From the perspective of comprehensive treatment methods for steelmaking smoke and dust, two concepts are proposed: treatment outside the furnace and treatment inside the furnace. The application and technical methods of steelmaking smoke are introduced, the treatment outside the furnace is described in detail, and the research progress of treatment inside the furnace is also described.
Keywords: steelmaking; smoke and dust; comprehensive control; application
The amount of smoke generated by converter steelmaking and electric furnace steelmaking is 7~15kg/t and 10~20kg/t respectively. Steelmaking smoke has the characteristics of large emission, wide range of pollution, complex types, and small particles. The task of controlling steelmaking smoke is heavy and difficult. However, from the perspective of comprehensive utilization of resources, steelmaking smoke is rich in components. Many fine dusts can often be used as raw materials and contain various metal elements, which can be recovered or directly recycled. The comprehensive treatment of steelmaking smoke and dust can basically be divided into treatment outside the furnace and treatment inside the furnace.
Treatment outside the furnace
Out-of-furnace treatment is the current mainstream trend, that is, smoke and dust are generated and discharged for treatment or recycling.
Recycling within the steel system
Application in sintering production
The iron content of converter dust sludge is about 56%, and the CaO and MgO contents are relatively high. The smoke and dust are generally purified by hydraulic dust collectors.
(1) Processing of converter dust and sludge. The hot gas ash larger than 100e is mixed into the converter sludge in proportion, and after mixing in a mixer, it absorbs moisture and generates steam. The lumpy dust sludge is softened and broken into loose pieces, and then enters the two-stage mixer for mixing through the conveyor belt to refine the particle size and produce powdery and granular materials.
(2) Production and use conditions. In sintering production, the general proportion of this processed material is 4%, which can effectively speed up the sintering speed. At the same time, the sintered mineral material produced has a low melting point, good sintering conditions, high yield, increased strength, drum index (>5mm) 80.67%, and low fuel consumption. decline.
(3) Economic benefits. Using processed materials to produce sinter can reduce the cost of sinter by 9.52 yuan per ton, save 17kg of solid fuel, improve the environment and reduce pollution.
Application in pellet production
For blast furnaces, if the total zinc content in the charge is too high, zinc deposits will be generated in the blast furnace, which will affect the operation of the blast furnace. Therefore, the zinc content in the raw materials must be limited. Dust with high zinc content cannot be used as blast furnace raw materials and can only be thrown away.
Junjin Factory in Japan chose RHF dust treatment technology. This process has a dezincification function, and the dezincification rate reaches 90%. During the production process, stable operation technologies such as granulation, shaping, reaction condition control and exhaust gas treatment of dust raw materials were developed, and the recycling of dust in the blast furnace was realized for the first time in the world. Converter dust is processed on dust treatment equipment with dezincification function, see Figure 1.
Figure 1 Process flow of dust recycling equipment
Since the dust recycling equipment was officially put into operation, the reduced pellets produced by it have been directly used in blast furnace production. With the increase in the use of reduced pellets, the blast furnace fuel ratio showed a downward trend, achieving the expected results. For every 30kg/t of reduced pellets used in the blast furnace, the fuel ratio can be reduced by approximately 7kg/t.
Application in electric arc furnace steelmaking
ASW Sheerness Steel Company in southern England is located in an area with very strict environmental protection requirements. In order to achieve the goal of “zero emissions”, the company recycles all the dust in the bag dust collector of the steelmaking plant and 5,000t of oily oxides produced during the rolling process of the steel rolling mill, and then blows them into a 90t electric arc furnace.
The proportion of the injection materials is: 20% to 80% of oily iron oxide scale, 0% to 60% of electric furnace ash, 15% to 25% of carbon powder, and 0% to 10% of lime.
Since the implementation of the process, not only has it not had any adverse effects on the electric furnace steelmaking process and steel quality, but it has increased steel production.
Extraction of valuable metals from stainless steel dust for recycling
The steelmaking dust produced during the smelting of stainless steel contains a large amount of nickel and chromium, which are precious metal resources that are scarce in our country. Dealing with these dusts directly during the steelmaking process is a self-reduction and self-recycling process. Recycling valuable metals is an economical and practical method.
Researchers at Central South University used an intermediate frequency furnace to simulate an electric arc furnace to explore the direct reduction recycling process. Mix the dust and the reducing agent carbon evenly, add appropriate binders to pelletize, and then return the pellets directly to the electric arc furnace during the reduction period of the steelmaking process, and further add a metal reducing agent to recover the difficult-to-restore components in the dust from the slag. Metal, so that the metal in the dust is recycled into steel products in the form of alloy elements. By detecting the gas phase components and melt temperature during the reduction process and adjusting the power supply to control the furnace temperature, the experimental conditions are close to the actual production.
Research has found that the reduction of Ni is completely completed by the reducing agent carbon in the pellets, and proceeds more thoroughly. The reduction of Fe and Cr is partly accomplished by carbon and partly by metal reducing agents from slag. The amount of metal reducing agent and lime that have a greater impact on the reduction and recovery of Fe and Cr is the amount of metal reducing agent and lime added. Therefore, in actual production, the amount of metal reducing agent and lime added can be appropriately increased.
The amount, especially increasing the amount of lime, can not only increase the recovery rate of valuable metals, but also reduce the sulfur content in steel.
Treatment and utilization outside the steel system
An economical treatment method for electric furnace dust with very low zinc content is not to recover the resource, but to stabilize the dust with a curing agent to form an impermeable solid, which is then landfilled. For example, after mixing lime, coal-fired fly ash and electric furnace dust, an impermeable matrix is formed to wrap the dust particles so that toxic substances in the dust cannot seep out. The landfill must meet the requirements of environmental regulations before it is landfilled.
The patented curing agent used by International Curing Co., Ltd. is a highly reactive complex. Whether it is oil sludge, mine tailings, sea sand, waste sludge, electroplating waste, etc., the solid inorganic materials obtained through the solidification process are non-toxic and stable materials without exudation.
Application in cement production
Using electric furnace dust removal ash instead of iron ore powder to produce cement can reduce production costs, save iron resources, prevent secondary pollution, and has high environmental and economic benefits.
In the 1990s, the iron oxide content of dust removal from Shanghai Wugang Electric Furnace was greater than 50%, and the composition fluctuated little, which met the technical conditions for iron oxide content to be greater than 40% as required for iron correction raw materials for cement production. Laboratory trial preparation and productivity tests show that the physical and chemical properties and hydration products of clinker fired with electric furnace ash are basically the same as those of normal clinker. The quality of the cement produced meets national standards. This type of electric furnace dust removal ash is used in cement production. It’s technically possible.
Application in crude extraction of valuable metals
Metals such as zinc, lead, and cadmium have low boiling points. Under high-temperature reduction conditions, their oxides are reduced and volatilized into metal vapor, which is discharged along with the flue gas, separating them from the solid phase body. In the gas phase, these metal vapors are easily re-oxidized by the carbon monoxide in the flue gas to form metal oxide particles, which are collected together with the flue dust in the flue gas treatment system. The zinc content in the collected smoke is about 50%, which can be sold as crude zinc or further processed into zinc oxide products.
The zinc extraction process flow is shown in Figure 2.
Figure 2 Reduction process of zinc-containing smoke and dust with carbon
Furnace treatment is still in the research stage, including research on new systems and equipment for furnace treatment and new processes for reducing smoke and dust emissions.
Japan’s JRCM develops an electric furnace system that does not produce dust
Nippon JRCM has been conducting development research on “dust-free electric furnace systems” since 1998. The test was successfully conducted on a 1 ton scale small-scale equipment. The separation and recovery rate of iron and zinc reached 99%, and a recycling system for the remaining 1% was established.
In March 2001, a small pilot plant was installed at the Chita Plant of Aichi Steel Co., Ltd. to conduct research on “making an electric furnace system that does not generate dust.” The system passes high-temperature electric furnace exhaust gas through a carbon filter and heavy metal condenser. Direct recovery of iron and zinc is a key technology in this process. In the carbon filter, reduction is used to reduce iron and zinc oxides and adsorb and separate iron. In the metal condenser of the subsequent process, the zinc component contained in the steam is adsorbed on the falling alumina balls to recover the metal zinc. The iron separation recovery rate in the carbon filter and the zinc separation recovery rate in the metal condenser are targeted to be above 80%. The ultimate goal is to achieve an iron and zinc separation and recovery efficiency of 99%.
Explore and develop new steelmaking processes that reduce dust generation
In recent years, the University of Science and Technology Beijing has been exploring new steelmaking processes that can reduce the generation of smoke and dust during the smelting process without affecting the normal progress of the smelting. It is also believed that during the oxygen blowing process, the local high temperature of the fire point in the jet area is the main reason for the generation of smoke. If the temperature of the oxygen jet fire point area can be reduced, the volatilization of iron and the emission of iron-containing smoke can be controlled. It was found in the experiment that as the proportion of carbon dioxide in the oxygen jet increases, the amount of smoke produced gradually decreases. When carbon dioxide reaches a certain value, smoke is basically not produced, and only high-temperature smoke is emitted.
If the process of using CO2 to reduce smoke and dust emissions in the steelmaking process is implemented, the amount of steelmaking smoke and dust will be reduced by 30%, that is, the emissions per ton of steel will be reduced by 5kg. Based on the domestic annual steel production of 500 million tons, the amount of smoke and dust generated will be reduced by 4 million tons, and the direct economic benefit will reach 16 billion yuan. If 3% CO2 is added to the injected oxygen, a 50-million-ton steel and iron and steel enterprise will recycle hundreds of thousands of tons of CO2. This will be a new energy-saving and environmentally friendly technology that kills two birds with one stone, and its economic and social benefits cannot be underestimated.
The comprehensive smelting of steelmaking smoke and dust by steel companies not only reduces pollution and improves the environment, but also creates considerable economic benefits by recycling the metal resources in the smoke and manufacturing various products, which is conducive to saving resources and protecting the environment. It deserves vigorous promotion.