To reduce the breakage rate of rice grinding mills, comprehensive measures should be taken from multiple aspects such as equipment debugging, raw material processing, process optimization, and daily maintenance. The specific methods and principle explanations are as follows:
Raw Material Pretreatment: Enhancing the Processing Adaptability of Paddy Rice
Moisture Content Control
Standard: The moisture content of paddy rice should be controlled at 14%–16%.
When the moisture content is too high, the husk becomes highly tough, easily adhering to the rice grains during milling and causing breakage.
When the moisture content is too low, the rice grains become brittle and prone to cracking.
Measures: Adjust the moisture content through dryers or natural sun-drying. Before processing, perform "conditioning" (spray a small amount of water and stack for 1–2 hours) to weaken the bonding force between the husk and rice grains, reducing milling resistance.
Screening and Grading
Use equipment such as vibrating screens and specific gravity stone separators to remove impurities, broken rice, and immature paddy. This prevents uneven force distribution during milling due to inconsistent raw material particle sizes, thereby reducing the risk of breakage.
Equipment Parameter Debugging: Optimizing the Core Rice Milling Process
Adjusting the Clearance between the Grinding Roller and Screen Cylinder
Principle:
A too-small clearance increases milling pressure, aggravating frictional breakage of rice grains.
A too-large clearance reduces whitening efficiency, requiring extended milling time and also increasing breakage.
Recommendations: Adjust according to the paddy variety.
For indica rice, the clearance is generally set at 1.5–3 mm.
For japonica rice, it can be appropriately increased to 2–4 mm.
New grinding rollers should use the larger value, which is gradually reduced as they wear.
Controlling the Rotation Speed and Linear Velocity of the Grinding Roller
Excessively high speed: Generates excessive frictional heat, hardening the rice grain surface and increasing internal stress, making them prone to breakage.
Excessively low speed: Inadequate whitening capability requires multiple milling passes, also increasing breakage.
Reference values:
For emery roller rice mills, the rotation speed is recommended at 600–800 r/min.
For iron roller rice mills, control it at 400–600 r/min, with the linear velocity maintained at 10–15 m/s.
Regulating Feed Flow and Pressure
The opening of the feed inlet gate should be uniform and stable to avoid pressure fluctuations in the milling chamber caused by inconsistent flow rates.
The rotation speed of the feed auger can be controlled by a frequency converter to maintain the flow at 80%–90% of the equipment's rated processing capacity, reducing breakage due to overload.
Process Optimization: Application of Graded Rice Milling and Auxiliary Devices
Adopting Multi-Stage Rice Milling Process
Single-stage rice milling: High pressure is required for one-time whitening, leading to a high breakage rate.
Multi-stage rice milling (such as two or three stages): The first stage gently mills to remove the husk, and the subsequent stages finely mill to remove the bran. Each stage has a clear division of labor, reducing the pressure of single-time milling. For example, adjusting the pressure of the first-stage rice mill to 30–40 kPa and the second stage to 20–30 kPa can reduce the breakage rate by 30%–50%.
Installing Spray or Ventilation Devices
Spray conditioning: Uniformly spray a small amount of water mist in the milling chamber (with moisture content controlled at 0.5%–1%), reducing the surface friction coefficient of rice grains and minimizing brittle breakage caused by high temperatures.
Negative pressure air suction: Use a fan to remove heat and broken rice generated during milling, preventing broken rice from repeated friction in the milling chamber while reducing the chamber temperature (it is recommended to control below 40°C).
Equipment Maintenance and Component Upgrades
Regular Inspection of Grinding Roller Wear
When the surface wear of the emery roller exceeds 1/3 or the grooves of the iron roller become shallow, replace them promptly. Otherwise, uneven milling force will cause concentrated stress on rice grains, increasing breakage. It is recommended to replace the grinding roller every 500–1,000 tons of paddy processed.
Optimizing Screen Configuration
The size of screen holes should match the rice grain size:
For indica rice, the screen hole diameter is generally 1.2–1.8 mm.
For japonica rice, it is 1.5–2.0 mm.
Too dense a screen will cause poor rice discharge, prolonging milling time; too sparse a screen will allow less broken rice to leak out, making it prone to repeated breakage in the milling chamber.
Calibrating the Transmission System
Check the tension of belts and lubrication of bearings to avoid pressure fluctuations in the milling chamber caused by equipment vibration or eccentric operation, which may lead to a localized increase in breakage rate.
Varieties and Operational Management
Selection of Breakage-Resistant Varieties
The breakage resistance of different paddy rice varieties varies significantly. For example, the compression strength of japonica rice is higher than that of indica rice. During processing, prioritize varieties with high horny rate and short-round grain shapes (such as Northeast japonica rice), which can reduce the breakage rate by approximately 20%.
Standardized Operator Training
Operators must master the sequence of "start the machine first, then feed materials; stop the machine first, then cut off materials" to avoid breakage caused by sudden pressure changes during no-load startup or material-carrying shutdown. Meanwhile, regularly record data on the rice mill's current, temperature, and breakage rate, and adjust parameters in a timely manner.
Application of New Technologies
Low-Temperature Rice Milling Technology
The temperature of the milling chamber is controlled at 25–30°C through a cooling system, reducing cracks in rice grains caused by thermal stress. This can decrease the breakage rate by 15%–25% while preserving more nutritional components.
Flexible Rice Milling Technology
Grinding rollers are made of elastic materials such as polyurethane, replacing rigid extrusion with flexible friction to reduce milling impact force. This technology is particularly suitable for processing high-quality rice or weak-quality rice.
Conclusion
To reduce the breakage rate of rice mills, optimization should be carried out across the entire chain of "raw materials - equipment - process - management". The core lies in balancing whitening efficiency with grain integrity. By controlling moisture content, adopting graded milling, precisely adjusting equipment parameters, and introducing new technologies, the breakage rate can be kept below 5% while ensuring whitening accuracy (the breakage rate of traditional processes is typically 8%–12%). It is recommended to conduct trials based on specific paddy varieties and equipment models to formulate personalized process plans.
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