Note: Descriptions are shown in the official language in which they were submitted.
4~Z I
1 BACKGROUND OF THE INVENTION
2 ` ~ l. 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 alkal~ metal constituents from spent solids produced
i during coal gasification and similar operations and their
8 reuse as constituents of the alkali metal-containg cata-
;
lysts.
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 the
13 gasification of coal and similar carbonaceous solids. The
14 use of such compounds in coal liquefaction, coal carboni-
zation, coal combustion and related processes has also
16 been proposed. To secure the higher reaction rates made
1i possible by the presence of the alkali metal compounds it
18 has been suggested that bituminous coal, subbituminous -
19 coal, lignite, petroleum coke, oil shale, organic wastes
~ and similar carbonaceous materials be mixed or impregnated
21 with potassium, cesium, sodium or lithium compounds, alone
22 or in combination with other metallic constituents, before
23 such materials are reacted with steam, hydrogen, oxy~en or
24 other agents at elevated temperatures to produce gaseous
and/or liquid effluents. Studies have shown that a wide
26 variety of different alkali metal compositions can be used
27 ~or this purpose, including both organic and inorganic
28 salts, oxides, hydroxides and the like.
29 In general the above-described studies indicate
that cesium compounds are the most effective gasification
~ 2 -
~19~
.
:' . ' '
1 catalysts followed by potassium, sodium and lithium com-
2 po~nds, in that order. Because of the relatively high
3 cost of cesium compounds and the low effectiveness of
4 lithium compounds, most of the experimental work in this
area that has been carried out in the past has been
6 directed toward the use of compounds of potassium and
7 sodium. This work has shown that the potassium compounds
.
` 3 are substantially more effective than the corresponding
. . .
' 9 sodium compounds. Attention has therefore been focused
... ~
on the use of potassium carbonate.
11 Coal gasification processes and similar opera-
12 tions carried out in the presence of alkali metal com-
13 pounds at high temperatures generally result in the forma-
14 tion of chars and alkali metal residues. The chars
normally include unconverted carbonaceous constituents of
16 the coal or other feed material and various inorganic con-
17 stituents generally referred to as ash. It is generally
18 advisable to withdraw a portion of the char from the
19 reaction zone during gasification and similar operations
in order to eliminate the ash and keep it from building
21 up within the reaction zone or other vessels in the system.
22 Elutriation methods and other techniques for separating
23 char particles of relatively high ash content and return-
24 ing particles of relatively low ash content to the
reaction zone in order to improve the utilization of car-
26 bon in such processes have been suggested.
27 In gasification and other processes referred to
28 above that utilize alkali metal-containing catalystS, the
29 cost of the alkali metal constituents is a significant
factor in determining the overall cost of the process.
1~9542
1 In order to maintain catalyst cost at reasonable levels,
2 it; is essential that the alkali metal constituents be
3 recovered and reused. There have been proposals for the
4 recovery of alkali metal constituents by leaching as
-5 they are withdrawn from the reaction zone with char
6 during operations of the type referred to above. Studies
~ 7 indicate that these constituents are generally present in
part as carbonates and other water soluble compounds
9 which can be recovered by water washing. Experience has
.
shown that only a portion of the potassium carbonate or
11 other alkali metal constituents is normally recovered and
12 that substantial quantities of makeup alkali metal com- ¦
13 pounds are therefore required. This adds appreciably to
the cost of such operations.
SUMMARY OF THE INVENTION
16 The present invention provides an improved pro-
17 cess for the recovery of alkali metal constituents from
18 char particles produced during coal gasification and other
19 conversion processes carried out in the presence of an
alkali metal-containing catalyst. In accordance with the
21 invention, it has now been found that increased amounts
22 f alkali metal constituents can be effectively recovered
23 from particles containing alkali metal residues produced
24 during coal gasification and related high temperature
conversion processes by mixing the particles with calcium
26 oxide or a solid calcium-containin~ compound that decom-
27 poses upon heating to form calcium oxide, and heating the
28 resultant mixture of solids to a temperature sufficiently
high to cause calcium oxide to react with water insoluble
alkali metal aluminosilicates in the alkali metal residues
-- 4 --
~i~gS4Z
1 to produce reaction products containing water insoluble
2 calcium silicates and water soluble alkali metal alumi-
,
3 nates. The reaction products are contacted with water,
4 which leaches the alkali metal aluminates and other water
soluble alkali metal constituents from the solids. The
6 pH of the resultant aqueous solution containing the water
7 soluble alkali metal aluminates is sufficiently lowered
8 to cause aluminum hydroxide to precipitate, thereby
9 forming a solution containing alkali metal constituents
substantially free of aluminum. These alkali metal con-
11 stituents are then used as at least a portion of the
12 alkali metal constituents which co~prise the alkali metal-
13 containing catalyst. Preferably, such use is achieved
14 by recycling the solution directly to the conversion pro-
cess. If desired, however, the alkali metal constituents
16 may first be recovered from the solution and then used in
7 the conversion process.
~8 The invention is based in part upon studies of
19 the reactions that catalysts containing alkali metal con-
stituents undergo during coal gasification and similar
21 operations. Coal and other carbonaceous solids used in
22 such operations normally contain mineral constituents
23 that are converted to ash during the gasification process.
24 Although the composition of ash varies, the principal con-
stituents, expressed as oxides, are generally silica,
26 alumina and ferric oxide. The alumina-is usually present
27 in the ash in the form of aluminosilicates. Studies have
28 indicated that at least a portion of the alkali metal com-
.
29 pounds, such as potassium carbonate, that are used as
gasiflcation catalyst constituents react with the alumino-
i~954Z
1 silicates and other ash constituents to form alkàli metal
2 residues containing water soluble alkali metal compounds
3 such as carbonates, sul~ates, sulfides, sulfites and the
4 like and water insoluble, catalytically inactive materials
such as potassium aluminosilicates and other alkali metal
6 aluminosilicates. Unless the alkali metal constituents in
7 these insoluble aluminosilicates can be reco~ered, they
8 are lost from the process and must be replaced by makeup
9 alkali metal compounds. The process of this invention
allows recovery of these alkali metai constituents and
11 thereby decreases the costs incurred by utilizing large
12 amounts of makeup alkali metal compounds. As a result,
13 the invention makes possible substantial savings in
14 gasification and other conversion operations carried out
in the presence of alkali metal-containing catalysts and
16 permits the generation of product gases and/or liquids at
7 significantly lower cost than would otherwise be the case.
18 BRIEF DESCRIPTION OF THE DRAWING
-
19 The drawing is a schematic flow diagram of a
~ catalytic coal gasification process in which alkali metal
21 constituents of the catalyst are recovered and reused in
22 the process.
23 DESCRIPTION OF THE PREFERRED EMBODIMENTS
24 The process depicted in the drawing is one for
the production of methane by the gasification of bitumi-
26 nous coal, subbituminous coal, lignite or similar carbo-
27 naceous solids with steam at high temperatures in the
28 presence of a carbon-alkali metal catalyst prepared by
. .
29 impregnatin~ the feed solids with a solution of an alkali
metal compound or a mixture of such compounds and there-
'
,
- 6 -
~11954Z
1 after heating the impregnated material to a temperature
2 sufficient to produce an interaction between the alkali
. . ,: . .
3 metal and-the carbon present. It will be understood that
4 the alkali metal recovery system disclosed is not res-
tricted to this particular gasification process and that
6 it can be employed in conjunction with any of a variety of
7 other conversion processes in which alkali metal compounds
; 8 or carbon-alkali metal catalysts are used to promote the
9 reaction of steam, hydrogen, oxygen or the like with car-
bonaceous feed materials to produce a char, coke or similar
11 solid product containing alkali metal residues from
12 which alkali metal compounds are recovered for reuse as
13 the catalyst or a constituent of the catalyst. It can
14 be employed, for example, for the recovery of alkali
~ metal compounds from various processes for the gasifica-
16 tion of coal, petroleum coke, lignite, organic waste
17 materials and similar solids feed streams which produce
. ~ ,
18 spent carbonaceous solids at temperatures below the ash
19 fusion point. Other conversion processes with which it
may be used include operations for the carbonization of
21 coal and similar feed solids, for the liquefaction o~
22 coal and related carbonaceous feed materials, for the
23 retorting of oil shale, for the partial combustion of
24 carbonaceous feed materials, and the like. Such pro-
cesses have been disclosed in the literature and will be
26 familiar to those skilled in the art
2? In the process depicted in the drawing, a solid
28 carbonaceous feed material such as bituminous coal, sub-
29 bituminous coal, lignite or the like that has been crushed
to a particle size of about 8 mesh or smaller on the U.S.
- 7 ~
54Z
1 Sieve Series Scale is passed into line lO from a feed pre-
2 ; ;paration plant or storage facility that is not shown in
the drawing. The solids introduced into line lO are fed
4 into a hopper or similar vessel ll from which they are
passed through line 12 into feed preparation zone 14.
6 This zone contains a screw conveyor or similar device 15
7 that is powered by a motor 16, a series of spray nozzles
8 or similar devices 17 for the spraying of alkali metal-
9 containing solution supplied through line 18 onto the
solids as they are moved through the preparation zone by
11 the conveyor, and a similar set of nozzles or the like
12 l9 for the introduction of steam into the preparation
13 zone. The steam, supplied through line 20, serves to
14 heat the impregnated solids and drive off the moisture.
Steam is withdrawn from zone 14 through line 21 and
16 passed to a condenser not shown, from which it may be
17 recovered for use as makeup water or the like. The
18 majority of the alkali metal-containing solution is
19 recycled through line 79 from the alkali metal recovery
section of the process, which is described in detail
21 hereafter. Any makeup solution required may be intro- ¦
22 duced into line 79 via line 13.
23 It is preferred that sufficient alkali metal-
24 containing solution be introduced into feed preparation
zone 14 to provide from about l to about 50 weight per-
26 cent of the alkali metal compound or mixture of such
. . ,
27 compounds on the coal or other carbonaceous solids. From
28 about l to about 15 weight percent is generally adequate.
29 The dried impregnated solid particles prepared in zone 14
are withdrawn through line 24 and passed to a closed
- 8
~9S4~i~
.,
1 hopper or similar vessel 25. From here they are dis-
2 charged through a star wheel feeder or equivalent device
3 26 in line 27 at an ele~ated pressure sufficient to per-
4 mit their entrainment into a stream of highpressure steam,
S recycle product gas, inert gas or other carrier gas intro-
6 duced into line 29 via line 28. The carrier gas and
7 entrained solids are passed through line 29 into manifold
; 8 30 and fed from the manifold through feed lines 31 and
; g nozzles, not shown in the drawing, into gasifier 32. In
lieu of or in addition to hopper 25 and star wheel feeder
11 26, the feed system may employ parallel lock hoppers,
12 pressurized hoppers, aerated standpipes operated in
13 series, or other apparatus to raise the input feed solids
14 stream to the required pressure level.
~ It is generally preferred to operate the
16 gasifier 32 at a pressure between about 500 and about
17 2000 psig. The carrier gas and entrained solids will
18 normally be introduced at a pressure somewhat in excess
19 Of the gasifier operating pressure. The carrier gas may
be preheated to a temperature in excess of about 300 F.
21 but below the initial softening point of the coal or
22 other feed material employed. Feed particles may be sus-
23 pended in the carrier gas in a concentration between about
24 0.2 and about 5.0 pounds of solid feed material per pound
of carrier gas. The optimum ratio for a particular system
26 will depend in part upon the feed particle size and den-
`27 sity, the molecular weight of the gas employed, the tem-
28 perature of the solid feed material and input gas stream,
29 the amount of alkali metal compound employed and other
~ factors. In general, ratios between about 0.5 and about
11195L~Z
1 4 0 pounds of solid feed material per pound of carrier
2 gas are pre~erred.
3 ~Gasifier 32 comprises a refractory-lined vessel
4 containing a fluidized bed of carbonaceous solids extend-
ing upward within the vessel above an internal grid or
6 similar distribution device not shown in the drawing. The
7 bed is maintained in the fluidized state by means of
8 steam introduced through line 33, manifold 34 and peri-
. 9 pherally spaced injection lines and nozzles 35 and by
means of recycle hydrogen and carbon monoxide introduced
11 through bottom inlet line 36. The particular in~ection
12 system shown in the drawing is not critical and hence
13 other methods for injecting the steam and recycle hydrogen
14 and carbon monoxide may be employed. In some instances,
~ for example, it may be preferred to introduce both the
16 steam and recycle gases through multiple nozzles to
17 obtain more uniform distribution of the injected fluid
18 and reduce the possibility of channeling and related
19 problems. The space velocity of the rising gases within
the fluidized bed will normally be between about 300 and
21 about 3000 volumes of steam and recycle hydrogen and
22 carbon monoxide per hour per volume of fluidized solids.
23 The injected steam reacts with carbon in the
24 feed material in the M uidized bed in gasifier 32 at a
temperature within the range between about 800 F. and
26 about 1600 F. and at a pressure between about 500 and
?7 about 2000 psig. Due to the equilibrium condition exist-
28 ing in the bed as a result of the presence of the carbon-
29 aikali metal catalyst and the recycle hydrogen and carbon
monoxide injected near the lower end of the bed, the
11~9~42:
1 reaction products will normally consist essentially of
2 m;ethane and carbon dioxide. Competing reactions, which
3 in the absence of the catalyst and the recycle gases ~
4 would ordinarily tend to produce additional hydrogen and
carbon monoxide, are suppressed. The ratio of methane to
6 carbon dioxide in the raw product gas thus formed will
7 preferably range from about l to about 1.4 moles per mole,
8 depending upon the amount of hydrogen and oxygen in the
... . . . .
9 feed coal-or other carbonaceous solids. The coal employed
may be considered as an oxygenated hydrocarbon for pur-
11 poses of describing the reaction. Wyodak coal, for
12 example, may be considered as having the approximate for-
13 mula CHo 84o 20' based on the ultimate analysis of mois-
14 ture and ash-free coal and neglecting nitrogen and sulfur.
~ The reaction of this coal with steam to produce methane
16 and carbon dioxide is as follows:
17 . 4 H20(g) + l-8 CHo.84o 20 o-8 C02 + CH4-
18 Under the same gasification conditions, coals of higher
19 oxygen content will normally produce lower methane to
carbon dioxide ratios and those of lower oxygen content
21 will yield higher methane to carbon dioxide ratios.
22 The gas leaving the fluidized bed in gasifier
23 32 passes through the upper section of the gasifier~
24 which serves as a disengagement zone where particles too
heavy to be entrained by the gas leaving the vessel are
26 returned to the bed. If desired, this disengagement
27 zone may include one or more cyclone separators or the
28 like for removing relatively large particles from the gas.
29 The gas withdrawn from the upper part of the gasifier
through line 37 will normally contain methane and carbon
111954Z
1 dioxide produced by reaction of the steam with carbon,
2 hvdrogen and carbon monoxide introduced into the gasifier
3 as recycle gas, unreacted steam, hydrogen sulfide, ammonia
4 and other contaminants formed from the sulfur and nitrogen
contained in the feed material, and entrained fines. This
6 gas is introduced into cyclone separator or similar device
7 38 for removal of the larger fines. The overhead gas
8 then passes through line 39 into a second separator 41
; 9 where smaller particles are removed. The gas from which
the solids have been separated is taken overhead from
11 separator 41 through line 42 and the fines are discharged
12 downward through dip legs 4O and 43. These fines may be
13 returned to the gasifier or passed to the alkali metal
! ~ 14 recovery section of the process as discussed hereafter.
~ After entrained solids have been separated
16 from the raw product gas as described above, the gas
17 stream may be passed through suitable heat exchange equip-
. ~ . .. .
18 ment for the recovery of heat and then processed for the
19 removal of acid gases. Once this has been accomplished,
the remaining gas, consisting primarily of methane,
21 hydrogen and carbon monoxide, may be cryogenically
22 separated into a product methane stream and a recycle
23 stream of hydrogen and carbon monoxide, which is re-
24 turned to the gasifier through line 36. Conventional
gas processing equipment can be used. Since a detailed
26 description of this downstream gas processing portion of
27 the process is not necessary for an understanding of the
28 invention, it has been omitted.
The fluidized bed in gasifier 32 is comprised
of char particles formed as the solid carbonaceous feed
- L2 -
~1954Z
1 material undergoes gasification. The composition of the
2 char particles will depend upon the amount of mineral
3 matter present in the carbonaceous material fed to the
4 gasifier, the amount of the alkali metal compound or mix-
ture of such compounds impregnated onto the feed material,
6 and the degree of gasification that the char particles
7 undergo while in the fluidized bed. The lighter char
8 particles, which will have a relatively high content of
... . .
9 carbonaceous material, will tend to remain in the upper
portion of the fluidized bed. The heavier char particles,
11 which will contain a relatively small amount of carbona-
12 ceous material and a relatively large amount of ash and
alkali metal residues will tend to migrate toward the
14 bottom of the fluidized bed. A portion of the heavier
~ char particles are normally withdrawn from the bottom
16 portion of the fluidized bed in order to eliminate ash
17 and thereby prevent it from buildling up within the gasi-
18 fier and other vessels in the system.
19 Since the cost of the alkali metal constituents
comprising the gasification catalyst is a significant fac-
21 tor in determining the overall cost of the gasification
22 process, it is particularly important that these consti-
23 tuents be recovered and reused. It has been proposed to
24 recover these alkali metal constituents as they are with-
drawn from the gasifier with the char particles by leach-
26 ing them out with water. Studies have indicated that
. , .
27 these alkali metal constituents are generally present in
28 part as carbonates, sulfates, sulfides, sulfites and
29 other water soluble compounds that can be recovered by
water washing. Experience has shown, however, that only
~.~1954Z :
1 a portion of the alkali metal carbonates or other alkali
2 ;~etal constituents is normally recovered and that a sub-
3 stantial quantity of makeup alkali metal compounds is
4 therefore required.
The process of this invention is based in part
6 upon studies of the reactions that catalysts containing
7 alkali metal constituents undergo during coal gasifica-
. . .. ~
8 tion and similar operations. Coal and other carbonaceous
,.. . ,
9 solids used in such operations normally contain mineral
i, . ~ 1
1~ constituents that are converted to ash during the gasi-
11 fication process. Although the composition of the ash
12 varies, the principle constituents, expressed as oxides, I
13 are generally silica, alumina and ferric oxide. The J
14 alumina is usually present in the ash in the form of
1~ aluminosilicates. Studies have indicated that at least
16 a portion of the alkali metal compounds, such as potas- -
17 sium carbonate, sodium carbonate and the like, that are
18 used as gasification catalyst constituents react with
19 the aluminosilicates and other ash constituents to form
alkali metal residues containing water soluble alkali
21 metal compounds such as carbonates, sulfates, sulfides,
22 sulfites and the like and water insoluble, catalytically
23 inacti~e materials such as potassium aluminosilicates
24 and other alkali metal aluminosilicates.
It has been found that from about lO to about
26 5O percent by weight of the potassium carbonate or other
27 alkali metal compound employed to impregnate coal or
28 similar feed material prior to gasification will react
29 with the aluminosilicates in the ash during gasification
to form water insoluble alkali metal aluminosilicates,
- 14 -
~19S4Z
1 which cannot normally be recovered from the ash by water
2 washing. Preliminary studies tend to indicate that when
3 potassium carbonate is utllized to impregnate the coal,
4 the major constituent of the water insoluble potassium
aluminosilicates produced is a synthetic kaliophilite,
which has the chemical formula KAlSiO4.
7 To improve the economics o~ the catalytic gasi-
8 fication process described above and other catalytic con-
..... . .
. 9 version processes where water insoluble alkali metal
residues are formed, it is desirable to recover as much
11 as possible of the alkali metal constituents from the 3
12 insoluble residues and reuse them as catalyst constituents
13 in the conversion process, thereby decreasing the amount
14 f costly makeup alkali metal compounds needed. It has
1~ been found that a substantial amount of the alkali metal
~6 constituents in the water insoluble alkali metal residues
17 withdrawn with the char and ash from the gasifier of the
18 above described process or the reaction zone of other
19 conversion processes can be recovered for reuse in the
conversion process by mixing the particles withdrawn
21 from the reaction zone with calcium oxide or a solid
22 calcium-containing compound that decomposes upon heating
23 to ~orm calcium oxide, and heating the resultant mixture
24 of solids to a temperature sufficiently high to cause
calcium oxide to react with water insoluble alkali metal
26 aluminosilicates in the alkali metal residues to produce
27 reaction products containing water soluble alkali metal
28 aluminates and water insoluble calcium silicates. The
29 reaction products are contacted with water, which leaches
the alkali metal aluminates and other water soluble alkali
- 15 -
~1954Z
1 metal constituents from the solids. The pH of the resul-
2 tant aqueous solution containing the water soluble alkali
3 metal aluminates is sufficiently lowered to cause aluminum
4 hydroxide to precipitate, thereby forming a solution con-
taining alkali metal constituents substantially free of
6 aiuminum. These alkali metal constituents are then used
7 in the conversion process as at least a portion of the
8 alkali metal constituents which comprise the alkali metal-
.,,
9 containing catalyst. Preferably, such use is achieved by
recycling the solution directly to the conversion process.
11 If desired, however, the alkali metal constituents may
12 first be recovered from the solution and then used in the
13 conversion process. -
14 Referring again to the drawing, char particles
1~ containing carbonaceous material, ash and alkali metal
16 residues are continuously withdrawn through line 44 from
1~ the bottom of the fluidized bed in gasifier 32. The par-
18 ticles flow downward through line 44 countercurrent to a
19 stream of steam or other elutriating gas introduced
- 20 through line 45. Here a preliminary separation of solids
21 based on differences in size and density takes place.
22 The lighter particles having a relatively large amount of
23 carbonaceous material tend to be returned to the gasifier
24 and the heavier particles having a relatively high content
of ash and alkali métal residues continue downward through
26 line 46 containing valve 55 into hopper 56. Char fines
27 recovered from the raw product gas through dip legs 4O
28 and 43, and line 57 may also be fed into the hopper.
29 Thè solid particles in hopper 56, which contain
both water soluble and water insoluble alkali metal resi-
- 16 - 1,
~19S4Z
1 dues, are passed into line 58 where they are mixed with a
2 calcium-containing compound introduced into line 58 from
.
3 hopper 59 via line 60. The solid calcium-containing com-
4 pound may be calcium oxide or any calcium compound that
decomposes to form calcium oxide when subjected to rela-
6 tively high temperatures~ The calcium-containing compound
7 may be inorganic or organic and may, for example, be cal-
8 cium hydroxide, calcium acetate, calcium oxalate, calcium
... .
- 9 formate, calcium carbonate, dolomite and the like. The
actual calcium-containing compound used will depend pri-
11 marily upon its availability and cost. The amount o~ the
12 calcium-containing compound required will depend in part
13 on the amount of silicates in the particulate matter with
' 14 which it is mixed. If desired, a mixture of two or more
1-S calcium-containing compounds may be used in lieu of a
16 single compound.
- .
17 The mixture of char particles containin~ the
18 alkali metal residues and the calcium-containing compound
19 is passed through line 61 into rotary kiln or similar
heating device 62 where it is subjected to temperatures
21 sufficiently high to cause the alkali metal aluminosili-
22 cates in the residues to react with calcium oxide, which
23 is formed by the thermal decomposition of the calcium-
24 containing compound unless that compound is calcium oxide
itself. The reaction converts the water insoluble alkali .
26 metal aluminosilicates into reaction products containing
27 water insoluble calcium silicates and water soluble alkali
28 metal aluminates. The reaction products are subsequently
29 treated, as described hereafter, to recover alkali metal
constituents that are recycled to the gasification
- 17 -
~954Z
1 process where they serve as at least a portion of the
2 ;-~;al~ali metal constituents which comprise the alkali
3 metal-containing catalyst.
4 The mixture of solids introduced into the
rotary kiln will normally be subjected to a temperature
6 ranging from about 1600 F. to about 2600 F. Preferably,
7 the mixture is heated to the sintering temperature~ which
8 ~ causes the surface of the particles to soften thereby in-
9 creasing the tendency of the particles to agglomerate or
stick together. Sintering imparts mobility to the calcium
11 ions present and apparently enables them to easily satu-
12 rate -the atomic structure of the alkali metal aluminosili-
13 cates and displace alkali metal constituents and alumina
14 to form water insoluble calcium silicates. Sintering
1~ may be effectively accomplished in a countercurrent rotary
16 kiln in which the fuel is passed through the kiln in a
17 direction opposite to that in which the mixture of solids
18 is passed. In lieu of the rotary kiln any furnace or
19 similar heating device may be used as long as the required
temperatures are obtainable. If desirable, the tempera-
21 ture in the heating device may be raised above the sin-
22 tering temperature to convert the mixture of solids into
23 a liquid mass in which the desired reactions will take
24 place more rapidly. This procedure, however, may not be
advantageous since the liquid upon cooling will form a
26 glass-like solid from which it may be difficult to water
27 leach soluble alkali metal constituents.
28 An example of one reaction that is believed to
29 take place in rotary kiln 62 is set forth below. The
symbol "M" is used to represent any alkali metal cation.
- 18 -
1119542
The actual alkali metal present will depend on the type
2 ;~;of alkali metal compound utilized as a constituent of
3 the al~ali metal-containing gasification catalyst.
4 ~AlSiO4 + 2CaO ~ MAlO2 + Ca2SiO4
As can be seen from the above equation, an alkali metal
6 aluminosilicate reacts wlth calcium oxide to produce an
7 alkali metal aluminate and dicalcium silicate. It will
8 be understood that the above equation represents only
~ one reaction that may occur in the rotary kiln. Other
reactions involving more complicated aluminosilicates
11 and other insoluble constituents of the alkali metal
12 residues may also take place.
13 The sintered mixture of solids from rotary kiln
14 62 is cooled and passed through line 63 to ball mill or
similar crushing device 64 where the solids are pulver-
16 ized, ground, or otherwise crushed to a size suitable
17. for water leaching. It is desirable to produce rela-
18 tively small particles since they will provide a high
19 surface area for more effective water leaching. The
actual size is determined in part by balancing the cost
21 of crushing with the effectiveness of the water leaching.
22 Preferably, the particles will be crushed to a size
23 smaller than about 60 mesh on the U.S. Sieve Series
24 Scale.
The crushed solids are removed from ball mill
26 64 and passed through line 65 to water wash zone 66,
27 which will normally comprise a multistage countercurrent
28 extraction system in which the solids are countercur-
29 rently contacted with water introduced through line 67.
The water leaches alkali metal aluminates and other
- 19 -
~9S4Z
il
1 water soluble alkali metal constituents from the solids,
2 ~ ; thereby producing an aqueous solution of such consti-
3 tuents, which is removed from the water wash zone
4 thrpugh line 68 and will normally have a pH of about
12.0 or less. Spent solids are removed from the water
6 wash zone via line 69 and will contain, among other sub-
7 stances, ash, calcium silicates and aluminum hydroxide.
8 These solids may be disposed of as land fill or further
9 processed to recover valuable components such as calcium
silicates, which may subsequently be used in the manu-
11 facture of cement.
12 The aqueous solution containing alkali metal
13 aluminates and other water soluble alkali metal consti-
tuents removed from water wash zone 66 via line 68 is
1~ passed through line 70 to contactor or similar vessel 71.
16 Here the pH of the solution is lowered to a value in the
17 range between about lO.0 and about 4.0, preferably between
18 about 9.0 and about 5.0, by contacting it with a carbon ¦
19 dioxide-containing gas. The aqueous solution is passed
downward through the contacting zone in contactor 71
21 where it comes in contact with an upflowing gas that con-
22 tains carbon dioxide. The carbon dioxide-containing gas
23 is injected into the bottom of the contactor through line
24 72. As the carbon dioxide gas rises upward through the
downflowing aqueous solution, the carbon dioxide in the
26 gas reacts with the alkali metal aluminates in the solu-
27 tion to form alkali metal carbonates and water insoluble
28 aluminum hydroxide. If the partial pressure of carbon
29 dioxide is sufficiently high and the temperature in the
contactor is low, alkali metal bicarbonates may also form.
- 20 -
l~l9S4~'
.. . .
1 - A gas depleted in carbon dioxide is withdrawn
2 --overhead of contactor 71 through line 73 and either
3 vented to the atmosphere, further processed for the
4 recovery and reuse of carbon dioxide, or otherwise dis-
posed of. Any carbon dioxide-containing gas, including
6 pure carbon dioxide and air, may be used. It is
: 7 preferred, however, to utiliz~ the flue gas produced
- 8 from the fuel combustion taking place in rotary kiln 62.
.... . !
9 The contacting vessel utilized does not necessarily have
, .. .
to be of the type shown in the drawing but may be any
- ~ 11 type of vessel that allows for fairly good contacting
12 between the carbon dioxide-containing gas and the aqueous
13 solution containing alkali metal aluminates. A tank in
14 which the carbon dioxide-containing gas is bubbled
i5 through the aqueous solution may be sufficient for pur-
16 poses of the invention.
17 The purpose of the above-described step of the
18 alkali metal recovery process is to lower the p~ of the
19 aqueous solution containing the alkali metal aluminates
so that substantially all of the aluminum is removed
21 from the solution in the form of a water insoluble pre-
22 cipitate of aluminum hydroxide, thereby leaving in solu- i
23 tion aluminum free alkali metal constituents that are
24 subsequently recovered and used as constituents of the
gasification catalyst. Removal of aluminum from the
26 alkali metal constituents before their use in the gasi-
27 fication catalyst is desirable to help avoid the possible
28 formation of additional alkali metal aluminosilicates in
29 the gasifier by the reaction of the aluminum with silica
in the feed material and alkali metal constituents of
- 21 -
1~1954Z
1 the catalyst. It will be understood that for purposes
2 .;of the invention any method of lowering pH may be used.
3 For example, instead of contacting the aqueous effluent
4 from water wash zone 66 with a carbon dioxide-containing
gas, the effluent may be mixed with sufficient quanti-
6 ties of sulfuric acid, nitric acid or the like to lower
7 the pH to the desired value.
8 Referring again to the drawing, the effluent
..... . .
9 from contacting vessel 71, which contains alkali metal
carbonates and other water soluble alkali metal consti-
11 tuents, and aluminum hydroxide, is withdrawn from the
12 bottom of the vessel through line 74 and passed to rotary
13 filter or similar liquids-solids separation device 75
where the solid aluminum hydroxide is separated from the
j~ aqueous solution and then passed through line 76 to
16 : rotary kiln or similar heating device 77 where it is cal-
17 cined at high temperatures to produce alumina, which is
18 recovered via line 78 and may be soId as a byproduct.
19 The sale of this material may produce an additional
return from the process and thus reduce the overall cost
21 of the product gas.
22 A solids-free aqueous solution containing
23 alkali metal carbonates and other water soluble alkali
24 metal compounds is removed from filter 75 through line
79 and recycled to feed preparation zone 14 via line 18
26 where the coal or similar carbonaceous feed material is
27 impregnated with the alkali metal compounds in the solu-
2~ tion. If the concentration of alkali metal compounds
29 in the recycle solution is undesirably low, the solution
may be concentrated by removing excess water before it
- 22 -
~9s~z
1 is returned to the feed preparation zone. It will be
2 understood that the exact alkali metal compound or com-
3 pounds present in the recycled solution will depend on
4 the substance used to lower the pH of the aqueous efflu-
.
ent from water wash zone 66. For example, if nitric acid
6 is used in lieu of a carbon dioxide-containing gas, the
7 recycled solution will contain alkali metal nitrates
8 instead of carbonates.
The embodiment of the invention shown in the
. , ,
drawing and discussed above is one that allows for the
11 recovery of alumina as a byproduct of the alkali metal
12 recovery process. If recovery of alumina is undesirable
13 for any reason, the embodiment of the invention depicted
14 in the drawing may be simplified by eliminating water
wash zone 66 and rotary kiln 81 and replacing contactor
16 71 with a series of stirred tanks through which water and
the crushed solids from ball mill 64 are passed counter-
:. . .
18 currently to the carbon dioxide-containing gas. The
1~ slurry effluent from the series of stirred tanks is then
passed to rotary filter 75 where the solids are separated
21 from the aqueous solution, which is recycled to the feed
22 preparation zone. The solids, which will contain, among
23 other substances, ash, calcium silicates and aluminum
24 hydroxide, may be used as landfill or otherwise disposed
of.
26 It will be apparent from the foregoing that the
-~27 process of the invention provides an improved alkali metal
28 recovery system, which makes it possible to significantly
29 increase the amount of alkali metal constituents that are
recovered from alkali metal residues produced during cata-
- 2~ -
11~954;~
,
1 lytic gasification and similar hi~h temperature catalytic
.2 .~onversion processes. As a result, the need for costly
3 makeup alkali metal compounds is reduced, thereby lowering
. 4 the overall cost of the conversion process.
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- 24 -
