Method for reducing chromium containing raw material

Abstract

An object of the present invention is to provide a method for reducing a chromium-containing material at a high chromium reduction degree. In the method of the present invention, a mixture of a feedstock containing chromium oxide and a carbonaceous reductant is heated and reduced by radiation heating in a moving hearth furnace. The average rate of raising the temperature of the mixture in the reduction is preferably 13.6° C. / s or higher in the period from the initiation of the radiation heating of the mixture until the mixture reaches 1,114° C.

Description

TECHNICAL FIELD [0001] The present invention belongs to the field of ferrochromium manufacturing technology and, specifically, relates to methods for reducing a chromium-containing material. BACKGROUND ART [0002] High-carbon ferrochromium (hereinafter simply referred to as “ferrochromium”) is manufactured by smelting and reducing chromium ore after pretreatment in a submerged arc electric furnace (hereinafter simply referred to as “electric furnace”). Examples of the pretreatment of the chromium ore include briquetting, sintering, pellet firing, and pellet prereduction. [0003] In pellet prereduction, chromium ore is pulverized with coke and is granulated to prepare green pellets, which are then subjected to reduction roasting in a rotary kiln at 1,300° C. or higher to provide prereduced pellets. The reduction degree of the prereduced pellets, which is 60% to 70% only with the internally added coke, reaches 80% in combination with externally added coke. This method therefore has a si...

AI Technical Summary

Benefits of technology

[0014] If the temperature of the mixture is rapidly raised in the moving hearth furnace, the reduction of chromium oxide can be allowed to start before the internally added carbonaceous material in the mixture is consumed in the reduction of iron oxide. Accordingly, the reduction of chromium oxide proceeds while the contact opportunity between chromium iron and the internally added carbonaceous material is maintained. This method can therefore provide a reduced mixture having a high chromium reduction degree. In particular, a moving hearth furnace in which a feedstock placed on the hearth is stationary is preferably used for the heating and reduction of the mixture. The use of such a furnace can significantly reduce the amount of dust produced and prevent dam rings due to dust deposited on the furnace wall. In addition, this furnace does not require extensive equipment as required for rotary kilns since the residence time of the mixture is uniform in the furnace. Accordingly, the equipment used is more compact and therefore provides the advantages of a smaller installation area and a less amount of heat dissipated.

[0026] If the carbonaceous atmosphere-adjusting agent is charged together with the mixture onto the hearth, volatile components devolatilized from the atmosphere-adjusting agent and gases such as CO and H2 produced in the solution loss reaction of CO2 and H2O contained in the atmosphere gas keep the vicinity of the mixture in a reducing atmosphere to prevent the reoxidation of the reduced mixture. The volatile components and the gases such as CO and H2 can also be used as fuels for the radiation heating in the moving hearth furnace to reduce the fuel consumption in the moving hearth furnace. In addition, the atmosphere-adjusting agent is converted into a carbon-based material that does not soften at high temperature after the devolatilization. This material can prevent the buildup of deposits on the hearth to reduce the load on a discharger that discharges the reduced mixture (or the reduced molten material or reduced solid) and the abrasion of members such as cutting edges. Furthermore, the carbon-based material discharged together with the reduced mixture (or the reduced molten material or reduced solid) can be used as a reductant and / or heat source in the following smelting step.

Problems solved by technology

Pellet prereduction is an excellent method with low power consumption; however, this method, involving the use of a rotary kiln for the pretreatment, has the following many problems unique to the rotary kiln.

Because the fundamental principle of the rotary kiln is based on the tumbling of feedstock, the rotary kiln disadvantageously produces a large amount of dust to readily cause dam rings therein.

In addition, the rotary kiln requires an excessive length due to variations in the residence time of the feedstock, thus involving a large equipment installation area and a large surface area.

Consequently, the rotary kiln disadvantageously dissipates a large amount of heat, leading to high fuel consumption.

Furthermore, a combination with externally added coke is disadvantageous in that it causes a large oxidation loss of the externally added coke in the rotary kiln.

As a result, the reduction of chromium oxide lags behind since chromium oxide is reduced less easily than iron oxide.

In the above methods, however, the internally added carbonaceous material starts to reduce iron oxide even at about 600° C. to 800° C. in the shaft preheater (while the carbonaceous material does not reduce chromium oxide at such temperatures).

On the other hand, increasing the amount of carbonaceous material added internally to maintain the contact opportunity causes the following typical problems: the green pellets disintegrate due to a decrease in strength to form deposits on the hearth; the dust loss from the rotary hearth furnace to the flue gas is increased; and the reduced pellets disintegrate, or otherwise their density decreases, to cause difficulty in dissolving in molten metal in an electric furnace, leading to a lower smelting yield.

Furthermore, the above methods make no mention of the heating temperature and temperature raising rate of the pellets and the above problem that the reduction of chromium oxide lags behind.

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