US2385955A - Manufacture of sulphite pulp - Google Patents

Description

Oct. 2, 1945.

G. H. TOMLINSON MANUFACTURE OF SULPHITE PULP Filed April 8, 1941 4 Sheets-Sheet l as@ ESE Oct. 2, 1945. G. H. ToMLlNsoN MANUFACTURE OF SULPHITE PULP Filed Apr-11 8 1941 4 Sheets-Sheet 2 Oct. 2, 1945. G TOMLINSQN 2,385,955

MANUFACTURE 0F SULPHITE PULP Filed April 8, 1941 4 Sheefcs-Sheet I5 .ITF ll' ATTORNEY.

Oct. 2, 1945. G. H. ToMLlNsoN MANUFACTURE OF SULPHITE PULP 4 'sheets-sheet 4 Filed April 8, 1941 MAX/MUM HMPERA TURF 0F ASH "/f.

1N VENTOR. @6o/ge H, Tm/'nsdn ATTORNEY.

Patented Oct. 2,y 1117117945 RLIANUFACTURE F SULPHITE PULP George H. Tomlinson, Westmount, Quebec, Canada Application April 8, 1941, Serial No. 387,474

13 Claims.

The general object of my invention is the proi vision of an improved system of manufacturing pulp from cellulosic fibrous material using a relatively pure magnesium base sulphite cooking liquor. A further object is the provision of an improved self-supporting cyclic system of recovering chemicals and heat from, and completely disposing of, the residual pulp liquor in a process of the character described. A further and more specic object is the provision of an improved process of treating the residual pulp liquor to recover magnesium and sulphur in a form permitting their economic reuse in the pulping process, and heat'in economic quantities.

'Ihe various features of novelty which characterize my invention are pointed out with partlcularity in the claims annexed to and forming a part of this specification. For a better understanding of the invention, its operating advantages and specific objects attained by its use, reference should be had to the accompanying drawings and descriptive matter in which I have illustrated and described a preferred embodiment of my invention.

Of the drawings:

Fig. 1 is a diagrammatic flow diagram of a cyclic sulphite pulp liquor recovery system embodying my invention;

Fig. 2 is a view similar to Fig. 1 in the form of a material balance showing the daily quantities involved in a plant having a production of 100 tons of pulp per day;

Fig. 3 is an enlarged sectional elevation of the recovery'furnace and boiler shown in Figs. 1 and 2: and

Fig. 4 is a curve sheet showing the relation between time, temperature and mean reactivity of the ash produced.

The cyclic recovery system shown in Fig. 1 is of the general character disclosed in my prior applications Serial No. 186,938, filed January 26, 1938, now U. S. Patent No. 2,285,876, and Serial No. 221,304, filed July 26, 1938, now U. S.,Patent No..2,238,456, of which this application is a continuation-in-part. InA such cyclic systems a cooking liquor consisting of a relatively pure acid g sulphite compound of magnesium, i. e. magnesium bisulphite, with an excess of sulphur dioxide, is supplied toa digester I0 from a cooking acid tank II. 'I'he digester I0 is shown as being equipped for indirect heating, permitting the Baume of the residual liquor following cooking to bemaintained at a higher value than would be possible if direct steaming were employed. A more economical evaporation of the liquor to the desired concentration for combustion is thus made possible.`

fWhen the cooking operation is completed the contents of the digester are discharged into a blow pit I2 from which gases are vented and the pulp and liquor pumped to suitable pulp washing equipment, such as rotary vacuum iilters I3 and I4 arranged in series.

Wash water from a hot water tank I5 is delivered to the pulp washer I4 as indicated and the filtrate discharged into a tank I6, from which a portion is delivered to the pulp washer I3 to serve as wash liquor therein. The filtrate from the second washer I4 is also employed to insure the correct consistency of the pulp going to that washer by returning a portion of the filtrate to the stock box of the washer. A further advantageous use of the ltrate is as a basis for the magnesia suspension employed in the absorption towers, even though this results in the circulation of a small amount of inert solids in solution in the total system, as the filtrate thus forms the basis of subsequent cooking liquor. The lltrate from the washer I3 is sent to an acid Waste liquor tank Il from which a portion is returned to the stock box of the washer I3, and another portion to the blow pit IZ to increase the fluidity of the pulp and thus facilitate its delivery to the pulp washers. Substantially all of the liquor and washer filtrate is thus maintained in the system, the only loss being that carried out by the pulp leaving the second washer I4 for subsequent treatment in the knotters, riiliers. bleachers and dryers.

The residual liquor in the tank Il will have a solid content of approximately 10%. This liquor is advantageously utilized as the wash liquor in a gas scrubber or spray tower I8 which receives the heating gases generated in the chemical recovery furnace, after a major portion of the solid chemicals have been separated therefrom, and prior to the absorption towers. 'I'he spraying of the liquor into intimate contact with the hot gases effects the liberation of free sulphur dioxide in the liquor which upon its liberation is carried with the heating gases for recovery in the subsequent absorption towers. It also results in the recovery of most of the small amount of solid chemicals present in the gases coming to the scrubber, and the magnesium oxide so recovered is used for the partial neutralization oi' the acid liquor. The amount of ash present in the gases can be controlled by varying the operation of the cyclones. Some evaporation of the'weak-acid liquor also results, reducing the amount of concentrating to be done in the evaporators and thus saving steam.

The partly neutralized and partly concentrated residual liquor is then pumped from the scrubber and the neutralization is then completed by the controlled addition of magnesium oxide to a neutralizing tank i9 and delivered to the neutral waste liquor tank 20. The neutralization is either completely or very largely accomplished by the use of recovered ash, but as indicated in Fig. 1, if magnesia is used as make-up to the system, the fresh magnesia may be introduced at this point. If magnesium sulphate is used as make-up to the system, this is preferably added to the liquor following its concentration, so that the inert sulphate will be reduced to highly reactive magnesium oxide on passing through the recovery furnace, and thus become immediately eilective as active chemical. If the sulphate were added to the cyclicsystem subsequent to the furnace, it would be an inert burden in the first batch of cooking liquor of which it formed a part. The acid residual liquor is highly corrosive,vhaving a pH of 2.5 to 3.0, and unless neutralization is efe fected the equipment handling the liquor must necessarily be made of relatively expensive corrosion-resisting alloy. Neutralization of the acid residual liquor prior to the delivery of the liquor to a multiple eil'ect evaporator is also important in that it eiects the combination of the free SO2 present, thus preventing its liberation in the evaporator, a condition which would adversely aifect the vacuum. The neutralizing magnesia dissolves readily, producing a slightly alkaline liquor hav ing a pH normally in the range of 7.2 to 8.5. No precipitation of organic or inorganic matey rial has been found to occur even at a pH of 8.5.

Further concentration ofthe neutralized residual liquor to a concentration of 50-60 percent solids (3140 Baum) is economically desirable, and this is attained by passing the neutralized liquor from the tank 20 to a multiple effect evaporator system shown as a six-stage evaporator 2|, in the rst stages of which the liquor is heated by exhaust steam from a back pressure turbine Per cent Free moisture 45.0 Carbon 25.04 Hydrogen 2.41` Sulphur f 2.751 Ash 6.87 Oxygen and nitrogen 17.93

B. t. u. per 1b., 41.60.

The concentrated liquor is then delivered to a storagetank 23 and supplied as required to the caustic magnesium oxide (MgO), and also more chemical recovery unit 2l and burned therein in suspension under self-sustaining combustion conditions to yield a dry ash consisting mainly of than enough steam to satisfy the requirements of the system. The construction and operation of the recovery unit is hereinafter set forth and is also disclosed and claimed in a co-pending application of Leslie S. Wilcoxson, Serial No. 347,944, illed July 27, 1940, now U. S. Patent No. 2,354,175.

The ash produced by the spray burning of the liquor leaves the recovery unit in suspension in the furnace gases which are then passed through suitable ash separating apparatus, such as a group of cyclone separators 25 arranged in parallel, from which the separated ash is withdrawn and delivered to an ash storage tank 26. 'I'he operation of the cyclones is particularly effective as the chemical is normally present in the-gases in particles of sulcient size to permit of their removal by such separating means. The conditions of the gases and chemicals when in the cyclones are such as to insure ease of operation and long life to that apparatus.

An induced draft fan 21 is located at the sas discharge end of the cyclones. This is an advantageous location for the fan as the solids in the gases have been reduced to a point suitable for goodfan operation and the temperature of the gases is suiliciently above the dew point of the .gases to definitely avoid any corrosive action on the contacted fan parts.

Most of the remaining ash in the gases is removed during the passage of the gases through the scrubber i8, as has been described. On leaving the scrubber the gases are delivered to serially connected absorption towers 28 and 29 for the recovery of the sulphur content of the gases. In the sulphur absorption towers, the gases are subjected to contact with a slurry consisting mainly of magnesia in suspension formed by mixing the ash from the tank 26 with the filtrate from the second pulp washer I4. The slurry is fed into the top of each absorption tower and passes downwardly over wooden slats therein in counter-current relation to the relatively low temperature ascending gas. During its passage through the towers the magnesia in suspension combines with the sulphur dioxide content of the gases and forms a solution of sulphited magnesium. Any sulphur trioxide present in the towers will combine to form magnesium sulphate in the resulting liquor which relation while forming the inert sulphate, retains the chemicals in the system for subsequent reduction and reuse. By the foregoing operations the gases passing from the upper end of the absorption tower 29 to the stack have had both their solid magnesium oxide and gaseous sulphur compounds eillciently removed and are consequently innocuous.

In order to have and maintain the greatest recovery efficiency in the absorption towers, a balance is required between the amount of magnesium oxide in the slurry and the amount of sulphur dioxide in the gases. The accumulation of calcium impurities in the system is controlled to cause the sulphited calcium to remain in suspension and the magnesium to go into solution as sulphite and bisulphite. The slurry fed to the towers is preferably a liquor of high alkalinity, i. e.. a liquor having a pH value of about 9.5, but as an increasing amount of sulphur dioxide is combined, the pH value falls, first gradually and then rapidly. As disclosed in my lsaid Patent No. 2,238,456, the pH value of the liquor leaving the .tion towers 29 and 28 is automatically controlled in response to variations in the percentage of SO2 in the gases passing to the absorption towers, by means of an automatic SO2 analyzer 3| and a rei mote control valve 32. A supplementary control temperature is reduced to facilitate the subsequent absorption of SO2 from the sulphur burners. The operation of the valve 32 in response to the SO2 analyzer is checked by having a pH control 33 at the discharge side of the cooler 34 act on the valve 32, preferably between the operating intervals of the SO2 analyzer. An SO2 recorder 35 is also employed.

The necessity for clean cooking liquor, in view oi' the possibility of some solids being present in the washer ltrate andthe collection of solids in the form of unburned carbon from the gases in the absorption tower, is provided for byiilterng the liquor in a suitable filter 40. By having the lter subsequent to the pH control operation, the calcium compounds in suspension in the liquor can be removed along with any carbon or other solid particles. The magnesium bisulphite liquor is fortified to the desired sulphur dioxide concentration by bringing sulphur dioxide, generated by the sulphur burners 4l and cooled in thef gas cooler l2, into contact with the liquor while passing through a gas absorption system 43. The fortifled liquor is then delivered to a tank 44 where "t is mixed with relief gases from the digester I before being delivered to the cooking acid storage tank il.

The high cost of chemicals involved in a pulping process employing a relatively pure magnesium base cooking liquor requires a cyclic recovery process having a high efficiency of recovery of the heat and chemical values of the residual liquor to be economic. magnesium oxide, must also be recovered in a form which permits their economic reuse in the lpulplng process and the heat values in the liquor ysis being:

Per cent CO2 17.7 SO: 0.6 Oz 2.40

Any attempt to recover the sulphur dioxide from The chemicals, sulphur and `passed through a liquor cooler 34 inwhich its quently be useless.

the gases under such conditions is'unusual because its recovery can only be effected in the presence of an extremely reactive reagent. An overheated or "dead-burned magnesia-would conse- In fact, the magnesia should preferably be more reactive than the best grades of commercial caustic magnesia to effect the desired amount of sulphur dioxide recovery under such conditions.

Magnesia is commercially produced by the calcination of magnesium compounds, such as magnesite, by coal or carbon mixed with the material. It is known that the reactive character of the resultant magnesia depends upon the `amount of carbon present in the mixture; and it is generally considered that for the production of caustic magnesia, the amount of coal or carbon used should not exceed about 20% of the weight of the magnesite. For the commercial production of dead-burned magnesia, it is the customary practice to mix from 30% to 50% coal with the magnesite.

In the representative analysis of the concentrated liquor given, the carbon is 25.04% and the ash (i. e. the magnesium compounds) 6.87%. Accordingly, if such liquor were evaporated to dryness before being introduced into a furnace and the dried solids then burned, the material would then include an amount of carbon over ten times the amount of carbon required for producing dead-burned magnesia. Complete combustion of the organic constituents of the residual liquor is highly desirable, both in the interests of thermal efficiency and to obtain an ash as free from impurities as possible in order to minimize the subsequent filtering operation.

In accordance with my recovery process, pulp residual liquor of the character described can be treated to obtain an ash containing a high percentage of magnesia of a high reactivity and free from carbon while maintaining desirable combustion conditions in the recovery furnace. It has been found that recovery furnace conditions suitable for obtaining certain of these characteristics are not suitable for obtaining others. For example, it is important that any magnesium compounds in the form of magnesium sulphate be reduced to magnesium oxide While in the furnace as the sulphate is of no value in the cooking liquor and would form a dead load of chemical circulating through the system. The greatest reduction of the sulphate will occur when a high temperature reducing atmosphere is maintained in the furuace. However, high temperature :conditions are not suitable for obtaining the highly reactive caustic magnesia desired, nor isa highly reducing atmosphere suitable for a complete combustion of the combustible portions of the liquor. Complete combustion of the combustible ymatter is favored by the use of a substantial amount of. excess air in the furnace, but such conditions tend to increase the percentage of sulphur dioxide (SO2) converted into sulphur trioxide (S03), this reaction being accelerated in the temperature range (l000-l200 F). While any sulphur trioxide in the heating gases can be recovered in tl'e absorption towers, it would combine with the magnesia in the slurry to form magnesium sulphate and add to the dead load of circulating chemicals. The production 'of caustic magnesia of the desired reactivity, i. a mean reactivity at least greater than 1.5 on an arbitrary scale which has been devised, on whichl ordinary commercial caustic magnesia has a mean reactivity of 2.1, requires careful control therein.

of the temperature and atmosphere conditions, velocityof gas flow in the furnace, and size of the ash particles. It has been found that furnace temperatures in the range of l8002400 F., and preferably 21002300 F., are most desirable with a rapid passage of the ash particles through the furnace sections having such temperatures, the permissible time of exposure decreasing as the temperature increases. If the exposure at these temperatures is more than momentary, the reactivity of the magnesia diminishes. and even dead burning is likely to occur. The relation of the ash solubility in acid relative to the time and temperature of exposure is illustrated in Fig. 4, in which the upper solid curve is for a one second exposure and the lower broken line curve for a two second exposure. All of these factors require consideration in the construction and operation of the recovery apparatus.

As shown in Fig. 3 the heat and chemical recovery apparatus comprises a combined furnace and steam boiler unit 24 having a furnace chamber 50 of rectangular horizontal and vertical cross-section formed by refractory walls consisting of a vertical front wall 5I, side walls 52,roof 53, floor 54, and bridge wall 55. A rectangularly shaped gas passage 56 connects the furnace chamber 50 with a vertically elongated narrow passage 51 at the rear side of the bridge wall 55, constituting an unobstructed open passbetween the chamber 50 and the convection heat absorbing section of the unit.

At the rear side of the passage 51 is arranged a bank of vertically disposed steam generating tubes 60 connected to a steam and water drum 6I and a lower water drum 62. The front row of tubes 60a are offset laterally and provided with integral metallic studs over the greater portion of their length with the intertube spaces filled with refractory to form a water cooled baille 63 extending downwardly from a point adjacent the drum 6| and defining the rear'wall of the passage 51. The remaining unstudded portions of alternate tubes 60B are bent in spaced relation to form a water tube screen 64 across the gas discharge opening at the rear side of the passage 51. A horizontally extending refractory baiile 65 forms the bottom of the gas passes within the tube bank. 'Ihe rear portion of the baffle 65 slopes downwardly to form one side of a hopper 65a having a bottom outlet for ash deposite A vertical refractory baille 66 terminates short of a horizontal refractory extension 63a of the tube baille 63. Another vertical refractory baffle 61 extends downwardly from the baille extension 63 to form a vertical gas pass 68 with a gas exit 69 at its lower end. Most of the steam generating tubes 60 are positioned within the gas pass 68. The two rearmost rows of tubes 60,are positioned between the baille 61 and the adjacent wall of the associated air heater. Access doors 50a and 51a are provided at the bottom of the various gas passages where ash may tend to collect.

The bailles 63 and 66 define a vertical gas flow passage 10 of substantially the same width as the gas passages 51 and 68. The space 10 is divided into three sections transversely of the unit by transversely spaced groups of vertical steam generating tubes li0D arranged in alignment between Ithe bailles 63 and 66. The tubes of each group have their lower ends connected to the water drum 62 and their upper ends connected to a corresponding short horizontal header 1l, connected to the drum 6l by tubes 12.

With the foregoing construction the heating gases leaving the lower end of the passage 51wil1 pass through the tube screen 64 into the lower end of the sections of the passage 10, all of the walls of which are thus defined 4by Water tubes. The temperature of the gases therein is rapidly reduced by radiation to and contact with the surrounding water tubes. The quenched condition of the heating gases in the passage 10 permits the installation of superheating surface in the upper portions of thev sections thereof, and, as shown, the superheating surface consists of small diameter multi-looped tubes 13 arranged in parallel vertical fiat coils and connected to superheater inlet and outlet headers 15 and 16 respectively. Tubes 11 connect the inlet header 15 to the steam space of the drum 6l.

The concentrated liquor from the liquor storage tank 23 is delivered by a pump 80 through a pipe line 8| t having a. return flow connection 62 to the tank) to the furnace. Separately controllable liquor nozzles 83 positioned in corresponding burner ports 84 in the furnace roof 53 are employed for introducing the liquor. An atomizing steam nozzle 85 alongside each liquor nozzle 83 is supplied with steam from the turbine 22, as indicated in Fig. l. Each pair of liquor and steam nozzles has an associated adjustable distributor block 86 carried by a. bracket supported on the nozzle assembly. Each block has a vertically arranged impact surface, against which the steam jet impacts and is deected across the converging liquor stream, breaking up the liquor stream into a relatively flat finely divided spray distributed substantially in a plane parallel and adjacent to the front wall of the furnace.

Combustion air is separately supplied to each burner port from a preheated air duct 90 by branch ducts 9| controlled by dampers 92. The du'ct 90 forms a branch of a main air duct 93 connected to the outlet of a tubular air heater 94 at the rear of the boiler bank. A forced draft fan |05 maintains a pressure air supply to the air heater and furnace, the fans 21 and |05 being manually or automatically regulated to maintain the desired furnace pressure and gas velocity throughout the unit.

In operation the furnace chamber 50 is initially heated to a predetermined temperature by auxiliary fuel, such as a wood fire on the furnace bottom 54 or auxiliary oil or gas burners temporarily inserted into the furnace. With the burner arrangement described the residual liquor introduced will burn in suspension while in a vertically elongated U-shaped path extending downwardly along the front wall 5I, across the furnace floor, and upwardly along the bridge wall 55 to the gas exit 56. The burning fuel particles and products of combustion then pass downwardly 4through the first open pass 51, and through the tube screen 64 into the various sections of the passage 10. The gas stream passes upwardly through the sections of the passage 10 in contact with the water tubes forming the walls thereof in an unobstructed flow until reaching thesuperheater tubes 13. The gases then flow along the superheater tubes over the upper end of the baile 66 and downwardly through the main generating section of the boiler longitudinally of the tubes 60. 'I'he gases pass across the lower egidtssosf the downcomer tubes |50c and out the gas e To permit the eiilcient recovery of magnesium and sulphur in a form permitting their economic reuse in the cyclic system, introduction of the chemicals inw the furnace in a concentrated liquor of the character described and maintenance of predetermined temperatures, atmospheres and gas velocities in different portions of the unit are of prime importance. A long flame combustion of the combustible constituents of the residual liquor under a reducing atmosphere while in intimate contact with the magnesium constituents to a location adjacent the bottom of the first open pass is considered desirable to avoid excessive furnace temperatures and possible dead-burning of the magnesia. For this purpose the total amount ci combustion air supplied to the furnace is only slightly in excess of the.

theoretical combustion requirements and that air is supplied to the furnace at widely spaced points along the flow path in predetermined proportions.

A total combustion air approximately 110% of the theoretical requirements has been found suitable, for example. About 80% of the theoretical amount is introduced through the conduits 9| and burner ports 04 along with the sprayedliquor to be incinerated. The reverbatory eil'ect of the U-shaped flame path in the furnace facilitates the dryiz distillation, ignition and combustion of the entering liquor.

'I'he liquor particles are burned in suspension as they pass downwardly along the front wall 5| and a considerable portion of the chemical'ash tends to separate from the burning fuel stream as the flame and gas stream turns acrossr the floor 54 and upwardly towards the gas exit 56. The

4separated chemical tends to deposit on the floor 50 and if Apermitted to so deposit and remain thereon would rapidly become dead-burned and substantially useless in the pulping process. A second supply of combustion air is delivered to the furnace from a main l branch duct 90 through horizontally arranged inlet passages 85 formed along the front wall 5| at the level of the furnace floor 54 and opening into a branch air duct 98 controlled by a damper 91. Normally, about 15% of the theoretical air supply is discharged by the inlets 05 horizontally across the furnace floor 54 to supply additional air for combustion and also cause any ash tending to deposit on the furnace door to be swept out of the furnace with the furnace gases. With the described amounts of combustion air supplied, a reducing atmosphere is maintained throughout the furnace chamber 50 and open pass 51, and after probably a break-down of the magnesium lignin sulphonate to magnesium sulphate, the magnesium sulphate is reduced to magnesium oxide with the release of sulphur dioxide and carbon dioxide approximately in accordance with the formula:

'I'he thick refractory walls of the furnace chamber with their substantial heat storage capacity contribute to the maintenance of uniform temperature conditions therein. The heating gases and suspended ash particles passing out through the gas exit 56 down through the passage 51 contain some unburned carbon particles and combustible gases, the combustion of which is completed in the lower part of the passage 51 and passage l0 by the introduction of a third air supply through a series of air inlet openings 98 opening through the rear side of the bridge wall l5. The air inlets 98 are connected to an air duct 09, having a control damper |00, through vertical passages |0| in the bridge wall and horizontal passages |02 below the furnace floor. About 15% of the theoretical air supply is delivered to the ports 00 and an oxidizing atmosphere thus maintained in the lower part of the passages l1 and l0. Combustion is completed and the temperature of the heating gases and suspended ash particles is rapidly reduced as the stream enters the passage 10 by radiation to the water cooled walls of that space and the subdividing tube groups 0b. The lowering of the temperature of the gases affords a safe metal temperature for the superheater tubes 13.

The furnace is thus designed with respect to shape and air admission to maintain a reducing atmosphere of gases in that portion of their travel to a location in the first open pass l1, where additional air is admitted to complete com.. bustion just prior to the gasventrance to the second open pass 10. In contrast to the mainly refractory construction of the first pass walls, the walls of the second pass are mainly water cooled. The time-temperature relation necessary to obtain the resulting magnesium oxide ash in a highly reactive condition best suited for the subsequent production of magnesium bisulphitecooking acid is thus obtained. While the addition of air and the production of an oxidizing atmosphere in the lower part of the first open pass insures completion of combustion, the reaction of the sulphur dioxide released to sulphur trioxide is minimized, even with the oxidizing atmosphere present, by passing the gases quickly through the temperature range (10001200 F.) in which this reaction is accelerated, by having the gases on completion of combustion enter the main tube bank pass 68 wherein the gas temperature is rapidly reduced through this range.

The chemical recovery unit described when used to treat the residual liquor from a pulp mill having a pulp capacity of 100 tons per day and with liquor of the analysis given, will receive approximately 19,300 1bs./hr. With the furnace construction, and distribution and gas velocity described herein, the gas temperature on leaving the furnace chamber 50 will be approximately 2250 F. and entering the screen 84 approximately 2l40 F.

The ash produced will be mainly in the form of light cenospheres and flakes having a density of from three to eight lbs/cu. ft., the density increasing as the reactivity decreases. 'I'he percentage of caustic magnesium oxide in the ash will depend upon the efficiency of reduction. Normally the caustic magnesium oxide content will be at least 70%, with the remainder mainly magnesium sulphate and carbonate. Very small percentages of inorganic impurities will also be present, these varying with the wood used.

The boiler heating surface is characterized by its simplicity of construction and ease in which it can be kept absolutely clean of ash. 'I'he furnace temperatures maintained are below the fusion temperature of the ash, and the tubes and ballies of the boiler are so arranged that any of the dry ash depositing thereon can be readily removed. All of the boiler tubes are vertical as well as the ballles defining the boiler passes. Both of the horizontal baflies 63 and 05 are readily cieanable and insure freedom of corrosion of the drum tube connections from the high sulphur 6 assauts gases would cause corrosion. The ability to maintain the boiler heating surface absolutely cle'an minimizes the draft loss and permits the use of high gas velocities, such as 50 feet per second, for example, consistent with the economical utilization of induced draft fan power requirements and consequently results in highly efficient heat transfer conditions. The rapid travel of the gases and suspended chemicals at such velocities is indicated by the fact that the length of the flow path from the spray nozzles to the bottom of the iirst open pass may be approximately 50 feet, for

- example.

On leaving the boiler, the heating gases pass upwardly, through the tubes of the air heater with the air to be preheated flowing downwardly around the tubes under the action of a forced draft fan |05. The gases then flow in parallel through the dust collecting cyclones forming the separating apparatus 25. Most of the chemical ash in suspension is separated at this point and collected in hoppers at the bottom of the cyclones. The gases then pass out through the induced draft fan 21 to the spray tower I8. 'Ihe spray tower I8 is divided into a pair of narrow and wide passes. The gases flow downwardly through the narrow passage in which they successively contact descending sprays of residual liquor delivered from the tank I1 to vertically spaced spray nozzles. The gases then flow upwardly through the wide pass to the absorption towers 28 and 29. A pump receives the liquor from the bottom of the spray tower and discharges it to the neutralizing tank.

While in accordance with the provisions of the statutes I have illustrated and describedvherein the best form of the invention now known to me, those skilled in the art will understand that changes may bev made without departing from the spirit of the invention covered by my claims,

and that certain features of the invention mayy sometimes be used to advantage without a corresponding use of other features.

I claim:

1. The method of treating the residual liquor resulting from the digestion of cellulosic fibrous material in a relatively pure magnesium base sulphite cooking liquor and separation from the pulp in a pulp washing system which comprises concentrating the liquor by evaporation, burning the combustible organic constituents of the concentrated liquor in a. furnace chamber in suspension therein under temperature conditions and for an interval suiiiciently brief to obtain a dry ash having a high percentage of caustic magnesia and combustion gases containing a low percentage of sulphur dioxide and substantially all of the ash in suspension therein, separating ash from the combustion gases, then passing unconcentrated liquor into intimate contact with the combustion gases for the recovery of additional ash from the gases and partial evaporation and neutralization of the unconcentrated liquor, completing the neutralization of the partially neutralized and evaporated liquor by the addition of caustic magnesia thereto prior to further evaporation thereof, mixing recovered ash with pulp washer filtrate to form an alkaline aqueous suspension, and passing the ash suspension through a gas absorption chamber in contact with the combustion gases to recover sulphur dioxide.

2. In the method of making pulp by the digestion of cellulosic fibrous material in a relatively pure magnesium base sulphite cooking liquor, the steps which comprise passing the cooked pulp through a multi-stage pulp washing system, concentrating the liquor from the ilrst washing stage to a predetermined concentration by evaporation, burning the combustible organic constituents of the concentrated liquor in a furnace chamber under conditions yielding a dry ash having a high percentage of caustic magnesia and combustion gases containing a low percentage of sulphur dioxide and substantially all of the ash in suspension therein, separating ash from the combustion gases, then passing unconcentrated liquor from the first washing stage into intimate contact with the combustion gases for the recovery of additional ash from the gases and partial evaporation and neutralization of the unconcentrated liquor, completing the neutralization of the partially neutralized and evaporated liquor by the addition of caustic magnesia thereto vprior to further evaporation thereof, mixing recovered ash with liquor from a subsequent washing stage to form an alkaline aqueous suspension, and passing the ash suspension through a gas absorption chamber in contact with the combustion gases to recover sulphur dioxide.

3. The method of treating the residual liquor resulting from the digestion of cellulosic ibrous material in a relatively pure magnesium base sulphite cooking liquor which comprises evaporating the liquor to a predetermined concentration in a multiple-effect evaporator, burning the combustible organic constituents of the concentrated liquor in a furnace chamber in suspension there- 'in under conditions yielding an ash having 'a high percentage of caustic magnesia, mixing recovered ash with residual liquor to neutralize the residual liquor before evaporation, and supplying ammonia to the vapor section of the evaporator to minimize corrosion therein due to acid constituents of the vapor generated therein.

40 4. The method of treating the residual liquor resulting from the digestion of cellulosic fibrous material in a relatively pure magnesium base sulphite cooking liquor and separation from the pulp in a pulp washing system which comprises concentrating the liquor by evaporation, burning the combustible organic constituents of the concentrated liquor in a furnace chamber in suspension therein under temperature conditions and for an interval sufficiently brief to obtain a dry ash having a high percentage of caustic magnesia and combustion gases containing a low percentage of sulphur dioxide and substantially all of the ash invsuspension therein, separating ash from the combustion gases, then passing unconcentrated liquor into intimate contact with the combustion gases for the recovery of additional ash from the gases and partial evaporation and neutralization of the unconcentrated liquor, completing the neutralization of the partially neutralized and evaporated liquor by the addition of caustic magnesia thereto prior to further evaporation thereof, and treating the separated ash to form fresh magnesium base cooking liquor.

5. A cyclic method of treating ligno-cellulosic material which comprises cooking the ligno-cellulosic material with a solution of magnesium bisulphite containing a substantial excess of free sulphur dioxide, releasing gases containing free sulphur dioxide during the cooking'operation, separating the acid waste liquor from the treated cellulosic material after completion of the cooking operation, neutralizing the acid waste liquor by the addition of caustic magnesia, evaporating and burning the neutralized waste liquor to produce an ash consisting mainly of magnesium oxide and combustion gases containing a low percentage of sulphur dioxide, adding a mag-I nesium compound to the waste liquor prior to its evaporation as required to make up chemical losses. making a slurry of the magnesium oxide ash, absorbing the sulphur dioxide in said comi bustion gases in said slurry to reform magnesium bisulphite to be used in a subsequent cooking op'- eration, making up losses of free sulphur dioxide from the system by fortifying said reformed magnesium bisulphite with a gas resulting from the burning of a sulphurous material and absorbing free sulphur dioxide from the gases released during cooking in said reformed magnesium bisulphite, and returning the reformed magnesium bisulphite containing the absorbed sulphur dioxide for further cooking of ligno-cellulosic material.

6. The process of treating the residual liquor resulting from the digestion of cellulosic fibrous material in a relatively pure magnesium base sulphite cooking liquor which comprises concentrating the residual liquor to a concentration within the range of 45-70% solids, spraying the concentrated liquor into a high temperature furnace chamber, burning the liquor so introduced therein to a dry unsintered solid residue containing a relatively high proportion of caustic magnesium oxide and to release sulphur dioxide, continuously removing the residue from the furnace chamber by notation in the gaseous products of combustion in such a brief period of time that the dwell of the residue within the combustion zone is insuillcient to effect dead-burning of the magnesium oxide in the residue, returning a portion of the residue removed to the residual liquor to neutralize the liquor before concentration, treating another portion of the residue removed with water to produce an alkaline liquor containing the recovered magnesium compounds, and absorbing sulphur dioxide from the gases after leaving the furnace chamber by contact with the alkaline liquor thus produced.

7. The process of treating the residual liquor resulting from the digestion of cellulosic fibrous material in a pure magnesium bisulphite cooking liquor which comprises concentrating the residual liquor, continuously spraying the concentrated liquor into a high temperature furnace chamber, dehydrating and burning the liquor so introduced therein while maintaining a reducing atmosphere and a furnace temperature below the fusion temperature of the non-combustible constituents of the liquor to yield a dry solid residue containing a. relatively high proportion of magnesium oxide and to release sulphur dioxide. continuously removing the residue from the furnace chamber by gas flotation, separating the residue from the gases after leaving the furnace chamber, returning a portion of the separated residue to the residual liquor to neutralize the liquor before concentration, treating another portion of the separated residue with water to produce an alkaline liquor containing the recovered magnesium compounds, absorbing sulphur dioxide from the gases after leaving the furnace chamber by contact with the alkaline liquor produced to make fresh magnesium bisulphite cooking liquor, and replenishing any sulphur and magnesium losses in the system by the addition of magnesium sulphate to the concentrated residual liquor before burning.

8. The process of treating residual liquor resulting from the digestion of cellulosic fibrous material in a relatively pure magnesium base sulphite cooking liquor which comprises concentrating the residual liquor, introducing the concentrated liquor into a primary combustion and reducing zone, burning the concentrated liq uor so introduced therein while maintainingv a temperature and atmosphere therein'yielding a dry ash containing a high percentage of reduced magnesium compounds and to release sulphurous gases, withdrawing the ash from the primary combustion and reducing zone through a secondary combustion zone by flotation in the combustion gases, supplying combustion air slightly in excess of the theoretical combustion requirements and proportioning the same between the zones to provide a strongly reducing atmosphere in the primary combustion and reducing zone and to provide a slightly oxidizing atmosphere in the secondary combustion zone suilicient to complete the combustion in suspension of any unburned combustibles in the combustion gases, recovering the sulphurous gases released by contact with an absorbing liquid including recovered ash, and adding free sulphur dioxide to the sulphited liquid to form fresh magnesium bisulphite cooking liquor. l

9. The process of treating residual liquor resulting from the digestion of cellulosic fibrous material in a relatively pure magnesium bisulphite cooking liquor which comprises concentrating the residual liquor, introducingl the concentrated liquor into a primary combustion and reducing zone in a iinely divided spray, burning the concentrated liquor so introduced therein in suspension while maintaining a temperature and atmosphere therein yielding a dry ash containing a high percentage of reduced magnesium compounds and to release sulphurous gases, withdrawing the ash from the primary combustion and reducing zone through a secondary combustion zone by flotation in the combustion gases,

supplying combustion air slightly in excess of the theoretical combustion requirements and proportioning the same between the zones to provide a. strongly reducing atmosphere in the primary combustion and reducing zone and to provide a slightly oxidizing atmosphere in the secondary combustion zone sufllcient to complete the combustion in suspension of any unburned combustibles in the combustion gases, recovering' the sulphurous gases released by contact with an absorbing liquid including recovered ash, filtering the sulphited liquid produced to eliminate any unburned carbon remaining, and adding free sulphur dioxide to the filtered liquid to form fresh magnesium bisulphite cooking liquor.

10. The process of treating the residual liquor resulting from the digestion of cellulosic brous material in a relatively pure magnesium base sulphite cooking liquor and separation from the pulp in a pulp washing system which comprises spraying the residual liquor into a high temperature furnace chamber, burning the residual liquor so introduced therein While maintaining a furnace temperature below the fusion temperature of the non-combustible constituents of the liquor to yield a dry unsintered solid residue containing a relatively high proportion of caustic magnesia and combustion gases containing a low percentage of sulphur dioxide, removing substantially all of the residue produced from the combustion zone by flotation in the combustion gases in such a brief period of time that the dwell of the residue within the combustion zone is insumcient to eiect dead-burning of the magnesia in the residue. mixing the solid residue vwith pulp washer nitrate to produce an alkaline aqueous suspension, and passing the suspensionproduced through a gas absorption chamber in contact with the combustion gases to recover sulphur dioxide. l

11. The process of treating the residual liquor resulting from the digestion of cellulosic iibrous brief period of time that the dwell of thel residue e within the combustion zone is insuilicient to effect dead-burning of the magnesia in the residue, mixing a portion of the recovered residue with the residual liquor to neutralize the residual liquor before its evaporation, forming an alkaline aqueous suspension of another portion of the recovered residue, and passing the suspension produced through `a gas absorption chamber in contact with gaseous products of combustion from the furnace chamber.

12. The process of treating the residual liquor resultingfrom the digestion of cellulosic fibrous .material in a relatively pure magnesium base sulphite cooking liquor and separation from the pulp in a pulp washing system which comprises burning the combustible organic constituents of the residual liquor in a furnace lchamber in suspension therein while maintaining a furnace temperaturebelow the fusion temperature of the non-combustible constituents of the liquor to yield a solid residue containing a relatively high proportion of caustic magnesia and combustion gases containing a low percentage of sulphur dioxide, removing the residue produced from the combustion zone by notation in the combustion gases in such a brief period of time that the dwell of the residue within ,the combustion zone is insumcient to eiIect dead-burning of the magnesia in the residue, forming an alkaline aqueous suspension of the solid residue, passing the aqueous suspension produced through a gas absorption chamber in contact with the combustion gases to recover sulphur dioxide.'and controlling the amount of solid residue supplied to the gas absorption chamber in response to variations in the sulphur dioxide content oi the combustion gases passing to the gas absorption chamber.

13. The process of treating the residual liquor resulting from the digestion of cellulosic fibrous material in a relatively pure magnesium base sulphite cooking liquor and separation from the pulp in a pulp washing system which comprises spraying the residual liquor into a high tempera-- ture furnace chamber, burning the residual liquor so introduced therein while maintaining a furnace temperature below the fusion temperature of the non-combustible constituents of the liquor to yield a dry unsintered solid residue containing a relatively high proportion of caustic magnesio. and combustion gases containing a low'percentage of sulphur dioxide, removing substantially all of the residue produced from the combustion zone by flotation in the combustion gases -in such a brief period of time that the dwell of the resi-,- due within the combustion zone is insufficient to eiect dead-burning of the magnesla in the 'resi due, mixing the solid residue with pulp washer illtrate to produce an alkaline aqueous suenension, passing the aqueous suspension produced through a gas absorption chamber in .contact with the combustion gases to recover sulphur dioxide, and controlling the amount of aqueous suspension supplied to the absorption chamber in response to variations in the sulphurl dioxide vcontent of the combustion gases passing to the absorption chamber.

GEORGE H. TOMLINSON;

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