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mixingtank

Control model for optimizing the set point of froth depth

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Flotation is an important part of coal preparation, and the flotation column is widely applied as efficient Flotation Equipment. This process is complex and affected by many factors, with the froth depth and reagent dosage being two of the most important and frequently manipulated variables. This paper proposes a new method of switching and optimizing control for the coal flotation process. A hybrid model is built and evaluated using industrial data. First, wavelet analysis and principal component analysis (PCA) are applied for signal pre-processing. Second, a control model for optimizing the set point of the froth depth is constructed based on fuzzy control, and a control model is designed to optimize the reagent dosages based on expert system. Finally, the least squares-support vector machine (LS-SVM) is used to identify the operating conditions of the flotation process and to select one of the two models (froth depth or reagent dosage) for subsequent operation according to the condition parameters. The hybrid model is developed and evaluated on an industrial coal flotation column and exhibits satisfactory performance.

Development and use of froth flotation as a beneficiation process has been ongoing since the first part of the last century. Initial study of the flotation concept was in the late 19th century. The basic process involves the selective coating of a particle's surface to alter or enhance its surface chemical characteristics. The flotation process is widely used for treating metallic and non-metallic ores. A greater tonnage of ore is treated by flotation than by any other single process. Practically all the metallic minerals are being recovered by the flotation process and the range of nonmetallic minerals is steadily being enlarged.

A fragrant aroma unlike anything that I had ever smelled was leading me to long banks of green and yellow machinery. I heard sloshing sounds like a long row of washing machines, all agitating at once. I peered into one of these odd looking contraptions. I saw a stream of bubbles with a gray metallic sheen overflowing quickly towards a collection point. Lead and silver minerals were coating these bubbles, and I marveled at the sight. We then went to another row of these machines, and I was greeted by the sight of bubbles covered in zinc sulfide flowing down a long groove. The floating metals ultimately sank downward in a large tank, were collected, then dried on a rotating drum. This was my first encounter with the critical processes known as froth flotation.

A new high-capacity flotation technology, the StackCell, has been developed as an alternative to both conventional and column fotation mchine. This technology makes use of a pre-aerated, high-shear feed canister that provides efficient bubble-particle contacting, thereby substantially shortening the residence time required for coal flotation. Other potential advantages of the process include low air pressure requirements, low capital and installation costs, and increased flexibility in plant retrofit applications.

In Flotation Machine, the operation takes place in a highly turbulent flow. Therefore, the modelling as well as the optimization of a flotation process necessitate the application of essential results of the statistical turbulence theory, where an extensive simplification of the complicated laws is typical for the application in processing. Three effects of turbulence are important in flotation: the turbulent transport phenomena (suspension of particles), the turbulent dispersion of air and the turbulent particle–bubble collisions. While the transport phenomena are mainly caused by the macroturbulence, the microturbulence controls the two last-named microprocesses. In the paper a brief introduction of the theoretical background is given as far as it is necessary for modelling. The effect of turbulence damping by fine particles is also discussed. Models of the microprocesses air dispersion and particle–bubble collisions are presented, and it is clearly demonstrated that the particle–bubble attachment almost exclusively occurs in the zone of high energy dissipation rates, i.e., in the impeller stream. Further on, it is shown that the entrainment of fine particles into the froth lamellae is a result of the suspension state and, therefore, can be influenced by the design of the turbulence generating system (impeller–stator system). Finally, it is demonstrated that there is no feasibility to achieve optimum hydrodynamics for all particle sizes simultaneously. For coarse particle flotation, the power input should be minimized (generation of coarser bubbles; stronger buoyancy and lower turbulent stresses acting on the particle–bubble agglomerates!). In contrast to this, fine particle flotation requires high turbulent collision rates, i.e., a higher power input.

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