Effect And Mechanism Of Iron Removal From Aluminum Melt By Boron Compounds

Iron is the most pervasive and deleterious element in Al and its alloys. Iron is difficult to be removed from Al alloys, which prevents the application of recycling Al scrap because the iron content will accumulate during the recycling process. The development of high quality Al alloys also needs the iron content in original Al materials be controlled under a low level. More efforts have been devoted on how to weaken the deleterious effect of iron (called iron phase"neutralization processing") than iron removal. So far the addition of Mn into Al-Si alloy is only the practical way to remove iron in experimental scale. The mechanism is the formation of primary Fe-rich intermetallics, generally primaryα-Al15(Fe, Mn)3Si2, then the primary Fe-rich intermetallics can be removed by filtration, centrifugal and electromagnetic separation processing. But Mn addition will result in serious Mn-Fe phases residue, which imperils the properties of Al alloy. So Mn addition processing isn't applicable.In order to meet the pervasive demand for iron removal in Al industry, a new method to remove Fe from aluminum by boron compounds processing is introduced in this dissertation. The effects of boron compounds on iron removal from Al melt are studied, and the mechanism is also clarified.In this study, the ternary phase diagrams of Al-Fe-B and Al-Fe-B₂O₃ were calculated. The phase diagrams clarified the phase formation at Al-rich corner, which was an important theory guidance for the iron removal by boron compounds. The effects of B, B₂O₃ and Na₂B₄O₇ on the iron phases in purity aluminum were studied by constant temperature holding protected by argon atmosphere, ICP-AES, Optical microscopy (OM), SEM/XRD and so on, which also proved the Al-rich corner of the calculated phase diagrams. The effects of Na₂B₄O₇ on elimination of iron in commercial purity aluminum with 0.14 wt.%, 0.66 wt.%, 1.08wt.% iron content were systemically studied by the way of flux refining, constant temperature holding, ultrasonic processing, tensile experiments and so on. The processing parameters were optimized. The mechanism of iron removal was established based on the combination of sludge and structure analysis and the calculated phase diagrams. The main works in this study are as the following:The calculated phase diagrams of Al-Fe-B and Al-Fe-B₂O₃ indicated that only AlB2 formed at the Al-rich corner of Al-Fe-B phase diagrams at the temperature from 680 oC to 1000 oC, and no iron borides formed. While at the Al-rich corner of Al-Fe-B₂O₃ phase diagram, the Fe2B formed with any Fe and B₂O₃ contents at the temperature from 680 oC to 900 oC.Only AlB2 formed and no iron borides were found after boron processing in Al melt with ¹ wt.% and 0.15 wt.% Fe, and iron concentration didn't decrease, which agreed with the calculated Al-Fe-B phase diagram. Fe2B was observed in the aluminum samples with the B₂O₃ addition of 7.61 wt.% or 0.33 wt.%, which proved the Al rich corner of the calculated phase diagram Al-Fe-B₂O₃. With the 1.5 wt.% addition of Na₂B₄O₇, Fe content decreased from 0.33 wt.% to 0.18 wt.% in Al melt, about 45 % Fe was removed. The XRD analysis of sludge proved the formation of Fe2B.A new flux, consisting of Na₂B₄O₇,KCl,NaCl, for iron removal from Al melt was invented. The main factors affecting the iron removal function of flux included Na₂B₄O₇ addition, holding time and processing temperature. Iron content decreased with the increasing of Na₂B₄O₇ addition and holding time. The iron content decreased from original 0.14 wt.%, 0.66 wt.%, 1.08 wt.% to 0.077 wt.%, 0.45 wt.%, 0.84 wt.% respectively, and the reduction rate was 45 %, 31.8 % and 22.2 % respectively. Increasing the processing temperature can increase iron reduction rate, but there were no significant changes from 720 oC to 900 oC. Usually, at 60min holding time, the iron content reached the lowest level. The appropriate processing parameters were 0.9 wt.% addition of Na₂B₄O₇, processing temperature ⁷20 oC and holding time 60 min.The extensibility of the aluminum increased firstly and then decreased rapidly with the increasing of Na₂B₄O₇ addition. The extensibility achieved optimal value when the addition of Na₂B₄O₇ was 0.3-0.5 wt.%. The extensibility of aluminum with 0.14 wt.%, 0.66 wt.%, 1.08 wt.% iron content can be improved 65 %, 70 %, 80 % higher than the original value. The tensile fracture showed characteristics of dimple. The extensibility decreased with the increase of Na₂B₄O₇ addition, and alumina were found at the tensile fracture, which indicated that the increase of Na₂B₄O₇ addition resulted in serious alumina pickup, and caused the extensibility reduction.The thermodynamic calculation proved that the iron cannot react with either the precipitated AlB2 or [B] in the liquid phase during the boron processing, while Fe2B can form spontaneously through the reaction between Na₂B₄O₇ and Fe in the Al melt under the experimental conditions. The mechanism of iron removal by Na₂B₄O₇ processing is that iron boride (Fe2B) forms, enters the flux and is removed with the sludge. The kinetic model of Na₂B₄O₇-containing flux purification process is also established, which can explain the relationship between iron content and processing time.