A method for removing phosphorus and fluorine from phosphogypsum in situ and for producing high-strength gypsum simultaneously

Abstract

The present application relates to a kind of in situ phosphogypsum P, F and the method for producing high-strength gypsum in cooperation, specific steps are as follows: (1) preheating: with flash-off tail gas preheating in situ phosphogypsum, after preheating in situ phosphogypsum is added into reaction kettle;(2) heat treatment: after feeding is finished, saturated steam is imported into reaction kettle, and the absolute pressure of outlet steam is controlled by outlet valve, when the pH of outlet steam condensate is ≥5.5, cut off steam and discharge remaining steam in reaction kettle;(3) flash-off: subsequently, phosphogypsum after heat treatment in step (2) is taken out and is subjected to flash-off, and high-strength gypsum powder is obtained.The method of the present application is that the pH of phosphogypsum after steam heat treatment is 6-6.5, the water-soluble P content is ≤0.01wt%, wherein the eutectic P is ≤0.003wt%, and the water-soluble F content is ≤0.01wt%.By steam dewatering and impurity removal, P and F in in situ phosphogypsum are removed, and the problem that P and F in phosphogypsum harm human health and cause pollution to soil and ecological environment is solved.

Description

TECHNICAL FIELD

[0001] The present application relates to the treatment of industrial solid waste, belonging to the field of environmental protection and governance, in particular to a method for in-situ phosphogypsum P, F removal and simultaneous production of high-strength gypsum. BACKGROUND

[0002] Phosphogypsum is a solid waste produced in the process of wet-process phosphoric acid, and its main component is calcium sulfate dihydrate. The composition of phosphogypsum is relatively complex, in addition to calcium sulfate, there are also incomplete decomposition of phosphate rock, residual phosphoric acid, fluoride, acid-insoluble substances, organic matter, etc., among which fluorine can harm human health, mainly manifested as yellowing of teeth and deformation of bones, etc., and phosphorus entering the soil will cause soil compaction and alkalization, and entering the water body will cause water eutrophication, leading to rapid reproduction of algae and causing fish death due to lack of oxygen in water, etc., therefore the existence of fluorine and phosphorus directly affects its resource utilization.

[0003] At present, the main ways to deal with phosphogypsum are comprehensive utilization and stockpiling, among which stockpiling is the main one. If it is stockpiled for a long time, the fluorine and phosphorus pollutants in phosphogypsum will pollute surface water, soil and groundwater. Therefore, the removal and upgrading of phosphogypsum is very important, and the phosphogypsum after removal can be widely used to produce building materials, improving the resource utilization rate of phosphogypsum.

[0004] At present, the main methods for removing phosphorus and fluorine from phosphogypsum at home and abroad are water washing purification, lime neutralization, acid leaching and calcination process. Water washing method can effectively remove soluble phosphorus and soluble fluorine in phosphogypsum, but it is difficult to solve the problem of eutectic phosphorus removal, and there is also the problem of system water balance caused by excessive water washing; calcination method can effectively remove eutectic phosphorus, but the energy consumption and cost are high. Acid leaching and lime removal method is a commonly used method, which has low cost and simple operation. Most of them use lime to cooperate with other substances to stabilize soluble phosphorus and fluorine. Table 1 is a comparison table of common phosphogypsum removal methods, and the specific methods include physical method, chemical method and physical-chemical comprehensive method.

[0005] Table 1

[0006]

[0007]

[0008] Dihydrate gypsum can be converted to hemihydrate gypsum under heating conditions, when dihydrate gypsum is heated in pressurized steam or in an acid or salt medium, it can be converted to α-hemihydrate gypsum, and when it is heated and dehydrated in a dry environment, it generates β-hemihydrate gypsum. At present, the main production methods of α-hemihydrate gypsum are: autoclaving method, hydrothermal method, normal pressure salt solution method, dry steaming method, etc., each has its own advantages and disadvantages, but it is proved that dihydrate gypsum will dehydrate and convert to α-hemihydrate gypsum in steam. SUMMARY

[0009] The purpose of the present application is to provide a method for in-situ dephosphorization and defluorination of phosphogypsum and co-production of high-strength gypsum to solve the problem of difficult resource utilization and serious environmental pollution caused by stockpiling in the prior art.

[0010] To achieve the above-mentioned purpose, the technical scheme of the present application is to provide a method for in-situ dephosphorization and defluorination of phosphogypsum and co-production of high-strength gypsum, which has the following specific steps:

[0011] (1) Preheating: the in-situ phosphogypsum is preheated with flash-off tail gas, and the preheated in-situ phosphogypsum is added into a reaction kettle;

[0012] (2) Heat treatment: after the feeding is completed, saturated steam is introduced into the reaction kettle, and the outlet steam absolute pressure is controlled by an outlet valve, and the steam condensate pH value is monitored; when the outlet steam condensate pH value is greater than or equal to 5.5, the steam inlet is cut off and the remaining steam in the reaction kettle is discharged;

[0013] (3) Flash-off: the phosphogypsum subjected to the heat treatment in step (2) is then taken out for flash-off to obtain high-strength gypsum powder.

[0014] Further, the in-situ phosphogypsum in step (1) is the gypsum solid produced in the phosphoric acid production process after washing and filtration, and has a free water content of 22-28%, a total P content of 0.8-1.2wt%, a total F content of 0.1-0.2wt%, a water-soluble P content of 0.2-0.5wt%, a eutectic P content of 0.1-0.3wt%, a water-soluble F content of 0.08-0.15wt%, and a pH value of 2.5-3.0.

[0015] Further, the temperature reached by the in-situ phosphogypsum flash-off tail gas preheating in step (1) is 60-80℃.

[0016] Further, the absolute pressure of the steam introduced into the reaction kettle after the feeding is completed in step (2) is 0.1-0.5MPa.

[0017] Further, the outlet steam absolute pressure is maintained at 0.05-0.1MPa by controlling the outlet steam absolute pressure through the outlet valve in step (2).

[0018] Further, during the heat treatment process, the dihydrate gypsum in the in-situ phosphogypsum is converted into alpha hemihydrate gypsum through dissolution-crystallization.

[0019] Further, the pH value of the phosphogypsum subjected to the heat treatment in step (2) is 6-6.5, the water-soluble P content is less than or equal to 0.01wt%, the eutectic P content is less than or equal to 0.003wt%, and the water-soluble F content is less than or equal to 0.01wt%.

[0020] Further, when the pH of the steam condensate to be exported in step (2) is greater than or equal to 5.5, the steam is cut off, and the remaining steam in the reaction kettle is discharged; the condensate containing phosphorus and fluorine after condensation is recovered and returned to the main production system.

[0021] Compared with the prior art, the present application has the following beneficial effects:

[0022] (1) The method for removing P and F from in-situ phosphogypsum and simultaneously producing high-strength gypsum provided by the present application has a pH of 6-6.5, a water-soluble P content of less than or equal to 0.01 wt%, and a water-soluble F content of less than or equal to 0.01 wt% after steam heat treatment. By removing P and F from in-situ phosphogypsum through steam dehydration, the problem of P and F in phosphogypsum polluting the soil and the ecological environment and harming human health is solved.

[0023] (2) The method for removing P and F from in-situ phosphogypsum and simultaneously producing high-strength gypsum provided by the present application obtains alpha hemihydrate gypsum while removing impurities, and the alpha hemihydrate gypsum is further prepared into high-strength gypsum by flash calcination. The performance of the obtained high-strength gypsum meets the requirements of JC / T2038-2010 alpha 40, which provides a guarantee for the subsequent application of phosphogypsum and can be widely used in various types of building gypsum and products, and used as a cementing material for repairing barren mountains and slopes. The problem of phosphogypsum accumulation is solved, and the phosphogypsum can be effectively recycled. BRIEF DESCRIPTION OF DRAWINGS

[0024] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the drawings needed in the embodiments will be briefly introduced as follows. Obviously, the drawings in the following description are only some embodiments described in the present application, and other drawings can be obtained by those skilled in the art without creative labor.

[0025] Figure 1 The flow chart of the method for removing P and F from in-situ phosphogypsum and simultaneously producing high-strength gypsum provided by the present application. DETAILED DESCRIPTION

[0026] The technical solutions of the present application will be described clearly and completely in the following specific embodiments.

[0027] The present application provides a method for removing P and F from in-situ phosphogypsum and simultaneously producing high-strength gypsum. The in-situ phosphogypsum is preheated, subjected to steam heat treatment, dehydrated and impurities-removed, and crystallized, and then subjected to flash calcination to prepare high-strength gypsum cementing material. The specific step flow is as shown in Figure 1 The method comprises the following processes.

[0028] (1) preheating: the in-situ phosphogypsum is preheated by flash-off tail gas to a temperature of 60-80 DEG C, and the preheated in-situ phosphogypsum is added into a reaction kettle;

[0029] The in-situ phosphogypsum in step (1) of the present application is the phosphogypsum solid produced in the process of phosphoric acid production after washing and filtration, and has a free water content of 22-28%, a total P content of 0.8-1.2 wt%, a total F content of 0.1-0.2 wt%, wherein the water-soluble P content is 0.2-0.5 wt%, the eutectic P content is 0.1-0.3 wt%, the water-soluble F content is 0.08-0.15 wt%, and the pH is 2.5-3.0.

[0030] (2) heat treatment: after the feeding is completed, saturated steam is introduced into the reaction kettle, the absolute pressure of the introduced steam is 0.1-0.5 MPa, and the outlet steam absolute pressure is maintained at 0.05-0.1 MPa by controlling the outlet valve, and the steam condensate pH is monitored; when the outlet steam condensate pH is greater than or equal to 5.5, the steam inlet is cut off and the remaining steam in the reaction kettle is discharged, and the remaining steam condensate containing phosphorus and fluorine is recovered and returned to the main production system.

[0031] In the steam heat treatment process, P and F in the in-situ phosphogypsum are removed into the steam, and at the same time, the dihydrate gypsum in the in-situ phosphogypsum is converted into alpha hemihydrate gypsum through dissolution-crystallization.

[0032] The phosphogypsum after the heat treatment of step (2) has a pH of 6-6.5, a water-soluble P content of less than or equal to 0.01 wt%, wherein the eutectic P is less than or equal to 0.003 wt%, and a water-soluble F content of less than or equal to 0.01 wt%.

[0033] (3) flash-off: the phosphogypsum after the heat treatment in step (2) is then taken out for flash-off to obtain high-strength gypsum powder. The obtained high-strength gypsum powder has properties meeting the requirements of JC / T 2038-2010 alpha 40. The physical and mechanical properties of the prepared high-strength gypsum are detected according to the JC / T 2038-2010 alpha high-strength gypsum standard.

[0034] In step (2) of the present application, the saturated steam heats the phosphogypsum in the process of pressure reduction to form non-saturated steam, and at the same time, the outlet steam pressure is controlled to ensure the temperature in the reaction kettle. In the vapor-liquid equilibrium phase, the stable point of the fine crystal and unstable crystallization of the in-situ phosphogypsum is utilized to create conditions for the dissolution-crystallization of the in-situ phosphogypsum into alpha hemihydrate gypsum, and at the same time, the crystalline phosphorus is released. On the other hand, by using the conversion of saturated steam into non-saturated steam and the conversion process of non-saturated steam into saturated steam, the P and F in the phosphogypsum are carried out of the reaction system through steam discharge, so as to achieve the purpose of removing P and F. The prepared alpha hemihydrate gypsum has low water-soluble P and F content, and therefore has superior physical and mechanical properties, which creates favorable conditions for the utilization of alpha hemihydrate gypsum.

[0035] The application is further described based on the above technical solutions through the following examples.

[0036] Example 1

[0037] (1) The in-situ phosphogypsum with a water content of 28wt%, a total P content of 0.8wt%, a total F content of 0.1wt%, a water-soluble P content of 0.2wt%, a eutectic P content of 0.1wt%, a water-soluble F content of 0.08wt% and a pH of 3.0 was preheated to 60°C by flash-off tail gas and loaded into a reaction kettle;

[0038] (2) After the loading was completed, saturated steam with an absolute pressure of 0.1 MPa was introduced into the reaction kettle, and the steam outlet pressure was controlled to be 0.05 MPa. When the pH of the outlet steam condensate water was monitored to be 5.5, the inlet steam was closed and the steam in the reaction kettle was discharged;

[0039] (3) The phosphogypsum after steam heat treatment was quickly taken out, and high-strength gypsum was obtained by flash-off.

[0040] The effect of the in-situ phosphogypsum after steam heat treatment purification in Example 1 and the physical and mechanical properties of the high-strength gypsum produced are shown in Tables 2 and 3, respectively.

[0041] Table 2

[0042] Condensate pH Gypsum pH after heat treatment Water-soluble P / wt% Eutectic P / wt% Water-soluble F / wt% 5.5 6.0 0.01 0.003 0.01

[0043] Table 3

[0044] Water for normal consistency Initial setting Final setting 2h Flexural strength (MPa) Dried compressive strength (MPa) 0.45 5′35″ 12′40″ 5.2 41.5

[0045] Example 2

[0046] (1) The in-situ phosphogypsum with a water content of 22wt%, a total P content of 1.2wt%, a total F content of 0.2wt%, a water-soluble P content of 0.5wt%, a eutectic P content of 0.3wt%, a water-soluble F content of 0.15wt% and a pH of 2.5 was preheated to 80°C by flash-off tail gas and loaded into a reaction kettle;

[0047] (2) After the loading was completed, saturated steam with an absolute pressure of 0.5 MPa was introduced into the reaction kettle, and the steam outlet pressure was controlled to be 0.1 MPa. When the pH of the outlet steam condensate water was monitored to be 5.8, the inlet steam was closed and the steam in the reaction kettle was discharged;

[0048] (3) The phosphogypsum after heat treatment was quickly taken out, and high-strength gypsum was obtained by flash-off.

[0049] The effect of the in-situ phosphogypsum after steam heat treatment purification in Example 2 and the physical and mechanical properties of the high-strength gypsum produced are shown in Tables 4 and 5, respectively.

[0050] Table 4

[0051] Condensate pH Gypsum pH after heat treatment Water-soluble P / wt% Eutectic P / wt% Water-soluble F / wt% 5.8 6.6 0.005 0.001 0.002

[0052] Table 5

[0053] Water for normal consistency Initial setting Final setting 2h Flexural strength (MPa) Dried compressive strength (MPa) 0.43 7′24″ 15′35″ 5.6 48.7

[0054] Example 3

[0055] (1) The in-situ phosphogypsum with a water content of 25wt%, a total P content of 1.0wt%, a total F content of 0.14wt%, a water-soluble P content of 0.32wt%, a eutectic P content of 0.2wt%, a water-soluble F content of 0.12wt%, and a pH of 2.8 was preheated to 70°C by flash gas and loaded into a reaction kettle;

[0056] (2) After the loading was completed, saturated steam with an absolute pressure of 0.3MPa was introduced into the reaction kettle, and the steam outlet pressure was controlled at 0.1MPa. When the pH of the outlet steam condensate was monitored to be 5.6, the inlet steam was closed and the steam in the reaction kettle was discharged,

[0057] (3) The heat-treated phosphogypsum was quickly taken out, and high-strength gypsum was obtained by flash.

[0058] The effect of the in-situ phosphogypsum of Example 3 after steam heat treatment purification and the production obtained are the physical and mechanical properties of high-strength gypsum in Tables 6 and 7, respectively.

[0059] Table 6

[0060] Condensate pH Gypsum pH after heat treatment Water-soluble P / wt% Eutectic P / wt% Water-soluble F / wt% 5.6 6.3 0.009 0.001 0.003

[0061] Table 7

[0062] Water for normal consistency Initial setting Final setting 2h Flexural strength (MPa) Dried compressive strength (MPa) 0.42 8′30″ 18′22″ 5.4 44.2

[0063] As can be seen from Tables 2-7, by the method of the present application, P and F in the in-situ phosphogypsum can be effectively removed, and the removal rate is very high. At the same time, the high-strength phosphogypsum prepared by flash after heat treatment and impurity removal has good gel effect, and the bending and compressive strengths are relatively large. Not only the harm to the human body or the environment caused by the accumulation of phosphogypsum is solved, but also the phosphogypsum can be effectively utilized.

[0064] The above-described examples are merely preferred embodiments of the present application and are not intended to limit the concept and scope of the present application. Various modifications and improvements to the technical solutions of the present application made by ordinary engineering technicians in the field without departing from the design concept of the present application shall fall within the protection scope of the present application. The technical content claimed by the present application has been fully recorded in the technical requirements.

Claims

1. A method for in-situ de-P, F of phosphogypsum and synergistically producing high-strength gypsum, characterized in that: The specific steps are as follows: (1) preheating: the in-situ phosphogypsum is preheated by flash combustion tail gas, and the preheated in-situ phosphogypsum is added into the reaction kettle; (2) heat treatment: after the feeding is completed, saturated steam is introduced into the reaction kettle, and the outlet steam absolute pressure is controlled through the outlet valve, and the steam condensate pH value is monitored, when the outlet steam condensate pH value is greater than or equal to 5.5, the steam inlet is cut off and the remaining steam in the reaction kettle is discharged; (3) flash combustion: the phosphogypsum treated by heat treatment in step (2) is taken out for flash combustion to obtain high-strength gypsum powder; the in-situ phosphogypsum in step (1) is the gypsum solid produced in the process of phosphoric acid production after washing and filtration, and has a free water content of 22-28%, a total P content of 0.8-1.2wt%, a total F content of 0.1-0.2wt%, wherein the water-soluble P content is 0.1-0.3wt%, the eutectic P content is 0.05-0.1wt%, the water-soluble F content is 0.01-0.02wt%, and the pH value is 2.5-3.0; the absolute pressure of the steam introduced into the reaction kettle after the feeding is completed in step (2) is 0.1-0.5 MPa; the outlet steam absolute pressure is controlled through the outlet valve in step (2) to maintain the outlet steam absolute pressure at 0.05-0.1 MPa.

2. The method for in-situ de-P, F of phosphogypsum and synergistically producing high-strength gypsum according to claim 1, characterized in that: The temperature reached by the in-situ phosphogypsum flash combustion tail gas preheating in step (1) is 60-80℃.

3. The method for in-situ de-P, F of phosphogypsum and synergistically producing high-strength gypsum according to claim 1, characterized in that: During the heat treatment process, the dihydrate gypsum in the in-situ phosphogypsum is converted into α hemihydrate gypsum through dissolution-crystallization.

4. The method for in-situ de-P and de-F of phosphogypsum and co-production of high-strength gypsum according to claim 1, characterized in that: The phosphogypsum treated by heat treatment in step (2) has a pH value of 6-6.5, a water-soluble P content of less than or equal to 0.01wt%, wherein the eutectic P is less than or equal to 0.003wt%, and a water-soluble F content of less than or equal to 0.01wt%.

5. The method for in-situ de-P, F of phosphogypsum and synergistically producing high-strength gypsum according to claim 1, characterized in that: When the outlet steam condensate pH value is greater than or equal to 5.5 in step (2), the steam inlet is cut off and the remaining steam in the reaction kettle is discharged, and the condensate containing phosphorus and fluorine after condensation is recovered and returned to the main production system.

Images