1. Field of the Invention
The present invention relates generally to the art of smelting and more particularly to a method for the sequential reduction of a starting ore material in a melting furnace having two melting zones divided by a barrier.
2. Description of the Prior Art
In the production of steel, various chemical materials are added to the molten ferrous metal to remove undesirable constituents, such as oxygen and sulphur, and to impart one or more desirable properties. These properties may include controlled grain size, improved mechanical strength, and corrosion resistance, among others. Such elements, called addition agents, may include various alloys of iron, termed ferroalloys. Two important addition agents are ferromanganese and ferrochromium which are commonly used to remove and control sulphur and to introduce the elements manganese and chromium into molten steel. Typically, the ferromanganese used as an addition agent has 78-84% manganese, a maximum of about 7.5% carbon, and smaller percentages of other elements. Ferrochromium, when it is used as an addition agent, has similar proportions of chromium and carbon. Ferromanganese and ferrochromium are normally produced by refining ferromanganese or ferrochromium ores having a starting manganese or chromium to iron ratio of about four or five to one.
In one method of producing silicomanganese, another addition agent, standard grade, high carbon ferromanganese is reduced from high-grade ore leaving a gangue-containing slag having up to about 50% manganese oxide therein. After cooling and crushing, this slag may be resmelted in a charge containing a lower grade of manganese, together with silica (which may be a constituent of the ore or may be in the form of quartz), a reductant in the form of carbon, such as coal or coke, and possibly additional fluxes such as lime and silica.
Various types of furnaces, including electric arc furnaces, have been employed in the prior art for such smelting operations. One prior art method of producing ferromanganese by reduction employed pairs of electric arc furnaces. The first furnace was charged with manganese bearing ore along with other materials such as carbon which are required for the reduction to be carried out in the smelting process. Certain of the intermediate products obtained in that furnace were then transported to a second furnace for further reduction to produce ferromanganese or silicomanganese along with other products.
Since the temperatures required for the reduction of ferromanganese and ferrochromium ores are relatively high, usually above at least 1250.degree. C., and since the heat transfer rate between bodies of disparate temperature is directly related to the temperature difference between the two bodies, it is advantageous from an energy conservation standpoint to retain any material being transported from a first stage furnace to a second stage furnace in a high temperature environment. In smelting processes using separate furnaces, the material was cooled and crushed prior to delivery to the second furnace. As a result, considerable heat was lost in the process, requiring the addition of the lost heat in the second furnace. Prior art smelting processes employing two furnaces also have higher manpower requirements. Because of energy, equipment and manpower costs, prior art processes are not normally employed for smelting low-grade ferromanganese or ferrochromium ores which have a manganese or chromium to iron ratio of between four and five to one.
Another disadvantage of prior art furnaces is that the carbon refractory brick used to line the vessel hearth was often absorbed into the product resulting in unpredictable variations in product chemistry as well as erosion of the refractory itself. A method which would overcome this and the above-mentioned disadvantages of the prior art would represent a significant advance in this technology.