Dephosphorization


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Dephosphorization

 

a set of physicochemical processes that facilitate the removal of phosphorus from a metal (cast iron or steel) during smelting. It is usually achieved by oxidizing the phosphorus to its pentoxide, P2O5, which passes into the slag, where it is permanently fixed as tetracalcium phosphate, 4CaOP2O5.

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At sufficiently high temperature dephosphorization of the metal proceeds very slowly, or does not proceed at all.
In melting of metal in furnaces with basic lining conditions of dephosphorization are the most favorable within the period of the charge melting, which is characterized by low temperature of the metal and its high oxidation.
Dephosphorization of the metal in the process of melting is more efficient if oxidizing atmosphere, high activity of iron oxides in the slag, sufficiently high basicity of the slag, presence of slags with insignificant content of phosphorus, replacement of the slag, and low temperature of the metal are ensured.
It is established that kinetics of dephosphorization is determined by content of oxygen in boundary layers of the metal, degree of oxygen transition to the contact surface, on which phosphorus oxidation reaction takes place, degree of phosphorus transition to the same surface, and reaction proper of phosphorus oxidation with formation of phosphoric anhydride.
Rate of dephosphorization is expressed by the following formula:
where dP/d[tau] is the rate of dephosphorization,%/s; [[O].sub.surf] is the content of oxygen on the contact surface metal-slag,%; [[O].sub.1] is the content of oxygen at the time [tau] in the metal,%; D is the coefficient of oxygen diffusion to the contact surface, [cm.sup.2]/s; V is the total volume of the molten metal, [cm.sup.3]; A is the area of contact surface, on which the reaction takes place, [cm.sup.2]; [delta] = 0.004 cm is the thickness of the metal diffusion layer.
Calculations, carried out by the authors, allowed drawing conclusion that the limiting stage of the metal dephosphorization is diffusion arrival of oxygen to the surface, on which the reaction proceeds.
So, certain regularities of the metal dephosphorization in blowing of the molten pool by gas-reactant mixtures may be extended to the process of liquid-phase melting.
For the purpose of studying efficiency of dephosphorization and desulfuration of a high-carbon melt by the refining slag a melting was carried out with application of the metal of melts 4.11-4.15 and the refining slag, containing 56% CaO, 27% FeO and 17% Ca[F.sub.2].
Process of ferronickel dephosphorization in basic converter during its blowing by oxygen under high-basicity slag with high content of (FeO) may be presented in general form by the following summarized reaction [5, 8]: