Power plants produce a significant amount of coal fly ash as a waste product from the burning of pulverized coal. In the United States alone, more than 80 million tons of coal fly ash is generated each year. The fly ash produced is a fine-grained, powdery particulate material. About 22 % of the total quantity produced, is used in construction-related applications (cement and concrete production, road construction materials, mineral filler in asphalt paving, etc.). Fly ash is useful in these applications because it is a siliceous or alumino-siliceous material that, in the presence of water, will combine with calcium hydroxide (from lime or ordinary Portland cement) to form cementitious compounds. Coal fly ash can also replace clay, sand, limestone and gravel. In this way the environment can be saved and the energy costs of mining such materials can be eliminated.
The largest single use of fly ash is as a mineral admixture in Portland cement. Fineness, loss on ignition, and chemical composition are the most important characteristics affecting its use in cement. Fly ash used in Portland cement must meet the requirements of ASTM C618 which specifies two classes of product, namely F and C. The chief difference between the two classes is the amount of calcium and silica, aluminium, and iron in the fly ash. The chemical composition of fly ash needs to be closely monitored to ensure a consistent, quality-controlled product.
XRF is an excellent method for the multi-elemental analysis of coal fly ash for the purposes of quality control, product development and product performance classification. It produces fast, cost-effective, precise and accurate data with a minimum of sample preparation and operator dependence.
Preparation of standards
Certified reference fly ash materials from different standardization bureaus (BCR, NIST, SABS and IRANT) were used to set up calibrations. 0.9 grams of each coal fly ash standard was mixed with 9.0 grams of lithium tetraborate (Li2B4O7) and fused into a bead (40 mm diameter). As a comparison, 12 g of each coal fly ash standard was mixed with 3 g of SpectroBlend binder (Chemplex Industries) and pressed into a pellet of 40 mm in diameter.
The power settings of the X-ray tube were between 24 kV / 100 mA (for Na2O, MgO, Al2O3, SiO2, P2O5, SO3, K2O and CaO) and 60 kV / 40 mA (for Fe2O3). Na and Mg in the standards were analyzed using a 700 µm collimator and a PX1 crystal. Al2O3 and SiO2 were analyzed with a special curved PE002 crystal, whereas for P2O5 and SO3 a special curved Ge111 crystal was used. The total measurement time per sample was less than 4 minutes.
Precision and instrument stability The precision, repeatability and reproducibility of the Zetium spectrometer is outstanding, not only for short-term measurements (20 consecutive measurements in this case, Table 1), but also for longer- term measurements (measurements carried out over a period of ten days in this case). For comparison, the counting statistical error (CSE) is also shown in Table 1. Twenty consecutive measurements of a coal fly ash standard (NIST SRM 1633b, fused bead) demonstrates standard deviations of 0.1 % relative for the major elements.
This level of precision is maintained for measurements carried out over a period of ten days, for example, Al2O3, SiO2 and Fe2O3 , illustrating the long-term stability of the system. Comparison of the precision measurements with the counting statistical error (theoretically, the minimum possible error) emphasizes the inherent stability of the instrument.
In Figure 1, the analytical precision of the Zetium spectrometer is illustrated graphically for CaO and K2O in NIST SRM 1633b (fused bead) without using any monitor correction.
Table 1. Analytical precision (measured on NIST SRM 1633b, fused bead)
Figure 1. Short- and long-term stability measurements of K2O and CaO in NIST 1633b coal fly ash standard (fused bead)
The accuracy of the calibrations is presented in Table 2 for both fused bead and pressed pellet standards. The calibration RMS value is a statistical comparison (1 sigma) of the certified chemical concentrations of the standards with the concentrations calculated by regression in the calibration procedure. Calibration plots for SiO2 in coal fly ash standards both as fused beads and as pressed powders give a graphic illustration of the accuracy of the method (Figures 2 and 3, respectively). Figures 4 and 5 show the calibration curves of CaO in the coal fly ash standards both as fused beads and as pressed powders, respectively. The accuracy of the calibrations obtained with pressed powders is not as good as those obtained with fused beads because of the sample heterogeneity (small particles) and the surface roughness (especially for Na2O and MgO).
Table 2. Calibration quality
Figure 2. Calibration graph for SiO2 (fused bead)
Figure 3. Calibration graph for SiO2 (pressed pellet)
Figure 4. Calibration graph for CaO (fused bead)
Figure 5. Calibration graph for CaO (pressed pellet)
Components typeset in bold were present in the spectrometer used to obtain the data in this note.
Summary and conclusions
The results clearly demonstrate that the Zetium spectrometer is capable of analyzing coal fly ash samples with a high precision. Measurements are accurate and fast and the method benefits from simple sample preparation.The stability of the Zetium spectrometer is such that individual calibrations can be used for months. Time-consuming re-calibrations are unnecessary and the resulting data are highly consistent over time.