CURRENT RESEARCH Rejection of Trace Metals from Coal During Beneficiation by Agglomeration C. Edward Capes,l Allan E. Mcllhinney, Douglas S. Russell, and Aurelio F. Sirianni
Division of Chemistry, National Research Council of Canada, Ottawa, Canada Examined was the rejection of trace quantities of heavy metals from coals during the process of fine grinding followed by selective oil agglomeration of the carbonaceous constituents. Six different steam and coking coals, mainly from Canadian and U.S. sources, were treated. The level of most heavy elements was generally low but, in light of the large throughputs of power plants, the potential environmental hazard may be significant, especially for those elements which are volatile or may form volatile compounds when fired. Many of the trace metals were substantially removed during agglomerative beneficiation of the coal. Some which are apparently in organic association in the coal remained in the agglomerated product. In addition to the reduced environmental hazard, a number of other advantages in upgrading coals prior to use are discussed. Metals emissions from coal-burning power plants are receiving increasing attention as potentially dangerous air pollutants. Although the levels of heavy elements in coals are generally very low [see, for example, the recently reported analyses of Brown et al. (1972)], the annual emission from a given plant may reach significant amounts when one considers that the largest of these plants being built today may consume on the order of 6 x lo6 tons of coal in one year. In conventional technology, it is thought that many of the trace metals are collected with the ash in electrostatic precipitators. This approach is unlikely to succeed, however, with volatile compounds. For example, one report (Billings and Matson, 1972) suggests that only about 10% of the mercury released in a coal-fired station remains with the furnace residual ash. An alternative approach is to remove as much as possible of the trace metals prior to combustion. This not only avoids the loss of those elements which are naturally volatile or are present as volatile compounds, but also avoids the possible high-temperature formation of other volatile compounds (e.g., carbonyls, halides) of the metals. The purpose of the present investigation was to examine the ability of oil agglomeration to remove heavy metals during coal beneficiation. This coal-cleaning method has been very effective (Sirianni et al., 1969; Capes et al. 1970, 1971). It consists of fine grinding in aqueous slurry followed by collection of the carbonaceous constituents by oil agglomeration. This is perhaps the only method available for recovering very fine particles from aqueous slurry economically on a large scale. To whom correspondence should be addressed
Experimental Coal Beneficiation. Six coals from different geographical locations were tested in the experiments, details of which are given in Table I. In all cases except experiments 8-10, the coal was ground as a 40-50 wt YO aqueous slurry in a ceramic-lined jar mill using stainless steel balls. This wet grinding resulted, typically, in size distributions containing 100% -200-mesh particles and 40-50% -325mesh particles. The samples in experiments 8-10 consisted of feed material for power stations and had been dry pulverized to approximately 100% finer than 60 mesh and 30% finer than 325 mesh. Each agglomeration experiment was done on 500 cm3 of a 10 wt YO aqueous slurry of the coal with mixing provided by a high-speed blender. From 10-30% by weight based on the dry solids of a light petroleum distillate was added and agitation was continued for 5 min to effect agglomeration of the carbonaceous constituents. The mixture was then poured onto a 100-mesh screen to allow the water containing the tailings to drain through while retaining the agglomerated coal. The two fractions were dried a t 110°C and sent for analyses. Table I indicates the ash rejection levels obtained. The recovery in the agglomerates of the combustible content of the feed exceeded 90% in every case. Heavy Metals Analyses. A variety of instrumental analytical methods were used in this study; the methods for the different elements are given in Table 11. Atomic absorption (AA) was used for the heavy metals wherever possible because of the better precision it provided. However, many of the metals of interest could not readily be determined in this way chiefly because they were present a t lower concentrations, and therefore methods which were much more sensitive, such as optical emission spectroscopy (OES) and spark source mass spectroscopy (SSMS), were employed. In the cases in which atomic absorption was applied, the organic matter present was destroyed by acid digestion. The powdered coal samples were digested with nitric and sulfuric acids for 1 hr in a flask fitted with a reflux condenser. Perchloric acid was added and the digestion was continued until a water-clear solution resulted. After appropriate dilution, the concentrations were measured on a Techtron A4 atomic absorption spectrophotometer. Calibration curves were prepared from standard samples in similar acid media, and since the digesting acids contained appreciable concentrations of these elements, blanks were deducted. The determination of mercury was a special case where great care was taken to prevent contamination from the laboratory atmosphere and from the reagents used. Moist Volume 8,Number 1, January 1974
35
100,
1
/ /
s
!
, /
/
0
I 20
/
'
I
I
40
80
1 60 % ASH REJECTiON
100
dc arc. Synthetic samples for calibration were prepared by mixing oxides of the major constituents with "G" standards (Spex Industries). To analyze the ashes by spark source mass spectroscopy, they were mixed with ultrapure graphite and compressed into small electrodes. These were sparked in the instrument and the results were computed from data from standards of similar composition. Mass balances were performed on every analytical measurement of the elements-i.e., using the agglomerates/ tailings ratios given in Table I, the concentration of a given element in the feed was back-calculated from the agglomerates and tailings analyses for that element. This calculated feed concentration was then compared with the measured value and only those which agreed within &30% are reported in Table 11. Overall, the mass balances were within these limits in 62% of the determinations. This rather low proportion reflects the difficulty in sampling and analyzing for materials which are present in low concentrations. For the three analytical methods used, the result was:
Figure 1. Average metal rejection in each experiment as function of ash rejection Numbers correspond
to experiments in Table I
samples were dried below 110°C to avoid vaporization of mercury. The coal was digested as described with additional precautions recommended by Rains (1971) and then determined by flameless atomic absorption according to his procedure. For those elements in which optical emission and spark source mass spectroscopy were employed, the analyses were mainly carried out on samples which had been carefully ashed a t a low temperature to prevent the loss of the more volatile elements. The powdered samples were heated in a furnace where the temperature was raised about lOO"C/hr and finally held a t 550°C overnight. No losses were detected when this ash was analyzed by atomic absorption and compared with the results from the wetashed samples. In addition, some unashed samples were examined spectrographically to check for losses on ashing. The optical emission results were obtained by firing 1IO-mg samples of ash in graphite electrodes using a 10-A
Method
Proportion of determinations in which calculated and measured feed levels agreed within =30%
AA 0 ES SSMS
75% 55% 46%
It is felt that these figures agree qualitatively with the relative precisions to be expected of the analytical techniques applied.
Results and Discussion In examining the results in Table 11, the very low levels of metals present in coals in many cases must be considered in relation to a large power station which may consume 6 x lo6 tons of coal each year. Thus, for example, two potentially volatile materials, such as arsenic and lead, may be emitted in quantities up to about 500 tons/ year from such a source. The potential hazard in even such low levels of pollutants becomes obvious, at least with regard to the local environment around the source. It is evident, from the average values of the ratio of
Table I. Summary of Experiments Feed coal Expt. no.
ASTM classification
Source
Agglomerates Wt. o/o total sulfur
-
Wt. o/o asha
Wt. fraction
Wt. 76 ash=
Wt. fraction
Wt. o/o asha
Wt % ash rejected in tailingsb
1
New Brunswick
H igh-volatile A bituminous
-8
24.4
0.74
4.8
0.26
82
86
2
New Brunswick
-8
22.7
0.74
4.1
0.26
76
87
3
New Brunswick
-8
22.0
0.78
6.0
0.22
79
79
4
New Brunswick
-8
24.7
0.72
6.6
0.28
79
82
5
New Brunswick
-8
21.1
0.79
5.3
0.21
91
82
6 7 8 9 10 11
Western Canada Western Canada U.S.,sample A U.S., sample B U.S., sample B U S . (Ohio), sample C
High-volatile A bituminous High-voltage A bituminous High-volatile A bituminous High-volatile A bituminous Low-volatile bituminous Low-volatile bituminous
0.5 0.5 2.6 2.6 2.5
13.3 13.3 11.0 8.9 13.5 11.5
0.97 0.93 0.96 0.93 0.89 0.85
8.4 7.3 7.5 3.3 3.8 3
0.03 0.07 0.04 0.07 0.11 0.15
-90 88 82 80 89 80
39 47 33 63 75 82
12
Brazil
2.2
29.9
0.74
8.3
0.26
86
79
...
...
...
High.volatile A bituminous
...
All analyses are o n moisture-free basis. n Loss on ignition a t 650-700°C. h Based on analyses of agglomerates and tailings and their weight fractions.
36
Tailings
~
Environmental Science & Technology
metal rejection to overall ash rejection, that many of the trace elements are concentrated in the tailings and are thus removed by fine grinding and oil agglomeration. A number of elements, however, are apparently associated with the organic constituents of coal and thus report with the agglomerated product. Oil agglomeration is hence not a satisfactory separation method for these constituents. Most prominent among these materials are barium, beryllium, boron, germanium, mercury, selenium, titanium, and zirconium. There is no apparent reason to explain the association of these particular elements with the carbonaceous fraction in coal although the results are consistent with previous work. Thus, Abernathy and Hattman (1970) reported that beryllium is mainly associated with the organic fraction in coals, while Ruch et al. (1971) reported that perhaps 50% of the mercury in coal is in organic association. Germanium is known to be generally present in the carbonaceous fraction of coals (Capes and Sutherland, 1966; Farnand and Puddington, 1969). Further information on the relationship between metal
rejection and overall ash rejection is given in Figure 1. The average metal rejection in each experiment for all the elements (other than those listed above and believed to be substantially in organic association) is seen to be approximately proportional to the ash rejection, the least squares constant of proportionality being 0.91. There is considerable scatter in the data which is no doubt related to the fact that although the majority of a given element may be present as inorganic material, a minor proportion is in organic association, the amount varying not only from element to element but also from coal to coal for a given element. Finally, some mention should be made of another class of potentially hazardous materials-those which may be naturally radioactive. A few spot analyses by mass spectrograph indicated, for example, uranium levels in the range 0.2-1 ppm and thorium levels in the range 0.5-4 ppm. As might be expected, these elements were considerably concentrated in the tailings after upgrading the coals by agglomeration.
Table II. Summary of Results 77" Element
Concentrations, ppmb
Element
Analytical method
No. of detnsa
Feed coal Av
Aluminum
0 ES
1
Antimony
SSMS
2
15,000 1.3
Arsenic
0 ES
3
72
Barium
0 ES
4
40
Be ry II i u rn
0 ES
Boron
0 ES
Cad mi u rn
SSMS 0ES
2
0.5
High, low
2.0 0.5 115 50 100 0 0.8
Av
10,000 0.7
15 38
7
21
50
0.7
AA
10
Copper
AA
9
301
Germanium
0 ES
1
14
Iron
AA
8
49,000
Lead
0 ES
5
75
Magnesium
0 ES
2
250
Manganese
AA
8
275
Mercury
AA
10
18.8
Molybdenum
0ES
Nickel
AA
Selenium Sodium
3
183
10
59
SSMS
1
0 ES
2
4 50
Tin
SSMS
1
65
Titanium
0 ES
2
425
Vanadium
OES
5
150
Zinc
AA
6
284
1690 25 800 42 23
171 14
Zirconium
0 ES
1
25
87,000 15,000 160 15 500 0 410 67 0.86 0.07 250 150 86 18 100
15,700 33 125
55 0.41 43 24 3 25
0
a b
1.2 50 3
0
1300 186
5
0.49
0.8 0.5 45 0 100
600 250 500 50 570 41
Av
100,000 7
20 400 61
488 25 566 21 22 5 30,000 7,700 105 10 250 0 152 17 0.91 0.05 100 10 56 6 50 0 550 250 150
100
0.03
25
385 25
8
av (std dev)
0.88 0.68
410 200 300 0 0.05
l.ll(0.18) 0.42(0.21) 0.02
0
32 2.0 4800 2837 1103
60 10 7690 100 4300 165
0
193,500 242 1500 935 1.2 667 204 20 500
0.49(0.26) 1.23 0.65 0.90(0.19) 0.79(0.15) 0
330,000 50,000 410 100 3000 0 1326 400 3.4 0.4 1000 500 325 111 1000
0.89(0.15) 0.90(0.25) 0.88 0.97(0.14) 0.51(0.34) l.Ol(O.24) 0.88(0.17) 0.44 1.48
0
170 710 410
10
144
High, low
??, ash, rejected,
5
270
0.1
5 1 1
High, low
rejected
Tailings
0
0.1
0.3 2200 652
Calcium Chromium
Agglomerates
1150
100
800 620 1000 150 4160 110
0.90 0.33 0.87(0.28) 0.84(0.29) 0.42
N o t replicate analyses b u t t h e number of measurements for which calculated a n d measured feed concentrations agreed within %30% All analyses are on a moisture-free basis.
(see text).
Volume 8, Number 1 , January 1974
37
Conclusion
The study generally supports the contention that much of the hazardous trace material in coals may be removed by fine grinding and agglomeration prior to firing. In addition to reducing this source of heavy metal discharge to the environment, this type of processing has a number of advantages over conventional operation. The cost and difficulty in disposing of large amounts of ash in the urban environment are avoided. Hidden costs due to the ash, such as reduced boiler capacity, wear on tubing and maintenance of the collection and ash-handling system are also reduced. Ash in the form of tailings can be handled more economically at the mine site and needless ash transportation to the power plant does not take place. Even marginal coals can be pelletized to a low-ash product which is readily handled and dust-free. In addition, there is the possibility of recovering appreciable amounts of metals with commercial value. In this study, tailings quite high in levels of copper, iron, lead, manganese, and zinc were produced. Although the metal content of the tailings may be below the amount ordinarily required for economic recovery, these low levels may justify recovery because of the proportion of mining and treating costs borne by the coal beneficiation operation. It must not be assumed that these metals could be recovered equally well from fly ash after firing since high-temperature complexes (e.g., spinels), difficult to recover and separate, may be produced. As was noted, low-temperature processing has the added advantage of removing for possible subsequent recovery materials which may be volatilized and not trapped in fly ash. Regarding the environmental impact of the oils used in agglomeration, checks have indicated that they are effectively adsorbed quantitatively by the fine, high-surface
area coal particles. Any traces of oil remaining in the tailings fraction could be reused by recycling the water after solids removal. Volatile components of the oil evaporated during thermal drying could be condensed from the dryer exit gases and reused if they constituted an air pollution problem. Oils of low volatility would remain in the agglomerates and increase the fuel value of the product. Acknowledgments
The authors thank I. E. Puddington for his continuing interest and suggestions, and E . C. Goodhue, A. Mykytiuk, and P. Tymchuk for their help in the chemical analyses. L i t e r a t u r e Cited Abernathy, R. F., Hattman, E . A,, L‘.S. Bur. Mines, Rep. Inuest. 7452 (November 1970). Billings, C. E., Matson W. R., Science, 176, 1232 (1972). Brown, R., Jacobs, M. L., Taylor, H. E., Amer. Lab. 29 (November 1972). Capes, C. E., McIlhinney, A. E., Coleman, R. D., Trans. AIME., 274,233 (1970). Capes, C. E., McIlhinney, A. E. Sirianni, A. F., Puddington, LE., Proc. 12th Bien. Conf., Inst. Briquet. Agglom., 53 (1971). Capes, C. E., Sutherland, J. P., Ind. Eng. Chem. Process Des. Deuelop., 5 , 330 (1966). Farnand, J . R., Puddington, I. E., Can. Mining Met. Bull., 62, 267 (1969). Rains, T. C., Nut. Bur. Stand., Washington, D.C., private communication (1971). Ruch, R. R., Gluskoter, H. J., Kennedy, E. J., Enuiron. Geol. Notes (Illinois State Geological Survey), No. 43, p 1 (February 1971). Sirianni, A. F., Capes, C. E., Puddington, I. E., Can. J. C h e m Eng., 47, 166 (1969).
Received for reuieu February I , 1973. Accepted September 5, 1973. NRCC AJo. 13,518.
Atrazine, Propachlor, and Diazinon Residues on Small Agricultural Watersheds Runoff Losses, Persistence, and Movement William F. Ritter,’ Howard P. Johnson, Walter G. Lovely,2 and Myron Molnau3 Agricultural Engineering Department, Iowa State University, Ames, Iowa 5001 0
In the past 10 years, many research workers have meaAtrazine (2-chloro-4-ethylamino-6-isopropylamino-s-sured the concentrations of organochlorine insecticides in triazine), propachlor (2-chloro-N-isopropylacetanilide), rivers. Only a few investigators have presented data for and diazinon (0,O-diethyl 0-(2-isopropyl-6-methyl-4-py- pesticide residues in surface runoff and sediment. White rimidinyl) phosphorothioate) losses in sediment and suret al. (1967) measured atrazine losses in water and sediface runoff were measured from four watersheds ranging ment from fallow plots of Cecil sandy loam soil (6.5% in size from 1.9-3.8 acres and located in the loessial soil slope). They found, using a rainfall simulator, that a 1-hr region of western Iowa. Two of the watersheds were plantstorm of 2.5 in. occurring 96 hr after atrazine was applied ed to ridged corn, and two were planted to surface-cona t 3 lb/A, resulted in atrazine losses of 18%. Most of the toured corn. Movement of atrazine, propachlor, and diazatrazine losses were associated with the water fraction beinon in the soil profile and degradation of these pesticides cause of the greater amounts of water lost. Car0 and Taywere measured. Pesticide losses were much greater from lor (1971) found that dieldrin losses in sediment and in the surface-contoured watersheds than the ridged waterrunoff water reached 2.2 and 0.0770,respectively, of the 5 shed. Significant amounts of surface-applied atrazine and propachlor were removed from the surface-contoured watersheds by storms occurring shortly after the pesticides were applied. Insignificant amounts of diazinon were rePresent address, Agricultural Engineering Department, University of Delaware, Xewark, Del. 19711. To whom corresponmoved in the surface runoff and sediment. Generally, pesdence should be addressed. ticide concentrations were higher in the sediment than in * Present address, ARS-USDA, Agricultural Engineering Dethe runoff water; however, greater total losses were associpartment, Iowa State University, Ames, Iowa 50010 Present address, Agricultural Engineering Department, Uniated with the greater volume of water. versity of Idaho, Moscow, Idaho 83843
38
E n v i r o n m e n t a l S c i e n c e &. T e c h n o l o g y
