RU2160711C1 - Method of treatment of sewage waters of urea production process - Google Patents

Abstract

FIELD: treatment of sewage waters containing ammonia and urea. SUBSTANCE: method consists in that sewage waters after hydrolysis and desorption are subjected to treatment with active chlorine with air supply at pH 6.5-8 in amount of 5-7 weight parts per weight part of ammonium nitrogen with subsequent anionation. Treatment with active chlorine may be accomplished with gaseous chloride or with calcium or sodium hypochlorite. EFFECT: higher efficiency of purification of sewage waters and attaining quality of purified sewage meeting the requirements for chemically treated water, for instance, for supply of medium pressure boilers with P=4 MPa (kgf/sq.cm), or possible use in systems of circulating water supply. 4 cl, 3 ex

Description

 The invention relates to the treatment of wastewater containing ammonia and urea, in particular wastewater from the production of urea, and can be used in the chemical industry.

 A known method of treating wastewater from the production of urea by hydrolysis of treated effluents by heating and pressure. When this hydrolysis is carried out in the presence of phosphoric acid, and the resulting solution is treated with alcohol or ketone, followed by separation of the precipitate (Aut. St. N 239863, MKI C 02 F 1/52, publ. 1969). This method is complex in technology and does not provide deep wastewater treatment, allowing its reuse.

A known method of treating wastewater from urea by sorption it on strongly acidic cation exchangers, for example KU-2, followed by their regeneration by strong inorganic acids (Auth. St. N 451641, M. Cl. C 02 C 5/08, publ. 1974 ) This method requires the use of expensive ion-exchange resins, a large amount of phosphoric acid for regeneration and leads to the formation of wastewater when washing the ion-exchange resins from regeneration solutions (the amount of urea in wastewater is 2.73 g / dm ³ , ammonia is 20 g / dm ³ ) .

 A known method of treating wastewater from urea production, including mixing circulating and fresh wastewater streams with air, evaporating, spraying and releasing air with water vapor into the atmosphere, the fresh wastewater stream being pre-mixed with the air stream leaving the evaporation zone (Auth. St. N 732212, M. C., C 02 C 5/00, publ. 1980).

A known method of treating wastewater from the production of urea by hydrolysis of treated effluents by thermal decomposition at elevated pressure in a cascade type column apparatus at a temperature of 150 - 262 ^o C and a pressure of 1.5 - 5.0 MPa uses sequential stages of the cascade in an amount of 4 - 30 (Patent RU N 2056408, M.C. C 07 C 273/04, publ. 1996). This method is energy intensive (high temperatures are used).

Known closest to the proposed method for wastewater treatment of urea production by hydrolysis of treated effluents by heating and pressure: at a temperature of 180 - 200 ^o C, a pressure of 15 - 20 atm, followed by steam blowing of ammonia and carbon dioxide. Wastewater with an ammonia content of 0.5 g / l is fed to ammonia by nitrogen or air at atmospheric pressure and a temperature of 100 ^o C. The residual ammonia content is 0.03 - 0.006 g / l (Aut. St. N 186891, M Cl. C 02 F 1/02, publ. 1966 Prototype).

 This method does not provide effective wastewater treatment, which excludes their reuse in the circulating water supply system or as feed water for boilers.

The objective of the invention is to increase the efficiency of wastewater treatment and to bring the quality of wastewater to the requirements of chemically treated water, for example, to power medium-pressure boilers P - 4 MPa (40 kgf / cm ² ) or the possibility of using the enterprise in the water recycling system.

 This problem is solved by the fact that in the method of treating wastewater from urea production by hydrolysis of the effluents to be purified by heating and pressure, the wastewater after hydrolysis is subjected to treatment with active chlorine followed by anionization, treatment with active chlorine is carried out at pH 6.5-8, and the amount of active chlorine is taken 5 to 7 parts by weight per 1 part by weight of ammonium nitrogen.

The method is as follows: wastewater containing ammonia and urea, enters the stripper of the first stage, where at a temperature of 130 - 145 ^o C, a pressure of 0.28 - 0.35 MPa from the solution, carbon dioxide, ammonia and water are released. Next, the waste water from the lower part of the stripper of the first stage is sent to the hydrolyzer, in the lower part of which "sharp" high-pressure steam is supplied. In the hydrolyzer at a temperature of no more than 190 ^o C and a pressure of 1.5 - 1.8 MPa, decomposition (hydrolysis) of urea, ammonia and carbon dioxide occurs. The liquid phase enters the second stage stripper, where ammonia and carbon dioxide are distilled off from the solution. Wastewater at the outlet of the desorption-hydrolysis unit contains urea with a mass concentration of 10-30 mg / dm ³ and ammonia with a mass concentration of 10-15 mg / dm ³ . Next, the wastewater is treated with active chlorine in an amount of 5 to 7 parts by weight of active chlorine per 1 part by weight of ammonium nitrogen at a pH of 6.5 to 8. For chlorination, for example, gaseous chlorine, calcium and sodium hypochlorites, which form hypochlorous acid in water, can be used. and hypochlorite ion. The sum of free Cl ₂ , HOCl, OCl and nitrogen bound in chloramine is called active chlorine. The process of oxidation of ammonium nitrogen by active chlorine proceeds by the reaction:
2NH ₄ + 2Cl ₂ = 2NH ₂ Cl = 4H ⁺ + 2Cl-
NH ₂ Cl + Cl ₂ = NHCl ₂ + H ⁺ + 3Cl-
NH ₂ Cl + NHCl ₂ = N ₂ + 3H ⁺ + 3Cl-
To speed up the chlorination process, air purging is used. Raising the pH above 8 leads to an increase in the consumption of active chlorine for the oxidation of ammonium nitrogen, and lowering the pH below 6.5 does not provide water quality. Chlorinated water is passed through a weakly basic anion exchange resin to remove chlorine ions. After chlorination with sodium hypochlorite or calcium hypochlorite, preliminary cationization is required.

Waste water after treatment has the following indicators:
Chlorides - Not more than 3 mg / dm ³
Alkalinity - Not more than 700 mcg-equiv / dm ³
Hardness - 10 μg-equiv / dm ³
Salinity - Not more than 300 mg / dm ³
Ammonia - Not more than 2 mg / dm ³
Urea - Lack
which corresponds to the requirements for the quality of chemically treated water for feeding medium-pressure boilers P - 4 MPa (40 kgf / cm ² ).

 The amount of active chlorine needed for the chlorination of wastewater and the pH value are established experimentally.

Example 1
Wastewater from urea production after desorption and hydrolysis, characterized by the following quality:
pH 7.2
Ammonia - 11.7 mg / dm ³
Urea - 28.3 mg / dm ³
Suspended Substances - No
The dry residue is 16 mg / DM ³
Chlorides - Lack
Sulphates - Lack
Phosphates - Absence
Iron - 0.04 mg / dm ³
Hardness - 0.02 mcg-equiv / dm ³
Alkalinity - 0.4 μg-equiv / dm ³
Calcium - 1.4 mg / dm ³
Magnesium - Lack
Copper - Lack
Nitrates - Lack
Nitrite - Lack
Silica - 0.04 mg / DM ³
Carbon Monoxide - Lack
treated with active chlorine. For the complete oxidation of ammonia and urea, 5 parts by weight of active chlorine per one part by weight of ammonia nitrogen are required. To speed up the oxidation process, an air purge is given. After oxidation, the urea content in wastewater is 0, and ammonia is also absent. The chloride content increases to 17.5 mg / dm ³ . To remove chlorides, wastewater is passed through anion exchange resin.

Example 2
Urea production wastewater containing:
pH - 8.0
Ammonia - 11.3 mg / dm ³
Urea - 12.1 mg / dm ³
(other quality indicators in connection with the content significantly lower than normalized are not given) are treated with active chlorine. The dose of active chlorine take 7 parts by weight per 1 part by weight of ammonium nitrogen. After oxidation, the ammonia content is absent, urea is absent, and the chloride content is 28.5 mg / l. To remove chlorides, wastewater is passed through anion exchange resin.

Example 3
Urea production wastewater containing:
pH 9.4
Ammonia - 25.4 mg / dm ³
Urea - 20 mg / dm ³
treated with active chlorine. In this example, at pH 9, for the complete removal of ammonia and urea, the ratio of active chlorine per one part by weight of ammonium nitrogen increases to 9 parts by weight. Those. with increasing pH of the test water, the dose of active chlorine increases, which is impractical, since it leads to an overrun of reagents.

 Lowering the pH below 6.5 is also impractical, since water becomes corrosive.

The examples show that, in comparison with the prototype, the degree of purification from ammonia and urea by the proposed method increases and amounts to 100%, which allows its reuse in recycled water supply or as feed water for medium-pressure boilers P - 4 MPa (40 kgm / cm ² ).

 The tests were carried out in the laboratory of Agro-Cherepovets CJSC. Preparatory work is being carried out for industrial use at ZAO Agro-Cherepovets.

Claims (4)

 1. The method of purification of wastewater from the production of urea by hydrolysis and desorption of treated effluents by heating and pressure, characterized in that the wastewater after hydrolysis and desorption is treated with active chlorine with air supply and subsequent anionization.  2. The method according to claim 1, characterized in that the treatment with active chlorine is carried out at a pH of 6.8 to 8, and the amount of active chlorine is 5 to 7 parts by weight. for 1 weight.h. ammonium nitrogen.  3. The method according to claim 1, characterized in that chlorine gas is used for treatment with active chlorine.  4. The method according to claim 1, characterized in that for the treatment with active chlorine, calcium or sodium hypochlorites are used, prior to wastewater treatment on anion exchangers, preliminary cationization is carried out.