Patent GB-748479-A (GB)
Title: Improvements in or relating to method of operating a fuel-fired calcining apparatus
IPC: B01J6/00
Filed: 1954-02-17
Abstract: <PICT:0748479/III/1> <PICT:0748479/III/2> <PICT:0748479/III/3> A fuel-fired calcining apparatus is employed to carry out the process of the parent Specification, wherein bath values which have become contaminated with carbon during aluminium reduction cell operation are treated to recover cryolite and alumina by pre-heating the contaminated bath values in particulate form in contact with a carbon dioxide-containing gas at a temperature below the fusion point of the contained sodium hydroxide (about 600 DEG F.) to convert all the hydroxide to carbonate, and heating the material at a higher temperature below the fusion point of the cryolite and alumina for a period sufficient to remove free and combined carbon.  The calciner may be a multihearth furnace, combined or not with a flash roaster, or a fluidized bed furnace. Fig. 1 shows a multihearth furnace of conventional type with air cooled rabble arms.  The material passes successively over hearths 11 to 19, heated by fuel supplied at points 30 to 33.  Combustion products are withdrawn through regulated ducts 34 and 35.  The conversion of hydroxide to carbonate occurs on the upper hearths 11 to 14 in the presence of carbon dioxide resulting from the combustion of fuel injected at 30, the temperature of hearths 11-14 not exceeding 600 DEG F.  Water is also introduced at 30 to react with any contained carbides or nitrides.  The lower burners 31 and 32 are supplied with fuel, water and air to produce a carbon monoxide containing gas which is drawn downwards to leave at duct 34, the carbon monoxide content being burned by extra air introduced at burner 33 to provide extra heat.  The temperature of the material in the lower hearths 15-18 rises to below 1150 DEG F. and hearth 19 is at 1150 DEG F.; the lowermost hearth 20 may function as a cooling zone.  Alternatively the flash calciner of Fig. 2 may be used wherein the material having been subjected to the first stage heating on hearths 51 to 54, is passed through pipe 66 to be entrained in an air stream from blower 69 for injection, preferably tangentially, into the space 64 of the flash calciner. This space is supplied with fuel and water, again preferably tangentially, at points 72 and the material leaves at 74.  As in Fig. 1, combustion products are withdrawn through ducts 75 and 81. In addition combustion products from the flash calciner 64 are removed by a pipe 76 and fed via valve 78 to the first stage heating zone at 79 thereby providing heat and carbon dioxide for the conversion.  Fluidized bed operation is shown in Fig. 3 where the material passes successively through fluidized beds 101 and 102 where the carbonate conversion is effected, then by down-comer 109 to to a calcining bed 103 and finally to a cooling bed 104.  Fuel is supplied to the calcining bed at 121 and fluidizing air for beds 104 and 103 from blower 113. Combustion products are withdrawn from above the bed 103 through line 123 and after passing through a dust settler 124, pass in part through pipe 127, heat exchanger 129 and pipe 144 to waste, and in part through pipe 125 to supply heat, carbon dioxide and some of the fluidizing gas for the upper beds 102, 101.  A portion of the cooled gases emerging from heat exchanger 129 are drawn off at 134 and passed, together with air, through blower 130 to fluidize the beds 102, 101.  Gases are removed from above bed 101 through dust collector 141 and pipe 143.  When the material treated is spent cell linings or cell skimmings the material after calcination contains excess alkali, and aluminium fluoride must be added to the calcine to form cryolite.  Such addition may be made to the reduction cell bath.