Raw Material Characteristics and Process Objectives
Understanding the properties of raw materials and setting clear process goals are the foundation of effectively using an impact crusher and vibrating screen combination to process asphalt pavement materials. The characteristics of Reclaimed Asphalt Pavement (RAP) directly determine the selection of equipment parameters and process steps, while process objectives ensure that the final product meets practical application requirements. By analyzing the physical and chemical properties of RAP, we can optimize the crushing and screening process to avoid problems such as material blocking or excessive heat generation. At the same time, clear quality indicators for finished products guide the entire production line to operate efficiently, balancing recycling efficiency and material performance.
This section will first explore the unique characteristics of RAP, including its asphalt content, moisture level, particle size distribution, and the properties of aged asphalt. Then, it will detail the key performance indicators (KPIs) that the finished product must meet, explaining how these indicators connect with actual engineering applications. This linkage between raw material characteristics and process objectives ensures that the entire process is both scientific and practical, laying a solid foundation for the efficient recycling of asphalt pavement materials.
Physicochemical Characteristics of RAP
Reclaimed Asphalt Pavement (RAP) has distinct physicochemical properties that affect its processing. One of the main features is its asphalt content, which typically ranges from 3% to 6%. This asphalt coating binds the aggregate particles together, giving RAP a certain degree of cohesiveness. The moisture content of RAP is generally required to be ≤3%; if it exceeds this limit, it may cause difficulties in crushing and screening, such as the agglomeration of fine particles. In terms of particle size, RAP usually has a range of 0–50 mm, containing both small fines and larger chunks, which requires the processing equipment to have good adaptability to different particle sizes.
Another critical characteristic of RAP is the aging of its asphalt binder. Aged asphalt shows an increase in softening point and a decrease in penetration, making it more brittle and less flexible. During processing, it is crucial to control the secondary heating temperature to below 120°C. Excessive temperature can further degrade the aged asphalt, reducing its residual binding capacity. The impact crusher is particularly suitable for RAP processing because its crushing principle relies on high-speed impact rather than friction, which minimizes heat generation, thus helping to maintain the stability of the aged asphalt's properties.
Finished Product Quality KPIs
The quality of the finished product processed from RAP is evaluated through a series of key performance indicators (KPIs) to ensure its suitability for reuse in road construction. One of the core indicators is the proportion of three graded materials: the 0–5 mm fraction should account for 25%±5%, the 5–15 mm fraction for 45%±5%, and the 15–30 mm fraction for 30%±5%. This specific distribution meets the requirements of different layers in road construction, where the finer fractions can be used as fillers, and the coarser ones as base materials, ensuring good structural performance of the recycled pavement.
In addition to particle size distribution, other important KPIs include the content of oversize particles, fine powders, and metal impurities. The proportion of oversize particles (larger than the target maximum size) must be less than 2% to avoid affecting the uniformity of the pavement structure. The content of fine powders smaller than 0.075 mm should be controlled below 10% because excessive fines can reduce the permeability of the asphalt mixture. Moreover, metal impurities, such as fragments from construction tools, must be less than 0.1% to prevent damage to subsequent construction equipment. These indicators are closely related to the adjustment of the discharge size of the impact crusher and the screening efficiency of the vibrating screen, reflecting the effectiveness of the entire process in construction and demolition waste recycling.
Core Equipment Selection and Layout
The selection and layout of core equipment—specifically the impact crusher, vibrating screen, and their overall arrangement—directly determine the efficiency and stability of the asphalt pavement material processing line. A well-chosen impact crusher with optimized parameters ensures effective crushing of RAP, while a properly configured vibrating screen guarantees precise separation of finished products. Meanwhile, a reasonable layout, especially for mobile systems, enhances flexibility and space utilization, making the process adaptable to different job sites, from urban road repair to large-scale recycling yards.
This section focuses on the key considerations in equipment selection: optimizing impact crusher parameters to improve asphalt stripping efficiency, configuring the vibrating screen for high separation accuracy, and designing a mobile closed-loop layout to balance productivity and mobility. Each aspect is tailored to the unique properties of RAP, ensuring that the equipment works in harmony to meet the finished product quality standards outlined earlier.
Parameter Optimization of Impact Crusher
The impact crusher, as the core crushing equipment, requires precise parameter setting to handle RAP effectively. A rotor diameter of Ø1,150 mm is chosen for its balance between crushing force and energy consumption, providing sufficient inertia to break down larger RAP chunks (up to 50 mm) while maintaining control over particle size. The adjustable rotational speed, ranging from 480 to 680 rpm, allows operators to adapt to varying RAP hardness and asphalt content—higher speeds (600–680 rpm) are ideal for more cohesive RAP with higher asphalt content, ensuring thorough fragmentation.
Material selection for key components further enhances performance. Using a ceramic blow bar instead of traditional metal ones reduces wear and minimizes heat generation, which is critical for preserving aged asphalt. Paired with a two-stage impact plate design, this configuration increases asphalt stripping efficiency by 20% compared to standard setups. The first stage breaks large particles, while the second stage refines the material and separates asphalt coatings from aggregates, ensuring the crushed product meets the required cleanliness for reuse.
Vibrating Screen Configuration
The vibrating screen plays a vital role in classifying the crushed RAP into the target particle size fractions. A three-layer screen design with mesh sizes of 10 mm, 15 mm, and 30 mm is employed to separate the material into 0–5 mm, 5–15 mm, and 15–30 mm fractions, aligning with the finished product KPIs. The screen operates with an amplitude of 6–8 mm, a range that balances effective particle stratification and energy efficiency—sufficient to dislodge trapped particles without excessive vibration that could damage the screen or cause material spillage.
To address the tendency of RAP to stick to screens due to its asphalt content, an anti-blinding ball deck is integrated into the screen design. This system uses small, free-moving balls that strike the screen surface during vibration, dislodging adhering particles and maintaining screen permeability. This feature, combined with the optimized amplitude, ensures a screening efficiency of ≥95%, minimizing the risk of oversized particles in the finished fractions and reducing the load on recirculation systems. Such configuration ensures the vibrating screen operates reliably even with RAP’s cohesive properties.
Mobile Closed-Loop Layout
The mobile closed-loop layout is designed for maximum flexibility and space efficiency, combining a track-mounted impact crusher, a track-mounted vibrating screen, and a return conveyor belt. This compact arrangement occupies less than 180 m², making it suitable for confined job sites such as urban road repair areas or small recycling yards. The track-mounted design allows easy movement between locations, eliminating the need for permanent infrastructure and reducing setup time.
This layout achieves a production capacity of 120 t/h, as detailed in crushing capacity guidelines, with a total power consumption of less than 350 kW, balancing productivity and energy efficiency. For added versatility, a diesel-electric hybrid power option is available—ideal for sites with limited grid access, where diesel power can be used, while electric mode reduces emissions in urban or environmentally sensitive areas. The closed-loop design, with the return belt recirculating oversize particles back to the mobile impact crusher, ensures all material is processed to meet the target size, maximizing resource utilization.
Four-Step Standardized Process Flow
A standardized four-step process flow ensures the efficient and consistent processing of asphalt pavement materials, from raw RAP to qualified finished products. Each step is designed to address specific challenges posed by RAP—such as impurities, moisture, and asphalt aging—while aligning with the quality KPIs. By breaking down the process into pre-screening, primary crushing, two-stage screening, and return material re-crushing, operators can maintain tight control over every stage, minimizing errors and maximizing resource utilization. This structured approach not only enhances productivity but also guarantees that the final product meets the strict specifications required for road construction and recycling applications.
The four steps work in sequence to transform raw RAP into graded aggregates: pre-screening removes large impurities and adjusts moisture; primary crushing breaks down materials and begins asphalt stripping; two-stage screening precisely classifies particles into target sizes; and return re-crushing ensures no oversize particles remain. Together, they form a closed-loop system that adapts to variations in RAP quality, ensuring stability even when handling inconsistent feed materials.
Pre-screening and Impurity Removal
The first step in the process is pre-screening and impurity removal, which lays the groundwork for smooth subsequent operations. A 150 mm grid screen installed at the front of the feeder is used to remove oversize RAP particles (larger than 150 mm), preventing them from entering the impact crusher and causing equipment damage or uneven crushing. Additionally, a magnetic separation drum is integrated to extract metal impurities such as steel bars or nails, which are commonly present in reclaimed pavement materials. This step directly contributes to meeting the finished product KPI of metal impurities<0.1%, protecting both the crusher and downstream equipment.
Moisture control is another critical aspect of pre-screening. When the moisture content of RAP exceeds 3%, a rotary dryer operating at 60°C is used for low-temperature dehumidification. This low temperature is carefully chosen to avoid heating the aged asphalt beyond safe limits, preventing premature softening or degradation. By reducing moisture to ≤3%, the process minimizes the risk of particle agglomeration during crushing and screening, ensuring that the feed size and condition are optimal for the next stage. This preparation step is vital for maintaining the efficiency of the entire production line.
Primary Crushing
Primary crushing is where RAP is first broken down into smaller particles, with the impact crusher set to specific parameters for optimal results. The closed side setting (CSS) of the impact crusher is fixed at 25 mm, a value chosen to balance crushing efficiency and particle size control. This setting ensures that after primary crushing, the P80 (the size at which 80% of particles are smaller) is approximately 20 mm, which is coarse enough to allow further processing but fine enough to start stripping the asphalt coating from the aggregates. This initial asphalt stripping sets the stage for better separation in subsequent steps.
To protect the aged asphalt from secondary aging, the crushing chamber is equipped with negative pressure ventilation, keeping the internal temperature below 50°C. This is critical because excessive heat can accelerate asphalt degradation, reducing its residual binding capacity. The crushing chamber design, combined with controlled ventilation, minimizes friction-induced heat, ensuring that the asphalt remains stable throughout the crushing process. This attention to temperature control aligns with the earlier requirement of limiting secondary heating to<120°C, preserving the material’s quality for reuse.
Two-Stage Screening
Two-stage screening refines the crushed material into the target particle size fractions, ensuring precise classification. The first stage of screening uses screens to separate particles into two streams: 0–15 mm and 15–30 mm. This initial split reduces the load on the second stage, which employs a high-frequency screen to further divide the 0–15 mm fraction into 0–5 mm and 5–15 mm. This two-step approach enhances screening accuracy, as high-frequency vibration in the second stage is particularly effective at separating fine particles, which are prone to sticking due to residual asphalt.
To maintain consistency, an online NIR (Near-Infrared) sensor is used to monitor moisture and oil content in real time. This data is fed back to the control system, which adjusts the screen’s tilt angle and vibration frequency accordingly. For example, if moisture levels rise slightly, increasing vibration frequency helps prevent particle adhesion to the screen mesh. This dynamic adjustment ensures that the screening efficiency remains high, aligning with the requirement of ≥95% efficiency and guaranteeing that each fraction meets its specified size range. The precision of this stage is key to achieving the finished product’s size distribution KPIs.
Return Material Re-crushing
Return material re-crushing ensures that any oversize particles from screening are reprocessed, completing the closed-loop system. Particles larger than 30 mm are conveyed back to the impact crusher via a return belt, creating a continuous cycle that eliminates missed oversize materials. The load rate of this return loop is controlled between 30–40% of the crusher’s capacity, balancing throughput with crushing effectiveness. This control keeps the F80 (the size at which 80% of the feed to the crusher is smaller) at approximately 25 mm, ensuring the crusher operates within its optimal range without being overloaded.
Typically, 3–4 cycles of this closed-loop re-crushing are sufficient to eliminate nearly all oversize particles, bringing the oversize content below the 2% KPI. This repeated processing not only improves particle size uniformity but also enhances asphalt stripping efficiency, as each pass through the crusher further loosens the asphalt coating from the aggregates. By maintaining this cycle, the process leverages the crushing ratio effectively, transforming even the most stubborn RAP particles into合格 (qualified) aggregates. This final step ensures that the entire process delivers consistent, high-quality results, ready for reuse in new pavement construction.