Application of vertical mill in quartz micro powder processing industry

Quartz powder (same as quartz sand) is also called silicon micropowder. Quartz sand is a hard, wear-resistant, chemically stable silicate mineral. Its main mineral component is SiO2. The color of quartz sand is milky white or colorless and translucent, with a hardness of 7, brittle and without cleavage, a shell-like fracture, and an oily luster. The density is 2.65, and the bulk density (20-200 mesh is 1.5). Its chemical, thermal and mechanical properties have obvious anisotropy. It is insoluble in acid, slightly soluble in KOH solution, and has a melting point of 1650℃. After processing, the quartz stone mined from the mine is generally called quartz sand with a fineness of more than 120 mesh (less than 120 mesh), and the product with a fineness of more than 120 mesh is called quartz powder.

High-quality quartz micropowder is widely used as a basic industrial raw material in glass, glass fiber, ceramics and refractory materials, metallurgy, construction, chemical industry, casting, grinding materials, electronics, optical fiber and other industries.

It is a difficult process to turn quartz sand into high-quality quartz micro-powder. At present, the main equipment for grinding quartz in China is Raymond mill and rod mill. The output of Raymond mill is only about 1 ton, with low efficiency, high energy consumption and difficult to control product particle size. The above parameters are not as good as Raymond mill when using rod mill.

Another problem is: with the increase of fineness in micro-powder, the iron content is also increasing, especially the rod mill has a very high iron content due to the invalid collision between the wear-resistant materials, and the wear-resistant material of Raymond mill is made of high manganese steel, which is weakly magnetic. In this case, it is difficult to remove the mechanical iron (the iron contained in quartz powder is a harmful impurity, and this industry has relevant requirements for this, see Table 1 for details). How to greatly increase the production capacity of quartz micro-powder mill while reducing the iron content, energy saving and consumption reduction have become technical problems that must be solved for large-scale production in this industry.

Introduction to vertical mill and its special applicability in the quartz micro powder industry

Aiming at the above problems, the LM vertical mill developed by us may help reduce the energy consumption per unit product for grinding a large amount of quartz. The LM vertical mill is a high-efficiency and energy-saving equipment for drying and grinding developed by Liming Heavy Industry on the basis of widely absorbing foreign advanced technologies and summarizing the application experience of vertical mills at home and abroad.

It has the advantages of the Loesche mill that can be turned over for maintenance, and the MPS roller sleeve that can be turned over for use and has a long life. It integrates fine crushing, drying, grinding, powder selection and transportation. It has the advantages of high grinding efficiency, low power consumption, large drying capacity, easy adjustment of product fineness, simple process flow, small footprint, low noise, no dust pollution, low wear, convenient maintenance and reliable operation.

 Chemical Composition Standards for Glass Industry

The structure of LM vertical mill is basically composed of reducer, machine body, grinding roller and grinding disc, separator, transmission arm, frame, hydraulic cylinder, limiter and other parts.

The main motor drives the grinding disc to rotate through the reducer, and hot air enters the vertical mill body from the air inlet. The material falls on the center of the grinding disc from the discharge port. Under the action of centrifugal force, the material moves evenly from the center of the grinding disc to the edge. When passing through the grinding roller area on the grinding disc, it is crushed by the grinding roller. Large pieces of material are directly crushed, and fine particles are squeezed to form a material bed for inter-particle crushing.

The crushed material continues to move to the edge of the grinding disc until it is taken away by the strong airflow at the wind ring, and the larger particles fall back to the grinding disc to continue to be crushed. When the material in the airflow passes through the upper separator, the coarse particles return to the grinding disc for grinding under the action of the rotor blades. Qualified fine powder is discharged from the mill with the airflow and collected by the system’s powder collector. The collected powder is the product of the vertical mill. When iron blocks and other debris mixed in the materials move to the edge of the grinding disc with the materials, they cannot be blown up by the wind due to their own weight and fall into the lower cavity of the mill. They are scraped into the slag discharge port by the scraper installed at the bottom of the grinding disc and discharged out of the machine.

Grinding material: quartz; grinding disc middle diameter: 1700mm; grinding roller diameter: 1400mm; width: 460mm; number of grinding rollers: 2; transmission power: 400kW; guaranteed output: 10-15t/h; product fineness: 100% pass of 100 mesh, 99% pass of 200 mesh, 99.9% pass of 325 mesh (45 μm).

Special applicability of quartz micro powder milling and design for this problem

The power system of the vertical mill refers to the grinding disc, main reducer, main motor and other three parts. The rotation speed of the grinding disc will be one of the focuses of our design. The vertical mill adopts the material bed grinding principle to grind the material. The grinding roller rolls and grinds the material falling on the grinding disc for many times. While the grinding disc rotates, the material moves outward due to the centrifugal force. The rotation speed of the grinding disc determines the movement speed and residence time of the material on the grinding disc. The faster the grinding disc rotates, the faster the material moves outward. The difficult-to-grind material may be transferred out of the grinding disc because it has not been ground to a reasonable fineness, so that the product does not meet the required fineness and the grinding efficiency is low; conversely, over-grinding will also make the grinding efficiency low.

Different grinding disc rotation speeds will also cause different mutual friction and movement trajectories between materials on the grinding disc. To improve the fineness of the grinding product and the grinding efficiency of the mill, the grinding disc rotation speed must be balanced and matched with the grinding speed of the material. Therefore, the grinding disc rotation speeds of different mill structures and mills grinding different materials are different.

The chemical composition and grinding work index of slag and quartz are similar, both in the range of 20-25kW/t. The finished fineness of quartz powder is slightly higher than that of slag. Therefore, the K value of the quartz vertical mill we designed should be 20% smaller than that of the raw mill, just like the slag mill. The grinding disc speed of the quartz vertical mill is about 20% lower than that of the raw mill of the same type and specification. The grinding disc speed is determined to be 38 rpm for this type of material under the condition of this fineness requirement.

When the quartz superfine grinding is carried out, the material on the grinding disc contains very fine 1 μm to 40 μm fine particles. During production, the material in the material bed is filled with a large amount of gas due to air flow, that is, the material bed has a tendency to fluidize, which makes it easy for the vertical mill to not effectively engage a large number of particle groups, causing the roller to slide or accumulate in front of the roller.

The grinding roller of the quartz vertical mill adopts a tire type, and the grinding disc lining adopts a bowl type. On the one hand, it is to avoid the edge wear effect of the grinding roller working surface. It can be turned over and reused, which prolongs the service life. The arc surface of the tire type grinding roller tends to be steep. The drum roller-bowl structure is used to exhaust air by itself, and the principle of single roller self-preparation of the material bed is realized to control the material bed and reduce vibration.

On the inside of the grinding roller, the gap between the roller and the disc is large, forming a material bed preparation area, and the material bed is loose. On the outside of the grinding roller, the gap between the roller and the disc is small, and the material bed is further compacted to form an efficient grinding area. During the entire grinding process, even if the surface area of ​​the finished product is large, it can be operated smoothly with low vibration.

On the other hand, there is a material blocking ring structure of a certain height on the edge of the grinding disc, which can effectively block the material and reduce the movement speed of the material on the grinding disc to achieve stable operation of the mill.

To solve this problem, on the one hand, it is required that the material on the grinding disc must be partially exhausted before entering the grinding area, so that the grinding roller and grinding disc can grind the material more smoothly and effectively.

The application industry of quartz micropowder has strict requirements on the iron content of micropowder, so the wear-resistant materials on the grinding roller and grinding disc of the vertical mill are all made of high-chromium cast iron. High-chromium cast iron is almost one of the most wear-resistant materials in the world among steel materials, and its economic efficiency is also quite good, so it is widely used in heavy industry and vertical mill industry. Moreover, this material is not as weakly magnetic as the high manganese steel used in Raymond mill. On the contrary, the high-chromium cast iron mixed in the quartz micropowder due to wear is easily attracted by the magnet, so even if there is a small amount of mechanical iron in the micropowder, it is easy to remove.

From the above principle, it can be seen that when the vertical mill is working, there is no direct contact with the wear-resistant parts in the grinding area, and the material and the wear-resistant parts are rolling friction, so the iron loss of its grinding is quite small. When the vertical mill is used to grind cement raw materials, the wear data of the roller sleeve and the grinding disc lining plate recognized by the industry is 5-6g/t. According to many years of experience in the grinding industry, the wear of grinding quartz powder will be between 50-60g/t in theory.

The total mass of our LM170 wear-resistant parts is about 6500Kg. Generally, the wear-resistant parts should not be used after 1/3 of the weight is worn. The total amount of quartz powder that can be ground by a set of wear-resistant parts is 39400 tons, and the estimated life is about 3800 hours. It is not difficult to calculate the iron content based on the above data, and the result is 0.006%. Assuming that the iron loss of the mill system friction is twice that of grinding, which is 0.012 (actually it is impossible to reach twice), the total iron loss will not exceed 0.02%, which is quite considerable. Therefore, the actual service life of the wear-resistant parts has become the key to verifying this design.

The main function of the separator is to separate qualified materials in time. When the traditional mill produces coarse quartz powder below 200 mesh, it uses an ordinary dynamic powder classifier. Although this powder classifier is more efficient, it is not suitable for the classification of finer quartz micro-powder products.

We carefully studied and analyzed the advanced and efficient combined powder classifiers, such as LV powder classifier, O-sepa powder classifier, SLS powder classifier, etc., and designed a suitable quartz vertical mill separator (dynamic and static combined powder classifier), focusing on the powder selection area and the control of particle movement, adjusting the spacing between the dynamic rotor and the guide blade, the dynamic rotor speed, the number of guide blades and the geometric size, and using an air-sealed structure for the rotor, which can effectively prevent large particles from entering the finished product, thereby reducing the circulation load and improving the grinding efficiency.

The crushed granular quartz is stored in the raw material bin, and a valve is installed under the bin. After being lifted to the height required by the process system by the bucket elevator, it falls onto the skin machine. After the final iron removal, it falls into the intermediate silo, and is closed and connected to the closed quantitative feeder at the bottom of the silo. The quantitative feeder and the vertical mill are also closed, so the materials in the intermediate silo can serve as a good medium for locking the wind. The closed quantitative feeder has four major functions: belt scale, material conveying, air locking, and quantitative feeding. The service life of the closed fixed plate. The finished quartz powder collected by the dust collector is sent to the quartz powder bin through the conveying device for use.

After the first LM170 quartz vertical mill was installed, it was successfully put into production after the equipment trial operation, the system supporting equipment adjustment and system parameter adjustment. In the initial operation of the equipment, the system operated smoothly and normally, the output was stable at more than 10t/h, the average power consumption of the system was less than 65kWh/t, and the other parameters were within the design range. The mechanical iron of the product was easy to remove, and the iron content met the requirements of the customer. Later, according to feedback, the average life of the wear-resistant parts was between 3000 and 3500 hours.

The operation effect of the LM170 quartz vertical fine mill shows that compared with the Raymond mill, the production capacity of a single machine has increased by 10 times, which meets the needs of large-scale industrial production of the quartz micro-powder industry. At the same time, the unit product energy reduction and equipment consumption are reduced by more than 40% compared with the Raymond mill grinding system. The vertical mill has such good adaptability for grinding quartz micro-powders with higher hardness and fineness. It is believed that it will surely promote other types of non-metallic mineral production enterprises to rapidly increase their scale and achieve low carbon emissions.