Test method for purity of carbon graphite materials

On the purity test of carbon graphite materials

Scotland wei once worked on the high purification project of carbon and graphite materials. At present, everyone's understanding of graphite ash test is not very unified. High quailty graphite electrodes for reference. Many concepts are entangled and communication is very difficult. Today, I found an email I sent in my team's internal email a few years ago, sorted it out and introduced the main test methods on the purity of carbon and graphite materials.

• Ⅰ. Clarify several concepts:

• Purity: the purity of graphite is the mass fraction of carbon element in graphite, which is usually expressed by numbers such as 99% and 99.9%, or several 9. 

• Ash: the residue of graphite after burning at high temperature to completely escape graphite and volatiles, that is, ash, which is usually expressed in %. There is a big gap between this concept and the actual situation. For example, the elements contained in graphite becomes the volatilization of sulfur dioxide gas when burning, and the existence of sulfur is not reflected in the ash content data. In addition, for example, iron element exists as a simple substance in graphite and becomes ferric oxide after burning, which increases the weight, so the statistics of such elements in ash data relatively large. Therefore, ash content is only a reference data, and there is a certain gap with the actual impurity content, and the gap level is related to the content of each element in graphite.

• Impurity content: the content of a non-carbon element in graphite. When it comes to impurity content, the impurity content of an element or the total impurity content should be taken. It is usually expressed in ppm, that is, the mass ratio of impurity elements in graphite, and the unit is one millionth. This concept is contrary to the concept of purity. At present, many people often say the concept of purity ppm.

• Dust generation: Japanese Shigeki Iyamoto mentioned this concept when he came to give a lecture, which is to put graphite in a sealed container, heated to a certain temperature, one end of the gas through high purity nitrogen, and then collect the gas at the other end, test the element content of the gas. Dust generation is different from other concepts. Firstly, there is no corresponding relationship between dust generation and impurity content in graphite. The elements in graphite with high impurity content in graphite may not volatilize at the service temperature, and if they do not volatilize, they will not affect the products in the furnace; Secondly, carbon is also a component of dust generation. If carbon can volatilize under service conditions, it can also increase the dust generation. This concept is closest to the user's experience, but it is difficult to test and requires special devices. Other units except Kureha have not proposed this concept.

  

• Ⅱ. About test methods

  Several methods have been done before, namely plasma emission spectroscopy (ICP), high resolution glow discharge mass spectrometry (GDMS) and combustion method.

• 1.Plasma emission spectroscopy (ICP)

  At present, plasma emission spectroscopy is the most accurate means to detect trace elements. Some elements can reach the PPB level, the detection limit is relatively low, and the detection limit of each element is different. The main principle is to pass the liquid sample into the test room through a conduit and bombard the sample with high-purity plasma, so as to make the energy transition of each element, different element transition will produce different spectrum, high-precision detector analysis of different spectral intensity to calibrate the content of each element indirectly, this method is also called ICP-AES;If each element content is calibrated according to each element quality spectrum, this method is also called ICP-MS. The accuracy and reliability of ICP-MS are better than ICP-AES. ICP method can be used to measure most elements except helium, neon, argon, krypton, xenon and other inert gases. According to the test principle, it can be found that the test results of this method actually correspond to the concept of impurity content mentioned above.

• Advantages: 1). High detection accuracy, can detect most of the impurity elements, the detection result is the impurity content.

• Disadvantages: 1). Difficult sample preparation. ICP method requires that the test sample must be a liquid sample. Due to the acid and alkali resistance and corrosion resistance of carbon materials, it is difficult to dissolve in a certain solution, so the difficulty of sample preparation has become the bottleneck restricting the purity of graphite materials analyzed by ICP method. A test was conducted in Shimadzu, Japan. At that time, the carbon felt sample was digested by microwave under high temperature and high pressure in strong acid to make the graphite trace Dissolve in strong acid, and then conduct ICP test. Shimazu's approach was to make several solutions of different concentrations, which had a linear relationship with the impurity content, to remove the impurities from the solution and normalize the data. Although this method is technically possible, few units with microwave digestion and ICP tester are found, and the sample preparation process requires very professional operation, which makes it difficult for domestic testing units to meet this requirement. Shimazu can only provide this initial test to customers who have purchased shimazu instruments.

• 2) High prices. At present, the price of ICP test for 4 elements is 80 yuan, and each additional element will be increased by 5 yuan, so the test cost is about 200 yuan. If you want to carry out different concentration test, you need to do at least 5 samples, so that is 1000 yuan, plus the sample preparation cost, the cost will be about 1500 yuan.

• 2.High resolution glow discharge mass spectrometry (GDMS)

  The principle of high-resolution glow discharge mass spectrometry is to apply a high temperature of thousands of degrees on the surface of graphite material to make the elements on the material surface evaporate instantaneously, and the electron energy outside the element core is excited to release the spectrum, and then calibrate the content of each element through analysis. This is the test method for the samples sent to Evans in America. This test also measures impurity content and can test all elements except C, H, O and N. The test cycle is about 7 working days. American Evans has also done nuclear industry with graphite material purity analysis. The limit of element detection is about 0.01 PPM.

• Advantages: 1). Simple sample preparation. High resolution glow discharge mass spectrometry can test solid samples, that is, processed graphite blocks can be tested directly. It avoids the pollution of materials caused by sample preparation and other processes.

  2). High accuracy. Due to reducing the impact of sample preparation on materials, the test accuracy is high.

• Disadvantages: 1). Expensive. The testing cost of each sample is 4200 yuan for all elements and 3500 yuan for 25 elements.

• 3.Combustion method

  This method tests the ash content, and the test principle is also consistent with the definition of ash, that is, the ash content is calculated by the ratio of the residue of graphite after burning to the original mass of graphite. This test method is a relatively rough statistical method, which cannot count volatile elements after burning and has statistical deviation for elements with larger mass after burning. Therefore, ash test can only be used as a rough reference, not as the basis of impurity content. However, at present, the test methods for graphite purity in China are basically combustion method.

• Advantages: 1). Simple test. Testing equipment and testing principle are relatively simple, there are electronic balance, crucible, muffle furnace can test ash content.

  2). Customer is relatively recognized. At present, most of the data the customer is exposed to is ash data. Although the customer has misunderstood this data, it can be recognized by the customer.

• Disadvantages: 1). Less testing basis. Although there are several ash test standards in China, the implementation details affecting the test accuracy are not described, and the standards are similar. The ash test accuracy is hundreds or thousands of PPM.

  2). Poor reproducibility of the results. Since ash test is a rough test method, the two test results may differ greatly.

• Ⅲ.About combustion ash test

  At present, there is no ash test standard for our high-purity isostatic pressing graphite products, so we can only use the existing standards as the minimum standard and refine them according to the characteristics of our materials.  The national standard for ash test error is basically hundreds or thousands of PPM, which is obviously not applicable to our graphite material with a purity of several or dozens of PPM. According to the national standard "Graphite Chemical Analysis Method" (GB/T 3521-2008), there are many domestic standards for the ash content of carbon or graphite materials, and they repeat each other. Sigli Publicity and national Graphite Product Supervision and Inspection Center, they all carry out ash content tests based on domestic standards and make corresponding adjustments. Strictly speaking, domestic test standards are not suitable for the materials we test, so we can only select one national standard as the standard framework, so as to reduce the ash test results to a small fluctuation range as much as possible and maximize the reproducibility of the results by determining the implementation rules.

  Therefore, the purity test depends on what data the customer needs. If it is simply to study the impact of non-carbon elements on the product during the use of carbon graphite materials, I suggest using the concept of dust generation. The specific test method needs to be communicated with the customer and then tested. If it is simply ash, it is recommended to use national standards and do it in the national graphite material supervision and testing center, contact us to get the related news.