Anal. Chem. 1900, 52, 929-935
929
Multitechnique Multielemental Analysis of Coal and Fly Ash R. A. Nadkarni Analytical Research Laboratory, Exxon Research and Engineering Company, P. 0. Box 4255, Bayiown, Texas 77520
The coal sample is first ashed with high temperature ashing or with RF plasma low temperature ashing. The coal ash or fly ash can be analyzed for major ash elements by fusing with lithium tetraborate in an automatic fusion device, the Claisse Fluxer. The ash samples are also dissolved in a Parr bomb in a mixture of aqua regia and HF. Subsequently, the solutions are analyzed for eight major (AI, Ca, Fe, K, Mg, Na, Si, and Ti) and 20 trace elements (As, B, Ba, Be, Cd, Co, Cr, Cu, Li, Mn, Mo, Ni, P, Pb, Sb, Se, Sr, U, V, and Zn) by inductively coupled plasma emission spectroscopy. Mercury in coal and fly ash is determined on a separate aliquot by the cold vapor atomic absorption technique. Fluorine and chlorine in the samples are determined by fusing with Na,CO, and Eschka mixture, respectively, and then measuring the two ions In solution with specific ion electrodes. Oxygen in the samples can be determined rapidly and nondestructively by 14-MeV neutron activation analysis. These methods have been tested by analyzing several NBS coal and fly ash standards with good accuracy and reproducibility.
Oil and gas a t present supply three-fourths of the energy used in the United States. In spite of the increasing conservation measures taken, our import of these materials is steadily on the rise. However, the supply of these hydrocarbon fuels worldwide is expected to decline in the foreseeable future. T h e only practical alternative energy supplies a t present are nuclear energy and coal. T h e world reserves of coal far exceed those of any other fossil fuel and are sufficient to support a massive increase in consumption well into the future. It is estimated that the United States has 1.7 trillion tons of coal ( I ) . Despite these massive reserves, however, the amount of coal used in this country has remained almost constant a t about 600 million tons per year for the past few years ( 2 ) . One of the chief reasons for this lack of coal utilization is the environmental concern as to how much increase in atmospheric pollution will take place by burning increased amounts of coal. Coal is a fairly "dirty" fuel and contains large amounts of many inorganic elements. At the same time, there are wide variations in the trace elements content of coal seams even within a single mine. Hence, it is important to have reliable analytical methods which can monitor the inorganic constituents a t various stages of coal production and utilization. Since as many elements as possible need to be monitored in the coal products, it is desirable to have multielement rather than single element techniques. T h e need for the standardization of the analytical methodology and availability of analytical standards was illustrated by the U S . Environmental Protection Agency's round robin analysis of four fuel matrices for 28 elements in nine laboratories (3). Same samples of coal, fly ash, fuel oil, and gasoline were analyzed for 28 elements in nine laboratories by a variety of analytical techniques - neutron activation analysis, atomic absorption spectrophotometry, spark source mass spectrometry, optical emission spectrometry, anodic stripping voltammetry, and X-ray fluorescence. Amongst the elements investigated were elements of environmental concern such as Hg, Be, Cd, P b , As, V, Mn, Cr, and F. There were large 0003-2700/80/0352-0929$01 .OO/O
variations in the reported concentrations of most elements in each of these four fuel matrices. Of the 28 elements examined, the agreement was within an order of magnitude for only seven elements: Si, Ca, S, Sr, Fe, Cr, and Ni in all four matrices. The wide range in reported concentrations indicates the errors introduced due to different sample preparation, interferences peculiar to an analytical technique and/or operator error. Since then the U.S. National Bureau of Standards has issued several coal, fly ash, and fuel oil standards certified for a number of elements. Practically all available analytical techniques have been employed in the analysis of coal products. Recent reviews of analytical methodology for coal products include those by Slates ( 4 ) ,Lyon ( 5 ) ,Babu, (6), and the recent book edited by Karr (7). The analytical techniques utilized include optical emission spectroscopy, spark source mass spectroscopy, neutron activation analysis, X-ray fluorescence, atomic absorption spectrometry, colorimetry, and ion-selective electrodes. In the present paper, we are describing our methods of multielement analysis of coal and fly ash utilizing inductively coupled plasma emission spectroscopy, neutron activation analysis, atomic absorption spectrometry, and ion-selective electrodes. These methods are being used on a large number of coal and fly ash samples on a routine basis a t Exxon Research and Engineering Company in Baytown, Texas.
EXPERIMENTAL Ashing of Coal. In the high temperature ashing, about 1-2 g of finely powdered coal is taken in a platinum crucible and gradually heated in a muffle furnace at 275 "C for an hour, at 550 "C for another hour, and finally at 750 "C for 1-2 more hours. In the low temperature ashing, about 1-2 g of finely powdered coal is taken in a Pyrex Petri dish and subjected to RF discharge oxygen plasma in a LTA asher for periods up to 24 h. Every few hours, the sample is stirred and weighed, so that a fresh surface is exposed to oxidation. The ashing is continued until a constant weight loss is obtained. Claisse Fluxer Automatic Fusion Device. This apparatus was purchased from Corporation Scientifique Claisse, Quebec, Canada, and is fitted with six 20-mL platinum--5% gold alloy crucibles and six 200-mL quartz beakers for solution preparation. The detailed procedure is described by Botto (8). Basically, the device can simultaneously fuse six samples in six platinum-gold alloy crucibles over an ai-propane flame with Li2B407.The finely powdered ash is mixed with 10 times its weight with lithium metaborate in a platinum crucible. About 50 mg of CsI is added to the fusion mixture as a nonwetting agent to prevent the molten fluxfrom adhering to the walls of the crucibles as well as to prevent incomplete transfer of the bead to the acid solution. When the fusion is over, the molten glasses containing the samples are cast into acid solutions and dissolved for solution analysis. The fusion procedure lasts about 6 min followed by 10 min of shaking the solutions to completely dissolve the fused beads into the acids. After the solution is diluted to volume (1 L), it is analyzed for major ash elements by ICPES. Phosphorous is determined by a separate molybdenum blue colorimetric procedure from an aliquot of the same solution. Parr Bombs. Parr Teflon acid digestion bombs were obtained from Parr Instrument Company, Moline, Ill. About 0.2 g of coal ash or fly ash is taken in 2 mL aqua regia and 2 mL HF in a Parr bomb, and heated in an oven at 110 "C for 1-2 h. After cooling, the bomb is opened and 1 g of boric acid is added to each sample and the samples are heated on a waterbath for 15 min. If any unburned carbon is visible at this stage, the solutions are filtered; 0 1980 American Chemical Society
930
ANALYTICAL Ci-IEMiSiRY, L’OL. 52, NO. 6, M A Y 1980
lie necessary in addition t o the K M n 0 4 oxidation.
Table I. Elements Determined b y ICPES
element
wavelength, n ni
Cu Fe Hg
338.29 308.22 193.70 249.77 455.40 234.86 393.3: 214.44 228.62 357.80 324.76 2 5 9.9 ‘1 253.65
K
7 66.49
Lk AI As .
n
-
Ba Ca cd co -
cr
dctPction upper limit ( 2 0 ) limit. PPm PPb 5tI 2.6 5.9 36 5.4 0.3 0.9
1.2 5.0 6.1 3.1
0.6 2.8
25
mtcrfercnces
100 300 50 100
-1 3-
‘0
400 100
200 50 190 60
Pt
cs. h‘e. Ti, -# 7#
Wt
Mn __ Mo Na Nb Ni __ P Ph Pt Sb
& Si Sn ‘Pi
TL
U V Zn a
279.55 257.61 202.03 589.00 f113.70
25 0.6
100
0 .4
50 400
19 2.2
75 5I 1
12
50
341.47
10
214.91 220.35 265.95 231.15 196.33 288.16 189.99
51 19
100 500 100 50 30
334.9tI 377.37 367.01 242.40 206.20
-
il
&I
39
55 15
24 1.2 27 63 14 3,;;
500 50 200
Nb
C o , Ii’n Ii‘C
u
CY, hlg, N b ,
u
130 50 50 50
20 9
Backpound corrected channels are unii~rlu!ecl
otherwise, the solutions are diiuted to 50 or 100 mI,. An aliquot is diluted further 100-fold, and both the solutions arz iindlyiled by ICPES, the first solution for trace elements and the secoiid for major elements. A blank is used throughout the analysis containing the same amounts of aqua regia, HF, and boric acid and is used for the blank correction in the ICPE,S measiirements. Inductively Coupled P l a s m a Emission Spectroscopy (ICPES). A most promising technique in recent years has been the development of plasma sources for emission spectroscopy. There are only four papers pablished so far on the appiications of ICPES in coal products analysis. Jarrell-Ash, on^ of the major suppliers of ICPES, has memured nearly 20 elements in coal ashes and fly ashes after acid dissolution ( 9 ) . Det.ails of our instrumentation are given by Rotto (10). A t the time of this work, our system was equipped with 32 elemental channels. .A list of these element channels, the wavelengths at which these elements are determined, the detection limits, linear range of analysis. ana :he interferences associated with these elemenrs are described iii Table I. Some of the channels are operated with a spectrum shifter for background correction. These channels are underlined in Table I. Atomic Absorption Spectrometer. An Instrumentation Laboratory IL-251 spectrometer was used wit,h a Jarrell-.‘\sh high intensity mercury hollow cathode lamp and a quartz absorption cell. In the analytical procedure developed, a sample of coal or fly ash is digested with aqua regia for 2 min a t 95 “c‘ followed by oxidation of organic materia: with KMnO,. Excess of K M n 0 4 in the solution is destroyed with hydroxylamine, and then the mercury in the solution is reduced to the elemental state wiih SnC12and is aerated from the solution into a quart7 absorption cell on an atomic absorption spectrophotometer, where the absorption of mercury radiation at 253.7 rim is measured. The method is applicable to coals. fly ash, sediment, or fiy ash ieachates. KMnO, is used to oxidize the orgairic mercury cornpounds in the sample. For some organic materials, K,S208oxidation ma?’
Ion-Selective Electrodes. Orion model 901 microprocessor ionanaly-zer, 94-09 fluoride electrode, 94-17 chloride elechde, and 90-02 double junction reference e!ectrode were used. Samples ere fused with anhydrous Na2C03a t 475 y at !000 “C for 15 mi::. The melt was diss;ilved in dilute H2S04,and the p H was adjusted to near 5.6 m d , efter the addition of ionic strength buffer, fluoride was determined hy a fluoride ion-selective electrode. In the chloride anaiysib, the coal i ) ~ ’fly ash sample was fused with Eschka mixture ai 675 “C for 2 h. The melt, was dissolved in 1:l HNO,, pH was adjusted to 3 anti, after the addition of NaNO, as ionic strength adjustor huffer, the ch!oride was determined with a chloride ion-selcctive electrode. Both fluorine and chlorine measurements were carried out by standard addition methods. Neutron Activation System. Texas Nuclear Corp. Model 9500 neutron generator capable of producing loi1 n/s was used in a concrete shielded building. The samples contained in 2/5-dra.m uo1::ethyIene vials are transferred through a s,’/y-inchplastic tubing by means nf compressed air. The irradiated sample is positioned in a Lucite holder between two 3 inch X 3 inch NaI(T1) detectors corlnected to a 490 Channel pulse height analyzer. Usually about n g r i i n of the sample is seaied in the polyethylene rabbit, irradiated for 15 s in the neutron beam, and after a 3-s delay in transit, is counted for 30 s. ,The details of the procedure are described .Asew’r~ere( 1 I ) . Reagents. All the acids such as HCI, FINO,, H2S0,, HF, and boric acid were of “Ultrex” quality from J. T. Baker Chemical Co. Other chemicals were of analytical reagent grade quality. Lleioriized water was obtained by purifying the house deionized u a t e r with a Mi!liq system frorn Millipore Corp. This system produced a water of 18 MR;cm specific resistivity and an ICPES anal>;sis of this water showed the presence of none Of the 32 eieniects thal iari be determined by our ICPES system. Thus, if any elements were present in it, they would be in sub-ppb amounts. Coal a n d Fly Ash Standards. Throughout this work, NUS standards 163 cnal and 1633 fly ash have been used. In addition tcl their original certified value? by NBS for 21 elsments, a large body of literatwe values (over 70 papers) exists for a total of about 60 elements. From this we have compiled the “best values” €or these t w u standards and these are the values which we have used tc compare with our experimental data. The obvious outliers have been omitted in calculat,ing an average value. With the hope that this compilation wili be helpful to others working in this field, I have included these “best values” for these two standards in Table IT. This compilation should be quite useful for checking the accuracy of the data obtained during the developmental stages of an:; analytical procedure by analyzing either or both of these standurds.
RESULTS AND DISCUSSION Ashing of the Goal. Practically all the chemical methods of coal analysis depend on the ashing of coal as a first step irwfore the dissolution; however, there are a few references where attempts have been made to dissolve the coal wit,hout any pretreatment. These include reflux heating of coal with oxidizing acid mixtures such as HC104 i HIQ4 (12) and “OB +- H,S04 ( 1 3 , or in a Parr bomb with aqua regia + HF ( 9 ) , or with fuming HN03 ( 1 4 ) . These methods have the disadvantage of either incomplete dissolution or losses of certain volatile slenierits due to too vigorous dissolution conditions, plus contamination from t h e acids used. We attempted to duplicate some of the above procedures by heating -,O.:! g of dried coal in aqua regia + HF in a Parr homh a t l i 0 “C for 2 h, filtering off the unreacted carbon, and diluting the solution t~ IIX) mT, with the addition of 1 g of boric acid. T h e results obt,ained showed poor recovery of many rlenients indicating incomplete dissolution. We also attempted t o dissolve coal and fly ash in a Parr bomb with fuming HNO, + HF, heated a t 150 “C for 2 h. T h e results of this analysis also indicate poor recoveries. In addition, the severe attack oil the metallic parts of the Parr bombs by fuming H N O j , and the difficulty of handling this corrosive
ANALYTICAL CHEMISTRY, VOL. 5 2 , NO. 6, MAY 1980
Table 11.'' Best Values for NBS Coal-1632 and Coal Fly Ash-1633 Coal SRM-1632
Fly Ash SRM-1633
1.78 ( 1 2 ) 0.41 ( 1 2 ) 0.87 i 0.03 0.29 ( 1 4 ) 0.16 ( 1 0 ) 1 . 0 3 (1) 0.038 (13) 8.03 (1) 1.35 ( 4 ) 3.38 ( 4 ) 0.096 (12) 56 (4) 5.9 i 0.6 0.92 ( 2 ) 40 ( 3 ) 342 ( 1 2 ) 1.45 ( 6 ) 17.7 ( 1 2 ) 0.19 i 0.03 19.6 ( 8 ) 962 ( 1 2 ) 5.78 ( 1 3 ) 20.2 i 0.5 1.52 ( 1 0 ) 182 2 1.43(3) 0.34 (11) 90 ( 2 ) 6.8 ( 7 ) 0.99 ( 7 ) 0.12 i 0.02 2.88 ( 5 )
12.5 ( 1 3 ) 4.60 (15) 6.18 ( 1 7 ) 1.68 (15) 1.55 (13) 0.04 (1) 0.32 (14) 0.25 (1) 0.50 ( 3 ) 20.5 ( 4 ) 0.72 ( 1 5 )
10.5 (10) 2 5 (1) 0.13 ( 6 ) 40t 3 3.82 ( 6 )
61 i 6 497 ( 2 ) 2690 ( 1 3 ) 11.9 ( 7 ) 8.6 ( 1 2 ) 1.45 i 0.06 149 ( 1 0 ) 39.3 (17) 131 t 2 8.34 (8) 128 * 5 2.74 ( 7 ) 20 (1) 7.94 (8) 0.14 i 0.01 2.7 ( 4 ) 0.27 ( 5 ) 84.3 (10) 0.98 ( 7 ) 493i 7 31 ( 4 ) 64 ( 4 ) 98i 3 880 (1) 70i 4 113 (13) 6.9 (11) 25.5 ( 9 ) 9 . 4 i 0.5 12.6 ( 1 0 ) 10.9 ( 4 ) 1410 ( 1 9 ) 1.98 ( 8 ) 1.98 ( 4 )
15i 1 71; 1 1 8 ( 2 ) 30 + 9 2 1 . 1 (11) 3.56 ( 1 0 ) 3.86 (11) 2.9 i 0.3 1.66 ( 6 ) 10.1 ( 2 ) 153(10j 0.25 (7 j 0.23 ( 3 ) 0.55 ( 2 ) 3.1 ( 9 ) 24 ( 9 ) 0.59 i 0.03 3.8 ( 3 ) 1.4 i 0 . 1 11.6 i 0.2 214 i 8 35i 3 0.71 ( 5 ) 4.3 ( 6 ) 7.6 ( 4 ) 62.9 (8) 0.78 ( 8 ) 6.55 ( 8 ) 37 2 4 210 t 20 303 ( 8 ) 45 ( 3 ) a Data with standard devlations are NBS certified values. Numbers in parentheses indicate the number of papers from which the mean was calculated. acid, makes the use of this procedure very unattractive. Thus, a t the moment we have abandoned the attempts to dissolve the coal without any pretreatment and concentrated on the dissolution procedures for coal ashes or fly ashes. For this procedure it is first necessary to ash the coal. There are two procedures available for this purpose - high temperature (HTA) and low temperature ashing (LTA).
931
Table 111. Comparison of HTA and LTA Ash of NBS Coal-1632 element, PPm Al, % As
Ba Be Ca, %
co Cr
cu Fe, % K, % Mg, 5%
Mn Mo Na Ni P
Pb
s, %
Si, % Sn Ti V
Zn
present 1.78 5.9 i 0.6 342 1.45 0.41 5.78 20.2 i 0.5 18i 2 0.87 i 0.03 0.29 0.16
40 * 3 3.82 380 15i 1 7 1 ; 118 30 i 9 1.35 3.38 10.1 960 35i 3 37 i 4
found
_
HTA 1.86
