MXPA96005042A - Method for the direct use of chromium mineral in the production of stainless steel - Google Patents

Abstract

The present invention relates to a method for the production of stainless steel by casting metal oxide in situ in a refining reactor, characterized in that it comprises the steps of: supplying a bath of a mixture of iron / slag in the refining reactor, the iron bath contains dissolved carbon, the reactor includes means for low stirring of the iron bath, loading a chromium metal bonded with oxygen in the iron bath, injecting an oxygen containing gas through a stirring means to effect the decarburization and vigorously stirring the iron bath, slag and oxygen-bonded metal whereby a chromium alloy bath is formed having the carbon reduced to its final specification, loading a metalloid reductant into the reactor and injecting a non-oxidizing gas through of a stirring medium to rinse the alloy bath to maintain a dynamic balance and maximize the performance of

Description

METHOD FOR THE DIRECT USE OF CHROMIUM MINERAL IN STAINLESS STEEL PRODUCTION BACKGROUND OF THE INVENTION The invention relates to a three-step process for the smelting and refining of chromite ore, to obtain chromium units during the manufacture of stainless steel. More particularly, an iron bath containing chromite ore, carbon and slagging agents, is melted and refined in a reactor that produces an intermediate iron-chromium base alloy bath, having a carbon content below the saturation of the carbon. The chrome-alloyed iron bath is subsequently decarburized to specification, and any remaining chromium oxide is reduced to a high chromium yield.

An industrial method of the prior art, for the manufacture of stainless steel, is by melting scrap containing chromium and ferrochrome, in a melting furnace such as an electric arc furnace, followed by decarburization while stirring the alloyed bath with chrome in a refining reactor. Typically, about 15% by weight of the chromium is reoxidized to the slag, as the thermodynamic activity of the carbon from the REF: 23058 bath decreases. The decarburization step is followed by a reduction step, wherein a metalloid reductant such as silicon or aluminum is charged, and high purity argon is injected to recover the Cr units of the bath from the chromium oxide. This is followed by a trimming addition of ferrochrome until the final specification of the alloy is reached.

By ferrochrome is meant an alloy containing 20 to 70% by weight of chromium and 4 to 8% of carbon, and the balance is essentially iron and impurities. The Cr units in ferrochrome are expensive, due to the confidence in the electricity and the high quality chromite concentrate that is used when it is manufactured by the conventional method. The metallurgical grade chromite is coked with coke in a submerged electric arc furnace and then cast in casting molds. Efficient casting requires that the load be adequately sized.

A recent innovation is the casting of liquid ferrochrome from lower quality chemical grade chromite or its concentrate, which is then added to an iron bath in a separate reactor to retinal it in stainless steel. The U.S. No. 4,565,574, describes a process for the production of liquid ferrochrome in a porous top and bottom converter, from chromium pellets containing carbon, preheated and prereduced. The pellets are prepared from pulverized coke and chemical grade chromite ore. The pellets are loaded in a rotary kiln, together with additional coke and lime for preheating and partial metallization. The hot pellets are then charged to the converter, equipped with a porous bottom nozzle protected with propane and an upper lancet through which oxygen is injected. The purpose of the lancet is mainly the combustion of carbon monoxide (CO) from the reduction of chromite to carbon dioxide (C02), thus delivering combustion heat into the slag layer that protects the bath of metal. The heat balance is such that a significant degree of post-combustion (> 30%) and a corresponding heat transfer efficiency are needed (> 85%), to ensure that sufficient heat is available for the endothermic reduction of the chromite by carbon up to crmo and iron. It is essential to sustain a rapid rate of reduction in the slag layer, to keep about 20% by weight of coke in the slag. The presence of coke in the slag also minimizes foaming. While the presence of coke in the slag layer also helps to minimize the reoxidation of chromium from the bath to the slag, it has the unfortunate consequence of dissolving the coke coal within the bath, until the saturation limit of the coal provided with the chromium content. It also requires a so-called strong agitation, to eliminate temperature differences between the slag and the bath, and to achieve a sufficient reduction kinetics. The degree of agitation is maintained below that which is thought to result in excessive wear of the refractory lining.

The U.S. 4,961,784, discloses a method for melting crude chromite ore in a converter with porous capacities in the upper, lower and lateral parts. A liquid ferrochrome having about 18% by weight of Cr and 6% by weight of C occurs in about one hour. After the molten iron is loaded into the converter, the raw chromite ore, coke and flux at room temperature are then added to the bath. A relatively large amount of sensible heat to bring the temperature of the charge materials to the bath temperature and a high heat of reaction for the highly endothermic reduction of the chromite by the carbon, define the great requirements of total heat. This is supplied mainly from a high degree of post combustion of CO from decarburization, to a high level of heat transfer efficiency. Oxygen for decarburization and that happens after the combustion, is injected through the upper lancet, while only CO and / or argon (Ar) or nitrogen (N2), are injected into the lateral and inferior nozzles. The lancet is immersed in a foaming scum containing a substantial carbonization for the stabilization of the foam. The lancet includes a nozzle design that delivers a jet of oxygen that does not penetrate through the slag for afterburning. In combination with the proper amount of side gas injection, the oxygen injection allows a degree of afterburning of at least 30% at an efficiency to be achieved of high average heat transfer of 85%.

Japanese patent application 58-117852, describes a method for the use of a porous cap and base converter having side blowing capabilities. The raw and fine chromite ore and coke are loaded into the molten metal. However, unlike the US patent U.S. 4,961,784, oxygen is blown through all three ports, and the oxygen injected from the top is blown relatively softly. After the casting period, a finishing period follows in which the oxygen injection continues only through the upper lancet, resulting in a chromium-iron alloy saturated with carbon, having from 2C to 32% by weight of chrome.

European patent application 330,483, teaches a method for the production of a chrome-iron bath saturated with carbon from cast stainless steel scrap, followed by the melting of partially reduced chromite pellets, in a converter with blowing capacities in the upper and lower part. Scrap, coke and the first fusion smelter are loaded into the converter. The heat generated by the decarburization of the first melt melts the scrap. Flux is added to neutralize the silicon dioxide (Si02) generated from the silicon contained in the scrap and the first fusion melt. After a period of about 30 minutes, the pellets of partially reduced chromite and the carbonaceous material are loaded into the converter. The oxygen blowing in the upper and lower parts continues for about 45 minutes, producing a saturated carbon bath containing about 15% by weight Cr and 5.5% by weight C. The use of expensive ferrochrome alloys is avoided .

The patent-American U.S. 5,302,184, describes a method for injecting a material containing alloy, flux and a carbonaceous material directly into the metal bath, to make liquid ferroalloys such as ferrochrome. Liquid iron is the foundry medium, which is agitated by the injection of a gas containing oxygen. The process can be continuous, where the objective is to control the oxygen potential that enters the system, depending on the metal oxide that will be reduced. This control is achieved by controlling the relative injection speeds of the key components. The carbon content is maintained between 3 and 12% by weight, by the addition or injection of a carbonaceous material. The oxygen is also injected to effect a very high degree of afterburning, between 40 and 60%. Due to the high degree of afterburning and agitation in the reaction chamber, metal droplets are continually exposed to a source of oxygen and heat and subjected to decarburization. These drops are returned to the bath, transferring much of the required heat and supplying a spent carbon metal, which then absorbs the carbon on contact with the carbonaceous material injected into the bath.

However, there remains the need to supply inexpensive metallic Cr units directly from the raw chromite ore or the chromite ore concentrate during the production of stainless steel instead of the expensive ferrochrome. The physicochemical and thermochemical processes involved in the prior art described above for chromite ore smelting, have inherent limitations that can only be optimally adjusted to a particular set of demands. A key limitation is the production of liquid ferrochrome with relatively high carbon. A high carbon content at or near the saturation of ferrochrome produced to be refined directly into stainless steel, requires either a long decarburization stage if it is the base alloy, or a larger casting facility if it is a master alloy that will feed various reactors of refining. Another important limitation is the high degree of afterburning required for the heat balance. While this may be desirable to increase the energy efficiency of the process, it may not be as economical. A high post-combustion can result in excessive wear of the refractory and a confidence in the carbonaceous material in excess, to maintain an acceptable yield of chromium, resulting in a high carbon product.

BRIEF DESCRIPTION OF THE INVENTION The main object of the invention is to produce inexpensive metallic Cr units, from crude chromite ore not expensive chemical or from chromite ore concentrate. Another object of the invention is the reduction of the chromite ore in a simple refining reactor containing molten iron.

Another object of the invention is that at least 20% of the total metal Cr units required in the specification for a stainless steel originate from the chromite ore.

Another object of the invention is that Cr's substantial metal units are required in the specification for a stainless steel that is essentially of chromite ore with less confidence in expensive ferrochrome.

Another object of the invention is to supply metallic Cr units required in the specification of a stainless steel from chromite ore in about the same total reduction, melting and melting time or marginally increased, as compared to conventional stainless steel processing.

Another object of the invention is to integrate the chromite reduction and smelting process with an existing melting furnace to supply molten iron with a minimum capital investment.

Another objective of the invention is that the chromite reduction and smelting process is adaptable to a small-scale casting or specialty facility, requiring a minimum capital investment and a marginally increased production time.

The invention relates to a process for the reduction of metal oxide, for supplying metallic Cr units during the production of a high chromium alloy bath for manufacturing stainless steel. The invention includes providing an iron / slag bath mixture within a reactor having means for stirring the iron bath. The iron bath contains dissolved carbon, chromium bonded with oxygen and iron metal and constituents that accompany the slag. A gas containing oxygen is injected through the agitation means to effect the decarburization and to vigorously stir the iron, slag and metal bath bonded to the oxygen, to form a chrome alloy bath. The oxygen content of the stirring gas decreases as the carbon content of the alloy bath approaches its final carbon specification. A reducing metalloid is then loaded into the reactor and a non-oxidizing gas is injected through the agitating means to rinse the alloy bath until a dynamic equilibrium is maintained and the chromium yield is maximized.

Another feature of the invention is that the reactor includes means for blowing oxygen gas at the top, with the discharge of a portion of the oxygen gas above the iron bath to effect the post-combustion of CO and H2, and the remaining oxygen gas is injected inside the iron bath to effect decarburization and generate CO.

Another feature of the invention is for the total gas passing through the blowing means and passing through the agitation means, being at least 0.5 NM3 / in / MT.

Another feature of the invention is for 30 to 60% of the total gas flowing into the reactor until it passes through the agitation means.

Another feature of the invention is for the degree of afterburning of CO and H2, being less than 50%.

Another feature of the invention is for the aforementioned stirring gas, which has an initial molar ratio of oxygen to non-oxidizing gas which follows the afterburning of 4/1, with a ratio decreasing to 1/3 by the end of the decarburization.

Another feature of the invention is for the temperature of the iron bath before blowing oxygen, which is at least 1,500 ° C.

Another feature of the invention is for the initial iron bath containing at least 0.5% by weight and until the saturation of the coal.

Another feature of the invention is for the chromium alloy bath containing 0.5-1.5% by weight of C and at least 2% by weight of Cr at the end of the afterburning.

Another feature of the invention is for the total chromium yield which is at least 70% at the end of the afterburning.

Another feature of the invention is for the aforementioned oxygen bonded metal, which is from the group consisting of crude chromite ore, chromite ore concentrate, partially metallized chromite ore and chromium oxide powder.

Another feature of the invention is for the metal bonded to the aforementioned oxygen to preheat to at least 1000 ° C.

Another feature of the invention includes adding a solid, a carbonaceous reductant and scoriating agents to the initial iron bath.

Another feature of the invention is for the carbonaceous reducer that includes solid carbon in an amount in excess of that required for premetalization of the metal bonded to oxygen.

Another characteristic of the invention is for the weight of the slag during the post-combustion, exclusive of Cr203 or FeO, not to exceed 400 kg / MT.

Another feature of the invention is to add a metalloid reducer to the initial iron bath.

Another feature of the invention is that at least 20% of the total metal Cr units of the carbon-chromium alloy bath are derived from the chromite ore.

Another feature of the invention is for the initial iron bath that is melted in an electric arc furnace from solid ferrous materials of the group consisting of scrap of carbon steel, scrap of stainless steel and waste of steel plants.

The advantages of the invention include an economical process to produce stainless steel using chemical grade, economical chromite ore and concentrates, which can melt and refine the steel in the same refining reactor and minimize the reoxidation of the chromium during the decarburization of the iron bath . Another advantage is being able to produce stainless steel using stainless steel scrap and expensive iron-chromium alloy as a secondary source of metallic Cr units. An additional advantage includes integrating the process with an existing electric arc furnace on a smaller, specialty scale or in a mini-casting facility with minimal capital investment.

The foregoing objects and other features and advantages of the invention will become apparent upon consideration of the detailed description and accompanying drawings.

BRIEF_PESCRIPTION OF THE DRAWINGS Fig. 1 illustrates schematically one embodiment of a reactor for use in the process of the invention, Fig. 2 illustrates schematically a lower portion of a lancet of the reactor of Fig. 1 including a gas passage for oxygen charging above an iron bath for combustion and another gas passage or passage for injecting oxygen into the bath of iron for decarburization, Fig. 3 illustrates a sectional view taken along line 3-3 of Fig. 2, gas passages or passages for discharging oxygen within the reactor, Fig. 4 schematically illustrates the% by weight of the cr bath during the conventional casting and refining of the stainless steel, FIG. 5 schematically illustrates the% by weight of the Cr bath during casting and refining when stainless steel is made in accordance with the present invention. _ DETAILED DESCRIPTION OF THE PREFERRED MODALITY An object of the invention is to derive the maximum metallic Cr units required when making a stainless steel from inexpensive sources of metals containing chromium bonded with oxygen or chromium oxides such as crude chromite ore, concentrate made of chromite ore, partially metallized chromite ore and waste from steel plants containing chromium oxide. Up to this point, as much as 90% of the metal Cr units can be derived from the chromite ore when AISI grade stainless steel is manufactured such as 409 and 50% of the Cr units when grades AISI 304 and 439 are made. It will be understood that some portion of the Cr units required of the stainless steel specification can be purchased from fillers containing chromium such as stainless steel scrap. It will be further understood that a minor amount of ferrochrome alloy can also be used as a final trim addition to adjust the bath specification to that required of the final specification of the alloy.

The invention relates to a three-step process for producing stainless steel directly from a chromium metal bonded with oxygen. After heating in an iron bath, the chromium metal bonded with oxygen is melted at least partially in a well-stirred slag / iron bath, to a low or medium carbon content and to an intermediate chromium run. The bath alloyed with chromium is then decarburized until the final specification of the bath. Since a portion of the chromium metal bonded with oxygen remains during casting, an important advantage of the process of this invention is that the reoxidation of the chromium in the bath during the decarburization is minimized. Hereinafter, the chromium-alloyed bath is further reduced with a metalloid reducer to obtain high chromium yields at low carbon contents by recovering the Cr metal units from the chromium oxide. The melting, decarburization and reduction occur in the same refining reactor. The entire process can be carried out in a melting furnace equipped with a melting furnace such as an electric arc furnace and a refining reactor, preferably retro-coupled with means for blowing by the top of oxygen gas such as an argon decarburizer. oxygen or an oxygen-vacuum decarburizer, thus recovering the capital costs.

An advantage of the present invention is to provide an inexpensive process for the manufacture of stainless steels in a refining reactor using less expensive and concentrated chromite ores that partially replace the relatively more expensive ferrochrome and stainless steel scrap. More specifically, the partially reduced preheated chromite is melted in an iron bath, which is refined directly to stainless steel in the same reactor. The process of the invention is such that it is economically implemented in an existing stainless steel foundry having a preferably back-coupled reactor with upper blowing means such as a lancet in an Oxygen-Argon Decarburizer (DOA), an Oxygen-Vacuum Decarburizer ( DOV), a Crusot-Loire-Uddeholm converter (CLU), or having installed a generic Upper and Lower Blower Refining Reactor (RRSI).

The basis of the present invention is the exploitation of the mixing capacity of the DOA, DOV, CLU or RRSI to facilitate the rapid reduction of the chromite ore, a metal oxide normally difficult to reduce. Coal is the main reducer in the early stages of smelting. Any of the silicon metalloids (Yes), aluminum (Al), titanium (Ti), magnesium (Mg) or calcium (Ca) can be a co-reducer in the early stages of casting, and the only reducer in the final stage of reduction. Preferred metalloid reductants are Si, Al or a mixture thereof.

The heat deficit which is generally incurred in the smelting of the chromite ore by coal is preferably covered by a combination of the following steps: preheating and partial metallization of the chromite charge, post-combustion of the gases resulting from the melting, e.g. ex. , CO, and / or the addition of a metalloid reducer. When the metalloid reducer such as Si and Al are the reductants during the final reduction, the heat generated by the exothermic reaction contributes significantly to the heat balance and is the only source of heat needed. In the early stage of partial melting when sufficient oxygen is injected, the heat generated by the combustion of the metalloids with oxygen and the exothermic reduction can compensate for one or a combination of post-combustion, or preheating or partial pre-metalization or loading. the chromite. However, an economic penalty may be incurred, since metalloids such as Si and Al are more expensive as reducers by weight of chromium reduced than coal. Moreover, its use, particularly in the early stage of smelting, depends on the degree of premetalization, which can substantially increase the weight of the slag, ultimately limiting the load of chromite by weight per tonne of alloy produced. In the present invention, the primary reducer is coal with metalloids possibly present in the first stage depending on the degree of heat balance. These metalloids, however, must be used in the final casting stage to improve the chromium yield in the absence of injected oxygen. In the final reduction stage, in spite of the absence of oxygen injected for the combustion of the metalloids, the exothermic reduction of the chromite by metalloids is generally sufficient to compensate the heat requirements, mainly called heat losses and sensible heat of a inert gas in agitation.

The process of the invention includes three distinct stages which are presented consecutively in a refining reactor 10 such as the RRSI illustrated schematically in FIG. 1. The reactor includes a refractory liner 12, final top or throat 14, a low or low end 16, oxygen blowing means such as a lancet 18 extended to a point just above the bath and means 22 such as the nozzle or porous stopper, mounted on or near the 16th of a reactor and extended to the refractory cover for agitation and a slag / iron slag mixture containing dissolved carbon. The iron bath can be covered by a thin layer of slag 26, depending on the weight of the slag.

The process of the invention will now be fully described. An oxygen-bonded chromium metal such as the preheated chromite mineral pre-metallized together with a carbonaceous reductant and slagging agents are charged into the reactor through the throat 14. The lower portion 20 of the lancet 18 is then lowered into the reactor but not within the iron / slag mixture 24 to inject oxygen gas into the reactor. The oxygen gas will preferably be pure oxygen. If the reactor does not include the lancet 18, reducing metalloids such as silicon will be added together with the carbonaceous reducer and with the scorifying agents to supply the necessary heat in the oxidation and reduction of chromite. FIG. 2 illustrates the lancet 18 which preferably includes a pair of channels having a common oxygen supply (not shown) in such a manner that the flow velocity of oxygen gas through one of the channels being injected into the bath 24 can be independent of the velocity of the oxygen gas flowing simultaneously through the other channel for the afterburning of CO to C02. Carbon monoxide afterburning of chromite reduction to carbon dioxide is needed to release heat within the slag layer and the slag / slag bath to ensure sufficient heat available for the endothermic reduction of the chromite by the carbon to chrome and iron. The oxygen gas 30 flows through the central channel 34 which is a compact directed jet injector which can penetrate through the slag layer 26 and into the bath 24 for decarburization of the molten iron. The oxygen gas 28 flows through another channel 32 is dispersed over and above the iron / slag bath 24 for the post-combustion of CO to C02 to auxiliary heat supply to the molten iron. The stirring means 22 is adapted to inject gas containing oxygen and non-oxidizing gas. The portion containing oxygen gas for agitation may include air, air enriched with oxygen, pure oxygen, water, steam or a combination of them as well as Ar, N2 or a mixture of them. The agitating means 22 can include one or more concentric tubes with gas containing oxygen flowing through the inner tube and a methane gas flowing through an outer tube. The methane gas functions as a nozzle cooler. With a lower injection of continuous gas a disintegration and fusion of the materials of the charge is created, a turbulent mixture of slag, molten iron and chromite grains. While the chromite is pre-reduced, the scorifying agents and the residual carbon dissolve all in contact with the hot iron / slag bath, the chromite not reduced in the charge will exist as small as solid grains dispersed in the slag / metal mixture. .

FIG. 3 illustrates the central channel 34 for injecting oxygen into the bath which is a metallic tube 35. The outer channel 32, for discharging the oxygen above the bath, includes four equally spaced annular nozzles 33 diverging outward at an angle β (FIG 2) about 45 ° relative to the central axis of the lancet 18. The nozzles 33 terminate in a conical transition section 21 of reduced thickness in the lower portion 20 of the lancet 18. The lancet 18 additionally includes a pair of concentric conduits 36 and 38 to transport a refrigerant through the lancet. To obtain a good afterburning, the interaction of the oxygen gas 30 passing through the nozzle 33 and the oxygen gas 28 passing through the tube 35 must be minimized. At this point, the oxygen gas 28 passing through the nozzle 33 must diverge downwards but away from the central axis of the vertical lancet at an angle ß * of at least about 30 °. Otherwise, to reduce the oxygen velocity into the reactor wall, the angle ß should not exceed 60 °. The flow rate of the oxygen gas 30 through the nozzle 33 is preferably controlled independently of the flow velocity of the oxygen gas 28 flowing through the tube 35. It is understood that more than four nozzles 33 or one tube 35 can be used depending of the requirements of oxygen flow velocity or reactor size.

The fusion and dissolution of the material load, the partial melting of the chromite by coal and the generation of heat by decarburization and afterburning are the key events of stage 1. Depending on the degree of premetalization of the chromite, the smelting proceeds to a Cr yield of at least 70%, possibly as high as 85% or higher, while the bath temperature remains unchanged from its initial temperature. The initial temperature of the iron bath can vary between approximately between 1500 ° C and 1750 ° C, preferably between 1600 ° C to 1650 ° C. The temperature is preferred to be below 1750 ° C due to the cost associated with the excess of refractory use in the RRSI.

The oxygen bound base metal of the invention can be crude chromite ore, a concentrate made of chromite ore and stainless steel plant waste containing chromium oxide. By chromite ore or chromite concentrate is meant a metal oxide containing between 25-55% by weight of Cr203 and the balance of FeO, MgO, SiO2. A103 and CaO. The weight ratio of Cr / Fe is preferably between 0.9 and 3.5, preferably 1.5 to 2.0. If the chromite is not partially metallized, the average size of the chromite grains should preferably be below the 50 mesh to decrease the melting time. The concentrate of size below the 100 mesh should preferably be injected to avoid excessive loss of fines. If the chromite is pre-metallized, the mineral must be pulverized to have a grain of 200 mesh size prior to agglomeration. Steel plant waste containing chromium oxide is understood as kiln dust, acid sludge, crust of rolling mills and the like.

If the raw chromite ore in the form of lump or coarse concentrate is used, the chromite can be mixed with a solid, carbonaceous reductants and scorifying agents, and charged dispersed into the reactor. Alternatively, the chromite ore can be pulverized and agglomerated as sinter or pelletized or injected directly into the iron / slag bath. If they are agglomerated, the carbonaceous reductant and the scorifying agents should preferably be mixed with the chromite grains pulverized and combined within the sinter or pelletized. The sinter or non-metallized pellets at room temperature (25 ° C) can be preheated and partially metallized in a rotary kiln or in a rotary kiln, or in similar furnaces capable of solid-state reduction of chromite by carbon solid to partially pre-metallize the chromite grains by the accompaniment of the carbonaceous reducer. If they are pre-metallized, the sinter or pellets should preferably be charged while still hot, immediately after being removed from the reduction furnace inside the reactor at a temperature above 1200 ° C after being transported to the RRSI. The discharge temperature in the reduction furnace must not exceed 1400 ° C due to damage to the refractories in the reduction furnace. Preferably, a mixture of pre-metallized and preheated carbon-chromite slagging agents are charged into the reactor with the chromite having a chromium metallization of at least 10%, an iron metallization of at least 50% and a temperature of at least 1000 ° C.

A carbonaceous reductant is a predominantly solid, carbon-containing material. This carbonaceous reductant may accompany the pre-metallized chromite in excess of that required for the pre-metalization process, or it may be charged separately to the iron bath in the reactor as in the case where the chromite is not metallized. If the iron is melted it is supplied upstream of the reactor in a melting furnace such as the Electric Arc Furnace (HAE), the carbonaceous reducer can be partially or completely loaded in the HAE. Suitable carbonaceous materials include coke, charcoal, petroleum coke, coal, graphite, low and medium volatility bituminous carbons, and anthracite carbons. Depending on the solid iron materials used to produce the iron bath, it is understood that the initial iron bath can contain enough dissolved carbon for the chromite smelting in Step 1 and it may not be necessary to add carbonaceous reducing material to the bath in the reactor.

Suitable scorifying agents for use in the invention include CaO, MgO, Al203, Si03 and CaF2. One or more scorifying agents can be added to the iron bath in the refining reactor, upstream in a melting or casting furnace, or upstream as such during the pelletization of the chromite. The use of scorifying agents is preferred to preferably maintain the slag in basicity and preferably the MgO / Al203 ratio, depending on the source of chromite and the degree of use of silicon or aluminum as reducing agents.

The iron bath may be formed in a blast furnace or in any other casting unit capable of supplying liquid iron from solid iron containing materials, including iron oxides. Alternatively, the iron bath can be formed by casting solid, scrap containing iron and the like, either in the refining reactor or preferably upstream of the reactor with a melting furnace, such as an HAE. The proper solid, scrap containing iron to form the iron bath includes scrap carbon steel, scrap stainless steel, iron carbide, direct reduction iron (FRD) or hot briquetted iron (FBC). In the case where the iron bath is produced by scrap cast in an HAE, the carbonaceous reducer and the scorifying agents may be dissolved or partly or completely melted in the iron bath while the iron bath is still in the HAE prior to being transferred to the reactor. Depending on the furnace and the type of iron-bearing materials used, the initial iron bath can contain 0-15% by weight of Cr, 0.5% by weight of C and above the carbon saturation.

All three stages of the invention will now be fully described in detail.

During Step 1 of the process of the invention, an iron bath at a temperature of at least 1500 ° C is supplied inside a refining reactor. The chromite ore is pulverized and mixed with an excess of solid carbon and scorifying agents. The mixture is agglomerated in pellets and the pellets are partially metallized in a rotary chimney furnace as described in US Patent No. 08/470311, filed on June 6, 1995, entitled "Method of Reduction of a Metal Oxide in a Rotary Fireplace Furnace Heated by a Flame Oxidizer ", incorporated here in the reference, after being pre-reduced to at least 10% chromium and 50% iron metallization, the pellets are loaded through the throat of the reactor while the temperature is raised to at least 1000 ° C, preferably to at least 1200 ° C. The oxygen gas is blown through the lancet, and the oxygen-containing gas is injected through the stirring means into the reactor. a total flow rate between 0.5 and 4 NM3 / min / MT, preferably at least at 2 NM3 / min / MT and most preferably at least 3 NM / min / MT.The percentage of total gas flowing into the reactor through the means of agi tion is between 30 and 60%. The gas injected through the agitation means includes a non-oxidizing gas in which the ratio of 02 / non-oxidizing gas is between 2 and 4. If the reactor is a CLU converter, the gas containing oxygen may include steam because in the reaction with the carbon dissolved in the iron bath H 2 H formed which reduces the partial pressure of CO and can be substituted mol to mol by Ar. If the reactor is an DOV, for stage 1 the reactor is operated much more like a DOA where the oxygen is blown through a top lancet through a lower nozzle accompanied by an inert gas. Acceptable non-oxidizing gases include inert gases such as Ar or N2, where Ar is preferred. Oxygen passing through the lancet serves two functions: supplying oxygen for decarburization and oxygen for secondary reactions, or for the degree of afterburning of CO and H2 that comes from the bath. Both combustion reactions supply heat to the iron bath, as the afterburner is generated twice as much heat as by decarburization per unit of oxygen consumed. The Post-combustion Degree (GPC) is defined by the gas coming from the reactor as: 100X (% CO2 +% H2O) / (% CO +% CO2 +% H2 +% H2O) In Step 1, the GPC is less than 50%, prably between 20 and 30% when used as a heat source. The total amount of pure oxygen gas as pure 02 to be supplied to the iron bath is calculated based on the heat and mass balances. The lancet nozzle is designed to simultaneously direct a portion of the oxygen gas above and above the bath via a broad jet specifically to affect post-combustion, and the remaining portion of oxygen gas, within the bath via a compact jet specifically for the bath. decarburization. The desired GPC of the waste gas is obtained by adjusting the shape of the nozzle, mainly affecting the angle of the broad jet and its momentum, as well as the height of the nozzle above the bath. It is important that the lancet nozzle is not positioned inside or through the iron / slag mixture to ensure that the portion of oxygen gas blown through the lancet burns above the bath. .. "The portion of heat generated by the afterburner, at a given GPC, that is truly captured or transferred to the bath, excluding that loss for free feeding and for the gas released is rred to as its Heat Transfer Efficiency (ETC). . An important feature of the invention is that the lancet should not be submerged within the bath to ensure that afterburning is present on the bath. Significantly less heat is available to be captured or transferred into the bath than if the lancet were submerged. As a result, the ETC of the present invention is expected to be 50% or less. This in contrast to the ETC achieved in the order of 80 to 90% when the lancet is submerged. The submerged lancet requires ample presence of solid carbon to prevent significant reoxidation of Cr and Fe from the chromium alloy bath for the slag and to prevent foaming scum. Stage 1 is continued, p. ex. passing the oxygen gas through the lancet accompanied by a low injection of oxygen-containing gas, until the content of carbon in the bath drops to no more than 1.5% by weight, prably less than 1.0% by weight of C, more prably less than 0.7% by weight and best as little as 0.5% sui. At this point, the Cr yield of the total chromium should be at least about 70% and the bath of the chromium alloy should contain at least 2% by weight of Cr and have a temperature not higher than 1750 ° C. Prably the yield of Cr should be at least about 70% and the bath of the alloy should contain at least 5% by weight of Cr and the best yield of Cr should be at least about 85% and the bath of the chromium alloy should contain at least 8% by weight of Cr.

Another important feature of the invention is to control the composition of the slag basicity and the MgO / Al203 ratio. The basicity of the slag is defined as the weight ratio of (% CaO +% MgO) /% Si02. This basicity of the slag must be at least 1.0, prably at least 1.5, more prably at least 2.0 and best at least 2.5. A higher basicity of the slag, however, should not exceed 3.0 because the slag becomes very viscous at high concentrations of CaO and MgO due to the increase in liquid temperature. Al203 present in the slag should prably be in the range of 15 to 25% by weight. In the same way the MgO should be in the range of 10 to 20% by weight and the MgO / Al203 ratio should be between 0.3 and 0.8.

Another important feature of the invention is the control of the specific weight of slag as slag Kg / TM of metal. If the slag weight becomes excessive, the effective mixing of the slag becomes very difficult. The weight of the slag, excluding the chromium oxide accumulated therein in steps 1 and 2, should not exceed 400 kg / MT of metal, and prably should not exceed 350 and more prably should not exceed 300. Generally, the slag is dragged into the bath during the vigorous mixing action of the gas injection through the bottom nozzle. As the slag weight increases much above 300 kg / MT of metal, a significant portion of the slag can coalesce as a slag layer, where the absence of mixing in the layer inhibits the kinetics of the reduction and the transfer of post-combustion heat. As a result, the weight of the slag can limit the amount of chromite ore charged for a given chromite chemistry.

During Step 2 of the process of the invention, the bath is decarburized to near the desired carbon specification, for the grade of stainless steel that is being produced. The beginning of this step is marked by the cessation of the passage of oxygen gas through the lancet and the start of the reduced injection of oxygen-containing gas through the agitation means. The process of decarburization in a DOA requires that a non-oxidizing gas, such as an inert gas such as Ar, be included with the oxygen-containing gas, wherein the ratio of 02 / Ar is systematically decreased. That is, the flow of inert gas in relation to the flow of oxygen increases. This procedure in the DOA preferably starts at a 02 / Ar ratio of about 4/1, which decreases step by step or continuously to a ratio of l / 1 over a period of 15 to 30 minutes. The chrome alloy bath is sampled, and then agitation of the decarburization is continued if necessary, up to about another 10 minutes at a 2/3 Ar / 1/3 ratio. Steel scrap can be added to the coal or scrap of stainless steel as a quencher if needed, to compensate for heat generation by decarburization after compensating for heat losses and sensible heat of the stirring gas, so as to maintain approximately constant bath temperature, preferably in the range of 1600 to 1650 ° C. If the reactor is a DOV, the agitation means are effected by a large drop in pressure. The dissolved oxygen becomes supersaturated and reacts with the residual carbon forming CO and therefore decarburizing the bath. The bath then becomes agitated by the CO that is vigorously dismissed.

Another important feature of the present invention is the absence of a significant re-oxidation of chromium to slag during stage 2. During the conventional decarburization of an alloy bath with chromium produced from ferrochrome scrap and stainless steel, as the carbon content decreases, the chromium and iron are oxidized to slag such as Cr203 (s), FeO. Cr203 (s), CrO (l) and FeO (l). This re-oxidation is the result of an increase in the partial pressure of oxygen, controlled by the carbon-oxygen equilibrium in the bath as the thermodynamic activity of coal decreases during decarburization, independently of a lower partial pressure of CO. Typically, at least 10% and as much as 30% of the chromium in the chromium alloy bath can be reoxidized in this manner, causing the chromium yield at this point to decrease significantly. A significant disadvantage inherent in prior art processes is illustrated schematically in Fig. 4. That is, as decarburization continues, the Cr content in the bath may drop from say, about 10% by weight to as much as as 7% by weight in number 42.

In contrast to the present invention, significant reoxidation of the chromium in the bath to the slag is evaded by the presence of unreduced chromite from step 1. Its presence maintains a higher thermodynamic activity of FeO. Cr203 (s) as well as Cr203 (s) and CrO (l) in the slag, thereby reducing the driving force to reoxidize the chromium despite the higher partial pressure of oxygen at the end of the decarburization. This is illustrated schematically as numeral 44 in Fig. 5, that is, the invention. This is also true in the case where the reactor is a DOV where the partial pressure of the CO is reduced by the vacuum, rather than by the dilution with Ar. Independently of the lower partial pressure of CO by the vacuum in the DOV, as the thermodynamic activity of the coal decreases, the activity of Cr203 tends to increase. As in the DOA, the presence of unreacted FeO.Cr203 from step 1 tends to maintain a high Cr203 activity, thereby minimizing further chromium oxidation. A limited amount of oxidation may occur at the end of the decarburization in Step 1 of the invention. Similarly, a limited amount of chromite smelting by carbon may occur at the beginning of step 2 of the invention. As a result, the chromium yield remains approximately the same at the end of stage 1 and approximately that which is normally found at the end of the decarburization in the routine practice of stainless steel refining.

Stage 3 Stage 3 of the process of the invention is also a reduction stage, but where one or more of the metalloids Si, Al, Ti, Mg or Ca are the reducing agents rather than the carbon. Also, a non-oxidizing gas such as Ar, preferably Ar of high purity, is injected through the agitating means to effect vigorous mixing upon contact of the reducer dissolved in the chrome alloy bath with the various oxides of chromium. and iron. These oxides reduce the dissolved metal, increasing the chromium yield generally up to more than 95%, depending on the balance or quasi-equilibrium chromium-chromium oxide. The maximum transfer of chromium from the slag to the metal is achieved under conditions of vigorous mixing of metal and slag, at a high basicity where equilibrium is reached. By quasi-equilibrium it is meant that the iron-slag interfacial movement is sufficient to result in < * a dynamic balance between the iron bath and the slag containing the chromium oxides, resulting in a thermal and chemical equilibrium that closely approximates between the iron and the slag.

The reduction of the chromite by these metalloid reducers is exothermic, compensating the heat losses and the sensible heat requirement of the stirring gas. Thermal bath adjustments can be made by adding coolants such as scrap steel or any required trimming additions. Trimming additions may include small amounts of stainless steel or ferrochrome scrap to cover the final chromium specification.

Pilot Tests of the Invention Cast iron was loaded into a preheated 1/2 ton pilot reactor, equipped with a commercial porous plug, through which argon flowed. The iron was melted in an air induction furnace with a capacity of 550 kg and bifurcated through a refractory tundish inside the reactor. The heats were derived as hot as possible, typically from 1700 to 1750 ° C, to overcome the relatively high thermal losses due to the small size of the heat and the high sensible heat requirements of the fillers. With a D-cast working lining and an alumina-supported liner in the pilot reactor, heat losses through the walls and open top represented only 9 ° C / min. The capacity of the reactor used for the pilot tests of the invention was limited to only the bottom agitation means, thus not allowing the option of decarburization and post-combustion from the blowing oxygen in the upper lancet.

Partially metallized chromite pellets containing carbon and scoriates were cold charged into the reactor containing the molten iron. Table I characterizes the pellets, where the subscripts "t" and "m" refer to "total" and "metallized".

Table I% Crt% Cr ~% Crffi /% Crt% Fe ~% Fero% Fem /% Fet% C 30.4 16.5 0.54 19.7 17.4 0.88 4 ~~ 7 % A1203% MgO% Si02% CaO% P% S 15.9 11.4 7.6 0.3 0.004 0.15 After the loading, the bath and the slag were sampled and the temperature was taken every two or three minutes for the duration of the test Table II provides the key conditions and results for the 12 tests of the invention. Table II Table II Test I illustrates that with carbon as a reducer, and in the absence of injected oxygen, a Cr yield of about 79% is achievable for 14 minutes, starting with pre-reduced chromite pellets at a metallization of 54% Cr. If silicon as well as carbon are the reducers, also in the absence of injected oxygen, the yield of Cr is considerably improved (up to 99%), in less than eight minutes, as indicated by Test II. Test III illustrates that when oxygen is injected (02 / Ar = 1.5), a high yield of Cr is still achievable in the presence of carbon and silicon, but now requiring twice as long (19 minutes). (Not shown for Test III in Table II, the Cr yield is about 90% at eight minutes in the assay).

Test IV essentially repeats Test III but for a lower ratio of 02 / Ar of the gas injected and a lower Si% in the bath. Around the same yield of Cr results (96%) for the equivalent casting time (the yield of Cr of 95% at 14 minutes of the test is not shown in Table II).

The Test. And demonstrates the negative effect of the higher slag weight. For about the same% Si and% C in the bath, and a 02 / Ar ratio of the gas injected as Test III, doubling the weight of the slag decreases the Cr yield from 99% to 19 minutes within the assay, up to around 84% at 22 minutes within Trial V.

Test VI also repeats Test VI, but for aluminum more than for silicon as the co-reductant. Test VII repeats the IV test but for a greater basicity of the slag, resulting in a moderately higher Cr yield. This shows that aluminum is as effective as silicon in achieving a high yield of Cr (98%).

Test VIII is compared to tests III and V, indicating the effects of the increased slag weight and a higher ratio of 02 / Ar of the injected gas as a deteriorating performance of Cr, up to 75%. It is noted that neither the V test nor the VIII test were followed by only an argon rinse. Tests IX and X, however, were followed by an Ar rinse of 3 to 5 minutes, which substantially improves the yield of Cr up to 94% and 98% respectively. In Test IX, the Cr content in the initial bath was about 10%, but the Scotland weight of only about 50 kg / MT and the ratio of 02 / Ar was relatively low. In the X test, the chromium was initially absent in the bath, as in all the other tests except the IX, but the level of slag was increased 6 so many to around 300 kg / MT and the ratio of 02 / Ar of the injected gas it was at its highest level of all trials, just below five. In this test, the pellets and scoriates were not all charged at once, but at 10 minute intervals to allow the pilot reactor in need of heat to be reheated by combustion of the silicon and the carbon following each charge. The last batch was loaded around 20 minutes from the end of the test, including the rinsing of Ar, clearly showing that a high yield of Cr (98%) is possible regardless of a very high initial ratio 02 / Ar of the stirring media and the high slag weight when following the rinsing of Ar, the latter corresponding to step 3 of the present invention.

Finally, tests XI and XII show at a low volume of slag, that a yield of Cr above 95% is achievable at a high ratio of 02 / Ar, if some silicon is present at the end of the test (about 0.3% by weight). But, test X shows that at a high volume of slag, a short argon is required to achieve a high Cr yield, despite the much higher silicon end content. During the XI and XII tests, some of the silicon (around three kg) was charged in the bath to generate the heat required by the combustion, but it was almost completely suppressed at the end of the test.

Examples for the Commercial Operation of the Invention The present invention can be used to produce a variety of stainless steels using a reactor such as the one illustrated in FIG. 1, where a range of metallic Cr units can come directly from the chromite ore. The chromium balance may come from an updraft stream of molten stainless steel scrap and possibly a lower amount of ferrochrome added as a clipping addition after the reduction has been completed. The number of units of metallic Cr directly derived from the chromite depends on the conditions of the process selected in the invention.

Ten examples are presented to illustrate the commercial applications proposed for the invention. Table III gives the operating conditions and the consequences of stage 1 of the invention where the key parameters are varied. Note that the application of the invention is not limited to the range of selected parameters. For example, the initial temperature of the iron bath can be a parameter, however in Table III, this is constant for all the given examples. Also for simplicity the examples are limited to the production of a base alloy containing 10% by weight of Cr. 0.05% by weight of C and the Fe balance. This base alloy corresponds closely to that of AISI 409 stainless steel which can be prepared from the base alloy by trimming additions. The invention can be used to obtain higher chromium contents in the bath, but it implies high slag contents by weight, which will limit the maximum achievable chromium content. In Table III, the alloy produced in step 1 varies in Cr content depending on the selected conditions. The differences in Cr content between the alloy produced in step 1 and the base alloy to be produced is adjusted by additions of ferrochrome in step 3.

Table lll EXAMPLES FOR THE FUNDING OF CROMITE IN STAGE 1 BASES - 1 TM ALLOY OF FAITH -Cr-C IN STAGE 1 EXAMPLES FOR THE CASTING OF CHROMIUM IN STAGE I - CONTINUATION BASES FOR STAGE 1 WITH A MIXTURE OF - 1 TM Fe-Cr-C In all the examples, the cast time to reach the proper Cr yield is taken to be less than or equal to the decarburization time: i? Ooin.

Note that the heat balance for Table III is maintained, as the parameters are changed for each example, by adjusting the percentage by weight of coal in the load or the initial weight% of C in the hot metal, which determines the decarburization speed.

Table IV gives examples of the Si and Cr balances for the three stages, the final production of the base alloy. It shows the consumptions of silicon and the% by weight of Cr resulting in the bath in each stage. The yield of Cr for stage 1, which depends on the degree of premetalization, is given in Table III. No additional chrome losses are assumed in the slag in step 2. For step 3, a Cr yield of 97% is assumed for all examples. Any deficiency of chromium needed to make the base alloy is supplied by additions of chromium in step 3.

Also shown in Table IV is an estimate of the savings in production costs calculated as a percentage of a baseline of production costs that refers to the conventional operation, where the units of metallic Cr have a price of $ 1.43 per Kg. Cr. In the production baseline, the Cr units of stainless steel scrap and ferrochrome are quoted in the same. For the examples of the invention, the price of the chromite ore, including transport, which is taken as $ 137.50 per MT of ore. Finally, the Si is assumed to be $ 0.88 per Kg. All the necessary costs in the production cost calculations are based on the prices assumed for the base line of operation. Table IV BALANCES OF Si and Cr FOR STAGE 1-3 BASES. MIX OF 1 MT FROM STAGE 2 Examples A and B In these two examples of the invention using a reactor such as that illustrated in FIG. 1 shows the impact of an increase in the chromium yield of the total chromium in stage 1 from 73% to 87%. All other parameters are constant, except the% of fixed carbon of the pre-reduced load of chromite. The degree of pre-combustion and the efficiency of heat transfer are established between 25 and 50%, respectively. The charge rate of chromite is also the same for both examples, which is at a level that results in the base alloy in stage 3.

The higher chromium yields correspond to an increased carbon requirement because more coal is needed to melt the chromite to achieve a higher chromium yield for the same degree of premetalization. Because the reduction of chromite by carbon is endothermic, the additional carbon has to be decarburized to satisfy the heat balance. This results in a longer time in the decarburization. Note for both examples, the decarburization time is excessively long, p. ex. , one hour, compared to the time required for simultaneous smelting as indicated from the result of the pilot test, p. ex. , approximately 20 minutes.

The level of chromium produced in stage 1 increases with the yield of Cr. The weight of the slag increases modestly but in no case much below 400 Kg of slag / "TM, and this is not a limiting factor. In step 3, more silicon is charged for example A than for example B to recover more unreduced chromite from stage 1 in the previous case. However, the impact on savings in production costs is modest, decreasing by approximately 2%.

Examples A and C In these two examples, the degree of premetalization of the chromite ore is varied to reflect the impact of pre-reduction. The prereduction stage can be in a high temperature furnace or a rotary chimney furnace where the chromite ore mixes with the carbonaceous and partially metallized material in the solid state. All other parameters are constant, except the fixed% carbon of the pre-reduced chromite charge. This decreases when the degree of metallization increases, less carbon is required when there is a reducer for casting. As a result, the decarburization time is substantially reduced.

In Examples A and C, the Cr yield of the molten chromite in step 1 is the same (70%). However, since the chromite was loaded in example C is more highly metallized than in example A, the net yield of Cr. For all chromium in step 1 is increased from 73% to 85%. As a result, the Cr level of the alloy produced in step 1 was increased. Also, in step 3, less silicon was needed to recover less of the unreduced chromite from stage 1. The degree of premetalization increased and accompanied with high Cr yield has a great impact on production costs. Example C shows the highest savings in production costs.

Examples C and D Example D is compared to example C, where the decaburization rate increases from 0.15% C / min to 0.12% C / min. The biggest impact in stage 1 is on decarburization time. As a result, decarburization decreases from 45 to 33 minutes and the% of fixed carbon with the chromite modestly decreases from 17.5 to 16.5% by weight.

In stage 3, approximately the same amount of silicon is consumed for the two examples, but the savings in production costs increase to the highest of all scenarios, p. ex. , 22%, mainly as a result of lower use of refractory.

Example C and E In these two examples, the% CPG is varied as a parameter, everything else remains constant. The increase in GPC from 25% to 30%, remaining ETC constant at 50%, has a modest impact on the carbon requirement for the heat balance and consequently, on the decarburization time. Also, from step 3 approximately the same amount of silicon is consumed for these two examples, the production cost savings increase very modestly by approximately 1/2%.

Examples C.F and G Example F is the first of many examples in Tabal III having silicon as a co-reductant in step 1, to be compared with example C, where all other key parameters are the same. The biggest impact is on the heat balance now dictating less carbon for decarburization heat and as a result, less ash accompaniment. This significantly lowers the decarburization time from about 45 minutes to about 29 minutes, a 35% reduction.

Surprisingly, the slag weight decreases modestly despite the contribution of additional SiO2 and CaO to the slag. However, there is significantly less slag from a lower carbon velocity.

The significantly higher use of silicon in Example F compared to Example C, however, does not adversely impact the savings in production cost, showing the same level. This is due to the lower time of decarburization, balancing the highest use of silicon to the price budgeted for silicon relative to chromium (budgeted approximately 60% of the price of Cr in ferrochrome, kg per kg).

In Example G, more silicon is substituted which replaces carbon as a reducing agent than in Example F, where 16.3 kg of Si are loaded in stage 1 compared to 9.6 kg of Si / TM. As a result, the% of fixed coal is reduced from 17.5% (Example C) to 12.6% (Example F) to 8.9% (Example G). Correspondingly, the decarburization time is reduced from 45 minutes (Example C) to 29 minutes (Example F) to 19 minutes (Example G). Although the total silicon consumption is substantially higher in Example G when compared to Example C, the production cost savings remain virtually unchanged at the budgeted price for Si relative to Cr.

Examples C. F and H While in both examples F and H, silicon is a coreductor together with carbon, GPC is zeroed for the latter, corresponding to the absence of any decarburization and afterburning of a higher lancet. Different from the examples A-G that use a superior lancet, example H corresponds to the case of no use of upper lancet for post combustion and decarburization. The rate of decarburization is reduced from 50% to 0.06% C / min because decarburization occurs using only low nozzles. To compensate for heat losses from afterburning in the heat balance, silicon consumption increases dramatically, with a modest increase in coal consumption. As a result, the weight of the slag is also increased substantially to 309 kg / MT. The decarburization time increases dramatically over one hour, increasing the heat load of heat losses. The combination of all these changes reduces the savings of the production cost to 14%.

Example I and J Examples I and J refer to significantly different process configurations compared to the previous examples. In example 1, the chromite is partially metallized but is cold released to an RRSI. This may correspond to the case where a pre-reducer is not located in the foundry. In Example J, the unreduced chromite concentrate is hot charged in the RRSI. This is the case where a cheap oven is simply used to preheat but does not metallize the loaded material. In both examples I and J, the GPC is again zero because an upper lancet is not used and the rate of decarburization is low, where the decarburization is totally via the low nozzles. Silicon is the main reducer along with the carbon with the latter being dissolved in the updraft of the iron metal charge in an HAE.

Both examples quickly reach a high volume of slag which limits the total weight load of the chromite. A slag volume of 300 kg / MT is taken as a limit for these two examples. Decarburization times are short, p. ex. , about 20-25 minutes, but can be extended by decreasing the ratio of Si / C reducers. Under the assumption that the chromium yield of 85 can be reached within the decarburization time, the chrome bath level of the chromite is significantly lower for these two examples, e.g. ex. , 1.5% and 5.1% for Example I and J, respectively. Because few chromium units per ton of 10% by weight Cr alloy are supplied in these examples of cheap chromite together with high silicon consumption, the savings in production costs decrease significantly. The savings in production costs are scarcely significant for example J and significantly negative for example I.

It is understood that the various modifications that can be made to the invention without leaving the spirit and scope thereof. Therefore, the limits of the invention should be determined from the appended claims.

It is noted that in relation to this date, the best method known to the applicant to carry out the present invention, is that which is clear from the present description.

Having described the invention as above, the content of the following is claimed as property

Claims (43)

1. A method for the production of stainless steel by casting the metal oxide in situ in a refining reactor, characterized in that it comprises the steps of: supplying a bath of an iron / slag mixture in the refining reactor, the iron bath contains dissolved carbon, the reactor includes means for low stirring of the iron bath, loading a chromium metal bonded with oxygen in the iron bath, injecting an oxygen containing gas through a stirring means to effect the decarburization and vigorously stirring the iron bath, slag, and oxygen bonded metal whereby a chromium alloy bath is formed having the carbon reduced to its final specification, loading a metalloid reducer into the reactor, and injecting a non-oxidizing gas through the reactor. a stirring medium for rinsing the alloy bath to maintain a dynamic equilibrium and maximize the chromium yield.

2. The method of claim 1 characterized in that the reactor includes means for upper oxygen blowing and the additional step of passing oxygen gas through the blowing means in the reactor, a portion of the oxygen gas is discharged above the iron bath to effect the afterburning of CO and H2 and the remaining oxygen gas is injected into the iron bath to decarburize carbon to CO in the iron bath.

3. The method of claim 2 characterized in that the oxygen gas is essentially pure oxygen.

4. The method of claim 1 characterized in that the gas containing the oxygen is from the group consisting essentially of air, oxygen enriched air, pure oxygen, water, steam or a mixture thereof.

5. The method of claim 4 characterized in that the gas containing the oxygen additionally includes Ar, N2 or a mixture thereof.

6. The method of claim 2, characterized in that the total specific flow of oxygen gas passing through the blowing means and the gas containing the oxygen passing through the agitation means is at least 0.5 NM 3 / min / MT. .

7. The method of claim 2 characterized in that the total specific flow of gas passing through the blowing means and the gas passing through the agitating means is from 2 to 4 NM / min / TM.

8. The method of claim 2 characterized in that 30-60% of the total specific flow of gas within the reactor is through the agitation means.

9. The method of claim 2 characterized in that the gas passing through the blowing means is essentially pure oxygen and the gas injected through the stirring means has oxygen in a molar ratio of non-oxidized gas of less than 4.

10. The method of claim 2 characterized in that the degree of afterburning of CO and H2 is less than 50%.

11. The method of claim 10 characterized in that the degree of afterburning of CO and H2 is 20-30%.

12. The method of claim 2 characterized in that during the post-combustion the initial molar ratio of oxygen to non-oxidized gas in the stirring gas is 4/1.

13. The method of claim 12 characterized in that the molar ratio of oxygen to non-oxidized gas with the oxygen in the stirring gas decreases to 1/1 for the end of the decarburization.

14. The method of claim 13 characterized in that the molar ratio of oxygen to non-oxidizing gas with the oxygen in the stirring gas decreases about 1/3 by the end of the decarburization.

15. The method of claim 2 characterized in that the temperature of the iron bath before blowing oxygen is from 1500 ° C to 1750 ° C.

16. The method of claim 2 characterized in that the alloy bath contains 0.5-1.5% by weight of

C, at least 2.0% by weight of Cr and the chromium yield of the total chromium is at least 70% at the end of the afterburning. "17. The method of claim 1, characterized in that the oxygen-bonded metal includes at least 10% metallized chromium oxide and at least 50% metallized iron oxide.

18. The method of claim 1 characterized in that the oxygen-bonded metal is a concentrate of chromite ore containing between 25 and 55% of Cr203, the balance of FeO, MgO, SiO2? A1203 and CaO and characterized in that the weight ratio of Cr to Fe is between 0.9 and 3.5.

19. The method of claim 1 characterized in that the oxygen bonded metal is preheated to at least 1000 ° C.

20. The method of claim 1 characterized in that the metal bonded with oxygen includes a carbonaceous reductant and metalloid reductant and slagging agents.

21. The method of claim 20 characterized in that the metalloid reductant is silicon.

22. The method of claim 1 characterized in that the oxygen-bonded metal contains solid carbon of an amount in excess of that required for the premetalization of the oxygen-bonded metal.

23. The method of claim 2 characterized in that at least one of the carbonaceous solid reducing agents and a metalloid reducer are added into the initial iron bath.

24. The method of claim 1, characterized in that the iron bath contains 0-15% by weight of Cr and 0.5% by weight of C up to the carbon saturation.

25. The method of claim 1 characterized in that the slag basicity is maintained between 1.0-3.0.

26. The method of claim 1 characterized in that the weight ratio of MgO / Al203 in the slag is maintained between 0.3-0.8.

27. The method of claim 1, characterized in that the weight of the slag, excluding Cr203 or FeO, during the post-combustion does not exceed 400 kg / MT.

28. The method of claim 27, characterized in that the weight of the slag does not exceed 300 kg / MT.

29. The method of claim 1 includes the additional step of adding ferrochrome to the alloy bath to achieve the final chromium specification.

30. The method of claim 1 characterized in that at least 20% of the total chromium of the final alloy bath is derived from the chromite of the material charge.

31. The method of claim 1 characterized in that the iron bath is melted from one or more solid iron materials from the group consisting of scrap of carbon steel, scrap of stainless steel, iron by direct reduction, hot iron briquette, carbide of iron and waste from steel plants.

32. The method of claim 31 includes a further step for supplying a foundry furnace for casting the iron bath from solid iron materials.

33. The method of claim 1 characterized in that the iron bath is produced in an iron melting furnace from solid iron or iron oxide containing materials.

34. The method of claim 1 characterized in that the oxygen-bonded metal is from the group consisting of crude chromite ore, chromite ore concentrate, metallized chromite ore and steel plant waste containing chromium oxide.

35. The method of claim 1 characterized in that the reactor is an DOV and the oxygen content of the stirring gas is reduced as the carbon content of the alloy bath reaches its final carbon specification.

36. The method of claim 1 characterized in that the reactor is a DOV and the carbon content of the alloy bath is reduced by reducing the partial pressure of oxygen in the bath.

37. The method of claim 2, characterized in that the oxygen flow rate of the post-combustion is controlled independently of the flow rate of oxygen decarburization.

38. The method of claim 2 characterized in that the blowing means includes a lancet having a pair of gas channels, the oxygen of the afterburner flows through one of the channels and the oxygen of the decarburization passes through the other of the channels .

39. The method of claim 37 characterized in that the post combustion oxygen channel includes a plurality of nozzles and the oxygen decarburizing channel includes a nozzle.

40. A method of producing stainless steel by melting the metal oxide in situ in a refining reactor with upper and lower blower, characterized in that it comprises the following steps: Step 1 - supplying a mixture of an iron bath containing carbon / slag in the reactor, the reactor includes means for the upper blowing of oxygen and means for the lower stirring of the iron bath, charging an oxygen-bonded metal, the carbonaceous material and the scorifying agents inside the reactor, passing oxygen gas through the blowing means, a portion of the oxygen gas is being discharged above the iron bath to carry out the post-combustion of CO and H2 and the remaining oxygen gas is being injected inside. ofJ. iron bath to decarburize the coal in the iron bath to CO, inject the gas containing oxygen through the agitation means to effect the decarburization in the iron bath and vigorously mix the iron of the bath, the slag, and the metal bonded with oxygen and thereby form a chrome alloy bath, Step 2 - discontinue the passage of oxygen gas through the blowing means and thereby stop the post-combustion and decarburization, and reduce the carbon content of the alloy bath to its final carbon specification, and Step 3 - charge a metalloid reducer into the reactor and inject a non-oxidizing gas through the stirring means to rinse the alloy bath until the dynamic equilibrium is sustained and the chromium yield is maximized.

41. A method for producing stainless steel by casting an insitu metal oxide in a refining reactor, characterized in that it comprises the steps of: providing a mixture in an iron / slag bath containing at least 0.5% by weight of C and having a temperature of at least 1500 ° C in the reactor the reactor includes upper means for oxygen ventilation and lower means for stirring molten steel, load chromite, carbonaceous material and scorifying agents inside the iron bath, passing oxygen through the blowing means, a portion of the oxygen gas is being discharged above the iron bath to carry out the post-combustion of CO and H2, characterized in that the degree of afterburning is less than 50% and injecting the remaining portion of oxygen into the iron bath to effect decarburization of the coal in the iron bath to CO, injecting the oxygen-containing gas through the agitation means to effect the decarburization in the iron bath and vigorously mixing the iron bath, the slag, and the chromite to decarburize the bath and thereby form a chromium alloy bath containing not more than 1.5% by weight of C, and at least 2% by weight of Cr and having a temperature not higher than 1750 ° C at the end of the afterburning, discontinue the passage of oxygen gas through the blowing means and thereby stop the post-combustion and decarburization, and then decrease the oxygen content of the stirring gas as the carbon content of the alloy bath reaches its carbon specification final, and loading a metalloid reducer into the reactor and injecting a non-oxidizing gas through the stirring means to rinse the alloy bath until the dynamic equilibrium is sustained and the chromium yield is maximized.

42. A method for producing stainless steel by casting an insitu metal oxide in a refining reactor, comprising the steps of: providing a mixture in an iron / slag bath containing at least 0.5% by weight of C and having a temperature of at least 1500 ° C in the reactor the reactor includes an upper lancet and stirring means of molten steel, loading a concentrate containing at least 25% by weight of Cr203 and at least 7% by weight of FeO, the chromium oxide of the concentrate is at least 10% metallized and the iron oxide of the concentrate is at least 50% metallized, pass oxygen gas through the lancet, the lancet discharges a portion of the oxygen above the bath to effect the post-combustion of CO and H2 characterized in that the degree of afterburning is less than 50% and the injection lancet of the remaining portion of the oxygen gas within the bath to effect decarburization, inject oxygen-containing gas through the agitation means to effect the decarburization in the iron bath and vigorously mixing the iron bath, the slag, the chromite concentrate to decarburize the bath and thereby forming a chromium alloy bath containing 0.5-1.5% by weight of C, and at least 5% by weight of Cr having a temperature not higher than 1750 ° C after the afterburning, discontinue the oxygen gas stage through the lancet and thereby cease the post-combustion and decarburization via the lancet, then reduce the oxygen content of the agitator gas as the carbon content of the alloy bath reaches its final carbon specification , Y load a metalloid reducer into the reactor and inject a non-oxidizing gas through the agitation means to rinse the alloy bath until the dynamic equilibrium is sustained characterized by at least 50% of the chromium in the alloy bath is from the chromite concentrate.

43. A method for producing stainless steel by casting a metal oxide in situ in a refining reactor, characterized in that it comprises the steps of: supplying a mixture in an iron / slag bath containing at least 0.5% by weight of C and having a temperature of at least 1500 ° C in the reactor, the reactor includes an upper lancet and means for lower agitation of the iron bath, loading a metal bonded with oxygen, a carbonaceous agent, at least one of the carbonaceous solids or metalloid reducing agents and slagging agents within the iron bath, pass oxygen gas through the lancet, the lancet has a couple of gas channels, the lancet discharges a portion of the oxygen gas through one of the channels above the bath to effect post-combustion of CO and H2 characterized in that the degree of afterburning is less than 30% and the injection lancet of the remaining portion of the gas oxygen through the other channels inside the bath to effect decarburization, injecting gas through the agitation means to effect the decarburization and vigorously mixing the iron bath, the slag, the oxygen-bonded metal and thereby forming a chrome alloy bath containing 0.5-1.5% by weight of C, and at least 8% by weight of Cr having a temperature not higher than 1750 ° C after the afterburning, the stirring gas is a mixture of oxygen and non-oxidizing gas, the specific total flow of oxygen gas flowing through the lancet and the gas containing the oxygen flows through the agitation means being from 2 to 4 NM3 / min / TM, discontinue the passage of oxygen gas through the lancet and thereby cease the post-combustion and decarburization via the lancet, and then decrease the oxygen content of the agitator gas as the content of the carbon in the alloy bath reaches its final specification. carbon, the molar ratio of oxygen to non-oxidizing gas in the stirring gas is not greater than 4/1 during the post-combustion, loading a metalloid reducer into the reactor, and injecting a non-oxidizing gas through the stirring means to rinse the alloy bath until the dynamic equilibrium is sustained characterized in that the chromium yield of the oxygen-bonded metal is at least 97 % and at least 80% of the chromium in the alloy bath is from the chromite concentrate. SUMMARY OF THE INVENTION A three-stage process to obtain metallic Cr units insitu during the production of stainless steel. The raw chromite ore or a concentrate produced from the chromite ore is mixed with a carbonaceous reductant and scorifying agents and placed in an iron bath to melt and refinish in a refining reactor. During the first stage, the partially metallized chromite is fused with carbon in the reactor which is blown up and down with oxygen gas and oxygen containing gas, respectively, to produce a chromium alloy bath having a carbon content well below saturation. In the second stage, the bath of the alloy is decarburized by agitation in the lower part with gas containing oxygen to the final carbon specification of the bath. In the third stage, the bath of the alloy is reduced by a metalloid reducer such as silicon or aluminum and is again stirred at the bottom but with a non-oxidizing gas to achieve a high chromium yield. The reactor includes a top lancet extended through the throat with a lower portion of the lancet extended to the point just above the bath and means such as nozzles or porous plug mounted on or near the bottom and extended through the lancet. of a refractory coating to stir the bath of. iron containing dissolved coal. The lancet includes a central channel for injecting a compact, jet-induced oxygen gas that can penetrate through the slag layer for decarburization of the iron bath and an external channel for discharging the oxygen gas above the CO and C02 afterburning bath. . The channel includes a plurality of equispaced annular divergent nozzles. The lancet also includes a pair of concentric conduits for conducting a cooling fluid.