Froth flotation in mineral processing plant

In a beaker of water, if air is introduced from the bottom, air bubbles are produced and rise to the surface of the water as the density of an air bubble is much less than that of water (Figure.1 A). Similarly air bubbles also rise if air is introduced in a pulp containing mineral particles. If a mineral particle of high density adheres to the air bubble, the air bubble along with the mineral particle rises to the surface because the apparent density of the air bubble and the adhered mineral particle is less than that of water. If many mineral particles are adhered to the air bubble, still the air bubble rises to the surface as the apparent density of the air bubble and adhered mineral particles is less than that of water due to the relatively large volume of the air bubble (Figure 1 B). This concept is the basis for froth flotation operation.

flotation machine

To use this concept or phenomenon, the following are required:

1 Method to make the mineral particles adhere to the air bubble.

2 Method to keep the air bubble alive when it reaches the surface of water. It is known that the air bubble collapses when it reaches the surface of water. When it is collapsed, the adhering mineral particles are dropped into the water. So the air bubble must be kept alive on the surface of the water for a sufficient length of

time for it to be removed from the surface.

The following are the methods adopted in practice:

1.To make the mineral particles adhere to the air bubble whilst it is rising through the pulp, the mineral particles are treated with suitable chemicals to acquire adhering properties.

2.To prevent the air bubble from collapse, some other chemicals are used to prevent the collapsing of the air bubble or in other words to increase the life of the air bubble.

The required mineral particles after adhering to the air bubble float to the surface along with the air bubble. The aggregation of several such mineral adhered air-bubbles forms the froth on the surface of the pulp (Figure 16.1 C). Hence this operation of mineral separation is named Froth flotation

Flotation is a method of separating the minerals in a relatively finely divided state.

It utilises the differences in physico-chemical surface properties of particles of various minerals.

froth flotation

Figure 1 Process of rising air bubbles and forming froth.

This method can only be applied to relatively fine particles (less than 150um). If the particles are too large, the adhesion between the particle and the bubble is less than the weight of the particle and the bubble drops the mineral particles. The air bubbles can only stick to the mineral particles if they can displace water from the mineral surface. This can only happen if the mineral is water repellent or hydrophobic. Air bubbles, after reaching the surface, can continue to hold the mineral particles if they can form a stable froth. If not, air bubbles will burst and drop the mineral particles. In order to achieve the favourable conditions for froth flotation, the pulp is treated with various chemical reagents known as flotation reagents. The chemicals used for treating the mineral particles to make them to adhere to the air bubbles are called collectors and the chemicals used to increase the life of the air bubbles are called frothers.

The majority of the ores mined at present require fine grinding for a high degree of liberation of valuable minerals, and thus the flotation becomes the only possible means of beneficiation for higher grades and recoveries. Flotation treatment is extensively applied to the concentration of metalliferous minerals, both sulphides and oxides. About 90% of the world’s important ores of lead, zinc and copper are upgraded by flotation operation. The flotation technique is also extensively used in upgrading of non-metallic ores like graphite, limestone, fluorite, fluorspar, clay, rock phosphate and coal.

FLOTATION REAGENTS

Flotation reagents are substances added to the ore pulp prior to or during flotation in order to make it possible to float valuable mineral particles and not to float the gangue mineral particles. Important flotation reagents are collectors, frothers, depressants, activators and pH regulators.

A Collector is a chemical reagent, either an acid, base or salt, and is hetero-polar in nature; the polar part of it has an affinity towards a specific mineral and the non-polar part has an affinity towards an air bubble. A small amount of collector is added to the pulp and agitated long enough that the polar part is adsorbed on to the mineral to be floated whilst the non-polar part is oriented outwards and makes the surface of mineral particles hydrophobic. The collector increases the contact angle of the valuable mineral particles.

Collectors are broadly classified as anionic, cationic and oily collectors. Anionic and cationic collectors are ionizable organic compounds. They are said to be an anionic or cationic collector whether the ion that carries hydrocarbon group is anion or cation. Oily collectors are oily liquids which spread out thinly on solid surfaces to cause bubble attachment.

Xanthates, Dithiophosphates (known as Aerofloats), Dithiocarbamates, Fatty acids and soaps are the important anionic collectors. Xanthates are the most widely used collectors for flotation of sulphide minerals. The general formula of sodium or potasium xanthate is:

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The R group, in the case of ethyl and isopropyl xanthates, are C2H5 and C3H7 respectively and is shown below:

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The general formulae for other anionic collectors are:

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Dithiophosphates and dithiocarbamates are used in the flotation of sulphide minerals; carboxylic collectors are used for flotation of non-sulphides and nonsilicates.

Cationic collectors are used for oxide and silicate minerals including quartz.

Amines are the most commonly used cationic collectors. Oily collectors normally used are petroleum products, blast furnace oils, coal-tar and wood-tar creosotes. They are used in flotation of oxidized metalliferous ores and gold ores.

A Frother is a chemical reagent and is heteropolar in nature; the polar part of it has an affinity for water and the non-polar part has an affinity for gas or repulsion for water. The frother acts upon the gas water interface. The addition of a frother decreases the surface tension of water and increases the life of bubbles produced.

The main objective of a frother is to permit the production of a sufficiently stable froth to hold the mineral particles that form a network around the bubbles until they are removed from the flotation unit.

As a result of the addition of a frother, the gas bubbles, formed under the surface of the water, are more or less completely lined with a monomolecular sheath of frother molecules which allows each bubble to come in contact with other bubbles.

This forms a froth. Thus a froth is simply a collection of bubbles.

Cresylic acid and pine oil are the most widely used frothers. A wide range of synthetic frothers are now in use in many plants. Methyl Iso-Butyl Carbinol (MIBC) is most important among the synthetic frothers. The following are the chemical formulae of these organic reagents.

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Eucalyptus oil, camphor oil and sagebrush oil are used when they are more cheaply available than the common frothing agents.

To have an independent control, the frother should not have a collecting property.

The reagents having both frothing and collecting properties are known as frother collectors.

The compounds like fatty acids, sulphonates and amines which are in use as collectors have also frothing properties. Kerosene is a frother-collector used in coal flotation.

Other chemical reagents, depressants, activators and pH regulators, called modifiers, are used extensively in flotation to modify the action of the collector, either by intensifying or reducing its water – repellent effect on the mineral surface. Thus they make collector action more selective towards certain minerals.

Depressants are inorganic chemicals. They react chemically with the mineral particle surfaces to produce insoluble protective coatings of a wettable nature making them non-floatable even in the presence of a proper collector. Thus formed protective coatings prevent the formation of collector film. Sodium or potassium cyanide is a powerful depressant for sphalerite and pyrite. A combination of sodium cyanide and zinc sulphate is more effective in depressing zinc sulphide minerals, sphalerite and marmatite. They also have a depressing action on pyrite. Lime is sometimes used to depress pyrite in sulphide flotation. Sodium or potassium dichromate is used to depress galena.

Sodium silicate is extensively used for the depression of silicates and quartz. Sodium silicate is much used as a dispersant for removing slimes from particle surfaces of sulphide minerals. Sulphuric acid is used to depress quartz in soap flotation.

Lactic acid is a powerful depressant in iron sulphide flotation. Metaphosphates are used as depressants for non-silicates like barite, fluorite and calcite.

Activators, generally inorganic compounds, can modify the surface of non-floatable or poorly floatable mineral particles by adsorption on particle surface so that the collector may film the particle and induce flotation. An example of this is the use of copper sulphate in the flotation of sphalerite. Copper sulphate dissociates into copper ions in solution and copper sulphide is formed at the surface of sphalerite.

Then it reacts with xanthate and forms insoluble copper xanthate which makes the sphalerite surface hydrophobic. In the flotation of lead-zinc ore, after lead flotation, the sphalerite is activated with copper sulphate and floated. Copper sulphate also activates depressed pyrite when added in sufficient quantity.

flotation reagent

Table1 Quantities of flotation reagents.

Sodium sulphide is used to activate oxide minerals of lead, zinc and copper such as cerussite, smithsonite and malachite. As sodium sulphide imparts sulphide surface to the mineral particles to facilitate for collector coating, this activator is also known as sulphidizer. Sodium sulphide is also used to float previously depressed pyrite.

It has dispersing and depressing effects on sulphide minerals when added in large quantities.

pH regulators are used to modify the alkalinity or acidity of a flotation circuit or in other words to control the pH of the pulp. The pH of the pulp has an important and sometimes very critical controlling effect on the action of the flotation reagents.

Common pH regulators are lime and soda ash for creating alkaline conditions, sulphuric and hydrochloric acids for creating acidic conditions.

The quantity of reagents used in flotation varies from ore to ore and day to day or hour to hour for one ore. Small quantities of reagents are normally required. Low and high quantities of the reagents are of no use. In certain cases, increasing quantities results in other effects. For example, use of increased quantities of collector tends to float other mineral particles (other than required) also. Table 1 shows the approximate quantities of various reagents used in flotation.

TYPES OF FLOTATION

As a mineral is selected and floated in flotation, it is called selective flotation. As a required mineral is selected and floated, it is also called direct flotation. When an unwanted mineral is selected and floated, in which case, sink is the required product, it is called reverse flotation. When an ore contains two or more valuable minerals, one valuable mineral is selected and floated first and second valuable mineral is floated from the tailings. This flotation is called differential flotation. Alternatively, when all the valuable minerals are selected and floated, it is called bulk flotation. The individual minerals are selected and floated one after the other from the floats.

FLOTATION MACHINES

A flotation machine is the equipment used to carry out flotation operation. It provides the hydrodynamic and mechanical conditions which effect the separation. Basically the flotation machine must include:

1 Means for receiving the pulp.

2 Means for agitation and mixing the pulp.

3 Means for settling and discharging the pulp.

4 Means for air introduction and dispersion.

5 Means for discharging the froth.

The most commonly used flotation machines are of two types, namely mechanical type and pneumatic type. In a mechanical type flotation machine a mechanically driven impeller agitates the pulp and disperses the incoming air into small bubbles.

flotation machine

The air may be drawn in by suction created by the impeller or may be introduced to the base of the impeller by an external blower. These flotation machines are often composed of several identical cells arranged in series in such a way that one cell receives the de-frothed pulp (tailing of preceding cell) as feed. Such a series of cells are called a bank. The Denver sub-aeration machine (Figure 2) is well known cell-tocell machine.

The pulp from the weir of the preceding cell flows through the feed pipe on to the rotating impeller. The positive suction created by the impeller draws air through a hollow standpipe, sheared into fine bubbles by impeller and intimately mixed with the pulp. The diffuser arranged around the impeller prevents the agitation and swirling of the pulp above the impeller. The stationary hood above the impeller prevents the pulp in the cell having direct contact with the impeller. The mixture of bubbles and pulp leaves the impeller and bubbles ascend with their mineral load to form the froth. As the bubbles move to the pulp level, they are carried to the overflow lip by the crowding action of succeeding bubbles and removed by froth paddles. Pulp from the cell flows over the adjustable weir on to the impeller of the next cell. Particles which are too heavy to flow over the weir are by-passed through sand relief ports.

Denver sub-aeration cell

Figure 2 Denver sub-aeration cell

In an open-flow type machine, intermediate partitions and weirs between cells are eliminated. The pulp is free to flow through the machine without interference.

In pneumatic machines, air is introduced through the porous bottom of the cell. The air used in these machines not only creates aeration and produces froth but also maintains suspension by circulation.

FLOTATION OPERATION

The flotation operation is generally carried out in three stages namely roughing, scavenging and cleaning, called a flotation circuit (Figure 3). Each stage consists of a bank of cells and the number of cells in a bank is primarily depends upon the residence time of the pulp in the cells and the required throughput. The reagent conditioned feed pulp is treated in a first bank of cells called roughers. The tailing from the rougher cells, which may still contain some valuable mineral particles, is treated in another bank of cells called scavengers. The concentrate from the rougher cells is further treated in a bank of cells called cleaners to obtain high grade final concentrate.

The scavenger concentrate and cleaner tailing are re-fed to the rougher bank to be treated with the fresh feed pulp. The scavenger tailing is the final tailing. It is to be noted that the pulp is treated in a conditioner, called conditioning, with necessary reagents like depressant and collector, prior to flotation in order to convert the mineral particles to respond readily in a flotation cell. Conditioning the pulp reduces its residence time in the flotation cell and hence the capacity of the cell increases.

flotation circuit

Figure 3 A typical flotation circuit

COLUMN FLOTATION

The column flotation technique, which is a recent development, uses the principle of counter current wash-water flow for better separation particularly when operating on fine materials. The flotation column is a simplest form of pneumatic type flotation machine. It consists of a tall cylindrical column having the height to diameter ratio of more than 10 (Figure 16.5). The reagent conditioned feed pulp enters somewhat at the middle of the column. Compressed air is admitted through a distributor near the bottom of the column. In the section below the feed point, called recovery section, the feed pulp travels downwards against the rising air bubbles. Valuable mineral particles adhere to the air bubbles and are transported to the top part of the column, called washing section.

flotation column

Gangue mineral particles that are loosely attached to the bubbles are washed down by water sprays and only clean froth rises and discharged through annular trough.

The basic advantage of column flotation is the production of high grade concentrate without the loss of recovery. A single flotation column can replace five to six stages of operations involving conventional cells and yet achieve better performance. There are considerable savings in reagent requirement. The column occupies less floor space.

flotation column

Figure 4 Flotation column

The flotation column is a simplest form of pneumatic type flotation machine. It consists of a tall cylindrical column having the height to diameter ratio of more than 10 (Figure 4). The reagent conditioned feed pulp enters somewhat at the middle of the column. Compressed air is admitted through a distributor near the bottom of the column. In the section below the feed point, called recovery section, the feed pulp travels downwards against the rising air bubbles. Valuable mineral particles adhere to the air bubbles and are transported to the top part of the column, called washing section.

Gangue mineral particles that are loosely attached to the bubbles are washed down by water sprays and only clean froth rises and discharged through annular trough.

The basic advantage of column flotation is the production of high grade concentrate without the loss of recovery. A single flotation column can replace five to six stages of operations involving conventional cells and yet achieve better performance. There are considerable savings in reagent requirement. The column occupies less floor space.

FLOTATION PRACTICE OF SULPHIDE ORES

Flotation operation was initially developed to treat the sulphides of lead, zinc and copper. In a typical flotation practice of lead-zinc ore, the different reagents used are sodium cyanide and zinc sulphate as depressants for pyrite and sphalerite, potassium ethyl xanthate as collector for lead circuit to float galena, sodium isopropyl xanthate in zinc circuit to float sphalerite, copper sulphate to activate already depressed sphalerite in zinc circuit, lime as pH regulator, and crysilic acid as frother in both circuits. In some plants, Methyl Iso-Butyl Carbinol (MIBC) is used as a frother. For the flotation of chalcopyrite (copper mineral in most of the copper concentrators), sodium isopropyl xanthate and pine oil are the collector and frother respectively in most of the plants. Soda ash is used as a pH regulator.


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