Summary：The processing technology of sand and gravel aggregates is the key to the treatment and utilization of tunnel slag, mainly including the selection of tunnel slag recovery, the selection and layout of sand and gravel processing systems, the processing technology of sand and gravel aggregates, wastewater treatment, dust and noise control, etc.
Utilization status of tunnel slag
1. What is tunnel slag?
Tunnel slag refers to the stone waste excavated during the tunnel excavation process.
2. Hazards of improper disposal of tunnel slag
During the excavation process of highways and high-speed railway tunnels, a large amount of tunnel slags are generated. Due to factors such as construction technology and organization, the tunnel slags cannot be reasonably utilized, and it is often necessary to build specialized spoil areas for disposal.
Occupy arable land and waste land resources
The arbitrary disposal of tunnel slags generated by tunnel excavation not only occupies a large amount of arable land, but also affects the function of the land, and the physical and chemical properties of the surface soil may change. At the same time, the residue of building materials may cause heavy metal pollution to the soil, significantly decrease the cultivability of cultivated land.
Increase the probability of flood disasters
The excavation of tunnel slag disturbs the surface area greatly, increasing the area of soil erosion that was originally severely eroded. If not treated and protected during the construction process, it will cause regional soil erosion and bring unstable factors to the safety of the main project, increasing the probability of flooding disasters along the river.
Economic resource waste
To meet the requirements of green construction, a large amount of tunnel slags generated during tunnel excavation needs to be treated. However, long-distance transportation not only increases project costs but also causes resource waste. Therefore, it is particularly important to properly treatment of abandoned tunnel slag in engineering.
3. Constraints on the preparation of sand from tunnel slag
The multi-deformation and non-selectivity of tunnel lithology
Compared with sand and gravel mine, the biggest drawback of using tunnel slag to produce machine-made sand is that the material is not selective. According to the planning schedule of the project, the slag is produced in the tunnel construction process, which means that the difference of the rocks may be relatively large, and the quality of machine-made sand is unstable. If the slag is produced by multiple tunnels, this situation will be more obvious.
Lack of reasonable evaluation of tunnel slag
Some engineering personnel may only have limited understanding of tunnel slag in terms of roadbed filling, and lack technical support and objective understanding of its application in concrete engineering, making it difficult to organize human, material, and financial resources to study and apply tunnel slag.
Lack of standardized processing technology
The composition of tunnel slag is complex, and the lithology of tunnel slag varies greatly in different regions. Currently, there is no standardized treatment plan and process, and customized treatment plans need to be designed based on the specific situation of different sites.
Applications of tunnel slag
1. making machine-made sand
According to the utilization principle of tunnel slag, the slag with higher strength can be preferentially used in the production of machine-made sand.
2. making rubbles
The secondary hard stone in tunnel slags can be considered for making rubbles, which can be used in pavement base, subbase or bridge and tunnel structure.
3. permeable materials
Soft stone and some secondary hard stone excavated from tunnel can be used for subgrade filling or permeable materials (slag breaking and slag cleaning) of roadbed and soft foundation.
4. subgrade filling
Tunnel excavation earth can be used for subgrade filling.
Key technologies for preparing sand and gravel from tunnel slag
The process of tunnel slag sand production mainly includes: analysis of the type and grade of tunnel surrounding rock → selection of tunnel slag recovery → supply and demand analysis of tunnel slag and sand stone → comparison and selection of sand and gravel processing sites → design of sand and gravel processing technology → selection of sand and gravel equipment → construction of sand and gravel processing sites, installation of equipment → quality inspection of sand and gravel aggregates → adjustment of equipment.
The processing technology of sand and gravel aggregates is the key to the treatment and utilization of tunnel slag, mainly including the selection of tunnel slag recovery, the selection and layout of sand and gravel processing systems, the processing technology of sand and gravel aggregates, wastewater treatment, dust and noise control, etc.
1. Analysis of tunnel surrounding rock types and grades
The rock type of surrounding rock is the key factor to determine whether the sand and gravel can be prepared. The grade of surrounding rock is mainly determined by the degree of fragmentation of tunnel slag and the type of surrounding rock. The surrounding rock with high strength can be used to prepare sand and gravel.
2. Selection of tunnel slag recovery
Tunnel slag has the following characteristics:
(1) The tunnel slag may come from different parts or units of the engineering project, and the fluctuation of lithology, compressive strength, weathering degree, etc. increases the diversity and complexity of the parent material, making it difficult to ensure the quality and stability of the parent material.
(2) There are many impurities such as mud and soil in the tunnel slag, and the cleanliness is low. Therefore, appropriate measures for removing impurities and soil need to be taken.
(3) The main method of engineering excavation is blasting. During tunnel excavation, due to the influence of the cross-sectional design size, the blasting surface is small and the blasting points are concentrated, resulting in a smaller average size of the blasting slag, with more powder and thicker powder coating.
According to the characteristics of tunnel slag, if all of them are mixed and stacked in the slag yard, it will cause instability of the parent material. Preliminary screening and classification are needed to minimize the quality fluctuation of the parent material from the source.
Effective measures to improve the quality of tunnel slag parent rock:
Firstly, before excavation, compare the on-site construction measurement data and the geological survey data to determine the corresponding lithology, strength, and weathering degree of different excavation parts, as well as whether they can be used as raw materials for preparing sand and gravel aggregates, so as to select the tunnel slag from the source.
Then, during the excavation process, appropriate screening is carried out on the tunnel slag, such as selecting rocks with good performance and high strength for processing sand and gravel aggregates. The excavated slag materials from crushed zones, muddy formations, and weak formations are not used for the preparation of sand and gravel aggregates.
Finally, the tunnel slag transported to the slag yard is classified and stacked according to its quality to ensure that the quality difference of the slag in the same pile is minimized, the performance is more stable, and it is easy to classify, process, and utilize.
3. Site selection and layout of sand and gravel processing system
There are mainly two kinds of sand and gravel processing systems: fixed and mobile. Currently, large and medium-sized systems mostly use fixed types. For small scaled sand and stone processing systems in linear engineering (such as railways, highways, etc.), mobile types should be used.
The mobile sand and gravel processing system adopts modular assembly, which flexibly combines crushing, screening, and sand making processes into one. It can be quickly transferred to production along with the project schedule and shorten the transportation distance between the various processes.
The site selection and layout of the sand and gravel processing system should comprehensively analyze the source of raw materials and the location of the mixing plant. Based on regional characteristics, surrounding environment, site size (considering a certain amount of finished material storage and tunnel slag storage), system scale and form, production process, and other factors, the ideal location should be selected from available sites, and reasonable planning should be carried out to meet the requirements of advanced technology, convenient construction, reliable operation, and good economy, safety and environmental protection.
4. Sand and gravel aggregate processing technology
The preparation of sand and gravel aggregates from tunnel slag involves crushing, screening, and sand making, with the main process being "more crushing and less grinding, replacing grinding with crushing, and combining crushing and grinding". The characteristics of the processing material directly affect the design of the sand and gravel aggregate processing process.
The number of crushing sections should be determined according to the lithology, hardness, feed particle size, required processing capacity of the tunnel slag, and combined with other factors for comprehensive analysis.
For rocks that are difficult to crush and have strong abrasiveness, such as basalt and granite, a 3-stage crushing process is usually used. For coarse crushing, jaw crusher or gyratory crusher is often used. For medium crushing, medium-sized cone crusher with a relatively large crushing ratio is used, while for fine crushing, a short head cone crusher is used.
For medium or fragile rocks like limestone and marble, two-stage or three-stage crushing process can be used. For coarse crushing, we can adopt impact crusher or hammer crusher that has relatively large crushing ratio. For medium and fine crushing, we recommend choose impact crusher or cone crusher.
There are three forms of crushing processing: open circuit, closed circuit, and segmented closed circuit:
When adopting open-circuit production, the process is simple, there is no cycle load, and the workshop layout is relatively simple, but the flexibility of grading adjustment is poor. After balancing, there may be some waste materials;
When adopting closed circuit production, the aggregate grading is easy to adjust, and the workshop layout is relatively concentrated. However, the process is complex, the cycle load is large, and the processing efficiency is low;
When adopting segmented closed circuit production, the adjustment of aggregate gradation is flexible, the cycle load is relatively small, but the number of workshops is relatively large, and the operation management is relatively complex.
Screening is the key factor to control the particle size of sand and gravel aggregates, and the tunnel slags are screened and graded after being crushed. The configuration of the vibrating screen should be determined based on the mud content, washability, required processing capacity, grading of the screened raw materials, discharge requirements, etc.
When calculating the screening processing capacity, the fluctuation of feed volume should be taken into account. Multi-layer screen should be calculated layer by layer, and the model should be selected according to the most unfavorable layer and the thickness of the material layer at the discharge end should be checked. It is required that the thickness of the material layer at the discharge end of the screen should not be greater than 3-6 times the size of the mesh hole (the smaller value should be taken when used for dehydration).
1) Sand making process
The production process of sand and gravel aggregates includes three methods: dry method, wet method, and a combination of dry and wet methods.
(1) Wet method production: suitable for situations when the raw materials contain too much mud or soft particles, and the content of fine aggregate stone powder is relatively high. Wet method production can be used to remove some stone powder.
The advantages are high screening efficiency, the surface of aggregate is clean, and no dust during the production process; the disadvantages are high water consumption, difficulty in wastewater treatment, severe loss of fine aggregate and stone powder, and difficulty in dehydration.
(2) Dry method production: mainly suitable for clean raw material and sand processing system with low sand formation rate of fine aggregate and low stone powder content.
The advantages are low water consumption, low loss of stone powder, and low or no wastewater treatment.
The disadvantage is that the dust is generally large, and the areas with high dust need to be sealed and equipped with dust removal equipment. When the raw material contains water, the fine aggregate is not easy to screen through.
(3) Dry and wet method combined production: generally refers to the production process that combines wet method production of coarse aggregate and dry method production of fine aggregate. This production method is mainly suitable for sand and gravel processing systems with high mud content in raw materials and low content of fine aggregate and stone powder.
The advantage is that it combines the advantages of dry and wet production, with less water consumption, less wastewater treatment, clean surface of coarse aggregate, less loss of fine aggregate stone powder, and less dust.
The disadvantage is that the raw materials need to be dehydrated before entering the vertical shaft impact crusher after being washed with water (the moisture content of the raw materials is generally not greater than 3%, otherwise it will seriously affect the sand making effect).
2) Sand making equipment
The selection of sand making equipment should be determined based on the characteristics of the material source, regional characteristics, production process, and discharge requirements. The mainstream sand making equipment in the current market are vertical shaft impact crusher and tower-like sand making system. Customers can also choose mobile crushing sand making equipment according to project progress and site conditions etc.
1. vertical shaft impact crusher
The VSI6X series vertical shaft impact crusher has optimized the structure of the crushing cavity, equipped with "rock on rock" and "rock on iron" crushing forms, and the "rock on rock" material lining and "rock on iron" impact block structure are specially designed according to the working state of the equipment, significantly improving the crushing efficiency of the equipment.
Generally, when the raw material is difficult to crush and has strong abrasiveness, the "rock on rock" crushing method should be chosen; when the raw material is medium fragile or fragile, and the abrasion is medium or weak, the "rock on iron" crushing method should be selected.
2. tower-like sand making system
The tower-like sand making system is a new type of sand making method and also a trend in the future development of the machine-made sand industry. In order to solve the problems of unreasonable grading, high powder and mud content, and substandard particle size of traditional machine-made sand, the VU Integrated Sand Making System adopts grinding technology and waterfall shaping technology, which makes the finished sand and gravel has reasonable grading and round particle shape, effectively reducing the specific surface area and porosity of coarse and fine aggregates. At the same time, adopting dry powder removal technology makes the powder content in the finished sand adjustable and controllable.
VU Integrated Sand Making System occupies a small area, adopts fully enclosed transportation, production and negative pressure de-dusting design, with low noise, no sewage, sludge and dust discharge, and meets the national environmental protection requirements.
3. mobile crushing and sand making machine
The K3 series mobile crushing and sand production line is equipped with a new type of host equipment, with full and strong speed and power, and stable and reliable operation;
Equipped with a sled type automatic lifting foundation, it allows for quick transfer and convenient installation;
After switching modes, it can also be used as a fixed line, making it an ideal choice for tunnel slag treatment.
5. Environmental protection measures
Sedimentation and solid-liquid separation are commonly used to treat the wastewater discharged in the process of sand and gravel processing.
Sedimentation treatment generally has two stages: pre sedimentation and sedimentation. The investment of this method is small and the operation is simple, but it covers a large area and is susceptible to climate restrictions.
In the solid-liquid separation method, the discharged wastewater is first put into the enrichment tank for concentration, and the waste slag that has reached a certain concentration is dehydrated mechanically. The overflow water of the enrichment tank enters the sedimentation tank for clarification. This treatment method occupies a small area and is not affected by climate conditions. The recycling rate can generally reach over 70%, but the engineering investment is relatively high.
At present, the wastewater treatment of sand and gravel processing systems usually adopts a combination of two methods: first separating a portion of coarse particles by sedimentation, and then using mechanical methods for dehydration after concentrating the fine particles. It can ensure the normal operation of the wastewater treatment system while also controlling costs.
The dust in the sand and gravel processing system mainly comes from crushing, screening and grading, material transfer, and feeding chute stage, which not only pollutes the environment but also affects the physical health of operators and surrounding residents. Generally, water spray dust removal, biological nanotechnology dust suppression and dust collection equipment are combined in the system.
The main measures for noise control in the sand and gravel processing system include:
- Select low noise equipment to reduce noise intensity;
- Select appropriate noise reduction materials to reduce noise;
- Use soundproofing materials to block transmission pathways or reduce noise intensity during transmission;
- Use personal protective equipment against noise, etc.
Analysis of mix ratio of concrete with tunnel-slag sand
1. Selection of preparation strength and water cement ratio
The strength and water cement ratio of machine-made sand concrete should meet relevant regulations.
2. Determination of unit water consumption
Compared to river sand concrete, machine-made sand concrete requires more water to achieve the same slump.
3. Determination of unit cement consumption
When preparing machine-made sand concrete with lower grade (C30 and below), to achieve required strength, the cement consumption does not need to be increased compared to river sand concrete.
4. Selection of sand rate
The selection of sand rate for machine made sand concrete is usually 2% -4% higher than that of river sand, or even higher. Due to the factors such as the gradation, appearance particles, fineness modulus, and stone powder content of the machine-made sand itself, the specific value need to be determined through further experiments.
Cases of tunnel slag treatment
1. Preparation of sand from tunnel slag of Chengdu-Kunming Railway
The main rocks in tunnel slag of this project are basalt and limestone. And this project is close to water source, there is sufficient water for production use.
1 vibrating feeder, 1 jaw crusher, 1 cone crusher, 1 vertical shaft impact crusher, 2 vibrating screens, 10 conveyor belts, 1 set of electrical cabinet and cable, 1 set of sand washing equipment, and 2 loaders.
①Considering that the tunnel requires 5~10mm gravel for shotcrete, the gravel is designed in 3 grading, with sizes of 5~10mm, 10~20mm, 16~31.5mm, and machine-made sand less than 4mm.
The mesh sizes are 4mm (steel mesh screen), 6mm (nylon mesh screen), 12mm (nylon mesh screen), 21mm (nylon mesh screen), and 32mm (steel mesh screen).
②Undersize material from 4mm mesh sized screen is machine-made sand. Adjust the speed of the sand making machine (the speed of the sand making machine is 1200r/min) to control the fineness modulus of the machine-made sand; Adjust the way of water volume of the sand washing machine to control the sand grain shape and stone powder content.
Practice has shown that increasing the content of stone powder can reduce the fineness modulus. However, in actual use, due to the large amount of stone powder and the excessive viscosity of sand, it is difficult to discharge the material from the batching hopper, and manual cleaning are required during batching.
③The 4~6mm gravel returns to the sand making machine, reduces content of particles below 5mm in the 5~10mm gravel, the particles on 6mm mesh sized screen is 5~10mm gravel, the particles on 12mm mesh sized screen is 5~10mm gravel, the particles on 21mm mesh sized screen is 16~31.5mm gravel.
2. Sand preparation from tunnel slag of Jiande-Jinhua Expressway
The surrounding rock of tunnels along the line is mainly tuff.
Raw material: tuff, tunnel slag
Production capacity: 260t/h
Equipment configuration: F5X vibrating feeder, PEW jaw crusher, HST single cylinder hydraulic cone crusher, VSI5X sand making machine, S5X vibrating screen and other supporting devices.
Finished sand and gravel: 0-5, 5-10, 10-20, 20-28mm
High quality: The high-end intelligent crushing and sand making equipment is the highlight and core of the entire project. The advanced hydraulic control technology and mature production process in the crushing section ensure the efficient and stable operation of the entire project; the finished machine-made sand produced by the sand making section has adjustable particle size distribution and controllable mud content, which can effectively improve the engineering quality.
High intelligence: This project is equipped with PLC control system, which can observe and control the operation status of the entire production line. The intelligent production workshop not only facilitates production operations, but also reduces manpower expenditure, which is conducive to project cost control.
High benefit: The project plans to use 250,000 cubic meters of machine-made sand. Calculated according to the market price of the project at that time, the market price of natural sand is as high as 280RMB per square meter, and the market price of mechanical sand is as high as 100RMB per cubic meter, with a difference of 180RMB per cubic meter. The cost can be saved about 45 million RMB with significant indirect economic benefits.