Costa-Rica Impact crusher

1. Working Principle and Applications of Impact Crushers

The working principle of an impact crusher is essentially the same as that of a hammer crusher; both utilise high-speed impact to crush materials. However, their structures and operating processes differ. The working principle of an impact crusher is illustrated in Figure 4.1. During the crushing process, the material follows a predetermined flow path between the first and second impact plates, undergoing repeated impacts over a specific duration and distance to achieve fragmentation. The grading screen below serves to determine the final particle size of the output. The initial crushing occurs under the impact of the hammer plates, followed by further fragmentation as the material is projected onto the impact plates; simultaneously, clusters of material collide with one another in mid-air to achieve pulverisation.

Compared with hammer crushers, impact crushers have the following differences:

1.1 In impact crushers, the impact plates and rotor are rigidly connected; the inertia of the entire rotor is utilised to impact the material, causing it not only to break but also to acquire significant velocity and kinetic energy. In hammer crushers, individual hammers strike the material to break it, resulting in limited velocity and kinetic energy being imparted to the material.

1.2 The crushing chamber of an impact crusher is larger, providing the material with sufficient space to move and ensuring it is fully subjected to the impact crushing action. The crushing chamber of a hammer crusher is smaller.

1.3 The impact plates of an impact crusher meet the incoming material from below to perform impact crushing, throwing it onto the upper impact plate. In contrast, a hammer crusher strikes the material in the direction of its fall.

1.4 Impact crushers generally do not have a screen at the bottom; the product particle size is determined by the speed of the impact plates and the clearance between them and the counter-plate or grading screen. Hammer crushers, on the other hand, rely on a screen to control the product particle size. Impact crushers possess a high impact crushing capacity and are increasingly widely used in industries such as building materials, metallurgy, mineral processing and chemicals. Impact crushers are particularly suitable for crushing brittle materials of medium hardness; they can be used for coarse, medium and fine crushing of materials such as limestone, coal, sandstone, cement clinker, iron ore, bauxite and molybdenum ore. Due to their significant crushing performance, impact crushers can replace jaw, roller and cone crushers in some small and medium-sized plants and mines.

2. Advantages and Disadvantages of Impact Crushers

Compared with other types of crushers, impact crushers offer the following advantages:

2.1 High crushing efficiency and low energy consumption, typically ranging from 0.5 to 1.3 kW·h/t. As the impact strength of materials is about ten times lower than their compressive strength, impact crushers consume one-third less energy than jaw crushers and between one-half and four-fifths less than roller crushers.

2.2 High crushing ratio, generally around 20, with some models reaching 50–60 or even higher. This reduces the number of crushing stages, simplifies the production process, saves on investment and lowers production costs.

2.3 The equipment has a simple structure, making it easy to manufacture, operate and maintain.

2.4 It features selective crushing, meaning that materials with high density produce smaller particles after crushing, whilst those with low density produce larger particles. This facilitates mineral separation.

2.5 The equipment is lightweight. It operates without significant unbalanced vibration and does not require a heavy foundation.

Disadvantages of impact crushers:

2.6 The hammers and impact plates wear out relatively quickly and require frequent replacement, particularly when crushing hard materials, in which case wear is accelerated.

2.7 During operation, there is significant noise and dust generation, and the final product may contain oversized particles. The equipment has poor adaptability to materials containing water or clay. In the southern regions during the rainy season, blockages are likely to occur, and clearing these blockages is extremely difficult.

3. Classification of impact crushers

Impact crushers can be classified into two types based on their structural characteristics: single-rotor and double-rotor.

3.1 Single-rotor impact crushers, as shown in Figure 4.2 (A–E), have a simple structure and are suitable for use in small and medium-sized plants and mines. They are classified by rotor rotation into unidirectional and bidirectional types. There are two structural configurations below the rotor: one with a grading screen and one without. Impact crushers equipped with a grading screen can control the particle size of the output, resulting in fewer oversized particles and a more uniform product size. The suspension points of the grading screen can be moved horizontally to adapt to various crushing conditions. The angle between its lower end and the rotor can be adjusted via an adjustment mechanism, thereby compensating for changes in the discharge gap caused by wear on the screen and hammers.

3.2 As shown in Figure 4.2 (F–1), twin-rotor impact crushers can be classified into three types based on the direction of rotation of the rotors.

3.2.1 Impact crushers with two rotors rotating in the same direction, as shown in Figures 4.2 (F) and (H), are equivalent to two single-rotor impact crushers operating in tandem, and can simultaneously perform coarse, medium and fine crushing operations. It offers a high crushing ratio, uniform product particle size and high production capacity, but also has high power consumption. The use of this machine can reduce the number of crushing stages and simplify the production process.

3.2.2 The impact crusher with two counter-rotating rotors, as shown in Figure 4.2 (G), is equivalent to two single-rotor impact crushers operating in parallel. It has a high production capacity, can crush large-sized materials, and can be used as a large-scale primary or secondary crusher.

3.2.3 The impact crusher with two counter-rotating rotors, as shown in Figure 4.2 (I), primarily utilises the mutual impact of material ejected from the two opposing rotors to achieve crushing. Consequently, it offers a high crushing ratio and relatively low metal wear. The specifications of an impact crusher are denoted by the rotor diameter D (m) and length L (m), i.e., DXL.

4. Main Components of Impact Crushers

4.1 Rotors

The structural forms of impact crusher rotors include integral, modular and welded types.

The rotors of impact crushers must possess sufficient mass to meet the requirements of crushing large-sized materials. Most crusher rotors adopt an integral cast steel structure; such rotors have high rotational inertia, are robust and durable, facilitate the mounting of impact plates, and can meet the demands of crushing operations. For small impact crushers, a welded steel plate structure may also be used. Such rotors are convenient to manufacture and easy to balance, but they have lower strength and durability.

4.2. Hammer Plates

The hammer plates of an impact crusher are all fixed to the rotor; they are a vital component of the crusher and must be securely installed, easy to replace, and manufactured from materials with good impact resistance. Domestically, they are predominantly forged from high-manganese steel, medium-carbon alloy steel, or bearing steel. Some manufacturers apply a layer of build-up welding using ‘UP WELDING 64’ or ‘UP WELDING 64A’ electrodes onto carbon steel impact plates, or use high-manganese steel electrodes to apply a layer of metal material onto impact plates made of high-manganese steel. The geometric shape of the hammer plates affects both the crusher’s production capacity and service life.

Figure 4.7 Structural configurations of impact plates

(a) Screw-fixed: (b), (c) Insertion-fixed: (d) Wedge-fixed: (e) Module and set screw-fixed

a. Fixed elongated impact plates: This configuration is currently the most widely used. However, as the bolts are exposed, they are prone to damage and have a short service life.

b & c. Impact plates inserted into a fixed rotor. Once worn, the plates can be turned over for reuse, thereby conserving steel. However, this design requires strict dimensional accuracy during manufacture; otherwise, disassembly and reassembly become difficult.

d. Impact plates comprising a guard plate and a striking plate. This structure is secured by wedges; no bolts are exposed on the striking plate surface, so only the guard plate needs to be replaced when worn. This complex structure is only suitable for large impact crushers.

e. Impact plates secured by modules and set screws. When the set screws are tightened, the impact plate is pushed outwards, interlocking tightly with the wedge surface. As the rotor rotates during operation, centrifugal force causes the hammer plates and modules to interlock more tightly with each rotation. This design is simple in construction, easy to maintain and replace, and operates reliably.

4.3 Impact Plates

The function of the impact plates is to withstand the impact and crushing of material ejected by the hammer plates, and to rebound the crushed material back into the crushing zone for further processing. The shape and structure of the impact plates have a significant influence on crushing efficiency.

There are many forms of impact plates, primarily of two types: angular and curved. Angular impact plates have a simple structure but cannot guarantee the most effective impact on the material; curved impact plates enable material blocks, after rebounding from the plate, to collide violently with one another at the centre of the arc, resulting in high crushing efficiency.

The rebound path is zigzag-shaped, with the material moving towards the discharge opening. This type is primarily used for the primary crushing of various friable materials. This configuration causes the material to return to the impact point in a backward motion during the crushing process, thereby increasing the number of impacts and yielding a finer particle size distribution.

Impact plates are currently manufactured predominantly from impact- and wear-resistant materials such as high-manganese steel.

5. Technical data