Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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BACKGROUND
It is well known that coal piles having at least one surface
exposed to the atmosphere suffer from a number of disadvantageous conditions.
For purposes of this specification and claims, the term "pile" encompasses
any cong10meration of coal lumps, e.~., stockpiled on the ground for use in
industrial plants, electric utilities, and the like; stowed in open bins,
railway freight cars, trucks, and the like; moved by conveyeri and the like.
Some problems which occur regularly in the storage and transporta- -
tion of coal include the following:
1. Excess moisture in the body of coal caused by excessive
humidity, rain, or snow results in-
a) loss of fuel value (by as much as 3% or more) due to loss
of heat to water evaporation during use;
b) Prevention of normal operation of grinding and pulver-
izing machinery which frequently cause outages in factory
or electric utility operation;
c) Water freezing which makes it very difficult to remove the
coal from the storage pile.
2. The free flow of air through the pile induces auto-oxidation
of the coal which can reduce fuel value by as much as 5% or more in a year.
Continued auto-oxidation can reach the point of spontaneous combustion,
particularly in low-rank coals.
3. -Dust and acid leachates emanating fronrcoal piles and conveyers
create air and soil pollution and loss of fuel value.
4. Wind and the motion of train or truck can result in:
a) physical losses of coal from the upper surface of the pile,
and
b) generation of coal fines which are not physically usable
as fuel in many facilities and must be segregated and
disposed of separately.
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Efforts have been made to reduce these problems, but with limited
success aside from the fact that they are excessively costly and none has
been able to eliminate most of the problems. Layering and colnpaction of
coal stores have been used to reduce void volume and minimize air circulation
with only partial success. Costly oil sprays, which have been tried to
reduce dusting and oxidation,'have been only partially successful. Exper-
iments have shown that spontaneous combustion can be eliminated by vapor
deposition of SiO2 on the surface of the coal, but such a solution is imprac-
tically costly.
Although certain of the coating compositions containing pitch,alphalt, tars, and the like are somewhat similar to compositions known to
the art for such use as roofing materials, mastic tile, the making of carbon
articles, e.g., electrodes, and sealing compositions, none has been disclosed
for the purpose of the present invention.
The coating of the present invention, when applied to the exposed
surface of a coal`pile (as defined supra), melds into a hard, continuous
coating which adheres well to the coal and, when applied to the entire
exposed surface, forms an air- and moisture-impervious layer which protects
the pile from the problems engendered by excessive moisture and air penetra-
tion. Additionally, it substantially eliminates dust and acid leachate
emanations and protects piles in transit from physical losses due to wind
and the motion of the vehicle and, because of the immobilizing effect of the
hard, continuous surface layer, minimizes generation of coal fines due to
frictional m~tion~within the pile during transit.
Thus the surface layers of the invention substantially eliminate
loss of fuel value, processing difficulties, and air and soil pollution by
substantially eliminating auto-oxidation, spontaneous combustion, excessive
moisture, dust emanation, obstructions to handling, wind and motion loss.
The coating composltions are low cost, particularly where the lower cost
binders and waste material fillers are employed. Their cost, furthermore,
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is substantially offset by their added fuel-value, since they contribute to
the combustion process.
SUMMARY
The coating compositions comprise a binder comprising wax, pitch,
asphalt, or tar with or without~.preferably with, at 1east a small proportion
of a synthetic organic polymer, dispersed therein, and a filler comprising
a particulate organic filler, such as coal fines, bagasse, seed hulls, saw-
dust, shredded paper, and the like.
The coatings are of a fluid or semi-fluid consistency, generally
produced by elevation of the temperature, so that they can be readily applied
to the exposed surface(s) of the coal pile. The coatings adhere well to the
exposed surface coal in the pile and harden into a tough continuous film of
a desired thickness, which, as aforedescribed, is substantially impervious
to air and moisture; contain any coal dust and fines within the pilei and
prevent physical loss of coal to wind or the motion of a carrying vehicle
in transit.
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, DETAILED DESCRIPTION
The coating includes an organic binder which can be wax, tar,
asphalt, pitch, or mixture thereof. These materials can be natural or
synthetic. The tars, p`itches, and asphalts can be, for example, from
nat-ural sources or the residues of petroleum refining or other chemical
processes. The waxes can be paraffin waxes of mineral origin, such as
petroleum, or of vegetable origin, such as ceresin, caudilla, or carnauba.
The paraffin waxes are preferred because of their greater availability and
relatively low cost, particularly in the case of such low-grade wax as
slack wax or the somewhat higher grade scale wax. Slack wax alone or,
preferably, in combination with scale wax, has been found particularly
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useful in practicing the invention. The scale wax produces a somewhat
harder, higher melting point, ar,d higher molecular weight mixture.
It has been found desirable, although not essential, to incorpor-
ate into the binder a minor proportion, preferably about 5 to 30%, of a
synthetic organic polymer since they result in a somewhat tougher or
stronger final coating whjc~ does not break easily; minimize stratification
or separation of the coating components until the coating solidifies; ana
may improve melt viscosities. The polymers are preferably thermoplastic.
They may, however, be thermosetting plastics in liquid state which harden
into solids by cure after application of the coating composition. Exdmples
of suitable polymers include, but are not limited to, polyalkylenes, poly-
esters, polybutadienes, polystyrene, and the like. In general, the poly-
alkylenes, such as polyethylene, polypropylene, and polybutylene, are
preferred. Polyethylene is particularly preferred.
The synthetic polymer is dispersed, preferably dissolved, in the
wax, tar, asphalt, or pitch, prior to incorporation of the filler, generally
at elevated temperatures.
The filler, as aforementioned, can be an organic particulate solid,
such as coal fines, shredded paper, seed hulls, bagasse, sa~dust, and the
like. ,They are preferably waste-type materials because of their low cost.
In general, coal fines are preferred because of their frequent on site avail-
ability. Size is not critical, though relatively fine comminution is desir-
able, e.g., about -10 to +200 Tyler mesh size. The filler is introduced into
the binder_in major proportion by weight, prefera~ly about 65 to 85~
In preparing the filled composition, the binder is generally heated
sufficiently to provide suffic;ent fluidity to facilitate uniform mixing and
coating of the filler. The particular mixing temperature is, of course,
determined by the particular binder. In some cases, the filler may also be
preheated to offset excessive temperature reduction of the binder during
mixing or to improve binder-wetting of the particles.
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The resulting coating compositions while at a temperature determined
by the particular ~luidity of the composition, e.g., ambient to about 170C
are generally relatively smoo~h, viscous materials and are usually, though
not necessarily, thixotropic. After application, they harden into tough solid
materials which require substantial force for breakage. They also have a
relatively low coefficient of. expansion which is partlcularly important as
a fissure preventative under widely varying ambient temperature conditions~
The compositions are applied to the exposed coal pile surface in at
least a semi-fluid state, if necessary, at elevated temperatures, e.g., about
12~C-170C, by any suitable method, such as pump spraying, gun sputtering,
and the like. It has been found that the deposited discrete gobs or droplets
of the coating, if properly applied to and distributed on the coal, flow
sufficiently to meld into a continuous~ coal-adherent coating. Any bare
spots subsequently found can be filled with added coating material which
weld to the original coating. The coatings can be of any desired thickness,
e.g., one-quarter inch or less to one inch or more. In general for purposes
of economy, the least thickness adequate for the purpose is desirable.
At the time the coal pile requires transfer or use, the coating can
be broken to pieces of desired size by the use of devices applying adequate
force and then be utilized as fuel together with the coal, including applica-
tion of coal grinding or pulverizing.
EXAMPLE
The following coating compositions were prepared. Percentages are
by weight.
A. Binder - 25%: PE* - lO%, slack wax - 90%
Filler - 75%: coal fines
B. Binder - ~8.~)%: PE - 15~, slack wax - 90%
Filler - 71.4%: coal fines
C. Binder - 25%: PE - 12%, slack wax - 44%, scale wax - 44%
~polyethylene
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In all of the above compositions, the coal fines were -35 to ~l20
Tyler mesh sizei the binder WdS heated to about l3~C; and the coal fines
were heated to somewhat above 100C. A11 of the resulting compositions were
smooth, semi-fluid, continuous, and thixotropic.
They were each applied at a temperature of about 135~C to a coal
pile by being ~ropped in gobs from a height of about 4 feet. The gobs on
the codl pile surface spread su~ficiently to meld into a continuous coati~ng
which adhered to the coal at the pile surface and hardened as it cooled into
a tough continuous coating, which required application of substantial force
to break. Because of the essentially thixotropic nature of the coating
composition, there was minimal tendency to sag or spread substantially below
the surface portion of the coal pi]e.
Although this invention has been described with reference to illus-
trative embodiments thereof, it will be apparent to those skilled in the art
that the principles of this invention can be embodied in other forms but
within the scope of the claims.
