- Email: [email protected]
Coal face and stockpile ash analyser for the coal mining industry
Applied Radiation and Isotopes 55 (2001) 407–412
Coal face and stockpile ash analyser for the coal mining industry M. Borsaru*, R. Dixon, A. Rojc, R. Stehle, Z. Jecny CSIRO Exploration and Mining, PO Box 883, Kenmore, Qld 4069, Australia Received 20 December 2000; received in revised form 5 February 2001; accepted 14 February 2001
Abstract A portable nucleonic instrument was developed for the determination of coal ash on the coal face or the surface of coal stockpiles. The instrument employs the backscattered gamma–gamma technique. There are two g-ray sources used in this instrument: a 1.1 MBq 133Ba source as the primary source of radiation and a 37 kBq 137Cs for gain stabilization. The instrument is commercially available. # 2001 Elsevier Science Ltd. All rights reserved.
1. Introduction Nucleonic techniques for monitoring coal quality are well established in the coal mining industry, both for online analysis and borehole logging. PGNAA (Prompt Gamma Neutron Activation Analysis), g-ray transmission, g-ray backscatter and natural gamma techniques are used in both applications. Although it is always preferable to use low activity neutron and g-ray sources for such applications, many of today’s nucleonic instruments used for on-stream analysis and borehole logging employ high activity sources. For example, commercial logging companies use borehole logging tools fitted with g-ray sources in excess of 3700 MBq. This is due to the fact that good shielding can be incorporated for on-stream analysis instruments and the borehole logging tools do not expose the personnel to radiation while in boreholes (the time when the tools are out of holes or the source out of the source holder is normally short). When the ash content of coal is measured on the coal face or in coal stockpiles, portable instruments are needed and they must be light. This means that high activity sources requiring heavy shielding cannot be used *Corresponding author. Tel.: +61-2-3327-4627; fax: +61-73327-4455. E-mail address: [email protected] (M. Borsaru).
for such applications and new nucleonic instruments employing low activity sources must be developed. The development of such instruments requires research and development; they are not a straightforward extension of those used for on-stream analysis or borehole logging applications. This paper describes a portable nucleonic instrument developed for determination of the ash content of coal at the coal face or in stockpiles.
2. Principle of ash determination The principle of ash determination is g-ray backscatter. 133Ba is the primary source of gamma radiation and 137Cs is used for spectrum gain stabilisation. The source–detector configuration was designed for 2p measurement and was described in an earlier publication (Borsaru et al., 1997). The lead shielding was redesigned so that its weight was reduced to 400 g, which is suitable for measurements taken on coal faces or the surface of coal stockpiles. The configuration can be easily modified for 4p measurements inside coal stockpiles. The ash content of coal is determined by measuring changes in the ‘‘equivalent’’ atomic number, Zeq , which is given by ffi sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi P 4:5 3:5 P i pi Zi =Ai ; ð1Þ Zeq ffi i pi Zi =Ai
0969-8043/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 9 - 8 0 4 3 ( 0 1 ) 0 0 0 7 2 - 0
408
M. Borsaru et al. / Applied Radiation and Isotopes 55 (2001) 407–412
where Ai is the atomic weight and pi and Zi are the weight fraction and atomic number of the ith elemental constituent of the coal. Changes in Zeq are measured either by the measurement of spectral intensity at low energies or Pz , which is the ratio between count rates recorded in two windows located in the high and low energy regions of the backscatter spectrum (Borsaru et al., 1985, 1997). It is therefore important that spectrometric measurements are made and that the whole backscattered spectrum is recorded. It is also essential to employ gain stabilization to decrease the error produced by gain drifts in the spectrum. This method assumes that the ash content of coal and Zeq are uniquely correlated which occurs when the chemical composition of the ash is stable. For coals with variable ash composition, Zeq will vary accordingly. This is especially the case when there is large variation in the ash components of ash with high atomic number, e.g. Ca, Ti or Fe. High variations in one of these components decreases the accuracy of ash determination. The reason is apparent from Eq. (1). For low-Z elements, Zi =Ai ffi 0:5 and Zeq can be approximated as qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi X Zeq ¼3:5 p Z3:5 : i i i
ð2Þ
Eq. (2) shows the strong dependence of the Zeq of the ash on the constituent elements with high atomic number. 133 Ba was chosen as the primary source of gamma radiation, because it emits g-rays of lower energy than other g-ray sources commonly used in nucleonic instruments, e.g. 60Co and 137Cs. The three prominent g-rays produced by 133Ba have energies of 80, 300 and 350 keV and there is also a 32 keV X-ray accompanying them. This increases the sensitivity of Zeq measurement. The reason is that the photoelectric absorption crosssection per atom as function of photon energy E is given by sðEÞ
3. The instrument The instrument is shown in Fig. 1 and is available commercially. It consists of a handheld measuring device and a display unit connected by a coiled cable approximately 2 m long. The system has an RS 232 port to connect to a PC for calibration, diagnostics and other control. A 40 2 line Alpha-numeric LCD is used for output of the ash content. The measuring device weighs 1.5 kg and houses a 37 dia 25 mm NaI(Tl) scintillation detector, a preamplifier and HV generator, and two 137 Cs and 133Ba g-ray microsources of activities 37 kBq and 1.1 MBq, respectively. The distance between the 133 Ba g-ray source and the end of the lead shielding was 25 mm. The display unit weighs 2.5 kg and houses the acquisition electronics, CPU, LCD and battery pack. The display unit provides safe docking for the handheld unit with operator protection from the 133Ba microsource (Fig. 2). Fig. 3 shows four spectra collected on two samples of crushed coal of 1.2 and 84% ash, one sample of sand and on the concrete floor. The coal and sand samples were placed in 4 l buckets and the measurements were taken on the top surfaces. The peak at around 29 keV is produced by the backscattered 32 keV X-rays releazed by the 133Ba source, and the count-rate is very sensitive to Zeq . This explains the much higher count-rate recorded on low ash coal than on high ash coal. The peak at around 65 keV is produced by the backscattered 80 keV g-rays from the 133Ba g-ray source. In order to prove this, a measurement was taken on a low ash coal sample with a Cd/Cu filter placed in front of the 133Ba source which absorbs the 32 keV X-rays and 80 keV g-rays produced by the source. Fig. 4 shows two spectra taken with the instrument, one spectrum being taken with the Cd/Cu filter in the path of the g-rays from the 133 Ba source. The two peaks below the 80 keV energy are not seen in the spectrum taken with the filter. The higher energy region above 80 keV (see Fig. 3), is due to the
Z4:5 ; En
where 2:54n43:5
ð3Þ
This formula shows that photoelectric absorption at low energies is substantial for heavy elements because of their high atomic number, but for light elements, it is much less because of the linear relationship between cross-section and Z 4:5 . This implies that the count rate measured in the low energy region of the backscattered spectrum is inversely correlated with Zeq . Hence, for coals with high ash (high Zeq ) the count rate is lower than for coals with low ash (low Zeq ).
Fig. 1. Measurement of coal ash on the coal face.
M. Borsaru et al. / Applied Radiation and Isotopes 55 (2001) 407–412
backscattered radiation from the 300 and 350 g-rays and is sensitive to changes in density. In contrast to the low energy region, the count rate in the high energy region is higher for the spectra taken from higher density samples. The sequence of count rates in the higher energy region shown in Fig. 3 for concrete, sand, high ash coal and low ash coal follows the sequence of densities. Linear regression analysis is used for determination of ash content. This entails setting windows in the low and high energy regions of the backscattered g-ray spectrum and fitting a linear regression model of the form: %ash ¼ a0 þ a1 X1 þ . . . þ an Xn ; where a0 ; a1 ; . . . ; an are constants and X1 ; X2 ; . . . ; Xn are variables corresponding to count rates recorded in the energy windows chosen, or ratios of the count rates.
409
One parameter which affects the accuracy of measuring ash content is the roughness of the coal face. To investigate this, measurements were taken on a block of graphite with the face analyser separated from the flat surface by multiples of 3 mm spacers. Fig. 5 shows the variation of the ratio of the count rates recorded in the energy windows 25.5–46.5 keV (low) and 105.5– 142.5 keV (high) with spacing, noting that this ratio is likely to be correlated with ash content. It is clear that the ratio is less sensitive to roughness if the spacing between the face analyser and the coal surface is more than 6 mm. Consequently, the face analyser is fitted with a spacer of 7 mm so that the face of the analyser does not touch the coal face. Another laboratory test was carried out to determine the thickness of coal interrogated by the analyser. The test consisted of calibrating the analyser on 20 samples of crushed coal and subsequently predicting the ash content of coal samples of variable thickness prepared from one of the samples used in the calibration (35 %ash). The samples were prepared in steps of 10 mm thickness and the results showed that the analyser underestimated the ash content if the depth of penetration was larger than the thickness of the coal sample. The analyser predicts the correct ash content when the thickness of coal reaches 60 mm.
4. Laboratory tests on coal samples collected at different coal mines
Fig. 2. The handheld measuring device and display unit of the coal ash face analyser
The accuracy of the instrument was assessed on crushed black coal samples collected from two coal mines in Queensland and one coal mine in New South
Fig. 3. Spectra recorded on two samples of crushed coal (84 and 1.2% ash), sand and concrete.
410
M. Borsaru et al. / Applied Radiation and Isotopes 55 (2001) 407–412
Fig. 4. Two spectra taken with the face analyser on a coal sample. A Cd/Cu filter was placed in front of the 133Ba g-ray source in one measurement.
Fig. 5. Ratio of total counts in energy windows 25.5–46.5 keV/105.5–142.5 keV vs spacing.
Wales. The instrument was also tested on samples of brown coal from Victoria. The decision to test the ash analyser on samples of crushed coal was based on the fact that the ash content of such samples can be determined with high accuracy by laboratory analysis. This would provide an indication of the accuracy of ash determination on the surface of coal stockpiles or on samples taken from coal stockpiles. The coal was collected from different areas of the mines to ensure that a wide range of ash content was
covered. After collection the coal was crushed, sampled and the ash content determined in the laboratory. Table 1 shows the RMS deviations between the laboratory assays and Face Analyser predictions, as well as the standard deviation of the ash content of the samples collected from each mine. The regression equations and the energy windows selected for the calibrations are shown in Table 2. The number of brown coal samples from Victoria shown in Table 1 is 24. Note that the initial number of
411
M. Borsaru et al. / Applied Radiation and Isotopes 55 (2001) 407–412 Table 1 Regression analysis results for samples of crushed coal collected from four mines Mine
No. of samples
Range (%ash)
RMS deviation (wt%)
Correlation coefficient
Standard deviation of ash content (wt%)
Queensland (1) Queensland (2) NSW Victoria
20 23 25 24
33–50.2 8.8–48.4 13.4–41.3 0.2–84.4
1.9 2.7 3.7 4.6
0.9 0.96 0.92 0.97
5.1 10.3 9 23.8
Table 2 Regression equations for the determination of %ash Mine
Regression equation
Queensland (1) Queensland (2) NSW Victoria
%ash= 14.9+25.5 Rat %ash= 46.3+36.5 Rat %ash= 25.3+24.3 Rat %ash=123.2 128.5 Rat
Ratios of energy windows (keV) A B C D
Rat Rat Rat Rat
A=(105 142.5)/(25.5 46.5) B=(105 142.5)/(25.5 46.5) C=(105 142.5)/(25.5 46.5) D=(21 42)/(112.5 187.5)
Fig. 6. Ash-laboratory assays vs coal face analyser predictions on crushed samples of black coal collected from Queensland (2) coal mine.
samples was 13, but the number of samples was artificially doubled by preparing two separate smaller 4 l samples from each larger coal sample (>10 l) collected at the mine. The artificial 13 samples were created by simply subdividing the original 13 samples into two samples each. One of the original 13 samples collected and its artificially created sister sample did not fit the calibration because the predicted ash content by the analyser was 15% ash above the laboratory assay. These samples were therefore rejected from the regres-
sion equation. One possible explanation for this large discrepancy was that the iron content of the ash was high. However, the chemical composition of the ash was not available, so this possible explanation could not be confirmed. The energy windows used in Ratio D for the brown coal from Victoria are slightly different than the energy windows used in the case of black coals from Queensland and NSW. It should be mentioned that the measurements on the brown coal were taken with a
412
M. Borsaru et al. / Applied Radiation and Isotopes 55 (2001) 407–412
different face analyser instrument and there are slight differences between instruments. The regression equation also has the signs of the constant and the ratio reversed by comparison to the other equations. This is due to the fact that the nominator and denominator for the ratio were interchanged. Fig. 6 shows the comparison of predicted ash content and ash content determined by laboratory analysis for the samples collected from Queensland (2) coal mine. This figure was selected to be shown because the ash content, as shown in Table 1, has a wider spread among the samples collected from the other mines.
5. Summary and conclusions The present work demonstrated that the Coal Face Ash Analyser, using two g-rays microsources of total activity less than 1.1 MBq, can measure the ash content of crushed coal samples. The instrument can be used for ash measurements on coal stockpiles or coal samples taken from coal stockpiles. The instrument can also be used to measure ash on coal faces (Borsaru et al., 1997). The accuracy of ash determination is not as good as that of laboratory measurements. However, the time per measurement is short, and by taking a large number of measurements the accuracy for the average is improved. The current time per measurement (20 s) can be
decreased to 15 s without increasing significantly the statistical error of measurement. The accuracy of ash determination is worse for coals with high variation in iron content. For such coals, the analyser may only be useful for differentiating between coal and similar looking coal sediments on the coal face. The instrument has been commercialised and is available from Scintrex/Auslog Pty Ltd, Queensland, Australia.
Acknowledgements The authors wish to thank Dr Ralph Holmes and Dr Bill Mathew for their valuable comments during the preparation of this manuscript. This work was supported under Australian Coal Association Research Program (ACARP).
References Borsaru, M., Ceravolo, C., Carson, G., Tchen, T., 1997. Low radioactivity portable coal face ash analyzer. Appl. Radiat. Isot. 48, 715–720. Borsaru, M., Charbucinski, J., Eisler, P.L., Youl, S.F., 1985. Determination of ash content in coal by borehole logging in dry boreholes using gamma-gamma methods. Geoexploration 23, 503–518.
Coal face and stockpile ash analyser for the coal mining industry M. Borsaru*, R. Dixon, A. Rojc, R. Stehle, Z. Jecny CSIRO Exploration and Mining, PO Box 883, Kenmore, Qld 4069, Australia Received 20 December 2000; received in revised form 5 February 2001; accepted 14 February 2001
Abstract A portable nucleonic instrument was developed for the determination of coal ash on the coal face or the surface of coal stockpiles. The instrument employs the backscattered gamma–gamma technique. There are two g-ray sources used in this instrument: a 1.1 MBq 133Ba source as the primary source of radiation and a 37 kBq 137Cs for gain stabilization. The instrument is commercially available. # 2001 Elsevier Science Ltd. All rights reserved.
1. Introduction Nucleonic techniques for monitoring coal quality are well established in the coal mining industry, both for online analysis and borehole logging. PGNAA (Prompt Gamma Neutron Activation Analysis), g-ray transmission, g-ray backscatter and natural gamma techniques are used in both applications. Although it is always preferable to use low activity neutron and g-ray sources for such applications, many of today’s nucleonic instruments used for on-stream analysis and borehole logging employ high activity sources. For example, commercial logging companies use borehole logging tools fitted with g-ray sources in excess of 3700 MBq. This is due to the fact that good shielding can be incorporated for on-stream analysis instruments and the borehole logging tools do not expose the personnel to radiation while in boreholes (the time when the tools are out of holes or the source out of the source holder is normally short). When the ash content of coal is measured on the coal face or in coal stockpiles, portable instruments are needed and they must be light. This means that high activity sources requiring heavy shielding cannot be used *Corresponding author. Tel.: +61-2-3327-4627; fax: +61-73327-4455. E-mail address: [email protected] (M. Borsaru).
for such applications and new nucleonic instruments employing low activity sources must be developed. The development of such instruments requires research and development; they are not a straightforward extension of those used for on-stream analysis or borehole logging applications. This paper describes a portable nucleonic instrument developed for determination of the ash content of coal at the coal face or in stockpiles.
2. Principle of ash determination The principle of ash determination is g-ray backscatter. 133Ba is the primary source of gamma radiation and 137Cs is used for spectrum gain stabilisation. The source–detector configuration was designed for 2p measurement and was described in an earlier publication (Borsaru et al., 1997). The lead shielding was redesigned so that its weight was reduced to 400 g, which is suitable for measurements taken on coal faces or the surface of coal stockpiles. The configuration can be easily modified for 4p measurements inside coal stockpiles. The ash content of coal is determined by measuring changes in the ‘‘equivalent’’ atomic number, Zeq , which is given by ffi sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi P 4:5 3:5 P i pi Zi =Ai ; ð1Þ Zeq ffi i pi Zi =Ai
0969-8043/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 9 - 8 0 4 3 ( 0 1 ) 0 0 0 7 2 - 0
408
M. Borsaru et al. / Applied Radiation and Isotopes 55 (2001) 407–412
where Ai is the atomic weight and pi and Zi are the weight fraction and atomic number of the ith elemental constituent of the coal. Changes in Zeq are measured either by the measurement of spectral intensity at low energies or Pz , which is the ratio between count rates recorded in two windows located in the high and low energy regions of the backscatter spectrum (Borsaru et al., 1985, 1997). It is therefore important that spectrometric measurements are made and that the whole backscattered spectrum is recorded. It is also essential to employ gain stabilization to decrease the error produced by gain drifts in the spectrum. This method assumes that the ash content of coal and Zeq are uniquely correlated which occurs when the chemical composition of the ash is stable. For coals with variable ash composition, Zeq will vary accordingly. This is especially the case when there is large variation in the ash components of ash with high atomic number, e.g. Ca, Ti or Fe. High variations in one of these components decreases the accuracy of ash determination. The reason is apparent from Eq. (1). For low-Z elements, Zi =Ai ffi 0:5 and Zeq can be approximated as qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi X Zeq ¼3:5 p Z3:5 : i i i
ð2Þ
Eq. (2) shows the strong dependence of the Zeq of the ash on the constituent elements with high atomic number. 133 Ba was chosen as the primary source of gamma radiation, because it emits g-rays of lower energy than other g-ray sources commonly used in nucleonic instruments, e.g. 60Co and 137Cs. The three prominent g-rays produced by 133Ba have energies of 80, 300 and 350 keV and there is also a 32 keV X-ray accompanying them. This increases the sensitivity of Zeq measurement. The reason is that the photoelectric absorption crosssection per atom as function of photon energy E is given by sðEÞ
3. The instrument The instrument is shown in Fig. 1 and is available commercially. It consists of a handheld measuring device and a display unit connected by a coiled cable approximately 2 m long. The system has an RS 232 port to connect to a PC for calibration, diagnostics and other control. A 40 2 line Alpha-numeric LCD is used for output of the ash content. The measuring device weighs 1.5 kg and houses a 37 dia 25 mm NaI(Tl) scintillation detector, a preamplifier and HV generator, and two 137 Cs and 133Ba g-ray microsources of activities 37 kBq and 1.1 MBq, respectively. The distance between the 133 Ba g-ray source and the end of the lead shielding was 25 mm. The display unit weighs 2.5 kg and houses the acquisition electronics, CPU, LCD and battery pack. The display unit provides safe docking for the handheld unit with operator protection from the 133Ba microsource (Fig. 2). Fig. 3 shows four spectra collected on two samples of crushed coal of 1.2 and 84% ash, one sample of sand and on the concrete floor. The coal and sand samples were placed in 4 l buckets and the measurements were taken on the top surfaces. The peak at around 29 keV is produced by the backscattered 32 keV X-rays releazed by the 133Ba source, and the count-rate is very sensitive to Zeq . This explains the much higher count-rate recorded on low ash coal than on high ash coal. The peak at around 65 keV is produced by the backscattered 80 keV g-rays from the 133Ba g-ray source. In order to prove this, a measurement was taken on a low ash coal sample with a Cd/Cu filter placed in front of the 133Ba source which absorbs the 32 keV X-rays and 80 keV g-rays produced by the source. Fig. 4 shows two spectra taken with the instrument, one spectrum being taken with the Cd/Cu filter in the path of the g-rays from the 133 Ba source. The two peaks below the 80 keV energy are not seen in the spectrum taken with the filter. The higher energy region above 80 keV (see Fig. 3), is due to the
Z4:5 ; En
where 2:54n43:5
ð3Þ
This formula shows that photoelectric absorption at low energies is substantial for heavy elements because of their high atomic number, but for light elements, it is much less because of the linear relationship between cross-section and Z 4:5 . This implies that the count rate measured in the low energy region of the backscattered spectrum is inversely correlated with Zeq . Hence, for coals with high ash (high Zeq ) the count rate is lower than for coals with low ash (low Zeq ).
Fig. 1. Measurement of coal ash on the coal face.
M. Borsaru et al. / Applied Radiation and Isotopes 55 (2001) 407–412
backscattered radiation from the 300 and 350 g-rays and is sensitive to changes in density. In contrast to the low energy region, the count rate in the high energy region is higher for the spectra taken from higher density samples. The sequence of count rates in the higher energy region shown in Fig. 3 for concrete, sand, high ash coal and low ash coal follows the sequence of densities. Linear regression analysis is used for determination of ash content. This entails setting windows in the low and high energy regions of the backscattered g-ray spectrum and fitting a linear regression model of the form: %ash ¼ a0 þ a1 X1 þ . . . þ an Xn ; where a0 ; a1 ; . . . ; an are constants and X1 ; X2 ; . . . ; Xn are variables corresponding to count rates recorded in the energy windows chosen, or ratios of the count rates.
409
One parameter which affects the accuracy of measuring ash content is the roughness of the coal face. To investigate this, measurements were taken on a block of graphite with the face analyser separated from the flat surface by multiples of 3 mm spacers. Fig. 5 shows the variation of the ratio of the count rates recorded in the energy windows 25.5–46.5 keV (low) and 105.5– 142.5 keV (high) with spacing, noting that this ratio is likely to be correlated with ash content. It is clear that the ratio is less sensitive to roughness if the spacing between the face analyser and the coal surface is more than 6 mm. Consequently, the face analyser is fitted with a spacer of 7 mm so that the face of the analyser does not touch the coal face. Another laboratory test was carried out to determine the thickness of coal interrogated by the analyser. The test consisted of calibrating the analyser on 20 samples of crushed coal and subsequently predicting the ash content of coal samples of variable thickness prepared from one of the samples used in the calibration (35 %ash). The samples were prepared in steps of 10 mm thickness and the results showed that the analyser underestimated the ash content if the depth of penetration was larger than the thickness of the coal sample. The analyser predicts the correct ash content when the thickness of coal reaches 60 mm.
4. Laboratory tests on coal samples collected at different coal mines
Fig. 2. The handheld measuring device and display unit of the coal ash face analyser
The accuracy of the instrument was assessed on crushed black coal samples collected from two coal mines in Queensland and one coal mine in New South
Fig. 3. Spectra recorded on two samples of crushed coal (84 and 1.2% ash), sand and concrete.
410
M. Borsaru et al. / Applied Radiation and Isotopes 55 (2001) 407–412
Fig. 4. Two spectra taken with the face analyser on a coal sample. A Cd/Cu filter was placed in front of the 133Ba g-ray source in one measurement.
Fig. 5. Ratio of total counts in energy windows 25.5–46.5 keV/105.5–142.5 keV vs spacing.
Wales. The instrument was also tested on samples of brown coal from Victoria. The decision to test the ash analyser on samples of crushed coal was based on the fact that the ash content of such samples can be determined with high accuracy by laboratory analysis. This would provide an indication of the accuracy of ash determination on the surface of coal stockpiles or on samples taken from coal stockpiles. The coal was collected from different areas of the mines to ensure that a wide range of ash content was
covered. After collection the coal was crushed, sampled and the ash content determined in the laboratory. Table 1 shows the RMS deviations between the laboratory assays and Face Analyser predictions, as well as the standard deviation of the ash content of the samples collected from each mine. The regression equations and the energy windows selected for the calibrations are shown in Table 2. The number of brown coal samples from Victoria shown in Table 1 is 24. Note that the initial number of
411
M. Borsaru et al. / Applied Radiation and Isotopes 55 (2001) 407–412 Table 1 Regression analysis results for samples of crushed coal collected from four mines Mine
No. of samples
Range (%ash)
RMS deviation (wt%)
Correlation coefficient
Standard deviation of ash content (wt%)
Queensland (1) Queensland (2) NSW Victoria
20 23 25 24
33–50.2 8.8–48.4 13.4–41.3 0.2–84.4
1.9 2.7 3.7 4.6
0.9 0.96 0.92 0.97
5.1 10.3 9 23.8
Table 2 Regression equations for the determination of %ash Mine
Regression equation
Queensland (1) Queensland (2) NSW Victoria
%ash= 14.9+25.5 Rat %ash= 46.3+36.5 Rat %ash= 25.3+24.3 Rat %ash=123.2 128.5 Rat
Ratios of energy windows (keV) A B C D
Rat Rat Rat Rat
A=(105 142.5)/(25.5 46.5) B=(105 142.5)/(25.5 46.5) C=(105 142.5)/(25.5 46.5) D=(21 42)/(112.5 187.5)
Fig. 6. Ash-laboratory assays vs coal face analyser predictions on crushed samples of black coal collected from Queensland (2) coal mine.
samples was 13, but the number of samples was artificially doubled by preparing two separate smaller 4 l samples from each larger coal sample (>10 l) collected at the mine. The artificial 13 samples were created by simply subdividing the original 13 samples into two samples each. One of the original 13 samples collected and its artificially created sister sample did not fit the calibration because the predicted ash content by the analyser was 15% ash above the laboratory assay. These samples were therefore rejected from the regres-
sion equation. One possible explanation for this large discrepancy was that the iron content of the ash was high. However, the chemical composition of the ash was not available, so this possible explanation could not be confirmed. The energy windows used in Ratio D for the brown coal from Victoria are slightly different than the energy windows used in the case of black coals from Queensland and NSW. It should be mentioned that the measurements on the brown coal were taken with a
412
M. Borsaru et al. / Applied Radiation and Isotopes 55 (2001) 407–412
different face analyser instrument and there are slight differences between instruments. The regression equation also has the signs of the constant and the ratio reversed by comparison to the other equations. This is due to the fact that the nominator and denominator for the ratio were interchanged. Fig. 6 shows the comparison of predicted ash content and ash content determined by laboratory analysis for the samples collected from Queensland (2) coal mine. This figure was selected to be shown because the ash content, as shown in Table 1, has a wider spread among the samples collected from the other mines.
5. Summary and conclusions The present work demonstrated that the Coal Face Ash Analyser, using two g-rays microsources of total activity less than 1.1 MBq, can measure the ash content of crushed coal samples. The instrument can be used for ash measurements on coal stockpiles or coal samples taken from coal stockpiles. The instrument can also be used to measure ash on coal faces (Borsaru et al., 1997). The accuracy of ash determination is not as good as that of laboratory measurements. However, the time per measurement is short, and by taking a large number of measurements the accuracy for the average is improved. The current time per measurement (20 s) can be
decreased to 15 s without increasing significantly the statistical error of measurement. The accuracy of ash determination is worse for coals with high variation in iron content. For such coals, the analyser may only be useful for differentiating between coal and similar looking coal sediments on the coal face. The instrument has been commercialised and is available from Scintrex/Auslog Pty Ltd, Queensland, Australia.
Acknowledgements The authors wish to thank Dr Ralph Holmes and Dr Bill Mathew for their valuable comments during the preparation of this manuscript. This work was supported under Australian Coal Association Research Program (ACARP).
References Borsaru, M., Ceravolo, C., Carson, G., Tchen, T., 1997. Low radioactivity portable coal face ash analyzer. Appl. Radiat. Isot. 48, 715–720. Borsaru, M., Charbucinski, J., Eisler, P.L., Youl, S.F., 1985. Determination of ash content in coal by borehole logging in dry boreholes using gamma-gamma methods. Geoexploration 23, 503–518.