JPS6144806B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for producing purified common salt using hydrogen chloride contained in incinerator exhaust gas such as municipal waste. Recently, in incinerators for municipal waste, a large amount of hydrogen chloride is contained in the exhaust gas due to vinyl chloride, etc. contained in the waste, which has become a major environmental problem. This hydrogen chloride gas is usually said to be several hundred ppm, but sometimes it exceeds 1000 ppm. The most efficient way to remove such hydrogen chloride is to wash it with a caustic soda solution, and from the perspective of recycling, the generated common salt is electrolyzed and reused as caustic soda and chlorine for vinyl chloride production. Since it can be produced, it can be said to be an extremely rational method. Currently, approximately 100 tons of municipal waste is generated per day for every 100,000 people, which is equivalent to 1 ton of salt. Therefore, the amount of table salt generated in Japan as a whole amounts to several hundred thousand tons per year, but Japan relies on imports for most of its industrial salt, and a large amount of vinyl chloride intermediates containing chlorine are also imported. Considering the current situation, this can be said to be a valuable resource recovery method. However, the crude salt solution (hereinafter referred to as smoke washing waste liquid) produced when the above exhaust gas is washed with caustic soda contains impurities that account for about 15% of various heavy metals and salt.
Because of the presence of mirabilite, there is a problem in how efficiently these can be removed to obtain high-quality common salt. Furthermore, the salt concentration in the smoke washing waste liquid is at most 15-20%, and a heat source for concentration is required, but the use of heavy oil or the like is economically disadvantageous. The present invention is intended to solve the above-mentioned problems. Specifically, the present invention uses a mixed solution of a caustic soda solution and slaked lime to scrub the flue gas from a municipal waste incinerator containing hydrogen chloride, and uses a mixture of a caustic soda solution and slaked lime, and After adjusting the weight ratio of slaked lime to 0.1 to 0.3 and removing impurities mainly consisting of calcium and heavy metals in the obtained smoke washing waste liquid by adding iron chloride, soda sulfide and soda carbonate, the above municipal waste incinerator is heated. This is a method for producing refined common salt, which is characterized by heating and concentrating the salt using the heat of combustion to precipitate salt crystals. The method of the present invention will be explained with reference to drawings (flow sheets).When exhaust gas from a municipal waste incinerator is washed with only a caustic soda solution, and when it is washed with a caustic soda solution and slaked lime as in the method of the present invention, the composition of the smoke washing waste liquid is determined by the type of garbage. Although there are some differences depending on the situation, typical examples are as follows.

[Table] In the method of the present invention, slaked lime is used in a weight ratio of 0.1 to 0.3, preferably 0.15 to 0.2, based on the pure caustic soda. If it is less than 0.1, the removal of sulfuric acid content in the exhaust gas is incomplete, and if it exceeds 0.3, calcium chloride is produced and salt is lost. The concentration of caustic soda solution is about 10-15% by weight, and nitrate lime is mixed and used. In this smoke washing process, it is possible to use chloride lime instead of slaked lime, but chloride lime is generally considerably more expensive than nitrate lime, has greater hygroscopicity, and is inconvenient to handle. For the reasons mentioned above, it is not appropriate to use lime chloride as a cleaning solution for exhaust gas from municipal waste incinerators, which generate a large amount of waste. This smoke washing waste liquid contains insoluble substances mainly composed of gypsum, so this is separated, iron chloride and sodium sulfide are added to the mother liquor, and if necessary, an alkali such as caustic soda is added to bring the pH to 10 to 10.5. When adjusted to , the impure heavy metals contained in the mother liquor precipitate as sulfides, which are separated by filtration using a filter press or the like.
Addition of iron chloride produces a floc-like iron hydroxide precipitate, which has the effect of co-precipitating fine heavy metal sulfide precipitates and making them easier to separate. Further, calcium ions remaining in the mother liquor are precipitated as calcium carbonate by adding sodium carbonate and separated by filtration using a filter press or the like. In addition, smoke washing waste fluid from municipal garbage incinerators often contains mercury, and in order to strictly purify this, the mother liquor must be finally removed by passing the mother liquor through a chelating resin or chelating agent-supported activated carbon. . In this way, a saline solution with very few impurities can be obtained, which can be heated and concentrated to precipitate salt crystals, which can then be centrifuged to obtain purified salt. At this time, in the method of the present invention, by using the combustion heat of the incinerator, no new heat source is required in the heating concentration step. The analytical value of the purified salt obtained in this way is 99.8% NaCl or more, Ca 0.001% or less, Mg 0.001% or less, SO ₄ 0.03% or less, which is not inferior to commercially available purified salt, and no heavy metals are detected. The mother liquor separated during centrifugation is a saturated saline solution, and the impurities are slightly more concentrated than the liquid before concentration, so the recovery rate of salt can be improved by returning it to the purification process along with the smoke washing waste liquid. I can do it. As described above, according to the method of the present invention, by replacing approximately 20% of the caustic soda solution with slaked lime as an exhaust gas cleaning solution for municipal waste incinerators, sulfuric acid radicals in the exhaust gas are removed, and the entire amount of expensive caustic soda is replaced with raw material salt. The washing waste liquid (salt water) is purified by applying the iron chloride-soda sulfide method, which is known as a wastewater treatment method, to remove impure heavy metals, and if necessary, rigorous purification using chelate resin is performed. Because this process is carried out, the precipitated salt does not contain any heavy metals. By utilizing the combustion heat of the incinerator, no separate heating energy is required for concentrating the purified brine. Various effects can be mentioned. Conventionally, such high-quality purified salt is produced by separating ordinary salt, which is less pure than purified salt, from bittern through ion-exchange membrane electrolysis and steam concentration from seawater, and then dissolving it in water and adding a purifying agent to precipitate impurities. However, the method of the present invention utilizes hydrogen chloride in the exhaust gas and the heat of combustion. Since refined common salt having the same purity can be produced by only one concentrated precipitation, expensive energy such as heavy oil or electrolytic power is not required at all, and it has great industrial utility value. In the following Examples and Comparative Examples, all percentages are based on weight. Example Mixed solution of 10% caustic soda and 1.7% slaked lime 210
Using municipal waste incinerator exhaust gas (temperature 60℃,
A smoke washing waste liquid containing precipitates mainly composed of calcium sulfate dihydrate was obtained by washing with 15,000 Nm ³ (HCl 1100 ppm, SO ₂ 100 ppm). Its composition is as follows. PH 6.5 Specific gravity 1.15 NaCl 15% Na ₂ SO ₄ 0.6% CaSO ₄ 2H ₂ O 2.5% CaCO ₃ 6000ppm Mg 60ppm Cd 1.1ppm Cr 1.3ppm Hg 5ppm Pb 50ppm Zn 350ppm As 0.1ppm This waste liquid is filtered with a filter press and mainly After separating the gypsum component, dichloride is added to the filtrate 100%.
Add 50g of iron and 3g of sodium sulfide to dissolve it, add a small amount of caustic soda solution to adjust the pH to 10, and then filter with a filter press to remove mainly heavy metals.
Furthermore, 300 g of soda carbonate is added to this mother liquor and dissolved, and then filtered again using a filter press. After passing this mother liquor through a chelate resin packed tower, steam from the above municipal waste incinerator waste heat boiler (steam recovery amount 110
100 kg) was placed in a concentrator using a heating source and concentrated at 115°C, and about 75 kg of water was evaporated to obtain a salt slurry 25. This slurry was centrifuged to separate the mother liquor, washed with 1 portion of water, and further centrifuged to obtain 10.5 kg of salt crystals. After drying this, it was analyzed and the following results were obtained. NaCl 99.8% Ca 0.005% Mg 0.003% SO ₄ 0.028% Heavy metals Not detected Comparative example Municipal garbage incinerator exhaust gas 15000Nm ³ same as Example
was washed with 12% caustic soda solution 210 to obtain a solution with the following composition. PH 5.8 Specific gravity 1.15 NaCl 15% Na ₂ SO ₄ 3% Precipitate 2000ppm CaCO ₃ 5000ppm Mg 40ppm Cd 1.1ppm Cr 1.3ppm Hg 5ppm Pb 40ppm Zn 360ppm As 0.1ppm After filtering 100 of this liquid with a filter press,
It was concentrated and solidified using a dryer that uses steam as a heat source. Analysis of this crystal after crushing _and drying revealed that it contained an extremely large amount of impurities, making it completely unsuitable for use as _an industrial salt.

[Brief explanation of the drawing]

The drawing is a flow sheet illustrating the method of the invention.

Claims (1)

[Claims] 1. When washing municipal waste incinerator exhaust gas containing hydrogen chloride, a mixed solution of caustic soda solution and slaked lime is used, and the weight ratio of slaked lime to the pure caustic soda content is 0.1 to 0.3, and the smoke washing waste liquid obtained is The method is characterized in that impurities mainly consisting of calcium and heavy metals are removed by adding iron chloride, sodium sulfide and soda carbonate, and then heated and concentrated using the combustion heat of the municipal waste incinerator to precipitate salt crystals. Manufacturing method of refined table salt.