The evaluation of the quality of the mixer (mixer) is based on different physical quantities:
A. Mixing uniformity: Analyze the physical quantity of the quality of the material, and obtain it through probability theory. The mixing uniformity is determined by the type of mixer.
B. Dead angle: refers to the physical phenomenon that the material cannot participate in the mixing in the mixing container. The percentage of dead angle is a physical quantity to evaluate the quality of the mixer.
C. Mixing time: the physical quantity for evaluating the mixing speed, which refers to the time from the start of mixing of different materials to the time when the mixing reaches the required uniformity. The mixing time is determined by the type and model of the mixer.
Basic principles of mixer (mixer) selection:
When selecting a mixer, first of all, it is necessary to understand the position of the mixing operation in the entire production process, the purpose of mixing, the required mixing degree, the physical properties and processing capacity of powder and granules, and other corresponding working conditions.
These working conditions mainly include:
1. Intermittent or continuous operation
2. Whether the material is shear sensitive
3. Whether it is corrosive or requires special structural materials
4. Whether to allow the reduction of the particle size
5. Heat removal or addition
6. Environmental Requirements
7. Maintenance and cleaning requirements of the mixer, etc.
At the same time, it is necessary to select the appropriate mixer type according to the characteristics of the mixer itself.
In general, the selection can be made according to the following three basic principles:
Select according to the requirements of the mixing process.
Selection according to the quality requirements of the mixture.
According to the mixed process cost selection.
1. Selection according to the requirements of the mixing process:
When selecting a mixer, it is rare to determine a mixer type solely on the basis of the quality of the mixture obtained in a series of independent experiments. Usually, the quality of the mixture and the compatibility of different mixer types to the process are considered together, and incompatible mixing equipment is eliminated first in the initial selection. These compatibilities mainly refer to the following:
product purity
In many industrial processes, the purity of the product is more important, in order to avoid contamination between batch operations, the mixing equipment must be thoroughly cleaned after each use. In order to adapt to this condition, the internal shape of the mixer should be smooth, the inner surface should be polished with high precision, and it should be easy to clean. To avoid possible contamination of the lubricating oil, bearings and seals should not come into direct contact with or rest on the mixture. Rotary drum mixers—that is, common double cone mixers, V-type mixers, etc., can meet the above requirements, have a simple shape, and the interior of the mixer has no contact with the rotating part.
The low-speed convective mixing equipment with complex impeller device not only has cleaning problems, but also all convective mixing equipment has the possibility of contact between the mixture and the lubrication surface, such as horizontal ribbon mixers, conical mixers, etc. In addition, many mixers are not suitable for processing environments where product specifications change frequently and require thorough cleaning.
Mixer Containment
When selecting a mixer, in addition to providing high-purity products, environmental issues such as dust spillage and environmental pollution must also be considered. From a health, safety and economic point of view, dust spillage should be minimized, and all powders should be sealed in a closed mixing container for operation. However, during the feeding and discharging operations, the closed container must be opened. At this time, some fine dust will overflow and disperse into the surrounding air, and corresponding measures should be taken to minimize pollution.
pulverization
Sometimes, the mixing process should avoid or promote the reduction of particle size, that is, the crushing effect of the mixer. Under normal circumstances, the "gentle" operation of the tumbler mixer and the low-speed convection mixer will not cause obvious pulverization; but the mixing equipment with high-speed impeller impacting particles or the mixing equipment with strong shearing effect has strong pulverization. effect. Therefore, if the crushing effect of the mixer is required to be small, the use of high-speed impeller mixers and mixing equipment with a small gap between the impeller and the inner wall of the mixer should be avoided.
temperature rise problem
Pulverizing mixers with a large energy input may produce an objective temperature rise in the mixture. If the preheating of the mixture in the mixer will cause decomposition, this type of mixing equipment should not be used. Among them, in individual cases, the temperature increase formed by the high-speed impeller mixer can be used to make a certain component of the mixture approach the melting point and ensure the physical bonding of the mixture components.
wear problem
For particles with strong grinding performance, the wear problem of the mixer is more important than the particle grinding, and even contact with the fly ash of such mixtures may cause bearing damage. In this regard, rotary drum mixing equipment is advantageous because the mixture does not come into contact with lubricated surfaces and the simple drum wall can be protected with a wear resistant lining coating.
Mixed wet powder
For the mixing of wet powder, convection mixing equipment that does not rely on natural circulation can be used, that is, forced convection mixing equipment. If the humidity is increased in the rotary drum mixer, in order to avoid agglomeration and agglomeration of a large amount of materials, a stirring impeller can be installed in the mixer.
continuous operation
During continuous mixing operation, the specifications of the mixture will not change frequently, and the cleaning function of the mixer is not very important. The key is the working time, with continuous charging and discharging functions.
In addition, the design standards of continuous mixing equipment are also quite different from batch mixing equipment. The main manifestations are: in the case of selecting different types of feeding control systems, continuous mixers sometimes expect a large amount of backmixing to occur inside the mixer.
2. Selection according to the quality requirements of the mixture:
The mixing quality of the mixture and the efficiency of the mixer complement each other. Only with the mixing quality of the mixture can the efficiency of the mixing equipment be achieved. A reasonable benchmark for comparing mixer efficiency is the quality of the balance mixture. The time required for different mixers to reach this equilibrium condition varies greatly, and a standard mixture can be used to comprehensively evaluate its efficiency by comparing the performance of each mixer.
The mixing equipment with diffusion mixing or shear mixing as the main mixing mechanism often has classification, while the mixing equipment with convective mixing as the main mixing mechanism is less classified. A mixer that relies on rotation or stirring, such as a rotating shell or a vertical stirring mixer, can produce a certain degree of classification, but a mixer with a shovel impeller such as a ribbon-mounted mixer can reduce the classification tendency.
In addition, the degree of classification also depends on the characteristics of the components. When the added material is too fine or wet to make the fluidity worse, the quality of the mixture produced by the mixer with different structures will not be much different. At this time, the structure selection will mainly be determined according to the price and process requirements. However, if the material has good fluidity and wide screening, it must be selected in a mixer that does not have a grading phenomenon to obtain a better mixing effect.
When choosing a mixer for viscous powder, the mixing rate should be mainly considered, which is more important than the quality of the balanced mixture. It should also be noted that the mixing speed of the same mixer in different directions may be different. For example, the mixing speed of the rotary drum mixer is relatively fast in the radial direction, while the mixing speed in the axial direction is relatively slow. However, for a mixture with good flow properties, the mixing rate has little to do with the initial feeding method due to the strong mobility of a single particle.
At the same time, when evaluating the performance of the mixer, the operation characteristic that the mixer must be completely empty cannot be ignored. This is because the emptying operation is often designed for the downward diffusion of particles on inclined surfaces and often involves shearing of the main body of the mixture. , and these factors will promote the classification, it is possible that the original high-quality mixture in the mixer will be destroyed by the discharge process.
Three, according to the mixed process cost selection:
If there are two or more mixers that can meet the requirements of the process and ensure the quality of the mixture, the choice of mixer will depend on the unit cost of the mixing operation. At this point, it is important to fully and correctly estimate the cost of mixing. Typically, powder blending represents a small percentage of total product production costs, and blending becomes a costly process only when production time is long due to out-of-specification products.
Therefore, choosing mixing equipment that both meets the quality specifications of the mixture and is well coordinated with the overall process is more important than small savings in operating costs.
The cost of the mixing process can be decomposed into three parts: A, investment depreciation; B, energy consumption; C, labor wages.
For the batch mixing process of powder and granular materials, the difference between the unit operating cost of using a convection mixer and a drum mixer is very small. In determining the total cost of the mixing process, in most cases, labor costs are dominant, and measures that can reduce labor content can reduce unit operating costs.
One of the measures to reduce the labor content is to reduce the operating frequency of the mixing cycle, that is, to appropriately increase the capacity of the low process flow, that is, to appropriately increase the installed capacity of the mixer. However, the labor savings will result in increased depreciation expense, so cycle time needs to be optimized. Appropriate over-capacity designs are possible, such as for lower mixing rate mixers.
The second measure to reduce labor content is to use continuous mixing operations. In this case, labor consumption is low, and the main role of the mixer is to act as a buffer vessel to smooth out fluctuations in the ingredient feed rates. And the continuous mixing operation will not increase the depreciation expense while reducing the labor cost. It should be emphasized, however, that when the focus is on controlling the flow of particles from the storage system to the equipment, the cost of ancillary equipment will rise.
