lQ~13~ CASE 5855 1 The present invention relates to an improved method of removing gaseous fluoride, condensable tars, particulate matter and the like from the cell off-gases from a Hall-Heroult-type aluminium reduction cell, partic- ularly the off-gases from a reduction cell with a Soderberg anode. The cell off-gases from an aluminium reduction ¦-- cell generally comprise a dilute mixture of air with gaseous fluorides, carbon dioxide, carbon monoxide, partic- ¦- 10 ulate matter and the like. The gaseous fluorides are - essentially HF. The particulate matter comprises finely divided alumina, carbon and other carbonaceous materials and also solid fluorides, such as cryolite (Na3AlF6), aluminium fluoride (AlF3), sodium fluoride (NaF), calcium fluoride (CaF2) and chiolite (Na5A13F4). Soderberg reduc- tion cells have one large anode which is baked in place from a paste of carbon aggregate and pitch or tar. me baking of the anode results in the evolution of consider- able amounts of tarry, carbonaceous materials, commonly termed "tar fogn. The carbonaceous materials evolving from vertical stud Soderberg anodes are sometimes of sufficient concentration to allow for the combustion of the tarry materials, but, because of the harsh environ- ment, maintaining burners in operating condition is very difficult. There is no practical way to economically burn the carbonaceous materials evolving from horizontal stud Soderberg anodes because of the low concentration of carbonaceous materials in the off-gases. The tar-contam- inated gas mixture evolving from a Soderberg reduction -- 2 -- .~,., - i ~ . : - :', '~ '. ~ ' ' . ' 1038i3~ 1 cell renders the subsequent treatment of the gas very difficult because much of the carbonaceous matter in the gas is sticky or condensable and thus tends to foul or plug any subsequent gas treating facility. Generally, two method~ have been employed over the years in treating the cell off-gases from an aluminium reduction cell to remove fluorides. The first involves the scrubbing of the cell off-gases with water to remove the fluorides. However, the wet method is not very desir- able because it more or less converts an air pollution problem into a water pollution problem due to the fact that an aqueous solution of fluoride is very difficult to discard without extensive treatment. Frequently, the wet method includes treatment with lime or limestone to react with the fluorides to form CaF2. The other method, a dry method, involves intimately contacting the cell off-gases with the alumina fed to the cell so as to sorb the gaseous fluoride in the cell off-gases onto the alumina surfaces. Up to 99.95~ of the gaseous fluoride evolving from the cell can be captured by this method. An additional advan- tage of the dry method is that all of the fluoride captured can be returned to the cell along with the cell feed. Several methods have been employed in contacting the al-~n~;na cell feed with the fluoride-ladened cell off-gases. 25 One method shown in Canadian Patent 613,352 and U.S. 2,875,844 is to introduce alumina into the moving stream of the cell off-gases and then subsequently removing the particulate matter including the alumina from the gaseous stream by a suitable means, such as a baghouse or an , 30 electrostatic precipitator. Another method shown in U.S. -- 3 -- 10;~t~13~ 2,934,405 and 3,503,184 is to pass the fluoride-containing off-gases through a bed of alumina. In the latter method, probably the most efficient, it is preferred to pass the fluoride-ladened cell off-gases through a bed of finely divided alumina and then subsequently removing any particulate matter including alumina by means of filter bags or an electrostatic precipitator. However, neither of these two dry methods have been employed to any significant extent with the off-gases from a horizontal stud Soderberg cell because the tarry, carbonaceous materials which evolve from the baking anode tend to foul and plug up any gas treatment ~ : equipment facilities that might be employed. Against this background, the present invention was developed. The invention as claimed herein is a method of removing gaseous fluoride and condensable carbonaceous matter from a gaseous stream containing same comprising introducing pulverulent alumina into a moving gaseous stream containing gaseous fluoride and condensable carbonaceous matter so as to entrain therein the alumina and to cause at least part of the carbonaceous matter to condense on the alumina; directing the alumina containing gaseous streaminto a chamber containing therein a foraminous plate and having pulverulent alumina disposed above the foraminous plate; passing the alumina- : containing gaseous stream through the foraminous plate so as to maintain the alumina above the plate in a turbulent condition, thereby removing gaseous fluoride from the gaseous stream; and subsequently removing essentially all particulate matter from the gaseous stream. The claimed method may also include discharging alumina above the foraminous plate from the chamber and recycling from about 10 to 80~ of the discharged alumina into the moving gaseous stream before the stream is passed through the foraminous plate. The recycled alumina may amount to about 10 to 100~ of the alumina entrained in the moving gaseous stream before the stream passes through the foraminous plate. In the claimed method, the particulate matter may be removed from the gaseous stream by passing the stream through a filtering surface. The alumina containing gaseous stream may be maintained at a velocity of about 50 to 150 feet per second. The gaseous stream containing fluoride and condensable carbonaceous matter may be the off-gas from an aluminium reduction cell having a horizontal stud Soderberg anode. The particulate matter removed from the gaseous stream may be returned to the turbulent mass of pulverulent alumina maintained above the foraminous plate. At least a portion of the alumina above the plate may be removed therefrom and treated at high temperature and under oxidizing conditions to remove carbonaceous matter from the alumina. Figure 1 is a side view with parts exposed for clarity of the gas treating system of the present invention. Figure lA is a top view of the foraminous plate in the reaction chamber. The present invention is directed to an improved method of dry scrubbing cell off-gases from an aluminium reduction cell and in particular a reduction cell with a Soderberg anode. In accordance with the present invention, the fluoride and tar-ladened off-gases from the cell are mixed with appropriate amounts of pulverulent alumina cell feed and then directed up through a foraminous plate in a reaction chamber above which is maintained a turbulent mass of pulverulent alumina. The gas passes through this mass of alumina to a dust collection system to remove essentially all particulate matter therefrom and then is vented to the -4a- . . 3~ 1 atmosphere or additional treatment facilities if desired. By introducing the a~umina cell feed into the gaseous stream prior to the reaction chamber, most of the condens- able carbonaceous or tarry materials in the cell off-ga~es will condense onto the alumina introduced and thereby -minimuze the fouling effect of the tar fog. Moreover, the alumina in the gaseous stream removes any build-up of tarry materials which may occur in the gas trea~ing unit. No substantial reduction is found in the amount of gaseous fluoride sorbed onto the surface of the alumina particles due to the condensation of the carbonaceous matter onto the alumina. The amount of alumina used for treating the gaseous stream can range from about 10 to 100% of the alumina required by the reduction cells. Referring to the drawing which illustrates a preferred embodiment, the reactor 10 generally comprises a chamber 11 within which is disposed a foraminous dis- persing plate 12. A plurality of filter bags 13 are dis- : posed therein a suitable distance abo~e the dispersing plate 12 and usually are provided with suitable agitating means (not sh~wn) so as to periodically shake the bags to remove any build-up of particulate matter on the collecting ; surfaces. The fluoride-ladened gases are directed to the reactor 10 from conduit 14 through conduits 15, 16, 17 and 18, as shown. The alumina cell feed is introduced into the conduit 14 from the alumina supply hopper 19. Discharge conduit 20 is provided to discharge the alumina from the reactor chamber 11. All of the alumina discharqed from the reactor can be fed to the reduction cell, but, 0 frequently, it is desirable to recycle portions of the -- 5 -- 1038i3~ 1 discharged alumina to the gaseous stream before the reactor 10 . In operation, the fluoride-tar ladened cell off- gases are withdrawn from the reduction cells and passed through conduit 14 wherein alumina, preferably fresh alumina, is dispersed therein from the supply hopper l9. This solid gaseous mixture is directed up through the dis- persing plate 12 and through the turbulent mass of pulver- ulent alumina which is maintained above the dispersing plate. The gaseous stream then passes through the filter bags 13 for particulate removal and then vented through conduit 21 to the atmosphere or other treatment facilities if desired. Periodically, the filter bags are agitated to remove build-up of particulate matter and to return same to the turbulent region. Initially, most of the condensable tars condense on the alumina particles when the alumina is introduced into conduit 14 but before the gases pass through the dispersing plate. By the time the gas passes through the turbulent region above the dispers- ing plate, it is essentially free of gaseous fluorides andthus can be passed to a suitable dust collection system, such as a baghouse or an electrostatic precipitator, for removing particulate matter. The rate of discharging alumina from the reaction chamber is preferably controlled so as to maintain a relatively constant pressure drop through the reactor. It might be expected that the openings in the foraminous plate would quickly erode due to the passage of highly abrasive alumina at high velocity through the openings. However, little erosion is found even over long 6 -- ., , -. . ~a\;3t~13~ 1 periods, such as 12 months or more. In addition, no significant build-up of tar or other carbonaceous matter is found on the gas-treating equipment. The tars which condense pose no problems within the reaction chamber, the filter bags or any subsequent handling facilities. m e alumina discharged from the reactor can be directed as is to the reduction cell as cell feed, but it may be preferred to pass the tar coated alumina through a combustion chamber or other suitable device to drive off and burn the carbonaceous contaminant material. Substan- tially little or no fluorides will be driven off from the alumina in such an operation, provided the temperatures are not excessive. The amount of alumina entrained in the fluoride- ladened gaseous stream prior to the reaction chamber is that necessary to provide a grain loading of about 1 to 200 grains per standard cubic foot (SCF), preferably from about 1-50 grains per SCF. To maintain particulate entrainment, the gas velocity in the stream should range from about 50 to 100 feet per second, preferably about 60 to 85 feet per second. ~he velocity of the gases through the dispersion plate must be sufficient to prevent any substantial quantity of alumina above the diffuser plate from falling through the openings and must be sufficient to maintain the alumina above the plate in a turbulent condition. The apertures in the foraminous plate generally have a diameter of about 0.1 to about 0.3 inch and are located on from about l-inch to 3-inch centers. Preferably, the alumina-gas mixture above the foraminous plate is a dense turbulent mass more or less undefined in shape but -- 7 -- 13~ 1 approaching a spouting bed-type configuration. The overall or superficial gas velocity through this turbulent region ranges from about 0.5 to 5 feet per second, preferably about 1 to 2 feet per second. Residence times for alumina in the reaction chamber range from about 5 minutes to 10 hours depending upon the load of alumina in the reactor and the recycle load of alumina to the reactor unit. For efficient fluoride removal, the reduction-grade alumina particles should range from about 44 to 149 microns in size. However, up to 8% can be less than 44 microns and up to about 4% can be above 149 microns. The alpha alumina content should be less than 30~. The method of the present invention provides for efficient fluoride, tar and solid particulate capture even though the fluoride, tar and particulate emissions from the cell vary considerably over a period of time due to changes in the operating characteristics of the cell. For example, the fluoride emissions increase considerably every time the crust (i.e., the frozen electrolyte on the surface of the cell) is broken, such as when feeding alumina to the cell. Copious amounts of fluoride also escape during anode effects which occur when there is a depletion of alumina dissolved in the elect~olyte. In horizontal stud Soderberg cells, tar emissions increase considerably during periods when the flexes are raised, this being the time when the electrical conductors or flexes which feed current to the cell are raised to the next higher row of studs and the lower row of studs is removed to prevent any contact thereof with the bath. As 0 an example of this variation, the total particulate 8 - .: . ~.U~ 1 loading of the cell off-gases can range from about 0.005 to over 0.2 grain/SCF. The tar fog can va~y from about 0.001 to about 0.02 grain/SCF. me gaseous fluorides can vary from about 0.005 to about 0.1 grain per standard cubic foot. Kowever, notwithstanding these variations, the method of the present invention consistently maintains a high fluoride removal. The examples in the table below are given to fur- ther illustrate an embodiment of the present invention. This data was collected for an experimental unit handling the off-gases of 9 horizontal stud Soderberg reduction cells (75 KA) employing the method of the present invention. m e data relate to the off-gases from all 9 cells. Gas temperatures in the reactor ranged from about 120 to 200F depending upon the ambient temperatures. The concentration values are mean values of the test period which ranged in duration from about 2 to 5 hours. The air-to-cloth ratio for th~ filter bags was 5.25 ACFM/ft2 of bag. ` _ g _ ~3B136 O r~N 1` O _I--I O U~ O~`O OOIt~ `~D OO O ~17 OO O~ O~ O~ N . a~ o u~N ~r O ON O ~r o1`0 00 ~D ~ ~r . ' ~ OO O -' ~O OO OGD N . a~ o o ~c~ z o oo o ~Or~ oo oa~ ~ u~ ~ o o o ~ ~ N O DN O _IN O NO1~ 00 0~ O~ ~ : `~D OO O ~D OO O~Da~ ~ .., N a~ o _I O1`0 0 0 U') a~ `o oo o ID OO O a~ . tD 1` ~ ' ~~ o~ U I ,~ - U~ 14 E........................... - U ~ o ~ U~ ~ o 0 O ~d ~ o o o o O ~ æ æ æ o h 0 0 - 10 - ,"'~' ~' ' - - ' . ~ . : , . - : . . ~ . -. . . . . :- , - , , . : ' ' ~0~ 1 It is recognized that various modifications and improvements can be made upon the present invention with- out departing from the spirit thereof and the scope of the appended claims. For example, when recycling alumina dis- charged from the reactor, all or portions of the fresh alumina could be introduced into the reactor above the foraminous plate rather than in the-gaseous stream before the reactor. Moreover, although the invention is described herein primarily as -a treatment for the off-gases from a horizontal stud Soderberg cell, it is fully applicable to any fluoride-containing gaseous stream which contains condensable or sticky carbonaceous materials. For example, the present method can be used with the off-gases from vertical stud-Soderberg cells, and it eliminates the problem of maintaining burners in such a harsh environment. : -- 11 --