Method for recovering nitric acid from photovoltaic panel cleaning waste liquid

Abstract

The invention discloses a method for recovering nitric acid from photovoltaic panel cleaning waste liquid, which comprises the following steps: adding silica powder into the photovoltaic panel cleaning waste liquid, and converting hydrofluoric acid into fluosilicic acid after full reaction; adding potassium nitrate into the converted mixed solution of the fluosilicic acid and the nitric acid, so that the fluosilicic acid forms a potassium fluosilicate form and precipitates are separated out, the precipitates are separated from the potassium fluosilicate and the nitric acid, and a superior potassium fluosilicate product meeting the industrial standard and a pure nitric acid solution are obtained; the pure nitric acid solution enters an extraction system and is subjected to multi-stage extraction; the loaded organic phase enters a rectification system to be rectified and desorbed, so that the extracting agent is regenerated and reused, and volatilized nitrogen dioxide is circularly absorbed by pure water to obtain nitric acid with the concentration of more than 68%.

Description

Technical Field

[0001] The invention belongs to a method for recovering nitric acid, and in particular relates to a method for recovering nitric acid from photovoltaic panel cleaning waste liquid. Background Art

[0002] Polysilicon and monocrystalline silicon are the basic materials of the photovoltaic industry. The heads and tails of polysilicon and monocrystalline drawn and cut silicon rods will cause surface contamination during transportation, storage and use; therefore, the surface must be cleaned before using these materials, mainly to remove organic matter, oxide layer and metal contamination to ensure surface quality.

[0003] During the cleaning process, a mixed solution of 68% nitric acid and 49% hydrofluoric acid is mainly used for soaking to remove impurities. After the mixed acid has been used in the cleaning tank for a certain period of time, the reduction in its concentration and purity will lead to a decrease in the cleaning effect, affecting the cleaning effect. The mixed acid in the tank needs to be discharged regularly and new acid added. The components of the discharged waste mixed acid are 35~40% nitric acid, 4~6% hydrofluoric acid, and 5% fluorosilicic acid.

[0004] At present, the research methods for treating such fluorine-containing and nitric acid-containing wastewater at home and abroad include chemical precipitation, adsorption, reverse osmosis, etc. Among them, chemical precipitation is the commonly used method in the industry. Its principle is to add calcium salt to the fluorine-containing and nitric acid-containing wastewater, and use hydrofluoric acid, fluorosilicic acid, nitric acid and Ca 2+ The waste acid generated by the single crystal silicon material cleaning process is removed by the principle of forming insoluble salt precipitation, and the filtered waste liquid is then added with a coagulant for deep purification, so as to meet the wastewater discharge standards. However, the waste acid from the cleaning of single crystal silicon materials contains a large amount of fluorine resources and nitric acid resources. If the chemical precipitation method is used, the valuable resources are converted into sludge with no economic utilization value, which not only causes a waste of resources, but also poses a risk of secondary pollution. In addition, there are few reports in the existing public technologies on methods for resource recycling of waste acid generated during the cleaning of single crystal silicon materials. Summary of the invention

[0005] The technical problem to be solved by the present invention is to address the deficiencies of the prior art. The present invention provides a method for recovering nitric acid from photovoltaic panel cleaning waste liquid. The method for recovering nitric acid from photovoltaic panel cleaning waste liquid comprises:

[0006] S1001, adding silicon powder to the photovoltaic panel cleaning waste liquid, and converting the hydrofluoric acid therein into fluorosilicic acid after sufficient reaction;

[0007] S1002, adding potassium nitrate to the mixed solution of the converted fluorosilicic acid and nitric acid, so that the fluorosilicic acid forms potassium fluorosilicate and precipitates, thereby separating it from the potassium fluorosilicate nitric acid, and obtaining high-quality potassium fluorosilicate and pure nitric acid solution that meet industry standards;

[0008] S1003, the pure nitric acid solution enters the extraction system and undergoes multi-stage extraction;

[0009] S1004, the loaded organic phase enters the distillation system again for distillation and de-extraction, so that the extractant can be regenerated and reused, and the volatilized nitrogen dioxide is recycled and absorbed by pure water to obtain nitric acid greater than 68%.

[0010] According to one embodiment of the present invention, 1.1 times the theoretical amount of potassium nitrate is added at room temperature.

[0011] According to one embodiment of the present invention, N235 is used as the extractant with a volume concentration of 10-30%, the rest is the diluent sulfonated kerosene and the modifier tributyl phosphate, the extraction phase ratio O / A is 1-4:1, and four-stage countercurrent extraction is performed with a mixing time of 2-5 minutes.

[0012] According to one embodiment of the present invention, the loaded organic phase enters the distillation tower, the bottom temperature is controlled to be 105-110 ° C, the top temperature is 90-95 ° C, and the bottom pressure is -30--40 KPa.

[0013] According to one embodiment of the present invention, the molar amount of the added silicon powder should be 1.1 to 1.2 times the molar amount required for complete reaction with hydrofluoric acid, so that the hydrofluoric acid is completely converted into fluorosilicic acid. DETAILED DESCRIPTION

[0014] To further illustrate the present invention, the technical solutions in the embodiments of the present invention are described clearly and completely below. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the protection scope of the present invention.

[0015] According to one aspect of the present invention, the present invention provides a method for recovering nitric acid from photovoltaic panel cleaning waste liquid, wherein the method for recovering nitric acid from photovoltaic panel cleaning waste liquid comprises:

[0016] S1001, adding a sufficient amount of silicon micropowder to the photovoltaic panel cleaning waste liquid, and converting the hydrofluoric acid therein into fluorosilicic acid after sufficient reaction; preferably, the molar amount of the added silicon micropowder should be 1.1 to 1.2 times the molar amount required for complete reaction with the hydrofluoric acid during the reaction, so that the hydrofluoric acid is completely converted into fluorosilicic acid.

[0017] S1002, adding potassium nitrate to the mixed solution of the converted fluorosilicic acid and nitric acid to precipitate the fluorosilicic acid in the form of potassium fluorosilicate, thereby separating the potassium fluorosilicate from the nitric acid and obtaining high-quality potassium fluorosilicate and pure nitric acid solution that meet industry standards;

[0018] S1003, the pure nitric acid solution enters the extraction system, and after multi-stage extraction, the nitric acid concentration in the residual solution is reduced to less than 1%, and the extraction rate reaches more than 97%;

[0019] S1004, the loaded organic phase enters the distillation system again for distillation and deextraction, so that the extractant can be regenerated and reused, and the volatilized nitrogen dioxide is circulated and absorbed by pure water to obtain nitric acid greater than 68%, which meets the concentration requirement for single crystal silicon material cleaning and realizes the recovery and reuse of nitric acid.

[0020] It is worth mentioning that in step S1001:

[0021] There is still 4-6% hydrofluoric acid remaining in the waste acid. Excess silicon micropowder is added to the hydrofluoric acid at room temperature to fully react for 2 hours to convert the hydrofluoric acid into fluorosilicic acid. The presence of hydrofluoric acid should be reduced as much as possible to ensure the purity of the subsequent nitric acid.

[0022] Reaction equation: SiO2+6HF=H2SiF6+2H2O;

[0023] In S1002:

[0024] The waste acid solution after the conversion contains only fluorosilicic acid and nitric acid. At this time, adding 1.1 times the theoretical amount of potassium nitrate at room temperature can completely react with the fluorosilicic acid therein to generate potassium fluorosilicate precipitation. The crude potassium fluorosilicate and pure nitric acid solution obtained after filtration are then washed with water to remove residual potassium nitrate. After filtration and drying, 99% potassium fluorosilicate can be obtained.

[0025] Reaction equation: H2SiF6+2KNO3=K2SiF6 +2HNO3.

[0026] In S1003:

[0027] The pure nitric acid solution enters the extraction system, with N235 as the extractant, the volume concentration is 10-30%, the rest is the diluent sulfonated kerosene and the modifier tributyl phosphate, the extraction phase O / A ratio is 1-(4:1), after four-stage countercurrent extraction, the mixing time is 2-5 minutes, the nitric acid concentration in the raffinate can be reduced to less than 1%, and the extraction rate of nitric acid can reach more than 97%.

[0028] In S1004:

[0029] The loaded organic phase enters the distillation tower, the tower bottom temperature is controlled to be 105-110°C, the tower top temperature is 90-95°C, and the tower bottom pressure is -30--40 KPa. The nitrate ions in the organic phase are thermally decomposed into nitrogen dioxide and volatilized, and enter the pure water absorption tower. After continuous circulation, absorption and concentration, 68% concentrated nitric acid is obtained, which meets the standard of nitric acid for cleaning single crystal silicon materials and can be directed back to the cleaning process. At the same time, the organic phase is also regenerated and returned to the extraction process for recycling.

[0030] Compared with the prior art, the present invention has the following beneficial effects:

[0031] The present invention adopts a composite process system of chemical reaction, extraction, distillation and purification to effectively and comprehensively recover the mixed waste acid containing hydrofluoric acid, fluorosilicic acid and nitric acid, thus creating considerable economic benefits and avoiding waste of resources.

[0032] The present invention effectively recycles waste acid, greatly reduces the amount of wastewater discharged in existing treatment technologies, and does not generate solid waste, thereby protecting the environment and effectively reducing the wastewater treatment costs of enterprises.

[0033] The concentration of the purified nitric acid solution is only about 35%. It is difficult to concentrate it to 68% by conventional evaporation and concentration. There are reports on the concentration process of nitric acid. Concentrated sulfuric acid is introduced as a water absorbent to evaporate the nitric acid. However, the introduction of concentrated sulfuric acid forms a mixed acid with the residual nitric acid at the bottom of the kettle, which affects the subsequent recycling of sulfuric acid and greatly increases the energy consumption cost and reuse effect. The present invention introduces an extraction process. After multi-stage extraction, the extraction rate of nitric acid can reach more than 97%. The loaded organic phase is subjected to rectification and pure water absorption, and the metal ion impurities can be effectively separated. After cyclic absorption, more than 68% concentrated nitric acid can be obtained, which meets the acid concentration requirements of the single crystal silicon material cleaning process.

[0034] Example 1

[0035] The waste acid discharged from the single crystal silicon material cleaning process is collected in a storage tank and then quantitatively pumped into the conversion reactor. Excess silicon micropowder is added to the hydrofluoric acid at room temperature for 2 hours to fully react with the hydrofluoric acid to convert the hydrofluoric acid into fluorosilicic acid. The presence of hydrofluoric acid should be reduced as much as possible to ensure the purity of the subsequent nitric acid. After filtration, the unreacted silicon slag is returned to the conversion reactor to continue the reaction. Only fluorosilicic acid and nitric acid exist in the converted waste acid solution. At this time, 1.1 times the theoretical amount of potassium nitrate is added at room temperature and stirred for 1 hour, which can completely react with the fluorosilicic acid therein to generate potassium fluorosilicate precipitation. The crude potassium fluorosilicate and pure nitric acid solution are obtained after filtration. The crude potassium fluorosilicate is then washed with water to remove the residual potassium nitrate. After filtration and drying, 99% potassium fluorosilicate can be obtained. The obtained pure nitric acid solution enters the four-stage centrifugal continuous extraction system, with N235 as the extractant, the volume concentration is 10~30%, the rest is the diluent sulfonated kerosene and the modifier tributyl phosphate, the extraction phase ratio O / A is 1~(4:1), after four-stage countercurrent extraction, the mixing time is 2~5min, the nitric acid concentration in the raffinate can be reduced to less than 1%, and the extraction rate of nitric acid can reach more than 97%. The loaded organic phase after extraction enters the distillation tower, the tower bottom temperature is controlled to be 105~110 °C, the tower top temperature is 90~95°C, and the tower bottom pressure is -30~-40 KPa. The nitrate ions in the organic phase are thermally decomposed into nitrogen dioxide and volatilized, and enter the pure water absorption tower. After continuous circulation, absorption and concentration, 68% concentrated nitric acid is obtained, which meets the standard of nitric acid for single crystal silicon material cleaning and can be directed back to the cleaning process. At the same time, the organic phase is also regenerated and returned to the extraction process for recycling.

[0036] It should be understood by those skilled in the art that the embodiments of the present invention described above are only examples and do not limit the present invention. The purpose of the present invention has been fully and effectively achieved. The functional and structural principles of the present invention have been demonstrated and explained in the embodiments, and the embodiments of the present invention may be deformed or modified in any way without departing from the principles.

Claims

1. A method for recovering nitric acid from photovoltaic panel cleaning waste liquid, characterized in that: The method for recovering nitric acid from photovoltaic panel cleaning waste liquid comprises: S1001, adding a sufficient amount of silicon powder to the photovoltaic panel cleaning waste liquid, so that the hydrofluoric acid therein is converted into fluorosilicic acid after sufficient reaction; S1002, adding potassium nitrate to the mixed solution of the converted fluorosilicic acid and nitric acid, so that the fluorosilicic acid forms potassium fluorosilicate and precipitates, thereby separating it from the potassium fluorosilicate nitric acid, and obtaining high-quality potassium fluorosilicate and pure nitric acid solution that meet industry standards; S1003, the pure nitric acid solution enters the extraction system and undergoes multi-stage extraction; S1004, the loaded organic phase enters the distillation system again for distillation and de-extraction, so that the extractant can be regenerated and reused, and the volatilized nitrogen dioxide is recycled and absorbed by pure water to obtain nitric acid greater than 68%.

2. The method for recovering nitric acid from photovoltaic panel cleaning waste liquid according to claim 1, characterized in that: Add 1.1 times the theoretical amount of potassium nitrate at room temperature.

3. The method for recovering nitric acid from photovoltaic panel cleaning waste liquid according to claim 1, characterized in that: N235 is used as the extractant with a volume concentration of 10~30%, the rest is the diluent sulfonated kerosene and the modifier tributyl phosphate, the extraction phase ratio O / A is 1~(4:1), and it undergoes four-stage countercurrent extraction with a mixing time of 2~5min.

4. The method for recovering nitric acid from photovoltaic panel cleaning waste liquid according to claim 1, characterized in that: The loaded organic phase enters the distillation tower, and the bottom temperature is controlled to be 105-110 °C, the top temperature is 90-95 °C, and the bottom pressure is -30--40 KPa.

5. The method for recovering nitric acid from photovoltaic panel cleaning waste liquid according to claim 1, characterized in that: The molar amount of silicon micropowder added should be 1.1 to 1.2 times the molar amount required for complete reaction with hydrofluoric acid, so that hydrofluoric acid is completely converted into fluorosilicic acid.