9 Claims, 4 Drawing Figures
Primary Examiner-Carl F. Dees
Attorney, Agent, or Firm-Sheridan, Ross & Fields
A process for improving coal wherein the raw coal is
reacted with substantially undecomposed iron carbonyl
which alters the apparent magnetic susceptibility
of certain impurity components contained in the
raw coal thereby permitting their removal by low intensity
magnetic separators. The process is especially
effective for removing pyrite from coal, while at the
same time reducing ash and increasing the calorific
value.
United States Patent [19]
Kindig et al.
[54] PROCESS FOR IMPROVING COAL
[75] Inventors: James K. Kindig, Arvada; Ronald L
Turner, Lakewood, both of Colo.
[73] Assignee: Hazen Research, Inc., Golden, Colo.
[22]. Filed: Mar. 25, 1974
[21] Appl. No.: 454,253
[52] U.S. CI. 44/1 R; 44/1 C
[51] Int. CI.2 CI0L 9/02
[58] Field of Search 44/1 R, I C,4-6;
201/17; 208/8; 75/2
[56] References Cited
UNITED STATES PATENTS
3,348,932 10/1967 Kukin 44/4
3.595,965
3,736,233
[57]
[ II] 3,938,966
[45] Feb. 17, 1976
7/1971 Franz et al. 201/17 X
5/1973 Sass et al. 201/17
ABSTRACT
u.s. Patent Feb. 17, 1976 Sheet 1 of 2 3,938,966
10
14
u.s. Patent Feb. 17, 1976 Sheet 2 of 2 3,938,966
22
~':::~~~~~~24
3,938,966
SUMMARY OF THE INVENTION
2
ntles are weakly paramagnetic; however, their paramagnetism
is not sufficient to economically effect a
separation from coal. However, effective beneficiation
of coals can be made if the apparent magnetic susceptibility
of pyrite or other impurities is increased. For
pyrite it has been estimated that a sufficient increase in
susceptibility can be achieved by converting less than
0.1 percent of pyrite in pyritic coal into ferromagnetic
compounds of iron. ("Magnetic Separation of Pyrite
from Coals," Bureau of Mines Report of Investigations
7181, P.l.)
In discussing the use of heat to enhance the paramagnetism
of pyrite it is stated in the above report (P.l)
that ferromagnetic compounds of iron are not formed
in significant quantities at temperatures below 400°C,
and that such conversion occursin sufficient quantities
to effect beneficiation only at temperatures greater
than 500°C. As this is above the combustion point of
coal, the use of heat to enhance magnetic susceptihility
does not appear feasible. Further, other methods for
enhancing the paramagnetism of. pyrite to permit its
separation from coal have not been encouraging.
Accordingly, it is a principal object of this invention
to provide an economically feasible method for improving
raw coal by enhancing the apparent magnetic
susceptibility of pyrite or other impurities associated
with but substantially liberated from the raw coal to the
point where these impurities can be successfully separated
from the coal by magnetic separators.
It has been found that pyrite reacts with iron carbonyls
to form one or more compounds different from
pyrite and having a magnetic susceptibility very much
greater than the original pyrite. Although iron pentacarbonyl
has proven effective in the reaction, it is obvious
that other carbonyls, such as iron nonacarbonyl or
a mixture of iron carbonyls would also be effective and
the term "iron carbonyl" as used herein includes all
carbonyls of iron and mixtures thereof. This discovery
can be used to alter the surface of the pyrite by applying
the carbonyl treatment so that the apparent magnetic
susceptibility of the pyrite is increased. Pyrite
particles that have been so treated can then be separated
by magnetic processing from other materials
which are inert to a surface treatment of iron carbonyl.
Such a process has wide application in the field of
mineral beneficiation.
The apparent magnetic susceptibility of pyrite as well
as other associated impurit~es in coal is increased to the
point where selective magnetic separation of these
impurities from the coal particles is feasible. The increase
is effected by contacting coal containing pyrite
or other impurities liberated from the coal with an iron
carbonyl like iron pentacatbonyl under conditions at
which ordinary pyrolyti-;: decomposition of the iron
carbonyl into metallic iron and carbon monoxide is not
appreciable. With pyrite a chemical reaction between
the iron carbonyl and the pyrite particles occurs to
60 form a replacement shell, on the surface of the pyrite
particles, of a material having a magnetic susceptibility
significantly greater than that of untreated pyrite. The
carbonyl treated coal product is then passed through a
magnetic separator for removal of the pyrite and impurity
particles.
It is desirable to have the coal comminuted finely
enough to give substantial liberation of impurities from
coal particles. The carbonyl is introduced as a vapor
1
PROCESS FOR IMPROVING COAL
BACKGROUND OF THE INVENTION
.With the present world-wide emphasis on the energy 5
crisis and the rapidly diminishing sources of oil, increased
attention by both government and private organizations
is being given to coal as a source of energy,
especially for the generation of electricity. For the year
1972 the annual consumption of coal in the United 10
States for the generation of electricity exceeded 348
million tons. This country has vast resources of coal for
development as other sources of energy diminish.
Depending upon their origin, coals contain varying
amounts of iron disulfide (iron disulfide is hereinafter 15
referred to as pyrite whether crystallized as pyrite or
marcasite) from which sulfur dioxide is formed as a
combustion product when coal is burned. This is a
tremendous disadvantage to the use of coal as an enerby
source, particularly in view of the present empha- 20
sis on pollution control as illustrated by present federal
emission control standards for sulfur dioxide. Illustrating
the enormity ofthe sulfur dioxide emission problem
is the fact that large transportation expenses are incurred
by coal users in transporting Western and Euro- 25
pean coal of relatively low sulfur content long distances
to supplant available high sulfur-containing coals in
order to make compliance with sulfur dioxide emission
standards possible when using coal as an energy source.
At this time there are no effective means available 30
which are commercially feasible for absorbing the large
amounts of sulfur dioxide emitted by the combustion of
coal to produce heat and electricity. Currently U.s.
utilities in burning about 395 million tons of coal a year
generate about 21 million tons of sulfur dioxide in the 35
process. One solution to the problem is to separate the
sulfur-bearing pyrite from the coal before it is burned.
Coals also contain, depending upon their origin, various
amounts and kinds of minerals which form ash
when the coal is burned. The ash also is a disadvantage 40
to the use of coal as an energy source, since it contributes
no energy value during combustion thereby diluting
the calorific value of the coal, causes a waste disposal
problem, and a potential air pollution problem.
The problem of separating pyrite or other impurities 45
from raw coal is not new and a number of methods
have been extensively tested over the years. Among
these are methods which employ the difference in specific
gravity between coal particles and the impurity
particles or differences in their surface, electrostatic, 50
chemical or magnetic properties. For one reason or
another difficulties are encountered in making an efficient
separation of pyrite or other impurities from coal
which has been ground finely enough to substantially
liberate impurity particles from coal particles. In water 55
systems this difficulty is related to the slow settling rate
of fine particles and in air systems to the large difference
in specific gravity between air and the particles.
However, for magnetic separations the magnetic attraction
force acting on small magnetic particles is
many times greater than the opposing separating force,
which is usually a hydraulic drag and/or gravity force.
For the separation of pyrite or other impurities from
raw coal the success of a magnetic process is dependent
on some effective pre-treatment process for selectively 65
enhancing the magnetic susceptibility of the pyrite or
impurity particles. Coal particles.aione are slightly
diamagnetic while pyrite and many other mineral impu3,938,966
BRIEF DESCRIPTION OF THE DRAWINGS
DESCRIPTION OF THE PREFERRED
EMBODIMENTS
3
into a reaction chamber containing the coal. These
carbonyl vapors can be carried into the chamber by a
gas, inert to the reaction, by first passing the gas over or
through a vessel holding liquid iron carbonyl.
The operation of the invention will be explained in
conjunction with the accompanying drawing showing
reproductions of photomicrographs of products obtained
in comparative tests using and not using the
process of the invention, the figures of the drawing
being described as follows:
FIG. 1 is a copy of a photomicrograph of an untreated
particle of native Colorado pyrite not associated
with coal measures;
FIG. 2 is a copy of a photomicrograph of a particle of
the same type pyrite altered by the carbonyl treatment
process of the invention;
FIG. 3 is a copy of a photomicrograph of a particle of
the same type pyrite in which the particle was first 20
given the carbonyl treatment of the process of the invention
to form an altered particle like that of FIG. 2
followed by further treatment not a part of the process
in which iron pentacarbonyl was thermally decomposed
to form the outer layer of iron; and
FIG. 4 is a copy of a photomicrograph of a particle
from an Iowa coal seam showing a locked coal and
pyrite particle which has received the carbonyl. treatment
of the process.
4
The process is applied by contacting the raw coal
which is liberated from pyrite or other impurities with
iron carbonyl under conditions where there is an insufficient
dissociation of carbonyl into metal and carbon
5 monoxide to cause substantial deposition of metal on
the coal particles. These conditions are determined by
the temperature, the type of carbonyl, pressure, gas
composition, etc. Ordinarily, the carbonyl gas is heated
to a temperature just below its decomposition tempera-
10 ture under the reaction conditions. Various types of
available equipment cana be used for contacting the
iron carbonyl and coal, such as, a rotating kiln used as
the reaction vessel with iron carbonyl vapors carried
into contact with the tumbling contents of the kiln by a
15 gas such as nitrogen which is inert to the reaction process.
The process must be carried out at a temperature
below the temperature of major decomposition of the
carbonyl under the reaction conditions so that there is
opportunity for the iron of the carbonyl to chemically
react with the pyrite particles. Obviously, ifthe temperature
is allowed to rise above the decomposition temperature
of the carbonyl for a sufficient time, the coal
will be coated with iron and the pyrite particles will
25 either react with or be coated with metallic iron to give
both types of particles high magnetic susceptibilities,
thus preventing their separation magnetically.
The amount of carbonyl used and the time of treatment
can be varied to affect the percent of pyrite re30
acted. The carbonyl must be in contact with the pyrite
particles a sufficient time for the outer shell of reacted
material to form on the particles. The thickness of this
The invention is especially useful for reducing the outer shell determines the extent to which the apparent
content of pyrite from coals containing these impuri- magnetic susceptibility is increased; judgment of optities.
The invention can be applied to coals of diverse 35 mum thickness is a balance between reaction rate of
origins and rank including coking, steam, and other shell formation and economics of the reaction process
coals as well as refuse from coal cleaning plants, and and magnetic separation process. Generally a reaction
the term "coal" as used herein includes all of these time not in excess of about two hours is adequate.
types of coal. Depending on adequate coal-pyrite liber- Analyses of the residual sulfur in a portion of treated
ation, pyrite removal approaching the theoretical limit 40 coal after magnetic separation of the pyrite will indiis
possible. cate optimum treating time, amount of carbonyl used,
The probable typical reaction which generates the and other reaction parameters necessary for obtaining
ferromagnetic species comprising the outer shell of coal containing permissible amounts of sulfur.
treated pyrite particles that enhances the apparent The invention is illustrated by the examples presmagentic
susceptibility of the pyrite particles is as fol- 45 ented below in which iron pentacarbonyl was reacted
lows: with iron disulfides of various origins either alone or
FeSz +
iron disulfide
(pyrite or marcasite)
X Fe(CO), Fe,,+x)SZ
iron carbonyl "iron-rich
disulfide"
+ 5X CO
carbon monoxide
EXAMPLE I.
mixed with coal.
The examples are illustrative of the invention but not
limiting thereof.
Initial experiments were made with an igneous pyrite
concentrate from Colorado in order to be dealing with
essentially a pure pyrite rather than a material contain-
60 ing mostly coal and only a little pyrite. This Colorado
pyrite was tested and found to be non-magnetic. A
sample of this Colorado pyrite was placed in a rotating
kiln. Iron carbonyl vapors, carried in argon, were
passed over the pyrite which was heated to a tempera-
65 ture of 195°C which is below the temperature where
metallic iron forms in abundance under the conditions
of the test. The treatment time was one hour, although
treatment times and temperatures will vary as ex-
The "iron-rich disulfide" forms as a replacement shell
around the pyrite grains and is highly magnetic.
For efficient separations of pyrite from coal, the coal 55
should be crushed to such fineness that pyrite particles
are free, or nearly free, from the coal particles. The
required fineness depends upon the size distribution of
the pyrite in the coal. A thorough treatment of the
subject for power plant coals is given in the article
entitled "Pyrite Size Distribution and Coal-Pyrite Particle
Association in Steam Coals," Bureau of Mines Report
of Investigation 7231. The requirement for pyrite
liberation applies to all types of physical separations
and so is not a disadvantage to this invention. Additionally,
present technology for coal-fired power plants
generally requires pulverizing the coal to 60-90 percent
minus 200 mesh before burning.
EXAMPLE 3
The process was also applied to' a bituminous coal
from central Pennsylvania. The coal was charged into a
lciln whiCh was then rotated. ;The introduction of iron
pentacarbonyl into the reaction zone was as described
in Example 2. The reaction zone was held. between
185°C and 195°C for one hour following whichthe kiln
was purged of carbonyl vapors by the inert gas and the
reaction zone cooled .to room temperature. Three
products were made by magnetic separation using magnets
of different field strengths,a "magnetic fraction,"
"weakly magnetic fraction,': and "noll"lIlagnetic fraction/'
with the "magnetic fraction"obtainedfrqm wet
processing. Two magnets were used inthesep~ratlon; a
l!1boratory Davis tube tester and a small, hand horseshoe,
Alnico magnet. These three products were.analyzed
for forms of sulfur, ash, and calorific value (Btu);
results are given in Table 1.
TABLE 1
3;93-8,966
5 ',~
plained above. The product from this run was highly rite that was. of sedimentary origin and deposited in a
magnetic. . coal matrix. Tlte raw coalfor this test was charged into
A polished section of the reacted material showed a a kiln which was then rotated. To introduce the iron
replacement shell of the newly formed compound pentacarbonyl into the reaction zone, an inert gas was
around the pyrite grain. No such shell was ·formed 5 passed through liquid iron pentacarbonylat room temaround
gangue particles. Frorp a microscopic study of Perature contained in a vessel outside the kiln with the
the section, it was obvious that the replacement shell gas carrying carbonyl vapors then being introduced
was not metallic iron but rather a reaction product of into the reaction zone of the kiln. The reaction zone
different color wh~ch has replaced the pyrite. was held between 185°C and 195°C for one hour, fol-
Referring to. FIG. 1 ofthe drawing, wherein the nu- 10 lowing which the kiln was purged of carbonyl vapors by
merall0 indicates a depiction ora photomicrograph of the insert gas and the reaction zone cooled to room
a sectioned particle ofuntreated Colorado pyrite, it will temperature.
be seen that the particle is of the. same material A polished section was prepared from the magnetic
throughout and there is no layer on the periphery ofthe fraction of material. obtained by processing the carparticle.
The particle .sho\\,ed no attraction to a low 15 bonyl treated coal with a low intensity. magnet. One
intensity magnet. In contrast, inspection of FIG.. 2, the particle from this polished section was photographed
same type illustrationof a particle ofthe sameJ:l1aterial ~d is depicted in FIG. 4 of the drawing. It will be seen
sectioned after treatment.by the process. of.the inven- that the particle is comprised of coal (20) locked to
tion, shows an outer replacementshell 14 around the pyrite (22). However, as was noted in FIG. 2 then" is a
periphery of the particle of a material ofan entirely 20 replacement shell (24) of different color and luster
(N'f~rent composition than that of the pyrite. particle. around the pyrite and this shell has even invaded the
This. replacement shell.had an entirely different color cracks.andfissl,lres in the pyrite. There is no evidence
and luster than that of the pyrite particle. There was a of ~ny iron deposition either around the pyrite or
definite line of demarcation between the shellanli the around any of the coal surface.
particle. The treated particle shown in FIG. 2 was at- 25
tracted to a low intensity magnet. . "
FIG. 3, the same type illustration as that ofthe other
figures, shows a particle 16 of the sarne type pyrite. AA
the parti~les of the first two figures. Tile particle16 was
first treated in accord~nc~with the process of the in- 30
vention witb undecomposed iron pentacarbonyl vapors
at a temperature of 190°<; to .form the outer replacement
shell 14 ofthe same composition as the.shell14 of
FIG. 2. The particle 16 with the replacement shell 14
on it was then funqer treated with the carl;!onyl.,~t 35
temperatures up to 225°C' to effect decomposition of
the carbonyl with the result that an outer shell or laYer
18 was deposited over the shell 14. and thisoutermos,t
layer 18 was readily recognizable as iron. The cle~vage
between layers 14 and 18 was very distinct and outer 40
layer 18, of course,had a different color, luster, arid
texture than layer 14. This illustrates what would happen
if the reaction conditions are such during the prac-
Organic
(%S)
Sulfur
Inorganic"
(%S)
Total
(%S)
Ash
(%)
Weight
(%)
ANALYSES OF PRODUCTS BOTH TREATED AND UNTREATED BY THE INVENTION
Coal Description: Lower Freeport Bituminous Coal from Pennsylvania.
Size treated. 14X200 Mesh, not all pyrite liberated at this size,
.Calorific
Value
(Btu)
Material Untreated by the Process
Raw Coal21 100.0 22.1 12,106 1.99 1.71
Material Treated by the Process
Clean Coal (non-magnetic fraction) 79.2 13.2 13,556 1.10 0.69
Middling (weakly magnetic fraction) 14.1 53.2 6.467 4.40 4.22
Refuse (magnetic fraction) 6.7 51.6 6,828 8.22 8.05
0.28
0.41
0.18
0.18
IIlnorganic sulfur is mostly pyritic sulfur plus a small amount of sulfur from the pyrite altered by the carbonyl treatment and any sulfate prese
0.01% for this coal. .
2/Not responsive to low intensity magnets.
tice of the process of the invention that the carbonyl
decomposes, i.e., the coal particles would become
coated with iron and selective magnetic separation of
the pyrite particles would not be possible.
EXAMPLE 2
The process was applied to an Iowa coal containing
7.8% pyrite sulfur, thus providing an example of a py-
As can be seen from Table 1, magnetic separation of
coal that did not receive the carbonyl treatment results
in no magnetic material and, therefore, no beneficiation
by magnetic processing. However, with treatment
65 and magnetic processing, two or more products may be
obtained depending on the operating conditions of the
magnetic separators. Results of a three-product sep~ration
are shown in Table 1. The process removed almost
3,938,966
7 8
70% of the pyritic sulfur. Not all pyritic sulfur was 3. The Flrocess of claim 2 in which the iron carbonyl
liberated at the size treated in this example so the 68% is iron pentacarbonyl. .
reduction may in fact represent all the liberated pyrite. 4. The process of claim 2 in which tM carbonyl is in
The process also reduced the ash from 22.1 to 13.2 gaseous form and is contacted with the coal in an inert
percent. This is a marked reduction in ash in the clean 5 carrier gas.
coal product, and it is a greater reduction than can be 5. A process for beneficiating coal associated with
attributed to the reduction in ash that occurs because impurities, such as, pyrite and ash.producing impuripyrite,
an ash-forming mineral, was removed from the ties, which comprises the steps of:
clean coal. It is not known at this time if the ash-form- a. reducing the coal and associated impurities to a
ing minerals are attracted to the magnet because they 10 fine particle size to liberate substantially all of the
are locked with pyrite particles or if their apparent impurities from the coal; .
magnetic susceptibility is increased by the carbonyl b. placing the mixture of coal and liberated impuritreatment.
In any event, there is a significant lowering ties in a gas treatment chamber;
ofash in the clean coal product. The table also reflects c. contacting an inert carrier gas with iron carbonyl
the concomitant improvement in the coal by the in- 15 vapor to incorporate the iron carbonyl vapor in the
crease in the Btu value of the clean coal resulting from carrier gas;
ash and sulfur reductions. Similar improvements would d. introducing the iron carbonyl vapor in the carrier
be observed with other tests which characterize the gas inb-said chamber under conditions which precoal,
for example, volatile matter, grindability, etc. clude· substantial decomposition of the iron car-
From the above, it will be seen that a process has 20 bonyl, and
been disclosed for improving coal by increasing the e. maintaining the iron carbonyl vapor in contact
apparent magnetic susceptibility of pyrite and other with said mixture for asufficient time for the undeimpurities
in the raw coal to a point that permits an composed iron carbonyl to react with the pyrite
economically feasible separation of a large percentage 25 particles. .
of these impurities from the coal by magnetic separa- 6. The process ofclaim 5 in which the temperature in
tion processes. the chamber is not in excess of about 250°C.
What is claimed is: 7. A process for beneficiating coal, including reduc-
1. A process for beneficiating coal, including reduc- ing sulfur and ash, increasing calorific value, and im·
ing sulfur and ash, increasing calorific value, and im- 30 proving other properties, which comprises contacting a
proving other properties, which comprises contacting a coal which contains impurities, such as pyrite or marcacoal
which contains impurities, such as pyrite or marca- site or other ash-forming minerals, which are substansite
or other ash-forming minerals, which are substan- tially liberated from the coal particles, with an iron
tially liberated from the coal particles, with an iron carbonyl in order to increase the apparent magnetic
carbonyl under reaction conditions which substantially 35 susceptibility of the impurities so that a magnetic sepapreclude
the general thermal dissociation of the car- ration between the coal and impurities may be effected.
bonyl into irori and carbon monoxide, in order to in- 8. The process of claim 7 performed under condicrease
the' apparent magnetic susceptibility of the im- tions to preclude coating of the coal particles with iron
purities sothat a magnetic separation between the coal from the carbonyl to make them ma~netic.
and impurities may be effected. 40 9. The process of claim 7 in which the treated coal is
2. The process of claim 1 in which the treated coal is subjected to a magnetic field to remove the impurities. '
subjected to a magnetic field to remove the impurities. * * * * *
45
50
55
60
65