NMR Can Identify the Purity of the Mineral Fluorspar

Flourite, also known as the mineral fluorspar, is the most valuable source of fluorine. The mineral’s purity — that is, calcium fluoride (CaF2) content — will establish its ultimate application in the aluminum, chemical, and steel sectors. The highest grade of fluorspar is used for producing hydrogen fluoride — a precursor to several major compounds including polymers and pharmaceuticals, for example, polytetrafluoroethylene (PTFE), commercially known as Teflon.

Therefore, it is vital to establish fluorspar’s fluorine content for both quality control and commercial purposes.

Advantages of NMR

In traditional wet chemical techniques used for determining CaF2, calcium is measured by complexometric titrimetry (ASTM E815), and fluoride is determined by ion chromatography or ion selective electrode. However, all these methods involve skilled chemists, time-intensive operations, and require the use of potentially dangerous chemicals, which have to be subsequently disposed, thus collectively contributing to the cost of the analysis.

The MQC benchtop nuclear magnetic resonance (NMR) analyzer provides a rapid, simple, and non-destructive technique for measuring fluorine in fluorspar, which could be employed for routine analysis in a production setting without any need for specialist operator training and extra chemicals.

NMR

Method

The analytical method is based on direct measurement of the nuclear magnetic resonance (NMR) signal of fluorine-19, which is known to have 100% natural abundance. It is believed that the quantified NMR response is linearly proportional to a material’s CaF2 content. The NMR signal, thus obtained, is normalized by the sample mass and the CaF2 content (weight-%) is subsequently computed using a suitable calibration curve.

Calibration

The MQC benchtop NMR analyzer can be calibrated using just two samples, provided the reference values are known to be precise. Conversely, it is initially suggested to calibrate the instrument by 3-6 — if possible more — standards with known CaF2 contents uniformly spread across the range of interest. Since NMR is a comparative method, it cannot be more precise than the reference method against which it is being evaluated; analysis of more reference samples also reduces errors.

NMR calibration for calcium fluoride in fluorspar

Figure 1. NMR calibration for calcium fluoride in fluorspar

Measurement

The powder sample is transferred to a tared sample tube and filled up to a specified height and subsequently weighed before NMR analysis. It takes 80 seconds to measure each sample.

Results

A comparison of the expected NMR values against the specified reference values is shown in Table 1. Deviations can be taken into consideration through an error in the reference values and also fluorine from sources other than CaF2 contributing to the NMR signal.

Table 1. NMR results predicted for calibration samples

Sample ID Given %CaF2 (wt.-%) Predicted %CaF2 (wt.-%) Difference %CaF2 (wt.-%)
FG1 81.9 83.5 +1.6
FG2 66.0 66.3 +0.3
FG3 11.3 12.6 +1.3
FG4 26.8 24.2 -2.6
FG5 47.7 45.1 -2.6
FG6 11.6 11.6 +0.0
FG7 10.7 12.5 +1.8
FG8 43.8 44.1 +0.3

 

In Table 2, the repeatability, or precision, of the NMR measurement of the same sample is shown to be excellent.

Table 2. Measurement repeatability of the same fluorspar sample

Repeat Measured CaF2 content, %
1 37.1
2 37.0
3 37.3
4 37.1
5 37.2
6 37.0
7 37.0
8 37.1
9 37.2
10 37.0
Mean % 37.1
Std. Dev. % 0.1

 

Conclusion

  • NMR is extremely stable over the long term and hardly requires any calibration adjustment
  • A main calibration can cover a concentration range between 0% and 100%
  • Measurement accuracy is good in comparison with chemical techniques
  • NMR is not sensitive to the air voids between the powder grains
  • Rapid sample measurement time
  • The NMR is a non-destructive technique, which means the same sample can be determined a number of times before being examined by other methods

Complete Package

Oxford Instruments provides a package specifically customized to suit the measurement of fluorine in fluorspar.

  • Oxford Instruments MQCF NMR analyzer
    • Probe for sample tubes of 26 mm diameter (14 mL sample volume)
    • 0.55 T (22 MHz) high-homogeneity magnet
    • Built-in flat-screen display
    • Built-in system controller (external PC is not needed)
  • MultiQuant software including RI Analysis, RI Calibration, and the EasyCal “Fluorine in Fluorspar” application, which guides users through the analysis and calibration procedures
    • Glass tubes
    • Test/tuning samples
    • Method sheet
    • User manual
  • Optional items are:
    • A dry heater and aluminum block with holes for conditioning the samples at 40 °C (optional)
    • A precision balance

Oxford Instruments MQCF NMR analyzer

This information has been sourced, reviewed and adapted from materials provided by Oxford Instruments Magnetic Resonance.

For more information on this source, please visit Oxford Instruments Magnetic Resonance.

Citations

Please use one of the following formats to cite this article in your essay, paper or report:

  • APA

    Oxford Instruments Magnetic Resonance. (2019, May 03). NMR Can Identify the Purity of the Mineral Fluorspar. AZoMining. Retrieved on May 20, 2019 from https://www.azomining.com/Article.aspx?ArticleID=1445.

  • MLA

    Oxford Instruments Magnetic Resonance. "NMR Can Identify the Purity of the Mineral Fluorspar". AZoMining. 20 May 2019. <https://www.azomining.com/Article.aspx?ArticleID=1445>.

  • Chicago

    Oxford Instruments Magnetic Resonance. "NMR Can Identify the Purity of the Mineral Fluorspar". AZoMining. https://www.azomining.com/Article.aspx?ArticleID=1445. (accessed May 20, 2019).

  • Harvard

    Oxford Instruments Magnetic Resonance. 2019. NMR Can Identify the Purity of the Mineral Fluorspar. AZoMining, viewed 20 May 2019, https://www.azomining.com/Article.aspx?ArticleID=1445.

Ask A Question

Do you have a question you'd like to ask regarding this article?

Leave your feedback
Submit