CN108706610B

CN108706610B - Method for recovering ammonia and high-quality gypsum from ammonium sulfate

Info

Publication number: CN108706610B
Application number: CN201810612036.3A
Authority: CN
Inventors: 成有为; 潘腾; 秦伟; 王丽军; 李希
Original assignee: Zhejiang University ZJU
Current assignee: Zhejiang University ZJU
Priority date: 2018-06-14
Filing date: 2018-06-14
Publication date: 2021-06-18
Anticipated expiration: 2038-06-14
Also published as: CN108706610A

Abstract

The invention discloses a method for recovering ammonia gas and high-quality gypsum from ammonium sulfate, which comprises the following steps: firstly, mixing and reacting a lime milk solution with ammonium sulfate (or an ammonium sulfate aqueous solution) to generate ammonia and calcium sulfate; the generated ammonia can be recovered from gas phase, and the ammonia is completely recovered after steam stripping of slurry rich in calcium sulfate crystals after reaction; calcium sulfate in the slurry is filtered, dried and dehydrated to recover a high-quality gypsum product. The technology adopts multi-stage steam stripping and ammonia distillation to ensure that the ammonia in the reaction mother liquor is completely recovered; the mixing and reaction in the technical process are operated under pressurization or micro positive pressure, the temperature in the subsequent processes of steam stripping, solid-liquid separation and the like is kept higher than 95 ℃, so that the dihydrate gypsum scab can be prevented, meanwhile, the gypsum crystal is mainly low-bound dihydrate gypsum, the process energy consumption is reduced, and the quality of the gypsum product is ensured. The technology can efficiently recover the ammonia gas and the gypsum product with low bound water by the ammonium sulfate, and has low process energy consumption and good economic benefit.

Description

Method for recovering ammonia and high-quality gypsum from ammonium sulfate

Technical Field

The invention relates to a method for recovering ammonia and high-quality gypsum from ammonium sulfate, in particular to a method for recycling ammonia in a chemical process taking ammonium sulfate as a byproduct.

Background

The ammonium sulfate is a byproduct in the chemical process, and particularly, in the caprolactam production process, about 1.5-2.0 tons of ammonium sulfate can be produced per 1 ton of caprolactam product. In recent years, the yield of ammonium sulfate has been greatly increased with the spread of ammonia desulfurization technology. Ammonium sulfate is mainly used as a nitrogen fertilizer, but long-term application causes soil acidification and the market is limited. Therefore, ammonium sulfate has been supplied for a long time and is low in price. 258kg of liquid ammonia is consumed for each ton of ammonium sulfate byproduct, calculated by the average price of 2017 years (3200 yuan/ton of liquid ammonia and 450 yuan/ton of ammonium sulfate), 825 yuan of liquid ammonia is consumed for each ton of ammonium sulfate production. The byproduct of the low-added-value ammonium sulfate consumes a large amount of expensive ammonia, which is obviously unreasonable. Therefore, recovery of ammonia gas from ammonium sulfate is desirable, and once ammonia is recycled, it is expected to greatly improve the economic efficiency of the relevant process.

So far, there are few reports on attempts to recover ammonia from ammonium sulfate, and the recovery of ammonia by pyrolysis of ammonium sulfate has been studied. Chentianlang (chemical research and application, 2002,14(6):737-739) investigated the pyrolysis rule of ammonium sulfate under different atmosphere conditions by a tube furnace experiment; the pyro-gravimetric method is adopted by Cao Hai et al (Proc. Chem. Engineers of colleges and universities, 2011,25(2): 341-. The results show that the ammonia recovery rate is low in the ammonium sulfate pyrolysis process, the temperature is high, the energy consumption is high, and the feasibility is not realized.

Patent CN207002285U provides a method for preparing potassium sulfate by using industrial by-product ammonium sulfate, mainly by using ammonium persulfate and potassium chloride to make metathesis reaction by heating, and removing ammonium chloride by vaporization to obtain the potassium sulfate product. The method can only be used as a method for treating the ammonium sulfate byproduct containing organic impurities, and because the process has high energy consumption and complex equipment, the obtained potassium sulfate and ammonium chloride have low additional value, and the large-scale application has no economic benefit.

Patent CN107686193A provides a method for recovering ammonium sulfate solid from high concentration ammonium sulfate wastewater, which utilizes the characteristic that ammonium sulfate is easily soluble in water and insoluble in ethanol at normal temperature and pressure, and mixes the ammonium sulfate wastewater with absolute ethanol to separate out ammonium sulfate solid from wastewater, and the recovery rate of ammonium sulfate can reach more than 92%. The absolute ethyl alcohol in the mother liquor can be concentrated by rectification and recycled. Compared with the existing sulfuric acid evaporation crystallization technology, the method is more energy-saving and has lower operation cost.

Ammonia recovery using strongly basic materials such as lime, NaOH or KOH to react with ammonium sulfate to produce the corresponding sulfate and ammonia is also obvious, but the use of expensive NaOH or KOH is not economically feasible. The only strong alkaline substances that can be used are inexpensive lime water, which can undergo a metathesis reaction with ammonium sulfate to produce calcium sulfate and ammonia:

(NH₄)₂SO₄+Ca(OH)₂→H₂O+2NH₃↑+CaSO₄↓ (1)

patent CN107935016A discloses a method for preparing alpha-hemihydrate gypsum from waste water containing ammonium sulfate: mixing the ammonium sulfate-containing wastewater with calcium hydroxide to perform double decomposition reaction to obtain a calcium sulfate crude product; mixing the calcium sulfate crude product with water, and adjusting the pH value to 6-8 by using sulfuric acid to obtain a dihydrate gypsum solution; the dihydrate gypsum solution and the composite crystal transformation agent are mixed to carry out phase change reaction, and the alpha-type hemihydrate gypsum is obtained. The reaction of aqueous ammonium sulfate with calcium hydroxide to form calcium sulfate and ammonia is obvious and the objective of this patent is to obtain an alpha hemihydrate product. In order to obtain the alpha-hemihydrate gypsum, a plurality of auxiliary chemical reagents are required to be added, including sulfuric acid for adjusting the pH value, aluminum potassium sulfate, sodium citrate and other crystal modifiers, and the like, so that the process is complex, the cost is high, and the industrial implementation is difficult. In particular, the patent does not recover the ammonia produced.

Disclosure of Invention

The invention provides a method for recovering ammonia and high-quality gypsum by using ammonium sulfate, aiming at solving the problem of the outlet of a large amount of byproduct ammonium sulfate in the industrial processes of caprolactam and the like. By adopting the technology, the ammonia which is fixed in the ammonium sulfate byproduct in a low-value mode can be recovered and recycled, the circulation of the ammonia is realized, the ammonia consumption in the process is reduced, and the economic benefit is improved.

The typical process of the method for recovering ammonia and high-quality gypsum from ammonium sulfate can refer to FIG. 6, and specifically comprises four steps of mixed reaction crystallization, stripping deamination, ammonia recovery and gypsum recovery:

1) firstly, feeding lime milk solution and ammonium sulfate into a reaction crystallizer for fully mixing and reacting to generate gas phase rich in ammonia and water slurry rich in calcium sulfate precipitate, carrying out steam stripping deamination on the slurry, and feeding the gas phase containing ammonia into an ammonia recovery step;

2) then, carrying out steam stripping deamination treatment on the water slurry rich in calcium sulfate precipitates generated by the reaction, sending ammonia-containing steam obtained by stripping to an ammonia recovery step, and sending the slurry subjected to deamination to a gypsum recovery step;

3) high-concentration ammonia water and ammonia gas are recovered from ammonia-containing steam generated by stripping deamination and ammonia-containing gas phase generated by reaction by rectification and condensation methods;

4) finally, high-quality gypsum products are recovered from the calcium sulfate-containing precipitate slurry after the stripping deamination, and the typical solid-liquid separation process comprises filtering, washing, drying and the like.

In the method for recovering ammonia and high-quality gypsum from ammonium sulfate, the ammonium sulfate raw material added into the reaction crystallizer can be selected from ammonium sulfate solid or ammonium sulfate aqueous solution. Because of the transportation requirement, especially to meet the requirement of fully mixing and reacting with the lime milk, the ammonium sulfate aqueous solution is preferably used as the raw material, and more preferably the saturated ammonium sulfate aqueous solution with the mass percent of more than 30wt percent is more preferred. In particular in caprolactam production plants, the ammonium sulphate content of the ammonium sulphate solution stream from the ammonium sulphate unit is about 40 to 60 wt.%, which stream can be used directly as the ammonium sulphate feed for the present technology.

In the method for recovering ammonia and high-quality gypsum from ammonium sulfate, lime milk solution from a lime milk preparation unit is pumped into a reaction crystallizer by a pump, and is fully mixed, reacted and crystallized with the ammonium sulfate solution, and the reaction crystallizer adopts a kettle type reactor with an intensified mixing device. In the reactor, ammonium ions in the dissolved ammonium sulfate are combined with hydroxide ions to generate free ammonia, and part of the free ammonia enters a gas phase in the form of ammonia gas; meanwhile, sulfate ions in the ammonium sulfate can be combined with calcium ions to generate calcium sulfate precipitates, and the precipitated calcium sulfate can be combined with water molecules in different proportions under different reaction temperature and pressure conditions. The main reactions involved are shown in formulas (1) to (4):

in the method for recovering ammonia and high-quality gypsum from ammonium sulfate, the kettle-type reactor is provided with a stirring device or a liquid-phase injection feeding device and the like, the mixing is strengthened by liquid-phase stirring, and simultaneously, the solid deposition and scabbing of the sulfuric acid are prevented. The reaction crystallizer is operated under the condition of pressurization, and the preferable temperature range is 90-150 ℃, so that the precipitated calcium sulfate crystals are mainly semi-hydrated gypsum or anhydrous gypsum or a mixture of the semi-hydrated gypsum and the anhydrous gypsum.

In the method for recovering ammonia and high-quality gypsum from ammonium sulfate, slurry from the bottom of a reaction crystallizer is rich in calcium sulfate precipitate particles, ammonia is also saturated and dissolved, and the dissolved ammonia needs to be removed by steam stripping. Steam stripping deamination is carried out in a multi-stage stripping tower, water slurry rich in calcium sulfate precipitate particles is added from the upper part of the tower, and steam is introduced from the bottom of the tower. The slurry and the steam are subjected to multistage countercurrent contact to remove dissolved ammonia, ammonia-containing steam collected from the tower top is sent to an ammonia recovery step to further recover high-concentration ammonia water and ammonia gas, and water slurry after complete deamination is sent to a subsequent gypsum recovery step from the tower bottom. In order to prevent calcium sulfate precipitation particle scabbing, the stripping tower needs to be operated at a temperature of more than 95 ℃, and the temperature control range of the bottom of the stripping tower is 100-170 ℃, preferably 120-150 ℃.

In the method for recovering ammonia and high-quality gypsum from ammonium sulfate, the theoretical gas-liquid equilibrium stage number of the multistage steam stripping deamination tower can be 2-20, and preferably 3-7 theoretical equilibrium stages. The tower plate member of the stripping tower needs to adopt an anti-scab design, and an optional anti-scab tower plate structure is a 'cone cap' type tower plate (shown in figure 5).

In the method for recovering ammonia and high-quality gypsum from ammonium sulfate, the recovered gas-phase ammonia is concentrated by adopting a multi-stage rectifying tower, and the rectifying tower can adopt a packed tower or a plate tower. The ammonia-containing steam from the top of the stripping deamination tower is added from the bottom of the rectifying tower, and the ammonia-containing gas phase from the reactive crystallizer is also added into the rectifying tower from a proper position of the tower; the low-concentration ammonia water obtained at the bottom of the tower can be added into a reactor to be used as dilution water and can also be used as reflux water of a stripping tower; condensing the gas phase at the top of the crystallization tower by using a condenser, and refluxing a condensate part; high-purity ammonia gas is recovered from the non-condensable gas, and high-concentration ammonia water is recovered from the condensate. The concentration of the recovered ammonia water can be adjusted by the reflux ratio of the condensate and the load of the condenser. The condensation temperature is low, the reflux ratio is large, ammonia water with high concentration can be obtained, the energy consumption of a system can be increased, and the reflux ratio of the condensate is selected from 0.1-3, preferably 0.5-1.5.

In the method for recovering ammonia and high-quality gypsum from ammonium sulfate, the steam stripping deamination tower and the ammonia rectifying tower can be integrally designed, namely two functions are realized in one tower device, and the integrated design can simplify the flow, reduce the number of devices and save land. The typical integration scheme is that an ammonia rectifying tower is arranged above a stripping deamination tower, namely slurry stripping is completed at the lower section of the tower, ammonia rectification is completed at the upper section of the tower, ammonia-containing steam at the stripping section directly enters the ammonia rectification for ammonia water rectification, a condenser can be arranged in the tower top, condensate directly flows back, and recovered ammonia gas and ammonia water are extracted from the tower top. A typical tower architecture for a unified integrated design is shown in fig. 5.

In the method for recovering ammonia and high-quality gypsum from ammonium sulfate, the slurry rich in calcium sulfate precipitate is subjected to stripping deamination, and then a high-quality gypsum product is obtained through a multi-stage solid-liquid separation process. The multi-stage solid-liquid separation process is carried out at the temperature higher than 100 ℃, so that the finally obtained gypsum product is mainly high-quality anhydrous gypsum, semi-hydrated gypsum or a mixture of the anhydrous gypsum and the semi-hydrated gypsum. The adopted multi-stage solid-liquid separation sequence can comprise the processes of concentration, filtration, washing and drying of a cyclone. Referring to fig. 6, after the slurry is concentrated by the cyclone, the clear liquid can be recycled to the reactor; filtering and washing the concentrated slurry, and drying a filter cake to obtain a gypsum product; part of the filtrate and the washing liquid are added into a lime emulsifying tank as make-up water, and part of the filtrate and the washing liquid can be discharged outside the medium area as wastewater for further treatment.

Compared with the prior art, the technology of the invention has the following obvious technical advantages:

1) the invention can recover two products of ammonia and high-quality gypsum from the ammonium sulfate byproduct;

2) the ammonia product obtained by the invention is high-concentration ammonia gas and ammonia.

3) The gypsum product obtained by the invention is low-bound water semi-hydrated gypsum or anhydrous gypsum, the generation of dihydrate gypsum is avoided in the intermediate process, the process is simple, and the energy consumption is low.

Drawings

FIG. 1 is a graph of the change in pH and ion concentration of a solution over time under typical conditions for a reactive crystallization process;

FIG. 2 is a comparison of XRD analysis results of gypsum crystalline products under different conditions, wherein A represents example 1; b-example 2; c-example 3; d-example 4; e-example 5;

FIG. 3 comparison of the results of particle size analysis of gypsum crystalline product under different conditions, A-example 1; b-example 2; c-example 3; d-example 4; e-example 5;

FIG. 4 is a comparison of gypsum crystal patterns under different conditions, wherein A-example 2 (calcium sulfate hemihydrate); b-example 4 (calcium sulfate dihydrate);

FIG. 5 is a schematic diagram of a structure of a stripping and rectifying integrated tower device;

FIG. 6 is a schematic process flow diagram for recovering ammonia and high quality gypsum from ammonium sulfate.

Detailed Description

Example 1

The crystallization rule of the mixing reaction of ammonium sulfate and lime milk is experimentally observed in a 2L stirring kettle. First, a stirred tank was charged with 900ml of 1.1mol/L Ca (OH)₂Mechanically stirring the solution, introducing heat-conducting oil into an outer jacket of the reactor to heat and control the temperature, wherein the temperature is controlled at 100 ℃; then 450ml of (NH) with a concentration of 2.0mol/L₄)₂SO₄The solution (21.2 wt%) was poured quickly into the reactor at one time, the lime milk was kept in excess, and mixed well by mechanical stirring. And after the reaction starts, sampling at regular time to determine the pH value, the calcium ion concentration and the sulfate ion concentration of the solution, and simultaneously, sampling and analyzing the crystal form and the particle size of the generated gypsum crystal. And blowing 50ml/min of nitrogen into the liquid phase in the reaction process, absorbing the generated gas phase by a dilute hydrochloric acid solution in a subsequent absorption bottle, and detecting the concentration of ammonium ions in the absorption bottle by timing sampling to determine the recovery rate of ammonia. Measuring the pH value by a pH meter; measuring the concentration of calcium ions by adopting an EDTA (ethylene diamine tetraacetic acid) complexation titration method, and measuring the concentration of sulfate ions by adopting a barium sulfate turbidimetry method; measuring absorption liquid by adopting nano reagent spectrophotometryThe ammonium ion concentration of (a); and (3) determining the particle size of gypsum crystal particles by using a Malvern laser particle size analyzer, and determining an XRD (X-ray diffraction) pattern by using an X-ray diffractometer to judge the type of gypsum crystals.

The pH and ion concentration of the solution of this example were plotted against time as shown in FIG. 1, and it can be seen that the sulfate ion concentration, calcium ion concentration and pH in the liquid phase did not change after 3min of mixing, which means that the reaction of ammonium sulfate and calcium hydroxide was a rapid equilibrium reaction in a short time (b)<3min) the liquid phase reaction had reached equilibrium. After the reaction time is 60min, the concentration of ammonium ions in the tail gas absorption bottle is analyzed, and the ammonia recovery rate is calculated to be 99.1 wt% by taking the added ammonium sulfate as a reference, which indicates that the amino can be completely recovered. The XRD pattern of calcium sulfate obtained after 60min of reaction is shown in FIG. 2 (A). As can be seen from FIG. 2(A), the obtained crystal form of calcium sulfate is hemihydrate gypsum (CaSO)₄·0.5H₂O). The particle size distribution is shown in FIG. 3(A), and the average particle size is 27.7 μm, while there is a large amount of fine particle size distribution of small particle size.

Example 2

The crystallization law of the mixing reaction of ammonium sulfate and lime milk was examined in a 2L stirred tank as in example 1, considering the reaction temperature also as 100 ℃, except that the lime milk was added to the ammonium sulfate solution at once under stirring. 450ml of (NH) with the concentration of 2.0mol/L is filled into a 2L stirring kettle₄)₂SO₄Solution (21.2 wt%) 900ml of Ca (OH) at a concentration of 1.1mol/L₂The solution was poured rapidly into the reactor at once, the lime milk was kept in excess, mechanical stirring was added to keep mixing well, and at the same time 50ml/min nitrogen was bubbled into the liquid phase. After the reaction time of 60min, a sample was taken to analyze the ammonium ion concentration in the tail gas absorption bottle, and the ammonia recovery rate was 98.5 wt% calculated based on the added ammonium sulfate, indicating that the ammonia was substantially completely recovered. And (4) sampling and analyzing the solid, determining the particle size by using a Malvern particle size analyzer, and determining an XRD (X-ray diffraction) pattern by using an X-ray diffractometer. The XRD pattern of calcium sulfate obtained after 60min of reaction is shown in FIG. 2 (B). As can be seen from FIG. 2(B), the obtained crystal form of calcium sulfate is hemihydrate gypsum (CaSO)₄·0.5H₂O). The particle size distribution is shown in FIG. 3(B), and the particles are stored simultaneouslyThe average grain size is 32.5 mu m when the grains are distributed in a large number of small-grain-size fine grains. The photomicrograph shows that, as shown in FIG. 4(A), the calcium sulfate hemihydrate clusters are aggregated and formed into short rod-shaped crystals.

Example 3

The crystallization law of the mixing reaction of ammonium sulfate and lime milk was experimentally examined in a 2L stirred tank as in example 1, with the difference that the reaction temperature and the feeding mode were adopted, the reaction temperature was 120 ℃, and the ammonium sulfate solution was continuously added into the lime milk by a pump under the stirring condition. A2-liter stirred tank was initially charged with 900ml of 1.1mol/L Ca (OH)₂The solution was then mixed with 450ml of 2.0mol/L (NH)₄)₂SO₄The solution (21.2 wt%) was continuously fed into the reactor at a flow rate of 50ml/min, the ammonium sulfate solution was added over 9 minutes, and mechanical stirring was added to keep the mixture well mixed while bubbling 50ml/min of nitrogen into the liquid phase. The heat conducting oil is introduced into the jacket of the reactor to heat and preserve heat, and the temperature is controlled at 120 ℃. After the reaction time of 60min, a sample was taken to analyze the ammonium ion concentration in the tail gas absorption bottle, and the ammonia recovery rate was 98.8 wt% calculated based on the added ammonium sulfate, indicating that the ammonia was substantially completely recovered. And (4) sampling and analyzing the solid, determining the particle size by using a Malvern particle size analyzer, and determining an XRD (X-ray diffraction) pattern by using an X-ray diffractometer. The XRD pattern of calcium sulfate obtained after 60min of reaction is shown in FIG. 2 (C). As can be seen from fig. 2(C), the obtained crystal form of calcium sulfate is calcium sulfate hemihydrate (CaSO)₄·0.5H₂O). The particle size distribution is shown in FIG. 3C, and the average particle size is 28.8. mu.m, and the small particle size is not much fine crystal. Comparing fig. 3(C), 3(B) and 3(a), it can be seen that under the conditions of example 3, the formation of fine crystals of the hemihydrate gypsum particles can be effectively reduced, which is beneficial to the subsequent filtering and washing processes.

Example 4

The crystallization law of the mixed reaction of ammonium sulfate and lime milk was examined experimentally in a 2L stirred tank as in example 1, except that a reaction crystallization temperature of 90 ℃ was used. First, a stirred tank was charged with 900ml of 1.1mol/L Ca (OH)₂Adding 450ml of 2.0mol/L ammonium sulfate solution (21.2 wt%) into the lime milk at one time under stirring, adding mechanical stirring to keep mixing uniformly, and simultaneously bubbling 50ml/min nitrogen into the liquid phaseAnd (4) qi. The heat conducting oil is introduced into the jacket of the reactor to heat and preserve heat, and the temperature is controlled at 90 ℃. After the reaction time of 60min, sampling and analyzing the ammonium ion concentration in the tail gas absorption bottle, and calculating by taking the added ammonium sulfate as a reference to obtain the ammonia recovery rate of 100.5 wt%, which indicates that the ammonia is completely recovered. And (4) sampling and analyzing the solid, determining the particle size by using a Malvern particle size analyzer, and determining an XRD (X-ray diffraction) pattern by using an X-ray diffractometer. The XRD pattern of calcium sulfate obtained after 60min of reaction is shown in FIG. 2 (D). As can be seen from FIG. 2(D), the obtained crystal form of calcium sulfate is dihydrate Gypsum (CaSO)₄·2H₂O). The particle size distribution was as shown in FIG. 3(D), and the average particle size was 29.1. mu.m. The micrographs are shown in FIG. 4(B), and the obtained calcium sulfate product, crystalline dihydrate gypsum, is a long and thin rod-like crystal.

Example 5

The crystallization law of the mixing reaction of ammonium sulfate and lime milk was examined experimentally in a 2L stirred tank as in example 4, except that different feeding methods were used. First, a stirred tank was charged with 900ml of 1.1mol/L Ca (OH)₂450ml of 2.0mol/L ammonium sulfate solution (21.2 wt%) is continuously added into the lime milk by a pump under the condition of stirring, the ammonium sulfate solution is added for 9 minutes, mechanical stirring is added to keep uniform mixing, and meanwhile 50ml/min nitrogen is blown into the liquid phase. The heat conducting oil is introduced into the jacket of the reactor to heat and preserve heat, and the temperature is controlled at 90 ℃. After the reaction time of 60min, sampling and analyzing the ammonium ion concentration in the tail gas absorption bottle, and calculating by taking the added ammonium sulfate as a reference to obtain that the ammonia recovery rate is 99.7 wt%, which indicates that the ammonia is basically and completely recovered. And (4) sampling and analyzing the solid, determining the particle size by using a Malvern particle size analyzer, and determining an XRD (X-ray diffraction) pattern by using an X-ray diffractometer. The XRD pattern of calcium sulfate obtained after 60min of reaction is shown in FIG. 2 (E). As can be seen from FIG. 2(E), the obtained crystal form of calcium sulfate is dihydrate Gypsum (CaSO)₄·2H₂O). The particle size distribution is shown in FIG. 3(E), and the average particle size is 32.2. mu.m.

It is understood from the comprehensive comparison of examples 1 to 5 that the ammonia in the ammonium sulfate can be completely converted and recovered under the condition that the lime milk is slightly excessive, and the recovery rate of the ammonia is close to 100%. The continuous addition of the aqueous ammonium sulfate solution is helpful to obtain calcium sulfate crystals with uniform particle size distribution. At a temperature higher than 100 ℃, the calcium sulfate crystals generated are mainly hemihydrate gypsum containing 0.5 crystal water, and at a temperature lower than 100 ℃, the calcium sulfate crystals obtained are mainly ammonium dihydrate gypsum containing 2 crystal water. Converting dihydrate gypsum into hemihydrate requires more energy to be consumed in subsequent processing. Therefore, in order to ensure the stability of the hemihydrate gypsum product, the reaction crystallization and the subsequent solid-liquid separation are carried out at a temperature of more than 100 ℃.

Example 6

By adopting the technology, a set of pilot plant with the annual treatment capacity of 1 ten thousand tons of ammonium sulfate is built on the site of a certain caprolactam industrial device, 1280 tons of ammonia is recycled annually, and 1.1 ten thousand tons of calcium sulfate hemihydrate is byproduct. The device main equipment includes: lime milk tank, reactor, ammonia recovery tower, condenser, swirler, filter and dryer, etc. the main process flow diagram is shown in FIG. 6.

Mixing and digesting quicklime and water in a lime milk tank to prepare lime milk, wherein the CaO content in the lime milk is 11.2 wt%, the lime milk treatment capacity is 5.0 ton/hr, the lime milk tank is a container with stirring, and the volume is 10m³. A saturated aqueous sulfuric acid solution (45.0 wt%) from an ammonium sulfate unit of a caprolactam device is fed into a reactor, and is stirred and mixed with lime milk (5.6 wt%) from a lime milk tank in the reactor for reaction, wherein the material flow rate of the added aqueous ammonium sulfate solution is 2.78 tons/hr, and the added lime milk amount is 5.0 tons/hr. The volume of the reactor with a multilayer stirring device is 20m³And the reaction crystallization temperature is 100-110 ℃, the micro-positive pressure operation is carried out, a gas phase washing and collecting device is arranged, gas generated by the reaction is sent to the rectifying section of the ammonia recovery tower from a top outlet to be used as gas phase feeding, and generated liquid phase slurry is sent to the ammonia recovery tower from the bottom of the reactor to be used as upper feeding of the stripping section. The ammonia recovery tower consists of an upper distillation section and a lower stripping section, wherein the diameter of the upper distillation section is 0.60m, the height of the upper distillation section is 3m, and regular packing is filled in the upper distillation section; the diameter of the lower stripping section is 0.80m, the height is 5m, and 8 mushroom cap-shaped tower plates are distributed. Feeding the ammonia-rich phase generated in the reaction crystallizer from the middle part of the distillation section at the upper part of the tower, matching a condenser at the top of the tower, refluxing condensate from the top of the distillation section, and realizing countercurrent contact between refluxed ammonia water and steam containing ammonia through the distillation section to realize the effectThe ammonia gas and aqueous ammonia were concentrated, and the ammonia gas and aqueous ammonia recovered from the column top in total of 246kg/hr (ammonia content: about 65.0 wt.%) were sent to a neutralization unit for caprolactam to be used as neutralized ammonia. The water slurry rich in calcium sulfate precipitate generated from the reactor is fed from the uppermost tray of the stripping section at the lower part of the ammonia recovery tower, and the ammonia-containing steam recovered from the top of the stripping section directly enters the distillation section at the upper part for recovering ammonia. Low-pressure steam (1.6bar) is introduced into the lower part of a stripping section at the bottom of the ammonia recovery tower for stripping deamination, the steam amount of the introduced stripping 1.6bar is 100-200 kg/hr, the temperature at the bottom of the tower is kept higher than 110 ℃, and the ammonia nitrogen content in residual liquid at the bottom of the tower is less than 20 ppm. The hot water slurry rich in calcium sulfate precipitate recovered from the bottom of the stripping section at the bottom of the ammonia recovery tower is pumped into a cyclone by a slurry pump, the concentrated phase of the calcium sulfate precipitate obtained by the cyclone is sent to the subsequent filtering step, and the water phase is returned to the reactor to be used as dilution water. Calcium sulfate hemihydrate (high quality gypsum) products are obtained after filtration, washing and drying. Ensuring the operation temperature of the processes of filtering, washing and the like to be higher than 95 ℃ and preventing scab and the generation of calcium sulfate dihydrate.

Claims

1. A method for recovering ammonia and high-quality gypsum from ammonium sulfate is characterized by comprising the following steps:

(1) feeding the lime milk solution and ammonium sulfate into a reaction crystallizer for full mixing and reaction to generate a gas phase rich in ammonia and a water slurry rich in calcium sulfate crystals;

the operating temperature range of the reaction crystallizer is 100-150 ℃;

(2) carrying out steam multistage stripping deamination on the water slurry rich in calcium sulfate crystals to obtain ammonia-containing steam and gypsum slurry;

in the step (2), the steam multi-stage steam stripping is carried out in a multi-stage stripping tower, water slurry is added from the upper part of the tower, steam is introduced into the bottom of the tower, and the slurry and the steam are subjected to multi-stage countercurrent contact to remove dissolved ammonia; in the stripping tower, the temperature range of the tower bottom is 100-160 ℃, and a tower plate member of the stripping tower adopts an anti-scarring design;

(3) rectifying the gas phase rich in ammonia obtained in the step (1) and the ammonia-containing steam obtained in the step (2) to recover high-concentration ammonia water and/or ammonia gas;

(4) recycling a high-quality gypsum product from the gypsum slurry obtained in the step (2) through a multi-stage solid-liquid separation process;

in the step (4), the multistage solid-liquid separation process of the gypsum slurry is carried out at 100-160 ℃, and the multistage solid-liquid separation process comprises the processes of concentration, filtration, washing and drying.

2. The method of claim 1, wherein the ammonium sulfate is selected from the group consisting of ammonium sulfate solid and aqueous ammonium sulfate solution.

3. The method as claimed in claim 2, wherein the ammonium sulfate is more than 20wt% ammonium sulfate aqueous solution.

4. The process of claim 3 wherein the ammonium sulfate is selected from streams that are byproducts of the caprolactam production process.

5. The method of claim 1, wherein in step (1), the reactive crystallizer is a tank reactor equipped with an intensive mixing means selected from stirring and liquid phase jet feeding.

6. The method of claim 1, wherein in step (3), the means for rectifying is selected from the group consisting of a packed column or a tray column; the ammonia-containing steam is added from the bottom of the rectifying tower, the tower top is condensed and refluxed, and high-concentration ammonia water and ammonia gas are recovered from the tower top.

7. The method according to claim 6, characterized in that the multistage stripping and ammonia rectification are integrated, the ammonia rectification column is arranged above the stripping column in an integrated column device, and the ammonia-containing steam of the stripping column directly enters the ammonia rectification column.