CA1045343A

CA1045343A - Dry scrubbing waste gases

Info

Publication number: CA1045343A
Application number: CA224,167A
Authority: CA
Inventors: Alvin W. Kielback
Original assignee: Alcan Research and Development Ltd
Current assignee: Alcan Research and Development Ltd
Priority date: 1974-04-10
Filing date: 1975-04-09
Publication date: 1979-01-02
Also published as: GB1501866A

Abstract

A B S T R A C T

In a procedure for dry scrubbing waste gases with finely divided soli? materials, the solids are separated from the gases by filters, from which the solids are recovered at intervals and transported by means of air slides. In order to prevent scale formation the air bled off iron the air slide casing is recirculated to the inlet of the air manifold of the air slide, since the air is partially desiccated in its passage through the solids carried by the air slide.

Description

3~ 3 ~ 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~
' ~JX/96~9 . ~ -~453~3 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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~5~3 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 Ql~ n 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 ,, . ., ~I
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~ 53~3 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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Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. Apparatus for dry scrubbing waste gas comprising means for injecting a dry particulate material into a stream of said waste 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 manifold 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.

2. Apparatus according to claim 1 further including means for recirculating filtered air to the air duct of said air slide.

3. Apparatus according to claim 1 wherein said separating means includes a plurality of bag filters, material collected on each of said bag filters being discharged to a common air slide.

4. Apparatus according to any of claims 1 to 3 in which the casing is provided with a series of air vents at longitudinally spaced intervals, each vent being provided with an air filter, a pump means being provided for recirculating the filtered air from such filter to the air duct of the air slide.

5. A method of removing one or 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 gas after contact therewith, dis-charging 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 at one or more points from said casing and filtering air thus vented.

6. A method according to claim 5 further including recirculating said filtered air to said air duct.