Main Equipment and Functions of the Grain Cleaning Section in a Steel Silo System

In modern steel silo systems, grain cleaning is a crucial "pre-processing stage," often referred to as the "quality gatekeeper" of grain storage. Its core purpose is to efficiently remove various impurities mixed in with the raw grain, ensuring the purity, safety, and stability of the grain entering the silo, laying a solid foundation for long-term safe storage.

Today, Shelley Storage will analyze the main equipment and functions of the grain cleaning stage in a steel silo system, following a process flow configuration of "initial cleaning → fine selection → grading → dust removal."

1. Core Cleaning Equipment (Sorted by Process Priority)

1.1. Initial Cleaning Equipment: Removing large particle impurities (first pretreatment step)

1.1.1 Vibrating Screen (High-Frequency Linear Vibrating Screen)

• Core Function: Removes impurities larger than grain grains (such as straw, corn cobs, large clods of soil, plastic film) and some medium-sized impurities (small stones, broken straw) from the raw grain, preventing blockage of subsequent equipment (such as elevator hoppers, chutes).
• Working Principle: A motor drives an eccentric block to generate high-frequency linear vibration. Grain and impurities separate on the screen surface due to inertia—grain falls through the screen holes, while large impurities are conveyed to the discharge port.
• Technical Parameters and Design Considerations:
  • Number of Screen Layers: 2-3 layers (upper layer coarse screen, 8-12mm aperture; lower layer fine screen, 2-4mm aperture, adjusted according to grain type: wheat 2.5-3mm, corn 3-4mm, rice 3.5-4.5mm).
  • Vibration Frequency: 1500-2000 r/min, amplitude 3-5mm (high frequency, low amplitude reduces grain breakage).
  • Processing Capacity: 50-500 t/h (matching the daily receiving capacity of a steel silo system; large steel silos typically have 2-4 units in parallel).
  • Industry Standard: Meets the cleaning requirement of "large impurity content ≤ 0.5%" in GB/T 11986-2008 "Soybeans for Feed".
• Application Connection: Usually installed downstream of the grain receiving port (such as after the unloading pit or before the bucket elevator), it is the core equipment of the primary cleaning section and needs to be equipped with an inspection door and a quick screen replacement mechanism.

1.1.2Cylindrical Primary Cleaning Screen (Rotary Cylindrical Screen)

• Core Function: Suitable for grains with high impurity content (such as corn and rice harvested directly from the field), it efficiently separates large impurities and some light impurities, and features large processing capacity and is not prone to clogging.
• Working Principle: The cylindrical screen rotates at a low speed of 15-25 r/min. The grain enters from one end and, during the tumbling process inside the cylinder, the grain particles fall into the receiving hopper through the screen holes, while large impurities are pushed to the other end by the spiral blades and discharged.
• Technical Advantages: The screen uses perforated plates or woven mesh, and the aperture can be customized according to the grain type; the cylinder has an angle of 3-5° with the horizontal plane to ensure smooth material flow; the processing capacity is 30% higher than that of a vibrating screen of the same specification, making it suitable for the continuous operation needs of large steel silos (single silo capacity ≥ 10000t).

1.2. Grain Processing Equipment: Removing Small and Medium-Sized Particle Impurities (Improving Grain Purity)

1.2.1 Gravity Grading Machine (Gravity Cleaner)

• Core Function: Based on the density difference between grains and impurities, this machine separates medium-sized impurities (such as small stones, metal particles, immature grains) and moldy particles, making it a key piece of equipment for improving grain quality.
• Working Principle: On an inclined screen surface, grains are processed by a combination of airflow (bottom-mounted air supply) and screen vibration, forming a layered structure of "upper layer of light impurities, middle layer of high-quality grains, and lower layer of heavy impurities." High-quality grains are discharged from the middle outlet of the screen, heavy impurities (stones, metal) are discharged from the bottom, and light impurities (shriveled grains, dust) are carried away by the airflow.
• Key technical parameters:
  • Airflow velocity: 1.2-2.5 m/s (adjusted according to grain density: wheat 1.8-2.2 m/s, corn 2.0-2.5 m/s).
  • Screen inclination angle: 12-18°, vibration frequency 800-1200 r/min.
  • Grading accuracy: Heavy impurity removal rate ≥98%, immature grain removal rate ≥85% (meets the high-quality grain standard of GB 1351-2008 "Wheat").
• Application scenario: Used for "refined cleaning" before storage in steel silos, especially suitable for the high-purity storage needs of seed grains and commercial grains.

1.2.2 Cylindrical cleaning screen (fine rotary screen)

• Core function: Further separates small impurities (such as fine sand, broken grains, and grass seeds) from grains, making up for the insufficient precision of the initial cleaning screen, and ensuring that the grains entering the steel silo have a "cleanliness ≥99%".
• Technical Features:
  • Number of screen layers: 3-4 layers, with gradually decreasing aperture (upper layer 4-5mm, middle layer 2-3mm, lower layer 1.5-2mm).
  • Rotation speed: 30-40r/min, using variable frequency speed control to adapt to the flowability of different grains.
  • Structural design: The screen is tensioned for easy disassembly and cleaning; equipped with an anti-clogging device to prevent fine impurities from clogging the screen holes.

1.3. Grading Equipment: Sorting by particle size (optimizing storage stability)

1.3.1 Grading Screen (Plane Rotary Grading Screen)

• Core Function: Grading the cleaned grains by particle size (e.g., wheat is divided into large, medium, and small grades, and corn is graded by particle size), avoiding "automatic grading" caused by mixed storage of grains of different sizes (large particles are in the upper part of the storage, and small particles are in the lower part, forming gap differences and easily causing condensation).
• Working Principle: Utilizing planar rotary motion (combining vibration and circular motion), grains are graded by particle size on the screen surface. The graded grains are then distributed to different storage compartments or different areas within the same steel silo.
• Technical Parameters: Grading accuracy deviation ≤2%, screen life ≥8000h, processing capacity matched to the primary cleaning equipment (usually 1.1 times the primary cleaning equipment's capacity to avoid bottlenecks).

1.3.2 Drum Grater (Suitable for lumpy grains such as corn and soybeans)

• Core Function: Separates grains by particle size through grading plates or screen holes within the drum, especially suitable for removing small broken grains from corn and shriveled grains from soybeans, improving storage uniformity.
• Application Advantages: Low grain breakage rate (≤0.3%), suitable for grains sensitive to breakage; can be linked with the zoning distribution system of the steel silo to achieve directional storage of graded grains.

1.4. Special Impurity Removal Equipment

1.4.1 Magnetic Separator (Permanent Magnet Drum/Electromagnetic Iron Separator)

• Core Function: Removes metallic impurities (iron nails, iron wires, metal fragments) from grains, protecting downstream equipment (such as the blades of screw conveyors and the discharge gate of steel silos) from impact damage, while preventing grain contamination caused by metallic impurities.
• Technical Requirements:
  • Magnetic field strength: ≥12000Gs (Gauss) to ensure the adsorption of fine metal particles.
  • Installation location: Usually after the primary cleaning screen and before the fine separation equipment, or at the discharge port of the bucket elevator (upstream of key equipment).
  • Structural design: Uses a drawer-type magnetic core, facilitating regular cleaning of adsorbed metallic impurities without affecting continuous operation.

1.4.2 Color Sorter (Photoelectric Color Sorter)

• Core Function: Removes moldy, insect-infested, and discolored grains (such as black moldy grains in wheat and yellowed grains in rice), improving the appearance quality and storage stability of grains (moldy grains carry mold spores, which can easily cause mold growth in the entire silo).
• Technical Parameters:
  • Detection Rate: ≥99.5% (moldy grains, discolored grains), False Selection Rate ≤0.1%.
  • Processing Capacity: 30-150t/h, using a CCD image sensor (resolution ≥2048 pixels), adaptable to color threshold adjustments for different grain types.
  • Application Scenarios: Essential equipment for high-end commercial grain and seed grain storage, especially suitable for steel silo systems for long-term storage (≥6 months).

2. Auxiliary Equipment: Ensuring Cleaning Effectiveness and Environmental Compliance

2.1. Dust Removal Equipment (Pulse Jet Bag Filter)

• Core Function: Collects dust generated during the cleaning process (such as grain skin, fine sand, and dust), preventing dust pollution of the environment and harm to operator health, while also preventing dust accumulation in the workshop and the risk of explosion (grain dust is combustible dust, explosion limit 10-20g/m3).
• Technical Details:
  • Filtration Area: Calculated based on the dust emission of the cleaning equipment (typically 15-20 m² filtration area per 100 t/h processing capacity).
  • Pulse Cleaning: Compressed air pulse cleaning is used, with a cleaning pressure of 0.4-0.6 MPa and a cleaning cycle of 30-60 s, ensuring a filtration efficiency ≥99.9%.
  • Environmental Standards: Emission concentration ≤10 mg/m³ (compliant with GB 16297-1996 "Integrated Emission Standard for Air Pollutants").
  • Linkage Design: Linked with the start/stop of equipment such as vibrating screens and classifiers to ensure no dust leakage.

2.2. Conveying and Unloading Equipment (Connecting Cleaning and Storage)

2.2.1 Bucket Elevator

• Function: Vertically conveys the cleaned grains to the feed inlet of the steel silo, serving as the core connecting equipment between the cleaning and storage sections.
• Design Highlights: Deep-bucket type hoppers are used (to prevent grain spillage); lifting speed is 1.2-1.8 m/s (adjusted according to grain type to prevent breakage); flexible connection is used with the discharge port of the cleaning equipment to reduce dust leakage.

2.2.2 Screw Conveyor (Horizontal/Inclined Conveying)

• Function: For horizontal/short-distance inclined conveying between cleaning equipment (e.g., primary screen → gravity classifier) ​​or between cleaning equipment and elevator; compact structure, suitable for workshop layout.
• Protection Requirements: Blades are made of wear-resistant steel plate (e.g., Mn13) to prevent impurities from wearing the blades; equipped with overload protection devices to prevent large impurities from jamming the equipment.

2.2.3 Unloader (Rotary Unloader/Impeller Feeder)

• Function: Controls the unloading speed of the cleaned grains to ensure smooth operation of subsequent conveying equipment; also acts as a seal to prevent air leakage caused by negative pressure in the dust collector.
• Technical Parameters: The unloading capacity should match the processing capacity of the cleaning equipment (usually 1.2 times the processing capacity), and the air leakage rate should be ≤5%.

3. Key Points for Equipment Selection and Process Integration (Industry Practical Standards)

3.1. Processing Capacity Matching Principle:

• The total processing capacity of the cleaning section equipment must be ≥ the daily receiving capacity of the steel silo system (usually with a 10-20% margin) to avoid raw grain accumulation. For example, a steel silo system with a daily receiving capacity of 500t requires cleaning equipment with a total processing capacity of 600t/h (e.g., two 300t/h vibrating screens in parallel).

3.2. Grain Type Compatibility:

• Wheat/Rice: Prioritize color sorters (to remove moldy grains) and high-frequency vibrating screens (to separate fine impurities).
• Corn: Prefer cylindrical primary cleaning screens (to handle high impurity content) and drum classifiers (to classify by particle size).
• Soybeans: Reduce high-intensity vibration equipment to avoid soybean breakage; low-amplitude vibrating screens + gravity classifiers can be used.

3.3. Storage Period Related Design:

• Short-term storage (≤3 months): The focus of cleaning is on removing large impurities and metallic impurities; the selection equipment can be simplified (e.g., color sorter can be omitted).
• Long-term storage (≥6 months): A gravity grader + color sorter must be configured to ensure grain cleanliness ≥99.5% and moldy grain content ≤0.1%, reducing storage risks.

3.4. Maintenance and Safety Requirements:

• All equipment must be equipped with inspection doors and emergency stop buttons; easily damaged parts such as screens and magnetic cores must be easily replaceable.
• Dust explosion risk prevention: Equipment grounding (anti-static), workshop ventilation (wind speed ≥0.5m/s), and a safe distance (≥5m) between dust collectors and cleaning equipment.

4. Industry Standards and Development Trends

4.1. Core Implementation Standards

• GB/T 11986-2008 Soybeans for Feed: After cleaning, large impurities ≤0.5%, small impurities ≤1.0%.
• GB 1351-2008 Wheat: Medium impurities ≤1.0%, moldy kernels ≤1.0%.
• GB/T 22104-2008 Corn: Total impurities ≤1.0%, broken kernels ≤5.0%.
• GB 12463-2009 General Technical Conditions for Packaging for the Transport of Dangerous Goods: Dust explosion-proof equipment must comply with Ex tDA21 IP65 standard.

4.2. Technological Development Trends

• Automated Integration: The cleaning equipment is linked with the steel silo's level sensors and PLC control system to achieve unmanned operation of "raw grain receiving → cleaning → storage".
• Intelligent Detection: The color sorter integrates AI image recognition technology, which can accurately distinguish different types of moldy and insect-damaged grains, increasing the detection rate to over 99.8%.
• Energy Saving and Consumption Reduction: Utilizing variable frequency motors (adjusting speed according to grain volume) and high-efficiency dust collectors (extending filter material lifespan to 12,000 hours), energy consumption per unit processing volume is reduced (currently, the industry advanced level is ≤0.3 kWh/t).

Through the scientific configuration of the above equipment, the grain cleaning section of the steel silo system can achieve the goals of "thorough impurity removal, minimal grain damage, environmental compliance, and stable operation," providing a high-quality raw grain foundation for subsequent storage. We hope that Shelley Grain Depot's explanation will be helpful when selecting grain steel silo cleaning equipment in the future.

Written by

Shandong Shelley Grain Steel Silo Co., Ltd

Editor Jin

WhatsApp : +86-18653877118

Email : shelley@cnshelley.com