Open Cut » Coal Extraction
The raw ash content of coal is an important parameter for determining coal quality and a number of techniques have been developed in the past for measurement in-situ, before mining (in boreholes or at the coal face) or after mining (on conveyor belts). These techniques are becoming more and more common in the coal mining industry. The present project deals with the coal ash determination at the coal face (in-situ).
The objectives of the project were:
To carry out research and development leading to the design and construction of a prototype coal face ash analyser using a gamma ray source of very low activity. The instrument was to be portable, weighing below 2.5kg and not expose the user to unacceptable levels of radiation.
To test the instrument at the pit face of a number of coal mines in order to differentiate between coal seams and 'look-alike' coal sediments. The accuracy of ash predictions of the instrument would be estimated during the field tests.
The work undertaken for this ACARP project was carried out by the CSIRO Division of Exploration and Mining.
The project consisted of three stages. A description of each stage is given below:
A laboratory investigation was carried out to establish the best shielding/configuration of the gamma-ray microsources in respect of the detector, the type of gamma-ray sources, detector and detector dimensions. Samples of coal, graphite, shale, concrete and other rocks were used in assessing the sensitivity of different type of configurations to differentiate between the samples.
Test apparatus was assembled using the data acquisition system developed for the borehole logging systems. Tests were then conducted at Curragh Mine in Queensland.
Following successful tests a prototype analyser was developed and constructed. The analyser consists of two units:
The measuring device, and
The data acquisition system with the power supply.
New software was developed for the data acquisition system. The analyser was field tested at Ravensworth Mine in NSW.
The present work confirms that the coal face analyser developed under the present grant can differentiate between coal seams and 'look-alike' sediments on the coal face. The analyser is capable, after calibration, of predicting the ash content of the coal seams.
The instrument must be calibrated for each type of coal as is the case of the 'Coalscan' on belt ash analyser. The accuracy of the ash determination depends on the variability of the ash constituents especially iron.
The prototype instrument is portable. The measuring device which must be held against the coal face during the measurement weighs less than 2.5kg. The time per measurement is 60 seconds. The instrument uses a radioactive source of such low activity (2.1MBq) that it does not expose the user to unacceptable levels of radiation.
The instrument is suitable for both open cut and underground coal mine applications. The prototype developed under the present ACARP grant can only be used in open cut mines. Different electronic circuits are required in underground coal mines to satisfy the requirement for intrinsic safety.
A prototype Coal Face Ash Analyser was designed and constructed under the present ACARP grant. The instrument uses a 1.8MBq 133Ba gamma-ray microsource as the primary source of radiation and a 0.35 MBq
Cs gamma-ray microsource for gain stabilisation. The instrument is portable, hand held and weighs approximately 2.5kg. The identification of coal and 'look alike- coal sediments on the coal face is based on the backscattered gamma-gamma technique.
The time per measurement chosen in the present work was 100 seconds but 60 seconds would be sufficient.
The Coal Face Ash Analyser does not require special shielding and does not expose the user to unacceptable levels of radiation.
The instrument was tested at two coal mines; Curragh Mine in Queensland and Ravensworth Mine in the Hunter Valley, NSW. The field tests proved that the Face Analyser is capable of determining the ash content of coal on the coal face. The analyser must be calibrated for each coal seam. The calibration is different for different types of coal.
The accuracy for ash prediction is influenced by the variation of the iron content in the coal. Fe is one of the major components of ash and has a higher Z number (Z=26) than Al (Z=13) and Si (Z=14), two other major components of coal ash. Based on the principle of the gamma-gamma technique presented earlier in this report, the variation in the concentration of the ash constituents with high Z number is the dominant effect in the backscattered spectrum recorded by the Face Analyser. The ash content of coal with a high concentration of Fe will be overestimated by the analyser.
Based on the results obtained in this work it is recommended that the development of an industrial prototype of the Face Analyser should occur. The industrial prototype should be developed for both open cut and underground applications.
The Face Analyser as developed to date, or the industrial prototype, must be tested at other coal mines for proper evaluation.
The commercialisation of the Coal Face Ash Analyser is strongly recommended.
[N.B. Editor's Note: This commercialisation has subsequently occurred.]
LOW ACTIVITY PROBE
The broad objective of this project was to develop in-situ minerals analysis equipment for the Australian coal mining industry. The equipment needed to be safe for personnel and safe for the mine environment, as regards to the effects of radiation.
The project's most important aim was to ensure that the borehole logging probe resulting from the research would operate satisfactorily using a radioactive source of extremely low activity.
The ZERO PROBE'S activity (1.1MBq) was well below the level of 3.7 MBq, specified by the NSW and Queensland state health authorities as the threshold activity above which a Licence for Possession, Use and Transport of Radioactive Substances in required.
The ZERO PROBE has been designed to provide spectrometric data from which information about coal seam delineation and raw ash content can be extracted to the desired level of accuracy. The probe was tested at three coal mines (in the NSW and Queensland). Operation of the ZERO PROBE at Bowen Basin and Hunter Valley black coal mines has shown that it can be successfully applied to problems of coal seam delineation and coal-ash determination
Operation of the ZERO PROBE at Bowen Basin and Hunter Valley black coal mines has shown that it can be successfully applied to problems of coal seam delineation and coal-ash determination. Coal seam delineations can be made of the same quality as for the SIROLOG gamma-gamma probe and commercial logging probes.
The standard deviation of the calibration data was almost identical at the three mines where the ZERO PROBE has undergone field tests. This standard deviation, equal to 2.7% ash is higher than the standard deviations obtained for the SIROLOG logging operations (1.5-2.2% ash). However, this lower accuracy is acceptable in many mining operations, where high accuracy is not essential.