Preparation
About half the coal presently mined in
the United States is cleaned mechanically to remove impurities and
supply a marketable product. Mechanical mining has increased the
proportion of fine coal and noncoal minerals in the product.
At the preparation plant run-of-mine
coal is usually given a preliminary size reduction with roll crushers
or rotary breakers. Large or heavy impurities are then removed by
hand picking or screening.
Tramp iron is usually removed by
magnets. Before washing, the coal may be given a preliminary size
fractionation by screening.
Nearly all preparation practices are
based on density differences between coal and its associated
impurities. Heavy-medium separators using magnetite or sand
suspensions in water come closest to ideal gravity separation
conditions. Mechanical devices include jigs, classifiers, washing
tables, cyclones, and centrifuges.
Fine coal, less than 1⁄4 in (6.3 mm)
is usually treated separately, and may be cleaned by froth flotation.
Dewatering of the washed and sized coal may be accomplished by
screening, centrifuging, or filtering, and finally, the fine coal may
be heated to complete the drying.
Before shipment the coal may be
dustproofed and freezeproofed with oil or salt. Removal of sulfur
from coal is an important aspect of preparation because of the role
of sulfur dioxide in air pollution.
Pyrite, the main inorganic sulfur
mineral, is partly removed along with other minerals in conventional
cleaning. Processes to improve pyrite removal are being developed.
These include magnetic and electrostatic separation, chemical
leaching, and specialized froth flotation.
Storage
Coal may heat spontaneously, with the
likelihood of self-heating greatest among coals of lowest rank. The
heating begins when freshly broken coal is exposed to air. The
process accelerates with increase in temperature, and active burning
will result if the heat from oxidation is not dissipated.
The finer sizes of coal, having more
surface area per unit weight than the larger sizes, are more
susceptible to spontaneous heating.
The prevention of spontaneous heating
in storage poses a problem of minimizing oxidation and of dissipating
any heat produced. Air may carry away heat, but it also brings oxygen
to create more heat.
Spontaneous heating can be prevented or
lessened by (1) storing coal underwater; (2) compressing the pile in
layers, as with a road roller, to retard access of air; (3) storing
large-size coal; (4) preventing any segregation of sizes in the pile;
(5) storing in small piles; (6) keeping the storage pile as low as
possible (6 ft is the limit for many coals); (7) keeping storage away
from any external sources of heat; (8) avoiding any draft of air
through the coal; (9) using older portions of the storage first and
avoiding accumulations of old coal in corners. It is desirable to
watch the temperature of the pile.
A thermometer inserted in an iron tube
driven into the coal pile will reveal the temperature. When the coal
reaches a temperature of 50°C (120°F), it should be moved.
Using water to put out a fire, although
effective for the moment, may only delay the necessity of moving the
coal. Furthermore, this may be dangerous because steam and coal can
react at high temperatures to form carbon monoxide and hydrogen.
No comments:
Post a Comment