Note: Descriptions are shown in the official language in which they were submitted.
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The invention relates to a new process for the removal of organic
compo~mds from the alum earth production cycle performed according to the
Bayer process.
The term "alum earth production according to the Bayer process"
refers both to the so-called European and the so-called American Bayer pro-
cesses.
The term "thin liquor" used in the specification and claims refers
to the mother liquor obtained after the separation of alum earth hydrate,
whereas the term "thick liquor" refers to the recirculated and concentrated
thin liquor. The term "digesting liquor" refers to the liquor ready for the
digestion of bauxite, which is to be introduced into the digesting appar-
atus. In some instances the thick l;quor is applied clirectly as digesting
liquor, whereas in other instances the thick liquor is subjected first to
other pre-treatments, such as purification, adjustment of concentration, etc.
In these latter instances the digesting liquor is different from the thick
liquor. The other expressions applied in the specification are the same as
generally used in connection with alum earth production.
Several difficulties are caused in the alum earth production cycle
by the organic impurities which enter the cycle in part with the raw material
and in part with other additives. Of the latter the additives applied in the
separation of red mud, added to the slurry in order to facilitate filtration
or sedimenting, are to be mentioned first.
The organic impurities are substances most diverse in chemical nat-
ure. The major problems arise in connection with humic acids, which increase
the viscosity of the circulating liquid, prevent the thin liquor from concen-
tration and increase the amount of heat necessary for evaporation. Humic
acids represent 2 to 10% of the total organic impurities calculated on the
basis of the carbon content. Oxalates represent another important group of
organic impurities. These compounds remain in the aluminate liquor until
precipitation, and they precipitate in major part together with alum earth
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hydrate as impurity. The amount of oxalates remaining in the mother liquor
is only 0.3 g/l. About 10 to 15% of the organic impurities (calculated on
the basis of the carbon content) consist of formates. The remaining part of
the impurities consists of various other organic compounds. Plour and starch
form the majority of additives introduced into the cycle; these are utilized
as sedimentation aids.
The organic impurities listed above may cause several problems in
the alum earth producing cycle, primarily because
- they worsen the sedimenting properties of red mud to a great extent,
- they decrease the efficiency of precipitation and make hard or even im-
possible the filtration of alum earth hydrate,
- due to excessive foaming they disturb the concentration of the liquors and
decrease heat transfer,
- they increase the solubility of carbonates to a great extent, strongly in-
hibiting thereby the separation of ballast salts.
Several methods have been elaborated for the removal of these in-
convenient impurities. An extensive review of the known methods is given in
the German Offenlegungsschrift No. 2,145,872. Such known methods are e.g.
slurrying bauxite with water before digestion and then separating the solid
phase from the liquids; roasting bauxite and other raw materials before di-
gestion; furthermore oxidative destruction with chlorine or sodium hypo-
chlorite.
With this latter method, however, the degree of destruction is in-
sufficient. This method cannot be applied in large-scale production, either,
since the additional process steps required to remove the organic impurities
greatly increase the production costs of alum earth. From economical aspects
the method disclosed in the cited reference appears to be more favourable.
According to this latter method sodium humates present in the thick liquor
of the Bayer process are converted into insoluble calcium compounds with lime
milk, removing thereby about 50 to 60% of humic acid impurities.
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A related technology is suggested in the German Offenlegungsschrift
No. 2~518J~31. In this method magnesium compounds are added to the liquor to
be treated, whereupon a mixture of Mg(OI-1)2 and Al(OH)3 separa~es, removing 10
to 25% of the organic substances present. This method, in contrast to that
utilizing calciwn hydroxide, can be applied for the treatment of both thick
liquor and thin liquor, and it improves the quality of the digesting liquor
substantially.
According to the Hungarian patent specification No. 1~6,023 solid
sodium hydroxide is added to the liquor to provide an extremely high alkali
concentration, thereby facilitating the decomposition of the organic compon-
ents. ~lowever, the above problems canrlot be avoided completely by this meth-
od either.
The United States patent specification No. 2,806,766 suggests a
completely new way for removing organic impurities. According to this method
the aluminate liquors are heated to 150 to 250C prior to their further pro-
cessing, and the organic compounds are separated in part together with the
salts which crystallize upon the decrease of solubility. No oxidizing agent
is applied in this method.
A part of the known processes mentioned above has the disadvantage
,~
of requiring specific apparatuses~ whereas the others cannot be utilized ef-
ficiently in the production of alum earth, since they do not enable the re-
~`,! moval of organic impurities to the required degree. Some of these latter
methods utilize rather specific reactions, thus they are not capable of de-
~! composing all the disturbing organic compounds or removing them completely
,, ~
P~ from the cycle. Thus there exists a need for a process which can be per-
!l formed easily and enables one to decompose and remove a substantial part of ~ -~
the organic impurities with quite diverse chemical structures.
The invention aims at the elaboration of an inexpensive and effi-
cient method for the removal of organic compounds appearing in the circulat-
ing liquids of the high-temperature digestion of aluminium-containing raw
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materials.
The new process of the invention ensures a very efficient removal
of the organic impurities from the alum earth production cycle. By this new
method the organic substances can be removed from the thin liquor and the
thick liquor as well. One can also proceed, however, so as to decompose or-
ganic compounds in the digestion liquor in a part withdrawn from the main
cycle, and remove the decomposition products in the separate steps performed
after digestion.
The invention is basecl on the recognition that a substantial part
of the organic impurities can be converted into oxalates and carbonates,
easily removable from the cycle, when oxidation is performed under appropri-
ate conditions.
Thus, this invention provides a process for the removal of organic
compounds from circulating liquors of an alum earth production cycle per-
formed according to the Bayer process, characterized in that
(a) oxygen or an oxygen-containing gas is introduced into the li-
quor to be recirculated that is obtained after precipitation until a partial
oxygen pressure of 3 to 30 atmospheres is attained and oxygen is finely dis-
persed in the liquor, thereafter pressure is decreased to atmospheric, and
the solid decomposition products of the organic impurities are separated from
the liquor; or
(b) a part of the liquor to be introduced into the digestion step
is heated to 120-350C oxygen or an oxygen-containing gas is introduced into
the liquor until a partial oxygen pressure of 3 to 30 atmospheres is attained
and oxygen is finely despersed into the liquid, thereafter the liquor, pre-
treated as described above, is combined in a digesting apparatus with a cir-
' culating slurry to be digested, and the solid decomposition products of the
organic impurities are removed from the aluminate liquor together with red
, mud.
According to the invention one proceeds as follows:
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~ a) the liquor to be recirculated, obtained after precipitation,
is optionally concent-rated and heated to 120-350C, preferably 210-300C,
oxygen or an oxygen-containing gas is introduced into the liquor until a
partial oxygen pressure of 3 to 30 atmospheres, preferably 10 to 25 atmos-
pheres, is attained, oxygen is finely dispersed in the liquor, and, if nec-
essary, oxygen is supplemented according to the consumption, thereafter pres-
sure is decreased to atmospheric, and the solid decomposition products of the
organic impurities are separated from the liquor; or
(b) a part of the liquor to be introduced into the digestion step
is heated to 120-350C, preferably 210-300C, oxygen or an oxygen-containing
gas is introduced into the liquor until a partial oxygen pressure of 3 to 30
atmospheresJ preferably 10 to 25 atmospheres, is attained, oxygen is finely
dispersed in the liquor, and, if necessary, the consumed amount of oxygen is
supplemented, thereafter the liquor, pre-treated as described above, is com-
bined in the digesting apparatus with the circulating slurry to be digested,
and to solid decomposition products of the organic impurities are removed
from the aluminate liquor together with red mud.
The oxidative decomposition of the organic impurities can be per-
formed in any apparatus used conventionally in the production of alum earth.
Thus an autocalve of any construction, equipped with an assembly for the in-
troduction of oxygen or an oxygen-containing gas, can be applied, provided
that the temperature and pressure conditions mentioned above can be main-
tained therein.
Oxygen or the oxygen-containing gas should be dispersed completely
in order to provide the greatest possible contacting area between the liquor
and the oxidizing agent.
Therefore the process is performed in an autoclave equipped with -
high-performance stirring means. Hollow stirrers (gas stirrers) proved to
be preferable. It is also preferred to apply flow-breaking means.
The oxidative destruction of organic impurities can also be per-
i"76~J
formed in the digesting apparatus itself. In this instance a part of the di-
gesting liquor is removed from the cycle, saturated with oxygen or an oxygen-
containing gas, and after this pre-treatment the liquor is introduced into
the digesting apparatus. When digestlon is conducted in a tube reactor,
specific means, filling agents, screens or baffle plates are applied to en-
sure a good distribution of the gas in the slurry to be digested.
The gaseous oxidizing agent is preferably circulated. The gaseous
oxidizing agent leaving the expansion vessel after the pressure decrease fol-
lowing oxidation is optionally pressurized and admixed with fresh oxidizing
agent, and then recirculated into the vessel wherein oxidative decomposition
is performed.
As oxidizing agent e.g. oxygen, air, oxygen-enriched air, or a sub-
stance capable of liberating oxygen can be applied.
Method a) of the invention enables one to remove the organic com-
pounds from both the thin liquor and the thick liquor of the cycle.
The rate of oxidation depends strongly on the temperature. At
180C only about 20 to 30% of the organic impurities can be destructed with-
in 30 to 60 minutes, whereas if the process is conducted at 260 to 280 C, 95
to 97% of the organic impurities decompose within the same period. Complete ;~-
oxidation can be achieved above 300C.
In method a) of the invention the solid decomposition products of
the organic impurities are separated together with the excess of soda, by a
conventional method for separating solids from liquids applied in the alum ~ ~
earth industry. ~-
If the organic impurities are decomposed in the thin liquor, the
separation of the decomposition products can be facilitated greatly by ex-
pansion and/or evaporation cooling. Under such conditions the alkali con-
centration increases, whereupon the solid decomposition products of the or-
ganic impurities precipitate more easily and can be removed.
If the oxidative decomposition of the organic impurities is per-
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formed during digestion, digestion is conducted pre-Eerably at 230 to 260C
under a partial oxygen pressure of about 10 atmospheres. In this case the
decomposition products appearing as solid substances need not be removed in
a separate step, since they can be separated from the aluminate liquor sim-
ultaneously with the removal of red mud.
In method a~ of the invention either the total amount of the cir-
culating liquor or a part thereof can be subjected to oxidative destruction.
The actual method of operation depends on the given technological condi-
tions.
Method b) of the invention is always performed so that a part of
the liquor ls withdrawn from the cycle, and, after treating it with the ox-
idizing agent, the liquor is fed back into the cycle. It should be noted
that in this instance oxidation is not necessarily terminated when the liquor
reenters the cycle, and oxidation may continue until the pressure of the sys-
tem is reduced to atmospheric.
In the process of the invention catalysts can be applied optionally
in order to facilitate the oxidative decomposition of the organic impurities.
As catalysts primarily metals with varying valences can be applied. Silver,
copper and cobalt proved to be particularly preferable.
The oxidation time may vary within wide limits, depending on the
various technological parameters and on the amount of the organic impurities
present. Oxidation is performed generally for about 5 minutes to about 3
hours, preferably for about 30 to 60 minutes. ;
Both the oxidation and the removal of the solid decomposition pro-
ducts of the organic impurities can be performed in the conventional appara-
tuses of alum earth production. Thus the process of the invention can be
introduced very easily into alum earth production. ;~
The most important advantage of the new process is that it allows
the organic impurities of the circulating liquors to be removed substantial-
ly. Humic acids, causing the most serious problems, can be removed practi-
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cally quantitatively. It is also surprising that after the removal of theimpurities according to the invention the aluminate liquor contains oxalates
in a very low concentration, causing no particular problems in the precipita-
tion step.
The invention is described in the following with reference to the
attached drawings.
Figure 1 is the scheme of method a).
Figure 2 is the scheme of method b).
The diagram shown in Figure 3 illustrates the percentage decrease
in organic carbon content of the liquor as a function of temperature. The
liquor was treated for one hour under a partial oxygen pressure of 25 atmos-
pheres. Further details of the process are given in Example Z.
The diagram shown in Figure 4 illustrates the percentage decrease
in organic carbon content of the liquor as a function of time. The liquor
was treated at 240C under a partial oxygen pressure of 25 atmospheres. Fur-
ther details of the process are given in Example 4.
The diagram shown in Figure 5 illustrates the percentage decrease
in organic carbon content of the liquor in the catalytic oxidative decomposi-
tion as described in Example 5, as a function of time. The liquor was treat-
20 ed at 240C under a partial oxygen pressure of 25 atmospheres.
It should be noted that Figures 1 and 2 contain at some places two
or three apparatuses with the same function, marked with the same reference
number. The invention is, however, not limited to the use of the specific
number of identical apparatuses shown in the figures.~ By including in the
figures more than one apparatuses with the same function we intended to ex-
press that according to a preferred method more than one of these apparatuses
are utilized in the process of the invention.
In Figure 1 the liquor arriving through line 1 is forwarded by pump
2 into pre-heating autoclaves 3, where it is pre-heated by the expansion va-
pours arriving through line ~. The liquor is then passed into autociiàves 5,
.. . . ................. , . . . . , ~ , .
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where it is heated to the temperature of oxidation by steam arriving through
fresh steam line 6. Oxygen is fed into the liquor through hollow shafts 7
installed into autoclaves 5. The liquor is then passed into autoclave 8, and,
after a repose period, into expansion vessels 9. In the expansion vessels
the oxygen separates from the liquor together wikh the vapours and enters the
bundle of heating -tubes mounted into autoclave 3 through vapour line 4. Ilere
the vapours condense, and oxygen is led through lines 11, together with the
condensed water, into separating vessel 12. Water is removed ~rom this ves-
sel through separators 13 into waste water conduit 14, whereas oxygen is
passed into oxygen collecting conduit 17 through lines 15 and valves 16. For
repeated use, oxygen is fed into oxygen pump 18, and the amount of oxygen is
supplemented to the required value from an external oxygen source (e.g. a
pressure bottle) through valve 20. The expanded liquor is led from expansion
vessels 9 through line 21 into container 22, from which it is passed by pump
23 onto salt filter 24. The salt-free liquor is removed through line 25,
whereas the salt is removed through line 26 for further use. When oxidation
is performed at 200C, it is preferred to adjust the pressure at the dis-
charge pipe end of pump 2 to 30 to ~0 kp/cm2, and to maintain a pressure of
32 to 45 kp/cm2 at the discharge pipe end of pump 18.
In Figure 2 the liquor arriving through line 1 is forwarded by pump
2 into pre-heating autoclaves 3, where it is heated to the required tempera-
ture by the expansion vapours arriving through line 4. The pre-heated liquor
is led from pre-heater 3 into autoclave 5, where it is heated to the tempera-
ture of oxidation, and oxygen or an oxygen-containing gas is introduced into
the liquor through hollow shaft 7. Thereafter the liquor is led into auto-
clave 30, where the liquor is combined with the pre-heated slurry to be di-
gested. The slurry to be digested is passed through line 27 and is pumped by
pump 28 through pre-heaters 29, also heated by the expansion vapours arriving
through line 4. The mixture of the slurry and the liquor is heated to the
temperature of digestion by the steam arriving through fresh steam line 6.
.. . . "
~37'~60
The slurry is then fed into autoclave 31, and~ after a repose period, into
expansion vessels 32. Nere the vapours and the oxygen separate from the ex-
panded slurry. The vapour - oxygen mixture is led through lines 4 into heat-
ing elements 10, mounted into autoclaves 3, or into heat exchangers 29, where
the vapours condense. The mixture of waste water and nxygen is led through
line 11 into separating vessel 12. Water is moved from this vessel through
separators 13 into waste water conduit 14, whereas oxygen enters oxygen col-
lecting conduit 17 through lines 15 and valves 16. The further treatment of
oxygen is the same as given in connection with Figure 1. The expanded slurry
10 is led from expansion vessel 32 through line 33 into diluting tank 34, where-
from it is removed for further processing.
When oxidation is performed at 200C, it is preferred to adjust the
temperature to 240 to 250C after autoclave 31, to adjust the pressure at the
discharge pipe end of pump 2 to 50 to 60 kp/cm2, and to maintain a pressure
of 5Z to 65 kp/cm at the discharge pipe end of pump 18.
The process of the invention is elucidated in detail by the aid of
the following non-limiting Examples. In the examples the symbol "Corg" re-
fers to the amount of organic compounds expressed as carhon~ and Na20c is the
caustic sodium content, i.e. the sum of the amount of sodium present in the
20 form of NaOH and NaAl~OH)4.
Example
A digesting liquor with a C rg content of 6.62 g/l, Na2Oc content
of 200 g/l and A12O3 content of 100 g/l was treated in the heat recuperative
equipment shown in Figure 1.
Oxygen gas was fed into the liquor, and the partial oxygen pressure
was adjusted to 25 atmospheres. The amount of oxygen consumed in the reac-
tion was supplemented continuously. I`he mixture of liquid and gas was heated
first to 120 C and then gradually to 300C, and maintained for one hour at
the final temperature.
In one series of tests no gas stirrer was applied (Figure 3, curve
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1), whereas in the otller series all autoclaves were equipped with gas stir-
ring means (Figure 3, curve 2).
l'he test results summarized in those curves indicate that at 240C
12% or 48%, respectively, of the original C content was oxidized into car-
bon dioxide (appearing as carbonate), whereas at 300C the ratio of oxidized
C was 32% or 92%, respectively. Consequently, the use of stirring means
increases the efficiency of decomposition to 3-4 times.
Example 2
Thick liquor originating from an alum earth plant processing karst-
bauxite (sample 1) and thick liquor originating from an alum earth plant pro-
; cessing laterite bauxite (sample 2) were oxidized with oxygen gas at a final
temperature of 180C or 250C in an autoclave equipped with gas stirring
means. The Corg content of the treated liquor, the amount of Corg removed in
salt form, furthermore the ratios of some characteristic fractions are listed
in Table 1.
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D O~1 ~ ~ Ou~
~C Cd O O O O O 1
0 ~ ~ ' ,~'
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r ~ oo o o
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~ ~ h ~ ~ Ln t` ~t
~ o
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~ O ~D ~O O NO
N ~ ~rlr l N OO CO O
r l O tl~N r-l
., ~ ~r~l O OO r~l O
4~ ~ ~ O O O O O O
, O e ~ ~ . O O ~D o
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00 0 00 ~ 1` .
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E~0 4
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R ~ OLn o O
o a~ 1~r~
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U~
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X ;~
O P~ O N N N O N
h a) ~ ^
h ~) h U U ~J o o
C) ~ r-l ~ O O O NIH O
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The data of Table 1 indicate that in samp:Le 1 the amount of humic
acids and oxalic acid decreases to one-tenth of the original already after a
treatment performed at 180C. When oxidation is per-formed at 250C, none of
the samples contain detectable amounts of humic acids and oxalic acid. It is
remarkable that the oxalate content of the salts separated from the liquor is
2 to 26 times higher than the original oxalate content of the liquor.
Example 3
A thin liquor with a Na2Oc content of 116 g/l, A12O3 content of 45
g/l and Corg content of 4.23 g/l was oxidized in an autoclave equipped with
gas stirring means for 60 minutes under a partial oxygen pressure of l0 atmos
pheres. The temperature was varied in the individual tests between 120 C and
300C. The amounts of organic impurities remaining in the liquor were as
follows:
TemperOa- 120 160 200 240 280 300
. . . _ n . .
Corg' g/l 3.8 3.6 2.9 1.3 0.7 0.1
The purified liquor was separated from the oxygen, then it was
passed directly to the evaporation step ~thereby utilizing the heat of the
mix~ure)J and the separated decomposi~ion products were removed in the usual
way together with the ballast salts.
Example 4
A thick liquor with a Corg content of 6.62 g/l, a Na2Oc content of
200 g/l and an A12O3 content of 100 g/l was heated in an autoclave to 240C.
Thereafter oxygen gas was introduced into the liquor until a partial oxygen
pressure of 25 atmospheres, thereby producing a gas-liosol. The amount of
oxygen consumed in the reaction was supplemented continuously. Oxidation was
conducted for 5, 10, 20, 30, or 60 minutes.
The separated salts, containing a substantial amount of organic sub-
stances, were filtered off after the treatment. The results of the tests are
" .,
7~7~3
summarized in Figure 4. Curve 1 of Figure 4 shows the amount of organic sub-
stance oxidized into carbonate, whereas curve 2 shows the amount of organic
substance oxidized and removed together with the salt (i.e. the total amount
of organic substance removed). These amounts are given in percentages re-
lated to the organic substance content of the starting liquor. 75 to 98% of
the total organic substance content of the separated salt was oxalate.
Figure 4 indicates that 48% of the total organic substance content
of the liquor can be removed within one hour.
Example 5
A digesting liquor with a C content of 6.62 g/l, a Na2Oc content
of 200 g/l and an A1203 content of 100 g/l was treated in an autoclave equip-
ped with gas stirring means.
The liquor was heated to 240C, and a solid catalyst containing 10
mg/l of copper, cobalt or silver was added. Oxygen gas was fed into the liq-
uor until a partial oxygen pressure of 25 atmospheres. The amount of oxygen
consumed in the reaction was supplemented continuously .
No catalyst was used in the comparative test ~Figure 5, curve 1).
; When comparing the results it appears that at 240C 24% of the initial C g
content is oxidized within 20 minutes when no catalyst is applied, whereas in
the presence of e.g. a copper catalyst (see curve 4) the amount of oxidized
Corg is 62%, i.e. a 2.6-fold increase in efficiency is caused by this cat-
alyst. When using silver (curve 2) or cobalt catalyst (curve 3), the in-
crease in oxidation efficiency is 1.2-fold or 1.3-fold, respectively, under
otherwise identical conditions (20 minutes at 240C).
Example 6
A thic~ liquor with a Na20c content of 280 g/l, A12O3 content of
140 g/l, Corg content of 9.30 g/l and Na2CO3 content of about 10% was treated
directly in the digesting step as shown in Figure 2. Thus, according to the
two-stream digesting process, the thick liquor was heated separately, and the
organic substances were destroyed in this heating step. Pressurized air,
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enriched ln oxygen, was introduced into the thick liquor until Q partial ox-
ygen pressure of 5 atmospheres. A gas-l:iosol was produced in the autoclave
by gas stirring means, and the partial oxygen pressure was maintained at a
constant value. The thick liquor was heated to 210C and maintained at this
temperature for 100 minutes in order to ensure a more perfect oxidation.
Thereafter the thick liquor stream was combined with the stream of bauxite
slurry. The oxygen-containing gas liberated in the expansion vessels was re-
circulated into the process. The oxygen-containing gas was kept in circula-
tion, and only the amount of oxygen consumed in the chemical react;on was
10 supplemented.
25% of the organic substance content of the thick liquor was oxid-
ized into carbonate, whereas the majority of the residual 75% was converted
into sodium salts of formic acid, oxalic acid and arornatic carboxylic acids.
These components in the amounts obtained disturb the Bayer cycle much less
than the original organic impurities.
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