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BAUXITE Model 2. Model 3. of pure water, yielding a set of sample wet masses {Mi} with moisture contents {mci}. This is best done in laboratory, under controlled conditions. The samples are subjected to a microwave analyzer, and the resulting set of microwave attenuations {αi} and phase-shifts {φi} are measured. After these laboratory tests, the set of experimental data values {mci, Mi, αi, φi} is statistically analyzed to detect and eliminate possible outliers. The remaining valid data set is then used in a multivariate regression to determine values of the parameters α0, b1, and b2 that best fit the experimental data. To improve measurement accuracy, some moisture analyzers have been designed to operate with two simultaneous microwave signals of different frequencies, leading to a more complex model, as seen in Model 2. Model 2 is used in applications in which the material mass M can be directly measured and provided for the model. If this is not the case, but the material density ρ is almost constant (has a small variance), Model 2 can be rewritten in terms of material level (bed depth) h, as seen in Model 3. Where: mc: moisture content by weight (%w) M: wet mass of material (Kg) ρ: material density (g/cm3) h: material level (mm) αH: higher frequency microwave amplitude attenuation (dB) φH: higher frequency microwave phaseshift (rad) αL: lower frequency microwave amplitude attenuation (dB) φL: lower frequency microwave phaseshift (rad) α0, bi, ci: model parameters The calibration of Models 2 and 3 follows the same guidelines as for the simpler Model 1, with the difference that a larger set of sample data values is needed for the multivariate regression calculations, due to the greater number of parameter values to be determined. and the phase-shift of a microwave signal passing through the material. A microwave moisture analyzer, shown in Figure 1, is an instrument that implements this principle. The analyzer has two microwave antennas—one for transmitting and one for receiving—which are placed around the material flow (normally in a conveyor belt) in a upper-lower orientation. The transmitting antenna generates a known reference microwave signal which passes through the material, is attenuated and phase-shifted, and then reaches the receiving antenna. By comparing the received signal with the emitted signal, the analyzer determines the overall attenuation and phase-shift of the microwaves due to their interactions with both the material and its water content. To distinguish the attenuation and phase-shift due to only the water content in the material, the analyzer needs information about the amount of material, which is provided by a "compensating signal" from an independent instrument. All the signal processing and model calculations are performed by a specialized algorithm implemented in the analyzer electronics, which also yields the moisture measurements to the plant control system. Some advantages of microwave moisture analyzers are: Representative non-contact measurement: Microwaves perform a reliable noncontact measurement because they are not subject to wear by contact. And, because they pass through the bulk of the material being measured, a very representative measurement can be obtained. Accurate measurement: Microwave moisture analyzers can reach accuracies from 0.5% to 0.1%w, for 1-standard-deviation confidence, when they are well designed and calibrated. This accuracy is very suitable for process control and production accounting in mineral applications. Wide measurement range: Microwave moisture analyzers can work within measurement ranges as wide as 0% to 90%w. Such a range can meet a large variety of applications. Suitable for harsh environments: Harsh industrial conditions are a major challenge for instrument reliability, mainly in mineral applications. Microwave moisture analyzers are usually robust and can handle severe applications, with proper installation design. One limitation of microwave moisture analyzers is: Hard-to-perform calibration: A welldesigned, installed, configured and calibrated microwave moisture analyzer system can be reliable for long periods. However, due to its relative complexity and the inherent need for material sampling, handling, and drying, calibration of such analyzers is a multi-step, hard-toperform and time-consuming task. This limitation becomes a problem when, for whatever reason, the analyzer requires frequent recalibrations. The best way to avoid this problem is through care in the design, installation, calibration and maintenance of the equipment. The HYDRO Bauxite Plant HYDRO's bauxite processing plant, located 60 km from Paragominas in Pará state, Brazil, is shown in Figure 2. Plant construction ran from August 2004 to December 2006, and commissioning began in March 2007. The plant receives raw bauxite ore from an open-pit mine, and performs a set of comminution and separation processes to produce bauxite slurry, which is pumped through a 243-km-long pipeline from the plant site to the HYDRO Alunorte alumina refinery, in the city of Barcarena, Pará. Microwave Moisture Analyzers for Mineral Applications An important attribute of microwaves is that they are highly interactive with water. The measuring principle of moisture content of a material relies on sensing the attenuation www.e-mj.com Figure 2—Aerial photo of the HYDRO Bauxite Processing Plant. JANUARY 2013 • E&MJ; 45