- Email: [email protected]
Low radioactivity portable coal face ash analyser
~
Pergamon
Appl. Radiat. lsot. Vol. 48, No. 6, pp. 715 720, 1997 ~, 1997 Elsevier Science Ltd. All rights reserved Printed in Great Britain P I I : $1D69-8043(96)00313-2 0969-8043/97 $17.00 + 0.00
Low Radioactivity Portable Coal Face Ash Analyser M. B O R S A R U I, C. C E R A V O L O 1, G. CARSON-" a n d T. T C H E N ~ ~CSIRO-Exploration and Mining, P.O. Box 883, Kenmore, Queensland 4069, Australia and 2CSIRO-Division of Petroleum Resources, P.O. Box 3000, Mt Waverley 31500 Australia (Received 18 February 1997) A coal face ash analyser based on gamma-gamma technique has been developed and tested at two coal mines. It is light in weight (2 kg) and uses two very low activity sources which ensure safe operation of this hand-held radiation instrument. The primary source of radiation is a 1.8 MBq )33Bagamma-ray source. Another 0.35 MBq ~31Csgamma-ray source is used for gain stabilization. At field tests at a Queensland coal mine, ash in the range 7-23% ash was determined with a root mean square deviation of 2.6%. At a New South Wales coal mine, ash in the range 4-31% was determined to 3.3%. © 1997 Elsevier Science Ltd
Introduction The coal mining industry requires in situ analysis of coal quality with short turn-around times at the final exploration and for pre-mine planning and production. Both borehole logging and coal face analysis fall in the category of in situ analysis. Borehole logging is routinely used in the coal mining industry for delineating the coal seams and for the determination of their ash. Both gammagamma and neutron-gamma techniques are used in borehole logging for coal. Logging is used in exploration as well as in mine production. Coal face analysis would be mainly applicable to the production phase in open-cut pits and underground. The problem for coal mining companies to avoid at the coal face is diluting the coal with waste, in situations where coal and waste are visually indistinguishable. The use of quantitative face ash analysers would permit this form of selective mining, The safety problems associated with a radioactive source are more stringent when using a hand-held instrument scanning the coal face, like the coal face analyser, than in using a logging tool which operates in the borehole. Coal ash determination on the coal face has received less attention than coal ash measurement in boreholes or on conveyor belts. The technical challenges are to design and construct a coal face analyser which is safe, accurate, portable and man~euvrable. Considerable research, based on neutron activation or beta particle and gamma-ray scattering, has been carried out and, in some cases, it has led to the development of various useful bulk sample analysers for coal. However, due to the constraints of source
shielding this approach is totally unsuited to hand-held monitors. Wesolinski and de Jesus (1991) developed a dual-beam coal face ash monitor. The ash monitor proved useful for providing an estimate of the ash concentration profile on a coal face and in locating the rock/coal seam interface in a short time. The accuracy of the analyser varies with the degree of variation of the iron content in the ash, which is an effect common to all methods dependent on gammaray scattering. The drawback of their instrument is that it uses two gamma-ray sources, having a disadvantageously high activity of 185 MBq. Another coal face ash analyser was developed by Borsaru et al. (1992). This instrument did not pose any radiation exposure or toxicity for personnel because it is based only on natural gamma-ray detection. However, its analysis for ash is only semi-quantitative and it cannot measure density. Further, it is too heavy (15 kg) to be handled conveniently by a single operator. This paper describes work undertaken on the development and construction of a prototype coal face ash analyser using two gamma-ray microsources: ~3~Ba and ~37Cs of activities 1.8 and 0.35 MBq, respectively. This face analyser does not need special shielding and the user is not exposed to unacceptable levels of radiation.
Methodology The gamma-gamma method for the determination o/ coal ash The determination of the ash content of coal is based on the backscattered gamma-gamma 715
716
M. Borsaru et al.
technique. The application of this technique to the determination of ash in coal seams intersected by boreholes was described in an earlier publication (Borsaru et al., 1985). The present work is an extension of this technique to measurements on the coal face. The theory of gamma-ray scattering indicates that the intensity of the backscattered gamma-ray spectrum at high energies (> 150 keV) is determined by density, and at low energies, by both density and the average atomic number of the scattering medium Zeq, The ratio of counts recorded in two windows, at high and low energies, is independent of density and is proportional to Z~4. In order to extract the most information from the backscattered gamma-ray spectrum, the gamma-gamma measurement must be spectrometric and automatic gain stabilization must be incorporated in the system. When the ash content and Z~q are correlated, ash content can be determined from the measurement of Zn. This is the case if the chemical composition of ash content is stable. However, an important source of error in the measurement of ash content of coal arises if the concentration of ash components of high Z atomic number, such as Fe, Ti, Ca, etc., shows considerable variation in the ash. In this case, the ash-density correlation is a better alternative for the determination of ash. The coal density, as mentioned above, is proportional to the count rate measured in a window in the high energy region of the backscattered spectrum. Source detector configuration
The source-detector configuration (Fig. 1) is designed for a 2n measurement. It is similar to that developed for the borehole logging low activity tool,
4n measurement (Borsaru and Ceravolo, 1994) and adapted in the present work for a 2n surface measurement. Forty millimetres of lead separates the L~3Ba gamma-ray source from the scintillation detector, ensuring that the gamma-ray detector only detects the backscattered gamma-rays from the coal face. The distance between the ~3Ba gamma-ray source and the end of the lead shielding was 30 ram. The source to detector distance in the present configuration is short in order to improve the counting statistics with the low activity gamma-ray source. A Cu-Cd filter lines the lead shielding to cut down the -73 and -87 keV X-rays generated in lead by the backscattered gamma-rays. Radiation sources and detector
~"Ba was chosen as the primary source of radiation because of the low energy gamma-rays produced by this source. The two prominent peaks in the gamma-ray spectrum of ~33Bahave energies of about 300 and 350 keV. Changes in the Zeq of the medium affect mostly the low energy region of the backscattered spectrum and the analyser would be more sensitive to changes of ash content of coal if a low energy primary source of radiation like ~33Bawere used. The ~37Cs source is the best choice for gain stabilization because it produces a strong peak of 662 keV. A 37 dia x 25 mm NaI(TI) detector was used in the present work.
Laboratory Tests Different configurations, detectors and gamma-ray sources were tested in the initial stage of the laboratory investigation. The tests were carried out
Fig. I. Schematic diagram of the coal face ash analyser.
Coal face ash analyser
717
3500
q.-
3000
g
2500
i
2000
-
..-...-~...-
o ,~llb "1.
15oo 0
o
.•
1000
\
500
1O0
200
300
400
500
A
600
700
800
Energy (keV)
Fig. 2. Typical gamma-ray spectrum recorded by the face analyser on a coal block. on coal blocks and different type of rocks. The configuration selected was the one which provided the highest sensitivity in differentiating between the models selected. Figure 2 shows a typical spectrum collected on a coal block. The spectrum was obtained with the 1.8 MBq 133Ba gamma-ray source and 37 dia × 25 mm NaI(TI) detector with a measurement time of 100 s. This translates in a source to centre of the NaI detector of 48 mm. The small peak at 660 keV corresponds to the gamma-rays produced by the t37Cs source used for gain stabilization. The spectrum was recorded in 512 channels with an energy dispersion of t.5 keV/channel. Figure 3 shows four spectra collected on graphite, crushed coal, concrete and a rock with high iron content. There is a noticeable difference between the spectra. The highest count rate in the low energy region of the spectra (below 100 keV) is given by the graphite block, followed by the crushed coal, concrete floor and the rock with high iron content. The explanation is that the graphite block, which is mostly pure carbon, has a lower Zeq number than the crushed coal of 15% ash (ash contains A1, Si, Fe, etc. which have much higher Z) and therefore there is less absorption of the low energy gamma-rays in graphite. The concrete floor has a higher Z0q number than both the graphite and the crushed coal and the count rate is lower. The rock with high content of iron ore has the highest Z,q number. In the high energy region of the backscattered spectrum (above 180 keV) the count rate is directly correlated to the density of the medium and the crushed coal gives the lowest count
rate. The ratio of the count rates recorded in two windows, in the high ( > 180 keV) and low ( < 90 keV) energy region of the spectrum, is sensitive to the amount of ash content in coal. Coal with high ash content has (in general) also higher density than coal with low ash content.
Field Tests The coal face ash analyser was tested at one mine in Queensland and one in New South Wales. The measurements were taken on the coal seam or on big coal blocks which had broken off the coal face. The time per measurement was 100 s. A coal sample weighing between ! and 2 kg was collected from each measurement point and sent for laboratory ash analysis. The spectra collected with the face analyser were regressed against the ash laboratory assays using multiple regression analysis. Field tests at the Queensland mine
Twenty-nine measurements were carried out at the coal mine in Queensland. The ash content varied between 6.7 and 22.9% ash. However, 26 samples were below 16% ash and only three samples were between 20 and 22.9% ash. Figure 4 shows three spectra corresponding to three measurements taken on the coal face. The measurements were taken on coal with low and medium ash content (3.7 and 21% ash) and shale. The figure shows the same pattern mentioned earlier in the section "Laboratory Tests": for shale, the lower count rate in the low energy region of the backscattered spectrum is due to the
M, Borsaru et al.
718 4000
3500 %
3000
x -
xxx xxx
2500 -
• concrete • graphite • rock high in iron x crushed coa
l
o
~
×× : ~ . 2000
× : ~%"
(~ 1500
×
~,
1000
S
.=
%
500
50
100
150
200
250
300
Energy (keY)
Fig. 3. Backscattered spectra collected by the face analyser on concrete, graphite, rock high in iron and crushed coal.
higher Z0q and the higher count rate in the high energy region is due to higher density. Figure 5 shows a cross-plot between the ash content of coal predicted by the regression equation versus laboratory assays for all the measurements. The root mean square deviation given by the regression equation was 2.6% ash with a correlation
coefficient of 80%. The standard deviation of the population was 4.3% ash. The regression equation for ash prediction was %ash = - 38.4 + 30.39 x rl where r I is the ratio of the count rates recorded in the energy windows 90-165 and 15-75 keV.
4000
3500 J
,
3000
~, 2500 - e - - 3.7%ash - i l - shele 21%ash
i
~ 2000
o
1500
1000
500
0
50
100
150
200
250
Energy (keY)
Fig. 4. Backscattered spectra collected by the face analyser during field tests on coal of 3.7% ash content, shale and coal of 21% ash content.
Coal face ash analyser
719
25
20
== 15 _>, e
< 10
5
5
10
15
25
20
% Ash (chemical)
Fig. 5. Ash-chemical versus face analyser prediction at the Queensland mine.
If only the samples with ash values below 13.8% ash were considered, the root mean square deviation given by the regression equation was 1.1% ash with a correlation coefficient of 80%. The standard deviation of the population was 1.8% ash.
Field tests at the N e w South Wales mine
Thirteen measurements were taken on the coal face. The ash content of the samples analysed in the laboratory varied from a minimum of 3.7 to a
30
25
20
8 is
=. <
10
10
15
20
25
30
% Ash (ohemloal)
Fig. 6. Ash-chemical versus face analyser prediction at the New South Wales mine.
35
720
M. Borsaru et al.
maximum of 30.6% ash. A two-parameter regression equation was used for ash prediction. The regression equation was %ash = - 139.1 + 144.786 × r2 - 175.498 × rl where r2 is the ratio of the count rates recorded in the energy windows 60-135 and 30-75 keV and rl is the same ratio as defined earlier. The root mean square deviation given by the regression equation was 3.3% ash. The standard deviation of the sample population was 8.5% ash. Figure 6 shows a cross-plot between the ash content of coal predicted by the regression equation versus laboratory assays for all the measurements. Comments on field trials The variations in high atomic number elements in the ash are the main cause of errors in ash determination found in the field trials. It is probable that iron variations in the ash have the largest effect on accuracy of ash determination, its atomic number Z being much greater than those of the more abundant elements in the ash, alumina and silica. The ash content of coal with a high concentration of iron will be overestimated by the analyser. The time for measurement chosen in the present work was 100 s. In practice, it was found that 60 s was sufficient. The instrument was found to differentiate well between coal and "look-alike" coal sediments on the coal face.
Summary and Conclusions A coal face ash analyser based on the gammagamma technique has been developed. The instrument is portable and uses a 1.8 MBq "3Ba gamma-ray microsource as the primary source of radiation and a 0.35 MBq ~'Cs gamma-ray microsource for gain stabilisation. 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, in Queensland and New South Wales. 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.
References Borsaru M., Charbucinski J., Eisler P. L. and Youl S. F. (1985) Determination of ash content in coal by borehole logging in dry boreholes using gamma-gamma methods. Geoexploration 23, 503-518. Borsaru M., Ceravolo C., Waddington P. and Wenhao Gu. (1992) A coal face ash analyser based on natural gamma-ray activity. Nucl. Geophys. 6, 383 390. Borsaru M. and Ceravolo C. (1993) A low activity spectrometric gamma-gamma borehole logging tool for the coal industry. Nucl. Geophys. 8, 343-350. Wesolinksi, E. S. and de Jesus, A. S. M. (1991) A portable coal face ash monitor based on dual energy gamma radiation. Nuclear Techniques in the Exploration and Exploitation of Energy and Mineral Resources, IAEA, Vienna, IAEA-SM-308/4, pp. 33~,6.
Pergamon
Appl. Radiat. lsot. Vol. 48, No. 6, pp. 715 720, 1997 ~, 1997 Elsevier Science Ltd. All rights reserved Printed in Great Britain P I I : $1D69-8043(96)00313-2 0969-8043/97 $17.00 + 0.00
Low Radioactivity Portable Coal Face Ash Analyser M. B O R S A R U I, C. C E R A V O L O 1, G. CARSON-" a n d T. T C H E N ~ ~CSIRO-Exploration and Mining, P.O. Box 883, Kenmore, Queensland 4069, Australia and 2CSIRO-Division of Petroleum Resources, P.O. Box 3000, Mt Waverley 31500 Australia (Received 18 February 1997) A coal face ash analyser based on gamma-gamma technique has been developed and tested at two coal mines. It is light in weight (2 kg) and uses two very low activity sources which ensure safe operation of this hand-held radiation instrument. The primary source of radiation is a 1.8 MBq )33Bagamma-ray source. Another 0.35 MBq ~31Csgamma-ray source is used for gain stabilization. At field tests at a Queensland coal mine, ash in the range 7-23% ash was determined with a root mean square deviation of 2.6%. At a New South Wales coal mine, ash in the range 4-31% was determined to 3.3%. © 1997 Elsevier Science Ltd
Introduction The coal mining industry requires in situ analysis of coal quality with short turn-around times at the final exploration and for pre-mine planning and production. Both borehole logging and coal face analysis fall in the category of in situ analysis. Borehole logging is routinely used in the coal mining industry for delineating the coal seams and for the determination of their ash. Both gammagamma and neutron-gamma techniques are used in borehole logging for coal. Logging is used in exploration as well as in mine production. Coal face analysis would be mainly applicable to the production phase in open-cut pits and underground. The problem for coal mining companies to avoid at the coal face is diluting the coal with waste, in situations where coal and waste are visually indistinguishable. The use of quantitative face ash analysers would permit this form of selective mining, The safety problems associated with a radioactive source are more stringent when using a hand-held instrument scanning the coal face, like the coal face analyser, than in using a logging tool which operates in the borehole. Coal ash determination on the coal face has received less attention than coal ash measurement in boreholes or on conveyor belts. The technical challenges are to design and construct a coal face analyser which is safe, accurate, portable and man~euvrable. Considerable research, based on neutron activation or beta particle and gamma-ray scattering, has been carried out and, in some cases, it has led to the development of various useful bulk sample analysers for coal. However, due to the constraints of source
shielding this approach is totally unsuited to hand-held monitors. Wesolinski and de Jesus (1991) developed a dual-beam coal face ash monitor. The ash monitor proved useful for providing an estimate of the ash concentration profile on a coal face and in locating the rock/coal seam interface in a short time. The accuracy of the analyser varies with the degree of variation of the iron content in the ash, which is an effect common to all methods dependent on gammaray scattering. The drawback of their instrument is that it uses two gamma-ray sources, having a disadvantageously high activity of 185 MBq. Another coal face ash analyser was developed by Borsaru et al. (1992). This instrument did not pose any radiation exposure or toxicity for personnel because it is based only on natural gamma-ray detection. However, its analysis for ash is only semi-quantitative and it cannot measure density. Further, it is too heavy (15 kg) to be handled conveniently by a single operator. This paper describes work undertaken on the development and construction of a prototype coal face ash analyser using two gamma-ray microsources: ~3~Ba and ~37Cs of activities 1.8 and 0.35 MBq, respectively. This face analyser does not need special shielding and the user is not exposed to unacceptable levels of radiation.
Methodology The gamma-gamma method for the determination o/ coal ash The determination of the ash content of coal is based on the backscattered gamma-gamma 715
716
M. Borsaru et al.
technique. The application of this technique to the determination of ash in coal seams intersected by boreholes was described in an earlier publication (Borsaru et al., 1985). The present work is an extension of this technique to measurements on the coal face. The theory of gamma-ray scattering indicates that the intensity of the backscattered gamma-ray spectrum at high energies (> 150 keV) is determined by density, and at low energies, by both density and the average atomic number of the scattering medium Zeq, The ratio of counts recorded in two windows, at high and low energies, is independent of density and is proportional to Z~4. In order to extract the most information from the backscattered gamma-ray spectrum, the gamma-gamma measurement must be spectrometric and automatic gain stabilization must be incorporated in the system. When the ash content and Z~q are correlated, ash content can be determined from the measurement of Zn. This is the case if the chemical composition of ash content is stable. However, an important source of error in the measurement of ash content of coal arises if the concentration of ash components of high Z atomic number, such as Fe, Ti, Ca, etc., shows considerable variation in the ash. In this case, the ash-density correlation is a better alternative for the determination of ash. The coal density, as mentioned above, is proportional to the count rate measured in a window in the high energy region of the backscattered spectrum. Source detector configuration
The source-detector configuration (Fig. 1) is designed for a 2n measurement. It is similar to that developed for the borehole logging low activity tool,
4n measurement (Borsaru and Ceravolo, 1994) and adapted in the present work for a 2n surface measurement. Forty millimetres of lead separates the L~3Ba gamma-ray source from the scintillation detector, ensuring that the gamma-ray detector only detects the backscattered gamma-rays from the coal face. The distance between the ~3Ba gamma-ray source and the end of the lead shielding was 30 ram. The source to detector distance in the present configuration is short in order to improve the counting statistics with the low activity gamma-ray source. A Cu-Cd filter lines the lead shielding to cut down the -73 and -87 keV X-rays generated in lead by the backscattered gamma-rays. Radiation sources and detector
~"Ba was chosen as the primary source of radiation because of the low energy gamma-rays produced by this source. The two prominent peaks in the gamma-ray spectrum of ~33Bahave energies of about 300 and 350 keV. Changes in the Zeq of the medium affect mostly the low energy region of the backscattered spectrum and the analyser would be more sensitive to changes of ash content of coal if a low energy primary source of radiation like ~33Bawere used. The ~37Cs source is the best choice for gain stabilization because it produces a strong peak of 662 keV. A 37 dia x 25 mm NaI(TI) detector was used in the present work.
Laboratory Tests Different configurations, detectors and gamma-ray sources were tested in the initial stage of the laboratory investigation. The tests were carried out
Fig. I. Schematic diagram of the coal face ash analyser.
Coal face ash analyser
717
3500
q.-
3000
g
2500
i
2000
-
..-...-~...-
o ,~llb "1.
15oo 0
o
.•
1000
\
500
1O0
200
300
400
500
A
600
700
800
Energy (keV)
Fig. 2. Typical gamma-ray spectrum recorded by the face analyser on a coal block. on coal blocks and different type of rocks. The configuration selected was the one which provided the highest sensitivity in differentiating between the models selected. Figure 2 shows a typical spectrum collected on a coal block. The spectrum was obtained with the 1.8 MBq 133Ba gamma-ray source and 37 dia × 25 mm NaI(TI) detector with a measurement time of 100 s. This translates in a source to centre of the NaI detector of 48 mm. The small peak at 660 keV corresponds to the gamma-rays produced by the t37Cs source used for gain stabilization. The spectrum was recorded in 512 channels with an energy dispersion of t.5 keV/channel. Figure 3 shows four spectra collected on graphite, crushed coal, concrete and a rock with high iron content. There is a noticeable difference between the spectra. The highest count rate in the low energy region of the spectra (below 100 keV) is given by the graphite block, followed by the crushed coal, concrete floor and the rock with high iron content. The explanation is that the graphite block, which is mostly pure carbon, has a lower Zeq number than the crushed coal of 15% ash (ash contains A1, Si, Fe, etc. which have much higher Z) and therefore there is less absorption of the low energy gamma-rays in graphite. The concrete floor has a higher Z0q number than both the graphite and the crushed coal and the count rate is lower. The rock with high content of iron ore has the highest Z,q number. In the high energy region of the backscattered spectrum (above 180 keV) the count rate is directly correlated to the density of the medium and the crushed coal gives the lowest count
rate. The ratio of the count rates recorded in two windows, in the high ( > 180 keV) and low ( < 90 keV) energy region of the spectrum, is sensitive to the amount of ash content in coal. Coal with high ash content has (in general) also higher density than coal with low ash content.
Field Tests The coal face ash analyser was tested at one mine in Queensland and one in New South Wales. The measurements were taken on the coal seam or on big coal blocks which had broken off the coal face. The time per measurement was 100 s. A coal sample weighing between ! and 2 kg was collected from each measurement point and sent for laboratory ash analysis. The spectra collected with the face analyser were regressed against the ash laboratory assays using multiple regression analysis. Field tests at the Queensland mine
Twenty-nine measurements were carried out at the coal mine in Queensland. The ash content varied between 6.7 and 22.9% ash. However, 26 samples were below 16% ash and only three samples were between 20 and 22.9% ash. Figure 4 shows three spectra corresponding to three measurements taken on the coal face. The measurements were taken on coal with low and medium ash content (3.7 and 21% ash) and shale. The figure shows the same pattern mentioned earlier in the section "Laboratory Tests": for shale, the lower count rate in the low energy region of the backscattered spectrum is due to the
M, Borsaru et al.
718 4000
3500 %
3000
x -
xxx xxx
2500 -
• concrete • graphite • rock high in iron x crushed coa
l
o
~
×× : ~ . 2000
× : ~%"
(~ 1500
×
~,
1000
S
.=
%
500
50
100
150
200
250
300
Energy (keY)
Fig. 3. Backscattered spectra collected by the face analyser on concrete, graphite, rock high in iron and crushed coal.
higher Z0q and the higher count rate in the high energy region is due to higher density. Figure 5 shows a cross-plot between the ash content of coal predicted by the regression equation versus laboratory assays for all the measurements. The root mean square deviation given by the regression equation was 2.6% ash with a correlation
coefficient of 80%. The standard deviation of the population was 4.3% ash. The regression equation for ash prediction was %ash = - 38.4 + 30.39 x rl where r I is the ratio of the count rates recorded in the energy windows 90-165 and 15-75 keV.
4000
3500 J
,
3000
~, 2500 - e - - 3.7%ash - i l - shele 21%ash
i
~ 2000
o
1500
1000
500
0
50
100
150
200
250
Energy (keY)
Fig. 4. Backscattered spectra collected by the face analyser during field tests on coal of 3.7% ash content, shale and coal of 21% ash content.
Coal face ash analyser
719
25
20
== 15 _>, e
< 10
5
5
10
15
25
20
% Ash (chemical)
Fig. 5. Ash-chemical versus face analyser prediction at the Queensland mine.
If only the samples with ash values below 13.8% ash were considered, the root mean square deviation given by the regression equation was 1.1% ash with a correlation coefficient of 80%. The standard deviation of the population was 1.8% ash.
Field tests at the N e w South Wales mine
Thirteen measurements were taken on the coal face. The ash content of the samples analysed in the laboratory varied from a minimum of 3.7 to a
30
25
20
8 is
=. <
10
10
15
20
25
30
% Ash (ohemloal)
Fig. 6. Ash-chemical versus face analyser prediction at the New South Wales mine.
35
720
M. Borsaru et al.
maximum of 30.6% ash. A two-parameter regression equation was used for ash prediction. The regression equation was %ash = - 139.1 + 144.786 × r2 - 175.498 × rl where r2 is the ratio of the count rates recorded in the energy windows 60-135 and 30-75 keV and rl is the same ratio as defined earlier. The root mean square deviation given by the regression equation was 3.3% ash. The standard deviation of the sample population was 8.5% ash. Figure 6 shows a cross-plot between the ash content of coal predicted by the regression equation versus laboratory assays for all the measurements. Comments on field trials The variations in high atomic number elements in the ash are the main cause of errors in ash determination found in the field trials. It is probable that iron variations in the ash have the largest effect on accuracy of ash determination, its atomic number Z being much greater than those of the more abundant elements in the ash, alumina and silica. The ash content of coal with a high concentration of iron will be overestimated by the analyser. The time for measurement chosen in the present work was 100 s. In practice, it was found that 60 s was sufficient. The instrument was found to differentiate well between coal and "look-alike" coal sediments on the coal face.
Summary and Conclusions A coal face ash analyser based on the gammagamma technique has been developed. The instrument is portable and uses a 1.8 MBq "3Ba gamma-ray microsource as the primary source of radiation and a 0.35 MBq ~'Cs gamma-ray microsource for gain stabilisation. 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, in Queensland and New South Wales. 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.
References Borsaru M., Charbucinski J., Eisler P. L. and Youl S. F. (1985) Determination of ash content in coal by borehole logging in dry boreholes using gamma-gamma methods. Geoexploration 23, 503-518. Borsaru M., Ceravolo C., Waddington P. and Wenhao Gu. (1992) A coal face ash analyser based on natural gamma-ray activity. Nucl. Geophys. 6, 383 390. Borsaru M. and Ceravolo C. (1993) A low activity spectrometric gamma-gamma borehole logging tool for the coal industry. Nucl. Geophys. 8, 343-350. Wesolinksi, E. S. and de Jesus, A. S. M. (1991) A portable coal face ash monitor based on dual energy gamma radiation. Nuclear Techniques in the Exploration and Exploitation of Energy and Mineral Resources, IAEA, Vienna, IAEA-SM-308/4, pp. 33~,6.