Theory And Process Research On Producing Stainless Steel Master Alloys In A Shaft Furnace

In order to implement the transition from the big iron & steel nation to the strong iron & steel nation and reply to the increasingly ardent competition of stainless steel market, It is necessary to develop a new producing process of stainless steel which accords with the situation of China and possesses independent knowledge property rights. Aim to help the project construction of 720-kiloton/y stainless steel at Shanghai No. 1 Iron and Steel Co. Ltd., Baosteel Group, and resolve the lack of chromium, a new process of producing stainless steel master alloys by smelting reduction in a shaft furnace has been put forward in this thesis. Therefore, fruitful research have been carried out according to the main line of practice and theory of directly producing master alloys by a shaft furnace.From the view of the preparation methods for stainless steel master alloys, some new developments of producing process and their technical characteristics have been introduced in chapter two. Production costs and economic benefits have been analyzed. It is obtained that the new process not only possesses advanced technology but also economical rationality under the background of using newly technology in a shaft furnace at present.In order to ensure the success of industrial test of directly producing master alloys in a shaft furnace, theoretical analysis, detailed process calculations and a series of process experiments were completed. Process calculations involved the charge calculation, material balance calculation, thermal balance calculation and theoretical coke ratio calculation. Process preparation experiments included the selection of slag styles and the determination of slag viscosity; the performance mensuration of charging material bearing chromite during smelting dropping; and the determination of liquidus line and the flowing property of master alloys. Being well arranged on-site, the industrial test producing stainless steel master alloys was successfully performed in a 255M3 BF by Shanghai University and Shanghai No. 1 Iron & Steel Co. Ltd., Shanghai Baosteel Group Corporation. It is the first time for China to industrially produce stainless steel master alloys in a shaft-type reactor. A great deal of data, with which can provide important technical and economical basis for the decision of the new process, were obtained by the industrial test.Nearly 1000 ton of stainless steel master alloys with chromium contents from 5% to 21.3% were trouble-freely produced during the period of nine trial days. The chromium recovery rate was up to 98.02%. The operation of BF was smooth. The slag and hot metal produced had good fluidity. By converter refining, continuous casting and rolling. 35 ton master alloys with 15.6% chromium were processed into stainless steel plates. Thus whole technological process for producing stainless steel started from the BF was established. The expected objective was achieved for the whole test.The physical chemistry changes at different parts of shaft furnace were simulated in laboratory. So the whole mechanism and regularity about chromite reduction havebeen obtained. The solid state reduction of chromite at the upside in shaft furnace and the smelting reduction of chromite during smelting dropping were studied in this thesis. The reduction mechanisms of chromite under the different conditions were elucidated.Being simulated the condition of lower temperature and appropriate atmosphere at the upside of the furnace, the positive role of iron ore addition on the carbothermic solid state reduction of chromite briquet was investigated by the mass spectrometer and the loss of weight analysis method. The effects of temperature, ore size, reductant, carbon addition as well as the ores' proportion on the reduction of chromite briquet were also studied.The results investigated show as follows: (1) The reduction of chromite was promoted by adding more iron ore in briquet. It was obviously observed that the edges of chromite in briquet which containing 1/5 of Australian chromite to iron ore were partly reduced in 25 minutes at 1100*C. (2) The effect of temperature on the solid reduction of admixture briquet was significant. The higher the temperature was, the higher the final reduction rate was. It was only the reduction of iron ores and iron oxides in chromite that could be completed at 1100'C when the briquet contained 2/5 of Australian chromtie to iron ore. It was found that the starting reduction temperature of chromium oxide in briquet would be higher than 1150°C at least. (3) The smaller the size of ore was, the faster the reduction speed and the final reduction rate got. But it was difficult to increase the reduction with further smaller size of ore. (4) The reducing characteristic of coke was superior to that of graphite. The reduction of chromite briquet was improved with the increasing of carbon content in briquet. But there existed an optimum content of carbon at a particular temperature. (5) The reducing characteristic of Australian chromite was better than that of Iranian.The reaction rate limiting steps of the solid-state reduction were also analyzed through experiment results. The rate was proposed to be controlled by Boudouard reaction in the initial stage of reduction; In the second stage, the rate was consideredto be mixed-controlled by Boudouard reaction and diffusion, in which the reductionmechanism was described with a rate equation as: ; and thediffusion of gaseous products was the rate limiting step in the latter stage. The results indicated that the apparent activation energy was about 72.2 kJ/mol for the briquet containing 2/5 of Australian chromtie to iron ore.The reduction mechanism of chromite during the smelting dropping was explored by means of optical micrograph analysis, scanning electron microscope (SEM) and energy dispersive X-ray analysis (EDAX) under the simulated condition in shaft furnace. The results indicated that the iron ore of the mixed charge would get reduced prior to chromite, while the reduction of the iron oxides in chromite was prior...