A method for treating coal gasification wastewater

Abstract

The present invention relates to a method for treating coal gasification wastewater. The method comprises adjusting the pH of the coal gasification wastewater to 10.0-12.0, adding a reagent to the coal gasification wastewater to convert calcium, magnesium, and silicon ions in the coal gasification wastewater into precipitates, and then passing the supernatant into a softening and deammonification tower. While removing ammonia, the sludge particles generated by softening are filtered and removed through an ultrafiltration membrane device at the bottom of the tower, thereby obtaining softened and deammonified wastewater and pure ammonia water or ammonium sulfate solution. The softened and deammonified wastewater is partially reused in the ash water system for flushing the gasifier lock hopper or slag tank, or is fed into a subsequent biochemical system for advanced treatment. Reuse of the softened and deammonified wastewater can avoid equipment clogging. After the coal gasification wastewater is treated by the method of the present invention and then subjected to biochemical treatment, the amount of additional carbon source required for denitrification in the subsequent biochemical system can be significantly reduced.

Description

Technical Field

[0001] The present invention relates to the field of sewage treatment, and in particular to a method for treating coal gasification wastewater. Background Art

[0002] The quality of coal gasification wastewater varies significantly depending on the gasification process. Low-temperature gasification technology produces wastewater characterized by high chemical oxygen demand, high levels of phenols, high levels of ammonia nitrogen, high levels of oil, and high turbidity. High-temperature coal-water slurry and dry powder pressurized gasification technology produces wastewater characterized by high levels of ammonia nitrogen and high hardness, but low levels of organic contamination and readily biodegradable organic matter. This technology is primarily targeted at coal gasification wastewater with high levels of ammonia nitrogen, high turbidity, high hardness, and low concentrations of organic matter.

[0003] As an important branch of new coal chemical industry, coal gasification chemical industry will produce a large amount of wastewater in actual operation, so it is necessary to treat coal gasification wastewater efficiently.

[0004] Coal gasification wastewater primarily refers to the gray water overflowing from the gasifier. This is actually the direct discharge of coal-rich ashwater from both the gasifier and scrubber, which contains excessive amounts of coal. Its primary characteristics are high hardness, high ammonia nitrogen content, and high levels of suspended solids. Currently, most ashwater systems face blockages in heat exchangers, scrubbers, and pipelines caused by these high hardness and suspended solids. Furthermore, flocculants are added to the sedimentation tank inlet during ashwater circulation to precipitate suspended solids, while dispersants must be added to the sedimentation tank outlet to prevent equipment and pipeline blockage. These two agents have opposing effects, making it difficult to find the right adjustment point, leading to unstable ashwater system operation. Discharge of this wastewater to sewage treatment plants also introduces a series of issues, including scaling and high ammonia nitrogen content. The addition of dispersants also increases the total phosphorus treatment load at the sewage treatment plant. The high ammonia nitrogen content in gasification wastewater often requires the addition of a large amount of carbon source to the sewage system for denitrification to control the TN of the sewage system's effluent to not exceed the standard; if the total nitrogen TN is not controlled, the nitrate in it will enter the subsequent zero-emission system to generate miscellaneous salt sodium nitrate (hazardous waste), affecting the operation of the zero-emission system and increasing the disposal cost of the miscellaneous salt.

[0005] Because coal gasification wastewater contains a certain amount of dispersant, which forms stable soluble complexes with calcium and magnesium ions, affecting the chemical reaction between the softening agent and calcium and magnesium, traditional softening methods (alkali-soda method, lime-soda method) are less effective in removing hardness from coal gasification wastewater. Existing treatment technologies for coal gasification wastewater generally involve directly adding a softening agent followed by chemical precipitation and then denitrification, without considering the impact of the dispersant in the water. Furthermore, the traditional precipitation separation process is ineffective and unstable. Separate precipitation separation and ammonia and nitrogen removal processes require a longer process flow, increasing treatment costs. Summary of the Invention

[0006] The operation of the gasification ash water system is faced with the blockage of heat exchangers, washing towers, pipelines, etc. caused by high hardness and high suspended matter. At the same time, when the gasification ash water is circulated internally, flocculants are added to the sedimentation tank inlet to precipitate suspended matter, and dispersants have to be added to the sedimentation tank outlet to prevent blockage of equipment and pipelines. The two agents have completely opposite effects, and it is difficult to find a suitable adjustment point, resulting in unstable operation of the gasification ash water system. The discharge of gasification wastewater to the sewage treatment plant brings a series of problems such as scaling and high ammonia nitrogen to the sewage system.

[0007] The present invention provides a method for treating coal gasification wastewater. After being treated by the method of the present invention, the coal gasification wastewater can be reused in the ash water system for flushing the lock hopper or slag pool of the gasifier, thereby avoiding fouling and clogging of the equipment. At the same time, after the coal gasification wastewater is treated by the method of the present invention, when it is subjected to biochemical treatment, the additional carbon source required for denitrification in the subsequent biochemical system can be greatly reduced.

[0008] The purpose of the present invention can be achieved by the following technical solutions:

[0009] The present invention provides a method for treating coal gasification wastewater, comprising the following steps:

[0010] (1) controlling or adjusting the pH value of the coal gasification wastewater to 10.0-12.0, adding a softening agent and a flocculant to the coal gasification wastewater in a dosing reaction box (2), and mixing them evenly using a stirrer (1) to convert calcium, magnesium, and silicon ions in the coal gasification wastewater into solid or colloidal particles;

[0011] (2) The supernatant after the treatment in step (1) enters the softening and deammonification tower through the mixed liquid feed pump (3), and while removing ammonia, the sludge particles generated by softening are filtered and removed through the ultrafiltration membrane device at the bottom of the tower to obtain softened and deammonified wastewater, and the ammonia is purified and absorbed using an ammonia absorption tank to obtain a pure ammonia solution or ammonium sulfate solution;

[0012] (3) The wastewater after softening and deammoniation is partially recycled to the ash water system for flushing the lock bucket of the gasifier or the slag pool, or the wastewater after softening and deammoniation is transported to the sewage treatment system for further biochemical treatment according to the design requirements. In one embodiment of the present invention, the coal gasification wastewater has the characteristics of high hardness, high ammonia nitrogen, high SiO2, and high salt. Compared with high-salt wastewater, the impurity components contained in coal gasification wastewater are more and more complex, and the direct addition of reagents has a poor effect on removing hardness.

[0013] In one embodiment of the present invention, the method for controlling or adjusting the pH value of the coal gasification wastewater to 10.0-12.0 in step (1) is to add a conventional pH regulator to the coal gasification wastewater. The purpose of adjusting the pH value of the coal gasification wastewater to 10.0-12.0 is to improve the softening and ammonia nitrogen removal effects in the softening and deamination process. When the pH is within the range of 10-12, ammonia nitrogen often exists in the form of free ammonia and is positively correlated with pH. When the pH is low, the softening and ammonia nitrogen removal effects are poor and the treatment effect cannot be achieved. If the pH value is too high, the dosage of the agent will be large, which increases the cost. Therefore, based on the special composition of the coal gasification wastewater, the present application controls or adjusts the pH value of the coal gasification wastewater to 10.0-12.0, preferably 10.0-11.0.

[0014] In one embodiment of the present invention, in step (1), a suitable amount of softening agent is added according to the composition of the coal gasification wastewater to convert calcium, magnesium and silicon ions in the coal gasification wastewater into precipitates.

[0015] In one embodiment of the present invention, in step (1), a softening agent and a flocculant are added to the coal gasification wastewater to convert the calcium ions in the coal gasification wastewater into calcium carbonate, the magnesium ions in the coal gasification wastewater into magnesium hydroxide, and the silicon ions in the coal gasification wastewater into silicate precipitates.

[0016] The softening agent includes sodium carbonate and sodium hydroxide, and the flocculant includes aluminum salt, iron salt and other agents.

[0017] The main function of the flocculant is to replace calcium and magnesium ions and combine with the dispersant through a complexation mechanism, thereby ensuring that the addition of sodium carbonate reagent to the coal gasification wastewater can well react according to theoretical approximate data, thereby generating effective solid or colloidal particles and removing calcium and magnesium hardness through membrane filtration;

[0018] The dosage of softening agent is 100-500ppm. The dosage of softening agent in coal gasification wastewater is adjusted according to the hardness of wastewater and scaling tendency of the system.

[0019] The dosage of the flocculant is 1-100 ppm, preferably 20-50 ppm.

[0020] The dosing reaction box in step (1) has a circular cone bottom structure and is equipped with a stirring device to ensure that the liquid and the added reagent are completely mixed and react evenly. After the reaction is complete, the sediment at the bottom is discharged by gravity and sent to the sludge treatment system for treatment, and the supernatant is sent to the softening and deamination tower.

[0021] In one embodiment of the present invention, in step (2), the softening and deamination tower is composed of a tower body, a tower plate, a membrane assembly, an aeration device, an ammonia absorption tank, and a water production system.

[0022] A membrane assembly is placed at the bottom of the softening deamination tower, and an aeration device is arranged at the bottom of the membrane assembly. Tower plates are staggered on the upper part of the tower body. The tower plates are provided with water retaining and evenly distributed water holes, which increase the contact area between the gas phase and the liquid phase and improve the mass transfer efficiency between the two phases. In addition, the temperature of the waste liquid entering the tower is 60-70°C, which can better maintain the escape velocity of ammonia and improve the removal rate of ammonia nitrogen.

[0023] The upper part of the tower body is made of lightweight material, which can be glass fiber reinforced plastic or other corrosion-resistant materials; the lower part is made of stainless steel, and the membrane component is installed in the lower part; the upper and lower parts of the tower body are connected by flange sealing, and the tower body is divided into two parts, which is more convenient for disassembly and assembly of the membrane component.

[0024] The trays are made of corrosion-resistant materials such as CPVC, ABS, and PP, or high-strength organic composite materials. The tray bottoms are supplemented with stainless steel reinforcement ribs and frames.

[0025] The tower plate is provided with water retaining means and evenly distributed water holes, thereby increasing the contact area between the gas phase and the liquid phase and improving the mass transfer efficiency between the two phases.

[0026] The water production system is led out from the membrane module.

[0027] The aeration device is used to aerate the wastewater within the softening deammonification tower. Aeration evenly mixes the liquid within the tower, flushes the membrane surface, and reduces membrane contamination. The wastewater inlet temperature is 60-70°C, and the pH is between 10-12. Ammonia nitrogen often exists as free ammonia, and the aeration device also improves the removal rate of ammonia nitrogen. Under alkaline conditions, at 60-70°C, the aeration device easily removes ammonia from the liquid phase. The distribution of ammonia across multiple tower plates increases the gas-liquid contact interface, further improving deammonification efficiency.

[0028] The ammonia absorption tank is connected to a fiberglass reinforced plastic pipe leading from the top of the tower body. Ammonia is fully absorbed in the ammonia absorption tank. Ammonia collected from the top of the tower is absorbed by dilute sulfuric acid or water in the ammonia absorption tank. A circulating pump is installed in the absorption tank to maintain a certain circulation ratio, so as to better distribute the absorption liquid and improve absorption efficiency.

[0029] When the ammonia absorption tank uses dilute sulfuric acid as the absorbent, the pH at the absorption endpoint is generally 3.5-5.5, preferably 4-5.

[0030] When water is used as the absorbent in the ammonia absorption tank, the concentration of the obtained ammonia water is generally 1-20%. The concentration range of the ammonia water can be determined according to the reuse requirements.

[0031] In one embodiment of the present invention, in step (2), the softening and deamination tower further includes a backwash system and a cleaning system, which are connected to the membrane assembly of the softening and deamination tower for backwashing or cleaning the membrane.

[0032] In one embodiment of the present invention, in step (2), the water production system of the softening and deamination tower includes a water production pipeline leading from the membrane module and a water production pump provided on the water production pipeline.

[0033] In one preferred embodiment of the present invention, a backwash system and a cleaning system are installed on the produced water pipeline. The backwash system includes a backwash tank and a backwash pipeline between the backwash tank and the produced water pipeline. Backwash water in the backwash tank is pumped through a backwash pump to the membrane separation device to clean the membranes and ensure membrane separation efficiency. As needed, the water in the backwash tank can be produced water or a regularly prepared acid, alkali, or sodium hypochlorite solution for backwashing.

[0034] In one embodiment of the present invention, the membrane component is made of a polymer material with excellent acid and alkali resistance, can withstand long-term operation in the pH range of 1-14, has no requirements for chloride ion concentration, and does not have chloride ion corrosion problems.

[0035] In one preferred embodiment of the present invention, the membrane assembly has a pore size of 0.05-0.1 μm. This range of pore sizes can be selected based on specific needs to accommodate a wide range of particle sizes. Utilizing the principle of sieving and filtration, the solid or colloidal particles generated by the reaction are directly trapped outside the membrane fibers, eliminating the need to consider the sedimentation time and water quality fluctuations that affect the precipitated particles in the traditional softening process (reaction-precipitation-filtration-ultrafiltration).

[0036] In one preferred embodiment of the present invention, in one embodiment of the present invention, in step (2), an absorption distributor is provided at the bottom of the ammonia absorption tank, and the normal operating liquid level is controlled to be approximately 2-3 m to improve the ammonia absorption efficiency; the absorbent is pure water or dilute sulfuric acid, and the pH at the absorption endpoint is controlled to allow for timely replenishment of the absorbent. The ammonia absorption tank is auxiliary provided with a circulation pump to ensure that the solution in the ammonia absorption tank is evenly mixed and reaches a certain reuse concentration, and is absorbed by the water or dilute sulfuric acid and converted into ammonia water or ammonium sulfate solution with a maximum concentration of 20%.

[0037] The precipitated suspended solids in the wastewater are removed through the softening and deammoniation process, and softening and deammoniation are carried out simultaneously to obtain softened and deammonified wastewater and pure ammonia water or ammonium sulfate solution.

[0038] In one embodiment of the present invention, in step (2), the gas-water ratio of the softening deamination tower is directly related to the ammonia nitrogen content in the wastewater, as well as the water temperature and pH. When the water temperature is in the range of 60-70°C and the pH is in the range of 10-12, the gas-water ratio of the softening deamination tower is generally 50-2:1, preferably 20-5:1.

[0039] In one embodiment of the present invention, in step (2), preferably, the backwash frequency of the softening deamination tower is 1 minute every 2 hours, the online cleaning cycle is 7 days, and the offline cleaning cycle is 6 months.

[0040] In one embodiment of the present invention, in step (3), most of the wastewater after softening and deammonification is recycled to the gray water system for flushing the lock hopper of the gasifier or the slag pool, and ammonia water or ammonium sulfate solution can be recovered.

[0041] In step (3), when the softened and deammonified wastewater is transported to the sewage treatment system for further biochemical treatment according to the design requirements, the ammonia nitrogen content in the produced water is greatly reduced compared with the original gasified wastewater, which can greatly reduce the treatment cost of the biochemical system (saving 40-50% of electricity consumption, extending the service life of the biochemical aerator, and preventing calcium and magnesium particles from clogging the aeration holes).

[0042] The ammonia water or ammonium sulfate solution obtained based on the method of the present invention has high purity and can be used for reproduction.

[0043] Compared with the prior art, the beneficial effects of the present invention are embodied in the following aspects:

[0044] 1) Softening and denitrification are concentrated in one device, achieving the effects of softening and denitrification simultaneously;

[0045] 2) Only adding softening agents to coal gasification wastewater cannot achieve the effect of removing hardness. This technology can treat complexed calcium and magnesium ions that are difficult to treat with traditional softening methods by adding flocculants at the same time;

[0046] 3) The total hardness of the effluent from the softening and deamination tower is ≤1mmol / L, the silicon removal rate is ≥80%, and the ammonia nitrogen removal rate is ≥80%. The water treatment effect is stable. The effluent from the softening and deamination tower is returned to the lock hopper of the gasifier for flushing or the slag pool to ensure stable operation of the system.

[0047] 4) After the total hardness of the effluent from the softening and deamination tower is reduced, the effluent enters the subsequent biochemical system, avoiding the fouling and clogging of the aeration equipment in the biochemical system and ensuring the stability of the biochemical system;

[0048] 5) The ammonia nitrogen concentration of the softening deammonification effluent is reduced, which greatly reduces the additional carbon source required for subsequent denitrification in the biochemical system and saves the operating cost of the biochemical system;

[0049] 6) The ammonia nitrogen concentration of the softened deammonification effluent is reduced, which reduces the tank capacity of the biochemical system and further saves costs;

[0050] 7) An aeration device is installed at the bottom of the softening deamination tower to reduce the contamination of the bottom membrane assembly. At the same time, under alkaline conditions, at 60-70°C, aeration can easily remove ammonia from the liquid phase, improving the deamination efficiency. The softening deamination tower is equipped with a multi-stage tray distribution to increase the gas-liquid contact interface, further improving the deamination efficiency.

[0051] 8) A gas collecting pipeline is set at the top of the softening deamination tower to connect to the ammonia absorption tank, and an absorption distributor is set at the bottom of the tank. The ammonia absorption tank uses dilute sulfuric acid or water to absorb the ammonia collected at the top of the tower, and a circulating pump is set in conjunction with it to maintain a certain circulation ratio in the absorption tank, so as to better evenly absorb the liquid and improve the absorption efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0052] In order to illustrate the present invention more clearly, the softening deamination tower in the accompanying drawings is briefly introduced below.

[0053] Figure 1 This is a schematic diagram of the softening deamination process;

[0054] Figure 1 Among them, 1. Agitator; 2. Dosing reaction box; 3. Mixed liquid feed pump; 4. Tower body; 5. Tower plate; 6. Membrane assembly; 7. Water production system; 8. Circulation pump; 9. Ammonia absorption tank; 10. Aeration device. DETAILED DESCRIPTION

[0055] The present invention is described in detail below with reference to specific embodiments.

[0056] The softening deamination process involved in the following examples is as follows Figure 1 As shown, it includes a stirrer 1, a dosing reaction box 2, a mixed liquid feed pump 3, a tower body 4, a tower plate 5, a membrane assembly 6, a water production system 7, a circulation pump 8, an ammonia absorption tank 9, and an aeration device 10;

[0057] The dosing reaction box has a circular cone bottom structure and is equipped with a stirring device to ensure that the liquid and the added reagent are completely mixed and react evenly. After the reaction is complete, the sediment at the bottom is discharged by gravity and sent to the sludge treatment system for treatment, and the supernatant is sent to the softening and deamination tower.

[0058] The softening and deamination tower consists of a tower body, tower plates, membrane components, an aeration device, an ammonia absorption tank, and a water production system.

[0059] A membrane assembly is placed at the bottom of the softening deamination tower, and an aeration device is arranged at the bottom of the membrane assembly. Tower plates are staggered on the upper part of the tower body. The tower plates are provided with water retaining and evenly distributed water holes, which increase the contact area between the gas phase and the liquid phase and improve the mass transfer efficiency between the two phases. In addition, the temperature of the waste liquid entering the tower is 60-70°C, which can better maintain the escape velocity of ammonia and improve the removal rate of ammonia nitrogen.

[0060] The upper part of the tower body is made of lightweight material, which can be glass fiber reinforced plastic or other corrosion-resistant materials; the lower part is made of stainless steel, and the membrane component is installed in the lower part; the upper and lower parts of the tower body are connected by flange sealing, and the tower body is divided into two parts, which is more convenient for disassembly and assembly of the membrane component.

[0061] The trays are made of corrosion-resistant organic composite materials such as CPVC, ABS, and PP, which are also relatively strong. The tray bottoms are supplemented with stainless steel reinforcement ribs and a frame.

[0062] The tower plate is provided with water retaining means and evenly distributed water holes, thereby increasing the contact area between the gas phase and the liquid phase and improving the mass transfer efficiency between the two phases.

[0063] The water production system is led out from the membrane module.

[0064] The aeration device is used to aerate the wastewater in the tower body of the softening and deammonification tower. The ammonia absorption tank is connected to the glass fiber reinforced plastic pipe leading out from the top of the tower body, and the ammonia is fully absorbed in the ammonia absorption tank.

[0065] The ammonia absorption tank is equipped with an absorption distributor at the bottom, and the normal operating liquid level is controlled at approximately 2-3 meters to improve ammonia absorption efficiency. The absorbent is pure water or dilute sulfuric acid, and the pH at the absorption endpoint is controlled to ensure timely replenishment of the absorbent. The ammonia absorption tank is equipped with an auxiliary circulation pump to ensure uniform mixing of the solution within the ammonia absorption tank and to achieve a certain reuse concentration. The solution is absorbed by the water or dilute sulfuric acid and converted into ammonia water or ammonium sulfate solution with a maximum concentration of 20%.

[0066] The ammonia absorption tank uses dilute sulfuric acid as the absorbent, and the absorption endpoint pH is generally 3.5-5.5, preferably 4-5.

[0067] The ammonia absorption tank uses water as the absorbent, and the concentration of the obtained ammonia water is generally 1-20%. The concentration range of the ammonia water can be determined according to the reuse requirements.

[0068] The backwash system and the cleaning system are connected to the membrane assembly and are used for backwashing or cleaning the membrane assembly.

[0069] Specifically, the water production system includes a water production line leading from the membrane assembly and a water production pump installed on the water production line. A backwash system and a cleaning system are installed on the water production line. The backwash system includes a backwash tank and a backwash line between the backwash tank and the water production line, and a backwash pump is installed on the backwash line. The cleaning system includes an acid metering tank, an acid dosing pump, an alkali metering tank, an alkali dosing pump, a sodium hypochlorite metering tank, and a sodium hypochlorite dosing pump. Backwash water in the backwash tank flows through the backwash pump to the membrane assembly, cleaning the membranes and ensuring membrane separation efficiency. As needed, the water in the backwash tank can be produced water or a regularly prepared acid, alkali, and sodium hypochlorite solution for backwashing.

[0070] In the following embodiments, the membrane assembly is made of a polymer material with excellent acid and alkali resistance, can withstand long-term operation in the pH range of 1-14, has no requirements for chloride ion concentration, and does not have chloride ion corrosion problems.

[0071] In the following examples, the membrane assembly has a pore size of 0.05-0.1 μm. Different pore size ranges can be selected based on specific needs to meet the requirements of a variety of particle sizes. Utilizing the principle of sieving and filtration, the particles generated by the reaction are directly trapped outside the membrane fibers, eliminating the need to consider the sedimentation time and water quality fluctuations that affect the precipitated particles in the traditional softening process (reaction-precipitation-filtration-ultrafiltration).

[0072] In order to facilitate the understanding of the present invention, the present invention will be described in more comprehensive and detailed manner below using "coal gasification wastewater of a chemical company" as an example, but this does not limit the present invention.

[0073] Example 1:

[0074] A method for treating coal gasification wastewater comprises the following steps:

[0075] (1) Take 40L of coal gasification wastewater;

[0076] (2) After adding 148.9 g of 30% sodium hydroxide solution to adjust the pH to 11, 20 mg / L of flocculant and 70 g of anhydrous sodium carbonate were added in a dosing reaction box and mixed thoroughly for reaction.

[0077] (3) The supernatant of the above mixed liquor is treated in a softening and deamination tower, and the parameters of the softening and deamination tower are controlled as follows: gas-water ratio 50:1, hydraulic retention time 60min.

[0078] (4) Determine the values of calcium, magnesium, silicon and ammonia nitrogen in the effluent.

[0079]

[0080]

[0081] From the data in the above table, it can be seen that after the coal gasification wastewater is softened and deammonified, the total hardness (calcium + magnesium) = 0.28 + 0.03 = 0.31 mmol / L, the hardness removal rate is 98%, the silicon removal rate is 84.1%, and the ammonia nitrogen removal rate is 82.2%.

[0082] Example 2: A method for treating coal gasification wastewater, comprising the following steps:

[0083] (1) Take 40L of coal gasification wastewater;

[0084] (2) After adding 156.3 g of 30% sodium hydroxide solution to adjust the pH to 10, 30 mg / L of flocculant and 70 g of anhydrous sodium carbonate were added in a dosing reaction box to fully react.

[0085] (3) The supernatant of the above mixed liquor is treated in a softening and deamination tower, and the parameters of the softening and deamination tower are controlled as follows: gas-water ratio 30:1, hydraulic retention time 90min.

[0086] (4) Determine the values of calcium, magnesium, silicon and ammonia nitrogen in the effluent.

[0087] Ca Mg Si NH3-N raw water 16.55mmol / L 0.76mmol / L 2.14mmol / L 20.79mmol / L Water 0.39mmol / L 0.01mmol / L 0.32mmol / L 3.25mmol / L Removal rate 85.0％ 84.4％

[0088] From the data in the above table, it can be seen that after the coal gasification wastewater is softened and deammonified, the total hardness (calcium + magnesium) = 0.39 + 0.01 = 0.4 mmol / L, the silicon removal rate = 85.0%, and the ammonia nitrogen removal rate = 84.4%.

[0089] Example 3:

[0090] A method for treating coal gasification wastewater comprises the following steps:

[0091] (1) Take 40L of coal gasification wastewater;

[0092] (2) After adding 30% sodium hydroxide solution to adjust the pH to 12, anhydrous sodium carbonate and 50 mg / L flocculant were added in a dosing reaction box for full reaction.

[0093] (3) The supernatant of the above mixed liquid is treated in a softening and deamination tower, and the parameters of the softening and deamination tower are controlled as follows: gas-water ratio 40:1, hydraulic retention time 30min.

[0094] (4) Determine the values of calcium, magnesium, silicon and ammonia nitrogen in the effluent.

[0095]

[0096]

[0097] From the data in the above table, it can be seen that after the coal gasification wastewater is softened and deammonified, the total hardness (calcium + magnesium) = 0.5 + 0.01 = 0.51 mmol / L, the silicon removal rate = 90.0%, and the ammonia nitrogen removal rate = 85%.

[0098] The above description of the embodiments is intended to facilitate understanding and use of the invention by those skilled in the art. It will be apparent that those skilled in the art can readily make various modifications to these embodiments and apply the general principles described herein to other embodiments without requiring inventive effort. Therefore, the present invention is not limited to the above-described embodiments. Improvements and modifications made by those skilled in the art based on the disclosure of the present invention, without departing from the scope of the present invention, should be within the scope of protection of the present invention.

Claims

1. A method for treating coal gasification wastewater, characterized in that: The following steps are involved: (1) controlling or adjusting the pH value of the coal gasification wastewater to 10.0-12.0, adding a softening agent and a flocculant to the coal gasification wastewater in a dosing reaction box (2), and mixing them evenly using a stirrer (1) to convert calcium, magnesium, and silicon ions in the coal gasification wastewater into solid or colloidal particles; (2) The supernatant after treatment in step (1) enters the softening and deammonification tower through the mixed liquid feed pump (3), and while removing ammonia from the supernatant, the sludge particles generated by softening are filtered and removed through the ultrafiltration membrane device at the bottom of the tower to obtain softened and deammonified wastewater; the ammonia is purified and absorbed using an ammonia absorption tank to obtain a pure ammonia solution or ammonium sulfate solution; (3) The wastewater after softening and deammoniation is partially reused in the gray water system for flushing the lock hopper of the gasifier or the slag pool, or the wastewater after softening and deammoniation is transported to the sewage treatment system for further biochemical treatment according to the design requirements; In step (1), a softening agent and a flocculant are added to the coal gasification wastewater under alkaline conditions of pH 10.0 to 12.0 to remove calcium, magnesium ions and scaling pollutants including silica from the wastewater. The sludge generated after softening is partially settled at the bottom of the dosing reaction box (2) and discharged to the sludge treatment system; and the supernatant of the dosing reaction box (2) enters the softening deamination tower for further solid-liquid separation and ammonia nitrogen removal. The softening agent includes sodium carbonate and sodium hydroxide, and the flocculant includes aluminum salt and iron salt. The dosage of the softening agent in the coal gasification wastewater is adjusted according to the hardness of the wastewater and the scaling tendency of the system, and is 100-500ppm; In step (2), the softening and deamination tower is composed of a tower body (4), a tower plate (5), a membrane assembly (6), an aeration device (10), an ammonia absorption tank (9), and a water production system (7); A membrane assembly (6) is placed at the bottom of the tower body (4) of the softening deamination tower, and an aeration device (10) is arranged at the bottom of the membrane assembly (6). Tower plates (5) are staggeredly installed on the upper part of the tower body (4). The tower plates (5) are provided with water retaining and evenly distributed water holes to increase the contact area between the gas phase and the liquid phase, and at the same time can improve the mass transfer efficiency between the two phases; The upper part of the tower body (4) is made of lightweight material, the lower part is made of stainless steel, and the membrane assembly (6) is installed in the lower part; the upper and lower parts of the tower body (4) are connected by flange sealing; the tower plate (5) is made of organic composite material; The membrane assembly (6) adopts an ultrafiltration membrane; The water production system (7) is led out from the membrane module (6); The aeration device (10) is used to perform aeration treatment on the wastewater in the tower body (4) of the softening and deammonification tower. The ammonia absorption tank (9) is connected to a pipeline leading out from the top of the tower body (4). Ammonia is fully absorbed in the ammonia absorption tank (9). The total hardness of the effluent from the softening and deamination tower is ≤1mmol / L, the silicon removal rate is ≥80%, and the ammonia nitrogen removal rate is ≥80%.

2. The method for treating coal gasification wastewater according to claim 1, characterized in that: In step (2), the softening and deamination tower further comprises a backwashing system and a cleaning system for the membrane assembly (6), which are used to backwash or clean the membrane assembly (6) at the bottom.

3. The method for treating coal gasification wastewater according to claim 1, characterized in that: An absorption distributor is provided at the bottom of the ammonia absorption tank (9), and the normal operating liquid level is controlled to be 2-3 m, so as to improve the absorption efficiency of ammonia. The absorbent in the ammonia absorption tank (9) is pure water or dilute sulfuric acid, and the pH at the absorption end point is controlled so as to replenish the absorbent in time.

4. The method for treating coal gasification wastewater according to claim 3, characterized in that: The ammonia absorption tank (9) is assisted by a circulation pump (8) to ensure that the solution in the ammonia absorption tank (9) is evenly mixed and reaches a certain reuse concentration; When the ammonia absorption tank (9) uses dilute sulfuric acid as the absorbent, the absorption endpoint pH is 3.5-5.5; When water is used as the absorbent in the ammonia absorption tank (9), the concentration of the obtained ammonia water is 1-20%.

5. The method for treating coal gasification wastewater according to claim 1, characterized in that: In step (2), the precipitated suspended solids in the wastewater are removed by a softening and deammoniation process, and the ammonia nitrogen is purified and absorbed simultaneously to obtain the softened and deammonified wastewater and a pure ammonia solution or ammonium sulfate solution.

6. The method for treating coal gasification wastewater according to claim 1, characterized in that: In step (2), when the water temperature of the supernatant is in the range of 60-70°C and the pH is in the range of 10-12, the gas-water ratio of the softening deamination tower is selected to be 50-2:

1.

7. The method for treating coal gasification wastewater according to claim 1, characterized in that: In step (2), the backwash frequency of the ultrafiltration membrane in the softening deamination tower is 1 minute every 2 hours, the online cleaning cycle is 30 days, and the offline cleaning cycle is 6 months.

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