COARSE AND FINE PARTICLE FLOTATION
Mineral particle size is an important parameter in froth flotation.In 1931,Gaudin,et al. showed that coarse and extremely fine particles are more difficult to recover by froth flotation as compared to intermediate size particles.This was approved by Morrisin 1952.Generally,the flotation recovery and efficiency deteriorate rapidly when operating in the extremely fine(< 10μm)or coarse particle size ranges( >250μm).For minerals such as phosphate,the existing conventional flotation practices are optimal only for the recovery of particles in the size range of about 45 to 250μm. The improved flotation performance of coarse and fine particles has long been a goal in the minerals processing industry.A lot of efforts have been made to overcome the inefficiencies associated with processes and equipment.
To overcome the limitations of traditional flotation cells in recovering coarse particles,a new separation device,the Eriez HydroFloat separator has been developed.The HydroFloat cell separates particles based on the apparent density differences between hydrophilic particles and particle-bubble aggregates after the selective attachment of air bubbles to the hydrophobic component of the feed stream.The HydroFloat separator is different from teeter-bed separators that are commonly used in minerals industry.
HydroFloat separators overcome the shortcomings of traditional teeter-bed separators and flotation cells by combining their advantages.Figure 1 schematically shows the full-scale HydroFloat separator,which consists of an upper separation chamber and a lower dewatering cone.The particles to be separated may be naturally hydrophobic or made hydrophobic through the addition of flotation collectors.Pulp feed enters near the top of the separation chamber.The HydroFloat separator operates like a traditional hindered-bed separator with the feed settling against an upward current of fluidization (teeter)water.The teeter water is supplied through a network of pipes that extend across the bottom of the entire cross-sectional area of the separation chamber.In addition to the teeter water,the HydroFloat separator is continuously aerated by injecting compressed air and a small amount of frothing agent into the fluidization water.The rising air bubbles will attach to the hydrophobic particles and reduce their effective density.The lighter bubble-particle aggregates rise to the top of the denser teeter bed and overflow the top of the separation chamber.Unlike flotation,the bubble-particle agglomerates do not need to have sufficient buoyancy to rise to the top of the cell because the teetering effect of the hindered bed forces the low-density agglomerates to overflow into the product launder.Hydrophilic particles that do not attach to the air bubbles settle down in the teeter bed and are eventually discharged at the bottom of dewatering cone.
For fine particle flotation,column flotation cells were introduced to the market place as devices capable of producing concentrates that were lower in impurities than those produced by other types of flotation machines.The ability to operate columns with deep froth beds and to wash the froth was the main reasons cited for the improved metallurgical performance.In recent years,many phosphate producers have installed column flotation systems as a means of boosting production whilst reducing operating costs.The size range of recoverable apatite particles has been extended from about 30μm down to 5μm through the introduction of column flotation.The high degree of selectivity achieved by this equipment has made it economical to treat material previously considered to be tailings.
The depletion of high grade reserves coupled with increasing market pressure for improved product quality has forced iron ore producers to re-examine their process flowsheets and evaluate alternate or supplemental processing routes.The requirement for higher quality pellets demands that the silica content be lowered to levels ranging from 2.0%SiO2 to below 1.0%SiO2.Reverse flotation (silica is floated away from the iron concentrate)has proven to be an economical and effective method for reducing the concentrate silica content to very low levels.Laboratory and commercial test-work has demonstrated some significant metallurgical and economic advantages when column flotation cells are used for this application.Excellent metallurgical performance[6-8]along with low capital and operating costshas made column flotation popular in the mineral processing industry.For iron ore applications,the ability to wash the froth has provided a means of obtaining low concentrate silica levels while keeping iron losses to a minimum.Recent cost comparisonshave shown that the cost of installing a column flotation circuit is typically 20%-30%less than an equivalent conventional flotation circuit but can be as much as 50%lower depending on the circuit and plant location.The Brazilian iron ore industry has led the world in adopting column flotation technology for reducing the silica content of iron pellets.CPT,a wholly owned subsidiary of Eriez Manufacturing,has been instrumental on supplying flotation columns to this industry.Several companies have installed,or are in the process of installing column cells into their process flowsheets.Samarco Mineracao S.A.,the first Brazilian producer to use column cells,installed columns to increase flotation capacity as part of a plant expansion programin 1990.Since that time,they have installed additional columns for the recovery of fine iron from the desliming circuit and for a recent plant expansion program.The application of column flotation for silica rejection is being actively investigated by several iron ore companies in Brazil,Canada,the United States,Venezuela,and India.