System for discharging ball mills

A rotary batch-type grinding mill for processing powder is provided with an improved system for discharging material under seal from the atmosphere. The discharge system comprises a sealable collection chute having a banana-like configuration secured on the mill shell at an outlet port in the shell and extending substantially parallel to the circumference of the shell, a grate which prevents discharge of the grinding media from the mill during unloading of material from the mill and an elastomeric faced sealing plate which covers the grate during operation of the mill. The sealing plate is operated from outside the sealed system so that discharge from the mill can be effected without breaking the seal and the sealing plate is so disposed that in the open position it does not interfere with the flow of powder out of the mill.

FIELD OF INVENTION 
This invention relates to an improved system for discharging rotary 
grinding mills under controlled environmental conditions. More 
particularly it relates to a system for discharging particulate material 
from batch-type, rotary mills under seal to the atmosphere. 
BACKGROUND OF INVENTION 
In milling certain types of materials it is often necessary or desirable to 
have a positive control of the atmosphere within the mill at all times. 
For example, readily oxidizable materials such as aluminum, titanium, 
magnesium, lithium and fine powders of many compositions are combustible 
or even explosive under certain conditions or they may be contaminated by 
the presence of air. In milling such materials the control of the 
atmosphere must extend to charging and discharging of the mill without 
opening the mill to air. 
The present invention is not restricted to any particular materials. 
However, it is described below with reference to metal powders which are 
readily oxidized and are prepared as dispersion strengthened materials or 
alloys by powder metallurgy routes. Of necessity the milling of such 
materials must be carried out in a controlled atmosphere. The environment 
in the mill may be, for example, inert or may contain low levels of 
oxygen, hydrogen or hydrocarbons. To obtain such an atmosphere it is 
generally necessary to seal the mill to air. 
The problems encountered in milling powders are particularly troublesome in 
mechanical alloying of readily oxidizable metals such as aluminum, 
magnesium and lithium. Mechanical alloying has been described in detail in 
the literature and in patents. U.S. Pat. Nos. 3,740,210, 3,816,080 and 
3,837,930, for example, involve the mechanical alloying of aluminum alloys 
and the composite materials containing aluminum. In practice of mechanical 
alloying the components of the product are charged in powder form into a 
high energy milling device such as a ball mill where, in an environment 
free of or reduced in amount of free or combined oxygen, the powders are 
ground down to a very fine size initially, prior to particle agglomeration 
in the latter stages of the process. This initial grinding increases the 
total surface area of the metallic powders significantly. Since any 
freshly exposed surface of the powder is not oxidized, it is very hungry 
for oxygen to the extent that the powders in this condition will burn 
and/or might explode spontaneously if exposed to air. Thus, any port in 
the mill, for example, for charge or discharge of powders, is a source of 
potential danger from the standpoint of the quality of the product 
produced and/or the possibility of fire and/or an explosion. To avoid 
problems of explosion, burning and contamination, the mill should be 
emptied while maintaining positive control of the environment in the mill 
and throughout the entire discharging system and with minimum retention of 
powder in the mill. 
It has been known to operate a rotary mill with a plug in an opening in the 
shell, the plug being replaceable with a grate during discharge. For 
protection of the environment during discharge, the shell is enclosed in a 
housing. When the milling cycle is finished the housing is opened to 
replace the plug with a grate, then the housing is closed for the 
discharge cycle. During the discharge cycle the discharge opening is 
rotated to the underside of the shell, thereby permitting the powder to 
run into the housing. The rotation for discharge of material can be 
repeated. This arrangement is not satisfactory. It opens the system to the 
atmosphere when the plug is replaced by the grate. Powder discharged from 
the shell tends to accumulate in the housing, thereby requiring cleaning 
of the housing after each run and further opening the system to air. 
Opening of the housing and accumulation of powder in the housing are 
sources of contamination of the powder discharged from the mill and to 
subsequent runs in the mill. A further serious problem is that when the 
shell rotates inside the housing the discharging powder may be in the 
explosion range in terms of concentration of various portions of powder 
discharged in any cycle. Another proposed method for discharge is by gas 
sweep through the mill to pick up particles and carry them to a 
classification system. This involves the use of a combination of devices 
such as drop-out chambers, cyclones, bag filters, blowers and the like. 
Since the powder conveyed is combustible and/or explosive, the gas sweep 
system poses a significant hazard. Furthermore, it is difficult to seal 
against infiltration of air and against leaks. It is also difficult to 
control the flow of powder in the discharge. 
In the present system the discharge of processed material, e.g. processed 
to an alloy powder, is essentially gravity-dependent, the material is not 
aerated, it is relatively easy to keep the entire system under sealed 
conditions throughout the milling and discharge cycles, and the mill is 
discharged with minimized retention of the discharge material in the mill. 
Further advantages of the present discharge system are that the 
maintenance of the system can be achieved with minimum disturbance to the 
mill, and it can be done completely from outside of the mill. 
The present discharge system is especially useful for mills having shells 
of up to about 2 to 3 feet in length, and it is possible to empty the mill 
substantially completely. 
The discharge system of the present invention can be incorporated into 
existing rotary mills, permitting them to be discharged under protective 
conditions. 
STATEMENT OF THE INVENTION 
In accordance with the present invention a batch-type grinding mill 
operable under seal to the atmosphere is provided with a system for 
discharging material from the mill, the discharge system permitting 
substantially complete emptying of the mill while maintaining the entire 
grinding system and discharge system under seal to the atmosphere, and the 
discharge system comprising: 
(A) a rotatably mounted shell having an outer side wall, means to rotate 
the shell, a plurality of grinding media within the shell, and at least 
one discharge orifice through the outer side wall of the shell; 
(B) blocking means securable across the discharge orifice for preventing 
passage of the grinding media outwardly therethrough; 
(C) at least one collection chute sealably secured on the outer side wall 
of the shell over the discharge orifice and extending adjacent and 
substantially parallel to the circumference of the outer side wall, said 
collection chute having at least one entry port and one unloading end, the 
entry port providing a sealed access to the chute for material from the 
discharge orifice and the unloading end providing an atmospherically 
sealable exit passage from the chute, said unloading end being adapted to 
receive a receptacle for discharge and removal of discharge material from 
the chute under sealed conditions; and 
(D) a discharge orifice seal assembly for each discharge orifice, said 
discharge orifice seal assembly forming a part of the collection chute and 
comprising a removable closure means for sealing the discharge orifice 
against the outward flow of material from the shell into the collection 
chute, and means to open and close the closure means without disturbing 
the environment in the mill. 
To balance the mill, balancing weights may be used or more than one chute 
can be installed. Each chute may accommodate more than one discharge 
orifice. However, for discharge of material under a controlled 
environment, the discharge orifice must empty into a chute. The blocking 
means over each discharge orifice, e.g. a grate, sieve or screen (herein 
after referred to generally as a grate), has openings sized to prevent the 
grinding media from outward discharge from the shell into the chute. The 
grates are sealably mounted across the orifice and may be located in the 
shell or in the discharge assembly, sealable in the discharge orifice 
during the discharge mode of the mill. The grinding media may be balls, 
pebbles, rods or any other appropriate device. 
During operation of the mill under seal to the atmosphere the closure means 
is secured over the discharge orifice in the shell. The collection chute 
(or chutes) is retained on the mill, preferably with its unloading end 
sealed to the atmosphere, e.g. with a valve and closure cap, as a back-up 
to the discharge port seal. It is possible to retain the collection chute 
on the mill during the processing mode because of the spatial relationship 
of the collection chute to the mill shell and the closure means which can 
be secured over each discharge orifice. 
To discharge material the rotation of the mill is stopped and the seal, 
e.g. a plate, across the orifice (and grate) is retracted. The mill is 
rotated a number of times (with the collection chute unloading port 
sealed). The discharge material, e.g. a processed powder, will collect in 
the chute. Since the powder will reverse direction in the chute at the end 
of each rotation a deflector plate is placed in the chute just ahead of 
the grate so that the powder "jumps" over the grate as it flows to the 
other end of the chute. Eventually the chute stagnates (no additional 
powder will accumulate in the chute), whatever enters the chute flows back 
through the grate. This stagnatory point depends on the cross-sectional 
area of the chute. The chute must be at least sufficiently long to hold 
the volume of powder collectable up to stagnation. After stagnation or 
complete discharge of the mill, the shell rotation is stopped, and an 
unloading receptacle, e.g. a sealed drain can, is connected to the end of 
the chute, e.g. by means of a valve arrangement and adapter. The chute 
collection container valves are opened and the powder flows into the 
unloading receptacle. The cycle of collection and draining is repeated 
until the mill is empty. 
The speed of discharge is complex. The amount of powder that can be 
collected in the chute depends on its cross-sectional area. Length is a 
factor, but beyond a certain point, stagnation occurs and further length 
adds nothing but weight and cost. Rotation speed during discharge is also 
a factor - at higher speeds discharge is faster. 
The material processed in the mill may comprises elements, compounds, 
mixtures, alloys, ceramics and combinations thereof. Examples of elements 
which may be present in major or minor amounts are nickel, copper, zinc, 
titanium, zirconium, niobium, molybdenum, vanadium, tin, aluminum, 
silicon, chromium, magnesium, lithium, iron, yttrium and rare earths; e.g. 
cerium and lanthanum; examples of compounds are oxides, nitrides and/or 
carbides of alimunim, magnesium, yttrium, cerium, silicon and lanthanum; 
examples of alloys are master alloys of aluminum-lithium and 
aluminum-magnesium. The present invention is particularly useful when the 
material to be processed must be charged to and/or processed in a mill 
under a controlled atmosphere. The present invention is particularly 
advantageous for processing in a ball mill metal powders which are readily 
oxidized and are prepared as dispersion strengthened materials or alloys 
by powder metallurgy routes. Of necessity the milling of such materials 
must be carried out in a controlled atmosphere, e.g., in a hermetically 
sealed or a purgative atmosphere, or in an environment of controlled gas 
or gas flow. However, it will be understood that the present invention is, 
generally, especially useful for processing in a mill any materials where 
a controlled atmoshpere is required or beneficial. For example, the 
present invention can be used advantageously for preparing by a powder 
metallurgy route dispersion strengthened alloys having, e.g., nickel, 
copper, iron, titanium, magnesium or aluminum as a major constituent.

DESCRIPTION OF A PREFERRED EMBODIMENT 
Referring to the drawing, FIGS. 1A and 1B show an end elevation 
diagrammatic view of ball mill 10, in the operation or collection mode in 
FIG. 1A, and in the unloading mode in FIG. 1B. The ball mill comprises 
support frame 11, rotatably mounted cylindrical shell 12 having an end 14 
and wall 15. Orifice 18 (not shown in FIGS. 1A or 1B) in wall 15 is 
provided for discharge of material from the shell. Charging means 13 is 
disposed as disclosed in U.S. Pat. No. 4,679,736. (Other suitable devices 
may be used.) The charging means is not fully shown in the drawing. The 
discharge assembly is shown in various modes of opeation and in various 
details in the drawing, as described below. The ball mill contains a 
plurality of balls (not shown), and the shell is rotated about a 
substantially horizontal axis on trunnion bearing 16 by a motor (not 
shown) through a gear reduction means (not shown). Means for establishing 
a controlled environment in the mill are not shown. 
Collection chute 20 has entry port 21 aligned with discharge orifice 18, 
and unloading end 22. Valve 23 on the unloading end serves to seal the 
chute and material therein from the atmosphere during the collection mode, 
and adapter 24 enables the sealed connection of the collection chute 20 
with an unloading receptacle 25, e.g. as shown in FIG. 1B. Incorporated in 
collection chute 20 is discharge orifice seal assembly 30, which is 
located in the collection chute so as to enable the sealing of discharge 
orifice 18 during the operation mode of the mill. Straps 26 and 27 assist 
in securing the collection chute on the shell wall but other appropriate 
means could be used. 
The "banana"-like structure of collection chute 20, having top and bottom 
walls 52a and 52b, is shown in FIG. 2A. Seal assembly 30 forms a part of 
the collection chute. Purge connection 50 can be used to purge the 
atmosphere in collection chute 20. As shown in FIGS. 2B and 3A seal 
assembly 30 for discharge orifice 18 is sealed to the mill shell wall 15 
by flange 31 and bolts 32. Orifice 18 is covered with grate 19 to prevent 
the balls from falling out of the mill. During milling grate 19 is sealed 
with seal plate 33 which is faced with elastomeric gasket 34. Elastomeric 
gasket 34 is held against grate 19 by stem 36, having threaded section 37 
on its end, and a portion 35 of stem 36 is operable outside the seal of 
the discharge system. As shown in FIGS. 3B and 3C the connection of seal 
plate 33 to stem 36 is fitted with thrust bearing 40, bearing seal 41 and 
split retaining clip 48 secured with screws 49. Stem 36 is slidably sealed 
to cover plate 38 with "O"-ring 39, or other appropriate sliding seal. The 
threaded section 37 of stem 36 is used to hold grate seal plate 33 in 
place during milling. To hold the grate seal plate 33 open during mill 
discharge, pin 42, tethered by cable 42a is inserted in hole 43 after 
retracting stem 36 past threaded section 46 of cover plate 38. Deflector 
plate 51 adjacent to the grate permits an increase in the amount of powder 
collected during a discharge cycle. 
FIG. 3A, section A--A of FIG. 2A, shows details of the discharge system at 
discharge orifice 18, with the closure means, i.e. and elastomeric faced 
metal plate, (seal plate 33) sealing the orifice. Grate 19 across orifice 
18 has openings 47 sized to retain the balls in the mill. 
FIG. 3B, section B--B of FIG. 3A, shows a section of the closure means held 
in the open position with pin 42 in hole 43 of stem 36. Top and bottom 
walls 52a and 52b, respectively, of the collection chute extend from both 
sides of the discharge seal assembly. 
The rotary mill 10 rotates about a substantially horizontal axis. During 
processing of material, e.g. metal powders (not shown), in the mill, the 
discharge and charging orifices in the rotary shell are sealed so that 
nothing can pass outwardly from the shell. In the closed, sealed condition 
elastomeric gasket 34 is pressed against grate 19 by means of threaded 
section in stem 36 of the sealing member, blocking the orifice 18 and 
thereby preventing powder from passing from the mill shell into the 
collection chute. The unloading end 22 of the collection chute is sealed 
by valve 23, e.g. a butterfly or other appropriate valve. thus, the entire 
discharge system is sealed to the atmosphere. For discharging powder to 
the collection chute, stem 36 in the seal assembly 30 is released by 
relieving the pressure at the threaded end 37 located through cover plate 
38 of the seal assembly. Plate 33 is raised by pulling stem 36 outwardly, 
sliding it through a sealing means 39 until the hole 43 in the rod is 
above the cover plate 38. Pin 42 is then inserted to retain the grate 
sealing means away from the grate see FIG. 3B. In the preferred embodiment 
in the open position plate 33 is flush against inner wall 44 of the cover 
plate 38. During rotation of the cylinder, e.g. in a counterclockwise 
direction (for the position shown for the chute in FIG. 1A), and powder is 
released through the grate 19 into the collection chute. 
To unload the powder, the shell is rotated so that the unloading end 22 of 
the collection chute faces directly normal to a horizontal base line. An 
unloading receptacle 25 under seal, e.g. with a valve 45 is fitted onto 
the unloading end 22 of the collection chute via adapter 24. The valve 45 
in the receptacle then the valve 23 in the unloading port of the 
collection chute can be opened, and under sealed condition the powder 
passes from the collection chute into the receptacle. Before removal of 
the receptacle the valves in the unloading receptacle and the unloading 
end of the collection chute are closed. 
The above operation can be repeated, if necessary, until the mill is empty. 
The discharge assembly hereinbefore described is characterized by a means 
to seal the mill from the atmosphere during operation of the mill, during 
discharge of material from the mill into a collection system attached to 
the mill with retention of the grinding media in the mill, and during and 
after unloading material from the collection system. One of the advantages 
of the system, apart from the seal to the atmosphere, is that the 
discharge system prevents fine powders from leading from the mill into the 
discharge system during processing of the powders in the mill. By 
operation of the present discharge system the processing conditions can be 
controlled, the quality of the product processed in the mill can be 
insured and hazardous conditions can be prevented. 
Although the present invention has been described in conjunction with 
preferred embodiments, it is to be understood that modifications and 
variations may be resorted to without departing from the spirit and scope 
of the invention, as those skilled in the art will readily understand. 
Such modifications and variations are considered to be within the purview 
and scope of the invention and appended claims.