Coal By-Products Using Borate Fusion

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:

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
Nebulizer	Seaspray
Spray chamber	Baffled Cyclonic
Injector	Alumina 2 mm DI
RF	1500 W
Argon flow	Plasma: 16 L/min Nebulizer: 0.6 to 0.8 L/min Auxiliary: 0.4 L/min
Sample flow rate	1.0 mL/min

Samples

Table 2: Certified Reference Materials Used to Validate the Developed Method
Identification	Matrix
NCS FC28127	Coal
VS-7177-95	Coal Ash
EOP 12-1-02	Coal Fly Ash

Table 3: Analytes of interest with selected wavelengths, Method Detection Limits (MDL) and viewing modes
Element	Wavelength (nm)	View	MDL (mg/L)
Al	237.313	Axial	0.005
Ca	422.673	Radial	0.05
Fe	238.863	Axial	0.02
K	766.490	Axial	0.005
Mg	279.077	Axial	0.005
Na	589.592	Radial	0.008
Si	252.851	Axial	0.1
Ti	337.279	Axial	0.002

Results

Table 4: Accuracy and precision measurements on VS-7177-95 (Coal Ash)
^Element	^{Wavelength (nm)}	^View	^{Average experimental values n=10 (%)}	^{Certified values (%)}	^{Accuracy (%)}	^{Precision (%)}	^MDL (%)
Al	237.313	Axial	13.7	14.33	96	0.6	0.0007
Ca	422.673	Radial	3.80	3.49	109	0.9	0.007
Fe	238.863	Axial	3.72	3.83	97	1	0.003
K	766.490	Axial	0.468	0.49	96	1	0.0007
Mg	279.077	Axial	0.838	0.893	94	1	0.0007
Na	589.592	Radial	0.112	0.104	108	0.7	0.001
Si	252.851	Axial	27.4	27.43	100	0.8	0.01
Ti	337.279	Axial	0.340	0.36	95	1	0.0003

Table 5: Accuracy and precision measurements on NCS FC28127 (Coal)
^Element	^{Wavelength (nm)}	^View	^{Average experimental values n=10 (%)}	^{Certified values (%)}	^{Accuracy (%)}	^{Precision (%)}	^MDL (%)
Al	237.313	Axial	3.36	3.47	97	0.7	0.0002
Ca	422.673	Radial	1.96	1.88	104	0.8	0.002
Fe	238.863	Axial	1.04	1.02	102	1	0.0007
K	766.490	Axial	0.286	0.29	99	1	0.0002
Mg	279.077	Axial	0.291	0.28	104	1	0.0002
Na	589.592	Radial	0.0504	0.052	97	0.7	0.0003
Si	252.851	Axial	5.67	5.61	101	0.8	0.004
Ti	337.279	Axial	1.70	0.18	95	1	0.00007

Table 6: Accuracy and precision measurements on EOP 12-1-02 (Coal Fly Ash)
^Element	^Wavelength^(nm)	^View	^Average^experimental values n=10 (%)	^Certified^values (%)	^Accuracy^(%)	^Precision^(%)	^MDL^(%)
Al	237.313	Axial	15.9	16.1	99	0.3	0.0007
Ca	422.673	Radial	1.55	1.49	104	0.6	0.007
Fe	238.863	Axial	5.35	5.17	103	0.6	0.003
K	766.490	Axial	0.671	0.651	103	1	0.0007
Mg	279.077	Axial	0.603	0.581	104	2	0.0007
Na	589.592	Radial	0.350	0.361	97	1	0.001
Si	252.851	Axial	23.1	22.9	101	0.6	0.01
Ti	337.279	Axial	3.50	3.61	97	1	0.0003

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.

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