Flotation process (sometimes called flotation separation) is a method of separation widely

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Oct 29, 2013 (3 years and 10 months ago)

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Flotation process

(sometimes called
flotation
separation
) is a method of
separation

widely
used in the
wastewater

treatment and
mineral
processing

industries.



Mechanism of froth flotation are:


Attachment of a specific mineral particle to
air bubbles


Being carried by the water in the
floth


Caught b/w particles in the froth


In flotation concentration mineral is
transferred to the froth leaving the gangue in
the pulp. This is direct flotation.


In reverse flotation, gangue is separated into
the froth fraction. Air bubbles can only stick
to the mineral particles if they can displace
water from the mineral surface which means
that the mineral has to be hydrophobic. Air
bubbles can continue to support the mineral
particles at the surface only if they can form a
stable froth which is achieved by using
floatation reagents.


Dissolved
air flotation



Induced gas flotation



Froth
flotation
, typical in the mineral
processing
industry


Dissolved
air flotation

(DAF) is a
water treatment

process that
clarifies
wastewaters

(or other waters) by the removal of
suspended matter such as oil or solids. The removal is achieved
by dissolving air in the water or wastewater under pressure and
then releasing the air at
atmospheric pressure

in a flotation tank
or basin. The released air forms tiny bubbles which adhere to the
suspended matter causing the suspended matter to float to the
surface of the water where it may then be removed by a
skimming device
.
[


Dissolved
air flotation is very widely used in treating the
industrial wastewater
effluents

from
oil refineries
,
petrochemical

and
chemical plants
,
natural gas processing plants
, paper mills,
general water treatment and similar industrial facilities. A very
similar process known as
induced gas flotation

is also used for
wastewater treatment.
Froth flotation

is commonly used in the
processing of mineral ores.


In the
oil industry
, dissolved gas flotation (DGF) units do not use
air as the flotation medium due to the explosion risk.
Natural gas

is used instead to create the bubbles.



The feed water to the DAF float tank is often (but not
always) dosed with a
coagulant

(such as ferric chloride or
aluminum sulfate) to
flocculate

the suspended matter.


A portion of the clarified effluent water leaving the DAF
tank is pumped into a small pressure vessel (called the air
drum) into which compressed air is also introduced. This
results in saturating the pressurized effluent water with
air. The air
-
saturated water stream is recycled to the front
of the float tank and flows through a pressure reduction
valve just as it enters the front of the float tank, which
results in the air being released in the form of tiny
bubbles. The bubbles adhere to the suspended matter,
causing the suspended mater to float to the surface and
form a froth layer which is then removed by a skimmer.
The froth
-
free water exits the float tank as the clarified
effluent from the DAF unit.
[1]


Some DAF unit designs utilize parallel plate packing
material, lamellas, to provide more separation surface and
therefore to enhance the separation efficiency of the unit.



Induced
Gas Flotation

(IGF) is a
water treatment

process that
clarifies
wastewaters

(or other waters) by the removal of
suspended matter such as oil or solids. The removal is achieved
by injecting air bubbles into the water or wastewater in a
flotation tank or basin. The small bubbles adhere to the
suspended matter causing the suspended matter to float to the
surface of the water where it may then be removed by a
skimming device.


Induced Gas Flotation is very widely used in treating the
industrial wastewater
effluents

from
oil refineries
,
petrochemical

and
chemical plants
,
natural gas processing plants

and similar
industrial facilities. A very similar process known as
dissolved air
flotation

is also used for waste water treatment.
Froth flotation

is
commonly used in the processing of mineral ores.


IGF Units in the
oil industry

do not use air as the flotation
medium due to the explosion risk. These IGF Units use
natural
gas

to create the bubbles.



The feed water is to the IGF float tank is often (but
not always) dosed with a
coagulant

(such as ferric
chloride or aluminum sulfate) to
flocculate

the
suspended matter.


The bubbles may be generated by an impeller,
eductors

or a
sparger
. The bubbles adhere to the
suspended matter, causing the suspended mater to
float to the surface and form a froth layer which is
then removed by a skimmer. The froth
-
free water
exits the float tank as the clarified effluent from the
IGF unit.
[1]


Some IGF unit designs utilize parallel plate packing
material to provide more separation surface and
therefore to enhance the separation efficiency of the
unit.




Froth

flotation

is a process for selectively
separating hydrophobic materials from
hydrophilic. This is used in several processing
industries. Historically this was first used in
the mining industry
.


Eg oxides
-
hematite,cassiterite;oxidised
-
malachite,cerussite;non

metallic
-
fluorite,phosphate,fine

coal;sulphides
-
copper,zinc,lead




The
flotation process is also widely used in
industrial waste water treatment plants, where it
removes fats, oil, grease and suspended solids
from waste water. These units are called
Dissolved air flotation

(DAF) units
.
In particular,
dissolved air flotation units are used in removing
oil from the wastewater
effluents

of
oil refineries
,
petrochemical

and
chemical plants
,
natural gas
processing plants

and similar industrial facilities.



Froth
flotation is one of the processes used
to recover
recycled paper
. In the
paper
industry

this step is called
deinking

or just
flotation. The target is to release and remove
the
hydrophobic

contaminants from the
recycled paper. The contaminants are mostly
printing ink

and
stickies
. Normally the setup
is a two stage system with 3,4 or 5 flotation
cells in series
[4]


Chemicals for deinking of recycled paper


pH control:
sodium silicate

and
sodium
hydroxide



Calcium

ion

source:
hard water
,
lime

or
calcium chloride



Collector:
fatty acid
, fatty acid
emulsion
, fatty
acid
soap

and/or
organo
-
modified
siloxane
[6]




Froth
flotation commences by
comminution

(that is,
crushing and grinding), which is used to increase the
surface area of the ore for subsequent processing and
break the rocks into the desired mineral and gangue in a
process known as liberation, which then has to be
separated from the desired mineral. 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. As an
example, pine oil is used to extract
copper

(see
copper
extraction
). This slurry (more properly called the
pulp
) of
hydrophobic mineral
-
bearing ore and
hydrophilic

gangue

is 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 properly called a
froth
). The froth is removed
and the concentrated mineral is further refined.



To
be effective on a given ore slurry, the surfactants are chosen based
upon their selective
wetting

of the types of particles to be separated. A
good surfactant candidate will
completely

wet one of the types of
particles, while partially wetting the other type, which allows bubbles to
attach to them and lift them into a froth. The wetting activity of a
surfactant on a particle can be quantified by measuring the
contact
angles

that the liquid/bubble interface makes with it. For complete
wetting the contact angle is zero.


Another consideration, especially important for heavy particles, is to
balance the weight of the particle with the surfactant adhesion and
buoyant forces of the bubbles that would lift it.


For typical values of metal densities and surface tensions, if the bubbles
are larger than the ore particles, and the particles are equal to or less
than 1

mm radius, then particles will rise into the froth layer if:
[5]


where is the radius of the particles, is the average
surface tension

between the three pairs of phases (particle, flotation solution, air), is the
mass density

of the particles, and is the acceleration of
gravity

(9.81
m/s
2
).


For particles larger than the bubbles, they too can rise into the froth,
each buoyed by a swarm of bubbles, under similar conditions as those
expressed in the inequality.
[5]



Diagram
of froth flotation cell. Numbered triangles show direction of
stream flow. A mixture of ore and water called pulp [1] enters the cell
from a conditioner, and flows to the bottom of the cell. Air [2] or
nitrogen is passed down a vertical impeller where shearing forces break
the air stream into small bubbles. The mineral concentrate froth is
collected from the top of the cell [3], while the pulp [4] flows to another
cell.


Flotation can be performed in rectangular or cylindrical mechanically
agitated cells or tanks, flotation columns, Jameson cells or deinking
flotation machines.


Mechanical cells use a large mixer and diffuser mechanism at the bottom
of the mixing tank to introduce air and provide mixing action. Flotation
columns use air
spargers

to introduce air at the bottom of a tall column
while introducing slurry above. The countercurrent motion of the slurry
flowing down and the air flowing up provides mixing action. Mechanical
cells generally have a higher throughput rate, but produce material that
is of lower quality, while flotation columns generally have a low
throughput rate but produce higher quality material.


The Jameson cell uses neither impellers nor
spargers
, instead combining
the slurry with air in a
downcomer

where high shear creates the
turbulent conditions required for bubble particle contacting.


s


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


Formation of a stable froth on the surface of the
flotation cell


Separation of the mineral laden froth from the bath
(flotation cell)



1) pneumatic machines either use air
entrained by turbulent pulp addition(cascade
cells), or more commonly air either blown in
or induced, in which case air must be
dispersed either by baffles or some form of
permeable base within the cell. It gives low
grade concentration &little operating trouble.


The davcra cell is used for roughing or
cleaning operations.


Most resent is floatation column.


Mechanical floatation characterized by a
mechanically driven impeller which agitates
the slurry and disperses the incoming air into
small bubbles.


Criterions for assessing cell performance are:


Metallurgical performance i.e. product
recovery & grade.


Capacity in
tonnes

treated per unit volume.


Power consumption/
tonne
.


economicali.e
. initial, operation &
maintenance cost







Numbered
triangles show direction of stream flow, Various flotation
reagents are added to a mixture of ore and water (called pulp) in a
conditioning tank. The flow rate and tank size are designed to give the
minerals enough time to be activated. The conditioner pulp [1] is fed to
a bank of rougher cells which remove most of the desired minerals as a
concentrate. The rougher pulp [2] passes to a bank of scavenger cells
where additional reagents may be added. The scavenger cell froth [3] is
usually returned to the rougher cells for additional treatment, but in
some cases may be sent to special cleaner cells. The scavenger pulp is
usually barren enough to be discarded as tails. More complex flotation
circuits have several sets of cleaner and re
-
cleaner cells, and
intermediate re
-
grinding of pulp or concentrate.



A
composition and process are provided for the recovery of the
values of zinc, molybdenum, copper, lead, iron (pyrite), and
iron
-
containing small amounts of gold or uranium, or both, from
ores comprising these mineral sulfides. The aqueous
composition is the impure form of an alkali metal alkyl
trithiocarbonate compound. The process comprises employing
said aqueous composition as a collection agent for the above
minerals in an ore recovery process.


A process for the separation of zinc values from lead values from
an ore comprising both is provided by employing an alkali metal
alkyl trithiocarbonate compound as a collection agent for zinc.


In addition, both a composition and process are provided for the
recovery of the values of iron, copper, and lead from ores
comprising these values. The composition consists essentially of
a dispersant and an impure form of an alkali metal alkyl
trithiocarbonate compound. The process comprises employing
this composition as a collection agent for the above minerals in
an ore recovery process.



Collectors


Collectors either chemically bond
(
chemisorption
) on a hydrophobic mineral
surface, or
adsorb

onto the surface in the
case of, for example, coal flotation through
physisorption
. Collectors increase the natural
hydrophobicity

of the surface, increasing the
separability

of the hydrophobic and
hydrophilic particles.



Xanthates


Potassium Amyl
Xanthate

(PAX)


Potassium Isobutyl
Xanthate

(PIBX)


Potassium Ethyl
Xanthate

(KEX)


Sodium Isobutyl
Xanthate

(SIBX)


Sodium Isopropyl
Xanthate

(SIPX)


Sodium Ethyl
Xanthate

(SEX)


Dithiophosphates


Thiocarbamates




Xanthogen

Formates




Thionocarbamates




Thiocarbanilide





Frothers

`
Frothers

produces stable bubbles for
hydrophobic particles to attach
to.work

in
liquid phase only and not to mineral surface.


It should have low collecting power.


Reduce surface
tention
.


Pine oil



Alcohols

(MIBC)


Polyglycols



Polyoxyparafins
|


Cresylic

Acid

(
Xylenol
)



Modifiers

`
Modifiers as activators, depressants or pH
modifiers. Alters selectivity of the collectors.


It intensifies or reduces their water repellant
effect on the mineral surface.


Activator
-
soluble salts which ionizes in solution


Eg activation of sphalerite by cu in solution


Depressant
-
are used to increase the selectivity of
floatation by rendering certain minerals
hydrophilic thus preventing their floatation.


Eg slime coating


Cationic

modifiers:


Ba
2+
, Ca
2+
, Cu
+
, Pb
2+
, Zn
2+
, Ag
+



Anionic

modifiers:


SiO
3
2
-
, PO
4
3
-
, CN
-
, CO
3
2
-
, S
2
-



Organic
modifers
:


Dextrin
,
starch
,
glue
, CMC



pH modifier floatation is carried out at alkali
pH because most collector are stable at
higher pH & corrosion of cells,
pipework

is
minimized.


pH

modifiers such as:


Lime

CaO



Soda ash

Na
2
CO
3



Caustic soda

NaOH



Acid

H
2
SO
4
,
HCl


Specific ore applications


Sulfide ores


Copper (see
copper extraction
)




Copper
-
Molybdenum




Lead
-
Zinc




Lead
-
Zinc
-
Iron




Copper
-
Lead
-
Zinc
-
Iron




Gold
-
Silver




Oxide Copper and Lead




Nickel




Nickel
-
Copper




Nonsulfide

ores


Fluorite




Tungsten




Lithium




Tantalum




Tin




Coal





Ore
Concentration by Froth Flotation


Remember that only 0.67% of the ore is copper.
The copper minerals and waste rock are
separated at the mill using froth flotation. The
copper ore slurry from the grinding mills is
mixed with milk of lime (simply water and
ground
-
up limestone) to give a basic pH, pine oil
(yes, it comes from trees
--

a by
-
product of
paper mills) to make bubbles, an alcohol to
strengthen the bubbles, and a collector chemical
called
potassium amyl
xanthate

(or the
potassium salt of an alkyl
dithiocarbonate
).


The
xanthates

are added to the slurry in relatively
small quantities.
Xanthate

is a long hydrocarbon
(5 carbons) chain molecule. One end of the chain
(the ionic
dithiocarbonate
) is polar and sticks to
sulfide minerals while the other end is
nonpolar
,
containing the hydrocarbon chain is hydrophobic
--

it hates being in the water and is attracted to
the
nonpolar

hydrocarbon pine oil molecules.



Raising the pH causes the polar end to ionize more
and to preferentially stick to chalcopyrite (CuFeS
2
)
and leave the pyrite (FeS
2
) alone. Air is blown into the
tanks and agitated like a giant blender, producing a
foamy froth. The chalcopyrite grains become coated
with
xanthate

molecules with their hydrophobic ends
waving around trying desperately to get out of the
water.



They attach themselves to the oily air bubbles which
become coated with chalcopyrite grains as they rise
to the surface and flow over the edge of the tank. In
this manner through a series of steps the copper ore
is concentrated to an eventual value of over 28%
copper. Waste rock particles do not adhere to the
bubbles and drop to the bottom of the tank. The
waste material that comes out of the bottom of the
tanks at the tail end of this process is called
"tailings." It is nothing more than ground
-
up rock
with the copper minerals removed.




The
bubbles that flow over the edge of the first set of
flotation tanks (inside the mill building) end up in this
thickener. By then the bubbles have broken and the
slurry is poured into the center of this round tank.
The solid material settles to the gently sloping
bottom and is pushed toward the center by a systems
of rakes that slowly revolve around the tank.


The thickened slurry is pumped back into the mill for
further processing. The clarified water flows under
the small dam, that you can see just inside the
perimeter of this tank, flows over the side, and is
pumped back to the mill for reuse.


That stuff floating on top is "almost money"
-

just
chalcopyrite that hasn't sunk in the tan yet. A water
spray on the opposite side of that radial walkway
helps sink it.



Text
edited by Rob
Vugteveen
, Director,
Asarco Mineral Discovery Center


The
graphic
down shows
an air bubble
surrounded by grains of chalcopyrite that has
been coated with
xanthate
. The pine oil acts
as a
frother

only, providing the air bubbles
that the
xanthate

sticks to.


The
earliest patent relating to the mineral flotation process
is that of Haynes in 1860.

his recognition of the
differences in
wettability

of various

minerals by water and
oil formed the basis for a number of

“oil” flotation
processes.

During the next fifty years, there principal
stages of flotation development occurred:


Bulk oil flotation

used the fact minerals of metallic
luster

are preferentially wetted by oil the presence of water


consequently passing into the interface between the oil
and water


while the water


wetted gangue (worthless
rock) drops out.

This process requires large amounts of oil


usually one part for each part of are.


Skin flotation

used the fact that when finely ground dry ore
was gently brought into contact with still water, the
metallic particles tended to float more than did the
gangue.

This process was developed between 1890
-
1915.

However, both skin and bulk oil flotation were made
obsolete by the

froth flotation process.



William
Haynes in 1869 patented a process for separating sulfide and gangue
minerals using oil and called it
bulk
-
oil flotation
.


The first successful commercial flotation process for mineral
sulphides

was
invented by Frank Elmore
[1]

who worked on the development with his brother,
Stanley. The
Glasdir

copper mine at
Llanellyd
, near
Dolgellau
, in North Wales was
bought in 1896 by the Elmore brothers in conjunction with their father, William
Elmore. In 1897, the Elmore brothers installed the world's first industrial size
commercial flotation process for mineral beneficiation at the
Glasdir

copper mine.
The process was not froth flotation but used oil to agglomerate
pulverised

sulphides

and buoy them to the surface, and was patented in 1898 with a
description of the process published in 1903 in the Engineering and Mining
Journal. By this time they had recognized the importance of air bubbles in
assisting the oil to carry away the mineral particles. The
Elmores

had formed a
company known as the Ore Concentration Syndicate Ltd to promote the
commercial use of the process worldwide. However developments elsewhere,
particularly in Australia by Minerals Separation Ltd, led to decades of hard fought
legal battles and litigations which, ultimately, were lost as the process was
superseded by more advanced techniques.


The modern
froth

flotation process was independently invented the early 1900s in
Australia by C.V Potter and around the same time by G.D
Delprat
[2]
. Initially,
naturally occurring chemicals such as
fatty acids

and oils were used as flotation
reagents

in a large quantity to increase the
hydrophobicity

of the valuable
minerals. Since then, the process has been adapted and applied to a wide variety
of materials to be separated, and additional collector agents, including
surfactants

and synthetic compounds have been adopted for various applications.


In the 1960s the froth flotation technique was adapted for
deinking

recycled
paper
.



^
a

b

Beychok
, Milton R. (1967).
Aqueous Wastes from
Petroleum and Petrochemical Plants

(1st ed.). John
Wiley & Sons.
LCCN

67019834.




^

Lawrence K. Wang, Yung
-
Tse

Hung, Howard H. Lo
and Constantine
Yapijakis

(2004).
Handbook of
Industrial and Hazardous Wastes Treatment

(2nd ed.).
CRC Press.
ISBN

0
-
8247
-
4114
-
5
.




^

Kiuru
, H.;
Vahala
, R.,
eds

(2000). "Dissolved air
flotation in water and waste water treatment".
International conference on DAF in water and waste
water treatment No. 4, Helsinki, Finland
. IWA
Publishing, London.
ISBN 1
-
900222
-
81
-
7
.




Flotation Concentration

Asarco Mining Operations in Arizona
,
Mission Mine,
Tucson
,
Text edited by Rob
Vugteveen
, Director,
Asarco Mineral Discovery Center



The Columbia Electronic Encyclopedia® Copyright © 2007,
Columbia University Press. Licensed from Columbia
University Press. All rights reserved.
www.cc.columbia.edu/cu/cup/


recovery process.


www.grandpolycoats.com


US Patent References:


Flotation reagents

Parlman

et al.
-

March, 1984
-

4439314


Froth flotation process and collector composition

Wiechers

-

July, 1980
-

4211644


ORE FLOTATION PROCESS WITH POLY(ETHYLENE
-
PROPYLENE)GLYCOL FROTHERS

Booth
-

July, 1971
-

3595390


Method of making tertiary alkyl
trithiocarbonates

Crouch et al.
-

June, 1952
-

2600737


Flotation reagent

Ott

-

June, 1940
-

2203739