flotation 21

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Any operation in which one solid is separated

from one another by floating one of them at or on

the surface of fluid (Brown, 1950)

Separation of mixed liberated particles in terms

of their wettability differences (Perry, 1997)

Based on the differences in the surface chemical

properties (i.e. wettability)


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One solid more readily adsorbs the water

phase, becomes surrounded by water, and

sinks

The other solid more readily adsorbs air and

becomes at least partially surrounded orcovered by air


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The average or bulk density

of the solid and adsorbed

air bubbles is less than thatof water

The whole mass of air and

solid floats to the surfaceto form a mineralized froth


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Prepare a water

suspension of a

mixture ofrelatively fine-sized particles

(smaller than 150micrometers)

Contact the

suspension with aswarm of airbubbles of air in asuitably designed

process vessel

Particles whichare readily wetted

by water - remain

in suspension

Particles that

are not

wetted by

water tendto attached to

air bubbles

The air bubbleslevitate (float) to

the top of the

process vessel

Collect in

a froth

layer


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Used in the mineral process industry to

concentrate mineral values (i.e. copper, lead,

zinc, molybdenum and nickel)

Used for the recovery of fine coal and for

the concentration of a wide range of mineralcommodities


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Wastewater treatment to remove particulate,

organic and biological contaminants

Extraction of metallic values

Removal of heavy metal compounds fromhydrometallurgical streams (precipitation

flotation)

Recovery of bitumen from tar sands


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Deinking of wastepaper

Recovery of solids from white waste inpaper making

Recovery of glass sands from industrial

wastes

Removal of impurities from peas


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Removal of ergot from rye

Separation of proteins from milk

Clarification of fruit juice

Extraction of soluble species (ion

flotation and foam fractionation)


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Rougher


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The first step Conditioning

The slurryor thepulp,consisting of particles to beseparated is fed in the

conditioning to which thenecessary flotationreagents are added

Particles are finely divided

material reduced by theball mill or other finecrusher, usually varying insize from 20 mesh to under200 mesh

Rougher


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The first step Conditioning

Purpose:

to create physical-

chemical conditions for

achieving appropriateselectivity between

particle species that are

to be separated

To cause complete

filming of the solid by

the reagent

Rougher


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The first step Flotation Reagent

Can be collectors,

frothers, and

modifiers


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Collectors

- surface-active agents that are added to the

flotation pulp, where they adsorb selectively on thesurface of the particles and render them

hydrophobic


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o Non-ionizing collectors

-practically insoluble in

water and cause theparticles to become

hydrophobic by covering

them with a thin film


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o Ionizing collectors

-dissociate into ions in waterare made up of complexheteropolar molecules in that the

molecule contains both a nonpolarhydrocarbon group withpronounced hydrophobicproperties and a polar group withhydrophilic properties

Anionic collectorsCationic collectors


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CollectorsDosage requirements depend on the mechanisms by which they interact with the

particle surface just to form monomolecular layer

RULE:High dosages are required for non-ionizing collectors and

physisorbing ionizing collectors (in the order of 0.1 to 1 g of reagent

per kg of solids)

Low dosages for chemisorbing ionizing collectors (0.01 to 0.1 g of

reagent per kg of solid)


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CollectorsDosage requirements depend on the mechanisms by which they interact with the

particle surface just to form monomolecular layer

NOTE:

Addition of excess quantities of a collector is not desirable because it

results in reducing the selectivity and increasing the cost


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Frothers

-surface-active agents added to the flotation pulp

primarily to stabilize the air bubbles for effective particle-bubble attachment, carryover of particle-laden bubbles to

the froth, and removal of the froth

-similar to ionizing collectors except that theyconcentrate primarily at the air-liquid interface


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Frothers

-i.e. pine oil, cresylic acid, polypropylene glycol, short-

chain alcohols and 5- to 8-carbon aliphatic alcohols

Dosage requirements are usually 0.01 to 0.1 g per kg of

solids


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ModifiersActivators- used to make a mineral surface

amenable to collector coating

pH regulators- used to control or adjust pH, a verycritical factor in many flotation separations


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ModifiersDepressants- assists in selectivity (sharpness of

separation) or stop unwanted minerals from

floating

Dispersants and flocculants- control slimes thatsometimes interfere with the selectivity and

increase reagent consumption


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ModifiersDosage requirement: Vary widely, ranging from as little as

0.01 to 0.1 g/kg to as high as 1 to 2 g/kg of solids,

depending upon the reagent and the metallurgical

problem


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Important phenomena that occur;

Solubility and dissociation of reagents in water

Change of pH of the suspension

Change of air-water surface tension

Physical and chemical adsorption of the dissolved species onthe solid surfaces due to hydrogen bond formation


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Important phenomena that occur;

Electrostatic interactions

Hydrophobic bonding

Chemical bond formation

Fixation of reagent species in the solid lattice


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o Involves relative interaction of three phases:solid, water and air

o Objective: To contact solid particlessuspended in water with air bubbles andcause a stable bubble-particle attachment


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Recall: Flotation depends upon the relativeadsorption or wetting of the solid surfacesby the fluid

This is controlled by surface or interfacialenergy


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Generally, The sum of the components of the interfacial

tensions equals to zero.

SG

SL

Gas

Solid

Liquid

Figure 96. Diagram of surface tensions involved in thee-phase contact.


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SG = SL + LG (cos )

where = interfacial tension of the solid-gas

(SG), solid-liquid (SL) and liquid-gas(LG),respectively

= contact angle

SG SL

Gas

Solid

Liquid


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When a solid particle attaches itself to abubble, there is a loss in surface energy Eper unit area of surface , equal to the loss inthe surface tension

E = -(SG SG + SL SL + LG LG)

E = SG SG + SL SL + LG LG


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Since,

SG

= SL

+ LG

(cos )

SL - SG = -LG (cos )

)( SG-LGSLSG

E

)( cos-1LGSG

E


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The second step Separation

Purpose:

To generate and

introduce air bubblesinto the process into the

process vesselRougher


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The second step Separation

Purpose:

Particles attached the air

bubbles are in mostapplication removed from

the process vessel

(flotation cell) as froth

Rougher


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Electrolytic or Electroflotation

Units Illustration

- based on the generation

of hydrogen and oxygenbubbles in a dilute aqueous

solution

- passes direct current

between two electrodes


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Electrolytic orElectroflotation Units Illustration

Choice of Electrode Materials

- aluminum- platinized titanium

- titanium coated

with lead dioxide- stainless steel


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Advantage- Bubble size tends to be the smallest(10 50 micrometer

-Very little turbulence

- attractive for the separation of small

particles and fragile flocs

-attractive for small installations in the

flowrate range of 10 20 cu.m/h

Electrolytic or

Electroflotation Units


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Disadvantage

- fouling of electrodes

- not suitable for potable watertreatment due to heavy metalcontamination

Electrolytic or



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Application

- effluent treatment

and sludge thickening

Electrolytic or



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Dissolved-air Flotation Unit Dissolved-air Flotation Unit

Saturation of processed stream

with air and generation of airbubbles by releasing pressure

Used to treat process streams

with low solid concentration

(0.01- 2% by vol)

No addition of frother-type

chemical reagents

Particle bubble contact

- achieved by direct

nucleation and growth of air

bubbles on the particles

Bubble size- range from 20

100 micrometers


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Dissolved-air Flotation Unit

Applications

- for sewage and water treatment of potable water

- treatment of slaughterhouse, poultry processing,

seafood processing, soap, and food processingwastes


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Two Main Types of Dissolved-air Flotation

Unit

1.Vacuum flotation

2.Pressure flotation


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Vacuum Flotation Unit

Saturation of process

stream with air at

atmospheric pressure and

introduction to the flotation

tank on which vacuum is

applied

Can be run only as a batch

process

Uses sophisticated

equipment to produce and

maintain vacuum


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Pressure flotation unit

Consists of pressurizingand aerating the process

stream and introducing it

into the flotation vessel

that is maintained atatmospheric pressure

Can be operated on a

continuous basis

Reduction of pressure-results in formation of fine

air bubbles

Pressurization can becarried out through full-

flow or split-flow pressure

flotation.


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Pressure flotation unit


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Dispersed-air Flotation Unit Generation of air bubbles by:

a. pneumatic

b. mechanical

Relatively large air bubbles (at least 1 mm in size)

Frothers are added to control the size and stability of air

bubbles


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- the equipment in which

the material is actually

separated or floated from theresidual tailings

- consists of a vessel provided

with a feed at one end, an

overflow for froth removal, anda discharge for tailing at the

opposite end

Flotation Cell


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Pneumatic Cells Mechanical Cell

- depend upon compressed air

for agitation

- relatively mild agitation

- produce a clean frothrelatively free from gangue

- more violent agitation, more

thorough flotation and tailingsmore nearly free from material

desired in the concentrate

- greater capacity for the same

volume


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Pneumatic Cells Mechanical Cell

- 50% longer contact time and

full conditioning of the pulpbefore flotation

- flotation columns

air-bubble generation is

accomplished by a gas-sparging system

- incorporate a mechanical

agitator that draws in air andbeats it into the pulp

- mechanical agitation and

aeration by means of a rotation

impeller on an upright shaft


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It is desired to recover lead from an orecontaining 10 percent lead sulfide (PbS) andthe balance assumed to be silica, 500 tons ofore being treated per 24-hr day.

It is assumed that the concentrate from asingle cell is of acceptable purity but thetailings are to be retreated in scavenger cellswith return of scavenger concentrate to therougher.


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Laboratory findings indicate that if water-to-solids ratio L/S =2 and the contact time is 8 min in the rougher and L/s = 4 for15 min in scavenger, with mechanically agitated machines ofthe Denver type, the following compositions will be found forthe various products.

The density of PbS and SiO2 are 7.5 and 2.65 g/cc, respectively

PbS SiO2Feed, a 10 % 90 %

Concentration, b 80 20

Rougher tailings, c 2 98

Scavenger concentrate, d 11 89

Final tailings 0.5 99.5


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Determine:a. Density for all solids

b. Mass of products

c. Volume of tanks

d. Number of cells and power requirementusing Denver No. 24 which have 50 cu. ft

of volume


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Determine:E. Volume of delivered air and power

requirement of air compressor when Air-Lift

Machine by the Southwestern Engineering Co.is used (Assume 75 cfm air/ft in

Rougher, 60 cfm air/ft in scavenger, and 25

percent longer contact time at 2 psi).


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Air delivered, cfm Approximate Horsepower at Pressures of2 psi 3 psi 4 psi 5 psi

500 6 9.5 13.5 17.51000 12 18.5 26 332000 24 38 54 703000 34 57 76 964000 46 76 105 1355000 58 91 125 1606000 70 120 168 2207000 83 135 190 2458000 96 165 225 310


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Size, cubic ft Horsepower Consumed per CellDenver Fagergren

10 112 1.2 1.8-2.018 1.424 2.2 3.5-4.040 3.2 550 4.2 670 8100 9

flotation 21

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