Dephosphorization


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The only element of metallurgical technology, carried out simultaneously with melting of the charge, remains dephosphorization, while all other processes are imposed on OFP (adjustment of the steel chemical composition with regulation of the content of separate elements and the melt temperature; refining of the metal from harmful impurities, gases, and non-metal inclusions; microalloying and modification of steel and control of morphology of non-metal inclusions; improvement of technical-economic parameters of metal production both in steelmaking conversion and within the whole cycle of steel production).
Each of the conventional methods of large-scale dephosphorization has its drawbacks: Using biological treatment, it is difficult to maintain consistently low phosphorus concentrations; coagulation-sedimentation produces a large amount of sludge which requires costly treatment and disposal.
Conventional methods of dephosphorization, however, generally recover phosphorus in the form of a mixture with many impurities.
This would entail an increase in iron ore production by approximately 28 million tonnes (of which 17 million tonnes to be achieved by 2008), which would come from restarting production in Liberian iron ore mines under Mittal Steel's management in 2007, increasing the open pit mining in Ukraine to increase raw ore production, similarly expanding its Bosnian mine, commissioning a pioneering project for dephosphorization of iron ore concentrate in Kazakhstan, and investment in new mines in Mexico.
That's why it is practically impossible to perform dephosphorization of metal in a furnace with acid lining.
At sufficiently high temperature dephosphorization of the metal proceeds very slowly, or does not proceed at all.
So, dephosphorization of ferronickel is a necessary technological operation, because in commercial ferronickel weight share of phosphorus should not exceed 0.
For reducing content of impurity elements, electric furnace (crude) ferronickel is subjected to three-stage refining: desulfuration by soda in ladle; oxidation refining in converter with acid (silica brick) lining for the purpose of reducing content of chromium and silicon; oxidation refining in converter with basic (periclase-carbon or periclase-chromite) lining for dephosphorization, decarburization, and final oxidation of silicon and chromium under basic slag.
So, heat of nickel phosphide formation is higher than that of iron phosphide, which is one of the reasons of complex thermodynamic conditions of ferronickel dephosphorization.