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Under the critical and harsh operating conditions of the underflow in flash drums during alumina production, ultrasonic acoustic impedance densitometers demonstrate overwhelming technical advantages over traditional Na22 (radioactive) densitometers. It fundamentally solves the problem of measurement distortion caused by numerous bubbles, and offers a comprehensive advantage in terms of safety, maintenance costs, and response speed. 1. Core advantages: Precisely overcoming the challenge of “bubble interference”. The bottom stream in the flash drum consists of a slurry where gas, liquid, and solid phases are intensely mixed under high temperature and pressure; it also contains a large number of secondary steam bubbles. This poses the biggest challenge for measurement technologies. ①Na22 densitometer: Its working principle is that the degree of attenuation of gamma rays after passing through a medium is related to the density of the medium. When there are numerous bubbles in the medium, the number of paths along which radiation travels through low-density gas increases, resulting in a decrease in overall attenuation. This causes the instrument to erroneously interpret this as a “decrease in overall density”. Consequently, the measured values become significantly lower than they should be and fluctuate wildly, failing to reflect the true solid content. ②Ultrasonic densitometer: It utilizes the principle of acoustic impedance and incorporates intelligent algorithms (such as chirp analysis). The reflection and propagation characteristics of sound waves at the interfaces between different media (gases, liquids, solids) are completely different. This technology can effectively identify and filter out the acoustic signals generated by bubbles, calculating only the true acoustic impedance of the solid-liquid mixture, thereby providing accurate and stable density data. 2. Safety and compliance: From “high-risk regulation” to “inherent safety”. This is the most fundamental difference between the two, and it is also the core driving force behind technological upgrades undertaken by enterprises today. ①Na22 densitometer: It contains a radioactive isotope source (gamma rays), posing a potential risk of leakage and representing a long-term hazard to personnel on site and the environment. Enterprises must obtain a “Radiation Safety Permit”, designate radiation control areas, conduct regular health examinations for personnel and equipment calibrations, and undergo inspections by environmental protection authorities; all these measures incur high management costs. When equipment is scrapped, the radioactive source must be professionally disposed of as hazardous waste; the procedures are complex and extremely costly. ②Ultrasonic densitometer: It uses sound waves for measurement, is completely radiation-free, and harmless to both humans and the environment. No special approvals, protective equipment, or dedicated personnel are required, thereby completely eliminating radiation-related legal compliance risks and management burdens. 3. Maintenance costs and service life: From “continuous investment” to “long-term stability”. Under conditions of high temperature, heavy wear, and frequent scaling, the maintenance costs and service life of equipment are of great importance. ①Na22 densitometer: The half-life of the Na22 radioactive source is approximately 2.6 years; its activity naturally decreases over time, leading to a decline in measurement accuracy. Consequently, the radioactive source must be replaced periodically (usually every 1–2 years) or a complex recalibration process must be carried out. This entails ongoing and significant costs. In the event of a failure, maintenance must be carried out by specialized agencies, which is a complex process that results in long downtime. ②Ultrasonic densitometer: No problem of radiation source decay; long lifespan of electronic components (up to 10 years or more). The probe is typically made of high-hardness, wear-resistant ceramic, which can withstand the intense scouring by slurry, offering a service life of 3–5 years. In daily use, it is essentially “maintenance-free”; only occasional checks of the probe are required. When installed with online ball valves, it allows for cleaning or replacement without shutting down production or emptying the pipeline, thus ensuring continuous operation. 4. Real-time response and process optimization ① Na22 densitometer: To achieve a stable counting rate, a time constant of 1–2 seconds is usually required; there is a delay in response, making it difficult to capture instantaneous density fluctuations, which hinders rapid closed-loop control. ②Ultrasonic densitometer: With a response time in the millisecond range, it can capture process fluctuations in real time. It provides fast and reliable data for precise control of the bottom stream discharge from the flash drum, helping to prevent pipe blockages and turbid fluid leakage, thereby optimizing the stability and efficiency of the entire digestion system. In summary, under the bottom flow condition in flash tanks, the ultrasonic impedance densitometer, thanks to its resistance to bubbles, intrinsic safety, low maintenance requirements, and rapid response time, has become an ideal alternative to Na22 densitometers. It not only resolves long-standing measurement problems but also brings significant safety and economic benefits to enterprises.