CN111547885A - Process for removing fluorine, controlling salt and recovering fluorine from silicon solar cell production wastewater - Google Patents
Process for removing fluorine, controlling salt and recovering fluorine from silicon solar cell production wastewater Download PDFInfo
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Abstract
The invention discloses a process for removing fluorine, controlling salt and recovering fluorine from silicon solar cell production wastewater, and belongs to the field of wastewater treatment. The method fully considers the characteristics of various waste water generated in the production process of the silicon solar cell, performs resource recovery on the concentrated acid waste water with high fluorine ion concentration, reduces the impact of high-concentration fluorine in the concentrated acid waste water on a fluorine removal system, and enables a single-stage fluorine removal process to meet the requirement of battery industrial waste water discharge by adjusting the addition amount of sulfuric acid. The calcium fluoride recovered by the method has remarkable economic benefit; the sulfuric acid reacts with the calcium hydroxide, other salt substances are not introduced, and the TDS content of the soluble total solids of the effluent is easy to control.
Description
Technical Field
The invention belongs to the technical field of wastewater treatment, and particularly relates to a process for removing fluorine, controlling salt and recovering fluorine from silicon solar cell production wastewater.
Background
Photovoltaic industrial workThe method is one of important development industries of new energy, and the rapid development of the method brings great economic benefits to the modern society and also brings new environmental problems. Taking the production process of silicon solar cells (monocrystalline silicon and polycrystalline silicon) in the midstream of the industry as an example, a large amount of hydrofluoric acid is used in the production process, so that industrial wastewater with high fluorine ion concentration is generated. The fluorine-containing wastewater is generated in the production links of wool making, acid cleaning, etching and the like, and mainly comprises four parts, namely concentrated acid wastewater (with small water quantity), dilute acid wastewater, concentrated alkali wastewater and dilute alkali wastewater. Wherein the content of fluoride ions in the polysilicon concentrated acid wastewater reaches 120000mg/L, and the content of fluoride ions in the monocrystalline silicon concentrated acid wastewater reaches 35000 mg/L; the content of fluorine ions in the polycrystalline silicon dilute acid wastewater is about 1000mg/L, and the content of fluorine ions in the monocrystalline silicon dilute acid wastewater is about 500 mg/L. The conventional etching equipment uses HF-HNO3In a corrosion system, the requirement can be met only when the weight of the corrosion system is reduced by 0.3g required by a PERC battery, the flatness and the polishing degree are required, the medicine consumption reaches 15 mL/piece, the cost of a single chip of an etching section is only 0.15 yuan/piece, and the cost is urgently reduced. Meanwhile, the high-concentration nitrogen-containing and fluorine-containing elements are discharged to cause great burden on wastewater treatment. At present, a newly expanded battery workshop basically takes a single crystal PERC battery technology as a main technology, develops a polishing etching technology of a NaOH-KOH alkaline system, greatly reduces the usage amount of a medicament, lowers the single-chip cost by more than 0.1 yuan compared with acid polishing, and reduces the cost by 70%. However, the polycrystalline silicon wafer is composed of a plurality of grains with different sizes, the crystal directions on the surface are randomly distributed, and the texturing can only be performed by adopting HF-HNO3A large amount of wastewater containing fluorine is generated and NO is generated3 -The nitrogen-containing wastewater.
A great problem in the treatment of wastewater from solar cell production is the problem of fluoride removal, and the discharge standard of pollutants in the battery industry (GB 30484 and 2013) further provides strict standards of the discharge concentration of fluoride ions of 10mg/L (existing enterprises), 8mg/L (newly-built enterprises) and 2mg/L (special discharge limit) in the solar cell industry. At present, for high-concentration fluorine-containing industrial wastewater, a lime precipitation method is generally adopted, namely lime is added into the wastewater, and Ca generated after dissolution is utilized2+With F in water-Reaction to form insoluble CaF2Precipitating to remove F in water-Removal (Ca (OH)2+2HF＝CaF2↓+2H2O). When calcium hydroxide is used for removing fluorine, the calcium hydroxide is generally added in a lime milk form due to low solubility, and the generated calcium fluoride is easily coated on the surface of unreacted calcium hydroxide, so that the utilization rate is low and the adding amount is large. CaCl2Because of high solubility in water, more Ca can be dissociated2+Therefore, calcium chloride is often used in large quantities to replace the amount of lime used in the engineering. The traditional chemical precipitation method can only reduce the fluoride concentration to 15-30mg/L generally, and can further reduce the fluoride concentration to 15mg/L by adding a flocculating agent and controlling proper reaction conditions, but still does not meet the discharge standard of pollutants in the battery industry<The emission limit of 10mg/L, and the deep fluorine removal through a secondary process. Patent CN 103373776B adopts Ca (OH)2Adjusting the pH value of the wastewater, and adding CaCl2PAM controls the fluorine ion at 30-40 mg/L, and then CaCl is added2Secondary defluorination of PAC and PAM is carried out to reach the standard and finally discharged; the patent CN 108689522A defluorination comprises two defluorination steps, wherein calcium hydroxide emulsion is added in the first defluorination step, and calcium carbonate is added in the second defluorination step. The separation difficulty of the precipitated product is reduced by using the mixed precipitator of calcium carbonate and calcium hydroxide, but the method is not suitable for treating mixed acid wastewater containing a large amount of fluoride ions because the calcium carbonate and H are mixed under an acidic condition+Reaction to produce a large amount of CO2Gas generates a large amount of foam, which affects the fluorine removal process and causes the fluorine removal efficiency to drop sharply. The secondary defluorination process aiming at the high-concentration fluorine-containing wastewater needs huge tank capacity for treatment, improves the civil engineering cost of the project, increases the usage amount of chemical agents and causes great waste of resources.
In addition, due to the fact that the salt content of the solar cell production wastewater is high, and a large amount of calcium salt, especially calcium chloride, is introduced into the front-end defluorination unit, certain negative effects can be brought to subsequent biological denitrification treatment, for example, the excessive chloride ion concentration can inhibit the growth of microorganisms, and influence the activity of activated sludge and the sedimentation effect of the sludge; the waste water containing high-concentration calcium magnesium salt is easy to scale, influences the use effect of the filler or the aeration equipment, and easily causes pipeline blockage and the like. The sanitary standard for drinking water (GB5749-2006) requires that the total soluble solids (TDS) of drinking tap water is less than or equal to 1000mg/L, the standard for discharging sewage into urban sewer water (GBT31962-2015) has the requirement of less than 2000mg/L for the total soluble solids, and the discharge standard for chlorides in Hebei province (DB 13831-2006) also defines the maximum discharge concentration limit for discharging the chlorides into the environment. Therefore, in the fluorine removal process of silicon solar cell production wastewater, sufficient attention needs to be paid to the control problem of the discharge salinity.
The large amount of calcium fluoride sludge generated in sewage treatment has potential hazard due to fluorine, and the "national hazardous waste record" published by the ministry of environmental protection and the national development committee in 2008 lists the fluorine-containing sludge generated in the fluorine-containing wastewater treatment in the hazardous waste investigation scope (HW 32). The sludge has complex components, high silicon content and low calcium fluoride content which is about 30 to 60 percent of the dry weight of the sludge under the influence of the water quality of the wastewater. Fluorine-containing sludge has great potential hazard, and if the treatment and the disposal are not proper, higher-concentration fluorine ions can be leached out along with rainwater to directly pollute surface water or nearby soil, and under the acid condition after landfill, the fluorine ions are more easily leached out to pollute underground water and soil. Therefore, the problem of disposing the fluorine-containing sludge generated by the treatment of the wastewater produced in the solar cell panel production is also a key point of pollution control in the industry, and the reduction of the generation of the sludge from the source is particularly important. From the aspect of resource utilization, the research on fluoride recycling technology in the wastewater is enhanced, and fluorine resources are recycled as much as possible, so that the yield of fluorine-containing sludge is effectively reduced, and the burden of sludge treatment and disposal in the subsequent links is reduced.
Disclosure of Invention
Aiming at the defects that a large amount of chemical reagents are required to be added during the mixed treatment of the fluorine-containing wastewater of the solar cell in the prior art, the fluorine removal efficiency is low, the sludge yield is high, the Total Dissolved Solids (TDS) content of effluent is high and the like, the invention aims to provide a process for efficiently removing fluorine and controlling salt of the fluorine-containing wastewater of the silicon solar cell, and the recovery of partial fluorine resources is realized while the fluorine-containing wastewater of the solar cell is treated efficiently and at low cost.
The invention conception of the invention is as follows: firstly, acid-washing concentrated acid wastewater is etchedClassifying and collecting concentrated acid wastewater, dilute acid wastewater, concentrated alkali wastewater and dilute alkali wastewater; because the concentration of the fluorine ions in the concentrated acid wastewater is higher, the concentrated acid wastewater is firstly treated: adding calcium salt into the concentrated acid wastewater to perform induced crystallization reaction to obtain calcium fluoride with higher purity for resource utilization, sending system effluent after crystallization reaction, the rest diluted acid wastewater, concentrated alkali wastewater and diluted alkali wastewater into a wastewater adjusting tank together to be mixed for homogenization and pH adjustment, and then sequentially adding Ca (OH) into the mixed wastewater2And removing fluorine from the emulsion, concentrated sulfuric acid, PAC and PAM, and then sending the emulsion, the concentrated sulfuric acid, the PAC and the PAM into a sedimentation tank for sedimentation treatment. The sedimentation tank delivery port can set up fluorinion and TDS on-line monitoring appearance, accessible backwash pump returns the effluent pump to the reaction tank when going out water unusual to through adding phosphate and sulphuric acid further strengthening the fluorine removal accuse salt.
In order to achieve the above purpose, the invention specifically adopts the following technical scheme:
a process for removing fluorine, controlling salt and recovering fluorine from silicon solar cell production wastewater comprises the following steps:
s1: classifying and collecting wastewater discharged from different procedures in the production process of the silicon solar cell;
s2: combining concentrated acid wastewater discharged from an acid washing process and concentrated acid wastewater discharged from an etching process into concentrated acid fluorine-containing wastewater, and adding excessive calcium salt solution into the concentrated acid fluorine-containing wastewater, wherein the adding amount of the calcium salt solution meets the condition that the total molar concentration ratio of calcium to fluorine in the wastewater is (0.6-0.75): 1; fully stirring and mixing, and standing to precipitate calcium fluoride crystals; extracting calcium fluoride slurry crystallized and precipitated at the bottom in the reaction area, carrying out solid-liquid separation on the calcium fluoride slurry, using liquid obtained after the solid-liquid separation and supernatant of the precipitate for wastewater regulation, and drying the solid to obtain dry calcium fluoride sludge for resource utilization;
s3: mixing the wastewater collected in the step S1 and discharged from other steps except the pickling step and the etching step with the liquid obtained by solid-liquid separation of the calcium fluoride slurry in the step S2 and the supernatant, and homogenizing to obtain mixed wastewater;
s4: adding excessive Ca (OH) to the mixed wastewater obtained in S32Emulsion, Ca (OH)2The addition amount of the emulsion meets the condition that the total molar concentration ratio of calcium to fluorine in the wastewater is (1.1-1.25): 1, and the calcium fluoride crystal precipitate is generated primarily after the emulsion is fully stirred and mixed;
s5: adding concentrated sulfuric acid into the mixed wastewater obtained in the step S4, adjusting the pH value of the mixed wastewater to 6.5-7, and dissociating suspended calcium hydroxide in the wastewater to generate Ca2+Further generating calcium fluoride crystal precipitate;
s6: adding a coagulant PAC and a coagulant aid PAM into the mixed wastewater obtained in the step S5 to enable the calcium fluoride crystal in the water to be precipitated and flocculate;
s7: adding sulfuric acid into the mixed wastewater obtained in the step S6 to enable excessive calcium ions in the water to react to generate calcium sulfate precipitate to be removed, wherein the adding amount of the sulfuric acid is 0.02-0.05 mol/L of the mixed wastewater; after fully mixing and precipitating, adding the coagulant PAC and the coagulant aid PAM again to flocculate the generated calcium sulfate precipitate;
s8: and (4) standing and precipitating the mixed wastewater obtained in the step S7, carrying out outward treatment on precipitated sludge, and inputting the supernatant into a subsequent biochemical system for treatment.
Preferably, in S2, the calcium salt solution is a calcium chloride solution.
Preferably, in S6, the addition amount of the coagulant PAC is 300-400 mg/L of the mixed wastewater, and the addition amount of the coagulant aid PAM is 2-5 mg/L of the mixed wastewater.
Preferably, in S7, the addition amount of the coagulant PAC is 100-200 mg/L of the mixed wastewater, and the addition amount of the coagulant aid PAM is 2-5 mg/L of the mixed wastewater.
Preferably, in the step S2, the concentrated acid fluorine-containing wastewater flows into the first reaction tank for treatment.
Preferably, the mixed wastewater in the step S3 is stored in a regulating reservoir and continuously pumped into a second reaction reservoir to perform the treatment of the steps S4 to S7, the second reaction reservoir is divided into four different cells according to the wastewater flow, and the wastewater sequentially flows through the four cells to perform the treatment of the steps S4, S5, S6 and S7.
Further, in the step S8, fluorine ion and TDS online monitoring is performed on the effluent of the sedimentation tank, when it is detected that the effluent does not meet the standard, the inflow of the adjustment tank is closed, the effluent of the sedimentation tank flows back to the second reaction tank to perform the steps S4-S7 again, and phosphate is further added in the treatment process to perform secondary enhanced fluorine removal and salt control.
Furthermore, when the secondary enhanced defluorination salt control treatment is carried out, the molar concentration ratio of the added phosphate to the fluorine ions in the effluent of the sedimentation tank is (0.2-0.4): 1.
further, the phosphate is KH2PO4、K2HPO4、K3PO4One kind of (1).
Preferably, the precipitated sludge generated in the steps S4-S8 is pumped into a sludge storage tank, is subjected to pressure filtration by a plate-and-frame filter press and then is transported outside.
Compared with the prior art, the invention has the following advantages: 1) the method has the advantages that the acid-washing concentrated acid wastewater, the etching concentrated acid wastewater, the dilute acid wastewater, the concentrated alkali wastewater and the dilute alkali wastewater are subjected to classified collection treatment, the set process is strong in pertinence, and the overall treatment difficulty and the operation cost are reduced; 2) the concentrated acid wastewater is crystallized and recycled, calcium fluoride with higher purity can be prepared, the amount of calcium fluoride materialized sludge generated by conventional mixed treatment is less, and the subsequent sludge treatment cost is reduced; 3) sulfuric acid is added into the grid 2 of the reaction tank, so that the pH value of the wastewater can be adjusted to CaF2The generated optimal reaction pH value can also be subjected to acid-base reaction with calcium hydroxide suspended in the wastewater to dissociate more effective Ca2+The utilization rate of the calcium hydroxide is improved; 4) hardness (Ca) in waste water of 4 th zone of reaction tank2+) The concentration of calcium ions in the effluent can be further reduced by adding the PAC and the PAM in an auxiliary way, and the treated effluent has low Total Dissolved Solids (TDS); 5) the reduction of the calcium chloride reduces the operation cost, does not generate too high chloride ion concentration, and avoids negative influence on the subsequent biological denitrification treatment; 6) the phosphate can enhance the removal effect of fluoride ions, not only participates in the reaction to generate calcium fluophosphate to directly precipitate the fluoride ions, but also can effectively promote the fluoride ions because the generated calcium fluophosphate and the generated calcium phosphate can play the role of crystal nucleusAnd (4) precipitating calcium chloride.
The method fully considers the characteristics of various waste water generated in the production process of the silicon solar cell, performs resource recovery on concentrated acid waste water with high fluorine ion concentration, reduces the impact of high-concentration fluorine in the concentrated acid waste water on a fluorine removal system, and enables a single-stage fluorine removal process to meet the requirement of battery industrial waste water discharge by adjusting the addition amount of sulfuric acid; the calcium fluoride sludge is reduced by more than 63 percent, and the economic benefit of the recovered calcium fluoride is obvious; the sulfuric acid reacts with the calcium hydroxide, other salt substances are not introduced, and the TDS content of the soluble total solids of the effluent is easy to control.
Drawings
FIG. 1 is a process flow chart of the efficient fluorine and salt removal and control process for fluorine-containing wastewater of silicon solar cells
FIG. 2 is a graph showing the effect of removing fluorine and controlling salt during the steady operation period of the process in the example.
Detailed Description
The following description of the embodiments of the present invention is provided in conjunction with specific embodiments, and other advantages and capabilities of the present invention will be readily apparent to those skilled in the art from the present disclosure. The invention is capable of other and different embodiments and of being practiced or of being carried out in various ways, and its several details are capable of modification in various respects, all without departing from the spirit and scope of the present invention.
Before the present embodiments are further described, it is to be understood that the scope of the invention is not limited to the particular embodiments described below; it is also to be understood that the terminology used in the examples of the invention is for the purpose of describing particular embodiments only, and is not intended to be limiting of the general inventive concept.
When numerical ranges are given in the examples, it should be understood that all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art, unless otherwise defined. In addition to the specific methods, devices, and materials used in the examples, any methods, devices, and materials similar or equivalent to those described in the examples may be used in the practice of the invention in addition to the specific methods, devices, and materials used in the examples, in keeping with the knowledge of one skilled in the art and with the description of the invention.
As shown in fig. 1, in a preferred embodiment of the present invention, a process for efficiently removing fluorine, controlling salt and recovering fluorine from silicon solar cell production wastewater is provided, which comprises the following steps:
step 1: collecting waste water: waste water of different procedures in the production process of the silicon solar cell is collected in a classified mode, wherein concentrated acid waste water with high fluorine concentration is generated in an acid washing procedure and an etching procedure, and diluted acid waste water, concentrated alkali waste water, diluted alkali waste water and the like with low fluorine concentration are generated in the other procedures. Therefore, the fluorine-containing wastewater of each path of the acid-washing concentrated acid wastewater, the etching concentrated acid wastewater, the dilute acid wastewater, the concentrated alkali wastewater and the dilute alkali wastewater needs to be classified and collected.
Step 2: and (3) fluorine recovery treatment of concentrated acid wastewater: the concentrated acid wastewater discharged in the acid washing process and the concentrated acid wastewater discharged in the etching process are combined into concentrated acid fluorine-containing wastewater to be treated together, the concentrated acid fluorine-containing wastewater is left by gravity to enter a first reaction tank, and the concentration of fluorine ions in the concentrated acid wastewater is measured in real time so as to facilitate subsequent feeding. And adding a calcium salt solution into the first reaction tank to perform a crystallization reaction, wherein the calcium salt comprises one or two of a calcium chloride solution and a calcium hydroxide solution, and the calcium chloride solution is preferably used. The calcium salt is added in an amount to maintain the total molar concentration ratio of calcium to fluorine in the first reaction tank at (0.60-0.75): 1. And starting a stirrer in the first reaction tank, stirring for 30min to fully mix, and standing for 1h to precipitate calcium fluoride crystals. And (3) periodically pumping out the crystallized calcium fluoride slurry from the bottom of the first reaction tank, conveying the crystallized calcium fluoride slurry to a calcium fluoride wet sludge storage tank, centrifuging the calcium fluoride slurry to perform solid-liquid separation, allowing the separated liquid to flow into a wastewater adjusting tank, and allowing the supernatant in the first reaction tank to flow into the wastewater adjusting tank. Drying the solid in the wet sludge storage tank to obtain calcium fluoride dry sludge, and conveying the calcium fluoride dry sludge to a dry sludge storage tank;
in addition, the first reaction tank can be replaced by a solid-liquid two-phase fluidized bed reactor. The concentrated acid fluorine-containing wastewater can be pumped into a solid-liquid two-phase fluidized bed reactor containing calcium fluoride crystal seeds, calcium salt solution is added into the fluidized bed reactor for induced crystallization reaction, the calcium salt comprises one or two of calcium chloride solution and calcium hydroxide solution, and the adding amount of the calcium salt keeps the total molar concentration ratio of calcium and fluorine in the wastewater in the first reaction tank to be (0.65-0.70): 1. And (4) periodically discharging the crystallization concentrated solution at the bottom of the reactor, drying to obtain calcium fluoride, and conveying to a dry sludge storage tank. And mixing the supernatant discharged from the reactor with other wastewater, and then sending the mixture into a second reaction tank.
And step 3: and (3) low-fluorine mixed wastewater treatment: and (2) feeding system effluent (including supernatant and a liquid part obtained by solid-liquid separation) after the crystallization reaction of the concentrated acid wastewater and the dilute acid wastewater, the concentrated alkali wastewater and the dilute alkali wastewater collected in the rest procedures in the step (1) into a wastewater adjusting tank, carrying out uniform mass mixing and pH adjustment, and measuring the concentration of fluorine ions in the mixed wastewater so as to control the feeding amount subsequently. The mixed wastewater in the regulating reservoir can be continuously pumped into a second reaction tank for treatment, and the water continuously enters the second reaction tank and continuously exits. The second reaction pool is internally provided with four cells which are a 1 st cell, a 2 nd cell, a 3 rd cell and a 4 th cell in sequence according to the flow, wastewater firstly enters the 1 st cell, then sequentially flows through the 2 nd cell, the 3 rd cell and the 4 th cell, and finally is discharged from the 4 th cell. The specific treatment process in the second reaction tank is as follows:
and 4, step 4: ca (OH)2Emulsion treatment: after the mixed wastewater enters the 1 st cell, according to Ca in the wastewater: ca (OH) is added to the mixture at a molar ratio of F of (1.1-1.25): 12Starting a stirrer to stir the emulsion for 30min to ensure that the emulsion is fully mixed, and quickly generating CaF in the 1 st area of the second reaction tank2And (4) precipitating. With Ca (OH)2The pH of the solution gradually increases with the addition of the emulsion, with the formation of CaF2The coating on the surface of the calcium hydroxide causes the calcium hydroxide to be difficult to dissolve, so that the calcium hydroxide needs to enter the 2 nd cell for further treatment.
And 5: concentrated sulfuric acid treatment: after the mixed wastewater enters the No. 2 cell, adding a proper amount of concentrated sulfuric acid into the No. 2 cell of the second reaction tank, and adjusting the pH value of the wastewater to CaF2The generated optimal reaction pH value is 6.5-7, and the added sulfuric acid and calcium hydroxide suspended in the wastewater are subjected to acid-base reaction to be dissociated into moreMuch available Ca2+Further removing F in the wastewater-. In practical application, due to fluoride (such as CaF)2) The alumen ustum is small, the sedimentation performance is poor, and the emission standard is difficult to reach in the conventional sedimentation equipment (the sedimentation time is about 1 h), so that the alumen ustum needs to enter a 3 rd cell for further treatment.
Step 6: flocculation treatment: therefore, after the 2 nd cell is subjected to a sufficient precipitation reaction, the wastewater enters a 3 rd cell of the second reaction tank, 300-400 mg/L of wastewater coagulant polyaluminium chloride (PAC) and 2-5 mg/L of wastewater coagulant aid Polyacrylamide (PAM) are added into the 3 rd cell, and the generated fluoride is further condensed into large particles to achieve the effect of efficient precipitation by combining the mechanisms of 'compressed double electric layers', 'electric neutralization', 'adsorption' of high-efficiency coagulants, and 'precipitation net-capturing', 'adsorption bridging' of high-molecular coagulant aids;
and 7: treating the salinity of the wastewater: because the components of the wastewater are complex, the salt content in the wastewater is high, the mutual restraint effect between ions is strong, the activity of fluorine ions and calcium ions is reduced, and the combination of the fluorine ions and the calcium ions is hindered. When the lime is used for treating the fluorine-containing wastewater, in order to ensure that the effluent reaches the standard, the lime addition amount far exceeds the theoretical ratio, so that the hardness (Ca) in the second reaction tank is easily caused2+) Is higher. Therefore, after the wastewater enters the 4 th cell of the second reaction tank, 0.02-0.05 mol/L sulfuric acid of the wastewater is added into the wastewater of the 4 th cell, sulfate radicals and calcium ions are utilized to react to generate calcium sulfate precipitate, meanwhile, 100-200 mg/L PAC of the wastewater and 2-5 mg/L PAM of the wastewater are added, and the generated calcium sulfate precipitate is subjected to flocculation precipitation, so that the concentration of calcium ions in the effluent is further reduced.
And 8: and (4) inputting the mixed wastewater discharged from the second reaction tank in the step (7) into a sedimentation tank, standing and precipitating, transporting the precipitated sludge out, and inputting the supernatant into a subsequent biochemical system for treatment.
In the process of step 8, the abnormal condition of the effluent needs to be monitored and processed, specifically: set up fluorinion and TDS on-line monitoring appearance at the sedimentation tank delivery port, when detecting out water unusual, close the equalizing basin and intake and open the backwash pump and go out sedimentation tank water storage in emergent pond, the second pump returnsAnd two reaction tanks, wherein phosphate is added into the 4 th cell to further strengthen the fluorine removal and salt control. The phosphate is KH2PO4、K2HPO4、K3PO4One of (1), amount of phosphate added: the molar concentration ratio of fluorine ions in the effluent of the sedimentation tank is (0.2-0.4): 1. meanwhile, sulfuric acid, PAC and PAM may be added in the 4 th cell according to the operation of step 7 to remove the generated precipitate.
In addition, because partial precipitated sludge exists at the bottom of the four cells of the second reaction tank in the treatment process, sludge conveying devices are arranged in the second reaction tank and the sedimentation tank respectively and comprise sludge pipelines and sludge pumps, sludge pipelines are arranged at the bottoms of the four cells of the second reaction tank and the bottom of the sedimentation tank respectively, the sludge pumps are started regularly to pump materialized sludge into a sludge storage tank, the sludge is transported outwards after the sludge is subjected to filter pressing by a plate-and-frame filter press, and filtrate flows back into the regulating tank.
Therefore, the method fully considers the characteristics of various waste water generated in the production process of the silicon solar cell, performs resource recovery on concentrated acid waste water with high fluorine ion concentration, reduces the impact of high-concentration fluorine in the concentrated acid waste water on a fluorine removal system, and enables a single-stage fluorine removal process to meet the requirement of battery industrial waste water discharge by adjusting the addition amount of sulfuric acid; and the low-fluorine wastewater is treated by adopting the calcium hydroxide emulsion, and the sulfuric acid reacts with the calcium hydroxide without introducing other salt substances, so that the TDS content of the soluble total solids of the effluent is easy to control. To further illustrate the advantages of the process, it is applied to a specific embodiment in order to better embody its technical effects.
Examples
A certain solar energy science and technology limited company is a medium-scale solar cell panel production enterprise, a 3 monocrystalline silicon solar cell panel production lines are newly built at the end of 2017, the annual energy production capacity is 480MW, imported production technology and imported production equipment are used, the automation level is high, and the quality and the quantity of production wastewater are shown in a table 1:
TABLE 1 quality and quantity of wastewater from solar cell panel production enterprise
As can be seen from the data in Table 1, the amount of concentrated acid wastewater (acid washing, etching) was 14.1t/d, which was 2.22% of the total amount of wastewater (634t/d), and the fluorine content was 69.7% of the total fluorine content of the wastewater. If the part of wastewater is mixed with other fluorine-containing wastewater, the concentration of fluorine ions in the mixed wastewater reaches 975mg/L, and no small impact is caused on a fluorine removal system; if not, the fluorine ion concentration of the mixed waste water is reduced to 363mg/L according to the removal rate of 90 percent of the fluorine ions of the concentrated acid waste water.
Based on the data analysis, the implementation steps of the process are as follows:
(1) and (3) respectively collecting the fluorine-containing wastewater of each path of acid-washing concentrated acid wastewater, etching concentrated acid wastewater, dilute acid wastewater, concentrated alkali wastewater and dilute alkali wastewater in a classified manner.
(2) And (3) fluorine recovery treatment of concentrated acid wastewater: the method comprises the steps of combining concentrated acid wastewater discharged in an acid pickling process and concentrated acid wastewater discharged in an etching process into concentrated acid fluorine-containing wastewater for treatment, enabling the concentrated acid fluorine-containing wastewater to automatically flow into a first reaction tank through gravity, measuring the fluorine ion concentration (the average concentration of fluorine ions is 30553mg/L) of the concentrated acid wastewater, adding 29.4g/L of calcium chloride solution into the first reaction tank for crystallization reaction, enabling the water inlet flow rate of the concentrated acid wastewater and the dosing flow rate of calcium chloride to be 1:4, simultaneously starting a stirrer to stir so as to enable the concentrated acid wastewater to be fully mixed, continuing stirring for 30min after the concentrated acid wastewater completely enters, and standing for 1h so as to enable calcium fluoride to be crystallized and precipitated. And (3) periodically pumping out the crystallized calcium fluoride slurry from the bottom of the first reaction tank, conveying the crystallized calcium fluoride slurry to a calcium fluoride wet sludge storage tank, centrifuging to perform solid-liquid separation, drying the solid to obtain a high-purity calcium fluoride product (0.837t/d) according to the removal rate of fluoride ions in concentrated acid wastewater of 90 percent, and allowing the separated liquid to flow into a wastewater adjusting tank.
(3) And (3) low-fluorine mixed wastewater treatment: sending system effluent (70.5t) after the crystallization reaction of the concentrated acid wastewater, the dilute acid wastewater, the concentrated alkali wastewater and the dilute alkali wastewater into a wastewater adjusting tank together for uniform mass mixing and pH adjustment, and determining that the average concentration of fluorine ions in the mixed wastewater is 288 mg/L; then pumping the mixed wastewater into a second reaction tankIn zone 1, 30 wt.% Ca (OH) was added to the wastewater in a molar ratio of Ca to F of 1.2:12Stirring the emulsion for 30min by starting a stirrer to fully mix the emulsion and rapidly generate CaF in the second reaction tank2And (4) precipitating. With Ca (OH)2The pH of the solution gradually increases with the addition of the emulsion, with the formation of CaF2Covering the surface of calcium hydroxide to cause the calcium hydroxide to be difficult to dissolve, adding a proper amount of concentrated sulfuric acid into the 2 nd cell of the second reaction tank, and adjusting the pH value of the wastewater to CaF2The generated optimal reaction pH value is 6.5-7, and the added sulfuric acid and calcium hydroxide suspended in the wastewater are subjected to acid-base reaction to dissociate more effective Ca2+Further removing F in the wastewater-. In practical application, due to fluoride (such as CaF)2) The alumen ustum is smaller, the sedimentation performance is poorer, after the full reaction, 400mg/L coagulant polyaluminium chloride (PAC) and 2.0mg/L coagulant aid Polyacrylamide (PAM) are added into the 3 rd cell of the second reaction tank, and the generated fluoride is further condensed into large particles to achieve the effect of high-efficiency sedimentation by combining the mechanisms of ' compressed double electric layers ', ' electric neutralization ', ' adsorption ' of high-efficiency coagulants, precipitation net trapping ', ' adsorption bridging ' of high-molecular coagulant aids and the like.
(4) As the lime addition amount far exceeds the theoretical ratio, the hardness (Ca) in the 4 th cell wastewater of the second reaction tank is caused2 +) Therefore, the sulfuric acid added into the wastewater of 0.035mol/L in the 4 th cell reacts with calcium ions to generate flocculent calcium sulfate precipitate, and the concentration of calcium ions in the effluent can be further reduced by adding 200mg/L PAC and 2.0mg/L PAM.
(5) And (3) inputting the mixed wastewater discharged from the second reaction tank into a sedimentation tank, standing and settling, transporting the settled sludge out, and inputting the supernatant into a subsequent biochemical system for treatment.
And meanwhile, the abnormal condition of the effluent needs to be monitored and treated: set up fluorinion and TDS on-line monitoring appearance at the sedimentation tank delivery port, when detecting out water unusual, close the equalizing basin and intake and open the backwash pump and deposit sedimentation tank water earlier in emergent pond, then the second reaction tank is returned to the pump, adds the phosphate and further strengthens removing fluorine in 4 th district check and controls the salt, and the phosphate is KH2PO4Amount of phosphate added: the molar concentration ratio of fluorine ions in the effluent of the sedimentation tank is 0.3: 1.
and a sludge conveying device is also arranged between the second reaction tank and the sedimentation tank, a sludge pump is periodically started to pump the materialized sludge into a sludge storage tank, the sludge is transported outwards after being subjected to filter pressing by a plate-and-frame filter press, and filtrate flows back into the regulating tank.
The effect of fluorine and salt removal during the stable operation of the process is shown in figure 2: during the monitoring period from 5 and 4 days in 2018 to 11 and 2 days in 2018, the concentration of fluorine ions in inlet water of the regulating reservoir fluctuates between 231 and 382mg/L, the average value is 305mg/L, the concentration of fluorine in outlet water after calcium salt physical and chemical precipitation treatment is reduced to 4.49 to 9.31mg/L, the average value is 6.97mg/L, the removal rate is about 97.7 percent, and the outlet water meets the concentration of fluorine ions in waste water discharged by newly-built solar cell production enterprises specified in the discharge standard of pollutants for the battery industry (GB 30484 laid-aside 2013)<Emission limit of 8 mg/L. 30 percent of calcium hydroxide emulsion is added into the second reaction tank of the engineering to be used as a reaction precipitator, calcium chloride with higher solubility commonly used in the engineering is not adopted, and Cl is mainly considered-The negative effect of the subsequent biological denitrification treatment. By other measures, such as adding sulfuric acid to react with calcium hydroxide suspended in the wastewater to dissociate more effective Ca2+And the utilization rate of the calcium hydroxide is improved. However, in order to ensure the defluorination effect, the amount of calcium salt precipitator added is often far higher than the theoretical value (the actual Ca/F molar concentration ratio is 1.2:1), so that the hardness (Ca/F molar concentration ratio) of the wastewater in the 4 th zone of the reaction tank is ensured2+) The concentration of calcium ions in the effluent can be further reduced by adding sulfuric acid to react with the calcium ions to generate calcium sulfate flocculent precipitates to remove redundant calcium ions, and the PAC and PAM are added in an auxiliary manner, so that the treatment effect is shown in figure 2. Cell inlet Ca of reaction tank 42+The concentration fluctuates between 1400 and 1900mg/L, the average value is 1714mg/L, and the Ca in the sedimentation tank is reacted by sulfuric acid2+The concentration is 140-250 mg/L, the average value is 188mg/L, and the removal rate is about 89%. The sulfuric acid is added mainly through the reaction of sulfate radical and excessive calcium ion in the waste water to produce calcium sulfate precipitate, and the hydrogen ion can neutralize the waste water alkali without introducing other salt matter, so that the treated water has Total Dissolved Solid (TDS) content higher than that of waste waterLow. The addition of other calcium ion precipitants (such as sodium carbonate) introduces other salt ions, resulting in higher TDS of the effluent.
Comparative example:
the following is a conventional treatment method for fluorine-containing wastewater of silicon solar cells in the prior art
(1) The water quality and the water quantity are the same as those of the embodiment; collecting acid-washing concentrated acid wastewater, etching concentrated acid wastewater, dilute acid wastewater, concentrated alkali wastewater and dilute alkali wastewater together to an adjusting tank for homogenizing;
(2) pumping the mixed wastewater into a first-stage reaction tank, adding 30% calcium hydroxide to adjust the pH value, adding a large amount of calcium chloride to perform a precipitation reaction in cooperation with PAC and PAM with certain concentrations, and allowing the mixed solution to enter a first-stage precipitation tank; the effluent of the first-stage sedimentation tank automatically flows into a second-stage reaction tank, and excessive calcium chloride is added to further remove fluoride ions. Sludge in the sedimentation tank is filter-pressed and transported outwards, and supernatant is sent into an adjusting tank;
(3) and (4) enabling supernatant in the sedimentation tank to enter a chemical softening tank, adding sodium carbonate to remove excessive calcium ions, and enabling effluent to enter a biochemical denitrification system for further treatment.
The theoretical yield of calcium fluoride sludge from which fluoride ions are removed per unit by precipitation (i.e. it is considered that all removed fluoride ions are converted into calcium fluoride, and the purity of calcium fluoride in sludge is 100%) is as follows: each 1mg of fluoride ion removed produced 2.05mg of dry sludge. The water amount of the concentrated acid wastewater is small (14.1t/d), impurities in the water are less, the purity of sludge calcium fluoride generated by induced crystallization is high (Zingiberaceae and the like, 2012), and impurities such as silicon, carbonate, sulfate radicals, organic matters and the like in the mixed wastewater are more, so that a large amount of impurities are settled and enter the sludge through reaction with calcium salt and fluorine ions and adsorption and net-catching effects of a flocculating agent. The sludge has complex components, high silicon content and low calcium fluoride content, and the calcium fluoride content is about 30-60% of the dry weight of the sludge (Aldaco R et al, 2008). The main operating parameter pairs for the specific examples of the invention and the comparative examples are shown in table 2:
TABLE 2 Main operating parameters of the examples and comparative examples
As can be seen from table 2: aiming at the fluorine-containing wastewater of the silicon solar cell, the invention classifies and collects the acid-washing concentrated acid wastewater, etching concentrated acid wastewater, dilute acid wastewater, concentrated alkali wastewater and dilute alkali wastewater, and the set process has strong pertinence and reduces the overall treatment difficulty and the operating cost; the concentrated acid wastewater is crystallized and recycled, so that calcium fluoride (95%) with higher purity can be prepared at a rate of 0.837t/d, and the annual income reaches 76 ten thousand yuan/year. Meanwhile, compared with the conventional mixed treatment, the amount of the calcium fluoride materialized sludge is less, the amount of the sludge is only 2.811t/d which is far less than the 7.539t/d of the comparative example, and the annual sludge disposal cost is reduced by 63 percent. The use of a large amount of calcium chloride can bring certain negative effects to the subsequent biological denitrification treatment, for example, the excessive chloride ion concentration can inhibit the growth of microorganisms and influence the activity of activated sludge and the sedimentation effect of the sludge. Concentrated acid wastewater and other fluorine-containing wastewater are classified, collected and treated, the use amount of calcium chloride is 1.13t/d which is far lower than that of 1.81t/d of a comparative example, the calcium chloride is reduced by 37.6 percent, the operation cost is reduced, the chloride ion concentration in effluent is only 50 percent of that of the comparative example, and the limit requirement of the total solids (TDS) of subsequent effluent is met.
The above-described embodiments are merely preferred embodiments of the present invention, which should not be construed as limiting the invention. Various changes and modifications may be made by one of ordinary skill in the pertinent art without departing from the spirit and scope of the present invention. Therefore, the technical scheme obtained by adopting the mode of equivalent replacement or equivalent transformation is within the protection scope of the invention.
Claims (10)
1. A process for removing fluorine, controlling salt and recovering fluorine from silicon solar cell production wastewater is characterized by comprising the following steps:
s1: classifying and collecting wastewater discharged from different procedures in the production process of the silicon solar cell;
s2: combining concentrated acid wastewater discharged from an acid washing process and concentrated acid wastewater discharged from an etching process into concentrated acid fluorine-containing wastewater, and adding excessive calcium salt solution into the concentrated acid fluorine-containing wastewater, wherein the adding amount of the calcium salt solution meets the condition that the total molar concentration ratio of calcium to fluorine in the wastewater is (0.6-0.75): 1; fully stirring and mixing, and standing to precipitate calcium fluoride crystals; extracting calcium fluoride slurry crystallized and precipitated at the bottom in the reaction area, carrying out solid-liquid separation on the calcium fluoride slurry, using liquid obtained after the solid-liquid separation and supernatant of the precipitate for wastewater regulation, and drying the solid to obtain dry calcium fluoride sludge for resource utilization;
s3: mixing the wastewater collected in the step S1 and discharged from other steps except the pickling step and the etching step with the liquid obtained by solid-liquid separation of the calcium fluoride slurry in the step S2 and the supernatant, and homogenizing to obtain mixed wastewater;
s4: adding excessive Ca (OH) to the mixed wastewater obtained in S32Emulsion, Ca (OH)2The addition amount of the emulsion meets the condition that the total molar concentration ratio of calcium to fluorine in the wastewater is (1.1-1.25): 1, and the calcium fluoride crystal precipitate is generated primarily after the emulsion is fully stirred and mixed;
s5: adding concentrated sulfuric acid into the mixed wastewater obtained in the step S4, adjusting the pH value of the mixed wastewater to 6.5-7, and dissociating suspended calcium hydroxide in the wastewater to generate Ca2+Further generating calcium fluoride crystal precipitate;
s6: adding a coagulant PAC and a coagulant aid PAM into the mixed wastewater obtained in the step S5 to enable the calcium fluoride crystal in the water to be precipitated and flocculate;
s7: adding sulfuric acid into the mixed wastewater obtained in the step S6 to enable excessive calcium ions in the water to react to generate calcium sulfate precipitate to be removed, wherein the adding amount of the sulfuric acid is 0.02-0.05 mol/L of the mixed wastewater; after fully mixing and precipitating, adding the coagulant PAC and the coagulant aid PAM again to flocculate the generated calcium sulfate precipitate;
s8: and (4) standing and precipitating the mixed wastewater obtained in the step S7, carrying out outward treatment on precipitated sludge, and inputting the supernatant into a subsequent biochemical system for treatment.
2. The process for fluorine removal, salt control and fluorine recovery for silicon solar cell production wastewater as claimed in claim 1, wherein in S2, the calcium salt solution is a calcium chloride solution.
3. The process for removing fluorine, controlling salt and recovering fluorine in the wastewater generated in the silicon solar cell production according to claim 1, wherein in S6, the addition amount of a coagulant PAC is 300-400 mg/L of the mixed wastewater, and the addition amount of a coagulant aid PAM is 2-5 mg/L of the mixed wastewater.
4. The process for removing fluorine, controlling salt and recovering fluorine in the wastewater generated in the silicon solar cell production according to claim 1, wherein in S7, the addition amount of a coagulant PAC is 100-200 mg/L of the mixed wastewater, and the addition amount of a coagulant aid PAM is 2-5 mg/L of the mixed wastewater.
5. The process for removing fluorine, controlling salt and recovering fluorine in the wastewater generated in the production of silicon solar cells as claimed in claim 1, wherein in the step S2, the concentrated acid fluorine-containing wastewater automatically flows into the first reaction tank for treatment.
6. The process for removing fluorine, controlling salt and recovering fluorine in the wastewater generated in the production of silicon solar cells as claimed in claim 1, wherein the mixed wastewater in S3 is stored in a regulating tank and continuously pumped into a second reaction tank for the treatment of the steps S4-S7, the second reaction tank is divided into four different cells according to the wastewater flow, and the wastewater sequentially flows through the four cells for the treatment of the steps S4, S5, S6 and S7.
7. The process for removing fluorine, controlling salt and recovering fluorine from wastewater generated in the production of silicon solar cells as claimed in claim 6, wherein in S8, the effluent of the sedimentation tank is monitored online by fluoride ions and TDS, when the effluent is detected to be not reaching the standard, the influent of the adjustment tank is closed, the effluent of the sedimentation tank is returned to the second reaction tank for the treatment of the steps S4-S7, and phosphate is further added in the treatment process for secondary intensified fluorine removal and salt control.
8. The process for removing fluorine, controlling salt and recovering fluorine from the wastewater generated in the production of silicon solar cells as claimed in claim 7, wherein the molar concentration ratio of the added phosphate to the fluorine ions in the effluent of the sedimentation tank is (0.2-0.4) when the secondary enhanced fluorine removal and salt control treatment is carried out: 1.
9. the process for removing fluorine, controlling salt and recovering fluorine in wastewater generated in the production of silicon solar cells as claimed in claim 7, wherein the phosphate is KH2PO4、K2HPO4、K3PO4One kind of (1).
10. The process for removing fluorine, controlling salt and recovering fluorine in the wastewater generated in the silicon solar cell production as claimed in claim 1, wherein the precipitated sludge generated in the steps S4-S8 is pumped into a sludge storage tank, and is subjected to pressure filtration by a plate-and-frame filter press and then is transported outside.
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