Note: Descriptions are shown in the official language in which they were submitted.
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~ he present invention relates to systems for dry
scrubbing waste gases and in particular to dry scrubbing
waste gases by contact with finely divided solid materia~s.
It is well k~own to treat the waste gases from
reduction cells for the production of ~u~}Y~ by contacting
the stream of waste gases with finely divided alumina to react
with reactive ~ases, particularly hydroge~ fluoride, contained
in such waste gases. In these known processes the alumina ~:~
entrained in the gas stream and after a predetermined contact
time the entrained solids are remGved by means of ~ filter~
from which the collected solids are recovered at intervals,
for example by pulsing the filter to shake off the collected
solids. ~he collected solids are preferably carried aw~y from
the region of the filter b,y means of an air slide and recover~d
for further use. The employment of a~ air slide to co~vey
~ material over a substan~ial distance involves bleeding off
i air at spaced intervals from ~he housin~ within which the air
slide is contained. Since this air contains substantlal
quantities of entrained finely di~ided solids, particularly
sol-ids in the sub-sieve size range, it i~ customary practice
to recycle this air to the waste gas stream both to ensure
recovery of the valuable entrained solids and to avoid
I atm~spheric pollution arising from discnarge into the
atmosphere.
i 25 ~ practical difficulty experienced with dry scrubbin~
systems of the abo~e type is the ~orma~ion o~ scale on all the
sta~ionary surfaces, such as the surfaGes o~ ~he filter boxe~
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and of the housing of the air slide, Such scale requires periodic removal
to ensure satisfac~ory operation of the system and a considerable expenditure
of time is involved in such removal of scale.
The causes of the formation of such scale are not clearly under~-
stood, bu~ examination of the scale appeiars to show tha~ it is formed by
the agglomeration of the finer par~icles of the alumina supplied to the waste
gas. It is currently believed that conditions of high atmospheric humidity `~
promotes scale formation. ~ -~
The present invention seeks to overcome ~he above noted drawbacks.
Accordingly the present invention provides apparatus for dry
scrubbing waste gas comprising meansfor injecting a dry particulate material
into a stream of said was~e gas, means for separating said particulate
material from said waste gas after contact therewith, means for discharging ;~
the separated particulate material from said separative means to an air
slide, said air slide comprising an inclined foraminous floor, which forms
part of the wall of an air duct an air man:ifold under said floor and a
casing enclosing the space above said floor, said casing being provided with
one or more air vents, and means for filtering air discharged from said
casing through said vent or vents.
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, 20 The present invention also provides a me~thod of removing one or ~
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more components from a waste gas derived from reduction cells for the
production of aluminum comprising injecting particulate alumina into a
stream of said waste gas, separating the particulate material from the waste
i gas after contac~ therewith, discharging the separated particulate alumina
to an air slide which comprises an inclined foraminous floor, which forms
part of the wall of an air duct under said floor and a casing enclosing
the space above said floor, passing a stream of air through said duct and
through said floor and alumina supported thereon, venting air a~ one or
` more points rom said casing and filtering air thus vented. ``
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Whereas the present invention has been particularly described with
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reference to the dry scrubbing of aluminum reduction cell waste gases
with alumina~ it is belie~ed to be equally applicable to other dry scrubbing
operations in which a finely divided solid material is brought int,o contact
with a stream of gas for the purpose of removing undesirable mate~ials
and/or valuable recoverable materials therefrom before discharge to
atmosphere.
The drawing is a partially sec~ioned side view of an apparatus
in accordance with the present invention.
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Referring now to the accompan~ing diagrammatic
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drawing a ~tream of waste gas from a line o~ u~Y}
reduction cells is carried in a conduit 1, into which
finely divided alumina is fed in metered ~uantit~ at a
supply station 2. ~he stream of air i~ the conduit 1 is led
into a fil-ter box ~ in which are suspended a group of ba~
~ilters 4. The waste gas passing upwardly through the bag
filters is discharged to atmosphereO ~he bag fil~ers 4 are
vibrated at intervals in the kno~m wa~ so that the material
collected thereby is shaken downwardly into the co~ical lowe~
section 5 of the filter box 3. Although only one is sho~,
; a large number, for exa~ple ~ortyeight, of filter boxes are
- connected so that their lower sections 5 lead into a common
air slide 6, via discharge gates (not shown) so -that the
material in each filter box may be discharged into the air
slide at suitable intervals when an appropriate qua~tity has
accumulated. As is usual, the air slide 6 includes an inclined
foraminous floor ?, located over a~ air manifold 8, which is
i maintained at an appropriate slightly superatmospheric pressure
to ensure a co~stant upward flow o~ air through the floor 7
The air slide 6 is housed within a housing 9 to prevent escape
I o~ dust material to àtmosphere. The air slide 6 carries
;, material ~rom the filter boxes 3 to a collec~or 10, from which
it ma~ be recycled to the alumina feed stations or removed ~or
a/~ ;n~
supply to alu~a~,~reduction cells.
As already explained, the air slide housing 9 is
provided with air vents 11 at longitudinal i~tervals. In
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accordance with the principles o:E the present inventlon each
; of these vents 11 are provided with a filter 12, which ma~ be
of similar con~truction and operation to the ~ilters 3. ~ach
vent 11 may be provided with an individual filter, as shown.
~ 5 Alternatively, in some circumstances, the strea~s of air from
; the vent~ 11 can be led to a sin~le filter. ~he stream of air
issuing from the filters 12 is preferably ~eturned to the air
manifold 8 cf the air slide by means of a blower 140
It will be appreciated that the alumina fed into the
waste gas stream at the station 2 consists o~ a wide ran~e of
particle si.zes. As already explained, a proportion of these
fines become entrained in the air bled off from the air slide
houslng through the vents 11. These en-train~d fines are, i~
accordance with the present invention, filtered off at the
filters 12 and may be disposed of in any appropriate m~nner~
. Preferably, however, they are not returned to the scrubbin~
circuit~
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