Experimental Study On Removal Of Trace Arsenic From Steel By Rare Earth Elements

Cleanliness control is an essential theme of improving the performance of steel products.With the recycling of scrap steel and the utilization of low-grade iron ore in China,the control of residual elements in steel has become one of the critical problems to improve the performance of steel products.Because our country has a large number of arsenic-containing iron ore,the problem of residual arsenic is very prominent.Since the oxidation potential of arsenic is weaker than that of iron,it is challenging to remove arsenic in the normal steelmaking process.Arsenic is easy to accumulate in the phase and grain boundaries,becoming a potential source of cracks,and jeopardizing the hot workability of continuous casting slabs and the mechanical properties of steel.Rare earth elements(REEs)can react with the residual element arsenic in steel to form high melting point phases,changing its occurrence form in steel,thereby reducing its harm.Based on the previous research of our team about the study of the control of arsenic in steel by using rare earth,the feasibility and mechanism of the removal of arsenic from molten steel were studied.Meanwhile,the competition mechanism of the reactions between the molten steel and refractories was discussed.In this work,the following conclusions were obtained.Adding lanthanum to steel made the generation of arsenic inclusions that significantly affected the distribution of arsenic in the ingot.The arsenic inclusions are mainly La-S-As,La-As,La As O₄,and their composite inclusions.La-S-As is generated in the liquid phase and is primarily distributed on the surface of the ingot,thereby significantly reducing the arsenic content in the steel.The reaction between lanthanum and the crucible dramatically affected the evolution of inclusions in steel during smelting.When carrying out the steelmaking process with alumina or magnesia crucibles,the existed inclusions in steel first changed from Si-Mn-Al-O inclusions to La inclusions with the continuous reaction of lanthanum with alumina or magnesia crucibles,and the inclusions finally changed to alumina-and magnesia-rich ones.It is feasible to remove arsenic from molten steel by adding lanthanum.The principle is to generate La-S-As in the molten steel and remove it by floating.This arsenic removal reaction is affected by the lanthanum-crucible reactions.The addition of 0.15%lanthanum resulted in a large number of La-S-As compounds that mostly floated up to the surface of the molten steel,thereby reducing the arsenic content in the steel significantly.The reactions of lanthanum and the crucible consumed the effective concentration of lanthanum,causing the reaction to generate La-S-As to move in the reverse direction and the partial decomposition of the generated La-S-As,which increased the arsenic content in the steel finally.Adding 0.05%La has no significant effect on the arsenic content in the steel.Compared with magnesia refractories,alumina refractories are more conducive to the removal of arsenic from molten steel by adding REEs.