Reactive phosphide inclusions in commercial ferrosilicon
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I. A. Previous Work
INTRODUCTION
THE tendency for ferrosilicon alloys to crumble and evolve toxic and flammable gases has been known since the beginning of the 20th century. Research conducted in 1910 in England and Sweden found that ferrosilicon evolved hydrogen (H2) and the toxic gas phosphine (PH3) when exposed to moisture.[1] The H2 evolution is a product of iron oxidation that occurs when the material is exposed to moisture. The work conducted in England and Sweden speculated that the PH3 evolution might be associated with the presence of reactive phosphides in the ferrosilicon (i.e., AlP and Ca3P2), although these phosphides were not directly observed.[1] The speculation that AlP and Ca3P2 were present in ferrosilicon was based on the observation that lowering the content of aluminum, calcium, and phosphorus minimized the evolution of PH3 from the final product. In most commercial ferrosilicon produced today, aluminum, calcium, and phosphorus are still present as impurities. As a result, some PH3 (but in very small amounts)[2] will evolve if the alloy is exposed to moisture. Unfortunately, more stringent environmental regulations have classified most FeSi alloys as ‘‘Hazardous When Wet,’’ despite the fact that the amount of PH3 evolved is far below the specified maximum permissible ranges.[2] Recent research has verified the existence of phosphide particles in synthetic ferrosilicon.[2,3] Investigators in the former Soviet Union[3] and, independently, at Michigan Technological University[2] identified phosphide particles in the microstructure of laboratory-produced ferrosilicon alloys containing greater than 0.1 wt pct (0.1 at. pct) phosQ.C. HORN, Graduate Student, R.W. HECKEL, Professor Emeritus, and C.L. NASSARALLA, Assistant Professor, are with the Department of Metallurgical and Materials Engineering, Michigan Technological University, Houghton, MI 49931. Manuscript submitted July 21, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS B
phorus. Researchers in the former Soviet Union identified phosphides of calcium and aluminum in synthetically produced Fe-45 wt pct Si and Fe-65 wt pct Si containing approximately 10 wt pct Mn and 0.13 to 1.2 wt pct P (0.13 to 1.2 at. pct P).[3] Phosphides were observed having both inclusion and film morphologies, the films being situated on grain boundaries. Investigators at Michigan Tech observed a phosphide particle composed of calcium and magnesium in a commercial Fe-75 wt pct Si alloy that had been laboratory remelted and enriched with phosphorus to 0.1 wt pct (0.1 at. pct).[2] The phosphide phase was inclusion like and was located on the interface between the silicon and z phases. Both investigations concluded that moisture penetrates the ferrosilicon alloy through cracks resulting from thermal stresses developed during cooling and reacts with the phosphides to form PH3. To date, phosphides have only been identified in artificially produced ferrosilicon containing higher amounts of phosphorus than what is typically found in commercial ferrosilicon alloys. The phosphor
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