Ensuring Consistent Product Quality Using Time Critical Analysis and XRF

By AZoM

Table of Contents

Introduction
     X-Ray Fluorescence (XRF)
Cement Analysis – A Time Critical Solution
     XRF Instrumentation
     Bench-Top EDXRF Systems
     Principal of Operation
     Sample Preparation
     Performance and Results
     Routine Analysis
     Data Storage and Output
     Remote Diagnostics
Conclusion
About Oxford Instruments Industrial Analysis

Introduction

In most industrial manufacturing processes, the key challenges faced by production are maximising production output and minimising expenditure, while at the same time enhancing quality and satisfying complicated, strict and varying regulatory requirements.

At each stage of the production process, from raw material supply to shipping of the finished product, time-dependent quality control analysis is needed in order to ensure product integrity and smooth production operations. Making sure that chemical specifications are met at each major stage will result in a predictable, reliable and high-quality product, thus enabling time and cost savings.

X-Ray Fluorescence (XRF)

An analytical technique that is gaining widespread acceptance is the X-ray fluorescence (XRF) analysis that is used in a large number of manufacturing industries. It provides quick and economic solutions to time critical analysis on easy to use instruments, at different stages in the production process.

In the following paper, XRF for an industrial mineral, e.g. cement analysis is discussed in detail. However all of the advantages described below also apply for a range of industrial processes. These include minerals such as silica sand, china clay, bauxite, limestone etc, petroleum products that include fuels, oils, lubricants, plastics such as polypropylene, PET, etc, healthcare products such as cosmetics and medical, chemicals such as catalysts and sealants and general analysis that includes environmental analysis and recycling.

Cement Analysis – A Time Critical Solution

X-ray fluorescence (XRF) is a popular analytical technique for the study of raw materials, intermediate and finished products enabling ease in sample preparation, speed and precision of analysis.

Bench-top, advanced XRF instruments, such as the X-Supreme8000, now provide a cost-efficient solution for ensuring consistent product quality. They offer superior performance analysis, with field proven high consistency and are run by production staff round the clock. These systems can be used both to measure the key elements that include magnesium, aluminium, silicon, calcium, iron in cement making materials at major points in the manufacturing process and/or as a backup to another XRF. Used either independently or as part of an automated system, XRF instruments ensure conformance to ISO standards that require compliance with 24/7 continuous product analysis. The benchtop X-Supreme8000 XRF analyzer is shown in Figure 1.

Figure 1. X-Superme8000 Benchtop XRF analyser

XRF Instrumentation

X-ray fluorescence spectrometry is accepted in the cement industry as a suitable analytical technique. The instruments are classified into two broad categories that include wavelength dispersive (WDXRF) and bench-top energy dispersive spectrometers (EDXRF).

Bench-Top EDXRF Systems

Bench-top EDXRF systems are compact, low-cost, single sample such as Lab-X3500 or multi-sample such as the X-Supreme8000 shown in Figure 1. These instruments are easy to install and are often situated in different locations, that include a quarry, grinding plant, blending site, at-line, control room, or in the laboratory.

Advanced EDXRF instruments offer a field proven tough and consistent solution. They use matched X-ray tube and solid state superior resolution, and high count-rate detectors for the elemental determination of low atomic number elements such as sodium in cement and chlorine in clinker, as well as the standard range of cement making elements, that include Mg, Al, Si, S, K, Ca and Fe.

Principal of Operation

X-ray fluorescence (XRF) takes place when elements in a sample are excited at an atomic level; when they return to their initial state they emit a characteristic X-ray photon. Each element emits a distinct X-ray energy so an XRF spectrometer consists of a source of excitation, and selective detection and quantification of the characteristic elemental X-ray photons. A calibration line is then utilized to perform quantitative analysis.

The Lab-X3500 and X-Supreme8000 are examples of EDXRF instruments that have been used successfully in cement analysis. Some of the benefits are of using these instruments are listed below:

  • Low power, high efficiency X-ray tubes are used for elemental excitation. Hence the X-ray tubes are only switched on when taking a measurement which leads to lower levels of heat generation with no need for external cooling. This results in highly steady, very consistent X-ray tubes, with long lifetimes, usually greater than ten years, resulting in low running and maintenance costs, giving an overall low cost of ownership.
  • Characteristic X-ray detection is accomplished using the latest detection technology, a silicon drift detector (SDD), which offers high spectral resolution. The integration of optimum detection and selective elemental excitation, results in optimal analysis speed and stable and consistent results.
  • Since dust ingress is a key factor affecting the instrument’s life span, the planned design of the X-Supreme causes cooling air to be circulated in a separate compartment such as a wind tunnel from the main spectrometer components. This feature along with the integral “industrial grade” PC that does not use fans, ensures a long operating life for the main hard disk and spectrometer.
  • An example of the X-ray spectrum that results from using this combination to measure Cl in clinker is shown in Figure 2. In this example two spectra are shown representing 0.014% Cl (red spectra) and 0.51% Cl (yellow spectra) in cement. This example shows that a good elemental separation from adjacent elements in the periodic table, in this case S and Cl can now be attained leading to low detection limits. Table 1 shows the results obtained from calibration for CI.

Table1. Results obtained from calibration for CI.(* The measurement time is a simultaneous time measuring Na, Mg, Al, Si, S and Cl)

Analyte Range (% m/m) Standard error of calibration (% m/m) Guaranteed limit of detection (3ó)(% m/m) Mid-range precision (95% confidence)(% m/m) Measurement time (seconds)*
Cl 0.014-0.51 0.006 0.005 0.0035 200

Figure 2. Two superimposed X-ray spectra of cement samples, with an X-ray Region of Interest (ROI) for Cl covering the X-ray energy range 2.55-2.71 keV.

Sample preparation

In regular production, control samples are often measured as pressed pellets. The powder sample is first ground in a swing mill to ensure a reliable particle size, and then compressed using a hydraulic press at 20 tons to produce a strong pellet. A 40mm sample holder then holds the pressed pellet as shown in Figure 3.

Figure 3. Standard 40mm sample holder with cement pellet in place

Performance and Results

For superior quality production control the instrument is first calibrated which involves measuring a range of well analyzed samples of known elemental content. In the determination of Cl in cement, an X-ray “Region of Interest” (ROI) is defined around the Cl peak and the X-ray intensity obtained for each calibration standard is measured. The calibration after applications of X-ray corrections is shown in Figure 4.

Figure 4. Calibration regression for Cl in Cement

For a comprehensive cement analysis the data in Table 2 shows the performance for the remaining cement elements.

Table 2. Typical calibration performance for full cement analysis (XSMET-03B)

Analyte Range (% m/m) Standard error of calibration (% m/m) Guaranteed limit of detection (3ó)(% m/m) Mid-range precision (95% confidence)(% m/m) Measurement time (minutes)
Na2O 0.02 – 1.07 0.04 0.021 0.012 ~ 7
MgO 0.81 – 4.48 0.06 0.015 0.03
Al2O3 3.9 – 7.1 0.1 n/a 0.03
SiO2 18.6 – 22.4 0.2 n/a 0.07
P2O5 0.02 – 0.31 0.009 0.005 0.003
SO3 2.1 – 4.6 0.1 n/a 0.011
K2O 0.09 – 1.23 0.04 0.005 0.011
CaO 57.6 – 67.9 0.5 n/a 0.08
TiO2 0.08 – 0.37 0.006 0.003 0.004
Cr2O3 0.002 – 0.06 0.003 0.001 0.001
Mn2O3 0.007 – 0.26 0.006 0.001 0.002
Fe2O3 0.15 – 3.1 0.06 0.003 0.008
ZnO 0.001 – 0.11 0.001 0.0006 0.001
SrO 0.02 – 0.64 0.004 0.002 0.001

The precision was calculated from 10 repeat measurements of NIST standards. The standards were chosen so that the analytes’ concentration matched the calibration mid-range. In addition from the X-Supreme8000’s instrument repeatability it can be seen to comply with the precision requirements of the ASTM C114 “Standard methods of chemical analysis of hydraulic cement”.

Routine analysis

On a 24/7 basis production personnel can load from one to ten samples onto the sample carousel, enter the sample data either by touch screen, mouse or bar code reader and the measurement begins. For a single measurement in simultaneous analysis mode provisional results on major elements are displayed after just five seconds of measurement. This gives a rapid assessment of elemental content, allowing confirmation of product quality.

In case several samples are being measured, the clear yellow/red/green labels for each sample shows that those samples that have already been measured and those awaiting analysis.

Once each measurement is done, customer specified “cement moduli calculations” are also performed on the final result. Examples include lime saturation factor (LSF), silica ratio (SR), alumina ratio (AR), and these are calculated and shown at the end of each measurement allowing optimization of the production process as shown in Figure 5.

Figure 5. Display of results and cement calculations displayed on the X-Supreme8000

Data Storage and Output

Once the results are accepted they can be printed and/or stored as specified by the manager in the analytical method. Results can be obtained remotely by other PC based systems for subsequent data storage or as part of a centralized quality control system.

Remote Diagnostics

In addition to the inherent high consistency of EDXRF, remote diagnostics can be performed allowing an on-going “health check” using in-built samples placed underneath the sample carousel. This leads to preventative maintenance ensuring that the instrument continues to perform to specification. Any servicing requiring additional components are then readily available to minimize downtime.

Conclusion

Cost-effective XRF analysis with field proven consistency can now be achieved with instruments located on or near a production site and operated by production staff providing round-the-clock analysis coverage ensuring consistent product quality by employing user-friendly, yet powerful and accurate XRF instruments such as the Lab-X3500 and X-Supreme8000 from Oxford Instruments.

About Oxford Instruments Industrial Analysis

Oxford Instruments Industrial Analysis provides desktop, portable and handheld XRF and OES materials identification and analysis systems, coating thickness measurement and gauging instrumentation to industrial customers with diverse needs.

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

For more information on this source, please visit Oxford Instruments Industrial Analysis.

Date Added: Sep 22, 2011 | Updated: Jun 11, 2013
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