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Characterization of refuse-derived fuel at various stages of processing
and Conservation, 11 (1985) 255-261 Elsevier Science Publishers B.V., Amsterdam -Printed
255
Resources
CHARACTERIZATION OF REFUSE-DERIVED STAGES OF PROCESSING
D.R. KIRKLIN,
P.H. DECKER
National
of Standards,
Bureau
in The Netherlands
FUEL AT VARIOUS
and E.S. DOMALSKI Washington,
D.C.
20234
(U.S.A.)
(Received March 28, 1984; accepted in revised form November 4, 1984)
ABSTRACT
The calorific value, moisture, ash, sulfur and chlorine contents were measured by the National Bureau of Standards (NBS) and the results analyzed to characterize refusederived fuel (RDF) as a function of the type of processing that the RDF had undergone. The RDF was separated from a municipal solid waste (MSW) sample from New Castle County, Delaware at the Bureau of Mines (BuM) pilot resource recovery plant in College Park, Maryland. The RDF streams were collected at seven points after various stages of processing through air classifiers and trommels. NBS characterized RDF as a function of the type of processing that the RDF had undergone. Air classifiers were very effective in separating the light components of RDF (i.e. paper and plastic films) from the heavier components of RDF. A trommel in the RDF separation scheme removes some of the undesirable characteristics of RDF, namely, the non-combustible, sulfur- and chlorine-containing components of RDF.
INTRODUCTION Refuse-derived fuels (RDF) are prepared from municipal solid waste (MSW) through various processing stages which separate the combustible fraction from the non-combustible fraction. The combustible fraction, commonly referred to as RDF, is separated from the non-combustible portion of MSW by a series of air classifiers and trommels. The U.S. Bureau of Mines (BuM) resource recovery plant in College Park, Maryland [I, 21 has been used to separate and evaluate the recoverable materials from MSW [3] . The National Bureau of Standards (NBS) obtained a laboratory sample of RDF from New Castle County, Delaware, that was processed through the Bureau of Mines resource recovery plant. The New Castle County sample was used to evaluate the effect of processing on RDF. RDF was selected from the seven RDF outputs of the Bureau of Mines resource recovery plant. The calorific value, moisture, ash, sulfur and chlorine contents were measured to characterize these properties as a function of the type of processing that the RDF had undergone.
256
Calorimetric measurements were made in the NBS 25gram-capacity bomb calorimeter [ 41. This bomb calorimeter was designed to make measurements on samples where heterogeneity of a small sample is a problem. RDF PROCESSING PLANT
An overall flow-diagram for the processing of municipal (MSW) at the U.S. Bureau of Mines resource recovery facility
solid waste is shown in
Fig. 1. Overall flow-diagram for processing MSW; Resource Bureau of Mines, College Park, Maryland.
Facility,
Recovery
U.S.
257
Fig. I [1,2]. The processing plant was designed to collect all recoverable fractions of MSW, such as the combustible fraction, metal fraction, mineral fraction and other valuable recoverable fractions. This report describes the characterization of the combustible fractions of MSW which were collected at the seven collection points; these are shown as circled numbers in Fig. 1. The seven NBS samples and their as-received masses are shown in Table 1. Material from cyclones 1 and 2 was placed in a trommel with 20 mm openings to separate the small-particle material from the large-particle material. The minus-20 mm material was collected on a 1.40 mm screen to remove some of the fines. Therefore, samples 1 and 2 received only one air-classification step and were collected in cyclone 1 followed by a trommeling step. The plus-20 mm RDF was reshredded and collected as Sample 1. The minus-20 mm RDF was collected on a 1.4 mm screen as sample no. 2. Samples 3 and 4 received two air-classification steps and were collected in cyclone 2 followed by processing through a trommel. The plus-20 mm RDF was re-shredded and collected as sample no. 3, while the minus-20 mm RDF was collected on a 1.4 mm screen as sample no. 4. Samples 5, 6 and 7 were obtained from the heavy fraction of the second air-classification step followed by processing through a trommel. The plus-20 mm RDF was re-shredded and air classified. The light fraction from this third air classifier was collected in cyclone 3 and became sample no. 5. The heavy fraction from the third air classifier was passed over an electrostatic separator to remove the non-magnetic metals. The combustible fraction from the electrostatic separator consisted of heavy organic material and was collected as sample no. 6. The minus-20 mm RDF from the trommel was processed through a mineral jig where the low-density organic waste was collected as sample no. 7. TABLE
1
NBS samples of RDF from New Castle Co., DE No.
Identification
As-received sample masses (g)
1
Cyclone no. 1, trommel oversize Cyclone no. 1, trommel undersize Cyclone no. 2, trommel oversize Cyclone no. 2, trommel undersize Cyclone no. 3, trommel oversize lights Heavy organics, trommel oversize heavies Jig overflow, trommel undersize
3268.3 2287.5 2672.5 2361.2 3540.3 3664.8 3203.1
2 3 4 5 6 7
The seven samples studied had received various stages of processing through air classifiers and trommels. Samples 1 - 5 consisted of the air classified RDF from cyclones 1, 2 and 3 with processing through a trommel to separate fines and other minerals. Samples 6 and 7 consisted of the heavy fraction of the RDF that fell from the air classifiers.
258 RDF SAMPLE PREPARATION
New Castle County, Delaware, delivered a refuse sample of approximately 2,300 kg (5,000 lb) to the pilot resource recovery plant operated by the U.S. Bureau of Mines in College Park, Maryland. The National Bureau of Standards (NBS) received a 21 kg aliquot of RDF which was obtained from processing the raw refuse from New Castle County. The 21 kg aliquot was made up of seven samples, of approximately 3 kg each, one from each of the seven RDF outputs described in the previous section. The Pittsburgh Energy Technology Center (now part of the Department of Energy but formerly part of the Bureau of Mines) also received an aliquot of the RDF (3) from New Castle County. Each of the seven separate NBS laboratory samples received the following sequential treatment: (1) Air-drying in a convection oven at 37 rt 3°C; (2) Milling to reduce particle size, (a) samples 1 - 7 were milled in a hammer mill to 6 mm (% inch) particle size; (b) the 6 mm RDF of samples 1 - 5 were milled further to 2 mm particle size in a Wiley mill; (3) Splitting to obtain a laboratory sample of workable size by successive coning and quartering; (4) Blending of the laboratory sample in a vee-blender to obtain a ‘homogenous’ sample; (5) Selecting analysis samples of approximately 2.5 or 25 grams, (a) 3 samples for calorific value measurements (25 grams), (b) 2 samples for residual moisture measurements (2.5 grams), (c) 2 samples for furnace ash measurements (2.5 grams), (6) Pelletizing of analysis samples with the exception of those from samples 6 and 7 which did not pelletize well, A Wiley mill was used to reduce the particle size from 6 mm to 2 mm in step (2) above. The cutting action of this mill was very effective on the light paper and plastic film components of RDF. However, it was considerably less effective in reducing the particle size of the denser combustible components of RDF such as wood and heavy plastic products. For this reason, samples 6 and 7 were only milled to 6 mm. DISCUSSION
Table 2 presents the NBS mean calorific values produced from the processing of MSW from New ific values (i.e. Higher Heating Values, HHV) are HHVl is the higher heating value on an as-received heating value on a moisture-free basis, and HHV3 on a moisture-free and ash-free basis. The HHV2
of the seven RDF samples Castle County. The calorpresented on three bases: basis, HHV2 is the higher is the higher heating value values for the seven RDF
259
sample streams varied from 14.189 to 21.916 MJ/kg. The percent standard deviation of the mean for these measurements ranged from 0.11 to 1.03. Three measurements were made on each of the seven RDF streams. Samples 1, 3 and 5 are identified as cyclone no. 1 --- trommel oversize, cyclone no. 2 -- trommel oversize, and cyclone no. 3 -- light trommel oversize, respectively. The HHV2 values for samples 1, 3 and 5 are approximately 20 MJ/kg, and compare well with the overall mass-weighted mean HHV2 value of 19.350 MJjkg. These light paper and plastic components of RDF had calorific values in the middle of the observed range. However, samples 6 and 7, which are identified as the heavy organic and jig overflow fractions, are composed of wood and heavy plastic materials that are part of the heavy fraction from the air classifiers. These heavy combustible components had HHV2’s somewhat higher than the HHV2’s of the light combustible fraction. In fact, the HHV3’s of samples 6 and ‘7 approach that of low-grade coals. TABLE
2
NBS calorific
value results
Sample no. and RDF sample stream
HHVl
HHV2
HHV3
(MJ/kg)
(MJ/W
WJ/kg)
Cyclone
no. 1 trommel
oversize
16.722
19.317
22.081
Cyclone
no. 1 trommel
undersize
12.198
14.189
21.532
Cyclone
no. 2 trommel
oversize
17.315
19.999
21.816
Cyclone
no. 2 trommel
undersize
12.465
16.342
21.229
Cyclone
no. 3 trommel
oversize
16.289
20.355
22.378
20.853
21.916
24.607
19.576
20.692
23.274
16.910
19.350
22.565
Heavy organics Jig overflow
trommel
trommel
Mass wtd. mean
oversize
undersize
lights heavies
Table 3 contains the moisture and ash contents measured for these seven RDF samples. The moisture content allows the conversion of all measured values to a moisture-free basis for comparison. Ash content is definitely characteristic of the processing procedures utilized. The higher ash contents of samples 2 and 4 indicate that the non-combustible components of RDF are generally of smaller particle size than the combustible components of RDF. Samples 2 and 4 are identified as cyclone no. 1 --- trommel undersize, and cyclone no. 2 -- trommel undersize. These samples have ash contents of 34.11 and 23.02%. Comparison of the ash from the seven samples shows that the ash is concentrated in the trammel undersize material.
TABLE NBS
3
analytical
results
Sample no. and RDF sample stream
Moisture (wt. %)
Asha (wt. %)
Sulfura (wt. a)
Chlorinea (wt. %)
1
Cyclone
no. 1 trommel
oversize
13.44
12.51
0.14
0.30b
2
Cyclone
no. 1 trommel
undersize
14.02
34.11
0.23
0.57
3
Cyclone
no. 2 trommel
oversize
13.42
8.31
0.23
0.40
4
Cyclone
no. 2 trommel
undersize
23.73
23.02
0.22
0.77
5
Cyclone
no. 3 trommel
oversize
19.98
9.03
0.12
0.39
6
Heavy organics
4.85
10.93
0.20
1.29
7
Jig overflow
5.40
11.09 ---
0.15
0.34
13.03
14.43
0.16
0.59
trommel
trommel
Mass wtd. mean
oversize
undersize
lights heavies
P
aMoisture-free basis. ‘Data from [ 31.
Table 3 also contains the sulfur and chlorine contents of the seven samples studied. Samples 2, 4 and 6 have the largest values for both sulfur and chlorine. Samples 2, 4 and 6 are identified as cyclone no. 1 - trommel undersize, cyclone no. 2 - trommel undersize, and heavy organics. Therefore, sulfur and chlorine are also concentrated in the small-particle components of RDF. Similar measurements [3] were performed by the Pittsburgh Energy Technology Center (PETC) on another aliquot of the RDF from New Castle County. The NBS results are in reasonable agreement with those of PETC. The results of NBS measurements show that the trommel is a good piece of processing equipment for separating the non-combustible fraction of RDF from the combustible fraction. In addition, the undesirable sulfur and chlorine are also concentrated in the trommel undersize material. The use of a trommel is beneficial in producing a cleaner and more environmentally desirable RDF. CONCLUSIONS
Some of the RDF properties have been characterized as a function of the type of processing that the RDF had undergone. An evaluation of the moisture-free calorific values (i.e. HHV2) showed the following characteristics: (1) The air-classified, trommel oversize RDF components (i.e. paper and
261
plastic films) have calorific values that compare on a relative basis to the mean values observed. (2) The heavy-organic and jig-overflow components of RDF (i.e. wood and heavy plastic products) have calorific values approaching that of low grade coals. (3) The NBS 25-gram-capacity bomb calorimeter is better suited for heterogeneous fuels because the larger sample size requires less particle size reduction. A trommel in the RDF separation scheme removes some of the undesirable characteristics of RDF, namely: (1) The non-combustible components of RDF were found to be concentrated in the trommel undersize material; and (2) Sulfur and chlorine were found to be concentrated in the trommel undersize and heavy organic components of RDF.
REFERENCES 1 Sullivan, P.M., Stanczyk, M.H. and Spendlove, M.J., 19’73. Bureau of Mines Report of Investigations 7760. 2 Phillips, T.A., 1977. Bureau of Mines Information Circular 8732. 3 DeCesare, R.S., Palumbo, F.J. and Sullivan, P.M., 1980. Bureau of Mines Report of Investigations 8429. 4 Kirklin, D.R. and Domalski, E.S., 1983. Resources and Conservation, 10: 177-187.
255
Resources
CHARACTERIZATION OF REFUSE-DERIVED STAGES OF PROCESSING
D.R. KIRKLIN,
P.H. DECKER
National
of Standards,
Bureau
in The Netherlands
FUEL AT VARIOUS
and E.S. DOMALSKI Washington,
D.C.
20234
(U.S.A.)
(Received March 28, 1984; accepted in revised form November 4, 1984)
ABSTRACT
The calorific value, moisture, ash, sulfur and chlorine contents were measured by the National Bureau of Standards (NBS) and the results analyzed to characterize refusederived fuel (RDF) as a function of the type of processing that the RDF had undergone. The RDF was separated from a municipal solid waste (MSW) sample from New Castle County, Delaware at the Bureau of Mines (BuM) pilot resource recovery plant in College Park, Maryland. The RDF streams were collected at seven points after various stages of processing through air classifiers and trommels. NBS characterized RDF as a function of the type of processing that the RDF had undergone. Air classifiers were very effective in separating the light components of RDF (i.e. paper and plastic films) from the heavier components of RDF. A trommel in the RDF separation scheme removes some of the undesirable characteristics of RDF, namely, the non-combustible, sulfur- and chlorine-containing components of RDF.
INTRODUCTION Refuse-derived fuels (RDF) are prepared from municipal solid waste (MSW) through various processing stages which separate the combustible fraction from the non-combustible fraction. The combustible fraction, commonly referred to as RDF, is separated from the non-combustible portion of MSW by a series of air classifiers and trommels. The U.S. Bureau of Mines (BuM) resource recovery plant in College Park, Maryland [I, 21 has been used to separate and evaluate the recoverable materials from MSW [3] . The National Bureau of Standards (NBS) obtained a laboratory sample of RDF from New Castle County, Delaware, that was processed through the Bureau of Mines resource recovery plant. The New Castle County sample was used to evaluate the effect of processing on RDF. RDF was selected from the seven RDF outputs of the Bureau of Mines resource recovery plant. The calorific value, moisture, ash, sulfur and chlorine contents were measured to characterize these properties as a function of the type of processing that the RDF had undergone.
256
Calorimetric measurements were made in the NBS 25gram-capacity bomb calorimeter [ 41. This bomb calorimeter was designed to make measurements on samples where heterogeneity of a small sample is a problem. RDF PROCESSING PLANT
An overall flow-diagram for the processing of municipal (MSW) at the U.S. Bureau of Mines resource recovery facility
solid waste is shown in
Fig. 1. Overall flow-diagram for processing MSW; Resource Bureau of Mines, College Park, Maryland.
Facility,
Recovery
U.S.
257
Fig. I [1,2]. The processing plant was designed to collect all recoverable fractions of MSW, such as the combustible fraction, metal fraction, mineral fraction and other valuable recoverable fractions. This report describes the characterization of the combustible fractions of MSW which were collected at the seven collection points; these are shown as circled numbers in Fig. 1. The seven NBS samples and their as-received masses are shown in Table 1. Material from cyclones 1 and 2 was placed in a trommel with 20 mm openings to separate the small-particle material from the large-particle material. The minus-20 mm material was collected on a 1.40 mm screen to remove some of the fines. Therefore, samples 1 and 2 received only one air-classification step and were collected in cyclone 1 followed by a trommeling step. The plus-20 mm RDF was reshredded and collected as Sample 1. The minus-20 mm RDF was collected on a 1.4 mm screen as sample no. 2. Samples 3 and 4 received two air-classification steps and were collected in cyclone 2 followed by processing through a trommel. The plus-20 mm RDF was re-shredded and collected as sample no. 3, while the minus-20 mm RDF was collected on a 1.4 mm screen as sample no. 4. Samples 5, 6 and 7 were obtained from the heavy fraction of the second air-classification step followed by processing through a trommel. The plus-20 mm RDF was re-shredded and air classified. The light fraction from this third air classifier was collected in cyclone 3 and became sample no. 5. The heavy fraction from the third air classifier was passed over an electrostatic separator to remove the non-magnetic metals. The combustible fraction from the electrostatic separator consisted of heavy organic material and was collected as sample no. 6. The minus-20 mm RDF from the trommel was processed through a mineral jig where the low-density organic waste was collected as sample no. 7. TABLE
1
NBS samples of RDF from New Castle Co., DE No.
Identification
As-received sample masses (g)
1
Cyclone no. 1, trommel oversize Cyclone no. 1, trommel undersize Cyclone no. 2, trommel oversize Cyclone no. 2, trommel undersize Cyclone no. 3, trommel oversize lights Heavy organics, trommel oversize heavies Jig overflow, trommel undersize
3268.3 2287.5 2672.5 2361.2 3540.3 3664.8 3203.1
2 3 4 5 6 7
The seven samples studied had received various stages of processing through air classifiers and trommels. Samples 1 - 5 consisted of the air classified RDF from cyclones 1, 2 and 3 with processing through a trommel to separate fines and other minerals. Samples 6 and 7 consisted of the heavy fraction of the RDF that fell from the air classifiers.
258 RDF SAMPLE PREPARATION
New Castle County, Delaware, delivered a refuse sample of approximately 2,300 kg (5,000 lb) to the pilot resource recovery plant operated by the U.S. Bureau of Mines in College Park, Maryland. The National Bureau of Standards (NBS) received a 21 kg aliquot of RDF which was obtained from processing the raw refuse from New Castle County. The 21 kg aliquot was made up of seven samples, of approximately 3 kg each, one from each of the seven RDF outputs described in the previous section. The Pittsburgh Energy Technology Center (now part of the Department of Energy but formerly part of the Bureau of Mines) also received an aliquot of the RDF (3) from New Castle County. Each of the seven separate NBS laboratory samples received the following sequential treatment: (1) Air-drying in a convection oven at 37 rt 3°C; (2) Milling to reduce particle size, (a) samples 1 - 7 were milled in a hammer mill to 6 mm (% inch) particle size; (b) the 6 mm RDF of samples 1 - 5 were milled further to 2 mm particle size in a Wiley mill; (3) Splitting to obtain a laboratory sample of workable size by successive coning and quartering; (4) Blending of the laboratory sample in a vee-blender to obtain a ‘homogenous’ sample; (5) Selecting analysis samples of approximately 2.5 or 25 grams, (a) 3 samples for calorific value measurements (25 grams), (b) 2 samples for residual moisture measurements (2.5 grams), (c) 2 samples for furnace ash measurements (2.5 grams), (6) Pelletizing of analysis samples with the exception of those from samples 6 and 7 which did not pelletize well, A Wiley mill was used to reduce the particle size from 6 mm to 2 mm in step (2) above. The cutting action of this mill was very effective on the light paper and plastic film components of RDF. However, it was considerably less effective in reducing the particle size of the denser combustible components of RDF such as wood and heavy plastic products. For this reason, samples 6 and 7 were only milled to 6 mm. DISCUSSION
Table 2 presents the NBS mean calorific values produced from the processing of MSW from New ific values (i.e. Higher Heating Values, HHV) are HHVl is the higher heating value on an as-received heating value on a moisture-free basis, and HHV3 on a moisture-free and ash-free basis. The HHV2
of the seven RDF samples Castle County. The calorpresented on three bases: basis, HHV2 is the higher is the higher heating value values for the seven RDF
259
sample streams varied from 14.189 to 21.916 MJ/kg. The percent standard deviation of the mean for these measurements ranged from 0.11 to 1.03. Three measurements were made on each of the seven RDF streams. Samples 1, 3 and 5 are identified as cyclone no. 1 --- trommel oversize, cyclone no. 2 -- trommel oversize, and cyclone no. 3 -- light trommel oversize, respectively. The HHV2 values for samples 1, 3 and 5 are approximately 20 MJ/kg, and compare well with the overall mass-weighted mean HHV2 value of 19.350 MJjkg. These light paper and plastic components of RDF had calorific values in the middle of the observed range. However, samples 6 and 7, which are identified as the heavy organic and jig overflow fractions, are composed of wood and heavy plastic materials that are part of the heavy fraction from the air classifiers. These heavy combustible components had HHV2’s somewhat higher than the HHV2’s of the light combustible fraction. In fact, the HHV3’s of samples 6 and ‘7 approach that of low-grade coals. TABLE
2
NBS calorific
value results
Sample no. and RDF sample stream
HHVl
HHV2
HHV3
(MJ/kg)
(MJ/W
WJ/kg)
Cyclone
no. 1 trommel
oversize
16.722
19.317
22.081
Cyclone
no. 1 trommel
undersize
12.198
14.189
21.532
Cyclone
no. 2 trommel
oversize
17.315
19.999
21.816
Cyclone
no. 2 trommel
undersize
12.465
16.342
21.229
Cyclone
no. 3 trommel
oversize
16.289
20.355
22.378
20.853
21.916
24.607
19.576
20.692
23.274
16.910
19.350
22.565
Heavy organics Jig overflow
trommel
trommel
Mass wtd. mean
oversize
undersize
lights heavies
Table 3 contains the moisture and ash contents measured for these seven RDF samples. The moisture content allows the conversion of all measured values to a moisture-free basis for comparison. Ash content is definitely characteristic of the processing procedures utilized. The higher ash contents of samples 2 and 4 indicate that the non-combustible components of RDF are generally of smaller particle size than the combustible components of RDF. Samples 2 and 4 are identified as cyclone no. 1 --- trommel undersize, and cyclone no. 2 -- trommel undersize. These samples have ash contents of 34.11 and 23.02%. Comparison of the ash from the seven samples shows that the ash is concentrated in the trammel undersize material.
TABLE NBS
3
analytical
results
Sample no. and RDF sample stream
Moisture (wt. %)
Asha (wt. %)
Sulfura (wt. a)
Chlorinea (wt. %)
1
Cyclone
no. 1 trommel
oversize
13.44
12.51
0.14
0.30b
2
Cyclone
no. 1 trommel
undersize
14.02
34.11
0.23
0.57
3
Cyclone
no. 2 trommel
oversize
13.42
8.31
0.23
0.40
4
Cyclone
no. 2 trommel
undersize
23.73
23.02
0.22
0.77
5
Cyclone
no. 3 trommel
oversize
19.98
9.03
0.12
0.39
6
Heavy organics
4.85
10.93
0.20
1.29
7
Jig overflow
5.40
11.09 ---
0.15
0.34
13.03
14.43
0.16
0.59
trommel
trommel
Mass wtd. mean
oversize
undersize
lights heavies
P
aMoisture-free basis. ‘Data from [ 31.
Table 3 also contains the sulfur and chlorine contents of the seven samples studied. Samples 2, 4 and 6 have the largest values for both sulfur and chlorine. Samples 2, 4 and 6 are identified as cyclone no. 1 - trommel undersize, cyclone no. 2 - trommel undersize, and heavy organics. Therefore, sulfur and chlorine are also concentrated in the small-particle components of RDF. Similar measurements [3] were performed by the Pittsburgh Energy Technology Center (PETC) on another aliquot of the RDF from New Castle County. The NBS results are in reasonable agreement with those of PETC. The results of NBS measurements show that the trommel is a good piece of processing equipment for separating the non-combustible fraction of RDF from the combustible fraction. In addition, the undesirable sulfur and chlorine are also concentrated in the trommel undersize material. The use of a trommel is beneficial in producing a cleaner and more environmentally desirable RDF. CONCLUSIONS
Some of the RDF properties have been characterized as a function of the type of processing that the RDF had undergone. An evaluation of the moisture-free calorific values (i.e. HHV2) showed the following characteristics: (1) The air-classified, trommel oversize RDF components (i.e. paper and
261
plastic films) have calorific values that compare on a relative basis to the mean values observed. (2) The heavy-organic and jig-overflow components of RDF (i.e. wood and heavy plastic products) have calorific values approaching that of low grade coals. (3) The NBS 25-gram-capacity bomb calorimeter is better suited for heterogeneous fuels because the larger sample size requires less particle size reduction. A trommel in the RDF separation scheme removes some of the undesirable characteristics of RDF, namely: (1) The non-combustible components of RDF were found to be concentrated in the trommel undersize material; and (2) Sulfur and chlorine were found to be concentrated in the trommel undersize and heavy organic components of RDF.
REFERENCES 1 Sullivan, P.M., Stanczyk, M.H. and Spendlove, M.J., 19’73. Bureau of Mines Report of Investigations 7760. 2 Phillips, T.A., 1977. Bureau of Mines Information Circular 8732. 3 DeCesare, R.S., Palumbo, F.J. and Sullivan, P.M., 1980. Bureau of Mines Report of Investigations 8429. 4 Kirklin, D.R. and Domalski, E.S., 1983. Resources and Conservation, 10: 177-187.