1. Compliance with Design Specifications of the Mixer
Core Requirement: The particle size of the feed must not exceed the maximum allowable particle size specified in the design parameters of the asphalt mixer.
For example, if the mixer is designed for a maximum particle size of 31.5 mm (common in medium and large projects), particles larger than this size will cause blockages in the feeding system, damage to the mixing drum, or uneven mixing.
Impact on Mixing Efficiency: Over-sized particles may lead to prolonged mixing time, increased energy consumption, and reduced production efficiency.
2. Particle Size Distribution (Gradation) Requirements
Gradation Compliance with Mix Design: The feed particles should conform to the gradation range specified in the asphalt mixture design (e.g., AC-16, SMA-13, etc.).
Example: For an AC-20 asphalt mixture, the particle size distribution of aggregates (such as stone, sand) should typically cover the range of 0–20 mm, with key control points at sieve sizes like 20 mm, 16 mm, 4.75 mm, and 0.075 mm.
Importance of Gradation: Reasonable gradation ensures the compactness, strength, and durability of the asphalt mixture. If the particle size is too uniform (e.g., lacking fine aggregates), the mixture may have poor cohesiveness and be prone to segregation.
3. Control of Over-Sized and Under-Sized Particles
Limiting Over-Sized Particles:
Over-sized particles (e.g., >110% of the maximum allowable size) must be screened out before feeding to prevent damage to the mixer's feeding hopper, conveyor belt, or mixing blade.
Measures: Install vibrating screens or grizzly screens in the aggregate production line to remove over-sized materials.
Control of Under-Sized Particles (Fine Powders):
Excessive fine powders (e.g., <0.075 mm) may increase the water absorption of the mixture, reduce its fluidity, and cause dust pollution during mixing.
Requirement: The content of fine powders should generally be controlled within 3%–5% (depending on the mixture type), and proper dust removal equipment (e.g., bag filters) should be used in the mixer.
4. Uniformity and Stability of Particle Size
Consistent Particle Size: The feed particle size should be as uniform as possible to avoid segregation during mixing and transportation.
Impact of Instability: If the particle size fluctuates significantly, the ratio of asphalt to aggregates may be unbalanced, affecting the performance of the mixture (e.g., excessive coarse aggregates may lead to poor compactability).
Stable Production Source: Aggregates should be sourced from stable quarries, and the crushing process (e.g., jaw crushers, cone crushers) should be standardized to ensure consistent particle shape and size.
5. Particle Shape and Surface Characteristics
Preferred Particle Shape: Aggregates should have a cubical shape with less needle-like or flaky particles.
Standard: The content of needle-like and flaky particles is usually required to be <15% (strict projects may require <10%). Excessive needle-like particles reduce the strength of the mixture.
Surface Roughness: Coarse aggregates should have rough surfaces to enhance the adhesion between asphalt and aggregates, while fine aggregates should be clean and free of soil impurities.
6. Moisture Content of Feed Materials
Impact of Moisture: Excessive moisture in aggregates (e.g., >3%) can cause particle agglomeration, affect the accuracy of the weighing system, and increase energy consumption during drying.
Control Requirement: The moisture content of aggregates is typically controlled below 1%–2%. Drying drums in the mixer should be used to remove moisture, and wet aggregates should be avoided during feeding.
7. Matching with Mixing Process Parameters
Drying and Heating Conditions: The particle size affects the heat transfer efficiency in the drying drum. Smaller particles heat up faster, while larger particles require longer heating time.
Adjustment: The rotation speed of the drying drum and the fuel supply should be adjusted according to the particle size to ensure that the aggregates reach the target temperature (usually 160–190°C).
Mixing Time: The particle size determines the optimal mixing time. For example, mixtures with larger particles may require a longer mixing cycle (e.g., 45–60 seconds) to ensure uniform coating of asphalt.
8. Safety and Equipment Protection
Preventing Equipment Damage: Avoid feeding hard stones or metal impurities that could damage the mixer's blades or internal components.
Dust and Noise Control: Proper particle size control reduces dust emissions (e.g., fine powders) and mechanical noise during feeding, meeting environmental protection requirements.