Review of Queensland underground coal mines’ stone dust application and sampling and analysis of roadway dust

Introduction

A coal dust explosion is one of the principal hazards in an underground coal mine. These mines must develop, establish and maintain an adequate and effective safety and health management system (SHMS) to ensure risk from such hazards is at an acceptable level. One of the primary controls against this hazard is the application of stone dust throughout mine roadways to reduce the potential for the propagation of an explosion.

While Queensland coal mining operates under a risk-based legislative framework, a coal dust explosion is one of a few hazards with prescriptive regulation. Sections 300, 301, 302 and 303 of the Queensland Coal Mine Safety and Health Regulation 2001 (CMSHR) provide specific requirements to control this principal hazard.

Also, Recognised Standard 5: Quality of incombustible dust, sampling and analysis of roadway dust in underground coal mines was developed by the Queensland Mines Inspectorate to provide guidance complying with the provisions of the CMSHR. This recognised standard aligns with the NSW document MDG 3006 Guideline for coal dust explosion prevention and suppression (PDF).

The CMSHR is silent on the requirement for stone dust barriers and Queensland coal mines have gradually discontinued the practice of installing different types of explosion barriers as a primary control to limit the propagation of an explosion in a mine. In the absence of explosion barriers, it is critical to ensure the effectiveness of the current practice of stone dusting by efficient monitoring through sampling procedures in line with the CMSHR and the recognised standard.

Recent incidents

Investigations into recent explosions in underground coal mines around the world have revealed that a lack of adequate and effective stone dusting on mine roads was primarily responsible for failure to contain/ limit the explosion and its devastation.

Further studies also highlighted the fact that quality of stone dust used in different mines throughout the USA did not comply with the statutory requirements.

Queensland stone dusting audit program

International incidents have prompted the Queensland Mines Inspectorate to audit the efficacy of coal dust explosion prevention and suppression systems in underground coal mines. The audit's primary objective was to review the status of SHMSs or practices, procedures and processes related to coal dust explosion prevention and suppression, identifying non-compliance with legislation and scope for SHMS improvement to ensure that risk from this hazard is at an acceptable level.

Audit findings

The auditing revealed a gross lack of understanding among various levels of management and supervision as to legislative requirements for sampling, analysis and application of stone dust. General erosion in the quality of sampling and analysis over a period of time was observed. Considering the requirements for roadway dust sampling in extensive out-bye areas, long and multiple gate-roads, sampling organisations in most cases were found lacking resources.

Some of the critical audit program observations on the SHMS in relation to sampling and analysis of roadway dust and application of stone dust are summarised below. These general observations may vary from mine to mine.

SHMS documentation

• The stated purpose of the SOP for stone dust application, sampling and analysis was to be a process to manage the hazard of stone dust application and sampling, when the purpose of the SOP should include the procedure for minimising risk of explosion and limiting propagation of explosions.
• The risk assessment for developing the SOP was either not conducted or it failed to identify the risks due to failure of different current control mechanisms, such as inadequate or ineffective SOP, inadequate organisation, a sampler's lack of competence, inadequate or ineffective stone dust application, non-compliance with stone dust quality specifications, delay in analysis of samples, absence of effective monitoring mechanisms, and so on.

Prevention of accumulation of float dust

• Provisions for minimising risk of a coal dust explosion, suppressing a coal dust explosion, and limiting its propagation to other parts of the mine were not included in the SOP.
• There was no systematic procedure in place for collection and removal of excessive coal dust from roadways.

Stone dust application

• No study on coal dust fall out had been carried out to decide the optimum rate of application of stone dust at different sections of mine roadways vis-à-vis source or generation rate of float dust.
• The SOP did not provide for rate of application of stone dust in development panels, longwall panels and outbye areas.
• Where a stone dust application rate was provided, the rate was based on past experience and results of sample analysis, not on a scientific study.
• Application rates per hour (for continuous miners 30 kg/hour and longwalls 150 kg/hour) should rather be based on quantity of float dust generated, which is directly proportional to volume of coal production.
• The stone dusting method, dusting frequency, triggers for application of stone dust etc. were not detailed in the SOP.
• In longwall returns, stone dust was mainly applied by hanging 1 tonne stone dust bags at 20m intervals/from POD duster in MG through compressed air pipe line, in LW MG by bulk dusting and conveyor road by trickle duster. Longwall returns were found to be grossly lacking in stone dust.

Sampling and analysis

• Gross lack of understanding of the intent and requirements of legislation
• SOP lacked detailed method of zoning and sub-zoning
• Provisions for dividing the mine into zones and sub-zones did not conform to the requirements of Recognised Standard 5.
• Many samplers were unaware of requirement of strip samples and the method of preparing composite samples for each zone.
• In most of the cases, samples were collected by spot sampling only
• For spot sampling, the size of each spot or interval between successive spots were not stipulated in the SOP and the total samples collected from each sub-zone were less than adequate for a true representation of the roadway dust samples in the sub-zone.
• Monthly strip samples were not collected from face zones (85%, see section 301, CMSHR) in longwall and development panels
• Spot samples were collected from the monthly or three-monthly zones instead of strip samples.
• Subzones of 200m in 80% zones were divided into three sections and samples were collected from one such section every month alternately to cover the subzone in three months. Whereas the requirement is that entire zone (80%) comprising of all subzones should be sampled monthly.
• Only one strip from one sub-zone was being collected instead of collecting at least 10 strip samples from each sub-zone as required
• The SOP did not provide for any sampling scheduling. A general work order for sampling was generated by the survey department. However, there was no spread-sheet or composite record for all the sample points and sampling date.
• There was no time limit stipulated for sample analysis and reporting. Normally weekly samples are analysed within 7 days of collection. All monthly and three monthly samples collected over a month period are sent to the laboratory once a month and the results are obtained within a week from the date of receipt of samples. For example, weekly samples received in December 2010 i.e. on 1/12, 9/12, 11/12, 14/12 were all reported on 9/02/11.
• There was no confirmatory sampling conducted after re-dusting a failed area.

Sampling plans

• The sampling plan was not properly maintained, colour coded or divided into zones and sub-zones as per Recognised Standard 5
• Colour coding on the sampling plan was not properly indexed
• Each zone of 200m is divided into three sub-zones A, B and C of equal length. Samples from these sub-zones (A, B or C) were collected every month for 80% zones. This is not in compliance with the Regulation.
• Face zones (85%) were often divided into subzones of 200m length instead of a maximum length of 100m as per Recognised Standard 5
• As required under section 303(2) of CMSHR, a sample's incombustible material content result was not marked on any of the mine plans sighted.

Record keeping

• Date of collection and analysis of samples were not recorded on the analysis results.
• Date of reporting of analysis results was more than 60 days after receipt of the sample in some cases. For any non-compliance event, risk of deficiency in treating roadway dust in any area would remain unidentified for more than two months and no corrective actions (re-dusting) would be taken for this long period.
• There was no record maintained regarding the generation of coal dust at different locations and the required application rate of stone dust.
• No record of re-dusting locations and results were observed.
• Sample locations were often not properly recorded on the analysis report.
• There was gross inconsistency in the labels of samples, sample results from the laboratory and the work orders for sampling.
• No spread sheet was maintained to keep the records of each sample's analysis result except the hard copy of the report provided by the laboratory.

Notice

• The SOP did not provide for notifying the ERZ controllers of analysis result of all samples. However, work orders for re-dusting after failed sample analysis results are displayed on the notice board in ERZ controllers (deputy's) room.

Organisational structure

• Sampling regimes in most of the mines were grossly inadequate considering the actual sampling work load as per the provisions of the CMSHR and the Recognised Standard 5.
• Face zones (85%) were mostly sampled by face deputies and outbye areas by designated samplers. However, 80% and 70% zones need to be sampled by strip sampling only and each sub-zone should have at least 10 sampling points, requiring preparation of a composite sample for each sub-zone. The amount of work required for extensive mines is huge. Existing arrangements in most of the mines, with one or two samplers for the entire area outbye of the face zones, were grossly inadequate.
• Competencies of the samplers also need to be ascertained before appointing them and there must be a clear understanding by the samplers regarding the requirement of the CMSHR and Recognised Standard 5.

Stone dust quality

• While there was no provision in the SOP for checking stone dust quality, as required by Recognised Standard 5, a statement of compliance from the supplier was obtained. There was no specific mention of each batch supplied. The statement was issued by the supplier for the stone dust supplied during the past year, leaving little scope for taking corrective action in the event of non-compliance to the quality requirement. The risk of ineffective application of stone dust then remains unidentified. You should ensure the quality before supply or application—each batch supplied should be checked for quality compliance and a certificate issued.
• There was no system for using an independent test laboratory to verify or cross check the quality of stone dust supplied.

Section 5.3 of the Recognised Standard 5 for quality of incombustible dust, sampling and analysis of roadway dust in underground coal mines provides details for dividing the zones into sub-zones to increase the representativeness of the sampling process.

Recommendations

• Underground coal mines must review their current SHMS/SOP in relation to prevention of coal dust accumulation on mine roads, application of stone dust and sampling and analysis of roadway dust to identify whether the requirements and intent of the legislation is being achieved (Sections 300, 301, 302 and 303 of CMSHR and the Recognised Standard 5).
• Table 1 summarises the requirements of sampling and analysis of roadway dust, and may be referred to during the review
• There must be a mechanism for monitoring and reviewing the efficacy of the roadway dust sampling and stone dust application practices, procedures and processes at each mine
• The mine must decide on the rate of application of stone dust for different sections of the mine roadways based on some scientific study. Such study should be conducted whenever there is some significant change in the operational condition likely to affect the generation of coal float dust.
• The SHMS should include provisions for verification or cross checking of quality of stone dust to ensure that the stone dust conforms to the specified standard in terms of physical properties and also silica content
• The dust treatment system should be adequate and effective to ensure that the dust contains adequate non-combustible material as required under the CMSHR. Longwall tail gate or return roadways are the most vulnerable areas in terms of deposition of float coal dust and poor accessibility for effective dust application. Suitable methods for application of an adequate quantity of stone dust throughout the period of longwall coal cutting should be in place to treat the coal dust effectively.
• Adequately resourced and effective organisational structure comprising of competent samplers and sampling coordinators for sampling and analysis of roadway dust should be maintained keeping in mind the actual requirement of sampling complying with the CMSHR and Recognised Standard 5
• The mine should maintain up-to-date mine plans showing the locations of different zones and sub-zones, preferably with colour coding, uniform and suitable nomenclature. Also maintain a spreadsheet of all sampling zones and sub-zones to raise the work orders for sampling.
• Documented records, preferably electronic, should be maintained regarding quantity of stone dust applied in different sections vis-à-vis coal production, date of collection and analysis of samples including method of sampling and analysis, percentage of non-combustibles for each sub-zone, corrective actions taken, if required, with date and time of re-dusting and so on.
• Review of the sampling records at regular intervals would be helpful in identifying proactive measures based on trends of the sampling results for different locations.
• Underground coal mines should consider additional controls against coal dust explosions, such as stone dust barriers.

Table 1. Sampling and analysis of roadway dust samples

Authorised by Andrew Clough - Chief Inspector of Coal Mines

Contact: minesafetyandhealth@dnrm.qld.gov.au

Issued by Queensland Department of Natural Resources and Mines