Method for treating phosphogypsum

Abstract

The application discloses a kind of phosphogypsum processing methods, belong to phosphorus chemical technology field.Method includes: S1: collecting pyrite sulfuric acid cooling wastewater, add quicklime or slaked lime and adjust pH to 7.2-7.5;S2: phosphogypsum is washed;S3: the cooling wastewater after adjusting pH is mixed with the phosphogypsum after washing, and the mass ratio is 5-8:1, after stirring 30-45min, stationary 45-120min, separate supernatant;S4: in the sediment after separating supernatant, add water and re-slurry to solid content is 23-28%, solid-liquid separation obtains the phosphogypsum after processing;Wherein, cooling wastewater comes from cooling slag machine and cylinder humidifier;Waste water after step S2 washing, supernatant of step S3 and liquid of step S4 solid-liquid separation are used as phosphogypsum backwater.Overphosphated phosphogypsum treats pyrite sulfuric acid cooling wastewater, washes out soluble phosphorus of phosphogypsum, and reduces the arsenic content and iron content of waste water.

Description

Technical Field

[0001] This invention belongs to the field of phosphorus chemical technology, and specifically relates to a method for treating phosphogypsum. Background Technology

[0002] The preparation process of phosphoric acid is as follows: phosphate rock slurry reacts with concentrated sulfuric acid to obtain phosphoric acid, with phosphogypsum and phosphogypsum-treated water as byproducts. The phosphogypsum-treated water is recycled. The amount of phosphogypsum produced is very large and usually cannot be treated, generally being landfilled or stockpiled. Concentrated sulfuric acid can be obtained from pyrite through roasting, purification, conversion, and absorption processes, as detailed in the patent application number CN201410478737.4. The roasting process generates a large amount of wastewater, primarily from the cooling and humidification processes of the hot slag.

[0003] For example, patent application number CN201110179254.0 discloses an integrated process for enhancing pelletizing by combining waste heat from hot slag with slag humidification. The hot slag discharged from the fluidized bed furnace enters a slag cooler. Inside the slag cooler, heat exchange between water and hot slag is used to lower the slag temperature to below 200°C. The slag is then transported to a slag intermediate silo, where slag from the fluidized bed furnace, waste heat boiler, and cyclone dust collector are combined. The combined slag then exits the slag intermediate silo and enters a cylindrical humidifier for water addition and further cooling. After being humidified to a moisture content of 14-17% by mass, the slag is then transported to a slag storage silo to further uniformly wet the surface of the slag, thereby improving its pelletizing properties.

[0004] The wastewater from slag coolers and cylindrical humidifiers has the following characteristics: 1. It has a relatively high temperature; 2. It is weakly acidic; 3. It contains a significant amount of particulate matter; 4. It contains high levels of arsenic and iron. This wastewater typically requires cooling and sedimentation treatment before being sent to the factory's wastewater treatment plant for further processing. Summary of the Invention

[0005] To address the aforementioned problems, this invention provides a method for treating phosphogypsum. By treating the cooling wastewater from the sulfuric acid production process using phosphogypsum, the soluble phosphorus in the phosphogypsum is washed out (fluorine is also washed out, but to a lesser extent than phosphorus), and the arsenic and iron content of the wastewater is reduced (both of which affect the quality of the prepared phosphoric acid). The technical solution is as follows: See Figure 1 This invention provides a method for treating phosphogypsum, the method comprising: S1: Collect the cooling wastewater from the sulfuric acid production process of pyrite, and add quicklime or slaked lime to adjust the pH value to 7.2-7.5 (adjust first to allow iron ions to react with the alkali and obtain iron salts with flocculation effect; experiments have shown that this is more effective than adding quicklime or slaked lime in step S3).

[0006] S2: Wash the phosphogypsum with water. The conditions for washing the phosphogypsum are as follows: mix the phosphogypsum and water at a mass ratio of 1:3-4, stir for 20-35 minutes, and then separate the solid and liquid. Washing with water can remove soluble impurities from the phosphogypsum and reduce the possibility of interaction with cooling wastewater.

[0007] S3: After adjusting the pH and cooling (to 40-55℃; high temperature can slightly improve the extraction effect of soluble phosphorus, but will significantly increase the F content of wastewater), mix the cooled wastewater after pH adjustment and cooling (to 40-55℃; high temperature can slightly improve the extraction effect of soluble phosphorus, but will significantly increase the amount of water used, which is not cost-effective). Stir for 30-45 minutes, let stand for 45-120 minutes, and separate the supernatant.

[0008] S4: Add water to the sediment after separating the supernatant and re-slurry until the solid content is 23-28%. Solid-liquid separation yields treated phosphogypsum (processed according to conventional procedures, such as landfilling, stockpiling, or sending to specialized solid waste treatment plants, etc.).

[0009] The cooling wastewater comes from the slag cooler and the cylindrical humidifier; the wastewater after washing in step S2, the supernatant in step S3, and the liquid from solid-liquid separation in step S4 are used as phosphogypsum return water.

[0010] Specifically, solid-liquid separation is performed by pressure filtration (steps S2 and S4).

[0011] Preferably, in step S3, after stirring for 30-40 minutes, the mixture is allowed to stand for 60-90 minutes.

[0012] The beneficial effects of the technical solution provided by the embodiments of the present invention are as follows: (1) Treat the cooling wastewater from sulfuric acid production from pyrite, and use the treated wastewater as phosphogypsum return water to reduce the amount of wastewater to be treated and discharged.

[0013] (2) It can reduce the arsenic content (removal rate greater than 60%) and iron content (removal rate greater than 85%) in cooling wastewater, reducing the impact on phosphoric acid production; and increase the pH value of the wastewater (by about 0.3-0.5), which is beneficial for the use of phosphogypsum recycled water.

[0014] (3) Further extraction of soluble phosphorus from phosphogypsum can be achieved by cooling wastewater. Water washing can only extract a portion of the soluble phosphorus. Increasing the washing time, amount, and number of washes has virtually no further effect on the extraction of soluble phosphorus.

[0015] (4) Although alkaline solution can increase the extraction of soluble phosphorus, the extraction effect is not as good as that of this patent, and the extraction effect of F is better than that of this patent; the high F content is not conducive to the use of phosphogypsum rehydrate.

[0016] (5) Both the water-washed and re-slurried phosphogypsum slurries can achieve good solid-liquid separation, thus improving the processing speed. Attached Figure Description

[0017] Figure 1 This is a flowchart of the phosphogypsum processing method provided by the present invention. Detailed Implementation

[0018] To make the objectives, technical solutions, and advantages of the present invention clearer, the present invention will be further described in detail below with reference to the accompanying drawings.

[0019] Example 1 Example 1 provides a method for treating phosphogypsum, the method comprising: S1: Collect the cooling wastewater from the production of sulfuric acid from pyrite, and add quicklime to adjust the pH value to 7.4.

[0020] S2: Wash the phosphogypsum with water. The conditions for washing the phosphogypsum with water are as follows: mix the phosphogypsum and water at a mass ratio of 1:4, stir for 30 minutes, and then separate the solid and liquid.

[0021] S3: The cooled wastewater after pH adjustment and cooling (cooling to 50℃ while simultaneously reacting) is mixed with the washed phosphogypsum at a mass ratio of 6:1. After stirring for 32 minutes, the mixture is allowed to stand for 60 minutes, and the supernatant is separated.

[0022] S4: Water is added to the sediment after separating the supernatant and the mixture is re-slurryed until the solid content is 25%. Solid-liquid separation yields the treated phosphogypsum.

[0023] The cooling wastewater comes from the slag cooler and the cylindrical humidifier; the parameters of the cooling wastewater are shown in Table 1. Table 1

[0024] The wastewater from step S2 (washing), the supernatant from step S3, and the liquid from solid-liquid separation in step S4 are used as phosphogypsum return water. The parameters of the mixture of the wastewater from step S2 (washing), the supernatant from step S3, and the liquid from solid-liquid separation in step S4 (approximately 1.8 times the mass of the cooling wastewater) are shown in Table 2. Table 2

[0025] The parameters of phosphogypsum for each step are shown in Table 3: Table 3

[0026] As shown in Table 3, washing with water alone does not yield good extraction results for soluble phosphorus. Re-slurrying only slightly improves the extraction efficiency. The recovery rates of both soluble phosphorus and fluorine in this patent are around 50%.

[0027] Table 4 can be obtained from Tables 1 and 2: Table 4

[0028] As shown in Table 4, the method of this patent can significantly reduce the concentration of arsenic and iron ions in the treated wastewater.

[0029] Example 2 Example 2 provides a method for treating phosphogypsum, the method comprising: S1: Collect the cooling wastewater from the sulfuric acid production process using pyrite, and add quicklime to adjust the pH value to 7.5.

[0030] S2: Wash the phosphogypsum with water. The conditions for washing the phosphogypsum with water are as follows: mix the phosphogypsum and water at a mass ratio of 1:3.6, stir for 35 minutes, and then separate the solid and liquid.

[0031] S3: Mix the cooled wastewater (after adjusting pH and cooling down to 50℃) with the washed phosphogypsum at a mass ratio of 7:1, stir for 35 minutes, let stand for 75 minutes, and then separate the supernatant.

[0032] S4: Water is added to the sediment after separating the supernatant and the mixture is re-slurryed until the solid content is 24%. Solid-liquid separation yields the treated phosphogypsum.

[0033] Comparative Example 1 Washing was performed only with water, following the procedure in Example 1, and the washing was repeated twice. The results are shown in Table 5. Table 5

[0034] As can be seen from Table 5, the effect of water washing is generally poor, and increasing the number of water washes does not improve the effect.

[0035] Comparative Example 2 Water washing was performed as in Example 1, followed by alkaline washing (the pH was adjusted to approximately 8 with quicklime; the amount added was adjusted as needed, following the existing alkaline washing process). The results are shown in Table 6. Table 6

[0036] As can be seen from Table 6, adding alkaline washing can further improve the effect on the basis of water washing, but the effect is not as good as that of this patent, and a large amount of fluoride will be washed out, significantly increasing the fluoride content in the wastewater.

[0037] Comparative Example 3 Step S2 (i.e., water washing) was omitted, and the rest of the process was the same as in Example 1. The results are shown in Table 7: Table 7

[0038] As can be seen from Tables 5 and 7, although water washing is not very effective, it is necessary. The applicant speculates that the composition of phosphogypsum is complex and its acidity is low (water washing can remove some impurities and reduce acidity), while the composition of cooling wastewater is also complex, and direct mixing would cause unpredictable reactions.

[0039] Comparative Example 4 In step S1, the pH value was adjusted to 8.5 with and without adding quicklime, respectively; the other processes were the same as in Example 1. The results are shown in Table 8: Table 8

[0040] As can be seen from Table 8, in step S1, pH value only has a good effect within a specific range.

[0041] Comparative Example 5 In step S1, no quicklime was added, but in step S3, the same amount of quicklime as in Example 1 was added; the rest of the process was the same as in Example 1. The results are shown in Table 9: Table 9

[0042] As can be seen from Table 9, adding quicklime to the cooling wastewater in advance is more effective than adding it to Comparative Example 5.

[0043] Comparative Example 6 In step S3, instead of using cooling wastewater, tap water and quicklime were used, and the pH was adjusted to 7.4; the other processes were the same as in Example 1. The results are shown in Table 10: Table 10

[0044] As can be seen from Tables 6 and 10, using lime water directly in step S3 is not very effective.

[0045] Comparative Example 7 Step S3 is omitted; the remaining processes are the same as in Example 1. The results are shown in Table 11. Table 11

[0046] As can be seen from Table 11, in step S3, although cooling wastewater increases the extraction efficiency of fluorine, its extraction efficiency for soluble P is significantly better than that for fluorine, which is beneficial for the use of phosphogypsum recycled water.

[0047] Comparative Example 8 Step S4 is omitted; the remaining processes are consistent with Example 1. The results are shown in Table 12. Table 12

[0048] As can be seen from Table 12, repulping can further improve the extraction effect of soluble phosphorus and significantly increase the filtration speed.

[0049] Comparative Example 9 The experiments were conducted with three methods: step S4 omitted; tap water and quicklime used, with the pH adjusted to 7.4; and tap water and quicklime used, with the pH adjusted to 7.4, but step S4 omitted. All other procedures were the same as in Example 1. The results are shown in Table 13. Table 13

[0050] As can be seen from Table 13, using cooling wastewater significantly increases the difficulty of filtration and requires re-slurrying.

[0051] Comparative Example 10 The solid-to-water ratio in step S2 was adjusted, while the rest of the process remained the same as in Example 1. The results are shown in Table 14. Table 14

[0052] As can be seen from Table 14, increasing the amount of water can improve the effect, but after the amount of water is increased to a certain extent, the improvement in effect is negligible.

[0053] Comparative Example 11 The solid-to-water ratio in step S3 was adjusted, while the rest of the process remained the same as in Example 1. The results are shown in Table 15. Table 15

[0054] As can be seen from Table 15, increasing the amount of water can improve the effect, but after the amount of water is increased to a certain extent, the improvement in effect is negligible.

[0055] Comparative Example 12 The temperature of the cooling water in step S3 was adjusted, and the rest of the process was the same as in Example 1. The results are shown in Table 16: Table 16

[0056] As can be seen from Table 16, the effect decreases at room temperature; high temperature can slightly improve the effect, but the improvement on F is more significant, which is more detrimental to the use of phosphogypsum rehydrate.

[0057] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for treating phosphogypsum, characterized in that, The method includes: S1: Collect the cooling wastewater from the production of sulfuric acid from pyrite, and add quicklime or hydrated lime to adjust the pH value to 7.2-7.5; S2: Wash the phosphogypsum with water. The conditions for washing the phosphogypsum with water are as follows: mix the phosphogypsum and water at a mass ratio of 1:3-4, stir, and then separate the solid and liquid. The stirring time for washing the phosphogypsum with water is 20-35 minutes. S3: Cool the cooling wastewater to 40-55℃, mix the pH-adjusted cooling wastewater with the washed phosphogypsum at a mass ratio of 5-8:1, stir for 30-45 minutes, let stand for 45-120 minutes, and separate the supernatant. S4: Add water to the sediment after separating the supernatant and re-slurry until the solid content is 23-28%. Solid-liquid separation yields the treated phosphogypsum. The cooling wastewater comes from the slag cooler and the cylindrical humidifier; the wastewater after washing in step S2, the supernatant in step S3, and the liquid from solid-liquid separation in step S4 are used as phosphogypsum return water.

2. The method for treating phosphogypsum according to claim 1, characterized in that, Solid-liquid separation is performed using pressure filtration.

3. The method for treating phosphogypsum according to claim 1, characterized in that, In step S3, stir for 30-40 minutes, then let stand for 60-90 minutes.

Images