Characterization of coal and its by-products using borate fusions and ICP-OES analyses

Coal has been used for centuries as a source of energy. It is the second sourse of energy produced and consumed in the world. The major producers are China, the United States, India, Australia, Russia, Indonesia and South Africa. In coal plants, two types of coal are produced,; thermal and metallurgical coal. Although the two are extracted substantially in the same way, there are significant differences in the end products and their uses. As its name suggests, the thermal coal, differing from the thermal coal in some physical aspects but also containing lower sulphur and phosphorus levels, is essential to global steelmakers.

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Introduction

Coal has been used for centuries as a source of energy. It is the second source of energy produced and consumed in the world. The major producers are China, the United States, India, Australia, Russia, Indonesia and South Africa. In coal plants, two types of coal are produced; thermal and metallurgical coal. Although the two are extracted substantially in the same way, there are significant differences in the end products and their uses. As its name suggests, the thermal coal is used for its heat generating capacities. The metallurgical coal, differing from the thermal coal in some physical aspects but also containing lower sulphur and phosphorus levels, is essential to global steelmakers.

Apart from the very tight control of sulphur and phosphorus levels from the coal, the other important quality analytical criteria are very little known. Several environmentalists would like to know that the future of coal as a source of heating is not so bright. However, the energy power of coal versus all other energy sources is so large that it is unthinkable to stop its use in the short or medium term. It is rather more sensible to learn how to control the use of coal and manage the recovery of derivative products, such as fly ash and coke, in order to reduce the environmental impact. Another advantage of the characterization of coal will be the prevention of major damages (erosion and corrosion) on the equipment caused by the deposition of minerals on the furnaces and kettles in the industries.

Both the industries that use coal as an energy source and the industries that use the derivative products need to characterize the major and minors elements of interest. A few standard methods including the ASTM D6357-00a, ASTM D6349-01 and the Australian standard AS 1038.14.1 -2003 use ICP-OES or AAS for this characterization. In the case of the ASTM D6357, ICP-MS or AAGFS are alternative analytical methods for very low concentration elements. The purpose of this project is to demonstrate that the criteria of accuracy and precision mentioned in these standard methods may be achieved using a dissolution method by borate fusion for analysis by ICP-OES. Borate fusions are a good alternative to other dissolution / digestion methods because of the rapidity of the dissolution and the absence of use of dangerous acids such as HF.

Method

Each sample was ashed in a porcelain crucible prior to being submitted to the fusion process according to this following ashing method in a Muffle Furnace: 

  1. Ramp up to 500°C in 1 hour
    2. Ramp up to 750°C in 2 hours
    3. Hold at 750°C for 2 hours 


The ashes were then fused following the procedure of mixing 0.300 g with 1.000 g of 98.5% Lithium Metaborate/1.5% Lithium Bromide flux (LiM/LiBr) and 0.150 g of Lithium Nitrate (LiNO3) in Pt/Au crucibles. The fusions were done using the Claisse® fully automatic M4™ Fluxer.

The complete process of fusion and dissolution took less than 15 minutes. The resulting solutions were then diluted up to 400 mL for subsequent analyses on a PerkinElmer® Optima 7300 DV ICP OES (Figure 2). Table 1 shows the operating parameters used on the spectrometer.

Table 1: Optima 7300 DV operating parameters
NebulizerSeaspray
Spray chamberBaffled Cyclonic
InjectorAlumina 2 mm DI
RF1500 W
Argon flowPlasma: 16 L/min
Nebulizer: 0.6 to 0.8 L/min
Auxiliary: 0.4 L/min
Sample flow rate1.0 mL/min

Samples 

Table 2: Certified Reference Materials Used to Validate the Developed Method
IdentificationMatrix
NCS FC28127Coal
VS-7177-95Coal Ash
EOP 12-1-02Coal Fly Ash
Table 3: Analytes of interest with selected wavelengths, Method Detection Limits (MDL) and viewing modes
ElementWavelength
(nm)
ViewMDL
(mg/L)
Al237.313Axial0.005
Ca422.673Radial0.05
Fe238.863Axial0.02
K766.490Axial0.005
Mg279.077Axial0.005
Na589.592Radial0.008
Si252.851
Axial0.1
Ti337.279Axial0.002

Results

Table 4: Accuracy and precision measurements on VS-7177-95 (Coal Ash)
ElementWavelength
(nm)
ViewAverage
experimental
values n=10 (%)
Certified
values
(%)
Accuracy
(%)  
Precision
(%)
MDL
(%) 
 
Al237.313Axial13.714.33960.60.0007
Ca422.673Radial
3.803.491090.90.007
Fe238.863Axial3.723.839710.003
K766.490Axial0.4680.499610.0007
Mg279.077Axial0.8380.8939410.0007
Na589.592Radial0.1120.1041080.70.001
Si252.851
Axial27.427.431000.80.01
Ti337.279Axial0.3400.369510.0003
Table 5: Accuracy and precision measurements on NCS FC28127 (Coal)
ElementWavelength
(nm)
ViewAverage
experimental 
values n=10 (%)
Certified
values 
(%)
Accuracy
(%)
Precision
(%)
MDL
(%)
Al237.313Axial3.363.4797
0.70.0002
Ca422.673Radial1.961.881040.80.002
Fe238.863Axial1.041.02
102
10.0007
K766.490Axial0.2860.299910.0002
Mg279.077Axial0.2910.2810410.0002
Na589.592Radial0.05040.052970.70.0003
Si252.851Axial5.67
5.611010.80.004
Ti337.279Axial1.700.18
9510.00007
Table 6: Accuracy and precision measurements on EOP 12-1-02 (Coal Fly Ash)
ElementWavelength 
(nm)
ViewAverage experimental

values n=10 (%)


Certified 
values  

(%)


Accuracy 
(%)
Precision 
(%)
MDL 
(%)
Al237.313Axial15.916.1990.30.0007
Ca422.673Radial1.551.491040.60.007
Fe238.863Axial5.355.171030.60.003
K766.490Axial0.6710.65110310.0007
Mg279.077Axial0.6030.581
10420.0007
Na589.592
Radial0.350
0.3619710.001
Si252.851Axial23.122.9
1010.60.01
Ti337.279Axial3.50
3.619710.0003

Discussion

Sample preparation by borate fusions takes only a few minutes and the dissolution is always complete. It offers various advantages versus other dissolution methods that are mostly brought on by their versatility, speed, cleanliness, temperature stability and reproducibility 

Using matrix matching and internal standards as well as using a strict interference management scheme, tables 4, 5 and 6 reveal RSDs that are lower than 2% for all elements of interest thus demonstrating good reproducibility during the entire sample preparation and analytical processes.

Conclusion

Borate fusions combined with the simultaneous ICP-OES have the analytical capabilities to perform the elemental analysis on metallurgical and thermal coal samples. The accuracy and precision results obtained with this method are in complete accordance with ASTM and ISO standards while offering the advantage of speed and safety over other aggressive acid digestion techniques.

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