TECHNOLOGY OF INTEGRATED PROCESSING OF STEEL-MELTING DUSTS

ABSTRACT

The processing of dust from steel-making furnaces requires special attention, since, firstly, it is impossible to simply use them as an additive to the charge (this will worsen the quality of steel); secondly, it is not subject to burial in the ground, since soil poisoning is possible; thirdly, it is advisable to extract valuable components (except for iron) contained in such dust from such dust.

АННОТАЦИЯ

Переработка пыли сталеплавильных печей требует особого внимания, так как, во-первых, просто использовать их в качестве добавки в шихту нельзя (это ухудшит качество стали); во-вторых, она не подлежит захоронению в земле, так как возможно отравление почвы; в-третьих, из такой пыли целесообразно извлекать содержащиеся в ней ценные (кроме железа) компоненты.

Ключевые слова: пыли сталеплавильных печей, улавливание, нефтяной кокс, спекание. 

Keywords: steel furnace dust, collection, petroleum coke, sintering.

Introduction. The introduction of modern technologies aimed at increasing the complexity of the use of raw materials, the use of resource and energy saving technologies, the utilization of all valuable components in waste and intermediate products of metallurgical production is caused by a decrease in the content of metals in processed ores, depletion of rich and easily opened ore deposits and increased requirements for environmental protection [1].

Ferrous metallurgy of the Republic of Uzbekistan is represented by such an enterprise as JSC "Uzmetkombinat" [2]. The main source of raw materials for steel smelting is iron-containing scrap of various compositions. Steel smelting at the enterprise is carried out in an arc steel-making furnace. Smelting produces a large amount of dust carried away by the waste gases. The dust of steel-making furnaces, in addition to iron-containing components, contains a significant amount of zinc in the form of zinc oxide (ZnO up to 17%) [3].

Scrap metal is the main source of impurities such as Zn and Pb entering steelmaking units (galvanized iron, brass and bronze parts, electrical cable sheaths, etc.).

The composition of the removed dust is significantly influenced by the composition of the charge [4]. This effect is especially noticeable when steel is smelted in electric arc furnaces. The processing of such dust requires special attention, since, firstly, it is impossible to simply use them as an additive in the charge (this will worsen the quality of the steel); secondly, it is not subject to burial in the ground, since soil poisoning is possible (burial in the ground is prohibited in a number of countries); thirdly, it is expedient to extract from such dust the valuable (except for iron) components contained in it [5].

Materials and research methods. The object of research was the dust of steel-making furnaces of JSC "Uzmetkombinat". To date, over 60,000 tons of zinc-containing dust from the gas cleaning plant of the ESPC have been accumulated in special warehouses. An additional 12,000 - 13,000 tons of zinc-containing dusts are generated annually [6].

The chemical composition of steel-making dusts obtained from JSC "Uzmetkombinat" is shown in Table 1.

Table 1.

Chemical composition of steel-making dusts

Petroleum coke of the Fergana Oil Refinery (FOR) was chosen as reducing agents for zinc oxide and iron oxides contained in steel-making dust [7].

Petroleum coke is a complex dispersed system in which the dispersed phase consists of crystalline formations (crystallites) of different sizes and ordering in the mutual arrangement of molecules and pores, and the dispersion medium is a continuous gaseous or liquid phase that fills the pores of crystallites, from which adsorption-solvation layers are formed, or solvate complexes. Despite the unequal preparation conditions, the crystallites have similar sizes and are packets of parallel layers (planes). Crystallite sizes (in nm): plane length a = 2.4-3.3, packet thickness c = 1.5 - 2.0, interplanar distance 0.345 - 0.347 [8].

Results and its discussion. To determine the optimal reductant and its consumption, experiments were carried out using petroleum coke from the Fergana Oil Refinery (FOR) and brown coal from the Angren open pit as reductants.

Based on the industrial experience of the process of Waelz zinc cakes and information obtained from the analysis of literature sources, it was decided to take the amount of coke from the FOR loaded into the charge equal to 40% of the dust mass (average carbon content in the coke is 95%).

When brown coal from the Angren open pit was used as a reducing agent, a lower carbon content in it (73.44%) was taken into account compared to coke, based on this, the amount of coal added to the charge was increased to 52% of the dust mass (95: 73.44 * 40 = 52).

The weighed amount of dust for the preparation of the charge with petroleum coke was 20 g, and the amount of added coke was 8 g.

The weighed amount of dust for the preparation of the charge with brown coal was also 20 g, and the amount of added coal was 10.4 g.

The prepared charge was loaded into a fireclay crucible and installed in a shaft furnace preheated to 1200 ° C and fired for 90 minutes. After a specified time, the crucibles with weighed portions were removed from the furnace and cooled to room temperature.

The residual content of zinc in the clinker and the degree of reduction of iron oxides were carried out on an atomic absorption analyzer in the laboratory "Physicochemical research methods" of the State Enterprise "Institute of Mineral Resources". The results of the analyzes are shown in table. 2.

Table 2.

Zinc content in the original sample and after firing

From the data given in the table it can be seen that when using coal as a reductant, the zinc content in clinker is lower than when using petroleum coke. Based on the results obtained, as well as taking into account the cost of petroleum coke (about 2.4 million soums per ton), which exceeds the cost of coal (about 0.5 million soums per ton) and the high sulfur content in coke, which is a harmful impurity for steel, it was accepted solution to use Angren brown coal as a reducing agent.

The results of the experiment show that the zinc content in the clinker remains high, this can be explained by many reasons, the main of which is the lack of reducing agent and insufficient firing temperature.

To determine the optimal amount of the reducing agent, it was decided to increase the consumption of the reducing agent (coal). Three samples were prepared with a coal content of 65, 80 and 90% of the dust mass in the charge. All three samples were fired under the same conditions (1200 ° C, 90 min). The firing results are shown in Table 3 and Fig. 1.

Table 3.

Zinc content in clinker versus the amount of reducing agent in the charge

Figure. 1. Dependence of the zinc content in clinker on the consumption of the reducing agent

From the research results given in Table 3 and Figure 1, it can be seen that with an increase in the amount of coal in the charge to 80-90% of the dust mass, the zinc content in the clinker decreased to 7.98%. The extraction of zinc in sublimates is 52.4% (16.78 - 7.98): 16.78 = 52.4).

The choice of the optimal temperature 1200 0С is explained by the fact that at this temperature the degree of zinc sublimation according to the reaction ZnO + Fe = FeO + Zn (g) has large values (Table 4.).  

Table 4.

Zinc vapor pressure during its sublimation

 Based on the data given in the table, it can be concluded that a further increase in temperature will favorably affect the stripping of zinc, however, the limitation of the temperature regime is caused by the presence of low-melting components, which, when reflowed, cover the pores of solid particles, causing an obstacle to the stripping of zinc into the gas phase.

Iron present in steelmaking dust during the reduction roasting process is also reduced from oxide compounds to metal. The reduction of iron oxides with carbon is described by the reactions:

3Fе₂О₃ + С = 2Fe₃O₄ + СО

Fe₃O₄ + С = 3FeO + СО

FeO+ С = Fe +СО

The resulting metallic iron during the reduction firing of steel-making dusts will increase the extraction of zinc into sublimates; it will restore zinc from oxide by the reaction:

ZnO + Fe = Zn + FeO

In addition, due to the carburization of iron at a temperature of 1150 ° C, cast iron will be formed. As a result of reductive roasting, the zinc content in the dust will decrease due to the distillation of zinc into the gas phase and a solid product, the so-called "clinker", is obtained, in which the bulk of the iron will be in metal form, which will allow it to be remelted in steel-making furnaces without preliminary preparation.

The results on the reduction of iron oxide when carrying out a reductive roasting with coal as a reducing agent are shown in Table 5.

Table 5.

Iron oxide reduction rates

Atomic absorption analysis determined the content of zinc in the sublimates. The zinc content in sublimates is 64.41%.

Conclusion. The problem of involving dust in the steelmaking industry is especially acute at the present time, when the enterprises of the ferrous metallurgy of the Republic are experiencing a shortage of raw materials. At present, the warehouses of Uzmetkombinat JSC have accumulated 60 thousand tons and annually generate 10 thousand tons of steel-making dusts with a ZnO content of up to 17%.

 One of the solutions to this problem may be to reduce the amount of zinc charged into the steelmaking furnace along with the raw materials.

The studies carried out have established that the stripping of zinc and the reduction of iron oxides, possibly by means of reductive roasting. The use of brown coal from the Angren deposit is proposed as a reducing agent.

For the practical implementation of the proposed technology, it is proposed to use the existing technology and hardware design of the Waelz zinc cakes used at the zinc plant of the Almalyk Mining and Metallurgical Combine.

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