阅读说明：本技术 多重结晶半水-二水生产湿法磷酸的方法及半水-二水湿法磷酸生产系统 (Method for producing wet-process phosphoric acid by multi-recrystallization semi-water-dihydrate and semi-water-dihydrate wet-process phosphoric acid production system ) 是由 郭国清 赵军 李志刚 杨培发 徐金桥 邹文敏 陈志华 严山 于 2021-04-02 设计创作，主要内容包括：本发明涉及一种多重结晶半水-二水法生产湿法磷酸的方法及其系统,解决了现有半水-二水湿法磷酸工艺中存在开车率低、硫酸消耗量大、产品质量和收率有待提高的问题。技术方案包括半水反应工序和二水反应工序,所述半水反应工序包括溶解、结晶、闪冷和半水过滤步骤,所述闪冷步骤中,出第一闪蒸冷却器的料浆先送入过滤给料槽与加入的磷矿反应,然后再送入半水过滤机进行过滤。本发明系统中所述半水反应系统包括串联的溶解槽组、结晶槽组、第一闪蒸冷却器和半水过滤机,所述第一闪蒸冷却器经过滤给料槽与半水过滤机连接,所述过滤给料槽设有磷矿加入口。本发明工艺简单、易于控制、硫酸消耗低、开车率高、产品酸质量好、收率高、水溶性P高。(The invention relates to a method and a system for producing wet-process phosphoric acid by a multi-crystallization semi-water-dihydrate method, which solve the problems of low start-up rate, large consumption of sulfuric acid and product quality and yield to be improved in the existing semi-water-dihydrate wet-process phosphoric acid process. The technical scheme includes a semi-water reaction process and a secondary water reaction process, wherein the semi-water reaction process comprises the steps of dissolving, crystallizing, flashing and semi-water filtering, and in the flashing step, slurry discharged from a first flashing cooler is firstly sent into a filtering and feeding trough to react with added phosphorite and then is sent into a semi-water filter to be filtered. The semi-water reaction system in the system comprises a dissolving tank group, a crystallizing tank group, a first flash evaporation cooler and a semi-water filter which are connected in series, wherein the first flash evaporation cooler is connected with the semi-water filter through a filtering and feeding tank, and the filtering and feeding tank is provided with a phosphate rock feeding port. The method has the advantages of simple process, easy control, low sulfuric acid consumption, high start rate, good acid quality of the product, high yield and high water solubility P.)

1. a method for producing wet-process phosphoric acid by a multi-crystallization semi-water-dihydrate method comprises a semi-water reaction process and a dihydrate reaction process, wherein the semi-water reaction process comprises the steps of dissolving, crystallizing, flashing and semi-water filtering, and is characterized in that slurry discharged from a first flashing cooler is firstly sent into a filtering and feeding tank to react with added phosphorite and then sent into a semi-water filter to be filtered.

2. The process for producing wet process phosphoric acid by multiple recrystallization hemihydrate-dihydrate method as set forth in claim 1, which comprisesCharacterized in that phosphorite is added into the filtering and feeding trough to react SO as to lead SO in slurry to be₄ ^2-The content is reduced to 0.8-1.2 wt%.

3. The method for producing wet-process phosphoric acid by the multiple-crystallization semi-water-dihydrate method according to claim 1 or 2, characterized in that excessive phosphate ore is added into the filter feeding trough for reaction for 1-3 hours.

4. The multiple crystallization semi-water-dihydrate process of claim 1 or 2 for producing wet-process phosphoric acid, wherein the temperature of the slurry exiting the first flash cooler is controlled to be 80-85 ℃.

5. The process for producing wet process phosphoric acid by the multiple crystallization hemihydrate-dihydrate method of claim 1, wherein three crystallization tanks are arranged in series in the crystallization step, wherein sulfuric acid is added into the first crystallization tank, part of slurry discharged from the second crystallization tank is sent to the second flash cooler for cooling and then recycled back to the first crystallization tank, part of slurry in the third crystallization tank is sent to the first flash cooler, and the rest of slurry is recycled to the first dissolving tank in the dissolving step.

6. The process for producing wet process phosphoric acid by the multiple crystallization hemihydrate-dihydrate method of claim 5, wherein sulfuric acid is added to the first crystallization tank to control SO in the first crystallization tank₄ ^2-The content is 3-4 wt%.

7. The process for the multiple recrystallization hemihydrate-dihydrate process of producing wet process phosphoric acid as set forth in claim 5, wherein said second flash cooler is a low-level flash cooler.

8. The method for producing wet-process phosphoric acid by the multi-crystallization semi-water-dihydrate method according to claim 5, wherein the temperature difference between the inlet and the outlet of the second flash cooler is controlled to be 1.0-1.5 ℃.

9. The process for producing wet process phosphoric acid by the multiple crystallization hemihydrate-dihydrate method of any one of claims 1-8, wherein the dihydrate reaction step comprises a conversion step and a dihydrate filtering step, wherein the conversion step comprises at least two conversion tanks connected in series, and the slurry discharged from the last conversion tank is sent to a third flash cooler for cooling and then recycled to be sent to the first conversion tank.

10. The multiple crystallization hemihydrate-dihydrate process of claim 9 wherein the third flash cooler is a low level flash cooler.

11. A semi-water-dihydrate wet process phosphoric acid production system for use in the method of any one of claims 1-10, comprising a semi-water reaction system and a dihydrate reaction system, wherein the semi-water reaction system comprises a dissolving tank set, a crystallizing tank set, a first flash evaporation cooler and a semi-water filter in series, wherein the first flash evaporation cooler is connected to the semi-water filter via a filter feed tank, and the filter feed tank is provided with a phosphate ore feeding port.

12. The system of claim 11 wherein the control of SO in the slurry in the filter feed tank is by the addition of phosphate ore₄ ^2-The content is reduced to 0.8-1.2 wt%.

13. The system for semi-water-dihydrate wet phosphoric acid production of claim 11, wherein the bank of crystallization tanks includes three crystallization tanks connected in series, the slurry outlet of the second crystallization tank is connected to the slurry inlet of the first crystallization tank via a second flash cooler, and the slurry outlet of the third crystallization tank is connected to the first dissolving tank and the first flash cooler of the dissolving tank bank, respectively.

14. The semi-water-dihydrate wet process phosphoric acid production system of claim 13, wherein the second flash cooler is a low-level flash cooler.

15. A semi-water-dihydrate wet phosphoric acid production system as claimed in any one of claims 10-14, wherein the secondary water reaction system comprises a conversion tank group and a secondary water filter which are connected in sequence, wherein the conversion tank group comprises at least two conversion tanks which are connected in series, and the slurry discharged from the last conversion tank is sent to a third flash cooler for cooling and then recycled to be sent to the first conversion tank.

16. The semi-water-dihydrate wet process phosphoric acid production system of claim 15, wherein the third flash cooler is a low-level flash cooler.

Technical Field

The invention relates to a phosphoric acid production process and a phosphoric acid production system in the field of chemical industry, in particular to a method for producing wet-process phosphoric acid by multi-recrystallization semi-water-dihydrate and a semi-water-dihydrate wet-process phosphoric acid production system.

Background

Phosphoric acid (H) by wet process₃PO₄) The method is an important chemical raw material, can produce various industrial grade, food grade and electronic grade phosphoric acid and related phosphates through deep processing treatment, is a key basic product, and is a basic method for preparing phosphoric acid by using sulfuric acid to decompose phosphorite.

The method for producing wet-process phosphoric acid by the semi-water-dihydrate method has the advantages of high yield, high product concentration, low energy consumption and the like, and is a widely popularized process; in the existing production device, the P is caused in the semi-water reaction₂O₅High concentration (-45 wt%), SO₄ ^2-High concentration (3-4 wt%) and the like, and is due to sesquioxide (Fe)₂O₃,Al₂O₃) With a following P₂O₅And SO₄ ^2-The increased concentration and increased solubility characteristics of the phosphate sesquioxide (Fe) product₂O₃,Al₂O₃) High, reduced water-soluble P content and SO₄ ^2-The concentration is high, which results in higher consumption of sulfuric acid and is not beneficial to the use of the product phosphoric acid.

On the other hand, in the existing production device, only one semi-water flash cooling system is arranged, and the system is an open circuit system, namely, circulation and semi-water filter filtration in the same crystallization tank form an open circuit; the temperature difference between the inlet and the outlet of the semi-water flash cooler is large (the temperature difference is 15 ℃), and the acid circulation quantity is small; specifically, the slurry in the crystallization tank is cooled by flash cooling equipment, one part of the slurry is sent to the same crystallization tank, and the other part of the slurry is sent to a semi-water filter for filtration, and the method is carried out in such a way thatThe following production problems: firstly, sulfuric acid is required to be added into a crystallization tank, due to the existence of a cooling circulation open circuit, the sulfuric acid is circularly pumped out to enter flash cooling equipment and a semi-water filter for filtration when the sulfuric acid is not added into the tank and reacts, and the unreacted sulfuric acid is filtered to enter product phosphoric acid to become free sulfuric acid, so that the reaction efficiency is influenced; secondly, because the flash cooling temperature difference is large (the temperature difference is 15 ℃), the acid circulation amount is small, and water evaporation can cause P in the flash cooler₂O₅The concentration is increased (compared with P in a crystallization tank)₂O₅High concentration of 2%), low temp. and high concentration of P after flash cooling₂O₅The temperature and P of the slurry in the crystallization tank are increased after the content of the slurry is returned to the crystallization tank₂O₅The concentration fluctuation is too large, thereby seriously influencing the reaction and crystallization, causing low driving rate and seriously influencing the yield.

Disclosure of Invention

The invention aims to solve the technical problems and provides a method for producing wet-process phosphoric acid by using multi-crystallization semi-water-dihydrate, which has the advantages of simple process, easy control, low sulfuric acid consumption, high start rate, good acid quality of products, high yield and high water solubility P.

The invention also provides a semi-water-dihydrate wet-process phosphoric acid production system which is used for the method and has the advantages of low energy consumption, low production and operation cost and good system stability.

The method comprises a semi-water reaction process and a dihydrate reaction process, wherein the semi-water reaction process comprises the steps of dissolving, crystallizing, flashing and semi-water filtering, and in the flashing step, slurry discharged from a first flashing cooler is firstly sent into a filtering and feeding tank to react with added phosphorite and then is sent into a semi-water filter to be filtered.

Control of SO in slurry in a filter feed trough by adding phosphate ore₄ ^2-The content is reduced to 0.8-1.2 wt%.

And adding excessive phosphate ore into the filtering and feeding trough for reaction, wherein the reaction time is 1-3 hours.

And controlling the temperature of the slurry of the first flash cooler to be 80-85 ℃.

And three crystallization tanks connected in series are arranged in the crystallization step, wherein sulfuric acid is added into the first crystallization tank, part of slurry led out from the second crystallization tank is sent to the second flash evaporation cooler for cooling and then is recycled and sent to the first crystallization tank, part of slurry in the third crystallization tank is sent to the first flash evaporation cooler, and the rest of slurry is recycled and sent to the first dissolving tank in the dissolving step.

Adding sulfuric acid into the first crystallizing tank, and controlling the liquid phase SO in the first crystallizing tank₄ ^2-3-4 wt% and the reaction temperature is 95-100 ℃.

The second flash cooler is a low-level flash cooler.

And controlling the temperature difference between the inlet and the outlet of the second flash evaporation cooler to be 1.0-1.5 ℃.

The secondary water reaction process comprises the steps of conversion and secondary water filtration, wherein the conversion step comprises at least two conversion tanks connected in series, and part of slurry led out from the last conversion tank is sent to a third flash cooler for cooling and then is recycled and sent back to the first conversion tank.

The third flash cooler is a low-level flash cooler.

The wet-process phosphoric acid production system comprises a semi-water reaction system and a dihydrate reaction system, wherein the semi-water reaction system comprises a dissolving tank group, a crystallizing tank group, a first flash evaporation cooler and a semi-water filter which are connected in series, the first flash evaporation cooler is connected with the semi-water filter through a filtering and feeding tank, and the filtering and feeding tank is provided with a phosphate rock feeding port.

Control of SO in slurry in a filter feed trough by adding phosphate ore₄ ^2-The content is reduced to 0.8-1.2 wt%.

The crystallizing tank group comprises three crystallizing tanks which are connected in series, the outlet of the circulating slurry of the second crystallizing tank is connected with the inlet of the circulating slurry of the first crystallizing tank through a second flash evaporator, and the outlet of the slurry of the third crystallizing tank is respectively connected with the first dissolving tank and the first flash evaporator of the dissolving tank group.

The second flash cooler is a low-level flash cooler.

The secondary water reaction system comprises a conversion tank group and a secondary water filter which are sequentially connected, wherein the conversion tank group comprises at least two conversion tanks which are connected in series, and part of slurry led out from the last conversion tank is sent to a third flash evaporation cooler for cooling and then is recycled and sent to the first conversion tank.

The third flash cooler is a low-level flash cooler.

In view of the problems in the background art, the inventor makes the following improvements:

1. the inventors have conducted intensive studies on the reaction step of semi-aqueous solution and found that SO is contained in the slurry in the crystallizer₄ ^2-The content is still higher and can reach 3-4 wt%, and on one hand, the part does not contain SO participating in the reaction₄ ^2-Presence, increasing the consumption of sulfuric acid for the reaction, with a consequent increase in costs, and, on the other hand, the semioxide (Fe)₂O₃,Al₂O₃) The solubility is increased along with the increase of the sulfate radical concentration, and the dissolved sesquioxide forms phosphate with limited solubility, so that the content of water-soluble P is reduced, and the application range is limited. In order to solve the technical problem, the inventor considers that slurry out of a crystallization tank is sent to a first flash evaporation cooler for cooling, is not directly sent to a semi-water filter, but is introduced into a filtering and feeding tank, meanwhile, phosphorite is innovatively added at the position, and the phosphorite and SO in the slurry are utilized₄ ^2-(about 3 to 4%) reacting, adding SO₄ ^2-The content is reduced to 0.8-1.2 wt%, and the slurry cooling is considered to generate a large amount of broken crystals after the first flash evaporation cooler, so that the broken crystals can continue to crystallize and grow with the generated phosphogypsum, multiple crystallization is realized, easy-to-filter polymerized spherical crystals are formed, the filtration yield is further improved, and the P in the semi-water slurry is reduced₂O₅Concentration is favorable for carrying out the reaction of the dihydrate; with SO₄ ^2-Decrease in concentration, sesquioxide (Fe)₂O₃,Al₂O₃) The solubility of the phosphate in the slurry can be reduced by more than 20 percent, and the supersaturated part of the phosphate formed by the sesquioxide is crystallized and precipitated, so that the water solubility P of the product phosphoric acid is improved due to the reduction of the sesquioxide, thereby being beneficial to subsequent utilization, achieving multiple purposes and generating unexpected technical effects; and controlling the appropriate excess (preferably 110-120 of theoretical amount) of the part of the added phosphorite% of the total amount of the semi-water slurry is filtered by a semi-water filter, and the part which is not completely reacted enters a subsequent reaction process of the semi-water slurry along with the filtered semi-water slurry by the semi-water filter, and can further react with added sulfuric acid in a first conversion tank to play roles of accelerating the dissolution of semi-water crystals, promoting the formation of the semi-water crystals and the like.

2. Aiming at the problems caused by open-circuit flash cooling system and circulation in the same crystallization tank in the semi-water reaction process in the existing device, the invention is specially provided with three crystallization tanks connected in series in the crystallization step, sulfuric acid is added into the first crystallization tank, and aiming at the problem that the temperature of slurry is raised by reaction heat release in the crystallization tanks, in order to prevent the temperature from being overhigh, a second flash cooler is arranged, partial slurry in the second crystallization tank is circularly extracted and returned to the first crystallization tank after being cooled by the second flash cooler, the slurry containing the crystals after the reaction is led out from the third crystallization tank, thus the inlet and the outlet of a sulfuric acid adding and circulating cooling channel and the slurry outlet sent to the subsequent process are respectively positioned in three different crystallization tanks and are not interfered with each other, the semi-water flash cooling system also forms a closed-circuit system, and the sufficient time for the sulfuric acid to be added into the slurry to react is ensured, the reaction crystallization time is ensured, and coarse crystals are obtained; furthermore, the circulating cooling channel is only arranged in the second and third crystallizing tanks, so that a low-level flash evaporator can be used for realizing cooling with large circulating capacity (the circulating ratio is about 35 times) and low temperature difference (the temperature difference is between 1.0 and 1.5 ℃), and the cooling mode is particularly suitable for the crystallizing step, so that P in slurry of the three crystallizing tanks is ensured to be in a liquid state₂O₅Content, SO₄ ^2-The temperature and the like are close to the same, the three-parameter fluctuation is effectively reduced, the stable reaction of the crystallization tank is facilitated, the crystallization effect and the operation of the filter are ensured, and the start rate and the yield of phosphoric acid are effectively improved.

3. At least two conversion tanks connected in series are also arranged in the conversion step in the secondary water reaction procedure, part of slurry led out from the last conversion tank is sent into a third flash evaporation cooler for cooling and then is recycled and sent into the first conversion tank, and the inlet and the outlet of the third flash evaporation cooler, particularly the inlet and the feeding port of sulfuric acid are respectively provided with different conversion tanks, so that the problems of insufficient reaction, temperature difference and material component fluctuation are solved.

4. In the invention, multiple crystallization is carried out by introducing a plurality of crystallization tanks, a filtering and feeding tank is additionally arranged, and phosphorite is added into the tanks for secondary reaction and crystallization, so that multiple crystallization is truly realized.

Has the advantages that:

the method introduces multiple crystallization in the process of producing wet-process phosphoric acid by semi-water-dihydrate, and effectively solves the problems of insufficient semi-water reaction, reaction temperature and P in the existing production device₂O₅The problems of large concentration fluctuation, high sulfuric acid consumption, large concentration of sesquioxide, low water-soluble P content, limited use of phosphoric acid and the like are solved, and sulfate radicals in the slurry are consumed by adding a filtering and feeding trough and adding phosphorite for reaction; three crystallization tanks are arranged and matched with an independent closed cooling circulation loop to reduce SO in the crystallization tanks₄ ^2-、P₂O₅And the temperature gradient difference is favorable for the stable reaction of the crystallization tank, and the crystallization effect and the operation of the filter are ensured₄ ^2-As low as 1%, low sesquioxide), good acid quality of the product, and high proportion of water-soluble P. The system of the invention is easy to be transformed and maintained, and has low energy consumption and low equipment investment and operation cost.

Drawings

FIG. 1 is a schematic of a hemihydrate process of the present invention.

FIG. 2 is a schematic diagram of a dihydrate reaction step of the present invention.

Wherein, 1-a first dissolving tank, 2-a second dissolving tank, 3-a first crystallizing tank, 4-a second crystallizing tank, 5-a third crystallizing tank, 6-a filtering and feeding tank, 7-a second flash evaporation cooler, 8-a first flash evaporation cooler, 9-a flash cooling circulating pump, 10-a semi-water flash cooling circulating pump, 11-a first filtering and feeding pump, 12-a mist separator, 13-an acid returning tank, 14-an acid returning pump, 15-a semi-water filter, 15.1-a filtering area, 15.2-a filter cake washing area, 15.3-a filter cloth washing area, 16-a first converting tank, 17-a second converting tank, 18-a third flash evaporation cooler, 19-a second water filtering and feeding pump, 20-a second water flash cooling circulating pump, 21-a second water filter, 22-a product acid pump, 23-slurry circulating pump, 24-vacuum pump and 25-vacuum pump.

Detailed Description

The system of the present invention is further explained below with reference to the accompanying drawings:

referring to fig. 1, the wet-process phosphoric acid production system comprises a semi-water reaction system and a dihydrate reaction system, wherein the semi-water reaction system comprises a dissolving tank group, a crystallizing tank group, a first flash evaporator cooler 8 and a semi-water filter 15 which are connected in series;

wherein the deep dissolving tank group comprises a first dissolving tank 1 and a second dissolving tank 2 which are connected in series;

the crystallizer bank is including first crystallizer 3, second crystallizer 4 and the third crystallizer 5 of establishing ties in proper order, the circulation ground paste export of second crystallizer 4 is through the circulation ground paste entry linkage of semiwater flash cooling circulating pump 10, second flash cooling ware 7 with first crystallizer 1, the ground paste export of third crystallizer 5 is through flashing cooling circulating pump 9 connection first flash cooling ware 8, the ground paste export of third crystallizer still connects first dissolving tank 1 through ground paste circulating pump 23.

First flash cooler 8 is connected with the feed inlet of semi-water filter 15 through filtering feed trough 6, first filtration feed pump 11, it is equipped with the phosphorus ore inlet to filter feed trough 6.

The filtrate collecting pipe below the filter cloth washing area 15.3 of the semi-water filter 15 is connected with a conversion tank group of a subsequent two-water reaction system, the filtrate below the filtering area 15.1 is taken as a product and is sent to a battery compartment area through a product acid pump 22, and the filtrate collecting pipe below the filter cake washing area 15.2 is respectively connected with the first dissolving tank 1 and the first crystallizing tank 3 through an acid returning tank 13 and an acid returning pump 14.

The secondary water reaction system comprises a conversion tank group and a secondary water filter 21 which are connected in sequence, in the embodiment, the conversion tank group comprises a first conversion tank 16 and a second conversion tank 17 which are connected in series, partial slurry led out from the second conversion tank 17 is sent to a third flash evaporator 18 through a secondary water flash cooling circulating pump 20 to be cooled and then is circulated and sent back to the first conversion tank 16, and partial slurry in the second conversion tank 17 is sent to the secondary water filter 21 through a secondary filtering feed pump 19.

The first flash cooler 8 and the third flash cooler 18 are low-level flash coolers, and the second flash cooler 7 is a high-level flash cooler.

The process comprises the following steps:

the method comprises a semi-water reaction process and a dihydrate reaction process,

referring to fig. 1, in the hemihydrate process:

adding phosphorite, circulation slurry (from the third crystallizing tank 5) and return acid (from the semi-water filter 15) into the first dissolving tank 1 to form reaction slurry, overflowing and reacting in the first and second dissolving tanks 1, 2 in sequence, and controlling SO in the dissolving tank₄ ²The concentration is negative, the solid content of the slurry is 33-35 wt%, and the liquid phase acid P₂O₅The concentration is 40-45 wt%, the reaction temperature is-98 ℃, and the phosphorite is decomposed into calcium dihydrogen phosphate and calcium sulfate hemihydrate;

the slurry discharged from the second dissolution tank 2 overflows into the first, second and third crystallization tanks 3, 4, 5 in sequence, sulfuric acid and return acid (return acid from the semi-water filter 15) are added into the first crystallization tank 3, and the excess sulfuric acid (SO) in the slurry₄ ^2-About 3-4%) to promote the growth of calcium sulfate hemihydrate crystals, the chemical reaction formula is as follows:

Ca₅F(PO₄)₃+6H₂SO₄+2 1/2H₂O＝3H₃PO₄+5CaSO4·1/2H₂O↓+HF↑+H₂SO₄

because the temperature of the slurry is raised due to the reaction heat release, in order to prevent the temperature from being overhigh, partial slurry in the second crystallization tank 4 is sent into a second flash evaporation cooler 7 through a semi-water flash cooling circulating pump 10 for circulation, the cooled slurry is returned to the first crystallization tank 3, the temperature difference between the inlet and the outlet of the second flash evaporation cooler 7 is 1.0-1.5 ℃, in the embodiment, the inlet temperature of the second flash evaporation cooler 7 is 100 ℃, the outlet temperature is 98.5 ℃, and the circulating slurry ratio is 35;

part of slurry after the reaction in the third crystallization tank 3 is sent to a first flash cooler 7 through a flash cooling circulating pump 9 for cooling, and then enters a filtering and feeding tank 6 after being cooled to 80-85 ℃ from 100 ℃, and excessive phosphorite (preferably 110-1 of theoretical amount) is added into the filtering and feeding tank 6 at the same time20 wt.%) reaction to consume excess sulfate and make the SO in the slurry₄ ^2-The content is reduced from 3-4 wt% to 0.8-1.2 wt%, and a large amount of crushed crystals are generated due to slurry cooling and continue to crystallize and grow with generated phosphogypsum, so that coarser and uniform crystals are obtained, and subsequent filtration is facilitated; the chemical reaction formula is as follows: 5H₂SO₄+2Ca₅F(PO₄)₃+2 1/2H₂O＝3H₃PO₄+5CaSO₄·1/2H₂O↓+HF↑+Ca₅F(PO₄)₃

Slurry which is taken out of the filtering and feeding tank 6 is sent into a semi-water filter 15, product phosphoric acid is obtained after filtering, filtrate in a filtering area of the semi-water filter 15 is defoamed by a mist separator 12 and is sent into an acid returning tank 13 together with filtrate in a filter cake washing area 15.2, the filtrate is used as returning acid and is sent into a first dissolving tank 1 and a first crystallizing tank 3 respectively through an acid returning pump 14, the filtrate below the filtering area 15.1 is used as a product and is sent to a boundary area through a product acid pump 22, and semi-water slurry which is collected in a filter cloth washing area of the semi-water filter 15 contains unreacted phosphorite in the filtering and feeding tank 6 and is sent into a secondary water reaction process together.

And the rest slurry after the reaction in the third crystallization tank 3 is circularly sent to the first dissolving tank 1 through a slurry circulating pump 23.

Referring to fig. 2, in the dihydrate reaction step, the hemihydrate slurry is added to the first conversion tank 16 together with sulfuric acid, and unreacted phosphate ore in the dihydrate slurry is reacted with sulfuric acid to increase P continuously₂O₅Yield; at the same time at low concentration P₂O₅Under the condition, the hemihydrate crystalline gypsum is hydrated to form dihydrate gypsum, the solid content of reaction slurry is controlled to be 30-33 wt%, and the liquid phase P₂O₅The concentration is controlled to be 16-18 wt%, and the temperature of the reaction tank is controlled to be 70-75 ℃. The chemical reaction formula is as follows:

Ca₅F(PO₄)₃+6H₂SO₄+2 1/2H₂O＝3H₃PO₄+5CaSO4·1/2H₂O↓+HF↑+H₂SO₄

CaSO₄·1/2H2O+1 1/2H₂O＝CaSO₄·2H₂O↓

the slurry overflows from the first conversion tank 16 to the second conversion tank 17 for continuous reaction, part of the slurry led out from the second conversion tank 17 is sent to the third flash evaporation cooler 18 through the secondary water flash cooling circulating pump 20 for cooling and then is circulated and sent back to the first conversion tank 16, the temperature difference of the slurry at the inlet and the outlet of the third flash evaporation cooler 18 is controlled to be 1 ℃, in the embodiment, the temperature of the slurry at the inlet is 72 ℃, the temperature of the slurry at the outlet is 71 ℃, the ratio of the circulating slurry is 20, the reacted slurry part in the second conversion tank 17 is sent to the secondary water filter 21 through the second filtering and feeding pump 19 for filtering to obtain the secondary water gypsum, and other steps are the same as the prior art. By way of example, the dihydrate peracid withdrawn from the filtration zone of the dihydrate filter 21 in this embodiment can be returned as wash liquor to the cake wash zone 15.2 of the hemihydrate filter 15; the secondary filtrate of the secondary water led out from the filter cake primary washing area of the secondary water filter 21 can be used as washing liquid to return to a slag hopper below the filter cloth washing area 15.3 of the semi-water filter 15.

Compared with the existing semi-water-dihydrate wet-process phosphoric acid process, the method of the invention has the advantages of P₂O₅High yield (P)₂O₅The yield is more than 99 percent), the consumption of sulfuric acid is low (can be reduced by more than 3.5 percent), the acid quality is good (the sulfate radical is as low as 0.8 to 1.2 weight percent, the sesquioxide is low), the water-soluble phosphorus content is high (more than 90 weight percent), the start rate is high, and the like, and the indexes of the phosphoric acid product obtained by a certain project are as follows: