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Flotation 

 

Summary

This review looks at the basic concepts of flotation, including their principle and usage. It also mentions suppliers, consultants, and some important books.

INTRODUCTION

Flotation is a mineral separation process done in the water medium. It is based on the difference in the surface properties of the mineral and gangue. The surface of the selected mineral is made hydrophobic (water repellent) by the use of selective reagents and these particles get attached to the air bubbles that are introduced in the system and collected as froth whereas the hydrophilic (wetted) particles are left behind in the slurry.

The separation process produce better results if the particles are well liberated which is achieved by grinding the ore. The recovery of the particles increases with the decrease in particle size to a limit and then it again recovery starts decreasing.


THEORY

Flotation is a physical-chemical process that uses the difference in the surface properties of the wanted and unwanted particles. It involves a three-phase interaction between solid, liquid and air.

For a given mineral system,

So, according to the equation, condition for hydrophobicity for,

That means

Where, WSL is the work of adhesion between the solid and the liquid whereas, WLL is the work of cohesion for water.

The work of adhesion of liquid to solid (WSL) can be divided as van der Waals dispersion forces contribution, hydrogen bond contribution and electrical forces contribution. Thus, depending upon the mineral particle, its molecular structure and its intermolecular forces, the work of adhesion varies and so does the contact angle, thus, making a particle more or less hydrophobic.


FLOTATION SURFACTANTS

Surfactants are the surface-active agents, which adsorb on the mineral surface and change the mineral characteristics. This can be classified into different groups:

1. Collectors: These are the chemicals that adsorb on the mineral surface and make it hydrophobic. According to Professor Laskowski, collectors can be classified as,

2. Activators: If the mineral surface doesn't adsorb a collector, special reagents are added to the system, which allow the collector to bind the mineral surface. An activator normally works by adsorbing onto the mineral surface, thereby providing sites for the adsorption of the collector species.

3. Depressants: The purpose of depressants is to inhibit or retard the flotation of a desired type of particle. It either inhibits the collector molecule from adsorbing onto the particle surface or to mask the adsorbed collector species from the bulk solution so that the particle does not display a hydrophobic property. Some examples are starch, tannin, silicates, Al salts and dextrin.

4. Dispersants: These are used to prevent fine particle aggregation. It gets adsorbed on the surface and thus increasing the strong columbic repulsion forces and preventing particles to aggregate. Some examples are Dextrin, starch, Guar gum etc.

5. pH Modifiers: Flotation process is carried out a given pH as it determines the electrical charge on the particle surface thus effecting the flotation. Mostly Lime, NaOH, Soda ash or sulfuric acid is used for controlling pH depending upon the requirements and mineral properties.

6. Frothers: Frothers are added to the system to change the bubble characteristics and stability of the foam/froth. Commonly used frother are pine oil, MIBC, polyglycols etc.


CLASSIFICATION OF MINERALS FOR FLOTATION

M. A. Eagles classified all minerals into different categories based on their surface properties:


FACTORS AFFECTING FLOTATION

There are numerous factors affecting flotation. Some important factors are listed below:
  • Particle size and density
  • Pulp density
  • Air bubble size
  • Agitation intensity
  • Conditioning time
  • Residence time
  • Nature of Particle (Particle surface charge)
  • pH of slurry
  • Type of reagent
  • Amount of reagents
  • Frother stability
  • Amount of Wash water

FLOTATION MACHINES

There are numerous types of flotation equipment available and that can widely be classified as mechanical and non-mechanical cells.

Mechanical cells use impellers to circulate the pulp and keep the solid suspended in the solution and to draw adequate quantity of air to create bubble. These cells can be subareation/self-aerating (self-air drawing) and supercharged/pneumo-mechanical cells (external blower). Example is Denver cells, Wemco cells etc.

Non-mechanical cells don't have the impeller. This can be Pneumatic cells which rely on compressed air to agitate the pulp and Air precipitation cells in which bubbles of air precipitate from solution which can be achieved by decreasing pressure above the pulp (Vacuum machines) or oversaturating the pulp with pressurized air. Very good examples are column cells and Jameson cells.

For more information about flotation cells, check the METSO flotation machines.


RECENT DEVELOPMENTS IN FLOTATION

Flotation technology is always evolving and new developments are made. Most of the results are based on the reagents developments. The development of better collector and frother are made. Also, the chemistry of reagents is been studied to check the effect and mechanism of actions.

In the world of bubble generation, more micro bubbles are trying to been generated. Decreasing the bubble size increase the efficiency of flotation. For more complex mineralogy, better techniques are been developed. Also, research is going on to make the flotation in seawater for copper flotation in Chile.

In term of Flotation equipment, the cell size is getting bigger. FLSmidth developed the Mechanical cells of 300m3 to 350m3 volume (Supercell). And research is also going on the applicability of column flotation as rougher cells.


SUPPLIERS

There are numerous numbers of flotation equipment suppliers are available. InfoMine's mining suppliers database has a long list of flotation cell suppliers.
  • Metso Flotation cell
  • FLSmidth
  • Machinery & Equipment Co., Inc
  • Global Sources
  • Alibaba.com
  • Outotec
  • Shanghai Minggong heavy Equipment Co. Ltd.
  • Hongxing Machinery, China
  • Delkorglobal
  • Eriez Flotation cells
  • Consep (Bateman Flotation cells)

BOOKS AND MAGAZINES

The best book available for the basic study for flotation is Mineral Processing Technology by B A Wills. Some others books for the flotation are: There are some very good information provided at the following links:

There are some conferences where a lot of information can be found about recent developments. Here are some of them:

Courses on flotation from EduMine include:


SOFTWARE

There are some software available for flotation machine design and process optimization.

CONSULTANTS

You can find a list of flotation consulting companies in the InfoMine consultants database. Some popular consultants are:

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