Analysis of steel slag treatment technology, present situation and utilization value

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Steel slag treatment technology, status quo and utilization value analysis

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As a by-product of steel production process, the output of steel slag is 10% - 15% of that of crude steel. In 2014, China's crude steel output was 820 million tons, which can be calculated that the steel slag production in 2014 was about 82-120 million tons. At present, about 70% of the steel slag is in the state of storage and landfill. If the steel slag can be recycled, not only a large number of valuable metals can be recovered, but also the environmental burden can be reduced. In the final analysis, the recycling of steel slag is how to effectively and green use steel slag tailings, how to quickly and economically digest free calcium oxide (f-CaO) and free magnesium oxide (f-mgo).

The treatment of steel slag is divided into two stages, that is, the treatment process from liquid slag to solid slag, i.e. primary treatment, and the separation and magnetic separation process of solid slag and slag steel are secondary treatment. After the in-depth investigation of steel slag treatment industry, this paper will respectively from the steel slag treatment industry environmental impact, steel slag primary treatment, secondary treatment and tailings utilization four parts to elaborate the steel slag recycling.

1. Environment of steel slag treatment industry

In China, the requirements for pollutant emission of steel slag treatment industry are higher and higher. Some traditional steel slag treatment methods with serious pollution are bound to face transformation and upgrading. High purity carbon dioxide, electric power or heat, shale oil, recycled asphalt concrete, cement (including cement clinker) produced by waste resources with rotary kiln process or purchased cement clinker by grinding process shall be subject to VAT refund policy. Among them, "cement produced by rotary kiln process or cement clinker purchased by grinding process" refers to that the proportion of waste slag mixed in cement production raw materials is no less than 30%. In order to further promote the comprehensive utilization of resources, the comprehensive utilization of steel slag resources and preferential tax policies are discussed. The new steel slag industry policy is under discussion and formulation, yet to be promulgated. The world metal guide will actively follow up the report.

Converter steelmaking process is dominant in China, most steel plants use converter steelmaking process, and electric furnace steelmaking is also in converter, hot metal ratio reaches 70%. At present, the "less slag smelting" process is emerging in domestic converter steelmaking. It is a process of slag retention. In the second furnace dephosphorization period, part of the slag containing impurities such as silica and phosphorus pentoxide is poured out, so that a small amount of raw materials are added in the decarburization period to form slag again, and the slag is retained and solidified at the end of smelting, so that the f-CaO in the slag can be fully utilized. At present, Shougang Qiangang is the representative enterprise of this process in China, and nearly 50% of its full variety series adopt "less slag smelting" process.

By comparing the steel slag composition of ordinary converter smelting process and "less slag smelting" process, it can be found that the content of f-CaO in steel slag by ordinary smelting is 5.7%, and that of steel slag with "less slag smelting" is only 2.1%. Therefore, the "less slag smelting" process can promote the recycling of steel slag.

2. Primary treatment process of steel slag

The conventional primary treatment methods of steel slag are mainly hot stewing decomposition method and mechanical crushing method. The principle of this kind of treatment process is that under high temperature conditions, the basic oxides such as calcium oxide and magnesium oxide in the steel slag react with water to form hydroxide, which can achieve the goal of granulation of steel slag by volume expansion and crushing. Mechanical crushing method, as the name implies, is through the effect of mechanical external force on the flow of steel slag impact, so that it is broken. The mechanical crushing methods include water quenching, air quenching, roller grinding and steel slag granulation.

According to the statistics of one-time slag treatment data of 129 large and medium-sized iron and steel enterprises in China, there are 59 enterprises adopting hot stewing method, accounting for 45.73%; 51 enterprises adopt the improved treatment process of hot splashing method, i.e. "hot splashing in pool", accounting for 39.53%; drum slag treatment method ranks third, accounting for 7.75%. It is worth mentioning that although the roller slag treatment method has a low share in China, it has been applied in POSCO steel works in South Korea, JSW in India, zhonglong in Taiwan and CSP in Brazil due to its advantages of fast processing speed and good environmental protection.

3. Present situation of secondary treatment of steel slag

The process of iron and steel slag (including iron and steel slag) crushing, secondary cleaning, and high-purity iron and steel slag cleaning process. The earlier secondary treatment process of steel slag is mainly simple crushing and screening. At present, it has been upgraded to multi-stage crushing, multi-stage screening and multi-stage magnetic separation. For example, Meishan Iron and Steel Co., Ltd. adopts 3-out-7-5-screening, Taiyuan Iron and Steel Co. Ltd. adopts 2-out-3-out-5-screening, all of which aim to fully recover metal in steel slag and make full use of tailings. However, with the increase of treatment process, equipment investment, plant investment and daily maintenance labor will be doubled. Therefore, crushing, screening and magnetic separation should also be adjusted according to the situation of primary steel slag treatment.

Steel slag fine crushing equipment such as inertia cone crusher, pulverizing equipment such as flexible vertical mill, cone mill, horizontal mill and roller mill are widely used. The particle size of steel slag after mechanical treatment is relatively small, so the investment of crushing, screening and magnetic separation equipment can be reduced in the secondary treatment. From the overall point of view, the domestic steel slag secondary treatment process has its own characteristics, which needs to be combined with the overall value advantage of steel slag primary treatment process to judge its advantages and disadvantages.

4. Analysis on utilization value of tailings

(1) Application direction of tailings

At present, there are many main ways to use steel slag. The areas with high added value of steel slag utilization include high skid resistant pavement, metallurgical flux and agricultural fertilizer, but their use quantity is relatively small; the added value of steel slag used for asphalt surface course and asphalt road base is also high, but the use quantity is also general; in the use field on the right side of asphalt aggregate, although the use value is general, the use quantity is relatively large Therefore, these fields are the main development direction of the application of steel slag tailings in China.

(2) Application status of tailings

The application fields of steel slag tailings in China include the preparation of modified binder from steel slag tailings, new slag pot protective agent, steel slag filler subsurface flow wetland, steel slag blending into coking coal, acid modified steel slag treatment, modified steel slag removal from modified steel slag, ceramic preparation from converter steel slag, etc. At present, the main application direction of steel slag in China is to produce steel slag powder and steel slag composite powder. The National Institute of science will be responsible for the drafting of the "composite slag and slag powder" standard. There are also many application areas worthy of steel slag enterprise development.

The effective utilization of the tailings is also affected by the content of f-CaO in the steel slag. For example, the industry standard for steel slag used as backfill and road base material clearly requires that the immersion expansion rate shall not be greater than 2%, and the metal iron content shall not be greater than 2%, and the steel slag particles shall also be clearly required. There are no strict requirements for the content of f-CaO in the standards of other steel slag application fields, so the steel slag treatment methods such as hot splashing method and roller method also have strong vitality. For example, Guangdong Yangchun Iron and Steel Co., Ltd. adopts the hot splashing process to treat steel slag, but there are many large cement enterprises around it, such as conch cement, China Resources Cement, chuntan cement, etc "Zero discharge" of steel slag has been achieved.

The application of steel slag tailings is also diversified. HANGGANG, represented by the hot splashing method, produced 195000 tons of steel slag powder and 315000 tons of tailings in 2014, including 100 tons of asphalt, 47000 tons of subgrade materials, 13000 tons of heavy sand, 200000 tons of cement concrete and 55000 tons of large tailings. Since the production line of roller press final grinding steel slag powder was put into operation in September 2014, slag company, represented by hot stewing method, has optimized the process parameters of producing steel slag powder with different specific surface area of 450-600m2 / kg, and achieved stable production with an hourly output of more than 80t. Steel slag powder and slag powder are used in the production of steel slag powder, and they are successfully applied in large-scale technical transformation project, high-strength continuous annealing project of cold rolling mill and reconstruction project of No.3 work area of steel-making. They all meet the strength requirements of design standard C30 concrete. The proportion of steel slag powder accounts for 15% - 30% of the cementitious materials of concrete; at the same time, the proportion of steel slag powder and slag powder in the industrial test of artificial reef reaches more than 90%. In the application of steel slag powder and steel slag powder in cement production, high grade 42.5 steel slag Portland cement can be prepared.

To sum up, the steel slag is only about 30-82 million tons in China. At present, the primary treatment of steel slag is mainly hot stewing and hot splashing process. Drum method is widely used in foreign countries. However, domestic steel slag secondary treatment methods have their own characteristics, and there are many processing equipment and processes, so it has strong selectivity. As the output of cement and steel slag is not matched, the output of steel slag in some areas is seriously saturated, but the supply in some areas is in short supply. Compared with the steel powder in China, the application of steel slag in China should be diversified. China's steel slag industry should actively promote the development of preferential policies.

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