JPS591835B2 - Recovery method for sodium sulfite cooking chemicals - Google Patents

Description

[Detailed description of the invention] The present invention relates to a method for recovering sodium sulfite cooking chemicals.

The SCP method and CGP method are representative pulp manufacturing methods that use sodium sulfite as a cooking chemical.These methods have higher pulp yields than the KP method, and the price of raw wood has increased in recent years. Therefore, it is advantageous for producing inexpensive pulp.

However, unlike the KP method, the SCP method and CGP method do not have established methods for recovering cooking chemicals, and factories implementing these methods are required to establish a closed system from the perspective of pollution problems. ing.

In response to this demand, as a method for recovering sodium sulfite in the SCP method and CGP method, a smelt consisting of the main components Na2S and Na2CO3 obtained by concentrating and burning cooking waste is directly oxidized to convert Na2S to Na2SO3, and then converted into this aqueous solution. Absorbs exhaust gas containing SO2 and converts it to Na2CO
The present inventors have proposed a method of converting and recovering 3 to Na2SO3 (see Japanese Patent Publication No. 14401/1983).

The above method discloses the principle of the recovery method of sodium sulfite in the SCP method, but for its industrialization, smelt must be prepared in addition to sodium carbonate and sodium sulfide, as well as sodium sulfate and trace amounts of sodium thiosulfate, sodium sulfite, sodium chloride, and sodium chloride. Since the smelt contains combustion carbon and there are various pulp cooking methods, the composition ratio of the components in the smelt differs in each factory, making it difficult to design equipment and complicating operation management. It was desired to find out how to reduce chemical loss in the entire process and maintain the material balance of the entire process.

The present invention was made in view of the above-mentioned current situation, and its purpose is to provide a method for recovering sodium sulfite cooking chemicals that solves the pollution problem and reduces chemical loss by collecting and regenerating the cooking chemicals used in the SCP method and CGP method. It is to provide.

To summarize the present invention, the method for recovering sodium sulfite cooking chemicals (first invention) of the present invention includes (a) completely dissolving smelt from a recovery boiler in a slurry circulation tank having a cooler with circulating slurry; Therefore, the proportion of soda carbonate in the total solid content of the slurry is lower than the proportion of soda carbonate in the total solid content of the smelt, and the total solid content concentration is 35%.
A first step in which a slurry of smelt containing ~70% by weight and a temperature of 55 to 90° C. is formed, and this slurry is circulated to the smelt receiving device while a portion of the slurry is extracted; (b) the slurry extracted from the slurry circulation tank; is separated into a liquid containing solids and fine crystals, and a part of the slurry and/or a part of the liquid separated from the solids is passed through a drum flaker to form flakes, and then the water content is 10 to 50% by weight. The solids and flakes are sent to an oxidation reactor so that the ratio of sulfur to sodium is equal to the ratio of sulfur to sodium in the smelt, while the liquid separated in the drum flaker is sent to a slurry circulation tank. and (c) the solid matter and flakes are powdered in an oxidation reactor consisting of a continuous discharge type main reactor and a side reactor in which the hot powder containing soda carbonate and sodium sulfite is forcibly stirred in advance. It is characterized by comprising a third step of oxygen oxidation in granular form, and
The method for recovering sodium sulfite cooking chemicals of the present invention (second invention) includes (a) completely dissolving smelt from a recovery boiler in a slurry circulation tank equipped with a cooler to reduce all solids in the slurry; The proportion of soda carbonate in the smelt is lower than the proportion of soda carbonate in the total solid content of the smelt, the total solid concentration is 35-70% by weight, and the temperature is 55-70% by weight.
A first step of forming a smelt slurry at 90°C and circulating this slurry to a smelt receiving device while extracting a part of it; (b) a slurry extracted from a slurry circulation tank containing solids and fine crystals; The liquid is separated, and a part of the slurry and/or a part of the liquid separated from the solids are passed through a drum flaker to form flakes.
The solids and flakes were sent to an oxidation reactor with a composition of 0 to 50% by weight, the ratio of sulfur to sodium being equal to the ratio of sulfur to sodium in the smelt, while they were separated in a drum flaker. 2nd step (c) of returning the liquid to the slurry circulation tank: The solids are collected in an oxidation reactor consisting of a continuous discharge type main reactor and a side reactor, in which hot powder and granules containing sodium carbonate and sodium sulfite are forcibly stirred in advance. and a third step in which the flakes are oxidized with oxygen in the form of powder and granules; (d) the powder and granules mainly composed of sodium sulfite and soda carbonate obtained in the third step are dissolved in water to form undissolved matter and a clear liquid; and (e) clarifying the insoluble matter separated in the previous step and the aqueous sodium sulfite solution obtained in the previous step. The 5th step of washing and dewatering the separated sludge is combined, the dilute solution separated in the 5th step is returned to the 1st step and/or the 3rd step, and the material balance of the entire process is calculated. It is characterized by being maintained.

According to the present invention, the ratio of sulfur content to sodium content in the solid material supplied to the oxidation reactor in the third step, that is, S/
It has been found that the conditions of the first, second and third steps can be set in relation to each other so that the Na2O is approximately equal to that of the smelt, and as a result the loss of cooking chemicals can be extremely suppressed.

In the general SCP method, the composition of smelt is Na2S
30-40%, Na2C0345-60%, Na2SO
45-10%, Na2S2O30-5%, others 2-4
%, and it is known that a reaction occurs when this smelt is oxidized with oxygen, but a reaction in addition to the above reaction was also confirmed through experiments by the present inventors.

Therefore, in such a complex reaction, Na2S
promotes the reaction in which Na2So3 is converted to Na2So3.
It is extremely difficult to suppress the reaction of conversion to SO4.

Moreover, the reaction requires the presence of water, and if the sulfur content is recovered in the form of Na2SO3, the S/N of the solid material supplied to the oxidation reactor in the third step is
It is reasonable to adjust a2O so that it is approximately equal to that of smelt, but due to the amount of water accompanying the solid material, S
/Na2O varies, and elucidation of this correlation is also extremely difficult.

According to the present invention, this problem has been solved by combining the first to third steps.

Next, each step of the present invention will be explained in detail.

In the first step, smelt from the recovery boiler flows into a smelt receiver equipped with a smelt chopper and a smelt screen.

The smelt is crushed by steam in the smelt hopper, and the inner wall of the smelt hopper is washed with circulating slurry from a slurry circulation tank to prevent smelt particles from adhering to the inner wall of the smelt hopper.

At this time, outside air may be sucked into the smelt hopper to oxidize a portion of the sodium sulfide in the circulating slurry to Na2S2O3 (hereinafter, this oxidation will be referred to as preoxidation).

The crushed smelt particles are separated by a smelt screen, and the small particles are flowed into a slurry circulation tank, while the coarse smelt lumps are dissolved in a green liquor tank attached to the slurry circulation tank, and the green liquor is used to wash the inner wall of the smelt hopper. and is sent to the slurry circulation tank as make-up water.

The smelt particles must be completely dissolved in the slurry circulation tank.

Incompletely dissolved means that Na2S, Na2Co3, etc. in the smelt is once dissolved and recrystallized to form a slurry, or is not completely dissolved and forms a slurry.

In the slurry circulation tank, the proportion of Na2CO3 in the total solid content of the slurry is equal to that of Na2C in the total solid content of the smelt.
A state must be maintained in which the percentage of O3 is lower than that of the slurry.
% by weight, preferably 45-60% by weight, slurry temperature 5
It is obtained by controlling the temperature to 5 to 90°C, preferably 75 to 83°C.

The slurry temperature is maintained by a cooler provided in the slurry circulation tank, such as a cooling coil or a cooling jacket.

The amount of slurry circulated between the slurry receiving device and the circulation tank is 20 to 200 times the amount of inflowing smelt, preferably about 1
It is 00 times more similar.

In the second step of the present invention, the slurry extracted from the first step is fed to the oxidation reactor in the third step at a concentration of S/Na2 equal to the ratio S/Na2O of the sulfur content and sodium content of the smelt.
The ratio of total solids to entrained moisture is adjusted to provide a composition of O and to satisfy the reaction conditions in the oxidation reactor.

Specifically, a part of the circulating slurry in the first step is sent to a head tank by a pump, and a fixed amount is supplied from the head tank to, for example, a screw decanter, so that the circulating slurry is reduced in moisture by 1 part.
The solids containing 0 to 50% by weight, preferably 15 to 35%, and the liquid containing fine crystals are separated, and the solids are sent to the oxidation reactor in the third step, while the liquid is sent to the slurry circulation tank in the first step. send.

Since this liquid has a high content of Na2S, when it is returned to the slurry circulation tank, N in the liquid phase of the slurry is
The content of a2S becomes higher than the S/Na2O of the smelt, and as a result, the S/Na2O in the solid separated from the slurry obtained in the circulation system of the first and second steps.
can be controlled to be approximately equal to that of smelt.

By the way, in cooking using sodium sulfite, the blending ratio of Na2S and Na2CO3 used varies depending on the cooking conditions, so the composition of the smelt obtained from the recovery boiler also changes.

Therefore, the general composition of the smelt is as described above, but in some cases the Na2S content may exceed 40% by weight.

In this case, in order to make the S/Na2O of the separated solid approximately equal to that of the smelt, a liquid with a high Na2S content is attached to the solid, or Na2S is in the liquid phase of the slurry.
The concentration has to be increased.

However, in the former case, it is very difficult to operate a solid-liquid separator, such as a screw decanter, and the liquid overflows to the separated solids, and when this is sent to the third step, an oxidation reaction occurs due to excess moisture. Conditions become extremely difficult to control.

On the other hand, in the latter case, the temperature must be raised due to the solubility of Na2S in the aqueous solution of the multi-component sodium compound that forms the smelt (however, at temperatures above 85°C, the temperature
-8 series stainless steel is also highly corrosion resistant and requires increased equipment costs due to the use of materials with high nickel content.

Therefore, as a means to make the S/Na2O of the solids supplied to the third step approximately equal to that of the smelt while maintaining the total solid concentration and temperature conditions of the slurry in the first step, another method is used to supply the insufficient sulfur content. A feeding device is required.

A feature of the present invention is that a drum flaker is used as such a supply device to cool and solidify a portion of the slurry supplied from the first step and/or a portion of the liquid separated by the screw decanter to form flakes. and the third layer with the solids.
The above objective can be easily achieved by taking additional means to supply the process.

Next, in the third step, the solids obtained in the second step and optionally the flakes are preliminarily treated with Na2Co3 and Na2SO.
The hot powder containing 3 is sent to an oxidation reactor where it is forcibly stirred.

The pre-existing thermal granules are the oxidation product of the solids in continuous operation, and the freshly supplied moisture is 10-50% by weight.
The solid material 1 is sprinkled with pre-existing hot powder and granules.

This reactor is, for example, a twin-screw kneader type mixer equipped with a weir and an overflow chute, and continuously discharges the oxidized products.

If Na2S remains unoxidized in this discharge, it is further sent to a side reactor.

If no Na2S remains, the side reactor may be omitted.

It is generally convenient to carry out oxidation using a main reactor and a side reactor, in which the solid content is oxidized by oxygen and N
a2S and Na2S2O3 (including from pre-oxidation) are converted to Na2SO3.

The temperature inside the reactor rises due to the heat of oxidation reaction, but is regulated by the entrained moisture.

The heat of reaction when Na2S is oxidized to Na2SO3 is 171
Kcal/? On the other hand, the reaction heat of oxidation to Na2S2O3 is 112K cal, "'mol. Therefore, in order to lower the reaction heat of the oxidation reactor, a process such as a smelt receiving device or a sludge IJ-circulation tank is required before the oxidation reaction is introduced. In this case, the amount of heat generated in the oxidation reactor can be reduced by performing preliminary oxidation.

Oxygen as an oxidizing agent is oxygen or air, and the temperature is 2 to 20 times, preferably about 10 times, the theoretical amount of oxygen for oxidizing Na2S to Na2SO3, and the temperature is ioo to 200°C, preferably about 1
The reaction temperature of the main reactor is maintained at 100-300°C, preferably 150-250°C, and the reaction time is 2-15 hours, preferably 3-6 hours.

In the low reaction temperature range, Na2S2O3 tends to remain, and in the high reaction temperature range, Na2SO4 tends to be produced.
This must be adjusted depending on the amount of moisture entrained.

In the main reactor, unreacted Na2S and Na2S2O3 are
The oxidation products of the main reactor containing Na2S produced by reaction with a OH and a little water are continuously discharged and fed to the side reactor.

The amount of oxygen in the side reactor is 10 to 30 times that of the main reactor.
%, temperature condition is 100-300℃, preferably 150℃
~250°C.

The remaining Na2S from this side reactor is Na2S
Converted to 03.

In the oxidation reaction, the reactions of formulas (5) and (6) require NaOH, but the reaction of formula (4) requires NaOH.
H is replenished.

However, NaOH reacts with CO2 in the oxidizing air to a small extent and is consumed as Na2CO3.
Since OH is insufficient, Na2S2O in the oxidation reactor
NaOH may be added to prevent 3 from remaining.

In addition, in this oxidation reaction, water is necessary for the reaction of formula (4) and for adjusting the reaction heat, and water is replenished as water contained in the solids and flakes supplied to the oxidation reactor, but sometimes there is insufficient water. In some cases, water is added to the oxidation reactor.

In particular, since water tends to be insufficient in the side reactor, residual Na2S can be reduced by adding water.

When operating the oxidation reactor, powder is also entrained in the exhaust air from the main reactor and side reactor, so the powder is collected with a cyclone separator or other dry separator, and the main reactor, case is returned to the side reactor.

This ensures the amount of powder in the main reactor, assists in the dispersion of the solid acid or flakes into powder, and furthermore uses a wet method to collect fine powder that cannot be separated by a cyclone separator, etc. A dust remover is installed, the collected powder is dissolved in water, and the aqueous solution is used to dissolve the almost completely oxidized powder discharged from the side reactor, and the concentration is 15 to 25% by weight and the temperature is 30%. An aqueous solution containing Na2CO3 and Na2SO3 as main components at a temperature of 0.degree. C. or higher can be recovered.

Another feature of the present invention is (d) dissolving the granular material mainly composed of sodium sulfite and soda carbonate obtained in the third step in water to separate the undissolved matter and the clear liquid; A fourth step of converting soda carbonate into sodium sulfite by absorbing exhaust gas containing sulfur dioxide into the liquid; and (e) clarifying the sludge of insoluble matter separated in the previous step and the aqueous sodium sulfite solution obtained in the previous step. The fifth step of washing and dewatering the sludge separated by the process is combined, and the dilute solution separated in the fifth step is returned to the first step and/or the third step to maintain the material balance of the entire process. exists in

(Second Invention) As mentioned above, it is known that exhaust gas containing SO□ is absorbed into an aqueous solution of Na2CO3 and Na2SO3 and recovered as Na2SO3.
It was confirmed that the granular material mainly composed of Na2CO3 and Na2sO3 obtained in the third step contained extremely little residual Na2S, and could be used as an S02 absorption liquid simply by clarifying this solution.

In addition, by removing carbon sludge, iron compounds, etc. during clarification, Na2S due to 02 contained in the exhaust gas is removed.
Conversion of O3 to Na2SO4 can be suppressed.

The clarification in the fourth step is carried out using, for example, a thickener.

The clarified liquid is sent to an S02 absorber or sulfite unit where it is contacted with the exhaust gas from the recovery boiler and/or supplemental other boiler exhaust gases.

Recovery boiler exhaust gas may contain a large amount of S03, which is a factor in the formation of Na2SO4, so it may be washed out with a water sprinkler tower valve.

This absorption liquid may contain H2 based on Na2S.
pH 7 to 8.5, preferably 7 to prevent the generation of S.
．． Adjusted to 5-8.

In the fifth step, the Na2SO3 obtained in the fourth step
Since insoluble components from exhaust gas are mixed into the aqueous solution,
Clarify with thickener etc.

At this time, the sludge discharged from the bottom of the thickener and the sludge of the thickener in the fourth step are sufficiently concentrated and contain a large amount of sodium salts.

Therefore, in order to recover these, they are washed with water, further concentrated using a thickener, etc., and the resulting sludge is further dehydrated using a filter.

This thickener supernatant liquid and the P liquid from the filter are used as replenishing liquids for the exhaust wet scrubber provided in the first step to prevent chemical loss and maintain the mass balance of the entire process of the present invention.

Next, a specific example of the present invention will be described with reference to the accompanying drawings.

The accompanying drawings are block flow diagrams illustrating one embodiment of the present invention.

The smelt 28 discharged from the recovery boiler 1 is transferred to the smelt receiver 2
flows into.

Slurry 29 is circulated into the smelt receiving device 2 from the slurry circulation tank 3 at a flow rate of about 100 times the amount of smelt 28, and the smelt 28 is mixed into the slurry.

At this time, steam or high-pressure air is blown onto the inflowing smelt to atomize it.

Furthermore, outside air 30 is forced through the smelt receiving device 2 to remove heat from the inflowing smelt and to pre-oxidize a portion of Na2S contained in the slurry.

The exhaust air 72 from the smelt receiving device 2 is discharged into the atmosphere from the duct 73 after the mist is removed by the wet dust remover 5.

Since the smelt flowing out of the smelt outlet of the recovery boiler 1 tends to cool and clump, it is sometimes necessary to remove the clumps manually, so that large clumps enter the smelt receiver 2.

If this large lump of smelt flows directly into the slurry circulation tank 3, it will be difficult to dissolve because the slurry concentration is high (
, it accumulates at the bottom of the tank and blocks the tank nozzle and piping, making it unfavorable for operation.

Therefore, in order to remove these smelt lumps, the smelt receiving device 2 is provided with a screen (not shown).

The slurry 31 containing the smelt from which large lumps have been removed through this screen enters the slurry circulation tank 3, where the smelt is gradually dissolved.

However, the composition of the liquid phase of the slurry is a multi-component sodium compound aqueous solution, and the components other than Na2S and NaOH are in a supersaturated state even with slight temperature changes, and the smelt is not completely dissolved.

In addition, although the slurry is small, there are temperature and concentration differences in various places, and due to the influence of this, new sodium salts are crystallized, and the solid phase in the slurry is composed of these crystallized single crystals or multiple crystals. It consists of salts, amorphous crystals that cannot be completely dissolved, and insoluble components such as unburned carbon.

On the other hand, the liquid phase of the slurry is mainly composed of Na2S, and also contains NaOH and NaSx produced by the above-mentioned preliminary oxidation, as well as Na2S203, Na2SO4,
Dissolves sodium salts such as Na2CO3 and NaCl.

In addition, the S /N a 20 of the total solid content in the slurry is
S/Na2 of the smelt due to the influence of the liquid returned from the second step
O sulfur content is extremely high.

In terms of specific numbers, the S/Na2O of normal smelt is 0.
．． It is often about 5, but the slurry is about 0.8 to 1.0.

The smelt lumps separated by the smelt receiver 2 flow into the green liquor tank 4 through a chute and are dissolved by a dilute aqueous solution 33 discharged from the wet dust remover 5.

The green liquor tank 4 is equipped with a stirrer to help dissolve the smelt lumps.

The concentration of the green liquor 34 in which the smelt lumps are dissolved is limited by the amount of water to be replenished from the water balance of the entire process, but is approximately 10% by weight.

The green liquor 34 is supplied to the slurry circulation tank 3 to control the slurry concentration.

The moisture replenished by the green liquor 34 is balanced by the moisture entering the main reactor 9 and the moisture discharged from the smelt receiver 2 together with the air, and the total solids concentration in the green liquor tank 4 is stabilized.

On the other hand, the slurry in the slurry circulation tank 3 is cooled by cooling water 35, and the temperature is 55-90°C, preferably 75-83°C.
kept at ℃.

The temperature of the cooling water 35 is preferably 5 to 40 degrees Celsius, preferably 10 to 20 degrees Celsius lower than the temperature of the slurry. Salt crystals precipitate and adhere, inhibiting heat transfer.

However, it is not enough to reduce the temperature difference to 5°C or less.

When cooling and air cooling are performed, oxygen is dissolved in the slurry to preoxidize the sodium sulfide.

The total solids concentration of the slurry in slurry circulation tank 3 is 35
~70% by weight, preferably 45-60% by weight.

As mentioned above, this slurry total solids concentration is maintained by the balance between the moisture in the green liquor 34, the moisture in the solids entering the main reactor 9, and the moisture evaporated in the smelt receiver 2.
If the total solids concentration of the slurry is 35% by weight or less, cooling becomes easier, but the solids separated during separation in the screw decanter 7 contain a very large amount of Na2CO3 and only a small amount of Na2S. The S/Na2O content of the smelt becomes unbalanced.

On the other hand, even if an attempt is made to cool and solidify the slurry or decanter P liquid by the drum flaker 8 and supply it to the main reactor 9, not only is the solidification insufficient, but also the amount of water brought in is too large and the reaction temperature in the main reactor 9 is too high. decreases and a satisfactory reaction no longer occurs.

Furthermore, if the total solids concentration of the slurry exceeds 70% by weight, cooling becomes difficult and it becomes impossible to maintain the slurry temperature at 90° C. or lower.

When the temperature of the slurry exceeds 90°C, the corrosion resistance of the 18-8 stainless steel used as the equipment material deteriorates, leading to corrosion, and the high concentration of the slurry can cause blockage of pipes. This makes stable operation difficult.

On the other hand, if the slurry temperature is below 55 degrees Celsius, the Na2S5 degree of the liquid phase of the slurry will not rise and there will be too much moisture in the main reactor 9.
Lower the temperature.

The slurry 36 that satisfies the above-mentioned concentration and temperature conditions in the slurry circulation tank 3 is sent to the head tank 6, and is supplied to the screw decanter 7 through a pipe 3γ that allows the slurry to flow down naturally to the screw decanter 7 while maintaining a constant liquid pressure.

The ratio of Na2S contained in smelt 28 varies depending on the ratio of Na2SO3 and Na2CO3 used for pulp cooking.

If the ratio of Na2S contained in the smelt is 40% or less, preferably 30% or less, by using only the screw decanter 7, the S/Na of the separated solid can be reduced.
2O can be easily controlled to be almost equal to that of smelt, but if it exceeds 30%, especially 40%, the above-mentioned S/ The Na2O becomes unbalanced, and as a result, the slurry 36 has a high total solids concentration,
The temperature also cannot be maintained below 90°C.

In such a case, the slurry 36 is sent to the head tank 6 and supplied to the drum flaker 8 via a pipe 38 that allows a portion of the slurry to flow down by gravity.

In this case, a portion of the slurry 36 is directly transferred to the drum flaker 8.
Alternatively, a part of the P liquid in the screw decanter 7 may be supplied to the drum flaker 8.

In this case, the line branches off a part of the sap 39 to line 40.
Supplied by

The slurry or P liquid supplied to the drum flaker 8 is supplied in an amount larger than the amount of flaky solids supplied to the main reactor 9, and the excess slurry or R liquid is returned to the slurry circulation tank 3 via a return line 41.

If you try to supply the same amount of flaky solids to the main reactor 9 to the drum flaker 8, you will have to control the flow rate of the slurry or P liquid, and the supply amount will not be that large and the concentration will be high. (Moreover, it is a fluid that tends to solidify when cooled, which is undesirable as it may clog pipes or valves.

Therefore, the slurry or P liquid to be supplied to the drum flaker 8 should be provided with as few valves as possible, the flow rate should be increased, and the amount of flaky solids to be supplied to the main reactor 9 will depend on the number of rotations of the drum flaker and the cooling water 42. Preferably controlled.

In addition, if necessary, part of the circulating slurry and/or part of the liquid returned to the slurry circulation tank may be added to the S/S in the solids.
It can also be sent directly to the oxidation reactor in response to changes in the Na2O composition.

The solids supplied to the main reactor 9 through the above operations depend on the amount of solids 43 separated in the screw decanter 7 and the flaky solids solidified by cooling in the drum flaker 8, depending on the sodium sulfide content ratio of the inflowing smelt. The amount of the substance 44 can be controlled so that the ratio is approximately equal to the ratio of the sulfur content to the sodium content of the smelt, that is, the ratio of S/Na2O.

Further, the moisture content of the solids supplied to the main reactor 9 is controlled to an amount most suitable for the reaction in the main reactor 9.

Therefore, the total solids concentration of the slurry 36 must be controlled within the aforementioned concentration range.

The main reactor 9 is a complete mixing type reactor with a two-shaft stirrer, and most of the solids supplied in advance in the second step have already finished reacting inside, and the water content is usually less than 5%. A large amount of granular material is present.

A solid substance 43 or a flake-like solid substance 44 is supplied thereto and quickly mixed and dispersed with the powder and granules, and the unreacted solid content has a greatly increased contact area with air, creating a state in which it is easy to react.

The air 45 supplied to the main reactor 9 is heated by the heater 14 to a temperature of 100 to 200°C, preferably about 150 to 185°C.

Note that the air 45 is heated by heating steam 47.

The amount of air 45 supplied to the main reactor 9 is 2 to 20 times, preferably about 10 times, the theoretical amount of oxygen assuming that the entire amount of Na2S contained in the smelt is oxidized in the main reactor 9.

The exhaust gas 48 contains moisture evaporated in the main reactor 9, and its dew point increases.

Therefore, if the amount of oxygen is less than twice the theoretical oxygen amount, the dew point is too high, and unless outside air is mixed in with an exhaust fan, etc., dew will condense in the exhaust duct, cyclone separator 11, etc., and the powder will be blown away with the exhaust from the main reactor 9. Resistant to adhesion and occlusion.

However, since the main reactor 9 is not sealed, drawing a large amount of outside air with an induction fan will lower the dew point and prevent clogging of the powder. (Since the turbulent action of the air 45 improves the contact between the powder and granules with the air, the supply amount is at least twice as high, preferably about 10 times as much, as described above.

On the other hand, increasing the amount by 20 times or more increases the steam consumption of the heating steam 47, the power cost of the fan, and the equipment cost, which is not commensurate with the improvement in the effect described above.

Since the powder and granules are strongly stirred in the main reactor 9, some of the scattered powder is discharged together with the exhaust gas 48, and the amount may exceed the amount of solids entering the main reactor 9 in some cases. As it is, the above-mentioned conditions suitable for the reaction cannot be maintained, so the powder collected by the cyclone separator 11 is sent back.

The powder return line 50 is for this purpose, and part of the powder in the exhaust gas 49 from the side reactor 10 is also included.

Since the main reactor is a complete mixing type reactor as described above, a part of the supplied solid content is short-circuited and is discharged unoxidized in the powder and granular material 51 that has exited the main reactor 9. However, there are some areas where the reaction time is not sufficient.

Therefore, the granular material 51 contains unoxidized sodium sulfide, and if left as is, Na 2 S 2 will be converted into Na 2 S 2 in the subsequent process.
There is also a possibility that 03 is generated or hydrogen sulfide is generated.

In order to prevent this, the side reactor 10 removes unreacted Na2S.
The purpose is to further oxidize the remaining Na2S and reduce the amount of residual Na2S to a sufficiently acceptable range.

The side reactor 10 may be of a piston flow type, in which the powder and granules are stirred by a stirrer and sent sequentially from the inlet to the outlet, to which oxidizing air 46 is supplied, and the reaction temperature is preferably 100 to 300°C. is oxidized at 150-250°C.

Of course, this reaction also requires a certain amount of water, but the powder 51 usually contains less than 5% water, and this amount is sufficient for oxidizing the residual Na2s.

However, if the reaction does not proceed due to insufficient water, water may be replenished by water spray or steam.

The exhaust gas 52 that has exited the cyclone separator 11 still contains fine powder particles and cannot be released into the atmosphere as it is, and also causes a loss of chemicals.

Therefore, it is collected by the wet dust remover 12 and released into the atmosphere from the exhaust pipe 53.

Since the powder in the exhaust gas 52 is easily soluble in water, a wet type dust remover is suitable, and the dust is dissolved by a dilute solution 71 or makeup water 74 and taken out as a dilute aqueous solution 55.

Since the dew point of the exhaust gas 52 has increased as described above, a dry dust removal type such as a bag filter is likely to clog and is not suitable.

The powder and granular material 56 that has finished the reaction in the side reactor 10 is transferred to the dissolution tank 1.
A dilute aqueous solution 55 sent to 3 and obtained by the wet dust remover 12
to form an aqueous solution 57 with a concentration of about 20% by weight.

The concentration of the aqueous solution 57 is related to the concentration of the chemical solution used in the subsequent cooking step; if it is too thin, it will contain too much water, while if it greatly exceeds 20% by weight, it will be unsuitable as an absorption liquid.

The aqueous solution 57 contains unburned carbon and other insoluble components, which, as described above, adversely affect the reaction of sodium carbonate to sulfite.

These undissolved components are sedimented and separated by the thickener 15 and separated into a clear liquid 58 and a sludge portion 59 discharged from the bottom.

Clarified liquid 58 is supplied to receiver tank 16 .

The liquid 58 supplied to the absorption device 17 is controlled by the pH of the absorption liquid as well as the level of the receiver tank 16.

Na2C contained in the liquid 58 supplied to the absorption device 17
O3 contacts and reacts with the exhaust gas 60 from the recovery boiler 1 or sulfur dioxide gas from another sulfur dioxide gas source 61, forming Na2S.
It becomes O3.

The amount of Na2CO3 converted into Na2SO3 varies depending on the chemical ratio used in the cooking 27, but it is not necessarily necessary to convert the entire amount of Na2CO3 into Na2SO3.

The sulfur dioxide gas content of the exhaust gas 62 leaving the absorption tower 17 falls within the regulation value and is released into the atmosphere.

The liquid 63 obtained by the absorption device 17 is clarified by a thickener 19, temporarily stored as a clarified liquid 64 in a clarified liquid receiving tank 20, and sent to a cooking (process) 27.

The sludge portion 59 of the thickener 15 is sent to the sludge receiving tank 21 via the sludge receiving tank 18, mixed together with the sludge 65 of the thickener 19, and washed with washing water 54.

The washed sludge 66 is further concentrated in a thickener 22, and the concentrated sludge 67 is received in a sludge receiving tank 24 and further dehydrated in a filter 25.

The dehydrated cake 68 is taken out of the system and treated in a sludge treatment 26.

The slough 69 of the filtration machine 25 and the supernatant liquid 70 of the thickener 22 are received in a receiving tank 23 and used as a dilute solution γ1 for make-up water in the wet dust removers 5 and 12*.

The water in this dilute solution 71 is used to dissolve the smelt in the first step, becomes evaporated water for heat removal, further becomes evaporated water to balance the reaction heat in the third step, and is used to remove chemicals from the system. prevent losses.

In this way, Na2SO3, which is the cooking chemical, can be efficiently recovered from the smelt obtained by concentrating and burning the waste liquid of pulp cooking using Na2SO3 as the cooking chemical without producing any waste liquid.

Next, the present invention will be described with reference to Examples, but the present invention is not limited thereto in any way.

Comparative Example 1 According to the method described in the above specific example, in the first step, the temperature of the circulating slurry was 83.5°C, the total solids concentration was 59.6
In the second step, only a screw decanter is used, and in the third step, the temperature of the main reactor is set to 190℃.
Each step was carried out with the temperature of the side reactor at 150°C.

Table 1 shows the composition of each process.8 The smelt flow rate is 2. Ot/hour, fresh water supply amount is 24
t/hour.

Example According to the method described in the above specific example, in the first step, the temperature of the circulating slurry was 78°C, the total solids concentration was 55.1,
In the second step, both a screw decanter and a drum flaker are used, and in the third step, the temperature of the main reactor is set to 210°C, and the liquid separated by the drum flaker is circulated as a slurry through a return line 41. tank 3
The temperature of the side reactor was set at 160° C., and water was injected into the side reactor at 50 l/hour to carry out each step.

Table 2 shows changes in composition in each step.

As is clear from Tables 1 and 2, according to the invention, both a screw decanter and a drum flaker are used, particularly in the second step, and the liquid separated in the flaker is returned to the slurry circulation tank. As a result, a flaky solid containing a high concentration of Na2S (high S/Na2O value) can be obtained.

The solids fed to the main reactor by the operation of the present invention are:
The amount of solids separated in the screw decanter and the amount of flaky solids cooled and solidified in the drum flaker are controlled by the Na2S content ratio of the inflowing smelt, and the smelt's sulfur content and sodium content are controlled. The ratio can be made equal to the ratio S/Na2O.

As explained above, the present invention has the advantage that it can be implemented as a closed system and that various processes can be easily managed based on compositional analysis of each process.

[Brief explanation of the drawing]

The accompanying drawing is a block flow diagram illustrating one embodiment of the invention. Explanation of symbols, 1...Recovery boiler, 2...
・Smelt receiver, meter...--Slurry circulation tank,
4...Green liquid tank, 5...Wet type dust remover, 6...Head tank, γ...Screw decanter 18...Drum breaker -19
...Main reactor, 10...Side reactor, 1
1...Cyclone separator, 12...
...Wet type dust remover, 13...Dissolution tank, 14...
... Heater, 15 ... Thickener, 16.
...Receiving tank, 17...Absorption device, 18...
・・・・Sludge receiving tank, 19・・・・−・Thickener,
20... Clear liquid receiving tank, 21... Sludge receiving tank, 22... Thickener, 23...
・Receiving tank, 24...Sludge receiving tank, 25...
...Filter, 26...Sludge treatment, 2γ...
... Cooking (process), 28 ... Smelt, 2
9...Slurry, 30...Outside air, 31
...Sura')-1,32...Chute, 33...Dilute aqueous solution, 34...Green liquid, 35...Cooling water , 36... Slurry, 37, 38... Piping for flowing down, 39-...
... P liquid 1.40 ... R liquid supply line, 41
... Return line, 42 ... Cooling material, 4
3... Solid matter, 44... Flakes, 4
5... Air, 46... Oxidizing air, 4
7... Steam for heating, 48, 49...
・Exhaust, 50... Powder return line, 51...
... Powder, 52... Waste gas, 53-...
・Exhaust pipe, 54...Washing water, 55...
Dilute aqueous solution, 56... Powder, 57...
・・Aqueous solution, 58・・Clear liquid, 59・・・・
・Sludge content, 60...Exhaust gas from 1, 61
...S02 gas source, 62 ...--exhaust gas,
63...Liquid, 64...Clear liquid, 65.
...--Sludge, 66...Mixed sludge,
67... Concentrated sludge, 68. Dehydrated cake, 69... - liquid, 70... Supernatant liquid, 71.
...Dilute solution, 12...Exhaust, 73...
...Exhaust port, γ4...Supplementary water.

Claims (1)

[Claims] 1(a) The smelt from the recovery boiler is completely dissolved by the circulating slurry in a slurry circulation tank having a cooler, so that the proportion of soda carbonate in the total solids of the slurry is reduced lower than the proportion of carbonated soda in minutes;
A first step of forming a smelt slurry with a total solids concentration of 35 to 70% by weight and a temperature of 55 to 90°C, circulating this slurry to a smelt receiving device, and extracting a part of it;
(b) Separate the slurry extracted from the slurry circulation tank into a liquid containing solids and fine crystals, and pass a portion of the slurry and/or a portion of the liquid separated from the solids through a drum flaker to form flakes. and then moisture 10-50
The solids and flakes are sent to the oxidation reactor so that the weight percentage and the ratio of sulfur to sodium are equal to the ratio of sulfur to sodium in the smelt, while the liquid separated in the drum flaker is made into a slurry. The second step is to return the solids and flakes to the circulation tank, and (c) the oxidation reactor, which consists of a continuous discharge type main reactor and a side reactor, in which the hot powder containing soda carbonate and sodium sulfite is forcibly stirred in advance. A method for recovering a sodium sulfite cooking chemical, comprising a third step of oxygen-oxidizing the sodium sulfite cooking chemical in the form of powder or granules. 2(a) The smelt from the recovery boiler is incompletely dissolved by the circulating slurry in a slurry circulation tank with a cooler, so that the proportion of soda carbonate in the total solid content of the slurry is reduced to the proportion of soda carbonate in the total solid content of the smelt. lower than the percentage (,
A first step of forming a smelt slurry with a total solids concentration of 35 to 70% by weight and a temperature of 55 to 90°C, circulating this slurry to a smelt receiving device, and extracting a part of it;
(b) Separate the slurry extracted from the slurry circulation tank into a liquid containing solids and fine crystals, and pass a part of the slurry and/or a part of the liquid separated from the solids to a drum flaker. Make it into flakes, then reduce the moisture content to 10-50
The solids and flakes are sent to the oxidation reactor so that the weight percentage and the ratio of sulfur to sodium are equal to the ratio of sulfur to sodium in the smelt, while the liquid separated in the drum flaker is made into a slurry. 2nd step (c) of returning to the circulation tank: solids and flakes are returned to the circulation tank in an oxidation reactor consisting of a continuous discharge type main reactor and a side reactor in which the hot powder containing soda carbonate and sodium sulfite is forcibly stirred. 3rd step of oxidizing the powder with oxygen in the form of powder and granules; (d) the powder and granules mainly composed of sodium sulfite and soda carbonate obtained in the 3rd step are dissolved in water to form undissolved matter and clear liquid; and (e) clarifying the insoluble matter separated in the previous step and the aqueous sodium sulfite solution obtained in the previous step. Combines the fifth step of washing and dewatering the separated sludge, returns the dilute solution separated in the fifth step to the first step and/or third step, and maintains the material balance of the entire process. A method for recovering a sodium sulfite cooking chemical.