Chemical Engineering August 2014 - 63

FIGURE 3. While the automation and control techniques for gravity separators are
quite advanced, a typical modern gravity separator, as shown here, has utilized the
same operating principles for nearly 100 years
Deck surface openings range from
¼ in. to 325 mesh.
Specific gravity separation
While most engineers in industries
that process dry materials know
what a gravity separator (Figure 2)
does, its work is difficult to define
with precision. The statement " a
separator classifies dry, free-flowing,
granular mixtures by weight or
bulk density or specific gravity " is
accurate. But a more precise definition
would add the qualification
" if all the particles in the mixture
are the same size and shape. " With
equal accuracy, the statement could
be turned upside down to read, " if
all the particles are the same shape
and specific gravity, the separator
will classify them according to size. "
Unfortunately, all of the particles in
any given mixture are never exactly
alike in size or shape. One way to
more clearly understand the specific
gravity theory is to understand
the particle's terminal velocity (Vt),
which is defined in Equation (3), in
terms of mass (m), gravity (g) and
density (ρ). Frontal area (Af) is the
size and coefficient of drag (Cd) relates
to the shape and surface texture
of a given particle.
Vt =
2
ρ fd
mg
AC
(3)
The terminal velocity of particles
has historically been an extremely
important topic - one could look
back to Galileo dropping two rocks
(one heavy and one lighter) from a
tower or to Newton's confirmation of
the theory of gravity to arrive at the
beginning of evaluations of a particle's
terminal velocity. What Galileo
had intended to demonstrate was
that the attraction of gravity acts
equally on all bodies, regardless of
size or weight. This being true, they
should, neglecting air resistance,
accelerate at the same rate and
thus travel equal distance in the
same time.
Had Galileo employed in his experiment
smaller weights, say rock
fragments 1/4 and 1/16 in. in diameter,
he would have found air resistance
hard to neglect. It is this resistance
in any fluid, whether liquid
or gas, to motion of a solid body, that
makes gravity separation possible.
Figure 3 shows the clear separation
of solid components in a gravity separator,
as metal pellets move through
the machine via gravity flow. Gravity
alone, with equal intensity on all
bodies regardless of size or weight,
would be of no use unless there were
some other resisting force, sensitive
to size and weight, to balance it. And
taken further to understand how a
gravity separator works, the stratification
upon which the separation
largely depends, occurs according
to the terminal velocity in air of the
particles composing the mixture.
Particles with higher terminal velocities
are " heavies " and those with
lower values are " lights. "
Dry separation equipment first
appeared over a century ago when
the fluidized-bed separator, then
called the specific gravity separator,
was invented by Edwin Steele
and Henry Sutton [1]. Constructed
of wood, early gravity separators
were originally developed to concentrate
gold and other metallic
ores without using water. By 1919,
when a patent application for the
technology was submitted, this
new separator had found its way
into many other dry materials markets,
including field seeds, peanuts,
peas, beans, corn, beach sands, coal,
cork, chemicals and many other
bulk solids.
The gravity separator (also
known as fluidized-bed separator,
air table or density separator)
makes a highly sensitive dry separation
on the basis of one of three
particle characteristics: density, size
or shape. When two of these characteristics
are controlled within
certain limits, the gravity separator
is unmatched in its ability to
separate a complex mixture into
a continuous gradation across the
range of differentiating characteristics
(light to heavy, fine to coarse, or
platy to granular), while permitting
the isolation of many intermediate
fractions between the two extremes.
The ability to produce intermediate
or " middling " fractions distinguishes
these machines from other
kinds of dry separation equipment.
This property and this property
alone, permits the development of
high-purity concentrations without
loss of efficiency in recovery. For
example, when processing copper
wire, a gravity separator will divide
insulation materials into copper,
insulation and copper-containing
insulation, so the latter can be reduced
further before being brought
back to the gravity separator, as
seen in Figure 4.
In addition to material density, the
relative size and shape of each component
of the mixture also bear on
the efficiency of the separation. Wide
variations in these material characteristics
can dramatically affect the
separation results. Where a wide
range of particle sizes is present,
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Chemical Engineering August 2014

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