Abstract: This article introduces Siemens' experience in applying various level measurement technologies to the first and second phases of Shandong Weiqiao Aluminum Industry, currently the largest single alumina project in China. This project is Siemens' largest procurement project for level instruments in China and even the world. Keywords: Alumina radar level gauge, ultrasonic level gauge, radio frequency admittance pulse continuous frequency modulation (FMCW) I. Background Shandong Weiqiao Aluminum Industry is currently the largest single alumina project in Asia under planning. Weiqiao Aluminum Industry plans to build a 10 million ton/year alumina production facility in five phases, with four production lines in each phase. The designed daily production capacity of each production line is 400,000 tons/year, and the actual daily production capacity is 500,000 tons/year. Currently, the first production line of the first phase has been put into operation since May, and the second to fourth production lines have begun trial operation and will soon reach full production capacity; the capacity will reach 2 million tons/year. With its good corporate image, pragmatic work attitude, and reliable product performance, Siemens defeated strong competitors and won the order for more than 100 radar and other level gauges for the first alumina production line of Weiqiao Aluminum Industry. In August 2006, Siemens once again won the favor of Weiqiao Aluminum, securing an order for nearly a thousand level gauges, primarily radar level gauges. This order is the largest level gauge order in Siemens' history. II. Siemens Level Solutions 1. Ultrasonic Level Gauges Ultrasonic level gauges are based on the echo ranging principle. Most ultrasonic level gauges use air-mediated sound, utilizing the reflection of ultrasonic pulses propagating in the air onto the measured object and receiving their echoes. The round-trip time of the ultrasonic pulse is proportional to the propagation distance. By measuring the travel time of the sound pulse, the level can be calculated. Siemens offers a wide variety of ultrasonic level gauges, among which the Probe LU integrated ultrasonic level gauge is one of the most popular due to its high cost-effectiveness. It offers 6m and 12m capacities to meet different range requirements; integrates state-of-the-art intelligent echo processing technology to effectively eliminate various interferences; and boasts unparalleled high precision: 0.15%, making it the only ultrasonic level gauge currently capable of achieving this precision requirement. Ultrasonic level gauges are mainly used for level measurement applications at normal temperature and pressure, such as various sewage tanks. 2. Radar level gauges: Radar level gauges are a level measurement method based on the echo ranging principle. It uses electromagnetic waves, and its propagation does not require a transmission medium, thus largely eliminating the need to consider the effects of volatile gases and vapors, temperature, pressure (vacuum), or even dust. In alumina plants, radar level gauges are currently the most widely used level measurement technology. Siemens' level solutions offer two radar measurement technologies to meet complex process requirements for different applications. One technology uses C-band microwave pulse measurement. Siemens' Sitrans Probe LR, LR200, and LR300 radar level gauges utilize this technology. Pulse radar level gauges, similar to ultrasonic technology, use the time difference principle to calculate the distance to the medium surface. The device transmits pulses of a fixed frequency, then receives and establishes an echo pattern. The signal propagation time is directly proportional to the distance to the medium. However, unlike ultrasound which uses sound waves, radar uses electromagnetic waves. It uses tens of thousands of pulses to "scan" the container and obtain a complete echo map. C-band microwaves generally refer to microwaves with a frequency of 6 GHz. Due to the use of a smaller microwave frequency, its wavelength is longer. It has the following advantages: better resistance to material buildup on the antenna, better performance on fluctuating liquid surfaces, and better foam penetration at lower frequencies. Therefore, C-band pulse radar is widely used in liquid measurement in various complex processes, such as: alkaline liquid level measurement with foamy surfaces, red mud tanks with severe material buildup, and settling tanks with large surface fluctuations and agitators. Siemens' Sitrans LR400 uses continuous frequency modulation (FMCW) measurement technology with K-band microwaves. FMCW microwave level gauges use linearly modulated high-frequency signals, typically 10 GHz (X-band) or 24 GHz (K-band) microwave signals. It is an indirect measurement method based on complex mathematical formulas, calculating the level distance from the spectrum. The antenna emits a linearly modulated continuous high-frequency microwave signal and scans, while simultaneously receiving the return signal. The frequency difference between the transmitted and returned microwave signals is proportional to the distance to the surface of the medium. Due to its short wavelength, it has the following advantages: a narrower beamwidth, allowing it to avoid obstacles; better reflection on inclined solid surfaces due to its shorter wavelength; and the ability to use a smaller antenna. The Sitrans LR400 is mainly used in applications with a large measurement range, such as separation tanks; and for measuring powdery solids with high dust levels, such as finished alumina silos (as shown in Figures 2 and 3), bauxite silos, and limestone powder silos. 3. Radio Frequency Admittance Level Gauge: Capacitive level gauges are instruments that measure level using the principle of capacitor measurement. Radio frequency admittance is a level control technology developed from capacitive level gauges, offering better anti-fouling properties, reliability, accuracy, and wider applicability. In radio frequency admittance, admittance is the reciprocal of impedance in electrical engineering; it is composed of resistive, capacitive, and inductive components. Radio frequency refers to the high-frequency radio wave spectrum, so radio frequency admittance can be understood as measuring admittance using high-frequency radio waves. Siemens' level products utilizing radio frequency admittance technology primarily include the CLS series level switches and the LC series level gauges. Compared to other companies' capacitance and radio frequency admittance measurement technologies, Siemens' radio frequency admittance level gauges employ a higher transmission frequency and utilize patented Active-Shield technology, thus offering the following advantages: high measurement accuracy; ability to measure media with dielectric constants as low as 1.5; effective protection against the effects of material buildup, foam, steam, and volatile gases; and the ability to measure various viscous conductive or non-conductive media. Less affected by installation location, radio frequency admittance technology is widely used in alumina plants for level measurement in tanks with significant foaming at the liquid level. Radar level gauges often lose signal due to their inability to penetrate thick foam, while radio frequency admittance is less affected. Examples include alkali tanks. There are also locations where installation is difficult due to narrow spaces, making them susceptible to various interferences, such as overflow tanks next to settling tanks. These overflow tanks have small diameters, and the overflow water often enters the radar beam, interfering with the normal operation of radar level gauges. Radio frequency admittance level gauges are relatively less affected by overflow water flow. III. Selection and Considerations for Level Gauges in Alumina Processes Most alumina processes currently use the Bayer process. A basic production process diagram is shown below: In alumina processes, level measurement mainly includes solid material level measurement and liquid level measurement. Different processes have different requirements for level measurement. The key is to select the most suitable level measurement technology and the most cost-effective solution. Requirements and considerations for solid material level measurement: 1. Measurement requirements and objectives. Based on the current requirements for solid material level measurement, it can be divided into two types: continuous measurement and point measurement. Point measurement is relatively simple; when the material level is higher or lower than a certain level, the level switch outputs an alarm signal (usually a relay action signal) to protect the silo from overflow or emptying. Siemens provides level switches such as the CLS series RF admittance level switches and LPS/LVS mechanical (rotary paddle/tuning fork) level switches. Continuous measurement requires the instrument to continuously monitor the material level to achieve storage management and improve production efficiency. Siemens provides various solutions such as the Sitrans LU series, Multiranger ultrasonic level gauges, and Sitrans LR400 solid-state radar to meet different application needs. 2. Characteristics of solid material level (particle size, dust, etc.). Generally speaking, based on the different particle sizes of different materials, solids can be simply divided into lumpy solids and powdery solids. For lumpy solids, such as limestone (lumps), ultrasonic level gauges can be used for measurement. Siemens' ultrasonic level measurement technology employs advanced technologies such as high frequency, acoustic impedance matching, and array systems to overcome the effects of ultrasonic energy attenuation caused by scattering from solid surfaces and small amounts of dust. This ensures high accuracy, cost-effectiveness, reliable performance, and long service life for the ultrasonic level measurement system. For applications involving powdery solids, such as bauxite (powder) silos, finished alumina (powder) silos, and limestone powder silos, the Sitrans LR400 solid-state radar is generally used, achieving good results. The Sitrans LR400 uses 24GHz FMCW microwave technology, which overcomes the significant effects of dust and produces ideal reflection effects on fine powder surfaces, resulting in the most reliable measurement results. 3. Installation Location and Silo Structure. For non-contact level measurement technologies like ultrasonic and radar, choosing a suitable installation location is crucial. Generally, the selected installation location should avoid strong interference such as the feed inlet, reinforcing rings within the silo, and steel beams; it should also avoid being installed in the center of the silo to prevent interference from multiple echoes. Therefore, it is best to study the structural diagram of the silo before choosing a suitable installation location to avoid some usage problems. For materials with a large angle of repose, choosing the aiming device option (accessory) will result in better reflection effects. It is worth mentioning that the Sitrans LR400 solid-state radar integrates the aiming device configuration, eliminating the need for separate selection. Liquid level measurement requirements and precautions: 1. Range and accuracy requirements. In liquid level measurement applications in alumina plants, most tanks and containers are within 10-20 meters in height, and the Probe LR, LR200, or Probe LU can meet the application range requirements. However, there are also some applications with large ranges, such as settling tanks with heights exceeding 35m, in which case the LR400 or Sitrans LU series (with a maximum range of 60m) can be selected to meet the large range application requirements. In liquid level measurement, accuracy requirements are generally divided into two types: trade settlement level (metering level) and process level. The accuracy requirement for trade settlement level gauges is ±1mm; while the accuracy requirement for process level gauges is not very high, generally only ±5~15mm is required. Conversely, process level gauges require higher reliability and stability because the operating conditions they are used in are much more complex. In the alumina process, the accuracy requirement for all level gauges is process level. For some more important process points, high-precision radar level gauges (generally with an accuracy of 0.1% or higher) are usually selected. 2. Process temperature and ambient temperature. Many level measurement points in the alumina process experience high process and ambient temperatures, with process temperatures in many locations even reaching 150~200ºC. For example, slurry tanks and dilution tanks. Generally, in these high-temperature applications, radar level gauges are used instead of ultrasonic level gauges. Antennas made of PTFE material (rod type and horn antenna) can be used in temperature environments up to 200 ºC, which can basically meet all measurement requirements in the alumina process. In many locations, the presence of high-temperature steam or high-temperature processes leads to very high flange temperatures for instruments, sometimes reaching 50-60ºC, or even 80-90ºC (as shown in Figure 7), such as hot water tanks and slurry tanks. In these cases, besides some level gauges that can withstand high ambient temperatures due to their integrated structure – Sitrans Probe LR/LU and LR200 both use an integrated structure and can withstand ambient temperatures of 80-85ºC – heat insulation gaskets are typically added between the flanges. In extreme high-temperature situations where a suitable installation location cannot be found – such as the raw material slurry level measurement point in the raw material grinding process section – Siemens designed a radar level gauge to be installed above the top of the container in Weiqiao Aluminum's alumina unit. Utilizing the principle of natural cooling through radiant heat, it avoids the high temperatures inside the container and uses acoustic intelligent trap echo processing software to eliminate interference from the top of the container, "penetrating" the container to measure the liquid level in the tank, achieving excellent results and solving a problem for the customer (as shown in Figure 8). 3. Dielectric constant and density of the measured medium. For radar and RF admittance level gauges, the dielectric constant of the measured medium is relatively important. Since the dielectric constant of most media in the alumina process is greater than 3, it has little impact on the choice of radar. However, RF admittance level gauges require a linear relationship between changes in liquid level and changes in the measured capacitance, thus requiring a stable dielectric constant. Therefore, RF admittance level gauges are unsuitable for applications where the dielectric constant changes significantly. For ultrasonic level gauges, the density of the measured medium is directly related to the amount of ultrasonic wave reflected energy. Therefore, the higher the density of the measured medium, the better the measurement effect. Thus, in conditions with significant volatile gases (fumes) or water vapor, the performance of ultrasonic level gauges will be affected. 4. Whether there is water vapor, material buildup, or scale in the tank or vessel. Although water vapor has a very small impact on radar waves, under normal circumstances, the action of water vapor (as shown in Figure 9) can lead to the formation of condensate, affecting the normal operation of the level gauge. Comparative experiments show that radar level gauges using rod antennas are less affected by condensate caused by water vapor. In many processes, material buildup frequently occurs during slurry level measurement, such as in red mud tanks. Since the dielectric constant of slurries is generally high, its impact cannot be ignored. Using a horn-shaped antenna can minimize the impact of material buildup. In extreme cases, a horn antenna with a self-cleaning device (water or compressed air) can be used to eliminate the effect. Severe scaling is a persistent challenge in alumina processes, especially in seed tank level measurement. Scaling in this process is very serious, and currently, it can only be removed manually from the level gauge probe, which is labor-intensive for on-site workers. However, non-contact radar level gauges represent a significant improvement over other contact measurement methods. 5. Agitator interference. Sitrans LU or LR level gauges, which integrate intelligent acoustic echo processing technology, can effectively overcome agitation interference, such as in decomposition tanks. 6. Corrosion compatibility. Radar level gauge antennas are generally made of PTFE and PPS, while ultrasonic probes are typically made of PVDF and ETFE. When selecting a level gauge, it is essential to consider the corrosion compatibility between the measured medium and volatile gases and the material of the level gauge probe. For example, for level measurements of sulfuric acid tanks and alkali tanks, relevant material corrosion compatibility data should be consulted based on the required concentration to determine the appropriate material. For media that produce corrosive volatile gases, such as fuming sulfuric acid, an integrated gasket option can be chosen to prevent corrosion and penetration of stainless steel flanges and transmitter heads. 7. The influence of installation location and container structure. Similarly, when selecting a suitable installation location, the center of the container should be avoided as much as possible to reduce multiple echo interference; avoid the feed inlet location and areas with strong fixed interference (such as: reinforcing rings, ladders, crossbeams, etc.) within the beam range; maintain a certain distance from the tank, generally following the 10:1 principle, that is, the distance from the tank wall should not be less than the height of the tank (range), and the optimal installation location is at 1/3 to 1/4 of the container diameter; the height of the container's installation riser should generally not exceed the length of the radar level gauge's shielding section, and the riser diameter should generally be greater than 50mm, with the optimal diameter being 150mm. 8. System grounding issues. In the application of the Weiqiao alumina plant, it was unexpectedly found that the radar level gauges in some process sections were affected by the surrounding frequency converter equipment. Observations showed that when some frequency converters started, the radar level gauges generated excessive noise, leading to attenuation or even loss of the echo signal. Initially, we designed and selected some filters to be installed in the circuit loop (as shown in Figures 10 and 11), which suppressed the noise and restored the use of the radar level gauges. Further investigation revealed that the interference was caused by poor grounding during the installation of the control system. Following our advice, the customer improved the grounding measures, resolving the issue. Our efforts earned the customer's goodwill. IV. Conclusion In the past two years, driven by both a widening domestic supply gap and rising international alumina prices, my country's alumina industry has experienced an investment boom, with Shandong, a major aluminum-producing province, leading the way. Especially with the impetus from several large private enterprises such as Weiqiao Group, non-Chinalco alumina production capacity has rapidly expanded, breaking Chinalco's monopoly. This inevitably leads to a decline in national alumina prices and fierce market competition. How to effectively control costs and improve production efficiency is a problem every alumina company must face: choosing the optimal, suitable, and mature automation system equipment and instrumentation solutions will greatly benefit users in achieving optimal automation control. In the alumina process, the application of level instruments is a crucial element. With technological advancements and accumulated experience, level measurement technology in alumina plants will become increasingly mature. References: 1. Yan Yongsheng. Alumina process level gauges and selection, Instrument and Meter User, 2004, No.3, Article number: 1671-1041 (2004) 03-0077-02, P77-78 2. Wang Lijie, Han Hongfang. Application of several liquid level gauges in settling tanks, Automation and Instrumentation, 2005, Issue 1, P48-49, 70 3. Hao Xianting, Shi Dengyue. Application of radar liquid level gauges in 42m settling tanks, Intelligent Instrumentation, 2004, Issue 1, P51-52 4. Xue Hui. Application of ultrasonic level gauges in engineering, Nonferrous Metallurgical Design and Research, June 2000, P15-17 5. Bi Shiwen et al., Alumina production process, Chemical Industry Press, February 2006