US3043681A

US3043681A - Metallurgical processes

Info

Publication number: US3043681A
Application number: US790002A
Authority: US
Inventors: Marvin J Udy; Frank C Senior
Original assignee: Strategic Materials Corp
Current assignee: Strategic Materials Corp
Priority date: 1959-01-29
Filing date: 1959-01-29
Publication date: 1962-07-10
Anticipated expiration: 1979-07-10

Description

July 10, 1962 M. J. UDY ET AL 3,

METALLURGICAL PROCESSES Filed Jan. 29, 1959 v 2 Sheets-Sheet 1 Ni- Cr- Fe- Mn, etc Natural or blended Oxidic Reduction Burden Essential Fluxes calcining w Unit 4-. l l I F' l Stabilized l free flowing l (Selective Reduction) Sinter carbonaceous Reductant First Stage 1 Furnace Ferronickel High- Iron Alloy Chromium 'T Slag Flux Adjustment '9 Second-Stage e Furnace Chrome-iron S r ni Steel Slag c b To Market ar orgaceous, i Flux Adjustment Non Rca'bonaceous i Oxidic Chrome e uctant A Third Stage Furnace J l Chrome-Iron Residual I Alloy Slag l to Waste l Alloying Ad Steel Refining Non carbonaceous Furmlce 1 Reductant (Return of oxidized Chromium to metallic Phasel NVENTORS Finished Slag Mmvi" v Steel to Waste 5 Frank -C. Senior July 10, 1962 M. J. UDY ET AL METALLURGICAL PROCESSES Filed Jan; 29, 1959 Ni Gr- Fe- Mn, etc.

2 SheetsSheet 2 Blended Oxidic Reduction Burden Essential Fluxes Calcining Unit Stabilized free-flowing (Direct Reduction) Sinter I carbonaceous Reductant First Stage Furnace carbonaceous or Non-carbonaceous 5 u m Grude Chrome-Iron Flux Adjustment Metallic Slag as required Steel Blend "'T" Ni -Cr-Mn, etc. T Oxidic Chrome J Additive Second-Stage Furnace l fegroghromium Residual T log to Waste :To Market I I I AdAlloying Steel Refining Non carbonaceous Furnace .l

Reductant (Return of oxidized Chromium to metallic Phase) Finished slug Steel to Waste to Mill INVENTORS or Market Marvin J. Udy

, Frank C. Senior ORNEY United States Patent 3,043,681 METALLURGICAL PROCESSES Marvin J. Udy, Niagara Falls, N.Y., and Frank C. Senior, Pittsburgh, Pa., assignors, by mesne assignments, to

Strategic Materials Corporation, New York, N.Y., a.

corporation of New York Filed Jan. 29, 1959, Ser. No. 796,902. 11 Claims. (Cl. 75-1335) type containing minor amounts of other metal oxide values that actually constitute potentially valuable alloying agents but which are generally viewed as deleterious contaminants or impurities from the standpoint of attempted use of such materials in conventional steel-making operations; or ('2) similarly constituted blends or mixtures of iron oxide-bearing ores or concentrates with one or more other marginal or sub-standard ores containing oxides of specific metals required for alloying with iron in the production of a desired grade or type of steel. Specifically, the invention involves the provision of unique processes for the direct treatment of either raw or previously concentrated materials consisting, for example, of relatively low-grade nickel-iron and nickel-chromiumiron ores of the nickel lateritic type, other relatively lowgrade nickel-bearing iron ores, high-iron chromite ores, nickel-bearing senpentines and similar nickel-bearing silicates such as \garnierite, etc., or mixtures or blends of two or more such materials possibly containing metallic components in the form of scrap, for the production and recovery of semi-steel as well as a vw'de variety of finished steels including, by way of illustration, straight ferriticor martensitic chromium'steels of the series 400 and 500 types, austenitic nickel-chromium stainless steels of the so-called Strauss or series 300, such as the popular 18-8 type, modified chromium-nickel steels such as the silicon-containing Rezistal type of austenitic alloys, lowchromium and chromium-vanadium toolsteels, manganese-chromium steels, and the like.

In copending U.S. application Serial No. 717,839, now U.S. Patent No. 2,953,451, granted September 20, 1960, entitled Metallurgical Process, which was filed by Marvin I. Udy and Murray C. Udy on February 27, 1958, there is described and claimed an improved process for the production and recovery of separate valuable commercial products consisting of ferronickel, iron and ferrochrornium from relatively low-grade nickel-bearing iron ores, nickel-laterites, nickel-bearing serpentines and other nickelabearing silicates. In essence, the process of said copending application involves an initial selective carbothermic reduction of an ore or concentrate of the general class described under such conditions of fiuxing and smelting as to produce a substantially chrome-free ferronickel alloy and an iron oxide-enriched molten slag product; followed by a second selective carbothermic reduction of the molten iron oxide-bearing slag recovered from the first stage for the production and recovery of metallic iron of low-nickel and low-chromium contents, and a controlled iron oxide-bearing residual slag containing the majority of the chromium oxide content of the original ore or concentrate. Depending upon the relative concentration of chromium oxide contained within the original ore, the process of said copending application further involves the treatment, preferably while still molten, of the final residual slag product by car-bothermic reduction for the separation and recovery of a chrome-iron alloy containing on the order of from 18 .to 50% chromium metal,

"ice

The processes of the present invention are based, at least in part, on the fundamental observation made during the course of treating substantial quantities of a lateritictype ore of combined iron-nickel-chromium composition via the general processing techniques of the aforementioned copending application, to the eifect that the natural ratio of the oxidic occurrence of these metals within such ores, which is normally on the order of from about two to three and one-half (23 /z) parts chromium to approximately each part nickel, with the major portion of the remainder being iron, compares favorably with the synthetic blending ratio for such alloying ingredients generally employed in steel-making for the production of heat and corrosion resisting steels of the chromium and nickel-chromium types. Of course, in accordance with conventional steel-making practices such alloying is normally effected by the use of substantially pure starting materials, i.e., either elemental metals or standard commercial ferroalloys of such metals which are produced from high-grade ores or concentrates is smelting operations that are generally conducted on a separate and distinct basis from the basic 'ironand steel-making operation, the reason being that it has not been possible heretofore to maintain grade standards or specifications with respect to' carbon and other elements, including the alloying .metals, per se, when attempting to proceed directly from a mixed oxidic or so-called low-grade reduction burden to an alloyed or even low-carbon steel. For example, it has been proposed heretofore to treat nickelcontaminated iron ores of the lateritic type in a preliminary selective smelting operation for removal of the nickel in the form of a ferronickel alloy, followed by conventional treatment of the resulting slag for the production and recovery of a ferrochromium alloy containing the remainder of the iron and substantially all of the chromium present within the original ore. This type of operation is inherently disadvantageous for the reason that substantial iron credits, over and above the iron required for alloying with the nickel content of the ores, must be recovered in alloyed form with chromium; an alloy which is often of no direct commercial value because its chromium content is excessive from the standpoint of routine steel operations, but usually insufiicient to justify use of the product as a raw material in chrome-steel operations. In effect, the dilution of chromium with iron, and vice versa, brought about by this method of treatment destroys the beneficial ratio of iron to alloying ingredients naturally present within such starting materials, with the result that the end-products are not readily adapted to use in combination for further processing towards the production of any variety of heat and corrosion resistant steels.

We have discovered that it is entirely possible, under the prescribed conditions of operation which are set forth in greaterdetail hereinafter, to efiect extraction of the metal values from a natural nickel-chromium-iron ore of the laterite type in the form of several different metallic products of high-purity, while enhancing rather than destroying the aforementioned beneficial ratio of iron to alloying ingredients, such that these intermediate metallic products may be recombined directly in controlled proportions for the production of either chromium or nickelchromium steels of special heat and corrosion resisting properties, that is, steels of the so-called stainless types. We have further found it to be possible, under the same conditions of operation, to compound and selectively 'reduce a synthetic oxidic reduction burden for the production of the same type of intermediate metal products of purity and composition adequate to permit their recombination in the production of such steels, among other standard grades, or, under slightly modified conditions of operation, to compound and directly reduce aoeaeer such a synthetic oxidic reduction burden for the direct production of such steels, by blending two or more relatively low-grade metal oxide-containing ores such, for example, as an iron-nickel serpentine of otherwise deficient chromium content with a low-grade chromite ore, or a garnierite with a high-iron chromite ore or with separate ores containing iron oxide and chromium oxide for eventual reduction and alloying with the nickel content of the garnierite, or a low-grade manganiferous ore and a high-iron chromite ore for the eventual production of a manganese oil-hardening die steel, etc. Significantly, in contrast to the highly iron-diluted ferro-chromium alloy necessarily produced in the second-stage of the above-described prior processing technique for the treatment of laterite ores, the extreme versatility of our invention with respect to end-product steel, stems principally from the fact that we are able to effect the separation and recovery of bulk iron from the original mixed oxidic reduction burden in the form of a high-purity iron product, or, more accurately, in the form of semi-steel, and to virtually any extent desired beyond that represented by the iron which is actually required in the ferroalloys of nickel and chromium, or other alloys, as ultimately used to introduce the desired alloying ingredients into the final steel product.

Briefly, one embodiment of the process of our invention involves an initial selective reduction of a natural ore of mixed oxide content, or a blended reduction burden of equivalent mixed oxide content, under such conditions as to produce and recover a substantially chrome-free low-carbon ferronickel alloy of nickel content closely adjusted for subsequent'controlled alloying applications in steel-making as, for example, within the range of from 3 to 35 percent Ni or higher, and an iron-enriched molten slag product which also contains the chromium oxide content of the original ore or blend of ores. This slag is then subjected to a second selective carbothermic reduction for the purpose of producing a metallic iron or semi-steel product virtually free of nickel and chromium or other minor alloying metals contained in the original charge, and a molten slag product of closely controlled residual iron oxide content containing substantially all of the chromium oxide content of the original charge. Thereafter, the latter slag is subjected to reduction smelting, or possibly adjusted first with respect to its chrome- 'iron ratio by the addition of chromite ore, and then reduced, to produce a ferro-chromium alloy of chromium content closely adjusted for subsequent alloying with the ferronickel and/ or quantities of the semi-steel in the pro duction of a finished steel of any desired specification. In this embodiment of our invention, the separate metallic products of the three selective reduction smelting operations, or any lesser combination of the same, are then blended, either in the molten stage or after cooling, and

reconstituted in appropriate proportions under action of a refining furnace to produce the particular type of steel desired.

In an alternate process of our invention, the initial oxidic reduction burden is very closely regulated by blending a plurality of difierent oxide-containing ores or concentrates to provide substantially the exact ratio of each metal required in the final steel product, and this steel blend is then subjected to a direct reduction smelting for the production and recovery of an alloyed semi-steel containing the iron and alloying metals in proportions corresponding to the specification for the finished steel product which is sought to be produced. In order to maintain the carbon at a minimum within the alloyed semi-steel produced by this direct reduction technique, the carbonaceous reducing agent is employed in an amount insufiicient to reduce all of the iron oxide of the original blend to the metallic state, such that a residual slag is produced containing a fixed proportion of both iron oxide and chromium oxide. This slag may be subsequently processed for the production and recovery of a marketable standard grade of ferrochromium. The lowcarbon alloyed semi-steel from the direct reduction is supplied to an electric or open hearth finishing furnace wherein decarburization is eifected with oxygen in conventional manner and adjustments made for silicon, and the like, or for any minor portion of the alloying metals which may be found to be slightly ofiE-standard in the crude steel. Conveniently, the chromium content of the end-product steel can be varied in the finishing process by re-oxidation into the slag. Alternatively,,if all of the chromium entering the crude alloy during the direct reduction is also desired in the finished steel, then it is usually necessary to employ a non-carbonaceous reducing agent following the decarburization to reduce any chromium that has been oxidized, back into the metal.

It is believed that the foregoing process measures may be best understood by reference to the following detailed description of the separate embodiments of our invention taken in conjunction with the accompanying drawings wherein:

FIG. 1 constitutes a schematic flow diagram or flow sheet illustrating the exact sequence of steps or operations involved in the selective reduction process of our invention, wherein the various alternative practices described hereinafter have been depicted by means of dottedline indications; and

FIG. 2 is a similar flow sheet illustrating the exact sequence of steps or operations involved in the direct reduction process of our invention.

In the general practice of the processes of our invention, the ore or concentrate or a blend of suitable ores or concentrates, may be calcined or otherwise treated,

initially, to provide a reduction burden of substantially constant composition, which is thereafter fluxed by the addition of selected base and acid constituents to form a charge capable of producing a relatively high-melting point slag in the subsequent selective or direct reduction smelting operation, as explained more fully hereinafter. Alternatively, in accordance with preferred operating technique, we can admix the raw ore with the necessary fluxing materials, and thereafter calcine the combined mass to produce a substantially stabilized reduction burden of predetermined base-acid ratio. In either event, we prefer to employ magnesia as the basic flux component with respect to silica, in lieu of conventional basic fluxes such as lime or limestone, according to the unique fluxing practices described and claimed in the aforementioned copending application; The magnesia may be employed as such, or in combination with calcium oxide in the form of a true dolomite of natural or simulated composition, or, most advantageously, we may employ a natural magnesium silicate such as serpentine for the combined flux and metal values contained therein, or other serpentinelike magnesium silicates in which some of the magnesium is replaced by a desiredalloying metal, such as the nickel of garnierite, and the like, or a magnesite in combination with silica can be employed to advantage. It is found that such magnesia-silica and magnesia-lime-silica flux systems are ideally suited for use in the production, settling and recovery of ferronickel as well as iron and the crude alloyed semi-steel from ores and synthetic blends of the type specified hereinbefore. Of course, conventional lime fluxing can be employed in conjunction with the processes of the invention, but not without some sacrifice in the overall efliciency and opera-ting characteristics of the processes.

In the latter connection, it should be explained that in the carbothermic processing of natural ores of the general class described as usually practiced heretofore, it is customary to flux the iron content of the ores with basic fluxes in the form of lime or limestone, virtually exclusively, with the result that the melting point ofthe resulting slags is generally too low to permit the production of high-grade ferronickel, other than possibly on a batch basis. Theoretically, the melting point of such slags can be raised to an effective Value for continuous operations by further increasing the concentration of lime therein, but it has been found that the quantity of additive-lime required to effectively raise the melting point, rather than lowering it, becomes excessive, resulting in dilution of the iron oxide content of the slags to an extent that they become impractical to treat on any economical basis. In addition, high-iron-calcium silicate slags of this general type tend to be extremely corrosive towards furnace linings and bottoms.

As described in substantial detail within said copending application Serial No. 717,839, it has now been established that magnesia-silica and magnesia-lime-silica flux systems of the types specified hereinafter, effectively overcome all of the major disadvantages inherent in similar operations conducted with lime and limestone fluxes. Thus, such slag systems can be maintained at the optimum high temperatures required for reduction and removal (i.e., tapping) of ferronickel (or iron or semi-steel) in the fluid, molten state without excessive power input to the furnace. Furthermore, the severe corrosive action normally to be expected from slags of equivalently high-iron contents is totally eliminated through use of such fluxing techniques, with the result that one can operate on a continuous basis, even with magnesite-lined furnaces, for example, without encountering excessive shut-down time for repair of furnace linings and bottoms. Of course, the advantages to be gained through permissive use of a non-carbonaceous furnace lining with respect to the quality of end-product iron or semi-steel, for example, will be obvious to any skilled technician. In addition, since one embodiment of our invention proceeds initially to the production of ferronickel, followed by the production of metallic iron through reprocessing of the slag remaining after recovery of the ferronickel, it is found that the use of the relatively lighter magnesia fluxes results in less dilution of the initial slag as compared with calcium oxide fluxes, thereby maintaining the iron oxide content of the slag relatively high, and simplifying ultimate recovery of metallic iron or semi-steel therefrom.

Of further significance .is the fact that While certain prior investigators have postulated that high-iron slag systems intended for use in conjunction with ores of the general class described hereinbefore should be of an acid nature to provide satisfactory operating conditions, it has been demonstrated in accordance with the process of said copending application Serial No. 717,839 that the magnesia-silica and magnesia-calcium oxide-silica slag compositions of the invention are ideally suited for the production of ferronickel, as well as iron and steel, from natural lateritic ores or equivalent synthetic charges, when proportioned within the range of approximately one to one and one-quarter (1.0-1.25) parts of basic constituents to each part of acid constituents, or more precisely, when comprised of from one to one and one-quarter parts by weight magnesia and other basic components such as calcium oxide (all calculated to MgO equivalency, but excluding iron oxide as a base), to each part by weight silica. In point of fact, while it has been shown that the 1.0l.25:1 base-acid ratios represent optimum operating conditions, the base-acid ratios of the slag systems of the present invention may be constituted anywhere within the range of from one to two-and-one-quarter (LO-2.25) parts by weight base (calculated as MgO and excluding iron oxide) to each part by weight silica, and entirely satisfactory results are obtained. In general, it is found that the base-acid ratios of the preferred slag systems of the invention can be varied anywhere within this wider range to achieve optimum operating conditions as a function of power input to the furnaces.

It is believed that such ratios of base components in the form of magnesia and magnesia-lime mixtures or com postions, in combination with the acid flux in the form of silica, render possible the reduction of the nickel oxide of the ores and settling of metallic nickel into the ferronickel 6 alloy in the presence of the otherwise high concentrations of iron (i.e., 35% and higher) within the residual slags. In order to insure good fluidity of such slags at high temperatures, and particularly the second stage slags which are processed for iron recovery in accordance with the selective reduction process of our invention, thereby facilitating complete settling of metallic values therefrom, one may add further quantities of fluxing components, maintaining the desired optimum base-acid ratios specified above, while providing a sufficient volume of slag to achieve this desired effect. For example, if the natural silica content of an ore is relatively low,'while one may practice magnesia or magnesia-calcium oxide fluxing to obtain a base-acid balance within the desired optimun. range, it may be that the overall volume of the resulting slag mass will be insuflicient to provide good settling action. Under these conditions, it is merely necessary to add synthetic slag of the same desired base-acid'ratio,

practicing volumetric dilution, so to speak, to whatever extent may be necessary or desirable. Ordinarily, the slag volume in the first stage will be found to be adequate following fluxing to establish the desired base-acid properties as specified above, but dilution by the addition of synthetic-slag may be desirable prior to treatment of the first stage residual slag for iron recovery in the second stage when processing reduction burdens by the selective reduction technique of the invention.

Provided a flux ratio within the preferred optimum range of from 1.()1.25 parts base (as MgO) to each part acid is established within the initial charge to the first stage for ferronickel or direct semi-steel production, then it should not be necessary to practice any further fluxing (other thansimple dilution) in subsequent smelting stages of the process, such as the second stage smelting for iron recovery via the selective reduction process, or ferrochromium production when practicing the direct reduction process of our invention. On the other hand, if fluxing of the initial charge for ferronickel or semi-steel recovery is practiced such as to establish a base-to-acid ratio of from 1.25 to 2.25 parts by weight base to each part by weight acid, i.e., ratios within the upper limits of the somewhat wider permissive range for our process as specified hereinbefore, it is usually advisable to add some additional silica, alumina or fluorspar to the second stage, since the slags might otherwise tend to become viscous upon reduction and removal of the major portion of their iron oxide content. In essence, these additions are controlled such as to establish the second stage slag at or near a base-acid ratio within the optimum range of 1.0 to 1.25 parts base to each part acid. The addition of an acid flux effectively reduces the concentration of base components to values within the optimum range. Quite naturally, however, it is most advantageous to adjust to the optimum range within the first stage so that the charge can be processed through both stages without intermediate fluxing between stages in both the selective and direct reduction techniques.

' In the actual practice of the processes of our invention, we prefer to flux the raw ore or ores with magnesia or magnesia and calcium oxide, together with any silica that may be required over and above the natural silica content of the ore or blended ores, to establish the prescribed baseacid ratio of from 1.0 to 2.25 parts by Weight base (taken as MgO) to each part by Weight acid. The resulting charge is then calcined by heating within any suitable apparatus, such as a rotary kiln, to establish it at the maximum possible temperature for a free-flowing consistency, without overheating to the extent that the charge will form rings within the kiln. Ordinarily, this can be accomplished by heating the charge to a temperature within the range 1100" to 1300 C. It is found that the stabilization of the ores provided by preliminary calcining is particularly necessary for subsequent selective reduction. Of course, any of the charges can be melted directly within an electric furnace, but we find that the use of a kiln with gas, oil, coal oreven waste gases from an electric furnace, provides a more economical operation as compared with the use of electrical energy exclusively. Furthermore, we prefer to operate within successive stages of the process of the invention with molten charge material recovered from a preceding stage in order to further economize on power consumption by avoiding the necessity for remelting the various reduction burdens. If desired, pre-reduction can be practiced to some extent Within the kiln such that a major portion of the ferric iron is reduced to the ferrous state, and a portion to the elemental state, consistent, of course, with the primary objective of maintaining maximum through-put in the kiln for supplying the electric furnaces or other smelting equipment on a continuous basis. In point of fact, in actual operation of the process of the invention via the direct reduction technique, it has been observed that substantially all of the nickel, most of the chromium as well as any manganese present within a blended reduction burden, and approximately 35-65% of the iron can be reduced in the calcining operation without impairing the operating capacityofthe kiln. In a similar manner, when the charge to the kiln contains reducing agent in an amount proportioned to effect ultimate selective reduction 7 of the nickel and only limited iron for alloying with the amount suflicient to effect reduction to the metallic state of all of the nickel oxide contained therein, and that portion only of the iron oxide required for the production of a ferro-nickel alloy of any desired nickel content. If the alloy is to be used for the production of 18-8 stainless steel, for example, the initial selective reduction may be conveniently controlled to provide a ferronickel alloy of approximately 15% nickel content for ultimate comhination with the ferrochrornium alloy produced in the third stage. If an alloy of lower nickel content is desired, alarger quantity of iron oxide is reduced into the alloy and, conversely, if more nickel is desired in the alloy, less 7 of the available iron oxide is;reduced in the initial smelting operation. This balance is largelya function of the amount of reductant used in the operation. Preferably, the reducing agent should be of such a size and density that it will be wetted by and react readily with the slag. For example, particle. sizes within the range of approximately V to /2" are entirelysatisfactory. In the absence of any pre-reduction in the kiln, the addition of the carbonaceous reducing material to the furnace burden should include an excess over and above that required t5 combine electric arc-resistance and slag-resistance heating, achieved through the m-aintenance of short arcs by positioning the electrode tips between approximately three 7 inches (3) below the surface of molten slag contained for production of the desired ferronickel alloy, in order to effect reduction of the major portion of the ferric iron content of the ore to the ferrous state, in which form it combines chemically Withthe flux components for the stages to effect complete settling of metallic shot from the molten slag, and to maintain a reservoir of molten material to insure a smooth cycle of operations. Preferably, the smelting practices employed in the electric furnacing of all materials according to our invention inthe furnace to approximately one-half inch /2) above the surface of said slag. Incoming charge material is supplied to the peripheral portions of the furnace chamber to maintain the arc zones free of accumulated charge, and heat enters the cool charge directly from the slag bath in contact therewith.

In the second stage of the selective reduction process of our invention, the molten iron-oxide slag recovered from the first stage is passed to a second electric furnace, wherein carbonaceous material of proper size and density is added in an amount sufficient to effect reduction to the metallic state of the major portion of the iron contained therein. In order to maintain this product free of chromium metal, a controlled portion of the total available iron oxide is left unreduced within the residual slag, under which condition substantially all of the chromite canbe kept in the slag rather than contaminating the larger proportions of iron which are reduced and recovered in elemental form. Thus, it has been found that the presence of as little as three percent (3%) by weight of iron oxide, to a of ten percent (10%) by Weight, and preferably about six to eight percent (68%) by weight, within the residual slag, will generally function to maintain the metallic iron product substantially free of excessive chromium metal. Accordingly, in the second stage smelting of the selective reduction process of our invention, the slag recovered from the first stage is smelted with deficient carbonaceous reducing material, such as to retain at least three to ten percent (310%) by weight of the iron oxide within the'slag remaining after recovery of the metallic iron. In addition, carbon Within the iron recovered in this stage should be kept at or below about two percent (2.0%), since at carbon concentrations above 2% the chromium will tend to come down into the iron. When employing a magnesia-lined furnace, carbon in the pig iron can be kept to values below approximately 0.5%, thereby producing, in effect, a medium carbon steel. As previously pointed out hereinbefore, the MgO-SiO slag systems effectively prevent attack of the MgO lining. On the other hand, control of carbon within the iron product can be readily maintained at or below approximately two percent (2.0%) even in a normal carbon-lined furnace.

In general, the second stage selective reduction for iron removal is effected in such manner as to adjust the chrome-iron ratio of the resulting slag-at'the level required for the ferrochromium alloy to be produced in the third stage of the process. By suitably regulating the amount of iron taken out in the second stage, to within the minimum of three percent (3%) iron oxide required to be left in the slag to insure the production of within the minimum of three percent (3%) iron oxide chromium oxide slag can be concentrated with respect to chromium at any value within the range of from 12-50% depending, of course, on the exact amount of iron oxide present within the slag, since these slags tend to increase in refractoriness as their iron oxide content decreases and their chromium oxide content increases. Thus, if the iron oxide is low in the slag then the chromium oxide can only be concentrated to values Within the range of from 12-25%, unless auxiliary fluxing is practiced, whereas with a high residual iron oxide content the chromium can be concentrated up to 50%. Alternatively, the chrome-iron ratio of the slag can be further adjusted by the addition of chromite ore directly to the slag with appropriate essential fluxes. Preferably, the flux system in the third stage of our selective reduction process is adjusted to provide for the eventual production (following reduction of the FeCr) of a residual waste slag containing about 1.5 parts base (as MgO) to each part acid (silica).

' Assuming that a ferrochrome alloy is desired from the third stage suitable for use in the production of standard .18-8 stainless steel when combined with a ferronickel alloy of 15% Ni produced in the first stage of the selective reduction, then the chrome-iron ratio of the second stage slag should be adjusted to yield a ferrochromium alloy of approximately 38% chromium and 58% iron, with the balance consisting of minor amounts of manganese, titanium, silicon, etc. On the other hand, if the nickel is only required in comparatively small percentages within a straight chromium steel, as, for example, concentrations below 2% for hardening purposes to form martensite and to enable the resulting alloy to be grain refined by heat treatment, it will be seen that substantially the same alloys could be modified for such an application by appropriate dilution with the unalloyed semi-steel recovered in the second stage of the process. It is the simultaneous availability of the separate chromium and nickel ferroalloys with unalloyed iron of low-carbon content, coupled, of course, with the inherent processing adjustability of the ferroalloys with respect to their nickel and chromium contents, which renders our process so flexible from the standpoint of end-product steel.

The chrome-iron slag from the second stage is passed into the third furnace, preferably in molten form, and subjected to reduction smelting to produce and recover a ferrochromiu m alloy corresponding to the chrome-iron ratio provided in the slag. Since a majority of the stainless steels are characterized by low-carbon specifications, the third stage reduction is preferably conducted in the presence of a non-carbonaceous reducing agent such as silicon, ferrosilicon, ferrochromium silicon, aluminum and aluminum silicon. For very low-carbon ferrochromium, we prefer to employ aluminum or ferrochromiu-m silicon as the reducing agent, and inthe latter instance, an alloy containing approximately 40-50% silicon. Of course, a portion or all of the ferrochromium may be reduced to standard high-carbon alloy depending on the particular use to which it is to be put. In the event that chromecontaining reductants are used in the third stage of smelting, the chrome-iron ratio of the slag to be reduced should be adjusted to allow for chromium entering the metal from the reducing agent.

The ferrochromium alloy recovered from the third stage is charged directly to a refining furnace of the electric or open-hearth type wherein it is blended with the ferronickel alloy from the first stage and/or portions of the semi-steel from the second stage in the respective proportions desired in the final steel product. The finishing furnace is operated in conventional fashion to effect decarburization of the metal and adjustments are made for silicon content and the like. Undesirable constituents can be removed during the refining smelt or other constituents may be added to obtain the desired final composition. Of course, chromiumadjustments can be made directly during the finishing heat by oxidizing any excess chromium into the slag. On the other hand, assuming a correct chromium balance has been obtained initially in the product recovered from the third stage, it is desirable, following the usual oxygen decarburization in the finishing furnace, to treat the melt with a non-carbonaceous reducing agent such as aluminum, ferrosilicon, silicochromium, etc., to return to the steel any chromium which has been oxidized into the slag during the oxidation reaction. The finished steel is then recovered from the steel furnace and treated by conventional methods for conversion to consumer products.

The direct reduction or single-stage process of our invention takes advantage of the fact that the unique smelting and fiuxing practices described hereinbefore permit close control of the metallic contents reducible out.of a mixed oxidic reduction burden, such that the oxide content of such a burden can be blended in the first instance to provide, upon subsequent direct reduction, a metallic steel blend which can then be finished to a desired grade of steel. For example, in situations requiring only ,300 or 400 series steels, with no particular requirement chromium oxide slag in which the chrome-iron ratio has been enhanced, via the large proportions of iron entering the steel blend, to permit processing for the production of a standard grade ferrochromium alloy. In a similar manner, the initial oxidic blend can be appropriately proportioned to provide on direct reduction, a metallic steel blend capable of being finished to any of the austenitic nickel-chromium stainless steels. In addition to the chromium-nickel steels, the oxidic blend can also be adjusted to provide a suitable source of other metallic alloying agents such as manganese, for example, by the addition of a manganiferous ore to the reduction burden. Essentially, the single stage direct reduction process finds preference over the three-stage selective reduction process in any situation in which by-product iron or semi-steel is not required, andwhere high-iron nickel ores of the laterite type can be replaced with low-iron,higher nickelbearing ores of the serpentine or garnierite types, among others.

In the practice of the direct reduction process, the blended oxidic charge to the first stage is preferably fluxed and calcined, initially, in exactly the same manner as described hereinbefore with respect to the fluxing and calcining of a selective reduction burden. The charge is then subjected to reduction smelting under action of combined arc-resistance slag-resistance heating and in the presence of sufiicient carbonaceous reducing agent to effect reduction to the metallic state of all of the nickel oxide contained therein and that portion only of the iron oxide and chromium oxide required to produce the desired steel blend or crude alloy for subsequent finishing to steel. While the reduction of iron oxide and chromium oxide in the first stage is controlled principally to secure a metallic product of proper iron-nickel-chromium content, by appropriate blending of oxidic constituents and controlled reduction of the iron and chromium, an effort also made to adjust the chrome-iron ratio within the residual slag to values of the order of 2.521, or higher provided supplemental fiuxing is practiced, such that this slag may be readily converted to a standard grade ferrochromium product in the second stage of smelting.

The metallic steel blend from the first stage is separated from its residual slag and charged to a steel furnace of the electric or open-hearth type wherein it is treated in exactly the same manner as theseparate endproducts produced by the selective reduction process, to produce the particular gradeof finished steel desired. The single-stage steel blend will generally be found to be of slightly higher carbon content as compared to the blended metallics Which are supplied to the finishing furnace from the selective reduction process, but it is found that the carbon content of the alloy may be readily adjusted to meet specifications in the finishingfurnace.

The iron oxide-chromiiu'n oxide slag is then transferred, preferably while'still molten, to a second furnace wherein it is reduced with a carbonaceous reducing agent to produce and recover a high-carbon standard grade ferrochromium, or with non-carbonaceous reducing agents to produce mediumor low-carbon ferrochromium. The slag phase in this operation is preferably adjusted to provide approximately equivalent parts by Weight of alumina, magnesia and silica, or a 1:1:1 ratio of these components, with any calcium oxide naturally present therein being calculated to magnesium oxide equivalency and considered as part of the total MgO requirements. if

necessary or desirable, part of the ferrochromium product from this stage may be used for alloying purposes with the first stage metallic alloy within the steel finishing frorna different ore source, and under such circumstances,

it is recommended that a procedure be adopted similar to the invention described and claimed in copending application Serial Number 731,993 of co-applicant Marvin I. Udy which was filed on April 30, 1958. In accordance with the process of said copending application, a natural chromite ore or concentrate of relatively low Cr:Fe ratio is beneficiated for ultimate use in the production of ferrochromium alloys by an initial smelting operation conducted for the selective reduction and removal of excess iron present in the ore or concentrate, with the production and recovery of metallic iron of controlled chromium content, and a molten slag product of relatively enriched chromium oxide content and containing iron in any desired proportion lower than that of the original raw charge material. Thus, if desired, the initial smelting operation with high-iron chrome ores can be controlled to produce a ferritic iron product containing approximately 5% chromium (chromesteel), or iron containing 12% or 18-20% chromium, or steel, or high-chromium irons containing 25% or more chromium, with the balance of the chromium and iron being recovered within a subsequent stage or stages as ferrochrornium alloys of standard grades. The nickel-free chrome-iron alloys produced in accordance with the process of said copending application are ideally suited for use in steel-making, and may be blended within the finishing furnace with pure elemental nickel or a high-grade ferronickel alloy from an independent source to produce nickel-chromium-iron steels, provided a reasonably good, natural source of nickel oxideis not readily available for processing in accordance with the selective and direct reduction techniques of our present invention.

-It is believed that the processes of the invention will be best understood by reference to the following specific examples illustrating the application of the foregoing principles and procedures to the production of steel from various different oxidic starting materials:

EXAMPLE I Lateritic OreSelective Reduction One-hundred (100) parts of a raw laterite ore are admixed with approximately 18 parts of a magnesite flux, and approximately 8 parts of a carbonaceous reductant, and calcined at a temperature above 1100 C. to provide a free-flowing stabilized reduction burden of the following compositionwith respect to essential constituents:

The foregoing charge is then smelted within an electric furnace in the presence of a carbonaceousreductant comprising coke, coal or lignite char having from 2-6 parts by weight of contained carbon.

There are recovered from the smelting furnace 4.2 parts by weight of a ferronickel alloy containing 12 nickel, 8484.5% iron, and from 0.13-0.20% cobalt, to-

gether with a molten slag of the following essential analyses:

Percent Ni 0.05

Fe 47.0 Cr 2.3

A1 0 1 7.0 MgO 16.90 CaO 4.7

The foregoing slag is run into a second furnace together with a carbonaceous reductant containing 8-11 parts by Weight of contained carbon. The slag is smelted at approximately 1500 C. until the test samples as determined by visual as well as laboratory means disclose the desired separation of the primary slag into the lowcarbon iron or semi-steel and the chrome-iron secondary slag. There are recovered from the furnace 41.7 parts by weight of a semi-steel product of the essential analyses:

' Percent C 0.19 Si 0.08 Cr Tr. Ni 0.15 Co 0.03

and a residual slag of the analysis:

Percent Fe 8.97 C-r 5.39 nMn 0.75 SiO 29.0 A1 0 13.0 CaO 14.6 MgO 22.4

The chrome-iron slag recovered from the second stage is thereafter run into a third furnace together with 6 parts by weight of non-carbonaceous reductant per 100 parts of slag in the form of a ferrochromium silicon alloy containing 42% Si. The slag is adjusted to approximately a 1:1 base-acid ratio, and smelted to producev 3.9 parts by weight of a low-carbon ferrochrome of the following essential analyses:

Percent Cr 38.0 Mn 2.0 Si 2.8 Fe 57.2

The entire quantity of the foregoing ferrochromium alloy and all of the ferronickel recovered in the first stage are introduced into an electric refining furnace and processed for the production of a stainless steel product of standard 188 composition.

EXAMPLE II Mixed Laterite and Chromite Ores-Selective Reduction 0.1% Si) and 422 pounds of slag containing 11.9% Fe and 4.3% Cr.

The chrome-iron slag from the second stage was transferred to a third electric furnace and admixed with pounds of a chromite ore containing 10.5% vFe and 1 3 from the first and third stages were transferred molten to a steel refining furnace and finished to yield 184 pounds of standard 18-8 stainless steel.

EXAMPLE III Mixed Serpentine and Chromium resDz rect Reduction One-thousand (1000) pounds of a serpentine containing 14.5% Fe, 1.6% Ni and 0.6% Cr were blended With 150 pounds of a chromium ore containing 10.5% Fe and 22.0% Cr, and the blend calcined to a constant composition. The burden was reduced directly with coke in an electric furnace to yield 195 pounds of a metallic steel blend analyzing 18.1% Cr, 8.05% Ni, 0.5% C and 0.1% Si. This metal product was subsequently finished to standard 18-8 stainless steel in an electric steel refining furnace.

This application constitutes a replacement continuation-in-part of our former copending application Serial No. 729,026 of April 16, 1958, which has since been abandoned in favor of the present disclosure.

Having thus described the subject matter of our invention, What it is desired to secure by Letters Patent is:

1. Process for the production of stainless steel from a relatively low-grade complex reduction burden selected from the group consisting of (1) a natural iron oxidebearing ore containing minor portions of nickel and chromium oxides and (2) a synthetic blend of a plurality of different natural ores constituted to provide a major proportion of iron oxide and minor portions of nickel and chromium oxides, said material being fiuxed and thereafter subjected to a first reduction smelting in the presence of a reducing agent in an amount just sufficient to effect the selective reduction to the metallic state of substantially all of the nickel oxide contained in said material and a controlled portion only of the iron oxide content of said material with the production of a molten metallic iron-nickel alloy and a molten iron oxide-bearing slag containing the chromium oxide content of said material, separating and recovering said iron-nickel alloy from said molten slag, subjecting said slag to a second reduction smelting in the presence of a reducing agent in an amount sufficient to effect the selective reduction to the metallic state of a controlled. portion of the residual iron oxide contained therein with the production of metallic iron and a molten iron oxide-chromium oxide slag, separating and recovering said metallic iron from said molten slag, subjecting the molten slag to a third reduction smelting in the presence ofa reducing agent in an amount sufficient to effect the reduction to the metallic state of the iron oxide and chromium oxide contained therein with the production of an iron-chromium alloy and a molten residual slag, separating and recovering said iron-chromium .alloy from said residual slag,- charging the separate iron-nickel and iron-chromium alloys to said refining furnace, and treating the alloys in said refining furnace by blending and smelting the same therein under action of said refining agent for the production and recovery of stainless steel.

2. Process for the production of stainless steel from a relatively low-grade complex reduction burden comprising a relatively lowsiron nickel-bearing ore and a low-grade chromite ore blended together in approximate proportions to provide nickel in an amount substantially equal to that desired in said stainless steel and iron and chromium in excess of that desired in said stainless steel, said burden being fiuxed and thereafter subjected to reduction smelting in the presence of a reducing agent in an amount surficient to effect the selective reduction to the metallic state of substantially all of the nickel oxide contained therein and controlled proportions of the iron oxide and chromium oxide contents thereof with the production of an alloyed metallic crude steel blend containing iron, nickel and chromium in approximate proportions corresponding to the desired composition of said stainless steel and a residual molten iron oxide-chromium oxide slag, separata ing and recovering said metallic crude steel blend from said molten slag, and treating said crude steel blend in said refining furnace by smelting the same therein under action of said refining agent for the production and recovery of stainless steel.

3. Theprocess as claimed in claim 2, that further cornprises subjecting said iron oxide-chromium oxide slag to reduction smelting in the presence of a reducing agent in an amount sutficient to effect reduction to the metallic state of the iron oxide and chromium oxide contained therein with the production of a ferrochromium alloy and a residual molten slag, and separating and recovering said ferrochromium alloy from said residual molten slag.

Process for the production of separate commercial products in the form of finished steel and low-carbon iron from a relatively low-grade iron ore comprising nickel oxide and chromium oxide diluted to relatively minor proportions principally by the iron oxide and gangue constituents of said ore, that comprises fiuxing said ore and thereafter subjecting the fiuxed ore, initially, and its suc-- cessive residual slags, thereafter, to three successive reduction smeltings each in the presence of a carefully controlled quantity of a reducing agent to effect the selective reduction to the metallic state and recovery, in succession, of (1st) an iron-nickel alloy of substantially enhanced nickel concentration as compared With the ironznickel ratio of the original ore, -(2nd) at low-carbon iron corresponding to the major portion of the iron content of the original ore, and (3rd) an iron-chromium alloy of substantially enhanced chromium concentration as compared with the ironzchromium ratio of the original ore; charging said iron-n-ickelralloy and iron-chromium alloy to a common steel finishing furnace, subjecting said blended alloys to smelting therein in the presence of a 6. Process for the production of finished steel from individual relatively low-grade ores including (1) a lowgrade chromite ore and (II) a low-iron nickel-bearing ore, that comprises blending said individual ores in respective proportions to providea mixed oxidic reduction burden containing (1) nickel in an amount approximately equal to that desired in the finished steel, and (2) iron and chromium in excess of that desired in said finished steel, fluxing said mixed reduction burden and thereafter subjecting the same to reduction smelting in the presence of a reducing agent in an amount controlled to promote the selective reduction to the metallic state of substantially all of the nickel contained therein and controlled portions only of the iron and chromium contents thereof corresponding to the approximate metallic iron and chromium requirements of said finished steel, separating and recovering a metallic phase comprising a crude steel blend containing nickel, chromium and iron in approximate proportions corresponding to the alloy composition of said finished steel and a slag phase containing the excess iron oxide and chromium oxide present Within said original ore blend, charging said crude steel blend to a steel finishing furnace together With any other essential alloying ingredients desired in said finished steel including minor metallic additions, as required, to adjust the iron, nickel and chromium ratio o-f'the crude steel blend to grade specifications, and smelting said charge in the presence of a refining agent within said finishing furnace for the production and recovery of finished steel.

7. The process as claimed in claim 6, that further comprises subjecting said iron oxide-chromium oxide slag concentrations.

phase to reduction smelting in the presence of a reducing agent in an amount suflicient to eifect reduction to the metallic state of the iron and chromium contained therein with the production and recovery of a ferrochrom-ium alloy, and charging limited portions of said ferroc-hromium alloy to said steel finishing furnace together with said crude steel blend to adjust the composition of the steel melt with respect to iron and chromium 8. Process for the production of separate commercial products in the ,form of finished steel and low-carbon iron from a laterite ore comprising nickel oxide and ohro. oxide diluted to relatively minor proportions prin cipally by the iron oxide and gangue constituents of said ore, that comprises, fiuxing said ore to a basic composition, subjecting said fluxed ore to a first reduction smelting inthe presence of a carbonaceous reducing agent in an amount sufficient to effect the selective reduction to the metallic state of substantially all of the nickel contained therein and sufficient iron to provide for the production of an iron-nickel alloy containing nickel at a concentration within the range 3-35 percent and a molten slag product containing the remainder of the iron oxide content of said ore together with the chromium oxide content thereof, subjecting said molten slag product to a second reduction smelting in the presence or" a carbonaceous reducing agent in an amount controlled to provide for the selective reduction to the metallic state of that portion of the iron oxide content thereof in excess of that required for eventual alloying with the chromium content of the ore to provide an iron-chromium alloy of chromium concentration within the range 12-50 percent, with the production of low-carbon metallic iron and a molten slag product containing the residual iron oxide content of said ore and substantially the entire chromium oxide content thereof, subjecting said molten slag product to a third reduction smelting in the presence of a reducing agent in an amount sufiicient to eflect reduction to the metallic state of the iron and chromium contents thereof with the production of an iron-chromium alloy of chromium concentration within said range of 12-50 percent, blending said iron-nickel and iron-chromium alloys within a common steel finishing furnace, and smelting said blended alloys therein in the presence of a finishing agent to efiect the production and recovery of a'finished nickelchromium steel.

, 9. The process as claimed in claim 8, that further comprises admixing theiron oxide-chromium oxide slag remaining from said second reduction smelting with controlled quantities of a low-grade chromite ore, and subjecting the admixed ore and slag to reduction smelting for the production and recovery of said iron-chromium alloy.

10. Process for the production of separate commercialproducts in the form of finished steel and low-carbon iron froma synthetic reduction burden consisting of a nickel laterite ore blended with a low-grade chromite ore to provide nickel oxide and chromium oxide diluted to relatively minor proportions principally by the iron oxide and gangue constituents of said ores, that comprises, fiuxing said reduction burden to a basic composition, subjecting said fluxed reduction burden to a first reduction smelting in the presence of acanbonaceous reducing agent in an amount suflicient to eifect the selective reduction to the metallic state of substantially all of the nickel contained therein and sufficient iron to provide for the production oi. an iron-nickel alloy containing nickel at a concentration within the range 3-35 percent and a molten slag product containing the remainder of the iron oxide content of said burden together with the chromium oxide content thereof, subjecting said molten slag product to a second reduction smelting inthe presence of a carbonaceous reducing agent in an amount controlled to provide for the selective reduction to the metallic state of that portion of the iron oxide content thereof in excess of that required for eventual alloying with ,the chromium content of the originalreduction burden to provide an iron-chromium alloy of chromium concentration within the range 12-50 percent, with the production of low-carbon metallic iron and a molten slag product containing the residual iron oxide content of said burden and substantially the entire chromium oxide content thereof, subjecting said molten slag product to a third reduction smelting in the presence of areducing agent in an amount sufficient to elfect re duction to the metallic state of the iron and chromium contents thereof with theproduction of an iron-chromium alloy of chromium concentration within said range of 12-50 percent, blending said iron-nickel and iron-chromium alloys Within-a common steel finishing furnace, and smelting said blended alloys therein in the presence of a finishing agent to effect the production and recovery of a finished nickel-chromium steel.

11.- Process for the production of finished steel from a mixed reduction burden consisting of a low-grade chromite ore and a nickel-bearing ore selected from the class consisting of serpentine and garnierite ores, that comprises blending said chromite ore and nickel-bearing ore in respective proportions to provide a mixed oxidic reduction burden containing in combination (1) nickel in an amount approximately equal to that desired in the said finished steel with the production of a metallic phase comprising a crude steel blend containing nickel, chromium and iron in approximate proportions corresponding to the alloy composition of said finished steel and a slag phase containing the excess iron oxide and chromium oxide present within said original mixed reduction burden, sepanating said metallic and slag phases and charging said crude steel blend to a steel finishing furnace together with any other essential alloying ingredients desired in said finished steel including minor metallic additions required to adjust the iron, nickel and chromium .ratio of the crude steel blend to grade specifications, and smelting said charge in the presence of a refining agent within said finishing furnace for the production and recovery of finished steel.

References Cited in the file of this patent UNITED STATES PATENTS 2,395,029 Baily Feb. 19, 1946 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,043,681 July 10, 1962 Marvin J. Udy et a1.

It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 2, line 20, for "is" read in column 5 line 73, strike out "the", first occurrence; column 8, line 56, strike out "within the minimum of three percent (3%)" and insert instead essentially chrome-free iron, the residual Signed and sealed this 13th day of November 1962..

SEAL) Lttest:

RNEST w. SWIDER DAVID A Lttesting Officer Commissioner of Patents

Claims

1. PROCESS FOR THE PRODUCTION OF STAINLESS STELL FROM A RELATIVELY LOW-GRADE COMPLEX REDUCTION BURDEN SELECTED FROM THE GROUP CONSISTING OF (1) A NATURAL IRON OXIDEBEARING ORE CONTAINING MINOR PORTIONS OF NICKEL AND CHROMIUM OXIDES AND (2) A SYNTHETIC BLEND OF A PLURALITY OF DIFFERENT NATURAL ORES CONSTITUTED TO PROVIDE A MAJOR PROPORTION OF IRON OXIDE AND MINOR PORTIONS OF NICKEL AND CHROMIUM OXIDES, SAID MATERIAL BEING FLUXED AND THEREAFTER SUBJECTED TO A FIRST REDUCTON SMELTING IN THE PRESENCE OF A REDUCING AGENT IN AN AMOUNT JUST SUFFICIENT TO EFFECT THE SELECTIVE REDUCTION TO THE METALLIC STATE OF SUBSTANTIALLY ALL OF THE NICKEL OXIDE CONTAINED IN SAID MATERIAL AND A CONTROLLED PORTION ONLY OF THE IRON OXIDE CONTENT OF SAID MATERIAL WITH THE PRODUCTION OF A MOLTEN METALLIC IRON-NICKEL ALLOY AND A MOLTEN IRON OXIDE-BEARING SLAG CONTAINING THE CHROMIUM OXIDE CONTENT OF SAID MATERIAL, SEPARATING AND RECOVERING SAID IRON-NICKEL ALLOY FROM SAID MOLTEN SLAG, SUBJECTING SAID SLAG TO A SECOND REDUCTION SMELTING IN THE PRESENCE OF A REDUCING AGENT IN AN AMOUNT SUFFICIENT TO EFFECT THE SELECTIVE REDUCTION TO THE METALLIC STATE OF A CONTROLLED PORTION OF THE RESIDUAL IRON OXIDE CONTAINED THEREIN WITH THE PRODUCTION OF METALLIC IRON AND A MOLTEN IRON OXIDE-CHROMIUM OXIDE SLAG, SEPARATING AND RECOVERING SAID METALLIC IRON FROM SAID MOLTEN SLAG, SUBJECTING THE MOLTEN SLAG TO A THIRD