Quantitative Evaluation of Phosphine Released from Ferrosilicon

Central Laboratory, Mitra S. K. Private Limited

Introduction

Phosphine (PH₃) formation from ferrosilicon (FeSi) is a critical safety and quality concern, as PH₃ is a highly toxic and reactive gas released when phosphide impurities in FeSi are exposed to moisture. Even at low concentrations, PH₃ poses significant health risks during the handling, transportation, and storage of FeSi materials. Reliable quantification of PH₃ potential is therefore essential for industrial quality control and safe operation. A robust analytical approach involves trapping PH₃ in silver nitrate (AgNO₃) solution, where it reacts to form insoluble silver phosphide (Ag₃P). The decrease in Ag concentration is then quantified using ICP-OES. This method provides high sensitivity and ensures complete immobilization of PH₃, making it well suited for routine monitoring.

Chemical Composition of the Fe–Si Sample

The FeSi material used in this study was analyzed for its major elemental components. The composition is summarized in Table 1.

Element	Composition (%)
Si	74.37
Al	1.23
Ca	1.80
P	0.03

Reaction Chemistry

Ferrosilicon contains minor phosphide phases formed during alloy production. Upon moisture exposure, these phosphides—primarily calcium phosphide (Ca₃P₂)—undergo hydrolysis and release phosphine gas:
Ca_3 P_2+6H_2 O→3Ca(OH)_2+2PH_3↑
The evolved PH₃ is immediately directed into the AgNO₃ trap, where silver ions react selectively with phosphine to form insoluble silver phosphide:
PH_3+3Ag^+→Ag_3 P↓+3H^+
This reaction ensures quantitative conversion of PH₃ into a stable solid phase (Ag₃P), enabling precise calculation of PH₃ from the decrease in Ag⁺ concentration measured by ICP-OES.

Results and Discussion

An appropriate amount of the FeSi sample was reacted under controlled wet conditions, and the released PH₃ was captured in AgNO₃ trapping solution of known strength. Following reaction, the entire trap solution was diluted to 50 mL for ICP-OES analysis.
Measured Ag concentrations (in diluted trap):

Condition	Ag Concentration (ppm)
After treatment with 250 mL water (No sample)	188
After treatment with sample + 250 mL water	174
The net Ag depletion due to PH₃ absorption	(188-174) = 14 ppm

Correcting for the dilution, the actual mass of Ag consumed was 0.70 mg. Based on the reaction stoichiometry of 3 Ag⁺ per 1 PH₃, the amount of phosphine trapped was calculated to be 0.0736 mg.
Relative to the sample mass, this corresponds to:
PH_3=26.2" " ppm" "
The PH₃ level is consistent with the low phosphide content expected for commercial-grade ferrosilicon. The combined trapping–ICP approach proved sensitive and precise, effectively capturing all PH₃ as solid Ag₃P and allowing accurate back-calculation through Ag quantification.

Conclusion

The AgNO₃ trapping and ICP-OES method successfully quantified the phosphine-generating potential of ferrosilicon. The FeSi sample exhibited 26.2 ppm PH₃, reflecting trace but measurable phosphide impurities. Owing to its high sensitivity, complete PH₃ capture, and good reproducibility, this method is well suited for routine monitoring of FeSi batches where phosphine formation poses operational and safety challenges.

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