[1]Patented Apr. 1% 1933 lt9O4tl7O UNITED'STATES PATENT OFFICE GEORGEA.RICHTER,OFBERLIN,NEWHA PSHIRE,ASSIGNOUTOBROWNCOXPANYOP EFRLIN, NEW HAXPSHIRF., A CORPORATION 07 XAINE CYCLIC PROCESS FOR THE PRODUCTION OF SULPH3[TE PULP Application Itled Tuly' 14,1925., Serial No. 43,481. This invention lias relation to the produetion of sulpliite ptilp and has for its salient object to provide an efficient an(i cyclic prbcess for such pulp productioii. In attaining i, this object, this invention provide@,3 for the utilization of the combustible content of the waste liqi)or for the generation of steam and power, for the recovery of the mineral or inorganic content of the waste liquor in the form 10 of chemicals which may be utilized in the formation of fresh sulphite co6king liquor, and for the production of such fresh cooking liquorinahighlyefficientmanner. TheproeeSS9 thus, also elimidates the necessity for the 15 pbllution of rivers and streams by the discharge of waste or spent liquor thereinto, a nuisance thereby being avoided which is practically always present in the manufacture of sulphite pulp so-called. Other advantages 20 attained in this invention will appear as the description proceeds. For the cooking of the wood in the liberation of the cellulose fiber according to the present invention, I employ an acid sulphite 25 liquor, comprising a soluble sulphite,preferablv sodium sulphite, and free SO, in thP, form Of .1 sulphurous acid. Other soluble sulphites such as potassium, lithium, zinc and magnesium, while ordinarily commercially niore 30 nsive than sodium. sulphite, may also be expe employed. Moreover the acid c-ooking liquor iiiay C'ontain varying proportions of combined and free S02 according to the character of the pulp which it is desired to prodiiee or 35 to economic expedienev in the production of the liquor. For example, in the prelparation of a pulp characterized bv its higli st-rength and pentosan content, as s"et forth in my applica.tion for U. S. Letters Patent Serial No. 40 7491,564, filed No. 12, 1924, a "4-4" acid liq-, uor, that is, a liquor containin(y about 4% f ree 47, combined Ln S02, may be employed. ' Or, if desired, an acid liquor cont-,.iining 8 @' , combined and 4,7o free, or a liquor having the 45 usiial S02content o@ Ca (HSOz@) 2.cooking liqtior and containing 17o combiiied and about 5% free S02 may be I)rel)ared. The wood to be treated is placed in a dige.,;ter whicli iiiay bB of the s-anie character 50 as that ordinarily employed in th,e manufacture of sulphite pulp. The aeid sulphite liquo r mty be heated directly, as by injecting steam into the digester, or if an indirect or "slow" cook is desired, i-n which case the coo' @ing liquor is not diluted by the steam con- 55 dens ate, the liquor may be heated indirectly as by steam coils disposed in the digester, or b y eirculatin@,r the liquor from the bottom of the dige@@tei ihrou(rh a heater, wherein it is heated by steam coils or other suitable 60 he@ ting surface, t,o the top of the digester. Afte r the cooking operation has been complete d, as ascertained by tests made in the usua l way from time to time , the digester is blow n into a blow pit and gases arising from (35 the discharged mass are passed through a cond enser wherein the steam is condensed by dire ct contact with watei@ and the chilled gase s comprising SO, pa.ssing from the condens er are utilized in the preparation of fresh 70 cook ing liquor. Th e spent liquor which is separated from the pulp and which contains sodium sulphite too,,,ether with combustible organic compou nds and spent sodium in the form of or- 75' (rani c sodium compounds is now treated in a LI manner to regenerate such e-ompounds economically in a form for utilization in the preparation of fresh acid cooking liquor. In such treatment, the waste liquor is evaporat- 80 ed to a certain degree of concentration and Is then delivered to a retort wherein its combusti ble content is burned or consumed and its inorganic mineral content smelted in a redu cing atn-los@@here and recovered in suit- E-5 able form for subseqiient treatinent for the prod uctioln of fresh acid cooking liquor, that is. substantially in the forrn of sulphite and carb onate. The sm.elt is dissolved in water and carbo,nated, as by bubbling carbon diox- 90 ide tbrough the aqueoug solution, thereby liberati ng hydro,-en sulphide, and producing a sodi um carbonate liquor which is then ae-idified with sulphur dioxide, @roducing a fresh 5 acid coaking liquor. 71-ie carbon dioxide lib_ 6rate d in the acidification of the carbonate liquo r is recovered and employed in the carbonation of other smelt liquor; and the hydro,- en sulphide liberated in such carbonaioxi 100 tion is recove,red, burned to!@ulphur d de
[2]2 2,904,170 and utilized in the preparation of the fresh acid.cooking liquor. The lo-ss of sodium constituent in the cooking operation and recovery cycle is made up by the addition of the nec?ssary sodium constituent in the form of sodlum compounds, e. g., sodium carbonate, in the treatment or neutralization of the spent liquor, for the production of the fresh acid cooking liquor; @nd the loss in suldhur di20 oxide is made up by the gases produced by the combustion of sulphur or sulphur-bearing@materials. In fact, after the process is in operation, practicallv the only raw materials which are needed for making up losses. 15 aye sulphur or sulphur-bearing material, and either sodium sulphate, sodium sulphite, sodium carbonate, caustic soda, or sodium salts Of a weak acid ' The invention may best be understood froirl 20 the following more detailed disclosure thereof when considered in conjunction with the accompan--ving dmwing, in which I have illustrated -aiagrammaticallv an'd conventionstlly var@ous instrumentalities employed in 25 the pro-cess.' Referring to the drawing, at 1 1 have ir)dica,ted the usual digester in which the cooking of the woocl chips (spruce for example) may be offected. In carrying out the fiber libera30 tion, the digester first is filled with wood chi-Ps and the necessary,quantity of freqh acid coo'king liquor (e. g.. sodium sulphite e-omprising, sa.y, 1% combined and 57o free SO,, though the p@oportion of combined and free 35 SOI, ma-.v be varied according to requirements) is theii pumped into the diaester. ,The digester cover is then tigbtly bolted or clamped on to the digester and, if a direct cook is beino, practiced, stp-am. is slowl-.v ad40 mitted into the digester until the temper . a,t-ure and pressure desired in cookin-v is reached, whe@eupon the cooking is continued for the requisite time period, tests being made in the iisual manner until the cooking or fiber liberationhasbeencomi)leted. Duringthecookin.,operation, the digester is relieved as ordi-. narily, from time to time, and the relief gases are utilize(I for @ a purpose subsequently exrio plailled. After the, coolicing operati'on' is completed, the contents of the digegter are blown through a blow pipe 3, valved as at 4, into a blow pit indicated at 2. The blowing operation'is accompanied by the formation and liberation of large quantities of steam or vapor a-nd sulphur d-oxide. . These vapors and gases are conducted from the blow pit through the c-onduit or stack 5 into the lower port-ion of a direct condenser or "blo-w pit tower" 'indicated as a whole at 6. This 60 eondenser I may be substantially like that illustrated in U. - S. Letters Pat@eiit No. 1,469,958, granted to me October 9, 1923, and may contain a.mass of inert interstitial mit65 terial, such as spiral br'icks, rocks, or the like, so that the vapors and gases are broken u' p into multitudinous streamlets in @ass' mg Upwardly through the interstitial 'material. Fresh cool water provided t,hrotigh a valved pipe 8 is sprayed into the top of the condenser and passing downwardly ihrough the inter-' TO stitial material therein, condenses the water vapor and cools the gas. Preferably the flow of water is so controlled that it leaves the condenser through a pipe 15, preferably discbarging into the blow pit, at a temperature slight-ly less than its boiling point and thus containing only a slight amount of sulphur dioxide. The cooled gases containing about 30@'o to 60@lo SO, are conducted from the top (f the condenser tower by a pipe 9 and may be utilized in the production of fresh cooking liquor, as will be explained. As the spent sulphite liquor -associated with the pulp in the blo@w pit is still acid, in order to avoid the necessity for a cid-proof apparatus in subsequent on, it i!s preferable to neutralize the to a point of slight alkalinity by an and, for this purpose I preferab'ly ut e an alkaline sol'ution of caustic soda'or sodium carbonate, or any other sodium salt of a weak acid, necessary to compensate for the losses encountered in the entire cycle, or if more alkali is necessary fo'r the neutralization than required for make_up losses, it is furnished from a point in 0.5. the cycle her@inlifter described.. In carrying 6ut the neutralization, the sulphite stock together with its associated spent liquor is passed from the blow pit 2 through an outlet pipe 15a into a tank 16 whieh I may term 130 for'convenienco, the neutralizing and pulp storage tank, into which the neutrajizing. liquor is fed through a pipe line II. It is now necessary to separate the pulp from the spent liquor and to wash th6 pulp 105 substantially free of liquor and entrained products of reaction prior to passage through the usual screen which are employed for refining.the pulp and removing the so-called screenings therefrom, which consist of knots, shives, specks, particles of brick, cement, etc. For this purpose the pulp and waste liquor are forced by a ump 12 through a pipe 13 to a washing maceine consisting of a seri-es of pulp washers indicated conventionally at 18. '15 The washing machin,e is preferably of the countercurrent type substantially like that illustrated, and a@scribed in U. S. Letters Patent No. 1,421,664, to Brown, Martinson, 'Moore & Park@6r, dated July 4,1922, although if desired the ordinary rotary vacuum type of washer may be em ioyed. In the washer of ihe patent referretto, the pulp is carried continuously throuoh tanks in one direction and t-he washing liquor is carried successively through such tanks in the opposite direction. The wash water is passed through a pipe 20 t6 the pulp delivered to the last washet of the series from which the washed Pulp is delivered. The washed,pulp.may then be de85 120
[3]1,904,170 livered to the usual screen (not shown) by esea es from the furnace. As indicated, an a conduit 10. The wash water containin 9 substantially all the spent liquor from the pulp'is discharo,,ed from th(, first pulp washer o@ the series -by a valved conduit 21 into a spent liquor storage tank 22 prior to the concentration thereof, from which it is drawn at tho-,desired rate fo@ evaporatihon and concentration. For the purpose , ave illustrated 13 conventionally at 23 a seres of multiple effeet evaporators, the details of construction of which it is unnecessary to describe. M7hile 1,, preferably employ multiple effect evaporators, any other form of evaporators for 1-- removing a part.of the aqueous content of the liquor may be utilized. The liquor is drawn from the tank 22 by the pump 90 and delivered through a pipe 91 into the first effect of the multiple effect evaporator. 2i From the last effect the,liquor, which has been concentrated, say, to, about 16' Baum6, and which contains about 257o solids, is passed through a pipe 93 into a storage tank 24, from which it is withdrawn for the com23 bustion'of the combustible content and the recolvery of the inorganic compounds in the for@i oi sulphide and carbonate. Th6 concentrated liquor in the tank 24.is now subjected both -to heat and pressure to 30 aenerate an explosive force therein, substantially as described in U. S. Letters Patent No. 1,326,414 to Moore & Quinn dated December 30, 1919. To this end the concentrated liquor is. drawn from the tanl@ 24 --5 through a,conduit 25 provided with a valve as at 28, by a pump 26, and it is, circulated through a circulation system comt)rising the pipe 27 and a heater 29. The liquor iii passing under pressure through the circulatino, @ystem is heated to a temperature above the oiling poiiat by the heater 29 and is delivered from the eirculatina system through a pipe or serie:s of valved pipes 30, terminating in nozzles 32 extendin(r into a coiiabined 7i boiler furnace and retort indicated as a whole at 31. In the present instance it i desired that the sodium compounds recovered from the furnace should contain sodium sulphide and sodium carbonate. If this recovery takes pla.ce in a, reducing atmosphere there is no difficulty in recovering the desired salts, although there m@.y be a slight accidental amount of adveiititious sodium sulphate and sodium sulphite. One may, however9 OPetC3 ate the furnace as most convenient, as the small amount of sodium sulphate is a harmless constituent and the sodium sulphite "S not at all undesirable. The furnace as shown is provided with a ci- floor 33 which preferabl.@, extends rearwardly and downwardlv @rom the front of the furnace into which project. the nozzles 32 for the ernission of the spent liquor. The floor at the rear w I all of the furnace is provided C5 with spouts 35 from which the molten effluent expf (osive force is generated in -the waste. liquo r which is then disrupted and its combusti ble content burned and its inorganic mine ral content sinelted in the reducing at- @3 mos @here of the furnace as it leaves the noz z es. The molten effluent from the furnace,'consistin g essentially of sodium sulphide and sodi um carbonate, is delivered from the spouts 75 35 to a di@splving tank 36 and is dissolved in the desired @Lmount of water supplied to that tank at se asonable temperatures from a suitable source through a water conduit 37 provide dwithavalveindicaiedat38. A-certain portio nof the alkaline smelt liquor may be emplo yed to neutralizethe spent liquor in the neutra lizing tank 16 when the alkaline solution containing sodium "make-up" compounds is insufficient for the purpose. In such case 85 the necessary amount of smelt liquor is aecordin lrly withdrawn from the dissolving tank a@d delivered through a valved pipe 96 into the pi.p@ 11, from which the neutralizing comp ound is discharged int-o the tank 16. While such procedure may be followed in certain - cases, owing to the'fact that tho smelt liquor reacts with, the free sulphur dioxide in the spent liquor to form sodium-,thiosulphate and perchance to precipitate sulphur, 05 such procedure is oiily optional, and I wi'l prese ntlydescribeapreferredprocedure. In the optional treatment, the spent liquor after evapo ra-tion is also reduced substantially to sodiu m sulpliide and sodium c@irbonate in the ,@iiielt ing retort, but there is also likely to be I)rese nt adventitious amounts of other compound s which ar6 not of advantage in the cool@ ing process. The molten effluent delivered from the fiirnace into the dissolving tank 105 I)rodu ces a hot alkaline solution which may contai n 6, certain a-mount of solid adventitious materi al. Such matter is preferably removed f rom the liquor by filtration prior to the carbonati on thereof. 110 T he carbonation is accomplishe I d either by bubbi.i g carbon dio:xide gas obtaiiied from any su able sotirce through the liquor 6r causing such liquor and carbon dioxide to pass in count ercurrent flow through a suitable tower 115 or series of towers 6ontainin,- inert interstitial iii@ter ial. 'The liquor containing this dissolved effluent from the combined boil@r and furnac e retort comprises, as has already been stated ' essentially sodium carbonate and so- 120 dium sulphide, and the carbon dioxide by reacti on with the sodium sulphide in solution conve r'ts the latt-er into sodium carbonate with the evo ution of hydrogen sulphide gas. In the present filstance, the alkaline liquor 12-5 is accordingly withdrawn for carbonation at the desired rate from the dissolving tank through a pipe 39, valved as at 40, b@y a circulating pump 41, and pumped successively througli a filter 44, and thence through a p' e 130 ip
[4]1,904,170 42 to the t6p of a carbonating tower indicated as a whole at 43 which is supplied with carbon dioxide at its lower portion. The tower is filled with inert interstitial material, such iLs spiral brick, iock, and the like, and thus offers a large surface of contact betweenthe alkaline liquor (*hich-might have a tendency of plugging LUP the interstices of the interstitial material if it were n6t filtered), and 70 the carbon dioxide. The carbon dioxide em-! ployed in the carbonation m'ay be waste gas, diluted with other gases, such as the ordinary furnace flue gas or the waste carbon dioxide gas obtained from lime kilns in the decom15 position of limestone in the fnanufacture of lime;.but, as waste gas is usuallv contaminated with soot or other like dirty ingredients which tend to contaminate the liquor and because of the low concentration of carbon di20 oxide therein, is not to be preferred in the actice of the process. Moreover, even if ,Phr 6 -as were scrubbed and cleaned, because of 'y the presence of a high pereenta ige of other inert gases, the hydrogen sulphi e liberated S5 in the carbonating proe ess would be diluted bv -q,he inert gases introduced in the carbonaiing system with the carbon dioxide, so that it is difficult to burn, and even when it -s subsequently burned in air an'd recovered , 09 in the form of sulphur dioxide, the sulphur dioxide is so dilute as to be of comparatively low value in the reparatioia oi fresh acia cooking liquor. 3;ence, according to @@,h-@ pres'ent invention I provide for the c,@)rbona@6, 0 tion of the alkaline. smelt liquor with clean :@nd substantially 100@'o carbon dioxide gas obtained ;_n the acidification of the sodium carbonate, as will presently be explained. By e@hploying such a, strong, clean carbon dide omide, a smaller carbonating apparatus is required due to the increased rate of carbona,W-ion, subsequent filtration of the liquor or purifi6ation and cooling of dilute waste are avoided, and a strong, substantially 100@10 '15 H,S gas is produced which may be burned in.atmospheric air to produce a sulphur dioxide gas which is almost as strong as sulphur burner gas and which may be employed together with the burnei gas in the acid system. 50 The strong carbon dioxide is passed, as already indicated, into the lower portion of the tower 43, is broken up into multitudinous streamlets by ihe inert interstitial mateiial, and comes into intimate contact with and in 55 countercurrent flow to the downwardky flowing alkalizie liquor distributed- at the top of the tower. The rate of feed of the alkaline liquor and the amount of carbon dioxide employed are controlled -by suitahle valves and @60 speed controlling devic . es for the pumps, so that a liquor which is substantially 100@o car-, bonated is produced without the introduction of excess carbon dioxide, and so thai the hydrogen sulphide which is liberated in -all parts of the tower and passes upwardly to the top, thereof is not diluted by excess carbon dioxide. The substantially 100,Yo hydrogen sulphide is conducted from the top of the tower through an outlet pipe 45 into a gas container or gasometer indicated conventionally at 46, from which it is drawn to be burned and recovered as sulphur dioxide. The hvdrozen sulphide is supplied from the gasometer at the desired pressure and velocity throu-h a gas pipe or pipes 47, provided 75 with iegi@lating valves as at,47a, and terrninating in nozzles, burners, or jets 48, ext,@nding into a gas furnace indibated as a whole at 50. The. hydrogen sulp@ide gas issuinl from the'nozzles is burned in the presence ol 80 air to form sulphur dioxide and water, the amount of air inttoduced into the furnace bp-ing accurately regulated for such combustion without the introduction of unnecessary diluting air. The resultant gas after cooling a -5 contains about 10,7o to 11@o SO, and may be commingled with the cooled and more or less purified sulphur burner gases, and utilized in the acid system, as will presently a@ppear. The sodium carbonate solution failing to go the bottom of the tower is now in a condition to be acidified with sulphur dioxide. As previously indicated, if the make-up alkaline solution in the neiitralizing tank is insufficient for the neutralization of th6 spent di- '-'s gesting liquor, alkali is provided from the cycle per se. The carbonate liquor is employed in assisting the neutralization, in pr@eference to the smelt liquor, and 7 have accordingly indicated a valved conduit 49- ZOO through which the portion of the liquor necessary for neutralization is bypassed from the main butl6t pipe line 53, to the pipe II, discharging n@utralizing liquor into- the neutralizing tank 16. The main portion of the 105 sodium cerbonate liquor, however, is withdrawn from the carbonating tower through the outlet pe 53 bv 4 pump 54 and conducted througtithe 55 to the top of an acidif@ing or "su@ipIrite" tower indicated as 110 a whole at 56. While, for simplicity, I have sho-@vn a single acidifying tower, it is to be understood that any suitable methods or means for the acidification of the sodium carbonate may be employed. For example, the 215 acidification may be effected as by bubbling sulphur dioxide through the aqueous solution of sodium carbonate or by passing the solution through two or more towers arranged'in series. Before ]?assage into the sul- 220 .@hiting tower, however, it may be desirable to bleed in a small quantity of chlorine into the pipe 55 through a pipe 55a to oxidize any adventitious small aniount of sodium Sulphide @ontained in the carbonate liquor due 125 to incomplete carbonation in the carbonating tower, to sodium sulphite or sulphate. This prevents the reaction of the sodium sul-; phide with sulphur dioxide in the acidifyinly or, sulphite tower to fprm sodium thiosui-'
[5]1,904,170 5 p@ate or the _precipitation of sulphur, which might give rise to objectionable mereaptan odors or discolor the pulp in the digestin 9 process. The sulphite tower is filled with inert interstitial material such as sp iral brick field rock, or the like, and the carbonate' liquor sprayed thereont6 passes dowilwardly through the tortuous passages in the interstitial mass, and comes into intimate contact 10 with sulphur dioxide passed into the bottolil of the tower'in countercurreiit flow tliereto. The sulphur dioxide emplued in the acidification of the carbonate may be derived frolii any suitable. source, as f6r example sulpliur 15 burner gas or blow pit gas,,but in the present case I preferably employ substantially 100@'o @ulphur dioxide gas obtained from th reliev. ing operation Of the sulphite digesters. Iii this connection, it may be stated that while I 20 have conveiationauy indicated one di,-ester, I desire to have it understood that the process is ordinarily practised where there is a battery of digesters in operation so that relief gas may be available practically, at all times. 25 The sulphur dioxide displaces the less strongly acidic CO@ radical in solution liberating carbon diox:ide gas in all parts of the tower. The substantially 10O To SO, gas is preferably so riegulated and coiatrolled that all the car30 bon dioxide is displaced from the carbonate solutionproduci'Lng a normal sulphite salt in solution coiatain'mg an immaterial amount of free SO,, since a free SO, content in such liquor may be subsequently produced by 35 weaker sulphur dioxide gas, as will be de. scribed. Since practically no dilute gas is introduced into the sulphite tower, the carbon dioxide liberated therein is also undiluted, and passes upwardly to the top thereof 40 and is employed in the carbonating tower 43. For this purpo@e the carbon dioxide is conducted from the top of the tower by the pipe 59 and is blown as by a fan blower 60 through a conduit 61 into a lower portion of the tower 45 43' In other words, the carbon dioxide serves as a darrier for the S02 to the sodiurn sulphide of the smelt solution and is again liberated as C02 to again pass through the same cycle. By omploying a substantially 10OTo 5o 8ulphur dioxide in acidification, the reaction with the carbonate solution takes place rnore rapidly, and a smaher acidifying apparatus is necessary than if a dilute sulphur dioxid(gas ivere employed in such acidification. 55 The relief gas from the dilrester employed in the acidification of the car-bonate liquor is -obtained, as indicated, during the relieving operation of the sulphite digester., The relief -gas, however, emerges hot and moist from 60 the digester so that provision must be made to separate the sulphur dioxide from the steam and entrained cooking liquo ' r prior to use in the acid system. On the drawing, I have conventionally indicated a relief pipe 65 95, provided with, a relief valve 96a throuih which 'the relief gases pass into, a trap or separator 97 wherem. the entrained digesting liq is separated fro'm the hot gas. The uor relief liquor" passes from the separaor tliro uoh the outlet pipe 62 and is discharged 70 In ilil,o ttie iieutilalizin- taiik 16 or the blow pit 2 for the recovery "of valuable ebilstituents. 'file hot alid ilioist relief gas passes from the sepa i,ator tlirougli aii exit pipe line 98 into aii iiidirect or surface eooler -indicatecl, as'a- 75 wliol e, colivelitiolially, as a double pipe worm or coil cooler 64, wherein the vapors are conueiis ed aiid the 6ulpliur dioxide cooled. Any other suitable cooling surf ace and method for cooling the reiiet gas may be employed, so as, for exaiiiple, the method and apparatus (tescri bed iii my application for U. S. f.,etters I'atelit Sci-ial i'@o. !46,363, filed April 28 1925 The cooled relief gas colisisting of substan-' tially 100@lo 80@ passes from the- cooler 64 es throuo , the c(,nduit 65 aiid divides into two branc hes, namely, the branch 66, valved as at 67 for supplyilag the acidifying tower with a predeterminec[ and regulated amount of strong sulphur dioxide as has beeii previously go indicat ed, and the branch 66a, valved at 68, for supplying the finished acid-making system with strong sulphur dioxide as will presently be -indicated. The strong normal soffium sulphite solu- 95 tion falling to the bottom of the acidifying tower is delivered to aiiother acidi:f3@ing sylstern wherein the sulphite liquor is again treate d with sulphur dioxide to build up the desire d free SO@ contejit therein. Any suit- 1.00 able system or method for acidification may be employed as by bubbling SO@ through the normal sulphite solution or by passing the solution successively through two or more towers arranged iii series. As shown, the 105 normal sulphite liquor is withdrawia :trom the acidifying tower through.the valved pipe TO and passed into the top of an acidifyirig. or "raw acid tower" 71, supplied with s,..@.- phur dioxide at its lower portion. The tower 110 'il is Jilled with inert interstitial or surface material and the liquor passes downwardly through tortuous or windiiig passages intimately contacting witli the sulphur dioxide passed upwardly in countercurrent' flow 115 thereto. The sulphur dioxide employed in the preparation of the raw acid liquor is preferably that obtained 1]D- the -combustion of sulphur or sulphur-bearing material and in the com- 120 bustion of the hydrogen sulphide from the carbonat ing tower, together with, if desired, the recovered blowpit sulphur dioxide gas. For this purpose, as indicated on the'drawing, sulphur or sulphur-bearing material is 125 burned as in a rotary sulphur burner 72, and the products of combustion pass therefrom thr6ugh a combustion chamber 73, wherein the conversion of the sulphur vapor to Sulphur dioxide is completed-to produce a gas 130
[6]i,904,170 comprising about 16@o sulphur dioxide, and thence through a cooling and scrubbing system indicated conventionally as a whole at 74 wherein the gases are more or less cooled and 'fled. The cooled burner gases are dra Uri wn Yirlom th6 cooling system through a conduit 75 by a blower 76 and forced through a e,)nduit 77 into thi@ lower portion of the raw acid tower 71. @10 Similarly, the gases resulting from the combustion of the hydrogen sulphide in the ,gas furnace 50 pass therefrom through a conduit 78 and a suitable coolin,g surf ace, as fol. example a double-pipe type worm cooler @is indicated conventionally at 79. The water va]?.or contained in the burned hydrogen sulphile gas is condensed resultin to I g in a 10@o 117o 90, gas which is drawn rom the cooler by a f o.n 80, and forced through the conduit -go 81 into the conduit 77 and into the tower 71 along with the sulphur burner gas. At ordinary temperature and atmospheric pressure, by re lating the flow of gases from the sulphurtlicurner, the hydrooen sulphide furnace, '25 an(d the gas obtainable :rrom the blow pit, and also the rate of flow of the normal sulphite solution delivered at the top of the tower 71, with a normal sulphite liquor containing 4 @'O combined SO,, a raw acid liquor containing 30 6@'o to 79'o , free 80@2 can be produced, and "With a,@ulphite liquor containing 17o combined S02, a raw acid liquor conta 'ng 2.5 @'o ln - , t free SO@@ can be produced. Any inr gases Fnabsorbed by the norinal sulphite solut',ion 3u, in the tower 71 are allowed to escape through --n outlet or vent 82. In certain cases it may be desirable to cook in an acid liquor having a very high free so@ content. -'In such cases 9 the raw acid co liquor produced in the tower 71 is withdrawn therefrom and passed -to a f ortifying system wherein the liquor is suitably treated with concentrated sulphur, dioxide. To this end thie liquor is conducted from the lower por7 -;15 tion of the tower by an outlet ]?ipe 83 and A loreed by a pump 84 through a pipe 85 to the top of a fortifying or finishing tower, indicated as a whole at 86, supplied with substantially 100@'o SO@ at its lower portion, as from 5o the branch line 66a previously referred to. The tower is filled with inert interstitial or surface material such as spiral brick, field rock, or the like. The relief gas may be commingle,d, if @desired, with a portion of the 55 recovered blowpit gas from the pipe 9 not required by the raw acid liquor system. The downwardly flowing raw acid liquor comes into intimate and c'ountercurrent contact with the upwardly flowing relief gas over 60 the large surf ace of interstitial material for a sufficient period of time to produce an acid liquor containing a maximUM S02 content at atmospheric pressure and ordinary temperature. Any uliabsorbed as at the top of the 60 relief tower is bypasseg through a conduit 87 and bled into the conduit 81 dischargirig into the raw acid tower. With a raw acid liquor cont-aining about l@'o combined SO,, by regulatirg the flow of relief gas into the tower and the amount of raw cid distributed 70 a at the top of th,e tower 86, a finished cooking acid liqiior containing 57o free SO,, may be produced, and similarly with a raw acid liquor containino, 47o combined SO@, a finishea acid coo@ing liquor Containing 7% -75 to 8@'o free SOI! may be produced. The finished acid liquor falling to the bottom of the tower 86 is withdrawn through an outlet pipe 88 and inay be delivered into storage tanks from which it ma be pumped for use 80 iri the dizesters. y Befor@'concluding it may be stated that in a plant where a "4-4" acid, that is, an acid containing 4@'o combined and 4@'o free SO,, is employed or, in any other case, where the $5 free SO, is eqtial to, or less ihan, but not in excess of the combined SOf!, the necessity for an acid finishin- tower is eliminated. i-tforeover in such cases 7 the necessity, for cooling the liqiior @ or the sulphur dioxide gas from 90 the various sources is also eliminated, and the sulphur dioxide zases from all the sources may be c 'e hot state and passed into ombined in the raw acid liquor tower, the amount of sodium carbonate liquor introduced into the 95 sulphite tower and the rate of flow of the sulphur dioxide gas passed thereinto being controlled and regulated to produce kn acid sulphite liquor of the desired strength. Such procedure is possible, owing to the fact that IGO sodium bisulphite is a stable compound and there is a tendency for its formation in solution everi. at an elevated temperature. Th-as, in such cases, less apparatus is needed than where a hi2-her free than combined SO, con- 05 tent is necessary in th@ finished acid cooking liquor6 Moreover, where a "4-4" or other acid cookiiig liquor having a hilrh combined SO, content is employed, a- substantial amount of spent,liquor and all the relief 110 liquor miy be reemploy-ed in digestion of other raw cellulosic material, since these liquors contain substantial amounts of unspent combined and free SO@, thus arowing a conservation of heat and chemicals to be prac- 215 ticed in con'nection with the process. However, as stated, the maximum amourit of free SO, is determined by the formula of the sodium acid sulphite: that is, the maximum free SO, conte@nt obtainable is equal to the 120 combined S02, plus the slight solubility of the.SO, at that particular elevated temperature. At the risli: pf repetition, it may be stated that while I ha@ve confined myse.1f to a niore 125 or less detailed description of a preferred procedure and a preferred apparatus theref or 5 this was by way of illustration only; and that this invention is capable of various changes and modifications which axe, within 130
[7]1,004,170 its spirit and scope as defined bv the appended clainis. It may be also ajain stated that the drawing is diagrammatic,and conventional, no attempt being made. to show the relative sizes or true location of the various instrumentalities. For the sake of clarity in the drawing, certain d@tails of construction have, been omitted, but it will of course be understood that valves for the pipe 10 lines, pumps, and fans for maintaining flow or circulation through the system, storage tanks, or any other construction will be supplied where necessary. 15 What I
