Solid Surfaces—Adsorption - Surface and Colloid Chemistry: Principles and Applications - page 127

5.8 aPPlIcatIonS

5.8.1 f l of T a T I of n of f S o l I d p a r T I c l e S T o l I q u I d S u r f a c e
In only rare cases does one find minerals or metals in pure form (such as gold).
The earth's surface consists of a variety of minerals (major components: iron, silica
oxides, calcium, magnesium, aluminum, chromium, cobalt, and titanium).
Minerals as found in nature are always mixed together (e.g., zinc sulfide and feld-
spar minerals). In order to separate zinc sulfide, the mixture is suspended in water,
and air bubbles are made to achieve separation. This process is called lotation ,
where an ore heavier than water is floated by bubbles.
Flotation is a technical process in which suspended particles are clarified by
allowing them to float to the surface of the liquid medium. The material can thus be
removed by skimming at the surface. This is economically much cheaper than any
other process. If the suspended particles are heavier than the liquid (such as miner-
als), gas (air, CO 2 , or other suitable gas) bubbles can enhance the flotation.
Froth flotation commences by comminution, which is used to increase the surface
area of the ore for subsequent processing and to break the rocks into the desired
mineral and gangue (which then have to be separated from the desired mineral); the
ore is ground into a fine powder. The desired mineral is rendered hydrophobic by the
addition of a surfactant or collector chemical; the particular chemical depends on the
mineral being refined. As an example, pine oil is used to extract copper. This slurry
(more properly called the pulp ) of hydrophobic mineral-bearing ore and hydrophilic
gangue is then introduced to a water bath that is aerated, creating bubbles. The hydro-
phobic grains of mineral-bearing ore escape the water by attaching to the air bubbles,
which rises to the surface, forming a foam (more properly called a froth ). The froth is
removed, and the concentrated mineral is further refined.
The flotation industry is a very important area in metallurgy and other related
processes. The flotation method is based on treating a suspension of minerals (rang-
ing in size from 10 μm to 50 μm) in the water phase to air (or some other gas) bubbles
(Figure 5.13).
Flotation leads to separation of ores from the mixtures. It has been suggested that,
among other surface forces, the contact angle plays an important role. The gas (air or
other gas) bubble as attached to the solid particle should have a large contact angle
for separation (Figure 5.14). Further, it should be stable at the surface.

Air Bubble

Mineral

FIGure 5.13

Flotation of mineral particles as aided by air bubbles.