Device and process for fluorine recovery from smoke after phosphorus absorption by hydration in kiln process for production of phosphoric acid
10232312 ยท 2019-03-19
Assignee
Inventors
Cpc classification
Y02P20/151
GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
B01D2257/204
PERFORMING OPERATIONS; TRANSPORTING
International classification
Abstract
A device and process for fluorine recovery from smoke after phosphorus absorption by hydration in KPA, wherein the device comprises a first-stage and second-stage fluorine absorption tower, which are both fluidized counter-current washing towers. The device according to the present invention has simple structure, low investment cost, high raw material utilization rate, and good fluorine recovery effects.
Claims
1. A process for recovering fluorine from a smoke generated from a Kiln Phosphoric Acid (KPA) process using a device, wherein phosphorus in form of P.sub.2O.sub.5 in the smoke has been absorbed by hydration before recovering fluorine from the smoke, wherein the device comprises a first-stage and second-stage fluorine absorption towers, wherein the first-stage and second-stage fluorine absorption towers both are fluidised counter-current washing towers, and the first-stage fluorine absorption tower mainly comprises a first-stage fluorosilicic acid washing pipe and a first-stage fluorosilicic acid separation tank, and the second-stage fluorine absorption tower mainly comprises a second-stage fluorosilicic acid washing pipe and a second-stage fluorosilicic acid separation tank, wherein the process comprises the following steps: (1) performing a first-stage fluorine recovery, wherein the smoke is introduced into a top of the first-stage fluorosilicic acid washing pipe, and a first fluorosilicic acid solution is introduced into a bottom of the first-stage fluorosilicic acid washing pipe from the first-stage fluorosilicic acid separation tank and is sprayed from the bottom to the top of the first-stage fluorosilicic acid washing pipe, such that the smoke is contacted with the first fluorosilicic acid solution by convection in the first-stage fluorosilicic acid washing pipe, wherein during contacting, mass transfer and heat transfer between the smoke and the first fluorosilicic acid solution occur, and meanwhile fluorine in form of SiF.sub.4 contained in the smoke reacts with water in the first fluorosilicic acid solution to form a fluorosilicic acid which dissolves in the first fluorosilicic acid solution, and an enthalpy of the smoke is partially transferred into the first fluorosilicic acid solution such that water in the first fluorosilicic acid solution becomes a steam by adiabatic evaporation; (2) performing a first-stage gas-liquid separation, wherein a gas and liquid from the first-stage fluorosilicic acid washing pipe are introduced into the first-stage fluorosilicic acid separation tank for gas-liquid separation, and a gas obtained by separation flows into the second-stage fluorosilicic acid washing pipe via a smoke outlet of the first-stage fluorine absorption tower, and a liquid remained in the first-stage fluorosilicic acid separation tank is introduced into the first-stage fluorosilicic acid washing pipe via a first circulating and conveying pipeline provided with a first circulating pump to perform the first-stage fluorine recovery; (3) performing a second-stage fluorine recovery, wherein a smoke from the first-stage gas-liquid separation is introduced into a top of the second-stage fluorosilicic acid washing pipe, a second fluorosilicic acid solution is introduced into a bottom of the second-stage fluorosilicic acid washing pipe from the second-stage fluorosilicic acid separation tank and is sprayed from the bottom to the top of the second-stage fluorosilicic acid washing pipe, such that the smoke from the first-stage gas-liquid separation is contacted with the second fluorosilicic acid solution by convection in the second-stage fluorosilicic acid washing pipe, wherein during contacting, mass transfer and heat transfer between the smoke and the second fluorosilicic acid solution occur, and meanwhile fluorine in form of SiF.sub.4 contained in the smoke reacts with water in the second fluorosilicic acid solution to form a fluorosilicic acid which dissolves in the second fluorosilicic acid solution, and an enthalpy of the smoke is partially transferred into the second fluorosilicic acid solution such that water in the second fluorosilicic acid solution becomes a steam; (4) performing a second-stage gas-liquid separation, wherein a gas and liquid from the second-stage fluorosilicic acid washing pipe are introduced to the second-stage fluorosilicic acid separation tank for gas-liquid separation, and a gas obtained by separation flows into a tail gas absorption tower to be further processed via a smoke outlet of the second-stage fluorine absorption tower, and a part of a liquid remained in the second-stage fluorosilicic acid separation tank is introduced into the second-stage fluorosilicic acid washing pipe via a second circulating and conveying pipeline provided with a second circulating pump to perform the second-stage fluorine recovery, and a part of the liquid remained in the second-stage fluorosilicic acid separation tank is introduced into the first-stage fluorosilicic acid separation tank.
2. The process according to claim 1, wherein in the step (1) a mass concentration and temperature of the first fluorosilicic acid solution in the first-stage fluorine recovery are in the range of 8 to 25 percent and 25 to 65 C., respectively, and a sprayed amount by L of the first fluorosilicic acid solution relative to a volume by m.sup.3 of the smoke introduced into the first-stage fluorosilicic acid washing pipe is controlled in the range of 3 L/m.sup.3 to 25 L/m.sup.3; and in the step (3) a mass concentration and temperature of the second fluorosilicic acid solution in the second-stage fluorine recovery are in the range of 0.5 to 5 percent and 25 to 60 C., respectively, and a sprayed amount by L of the second fluorosilicic acid relative to a volume by m.sup.3 of the smoke introduced into the second-stage fluorosilicic acid washing pipe is controlled in the range of 3 L/m.sup.3 to 25 L/m.sup.3.
3. The process according to claim 2, wherein in the step (1) the mass concentration and temperature of the first fluorosilicic acid solution in the first-stage fluorine recovery are in the range of 10 to 20 percent and 50 to 65 C., respectively, and the sprayed amount by L of the first fluorosilicic acid solution relative to the volume by m.sup.3 of the smoke introduced into the first-stage fluorosilicic acid washing pipe is controlled in the range of 3 L/m.sup.3 to 6 L/m.sup.3; and in the step (3) the mass concentration and temperature of the second fluorosilicic acid solution in the second-stage fluorine recovery are in the range of 0.5 to 5 percent and 45 to 60 C., respectively, and the sprayed amount by L of the second fluorosilicic acid solution relative to the volume by m.sup.3 of the smoke introduced into the second-stage fluorosilicic acid washing pipe is controlled in the range of 3 L/m.sup.3 to 6 L/m.sup.3.
4. The process according to claim 1, wherein the step (3) further comprises cooling the second fluorosilicic acid solution before introducing it into the second-stage fluorosilicic acid washing pipe by a fluorosilicic acid cooler, wherein a temperature of a smoke processed by the step (3) decreases below 60 C.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
(1)
(2)
(3)
(4)
(5)
LISTING OF PARTS
(6) The reference number 1 denotes a hydration tower; 11 a smoke inlet; 12 a smoke outlet; 13 a spraying device; 14 a liquid inlet; 15 a liquid outlet; 16 an acid storage tank; 17 a water-cooling system; 18 an acid cooler; 2 a circulating pump; 21 a pressure filter; 22 a packing filter; 23 a phosphoric acid refining equipment; 24 a concentrated phosphoric acid spraying layer; 25 a dilute phosphoric acid spraying layer; 3 a phosphoric acid mist absorption tower; 31 a washing pipe; 32 a separation tank; 33 an acid outlet; 34 an acid inlet; 35 a nozzle; 4 a demisting separation tower; 41 an online water flushing device; 42 a mesh demister; 43 a phosphoric acid drop collection structure; 5 a first-stage fluorine absorption tower; 51 a fluorosilicic acid washing pipe; 52 a fluorosilicic acid separation tank; 53 a fluorosilicic acid liquid outlet; 54 a fluorosilicic acid refining equipment; 6 a second-stage fluorine absorption tower; 61 a second-stage fluorosilicic acid washing pipe; 62 a second-stage fluorosilicic acid separation tank; 63 a fluorosilicic acid cooler; 7 a tail gas absorption tower; 8 a draught fan.
DETAILED DESCRIPTION
(7) The embodiments of the present invention are described more fully hereinafter with reference to the accompanying drawings, which form a part hereof, and which show, by way of illustration, specific exemplary embodiments by which the invention may be practiced. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be through and complete, and will fully convey the scope of the invention to those skilled in the art. Among other things, the present invention may be embodied as systems, methods or devices. The following detailed description should not to be taken in a limiting sense.
(8) Throughout the specification and claims, the following terms take the meanings explicitly associated herein, unless the context clearly dictates otherwise. The phrase in one embodiment as used herein does not necessarily refer to the same embodiment, though it may. Furthermore, the phrase in another embodiment as used herein does not necessarily refer to a different embodiment, although it may. Thus, as described below, various embodiments of the invention may be readily combined, without departing from the scope or spirit of the invention.
(9) In addition, as used herein, the term or is an inclusive or operator, and is equivalent to the term and/or, unless the context clearly dictates otherwise. The term based on is not exclusive and allows for being based on additional factors not described, unless the context clearly dictates otherwise. In addition, throughout the specification, the meaning of a, an, and the include plural references. The meaning of in includes in and on. The term coupled implies that the elements may be directly connected together or may be coupled through one or more intervening elements. Further reference may be made to an embodiment where a component is implemented and multiple like or identical components are implemented.
(10) While the embodiments make reference to certain events this is not intended to be a limitation of the embodiments of the present invention and such is equally applicable to any event where goods or services are offered to a consumer.
(11) The present invention provides a device for fluorine recovery from smoke after phosphorus absorption by hydration in KPA comprising a first-stage and second-stage fluorine absorption tower, which are both fluidised counter-current washing towers. The first-stage fluorine absorption tower mainly comprises a fluorosilicic acid washing pipe and a fluorosilicic acid separation tank; wherein the fluorosilicic acid washing pipe inlet and outlet are connected to a conveying pipeline of smoke after phosphorus absorption by hydration and the middle part of the fluorosilicic acid separation tank, respectively; the top of the separation tank is provided with a smoke outlet, and at the bottom thereof a fluorosilicic acid liquid outlet is connected to a nozzle in the fluorosilicic acid washing pipe via a circulating and conveying pipeline which is provided with a circulating pump.
(12) The second-stage fluorine absorption tower mainly comprises a second-stage fluorosilicic acid washing pipe and a second-stage fluorosilicic acid separation tank. The smoke outlet of the first-stage fluorine absorption tower is connected to the inlet of second-stage fluorosilicic acid washing pipe via a pipeline and the outlet of fluorosilicic acid washing pipe is connected to the middle part of second-stage fluorosilicic acid separation tank. The top of the second-stage fluorosilicic acid separation tank is provided with a defoaming layer and a smoke outlet, and at the bottom thereof a fluorosilicic acid liquid outlet is in communication with a nozzle in the second-stage fluorosilicic acid washing pipe and first-stage fluorosilicic acid separation tank via a circulating and conveying pipeline which is provided with a circulating pump.
(13) In an embodiment of the present invention, the circulating and conveying pipeline in second-stage fluorine absorption tower is provided with a fluorosilicic acid cooler. The outlet of the fluorosilicic acid cooler is divided into two paths, one is connected to a nozzle in the second-stage fluorosilicic acid washing pipe and another is in communication with the spraying layer which is at the top of second-stage fluorosilicic acid separation tank.
(14) In an embodiment of the present invention, the smoke inlet of the second-stage fluorine absorption tower is connected to a tail gas absorption tower which is an empty spraying tower. The top of the tail gas absorption tower is provided with a smoke outlet, and the upper part of inside thereof is provided with a spraying layer, and the bottom thereof is provided with an alkali absorption tank, and the outlet of the alkali absorption tank is connected to a spraying layer in the tail gas absorption tower via a circulating and conveying pipeline which has a circulating pump.
(15) The present invention also provides a process for fluorine recovery from smoke after phosphorus absorption by hydration in KPA using the device described above which comprises the following steps:
(16) (1) The first-stage fluorine recovery: contacting the smoke after phosphorus absorption by hydration flowing downward in the fluorosilicic acid washing pipe in the first-stage fluorine absorption tower with circulating fluorosilicic acid sprayed upward by a nozzle, and forming fluorosilicic acid after mass transfer and heat transfer between said smoke and circulating fluorosilicic acid, and the enthalpy in the smoke is partially transferred into the steam by adiabatic evaporation of water in circulating fluorosilicic acid solution;
(17) (2) The first-stage gas-liquid separation: conveying the gas and liquid in the fluorosilicic acid washing pipe to the fluorosilicic acid separation tank for gas-liquid separation, and the gas obtained by separation flows into second-stage fluorosilicic acid washing pipe of the second-stage fluorine absorption tower via the smoke outlet of first-stage fluorine absorption tower, and conveying the remaining liquid in the fluorosilicic acid separation tank to fluorosilicic acid washing pipe for the operation in step (1) via a circulating and conveying pipeline, wherein the circulating and conveying pipeline has a circulating pump;
(18) (3) The second-stage fluorine recovery: contacting the smoke flowing downward in the second-stage fluorosilicic acid washing pipe with circulating fluorosilicic acid sprayed upward by a nozzle, and forming fluorosilicic acid after mass transfer and heat transfer between said smoke and circulating fluorosilicic acid, and the enthalpy in the smoke is partially transferred into circulating fluorosilicic acid solution by heat transfer;
(19) (4) The second-stage gas-liquid separation: conveying the gas and liquid in the second-stage fluorosilicic acid washing pipe to the second-stage fluorosilicic acid separation tank for gas-liquid separation, and the gas obtained by separation flows into the tail gas absorption tower to be further processed via the smoke outlet of second-stage fluorine absorption tower, and the remaining liquid in the fluorosilicic acid separation tank is partially transferred to second-stage fluorosilicic acid washing pipe for the operation in step (3) by a circulating pump and the rest of the liquid said above is transferred to fluorosilicic acid separation tank of the first-stage fluorine absorption tower;
(20) (5) As the fluorosilicic acid solution in the first-stage fluorine absorption tower keeps increasing, removing silica gel in the extra fluorosilicic acid solution by filtered, and obtaining a byproduct of fluorosilicic acid.
(21) In an embodiment of the present invention, in the step (1) the mass concentration and temperature of circulating fluorosilicic acid solution in first-stage fluorine recovery are in the range of 8 to 25 percent (especially, 10 to 20 percent) and 25 C. to 65 C. (especially, 50 C. to 65 C.), respectively, and the spraying liquid-gas ratio is controlled in the range of 3 L/m.sup.3 to 25 L/m.sup.3 (more particularly, 3 L/m.sup.3 to 6 L/m.sup.3); in the step (3), the mass concentration and temperature of circulating fluorosilicic acid solution in second-stage fluorine recovery are in the range of 0.5 to 5 percent and 25 C. to 60 C. (especially, 45 C. to 60 C.), respectively, and the spraying liquid-gas ratio is controlled in the range of 3 L/m.sup.3 to 25 L/m.sup.3 (especially, 3 L/m.sup.3 to 6 L/m.sup.3). In an embodiment of the present invention, in the step (3) the circulating fluorosilicic acid solution in the second-stage fluorosilicic acid washing pipe is cooled by the fluorosilicic acid cooler, and the temperature of smoke which has been processed by step (3) decreases below 60 C.
(22) In an embodiment of the present invention, the said smoke after phosphorus absorption by hydration is obtained after hydration absorption in phosphoric acid production system which comprises a hydration tower, a circulating and spraying system for acid, a phosphoric acid mist absorption tower and a demisting separation tower; In an embodiment of the present invention, in step (1) the fluorine in the smoke (mainly SiF.sub.4), reacts with water in circulating fluorosilicic acid solution to form the said fluorosilicic acid.
(23) The said hydration tower is an empty spraying tower, and the lower part of the hydration tower is provided with a smoke inlet for the smoke exiting the kiln, and the top thereof is provided with a smoke outlet, and the bottom thereof is provided with a liquid inlet of the said circulating and spraying system for acid, and the outlet of the circulating and spraying system for acid is connected to the inlet pipe of a spraying device disposed in a chamber which is above the smoke inlet, and the circulating and spraying system for acid is also provided with an acid storage tank and a circulating pump; The said phosphoric acid mist absorption tower is a fluidised counter-current washing tower which mainly comprises a washing pipe and a separation tank, the inlet and outlet of the washing pipe are connected to a smoke outlet of the said hydration tower and the middle part of the separation tank, respectively, and the top of the separation tank is provided with a smoke outlet, and at the bottom thereof an acid outlet is connected to a nozzle in washing pipe via a circulating and conveying pipeline which is provided with a circulating pump; A smoke outlet of the said phosphoric acid mist absorption tower is connected to the lower part of the said demisting separation tower which is provided with an online water flushing device, and the top of the demisting separation tower is provided with a smoke outlet which is used for exhausting of the smoke after phosphorus absorption by hydration, and at the bottom thereof an acid outlet is connected to an acid inlet of the phosphoric acid mist absorption tower via a pipeline.
(24) In an embodiment of the present invention, the hydration tower comprises a cooling system, wherein the cooling system comprises the following structure a) and/or b):
(25) a), the outside wall of the chamber in hydration tower is coated by a water-cooling system;
(26) b), on the position near a liquid inlet in the circulating and spraying system for acid is provided with an acid cooler.
(27) In an embodiment of the present invention, the upper part of the demisting separation tower is provided with a mesh demister, and the lower part thereof is provided with a phosphoric acid drop collection structure which is similar to a cyclone duster, and an online water flushing device is installed above the mesh demister.
(28) In an embodiment of the present invention, the spraying device comprises at least two spraying layers which are located at different heights in the chamber of hydration tower, respectively, and the spraying device comprises at least a concentrated phosphoric acid spraying layer and at least a dilute phosphoric acid spraying layer, and the concentrated phosphoric acid spraying layer is located above the dilute phosphoric acid spraying layer; and a liquid inlet of concentrated phosphoric acid spraying layer is connected to the circulating and spraying system for acid; and a liquid inlet of dilute phosphoric acid spraying layer is in communication with a circulating and conveying pipeline in the phosphoric acid mist absorption tower; the conveying pipeline behind the circulating pump in the circulating and spraying system for acid is connected to an acid inlet of the phosphoric acid mist absorption tower via a branch pipe.
(29) The structure of the entire device for fluorine recovery according to the present invention is much more simplified and rational after lots of work in the improvement and optimization of the structure and connection relationship, and the device said satisfies the requirements of the process for phosphorus absorption by hydration. The device for fluorine recovery according to the present invention also has a strong adaptability for the adjustment and optimization of the integrated performance of the device according to the specific requirements of economy, environmental performance, investment costs of the process.
(30) The device for fluorine recovery according to the present invention has a greatly simplified system structure, and decreased cost of investment, operating and maintenance for device with the same functionality effect.
(31) In the preferred solution of the present invention, the optimized limitation of preparation source of smoke after phosphorus absorption by hydration ensures the absorption of both P.sub.2O.sub.5 and fluorine in the smoke exiting the kiln and the effective cooperation of process for phosphorus absorption by hydration and the process for fluorine recovery. The main product of phosphoric acid and byproduct of fluorosilicic acid obtained have a great value which ensures a much more effective utilization of raw material source and improvement of the economic benefits of KPA.
(32) In the preferred solution of the present invention, the nearly zero emissions of waste gas, waste materials and waste liquid in preferred solution of the present invention make the process environment friendly.
(33) The device provided by the present invention is fully applicable for direct production of phosphoric acid using low-grade phosphate ore which will be of great importance on the effective utilization of low-grade phosphate ore.
EXAMPLES
Example 1
(34) A Device for Fluorine Recovery from Smoke after Phosphorus Absorption by Hydration in KPA
(35) As shown in
(36) The main structure of the second-stage fluorine absorption tower 6 is similar to that of the first-stage fluorine absorption tower 5, which mainly comprises a second-stage fluorosilicic acid washing pipe 61 and a second-stage fluorosilicic acid separation tank 62. A smoke outlet 12 of the first-stage fluorine absorption tower 5 is connected to an inlet of second-stage fluorosilicic acid washing pipe 61 via a pipeline and an outlet of fluorosilicic acid washing pipe 61 is connected to the middle part of second-stage fluorosilicic acid separation tank 62. The top of the second-stage fluorosilicic acid separation tank 62 is provided with a defoaming layer and a smoke outlet 12, and at the bottom thereof a fluorosilicic acid liquid outlet 53 is in communication with a nozzle 35 in the second-stage fluorosilicic acid washing pipe 61 via a circulating and conveying pipeline which is provided with a circulating pump 2.
(37) A circulating and conveying pipeline of second-stage fluorine absorption tower 6 is provided with a fluorosilicic acid cooler 63. An inlet of the fluorosilicic acid cooler 63 is connected to a circulating pump 2 and the outlet is divided into two paths, one is connected to a nozzle 35 in the second-stage fluorosilicic acid washing pipe 61 and another is in communication with a spraying layer which is at the top of second-stage fluorosilicic acid separation tank 62, and second-stage fluorosilicic acid separation tank 62 is also used as an acid circulating tank for the circulating and conveying pipeline. An outlet of the circulating pump 2 in second-stage fluorine absorption tower 6 is connected to a liquid inlet of the fluorosilicic acid separation tank 52 in the first-stage fluorine absorption tower 5 via a branch pipe, and thus the extra fluorosilicic acid in second-stage fluorine absorption tower 6 can be transferred into the first-stage fluorine absorption tower 5.
(38) For the emission on standard of all pollutant, a tail gas absorption tower 7 which is an empty spraying tower is installed in the device for fluorine recovery. A smoke inlet 11 of the tail gas absorption tower 7 is connected to a smoke outlet 12 of the second-stage fluorine absorption tower 6. The top of the tail gas absorption tower 7 is provided with a smoke outlet 12, and the upper part of the inside thereof is provided with a spraying layer, and the bottom thereof is provided with an alkali absorption tank (sodium hydroxide). An outlet of the alkali absorption tank is connected to spraying layers in the tail gas absorption tower 7 via a circulating and conveying pipeline which is provided with a circulating pump 2 to from a circulating and spraying system for the absorption of the tail gas.
(39) The said fluorosilicic acid liquid outlet 53 is connected to an external fluorosilicic acid refining equipment 54 (fluoride salts processing equipment) via a pipe which is provided with a feeding pump. The fluorosilicic acid must be pressure filtered by a pressure filter 21 before flowing into the fluorosilicic acid refining equipment 54, and an overflow outlet of the pressure filter 21 is connected to the fluorosilicic acid refining equipment 54.
Example 2
(40) A Process for Fluorine Recovery from Smoke after Phosphorus Absorption by Hydration in KPA
(41) The process for fluorine recovery using the device described in example 1 comprises: (1) the first-stage fluorine recovery: the smoke after phosphorus absorption by hydration is conveyed to the fluorosilicic acid washing pipe 51 in the first-stage fluorine absorption tower 5, and the major fluorine (mainly SiF.sub.4) in the smoke flowing downward contacts completely with the circulating fluorosilicic acid solution (with a mass concentration of 10 to 20 percent, a temperature of 25 C. to 65 C. and a spraying liquid-gas ratio of 3 L/m.sup.3 to 25 L/m.sup.3) spraying upward from the nozzle 35, and the thus mass transfer and heat transfer between said smoke and the circulating fluorosilicic acid solution occur, and chemical reaction of said smoke with the circulating fluorosilicic acid solution takes place to form fluorosilicic acid, and most of the enthalpy in the smoke is transferred into circulating fluorosilicic acid solution by heat transfer; the temperature of the smoke is further decreased to a range of 50 C. to 70 C. by adiabatic evaporation of water in circulating fluorosilicic acid solution and heat transfer to circulating fluorosilicic acid solution from the smoke; (2) the first-stage gas-liquid separation: the gas and liquid in the fluorosilicic acid washing pipe 51 is all conveyed to the fluorosilicic acid separation tank 52 for gas-liquid separation, and the gas obtained by separation flows into second-stage fluorosilicic acid washing pipe 61 of the second-stage fluorine absorption tower 6 via the smoke outlet of first-stage fluorine absorption tower 5, and the remaining liquid in the fluorosilicic acid separation tank 52 is conveyed to fluorosilicic acid washing pipe 51 for the operation in step (1) via a circulating and conveying pipeline which is provided with a circulating pump 2;
(42) (3) the second-stage fluorine recovery: the smoke (most remaining fluorine-containing substance is mainly SiF.sub.4) flowing downward in the second-stage fluorosilicic acid washing pipe 61 contacts completely with circulating fluorosilicic acid solution (with a mass concentration of 0.5 to 5 percent, a temperature of 25 C. to 60 C. and a spraying liquid-gas ratio of 3 L/m.sup.3 to 25 L/m.sup.3) sprayed upward by the nozzle, and then the mass transfer and heat transfer between said smoke and circulating fluorosilicic acid solution occur, and chemical reaction of said smoke with circulating fluorosilicic acid solution occurs to form fluorosilicic acid, and the enthalpy in the smoke is partially transferred into circulating fluorosilicic acid solution by heat transfer; the temperature of the smoke processed according to step (3) is further decreased below 60 C.;
(43) (4) the second-stage gas-liquid separation: the gas and liquid in the second-stage fluorosilicic acid washing pipe 61 is all transferred to the second-stage fluorosilicic acid separation tank 62 for gas-liquid separation, and the gas obtained by separation flows into the tail gas absorption tower 7 to be further processed via the smoke outlet of second-stage fluorine absorption tower 6, and the remaining liquid in the fluorosilicic acid separation tank 62 is partially transferred to second-stage fluorosilicic acid washing pipe 62 for the operation in step (3) via a circulating and conveying pipe which is provided with a circulating pump 2. The circulating fluorosilicic acid solution in the second-stage fluorosilicic acid washing pipe 61 is cooled by a fluorosilicic acid cooler 63 which is installed on the circulating and conveying pipe, and a portion of the rest circulating fluorosilicic acid solution is discharged directly into the fluorosilicic acid separation tank 52 in the first-stage fluorine absorption tower 5;
(44) (5) the fluorine in the smoke is accumulated in the circulating fluorosilicic acid solution in the first-stage fluorine absorption tower 5 and second-stage fluorine absorption tower 6. The extra fluorosilicic acid solution in the second-stage fluorine absorption tower 6 is discharged into the first-stage fluorine absorption tower 5, and the extra fluorosilicic acid solution in the first-stage fluorine absorption tower 5 is conveyed to the pressure filter 21 via a feeding pump to be pressure filtered. The filtrate obtained is conveyed to the fluorosilicic acid purification process to form the fluorosilicic acid product or be further processed to form fluorine salts product; the filtration residue which is silica gel is a byproduct after being washed and purified. The smoke in the tail gas absorption tower 7 moving upward comes into a counter-current contact with the circulating absorption solution spraying downward, and the absorption tank at the bottom of the tail gas absorption tower 7 is connected to a spraying layers in the tower via a circulating pump 2 to form a circulating and spraying system; the constant addition of dilute alkali solution (sodium hydroxide solution) is necessary for maintaining a pH value higher than 8 to ensure the absorption capacity of the absorption solution, and the absorption solution needs to be discharged constantly for wastewater treatment because of the addition of dilute alkali solution and accumulation of impurities absorbed, such as P.sub.2O.sub.5 and fluorine, in the smoke, and the water after treatment and recovery is used again in KPA; the smoke is further washed and purified by absorption of remaining pollutant (P.sub.2O.sub.5, SiF.sub.4 and dust etc.) in the smoke to reach the natural discharge standard, and then the smoke is discharged into the chimney by a draught fan.
(45) The said smoke after phosphorus absorption by hydration is obtained after hydration absorption in phosphoric acid production system shown in
(46) The hydration tower 1 is an empty spraying tower, and the lower part of the hydration tower 1 is provided with a smoke inlet 11 for the smoke exiting the kiln, and the top thereof is provided with a smoke outlet 12 for the smoke after phosphorus absorption by hydration, and the bottom thereof is provided with a liquid inlet 14 of the circulating and spraying system for acid, and an outlet 15 of the circulating and spraying system for acid is connected to an inlet pipe of a spraying device 13 which is in a chamber which is above the smoke inlet 11, and the circulating and spraying system for acid is also provided with an acid storage tank 16 and a circulating pump 2. In the present embodiment, the outside wall of the chamber in hydration tower 1 is coated by a water-cooling system 17, and the cool water flows into the water-cooling system 17 through the inlet at the bottom and out through the outlet at the top. Additionally, the position near the liquid inlet 14 in the circulating and spraying system for acid is provided with an acid cooler 18; and an outlet of the acid cooler 18 is connected to an inlet of the acid storage tank 16, and an outlet of the acid storage tank 16 is in communication with a liquid inlet of the spraying device 13 to form a circulating and spraying system for acid.
(47) The phosphoric acid mist absorption tower 3 is an effective fluidised counter-current washing tower which mainly comprises a washing pipe 31 and a separation tank 32, and the inlet and outlet of the washing pipe 31 are connected to a smoke outlet 12 of the hydration tower 1 and the middle part of the separation tank 32, respectively, and the top of the separation tank is provided with a smoke outlet 12, and at the bottom thereof an acid outlet 33 is connected to a nozzle 35 in washing pipe 31 via a circulating and conveying pipeline which is provided a circulating pump 2 (see
(48) For the acid crossflow between the hydration tower 1 and the phosphoric acid mist absorption tower 3, three spraying layers in spraying device 13 which are located at different heights in the chamber of the hydration tower 1 are installed, and the said three spraying layers comprises a dilute phosphoric acid spraying layer 25 and two concentrated phosphoric acid spraying layers 24 (see
(49) The smoke outlet 12 of the phosphoric acid mist absorption tower 3 is connected to the lower part of the demisting separation tower 4, and the top of the demisting separation tower 4 is provided with a smoke outlet 12 which is used for exhausting of the smoke after phosphorus absorption by hydration, and at the bottom thereof an acid outlet 33 is connected to an acid inlet 34 of the phosphoric acid mist absorption tower 3 via a pipeline. An online water flushing device is installed in the demisting separation tower 4, and the water added into the online water flushing device is also used as supplied water to the entire process for production of phosphoric acid by hydration absorption, and said water returns to the phosphoric acid mist absorption tower 3 and the hydration tower 1 step-by-step via a pipeline. The upper part of the demisting separation tower 4 is installed with a mesh demister 42, and the lower part thereof is provided with a phosphoric acid drop collection structure 43 which is similar to a cyclone duster, and the online water flushing device 41 is installed above the mesh demister 42.
Example 3
(50) The Mechanism of the Production System of Phosphoric Acid
(51) 1. The Absorption of P.sub.2O.sub.5 by Hydration in Hydration Tower:
(52) The smoke contains P.sub.2O.sub.5 and fluorine (a particular case is the smoke in the KPA with a temperature higher than 500 C. and a content of P.sub.2O.sub.5 of 80 g/Nm.sup.3) is pumped into the tower via the smoke inlet 11 at the lower part of the hydration tower 1 following the starting of the circulating pump 2 in the circulating and spraying system for acid, and thus the spraying of the concentrated phosphoric acid in the hydration tower 1 from the upper and middle concentrated phosphoric acid spraying layers 24 with the spraying of the concentrated phosphoric acid from a portion of nozzles in the upper concentrated phosphoric acid spraying layer 24 to the inner wall of the tower in an oblique direction, and the concentrated phosphoric acid is sprayed from nozzles in other concentrated phosphoric acid spraying layers 24 in a vertical direction. The phosphoric acid is sprayed from the nozzles in the middle and lower spraying layers in a vertical direction. The transfer of mass and heat takes place after a complete contact of the spraying concentrated phosphoric acid and the smoke containing P.sub.2O.sub.5 and fluorine fed into the tower, and P.sub.2O.sub.5 in the smoke reacts with the water in the concentrated phosphoric acid to form phosphoric acid, and over half the phosphoric acid obtained in the chemical reaction is absorbed by the spraying liquid and the other is remained in the gas in form of phosphoric acid mist, however, the fluorine (mainly SiF.sub.4 and HF etc.) in the smoke is hardly absorbed by the spraying liquid under a condition with concentrated phosphoric acid and a high temperature; and the temperature of the smoke in decreased to a range of 75 C. to 130 C. after the heat transfer between the smoke and circulating and spraying concentrated phosphoric acid of lower temperature and cooling of the smoke by the water-cooling system 17 in the hydration tower 1, and the temperature of the circulating concentrated phosphoric acid out of the hydration tower 1 is increased to a range of 70 C. to 95 C. The mass concentration of the circulating and spraying concentrated phosphoric acid is adjustable in the range of 60 to 90 percent (70 to 85 percent in the present embodiment), and the temperature of concentrated phosphoric acid fed into the hydration tower is controlled in the range of 50 C. to 80 C., and the spraying liquid-gas ratio can be adjusted in the range of 3 L/m.sup.3 to 20 L/m.sup.3 according to the content of water in the smoke. The phosphoric acid mist is hardly subsided in the hydration tower 1 and then exhausted with the smoke exiting the hydration tower 1. The hydration tower 1 has the function of both cooling of the smoke and absorption of P.sub.2O.sub.5 by hydration, and the chemical reaction in hydration tower 1 is as follows:
(53) P.sub.2O.sub.5+3H.sub.2O=2H.sub.3PO.sub.4
(54) After being sprayed, the concentrated phosphoric acid in the hydration tower 1 flows into the circulating and spraying system for acid via the liquid inlet 14 and then into the acid cooler 18 which is an agitating tank with a heat exchange plate which is made of several stainless steel tubes, and inside the stainless steel tubes is fed the circulating and cooling water. The forced convection heat transfer between the phosphoric acid flowing into the acid cooler 18 and the heat exchange plate with stirring of the liquid occurs, and with an increased efficiency of heat transfer, the heat in the circulating concentrated phosphoric acid is constantly transferred by the circulating and cooling water with the transfer of a portion of enthalpy in the concentrated phosphoric acid to the circulating and cooling water in the acid cooler 18. The circulating acid out from the outlet of the acid cooler 18 flows into the acid storage tank 16 and then is sent back again to the nozzles in the upper and middle circulating and spraying layers by the circulating pump 2 for circulating and spraying.
(55) 2. The Absorption of Phosphoric Acid Mist in the Phosphoric Acid Mist Absorption Tower:
(56) The gas exhausted from the smoke outlet 12 at the top of the hydration tower 1 is conveyed to the washing pipe 31 in phosphoric acid mist absorption tower 3, which is a fluidised counter-current washing tower. The circulating dilute phosphoric acid sprayed upward in the washing pipe 31 collides and contacts with the smoke flow running downward with a high speed to form an intensive turbulent area in the gas-liquid interface area, and the smoke passes through the stable foam zone (foam column) with a certain height which is formed after the balance of fluid momentum and contacts with large-area phosphoric acid solution surface which is constantly updated, and the capture, collection and polymerization of particles and transmission of heat take place in the foam zone. The most of phosphoric acid mist in the smoke is absorbed by circulating dilute phosphoric acid, and the superficial velocity of the smoke and liquid-gas ratio in the absorption zone are in the range of 10 m/s to 30 m/s and 3 L/m.sup.3 to 25 L/m.sup.3, respectively. The temperature of the smoke is further decreased to a range of 60 C. to 75 C. by adiabatic evaporation of water in circulating dilute phosphoric acid solution. Compared with the traditional Venturi demister in a hot process for production of phosphoric acid, the utilization of phosphoric acid mist absorption tower according to the present invention can make both the dynamic pressure head loss of the device and energy consumption of the acid storage installation reduced with the same removal effect of mist.
(57) The circulating and spraying acid in phosphoric acid mist absorption tower 3 is dilute phosphoric acid with a mass concentration of 10 to 50 percent. The gas and liquid in the washing pipe 31 are transferred to the separation tank 32 located at the lower part of the tower for gas-liquid separation, and the circulating acid obtained by separation is remained in the separation tank 32 which is also used as a circulating acid tank. The dilute phosphoric acid can be sent back by the circulating pump 2 to the washing pipe 31 or the dilute phosphoric acid spraying layer 25 in the hydration tower 1 for acid crossflow according to the actual requirements.
(58) 3. The Absorption of Phosphoric Acid Mist in Demisting Separation Tower:
(59) The smoke exhausted from the smoke outlet 12 in the phosphoric acid mist absorption tower 3 is conveyed to the demisting separation tower 4 for further separation of gas and liquid to further remove phosphoric acid mist in the smoke. The lower part of the demisting separation tower 4 is provided with a phosphoric acid drop collection structure 43 which is similar to a cyclone duster, and the upper part thereof is provided with a mesh demister 42, and grown phosphoric acid drop is separated by centrifugation and collected in the phosphoric acid drop collection structure 43, and the phosphoric acid drop which is not grown up is further separated and collected by the mesh demister 42 to ensure a direct yield of P.sub.2O.sub.5 in the device; and the smoke after phosphorus absorption by hydration exhausted from the demisting separation tower 4 is conveyed to the device for fluorine recovery.
(60) The process of absorption by hydration requires a constant supply of water because of the consumption of water in the chemical reaction with P.sub.2O.sub.5 in the process for absorption of phosphoric acid by hydration and the evaporation of water in the spraying acid in cooling process of the smoke. In the present embodiment, all of the supplied water is fed into the smoke outlet 12 in the demisting separation tower 4 and thus the online water flushing device 41 is used as both a device for supplying water and a device for flushing the mesh demister in the demisting separation tower 4. The concentration of the circulating acid in the phosphoric acid mist absorption tower 3 will be decreased gradually because all the supplied water is added into the demisting separation tower 4, and the base solution in the demisting separation tower 4 is sent back to the phosphoric acid mist absorption tower 3 from the acid inlet 34, and on the other hand, the concentration of the circulating acid in the hydration tower 1 will be increased gradually because of the constant absorption of P.sub.2O.sub.5 in the smoke, therefore, the crossflow between the circulating acid in the hydration tower 1 and the phosphoric acid mist absorption tower 3 is necessary for the stabilization of the concentration of the respective circulating acid, and the acid for crossflow in the hydration tower 1 needs to be cleared and filtered by the packing filter 22 firstly and then conveyed to the phosphoric acid mist absorption tower 3, and the acid for crossflow in the phosphoric acid mist absorption tower 3 is directly leaded from the outlet of the circulating pump 2 in the phosphoric acid mist absorption tower 3 to the hydration tower 1, and the extra phosphoric acid in the process system (according to the based on the material balance) is conveyed to the refining process from the outlet of supernatant in the packing filter 22 to form the concentrated phosphoric acid product after the addition of activated carbon, diatomaceous earth, and barium salt for the removal of the color the crude phosphoric acid and sulfate ion and filtration by the plate and frame pressure filter. Furthermore, the most of the solid particles, such as dust, in the smoke is transferred into the circulating phosphoric acid solution and then enriched in the underflow in the packing filter 22 which is discharged regularly into the pressure filter 21 for filtration, and the filtrate obtained is sent back to the acid.