Practice of the first stage of magnetic separation of steel slag in Honggang

　　Abstract: steel slag contains not only high iron content, but also high Cao and MgO, which are indispensable elements in iron and steel production. The resource utilization of steel slag, on the one hand, can reduce the production cost, on the other hand, is conducive to reducing environmental pollution. Through reforming the first stage of magnetic separation of steel slag, Honggang has improved the effect of separation and enrichment of steel slag and made effective use of resources.

　　Key words: steel slag; Magnetic separation; Existing problems; Process transformation; effect
 

　　1、Preface

　　Steel slag is the molten slag discharged from the steelmaking process. It is an important by-product with high iron content produced in the metallurgical production process. Steel slag contains not only iron, but also high Cao and MgO. [1] These are indispensable elements for steel production. In the past, there were some difficulties in the processing, detection and use of steel slag, so most of them were directly abandoned as waste in steel production. In recent years, with the gradual depletion of iron resources, the cost of mining has risen sharply. Many steel mills are exploring the resource utilization technology of steel slag. Through the recycling of steel slag, on the one hand, the production cost can be reduced, on the other hand, it is conducive to reducing environmental pollution. In the continuous research and practice, certain breakthroughs have been made in processing technology and comprehensive utilization, and the vast majority of steel mills have basically achieved zero discharge of steel slag.

　　The converter of steel-making slag per ton in iron and steel enterprises is about 100 ~ 150kg. [2] Honggang currently produces 250000-300000 tons of steel slag annually. The steel slag magnetic separation system was put into use in may2009, with an annual steel slag treatment capacity of 260000 tons and a daily steel slag treatment capacity of 750 tons. The process flow is set in two sections: primary processing, sorting and magnetic separation in section I; In Section II, the low-grade fine-grained steel slag is subject to fine grinding and magnetic separation. After the primary processing, sorting and magnetic separation of section I, some slag steel is directly used for steelmaking, and some products need to enter section II for fine grinding and magnetic separation. The whole magnetic separation process system was originally considered to be reformed. However, due to the use of grinding, humidifying, magnetic separator and spiral classifier for fine grinding and sorting in the process of section II, there is a lack of reliable sewage and sludge treatment system in the design, and it is difficult to discharge the sludge, so continuous production cannot be realized, and the downstream enterprises have reliable processing systems. After demonstration, the necessity and urgency of section II reconstruction is insufficient, and it is temporarily idle. The process transformation was carried out for section I.
 

　　2. Production process and main problems of section I

　　2.1 original design production process of section I

　　The production process of section I in the original design is as follows: the steel slag is transported from steelmaking to the magnetic separation yard for a certain period of time, fully digested, and then loaded through the feeding grid after the separation of slag and iron. The screened objects (large blocks) are transported by the loader to the side for processing by the metal processing team. The screened objects are transported by the 1\belt conveyor, and the iron separator above the 1\belt conveyor is used for primary magnetic separation, and the permanent magnet drum at the head of the 1\\\, The primary magnetic separation products are directly stacked at the stacking point under the iron separator for steelmaking. The secondary magnetic separation products are transported to the designated stacking point by 3# belt conveyor and transported to the metal processing team for classification screening. After secondary magnetic separation, the waste part is screened by large blocks, and the lower part of the screen is transported to the designated stacking site by 4\. The upper part of the screen is returned to the process for magnetic separation after passing through the jaw crusher. As shown in Figure 1:

　　2.2 main problems

　　According to the application requirements of steelmaking and several years of production practice, there are many problems in this process, mainly including the following:

　　2.2.1 the process design is not perfect, and the product cannot meet the use requirements

　　It can be seen from the process flow chart that the large slag steel that cannot enter the production process from the feed grid and the slag steel from the No. 3 belt need to be re transported to other places for processing and purification before use. The back and forth tossing not only increases the handling, transportation and processing costs, but also increases the management links.

　　2.2.2 too few magnetic separation times and high slag content in magnetic separation products

　　The steel slag not absorbed by the iron separator only completes the whole magnetic separation process through two-stage magnetic separation. The TFE content of magnetic separation powder is only 30%-40%, and the TFE content of magnetic separation small pieces is 45%-55%. The iron grade cannot meet the use requirements.

　　2.2.3 the particle size of magnetic separation product of iron separator cannot meet the requirements for steelmaking

　　Generally, the products of magnetic separation of iron separator are directly returned to steelmaking for use. Due to the failure to pass the screening for particle size classification, the finished products contain a large amount of small particle slag steel, which can not be effectively used during steelmaking.

　　2.2.4 high iron content and low recovery rate in tailings

　　Due to too few magnetic separation stages, many slag steels with high iron content in the tailings are discarded before they are detected, and the magnetic iron and total iron content in the tailings are high. It does not meet the discharge standard of tailings.
 

　　3. Production process configuration after section I reconstruction

　　The purpose of steel slag treatment is to separate slag and iron as much as possible and recover more effective resources. The treatment method is mainly through crushing, screening, magnetic separation and other means to recover as much metal materials in the steel slag as possible, so as to maximize the economic benefits of steel slag recovery. According to the analysis of existing technical documents and actual production experience, the original section I process has only two-stage magnetic separation and one-stage screening. To improve the iron grade of steel slag products, it is necessary to increase the magnetic separation stages, screening times and crushing efficiency. The specific production process configuration after transformation is shown in Figure 2:

　　1. Extend the 3# belt conveyor to the position of the front stacking retaining wall. An adjustable permanent magnetic drum with a diameter of 300mm and a magnetic flux of 1600gs is installed at the head of 3\\\belt conveyor. After passing through the magnetic drum, the discarded part is treated as tail slag. The magnetic separation products are screened. The part greater than 10mm is used as No. 6 slag steel for steelmaking, and the part less than 10mm is used as magnetic separation powder for sintering.

　　2. An electric vibrating grid screen with a hole diameter of 16mm is installed under the secondary magnetic separation product (the magnetic separation product of the head wheel of the No. 1 belt conveyor), and a underpants guide groove is made under the grid screen to guide the material on the screen to the 5\belt conveyor and the material under the screen to the 3\belt conveyor.

　　3. Move the 5# belt conveyor to be parallel to the 3\belt conveyor and used as the treatment equipment for the screening products of the No. 1 belt head pulley. An adjustable permanent magnet roller with a magnetic flux of 2500gs is used at the head wheel position to replace the original ordinary roller. A retaining wall is set under the head wheel to separate the thrown wastes from the magnetic separation products. No. 5 belt magnetic separation product is named No. 4 slag steel.

　　4. A 20mm size vibrating screen shall be installed below the iron separator. The material on the screen is determined as No. 2 slag steel for steelmaking, and the material under the screen is determined as No. 3 slag steel for steelmaking after briquetting.

　　5. Extend the 4# belt conveyor until the head wheel is flush with the No. 3 belt conveyor. The head wheel adopts a magnetic adjustable permanent magnet drum with a magnetic field strength of 800gs. A retaining wall is set under the head wheel to separate the magnetic separation products from the tailings.

　　6. The large slag steel that cannot enter the production process from the feed grid is directly crushed by the excavator crushing head on site. The crushed large slag steel is used for steelmaking after mechanical sorting. The remaining slag steel with low iron content is re sorted by magnetic separation process.

　　4. Transformation effect

　　4.1 from two-stage magnetic separation to five-stage magnetic separation, the number of magnetic separation products is increased from three to eight, which are used according to different iron grades, which is conducive to the accurate control of the next process. As shown in the table:

　　Table 1 indicators of magnetic separation products after transformation

　　4.2 the grade of slag steel and steel slag magnetic separation powder is improved, and the harmful elements are reduced, which is conducive to the reduction of resource consumption and energy consumption.

　　4.3 increase the metal recovery rate of steel slag, improve the internal recovery and utilization rate of steel slag resources, and reduce the production cost of steelmaking to a certain extent.

　　4.4 the steel slag products of the whole steel slag magnetic separation plant do not need to be re transported for processing. Referring to the outsourcing processing cost of RMB 2.5 million in 2016 and RMB 3million in 2017, the outsourcing processing cost of RMB 2.5 million can be saved every year.

　　4.5 the production of steel slag is generally reduced, which alleviates the problems of environmental pollution and resource waste caused by steel slag discharge, and reduces the operation cost at the same time.

　　4.6 in terms of environmental protection benefits, through the transformation, the produced slag steel can be returned to steelmaking, and the steel slag magnetic separation powder can be returned to sintering. Export the produced tailings, reduce the stacking of waste slag, improve the utilization rate of iron resources, and effectively reduce the impact of steel slag stacking on the ecological environment.
 

　　5、Conclusion

　　5.1 by carefully studying the successful experience of other enterprises and combining with their own production practice, the transformation puts forward plans, and continuously adjusts and optimizes in production, the overall process flow is reasonable, simple, efficient, low investment, and the effect is obvious.

　　5.2 some management adjustments are made through process transformation and in combination with the actual situation, such as the on-site processing of large slag steel and the storage of incoming steel slag in two piles, the separation of incoming and outgoing steel slag, the use of one pile and the construction of another pile, the average storage time of steel slag on the storage yard before use is not less than one week, reducing the stock of slag steel pile, and realizing the use with production. The management process of steel slag magnetic separation system is more standardized.

　　5.3 steel slag magnetic separation section I process transformation practice: by combining the original process, three-stage magnetic separation, three-stage screening and one-stage crushing are added, the process design is improved, multi-stage separation is realized, the products are enriched, the process requirements are met, and the metal recovery rate is improved.

　　5.4 the technological transformation of magnetic separation of steel slag can promote the development of circular economy, improve the resource utilization of steel slag, save mineral resources and protect the ecological environment.

　　5.5 in terms of section II and comprehensive utilization of tailings, downstream enterprises have more mature production processes, lower production costs and more complete resource treatment channels than Honggang. They can make full use of the cooperative relationship to achieve 100% recovery of resources and achieve mutual benefit and win-win results.