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Preliminary research on the primary cleaning process in steel silo project

With the extensive application of steel silos in grain processing plants, oil plants, ports and other industries, the safety of grain storage has also received increasing attention. The safe storage of foodstuffs (grains, oil, etc.) is mainly affected by the following factors: 1. The moisture content of the food 2. The temperature of the food 3. The impurity status of the food 4. Other factors (eg pests, etc.) Indicators are the key to safe grain storage. However, the above indicators are actually related to each other. In general, when the grain moisture content is high, the grain temperature tends to rise, and the elevated grain temperature in turn aggravates the respiration of grain grains, thereby further increasing grain moisture, thus creating a vicious circle that causes storage. Rapid food warming eventually led to a serious decline in food quality. In addition, if there are many impurities in the food, even if the overall moisture content of the grain is not high, the moisture content of the grain at the site where impurities are concentrated may exceed the standard, thus causing food warming locally. If the site of warming is at the bottom of the grain pile, the hot and humid air will go upwards, which will cause the entire grain pile to warm up quickly, resulting in a decline in grain quality.

From the engineering practices of the steel silos that we have come into contact with, the production companies and grain transit, storage and transportation units have paid more attention to the issue of grain moisture content. The loss of stored grain due to high food moisture is not too much. The impact of impurities in food is relatively large. This is because of the characteristics of the steel silo itself. Due to the high general height of steel silos (for large steel silos, the height can reach 26m, the overall height can reach 34m or more), and the center feed method is generally used. This causes the feed stream to be affected by the air flow. The result is a serious grain grading: from the center of the silo to the edge of the silo, multiple cylindrical contaminated areas are formed. These contaminated areas are prone to moisture absorption and mold, which can easily increase mold temperature, thereby affecting the quality of stored grain in the entire silo. Different kinds of impurities have different effects on stored grain: light and miscellaneous (such as: corn peel in corn). These impurities have high water absorption. If the content of such impurities in food is too high, it may easily cause local moisture in food. Too high, causing the grain piles to heat up. Small impurities (eg, dust, chopped grains), the water absorption rate of these impurities is generally high, and because these impurities are small in size, the voids in the grain piles are often filled, which affects the air circulation in the grain piles, further causing food inflows. The pile warms up. Large impurities (such as: stones, soybean meal, corn cobs), these impurities often have little impact on the storage of grain, but on the normal operation of the transport equipment. From the above analysis, it can be seen that reducing impurities in food is a key factor in safe grain storage. In particular, it should be cleaned of light and small impurities. Therefore, at least one primary cleaning process is generally deployed in the relatively perfect steel silo feeding section. A typical steel silo process flow is shown in the figure: As shown in the figure, the grain is fed into the warehouse after the primary cleaning and the iron remover, and a set of dust removal air nets is also provided for the primary cleaning. Here, the primary cleaning screen is responsible for removing the impurities in the grain, and the dust removal wind screen can remove most of the light impurities. The iron remover can remove magnetic metals (such as iron scraps, nails) from the grain, in order to protect the conveying equipment. Therefore, the iron remover may not be counted as a primary cleaning device here.

Commonly used primary cleaning screens mainly include: grid plates, mesh belt preliminary cleaning screens, cylinder primary cleaning screens, double-layer cylinder primary cleaning screens, and the like. Among them, the grid plate is generally placed on the feeding port, which not only plays a supporting role but also removes some of the larger impurities, such as: stones, linen pieces, and the like. The mesh belt primary cleaning screen is only equivalent to a layer of improved grating board, and only impurities such as stones, hemp chips, and corn cobs can be cleaned out, and the cleaning effect for light and miscellaneous items is relatively poor. So it is less used now. The initial clean screen of the cylinder is relatively compact in shape, which can clean most of the impurities and a small amount of miscellaneous and light impurities. However, the output of the primary cleaning screen of the cylinder is small, and generally it is only suitable for applications where the yield is below 80t/h. The double-layer cylinder primary cleaning screen has two screen tubes inside and outside, which can effectively clean large and small miscellaneous items. If a good dust-collecting air pattern is also equipped at the same time, it can effectively remove most of the light miscellaneous items. At the same time, the output of the equipment is large. (100-300t/h), so it is a good initial cleaning equipment. It is recommended to use it.

The mistakes often made in the use of primary-cleaning screens in the project are the ignorance of the corresponding dust removal wind screens. In fact, the deployment of wind nets in the primary cleaning process is critical. Here, the role of wind nets mainly includes: cleaning the screen surface to prevent obstruction of the screen holes and ensuring the normal operation of the equipment; sucking out a small amount of miscellaneous and most of the light and miscellaneous, and cooperate with the appropriate dust remover to play an auxiliary cleaning effect; Clean the screen to maintain a certain negative pressure to avoid dust spills. As a large amount of dust is sucked out from the primary cleaning screen, the dust concentration in the air flow is high, causing a great load on the dust removal equipment. Therefore, in many cases, it is difficult to meet the requirements with only one dust removal device. If the conditions permit, it is recommended to use secondary dust removal as much as possible: the first dust-removal equipment is used for the brakes, and the second dust-removal equipment is used for the bag filter. This can not only achieve a good dust removal effect, but also reduce the load of the bag filter to achieve the purpose of protecting the bag filter. In addition, in the engineering practice, it often happens that the conveying equipment or the cleaning equipment is tangled and pulled by the rope. For this purpose, a rope removal process may be added prior to the initial cleaning. There are already some manufacturers that produce "rope machines". The effect of intercepting the ropes can also be achieved.

However, it is necessary to manually clean the ropes intercepted in the equipment and it cannot be fully automated. Whether it can be improved from the principle of de-roping in order to fully automate the de-roping process is a research topic. Some users saw that the raw materials that were cleaned by the initial clean-up were not directly used for workshop production, and they needed further clean-up. Therefore, it is considered that the effect of the primary cleaning screen is not good. In fact, from the above analysis, we can already know that the purpose of the early Qing Dynasty was not to completely remove the impurities in the raw materials to meet the production requirements. The purpose of the early Qing Dynasty was only to protect the equipment and ensure the safety of stored grain. If you want to completely remove the impurities in the raw materials, you often need multiple cleaning equipment to cooperate. For example, for the cleaning process of flour mills, there are often: three sieves, two dozens, two stones, one selection, ie: three sieves, Two wheat, two stones, and one selection). If you require a primary cleaning sieve to meet such high requirements, it may seem too harsh for the primary cleaning sieve.

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