A demonstration of the cessation of spontaneous combustion in a coal overburden spoil pile

Author(s): 
A Garvie (SRK Consulting), K Donaldson (Flinders Power), B Williams (Flinders Power), J Chapman (SRK Consulting)
Date: 
Wednesday, September 4, 2019
First presented: 
Mine Closure 2019
Type: 
Published paper
Category: 
Environmental/Social

The Leigh Creek Coal Mine, located approximately 500 km north of Adelaide in South Australia, operated between 1944 and 2015. During operations localised spontaneous combustion occurred on numerous occasions in the overburden spoil piles containing relatively small volumes of coal (about 0.8%). The majority of the overburden sequence consisted of sandstones and carbonaceous mudstones. Samples of overburden rocks had total sulfur and total organic carbon present at up to 2.2 and 14 wt% respectively.

At the end of mining surface temperatures of up to 200°C were measured and laboratory testing demonstrated the potential for self-heating and spontaneous combustion of both coal and overburden material in the spoil piles. The management strategy selected for preventing spontaneous combustion post closure included reducing the batter angle of the spoil pile and covering with a layer of fine inert material.
 
A trial of the management strategy was established in June 2017 in a location that was actively combusting immediately prior to reducing the batter slopes and placement of the inert cover. Measurements of temperature and oxygen and carbon dioxide concentrations in the spoil pile over 20 months show that:
  • most oxygen is consumed in the top 1 m of waste,
  • carbon dioxide production rates have decreased and are consistent with historic combustion and ongoing oxidation of the carbonaceous mudstone,
  • overall there has been a net heat loss from the spoil pile and
  • temperatures have decreased below the threshold required for the initiation of spontaneous combustion.
Spoil pile conditions before and after the implementation of the management strategy are presented and an assessment of the effectiveness of the management strategy is provided.

 

Feature Author

Andrew Garvie

Andrew Garvie has more than 24 years’ experience providing scientific and technical assessments in acid and metalliferous drainage (AMD) and heap leach oxidation.  More recently he has undertaken assessments of self-heating and the potential for spontaneous combustion of coal wastes and carbonaceous black shales associated with sulfide minerals.  Studies have included the quantification of physical processes that support the oxidation of mine wastes and heap leach piles by measurement and predictive modelling.  He has assessed strategies used at mines to control oxygen supply and water flux into dumps and heap leach piles using the same methods.  Andrew’s experience includes use of geostatistics to assess the adequacy of sampling, geochemical characterisation to examine the potential of mine wastes to produce AMD, and assessment of contributors to pit lake water quality, including wall rock oxidation and in-pit waste rock disposal.  He has applied his understanding of the above processes to the development of conceptual waste landform closure strategies for the control of AMD production and spontaneous combustion.

Principal Consultant (Geochemistry)
PhD (Physics), MAusIMM
SRK Sydney
John Chapman

John Chapman has more than 26 years of relevant experience and has developed multi-disciplinary skills relating to mining environmental impacts assessment and environmental controls.  He is a recognised expert in ARD assessment and prediction, mine waste characterisation and management, and mine closure with recent project experience in Australia, South East Asia, Canada, USA and Europe.  John has facilitated enterprise-wide risk assessment workshops, undertaken due diligence and environmental risk assessments for a wide range of mining projects.  Recent projects include detailed geochemical assessment of the tailings properties for the Olympic Dam Project in SA, design and development of the geochemical characterisation program for the Cannington Life Extension project in QLD as well as for the Yeelirrie Project in WA.

Principal Consultant (Geochemistry and Environment)
MSc (Chemical Engineering), P Eng (British Columbia), P.Eng (Yukon Territory)
SRK Brisbane
SRK Worldwide