US2394052A - Method of manufacturing magnesium powder - Google Patents

Description

flares, incendiary and tracer Patented Feb.--5," 1946 METHOD OF 2,394,952 MANUFACTURING MAGNESIUM POWDER Harold E. Hall, Short Hills, and Alexander F.

Knoll, Westileld, N. J., assignors to Metals Disintegrating Company, Inc., Elizabeth, N. J., a corporation of New Jersey No Drawing. Application April 1, 1942, Serial No. 437,226

Claims. (c1. 241-) This invention relates to a metal powder, e. g., magnesium powder having form and particle size such that it is fitted for use as a component of bullets and shot, pyrotechnic signals, flashlight bombs, and the like devices, and as a reagent in organic synthesis.

In accordance with this invention it is contemplated to produce a powder that, while exhibiting a utility-value for the purposes above mentioned, not substantially less, and in some cases even greater, than previously known and standard powders, is free from the notorious explosion hazards of such standard powders, to the degree that it may fairly be considered a new product. And it is incidentally a product which is adapted for incorporation in some of the devices referred to, without the need for the preliminary treatment required where such standard powders are used.

Incidentally, the invention includes the idea of packaging a given quantity of the .powder of desired characteristics as to particle size and distribution, in a container that may be readily and safely handled during certain otherwise dangerous stages of manufacturing process and during ordinary transportation and handling of the finished product.

In another or its aspects the invention involves new methods of reducing the massive metal to through all stages of manufacture, transportation and ultimate utilization, with a minimum of hazard heretofore unattained.

Other objectsand aims of the invention, will appear in the course of the following description, in which are set forth the characteristics of the products referred to, and the steps of process and the operating conditions employed in the manufacture.

At the present time there are three magnesium powders commonly used for military purposes, known as grades A, B, and C, respectively. Of these, grade A is minus 80 mesh and finer, with a minimum metal content of 93%. Grade B answers the requirement of all particles through a 115 mesh screen, with a minimum of 90% through a 200 mesh screen and a minimum metal. content of 92%. Grade C is identified by the fact that a maximum of 12% of the particles are held on a 48 mesh screen, a minimum of 76% pass through a 48 mesh screen but remain on and a 100 mesh screen, a maximum of 12% pass through a 100 mesh screen, with a minimum metalcontent of 96%.

There is also a powder known as Photoflash," of

which a minimum of 60% passes through a 200 mesh screen, and all of it passes through an 80 mesh screen, with minimum metal content of 95%.

Any means for mechanically removing from the massive metal a usable powder particle, results in the production of a variety of particle sizes. In

some instances the intended use of powders so produced allows a considerable range of particle sizes. Where military use is contemplated, grade A, above, is the only grade in this category. If the manufacturer has sufiicient diversity of demand for his .product so that it is profitable to classify his hetero-sized product, and allocate the fractions so classified among the various grades, he may readily supply the several grades above specifled,-so far as particle size is concerned. It has been demonstrated that the finer grades, 1. e., grade B and Photoflash, result casually during the manufacture of the coarser grade C or of the most hetero-sized material, grade A. It is one of the interesting and important features of the present invention that it makes possible, if desired, the manufacture of the grade B material in quantity but little short of the total starting material, and as the main rather than as the byproduct. And another feature is that the pro- ,the desired comminute form, and of handling it portions of various usable particle sizes in a given mass as the comminuting operation is finished may be to an extent regulatablypredetermined; this, of course, has a distinct bearing upon the yield of any particular grade that may be desired.

Manufacture begins with the removal from massive high purity magnesium metal, in ingot,

bar, rod, billet, stick or other suitable form, of

chips or turnings, as by means of appropriately designed filing machines. The design and/or coarseness of file used produces particles of vary-' ing bulking density, with important effect upon the ease with which the various grades of finished powder are produced in subsequent operations.

The material to be filed is fed against the-reciprocating file by suitable means, and speed of reciprocation of the file, and pressure of the metal against the file are regulated in view of the desired volume of production and the suitability of the product for the subsequent operations re- The filing operation '(and this is an important feature of the invention) is conducted under a heavy stream of coolant, e. g., mineral spirit or other high flash petroleum solvent. This "wet filing operation results in increased production and diminution of hazard as compared with similar operations sired size distribution,

ground terminal side, and after passing over the moist In these, even when 2 conducted under dry conditions; and the liquid coolant-also serves as a vehicle to carry the filedoff particles into a hopper or receptacle, whence they proceed to a so-called disc mill or'equivalent device, by means of which many or most of the I means are taken to filed-off particles are reduced in size. A disc mill I I comprises two relatively rotatable grinding plates operating in a bath of coolant such as already described; The'design of the plates, the spacing between them, and the relativegspeed of rotation determine the degree to whichthe filed particles are broken along chatter planes and the degree to which the curved particles thus produced are flattened, or even reduced in thickness. factors are controlled so as to produce a major enumerated above. magnesium .in partial suspension in the coolant is pumpable by standard methods. It is, therefore, lifted to a screen remove oversize particles, these being diverted to another disc mill and aftera second milling pumped back into the stream going to the screen. The undersize from this screen is conducted to any standard wet classifying device, such as a wet These dried. When this moved from the prod ct forms an explosive mixture with the air, recirculation is practised and ensure that the vapor concentration is always below the explosive limit. It is preferred, however, to pay the additional cost in size of equipment installed,' and amount of steam consumed, so as tialexplosive spirit-airmixtures are supplied to the material being dried.

Passage of heated fresh air is continued until the powder contained-in the trays is thoroughly state is attained the passage of heated air is stopped and the passage of air, at

' atmos heric temperature, or slightly thereunder,

having a mesh suitable to screen or a wet centrifugal classifier, where the -200 material, 1. e., suspension in the coolant, being caught in hanging filter bags, the coolant passing through the bags and going for re-use in the filing and/or disc-milling operation.

The material h d on the 200 mesh screen or precipitated in the wet classifier is now of a dein accordance with and because of the indicated controls of the filing and Photoflash, is removed in by gravity to a spirit reservoir.

disc-milling operations, to answer the require- (1) grade C, plus (2) grade for grade B. However, a

ments of a mixture of A, and (3) mill feed portion of the -200 mesh material previously.

screened out will have to the grade A component.

In any case the material (except-the mill feed for grade B) is centrifugedto a definite metal content and then filled into bags, like those just referred to in connection with the separating out of Photoflash, the rial such as what is known as mole-s "napped twill," with the nap on the inner surface of the bag, substantially dust-tight to the minimum size particles fed into them, but permitting the passage of the coolant and/or coolant fumes, the filtered-out coolant going to a reservoir for re-use. These bags are to hold a measured quantity of material, and are sealed against loss of metal. When the material is, wanted in the truly dry state, the sealed bags are brought to a'drying compartment, where they are placed upon perforated or expanded metal trays which are electrically connected to a to prevent the accumulation of be returned (added) to terial is exhausted sufilciently removed the recirculation through the compartment of air laden with spiritvapors. This is an ineffiwet bags being of textile matej ,whlch the metal powder is is obvious that the cient drying process, as such, due to the fact that the warm air is .not recirculated, which is the practise in other commercial drying installations.

the liquid which is being reliquid. Subsequent ,will leave an increasingly. concentrated oil-liq- 'uid solution until, with solvent or spirit is le'ssened by is commenced, until the material contained in the bags is brought to atmospheric-temperature. When this condition is attained the cooled, dried, material, still in the closed bags, is placed in appropriate metal containers, inwhich it is shipped to the consumer.

We have referred to a process for 'the production of grades A and C magnesium powder .in a form more useful to the loaders of pyrotechnic components, to which their use is almost solely confined, than is the the military establishments that developed the munitions in which they are used. In our proc-' ess werecognize the necessity, on the part of the users of these grades of powder, to coat the dry powder with oil, usually linseed oil or castor oil, before incorporating them into the other ingredients of the pyrotechnic charge. It is well known that such oik are characterised by their higher surface tension and viscosity, which make them prone to remain in spherical globules when in contact with If, however, the metal intended or soughtto apply the oil, are first thoroughly wet with a liquid in which that oil is soluble, as happens when material is processed by wet filing and discgrinding as above set forth, that metal surface so wetted will muchmore readily receive a uniform coating of the solution of the oil in the same evaporation of the solvent the evaporationof the last traces of the solvent, the solute will be uniformly distributed over the surface of the particle which was originally wetted with the solvent liquid. 4

Thus may be accomplished a more thorough. imiform and satisfactory oil'coating of the metal powder particles than is possible by any attempts to oil-coat metal powder that haseither been produced dry, or has been produced wet and subsequently dried. It is contemplated, therefore. to take metalpowder that has been comminuted in the presenceof a spirit or other aqueous coolant while still wet therewith, and either before or during or after the comminution of the metal to add to the spirit the oil with eventually to be coated. It is preferred to make this addition in the form of a solution of the oil in the spirit after centrifuging or minuted metal to a known spirit content, but it oil could be added at other stages of the process. This point, i. e., after centrifuging and before bagging, is chosen because a definite percentage of oil is usually specifled, and the time consumed in removal of the the lessening of before the powder the spirit content of the metal goes to the drying compartment. The control to be sure thatnopotenm dry form now specified by' suitable nonotherwise bringing the com- 'of the oil content is also more sure by this means.

Powder wet with a solution of oil in spirit, as distinguished from powder wet only with a readily volatile spirit, is relatively non-dusting.

' (and, therefore, not so liable to the hazard of explosion) after the spirit has been evaporated. The drying and packaging of oil-coated powder is less involved, therefore, than the similar operation on uncoated, or pure, powder. In this case, bagging in sealed bags may be unnecessary and the removal of the spirit or solvent may be accomplished in pans having a cover of the aforesaid mole-skin cloth, placed in the trays oi'the' drying coml'iartments. Packingentails only the,

removal of the cover and the scooping'up of the solvent-free oil-coated powder into metal containers.

While the advantages of producing-a .coating it were in the free molecular state oi. 02. Thus.

magnesium milled in an atmosphere of CO2 and on powder by the above means are theoretically N: for two hours (impact milling)v showed these results. y i

Metal 00:

= Percent Percent Percent V stillt.'---.. .L 99 12 I 88 Finish (two hours) 91 1 v t also appears that this utilization of the normally fixed oxygen content of oxides in the oxidation of metals is catalysed by thecold works ing otthe ductile metal in certain zones of the ball milling operations'and not in others.

Thus, coarse magnesium or 95% metal content milled at room temperature in aCOz or air atmosphere was reduced to all 200 mesh material of 91% metal content in two hours. Yet -200 mesh material of 95% 'metal content is stable to air if not worked. Even on standing for weeks it shows no significant reduction in -sn etal content. It is believed that the greater I Candle- Coating method power dcyeloped The instant process 885, 000 Coating of dry powders 850, 000

I Grade B powder-This material is 90% 2oo mesh of a minimum metallic content of 92% Mg. As previously stated, a process for the manuiactureof this product in substantial percentages of the total amount of metal comminuted to obtain it, has long been needed. Three factors have obstructed the high percentage production or this grade. First, is the tendency of magne-' slum to oxidize readily in presence of oxygenbearing gases, such as air, water vapor, or CO2,

.when its particle size is reduced by cold working.

Second, is the tendency of particlesoiductile metals to weld together when milled in' complete absence of oxygenbearing gases, e. g., in

nitrogen formation of explosive metal-air mixtures as the Third, is the greater tendency toparticles decrease in size, when these particles are free to move in the air, i. e., in any dry'proc-' ess. For variousreasons, including safety, therefore, a wet ball-milling procedure was developed, in order to produce a powder of the required degree of fineness and metallic content; these fac-,

tors had to be considered: v

(1) The atmosphere'of the ball mill.

(2) The degree and rate of cold working produced in the wet ball milling operation. This involves diameter of millQspeed of rotation of the mill, size and weight of balls, etc.

(3) The particle size of the mill feed, with particular attention being thickness.

(4) The composition of the charge, I. e., the

- ratioof metal to weight of balls and'to weight of spirit.

The atmosphere of the ball mill must be such that the available oxygen content in a sealed mill is less than that required for the oxidation of sufficient metal to reduce the metallic content paid to the particle chemical activity of they metal while it is being milled is due to the formation of the highly coldworked 'Beilby layers of metal, which exhibit some of the properties of the liquid phase of the metal in question in being. more reactive than unstrained metal. In any event, magnesium can be oxidized in air atmosphere; at room temperature when suiiiciently severe cold work is done upon the metalparticles. One can grind the same metal particles in the same gaseous atmosphere with reduced oxidation it the grinding is done in the zone of attrition rather than the zone of impact in the bal mill. This is'indicated by the fact that -200 mesh material produced by disc-milling in air atmosphere, a typical attrition method, always has a metal content above whereas all impact ball milled material is below this.

As one would expect, the grinding is less eiflcient in the former case but alsothe lower efficiency must be accepted if-the metal content is to be held above 92%. 'It is, therefore, recommended to mill in an atmosphere consisting of a mixture 0f N2 and CO: and/or Orand H2O held to the specified limits or any combination of them; thus it is recommended that the combined quantities of Oz, H20, and CO2 present in the milling circuit, computed to M80, Mg (0H2) and MgCOa, be maintained at not in excess of 8% 'MgO-l-Mg(0H2) +MgC0a, based on the weight of metal charged into the mill, the mill circuit being closed. C02 and/or 02 are necessary ingredients in the mill atmosphere in order for grinding to the required 200 mesh to take place. It is found that increasingly fine grinding takes place with an accompanying increase in oxide content of the powder as the percentage of CO2, 02 or H2O increases, and that as Nz percentage approaches the tendency for the material to flake rather than grind increases with increasing tendency'to retain the original metallic content without oxidation. v

For example, coarse magnesium filings milled three hours in nitrogen gave only 8% 200 material. Same type of feed milled three hours in 12% Cori-88% N2 atmosphere gave 78% 200 material.- Same type of feed milled three hours tendencies materially so that under some atmospheric conditions and of metal content of original,

mill feed, milling in the range of attrition is necessary in order to preserve the metallic content'as specified, under other conditions of mill atmosphere and mill-feed characteristics, impact milling is possible with a preservation of metallic content.

Thus if the original feed used in ball milling for grade B is sumciently reduced by other methods (filing and/or disc-grinding) methods which do not work the surface enough 'to induce or promote rapid oxidation, ball milling in the zone of impact will give a very satisfactory product. Thus 100 pounds of feed of 99% plus metal content milled in a 3 ft. diameter x 9 ft. long mill with 5,000 pounds of balls at 4''! R. P. M., with 25 gallons of mineral spirits, in an atmosphere of air, mill sealed, temperature 43 0., barometric pressure of 30" Hg gave a 97% yield of grade B material of 93% metal content, density 0.60, flow 10 grams in 90 sec., Metals Disintegrat- .ing Company Flow-Tap meter.

Grades A and Photoflash" can be prepared in a similar way, thus grinding 100 pounds of magnesium under the same conditions as those for grade B, given above, for three and one-half hours gave a, 95% yield of Photoflash (62% 200 mesh) of 95.5%. metal content.

A similar run made with 150 pounds of filed and disc ground metal for two hours gave an 80% yield of grade A of 94% metal content.

Control of particle sizerange, or the des- 'ignated grades of material can be achieved by making use of the discovery that comminution of malleable metals can be achieved only-inthe presence of gases or vapors which react chemically with the metalundergoing comminution.

Therefore, the degreeto which magnesium metal can be subdivided in a closed ball mill can be controlled either-{by adjusting the amount of oxygen,"c'arbon dioxide or other reactive gas in the mill atmosphere, or by adjusting the weight of metal charged into the mill when the atmosphere introduced into-the mill is one of constant composition suchas air. with such-control-of disintegrating conditions, the amount. ofsubdivision achieved-is, largely independent of time. Thus in two-grinds in whichmagnesiumlwa's milled in a sealed mill in'air under identical conditions-other'than time of grinding one grind of six hours gave an 85% yield of 200 mesh Carbon dioxide, oxygen necessary for comminution of magnesium metal, has the capacity for causing mag nesium particles to adhere to the steel balls andmill walls. This armouring effect improves greatly the emciency of grinding and reduces contamination of the product with iron, etc., from mill balls and walls.

Thus, showing efliciency of CO; as grinding atmosphere, consider the following comparison of grinding efliciency and oxidation of metal in milling magnesium in air and carbon dioxide at- I mospheres, respectively:

Mill 36" diameter, 6" deep Balls v 280 lbs., Feed 4 lbs., coarse Mg fillings Vehicle 1 gal. mineral spirits spppfl Q R- P} M- Temperatur C.

. Yield 6r Atmosphere in sealed mill 333]: $335 product Hoar; Percent v Percent 90% Nrl-l 00s.... 2 07 r+20 0 2 19 94 90 ,oo,+1o% air..-. 1 p 90 88 Some explanation of the technique of ballmilling and of thedifference between attrition and impact, between the effects of cascading of balls and cataracting of balls, is set forth in U. S.

Patent No. 2,002,891, to Hall.

Where particular materials or forms of, ap-

paratus are mentioned inthe specification, itis to be understood that materials or apparatus equivalent for the purposes of our inventiou'lare contemplated and may be substituted, except where the language of the claims clearly for' bids.

' And wherein the specification it is stated h t certain .procedure, involving certain materials", temperatures, etc., is followed, reference is-Lima plied to typical examples given in thespe'cifle. cation, not as limitative of the claims but illustrating the technique to be followed.

We "claim: l. The method of producing comm'inutedmet'al of such chemical characteristics; and particle size that when dusting thereof occurs in certain magnesium, another of eighteen hours gave 95% of, 200 mesh-magnesium. In both grinds the oxygen was completely =consumed. Use 'ofthis fact, i. e"., the; dependency of comminution onthe metal-chargeand-reactive-gas relation,- al- 1ows..control of degree of comminution or of the designated grades.

. Thus in a ballill ft. multimeter and 9 ft.

long, containing 5,000 pounds of. balls, air -atmosphere. sealed .in; pounds oi-f'magnesium filings" and 25 gallonsof mineralspiritslmilled six hours. at47 Rf-P. M. gave' 95%- yieldfoi grade magnesium enumerated above, the preferred at-- mosphere-for ball milling is air, where-fine and v efllcient grinding is desired and a low metallic content (say about 85%) can be'tolerated, an atmosphere of carbon dioxide will give the best results. V

atmospheres there is danger of. explosiomjvhich consists in subjecting the massivmetal to ;the action of a disintegrator effective to produce .,and

remove from the mass a succession of particles of size within a given range, protecting the pa ticles, as they are produced, by a volatilefli'quid' hydrocarbon coolant,.removing coolant andfillf ,ing the still wet. material into a dustproof ba g that is pervious tothe coolant and to; the 'fu'm'es thereof, sealing th bag, andaccomplishing the further removal of coolant.

2. .The method of producing comminuted; metal of such chemical characteristics and particle size that-- when dusting thereof 0ccui s;in certain atmospheres there is danger of explosion, which consists in subjecting the massive. metal to the action of a disintegrator effective toproduce and remove from the mass a succession of particles of size within a given range, protecting the particles, as they are produced, by a volatile liquid a hydrocarbon coolant, removing coolant and filling the still wet material into a dustproof bag that is pervious to the coolant and to the fumes '7 thereof, sealing the bag, and accomplishing the in addition" to furnishing the further removal of coolant and thereafter drying the bag and its contents by means of moving heated air.

3. Th method of producing comminuted metal of such chemical characteristics and particle size that when dusting thereof occurs in certain at msopheres there is danger of explosion, which consists in subjecting the massive metalto the action of a disintegrator effective to produce and remove from the'mass a succession of particles of size within a given range, protecting the particles, as they are produced, by a volatile liquid hydrocarbon coolant, removing coolant, adding a solution of oil in coolant liquid, mixing and filling the still wet material into a dust-proof container that is pervious to the coolant and to the fumes thereof, and thereafter substantially completely removing the coolant and leaving the metal particles coated with the oil.

' 4. A method of producing comminuted magnesium and controlling the explosive tendency of said metal which comprises in stated order the steps or subjecting massive metal to the action of an abrader to produce and remove from the mass a succession of particles, mixing said particles as they are produced with a volatile liquid hydrocarbon, transporting said particles in said hydrocarbon into and through at least one turther disintegrating step, draining a portion of the hydrocarbon from'the disintegrated particles, charging the remaining hydrocarbon-particle mass into bags pervious to the hydrocarbon andiumes thereof, and thereafter drying the bag and its contents by means of moving heated air. I

5. A method of producing comminuted magnesium and controlling the explosivetendency of said metal which comprises in stated order the steps of subjecting massive metal to the action of an abrader to produce and remove .from the mass a succession of particles, mixing said particles as they are produced with a volatile liquid hydrocarbon,- transporting said particles in said hydrocarbon into and through at least one further disintegrating step, transporting said particles in said hydrocarbon on and through classifying screens, draining a portion of the hydrocarbon from the disintegrated particles, charging the remaining hydrocarbon-particle mass into bags pervio'us tothe hydrocarbon and fumes thereof. and thereafter drying the bag and its contents by means of moving heated air;

HAROLD E. HALL. v ALEXANDER F. KNOLL.