F R OTH F

LO AT NT I O

PREPARED BY:-

SWAPNIL

NIGAM

GUIDED BY:-PROF. M.K.MANOJ

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.

FLOTATION is a physico-chemical separation process that utilizes the difference in the surface properties of the valuable and gangue minerals.

FROTH FLOTATION involves three different phases- solid (fine ore powder), liquid (water) and froth.

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.

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.

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.

AIR IN

MINERALISED FROTH

PULP

AIR BUBBLE

CELL

AGITATOR

MINERALS’ PARTICLES ATTACHED TO BUBBLE

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.

BUBBLE

SOLID

WATER

ΓW/A

ΓS/WΓS/A

θ

ACCORDING TO YOUNG-DUPRE EQUATION,

ΓW/A COSθ ΓS/WΓS/A= -

WHERE, in the above equation,

ΓW/A, ΓS/A, ΓS/W

are the SURFACE ENERGIES between water-air, solid-air, solid-water interface and θ is the CONTACT ANGLE.

NOW, let WS/A is the WORK OF ADHESION, i.e. , the force required to break the particle-bubble interface, THEN,

WS/A = ΓW/A + ΓS/W – ΓS/A

WS/A = ΓW/A - ΓW/A COS θ

WS/A = ΓW/A (1- COS θ)

From the above Equation, it can be seen that, Greater the Contact Angle ; Greater is the WS/A , i.e. WORK OF ADHESION between particle and bubble and thus more resilient is the system to the disruptive forces. Contact Angle around 90° is sufficient. If the bubbles are large enough in SIZE relative to the particles, thereby increasing the surface area of the bubble, which causes more fluid to enter into the froth, which leads to Entrainment. Therefore, the bubble diameter must be comparable to the particle diameter, to ensure a good contact between them.

Also, the stability of the froth must be not too high, as it can lead to the formation of persistent Foam, which is difficult to convey and pump through plants.

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

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,

NON-IONISING IONISING

ANIONIC CATIONIC

OXYHYDRYL SULPHYHYDRYL

SULPHATES SULPHONATES

DITHIOPHOSPHATESXANTHATES

CARBONATES

(LIQUID NON-POLAR HYDROCARBONS, INSOLUBLE IN WATER.)

There are two ways of adsorption of collectors,

1. CHEMISORPTION

2. PHYSISORPTION

In Chemisorption,

Ions or molecules from solution forms irreversible bonds with the surface, through chemical reaction.

It is a highly specific process and thus more selective.

In Physisorption,

Ions or molecules from solutions reversibly attach to the surface, either by electrostatic attraction or van der Waals bonding.

It is less selective.

POLAR GROUP

NON-POLAR GROUP

MINERAL

The Collectors should be used in very small concentration, because,

It adversely affects the recovery of the valuables, due to the development of multi-collector layers on the surface, thereby reducing the proportion of the hydrocarbon part oriented toward the bulk solution, which reduces the hydrophobicity.

It increases the cost.

It tends to float other minerals thus, reducing selectivity.

Long chain collectors should be used, so as to increase the floatability of the surface. Generally, the chain length is limited to two to five, since solubility in water decreases with the increase in chain length. Also, branched chains have higher solubility then straight.

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:-

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.

POLAR GROUP

NON-POLAR GROUP

CATIONANION

Sulphonates and Sulphates (OXYHYDRYL):

These have lower collecting power and have greater selectivity.

They are used for flotation of Barite, celestite, fluorite, apatite, chromite, cassiterite, mica, kyanite and scheelite.

Xanthates (SULPHYHYDRYL):

They are most widely used THIOL collectors. Also called Xanthogenates.

They are formed by reacting Alkali Hydroxides (eg. KOH), Carbon Disulphide (CS2) and Alcohol (ROH).

They contain normally 1 to 6 Carbon atoms.

Sodium (the cationic part) Alkyl Xanthates decreases in efficacy with age.

CATIONANION

POLAR GROUPNON-POLAR GROUP

Adsorb chemically on the sulfide mineral surface and form insoluble metal Xanthates

Used for collection of oxidised ores like malachite, cerrusite, anglesite and native minerals like gold , silver.

Dithiophosphates (SULPHYHYDRYL):

Comparatively weak collectors. Possess pentavalent Phosphorous in the polar group.

Also called Aerofloat Collector. They are effective selective collectors for Copper sulfide minerals.

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.

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.

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.

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.

Hence, Copper Sulphate is used as an Activator in this case. The reaction proceeds as,

The Copper Sulphide film on the mineral surface now allows a stable and easy flotation of the Sphalerite ore by the Xanthate collector, as the Copper Xanthate compound so formed is insoluble in water.

The oxidised minerals such as Cerrusite, Smithsonite, Azurite and Malachite require Sodium hydrosulphide and Sodium sulphide as the Activators. The amount of these Activators depends on the pH of the solution.

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 sulphate etc.]

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

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,

The dissociation constant of the first reaction is high as compared to that of the second, and thus, the alkalinity of the solution increases and the free HCN decreases thereby, producing more and more CN- ions. The major function of alkali is to control the amount of CN- ions, available for the dissolution of Copper to Cupro-cyanide,

CUPRO-CYANIDE (soluble complex)

Also, cyanide ions can react with the Metal Xanthates to form soluble complexes, thereby preventing the adsorption of Xanthates on the mineral surface (in the above case Zn mineral), although this cannot occur until the metal ions (in the above case Copper) have been complexed. In the above example, unless the ratio of CN-to Cu2+ is greater than 3:1, the prevention of Xanthate adsorption cannot occur.

The more soluble a Metal Xanthate is in cyanide, less is the attachment of Xanthate to that mineral.

The depressive effect of cyanide depends on its concentration, the collector concentration and the hydrocarbon chain length. Longer the chain, greater is the solubility of the metal Xanthate in cyanide and greater will be the amount of cyanide reqd..

Zinc sulphate is also used in some cases.

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.

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.

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

LLEC

TO

R’S

C

ON

CEN

TR

ATIO

N (

in m

g/L

)

pH VALUE

The role of Bubble is one of the major components in the science of flotation process.

GORAIN showed that the First order rate constant (k) for various industrial flotation cells depends on,

1.Feed ore Floatability (P),

2. Bubble Surface Area Flux (Sb ),

3. Recovery in the froth phase (Rf ).

They are numerically related as,

k = P RfSb

The particle Floatability depends on the Degree of Hydrophobicity, the bubble Surface Area Flux is a key driver within the pulp zone of a flotation unit and the Froth Recovery describes the froth zone performance.

The bubble Surface Area Flux is the rate at which a bubble surface moves through a cell per unit of the cell cross-sectional area.

Sb=

Where,

Jg = Superficial Gas Velocity (m/s)

and db = Sauter Mean Bubble Diameter (m)

The industrial Flotation practice requires several stages to produce market quality products. These stages are combined in various methods and are called the FLOTATION CIRCUITS.

A preliminary Laboratory test work is carried out before the built-up of a Circuit for a specific ore, in order to determine the suitable reagent and the size of the plant for given throughput and the flowsheet and peripheral data.

Also, along with the Laboratory test work, another test called Pilot Plant Test work is also carried out in order to determine, the continuous operating data for the design (planned on the basis of Lab test work), the comparison in equipment cost and performance etc.

The commercial Flotation is a continuous practice. CELLS are arranged in series forming a BANK. The pulp enters the first Cell and give some of its valuable minerals in the froth; the overflow from this cell passes to the second cell, where more mineralised forth is removed and so on down the tank, until the barren tailings overflow the last Cell of Bank.

The Froth column in the first few Cells is kept high, since there are plenty of hydrophobic particles to sustain it. The Pulp level is increased from cell to cell as the Pulp gets depleted in the floated mineral by progressively raising the Cell Tailings weir Height. The last few Cells (known as SCAVENGERS) contain low-grade ores called the Middlings, which are returned to the head of the system. This s the process in a SIMPLE FLOTATION CIRCUIT, as shown in figure,

This FLOWSHEET representing the Basic or Simple Flotation Ckt. can be operated only when the gangue is relatively unfloatable.

These are of two types,

Pneumatic

Mechanical

PNEUMATIC MACHINES are those which either use the air entrained during pulp addition or the air blown in or induced. Generally, give a low-grade concentrate and little operating trouble. Eg. are the Davcra Cell, Flotation columns and the Jameson Cell.

1.Floatation Columns work on the principle of countercurrent flow of air bubbles and solid particles, which is accentuated by the adding washwater at top.

2.They are about 12m high with diameters upto 3.5m .

3. Jameson Cell was developed by Mount Isa Mines Ltd. and University of Newcastle, Australia, with an advantage of the ability to produce cleaner concentrates in a single stage. The overall height is reduced to 1m.

MECHANICAL MACHINES are characterized by mechanically driven impeller which agitates the slurry and disperses the incoming air into small bubbles.

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”