Flotation
Flotation is a method of concentrating solid minerals in a relatively
finely divided state. It is essentially a method of gravity concentration in
water in which the effective specific gravity of certain of the ore minerals
is substantially decreased by causing air bubbles to attach more or less
tenaciously to particles of that particular mineral, whereupon they float on
the separating medium while the unaffected minerals sink. The lightening
may be insufficient to cause: actual buoying to the surface but yet enough
to cause differential travel in a gravity separator such as a shaking table or
hydraulic classifier. When the selected mineral is separated in the form of
froth the operation is called froth flotation. This is the usual method.
Steps of Froth-Flotation Method. In essential outline the ordinary
simple froth-flotation operation comprises the following steps: 1) Grinding
the ore in water to a maximum size of 35 or 48-m.; 2) Dilution to a pulp
consistency of 15 to 35 per cent solids; 3) Addition to the pulp of small
quantities of one or more various inorganic conditioning agents, which
have a number of (unctions; 4) Addition of a collector reagent which has
the function of coating the mineral to be floated with a water-repellent
film; 5) Addition of a frothing agent which imparts persistence to bubbles
when they reach the surface; 6) Aeration either by agitation, or by air
Injection as through the porous bottom of the containing tank, or through
pipes, during which the coated mineral particles become more or loss
firmly attached to gas bubbles; 7) Separation of mineral-bearing troth from
a liquid pulp containing residual particles which did not lake on collector
coatings.
These steps frequently follow the sequence stated, but the conditioning
agent, and sometimes the collector too, may be added to the grinding mill;
dilution is usually effected in the classifier; the frothing may be added in
the flotation machine where aeration is going, or it may lie added in the
classifier or even in the ball mill.
Principles of Flotation. A dotation machine and its contents, in
continuous operation, comprise a system in a rough sort of dynamic and
chemical equilibrium. Into this system ore, water, air, and a variety of
chemicals in small quantities are introduced continuously; from it flow
continuously two or more streams of products which differ obviously in
physical state and somewhat less apparently in chemical composition. The
overflow stream is usually a froth carrying a load of solid which is
different in mineralogical character from that of the other product streams
and of the feed stream. The underflows are suspensions of the residue of
the feed solid in the balance of the feed water.
Determination of the distribution of the added chemicals in these
product streams has been the key to the understanding and control of the
process. From such study it has developed that the added reagents have a
variety of functions. These have been assigned descriptive names as
follows:
1. Collection is a selective change in the surface of one mineral species
or genus present in the ore, as a result of which the changed mineral is
rendered water repellent, the other mineral species present remaining
unaffected and water-wet. Reagents which effect collection are called
collectors, or collecting agents, and less frequently, promotors.
Collection is performed for the purpose of producing selective waterrepellent coatings on the mineral particles which are to be separated from
the pulp, in order that gas bubbles may cling to these particles. The
coatings are of hydrocarbon or hydrocarbonlike nature. In the simplest
case all that is necessary to effect collection is to add to the pulp a small
amount of one of a certain class of organic chemicals (collectors), and to
agitate the mixture long enough to effect thorough dispersion and permit
the desired chemical reaction. If the pulp contains substances that interfere
with the reaction, or lacks other substances that facilitate it, remedial
treatment is required.
2. Conditioning comprises changes in mineral surfaces and in the
composition of the aqueous solution. Such changes in surface as aid
collection of the changed particle are called activation.
Surface changes that prevent collection are called depression; changes
in the solution which do not at the same time effect changes in particle
surfaces are designed to protect other reagents and are, therefore, called
protection. Other treatments of the flotation pulp such as heating,
desliming, dewatering, etc., which contribute to the success of flotation,
are also usually classed under the head of conditioning.
3. Levitation is the act of lightening collector-coated particles by
causing them to become attached to air bubbles. By extension the term is
also applied to flotation at the upper surface of the suspending medium,
and to attachment to droplets of a liquid lighter than and immiscible with
the suspending liquid.
4. Frothing is the act of producing a collection of bubbles froth at the
surface of the suspending medium. It is usually implied that the froth
carries a load of levitated solid.