Patent GB-445711-A (GB)
Title: Improvements in and relating to the recovery of sulphur dioxide from gas mixtures
IPC: C01B17/60
Filed: 1934-10-16
Abstract: <PICT:0445711/III/1> Sulphur dioxide is recovered from gases containing it by contacting the gases with a solution containing basic aluminium sulphate, the sulphur dioxide being expelled and the absorbent being regenerated by heating and/or reduction of pressure, with or without contact with steam or air.  Preferably, the excess of Al2O3 over that required to form the normal sulphate expressed as a percentage of the total Al2O3 should lie between 35--50 per cent. The solution may be prepared by adding lime, chalk or calcium sulphite to a solution of aluminium sulphate and removing the precipitated gypsum, by roasting aluminium sulphate to remove a portion of the sulphur trioxide and dissolving the residue in water, or by dissolving excess of pure metallic aluminium, preferably amalgamated, in dilute sulphuric acid.  When the initial gases contain oxygen, the accumulation of sulphate in the circulating liquors and consequent decrease in basicity can be controlled by the addition of chalk, lime or calcium sulphite to precipitate gypsum, by the addition of barium carbonate to precipitate barium sulphate, and/or by the addition of metallic aluminium (with or without mercury).  A portion of the circulating liquor may be treated as above, the treatment being sufficient to restore the basicity of the whole solution to the desired value after the return of the treated portion.  In the treated portion the excess of Al2O3 should not rise above 55 per cent.  The solution in addition to basic aluminium sulphate may contain neutral salts such as magnesium sulphate, or small amounts of iron or heavy metal salts.  After continued use, precipitation of insoluble basic aluminium sulphates is liable to occur.  This may be avoided by one or more of the following means, by avoiding the maintenance of the solution for long periods of time at a raised temperature, e.g. above 70 DEG  C., especially when the solution has been freed from sulphur dioxide; by avoiding the presence of alkali metal salts in the solution; by adding to the solution certain substances such as phosphoric acid and/or glycerol or glyceryl phosphates or according to the third Provisional Specification, ferrous sulphate; by periodically or continuously removing solid particles from the solution by filtering, settling or centrifuging or by regenerating the absorption liquor by heating it while in a non-stagnant state, e.g. in a film evaporator.  As shown, gases containing sulphur dioxide pass up towers 2 and 4 in sequence, the stripped gas leaving at 6 while fresh absorbent liquor is fed in at 7 passing through the towers in countercurrent to the gases and through an intermediate cooler 9.  The spent liquor withdrawn from the tower 2 by means of a pump 12 passes through a heat-exchanger 13 and pipe 14 to a tower 15 into which it is sprayed meeting a mixture of steam and sulphur dioxide from a boiler 16.  The liquor on leaving the tower passes to a film type boiler 16, steam-jacketed at 18, the liquor flowing down steam heated elements to a sump 19. Concentrated sulphur dioxide containing some steam passes off at 21.  On leaving the sump 19 the liquor is cooled by admixture with cool liquor from a pipe 22 and then passes via pipe 24 to the heat-exchanger 13 for further cooling.  It is then pumped by pump 26 through a cooler 27 for introduction into the tower 4 at 7. The apparatus should be constructed of non-corroding material, e.g. steels of low carbon content containing chromium nickel and molybdenum with or without a little titanium.ALSO:In absorbing sulphur dioxide from gases by means of solutions of basic aluminium sulphate and recovering the sulphur dioxide by heating and /or reduction of pressure, the apparatus may be constructed of steels of low carbon content containing chromium, nickel and molybdenum with or without a little titanium a suitable steel containing: carbon not more than 0,06 per cent, chromium 17,5-19,5 per cent, nickel 8,0-10,0 per cent, molybdenum 4,0-4,5 per cent and titanium not less than 0,25 per cent.