RU2798466C1 - Method for producing para-aminophenol and zinc acetate by reduction of para-nitrosophenol with metallic zinc in acetic acid medium - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: new method for the production of para-aminophenol and zinc acetate by reducing para-nitrosophenol with metallic zinc in an acetic acid medium. This method includes the following steps: a) preparing a solution of para-nitrosophenol in acetic acid; b) feeding the solution from step a) into the reactor with an inert atmosphere with stirring and forced maintenance of the temperature in the range from 40 to 55°C; c) dosing zinc metal powder into the reactor at a controlled feed rate; d) stirring the reaction mixture until the reaction is complete while maintaining the temperature in the range from 40 to 55°C; e) cooling the reaction mixture to 3-7°C for complete isolation of finely crystalline zinc acetate from the reaction solution; f) withdrawing the reaction mixture from the reactor into a nutsche filter; g) separating solid zinc acetate as retentate and a solution of para-aminophenol in acetic acid as permeate; h) washing the zinc acetate on the filter with glacial acetic acid; i) withdrawing the solution of para-aminophenol in acetic acid from the nutsche filter and obtaining the final product of para-aminophenol either individually or as a solution; j) drying the solid zinc acetate and discharging it from the nutsche filter to obtain the final zinc acetate product.

EFFECT: cheap and readily available reagents, as well as a decrease in the amount of waste for disposal, and allows, without the use of special equipment, to obtain and isolate several economically significant products that are hardwarily and technologically simple, providing high purity and modern yield.

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Description

The present invention relates to the field of chemical technology, in particular to a new method for the production of para-aminophenol and zinc acetate by reducing para-nitrosophenol with zinc metal in an acetic acid medium.

Para-aminophenol is a valuable product of organic synthesis. This compound serves as a raw material for the production of drugs (paracetamol), dyes, and is used as an antioxidant in the production of rubber. Therefore, there is still considerable interest in the research and production areas in the development and implementation of new effective methods for the synthesis of para-aminophenol that meet the modern realities of constantly rising raw material prices and tightening requirements for ensuring environmental friendliness of production.

At present, there are several approaches to the industrial production of para-aminophenol in the prior art, where the main method of synthesis is the reduction of para-nitro- or para-nitrosophenol.

Several approaches to the implementation of this chemical transformation have been described.

Catalytic hydrogenation with hydrogen has become widespread (RU2461543, CN104356007A, CN104447362A, WO2021219647A, CN112279781A, SU536167A1, Green Chem., 2019, 21, 6381-6389). Hydrogen reduction of para-nitro or para-nitrosophenol results in a high quality product and is relatively easy, but requires expensive hydrogenation catalysts. It is proposed to use deposited palladium on carbon, platinum or activated metallic nickel. Such catalysts withstand a limited number of synthetic cycles, after which they need to be replaced. The regeneration of such catalysts is extremely difficult. Raney nickel is pyrophoric, given that the reduction is proposed to be carried out in an environment of organic solvents (alcohols, ethyl acetate), then increased safety requirements are imposed on the work. For the processes described, gaseous hydrogen is used under pressure up to 20-30 atm, which also requires special precautions and special hardware design.

The electrolytic reduction of para-nitrosophenol in a weak solution of sulfuric acid (DE2256003A1) is described. As a result, a high quality product can be obtained at a relatively low cost. However, this method requires complex hardware design, and the selection of optimal technological parameters (current density, reagent concentrations) during the reaction is difficult. The result is strongly influenced by the choice of material for the electrodes.

As reagents for the "sulfide" method, sodium sulfides and polysulfides, hydrogen sulfide, sulfur dioxide, dithionite, sodium bisulfite or metabisulfite (CN103772217A, CN104356007A, US377256, US3223727) can be used. The use of sulfides has a number of advantages: the cheapness of reagents and the technically uncomplicated isolation of the product. However, due to the low chemical activity of the reducing agent, a significant excess of the reducing agent is required to carry out this transformation. As a result, a large amount of liquid waste is generated, which must be disposed of. Also, the reaction is accompanied by the release of a noticeable amount of toxic hydrogen sulfide, which requires additional devices to capture and neutralize it, and strict control over the state of the environment.

Reduction with metals in an acidic medium is a well-established laboratory method for the reduction of aromatic nitro and nitroso compounds. The industrial application of this technique is also described (CN1562959A, CN105218406A, CS203892B1). The metals commonly used are carbonyl iron and zinc powder.

When carrying out the reduction reaction of para-nitrophenol with iron (CN1562959A) or zinc (CN105218406A) in hydrochloric acid, the target para-aminophenol is formed as a hydrochloride. If it is necessary to isolate the product in the form of a free base, an additional neutralization step is needed. This requires the use of additional reagents and creates excess waste.

When using iron in acetic acid to reduce nitroaromatic compounds, the reaction proceeds with great heating, thickens strongly, and an amorphous mass of iron salts is formed as a waste, which makes it extremely difficult to completely isolate the product (WO2004014382A1). These factors make it problematic to scale up such a process.

Para-nitrosophenol as a starting compound for the production of para-aminophenol is used much less frequently, since it is not a sufficiently stable substance. Patent CS203892B1 proposes to reduce para-nitrosophenol with iron in hydrochloric acid, however, in this case, iron compounds that are difficult to separate at this stage are also formed, and therefore para-aminophenol is not isolated individually, and the reaction mixture is subjected to further transformations.

Thus, the prior art does not describe the use of metallic zinc in an acetic acid medium for the reduction of nitroso compounds. Although using this technique, a useful by-product, zinc acetate, is formed.

Zinc acetate dihydrate is a valuable substance used as a food additive as a flavor and aroma enhancer (E650), as a source of zinc, an essential trace element for the body, and is also a component in the production of cosmetics. In addition, zinc acetate is used as a starting compound for obtaining nano-structured zinc oxide and obtaining other zinc salts.

The main method for obtaining dihydrate zinc acetate is the dissolution of zinc oxide or hydroxide in acetic acid in the presence of water (RU2483056C2, CN1059196C, CN1696099A, TWI243814B). For the technical implementation of this process, specialized equipment is described (CN212041433U, CN209714555U).

Thus, at present, there is still a significant need to develop new methods for obtaining a para-aminophenol product valuable in chemical technology, combined with the production of other valuable products, characterized by a reduced amount of recyclable waste, an energy-efficient process, and also eliminating or at least minimizing the above disadvantages of the prior art and providing the person skilled in the art with a wider choice of means and technologies for achieving the necessary characteristics of para-aminophenol, in particular for use in further synthetic chains.

Therefore, the objective of the present invention was to provide a new method for the combined production of para-aminophenol and zinc acetate, characterized by ease of implementation from the hardware and technological points of view, the availability and low cost of the starting reagents and low energy consumption, as well as more favorable economic profitability due to the simultaneous production several valuable products, while providing reproducible end products with modern yields and high purity without additional purification steps.

This problem was solved by a new method for the combined production of para-aminophenol and zinc acetate by reducing para-nitrosophenol with metallic zinc in an acetic acid medium, which includes the following stages:

a) preparation of a solution of para-nitrosophenol in acetic acid;

b) feeding the solution from step a) into the reactor with an inert atmosphere with stirring and forced maintenance of the temperature in the range from 40 to 55°C;

c) dosing zinc metal powder into the reactor at a controlled feed rate;

d) stirring the reaction mixture until the reaction is complete while maintaining the temperature in the range from 40 to 55°C;

e) cooling the reaction mixture to 3 - 7°C for complete isolation of fine crystalline zinc acetate from the reaction solution;

f) withdrawing the reaction mixture from the reactor into a suction filter;

g) separating solid zinc acetate as retentate and a solution of para-aminophenol in acetic acid as permeate;

h) washing the zinc acetate on the filter with glacial acetic acid;

i) withdrawing the solution of para-aminophenol in acetic acid from the suction filter and obtaining the final product of para-aminophenol, either individually or as a solution;

j) drying the solid zinc acetate and discharging it from the suction filter to obtain the finished product zinc acetate dihydrate

As previously shown in the review of the prior art, several approaches are known for the industrial production of para-aminophenol, with para-nitrophenol being the main precursor for its production. The use of para-nitrosophenol has a very limited distribution.

This is due to the fact that para-nitrophenol as a starting material for the reduction reaction to para-aminophenol looks more attractive due to its greater chemical stability. However, para-nitrophenol is a less accessible and more expensive raw material for production, since it cannot be obtained in high yield directly from phenol by the nitration reaction due to the low regioselectivity of this reaction.

para-Nitrosophenol, in turn, can be obtained directly from phenol in high yield and selectivity, but it has a lower chemical stability, which imposes special requirements for the implementation of the process of its reduction: sufficiently mild process conditions must be provided to exclude the formation of by-products and, as a result, guarantee the required purity of the final product. Therefore, when such a reduction method is discovered, the general synthetic approach based on the nitrosation of phenol and the reduction of para-nitrosophenol to para-aminophenol becomes significantly more profitable due to the almost doubling of the total yield of the final product.

The inventors have succeeded in finding a method for the reduction of para-nitrosophenol that meets the necessary requirements.

The proposed method according to the invention is based on the reduction reaction of para-nitrosophenol with hydrogen released during the interaction of metallic zinc with acetic acid, accompanied by the formation of zinc acetate.

This method, according to the invention, allows you to realize the following advantages:

• the use of metallic zinc allows for a controlled exothermic reaction without the risk of dangerous overheating; during the reaction, heating of the chemical apparatus is only necessary at the initial stage, thus increasing the overall energy efficiency of the process. In contrast to the use of metallic iron as a reducing agent, during the reaction using metallic zinc powder, there are no critical density jumps in the reaction medium, which allows the mass and heat transfer processes to proceed efficiently, the resulting crystalline precipitate of zinc acetate coagulates into fairly large aggregates, no cooling of the reaction mass, which greatly facilitates filtration and ensures complete separation of the liquid phase without loss of dissolved para-aminophenol.

• the reduction process of p-nitrosophenol accelerates the interaction of zinc metal with acetic acid, so zinc acetate as a valuable by-product in the synthesis of para-aminophenol can be obtained much faster than in the case of a separate synthesis, in which metal zinc reacts with acetic acid without the presence of any other substances.

• para-aminophenol during the process can be obtained both in individual form and in the form of a solution, which facilitates its introduction to subsequent stages of chemical transformations.

• it is possible to introduce wet para-nitrosophenol into the reaction. Wet para-nitrosophenol has greater chemical stability, as oligomerization processes are inhibited.

• the method allows to obtain two valuable products in parallel: para-aminophenol and zinc acetate, which reduces the amount of waste to be disposed of and increases the economic viability of the method.

Thus, the presented new method, according to the invention, is characterized by cheap and readily available reagents, as well as a decrease in the amount of waste sent for disposal, allows, without the use of special equipment, to obtain and isolate several economically significant products, which ensures high purity, without the use of special equipment. and modern outlet.

Detailed description of the essence of the technical solution

The figure 1 shows one of the options for a possible installation scheme for implementing the method according to the invention

The following detailed description of the method is given for the sake of clarity with reference to FIG. 1 installation option, however, does not impose any restrictions on the hardware implementation of the method proposed in the invention.

In step a) of the process according to the invention, a solution of p-nitrosophenol in acetic acid is prepared in a container (E2.1). Preferably, a saturated solution of p-nitrosophenol in acetic acid is prepared to maximize productivity and reduce liquid waste, however, according to the invention, the use of a more dilute solution is also possible. In addition, para-nitrosophenol can be used to obtain a solution, both in a dry and wet state. However, too much water in p-nitrosophenol reduces the overall solubility of p-nitrosophenol in acetic acid, which lowers the overall efficiency of the process; therefore, acceptable in terms of the present invention, the range of moisture used para-nitrosophenol is the range of water content from 0 to 50 wt. -%, based on the mass of para-nitrosophenol. Moreover, the use of para-nitrosophenol in the wet state is preferable, since it has greater chemical stability, since in this case oligomerization processes are inhibited.

At the next technological stage b) using a peristaltic pump (H2.1) from the tank (E2.1) is loaded into the reactor (P2.1) prepared in stage a) solution. A weak current of inert gas is connected to purge the system to prevent the accumulation of hydrogen released during the interaction and the possible formation of a dangerous hydrogen-air mixture in the free space above the liquid.

With active stirring with a stirrer M2.1, preferably with a top drive, stage c) is started, at which, using a dosing screw, zinc metal powder is dosed from the hopper (B2.1) at a controlled rate, focusing on the intensity of the flow of the initiated interaction, the rate of heat release and the amount of solid phase in the reaction mixture. When dosing metal zinc powder, it is necessary to ensure its uniform distribution over the reaction volume, to exclude reagent clumping, i.e. to avoid a large concentration gradient and the occurrence of local overheating. In addition, since the interaction occurs with heating of the reaction mixture in the reactor, forced maintenance of the temperature in the range from 40 to 55 °C is carried out. Temperature control is carried out using a sensor (TT2.1), when the temperature rises above 50 ° C, cooling is connected using a cooling coil. During the reaction, an abundant precipitation of a finely crystalline precipitate of zinc acetate is observed.

After complete addition of the calculated amount of zinc in stage d), the reaction mixture in the reactor (R2.1) is further stirred until the reaction is complete, not allowing the temperature to rise above 55°C.

At the next stage f) the reaction mixture is cooled to 3-7 °C to completely precipitate zinc acetate from the solution, and the mixture is kept at this temperature for 20-90 minutes with constant stirring, and the intensity of mixing and the design of the stirrer blades should be such so as not to allow the formation of a layer of sediment at the bottom of the reactor during the entire process. The authors found that in order to best counteract the settling of the solid fraction during the implementation of the method according to the invention to the bottom of the reactor, the upward flow created by the agitator should be directed vertically along the axis of the reactor. In particular, the authors managed to ensure such an operating mode by means of a paddle mixer with flat blades inclined at an angle of 30° to 60° to the vertical. In the preferred embodiment, the 45° pitch of the blades creates sufficient flow with less energy. The stirrer diameter is preferably 25 to 60% of the reaction vessel diameter, particularly preferably 40%. The number of agitator blades is defined as the ratio of the height of the reaction liquid level in the reactor to the diameter of the agitator, rounded up, and ranges from 3 to 6, preferably 3. The height of the agitator blade is 0.2 of the agitator diameter.

Then, in step f), the cold reaction mixture is transferred by gravity from the reactor (R2.1) to the suction filter (F2.1) by opening tap 1.

By opening tap 3, the lower part of the suction filter is connected to the vacuum system, and filtration is carried out. In this case, a solution of para-aminophenol in acetic acid passes as a permeate to the lower part of the filter, and a solid residue of finely crystalline zinc acetate remains in the upper part as a retentate.

The distribution system used in the developed installation allows easy and flexible redirection of the flow of working fluids, using only one pump (H2.2).

At stage h) a container with glacial acetic acid (E2.2) is connected with a system of taps to the upper part of the suction filter (position of taps: 9 open; 10 closed; 2 open from left to right; 5 open; 6 closed; 7 open; 8 closed ) and using a pump (H2.2) to wash the zinc acetate from the residual para-aminophenol from the tank (E2.2), glacial acetic acid is served in three separate portions. During the supply of acetic acid for washing, the vacuum from the filter (F2.1) is turned off by tap 3, the precipitate is mixed with a manual stirrer (MP2.1) and the vacuum is reconnected by tap 3, filtering.

The resulting solution of para-aminophenol at the stage of method i) is removed from the bottom of the suction filter (F2.1) using a pump (H2.2) along the external line leading to the acetic acid evaporation apparatus (position of taps: 2 open from left to top; 5 open; 6 closed; 7 closed; 8 open).

In an evaporator (not shown in Figure 1), the acetic acid solution is concentrated to a concentration of >80% p-aminophenol by evaporation at reduced pressure and temperature, the solid product is precipitated by adding cold water to the evaporation residue, the technical grade solid para-aminophenol is separated by filtration and recrystallized from ethyl alcohol, resulting in para-aminophenol as the finished purified product.

The separated acetic acid can be reused in step a) to prepare a para-nitrosophenol solution or to wash the zinc acetate precipitate in step h).

In addition, para-aminophenol can be used without isolation as a solution in acetic acid for subsequent transformations, including for the acylation reaction, for which a solution of para-aminophenol in acetic acid is most preferred. In this case, the solution from the bottom of the suction filter is removed using a pump (H2.2) along an external line leading to the next stage of chemical transformations.

After step i), the wet zinc acetate residue can be optionally washed with an organic solvent to remove residual acetic acid, further purification, and accelerate subsequent drying. To do this, the distribution system connects the upper part of the suction filter with a container with a solvent for washing (E2.3) (position of valves: 9 closed; 10 open; 2 open from left to right; 5 open; 6 closed; 7 open; 8 closed). To remove the solvent from the bottom of the suction filter (F2.1), it is connected to a container for storing liquid waste (E2.4) (the position of the taps: 2 is open from the left-up; 5 is open; 6 is open; 7 is closed; 8 is closed). Pump the flushing solvent (H2.2) into a container (E2.3).

Water-miscible organic solvents in which zinc acetate is sparingly soluble, such as methyl alcohol, acetone, dioxane and mixtures thereof, can be used as the washing solvent.

Next, in process step j), the washed zinc acetate on the filter is dried either in a stream of air (tap position: 2 closed; 3 open; 4 open) or using vacuum (tap position: 2 closed; 3 open; 4 closed; 7 closed), and discharged from the suction filter to obtain solid zinc acetate as a finished product.

As used herein, the term "zinc acetate" includes both anhydrous zinc acetate and any of its hydrate and/or solvate forms. Preferably, in the process according to the invention, the zinc acetate is obtained in hydrated form, in particular as zinc acetate dihydrate.

Legend:

P2.1 Reactor

F2.1 Nutsch filter

B2.1 Hopper with dosing auger

L2.1 Loading hatch for maintenance of the reactor R2.1

M2.1 Mechanical stirrer

MP2.1 Agitator with manual drive for stirring sediment in the suction filter

H2.1 Peristaltic pump for supplying a solution of para-nitrosophenol to the reactor P2.1

H2.2 Peristaltic pump for supplying liquids and solutions to a distributor manifold

E2.1 Storage container for para-nitrosophenol solution

E2.2 Glacial acetic acid storage container

E2.3 Solvent storage container for washing the sediment in the suction filter

E2.4 Container for storing process drains for further processing and disposal

ТТ2.1 Reactor temperature sensor

1 Valve to completely drain the contents of the reactor into the suction filter

2 Three-way valve for adjusting the outlet flows from the suction filter and transporting liquids from containers E2.2, E2.3, E2.4

3 Valve for connecting the lower part of the suction filter to the vacuum system

4 Valve for connection to the atmosphere of the upper part of the suction filter

5 Valve for disconnecting the pump H2.2 from the distribution manifold

6 Distributor manifold valve directing the flow to the drain tank E2.4

7 Distributor valve, directing the flow to the top of the suction filter F2.1

8 Distributor manifold valve directing flow to further storage or next process step

9 Shut-off valve on the storage tank for glacial acetic acid E2.2

10 Shut-off valve on the solvent storage tank for washing the sediment in the suction filter E2.3

The invention is further explained in more detail with the help of exemplary embodiments, which, however, do not limit the scope of the present invention.

EXAMPLES

Example 1 Combined synthesis of para-aminophenol and zinc acetate dihydrate to obtain individual crystalline substances

To carry out the synthesis, a laboratory setup was used, created in accordance with the scheme shown in figure 1.

The following operating parameters and flows were used:

Total reactor volume 35 l

Reactor working volume 27 l

Capacity E2.1 30 l

Capacity E2.2 20 l

Capacity E2.3 20 l

Capacity E2.4 50 l

Pump capacity H2.1 ~1 l/min

Pump capacity H2.2 ~1 l/min

Screw capacity B2.1 42 g/min

The initial reagents used were p-nitrosophenol obtained in a laboratory by phenol nitrosation (purity 97% (HPLC)), commercial zinc metal powder (PCR-1 grade), glacial acetic acid, and deionized water.

Synthesis technique:

In vessel E2.1, 1183 g of wet p-nitrosophenol (water content 1.5 l) were dissolved in 14 l of glacial acetic acid. By means of a peristaltic pump (H2.1), the prepared solution was fed into the reactor (R2.1), equipped with a top-driven agitator (M2.1), a cooling coil with temperature control using a sensor (TT2.1), an inert gas supply and communication with external atmosphere for the possibility of purge, loading hatch (L2.1). After the complete introduction of the para-nitrosophenol solution, active mixing was turned on and 1260 g of PCR-1 metal zinc powder were dosed from the hopper (B2.1) using a dosing screw at a rate of 42 g/min. When the temperature rose above 50°C, cooling was connected using a cooling coil and the temperature was forcibly maintained in the range of 45–51°C. Abundant precipitation of a finely crystalline precipitate of zinc acetate was observed. After complete addition of the calculated amount of zinc (30 min), the reaction mixture in the reactor (P2.1) was stirred for an additional 1 hour to complete the reaction, not allowing the temperature to rise above 55°C. After that, the reaction mixture was cooled to 5°C for a more complete extraction of zinc acetate from the solution, and the reaction mixture was kept at this temperature for 30 minutes.

The cooled reaction mixture was transferred by gravity from the reactor (R2.1) to the suction filter (F2.1). The lower part of the suction filter was connected to the vacuum system and filtering was carried out. In this case, a solution of para-aminophenol in acetic acid passed into the lower part of the filter, and a solid residue of finely crystalline zinc acetate remained in the upper part. The suction filter was disconnected from the vacuum system and three separate portions (2 L, 2 L, 1 L) were supplied with glacial acetic acid from the tank (E2.2) using a pump (H2.2) to wash zinc acetate from residual para-aminophenol. After each batch, the sediment was stirred with a manual stirrer (MP2.1), and by connecting the vacuum with tap 3, filtration was performed. Then the suction filter was disconnected from the vacuum system, the connection with the external atmosphere was restored, and the resulting para-aminophenol solution was removed from the bottom of the suction filter (F2.1) using a pump (H2.2) into an industrial rotary evaporator with an evaporation flask volume of 10 L and the possibility of reloading the solution in the process of evaporation for continuous operation. In a rotary evaporator, acetic acid was removed at a temperature not exceeding 60°C, the evaporation residue was recrystallized from the minimum required amount of pure deoxygenated water to form a pure solution (11 L) with the addition of 3 g of sodium dithionite. After complete dissolution, the solution was cooled to 5°С, the precipitate of pure para-aminophenol was filtered off and dried in a vacuum oven. The separated acetic acid (13 L) was collected and sent to the container E2.1 to prepare a new solution of p-nitrosophenol.

As a result, 938 g of crystalline para-aminophenol was obtained (purity >99% (HPLC), yield 89.6% of theory).

¹ H NMR (300 MHz, DMSO-d6) δ 4.36 (s, 2H), 6.46 (d, J = 8.7 Hz, 2H), 6.52 (d, J = 8.7 Hz, 2H), 8.32 (s, 1H).

¹³ C NMR (300 MHz, DMSO-d6) δ 115.8, 116.1, 141.0, 148.8.

The wet zinc acetate residue was washed with an additional 1.5 L of cold methanol to remove residual acetic acid and re-filtered using vacuum.

The filter residue was dried in a stream of air for 30 min and unloaded from the filter.

This yielded 4030 g of solid zinc acetate dihydrate (>95% pure by NMR and elemental analysis, 95.3% of theory yield).

¹ H NMR (300 MHz, D ₂ O) δ 1.79 (s, 6H).

Example 2 Combined synthesis of para-aminophenol and zinc acetate dihydrate to obtain para-aminophenol as a solution in acetic acid

The example was carried out in accordance with the modes and methodology presented in example 1, with the only difference that the resulting solution of para-aminophenol in acetic acid was removed from the bottom of the suction filter (F2.1) using a pump (H2.2) along the outer lines into a storage vessel, from which they were subsequently sent to an additional reactor for carrying out the acylation reaction of para-aminophenol to obtain paracetamol.

The result was 14 L of a solution of para-aminophenol in acetic acid (para-aminophenol content 1038 g, 7.5% wt., yield 99.0% of theory) and 3985 g of solid zinc acetate dihydrate (>95% purity, yield 94.2% of theory).

Thus, as can be seen from the examples presented, the proposed method according to the invention leads to the simultaneous production of economically significant products: para-aminophenol and zinc acetate, with modern yields and excellent purity indicators using easy-to-perform stages carried out on standard chemical apparatuses, suitable for further industrial scale-up, while the amount of waste removed is reduced to a near minimum.

Claims (16)

1. A method for producing para-aminophenol and zinc acetate by reducing para-nitrosophenol with metallic zinc in an acetic acid medium, comprising the following steps: a) preparation of a solution of para-nitrosophenol in acetic acid; b) feeding the solution from step a) into the reactor with an inert atmosphere with stirring and forced maintenance of the temperature in the range from 40 to 55°C; c) dosing zinc metal powder into the reactor at a controlled feed rate; d) stirring the reaction mixture until the reaction is complete while maintaining the temperature in the range from 40 to 55°C; e) cooling the reaction mixture to 3-7°C for complete separation of fine crystalline zinc acetate from the reaction solution; f) withdrawing the reaction mixture from the reactor into a suction filter; g) separating solid zinc acetate as retentate and a solution of para-aminophenol in acetic acid as permeate; h) washing the zinc acetate on the filter with glacial acetic acid; i) withdrawing the solution of para-aminophenol in acetic acid from the suction filter and obtaining the final product of para-aminophenol either individually or as a solution; j) drying the solid zinc acetate and discharging it from the suction filter to obtain the final zinc acetate product. 2. Method according to claim 1, characterized in that the solution in step a) is prepared from wet para-nitrosophenol and glacial acetic acid. 3. The method according to p. 1, characterized in that the temperature in stage c) is maintained in the range from 45 to 50°C. 4. The method according to claim 1, characterized in that after step i) the zinc acetate is additionally washed with an organic solvent to remove residual acetic acid. 5. The method according to claim 4, characterized in that water-miscible organic solvents are used as the organic solvent, in which zinc acetate is sparingly soluble, preferably methyl alcohol, acetone, dioxane or mixtures thereof. 6. The method according to one of paragraphs. 1-5, characterized in that zinc acetate is obtained in hydrated form, in particular in the form of zinc acetate dihydrate.

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