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l Feb/[8, 1938.
c. o. TURNER Er AL
CONTINUOUS DIGESTION PROCESS
Filed Feb. l, 1935
7
2,107,919
Planear-gb. s,
l
l
2,107,919
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I
2,107,919
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Helvlnl’owcllwm stipula, Mo..
to Aluminum Company of America, Pittsburgh',
Pa., a corporation of Pennsylvania
,
y
Application February 1, ms, sci-uu N0. 4.552
150mm».
This invention relates to the extraction of a
solid with a solvent in a process of continuous
^ pressure digestion. It relates particularly to, and
_has as its chief object, the provision of an im
5 proved method of continuously extracting theA
alumina from> alumina-bearing materials by> pres~
sure digestion with caustic liquor, and an im
proved system adapted to carry out such a
process.
`
. 10 .,'I'he'copending application of -Weigel,v Serial
No. 561,081, discloses a continuous method of
digesting bauxite with caustic liquor. 'I'his con
tinuous processl consists broadly in heating the
mixture of bauxite and caustic liquor insuccessive
-15 stages'of increasing temperature and pressure
until the desired digestion temperature and pres
sure are attained, holding until the digestion is
complete, and then cooling in successive stages
of decreasing temperature and` pressure. In this
' 20. process, steam released in the cooling stages is in
Jected in some of the preliminary heating stages,
thus materially increasing the utilization Aof the
heat supplied to the system. Difference in pres
sure throughout the system is secured as'a- rule
lrvdrostatically by positioning -the component
parts of the system in a plurality of diñerent
elevations. 'I‘his invention allowed materially
increased -thermal and material eillciencies to be
secured and was a great advance over the older
batch system.
’
,
The present invention is similar to the above
described process of Weigel in that it is capable
of continuous operation through holding and
coolingvstages and in that heat released from the
«~- cooling stages may be utilized in the digestion.
Our invention contemplates an essentially simpli
iled process with provision for carrying it out in
a system constructed substantially on one level
with few moving parts and requiring a minimum
of capital expense and operating labor. Provision
is made for emciently heating the digestion with
steam without direct contact so that dilution due
to Vthe condensation of the steam is avoided.
' Owing to the elimination of the direct injection
of steam as a heating means it has been found
possible to usefully employ very low pressure
>steam for heating, as there is no direct mechani
cal relationship between the pressure in the steam
system 4and the pressure in the digesting system.
This gives a greater ñexibillty of control than
would otherwise be possible. With independent
control over the temperatures and pressures and
working with a substantially uniform flow of un
Si
diluted digest mixture it has been found practical
to make a separation of the caustic digest liquor
(Cl. 23-52)
from the solid residue after dlscstion at an ele
vated temperature equal to or greater than the
normal boiling point of the liquor under atmos
pheric pressure, as will be further described here
inbelow.
.
~
o
It has long been a desideratum to apply steam
heat indirectly to a batch pressure digestion to
avoid the dilution arising from heating by direct
injection of steam.. This has proved diillcult, as
the digestion mixture precipitates solidson the 10
heated surfaces, which seriously interfere with
the transfer of heat. In a batch process this dim
culty may be overcome to a certain extent by
employing high pressure steam and carefully
cleaning the heating surfaces between batches. l5
In a continuous .heat regenerative- digestion
process this problem has presented a serious dim
cuity because. no' opportunity is afforded for
cleaning the- heating surfaces and a layer of solid
material of increasing thickness is continually 20
formed. Furthermore, lin a heat regenerative
system the dißerence in temperature between the
material to be heated and the heating medium is
small, and a material resistance to heat ilow'or
transfer between-the two substantially negatives 25
the advantages of such a regenerative process. v
In our solution of-this problem it was found that
the caustic liquor, which at times contains small
amounts of alumina and other substances in solu
tion from prior treatments, may be satisfactorily
superheated in respect to the desired digestion
temperatures in known types of heaters, such as
a straight tube heat exchanger, without the for
mation of scale on the heater` walls. vIt was also
found that a substantially complete extraction of 35
all available alumina from a1umina~bearing
material might be made by treating such mate
rial with superheated caustic liquor to produce
a digestion mixture at a'predetermined digestion
temperature. In actual practice it has been 40
found desirable to wet the alumina-bearing mate-_
rial with sufllcient caustic liquor to form a thick
slurry which can be conveniently introduced into
the digestion process under pressure, as by pump
ing.
»
’
In the preferred practice of our invention the 45
caustic liquor is divided. This liquor may be at
room temperature or at a somewhat elevated tem- '
perature, depending upon the general operation
-of which this process may form a part. One por- 50
tion is passed under pressure through 'a step of
indirect heating, which may consist of several
stages of increasing temperature until the liquor
has been given a superheat with respect to the
contemplated digestion temperature. The other 55
aimais
.
portion, preferably the smaller portion, is added
I4 which may be a tubular heater, as indicated,
to the alumina-bearing material to form a ,thick
slurry which is subsequently maintained under
pressure. -The two portions are then combined
under pressure and the superheat of the liquor
through which it passes and is partially brought
to a superheated condition by heat supplied by
portion is absorbed by the slurry portion to pro
duce a digestion mixture having a temperature
predetermined by the degree of lsuperheat in the
liquor. The digestion mixture'is passed through
10 a plurality of holding stages under these condi
tions of `temperature and pressure until substan
` tially all of the available alumina is dissolved by
the caustic liquor. Under kthese conditions the
digest solution is rendered stable and there is no
15' precipitation of alumina, which assures a. maxi
mum yield.
The mixture is then passed through`
a plurality of cooling stages. Throughout the
' entire cycle of operation a constant flow of diges
tion mixture from step to step and from stage to
During each
cooling stage heat is absorbed in the evaporation
of water/from the liquor under reduced pressure.
This heat may be applied to the liquor-heating
stages and, owing to the use of indirect heating,
steam generated atsubstantially any pressure in
20 stage is preferably maintained.
the cooling stages may be efficiently utilized in
supplying heatto the heating steps. Even very
low pressure exhaust steam may be used 4in the
initial stages of liquor heating.
30
i
»
For the purpose of explanation, reference will
be had to the drawing which forms a part of this
specification and which represents, in schematic
outline, a flow sheet of a process vembodying
.this invention in the extraction of alumina from
35 alumina-bearing material.
Referring. to the drawing, the apparatus
which may be used in the practice of our inven
tion comprises proportioning feed hopper I for
the alumina-bearing material and caustic liquor
40 proportioning tank 2, slurry mixer 4, surge tank
6, tubular liquor heaters I4, I5, I6 and I1, diges
ters II, 2I, 22, 23, 24 and 25, cooling chambers
26, 21 and 28, thickener 44, and tank-connecting
and mixture-transferring means hereinafter re
45 ferred to. The slurry mixer andthe digesters
are equipped with revolving stirring arms 52 to
prevent the solids settling from the liquor. It
will be understood that while the process will be
described as having specifically _four heating
50 stages and three cooling stages and six digesting
vessels, this is merely by way of example, and
fewer or more stages may be used to best satisfy
any particular condition.
„
‘
The reagents, that is, alumina-bearing mate
Irial, which may be bauxite, and caustic liquor,
by which is meant solutions 'of potassium or
l sodium hydroxide which may contain in addi
tion small amounts of other substances, such as
carbonates and aluminates, are drawn continu
60 ously at a constant rate or in batches from the
tanks I and 2 respectively. The bauxite passes
low pressure steam through the tube walls. In
practice it is desirable to use oversize pumps and
reduce the flow by valves- in the pump discharge
line, such as 9a and I2a. From the heater I4
the liquor passes through similar heaters I5 and
I6 which are heated progressively by somewhat
higher pressures of steam to higher tempera 10
tures, after whichA the liquor finally passes
through a heater 'I1 which is heated with still
higher pressure steam, which lraises the tempera- ,
ture of the liquor to a superheat with respect to
the other temperatures employed in the process. 15v
The superheated liquor from l1 passes through.
pipe line I8 into the digester II. The superheat
of the liquor is given up to the slurryl from mixer
4 and may be adjusted to give the desired diges-tion temperature by means of the degree of__su 20
perheat, which in turn is regulated by the ther
mostatic steam valve I9 on the steam supply
l line 20.
From the digestion vessel I I the mixture vpasses
through similar digesters 2|, 22, 23, 24 and 25 25
under substantially the conditions of tempera
ture and pressure maintained in digester II.
During this passage the alumina is in a large
part extracted in digesters II and 2|.> Digesters
2|, 22, 23, 24 and 25 serve aslholding digesters to 30
allow sufficient time to elapse to substantially
complete the digestion or solution of all of the
available alumina andl to stabilize the digestion
mixture.
v
_
`
The digestion mixture, which now consists of
a caustic aluminate solution and the insoluble
residue from the bauxite, passes from digester
25 to the cooling tanks or blow-off- tanks 26, 21
and 28.
The digestion mixture is released con
tinually from digester 25 through the pipe line 40
29 and the control or blow-off valve 30 to tank
26. 'I’he pressure'inV tank 26 is lower than the
digestion pressure and steam isv released. The
steam passes through pipe 3I'and control valve
32 to the tubular heater I6. The pressure in
tank 26 is largely controlled or set by valve 32
and is higher than the pressure in tank, 21. The
mixture therefore flows from tank 26 to tank
21 through the loop pipe seal 33 and the valve
34. Steam is again released at lower pressures
and passes through the pipe line 35 and valve
36 to heater I5. The cooling mixture is .still
under substantial pressure and ñows through
loop seal pipe line 31 to tank 28. Steam is re
leased in this tank and is led to heater I4 through
the pipe 38 and the control valve 39. The mix
ture is drawn from tank 28 by a pump 48 through
pipe 4I and delivered to ñlters for separating the
residue from the liquor, not shown, which is a
step preliminary to treating the caustic liquor 60
through the proportioning apparatus 3 directly
for the recovery of alumina therefrom in known
manner, which treatment forms no part` of this
into a slurry mixer 4. The caustic liquor passes ’
invention.
through the pipe line 5 which has two branches,
l65 onelleading to the slurry mixer 4 and the other
to the clear liquor surge tank 6. The two branch
lines are equipped with proportioning orificesr 1
‘and 8 which permit a measured flow of both
streams. Orifice 1 preferably discharges about
70 one-tenth of the liquor and orifice 8 about
nine-tenths. A pump 9 draws thev slurry from
.the mixer 4 and forces it through the pipe line
I0 into a pressure digesting vessel Il. Pump
I2 forces clear caustic liquor- from the surge
75 tank s through the une
la to the heater'
-
'
In this process the mixture of bauxite and
liquor is digested without dilutionfrom the con 65
densate of heating steamand,V in fact, a certain
not unsubstantial amount of concentration is ef
fected by the amount of steam released in the
blow-off tanks. It is sometimes desirable, for
instance, in order to reduce capital: expense, to 70
heat the -mixture of caustic liquor from the re
generative heaters I4, I5 and I6 and slurry from
mixer 4 by the direct injection of high pressure
steam in the digester II` and thereby eliminate
the 'superheater I1. In this case the digestion u
2,107,919
temperature is controlled by the thermostatic
valve 42 in the steam supply line 26. In this
modiñcation of our invention all -of the advan
3
through the discharge pipe 49 by any suitable
form of sludge pump,
not shown. The clear liq
ucr overflows into the tank 50 and flows through
are preserved unaffected, and because of the fact - the valve 46 into the loop seal pipe line 33 to tank
that the digestion mixture is partially heated 21. The pipes 5I equalize the pressure through
out the thickening steps and maintain a pres
indirectly and to a degree concentrated, satis
sure substantially equal to the pressure of tank
factoryoperation may be secured by the injec
tion of high pressure steam to effect the last 26, and the thickener is of course thus main
stage ofheating.- If desired, a combination of tained under approximately the temperature of
tank 26. _The conditions of thickening above set 10
both means of heating may be used. 1
Primarily,l the flow of materials through the forth have been found to materially increase the
process is controlled by the pumps 9 and I2 and settling rate of ,the solid material from the mix
tages of the various steps of heat regeneration
is
valves 9a and I2a, which maintain a pressure
at least Aequal to the pressure in the digesters,
and the blow-off valve 3II. Secondarily, the
ñow is `controlled by the valves 32, 36 and 39
and the pumplw. `If desired, the elevation of
the blow-off tanks 26, 21 and 23 may be adjusted
to secure the desired action through the loop
seals 33 and 31. The temperature of the process
is controlled primarily by valves I9 and 42 which
regulate the primary source of heat; Secondar
ily, the temperature is controlled through the
ture, which serves to decrease the size of the
thickener or settling equipment. For example,
the settling rate may be increased ñfty per cent
or more by elevating the tempera-ture. 'I'he tem
perature is preferably increased
above the nor- '
mal boiling point of the liquid at atmospheric
pressure.' This remarkable change in settling
rate is thought to be due principally to a change
in viscosity of the liquor with temperature, but "
it is not desired to limit the fact in any way by
this explanation.
,
_
.
valves 32, 36 and 39 and the orifices 1 and 3.
It will be noted that in the operation of this
the process. The digesters are kept filled with
material at all times by the pumps 9 andl I2 and
the blow-off valve 30. The range of possible ad
to apply heat at one point, heater I1 or digester`
II, which is done with steam pressurel as from
the steam supply line 23, which in turn may be
connected to any source of steam >oi' sumcient
They ñow of materials is continuous throughout '_ process now being described it is necessary only
justmentsof valves 32, 36 and 39 is very wide,
as the steam entering the heaters I4, I5 vand I6
has no mechanical relation to the pressure main
tained on the digestion solution, and the heat
can therefore be supplied by steam at -any de
sired pressure having the desired temperature
within the range of the pressure of the released
steam. This gives the utmost flexibility in con
trolling the conditions of material flow, temper
atur'e and pressure.
40
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«
,.
In such a process having a continuous flow of
materials at predetermined controllable pres
sures and temperatures as described, it has been
found practical to insert a step of thickening or
separation of the solid residue dispersed in the
pressure.
.
'
~
Although we have described our invention with
reference to .a particular embodiment thereof,
it will be apparent that it is by no means limited
to the details of operation, temperature, pres
sure, composition or substances described` but
may be> utilized to advantage in any and all
similar methods of pressure digesting aluminous
material or other solids with a liquid.
We claim:
1. A method of continuously pressure digest
ing solids with a solvent, comprising forming a
slurry of the solids with some of the solvent, and
indirectly heating another portion of. the solvent
a plurality .of successive stages of increasing
caustic aluminate liquor after the digestion is in
temperature to> a predetermined superheat with
complete. The residue may be, therefore, in this respect to the slurry, combining the slurry and
case, discharged separately from the process as '
a thick sludge containing a small amount of liq
uor and the extract liquor discharged substan
.50 tially clear an'd containing very little residue.
.
the solvent under pressure to obtain a predeter
mined digestion temperature, digesting the mix
ture in a plurality of stages, then cooling the
mixture in a plurality of separate stages of de
The technical prerequisites for such a separation
are that the ñow of material through the thick
ener be constant, or'nearly so, and that- the
creasing. temperature and pressure, controlling
treated in the thickener be small, and`v that the
settling rate be rapid; otherwise, the size of thick
por- into heat exchangers which prevent contact
the decrease in pressure between successive cool
ing stages so' as to release vapor from the sol
- temperature and the c'oncentration of dissolved
vvent in such- quantity that it is adaptedA tov sup
55 material be such that no precipitation occurs in
A
vply
heat to several. of the solvent-heating stages.
1. the thickener. The practical prerequisite is that
heating of the solvent in several such stages
in addition the amount (i. e., the volume) to be the
being accomplished by passing said released va
65
between said vapor and said solvent, and main
ener necessary may become larger than can be
taining a flow of materialsthroughout all the
economically operated under the» conditions of. steps
of the process. .
~
temperature and pressure, or the time required
2.
A
method
of
continuously
extracting
alu
may be so long as to negative any advantage
mina from aluminous material, which comprises
in its use. In -the case of vextracting alumina forming a slurry of said aluminous material and
with caustic liquor as described, the.thickening
step may .be placed, for example, Vbetween the
ñrst two stages of cooling, that is, between blow
olf tanks 26 and 21. This is shown in the draw
` ing. The enclosed thickener 44 may be connected
70 in the system in the-„loop seal pipe line 33 by
caustic liquor, indirectly heating `a further
amount of caustic liquor in a plurality of suc
65
cessive lseparate stages of increasing tempera
tures to a temperaturey such that when com
bined with the slurry a predetermined tempera
closing valve 34 and opening valves 45, 46 and 41. ture of the mixture is attained, combining the 70
and the heated caustic liquor under pres
This -mixture discharged from tank 26 ñows slurry
sure, and digesting under suitable conditions of
through valve 45 andV pipe 43 into the thickener temperature
and pressure until substantially all »
below the liquid level where the residue settles
76 in the cone bottom and is drawn oil.' as a sludgeA of the alumina is in solution, then cooling the
mixture in a plurality of» separate stages of de
76
4
2,107,919
creasing temperature and pressure, predetermin
ing the decrease in pressure between successive
cooling stages so as to release steam from the
caustic liquor in such quantity that it is adapted
to supply heat for several of the caustic liquor
heating stages by passing said released- steam
into heat exchangers which prevent contact be
tween said steam and said caustic liquor, and
maintaining a ñow of materials- throughout all
10 the steps of the process.
3. A method of continuously extracting alu
mina from. aluminous material, which comprises
forming a slurry of said aluminous material and
caustic liquor, indirectly heating a further
to a temperature higher than that of the slurry
such that~ when combined with said slurry the
temperature of the resultant mixture is increased
to a predetermined digestion temperature, com
bining said slurry and heated caustic liquor under
pressure, digesting the mixture under controlled
conditions of temperature and pressure until sub
stantially all of the alumina is in solution, and `
maintaining a ilow of materials throughout all
10
the steps of the process.
'7. vIn apparatus for extracting solids with sol
vents by continuous pressure digestion, the com
bination comprising a mixing unit adapted to
form a pumpable slurry from a solid to be ex
tracted and a solvent, a plurality of digesting
tanks,
means for continuously supplying said
' sive separate stages of increasing temperature
slurry tov said digesting tanks, means forl continu
to a suiilcient temperature so that when com
ously supplying additional solvent to said digest
bined with the _slurry a predetermined tempera
ture of the mixture is attained, combining the- ing tanks in an amount having a predetermined
ratio to the amount supplied to said mixing unit,
20 slurry fand the heated caustic liquor and digest
ing under suitable conditions. of temperature and a plurality of> heat exchange units adapted to '
pressure until substantially all of the alumina raise the temperature of said additional solvent
in progressive steps, a plurality of cooling cham
is in solution, then cooling the mixture in a plu
amount of caustic liquor in a plurality of succes
rality of separate stages of decreasing tempera
bers connected in series, transfer means adapted
steam is released from the caustic liquor in such
means for regulating the pressure in the digest
to deliver solution and undissolved solids from 25
25 ture and pressure, the decrease in pressure be-> » said digesting tanks to said 'cooling chambers,
tween successive cooling stages being such that
quantity that it is adapted to supply heat for'
several of the caustic liquor heating stages, the
'30 heating of the caustic liquor in several such
>stages being accomplished 'by passing said re
leased steam and caustic liquor into heat ex
changers which prevent contact between said
steam and said caustic liquor, separating by sed
35 imentation the undissolved portion from the liq
uor solution at a temperature higher than the
ing tanks, control means between adjacent cool
` ing chambers in said series adapted to maintain
progressively decreasing pressures in said _cham
30
bers, and means for conveying the vapor released
in atleast Ione of said cooling chambers to at
least one of said heat exchange units.
'
8. In apparatus for extracting solids with sol
vents by continuous pressure digestion, the com
35
bination comprising a surge tank for solvent, a
boiling point of said liquor at atmospheric pres
mixing unit adapted to form a pumpable slurry
minous material, which comprises, in combina
unit, means for supplying solvent to said surge
sure„ and maintaining a ì ilow of materials , from a solid to be extracted and a solvent, means
for supplying said solid to said mixing unit,
throughout all the steps of the process.
'
4. Apparatus for extracting alumina from alu- g means for supplying said solvent to said mixing
tion, means for forming a slurry of aluminous
material and caustic liquor, means for indirectly
45
heating caustic liquor in a plurality of separate
stages of increasing temperature, means for com
bining and digesting the slurry and caustic liquor
under controlled conditions of temperature and
pressure, means for cooling the digestion product
in a plurality of successive stages of decreasing
50 temperatures and pressures, means >for indirect
heat exchange between said heating and cooling
means, and means for maintaining a constant
ilow of material throughout.
5. Apparatus for extracting alumina from alu
55 minous material, which comprises, in combina
tank, a plurality of digesting tanks connected in
series, means for continuously supplying said
slurry to the ñrst of said series of digesting tanks
from said mixingunit, means for continuously
supplying additional solvent to the first of said
series of digesting tanks from said surge tank in
an amount- Ihaving a predetermined ratio to the
amount supplied to said mixing unit, a plurality
of heat exchange units- interposed in the supply
means for additional solvent and adapted to raise
the temperature of said additional solvent in
progressive steps, a plurality of cooling chambers
connected in series, transfer means adapted to
deliver solutionand undissolved solids from the
tion, means for forming a slurry of aluminous last of said series of digesting tanks to the iìrst
material and caustic liquor, means for indirectly~ ‘of said series of cooling chambers and including
-heating caustic liquor in a plurality of separate- means for regulating pressure in the digesting
tanks, control _means between adjacent cooling
-stages of increasing temperature, means for con
chambers in said series adapted to maintain pres 60
and for digesting the mixture under controlled sures in said chambers decreasing progressively
conditions of temperature and pressure,V means _in steps from the digestion pressure, and means
for cooling the digestion product in at least two for conveying vapor released by the decrease of
stages of decreasing temperatures and pressures, pressure in at least one of said cooling chambers
65
means for indirect heat exchange between said to at least one of said heat exchange units.
9. In apparatus for extracting solidsA with sol
heating and cooling means, means for separating
vents by continuous pressure digestion, the com
insoluble portions by sedimentation from the di
bination comprising a mixing unit adapted to
gested liquor solution, and means for maintain
form a pumpable slurry from a solid to be ex
ing a constant ilow of material throughout.
tracted and a solvent, a plurality of digesting 70
6. A method of continuouslyextracting alu
mina from aluminous materials with caustic tanks, means for continuously supplying slurry
- liquor, which comprises dividing the' caustic to said'digesting tanks from said mixing unit,
liquor into major and minor portions, forming a means for continuously supplying additional sol
yslurry of the aluminous material with the minor vent to said digesting tanks, a plurality of cooling
chambers connected in series, transfer means 'ItA
76 portion and heating the major portion indirectly
60
trollably combining the slurryand caustic liquor
2,107,919
adapted to deliver _solution and undissolved solids
fromsaid ldigesting tanks tothe ñrst of said series
of cooling chambers, means for regulating the
pressure in the digesting tanks, control means
between adjacent cooling chambers in said series
adaptedvto maintain-pressures in said chambers
tity oi! caustic liquor by indirect contact with
steam, digesting said slurry and heated caustic
liquor in a closed vessel maintained at the desired
superatmospheric pressure and elevated temper
ature by direct contact with steam to dissolve alu
mina, reducing the pressure of the resulting so~
decreasing in steps from the digestion pressure, lution and withdrawing steam liberated thereby,
and returning the steam to indirectly heat'a fur
and a closed sedimentation tank positioned be
ther quantity of caustic liquor.
tween and operatively connected with two adja
13. ’I'he method of extracting alumina from
cent cooling chambers in said series and adapted »
aluminous material, comprising digesting alumi
to separate solids from solution under substan
nous material' with preheated caustic liquor in a
tially the conditions of superatmospheric pres
sure and elevated temperature maintained in the closed vessel, maintaining the pressure above at
mospheric and the temperature above the boiling
ilrst of said connected cooling chambers.
10. In apparatus for extracting solids with a l point at iatmospheric pressure by injecting steam
15
solvent by continuous pressure digestion, the into said vessel, separating the resulting solution
‘ combination comprising a mixing tank adapted from undissolved residues by sedimentation at
vto Iorm a slurry of solid and solvent, at least one elevated temperature and superatmospheric pres
heat exchange unit, at least one digesting unit, ' sure, reducing the pressure on said solution by
at least one cooling chamber, controlled means withdrawing vapor liberated thereby, and em 20
for delivering slurry from said mixing tank to
ploying said vapor to preheat a further quantity
'said digesting unit, means for passing a controlled , of said caustic liquor.4
quantity of ,additional solvent through saidv heat
exchange unit and into said digesting unit at a
25 controlled temperature and pressure, and means
for delivering material from said digesting unit
to said cooling chamber- while maintaining-the
desiredvpressure in the digesting unit.
11. The method of extracting alumina from
aluminous material >with caustic liquor, compris
ing mixing part of the caustic liquor with alumi
"' nous material to form a slurry, heating a further
quantity of the caustic liquor, mixing the heated
liquicr with the slurry, digesting the mixture at
35 an elevated temperature and superatmospheric
pressure, and separating >the resulting solution
from undissolved residues by sedimentation at a
temperature -above the boiling point of
tion at atmospheric pressure.
40
'
12. The method of extracting alumina from
aluminous material with causticliquor, compris
ing mixing solid aluminous material with caustic '
liquor to form a slurry, `heating a further quan
` 14. The methodvof extracting alumina from
aluminous material, comprising digesting alumi
nous material with caustic liquor atsuperat 25
mospheric pressure and at a temperature higher
than the boiling point of the liquor at atmos
pheric pressure, land. separating the resulting so
lution from undissolved residues by sedimenta
tion at elevated temperature and superatmos 30
pheric pressure.
`
,
.
15. The method of extracting alumina from
aluminous material, comprising digesting alum!
nous material with caustic liquor at superat»
mcspheric pressure and at a temperature higher 35
than the boiling point of the liquor at atmos
pheric pressure, and maintainingv the resulting
solution and undissolved residues at superatmos
pheric pressure and elevated temperature until
said residues are separated'from the solutionby
sedimentation.
CARRELL ORISON TURNER.“
MELVIN POWELL WEIGEL.
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