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BAUXITE Microwave-based Moisture Measurement of Bauxite Ore on Conveyor Belts Field-proven analyzers offer improved accuracy, a wide measurement range suitable for many applications, and dependable performance in harsh environments By Sidney A.A. Viana Production and quality control of any industrial plant rely on the use of suitable field instruments and analyzers to measure major process variables. In mineral processing plants, an important process variable is the moisture content of bulk ores, which represents the amount of water naturally absorbed by the ore, the measurement of which is necessary to determine the mass of the ore alone, referred to as dry mass. The classic way to measure moisture content of ores is by drying them under specific conditions and then comparing their initial wet mass to their final dry mass. Since this requires the ore to be sampled, handled and dried, it is usually tedious and time-consuming. However, there are now field analyzers for on-line measurement of moisture content that can be used to advantage over the classic method, provided they are properly designed for the application. This article describes the application of a microwave-based analyzer for on-line measurement of moisture content of bauxite ore on a conveyor belt at HYDRO's bauxite processing plant in Brazil. The equipment was the first-ever, fully operating online moisture analyzer installed at the plant. The results and benefits of using the analyzer are also discussed. Finding a Better Way Moisture content refers to the relative amount of free water contained in a materi- al. Moisture analysis relates to a variety of methods for measuring moisture content. The classic method for measuring moisture in solid or semi-solid materials is referred to as loss on drying (LoD). With this method, a sample of wet material is weighed, heated for an appropriate time to become dry, and then re-weighed. By comparing its initial wet mass to its final dry mass, moisture content is then determined. This method provides good results and is used as a reference method, however, for industrial applications it is considered difficult and time-consuming due to the need for sampling, handling, preparation and drying. An on-line moisture analyzer is a device that permits direct measurement of moisture content for specific applications by using methods others than LoD. Many industrial processes may call for the use of on-line moisture content measurement, for example: measuring coffee freshness (food process), determining the composition of paint (chemical process), and determining the water content in ores (mineral process). The relevance of use of a moisture analyzer depends on the nature and objectives of the process. Some key applications of such analyzers in mineral processes are: Equalization of bulk ore masses in dry basis for mass balance and production accounting. Operating control of dilution water in SAG and ball mills. Figure 1—Typical architecture of a microwave-based on-line moisture analyzer for belt conveyor applications. 44 E&MJ; • JANUARY 2013 Operating control of filtration, dewatering, drying and pelletizing processes. Monitoring the moisture content of ores regarding an upper limit, in ship loading operations. This paper outlines the application of an on-line moisture analyzer in a bauxite ore processing plant. Models for Microwave Moisture Measurement Microwaves are electromagnetic waves with frequencies in the range 0.3–300 GHz. When a microwave passes through any material, its energy is partially dissipated (absorbed) by the material, causing a decrease in wave intensity (attenuation) and an offset in wave displacement (phase shift). The degree by which the wave is attenuated and phase-shifted by a specific material depends on the wave frequency and on the electrical properties of the material, such as its "dielectric constant." The higher the dielectric constant of a material, the higher the energy dissipation (attenuation and phase shift) of microwave passing through it. Water has a higher dielectric constant than many common materials, and is highly interactive with microwaves. The basic model equation for microwave-based moisture measurement can been seen in Model 1. Model 1. Where: mc: moisture content by weight (%w) M: wet mass of material (Kg) α: microwave amplitude attenuation (dB) φ: microwave phase-shift (rad) α0, bi: model parameters The model does not consider ore density explicitly. Although different densities change microwave amplitude and phase in different ways, this effect is embodied by the mass M. The calibration of the model above is developed as follows: ore samples are dried, weighed, and then artificially moistened by the addition of known masses www.e-mj.com