Digestion method of silicon dioxide

Abstract

The invention discloses a silicon dioxide digestion method which comprises the following steps: adding a silicon dioxide sample into a pre-cooled ammonium fluoride and phosphoric acid solution, gradually heating to dissolve silicon dioxide, continuously heating to remove phosphoric acid, and finally adding pure water to a constant volume. Hydrofluoric acid is replaced by ammonium fluoride which is low in toxicity and not easy to volatilize, so that the risk of operation is reduced; ammonium fluoride is a stable white crystal and is more beneficial to long-term storage than hydrofluoric acid.

Description

Technical Field

[0001] This invention relates to the field of inductively coupled plasma atomic emission spectrometry (ICP) measurement technology, and more specifically, to the determination of metallic impurity elements in silica, particularly a method for digesting silica and a method for preparing water-soluble samples of metallic impurities in silica. Background Technology

[0002] The standard method for preparing silica samples to be suitable for atomic emission spectroscopy is to add hydrofluoric acid and carefully heat it (GB / T3284-2015, Section 8). Because concentrated hydrofluoric acid solutions are highly volatile and must be stored in plastic bottles, they are prone to leakage during long-term storage; personal injury accidents involving hydrofluoric acid are frequent, so many elemental analysis laboratories no longer stock hydrofluoric acid or operators are unwilling to use it, making the detection of silica-containing materials difficult.

[0003] Substances that can ionize fluoride ions all have a certain degree of toxicity. As the size of the fluoride ion increases, its ability to penetrate the skin decreases, and the possibility of serious injury from direct contact also decreases. Therefore, the contact toxicity of substances such as sodium fluoride and ammonium fluoride is less than that of hydrofluoric acid. Similar to hydrofluoric acid, ammonium fluoride can also react with silica to obtain a solution, in which the recovery rate of silica can reach 99.2% (Zhao Yue, Liu Jingdang, Guo Changlai, et al. Experimental study on the removal of silica from graphite by ammonium fluoride [J]. Nonmetallic Minerals, 2017, 40(6):3.).

[0004] Against the backdrop of the above-mentioned technologies, we provide a method for digesting silica to address the problem of the large-scale use of volatile and toxic hydrofluoric acid when digesting silica samples into samples suitable for ICP testing. Summary of the Invention

[0005] To address the problem of the large amount of hydrofluoric acid used when digesting silica samples into samples suitable for ICP testing, this invention provides a method for digesting silica.

[0006] Another objective of this invention is to provide an application of a method for digesting silica.

[0007] The technical solution provided by this invention is: a method for digesting silica, comprising the following steps: (1) Dissolve ammonium fluoride in cold water; (2) Add silica powder to pre-cooled concentrated phosphoric acid and carefully add the above-mentioned cold ammonium fluoride solution; (3) Gradually heat under ventilated conditions to allow the silica to react and dissolve; (4) Further heating removes excess phosphoric acid; (5) Cooling reaction, add pure water to form a sample suitable for atomic emission spectrometer measurement.

[0008] Preferably, in step (2), superior grade azeotropic phosphoric acid (85% phosphoric acid content) is used.

[0009] Preferably, the silica sample added in step (2) has a fineness of 200 mesh or higher.

[0010] Adding phosphoric acid to the reaction allows the sparingly soluble salts calcium fluoride and magnesium fluoride to dissolve under neutral conditions, preventing incomplete reactions. Pre-cooling the reaction solution prevents the spillage of volatile hydrofluoric acid, further protecting the operator. At 160°C, phosphoric acid forms an azeotrope with water and leaves the system. Removing excess phosphoric acid prevents flameout malfunctions in the inductively coupled plasma atomic emission spectrometer. The main reaction equation occurring in the experiment can be expressed as: SiO2+ 4NH4F + 4H3PO4→ SiF4↑+ 4NH4H2PO4+ 2H2O (1) The departure of SiF4 propelled the reaction to the right. Because the ionization constant of primary phosphate is greater than that of hydrofluoric acid, and side reactions exist under acidic conditions: NH4F + H3PO4 → HF + NH4H2PO4 (2) Under heating conditions, HF will escape.

[0011] To ensure the advantages of the main reaction, the silica sample should be crushed and ground to a fineness of 200 mesh or higher.

[0012] This invention also provides a method for preparing water-soluble samples of metallic impurities in silicon dioxide.

[0013] This invention discloses a method for digesting silica. A silica sample is added to a pre-cooled solution of ammonium fluoride and phosphoric acid, and the solution is gradually heated to dissolve the silica. Heating continues to remove the phosphoric acid, and finally, pure water is added to bring the volume to a final level. Ammonium fluoride, which is less toxic and less volatile, is used instead of hydrofluoric acid, reducing the operational risks. Ammonium fluoride is also a stable white crystal, making it more suitable for long-term storage than hydrofluoric acid.

[0014] Advantages of this invention: (1) The method of the present invention does not require operators to come into contact with volatile hydrofluoric acid, and is not likely to cause personal danger; (2) The required reagents are stable and easy to store for a long time. Detailed Implementation

[0015] The present invention will be further illustrated below with reference to specific embodiments. Unless otherwise specified, the experimental methods used in the following embodiments are conventional methods. Unless otherwise specified, the materials and reagents used in the following embodiments are commercially available. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the invention.

[0016] Example 1 Determination of impurity metal elements in vein quartz: 10 g of vein quartz ore was mechanically crushed and ground to 200 mesh, placed in a 100 mL beaker, and 50 mL of water and 1 mL of 68% analytical grade nitric acid were added. After boiling for 1 h, the dilute nitric acid was discarded, and the ore was dried at 120℃ for 3 h and then mixed thoroughly. This sample was used to determine the impurity metal elements present. (1) Preparation of precooling solution Add 2 mL of commercially available azeotropic phosphoric acid of analytical grade (85%) to a 100 mL polytetrafluoroethylene beaker and cool it to -20°C in a refrigerator. Add 20 mL of pure water and 1.0 g of analytical grade NH4F to another plastic beaker, stir with a plastic rod to dissolve, and then cool it to 4°C in a refrigerator. (2) Add the sample Remove the polytetrafluoroethylene beaker containing pre-cooled phosphoric acid from the refrigerator in step (I) and immediately and accurately weigh 250 mg of vein quartz sample and add it to the beaker. Carefully pour the NH4F solution pre-cooled to 4°C into the beaker along the beaker wall to obtain the reaction solution. Place the beaker containing the reaction solution on a graphite heating plate in a fume hood. (3) Digestion reaction Set the graphite heating plate to 55°C and maintain it for 1 hour; a few bubbles were released in the reaction solution; then set the graphite heating plate to 95°C, and bubbles continued to be generated in the reaction solution, and the sample gradually dissolved. Maintain heating for 2 hours. (4) Remove water and phosphoric acid The graphite heating plate was set to 110°C, and white mist was generated on the surface of the reaction solution. The temperature was maintained until the mist stopped being generated. Then the graphite heating plate was set to 175°C, and the heating was stopped after 1 hour. The reaction solution was cooled to obtain a white solid. Pure water was added to dissolve the white solid and the volume was adjusted to 50 mL. (5) Determine the concentration of metallic impurities.

[0017] The concentration of metallic impurities can be determined by setting the conditions according to GB / T3284-2015, and the elements Al, Ca, Cd, Co, Cr, Cu, Fe, Hg, K, Li, Mg, Mn, Mo, Na, Ni, Pb, and Zn can be determined simultaneously.

[0018] In this embodiment, vein quartz is a high-purity silica ore that is widely used in the semiconductor and photovoltaic industries to extract high-purity, non-radioactive silicon.

[0019] Example 2 Determination of the content of metallic impurities in silicon-carbon samples: (1) Preparation of precooling solution Add 1 mL of commercially available azeotropic phosphoric acid of superior purity (85% phosphoric acid concentration) to a 100 mL polytetrafluoroethylene beaker and cool it to -20°C in a refrigerator; add 20 mL of pure water and 0.5 g of analytical grade NH4F to another plastic beaker, stir with a plastic rod to dissolve, and then cool it to 4°C in a refrigerator. (2) Add the sample Take the polytetrafluoroethylene beaker containing pre-cooled phosphoric acid from the refrigerator in step (1) and immediately weigh 1.000 g of sample and add it to the beaker. Carefully pour the NH4F solution pre-cooled to 4°C into the beaker along the beaker wall to obtain the reaction solution. Place the beaker containing the reaction solution on the graphite heating plate in the fume hood. (3) Digestion reaction Set the graphite heating plate to 55°C and maintain it for 1 hour; then set the graphite heating plate to 95°C and maintain it for 2 hours. (4) Remove water and phosphoric acid Set the heating plate to 110°C. White mist will appear on the surface of the reaction solution. Maintain the temperature until the mist no longer appears. Then set the heating plate to 175°C and heat for 1 hour before stopping the heating. Filter to remove insoluble carbon particles, add pure water and make up to 50 mL. (5) Determine the concentration of metallic impurities The concentration of metallic impurities can be determined by setting conditions according to GB / T38823-2020, and elements such as Al, Co, Cr, Cu, Fe, Hg, K, Li, Mg, Mn, Mo, Na, Ni, Pb, and Zn can be determined simultaneously.

[0020] In this embodiment, silicon-carbon is a type of battery anode material composed of silicon oxide and carbon materials. It consists of uniform submicron particles and is dried at 120°C for 3 hours as a sample.

[0021] The above embodiments are merely examples to clearly illustrate the present invention and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A method for digesting silica, characterized in that, Includes the following steps: (1) Dissolve ammonium fluoride (NH4F) in cold water to obtain a cold solution of ammonium fluoride; (2) Add the silica powder sample to the pre-cooled concentrated phosphoric acid, and carefully add the above-mentioned cold ammonium fluoride solution; (3) Gradually heat under ventilated conditions to allow the silica to react and dissolve; (4) Further heating removes excess phosphoric acid; (5) Cooling reaction, add pure water to dissolve and form a sample suitable for inductively coupled plasma atomic emission spectrometry.

2. The method for digesting silica according to claim 1, characterized in that, The fineness of the measured silica powder sample is greater than 200 mesh.

3. The method for digesting silica according to claim 1, characterized in that, In step (2), superior grade azeotropic phosphoric acid with a phosphoric acid content of 85% is used.

4. An application of the silica digestion method according to any one of claims 1-3, characterized in that, Determination of impurity metal elements in vein quartz: 10 g of vein quartz ore was mechanically crushed and ground to 200 mesh, placed in a 100 mL beaker, and 50 mL of water and 1 mL of 68% analytical grade nitric acid were added. The mixture was boiled for 1 h, then the dilute nitric acid was discarded. The ore was dried at 120℃ for 3 h and then mixed thoroughly. This sample was used to determine the impurity metal elements. The procedure was as follows: (1) Preparation of precooling solution Add 2 mL of commercially available azeotropic phosphoric acid of superior purity (85% concentration) to a 100 mL polytetrafluoroethylene beaker and cool it to -20°C in a refrigerator to obtain pre-cooled phosphoric acid; add 20 mL of pure water and 1.0 g of analytical grade NH4F to another plastic beaker, stir with a plastic rod to dissolve, and then cool it to 4°C in a refrigerator. (2) Add the sample Take the polytetrafluoroethylene beaker containing pre-cooled phosphoric acid from the refrigerator in step (1) and immediately weigh 250 mg of vein quartz sample into it. Carefully pour the NH4F solution pre-cooled to 4°C into it along the beaker wall to obtain the reaction solution. Place the beaker containing the reaction solution on the graphite heating plate in the fume hood. (3) Digestion reaction Set the graphite heating plate to 55°C and maintain it for 1 hour. A few bubbles were released in the reaction solution. Then set the graphite heating plate to 95°C. Bubbles continued to be generated in the reaction solution, and the sample gradually dissolved. Maintain heating for 2 hours. (4) Remove water and phosphoric acid The graphite heating plate was set to 110℃, and white mist was generated on the surface of the reaction liquid. The temperature was maintained until the mist stopped being generated. Then the graphite heating plate was set to 175℃ and the heating was stopped after 1 hour. The reaction liquid was cooled to obtain a white solid. Pure water was added to dissolve the white solid and the volume was adjusted to 50 mL to obtain a sample suitable for inductively coupled plasma atomic emission spectrometry. (5) Determine the concentration of metallic impurities The concentration of metallic impurities was determined according to the conditions set in GB / T3284-2015, and the elements Al, Ca, Cd, Co, Cr, Cu, Fe, Hg, K, Li, Mg, Mn, Mo, Na, Ni, Pb, and Zn were determined simultaneously.

5. An application of the silica digestion method according to any one of claims 1-3, characterized in that, The content of metal impurities in silicon-carbon samples was determined. Silicon-carbon is a type of battery anode material composed of silicon oxide and carbon materials. It consists of uniform submicron particles and was dried at 120°C for 3 h as the sample. (1) Preparation of precooling solution Add 1 mL of commercially available azeotropic phosphoric acid of superior purity (85% phosphoric acid concentration) to a 100 mL polytetrafluoroethylene beaker and cool it to -20°C in a refrigerator. Add 20 mL of pure water and 0.5 g of analytical grade NH4F to another plastic beaker, stir with a plastic rod to dissolve, and then cool it to 4°C in a refrigerator. (2) Add the sample Take the polytetrafluoroethylene beaker containing pre-cooled phosphoric acid from the refrigerator in step (1) and immediately weigh 1.000 g of sample and add it to the beaker. Carefully pour the NH4F solution pre-cooled to 4 degrees Celsius into the beaker along the beaker wall and place the beaker on the graphite heating plate in the fume hood. (3) Digestion reaction Set the graphite heating plate to 55°C and maintain it for 1 hour, then set the graphite heating plate to 95°C and maintain heating for 2 hours. (4) Remove water and phosphoric acid Set the heating plate to 110°C. White mist will be generated on the surface of the reaction liquid. Maintain the temperature until the mist no longer occurs. Then set the heating plate to 175°C and keep heating for 1 hour. Stop heating, filter to remove insoluble carbon particles, add pure water and make up to 50 mL to obtain a sample suitable for inductively coupled plasma atomic emission spectrometry. (5) Determine the concentration of metallic impurities The concentration of metallic impurities was determined according to the conditions set in GB / T38823-2020, and the elements Al, Co, Cr, Cu, Fe, Hg, K, Li, Mg, Mn, Mo, Na, Ni, Pb and Zn were determined simultaneously.