Method for recovering fluorine and silicon resources from phosphorus-containing acidic wastewater

Abstract

The present application relates to the technical field of wastewater treatment, and particularly relates to a method for recycling fluorine and silicon resources in phosphorus-containing acidic wastewater. The present application uses a coagulation coupling chemical precipitation method to remove fluorine and silicon in the phosphorus-containing acidic wastewater, and then processes the waste residue to recycle the fluorine and silicon resources respectively. Through the coagulation coupling chemical precipitation method, the removal rate of silicon in the acidic wastewater can be more than 80%, and the removal rate of fluorine can be more than 90%; through the purification of the precipitation, calcium fluoride with a purity of more than 78% can be obtained. The pH value of the treated acidic wastewater is basically unchanged, the loss rate of phosphorus is 0-10%, the treatment effect is good, the investment is small, the operation is simple, the present application is easy to implement, and the present application is suitable for a wide range of applications.

Description

Technical Field

[0001] This invention relates to the field of wastewater treatment technology, and more particularly to a method for recovering fluorine and silicon resources from phosphoric acid-containing wastewater. Background Technology

[0002] Fluorine in the natural environment exists primarily in the form of fluorite and associated phosphate rock. As of 2009, the world's basic fluorite reserves reached 470 million tons, with proven recoverable reserves of 230 million tons (calculated as 100% CaF2). Currently, the United States, Japan, and Western Europe, as major fluorite consumers, have exhausted their fluorite resources. In contrast, my country's fluorite mining situation is characterized by a scarcity of high-grade ore, a prevalence of low-grade ore, a large amount of difficult-to-process ore, and a shortage of easily processed ore. Furthermore, my country's recoverable fluorite reserves are less than one-tenth of the global total, but its production accounts for approximately 60% of global production, indicating a severe problem of over-exploitation.

[0003] In the production of wet-process phosphoric acid, the byproduct phosphogypsum is typically separated by a wet slag removal method, then sent to a slurry re-slurry tank for further slurrying; finally, it is transported via pipeline to a phosphogypsum slag yard for storage. After settling, most of the soluble phosphorus, soluble fluoride ions, and soluble silicon elements contained in the phosphogypsum dissolve into the water, forming acidic wastewater containing large amounts of phosphorus, fluoride, and silicon. During the reuse of this wastewater as process water, phosphorus can be reused in the production of wet-process phosphoric acid, while fluoride and silicon increase the burden on the purification of wet-process phosphoric acid products. Simultaneously, continuous circulation leads to a continuous increase in the fluoride and silicon content in the slag yard return water. When this exceeds the solubility of calcium fluoride and calcium fluorosilicate (magnesium, potassium, etc.), fluoride ions will deposit on the pipe walls in the form of scale, hindering fluid transport and indirectly wasting fluoride resources from phosphate rock. In the semiconductor industry, wet etching is commonly used in the fabrication of photovoltaic materials and chips. Wet etching uses an etchant to react with a thin film to remove the parts that need to be etched. It is a relatively simple and low-cost method that can usually be used at room temperature. Fluorosilicic acid is a commonly used etchant for silicon dioxide, and phosphoric acid is a commonly used etchant for silicon nitride. Wet etching processes inevitably generate acidic wastewater containing phosphorus, fluorine, and silicon during production. If this wastewater is not treated, it poses a potential risk of environmental pollution; conversely, failing to recover fluorine and silicon resources also results in a waste of these resources.

[0004] There are many methods for treating fluoride and silicon-containing wastewater both domestically and internationally, such as chemical precipitation, coagulation sedimentation, adsorption, reverse osmosis, ion exchange, electrodialysis, and electrocoagulation. Chemical precipitation and coagulation sedimentation are traditional water treatment methods. The most common chemical reagents for fluoride removal are Ca(OH)₂ / CaO / CaCO₃, while reagents for silicon removal include sodium hydroxide / sodium carbonate and aluminum sulfate. Common flocculants are divided into two main categories: aluminum salts and iron salts; common coagulant aids are polyacrylamide (PAM) and polyaluminum chloride (PAC). Through chemical reactions and coagulation sedimentation, fluoride and silicon are separated in the form of CaF₂, CaSiF₆, and silicates. A disadvantage is the generation of large amounts of sludge, which increases the difficulty of subsequent treatment. Fluoride-containing filter residue is treated as waste, resulting in significant resource waste. In addition to relatively inexpensive adsorbents such as traditional activated carbon and molecular sieves, various new adsorbents are constantly being synthesized and applied in adsorption methods. The biggest problem with adsorption is that adsorption is easy, but desorption is difficult. Therefore, it is usually only suitable for treating wastewater or drinking water with low fluoride and silicon content. Furthermore, the cost of adsorption, especially the cost of novel synthetic adsorbents, may not be cost-effective for treating industrial wastewater. Membrane separation technology has wide applications in separation, concentration, and extraction. In the treatment of fluoride and silicon-containing wastewater, reverse osmosis and electrodialysis membrane separation are currently the most studied methods. Existing research shows that high energy consumption, demanding operating conditions, high membrane costs, and susceptibility to fouling limit the application of this technology in high-fluoride industrial wastewater. The principle of electrodialysis is to utilize the selective permeability of ion exchange membranes under an applied DC electric field to allow the directional migration of anions and cations in water. Ion exchange membranes are formed from ion exchange resins, so electrodialysis is actually another application of ion exchange resin methods. Electrodialysis has many applications in seawater desalination, but its selectivity is low for treating high-fluoride and silicon-containing wastewater. Electrocoagulation, a combination of electrolysis and flocculation, is used in wastewater defluorination. It mainly utilizes aluminum or iron electrodes under a DC electric field, where dissolved aluminum or iron ions hydrolyze in water to form different forms of hydroxides, which adsorb fluoride and silicon in the water. Electrocondensation is simple and safe to operate, but it is energy-intensive; when the fluorine concentration is high, the electrodes are easily passivated and new aluminum or iron ions are introduced.

[0005] The methods mentioned above are mostly based on the perspective of ensuring that industrial wastewater meets discharge standards. Therefore, the pH value of the treated wastewater is generally between 7 and 9, the loss rate of phosphorus, fluorine and silicon is very high, and the recycling rate of the treated waste residue is not high. Summary of the Invention

[0006] In view of this, the technical problem to be solved by the present invention is to provide a method for effectively recovering fluorine and silicon resources from phosphoric acid-containing wastewater with low cost, simple process, and minimal loss of phosphorus resources.

[0007] This invention provides a method for recovering fluorine and silicon resources from phosphoric acid-containing wastewater, comprising the following steps:

[0008] A) Acidic wastewater is treated with coagulants to remove silica, resulting in primary treated water and coagulated precipitate;

[0009] B) Wash the coagulated precipitate to obtain the silica gel product;

[0010] The primary treated water was defluorinated using a precipitant to obtain secondary treated water and crude calcium fluoride.

[0011] C) The crude calcium fluoride is purified to obtain the calcium fluoride product.

[0012] Preferably, in step A), the acidic wastewater has a phosphorus content of 3000 mg / L or more, a fluoride content of 500 mg / L or more, a silicon content of 200 mg / L or more, and a pH value of 1 to 4.

[0013] Preferably, in step A), the coagulant includes one or two of polyaluminum chloride, polyaluminum sulfate, polyferric chloride, polyferric sulfate, polyacrylamide, potassium aluminum sulfate dodecahydrate, ferric sulfate, ferric chloride, and ferrous ammonium sulfate.

[0014] In the acidic wastewater, the dosage of coagulant is 0.2–5 g / L.

[0015] Preferably, in step A), the silicon removal process includes:

[0016] After mixing the coagulant and acidic wastewater, let it stand to settle, and then separate the solid and liquid components.

[0017] The stirring and mixing temperature is 0–50℃, the rotation speed is 100–500 r / min, and the time is 5–30 min;

[0018] The settling time is 10–30 minutes;

[0019] In the primary treated water, the silicon removal rate is over 70%.

[0020] Preferably, in step B), the cleaning agent used for washing includes sodium hydroxide solution and / or sodium carbonate solution;

[0021] The dosage of cleaning agent in the coagulated precipitate is 0.1–10 g / g;

[0022] The washing is carried out under stirring conditions; the stirring speed is 100-500 rpm. The washing temperature is 0-50℃ and the washing time is 10-60 min.

[0023] Preferably, in step B), the precipitant includes one or two of CaCO3, Ca(OH)2, CaO, CaCl2, calcium citrate, calcium oxalate, and calcium lactate;

[0024] In the primary treated water, the dosage of the precipitant is 1–10 g / L.

[0025] Preferably, in step B), the defluorination treatment includes:

[0026] The precipitant was stirred and mixed with the primary treated water, and after settling, solid-liquid separation was performed.

[0027] The stirring speed is 100-500 rpm, the temperature is 0-50℃, and the time is 10-120 min;

[0028] The settling time is 0.2 to 72 hours.

[0029] Preferably, in step B), the phosphorus loss rate in the secondary treated water is less than 5%, the total fluoride recovery rate is more than 70%, and the total silicon removal rate is more than 80%.

[0030] Preferably, in step C), the purification reagent used in the purification process is one or two of the following: purified water, tap water, the secondary treated water, sulfuric acid, hydrochloric acid, nitric acid, phosphoric acid, fluorosilicic acid, citric acid, lactic acid, and ascorbic acid.

[0031] The ratio of crude calcium fluoride to purification reagent is 1–50 g / L.

[0032] Preferably, in step C), the purification process includes:

[0033] After mixing the crude calcium fluoride with the purification reagent, solid-liquid separation was performed.

[0034] The stirring speed is 100-500 rpm and the time is 5-120 min.

[0035] This invention provides a method for recovering fluorine and silicon resources from phosphoric acid-containing wastewater, comprising the following steps: A) treating acidic wastewater with a coagulant to remove silicon, obtaining primary treated water and coagulated precipitate; B) washing the coagulated precipitate to obtain silica gel product; treating the primary treated water with a precipitant to remove fluoride, obtaining secondary treated water and crude calcium fluoride; C) purifying the crude calcium fluoride to obtain calcium fluoride product. This invention utilizes a coagulation-coupled chemical precipitation method to first remove fluorine and silicon from phosphoric acid-containing wastewater, and then treat the waste residue, thereby recovering fluorine and silicon resources separately. By coupling coagulation with chemical precipitation, the removal rate of silicon in acidic wastewater can be above 80%, and the removal rate of fluoride above 90%; by purifying the precipitate, calcium fluoride with a purity of above 78% can be obtained. The pH value of the acidic wastewater treated by this invention remains acidic, the phosphorus loss rate is 0-10%, the treatment effect is good, the investment is low, the operation is simple, easy to implement, and the application range is wide. Attached Figure Description

[0036] Figure 1 A process flow diagram for the recovery of fluorine and silicon resources from phosphoric acid-containing wastewater is provided as an embodiment of the present invention. Detailed Implementation

[0037] The technical solution of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0038] This invention provides a method for recovering fluorine and silicon resources from phosphoric acid-containing wastewater, comprising the following steps:

[0039] A) Acidic wastewater is treated with coagulants to remove silica, resulting in primary treated water and coagulated precipitate;

[0040] B) Wash the coagulated precipitate to obtain the silica gel product;

[0041] The primary treated water was defluorinated using a precipitant to obtain secondary treated water and crude calcium fluoride.

[0042] C) The crude calcium fluoride is purified to obtain the calcium fluoride product.

[0043] In step A):

[0044] A coagulant was used to remove silica from acidic wastewater, resulting in primary treated water and coagulated precipitate.

[0045] In some embodiments of the present invention, the acidic wastewater comes from phosphoric acid-containing industrial wastewater generated by phosphoric acid chemical enterprises producing phosphate fertilizers, phosphoric acid, phosphate salts, etc., or phosphate ore beneficiation enterprises, or new energy enterprises, or photovoltaic material and chip manufacturing and processing enterprises; the acidic wastewater has a phosphorus content of more than 3000 mg / L, a fluorine content of more than 500 mg / L, a silicon content of more than 200 mg / L, and a pH value of 1 to 4.

[0046] In some embodiments of the present invention, the coagulant is one or two of polyaluminum chloride (PAC), polyaluminum sulfate, polyferric chloride, polyferric sulfate, polyacrylamide (PAM), potassium aluminum sulfate dodecahydrate, ferric sulfate, ferric chloride, and ferrous ammonium sulfate. The dosage of the coagulant in the acidic wastewater is 0.2–5 g / L, for example, 0.2 g / L or 4 g / L.

[0047] In some embodiments of the present invention, the silicon removal process includes:

[0048] After mixing the coagulant and acidic wastewater, the mixture is allowed to settle and then the solid and liquid are separated.

[0049] The mixing temperature is 0–50°C, for example, 25°C; the rotation speed is 100–500 r / min, for example, 150 r / min; and the time is 5–30 min, for example, 5 min. The mixing is carried out in a coagulation treatment tank.

[0050] The settling time is 10–30 minutes. The settling is carried out in the first settling tank.

[0051] The resulting primary treated water has a pH value that is basically the same as that of the acidic wastewater, and the silicon removal rate is over 70%.

[0052] The solid-liquid separation method is filtration.

[0053] In step B):

[0054] The coagulated precipitate was washed to obtain a silica gel product;

[0055] The primary treated water was defluorinated using a precipitant to obtain secondary treated water and crude calcium fluoride.

[0056] In some embodiments of the present invention, the cleaning agent used for washing includes sodium hydroxide solution and / or sodium carbonate solution. The concentration of the sodium hydroxide solution is 0.08–1 mol / L, for example, 0.1 mol / L or 1 mol / L. The dosage of the cleaning agent in the coagulated precipitate is 0.1–10 g / g, for example, 0.4 g / g or 10 g / g. The washing is carried out under stirring conditions. The stirring speed is 100–500 rpm. The washing temperature is 0–50°C, and the washing time is 10–60 min.

[0057] After washing, silicone products and wash water are obtained. The wash water is treated with lime, quicklime, etc., before being discharged.

[0058] In some embodiments of the present invention, the precipitant is one or two of organic and inorganic calcium salts; specifically, one or two of CaCO3, Ca(OH)2, CaO, CaCl2, calcium citrate, calcium oxalate, and calcium lactate. In the primary treated water, the dosage of the precipitant is 1–10 g / L, for example, 5 g / L.

[0059] In some embodiments of the present invention, the defluorination treatment includes:

[0060] The precipitant is stirred and mixed with the primary treated water, and after standing and settling, the solid and liquid are separated.

[0061] The stirring and mixing speed is 100–500 rpm, for example, 150 r / min; the temperature is 0–50℃, for example, 25℃; and the time is 10–120 min, for example, 90 min. The stirring and mixing is carried out in a chemical precipitation tank.

[0062] The settling time is 0.2 to 72 hours, for example, 72 hours or 48 hours. The settling takes place in the second sedimentation tank.

[0063] The solid-liquid separation method is filtration.

[0064] In step C):

[0065] The crude calcium fluoride is purified to obtain the calcium fluoride product.

[0066] In some embodiments of the present invention, the purification reagent used in the purification process is one or two of the following: purified water, tap water, the secondary treated water, sulfuric acid, hydrochloric acid, nitric acid, phosphoric acid, fluorosilicic acid, citric acid, lactic acid, and ascorbic acid. The ratio of the crude calcium fluoride to the purification reagent is 1–50 g / L, for example, 50 g / L or 10 g / L.

[0067] In some embodiments of the present invention, the purification process includes:

[0068] After mixing the crude calcium fluoride with the purification reagent, the solid and liquid phases were separated.

[0069] The stirring speed is 100-500 rpm, for example 200 r / min; the time is 5-120 min, for example 5 min.

[0070] After solid-liquid separation, the process further includes drying.

[0071] The purity of the calcium fluoride product is above 78%.

[0072] Figure 1 A process flow diagram for the recovery of fluorine and silicon resources from phosphoric acid-containing wastewater is provided as an embodiment of the present invention.

[0073] Compared with the original acidic wastewater, the acidic wastewater treated by this invention (i.e., secondary treated water) still has an acidic pH value, a phosphorus loss rate of less than 5%, a total fluoride removal rate of more than 90%, and a total silicon removal rate of more than 80%.

[0074] The recycling method provided by this invention has good treatment effect, low investment, simple operation, easy implementation, and wide applicability.

[0075] The present invention does not impose any special restrictions on the source of the raw materials used above, and they can be commercially available.

[0076] To further illustrate the present invention, the following detailed description of a method for recovering fluorine and silicon resources from phosphoric acid-containing wastewater provided by the present invention is provided in conjunction with embodiments, but it should not be construed as limiting the scope of protection of the present invention.

[0077] Example 1

[0078] Methods for recovering fluorine and silicon resources from phosphoric acid-containing wastewater:

[0079] 1) Take 1L of acidic wastewater with a pH of 2.35, a fluoride content of 2202 mg / L, a phosphorus content of 7810 mg / L, and a silicon content of 663 mg / L. Add 100mL of pre-prepared PAM with a mass concentration of 2‰ (the amount of PAM in the acidic wastewater is 0.2 g / L). Stir and mix at room temperature (25℃) and 150 rpm for 5 minutes. After standing and settling for 30 minutes, filter to obtain primary treated water and coagulated precipitate. The primary treated water has a fluoride content of 1781 mg / L, a phosphorus content of 7125 mg / L, and a silicon content of 180 mg / L.

[0080] 2) Add 0.1 mol / L NaOH solution to the coagulated precipitate as a cleaning agent (the amount of cleaning agent added to the coagulated precipitate is 0.4 g / g) to obtain sodium silicate solution (silica gel product) with a silicon content of 432 mg / L.

[0081] CaCl2 was added as a precipitant to the primary treated water (the dosage of the precipitant in the primary treated water was 5 g / L). The mixture was stirred for 1.5 h at room temperature (25℃) with a mechanical stirrer at 150 r / min. After standing for 3 days, it was filtered to obtain secondary treated water and 4.51 g of fluoride-containing precipitate (crude calcium fluoride). The secondary treated water contained 87.7 mg / L of sulfur (F), 5818 mg / L of phosphorus (P), and 102 mg / L of silicon (Si). The fluoride-containing precipitate contained 51.5% CaF2 and 6.9% P by mass.

[0082] 3) 90 mL of 3% hydrochloric acid was added to the fluoride precipitate as a purification reagent (the ratio of crude calcium fluoride to purification reagent was 50 g / L). The stirring speed and time were 200 r / min and 5 min, respectively. After filtration, 2.83 g of precipitate was obtained. The calcium fluoride content in the precipitate was 78.3%. The filtrate (wash water) was recovered and mixed into the secondary treated water to obtain permeable water, which contained 7419 g / L of phosphorus, 71 mg / L of phosphorus, 113 mg / L of silicon, and had a pH of 2.77. The fluoride removal rate was calculated to be 97%, the silicon removal rate to be 83%, and the phosphorus loss rate to be 5%.

[0083] Example 2

[0084] Methods for recovering fluorine and silicon resources from phosphoric acid-containing wastewater:

[0085] 1) Take 1L of acidic wastewater with a pH of 2.35, a fluoride content of 2202mg / L, a phosphorus content of 7810mg / L, and a silicon content of 663mg / L. Add 80mL of pre-prepared PAM (4g / L of coagulant in the acidic wastewater) with a mass concentration of 5‰. Stir and mix at room temperature (25℃) and 150r / min for 5min. After standing and settling for 30min, filter to obtain primary treated water and coagulated precipitate. The primary treated water has a fluoride content of 1792mg / L, a phosphorus content of 7229mg / L, and a silicon content of 304mg / L.

[0086] 2) Add 10 mL of 1 mol / L NaOH solution to the coagulated precipitate as a cleaning agent (the amount of cleaning agent added to the coagulated precipitate is 10 g / g) to obtain sodium silicate solution (silica gel product) with a silicon content of 316 mg / L.

[0087] CaO was added as a precipitant to the primary treated water (the dosage of the precipitant in the primary treated water was 5 g / L). The mixture was stirred for 1.5 h at room temperature (25℃) with a mechanical stirrer at 150 r / min. After standing for 2 days, it was filtered to obtain secondary treated water and 5.53 g of fluoride-containing precipitate (crude calcium fluoride). The secondary treated water contained 58 mg / L of sulfur (F), 5397 mg / L of phosphorus (P), and 165.4 mg / L of silicon (Si). The fluoride-containing precipitate contained 50.5% CaF₂ and 8.1% P by mass.

[0088] 3) 75 mL of 2% hydrochloric acid was added to the fluoride precipitate as a purification reagent (the ratio of crude calcium fluoride to purification reagent was 10 g / L). The stirring speed and time were 200 r / min and 5 min, respectively. After filtration, 3.15 g of precipitate was obtained. The calcium fluoride content in the precipitate was 83.9%. The filtrate (wash water) was recovered and mixed into the secondary treated water to obtain permeable water, which contained 7130 g / L of phosphorus, 136 mg / L of phosphorus, 130 mg / L of sulfur, and a pH of 2.98. The fluoride removal rate was calculated to be 94%, the silicon removal rate to be 80%, and the phosphorus loss rate to be 9%.

[0089] The descriptions of the above embodiments are merely illustrative of the methods and core ideas of the present invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the present invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for recovering fluorine and silicon resources from phosphoric acid-containing wastewater, comprising the following steps: A) Acidic wastewater is treated with coagulants to remove silica, resulting in primary treated water and coagulated precipitate; The acidic wastewater contains phosphorus at a level of 3000 mg / L or higher, fluorine at a level of 500 mg / L or higher, silicon at a level of 200 mg / L or higher, and has a pH value of 1 to 4. The coagulant includes one or two of the following: polyaluminum chloride, polyaluminum sulfate, polyferric chloride, polyferric sulfate, polyacrylamide, potassium aluminum sulfate dodecahydrate, ferric sulfate, ferric chloride, and ferrous ammonium sulfate. In the acidic wastewater, the dosage of coagulant is 0.2~5 g / L; B) Wash the coagulated precipitate to obtain a silica gel product; The primary treated water was defluorinated using a precipitant to obtain secondary treated water and crude calcium fluoride. The precipitant includes one or two of CaCO3, Ca(OH)2, CaO, CaCl2, calcium citrate, calcium oxalate, and calcium lactate; In the primary treated water, the dosage of the precipitant is 1~10 g / L; In the secondary treated water, the phosphorus loss rate is 0-10%, the total fluoride recovery rate is over 70%, and the total silicon removal rate is over 80%. C) The crude calcium fluoride is purified to obtain the calcium fluoride product.

2. The recycling method according to claim 1, characterized in that, In step A), the silicon removal process includes: After mixing the coagulant and acidic wastewater, let it stand to settle, and then separate the solid and liquid components. The stirring and mixing temperature is 0~50℃, the rotation speed is 100~500 r / min, and the time is 5~30 min; The settling time is 10-30 minutes. In the primary treated water, the silicon removal rate is over 70%.

3. The recycling method according to claim 1, characterized in that, In step B), the cleaning agent used for washing includes sodium hydroxide solution and / or sodium carbonate solution; The dosage of cleaning agent in the coagulated precipitate is 0.1~10 g / g; The washing is carried out under stirring conditions; the stirring speed is 100~500 rpm; the washing temperature is 0~50℃ and the washing time is 10~60 min.

4. The recycling method according to claim 1, characterized in that, In step B), the defluorination treatment includes: The precipitant was stirred and mixed with the primary treated water, and after settling, solid-liquid separation was performed. The stirring and mixing speed is 100~500 rpm, the temperature is 0~50℃, and the time is 10~120 min; The settling time is 0.2 to 72 hours.

5. The recycling method according to claim 1, characterized in that, In step B), the phosphorus loss rate in the secondary treated water is less than 5%.

6. The recycling method according to claim 1, characterized in that, In step C), the purification reagent used in the purification process is one or two of the following: purified water, tap water, the secondary treated water, sulfuric acid, hydrochloric acid, nitric acid, phosphoric acid, fluorosilicic acid, citric acid, lactic acid, and ascorbic acid. The ratio of crude calcium fluoride to purification reagent is 1~50 g / L.

7. The recycling method according to claim 1, characterized in that, In step C), the purification process includes: After mixing the crude calcium fluoride with the purification reagent, solid-liquid separation was performed. The stirring speed is 100~500 rpm, and the time is 5~120 min.

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