According to the requirement of green design of concrete mixing plant, the design concept and the way to realize the green process of concrete mixing plant are analyzed, and the recycling and utilization of wastewater, the treatment and application of solid waste, and the measures of dust and noise suppression in the concrete production process are described, which provide reference for the green design and production of concrete mixing plant.
Resolutely fight the battle of blue sky, accelerate the improvement of the ecological environment, control the concentration of fine particulate matter (PM2.5) in key areas, not only to Jinshan Yinshan Mountain, but also to green water and green mountains, is the people’s urgent desire, but also the inherent requirements of sustainable development. Based on this, different regions have issued documents to regulate the green production of concrete enterprises, the deadline for the environmental protection of the existing mixing plant renovation and upgrading of the new mixing plant set up environmental protection design access threshold, the concrete industry into a new era of green, environmental protection and sustainable development. The concepts of “green low-carbon”, “energy conservation and emission reduction” and “sustainable development” have become the environmental coordination development concepts for the greening and environmental protection renovation of traditional mixing plants and the greening and environmental protection of new concrete mixing plants. The entry point of the mixing plant.
In the mid-1980s, with the rapid development of the national economy, under a series of national policies and measures to support, the concrete mixing plant has been developed rapidly and production efficiency has been improved, but at the same time, the concrete mixing plant has gradually become a typical representative of the “dirty, messy and poor” image of wastewater flow and dust. In 2017, localities issued the Blue Sky Action implementation plan notice, requiring the implementation of urban dust pollution prevention and control projects, special actions to prevent and control dust pollution, requiring concrete mixing plants to strictly implement construction dust pollution prevention and control measures, strengthening the renovation of concrete mixing plants and long-term supervision, so that dust pollution prevention and control of the source gatekeeper, process control, end governance of the whole process of supervision mechanism. According to the principle and tenet of “no dust pollution, low noise production and zero waste emission”, the country has ushered in the climax of environmental protection renovation, upgrading and construction of concrete mixing plant.
From the perspective of the development of the existing mixing plant industry, the environmental issues involved in the construction of new and rebuilt concrete mixing plants include.
(1). Dust pollution, wastewater discharge and noise pollution from the production process.
(2). Overconsumption of non-renewable resources and irrational use of recycled aggregates and waste resources.
(3). Construction waste, such as waste wet concrete, cannot be effectively utilized and treated in a timely manner.
This paper focuses on the recycling and utilization of wastewater, the treatment and application of solid waste, as well as the dust and noise control measures (hereinafter, dust, wastewater, solid waste and noise of the mixing plant are referred to as “four wastes”).
Source and treatment of the four wastes of the mixing plant
- Wastewater recycling and utilization
(1). Domestic sewage. If there is no municipal sewage network around the mixing plant, it should be discharged into the self-built septic tank and disposed by professional units on a regular basis.
(2). Production wastewater. The production waste water comes from the mixing truck churning, and the production waste water is discharged into the septic tank.
Mixing tank cleaning wastewater, mixer cleaning wastewater. The mixing transport truck and mixer should be cleaned after use to prevent the inner wall from caking and affecting production and re-use. The wastewater containing sand, stone and cement slurry is poured into the feed hopper of the sand and stone separator after the mixing truck is poured into the washing platform and rolled with water. The overflow slurry is diverted to the slurry recovery system through the drainage ditch, and then reused in production through the step-by-step sedimentation and clarification of the slurry recovery system. The recycling of production wastewater of the mixing plant realizes zero discharge of wastewater and waste residue, which not only can avoid waste of resources, but also can reduce the actual production cost of the concrete mixing plant, make up for the shortage of large area due to many sedimentation ponds, so as to meet the requirements of green environmental protection mixing plant. Zhang Dongsheng pointed out that when no more than 4% of wastewater with a concentration of 30% was used in the production of C30 and below low-strength concrete, the slump loss of fresh concrete is slightly larger and the strength of test blocks is comparable to that of concrete produced with all clear water.
(3). Site wastewater. The site wastewater comes from rainwater, site wash water, and vehicle exterior wash water. The wastewater is diverted to the wastewater recycling system through the drainage ditch in the plant, and the wastewater is used for greening of the plant, washing of the site, washing of the exterior of vehicles or for production after passing the test.
When the production wastewater and site wastewater are used for concrete production instead of other concrete mixing water, the water quality should meet the requirements of the current industry standard JGJ 63-2006 “Concrete Water Use Standard”, and the mixing amount should be determined by trial mixing of concrete.
In summary, reducing the discharge of waste water and waste materials of concrete mixing plant and realizing the recovery and recycling of sand, stone, production wastewater and site water can save resources, reduce emissions, lower production costs and indirectly improve production and operation benefits.
- Dust source and treatment
(1). Powder spillage dust. The dust mainly appears in the powder conveying process, powder conveying truck to the powder tank after the end of the powder tank conveying interface, powder tank top of the dust. When the power source of powder conveying is high pressure air, the pressure in the silo is higher than the atmospheric pressure, and the compressed air is released through the filter layer of the dust collector on the top of the silo. When the filter element is blocked by dust or the ventilation is not smooth, the compressed air in the silo is unloaded by the safety relief valve, and when the safety valve is opened, the air with dust runs out from the safety valve, causing dust on the top of the silo. When the dust collector filter is blocked and the powder is pumped, the pressure safety valve will fail, the pressure in the warehouse will rise, and the weak part of the warehouse top will be deformed or destroyed due to high pressure, and the powder will be sprayed from the tank, forming a big pollution source and causing the burst warehouse accident. Therefore, in order to ensure safe production, set material level indicator in the powder silo, cooperate with the safety valve to protect the silo pressure in a safe range, to avoid the damage of the silo and its accessories, to prevent explosive silo accidents. In addition, the correct use of dust collector equipment, strengthen the maintenance and repair of dust collector equipment can also reduce the generation of dust: before conveying the powder, start the vibrator of dust collector on the top of the tank for 1~2 min, vibrate off the dust collector filter on the accumulated dust; after conveying, open the vibrator of dust collector on the top of the tank for 1~2 min, vibrate off the accumulated dust; in addition, the dust collector filter and safety valve should be cleaned regularly to protect the dust collection effect and equipment safety.
(2). Weighing equipment dust. Dust from weighing equipment includes dust from powder weighing and aggregate weighing. Dust collector should be installed at the powder weighing equipment, and should be checked, cleaned and maintained regularly to reduce the dust generation. Dust at the aggregate weighing equipment can be sprayed before weighing equipment to suppress dust.
(3). Aggregate yard and site dust. Aggregate yard and site dust, including sand and gravel aggregate unloading and transportation of dust, loading and unloading vehicles through or wind when the ground area dust dust, the presence of solid waste on the site after the surface drying dust and dust generated by the site vehicle driving excitement. The way to deal with the dust is to use high-pressure fog spray equipment, install high-pressure spray system on the top of the aggregate yard plant, reasonable arrangement of fog pile in the plant. Both use high-pressure pump to spray water atomization from the nozzle, atomized particles floating in the air, and dust particles in the air fusion, increase the dust weight, accelerate the settlement of dust, to increase air humidity and dust reduction role, the device has the advantage of energy and water saving, will not cause secondary pollution. Mobile sweeping and sprinkling equipment is installed in the site to clean the dust in the site. Professional washing facilities for mixing vehicles are installed in the plant to wash the chassis and wheels of the vehicles in and out of the mixing plant to prevent the vehicles from bringing in or taking out mud.The slurry and other pollution caused.
In summary, the mixing plant should strictly implement dust pollution prevention and control work to reduce environmental pollution and improve environmental ecological quality.
- Treatment and Application of Solid Waste
The solid waste of the mixing plant comes from the crushed concrete specimens after strength testing in the laboratory, the condensation waste cleaned inside the mixer, the condensation waste cleaned inside the tank of the mixer transport truck, and the hardened solid waste cleaned in the sewage settling tank. For these solid wastes, suitable crushers can be selected to crush and process them into aggregate resources with a particle size of 5~31 mm, which can be effectively used in concrete production. Wu Ziqiang et al. showed that it is feasible to crush the waste concrete specimens and make recycled aggregate to replace the crushed stone in ordinary concrete, and the strength of the prepared concrete can still meet the requirements.
In summary, the processing and reuse of solid waste has achieved the triple effect of environmental protection and energy saving, as well as cost and resource saving.
- Treatment of noise
The noise from concrete mixing plant includes the noise from vehicles such as on-site powder delivery trucks, loaders and concrete trucks, and the production noise from mixers and air compressors.
As the biggest noise source of the mixing plant, the sound of the powder conveying compressor can be more than 100 dB (sound of air pressure rig) during normal operation, which seriously affects human health. Nowadays, the mixing plant can adopt a low-pressure powder conveying system, which can reduce the noise from 100 dB to less than 70 dB (street noise).
Another major source of noise is loaders. Loaders work hard and can be up to 85 dB (the noise of a busy street). Prolonged exposure to this environment causes gradual damage to the nerve cells. The main working area of the loader is the aggregate yard, and when the aggregate yard is closed, it can partially reduce the noise. The new mixing plant can be used to change the aggregate stacking and loading method to reduce the use of loader: the aggregate stacking yard uses belt conveyor to stack the aggregate, the incoming aggregate is directly unloaded to the hopper, and then transported to the yard by belt conveyor, after which the loader is used for batching bin loading; the batching bin can be loaded by ground close method, and the loader can assist in pushing it to the ground close catchment. Both of these two bunker loading methods need loader, but do not need loader to lift stacking, greatly reducing the use of loader frequency, reducing the noise generation. The other is a high aggregate silo loading method, as shown in Figure 2, the bottom of the silo is equipped with an unloading port and aggregate weighing transport device, the aggregate after unloading and weighing directly into production through the belt, can be completely separated from the use of the loader, reducing the noise of the loader in the field. Noise from transport vehicles is reduced by slowing down the vehicle and other management methods.
Production concrete noise, although unavoidable, can be modified and designed to reduce noise harm to humans through the effective separation of production areas, office areas and living areas, green belt noise reduction and noise attenuation or the addition of sound barriers.
In order to win the battle of blue sky and provide a suitable living environment for mankind, the construction of environment-friendly concrete mixing plant has been an important reform in the production mode of the construction industry. Green and environmentally friendly concrete batching plant manages the production facilities such as sand and gravel, powder silo, etc. in a completely closed way, and through the reasonable recycling of solid waste and liquid waste, and strengthening the renovation and maintenance of high-pressure spraying and dust collection equipment, it can reduce pollution and save energy, which is an urgent need to cope with the global climate change. The new green concrete mixing plant needs to show the new appearance of “neat and beautiful appearance, noise reduction, clean site, zero emission of wastewater recycling”. To enhance the environmental controllability of the concrete mixing plant in terms of dust, waste water, solid waste and noise, to solve the problem of serious pollution and waste of resources, to promote the integration with the green development of modern urbanization, to make the concrete mixing plant move to the new development stage of green factory, and to realize the optimal transformation of the industry to green industrialization.