Patent GB-1348548-A (GB)
Title: Pollution controlled polysulphide recovery process
IPC: D21C11/04
Filed: 1971-08-03
Abstract: 1348548 Polysulphide pulping process; sodium polysulphides; sulphur COMBUSTION ENG Inc 3 Aug 1971 [3 Aug 1970] 36352/71 Heading C1A Spent liquor containing Na 2 S and Na 2 CO 3  in a polysulphide pulping process is (a) washed from digested pulp in an oxygen free atmosphere (20) (Fig. 1), (b) freed from soap by the addition of sodium bicarbonate (44) the spent liquor being then (c) carbonated with a CO 2  containing gas stream (50), (d) stripped of H 2 S by a flue gas containing CO 2  and inerts (52), (e) contacted with a free oxygen containing gas stream (54), (f) treated with NaOH to make up sodium content and (g) processed by steps including evaporation and burning (57, 58, 60) to produce a white liquor containing NaOH and Na 2 S, the white liquor is then being reacted with sulphur 84 to produce a polysulphide cooking liquor whereby sulphur is produced by collecting the effluent gas streams from steps (c), (d) and (e) into a combined gas stream which is then contacted with a liquid catalyst (76) whereby H 2 S is oxidized to sulphur by means of oxygen present in the stream, the sulphur being separated off (78). In (c) Na 2 CO 3  is converted to NaHCO 3  which reacts with Na 2 S to form H 2 S, in (d) the CO 2  reacts with any Na 2 CO 3  present to produce additional NaHCO 3  and in (e) residual NaSH is oxidized to Na 2 S 2 O 3  and NaHCO 3  decomposed to Na 2 CO 3 , the effluent stream containing residual oxygen. (I) White liquor production.-Washing may be carried out in a closed hood apparatus (cf Fig. 2 in the Specification). The spent liquor from the washing operation may be pre-evaporated in 38, which may consist of the last four stages of a 6-stage multiple stage evaporation system (cf Fig. 3 of the Specification), to a 25-30% dry solids concentration. The NaHCO 3  added to the soap skimming operation in 44 may be recycled carbonated spent liquor (e.g. 50-400% of main liquor stream may be recycled) from 50. Carbonated liquor may be also recycled to the inlet of the carbonating tower 50. Lime kiln flue gas or chemical recovery furnace flue gas may be used in the carbonating tower 50 and stripping tower 52. White liquor from 67 may be recycled to further augment the pH set up by the NaOH added on make up to ensure that lignin is not precipitated during the subsequent evaporation steps in 57, 58 and 60. Any Na 2 S present in the white liquor may be oxidized to Na 2 S 2 O 3  in the oxidizing tower 56, some main liquor being added to the recycled white liquor stream prior to 56 to act as catalyst. Liquor from 54 may be evaporated in 57, e.g. the first 2 stages of a 6 stage multiple stage evaporator, to a concentration of at least 50-55% dry solids and then in a direct contact evaporator 58, e.g. a cascade evaporator employing flue gases from the recovery boiler 60 or air heated by heat exchange with the flue gases to about 70% dry solids. White liquor may be made in conventional manner from the recovery furnace 60, e.g. as in the Kraft process employing the steps denoted by 62, 64, 66, 68, 70, 72, 74. (II) Sulphur recovery.-The collected gases from 50, 52 and 54 which may be combined with those from 18, 38 and 56 containing 1-3% H 2 S and 3-10% O 2  possibly with NH 3  for adjustment of pH, may be passed to 76 which may be at 170‹ to 190‹ F. The liquid catalyst may be Fe(X)Cl 3  where X=amino complex chelating group or a Na 2 CO 3  solution containing K 3 Fe(CN) 6  or sodium dithioarsenate solution. Some of the liquid catalyst and sulphur leaving 76 may be injected with steam and the resultant molten sulphur separated off at 78, steam being flashed from recycled liquid catalyst at 80. The sulphur from 78 together with molten make up sulphur may be subjected to cooling, prilling and crushing in the sulphur preparation section 82 which may include recovery means for any dimethyl sulphide present. (III) Polysulphide liquor production.-Sulphur along with white liquor may be metered along to 84 to produce polysulphide Na 2 S n+1  (n normally 1-4). Polysulphide may also be produced by reacting the molten sulphur directly with the white liquor.