Treatment process and system for phosphorus- and fluorine-containing wastewater in wet-process phosphoric acid production

Abstract

The present invention relates to a treatment process and system for phosphorus-fluorine-containing wastewater in wet-process phosphoric acid production, which solves the problems of long cycle, high equipment investment and operation cost, and removal efficiency that exist in existing phosphorus-fluorine-containing wastewater treatment. The technical solution includes at least two stages of neutralization and sedimentation equipment connected in series, and the neutralization and sedimentation equipment consists of a neutralization tank and a sedimentation tank provided with an agitator, characterized in that the upper section of the neutralization tank is provided with an overflow outlet, and the cone section is provided with a wastewater inlet and an additive inlet; the upper section of the sedimentation tank is provided with an overflow inlet and a clear liquid outlet, and the cone bottom is provided with a slurry outlet, and the sedimentation tank is composed of a cylindrical section, a rake bucket section, and a deep cone section from top to bottom. The present invention has a simple structure, can quickly and evenly mix materials, has high reaction efficiency, a short process cycle, low energy consumption, is easy to maintain and clean, and has low investment and operation costs.

Description

Technical Field

[0001] The present invention relates to the field of wastewater treatment, in particular to a treatment process and system for phosphorus- and fluorine-containing wastewater in wet-process phosphoric acid production. Background Art

[0002] Currently, in typical phosphorus chemical enterprises, the reslurry tank of the phosphoric acid unit enters the slag yard. The slag yard return water and the acid circulating water station drainage are combined and then enter the phosphoric acid unit. Because the gypsum slurry in the reslurry tank contains a small amount of P2O5 and F, after long-term operation, the P2O5 and F content in the slag yard return water and the acid circulating water station drainage gradually accumulates, reaching 0.8-1.0% P2O5 and 3000-5000ppm F.

[0003] Wastewater treatment for phosphorus chemical companies has always been a difficult problem. On the one hand, due to the recycling of slag yard return water and acidic circulating water station drainage, the phosphorus and fluorine content in the slag yard water and acidic circulating water of the acidic circulating water station gradually accumulates and continues to rise, posing a risk of environmental pollution to the water bodies around the slag yard; the increased F content in the acidic circulating water station leads to a deterioration of the production environment and a decline in occupational hygiene, affecting the health of employees; the phosphorus and fluorine content in the slag yard return water and acidic circulating water leads to a waste of phosphorus resources; at the same time, a large amount of fresh water is required for tail gas washing of phosphoric acid and phosphate fertilizer equipment, and the slag yard return water and acidic circulating water drainage are not suitable for tail gas washing due to their high phosphorus and fluorine content.

[0004] For this type of phosphorus-fluorine-containing wastewater, a multi-stage neutralization and precipitation method is used for treatment. For example, patent document No. 104445730B discloses a comprehensive utilization method for phosphorus-fluorine-containing wastewater. By neutralizing the phosphorus-fluorine-containing wastewater twice and controlling the end-point pH value after each neutralization, calcium fluoride in the wastewater is separated according to the solubility product of calcium fluoride and calcium phosphate. This method not only maximizes the recovery and utilization of phosphorus and fluorine resources, but also produces a by-product of silica gel, thereby increasing the added value of the product and reducing the cost of treating phosphorus-fluorine-containing wastewater. For example, Publication No. 105712529A discloses a method for treating high-concentration acidic phosphorus and fluorine wastewater, which adopts three-stage neutralization + three-stage flocculation and sedimentation to treat the wastewater, and treats the wastewater by controlling the process conditions of each stage of neutralization reaction. The specific methods include: ① Controlling the process indicators of the first-stage neutralization: pH is 2.4-4.0, reaction time ≮40min, temperature ≮40°C; ② Controlling the process indicators of the second-stage neutralization: pH is 5.0-7.0, reaction time ≮50min, water temperature ≮30°C; ③ Controlling the process indicators of the third-stage neutralization: pH is 9.6-10.5, reaction time ≮40min, water temperature ≮30°C.

[0005] It can be seen that in the existing technology, the treatment of phosphorus-fluorine-containing wastewater mainly focuses on the control of reaction conditions. However, in fact, the existing treatment of phosphorus-fluorine-containing wastewater still has the following problems: (1) The neutralization tank is usually top-fed. This feeding method is not conducive to the rapid dispersion and uniform mixing of materials in the neutralization tank, which greatly affects the reaction efficiency. If it cannot be mixed quickly, it is very likely that it will not have time to react and will flow into the sedimentation tank from the overflow port; (2) In order to draw out slurry with higher solid content and reduce the burden on subsequent equipment such as filter presses, the cone of the sedimentation tank is usually a deep cone. This deep cone requires a longer sedimentation time, a greater depth, and a low sedimentation rate, which leads to a long operating cycle and a high equipment height, which increases investment and operating costs and makes maintenance and cleaning inconvenient. Summary of the Invention

[0006] The purpose of the present invention is to solve the above technical problems and provide a treatment system for phosphorus-fluorine-containing wastewater in wet-process phosphoric acid production with a simple structure, rapid and uniform mixing of materials, high reaction efficiency, short process cycle, low energy consumption, easy maintenance and cleaning, and low investment and operating costs.

[0007] The present invention also provides a process for treating phosphorus- and fluorine-containing wastewater in wet-process phosphoric acid production using the above-mentioned treatment system.

[0008] The system of the present invention includes at least two stages of neutralization and sedimentation equipment connected in series, and the neutralization and sedimentation equipment consists of a neutralization tank equipped with a stirrer and a sedimentation tank. The upper section of the neutralization tank is provided with an overflow outlet, and the cone section is provided with a wastewater inlet and an additive inlet; the upper section of the sedimentation tank is provided with an overflow inlet and a clear liquid outlet, and the cone bottom is provided with a slurry outlet. The sedimentation tank consists of a cylindrical section, a rake bucket section and a deep cone section from top to bottom.

[0009] A first insertion pipe connected to the wastewater inlet is provided in the cone portion of the neutralization tank. The first insertion pipe is a cross structure, and holes are evenly opened on the pipe surface.

[0010] The depth of the first insertion tube is 20-40% of the depth of the cone portion.

[0011] The additive inlet includes at least a slaked lime milk inlet, which includes a first slaked lime milk inlet located at the bottom end of the cone and a second slaked lime milk inlet in the middle of the cone. A second insertion tube connected to the second slaked lime milk inlet is provided in the cone of the neutralization tank. The second insertion tube has a cross structure and holes are evenly opened on the surface of the pipe.

[0012] The depth of the second insertion tube is 80-90% of the depth of the cone

[0013] The additive inlet also includes a dilute phosphoric acid inlet.

[0014] The rake bucket section of the sedimentation trough has a rotating rake structure, the cone angle of which is 120°-150°, and the cone angle of the deep cone section is 10°-20°.

[0015] The depth ratio of the rotating rake section to the deep cone section is 1:3-1:6.

[0016] The present invention provides a treatment process for phosphorus-fluorine-containing wastewater in wet-process phosphoric acid production, comprising the steps of: feeding the phosphorus-fluorine-containing wastewater into the above-mentioned treatment system for treatment, wherein the phosphorus-fluorine-containing wastewater first enters a primary neutralization tank in a primary neutralization and sedimentation device through a wastewater inlet at a conical portion, and simultaneously introducing slaked lime milk and dilute phosphoric acid through an additive inlet at the conical portion; the wastewater, slaked lime milk, and dilute phosphoric acid react in the primary neutralization tank; under the action of an agitator, the reacted slurry overflows through an overflow outlet at an upper section and enters a primary sedimentation tank for sedimentation; solid matter in the slurry first rapidly settles in a rotating rake section, then is further collected downward through a deep conical section, and finally discharged through the bottom of the cone; and the primary clear liquid at the upper section of the primary sedimentation tank is fed into a secondary neutralization and sedimentation device;

[0017] The primary clear liquid first enters the secondary neutralization tank in the secondary neutralization and sedimentation equipment through the wastewater inlet of the cone, and at the same time, the slaked lime milk is introduced through the additive inlet of the cone. The wastewater and slaked lime milk react in the secondary neutralization tank. Under the action of the agitator, the reacted slurry overflows from the overflow outlet of the upper section into the secondary sedimentation tank for sedimentation. The solid matter in the slurry first settles rapidly through the rotating rake section, and then is further collected through the deep cone section downward and finally discharged through the bottom of the cone. The secondary clear liquid in the upper section of the secondary sedimentation tank is sent out as recycled water.

[0018] In the primary intermediate tank and the secondary neutralization tank, the phosphorus-fluorine-containing wastewater or the primary clear liquid is introduced into the cone portion through the first insertion pipe from the corresponding wastewater inlet; after adding dilute phosphoric acid, the P2O5 concentration in the slurry in the primary neutralization tank is controlled to reach 1-5%wt.

[0019] The additive inlet includes a first slaked lime milk inlet located at the bottom end of the cone and a second slaked lime milk inlet located in the middle section of the cone. The slaked lime milk is divided into two streams, one stream is fed in from the first slaked lime milk inlet at the bottom end of the cone, and the other stream is fed in from the second slaked lime milk inlet in the middle section of the cone through the second insertion pipe.

[0020] The mass ratio of the first stream of slaked lime milk to the second stream of slaked lime milk is 60%:40%.

[0021] In view of the problems existing in the background technology, the inventors have made the following improvements:

[0022] (1) All material inlets are changed from the top of the neutralization tank to the cone, and the bottom feeding method is adopted. On the one hand, the overflow outlet is avoided, and on the other hand, the bottom feeding is easier to disperse quickly and evenly under the action of the agitator. Furthermore, considering the problem that the amount of wastewater introduced into the neutralization tank is large and difficult to disperse quickly, a special insertion pipe structure is adopted, namely a cross-cross structure. By uniformly opening holes on the surface of the pipe, the introduced wastewater can be quickly and evenly dispersed, and the dispersion efficiency is greatly improved, thereby increasing the reaction contact surface, accelerating the reaction efficiency, and reducing the reaction residence time. Furthermore, the slaked lime milk is also divided into two streams and fed in. One stream is directly introduced from the bottom of the cone, and the other stream is introduced through the second insertion pipe of the cross-cross structure in accordance with the wastewater introduction method. By controlling the second insertion pipe to be higher than the first insertion pipe, the wastewater can be quickly mixed and reacted with the slaked lime milk above and below when it is introduced, ensuring sufficient reaction and greatly reducing the reaction residence time.

[0023] 2) A two-stage cone bottom structure is creatively adopted in the sedimentation tank, that is, a rake bucket is used in the first stage and a deep cone is used in the first stage. Combining the advantages of the two, the rake bucket structure is used to improve the material sedimentation rate, and the sedimentation material is quickly pushed into the deep cone. The deep cone is then used to increase the solid content of the discharged material, thereby reducing the filter pressure load of subsequent equipment. The above structure can greatly reduce the depth of the deep cone, the overall height of the sedimentation tank can be reduced by more than 10-30%, and the sedimentation rate is increased by more than 20-40%, with significant technical effects. Preferably, the cone angle of the rake bucket section is 120°-150° to meet the sedimentation rate; the cone angle of the deep cone section is 10°-20° to increase the solid content of the bottom slurry; the depth ratio of the rotating rake section to the deep cone section is 1:3-1:6. If the ratio is too large, the equipment height will be larger and the civil engineering investment will increase. If the ratio is too small, the solid content of the bottom slurry will be too low.

[0024] The two-stage overflow neutralization and sedimentation method of the present invention effectively ensures rapid and sufficient material reaction, accelerates sedimentation rate, shortens production cycle, reduces the load on subsequent processes, and improves phosphorus and fluorine removal efficiency. The system structure of the present invention is extremely simple and compact, with a low frame installation height, low investment and operating costs, and easy maintenance and cleaning. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 It is a structural schematic diagram of the present invention.

[0026] Figure 2 Schematic top view of the first insertion tube.

[0027] Figure 3 Schematic top view of the second insertion tube.

[0028] Figure 4 Schematic diagram of the installation of the first insertion tube and the second insertion tube.

[0029] Among them, 1-first-level neutralization tank, 1.1-wastewater inlet, 1.2-overflow outlet, 1.3-first lime milk inlet, 1.4-second lime milk inlet, 1.5-dilute phosphoric acid inlet, 2-agitator, 3-rotating rake, 4-first-level sedimentation tank, 4.1-cylinder section, 4.2-rake bucket section, 4.3-deep cone section, 4.4-overflow inlet, 4.5-clear liquid outlet, 4.6-slurry outlet, 5-secondary neutralization tank, 6-first insertion pipe, 7-secondary sedimentation tank, 8-second insertion pipe. DETAILED DESCRIPTION

[0030] The present invention will be further explained below with reference to the accompanying drawings:

[0031] See also Figure 1-Figure 3 The system of the present invention includes at least two stages of neutralization and sedimentation equipment connected in series. The structures of the two-stage neutralization equipment are basically the same. Taking the first-stage neutralization and sedimentation equipment as an example, it includes a first-stage neutralization tank 1 provided with a stirrer 2 and a first-stage sedimentation tank 4. The upper section of the neutralization tank 1 is provided with an overflow outlet 1.2, and the cone is provided with a wastewater inlet 1.1 and an additive inlet. The additive inlet includes a first slaked lime milk inlet 1.3 located at the bottom of the cone and a second slaked lime milk inlet 1.4 and a dilute phosphoric acid inlet 1.5 in the middle section of the cone; a first insertion tube 6 connected to the wastewater inlet 1.1 and a second insertion tube 8 connected to the second slaked lime milk inlet 1.4 are provided in the cone of the first-stage neutralization tank 1. Both the first and second insertion tubes are cross structures, and holes are evenly opened on the pipe surface. The second insertion tube 8 is higher than the first insertion tube 6. Preferably, see Figure 4 The depth of the first insertion tube 8 is 20-40% of the cone depth h, and the depth of the second insertion tube 8 is 80-90% of the cone depth h.

[0032] The primary sedimentation tank 4 consists, from top to bottom, of a cylindrical section 4.1, a rake bucket section 4.2, and a deep cone section 4.3. The rake bucket section 4.2 has a cone angle of 120°-150° and includes a rotating rake 3 driven by a drive device. The deep cone section has a cone angle of 10°-20°. Preferably, the depth ratio of the rotating rake section 4.2 to the deep cone section 4.3 is 1:3-1:6. Within this range, the height of the equipment can be minimized while effectively increasing the sedimentation rate. The upper section of the cylindrical section 4.1 is provided with an overflow inlet 4.4 and a clear liquid outlet 4.5, and the bottom of the deep cone section is provided with a slurry outlet 4.6.

[0033] The structures of the primary neutralization and sedimentation equipment and the secondary neutralization and sedimentation equipment are basically the same, the only difference being that the secondary neutralization tank 5 is not provided with the dilute phosphoric acid inlet 1.5.

[0034] Process:

[0035] In this embodiment, the phosphorus-fluorine-containing wastewater can be slag yard return water and / or acid circulating water station drainage. The phosphorus-fluorine-containing wastewater is evenly sprayed out through the first insertion pipe 6 through the wastewater inlet 1.1 of the primary neutralization tank 1. At the same time, the slaked lime milk is divided into two streams, which are respectively added through the first slaked lime milk inlet 1.3 and the second slaked lime milk inlet 1.4. The second stream of slaked lime milk is evenly sprayed out through the second insertion pipe 8. At the same time, dilute phosphoric acid is added through the dilute phosphoric acid inlet 1.5 in the middle section of the cone. When the phosphorus-fluorine-containing wastewater enters the bottom of the primary neutralization tank 1, it will be quickly and evenly sprayed out through the first insertion pipe 6. At the same time, it will be quickly and evenly mixed with the slaked lime milk and dilute phosphoric acid evenly sprayed out from the upper part and introduced from the lower part. Under the action of the agitator 2, it will continuously react to generate C After adding dilute phosphoric acid, the P2O5 concentration in the slurry in the primary neutralization tank is controlled to reach 1-5%wt and the pH is 2-3, which helps to increase the neutralization rate and accelerate the neutralization reaction. The reacted slurry flows out of the overflow outlet 1.2 and enters the primary settling tank 4 from the top. There, it settles. Under the action of the rake 3 in the rake bucket section 4.2, the high-solids particles in the slurry are rapidly settled and continuously pushed into the deep cone section 4.3. In the deep cone section 4.3, further solid-liquid separation is carried out. Finally, the high-solids slurry is drawn out from the bottom of the deep cone section 4.3 to downstream processes, such as the reslurry tank of the phosphoric acid unit.

[0036] The primary clear liquid at the top of the primary sedimentation tank 4 first enters the secondary neutralization tank 5 in the secondary neutralization sedimentation equipment, and enters from the wastewater inlet of its cone. At the same time, the slaked lime milk is also divided into two streams and introduced from the cone. The wastewater and slaked lime milk react in the secondary neutralization tank 5 under the action of the agitator 2 to generate DCP, and the pH is controlled to be 3-5. The slurry after the reaction overflows from the overflow outlet of the upper section and enters the secondary sedimentation tank 7 for sedimentation. The solid matter in the slurry is first rapidly settled by the rotary rake section, and then further collected by the deep cone section downward and finally discharged from the bottom of the cone. The secondary clear liquid in the upper section of the secondary sedimentation tank 7 is sent as recycled water for tail washing of phosphoric acid, MAP, DAP, NPK and other devices or as tail washing make-up water for acidic circulating water station, acidic circulating water station make-up water instead of fresh water, thereby reducing the amount of fresh water. The high-solid slurry discharged from the bottom is sent to the downstream process as DCP slurry.

[0037] Compared with the traditional wastewater treatment system which also uses two-stage neutralization and sedimentation equipment in series, with top feeding in the neutralization tank and a deep cone structure at the bottom of the sedimentation tank, the comparative results of the treatment of phosphorus-fluorine-containing wastewater using the method of the present invention are as follows:

[0038]

[0039] Note: The wastewater contains P2O5 8000ppm, F 3000ppm, and the wastewater volume is 120m 3 / h.

Claims

1. A system for treating phosphorus-fluorine-containing wastewater in wet-process phosphoric acid production, comprising at least two stages of neutralization and sedimentation equipment connected in series, wherein the neutralization and sedimentation equipment comprises a neutralization tank equipped with an agitator and a sedimentation tank, characterized in that: The upper section of the neutralization tank is provided with an overflow outlet, and the cone section is provided with a wastewater inlet and an additive inlet; the upper section of the sedimentation tank is provided with an overflow inlet and a clear liquid outlet, and the cone bottom is provided with a slurry outlet. The sedimentation tank is composed of a cylindrical section, a rake bucket section and a deep cone section from top to bottom; The cone portion of the neutralization tank is provided with a first insertion tube connected to the wastewater inlet, the first insertion tube is a cross-shaped structure, and the surface of the pipe is evenly opened; The additive inlet includes at least a slaked lime milk inlet, which includes a first slaked lime milk inlet located at the bottom end of the cone and a second slaked lime milk inlet located in the middle of the cone. A second insertion pipe connected to the second slaked lime milk inlet is provided in the cone of the neutralization tank. The second insertion pipe has a cross structure and holes are evenly opened on the pipe surface. The depth of the first insertion tube is 20-40% of the depth of the cone portion, the depth of the second insertion tube is 80-90% of the depth of the cone portion, and the second insertion tube is higher than the first insertion tube.

2. The system for treating phosphorus-fluorine-containing wastewater in wet-process phosphoric acid production according to claim 1, characterized in that: The additive inlet also includes a dilute phosphoric acid inlet.

3. The system for treating phosphorus-fluorine-containing wastewater in wet-process phosphoric acid production according to claim 1, characterized in that: The rake bucket section of the sedimentation trough has a rotating rake structure, the cone angle of which is 120°-150°, and the cone angle of the deep cone section is 10°-20°.

4. The system for treating phosphorus-fluorine-containing wastewater in wet-process phosphoric acid production according to claim 3, characterized in that: The depth ratio of the rotating rake section to the deep cone section is 1:3-1:

6.

5. A process for treating phosphorus-fluorine-containing wastewater in wet-process phosphoric acid production, characterized in that: The phosphorus-fluorine-containing wastewater is sent to the treatment system according to any one of claims 1 to 4 for treatment, wherein the phosphorus-fluorine-containing wastewater first enters the primary neutralization tank in the primary neutralization and sedimentation equipment through the wastewater inlet of the cone, and at the same time, slaked lime milk and dilute phosphoric acid are introduced through the additive inlet of the cone, and the wastewater, slaked lime milk and dilute phosphoric acid react in the primary neutralization tank. Under the action of the agitator, the slurry after the reaction overflows from the overflow outlet of the upper section into the primary sedimentation tank for sedimentation, and the solid matter in the slurry is first rapidly settled in the rotating rake section, and then further collected through the deep cone section downward and finally discharged through the bottom of the cone, and the primary clear liquid in the upper section of the primary sedimentation tank is sent to the secondary neutralization and sedimentation equipment; The primary clear liquid first enters the secondary neutralization tank in the secondary neutralization and sedimentation equipment through the wastewater inlet at the cone. At the same time, slaked lime milk is introduced through the additive inlet at the cone. The wastewater and slaked lime milk react in the secondary neutralization tank. Under the action of the agitator, the reacted slurry overflows from the overflow outlet of the upper section into the secondary sedimentation tank for sedimentation. The solid matter in the slurry first settles rapidly in the rotating rake section, then flows downward to the deep cone section for further collection and finally discharged from the bottom of the cone. The secondary clear liquid in the upper section of the secondary sedimentation tank is sent out as recycled water. In the primary neutralization tank and the secondary neutralization tank, the phosphorus-fluorine-containing wastewater or the primary clear liquid is introduced into the cone portion from the corresponding wastewater inlet via the first insertion tube; the additive inlet includes a first slaked lime milk inlet located at the bottom end of the cone portion and a second slaked lime milk inlet in the middle section of the cone portion, and the slaked lime milk is divided into two streams, one stream is fed into the cone portion from the first slaked lime milk inlet at the bottom end of the cone portion, and the other stream is fed into the cone portion from the second slaked lime milk inlet in the middle section of the cone portion via the second insertion tube; after the addition of dilute phosphoric acid, the P2O5 concentration in the slurry in the primary neutralization tank is controlled to reach 1-5%wt.

6. The process for treating phosphorus-fluorine-containing wastewater in wet-process phosphoric acid production according to claim 5, characterized in that: The mass ratio of the first stream of slaked lime milk to the second stream of slaked lime milk is 60%:40%.

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