Discussion on the rationality of wet method for testing the dust content of chrysotile asbestos fiber

Abstract: This paper describes the wet screening of asbestos by comparing the screening principle, the screen area, the screening medium, the pore size of the sieve, the permeability of the material, etc. in the method of wet test and dry test for asbestos fiber dust content. The scientific and rationality of fiber dust content. And put forward ideas and insights to solve the long time of wet test.
The new asbestos national standard GB/T 8071-2001 "Chrysotile" was implemented on October 1, 2001. In order to implement the standards, we must make great efforts to publicize the rationality of using the wet test method to check the quality of asbestos. We should be aware that the level of awareness and acceptance of wet testing by asbestos production and use companies is a key and difficult point in implementing standards. To this end, the author attempts to compare the advantages and disadvantages of the dry test and the wet test to eliminate the misunderstanding and incomprehension of the wet method for a long time, improve the enthusiasm of implementing the GB/T 8071-2001 "Chrysotile" national standard, and promote asbestos. Advances in inspection technology. It is well known that asbestos fibers undergo a series of crushing, loosening, screening and other operations in the beneficiation process to be selected. In these processes, the loosened asbestos fibers and the dust generated during the crushing are strongly adsorbed by electrostatic attraction and the like, and the dust is firmly adsorbed on the surface of the asbestos fibers. Even a portion of the short fibers are adsorbed on the long fibers, creating a so-called "bridge" phenomenon. In the purification process of asbestos air selection, asbestos fibers have to be cleaned and removed many times, but these strongly adsorbed dusts and fibers are difficult to be completely separated. At present, the dry test of asbestos fiber quality can be said to be a continuation of the beneficiation purification operation, and the dust and fiber which are strongly adsorbed cannot be completely separated in the screening. Therefore, the dust content measured by the dry method cannot represent the true dust content in the asbestos fiber. This is not the case with wet testing. The wet test is to screen the asbestos fibers after being sufficiently dispersed in an aqueous medium. The medium water can eliminate or balance the static electricity on the asbestos fibers and dust, and damage the adsorption between the asbestos fibers and the dust. Strong agitation or strong rinsing during the test destroys the “bridge” phenomenon, so that the dust in the fiber mass is dispersed and separated to form a certain concentration of suspension. When the suspension is sifted through the screen, the dust passes through the screen under the action of hydraulic force, and the fibers remain on the screen to achieve the separation purpose. The resulting fiber is clean and the dust content is more accurate. This is one of the reasons why the wet test is scientific and reasonable.
In the current popular dry test, the mesh aperture of the asbestos fiber and dust is 0.4 mm. Quite in English 40 mesh, the screen mesh area accounts for 38% of the total screen area. The sieve used for the wet test has a pore size of 0.075 mm, which is equivalent to 200 mesh, and the mesh area accounts for 37% of the total screen area. This shows that the sieve diameter of the dry test is about 5 times larger than that of the wet test sieve when the area of ​​the sieve hole is equal to the total screen area, which means that the dry test has a lot of dust content. short fibre. According to the composite reinforcement theory, materials with a long-diameter ratio greater than 10:1 have a reinforcing effect, and materials with a long-diameter ratio greater than 20:1 have a significant reinforcing effect in the composite. In the American ASTM standard and the GB/T 8071-2001 asbestos national standard, a fiber serpentine with an aspect ratio greater than 10:1 and a maximum transverse dimension of less than 0.1 mm is defined as asbestos fibers. In the dry test, materials smaller than 0.4 mm are metered as dust. Assuming that the material passing through is 0.4 mm in length, the material having a diameter of less than 0.04 mm in these materials is asbestos fiber according to the above aspect ratio. Even if the diameter is not less than 0.04 mm, we can further loosen it to less than 0.04 mm. These materials should also be counted as asbestos fibers. We use the dry test to determine the quality of asbestos, and they are still lost in the dressing. Isn't this a waste of resources? With the current beneficiation technology , it is entirely possible to loosen asbestos fibers to a diameter of less than 0.04 mm. This part of the material is completely usable. The use of fiber release equipment in asbestos beneficiation abroad is for this purpose. Compared with China's asbestos beneficiation process, some of the concentrating plants have not even broken and unpacked cotton equipment, and there is no more equipment for releasing the tailings. The tailings contain a large amount of undissociated and undissolved asbestos. Shaanxi Hanyuan Company, Gansu Xinyuan Company and Bazhou Jianghai Company use Li Kewen asbestos water selection patent technology to recover asbestos fiber from asbestos tailings to produce grade 5 cotton. This is a good proof. This is the second reason why the wet test is more scientific than the dry test. [next]
It is often said now: "The amount of wet test is too small, and there is no dry test." The author thinks this is a misunderstanding. The amount of sample used for the dry test is 500 grams, and the wet test sample is only 10 to 20 grams. After a simple numerical comparison, the amount of sample tested by the wet method is indeed too small. However, the test samples are still a small fraction of the asbestos extracted after all. If the inspection batch is 20 tons, 50 tons and the sample volume is 500 grams, 10 grams to compare, 500 grams, 10 grams can be seen as the same order of magnitude, the relative difference is not so significant. Moreover, the 500 gram and 10 gram samples were strictly prepared from the mixed samples of more than 3 kg according to GB/T 8072 national standard. If you carefully consider GB/T8071, GB/T8072 and GB/T6646 and other national standards, you will find that these standards are more detailed and scientific than the sampling, sample preparation and inspection methods of the previous 54-61 standard. It is precisely because of these detailed and scientific methods that the data of the wet test is scientific, accurate and reasonable than the dry test. If we still use a rough sampling method, sample preparation method to prepare samples for wet testing. The test results are definitely not accurate. Similarly, using such a rough method to prepare samples for dry testing, the results are not accurate. The results of the dry test results of many of our companies are poorly reproducible (one time and one time, one screen and one sieve, one person and one person's operation results greatly differ from the error). Is there no sampling, sample preparation, and inspection operation? Do you not be careful about this factor?
The area of ​​the screen used for the dry test is about 2,300 square centimeters, which is 7.4 times the area of ​​the screen used for the wet test (about 310 square centimeters), and the dry test of the sample is 50 times that of the wet test. Once contrasted, the dry screening probability of each fiber is much less than the wet test. In addition, the dust is permeable during the screening process: during the wet inspection process, the dust is fully dispersed in the aqueous medium and sifted under strong hydraulic force; during the dry inspection process, the dust must overcome the electrostatic adsorption and Overcoming the "bridge" between fibers, through the action of gravity and vibration force through a certain thickness of the sample through the sieve, the through-screen process is much more difficult than the wet test. This further illustrates that the probability of dust permeable screening during wet testing is much greater than the dry test. The above argument is the third reason for the scientific and reasonable reason for the wet test technique.
The statement that “wet test is too long to guide asbestos production in time” just points out the shortcomings of the current wet test. However, we can see from the analysis that the reason for the long wet test time is that the sample takes a long time to dry. There are two ways to solve this problem: one is to find a good drying method that is very short in use; the other is to directly obtain the result without drying the sample. The first approach, at the current state of the art, is difficult to use with very short drying methods to meet our requirements. The second way is to avoid drying. The author has tried this way. We can think that the process and ore properties of asbestos production enterprises are stable, so the full bottom content of the dry classification should be related to the dust content of the wet test. That is to say, we can use mathematical statistics to find a formula to calculate the dust content based on the dry bottom content of the dry classification. Suffering from the lack of more data of a certain enterprise, the author used the data of China's asbestos inspection from 1985 to 1987 to calculate a formula, which proves that there is a correlation between the two, and the correlation coefficient is about 0.5. If you use two sets of data accumulated by a particular company, the correlation coefficient will be close to 1, and the calculated result will be more accurate. In this way, the problem of guiding production at present can be better solved. In addition, through the efforts of the industry peers, we can certainly find a good way to overcome the shortcomings of the wet test. Let us work hard! [next]
In summary, it is scientific and reasonable to use wet method to test the content of chrysotile fiber dust. The technical improvement of China's asbestos production process is of positive significance, and the economic benefits generated are incalculable. Defects in the length of the wet test can be solved through our efforts. Therefore, in today's environmental awareness of the whole people, under the policy of safe and rational use of chrysotile asbestos, it is inappropriate for us to refuse to use scientific and environmentally friendly wet test methods.

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