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Raw Mill Control by the SpectraFlow Air Slide On-Line Analyzer
Tobias Füeg, Head of Development, SpectraFlow Analytics Ltd
Mike Ricci, Technical Sales Specialist, CEM Specialties Inc
Today cement producers optimize their operational costs by consuming more economical, alternative raw materials, either in the pre-blending bed or as additives at the raw mill. These alternative raw materials (e.g., contaminated soils, oil refinery catalyst fines, trash incinerator ash, or coal boiler ash) can be contaminated with undesirable elements, or can be very inconsistent in their chemical composition. The plant must maintain strict control of the product, and must take immediate corrective action, between the raw mill and the homogenization silos, whenever the chemistry exceeds quality standards.
The raw mill offers the last chance to actively correct the chemistry of the kiln feed. Traditional sampling systems, which normally provide a composite analysis over one hour, are too slow to compensate for the variations in the raw materials. Slow response from the sampling system causes the following problems:
• High standard deviation for the kiln feed LSF and Silica Modulus
• Frequent violation of the limits of undesirable elements
• Under utilization of the optimal amount of alternative raw materials
• Overly conservative adherence to alkali standards
Only the tuned combination of an expert system, with an accurate and reliable on-line analyzer, can optimize the raw mix with sufficient confidence to compete in today’s quality conscience markets. SpectraFlow Analytics and CEM Specialties offer just such an optimization package consisting of an on-line analyzer and control software.
Several manufacturers offer on-line bulk material analyzers using radioactive excitation for measuring the rock chemistry before the raw mill. But the reliable supply of the radioactive isotopes, safety concerns, and the overwhelming paperwork required for licenses have become onerous. Additionally, manufacturers of radioactive on-line analyzers have designed these machines for installation on conveyor belts before the raw mill, not on air slides after the raw mill. Therefore, the direct measurement of the final product, from the raw mill to the homogenization silos, is not possible. This issue becomes important as soon as the plant feeds alternative raw materials directly into the raw mill or into the air slide behind the raw mill. SpectraFlow Analytics has developed the On-line Air Slide Analyzer to overcome these limitations, by measuring the product after the raw mill using Near Infrared.
In this paper CEM Specialties and SpectraFlow Analytics will discuss the results and benefits of a turnkey control solution, for a cement plant’s vertical raw mill, commissioned in Switzerland in 2012. This solution includes SpectraFlow Analytics’ Air Slide On-line Analyzer System, an environmentally friendly, non-radioactive instrument, enhanced by powerful control software, to optimize the raw mill additive feeder set points in real time.
How does the SpectraFlow Air Slide On-line Analyzer work, based on NIR technology?
The Near Infrared (NIR) technology utilizes a wide range of infrared spectra, which a visible light source provides. The emitted NIR interacts with the target raw meal as it passes through the air slide. The NIR excites vibrational oscillations of the molecular bonds in each mineral species, which produces reflection and absorption spectra containing mineralogical information. Figure 1 shows a typical set of absorbance spectra, received from raw meal.
Figure 1: A typical wide range infrared spectrum
The Swiss plant installed the SpectraFlow Air Slide On-line Analyzer above a rectangular opening (200mm x 200mm) in the air slide. The control cabinet for the analyzer can be located up to 20 meters away from the location of the analyzer. Figure 2 shows the setup of the SpectraFlow Air Slide On-line Analyzer installation at the cement plant in Switzerland.
Figure 2: The installation of the analyzer
The measuring head of the analyzer covers the air slide completely. If plant engineers must open the measuring head for maintenance activity (e.g., cleaning the glass, standardizing on the reference, or changing a light bulb), a knife gate automatically closes the entrance to the air slide.
The unique design and setup of the SpectraFlow Air Slide On-line Analyzer reduces the maintenance effort and minimizes the parts costs in this harsh environment, relative to radioactive on-line analyzers. The maintenance work consists only of three activities:
• Cleaning the two light sources inside the measuring head with compressed air. This action is performed normally once per week, depending on the operation condition of the air slide.
• Standardize the analyzer with an easy to handle reference plate. This procedure is normally conducted every two months.
• Replace two light bulbs, maximum two times per year.
None of the above mentioned maintenance activities require any special skills. Plant personnel can perform all of the required maintenance work. No special training, licensing, or certification is required.
Figure 3: Measuring head with maintenance hatch removed
In the following example, SpectraFlow Analytics factory calibrated the Air Slide On-line Analyzer with raw meal samples from the Swiss cement plant. The samples represented the full range of material that the analyzer would encounter at the plant. In NIR spectroscopy, we create an analytical mathematical model to represent the chemical fluctuations in the raw meal.
Figure 4: Calibration for CaO Table 1: Calibration Ranges
SpectraFlow Analytics developed the analytical model using the standard method of partial least squares, multivariate linear regression (PLS1). This technique requires a sufficiently large training set, in this case 60 samples of raw meal from the plant. For each of the major constituents (CaO, SiO2, Al2O3, and Fe2O3) we compiled a separate model using PLS1, with a customized preprocessing to optimize the respective constituent. Every 100 milliseconds the analyzer acquires a full spectrum. SpectraFlow accumulates NIR spectra over one minute and reports the average minute analysis to the control software.
In the past, the mechanical sampling station, located after the vertical raw mill, provided the quality input for the feeder control software. This sampling station accumulated 40 minute composites, and then transported the sample via pneumatic tubes to the laboratory, where a robotic system automatically analyzed the sample by XRF. This transport time meant that the control software received the XRF result roughly one hour later than the SpectraFlow result. The following figures illustrate the effect of using the SpectraFlow Air Slide Analyzer as input to the control software.
In Figure 5 below, each vertical dashed line represents 15 hours. The graph spans 2.5 days from June 10 to June 12, 2012. The orange line represents one minute updates from SpectraFlow. The yellow line represents one hour updates from the lab XRF. Close inspection of Figure 5 reveals that the yellow XRF peaks and troughs are shifted about one hour to the right of the orange SpectraFlow peaks and troughs. This shift represents the time delay in the lab based XRF control system.
The set point in Figure 5 is 98 LSF, during two days of plant operation. The appearance of the horizontal blue line indicates that the plant operators have switched control over to SpectraFlow. The mechanical sampling system (blue line absent) produced considerable LSF variation between 90 and 115, with a one sigma standard deviation of ± 4.0. Due to the time delay in the mechanical sampling system, the plant cannot reduce this variability further. The one hour time delay in the lab based XRF system causes the control software to make inappropriate decisions that are always out of phase with the process. Once the input to the control software switched to the SpectraFlow On-line Analyzer (blue line present), the LSF variation decreased significantly to within a range of 95 to 102, with a one sigma standard deviation of ± 1.5.
The blue line indicates automatic control based on SpectraFlow.
No blue line means automatic control based on sampling station and XRF.
The yellow line indicates measurement by sampling station and XRF (1 value each hour).
The orange line indicates measurement by SpectraFlow (1 value each minute).
Figure 5: Control software input change from sampling station to SpectraFlow
Figure 6 demonstrates in more detail how the control software, based on real time SpectraFlow measurements, reacted to compensate for the variations at the end of a pre-homogenization stockpile. The rapid drop in the LSF value (orange line) reflects the start of the new stockpile. The control software started to increase the high lime additive feeder (gray line) within minutes to compensate for the change. Within one hour the SpectraFlow based control system pulled the LSF back up to the target value.
The vertical white line indicates the start time of the new stockpile.
Orange line is LSF.
The gray line indicates the reaction of the lime additive feeder.
Figure 6: Automatic compensation for a stockpile change
This example clearly demonstrated that, compared to mechanical sampling system control, the SpectraFlow Air Slide On-line Analyzer can reduce the variability of the raw meal LSF by more than 60%. However, the reduction of the LSF variation depends not only on the performance of the analyzer or the control software. Feeder calibration, chemical variability of the additives, availability of additives, material handling problems in feed bins, and constraints on target parameters also contribute significantly to the overall system performance. Another often neglected area is the impact on the maintenance and service costs. By using a Spectra Flow Air Slide On-line Analyzer, the plant can extend the time of the sampling interval and significantly reduce the wear and tear on the mechanical sampling station parts.
In summary, an expert system for feeder control, based on the SpectraFlow Air Slide Analyzer, leads to the following benefits:
• Significant reduction in the variability of the raw meal LSF
• Elimination of the process time delay introduced by mechanical sampling and lab based XRF systems
• Rapid compensation for excursion in chemistry that occur when the cement plant changes pre-homogenization stockpiles
• Accurate measurement of alternative raw materials introduced directly into the raw mill
• Lower maintenance and service costs for sampling stations due to reduced or eliminated sampling
• Reduced maintenance costs relative to nuclear on-line analyzers