US3927836A - Fine flaked aluminum manufacture - Google Patents

Abstract

The invention relates to an improved method and apparatus for comminution or milling of aluminum in 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

United States Patent Thurgood et al.

[ Dec. 23, 1975 FINE FLAKED ALUMINUM 1930,684 10/1933 Kramer 241/52 x MANUFACTURE 2.778577 1/1957 Linke 241/175 x 2.8l9,849 1/1958 Becker r 24l/l75 Inventors: J- Rand Thurgood, e Jordan; 3.084.876 4/l963 Podmore 241/59 x Robert B. Clay, Bount1ful, both of 3,709,439 1/1973 Tundermann et a]. 24l/30 X Utah 3,744,726 7/l973 Groszek............r................... 24l/3O [73] Assignee: lreco Chemicals, Salt Lake City,

U h Primary ExaminerGranv1lle Y. Custer, Jr.

At! A t, F R bert A. Bin ham 122 Filed: July 23, 1974 0 g [21] Appl. No.: 491,028 [57] ABSTRACT [52] us. CL 241/19, 241/24, 241/30 The invention relates to an improved method and ap- [51] Int Cl 2 B02C 17/04 paratus for comminution or milling of aluminum in [58] Fieid s e a l6 I9 24 granulated or atomized form into fine flaked particles 241/25 30 59 ,5 ,5 l 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 [56] References Cited UNITED STATES PATENTS 9 Claims, 1 Drawing Figure 1,92Q,234 8/1933 Arthur 24l/25 X US. Patent Dec. 23, 1975 3,927,836

FINE FLAKED ALUMINUM MANUFACTURE The present invention relates to an improved method and apparatus for the safe comminution or milling of aluminum in granulated or atomized form into fine flaked particles and to the particles 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, the method and apparatus comprise a closed system wherein the granulated aluminum is continuously supplied in fresh amounts to the vibratory mill, milled and the finished, fine flaked particles are continuously carried off or gas-swept 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 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 diameterzthickness ratio of greater than about 10.

BACKGROUND OF THE INVENTION Fine flaked aluminum powder or particles have a variety of uses such as for paint pigments (particularly that having good color and brilliancy) and in chemical, explosive and pyrotechnic applications. In the production of fine flaked aluminum particles from an aluminum feedstock 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 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 into tiny, flake-like particles and were generally employed until the early 1930s. The stamp mill consisted generally of a series of steel hammers raised 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 lubricant 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 stamping 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 Hametag 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. Pat. No. 1,832,868. Additional U.S. Pat. Nos. disclosing improvements and modifications of this process are 1,930,684, 1,932,741, 2,112,497, and 2,136,445. The Hametag 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 th 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 flaked 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 evaporation. 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 convenient 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 mode of milling is quite different. 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 gs (g is the value of acceleration of a body due to gravitation). For use in the present invention, vibratory mills should be capable of producing accelerations of greater than 1 g, preferably of approximately at least 5 g's, and most preferably at least 10 gs. Vibratory mills are more efficient when running at or near their resonance frequency in which 3 case comparable production rates can be obtained at lower energy consumptions that if the mills are not so operated.

The vibratory mill consists, in general, of a mill body containing a grinding media such as steel balls. The mill body is supported upon springs or similar resilient members which allow an oscillatory movement. The mill is furnished with some means of maintaining a forced vibration, this usually being a mechanical means consisting of rotating out-of-balance weights. Vibration of the mill by electro-mechanical means, such as a 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. Vibratory ball mills have been commercially available for more than years but to our knowledge have never been used to mill or comminute aluminum in granule or atomized form into fine flaked particles in a continuous, dry, gas-swept process.

Aluminum, Paint and Powder by Juneius David Edwards and Robert I. Wray, Reynolds Publishing Company, New York, 1955, discloses on page 6 that the production of finely 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 rotary ball mill due to increased acceleration 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. Although it is possible that certain rotary-milled pigment grades may have better color and brightness than those vibratory-milled, vibratorymilled particles generally can obtain comparable color and brightness to even these particles through use of a conventional polisher such as described in U.S. Pat. No. 2,1 12,497. In fact, such polishing is generally employed even when rotary mills are employed. Thus the dry processing in a vibratory ball mill of fine flaked aluminum powder having good color and brilliancy has been found to be significantly more economical in cost than the manufacturing of such particles in a rotary ball mill operation.

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 filling associated with a vibration mill for efficient working, such a system is impracticable." 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 very 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.

Fine flaked aluminum is used extensively to fuel and [or sensitize explosive compositions. The present invention is particularly advantageous 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 explosives sensitivity to detonation and its energy production. Furthermore, when employed in a slurry explosive composition having a continuous aqueous fluid phase, such particles containing a coating which renders them repellant to the fluid phase have been found to be extremely effective as sensitizers, i.e., they greatly facilitate ease of detonation of the composition. For example, U.S. Pat. No. 3,249,474 discloses that aluminum particles used in slurry explosive compositions and having a coating which renders them hydrophobic and repellant to the fluid phase of such compositions are much more effective than those not having such a coating. The patent discloses the now well-known fact that if the surfaces of the aluminum particles are not effectively 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 compositions. The term coated" surface or particles will hereafter refer to particles having a coating which renders them hydrophobic and repellant to the liquid phase or menstruum of he 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 alcohols, glycols, amides and analogous nigrogencontaining liquids; and water-immiscible organic liquids such as petroleum distillates and diesel fuels.

U.S. Pat. No. 3,367,805 discloses that extremely high sensitization of slurry explosive compositions can be obtained by employing only relatively small amounts (generally substantially less than 5%) of very finely divided aluminum particles having a coated surface and, in addition, a relatively high surface area. This patent discloses that small amounts of coated aluminum particles having a surface area of at least 0.5 mlgm 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 compositions 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 essential for 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 sensitizers or coarser aluminum having a lower surface area. it is therefore 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 characteristics 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 m /gm or greater and preferably l.0 m lgm and greater, and, additionally, preferably having a particle size such that most or all of the particles will pass through a 200 Tyler mesh screen and preferably a majority of those will pass through a 325 Tyler mesh screen.

SUMMARY OF THE lNVENTlON The present invention relates to a method and apparatus for producing fine flaked aluminum particles having outstanding characteristics 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 specific energy consumptions per 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 comprises 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 accordance with the method and apparatus of this invention.

DESCRlPTlON 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 feeds a fresh supply of aluminum particles (in granule or atomized form) to and continually withdraws fmished aluminum particles from the mill. The finished particles are carried from the mill by passing inert gas through the mill and are then separated from the gas. More specifically, 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 input to the mill 1, an oxygen gas monitor 12, an oxygen gas source 13, and a non-oxygen or inert gas source 14.

in operation, a fresh supply of aluminum particles are continuously fed to the mill 1 by the feeder 6. The

feeder 6 can be any conventional 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 motondriven, bearing-supported eccentric shaft or shafts which impart the oscillatory motion to the mill body. The mill body is preferably affixed to a stationary base by spring mountings. Any compatible combination of vibratory 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 intensified 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. The inert gas enters the mill through conduit 9 and leaves with the entrained particles through conduit 5. The flow rate of the inert gas is preferably sufficient to carry off some particles which have greater mass than those desired as 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 preferably 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 pressurized 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 particle 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 following examples and further elaborating disclosure.

PREFERRED OPERATING PARAMETERS AND EXAMPLE First presented is a description of general operating and design parameters which 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 additional operating and design parameters of similarly general applicability 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 is 5 in the down mode wherein gravity in addition to gassweeping aids the carriage of milled particles from the vibratory mill. Positioning of discharge conduit 5 is a matter of preference and is not critical to the invention.

The inert gas should have an oxygen content of about 0.5 to about 7% by volume and preferably about i to about 3% by volume. Control of the oxygen level is important both 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, however then with regard to explosive sensitizer grade particles, the available aluminum for reaction and energy production is correspondingly reduced. 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) coatmgs.

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 particles during milling to prevent them from welding 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 materials can be used such as normally solid fatty acids in addition to stearic; their derivatives, for example, calcium stearate; high melting point waxes; asphalting materials; silicone materials and combinations of the above. The particular coating employed 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.

Another reason for controlling the temperature within the chamber is than an overly high 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, 1200 rpm motors. This mill has a grinding chamber or mill body volume of 12.3 ft. The mill is charged with ball-shaped grinding media of approximately 3/ l6 to l 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 will 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 ft lmin. to about 3000 ft"/min., and the rate is somewhat dependent upon the mills supporting 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 Particle Feedstock and Rate atomized. 35% +100, l00 Tyler mesh; approx. Bl lbslhr Inert Gas Flow Rate 450 ftlmin.

lnertGas Oxygen Level 1.5%

Coating Material and stearic acid; approx. 3% of Feed Rate 8| lbs/hr Inert Gas Exit Temp- 58C, 62C

erature Average Residence Time of Particle Mill Filling (Balls) Ball Type and Size approx. 3/4 hr (includes recycle time) approx.

steel, 3/4" 500 35., 1/2" 1000 lbs., 3/16" 1200 lbs.

Particles produced from two separate runs were tested as shown in Example l 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.

EXAMPLE 1 ingredients (parts by weight): Mix 1 Mix 2 Mix 3 Solution containing:

Ammonium Nitrate 39.3 39.3 39.3

Fertilizer Grade Calcium Nitrate 32.1 32.l 32.1

Thiourea .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/Hp crosslinking agent .3 3 .3 Sodium nitrite/1L0 gassing agent .2 2 .2 Detonation results at C (Air Gap Test) p (gm/cc) 1.01 0.97 1.01

Detonated (1% Dia, No. 8 cap) 6" 6" 6" Failed 7" 7" 7" The above air gap test results indicate the relative sensitivities of the compositions. The air gap test was conducted by axially positioning in a line two cylindrical charges of 1 :6 inches in diameter and having the composition indicated but spaced apart from each other at the distance indicated. One of the charges was then detonated by a blasting cap. Detonation of the other charge either occurred or failed as indicated depending upon the charges 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 1.5 mlgm as comparable to about 1.4 mlgm for the paint grade particles. The particle size of the milled particles was l00%-150 Tyler mesh and 86%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 characteristics to commercially available paint grade aluminum. Furthermore, the production rate of the particles, 81 lbs/hr, is higher and the specific energy consumption of the mill (excluding other apparatus), 0.6 kw-hr/lb of product (based on a measured 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 AllisChalmers vibratory VBM 1518 mill driven by two 7% hp 1200 rpm motors. This mill has a grinding chamber volume of 1.6 ft". The apparatus employed a cyclone classifier, and a filter was used to collect the classified particles 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 Example 1. The operating parameters were as follows:

Aluminum Particle atomized, 26.5 lbs/hr Feedstock and Rate Inert Gas Oxygen Level 1.5-2.5%

Coating Material and stearic acid. 0.6 lbs/hr Feed Rate Inert Gas Exit Temperature 50-60C Average Residence approx. 1 hr Time of Particle Mill Filling 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 compositions and results are as follows:

EXAMPLE 2 Ingredients (pans by weight): Mix 1 Mix 2 Solution containing:

Ammonium Nitrate Fertilizer Grade Calcium Nitrate I Thiourea Guar gum derivative Ethylene glycol Starch Paint grade aluminum (U.S.

Bronze L-684) Vibratory milled aluminum Sodium dichromate/H,O crosslinking agent Sodium nitrite/Hp gassing agent Detonation results at 5C (Air Gap Test) gum/cc) etonated (2" Dia. No. 8 cap) Failed commercially available paint grade aluminum to which they were compared.

The milled particles produced by the method and apparatus of the above examples were found to have a stearic acid coating 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 in 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% wetting which was the same degree of wetting observed with the commercially available paint grade aluminum.

When examined under a Cambridge Mark 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 particles 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 of 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 that 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 not critical. The first example employed a grinding chamber having a volume of aproximately 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 aluminum 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 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 12 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 skilled 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.

What is claimed is:

1. In a method for producing fine flaked aluminum particles comprising:

a. milling aluminum in granulated or atomized form in a ball mill,

b. continually supplying fresh amounts of the aluminum in granulated or atomized form to the ball mill,

c. continually withdrawing milled fine flaked particles from the ball mill by passing an inert gas through the mill with a flow rate sufficient to carry the milled fine flaked particles from the mill, and

d. separating the milled fine flaked particles from the inert gas;

the improvement which comprises milling the particles in a vibratory ball mill.

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

3. A method according to claim 2 wherein said vibratory ball mill is capable of producing accelerations of the grinding media within the mill of at least 5 gs.

4. A method according to claim 1 which comprises continually supplying to the vibratory mill fresh amounts of a coating material for coating the milled particles and maintaining the temperature of the inert gas within the mill at a degree or range depending on the coating material employed such that the coating material will effectively 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 fine flaked particles from unfinished particles which may be carried from the vibratory mill along with the fine flaked particles and returning the separated, unifinished 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 fine flaked particles is accomplished by a centrifugal classifier.

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

Claims (9)

1. IN A METHOD FOR PRODUCING FINE FLAKED ALUMINUM PARTICLES COMPRISING: A. MILLING ALUMINUM IN GRANULATED OR ATOMIZED FORM IN A BALL MILL, B. CONTINUALLY SUPPLYING FRESH AMOUNTS OF THE ALUMINUM IN GRANULATED OR ATOMIZED FORM TO THE BALL MILL, C. CONTINUALLY WITHDRAWING MILLED FINE FLAKED PARTICLES FROM THE BALL MILL, AND A FLOW RATE SUFFICIENT TO CARRY THE MILLED FINE FLAKED PARTICLES FROM THE CILL, AND D. SEPARATING THE MILLED FINE FLAKED PARTICLES FROM THE INERT GAS, THE IMPROVEMENT WHICH COMPRISES MILLING THE PARTICLES IN A VIBRATORY BALL MILL.

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

3. A method according to claim 2 wherein said vibratory ball mill is 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 continually supplying to the vibratory mill fresh amounts of a coating material for coating the milled particles and maintaining the temperature of the inert gas within the mill at a degree or range depending on the coating material employed such that the coating material will effectively 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 fine flaked particles from unfinished Particles which may be carried from the vibratory mill along with the fine flaked particles and returning the separated, unifinished 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 fine flaked particles is accomplished by a centrifugal classifier.

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

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