US2488926A

US2488926A -

Info

Publication number: US2488926A
Authority: US
Publication date: 1949-11-22
Anticipated expiration: 1966-11-22

Description

Patented Nov. 22, 1949 PROCESS OF DESULFURIZING STAINLESS STEEL GRINDING DUST Jean Marcus Mousson, Baltimore, Md, assignor,

by mesne assignments, to Armco Steel Corporation, a corporation of Ohio No Drawing. Application (lctober 30, 1944, Serial No. 561,147

Claims. (Cl. IS-0.5)

This invention relates to the de-sulphurization of metalliferous particles and more particularly to a method of de-sulphurizing alloy steel grinding dust and to de-sulphurized products which result from the same.

An object of my invention is the provision of a simple, highly effective and thoroughly practical method for substantially eliminating sulphur from any of a wide variety of sulphur-bearing stainless steel grinding dusts, which method in practice is economical, and which reliably and consistently gives final products low in sulphur content.

A further object of my invention is the provision of de-sulphurized products of stainless steel grinding dust which comprise a source of metal suitable for reclamation in stainless steel melting practices.

Other objects of my invention in part will be obvious and in part pointed out hereinafter.

The invention accordingly consists in the combination of elements, composition of materials, and conditions oi operation, in the several steps, and in the relation of each of the same to one or more of the others as described herein, the scope of the application of which is indicated in the following claims.

As conducive to a clearer understanding of certain features or my invention it may be noted at this point that stainless steel by definition is a low-carbon alloy steel which comprises to 35% or more chromium, with or without nickel, and with or without supplemental additions of manganese, silicon, cobalt, copper, molybdenum, tungsten, vanadium, columbium, titanium, sulphur, or the like for special purposes, and a balance which is substantially all iron.

In the manufacture or stainless steel articles and products, it frequently occurs that the metal is subjected to grinding operations on one or more grinding implements as in a grinding room set aside for the purpose. As a result of the grinding operations more than a negligible quantity of grinding dust, comprising stainless steel particles and usually abrasive from the grinding means, is accumulated, particularly when the operations are conducted on a large scale.

It has become a growing practice in the stainless steel industries to recover stainless steel grinding dust as an available source of metallic values for remelting and reclamation. Through the adoption of the practice, however, a number of dificulties have been recognized which have 2 to do with the economical recovery of usable qualities of dust. In this connection, it should be understood that stainless steel grinding dust, particularly in crude state as produced by grinding stainless steel ingots, billets, bars, sheet bars,

or the like, generally is sulphur-bearing. The

sulphur is present in the metallic portion of the grinding dust and/or is in a fine film formed on the dust particles during grinding. A further possibility is that sulphur may be introduced along with binder material worn from the abrading instrument. In any event the sulphur constituent unless reduced to a negligible quantity imposes a serious limitation on possible uses of stainless steel grinding dust as a raw source of metal in stainless steel melting practices.

A number of recovery methods heretofore practiced, such as certain electrostatic separation methods or other methods as those including magnetic separation of the dust constituents usually do not serve to eliminate sulphur content to a satisfactory degree. In fact, the methods are intended primarily for use in separating the metallic and non-metallic dust particles and in this connection frequently are only effective to a degree. Even when efiicient separation is achieved any sulphur bearing film covering the metallic particles as a result of grinding still remains along with any sulphur in the metallic particles themselves.

An outstanding object Of my invention accordingly is the provision or" a method for directly and efficiently de-sulphurizing stainless steel grinding dust, in which positive operational steps are involved for achieving a grinding dust product suitable for use as a source of metal in stainless steel melting practices.

Referring now more particularly to the practice of my invention 1 de-sulphurize stainless steel grindug dust comprising metallic particles or a mixture of metallic and non-metallic particles, as for example dust so recovered as to include a controlled grade or class of stainless steel metal values such as set forth in my copending application for patent Serial No. 566,450, filed December e, 1944, now abandoned, and entitled Grinding dust segregation, by subjecting the dust to sintering treatment. With the treatment, all constituents of the dust are effectively de-sulp-hurized without any need for separation of metallics and non-metallics tered product is highly suitable for use as a source of metal in stainless steel melting prac- The resulting sin-' tices, especially when the introduction of substantial amounts of sulphur to the melt is to be avoided.

As illustrative of the practice of my invention, I provide a batch of stainless steel grinding dust recovered from grinding various grades of straight chromium stainless steel on an abrading wheel; for example, dust of a segregated character more particularly referred to in my copending application for patent mentioned hereinbefore. Let it be assumed that the dust by weight includes 0.2% to 0.5% sulphur. Then to effect an initial and substantial removal of sulphur from either metallic or non-metallic constituents of the dust or from both, as the case may be, I form a bed of the dust mixture, which preferably is damp for example as a result of dampening with water and free of any admixture of fuel, as on a grate or grates in a suitable furnace or sintering machine and there ignite and sinter the bedded dust such as with a direct gas or oil flame in an atmosphere which supports combustion both of the metallic dust elements and sulphur present, illustratively in a current of air continuously drawn or otherwise induced through the burning dust. Once the dust is ignited it is preferred to carry combustion of the same to conclusion in the presence of the continuously induced air. An intense heat usually somewhere in the vicinity of 2700" F. and 2900 F. is given off by the ignited grindings which ensures eifective sintering. The use of a down-draft sintering machine of endless conveyer type for the sintering is preferred both for efiicient de-sulphurization and for continuous production of de-sulphurized material.

Through treatment of the character just described, the sulphur-bearing stainless steel grinding dust is converted by ignition into a fused or sintered and substantially de-sulphuriz'ed mass, which as recovered from the sintering apparatus still includes illustratively about 0.1% sulphur by weight.

The appearance of the ignited and burned out grinding dust usually is that of a blackened, fused or partly fused porous cake which may or may not be friable. Usually the cake is hard though brittle. A microscopic examination of the cake ordinarily reveals numerous granules or flakes of a black oxide which are mixed with a quantity of small or large pellets of metal. These pellets are formed by the intense heat which melts the grindings especially when curled metal grindings were initially present in the dust. A chemical analysis of these pellets show them usually to be high in sulphur content, also sometimes high in phosphorus or carbon. If the pellets of metal do include an excess of sulphur, carbon or phosphorus it is advantageous to process the initially sintered material still further. Under certain conditions, a second operation may not be needed in which event the initially sintered material is ready for use or sale.

In preparation for the achievement of further de-sulphurization, I crush the initially sintered mass preferably to finely divided or pulverized condition as by grinding the same in suitable grinding apparatus as to a size which will pass through a 6 mesh to 20 mesh screen. To the crushed material I add fuel such as coke, sawdust, powdered coal illustratively of 2 mesh to 8 mesh anthracite grade, or fuel oil, in sufficient quantity to form a charge for re-sintering. Usually from about 1% to approximately 6% fuel by weight is sulhcient when the fuel employed is coal and/or oil or the equivalent in heat output. The fuel and the initially sintered material advantageously are thoroughly mixed together to ensure ample distribution of fuel throughout the mixture. Following this the mixture preferably is dampened as with water and then is ignited and sintered, while exposed to air or to some other usable atmosphere which will support combustion of the fuel, in suitable sintering apparatus such as in the apparatus employed for the initial sintering and de-sulphurizing treatment. It is preferable to force the air or other gas continuously through the sintering mass as in the initial sintering treatment. Finally, by continued heating in the manner described, a caked re-sintered rich product source of chromium and iron is obtained which usually is low in carbon and phosphorus and which by weight of product illustratively includes from about 0.004% to approximately 0.02% sulphur. The resulting product is found to have assumed once again the character of a porous cake which, like that of the first sintering operation is more or less hard and brittle. Unlike most first products, however, the re-sintered products are substantially free of metallic particles, which metallics now are wholly or nearly wholly in the form of oxides. This product is recovered from the sintering apparatus and is ready for use or for sale.

As a further illustrative example of the quality of dust which I treat and of the character of sintered product obtained, a 9700 pound lot of raw grinding dust consisting of 0.383% carbon, 0.030% phosphorus, 0.204% sulphur, 12.80% chromium, 4.52% nickel, the balance being substantially all iron and the usual impurities, is prepared, ignited and initially siritered with a monometer air pressure of about 5.5 to approximately 7.5 inches being maintained through the dust. The product or this first sintering operation is found to include for example 0.039% carbon, 0.012% phosphorus, 0.047% sulphur, 13.07% chromium and 3.64% nickel. The material then is crushed to a particl size which varies from about 4 mesh size so that the entire mixture when moistened with water includes by weight about 3% coal; The moist dust and fuel mixture is ignited and burned in a bed as described hereinbefore until combustion is substantially complete; this second sintering operation being achieved while maintaining a monometer air pressure of about 3.5 to approximately 5.5 inches through the bed. A hard, black and porous final product is obtained which for example includes 0.033% carbon, 0.004% phosphorus, 0.014% sulphur, 12.18% chromium and 3.38% nickel.

Other illustrative lots of grinding dust treated substantially as just described are identified below so as to give a comparison between original sulphur content and residual sulphur content after first and second sintering treatments. At the same time it will be seen that a similar comparison may be made for chromium, nickel, phosphorus and in part for carbon:

Elements included in original grinding dust Lot 0 P S Cr Ni 1 Consistent carbon analyses were not achieved with the samples tested.

The process of my invention is highly useful for the provision, from sulphur-bearing stainless steel grinding dust containing metallic and nonmetallic particles, a substantially sulphur-free product in which residuals of the metallic dust particles are present under such conditions as to afford a valuable source of metal. In the practice of my process the utility of product usually is increased by employing stainless steel grinding dust which includes metallic particles of controlled grade, as of a grade described in my copending application for patent referred to hereinbefore. Of further importance the process enables the direct provision of useful combustion products of grinding dust without need for preliminary separation of metallic and nonmetallic dust values and makes the reclamation of stainless steel practical.

The doubly sintered products which I provide are rich in oxides of metallic values of stainless steel grinding dust and are substantially free of such sulphur as was initially present in either or both metallic and non-metallic values of the dust or such sulphur as was initially included in a film of coating as of oil derived from grinding oily stainless steel Work. In addition the products include residuals of the non-metallic values of stainless steel grinding dust such as residuals of abrasive and of binder material for the abrasive. My doubly sintered products provide a valuable source of metallic elements essential to the production of stainless steel, and, therefore, are used as furance charge material in stainless steel melting practices. The residuals of non-metallics referred to, though present in my sintered products melt down and readily slag out in the stainless steel production furnace without harming the furnace bath.

The finally sintered products obtained, being in oxide form, are highly useful in certain melting techniques as oxidizers, as for effecting substantial removal of carbon from stainless steel baths. To illustrate further, the sintered products may be used to replace in whole or in part mill scales or iron oxides in the process disclosed in Patent 1,925,182, issued to Alexander L. Feild and entitled, Process for the manufacture of rustless iron.

Thus it will be seen that in this invention there are provided both a process for ensuring substantially complete removal of sulphur from sulphur-bearing stainless steel grinding dust and on the other hand substantially sulphur-free products of stainless steel grinding dust, in which the various objects hereinbefor noted together with many thoroughly practical advantages are successfully achieved. It will also be seen that the process serves reliably with a minimum of operational steps to condition stainless steel grinding dust for use as a raw material which is substantially free of sources of sulphur and that the process is successfully performed with a minimum of apparatus and attendant labor. It will further be seen that the sintered products which I provide, conveniently are handled and used in the physically caked condition which prevails after sintering and that the products at least at times are provided from predetermined grades of dust to ensure a yield of metals necessary to the production of certain grades of stainless steel without introducing substantial amounts of other metals to contaminate the bath.

It will be understood that the sintered lowsulphur products in my invention may be used directly as furnace charge materials either in the form of sinter cake or in crushed condition, as desired, and that in certain instances the doubly sintered products may be subjected to further sintering as to drive off sulphur although this usually is not necessary.

The sintered products may, if desired, be mixed or combined with certain additional materials such as bath-fluidifying or slagging agents or additional sources of bath-forming ingredients of substantially sulphur-free grade before actual use in the melting furnace or before sale as a merchantable commodity.

The invention also embodies the practice of my sintering process on certain sulphur-bearing non-ferrous alloy or high-alloy steel grinding dusts apart from dusts of stainless steel, especially for the obtainment of substantially sulphur-free products rich in residuals of metallic dust values for remelting in the production of ZIOIl-IGITOHS alloys or alloy steels as the case may As many possible embodiments may be made of my invention and as many changes may be made in the embodiments hereinbefore set forth, it is to be understood that all matter described herein is to be interpreted as illustrative and not as a limitation.

I claim:

1. In a method of preparing stainless steel grinding dust for recovering metallic values therefrom, the art which includes heating the dust to sintering temperature of about 2700 F. to about 2900 F. in an atmosphere of air thus forming a substantially de-sulphurized sinter, and subjecting the sintered material to such resintering by heating to sintering temperature of about 270il F. to about 2900 F. in an atmosphere of air as to form a product with sulphur content not exceeding 0.02%.

2. In a method of preparing stainless steel grinding dust for recovering metallic values therefrom, the art which includes heating the dust to sintering temperature of about 2700 F. to 2900 F. in an atmosphere of air thus forming a substantially de-sulphurized sinter, crushing the sinter, and subjecting the crushed material in admixture With carbonaceous fuel to such re-sintering at sintering temperature in an atmosphere or" air as to form a product with sulphur content not exceeding 0.02%.

3. In a method of preparing stainless steel grinding dust for recovering metallic values therefrom, the art which includes heating the dust to sintering temperature of about 2700 F. to 2900 F. while directing air into the same thus forming a substantially de-sulphurized sinter, crushing the sinter, and subjecting the crushed material in admixture with at least one fuel of '7 thegroup-consisting of fuel'oil and powderedccal to such re-sintering at sintering temperature in an atmosphere of air as to form a product with sulphur contentnot exceeding 0.02

4. 111 a method of the character described, the art which includes providing sulphur bearing stainless steel grinding 'dust consisting of particles of predetermined 'metalh'c grade and a remainder substantially all non-metallic particles, heating the dust to 'sintering temperature of about 2700" Fto 2900 F. in an atmosphere of air at 'a'monometer pressure of about 5.5 to 7.5 inches thus forming a substantially de-sulphurized sinter, crushing the sinter, and subjecting the sintered material to such re-sintering at a temperature of about 2700 F. to 2900 F. in an atmosphere of air at a monometer pressure of about 3.5'to 5.5 inches as to form a product with sulphur content not exceeding 0.02%.

5.111 a method of preparing stainless steel grinding dust for recovering metallic values therefrom, the art which includes heating the 'dustto 'sintering temperatures of about 2700 F. to 2900 F. in a current of air at a monometer pressureof about 5.5 to 7.5 inches, crushing the sintered materialto about 6 to 20 mesh, adding about 1% to 6% carbonaceous fuel to the same, and re-sintering in a current of air at a monometer pressure of about'3.5 to 5.5 inches to achieve aproduct having a sulphur content not exceedmg 0.02%.

JEAN MARCUS MOUSSON.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS OTHER REFERENCES Chemical Abstracts, 1939, vol 33, page 9191.