Specialized metallurgical processes – compositions for use therei – Processes – Producing or treating free metal
Reexamination Certificate
2000-02-09
2001-07-17
Andrews, Melvyn (Department: 1742)
Specialized metallurgical processes, compositions for use therei
Processes
Producing or treating free metal
C075S569000, C420S029000, C420S590000
Reexamination Certificate
active
06261339
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a method for desiliconizing pig iron prior to subsequent refining to steel.
2. Description of the Related Art
Blast furnace pig iron, as a rule, contains approximately 0.4 to 2.8% silicon and more than 4% by weight carbon. In a refining procedure, silicon is oxidized to SiO
2
and carbon to carbon monoxide, wherein also an iron burn-up caused by the supply of oxygen can, of course, not be prevented in the course of the refining process. The slag forming during the refining of steel, as a rule, occurs as a strongly basic slag, since an accordingly high lime batch has to be charged, which is partially neutralized by the SiO
2
formed. The high basicity of such steel slags as well as the high portion of iron and chromium oxides as well as, optionally, heavy metal oxides do not readily allow for the direct use of steel slags for reasons of a potential toxicity. While blast furnace slag exhibits favorable hydraulic properties and a substantially reduced content of iron oxide and hence may more easily be supplied to an exploitation as a basic construction material, the disposal of steel works slags becomes more and more difficult, because such steel works slags in the composition as obtained, i.e., without subsequent metallurgical processing, are not readily usable for structural purposes or the like. The metallurgical processing of steel works slags usually involves a high energy consumption and high investment costs.
From EP 666 930 B1, a process has already become known, in which steel slag has been used for refining pig iron. In that known process, the high content of iron oxide of the molten steel slag is utilized for refining molten pig iron having relatively high carbon and silicon contents. Thereby, iron oxide reacts with carbon or iron carbide to form iron and carbon monoxide, whereas the iron oxide of the slag reacts with the silicon of the pig iron bath to form iron and SiO
2
.
These reactions are relatively strongly exothermic, and with a long reaction time as in correspondence with the known proposal it is immediately feasible to recover crude steel, wherein the slag parameters may be optimized with a view to the subsequent use within the scope of hydraulically active binders by admixing additives such as CaCO
3
, Al
2
O
3
and/or SiO
2
, particularly in separate converters.
When refining pig iron, in particular when refining highly silicon-containing pig iron, a quite considerable amount of SiO
2
is formed, which leads to an increase in the lime batch required for the refining process.
SUMMARY OF THE INVENTION
The invention aims to better utilize the heat balance of a reaction of pig iron with steel slag and, at the same time, provide a starting product for subsequent refining, with which the amount of slag for refining and, in particular, the lime batch can be reduced while simultaneously enhancing the iron output. To solve this object, the method according to the invention essentially consists in that molten steel slag after blast furnace tapping is charged onto the tapped pig iron, and optionally onto the blast furnace slag bath, in an amount corresponding to the Si content of the pig iron and the Fe and/or Cr oxide contents of the slag, which amount is calculated such that the Si content of the pig iron is reduced to below 0.5% by weight at a simultaneous increase in the SiO
2
content of the slag while reducing to liquid metals the Fe and/or Cr oxides of the slag, and that the slag-iron bath temperature is maintained at below 1500° C. Due to the fact that the steel slag charge is reduced, or adapted, in consideration of the specific analytical values and merely is effected to an extent that brings about a major decrease of the silicon content of the pig iron, it has become feasible to reduce the amount of silicon dioxide forming during the subsequent refining of steel and, hence, reliably keep the required basicity of the steel slags by the admixture of a reduced lime batch. By charging the steel slag directly onto the molten pig iron drawn off the blast furnace and optionally onto the blast furnace slag bath floating thereon, the blast furnace slag optionally entrained may be used for diluting and hence neutralizing the oxidative reaction process. The limitation to the refining of silicon without remarkably lowering the carbon content in the following allows for the conventional production of steel without any substantial modification except for a reduction of the lime batch. Simultaneously with the lowering of the silicon content, the iron and chromium oxide contents of the deposited steel slag may be reduced by way of an exothermic reaction, thus presenting an extremely advantageous overall heat balance. In such a reaction carried out, for instance, in a ladle, which may thus be realized at low investment costs, approximately 610 MJ/ton slag may be recovered in the form of heat during silicon refining. The thermal losses of a ladle occurring due to convection or radiation amount to about 160 MJ/ton slag, thus resulting in an overall heat surplus of about 450 MJ/ton slag. On account of this heat balance, it is, for instance, feasible to add to the pig iron, prior to its introduction into the refining converter, also additives such as, for instance, Al
2
O
3
in solid form, which will subsequently substantially improve the composition of the slag for its use as a loading material for hydraulic binders. In order to heat 100 kg Al
2
O
3
additive per ton of slag, about 200 MJ of heat are required, which may be completely covered by the above heat balance. The yet remaining surplus enthalpy may, for instance, be used to add scrap already in the mixer or in the ladle, in particular a torpedo ladle, or to do without an additional heating at a pig iron mixer. The adaptation of the charged amount of steel slag to the desired final value of the silicon content, thus, allows for a particularly economic mode of procedure, in which the subsequent steel production process will not be affected in any manner whatsoever, but rather enhanced due to the low amounts of SiO
2
during steel refining. The simultaneous mixing with optionally present blast furnace slag in the pig iron chute or pig iron mixer, or in a ladle, due to the dilution thereby obtained causes a reduction of the reaction speed and, hence, a reduction of the refining enthalpy, while nevertheless improving the quality of the slag already enhanced by the possibility of reducing iron and chromium oxides to liquid metals, and facilitating its disposal or further utilization.
An essential prerequisite for the economic feasibility of the method according to the invention is the fact that the slag-iron bath temperature is maintained at below 1500° C. Above that temperature, carbon tends to react with the iron oxide of the steel slag, because above that temperature carbon becomes less noble than silicon. It is only the limitation of the temperatures to 1500° C. that enables an economically expedient desiliconization, preventing the otherwise possible explosion-like eruptions of the slag-iron bath due to the spontaneous, uncontrollable formation of carbon monoxide. Thus, temperature control is of decisive importance to the realization of the method according to the invention, being accordingly feasible by the addition of scrap, the addition of cold slag, the removal of heat, the introduction of gas or the like, as will be explained in more detail below.
Advantageously, the method according to the invention is carried out in a manner that the Si content of the pig iron is reduced to below 0.2% by weight.
As already mentioned in the beginning, the method according to the invention may be carried out without great expenses in terms of plant technology, the method preferably being carried out such that the steel slag is charged into a pig iron mixer or ladle, in particular a torpedo ladle.
In order to ensure that the energy balance will remain positive and an accordingly high amount of liquid steel slag can be disposed of or further processed, it
"Holderbank" Financiere Glarus AG
Andrews Melvyn
Pillsbury & Winthrop LLP
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