Unique, Flexible, CrossBelt Analyzer for Raw Material Quality Control

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1 Unique, Flexible, CrossBelt Analyzer for Raw Material Quality Control Stephan Nel Thermo Fisher Scientific INTRODUCTION Today s global economic growth fuels demand for increases in cement production. Infrastructure projects such as bridges, roads, dams, office towers and residential dwellings are at the center of this demand. However, at the same time, enhanced pressures to use the earth s finite resources more efficiently are present. The need to produce high quality cement, in high volumes, while closely controlling waste, cost and environmental impact has never been more critical to the cement producer. This drive towards higher production at higher efficiencies has led cement manufacturers to increasingly use innovative technologies within their processes. One such unique technology is continuous, raw materials analysis using Prompt Gamma Neutron Activation Analysis or PGNAA for short. These instruments, commonly called CrossBelt analyzers, easily integrate into existing or new conveyor belt lines and measure the composition of the entire raw materials stream while they travel through the system. A CrossBelt analyzer typically provides information on material chemistry every minute and this rapid and timely information on material chemistry allows for changes and adjustments to be made to the process quickly and accurately. Such a high frequency control strategy provides a reliable means to achieve consistent raw material quality with minimal chemistry deviation. It is well known that kiln feed chemistry that is tightly controlled, with low variability, has lower energy requirements per ton of clinker, minimizes waste and off specification product while at the same time creates stable process operations. These end results are exactly what cement manufactures seek in order to reach their goals of high efficiency and high production rates. The realization of the valuable benefits that a CrossBelt analyzer can bring to a cement process has led to the popular adoption of these systems throughout the industry. One key aspect of any CrossBelt installation is the fact that these online analyzers require neutrons for their analysis technique. Historically the simplest, most reliable and safest source of neutrons has been the radio-isotope Californium-252 ( 252 Cf). However, there also has been some level of market interest in an alternate form of a neutron source. With a degree of complexity, neutrons can be electronically generated by a compact type of linear accelerator that employs the use of the isotopes Tritium ( 3 H) and Deuterium ( 2 H) to generate neutrons. These compact linear accelerators that generate neutrons are called specifically Neutron Generators. A review of a unique CrossBelt analyzer option available in the market place today, that can use either, 252 Cf, or a Neutron Generator, for its source of neutrons is provided here along with an example of an installation of this unique system.

2 NEUTRON ACTIVATION ANALYSIS A REVIEW PGNAA for online analysis was introduced to the cement industry in the mid 1980s by Thermo Fisher Scientific. The technique requires a source of neutrons to interact with the nuclei of the raw materials of interest. The interaction of these neutrons with the nucleus of the atoms of the raw materials generates a gamma-ray that is characteristic of the element from which it came. For example Calcium (Ca), Silicon (Si), Aluminum (Al), etc. each all have unique gamma-ray energies. (See Figure 1). The gamma-rays are counted and their energies sorted by a detection mechanism. A spectrum of energies is created and the spectrum analyzed to give the percent concentrations of the elements in the raw materials. (See Figure 2). Figure 1 Figure Cf Mentioned earlier in this paper, the predominant source for neutrons to enable PGNAA online analysis is the radioisotope 252 Cf. (See figure 3). Greater than 90% of all installed online analyzers globally utilize 252 Cf as their source of neutrons. 252 Cf is a man made radio-isotope that is manufactured by special high flux reactors located in the United States and Russia. The capacity of these reactors to produce 252 Cf far outweighs any, and all, industry needs for the isotope so 252 Cf supply is very plentiful. While government run reactor facilities manufacture 252 Cf, civilian run organizations perform the process to encapsulate the isotope for use in online analyzers. In the past, only a limited number of encapsulation vendors were available however, within the past 3 years, several new additional vendors have entered the market to supply neutron sources for CrossBelt analyzers. This increase in encapsulation vendors has created a much larger supply pipeline for 252 Cf than has existed in the past and has significantly increased availability and reduced order backlogs. 252 Cf is compact, cost effective and unequaled in its reliability as a source of neutrons. It is packaged in safe, doubly encapsulated zirconium alloy containers and fissions spontaneously and continuously, emitting neutrons over a

defined spectrum of energies. The average energy of neutrons from a 252 Cf source is 2.1 MeV while the most probable energy, within the spectrum, is 0.7 MeV.

As well, it is less burdensome to provide adequate safety shielding for neutrons in this energy range than for neutrons in higher energy ranges. Figure 3.

After approximately one half-life, additional sources are added. As well, being a spontaneously fissioning isotope, it continuously emits neutrons, whether the conveyor is running or not.

3 defined spectrum of energies. The average energy of neutrons from a 252 Cf source is 2.1 MeV while the most probable energy, within the spectrum, is 0.7 MeV. These energy levels are suited quite well for PGNAA because low energies, < ev, are required for the neutron activation process to take place. As well, it is less burdensome to provide adequate safety shielding for neutrons in this energy range than for neutrons in higher energy ranges. Figure 3. A couple of notable points associated with 252Cf sources are that due to a half life of 2.6 years, it is required to be replenished periodically to maintain analyzer precision. After approximately one half-life, additional sources are added. As well, being a spontaneously fissioning isotope, it continuously emits neutrons, whether the conveyor is running or not. Neutron Generators Although 252 Cf would seem to be the ideal source of neutrons to use with PGNAA online analyzers, an alternate source does exist. Neutrons can be generated electronically through the use of a compact linear accelerator that employs the isotopes Tritium ( 3 H) and Deuterium ( 2 H). This special linear accelerator is termed a Neutron Generator and some CrossBelt analyzer vendors have responded to market interest with options for their implementation as a source of neutrons. (See figure 4). Deuterium/Tritium Neutron Generators used in online analyzers emit neutrons of a single energy level at 14 MeV. This level of neutron energy is much higher than the energy spectrum emitted by 252 Cf and as noted previously requires additional efforts to adequately shield for safety. As well, in order to lower the energy level of 14 MeV neutrons down to energies usable for prompt gamma reactions to take place, additional efforts are needed (over a 252 Cf based online analyzer). Figure 4. Figure 5. To function, these compact linear accelerators employ the use of an ion source, ion target, beam focusing components, acceleration tube, high voltage power supply and control electronics. (See figure 5). Ions created at the ion source are accelerated and focused onto the ion target. When the ions strike the target with high energy, the resulting reaction liberates a neutron and an isotope of He. The ion impact and subsequent reaction causes the ion target to erode over time and Helium to build up in the acceleration tube. As the ion target erodes, control electronics maintain neutron output by adjusting accelerating voltage, helium capture and other parameters.

4 At some point the ion target erodes so much that it is no longer capable of producing enough neutrons and the generator tube must be replaced. The period of time between when a new acceleration tube is started and when it fails is referred to as the lifetime of the generator tube. The lifetime of an accelerator tube can be variable and is somewhat unpredictable. However, it is fairly correlated with the neutron output rate (neutron flux) of the generator over time. A high neutron output rate will generally indicate a shorter lifetime whereas a lower neutron output rate will generally indicate a longer lifetime. This is an important point to consider when looking at the application of an online analyzer within a cement process. Neutron output rate or neutron flux in an online analyzer is directly correlated to analyzer repeatability. A high neutron flux delivers better repeatability (higher performance) than a lower neutron flux. One can immediately understand a conflict, or rather compromise, here. High analytical performance in an online analyzer with a neutron generator can come at the cost of lifetime of the tube. This point will be discussed in a little more detail below. APPLICATION CONSIDERATIONS FOR ONLINE ANALYZER PROJECTS The use of online analysis within a cement process generally can be categorized into three main applications: 1. Material Sorting 2. Pre-Blending Stockpile Monitoring & Control 3. Raw Mix Control Each application has its own set of unique control requirements and analytical performance needs. Many factors are considered when evaluating an online analyzer project. Now, an additional option, the potential for an alternate source of neutrons can be added. For example, a material sorting application must have the ability to quickly and accurately determine if a raw material chemistry parameter is trending out of quality control specifications and then alarm the user or change a gate position to divert the material to an alternate location. Control actions must happen quickly in this application. This would tend to indicate an analyzer with good repeatability/high performance be used. This would mean a fairly high neutron flux (high source strength in the 252 Cf case or high output rate in the neutron generator case). Whereas a Pre-Blending Stockpile application aims to ensure that the material pre-blend remains within quality parameters over the entire stockpile. Since the amount of material in a blending bed can be quite significant and the ability to modify quarry operations takes some time, control actions in this application are much slower. In a preblending stockpile application, slower control adjustments generally can be made and the use of time averaging can be employed. This tends to indicate an analyzer with lower repeatability/lower performance could be used. This would mean a lower neutron flux could be used (lower source strength in the 252 Cf case or lower output rate in the neutron generator case). In both the material sorting and pre-blending stockpile applications mentioned, the analyzer may only need to be used during a limited amount of time such as when the quarry is operational. Conversely a raw mix control application, in most instances, will need to run continuously, constantly maintaining high frequency control of the raw material additive bins. The system in this application will be required to make control decisions every minute and therefore the performance of the analyzer repeatability must be very good. This would require a relatively high neutron flux One can begin to understand that the Pre-Blending Stockpile application that only runs 8-10 hours/day, 5 days/week and only needs a lower neutron flux for adequate performance may be more suited to employ a Neutron Generator than a Raw Mix Control application that operates 24 hours/day, 7 days a week at a high neutron flux. Because of the relationship between neutron output and tube life, one can begin to understand that the Pre-Blending Stockpile application may be more suited to employ a neutron generator than a Raw Mix Control application. That outcome derives from the fact that Pre-Blending Stockpile analyzers may only operate hours per week with a

5 lower neutron flux while raw mix analyzers may operate 168 hours per week at a higher neutron flux. With the 24/7 duty cycle of raw mix analyzers coupled with the need for higher performance (higher neutron flux), the expected tube life can be as much as 5-6 times shorter than found in stockpile applications. When evaluating online analyzer applications for the use of a neutron generator, average life and expected replacement frequency should be taken into account. In essence, turning up the output of the generator to improve repeatability performance is at the expense of tube life. 252 Cf and NEUTRON GENERATORS: A COMPARITIVE VIEW 252 Cf has been the preferred choice for neutrons among most online analyzer owners for various reasons, chief among these are low cost, high reliability and simplicity. On the other hand, neutron generators offer some advantages that 252 Cf does not. The most interesting, and frequently quoted is for safety reasons, due to the ability to turn the generator off when not in use. However, this safety advantage is offset to a degree by the increased energy level of the neutrons produced when the generator is operational. Unless appropriately shielded, the higher energy neutrons can penetrate further with potential for increased risk of harm than neutrons of lower energy 252 Cf. Another advantage is the ease of transportation of the neutron generator and documentation required to operate the tube. While each country is different, the transportation arrangements, regulations and documentation associated with a neutron generator option may be simpler to comply with than with 252 Cf. Each country is different here though and must be investigated on a case-by-case basis. (See figure 6). 252 Cf Neutron Generator Small physical size No failure modes/unmatched reliability Low cost per neutron Less shielding required 30 yrs field experience Over 700 operating systems No possibility of unexpected failure No need for regulated isotopes Technology well understood and decades old Ability to turn off to save tube life Ability to turn off for safety/maintenance reasons Figure 6. A UNIQUE ONLINE ANALYZER OPTION 252 Cf or NEUTRON GENERATOR Currently there are four companies supplying online analyzers to the cement industry that use neutron interrogation techniques. Two of these companies manufacture 252Cf based systems only, one manufactures neutron-generator based systems only and one company manufactures both types of systems. The company that manufactures both types of systems does so somewhat uniquely. Of the two models of online analyzers they have available, one is capable of utilizing neutrons from either 252Cf or from a Neutron Generator, depending on how the system is configured. At any time the end user desires, the system can be re-configured to operate with the alternate source of neutrons. The key aspect of this product is its Flexibility to adapt to meet the needs of a particular cement manufacturer. The end user decides which neutron source they wish to use at the time of purchase however, during the lifetime of the product; they can elect to switch to the other source if so desired. The system is also available customizable options such as the ability to specify the number of detectors, 1 to 4, depending on the performance requirement of the application.

The system is able to accommodate belt width between 0.6 and 1.4 meters and its tunnel opening can be configured to match most all material particle sizes and plant production rates.

changing market conditions gives the analyzer owner a unique tool. CASE STUDY: LEHIGH (HEIDELBERG) MASON CITY Lehigh Cement s Mason City plant located near Mason City, Iowa in the U.S.A (See figure 7) was in need of process improvements in their quarry to reduce waste and increase quarry lifetime.

6 The system is able to accommodate belt width between 0.6 and 1.4 meters and its tunnel opening can be configured to match most all material particle sizes and plant production rates. Given the difficulty in predicting the long-term costs, reliability and availability of the different neutron source options, the flexibility of this unique system and its ability to adapt to changing market conditions gives the analyzer owner a unique tool. CASE STUDY: LEHIGH (HEIDELBERG) MASON CITY Lehigh Cement s Mason City plant located near Mason City, Iowa in the U.S.A (See figure 7) was in need of process improvements in their quarry to reduce waste and increase quarry lifetime. The quarry at Mason City was highly heterogeneous with areas of significant MgO throughout. The existing process methods of drill hole sampling made it quite difficult to adequately understand and efficiently mine the quarry. With a somewhat limited understanding of the quarry, the plant unfortunately had to waste limestone that otherwise could be used if a better, higher frequency understanding, of the quarry chemistry and current mining process was at hand. Figure 7. Lehigh decided that the best way to gain this better understanding of their quarry was through the implementation of an online analyzer between the crusher and the pre-blending stockpile. (See figure 8). The information provided by the analyzer would help them better understand their quarry structure and also allow them to accurately and precisely monitor the quantity of MgO in the Pre-Blending stockpile. Their goal was the ability to increase the concentration of MgO in their pile without exceeding the maximum quality control set points with respect to MgO. This would allow the use of more materials that were previously considered too high in MgO. Of course, having the analyzer also allowed Lehigh to monitor and control the other main raw materials going to the stockpile. Lehigh evaluated all analyzers on the market and the global practices of its parent company, Heidelberg Cement, has an active practice of considering new and flexible technologies. The selection of the new system capable of running either 252Cf or a Neutron Generator was an easy choice for Lehigh and the system was delivered in April of Figure 8.

7 Lehigh opted to configure the system for a neutron generator at start-up and continues to run the system in this mode, with the neutron generator instead of 252Cf. The positive impact that the system had on their process was immediate. They have been able to increase the amount of MgO content of the materials going to the stockpile without exceeding the quality limits set for MgO in their clinker. In 2009 the average MgO content in their clinker was running 2.07%. After the installation and implementation of the Flexible online analyzer running the neutron generator, the average MgO content in the clinker was raised to 3.03%. (See figure 9). This still meets the quality specification for the amount of MgO in their clinker. Figure 9. One of the concerns that Lehigh had at the onset of the project was the uptime and availability of the Flexible system running with the neutron generator. The uptime of the generator has been high and has met the needs of Lehigh s requirements. CONCLUSION As the global market place becomes increasingly competitive and pressures to use earth s finite resources more efficiently, cement producers continue to explore new technologies to increase their productivity, lower costs and reduce waste. CrossBelt Online analyzers are one means that can help cement manufacturers meet their goals. As this type of online analysis technology has progressed new options have become available and there is a growing awareness of a second viable means for neutrons to be used with these instruments. One supplier of online analyzers has available a system that can use either neutron generators or 252Cf for their source of neutrons and these instruments have been successfully implemented into cement processes across the globe. REFERENCES Irvin W. Osborne-Lee, C.W. Alexander, Oak Ridge National Laboratory (1996) Californium-252: A Remarkable Versatile Radioisotope David Chichester, James D. Simpson, American Institute of Physics, The Industrial Physicist Journal (January 2004), Compact Accelerator Neutron Generators Darrell Leetham, Richard Woodward, 2010 IEEE-PCA Conference Record (2010), Online Analyzer: Cf-252 Supply Chain Update and Risk Mitigation Kevin Gordon, Jeff Hook, World Cement, May 2011 Edition, The Mason City Experience

Abstract. Introduction - Neutrons for Online Analysis in the Cement Industry

Abstract. Introduction - Neutrons for Online Analysis in the Cement Industry Online Analyzers: Cf-252 Supply Chain Update and Risk Mitigation IEEE-IAS/PCA 52 nd Cement Industry Technical Conference March 31, 2010 Darrell Leetham, Richard Woodward, of Thermo Fisher Scientific Abstract