Note: Descriptions are shown in the official language in which they were submitted.
17,~98
~3ACKGRO[~ND OF THE INVENTION
___
2 1. Field of the Invention: This invention
__
3 relates to the conversion of coal and similar carbona-
4 ceous solids in the presence of alkali metal-containing
catalysts and is particularly concerned with the recovery
6 of alkali metal constituents from spent solids produced
7 during coal gasification and similar operations and their
8 reuse as constituents of the alkali metal-containing
9 catalysts.
2. Description of the Prior Art: Potassium
11 carbonate, cesium carbonate and other alkali metal com-
12 pounds have been recognized as useful catalysts for
13 the gasification of coal and similar carbonaceous solids.
14 The use of such compounds in coal liquefaction, coal
carbonization, coal combustion and related proeesses has
16 also been proposed. To secure the higher reaction rates
17 made possible by the presence of the alkali metal compounds
18 it has been suggested that bituminous coal, subbituminous
19 eoal, lignite, petroleum coke, oil shale, organic wastes
and similar carbonaceous materials be mixed or impreg-
21 nated with potassium, cesium, sodium or lithium compounds,
22 alone or in combination with other metallic constituents,
23 before such materials are reacted with steam, hydrogen,
24 oxygen or other agents at elevated temperatures to produce
gaseous and/or liquid effluents. Studies have shown that
26 a wide variety of different alkali metal compositions can
27 be used for this purpose, including both organic and
28 inorganic salts, oxides, hydroxides and the like.
29 In general the above-described studies indicate
3~ that cesium compounds are the most effective gasification
31 catalysts followed by potassium, sodium and lithium
32 compounds, in that order. Because of the relatively high
7~B
1 cost of cesium compounds and the low effectiveness of
2 lithium compounds, most of the experimental work in
3 this area that has been carried out in the past has
4 been directed toward the use of compounds of potassium
and sodium. This work has shown that the potassium com-
6 pounds are substantially more effective than the
7 corresponding sodium compounds. Attention has therefore
8 been focused on the use of potassium carbonate.
9 Coal gasification processes and similar opera-
tions carried out in the presence of alkali metal
11 compounds at high temperatures generally result in the
12 formation of chars and alkali metal residues. The chars
13 normally include unconverted carbonaceous constituents
14 of the coal or other eed material and various inorganic
constituents generally referred to as ash. It is gener-
16 ally advisable to withdraw a portion of the char from
17 the reaction zone during gasification and similar opera-
18 tions in order to eliminate the ash and keep it from
19 building up within the reaction zone or other vessels in
thé system. Elutriation methods and other techniques for
21 separating char particles of relatively high ash content
22 and returning particles of relatively low ash content to
23 the reaction zone in order to improve the utilization of
24 carbon in such processes have been suggested.
In gasification and other processes referred to
26 above that utilize alkali m0tal-containing catalysts,
27 the cost of the alkali metal constituents is a significant
28 factor in determining the overall cost of the process. In
29 order to maintain catalyst cost at reasonable levels, it
is essential that the alkali metal constituents be re-
31 covered and reused. There have been proposals for the
32 recovery of alkali metal constituents by :Leaching as
--3--
7~
1 they are withdrawn frorn the reaction zone with char
2 during operations of the type referred to above.
3 Studies indicate that these constituents are generally
present in part as carbonates and other water-soluble
compounds which can be recovered by water washing.
6 Experience has shown that only a portion of the potas-
7 sium carbonate or other alkali metal constituents is
8 normally recovered and that substantial quantities of
9 makeup alkali metal compounds are therefore required.
This adds appreciably to the cost of such operations.
ll SUMMARY OF THE INVENTION
12 The present invention provides an improved
13 process for the recovery of alkali metal constituents
1~ from mixtures of char, ash and alkali metal residues
produced during coal gasification and other conversion
16 processes carried out in the presence of an alkali
17 metal containing catalyst. In accordance with the
18 invention, it has now been found that increased amounts
19 of alkali metal consti~uents can be effective~y re-
covered from particles containing alkali metal residues
21 produced during coal gasification and related high
22 temperature conversion processes by contacting the
23 particles with a calcium or magnesium-containing
2~ compound at a temperature sufficiently high, normally
above 1600F, to convert water-insoluble alkali metal
26 constituents in the alkali metal residues into water-
27 soluble alkali metal constituents. It is believed that
28 the high temperature enables the calcium or magnesium-
29 containing compound to react with the water-insoluble
constituents, such as alkali metal aluminosilicates, in
31 the alkali metal residues to produce reaction products
32 containing water-insoluble compounds such as calcium
_~ _
79~
1 silicates and water-soluble alkali metal constituents.
2 The reaction products are contacted with wa~er, which
3 leaches the water-soluble alkali metal constituents
4 from the solids thereby forminq an aqueous solution.
The water-soluble alkali metal constituents in this
6 aqueous solution are then used as at least a portion of
7 the alkali metal constituents ~hich comprise the alkali
8 metal-containing catalyst. Preferably, such use is
9 achieved by recycling the solution directly to the
conversion process. If desired, however, the alkali
11 metal constituents may first be recovered from the
12 solution ànd then used in the conversion process.
13 The aqueous solution produced in the leaching step may
14 contain a substantial amount of water-soluble alkali
metal aluminates. If such is the case, it will
16 normally be desirable to lower the pH of the solution
17 to precipitate aluminum in the form of aluminum hydrox-
18 ide before the solution is recycled to the conversion
lg process.
The invention is based in part upon studies
21 of the reactions that catalysts containing alkali metal
22 constituents undergo during coal gasification and
23 similar operations. Coal and other carbonaceous solids
24 used in such operations normally contain mineral
constituents that are converted to ash during the
26 gasification process. Although the composition of ash
27 varies, the prinicipal consti~uents, expressed as
28 oxides, are generally silica, alumina and ferric
29 oxide. The alumina is usually present in the ash in the
form of aluminosilicates. Studies have indicated that
31 at least a portion of the alkali metal compounds, such
32 as potassium carbonate, that are used as gasification
--5--
i7~8
1 catalyst constituents react with the aluminosilicates
2 and other ash constituents to form alkali metal
3 residues containing water-soluble alkali metal com-
4 pounds such as carbonates, sulfates, sulfides and the
like and water-insoluble, catalytlcally inactive
6 materials such as alkali metal aluminosilicates.
7 Unless the alkali metal constituents in the insoluble
8 alkali metal residues can be recovered, they~are lost
9 from the process and must be replaced by makeup alkali
lG metal compounds. The process of this invention allows
11 recovery of these alkali metal constituents and thereby
12 decreases the costs incurred by utilizing large amounts
13 of makeup alkali metal compounds. As a result, the
1~ invention makes possible substantial sa~ings in gasifi-
cation and other conversion operations carried out in
16 the presence of alkali metal-containing catalysts and
17 permits the generation of product gases and/or liquids
18 at significantly lower cost than would otherwise be the
19 case.
BRIEF DESCRIPTION OF THE DRAWI~G
21 The drawing is a schematic flow diagram of a
22 catalytic coal gasification process in which alkali
23 metal constituents of the catalyst are recovered and
24 reused in the process.
DESCRIPTION OF THE PREFERRED EMBODI~IENTS
26 The process depicted in the drawing is one
27 for the production of methane by the gasification of
28 bituminous coal, subbituminous coal, lignite or similar
29 carbonaceous solids with steam at high temperatures in
the presence of a carbon alkali metal catalyst prepared
31 by impregnating the feed solids with a solution of an
32 alkali metal compound or a mixture of such compounds
--6--
7~3
1 and thereaftec heating the impregnated material to a
2 temperature sufficient to produce an interaction
3 between the alkali metal and the carbon present. It
will be understood that the allcali metal recovery
system disclosed is not restricted to this particalar
6 gasification process and that it can be employed in
7 conjunction with any of a variety of other conversion
- 8 processes in which alkali metal compounds or carbon-
9 alkali metal catalysts are used to promote the reaction
of steam, hydrogen, oxygen or the like with carbona-
11 ceous feed materials to produce a char, coke or similar
12 solid product containing alkali metal residues from
13 which alkali metal compounds are recovered for reuse
14 as the catalyst or a constituent of the catalyst. It
can be employed, for example, for the recovery of
16 alkali metal compounds from various processes for the
17 gasification of coal, petroleum coke, lignite, organic
18 waste materials and similar solids feed streams which
lg produce spent carbonaceous solids. Other conversion
processes with which it may be used include operations
21 for the carbonization of coal and similar feed solids,
22 for the liq~efaction of coal and related carbonaceous
23 feed materials, for the retorting of oil shale, for the
24 partial combustion of carbonaceous feed materials, and
the like. Such processes have been disclosed in the
26 literature and will be familiar to those skilled in the
27 art.
28 In the process depicted in the drawing, a
29 solid carbonaceous feed material such as bituminous
coal, subbituminous coal, lignite or the like that has
31 been crushed to a particle si~e of about 8 mesh or
32 smaller on the U.S. Sieve Series Scale is passed into
--7
7~1~
1 line 10 from a feed preparation plant or storage
2 facility that is not shown in the drawing. The solids
3 introduced into line 10 are fed into a hopper or
4 similar vessel 11 from which they are passed through
line 12 into feed preparation zone 14. This zone
6 contains a screw conveyor or similar device, not shown
7 in the drawing, that is powered by a motor 16, a series
8 of spray nozzles or similar devices 17 for the spraying
9 of alkali metal containing solution supplied through
line 18 onto the solids as they are moved through the
11 preparation zone by the conveyor, and a sirnilar set of
12 nozzles or the like 19 for the introduction of steam
13 into the preparation zone. The stearn, supplied through
14 line 20, ser~es to heat the impregnated solids and
drive off the moisture. Steam is withdrawn from zone
16 14 through line 21 and passed to a condenser not shown,
17 from which it may be recovered for use as makeup water
18 or the like. The maj,ority of the alkali metal-contain-
19 ing solution is recycled through line 79 from the
a]kali metal recovery section of the process, which is
21 described in detail hereafter. Any makeup solution
22 required may be introduced into line 79 via line
23 13.
24 It is preferred that sufficient alkali
metal-containing solution be introduced into feed
26 preparation zone 14 to provide from about 1 to about
27 50 weight percent of the alkali metal compound or
28 mixture of such compounds on the coal or other carbona-
29 ceous solids. From about 1 to about 1'5 weight percent
is generally adequate. The dried impregnated solid
31 particles prepared in zone 14 are withdrawn through
32 line 24 and passed to a closed hopper or similar vessel
--8--
7~3
1 25. From here they are discharged through a star wheel
2 feeder or equivalent device 26 in line 27~at an el-
3 evated pressure sufficient to permit their entrainment
4 into a stream of high pressure steam, recycle product
gas, inert gas or other carrier gas introduced into
6 line 29 via line 28. The carrier gas and entrainçd
7 solids are passed through line 29 into manifold 30 and
8 fed from the manifold through feed lines 31 and noz-
9 zles, not shown in the drawing, into gasifier 32. In
lieu of or in-addition to hopper 25 and star wheel
11 feeder 26, the feed system may employ parallel lock
12 hoppers, pressurized hoppers, aerated standpipes
13 operated in series, or other apparatus to raise the
14 input feed solids stream to the required pressure
level.
16 It is generally preferred to operate the
17 gasifier 32 at a pressure between about 300 and about
18 2000 psig. The carrier gas and entrained solids will
19 normally be introduced at a pressure somewhat in excess
of the gasifier operating pressure. The carrier gas
21 may be preheated to a temperature in excess of about
22 300F but below the initial softening point of the coal
23 or other feed material employed. Feed particles may be
24 suspended in the carrier gas in a concentration between
about 0.2 and about 5.0 pounds of solid feed material
26 per pound of carrier gas. The optimum ratio for a
27 particular system will depend in part upon the feed
28 particle size and density, the molecular weight of the
29 gas employed, the temperature of the solid feed mate-
rial and input gas stream, the amount of alkali metal
31 compound employed and other factors. In general,
32 ratios between about 0.5 and about 4.0 pounds of solid
_g_
3t7~3
1 feed material per pound of carrier gas are preferred.
2 Gasifier 32 cornprises a refractory lined
3 vessel containing a fluidized bed of carbonaceous
4 solids extending upward within the vessel above an
internal grid or similar distribution device not shown
6 in the drawing. The bed is maintained in the fluidized
7 state by means of steam introduced through line 33,
8 manifold 34 and peripherally spaced injection lines and
9 nozzles 35 and by means of recycle hydrogen and carbon
monoxide introduced through bottom inlet line 36. The
11 particular injection system shown in the drawing is
12 not critical and hence other methods for injecting
13 the stearn and recycle hydrogen and carbon monoxide may
14 be employed. In some instances, for example, it may be
preferred to introduce both the steam and recycle gases
16 through multiple nozzles to obtain more uniform distribu-
17 tion of the injected fluid and reduce the possi-
18 bility of channeling and related problems. The space
19 velocity of the rising gases within the fluidized bed
will normally be between about 300 and about 3000
21 volumes of steam and recycle hydrogen and carbon
22 monoxide per hour per volume of fluidized solids.
23 The injected steam reacts with carbon in
24 the feed material in the fluidized bed in gasifier 32
at a temperature within the range between about 800F
26 and about 1600F and at a pressure between about 300
27 and about 2000 psig. Due to the equilibrium condition
28 existing in the bed as a result of the presence of the
29 carbon-alkali metal catalyst and the recycle hydrogen
and carbon monoxide injected near the lower end of the
31 bed, the reaction products will normally consist
32 essentially of methane and carbon dioxide. Competing
- 1 0 -
1 reactions, which in the absence of the ca~alyst and the
2 recycle gases would ordinarily tend to produce addi-
3 tional hydrogen and ca-rbon monoxide, are suppressed.
4 The ratio of methane to carbon dioxide in the raw
product gas thus forrned will preferably range from
6 about 1 to about 1.4 moles per mole, depending upon the
7 amount of hydrogen and oxygen in the feed coal or other
8 carbonaceous solids. The coal employed may be consid-
9 ered as an oxygenated hydrocarbon for purposes of
describing the reaction. Wyodak coal, for example, may11 be considered as having the approximate formula
12 CH0.8~0.20r based on the ultimate analysis of
13 moisture and ash-free coal and neglecting nitrogen and
1~ sulfur. The reaction of this coal with steam to
produce methane and carbon dioxide is as follows:
16 1-24 H2O(g) + 1-8 CHo.84oo.2o -~ 0.8 CO2 + CH4.
17 Under the same gasification conditions, coals of higher
18 oxygen content will normally produce lower methane to
19 carbon dioxide ratios and those of lower oxygen content
will yield higher methane to carbon dio~ide ratios.
21 The gas leaving the fluidized bed in gasifier
22 32 passes through the upper section of the gasifier,
23 which serves as a disengagement zone where particles
24 too heavy to be entrained by the gas leaving the vessel
are returned to the bed. If desired, this disengage-
26 ment zone may include one or more cyclone separators or27 the like for removing relatively large particles from
28 the gas. The gas withdrawn from the upper part of the
29 gasifier through line 37 will normally contain methane
and carbon dioxide produced by reaction of the steam
31 with carbon, hydrogen and carbon monoxide introduced
32 into the gasifier as recycle gas, unreacted steam,
~,~,f~J~5~'7~'~
hydrogen sulfide, ammonia and other contaminants
2 formed from the sulfur and nitrogen contained in the
3 feed material, and entrained fines. This gas is
4 introduced into cyclone separator or similar device 38
5 for rernoval of the larger fines. The overhead gas then
6 passes through line 39 into a second separator 41 where
7 smaller particles are removed. The gas from which the
8 solids have been separated is taken overhead from
9 separator 41 through line 42 and the fines are dis-
. 10 charged downward through dip legs 40 and 43. These
11 fines may be returned to the gasifier 0 passed to the
12 alkali metal recovery section of the process as discuss-
13 ed hereafter.
lq After entrained solids have been separated
15 from the raw product gas as described.above, the gas
16 stream may be passed through suitable heat exchange
17 equipment for the recovery of heat and then processed
18 for the removal of acid gases. Once this has been
19 accomplished, the remaining gas, consisting primarily
20 of methane, hydrogen and carbon monoxide, may be
21 cryogenically. separated into a product methane stream
22 and a recycle stream of hydrogen and carbon monoxide,
23 which is returned to the gasifier through line 36.
24 Conventional gas processing equipment can be used.
25 Since a detailed description of this downstream gas
26 processing portion of the process is not necessary for
27 an understanding of the invention, it has been omitted.
28 The fluidized bed in gasifier 32 is comprised
29 of char particles formed as the solid carbonaceous feed
30 material undergoes gasification. The composition of
31 the char particles will depend upon the amount of
32 mineral matter present in the carbonaceous material fed
. --12--
3~?~
1 to the gasifier, the amount of the alkali metal compound
2 or mixture of such compounds impregnated onto the feed
3 material, and the degree of gasification that the char
~ particles undergo while in the fluidi~ed bed. The lighter
char particles, which will have a relatively high content
6 of carbonaceous material, will tend to remain in the upper
7 portion of the fluidized bed. The heavier char particles,
8 which will contain a relatively small amount of carbona-
9 ceous material and a relatively large amount of ash and
alkali metal residues will tend to migrate toward the
11 bottom of the fluidized bed. A portion of the heavier char
12- particles are normally withdrawn from the bottom portion of
13 the fluidized bed in order to elirninate ash and thereby
14 prevent it from building up within the gasifier and
other vessels in the system.
16 The process of this invention is based in part
17 upon the fact that the alkali metal constituents of the
18 gasification catalyst react with the mineral constituents
19 of the coal and other carbonaceous solids during the
gasification process. Studies have indicated that at least
21 a portion of the alkali metal compounds, such as potassium
22 carbonate, sodium carbonate and the like, that are used as
23 gasification catalyst constituents react with the alumino-
24 silicates and other ash constituents to form alkali metal
residues containing water soluble alkall,metal compounds
26 such as carbonates, sulfates, sulfides and the like and
27 catalytically inactive materials such as alkali metal
28 aluminosilicates and other water-insoluble compounds.
29 It has been found that from about lO to about
50 percent by weight of the potassium carbonate or oth~er
31 alkali metal compound employed to impregnate coal or
32 similar feed material prior to gasification will react
-13-
1 with the aluminosilicates and other ash constituents
2 during gasification to form alkali metal aluminosilicat~s
3 and other water-insoluble compounds which cannot normally
4 be recovered from the ash by water washing. Preliminary
studies tend to indicate that when potassium carbonate
6 is utilized to impregnate the coal, the major consti-
7 tuent of the water-insoluble portion of the alkali metal
8 residues produced is a synthetic kaliophilite, which has
9 the chemical formula KAlSiO4.
To improve the economics of the catalytic
11 gasification process described above and other catalytic
12 conversion processes where water-insoluble alkali metal
13 residues are formed, it is desirable to recover as much
14 as possible of the alkali metal constituents from the
insoluble residues and reuse them as catalyst consti-
16 tuents in the conversion process, thereby decreasing the
17 amount of costly makeup alkali metal compounds needed.
18 It has been found that a substantial amount of the alkali
19 metal constituents in the water-insoluble alkali metal
residues withdrawn with the char and ash from the gasi-
21 fier of the above described process or the reaction zone
22 of other conversion processes can be recovered for reuse
23 in the conversion process by mixing the particles with-
24 drawn from the reaction zone with a calcium or a magnesi
containing compound and heating the resultant mixture
26 in the absence of liquid water to a relatively high tempera-
27 ture. The heating process converts the water-insoluble
28 alkali metal constituents in the alkali metal residues ir.to
29 wa-ter-soluble constituents and thereby results in the
formation of reaction products containing water-soluble
31 alkali metal constituents that are free of water. The
32 reaction products are contacted with water, which leaches
-14-
-
1 the water soluble alkali metal constituents from the solids
2 thereby forming an aqueous solution. ~he water-soluble
3 alkali metal constituents in this solution are then
4 used in the conversion process as at least a portion of the
alkali metal constituents which comprise the alkali metal-
6 containing catalyst. Preferably, such use is achieved by
7 recycling the aqueous solution directly to the conversion
8 process. If desired, however, the alkali metal constit-
9 uents may first be recovered from the solution and~then
used in the conversion process. The aqueous solution
11 produced in the leaching step may contain a substantial
12 amount of water-soluble alkali metal aluminates. If such
13 is the case, it will normally be desirable to remove the
14 aluminum from the aqueous solution before it is recycled to
lS the conversion process. This may be accomplished by
16 sufficiently lowering the pH of the solution to precipitate
17 aluminum hydroxide.
18 Referring again to the drawing, char particles
19 containing carbonaceous materiall ash and alkali metal
residues are continuously withdrawn through line 44 from
21 the bottom of the fluidized bed in gasifier 32. The
22 particles flow downward through line 44 countercurrent
23 to a stream of steam or other elutriating gas introduced
24 through line 45. ~lere a preliminary separation of solid~
based on differences in size and density takes place. T:~e
26 lighter particles having a relatively large amount of
27 carbonaceous material tend to be returned to the gasifier
28 and the heavier particles having a relatively high cont~t
29 of ash and alkali metal residues continue downward throu~h
line 46 containing valve 55 into hopper 56. Char fines
31 recovered from the raw product gas through dip legs 40 and
32 43, and line 47 may also be fed into the hopper.
-15-
9i7~
1 The solid particles in hopper 56, which contain
2 both water-soluble and water-insoluble alkali rnetal
3 residues, are passed into line 58 where they are mixed
4 with a calcium or magnesium-containing compound introduced
into line 58 from hopper 59 via line 60. The calcium-
6 containing compound may be calcium oxide or any calcium
7 compound that decomposes in air to form calcium oxide when
- 8 subjected to relatively high temperatures. The calcium-
9 containing compound may be inorganic or organic and may,
for example, be calcium hydroxide, calcium acetate, calcium
11 oxalatel calcium formate, calcium carbonate, dolomite and
12 the like. Similarly, the ma~nesium-containing compound may
13 be magnesium oxide or any magnesium compound that de-
14 composes in air to form rnagnesillm oxide when subjected to
relatively high temperatures. The magnesium-containing
16 compound may be inorganic or organic and may, for example,
17 be magnesium hydroxide, magnesium acetate, magnesium
18 oxalate, magnesium formate, magnesium carbonate, dolomite
19 and the like. The actual calcium or magnesium-containing
compound used will depend primarily upon its availability
21 and cost. The amount of the calcium or magnesium-contain-
22 ing compound required will depend in part on the amount of
23 silicates in the particulate matter with which it is
24 mixed. If desired, a mixture of two or more calcium or
magnesium-containing compounds may be used in lieu of a
26 single compound.
27 The mixture of char particles containing the
28 alkali metal residues and the calcium or magnesium-contain-
29 ing compound is passed through line 61 into rotary kiln or
similar heating device 62 where it is subjected to tempera-
31 tures sufficiently high to cause the alkali metal alumino-
32 silicates and other water insoluble alkali metal compounds
-16-
1 in the residues to react with calcium or magnesium from the
2 calcium or magnesium-eontaining compound. Ti-e reaction
3 converts the water-insoluble alkali metal compounds into
4 reaction produets containing water-soluble alkali metal
constituents and water-insoluble compounds. The reaction
6 products are subsequently treated, as described hereafter,
7 to recover the water-soluble alkali metal constituents,
8 which are recycled to the gasification process where they
9 serve as at least a portion of the alkali metal consti-
tuents which eomprise the alkali metal-containing-catalyst.
11 The mixture of solids introduced into the
12 rotary kiln will normally be subjected to a temperature
13 ranging from about 1600F to about 2600F. Preferably,
1~ the mixture is heated to the sintering ternpe~ature,
whieh causes the surfaee of the partieles to soften
16 thereby increasing the tendeney of the partieles to
17 agglomerate or stiek together. Sintering imparts mobility
18 to the ealeium or magnesium ions present and apparently
19 enables them to easily saturate the atomic strueture of
the alkali metal aluminosilieates and displace alkali `
21 metal eonstituents. Sintering may be effeetively aeeom-
22 plished in a eountereurrent rotary kiln in whieh the fuel
23 is passed through the kiln in a direetion opposite to that
24 in whieh the mixture of solids is passed. In lieu of the
rotary klln, any furnaee or similar heating deviee may be
26 used as long as the required temperatures are obtainable.
27 If desirable, the temperature in the heating deviee may be
28 raised above the sintering temperature to convert the
29 mixture of solids into a liquid mass in which the desired
reactions will take plaee more rapidly. This proeedure,
31 however, may not be a~vantageous sinee the liquid upon
32 eooling will form a glass-like solid feom which it may be
-17-
~7~
1 difficult to water leach soluble alkali metal constit~ents.
2 The actual reactions that take place in rotary
3 kiln 62 to convert the water-insoluble compounds in the
4 alkali metal residues into water-soluble alkali metal
constituents are not completely understood. Apparently,
6 the calcium or magnesium ions from the calcium or mag-
7 nesium compound displace or liberate water-soluble alkali
8 metal constituents from the water-insoluble compounds in
9 the alkali metal residues. The liberation of these
water-soluble constituents is accompanied by the forma-
11 tion of solid water-insoluble calcium or magnesium constit-
12 uents such as calcium or magnesium silicatest aluminates,
13 aluminosilicates and other insoluble compoundst depending
14 upon the composition of the alkali metal resid~es.
The sintered mixture of solids from rotary kiln 62
16 is cooled and passed through line 63 to ball mill or
17 similar crushing device 64 where the solids are pulver-
18 ized, ground, or otherwise crushed to a size suitable
19 for water leaching. It is desirable to produce relatively
small particles since they will provide a high surface
21 area for more effective water leaching. The actual size
22 is determined in part by balancing the cost of crushing
23 with the effectiveness of the water leaching. Preferably,
24 the particles will be crushed to a size smaller than
about 60 mesh on the U.S. Sieve Series Scale.
26 The crushed solids are removed from ball mil~
27 64 and passed through line 65 to water wash zone 66,
28 which will normally comprise a multistage counterc~rrent
29 extraction system in which the solids are countercurrently
contacted with water introduced through line 67. The
31 water leaches water-solubIe alkali metal constituents from
32 the solids, thereby producing an aqueous solution of such
1 constituents, which is removed from the water wash zone
2 through line 68. Spent solids are removed from the water
3 wash zone via line 69 and will contain, among other sub-
4 stances, ash, and various types of calciurn or magnesium
silicates. These solids may be disposed of as land fill,
6 used for construction purposes, or employed in other
7 applications.
8 The aqueous solution removed from water wash
9 zone 66 via line 68 will contain water-soluble alkali
metal constituents. These constituents will normally be
11 comprised of alkali metal hydroxides and alkali metal
12 aluminates. If the solution contains only a small amount
13 of alkali metal aluminates, it can be recycled to feed
14 preparation zone 14 where the coal or similar carbona-
ceous feed material is impreynated with the alkali metal
16 constituents. The recycle of the so]ution may be accom-
17 plished by closing valve 70 and passing the solution
18 into line 71, through valve 72, into line 73 and- through
19 lines 79 and 18. If, however, the solution in line 68
contains a substantial amount of alkali metal aluminates, --
21 it will normally be desirable to remove the aluminum from
22 solution before it is recycled to the feed preparation
23 zone. Removal of the aluminum is desirable because it
24 may form additional alkali metal aluminosilicates in the
gasifier by reacting with silica in the feed material and
26 alkali metal constltuents of the catalyst. If removal
27 of aluminum is desired, valve 72 is closed and the solu-
28 tion is passed from line 68 through valve 70 and line 74
29 into contactor or similar vessel 75.
In contactor 75 the pH of the-solution i5
31 lowered to a value in the range between about 10.0 and
32 about 4.0, preferably between about 9.0 and about 5.0, by
--19--
1 contacting it with a carbon dioxide-containing gas. Tne
2 aqueous solution is passed downward through the contacting
3 zone in contactor 75 where it comes in contact with an
4 upflowing gas that contains carbon dioxide. The carbor,
dioxide-containing gas is injected into the bottom of the
6 contactor through line 76. As the carbon dioxide gas rises
7 upward through the downflowing aqueous solution, the
8 carbon dioxide in the gas reacts with the alkali metal
9 aluminates in the solution to form alkali metal carbon-
ates and water-insoluble aluminum hydroxide. If the
11 partial pressure of carbon dioxide is sufficiently high
12 and the temperature in the contactor is low, alkali metal
13 bicarbonates may also form.
14 A gas depleted in carbon dioxide is withdra~n
overhead of contactor 75 through line 77 and either
16 vented ko the atmosphere, further processed for the
1i7 recovery and reuse of carbon dioxide, or otherwise dis-
18 posed of. Any carbon dioxide-containing gas, includins
19 pure carbon dioxide and air, may be used. It is preferred,
however, to utilize the flue gas produced from the fuel
21 combustion taking place in rotary kiln 62. The contact-
22 ing vessel utilized does not necessarily have to be the
23 type shown in the drawing but may be any type of vessel
24 that allows for fairly good contacting between the carbon
dioxide-containing gas and the aqueous solution contair.ing
26 alkali metal aluminates. A tank in which the carbon
27 dioxide-containing gas is bubbled through the aqueous
28 solution may be sufficient for purposes of the inventicn.
29 The purpose of the above-described step of tne
alkali metal recovery process is to lower the pH of the
31 aqueous solution containing the alkali metal aluminates
32 so that substantially all of the aluminum is removed
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1 from the solution in the form of a water-insoluble precipi-
2 tate of aluminum hydroxide, thereby leaving in solution
3 aluminum~free alkali metal constituents that are subse-
4 quently recovered and used as constituents of the gasifi-
cation catalyst. As mentioned previously, removal of
6 aluminum from the alkali metal constituents before their
7 use in the gasification catalyst is desirable to help avoid
8 the possible formation of additional alkali metal alumino-
9 silicates in the gasifier by the reaction of the aluminum
with silica in the feed material and alkali metal consti-
11 tuents of the catalyst. It will be understood that for
12 purposes of the invention any method of lowering pH may be
13 used. For example, instead of contacting the aqueous
14 solution from water wash zone 66 with a carbon dioxide-
containing gas, the solutlon may be mixed with sufficient
16 quantities of sulfuric acid, nitric acid, formic acid or
17 the like to lower the pH to the desired value.
18 Referring again to the drawing, the effluent
19 from contacting vessel 75, which contains alkali metal
carbonates and other water-soluble alkali metal consti-
21 tuents, and aluminum hydroxide, is withdrawn from the
22 bottom of the vessel through line 78 and passed to rotary
23 filter or other liquid-solids separation device 80 where
24 the solid aluminum hydroxide is separated from the aqueous
solution and then passed through line 81 to rotary kiln or
26 similar heating device 82 where it is calcined at high
27 temperatures to produce alumina, which is recovered via
28 line 83 and may be sold as a byproduct. The sale of t~is
29 material may produce additional return from the process and
thus reduce the overall cost of the product gas.
31 A substantially solids-free aqueous solution
32 containing al~ali metal carbonates and other water-soluble
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37~3
1 alkali metal compounds is removed from ~ilter 80 through
2 line 84 and recycled through lines 79 and 18 to feed
3 preparation zone 14 where the coal or similar carbonaceous
4 feed material is impregnated with the alkali metal com-
pounds in the solution. If the concentration of alkali
6 metal compounds in the recycle solution is undesirably low,
7 the solution may be concentrated by removing excess water
8 before it is returned to the feed preparation zone. It
9 will be understood that the exact alkali metal compound
or compounds present in the recycled solution will depend
11 on the substance used to lower the pH of the aqueous
12 effluent from water wash zone 66. For example, if nitric
13 acid is used in lieu of a carbon dioxide-containing gas,
1~ the recycled solution will contain alkali metal nitrates
instead of carbonates.
16 In the embodiments of the invention discussed
17 above and shown in the drawing, the calcium or magnesium
18 compound is mixed with the particles containing alkali
19 metal residues after those particles are removed from the
gasifier or similar conversion zone. It will be understood
21 that the process of the invention is not restricted to this
22 method of supplying the calcium or magnesium-containing
23 compound but also encompases the situation where the
24 compound is added prior to or during gasification. This
may be accomplished by mixing the calcium or magnesium
26 compound with the coal or other carbonaceous feed material,
27 or impregnating the coal with a solution of the compound~
28 The embodiment of the invention which includes
29 the pH adjustment step is one that allows for the recovery
of alumina as a byproduct of the alkali metal recovery
31 process. If recovery of alumina is undesirable for any
32 reason, this embodiment of the invention may be simplified
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;?J~
l by elirninating contactor 75, rotary filter 80 and rotary2 kiln ~2, and injecting the carbon dioxide-containing
3 gas or other acidifying agent directly into water wash zone
4 66 to lower the pH and thereby precipitate aluminum hydrox-
ide. The solid aluminum hydroxide will be removed from the
6 water wash zone through line 69 along with other solids and
7 the aqueous solution containing water soluble alkali metal
8 constituents withdrawn from water wash zone 66 through line
9 68 is passed to feed preparation zone 19.
It will be apparent from the foregoing that
11 the process of the invention provides an impEoved alkali
12 metal recovery system, which makes it possible to signifi-
13 cantly increase the amount o alkali metal constituents
14 that are recovered from alkali metal residues produced
during catalytic gasification and similar high tempera-
16 ture catalytic conversion processes. As a result, the
17 need for costly makeup alkali metal compounds is reducea,
18 thereby lowering the overall cost of the conversion process
