JPH0551524B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Field of Application] The present invention relates to a method for thermal decomposition of alkali sulfur compounds, and in particular provides a method that significantly improves the prior art in the recovery of hydrogen sulfide and alkali components. The method of decomposing alkali sulfur compounds, recovering alkali metals as alkali carbonates, and recovering sulfur as hydrogen sulfide involves separating compounds combined with organic matter and recovering cooking chemicals, such as in the treatment of pulp cooking liquor. It is widely used to decompose sodium sulfite and mirabilite obtained from dry or hot flue gas desulfurization equipment. The recovered alkali carbonates are recycled to the desulfurization process, while
The generated hydrogen sulfide is used to produce elemental sulfur and sulfuric acid by known methods, or used to regenerate seeds generated during MHD power generation. Alternatively, hydrogen sulfide can be sent to a combustion process to produce sulfur dioxide gas, and simultaneously reacted with the resulting alkali carbonate to be reused as an alkaline sulfite solution for pulp cooking. [Prior Art] One of the conventional technologies is the Tampera method, which is a known method for recovering neutral sulfite pulp (NSSCP) cooking liquor in combination with a heat recovery boiler. This Tampera method oxidizes and burns cooking liquor containing sodium sulfide and sodium sulfite to obtain sodium sulfide and sodium carbonate, which are dissolved in water and pre-carbonated to produce a reaction solution containing NaHS and _NaHCO3 . do. This reaction solution was mixed with Na ₂ CO ₃ and CO ₂
The solid sodium bicarbonate obtained in a separate reaction step is reacted with H 2 S and Na 2 CO 3 to produce H ₂ S and Na ₂ CO ₃ . The H ₂ S is burned to form SO ₂ and this is reacted with Na ₂ CO ₃ obtained in the above step to obtain sulfite. The main reaction for obtaining alkali carbonate and hydrogen sulfide from alkali sulfide according to the conventional method can be expressed by the following reaction formula. (1) Na ₂ S + CO ₂ + H ₂ O → NaHS + NaHCO ₃ (carbonation tower) (2) NaHS + NaHCO ₃ → Na ₂ CO ₃ + H ₂ S (reaction tower) (3) Na ₂ CO ₃ + CO ₂ + H ₂ O → 2NaHCO ₃ (Bicarbonation tower) There is also a reductive decomposition method known as the SCA process. The characteristics of this method are that an alkali sulfur compound is heated at 700 to 780°C in a reducing atmosphere to decompose it into alkali carbonate, solid carbon, and hydrogen sulfide, and then the solid alkali carbonate is separated from the gas and dissolved. In this method, hydrogen sulfide is burned in a boiler to obtain SO ₂ , which is then reacted with the aqueous alkali carbonate solution and recovered as alkali sulfite. [Problems to be solved by the invention] In general, a reaction method such as the Tampera method requires a combustion furnace, and also requires a complicated process and a large amount of steam to obtain crystals of baking soda. Equipment costs are high and it is not economical in terms of energy. In addition, since the decomposition temperature is relatively low in the SCA method,
All of the carbon in the feed is not decomposed, and some of it is precipitated as solid carbon. Therefore, filtration equipment is required for separation from the alkali carbonate, and a new furnace is required for combustion of the resulting carbon. In order to recover heat from the combustion furnace, it is necessary to install a waste heat boiler with complicated operation, and the wet cake of carbon has a moisture content of about 80% and must be dried before it can be used as a fuel. If this carbon is not reused as waste, not only will the valuable organic matter in the pulp cooking liquor not be used effectively as fuel, but also the alkali and sulfur content attached to the carbon will be lost. Therefore, the SCA process is uneconomical in terms of equipment costs and energy costs. [Means for Solving the Problems] The present invention eliminates the drawbacks of the prior art, completely decomposes organic substances in the feed liquid, does not generate carbon, and minimizes the loss of valuable chemicals. The present invention provides a method for thermally decomposing an alkali sulfur compound. According to the present invention, unlike the SCA method or Tampera method, the alkali sulfur compound is basically thermally decomposed in a reducing atmosphere at a relatively high temperature. That is, the present invention is characterized in that a solid material, aqueous solution, or slurry containing an alkali sulfur compound is thermally decomposed at a temperature of 800° C. or higher in a reducing atmosphere. As a result, all the inorganic salts contained in the raw materials are carbonated in the molten state,
All organic matter, including carbon, is volatilized as a gas, and sulfur is converted to hydrogen sulfide. On the other hand, the carbonated melt contains alkali sulfide produced by a side reaction due to the presence of hydrogen sulfide, which reacts with CO ₂ and alkali carbonate produced by decomposition in the stripping tower to form alkali sulfide. A portion of the alkali carbonate is converted to alkali bicarbonate into alkali sulfide. Inside the emission tower, these react with each other.
Generate H ₂ S. Furthermore, the reaction solution containing unreacted components is
A heating reaction is carried out in a secondary stripping tower with a low partial pressure of H ₂ S,
As a result, an alkaline carbonate solution substantially free of alkali sulfide is recovered from the bottom of the column, and hydrogen sulfide is recovered from the top of the column. According to the present invention, since the decomposition is carried out at high temperatures, the amount of solid carbon generated by gasification of carbon is extremely small, and troubles caused by solid carbon do not occur, and the reaction liquid is of high purity and is free from environmental pollution and pharmaceutical agents. A lossless method is provided. Furthermore, it is economically advantageous since there is no need to separate the alkali carbonate into crystals during the reaction process, and there is no evaporation process that uses a large amount of steam. Furthermore, the reaction process of the present invention will be explained in detail by taking as an example the thermal decomposition treatment of sulfur oxide of Na. When sulfur oxides of Na are thermally decomposed in a high-temperature reducing atmosphere in a reactor, the higher the reaction temperature, the faster the reduction of the oxides, so the sulfur oxides of Na are almost completely converted into sodium carbonate and sulfurized. Conversion to hydrogen requires treatment at a temperature range of at least 800°C, preferably 900-1100°C. It is difficult to instantly and completely separate Na ₂ CO ₃ and H ₂ S produced under these conditions in a molten state.
In the present invention, the entire reaction gas is brought into contact with water in a washing tower and separated as an aqueous solution of sodium carbonate. In this case, the most difficult problem is that the main component of the melt, sodium carbonate, and the reaction product, hydrogen sulfide, react back in the reactor to form Na ₂ S. When Na ₂ S is generated by such a reaction, Na ₂ S is present in the resulting sodium carbonate solution, and hydrogen sulfide is generated when the sodium carbonate solution is reused, for example, for pulp cooking. Since decomposition is reduced, the amount of circulation when reusing alkali increases, which is not economical. Furthermore, when hydrogen sulfide obtained by the method of the present invention is burned to produce sulfur dioxide gas and this is brought into contact with sodium carbonate to obtain sodium sulfite, if an aqueous sodium carbonate solution containing Na ₂ S is used, SO ₂ as the absorption of
H ₂ S is released. This results in reduced drug recovery and air pollution problems. Such a reverse reaction, Na ₂ CO ₃ + H ₂ S → Na ₂ S,
Since this is unavoidable in operation, in the present invention, the liquid obtained in the contact type washing tower is brought into contact with the washing tower exhaust gas containing carbon dioxide gas in a multistage countercurrent manner to carry out the following reaction. H ₂ S+ (Na ₂ S + CO ₂ + Na ₂ CO ₃ + H ₂ O)
〓NaHS+NaHCO ₃ +H ₂ S↑ (1) In the present invention, this reaction is carried out under normal pressure and in the presence of hydrogen sulfide in the gas phase. Therefore, the conditions for the reaction to move from the right-hand side to the left-hand side are usually not satisfied, and the reaction proceeds to the right-hand side. When the aqueous solution of the reaction product is taken out from the bottom of the multistage countercurrent tower and reacted under heating in a secondary stripping tower where the partial pressure of hydrogen sulfide is low, the reaction of the following formula proceeds. NaHS + NaHCO ₃ → Na ₂ CO ₃ + H ₂ S↑ (2) This reaction is suitably carried out under vacuum and in the presence of a small amount of water vapor. Since the method of the present invention effectively utilizes carbon dioxide gas in the cracked gas, unlike the Tampera method, no separate gas source is required. On the other hand, NaHS in the reaction solution
Since the reaction is carried out using NaHCO ₃ under conditions of low H ₂ S partial pressure, there is no need to recycle sodium bicarbonate as a solid substance, and equipment costs and liquid costs are reduced.
It is extremely economical. FIG. 1 is an example of a flow sheet for carrying out the method of the present invention. The reaction tower 1 has a raw material supply line 11 and an air supply line 12, in which high-temperature reductive decomposition is performed. The reaction product is sent to washing column 2 where it is contacted with water from line 13.
The reaction product is dissolved by the washing water, and the resulting aqueous solution is supplied to the top of the primary stripping tower 3 via line 14, and from the bottom, the washed solution is sent from the top of the washing tower 2 via line 15. Gas is supplied and brought into contact in a countercurrent manner. In the primary stripping tower, it is preferable to provide several porous layers so that the reaction of formula (1) can be sufficiently achieved. H ₂ S is thus removed from the top of the primary stripper via line 17 in relatively pure form. The reaction liquid is supplied to the top portion of the multi-stage secondary stripping tower 4 via a line 16. Secondary emission tower 4
A vacuum pump 6 is installed at the top of the column via a condenser 5, and the above formula in the stripping column is
The reaction (2) is carried out under conditions of low H ₂ S partial pressure (80-85℃, 300℃
~400 mmHg), and a heating medium 24 such as steam is introduced at the bottom of the column to ensure that the reaction is sufficiently completed while the liquid is descending within the column. The H ₂ S gas produced in this reaction is drawn in and cooled by a condenser 5, and in a dehydrated form passes through a line 20 and is combined with the H ₂ S gas from the top of the primary stripping tower. On the other hand, the reaction liquid taken out from the bottom of the secondary stripping tower is an aqueous solution whose main component is sodium carbonate produced by the reaction. The basis of the method of the present invention is mainly the process of generating H ₂ S, but the H ₂ S from each separated column is sent to the combustion furnace 7, air is introduced from 25 and combusted, and this combustion gas is If it is sent to the SO ₂ absorption tower 8 and brought into contact with the sodium carbonate solution flowing out from the secondary stripping tower, the alkali can be recovered as sodium sulfite. In addition, the recovered H ₂ S can be used for other purposes such as producing elemental sulfur. Furthermore, in the method of the present invention, a waste heat boiler can be placed in parallel with the reactor, as shown in FIG. 2, from the viewpoint of heat recovery. In FIG. 2, 31 is a waste heat boiler, and 30 is a melt receiving tank. Dissolved water is added to this receiver tank to create a new dissolved solution, and the high-temperature gas is passed through a waste heat boiler to recover heat, then supplied to the lower part of the primary stripping tower, and the dissolved solution is supplied to the top of the primary stripping tower. Heat recovery is performed. In addition, as shown in FIG. 3, the reaction gas is sent to the cooling tower 41 to obtain high-temperature gas, which is sent to the heat exchanger 42 of the evaporator 43 and converted into the liquid to be evaporated with the condensate during dehumidification. Heat can also be applied to cause the solution to be concentrated in the stripping tower 3. Furthermore, it is also possible to recover steam by using the combustion furnace 7 as a gas-fired boiler. [Example] Using neutral sulfite pulp cooking liquor as a feedstock, the high temperature reductive decomposition method of the present invention was carried out using the apparatus shown in FIG. The concentrated composition of the stock solution is as follows. Liquid Concentration 55% by weight Composition Na 13.0% by weight S 8.8 C 30.3 H 3.2 O 45.0 Na ₂ SO ₄ 10.0 Calorific value 3000 Kcal/Kg The reaction tower has a diameter of 4.4 m and a height of 4.5 m. The above condensed raw material was supplied from the top of the reaction tower at a rate of 6000 Kg/H, and the amount of air was set to 9400 Nm ³ /H to prevent excessive reaction.
The reaction was carried out at 900°C. The gas and melt thus obtained are brought into contact with a cleaning liquid in a cleaning column. The composition of the aqueous solution obtained from the washing tower was as follows. Na ₂ CO ₃ 57.8% by weight Na ₂ S 35.3 〃 Na ₂ SO ₄ 6.8 〃 This aqueous solution is supplied to the top of the primary stripping tower, which has a diameter of 2.7 m and a height of 12 m and has six perforated plates, and is washed at the bottom. Supply gas from the tower. In the primary stripping tower, the reaction of formula (1) proceeded, and a solution having the composition (dry weight) shown in the table below was obtained. Na ₂ CO ₃ 80.7% by weight Na ₂ S 11.7 〃 Na ₂ SO ₄ 7.6 〃 Next, the obtained aqueous solution was placed in a container with a diameter of 12 m and a height of
4.5 m and fed to the top of a secondary stripping tower with 10 stages of perforated plates. 600-800 from the bottom of the secondary emission tower
Kg/H of steam is supplied, where equation (2),
The reaction is completed. The composition of the bottom liquid obtained (in terms of dry matter) was as follows. Na ₂ CO ₃ 92.0% by weight Na ₂ S 1.5 〃 Na ₂ SO ₄ 6.5 〃 The composition of the gas from the primary stripping tower is expressed in volume %: CO ₂ 14.9%, CO 1.4%, H ₂ 8.8%,
_H2S 1.6%, _CH4 0.1%, _N2 58.1%, _H2O
It was 12.1%, and the outflow amount was 13000Nm ³ /H. The sodium carbonate solution obtained as described above has a low Na ₂ S content, and when sent to the absorption process, there are problems with the SO ₂ absorption process due to the presence of sodium sulfide and when the produced sulfite is used for pulp cooking. It has been found that all the problems of the prior art are solved without causing any odor problems. [Effects of the invention] The effects of the method of the invention can be obtained when a waste heat boiler and a gas boiler are installed.
The results of a comparison with the conventional SCA method in the NSSCP200T/D plant are as follows.

[Table] As described above, the economical advantage of the method of the present invention has been revealed. In addition, the reduction in power consumption in the table above is mainly due to the fact that almost no carbon is generated in the method of the present invention.
This is because no filtration equipment is required. Furthermore, an advantage of the method of the present invention is that the amount of Glauber's salt in the recovered liquid is small. This is because, according to the reaction method of the present invention, most of the mirabilite in the feed liquid is decomposed into sodium carbonate, and the amount of mirabilite in the recovered chemical solution is extremely small. Therefore, there is no trouble in the evaporation operation due to precipitation of Glauber's salt in the black liquor concentration process, and the final concentration can be made higher than in the conventional method.

[Brief explanation of drawings]

FIG. 1 is an example of a flow sheet showing the configuration of an apparatus for implementing the present invention. Figure 2 is a part of a flow sheet when a waste heat boiler is installed for heat recovery. FIG. 3 is part of another flow sheet involving heat recovery. 1... Reaction tower, 2... Washing tower, 3... Primary stripping tower, 4... Secondary stripping tower, 5... Condenser, 6
... Vacuum pump, 7 ... Combustion furnace, 8 ... SO ₂ absorption tower, 30 ... Melt receiving tank, 31 ... Waste heat boiler, 41 ... Cooling tower, 42 ... Heat exchanger, 43 ...
...Evaporator, 44...Condenser.

Claims (1)

[Claims] 1. A solid substance, aqueous solution or slurry containing an alkali sulfur compound is thermally decomposed at a temperature of 800°C or higher in a reducing atmosphere, the resulting melt is dissolved in water, and the resulting aqueous solution and the heat The alkali sulfide in the solution is converted into alkali hydrogen sulfide and a portion of the alkali carbonate into alkali bicarbonate by contacting the gas stream containing carbon dioxide gas generated during decomposition, and these are converted into H ₂
Reacting with each other under conditions of reduced S partial pressure,
A method for thermally decomposing an alkali sulfur compound, which comprises recovering generated hydrogen sulfide gas and an alkali carbonate solution. 2. The method according to claim 1, further comprising the step of burning the recovered hydrogen sulfide to form sulfur dioxide gas, reacting it with the recovered alkali carbonate solution, and recovering the resulting alkali sulfite. Method.