Method for treating desulfurization waste liquid by one-step nitration and oxidation method

Abstract

The invention relates to a method for treating desulfurization waste liquid by a one-step nitration oxidation method, which comprises the following steps of: feeding the desulfurization waste liquid from a desulfurization system into a raw material tank, and mixing the desulfurization waste liquid with nitric acid and stronger ammonia water from ammonia distillation in the raw material tank to form a mixed solution with free ammonia concentration of 10g / L; increasing the pressure of the mixed solution through a high-pressure pump, mixing the mixed solution with compressed air, feeding the mixed solution into a first-stage heat exchanger and a second-stage heat exchanger, carrying out primary reaction and heat release in the first-stage heat exchanger and the second-stage heat exchanger to form a primary oxidation solution, heating the primary oxidation solution through an electromagnetic heater, heating the primary oxidation solution, controlling the temperature of the primary oxidation solution at 260-270 DEG C, and feeding the primary oxidation solution into a reactor; according to the method, ammonium thiosulfate in the desulfurization waste liquid is oxidized into ammonium sulfate and ammonium thiocyanate in the desulfurization waste liquid is oxidized into sulfuric acid by utilizing the characteristic that nitric acid and nitrate have the same oxidability under the conditions of high temperature and high pressure, so that the conversion rate of ammonium thiocyanate is increased, and the situation that the desulfurization waste liquid becomes hazardous waste due to the fact that the content of ammonium thiocyanate in the desulfurization waste liquid is too high is avoided; and recycling of resources is realized.

Description

Technical Field

[0001] The present invention relates to the technical field of treating desulfurized waste liquid from coke oven gas, and particularly to a method for treating desulfurized waste liquid by a one-step nitrification oxidation method. Background Art

[0002] At present, in the coking gas desulfurization process, there are generally two types: one is the external sodium carbonate wet desulfurization process such as the ADA method; the other is the pre-positioned wet ammonia desulfurization process using ammonia in coke oven gas as the alkali source, such as the HPF method, picric acid method, AS method, etc. Among them, the pre-positioned wet ammonia desulfurization process, due to using ammonia in gas as the alkali source, has the characteristics of simple operation and low investment cost, and is widely used in coking gas desulfurization.

[0003] In the pre-positioned wet ammonia desulfurization process, during the oxidation and regeneration of the desulfurization liquid, sulfur-containing desulfurized waste liquid will be generated, mainly containing sulfur-containing by-products such as ammonium thiosulfate, ammonium thiocyanate, and ammonium sulfate. At the same time, a large amount of dust and organic oils in the gas also enter the desulfurized waste liquid. When the content of the three salts > 300 g / L, it will seriously affect the desulfurization efficiency. Therefore, the desulfurized waste liquid of this method must be effectively treated.

[0004] The main methods for treating the sulfur-containing waste liquid in the pre-positioned wet ammonia desulfurization are mainly salt extraction and stepwise oxidation method.

[0005] Salt extraction mainly includes extracting mixed salts and refined salts. For extracting mixed salts, the desulfurized waste liquid is directly concentrated, and by cooling etc., the salts in the desulfurized waste liquid reach saturation to produce mixed salts. The important problem is that the produced mixed salts still belong to hazardous waste and the treatment cost is high. For extracting refined salts, mainly the fractional crystallization method is used. After concentrating the desulfurized waste liquid, by the fractional crystallization method, two salts, ammonium thiocyanate and ammonium thiosulfate, are extracted. Since the purity of ammonium thiosulfate is not high and the economic benefit is poor, some manufacturers oxidize it to ammonium sulfate and then sell it.

[0006] The stepwise oxidation method is to perform the first step of oxidation on the desulfurized waste liquid through dilute sulfuric acid or air to decompose ammonium thiosulfate in it to produce ammonium sulfate and sulfur. The second step of oxidation is to oxidize ammonium thiocyanate in it to ammonium sulfate, decolorize it at normal temperature and pressure by adding a catalyst and activated carbon. The finally formed waste liquid is sent to the coking ammonium sulfate production section to produce ammonium sulfate. Since ammonium thiocyanate is not completely oxidized at normal temperature and pressure, other harmful components in the waste liquid such as phenol are also likely to generate highly toxic hydrogenated gases, which are difficult to treat, resulting in a greater impact on the quality of the produced ammonium sulfate solution. Summary of the Invention

[0007] The present invention aims to at least partly solve one of the technical problems in the related technologies. For this purpose, the present invention proposes a method for treating desulfurized waste liquid by a one-step nitrification oxidation method.

[0008] The technical solution adopted by the present invention to solve its technical problems is: a method for treating desulfurized waste liquid by a one-step nitrification and oxidation method, comprising the following steps:

[0009] (1) Feed the desulfurized waste liquid from the desulfurization system into the raw material tank. The desulfurized waste liquid contains free ammonia, ammonium thiocyanate, ammonium thiosulfate and ammonium sulfate. In the raw material tank, the desulfurized waste liquid is mixed with nitric acid and 12-20% concentrated ammonia water from ammonia distillation to form a mixed solution with a free ammonia concentration of 10 g / L.

[0010] (2) The pressure of the mixed solution is increased to 6-7 mpa by a high-pressure pump, and then mixed with compressed air and sent to a primary and secondary heat exchanger. After preliminary reaction and heat release in the primary and secondary heat exchanger, a preliminary oxidation liquid is formed. The preliminary oxidation liquid is heated by an electromagnetic heater. After heating and controlling the temperature of the preliminary oxidation liquid at 260-270 °C, it enters the reactor.

[0011] (3) After sufficient reaction in the reactor, ammonium thiocyanate in the mixed solution reacts with oxygen in the compressed air under the action of a nitric acid catalyst to generate a secondary oxidation liquid containing ammonium sulfate. After the secondary oxidation liquid enters the cooler to cool down, it enters the decolorization kettle. In the decolorization kettle, it is stirred and adsorbed with activated carbon powder. After the activated carbon powder is filtered by a filter press, the secondary oxidation liquid enters the oxidation liquid tank and is sent to produce ammonium sulfate, and the waste activated carbon particles are sent to the coking coal yard for blending coal use.

[0012] (4) The gas at the top of the reactor, after being separated by a gas-liquid separator, the separated liquid is sent to the tail gas scrubber, the gas phase enters the waste heat boiler to recover heat, is cooled by a cooler, enters the low-pressure oxidation tank, and the gas fully oxidized in the low-pressure oxidation tank is fully washed in the tail gas scrubber to remove nitrogen monoxide and nitrogen dioxide gases in the tail gas, forming a nitric acid and nitrous acid solution, which is sent back to the raw material tank to reduce nitric acid consumption. The purified tail gas is discharged through a discharge chimney.

[0013] Further, in step (1), the temperature of the desulfurized waste liquid is 30-35 °C, and the ratio of ammonium thiocyanate: ammonium thiosulfate: ammonium sulfate in the desulfurized waste liquid is 1:1:0.2. After the desulfurized waste liquid is mixed with the low-pressure nitric acid-containing oxidation liquid from the tail gas scrubber, the nitric acid content in the solution in the raw material tank is configured to be more than 2.5 g / L, and the free ammonia concentration in the raw material tank is configured to be more than 10 g / L by using 16% concentrated ammonia water from ammonia distillation to form a mixed solution.

[0014] Further, in step (2), the pressure of the mixed solution is increased to 7 mpa by a high-pressure pump. After being preliminarily mixed with compressed air according to a molar ratio of 1:0.8, it is sent to a primary-secondary heat exchanger. In the primary heat exchanger, the temperature is raised to above 160 °C. Ammonium thiosulfate reacts at this temperature to form ammonium sulfate, forming a preliminary oxidation solution and releasing heat. In the secondary heat exchanger, the preliminary oxidation solution is heated to 260 - 270 °C by exchanging heat with the secondary oxidation solution coming out of the reactor. Then, the preliminary oxidation solution is adjusted to 270 °C by an electromagnetic heater and enters the reactor. The preliminary oxidation solution stays in the reactor for 2 h at a temperature of 270 - 273 °C. In the reactor, nitric acid acts as a catalyst, and ammonium thiocyanate in the preliminary oxidation solution reacts with oxygen to form a secondary oxidation solution containing ammonium sulfate. The reaction efficiency of ammonium thiocyanate reaches over 99.6%. The ammonium thiocyanate content is monitored at the reactor outlet and is <0.5 g / L. At the same time, the oxygen content is monitored at the gas phase outlet of the reactor. By controlling the amount of compressed air mixed in, the oxygen content in the gas phase at the gas phase outlet of the reactor is controlled to be 4 - 6%.

[0015] Further, in step (3), after the reactor has fully reacted, the secondary oxidation solution enters a cooler and is cooled to 50 - 70 °C, and then enters a decolorization kettle. In the decolorization kettle, activated carbon powder is added at a ratio of 2.5 kg / t. After 4 h of sufficient stirring and adsorption, the activated carbon powder is filtered by a filter press. The oxidation solution enters an oxidation solution tank and is sent to produce ammonium sulfate, and the waste activated carbon particles are sent to the coking coal yard for coal blending and use.

[0016] Further, in step (4), the gas at the top of the reactor is depressurized to 0.2 - 0.3 mpa, and after being separated by a gas-liquid separator, the separated liquid is sent to a tail gas scrubber. The gas phase enters a waste heat boiler to recover heat, and the temperature drops to 150 °C. It is cooled to room temperature by a cooler using circulating water and then enters a low-pressure oxidation tank. It stays in the low-pressure oxidation tank for 40 - 50 s, so that nitric oxide generated due to denitrification oxidation in the tail gas reacts fully with the oxygen in the tail gas to form nitrogen dioxide. Then, it is fully washed in the tail gas scrubber. 16% ammonia distillation ammonia water is supplemented in the tail gas scrubber. In an alkaline environment, nitric oxide and nitrogen dioxide gases in the tail gas form nitric acid and nitrous acid solutions and are sent back to the raw material tank to reduce nitric acid consumption. The purified tail gas is discharged through a discharge chimney.

[0017] The beneficial effects of the present invention are as follows: The present invention utilizes the characteristic that nitric acid and nitrate salts also have oxidizing properties under high temperature and high pressure conditions to oxidize ammonium thiosulfate in the desulfurized waste liquid into ammonium sulfate and ammonium thiocyanate into ammonium sulfate, improving the conversion rate of ammonium thiocyanate, avoiding the situation that the desulfurized waste liquid becomes a hazardous waste due to excessive ammonium thiocyanate content in the desulfurized waste liquid, realizing the recycling of resources, and nitric oxide and nitrogen dioxide formed during the oxidation of the desulfurized waste liquid are absorbed by ammonia water solution to form ammonium nitrate solution for recycling, reducing operation and procurement costs. Description of the Drawings

[0018] Figure 1 It is a schematic flow diagram of the method of the present invention. Detailed Embodiments

[0019] The following describes in detail the embodiments of the present invention. Examples of the embodiments are shown in the drawings, where the same or similar reference numerals represent the same or similar elements or elements with the same or similar functions throughout. The embodiments described below by referring to the drawings are exemplary and are only used to explain the present invention and should not be construed as a limitation to the present invention.

[0020] In the description of the present invention, it should be understood that the orientation or positional relationships indicated by the terms "center", "longitudinal", "transverse", "length", "width", "thickness", "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", "clockwise", "counterclockwise", etc. are based on the orientation or positional relationships shown in the drawings, and are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and thus should not be construed as a limitation to the present invention. In addition, the terms "first" and "second" are only used for descriptive purposes and should not be construed as indicating or implying relative importance or implicitly indicating the number of the technical features indicated. Thus, the features defined with "first" and "second" may explicitly or implicitly include one or more of the said features. In the description of the present invention, "a plurality" means two or more unless otherwise specifically defined.

[0021] As Figure 1 shown, a method for treating desulfurized waste liquid by a one-step nitrification oxidation method includes the following steps:

[0022] (1) 20 t / d of desulfurized waste liquid from the desulfurization system enters the raw material tank. The temperature of the desulfurized waste liquid is 35°C. The mass concentration of free ammonia in the desulfurized waste liquid is 8-14%. The mass ratio of ammonium thiocyanate: ammonium thiosulfate: ammonium sulfate in the desulfurized waste liquid is 1:1:0.2. After the desulfurized waste liquid is mixed with the low-pressure nitrated oxidation liquid from the tail gas scrubbing tower, the nitrate content in the solution in the raw material tank is configured to be more than 2.5 g / L by using the nitric acid in the low-pressure nitrated oxidation liquid. In this embodiment, 0.1 t of 50% industrial nitric acid needs to be added. Using 16% concentrated ammonia water from ammonia distillation, the free ammonia concentration in the raw material tank is configured to be a mixed solution of more than 10 g / L.

[0023] (2) The pressure of the mixed solution is increased to 7 MPa by a high-pressure pump. After being preliminarily mixed with compressed air at a molar ratio of 1:0.8, it is sent to a primary and secondary heat exchanger. In the primary heat exchanger, the mixed solution exchanges heat and its temperature rises above 160 °C. Ammonium thiosulfate in the mixed solution reacts at this temperature to form ammonium sulfate and releases heat. The specific reaction formula for ammonium thiosulfate to form ammonium sulfate is: (NH4)2S2O3 + 2O2 + 2NH3 + H2O = 2(NH4)2SO 4， ;

[0024] After the mixed solution releases heat during the preliminary reaction in the secondary heat exchanger, a preliminary oxidation liquid is formed. The preliminary oxidation liquid is heated by an electromagnetic heater, and its temperature is controlled at 260 - 270 °C after heating and then enters the reactor. The preliminary oxidation liquid maintains a temperature of 270 - 273 °C in the reactor and stays for 2 h of reaction time. In the reactor, nitric acid acts as a catalyst, and ammonium thiocyanate in the preliminary oxidation liquid reacts with oxygen in the compressed air to generate a secondary oxidation liquid containing ammonium sulfate. In the reactor, the reaction efficiency of ammonium thiocyanate reaches over 99.6%. The reaction formula for ammonium thiocyanate to react with oxygen in the compressed air is: 2NH4SCN + 4HNO3 = (NH4)2SO4 + 2NO2 + 2CO2 + 2H20;

[0025] And in the secondary heat exchanger, after the mixed solution and compressed air are mixed at a molar ratio of 1:0.8, they exchange heat with the secondary oxidation liquid coming out of the reactor and the temperature rises to 260 - 270 °C, realizing the recovery of the heat of the secondary oxidation liquid and reducing the power consumption of the electromagnetic heater for heating.

[0026] (3) After ammonium thiocyanate reacts fully in the reactor, the secondary oxidation liquid exchanges heat with the mixed solution in the secondary heat exchanger, and then the secondary oxidation liquid enters the cooler and is cooled to 50 - 70 °C, and then enters the decolorization kettle. In the decolorization kettle, activated carbon powder is added at a ratio of 2.5 kg / t. After 4 h of sufficient stirring and adsorption, the activated carbon powder is filtered by a filter press. Then, the secondary oxidation liquid enters the oxidation liquid tank and is sent to produce ammonium sulfate, and the waste activated carbon particles are sent to the coking coal yard for coal blending and use.

[0027] (4) After the gas at the top of the reactor is depressurized to 0.2 MPa, it is separated by a gas-liquid separator. The separated liquid is sent to the tail gas scrubbing tower, and the gas phase enters the waste heat boiler to recover heat. After the temperature drops to 150 °C, it is cooled to room temperature by a cooler and then enters the low-pressure oxidation tank. It stays in the low-pressure oxidation tank for 40 - 50 s, so that the nitric oxide generated due to denitrification oxidation in the tail gas reacts fully with the oxygen in the tail gas to generate nitrogen dioxide. Then, it is fully washed in the tail gas scrubbing tower. 16% of ammonia-steam ammonia water is added to the tail gas scrubbing tower. In an alkaline environment, nitric oxide and nitrogen dioxide gases in the tail gas form nitric acid and nitrous acid solutions, which are sent back to the raw material tank to reduce nitric acid consumption. The purified tail gas is discharged through the vent stack. In this embodiment, the specific formula for the cyclic production process of nitrate is: 2N02 = 2NO + O2; 4NO + 2O2 + H2O = 2HNO3; 4NO2 + O2 = 2N2O5; N2O5 + H2O = 2HNO3.

[0028] By detecting the nitrate content of the low-pressure nitrate-containing oxidation liquid coming out of the tail gas scrubbing tower, it is required that the nitrate content of the low-pressure nitrate-containing oxidation liquid < 0.5 mg / L. Due to different desulfurization processes, the ability of the desulfurization system to produce desulfurization waste liquid to consume nitrate is different. It is necessary to adjust the amount of nitric acid configured in the raw material tank according to the nitrate content of the low-pressure nitrate-containing oxidation liquid coming out of the tail gas scrubbing tower.

[0029] The contents of each component in the secondary oxidation liquid after the reaction in the reactor are as follows: ammonium thiocyanate 0.0172%, that is, the conversion rate of ammonium thiocyanate > 99.9%; the content of ammonium thiocyanate < 0.5 g / L; the conversion rate of ammonium thiosulfate 100%; the nitrate concentration < 0.5 g / L; the free acid content 1 - 3%.

[0030] The present invention utilizes the characteristic that nitric acid and nitrates also have oxidizing properties under high temperature and high pressure conditions to oxidize ammonium thiosulfate in the desulfurization waste liquid into ammonium sulfate and oxidize ammonium thiocyanate into sulfuric acid, improving the conversion rate of ammonium thiocyanate and avoiding the situation that the desulfurization waste liquid becomes a hazardous waste due to too high ammonium thiocyanate content in the desulfurization waste liquid, realizing the recycling of resources. Moreover, nitric oxide and nitrogen dioxide formed during the oxidation of the desulfurization waste liquid are absorbed by the ammonia water solution to form ammonium nitrate solution for recycling, reducing the operation and procurement costs.

[0031] In the description of this specification, the description with reference to terms such as "one embodiment", "certain embodiments", "illustrative embodiments", "examples", "specific examples" or "some examples" means that the specific features, structures, materials or characteristics described in connection with the embodiments or examples are included in at least one embodiment or example of the present invention. In this specification, the schematic representations of the above terms do not necessarily refer to the same embodiments or examples. Moreover, the specific features, structures, materials or characteristics described can be combined in a suitable manner in any one or more embodiments or examples.

[0032] In summary, although the present invention has been disclosed above in preferred embodiments, the above preferred embodiments are not intended to limit the present invention. Those of ordinary skill in the art can make various changes and modifications without departing from the spirit and scope of the present invention. Therefore, the protection scope of the present invention shall be subject to the scope defined by the claims.

Claims

1. A method for treating desulfurization waste liquid by a one-step nitrification and oxidation method, characterized in that, It includes the following steps: (1) Feed the desulfurized waste liquid from the self-desulfurization system into the raw material tank. The desulfurized waste liquid contains free ammonia, ammonium thiocyanate, ammonium thiosulfate and ammonium sulfate. In the raw material tank, the desulfurized waste liquid is mixed with nitric acid and 12-20% concentrated ammonia water from ammonia distillation to form a mixed solution with a free ammonia concentration of 10 g / L; (2) Use a high-pressure pump to increase the pressure of the mixed solution to 6-7 mpa, then mix it with compressed air and send it to a primary and secondary heat exchanger. After preliminary reaction and heat release in the primary and secondary heat exchanger, a preliminary oxidation liquid is formed. The preliminary oxidation liquid is heated by an electromagnetic heater. After heating and controlling the temperature of the preliminary oxidation liquid at 260-270 °C, it enters the reactor; (3) After sufficient reaction in the reactor, ammonium thiocyanate in the mixed solution reacts with oxygen in the compressed air under the action of a nitric acid catalyst to generate a secondary oxidation liquid containing ammonium sulfate. After the secondary oxidation liquid is cooled in a cooler, it enters the decolorization kettle. In the decolorization kettle, it is stirred and adsorbed with activated carbon powder. After the activated carbon powder is pressure-filtered by a filter press, the secondary oxidation liquid enters the oxidation liquid tank and is sent to produce ammonium sulfate, and the waste activated carbon particles are sent to the coking coal yard for coal blending; (4) The gas at the top of the reactor is separated by a gas-liquid separator. The separated liquid is sent to the tail gas scrubbing tower. The gas phase enters the waste heat boiler to recover heat, is cooled by a cooler, and enters the low-pressure oxidation tank. The gas fully oxidized in the low-pressure oxidation tank is fully washed in the tail gas scrubbing tower to remove nitrogen monoxide and nitrogen dioxide gases in the tail gas, forming nitric acid and nitrous acid solutions, which are sent back to the raw material tank to reduce nitric acid consumption. The purified tail gas is discharged through a discharge chimney.

2. The method for treating desulfurization waste liquid by the one-step nitrification and oxidation method according to claim 1, characterized in that, In step (1), the temperature of the desulfurized waste liquid is 30-35 °C, and the ratio of ammonium thiocyanate: ammonium thiosulfate: ammonium sulfate in the desulfurized waste liquid is 1:1:0.

2. After the desulfurized waste liquid is mixed with the low-pressure nitric acid-containing oxidation liquid from the tail gas scrubbing tower, nitric acid is used to configure the nitrate content (calculated as nitrate ion NO3-) in the solution in the raw material tank to be above 2.5 g / L, and 16% concentrated ammonia water from ammonia distillation is used to configure a mixed solution with a free ammonia concentration of above 10 g / L in the raw material tank.

3. The method for treating desulfurized waste liquid by the one-step nitrification and oxidation method according to claim 1, wherein In step (2), the pressure of the mixed solution is increased to 7 MPa by a high-pressure pump. After being preliminarily mixed with compressed air at a molar ratio of 1:0.8, it is sent to a primary-secondary heat exchanger. In the primary heat exchanger, the temperature is raised to above 160 °C. Ammonium thiosulfate reacts at this temperature to form ammonium sulfate, forming a preliminary oxidation solution and releasing heat. In the secondary heat exchanger, the preliminary oxidation solution is heated to 260 - 270 °C by exchanging heat with the secondary oxidation solution coming out of the reactor. Then, the preliminary oxidation solution is adjusted to 270 °C by an electromagnetic heater and enters the reactor. The preliminary oxidation solution stays in the reactor for 2 h at a temperature of 270 - 273 °C. In the reactor, nitric acid acts as a catalyst, and ammonium thiocyanate in the preliminary oxidation solution reacts with oxygen to generate a secondary oxidation solution containing ammonium sulfate. The reaction efficiency of ammonium thiocyanate reaches over 99.6%. The content of ammonium thiocyanate is monitored at the reactor outlet and is <0.5 g / L. At the same time, the oxygen content is monitored at the gas phase outlet of the reactor. By controlling the amount of compressed air mixed in, the oxygen content in the gas phase at the gas phase outlet of the reactor is controlled to be 4 - 6%.

4. The method for treating desulfurization waste liquid by the one-step nitrification and oxidation method according to claim 1, wherein, In step (3), after the reaction in the reactor is completed, the secondary oxidation solution enters a cooler and is cooled to 50 - 70 °C, and then enters a decolorization kettle. Activated carbon powder is added at a ratio of 2.5 kg / t in the decolorization kettle. After 4 h of sufficient stirring and adsorption, the activated carbon powder is filtered by a filter press. The oxidation solution enters an oxidation solution tank and is sent to produce ammonium sulfate, and the waste activated carbon particles are sent to the coking coal yard for coal blending and use.

5. The method for treating desulfurized waste liquid by the one-step nitrification and oxidation method according to claim 1, wherein In step (4), The gas at the top of the reactor is depressurized to 0.2 - 0.3 MPa, and after being separated by a gas-liquid separator, the separated liquid is sent to a tail gas scrubbing tower. The gas phase enters a waste heat boiler to recover heat, and the temperature drops to 150 °C. After being cooled to room temperature by a cooler with circulating water, it enters a low-pressure oxidation tank and stays for 40 - 50 s. Nitric oxide generated due to denitrification oxidation in the tail gas reacts fully with the oxygen in the tail gas to form nitrogen dioxide. Then, it is fully washed in the tail gas scrubbing tower. 16% of ammonia distillation ammonia water is added in the tail gas scrubbing tower. In an alkaline environment, nitric oxide and nitrogen dioxide gases in the tail gas form nitric acid and nitrous acid solutions, which are sent back to the raw material tank to reduce nitric acid consumption. The purified tail gas is discharged through a discharge chimney.

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