Note: Descriptions are shown in the official language in which they were submitted.
TWO STAGE BATCH l,IQUEFACTION PROCESS AND APPARATUS
This invention relates to converting pulverulent raw materials
into a liquefied state as a first step in a melting process. The inventiGn
is particularly applicable to melting glass, including flat glass, con-
tainer glass, fiber glass and sodium silicate glass. But the invention is
applicable to other processes that involve thermally converting a generally
flowable, essentially solid state feed material into a molten fluid. These
other processes may include metallurgical smelting type operations and fus-
ing of single or multiple component ceramics, metals or other materials.
It has long been recognized that exhaust gas from a combustion
heated glass melting furnace or other melting process furnace contains
large amounts of thermal energy that can be recovered to improve the over-
all efficiency of the process. Conventionally~ regenerators and recup-
erators have been employed to recover heat from melting process furnaces
but their efficiency is less than would be desired. Furthermore, regen-
erators and recuperators are large, costly and susceptible to deteriora-
tion. Instead of preheating combustion air by way of regenerators or
- recuperators, it has been proposed to recover waste heat by means of the
feed material. These proposals have been hampered by the fact that some
particulate feed materials tend to become entrained in the exhaust gas
stream. To overcome this problem, it has been proposed to agglomerate
(e.g., pelletize) fine feed materials and to contact the agglomerated
feed materials with the exhaust gas stream. However, it has been ~ound
that the cost of agglomerating feed materials can significantly diminish
the economic advantages of heat recovery, and in some cases the entrainment
r
of particulates is not entirely avoided by the use of agglomerated batch
materials. Accordingly, one aspect of the present invention rela~es to
recovering waste heat by preheating the batch materials while avoiding the
drawbacks described above.
Dusting of dry pulverulent batch materials is also a problem when
feeding the batch materials to a conventional melting furnace. It is a
common expedient to overcome this problem by wetting the batch (e.g., with
water). But, preheating the batch to any significant extent precludes
maintaining the batch in a wetted condition.
Although waste heat from melting furnaces is often available at
temperatures that could effect useful reactions in the batch materials, pre-
heating of batch materials is often limited to lower temperatures because
at higher temperatures the onset of melting of some batch materials can
lead to clogging of the preheating apparatus. For example, it would be
desirable to convert the carbonate batch materials typically employed in
commercial glass melting operations to the respective oxides by preheating
the batch to the calcining temperature of the materials, but heretofore, pre-
heating has been generally considered limited to the relatively low melting
temperature of the sodium carbonate component of the batch thereby preclud-
ing calcining of the calcium carbonate and magnesium carbonate constituents.
Another aspect of the invention deals with overcoming this problem.
In U.S. Patent No. 4,381,934 of Kunkle and ~atesa there is dis-
closed an intensified batch liquefying process in which large volumes of
batch are efficientIy liquefied in a relatively small space. This type of
process, particularly when using intensified heat sources, produces rela-
tively small volumes of exhaust gases, but it would be desirable to recover
the heat from the exhaust gases to further improve the efficiency of such
processes. It would be particularly desirable to recover the heat directly
into the batch feed stream.
~2~
In melting pulverulent materials, gases can become entrapped in the
molten product, and the presence of gaseous lnclusions may be considered
undesirable, par~icularly in the case of transparent glass. elimination of
certain gases from the molten material may be more difficult than othars
(e.g., nitro~en in the case of molten gla~s). It would be highly desirable to
eliminate these gases from the batch as part of a batch pretreatment so that
they do not become subsequently entrapped in the melt.
U.S. Patent Nos. 3,508,7b2 and 3,607,190 show direct preheatin~ of
glass batch materials. ~oth maintain temperatures below that at which
caLcining takes place. U.S. Patent ~o. 3,082,102 discloses prereacting
pellets of glass batch and indicates that temperatures should be maintained
below those at which sintering of the materials taXes place.
Summarv of the Invention
In the present invention, wasts heat from a combustlon fired meltin~
process is recovered by passing the exhaust gas stream into direct contact
with loose batch materials being fed to the melting proces In one aspect of
the invention, which is the subject of copending Canadian application 457,070,
carbonate containing batch materials are preheated to a calcining temperature
so as to decompose the carbonates to oxides in the absence of relatively low
melting temperature constituents of the batch which are fed in a separate
stream to the melter. In a soda-lime glass melting operation, the materials
subjected to the calcining preheatin~ include limestone and/or dolomite,
whereas the sodium source, such as soda ash and~or caustic soda, would bypass
the calcining preheatin~. Sand may also be included in the portion of batch
that is preheated to calcining temperatures. In the preferred embodiments,
the combining of the batch portions is completed by a melting vessel that
provides active mixin~ of the separate feed streams.
Another aspect of the invention> which is a subject of copending
Csnadian application S.~. 457,131, deals with the problem of psrticulate batch
materials being entrained in an exhaust gas stream when heat is being
recovered by direct contact therebetween. ~y providing sufficient
countercurrent contact between the exhaust gas and the batch materials, the
temperature of the exhaust gas can be reduced to the dew point of the water in
the products of the combustion, thereby produc~ng a wet conditlon at the cold
end of the heat recovery vessel. ~hi wet condition has been found to c~use
entrapment of a substantial portion of particulates in ths exhaust gas
stream. This includes partlculates from the meltin~ veRsel as well as
particulates entrained from the batch during preheating. It is preferred to
use oxy~en wholly or partially in place of 8ir in the combustlon means heating
the melting vessel, thereby reducing the volume of the exhaust gas stream.
The lower exhaust gas volume, in turn, results in a grester concentration of
water vapor per unit volume of exhaust gas, less entrainment of batch due to
lower velocity of the exhaust ~as stream, and a more effective wetting of the
batch due to the hi~her dew point at ths higher water vapor concentration.
When the combustion means of the meltin~ vessel iR fired with oxygen
rather than air, a substantial elimlnat;on of nitrogen from the exhaust gas
stream results. Since nitrogen bubbles are relatively difficult to remove
from molten glass, its elimination from the melting vessel ~s benefLc~al.
Another aspect of the present invention, which is the sub~ect of this
application, involves an additional benefit from a substantially nitrogen-free
exhaust gas stream, whereby preheatin~ of batch materials by d~rect contact
with the substantial nitrogen-free exh~ust gas stream results ~n str~pplng of
air from the batch materlals, thereby prevent~n~ introduction of entrapped
nitrogen into the melter.
4 --
~.~249~
Preheating batch materials in accordance with the presen~ inven-
tion is particularly advantageous in combination with the ablation enhancing
batch liquefying arrangements described in aforesaid U.S. Patent No.
4,381,934. Preheating batch reduces the heat requirement for liquefying
the batch which theoretically should yield a greater throughput of a
melter. But this theoretical advantage may not be fully attained in a
conventional melter because it has been found that a significant overall
rate limiting step is the runoff of liquefied batch from the mass of batch
being melted. The batch liquefication arrangements of U.S. Patent No.
lO 4,381,934 are adapted to enhance this runoff or "ablation" of liquefied
batch by providing a sloped melting surface and rapid draining of the
liquefied material, and thus these arrangements are particularly suited to
take advantage of the higher throughputs that can be achieved by batch pre-
heating. ~or those embodiments of the present invention that entail oxygen
~ ~ fired combustion, ehe batch liquefying arrangements of U.S. Patent No.
; 4,3~1,934 are particularly advantageous. In particular, those embodiments
wherein a layer of batch encircles the heat source are well suited for use
with the high temperatures produced by oxygen fired combustion. Oxygen
firing in such an arrangement produces a relatively small volume, high
temperature exhaust gas stream compared to a conventional glass melting
furnace and this small volume, high temperature exhaust gas stream is par-
ticularly well suited for the heat recovery and emissions control purposes
of the present invention. Other high temperature heat sources that sub-
stantially reduce the volume of nitrogen from the exhaust stream also lend
themselves to the present invention.
The batch liquefying arrangements of U. S. Patent No. 4,381,934
are capable of receiving dry, pulverulent batch materials. Therefore, in
accordance with the present invention, dry preheated batch may be fed
directly to the liquefying stage.
-- 5 --
3~
The invention may al90 be characterized as a two stage batch
liquefaction process which is a sub~ect of copending Canadlan applicstion S N.
4S7,131 and whereln batch i8 t~ansported relatively 810wly ~n a fir~t zone as
it is heated to a temperature approachln~ inclpient liquefaction and the bstch
is then transferred to a second zone where the preheated batch is dspo~ted
onto a sloped surface and rapidly liquefied by intense heating, whereby the
liquefied batch flows out of the second zone. This proces~ segregates the
batch liquefaction operation into a pre-liquefaction stags and a liqusfaction
sta~e, and provides each stage with condltions that maximize the efflclency of
each sta~e. In the first zone there are no essentlal time restraints ln
transferring heat to the batch as long as the batch remalns freeflowing.
Therefore, the flrst zone may bc relatively extensive in s~ze, may utilize
relatively low grade heat, and preferably provides agitation to the batch in
order to heat the batch throughout. The second sta~e is adapted to apply
intense heat to the batch in a relatîvely small space to rapidly convert the
batch to a liquefied state. Support for the batch in the second sta~e is
steeply sloped so as to enhsnce rapid run-off and high throughput, thereby
minimizing the size of the intensely heated zone. The first zone, on the
other hand, preferably employs wa~te heat from the second zona, and therefore
batch is conveyed throu~h the first zone preferably at a relatively slow rate
so as to maximize transfer of heat from the exhaust gas to th~ batch. The
relati~ely slow material transport in the first stage can be ma~ntalned only
~: 80 long as the batch remains freeflowin~, beeause when the batch temperature
approaches the melting point of one of its constltuents the creation of a
liquid phase results in cohesion between batch partlcles, wh~ch in turn
result~ in detri~ental ag~lo~erating of the batch, ~hlch can cau~e plugging of
the first zone and uneven melting in the second zone. Therafore, 8S the batch
approaches this condition the batch is transferred to the steeply sloped
support of the second zone and rapidly brought to the liquefied state.
~.
'122~2~
In the preferred embodiment, both stages are rotating vessels.
The second stage is characterized by speed of rotation of sufficient magni-
tude that centrifugal force on the batch is significant and serves to main-
tain the batch as a lining on the interior of the second stage vessel. The
speed of rotation of the first stage vessel, on the other hand, i5 below
that at which centrifugal force on the batch is significant, but merely
produces tumbling of the batch. Other distinguishing characteristics of
the two stages in at least some of the embodiments are that the first stage
is heated primarily by convection, and that the second stage is heated
primarily by radiation.
The Drawings
Figure 1 is a side elevation, partly broken away, of a preferred
embodiment of a two-stage batch liquefaction arrangement in accordance with
the present invention.
Figure 2 is a cross-section taken along line 2-2 of Figure 1.
Figure 3 is an enlarged cross-sectional view of the transition-
; between first and second stages of the arrangement shown in Figure 1.
Figure 4 is a top elevational view of the arrangement of Figure 3having elements removed for clarity.
Detailed Description
In order to provide a more complete understanding of the inven-
tion, a specific preferred embodiment will be described in detail, which
embodiment includes a rotary kiln preheater in combination with a rotary
liquefying vessel. It should be understood that the concepts of the
, ~ .
present invention are not limited to the specific apparatus described
and that other preheating means and liquefying means may be employed.
- 7 -
12;2~
Also, while the present invention has been found to be particularly
advantageous in the melting of glass and the detailed description will
relate specifically to melting glass, the invention may be practiced with
other materials that are thermally liquefied from a pulverulent feed
material.
In Figure 1 there is shown a rotary kiln 10 passing batch and
exhaust gases countercurrent to one another and feeding the preheated batch
to a liquefaction vessel 12. Batch materials are fed to the inlet or
; "cold" end of the rotary kiln 10 from a continuous weighing and metering
device 13 through a delivery tube 14. A single premixed stream of batch
materials may be fed to the metering device 13, or individual constituents
~ay be separately metered and delivered to the inlet of the rotary kiln,
since the rotary kiln itself serves as a batch mixer. An example of a
batch formula employed in the commercial manufacture of flat glass is the
following:
Sand 1,000 parts by weight
soda ash 313.5
limestone 84
dolomite 242
rouge 0.75
~; Other mineral sources are sometimes employed as alternatives or
additives. Commonly, the batch also includes a substantial amount of
~ cullet (scrap glass) typically on the order to 20 to 40 percent of the
; total batch weight. However, the process and apparatus disclosed can
accommodate any amount of cullet, including 100 percent cullet. The above
batch formula yields approximately the following glass:
.
9~
sio2 73.10 percent by weight
Na20 13.75 percent
CaO 8.85 percent
MgO 3.85 percent
A1203 0.10 percent
Fe203 0.10 percent
The open end of the rotary kiln at the cold end is enclosed by
an exhaust box 15 which directs exhaust gases leaving the rotary kiln to
a duct 16. The duct 16 may lead to a fan (not shown) which provides
the draft for drawing the exhaust gases through the rotary kiln and for
discharging the exhaust gases to the atmosphere through a stack (not
shown).
In those embodiments where calcining is to take place in the
rotary kiln, the relatively low temperature constituents of the batch, such
as the soda ash and the cullet, are not fed to the rotary kiln but are fed
directly to the liquefaction vessel 12. Commercially available limestone
and dolomite are not chemically pure or uniform, and therefore calcining of
limestone and dolomite takes place over a range of temperatures, but a tem-
perature above 1600F. (870C.) has been found desirable to effect thorough
calcining. Soda ash with a melting temperature of 1564 F. (~51 C.) is
omitted from the materials passed through the rotary kiln when operating
in the calcining mode to avoid melting of the soda ash which would cause
; the batch particles to adhere to one another. Caustic soda, which is some-
times substituted for soda ash as a source of sodium in glassmaking, like-
wise has a melting temperature below the preferred calcining temperatures,
and thus it is excluded from the materials being fed through the rotary
kiln to avoid forming a liquid phase at the hot end thereof when calcining.
~l~X~2~
Calcining has the advantage of reducing the heat load on the liquefaction
vessel, but calcining is also advantageous in that it eliminates chemically
contained carbon dioxide from the melting materials before the batch mate-
rials are liquefied, thereby eliminating generation of carbon dioxide
bubbles in molten glass. Accordingly, when the sodium source naterial is
fed separately to the liquefaction vessel, it is preferred to employ the
carbon dioxide-free caustic soda source rather than soda ash. Flat glass
cullet begins to soften at around 1200F. (650DC.), and therefore it, too,
should bypass the calcining process. On the other hand, sand, the major
ingredient of the batch raw materials, may advantageously accompany the
limestone and dolomite through the rotary kiln at calcining temperatures.
In addition to being preheated, it has been found that the sand assists in
keeping the limestone and dolomite in 2 free-flowing condition. With the
sodium source absent, a glass batch may be heated to about 2400F. (1300DC.)
without agglomerating the batch.
The rotary kiln 10 is generally of conventional design consist-
ing basically of a cylindrical steel shell 20 mounted for rotation about the
cylindrical axis which is inclined slightly from horizontal so as to convey
the batch material from the cold end toward the hot end by gravity and tumbl-
ing. The residence time of material in the rotary kiln may be related to thebasic kiln parameters in accordance with the following empirical relacionship:
t = 2.28 L/NDS
where:
t = residence time, minutes
D = diameter, feet
L = length, feet
N = rotational speed, rpm
S = slope, inches per foot.
-- 10 --
~L2~9~
The residence time in the rotary kiln will depend upon the
desired throughput of the melting operation and the amount of heat desired
to be transferred from the exhaust gases to the solid materials. From
these considerations and the above relationship, a suitable rotary kiln can
be designed for the purposes of the present invention.
To prevent heat loss, the rotary kiln 10 is preferably insulated.
The insulation may be a refractory wool blanket applied to the exterior
of the steel shell 20, or the insulation may be in the form of a ceramic
liner on the interior of the shell 20 as shown in portions of the rotary
kiln depicted in Figure 1. The section of the rotary kiln near the hot end
is shown provided with a refractory lining 21 which may be preferred for
high-temperature durability in the hotter regions of the kiln, particularly
when operating at calcining temperatures. Otherwise, it is preferred to
provide a layer of insulation 22 between the outer steel casing 20 and an
inner steel lining 23 as shown in the middle portion of the rotary kiln of
Figure 1. Where thermal durability is not a concern, the steel lining is
preferred for glassmaking because it has less of a tendency to erode contami-
nating materials into the throughput stream than does a refractory lining.
Heat transfer in the rotary kiln is influenced by agitation of
the batch materials which in turn is a function of the speed of rotation of
the kiln, and it can be enhanced by the interior construction of the kiln.
A wide variety of heat transfer improving baffle means are known to those
`~ skilled in the rotary kiln art and may be employed to advantage in the
present invention. Several examples are illustrated in Figure 1. Arranged
in spiral fashion around the interior of the kiln near the hot end are a
plurality of lifters 25 which are ceramic projections (metal,in the case of
a metal-lined section) extending radially inward from the inside walls of
-- 11 --
3~4
the kiln and which serve to lift the batch material and cause it to drop
throu~h the hot ~ases. Another example of a contact area extending
srran8ement is shown in a center section of the ~iln 10 where a plurality of
metal plates 26 e~tend radially between the metal liner 23 and a central tube
27 as can be seen in the cross-sectionsl view of Fi~ure 2. The cola end of
the kiln is preferably festooned wlth d plurality of chains 28 which serve to
break up any ag~lomerations of the batch that may occur ln that sect~on due to
the condensation of water vapor, to increase the area for heat exchange and
act as wet entrailment collectors. The amount of agitation provldeA by baffle
arran~ements or speed of rotation should not be so ~reat as to induce
excessive amounts of entralnment of the batch in the exhaust gas stream.
Another factor in minimizing entrainment is the diameter of the rotary kiln,
which should ge of sufficient magn~tude to avoid undue ~as velocities at
anticipated exhaust gas volume flow rates for a given operatlons.
The liquefaction stage 12 is of the type disclosed in U. S. Patent No.
4,381,934. The preferred em~odiment is that in which a stable layer of the
batch material is rotated about a heated central cavity wlth a 3ubstantially
vertical axis of rotation. The llquefaction means 12 shown here in the
drawings is an improved verslon. The liquefaction vessel comprls2s a ~teel
drum 35, which as shown in Figure 3 may be provided wlth steppQd ~ides so as
to decrease the amount of mass being rotated. The drum, however, ould have
strai~ht cylindr~cal sides or it could be provlded wl~h a conical shape. The
drum 35 is supported on a circular ~rame 3~ which is, in ~urn, mounted for
rotation about 8 generally YertiCal axis correspondin~ to the centerline of
the drum on a plurality of support rollers 37 and aligning rollers 38. A
bottom section 39 houses an outlet assem~ly which may be detached from
- 12 -
the remainder of the drum. The housing 39 may be lined with an annulus of
refractory material 40 such as castable refractory cement in which is seated
a ring-like bushing 41 of erosion resistant refractory. The bushing 41
may be comprised of a plurality of cut pieces of ceramic. An open center
42 in the bushing 41 comprises the outlet opening from the liquefaction
chamber. An upwardly domed refractory lid 43 is provided with stationary
support by way of a circular frame member 44. The lid includes an opening --
45 for inserting a burner 46. The burner 46 is preferably a wide angled
multiport burner and is preferably fired with oxygen and a gaseous fuel
such as methane. In order to eliminate carbon dioxide from the system, the
burner 46 could be fired with oxygen and hydrogen.
Exhaust gases escape upwardly through an opening 47 through the
lid and into an exhaust duct 48. Batch materials may be fed into the
liquefaction vessel through the opening 47, and a feed chute 50 is provided
for this purpose. Upper and lower water seals 51 and 52, respectively, may
be provided to isolate the interior of the liquefaction chamber from the
exterior ambient conditions and to trap any dust or vapors that may escape
from the vessel.
Within the liquefaction vessel a stable layer of unmelted batch
53 is maintained on the walls of the drum 35 encircling the central cavity
within which combustion takes place. The stable layer 53 may be initially
established in the desired parabloid shape by feeding batch into the drum 35
as it rotates without being heated. This initial batch may be wetted with
water to assist in establishing the stable layer. During melting, continuous
feeding of batch through the chute 50 causes the batch to become distributed
over the surface of the stable batch layer as the drum rotates. Heat from
the burner 46 causes a transient layer 54 of the batch to become liquefied
- 13 -
~'X~
and to flow on the stable layer 53 downwardly through the bGttom opening 42.
The liquefied batch then flows out of the liquefaction vessel and may be
collected in a vessel 55 below the liquefaction chamber for further process-
ing as needed.
In at least the upper portions of the liquefaction chamber the
transient layer of melting batch is supported at an acute angle to vertical.
The particulate batch material has a frictional resistance to flowing down
the slope, but as soon as the materials become liquefied by the thermal
energy this resistance is substantially reduced and the liquefied material
immediately flows downwardly and freshly fed batch material takes its place
in the transient layer. The rotation of the liquefaction vessel helps
maintain the sloped melting surface. The contour of the surface of the
stable layer 53 can be estimated from the theoretical relationship between
the speed of rotation and the shape assumed by loose dry batch as follows:
H = ~R + (2~2n2R2)/g
where:
H = the elevation of a point on the batch surface in
the direction parallel to the axis of rotation;
R = the radial distance of that point on the axis of
rotation;
~= a friction factor;
n= angular velocity; and
g = the acceleration of gravity.
The friction factor may be taken as the tangent of the angle of
repcse, which for dry glass batch is typically about 35 . The above equa-
tion may be employed to select suitable dimensions for the rotary vessel
at a selected speed of rotation, or conversely for determining a suitable
speed of rotation for a given vessel.
1~4~3;24
The liquefied batch of the transient layer 54 flows on a .support
surface that is comprised substantially of only additional batch materials,
and thus contaminating contact with refractories is avoided. Furthermore,
because glass batch is a good heat insulator, providing the stable batch
layer 53 with sufficient thickness protects any underlying support struc- -
ture from thermal deterioration. Because the vessel can thus be protected
thermally as well as from contact with corrosive molten materials, the
materials requirements can be greatly relaxed, even permitting the use of
mild steel for the drum 35. The economies thus achieved in furnace con-
seruction can be substantial. Furthermore, because the drum 35 is protected
by the insulating effect of the stable batch layer, no cooling of the
exterior is required, thereby avoiding extraction of useful heat from the
melting process. Additionally because the insulating noncontaminating
batch lining encircles the heated cavity, heat sources may be employed that
operate at considerably higher temperatures than can be used in conventional
refractory melting furnaces, e.g., oxygen fired combustion burners, plasma
torches or electric arcs.
The temperature in which the batch becomes flowable will depend
on the particular batch formula, especially the amount and melting tempera-
ture of the lowest melting temperaeure ingredients. A conventional flatglass batch containing soda ash has been found to liquefy at about 2000F.
(1090C.) to 2100F. (1150C.). The liquefied material drains from the
liquefaction vessel as soon as it reaches the fluid state, and therefore
the fluid draining from the liquefaction zone has a nearly uniform tempera-
~ ture close to the liquefying temperature of the particular batch formula.
; Some additional heat is usually imparted to the liquefied material as it
is flowing from the vessel, and therefore conventional flat glass batch is
:
- 15 ~
found typically to fLow from the ves6el at a temperature of about 2100~.
(1150C.) to about 2300F. (1260C.). Becnuse heut is transported out of the
liquefaction vessel at the liquefylng temperature, which is considerably lower
than the temperatures attained in a convent~onal ~lass melter, the temperature
of the liquefaction vessel may be maintained relatively low regardless of the
temperature of the heat source. Accordingly, the advantage of ~reater heat
fluxes afforded by higher temperature heat sources can be advanta~eously
employed without providin~ exotic containment means. The use of the
above-noted high temperature hea~ sources i5 also advantageous for the sake of
reducing the volume of exhaust ~ase~ by elimination of alr-borne nltrogen.
The absence of nitro~en also is favorable for preventin~ nitrogen bubbles ln
the ~lass. Optionally, use of a plasma torch with a su~table carrier gas can
prov;de a carbon dioxide-free environment in the liquefaction chamber,
particularly in comb~nation with the use of calcined batch. The use of a
plasma torch is the subject matter of commonly-assi~ned U. S. Patent 4,545,798
of J. M. Uatesa. A carbon dioxide~free environment may also be provided by
the combustion of oxy~en and hydrogen. Another advantage of nitrogen
elimination is that the emissivity (i.e., the efficiency with which a gas
radiates its thermal ener~y) of nitro~en i8 much lower than the emissivity of
carbon dioxide and water. Therefore, excludin~ nitrogen avoids diluting the
carbon dioxide and/or water produced by the combustion and intensifies the
release of energy. It should be understood that the benefits of nltrogen
exclusion can be attained by de~rees, and that partial excluslon can yield
useful improvements.
It is desirable for the thermal conductivity of the material
employed as the stable layer 53 to be relatively low 50 that practical
2~
thicknesses of the layer may be employed while avoiding the need for waste-
ful forced cooling of the vessel exterior. In general, granular or pulveru-
lent mineral source raw materials provide good thermal insulation, but in
some cases it may be possible to use an intermediate or product of the
melting process as a non-contaminating stable layer, e.g., in a glassmaking
process pulverized cullet (scrap glass) could constitute the stable layer
although a thicker layer would be required due to the higher conductivity
of glass as compared to glass batch. In metallurgical processes on the
other hand, using a metallic product as the stable layer would entail unduly
large thicknesses to provide thermal protection to the vessel, but some ore
materials may be satisfactory as insulating layers. The stable layer is
; preferably of essentially the same composition as the material being proc-
essed. However, it should be understood that precursor or derivative mate-
rials would be considered "essentially the same composition" in this context.
In other words, the stable layer could be the raw material, the product
material, an intermediate or a different form or mixture thereof as long as
it melts or reac~s to form a substance that does not introduce significant
amounts of foreign constituents into the throughput stream. It shoul~d also
be evident that this compositional requirement of the stable layer need
apply only to surface portions that actually contact the throughput stream
and to portions just under the surface that may occasionally erode into the
throughput stream. Therefore, an equivalent arrangement might employ a
different material in portions of the stable layer below the level at which
erosion is likely to occur. Since this subsurface portion would serve
:~ ~ primarily as insulation to protect the vessel, it could be composed of a
~; material selected for its thermal insulating properties (e.g., sand or
ceramic particles) although it should be sufficiently compatible composi-
tionally to not contaminate the surface layer at the temperatures involved.
:
It should be understood that the terms "transient" and "stable~'
are relative, and that a distinct physical demarcation between the transient
and stable layers may not always be identifiable. me use of the terms
"transient" and "stable" is not intended to preclude the possibility that
minor fluctuation of the interface therebetween may occur. The basic dis- -
tinction is that the region that is described as the transient layer is
characterized by melting and flowing, whereas the region termed the stable
layer, in at least its major portion, does not participate in the melting
and flowing of the throughput stream. Although the transient layer is said
to be on the stable layer, one might theoretically define an intermediate
layer therebetween, and it should be understood that that possibility is
intended to be included.
In some cases the preheated batch could be fed directly from
the exit of the rotary kiln 10 into the liquefaction chamber 12, but the
arrangment of Figure 3, wherein the rotary kiln and the liquefaction vessel
are somewhat separated from each other, is preferred. Putting the end of
the rotary kiln in direct communication with the opening 47 of the lique-
faction vessel has been found to cause caking of batch in the rotary kiln,
probably due to radiant energy from the liquefaction vessel overheating the
batch materials in the rotary kiln. This problem is overcome by providing
a transition between the rotary kiln and the liquefaction chamber which
comprises the chute 50 to carry batch therebetween and a nose piece exten-
sion 61 of the exhaust duct 48. The nose piece 61 directs the exhaust gases
into an upper portion of the rotary kiln away from ~.he glass batch 60 and
serves to shade the rotary kiln from radiant heat transfer from the lique-
faction vessel. An adjustable damper 62 may be provided in the exhaust pas-
sage of the transition section for the purpose of controlling the pressure
~'~2~9~
in the liquefaction chamber. In the event that boosting the temperature of
the exhaust gas is required for calcining the batch materials in the rotary
kiln, the nose piece 61 may be provided with openings 63 (shown in phantom
in Figure 3) for ;nserting auxiliary burners 64 (shown in Figure 4).
The batch chute 50 may be provided with a cool front plate 70
facing the exhaust gas stream so as to prevent the batch from becoming
overheated and plugging the chute. Other features of the batch chute 50
include a pivotable water-cooled baffle 71 at the exit end of the chute,
which serves to adjustably direct the falling batch onto the desired por-
tion of the stable batch layer 53 in the liquefaction chamber. A ceramicplate 72 may also be provided at the exit end of the chute 50 so as to
direct the falling batch away from the upper rim area of the drum 35. In
the event of an emergency need to stop feeding batch material to the lique-
faction chamber, a pivotable diverting gate 73 is provided at the en~rance
to the batch chute 50 and can be employed to divert the batch coming from
the rotary kiln to an escape chute 74. A seal between the rotary kiln and
the transition section may be provided by Teflon synthetic polymer or
graphite seals 75 bearing against a circular fin 76 extending radially from
the rotary kiln shell 20. The seal arrangement may be enclosed by an annu-
lar housing 77.
Referring to Figure 4 there is shown an auxiliary feeder 80 whichmay be provided to feed materials to the liquefaction chamber 12 which have
not been preheated or have been preheated independently from the batch
being preheated in the rotary kiln 10. For example, the auxiliary feeder 80
may be used to feed soda ash, caustic soda or cullet to the liquefaction
; ~ chamber, particularly when the kiln is being used to calcine the calcium
and/or magnesium carbonates. The auxiliary feeder 80 may be a conventional
Q ~ ~ ~
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2~
screw-type feeder comprised of a hopper 81, motor 82, and screw barrel 83
and may deposit materials through the opening 47 in the liquefaction
chamber lid onto the upper edge of the stable batch layer 53. A plurality
of auxiliary feeders similar to the feeder 80 may be employed if it is
desired to feed a plurality of diverse materials separately at the entrance
of the liquefaction chamber.
The slope of the rotary kiln may be selected in accordance with
the desire~ residence time of the material in the rotary kiln, and appro-
priate support means could be provided to make the angle variable. However,
it is preferred to provide a fixed inclination of about 2 (1 to 5) with
respect to horizontal and to control the residence time by varying the
speed of rotation of the rotary kiln. The speed of rotation of the rotary
kiln is in the range where the batch material tumbles in a lower portion of
the rotary kiln without significant portions of the batch being carried by
centrifugal force around the inner circumference of the kiln.
Glass batch at the hot end of the rotary kiln, when preheated to
about 900F. (480C.), assumes a semi-tacky condition that resists dusting.
Up to a temperature of about 1400F. (760C.) this tackiness does not lead
to undesirable agglomerating or plugging. Glass batches without soda ash
or an equivalent sodium source attain a similar condition when heated to
the vicinity of the calcining temperature of the limestone and dolomite.
This tacky condition is advantageous for feeding the batch to the liquefac-
tion chamber with very little dusting. Also, it is.advantageous that the
onset of this tacky condition corresponds with the transfer of the batch
from a substantially horizontal support in the rotary kiln to a nearly
vertical support in the liquefaction chamber thereby assuring a continuous
and uniform flow of material through the system.
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~224~;24
The following is an example of an arrangement substantiall~ as
shown in the drawings and described hereinabove, wherein standard flat
glass batch including soda ash was processed at a rate of 30 tons per day
t27 metric tons per day). The rotary kiln was 49 feet (15 meters) in length
and had an inside diameter of 2.5 feet (76 centimeters). The internal area
of the kiln was 502 square feet (46.6 square meters). The rotary kiln was
rotated at about 3 rpm and had an incline of 2. The liquefaction vessel
had an inside diameter of 51 inches (130 centimeters) and was rotated at
about 32 rpm. The liquefaction chamber was heated by a burner fueled with
methane and oxygen to produce a heat input of about 3 million BTU per hour
(0.878 million watts) which produced an exhaust gas volume at standard
temperature and pressure of 33,7~2 cubic feet per hour (957 cubic meters
per hour). The exhaust gas entered the rotary kiln at 1631F. (88~C.) and
left the rotary kiln at 474F. (246C.). The glass batch was fed into the
rotary kiln at 60F. (16C.) and was heated to 1100F. (593 C.) in the
rotary kiln. Liquefied batch flowing from the liquefaction chamber was at
a temperature of 2257F. (1236C.).
Other modifications and variations as would be obvious to those
of skill in the art may be resorted to-without departing from the scope of
the invention as defined by the claims which follow.
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