The most extensive and in-depth h-extraction method ion delay method and bookmark1 At present, China's acrylic fiber production industry has 8 kinds of solvent 8 process routes, of which DMF (dimethylformamide) dry method, NaSCN-step method and two-step method are The three methods that apply the most. The process route with NaSCN as solvent accounts for more than half of the total, reaching 57%, and China will focus on the development of NaSCN as solvent in the future. Therefore, in-depth study of NaSCN as a solvent is particularly important. NaSCN is a one-step or two-step method. The amount of NaSCN is very large, so it needs to be recycled in the system, but with the addition of the number of cycles, the solvent The impurity content is also correspondingly strong. In order to maintain a low level of impurity content, it is required to continuously purify the solvent. The level of solvent purification technology directly affects the quality of the acrylic fiber, and the production cost of the solvent and the degree of harm to the environment are the entire production level of the acrylic fiber. An important sign This paper introduces some NaSCN purification methods used in the production process of acrylic fiber at home and abroad, and compares the process characteristics. It provides 1NaSCN purification method for selecting the appropriate NaSCN purification process. At present, the production process of acrylic fiber using NaSCN as solvent in the world. Basically from three companies: Britain's Courtaulds (one-step) company, the United States ACC company and Japan's Exlan company (two-step method) solvent NaSCN purification method is also relatively concentrated in these three companies, especially with Exlan research The ion exchange method is currently used in several purification methods in the world, and gel chromatography has broad application prospects because of its unique advantages. The above several methods are applied in China.
The main technical feature of the 11 adsorption method is the combination of adsorption and concentration to purify NaSCN. First adjust the pH value of NaSCN solution with NttOH, and add Ba(N3)2 activated carbon or diatomaceous earth. After stirring the slurry for 1h, some S42-forms in the solution form flocculent precipitate, and the organic matter is adsorbed to the adsorbent and filtered. A sodium thiocyanate solution containing a small amount of sulfate was obtained, and the solution was further concentrated to a concentrated solution containing 59% NaSCN, and Na2SO4 was further precipitated, and the solid was filtered off to obtain a purified NaSCN concentrated solution.
The process basically does not consume other organic solvents, the operation is relatively simple, the gettering effect is relatively pregnant, but the amount of adsorbent (activated carbon or diatomaceous earth) consumed at one time is large, and the old adsorbent containing NaSCN is difficult to be incinerated, and is easy to form. Another patent proposed to pass the impurity-containing NaSCN solution through a sulphate mud bed to precipitate and remove sulfate. However, this method cannot remove other organic impurities, and the resulting toxic waste residue is difficult to handle. The production is of little significance. Exlan, Japan An improved technique of the process is proposed. The diluted solution is adjusted to pH value and directly sent to a fixed bed of activated carbon to remove non-volatile impurities (NVI) and iron ions, and then concentrated to a concentrated solution containing NaSCN 5 or more. The saturated Na2SO4 crystallized and was removed by filtration. When the activated carbon is saturated, the advantage of using NaOH solution for regeneration and fixed bed adsorption is that the process is compact, the equipment is simple, and the automatic control and operation of activated carbon can be recycled. However, this method removes ionic impurities, such as iron ions and some organic ions, which is not thorough enough to meet the purity requirements of the one-step process. It is only suitable for solvent purification in the two-step process. 1.2 Extraction The extraction method was first seen in the British patent. About 13% NaSCN mixed solution and 50% H2SO4 were mixed into the upper part of the extraction column, and reverse-extracted with isopropyl ether from the bottom of the column. The extracted phase was separated into the decantation tank to separate the ether phase containing HSCN, and cooled to neutralize. At the bottom of the column, the neutralization reaction is carried out with 22% NaOH and an equal amount of deionized water added from the upper part of the column, and the purified NaSCN solution is obtained at the bottom of the neutralization column at a concentration of about 22. The advantage of the method is the removal rate and recovery. The rate is high, the process is mature, and the product quality is good. The main disadvantage is that it consumes a large amount of high-purity acid, alkali and isopropyl ether. When the concentration of thiocyanate produced in the extraction process exceeds 5%, it is easily decomposed to produce highly toxic cyanic acid, and the boiling point of isopropyl ether is low, and it is easy to burn. Explosion, so the explosion-proof requirements of the production site are very high, a large amount of acidic wastewater is discharged during production, and the loss of isopropyl ether and NaSCN is large in the process, which makes it difficult to treat sewage. It is difficult to reach the industrial emission standard. 1.3 Ion Exchange Method Japan Toyo Spinning Company Process proposed in 1959: In the first step, the crude sodium thiocyanate solution was passed through a cation exchange resin containing Amberlite IR-120, and Na+ was exchanged on the resin, and NaSCN was converted to HSCN. The second step was carried out in the previous step. The solution was passed through an Amberlite IRB anion exchange column to adsorb SCN-; then SCN- was displaced with NaOH solution to obtain NaSCN solution. The NaOH in the recovery solution was removed by a weakly basic ion exchange resin. Exlan Corporation of Japan was proposed in the patent in 1975 as follows. Process: The solution first generates HSCN through a hydrogen-type cation exchange resin (such as Diaion SK-1B), and then distills off the HSCN under reduced pressure to remove the HSCN vapor condensation of the impurity component. After cooling to less than 5 C, a sodium thiocyanate recovery liquid was formed through a monosodium resin column at a concentration of about 5%.
Shanghai Petrochemical Co., Ltd. has also developed its own technology to remove non-volatile impurities (NVI) in the sodium thiocyanate using Lewatit Mp62 macroporous phenethyl weak alkaline anion exchange resin. The ion exchange process is as follows: The removal rate and NaSCN recovery rate are high, but its biggest disadvantage is that it needs to be regenerated with acid and alkali, which consumes chemical reagents and easily causes secondary environmental pollution. It is economically uneconomical. 14 ion retardation resin method ion retardation resin method is ACC company. Invented in the 1980s, it uses a unique resin-ion retardation resin (such as Dow's Retardion 11A8) ion retardation resin, also known as snake cage resin, which was first synthesized by Dow Chemical Company of the United States and refers to a specific charge exchange. In a functionally crosslinked polymer (cage), a special type of ion exchange resin physically trapped with a linear polymer (snake) having oppositely charged functional groups.
The delayed resin has two opposite functional groups of yin and yang, which can attract each other and neutralize part of the charge. Therefore, the mechanism of action is ion block rather than ion exchange. Since the mutually paired functional groups have a weak adsorption force to the blocked ions, they can be regenerated by washing with water. When the NaSCN solution passes through the retardation resin, the SCN- electrode is very strong and is firmly adsorbed in the cage. Other ions are distributed on the snake according to the electrode polarity. When eluted with water, these ions are orderedly replaced. Due to the optimistic prospects of ion-delayed resin in electrolyte separation and industrial application, Zhu Changying synthesized an ion-delaying resin with similar properties to Retrodion 11A-8, and evaluated the separation performance of impurities in acrylic spinning bath. Daqing Petrochemical Complex Acrylic fiber factory introduced ACC's NaSCN-step method technology in the late 1980s. The solvent NaSCN purification technology uses delayed resin method. The resin used is Dow Chemical Company's Retroardion 11A~8, and the single tower filling volume is 4.8m3. The patented technology was set up in Fushun Chemical Fiber Factory for 86 years. The ctM impurity and the sulphuric acid sputum I can't be cut off. It needs to be used as part of the i.netbookmark2 ion retardation resin, which makes the solvent purification process more simple, and the equipment investment is more solvent. The extraction method is greatly reduced. There is no need for chemical reagents and no pollution to the environment. The shortcomings of this technology are: the exchange capacity of the resin is small, the production capacity is low; the elution is slow, the concentration of the recovered liquid is dilute, and the energy concentration is again concentrated; the peak shape of the chromatogram is severe (especially when the pH value is lower than 4~5) The loss of sodium thiocyanate is the largest than that of the a-extraction method, and the process route is the longest. The formed ions are easily evaded by the phlegm of the phlegm-retaining medium, and the r is highly toxic and severe. In addition, consumption. In addition, if the material contains copper, iron, manganese ions, and impurities such as iodine, perchlorate, silicate, etc., it is easy to poison the resin. After a period of operation, it is still necessary to regenerate 1.5 strong gel with strong acid and alkali. Glue chromatography is based on the hydrodynamic volume of solute molecules. It is inside the gel media particles and contains very rich polydisperse micropores. Smaller molecules can enter more or all of the micropores. The molecules can only enter part of the micropores or even completely repel outside the pores. Therefore, when molecules of different sizes flow through the gel column, the small molecules are retained and the macromolecules are first excluded, and the gel pores play the role of screening the particle size of the material.
Depending on the polarity of the substances constituting the gel, it can be divided into two types: hydrophilic gels and hydrophobic gels. The hydrophilic gels have the ability to "sieve" inorganic salts, especially glucose gels. In the past, many scholars have carried out research in this area. It is generally believed that in addition to the volume exclusion effect, the mechanism of separation of inorganic salts by gel chromatography also has secondary effects such as ion exclusion, physical adsorption, and zwitterionic exchange. Benefits from the separation of inorganic salts SCN- is a very typical ion, which has a relatively obvious adsorption tendency in the gel, and the partition coefficient is often greater than 1, which is beneficial to the separation from other inorganic salts. Some scholars believe that the reason why SCN-ion can be exceptionally retained in the gel may also be due to the hydration degree and the thermodynamic properties of the hydration process. In view of the above understanding, the Japanese Exlan company, Xia Yuxing, etc., first proposed the use of cross-linked dextran. The gel-like purification of the acrylic solvent sodium thiocyanate solution. In 1983 and 1984, they applied for patents on gel regeneration methods in Japan and the United States, and applied for the best method for mixing old and new gels. It is expressly mentioned in the patent that these old glucan gels have been used continuously in solvent purification plants for up to five years. It shows that the gel chromatography purification technology is relatively mature, and Zhejiang University has done systematic research on gel chromatography purification technology, and applied it to industrial devices. Since its completion in 1997, it has performed well and its impurity removal. Rate and NaSCN recovery rate of 94% and 99% respectively. From the preliminary development work of gel chromatography and its industrial operation, the technology has the advantages of simple process, good impurity removal effect and low operation cost. Consuming any chemical reagent, neither introducing impurities nor decomposing sodium thiocyanate, causing environmental pollution. Although imported gels are expensive, they are not needed for long-term use due to high-valent cations or complex formation. The regeneration of chemical reagents makes the service life of the gel much longer than that of the ion exchange resin. Therefore, it is an ideal NaSCN solvent purification method. 16 Other purification methods In addition to the above several solvent purification methods, some purification methods have been proposed, such as Crystallization method and electrodialysis method, such as crystallization electrodialysis, reverse osmosis membrane filtration, simulated moving bed method of amphoteric resin Has been tested in domestic acrylic fiber companies, the results show that there is no value for promotion and application. The simulated moving bed method uses an ion-delayed resin with an impurity removal rate of 8 (% to 90%, and a product purity of 98% 3% to 5%. The simulated moving bed does not change the inherent incomplete separation of the ion-delay resin. The loss of NaSCN is still very large, and it will still pollute the environment due to excessive NaSCN in the discharged wastewater. In 1992, Exlan proposed to remove the coloring impurities in the recovered solvent with reverse osmosis membrane, the osmotic pressure is less than 20MPa, and the membrane material is available. Cross-linked polyvinyl alcohol, polyolefin, polysulfone polyamide and cellulose acetate, the reverse osmosis membrane has a NaCl rejection of 10% to 97%, preferably in the range of 3 to 60%. Soon, Exlan added a similar The patent extends the treatment to the purification of thiocyanate-containing solutions, including the purification of coke oven gas desulfurization by-product ammonium thiocyanate solution. However, neither of these patents evaluates the recovery rate of thiocyanate. There have been no reports of industrial applications, and the economics of its operation and the reliability of equipment operation (mainly the reliability of reverse osmosis membranes) have doubts. 2 Comparison of purification methods In order to facilitate the selection, in the comprehensive technical aspects On the basis of the above, the comparison of the above-mentioned main NaSCN purification methods is listed in Table 1. Table 1 Comparison of different purification methods Purification method Process equipment cost removal rate, recovery rate, %% operating cost Environmental pollution adsorption method Medium small larger and heavier The extraction method is complicated and large. The ion exchange method is complicated. The heavier ion delay method is smaller. The lighter and lighter gel chromatography is simpler and smaller. The amount of acid-base ether is almost large, and the NaSCN loss is large. Exceeding the standard, causing secondary pollution, the process is relatively backward.
The advantage of the ion exchange resin method is that the process is simple, the specifications of the anion-cation exchange resin are large and the options are large, and the quality of the domestic resin is stable and reliable, and the replacement is convenient, so the application value is obtained. However, due to the limited exchange capacity, when the NaSCN solution with a large concentration is processed, the acid-base regeneration is frequent, the consumption is also large, and the economical operation is far less than the ion retardation resin method.
The advantage of the ion retardation resin method is that it is simpler than the ion exchange resin method. The main problem of the resin is that it is easily contaminated by high-valent ions, and the activity is gradually reduced to lose the separation ability. Therefore, it is necessary to periodically regenerate with sodium metabisulfite or strong acid and alkali, which is easy to cause two. Sub-environmental pollution, which limits the industrial application of this method. Activated carbon adsorption is the most economical method of initial investment in all purification methods. The main reason is that the price of activated carbon is relatively cheap, but the problem is that acid-base regeneration is still needed. It is also difficult to adsorb and remove strong ionic coloring components. This incomplete method of removing impurities is only suitable for the two-step process.
The process of gel chromatography is the simplest, does not consume any chemical reagents, and the complete recovery rate is extremely high. The concentration of NaSCN in wastewater can be controlled at a very low level without special treatment, so it is an ideal purification method. The shortcoming of green technology gel chromatography is that the price of the gel is relatively expensive. If the gel is developed and produced domestically like the ion retardation resin, the prospect will be more pregnant.
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