A sample pretreatment method for the determination of impurity element content in high-purity graphite

Abstract

The present invention provides a sample pretreatment method for determining the impurity element content in high-purity graphite. The method comprises the following steps: placing the high-purity graphite sample in a high-purity quartz crucible, and then placing the high-purity quartz crucible on a high-purity quartz backing plate for high-temperature ashing to obtain an ashing residue; adding nitric acid into the high-purity quartz crucible to soak and pre-decompose the ashing residue to obtain a pretreated residue acid solution; transferring the pretreated residue acid solution from the high-purity quartz crucible to a polytetrafluoroethylene beaker, then adding hydrofluoric acid and perchloric acid, and heating until white smoke disappears completely; removing the beaker, cooling it to about 50 °C at room temperature, adding nitric acid and water, heating to slight boiling and then cooling. The present invention greatly reduces the contamination of the sample by the vessel, avoids the mass loss and contact contamination of the residue caused during the transfer of the ashing residue. The sample pretreatment method is simple and fast, and the cost is greatly reduced compared with the traditional method using a platinum crucible. The measurement result is stable and the accuracy is high.

Description

Technical Field

[0001] The present invention relates to the technical field of high-purity graphite, and particularly to a sample pretreatment method for determining the content of impurity elements in high-purity graphite. Background Art

[0002] High-purity graphite, as an excellent functional material and basic material, is widely used in high-tech fields such as the machinery industry, electronics industry, aerospace industry, and nuclear industry. Among them, impurity elements have a great impact on the performance of deep-processed products of high-purity graphite, and different industries have different requirements for the content of impurity elements in high-purity graphite. For example, high-purity graphite is an important and essential material in the production of semiconductor polysilicon. Impurities in high-purity graphite will react with chlorosilane at high temperatures, affecting the quality of the product; high-purity graphite, as the matrix material of nuclear-pure graphite, has very strict restrictions on impurity elements such as B, Li, Cd, etc. with a large neutron absorption cross-section. Therefore, it is particularly important to accurately determine the content of impurity elements in high-purity graphite.

[0003] The key points and difficulties in determining the content of impurity elements in high-purity graphite are that the sample is easily contaminated by the environment, utensils, etc. during the pretreatment process, and the residue after high-temperature ashing is easily lost during the transfer to another utensil. Therefore, minimizing or avoiding the contamination of the sample during the high-temperature ashing process and achieving the quantitative and lossless transfer of the ashing residue are one of the key points and difficulties affecting the accuracy of the measurement results.

[0004] For the determination of the content of impurity elements in high-purity graphite, a platinum crucible with a purity of 99.9% is usually used to perform high-temperature ashing on high-purity graphite samples. However, it is found in the actual detection process that there are certain limitations in using a platinum crucible for high-temperature ashing and decomposing the residue of high-purity graphite samples: First, enriching the ash requires high-temperature roasting, but at high temperatures, part of the ash will adhere to the platinum pot and is difficult to separate; second, a large amount of carbon in high-purity graphite undergoes a slow oxidation reaction with the platinum crucible at high temperatures, easily making the platinum pot brittle and fragile, reducing the service life of the platinum pot and increasing the detection cost; and the cost of the platinum pot is relatively high, which is not conducive to the batch determination of samples; third, although the platinum pot reaches a purity of 99.9%, if the ashing residue is directly alkali-fused or acid-decomposed in the platinum crucible, the impurity elements in the platinum pot may transfer to the ashing residue, affecting the accuracy of the measurement results, especially for the determination of trace and ultra-trace elements in high-purity graphite; if the ashing residue is transferred to a polytetrafluoroethylene beaker and decomposed with a mixed acid, it is necessary to brush it from the platinum pot to the polytetrafluoroethylene beaker, and the transfer is likely to cause mass loss of the residue and secondary contamination of the residue by the brush; fourth, when roasting the sample at high temperature in the furnace, the crucible containing the sample needs to be placed on a high-temperature resistant backing plate. The commonly used backing plate is made of high-temperature refractory materials and contains a large amount of impurity elements such as sodium, magnesium, aluminum, titanium, heavy metals, and rare earths, which will also affect the accuracy of the measurement results.

[0005] In addition, for the pretreatment of high-purity graphite samples, in the prior art, a mixed acid solution of HNO 3 -HClO 4 -H 2 SO 4 is added to a flask and heated with condensation reflux for the pretreatment of high-purity graphite samples. However, this method cannot completely decompose the high-purity graphite samples, and only some impurity elements in the graphite can be obtained. Moreover, at a temperature of 220 °C, the impurity elements of glassware are easily dissolved into the acid solution, which will affect the accuracy of the measurement results; there is also a method of putting high-purity graphite into a platinum crucible for high-temperature ashing, then decomposing the ash with a mixed alkali of sodium tetraborate and anhydrous sodium carbonate, dissolving the melt with hydrochloric acid, and finally making up the volume for determination. In addition to the disadvantages brought by using a platinum crucible, the mixed alkali used will introduce a certain amount of impurities, affecting the accuracy of the determination results of trace, especially ultra-trace impurity elements in high-purity graphite.

[0006] Therefore, the existing pretreatment techniques for high-purity graphite samples can no longer meet the requirements of analysis and testing for the determination of trace and ultra-trace elements in high-purity graphite, and it is necessary to improve and innovate the existing sample pretreatment techniques. SUMMARY OF THE INVENTION

[0007] The present invention provides a sample pretreatment method for determining the content of impurity elements in high-purity graphite. A high-purity quartz crucible with a purity of more than 99.9999% and high temperature resistance is used to replace the traditional platinum / porcelain crucible as the high-temperature roasting dish for high-purity graphite; a high-purity quartz backing plate with a purity of more than 99.9999% and high temperature resistance is used to replace the traditional refractory backing plate, which can greatly reduce the pollution of the high-purity graphite sample by the ware; the wet method for transferring and ashing the residue is adopted, which avoids the mass loss and contact pollution of the residue caused by the traditional dry method for transferring and ashing the residue, and greatly improves the accuracy of the analysis results.

[0008] The technical solution of the present invention is realized as follows: A sample pretreatment method for determining the content of impurity elements in high-purity graphite includes the following steps:

[0009] (1) Place 5-10 g of high-purity graphite sample in a high-purity quartz crucible, and place the high-purity quartz crucible on a high-purity quartz backing plate for high-temperature ashing to obtain an ashed residue;

[0010] (2) Add 5-10 mL of nitric acid into the high-purity quartz crucible, and soak and pre-decompose the ashed residue at room temperature to obtain a pre-treated residue acid solution;

[0011] (3) Transfer the pretreated residue acid solution from the high-purity quartz crucible to a polytetrafluoroethylene beaker. Use a PFA dropper to suck 1 mL of high-purity water, rinse the crucible wall in small amounts and multiple times, then add 5 - 10 mL of hydrofluoric acid and 1 mL of perchloric acid, and heat until white smoke disappears completely.

[0012] (4) Remove the polytetrafluoroethylene beaker, cool it to 45 - 55 °C at room temperature, then add 1 mL of nitric acid and 5 - 10 mL of water, heat to slightly boiling and then cool.

[0013] Furthermore, the purity of both the high-purity quartz crucible and the high-purity quartz backing plate is above 99.9999%.

[0014] Furthermore, in step (1), the method of high-temperature ashing is as follows: Place the high-purity quartz crucible on the high-purity quartz backing plate, put it in a muffle furnace at 850 °C - 1100 °C and burn for 2 - 5 h until the high-purity graphite sample is completely oxidized. Take out the high-purity quartz crucible and cool it to obtain the ashed residue.

[0015] Furthermore, in step (2), after adding 5 - 10 mL of nitric acid along the inner wall of the high-purity quartz crucible in a circular motion, then use a PFA dropper to add 1.5 - 2.5 mL of water along the inner wall in a circular motion to rinse the inner wall and gently shake well. Cold soak the ashed residue in the acid solution at room temperature for 30 min.

[0016] Furthermore, during the cold soaking process, gently stir the mixture of the ashed residue and nitric acid with a round-tipped polytetrafluoroethylene rod to prevent the residue from depositing at the bottom of the crucible.

[0017] Furthermore, in step (3), the heating method is as follows: Cover the polytetrafluoroethylene beaker, place it on a high-purity graphite hot plate, heat to 120 °C - 150 °C and boil for 10 min, then remove the lid and continue to heat the residue acid solution at 150 °C - 200 °C until white smoke disappears completely.

[0018] Advantages of the invention:

[0019] The present invention uses a high-purity quartz crucible and a high-purity quartz backing plate for the high-temperature ashing of high-purity graphite samples, greatly reducing the contamination of the sample by the vessel, and the cost of the high-purity quartz crucible is low, less than one-twentieth of the cost of the platinum crucible; the method of the present invention uses nitric acid to pretreat the ashed residue, and then quantitatively transfers all the ashed residue with the acid solution, avoiding the mass loss caused by using a brush to transfer the ashed residue in the traditional method and the contact contamination of the brush to the residue, and then completely digesting it with a mixed acid; after the ashed residue is soaked in nitric acid at room temperature first, it is more easily decomposed completely quickly, and the time for the residue to be thermally decomposed by the mixed acid is shortened by about 20 min. Detailed implementation manners

[0020] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0021] A sample pretreatment method for determining the content of impurity elements in high-purity graphite, comprising the following steps:

[0022] (1) Place 5-10 g of high-purity graphite sample in a high-purity quartz crucible, and perform high-temperature ashing to obtain an ashing residue;

[0023] (2) Add 5-10 mL of nitric acid into the high-purity quartz crucible, soak and pre-decompose the ashing residue to obtain a pretreated residue acid solution;

[0024] (3) Transfer the pretreated residue acid solution from the high-purity quartz crucible to a polytetrafluoroethylene beaker, then add 5-10 mL of hydrofluoric acid and 1 mL of perchloric acid, and heat until white smoke disappears completely;

[0025] (4) Remove the beaker, cool it to room temperature, and when it is slightly cooled to 45-55 °C, add 1 mL of nitric acid and 5-10 mL of water, heat the solution to slightly boiling and then cool it.

[0026] The purity of both the high-purity quartz crucible and the high-purity quartz backing plate is above 99.9999%.

[0027] In step (1), the method of high-temperature ashing is as follows: Place the high-purity quartz crucible on a high-purity quartz backing plate, place it in a muffle furnace at 850 °C - 1100 °C and burn for 2-5 h until the high-purity graphite sample is completely oxidized, take out the high-purity quartz crucible and cool it to obtain an ashing residue.

[0028] In step (2), after adding 5-10 mL of nitric acid along the inner wall of the high-purity quartz crucible in a circular motion, then add 1.5-2.5 mL of water along the inner wall with a PFA dropper in a circular motion to rinse the inner wall and gently shake it evenly. The soaking is carried out by cold soaking at room temperature, and the cold soaking time is 30 min.

[0029] During the cold soaking process, gently stir the mixture of the ashing residue and nitric acid with a polytetrafluoroethylene rod to prevent the residue from depositing at the bottom of the crucible.

[0030] In step (3), the heating method is as follows: Cover the polytetrafluoroethylene beaker, place it on a high-purity graphite hot plate, heat it to 120 °C - 150 °C and boil for 10 min, then remove the cover, and continue to heat the residue acid solution at 150 °C - 200 °C until white smoke disappears completely.

[0031] Add 50 mL of (1+9) nitric acid to a platinum crucible and a high-purity quartz crucible respectively. After cold soaking at room temperature for 30 min, then heat them on a hot plate at 180 °C for 30 min. The dissolution of impurities in the two crucibles is shown in Table 1: It can be seen from Table 1 that in the acid solution, the dissolution amount and dissolution rate of impurity elements in the platinum crucible are much higher than those in the high-purity quartz crucible.

[0032] Table 1 Dissolution of impurities in different crucibles in (1+9) nitric acid (μg / L)

[0033]

[0034]

[0035] Example 1

[0036] A sample pretreatment method for determining the content of impurity elements in high-purity graphite, comprising the following steps:

[0037] (1) Weigh 5 g (accurate to 0.0001 g) of high-purity graphite sample dried at 105 °C into a high-purity quartz crucible. Then place the crucible containing the sample on a high-purity quartz backing plate and put it into a muffle furnace at 850 °C for 3 h until the high-purity graphite sample is completely oxidized. Take out the crucible and cool it to obtain an ashed residue;

[0038] (2) Add 5 mL of nitric acid along the inner wall of the crucible in a circular motion, and then use a PFA dropper to add 1.5 - 2.5 mL of water along the inner wall of the crucible to rinse the crucible wall in a circular motion. Gently shake it evenly. After cold soaking at room temperature for 30 min (when cold soaking, stir the solution several times with a round-tipped polytetrafluoroethylene rod), obtain a pretreated residue acid solution;

[0039] (3) Completely and quantitatively transfer the pretreated residue acid solution to a polytetrafluoroethylene beaker, add 5 mL of hydrofluoric acid and 1 mL of perchloric acid. Then place the polytetrafluoroethylene beaker on a high-purity graphite hot plate and heat it to boil at 120 °C - 150 °C for 10 min. Then remove the lid and continue to heat the residue acid solution at 150 °C - 200 °C until white smoke disappears completely;

[0040] (4) Take down the polytetrafluoroethylene beaker and cool it to about 50 °C at room temperature. Add 1 mL of nitric acid and about 10 mL of water, cover the lid, place it on a high-purity graphite hot plate and heat it to slightly boiling. Take down the polytetrafluoroethylene beaker. After cooling, transfer it to a 50 mL polyethylene colorimetric tube, shake it evenly, and wait for measurement.

[0041] Comparative Example 1

[0042] This example is basically the same as Example 1, the difference is that: after adding nitric acid to the high-purity quartz crucible in step (2), without cold soaking at room temperature, directly transfer the residue acid solution to a polytetrafluoroethylene beaker and carry out steps (3) and (4).

[0043] Table 2 Influence of Different Acid Dissolution Methods on the Decomposition Effect of Ashing Residues

[0044]

[0045] In order to completely decompose the ashing residue with acid, in Example 1, after obtaining the ashing residue by high-temperature calcination of the sample, 5 mL of nitric acid was first added to a high-purity quartz crucible, and then the ashing residue was fully soaked at room temperature for pre-decomposition. Then, the residue was completely transferred to a polytetrafluoroethylene beaker together with the acid solution, avoiding the mass loss during the transfer of the ashing residue. Moreover, after the residue was soaked in nitric acid at room temperature first, it was more easily decomposed by hydrofluoric acid. As shown in Table 2, the time for the acid thermal decomposition of the residue was shortened by about 20 min, and the decomposition was complete, reducing the exposure time of the acid solution in the air during thermal decomposition, and thus reducing the environmental pollution to the acid solution. In Comparative Example 1, after obtaining the ashing residue by high-temperature calcination of the sample, nitric acid was added to the high-purity quartz crucible. The ashing residue was directly transferred to a polytetrafluoroethylene beaker together with the acid solution without cold soaking in nitric acid, and mixed acid was added for thermal decomposition. After the white smoke disappeared completely, residues remained.

[0046] For the test solution after the treatment of the high-purity graphite sample in Example 1, the content of 15 impurity elements was measured by inductively coupled plasma mass spectrometry (ICP-MS). The relative standard deviation (RSD, n = 4) of the results was 1.36% - 4.67%, and the recovery rate was 95% - 110%. The results are shown in Table 4, and the working parameters of the ICP-MS instrument are shown in Table 3.

[0047] Table 3 Working Parameters of ICP-MS Instrument

[0048] Instrument parameters Set value Instrument parameters Set value Incident power (W) 1550 Dwell time (ms / point) 10 Cooling gas flow rate (L / min) 14.0 Number of scans (n) 50 Nebulizing gas flow rate (L / min) 1.06 Reading channel 1 Auxiliary gas flow rate (L / min) 0.90 Measurement time / s 50 Sampling depth (mm) 5.0 Data acquisition method Peak skipping

[0049] Table 4 Results of Precision and Standard Addition Recovery Tests (Number of Determinations n = 4)

[0050]

[0051] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A sample pretreatment method for determining the content of impurity elements in high-purity graphite, characterized in that, it includes the following steps: (1) Place 5-10 g of high-purity graphite sample in a high-purity quartz crucible, and place the high-purity quartz crucible on a high-purity quartz backing plate for high-temperature ashing to obtain an ashing residue; (2) Add 5-10 mL of nitric acid into the high-purity quartz crucible to soak and pre-decompose the ashing residue to obtain a pretreated residue acid solution; (3) Transfer the pretreated residue acid solution from the high-purity quartz crucible to a polytetrafluoroethylene beaker, then add 5-10 mL of hydrofluoric acid and 1 mL of perchloric acid, and heat until white smoke disappears completely; (4) Remove the polytetrafluoroethylene beaker, cool it to 45-55 °C at room temperature, then add 1 mL of nitric acid and 5-10 mL of water, heat to slightly boiling and then cool.

2. The sample pretreatment method for determining the content of impurity elements in high-purity graphite according to claim 1, characterized in that, the purity of both the high-purity quartz crucible and the high-purity quartz backing plate is above 99.9999%.

3. The sample pretreatment method for determining the content of impurity elements in high-purity graphite according to claim 1, characterized in that, in step (1), the method of high-temperature ashing is as follows: place the high-purity quartz crucible on a high-purity quartz backing plate, place it in a muffle furnace at 850 °C - 1100 °C and burn for 2-5 h until the high-purity graphite sample is completely oxidized, take out the high-purity quartz crucible and cool it to obtain an ashing residue.

4. The sample pretreatment method for determining the content of impurity elements in high-purity graphite according to any one of claims 1-3, characterized in that, in step (2), after adding 5-10 mL of nitric acid along the inner wall of the high-purity quartz crucible, then add 1.5-2.5 mL of water along the inner wall to rinse the inner wall and shake the crucible, and soak the ashing residue in the acid solution at room temperature for 30 min.

5. The sample pretreatment method for determining the content of impurity elements in high-purity graphite according to claim 4, characterized in that, during the cold soaking process, stir the mixed solution of the ashing residue and nitric acid with a polytetrafluoroethylene rod to prevent the residue from depositing at the bottom of the crucible.

6. The sample pretreatment method for determining the content of impurity elements in high-purity graphite according to claim 1, characterized in that, in step (3), the heating method is as follows: cover the polytetrafluoroethylene beaker and place it on a high-purity graphite hot plate, then heat to 120 °C - 150 °C and boil for 10 min, then remove the cover, and continue to heat the residue acid solution at 150 °C - 200 °C until white smoke disappears completely.

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