Fundamental Research To Obtain Nugget Iron From Iron-bearing Solid Waste Through Rotary Hearth Furnace

In the steel industry,there has been an increasing amount of iron-bearing solid waste.Aiming at engineering application,a technical process of producing nugget iron using iron-bearing waste in the rotary hearth furnace?RHF?has been studied.In this paper,the special emphasis has been placed on three typical kinds of iron-bearing solid waste:stainless steel dust including valuable elements?Cr,Ni?,gerenral iron-bearing dust with universal significance and sulfate slag as an industrial by-product.Firstly,a relevant fundamental research has been carried out to achieve the best technical parameters for producing nugget iron.Furthermore,from the perspective of engineering application and based on the above research results,a calculation model for the nugget iron production process in the RHF has been built.This model has been used to analyze the comprehensive utilization of energy in RHF.The results show that it can meet the needs of project implementation and industrial production.Finaly,the current process of RHF has been briefly analyzed,and some feasible innovation thoughts for process integration of RHF have been put forward,which can adapt to different raw materials.Centering on the core of nugget iron preparation technology,a high temperature self-reduction process with high basicity and carbon addition has been employed in present study.Based on characterization analysis of different raw materials,theoretical calculation of mass balance,experimental batching and pelletizing test,testing and analysis on experimental phenomena and final collected products have been conducted.The main results are as follows:Through self-reduction study of stainless steel dust,it is found that when the internal carbon ratio increases to 1.1,the reduction potential inside the pellet gradually increases;iron oxides can be totally reduced;more metal single phases precipitate and they gradually shrink from net shape to ball shape;a largest mass percentage of nugget iron can be achieved.It has been proved that the reduction temperature plays a positive role in promoting iron grains to grow up.When the reduction temperature is set at1380?¹450?,metal carburization reaction gradually occurs,which lead to low melting point alloy phase and decreasing the melting temperature of iron grains.In that case,iron grains are easy to form a molten or sub-molten state and gather into granular particles under the effect of surface tension of metal phase.When the basicity of slag phase is 2.5,the theoretical slag phase composition is distributed in the precipitated phase area of calcium orthosilicate?2CaO?SiO₂?.After reduction reaction,the slag and nugget iron can be naturally separated.Under the above optimal combination parameters,the yield rate of metal iron and nickel can get to at least 95%,that of metal chromium can get to at least80%,the percentage of iron particles with the size of 5²0mm is over 70%,the dephosphorization rate is over 75%,and the desulfurization rate is over 95%.According to the above parameters,the atmosphere experiment has been conducted in the rotary hearth furnace?RHF?.It is found that when the reduction products slowly cool to below700?,the natural separation between iron grains and slag phase can be achieved,the percentage of iron grains with the size of over 5 mm is above 80%,the yield rates of metallic iron,nickel and chromium are more than 98%,97%and 93%,respectively.The overall dephosphorization rate can reach 70%,and the overall desulphurization rate can reach 95%.Through self-reduction study of iron-bearing dust in plain carbon steel,it is found that higher or lower internal carbon ratio and slag basicity in the pellet are non-conducive to the growth and accumulation of grain iron,and the optimal internal carbon ratio is 1.1.When the reduction temperature is at 1350?¹400?,the ratio of the iron grain with the size of above 5mm can reach over 80%.Only if the theoretical basicity of slag phase is higher than 1.8,can the phase of 2CaO?SiO₂ be fully formed during the process of self-reduction at high temperature through solid phase reaction,and can the slag and nugget iron be completely and naturally separated after the reduction products are cooled.The high temperature self-reduction of carbon-containing pellets with high basicity has excellent desulfurization conditions.Meanwhile,due to the inhibiting effect of high-basicity slag on the reduction of P2O5 by carbon,the high-alkalinity interior carbon distribution pellets have a certain ability of dephosphorization during its high temperature self-reduction.With the optimized combination of parameters?carbon ratio is 1.1,slag basicity is 2.0 and reduction temperature is 1350??,the metallic iron yield is above 95%,the proporation of iron grains with the size of 5²0 mm is over 80%,and the desulfurization efficiency is over 85%.According to the above parameters,the atmosphere simulation experiment has been conducted in a rotary hearth furnace.With a low air excess coefficient and at the reduction temperature of 1380?,the iron grains with the size of over5 mm can accout for over 80%,the metallic iron yield is over 95%,the overall desulphurization rate is over 95%,and the overall dephosphorization rate is about 10%.Through self-reduction study of sulficate slag,it is found that high reduction temperature is the key factor for the formation of big-sized nugget.When the slag phase basicity is 2.5 and the reduction temperature is 1250?,a large number of small-sized low-quality iron grains form.Only when the temperature reaches 1500?,can big-sized nugget with the percentage of over 60%be obtained.The metal iron content in large particles is94%⁹5%,while that in small particles is 82⁹2%.The average iron content of both particles is over 90%.Based on the above fundamental research parameters,the calculation model of the RHF process has been built,and a calculation software of the RHF process has been developed to meet the requirements of engineering design and industrial production.According to the analysis on the energy comprehensive utilization,it is found that about30%of energy is carried away by flue gas in nugget iron production,which needs to be fully utilized to achieve the purpose of energy saving and consumption reduction.Finally,the key facilities of the RHF for the nugget iron process are analyzed from the perspective of engineering application,and the technological processes to deal with different characteristics of raw material are put forward and will be testified by further industrial tests.