SE177250C1 - - Google Patents

Description

Inventors: KR Gray, HL Crosby, and JC Steinberg Priority set forth May 13, 1953 (United States of America) This invention relates to the digestion of wood chips with an acidic sodium sulfite coquatase and is intended to recover the chemicals from the sulfite liquor. reuse in the process.

The method according to the invention is above all advantageous in that it makes it possible to utilize the soda in the melt obtained during the combustion of the liquor, partly for the production of new cooking liquor, and partly as a flux for the small malt, so that it comes. to be able to flow out easily, from the soda oven.

The process has significant advantages in that in the melting of sodium chemicals a more rigid part ay, the sulfur is retained, which would otherwise be lost with the exhaust gases such as SO2 in very dilute form. According to the invention, the sulfur retained in the melt is recovered in a concentrated, easily usable form. By centralizing the strongly acidic aviuten evaporation according to the invention, the need for an expensive corrosion-resistant apparatus is eliminated.

Previously, it was not considered economically viable to produce acidic sodium sulphite pulp, due to the very high cost of the required sodium salts compared to calcium salts and also due to the impossibility of recovering the other chemicals in a form usable for the process. Although acidic sodium sulphite liquor can be easily evaporated and the evaporated liquor is burned under reducing conditions, due to the presence of sulfur in molar excess in relation to sodium in the liquor, the inorganic residue of the sodium chemicals becomes extremely corrosive at the high temperatures in the soda oven. The melt will also have such a high melting point that, due to its fidelity, it will be the answer to drop the Iran furnace. life with the view of the molar excess of sulfur Over sodium in acid sodium sulphite liquor goes more On half of the sulfur in the liquor lost up in the chimney in such a dilution and in such contaminated form that it would not prove economically viable to recover the sulfur. The acid sulphite liquor Or with a wear low pH value of 2-3 is also highly corrosive, which is why the construction material required for the devices will be very expensive and will involve a significant capital investment.

In addition to the difficulties of recovering the sodium chemicals from the furnace in the form of a liquid melt, the method has hitherto not been able to be used in practice for converting such sulphide-containing melts into a form suitable for the production of acid sulphite coquatase.

The method according to the invention overcomes the existing responsibilities, and one obtains at its disposal an efficient and unchangeable method of recovering the sodium chemicals for reuse in the process. In addition, the water recipient is not contaminated with sewage. Yid the combustion of the, organic material in the liquor is recovered a heat mantle, which can state. to about 440 0-5 040 heal / kg dry organic solid.

In the production of pulp by the acidic sodium sulphite process, the predominant reaction is the conversion of the insoluble lignin in the wood into soluble sodium lignosulphonate, which dissolves in the sulphite liquor. The combustion of the liquor resulted in large amounts of sulfur being lost in volatile form (ie sometimes sulfur dioxide) and a melt with a high sulphide content is formed. This combustion of the sodium lignosulfonate is assumed to proceed according to the following reaction formula, assuming that all available sodium will combine with the sulfur: 2 Lig-SONa Combustion under Na 2 SO 2 (+ CO 2), reducing conditions 2 - In practice, the part of the sulfur, which is lost as diluted sulfur dioxide in the chimney, becomes larger than according to the above-mentioned formula, because the sulphite liquor is very acidic and contains free lignosulphonic acids, in addition to sodium lignosulphonate, and it can luneItalia smaller amounts of sulfur dioxide, which has not been evaporated.

In practice, more sulfur will be lost than from the formula above, because the sodium in the sodium titanosulfonate does not completely and the hall will precipitate sodium sulphide, titanium also a certain amount of sodium carbonate under the release of air sulfur dioxide, which is lost in the chimney gases. The formation of sodium sulphide will, however, dominate, especially with regard to the excessively large excess of sulfur in relation to sodium, so that a melt with a high content of sodium sulphide will be formed.

That a very high content of sodium sulfide will result in a mixture with a high melting point is shown in Table 1 below, which shows the melting temperatures of different mixtures of Na2S and Na2CO2. I col. 3 hate the mixtures according to carbon. 1 has been recalculated to express the total amount of sodium present in the form of Na2S, which for this binary system Or is equivalent to the sulfidity. Under reducing conditions in the furnace, and if one disregards any loss of air sulfur in the chimney gases, one mole of Na 2 SO 3 will theoretically form one mole of Na 2 S in the melt.

Table 1.

Percent Na2S in Na2S-Na2CO3 mixture 0 point * C 852 Percent Na2S 6 88.0 S'S 19.3 8036.7 38 800 45.4 42.7950.0 857, 60 867.0 70 890 76.0 7980.3 82 970-990 86.0 1001180100 In current smelting operations, the smalt will contain significant amounts of sodium sulphate and smaller amounts of other sodium salts. This will tend towards slightly lower melting points On what is stated above for mixtures of pure soda and sodium sulphide. A melt containing 45.0% Na 2 S, 45.0% NaO 2 O 2 and 10% Na 2 SO 4 showed, for example, the melting point 738 ° C. An nail melt containing 70.0% Na 2 S, 20% Na 2 CO 3 oak 10.0% Na 2 SO 4, had a melting point of 777 ° C.

The term «sulfidity» refers to the molar ratio between Na2S and the sum of Na2S Na2CO3.

In order to be able to produce a slurry with the aid of soda ash, as Or, so lattfluten, that it can be easily drained & On a soda furnace air usual type, one must .treat the sulphite liquor with soda in sufficient quantity, father to (1) neutralize free lignosulfonic acids and free sulfur dioxide, (2) producing an excess of soda in the liquor in an amount equivalent to the sulfur dioxide formed during the combustion of the liquor (according to the above reaction formula), i.e. corresponding to a melt with 100% sulfidity, and (3) add soda in additional surplus to obtain a liquid melt with soda content.

Table 1 above shows that optimal results are obtained if soda is present in a sufficient excess for the smelt to give a sulphidity of about 50%. As a result, however, in practice some of them will prove suitable to add a sufficient amount of soda to the finished furnace treatment or incineration, in order to obtain a melt with about 25 sulphidity or other layers. The reason for this is not fully clarified, but should probably be attributed to the fact that, in addition to the excess of soda and sodium sulphide, there will always be present smaller amounts of other sodium chemicals, e.g. sodium sulphate, thiosulphate and if the pulpwood has been soaked in salt water - sodium chloride. Delta will change the properties of the melt.

According to a previously unpublished proposal, it is possible to convert a sodium sulphide-containing melt into a sulphide-free soda solution in the solution phase. Although such a soda ash is applied to the production of 'acid sulphite boiling water', some similarities may arise with adding such a soda material to the effluent to neutralize it and reduce the sulphidity of the treatment in the soda furnace. Depending on the limited solubility of the sodium salts of the carbon dioxide and the large amount of soda ash required, an additional amount of water would be required to be added to the iron side with the chemicals, which would require a considerable further evaporation. For example, if a soda solution containing 100 g / l (which can be easily obtained by carbonating a sodium sulfide-containing melt dissolved in water) is used, it would be necessary to add 9 kg of water to the usual kg of carbonate. This shows that it would be a very star advantage if the soda could be added in solid form.

The present invention provides a simple and useful method for converting the sodium content of an acidic sodium sulfite liquor to a liquid melt having a high content of sodium carbonate and sodium sulfide, with consequent recovery of heat and recovery of the melt from a substantial portion of the liquor from the liquor. 'the beginning present sulfur. The invention further provides a simple method of converting this melt to solid sodium carbonate of such high purity that it can be used for the preparation of acidic sodium sulphite cocaine and which is also practically anhydrous, so that it can be used. is to neutralize the liquor and lower its sulfidity, without diluting it economically. At the same time, the sulphide sulfur is recovered in the melt in the form of concentrated sulfur cotton, which can be easily burned to sulfur dioxide with such a concentration that it can be used easily and conveniently in the production of acid sulphite coke water.

The process according to the invention is characterized in that the melt is dissolved in such a limited amount of water that essentially only sodium sulfide is dissolved while forming a suspension of sodium carbonate crystals and a minor amount of sodium sulfate crystals in a concentrated solution of sodium sodium sulfate and sodium sulfate crystalline sodium sulfate , salts some of these crystals to the sulphite liquor, before burning them to lower the melting point of the inorganic constituents.

An important step in the process is often to form a liquid melt, which is cooled to staining, before it is dissolved with water. The melt was suitably continued with a solution of sodium carbonate and sodium sulphate containing such a limited amount of water that a concentrated solution of sodium sulphide was formed, and that sodium carbonate and sodium sulphate were separated and formed a suspension.

According to a more complete, driven process according to the invention, the solution of the melt is subjected to carbonation, maintaining at least four interdependent circulation systems consisting in (1) returning liquor, which has been subjected to evaporation, to the melt to form a concentrated suspension consisting of sodium sulphide solution. and sodium carbonate crystals, (2) separates a part of the sodium carbonate crystals instead of leading it to the spent pre-combustion as a flux to lower the melting temperature of the inorganic constituents, (3) the other part of the sodium carbonate crystals leads to the preparation step on a sodium basis and (4) oxidizes the hydrogen sulphide to sulfur dioxide and leads the sulfur dioxide to the preparation step for acid-based acid sulphite coke ash and returns the sodium-based acid sulphite boiling water to the digestion step.

In the carbonation with evaporation of hydrogen sulphide, the sodium sulphide solution is treated with carbon dioxide under, a pressure of at least 1.1 hp / cm 2 absolute pressure, preferably yid 501500 C, and the hydrogen sulphide is then evaporated at a vacuum of 127-736 Torr, preferably yid 65-1 ° C.

Although it usually proves sufficient to carbonate the sodium sulphide solution in a single strut under pressure with subsequent vacuum stripping, according to the invention the carbonation under pressure can be carried out in several struts, mart and a resultant of a vacuum stripping.

According to an advantageous embodiment of the process according to the invention, the liquor is evaporated and burned under reducing conditions to form a free-flowing melt, which is partially dissolved in such a limited amount of water, preferably a mother liquor obtained from the stripping step, which contains soda and sodium sulphide, slurry of sodium carbonate and sodium sulphate crystals is obtained in a concentrated sodium sulphide solution, which, as described above, is separated after the separation of the crystals is carbonated with subsequent evaporation of the sulfur-velocity formed during the carbonation.

The process according to the invention has the significant advantage of obtaining a narrow, melting point of Mars, which is relatively low thanks to the presence of sodium carbonate as a flux, which is formed during the carbonation. Another advantage is that a concentrated solution of sodium sulfide is obtained, from which sulfur can be recovered efficiently and in high concentration for preparation in boiling water. Other advantages have emerged from the Riljande heskrift.

In the accompanying drawing, Fig. 1 shows the four important, interdependent circulation systems in the method according to the invention, and Fig. 2, an arrangement with apparatuses which are suitable for carrying out the method according to the invention. The sulphite liquor, if it was evaporated and burned in a soda furnace in the usual way, would form a melt with a high sinus point, which would be the answer to the waves from the melting furnace. According to the present invention, the melting point of the melt was lowered by all solid soda frail. The suspension was added to the sulfite liquor by cycle in Fig. 1. The resulting low melting melt is dissolved in the least possible amount of water, preferably by forced mother liquor from cycle 2 was added to form a suspension. of sodium carbonate crystals in a concentrated solution of sodium sulfide. By using a limited amount of water, and especially by using the stripped mother liquor, which is already rich in sodium carbonate and sodium sulfide, one can achieve practical separation of sodium carbonate crystals for use in the process, and moreover, obtain a concentrated solution of sodium sulfide Iran. which sulfur is recovered. The other part of the sodium carbonate crystals is used in the preparation of the acid sulphite boiling liquor in cycle 3, where it is reacted with sulfur dioxide from cycle 4 to form an acidic sodium sulphite liquor, which Or uses directly in the boiling process. In this case, the circulatory systems cooperate to efficiently recover sodium chemicals by digesting pulp through the acidic sodium sulphite process.

In the device shown in Fig. 2, the pulp is digested by boiling with acid sodium sulphite boiling water in a digester 1. The effluent from the digester is treated at 2 with solid sodium carbonate, which releases carbon dioxide, which can later be used in the carbonation, and the treated liquor can be clarified at 3 sludge removal. The treated liquor (whether or not it has been clarified) is evaporated, preferably to at least 50% solids content, e.g. in multiple power evaporator 4 (or in multiple power evaporator and subsequent evaporation in direct contact with exhaust gases, eg in cascade or disc evaporator), and the concentrate is burned in a conventional furnace 5, e.g. of the same kind as the sulfur ovens of the sulfur factories. The furnace is operated under reducing conditions, and the obtained sodium sulphide with a worn content of soda, which gives a melt with a low melting point, is drained in a slightly liquid state from the furnace to the loser 6 (in this case a container where soda crystals are formed). The low sulfidity of the sinter tends to reduce any risk of explosion as a result of until the melt is added to the solution liquid (which may be water or preferably mother liquor from the stripper). It is highly concentrated and may contain as a solid component, for example, a mixture of about 70% soda and 30% sodium sulfide.

Alternatively, after draining from the oven, the melt can be cooled and, after solidification, added to the ladle or water. The cooling would eliminate the need to add excess water with consequent warning losses in passing. In each case only a limited amount of water was added, the melt (as an example: to some melt was added some water with the male to the evaporation), so that essentially all the sodium sulphide was read without significant dissolution of the sediment. incomplete reduction, a certain amount of sodium sulfate will be present in the melt. This sodium sulphate will for the most part crystallize out at the same time as the excess soda ash and together with this form a suspension in a concentrated solution of sodium sulphide. The suspension is filtered in a straight filter 7, the sodium sulphide solution is passed to a carbonator 8 together with a little additive water,, and the solid constituents in the suspension, i.e. soda and sodium sulfate, are divided into parts. One part is mixed with the sulphite liquor to neutralize and lower the sufficiency, while the other part in a scrubber 9 is washed with water, which may contain a little soda and sodium sulphate, after which the sodium sulphide-containing water is passed to the filter, while the washed the soda crystals and the sodium sulphate crystals are dissolved in water in the boiling water boiler 11, a compound with sulfur dioxide is formed to form acidic sodium sulphite liquor, which is led to the digester 1 for boiling the chips.

The concentrated loading of sodium sulfide. from, the filter 7, which may have a sulfidity of about 90%, is passed to the carbonator 8, where it is treated with carbon dioxide under pressure, and then passed to a vacuum stripper 12 to recover sulfurate and to form of a mother liquor with holy sulfidity, i.e. containing solids in a ratio of about 70 Vo soda and 30% sodium sulfide.

The stripping can be carried out in a device of the usual type, e.g. a filler column, which is arranged to operate at a vacuum of 127-736 Torr. As the previous carbonation was carried out under pressure, an explosion-like development of sulfur waffle occurs in a heap-concentrated form but with insignificant dilution with carbon dioxide, which is formed by bicarbonate probing.

According to the preferred embodiment of the process, the sulphide-containing weld is preferably carbonated in countercurrent with a carbon dioxide-containing gas under pressure. The subsequent evaporation is carried out with the aid of water vapor at a vacuum of 127-736 Torr, preferably at least 508 Torr to remove volatile sulfur water in concentrated form, whereby the sulphide content of the melt solution is substantially reduced. According to this preferred method, the carbonation is usually carried out at 50-1 ° C and 1.4-11.6 kp / cm 2 absolute pressure, and on such a salt that the molar ratio between absorbed carbon dioxide and total titratable alkali in the solution will to lie in the range 0; 6-1.5. In that the carbonation is carried out at elevated temperature and is thus used in reaction with sodium hydrosulfide in the stripping step, relatively concentrated solutions of sodium salts (ie of the order of 100 g / l, stripped as Na2 O) can be treated without the formation of precipitates.

By "totally titratable, alkali", above is meant the basicity obtained by titration with methyl orange as indicator. When titrating soda malt, the titration will include the sulfide, carbonate, and sodium hydroxide and half, of sufficiency but will not include such salts as thiosulfate, sulfate and chlorides.

Under the stripping conditions according to the invention, ie. Release of hydrogen sulphide from the marine solution, which has been carbon dioxide-treated under pressure. In a vacuum, whereby the solution is brought into contact with steam at low temperature, very little bicarbonate decomposition takes place, with the consequence that the evolved gas consists mainly of hydrogen sulphide and water vapor. The steam is used as a propellant, since a recovery of concentrated sulfur gas can be easily achieved by condensing the steam from the effluent Mouth of steam and sulfur. The use of high pressure during the stripping, as applied to this operation, is an important technical feature of the stripping process. Reduced pressure significantly improves the conversion and clearly reduces the carbon dioxide losses from the bicarbonate subdivision.

Regardless of whether pure carbon dioxide or carbon dioxide-containing ragas have been used in the carbonation step, it is not necessary and, not least, to try to remove all the sulfur water in the subsequent propulsion operation, with regard to the fact that it will require an elite. the greater the consumption, the more it approaches the end. liarbonatization and a-vdrivning.en can be carried out In appliances air for gas absorption and evaporation usual types, e.g. filler body columns, sldv columns, ‘sprinkle columns’ and continuous liquid phase columns. Devices for dispersing the gas at the inlet can be used to advantage with the carbonation.

When using carbon dioxide-containing gas for carbonation, a slightly higher working pressure or larger equipment is required than when using a carbon dioxide belt. If a part of the solid soda produced is to be used for the production of acid sulphite cocaine, the carbonate should have a high degree of purity, for which it is then advisable to purify the exhaust gas by removing suspended material and washing away all sulfur dioxide and others in it. onskvarda f Ororeningar. Lime kiln gas, which is liberated from particulate matter, can be used to advantage, as it normally contains 30-45% carbon dioxide.

In the case of use of emitted carbon dioxide (eg flue gases or lime kiln gases) in the carbonation under pressure, it is most appropriate to use the countertron principle. In doing so, it comes from the top, of the carbonating apparatus, emitting the gas with high-alkaline solutions (ie narrow solutions). Probably due to the high alkali content of the solutions in the middle of the gas outlet. The amount of sulfur water, which leaves the carbonator in the diluted form, is very small. On the basis of this, the predominant amount of the sulfur water formed is released during the evaporation in a highly concentrated form.

If carbon dioxide is used for the carbonation, the formation of air thiosulphate can be disregarded. When using air-diluted oxygen-containing carbon dioxide gas (eg ragas), thiosulphate is formed in unexpectedly small amounts, which, therefore, does not accumulate quickly in the system. The small amount of thiosulphate formed during the carbonation by the use of flue gas, which contains intl oxygen, is extremely small in comparison with the very large amounts of thiosulphate, which is formed during carbonation and solid smelting with carbon dioxide-containing flue gases.

The solution obtained from the carbonation is introduced into a stripper, in which a pressure is maintained which is substantially lower than the carbonation pressure and lower than the pressure of the water at the carbonation temperature, whereby a vigorous and sudden release of the water-hinge occurs during development of only sulfurate. small amount of carbon dioxide. It has been found expedient to allow this rapid release to take place at the top of a column, after which the remaining liquid flows down through the column, the bottom of which meets the water vapor fed from below. This effectively removes the sulfur water formed by the yid process.

The sulfur water recovered from the vacuum evaporation will contain flakes indicating which map is being condensed, thereby obtaining sulfur water of high concentration. By burning with lull, the sulfur fat can be easily oxidized to sulfur dioxide for use in the production of acid sulphite coke water. This sulfur dioxide is absorbed in a solution of sodium carbonate in the form of a separate suspension from the loser, for the preparation of acidic sodium sulphite coke water for digestion into mass.

Claims (5)

Patent and practice: 1. Set to treat the liquor in pulp preparation by digestion of wood chips with acidic sodium sulphite liquor comprising the means of evaporating the sulphite liquor and burning the obtained concentrate under reducing conditions to form a narrow, containing sodium sulphide, dissolving the molten water in such water. that mainly only sodium sulphide is in solution to form a suspension of sodium carbonate crystals and a minor amount of sodium sulphate crystals in a concentrated solution of sodium sulphide and to separate the sodium carbonate and sodium sulphate crystals, characterized in that the sodium sulphate is separated from the sulphite and the sodium sulfate crystals for Mt lower the melting point of the inorganic constituents formed by the burning of the sulfite liquor. Salt according to claim 1, characterized in that a liquid melt is formed which is cooled to solidification before being dissolved with water. 3. According to claim 1 or 2, characterized in that a solution of sodium carbonate and sodium sulphate, containing a limited amount of water, is added to the melt, to form a concentrated sodium sulphide solution in which sodium carbonate and sodium sulphate crystals are separated and formed. . 6— - 4. A salt according to claim 1, 2 or 3, characterized in that the solution of the melt is subjected to carbonation during evaporation of sulfur water, maintaining at least four interdependent circulatory systems, consisting in returning (1) liquor which has been subjected to evaporation, to the melt to form a concentrated suspension consisting of sodium sulfide solution and sodium carbonate crystals, (2) separates a part of the sodium carbonate crystals and leads them to the spent pre-combustion to lower the melting temperature of the inorganic constituents as flux, (3) conducts the other part separating the sodium carbonate crystals to the sodium sulphite boiling solution of sodium sulfate and (4) oxidizing the hydrogen sulfide to sulfur dioxide, and passing the sulfur dioxide to the acidic sodium sulphate boiling preparation stage of Saint Leif Or the acidic sodium sulfate boiling water to the digestion step. 5. A kit according to claim 1, characterized in that the portion of the sodium carbonate and sodium sulfate crystals which was not added to the sulfite liquor prior to its combustion is reacted with SO 2 to form escaping digestion liquid.