CA1059967A - Fine flaked aluminum manufacture - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

The invention relates to an improved method and apparatus for comminution or milling of aluminum is granulated or atomized form into fine flaked particles and to the particles themselves. The milling is accomplished in a continuous, dry, gas-swept process wherein the aluminum is milled in a vibratory ball mill.

Description

~059967 The present invention relates to an improved method and apparatus for the safe comminution or milling of aluminum in granu-lated or atomized form into fine flaked particles and to the parti-cles themselves. More particularly, the present invention relates to a continuous, dry, gas-swept process wherein the aluminum is milled in a vibratory ball mill. -Preferably, th~ method and apparatus com-prise a closed system wherein the granulated aluminum is continuously supplied in fresh amounts to the vibratory mill, milled and the fin-ished, fine flaked particles are continuously carried off or gas-s~ept by an inert gas which is passed through the mill. The swept particles are separated from the inert gas and collected as finished product.
The cleansed inert gas preferably is then recycled through the mill.
This method and apparatus is particularly suited to the production of fine flaked aluminum particles which are useful as highly effective I sensitizers for explosive compositions.
With regard to the present invention, ''fine flaked particles"
are defined as those particles having a size of less than about 20 Tyler mesh and greater than about 1 micron and having a diameter:thickeness ratio of greater than about 10.
BACKGROUND OF THE INVENTIO~I
Fine flaked aluminum powder or particles have a variety of uses such as for paint pigments (particularly that having good color and brilljancy) and in chemical, explosive and pyrotechnic applications. In the production of fine flaked aluminum particles from an aluminum feed-stock generally in granule or atomized form, the cost of the size reduction of the particles, i.e., the cost of the actual milling operation, generally represents a major fraction of the total cost of processing the particles. Thus an improved method of carrying out the milling operation is of considerable interest. The present invention relates to an improved method and apparatus for carrying out the milling operation which method and apparatus is significantly more efficient and economical

-2-lOS9967 in cost than any method or apparatus heretofore employed.
Mechanical methods of making aluminum powder or fine flaked particles were first developed in the nineteenth century. The first methods involved the use of stamp mills for beating aluminum ;nto tiny, flake-like particles and were generally employed unti1 the early 1930's.
The stamp mill consisted~generally of a series of steel hammers rai~ed by cam action and allowed to fall on a suitably enclosed steel base. In order to prevent welding together of the aluminum pieces, a small amount of lubrfcant was used to coat the pieces during the stamping operation.
Various lubricants were used such as tallow, stearic acid, olive oil, rape oil, etc.
In the 1930's, a more efficient process was developed. This process is commonly referred to as the Hametag process and employs a cylindrical rotary ball mill in which the balls, continually carried up the side of the mill and dropped on the rest of the charge, provide the stampin~ or hammering effect necessary for the production of aluminum flakes. Aluminum in granulated form is continuously fed into one end of the ball mill, and a lubricant is added to the charge to keep the flakes from welding together and to impart a desired coating to the finished particles, A current of inert gas (gas sufficiently low in oxygen to eliminate the possibility of explosion in the aluminum dust cloud in the mill and various connected equipment) circulates through the mill and carries the flakes into a separator or series of separators wherein the finished, fine flakes are removed from the gas which is then recirculated all in a closed system. Provision is usually made for separating the carried off finished flakes from unfinished flakes of generally greater size and mass which may have also been carried off by the gas stream.
The separated, unfinished flakes are then returned to the mill for further milling This ~lanletag or rotary ball mill process provided for greatly increased production rates over the earlier stamp mill processes and is described more fully in U.S. Patent No. 1,832,868. ~dditional U.S.
Pat~nts disclosing improvements and modifications of this process are Numbers 1,930,684, 1,932,741, 2,112,497, and 2,136,445. The ~lametag process is carried out ''dry" (meaning that the aluminum particles are not carried as a slurry in a liquid medium but rather are carried throughout the process in separate, particulate or "dry" form). The finished, fine flakes of aluminum can then, if desired, be transferred to a polisher for imparting to the flakes improved color and brilliancy.
Another milling or communition process is commonly referred to as the Hall process. This process is a "wet" process and employs a charge of finely divided aluminum as a starting material which, together with the steel balls, is mixed with sufficient mineral spirits to produce a sludge of creamy consistency. Because there is no dust within, the mill can be operated without the introduction of a special gaseous atmosphere. After the aluminum has been properly ~laked the volatile liquid can be removed by filtration and evaporation. In order to obtain dry, flaked powder, this wet process requires additional steps, i.e., filtration and evapora-tion. The wet process is particularly useful, however, where the finished product can be used directly without drying and in the form of aluminum paste. For example, this paste can be used directly in the manufacture of paints containing fine flaked "paint grade'' aluminum.
Where a dry, particulate finished product is desired as in the present invention, it is generally more economical in cost and more conven-ient to employ a dry process rather than a wet process since the latter requires additional steps. The present invention involves a dry process.
In essence, the Method and apparatus of the present invention resemble the Hametag process except that the rotary ball mill is replaced by a vibratory ball mill. Thus the actual nlode of milling is quite dif-ferent. The use of a vibratory ball mill can greatly reduce specific energy consumption per unit of production and greatly increase production rate per unit of grinding chamber volume. The increased efficiency of the vibratory ball mill results from its ability to develop accelerations of the grinding media having magnitudes greater than the gravitational field of the earth which field provides for acceleration of the grinding media in a rotary ball mill. Although vibratory mills can be designed to any capability, most commercially used mills develop accelerations of about

3 to about 15 g's (9 is the value of acceleration of a body due to gravi-tation). For use in the present invention, vibratory mills should be capable of producing accelërations of greater than 1 9, preferably of .
approximately at ieast 5 g's, and most preferably at least 10 g's. Vibra-tory mills are more efficient when running at or near their resonance frequency in which case comparable production rates can be obtained at lower energy consumptions than if the mills are not so operated.
The vibr~tory mill consists, in general, of a mill body contain-ing a grinding media such as steel balls. The mill body is supported upon springs or similar resilient rnembers which allow an oscillatory movement.
The mill is furnished with some means of maintain~ng a forced vibration, this usually being a mechanical means consisting of rotating out-of-balance weights. Yibration of the mill by electro-mechanical means, such as a .
i solenoid energized by alternating current, is clearly possible. Vibration Mills and Vibration Milling by H. E. Rose and R. M. E. Sullivan, Constable and Company, Ltd., London, 1961, describes different types of vibratory mills. Yibratory ball mills have been commercially available for more than 15 years but to our knowledge have never been used to mill or comminute t aluminum in granule or atomized form into fine flaked particles in a con-tinuous, dry, gas-swept process.
Aluminum, Paint and Powder by ~uneius David Edwards and Robert I. Wray, Reynolds Pulbishing Company, New York, 1955, discloses on page 6 that the production of fine1y flaked aluminum powder in ball mills must be operated in such a way that abrasion and grinding are reduced to a minimum; and the aluminum particles are beaten and burnished into bright, tiny metallic flakes. A vibratory mill probably has greatly increased abrasion and grinding vis-a-vis a rotary ball mill due to increased ac-celeration forces, and therefore would not be expected to produce bright flakes. It has been surprisingly found in the present invention, however, that a vibratory mill can be successfully employed in the production of such particles and at a rate per unit of grinding chamber volume higher and a specific energy consumption per unit of production lower than the respective rate and consumption experienced with a rotary ball mill. Al-tho~gh it is possible that certain rotary-milled pigment grades may have better color and brightness than those vibratory-milled, vibratory-milled particles generally can obtain comparable color and brightness to even these particles through use of a conventional polisher such as described ~n U.S. Patent No. 2,112,497. In fact, such polishing is generally employed even when rotary mills are employed. Thus the dry processing in a vibra-tory ball mill of fine flaked aluminum powder having good color and bril-liancy has been found to be significantly more economical in cost than the manufacturing of such particles in a rotary ball mill operation.
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The present invention employs the vibratory mill in a dry, gas-swept process similar, as mentioned, to the Hametag process. The effective use of gas-sweeping with a vibratory mill is surprising in view of the disclosure on pages 40 and 41 of the above Rose and Sullivan reference wherein it is stated that the "air-sweeping of a vibration mill, a common arrangement with conventional 'tumbling' mills, is practically unknown It is probable that, owing to the high charge ~illing associated with a vibration mill for efficient working, such a system is impracti-cable." This reference indicates a "very high filling" to be 80% (the grinding media fill 80% of the grinding chamber volume). It has been contrarily found in the present invention that a vibratory mill having even such a "~ery high filling" can be effectively gas-swept and is thus efficient and practicable.
~ Thus in view of the prior art the present invention provides a surprising and unexpected means of efficiently producing the fine flaked aluminum particles.
f Fine flaked aluminum is used extensively to fuel and/or sensitize- explosive compositions. The present invention is particularly advantage-ous for the production of fine flaked particles for such use. Fine flaked aluminum will react exothermally with oxygen-containing ingredients to increase and enhance an explosive's sensitivity to detonation and its .
; ~ energy production. Furthermore, when employed in a slurry explosive composition having a continuous aqueous fluid phase, such particles con-( taining a coating which renders them repellant to the fluid phase hàve been found to be extremely effective as sensitizers, i.e., they greatly i . . . . .
facilitate ease of detonation of the composition. For example, U.S. Patent No. 3,249,474 discloses that aluminum particles used in slurry explosive .
compositions and having a coating which renders them hydrophobic and re-pellant to the fluid phase of such compositions are much more effectfve than those not having such a coating. This patent discloses the now well-known fact that if the surfaces of the aluminum particles are not effective-ly coated they become wetted by the continuous liquid menstruum of the composition and lose their sensitizing effectiveness. It is therefore important to use appropriately coated aluminum particles in slurry explosive compositjons. The term "coated" surface or particles will hereafter refer to par~icles having a "coating" which renders them hydrophabic and repellant to the liquid phase or menstruum of the composition which usually comprises an aqueous phase containing dissolved inorganic oxidizer salts and other electrolytes and such liquids as water-miscible organic liquids such as ~lcohols, glycols. amides and analogous nigrogen-contain-i!lg liquids; and ~later-immiscible organic liquids such as petroleum . .
dist;llates and d;esel fuels.
U.S. Patent No. 3,367,805 discloses that extremely hi~h sensiti-zation of slurry explosive compositions can be obtained by employing only relitively small amounts (generally substantially less than 5%) of very finely divided aluminum particles haYing a coated surface and, in add;tion, a relatively high surface area. This patent discloses that small amounts of coated aluminum particles having a surface area of at least 0.5 m2/gm provide high sensitization. Commercially available paint grade aluminum powders were found to have such characteristics and thus are now commonly employed in slurry explosive compositions for their sensitizing ability.
Aluminum particles for use in slurry explosive colnpositions primarily as sensitizers and which particles will hereafter be referred to as ''explosive sensitizer grade" should be fine, have high surface area and be appropriately coated. Thus color and brilliancy~ generally essen-tial ~or paint grade, are not in and of themselves of primary concern for explosive sensitizer grade. In explosive compositions requiring high sensitization, the amount of paint grade aluminum employed is limited .primarily by its relative cost as compared to other ingredients or sen-sitizers or coarser aluminum having a lower surface area. It is there-fore highly beneficial to produce at least cost,aluminum particles having properties equivalent to or better than paint grade for use in slurry explosive compositions. The present invention relates to a method and apparatus for producing such particles having the necessary character-istics at a cost significantly lower than that for the production of paint grade particles.
With regard to the present invention, explosive sensitizer grade particles are defined as those coated particles having a surface area of 0.5 m2/gm or greater and preferably 1.0 m2/gm and greater, and, addition-ally, preferably having a particle size such that most or all of the par-105~967 ticles will pass through a 200 Tyler mesh screen and pr~ferably a majority of those will pass through a 325 Tyler mesn screen.
SUMMARY OF THE INVENTION
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The present invention relates to a method and apparatus forproducing fine flaked aluminum particles having outstanding character-istics and to the particles produced by such method and apparatus. The method and apparatus produce fine flaked particles at significantly higher rates per unit of grinding chamber volume and at lower speci-fic energy consumptions pér unit of production and thus at a more economical cost than heretofore possible. Specifically, the invention relates to an improved method for the production of fine flaked aluminum particles in a continuous, dry, gas-swept process, in which the aluminum is milled in a vibratory ball mill. The method comprises the steps of milling the particles in a vibratory ball mill, continually supplying fresh amounts of unmilled particles to the vibratory ball mill, passing an inert gas through the mill with a flow rate sufficient to carry the milled or finished particles from the mill, and separating the finished particles from the inert gas. Preferably, the inert gas is recycled through the vibratory ball mill in a closed system. The apparatus com-prises a vibratory ball mill for milling the particles and appropriate means for performing the other above-mentioned steps of the method. A
preferred use of the invention is for the production of fine flaked particles of explosive sensitizer grade. The particles produced by the method and apparatus of the present invention microscopically differ in some respects from those produced in a rotary ball mill.
The single FIGURE illustrates a production flow scheme in accor-dance with the method and apparatus of this invention.
DESCRIPTION OF A PREFERRED EMBODIMENT
In order to describe more fully the present invention, reference is made to the accompanying figure which discloses a vibratory ball mill 1 connected to apparatus which continuously ~eeds a fresh supply of aluminum partjcles (in granule or atomized form) to and continually withdraws fin-g ished aluminum particles from the mill. The finished particles are carriedfrom the mill by passing inert gas through the mill and are then separ-ated from the gas. More specificalty, the apparatus consists of the vibratory ball mill 1, a classifier 2, a collector 3, a blower 4, an aluminum particle feeder 6, a coating feeder 7, a conduit 5 leading from the mill 1 to the classifier 2, a conduit 8 leading from the classifier 2 to the collector 3, a conduit 9 leading from the collector 3 to and from the blower 4 and back to the mill 1, a receiver 10 for the finished particles, a conduit 11 leading from the classifier 2 to conduit 9 near the tnput to the mill 1, an oxygen gas monitor 12, an oxy~en gas source 13, and a non-oxygen or inert gas source 14.
In operation, a fresh supply of aluminum particles are continu-ously fed to the mill 1 by the feeder 6. The feeder 6 can be any conven-tional feeder such as a pneumatic means, auger or vibrator. The vibratory mill 1 is powered by a conventional power source (not shown) which is preferably connected to the mill through an appropriate motor-driven, bearing-supported eccentric shaft or shafts which impart the oscillatory motion to the mill body. The mill body is preferably affixed to a sta-tionary base by sprin~ mountings. Any compatible combination of vibra-tory mill, driving means and support means can be used. The mill body contains grinding media, preferably forged steel balls although media of other material can be used, filled to the desired volume fraction of the body or grinding chamber. During operation of the mill, the grinding media grind or mill the aluminum particles as the media collide with each other and with the internal surfaces of the grinding chamber thus producing an inteilsified impact/shear/attrition environment action. An inert gas is continually swept or passed through the ball mill by the blower 4 and carries the finished particles from the mill. ~he inert gas enters the mill through conduit 9 and leaves with the entrained particles through condujt 5. The flow rate of the inert gas is preferably sufficient to carry ofF some particles which have greater mass than those desired as ~059967 final product. The gas/particle output from the mill then goes to the classifier 2 which separates the finished from the unfinished particles of greater mass and returns the unfinished particles to the vibratory mill through conduit 11 for continued milling. The classifier 2 pre-ferably operates on a mass basis (although it can operate on a size basis) wherein the finished, fine flaked aluminum particles of lesser mass are allowed to flow out of the classifier through conduit 8 and the unfinished particles of greater mass are removed from the inert gas stream. The gas/particle stream exiting the classifier 2 through conduit 8 then enters collector 3 wherein all of the particles are removed from the gas. The inert gas then continues by suction force to blower 4 wherein it is pres-suri~ed and recirculated to the vibratory ball mill 1 through conduit 9.
The inert gas thus circulates in a closed system.
A coating/lubricant for the fine flaked aluminum particles is most always desired and is introduced to the vibratory ball mill by any appropriate conventional feeder means 7.
Although all of the above steps are simultaneously occurring during the continuous operation of the production process, the individual steps are performed upon a given particle essentially in the sequence described above.
The present invention can be better understood by reference to the fol10wing examples and further elaborating disclosure.
Preferred Operating Parameters and Example First presented is a description of general operating and design parameters whlch are applicable to a vibratory mill process and apparatus in accordance with the present invention of essentially any desired magnitude and design. This description is followed by an example disclosing a preferred mill process and apparatus to which the general operating and design parameters are applied. The example also discloses ~059967 additional operating and design parameters of s;milarly general appli-~cability but which are better illustrated by reference to a vibratory mill process and apparatus of a given magnitude and design.
The output of the mill, conduit 5, can be positioned either in an up or down mode., In the up mode disclosed in the figure, the particles carried or swept from the mill to the classifier by the inert gas stream have generally less mass than those that are carried if conduit 5 is in the down mode wherein gravity in addition to gas-sweeping aids the carriage of milled particles from the v;bratory mill.
Positioning of discharge conduit 5 is a matter of preference and is not critical to the invention.
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The inert gas should have an oxygen content of about .5 to about 7% by volume and'preferably about 1 to about 3% by volume!
Control of the oxygen level is important bot,h to prevent unwanted combustion or explosion of the aluminum particles and to control the degree of surface oxidation of the aluminum particles. It is desirable that the particles obtain a protective aluminum oxide coating but in a much more controlled environment than would be present if a gas of higher oxygen content such as air were used as the sweeping medium.
The oxide coating prevents the particles from being pyrophoric. If the coating becomes too thick, howevers then with regard to explosive sensitizer grade particles, the available aluminum for reaction and energy production is correspond;ngly reduce~. A thick coating also generally reduces brilliancy. It is generally preferable that 90% by weight of the particles comprise aluminum and thus that less than 10%
comprise aluminum oxide and hydrophobic (described below) coatings.
As mentioned previously, it is essential that fine flaked aluminum particles of explosive sensitizer grade have a coating to prevent them from being wetted by the liquid menstruum throughout which they are dispersed. It is also desirable,to provide a lubricant for the 105g967 particles during milling to prevent them from ~Jelding or agglomerating together. Conveniently, coating and lubrication can usually be provided by the same material. Although stearic acid is the preferred coating and lubricant, other mater;als can be used such as normally solid fatty acids in addition to stearici their der;vatives, for example, calcium stearate; high melting point waxes; asphalting materials; silicone materials and combinations of the above. Th~
particular coating employèd largely depends upon its compatibility with the particular liquid menstruum employed. The desired amount of coating on a particle varies according to the particular coating used but is generally less than 4 or 5% by weight although it may be as high as 10% or more. For a stearic acid coating, the amount is preferably 2~3% by weight.
For successful application of the coating or lubricant to the surfaces of the aluminum particles, the temperature within the mill (preferably determined by the temperature of the ex;t inert gas) should be maintained at a degree or range necessary to provide for an effective deposition of the coating on the surface of the particle.
The desired temperature depends upon the particular coating used and can be determined experimentally. For stearic acid, the temperature is preferably maintained at approx;mately 60-80C.
Another reason for controlling the temperature within the chamber is that an overly h;gh temperature may cause excessive oxidation of the particles as determined by the end use desired.
Thus temperature control is important in properly employing the present invention.
As an example of the present invention, a preferred vibratory ball mill is the Allis-Chalmers VBM 3034 mill which is driven by two 50 hp, l200 rpm motors. This mill has a grinding chamber or mill body ~59g67 volume of 12.3 ft3. The mill is charged with ball-shaped grinding media of approx;mately 3/16 to 1 in. mean diameter. The mill filling is preferably approximately 60 to 80% of the total chamber volume. The grinding media may be a distribution of sizes as desired for enhanced grinding effectiveness and such distribution can be experimentally determined to provide finished, flaked particles having desired physical characteristics. The media size "ill generally desirably vary with the size of the mill as illustrated in the examples below.
The feed rate of aluminum particles to this mill can range from as low as 50 lbs/hr to upwards of possibly one ton/hr and is preferably from about 80 lbs/hr to about 300 lbs/hr.
The inert gas should have a flow rate of from about 300 ft3/min.
to about 3000 ft3/min., and the rate is somewhat dependent upon the mill's s;upporting equipment. The inert gas flow rate can be varied according to the feed rate of the aluminum particles and the desired nature of the finished, fine flaked particles. The blower 4 must be capable of accomodating the desired inert gas flow rate.
The classifier 2 used with this mill is a centrifugal classifier.
Any conventional classifier such as an air separator, screen, screw or cyclone, can be used. The inert gas flow rate and classifier can be coordinatingly operated to recycle to the vibratory mill a desired amount of particles of desired mass which are carried from the mill by the gas stream.
The above-described apparatus was operated in the following manner:*
Aluminum Part;cle atomized, 35% +100, 65X -100 Feedstock and Rate Tyler mesh; approx. 81 lbs/hr Inert Gas Flow Rate 450 ft /min.
Inert Gas Oxygen Level 1.5%
Coating Material and stearic acid; approx. 3% of feed Rate 81 lbs/hr Inert Gas Exit Temperature 5~C, 62DC
Average Residence approx. 3/4 hr (includes Time of Part;cle recycle time) Mill Filling (Balls) approx. 75%
Ball Type and Size steel, 3/4" - 500 lbs., 1/2" - 1000 lbs., 3/16" -1200 lbs.
*Particles produced from two separate runs were tested as shown in Example 1 below. The parameters given here apply to both runs unless two numbers are given, in which case the first number applies to Mix 2 and the second to Mix 3 of the example.
Fine flaked particles of explosivq sensitizer grade were produced and then used to formulate two identical slurry explosive compositions. A third identical compos;tion was formulated but with an equivalent amount of commercially available paint grade aluminum replacing the milled aluminum. The compositions ~ere then detonated and their respective sensitivities compared. The composition formulations and results of the detonations are as follo~s:

Ingredients (parts by weight):
Mix 1 Mix 2 Mix 3 Solution containing:
Ammonium Nitrate 39.3 3g.3 39 3 Fertilizer Grade Calcium Nitrate 32.1 32.1 32.1 H 0 12.0 12.0 12.0 Tniourea .2 .2 .2 Guar gum derivative .9 .9 .9 Ethylene glycol 5.0 5.0 5.0 Starch 6.5 6.5 6.5 Paint grade aluminum (Alcoa 2003)3. 5 Vibratory milled aluminum --- 3.5 __-Vibratory milled aluminum --- --~ 3.5 Sodium dichromate/H20 cross-linking agent .3 .3 .3 Sodium nitrite/H20 gassing agent- .2 .2 .2 Detonat;on results at 20DC (Air Gap Test) e (gm/cc) 1.01 0.97 1.01 Detonated (1 1/2" Dia, #8 cap) 6" 6" 6"
Failed The above air gap test results indicate the relative sensitivi-ties of the compositions. The air gap test was conducted by axially :~059967 diameter positioning in a line two cylîndrical charges of 1 1/2" in ~; and having the composition indicated but spaced apart from each other at the distance indicated. One of the charges was then detonated by a blast;ng cap. Detonation of the other charge either occurred or failed as indicated depending upon the charge's sensitivity to detonation by the shock wave produced from the first detonated charge. The greater the distance or air gap over which a detonation occurred directly correlates with the greater the sensitivity of the charge. Thus, it is readily observed from the above tests that the aluminum manufactured by the above-described process was at least equivalent to paint grade aluminum in sensitizing explosive compositions.
The surface area of the milled particles was approximately l.S m2/gm as comparable to about 1.4 m2/gm for the paint grade particles.
The particle size of the milled particles was 100%-150 Tyler mesh and ~6%-325 mesh, as comparable to the paint grade aluminum particle size of 100% -150 and 91% -325.
Thus it can readily be seen that the above apparatus and method produce explosive sensitizer grade aluminum of at least comparable effectiveness and physical character;stics to commercially available paint grade aluminum. Furthermore, the production rate of the particles, 1 lbs/hr, is higher and the specific energy consumption of the mill a measured (excluding other apparatus), .6 kw-hr/lb of product (based on~2/3 of rated current load to mill motors), is lower than the respective figures for paint grade aluminum produced in a rotary ball mill of comparable chamber volume since the rotary ball mill is less efficient as explained above. Thus milling costs are significantly reduced.
A similar but smaller and much more simplified apparatus than that described in Example 1 was employed in the production of fine flaked aluminum particles. This apparatus consisted of an All k - !
Chalmers vibratory VBM 1518 mill driven by two 7-l/2 hp 1200 rpm !
motors. This mlll has a grind1ng chamber volume of 1.6 ft . The apparatus employed a cyclone classifier, and a filter was used to collect the classified ~articles carried from the vibratory mill by the inert gas. Finished particles were continuously withdrawn from the mill by the inert gas but the aluminum feedstock was fed on a step or semi-continuous basis. With regard to the present invention, the term "continuous" is not strictly applied and the feed or even the withdrawal may be accomplished in a semi-continuous manner as the feed [ was in this example. The collected particles were then sifted to obtain particles having the approximate size distribution of the commercially available paint grade aluminum used in Examp1e l. The operating parameters were as follows:
Aluminum Particle atomized, Z6.5 lbs/hr Feedstock and Rate ~ ~ ~ Inert Gas Oxygen Level 1.5-2.5%
f ~ ~ Coating Material and - stearic acid, 0,6 lbs/hr Feed Rate Inert Gas Exit Temperature 50-60C
Average Residence approx. l hr Time of Particle Mill Filling 80X
Ball Type and Size steel, 3/16"
Slurry explosive compositions were prepared and detonation - ' .
tests identical to those for the first example were conducted. The composit1ons and results are as follows:

. - ~

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Ingredients (parts by weight):
Mix 1 Mix 2 Solution containing: .
Ammonium Nitrate 39.3 39.3 Fertilizer Grade Calcium Nitrate 32.1 32.1 H70 12.0 12.0 Thiourea 0.2 0.2 Guar gum derivative 0.9 0.9 Ethylene glycol S.0 5.0 Starch ~6;5 6.5 Paint grade aluminum (U.S. 3.
Bronze~L-684) ~ibratory milled aluminum --- 3.5 Sodium dichromate/H 0 cross- 0.3 0.3 linking agent 2 Sodium nitrite/H20 gassing agent0.~ 0.2 Detonation results at 5C (Air Gap Test) ~ (gm/cc) 1.08 1.04 Detonated 6" 6"
(2" Dia, #8 cap) Failed 7" 7"
The above detonation results for both examples illustrate that the milled aluminum was at least as effective a sensitizer as the commercially available paint grade aluminum.
The f;ne flaked alum;num particles produced by the method and apparatus of the above examples were found to be comparable not only in particle size and surface area but also in color and brightness to the commercially available paint grade aluminum to which they were compared.
The milled particles produced by the method and apparatus of the above examples wPre found to have a stearic acid coatiny equivalent to that for the commercially available paint grade aluminum as determined by the degree of wetting observed when the particles were shaken for three minutes ;n an aqueous solution similar to that employed in the examples. The unwetted particles floated on top of the liquid whereas the wetted particles did not. The milled particles were observed to have less than 1% wett;ng which was the same degree of wetting observed with the commercially available paint grade aluminum.

When examined under a Cambridge Mark 2a Stereoscan scanning electron microscope at a magnification power of 500, the milled particles were found to have some apparent microscopic differences in appearance from examined, comparable paint grade partic1es milled in either wet or dry rotary ball mills. Such differences are not surprising, however~ since the acceleration in the vibratory mill is 15 times that of a rotary mill, and the residence time of particles milled in a vibratory mill is much shorter. The milled particles were observed to be generally discrete and separate whereas the rotary milled paint grade particles generally tended to be composed of pieces or flakes of various sizes welded or agglomerated together.
This difference may be important since particles composed of welded pieces or flakes appear to have no advantages and may have disadvantages such as if separation occurs thereby exposing poorly coated surfaces.
Another observed though less distinct microscopic difference is that the vibratory milled particles appear to have more jagged or sharply defined edges than those rotary milled, and, at least in comparison to some paint grade particles, appear to have a better stearic acid coating along these edges. Thus the milled particles of the present invention are at least comparable macroscopically to commercially available paint grade, and, in addition, they appear to have some microscopic differences which may make them superior at least for explosive sensitization purposes.
The above two examples illustrate that the size of the apparatus employed in the present invention can be varied significantly and thus is ~ot critical. The first example employed a grind;ng chamber having a volume of approximately nine times that employed in the second.
The important concept of the present invention is the use of a vibratory mill which can impart greatly increased abrasion and grinding action to the alum;num particles over that available from a rotary ball mill. The size of the mill can be varied as desired. The essential requirements in addition to the use of a vibratory mill are only that 1059~67 the mill be operated in a dry, gas swept process. The operating parameters can be reasonably adjusted by those skilled in the art to achieve varied and desired results. The apparatus employed for temperature control and for classifying, separating, feeding, circulating the inert gas, and other operations associated with the preferred closed system of operation, is conventional and can be easily varied or modified by those sk;lled in the art as desired.
While the present invention has been described with reference to certain illustrative examples and preferred embodiments, various modifications are intended to be within the scope of the invention as set forth in the appended claims.
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Claims (9)

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

1. In a method for producing fine flaked aluminum particles comprising:
(a) milling granulated or atomized aluminum particles in a ball mill, (b) continually supplying fresh amounts of unmilled aluminum particles to the mill, (c) continually withdrawing milled fine flaked aluminum particles from the mill, (d) passing an inert gas through the mill with a flow rate sufficient to carry the milled particles from the mill, and (e) separating the milled particles from the inert gas, the improvement which comprises milling the granu-lated or atomized aluminum particles in a vibratory ball mill.

2. A method according to claim 1 wherein the milling is accomplished in a vibratory ball mill capable of producing accelerations of the grinding media within the mill of greater than 1 g.

3. A method according to claim 2 wherein the milling is accomplished in a vibratory ball mill capable of producing accelerations of the grinding media within the mill of at least 5 g's.

4. A method according to claim 1 which comprises con-tinually supplying to the vibratory mill fresh amounts of a desired material for coating the milled particles and main-taining the temperature of the inert gas within the mill at a degree or range such that the coating material may effec-tively coat the particles.

5. A method according to claim 4 wherein the fine flaked particles are explosive sensitizer grade.

6. A method according to claim 1 comprising recycling the inert gas back through the vibratory mill in a closed system.

7. A method according to claim 1 comprising separating finished particles from unfinished particles which may be carried from the vibratory mill along with the finished particles and returning the separated, unfinished particles to the vibratory mill for further milling.

8. A method according to claim 7 wherein the step of separating the unfinished particles from the milled particles is accomplished by a centrifugal classifier.

9. A method according to claim 1 wherein the step of separating the milled particles from the inert gas is accomplished by a filter.