Flotation Basics

Froth Floatation
Floatation is an extractive process where various minerals can be selectively extracted. For example,
in poly-metal ores such as Pb-Zn-Cu, floatation allows separate extraction of Pb, Cu and Zn.
Floatation was originally intended to “mine” tailings (in the early days), which used to be very rich
by current standards.
Three basic mechanisms of froth floatation: i) attachment of a specific mineral particle to air bubbles
ii) being carried by the water in the froth and iii) caught between particles in the froth
Froth Floatation:
• Takes advantage of surface properties of minerals
Chemistry
Froth
Flotation
System
Equipment Operational conditions

• Mineralogy
• Feed rate
• PSD
• Temperature
• pH
A few definitions:
Hydrophobic: repels water. Such minerals will attach to air rather than water
Hydrophyllic: attracted to water. Such minerals will attach to water rather than air.
Collectors: Render minerals hydrophobic by attaching to the surface of mineral particles.
Frothers: Produces stable bubbles for hydrophobic particles to attach to.

Modifiers: Activators, depressants or pH modifiers. Alters selectivity of the collectors.
Steps involved mineral recovery through floatation
1. Liberation
2. Establish hydrophobic film on minerals that are not naturally hydrophobic.
3. Promote bubble formation.
Since water is a polar molecule, polar mineral surfaces tend to be hydrophyllic. Whereas,
nonpolar mineral surfaces tend to be hydrophobic. The goal is to make the mineral
surface hydrophobic so the minerals will attach to the bubbles in the froth.
Degree of polarity of mineral types:

Sulfides < sulfates < carbonates < phosphates < oxides < hydroxides < silicates.
Types of nonpolar minerals include:
Coal, oil, molybdenite, diamond, talc, gold
Collecters, and frothers, are organic molecules containing a polar head and a nonpolar
tail, as shown in figure 1.
Figure 1: collector structure and type.
Molecules, such as the one shown in fig 1, are polar due to localization of electrons
around one part of the molecule. In this case, the carboxyl group is an anionic polar end.
The oxygen atoms are more electronegative than the carbons to which they are attached.
So electrons become localized around the oxygen atoms. The organic tail, often
delineated with a (R) does have a slight positive charge. However, the tail is so large that
the charge becomes very dispersed throughout the tail, rendering it essentially nonpolar.
The longer the tail is the more “nonpolar” the tail will be. So water repulsion increases as
tail increases. BUT, solubility decreases (though not for soaps). The carboxylate
collectors, as in figure 1 are strong collectors, but they have low selectivity. The sodium
oleate is, by the way, the same molecule one would find in lye soap.
A collector’s polar head will attach to a polar mineral surface, leaving the nonpolar tails
toward the surrounding water molecules, rendering the mineral hydrophobic, as shown in
figure 2, below. It should be more accurate to say that hydrophobic substances are not
repelled by water, but that water is much more attracted to itself than to the hydrophobic
substance. Thus, water displaces the hydrophobic substance.
Fig 2: interaction of collector with mineral surface.
Types of anionic collectors include:

• Oxyhydryl (organic acid or sulfo acid group)
¾ Carboxylic ---
Derived from fatty acids from vegetable/animal oil. Lye soap is an example carboxylate.
These are strong collectors but have poor selectivity.
¾ Sulphate -----------------
¾ Sulphonate ----
• Sulphydryl (Most popular, due to higher selectivity)
¾ Xanthates --- ------ (most commonly used)
Used for sulfide ores (such as galena, sphalerite and chalcopyrite). Have low
solubility for Cu, Pb, Hg and Ag. Good for Zn and Fe. They oxidize easily
making storage difficult after a few months. Oxidation also reduces selectivity.
They form insoluble salts with ions of Cu, Pb and heavy metals. Reduces
effectiveness of collector. This can be prevented by maintaining alkaline
conditions to precipitate heavy metals.
¾ Dithiophosphates -----------
o Dosage of xanthate should be 0.02 to 0.2 pounds/ ton

Cationic Collectors -------
Are amines (i.e. organic compound that contains N). They attach based on
electrostatic attraction rather than based on chemistry. Hence weaker than anionic
collectors. Very sensitive to pH. Best in mildly acidic solutions. Used for floating
oxides and carbonates.
Table 1: collectors and their uses.
Frothers:
When the mineral surfaces have been rendered hydrophobic by the collector, the frother
is meant to produce a stable bubble. Frothers also act as collectors, which can lead to
decreased selectivity. Some collectors are such good frothers as well that they stabilize
the froth so much as to reduce their transportability. Too stable a froth also results in too
much frothing and bubble buildup downstream. Example frothers include pine oil and
MIBC (methyl isobutyl carbinol).
Properties of Frothers
• Contain an OH-
• Have same collecting power as collectors
• Use 0.01 to 0.65 pounds/ ton
Things to keep in mind about frothers:

• Control bubble size
• Coarse particles recovered by multiple SMALL bubble attachment
• Coarse particles are not recovered by big bubbles.
Contact Angle
Contact angle (θ) is directly related to the hydrophobicity of the mineral surface. It is the
angle produced between the flat mineral surface and the air bubble, as shown bellow.
Table 2: Contact angles

AIR
Material θ θ
Parafin 105
Graphite 86
Talc 88
Sulfur 85
Moly 75
Quartz 0
Figure 3: Contact angle

pH dependence:
If possible, flotation is carried out at alkali pH’s (above 7) because most collectors
are stable at higher pH’s. pH modifiers are used in large quantities and so their
contribution to cost can be very high.
• Xanthates break down between pH (4 & 5)
• Higher pH’s minimize corrosion of cells and plumbing.
Each mineral has a pH above which it will not float called the critical pH. The
critical pH is dependant on the concentration of the collector.
Chemicals for RAISING pH:

• CaO, NaOH, NaCO3
Chemicals used for LOWERING pH:

• H2SO4, HCl
Fig 4: critical pH vs. collector concentration
Figure 4 shows critical pH vs collector concentration for three minerals. If we want to

float chalcopyrite from Galena at a collector concentration of 50 mg/liter, a pH of 8
to 9 would work well. AT a pH of 8, Chalcopyrite will float. Pyrite and galena will not.
Lime is relatively cheap and the most common agent for raising the pH. In the
example of figure 4, we can say that lime acts as a depressant for Galena and
Pyrite. Hence, soda ash is used to modify pH when galena needs to be floated.
Some depressants include:
• Lime- (for pyrite, free gold)
• CN- --(for Pyrite)
• Potassium chromate
• Potassium permanganate.
Practical solutions:
Pb-Zn floatation: Cu- Zn Floatation:
• Lean xanthate addition CN-
• PH~7 to 8 ¾ added to consume SO4 in
¾ Float galena solution.
• PH~ 4 to 5 (add H2SO4)
• CuSO4 activation PH~ 9 to 10 (CaO)
• Xanthate, dithiophosphate Add Xanthate
¾ Float sphalerite ¾ Float copper minerals
PH ~4 to 5
¾ Destroy Xanthate
Activate with CuSO4

Add Xanthate- dithiophosphate
¾ Float sphalerite
The required volume of a flotation tank is given by:
V=T*R*A*C
Where:
• V is in ft3,
• T is design TPH of solids (not slurry)
• R is residence time as determined in the lab.
• A is an aeration factor.
¾ If one expects 15% air in the tank, then the rest is 85%. The aeration
factor (A) is 1/0.85= 1.18.
• C is CFM of slurry per TPH solids

Flotation Basics

Uploaded by

Copyright:

Available Formats

Flotation Basics

Uploaded by

Document Information

Copyright

Available Formats

Share this document

Share or Embed Document

Sharing Options

Did you find this document useful?

Is this content inappropriate?

Copyright:

Available Formats

Flotation Basics

Uploaded by

Copyright:

Available Formats

What are the basic mechanisms of froth floatation?

What are the basic mechanisms of froth floatation?

What are collectors and frothers and how do they work?

What are collectors and frothers and how do they work?

Froth Floatation

• Takes advantage of surface properties of minerals

Equipment Operational conditions

Collectors: Render minerals hydrophobic by attaching to the surface of mineral particles.

Frothers: Produces stable bubbles for hydrophobic particles to attach to.

Degree of polarity of mineral types:

Types of nonpolar minerals include:

Coal, oil, molybdenite, diamond, talc, gold

Types of anionic collectors include:

• Sulphydryl (Most popular, due to higher selectivity)

¾ Xanthates --- ------ (most commonly used)

o Dosage of xanthate should be 0.02 to 0.2 pounds/ ton

Things to keep in mind about frothers:

Table 2: Contact angles

Figure 3: Contact angle

Chemicals for RAISING pH:

Chemicals used for LOWERING pH:

Fig 4: critical pH vs. collector concentration

Figure 4 shows critical pH vs collector concentration for three minerals. If we want to

 Activate with CuSO4

You might also like

Activate with CuSO4