The reduction of iron ores usually is carried out in a blast furnace whereat iron ore, if desired flux materials, and coke are fed through the blast furnace mouth into the shaft, and air of combustion (blast air) is supplied through the tuyeres down in the hearth. In the classical blast furnace process, the whole energy for melting and reduction is supplied by the coke, and therefore large quantities of coke are needed. Especially in countries without own deposits of cokable coal, this need of coke is disadvantageous. It is already known to cut the need of coke by feeding liquid or gaseous hydrocarbons into the lower shaft portion or into the boshes of the blast furnace, respectively. According to this known method, the hydrocarbons are blown into the blast furnace either through the tuyeres or through separate nozzles. In this connection, it has been suggested that the liquid or gaseous hydrocarbons be supplied to the blast furnace in partially combusted condition whereat CO and H.sub.2 resulting from the partial combustion take over part of the reduction work. Thus it is possible to cut the need of coke, but there are still considerable quantities of coke needed for the operation of the blast furnace. Even if the reducing gases are introduced in preheated condition, e.g. at a temperature ranging from 950.degree. C (1742.degree. F) to 1000.degree. C (1832.degree. F), and transfer their heat to the ore, the greater part of the energy for melting and reduction in such a process is still supplied by the coke.
In all processes in which iron ore and coke are fed through the blast furnace mouth whereby the iron ore and the coke in the blast furnace shaft descend in uniflow in proportion to the proceeding of the melting process, the so-called Boudouard reaction takes place, which is a reaction of the oxygen of the carbon dioxide with the carbon of the coke. Thereby, a re-reduction of CO.sub.2 takes place at temperatures of above 1000.degree. C (1832.degree. F), and this re-reduction represents a considerable loss.
It is the principal object of the present invention to provide an improved method of reducing iron ores in a shaft furnace by which the above-mentioned disadvantages of the presently known processes are being overcome.
In accordance with one aspect of this invention there is provided a method for the reduction of ores, especially iron ores, in a shaft furnace which includes feeding ore and, if desired, slag-forming or flux materials, without any fuel or coke, respectively, into the upper portion of the shaft furnace, and feeding reducing gases originating from a partial combustion together with their combustion heat and additional heat, and, if desired, solid carbon in powder form, especially soot originating from the partial combustion, into the lower portion of the shaft furnace. It is, therefore, an essential features of the present invention that no fuel be supplied to the burden through the blast furnace mouth, so that upon descent of the burden no carbon is generated and therefore no Boudouard reaction can take place.
According to a further aspect of the invention, said partial combustion is being carried out immediately before introduction of the fuel into the shaft furnace, so that the heat of combustion can be utilized in a simple manner. The partial combustion could, however, also be carried out aside from the shaft furnace, and the heat of combustion be transferred indirectly to the blast furnace process. For increasing the temperature, it is expedient to carry out the partial combustion with preheated blast air. Furthermore, an increase of temperature can be obtained by using oxygen instead of air for the partial combustion, so that there is no nitrogen ballast to be heated.
Fuels which may be used in the partial combustion process are liquid and/or gaseous hydrocarbons and/or solid fuels. Since the heat of the partial combustion process is not sufficient for melting down iron, additional heat is supplied within the range of the zone where the partially combusted gases are being introduced. According to another aspect of the invention, said additional heat is generated by means of electrical energy such as plasma or flaming arcs or tension fields, respectively, within the burners, and/or heat energy is supplied generated by electric arcs or induction. By supplying additional heat, the requirement of heat energy for melting and reduction work and the like can be fulfilled without impairing the necessary reduction work of the introduced gases in any way.
Since no coke or other fuel is being charged through the blast furnace mouth, and as the burden consists only of ore and, if desired, slag-forming or flux materials, any re-reduction (Boudouard reaction) of carbon dioxide which causes losses in known processes is completely avoided. In the process of the present invention, only peheated reducing gases, i.e. carbon monoxide and hydrogen, developed by a partial combustion process, flow toward the descending burden column and effect the reduction. The soot generated as a product of partial combustion is being used for direct reduction and for carburization of the iron bath. By controlling the amount of soot to be developed, the grade of carburization of the iron bath can be set, and thereby a final product may be obtained which is in the range between pig iron and purified steel. The amount of soot to be developed and, as a consequence thereof, the carburization grade of the iron bath, may be controlled in an easy manner by regulating the oxygen supply to the partial combustion process. According to still another aspect of the invention, the partially combusted gases are introduced into the lower portion of the shaft furnace at different heights, if desired containing different amounts of oxygen and having different temperatures. Thereby, the process can be adapted optimally to the required course of temperatures and reductions in the furnace.
The present invention makes possible a blast furnace process without the necessity of using any coke which is of great importance for countries which do not have any, or only small, deposits of cokable coal. Besides, the economy of the blast furnace process is being improved by avoiding the Boudouard reaction, and an essential advantage of the present invention is the possibility of controlling the process at any time. The process is controllable due to the fact that the burden column contains only ore but no coke, and that the supply of reducing gases can be regulated precisely at any time whereat, particularly if additional heat is being supplied by means of electrical energy, the heat supply can be regulated precisely, too.
An apparatus for carrying out the process is characterized by the following important feature of the invention: a shaft furnace having cracking burners in its lower portion (in the present case the term "cracking burner" shall have the meaning that in such a burner the fuel is being combusted only partially to reducing gases). These cracking burners may be regulated for producing the desired reducing gases or soot, respectively. In accordance with another aspect of this invention, these cracking burners are arranged at different heights, whereat it is advantageous if the lower burners primarily generate heat and the upper burners primarily generate reducing gas and soot.
When carrying out the method of the present invention in a blast furnace of the well-known type, only ore and, if desired, slag-forming or flux materials are charged through the blast furnace mouth, while the partially combusted gases are fed into the boshes and/or into the hearth of the furnace. A blast furnace of the well-known type is, however, calculated for ore and coke to be supplied through the blast furnace mouth, and therefore it seems expedient to construct a special shaft furnace for the purposes of the present invention, wherein the lower portion, where the cracking burners are arranged, is enlarged as compared with the shaft, its cross-sectional area being expediently equal to at least one and a half times, preferably about two to four times, the cross-sectional area of the shaft.