XRF analysis for elemental determination of ferrochromium alloys using borate fusion

Introduction

Over 80% of the world’s ferrochrome is utilized in the production of stainless steel. It is essential to determine the concentration of chrome (Cr) in this alloy since its high corrosion resistance properties make it a major element in the production of stainless steel. The presence of some penalty elements at trace levels such as sulfur (S), phosphorus (P), silicon (Si) and titanium needs to be known since it affects the steel’s quality and decreases its value.

Fast, accurate and reliable sample preparation and elemental analysis allow direct feedback on changing conditions during the carbothermic reduction process and enable constant premium quality of the ferrochrome output that will result in premium prices. This application note presents a robust method to prepare FeCr samples by borate fusion as well as an extremely precise XRF method to determine Cr as a major element and other low concentration elements required in international standard norms.

Challenges

• Prepare alloy samples with the use of oxidizers.
• Obtain accurate and precise results for non-oxidized samples processed by fusion.
• Determine low-phosphorus and other minor components in ferrochromium alloys. 

Benefits

• Robust method to determine Cr as well as P and S at low concentrations from low to high carbon content alloys.
• Obtain repeatable and stable results. High throughput
• Breakthrough in sample preparation by fusion for XRF analysis.

Apparatus and instrumental conditions

A Claisse^® TheOx^® Advanced fusion instrument was used to prepare lithium borate glass disks for the calibration curve. The fusion instrument has ceramic parts compatible with all the previous versions of TheOx instruments. These parts are resistant to high temperatures. The life of the fusion instrument is then increased. 
XRF measurements were performed using a Malvern Panalytical sequential wavelength dispersive X-ray fluorescence instrument.

Global sample preparation

All samples were prepared using a pure grade (99.98+%) pre-fused lithium borate flux with specific composition. Additionally, sodium salts were used as oxidizers to fully oxidize samples. Samples and oxidizers must be grounded at 150 μm or below for optimal results. 
The sample was weighed in the same 95% Pt – 5% Au crucible in which oxidizers and lithium borate flux were weighed. 
The developed fusion program for FeCr samples includes different ramps to smoothly increase the fusion temperature to reach the highest oxidation state, ensure an optimal fusion program as wel as reliable XRF results with a measuring time of six (6) min. The sample was then poured into a Pt/Au mold with diameter of 40 mm and has an approximate weight of 9.1g.

Calibration curve and preparation of control samples

Sixteen (16) FeCr certified reference materials (CRMs) were selected for the calibration curve and they were all prepared in duplicates. 
Calibration curves for Cr, Si and P are shown in Fig 1,2 and 3. Three (3) FeCr CRMs were used as control samples and five (5) replicates were produced for each sample. In order to verify the precision between the XRF measurements and the stability of the sample preparation, ten (10) replicates of the same sample were prepared.

Figure 1. Calibration graph for Cr

Figure 2. Calibration graph for P

Figure 3. Calibration graph for Si

Results and discussions

The accuracy of the calibrations is shown in Table 1. The RMS (root mean square) and the lower limits of detections (LLD) gives an overview of the elements of interest with the ranges of analysis covered in the current application.

Table 1. Calibration values parameters

Calibration parameter	Min concentration (%)	Max concentration (%)	RMS (abs %)	LLD (ppm)
Cr	49.05	99.95	0.668	N/A
Fe	0.003	38.07	0.641	N/A
Co	0.0002	0.0622	0.002	60
Mn	0.014	1.25	0.013	120
Ni	0.002	0.43	0.015	50
P	0.037	0.153	0.002	130
S	0.0002	0.067	0.005	30
S	0.153	4.69	0.060	270
Ti	0.0002	0.41	0.011	230
V	0.0008	0.32	0.004	270

Precision and instrument stability

Eleven (11) analytes were determined due to their importance in the FeCr industry. A low carbon alloy (BCS-CRM 203/6), a medium carbon alloy (NCS HC 25651) and a high carbon alloy (BCS-CRM 204/6) were used to verify the precision of the fusion method. Table 2 shows a good agreement with the certified values and the RMS as well.

Table 2. Results of control samples vs certified results

BCS-CRM 203/6	Cr (%)	Fe (%)	Co (%)	Mn (%)	Ni (%)	P (%)	S (%)	Si (%)	Ti (%)	V (%)
Certified value (%)	71.00	N/A	0.044	0.153	0.217	0.019	0.004	0.38	N/A	0.073
Mean (%)	70.97	N/A	0.046	0.158	0.219	0.013	0.002	0.38	N/A	0.075
RMS (%)	0.398	N/A	0.002	0.004	0.005	0.003	0.001	0.040	N/A	0.006
BCS-CRM 204/6	Cr (%)	Fe (%)	Co (%)	Mn (%)	Ni (%)	P (%)	S (%)	Si (%)	Ti (%)	V (%)
Certified value (%)	68.76	N/A	0.036	0.225	0.289	0.016	0.019	0.72	0.057	0.094
Mean (%)	68.74	N/A	0.038	0.236	0.297	0.018	0.018	0.69	0.053	0.101
RMS (%)	0.286	N/A	0.006	0.007	0.026	0.001	0.001	0.018	0.008	0.012
NCS HC25625A	Cr (%)	Fe (%)	Co (%)	Mn (%)	Ni (%)	P (%)	S (%)	Si (%)	Ti (%)	V (%)
Certified value (%)	63.31	31.56	N/A	0.47	N/A	0.023	0.047	2.04	N/A	N/A
Mean (%)	63.38	31.58	N/A	0.49	N/A	0.027	0.049	2.08	N/A	N/A
RMS (%)	0.143	0.093	N/A	0.002	N/A	0.006	0.002	0.048	N/A	N/A

A test was performed to compare the precision of the XRF measurements with the stability of the sample preparation using TheOx Advanced (total ceramic option) during the borate fusion process. Table 3 shows the CRM SL-18 average concentrations, the absolute RMS of the 10 glass beads and one ferrochromium sample.

Table 3. Precision of measurements vs stability of sample preparation

Element	Certified value (%)	Mean of 10 measurements (%)	Mean of 10 replicates (%)	RMS of 10 measurements (%)	RMS of 10 replicates (%)
Cr	70.28	70.37	70.12	0.376	0.044
Fe	27.09	27.95	27.81	0.192	0.036
Co	0.050	0.050	0.049	0.002	0.002
Mg	0.015	0.014	0.022	0.028	0.018
Mn	0.209	0.238	0.237	0.003	0.005
Ni	0.369	0.356	0.365	0.006	0.002
P	0.026	0.022	0.025	0.002	0.003
S	0.004	0.005	0.015	0.004	0.001
Si	0.298	0.302	0.322	0.012	0.004
V	0.061	0.060	0.063	0.004	0.007

Conclusion 

A robust borate fusion method was developed for the determination of eleven (11) analytes in ferrochromium alloys using a TheOx Advanced instrument that withstands high temperatures. Even though FeCr samples have coarse granulometry, they have been fused with subsequent phases of oxidation without damaging the platinumware. We can see that there are highly repeatable analytical results for XRF analysis that is a key to determine physical quality of ferrochrome alloys.