The variability of RAP is an important factor affecting the proportion of recycled materials in the mixed reclaimed asphalt mixture, the addition of new asphalt, new aggregates and regenerants, and the quality and stability of recycled asphalt mixture. Measures such as secondary crushing, reasonable screening and storage reduce the impact.
1. Method of recovering RAP
There are two common methods for recycling RAP: one is to loosen the old road surface with the loosening equipment (Figure 1); the other is to use the milling machine for cold milling.
China's asphalt pavement surface layer is generally two or three layers. The old road loosening and crushing method will mix the three layers of mixed materials and recover it, which is difficult to be layered. Therefore, this recycling method is not conducive to the regeneration of the mixture, the scope of use is limited, and the material must be pretreated by using a crushing and screening device before regeneration.
The main advantage of this method of milling and recovering RAP by milling machine is that the asphalt mixture after milling does not need secondary crushing in one case; the disadvantage is that the content of fine aggregate is increased during the milling process. If the layered milling is not possible, the milling material needs to be pretreated with the crushing and screening equipment.
2 Common crushing equipment technical problems
Observing the RAP crushing and screening equipment used in the industry, the RAP is broken mainly by translation or directly using the ore crusher technology. Common methods are: hammer crusher, impact crusher, jaw crusher, roller crusher.
The hammer crusher is equipped with several weights on its transmission shaft at a certain interval. The rotation of the transmission shaft drives the impact force generated by the rotation of the weight to crush the RAP whose size exceeds the limit. The RAP that meets the size requirement will be from the gap of the weight. Passed in. The biggest advantage of the counter crusher and the hammer crusher is that the RAP that exceeds the size can be broken by the parameter adjustment. However, because the rotor speed is high, it is often easier to break the stone and destroy the original grade, and at the same time, because the speed is too high. The technical requirements for mechanical transmission and wear parts are relatively high, and the cost of maintenance at later stages is relatively high (Figure 3). Due to the extrusion crushing principle, the crocodile crusher is not suitable for the crushing processing of the mixture.
The working principle of the roller crusher is to bring the material entering the crusher into the crushing chamber by the friction between the rollers by the rotating roller (Fig. 4). The deeper the belt is, the distance between the rollers is deeper. The smaller, the purpose of breaking. The broken material is discharged from the bottom. The crushing process of the roller mainly relies on the pressing action and the shearing action. This method is most suitable for the characteristics of the RAP material, and it is not easy to destroy the grading of the material. And almost no dust is generated during the on-site work, and the roller speed is low, which effectively reduces the wear of the teeth.
It can be seen from the working principle of the above several crushers that, due to the lower rotation speed of the roller crusher, the degree of damage to the RAP grade is lower, and at the same time, due to the low rotation speed, the dust generation during the crushing process is less, and the equipment is less. The wear is slower, the use and maintenance costs are lower than others, so this crushing method is very suitable for RAP crushing.
Fujian Tietuo Machinery Co., Ltd. combines the design experience of RAP regenerative equipment for many years, and at the same time, the above-mentioned roller crusher has developed a crusher specially designed for RAP materials. In order to meet the different requirements of RAP crushing under different working conditions, Tietuo Machinery has developed two-stage crushing equipment for this purpose, one for single-roller and two for two-roller. Among them, the single roll crusher is very suitable for the super large RAP material recovered by the crushing machinery. The double roll crusher is suitable for small pieces of RAP material recovered by the crusher milling machine or processed by the primary crusher.
The main wear parts on the single roll crusher adopt the standard cutter head of the milling machine, which has good wear resistance and convenient replacement. The rear baffle of the crushing machine is provided with a Spring tensioning device. When the hard material cannot be broken, the baffle automatically pops open to prevent motor overload or mechanical damage (see Figure 5). It is more suitable for the super-fast RAP obtained by mechanical dig, so it is often used as a first-class crusher.
The single-roll crusher is also equipped with a remote control box to remotely control the start and stop of the single-roller primary crusher and the movement of the hydraulic pusher.
The double-roller crusher is provided with two sets of crushing rolls, and the crushing teeth on the top are arranged to cross each other (as shown in Fig. 7). One of the crushing rolls supports a spring tensioning device on the supporting frame, and the two geared motors are independently driven with sufficient power. .
The adjustment of the gap between the crushing rolls can be realized by adjusting the spring tensioning device, thereby realizing the adjustment of the particle size and the output of the crushed material. This gap can be flexibly adjusted according to the actual particle size of the waste asphalt mixture. At the same time, this function can prevent the instantaneous equipment damage caused by superhard foreign matter entering the crusher.
Compared with the conventional impact crusher, the roller crusher can effectively reduce the degree of destruction of the aggregate in the RAP. Reducing the content of fine materials in the finished product, since the rotation speed is much lower than that of the counter-attack type, the wear is small and the wearing parts have a long service life.
3 Screening equipment working principle and technical problems
Theoretically, if the specifications of the old material screening are narrower, the range of raw material particle size is narrower, and the degree of RAP segregation and variability in the feeding process is relatively reduced. It is recommended to select the sieve holes as follows: The grading range is divided into 4 to 5 types of materials, 4.75 mm or less, 9.5 to 4.75 mm, 13.2 to 9.5 mm, 13.2 to 19.0 mm, and 19.0 mm or more. The RAP material below 4.75mm has high asphalt content, high powder content, low aggregate content and highest regeneration value, which has great influence on the gradation of recycled mixture.
Common RAP screening equipment generally adopts the method of linear sieve, and the vibration frequency generally does not exceed 25Hz, but the experiment found that the existing screening technology for the RAP with particle size less than 10mm is lower due to the lower vibration frequency, the aggregate passes through. When the mesh is small, the kinetic energy is small, and the relative speed of the mesh is perpendicular to the direction of the screen surface, and the friction is small. Therefore, the asphalt on the surface of the RAP is easily adhered to the mesh, causing the mesh to gradually become smaller and sieved. Reduced efficiency. In response to this problem discovered and proposed by the customer, Tietuo Machinery has designed a high-frequency vibrating screen to minimize the problem of fine aggregate adhering to the mesh by increasing the vibration frequency during screening. At present, nearly ten sets have been sold. The test of the construction site, using the principle of the vibrating screen to increase the vibration frequency, combined with the anti-blocking screen technology, can solve this industry problem.
At present, the high-frequency vibrating screen designed by Tietuo Machinery has a frequency of nearly 50Hz, and is driven by an imported vibration motor. It is maintenance-free and can be equipped with a single-layer (or multi-layer) screen. The screen is spring-tensioned for easy maintenance and replacement. The structure is a high-frequency vibrating mesh screen, and the sieve body does not vibrate during operation, and only the screen mesh vibrates.
4 Conclusion
(1) The variability of RAP is an important factor affecting the proportion of recycled materials in the mixed reclaimed asphalt mixture, the addition of new asphalt, new aggregates and regenerants, and the quality and stability of recycled asphalt mixture. Measures such as secondary crushing, reasonable screening and storage of RAP reduce its impact.
(2) The roller crusher has a low spindle speed and low damage to the RAP grade. At the same time, due to the low speed, the dust generation is less during the crushing process, the equipment wears more slowly, and the use and maintenance cost is better than the counterattack. The hammer type and jaw crusher are low, so this crushing method is very suitable for crushing block or pellet RAP.
(3) Increasing the vibration frequency of the screen can increase the relative speed of the screen and the material in the direction perpendicular to the screen surface, increase the impact between the two, reduce the degree of adhesion of the RAP surface asphalt to the screen, and then cooperate Anti-blocking screen technology can solve the technical difficulties of RAP fine aggregate screening.
In short, as far as the current status of domestic pavement waste materials recycling technology is concerned, for engineering projects with RAP high reference regeneration technology requirements, suitable crushing and screening equipment must be provided to ensure the construction quality.
The combination of Tietuo Machinery and the design experience of RAP regenerative equipment for many years is a single-roller, two-stage two-roller crusher and its supporting high-frequency vibrating mesh screen. Suitable for material pretreatment process of high reference RAP mixture.
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