Influence Of Scrap On The Bath Stirring Of Bottom-Blowing For A Converter

The main raw materials for BOF steelmaking are iron and scrap.Nowadays,the production cost of molten iron has gradually increased with the increasing consumption of ore and coal resources and high environmental protection pressure.It is put forward to new requirements for converter to increase scrap ratio.The scrap ratio increasing not only affects the heat balance of the converter,but also affects the flow of the bath,and stirring of the bath would affect the smelting process.Consequently,the current paper focuses on the effects of scrap on the fluid flow characteristics of the bath based on physical and numerical simulation methods for a a 250 t-converter.The results is expected to provide some references for the process design with high scrap ratios..Firstly,the effects of volume,category and distribution of steel scrap on the bath stirring were investigated by the physical model.It was found that the existing of steel scrap influenced the bath stirring during the blowing process.Different bath stirring can be observed when light and heavy steel scrap were added into the bath.Generally,the mixing time increased when the light scrap volume increased in the bath.Comparing to the mixing time without steel scrap addition,the mixing time increased 48.60% and134.70% when 20 and 60 t steel scrap were added into the bath with the bottom flow rate of 50 L/min,respectively.However,different change trends can be observed when applying heavy steel scrap,which distribution affected the bath stirring,in turn affected the mixing time of the bath.For the heavy steel scrap,the mixing time increased when the scrap volume decreased or the bottom blowing flow rate increased when the scrap was centralized distributed.Excessive bottom blowing flow rates would react against the decreasing of the mixing time when the flow rate was more than 40 L/min.For a uniform distribution of heavy scrap(40 and 60 t)in the bath,the mixing time decreased30.13% and 12.93% compared to that of adding 20 t steel scraps when the bottom blowing flow rate was 25 L/min.An unsymmetrical flow which accelerated the horizontal flow in the bath can be acquired when the heavy steel scrap was one-side distributed.This was positive to increase the efficiency of the bath stirring.With the same bottom blowing flow rate(25 L/min)and scrap amount(40 t),the mixing time with uniform and one-side distribution of scrap decreased 38.87% and 41.01%compared to centralized distribution of scrap in the bath,respectively.Secondly,the effects of scrap volume on the velocity distribution of the gas-liquid plume,“dead zone” and “low-velocity zone” volume and the transfer index of the bottom blowing were studied with the help of mathematical model which is built based on the physical model.The results showed that the mixing time of the bath is a decreasing function of the bottom blowing flow rate when the scrap was added into the bath while over-volume of the scrap is not favorable for decreasing the mixing time.With increasing the scrap volume,the plume formed in the bottom blowing moves towards to the bath wall.Meanwhile,the maximum velocity of the plume increased from 0.24 m/s to 0.40m/s when the bottom flow rate was 50 L/min.The volume ratio of the “low-velocity zone” was decreased when increasing the scrap volume.The ratio can be decreased 72.31% when 40 t scrap was added into the bath compared to that of no scrap addition.In addition,the kinetic energy of the bath was increased when the bottom flow rate was increased while the energy transfer index was decreased.Specifically,the transfer indexes were decreased 2.98%,6.27% and 8.68% when the bottom flow rate were increased to 25,40 and 50 L/min,respectively,compared to that of 15 L/min when 60 t scrap was added.The effects of the scrap volume on the energy transfer index was also investigated and the results showed that increasing the scrap volume is benefit to increase the energy transfer index for the bath and large volume of the scrap greatly increased the energy transfer index.When the bottom flow rate was25 L/min,the energy transfer index was increased 2.48%,1.72%,25.19% and 41.41%when the scrap volume were increased to 10,20,40 and 60 t,respectively.During the converter smelting process,part of the bottom blowing occurs asymmetrical gas blowing due to operation or other reasons,this will result in an asymmetric flow of the bath,and the distribution of scrap in the bath will also affect the fluid flowing.Therefore,this section focuses on the fluid flowing in the gap between the scarp,which based on an asymmetric flow of the bath.It found that the asymmetric flow in the bath is beneficial to promote the flow of bath in the gap between the scarp.For the condition of asymmetric blowing in the bath,a significant horizontal flow phenomenon starts to occur in the bath when bottom blowing flow on one side decreases.The volume of “low-velocity zone” quickly increased in the gap area between scrap.And the horizontal velocity of the fluid between the scrap gaps increases when the asymmetric flowing increased.The vertical gap flow between the scraps decreases when the bottom flow rate decreased.