Mineral Processing – lab exercise
Coal flotation
1. Flotation
Flotation is one of the many methods of separation. It could be used for
separation of phases for instance to remove solid particles or oil drops from water.
More frequently flotation is used for separation of particles having different
hydrophobicities. Hydrophobicity is a feature of material characterizing its ability to
be wetted with a liquid in the presence of a gas phase. Solids, which can be easily
wetted with water, are called hydrophilic while solids with limited affinity for wetting
are called hydrophobic. As a result of hydrophobicity particles adhere to the gas
bubble forming a particle-air aggregate which is lighter than water, and travels
upwards to the surface of water (Fig. 1.). The hydrophilic particles do not adhere to the
bubbles and fall down to the bottom of the flotation tank.
water
gas bubble
intergrowths
hydrophobic
particle
hydrophilic particle
Fig. 1. Flotation
The hydrophilic-hydrophobic character of materials result from their
physicochemical properties or more precisely from a balance of forces operating at the
three solid-water, water-gas and solid-gas interfaces. These forces make the bubble to
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assume the shape and angle with the solid surface leading to minimization of the total
energy of the system. That angle is called the contact angle  (Fig.2.)
pęcherzyk

woda
ziarno
Fig. 2. Contact angle
and it is govern by the Young equation:
sg = sl + lg cos 
(1)
where:
sg - solid – gas interfacial tension (in mN/m or mJ/m2)
sl - solid – liquid interfacial tension
lg - liquid – gas interfacial tension
 - contact angle (in degrees).
A thermodynamic analysis of the flotation system indicates that the main
parameter of flotation is a combination of contact angle and surface tension:
Gf =lg (cos  - 1)
(2)
where:
Gf – Gibbs thermodynamic potential (free enthalpy) of flotation, mJ/m2.
When the surface tension of a given flotation system is constant, the contact
angle becomes the main parameter of flotation as well as a measure of hydrophobicity
of materials. The contact angle is measured between the lines drawn from the point
when the all three phase meet (Fig.2). It is a common practice to measure the angle of
contact between the liquid-gas and solid-liquid interfaces through the liquid phase.
Hydrophobicity of selected materials is given in Table 1.
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Table 1..Hydrohobicity of materials. Contact angle is in degrees and is based on flotometric
measurements
Strongly hydrophobic*
Material
1
Paraffin
CnH2n+2
hydrophobic

Material
3
2
90+ sulfides

Material
5
4
44–0 fluorite, CaF2
silicon carbide
SiC
hydrophilic**
=0
Material

6
7
gypsum
10–13
CaSO4·2H2O
weakly hydrophobic
Teflon, C2F4
90+
27,6 arsenic, As2O3
Sulfur, S
63,2 coal
Merkury, Hg
45,6 indum, In
Ge
Si
Talc
39,7 jodargyryt, AgI 23,5 diament, C
35,4 cassiterite, SnO2 22– tin, Sn
35,2 silver, Ag
14 boric acid, H3BO3
ilmenite, Fe
14 graphite, C
molibdenite,
5,9+ PbJ2
MoS2
gold, Au
26–0 perovskite, CaTiO3
25
szelit, CaWO4
barite, BaSO4
corundum, Al2O3
HgO
HgJ2
copper, Cu
9,3 ironsilicon
9
9
7,9
7,5
6,4
6,2+
6
dolomite
CaMg(CO3)2
magnetite
Fe3O4
halite, NaCl
brawn coal
kaolinite
hematite, Fe2O3
quartz, SiO2
5
calcite, CaCO3
anhydrite,
5
CaSO4
4 bones
3,3 turmaline
3 vegetables
3 iron, Fe
amber
ice, D2O
* Flotometric method is able to measure contact angles smaller than 90 o.
** Other hydrophilic materials: chromite, malachite, smitsonite, azurite, rutile, zircon, mica.
It results from Table 1 that most solid materials are only slightly hydrophobic or
hydrophilic. For these materials to float an application of a collector is necessary.
Typical collectors used for rendering certain groups of minerals hydrophobic are given
in Table 2.
Table 2. Classification of minerals according to their flotation properties
Class
Non-metals and solids with
significant natural hydrophobicity
Example
sulfur, graphite, coal, talc
Applied collectors
hydrocarbons, nonionic liquids
insoluble in water
Native metals and sulfides
gold, chalcocite
chalcopyrite, galena
sfalerite
cerusyte, smitsonite
xanthates, aerofloats
Oxidized minerals of non-ferrous
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xanthates (after sulfidization)
metals
malachite, tenorite, cuprite
Oxides, hydroxides and silicates
hematite, ilmenite
corundum, cassiterite
chromite, feldspars
kaolinite
fluorite, barite, calcite,
apatite, dolomite
halite, silvinite, carnalite
kiserite
Sparingly soluble salts
Soluble salts
siarczkowaniu), anionic and
cationic
anionic and cationic (with and
without activation using metal
ions)
anionic and cationic
cationic, seldom anionic
For successful froth flotation we need not only collector but also a frother. The
role of frother is to keep the floating particles in the most upper layer called froth for
easy removal of floating particles from the flotation system. The reagents used as
frothers are presented in Table 3.
Table 3. Flotation frothers
Group
1. aliphatic alcohols
a) linear
Frother
from amyl to decanol
iso-amyl
methyloisobutylocarbinol
diacetone
b) branched
c) with additional group
2. Cyclic
a) linear
b) branched
cyclohexanol
terpineol
cresols
xylenols
1,1,3-trietoxybuthane
R(X)nOH
R=H lub CnH2n+1
X=EO (ethylene oxide), PO (prophylene oxide)
BO (buthylene oxide) from 3 to 7
3. Aromatic
4. Alkoxy-hydrocarbons
5. Polyglycols
The structure of froth is presented in Fig. 3
froth
pulp
flot4
4
Fig. 3. Schematic presentation of froth and its structure with changes with the
position (height) in the flotation cell. Not to scale (magnification of upper layers is
greater)
Third group of reagents used in flotation is called the modifying reagents. The
group can be further divided into activators, depressant, pH, Eh, aggregation
regulators, etc.
2. Coal
Coal is an organic material of complex nature and structure. It does not have a
specific chemical formula. It contains many functional groups and units, mostly
aromatic, then aliphatic, and some oxygen while other elements such as S and N are in
smaller amounts. A very simplified formula of coal is given in Fig. 4.
H
H
H
H2C
H
H
H3C - CH
OH
CH2
weg2
Fig. 4. Simplified coal structure indicating typical chemical groups present in the coal.
Oxygen present in coal can exist as =CO, -COOH, -O-CH3, COC groups.
Coalification of coal is a process leading to an increase of carbon content in the
carbonaceous matter. The final stage of coalification is the formation of graphite (Fig.
5).
5
0
100%
20
60
40
60
n
40
e
yg
ox
hy
dr
o
ge
n
80
cellulose
brown
80
100%
20
hard coal
lignin
antracite
graphite
0
20
40
60
80
100%
carbon
Fig. 5. Coalification leads to increase in carbon content. The final product is graphite
The hydrophobicity of coal depends on its coalification degree(Fig. 6.)
contact angle, degree
80
60
40
coal
20
0
80
82 84 86 88 90 92
carbon content in coal , %
94
Fig. 6. Influence of coal coalification on contact angle (sessile drop method)
The contact angle of coal is greatly influenced by the method of measurement
(Fig. 7).
contact angle, degree
80
70
sessile drop
60
50
40
bubble attachment
30
captive bubble
r
20
10
flotometry
0
0
2
4
6
8
10
moisture content, %
6
12
14
Fig.7. Contact angle of coal depends on method of measurement (r – receding contact
angle)
Coal is subjected to flotation to separate carbonaceous (combustible) matter
from ash forming minerals. Usually only fine coal particles (size below 0.5 mm) is
used for flotation while greater particle are processed by other separation techniques,
mostly gravity methods. Coal contains from few to several percent of ash.
Apolar oils (mostly fuel oil) are used for flotation of coal. Flotation is possible
due to spreading of apolar oils on non-polar sites of coal. The adsorption is physical in
nature and is due to the van der Waals forces. Adsorption increases with hydrophobic
of coal.
Any reagent mentioned in Table 3 can be used in flotation of coal as frother. In
most cases, the frother is a mixture of reagents, frequently waste products. Their
commercial names and composition are subject to changes and modifications. The
main components of the frother mixture are higher aliphatic alcohols and alkyl
polyethoxy ethers.
Some coals are difficult to float. They need additional special reagent called
promoters (usually special alkyl polyethoxy ethers).
Flotation of coal increases in the presence of salts. The salts as a rule increase
the kinetics of flotation. A special addition of salt for coal flotation is not practiced due
to corrosion problems.
EXPERIMENTAL PART
Flotation of coal in Denver or Mechanobr laboratory flotation machine
1. Preparation of materials, reagents and equipment
a) Preparation of coal
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Use finely ground coal, which was previously ground by technicians in stages in
different grinding machines. Weigh 300 gram of the coal with a technical balance with
one decimal point accuracy.
b) Preparation of porcelain crucibles. The crucibles should be clean, dry, and stored in
a desiccator to keep their mass constant. They should be marked with a number on the
outside of the bottom. If they are not in desiccator, the mass of the crucible can be
slightly greater than real one due to moisture adsorption. The mass of the crucible
should be known with a 4-decimal-points accuracy.
c) Preparation of flotation collector
Fuel oil, dispersed in water, will be used as the collector (hydrophobization agent).
You will need 300 mg of fuel oil per 1kg of impure coal. Calculate how much oil in
cm3 is needed for flotation of 300 g of coal knowing that the density of fuel oil is
0.9g/cm3. Insert the oil with a pipette into a 400 cm3 beaker containing 100 cm3 of
water. Use ultrasonic treatment for about 30 seconds to prepare the emulsion. Prepare
emulsion shortly before use in flotation. Otherwise the emulsion will separate into oil
on top of water surface.
c) Preparation of frother
Use - terpineol as a frother. You will need to use 150 mg/kg of the frother. The
stock solution of - terpineol is 0.1% solution. Calculate how many cubic centimeters
of the 0.1% solution of - terpineol aqueous solution you will need for flotation.
Measure that value with a small graduated cylinder and get it ready.
d) Preparation of flotation machine
The laboratory flotation machines should be ready for experiment. Make sure
that you use 1.5 dcm3 metal flotation cell for flotation in Denver and 1.0 dcm3 plastic
cell for Mechanobr machine. Read the safety rules for operating the machine. Prepare
6 glass containers (5 having at least 0.5 dcm3 in volume for collecting the flotation
products and one larger (~2 dm3 for the tailing).
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2. Running the fractionating flotation test
Fill the flotation cell with 1.2 dm3 of water. Start the flotation machine with the
air valve closed and add the coal. Stir the flotation system for 5 minutes for wetting the
coal. Next, add freshly well-dispersed fuel oil emulsion and stir the system for
additional 3 minutes. Finally add the frother aqueous solution and stir the pulp for 1
min. After that slowly open the air valve and start to collect the first concentrate in the
first glass container or a pan. Make sure that you collect not too much of the
concentrate (about 30-50 g). Collect second concentrate of coal into a subsequent
container for such time intervals so each one will contain approximately similar
amounts of the solids. Collect no less than 4 products. When the flotation froth become
empty (no solids can be filled when the froth is tested with fingers) stop the flotation
machine. The remaining material in the cell is the tailing of flotation.
3.Evaluation of flotation performance
a) Determination of the yield of flotation products
Remove water from each flotation product by filtration in a Buchner funnel
under vacuum with a water pump. The moist filter cake of all products should be dried
in an oven at 105C. To speed up the evaluation of yield you may weigh the moist
products with a technical balance and then, assuming identical moisture of each
sample calculate the dried mass of the samples. Take a few gram-sample from each
moist product, spread it on a bottom of a flat glass container and put them into oven at
105o C for 5 minutes to dry the sample. Take about 0.5-g sample and weigh it very
precisely with an electronic balance with 4 decimal points accuracy. Put the samples to
pre-weighed crucible.
b) Determination of the ash and carbonaceous matter content in the flotation product.
Dry coal consists of carbonaceous matter and ash forming minerals. Burning
coal leaves ash in the container. The ash content in coal should be determined applying
appropriate standard procedures. The American procedure (ASTM D 3174) requires
that about 1 g coal to be put into a porcelain crucible and introduced to a roomtemperature furnace and heat up to 450-500o C within 1 hour and within two hours to
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700-750o C, and kept at this temperature for 2 hours. Next the furnace is turned off and
allowed to cool down. Burning should be performed in the presence of sufficient
amount of air. Calculate the content of ash in each product of flotation as an average
value of two ash determinations.
c) Evaluation of flotation performance
Having the yield of flotation products and their quality (ash + carbonaceous
matter content=100%) calculate cumulative yields, cumulative contents, and
cumulative recoveries using a balance sheet discussed in the mineral processing class.
Draw three different upgrading curves, for instance the Mayer (cumulative recovery
versus cumulative yield), Hall (cum. recovery versus cum. content), and Fuerstenau
(cum recovery versus cum. recovery) upgrading curves. Plot also the no-separation
and ideal separation lines in the graphs. Evaluate the degree of separation basing on
these lines and your upgrading curve. Predict values of separation parameters for the
case when recovery of the carbonaceous matter is 90%.
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