FLOTATION

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Presents by Group 6

History

Introduction

Basic Principles

Mechanics of Flotation

Chemicals of Flotation

Advantages and Disadvantages

References

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HISTORY

1869 - WILLIAM HAYNES patented a process of

separation of minerals and gangue using oil, which he

called “Bulk Flotation”.

1896-97 - FRANK ELMORE and STANLEY ELMORE

set up the “Glasdir Copper Mine” at Llanelltyd, Dolgellau

in North Wales, where they carried out the world’s first

commercial Flotation process in 1897.

1900(s) - In this era, C.V.PATTER and

G.D.DELPRAT independently, in Australia,

invented the “Modern Froth Flotation” process, in

which initially fatty acids and oil were the flotation

reagents to enhance hydrophobicity of the valuable

minerals.3

INTRODUCTION

Flotation is the separation of particles from a mixture by

causing some particles to collect on the surface of bubbles.

Floatation is an extractive process where various minerals

can be selectively extracted. For example, in poly-metal ores

such as Pb-Zn-Cu, floatation allows separate extraction of Pb,

Cu and Zn.

The process of separation of mineral includes three important mechanisms :

1. TRUE FLOTATION, i.e., selective attachment to air bubbles.

2. ENTRAINMENT in the water which passes through the froth.

3. AGGREGATION, i.e. physical entrapment between the particles in froth.

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CONT..

True flotation dominates the recovery of the valuable minerals and the other two decide the separation efficiency between the valuable and the gangue.

FLOTATION process can be applied to relatively fine particles, because if the particles are coarse and heavy, their weight will be greater than the adhesion between the particle and the air bubble and the particle will detach from the bubble.

There are two ways of flotation :

1. DIRECT FLOTATION- In which the mineral is attached to the froth and the gangue remains in the tailing.

2. REVERSE FLOTATION- in which the gangue is attached to the froth and minerals remain in tailing. 5

BASIC PRINCIPLES

This process commences with Comminution (to increase the surface area of the ore).

The ore is ground to fine powder and wetted with water to form a Slurry.

A Surfactant chemical (known as COLLECTOR) is mixed with slurry to render the desired mineral HYDROPHOBIC.

This slurry (now PULP) is then placed in the water bath containing FROTHER, which is aerated to create bubbles.

The desired mineral escape water by getting attached to the air bubbles, which rise to the surface and form what is called FROTH. This Froth is then removed and the concentrated mineral is refined. 6

AIR IN

MINERALISED

FROTH

PULP

AIR

BUBBLECELL

AGITATOR

MINERALS’ PARTICLES ATTACHED TO BUBBLE7

MECHANICS OF FLOTATION

The basis of Froth Flotation is the difference in the WETTABILITY of the mineral and gangue particles.

On the basis of Wettability of particles are classified as HYDROPHOBIC and HYDROPHILIC.

The valuable minerals can attach to the air bubbles , only if they are Hydrophobic. Once they reach the surface, due to the buoyancy of the air bubbles, the particle-bubble contact can sustain only if they form a stable froth.

The stability of the froth depends on the strength of the attachment of the bubble to the mineral surface. This strength can be estimated with the help of YOUNG-DUPRE EQUATION, which relates the strength of attachment to the interfacial energies. 8

Floatation Column Typical industrial column cell

comprises a steel tank equipped with a feed inlet pipe near the top of the column, a system of internal and external launders to collect and remove froth and a slurry outlet near the bottom of the tank to remove non-floating material.

Drain nozzle and a series of re-pulping nozzles are also located atthe bottom

Depending on the tank geometry, it may contain several internal baffles to control the mixing characteristics within the tank.

A gas sparging system used togenerate the bubbles required for the flotation process is located at the bottom of the vessel. A froth washing system, used to cleanse the froth of entrained impurities, is installed at the top of the tank.

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Cont… This process is used for

sulphide ores. Oils can wet sulphides. Oil floats on water. Sulphide ores are first ground to powder and water is added.

Then pine oil is added and the emulsion is agitated by passing compressed air. Oil and froth float on the surface along with the sulphide ore.

The gangue particles being insoluble in oil remain at the bottom of the water tank. The froth is removed and allowed to settle down.

This is called the froth-floating process. This process is used for sulphide ores of Cu, Pb and Zn 10

CHEMICALS OF FLOTATION

Chemicals are required,

1. To control the relative Hydrophobicities between the particles.

2. To maintain proper froth characteristics.

The different types of chemicals involved are

COLLECTORS

FROTHERS

REGULATORS,

ACTIVATORS

DEPRESSANTS

pH MODIFIERS 11

Collectors

These are Organic compounds used for enhancing the Hydrophobicities of the selected minerals, by Adsorption of its molecules or ions to the mineral surface and reducing the stability of the hydrated surface separating the mineral surface and air bubble.

They are added to the Pulp, and sufficient time for adsorption is provided during agitation. This period is known as the CONDITIONING PERIOD.

The different types of Collectors are tabulated as follows,

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NON-IONISING IONISING

ANIONIC CATIONIC

(LIQUID NON-POLAR HYDROCARBONS,

INSOLUBLE IN WATER.)

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These are required to enhance the hydrophobicities of the partially hydrophobic minerals surfaces (e.g. coal), by selectively adsorbing on their surface.

Fuel and Kerosene oil are some of the non-ionic collectors.

These have complex molecules, which are assymetric in nature and are Heteropolar, i.e., molecules have a non-polar hydrocarbon group (which is water repellant in nature) and a polar group (which reacts with water).

Ionic collectors are classed into:-

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These collectors possess non-polar and a polar group in the Anionic part, and the Cationic part has no significant role in the reagent-surface reaction.

Carboxylates (OXYHYDRYL):-

These are also called Fatty Acids or Soaps.

Examples are salts of oleic acid and linoleic acid.

Soaps have an advantage over other ionic collectors that though they have long carbon chains, they are soluble in water.

These are strong in nature and have low selectivity.

They are used for flotation of Ca, Ba, Sr, Mg and salts of alkali and alkaline earth metals. 15

POLAR

GROUP

NON-POLAR GROUP

CATIONANION

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CATIONANION

POLAR GROUPNON-POLAR GROUP

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These collectors have the Cationic part as their significant role player in the reagent-surface reaction.

The Polar group is based on PENTAVALENT NITROGEN (commonly amines).

They follow the principle of Physisorption and attach to the mineral surface through electrostatic attraction. Hence, they are weak collectors.

Active in slightly acidic solutions and inactive in strongly alkaline and acidic media.

There requirement can be reduced by adding a non-polar agent (eg. Kerosene), that gets pre-adsorbed.

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Frothers These are heteropolar surface-active reagents capable

of being adsorbed on the air-water interface.

This chemical has the following functions:

1.To stabilize the formation of bubble in the Pulp phase.

2.To create a stable froth to allow selective drainage from the froth of entrained gangue .

To increase the flotation kinetics.

A good Frother should have negligible collecting properties and should form such a froth, which is stable enough to transfer of floated mineral from cell to the collecting launder. They should have enough solubility in water, so that they are evenly distributed & effective. 19

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The most effective frothers include Hydroxyl, Carboxyl, Carbonyl, Amino group and Sulpho group in their composition. Alcohols having no collector properties is preferred over other frothers.

There are two types of frothers,

NATURAL (eg. Pine Oil, Cresol etc.)

SYNTHETIC (eg. MIBC [Methyl IsoButyl Carbinol], Cytec Oreprep 549)

The synthetic frothers are much stable in their composition and thus advantageous over the natural.

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These reagents Activate the mineral surface towards the action of the Collectors, by altering their chemical properties. Thus, they are referred as the FRIENDS of Collectors.

They are soluble salts which get ionised easily and the ions react with the mineral surface.

A classical eg. of Activation is in case of the SPHALERITE ORE.

Xanthates cannot effectively float the ZnS ore particles, due to the formation of readily soluble Zinc- Xanthate compound on the surface.

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These reagents Deactivate the mineral surface towards the action of Collectors, by altering their chemical properties. Hence, they are referred as the ENEMIES of the Collectors.

Their typical use is to increase the Selectivity of flotation, by preventing one mineral from flotation while allowing other mineral to float unimpeded.

There are mainly two categories of depressants,

INORGANIC [eg. Sodium cyanide, Zinc sulphateetc.]

ORGANIC (or POLYMERIC) [eg. Starch, tannin, Quebracho, Dextrin etc.]

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Cyanides are the most commonly used in the selective flotation of Lead-Copper-Zinc and Copper-Zinc systems as Depressants for Sphalerite and pyrite ores.

An eg. of the Cu-Zn system can be considered to understand the action of a Depressant,

The Cu ions present in the mineral leads to unintetional activation of the Sphalerite (i.e. Zn mineral) and thereby preventing selective flotation.

Hence, Sodium Cyanide is added to Desorb the surface Copper and react with the Copper ions in the solution to form soluble complexes.

The reactions proceed as follows, 24

These are advantageous as they are less hazardous than the inorganic ones.

They do not ionize in the solution, but prevent flotation forming a thin coating over the mineral particles.

They are used in less amounts for depressing Talc, Graphite and Calcite.

Starch and Dextrin also act as supplementary Lead depressants in Copper-Lead systems.

Other applications include the selective depression of polymetallic sulphide ores in the processing of iron ore.

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The selectivity in complex flotation processes is dependent on a delicate balance between the reagent concentration and the pH.

This pH factor is modified with the help of the substances called pH MODIFIERS. Alkalinity in a solution is maintained by the addition of Lime, Sodium carbonate, and to a lesser extent NaOH and Ammonia. Sulphurous and Sulphuric acids are used to lower the pH.

Lime is widely used in the form of Milk of Lime, to maintain the pulp alkalinity. It is added to the slurry prior to the flotation. It precipitates heavy metal ions from the solution thereby acting as Deactivators. But they dissociate to a greater extent into OH- and Hydrogen ions, which further modify the zeta potential and hence, the floatability of the mineral.

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For any concentration of Collector, there is a CRITICAL pH value, below which a given mineral will float and above which it will not.

This critical value depends on the mineral’s nature, collector’s nature and concentration, and the temperature.

CO

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CO

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(in

mg/L

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pH VALUE

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ADVANTAGES OF FLOTATION OVER SEDIMENTATION

High rise velocity permits small tankage.

Ability to handle variable solids loading (can adjust

air flow).

Can provide high float concentration (good

thickening).

Can remove low density particles which would

require long settling periods.

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DISADVANTAGES

Capital costs

Energy, operating costs

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REFERENCES

Wills, B.A. and Napier-Munn Tim, “Mineral Processing Technology”, Elsevier(2005)

Kavatra, S.K., “Flotation Fundamentals”

Ray, H.S. and Sridhar, R., “Extraction of Non-Ferrous Metals”

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