Method for recycling rare earth oxalic acid precipitation mother liquor
By using oxalic acid as a precipitant and multiple pH adjustments, the problems of resource waste and environmental pollution from rare earth oxalic acid precipitation mother liquor have been solved, achieving efficient recovery of rare earth and zero resource discharge, reducing processing costs and improving economic benefits.
Patent Information
- Application Number
- CN202511581150.0
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2025-10-31
- Publication Date
- 2026-01-16
AI Technical Summary
Existing rare earth oxalic acid precipitation mother liquor treatment technologies suffer from resource waste and environmental pollution. Traditional treatment methods neglect the resource recycling value of wastewater and result in the accumulation of harmful ions and high treatment costs.
Oxalic acid precipitant is used for dissolution and precipitation reaction. Combined with multiple pH adjustments and pressure filtration, calcium ions react with oxalic acid and fluoride ions to form a precipitate. Rare earth and ammonium chloride are recovered by evaporation and crystallization, thus realizing the resource utilization of rare earth oxalic acid precipitation mother liquor.
It achieves efficient rare earth recovery and zero resource emissions, reduces the amount of neutralization residue to be treated, lowers treatment costs, and transforms wastewater into usable resources, thereby improving economic efficiency.
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Figure CN121342048A_ABST
Abstract
Description
TECHNICAL FIELD
[0001] The present application relates to a rare earth oxalate precipitation mother liquor treatment process, more particularly to a method for resource utilization of a rare earth oxalate precipitation mother liquor. BACKGROUND
[0002] Due to the high recovery rate, good crystal form, easy filtration and other advantages of using oxalic acid to precipitate rare earths, oxalic acid is widely used for precipitating rare earths. In this process, an excess of oxalic acid is usually added to ensure the recovery rate of rare earths, resulting in the supernatant of the precipitation, i.e. the rare earth oxalate precipitation mother liquor (oxalate precipitation wastewater, oxalate precipitation mother liquor), containing a large amount of hydrochloric acid and oxalic acid, wherein the content of hydrochloric acid is about 2.5 mol / L, the content of oxalic acid is about 0.2 mol / L, and it also contains Fe, Al, Ca, Mg and other metal impurities. The main characteristics of the oxalate precipitation wastewater are strong acidity, high corrosion, high COD, and it has the characteristics of complex composition and difficult treatment.
[0003] Currently, the treatment technologies for the oxalate precipitation wastewater include direct reuse, neutralization and precipitation, extraction method, etc. The direct reuse method uses the oxalate precipitation wastewater to dissolve the rare earth neutralization residue, which can fully utilize the hydrochloric acid in the wastewater, and also can reuse the oxalate and part of the rare earth elements. The direct reuse method is simple in operation and has high utilization rate of hydrochloric acid and oxalic acid, but the recycling will cause the enrichment of harmful ions, affecting the product quality, which is the main factor restricting the application of this method.
[0004] The precipitation method uses calcium hydroxide to neutralize the acidic oxalate precipitation mother liquor, and at the same time, the oxalate is precipitated and removed in the form of calcium oxalate; or an excess of rare earth oxide is added to the oxalate precipitation mother liquor, so that the oxalate is precipitated in the form of rare earth oxalate. Although the precipitation method is simple in operation, it has large amount of reagents, large amount of drainage and a lot of solid waste.
[0005] The extraction method separates hydrochloric acid and oxalic acid by taking advantage of their different solubilities in organic solvents, and there are two ways of extracting oxalic acid and extracting hydrochloric acid. The extractant used in the extraction of oxalic acid process is mainly phosphorus extractant, and sulfonated kerosene is used as the solvent. This technology can make the extraction rate of oxalic acid and hydrochloric acid reach more than 90%, but the concentration of the extracted hydrochloric acid and oxalic acid is low, which is not suitable for direct reuse, and at the same time, it also produces extraction wastewater.
[0006] Under the background of increasing resource shortage and increasingly strict environmental protection requirements, the treatment and resource utilization of industrial wastewater has become a key link to realize sustainable development. The discharge of oxalate precipitation wastewater, which is generated in the rare earth smelting process, not only causes waste of water resources, but also may have negative impact on the environment. The traditional treatment method of oxalate precipitation wastewater ignores the potential resource recycling value of the wastewater. At the same time, in the rare earth smelting process, the consumption of water resources is large, and the cost of obtaining fresh water resources is increasing, while the environmental protection pressure of wastewater discharge is also increasing.
[0007] If the grass sinking wastewater can be effectively treated and reused in the rare earth smelting process, not only the production cost can be reduced, but also the pollution to the environment can be reduced, realizing the win-win of economic benefit and environmental benefit. Therefore, the feasibility of grass sinking wastewater resource utilization needs to be explored. SUMMARY
[0008] The present application aims at the deficiencies of the prior art, and provides a rare earth oxalate precipitation mother liquor resource utilization method with novel process, lower treatment cost and good effect.
[0009] The technical scheme of the present application is implemented as follows: a rare earth oxalate precipitation mother liquor resource utilization method, comprising the following steps:
[0010] (1) adding an oxalate precipitant: adding an oxalate precipitant into the rare earth oxalate precipitation mother liquor, performing dissolution and precipitation reaction for 10-30 min, and stirring at a speed of 100-500 rpm to obtain a pretreated oxalate precipitation mother liquor;
[0011] (2) adjusting the pH value once: adjusting the pH value of the pretreated oxalate precipitation mother liquor obtained in step (1) to 2-5, and standing for crystallization for 1-24 h to obtain an oxalate precipitation mother liquor after pH adjustment;
[0012] (3) primary pressure filtration: performing pressure filtration treatment on the oxalate precipitation mother liquor after pH adjustment obtained in step (2) to separate out a rare earth enrichment and pressure filtration wastewater;
[0013] (4) adjusting the pH value twice: adjusting the pH value of the pressure filtration wastewater obtained in step (3) to 3-8 to obtain pressure filtration wastewater after pH adjustment;
[0014] (5) secondary pressure filtration: performing pressure filtration treatment on the pressure filtration wastewater after pH adjustment obtained in step (4) to separate out neutralization residue and an ammonium chloride solution;
[0015] (6) evaporation crystallization: conveying the ammonium chloride solution obtained in step (5) to a horizontal-tube falling-film evaporation device to perform evaporation crystallization, and separating out ammonium chloride crystals in a centrifuge to recover condensate water.
[0016] In the above-mentioned rare earth oxalate precipitation mother liquor resource utilization method, the amount of the oxalate precipitant added in step (1) is 10-50 kg / m 3 .
[0017] In the above-mentioned rare earth oxalate precipitation mother liquor resource utilization method, the oxalate precipitant in step (1) is one or more of calcium hydroxide, calcium chloride, calcium carbonate and calcium oxide in any mixture.
[0018] The method for recycling the rare earth oxalate precipitation mother liquor comprises the following steps: (1) preparing a rare earth oxalate precipitation mother liquor; (2) adjusting the pH value of the rare earth oxalate precipitation mother liquor; (3) filtering the rare earth oxalate precipitation mother liquor; (4) adjusting the pH value of the filtered rare earth oxalate precipitation mother liquor; and (5) recycling the rare earth oxalate precipitation mother liquor. 3 The pH adjusting agent is ammonia water, ammonium bicarbonate or ammonium carbonate.
[0019] The pH adjusting agent is ammonia water, ammonium bicarbonate or ammonium carbonate.
[0020] The pH adjusting agent is ammonia water, ammonium bicarbonate or ammonium carbonate.
[0021] The method for recycling the rare earth oxalate precipitation mother liquor comprises the following steps: (1) preparing a rare earth oxalate precipitation mother liquor; (2) adjusting the pH value of the rare earth oxalate precipitation mother liquor; (3) filtering the rare earth oxalate precipitation mother liquor; (4) adjusting the pH value of the filtered rare earth oxalate precipitation mother liquor; and (5) recycling the rare earth oxalate precipitation mother liquor. 3 The pH adjusting agent is ammonia water, ammonium bicarbonate or ammonium carbonate.
[0022] The pH adjusting agent is ammonia water, ammonium bicarbonate or ammonium carbonate.
[0023] The method for recycling the rare earth oxalate precipitation mother liquor comprises the following steps: (1) preparing a rare earth oxalate precipitation mother liquor; (2) adjusting the pH value of the rare earth oxalate precipitation mother liquor; (3) filtering the rare earth oxalate precipitation mother liquor; (4) adjusting the pH value of the filtered rare earth oxalate precipitation mother liquor; and (5) recycling the rare earth oxalate precipitation mother liquor.
[0024] The method for recycling the rare earth oxalate precipitation mother liquor comprises the following steps: (1) preparing a rare earth oxalate precipitation mother liquor; (2) adjusting the pH value of the rare earth oxalate precipitation mother liquor; (3) filtering the rare earth oxalate precipitation mother liquor; (4) adjusting the pH value of the filtered rare earth oxalate precipitation mother liquor; and (5) recycling the rare earth oxalate precipitation mother liquor.
[0025] The ammonium chloride solution is subjected to an evaporation crystallization step, realizing zero discharge of the rare earth oxalate precipitation mother liquor, and recovering ammonium chloride crystals and condensate water. After pH adjustment and pressure filtration, the main component of the ammonium chloride solution obtained is ammonium chloride, and the content of the remaining impurities is within an acceptable range, so the ammonium chloride crystals are sellable agricultural-grade ammonium chloride, and the condensate water can be used to configure a specific concentration of a medicament for a rare earth smelting process. BRIEF DESCRIPTION OF DRAWINGS
[0026] The application will be further described in conjunction with the embodiments in the accompanying drawings, but does not constitute any limitation on the application.
[0027] Figure 1 The figure is a process flow diagram of the application. DETAILED DESCRIPTION
[0028] Referring to Figure 1 The method for resource utilization of the rare earth oxalate precipitation mother liquor of the application comprises the following steps:
[0029] (1) Adding an oxalate precipitant: 10-50 kg / m 3 of the oxalate precipitant is added to the rare earth oxalate precipitation mother liquor to perform a dissolution and precipitation reaction for 10-30 min at a stirring rate of 100-500 rpm, to obtain a pretreated oxalate precipitation mother liquor; the oxalate precipitant is one or more of any mixture of calcium hydroxide, calcium chloride, calcium carbonate and calcium oxide.
[0030] (2) Adjusting the pH once: the pretreated oxalate precipitation mother liquor obtained in step (1) is slowly added to 10-200 kg / m 3 of a pH adjuster to perform a dissolution and precipitation reaction for 30-180 min at a stirring rate of 100-500 rpm, to adjust the pH value of the wastewater to 2-5, and to stand for crystallization for 1-24 h, to obtain the oxalate precipitation mother liquor after pH adjustment. The pH adjuster is one or more of any mixture of ammonia, ammonium bicarbonate and ammonium carbonate. Preferably, the standing crystallization time is 12-16 h.
[0031] (3) Pressure filtration once: the oxalate precipitation mother liquor after pH adjustment obtained in step (2) is subjected to pressure filtration treatment, to separate out a rare earth concentrate and pressure filtration wastewater.
[0032] (4) Adjusting the pH twice: the pressure filtration wastewater obtained in step (3) is slowly added to 10-50 kg / m 3 of a pH adjuster to perform a dissolution and precipitation reaction for 30-180 min at a stirring rate of 100-500 rpm, to adjust the pH value of the wastewater to 3-8, to obtain the pressure filtration wastewater after pH adjustment. The pH adjuster is one or more of any mixture of ammonia, ammonium bicarbonate and ammonium carbonate.
[0033] (5) Secondary pressure filtration: the pH-adjusted pressure filtration wastewater obtained in step (4) is subjected to pressure filtration treatment to separate neutralization residue and ammonium chloride solution.
[0034] (6) Evaporative crystallization: the ammonium chloride solution obtained in step (5) is transported to a horizontal-tube falling-film evaporation device for evaporative crystallization, and ammonium chloride crystals are separated into a centrifuge, and condensate water is recovered.
[0035] Example 1:
[0036] The oxalic acid concentration of the rare earth oxalic acid precipitation mother liquor is 10.5 g / L, the fluorine ion concentration is 860 ppm, and the aluminum ion concentration is 2300 ppm.
[0037] (1) Adding oxalic acid precipitant: 12 kg / m 3 Calcium hydroxide is added to the rare earth oxalic acid precipitation mother liquor to perform dissolution and precipitation reaction for 15 min at a stirring rate of 100 rpm to obtain pretreated oxalic acid precipitation mother liquor.
[0038] (2) First pH adjustment: the pretreated oxalic acid precipitation mother liquor obtained in step (1) is slowly added to 10 kg / m 3 Ammonium bicarbonate is added to perform dissolution and precipitation reaction for 30 min at a stirring rate of 100 rpm to adjust the pH value of the wastewater to 2, and crystallization is performed for 1 h to obtain pH-adjusted oxalic acid precipitation mother liquor.
[0039] (3) First pressure filtration: the pH-adjusted oxalic acid precipitation mother liquor obtained in step (2) is subjected to pressure filtration treatment to separate rare earth enrichment and pressure filtration wastewater.
[0040] (4) Second pH adjustment: the pressure filtration wastewater obtained in step (3) is slowly added to 50 kg / m 3 Ammonia is added to perform dissolution and precipitation reaction for 180 min at a stirring rate of 500 rpm to adjust the pH value of the wastewater to 3 to obtain pH-adjusted pressure filtration wastewater.
[0041] (5) Secondary pressure filtration: the pH-adjusted pressure filtration wastewater obtained in step (4) is subjected to pressure filtration treatment to separate neutralization residue and ammonium chloride solution.
[0042] (6) Evaporative crystallization: the ammonium chloride solution obtained in step (5) is transported to a horizontal-tube falling-film evaporation device for evaporative crystallization, and ammonium chloride crystals are separated into a centrifuge, and condensate water is recovered.
[0043] After drying, the rare earth content of the rare earth enrichment is 5.2%, and the fluorine ion, aluminum ion, and oxalic acid concentrations in the condensate water are all less than 100 ppm.
[0044] Example 2:
[0045] The oxalic acid concentration of the rare earth oxalate precipitation mother liquor is 6.2 g / L, the fluorine ion concentration is 930 ppm, and the aluminum ion concentration is 1600 ppm.
[0046] (1) Adding oxalic acid precipitant: 50 kg / m 3 Calcium chloride is added to the rare earth oxalate precipitation mother liquor, and dissolution and precipitation reaction is carried out for 30 min, with a stirring rate of 500 rpm, to obtain a pretreated oxalate precipitation mother liquor.
[0047] (2) First pH adjustment: The pretreated oxalate precipitation mother liquor obtained in step (1) is slowly added to 200 kg / m 3 Ammonia water adjusting agent is added to the pretreated oxalate precipitation mother liquor, and dissolution and precipitation reaction is carried out for 180 min, with a stirring rate of 500 rpm, to adjust the pH value of the wastewater to 5, and crystallization is carried out for 24 h, to obtain an oxalate precipitation mother liquor after pH adjustment.
[0048] (3) First pressure filtration: The oxalate precipitation mother liquor after pH adjustment obtained in step (2) is subjected to pressure filtration treatment, to separate rare earth enrichment and pressure filtration wastewater.
[0049] (4) Second pH adjustment: The pressure filtration wastewater obtained in step (3) is slowly added to 10 kg / m 3 Ammonium carbonate is added to the pressure filtration wastewater, and dissolution and precipitation reaction is carried out for 30 min, with a stirring rate of 100 rpm, to adjust the pH value of the wastewater to 8, to obtain pressure filtration wastewater after pH adjustment.
[0050] (5) Second pressure filtration: The pressure filtration wastewater after pH adjustment obtained in step (4) is subjected to pressure filtration treatment, to separate neutralization residue and ammonium chloride solution.
[0051] (6) Evaporation crystallization: The ammonium chloride solution obtained in step (5) is transported to a horizontal tube falling film evaporation device for evaporation crystallization, and ammonium chloride crystals are separated in a centrifuge, and condensate water is recovered.
[0052] The rare earth content of the rare earth enrichment after drying is 6.7%, and the fluorine ion concentration, aluminum ion concentration and oxalic acid concentration in the condensate water are all less than 100 ppm.
[0053] Example 3:
[0054] The oxalic acid concentration of the rare earth oxalate precipitation mother liquor is 6.2 g / L, the fluorine ion concentration is 680 ppm, and the aluminum ion concentration is 3200 ppm.
[0055] (1) Adding oxalic acid precipitant: 35 kg / m 3 Calcium oxide and calcium carbonate are added to the rare earth oxalate precipitation mother liquor, and dissolution and precipitation reaction is carried out for 20 min, with a stirring rate of 300 rpm, to obtain a pretreated oxalate precipitation mother liquor.
[0056] (2) One-time pH adjustment: the pretreated grass sinking mother liquor obtained in step (1) is slowly added to 130 kg / m 3 ammonium carbonate, and a dissolution and precipitation reaction is carried out for 120 min at a stirring rate of 300 rpm, the pH value of the wastewater is adjusted to 4, and crystallization is carried out for 16 h to obtain the pH-adjusted grass sinking mother liquor.
[0057] (3) One-time pressure filtration: the pH-adjusted grass sinking mother liquor obtained in step (2) is subjected to pressure filtration treatment to separate out rare earth enrichment and pressure filtration wastewater.
[0058] (4) Secondary pH adjustment: the pressure filtration wastewater obtained in step (3) is slowly added to 30 kg / m3ammonia water, and a dissolution and precipitation reaction is carried out for 130 min at a stirring rate of 300 rpm, the pH value of the wastewater is adjusted to 6 to obtain the pH-adjusted pressure filtration wastewater.
[0059] (5) Secondary pressure filtration: the pH-adjusted pressure filtration wastewater obtained in step (4) is subjected to pressure filtration treatment to separate out neutralization slag and ammonium chloride solution.
[0060] (6) Evaporative crystallization: the ammonium chloride solution obtained in step (5) is transported to a horizontal-tube falling-film evaporation device for evaporative crystallization, and ammonium chloride crystals are separated out in a centrifuge, and condensate water is recovered.
[0061] After drying, the rare earth content of the rare earth enrichment is 5.7%, and the concentrations of fluoride ions, aluminum ions and oxalic acid in the condensate water are all less than 100 ppm.
[0062] Example 4:
[0063] The oxalic acid concentration of the rare earth oxalate precipitation mother liquor is 6.2 g / L, the fluoride ion concentration is 930 ppm, and the aluminum ion concentration is 1600 ppm.
[0064] (1) Adding an oxalic acid precipitant: 10 kg / m 3 calcium oxide is added to the rare earth oxalate precipitation mother liquor, and a dissolution and precipitation reaction is carried out for 10 min at a stirring rate of 270 rpm to obtain a pretreated grass sinking mother liquor.
[0065] (2) One-time pH adjustment: the pretreated grass sinking mother liquor obtained in step (1) is slowly added to 142 kg / m 3 ammonium bicarbonate, and a dissolution and precipitation reaction is carried out for 150 min at a stirring rate of 400 rpm, the pH value of the wastewater is adjusted to 5, and crystallization is carried out for 24 h to obtain the pH-adjusted grass sinking mother liquor.
[0066] (3) One-time pressure filtration: the pH-adjusted grass sinking mother liquor obtained in step (2) is subjected to pressure filtration treatment to separate out rare earth enrichment and pressure filtration wastewater.
[0067] (4) Secondary pH adjustment: slowly add the pressure filtration wastewater obtained in step (3) into 10 kg / m 3 ammonia water, and carry out dissolution and precipitation reaction for 60 min, with a stirring speed of 300 rpm, and adjust the pH value of the wastewater to 7 to obtain the pH-adjusted pressure filtration wastewater.
[0068] (5) Secondary pressure filtration: carry out pressure filtration treatment on the pH-adjusted pressure filtration wastewater obtained in step (4) to separate out neutralization residue and ammonium chloride solution.
[0069] (6) Evaporation crystallization: deliver the ammonium chloride solution obtained in step (5) to a horizontal-tube falling-film evaporation device to carry out evaporation crystallization, separate out ammonium chloride crystals in a centrifuge, and recover condensate water.
[0070] After drying, the rare earth content of the rare earth enrichment material is 7.9%, and the concentrations of fluorine ions, aluminum ions and oxalic acid in the condensate water are all less than 100 ppm. Experimental Example
[0071] Randomly use the process of Example 4 of the present application and the traditional process using lime neutralization to treat 1 cubic meter of grass sediment mother liquor, and the results are as follows:
[0072] Table 1 Treatment cost table of 1 cubic meter of grass sediment mother liquor
[0073] Project name Lime neutralization Example 4 process Remarks Lime amount (kg) 72.2 10 Lime cost (yuan) 9.24 1.28 128 yuan / ton Ammonium bicarbonate amount (kg) / 133 Ammonium bicarbonate cost (yuan) / 86.4-127.4 958 yuan / ton, can buy low-cost carbon ammonium to 650 yuan Ammonia amount (L) / 10 Ammonia cost (yuan) / 11.06 1106 yuan / cubic Solid waste residue weight (kg) 86 76 Solid waste residue treatment fee (yuan) 29.2 25.8 340 yuan / ton Ammonium chloride MVR cost (yuan) 0 110 110 yuan / cubic Calcium chloride MVR cost (yuan) 200 0 200 yuan / cubic Calcium chloride yield (kg) 247 / Calcium chloride selling price (yuan) 0 / Offset cost Ammonium chloride yield (kg) / 89.9 Ammonium chloride selling price (yuan) / 27 Sell 300 yuan / ton Recovered rare earth weight (kg) / 0.67 Contains 2.83% rare earth after burning Rare earth value / 122.6 Rare earth raw ore 183,000 yuan / ton Ozone electricity / / Literature data Total cost (yuan) 238.4 84.9-125.9
[0074] As can be seen from Table 1, compared with the existing traditional lime neutralization process, the process of the present application not only can save a large amount of lime consumption, but also can save the equipment cost, worker wages and accessory cost for maintaining the calcium chloride MVR system, and can play the scale effect of the ammonium chloride MVR system, thereby greatly reducing the treatment cost of the grass sediment mother liquor. At the same time, the process of the present application can also increase the annual output of REO (rare earth oxide) by about 5 tons, thereby further improving the economic benefit of the process.
[0075] The above examples are preferred embodiments of the present application, which are only used to facilitate the description of the present application and do not limit the present application in any form. Any person skilled in the art can make partial changes or modifications to equivalent embodiments within the scope of the technical features of the present application without departing from the technical features of the present application, and such equivalent embodiments still belong to the scope of the technical features of the present application.
Claims
1. A method for resource utilization of rare earth oxalate precipitation mother liquor, characterized in that, The method comprises the following steps: (1) adding oxalic acid precipitant: adding oxalic acid precipitant into the rare earth oxalate precipitation mother liquor, and performing dissolution and precipitation reaction for 10-30 min at a stirring speed of 100-500 rpm to obtain a pretreated oxalate precipitation mother liquor; (2) adjusting pH value for the first time: adjusting the pH value of the pretreated oxalate precipitation mother liquor obtained in step (1) to 2-5, and standing for crystallization for 1-24 h to obtain an oxalate precipitation mother liquor after pH adjustment; (3) performing pressure filtration for the first time: performing pressure filtration treatment on the oxalate precipitation mother liquor after pH adjustment obtained in step (2) to separate out rare earth enrichment and pressure filtration wastewater; (4) adjusting pH value for the second time: adjusting the pH value of the pressure filtration wastewater obtained in step (3) to 3-8 to obtain pressure filtration wastewater after pH adjustment; (5) performing pressure filtration for the second time: performing pressure filtration treatment on the pressure filtration wastewater after pH adjustment obtained in step (4) to separate out neutralization residue and ammonium chloride solution; (6) evaporation crystallization: delivering the ammonium chloride solution obtained in step (5) to a horizontal-tube falling-film evaporation device to perform evaporation crystallization, and separating out ammonium chloride crystals in a centrifuge, and recovering condensate water.
2. The method according to claim 1, characterized in that, The amount of the oxalic acid precipitant added in step (1) is 10-50 kg / m 3 .
3. The method according to claim 1, characterized in that, The oxalic acid precipitant in step (1) is any mixture of one or more of calcium hydroxide, calcium chloride, calcium carbonate and calcium oxide.
4. The method for resource utilization of rare earth oxalate precipitation mother liquor according to claim 1, characterized in that, Step (2) is specifically as follows: once pH adjustment: the pretreated grass sediment mother liquor obtained in step (1) is slowly added into 10-200 kg / m 3 pH regulator, and dissolution and precipitation reaction is carried out for 30-180 min, the stirring rate is 100-500 rpm, the pH value of the wastewater is adjusted to 2-5, and crystallization is carried out for 1-24 h to obtain the pH-adjusted grass sediment mother liquor.
5. The method according to claim 4, characterized in that, The pH adjusting agent in step (2) is any mixture of one or more of ammonia water, ammonium bicarbonate and ammonium carbonate.
6. The method according to claim 1 or 4, characterized in that, The standing crystallization time is 12-16 h.
7. The method according to claim 1, characterized in that, Step (4) is specifically: secondary pH adjustment: slowly add the pressure filtration wastewater obtained in step (3) to 10-50 kg / m 3 pH adjusting agent, carry out dissolution and precipitation reaction for 30-180 min, the stirring rate is 100-500 rpm, the wastewater pH value is adjusted to 3-8, and the pH-adjusted pressure filtration wastewater is obtained.
8. The method according to claim 7, characterized in that, The pH adjusting agent in step (4) is any mixture of one or more of ammonia water, ammonium bicarbonate and ammonium carbonate.