despite application of large quantities of stone dust. Consideration should be given to investigations of the effect of relative particle size differences between stone dust and coal dust on the prevention of explosion propagation.

It was also noted that respirable dust was approximately 10% of the total airborne dust in the samples analysed.

Of most significance was the calculation of estimated minimum stone dust application rates. Based on the average dust concentrations observed, the average coal dust burden entering the panel returns was estimated and used to estimate minimum stone dust application rates. For three mines in which sampling was undertaken this varied from about 17 to 150kg/hour, depending upon the volatile content.

An estimate of the current rule of thumb practice suggests that current application rates are about 20kg/hour, at the lower end of the calculated requirements. This is of some considerable concern and may indicate that current stone dusting practices are inadequate for the level of airborne coal dust carried into the returns from a longwall face. The level of hazard that this represents is significant and is worthy of further investigation.

The use of water sprays as a means of dust suppression was investigated by examining the operation of two types of spray under varying conditions of airflow, dust concentration and water flow. The dust suppression efficiencies were found to be similar under similar conditions with efficiencies of 75-85%. The major differences were in the water flow rates and pressures required to operate the sprays.

The TF spiral spray, manufactured by Bete, .requires a large flow rate of 40 to 60 litres/minute at low pressures of 0.6 to 1.1 bar.

The Conflow drum spray and the Conflow drum spray, manufactured by Senior operates at 5 litres/minute but requires a water pressure of 12 bar. Operationally the drum spray would be prone to blockage due to the very small orifice size of 1/16", compared with a minimum orifice size of 5mm on the spiral spray.


Areas suggested for further investigation include:

  • continued development of instrumentation for the instantaneous and continuous determination of total airborne concentrations;
  • investigation of the occurrence of peak dust concentrations and their origins;
  • investigation of the settlement characteristics of coal dust and stone dust in face returns roadways; and
  • the factors affecting the behaviour of stone dust as an effective means of explosion suppression such as relative particle size, and settlement characteristics.

There is evidence from this study that the average coal dust particle size is considerably less than the maximum stone dust size. The effect of this on the efficiency of stone dust inertisation is not clear and would be an area of considerable interest. This would also require a better knowledge of the stone dust particle size distribution and the relative depositional rates of stone dust and coal dust occurring in the panel return roadways



Health and safety, productivity and environment initiatives.

Recently Completed Projects

C25060Development Of Borehole Shear Monitoring Device For Routine Application In Coal Mine Roadways

This project outlines the development of a cost effective, routine s...

C26056Optimisation Of Low And High Pressure Longwall Hydraulic Systems

This project aimed to establish if longwalls with Hi-Set systems cou...

C27007Assessment Of Pyrite Coal Dust Induced Pneumoconiosis

There have been recent concerns regarding the re-identification of C...


Open Cut

Safety, productivity and the right to operate are priorities for open cut mine research.

Recently Completed Projects

C25031Developing Closure Criteria For River Diversions: An Alternative To Reference Sites

The use of reference sites for establishing closure criteria in area...

C25025Guidelines For Estimating Coal Measure Rock Mass Strength From Laboratory Properties - Report A Empirical Approach And Report B Synthetic Rock Mass Models

This report combined different approaches to investigate the estimat...

C27074Tyre Integrity Monitoring

Driving mine trucks with underinflated and overloaded tyres subjects...

Open Cut

Coal Preparation

Maximising throughput and yield while minimising costs and emissions.

Recently Completed Projects

C6046Optimising Coal Fragmentation For Improved Recovery

Uncontrolled fragmentation, in particular that resulting in the gene...

C25019Adaptation Of Coal Grain Analysis To Improve Flotation Yield Estimation

This project involved sampling of full-scale flotation circuits at ...

C27033Comprehensive Flotation Model Using CGA Particle Surface Composition

The aim of this project was to adapt existing particle based flotat...

Coal Preparation

Technical Market Support

Market acceptance and emphasising the advantages of Australian coals.

Recently Completed Projects

C27036Improved Understanding Of Coke Quality Using 3D Immersive Visualisation And Statistical Characterisation Of Microstructure Properties

This project carried out an explorative study into the use of a 3D v...

C27047Combustion Characteristics Of Australian Export Thermal Coal Using Advanced Imaging Techniques

During pulverised fuel combustion, coal particles are rapidly pyroly...

C25045Assessment Of In Situ High Temperature Strength Of Cokes

The research for this project was undertaken in three stages.


Technical Market Support

Mine Site Greenhouse Gas Mitigation

Mitigating greenhouse gas emissions from the production of coal.

Recently Completed Projects

C26004CFD Modelling Of Reverse Thermal Oxidisers For VAM Abatement - CFD Modelling Of Fixed-Bed RTO Devices

The project is part of a larger multi‐phase program of study a...

C27058Technological Assessment Of A Recycle Reactor For VAM Abatement

Underground coal mining emits high volumes of methane, diluted in ve...

C27008Selective Absorption Of Methane By Ionic Liquids

The connection of a ventilation air methane (VAM) abatement plant di...

Mine Site Greenhouse Gas Mitigation

Low Emission Coal Use

Step-change technologies aimed at reducing greenhouse gas emissions.

Recently Completed Projects

C17060BGasification Of Australian Coals

Four Australian coals were trialled in the Siemens 5 MWth pilot scale ga...

C17060AOxyfuel Technology For Carbon Capture And Storage Critical Clean Coal Technology - Interim Support

The status of oxy-fuel technology for first-generation plant is indicate...

C18007Review Of Underground Coal Gasification

This report consists of a broad review of underground coal gasification,...

Low Emission Coal Use

Mining And The Community

The relationship between mines and the local community.

Recently Completed Projects

C16027Assessing Housing And Labour Market Impacts Of Mining Developments In Bowen Basin Communities

The focus of this ACARP-funded project has been to identify a number...

C22029Understanding And Managing Cumulative Impacts Of Coal Mining And Other Land Uses In Regions With Diversified Economies

The coal industry operates in the context of competing land-uses that sh...

C23016Approval And Planning Assessment Of Black Coal Mines In NSW And Qld: A Review Of Economic Assessment Techniques

This reports on issues surrounding economic assessment and analysis ...

Mining And The Community


National Energy Research,Development & Demonstration Council (NERDDC) reports - pre 1992.

Recently Completed Projects

1609-C1609Self Heating of Spoil Piles from Open Cut Coal Mines

Self Heating of Spoil Piles from Open Cut Coal Mines

1301-C1301Stress Control Methods for Optimised Development...

Stress Control Methods for Optimised Development and Extraction Operations

0033-C1356Commissioned Report: Australian Thermal Coals...

Commissioned Report: Australian Thermal Coals - An Industry Handbook