Method for treating p-hydroxybenzonitrile production wastewater

Abstract

The invention provides a method for treating p-hydroxybenzonitrile production wastewater. The method comprises the steps of alkali adjustment, air stripping, absorption, neutralization, concentration, centrifugation and the like. On the basis of a traditional ammonia stripping device, a neutralizing device of hydrochloric acid and a concentrating device of ammonium chloride are introduced, the neutralizing device of hydrochloric acid can directly absorb ammonia gas generated by the ammonia stripping device to generate an ammonium chloride byproduct, and the concentrating device of ammonium chloride can form high-temperature water vapor in the process of concentrating an ammonium chloride solution; and meanwhile, high-temperature water steam is introduced into the ammonia stripping device to serve as a heat source, so that extra energy consumption required by the operation of the ammonia stripping device is greatly reduced, and the economical efficiency and the environmental protection property of system operation are improved. According to the method, only the cheap industrial byproduct hydrochloric acid is introduced as a new raw material, so that the value of the byproduct is greatly improved, the economic benefit is integrally improved, meanwhile, high-temperature steam generated in the ammonium chloride production process can also be used as a heat source of an ammonia stripping device, and the cost is further reduced.

Description

Technical Field

[0001] The invention relates to the technical field of chemical synthesis, and in particular to a method for treating wastewater produced by p-hydroxybenzonitrile. Background Art

[0002] 4-Cyanophenol is an organic compound, which is mostly used as an intermediate of organophosphorus insecticides cypermethrin and benzonitrile. It can also be used to synthesize liquid crystal materials, pesticides, fragrances, etc. There are many preparation methods, such as ammoniation of p-hydroxybenzoic acid, ammoniation of methyl p-hydroxybenzoate, ammoniation of p-hydroxybenzaldehyde, diazonium salt method, etc. At present, the mainstream synthesis process of p-hydroxybenzonitrile in China is to melt and react p-hydroxybenzoic acid, urea, and phosphorus pentasulfide, and then distill under reduced pressure, wash, centrifuge, and dry to obtain the product, with a yield of 75%. Its main reaction mechanism is that urea reacts with p-hydroxybenzoic acid to form amide, and then dehydrates intramolecularly under the conditions of dehydrating agent and high temperature reaction to form p-hydroxybenzonitrile.

[0003] Wastewater is inevitably produced in the production process of p-hydroxybenzonitrile. Since one of its main raw materials is urea, the wastewater contains a large amount of ammonia nitrogen substances. The current conventional treatment method is to use an ammonia stripping device to treat the wastewater, separate and recycle the ammonia nitrogen materials to form a by-product ammonia water, thereby obtaining certain economic benefits and reducing costs. However, the current ammonia stripping device has a large energy consumption. For every 60 tons of p-hydroxybenzonitrile wastewater treated, about 6.0 tons of liquid caustic soda and about 9.0 to 10.0 tons of steam are consumed, while only about 4.0 to 4.5 tons of 20% ammonia water can be produced as a by-product. During its production and operation, the consumption of liquid caustic soda and steam is large, the system operation energy consumption is high, and the by-product output is insufficient, resulting in low overall economic benefits. Therefore, a better method for treating p-hydroxybenzonitrile production wastewater is needed. Summary of the invention

[0004] In view of the deficiencies in the prior art, the present invention provides a method for treating p-hydroxybenzonitrile production wastewater, which solves the problems in the prior art of high raw material consumption, system operation energy consumption, insufficient by-product output, and low overall economic benefits.

[0005] According to an embodiment of the present invention, a method for treating p-hydroxybenzonitrile production wastewater comprises the following steps:

[0006] S1. Alkali adjustment: put the p-hydroxybenzonitrile wastewater into the alkali liquid pool, add 32% liquid alkali to adjust the pH value to 9-9.5, so that the raw water reaches the stripping balance and improves the removal rate;

[0007] S2, stripping: the solution after the reduction is pumped into the ammonia stripping tower through the preheater, and the tower bottom is heated with steam while evacuating, and alkali is added to increase the pH of the solution in the ammonia stripping tower, so that the ammonia nitrogen concentration in the tower bottom water is not higher than 500 mg / l, and the pH is 9-9.5, and the equilibrium removal rate is increased, so that the ammonia nitrogen in the tower bottom water is ≤500ppm, and the pH is ≥8.5;

[0008] S3, absorption: the solution blown out is pumped into the absorption kettle, and the circulating water pump and condensed water are turned on for circulating absorption. When the ammonia concentration is ≥20%, it is transferred to the neutralization kettle for standby use or pumped into the ammonia tank;

[0009] S4, neutralization: start the neutralization kettle and stir, add 30% hydrochloric acid by mass, adjust pH=6.5, after pH adjustment, insert the filter, open the vacuum of the concentration kettle, and draw the filtered solution into the concentration kettle for standby use;

[0010] S5, concentration: start the concentration kettle and stir, heat and concentrate the solution in the concentration kettle, and pass the water vapor generated by the concentration into the ammonia stripping tower to heat the ammonia stripping tower as a heat source; at the same time, observe the concentration change of the ammonium chloride solution in the concentration kettle. When the ammonium chloride solution turns white and a large amount of crystals precipitate, close the steam heating valve to stop heating, and at the same time, open the jacket cooling water to cool the concentration kettle. When the temperature drops below 40°C, stop cooling and prepare for discharging centrifugation;

[0011] S6. Centrifugation: After cooling, open the bottom valve of the concentration kettle to release the crude ammonium chloride, use a 400-mesh filter bag for centrifugal separation, and recycle the mother liquor after recovery. The solid filter cake obtained by centrifugation is the ammonium chloride crystal product.

[0012] Preferably, in step S2, the liquid level in the ammonia stripping tower is controlled at 1300 / 2200-1800 / 2200 mm, and the vacuum degree is 0.04-0.07 MPa; at the same time, the bottom temperature is 85-100°C, and the top temperature is 65-75°C; the reflux rate of the ammonia stripping tower is 150-300 L / h, and the inlet and outlet water flow rates are 2000-2500 L / h.

[0013] Preferably, in step S3, the cyclic absorption temperature is ≤40°C.

[0014] Furthermore, in step S4, the flow rate of adding hydrochloric acid is reduced at the end of the neutralization reaction to avoid excessive addition of hydrochloric acid, which makes the solution strongly acidic.

[0015] Furthermore, in step S4, the filter paper and filter cloth need to be checked before each filtration. If the filter paper is damaged, it should be replaced immediately. The filter cloth needs to be rinsed with steam after every two filtrations.

[0016] Preferably, in step S5, the reaction temperature of the concentration kettle is 110-120°C.

[0017] Furthermore, in step S6, before the centrifugation starts, the low speed switch is turned on, the material is discharged, and the centrifuge window is observed. When the material approaches the rinse pipe or the feed pipe, the discharge valve is closed, the centrifuge is started, and the centrifuge is centrifuged at medium speed for 30 minutes.

[0018] Further, in step S6, after the deliquoring is completed, the centrifuge is turned off, and the flat-plate four-legged centrifuge performs energy consumption braking until the drum stops rotating. After the flat-plate four-legged centrifuge stops rotating, the locking handle is loosened to open the centrifuge shell flap, and the filter cake is taken out together with the filter bag, and then the filter cake is poured into the material bag and the filter bag is checked to see if it is intact, and it is re-laid, the shell flap is covered and the handle is locked to start the next working cycle; when the operation is completed, the inner wall of the drum, the inner wall of the shell cylinder, and the outer wall of the drum of the flat-plate four-legged centrifuge need to be manually cleaned to prevent severe rust before the next use.

[0019] Furthermore, in step S6, after centrifugation, the filter bag and the centrifuge are cleaned and dried, and the ammonium chloride crystals obtained by centrifugation are sampled and sent for inspection, and the moisture, content and nitrogen content are measured, and the weight is recorded.

[0020] The technical principle of the present invention is: deammoniation is carried out by vacuum distillation. Under the action of sufficient alkali, the ammonia nitrogen of the extracted phenol wastewater is completely converted into free ammonia, and the wastewater is metered and pumped into the distillation tower. The wastewater is gradually dropped to the tower bottom through the stripping section. Low-pressure steam is directly heated in the tower bottom. Ammonia is separated at a certain temperature and corresponding vacuum degree. The gas ammonia concentration in the distillation section increases with the vertical height. The gas ammonia concentration reaches a peak value when the tower is discharged. The gas ammonia comes out from the top of the tower with part of the water vapor, and is cooled by the tower top condenser. The condensate (low-concentration ammonia water) formed by the water vapor is pumped to the top of the tower for total reflux. The gas ammonia is absorbed by the water jet pump of the ammonia absorption tank to become ammonia water. The ammonia water is neutralized with hydrochloric acid, concentrated, and centrifuged to make the finished ammonium chloride product. At the same time, after the ammonia water is separated, the water outlet from the tower bottom has met the qualified standard of ammonia nitrogen concentration, and can be sent to the sewage station for further treatment.

[0021] Compared with the prior art, the present invention has the following beneficial effects:

[0022] 1. The present invention introduces a hydrochloric acid neutralization device and an ammonium chloride concentration device on the basis of a traditional ammonia stripping device. The former can directly absorb the ammonia gas generated by the ammonia stripping device to generate ammonium chloride by-product, while the latter can form high-temperature water vapor in the process of concentrating the ammonium chloride solution. At the same time, the high-temperature water vapor is introduced into the ammonia stripping device as a heat source, thereby greatly reducing the additional energy consumption required for the operation of the ammonia stripping device, improving the economy and environmental protection of the system operation, realizing the effective utilization of resources and the improvement of economic benefits, and providing a useful reference for the green development of the chemical industry;

[0023] 2. The present invention only introduces cheap industrial by-product hydrochloric acid as a new raw material, and can process the original ammonia product into a new ammonium chloride product, which greatly increases the value of the by-product and improves the overall economic benefits. At the same time, the high-temperature steam generated in the process of producing ammonium chloride can also be used as a heat source for the ammonia stripping device, further reducing the cost. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 The process flow chart of an embodiment of the present invention. DETAILED DESCRIPTION

[0025] The technical solution of the present invention is further described below in conjunction with the accompanying drawings and embodiments.

[0026] Embodiment 1:

[0027] The method for treating the wastewater produced by the production of p-hydroxybenzonitrile in this embodiment is as follows:

[0028] (1) Alkali adjustment: The p-hydroxybenzonitrile wastewater after extraction enters the alkali solution pool, and 32% liquid alkali is added to adjust the pH value to 9.5, so that the raw water reaches the stripping balance and the removal rate is improved.

[0029] (2) Stripping: After adjusting the pH, the raw water is pumped into the negative pressure ammonia stripping tower through the preheater, evacuated and heated with steam. Add an appropriate amount of caustic soda to increase the pH of the raw water in the ammonia stripping tower and increase the equilibrium removal rate. The liquid level in the ammonia stripping tower is controlled at 1300 / 2200mm, and the vacuum degree is 0.04MPa; at the same time, the tower bottom temperature is 85℃, and the tower top temperature is 65℃; the ammonia stripping tower has a reflux rate of 150L / h and an inlet and outlet water flow rate of 2000L / h.

[0030] (3) Absorption: Pump 1 ton of soft water into the absorption kettle, turn on the circulating water pump and condensed water for circulating absorption. When the ammonia concentration is ≥20%, transfer it to the neutralization kettle for standby use or pump it into the ammonia tank. The circulating absorption temperature is 40℃.

[0031] (4) Neutralization: Open the neutralization kettle and stir, half-open the hydrochloric acid valve, observe the temperature change to prevent spraying, adjust pH = 6.5, use (5.5, 9.0) precision pH test paper, and after pH adjustment, inject into the filter, open the vacuum of the concentration kettle, and pump the filtered solution into the concentration kettle for standby. 2 tons of 20% concentration ammonia water are injected into the neutralization kettle; the acid can be half-opened in the early stage, and the flow rate can be appropriately reduced in the later stage; the solution cannot be adjusted to strong acid; the filter is padded with filter paper and filter cloth, and the filter paper is checked for damage every time filtering. If damaged, it is replaced immediately, and the filter cloth is rinsed clean with steam every two times; the filter liquid level should not be too high to prevent the filter plate from being crushed.

[0032] (5) Concentration: Start stirring the concentration kettle, heat and concentrate the solution in the concentration kettle, and pass the water vapor generated by the concentration into the ammonia stripping tower kettle as a heat source to heat the ammonia stripping; at the same time, observe the concentration change of the ammonium chloride solution in the concentration kettle. When the ammonium chloride solution turns white and is accompanied by the precipitation of a large number of crystals, close the steam heating valve to stop heating, and at the same time open the jacket cooling water to cool the concentration kettle. When the temperature drops below 40 degrees, stop cooling and prepare for centrifugation. The opening of the inlet and outlet steam valves, the opening of the ammonia stripping steam inlet valve, and the temperature of the concentration kettle are controlled at 110°C.

[0033] (6) Centrifugation: After cooling, open the bottom valve of the concentration kettle to release the crude ammonium chloride, and use a 400-mesh filter bag for centrifugal separation. First, turn on the low-speed switch to start discharging. Observe the centrifuge window during discharging. When the material approaches the rinse pipe or the feed pipe, close the discharge valve, turn on the medium-speed centrifuge button and start the centrifugation operation for 30 minutes. After the deliquescence is completed, press the stop button, and the flat-plate four-legged centrifuge will perform energy consumption braking until the drum stops rotating. After the flat-plate four-legged centrifuge stops rotating, loosen the locking handle to open the centrifuge shell flap, and take out the filter cake together with the filter bag. After pouring the filter cake into the material bag, check whether the filter bag is intact, and re-lay it, cover the shell flap and lock the handle to start the next working cycle. When the operation is completed, the inner wall of the drum, the inner wall of the shell cylinder, and the outer wall of the drum of the flat-plate four-legged centrifuge need to be manually cleaned to prevent severe rust before next use. The centrifugal mother liquor is recycled and reused, and ammonium chloride crystals are obtained after centrifugation. The filter bags and centrifuges are cleaned and dried after centrifuging each batch of materials. The ammonium chloride crystals centrifuged are sampled and sent for inspection to measure the moisture, content and nitrogen content, and the weight is recorded.

[0034] Embodiment 2:

[0035] The method for treating the wastewater produced by the production of p-hydroxybenzonitrile in this embodiment is as follows:

[0036] (1) Alkali adjustment: The p-hydroxybenzonitrile wastewater after extraction enters the alkali solution pool, and 32% liquid alkali is added to adjust the pH value to 9.5, so that the raw water reaches the stripping balance and the removal rate is improved.

[0037] (2) Stripping: After adjusting the pH, the raw water is pumped into the negative pressure ammonia stripping tower through the preheater, evacuated and heated with steam. Add an appropriate amount of caustic soda to increase the pH of the raw water in the ammonia stripping tower and increase the equilibrium removal rate. The liquid level in the ammonia stripping tower is controlled at 1800 / 2200mm, and the vacuum degree is 0.07MPa; at the same time, the tower bottom temperature is 100℃, and the tower top temperature is 75℃; the ammonia stripping tower has a reflux rate of 1300L / h and an inlet and outlet water flow rate of 2500L / h.

[0038] (3) Absorption: Pump 1 ton of soft water into the absorption kettle, turn on the circulating water pump and condensed water for circulating absorption. When the ammonia concentration is ≥20%, transfer it to the neutralization kettle for standby use or pump it into the ammonia tank. The circulating absorption temperature is 38°C.

[0039] (4) Neutralization: Start the neutralization kettle and stir, half-open the hydrochloric acid valve, observe the temperature change to prevent spraying, adjust the pH to 6.5, use (5.5, 9.0) precision pH test paper, and after the pH adjustment is completed, insert the filter, open the vacuum of the concentration kettle, and draw the filtered solution into the concentration kettle for standby use.

[0040] (5) Concentration: Start stirring the concentration kettle, heat and concentrate the solution in the concentration kettle, and pass the water vapor generated by concentration into the ammonia stripping tower kettle as a heat source to heat the ammonia stripping; at the same time, observe the concentration change of ammonium chloride solution in the concentration kettle. When the ammonium chloride solution turns white and is accompanied by a large amount of crystal precipitation, close the steam heating valve to stop heating, and at the same time open the jacket cooling water to cool the concentration kettle. When the temperature drops below 40 degrees, stop cooling and prepare for discharging centrifugation. The opening of the inlet and outlet steam valves, the opening of the ammonia stripping steam inlet valve, and the temperature of the concentration kettle are controlled at 120°C.

[0041] (6) Centrifugation: The centrifugation process is the same as that in Example 1 and will not be described in detail here.

[0042] Embodiment 3:

[0043] The method for treating the wastewater produced by the production of p-hydroxybenzonitrile in this embodiment is as follows:

[0044] (1) Alkali adjustment: The p-hydroxybenzonitrile wastewater after extraction enters the alkali solution pool, and 32% liquid alkali is added to adjust the pH value to 9.5, so that the raw water reaches the stripping balance and the removal rate is improved.

[0045] (2) Stripping: After adjusting the pH, the raw water is pumped into the negative pressure ammonia stripping tower through the preheater, evacuated and heated with steam. Add an appropriate amount of caustic soda to increase the pH of the raw water in the ammonia stripping tower and increase the equilibrium removal rate. The liquid level in the ammonia stripping tower is controlled at 1400 / 2200mm, and the vacuum degree is 0.05MPa; at the same time, the tower bottom temperature is 90℃, and the tower top temperature is 70℃; the ammonia stripping tower has a reflux rate of 225L / h, and an inlet and outlet water flow rate of 2250L / h.

[0046] (3) Absorption: Pump 1 ton of soft water into the absorption kettle, turn on the circulating water pump and condensed water for circulating absorption. When the ammonia concentration is ≥20%, transfer it to the neutralization kettle for standby use or pump it into the ammonia tank. The circulating absorption temperature is ≤39℃.

[0047] (4) Neutralization: Start the neutralization kettle and stir, half-open the hydrochloric acid valve, observe the temperature change to prevent spraying, adjust the pH to 6.5, use (5.5, 9.0) precision pH test paper, and after the pH adjustment is completed, insert the filter, open the vacuum of the concentration kettle, and draw the filtered solution into the concentration kettle for standby use.

[0048] (5) Concentration: Start stirring the concentration kettle, heat and concentrate the solution in the concentration kettle, and pass the water vapor generated by concentration into the ammonia stripping tower kettle as a heat source to heat the ammonia stripping; at the same time, observe the concentration change of the ammonium chloride solution in the concentration kettle. When the ammonium chloride solution turns white and is accompanied by the precipitation of a large number of crystals, close the steam heating valve to stop heating, and at the same time open the jacket cooling water to cool the concentration kettle. When the temperature drops below 40 degrees, stop cooling and prepare for discharging centrifugation. The opening of the inlet and outlet steam valves, the opening of the ammonia stripping steam inlet valve, and the temperature of the concentration kettle are controlled at 115°C.

[0049] (6) Centrifugation: The centrifugation process is the same as that in Example 1 and will not be described in detail here.

[0050] Embodiment 4:

[0051] The method for treating the wastewater produced by the production of p-hydroxybenzonitrile in this embodiment is as follows:

[0052] (1) Alkali adjustment: The p-hydroxybenzonitrile wastewater after extraction enters the alkali solution pool, and 32% liquid alkali is added to adjust the pH value to 9.5, so that the raw water reaches the stripping balance and the removal rate is improved.

[0053] (2) Stripping: After adjusting the pH, the raw water is pumped into the negative pressure ammonia stripping tower through the preheater, evacuated and heated with steam. Add an appropriate amount of caustic soda to increase the pH of the raw water in the ammonia stripping tower and increase the equilibrium removal rate. The liquid level in the ammonia stripping tower is controlled at 1600 / 2200mm, and the vacuum degree is 0.06MPa; at the same time, the tower bottom temperature is 92℃, and the tower top temperature is 73℃; the ammonia stripping tower has a reflux rate of 250L / h and an inlet and outlet water flow rate of 2400L / h.

[0054] (3) Absorption: Pour 1 ton of soft water into the absorption kettle, turn on the circulating water pump and condensed water for circulating absorption. When the ammonia concentration is ≥20%, transfer it to the neutralization kettle for standby use or pump it into the ammonia tank. The circulating absorption temperature is 36°C.

[0055] (4) Neutralization: Start the neutralization kettle and stir, half-open the hydrochloric acid valve, observe the temperature change to prevent spraying, adjust the pH to 6.5, use (5.5, 9.0) precision pH test paper, and after the pH adjustment is completed, insert the filter, open the vacuum of the concentration kettle, and draw the filtered solution into the concentration kettle for standby use.

[0056] (5) Concentration: Start stirring the concentration kettle, heat and concentrate the solution in the concentration kettle, and pass the water vapor generated by the concentration into the ammonia stripping tower kettle as a heat source to heat the ammonia stripping; at the same time, observe the concentration change of the ammonium chloride solution in the concentration kettle. When the ammonium chloride solution turns white and is accompanied by the precipitation of a large number of crystals, close the steam heating valve to stop heating, and at the same time open the jacket cooling water to cool the concentration kettle. When the temperature drops below 40 degrees, stop cooling and prepare for centrifugation. The opening of the inlet and outlet steam valves, the opening of the ammonia stripping steam inlet valve, and the temperature of the concentration kettle are controlled at 118°C.

[0057] (6) Centrifugation: The centrifugation process is the same as that in Example 1 and will not be described in detail here.

[0058] Embodiment 5:

[0059] The method for treating the wastewater produced by the production of p-hydroxybenzonitrile in this embodiment is as follows:

[0060] (1) Alkali adjustment: The p-hydroxybenzonitrile wastewater after extraction enters the alkali solution pool, and 32% liquid alkali is added to adjust the pH value to 9.5, so that the raw water reaches the stripping balance and the removal rate is improved.

[0061] (2) Stripping: After adjusting the pH, the raw water is pumped into the negative pressure ammonia stripping tower through the preheater, evacuated and heated with steam. Add an appropriate amount of caustic soda to increase the pH of the raw water in the ammonia stripping tower and increase the equilibrium removal rate. The liquid level in the ammonia stripping tower is controlled at 1700 / 2200mm, and the vacuum degree is 0.05MPa; at the same time, the tower bottom temperature is 88℃, and the tower top temperature is 69℃; the ammonia stripping tower has a reflux rate of 290L / h and an inlet and outlet water flow rate of 2350L / h.

[0062] (3) Absorption: Pour 1 ton of soft water into the absorption kettle, turn on the circulating water pump and condensed water for circulating absorption. When the ammonia concentration is ≥20%, transfer it to the neutralization kettle for standby use or pump it into the ammonia tank. The circulating absorption temperature is 39°C.

[0063] (4) Neutralization: Start the neutralization kettle and stir, half-open the hydrochloric acid valve, observe the temperature change to prevent spraying, adjust the pH to 6.5, use (5.5, 9.0) precision pH test paper, and after the pH adjustment is completed, insert the filter, open the vacuum of the concentration kettle, and draw the filtered solution into the concentration kettle for standby use.

[0064] (5) Concentration: Start stirring the concentration kettle, heat and concentrate the solution in the concentration kettle, and pass the water vapor generated by the concentration into the ammonia stripping tower kettle as a heat source to heat the ammonia stripping; at the same time, observe the concentration change of the ammonium chloride solution in the concentration kettle. When the ammonium chloride solution turns white and is accompanied by the precipitation of a large number of crystals, close the steam heating valve to stop heating, and at the same time open the jacket cooling water to cool the concentration kettle. When the temperature drops below 40 degrees, stop cooling and prepare for centrifugation. The opening of the inlet and outlet steam valves, the opening of the ammonia stripping steam inlet valve, and the temperature of the concentration kettle are controlled at 114°C.

[0065] (6) Centrifugation: The centrifugation process is the same as that in Example 1 and will not be described in detail here.

[0066] The quality of the hydroxybenzonitrile production wastewater added in Examples 1-5 is different, and the treatment results are shown in Table 1:

[0067]

[0068] Table 1

[0069] As can be seen from Table 1, the present invention substantially converts ammonia water in wastewater into high-purity ammonium chloride, and has effective economic benefits.

[0070] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solution of the present invention can be modified or replaced by equivalents without departing from the purpose and scope of the technical solution of the present invention, which should be included in the scope of the claims of the present invention.

Claims

1. A method for treating wastewater from the production of p-hydroxybenzonitrile, characterized in that: The steps include: S1. Alkali adjustment: put the p-hydroxybenzonitrile wastewater into the alkali liquid pool, add 32% liquid alkali to adjust the pH value to 9-9.5, so that the raw water reaches the stripping balance and improves the removal rate; S2, stripping: the solution after the reduction is pumped into the ammonia stripping tower through the preheater, and the tower bottom is heated with steam while evacuating, and alkali is added to increase the pH of the solution in the ammonia stripping tower, so that the ammonia nitrogen concentration in the tower bottom water is not higher than 500 mg / l, and the pH is 9-9.5, and the equilibrium removal rate is increased, so that the ammonia nitrogen in the tower bottom water is ≤500ppm, and the pH is ≥8.5; S3, absorption: the solution blown out is pumped into the absorption kettle, and the circulating water pump and condensed water are turned on for circulating absorption. When the ammonia concentration is ≥20%, it is transferred to the neutralization kettle for standby use or pumped into the ammonia tank; S4, neutralization: start the neutralization kettle and stir, add 30% hydrochloric acid by mass, adjust pH=6.5, after pH adjustment, insert the filter, open the vacuum of the concentration kettle, and draw the filtered solution into the concentration kettle for standby use; S5, concentration: start the concentration kettle and stir, heat and concentrate the solution in the concentration kettle, and pass the water vapor generated by the concentration into the ammonia stripping tower to heat the ammonia stripping tower as a heat source; at the same time, observe the concentration change of the ammonium chloride solution in the concentration kettle. When the ammonium chloride solution turns white and a large amount of crystals precipitate, close the steam heating valve to stop heating, and at the same time, open the jacket cooling water to cool the concentration kettle. When the temperature drops below 40°C, stop cooling and prepare for discharging centrifugation; S6. Centrifugation: After cooling, open the bottom valve of the concentration kettle to release the crude ammonium chloride, use a 400-mesh filter bag for centrifugal separation, and recycle the mother liquor after recovery. The solid filter cake obtained by centrifugation is the ammonium chloride crystal product.

2. A method for treating p-hydroxybenzonitrile production wastewater as claimed in claim 1, characterized in that: In step S2, the liquid level in the ammonia stripping tower is controlled at 1300 / 2200-1800 / 2200 mm, and the vacuum degree is 0.04-0.07 MPa; at the same time, the bottom temperature is 85-100°C, and the top temperature is 65-75°C; the reflux rate of the ammonia stripping tower is 150-300 L / h, and the inlet and outlet water flow rates are 2000-2500 L / h.

3. A method for treating p-hydroxybenzonitrile production wastewater as claimed in claim 1, characterized in that: In the step S3, the circulating absorption temperature is ≤40°C.

4. A method for treating p-hydroxybenzonitrile production wastewater as claimed in claim 1, characterized in that: In step S4, the flow rate of adding hydrochloric acid is reduced at the end of the neutralization reaction to avoid excessive addition of hydrochloric acid, which makes the solution strongly acidic.

5. A method for treating p-hydroxybenzonitrile production wastewater as claimed in claim 1, characterized in that: In step S4, the filter paper and filter cloth need to be checked before each filtration. If the filter paper is damaged, it should be replaced immediately. The filter cloth needs to be rinsed with steam after every two filtrations.

6. A method for treating p-hydroxybenzonitrile production wastewater as claimed in claim 1, characterized in that: In step S5, the reaction temperature of the concentration kettle is 110-120°C.

7. A method for treating p-hydroxybenzonitrile production wastewater as claimed in claim 1, characterized in that: In step S6, before the centrifugation starts, the low speed switch is turned on, the material is discharged, and the centrifuge window is observed. When the material approaches the rinse pipe or the feed pipe opening, the discharge valve is closed, the centrifuge is started, and the centrifuge is centrifuged at medium speed for 30 minutes.

8. A method for treating p-hydroxybenzonitrile production wastewater as claimed in claim 1, characterized in that: In the step S6, after the deliquoring is completed, the centrifuge is turned off, and the flat-plate four-legged centrifuge performs energy consumption braking until the drum stops rotating. After the flat-plate four-legged centrifuge stops rotating, the locking handle is loosened to open the centrifuge shell flap, and the filter cake is taken out together with the filter bag, and then the filter cake is poured into the material bag and the filter bag is checked to see if it is intact, and it is re-laid, the shell flap is covered and the handle is locked to start the next working cycle; when the operation is completed, the inner wall of the drum, the inner wall of the shell cylinder, and the outer wall of the drum of the flat-plate four-legged centrifuge need to be manually cleaned to prevent severe rust before the next use.

9. A method for treating p-hydroxybenzonitrile production wastewater as claimed in claim 1, characterized in that: In step S6, after centrifugation, the filter bag and the centrifuge are cleaned and dried, and the ammonium chloride crystals obtained by centrifugation are sampled and sent for inspection to measure the moisture, content and nitrogen content, and the weight is recorded.

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