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NEAR INFRARED TECHNOLOGY
Rails to hold 2 lamps as a NIR source
SpectraFlow is a unique new application of a known technology that has proven to meet all the analysis requirements throughout the cement manufacturing process.
SpectraFlow utilizes the Near Infra Red (NIR) spectra which are provided by stabilized white light sources (lamps). When NIR waves hit the target bulk material, which shall be analyzed as it passes by underneath the analyzer it excites vibrational oscillations of the molecular bonds in the monitored material. The result is an absorption spectrum, which is characteristic for the molecules and mineral phases of which the analyzed material consists.
The key to the analysis is Beer’s law of NIR Spectroscopy, which for a multitude of constituents has the form
ai is the absorbance coefficient of the i-th constituent
bi is the penetration depth of the i-th constituent
ci is the concentration of the i-th constituents
Absorbance is defined by
What Beer’s law basically states is that the absorbance spectra are directly proportional to the concentration of the constituent of interest and that in case of material, which consists of several constituents the total absorbance is a linear combination of the absorbance spectra of the individual constituents. The value Intensity emitted is defined prior to the on line analysis by putting an “ideal white body” at a defined position underneath the spectrometer and acquiring its spectrum.
The lamps are commercially available spots, which have a special reflecting mirror with a very good parabolic shape in order to have an almost perfectly parallel beam emitted. They are operated at 80 % of their rated power, which significantly increases the lifetime of the lamps. The angle of the lamp holders can be changed. This shall help to adapt the optimum position and angle at site. Once this position has been found it can be fixed by screws to avoid accidental changes.
Figure 1 Lamps and Illumination head setup
SpectraFlow uses its long used ABB Fourier Transform Infra Red (FTIR) spectrometer to split up the incoming NIR into its individual wavelengths. The heart of the spectrometer is a wishbone scan arm Michelson interferometer. The basic operating principle of this type of spectrometer is shown in figure 2
Figure 2 operating principle of the wishbone scan arm interferometer
The movement of the cube corner mirror and the beam splitting results in addition and subtraction of the split beam. The detector then receives an interferogram, which is characteristic for the incoming beam. The interfogram is then subjected to a Digital Fourier Transform. Thus the overlaid wishbone arm oscillation is removed from the interferogram and the pure spectrum is obtained. This means the complete spectrum, which covers the complete used wavelength range is obtained by a single step.
For the on line analysis of bulk material the NIR wavelength range from 700 nanometers to 2500 nanometers respectively from 14000 wave numbers to 4000 wave numbers is used.
SpectraFlow measures raw material on the conveyor belt after a crusher, as well as power in an airslide with a very high accuracy. As SpectraFlow is independent of the belt load no belt load compensation is necessary and with its high frequency measurement also no running averages are needed. Every single measurement represents the chemical composition of the raw material over the last minute. As SpectraFlow penetrates the molecules also a very accurate moisture measurement is possible. In FIgure 3 a comparison with a PGNAA (belt conveyor) and Figure 4 a comparison with a XRF (airslide) is shown
Figure 3 Performance comparison SpectraFlow and PGNAA
Figure 4 Performance comparison SpectraFlow and XRF