Thickeners
In
beneficiation plant, a conventional thickener
consists of a circular tank with a central feedwell and peripheral weir
overflow. Bottom slope will have a ratio of 1:12 up to 3:12 depending primarily
on tank diameter and particle size distribution. For large-diameter units or
for feeds containing a high percentage of coarser, fast-settling solids, a
double slope is used. The inner third (approximately) of the diameter might
have a 2:12 or 3:12 slope and the outer portion a 1:12 slope. (The use of
rectangular and square tanks in coal processing is gradually decreasing.)
Most
thickeners will run at 0%–20% of rated torque and as such will have a service life
of 20 years or more. The great amount of extra torque is called into play in an
upset condition (such as excessively coarse solids, underflow pump shutdown,
excessive tonnages, or foreign object) to prevent a shutdown and the need to
dig out the unit. The overflow is usually recycled back to processing for
reuse. If it is to be disposed of in a public water body, additional
clarification and other treatment may be required to meet regulations.
There are
three basic types of thickeners: bridge, center pier, and traction.:
The
bridge thickener supports the drivehead, centershaft, and rake mechanism from a
bridge across the tank diameter. Normally, the bridge thickener is the most
economical only up to tank diameters of 100 ft, but bridges have been built as
long as 140 ft. For tanks larger than 100 ft in diameter, a center pier usually
is less expensive. In a center pier thickener, a reinforced concrete or steel
center pier supports the drivehead and rake mechanism, and a cage connects the
two items (Figure 15). The underflow collects in a circular trough around the
center pier, and trough scrapers move the sludge. Normally, two or more ports
are used to connect to the pump suction manifold.
Both of
these units can be fitted with a lifting device, and the units can be covered
if required for process reasons. They normally have two long arms and an additional
two short arms if the raking load is high.
The
traction thickener employs a wheel drive that normally rides on a rail on the
side wall of the thickener and pulls one long arm. Usually, three short arms
are also used. Because no lift mechanism can be used, the installed torque is
normally higher than in other types of thickeners. Power enters the center of
the machine and connects through a commutator to the peripheral drive. This
type of unit is
not
widely employed but finds its greatest application in milder climates. The rail
surface must be at a
constant
elevation, and the unit is more expensive to cover.
A
thickener may be modified to produce a high-rate unit. When flocculants are
added, a relatively small amount of high-viscosity, low-specific-gravity fluid
(the flocculant) must be mixed with a large amount of feed slurry of higher
specific gravity that can also have a high viscosity. As the flocculant adsorbs
onto “what it sees,” slurry particles can be unevenly flocculated unless mixing
is relatively quick and thorough. Inserting flocculant into a launder with
baffles or a pipe in turbulent flow does not guarantee complete mixing. Some
slurries will require more shear or even more detention time than others to
obtain best results, although exposing the slurry to excessive shear or pumping
after flocculation will normally degrade the floc.
In one
type of high-rate unit, a relatively small diameter feedwell with internal
annular baffles may have up to three compartments (Figure 2). Various agitation
methods in the three compartments rapidly mix flocculant and slurry with
minimal hydraulic shear. A feed pipe permits flocculant to enter each
compartment on a controlled basis. About half usually enters in the top
compartment, and the rest is divided equally in the remaining ones. Baffling
also permits plug flow through the feedwell, and after exiting from the bottom,
the flocculated pulp falls to its specific gravity without further shear. Thus,
no floc degradation occurs.
When
flocculation is optimized, the area required by a thickener can be reduced to
only one-third to one-tenth of that required by conventional units. At the same
time, the unit must be more highly instrumented. These thickeners are used only
if flocculation is practiced.
Thickeners
have the double advantage of requiring relatively low horsepower and allowing
the construction of very large units (units of 750 ft in diameter have been
constructed), meaning that they have a large capacity. Very little operating
labor is required and maintenance is low.
评论
发表评论