US2638424A - Method of processing metal powders - Google Patents

Method of processing metal powders Download PDF

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US2638424A
US2638424A US646554A US64655446A US2638424A US 2638424 A US2638424 A US 2638424A US 646554 A US646554 A US 646554A US 64655446 A US64655446 A US 64655446A US 2638424 A US2638424 A US 2638424A
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particles
acid
nitric acid
metal
powder
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US646554A
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Fritz J Hansgirg
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American Electro Metal Corp
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American Electro Metal Corp
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    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B22—CASTING; POWDER METALLURGY
    • B22F—WORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER; APPARATUS OR DEVICES SPECIALLY ADAPTED FOR METALLIC POWDER
    • B22F1/00—Metallic powder; Treatment of metallic powder, e.g. to facilitate working or to improve properties
    • B22F1/14—Treatment of metallic powder
    • B22F1/145—Chemical treatment, e.g. passivation or decarburisation

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  • ethemhi eestoiathwinventimmill be more cleanse,.- undaretaodz when minut-ion proceed witlxtrefonmatmths ixxovmioh 2V Fis. lxllastrates schematically 0n hand 0f a HOW sheet a lprocess according to the invention snitedafgoi; mass producton,y and Fig. 2 shows scnematicallyiandin cross-section a Vacuum drying L no usable with the invention.
  • the metal powder is treatedwithnitric acid (NHOa) of suitable hignirconcentration so as yto remove from the powderyparticles-tlie objectionable oxide and/or nitride filmsrapidly; this erTect manifests itself by a cleanvanl metallic, shiny appearance of theftljeated particles.v
  • NHOa nitric acid
  • thoselayers are also cleaned in tneaforementioned sense .so that they stayirninline againstftheattacks of oxygen-or nitrogen ysuch as contained.
  • thev reaction between the nitric acid and powdery metal particles is interrupted quickly in Qrdertoavoid losseso the metall substance by the.l continued attack; thereon of the concentrated nitric acid.
  • the nitric acid can be removed immediately aiter the desired effect of the, inyention is obtained., for instance by sepaeantinsthelatter Orrin a eentrifueine process.
  • a suspension of magnesium powder in any suitable vehicle or suspension medium is prepared. Water is preferred as such suspension medium.
  • magnesium powder I4 is fed continuously from hop-- per I5 into tank IO at a predeterminable rate or amount per unit of time, which can be adjusted for instance by regulating the speed of a rotating feeder I6 arranged near the bottom and outlet oi hopper I5. Water is supplied to tank I 0 through pipe I? at a predeterminable rate perv time unit adjusted by adjustable valve 29. Thereby any desired ratio of magnesium powder and water as suspension medium can be adjusted and maintained during operation, and a suiicient supply of that suspension provided.
  • air can be injected into the water within tank lil as by an air supply line indicated by the dotted lines l3-I, or mechanical stirring means provided as indicated by stirl rer II on shaft i2 of an electric motor I3.
  • the suspension or slurry thus formed is withdrawn from tank IU through pipe I8 into reaction chamber I9 at a predeterminable rate which can be adjusted by any suitable means, such as adjustable valve 2I.
  • Tank I5 is arranged at a suitable highe'i ⁇ level than reaction chamber i9, so as to secure by action of gravity the desired flow of the suspension or slurry into the reaction chamber, and for other purposes to be explained hereinafter.
  • a supply of nitric acid of suitable high concentration is maintained in tank 22, and a Ineasured quantity per time unit of the acid is withdrawn from tank 22 through valve 23, pipe 24 and another adjustable valve 25 into nozzle 26 arranged within reaction chamber I9 so that the discharge end 2 of nozzle 28 is at a lower level than the discharge aperture of pipe I8 into chamber I9.
  • Nozzle 2t is of any suitable and known type and, if desired, it may be of a known type,
  • nozzle or similar discharge devices can be used.
  • the suspension o1 ⁇ slurry continuously formed in tank il] and the highly concentrated nitric acid continuously supplied to tank 22 are discharged and injected, respectively, into chamber I9 at predeterminable and adjustable rates.
  • the concentrated nitric acid injected into the chamber space admixes rapidly with the watery slurry or suspension of magnesium powder simultaneously discharged into chamber i9.
  • the concentration of the injected acid is such that its slight dilution by the suspension-water does not inhibit or substantiallyk reduce its instantaneous and vehement reaction with the magnesium powder.
  • cooling means can be provided in such case, such as a jacket 59 around chamber I9 through which cooling water is passed in the manner indicated in the drawing.
  • the turbulent nitrogen-oxide gases developing during the reaction secure the contact of all the particle surfaces by the acid, and the removal of oxide and nitride lms in those surfaces and the cleaning of the latter is therefore completed after the particles and acid have travelled a certain path or distance downwardly below the discharge end 27 of nozzle 26.
  • the mixture of the powder and acid could be discharged into a relatively large pool of continuously replenished water which dilutes the acid sufficiently, or the powder and acid could be centrifugally separated in any known way.
  • another nozzle 28 is arranged within chamber I9.
  • Nozzle 28 discharges downwardly and is connected with pipe 29 passing through the wall-of chamber VI9 to the outside. Water is supplied through pipe 29 from a source not shown, of a predeterminable quantity per time unit adjusted by adjustable Valve 30.
  • Nozzle 23 can be of any suitable type and supplies water in suiiiciently large quantity so that upon itsvrapid and intimate admixture with the nitric acid arriving below nozzle 28, the acid is diluted suiciently so'that its reaction with the magnesium powder is linterrupted or at least the velocity of that reaction is reduced sufficiently so that no objectionable losses of magnesium occur thereafter.
  • the distance between nozzles 26 and 28 depends on the now rate and concentration of the suspension or slurry of magnesium and on the flow rate and concentration of the nitric acid,V all of which are adjustable at the start of and during operation in the manner hereinbefore described.
  • the optimum distance between those nozzlesv is chosen so as to cause complete removal of objectionable surface lms from the metal particles. No general rule can be given for this purpose because of the varying factors stated and the further ⁇ fact that average particle size and kind of the metal powder or mixture of such powders comes into play too.
  • the nitrogen-oxide gases can penetrate that seal because the pressure in the liquid ⁇ c i'ilurnri between the upper end of the side wall of pot "5i and the lower end of chamber i3 is far smaller than the pressure in pipes i8 and 24, respectively.
  • Pot V5i is illled almost immediately after operation 'has started, andthe surously supplied mixture of 'magnesium powvder and diluted nitric acid ilows over the upper rim of pot 5I into a settling tank 3i.
  • the cleaned magnesium powder in the overowing mixture will always be covered vfully by the lliquid consisting of nitric acid and water, and detrimental access of air to those particles is thereby prevented.
  • the magnesium powder collected above bottom 33 forms a slurry still admixed with diluted nitric acid; itis withdrawn from tank 33 at proper time intervals, as explained later, through valve 36 into a vacuum lter, such asa washing and ltering vessel or tank 31 provided with a downwardly tapering bottom 33.
  • a filter 33 is arranged in vessel 31 above and spaced from bottom 38. Clean washing water is poured or sprinkled through pipe 40 upon the slurry deposited on lter 33 and accumulates above the powder to the level 44 of an overflow (trough) 4i from which it is withdrawn in the manner shown.
  • Vessel 31 communicates through an aperture in its bottom 38 and connecting pipe with another conilned space 42 thereunder in which a vacuum is produced and maintained by pipe 43 connected with a vacuum pump (not shown).
  • a vacuum pump not shown
  • a great part of the liquid consisting of washing water, admixed with the magnesium powder above filter 33 is sucked away through filter 39 and the magnesium powder slurry more concentrated to form a. cake which still contains but little liquid. Any access of air to powdery particles of the cake is prevented by the layer of washing Water collected above it. Any liquid collecting in vessel 42 can be withdrawn therefrom through a .pipe and valve 50.
  • valve 3i is closed and the filter cake allowed to remain until the water entering through the pipe 40 has thoroughly cleaned the surfaces of the metal particles in the cake.
  • the filter cake is then removed from vessel 31 and transferred into a dryer.
  • the valve 36 may then be opened to allow another quantity of slurry ⁇ to enter the vessel 31 to form a new filter cake.
  • the extraction of the water stili containedin the .filter cake can be accomplished by treating it with ⁇ a suitable ⁇ extraction medium, such as acetone.
  • the .extraction of the water and the drying of the lter cake may 'be eifected in Ia vacuum dryer of any known suitable type.
  • Fig. 2 indicates, by way of example, one form of a vacuum dryer having a revolvable drum '46 in which'vacuumiisproducedby means of pipe 4B projecting into the vinterior ofthe drum land connected with a Vacuum pump (not shown).
  • the vacuum produced within vdrum 4t should be oi a. pressure equall'ing or below 4to 5 millimeters mercury column absolute.
  • the filter cake While the filter cake is being dried, it breaks up into powder; after the powder vis completely dried, the vacuum can be released and the powder removed from the drum.
  • the completely dry powder is clean ,and shiny, and its surface layers are resistant to contamination in the aforementioned sense for a limited period of time. 'The powder can therefore be manipulated, such as compacted for metallurgicalpurposes, without the danger of the formation of objectionable oxide or nitride lms thereon.
  • an inert gas can be admitted into drum 46 immediately after the vacuum has been released, for instance, very dry hydrogen or an inert gas like helium, argon or neon.
  • the 'inert gas may be supplied through the same tube 45 through which the drum 46 is evacuated.
  • dry nitrogen can be used as protective gas and admitted into the drum after the vacuum has been released.
  • the powder and inert gas of the types mentioned can be filled into containers or cans which are air-tightly sealed thereafter.
  • an inert gas such as helium, etc.
  • a layer of that gas may be adsorbed by the dry powder particles which additionally protects the latter against Voxidation for some time thereafter.
  • a protective gas such las very dry hydrogen of a dew point far below minus 40 C., or helium, argon or neon, even such superficial oxidation or nitrid'ing can be prevented. Since compacts of high density can be produced of 'the metals herein concerned by relatively low compaction pressures, no danger of undue absorption of the protective gas or penetration of oxygen or nitrogen into the interior of the compact exists. Hence the final product obtained Aby sintering Will be of greatest purity, homogeneity and density and therefore exhibit unusual or heretofore unknown physical properties.
  • the powder processed according to the invention is adm'ixed with other clean powders in order to obtain from the powdery mixture a desired metal alloy, or if a mixture of those powders is processed, bodies or even ingots compacted and sintered therefrom will exhibit outstanding properties due to their homogeneity, density and purity.
  • casting of magnesium, aluminum, or alloys of those metals requires a large riser because of the high shrinkage of those metals and alloys, and their production is therefore connected with extensive melting losses.
  • the control of grain size vis equally difiicult in casting those metals and alloys, and the formation of blisters and blow holes therein is hard to avoid, if possible at all.
  • the properties of the alloys depend greatly on the purity and exact ratio of the alloying constituents therein which to obtain 'is extremely diicult in casting processes. Therefore, if ready to use bodies, or ingots for further working and shaping are made by powder metallurgical processes from powder or powdery mixtures containing metal powder processed according to the invention, all these difficulties can be obviated. However, these aspects of the use of powders processed according to the invention do not form a subject matter thereof and are mentioned only to illustrate the high utility and novel effects of powders processed according to the invention.
  • any number of them can be arranged discharging at the same lor slightly different levels, and the supply to them of the slurry and nitric acid, respectively, can be regulated simultaneously or independently.
  • a vacuum lter as exemplied hereinbefore, any other type of filter can be used, such as a continuous working filter.
  • their essential operation should consist in reacting upon the metal powder or mixture of such powders with nitric acid of suiiiciently high concentration so as to rapidly remove from the powdery particles oxide and nitride films and also clean the surface layers of the particles; to interrupt this reaction immediately after these effects are obtained in order to avoid losses of metal substance; to wash the cleaned powdery particles and to dry them in the absence of air, i. e. in a sufficiently high vacuum and/or in the presence of dry and inert gas (e. g. helium), not shown; and to store the processed dry powder, 'if required, in air-tight containers and advantage- 'ously in the presence of an inert gas until the powder is further processed or used, particularly in a powder metallurgical process.
  • dry and inert gas e. g. helium
  • What I claim is: l.
  • the method of cleaning light powdery particles of metal selected from the class consisting ⁇ of aluminum and magnesium and having metal compound surface impurities which are removable by nitric acid which comprises the steps -of reacting upon a uid mixture containing water and said powdery particles with concentrated nitric acid so as to remove metal compounds from the surfaces of said particles and thus produce on said particles cleaned surface layers, effectively interrupting immediately thereafter said reaction by effectively removing the powdery particles from the action of the concentrated acid, and thereafter separating said particles from the resulting mixture under conditions excluding the access of air.
  • the method of cleaning light powdery particles of metal selected from the class consisting of aluminum and magnesium and having metal compound surface impurities which are removable by nitric acid which comprises the steps of reacting upon a fluid mixture containing water and the powdery particles with concentrated nitric acid so as to remove metal compounds from their surfaces and thus produce cleaned surface layers, and effectively interrupting immediately thereafter said reaction by diluting the acid and separating said particles from the resultant mixture under conditions excluding the access of air.
  • the method of cleaning light powdery particles of metal selected from the class consisting of aluminum and magnesium and having metal compound surface impurities which are removable by nitric acid which comprises the steps of reacting upon a fluid mixture containing water and the powdery particles with concentrated nitric acid so as to remove metal compounds from their surfaces and thus produce cleaned surface layers, effectively interrupting immediately thereafter said reaction, thereafter adding water to the resulting mixture, and washing and drying the particles under conditions excluding the access of air.
  • the method of cleaning light powdery particles of metal selected from the class consisting of aluminum and magnesium and having metal compound surface impurities which are removable by nitric acid which comprises the steps of reacting upon a mixture of water and the powdery particles with concentrated nitric acid so as to remove metal compounds from the surfaces and thus produce cleaned surface layers on said particles, effectively interrupting immediately thereafter said reaction, thereafter adding a washing agent to the resulting mixture and washing the particles with said washing agent so as to protect said particles against the access of air, and removing the washing agent and other mixture ingredients still adhering to the particles by treating them with a volatile solvent medium.
  • the method of cleaning light powdery particles of metal selected from the class consisting of aluminum and magnesium and having metal compound surface impurities which are removable by nitric acid which comprises the steps of reacting upon a fluid mixture containing water and the powdery particles with concentrated nitric acid so as to remove metal compounds from their surfaces and thus produce cleaned surface layers, effectively interrupting thereafter said reaction, thereafter adding a washing agent to the resulting mixture and washing the particles under cover of a washing agent on a vacuum lter, and drying the wet filter cake in a nonoxidizing atmosphere.
  • the method. of cleaninglight powdery particles of metal selected from the classv consisting of aluminum and magnesium vand having metalcompound surface impuritiesA which. are removable. by nitric acid which comprises the steps 0f suspending the metal-powder in water so as to form a slurry, admixing with said slurry niacid of highconcentration and in an amount sumcient to secure an intense reaction of said acid upon the metal particles and thereby to remove metal compound impurities on the surfaces of said particles and to thus produce cleaned surface layers on said particles, and effectively interrupting immediately thereafter said reaction by diluting said acid.
  • the method of cleaning light powdery particles of metal selected from the class consisting of aluminum and magnesium and having metal compound surface impurities which are removable by nitric acid which-comprises the steps of suspending the metal powder in water so as to form a slurry, admixi'ng with said slurry nitric acid of high concentration andv in an amount mmcient to secure an intense reaction of said acid upon the metal particles and thereby to remove metal compound impurities on the surfaces of said particlesv and to thus produce cleaned surface layersr on said particles, effectively interrupting thereafter: said reaction by effectively removing said particles' from the aetion of said concentrated acid, substantially separating said acid and particles, washing the particles with and separatingr them thereafter from a washing agent, and drying the washed particles.
  • substantially separating the acid and particles by allowing the latter to settle as slurry, washing the particles of said slurry with water and removing a substantial portion of said washing water by filtering, and thereafter drying the particles, said settling, washing, filtering and drying performed under conditions excluding the access of air to the particles until they are completely dry.
  • the method of cleaning light powdery particles of metal selected from the class consisting of aluminum and magnesium and having metal compound surface impurities which are removable by nitric acid which comprises, in a continuous process, the steps of supplying continuously at predetermined rates and mixing metal powder and water so as to form a slurry thereof, discharging said slurry into confined space, admixing concentrated nitric acid with said slurry within said space so that the acid reacts upon the metal particles and removes from their surfaces metal compound impurities and thus produces cleaned surface layers while said particles and acid move downwardly through said space, discharging water into said space substantially at the place where said reaction has just been completed so as to dilute the acid and substantially reduce the velocity of said reaction, discharging the mixture of diluted acid and cleaned particles lfrom said ,space under conditions per- 10 mitting gases developing during said reaction ⁇ to escape, land thereafter subjecting the resulting mixture to a treatment ⁇ including washing of said particles and separation of said pas-ticlesfrom the resulting mixture under conditions excluding access of air to said particles.
  • said. treatment including collecting said mixture and allowing it to rest so that saidv particles separato substantially from ⁇ the diluted acid and form an intermediate slurry, subjecting said intermediate slurry to a washing treatment, increasing the concentration of the washed. slurry, and treatit to obtain ciean dry metal powder, said treatment including withdrawing at predetermined intervals the settled intermediate slurry for subsequent washing and drying.
  • said treatment including collecting said mixture and allowing it to rest so that said particles separate substantially from the diluted acid and form an intermediate slurry, subjecting said intermediate slurry to a washing treatment, increasing the concentration of the washed slurry, and treating it to obtain clean dry metal, said treatment also including, increasing the concentration of the withdrawn intermediate slurry by a vacuum filter treatment.
  • said treatment including collecting said mixture and allowing it to rest so that said particles separate substantially from the diluted acid and form an intermediate slurry, subjecting said intermediate slurry to a washing treatment, increasing the concentration of the washed slurry, and treating it to obtain clean dry metal, said treatment also including filling the dried, clean metal powder immediately upon completion of said treatment into a container and thereafter sealing the latter air-tightly.
  • said treatment including collecting said mixture and allowing it to rest so that said particles separate substantially from the diluted acid and form an intermediate slurry, subjecting said intermediate slurry to a washing treatment, increasing the concentration of the Washed slurry, and
  • treatment also including filling the dried, clean metal powder immediately upon completion of said treatment into a container in the presence of an inert gas, and thereafter sealing said container air-tightly.
  • the method of cleaning light powdery particles of metal selected from the class consisting of aluminum and magnesium and having metal compound surface impurities which are removable by nitric acid which comprises the steps of reacting upon a fluid mixture containing water and 'said powdery particles with concentrated nitric acid so as to remove metal compounds of the surfaces of said particles and thus produce on said particles cleaned surface layers, effectively interrupting immediately thereafter said reaction by centrifugally separating said particles from said acid, and thereafter separating said particles from the resultingmixture under conditions excluding the access of air.
  • the method of cleaning light powdery particles of metal selected from the class consisting of aluminum and magnesium and having metal compound surface impurities which are removable by nitric acid which comprises the steps of suspending the metal powder in Water so as to form a slurry, admixing with said slurry nitric acid of high concentration and in an amount suflicient to secure an intense reaction of said acid upon the metal particles and thereby to remove metal compounds formed on the surfaces of said particles to thus produce cleaned surface layers on said particles, and effectively interrupting immediately thereafter said reaction by effectively removing the powdery particles from the action of the concentrated acid.
  • the method of cleaning light powdery particles of metal selected from the class consisting of aluminum and magnesium and having metal compound surface impurities which are removable by nitric acid which comprises the steps of suspending the metal powder in water so as to form a slurry, admixing with said slurry nitric acid of high concentration and in an amount sufficient to secure an intense reaction of said acid upon the metal particles and thereby to remove metal compounds formed on the surface of said particles for producing cleaned surface layers on said particles, and effectively interrupting immediately thereafter said reaction by centrifugally separating said particles from said acid.

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  • Chemical Kinetics & Catalysis (AREA)
  • General Chemical & Material Sciences (AREA)
  • Manufacture Of Metal Powder And Suspensions Thereof (AREA)

Description

May l2, 1953 F. J. HANsGlRG METHOD oF PROCESSING METAL PownERs Filed Feb. 9, 194e w w M .i ,e f4 M E p W d r w M W E, M 0 w t l 7 aa Z 3 WW m 3 f TM mr um n m u a E J M uv 0 f Il' V L. w Z H27 9 r ,f., z :s 3 ,0, f 6 im w3 M xy Eph v l 4J 0 W n y Patented May l2, 1953 Q'FFI-CE assenze..
METHODQEPROGESSING METAL POWDERS.
February 9, 13416', Senial No., 646,554- c tion orfexnlosionplnzerdt Theims of. auch; as oxides end nitrdes, ,fermedlinlthe Isurface-il.tiefereof the powdery, metal carnales y-erehierly; obj ectionable for manylapplteaticnseetithe powders. in-that the products made therefwmmraheteroseneeusdue to the inclusion fheinfihQSe-Qompund- If me nowderszam used yfor powder metallurgical y purposes the flmsof theeompoundsstated, render it diicult to obtain a coherent compacted bodrandtefulLw-sinter it.. ',lhefemtereefhediee are hetereseneouar., or insumeienn density; and ccusanientlsrl ct interim Rhysinalf prenantes@- It. therefore an. Ohieet cet; the inrentiento process .metal .y powders 'QI aluminum-orf. meeneslum.` solas .to .remoref-suroeerfilms er. @bioeticaable compo,umtsffromr ther-rpertclesfinan-ecenomical and .eieotivo-,mnnm n It isanothembieetonthe invention tenroeees powders ofthe kind hemnswmmedt. Kl-t remore from their; martiennesunerfleteliiilrnsi-y of obieetionahle. comanda, :suelah es. oxides.: and nitrdes. andato immunise-vthe-,sorfaee lareraof the cleaneaperticles surement-,attacks by t1.1e. ncreeno1A nirwana eeneinedinlthe surrounding, air, atleast-.for a limited mcricepofftime.
It is. further.l objecttof the., invention te; remove objectionable-compounds., nrimrilronides and frritrdes;` formed by;I contesa: with air infierir- Iacelayeraoifnowdery metal particles? Qffaluminum or magnesium man economical. ena-emotive Process. SuitedY for; mais: wwwmain It-xsstill a further atleet@cffftberinrentimeto reduce thereuraoazleyerseof porselein*t nartieles. of metan herein concerned? te their; metallic ifstate by removing; therefrom-:..obieotimable communes premouslyf forme@ therein bs; contacta vwitlr-` air or, otherwise. andztcrstebilize 4or;rllmmvcrirc against Subsequent attackszofmin those entferne lwersat least fon-a limited of; time; in.- an eetive ind eeonomicalmsolssauitedf; mossenroduc- These and. ethemhi eestoiathwinventimmill be more cleanse,.- undaretaodz when minut-ion proceed witlxtrefonmatmths ixxovmioh 2V Fis. lxllastrates schematically 0n hand 0f a HOW sheet a lprocess according to the invention snitedafgoi; mass producton,y and Fig. 2 shows scnematicallyiandin cross-section a Vacuum drying L no usable with the invention.
Acco ding tothe invention, lthe powder of a metateueh. er magnesium, prepared enr known and. Sui-table manner and the particles of? which contain in their surface layers objectionableffcompounds, primarily oxides and/.or nitrideauis subjected to a chemical treatment or washing, process whichy removes the objectionablecompoiindsfrom theparticle surfaces and leaves the surfaces clean and resistant to corrosion and other chemical reaction with oxygen 0r nitrogen contained in the surrounding air, and atlvleastffor a` limited period of time. To this endl according teY the invention. the metal powder is treatedwithnitric acid (NHOa) of suitable hignirconcentration so as yto remove from the powderyparticles-tlie objectionable oxide and/or nitride filmsrapidly; this erTect manifests itself by a cleanvanl metallic, shiny appearance of theftljeated particles.v By the action of the high- 1y cqncentratedinitric acid upon the surface layers of the Vparticles from which those compounds are beingrernoyed, thoselayers are also cleaned in tneaforementioned sense .so that they stayirninline againstftheattacks of oxygen-or nitrogen ysuch as contained. in the surrounding air, or their v t calativity with respect to that oxygen ornitigogen is substantially reduced at least for a, limited time periodwhich suffices for manipulating4 or other subsequent treatment of the powder inpowder metallurgical and other metallureieal processes,-
Afterthese effects of theinvention are obtained,A thev reaction between the nitric acid and powdery metal particles is interrupted quickly in Qrdertoavoid losseso the metall substance by the.l continued attack; thereon of the concentrated nitric acid. To. this end the nitric acid can be removed immediately aiter the desired effect of the, inyention is obtained., for instance by sepaeantinsthelatter Orrin a eentrifueine process.
Another and more convenient wer, Dertienlarly sul blefor mess production, consiste in iow be described with reference to Fig. l and in its application to magnesium powder of any origin, although it should be understood that the same process can be applied to powder of any other metal or to a mixture in desired and predetermined ratio of powders of different kinds of metals.
In a tank or container I a suspension of magnesium powder in any suitable vehicle or suspension medium is prepared. Water is preferred as such suspension medium. To this end, magnesium powder I4 is fed continuously from hop-- per I5 into tank IO at a predeterminable rate or amount per unit of time, which can be adjusted for instance by regulating the speed of a rotating feeder I6 arranged near the bottom and outlet oi hopper I5. Water is supplied to tank I 0 through pipe I? at a predeterminable rate perv time unit adjusted by adjustable valve 29. Thereby any desired ratio of magnesium powder and water as suspension medium can be adjusted and maintained during operation, and a suiicient supply of that suspension provided. In order to suspend the magnesium powder in the water and maintain it suspended therein, air can be injected into the water within tank lil as by an air supply line indicated by the dotted lines l3-I, or mechanical stirring means provided as indicated by stirl rer II on shaft i2 of an electric motor I3.
The suspension or slurry thus formed is withdrawn from tank IU through pipe I8 into reaction chamber I9 at a predeterminable rate which can be adjusted by any suitable means, such as adjustable valve 2I.
Tank I5 is arranged at a suitable highe'i` level than reaction chamber i9, so as to secure by action of gravity the desired flow of the suspension or slurry into the reaction chamber, and for other purposes to be explained hereinafter. A supply of nitric acid of suitable high concentration is maintained in tank 22, and a Ineasured quantity per time unit of the acid is withdrawn from tank 22 through valve 23, pipe 24 and another adjustable valve 25 into nozzle 26 arranged within reaction chamber I9 so that the discharge end 2 of nozzle 28 is at a lower level than the discharge aperture of pipe I8 into chamber I9. Nozzle 2t is of any suitable and known type and, if desired, it may be of a known type,
not shown, arranged to impart to the jet or spray of acid a rotation movement. More than one nozzle or similar discharge devices can be used.
Upon proper adjustment of the adjustable valves 2| and 25 and after opening' of valve 23, the suspension o1` slurry continuously formed in tank il] and the highly concentrated nitric acid continuously supplied to tank 22 are discharged and injected, respectively, into chamber I9 at predeterminable and adjustable rates. The concentrated nitric acid injected into the chamber space admixes rapidly with the watery slurry or suspension of magnesium powder simultaneously discharged into chamber i9. The concentration of the injected acid is such that its slight dilution by the suspension-water does not inhibit or substantiallyk reduce its instantaneous and vehement reaction with the magnesium powder. Nitrogen-oxide gases develop turbulently by this reaction but their escape through either pipe I8 or 24 is prevented by the sufliciently high` pressure in the slurry and acid, respectively, flowing through those pipes and due to the arrangement of tanks i0 and 22 at suitable higher levels than that of reaction chamber I9.
Considerable heat may be developed by-'this 4 reaction, depending on the relative amounts of acid and magnesium powder, and cooling means can be provided in such case, such as a jacket 59 around chamber I9 through which cooling water is passed in the manner indicated in the drawing.
While the powdery particles and acid travel downwardly through the space of chamber I9,
"the turbulent nitrogen-oxide gases developing during the reaction secure the contact of all the particle surfaces by the acid, and the removal of oxide and nitride lms in those surfaces and the cleaning of the latter is therefore completed after the particles and acid have travelled a certain path or distance downwardly below the discharge end 27 of nozzle 26. In order to interrupt the reaction at this point and to avoid losses of magnesium metal, the mixture of the powder and acid could be discharged into a relatively large pool of continuously replenished water which dilutes the acid sufficiently, or the powder and acid could be centrifugally separated in any known way. In Fig.` 1, at the distance below nozzle 26 where the reaction accordingI to the invention is completed, another nozzle 28 is arranged within chamber I9. Nozzle 28 discharges downwardly and is connected with pipe 29 passing through the wall-of chamber VI9 to the outside. Water is supplied through pipe 29 from a source not shown, of a predeterminable quantity per time unit adjusted by adjustable Valve 30. Nozzle 23 can be of any suitable type and supplies water in suiiiciently large quantity so that upon itsvrapid and intimate admixture with the nitric acid arriving below nozzle 28, the acid is diluted suiciently so'that its reaction with the magnesium powder is linterrupted or at least the velocity of that reaction is reduced sufficiently so that no objectionable losses of magnesium occur thereafter.
It will be appreciated that the distance between nozzles 26 and 28 depends on the now rate and concentration of the suspension or slurry of magnesium and on the flow rate and concentration of the nitric acid,V all of which are adjustable at the start of and during operation in the manner hereinbefore described. The optimum distance between those nozzlesv is chosen so as to cause complete removal of objectionable surface lms from the metal particles. No general rule can be given for this purpose because of the varying factors stated and the further` fact that average particle size and kind of the metal powder or mixture of such powders comes into play too. However, after such optimum distance has once been established for a given kind of powder or mixture of powders, continuous operation in the chamber I9 in mass production can be maintained by adjusting the speed of feeder I6, `the amount of water admixed thereby by means of adjustable valve 20, the amount of the suspension or slurry discharged into chamber I9 by means of adjustable valve 2 I, and the amount of concentrated nitric acid injected through nozzle 26 by means of adjustable valve 25. Thereby unavoidable variations of the average particle size and other properties of commercially obtainable metal powders can be taken care of during operation without changing the distance between nozzles 26 and 28 or of other dimensions and ar- Y rangements of the reaction chamber I9.
The rapid or almost instantaneous admixture of the diluting water and nitric acid is enhanced by the nitrogen-oxide gases previously developed within chamber I9 which are forced to escape -acecica thrmighthe lower opcnend of the chamber. This end is arranged within a pct 5i the bottom -of which is at some distance below the open end o! chamber I! and the side wall of which fis Asimceti from and extends considerably above that end. Asa consequence, the mixture of cleaned magnesium powder and diluted nitric iacid will collect in pot 5i to the level of `its upper `rim and thereby form a seal for the lower open end o! the chamber. The nitrogen-oxide gases can penetrate that seal because the pressure in the liquid `c i'ilurnri between the upper end of the side wall of pot "5i and the lower end of chamber i3 is far smaller than the pressure in pipes i8 and 24, respectively. Pot V5i is illled almost immediately after operation 'has started, andthe connuously supplied mixture of 'magnesium powvder and diluted nitric acid ilows over the upper rim of pot 5I into a settling tank 3i. The cleaned magnesium powder in the overowing mixture will always be covered vfully by the lliquid consisting of nitric acid and water, and detrimental access of air to those particles is thereby prevented.
The slurry of magnesium particles and highly diluted nitric acid collects in tank 3i Since the nitrogen gases escaped previously in the manner hereinbefore described, this mixture cornes to a rest within tank 3| and segregates. The heavier magnesium powder sinks toward and accumulates upon the downwardly tapering bottom 33 of tank 31 up to level 34, whereas the diluted nitric acid forms a liquid layer on top of the settled magnesium powder to the level 32. Inside tank 3i and Vnear its upper open end, a trough 35 is arranged acting as an overflow for the diluted nitric acid which is withdrawn from trough 35 in the manner shovm.
The magnesium powder collected above bottom 33 forms a slurry still admixed with diluted nitric acid; itis withdrawn from tank 33 at proper time intervals, as explained later, through valve 36 into a vacuum lter, such asa washing and ltering vessel or tank 31 provided with a downwardly tapering bottom 33. A filter 33 is arranged in vessel 31 above and spaced from bottom 38. Clean washing water is poured or sprinkled through pipe 40 upon the slurry deposited on lter 33 and accumulates above the powder to the level 44 of an overflow (trough) 4i from which it is withdrawn in the manner shown. Vessel 31 communicates through an aperture in its bottom 38 and connecting pipe with another conilned space 42 thereunder in which a vacuum is produced and maintained by pipe 43 connected with a vacuum pump (not shown). Thereby a great part of the liquid consisting of washing water, admixed with the magnesium powder above filter 33 is sucked away through filter 39 and the magnesium powder slurry more concentrated to form a. cake which still contains but little liquid. Any access of air to powdery particles of the cake is prevented by the layer of washing Water collected above it. Any liquid collecting in vessel 42 can be withdrawn therefrom through a .pipe and valve 50. After a lter cake of sufficient height has been built up, the valve 3i is closed and the filter cake allowed to remain until the water entering through the pipe 40 has thoroughly cleaned the surfaces of the metal particles in the cake. The filter cake is then removed from vessel 31 and transferred into a dryer. The valve 36 may then be opened to allow another quantity of slurry `to enter the vessel 31 to form a new filter cake. The extraction of the water stili containedin the .filter cake can be accomplished by treating it with `a suitable `extraction medium, such as acetone. The .extraction of the water and the drying of the lter cake may 'be eifected in Ia vacuum dryer of any known suitable type. Whichever lrind of dryer and means for transferring "the cake into it are used, it is essential that the magnesium particles of the cake are wet luntil they are placed 'into the dryer, so that access of air to the magnesium particles is prevented which, before 'the latter are completely dried, could 'react with the particle surfaces `in `spite o'f their thorough ycleanliless.
Fig. 2 indicates, by way of example, one form of a vacuum dryer having a revolvable drum '46 in which'vacuumiisproducedby means of pipe 4B projecting into the vinterior ofthe drum land connected with a Vacuum pump (not shown). A mild heat, up to about 50 C.,'isproduced within the drum, for instance, by means of gas flames 4B of a gas burner 41. Thereby the .strong cooling effect resulting from 'the evaporation in vacuo of the liquid adhering to the magnesium powder can be compensated to desired extent. The vacuum produced within vdrum 4t should be oi a. pressure equall'ing or below 4to 5 millimeters mercury column absolute. While the filter cake is being dried, it breaks up into powder; after the powder vis completely dried, the vacuum can be released and the powder removed from the drum. The completely dry powder is clean ,and shiny, and its surface layers are resistant to contamination in the aforementioned sense for a limited period of time. 'The powder can therefore be manipulated, such as compacted for metallurgicalpurposes, without the danger of the formation of objectionable oxide or nitride lms thereon.
lf the dry powder is to be stored or shipped, an inert gas can be admitted into drum 46 immediately after the vacuum has been released, for instance, very dry hydrogen or an inert gas like helium, argon or neon. The 'inert gas may be supplied through the same tube 45 through which the drum 46 is evacuated. In certain cases and if the interior of the drum and the powder ltherein have been permitted to coo1 to room temperature while the vacuum is on, dry nitrogen can be used as protective gas and admitted into the drum after the vacuum has been released. The powder and inert gas of the types mentioned can be filled into containers or cans which are air-tightly sealed thereafter.
if an inert gas, such as helium, etc., has been admitted into the drum, a layer of that gas may be adsorbed by the dry powder particles which additionally protects the latter against Voxidation for some time thereafter.
use of a protective gas, such las very dry hydrogen of a dew point far below minus 40 C., or helium, argon or neon, even such superficial oxidation or nitrid'ing can be prevented. Since compacts of high density can be produced of 'the metals herein concerned by relatively low compaction pressures, no danger of undue absorption of the protective gas or penetration of oxygen or nitrogen into the interior of the compact exists. Hence the final product obtained Aby sintering Will be of greatest purity, homogeneity and density and therefore exhibit unusual or heretofore unknown physical properties. The same advantages are obtained if the powder processed according to the invention is adm'ixed with other clean powders in order to obtain from the powdery mixture a desired metal alloy, or if a mixture of those powders is processed, bodies or even ingots compacted and sintered therefrom will exhibit outstanding properties due to their homogeneity, density and purity. It is well known in the art that casting of magnesium, aluminum, or alloys of those metals requires a large riser because of the high shrinkage of those metals and alloys, and their production is therefore connected with extensive melting losses. The control of grain size vis equally difiicult in casting those metals and alloys, and the formation of blisters and blow holes therein is hard to avoid, if possible at all. The properties of the alloys depend greatly on the purity and exact ratio of the alloying constituents therein which to obtain 'is extremely diicult in casting processes. Therefore, if ready to use bodies, or ingots for further working and shaping are made by powder metallurgical processes from powder or powdery mixtures containing metal powder processed according to the invention, all these difficulties can be obviated. However, these aspects of the use of powders processed according to the invention do not form a subject matter thereof and are mentioned only to illustrate the high utility and novel effects of powders processed according to the invention.
It should be understood that the invention is not limited to any of the embodiments herein exemplified and illustrated. Thus, for instance, instead of single nozzles 26 and 2B, any number of them can be arranged discharging at the same lor slightly different levels, and the supply to them of the slurry and nitric acid, respectively, can be regulated simultaneously or independently. Instead of a vacuum lter as exemplied hereinbefore, any other type of filter can be used, such as a continuous working filter. Whatever the implements and equipment used for practicing the invention may be, their essential operation should consist in reacting upon the metal powder or mixture of such powders with nitric acid of suiiiciently high concentration so as to rapidly remove from the powdery particles oxide and nitride films and also clean the surface layers of the particles; to interrupt this reaction immediately after these effects are obtained in order to avoid losses of metal substance; to wash the cleaned powdery particles and to dry them in the absence of air, i. e. in a sufficiently high vacuum and/or in the presence of dry and inert gas (e. g. helium), not shown; and to store the processed dry powder, 'if required, in air-tight containers and advantage- 'ously in the presence of an inert gas until the powder is further processed or used, particularly in a powder metallurgical process.
What I claim is: l. The method of cleaning light powdery particles of metal selected from the class consisting `of aluminum and magnesium and having metal compound surface impurities which are removable by nitric acid, which comprises the steps -of reacting upon a uid mixture containing water and said powdery particles with concentrated nitric acid so as to remove metal compounds from the surfaces of said particles and thus produce on said particles cleaned surface layers, effectively interrupting immediately thereafter said reaction by effectively removing the powdery particles from the action of the concentrated acid, and thereafter separating said particles from the resulting mixture under conditions excluding the access of air.
2. The method of cleaning light powdery particles of metal selected from the class consisting of aluminum and magnesium and having metal compound surface impurities which are removable by nitric acid, which comprises the steps of reacting upon a fluid mixture containing water and the powdery particles with concentrated nitric acid so as to remove metal compounds from their surfaces and thus produce cleaned surface layers, and effectively interrupting immediately thereafter said reaction by diluting the acid and separating said particles from the resultant mixture under conditions excluding the access of air.
3. The method of cleaning light powdery particles of metal selected from the class consisting of aluminum and magnesium and having metal compound surface impurities which are removable by nitric acid, which comprises the steps of reacting upon a fluid mixture containing water and the powdery particles with concentrated nitric acid so as to remove metal compounds from their surfaces and thus produce cleaned surface layers, effectively interrupting immediately thereafter said reaction, thereafter adding water to the resulting mixture, and washing and drying the particles under conditions excluding the access of air.
4. The method of cleaning light powdery particles of metal selected from the class consisting of aluminum and magnesium and having metal compound surface impurities which are removable by nitric acid, which comprises the steps of reacting upon a mixture of water and the powdery particles with concentrated nitric acid so as to remove metal compounds from the surfaces and thus produce cleaned surface layers on said particles, effectively interrupting immediately thereafter said reaction, thereafter adding a washing agent to the resulting mixture and washing the particles with said washing agent so as to protect said particles against the access of air, and removing the washing agent and other mixture ingredients still adhering to the particles by treating them with a volatile solvent medium.
5. The method of cleaning light powdery particles of metal selected from the class consisting of aluminum and magnesium and having metal compound surface impurities which are removable by nitric acid, which comprises the steps of reacting upon a fluid mixture containing water and the powdery particles with concentrated nitric acid so as to remove metal compounds from their surfaces and thus produce cleaned surface layers, effectively interrupting thereafter said reaction, thereafter adding a washing agent to the resulting mixture and washing the particles under cover of a washing agent on a vacuum lter, and drying the wet filter cake in a nonoxidizing atmosphere.
6. In a method as set forth in claim 5, drying the wet filter cake in the presence of an inert gas.
7. In a method as set forth in claim 5, drying the wet filter cake in the presence of an inert gna` selectedfrom group consisting of helium, mlm and neon.
8. In a method as setforth in claim 5, drying said filter cake in a vacuum the pressure of which does not exceed about 5V millimeters mercury column absolute.
9. In a method as set forth in claim 5, drying the lter cake aty a temperature up to about 59 C;
10. The method. of cleaninglight powdery particles of metal selected from the classv consisting of aluminum and magnesium vand having metalcompound surface impuritiesA which. are removable. by nitric acid, which comprises the steps 0f suspending the metal-powder in water so as to form a slurry, admixing with said slurry niacid of highconcentration and in an amount sumcient to secure an intense reaction of said acid upon the metal particles and thereby to remove metal compound impurities on the surfaces of said particles and to thus produce cleaned surface layers on said particles, and effectively interrupting immediately thereafter said reaction by diluting said acid.
1l. The method of cleaning light powdery particles of metal selected from the class consisting of aluminum and magnesium and having metal compound surface impurities which are removable by nitric acid, which-comprises the steps of suspending the metal powder in water so as to form a slurry, admixi'ng with said slurry nitric acid of high concentration andv in an amount mmcient to secure an intense reaction of said acid upon the metal particles and thereby to remove metal compound impurities on the surfaces of said particlesv and to thus produce cleaned surface layersr on said particles, effectively interrupting thereafter: said reaction by effectively removing said particles' from the aetion of said concentrated acid, substantially separating said acid and particles, washing the particles with and separatingr them thereafter from a washing agent, and drying the washed particles.
l2. In a method as set forth in claim 11, substantially separating the acid and particles by allowing the latter to settle as slurry, washing the particles of said slurry with water and removing a substantial portion of said washing water by filtering, and thereafter drying the particles, said settling, washing, filtering and drying performed under conditions excluding the access of air to the particles until they are completely dry.
13. The method of cleaning light powdery particles of metal selected from the class consisting of aluminum and magnesium and having metal compound surface impurities which are removable by nitric acid, which comprises, in a continuous process, the steps of supplying continuously at predetermined rates and mixing metal powder and water so as to form a slurry thereof, discharging said slurry into confined space, admixing concentrated nitric acid with said slurry within said space so that the acid reacts upon the metal particles and removes from their surfaces metal compound impurities and thus produces cleaned surface layers while said particles and acid move downwardly through said space, discharging water into said space substantially at the place where said reaction has just been completed so as to dilute the acid and substantially reduce the velocity of said reaction, discharging the mixture of diluted acid and cleaned particles lfrom said ,space under conditions per- 10 mitting gases developing during said reaction` to escape, land thereafter subjecting the resulting mixture to a treatment` including washing of said particles and separation of said pas-ticlesfrom the resulting mixture under conditions excluding access of air to said particles.
14 In a method as set forth in claim i3, suppiying said initial suspension or slurry and said concentrated nitric acid from levels considerably higherthan the levels at which said suspension and acid are discharged into said confined space,
i5,. In a method as set forth in claim 13, adjusting independently the quantities per time unit of metal powder and suspension water to form said initial slurry and the quantity per time unit of said initial slurry fed into said conned spa-ce, and also adjusting independently the quantity per time unit of concentrated nitric acid. supplied to said confined space so as f to obtain and maintain a predetermined optimum ratio of said acid and powdery particles for saidl reaction.
16. In a method as set forth in. claim 13, ab stracting heat from` said confined space While i said reaction proceeds..
17. In a method as set forth in claim 13, admixing by injection under pressure said nitric acid with. said initial suspension or slurry in said conned space.
1.8. In a method as set forth in claim 13,. admixing by injection under pressure the diluting water with said mixture of acid and slurry within said confined space after said reaction has been. completed.
-l9. In a method as set forth in claim 1.3, said. treatment including collecting said mixture and allowing it to rest so that saidv particles separato substantially from` the diluted acid and form an intermediate slurry, subjecting said intermediate slurry to a washing treatment, increasing the concentration of the washed. slurry, and treatit to obtain ciean dry metal powder, said treatment including withdrawing at predetermined intervals the settled intermediate slurry for subsequent washing and drying.
20. In a method as set forth in claim 13 said treatment including collecting said mixture and allowing it to rest so that said particles separate substantially from the diluted acid and form an intermediate slurry, subjecting said intermediate slurry to a washing treatment, increasing the concentration of the washed slurry, and treating it to obtain clean dry metal, said treatment also including, increasing the concentration of the withdrawn intermediate slurry by a vacuum filter treatment.
21. In a method as set forth in claim 13 said treatment including collecting said mixture and allowing it to rest so that said particles separate substantially from the diluted acid and form an intermediate slurry, subjecting said intermediate slurry to a washing treatment, increasing the concentration of the washed slurry, and treating it to obtain clean dry metal, said treatment also including filling the dried, clean metal powder immediately upon completion of said treatment into a container and thereafter sealing the latter air-tightly.
22. In a method as set forth in claim 13 said treatment including collecting said mixture and allowing it to rest so that said particles separate substantially from the diluted acid and form an intermediate slurry, subjecting said intermediate slurry to a washing treatment, increasing the concentration of the Washed slurry, and
treating it to obtain clean dry metal, said treatment also including filling the dried, clean metal powder immediately upon completion of said treatment into a container in the presence of an inert gas, and thereafter sealing said container air-tightly.
23. The method of cleaning light powdery particles of metal selected from the class consisting of aluminum and magnesium and having metal compound surface impurities which are removable by nitric acid, which comprises the steps of reacting upon a fluid mixture containing water and 'said powdery particles with concentrated nitric acid so as to remove metal compounds of the surfaces of said particles and thus produce on said particles cleaned surface layers, effectively interrupting immediately thereafter said reaction by centrifugally separating said particles from said acid, and thereafter separating said particles from the resultingmixture under conditions excluding the access of air.
24. In a method as set forth in claim 5, drying the wet filter cake under exclusion of air.
25. The method of cleaning light powdery particles of metal selected from the class consisting of aluminum and magnesium and having metal compound surface impurities which are removable by nitric acid, which comprises the steps of suspending the metal powder in Water so as to form a slurry, admixing with said slurry nitric acid of high concentration and in an amount suflicient to secure an intense reaction of said acid upon the metal particles and thereby to remove metal compounds formed on the surfaces of said particles to thus produce cleaned surface layers on said particles, and effectively interrupting immediately thereafter said reaction by effectively removing the powdery particles from the action of the concentrated acid.
26. The method of cleaning light powdery particles of metal selected from the class consisting of aluminum and magnesium and having metal compound surface impurities which are removable by nitric acid, Which comprises the steps of suspending the metal powder in water so as to form a slurry, admixing with said slurry nitric acid of high concentration and in an amount sufficient to secure an intense reaction of said acid upon the metal particles and thereby to remove metal compounds formed on the surface of said particles for producing cleaned surface layers on said particles, and effectively interrupting immediately thereafter said reaction by centrifugally separating said particles from said acid.
27. In a method as set forth in claim l2, effectively interrupting said reaction by diluting said acid.
FRITZ J. HANSGIRG,
References Cited in the le of this patent v UNITED STATES PATENTS Germany Dec. 13, 1928

Claims (1)

1. THE METHOD OF CLEANING LIGHT POWDERY PARTICLES OF METAL SELECTED FROM THE CLASS CONSISTING OF ALUMINUM AND MAGNESIUM AND HAVING METAL COMPOUND SURFACE IMPURITIES WHICH ARE REMOVABLE BY NITRIC ACID, WHICH COMPRISES THE STEPS OF REACTING UPON A FLUID MIXTURE CONTAINING WATER AND SAID POWDERY PARTICLES WITH CONCENTRATED NITRIC ACID SO AS TO REMOVE METAL COMPOUNDS FROM THE SURFACES OF SAID PARTICLES AND THUS PRODUCE ON SAID PARTICLES CLEANED SURFACE LAYERS, EFFECTIVELY INTERRUPTING IMMEDIATELY THEREAFTER SAID REACTION BY EFFECTIVELY REMOVING THE POWDERY PARTICLES FROM THE ACTION OF THE CONCENTRATED ACID, AND THEREAFTER SEPARATING SAID PARTICLES FROM THE RESULTING MIXTURE UNDER CONDITIONS EXCLUDING THE ACCESS OF AIR.
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US4810288A (en) * 1987-09-01 1989-03-07 United Technologies Corporation Method and apparatus for making metal powder
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