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MEI Online: Froth Flotation: Conference Reports: Flotation and Flocculation

 
 

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Flotation and Flocculation, from Fundamentals to Applications
Strategic Conference and Workshop
Kailua-Kona, Hawaii, USA, 28 July-2 August, 2002

The meeting was held at the Ohana Keauhou Beach Resort, a beautiful and quiet place located in the Big Island of Hawaii. This conference and workshop dealt with cutting edge research in the science and engineering of two of the world’s key separation processes. Flotation and flocculation are extensively used in the material supply chain for technologies that are relevant to telecommunications, fuel cells, ceramics, waste removal/recycling processes and the minerals industry, for example.

It was an appropriate opportunity to combine the recent advances on process fundamentals with performance and design of industrial flotation/flocculation cells, at a focused conference involving some of the world’s leading researchers, and practitioners, drawn from academia, research institutions and industry. A total of 54 participants from 12 countries attended the oral presentations and panel discussions. The four day conference included 10 plenary lectures of keynote speakers and 20 presentations of target speakers.

1.- The first day was devoted to bridging flocculation in wastewater and fine particle treatment processes.

Dynamic monitoring of floc formation, breakage and re-growth has shown a more detailed information on these processes than a simple jar test procedure. Polyelectrolyte-assisted separation of suspended solids from aqueous phases and their subsequent dewatering was used to gain a perspective on the current state of knowledge of flocculation and dewatering. According to the author, this assessment was meant to raise some dialog on opportunities for improving the understanding, and to provoke responses to the question “do we yet know enough to be dangerous or helpful on the industrial scale ?". Upcoming flocculation and flotation techniques and equipment as well as their potential in different science and engineering fields, namely in the treatment of liquid effluents and recovery of mineral fine particles were analyzed. Critical issues on dispersion/flocculation and selective flocculation studies as well as stability and kinetics of flocculation by polymers were also matter of discussion.

Unconventional flocculation and flotation techniques applied to mineral processing were reviewed as potential alternatives to recover fines and ultrafines particles. A case of study was the first plant installed in Chile for mining effluent treatment by DAF (dispersed air flotation).

2.- Day two was focused on advances on bubble-particle encounters/models in flotation.

Experimental observation has shown evidence that two mechanisms can be responsible of rupture process of liquid films: the growing fluctuation waves on fluid interfaces and nucleation inside the film. It seems reasonable that collection of hydrophobic particles will preferably occur through micro-bubble formation on the mineral rather than attachment of existing bubbles. Thus, tiny bubbles forming on the particle surface facilitate the attachment of larger bubbles and in this way activate flotation by additionally cluster formation.

A general flotation model that allows to relate turbulence and collision to attachment and detachment has been developed. On the other hand, force measurements using an AFM (atomic-force-microscope) has been used to study the particle-particle and bubble-particle interactions, curves for Ea (energy barrier) for the coagulation and flotation are of the same shape, suggesting that both are controlled by the same mechanism, i.e., hydrophobic interaction. Also, a colloidal probe technique was used to measure forces to study the boundary slip of water on hydrophilic surfaces and electro-kinetic effects.

In a laboratory flotation column, it was observed that there exists a particle contact angle for which froth phase recovery is maximum (i.e. contact angles around 63º). This finding confirms that particle hydrophobicity plays a key role in bubble coalescence and froth stability. However, the interactive effects of particle characteristics such as hydrophobicity, particle size and shape, on froth stability and froth recovery remains as a challenge to froth phase modeling.

Analysis of the flotation kinetic process, using a first-order attachment-detachment model, suggests that an improved flotation cell would be sparged with small bubbles and operated at lower, more uniform turbulent energy dissipation rates.

Studies on particles movement near bubbles using high speed video microscopy showed that the bubble-particle sliding attachment interaction is not satisfactorily described by the available theories, and that the water film was probably ruptured due to the submicrometer-sized gas cavities existing at the hydrophobic particle-water interface.

The attachment efficiency of quartz particles with nitrogen bubbles was estimated, at different operating conditions, from experimental collection efficiency measurements and collision efficiency calculated with the Generalized Sutherland Equation model (GSE).

3.- Day three covered the area of flotation cells and their operation.

Experimental studies in a laboratory flotation column have shown that the froth phase is an efficient collection zone as well, especially if hydrophobic particles are fed into the froth, avoiding the turbulence of the pulp. Thus the flotation particle size range can be extended greatly, especially for collection of coarse particles. A model for drainage in a rising froth was developed and tested with fluorescein tracer in a 2-dimensional froth column.

Separation performance strongly relies on froth stability, in particular the grade, which depends critically on bubble coalescence and bursting. A methodology for calculating the stability of froths using information about the stability of individual films has been proposed.

From laboratory and industrial tests it was observed that the quality of the top of the froth corresponds to the maximum attainable grade of the ore being floated. This parameter has been used to characterize the grade-recovery curves in laboratory batch and continuous plant operations .

The loss of considerable amounts of valuable minerals in tailings generated in flotation processes are strongly related to the presence of fine particles (i.e. less than 10-20 microns). Now because of depletion of old deposits, there are more operations with low mineral grades and or fine disseminated ores, then the task of fine particles flotation becomes more relevant and attractive. It is believed that, the problem of selective flotation of fine minerals can be partially solved with the help of “turbulent microflotation”, using the experience from flotation water purification.

Dissolved air flotation DAF, using microbubbles of less than 100 microns, is used for removal of suspended solids from water and waste water treatment. This means that suspended solids down to a particle size of 5-10 microns are removed effectively from water. Recent developments using turbulent flow conditions, in DAF processes, showed a better control of hydraulics in comparison with conventional DAF units operating with laminar flow.

The current status of column flotation regarding practical aspects on process design, modeling and control was discussed. New applications of pneumatic flotation, i.e. Jameson cell, for waste water treatments in food factories were also presented.

4.- Day four covered the topic of diagnostic process chemistry in flotation and flocculation.

Statistical methods based on time of flight secondary ion mass spectrometry (ToF-SIMS) has been applied for diagnostic surface analysis in sulfide flotation. Recently in-situ FTIR/IRS technique has further extended the understanding of adsorption phenomena and the state of the adsorbed collector in flotation chemistry of non-sulfide minerals, such as semi-soluble salt minerals such as phosphate minerals, fluorite, calcite and barite. Dynamic surface tension measurements (DST) have been applied to study the adsorption of different types of surfactants and polymers at the air/water interface, using the maximum bubble pressure method (MBPM).

Practical aspects of the effect of electrochemical conditions in grinding mills on the flotation process has been evaluated at laboratory scale. Particularly the reduction of anodic dissolution of iron by using grinding balls with elevated chromium levels. For example, the replacement of iron or steel media containing elevated chromium levels is relatively common in tower mills used for regrinding in Australia.

Finally, the need of flocculation for particle size enlargement in mineral processing circuits was analyzed. The application is required to improve both the particles separation, by selective aggregation, and dewatering using non-selective aggregation.

In conclusion, despite the significant advances on process fundamental knowledge that have taken place alongside with major engineering investigations of the performance and design of industrial flotation/flocculation cells, further work has to be done to answer some key questions, i.e. metallurgical scale-up, froth transportation and ultrafine and coarse particle recovery, specially in large flotation cells. Thus, new efforts must be oriented to fill the gap between scientific knowledge and industrial requirement.

At the time the conference was running the eruption of a volcano located around 120 km was at the center of the news. After the conference, tours to see the volcano's area and other beautiful landscapes closed a fruitful week.

Dr. J.B. Yianatos, Santa Maria University, Chile, Juan.Yianatos@pqui.utfsm.cl

 

 

   

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