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Quantitative determination of the mineral-matter content of coal by a radiofrequency -oxidation technique
Quantitative determination of the mineralmatter content of coal by a radiofrequency -oxidation technique Frank W. Frazer and Charles B. Belcher The Broken Hill Proprietary Co. Ltd, Central Research Laboratories, Shortland, New South Wales 2307, Australia (Received 7 August 19721
The development of a low-temperature (=150°C) radiofrequency-oxidation technique as a routine laboratory method for the quantitative determination of the mineral-matter content of coal is described. The main advantage of the method over air-oxidation (370°C) and acid-extraction methods is that isolation of the unaltered mineral matter permits a more accurate expression of coal analyses on a dry mineral-matter-free basis. Comparison with the air-oxidation method has demonstrated the superiority of the radiofrequencyoxidation method for retention of carbonate and sulphide minerals; reproducibility is generally similar. The radiofrequency-oxidation method yields higher results for the percentage of mineral matter than the air-oxidation method. The differences between the combined water contents of the mineral-matter samples prepared by the two methods indicate that the air-oxidation method partly dehydrates clay minerals and that the radiofrequency-oxidation method produces results closer to the true mineral-matter contents. Other advantages - independence of extra analytical determinations, lower elapsed time and labour costs, applicability to a wider range of coals - are detailed in the Conclusions.
The quantitative determination of mineral-matter content is essential for predicting the utilization and processing
characteristics of high-mineral-matter (>l%) Australian coalslT2. The precise ultimate analysis of a coal requires determination of the mineral-matter content, so that the coal analyses can be converted to a dry mineral-matter-free (dmmf) basis. Other properties such as volatile matter and calorific value are also required on a dmmf basis, so that coal may be classified within such systems as those of Seyler3, UNECE4, ASTM’ and NCB6. The determination of ash always underestimates the mineral-matter content of coal, because water of hydration is evolved from clays, and carbonate and sulphide minerals decompose. Early methods for calculating the mineralmatter content were based on determination of the percentage ash at 775°C and applying empirically determined correction factors to allow for the decomposition of various minerals. Thus formulae proposed by Parr and Wheeler7, King, Maries and Crossley’ and various modifications’-” have been incorporated in standards4$5712, and involve assumptions about the average quantity of water of constitution of clay minerals, and the behaviour of carbonate and sulphide minerals on ashing. The formulae have been used with reasonable success for some European”~‘“r4 and South African” coals; but their application to certain North American1617 and Australian’>’ coals, which contain widelydiffering types of mineral matter, is unreliable. The direct determination of mineral matter may be made by digestion with hydrochloric and hydrofluoric acids, and weighing the mineral-matter-free residue2~‘3~14~18. The disadvantages are that pyrite is not extracted and the residue contains adsorbed hydrochloric acid, necessitating supple-
mentary analyses to determine correction factors for the mineral-matter content. We have also found that the method yields unacceptably high results for the mineralmatter contents of lower-rank bituminous coals, which is attributed to solubility of some of the coal material during the acid digestion process. Alternatively the mineral-matter content may be determined directly by a low-temperature air-oxidation method at 370-380°C 1Y519Y20.Oxidation may take more than 200 h and the residue comprising the relatively unaltered mineral matter is weighed. A further development of the method at these laboratories improved precision by allowing the mineral-matter residue to equilibrate with the laboratory atmosphere, before weighing and determining the moisture concurrently. Pyrite and siderite decompose in this process and corrections based on determination of pyritic sulphur and carbonate contents are necessary. Certain clays (particularly montmorillonite) partially dehydrate during the process although a rehydration step restores reversibly lost water of hydration. Generally air oxidation yields more reliable results than acid digestion for the high-mineral-matter (>l%) Australian coals which are frequently encountered. Secondary advantages of the air-oxidation method are that: isolation of the mineral-matter residue permits subsequent analysis of moisture and water of constitution for correction of organic hydrogen to dmmf basis; the mineral matter is in a suitable form for elemental analysis; and the mineral constituents, which are important in coal washing, coke making and blast-furnace operation, may be studied. During the period 1960- 1972, I5 000 survey and research coal samples
FUEL, 1973, Vol. 52, January
41
r. f. oxidation: F. W. Frazer and C. B. Belcher have been analysed by the air-oxidation method’ in the various laboratories of this company. The development of commercial apparatus for producing radiofrequency (RF)-excited gases (plasma) at low pressures21~22 has enabled a new method to be developed for the low-temperature oxidation of organic matter. This apparatus when used with oxygen permits the oxidation of coal material in the range 150-200°C ie, “‘, which is below the decomposition temperatures of most minerals associated with coal. RF oxidation has been applied previously to coal samples. Gluskoter’6 oxidized bituminous coals from the USA and found that the mineral-matter residues weighed significantly more than those estimated from the Parr and Wheeler’ correction formula for ash to mineral matter. Calcium montmorillonite and gypsum were partially dehydrated and a major portion of the chlorine and some sulphur were lost during the process. Estep, Kovach and Karr23 used this technique to oxidize coal samples prior to mineralogical analysis by infra-red spectroscopy. O’Gorman and Walker” RF oxidized a number of USA coal samples prior to traceelement analysis and determination of the mineral constituents by a combination of X-ray diffraction, i&a-red spectroscopy and chemical methods; the mineral-matter yields showed a wide variation from those calculated by the Parr and Wheeler’ formula, and significant quantities of sulphates and nitrates were synthesized in a lignite sample during the oxidation procedure. These investigators were not primarily interested in the quantitative determination of the percentage mineral matter but rather in the determination of the various mineralogical components. This paper discusses the development of the RF technique for use as a routine laboratory method for the quantitative determination of the mineral matter in coal.
DESCRIPTION
OF EQUIPMENT
An International Plasma Corporation IPC-1101 apparatus (California, USA) was used. The generator incorporates a crystal-controlled oscillator which operates at a frequency of 13.56 MHz and delivers a maximum of 1000 W RF energy with an efficiency (discharge power/reflected power) exceeding 95%. Oxidation is effected in six borosilicate chambers of length 152 mm and diameter 76 mm, each of which is surrounded by capacitive plates to discharge the RF energy (maximum 167 W per chamber). A two-stage mechanical vacuum pump with ballast (freeair displacement 300 1 min-1) is used to produce a chamber pressure of approximately 100 N rnp2 when commercial oxygen flows through the apparatus at 100 cm3 min-l (NTP). Haldenwanger (Berlin-Spandau) Pythagoras grade combustion boats, catalogue No.30, 102 X 19 X 12 mm are used to contain the coal samples. The effective surface area is approximately 700 mm2 which with 0.5 g coal gives a sample layer density of O-7 mg mme2. Glass rod frameworks are fitted within each chamber, allowing ten boats per chamber to be accommodated.
EXPERIMENTAL
bituminous and contain lo-25% mineral matter. No difficulty was experienced when the method was applied to a range of Northern Hemisphere coals. Samples are ground to pass a 105 pm IS0 sieve and equilibrated by exposure of a layer density not exceeding l-5 mgmrn2 to the air-conditioned laboratory atmosphere. Sample weights of 0.5 g are normally used for coals containing lo-25% mineral matter and the moisture is deter: mined concurrently24. Duplicate samples are weighed and distributed over the entire base of the boat to maximize the surface area of sample that can be presented to the plasma. As required, further samples are processed for elemental and mineralogical analyses. The samples are placed in the chambers which are then evacuated to 10 N m-2. Evacuation slows as adsorbed moisture is evolved from the samples, but the establishment of vacuum may be hastened by applying minimal RF power (total power of 50-100 W) once the pressure is below 150 N mp2, at which stage the plasma can be easily initiated. After 1 h, when most of the moisture has been expelled, the total RF power is increased to 200 W and a total oxygen flow rate of 200 cm3 min-r is set. As oxidation proceeds a bluish plasma is produced by the excited carbon dioxide species. After 48 h the plasma changes to the pink colour of excited oxygen species indicating that oxidation has virtually stopped. The samples are removed and raked with a spatula to expose unoxidized coal. The samples are replaced, the chambers evacuated and the system set at 150 W RF power and 100 cm3 min-l oxygen. After 24 h, the raking of the samples is repeated prior to a final oxidation stage of 24 h at 100 W RF power and 70 cm3 min-’ oxygen. The oxidized samples are removed from the chambers and equilibrated overnight with the laboratory atmosphere. Concurrent weighing of the residue and analyses for moisture, total water and carbon are carried out. csllcula tion
The percentage mineral matter (dry basis) obtained by RF oxidation (% &i?&FO) is determined by the relation
%blmRFO = 100
-
f'k!Oii~o -
cm)
(ml - H20-1 where ml = mass of air-dried coal sample; m2 = mass of residue after RF-oxidation and equilibration with laboratory atmosphere; HzO- = mass of moisture in air-dried coal sample; H20&0 = mass of moisture in equilibrated RFoxidized residue; C, = mass of residual organic carbon in equilibrated RF-oxidized residue. The residual organic carbon content of the mineral matter is determined as the difference between the total carbon (combustion at 1350°C) of the mineral matter and the carbon equivalent of carbonates present (acid dissolution). Strictly the mass of residual coal rather than residual carbon should be used in the calculation; however, since the levels of residual carbon comprise
METHOD DISCUSSION
The following method has been developed for the New South Wales and Queensland coals. Although ranging from lignite to anthracite, some 95% of the coals received are
42
(m2
FUEL, 1973, Vol. 52, January
OF EXPERIMENTAL
METHOD
A method for the quantitative determination of the mineralmatter content of coal should satisfy several requirements.
F. W. Frazer and C. B. Belcher: Determination
A reasonable oxidation rate coupled with minimum sample attention should be achieved. For quantitative results the method should be technically sound and be supported with precise sampling and sound laboratory techniques; the almost complete removal of organic material (residual carbon
Oxidation rate RF-oxidation studies of Union Carbide Corporation SP-2 carbon powder (ash
Sample attention Minimum sample attention during oxidation is desirable; both from labour considerations, where removing, raking and replacing sixty samples is tedious, and from analytical considerations, where risk of sample loss or contamination is increased with each raking operation. Other workers16*” have found that raking the sample at six intervals was necessary to achieve oxidation; however, sample layer densities were not discussed. Our early experiments aimed at increasing the effective
of mineral matter
in coal by r. f. oxidation
depth of oxidation by using high powers and oxygen flow rates. This was not achieved because sintering of the ash increased the enclosure of unoxidized coal particles. These effects became noticeable at RF powers of 700 to 1000 W and corresponding total oxygen flow rates of 700 to 1000 cm3 min-l. Experiments showed that greater depth of oxidation was possible at low powers and oxygen flow rates. The depth of oxidation was also determined by the pressure of the gas, but it became impracticable to work at pressures below about 70 N m-2, which corresponded to a flow rate of 70 cm3 min-l, because the rate of oxidation Thus optimum oxidation conditions became too slow. required the passage of as much oxygen as possible through the chambers at the lowest possible pressure. These experiments used a 300 1 min-l vacuum pump compared with the 150 1 min-l pump normally supplied. Accordingly the three-stage method described above was devised. An estimated 60-70% of the organic material is removed during the first stage, 2&3O% at the second stage and the remainder in the final stage.
Decomposition or alteration of minerals The sample temperatures attained during RF oxidation depend on factors such as RF power, oxygen flow rate, composition of the samples and the quantities of the various atomic, ionic and molecular species in the chamber atmosphere. The difficulties involved in measurement of the temperature of the sample have been outlined by other workers. Estep et aZ23used an infra-red radiation pyrometer to show that the sample temperature during RF oxidation was 145°C. O’Gorman and Walker” used Tempilsticks indicating that the maximum temperature attained was between 149 and 163’C. Gluskoter16 showed that gypsum was converted to hemihydrate during RF oxidation indicating that the sample temperatures attained were certainly below 190°C and may have been appreciably lower. Temperatures from 140 to 170°C were observed in the present work with a thermistor embedded below the sample surface; measurements were taken immediately after interruption of the RF power. A number of tests were made of the effect of RF oxidation on relatively pure samples of siderite, pyrite and montmorillonite in the absence of coal. Siderite, which decomposes to hematite in the air-oxidation method, did not decompose even at 1000 W RF power and 1000 cm3 min-l oxygen flow rate. Pyrite decomposed to hematite under these conditions; however at lower powers and oxygen flow rates (300 W, 300 cm3 min-‘) pyrite was stable. Kaolinite was quite stable. Calcium montmorillonite partially dehydrated at powers greater than 500 W; X-ray diffraction analysis confirmed a decrease in basal spacing (loss of interlayer water) from 1.45 to 1.15 nm (14.5 to 11.5 a). For the experimental conditions selected, montmorillonite appeared quite stable. Studies on relatively pure minerals from different localities have limited value in determining the effects of RF-oxidation on the various minerals associated with coal samples, because the temperature of decomposition of a mineral may be dependent on a number of factors such as crystallite size and the presence of impurities or other minerals. Our experiments with processing a range of coal samples confirm that 500 W RF-power and 500 cm3 mine1 oxygen will not decompose carbonates, and a maximum of about 300 W and 300 cm3 min-’ oxygen will not oxidize
FUEL, 1973, Vol. 52, January
43
Determination of mineral matter in coal by
r. f. oxidation: F. W. Frazer and C. B. Belcher
pyrite or cause irreversible loss of water from montmorillonite or other clays. Ideally, in the preparation of mineral matter, the organic sulphur should be oxidized and evolved with the other organic constituents; however, in both RF- and airoxidation methods, some organic sulphur is converted to sulphate in the mineral matter. The quantities of retained organic sulphur in the mineral matter are generally about 005% with respect to the original coal sample. The conversion of organic sulphur to sulphates appears to be more significant at low powers, and the oxidation of coals at 50 W and 20 cm? mine1 oxygen, which took one .week, produced the highest levels (=0*2%) of retained organic sulphur in the form of sulphates. The dehydration of gypsum to hemihydrate during RFoxidation of coal samples has been reportedi and our experiments confirm this observation. We have found however that hemihydrate may be reconverted to gypsum by a rehydration step consisting of exposure overnight to saturated water vapour at room temperature. Samples containing significant gypsum contents are rarely encountered in the NSW and Queensland bituminous coals and this step is normally omitted. Loss of chlorine has also been reportedi in the RFoxidation process, but chlorine contents are generally less than 0.05% in the NSW and Queensland bituminous coals and any chlorine loss would be insignificant. A number of samples of coal mixed with sodium chloride (from 0.2 to 50% NaCl) were RF-oxidized and analysis of the residues showed that at least 90% of the chlorine was retained. The presence of synthesized nitrates in RF-oxidized lignite samples has been reported previously”. Our experiments with a range of bituminous coals showed levels of nitrate nitrogen in the mineral-matter residues ranging from 0003% to 0.01% (with respect to the original coal sample) indicating that significant quantities of nitrates were not synthesized. We have not RF-oxidized any lignite samples and the method may have limited application to these lower-rank coals if significant quantities of nitrates are synthesized in the process.
costs Although the RF and 370°C air-oxidation methods have equivalent labour requirements for actual processing of the samples, we estimate that the RF method has a 35% lower labour component for a percentage mineral-matter analysis, as two samples only need be processed compared with three for the air-oxidation method. The third sample is analysed for the carbon dioxide content of the mineral matter in the air-oxidation method; this step is not necessary in the RFoxidation method, as carbonates are quantitatively retained. Capital and maintenance costs for a RF-oxidation apparatus are higher than for a 370°C air oven but running costs are similar.
R ESU LTS Repeatability tests for a Wongawilli-seam coal (SC1 14) containing 20% mineral matter showed a standard deviation of 0.10% for both the RF- and air-oxidation methods. By comparison the IS0 standard method for the determination of the percentage ash of coalz6 has a maximum acceptable standard deviation of 0.14% for a repeatability test at the
44
FUEL, 1973, Vol. 52, January
20% ash level.
Some 1500 coal samples from the NSW and Queensland coalfields together with selected Northern Hemisphere samples have been analysed by the RF-oxidation method during the last two years. A selection of these analyses covering a wide range of mineral-matter content from various rank seams and which are not necessarily representative of the seams, but which emphasize the specific advantages of RF-oxidation, is given in Table 1. Comparison of the percentage mineral matter obtained by the two methods shows that the RF-oxidation analyses are 0.2-O-4% higher than the air-oxidation analyses, and that the respective combined water contents (HzOr) are 0*03-0.3% higher. These comparisons confirm that the airoxidation method partly dehydrates clay minerals (which contribute virtually all the combined water content) and produces results for the mineral-matter content which are biased low. X-ray diffraction analyses of the samples in Table 1 showed that kaolinite is the major clay mineral, indicating that kaolinite may partly dehydrate in the airoxidation method, although the possibility of the presence of poorly crystalline clay minerals, not readily identifiable by X-ray diffraction analysis, is noted. Investigations of the decompositions of clay minerals2T28 have shown that hydroxyl water is evolved from most clay minerals upon prolonged heating at 4OO’C and that the decomposition temperature is strongly dependent on the crystallinity of the mineral. The mineral matter to ash ratios vary from 1.07 to 1.17 for the RF-oxidation and 1.06 to 1 *15 for the air-oxidation method. The higher mineral-matter and combined-water contents in the RF-oxidation analyses would result in higher values for carbon (dmmf) and lower hydrogen (dmmf), and corresponding higher rank of the coal. The residual carbon contents of the RF-oxidized samples are generally lower than O.l%, which was the previous acceptability standard for the air-oxidation method’. No difficulties were experienced with the RF oxidation of the anthracite sample (SC1 11); in contrast, an air-oxidation period of five weeks at 370°C was necessary to reduce the organic carbon to 0.2% and there is evidence of considerable combined water loss, as shown in Table 1. This demonstrates the advantage of the RF method for oxidizing higher-rank coals. A good comparison between the carbonate (COi) analyses in the coal and the mineral matter is indicated, whereas the air-oxidized mineral matter shows considerable destruction of carbonates. Comparison of the total sulphur contents of the RF- and air-oxidized mineral matters shows that in most instances there is surplus of sulphur (O-01 to 0.1%) over that attributable to the pyritic and sulphate sulphur content of the coal. X-ray diffraction analysis of the mineral matter shows that CaSQd,$H20 (hemihydrate) is synthesized by oxidation during both methods. The source of the calcium is not certain but we have noted (air-oxidation method) the decomposition of calcite to CaS04.sH20 in a coal sample containing large quantities of pyrite. The differing effects of the RF- and air-oxidation methods on samples containing significant quantities of pyrite were examined on selected samples of Bowen Basin The mineral-matter contents ranged (Queensland) coal. from 32 to 54% and the samples were analysed for pyritic and sulphate sulphur in the coal, the RF-oxidized and the air-oxidized mineral matters. The results in TabZe 2 show good agreement between the pyritic sulphur contents of
F. W. Frazer and C. B. Belcher: Determination Table 1
Comparative
analyses (56 dry basis coal) for RF and 370°C
air-oxidized
of mineral matter in coal by r. f. oxidation
coal samples
Sample No.
SC1 14
SC115
SC1 16
SC117
16666
+8982
SC111
Coalfield
Sth NSW
Nth NSW
Nth NSW
Nth NSW
Old
Sth NSW
Sth Wales
Seam
Wongawilli
Big Ben
Greta
Borehole
Blackwater
Bulli
-
K, Q. S,
K,M-I.Q.
K, M-l,
K.
K. Q. I=
K Q. S
C. AP
S. AP
M. Q, F, An
Minerals in coal’
Volatile
matter+
Ash MMRFO MM3700 MMRFQ/ash MM37oPlash
%o+R
ratio ratio
FO
H20+370°
Res CRFO Res C37co CO2 (coal) co2 RF0 co2
370°
Pyritic S (coal) Sulphate S (coall Total S,QFO Total S3700 l
Q, S
Q, P
29.2
41.7
445
37.8
27.1
23.4
2.7
la.3 20.45
26.9 30.25
11-5 13.4
19.3 21.7
9.4 10.8
10.5 11.7
12.8 i 3.8
20.25 1.12 1.11 1.60 1.40
30.0 1.12 1.12 2.15 1 .a9
13.2 1.17 1.15 1.23 1.03
21.5 1.12 1.11 1.11 0.92
10.65 1.15 1.13 0.76 0.60
11.6 1.11 1.10 0.94 0.88
13.55 1 .OB 1.06 1.11 0.79
0.07
0.10
0.06
0.07
0.03
0.06
0.03
0.02 0.58 0.53 0.03 0.01 0.01 0.02 0.01
0.04 0.24 0.23 0.20 0.31 0.16 0.50 0.32
0.03 0.11 0.06 0.02 0.25 0.08 0.45 0.17
0.02 1.72 1.60 0.10 0.06 0.02 0.20 0.23
0.07 0.57 0.53 0.12 0.01 0.01 0.05 094
0.06 0.31 0.33 0.19 0.01 0.01 094 0.03
0.29 0.16 0.06 0.05 0.01 0.03 0.04 0.05
Minerals are listed in order major-minor
as estimated
An = anatase. AP = apatite, C = calcite, F = feldspar, S = siderite t dmmf corrected for CO2
by X-ray diffraction
K = kaolinite.
M = mica, M-l
Table 2 Comparative pyritic and sulphate sulphur contents /% dry basis coal) of RF- and 370°C air-oxidized mineral matters from highly-pyritic-coal samples Sample No.
18126
18379
18304
18346
18382
MMRFO (%)
32.5 1.28 0.02 1.28 0.28 1.02 0.25
49.7 1.11 0.03 1.02 0.25 0.32 0.22
35.7 6.75 0.44 6.7 0.63 2.43 1.41
39.7 13-7 0.47 13.7 0.77 7.55 2.90
53.9 la.7 0.50 i a.3 0.78 11.6 6.8
Pyritic S (coal) Sulphate S (coal) Pyritic SRFO Sulphate SRFQ Pyritic S37oo Sulphate S3700
K
the coal and the RF-oxidized mineral matter, indicating that pyrite is quantitatively retained during this technique. The pyritic sulphur contents of the air-oxidized material show that appreciable quantities (25-75%) of pyrite decompose; however, rather than the reported oxidation to hematite112, the major reaction appears to be formation of sulphate. X-ray diffraction analysis of the air-oxidized mineral matter shows that the major phases synthesized are Fe3(SO&oH)5.2H20 (carphosiderite) and FeSOd.tiHzO (szomolnokite) with lesser quantities of hemihydrate and hematite. The presence of significant quantities of synthesized sulphates in the air-oxidation method clearly shows the limitations of the pyritic sulphur correction based on quantitative oxidation of pyrite to hematite’y2, and is probably responsible for the difficulties experienced by Brown et al2 in obtaining quantitative results for the highlypyritic Greta seam coals. Fortunately the important NSW and Queensland coalfields have relatively low pyriticsulphur contents and this has made possible the successful application of the air-oxidation method for Australian
= mixed layer montmorillonite-illite,
P = pyrite,
0 = quartz,
coals. British and European coals, which often contain pyrite as a major mineral, may have caused the limited acceptance of the air-oxidation method by these countries. However the problem of pyrite oxidation does not occur with the RF-oxidized mineral matter and this method should have application to a wider range of coals than the air-oxidation method.
CONCLUSIONS The RF-oxidation method retains all the inherent advantages of the air-oxidation method and eliminates the disadvantages of applying correction factors based on extra analytical determinations. The reproducibility of the mineral-matter yields and levels of residual carbon and retained organic sulphur are similar for both methods. The shorter RF-oxidation time of 100 h maximum is advantageous in allowing a total analysis to be completed with less total elapsed time than the air-oxidation method. Elimination of the carbon dioxide analysis of the mineral matter in the RF-oxidation method results in a 35% reduction in labour costs compared with the air-oxidation method. The RF-oxidation method can quantitatively oxidize anthracite and coals with high pyrite contents and hence has application to a wider range of coals than the airoxidation method.
ACKNOWLEDGEMENTS The authors acknowledge the permission of The Broken Hill Proprietary Co. Ltd to publish this work.
FUEL, 1973, Vol. 52, January
45
Determination
of mineral matter in coal by r. f. oxidation: F. W. Frazer and C. B. Belcher
REFERENCES 1 2 3 4 5 6 I 8 9 10 11 12 13
46
Brown, N. A., Belcher, C. B. and Callcott, T. G. J. Inst. Fuel 196538,198 Brown, H. R., Durie, R. A. and Schafer, H. N. S. Fuel, Lond 1959, 38, 295 Seyler, C. A. Proc. S. WaksZnst.Engrs 1948,63, 213 United Nations Economic Commission for Europe International Chssstficationof Hard Coals by Type, &X)247, E/ECE/COAL/llO. Pub. Sales No.1956 llE.4. 1956 American Society for Testing and Materials, kS78f HeslgnarionD388:66, 1971, Pt 19,57 UK National Coal Board Scientific Dept. Coal Survey /UK) 1964 Parr, S. W. and Wheeler, W. F. Univ.Illinois BulL 1909, 37, 1 King, J. G., Maries, M. B. and Crossley, H. E. J. SOC. them Ind., Lond. l936,55,217T Parr, S. W. IllinoisState GeoL Survey Bull. No.3, 1916 Brown, R. L., Caldwell, R. L. and Fereday, F. Fuel, Lond. 1952, 31, 261 MilIott, J. O’N. Fuel, Lond. 1958, 37, 71 British Standards Institution, BS 1016: Part 16: 1971 Radmacher, W. and Mohrhauer, P. Brennsf-Chem 1955, 36, 236
FUEL, 1973, Vol. 52, January
14 15 16 17 18 19 20 21 22 23 24 25 26 27 28
Bishop, M. andward, D. L. Fuel, Lond. 1958, 37, 191 Savage, W. H. D. Sth Afr. Chem Process. Dec.-Jan., 196768, 2, 177 Gluskoter, H. J. Fuel, Lond. 1965,44, 285 O’Gorman, J. V. and Walker. P. L. Fuel. Lond. 1971. 50. 135 Intemah’onalOrganlzationfor Standnrdisation,Recommendation R60.2, 1967 Hicks, D. and Nagelschmidt, G. Spec. Rep. Ser. Med. Res Cow, Lond No.244, Sect.F, 1943 Nelson, J. B. BCURA Monthly Bull. 1953, 17, 41 Gleit, C. E. and Holland, W. D. Analyt. Chem. 1962, 34, 1454 Gleit, C. E. Am. J. Med. Elec. 1963, 2, 112 Estep, P. A., Kovach, J. J. and Karr, C. Analyt. Chem. 1968, 40, 358 InternationalOrganizationfor Standardisation,Recommendation R331, 1963 Bersin, R. L. Znt. Pkwma Bull, 1 (3), International Plasma Corporation, California, 1968 InternationalOrganizationfor Standardisation,Recommendation RI1 71, 1970 ‘The Differential Thermal Investigation of Clays’ (Ed. R. C. Mackenzie), Mineralogical Society, London, 1957 Warshaw, C. M., Rosenberg, P. E. and Roy, R. Cluy Min. Bull. 1960,23,113
The development of a low-temperature (=150°C) radiofrequency-oxidation technique as a routine laboratory method for the quantitative determination of the mineral-matter content of coal is described. The main advantage of the method over air-oxidation (370°C) and acid-extraction methods is that isolation of the unaltered mineral matter permits a more accurate expression of coal analyses on a dry mineral-matter-free basis. Comparison with the air-oxidation method has demonstrated the superiority of the radiofrequencyoxidation method for retention of carbonate and sulphide minerals; reproducibility is generally similar. The radiofrequency-oxidation method yields higher results for the percentage of mineral matter than the air-oxidation method. The differences between the combined water contents of the mineral-matter samples prepared by the two methods indicate that the air-oxidation method partly dehydrates clay minerals and that the radiofrequency-oxidation method produces results closer to the true mineral-matter contents. Other advantages - independence of extra analytical determinations, lower elapsed time and labour costs, applicability to a wider range of coals - are detailed in the Conclusions.
The quantitative determination of mineral-matter content is essential for predicting the utilization and processing
characteristics of high-mineral-matter (>l%) Australian coalslT2. The precise ultimate analysis of a coal requires determination of the mineral-matter content, so that the coal analyses can be converted to a dry mineral-matter-free (dmmf) basis. Other properties such as volatile matter and calorific value are also required on a dmmf basis, so that coal may be classified within such systems as those of Seyler3, UNECE4, ASTM’ and NCB6. The determination of ash always underestimates the mineral-matter content of coal, because water of hydration is evolved from clays, and carbonate and sulphide minerals decompose. Early methods for calculating the mineralmatter content were based on determination of the percentage ash at 775°C and applying empirically determined correction factors to allow for the decomposition of various minerals. Thus formulae proposed by Parr and Wheeler7, King, Maries and Crossley’ and various modifications’-” have been incorporated in standards4$5712, and involve assumptions about the average quantity of water of constitution of clay minerals, and the behaviour of carbonate and sulphide minerals on ashing. The formulae have been used with reasonable success for some European”~‘“r4 and South African” coals; but their application to certain North American1617 and Australian’>’ coals, which contain widelydiffering types of mineral matter, is unreliable. The direct determination of mineral matter may be made by digestion with hydrochloric and hydrofluoric acids, and weighing the mineral-matter-free residue2~‘3~14~18. The disadvantages are that pyrite is not extracted and the residue contains adsorbed hydrochloric acid, necessitating supple-
mentary analyses to determine correction factors for the mineral-matter content. We have also found that the method yields unacceptably high results for the mineralmatter contents of lower-rank bituminous coals, which is attributed to solubility of some of the coal material during the acid digestion process. Alternatively the mineral-matter content may be determined directly by a low-temperature air-oxidation method at 370-380°C 1Y519Y20.Oxidation may take more than 200 h and the residue comprising the relatively unaltered mineral matter is weighed. A further development of the method at these laboratories improved precision by allowing the mineral-matter residue to equilibrate with the laboratory atmosphere, before weighing and determining the moisture concurrently. Pyrite and siderite decompose in this process and corrections based on determination of pyritic sulphur and carbonate contents are necessary. Certain clays (particularly montmorillonite) partially dehydrate during the process although a rehydration step restores reversibly lost water of hydration. Generally air oxidation yields more reliable results than acid digestion for the high-mineral-matter (>l%) Australian coals which are frequently encountered. Secondary advantages of the air-oxidation method are that: isolation of the mineral-matter residue permits subsequent analysis of moisture and water of constitution for correction of organic hydrogen to dmmf basis; the mineral matter is in a suitable form for elemental analysis; and the mineral constituents, which are important in coal washing, coke making and blast-furnace operation, may be studied. During the period 1960- 1972, I5 000 survey and research coal samples
FUEL, 1973, Vol. 52, January
41
r. f. oxidation: F. W. Frazer and C. B. Belcher have been analysed by the air-oxidation method’ in the various laboratories of this company. The development of commercial apparatus for producing radiofrequency (RF)-excited gases (plasma) at low pressures21~22 has enabled a new method to be developed for the low-temperature oxidation of organic matter. This apparatus when used with oxygen permits the oxidation of coal material in the range 150-200°C ie, “‘, which is below the decomposition temperatures of most minerals associated with coal. RF oxidation has been applied previously to coal samples. Gluskoter’6 oxidized bituminous coals from the USA and found that the mineral-matter residues weighed significantly more than those estimated from the Parr and Wheeler’ correction formula for ash to mineral matter. Calcium montmorillonite and gypsum were partially dehydrated and a major portion of the chlorine and some sulphur were lost during the process. Estep, Kovach and Karr23 used this technique to oxidize coal samples prior to mineralogical analysis by infra-red spectroscopy. O’Gorman and Walker” RF oxidized a number of USA coal samples prior to traceelement analysis and determination of the mineral constituents by a combination of X-ray diffraction, i&a-red spectroscopy and chemical methods; the mineral-matter yields showed a wide variation from those calculated by the Parr and Wheeler’ formula, and significant quantities of sulphates and nitrates were synthesized in a lignite sample during the oxidation procedure. These investigators were not primarily interested in the quantitative determination of the percentage mineral matter but rather in the determination of the various mineralogical components. This paper discusses the development of the RF technique for use as a routine laboratory method for the quantitative determination of the mineral matter in coal.
DESCRIPTION
OF EQUIPMENT
An International Plasma Corporation IPC-1101 apparatus (California, USA) was used. The generator incorporates a crystal-controlled oscillator which operates at a frequency of 13.56 MHz and delivers a maximum of 1000 W RF energy with an efficiency (discharge power/reflected power) exceeding 95%. Oxidation is effected in six borosilicate chambers of length 152 mm and diameter 76 mm, each of which is surrounded by capacitive plates to discharge the RF energy (maximum 167 W per chamber). A two-stage mechanical vacuum pump with ballast (freeair displacement 300 1 min-1) is used to produce a chamber pressure of approximately 100 N rnp2 when commercial oxygen flows through the apparatus at 100 cm3 min-l (NTP). Haldenwanger (Berlin-Spandau) Pythagoras grade combustion boats, catalogue No.30, 102 X 19 X 12 mm are used to contain the coal samples. The effective surface area is approximately 700 mm2 which with 0.5 g coal gives a sample layer density of O-7 mg mme2. Glass rod frameworks are fitted within each chamber, allowing ten boats per chamber to be accommodated.
EXPERIMENTAL
bituminous and contain lo-25% mineral matter. No difficulty was experienced when the method was applied to a range of Northern Hemisphere coals. Samples are ground to pass a 105 pm IS0 sieve and equilibrated by exposure of a layer density not exceeding l-5 mgmrn2 to the air-conditioned laboratory atmosphere. Sample weights of 0.5 g are normally used for coals containing lo-25% mineral matter and the moisture is deter: mined concurrently24. Duplicate samples are weighed and distributed over the entire base of the boat to maximize the surface area of sample that can be presented to the plasma. As required, further samples are processed for elemental and mineralogical analyses. The samples are placed in the chambers which are then evacuated to 10 N m-2. Evacuation slows as adsorbed moisture is evolved from the samples, but the establishment of vacuum may be hastened by applying minimal RF power (total power of 50-100 W) once the pressure is below 150 N mp2, at which stage the plasma can be easily initiated. After 1 h, when most of the moisture has been expelled, the total RF power is increased to 200 W and a total oxygen flow rate of 200 cm3 min-r is set. As oxidation proceeds a bluish plasma is produced by the excited carbon dioxide species. After 48 h the plasma changes to the pink colour of excited oxygen species indicating that oxidation has virtually stopped. The samples are removed and raked with a spatula to expose unoxidized coal. The samples are replaced, the chambers evacuated and the system set at 150 W RF power and 100 cm3 min-l oxygen. After 24 h, the raking of the samples is repeated prior to a final oxidation stage of 24 h at 100 W RF power and 70 cm3 min-’ oxygen. The oxidized samples are removed from the chambers and equilibrated overnight with the laboratory atmosphere. Concurrent weighing of the residue and analyses for moisture, total water and carbon are carried out. csllcula tion
The percentage mineral matter (dry basis) obtained by RF oxidation (% &i?&FO) is determined by the relation
%blmRFO = 100
-
f'k!Oii~o -
cm)
(ml - H20-1 where ml = mass of air-dried coal sample; m2 = mass of residue after RF-oxidation and equilibration with laboratory atmosphere; HzO- = mass of moisture in air-dried coal sample; H20&0 = mass of moisture in equilibrated RFoxidized residue; C, = mass of residual organic carbon in equilibrated RF-oxidized residue. The residual organic carbon content of the mineral matter is determined as the difference between the total carbon (combustion at 1350°C) of the mineral matter and the carbon equivalent of carbonates present (acid dissolution). Strictly the mass of residual coal rather than residual carbon should be used in the calculation; however, since the levels of residual carbon comprise
METHOD DISCUSSION
The following method has been developed for the New South Wales and Queensland coals. Although ranging from lignite to anthracite, some 95% of the coals received are
42
(m2
FUEL, 1973, Vol. 52, January
OF EXPERIMENTAL
METHOD
A method for the quantitative determination of the mineralmatter content of coal should satisfy several requirements.
F. W. Frazer and C. B. Belcher: Determination
A reasonable oxidation rate coupled with minimum sample attention should be achieved. For quantitative results the method should be technically sound and be supported with precise sampling and sound laboratory techniques; the almost complete removal of organic material (residual carbon
Oxidation rate RF-oxidation studies of Union Carbide Corporation SP-2 carbon powder (ash
Sample attention Minimum sample attention during oxidation is desirable; both from labour considerations, where removing, raking and replacing sixty samples is tedious, and from analytical considerations, where risk of sample loss or contamination is increased with each raking operation. Other workers16*” have found that raking the sample at six intervals was necessary to achieve oxidation; however, sample layer densities were not discussed. Our early experiments aimed at increasing the effective
of mineral matter
in coal by r. f. oxidation
depth of oxidation by using high powers and oxygen flow rates. This was not achieved because sintering of the ash increased the enclosure of unoxidized coal particles. These effects became noticeable at RF powers of 700 to 1000 W and corresponding total oxygen flow rates of 700 to 1000 cm3 min-l. Experiments showed that greater depth of oxidation was possible at low powers and oxygen flow rates. The depth of oxidation was also determined by the pressure of the gas, but it became impracticable to work at pressures below about 70 N m-2, which corresponded to a flow rate of 70 cm3 min-l, because the rate of oxidation Thus optimum oxidation conditions became too slow. required the passage of as much oxygen as possible through the chambers at the lowest possible pressure. These experiments used a 300 1 min-l vacuum pump compared with the 150 1 min-l pump normally supplied. Accordingly the three-stage method described above was devised. An estimated 60-70% of the organic material is removed during the first stage, 2&3O% at the second stage and the remainder in the final stage.
Decomposition or alteration of minerals The sample temperatures attained during RF oxidation depend on factors such as RF power, oxygen flow rate, composition of the samples and the quantities of the various atomic, ionic and molecular species in the chamber atmosphere. The difficulties involved in measurement of the temperature of the sample have been outlined by other workers. Estep et aZ23used an infra-red radiation pyrometer to show that the sample temperature during RF oxidation was 145°C. O’Gorman and Walker” used Tempilsticks indicating that the maximum temperature attained was between 149 and 163’C. Gluskoter16 showed that gypsum was converted to hemihydrate during RF oxidation indicating that the sample temperatures attained were certainly below 190°C and may have been appreciably lower. Temperatures from 140 to 170°C were observed in the present work with a thermistor embedded below the sample surface; measurements were taken immediately after interruption of the RF power. A number of tests were made of the effect of RF oxidation on relatively pure samples of siderite, pyrite and montmorillonite in the absence of coal. Siderite, which decomposes to hematite in the air-oxidation method, did not decompose even at 1000 W RF power and 1000 cm3 min-l oxygen flow rate. Pyrite decomposed to hematite under these conditions; however at lower powers and oxygen flow rates (300 W, 300 cm3 min-‘) pyrite was stable. Kaolinite was quite stable. Calcium montmorillonite partially dehydrated at powers greater than 500 W; X-ray diffraction analysis confirmed a decrease in basal spacing (loss of interlayer water) from 1.45 to 1.15 nm (14.5 to 11.5 a). For the experimental conditions selected, montmorillonite appeared quite stable. Studies on relatively pure minerals from different localities have limited value in determining the effects of RF-oxidation on the various minerals associated with coal samples, because the temperature of decomposition of a mineral may be dependent on a number of factors such as crystallite size and the presence of impurities or other minerals. Our experiments with processing a range of coal samples confirm that 500 W RF-power and 500 cm3 mine1 oxygen will not decompose carbonates, and a maximum of about 300 W and 300 cm3 min-’ oxygen will not oxidize
FUEL, 1973, Vol. 52, January
43
Determination of mineral matter in coal by
r. f. oxidation: F. W. Frazer and C. B. Belcher
pyrite or cause irreversible loss of water from montmorillonite or other clays. Ideally, in the preparation of mineral matter, the organic sulphur should be oxidized and evolved with the other organic constituents; however, in both RF- and airoxidation methods, some organic sulphur is converted to sulphate in the mineral matter. The quantities of retained organic sulphur in the mineral matter are generally about 005% with respect to the original coal sample. The conversion of organic sulphur to sulphates appears to be more significant at low powers, and the oxidation of coals at 50 W and 20 cm? mine1 oxygen, which took one .week, produced the highest levels (=0*2%) of retained organic sulphur in the form of sulphates. The dehydration of gypsum to hemihydrate during RFoxidation of coal samples has been reportedi and our experiments confirm this observation. We have found however that hemihydrate may be reconverted to gypsum by a rehydration step consisting of exposure overnight to saturated water vapour at room temperature. Samples containing significant gypsum contents are rarely encountered in the NSW and Queensland bituminous coals and this step is normally omitted. Loss of chlorine has also been reportedi in the RFoxidation process, but chlorine contents are generally less than 0.05% in the NSW and Queensland bituminous coals and any chlorine loss would be insignificant. A number of samples of coal mixed with sodium chloride (from 0.2 to 50% NaCl) were RF-oxidized and analysis of the residues showed that at least 90% of the chlorine was retained. The presence of synthesized nitrates in RF-oxidized lignite samples has been reported previously”. Our experiments with a range of bituminous coals showed levels of nitrate nitrogen in the mineral-matter residues ranging from 0003% to 0.01% (with respect to the original coal sample) indicating that significant quantities of nitrates were not synthesized. We have not RF-oxidized any lignite samples and the method may have limited application to these lower-rank coals if significant quantities of nitrates are synthesized in the process.
costs Although the RF and 370°C air-oxidation methods have equivalent labour requirements for actual processing of the samples, we estimate that the RF method has a 35% lower labour component for a percentage mineral-matter analysis, as two samples only need be processed compared with three for the air-oxidation method. The third sample is analysed for the carbon dioxide content of the mineral matter in the air-oxidation method; this step is not necessary in the RFoxidation method, as carbonates are quantitatively retained. Capital and maintenance costs for a RF-oxidation apparatus are higher than for a 370°C air oven but running costs are similar.
R ESU LTS Repeatability tests for a Wongawilli-seam coal (SC1 14) containing 20% mineral matter showed a standard deviation of 0.10% for both the RF- and air-oxidation methods. By comparison the IS0 standard method for the determination of the percentage ash of coalz6 has a maximum acceptable standard deviation of 0.14% for a repeatability test at the
44
FUEL, 1973, Vol. 52, January
20% ash level.
Some 1500 coal samples from the NSW and Queensland coalfields together with selected Northern Hemisphere samples have been analysed by the RF-oxidation method during the last two years. A selection of these analyses covering a wide range of mineral-matter content from various rank seams and which are not necessarily representative of the seams, but which emphasize the specific advantages of RF-oxidation, is given in Table 1. Comparison of the percentage mineral matter obtained by the two methods shows that the RF-oxidation analyses are 0.2-O-4% higher than the air-oxidation analyses, and that the respective combined water contents (HzOr) are 0*03-0.3% higher. These comparisons confirm that the airoxidation method partly dehydrates clay minerals (which contribute virtually all the combined water content) and produces results for the mineral-matter content which are biased low. X-ray diffraction analyses of the samples in Table 1 showed that kaolinite is the major clay mineral, indicating that kaolinite may partly dehydrate in the airoxidation method, although the possibility of the presence of poorly crystalline clay minerals, not readily identifiable by X-ray diffraction analysis, is noted. Investigations of the decompositions of clay minerals2T28 have shown that hydroxyl water is evolved from most clay minerals upon prolonged heating at 4OO’C and that the decomposition temperature is strongly dependent on the crystallinity of the mineral. The mineral matter to ash ratios vary from 1.07 to 1.17 for the RF-oxidation and 1.06 to 1 *15 for the air-oxidation method. The higher mineral-matter and combined-water contents in the RF-oxidation analyses would result in higher values for carbon (dmmf) and lower hydrogen (dmmf), and corresponding higher rank of the coal. The residual carbon contents of the RF-oxidized samples are generally lower than O.l%, which was the previous acceptability standard for the air-oxidation method’. No difficulties were experienced with the RF oxidation of the anthracite sample (SC1 11); in contrast, an air-oxidation period of five weeks at 370°C was necessary to reduce the organic carbon to 0.2% and there is evidence of considerable combined water loss, as shown in Table 1. This demonstrates the advantage of the RF method for oxidizing higher-rank coals. A good comparison between the carbonate (COi) analyses in the coal and the mineral matter is indicated, whereas the air-oxidized mineral matter shows considerable destruction of carbonates. Comparison of the total sulphur contents of the RF- and air-oxidized mineral matters shows that in most instances there is surplus of sulphur (O-01 to 0.1%) over that attributable to the pyritic and sulphate sulphur content of the coal. X-ray diffraction analysis of the mineral matter shows that CaSQd,$H20 (hemihydrate) is synthesized by oxidation during both methods. The source of the calcium is not certain but we have noted (air-oxidation method) the decomposition of calcite to CaS04.sH20 in a coal sample containing large quantities of pyrite. The differing effects of the RF- and air-oxidation methods on samples containing significant quantities of pyrite were examined on selected samples of Bowen Basin The mineral-matter contents ranged (Queensland) coal. from 32 to 54% and the samples were analysed for pyritic and sulphate sulphur in the coal, the RF-oxidized and the air-oxidized mineral matters. The results in TabZe 2 show good agreement between the pyritic sulphur contents of
F. W. Frazer and C. B. Belcher: Determination Table 1
Comparative
analyses (56 dry basis coal) for RF and 370°C
air-oxidized
of mineral matter in coal by r. f. oxidation
coal samples
Sample No.
SC1 14
SC115
SC1 16
SC117
16666
+8982
SC111
Coalfield
Sth NSW
Nth NSW
Nth NSW
Nth NSW
Old
Sth NSW
Sth Wales
Seam
Wongawilli
Big Ben
Greta
Borehole
Blackwater
Bulli
-
K, Q. S,
K,M-I.Q.
K, M-l,
K.
K. Q. I=
K Q. S
C. AP
S. AP
M. Q, F, An
Minerals in coal’
Volatile
matter+
Ash MMRFO MM3700 MMRFQ/ash MM37oPlash
%o+R
ratio ratio
FO
H20+370°
Res CRFO Res C37co CO2 (coal) co2 RF0 co2
370°
Pyritic S (coal) Sulphate S (coall Total S,QFO Total S3700 l
Q, S
Q, P
29.2
41.7
445
37.8
27.1
23.4
2.7
la.3 20.45
26.9 30.25
11-5 13.4
19.3 21.7
9.4 10.8
10.5 11.7
12.8 i 3.8
20.25 1.12 1.11 1.60 1.40
30.0 1.12 1.12 2.15 1 .a9
13.2 1.17 1.15 1.23 1.03
21.5 1.12 1.11 1.11 0.92
10.65 1.15 1.13 0.76 0.60
11.6 1.11 1.10 0.94 0.88
13.55 1 .OB 1.06 1.11 0.79
0.07
0.10
0.06
0.07
0.03
0.06
0.03
0.02 0.58 0.53 0.03 0.01 0.01 0.02 0.01
0.04 0.24 0.23 0.20 0.31 0.16 0.50 0.32
0.03 0.11 0.06 0.02 0.25 0.08 0.45 0.17
0.02 1.72 1.60 0.10 0.06 0.02 0.20 0.23
0.07 0.57 0.53 0.12 0.01 0.01 0.05 094
0.06 0.31 0.33 0.19 0.01 0.01 094 0.03
0.29 0.16 0.06 0.05 0.01 0.03 0.04 0.05
Minerals are listed in order major-minor
as estimated
An = anatase. AP = apatite, C = calcite, F = feldspar, S = siderite t dmmf corrected for CO2
by X-ray diffraction
K = kaolinite.
M = mica, M-l
Table 2 Comparative pyritic and sulphate sulphur contents /% dry basis coal) of RF- and 370°C air-oxidized mineral matters from highly-pyritic-coal samples Sample No.
18126
18379
18304
18346
18382
MMRFO (%)
32.5 1.28 0.02 1.28 0.28 1.02 0.25
49.7 1.11 0.03 1.02 0.25 0.32 0.22
35.7 6.75 0.44 6.7 0.63 2.43 1.41
39.7 13-7 0.47 13.7 0.77 7.55 2.90
53.9 la.7 0.50 i a.3 0.78 11.6 6.8
Pyritic S (coal) Sulphate S (coal) Pyritic SRFO Sulphate SRFQ Pyritic S37oo Sulphate S3700
K
the coal and the RF-oxidized mineral matter, indicating that pyrite is quantitatively retained during this technique. The pyritic sulphur contents of the air-oxidized material show that appreciable quantities (25-75%) of pyrite decompose; however, rather than the reported oxidation to hematite112, the major reaction appears to be formation of sulphate. X-ray diffraction analysis of the air-oxidized mineral matter shows that the major phases synthesized are Fe3(SO&oH)5.2H20 (carphosiderite) and FeSOd.tiHzO (szomolnokite) with lesser quantities of hemihydrate and hematite. The presence of significant quantities of synthesized sulphates in the air-oxidation method clearly shows the limitations of the pyritic sulphur correction based on quantitative oxidation of pyrite to hematite’y2, and is probably responsible for the difficulties experienced by Brown et al2 in obtaining quantitative results for the highlypyritic Greta seam coals. Fortunately the important NSW and Queensland coalfields have relatively low pyriticsulphur contents and this has made possible the successful application of the air-oxidation method for Australian
= mixed layer montmorillonite-illite,
P = pyrite,
0 = quartz,
coals. British and European coals, which often contain pyrite as a major mineral, may have caused the limited acceptance of the air-oxidation method by these countries. However the problem of pyrite oxidation does not occur with the RF-oxidized mineral matter and this method should have application to a wider range of coals than the air-oxidation method.
CONCLUSIONS The RF-oxidation method retains all the inherent advantages of the air-oxidation method and eliminates the disadvantages of applying correction factors based on extra analytical determinations. The reproducibility of the mineral-matter yields and levels of residual carbon and retained organic sulphur are similar for both methods. The shorter RF-oxidation time of 100 h maximum is advantageous in allowing a total analysis to be completed with less total elapsed time than the air-oxidation method. Elimination of the carbon dioxide analysis of the mineral matter in the RF-oxidation method results in a 35% reduction in labour costs compared with the air-oxidation method. The RF-oxidation method can quantitatively oxidize anthracite and coals with high pyrite contents and hence has application to a wider range of coals than the airoxidation method.
ACKNOWLEDGEMENTS The authors acknowledge the permission of The Broken Hill Proprietary Co. Ltd to publish this work.
FUEL, 1973, Vol. 52, January
45
Determination
of mineral matter in coal by r. f. oxidation: F. W. Frazer and C. B. Belcher
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