Performance comparison of limestone superfine grinding equipment

In order to more effectively react with molten iron to achieve the purpose of desulfurization, dephosphorization and slag formation, limestone raw materials are required to have: low S content (≤0.015%); high activity (>350mL); particle size control at 0.01~1mm; It is guaranteed to have good ductility and fluidity, and overcome the shortcomings of easy moisture absorption, difficult storage and transportation. In addition, the refining of the furnace often uses spray metallurgy to spray the lime powder. It is also required that the lime powder has fine and uniform particle size, high activity, waterproofing, low S content and surface passivation, which is convenient for storage and transportation.
For the requirements of lime powder in hot metal pretreatment and refining outside the furnace, the milling equipment is used to process lime powder in the process design. The milling effects of several different milling equipments are introduced and compared.
1 Raymond Mill
Raymond mill is also called hanging roller mill or pendulum mill. It is a commonly used grinding equipment, mainly used in metallurgy, building materials, chemicals, mining and other mineral products. The grinding range is a variety of non-flammable and explosive mineral products with a hardness below 7 and a humidity below 6%. Grinding product size range (0.125~0.044mm, ie 100 mesh ~ 325 mesh, individual can reach 500 mesh) Adjust according to different requirements. The Raymond mill structure is mainly composed of a main machine, an analyzer, a fan, a finished cyclone separator, a fine powder cyclone separator and a duct.
Working principle of Raymond grinding machine: The block material with the feed size allowed by the mill is uniformly and continuously fed into the main machine through the feeding equipment for grinding. The fine powder after grinding is taken out by the circulating airflow of the blower and placed. The analyzer above the host is categorized. The fine-grained powder enters the large cyclone collector with the wind flow, and is discharged after being discharged through the powder discharge pipe. The airflow flows into the blower from the return duct at the upper end of the large cyclone collector. The entire air duct system is closed and mostly flows under negative pressure.
Since the material to be ground contains moisture, it evaporates into gas during grinding and gas leaking into the duct under the action of negative pressure in the entire duct, resulting in an increase in the amount of air in the circulating air passage. This increased air volume is introduced into the small cyclone collector from the residual air duct between the blower and the main unit. A number of fine powders are introduced along with the wind flow, and after being collected by the small cyclone, they are discharged by another powder discharging pipe, and the gas is discharged into the atmosphere through the exhaust pipe at the upper end of the small cyclone collector.
Raymond grinding machine structural features: three-dimensional structure, small footprint, strong set, from the block to the finished powder independently into a production system; finished powder fine uniformity, screening rate of 99%, this is the other Milling equipment is difficult to have. The transmission adopts closed gear box and pulley, which has stable transmission and reliable operation. The important parts are made of high-quality steel. The wear parts are made of high-performance wear-resistant materials. The whole machine has high wear resistance and reliable operation. The electrical system adopts centralized control. The milling workshop can basically achieve unmanned operation and is easy to maintain. It is worth noting that Raymond Mill is sensitive to the amount of feed. If the feed is insufficient and the equipment is not running at full load, there will be a large noise, and the main body of the equipment and the equipment base will generate a large amplitude of vibration.
During the investigation, it was found that some of the manufacturers who used the Raymond mill to prepare lime powder removed one of the suspension rolls, and some removed the small cyclone. The main reasons are as follows: (1) The raw material lime itself has a low water content, and the steam generated during the grinding process is less, which has little effect on the gas pressure in the mill; 2 the required finished lime only requires a certain particle size (usually <1 mm to reach 100). %) Yes, without grading, fine powder can also be used as a finished product; 3 local production plants are not required to be environmentally friendly.
2 ball mill
Ball mills are also the grinding equipment commonly used in production. It is used to grind various raw materials, clinker, waste bricks, etc. Sometimes two or more kinds of raw materials are ground into a fine grinding, which plays a role of both grinding and highly uniform mixing. The ball mill is composed of two parts, a mill body and a transmission, and its structure is shown in Fig. 1.

Figure 1 Ball mill structure

To produce products of the required particle size with high efficiency and high quality, the following problems should be paid attention to during production: 1 selection of material and waveform of the lining; 2 selection of material, weight, size and size of the grinding body; 3 reasonable control Feeding particle size, feeding amount and discharge granularity; 4 Strictly control the moisture of the feed. If the feed moisture is too large, consider increasing the suction of the mill, or even using a hot air blow to the ball mill.
In the production of passivated lime, it is necessary to cooperate with the classifier to discharge. At the same time, since the grinding degree of the ball mill is usually only <0.088mm or less, the material after passing through the classifier is divided into two parts: one part meets the higher fineness requirement ( For example, <40μm) is the finished product, and directly enters the finished product warehouse; the other part cannot meet the fineness requirement, and returns to the ball mill for secondary grinding. This kind of process is called circulation grinding, which has the advantages of reducing the phenomenon of over-grinding, avoiding the aggregation of fine powder, sticking to the warehouse and sticking the abrasive body, improving the grinding efficiency and saving energy.
3 graded impact mill
The graded impact mill is widely used in the ultra-fine processing of minerals such as hard kaolin , talc , graphite , calcite , wollastonite , barite , pyrophyllite and Chinese herbal medicines and raw materials. The equipment is suitable for Mohs hardness. Processed for about 5 materials.
Performance characteristics of the graded impact mill: 1 Suitable for soft objects with a Mohs hardness of less than 5; 2 The particle size range is finer than ordinary mechanical grinding, and can produce products with steep particle size distribution curves.
Problems with graded impact mills:
A. In the transition hopper below the dust collector, the material has a sticky wall phenomenon;
B. The system in the test is an open circuit, that is, the air introduced by the dust collector fan will increase the moisture in the system. Due to the characteristics of metallurgical lime, in actual production, the system is required to adopt a closed circuit to avoid moisture in the product, and to consider the ventilation and dust removal of the finished silo into the closed loop system.
4 device performance comparison
Various grinding equipments have their own advantages and disadvantages. The comparison results are shown in Table 1.

5 conclusions
In the limestone powder preparation system process design, Raymond mill powder is usually used. However, in the foreign-related projects with relatively high environmental protection requirements, the system's milling equipment needs to be carefully selected. The test operation of the relevant equipment was made and the performance comparison of the above equipment was made for reference in the design.

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