A study of turboflotation, a novel high intensity froth flotation system

Ofori, Philip Kwadwo. (2003). A study of turboflotation, a novel high intensity froth flotation system PhD Thesis, School of Engineering, The University of Queensland.

       
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Author Ofori, Philip Kwadwo.
Thesis Title A study of turboflotation, a novel high intensity froth flotation system
School, Centre or Institute School of Engineering
Institution The University of Queensland
Publication date 2003
Thesis type PhD Thesis
Supervisor Howes, Tony
Firth, Bruce
Total pages 240
Collection year 2003
Language eng
Subjects L
290000 Engineering and Technology
670000 - Manufacturing
Formatted abstract TurboFlotation is a novel fi-oth flotation system in which the flotation sub-processes of aeration, particle-bubble contacting and fi-oth-pulp separation are intensified in order to achieve increased process efficiency. The reactor-separator concept is used. The process intensification is achieved by utilising a vertical upward jet ejector, a static mixer and a centrifugal separator.

The critical hydrodynamic parameters of the high intensity sub-process units used in the aeration, mixing and separation steps in a novel fast flotation system have been systematically studied and reported in this dissertation. The study is important in providing insights into this multiphase flow system, which coupled with innovative process design concepts have provided the opportunity to develop an elegant, cost effective, and compact high intensity flotation process. There is the prospect of achieving large reductions in the physical size of flotation and other process plants while maintaining production targets and product quality. The outcome would be reduced capital and operating cost and energy savings. The important hydrodynamic phenomena in the sub-process units are described in this dissertation, and the feasibility and advantages of this approach to froth flotation demonstrated.

The factors governing the air entrainment by the liquid jet ejector have been examined by varying the nozzle geometry, jet velocity and operating conditions. Two approaches have been used in the organization and analysis of the data. A dimensional analysis approach showed that air entrainment capacity before the onset of bubble to slug flow transition, was dependent on jet Froude number and ejector geometry. An analysis of the jet break-up and air entrainment was also conducted by application of the laws of conservation of mass, momentum and energy to obtain an expression for the mixing energy dissipation rate. The mixing energy dissipation rate was then compared with the total surface energy of bubbles generated to obtain a model that defines the transition boundary from bubble flow to slug/chum flow. For a given air addition rate, the maximum stable bubble size could also be predicted by the model. The model provides a good fit to the experimental data.

For a more complete picture of the resulting multiphase flow system, a detailed study of the internal structure of the flow was undertaken. Examination of the structure of the multiphase flow produced by the ejector and its suitability for the flotation application, in terms of interfacial mass transport capacity was carried out. Parameters selected for study included, bubble chord length distribution, gas void fraction, bubble surface area flux and Sauter mean bubble diameter using a dual-sensor resistivity probe. It was found that the void fraction peaked near the pipe wall in the absence of a frothing agent due to the wide range of bubble sizes produced. When a frothing agent was added, a more uniform void fraction distribution was observed.

A study was also undertaken to examine the effectiveness of the static inline mixer in promoting the mass exchange at the air-slurry interfaces to aid in the improved design of the TurboFlotation unit and similar systems. Liquid side mass transfer coefficients were determined for air-water flow in the static mixer as well as in the tube without any mixing elements. A correlation was developed to determine the volumetric mass transfer coefficient from the power input per unit mixer volume and gas void fraction. The effectiveness of the static mixer for turbulence and mass transfer augmentation in this application was confirmed.

A study of the liquid and gas phase dispersion in the separator of the TurboFlotation units was undertaken using residence time distribution techniques. It is highly desirable to minimize the residence time in the separator; however it is important to ensure that recovery and selectivity are not compromised. A model consisting of a small plug flow section in series with a modified tanks-in-series model adequately described the liquid phase residence time distribution (RTD). The model gives a good description of the measured RTD curves. The gas phase dispersion was adequately described by the axial dispersion model. Factors that controlled the gas-phase dispersion were identified as the liquid superficial velocity, diameter of the separator and concentration of frothing agent. Gas-phase dispersion increased with increasing liquid superficial velocity and increasing separator size.

A strategy for scale-up of the TurboFlotation cell is proposed based on geometric similarity of the separator, constant mean residence time in the separator and constant separation force in the separator. For the inline static mixer, constant power per unit volume is used as the scale-up criterion with the aim of attaining similar mass transfer rates. The jet ejector scale-up was based on maintaining constant non-aerated Froude number and specific mixing energy dissipation rates. On the basis of this scale-up strategy and the process data of a 0.09 m diameter pilot plant, a 0.3 m diameter development-scale unit was designed and tested. Metallurgical performance equivalent to that achieved by conventional flotation systems was achieved at an order of magnitude higher capacity than conventional technology. The design and operating data acquired formed the basis of scale-up to a 1 m diameter unit with a capacity of 600 m3/h. Testing of this unit at a mine site produced reasonable metallurgical results. The technical feasibility of the high capacity TurboFlotation system has been demonstrated on a large scale.
Keyword Flotation
Separation (Technology)

 
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