Study On The Direct Recycling Of Stainless Steelmaking Dust

Stainless steelmaking dust contains various heavy metals. It makes the environment polluted and metal value lost to dispose or landfill the dust in a traditional way. Direct recycling of stainless steelmaking dust was put forward after the initial investigation, in which the pellets were produced after mixing the dust and reducing agent carbon, and then added to the electric arc furnace used for the smelting of stainless steels. The metals present in the dust were reduced and recovered to the liquid steel in the form of alloying elements. This study was completed under the financial supports of Natural Science Fundation of China project "Study on the mechanism of direct reduction of stainless steelmaking dust", of Hunan Scienctific and Technological Key project "EAF Steelmaking and treatment of the dust", of Natural Sciences and Engineering Research Council of Canada project "Direct recycling of EAF stainless steel dust". The research contents dealt with following three aspects: (1) The forming mechanism of stainless steelmaking dust and he effects of adding pellets to the furnace on the smeling system; (2) Foundamental study on direct recycling of stainless steelmaking dust; (3) Study on the new technology of direct recycling of stainless steelmaking dust. The achievement of this study solved the some key academic and technic problems for the application of the new technology in practice and following progress was achieved.High speed camera work was taken to observe the bubble break on the surface of molten slag in the experiments and the dust collected was detected for the chemical elements, spacies and morphology. The properties of film drop and jet drop from the bubble break was commanded and the effect of bubble diameter on the size and amount of drops was determined. The formation procedure of the dust and the physical/chemical changes in the procedure were ascertained according to the chemical composition, phase structure and distribution of particulate size of the dust, and following that the physical and chemical characteristics of the dust were made clear. The procedure of the dust formation was classified into two stages, precursor appearance and accumulation of small particulates. It could delive the data for the separation of different componential dusts to treat in different ways and the equipment alteration of the dust collection.According to the thermodynamic calculation on the stainless steelmaking dust, the nickle can be easily reduced, and then is the iron, the chromium is the most difficult metal reduced in the direct recycling. At the same time of the reduction of above three metals, the lead and zinc present in the dust are reduced, but the reduction of Al₂O₃,CaO,MgO,MnO,SiO₂ will not happen. All phases and stable regins are indicated and the effect of carbon content in the pellet on the carbides apparing in the direct recycling is determined according to the chemical reactions in the process. The theories of dirct recycling of stainless steelmaking dust are detected in this research. The experimental results show that nickel and iron in the dust are easily reduced whereas chromium is hard. The kinetics research on the reduction of the dust indicates that the thermal reduction of pellet by carbon can be classified into three steps, as well as the control factors in each step and the reduction characteristics of metal oxides in the dust.It is possible to bring non-metallic inclusions into the molten steel in the direct recycling process by adding pellets to the furnace, which will affect the quality of stainless steel product. The incllusion remove depends on the ability of bubble to bring the inclusions pass through the interface of slag and steel. Water model and high temperature steel/slag model were setup to investigate the behavior of bubble on the interface. It was found that the bubble rest on the interface was dependent on the chemical composition of the slag, the interfacial tension of slag and steel, the size of bubble, the temperature of the melts. The rest time of bubble at the interface was prolonged with the increase of bubble diameter and shortened with in increase of the temperature. Behavior of the bubble resting on the interface of slag/steel was commanded through the experiments and it could delive the data for the improvement of the bubble to take the inclusions passing through the interface.To decrease the power and material consumption by improving the foamability of slag in steelmaking by electric arc furnace is the objective of metallurgists for a long time. The experimental results show the foamability of slag holds the line when the Cr₂O₃ content in the slag is below 16%, but the foamability will sharply decrease while the content is over this amount which is the solubility of Cr₂O₃ in the slag. Slag foaming depended on the amount of CO occurred in the smelting and the reduction of FeO was much faster than that of Cr₂O₃ so that the reduction of FeO produced much more amount of CO, which was the reason that the foamability of slag in carbon steelmaking was higher that that in stainless steelmaking. Direct recycling of stainless steelmaking dust was benefic to the slag foaming because a certain amount of CO was produced by the addition of pellet to the furnace. Furthermore the calcium carbonate or calcium nitrate introduction in the direct recycling produced a lot amount of CO by thermal decomposition. In other word, iner gas sources were introduced in this way. It was made clear that poor foamability of stainless steelmaking slag based on this study and the way to improve the foamability was found. The avhievement of the research is helpful for the improvement of steelmaking ability of electric arc fumace.It is the first time by the experiments to uncover the micro-process of the immerging pellet into the molten slag and to describe the solid slag shell formed and developed on the surface of the pellet and then melted down and penetrated to the inner part of the pellet. The lifetime of the solid slag shell and the mechanism of pellet melting are determined. The content of iron powder in the pellet can speed up the melting of the pellet by accelerating heat transfer. The position of pellet and the volume percentage of pellet in the slag and steel are confirmed. The method to calculate the interfacial intension and the factors affecting it are defined. This research makes the mechamism of direct recycling be deeply understood.The mathematic models for the remove of phosphor and sulfur from liquid steel were setup in this study. The transfer rate of phosphor and sulfur was the contol element of the process and the transfer coefficients for them in the initial and late stages of the process were ascertained. The functions of [P%], (P%),[S%] and(S%)with the time were established. FeO contributed to the remove of phosphor and sulfur and the content of FeO in the slag changed with the time was described. This research could delive the data for the control of phosphor and sulfur in the steel of dirct recycling.Pelletization is the first procedure of the direct recycling of stainless steelmaking dust. The strength and property of pellets heavily influence the recovery of metals present in the dust. It was found very difficult to make pellets with the dust, and only lignosulfonate used as the binder could strong pellets been made for the direct recycling of stainless steelmaking dust. The sulfur remove must be taken because element sulfur present in the lignosulfonate, the parameters of pelletization are determined and the research of pelletization is helpful for the farther work on the direct recycling of stainless steelmaking dust. An induction furnace was used to simulate the direc recycling of stainless steelmaking dust instead of electric arc furnace. The experimental results confirmed the metal value present in the dust could recoverd. The recoveries of nickel and iron were over 96% and the recovery of chromium was over 94% in the experiments. The direct recycling of stainless steelmaking dust would not affect the quality of steel product by the control of process parameters for the remove of phosphor and sulfur. Pre-reduction of pellet would go against the recover of metals and the pellet should add to the furnace in the reduction period of steelmaking after natural dryness. Metal reducing agents might be added to adjust the slag structure for the improvement of metal recoveries before the end of direct recycling of stainless steelmaking dust.For the discarding dust separated from the direct recycling process, the thermal solidification was taken after mixing with cheap and easily available clay, which was satisfied with standard toxicity characteristics leaching procedure. The mixture of 50% of dust and 50% of clay was treated at 1100℃for 15min and the product beared high thermal stability and could be used as architectural materials.In a word, the direct recycling of stainless steelmaking dust can fully make use of equipment in the steel plants and this new technology has the advantages of low investment and low cost for the operation.