United States Patent [19]
Kindig et at
[11] 3,961,914
[45] June 8, 1976
[52] U.S. CI... 44/1 R; 44/6
[51] Int.CI.2 ClOL9/00
[58] Field of Search 44/1 R, I F, I G,6,
44/l6 C, 16 E; 252/446
[54] PROCESS FOR TREATING COAL TO MAKE
IT RESISTANT TO SPONTANEOUS
COMBUSTION
[75] Inventors: James K. Kindig, Arvada; Ronald L.
Turner, Lakewood, both of Colo.
[73] Assignee: Hazen Research, Inc., Golden, Colo.
[22] Filed: July 26, 1974
[21] Appl. No.: 492,257
2,991,201
3,434,947
3,684,490
3,781,405
[56]
869,043
1,496,004
1,670,865
1,834,960
1,973,300
2,347,140
References Cited
UNITED STATES PATENTS
10/1907 Arden 75/42
6/1924 Laist 423/1 09
5/1928 Miller. 44/16 E
12/1931 Mitchel!.. 423/1 06
9/1934 Thompson, Jr. 423/544
4/1944 Weimer 44/1 R
7/1961 Joyce 252/446 X
3/1969 Steintuent... 423/140
8/1972 Steintueit... 423/1 09
12/1973 Allan et al. 423/633
OTHER PUBLICATIONS
Rastas, J. et aI., Treatment ofIron Residues in the Electrolytic
Zinc Process, TMS Paper Selection A73-11,
The Metallurgical Society of Aime, NY, NY.
Primary Examiner-Carl F. Dees
Attorney, Agent, or Firm-Sheridan, Ross & Fields
[57] ABSTRACT
Coal particles are made resistant to spontaneous combustion
by coating them with silicon dioxide by a
vapor deposition process in which the particles in a
coating chamber are contacted at a temperature that
promotes the deposition on the particles of the reaction
products between silicon tetrachloride vapor and
water, the coating step followed by elevating the
chamber temperature to drive off water vapor and hydrochloric
acid.
16 Claims, 1 Drawing Figure
u.s. Patent June 8, 1976 3,961,914
TIME - TEMPERATURE RESPONSE OF COAL SUBJECTED TO
HOT WATER-SATURATED OXYGEN
LL
o
w
a::
::::>
~
a::
w
Q.
~
w
I-
...J
<l: oU
DRIED UNCOATED COAL
DRIED COATED COAL
50
O~_----I~_---L._--L..-_-1-__..l-_--'Lo.,,-----L--~
TIME (HOURS)
3,961,914
SUMMARY OF THE INVENTION
DESCRIPTION OF THE PREFERRED
EMBODIMENTS
SiC!. + 2H20 -. Si02 + 4 Hel
BRIEF DESCRIPTION OF THE DRAWING
The single drawing is a time-temperature graph
showing the comparative responses of dried coated and
uncoated coal subjected to hot water-saturated oxygen.
The reactants, silicon tetrachloride, which is introduced
into the chamber in a gas stream, and water,
which may be introduced into the chamber as a separate
gas stream or may be water associated with the
coal already in the chamber, combine to form interme-
2
ous combustion; however, the treatment requires addition
of water which materially offsets the advantages of
drying.
U.S. Pat. No. 3,014,815 discloses a process for coat-
5 ing articles with a metal oxide by introducing into a
coating chamber containing the articles a hydrolysable
metal compound" which may be the chloride of the
metal, oxygen, and hydrogen or a compound which
produces hydrogen. The chloride is hydrolysed by the
water formed to produce the metal oxide with which
the article is coated. The patent does not disclose coating
coal and does not disclose tlhe use of silicon dioxide
as coating material. Further, and most important, the
source of water required for the process is oxygen and
a compound which reacts with oxygen to form water;
the formation of water by this method requires heating
the articles to at least approximately 1112°F (600°C).
The principal object of the present invention is to
improve coal by making it less susceptible to spontaneous
combustion or even entirely resistant to spontaneous
combustion. A more particular object is to provide
a process for coating particulate coal with an incombustible
coating.
Another object of this invention is to provide an
incombustible coating on the dried coal particles to
make them substantially resistant to spontaneous combustion.
Additionally the invention helps allay the dust
30 problem and slows down the reabsorption of water
vapor.
The above and other objects are accomplished by
coating coal particles with silicon dioxide in a vapor
deposition process in which the particles in a coating
chamber are contacted at or below room temperature
either with silicon tetrachloride vapor and water vapor
introduced into the chamber in separate gas streams or
with silicon tetrachloride introduced in a separate
stream which interacts with water associated with the
coal in order to coat the surface and interstices of the
coal with the reaction products of the gases, the coating
step followed by elevating the chamber temperature to
drive off water vapor and hydrochloric acid.
20
In accordance with the invention an extremely thin
inert coating can be applied to the coal particles to
exclude air from the coated part of the coal thereby
inhibiting air oxidation and spontaneous burning. This
inhibiting coating will last until the coal reaches the
power plant and is pulverized just before firing. The net
chemical reaction by which the coating material, sili-
60 con dioxide, is produced in the reaction chamber is as
follows:
1
PROCESS FOR TREATING COAL TO MAKE IT
RESISTANT TO SPONTANEOUS COMBUSTION
BACKGROUND OF THE INVENTION
I. Field of the Invention
The invention relates generally to a process for improving
coal to make it less susceptible to spontaneous
combustion. Particularly, the process of the invention
relates to coating coal particles with an incombustible 10
coating, specifically, silicon dioxide.
2. Status of the Industry
The rapid growth of electrical demand in the United
States, the concern for low sulfur dioxide emissions
from power plants, and the national goal for energy 15
self-sufficiency have all fostered an unprecedented
boom in Western coal. This low-rank, low sulfur, highmoisture
coal is now shipped by unit trains carrying
10,000 tons of coal thousands of miles to Eastern and
Southern electrical utilities.
There are certain disadvantages in shipping and storing
low rank coal. First of all, Western coals frequently
contain 25% or more inherent moisture, and the shipping
cost for transporting this much moisture in large
shipments is significant. Secondly, all coals and espe- 25
cially low-rank coals have a tendency to ignite spontaneously
during shipping or storage. Thirdly, low-rank
coals are usually quite dusty, even through they may
contain 25% inherent moisture.
3. Prior Art
In an article entitled "Self-Heating of Carbonaceous
Materials" by F. L. Shea, Jr., and H. L. Hsu, Great
Lakes Research Corporation, Elizabethton, Tennessee,
the authors reported the development of a simple test
for the determination of the self-heating rates of vari- 35
ous carbonaceous materials. They state that their test,
which provides for contact between oxygen at 150°F
(66°C) saturated with water vapor and materials which
contain sufficient hydrocarbons to combine with oxygen
at around 300°F (149°C) and thus ignite at rela- 40
tively low temperatures, provides a good measure of
the liability of the material to spontaneously ignite. The
effect of moisture in the self-heating process is extremely
important. This was well illustrated by the
behavior of raw lignite which, in the presencee of dry 45
oxygen at 150°F (66°C) experienced a temperature rise
of only 18°F (10°C) inabout five hours while in the
presence of oxygen saturated with water vapor at 150°F
(66°C) the rate of temperature rise was on the order of
tenfold greater, with the lignite igniting in less than 5 50
hours.
U.S. Pat. No. 3,723,079 discloses a process for "stabilizing"
dried lignitic and subbituminous coals against
spontaneous combustion, in which the dried coal is
treated at about 347°F (175°C) to 437°F (225°C) with 55
oxygen followed by some rehydration of the oxygentreated
coal with water. The test, which the inventors
used to measure the stability of the coals treated by
their process described in the above patent, was essentially
the same test as developed by Shea and Hsu as
described above.
While coal can be thermally dried below its inherent
moisture value, this has disadvantages. The product has
perhaps a even greater tendency toward spontaneous
ignition than undried coal, it is dustier, and under 65
humid conditions it will reabsorb moisture.
U.S. Pat. No. 3,723,079 described previously, discloses
a process for stabilizing a coal against spontane3,961,914
3
diate reaction products. The temperature of the coal is
then elevated to drive off hydrochloric acid and water
of hydration leaving a vitreous silica which adheres to
the coal particles and closes most of the pores, cracks
andd fissures therein.
The invention is illustrated by the following illustrative
examples which are in no way limiting of the invention.
4
heated continuously and ignited while the coated coal
reached a maximum temperature which then steadily
decreased, indicating that combustion would never
occur under conditions of the test. The coal used in this
5 example was from the same source as described in
Example I.
OTHER EXPERIMENTAL RESULTS
Hot Gas Phase Reactions
PROCESS ADVANTAGES
10
High Temperature Drying
A sample of undried coal was coated as described in
Example 2. During the heating step when moisture is
driven from the coal and the hydrated silica is dehydrated
the temperature was raised to 572°F (300°C)
15 followed by cooling. This coated coal was then tested
for self heating, and the results were that the coated
coal heated continuously up to ignition. Apparently the
high drying temperature causes the coal structure to be
expanded such that the coating integrity is destroyed
20 and the hot moisture-laden oxygen of the self-heating
test brings on ignition. A temperature range between
about 212°F (100°C) to 572°F (300°C) is suitable for
driving off water and hydrochloric acid vapors.
The primary economic advantage of shipping dried
coal occurs because of a reduced shipping weight resulting
in lower total shipping costs or other costs based
upon shipping weight. Because of moisture removal in
drying coal the calorific value of the coal is raised from
8500 to I 1,300 BTU for a typical sub-bituminous coal
while at the same time the volume required to store one
million BTU's is reduced about 25 percent. This latter
savings in volume can be quite important from a transportation
standpoint because of the high capital expense
of railroad cars. The process makes the coal
resistant to self-ignition. It also makes the coal less
dusty, and this has been observed in handling the
coated and uncoated coal. By microscopic observation
65 we have observed that many small coal grains adhere to
larger grains on the coated coals while this is very much
less evident in uncoated coals. The coating on the coal
can be expected to retard the reabsorption of moisture
Experimentation showed that silicon dioxide will not
deposit directly on coal which is at a temperature of
392°F (200°C) from the gas phase reaction of silicon
tetrachloride and water to any appreciable extent.
Gas Contact for Coating
Introduction of water vapor and silicon tetrachloride
vapor into the coating chamber in separate streams
prevents premature hydrolysis of the silicon tetrachloride.
In addition, it has been found that it is possible to
prevent premature condensation of the intermediate
reaction product by passing the mixed water and silicon
tetrachloride vapors through a tube heated to a temperature
above about 302°F (150°C). The fact that the
coal is at a low temperature and is being tumbled in the
rotary kiln as the separate streams of reactants are
introduced ensures that practically all of the reaction
occurs near the coal. Likewise, when silicon tetrachlo-
45 ride is vaporized prior to contact with the undried coal
particles and the source of water is the coal itself, these
conditions ensure that the reaction will occur at the
coal particles.
EXAMPLE I
EXAMPLE 2
A sample of dried coal was treated at room temperature
in a rotary glass kiln by passing over it a stream of
air carrying vapors of silicon tetrachloride and water,
introduced by separate injectors. Following the initial
low-temperature gas phase deposition, the temperature
inside the kiln was raised to 482°F (250°C), and the
coated coal was allowed to cool for subsequent testing.
When examined microscopically the coal particles
were observed to have a coating of vitreous silica adhering
to them which closed the pores, cracks and
fissures in the particles. The coating was quite thin,
varying up to a few microns.
Comparative tests of the self-heating characteristics
of coated coal and uncoated coal were made using a
testing apparatus substantially the same as that de- 25
scribed in the article by Shea and Hsu (supra) and also
as was used by Seitzer in evaluating the effectiveness of
his process for stabilizing coal against spontaneous
heating, which process is covered in U.S. Pat. No.
3,723,079 (supra). The results of the comparative tests 30
of coated and uncoated coal are illustrated in the graph
of the drawing in which temperature in degrees F. is
plotted on the ordinate against time in hours on the
abscissa. It will be noted that about 200°F (93°C) no
further self-heating is noted in the dried coated coal 35
while at about this temperature the dried uncoated coal
underwent rapid self-heating and spontaneously combusted
at about 350°F (177°C). The initial rise in temperature
of both coated coal and uncoated coal is
caused by the heat exchange from the water-saturated 40
pure oxygen introduced at a temperature of 150°F
(66°C). The samples used in the test were taken from a
27% moisture Wyodak coal, a subbituminous coal from
the Powder River Basin in Wyoming.
A sample of raw coal (undried) containing about
27% moisture was treated below room temperature in a
rotating glass kiln by passing over it a stream of nitro- 50
gen carrying vapors of silicon tetrachloride at room
temperature. Water for the reaction was supplied by
the coal itself. The coal warmed slightly while the silicon
tetrachloride was being admitted to the chamber;
this most likely was heat from the hydration of the 55
silicon tetrachloride by water from the coal. Following
the initial low-temperature gas phase deposition, the
temperature in the kiln was raised to 392°F (200°C) in
order to (I) dry the coal, and (2) dehydrate the coating
on the coal to give it its protective coating. At a differ- 60
ent time a second sample of the same raw (undried)
coal was placed in the same rotating glass kiln; however,
it was not subjected to the coating process. It was,
however, dried by heating in the same fashion as was
used for drying the coated coal.
Comparative tests of the self-heating characteristics
of coated and uncoated coal were made as described in
Example I. The uncoated coal which was dried self
3,961,914
6
form an intermediate reaction product on the surface
of the coal;
c. permitting the intermediate reaction product between
the silicon tetrachloride and water to form a
coating on the particles; and
d. raising the temperature in the chamber to drive off
water and hydrochloric acid as vapors.
9. The process of claim 8 in which the coal particles
are undried coal particles and the water combining
10 with the silicon tetrachloride is derived from the undried
coal particles.
10. The process of claim 8 ill which the silicon tetrachloride
is introduced into the coating chamber at a
temperature not below the temperature of the coal
15 particles.
11. The process of claim 8 in which the reaction
between silicon tetrachloride and water in the coating
chamber takes place at a temperature below about
20 212°F (100°C).
12. The process of claim 9 in which in step (d) the
temperature in the coating chamber is raised to within
a range of about 212°F (100°C) to S72°F (300°C).
13. The process of claim 8 iIll which the coal particles
25 are dried coal particles and the water combining with
the silicon tetrachloride is introduced into the chamber
in a separate stream.
14. The process of claim 1 in which the water and
silicon tetrachloride vapors are introduced through a
30 conduit heated to at least about 302°F ( ISO°C).
15. A new product, coal coated with an adherent
coating of silicon dioxide.
16. The new product of claim 15 in which the coal is
in particulate form.
* * * * *
5
from the atmosphere; this effect is apparent from results
of the self-heatiing test data.
From the above description of the invention, it is
apparent that a process has been developed for improving
coal which renders the coal resistant to spontane- 5
ous ignition with many other attendant advantages.
What is claimed is:
1. A process for making coal resistant to spontaneous
combustion which comprises coating the coal in particulate
form with a thin coating of silicon dioxide.
2. The pr9cess of claim 1 in which the silicon dioxide
is formed in situ in the presence of the coal particles by
the reaction of silicon tetrachloride and water.
3. The process of claim 2 in which the silicon tetrachloride
and water are introduced into the coating
chamber in separate gas streams.
4. The process of claim 3 in which each gas stream is
an air stream.
5. The process of claim 2 in which the water combining
with the silicon tetrachloride is derived from the
coal.
6. The process of claim 5 in which the silicon tetrachloride
is introduced into the coating chamber in a gas
stream.
7. The process of claim 6 in which the gas stream is
an air stream.
8. The process of claim 1 in which the coal particles
are coated in accordance with the following process
steps:
a. introducing coal particles into a coating chamber;
b. introducing silicon tetrachloride into the chamber
under conditions that will promote its combination
with water either introduced into the chamber in a
separate stream or derived from the coal itself to 35
40
45
50
55
60
65
;marg�4Pot0�(D�t;line-height: normal;mso-pagination:none;mso-layout-grid-align:none;text-autospace:none'>injected into the jacket for the heating cycle. The speed
of the agitator is that at which the particles are kept in
suspension and a homogeneous slurry is maintained. At
This example shows the process is effective for zinc
concentrates and shows the deleterious effect of zinc
and iron ions on yield.
EXAMPLE VII (See Example III)
About 270 grams of a concentrate produced by the
oxidation of chalcopyrite-containing sulfur 48%,chalcopyrite
12%, pyrite and other minerals 40%, was 50
washed, slurried in I liter of water and held at 135°C ±
5° for the times shown. The slurry was cooled and
screened on a 100 mesh U.S. Standard screen.
Test I Test 2 Test 3
Additive 2 gm NaOH 2 gm NaOH 2 gm NaOH
(.74 gm/lOO gm of (.74 gm/lOO gm of (.74 gm/lOO gm
feed) feed) of feed)
Time 30 min. 60 min. 235 min.
Product
(o/c sulfur) 88.5 90.9 86.7
Yield (%) 23.0 19.0 59.0
This example shows that while a ,small amount of 65
coalescence occurs very quickly that times in excess of
I hour are necessary to obtain a high degree of coale~cence.
the end of the heating cycle, steam was shut off, cooling
water was admitted to the jacket, the autoclave cooled
down, and when the temperature dropped below 80°C
the' agitator was slowed down, the bottom valve opened
and the autoclave 'dischargedtoa screen. The sulfur
3,939,256
9
product was removed as the oversize + 65 .mesh, and
the fine materials or the tailings proceeded on to a
thickener. The sulfur product averaged around 96%
elemental sulfur with a recovery of over 85%. The
particles were greenish-yellow in color and irregularly 5
shaped.
What is claimed is:
l. A process for the recovery of elemental sulfur
from mixtures in which it is present with soluble calcium
compound impurities which comprises: 10
a. contacting the mixture with water to solubilize
calcium ions;
b. separating the solids content of the treated mixture
from the liquid content and washing the solids
15
20
25
30
35
40
45
50
55
60
65
10
content to remove said solubilized calcium ions
from the solids content;
c. forming a water slurry of said solids content;
d. adding to the slurry a surface modifying additive
selected from the group consisting of alkali metal
hydroxides, alkali metal carbonates, and mixtures
thereof to produce an alkaline slurry pH of at least
about 9;
e. heating the slurry to at least the melting point of
sulfur for a period sufficient to coalesce substantially
all of the sulfur particles, and
f. recovering the coalesced sulfur from the slurry.
* * * * *