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Design criteria and equipment selection skills of sand and gravel production line

1. Scheme design of crushing sand making line

The scheme design mainly includes three stages: process design, plane layout design and equipment selection design.

1.1 process design

Under the condition that the requirements of system feed and final finished product are very clear, the process route to realize crushing and screening can be multi scheme. The number and type selection of equipment selected in different schemes are different, so the initial investment cost and future operation cost of scheme implementation will be different. Designers, investors and operators must fully discuss and be practical, Weigh the advantages and disadvantages to determine the better process scheme.

1.2 layout design

When the main equipment determined according to the process flow design is arranged in the plane according to the user’s terrain, the following aspects shall be considered:

(1) Distance between raw material mine and feed inlet of production line, feed inlet site and drop height, equipment layout site, stockyard and material output mode;

(2) Under the condition of smooth material flow, set as few and short belt conveyors as possible;

(3) Meet the design of intermediate stockyard and finished product stockyard for operation and product transportation, and make full use of the site;

(4) The operation and maintenance of machinery and the operation position and communication of electric control are convenient.

After the plane layout design is completed, preliminarily determine all equipment, including transportation equipment, storage equipment, electrical control, etc.

1.3 equipment selection and design

There are three types of combined crushing and screening equipment: fixed, semi mobile (or sled) and mobile. According to the moving mode, the mobile crushing station is divided into tire type and crawler type (self-propelled). These three types can be used completely independently or mixed. For example, the primary crushing unit is mobile, which is convenient for the nearby crushing of the feed from multiple ore sources, and then transported to a fixed place by belt conveyor, while the secondary, tertiary crushing and screening units are fixed. The type of a gravel yard shall be determined according to the frequency of equipment movement during the operation of the gravel yard. Self propelled equipment is suitable for particularly frequent situations. The most expensive ones are tire type and semi mobile type. The advantages are that these types of equipment have short installation cycle, less civil work and fast operation.

2. Design criteria and equipment comparison of crushing and sand making line

Various sand and gravel yards are completely different in terms of rock type, treatment capacity and requirements for sand and gravel products. Therefore, the crushing and screening equipment selected in the design is also different.

2.1 initial breaking unit

(1) At present, there are three types of primary crushers: jaw crusher, counterattack crusher and cycle crusher.

As the initial breaking, impact breaking is only applicable to the treatment of medium soft rock, such as limestone, so its application scope is limited.

The maximum allowable side length of large-size jaw crusher can be up to 1m, which has become the most used model of primary crusher. The selection depends on two items: * is whether the maximum allowable feed particle size meets the requirements; The second is to determine whether the processing capacity of the discharge port size under the discharge particle size meets the system requirements.

(2) Whether a feeder or bar screen is set before the initial breaker depends on the scale of the production line. The reasons are as follows:

① Since the jaw fracture is not allowed to be started in full, and the feeder can be started with load, the feeder controls the feeding in the previous process. Once the feeder is shut down abnormally, the storage of jaw fracture can be reduced and easy to recover;

② The feeder changes the feeding of intermittent dump trucks and loaders into continuous feeding to jaw crusher, reduces the fluctuation of jaw crusher load, and is conducive to prolonging the service life of the machine;

③ Often, the size of truck feeding is uneven, sometimes large and sometimes small. When there are many large pieces of feeding, the jaw crusher has a large load and the crushing speed is slow. On the contrary, it is fast. The feeder can adjust the feeding speed so that the jaw crusher can feed less when the load is large and more when the crushing speed is fast, which is also conducive to the improvement of average processing capacity.

(3) Generally, there are four types of feeders to choose from: bar screen, chain plate conveyor, motor vibration feeder and inertia vibration feeder. The chain plate conveyor is heavy and expensive. The allowable feeding of motor vibration feeder is small, and neither of them is equipped with screening device, so the scope of use is limited.

(4) The inertial vibrating feeder is usually installed horizontally, and the required drop height is less than the bar screen, so it is suitable for use in the primary breaking unit.

(5) The feed hopper of the feeder is not only matched with the feeder, but also determined by the feeding mode of the user. The dump truck usually adopts end feeding, while the loader adopts side feeding. Its feed hopper design is different, and the effective volume of the feed hopper shall be 1 ~ 1.5 times greater than that of the feed truck body.

2.2 there are three main types of secondary crushing equipment of secondary crushing unit: fine crushing, jaw crushing, cone crushing and impact crushing.

(1) In the past, fine crushing was common in small and medium-sized sand and gravel yards. Due to the small processing capacity and too many needle and flake materials in the discharge, it has been gradually replaced by cone crushing and counterattack crushing.

(2) Due to the large crushing ratio and less needle and flake particles, impact breaking has been widely used in sand quarries, especially highway pavement quarries in recent years.

The impact crusher has two significant weaknesses:

First, under the same processing capacity and similar particle size of incoming and outgoing materials, its installed capacity will be greater than that of cone crushing and jaw crushing, because it mainly adopts impact crushing, and the blast effect will cause large invalid energy loss during high-speed rotation;

Second, the wear of vulnerable parts is fast. Under the same treatment conditions, it is often more than three times shorter than that of cone crusher and jaw crusher, and the operation cost is high.

In addition, it has two other characteristics: first, there are many fine particles in the discharge, which is popular in some applications, such as manual sand making, while it becomes a disadvantage in others; The second is its selective crushing function. Its crushing force can be controlled through transmission power, rotor quality and speed, so as to choose to crush softer materials without crushing hard materials, which is convenient for subsequent separation.

(3) Cone crusher is a secondary crusher widely used in sand and gravel yards at home and abroad. Its different specifications and different cavity shapes of the same specification can meet the requirements of different treatment conditions, better close to the needs of the process flow, and it has stable operation and long service life of vulnerable parts. There are two weaknesses of cone breaking:

First, the operation is relatively complex. No matter what kind of cone break, it has a hydraulic and lubrication system to adjust the running state and cool the heating of the bearing;

Second, when crushing some materials (such as metamorphic rock), due to the large crack anisotropy of the rock itself, the percentage of needle and flake discharged is high.

2.3 three breaker unit

The commonly used three breaking units are cone breaking (short head type) and vertical shaft impact breaking (sand making machine).

(1) When the total crushing ratio of the whole crushing and screening combined equipment is large, the second stage crushing cannot be achieved, and the third stage crushing will be designed. For the cone crusher, the second crusher usually adopts the standard cavity type, while the third crusher adopts the short head cavity type.

(2) Vertical shaft impact crusher (sand making machine) has developed rapidly and has become a common equipment for sand making, shaping and three breaking. By adjusting the rotor structure, rotating speed and motor power, the discharge particle size can be controlled. The rock flow is particularly smooth and the processing capacity is large. Vertical shaft impact crusher is not only a kind of sand making machine, but also has become a development trend in tertiary crushing and even secondary crushing.

2.4 for the pre screening unit and the finished product screening unit, in the graded crushing process, the pre screening machine inserted in the middle of the front and rear crushing processes has two functions:

First, it can reduce the processing capacity of subsequent crushing process. The pre screening machine separates the materials whose discharge capacity after the previous crushing is less than the particle size of the subsequent crushing discharge, so as to reduce the proportion of fine particles in the subsequent crushing discharge;

Second, some large-scale product materials can be obtained by screening. Because the price of vibrating screen is lower than that of crusher, “more screen and less breaking” is a common method in * design. The working condition of the pre screening machine is characterized by large feed particle size and large throughput, so the screen mesh is also large, and the screening efficiency is not required to be very high (and it is not easy to produce material blockage). Therefore, in addition to the circular vibrating screen, the equal thickness screen and resonance screen can also be selected. The finished product screen is used for screening and grading the product materials in the sand quarry. Whether the screen is clean or not directly affects the product quality of the sand quarry. Generally, the fixed screening efficiency is more than 90%, and the screen mesh is set according to the particle size of the finished material. In addition to the circular vibrating screen, the triaxial elliptical screen can also be selected.

2.5 the machine-made sand products of the cleaning unit must be washed by water. The cleaning of sand and stone products can remove the mixed soil and other impurities, and control the content of fine powder. The cleaned sand and stone as concrete aggregate can improve the quality of concrete and reduce the amount of water. Therefore, it will be more and more common to use cleaning units in sand and gravel yards. There are two methods for sand and stone cleaning: if only the fine powder in the finished material is controlled, it can be cleaned on the vibrating screen. The cleaned water enters the sand and stone cleaning machine together with the fine particle material smaller than the lower screen to separate the water and fine powder from the sand and stone to obtain the required sand and stone. Water and fine powder are recycled after being separated by sedimentation and dehydration. It can also be cleaned in the sand and stone washing machine (i.e. not on the vibrating screen). At this time, according to the amount of fine powder in the finished material, the speed of the sand and stone washing machine and the overflow water volume shall be controlled to control the washing and storage amount of fine powder. If the clay adhered to the sand and stone is mainly cleaned, the clay adhered to the stone must be scraped off with a gravel or rock cleaner before the stone is crushed to fine powder, so as to ensure the quality of the sand and stone after subsequent crushing and screening. This kind of equipment is usually set before the vibrating screen, which is cleaned before screening.

When controlling the amount of sand and stone cleaning and recovering fine powder as much as possible, the external sand and stone yard adopts a hydraulic classifier, which is added between the vibrating screen and the sand and stone cleaning machine to adjust the gradation of sand and make it meet relevant standards. It is rarely used in China in this regard. A large area of three-stage precipitator must be prepared to recover a large amount of lost fine powder, or a large-scale dehydration and recovery equipment must be prepared, otherwise its discharge will cause great environmental pollution.

2.6 intermediate silos and finished product stockyards are large-scale sand and gravel yards. In order to improve the operation rate, intermediate silos are often set between the primary crusher and the secondary crusher. The intermediate silos can store a considerable amount of materials, so that the whole system is divided into front and rear sections. Advantages of intermediate silo: 1) when the equipment in the current section cannot operate normally due to mining reasons, transportation reasons or maintenance equipment, the equipment in the later section can operate normally for several hours or even days by relying on the inventory of intermediate silo. 2) The operation time can also be separated. Generally, due to the large feed block of the mine and the large equipment configuration specification, the production time does not need to be too long to meet the daily output, while the subsequent equipment does not need to be configured too large, and the daily start-up time can be increased. In this way, the existence of intermediate silo can make the front and rear sections adopt different operation time.

There are many structural forms of intermediate stockyard. The common form is to use the ground to pile materials, excavate underground passages, and use feeders and belt conveyors to transport materials from underground. Due to the limitation of terrain and investment, the output stored for 1 ~ 2 days is generally suitable. The capacity of the stockyard for finished products of different specifications shall be proportional to the percentage of the finished products in the total output. The layout of the finished product stockyard depends on the user’s finished product output mode, such as loader + dump truck, which is different from a general conveyor belt to the wharf for loading or railway loading.

2.7 electric control equipment

The driving of combined crushing and screening equipment varies due to different types: the self-propelled crushing station basically adopts the driving mode of diesel engine + hydraulic station, that is, the main engine is directly driven by diesel engine, and other equipment such as feeder, vibrating screen, belt conveyor and traveling mechanism are hydraulically driven, which is equipped with electric control equipment in this driving mode. The above methods may be adopted for the mobile tire crushing station, or the power supply method of diesel generator set may be adopted. Fixed or semi mobile combined equipment, in addition to diesel generator set, adopts power grid for power supply.

The common feature of all kinds of crushers is that the static inertia of moving parts is very large, so its motor has large installed capacity and large starting current. Foreign countries basically adopt soft start mode to reduce the impact on the power grid and protect the motor. The whole set of combined equipment includes more than a dozen motors, voltage and current control of host motor, variable frequency speed control of feeder, etc. From the electrical interlocking between the main motor of a single equipment and the lubricating hydraulic equipment in the control of temperature and pressure to the control of the equipment switching program before and after the whole line, it needs to be implemented by electronic control setting.

Post time: Aug-17-2021

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