CA1119822A - Method for beneficiating a waste product and the metallic abrasive material produced - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
A method for beneficiating scarfer spittings to produce metallic abrasives useful in blast cleaning metallic and non-metallic surfaces includes: separating all the foreign matter from the scarfer spittings; separating the plus 1/4 inch (6.35 mm) spittings from the minus 1/4 inch (6.35 mm) spittings; treating the minus 1/4 inch (6.35 mm) spittings in a grinding mill to remove brittle shells of iron oxides from the surfaces of metallic cores; separating the iron oxides from the metallic cores and size-grading the metallic cores .
The metallic cores are characterized by having a microstructure of untempered lath-like martensite substan-tially free from intergranular and intragranular cracking, a hardness of Rc 20 to 35, a grain size of between about 3 and 4, and an impact toughness at least equivalent to commercially available metallic abrasives.

Description

11198:~Z `

Background of the Invention This invention is directed to a method for bene-ficiating scarfer spittings to produce a product which can be used as size-graded metallic abrasives in machine or manual blast cleaning the surfaces of metals and non-metals.
Scarfer spittings are produced during scarfing of the surface of semi-finished steel products such as blooms, slabs, billets and bars to remove defects. During scarfing, the surface of the steel is heated to a molten temperature by gas torches in order to eliminate surface defects. The molten metal thus produced is customarily removed by high pressure water jets which impinge upon the surface of the workpiece immediately following passage of the gas torches.
The molten metal removed from the surface of the product solidifies in the form of generally spherical-like particles having a wide range of sizes, for example larger than 2 inches (50.8 mm) to less than #100 sieve size. The solidified particles, or scarfer spittings, are comprised of metallic cores having substantially the same chemical composition as the steel which had been scarfed, enclosed in brittle shells which are substantially iron oxides. The scarfer spittings are usually collected in a water bath. Scarfer spittings have been used in the past as a portion of the charge to sintering strands to reclaim the iron which they contain.

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However, only the larger particles can be used in this manner.
The!refore a large portion of the finer particles must be either stored or discarded. In recent years, increased emphasis on the surface cleanliness of steel has resulted in an increase in the use of automatic scarfing machines to scarf the steel surfaces. Because of the use of automatic scarfing machines, the volume of scarfer spittings produced in a steel plant has increased. At the present time, the scarfer spittings are a waste product for which no good use has been found.
In accordance with the invention we provide a size-graded steel abrasive material for blast cleaning metallic and non-metallic surfaces, characterized by having a hardness within the range of Rc 20 to 35, a microstructure comprised of untempered lath-like martensite substantially free from inter-granular and intragranular cracking, a grain size of about 3 to 4 and having animpactroughness equivalent to that of higher carbon and alloy grades of metallic abrasives.
The size-graded steel abrasive material is produced from scarfer spittings, a steel plant waste product. The size-graded metallic abrasives of the invention are useful in machine or manual blast cleaning of metallic and nonmetallic surfaces.
We also provide in accordance with the invention a method of producing size-graded steel abrasives from scarfer spittings comprised of steel cores enclosed in shells of iron oxides, the scarfer fitting being within a size range of plus

2 inches (50.8 mm) to minus #100 sieve size, characterized by ~ 8'~2 screening said scarfer spittings in a first screening step to separate all plus l/4 inch (6.35 mm) spittings from all minus 1/4 inch (6.35 mm) spittings and to remove any foreign matter contained therein, charging said minus 1/4 inch (6.35 mm) spit-tings into a grinding mill, removing said shells of iron oxidesfrom around said cores of said minus lt4 inch (6.35 mm) spittings in said mill to produce a mixture of steel cores and fragmented shells of iron oxides, screening the mixture thus praduced in a second screening step to separate said steel cores from the iron lO ' oxide shells drying said steel cores, and screening said steeL
cores in a third screening step to produce a size-graded metallic abrasive product.
In a specific a~pect o our method, the mixture of metallic cores and the fine particles of 'the shells is screened to separate the cores from the fine particles of shells and to make a size separation on a ~35 mesh sieve size. The plus ~35 sieve size fraction, comprised of metallic cores, is stored. The minus ~35 sieve size fraction, comprised o metallic cores and particles - ~f shells, is screened on-a ~100 sieve to separate substantially all the pius ~lOO sieve size cores from the minus ~100 sieve si~e cores and particles of shells. The plus ~100 sieve size cores are mixed with the plus ~35 sieve size cores. The minus ~lOO
sieve size cores and particles of shells are recycled to the steel plant. The mixture of plus #35 sieve size and plus ~100 sieve size cores are dried in a rotary drier and are then screened on a series of sieves into a plurality of sizes useful as size-graded metallic abrasives.
- Objects and advantages of the invention will become apparent from the following disclosure taken in conjunction with the accompanying drawings, in which:
FIGU~E 1 is a reproduction of a photomicrograph taken at lOO m~gnifi-cations of metallic cores prepared by the method of the invention.
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FIGURE 2 is a reproduction of a photomicrograph talcen at 100 magnifications of prior art metallic abrasives.

Preferred mbodiment of the Invention It has been discovered that scarfer spittings, which are a steel plant waste product having a wide range of sizes and comprised of a dual struct~re of metallic cores enclosed in shells of iron oxides can be beneficiated to remove the shells and the freed cores can be used as size-graded metallic abrasives to blast clean metallic and non-metallic surfaces. The metallic cores are separated into ; various sizes on a series of sieves to produce size-graded abrasives which meet the requirements of SAE Shot and Grit Specifications J444.
In the preferred embodiment of the invention the scarfer spittings, which range in size from plus 2 inches (50.8 mm) to minus #100 sieve size, are screened in a first screening step to separate substantially all the plus 1/4 inch (6.35 mm) spittings and all foreign matter which had been col-lected with the scarfer spittings, from substantially all the minus 1/4 inch (6.35 mm) spittings. The plus 1~4 inch (6.35 mm) fraction of spittings are separated from the foreign matter and are recycled in the steel plant to recover the iron which they contain. The foreign matter is discarded. The minus 1/4 inch (6.35 mm) fraction of spittings are charged into a rotating continuous wet grinding mill which contains a grinding media, such as steel, iron or ceramic balls or pebbles ranging in size from 1/4 inch (6.35 mm) to 1-1/2 inches (38.1 mm) in diameter. The size and weight of the Z
~;
ll grindlng medla selected for use ln the grlnding mlll is o~ ~ufficlent slze and welght and of a type to fracture the brlttle shell~ of lron oxldes into relatively fine l¦partic].es, whlch break away from the metalllc cores, without ~¦materlally affecting the ~hapes or the ~urface~ of the cores. The mlnu~ 1/4 inch (6.35 mm) ~pittin~a and the grlnding medla are tumbled for a time, usually not le~a than about eight minutes, to cause the brlttle shells of iron oxlde to ~racture into flne particle~. About 20 welght ¦percent of the cores freed ~rom the ~hells have a generally ¦spherlcal shape and a relatively smooth surface. The remalnder lor about 80 ~el~ht percen~ of the core~ ha~e irregular non-~ angular ~urface~. The mixture of core~ and fine particles ¦ of lron oxides ~ormed in the mlll together with a portlon of-¦ grlnding medla which breaks down durlng ~ervice, 15 rèmovedfrom the mlll through a dlscharge screen havlng appropriate l ope~in~s, for example a #16 sieve size. The mixture is I ¦screened in a second wet screenlng step to separate sub-stantlally all the plus ~lO0 sieve slze cores ~rom the minu~
¦ #100 sleve size cores and partlcles o~ shell B and grlndlng ¦ medla. For ease of operation and to prevent overloadlng of ¦ the screens, the mlxture 18 pre~erably flrst screened on a ¦ #35 sleve slze to separate the plus ff35 ~leve ~ize cores ¦ ~hich compri~e about 40 welght percent o~ the feed to the 25 ¦ mill from the minus #35 sleve ~ize cores, particles of ~ shell~ and the grindlng medla. An lnsi~nlficant quantlty of ¦ grindlng media may remaln in the #35 sieve slze. The #35 ~leve ~lze ~raction is ~tored in a storage bln or hopper.
¦ ~he minus #35 sieve size fraction of cores and partlcles of lll9b2z shells and any broken grlndlng media remalning in the rraction are then ~creened on a #100 ~leve to make the de~lred ~eparatlon at ~100 ~leve ~ize. All the plu~ #100 siève ~lze fraction Or core~ which comprlse about 35 welght 5 1l percent of the feed to the mill are mlxed wlth the plu8 ~35 sieve ~ize core~ in the stora~e bin or hopper. The minus ¦l#100 sleve 3ize fraction of particle~ Or shells and core~, which compri~e about 25 we~ght percent of the feed are ¦re¢ycled to the ~intering plant to reclaim the iron which 1~ Ithey contain. Prior to separatlng the metallic core~ lnto Ivarious ~12e~ of slze-graded metallic abraslve~ they are ~drled ln a ~uitable dryer, for example a fluld bed dryer or a rotary dryer at a temperature of not le~ t`han about 300F
(149~). The drled metalllc cores are then separated or slze graded on a seriea of ~creens into varlouR ~lze~ to produce ~i~e-graded metallic abraaive~ accordin~ to SAE Shot and Grit Specificatlon J444. In the~e ~peclflcatlons and Iclaim~ whenever ~creen or ~leve sizes are u~ed such ~creen ¦and ~ieve ~l~e~ are Unlted State3 Sleve Serie~. In the -- 20 jmethod of the lnvention ln ~Ihich a contlnuou~ mill i3 u8ed to bene~iclate the ~plttln~s described above, the minu~ 1/4 lnch (6.35 mm) ~pittlngs are continuously fed lnto the mlll at a rate ~ufflcient to obtain maxlmum throughput. Grindlng media are a~ded from time to tlme to maintain the propèr ratlo be~ween the ~pittlng~ and ~rinding medla to malntaln maximum efPlclency o~ grlndln~ and throughput.
¦ While a method for beneflclating ~carfer ~plttlngs to produoe slze-graded metalllo abrQslves ln whloh Q wet ll . I .

1.' 1119~

continuoua grlndin~ mill u~ing ~teel or ceramic and the llke balls or pebble~ has been described, lt ls withln the scope o~ thl~ inventlon to u~e a batch-type grlnd1ng mlll in llwhich the ~rlndlng media may be steel or ceramic balls or 5 1I pebbles or the like or to u~e an autogeneous mlll in which the ~car~er ~pittln~s are used as both the material to be llbene~lclated and the grindin~ medla requlred to bene~lciate ¦~the spitting~. In the batch method, a quantity o~ a mixture containing desired amount~ of splttings and grinding medla are charged into the mill. The mill is operated for a period of time, not lesa than eight mlnutes. ~he mill is stopped and the mixture or batch in the mlll i~ removed and proce~sed a3 described above and another batch ls charged lnto the mill.
In an autogenou~ mill, the ~plttlngs ~all upon each other with sufficient force to fracture the brittle shells of lron oxides. Dur1ng tumbling, the spittlngs rub ¦ agalnst each other and a~ a reault ~ome o~ the shells o~
I lron oxlde~ are removed by abrasion. The metallic core~ and ;20 the partlcles of shells are discharged ~rom the mill through~
a screen havlng suitable openlng~. The discharged material is treated in the manner described above ~or the operatlon ¦ of the contlnuous wet mlll to separa~e the metallic core~ -¦ ~rom the ~artlcles o~ shells and to produce slze-graded shot ¦ and ~rlt abra~ives.
¦ ~he spittin~s may be bene~iciated in an lmpact mlll ln which the ~plttings are hurled agalnat a target wlth ¦ sufficient force to ~racture the shell~ in a one-polnt 111~8'~2 .
impact The ~pittings may be recycled to the mill a suf~lclent number Gf tlmes to remove the shells or a series ~of mills may be used.
I Any Or the above mills may be used dry, however, 5 1l in a dry operation, copious amounts o~ du~t are generated.
~It ls therefore preferred to wet grlnd and wet screen to ¦~prevent e~ce~slve generation of dust.
It has been found that ln any of the above processes, except the lmpact mill but in particular uhen an autogeneous o ! mill ls used, small areas o~ iron oxides may adhere to the ~ur~aces of the smaller slze~ core3, that is, core~ whlch are suf~l¢lently small to pass a #40 sieYe size. The smaller cores are therefore preferably subJected to a flnal bene~lciating ~tep in whlch they are impacted by a one-point lmpact in an lmpact mlll to remove any iron oxides which may adhere to the 3urfaces of the cores. `
The metallic core3 have sub~tantlally the same composlti~n a~ ~he ~teel which ha~ been scarfed. Such ~;
composition~ can be any AISI carbon or alloy ~rade of steel but are generally Or the low carbon grades havlng a typical chemlcal ¢ompositlon of 0.03 to 0. o8 wei~ht percent carbon, .lO to .30 welght percent mangane~e, under .02 sul~ur, under .01 phosphoru~, under .02 silicon and the remainder sub-~tantlally lron and incidental lmpurltles usually assoclated with such grade~ of steel. Slnce the parti¢les of scar~er spittings are customarily quenched and cooled ln water whlle theg are at a relatively hlgh temperature, the microstructures and hardnQss o~ the particles are usually that o~ comparable ~rade~ o~ water quenched steeI.

1119~22 Turning to the FIGURES 1 and 2, a typical microstructure of the metallic cores produced by the method of the invention is shown in FIGURE 1 and a typical micro-structure of a commercially available steel abrasive is shown in FIGURE 2. The cores and abrasives have a size of minus #20, plus #40 sieve size. The microstructures of the metallic cores shown in FIGURE 1 comprise lath-like untempered martensite substantially devoid of any retained austenite, devoid of any intergranular or intragranular cracking and having a grain size of between 3 and 4 as determined by ASTM
E112-63 "Estimating the Average Grain Size of Metals", plate 1.
The microstructures of the commercially available metallic abrasive, shown in FIGURE 2, comprise plate-like tempered martensite with areas of alloy segregation and carbides and intragranular microcracks extending across the plates of untempered martensite and a grain size of about 7-8.
Typical chemical compositions of the cores and commercial abrasives are shown below:

ElementCores Abrasives ; 20 (Weight Percent) Carbon .o6 .97 Manganese .10 .96 Phosphorus .014 .017 Sulfur .012 .031 Silicon less than .93 .01 Iron 96.5 95.8 P.emainder incidental impurities and oxygen.

~'' The hardness of the metallic cores was between Rc 20 and 35 with segregate areas of about Rc 45~5 while the commercially available steel abrasive had a hardness of Rc 45 to 50. The intragranular microcracks in the com-mercially available steel abrasives can act as stress pointscausing transverse cracking across the grains of the abrasives leading to early failure of the abrasives when used in the high stress process of blast cleaning metallic and non-metallic surfaces.
The metallic cores prepared by the method of the invention can be used to machine or manually blast clean the surface of metallic and non-metallic material. The time required to machine blast clean the surface of a ferrous metal with the metallic cores is somewhat shorter than the time required when using conventional steel shot or grit of comparable size. The presence of both substantially smooth surfaced spherical metallic cores and irregularly shaped metallic cores results in a machine blast cleaned surface which has a surface profile intermediate between the surface profile formed by using steel shot or the surface profile formed by using steel grit.
The breakdown rate or impact resistance of the metallic cores prepared by the method herein described and a commercially available metallic abrasive grit having a RC45-50 were compared as described in "Metallic Shot and Grit Mechanical Testing - SAE 445A" appearing in the SAE Handbooks 1976, of the Society of Automotive Engineers, dated 1976. The test may be conducted on an Ervin Test machine as outlined and shown in Bulletin 644 of the Alloy Metal Abrasives Division X

of Ervin Industries, 121 S. Division Street, Ann Arbor, Michigan. In the test, a measured amount of a screened metallic abrasive of known size is prepared. One hundred grams of the abrasive is charged into the test machine. The test machine has a throwing arm which rotates at 6900 revolu-tions per minute, and an anvil and recirculating device which rotate around the throwing wheel on the same axis at 25 revolutions per minute. Each particle of abrasive is subjected to one impact each time the anvil and recirculat-ing device rotate. The number of rotations are counted toshow the impacts the abrasive can absorb.
The 100 grams (0.22 pounds) of abrasives are sub-Jected to a number of impacts. The machine is stopped and the abrasive particles are removed from the machine and carefully screened to remove all the fine particles from the sample. The remaining abrasives are weighed and a sufficient amount of fresh abrasive needed to bring the sample up to 100 grams (0.22 pounds) is added. The 100 gram (0.22 pounds) sample is then subject to another known plurality of impacts and the procedure is repeated until about 100 weight percent ; of the test abrasives have been replaced. The results of the comparison tests on a G-40 grit are shown below:

No. of Cumulative Weight Impacts Weight Percent Loss Percent Loss Commer- Metallic Commer- Metallic cial Cores cial Cores 250 16.8 4.6 16.8 4.6 500 lo.o 6.4 26.8 ll.o 750 8.7 7.5 35.5 18.5 8.3 g . o 43.8 27.5 1250 8.1 lo . l 51.9 37.6 lo1500 9.2 lo .0 61.1 47.6 1750 8.8 11.2 69.9 58.8 2000 9.7 11.5 79.6 70.3 2250 9.5 11.6 89.1 81.9 2500 lo . o 11.6 99.1 93.5 After 2500 impacts almost 100 weight percent of the original amount of the commercial metallic grit abrasive had been replaced whereas only 93.5 weight percent of the metallic cores had been replaced, indicating that the metallic cores herein described were more resistant to 0 impact than the same size commercially available steel grit.
In an example of the invention, 661ooo pounds (29,937.1 kilograms) of scarfer spittings were screened on a 1/4 inch ( 6.35 mm) screen to separate plus 1/4 inch ( 6.35 mm) spittings and foreign matter from minus 1/4 inch (6.35 mm) 25 spittings. Less than 1 weight percent of the total feed was larger than 1/4 inch ( 6.35 mm). The remaining 65,500 pounds (29.710.3 kilograms) passed through the screen. These relatively finer particles were charged at a rate of 1500 pounds per hour (680.4 kilograms per hour) along with 40 30 gallons per hour (151.4 liters per hour) water into a wet mill containing 1000 pounds ( 453.6 kilograms) of steel balls whlch ran~ed in size from 1/4 lnch (6.35 mm) to one inch (25.4 mm ln dlameter. ~he re~ultant mixture of spittln~s and water ~wa~ continuou~ly dl~charged from the mlll onto a ~35 sieve.
About 40 weight percent of the spittlngs feed was retained on the aleve. The remalnlng 60 wel~ht percent pa~sed through the sieve. The chemlcal analysea of the plu9 #35 sleve product are a~ follow:

! Product Reta~ned Product Passing '' on #35 Sieve ` Through ~35 Sieve lO ,I Element Welght Percent Wei~ht Percent _ I, C 0-0S ~4 ¦i Mn 0.10 0.49 P 0.017 0.013 I S 0.010 0.016 15 ll Sl <0. 01 0. ~3 llll Fe 96~3 78.8 Il Remainder inclden~al lmpurltie~ and oxygen The metalllc core~ retained on the #35 sieve were dried lland sl~ed into typical SAE cast steel Bhot alze rangea. The ; 20 ill particles pasaing throu~h the #35 sieve were further screened on a #lO0 ~ieve to remove the fine oxide shells. The produot retalned on the #lO0 ~leve was drled and pas~ed through a I Ijdry lmpact mlll to remove addltlonal oxldes from the finer ;metalllc cores. The product from the dr~ impact mill was 25 l¦ screened on a fi70 and #100 sleve.

! The chemistry of the products retalned on the #70 ~and #lO0 aievea i8 as follow~:

~I
., . 11 Product Retained Product Passing on #70 & #100 Through #100 Sieve Sieve Element Weight Percent Weight Percent C 0.05 0.09 Mn 0.10 0.53 P 0.017 0.013 S 0.010 0.016 Si <0.01 0.04 ~e 96.3 74.9 Remainder incidental impurities and oxygen.
Microscopic examination of a representative sample of the metallic cores was made at 100 diameters. The microstructure consisted of untempered lath-like martensite. No evidence of intragranular cracks was seen. The hardness was between 28 and 32 Rc.
The metallic cores were size~graded on a series of sieves. The weight percent of the cores retained on each sieve is shown below:
Sieve # Weight Percent 6 0.9 ~ 8 1.6 - 12 2.1 14.7 7 8.4 100 4.1 The remaining 59.2 weight percent consisted of particles of shells ~ 82 Representative ~amples of the me~alllc core~ were subJected to a durability te~t in an Ervin breakdown test.
Sample~ of the metalllc cores were recycled between 2700 and ~3O00 tlme~ in the te~t. The re~ults compare favorably with 'Icommercially avallable steel shot and gr~t slnce only the best grade~ of these abraslves gave slmllar value~ ln the Ervin breakdown te~t.
The cleaning actlon of metallic cores produced by I'beneficiating scarfer ~plttin~s wa~ compared to the cleanlng action of standard SAE 280 grade steel ~hot. To compare the cleanlng action of the sbraslve~, the ~urface o~ a ~teel plate 4 feet by 8 feet by 3/8 lnch (1.22 meters by 2.44 meter~ by 9.5 mlllimeters) covered with mill scale and patches o~ ru~t was divided into two equal parts. One part wa~ cleaned wlth the metallic cores o~ the invention and the other part was cleaned with the steel shot. The abra~ives were impinged onto the surfaces to be cleaned throu~h a hand-held compressed air nozzle having an opening of 1~4 inch (6.35 mm) at a pre~ure of 100 pounds per ~quare inch (7.O3 kilo~rams per aquare centlmeter). The nozzle was held about 8 to 12 inche~ ( 20.32 to 30.48 centimeters) away ~rom ¦the ~urfaces of the steel plate in a position perpendlcular Ito the surfaces. The results of the cleaning action are ~shown below in Table I:
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~1 lll X h h ! ol ~ P~^ ~

~D Sr'~ 1 o ~ 'ô ~
,,~o ~ ~
^
h ~ ~ b ~ bl~ ~o~
o ~ ~
N

X ' ::' C

~ O ~ C
' ' ' O U~ S; o : ~ . ~a "
~.

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Whlle the mean~ for e~fecting the inventlon have been speclrlcally descrlbed hereinabove with some speclrlclty~ it will be understood that other equlvalent ;means may be resorted to ln order to accompll h the same ¦re~ults.

,,1 I

Claims (9)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS::

1. A size-graded steel abrasive material for blast cleaning metallic and non-metallic surfaces, characterized by having a hardness within the range of Rc 20 to 35, a micro-structure comprised of untempered lath-like martensite substantially free from intergranular and intragranular cracking, a grain size of about 3 to 4 and having an impact thoughness equivalent to that of higher carbon and alloy grades of metallic abrasives.

2. The material of claim 1, characterized by having about 20 weight percent spherical particles with smooth surfaces and about 80 weight percent particles having irregular surfaces,

3. A method of producing size-graded steel abrasives from scarfer spittings comprised of steel cores enclosed in shells of iron oxides, the scarfer spittings being within a size range of plus 2 inches (50.8 mm) to minus #100 sieve size, characterized by screening said scarfer spittings in a first screening step to separate all plus 1/4 inch (6.35 mm) spittings from all minus 1/4 inch (6.35 mm) spittings and to remove any foreign matter contained therein, charging said minus 1/4 inch (6.35 mm) spittings into a grinding mill, removing said shells of iron oxides from around said cores of said minus 1/4 inch (6,35 mm) spittings in said mill to produce a mixture of steel cores and fragmented shells of iron oxides, screening the mixture thus produced in a second screening step to separate said steel cores from the iron oxide shells, drying said steel cores, and screening said steel cores in a third screening step to produce a size-graded metallic abrasive product.

4. A method according to claim 3, characterized in that the plus 1/4 inch (6.35 mm) fraction is recycled to recover iron contained therein and that the shells of iron oxide are removed from around the cores of the minus 1/4 inch (6.35 mm) fraction by rotating said grinding mill for a time sufficient to fracture the brittle shells thereby to remove them from the surfaces of said cores, the fractured brittle shells and said cores being then discharged from said grinding mill for initially screening on a #35 sieve, the plus #35 sieve size fraction being stored and the minus #35 sieve size fraction being further screened on a #100 sieve to make a size separation at #100 sieve size with the plus #100 sieve size being stored, recycling the minus #100 sieve size fraction to recover the iron contained therein, mixing the stored plus #100 sieve size fraction with the stored plus #35 sieve size fraction, and screening the mixture of the plus #100 and the plus #35 sieve size fractions on a series of screens to produce said size-graded steel abrasives.

5. The method according to claim 4, characterized in that prior to the last-named screening step the plus #100 and the plus #35 sieve size fractions are charged into a dryer wherein the mixture is dried at about 300°F (149°C) to remove all the moisture from said mixture.

6. The method according to claim 3, characterized in that the grinding mill is a ball mill, a pebble mill or an autogenous mill.

7. The method of any one of claims 4 to 6, characterized in that water is added to said mill during rotation thereof to fracture the brittle shells.

8. The method of any one of claims 4 to 6, characterized in that the grinding medium is steel or ceramic balls, or steel or ceramic pebbles.

9. The method of any one of claims 4 to 6, characterized in that it is a continuous or a batch method, the method when continuous utilizing a continuous grinding mill and the method when carried out as a batch method utilizing a batch mill.