Smashing is an art--a discussion on the purchase of laboratory pulverizers

More and more facts prove that the error of analysis mainly comes from sample preparation. The sampling and sample preparation technology of solid samples has a considerable impact on the results of the whole analysis. This article will start from different application fields, talk about the selection points and precautions of grinding and pulverizing instruments, and hope to work on your experiment. Helpful, if there is any inconvenience, please correct me.

Why should we pay attention to the sample preparation technology? Why do you need a professional pulverizer instead of manual operation? That is because, with the continuous improvement and development of analytical methods, the sensitivity of analytical instruments is getting higher and higher, the detection limit is getting lower and lower, and the sample size for analysis is only a few grams or even tens of milligrams or less. The uniformity and representativeness of the sample preparation put forward greater requirements. In theory, the smaller the sample particle, the better its uniformity and representativeness. Secondly, the manual sample preparation is very tiring and inefficient, especially for a certain Some samples, such as high hardness alloy ore or heat sensitive materials, can not be crushed into powder by hand. In addition, the sample preparation process should avoid the introduction of the elements or impurities to be tested, which also requires you to Use a professional crushing and grinding instrument.

The principle of pulverization varies from sample to sample of different nature. For example, for smashing of hard samples, extrusion, collision, friction or interparticle force is generally used; for soft samples, cutting or shearing is usually used.

Therefore, it is first necessary to understand the soft and hard properties of the sample to select a suitable principle of the pulverizer. For example, for soft or elastic samples, it is not suitable for pulverizing with a pulverizer of the extrusion principle. The so-called softness and hardness here does not mean "feel". You can choose it in a simple way. If your sample is suitable for cutting with scissors, then these samples are soft samples in the smash, such as plastic, rubber, textiles; if your sample is suitable for smashing with a hammer, then Such samples are hard samples in pulverization, such as glass, ceramics, ore. Of course, with the development of technology, many pulverizers do not use a single pulverization principle, and often use 2-3 kinds of pulverization principles in combination, which further expands the scope of application of the instrument, such as vibrating disc grinder, which is a An instrument that is very suitable for sample preparation before spectroscopic analysis (especially XRF), often grinding the sample to 200 mesh (75um) in a very short time (such as 2-3min), its pulverization principle not only has the pressing force It also has frictional force, which is very suitable for the preparation of hard samples such as ore samples. For example, in the ultra-centrifugal grinding machine, the particles are not only cut by the rotating knife and the ring screen which are rotated at a high speed, but also hit by the rotary knives. It is not only suitable for soft samples such as Chinese medicine, wood, etc., but also for hard samples in feed, coal, soil, etc. The following table shows the pulverization principle and application range of some common laboratory pulverizers.

After confirming the type of crusher, it is important to choose the right grinding accessories. The cost of the instrument is often related to the choice of grinding accessories. The same model can be equipped with different grinding accessories. The price may differ by 2-3 times. . The most common grinding materials are stainless steel, hard steel, pure titanium, agate, zirconia and tungsten carbide. Stainless steel or hard steel can produce large pulverization energy due to its high density. The price is relatively low and suitable for general use. The smash application. Pure titanium, agate, zirconia and tungsten carbide materials are generally considered to prevent the introduction of heavy metal contamination (mainly Fe and Cr), making them ideal for subsequent sample analysis of spectral analysis such as AAS, ICP or XRF. The choice of grinding accessories is based on the hardness and wear resistance of the material, the cost, and the impurities that may be introduced. For geological metallurgical applications, we recommend tungsten carbide grinding tanks because tungsten carbide is high in hardness, high in density and does not introduce iron or chromium contamination! Such users used to use agate cans. Although there is no metal contamination, the hardness and density of agate are not suitable for crushing samples such as alloy ore. Instead, it is a good choice for grinding soil samples. The figure below is a comparison table of hardness of different hardness units. In principle, the hardness of the sample to be treated should be less than the hardness of the sample of the selected grinding accessory.

For some samples or applications that are difficult to handle, such as heat sensitive samples or applications in the biological field, some special pretreatment methods, cryogenic comminution, are required. Liquid nitrogen is one of the most common cryogenic grinding aids because the sample becomes brittle like glass in liquid nitrogen, making the pulverization process easier. Such samples such as plastics and rubber are pulverized at room temperature. It will soften due to heat and cannot be ground to a powder. For biological samples, if the low temperature pulverization technique is not used, the activity is easily broken. The sample can usually be pre-cooled with liquid nitrogen. The sample or the grinding tank containing the sample is cooled in liquid nitrogen. After the temperature is equilibrated, it is pulverized by the instrument. Now there is a fully automatic freeze-grinding instrument that allows the sample to be sampled. It is always pulverized under liquid nitrogen to ensure that the sample will not be denatured, and it can be pulverized. The pre-cooling of the instrument and the addition of liquid nitrogen are completely controlled by the program, and the operation is safer and more convenient.

Based on the above-mentioned principle, how to choose a crusher suitable for your own application? Proceed as follows:
1. Identify the name and nature of the sample being processed -- is it a soft sample or a hard sample? Does it belong to the category of cryogenic pulverization or the category of water-containing oily samples?
2. Set the target size after crushing -- For physical and chemical analysis, the appropriate sample size is as follows:
Microwave digestion: Sonotype extraction below 500um: Infrared spectrum below 500um: below 150um
X fluorescence spectrum: below 100um
3. Select the appropriate grinding accessories -- if the subsequent analysis is AAS or ICP, it is recommended to use tungsten carbide grinding pot or titanium rotating knife. It is required to use liquid nitrogen for low temperature crushing. It is recommended to use stainless steel grinding tank.
4. Defining the amount of processing for a single pulverization -- in fact, for the sample preparation of physical and chemical analysis, 10 grams or tens of grams of sample is sufficient, the processing volume of the general sample is 1/3 of the volume of the selected grinding tank. Left and right, ie sample mass = grinding tank volume x 1/3 x sample density.

At the same time, you need to pay attention to the following points:
1. Crushing and grinding is not easy to imagine. It generally consists of two steps: primary pulverization and fine grinding. Primary pulverization refers to the preparation of samples with a sample size of several tens of millimeters into a few millimeters. Fine grinding refers to A few millimeters of sample is prepared to meet the requirements of physical and chemical analysis. Strictly speaking, the usual sample preparation technology requires more than two instruments to complete the whole process. For example, for ore samples, a jaw crusher and a vibrating plate are used. The combination of grinders can produce block samples below 200 mesh; for example, the combination of heavy-duty cutting pulverizers and ultra-centrifugal grinders is ideal for most soft, tough, fibrous samples such as plastics, leather, textiles, Chinese medicines. Preparation of the sample.
2. The size of the sample is not as fine as possible, but it is fine enough. The finer the grinding, the more energy is needed and the longer the time. In fact, some physical and chemical analysis only requires the sample to be pulverized, so you can use the sample size of the instrument as one of the indicators to measure the performance of the instrument, but never Blindly value the parameters on the catalog and ignore the actual application requirements, the so-called "too much".
3. The crusher has domestic brands and imported brands. In addition to comparing the size of the two samples, it is more important to compare the work efficiency, operation comfort and scope of use. For ordinary samples, the domestic crusher It can also meet the application, but its pulverization efficiency, such as pulverization time and sample passing rate, may be poor. In addition, the appearance and handling of the instrument are not as good as imported products, and impurities will be introduced during grinding. The imported pulverizer is in the production process. There are certain advantages in the selection surface and application fields of the grinding accessories.
4. Cleaning and anti-cross contamination of the instrument. The cleaning of the pulverizer has always been one of the main concerns of users. Users certainly do not want cross-contamination between samples during pulverization. The solution to this problem is mainly based on the design and application skills of the instrument. At present, it should be said that domestically produced instruments are still There are relatively large defects. But from another perspective, the cleaning of the instrument is also one of the aspects of sample preparation, and the operator should not take it too seriously. One experience is that the dryer the sample, the better the grinding, otherwise the sample will be wet, which will cause more trouble for cleaning.
5. There is no universal crusher! In fact, there is indeed a “universal crusher” in the commercialized instrument. In fact, it is also clear that all users do not have a “universal crusher”. The ideal pulverizer for the operator may be: an instrument that can smash the ore, pulverize the plastic, and perform low-temperature grinding. The bulk sample can be ground to less than 100 um at a time, and the processing capacity can be large or small. And it is easy to clean, no cross-contamination, and the price is low. It is a pity that there is no such instrument at present, and maybe there will be in the future. Perhaps this will be the direction of all manufacturers.

Smashing is an art. The purchase of shredders is actually not a clear description of the words. This article aims to attract the attention of the majority of experimental workers on the pretreatment of solid samples, especially the sample preparation technology. Choosing a professional crushing instrument will definitely make your experiment work more effective!