Speeding Up the Utilization of Steel Slag and Practicing the New Green Development Model
To practice the new green development mode is a long-term plan for the transformation of China's green economy. Correctly handle the current economic development and ecological environment protection, we should abandon the existing model of relying on increased consumption of material resources, extensive scale expansion, high energy consumption and high emission industry, which has caused damage or destruction of the ecological environment development. It is the most prominent structural contradictions of development for enterprises and national economic, which is inseparable from the concept of ideology and development.
In the face of the grim situation of resource constraints, environmental pollution and ecosystem degradation, heavy industries in China including iron and steel enterprises must speed up the industrial solid waste resource recycling, meanwhile status change of consumption of fossil resources, occupy of farmland and ecological environment damage are urgent.
Steel slag produced by steel enterprises during steelmaking is one of the largest solid wastes produced in the steel industry, with little use efficiency, and its proportion of crude steel yield is about 12% ~ 15%., China's crude steel yield in 2016 was 808 million tons, in which steel slag was nearly 105 million tons, with 13 thousand acres new occupation of farmland or land. Furthermore, national steel slag accumulation stockpiling of steel slag is nearly 110 million tons, with 138 thousand acres occupation of farmland or land. The continuous increase of steel slag piles every year makes the surrounding environment pollution and security risks increasing day by day. There are Zinc, Chromium, Cadmium, Arsenic and other metal elements in slag, they can get into soil and destroy structure of plants after rain, or get into the rivers or seep into the ground, which can cause serious pollution to water source. Meanwhile, slag dusts cause serious air pollution which is harmful to human health.
At present, the comprehensive utilization rate of steel slag in China is only 10%, much lower than that of developed countries, which is the main cause of environmental pollution, land occupation and waste of resources. At present, 40.2% steel slag is used for building roads, 32.3% for engineering backfill, 19.4% recycled for sintering, 6. 5% for manufacturing cement, and 1.6% for other building materials. The traditional extensive steel slag processing leads waste of renewable resources and secondary pollution to environment. The main cause of this situation is that the large iron and steel enterprises cannot solve the technical problems of steel slag with high added value of more comprehensive utilization, this is because the chemical composition and mineral composition of the slag is volatile, with slow hydration, low early strength and poor stability, hard to grinding, as well as difficulties in pretreatment of raw materials etc.
With the continuous development of urbanization, agriculture and ecological security of China, the demand of energy resources in China has a rigid growth. We need high value-added utilization of bulk solid waste to reduce the petroleum resource demand, promote the construction of ecological civilization. Therefore, adopting the current domestic large-scale and key innovation technology and equipment to stimulate the activity of steel slag, and using potential hydration activity of cementing material to realize high value-added recycling, is a feasible way to ease the constraints of resources, and protect the ecological environment.
There are several ways to recycle steel slag with high added value:
1) The extracted particle grade of the steel slag is greater than 85%, which can be directly returned for steelmaking. The magnetic powder grade is more than 63%, which can be directly used to return for sintering. The iron content of tailings is less than 1% and can be used for materials, which realizes 100% comprehensive utilization of steel slags;
2) The steel slag powder can be used in concrete production, utilization of steel slag composite powder (fly ash, slag powder and steel slag powder) instead of 20% to 50% of Portland cement, to formulate the excellent performance of the C20 ~ C60 concrete with different strength grade;
3) Portland cement mixed with a certain proportion of steel slag powder can produce steel slag Portland cement, low heat slag cement, slag road cement, slag masonry cement etc. 30% ~ 40% cement clinker mixed with a large proportion of steel slag composite powder can produce PC32.5 grade green composite cement;
4) Developing steel slag based expansive agent to prepare medium-low grade mass concrete. The adiabatic maximum temperature rise can be controlled under 500°C, and anti-permeability level is more than P40, which can inhibit the delayed ettringite formation in mass concrete, and effectively control the temperature rise caused by concrete cracking.
5) The mineral admixture with high activity and compensation shrinkage function can be developed by using steel slag powder, and the 28d activity index is higher than 65%. When the amount of admixture reaches more than 20%, the concrete can produce micro expansion, and the expansion rate begins to be stable at 90d, and the value is (1~3) ×10-4;
6) The stabilized treated steel slag cement can be used for road cushion, base and surface layer, as well as asphalt concrete pavement, to improve the compressive strength, abrasion resistance, flexural strength, flexural properties change of road sedimentation;
7) The steel slag can be produced for special bricks. Using stabilized treated steel slag and granulated furnace slag as the main raw material mixed with a small amount of activator to produce brick, floor tile, building block and other building materials as well as new wall materials. Its strength and durability is higher than the clay brick.
8) There is rich content of rare earth elements in steel slags such as Niobium, Tantalum, Zirconium and hafnium etc. Niobium can be extracted from the waste steel slags for preparation of high strength low alloy steel.
Taking an annual yield of 350 thousand tons of steel slag powder production line for example, it can not only improve the environmental pollution status, but also produce the following economic, social and environmental benefits:
1)Economic Benefits
① Benefits of production of steel slag powder:
According to the market price
Steel slag powder: 100 RMB/ton
Iron powder: 900 RMB/ton
Steel particles: 1200 ~ 2400 RMB/ton.
One ton of steel slag can produce 0.79t slag powder, 0.145t of iron powder and 0.065t particles. The revenue of slag after processing is below:
100 RMB/t × 0.79t +900 RMB/t × 0.145t +2100 RMB/t ×0.065 t =346.00RMB.
In addition to the direct material consumption, labor, other direct costs and manufacturing expenses to produce steel slag powder, the actual treatment revenue of steel slag is about 180 RMB/t and the annual profit is
180 RMB/t × 350 thousand tons =63 million RMB.
② The iron recovery benefits:
The traditional way of removing iron from steel slag is relatively extensive, and the metal iron in the slag cannot be recovered efficiently. Therefore, the content of metallic iron in the slag powder is generally between 2% and 4%, and some even exceed 5%. Taking an annual yield of 350 thousand tons of steel slag powder production line for an example, the content of metallic iron in steel slag powder can be reduced from 4% down to 0.8%. So that 11,200 tons of metallic iron can be recycled from steel slag powder per year, and the annual profit is
750 RMB/t × 11,200 t =8.4 million RMB.
2)Social and Environmental Effects
An annual yield of steel slag powder:
0.79t Powder/Slag × 350 thousand tons Slag =276,500 tons.
Using steel slag composite powder instead of 20% to 50% of the ordinary Portland cement for preparation of C20 ~ C60 concrete with different strength grade, 55.3 thousand tons to 138.3 thousand tons of Portland cement can be saved every year, saving 67.7 thousand tons of cement clinker, 7.4 thousand tons of coal consumption of 7, reducing 67 thousand tons of CO2emissions of about and 43.75 acres of farmland or land occupation every year.
Based on high value-added recycling of the steel slag resources, we can implement cleaner production in iron and steel enterprises, promote symbiosis coupling between enterprises and industries to form a loop link industrial system. The cleaning of nonferrous smelting slag can transfer pollutant source management to resource and energy saving, and recycling high value utilization of industrial solid waste, which complies with the new natural green development pattern of ecological civilization.
The above new process of annual yield of 350 thousand tons of steel slag powder production line is designed by Hubei University Tianshu Industrial Research and Design Institute of New Energy Materials (Tianshu IRDI) with independent intellectual property rights of the “New technology of high efficient extraction, harmless cleaning and disposal of valuable metals from steel slag regeneration resources”. This process was supported by Science and Technology Department of Hubei province, China in 2013 “New Key Products and Technology Research and Development Project”, and it has been applied in the Shandong Shunxing New Building Materials Co., Ltd, Quantou Group Zaozhuang Jinqiao Rotary Klin Cements Co., Ltd, Shenxian Great Wall Building Materials Co., Ltd etc., realizing industrialized production, promoting transformation and upgrading of enterprises.