lec#5 ms - flotation

8/3/2019 Lec#5 MS - Flotation

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C HEMICAL P ROCESS I NDUSTRIES

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` The process of separation of mixed liberated particles from each other if there are sufficient differences in their wet-ability is called flotation.

P RINCIPLE

F roth F lotation commences by Comminution (that is, crushing and grinding),

which is used to increase the surface area of the ore for subsequent processingand break the rocks into the desired mineral and gangue in a process known asliberation, which then has to be separated from the desired mineral.


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` The ore is ground into a fine powder and mixed with water to form a slurry.

`

The desired mineral is rendered hydrophobic by the addition of a surfactant or collector chemical.` The particular chemical depends on which mineral is being refined.` Example: pine oil is used to extract copper

` This slurry (more properly called the pulp) of ` hydrophobic mineral-bearing ore` hydrophilic gangueis then introduced to a water bath which is aerated, creating bubbles.

` The hydrophobic grains of mineral-bearing ore escape the water by attaching to the

air bubbles, which rise to the surface, forming a foam or a scum (more properlycalled a froth).

` The froth is removed and the concentrated mineral is further refined.


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` The floatation process operates by preparing a water suspension of amixture of relatively fine sized particles (smaller than 150 micro meters)and by contacting the suspension with a swarm of air bubbles in a suitablydesigned process vessel.

` Particles that are not wetted by water (hydrophobic) tend to be attached toair bubbles levitate (float) to the top of the process vessel and collect in theforth layer.

BASE O F SEPARATION` Thus difference in the surface chemical properties of the solids are the

basis of separation by flotation.

` Surfaces that do not have strong surface chemical bonds, that were broken,tend to be non-polar and are not readily wetted.

` e.g. Graphite and Talc.` These solids are naturally floatable.


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` Physical & Chemical nature of the material` Pre-treatment (grinding etc.)` Water chemistry` Reagents added` Type of flotation cell used


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M INERAL APPLICATIONS` The floatation process is most widely used in the mineral process industry

to concentrate mineral values in the ores. M ost of the worlds

` are produced from ores that are concentrated first by flotation.

In addition, floatation is commonly used` for the recovery of fine coal` for the concentration of wide range of mineral commodities including

fluorspar, mica, spodumene, bastnaesite, calcite, granite and talc.

Copper Lead

ZincM olybdenum

Nickel


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` WASTEWATER TREAT M ENT to remove particulate, organic and biologicalcontaminants.` Extraction of metallic values from hydrometallurgical streams by

particulate floatation.` De-inking of waste paper.

` Recovery of bitumen from tar sand.` Recovery of solids from white water in paper making.` Recovery of glass water from industrial wastes.` Removal of impurities from peas.` Separation of proteins from milk.` Clarification of fruit juices.` Extraction of soluble particles


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The following steps are followed:` Grinding to liberate the mineral particles` Reagent conditioning to achieve hydrophobic surface charges on the

desired particles.` Collection and upward transport by bubbles in an intimate contact with

air or nitrogen` F ormation of a stable froth on the surface of the flotation cell` Separation of the mineral laden froth from the bath (flotation cell)


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Unit operation of flotation is based on two major steps:

CONDITIONING` is basically a pre treatment step. the slurry or the pulp, consisting of

particles to be separated, is fed to the conditioning unit, to which thenecessary flotation reagents are added.

` The main purpose of conditioning step is to create physical-chemical conditioning or the process of chemical changes necessary for forth

flotation.` The conditioner may be soluble, insoluble, and partially soluble.

SEPARATION` is then intended to generate and introduce air bubbles into the process

vessel for contacting them with particulate species.` Particles attached to the air bubbles are in most applications removed from

the process vessel as froth.


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accordingly the unit operation of flotation is referred to as F ROTHF LOTATION.

In the mineral industry:` The froth overflow stream is called C ONCENTRATE` The slurry underflow is called T AILINGS .

depending on the application, these two steps may be carried out in twodistinctly different process units or in one combined unit.


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` Ores must be ground to a point of complete or nearly complete liberation.Even tough it might possibly be accomplished by coarse crushing.,grinding. Grinding is done in closed circuit with classifier.

` In many instances, superior flotation results are obtained by conditioning

the ore with the reagents before the flotation step.

` Oily type collectors are sometimes added to the grinding circuit to ensuredispersion.

` F lotation machines are built in multiple units. Common practice is to feedthe pulp to several cells known as roughers. Which produce a barrentailing. The concentrate is treated in cleaner cells and re-cleaner cells for final concentration.


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` Ore F lotation` M acro F lotation` M icro F lotation` Adsorbing Colloid F lotation` M olecular F lotation` Ion F lotation` Precipitate F lotation

M ACROSCOPIC F LOTATION

` type of flotation which deals with the removal of M acroscopic Particles.

M ICROSCOPIC F LOTATION

` type of flotation which deals with the removal of M icroscopic Particles particularly colloid or microorganisms.

` It is also called Colloid F lotation.


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ADSORBING COLLOID F LOTATION` type of flotation which deals with the removal of dissolved material in

piggyback fashions by adsorptions on colloidal particles is called adsorbingcolloid flotation

M OLEC U LAR F LOTATION

` type of flotation which deals with the removal of surface inactive moleculesthrough the use of collector (surfactant) which yields an insoluble product.

ION F LOTATION` type of flotation which deals with the removal of surface inactive ions through

the use of collector which yields an insoluble product; especially a removal of scum.

PRECIPITATE F LOTATION` type of flotation in which the precipitate is removed by a collector which is not

precipitating agent.


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Three types of chemical reagents used during the froth flotation process` Collectors` F rothers` M odifiers


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`

These are surface active agents that are added to the flotation pulp wherethey adsorbs selectively on the surface of the particles and render themhydrophobic. Collectors

` increase the natural hydrophobicity of the surface.` increase the separability of the hydrophobic and hydrophilic particles.

Caution:` Collectors are added in a certain amount. Addition of excess quantities of a

collector is not desirable because it results in reducing the selectivity andincreasing the cost.

TYPES O F COLLECTORS :` Ionizing Collector ` Non Ionizing Collector


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NON IONIZING COLLECTOR ` The non ionizing collectors are practically insoluble in water and cause the

particles to become hydrophobic by covering them with a thin film.

IONIZING COLLECTOR ` The ionizing collector adsorb either physically or chemically on the

particles surface and can further be classifiedAnionic & Cationic Collectors

` depending upon the nature of the non polar hydrocarbon group.

common examples of the ionizing collectors include:F atty AcidsLong Chain SulphatesSulphonates

AminesXanthatesDithiophosphates


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These are also surface active agents & are added to the flotation pulp primarily` to stabilize air bubbles for effective particle bubble attachment and` removal of the froth.

` The F rother action is similar to the ionizing collectors except that thayconcentrate primarily at air-liquid interface.

Commonly used F rothers are` Pine Oil` Short Chain Alcohol` Polypropylene Glycol` Cresylic Acid


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` Modifiers adsorb on the surface of particles or react chemically at thesurface and prevent the adsorption of the collector or promoter.

F lotation modifiers include several classes of chemicals` Activator

` pH regulators` Depressants` F locculants


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ACTIVATORS` Enhance the flotation by collectors that will not float in their absence e.g.

calcium activated flotation of Quartz using Oleate

PH R EG U LATORS` Regulators such as lime, caustic soda, soda ash & sulfuric acid are used to

control or adjust pH , a very critical factor in many flotation separations.

DEPRESSANTS` prevent the adsorption of collectors` and help in selectivity or stop unwanted mineral from floating.

Typical are` Sodium to depress Pyrite,` Zinc Sulphate to depress ZnS` Lime to depress Pyrite` Lignin Sulfonates to depress Graphite


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`

Various type of flotation machine designs, can be classified into differentcategories. on the methods used for the generation and introduction of air bubbles into the equipment .

` Each of the techniques has its own advantage and limitations. These must be considered carefully in selecting the equipment for a specificapplication. F ollowing are some different flotation units used as flotationequipment.


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Electrolytic or Electro- F lotation is based on the generation of hydrogen and oxygen bubblesin a dilute aqueous solution by passing direct current between two electrodes.` a low voltage potential of 5 to 10 volts is supplied to the electrodes.` Power consumption is in the range of 0.5 to 0.7 kW/m of F lotation tank surface,

depending in the` conductivity of the liquid

` distance between the electrodes.

DRAW -BACKS` The main drawback of the Electro- F lotation units is associated with the electrodes in

terms of ` their fouling requiring mechanical energy` their consumption needing replacement at frequent intervals.

` Electro- F lotation is not expected to suitable for potable water treatment because of the possibly heavy metals contamination that can arise due to dissolution of the electrodes.


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The two main types of Dissolved Air F

lotation processes are:` Vacuum F lotation` Pressure F lotation [most widely used]

V ACUUM F LOTATION` In Vacuum F lotation the process stream is saturated with air at atmospheric

pressure and introduced to the flotation tank on which a vacuum is appliedgiving rise to the generation of the air bubbles.

LIM ITATIONS` The process can be run only as a batch process and requires sophisticated

equipment to produce and maintain the vacuum. By and large the amountof air released during F lotation is limited by the vacuum achievable.


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P R ESSU R E F LOTATION` In contrast to vacuum flotation dissolved air flotation unit can be operated on

continued basis by the application of pressure.` This consist of pressurizing and aerating process stream and introducing it into

the flotation vessel that is maintained at low atmospheric pressure.` The sudden reduction of pressure results in the formation of fine air bubbles

and the collection of fine particulates to be floated and removed as sludge.

SPLIT F LOTATION SYSTE M` Pressurization could be carried out on the entire feed stream or a fraction of the

feed stream while the remainder is introduced directly without aeration into theflotation tank.

` The split flotation system offers a cost saving over the full flow units` in both cases however, the solid particles in the feed stream are flocculated

before introducing in the flotation tank ` The high shear during pressurization, aeration and pressure release can destroy

flocs.


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The dissolved air flotation process is more commonly used for ` Sewage and Potable Water Treatment.` Treatment of Slaughter House` Poultry Processing` Sea F ood Processing` Waste F ood Processing


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` Dispersed air flotation involves the generation of air bubbles either pneumatically or bymechanical means

` in both cases relatively large air bubbles are generated` to control the size and stability of air bubbles frothers are added to the flotation devices.

` M echanical flotation machines are most commonly used in mineral industry` Pneumatic Column Type U nits are gaining popularity recently.

M ECHANICAL CELLS` In a typical installation a number of flotation cells are connected in series such that

` Overflow each cell O U TPU T F ROTH goes into a launder

` U nderflow from one cell goes to the next one.` The cell design may be such that the flow of slurry from one cell to another can neither

be restricted by weirs or unrestricted.` The mechanical cells that are most widely used today in Sulfide, Coal and Non- M etallic

flotation operation are made by Fagergren, Denver & Outokumpy.


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` These machines provide mechanical agitation and aeration by means of arotation impeller on an upright shaft.

` in addition the agitator and Denver cells also utilize air from a blower tohelp aerate the pulp.

A D V ANTAGES [L A R GE SCALE F LOTATION ]

Large scale flotation machine offer advantages of ` lower installed cost` lower operating cost` lower floor space requirement` The use of large flotation cells does enable a few number of parallel rows

and thereby permits a reduction in pumps piping and other auxiliaries.

However it should be noted that large flotation cells do not permit` reduction in the number of cells in a series.


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F lotation columns belong to the class of pneumatic devices in which` air bubble generation is accomplished by a gas sparging system` no mechanical agitation is employed.

I NP U TS TO THE COLUM N I NCL U DE:` pre conditioned slurry feed [two third of height from the bottom]` air [from the bottom region]` wash water spray [at the top of the column]

OU TPU TS:` Overflow from the top of the tank, froth consisting of hydrophobic particles` U nderflow from the bottom of the tank, carrying the non floatable hydrophilic

particles.

U SES :` Iron Ore Copper, Lead Zinc & Coal F lotation processes.


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The main advantages of the flotation column technology include :` Improved Recovery` Optimized Grade` Increased Throughput` Low Capital Cost

` Low Operating Cost` Low plant floor space requirements` Easy adaptability to automatic control` Simplicity in construction` F lexibility in operation.` Improved Separation performance particularly for fine materials.


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` Column flotation cells better suited to flotation of fines

lec#5 ms - flotation

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