Method for preparing ionic membrane caustic soda from waste salt

Abstract

The invention belongs to the technical field of caustic soda preparation through waste salt treatment, and discloses a method for preparing ionic membrane caustic soda by using waste salt, which comprises the following steps: screening the waste salt to remove impurities, and crushing to 80-120 meshes; ozone is adopted to carry out oxidation pretreatment on the crushed waste salt, the reaction temperature is 30-50 DEG C, and the reaction time is 30-60 min; carrying out microwave pyrolysis on the waste salt subjected to oxidation pretreatment, introducing inert gas for protection at the same time, and carrying out pyrolysis at 400-800 DEG C for 20-60 minutes; and preparing the waste salt subjected to microwave treatment into a saturated saline solution, and precisely filtering to obtain the saline water for preparing the caustic soda by the ionic membrane. Ozone oxidation pretreatment and microwave pyrolysis are cooperated, so that the purity of the waste salt can be effectively improved, and the waste salt can be used for a process for preparing caustic soda through ionic membrane exchange electrolysis.

Description

Technical Field

[0001] The invention belongs to the technical field of preparing caustic soda by treating waste salt, and in particular relates to a method for preparing ion membrane caustic soda by using waste salt. Background Art

[0002] The statements herein merely provide background information related to the present invention and do not necessarily constitute prior art.

[0003] In modern industrial production, the amount of waste salt generated is increasing day by day, and the sources are wide-ranging. For example, in some chemical production processes such as dyes, pesticides, pharmaceuticals and other industries, waste salt is produced in large quantities as a by-product. These waste salts have complex compositions and often contain organic matter, heavy metal ions and many other impurities. They are not only difficult to treat effectively, but also cause serious environmental pollution if not properly disposed of. Traditional waste salt treatment methods have many limitations. For example, landfilling not only occupies a large amount of land resources, but may also pollute the soil and groundwater due to the leakage of harmful substances in the waste salt; incineration can remove some organic matter, but it is costly and prone to secondary pollution.

[0004] The process of preparing caustic soda by ion membrane is a process of using a cation exchange membrane electrolyzer to electrolyze a salt solution to produce chlorine, hydrogen and high-purity caustic soda. This process has high requirements for the purity of the raw salt (NaCl purity must reach more than 99.5%, and the content of key impurities such as calcium, magnesium, sulfate, heavy metals, etc. must be controlled below the ppm level). Impurities need to be removed as much as possible to ensure the service life of the cation exchange membrane and the quality of the caustic soda product. However, the raw salt prepared by the existing waste salt treatment method still contains many impurities, and the purity is difficult to meet the requirements, and thus it is difficult to use it in the ion membrane preparation of caustic soda process. Summary of the invention

[0005] In view of the shortcomings of the prior art, the purpose of the present invention is to provide a method for preparing ion-exchange membrane caustic soda using waste salt to overcome the shortcomings of the prior art and solve the problem that waste salt is difficult to prepare caustic soda by ion-exchange membrane after conventional treatment.

[0006] In order to achieve the above object, the present invention is implemented through the following technical solutions: A method for preparing ion-exchange membrane caustic soda using waste salt comprises the following steps: After screening the waste salt to remove impurities, crush it into 80-120 mesh; The crushed waste salt is pre-treated with ozone, the reaction temperature is 30-50°C, and the reaction time is 30-60min; The waste salt after oxidation pretreatment is subjected to microwave pyrolysis, while passing inert gas protection at 400-800°C for 20-60 minutes; The waste salt after microwave treatment is prepared into a saturated salt solution, and after precision filtration, the salt water for preparing caustic soda by ion membrane is obtained; The obtained brine is electrolyzed by ion membrane electrolysis to prepare caustic soda.

[0007] In the prior art, waste salt is generally treated by microwave pyrolysis, but the inventors have found that some macromolecular organic impurities in waste salt, such as aromatic compounds, chlorinated organic matter, and refractory organic matter, are difficult to completely decompose through microwave pyrolysis alone, and will continue to remain in the waste salt as impurities (such as small molecular organic matter or coke). The treated waste salt is dissolved in water to prepare brine. The impurities remaining in the waste salt have a high solubility in brine and can be dissolved in water in the form of molecules or ions to form a homogeneous solution. The pore size of the precision filter is usually micrometer or nanometer, and the molecular size of the soluble organic matter is much smaller than the pore size of the precision filter, so the precision filter cannot effectively remove the soluble organic matter. If it is applied to ion exchange membrane electrolysis to produce caustic soda, it will contaminate the ion exchange membrane, affect its service life, and seriously affect the purity of the caustic soda.

[0008] The inventors have discovered through experiments that by pre-treating the waste salt with ozone before subjecting it to microwave pyrolysis treatment, some easily degradable substances can be oxidized into small molecular organic or inorganic substances for removal, and the chemical structure of difficult-to-degrade organic matter can be destroyed, making it easier to decompose during microwave pyrolysis. The content of soluble organic matter in the waste salt is greatly reduced, and residual impurities can be effectively removed through a precision filter, thereby meeting the requirements for ion exchange membrane electrolysis to produce caustic soda.

[0009] Moreover, ozone oxidation pretreatment can also remove some heavy metal ions in waste salt and reduce its volatilization and residue during microwave pyrolysis (if heavy metals are contained in the microwave pyrolysis process, the volatilized heavy metals will enter the tail gas. If they are not effectively treated, they may be discharged into the atmosphere, causing environmental pollution; they may condense on the inner wall of the equipment, causing equipment corrosion and blockage, affecting equipment life and operating efficiency; if heavy metals are volatilized and re-condensed and mixed into the regenerated salt, the purity of the regenerated salt will decrease, affecting the subsequent ion membrane electrolysis process for preparing caustic soda and the product quality). Therefore, ozone oxidation pretreatment and microwave pyrolysis can effectively improve the purity of waste salt and make it usable in the process of preparing caustic soda by ion membrane exchange electrolysis.

[0010] In some embodiments, the waste salt is waste salt generated in the production process of chlor-alkali chemical industry or waste salt generated in the production process of polycarbonate. Such waste salt contains a large amount of refractory organic matter, such as aromatic compounds (benzene series), chlorinated organic matter (chlorobenzene, dichloromethane, etc.), and a small amount of heavy metal ions (such as Fe, Mn, etc.).

[0011] In some embodiments, during the oxidation pretreatment process, the concentration of ozone is 80-200 mg / L. If the ozone concentration is too low, the oxidation rate is slow, the treatment efficiency is low, and the oxidation reaction may remain at the intermediate product stage, making it difficult to completely degrade complex organic matter, which may result in some organic matter not being completely oxidized and residual intermediate products. If the ozone concentration is too high, energy consumption increases significantly, and unnecessary side reactions may be triggered (such as ozone decomposition to generate oxygen), resulting in reduced ozone utilization.

[0012] In some embodiments, during the ozone oxidation pretreatment of waste salt, a catalyst is added to catalyze the efficiency of ozone oxidation and decomposition of organic matter.

[0013] Preferably, the catalyst is TiO2, Fe2O3 or activated carbon.

[0014] Further preferably, when the catalyst is TiO2 or Fe2O3 powder, the catalyst powder is evenly dispersed in the waste salt at 0.1%-0.5% of the mass of the waste salt; When the catalyst is activated carbon, activated carbon particles are added to the waste salt at 0.5%-1% of the mass of the waste salt, or loaded into a porous container.

[0015] Use mechanical stirring or ultrasonic dispersion to ensure uniform mixing.

[0016] The activated carbon is packed into a porous container, such as a mesh bag, to facilitate subsequent separation of the activated carbon from the salt.

[0017] The particle size of activated carbon is 1-2mm; TiO2 and Fe2O3 powders are nano-scale catalysts. Nano-scale catalyst particles can be removed by sedimentation, centrifugal separation or precision filtration (such as 0.1μm microfiltration membrane).

[0018] The activated carbon can be removed by filtering through a sieve (pore size 1-2 mm) or by flotation separation.

[0019] In some embodiments, the heating rate during microwave pyrolysis is 5-10° C. / min.

[0020] Microwave pyrolysis can further decompose residual organic matter and other impurities in waste salt through rapid and selective heating, while avoiding the repolymerization of certain organic matter or the generation of secondary pollutants at high temperatures.

[0021] In some embodiments, the inert gas is nitrogen or argon.

[0022] In some embodiments, the microwave frequency of the microwave pyrolysis is 2400-2500 Hz.

[0023] Preferably, the microwave frequency is 2450 Hz.

[0024] In some embodiments, the temperature of microwave pyrolysis is 500-700°C.

[0025] In some embodiments, the precision of the precision filtration is 0.1 μm. The precision filter uses multiple layers of filter media, such as activated carbon to absorb organic matter, quartz sand to intercept tiny particles, and ultrafiltration membrane to further intercept tiny impurities.

[0026] In some embodiments, during the electrolysis process, the temperature of the electrolytic cell is 80-90°C, and the current density is 3000A / m²-5000A / m².

[0027] The beneficial effects achieved by one or more embodiments of the present invention are as follows: Steps such as ozone oxidation pretreatment and microwave pyrolysis can effectively decompose organic matter and other impurities in waste salt. Through precise microwave pyrolysis parameter settings and a complete microwave pyrolysis treatment process, the organic impurities in the waste salt can be fully decomposed and volatilized, greatly improving the purity of the waste salt. Industrial waste salt is successfully converted into high-quality ion membrane caustic soda to prepare raw salt, achieving efficient recycling of resources, reducing the company's waste salt treatment costs, alleviating environmental pressure, and having significant economic and environmental benefits.

[0028] The microwave pyrolysis process can effectively remove impurities from waste salt and stably achieve the high purity standard required by the ion membrane, providing a reliable guarantee for the subsequent caustic soda preparation. This not only prolongs the life of the ion membrane, reduces equipment maintenance costs, ensures the stability of caustic soda quality, and enhances the market competitiveness of products, but also broadens the source of raw materials for caustic soda production, reduces raw material costs and supply risks, and effectively promotes the sustainable development of the chemical industry.

[0029] Compared with the traditional waste salt treatment method, the present invention avoids the waste of land resources and soil pollution caused by landfill, as well as environmental problems such as harmful gas emissions caused by incineration. At the same time, in the entire process, through reasonable process design and operation control, energy consumption and waste generation are reduced, which is in line with the development concept of modern green chemical production. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] The accompanying drawings in the specification, which constitute a part of the present invention, are used to provide a further understanding of the present invention. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute improper limitations on the present invention.

[0031] Figure 1 This is a process flow chart of preparing ion-exchange membrane caustic soda using waste salt according to an embodiment of the present invention. DETAILED DESCRIPTION

[0032] It should be noted that the following detailed descriptions are illustrative and intended to provide further explanation of the present invention. Unless otherwise specified, all technical and scientific terms used in the present invention have the same meanings as those commonly understood by those skilled in the art to which the present invention belongs.

[0033] The present invention is further described in detail below in conjunction with specific embodiments.

[0034] Example 1 Waste salt with zero discharge from wastewater in the brine refining process of chlor-alkali chemical enterprises was screened by a vibrating screen and then crushed to 100 mesh using an XF-100 crusher. The impurity composition of the waste salt is as follows: organic impurities (mainly aromatic compounds (such as benzene series), chlorinated organic matter (such as chlorobenzene, dichloromethane, etc.)) content is 4.7%, inorganic matter (including metal ions such as calcium, magnesium, and iron) content is 4.7%, and other impurities (such as sulfates, silicates, etc.) content is 0.7%.

[0035] The crushed waste salt was placed in a stainless steel reaction container, ozone gas with a concentration of 100 mg / L was introduced, nano-scale catalyst Fe2O3 powder was added, the reaction time was controlled to 45 minutes, and the temperature was maintained at 40°C.

[0036] The waste salt after ozone oxidation pretreatment was continuously added to the MW-10 microwave reactor. The microwave frequency was selected to be 2450 Hz, the microwave heating temperature was set to 600 °C, the heating time was 30 min, and nitrogen was introduced at a flow rate of 4 m³ / h for protection to obtain microwave pyrolysis regeneration salt.

[0037] Organic matter: After ozone oxidation and microwave pyrolysis, the removal rate of macromolecular organic matter (such as aromatic compounds and chlorinated organic matter) is significantly improved, and the residual organic matter content is reduced to 0.3%.

[0038] Inorganic matter: The content of metal ions such as calcium, magnesium, and iron is reduced to 0.15%.

[0039] Other impurities: The content of sulfate, silicate, etc. is reduced to below 0.1%.

[0040] The microwave pyrolysis regeneration salt is prepared into saturated brine, filtered through a precision filtration device model JL-01 (using a multi-layer filter medium of activated carbon, quartz sand and ultrafiltration membrane), and then enters the subsequent conventional ion membrane electrolysis and caustic soda purification process to finally obtain a caustic soda product with a purity of 98.5%.

[0041] Example 2 The difference from Example 1 is that the microwave pyrolysis time is 60 min, and the rest is the same as Example 1.

[0042] Example 3 Waste salt from the polycarbonate production process of a chemical company was taken and crushed to 80 meshes with an XF-120 crusher after screening. The types and contents of impurities in the initial waste salt: organic matter: mainly organic solvents (such as dichloromethane, phenol, etc.) remaining in the polycarbonate production process, with a content of 5.1%; inorganic matter: including metal ions such as calcium, magnesium, and iron, with a content of 1.6%; other impurities: such as a small amount of carbonates, silicates, etc., with a content of 0.5%.

[0043] The crushed polycarbonate waste salt was placed in a stainless steel reaction container, ozone gas with a concentration of 120 mg / L was introduced, nano-scale catalyst TiO2 powder was added, the reaction time was controlled to 50 minutes, and the temperature was maintained at 45°C.

[0044] The pretreated waste salt was continuously added to the MW-10 microwave reactor, the microwave frequency was selected to be 2450 Hz, the microwave heating temperature was set to 550 °C, the heating time was 25 min, and nitrogen was introduced at a flow rate of 5 m³ / h for protection to obtain microwave pyrolysis regeneration salt.

[0045] Organic matter: After ozone oxidation and microwave pyrolysis, the residual organic solvent is significantly reduced, and the residual organic matter content is reduced to 0.4%. Inorganic matter: The content of metal ions such as calcium, magnesium, and iron is reduced to 0.11%. Other impurities: The content of carbonates, silicates, etc. is reduced to less than 0.1%.

[0046] The microwave pyrolysis regeneration salt is prepared into saturated brine and filtered through JL-02 (which also uses multi-layer filter media), and then enters the subsequent conventional electrolysis and purification process to obtain a caustic soda product with a purity of 98%.

[0047] Comparative Example 1 Compared with Example 1, the difference is that the step of ozone oxidation pretreatment is omitted, and the rest is the same as Example 1.

[0048] Since ozone oxidation pretreatment was not performed, the macromolecular organic matter (such as aromatic compounds and chlorinated organic matter) in the waste salt was not effectively degraded, resulting in a high organic matter content of 1.8% in the regenerated salt after microwave pyrolysis. The contents of inorganic matter and other impurities were similar to those in Example 1, at 0.17% and 0.1%, respectively.

[0049] The purity of the final caustic soda product is relatively low, at 95.8%, and a small amount of organic impurities may remain in the product, affecting product quality.

[0050] Comparative Example 2 Compared with Example 1, the difference is that the microwave pyrolysis step is omitted, and the rest is the same as Example 1.

[0051] Table 1 is a comparison of the effects of ozone pretreatment + microwave pyrolysis and microwave pyrolysis alone

[0052] As can be seen from Table 1, the following limitations can be overcome by combining ozone pretreatment with microwave pyrolysis: macromolecular organic matter (such as aromatic compounds, chlorinated organic matter, etc.) is difficult to completely decompose during microwave pyrolysis and is prone to remain or generate secondary pollutants; certain refractory organic matter requires higher temperature and longer processing time during microwave pyrolysis, which increases energy consumption and cost; microwave pyrolysis alone has limited effect on the removal of heavy metal ions, and it is difficult to meet the high purity requirements of raw salt for the preparation of ion membrane caustic soda.

[0053] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A method for preparing ion-exchange membrane caustic soda using waste salt, characterized in that: The steps include: After screening the waste salt to remove impurities, crush it into 80-120 mesh; The crushed waste salt is pre-treated with ozone, the reaction temperature is 30-50°C, and the reaction time is 30-60min; The waste salt after oxidation pretreatment is subjected to microwave pyrolysis, while passing inert gas protection at 400-800°C for 20-60 minutes; The waste salt after microwave treatment is prepared into a saturated salt solution, and after precision filtration, the salt water for preparing caustic soda by ion membrane is obtained; The obtained brine is electrolyzed by ion membrane electrolysis to prepare caustic soda.

2. The method for preparing ion-exchange membrane caustic soda using waste salt according to claim 1, characterized in that: During the oxidation pretreatment process, the ozone concentration is 80-200 mg / L.

3. The method for preparing ion-exchange membrane caustic soda using waste salt according to claim 1, characterized in that: During the process of pre-oxidation treatment of waste salt with ozone, a catalyst is added; the catalyst is TiO2, Fe2O3 or activated carbon.

4. The method for preparing ion-exchange membrane caustic soda using waste salt according to claim 3, characterized in that: When the catalyst is TiO2 or Fe2O3 powder, the catalyst powder is evenly dispersed in the waste salt at 0.1%-0.5% of the mass of the waste salt.

5. The method for preparing ion-exchange membrane caustic soda using waste salt according to claim 3, characterized in that: When the catalyst is activated carbon, activated carbon particles are added to the waste salt at 0.5%-1% of the mass of the waste salt, or loaded into a porous container.

6. The method for preparing ion-exchange membrane caustic soda using waste salt according to claim 1, characterized in that: The microwave frequency of the microwave pyrolysis is 2400-2500 Hz.

7. The method for preparing ion-exchange membrane caustic soda using waste salt according to claim 1, characterized in that: The temperature of microwave pyrolysis is 500-700°C.

8. The method for preparing ion-exchange membrane caustic soda using waste salt according to claim 1, characterized in that: The filtration accuracy of the precision filtration is 0.1 μm.

9. The method for preparing ion-exchange membrane caustic soda using waste salt according to claim 1, characterized in that: The method also includes the step of electrolyzing the obtained brine by ion membrane electrolysis to prepare caustic soda.

10. The method for preparing ion-exchange membrane caustic soda using waste salt according to claim 1, characterized in that: During the electrolysis process, the temperature of the electrolytic cell is 80-90°C and the current density is 3000A / m²-5000A / m².

Images