Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
A 7542
2~847 ~V
~etalloPsellschaft PG Frankfurt, dated January 22, 199Z
~euterweg 14
6Q00 Frankfurt-on-Main 1
Case ~!o. 88 00 84
Process for the nisposal of Resioual Materials l.lhich
Conbain fluorine- an~ Cyanide-containino ComDounds
nEscRIpTIo~l
Some residual materials contain water-soluble
fluorides and cyani~lles as well as nitro~en co~oounds an~
for this reason cannot readily be dumoed. ~uch a hazar~
dou9 residual ~at.erial is p~rticularly constituted b~
the material broken out from Furnaces used fnr the elec
trolytic nro~uctinn of aluminum, which material consiats
of the refractDry linin~ an~ L~f carhonaceoLIs cathsr~e ~na-
terisl, or ma~ he sonstitute~ b\l the dust which is Forme~
in the electrolytic oroduction of alu~inum. For a safe
~u~noino, the blater-soluble comuounds must be converted to
water~insoluble comoounds.
Fro~ JP-4~50 75 564 it is known to mix ~.~aste
naterials which contain water~soluble Fluorine co~oounds,
such as lininqs of electrolytic cells for the oraduction
of alurninum, with calciurn salts, mainly calcium chloride
and calcium sulfatel and to burn the resultinn mixtures
in an oxidizinQ atrnosohere at 50U to goOaC. Electric retort
furnaces and rotarv kilns are ~entioned as burninn units.
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In that process the water-soluble Fluorides are conver-
ted to water-inYoluble calcium flu~rlde within a treatinq
time of 30 to 60 minutes, an~ the nitrogen compound~ con-
tained in the waste materials are converted to N0 , which
i9 emitted with the exhaust gas. H01 gas will also be
formed if calcium chloride is used.
From CE, ~1arch 19a6, oages 3L to ~8, and From
"Poplication of the transoortable circulating bed com-
bustor for the treatment of hazardous waste", Presenta-
tion at the 79th Annual Meeting of the Air Pollution Oon-
trol nssociation, ~1inneaoDlis, ~innesota, June 22-27, 19~6,
it is known that after an addition of limestone ta the
material broken ou-t From furnaces used for the electro-
lytic production of aluminum said material can be burnt in
a circulatinq flui~ized bed under oxidizing conditions at
temoeratures from 790 to 870~C. The use of limestone will
result in the for~ation of a molten phase, i.e., in an
aqglomeration and formation of crusts.
It is an ob~ect of the invention to convert
the l~ater-snluble f~uorides in the resirJual materials as
comDletelv as possible to watrJr-insoluble comooun~s, to
l~ecomnose t,he cvanides as completel~/ as oogsible, an~
also substantiallv to avoid a formation of ~x derived
from the nitro~en ~oound3.
That object is accomplished in accordance with
the invention by a process which serves for the ~isposal
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of resldual materials which contain fluorine- and cya-
nide-containing compounds ano which is characterized in
that
a) the residual materials are thermally treated
after an addition of calcium and/or magnesium,
b) the thermal treatment is efFected in a two-stage
fluidizeo bed system,
c) a temoerature from 650 to 900C is maintained
in the two-stage fluidized bed systern,
d) the first stane of the fluidized bed svstem is
,slightly
ooerat~d under'reducino conditions with an air ratio C1,
e) th~ secwnd staoe of the Flui~ized berl system i5
Dperated under oxidizing conditians with an oxyqen content
~ 2~,'c by volume, a~d
f) the treated material is withdrawn from the se-
cond staqeO
The sulfates are used at a hyperstolchiometric
rate as regaros the formation of CaFz or ~F2 From the
water-soluble fluorides. Inexoensive waste gypsum can ~e-
sirably be used as calcium sulFate and will thus be dis-
Dosed oF at the same tirne. The tl,Jo-stao,e fluidize~ bed
system may consist of a circulatino, Fluidized bed system,
in which the First fluidized bed is Formed in the lawer
oart of the Fluidized bed reactor bv means of the fluidiz-
ing air flowino throunh the bottDm and the secon~ Fluidized
bed is formed in an uooer part of the Fluidized bed reactor
~ 4 ~ 2~8~7~0
by means of secondary air and optionally tertiary air.
Alternatively, the two-stage fluidized beri system may
comprise two separate orthodox fluidlzed be~ units and in
that case the solids and the gas are withdrawn from the
first fluidized bed unit and supplieo to the second flui-
dized bed unit. The temoerature in the First fluidized
bed will alwa~s lie in the lower oart of the temperature
ranoe stated because en~othermic reactions are performed
in the first fluidized bed awinq to the deficiency oF
oxygen. P9 the residual material i9 charged into a circu-
lating fluidized bed system Lt has a particle size oelow
~ m, preferably below 1 mm, and as the material is charqed
into an orthodox fluidized bed system it has a particle
size below 3 mm, preferably balo~ ~ rnm. The ma-terial is
always charoecl into the first staqe. The resiriual material
rnay be suoDlied in 3 solid state or as a slurry. In case
of a orocessinr-~ of material broken out from furnaces for
the electrolytic production of aluminu~ the feed material
will contain in most cases carbon in such an amount that
its combustion will generate the reaction heat which i9
required. If the resi~ual material rloes not contain enouclh
carbon, the additional amount required will be su~plierl as
solid, liquid or gaseous Fuel. ~n ootimum temoerature for
the reaction is suitablv adjusted bv 3 suitable coolino
of the aolids and where twa orthodox fluidiz2d bed urlits
- 5 - 2~8~7~0
are employeo that coolinq will be eFFected between said
units whereas in case oF a circulating fluidized bed
system the solids seDarated from the d;scharged suspe~-
sion will be cooled before they are recyc:led to the flui-
dized bed reactor.
The advanta~es afforded by the invention reside
in that the water-soluble Fluorine compounds are substan-
tially converted to oater-insoluble compounds, the cyanides
are substantially decomposed and the resulting exhaust gas
has a low content of N0 and this is accomDlished without
an occurrence of aoolomnration and a formation of crusts,
Pccordino to a oreferred feature a temperature
oF 750 to ~50C is maintained in the two-stage fluidized
becl system. Pn effective conversion to water-lnsoluble
fluorides and an effective decomoosition of the cyanides
in conjunction with a formation of ~lO at a low rate are
achieved at temperatures in that range and HF will be
even
emitted at a low rate/in the oresence of water.
According to a preFerred feature the first stage
of the fluioized bed system is operated with an air ratio
of 0.7 to 0.9. This will result in an emission of ~10x at
a very low rate.
According to a oreferred feature the second stacle
of the fluioize~ bed svstem is ooerated with an oxyclen
content of 6 to 10~', bv volume. This will result in a very
%0~710
effective decomposition oF the cyanioes and in an effec-
tive cnnversion tu water-lnsoluble fluQrides and in an
emlssion of ~Ix at a very law rate.
According to a preferred feature, sulFates of
calcium and/or of mar~nesium are added at a rate which i9
1.2 to 1.5 times the rate which is stoichiornetrically re-
quired for thr-~ conversion of the water-soluble fluorides
contained in the residual ~aterial to CaF2 or MgF2, In
that case the addition of the sulfates at a relatively low
rate will result in an efFective cQnversion to water-in-
soluble fluorides.
Addording to a preferrecl feature in case of the
processing of residual materials having a high SiO2 con-
tent an Al203 content of at leas-t 15% by weioht i5 adjusted
in the mixture by an addition of substances which contain
Qlz03. That addition will be requried if the SiOz content
i5 about 30~ ar more. The material which contains Al203
may consist of Al203, bauxite, clay, flue dust t sodium si-
licate or the like. This will avoid the formation of a rnol
ten phase in the presence of a high S~02 content.
~ccording to a ~referred featur~ the two-star~e
/is effected and
treat~ent in a circulatino fluidized bed svstern/is accornDa-
nied by a supolv of secondary air and ootionally tertiary
air to the uoper part of the fluidize~ bed reactor. The
fluidized bed system consists of a fluidized bed reactor,
a recycle cyclone and a recycle line. I~hereas in an
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20~471 ~
"orthodox" Fluidlzed bed 9y9tem a d~n~e phase i~ ~epa-
rated by a distinct density step ~rom the ouerlying gas
space, a circulating fluidized bed system involves states
of distribution having no deFined boundary layer. There
i9 no density steo between a dense ,ohase and an overlying
dust soace but in the fluldized bed reactor the solids
concentration decreases qradually From bottom to too. A
gas-solids suspension is discharged from the upper part
of the reactor. The followin~ ranges will be obtained if
the operating conditions are defineo by the Froude and
Archimedes numbers: g
û.1 < 3/4 x Fr2 x ~ ~ 10
an~0.01 ~ ar C 10Q
wherein 3
d x q (~
~r = D
J g X .~) 2
FrZ = u
g x dk
and
u = the relative oas velocity in m/sec.
ar = the Prchimedes number
Fr = the Froude number
~;9 = the densitv of the gas in kn/mZ
~k = the density of the solid particle in kq/mZ
dk = the diameter of thesolidspherical particle in m
~ = the kinematic viscosity in mZ/sec.
g - the constant o~ aravitation in m/secZ.
2~47~
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The suspen3ion discharged form the fluidized bed
reastnr i~ supplied to the recycle cyclone of the circu-
lating fluldized bed system and solids are subatantially
removed in that cyclone and are recycled to the flui-
dized bed reactor of such a rate that the amount of solids
within the circulatinn fluidized bed system per hour is
at lea~t four times the b~eight of the solids contained in
the fluidized bed reactor. The solids which have been se-
parated and are to be recycled are recycled to the first
staqe of the fluidized bed reactor. The first stage ex-
tends form the bottom of the fluidized bed reactor over
about 20 to 4U~ of the total height oF the fluidized bed
reactor. In the circulating fluidized bed system the re-
quired operating parameters can be very exactly maintained
so that particularly good results can be oroduced.
According to a preferred feature the solids to
be recyoled are cooled in a fluidized bed cooler before
they are charged into the fluidized bed reactor. The flui-
dized bed cooler consists of an orthodox fulidized bed
unit. The heated fluidizing air leaving the fluidized bed
cooler may be supplied to the unit for purifying the ex-
hauat qas that has leFt the circulating Fluidized bed sys-
tem, or may be supplied as secondary air to the Fluidized
bed reactor. The coolinn in the fluidized bed cooler per-
mits an effective and ~imple automatic con-trol of the tem-
perature in the circulatinq fluidized bed system.
- ~ 208~7~
~ ccording to a preferred feature the solids
which have been withdrawn are aFtertreated by being slur-
ried. 50 much water is added that a transportakion withoLIt
a raising of dust and/or a dumping without a raising of
dust will be possible. The ~lurrying will also result in
a ~urther decrease of the solid fluorine compound~. If
the thermally treated solids are aftertreated it will be
preferable to add a part of the calcium ~ulfate or magne-
sium sulfate during the aftertreatment rather than during
the thermal treatment.
The ~nveotion will be explained more in detail
with reFerence to a flow scheme and to examples.
Figure 1 i9 a Flow scheme of a plank cnmprising
a circulating fluirlized bed sy~tem.
The Fluidizecl bed system comprises a fluidized
bed reactor 1, a recycle cyclone 2 and a recycle line 3.
Material broken out of a furnace and having a suitable
particle size i3 charged through line 4 into a mixer ~,
which is supplied with calcium suifate through line 6 and
with an Alz03-containing material khrouqh line 7. Through
line U the mixture is charged into the lo~er part of the
Fluidized bed reactor 1. Fluidizing air is supplied to
the reactor through its bottom from line 9 and in the lower
part of the Fluidized bed reactor is used to form the first
fluidized bed. 5econdary air from line 10 and tertiary air
from line 11 are suoPlied to the upper part of the fluidized
bed reactor and in said upper part are used to form the
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second fluidized bed. The gas-salids suspension is conduc-
ted in line 1Z from the Fluidized bed reactor 1 to the
recycle cyclone Z. ~he collected solids are recycled in
the recycle line 3 to the lower part of the Fluidized
bed reactor. All or part of the separated solids can be
supplied through line 13 to the fluidized bed cooler 14,
which consists of an orthodox fluidized bed unit and is
supplied with fluidizing air through line 15. The cooled
solids are recycled in line 16 to the lower part of the
Fluidized bed reactor 1. From the separating cyclone Z
the gas phase is conducted in line 17 to a second cyclone
1B, from which the collected solids are recycled in line
19 to the lower part oF the Fluidized bed reactor 1. The
gas phase From the cyclone 1e i9 conducted in line Z0 to
the venturi scrubber Z1. Dust-containing fluidized air
which has been heated in the Fluidized bed cooler 14 is
supplied in line ZZ also to the venturi scruboer Z1, from
which the gas is supplied in line 23 to a wet-proces~ de-
duster Z4. The puriFied gas is discharged through line
75. The sludge Formed in the wet-orocess deduster is sup-
plied in line 26 to the sedimentatian tank Z7. The over-
Flow is suoolied ln line ZB to the venturi scrubber Z1 and
the underflow is supplied in line 29 to the stirred tank
30, which is supplied with calcium sulfate From line 31.
The suspension is supplied in line 3Z to the mixer 33,
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2~8~710
which i9 supplied in line 34 with separated salids from
the fluidized bed cooler. The separated solids are after-
treated in the stirred tank 30 and in the mixer 33. The
solids are conducted in line 35 to the dump.
In the following table the results are stated
which have been produced by a treatment of material which
has been broken out from a furnace used in the electroly-
tic production of aluminum. In experiments 7 and ~ the
material broken out from the furnace and having the sta-
ted comoosition was used as feed material without an addi-
tive. In experiments 1 to 6 an identical material bro~en
out from the furnace was mixed with AlzO~ In experiments
9 ta 17 a different material broken out from a furnace and
having the stated composition was usPd without an addition
of Plz03-containinq substances. In all cases, calcium 5ul-
fate or calcium carbonate was added in a molar ratio of
Ca:F oF 0.6. The data for exoeriments 1 to 3 and 8 reflect
the results of control exoeriments in which calicum sulfate
was not added. The data for experiments 5 and 6 reflect
the results produced when calcium carbonate rather than
calcium sulfate was added. The data for experiments 7 and S
reflect the results produced when the m~terial broken out
from the furnace had a very high SiOz content. In the column
"Solubility" the solubility is stated which was obtained by
a leaching accordinn to DEV-S4. In experiment 12, 5% CaS04
were added during the thermal treatment and Z.7% CaS04
were added durin~ the aftertreatment.
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