Coal, Coal Ash and Coal Fly Ash using Borate Fusion for ICP Analysis

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.

To read the full application note, please login or create a free account.

Introduction

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].

Method

Apparatus and Instrumental Conditions

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.

Table 1: Optima 7300 DV operating parameters
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

Global Sample Preparation Method

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:

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

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.

Results

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

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

Table 2: Method detection limit obtained by wavelength used and type of view
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

Accuracy and Precision

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.

Table 3: Precision and accuracy in brown coal fly ash with reference material EOP 12-1-02
Elements	Certified values (%)	Experimental values (%)	Precision t_{0,975 ; 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

Table 4: Precision and accuracy in coal ash with reference material NCS FC28127
Elements	Certified values (%)	Experimental values (%)	Precision t_{0,975 ; 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

Table 5: Precision and accuracy in coal with reference material VS 7177-95
Elements	Certified values (%)*	Experimental values (%) n=10	Precision t_0,975;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

^{*Measured as Al but reported as Al2O3 on the certificate of analysis provided with material}

Recovery

Table 6 shows the recovery values obtained in different matrixes. Recovery was calculated on five replicates in each of the certified reference materials used.

Table 6: Recovery obtained in coal, ash and fly ash
^Elements	^{EOP 12-1-02 (Brown coal fly ash)}			^{NCS FC28127 (Coal ash)}			^{VS 7177-95 (Coal)}
^Elements	^{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

Conclusions

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.

References

[ 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.

Characterization of coal, coal ash and coal fly ash using borate fusion for ICP analysis