“A reasonable roller speed and surface condition of the roller will affect whether the material enters the grinding zone accurately. If the feeding is inaccurate, the material will collide and splash between the two rollers and cannot directly enter the grinding zone, thereby affecting the yield and the grinding effect. However, if the feeding speed is too fast, it will be beneficial to the improvement of production, but it will result in inaccurate feeding and accumulation of materials in the grinding zone.”
Double-lion Grain & Oil Machinery
Wheat milling is the use of grinding, screening, flour cleaning and other equipment to put the net wheat cortex and endosperm separation, and the endosperm milled into flour, or through the processing of flour mixture to make a variety of different grades and uses of finished wheat flour. Although a variety of methods are used for milling, the current method of milling in the world is to crush the wheat grain, grind it step by step, and separate the wheat bran and endosperm by multiple sieve treatments. The commonly used grinding equipment is a roller mill. The auxiliary grinding equipment includes an impact mill and a loose powder machine.
Flour Milling Theory Brief Introduction
Grinding and sieving milling method mainly uses the difference in the strength of wheat endosperm and cortex to separate the cortex from the endosperm. However, the current milling technology cannot achieve the purpose with a simple method, and the milling method must be carried out by sub-system by way of grinding. Through long-term flour milling production practices, people have recognized and summarized that the milling process has the following basic rules:
- After each grinding and sifting of wheat, some wheat flour is obtained, and various intermediate products with different qualities and grain sizes are also obtained.
2.After grinding, the average particle size of the skin layer is larger than the average particle size of the endosperm. Therefore, among the various intermediate products obtained after the screening, the particle size has good quality, and the large particle size has poor quality.
3.Various intermediate products are ground according to different quality and granularity, which is beneficial to improve the quality and grinding effect of wheat flour.
4.The same kind of material, the intensity of grinding is lower than the quality of wheat flour obtained by mild grinding.
- The quality of the wheat flour extracted from each system is different, and the quality of the former is generally better than that of the back, and the quality of the heart grinding is better than that of the leather mill.
System Setting During Milling
In the powder road, the process system composed of equipment handling similar materials is called a system, and usually a multi-channel processing equipment should be set up in a system.
The milling process is generally equipped with systems such as skin mills, heart mills, slag mills, and flour cleaning. The skin grinding and heart grinding systems are the two basic systems of the milling process, each of which is equipped with a certain amount of grinding and screening equipment. The main role of each system is:
- Skin Milling System: Under the premise of keeping wheat bran intact, strip the wheat, scrape the endosperm on the cortex one by one, and extract high-quality endosperm granules and wheat flour of a certain quality and quantity.
- Slag Milling System: The bran-bearing granules provided on the anterior and middle roads were lightly ground to separate the cortex from the endosperm, so that a pure wheat heart was sent to the heart-milling powder.
- Heart Milling System: The purer endosperm particles provided by each system were grinded one-by-one into wheat flour with a certain degree of fineness, and bran was proposed.
- Tail Milling System: Located in the middle and posterior segment of the heart-milling system, it specifically treats bran-containing wheat bran, which is separated from the heart-grinding system, and produces wheat flour.
- Flour Cleaning System: The wheat mill residue, wheat kernel, and coarse meal extracted from the former mill and other systems are purified and classified, and then sent to the corresponding grinding system for processing.
- Flour Mixing System: Different wheat flours were stored separately, and then they were mixed, nutritionally fortified and mixed according to a certain ratio, and they were formulated into various types of finished wheat flour.
MILLING EQUIPMENT FACTORS
There are mainly the following aspects influencing the equipment.
(1)Feeding effect – The feeding effect mainly depends on the following aspects:
①Feed roller parameters selection
A reasonable roller speed and surface condition of the roller will affect whether the material enters the grinding zone accurately. If the feeding is inaccurate, the material will collide and splash between the two rollers and cannot directly enter the grinding zone, thereby affecting the yield and the grinding effect. However, if the feeding speed is too fast, it will be beneficial to the improvement of production, but it will result in inaccurate feeding and accumulation of materials in the grinding zone. In severe cases, it will cause plugging.
②The uniform distribution of the material in the full length of the roller
If the material distribution in the grinding area is not uniform, not only the grinding effect and equipment utilization rate are reduced, but also the grinding roll at the thick material is likely to be locally worn, making the rolling distance of the entire grinding roller length inconsistent. Therefore, it is important to carefully adjust the working gap of the feed valve and master the minimum material level in the feed cylinder.
(2)Roller cooling and cleaning
In the continuous operation of the milling machine, the temperature of the roller body will increase, especially the temperature rise of the roller will be higher, which will easily cause the evaporation and condensation of moisture, which will reduce the screen and grinding effect, and thus affect the yield and the quality of the wheat flour.
When pneumatic conveying is used, the roller can be air-cooled, but the picker for the device’s strong suction should be selected. In order to further reduce the temperature of the grinding roller, the water cooling method of the roller can be considered. That is, the water inlet and outlet pipes are set at both ends of the roller shaft, and the heat is taken away by the cold water flowing through the roller body to reduce the temperature of the roller body, the bearing and the material.
In the milling process, the phenomenon that the powdery material adheres to the surface of the grinding roller is called a winding roller, which affects the grinding of the material and generates vibrations of the machine body in severe cases. Therefore, it is necessary to keep the roller cleaning mechanism working properly, and pay attention to the pressure between the rollers should not be too large, the moisture into the grinding water should not be too high.
(3)The oldness of grinding teeth
Newly drawn rolls have sharper tooth angles, stronger grinding effect, higher stripping rate, brittle brittleness, more granular materials, less fines, low power consumption, and high flow rates. However, after a period of use, the tooth angle becomes dull, the grinding effect decreases, the output decreases, the bran flakes in the milled material are large, the wheat flour is large, the slag grain is reduced, the grinding temperature is increased, and the power consumption is particularly high. Therefore, the old and new grinding teeth will affect the balance of grinding effect and powder load. In order to balance the load and stabilize the production, the rollers must be changed in batches according to the wear of the rollers.
(4)Number of teeth and tooth angle
It is determined by the particle size of the grinding material, the nature of the material, and the degree of crushing required.
(1)Rolling distance-The rolling distance is the main operating factor that determines the grinding effect. The rolling distance adjustment is the main operation content of the grinding machine. In actual production, the technical characteristics of the powder path and the grinding roller are generally relatively stable, and when the conditions of the raw material, moisture, climatic conditions, grinding teeth are changed, the grinding effect is mainly corrected by adjusting the rolling distance.
When the 1B rolling distance is 0.9mm, the peeling rate is 20%; when the rolling distance is 0.7mm, the peeling rate is increased to 35%; when the rolling distance is reduced to 0.5mm, the peeling rate is 75%. The rolling distance is in the range of 0.5-0.7mm, which has the greatest influence on the stripping rate.
(2)Flow rate-The unit flow rate of the flour mill is usually expressed in units of contact length of the roller unit, ie kg/cm. d. When the flow rate is too large, the homogeneity of material crushing will be reduced. If the flow rate is too small, the feeding state is not normal and the equipment operation is unstable.
The 1B mill will control the entire powder flow. In addition to the final equipment, the grinding effect of each milling machine and the screening effect of the corresponding screening equipment will affect the working status of subsequent equipment.
When the flow rate is large, the rotation speed of the grinding roller can be appropriately increased. However, when the rotation speed is high, the loss and vibration of the equipment will increase.
Mill Machine Operation and Maintenance
The operation requirements of the mill machine are high, the equipment structure is more complex, and the mill machine must be carefully operated and properly maintained. Focus on the following three areas of work.
1.Ensure the normal operation of the equipment
First, always check the bearing temperature. If the temperature is too high, check whether the lubrication and transmission parts are normal and whether the rolling distance is too tight. Thoroughly overhaul and maintain the bearing every six months.
Second, check the adjustment of the tension of the belt once every three months.
Third, the mill must start the motor in the off-state. For a pneumatic milling machine, start the air source before starting the motor, withdraw the working pressure of the compressed air to 0.6 MPa, and switch on the air control system of the device.
Fourthly, after starting up, it is necessary to constantly check whether the pneumatic components, gas lines, and joints in the air path are leaky or damaged. At the same time, check whether the air supply pressure meets the requirements.
The role of the oil mist device in the gas path is to provide the necessary lubricant for the pneumatic components. Use 20# spindle oil. The amount of oil should be checked frequently. The oil volume is generally 4/5 of the oil cup, and the amount of oil drops is 5 drops in minutes. The function of the water separator is to separate the residual water and impurities from the air flow. It is necessary to check the water situation frequently. When there is water accumulation, the drain handle can be pulled out.
Fifth, the timing belts used in pairs must be marked when they are removed for maintenance so that they can be restored when installed.
Sixth, check the feeding mechanism, roller cleaning device, collecting hopper, and discharging conditions at any time during work. The feeding sensing mechanism must be flexible and accurate; the accumulated powder in the cleaning brush is often removed to ensure that it works properly; the material collection in the collecting hopper is checked at any time, so that the discharging is smooth and clogging is prevented.
Seventh, the feed chute of the mill should maintain a vertical section of 500 mm or more, so that the sensing plate is pressed evenly, and the feed roll is easily screeded. Pay attention to remove the impurities hanging on the sensor board so as not to affect its sensitivity.
- Maintain the milling effect
First, for the skin mill, the grinding effect is usually judged by observing the size of the bran in the milled material and the powder content; the heart mill is mainly determined by observing the amount of the powder, and judging by the thickness of the hand. Standard methods are used to check the rate of scraping and the rate of powder extraction. The same roll of each pair of rollers should generally maintain the flow-rate, close to the grinding effect.
Second, it is necessary to keep the flow rate stable. When the incoming material flow is too large or too small, check the working status of the front equipment.
Third, prevent the material from passing through both ends of the roller without being ground. If this phenomenon is found, the roller shield should be overhauled during shutdown.
Fourth, to prevent the shredder phenomenon that occurs during skin grinding. The main reasons for shredding are: too tight rolling distance, too small tooth angle, too low flow rate, and grinding tooth characteristics that are incompatible with the particle size and flow rate of the material.
Fifth, to prevent the material from being wound due to high flow rates, excessively turbid grinding teeth, and reduced speed ratio caused by the failure of the constant speed mechanism.
Sixth, there should be no condensation of water vapor on the inner wall of the pulverizer. There should be no dust flying outside the mill door. The temperature of the grinding material must not exceed 50°C. Otherwise check the working status of the feeder.
Seventh, when installing the roller, the two rollers must be carefully aligned in parallel. This is an important measure to ensure the consistent grinding effect of the entire length of the roller.
3.Prevent and correctly handle faults
First, when the incoming material is interrupted or suddenly stopped, it should be noted whether the device is off the brake.
Second, if the hopper is blocked, it is very easy to cause damage to the motor. If a blockage occurs, the roller should immediately roll back and clear it in time.
Third, if there are foreign objects in the feeding valve, it is necessary to retract the roller before trying to remove the foreign object.
Fourth, the tension of the roller protection spring must be appropriate, and the specification of the safety pin or the safety washer must not be arbitrarily increased to ensure the safe operation of the equipment.
Fifth, in the course of operation, it is not allowed to arbitrarily adjust the minimum distance limit device to ensure the grinding effect and protect the roller.
Sixth, after overhaul, the scattered materials should be concentrated in one place. After magnetic separation and screening, the materials can be returned to prevent the impurities from damaging the equipment.
Worked as marketing manager in mechanical industry for 7 years, Sophia Ren is now general manager of international trade department in Double-lion Machinery. Mrs. Ren has successfully supervised many turnkey projects in China, India, Bangladesh, South Africa and America with daily capacities ranging from 20 tons to 200 tons. She has published articles in different journals on her field of expertise.
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