Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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Method For Treating Carbonaceous Materials
Technical Field
The invention relates to methods for treating carbonaceous materials to remove
or
substantially decrease the amount of non-carbonaceous impurities therein.
s Background of the Invention
United States Patent No. 4,780,112 describes a process for treating carbon to
reduce
the ash therein. The process involves treating the carbon with an aqueous
solution of
hydrofluorosilicic acid (H2SiF6) and hydrofluoric acid (HF), whereby metal
oxides in the
carbon are converted to metal fluorides and/or metal fluorosilicates, from
which carbon is
io then separated. The process described in Uuted States Patent No. 4,780,112
is effective
for removal of metal oxides from carbon, but the present inventor has
surprisingly
discovered that when carbon that includes sulfur-containing impurities is
treated by the
process of United States Patent No. 4,780,112, the purified carbon is still
contaminated
with sulfur. The present inventor has surprisingly discovered that the
remaining sulfur is
is present as elemental sulfur, which in some circumstances is visible when
the carbon is
viewed under a microscope.
The presence of sulfur in carbon that is intended to be used as a fuel is
undesirable,
since combustion of the carbon will lead to conversion of the sulfur into
sulfur oxides. As
a result, the flue gases generated by the combustion of the carbon need to be
scrubbed or
ao otherwise substantially freed of the sulfur oxides before they can be
discharged into the
atmosphere if release of sulfur oxides into the environment is to be avoided.
Accordingly, there is a need for an improved process for treating carbonaceous
materials to decrease the amount of non-carbonaceous impurities therein, and
in particular
there is a need for an improved process for removing or at least substantially
reducing the
zs amount of sulfur in carbonaceous materials.
Surprisingly, the present inventor has discovered that the amount of sulfur-
containing impurities in carbonaceous materials can be substantially decreased
by a
process which involves treating the carbonaceous materials with an aqueous
solution of
hydrofluorosilicic acid, or with an organic solvent capable of dissolving
elemental sulfur.
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Summary of the Invention
According to a first embodiment of the invention there is provided a process
for
reducing the amount of sulfur-containing impurities in carbonaceous materials,
comprising (a) contacting the materials with an aqueous solution of
hydrofluorosilicic
s acid in the absence of hydrogen fluoride under conditions wherein at least
some of the
sulfur-containing impurities react with the hydrofluorosilicic acid to form
reaction
products, and (b) separating the reaction products from the carbonaceous
materials.
According to a second embodiment of the invention there is provided a process
for
reducing the amount of sulfur-containing impurities in carbonaceous materials,
io comprising
(a) contacting the materials with an aqueous solution of hydrofluorosilicic
acid in
the absence of hydrogen fluoride under conditions wherein at least some of the
sulfur-
contaiung impurities react with the hydrofluorosilicic acid to form reaction
products;
(b) separating the reaction products and the hydrofluorosilicic acid from the
is carbonaceous materials and subsequently
(c) treating the carbonaceous materials with a fluorine acid solution which
comprises an aqueous solution of hydrofluorosilicic acid and hydrogen
fluoride.
According to a third embodiment of the invention there is provided a process
for
reducing the amount of sulfur-containing impurities in carbonaceous materials,
zo comprising treating the carbonaceous materials with a fluorine acid
solution which
comprises an aqueous solution of hydrofluorosilicic acid and hydrogen
fluoride,
separating the carbonaceous materials from the aqueous solution of
hydrofluorosilicic
acid and hydrogen fluoride, and then contacting the carbonaceous materials
with an
organic solvent capable of dissolving elemental sulfur.
as As used herein, the term "carbonaceous materials" is to be understood to
mean
materials which consist predominantly of elemental carbon. Examples of
carbonaceous
materials include coal including brown coal, coke, lignite, anthracite,
charcoal, graphite
and the like.
As used herein, unless the context clearly indicates otherwise, the words
30 "comprise", "comprises", "comprising" or other variations thereof shall be
understood as
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meaning that the stated integer or integers is or are included but that other
integers are not
necessarily excluded from being present.
Detailed Description of the Invention
In the processes of the first and second embodiments of the invention, the
s concentration of hydrofluorosilicic acid in the step of contacting the
materials with an
aqueous solution of hydrofluorosilicic acid under conditions wherein at least
some of the
sulfur-containing impurities react with the hydrofluorosilicic acid to form
reaction
products may be in the range of 27% to 37% (wlv or w/w or v/w). The
concentration of
hydrofluorosilicic acid in the step of contacting the materials with an
aqueous solution of
io hydrofluorosilicic acid under conditions wherein at least some of the
sulfur-containing
impurities react with the hydrofluorosilicic acid to form reaction products is
typically in
the range of 28% to 36%, more typically about 32%(w/v or w/w or v/w). The
process is
usually carned out at atmospheric pressure, but the pressure can also be above
or below
atmospheric. The temperature may be in the range 28 to 75°C. Typically,
the temperature
is is in the range of 30 to 70 °C, more usually 30 to 40 °C. The
reaction time may be in the
range 8 to 120 minutes. The reaction time is typically from 10 to 100 minutes,
more
usually 15 to 30 minutes, still more usually 12 to 16 minutes. The minimum
quantity of
aqueous hydrofluorosilicic acid employed is typically enough to enable the
mixture of it
and the carbonaceous materials to be stirred in the acid. Usually, the
carbonaceous
ao materials are mixed with at least about twice their weight of the aqueous
hydrofluorosilicic acid. More usually, the aqueous hydrofluorosilicic acid is
present in an
amount of from about 70% to 90% by weight, relative to the total weight of the
mixture,
still more usually about 70% to 80% by weight of the total weight of the
mixture.
In step (a) of the processes of the first and second embodiments of the
invention,
as many metal oxides and some metals present in the carbonaceous materials are
converted,
at least partially, into the corresponding metal fluorosilicates, with water
being the other
product. Examples of metals or metal oxides converted to their fluorosilicates
are nickel,
aluminium, calcium, and mercury and their oxides. Sulfur compounds present are
converted, under the reaction conditions, to sulfur dioxide and/or sulfur
tetrafluoride.
so After step (a) of the processes of the first and second embodiments of the
invention,
relatively purified carbonaceous materials remain mixed with an aqueous
solution
containing dissolved metal fluorosilicates. Suitably, this mixture of
carbonaceous
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materials and metal fluorosilicates may be filtered or centrifuged to separate
the relatively
purified carbonaceous materials. Optionally, the filtered relatively purified
carbonaceous
materials may be treated with further aqueous hydrofluorosilicic acid,
typically having a
concentration of 32% by weight of hydrofluorosilicic acid, to wash out any
residual metal
s fluorosilicates. Separation of the remaining carbonaceous materials from the
aqueous
phase, and optionally washing the carbonaceous materials, affords a partially
purified
carbonaceous material which has a lower content of sulfur and metals compared
to the
original material. The principal impurities typically present in the partially
purified
carbonaceous materials at this stage are silica and iron sulfide.
io The partially purified carbonaceous material may be further purified to
remove
other impurities that are not removed in step (a). Thus, the process of the
second
embodiment provides such a process. In the process of the second embodiment,
step (c)
is typically a process in accordance with United States Patent No. 4,780,112,
the
disclosure of which is incorporated herein by reference. Similarly, in the
process of the
is third embodiment, the steps of treating the carbonaceous materials with a
fluorine acid
solution which comprises an aqueous solution of hydrofluorosilicic acid and
hydrogen
fluoride, and separating the carbonaceous materials from the aqueous solution
of
hydrofluorosilicic acid and hydrogen fluoride may be a process as described in
United
States Patent No. 4,780,112.
ao In step (c) of the process of the second embodiment, and in the process of
the third
embodiment, the fluorine acid solution may have a composition lying between
the
following compositions: 4% w/w H2SiF6, 92% w/w HzO, 4% w/w HF and 35% w/w
HZSiF6, 30% w/w HaO, 35% HF. In step (c) of the process of the second
embodiment,
and in the process of the third embodiment, the fluorine acid solution
typically has a
as composition lying between the following compositions: 5% w/w HaSiF6, 90%
w/w H20,
5% w/w HF and 34% w/w H2SiF6, 32% w/w H20, 34% HF. More typically the
composition of the fluorine acid solution is about 25% w/w HZSiF6, 50% w/w
HZO, 25%
w/w HF. This step is conveniently carried out in two stages as described in US
Patent
No. 4,780,112. That is, the first stage is conveniently carried out in a
stirred reactor at a
3o pressure of approximately 100kPa and a temperature of 40-60°C, and
the second stage is
conveniently carried out in a tubular reactor at a pressure within the range
of about 340 to
480 kPa and a temperature of 65°C to 80°C, more usually about
70°C. Typically, the
temperature is maintained at this value by the exotherm of the reaction
between silica
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present in the carbonaceous materials and hydrogen fluoride. In step (c) the
minimum
quantity of the fluorine acid solution employed is typically enough to enable
the mixture
of it and the carbonaceous materials to be stirred. Usually, the carbonaceous
materials are
mixed with at least about twice their weight of the fluorine acid solution.
More usually,
s the fluorine acid solution is present in an amount of from about 70% to 90%
by weight,
relative to the total weight of the mixture, still more usually about 70% to
80% by weight
of the total mixture.
Suitably, in step (c) of the process of the second embodiment, and in the
process of
the third embodiment, after being mixed with aqueous hydrofluorosilicic acid
and
io hydrogen fluoride, the mixture of the carbonaceous material and the
fluorine acid solution
may be ultrasonically agitated as described in United States Patent No.
4,780,112, in
order for any unreacted ferrous sulfide (which is relatively inert to HF and
SiF4) or other
relatively dense impurities to be capable of being separated from the bulk of
the relatively
purified carbonaceous material, which is less dense than the ferrous sulfide
and the
is aqueous phase. The purified carbonaceous material may be separated from the
aqueous
phase, optionally washed with aqueous HaSiF6, separated, dried to remove
excess water
(at about 100-110°C) and heated to a temperature within the range of
about 250°C to
400°C, or 280°C to 340°C, typically about 310°C,
to evaporate any residual
hydrofluorosilicic acid remaining on the carbonaceous material, before being
used for any
ao desired purpose, such as for a fuel. HF and SiF4 gases and water vapour are
typically
evolved during this drying step.
Aqueous fluorine acid separated from carbonaceous materials after it has been
contacted with them is relatively enriched in SiF4 and depleted in HF,
compared to the
fluorine acid solution before it is contacted with the carbonaceous materials,
as a result of
zs the reaction:
Si02 + 4HF ~ SiF4 + 2Ha0.
This spent aqueous phase, if is recycled to the step of being contacted with
the
relatively purified carbonaceous material, thus tends to reach a point where
it becomes
saturated with respect to SiF4, at which point any further SiF4 generated as a
result of
3o further reaction is evolved as a gas. Conveniently, the reactor in which
the fluorine acid
solution is contacted with the relatively purified carbonaceous materials
includes a means
for SiF4 to be removed from it. Suitably, the spent aqueous phase from this
step may be
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directed to a holding vessel where any excess SiF4 is vented from it. The
concentration of
HF in the spent aqueous phase may be increased by directing a gaseous mixture
of HF
and SiF4 into the vessel, whereby the HF is absorbed and the SiF4 passes
through. Vented
SiF4 is conveniently directed to a hydrolyser where it is treated with water
to produce
s H2SiF6, and SiOz according to the equation
3SiF4 + 2H20 -~ 2H2SiF6 + SiOz
The silica so produced may be separated from the acid by filtration or any
other
convenient means. The acid produced in this way is conveniently used in step
(a) of the
processes of the first and second embodiments.
io Advantageously, aqueous streams of hydrofluorosilicic acid with or without
hydrofluoric acid present, that are generated in processing steps associated
with the
processes of the present invention, may be directed to an acid still in which
the streams
are combined and distilled. A gaseous mixture of water, HF and SiF4 is
distilled from the
still, these substances being more volatile than the 32% w/w aqueous
hydrofluorosilicic
is acid azeotrope. The gaseous mixture of water, HF and SiF4 can be directed
first to a
dehydrating system for removal of water and then the resultant dehydrated
gaseous
mixture of the HF and SiF4 may be separated by directing it to a holding
vessel which
contains a solution of H2SiF6 that is saturated with respect to SiF4, as
described above.
Suitably, the dehydration step for a gaseous mixture of water, HF and SiF4
zo comprises contacting the gases with a sufficient quantity of anhydrous
metal fluoride such
as A1F3 to absorb all the water present. Other metal fluorides that can be
used include
zinc fluoride and ferrous fluoride. Substantially anhydrous gases may be
obtained in this
way, together with a hydrated metal fluoride, which may be separated from the
anhydrous
gases and heated to regenerate substantially anhydrous metal fluoride for
recycling to the
zs dehydration step.
In one form of the processes of the first and second embodiments, reaction
products
separated from carbonaceous materials in step (b) consist of sulfur dioxide
and metal
fluorosilicates dissolved or suspended in aqueous HZSiF6. Gaseous HCI, derived
from
inorganic or organic chloride in the carbonaceous material, may also be
present. Suitably,
3o these reaction products are directed to a still where they axe heated so as
to cause gaseous
HF, SiF4, steam, HCl and sulfur dioxide to be evolved, and so as to cause any
metal
fluorosilicates present to be concentrated above their solubility limit and to
separate as
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solids, which can be removed from the still for disposal or re-processing. The
gaseous
mixture leaving this still may suitably be dehydrated by a process as
described above, by
contacting it with anhydrous aluminium fluoride, and subsequently passed to an
activated
carbon filter for removal of sulfur oxides and HCl. The remaining HF and SiF4
gases,
s dried and freed of sulfur dioxide, may be directed to the holding vessel for
spent aqueous
phase from step (c) of a process of the second embodiment, for absorption of
the HF.
In the process of the third embodiment the process may further comprise after
the
separating:
washing the carbonaceous materials to remove any residual acid; and
io optionally drying the carbonaceous materials prior to the contacting.
The washing may be with water. The drying may take place at a temperature
within the
range 100-120°C, typically at 110°C.
In a process of the third embodiment, the organic solvent capable of
dissolving
elemental sulfur is typically ethanol, benzene, carbon disulfide, either or
carbon
is tetrachloride, or a mixture of two or more of these or other suitable
solvent capable of
dissolving elemental sulfur. Typically, the solvent is ethanol. The step of
contacting the
carbonaceous materials with the organic solvent is typically carried out at
ambient
temperature and atmospheric pressure, but elevated temperatures (e.g. in the
range of 30 -
90°C or elevated pressures (e.g. in the range of 1.01 -5 atm or 1.2-2.5
atm), or both, may
ao also be used. The quantity of solvent used is not critical, but a minimum
quantity for
practical purposes is a quantity sufficient to enable the mixture to be
stirred or agitated.
Suitably, in the process of the third embodiment the organic solvent is
contacted
with the carbonaceous materials for sufficient time for at least some of the
elemental
sulfur, which will be present in it after the step of treating the
carbonaceous material with
as the fluorine acid solution, to be dissolved. Conveniently, the solvent is
separated from the
carbonaceous materials after this time, and is distilled to recover as much as
possible for
reuse. The treated carbonaceous materials may also be treated to remove any
residual
solvent, although if the solvent includes no halogen or sulfur atoms, this
step can be
omitted. Removal of the residual solvent can be by any convenient means such
as air
3o blowing or heating (eg at a temperature within the range of 30-100°C
the temperature
chosen being dependent on the solvent).
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The step of separating in the embodiments of the invention may comprise
filtering,
separating by centrifuging or other suitable separating means.
The processes of the present invention provide several advantages over prior
art
processes. In addition to providing carbonaceous materials that have
significantly lower
s levels of sulfur than treated carbonaceous materials obtainable by the
process of United
States Patent No. 4,780,112, processes of the present invention can also
result in removal
or partial removal of other undesirable substances in carbonaceous materials
such as
silica, metal oxides and metal sulfides, metals such as mercury and
radioactive elements,
and inorganic chlorides. For example, if coal contained sulphur at about 8wt%
it may be
io possible to remove this sulphur to a lower level (e.g. about or less than
2wt% or about or
less than 1 wt% or about or less than O.Swt%) by subjecting the coal to one
(or more)
cycles of the processes of the first to third embodiments. Removal of
inorganic chlorides,
mercury and radioactive elements, in particular, is more effective with a
process of the
second embodiment, than with the process of United States Patent No.
4,780,112.
is Further, processes of the present invention can lower the levels of bound
oxygen in the
carbonaceous materials and, when applied to coal, can result in an increase in
its calorific
value, typically by 3-4%.
In the first to third embodiments the carbonaceous materials may reduced prior
to
the treating step to granular form which is less than about 4, 3, 2, 1.75,
1.5, 1.25, 1 or 0.75
ao mm in particle size. At least 80wt%, 85wt%, 90wt%, or 95wt% of the granular
particles
may be in the range of 5-0.25 mm, 4-0.25 mm, 3-0.25 mm, 2-0.25 mm, or 1-0.25
mm,
for example. Alternatively, the carbonaceous material may be treated in its
raw form. If
the carbonaceous material contains excess moisture it may be dried (e.g. at 60-
120°C or
100-120°C) prior to processing so as to remove excess moisture. The
drying may be
as conducted for long enough so as to result in an inherent moisture content
of carbonaceous
material in the range 3 to 8% w/w, more usually 3 to 5% w/w, for example. Some
coals
such as lignite which have a high water content usually have to be predried
prior to
processing. The carbonaceous material may be air dried (e.g. at 60-
120°C or 100-120°C)
prior to processing by passing hot air over the carbonaceous material, for
example. The
so temperature of the hot air used for drying the carbonaceous material is
lower than that
which would cause the carbonaceous material to combust.
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Brief Description of the Drawings
Figure 1 is a schematic block diagram of a system for purification and
combustion
of carbonaceous material, incorporating a process in accordance with the
present
invention.
s Figure 2 is a schematic block diagram of a still and associated plant for
processing
an aqueous solution or suspension produced by step (a) of a process of the
first or second
embodiments of the invention.
Figure 3 is a schematic block diagram of a system for the treatment of
carbonaceous
materials with a solvent for removal of elemental sulfur, as part of a process
of the third
io embodiment of the invention.
Best Method of Carrying out the Invention
Figure 1 illustrates in schematic block diagram form a system 10 for
purification
and combustion of carbonaceous materials, incorporating a process in
accordance with
the present invention.
is Refernng to Figure 1, system 10 includes hopper 20 for holding impure
carbonaceous materials which have been reduced to granular form, preferably
substantially spherical particles and preferably less than about 2mm in
particle size.
Associated with hopper 20 is feed unit 25 for conveying carbonaceous materials
from
hopper 20 to purification reactor 30.
ao Purification reactor 30 is positioned to receive carbonaceous materials
from feed
unit 25. Purification reactor 30 is also equipped with line 24 to admit an
aqueous solution
of approximately 32%w/w H2SiF6 from a hydrolyser 32. Purification reactor 30
may be a
flow through reactor or a stirred or rotating reactor. Typically, purification
reactor 30 is a
rotating drum reactor. It is also equipped with line 26 for transfer of the
contents of
as reactor 30, after the carbonaceous material has been in contact with the
aqueous HaSiF6
for a suitable time, to filter 50. Filter 50 is suitably a belt filter and is
equipped with line
51 to conduct separated liquids away from filter 50, and conveyor 52 whereby
separated
solids from filter 50 are transferred to silica removal reactor 55. Reactor 55
is equipped
with line 58 for admitting an aqueous fluorine acid solution of HF and H2SiF6
from HF
3o absorber 54, and vent line 59 which communicates with hydrolyser 32.
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A bottom outlet of reactor 55 communicates via pump 56 and line 57 with a two-
stage tubular reactor 65A, 65B, the first stage 65A of which is capable of
being agitated
ultrasonically. The distal end of reactor 65B discharges into separator 16
which is
equipped with takeoffs 66 and 67 adjacent its upper and lower ends
respectively. Upper
s takeoff 66 communicates with centrifuge or belt filter 70 which is capable
of separating
solid carbonaceous material from aqueous solution. The liquid removal side of
centrifuge
or belt filter 70 is equipped with line 69 that leads to HF absorber 54, and
the solids
removal side of centrifuge or belt filter 70 discharges to a system of mixers
and separators
for washing.
io The mixer/separator system consists of three mixing tanks 71, 73 and 75 and
three
separators, such as centrifuges or belt filters, 72, 74 and 76 arranged so
that carbonaceous
materials can flow sequentially from mixing tank 71 to separator 72, then to
mixing tank
73 followed by separator 74, then to mixing tank 75 and separator 76. The
system is
arranged so that aqueous phase moves essentially counterflow to the solids.
is The solids exit of final separator 76 is connected to a drying system which
consists
of mixing vessel 77, tubular reactor 78 and solids separator 79. The liquid
exit of the
mixer/separator system is from separator 72 and communicates with a still 80.
Separator
79 has a vapour off take that also communicates with still 80, which is
equipped with a
jacket heater, vapour outlet 81 and a bottom outlet leading to solids
separator 98.
ao Optionally, a solvent extraction system such as described below with
reference to
Figure 3 may be installed between the solids exit of separator 76 and mixing
vessel 77, as
shown in phantom in Figure 1.
Vapour outlet 81 of still 80 is connected via pressure fan 82 and mixer 83 to
gas
dehydration reactor 84. Mixer 83 is also equipped with a connection (not
shown)
Zs whereby hot gases can be admitted to it. Downstream of dehydration reactor
84 is
separator 86 with anhydrous gas takeoff 87 which is connected to HF absorber
54.
Separator 86 is also connected to solids transfer line 88 which communicates
with
fluoride drier 89. Fluoride drier 89 is equipped with water removal lines 91a,
91b and
fluoride supply line 90 for transferring substantially anhydrous metal
fluorides) from
3o drier 89 to mixer 83.
When system 10 is in use, carbonaceous material from hopper 20 is transferred
via
feed unit 25 to reactor 30. Suitably, the transfer of carbonaceous material
via feed unit 25
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is by a system of a plurality of disks within a tube or pipe, the disks being
approximately
the internal diameter of the tube or pipe and connected by a cable whereby
they can be
drawn through the tube or pipe. A suitable system is marketed under the name
"Floveyer" by GPM Australia Pty Ltd of Leichardt, New South Wales. The
transfer of
s material may be continuous or batchwise. Also supplied to reactor 30 is
aqueous H2SiF6,
from hydrolyser 32 via line 24. Reactor 30 is typically at a temperature of
about 30°C
and atmospheric pressure.
Carbonaceous material is contacted with the aqueous H2SiF6 in reactor 30 for a
time
sufficient for at least some of any sulfur-containing impurities in the
carbonaceous
io material to react and dissolve. This may be achieved in a flow-through
reactor by
controlling the flow rate of the reactant aqueous solution to provide a
sufficient residence
time in reactor 30. Alternatively, the process may be carried out batchwise,
with
sufficient time being allowed for reaction of each batch. Typically a suitable
reaction
time is in the range of 10 to 100 minutes, more typically 15 to 30 minutes,
still more
is typically 12 to 16 minutes.
The mixture of aqueous acid and carbonaceous materials from reactor 30 is
transferred via line 26 to filter 50 in which the aqueous phase containing
aqueous
hydrofluorosilicic acid and dissolved metal fluorosilicates, and the like, is
separated from
partially purified carbonaceous materials. The aqueous phase is transferred by
a line 51
ao to still 110 (not shown in Figure 1) for separation of metal fluorides as
described in more
detail below with reference to Figure 2.
Partially purified carbonaceous material is transferred via conveyer 52 to
reactor 55
where it is mixed with an aqueous fluorine acid solution comprising aqueous
hydrofluorosilicic acid and hydrogen fluoride so that partially purified
carbonaceous
as materials from purification reactor 30 can remain in contact with the
aqueous fluorine
acid solution for a sufficient time for at least some of any silica in the
partially purified
carbonaceous material to be dissolved. Reactor SS is typically maintained at a
pressure in
the range of about 100-135 kPa and a temperature of about 70°C.
Residence time of the
carbonaceous material in reactor 55 is typically from 10 to 20 minutes, more
typically
3o about 15 minutes.
From reactor 55 the mixture of the carbonaceous material and aqueous fluorine
acid
solution is passed via pump 56 to first stage tubular reactor 65A and thence
to second
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stage 65B. The temperature in tubular reactor 65A, 65B is typically about
70°C and the
pressure is typically from 350 to 500 kPa. In first stage reactor 65A the
suspension of
carbonaceous material in aqueous acid is agitated sufficiently for any FeS and
other
relatively dense material present to be separable at separator 16 at the end
of second stage
s reactor 65B. In second stage tubular reactor 65B, the mixture is not
ultrasonically
agitated. From a lower portion of separator 16 a slurry of solids which are
rich in FeS is
removed via line 67. A slurry of carbonaceous material in aqueous
hydrofluorosilicic
acid is removed from an upper portion of separator 16 via line 66 and
transferred to
centrifuge or belt filter 70 where aqueous acid is removed, leaving a
carbonaceous
io material stream to be transferred to the washer/separator system.
lil this system, carbonaceous material is washed with aqueous
hydrofluorosilicic
acid which flows through the system in the opposite direction to the direction
of flow of
the carbonaceous materials. That is, the fresh supply of aqueous
hydrofluorosilicic acid is
supplied from hydrolyser 32 to mixing tank 75 where it mixes with carbonaceous
material
is and is separated in separator 76. From separator 76 the aqueous phase is
transferred to
mixing tank 73 where it is mixed with carbonaceous material entering that
mixing tank,
and separated therefrom in separator 74. The aqueous phase separated in
separator 74 is
transferred to mixing tank 71 where it is mixed with carbonaceous material
leaving
centrifuge or belt filter 70. The solids and liquids in mixing tank 71 are
separated in
zo separator 72, the solids being transferred to mixing tank 73 and the
liquids being
transferred to still 80. Solids leaving separator 76 are thus washed solids,
and liquid
leaving separator 72 is relatively impure.
Carbonaceous material leaving the final separator 76 in the sequence of
vessels is
admitted (optionally via a solvent extraction system) to a drying system which
consists of
as mixing vessel 77 and steel tube reactor 78. The carbonaceous material
entering mixing
vessel 77 is mixed with oxygen-depleted combustion gases and transferred to
reactor 78
where it is baked under inert atmosphere, typically at about 310°C, to
remove the
remaining hydrofluorosilicic acid from the surface of the carbonaceous
material. The
hydrofluorosilicic acid is removed as gaseous hydrogen fluoride and silicon
tetrafluoride,
so together with steam, which gases are directed to still 80 after the gases
and the dried
solids are separated in separator 79. Dried solids exiting separator 79 are
purified
carbonaceous materials which are suitable for use as a combustible fuel.
System 10
further includes carbonaceous materials storage container 93 from which dried
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13
carbonaceous material can be supplied to furnace and gas turbine system 95.
Optionally,
system 10 includes a solvent extraction stage as described below with
reference to Figure
3, between separator 79 and storage container 93 as illustrated in phantom in
Figure 1.
Aqueous phase removed from centrifuge or belt filter 70 is passed to HF
absorber
s 54 where gases from drier 84 and separator 86 are admitted for absorption of
HF to
generate the fluorine acid solution to be supplied to silica removal reactor
55. Also
supplied to HF absorber 54, via line 53, and HF and SiF4 gases from system 100
as
illustrated in Figure 2 and described in more detail below. Gases leaving HF
absorber 54
pass to hydrolyser 32 to which water 36 is added in sufficient amount to
produce aqueous
io HZSiF6 of the desired concentration for use in reactor 30. Silica generated
in hydrolyser
32 is removed via a bottom outlet.
Aqueous acid leaving the washer/separator system at separator 72 is
transferred to
still 80 where it is heated to sufficient temperature (typically 105 to 110
°C) to cause
hydrogen fluoride and silicon tetrafluoride gases to be liberated from the
aqueous solution
is and any metal fluorides that had been contained in the aqueous phase to
separate out as
solids. It will be appreciated that the pressure difference across fan 82 will
affect the
pressure in still 80 and hence its temperature. The separated solids are
removed from still
80 via separator 98. Still 80 is typically heated by exhaust gas from gas
turbine 85.
Vapours from mixing vessel 77 and separator 79 are typically returned to still
80 and
zo provide a further source of heat.
Gases leaving still 80 are passed via line 81 and pressure fan 82 to mixer 83
in
which they are mixed with substantially anhydrous A1F3. The mixture is passed
through
tubular dehydration reactor 84 leading to removal of substantially all the
water from the
gaseous phase, thereby producing a substantially anhydrous gaseous mixture of
HF and
zs SiF4 which is transferred from dehydration reactor 84 to HF absorber 54 via
line 87.
Moist A1F3 produced in dehydration reactor 84 is transferred to A1F3 drier 89
in which the
moist A1F3 is heated. Water vapour generated by this heating is removed at 91
a and 91b,
and substantially anhydrous A1F3 is recycled via line 90 to mixer 83. Exhaust
gases from
gas turbine 95 are conveniently used for the purpose of heating drier 89.
3o Figure 2 illustrates in schematic block diagram form a system 100
comprising a still
and associated plant for processing an aqueous solution or suspension produced
by step
(a) of a process of the first or second embodiments of the invention.
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Referring to Figure 2, system 100 includes still 1 IO equipped with supply
line 1 I5
communicating with filter 50 as illustrated in Figure 1. Still 110 is also
equipped with
jacket heater 112, vapour outlet 120 and a bottom outlet connected to level
controlled
separator 150. Gas outlet 120 communicates via pressure fan 125 to water
removal
s system 130, the gas outlet of which is connected to a pair of activated
carbon filters 135,
136 which are connected to steam condenser 140. Condenser 140 is equipped with
vent
145 and drain 146. Carbon filters 135, 136 are respectively equipped with gas
outlets 138
and 139, and are connected to steam supply line 133.
Tn use, aqueous phase leaving reactor 30 as illustrated in Figure l and
separated
io from solids at filter 50 is admitted to still 110 via line 115, and still
110 is heated by
jacket heater 112 to a temperature sufficient for gases comprising HF, SiF4,
sulfur dioxide
and water vapour to be evolved from still 110 and leave via outlet 120. These
gases axe
pressurised by fan 125, typically to a pressure in the range of about 70-140
kPa, and
passed into a water removal system 130 including anhydrous aluminium fluoride,
as
is described above with reference to Figure 1. The temperature of still 110 is
dependent on
the pressure generated by fan 125, but is typically in the range of 105 to 110
°C. In water
removal system 130, water vapour is substantially removed and substantially
anhydrous
gases leave the water removal system and are admitted to one or the other of
activated
carbon filters 135, 136. As the gases pass through the activated carbon
filter, sulfur
ao dioxide and certain other gases that may be present, such as HCI, are
absorbed by the
activated carbon, generating a stream of gaseous HF and SiF4 which is removed
at gas
outlet 138 or 139 and transferred to HF absorber 54 of system 10 as shown in
Figure I,
via line 53 thereof. Conveniently, activated carbon filters 135, 136 are used
in tandem so
that one of the activated carbon filters is on-stream and being contacted with
gases
as leaving water removal system 130 while the other activated carbon filter is
off stream and
is being heated to desorb sulfur dioxide and other absorbed species such as
hydrogen
chloride. The heating is by means of steam admitted via line 133. The desorbed
species
are transferred from the activated carbon filter which is being cleaned in
this way to steam
condenser 140 where the steam is condensed and removed, together with
dissolved SOZ
so and any HCl present, via drain 146.
Liquids in still 110 become more concentrated as a result of the heating and
evaporation of gases therefrom, until a point where dissolved inorganics in
the liquids
exceed their solubility limit. Inorganic solids accumulating in still 110 can
be removed
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from the bottom outlet of the still and passed to a level controlled separator
150 from
which solids can be separated from the liquid phase by any convenient means
and can be
directed either to disposal or to a reprocessing plant to obtain useful
materials therefrom.
The separated liquids can be returned to still 110.
s Figure 3 illustrates in schematic form a system 200 for treatment of
partially
purified carbonaceous materials with a solvent capable of dissolving elemental
sulfur, in
accordance with a process of the third embodiment of the invention.
Referring to Figure 3, system 200 includes treatment vessel 210 which is
equipped
with carbonaceous material inlet 215 and solvent inlet 216, as well as outlet
218 to permit
io transfer of carbonaceous material and solvent from treatment vessel 210 to
solid/liquid
separator 220. Separator 220 may be any convenient form of separator such as
filter or
centrifuge, or settler. Separator 220 is equipped with a solids removal outlet
connected to
stripper 230 and a liquids outlet 225 connected to a still (not shown).
Stripper 230 is
equipped with a heater (not shown), vapour off take line 237 and solids outlet
235.
is When system 200 is in use, carbonaceous material which has been treated
with a
fluorine acid solution as described, for example, in United States Patent No.
4,780,112,
and solvent are charged into treatment vessel 210 where they are mixed and
allowed to
remain in contact for sufficient time for at least part of any elemental
sulfur present in the
carbonaceous materials to be dissolved by the solvent. The solvent is
typically ethanol,
ao but may be any other solvent which is capable of dissolving elemental
sulfur, or a mixture
of such solvents. The treatment in the treatment vessel 210 is typically at
ambient
temperature and atmospheric pressure. After an appropriate contact time, the
contents of
treatment vessel 210 are conveyed via bottom outlet 218 to separator 220 in
which the
solids phase is separated from the solvent phase. The solids phase is
transferred to
zs stripper 230 where it is heated, causing residual solvent to evaporate.
Suitably, the
temperature of heating is at or about the boiling point of the solvent used.
After sufficient
heating time to cause substantially all of the residual solvent to evaporate
from the
carbonaceous material in stripper 230, the dried carbonaceous material is
discharged via
outlet 235 for further processing or for use.
3o Liquids leaving separator 220 and vapour leaving stripper 230 may be passed
to a
solvent still (not shown) in which the solvent is distilled for recovery and
reuse, the other
major product in the still being elemental sulfur which is removed for
disposal or sale.
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Example
Coal samples processed by a process as described in United States Patent No.
4,780,112 were dried and examined under an electron microscope. They were
observed
to contain sulfur in two forms, pyrite and elemental sulfur.
s A raw high-sulfur coal sample was treated with about twice its weight of 32%
w/w
aqueous hydrofluorosilicic acid for 30 minutes at ambient temperature, then
dried and
treated with an aqueous fluorine acid solution as described in United States
Patent No.
4,780,112. After separation of the solids they were again dried and examined
under the
electron microscope. No elemental sulfur was visible.
