CN1217921C

CN1217921C - Recovery method of heavily polluated substance produced during production of foaming agent and device

Info

Publication number: CN1217921C
Application number: CN03132082.1A
Authority: CN
Inventors: 郑守樽; 郑敏榕
Original assignee: Individual
Current assignee: Individual
Priority date: 2003-07-15
Filing date: 2003-07-15
Publication date: 2005-09-07
Anticipated expiration: 2023-07-15
Also published as: CN1513835A

Abstract

The present invention relates to a method for recovering solid matter by evaporating and freezing crystal from a liquid and a device thereof. The recovery method of the severe pollutant in the production process of the ADC foaming agent comprises: (1) hydrazine solution and urea are condensed in a condensing pot and filtrated to obtain the first press cake (biurea) and the first filter liquor; (2) the first filter liquor is evaporated in an evaporating pot, then the second press cake (sodium chloride) and the second filter liquor are prepared from the first filter liquor which is centrifugated until the volume of the first filter liquor is reduced by 30 to 60% in a centrifuge; the second filter liquor is frozen in a freezing pot, then the third press cake (ammonium chloride) and the third filter liquor are prepared from the second filter liquor which is centrifugated in the centrifuge at the temperature of 10 to-5 DEG C; the third filter liquor is evaporated and frozen to crystallize the sodium chloride and the ammonium chloride repeatedly in the evaporating pot and the freezing pot, then the mother liquor is put in the condensing pot when the hydrazine content in the mother liquor is more than 1%. The whole production process has the characteristics of no waste effluent and good environmental protection benefit, and the value of the recovered product is high. The method saves all equipment and operating cost for treating the waste such as ammoniacal nitrogen, etc., and saves the cost.

Description

Recovery of serious pollutants generated during production of ADC foaming agent

Technical Field

The invention relates to a method and a device for recovering solid substances from liquid through evaporation and freezing crystallization, in particular to a method and a device for recovering serious pollutants generated in the production of ADC foaming agent.

Background

ADC foaming Agent (ADC) is widely used in plastic rubber industry, and has the widest application and the largest output in the foaming agent. However, the development of ADC production in China is hindered by the two problems of raw material sources, namely the oxidant sodium bichromate and the pure hydrazine hydrate. Therefore, the inventor of the presentinvention studied ADC production by chlorine oxidation instead of sodium bichromate in the first chemical plant of Fuzhou in 1966, and proposed ADC production by crude hydrazine instead of pure hydrazine in 1970, wherein the production process is as follows:

1. preparation of sodium hypochlorite with sodium hydroxide and chlorine:

--(1)

2. preparing hydrazine hydrate (also called crude hydrazine) by using sodium hypochlorite, excessive urea and excessive sodium hydroxide:

--(2)

3. preparation of biurea by condensation of hydrazine hydrate with excess urea:

--(3)

or

Product of this reaction (NH)₄)₂SO₄Also known as ammoniacal nitrogen NH₃-N，

4. Preparing an ADC foaming agent by oxidizing biurea and chlorine gas:

--(4)

it should be noted that, the pure hydrazine hydrate, one of the two problems mentioned above, is difficult to supply because the production of pure hydrazine hydrate is performed by the reaction of the formula (1) and the formula (2), and then crude hydrazine with hydrazine content of only about 3% is obtained, and the pure hydrazine hydrate contains many soluble impurities, such as by-products of sodium chloride 10-15%, sodium carbonate 4-10%, sodium hydroxide 1-5% and redundant urea and ammonia, and because hydrazine hydrate has a boiling point of 114 ℃, it is difficult to evaporate and concentrate hydrazine from crude hydrazine, and a five-layer evaporation tank is required.

The inventor of the present invention analyzed and researched whether various soluble impurities in crude hydrazine influence the reaction of the formula (3) in 1970 in detail, and after the biurea and ADC are washed by two filtration, the impurities can be removed, and the impurities can not influence the reaction of the formula (3), and also can not influence the purity and quality of the product, so that the invention provides a new process for producing ADC by using crude hydrazine instead of pure hydrazine at home and abroad, and a large number of tests and argumentations and large-scale production practices prove that the new process can meet the production requirements of ADC in terms of yield, saves a whole set of evaporation, fractionation and purification equipment and operation procedures for processing and refining pure hydrazine, greatly reduces energy consumption, reduces the loss of one-step evaporation hydrazine, reduces the treatment of hydrazine-containing waste residue, greatly reduces the cost, has international competitive power, and writes a factory for meeting the urgent needs of ADC market in order to meet the urgent needs of ADC market, and successfully creates hydrazine hydrate in Fuzhou, The new process for producing the ADC foaming agent recovers three byproducts of ammonium sulfate, hydrochloric acid and soda ash, is published in the fourth journal of 1972 of Fuzhou science and technology, is given without compensation in the chemical industry of China, is popularized and applied to form a unique process route of China, the yield is changed from dozens of tons produced in the country to tens of thousands of tons produced in the year, and China is also changed from a large number of imports of ADC to a large number of exports.

However, when crude hydrazine is used for producing ADC foaming agent by condensation, because the condensation reaction of formula (3) can only be carried out under slightly acidic or slightly alkaline conditions, otherwise, urea is subjected to a large amount of hydrolysis loss during condensation at 110 ℃ to cause difficult reaction, sulfuric acid must be added before and during the condensation reaction to neutralize the crude hydrazine solution and adjust the pH value, so that the condensation reaction can be rapidly and smoothly carried out under high-temperature conditions:

--(5)

--(6)

--(7)

--(8)

for the treatment of acid mist pollution, the inventor has thoroughly solved the problem in the owned Chinese patent ZL96107542.2 (grant publication No. CN 1095390C). In the mother liquor, pollutants such as ammoniacal nitrogen, sodium chloride and sodium sulfate exist, and in the seventies and eighty years of the last century, the mother liquor is a good liquid fertilizer for farmers lack of fertilizer and is in short supply. However, in the mid-eighties of the last century, a large amount of fertilizers are fully supplied and the price is reduced, so that a large amount of condensation mother liquor is directly discharged as waste liquor, and great pollution is caused to the surrounding environment.

With the publication of the state health care law and the emission standard, if the pollution problem of ammoniacal nitrogen in the ADC production process is not solved, the unique process route in China is rejected, and if the requirement of the environment protection law is executed, 241 kilograms of ammoniacal nitrogen as a byproduct of 1 ton of ADC product is the discharge capacity of 16000 cubic meters which is equivalent to the national allowable emission standard (15mg/l), and manufacturers who do not pay discharge fees or change the process route independently need to close due to loss.

However, even if some enterprises just want to treat the three pollutants of ammonium chloride, sodium sulfate and sodium chloride in the condensation mother liquor, the method still has great technical problems because a large amount of sodium sulfate is simultaneously separated out when the sodium chloride is recovered from the condensation mother liquor by an evaporation method to form mixed salt which cannot become a product; when the ammonium chloride is recovered by a freezing method, a large amount of sodium sulfate is simultaneously crystallized and separated out, so that the purity of the ammonium chloride is greatly reduced, and the ammonium chloride is difficult to separate and purify.

In order to overcome the interference of sodium sulfate on the recovery of sodium chloride and ammonium chloride, hydrochloric acid is proposed to replace sulfuric acid for neutralization and condensation reaction pH adjustment, so that the above-mentioned reactions (5) to (8) do not occur, and the following reactions occur instead:

--(9)

--(10)

the only serious contaminants produced when producing ADC blowing agents are ammonium chloride and sodium chloride.

Disclosure of Invention

The invention aims to solve the problem of serious pollution in the production of an ADC foaming agent by using crude hydrazine in the prior art, and provides a method and a device for recovering serious pollutants generated in the production of the ADC foaming agent, which can comprehensively recycle by-product pollutants in the production process of the ADC foaming agent, thoroughly eliminate pollution and reduce the production cost of the ADC foaming agent.

The purpose of the invention is realized through the following scheme.

The method for recovering serious pollutants generated in the production of ADC (azodicarbonamide) foaming agent is characterized by comprising the following steps:

1. providing a hydrazine solution obtained by neutralizing crude hydrazine with hydrochloric acid, a condensation kettle,

(1) adding urea into hydrazine solution and carrying out condensation reaction in a condensation pot,

(2) filtering the condensation reaction liquid, recovering the first filter cake biurea and the first filtrate,

2. provides an evaporating pot, a freezing pot,

(1) introducing the first filtrate into an evaporation tank for evaporating and crystallizing sodium chloride,

(2) introducing the first filtrate into a centrifuge for centrifugation when the volume of the first filtrate is reduced by 30-60%, recovering the second filter cake sodium chloride and the second filtrate,

(3) the second filtrate is pumped into a freezing pot to freeze and crystallize ammonium chloride,

(4) introducing the second filtrate into a centrifuge for centrifugation when the temperature of the second filtrate in the freezing pot reaches 10-5 ℃, recovering third filter cake ammonium chloride and third filtrate,

(5) and repeatedly introducing the third filtrate into an evaporation tank for evaporating and crystallizing sodium chloride and freezing crystal ammonium chloride in a freezing pot until the hydrazine content in the mother liquor is more than 1%, and then returning the mother liquor to the condensation pot for condensation.

The following reaction in the conventional art:

1. preparation of sodium hypochlorite with sodium hydroxide and chlorine:

--(1)

NaOCl+NH₂CONH₂+2NaOH＝N₂H₄H₂O+NaCl+Na₂CO₃--(2)

3. adding hydrochloric acid for neutralization:

Na₂CO₃+2HCl＝2NaCl+H₂O+CO₂--(9)

NaOH+HCl＝NaCl+H₂O--(10)

the resulting reaction solution is called hydrazine solution, which contains by-product sodium chloride, and excess urea and side-reaction ammonium salt in addition to the main product hydrazine hydrate.

Firstly, carrying out condensation reaction on hydrazine solution and urea in a condensation pot by adopting a conventional method, and filtering condensation reaction liquid, wherein a first filter cake is biurea, and a first filtrate contains ammonium chloride, sodium chloride, urea and a small amount of hydrazine.

Secondly, evaporating and crystallizing the first filtrate in an evaporating pot to recover sodium chloride, wherein the sodium chloride is easy to separate and purify the sodium chloride from the ammonium chlorideby an evaporating and crystallizing separation method because the solubility of the sodium chloride is small along with the change of temperature and the slope of a curve is low, and the solubility of the ammonium chloride is large along with the change of temperature and the slope of the curve is large, so that the purity of the separated sodium chloride is ensured as long as the ammonium chloride is not separated out when the volume of the first filtrate is controlled to evaporate 30-60%.

The best condition for judging the crystallization of the sodium chloride is selected as follows: when the volume of the first filtrate in the evaporation tank is reduced by 50%, the first filtrate is introduced into a centrifuge for centrifugation.

And because the chloride ions of the ammonium chloride with high solubility promote the solubility of the sodium chloride in the solution to be smaller, more sodium chloride can be separated out.

And centrifuging the sodium chloride precipitated by evaporation crystallization by using a centrifugal machine to obtain a second filter cake sodium chloride, wherein the second filtrate also contains ammonium chloride, sodium chloride, urea and a small amount of hydrazine.

And thirdly, freezing and crystallizing the second filtrate in a freezing pot by adopting a freezing method to obtain third filter cake ammonium chloride, wherein the solubility of the ammonium chloride is greatly reduced when the temperature is low, so that a large amount of ammonium chloride is separated out, chloride ions are continuously reduced, the solubility of sodium chloride is higher, and the separation of the sodium chloride cannot occur, so that the ammonium chloride with higher purity can be obtained by introducing the second filtrate into a centrifuge for centrifugally recovering the ammonium chloride when the temperature is 10-5 ℃ in the freezing pot, and the ammonium chloride can reach the standard of agricultural ammonium chloride by simple washing.

The best condition for judging the crystallization of the ammonium chloride is selected as follows: introducing into a centrifuge for centrifuging when the temperature of the third filtrate in the freezing pot reaches 0 ℃.

The third filtrate also contains sodium chloride and ammonium chloride which can be recovered by evaporating and freezing for many times, and when the hydrazine content in the residual solution is more than 1 percent (the hydrazine can be used for condensing biurea), the third filtrate returns to the condensation process for recycling, so that the purposes of avoiding the hydrazine polluting the environment and making the best use of the hydrazine are achieved, the cost is saved, and the waste is avoided. Thus, the whole closed cycle is established, the comprehensive recycling of the byproduct pollutants is realized, and the pollution is thoroughly eliminated.

The present invention relates to equipment such as a freezing pot, a centrifugal machine, an evaporating pot, a condensation pot, etc., which are all provided in the prior art.

The invention separately recovers the byproducts in the production process of the ADC foaming agent, so that the byproducts are easy to separate and purify, particularly the ammonia nitrogen which seriously pollutes the environment is recovered, a closed cycle is established, and the pollution is eliminated.

It is particularly emphasized that the use of hydrochloric acid in place of sulfuric acid for neutralization and pH adjustment in the condensation reaction according to the invention has the following advantages: the residual hydrochloric acid in a chlor-alkali plant and the hydrochloric acid byproduct in the formula (4) can be fully utilized, the conventional sodium sulfaterecovery equipment and operation can be reduced, and particularly, the ammonium chloride product can reach the standard of agricultural ammonium chloride because the sodium sulfate does not interfere the ammonium chloride recovery.

The whole recovery process of the invention does not need any raw and auxiliary materials, the process is simple and reasonable, and the equipment investment is saved; the whole production process has no waste discharge (ammoniacal nitrogen and sodium chloride), fully meets the requirements of environmental protection law on waste discharge, and has the best environmental protection benefit; the method changes waste into valuable, and the recovered ammonium chloride and sodium chloride can create good economic benefit and social benefit, which is an unexpected effect of the invention. On the other hand, the method saves all equipment and operation cost for treating wastes such as ammoniacal nitrogen and the like, and further reduces the cost.

The method also comprises the following steps when the second filter cake sodium chloride and the third filter cake ammonium chloride are recovered:

1. the second filter cake is washed with hot water in a centrifuge,

2. stopping the machine and discharging when the content of ammonium chloride in the second filter cake is lower than 0.05%.

The invention further resides in

1. The third filter cake is washed with cold water in a centrifuge,

2. and stopping the machine to discharge when the content of the sodium chloride in the third filter cake is lower than 2.5 percent.

The process aims to further purify the sodium chloride and the ammonium chloride, so that the quality of the sodium chloride and the ammonium chloride is further improved, and the economic benefit of the sodium chloride and the ammonium chloride is further improved.

The filtrate and washing liquid in the invention can be returned to an evaporating pot and a freezing pot for repeated evaporation and freezing crystallization to obtain sodium chloride and ammonium chloride.

The second filter cake and the third filter cake are washed to obtain purified products, so that the products obtained by the process of the invention reach the standard of like products, and serious pollutants in the prior art are changed into available fertilizers of ammonium chloride and sodium chloride, so that harm is turned into benefit, and waste is turned into wealth.

The purpose of the invention, namely providing a device for recovering pollutants in the production process of ADC foaming agent, can be realized by the following scheme.

Recovery unit of pollutant in ADC foamer production process, including the condensation pot, its main points lie in, still include neutralization hydrazine metering tank, centrifuge B, the evaporating pot, centrifuge C, freezing pot and centrifuge D, neutralization hydrazine metering tank, the condensation pot, centrifuge B, the evaporating pot, centrifuge C, freezing pot and centrifuge D's business turn over material mouth communicates in proper order, and centrifuge B, centrifuge C and centrifuge D's solid discharge gate respectively with the biurea storage tank, sodium chloride storage tank and ammonium chloride storage tank intercommunication, centrifuge D's filtrating discharge gate and evaporating pot and neutralization hydrazine metering tank intercommunication.

The above devices are conventional devices, and the connection mode of the devices is different from the prior art, so that the complete device achieves beneficial technical effects when used for recovering ammonium chloride and sodium chloride pollutants in the ADC foaming agent production process.

The circulation of material can be realized through the charge pump between each equipment of above-mentioned device, and some equipment are like then multiplicable agitator stirs the material with hydrazine metering tank, freezing pot etc..

The key point of the method for recovering the serious pollutants generated in the production of the ADC foaming agent is that the method can be further refined into the following steps:

(1) providing a crude hydrazine solution neutralized by hydrochloric acid, a condensation kettle,

① the hydrazine solution is introduced into a condensation kettle and added with urea for condensation reaction,

② filtering the condensation reaction liquid, recovering the first filter cake biurea and the first filtrate,

(2) provides an evaporating pot, a centrifuge, a dissolving pot,

(3) introducing the first filtrate into an evaporation tank and a centrifuge, evaporating for multiple times, filtering to obtain mixed salt, or solarizing to obtain mixed salt,

(4) heating and dissolving the mixed salt in a solvent in a dissolving pot until ammonium chloride is dissolved to be nearly saturated to obtain sodium chloride crystal mixed solution,

(5) there is provided a freezing pot having a plurality of freezing chambers,

(6) the mixed solution is led into a centrifuge for centrifugation to obtain a second filter cake of sodium chloride, a second filtrate of ammonium chloride and sodium chloride solution,

(7) the second filtrate is pumped into a freezing pot to freeze and crystallize ammonium chloride,

(8) introducing the filtrate into a centrifuge for centrifugation when the temperature of the filtrate in the freezing pot reaches 10-5 ℃ to obtain a third filter cake ammonium chloride and a third filtrate,

(9) and (4) repeatedly carrying out the steps (4) to (8) on the third filtrate until the hydrazine content in the mother liquor is more than 1%, and then sending the mother liquor back to the condensation kettle for condensation.

The solvent in the process can be third filtrate, mother liquor and even water. If the third filtrate is still remained after reaching the dissolving pot, the redundant third filtrate can be returned to the evaporating pot to recycle the mixed salt.

The process provides a method for recovering sodium chloride and ammonium chloride from a first filtrate obtained after condensation reaction, which comprises the steps of firstly removing water to obtain a mixed salt, then firstly dissolving ammonium chloride according to different solubilities of the sodium chloride and the ammonium chloride to recover the sodium chloride, and finally freezing to recover the ammonium chloride. The method is also particularly suitable for dry climates in northern winter, because the beach drying method can be adopted to obtain mixed salt, and even the freezing pot can not be used, and the natural freezing method can be directly adopted to obtain ammonium chloride. In this way, a lot of costs can be saved.

Compared with the prior art, the invention has the following advantages: the whole recovery process does not need any raw and auxiliary materials, the process is simple and reasonable, and the equipment investment is saved; the whole production process has no waste discharge, and has good environmental protection benefit; the method has the advantages that waste materials are changed into valuable materials, the recovered ammonium chloride and sodium chloride can create good economic benefit and social benefit, all equipment and operation cost for treating wastes such as ammoniacal nitrogen and the like are saved, and the cost is saved.

Drawings

FIG. 1 is a process flow diagram of example 1 of the present invention.

FIG. 2 is a process flow diagram of the preferred embodiment of the present invention in accordance with example 2.

Detailed Description

The present invention will be described in more detail with reference to examples.

The device for recovering the pollutants in the ADC foaming agent production process shown in figure 1 comprises the following equipment: the device also comprises a hydrazine neutralization metering tank 1, a condensation kettle 2, a centrifuge B, an evaporation tank 3, a centrifuge C, a freezing kettle 4, a centrifuge D, a biurea storage tank 5, a sodium chloride storage tank 6, an ammonium chloride storage tank 7, a stirrer 8, a feeding pump 9 and a valve 10.

The connection of these devices is as follows: the neutralization hydrazine metering tank 1 is provided with a stirrer 81 and is communicated with a condensation pot 2 through a feeding pump 91 and a valve 101, the condensation pot 2 is communicated with a centrifuge B through a feeding pump 92 and a valve 102, a solid discharge port of the centrifuge B is communicated with a biurea storage tank 5, a filtrate discharge port of the centrifuge B is communicated with an evaporation tank 3 through a feeding pump 93 and a valve 103, the evaporation tank 3 is communicated with a centrifuge C through a feeding pump 94 and a valve 104, the solid discharge port of the centrifugeC is communicated with a sodium chloride storage tank 6, the filtrate discharge port of the centrifuge C is communicated with a freezing pot 4 through a feeding pump 95 and a valve 105, the freezing pot 4 is provided with a stirrer 82, the freezing pot 4 is communicated with a centrifuge D through a feeding pump 96 and a valve 106, the solid discharge port of the centrifuge D is communicated with an ammonium chloride storage tank 7, the filtrate discharge port of the centrifuge D is communicated with the evaporation tank 3, and the discharge port of the centrifuge D is.

According to the recovery device for the serious pollutants generated in the ADC foaming agent production, the following recovery method for the serious pollutants generated in the ADC foaming agent production is adopted, and the recovery method comprises the following steps:

1. providing a crude hydrazine solution neutralized by hydrochloric acid, storing the hydrazine solution in a hydrazine neutralization metering tank 1, opening a stirrer 81, a valve 101 and a feed pump 91, introducing the hydrazine solution into a condensation pot 2, stopping the feed pump 91 and a closing valve 101, starting a centrifuge B, a valve 102 and a feed pump 92 when the condensation reaction is finished, centrifugally filtering the condensation reaction solution, stopping the feed pump 92, the closing valve 102 and the centrifuge B when the first filter cake biurea accounts for 70-80% of the capacity of the centrifuge B, and discharging the first filter cake to a biurea storage tank 5.

2. Recovering sodium chloride by an evaporation method:

opening a valve 103 and a feed pump 93, feeding the first filtrate to an evaporation tank 3, feeding to a certain height, stopping the feed pump 93 and the valve 103, starting evaporation operation until the volume of the first filtrate is reduced by 30%, a large amount of sodium chloride is crystallized and separated out, opening a centrifugal machine C, a valve 104 and a feed pump 94 when ammonium chloride is nearly saturated, performing spin-drying dehydration, stopping the feed pump 94, the valve 104 and the centrifugal machine C when the second filter cake sodium chloride accounts for 70-80% of the capacity of the centrifugal machine C, and discharging the second filter cake to a sodium chloride storage tank 6.

3. Ammonium chloride recovery by freezing:

opening a valve 105 and a feed pump 95, feeding the second filtrate into a freezing pot 4, stopping feeding the second filtrate to 80%, stopping the feed pump 95 and a closing valve 105, opening a stirrer 82 of the freezing pot 4, starting the cold liquid for freezing, stopping the cold liquid when the temperature of the second filtrate reaches 10 ℃, starting a centrifuge D, a valve 106 and a feed pump 96, performing spin-drying dehydration, stopping the feed pump 96, the closing valve 106 and the centrifuge D when the third filter cake ammonium chloride accounts for 70-80% of the capacity of the centrifuge D, discharging the third filter cake to an ammonium chloride storage tank 7, repeatedly introducing the third filtrate into an evaporation tank 3 through the feed pump 97 and the valve 107 for evaporating and crystallizing sodium chloride and freezing the ammonium chloride in the freezing pot 4 for cold crystallization, and returning the mother liquor to a neutralization hydrazine metering tank 1 after the hydrazine content in the mother liquor is more than or equal to 1%.

The parts not described in the present embodiment are the same as those in the prior art.

Example 2:

the method for recovering the pollutants in the ADC foaming agent production process shown in figure 2,

1. when sodium chloride is recovered by an evaporation method, a large amount of sodium chloride is crystallized and separated out when the volume of first filtrate in an evaporation tank 3 is reduced by 60%, ammonium chloride is nearly saturated, the solution is introduced into a centrifugal machine for centrifugation, a centrifugal machine C, a valve 104 and a feeding pump 94 are started for spin-drying and dehydration, when the volume of second filter cake sodium chloride accounts for 70-80% of the volume of the centrifugal machine C, the feeding pump 94 and the valve 104 are stopped, the second filter cake is washed by hot water in the centrifugal machine C until the content of ammonium chloride in the second filter cake is lower than 0.05%, the centrifugal machine C is stopped, and the second filter cake is discharged to a sodium chloride storage tank 6;

2. when the ammonium chloride is recovered by freezing, introducing the second filtrate into a freezing pot 4 to-5 ℃ for centrifuging, stopping cooling liquid, starting a centrifuge D, a valve 106 and a feed pump 96, performing spin-drying dehydration, stopping the feed pump 96 and closing the valve 106 when the third filter cake ammonium chloride accounts for 70-80% of the capacity of the centrifuge D, washing the third filter cake with cold water in the centrifuge D until the content of sodium chloride in the third filter cake is lower than 2.5%, stopping the centrifuge D, and discharging the third filter cake to an ammonium chloride storage tank 7.

The recovery apparatus and the remaining steps in this example were the same as in example 1.

The best embodiment is as follows:

1. when sodium chloride is recovered by an evaporation method, introducing the sodium chloride into a centrifugal machine for centrifugation when the volume of the first filtrate in the evaporation tank 3 is reduced by 50%;

2. when ammonium chloride is recovered by freezing, the second filtrate is introduced into a centrifuge for centrifugation until the temperature of the second filtrate in the freezing pot 4 reaches 0 ℃.

The present example is the same as example 1 except for the above-mentioned portions.

Claims

1. A method for recovering serious pollutants generated in the production of azodicarbonamide foaming agent is characterized by comprising the following steps:

(1) providing a hydrazine solution obtained by neutralizing crude hydrazine with hydrochloric acid, a condensation kettle,

① adding urea into hydrazine solution and carrying out condensation reaction in a condensation pot,

(2) provides an evaporating pot, a freezing pot,

① the first filtrate is introduced into an evaporator to evaporate and crystallize sodium chloride,

② introducing the first filtrate into a centrifuge for centrifugation when the volume of the first filtrate is reduced by 30-60%, recovering the second filter cake sodium chloride and the second filtrate,

③ the second filtrate is pumped into a freezing pot to freeze and crystallize ammonium chloride,

④ introducing the second filtrate into a centrifuge for centrifugation at 10-5 deg.C to recover the third filter cake ammonium chloride and the third filtrate,

⑤ introducing the third filtrate into an evaporator for several times to evaporate and crystallize sodium chloride and freeze crystal ammonium chloride in a freezing pot until the hydrazine content in the mother liquor is more than 1%, and returning the mother liquor to the condensation pot for condensation.

2. The method for recovering severe contaminants generated during the production of azodicarbonamide as a foaming agent as recited in claim 1 wherein the first filtrate is directed to a centrifuge for centrifugation by 50% reduction of the first filtrate volume in the evaporator.

3. The method for recovering serious contaminants generated during the production of azodicarbonamide as foaming agent as recited in claim 1, wherein the second filtrate is introduced into the centrifuge for centrifugation at a temperature of 0 ℃ to the temperature of the second filtrate in the freezing pot.

4. The method of claim 1 for recovering serious contaminants generated during the production of azodicarbonamide blowing agents, further comprising the steps of:

(1) the second filter cake is washed with hot water in a centrifuge,

(2) stopping the machine and discharging when the content of ammonium chloride in the second filter cake is lower than 0.05%.

5. The method of claim 1 for recovering serious contaminants generated during the production of azodicarbonamide blowing agents, further comprising the steps of:

(1) the third filter cake is washed with cold water in a centrifuge,

(2) and stopping the machine to discharge when the content of the sodium chloride in the third filter cake is lower than 2.5 percent.

6. A method for recovering serious pollutants generated in the production of azodicarbonamide foaming agent is characterized by comprising the following steps:

(2) provides an evaporating pot, a centrifuge, a dissolving pot,

(5) the mixed solution is led into a centrifuge for centrifugation to obtain a second filter cake of sodium chloride, a second filtrate of ammonium chloride and sodium chloride solution,

(6) there is provided a freezing pot having a plurality of freezing chambers,