Therefore, in terms of ultra-fine pulverization work, wet processing has higher efficiency than dry processing and low energy consumption per unit product. In addition to a colloid mill, a high pressure jet mill, non-metallic mineral wet superfine grinding they use stirring mill, a sand mill type ultrafine grinding media milling equipment.
Since the colloid mill and high-pressure jet mill process configuration is relatively simple, the following focuses on the superfine pulverization process of medium grinding such as agitating mill and sand mill.
Stirring mill superfine pulverization process
The wet agitating mill superfine pulverizing process is mainly composed of a wet agitating mill and its corresponding slurry tank. The raw material (dry powder) is adjusted into a slurry of a certain concentration or solid-liquid ratio by adding water and a dispersing agent to the slurry tank, and is pumped into the stirring mill through the slurry tank for grinding. The number of grinding stages depends on the feed size and the product fineness requirements.
In practice, one agitator mill (one mill) can be used, or two, three or more agitator mills can be used for series grinding. After polishing slurry into the reservoir tank and the slurry after iron contamination substance and ferrous impurities removed by a magnetic separator and concentrated. If the production line is built close to the user, it can be sent directly to the user through the pipeline or tank; if it is far away, the concentrated slurry is dried and dehydrated, and then depolymerized (granules produced during the drying process) Agglomerates) and packaging.
â–³The above figure is a typical three-stage continuous stirring mill heavy calcium carbonate superfine pulverization process, which is mainly composed of a three-stage wet mixing mill or a grinding and stripping machine and corresponding storage tanks and pumps. The raw material is adjusted into a slurry having a certain concentration or a solid-liquid ratio by adding water and a dispersing agent to the slurry tank 1, and then fed into the slurry tank 2, and pumped into the stirring mill I 3 through the slurry tank 2 for grinding. After the grinding and grinding of the slurry by the agitator mill, the grinding medium is separated and fed into the slurry tank 4, and pumped into the stirring mill II 5 through the slurry tank 4 for the second (stage) grinding; after the second grinding After separating the grinding medium, it enters the slurry tank 6, and then is pumped into the stirring mill III 7 for the third (stage) grinding; after the third grinding, the slurry enters the slurry tank 8, and the magnetic separator is used to remove the iron. Pollution and iron-containing impurities. If the line is built close to the user or closer to the user, it can be sent directly to the user with a pipe or tank. If it is far away, the slurry is dewatered and then broken up and packaged.
The main process factors affecting the ultra-fine pulverization of wet agitating mill are the particle size and distribution of the raw materials, the density, diameter and filling amount of the medium; the rotational speed or linear velocity of the agitating mill; the residence time of the material in the agitating mill; the slurry concentration and The variety and amount of grinding aids or dispersing agents.
(1) Density, diameter and filling amount of the medium
The density and particle size of the medium affect the particle size of the product and the energy utilization of the mill. Studies have shown that when talc is ground by agitating mill, the zirconia balls (6.31 g/cm3) with higher density are more efficient than the glass balls (2.5 g/cm3) with lower density, and the glass balls (3 mm) with smaller diameter are compared. A large diameter glass ball (5mm) has a good grinding effect. However, under certain product fineness requirements, the optimum media density and media diameter are related to the type of material being ground and the particle size of the feedstock, the concentration of the slurry, and the rotational speed of the mill.
The amount of media filled has an impact on product fineness and yield. Under a certain range and a certain amount of material throughput, an appropriate increase in the filling amount can increase the grinding strength, increase the yield and fineness. However, when the amount of medium filling is too large, energy consumption increases and energy utilization decreases.
(2) Mill speed
The mill speed or agitation speed has a large impact on product fineness, yield and energy efficiency. Tests on horizontal agitating mills have shown that the agitation speed is increased and the fine fraction yield is increased.
Higher speeds increase the proportion of fine-grained materials in the product. However, from the perspective of energy utilization, the higher the higher the better, the research shows that the input power of the mill increases with the increase of the mill speed, and the excessive speed causes the energy utilization to decrease. Energy utilization is higher at relatively low speeds.
Therefore, the stirring speed of the agitating mill should not be too high. However, under the continuous pulverization process, the excessively low agitation speed tends to cause a part of the material to be discharged without being subjected to sufficient grinding and pulverization, so that the particle size distribution of the product becomes coarse.
(3) residence time of materials in the mill
The residence time of the material in the mill has a significant impact on product fineness and yield when equipment and other process conditions are constant. In order to ensure a certain product fineness, it is necessary to ensure a certain residence time. The residence time is related to the concentration of the slurry (or solid-liquid ratio), the feed rate, the mill speed, the height of the mill barrel (vertical) or the length (horizontal), and the degree of dispersion of the material.
In general, the longer the residence time of the material in the mill, the finer the particle size of the product, but the lower the yield; on the contrary, the particle size becomes coarser and the yield increases.
(4) Slurry concentration
In the ultrafine pulverization process of the agitating mill, the slurry concentration is a very important process parameter or a factor affecting the ultrafine pulverization effect and energy utilization.
Properly increasing the slurry concentration helps to increase the number of grades of -10 μm particles and improve energy efficiency. However, as the slurry concentration increases, the viscosity of the system increases and the fluidity of the slurry deteriorates. The material to be ground tends to adhere to the surface of the medium to weaken the grinding and impact of the medium on the material. Therefore, too high a slurry concentration tends to result in a decrease in pulverization efficiency and energy utilization, a coarsening of the product grain size, or a decrease in yield.
The optimum slurry concentration is related to the media density, diameter, required product fineness, grinding aid, and material type and feed size. Can be referred to industrial production examples of similar materials or selected by experiment.
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(5) grinding aid or dispersant
Grinding aids or dispersants mainly affect or increase product fineness, yield or reduce energy consumption per unit by reducing the strength of the particulate material, reducing the viscosity of the slurry, increasing the dispersibility of the material and promoting the fluidity of the slurry.
In general, the use of dispersants only has a significant effect when the slurry concentration reaches a higher value. That is, the slurry concentration at the time of ultrafine pulverization of the wet stirring mill can be increased by adding a dispersing agent or a grinding aid.
Sand mill superfine pulverization process
Sand mills are mainly used for wet grinding. Taking a horizontal sand mill as an example, the grinding process generally includes: dissolving-dispersing (pre-treatment)-grinding-sieving and the like. According to the series horizontal sand mill, it can be divided into one machine one tank, one machine two tanks, multiple machines (more than three sand mills) in series of ultra-fine grinding process.
△The above picture shows the process flow of 12,000 tons of paper-making paint and filler-grade superfine heavy calcium carbonate in a factory in China. It consists mainly of ingredients, intermediate slow storage, horizontal sand mill and separation screen. The volume of the cylinder of the sand mill is 1000L. The feed size was d97 ≤ 45 μm (or all less than 70 μm). The product fineness is: paint grade d90 ≤ 2μm; filler grade d97 ≤ 4μm (d60 ≤ 2μm). The size of the grinding media is: Class I grinding 1.0-1.6 mm; Class II grinding 0.8-1.2 mm.
(1) One machine and one can process
The slurry is circulated and ground in a sand mill and a storage tank until the fineness of the product reaches the requirement. The process is simple, the equipment is small, the floor space is small, and the operation method is simple.
However, since the ground slurry is easily recirculated into the storage tank and mixed with the original slurry, and there is a "dead angle", the finished product has a non-uniform particle size, so it is only suitable for the case where the product particle size distribution is not strict.
(2) One machine and two cans process
The slurry was circulated and ground in a sand mill and two storage tanks. Although this process is more complicated than one machine and one can process, the cycle grinding level is clear and the product granularity is uniform.
(3) Three-machine series process
Ultra-fine pulverization process using three closed sand mills and two storage tanks in series for continuous grinding. In this process, the three sand mills in series use different speeds and grinding media. Gradually decreasing from the first stage, the discharge size is getting finer and finer.
The three-machine series process can shorten the grinding time and improve the grinding efficiency; the grinding product has fine and uniform grain size. However, there are many equipments and a large area.
Process design and equipment selection principle
(1) Satisfy the product program (product quality and quantity)
This is a principle that must be followed first in process design and equipment selection. The product program includes quantity (annual production) and specifications (quality). Product quality requirements include product fineness (particle size and particle size distribution), particle shape, surface properties and purity.
(2) Saving investment
The amount of investment directly affects the future economic benefits of the plant. Therefore, under the premise of meeting the aforementioned design product program, investment should be saved as much as possible. For process design, equipment investment is the main component of the investment amount. Therefore, it is necessary to choose a technology that is more economical to invest on the basis of satisfying the product program and the comparison of comprehensive performance indicators.
(3) Reduce energy consumption
Energy consumption is one of the most important production costs of ultra-fine powder processing plants, which directly affects the market competitiveness of future products and the economic benefits of enterprises. Therefore, on the basis of ensuring comprehensive analysis and comparison of product program and investment and benefit, we must select equipment with high efficiency and low energy consumption and simplify the process as much as possible to reduce the total installed capacity (total installed power) of the entire production line and reduce the unit product. Energy consumption.
(4) Reduce wear
In the case of ultrafine powder addition, the abrasion of the pulverizing and grading equipment and the consumption of the grinding medium are unavoidable. These wears not only increase the production cost, but also the pollutant material, which leads to the decrease of the purity of the pulverized material and affects the quality of the final powder product. Therefore, in the ultra-fine pulverization process design and equipment selection must pay attention to minimize wear and grinding media consumption.
(5) Meet environmental regulations
Environmental protection laws and standards are mandatory, such as the Air Pollution Prevention and Control Law, the Water Pollution Prevention and Control Law, and the Noise Pollution Prevention and Control Law. Therefore, in the ultra-fine pulverization process design and equipment selection must have environmental protection measures, otherwise the only end is to shut down.
(6) Meet labor safety and health regulations
Labor safety and health regulations, such as building design fire codes and lightning protection specifications, industrial enterprise design hygiene standards and lighting design standards, heating and ventilation and air conditioning design specifications are also mandatory, therefore, in the ultra-fine crushing process design and equipment selection The type must be considered at the same time.
The forced circulation pump does not rely on the centrifugal force of the impeller to the liquid, but uses the thrust of the rotating impeller blades to make the sent liquid flow along the direction of the pump axis. When the pump shaft is driven by the motor to rotate, because the blade has a certain helix angle with the axis of the pump shaft, a thrust (or lift) is generated on the liquid, and the liquid is pushed out and discharged along the discharge pipe. When the liquid is pushed out, a partial vacuum is formed at the original position, and the liquid outside will be sucked into the impeller along the inlet pipe under the action of atmospheric pressure. As long as the impeller keeps rotating, the pump can continuously suck in and discharge liquid. The forced circulation pump works by using the horizontal thrust along the direction of the pump axis generated by the rotation of the impeller, so it is also called a horizontal axial flow pump. Mainly used in the evaporation of diaphragm method caustic soda, phosphoric acid, vacuum salt production, lactic acid, calcium lactate, alumina, titanium dioxide, calcium chloride, ammonium chloride, sodium chlorate, sugar, molten salt, paper, waste water and other industries. Concentration and cooling, in order to improve the production capacity of the equipment, increase the heat transfer coefficient of the heat exchanger for forced circulation. Therefore, it can also be called an axial flow evaporative circulation pump.
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