US3133789A

US3133789A - Chemical recovery of waste liquors

Info

Publication number: US3133789A
Application number: US109128A
Authority: US
Inventors: Salvatore A Guerrieri
Original assignee: Lummus Co
Current assignee: CB&I Technology Inc
Priority date: 1961-05-10
Filing date: 1961-05-10
Publication date: 1964-05-19
Anticipated expiration: 1981-05-19
Also published as: NO117826B; AT256608B

Description

INVENTOR m'eij May 19, 1964 sjA. GUERRIERI CHEMICAL RECOVERY OF WASTE LIQUORS Filed May 10. 1961 Z'oSlack Salvatore J1. filler AG E N T ya. a w a a W mama Z 5 w, r m a w Tam, W2 1 5Z1 V m, 2 M w mm w m a H A a? z w M; 4% 2 w 1 o 5 a W 6 M m 4 sg WW 0 a 0% *F/ 7 5 6 ml 2 Al M t; w m J kkwng d M w M i United States Patent 3,133,789 CHEMHIAL REQQVERY 6F WASTE LIQUQRS Salvatore A. Guerrieri, Scarsdaie, N.Y., assignor to The Lummus Company, New York, N.Y., a corporation of Delaware Filed May 16, 1961, Ser. No. 109,128 11 Claims. ((31. 2348) The present invention relates to a process for recovering chemicals from the waste liquors produced in the wood pulping industry. In its more specific aspects, this invention relates to a continuous process for the preparation of delignification liquors from sodium base waste sulfite liquors for reuse in the pulping processes.

It is well known that most paper and paper products are manufactured from wood and/ or other cellulose materials which have been converted into pulp. Pulp may be prepared by several processes including mechanical, chemical and semi-chemical processes. Mechanical processes are based upon the physical reduction of cellulosic material to a fibrous state whereas the chemical processes, such as the acid sulfite, sulfate and soda processes, are based upon the chemical removal of the ligneous compounds contained in the cellulosic material. While chemical and mechanical processes have been widely utilized, it has only been in recent years that semi-chemical processes have achieved substantial importance, even though such processes have a substantially higher pulp yield. Semi-chemical processes normally comprise the steps of reacting the cellulosic material with a chemical liquor to partially remove the ligneous compounds and thereafter subjecting the partially delignified cellulosic material to a mechanical process to complete fiberization.

A residual waste liquor from a sodium sulfite or sodium bisulfite delignification contains sulfur dioxide bearing compounds of sodium, principally as ligno-sulfonic compounds or other organic sulfonic compounds formed by the action of the cooking liquor on the organic substances present in the ligneous cellulosic material, together with some residual sulfite compound or compounds. It also ordinarily contains other organic compounds derived from the cellulosic material or formed by the action of the cooking liquor on constituents of such material including, for example, sugars or other carbohydrates, and various organic acids, such as formic or acetic acid or the salts thereof.

The present invention is applicable to the treatment of the residual liquor obtained from delignifying processes utilizing various types of sodium base sulfite chemicals such as the sodium base acid sulfite processes, in which the cooking liquor contains sodium acid sulfite (NaHSO and generally free sulfurous acid, whereas the residual liquor contains some sodium acid sulfite together with ligno-sulfonic compounds and other organic compounds as mentioned hereinabove, and the neutral sodium base sulfiite processes in which the cooking liquor contains sodium sulfite (Na SO and one or more alkaline compounds of sodium such as sodium bicarbonate and/or sodium carbonate, whereas the residual liquor contains sodium sulfite with some of the other alkaline sodium compounds together with ligno-sulfonic compounds and other organic compounds as mentioned hereinbefore.

It is generally known that the products obtained from the combustion of a concentrated waste sulfite liquor contains in addition to sodium carbonate substantial quantities of sodium sulfide which should be converted if these products are to be treated to regenerate a sodium sulfite cooking liquor. This conversion step should be performed in such a manner as to minimize the formation of the undesirable sodium compounds, such as sodium thiosulfate and sodium polysulfides which may be formed in the presence of residual sodium sulfide. Sodium thiosulfate and polysulfides cause great difiiculties in subsequent digestion cycles, even though present in relatively small amounts, since sodium sulfide, sodium thiosulfate and sodium polysulfides in the presence of sulfur dioxide form free sulfur which has a deleterious effect on the pulp product obtained from an acid delignification, particularly in subsequent bleaching operations.

It is known to concentrate waste liquors from delignification processes used in the pulping industry, to burn and to smelt such liquor in a primary recovery unit, and to dissolve the smelt prior to subjecting it to the action of other chemicals. As a result of the general complexities of such processes, notwithstanding the simple chemistry involved, the prior art recovery and conversion plants are relatively complicated and expensive.

It is a principal object of this invention to provide an improved process for treating waste delignifying liquors so as to permit reuse of the chemicals therein in subsequent delignification cycles.

Another object of this invention is to provide a novel method of treating the smelt formed by burning such waste liquors to convert the sodium sulfide content thereof to sodium carbonate.

These and other objects, and a fuller understanding of the invention may be had by referring to the following description taken in conjunction with the accompanying drawing, in which the figure is a schematic flow diagram illustrating a preferred embodiment of my in vention.

I propose to treat in a reactor the smelt (formed by burning the concentrated black liquor) with carbon dioxide and water vapor at a temperature above the melting point of sodium carbonate, i.e. 851 C. While the smelt withdrawn from a recovery furnace is generally in the molten state, the smelt may be introduced into the reactor in the solid state and will pass into the molten state during the conversion of the sodium sulfide to sodium carbonate at the temperature maintained in the reactor. However, care must be taken not to cause the molten smelt obtained from the furnace to be contacted with air, and further, the smelt in the solid state must be at a temperature sufficient whereby the heat of reaction will maintain the converter at appropriate reaction conditions. It is important that the temperature in the reactor be maintained above the melting point of sodium carbonate to avoid clogging up the reactor. Preferably, the temperature in the reactor is maintained substantially above the melting point of sodium carbonate so as to convert substantially all of the sodium sulfide in the smelt to sodium carbonate. Further, with high conversion temperatures, the rate of reaction will be higher, thus resulting in smaller sized equipment. The thus treated smelt is thereafter passed through a series of processing steps whereby cooking liquors are formed for use in delignifying cellulosic materials.

Referring now to the drawing, and more particularly to FIGURE 1, the preferred embodiment comprises the following principal components, namely a primary recovery furnace 10 for burning the concentrated waste or black liquor, a converter 11 in which sodium sulfide in the molten smelt received from the furnace is subjected to chemical reactions, a dissolver 12 and a clarifier 13 adapted to recover sodium carbonate from the smelt leaving the converter, and carbon dioxide and sulfur

dioxide recovery units

15 and 17, respectively.

Concentrated black liquor is fed into the recovery furnace 10 through line 19. In the furnace lit the black liquor is burned to form gaseous products and a smelt primarily containing sodium carbonate and sodium sulfide. The temperature in the furnace 10 is maintained at a value at which the concentrated black liquor will burn in the presence of a combustion supporting medium.

Combustion air and make-up sulfur for the system are admitted to the furnace through

lines

21 and 21, respectively. The gaseous products formed in the converter 11 consisting primarily of hydrogen sulfide, carbon dioxide and water vapors may also be passed into the furnace 10 via line 22 wherein the hydrogen sulfide is converted to sulfur dioxide. The gaseous products leaving the furnace 10 are passed through line 23 to sulfur dioxide and carbon

dioxide recovery units

15 and 17 which will be described in detail subsequently. The combustion gases in line 23 consist primarily of carbon dioxide, water vapor, sulfur dioxide, and nitrogen.

In accordance with one embodiment of my invention, the smelt in the converter is reacted with carbon dioxide and water vapor introduced through line 24 and which is heated prior to introduction in a suitable manner (not shown). The conversion of sodium sulfide to sodium carbonate may be represented by the following reactions:

The conversion of the sodium sulfide to sodium carbonate is represented by the overall Equation 2 above. It will be appreciated that the above equations represent the primary reactions taking place. It is obvious that insignificant side reactions will also take place, such as, for example, the formation of minor amounts of sodium sulfate. The converter 11 is maintained at a temperature above the melting point of sodium carbonate (i.e. 851 C.) and preferably at a substantially higher temperature above the melting point for efficient and effective operation.

The gaseous products formed in the converter 11 are passed into the furnace 10 through line 22 as previously stated. Alternatively, these gases may be vented to an auxiliary heat exchange unit to undergo combustion and heat recovery.

The thus treated smelt mainly consisting of sodium carbonate is passed to dissolver 12 through line 25 wherein water is added to form a solution which is known in the art as green liquor. The green liquor (sodium carbonate solution) is withdrawn from dissolver 12 through line 26 and is passed to the clarifier 13.

The sodium carbonate solution in clarifier 13 may be passed partially through line 27 directly to the second stage of a two-stage delignification cycle, partially through

lines

28 and 29 to the carbon dioxide make-up unit 17, and partially to a sulfite reactor 16 via

lines

30 and 31. The use of the sodium carbonate solution in the latter two instances will be presently described in detail.

Although only one converter 11 has been shown, it is obvious that either a plurality of converters or a multistage converter may be utilized. Similarly, it is obvious that the carbon dioxide and water vapor introduced into the converter 11 which is shown entering through line 24 may enter via separate lines.

The carbon dioxide and the steam used in the converter 11 may be supplied from any suitable source. For purposes of economy it is preferable to supply these reactants from sources within the instant system or from related sources within the plant. For example, gaseous carbon dioxide is withdrawn from the reactor 16, a reboiler 18, and a blow gas scrubber 14 and may be passed to converter 11 through

lines

32, 33, and 34, respectively. Blow gas primarily containing carbon dioxide and sulfur dioxide is introduced into the scrubber 14 from the digesters (not shown). In the scrubber 14, a sodium sulfite solution is passed in counter-current relation to the blow gas whereby the sulfur dioxide contained therein reacts with a portion of sodium sulfite to form sodium bisulfite in accordance with Equation 3.

The sodium sulfite solution is introduced into scrubber 14 through line 35 and is obtained from reactor 16. The sodium sulfite-sodium bisulfite solution formed in the scrubber 14 is withdrawn through line 36 and is passed to sulfur dioxide recovery tower 15 through line 37.

The combustion products leaving the recovery furnace 10 may be vented to the atmosphere through a stack (not shown). However, it is preferable from an economic stand-point to pass these gases through various processing units to recover the sulfur dioxide and/ or carbon dioxide contents thereof.

In the sulfur dioxide recovery unit 15, the sulfur dioxide in the combustion gas flowing in the line 23 is removed by passing the combustion gases counter-current to a sodium sulfite-sodium bisulfite solution in accordance with Equation 3 above. A portion of the sodium bisulfite solution formed is passed to a digester in another part of the plant through line 38, with the remaining portion being passed to the reactor 16 through

lines

39 and 31.

In the sulfite reactor 16, the sodium bisulfite solution is contacted with the sodium carbonate solution from clarifier 13 to form a sodium sulfite solution in accordance with the following equation:

The sodium sulfite solution is transferred through line 35 to scrubber 14 and/or through line 37 to the recovery unit 15 for use in recovering the sulfur dioxide content of the furnace off-gas according to Equation 3 above. The carbon dioxide produced in reactor 16 is passed via

lines

32 and 24 to converter 11.

After the combustion gases from the furnace 10 have passed through the sulfur dioxide recovery unit 15, they are fed to the carbon dioxide make-up unit 17 prior to being exhausted to the atmosphere. In the carbon dioxide make-up or recovery unit 17, the combustion gases are passed in counter-current relation to a sodium carbonate solution entering the top of the unit through

lines

28 and 29. Carbon dioxide is removed from the combustion gases by the formation of bicarbonate according to Equation 5 below.

The sodium bicarbonate solution is removed from the unit 17 through line 40 and passed to reboiler 18 wherein the solution is heated to form carbon dioxide and sodium carbonate in accordance with the reverse reaction shown by Equation 5. The sodium carbonate solution is passed to a digester as a second-stage cooking liquor through line 41 or is recycled to the unit 17 through line 29. The steam and carbon dioxide which are formed are sent to the converter 11 through

lines

33 and 24.

Although the sulfur dioxide recovery unit 15 and the carbon dioxide recovery unit 17 are shown as superimposed, it is obvious that these units may be separated.

The feasibility of the process is verified by the results of the laboratory scale experiment described below.

A mixture of 50% sodium carbonate and 50% sodium sulfide was placed in a cylindrical stainless steel cylinder about 1% inches in diameter and 8 to 10 inches long and this was placed in an electrical furnace. Heat was applied and when a temperature of about 1500 F. was reached, the mixture was a molten fluid mass. At this point, a mixture of CO and water vapor was introduced through a tube leading to the bottom of the cylindrical vessel and H 8 was driven off. Since this reaction was carried out in the open, the hydrogen sulfide formed, burned to sulfur dioxide. As the reaction proceeded and the ratio of sodium sulfide to sodium carbonate decreased, the melting point of the mixture increased and a cake of essentially sodium carbonate began to form in the tube. After some 15 or 20 minutes the resulting cake was so extensive that gas fiow ceased and the experiment was discontinued. The resulting product was cooled, dissolved in water and analyzed for sodium sulfide and, sodium sulfide content was found to be of the order of magnitude of 3% indicating a conversion of approximately 90%.

The above described process may be modified by supplying air or oxygen) to the converter 11 through the optional air inlet 41. In such a case, the hydrogen sulfide formed in the converter 11 will be oxidized primarily to sulfur dioxide in the converter 11 rather than in the furnace 10. The reaction will proceed according to the following equation:

It is also within the scope of the instant invention to attain the objectives thereof without passing carbon dioxide into direct contact with the molten carbonate and sodium sulfide in converter 11. In such case, the reaction in converter 11 would proceed according to Equation 1. The resultant reaction products may be used directly or may be carbonized by adding a carbonating agent, either carbon dioxide or carbonic acid at a subsequent point in the system, for example, at the dissolver 12. Reaction (1a) above would then take place with a resultant formation of sodium carbonate.

In a further modification, only a carbonating agent is added to the melt in the converter 11.

For the sake of simplifying the description, various elements of a conventional nature, such as control valves, instruments, and the like, have been omitted from the foregoing detailed description of the invention.

Obviously, many modifications and variations of the invention as hereinabove set forth may be made without departing from the spirit and scope thereof and, therefore, only such limitations should be imposed as are indicated in the appended claims.

I claim:

1. In a process for treating sodium base waste liquors obtained from a wood pulping process, the steps of burning said Waste liquors to form a smelt containing sodium sulfide, contacting the resulting smelt with carbon dioxide and steam to convert sodium sulfide to sodium carbonate, and maintaining the temperature of said conversion above 851 C.

2. In a process for recovering a solution containing sodium carbonate substantially free from sodium sulfide from a sodium base waste liquor from a wood pulping process, the steps of burning said waste liquor to obtain a smelt containing sodium sulfide, contacting said smelt with carbon dioxide and steam to convert sodium sulfide to sodium carbonate, maintaining the temperature of said conversion above 851 C., removing the gaseous products formed, and dissolving the remaining products to form said solution.

3. In a process for treating sodium base waste liquors from a wood pulping process, the steps of burning said waste liquors to form a smelt primarily comprised of sodium sulfide and sodium carbonate, removing the gases formed during this heat treatment, contacting said smelt with carbon dioxide and steam to convert sodium sulfide to sodium carbonate, maintaining said smelt in a substantially molten state during the above step, removing the gaseous products of reaction, and dissolving the thus treated smelt to form a solution primarily comprised of sodium carbonate.

4. A process as defined in claim 3, further including the step of clarifying said solution.

5. In a process for treating sodium base waste liquors, the steps of burning said liquors in a furnace to form a smelt primarily comprised of sodium sulfide and sodium carbonate, passing said smelt to a reactor, contacting in said reactor said smelt with carbon dioxide and water vapor, maintaining the temperature of said contact above 851 C., passing the gaseous products of the reaction formed in said reactor to said furnace, dissolving the thus treated smelt to form a solution primarily comprised of sodium carbonate, and clarifying said solution.

6. A process as defined in claim 5, further including the steps of burning said gaseous products in said furnace, and passing the gaseous combustion products from said furnace to a sulfur dioxide recovery unit.

7. A process as defined in claim 6, further including the step of passing the gaseous combustion products from the furnace to a carbon dioxide recovery unit.

8. A process as defined in claim 7, further including the step of recycling carbon dioxide recovered in the carbon dioxide recovery unit to said reactor.

9. A process as defined in claim 8, further including the step of utilizing a portion of the clarified solution in the carbon dioxide recovery unit.

10. In a process for recovering chemicals from sodium base waste liquors, the steps of burning said liquors to form a smelt primarily comprised of sodium carbonate and sodium sulfide, contacting said smelt with carbon dioxide, steam and air, to convert sodium sulfide to sodium carbonate, maintaining the temperature of said conversion above 851 C.

11. In the recovery of chemicals from the soda base sulfite pulping process wherein sodium base waste liquor is burned in a recovery furnace forming smelt containing sodium sulfide, the sodium sulfide being converted to sodium carbonate which is treated to form sodium sulfite or bisulfite for use in the digesters, the combination with said soda base sulfite pulping process of the steps com prising:

(a) contacting said smelt containing sodium sulfide with hot steam and carbon dioxide at a temperature above the melting point of sodium carbonate in a reaction zone thereby converting substantially all of said sodium sulfide to molten sodium carbonate;

(b) taking hydrogen sulfide, residual carbon dioxide and steam overhead;

(0) contacting said hydrogen sulfide, residual carbon dioxide and steam in a furnace zone with combustion air to convert said hydrogen sulfide to sulfur dioxide;

(d) reacting said sulfur dioxide in a sulphiting zone with sodium sulfite to form bisulfite;

(e) withdrawing said molten sodium carbonate from said reaction zone;

(f) introducing said molten sodium carbonate into a dissolving zone and there dissolving said molten sodium carbonate in water to form an aqueous solution of sodium carbonate;

(g) employing a first portion of said aqueous solution of sodium carbonate as cooking liquor in the second stage of a two stage delignification process, a second portion for reaction with sodium bisulfite to form sodium sulfite for use in sulfur dioxide recovery and the final portion for carbon dioxide make-up, the carbon dioxide released in the process being returned to said reaction zone.

References Cited in the file of this patent UNITED STATES PATENTS

Claims

1. IN A PROCESS FOR TREATING SODIUJM BASE WASTE LIQUORS OBTAINED FROM A WOOD PULPING PROCESS, THE STEPS OF BURNING SAID WASTE LIQUORS TO FORM A SMELT CONTAINING SODIUM SULFIDE, CONTACTING THE RESULTING SMELT WITH CARBON DIOXIDE AND STEAM TO CONVERT SODIUM SULFIDE TO SODIUM CARBONATE, AND MAINTAINING THE TEMPERATURE OF SAID CONVERSION ABOVE 851*C.