1. Field of the Invention
The present invention relates to a method of producing stainless steel by re-using waste materials such as dust and scales produced in a stainless steel producing process.
2. Description of the Related Art
Stainless steel is generally produced by melting scraps, and raw materials such as Fe—Cr, Fe—Ni, and Ni metal in an electric furnace, and then refining molten steel by a refining furnace (stainless steel producing process). Conventionally, the electric furnace corresponds to a melting period in which raw materials are melted, but an oxidation period (referred to as a “pre-decarbonization period”) in which molten steel is decarbonized by oxygen blowing may be further provided. The refining furnace corresponds to the oxidation period in which molten steel is decarbonized by oxygen blowing, a reduction period in which Cr oxidized in the oxidation period and transferred into slag is re-reduced to a metal which is recovered to the molten steel, and a finish refining period in which the molten steel is deoxidized, and the steel components and temperature are controlled. The exhaust gas discharged from the electric furnace and refining furnace (VOD, RH, AOD, MRP, etc.) contains dust. Since the dust contains components such as Fe, Ni, Cr, and the like, the dust is preferably re-used as a raw material. Moreover, the dust contains Cr6+, and thus a great cost is required for disposing the dust. Therefore, from an economical viewpoint, it is desirable to recycle the dust. However, the dust contains Zn mainly derived from scraps, and thus when the dust is returned to the electric furnace and refining furnace without any treatment, Zn is reduced and evaporated, and scattered in the exhaust gas, thereby concentrating Zn in the dust. The dust containing concentrated Zn adheres to the inner surfaces of a throat and exhaust gas piping to cause the problem of coating the inner surfaces. In addition, oxidation and reduction of Zn are repeated at each time of recycle of dust, thereby causing the problem of deteriorating energy efficiency. Reduction of Zn contained in the dust is an endothermic reaction, and is effected in a furnace, thereby consuming heat energy. On the other hand, oxidation of Zn is an exothermic reaction and is effected in an exhaust gas system, thereby uselessly discharging most of the heat energy of the exhaust gas.
Therefore, a method has been proposed in which dust is re-used after it is reduced in a process apart from the stainless steel producing process, and then returned to the stainless steel producing process.
(Prior Art 1)
Japanese Unexamined Patent Application Publication No. 56-93834 discloses a method in which a carbonaceous reducing agent is added to mill scales, dust and sludge, the resultant mixture is pelletized, and heated and reduced in a rotary hearth furnace to produce metal-containing pellets, and then the metal-containing pellets are melted by an electric arc furnace for producing pig iron to separate and recover valuable metals such as Fe, Ni, Cr, Mo, etc. The recovered valuable metals are contained in a molten metal, and the molten metal is poured into a mold of a continuous casting machine from the electric arc furnace to form metal lumps. This publication discloses an example (example III) in which metal lumps containing 2.95% by mass of carbon are added to an electric arc furnace for producing stainless steel.
(Prior Art 2)
Japanese Unexamined Patent Application Publication No. 9-209047 discloses a method of re-using a waste material of a stainless steel producing process, the method comprising a pelletization step of pelletizing a mixture of coke and a chromium-containing blend obtained by adding an appropriate amount of chromium ore to a chromium-containing waste material produced in the stainless steel producing process to produce pellets, a reduction step of heating, by a combustion gas, the pellets allowed to stand on a hearth of a rotary hearth furnace to produce chromium-containing iron pellets with minimizing breakdown and fine generation, a waste heat recovering step of recovering, as steam, sensible heat possessed by an exhaust gas of the reduction step, and a zinc-containing dust recovering step of separating and collecting zinc-containing dust produced in the reduction step and contained in the exhaust gas discharged from the waste heat recovering step to recover the zinc-containing dust. This publication also discloses an example in which chromium-containing iron pellets are melted in an electric furnace, and used as a part of raw materials for producing chromium-containing pig iron.
In the above-described prior arts 1 and 2, in heating the pellets in the rotary hearth furnace, Zn contained in the pellets is reduced with the carbonaceous reducing agent, and evaporated and removed from the pellets. Therefore, even if the pellets after reduction are supplied to the electric furnace, the dust is not enriched with Zn, thereby preventing the problem of coating in an exhaust gas system.
However, in the above-described prior arts 1 and 2, the pellets after reduction are charged into the electric melting furnace, and used for producing chromium-containing pig iron having a high carbon content. Therefore, the content of carbon remaining in the pellets (metal lumps or chromium-containing iron pellets) after reduction is relatively high. Namely, as described above, in the example of the prior art 1, the carbon content of the metal lumps is 2.95% by mass. In example 2 of the prior art 2, the carbon content of the chromium-containing iron pellets is not specified, but 4.7% by mass of carbon is present in 125 parts by mass of chromium-containing pig iron, and the pig iron is produced from 211 parts by mass of chromium-containing iron pellets. Therefore, the carbon content of the chromium-containing iron pellets, which is estimated in consideration of the carbon content consumed by chromium reduction in the electric furnace, is 2.8% by mass or more. The chromium-containing pig iron is decarbonized to a target carbon level in a next oxidation period, and then reduced and finish-refined to produce stainless steel. Since decarbonization is performed by blowing oxygen into molten steel, Cr is oxidized with the progress of decarbonization, and is transferred into slag. After decarbonization is completed, Fe—Si is added as a reducing agent in the reduction period to reduce a Cr oxide to return the oxide to metal Cr, thereby recovering Cr in the molten steel.
However, Cr contained in the chromium-containing iron pellets is not sufficiently reduced by heating in the rotary hearth furnace (generally, a Cr metallization degree is about 40% or less), and most of Cr remains in an oxide form. The unreduced Cr oxide is metallized by reduction with carbon remaining in the pellets and carbon contained in the molten steel in the electric furnace in the melting period, and recovered in the molten steel. However, a part of the Cr oxide remains in the slag and is discarded together with the slag (electric furnace slag). Cr recovered in the molten steel is partially oxidized in a subsequent oxidation period (or pre-decarbonization period), and transferred into the slag. The Cr in the slag is again reduced in a subsequent reduction period and recovered in the molten steel, but a part of Cr remains in the slag, and is discarded together with the slag (refining furnace slag). In this way, the unreduced Cr oxide contained in the chromium-containing iron pellets is reduced in the melting period, and then oxidized in the oxidation period (or pre-decarbonization period), and further reduced in the reduction period. Therefore, an endothermic reduction reaction requires excess reduction energy, thereby causing an energy loss. Also, Cr remains in both the electric furnace slag and the refining furnace slag, thereby causing the problem of a low yield of Cr recovered to the molten steel.