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نمایش همه

TECHNOLOGY FOR PRODUCING FERRO MOLYBDENUM

TECHNOLOGY FOR PRODUCING FERRO MOLYBDENUM,فرومولیبدن

Standard ferro molybdenum alloy has about 60% Mo, although grades with 50% to 58% Mo have been also produced on request.as

Also, ferro molybdenum with increased silicon content ) (<15% to 20% Si, >45% to 50% Mo) has been produced (Voronov et al., 2000).as

As the MO-Fe phase diagram show ( see Fig .12.1 ) , such an alloy has a liguidus temperature of over 1800_C , but the first liquid phase forms around 1540_C ( decomposition of FeMo siga phase ) . as

A high melting temperature and high density of the alloys (9 to 9.3 g/cm3) represent a challenge of the addition of FeMo lumps into molten steel, although this challenge is less  critical than it is for ferro tungsten. Gourtsoyannis et al. ( 1984 ) reported on the dissolution of molybdenum and ferro molybdenum in liquid steel . as

Pure Mo dissolves in stagnant steel baths at 1600_C at a rate of 0.4 mm/min, whereas the rate measured in an inductively stirred bath was 0.7 mm/ min. By extrapolation it was calculated that ~1.2 min is needed for the dissolution of a 2” FeMo lump in a stirred steel bath at 1600_C.as

Other impurities in ferromolybdenum are usually tungsten (0.3% to 0.8%), silicon (<0.5% for the highest grade and <3% for the lowest grade), copper (<0.5% and <2%, respectively), phosphorus (<0.05%), and sulfur (<0.15%).as

Nonferrous metals accompanying molybdenum in minerals and concentrates(As, Sn, Sb, Pb, Zn, Bi) normally should not exceed 0.005% to 0.01% each (Gasik, et al., 2009; Voronov et al., 2000). Carbon content is limited to 0.05% for the highest grade and 0.5% for the lowest grade of the ferro alloy.ashigh

The smelting of FeMo is based on the reduction of molybdenum oxides with silicon and aluminum off-furnace, as the process is exothermic and the alloy ingot is solidified at the end of the smelting hearth.as

The following charge compositions have been employed commercially for FeMo processing (Table 12.1).as

At the Chelyabinsk metallurgical plant in Russia, the alloy is produced from the charge consisting of roasted molybdenum concentrate (50% to 55% Mo), granulated aluminum, iron ore and chips, and lime. The main reductant is FeSiAl alloy (60% to 65% Si and 10% to 12% Al), especially designed for this type of process to provide optimal heat release during reactions.a

The smelting hearth is charged and covered and the charge is ignited. This process of ferro molybdenum smelting consists of two periods: the first includes the reaction progress and gas and solid separation; the second consists of the separation of reaction products (metal and slag).as

The duration of the first period is regulated according to the specific heat release of the charge (kJ/kg of charge) based on industrial practice. It usually varies between 10 min (~2100_C) and 50 min (~1760_C).as

Temperature of the melt in the first period determines the time of the second period (30 to 50 min) and the final result of melting. This time is needed to allow metal droplets to coagulate and to precipitate at the bottom of the hearth.as

The liquid slag is tapped at the end of the process into the slag ladle via skimmer. Even though slag has a much lower density than ferromolybdenum alloy, metal droplets are not easily separated because of the high slag viscosity (Ivanov et al., 1984; Voronov et al., 2000) when slag has 60% to 70% SiO2, 9% to 13% Al2O3, 7% to 11% FeO, and 6% to 9% CaO.as

The highest metal content in the slag is in the skimmer and stuck to the walls of the smelting hearth (slags with >0.3% Mo are recycled back to the process with ferro molybdenum scrap).as

When processing low-grade concentrates (25% to 40% Mo), there is not enough exothermal heat to achieve reasonable amounts of reductant (Al and Si); in this case the process should be carried out in an electric furnace(Voronov et al., 2000).as

 The charge is formed from low-grade concentrate, recycled slags, FeMo scrap, lime, and 65% Si ferrosilicon. Smelting in a 3.5 MVA furnace is carried out for 90 minutes (first period) with 30 minutes soaking for droplets removal

The best results were obtained with slags having CaO/SiO2 ¼ ۰٫۸۵, giving <0.25% Mo in the slag and 50% to 51% Mo in the alloy.as

Although the molybdenum content is lower than in standard alloy, the process was reported to be efficient, as more expensive aluminum or FeSiAl alloy is not used.a

TECHNOLOGY FOR PRODUCING FERRO MOLYBDENUM, تکنولوژی ساخت فرو مولیبدن,فروآلیاژ

All smelting processes release a substantial amount of fumes (~3% of used concentrate) containing MoO3 and nonferrous metals; therefore, collected dust recycling is important.a

Typical dust composition as captured by electric filters has 10% to 15% Mo, <4% Bi, <10% Pb, <10% Zn, <0.5% Cu, 15% to 18% SiO2, 10% to 12% FeO, <2% CaO, <5% MgO, and <7 Al2O3 as well as other impurities (As, Sn, Re, Cd, Se, Te, Ag, Au, etc.).a

Rhenium and noble metals have their special extracting technology. Collected dust was pelletized with coke, lime, and fluorspar and fed into a small furnace (0.5 MVA).a

The addition of lime combines molybdenum into CaMoO4, decreasing losses with vapors and increasing the reduction temperature (so molybdenum retains in the slag phase).a

The slag contains 10% to 13% Mo, <2% Pb, <6% Zn, and <0.1% Bi. The by-product alloy has essentially bismuth (40% to 43% Bi) and lead (57% to60% Pb) with little molybdenum (<0.01% Mo). In this way both slags and dust from the FeMo smelting are recycled.a

 

REFERENCES

Gasik, M.I., Lyakishev, N.P., Gasik, M.M., 2009. Physical chemistry and technology of ferro alloys [in Ukrainian]. Sistemnye Tehnologii, Dnipropetrovsk, 494 pp.a

Gourtsoyannis, L., Guthrie, R.I.L., Ratz, G.A., 1984. The dissolution of ferromolybdenum, ferro niobium and rare earth (lanthanide) silicide in cast iron and steel melts. Proceedings of the42nd Electric Furnace Conference, Toronto, Canada. 119–۱۳۲٫a

 

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