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BAUXITE (a) (b) Figure 3—Relationship between attenuation (α) and phase-shift (φ) per unit mass and the moisture (mc) from experimental data used in the pre-calibration of the moisture analyzer. Measuring the moisture content of the ore at the input of grinding circuits allows for accounting of the actual dry mass fed to the circuits, from which the mass balance and the mass recovery of the circuits can be computed. The mass recovery, given by the ratio between the input dry ore fed to the grinding circuits and their output slurry mass produced, is a key performance indicator of the plant, since it indicates how much of the total input ore has been converted into product slurry. The moisture analyzer was originally installed on belt conveyor TC-121-02 (See Figure 2) in order to take advantage of an existing belt scale on this conveyor, to provide the compensating signal required by the analyzer. However, after the analyzer had been installed, its moisture measurements were found unreliable when compared with laboratory sample values. Further investigation of the entire system indicated incorrect belt scale measurements were caused by mechanical misalignment in the conveyor structure and belt sliding, hence compromising the moisture measurements. To overcome those problems, two steps were taken: to relocate the analyzer to another conveyor, TC-123-02 (See Figure 2), still at the input of a grinding circuit; and to use another compensating method for the analyzer, to avoid it being "slave" of unreliable belt scales. The new method chosen was material level compensation. Calibration and Installation of the Moisture Analyzer The first step in the design of a moisture analyzer system is the selection of the model that best suits the intended application. All further work involving the building and calibration of the system will depend on the chosen model. It is also necessary to define the moisture range in which the analyzer is intended to work because the set of sample moisture values to be used in the model building must cover appropriately the intended moisture range of the application. In this project, this was done by checking historical moisture data from the plant laboratory database, for the conveyor on which the moisture analyzer would be installed. It was found that the nominal moisture range of the ore was 10.5–13.5%w, with eventual higher values of no more than 17.5%w. Hence, the intended calibration range of the analyzer was chosen as 0–18%w. The analyzer was originally delivered to work with Model 2. However, the compensating method would be changed from material load compensation to material level compensation, and hence Model 3 had to be developed. Development of a new model is done in three steps. Figure 4—Installation of the moisture analyzer system on the conveyor: electronics cabinet, upper microwave antenna (the bottom antenna is not visible), and material level sensor. 46 E&MJ; • JANUARY 2013 www.e-mj.com