Coal is the most abundant fossil fuel in the world. It has been used by humans for centuries and is still widely used today to produce electricity, steel and industry materials such as cement.
It is essential for industries to characterize coal to fully understand its use as well as to manage the recovery of its derivative products in order to reduce the environmental impact. Characterization also prevents damages on industrial equipment caused by mineral deposit in furnaces and kettles.
The purpose of this project is to demonstrate that precision and accuracy criteria in standard methods such as ASTM D6357, ASTM D6349 and AS 1038.14 1-2003 can be met by using borate fusion as a dissolution method for ICP-OES analysis. This preparation step will be facilitated by using LeNeo fusion instrument.
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Coal is the most abundant fossil fuel in the world [1]. It has been used by humans for centuries and is still widely used today to produce electricity, steel and industry materials such as cement [2].
It is essential for industries to characterize coal to fully understand its use as well as to manage the recovery of its derivative products in order to reduce the environmental impact. Characterization also prevents damages on industrial equipment caused by mineral deposit in furnaces and kettles [3].
The purpose of this project is to demonstrate that precision and accuracy criteria in standard methods such as ASTM D6357, ASTM D6349 and AS 1038.14 1-2003 can be met by using borate fusion as a dissolution method for ICP-OES analysis. This preparation step will be facilitated by using LeNeo® fusion instrument.
An automatic LeNeo fusion instrument designed by Claisse was used to generate borate solutions. Its resistance-based electric system, excellent insulation properties and preset fusion programs allow a uniform heating, thus providing repeatable and reproducible fusion conditions and a perfect retention of volatile elements.
A Fisher Scientific Isotemp® muffle furnace was used for the LOI determinations and the preparation of ignited samples.
A Perkin Elmer® Optima® 7300 DV ICP-OES spectrometer was used to collect the data. The operating parameters used on the spectrometer are shown in Table 1.
| Nebulizer | Gem Tip Cross flow | Argon flow | Plasma: 16 L/min
Nebulizer: 0.8 L/min Auxiliary: 0.4 L/min |
| Spray chamber | Scott | ||
| Injector | Alumina 2 mm i.d. | ||
| RF | 1500 W | Sample flow rate | 1.0 mL/min |
Before being submitted to the fusion process, each sample was ashed in a Pt/Au crucible in the muffle furnace according to this following ashing method:
The ashes were then fused following the procedure of mixing 0.150 g of lithium nitrate (LiNO3) and 1.000 g of lithium metaborate/1.5% lithium bromide (LiM/LiBr) flux in Pt/Au crucibles. A fully automatic LeNeo instrument was used to fuse the samples.
The complete process of fusion and dissolution took less than 15 minutes. The resulting solutions were then diluted up to 200 mL in 10% nitric acid for subsequent analyses on a Perkin Elmer Optima 7300 DV ICP-OES.
High lithium matrix can cause signal suppression or signal increase for some elements.
In order to counteract this effect, matrix matching and internal standards were used. The dilution of sample prevented any clogging caused by a high salt matrix. The calibration solutions of each element were done in a LiM/LiBr 1.5% matrix, just like the sample. Calibration curves were done using a concentration range close to the sample concentrations.
Sample concentrations are within the calibration range. Five points including the blank were used for each element. The correlation of each curve was higher than 0.999.
Method detection limits (MDLs) were based on 10 replicate measurements of a series of low diluted sample solutions. The MDL was calculated by multiplying the standard deviation of the 10 replicate measurements by three.
MDL= 3 x S10
| Elements | Wavelength (nm) | View | MDL (mg/L) |
|---|---|---|---|
| Al | 237.313 | Axial | 0.07 |
| Ba | 413.065 | Axial | 0.006 |
| Ca | 422.673 | Radial | 0.1 |
| Fe | 238.863 | Axial | 0.02
|
| K | 766.490 | Axial | 0.002
|
| Mg | 279.077
| Axial | 0.007
|
| Mn | 293.305 | Axial | 0.001 |
| Na | 589.592 | Radial | 0.05
|
| P | 177.434 | Axial | 0.03 |
| S | 181.975
| Axial | 0.06 |
| Si | 252.851 | Radial | 0.07 |
| Sr | 460.733 | Axial | 0.002 |
| Ti | 337.279 | Axial | 0.002 |
| Zn | 213.857 | Axial | 0.003 |
The following tables show the precision and accuracy obtained in coal and fly ash with three different certified reference materials (CRMs) (10 replicates for each CRM). The precision expresses the closeness of the results obtained in a series of 10 measurements made with the same sample while the accuracy is the proximity of measurement results to the true value. Both were calculated based on the certified values of the following certified reference materials: EOP-12-1-02, NCS FC28127 and VS 7177-95.
| Elements | Certified values (%) | Experimental values (%) | Precision
t0,975 ; 9 (%) | Accuracy
(%) | RSD
(%) |
|---|---|---|---|---|---|
| Al | 16.1
| 16.3 | 0.1 | 98.7
| 0.5
|
| Ca | 1.49 | 1.44
| 0.04
| 96.5
| 3.9 |
| Fe | 5.17 | 5.21
| 0.02 | 99.2 | 0.5 |
| K | 0.651 | 0.630
| 0.011 | 96.7 | 2.4 |
| Mg | 0.581
| 0.623 | 0.002 | 93.0 | 0.5 |
| Na
| 0.361
| 0.37 | 0.01
| 97.6 | 3.5 |
| Si
| 22.9
| 23.8 | 0.4
| 96.3 | 3.7 |
| Ti | 3.61 | 3.44
| 0.01
| 95.3 | 0.5 |
| Elements | Certified values (%) | Experimental values (%) | Precision
t0,975 ; 9 (%) | Accuracy
(%) | RSD
(%) |
|---|---|---|---|---|---|
| Al | 3.47 | 3.37 | 0.03 | 97.1
| 1.2 |
| Ca | 1.88
| 1.89 | 0.04 | 99.3 | 2.8 |
| Fe | 1.02 | 1.05 | 0.01 | 97.4
| 1.0
|
| K | 0.29 | 0.281 | 0.005 | 97.0 | 2.5 |
| Mg | 0.28
| 0.294 | 0.002 | 94.7 | 0.8 |
| Na | 0.052 | 0.052 | 0.001 | 99.6 | 2.2 |
| Si | 5.61 | 6.05 | 0.06 | 92.2 | 1.3
|
| Ti | 0.18 | 0.173 | 0.002 | 96.4 | 1.4 |
| Elements | Certified values (%)* | Experimental values (%) n=10 | Precision t0,975;9 (%) | Accuracy
(%) | RSD
%) |
|---|---|---|---|---|---|
| Al | 14.33 | 14.0 | 0.1 | 97.7 | 1.3 |
| Ca | 3.49 | 3.70 | 0.06 | 94.0 | 2.4 |
| Fe | 3.83 | 3.73 | 0.03 | 97.3 | 1.0 |
| K | 0.49 | 3.73 | 0.006 | 91.0 | 1.9 |
| Mg | 0.893
| 0.874 | 0.009 | 97.9 | 1.5 |
| Na | 0.10 | 0.11 | 0.01 | 97.6 | 3.1 |
| Si | 27.43 | 28.6 | 0.4 | 95.7 | 2.1 |
| Ti | 0.36 | 0.357 | 0.003 | 99.3 | 1.2 |
Table 6 shows the recovery values obtained in different matrixes. Recovery was calculated on five replicates in each of the certified reference materials used.
| Elements | EOP 12-1-02
(Brown coal fly ash) | NCS FC28127
(Coal ash) | VS 7177-95
(Coal) | ||||||
|---|---|---|---|---|---|---|---|---|---|
| Spiked values
(mg/L) | Recovery
(%) | RSD
(%) | Spiked values
(mg/L) | Recovery
(%) | RSD
(%) | Spiked values
(mg/L) | Recovery
(%) | RSD
(%) | |
| Ba
| 1 | 102 | 0.4 | 1 | 102 | 1 | 1 | 103 | 0.2 |
| K | 3 | 91 | 3 | 5 | 102 | 2 | 3 | 91 | 4 |
| Mn | 1 | 102
| 1 | 5 | 100 | 0.7
| 1 | 104 | 0.6 |
| P | 1 | 99 | 1 | 1 | 103 | 1 | 1 | 101 | 3
|
| 1 | 97
| 3 | 5 | 100 | 1 | 1 | 96 | 3 | |
| Sr
| 1 | 96 | 1 | 1 | 100 | 0.9 | 1 | 98 | 3 |
| Zn
| 1 | 100 | 0.7 | 1 | 101 | 0.9 | 1 | 101 | 1 |
The results presented in the previous tables indicate that sample preparation by borate fusion followed by ICP-OES analysis is an effective method to analyse coal and fly ash. The accuracy obtained (between 91.0 and 99.6%) combined with an excellent recovery (100% ± 3% for most elements with a relative standard deviation below 3% for all elements, except for two of them) show that the method is highly efficient. The method also showed good precision, thus proving its receptivity. This demonstrates that the use of LeNeo fusion instrument leads to reproducible and efficient methods, despite the sample dissolution to reduce its salt content.
[ 1 ] Coal Association of Canada. “About Coal”. Retrieved from the Website www.coal.ca. Calgary, Alberta. 2015.
[ 2 ] Coal Association of Canada. “Module 1: Coal Evolution”. Retrieved from the official Website at http://www.coal.ca/wp-content/uploads/2012/04/module1_evolution.pdf. Calgary, Alberta. 2003. 12pp.
[ 3 ] PITRE, J. and BÉDARD, M. “Characterization of Coal and its By-products Using Borate Fusion and ICP-OES Analysis”. ICP-OES application note. 2014.
