During the steel making processes, a large amount of steel-making slag is generated. Although the steel-making slag contains, for example, P and metal components such as Fe and Mn, it also contains a large amount of CaO, which leads to expansion and collapse. This has restricted the steel-making slag to be used as, for example, a material for roadbed or aggregate. However, in recent years, resources have been increasingly recycled, and a large number of methods for recovering the valuable substances from the steel-making slag has been disclosed.
Patent Document 1 discloses a method of processing iron and steel slag, which includes adding iron and steel slag generated during melting and making iron and steel, to molten iron and steel liquid in the smelting furnace, further adding heat and reducing agents, moving Fe, Mn, and P to the molten liquid while altering the iron and steel slag to obtain altered slag, and then, moving Mn and P in the molten liquid into the slag. However, this processing method requires batch processing to be continuously applied several times until a slag with predetermined components can be obtained, and hence, results in poor working efficiency.
Patent Document 2 discloses a method, which includes: supplying steel slags having iron oxide contents of more than 5 wt % onto a steel bath having a carbon content of less than 1.5 wt %; then introducing carbon or carbon carriers to carbonize the steel bath to obtain the steel bath having a carbon content of more than 2.0 wt %; and then performing reduction processing.
However, with the method described in Patent Document 2, the concentration of C (carbon concentration) in the molten iron is set to less than 1.5 wt % at the time of inserting the molten slag to suppress the discharge of the large amount of gas, and the concentration of C is increased to more than 2.0 wt % at the time of performing smelting reduction, thereby performing desired reduction. Thus, a process of de-carbonization in conjunction with an increase in temperatures and a process of addition of carbon for reduction are repeated, which results in batch processing. As a result, working efficiency deteriorates. It should be noted that, since the method described in Patent Document 2 increases the concentration of C to more than 2.0 wt % at the time of performing the reduction processing, it is considered that this method promotes the reduction reaction mainly through the reaction between slag and metal.
Further, in the method described in Patent Document 2, the carbon material is used as the heat source as well as the reducing agent, and hence, the amount of exhaust gas increases. Thus, it is assumed that the thermal efficiency deteriorates, and the amount of dust generated increases.
Non-Patent Document 1 discloses results of reduction tests in which steel-making slag powder, carbon material powder, and slag-modifying agent powder are inserted through a hollow electrode into an electric furnace. However, in the reduction tests described in Non-Patent Document 1, the test is performed in an electric furnace by processing the cold steel-making slag, which has been solidified and crushed, and hence, the energy-consumption rate is large.
Further, Patent Document 3 discloses a technique of recovering valuable metals by reducing molten slags generated during smelting of non-ferrous metals using carbonaceous reducing agents in an open-type direct-current electric furnace to separate it into a metal phase and a slag phase. However, the method described in Patent Document 3 also involves a batch process with an electric furnace using the cold slag as the target of the processing, and hence, the energy-consumption rate is large.
As described above, these methods of recovering valuable components from the slags each have a problem of poor working efficiency or large energy-consumption rate.