GB2255983A

GB2255983A - Stirring metal melts with methane.

Info

Publication number: GB2255983A
Application number: GB9113854A
Authority: GB
Inventors: Navarro Arturo Lazcano; Gutierrez Gregorio Vargas; Cabrera Carlos Maroto
Original assignee: MEXICANO INVESTIGACION; Instituto Mexicano de Investigaciones Siderurgicas
Current assignee: MEXICANO INVESTIGACION; Instituto Mexicano de Investigaciones Siderurgicas
Priority date: 1991-05-24
Filing date: 1991-06-27
Publication date: 1992-11-25
Also published as: GB9113854D0

Abstract

The present invention relates to the improvement of metals' melting and refining processes by natural gas injection through the bottom of metallurgical reactors. It is upon the lab and semi-industrial plant experience of the inventors of the present invention, that it was found that during the injection of natural gas into liquid metals, there is substantially no cracking of methane under usual conditions of pressure and temperature in the refining processes. In such conditions, natural gas can be used as inert gas in metallurgical processes with technical and economical advantages over inert gases, introducing it through the bottom or at the surface of the liquid bath.

Description

PROCESS TO IMPROVE THE PYROMETALLURGY OF METALS BY NATURAL GAS INJECTION THROUGH THE BOTTOM OF MELTING AND/OR REFINING METALLURGICAL REACTORS In the metallurgy of the liquid state, natural gas has been used mainly for: copper de-oxidation by means of the injection of reformed natural gas (CO+H2); annular cooling of the oxygen injection tuyeres installed in the bottom of metallurgical vessels, and in the desulphurization of pig iron, as a carrier gas for the desulphurization agents.

In the process of this invention, natural gas is used as a stirring gas in substitution of inert gases with the advantage of lower cost.

In the melting operations and by high concentration of the stirring energy, the heat transfer rate is increased between the solid charge and liquid metal previously melted, giving as result a reduction in: energy consumption, process time and oxygen consumption by lance or burners. Additional advantages of using natural gas bottom stirring during the refining periods, are the process conditions, reducing or oxidizing, that can be manipulated according to the slag type used in the metallurgical process: 1. OXIDIZING CONDITIONS.

An oxidizing slag is used in combination with natural gas injection through the bottom of the reactor. By a vigorous stirring, the metal-slag and metal-atmosphere interphase area is increased resulting in elimination from the molten metal of oxidizable elements (C, Mn, P, Al) to lower levels than those obtained in conventional practices and being the oxidation level of the molten metal lower than the obtained in said conventional practices.

2. REDUCING CONDITIONS.

A reducing slag is used in combination with natural gas injection through the bottom of the reactor or at the surface of the molten bath By a vigorous stirring, the metal-slag interphase area is increased, enhancing the transfer, from metal to slag, of impurities which require reducing conditions to be incorporated in the slag.

The natural gas is burned out in the bath surface, consuming the atmosphere's oxygen and increasing the efficiency of the reducing conditions. The levels of impurities removal produced by this practice are highly satisfactories when compared with those obtained in conventional practices.

The residence time of natural gas inside the molten metal is small enough, so that there is substantially no cracking of methane during the gas ascension.

However, at reduced gas flow rates, it is possible that some methane cracking occurs near the surface bath in a small amount.

This case has two benefits: It produces a reducing atmosphere (H2), and the carbon deposition produces an additional stirring known as molten metal "boiling".

A percentage of natural gas is endothermically cracked around the hot face of the injection element. This endothermal process provokes solidification of metal around the injection element, forming a "mushroom" that protects the injection element against wearing.

Natural gas injection through the bottom of metallurgical reactors is done through ceramic, gas permeable devices of the type which are not penetrated by the molten metal even when the gas flow is cut-off. These elements should be used when the metallurgical process requires operation with some non-gas injection stages. When the above is not a requirement, i.e., when the reactor can operate with gas injection continuously, tuyere type injection elements can be installed. Such injection elements are installed as plugs in the bottom of the metallurgical reactor and its arrangement depends on the number of them to be installed, on the physical reactor configuration and on the stirring pattern desired.

The natural gas injection practice, according to the present invention, can be applied in different areas of the metallurgical processes. The following examples of the present invention are illustrative of the application of the method in steelmaking practices: EXAMPLE 1.

Steelmaking by combined blowing in oxygen converters.

This process consists basically of oxygen injection with a lance, such as in the conventional practice, and bottom injection of natural gas through ceramic gas permeable devices, porous plugs or tuyeres. The main objective of the process is to increase the efficiency of the reactions between metal, slag and gases (injected oxygen and the atmosphere). In this case, because the initial carbon content is high, its elimination is conducted in two steps to speed up the process: A. Bottom injection of natural gas and oxygen by lance as in the conventional practice. When the carbon in the metal has values around 0.1%, the injection of oxygen by lance is stopped and the process then continues with the next step.

B. Bottom injection of natural gas at the same flow rate as the conentional practice using inert gas.

The molten bath stirring produced by natural gas injection is high enough to expose all the metallic mass to the oxidizing slag and the atmosphere. Under such conditions, the metallic surface has such oxygen potential that it is possible to decarburize to levels of 0.002%C.

EXAMPLE 2.

Ladle treatment.

In ladle metallurgy, natural gas injection can be applied in three basic operations: A. Deep decarburization. Starting with low carbon contents (0.1%C), it is possible to get carbon levels of 0.002% only with bottom natural gas injection.

B. Desulphurization. The bottom natural gas injection in combination with an appropriate reducing slag and a good temperature control (ladle reheating) is an efficient and fast desulphurization practice. By this practice it is possible to get levels of sulphur of 0.002% S in the liquid metal.

C. Vacuum degassing. The elimination of dissolved gases and the flotation of inclusions is carried out by bottom natural gas injection at the same flow rate levels as in the conventional practice using inert gas.

In addition to the economical advantage that represents the inert gases substitution, natural gas is burned out at the bath surface, giving up the combustion enthalpy. With some steel grades, particularly when the dissolved oxygen level in the melt is very low, it is recommended a final Ar bubbling stage to eliminate some possible hydrogen pick-up.

EXAMPLE 3.

Pig iron desulphurization.

Natural gas is bottom injected in a metallurgical vessel which can be a transfer ladle or a transfer car in combination with desulphurization agent additions.

This practice increases the sulphur rate transfer to the slag and reduces the time of treatment, thereby increasing the desulphurizing agents yield due to their lower oxidation level because of natural gas burning at the surface bath that consumes oxygen.

EXAMPLE 4.

Electric arc furnace steelmaking.

The bottom injection of natural gas can be applied in the EAF process with different purposes according to the steelmaking stage: A. Melting period: Natural gas is bottom injected at high levels of stirring power since the beginning of the process.

The great heat transfer achieved between the solid charge and the metal that is being melted, reduces the energy consumption and the operation time, reducing also the oxygen usage by lance or burners.

B. Oxidizing period: the bottom natural gas injection is done in combination with an appropriate oxidizing slag. The impurities' levels that are eliminated in oxidizing conditions (carbon and phosphorous), obtained by this practice, can be as low as 0.002% due to the high metal-slag contact that is produced by natural gas injection.

C. Reducing period. Bottom natural gas injection is done in combination with an appropriate reducing slag. The desulphurization level that is obtained by this practice is higher and faster than the obtained by conventional practices. In addition, the product is cleaner due to the inclusions flotation and due to less refractory wear because of the reduced time of the operation.

D. Chemical adjustment period: the bottom natural gas injection is done at stirring power levels that are enough to produce thermal and chemical homogenization of the liquid bath. By this practice it is possible to obtain greater alloy additions' yielding, as compared with conventional practices, as well as lower levels of dissolved oxygen in the liquid metal.

In the different stages of the electric arc furnace process and due to the reducing characteristics of natural gas and because natural gas is burned out on the surface bath, it is possible to obtain, with respect to the conventional practices, the following additional advantages: lower electrode consumption and lower dissolved oxygen in the liquid bath that results in higher metallic yield and lower deoxidants consumption.

In the copper metallurgy, the natural gas bottom injection practice can be applied as indicated in the electric arc furnace example, particularly during the stages of: melting, oxidizing refining (with simultaneous oxygen injection by lance) and reducing refining.

Although the present invention has been described, it is to be understood that modifications and variations may be resorted to, without departing from the spirit of the invention. Such modifications and variations are considered to be within the scope of the present invention as defined by the appended claims.

What is claimed as new and desired to be protected by Letters Patent is set forth in the appended claims:

Claims

CLAIMS 1. A process to improve the melting of metals contained as solid charge in a metallurgical reactor by introducing a stirring gas through the bottom of said metallurgical reactor, to increase interphase contact between liquid metal that is being melted and the solid charge, and to reduce: the operating time, energy consumption and usage of oxygen by lance or burners, the improvement comprising the following operations: Introducing natural gas through a gas injection device which is installed in the metallurgical reactor bottom in which the solid charge and fluxes are contained, starting the melting period by the conventional method of the metallurgical reactor which contains said solid charge and fluxes; vigorously stirring the molten metal which is being melted by the introduction of natural gas at a flow rate that is sufficient to effect an elevated kinetic stirring energy concentration, increasing the interphase contact area between the liquid metal that is being melted and the solid charge, being the natural gas flow rate sufficient to avoid a substantial cracking of methane inside the liquid metal.
2. A process for refining liquid metals that are contained in a metallurgical reactor that includes molten metal and slag by introducing a stirring gas into the molten metal to increase interphase contact between the molten metal and the slag and to reduce the amounts of undesirable elements in the molten metal, the improvement comprising: introducing natural gas into the molten metal through a gas injection device being secured in the bottom of a metallurgical reactor in which the molten metal and slag are contained, vigorously stirring the molten metal and slag contained in the metallurgical reactor by the introduction of the natural gas at a flow rate that is sufficient to effect a high stirring kinetic energy concentration increasing the molten metal-slag and the molten metal-atmosphere interphase areas to eliminate undesired elements from the molten metal and that is sufficient to avoid substantial cracking of methane within the molten metal, and wherein the metallurgical reactor includes a slag with oxidizing characteristics, provoking that the liquid metal reduces its content of impurities of the type that requires oxidizing conditions to be incorporated in the slag.
3. A method for refining liquid metals that are contained in a metallurgical reactor that includes molten metal and slag by introducing a stirring gas into the molten metal to increase the interphase contact between the molten metal and the slag and to reduce the amounts of undesirable elements in the molten metal, the improvement comprising: introducing natural gas into the molten metal through a gas injection device being secured in the bottom of a metallurgical reactor in which the molten metal and slag are contained, vigorously stirring the molten metal and slag contained in the metallurgical reactor by the introduction of the natural gas at a flow rate that is sufficient to effect a high stirring kinetic energy concentration to increase the molten metal-slag and the molten metal-atmosphere interphase area to eliminate undesired elements from the molten metal and that is sufficient to avoid substantial cracking of methane within the molten metal, and wherein the natural gas is burned at the molten metal surface to consume oxygen and to increase reducing conditions in the atmosphere above the molten metal, and wherein the metallurgical reactor includes a slag with reducing characteristics, provoking that the liquid metal reduces its content of impurities of the type that requires reducing conditions to be incorporated in the slag.
4. A method as claimed in claim 2, wherein the process includes the step of injecting oxygen by lance simultaneously with the introduction, through the bottom, of natural gas until the main impurity level (carbon) in the molten bath has been reduced to about 0.1%, and then terminating the oxygen by lance while maintaining the flow of natural gas to further deoxidize the molten metal.
5. A method as claimed in claims 1, 2 and 3, wherein the process is conducted in an electric arc furnace.
6. A method as claimed in claim 3 wherein the process is conducted in ladle transfer or in a transfer car.
7. A method as claimed in claims 1, 2, 3 and 4 wherein the process is applied in the copper metallurgy.
8. A process as claimed in claim 1 or a method as claimed in claim 3, substantially as herein described.