Apparatus for dry placer mining

A dry placer mining machine and a belt assembly for use therewith. The machine concentrates metallic constituents from a gravel mix by fluidizing the mix with air which passes upwardly through the belt assembly, and moving the fluidized mix over the belt and applying an electrostatic charge thereto. The belt assembly is made up of a composite fabric belt member and a plurality of riffle members which extend transversely across this. The composite fabric belt member is constructed of non-conductive materials so as to minimize dissipation of the electrostatic charge, and this is made up of a finely woven cloth top layer, a reticulated foam middle layer, and a coarse mesh lower layer. The riffle members, in turn, are provided with insulation for preventing the electrostatic charge from being conducted away from the fabric belt member. The belt assembly is driven over the open upper end of a plenum chamber, and air pressure is supplied to this through a blower and ducting. The internal surfaces of these components are coated with an insulating material which enhances the build-up of electrostatic charge on the airflow.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates generally to apparatus for dry placer mining, 
and, more particularly, to such apparatus for concentrating recoverable 
metallic products from gravel particulate wherein the separatory effects 
are the consequence of fluidization of the gravel and electrostatic 
retention of the metallic constituent on a moving belt. 
2. Background Art 
Dry placer mining, particularly for mining gold and silver from gravel 
deposits, has been known for many years. In an earlier application, which 
issued May 29, 1984 as U.S. Pat. No. 4,451,357 (hereby incorporated in its 
entirety by reference), applicant disclosed a machine which has proven 
highly successful for such mining operations. This is a compact, durable, 
and highly efficient device which concentrates metallic products from 
gravel particulate by employing a fluidizing air stream and an endless 
belt which is electrostatically charged by the air stream. 
Inasmuch as the overall configuration of this earlier machine is somewhat 
similar to that of the improved machine of the present invention (and the 
improvements of the present invention are in some respects relative to the 
former), a general overview of these systems will be provided here. 
Accordingly, turning to FIG. 1, this shows a dry placer mining machine, 
designated generally as 10, which comprises generally a frame 12 which 
supports and guides an endless separation belt 14 about a closed, 
generally trapezoidal path surrounding a fluidization means in the form of 
an air box or plenum chamber, this being designated generally as 16. The 
separation belt 14 is caused to traverse the closed path by means of a 
drive system, designated generally as 18. As the belt moves about its 
path, ore or a particulate gravel mix containing metallic constituents is 
charged to the upper face of machine 10 along an upwardly inclined segment 
of the trapezoidal path, designated 20 in FIG. 1, while fluidization gas 
(most conveniently, air drawn from the surrounding atmosphere) is passed 
upwardly from the open mouth of plenum chamber 16 through the separation 
belt 14. As the belt moves upwardly along the inclined path, as 
represented by the arrow in FIG. 1, the gravel and sand components of the 
charge are fluidized by the air passing through the belt. During this 
fluidization, as a consequence of the structure of the belt 14 and its 
relationship to the plenum chamber 16, an electrostatic charge is 
developed proximate the separation belt 14. As a result of the combination 
of the fluidization of the gravel and the electrostatic charge which is 
developed on or proximate the belt, the metallic constituents are retained 
on separation belt 14 while the gravel is fluidized and flows downwardly 
across the inclined slope of the belt. The retained product is thereby 
enriched or concentrated in the relative proportion of metallic 
constituents vis-a-vis the original gravel. The concentrate, which 
includes heavy or dark sands, may then receive further refining treatment. 
Since machine 10, at least in its smaller embodiments, is intended to be 
portable, the frame 12 is comprised of a base plate 22 which provides a 
solid foundation, and an upper frame structure designated generally as 24, 
this being pivotally mounted on the base plate 22. The plate 22 includes a 
pair of stanchion members 26, and these support the upper frame 24 about 
pivot pins 28 so that the frame 24 and associated portions of machine 10 
may pivot from a transportation configuration, where the face 20 is in a 
generally horizontal orientation, to an inclined operational configuration 
such as that shown in FIG. 1. Any convenient means for pivoting the frame 
on the pivot pins may be employed; FIG. 1 shows a pair of hydraulic 
cylinders 30 which are conveniently employed to serve this purpose. 
Whatever means are used to apply the pivoting force, it is desirable that 
the same be capable of providing an adjustable angle of inclination to the 
upper face 20 of the belt, since this angle is an important factor in 
establishing the residence time of the gravel on the belt during the 
separation process. 
The construction of belt 14 is of manifest importance to the efficient 
separation of the metallic constituents of the gravel charge. In the 
machine which was disclosed by the above-referenced earlier patent, the 
belt comprised three components, namely a woven metal mesh belt, a 
plurality of round riffle members, and a fabric member. The woven mesh 
belt was a steel mesh web which made up a series of upstanding loop 
elements, through which the round riffle members were inserted so as to 
extend transversely with respect to the upwardly inclined path of the 
belt, generally as shown in FIG. 1, so that these defined a series of 
transverse collection zones. The riffle members were relatively small 
diameter (i.e., 5/8 inch) steel rods, positioned within the mesh-to yield 
collection zones about 31/2 inches wide. The fabric member (which was 
positioned beneath the riffle members) comprised an intermediate layer of 
an air pervious polyester foam, sandwiched between lower and upper fabric 
layers. The upper fabric layer was made of a relatively fine woven 
cotton/polyester blend broadcloth, while the lower layer was made from a 
synthetic fiber fabric or batting having a fairly open weave, such as a 
coarse woven polyester batting of fibers such as those sold under the name 
Orlon.TM.. 
As noted at the outset, this previous machine has for the most part proven 
highly successful in the field. Nevertheless, its belt assembly 
(constructed in the manner described above) has exhibited a number of 
inefficiencies. Perhaps foremost amongst these has been the tendency for 
the electrostatic charge to dissipate from the belt assembly, thus 
reducing the ability of the belt to retain sufficient charge to 
efficiently separate the constituents of the gravel charge, and so this 
has led to the need to provide an auxiliary electrostatic charge 
generator. It has been discovered that this tendency to dissipate the 
electrostatic charge is linked to the use of various uninsulated metallic 
components in the construction of the belt, the steel rods forming the 
riffle members and the steel mesh web are excellent conductors which tend 
to quickly dissipate the charge, and the "flights" which form the 
upstanding borders of the belt assembly have also been constructed out of 
steel. Furthermore, it was found that the cotton/polyester blend 
broadcloth material which was used to form the uppermost layer of the 
fabric member of the belt, while performing admirably under very dry 
conditions, also tended to cause dissipation of the charge when conditions 
became damp, this apparently occurring due to absorption of water by the 
cotton component of the material. 
Another inefficiency which was observed with the belt having the 
construction described above relates to the relatively small diameter of 
the riffle members which have previously been employed. In operation, it 
was noted that as the metallic constituents were collected on the belt, 
these tended to accumulate in "pockets" which were formed at the lower 
edge of each collection zone where the underlying fabric member met the 
riffle; in practice it was found that these pockets were insufficient in 
size and so tended to quickly overfill and lose their ability to retain 
the metallic particles. 
Other difficulties which were encountered with the belt having the 
construction described above stemmed from the arrangement of the 
upstanding plates or "flights" which formed the borders of the belt. By 
way of illustration, the corresponding flights in the machine of the 
present invention are designated generally by reference numeral 32 in. 
FIG. 1. As was noted above, in the earlier machine these flights consisted 
of a series of thin metal plates, which aggravated the dissipation of the 
electrostatic charge from the belt. Another problem which was encountered 
with the metal flights was that these presented a serious safety hazard, 
inasmuch as gaps open and close between the adjacent flights where these 
pass over the sprockets at the ends of the inclined run of the belt, and 
an operator's hand or other body part might accidentally be received in 
these gaps and be severely cut. 
It was also found to be difficult to form an effective seal to prevent the 
escape of air from the plenum chamber when using the belt having the prior 
construction. In an attempt to overcome this problem, an additional 
elongate sealing member (typically round or half-round in cross-section) 
was installed along the edge of the plenum, and then this was permanently 
compressed against the edge of the belt so as to form a seal; not only has 
this approach proven marginally effective in terms of forming the desired 
airtight seal, but the additional drag and friction which was generated 
due to the need to keep the sealing member partially compressed hindered 
the smooth operation of the machine, and in severe cases could cause 
tearing of the belt. 
Other inefficiencies which were encountered with the previously-existing 
type of machine included the tendency of excessive undesirable mineral 
dust and other non-metallic particulate matter to accumulate on the belt, 
whether due to the electrostatic charge or simple adhesion, so that this 
was collected with the metallic constituents and so diluted the 
concentration of the recovered material. Also, in many applications it was 
found necessary to install a supplemental electrostatic charge generator 
in the plenum chamber inorderto augment the buildup of the electrostatic 
charge which occurs as the air passes through the belt material, reducing 
the overall economy of the operation; as was noted above, the need for 
this auxiliary generator was increased by the tendency of the belt to 
dissipate its electrostatic charge. 
Accordingly, there exists a need for a dry placer mining machine of the 
type described above, wherein the construction of the moving belt of the 
machine reduces or eliminates dissipation of the electrostatic charge 
which is imparted thereto. 
Furthermore, there exists a need for a machine having such a belt in which 
increased collection areas are formed where the riffle members and the 
fabric member of the belt meet at the lower edges of the collection zones. 
Furthermore, there exists a need for a machine having such a belt in which 
the flights at the borders of the belt have a construction which reduces 
or eliminates the possibility of personnel being cut thereby. 
Furthermore, there exists a need for a machine having such a belt in which 
the belt forms an effective seal to prevent escape of air along the edges 
of the plenum chamber without generating excessive friction and drag 
between the belt and chamber. 
Furthermore, there exists a need for such a machine having means for 
dislodging and removing excess accumulations of undesirable mineral dust 
and other particulate matter from the surface of the belt prior to the 
concentrated metallic constituents being collected therefrom. 
Still further, there exists a need for such a machine which is constructed 
so as to eliminate the necessity of using an auxiliary electrostatic 
charge generator to provide sufficient electrostatic charge on the moving 
belt assembly. 
SUMMARY OF THE INVENTION 
The present invention has solved the problems cited above, and comprises 
generally an improved belt assembly for use in a dry placer mining machine 
which concentrates metallic constituents from a gravel mix by fluidizing 
the mix with a gas and separating the metallic constituents from the mix 
by moving the fluidized mix over the belt assembly and applying an 
electrostatic charge thereto. The belt assembly comprises broadly a 
composite fabric belt member which is configured so that the electrostatic 
charge is established on the belt member as the fluidizing gas passes 
therethrough, this fabric belt member being constructed substantially 
entirely of non-conductive materials so as to minimize dissipation of the 
electrostatic charge, with a plurality of riffle members extending 
transversely across the fabric belt member in spaced, generally parallel 
relationships so as to define a series of collection zones, each of these 
riffle members being attached to the fabric belt member and comprising 
insulating means for preventing dissipation of the electrostatic charge 
due to this charge being conducted away from the fabric belt member 
through the riffle members. 
The composite fabric belt member may comprise an upper layer of relatively 
finely woven cloth, this being non-conductive in character and resistant 
to absorption of moisture so that it retains its non-conductive character 
under damp operating conditions, a middle layer of air pervious foam, this 
also being non-conductive in character and further having pores sized 
generally larger than the openings in the finely woven cloth, and a lower 
layer of relatively coarsely woven cloth, this once again being 
non-conductive in character and having openings sized generally larger 
than the pores of the middle layer of foam, whereby the composite fabric 
belt member presents a gradient from bottom to top, corresponding to the 
direction in which the fluidization gas passes therethrough. 
The relatively finely woven cloth of the upper layer may be 100% polyester 
cloth, such as a broadcloth having a weave of about 200 threads per inch. 
The coarsely woven cloth of the lower layer may preferably be more 
resistant to stretching than the middle and upper layers, so that this 
forms a backing which imparts strength to the belt assembly during 
operation, and this may be formed from a coarsely woven vinyl coated 
polyester mesh. The middle layer of foam between these upper and lower 
layers, in turn, may preferably be a reticulated foam, such as polyester 
foam, in which the pores are of substantially uniform size so that the 
flow of the fluidization gas is evenly distributed through the fabric belt 
member by the foam layer. 
The insulating means of each riffle member may comprise a sleeve of 
insulating material which surrounds the riffle member, and this may be a 
sleeve of polyurethane tubing which surrounds a metallic rod which forms a 
core member of the riffle member. A metallic sleeve member may be 
positioned concentrically intermediate the core member and the outer 
sleeve member for providing the riffle member with additional strength and 
diameter, and an inner sleeve member of insulating material may be 
positioned concentrically intermediate the metallic sleeve and the core 
member so as to provide an additional insulating layer between these. 
The belt assembly may further comprise a plurality of flights positioned 
intermediate the edge of the fabric belt member and a drive chain which is 
mounted to the ends of the riffle members, so that these flights form an 
upstanding border along the edge of the belt member. The flights are 
formed of a flexible, resilient material so as to avoid posing a hazard to 
personnel where gaps between adjacent flights open and close during 
operation of the machine. The flights each have first and second 
cooperating bores which are configured to receive outer ends of adjacent 
core rods of the riffle members, and preferably, at least one of the 
cooperative bores is elongated so as to permit a predetermined amount of 
movement of the adjacent riffle members toward and away from one another 
during operation of the machine without requiring deformation of the 
flights. 
A dry placer mining machine is also provided for concentrating metallic 
constituents from a gravel mix by fluidizing the mix with a gas and 
separating the metallic constituents therefrom by moving the fluidized mix 
over a belt and applying an electrostatic charge thereto. The machine 
comprises a frame for supporting and guiding an endless separation belt 
along a closed path having an upwardly inclined segment for receiving a 
gravel mix containing a low concentration of metallic constituents. The 
endless separation belt assembly comprises a composite fabric belt member 
which is configured so that the electrostatic charge is established 
thereon as a fluidizing gas passes therethrough, this fabric belt member 
being constructed substantially entirely of non-conductive material so as 
to minimize dissipation of the electrostatic charge. A plurality of riffle 
members extend transversely across the fabric belt member in spaced, 
generally parallel relationship so as to define a series of collection 
zones, each riffle member being attached to the fabric belt member and 
comprising insulating means for preventing dissipation of the 
electrostatic charge due to this being conducted away from the fabric belt 
member through the riffle members. Fluidizing means are provided for 
passing the fluidizing gas upwardly through the fabric belt member 
substantially uniformly along and about the upwardly inclined segment of 
the belt so as to fluidize the gravel mix and establish the electrostatic 
charge on the belt assembly, and drive means are provided for driving the 
belt assembly along the closed path so that the fluidized gravel mix moves 
over the belt assembly and the electrostatic charge effectuates a 
substantial separation of the metallic constituents from the gravel mix 
and retention of the metallic constituents proximate the riffle members, 
whereby the metallic constituents are concentrated for collection. 
The machine may further comprise means for removing accumulations of 
non-metallic particulates from the belt assembly prior to collection of 
the concentrated metallic constituents, and this means may comprise nozzle 
means for directing a flow of compressed gas towards the belt proximate an 
upper end of the inclined segment so as to dislodge the accumulations 
therefrom, and means for supplying the compressed gas to the nozzle means. 
The nozzle means may comprise a plurality of nozzles spaced across the 
width of the belt assembly proximate the upper end of the inclined 
segment, and these nozzles may be in communication with a manifold tube 
which extends across the width of the belt. The compressed gas may be 
supplied by means for connecting the manifold tube in fluid communication 
with the same means which passes fluidizing gas through the separation 
belt assembly. 
The interior portions of the means for passing fluidizing gas through the 
belt assembly which are subject to contact with the fluidizing gas may be 
coated with an insulating material which is configured to build up an 
electrostatic charge on the gas, so as to supplement the electrostatic 
charge which is established on the belt assembly. This insulating material 
may comprise a polyurethane coating. 
Other features and advantages, and a full appreciation of the structure and 
utility of the invention, will be gained upon an examination of the 
detailed description of the invention, taken in conjunction with the 
accompanying drawings.

DETAILED DESCRIPTION 
a. Overview 
The present invention relates generally to apparatus for dry placer mining 
and, more specifically, to such apparatus wherein separation of valuable, 
metallic products such as gold or silver from gravel particulate 
containing the same is affectuated under the combined influences of 
fluidization of the gravel and electrostatic retention of the metallic 
product on a traveling separation belt. Accordingly, the invention will 
now be described with reference to certain preferred embodiments within 
the aforementioned context, although those skilled in the art will fully 
appreciate that Such a description is meant to be exemplary only and 
should not be deemed limitative. For example, it may be desirable in some 
applications to utilize such an apparatus to segregate undesirable 
metallic constituents (e.g., iron) from the gravel or other mineral 
particulate material, such as in the preparation of the gravel for use in 
cement mixes. 
Turning now to the drawings, in all of which like parts are identified by 
like reference numerals, a dry placer mining apparatus incorporating the 
present invention is shown in FIG. 1. As noted above, this comprises 
generally a frame 12 made up of a base plate 22 and a pivotable upper 
frame 24, the belt assembly 14 being driven about the generally 
trapezoidal, closed path defined by the upper frame by a drive system 18. 
As noted above, the upwardly inclined segment 20 of belt 14 extends over 
the air plenum chamber 16. Air, or other suitable fluidizing gas, is 
supplied to the interior of plenum chamber 16 through a duct 34 by a fan 
assembly (not shown in FIG. 1, but shown in FIG. 7) in the direction 
indicated generally by arrow 36. Both the duct and the interior of the 
plenum chamber include a series of baffle plates or deflectors 38, some of 
which are shown in FIG. 1, which serve to distribute the flow of 
fluidizing air uniformly throughout the plenum so that the air which is 
directed upwardly through the belt is substantially uniform along and 
across the inclined segment 20. 
As noted above, belt 14 is moved about its closed path by drive system 18. 
Preferably, this comprises a motor 42 (such as a suitable electric motor) 
which operates through a first belt 44, reduction gear 46, and second belt 
48, to drive a pulley 50 at the upper end of the inclined upper frame 24. 
Pulley 50 is connected via an axle 56 to a pair of drive sprockets 52 (one 
only shown in FIG. 1), each of which engages a drive chain 54 which is 
mounted along the edge of the belt assembly. Similarly, a pair of idler 
sprockets 58 engage drive chains 54 at the lower end of inclined segment 
20, these also being interconnected by a second axle 60 which is pivotally 
mounted to frame 24. As sprockets 52 rotate in the direction indicated by 
arrow 62 in FIG. 2, they draw the belt assembly 14 up the inclined segment 
20, in the direction indicated by arrow 64 in FIG. 1. As the belt reaches 
the upper end of the inclined segment, and rounds the "corner" at the 
drive sprockets 52 (see FIG. 2) to begin the return run of the closed 
loop, gaps 66 open and close between the upper ends of the overlapped row 
of flights 32 which make up the segmented border of the belt assembly. It 
is these gaps which have previously presented a personnel hazard, due to 
the possibility of hands and so forth being accidentally received in these 
gaps and becoming cut, this problem having been overcome in the present 
invention by the adoption of flights which are constructed of a suitable 
semi-rigid, yet resilient material, as will be described below. 
FIG. 3 illustrates the overall operation of machine 10. With the belt 
moving in the direction indicated by arrow 68 in FIG. 3, the screened 
gravel, preferably having an average size of less than 1/4 inch, is 
conveyed to a hopper 72 by suitable transport means, such as a conveyor 
belt 74. The hopper distributes the gravel mix uniformly across the belt 
14 near the upper end of the inclined segment 20. Fan 76 (see FIG. 7) 
simultaneously provides fluidization air to plenum chamber 16; this air 
flows through the chamber generally in the directions indicated by arrows 
78, and then through belt 14 in an upward direction. As the belt moves, 
the gravel charge thereon is fluidized by means of the upwardly directed 
pressurized air flowing through the belt, which classifies the lighter 
components from the more dense "dark" sands and metallic constituents. The 
fluidized portion of the gravel falls by gravity just above the surface of 
the belt 14, generally in the directions indicated by arrows 79, and as is 
shown diagrammatically in FIG. 6. As the gravel tumbles over the riffles, 
the air causes a turbulent flow of gravel which follows a somewhat 
elliptical or oval path. All this activity, with air passing through the 
composite fabric which makes up the fabric member of belt 14, creates an 
electrostatic potential or charge imbalance proximate the belt 14. This 
electrostatic potential will have little or no effect on the gravel, but 
will result in an attraction and retention of the metallic constituents 
proximate the belt 14. In practice, it is found that the metallic 
constituents tend to congregate at the upstream side of the riffle members 
80 and, more particularly, on the underside of the curved surface thereof 
such as is indicated at 82 in FIG., 6, which, by virtue of the curved 
profile and underlying fabric member, provides a closed collection space 
or "pocket" which traps the retained material. As the belt 14 continues 
its upward travel, with the separated or concentrated metallic 
constituents retained thereon along with dark sands, the gravel which has 
had these metallic constituents removed therefrom falls from the bottom of 
the device as tailings 84. When the belt reaches the uppermost part of the 
inclined segment 20 and begins its downward decent on the return side, the 
metallic constituents and dense particulate will tend to be dislodged, and 
will fall, as indicated generally at 88 so as to be deposited on a second 
moving conveyor belt 90, which discharges these concentrates to a pan or 
container, or possibly to yet another conveyor belt 92 which carries the 
concentrates away to such a container. Depending on the ambient 
conditions, it sometimes occurs that the residual electrostatic potential 
on the belt 14 retains the metallic particles even during this downward 
decent, so that not all of the particles are dislodged for collection. 
Accordingly, it is generally advantageous to include a "bump bar" or small 
magnetic vibrator at or near the point where the belt first leaves the 
drive sprocket array region to bump or vibrate the belt and assist in the 
dislodgement of the particulate. 
b. Fabric Member of Belt Assembly 
As noted above, the construction of the separatory belt is important to the 
efficiency of the separation of the metallic constituents from the gravel 
charge; turning now to FIG. 4, this shows a portion of this belt assembly 
which, in accordance with the present invention, incorporates several 
features which significantly improve its performance. First amongst these 
is the construction of the fabric member 100, which provides both improved 
electrostatic charge retention characteristics and greater durability and 
resistance to stretch in service. This fabric member resembles the 
corresponding element which was employed in the earlier constructions of 
belt, in that it is made up of an intermediate layer of air pervious foam 
sandwiched between upper and lower layers of fabric. In accordance with 
the present invention, however, this intermediate foam layer 102 is 
preferably made from a reticulated foam material (i.e., having pores of 
uniform size), most preferably polyester foam, this having been found to 
provide much more even distribution of the air which passes through the 
fabric member; a reticulated polyester foam having a porosity of about 60 
ppi has been found eminently suitable for this application. 
The upper layer 104, in turn, is preferably made from a 100% polyester 
cloth having a relatively fine weave, such as polyester broadcloth having 
a weave of 200 threads per inch; by avoiding the use of cotton in this 
fabric layer, the charge dissipation problems which were exhibited by the 
earlier cotton/polyester blend material due to water absorption under damp 
conditions have been eliminated. Finally, the lower layer 106 is 
preferably made of an open weave, heavy-duty vinyl coated polyester 
fabric, such as that sold under the name Phifertex.TM. by Philet Wire 
Products, Inc. of Tuscaloosa, Ala., this exemplary material being 
constructed of a heavy, 25 mil polyester yarn using a 17.times.12 mesh. 
This fabric been found to exhibit high breaking strength and excellent 
resistance to stretch during use, and thus forms a high-strength backing 
for the fabric member; this has made it possible to eliminate the steel 
webbing which was needed in previous constructions of the belt assembly. 
Furthermore, due to its construction of vinyl and polyester, this lower 
layer provides additional protection against dissipation of the 
electrostatic charge on the belt assembly. 
Thus constructed, the composite fabric member 100 presents a gradient in 
the pore size or dimensions of the air passageways from bottom to top, 
corresponding to the direction which the fluidization air flows through 
the belt 14. This construction has been determined empirically to provide 
very good airflow characteristics while maximizing the electrostatic 
charge which is a principle factor in the efficiency of separation of the 
metallic constituents from the gravel charge. When subjected to a 
fluidization airflow passing upwardly through the fabric member (a 
variable static pressure averaging about 6.2 psig of dry air has been 
found suitable), a turbulent airflow within the mix is established, 
imparting a charge which attracts the metallic constituents to the belt 
and riffles. The composite construction which is used in this fabric 
member, and particularly the foamed polymer layer, has been found to 
increase the ability of the fluidization air to impart this charge, while 
the overall structural configuration of the belt 14 has been seen to yield 
an oval turbulence pattern within the mix due to the belt weave, thereby 
creating an alternating field of twisting the dipoles in one direction and 
causing an energy loss, all of which results in hysteresis and retention 
of the metallic particles on the belt 14. 
c. Riffle Member of Belt Assembly 
A plurality of transversely extending riffle members 80 are positioned on 
top of fabric member 100, again in a manner somewhat resembling the 
earlier configuration described above. These are spaced apart with respect 
to the direction of motion of the belt, so as to form a plurality of 
collection zones 110 between them. However, these riffle members 80, 
rather than simply being the small diameter steel rods which were used in 
the earlier versions of the belt assembly, are constructed of several 
concentrically-disposed layers or members so as to provide several 
advantages over the previously known arrangements. 
In the embodiment which is illustrated, each riffle member 80 comprises a 
central steel rod 112 which forms a structural core member about which the 
remainder of the riffle member is built. A first insulating sleeve 114 is 
positioned concentrically about the steel core: primarily, this serves as 
a first electrical insulator for insulating the inner Steel core from the 
outer surface of the riffle member so as to prevent dissipation of the 
electrostatic charge on the belt, and secondly this serves as a spacer for 
supporting the surrounding portions of the riffle member and increasing 
the diameter thereof; polyurethane tubing has been found to be an 
eminently suitable material for constructing this sleeve. 
The next concentric member is primarily structural in nature, and, in the 
embodiment illustrated, is made up of inner and outer metallic (e.g., 
steel) tubes 116 and 118 disposed in concentric relationship; these tubes 
serve mainly to impart additional structural rigidity to the riffle member 
as this extends transversely across the belt, so much so that it has been 
found to be possible to slim,hate the central support structure which was 
incorporated in belt arrangements of the earlier machines described above. 
The use of two concentric tubes rather than one provides maximum 
additional strength at minimal expense in terms of weight, and these again 
serve again to increase the diameter of the riffle member. Inexpensive 
steel conduit has been found to be a highly suitable material for 
fabricating these inner and outer tubes. 
The final concentric layer making up each of the riffle members is provided 
by an outer insulating sleeve 120, such as polyurethane tubing again, this 
preferably being ultraviolet treated so as to enhance the durability of 
the structure in a field environment. The main purpose of this outer 
polyurethane tube is to provide the primary insulator for preventing 
dissipation of the electrostatic charge on belt 14, being that this outer 
sleeve forms the contact surface between the riffle and the fabric member 
of the belt. It will thus be observed that the inner and outer 
polyurethane sleeves provide two layers of insulation between the outer 
surface of the riffle member and its inner core. Hence, even though the 
inner steel core 112 may be attached directly to the metallic drive chain 
54 (as will be described below), so that this would otherwise form a 
conductive path by which the charge might be dissipated, the two separate 
insulating layers provide an effective shield against such dissipation. 
So as to further enhance the resistance of the belt structure to 
dissipation of the electrostatic charge, the coiled steel webbing which 
was formerly employed for attaching the riffle members to the fabric 
member of the belt has been eliminated in the present invention, and this 
has been replaced by a series of non-conducting cord loops which are 
spaced along the length of each riffle member. Preferably, these loops are 
made of short lengths of polyester cord 122, and, as is shown in FIG. 5, 
these extend over the tops of each of the cylindrical riffle member 80, 
and then through the underlying fabric member 100, with the ends of these 
cords being secured together by knots 124 or other suitable securing 
means. It will be appreciated that the riffle members 80 are thus securely 
attached to the fabric member of the belt. 
In an exemplary construction as described above, the riffle members 80 may 
each have an external diameter of approximately 11/4 inch, these being 
mounted to the fabric member of the belt on approximately 4 inch centers, 
thus forming collection zones 110 which are approximately 23/4 inches wide 
between adjacent riffle members. As was noted above, a significant 
advantage which is achieved by this construction is the formation of 
significantly enlarged "pockets" 82 where the downstream edges of the 
collection zones meet the upstream edges of the riffle members, this being 
best shown in FIG. 6. As previously discussed, this collection "pocket" is 
formed on the underside of the curved surface of the riffle, which, by 
virtue of the curved profile and underlying fabric, provides a closed 
collection space for trapping the retained material. The increased 
diameter of the riffles constructed in accordance with the present 
invention provides collection areas which are roughly trebled in size 
relative to those which were provided by the prior construction described 
above; furthermore, the increased radius of the riffle members yields a 
corresponding increase in the depth of the collection "pockets", so that 
these much more effectively retain the metallic particles once they have 
been collected therein. 
FIG. 6 provides an illustration of how the fluidization air passes upwardly 
through the fabric member 100 of belt 14, in the direction indicated by 
arrows 126, so as to fluidize the charge of gravel 128 on the belt member, 
with the result that this moves down the upwardly inclined segment of the 
belt and the metallic constituents 130 are retained in these collection 
areas 82. 
d. Edge Flights of Belt Assembly 
Returning now to FIG. 4, this also shows the manner in which the riffle 
members are attached to the drive chain 54 so as to enable the drive 
system to engage the belt, as well as the arrangement of the edge flights 
which form the upstanding border of the belt assembly. As is shown, each 
of these flights 32 comprises a generally rectangular plate-like 
structure, this being made of a suitable resilient material, such as 
fiberglass-reinforced vinyl or rubber, this being selected so as to be 
sufficiently rigid to control and guide the gravel charge as this flows 
along the belt assembly, yet also so as to be sufficiently flexible to 
minimize the previously-described safety hazards by flexing laterally in 
the event that a person's hand or other body part is accidentally caught 
between adjacent flights. 
Bores are formed through the two lower corners of each of the flights 32: 
the first of these bores, as indicated by reference numeral 134, is 
generally circular and has a diameter which corresponds to that of the 
outer steel sleeve 118 of the riffle member; the second bore 136 is 
generally oblong in configuration, and has a width which corresponds 
generally to the diameter of the inner steel core 112 of the riffle 
member. Thus, as can be seen in FIG. 4, each flight 32 fits over the ends 
of two adjacent riffle members. At one corner (normally the trailing) the 
large circular bore 134 slips over the outer steel sleeve of the riffle 
member and abuts the outer end of the polyurethane sleeve 120, this being 
spaced inwardly from the ends of the inner steel and polyurethane sleeves 
of the riffle member so as to form what amounts to a radially-extending 
shoulder about a central boss, as indicated generally at 138 in FIG. 4. 
The distance by which the outer polyurethane sleeve is cut back preferably 
corresponds to the thickness of the flights, so that when the boss at the 
end of the riffle member is slipped through the bore 134, the outer ends 
of the inner steel and polyurethane sleeves line up flush with the outer 
face of the flight when the shoulder formed by the outer polyurethane tube 
abuts the inner face of the flight, and the inner steel core 112 extends 
outwardly beyond this. Thus, when the next flight 32 is slipped over the 
protruding end of the inner steel rod of the riffle member, so that this 
passes through oblong bore 136, the inner face of that flight will fit 
tight against the outer face of the first flight where these overlap, 
forming a seal between the two adjacent flights to prevent the escape of 
material from the sides of the belt. 
The elongated oval bore 136 serves to permit relative movement of adjacent 
riffle members 80 when these pass over the sprocket assemblies at the end 
of the upwardly inclined segment of the belt. In other words, the slot 
permits the central rod 112 to work back and forth therein as the belt 
rounds the corners at sprockets 52 and 58, thus permitting the distance 
between adjacent riffle members to be lengthened and foreshortened as 
necessary without requiring the plate-like edge flights 32 to compress or 
flex. 
As is shown in FIG. 4, the core rods 112 of the riffle members 80 protrude 
through the holes in the edge flights 32 and extend laterally therefrom. 
Each of these protruding ends is received in a corresponding bore 140 in 
drive chain 54. These bores 140 pass through the pairs of inner and outer 
side plates 142, 144, and the central roller 146, which make up each link 
of the drive chain. The outer ends of the rods 112 protrude laterally 
beyond the outer edge of the drive chain, and these are secured thereto by 
any suitable means, such as by spot welding the ends of the rods to the 
outermost side plates. Preferably, the side plates and rollers of the 
drive chain 54 are sized so that at least one other roller member 146a is 
positioned between each of those which is mounted to a rod 112, this being 
connected to the side plates of the drive chain by a pivot pin 148 so as 
to permit the links of the drive chain to flex where this extends around 
bends in the system, as at the drive sprockets and elsewhere. In an 
exemplary construction, the links in the drive chain may each be about two 
inches in length, with the rollers being about 11/8 inch in diameter and 
5/8 inch in width. 
e. Edge Seal Structure 
Another feature of the present invention is that this is provided with 
means for forming a seal between the edges of the belt assembly and the 
air plenum chamber without developing excessive friction and drag between 
these parts. FIG. 5 shows the manner in which this seal is achieved. 
The edge of the fabric member lee of the belt assembly is provided with an 
abrasion resistant sealing strip 152. This may be formed by folding a 
strip of resilient, yet durable and abrasion-resistant fabric--such as a 
polyester fabric having a smooth, continuous vinyl coating--over the edges 
of the foam and upper and lower fabric layers of the fabric member, and 
then stitching through this to retain the edge strip in place. The lower 
surface of edge strip 152 slidingly abuts the upper surface of a seal 
member 154, which is mounted within the upstanding rim 155 of plenum 
chamber 16 and extends longitudinally along this. In the embodiment 
illustrated, this seal member 154 is formed by a steel bar or rod 156 
which is welded along the inner surface 157 of the rim of the plenum 
chamber at bead 158. The lower surface of the edge strip rides along a 
low-friction surface formed on the seal member by a low-friction sleeve 
159 which is fitted around rod 156. PVC tubing has been found to be an 
eminently suitable material for forming this sleeve 159, this being split 
longitudinally and then slipped over the support rod of the seal member; 
in this regard, it has been found advantageous to use square cross-section 
bar stock to form rod 156, since the corners on this engage the inner 
surface of the PVC tube to keep this from twisting and working on the rod, 
and also, one edge of the stock forms a convenient attachment portion for 
mating with bead 158. For example, it has been found suitable in some 
embodiments of the present to form the core of 3/8 inch square soft iron 
rod, with 1/2 inch PVC water pipe then being split and fitted over this. 
The upper surface of the edge strip on the fabric member fits against the 
undersides of the ends of the riffle members 80, the fabric member of the 
belt assembly being secured to these members by the cord loops described 
above. These components are configured so that when the belt assembly is 
installed over the plenum chamber 16, there are normally (i.e., in the 
absence of a load) relatively loose tolerances between the edge strip of 
the fabric member and the sealing member inside the mouth of the box, thus 
virtually eliminating friction or drag between the belt assembly and the 
sealing member under such conditions; for example, there may be a vertical 
gap or range of movement on the order of 1/16 inch between these members 
in the absence of a load. Then, when a load of gravel is charged onto the 
upper surface of the belt assembly, the weight of this depresses the whole 
of the assembly downwardly slightly, and especially the portions of the 
edge of the fabric belt member which are between the riffle members. This 
causes the edge strip of the fabric member to move downwardly into sliding 
engagement with the seal member 154, as is shown in FIG. 5, substantially 
along the entire length of each edge of the mouth of the plenum chamber. 
This sliding engagement of the vinyl edge strip and the PVC pipe forms a 
highly effective seal for preventing the escape of air along the edges of 
the plenum chamber, without generating excessive friction and drag between 
these two components. Furthermore, this arrangement eliminates the need to 
"preload", compress, or otherwise force sealing members into engagement 
with one another in these areas to form the necessary seal. The 
effectiveness of the seal is enhanced by the close fit of the outer 
surface of the downwardly extending edges of the flights against the inner 
surface 157 of the lip of the plenum chamber, and the relatively close fit 
of the edge strip of the fabric member against the inner surfaces of these 
portions of the flights. 
As a further refinement, the transverse edges of the upper mouth of the 
plenum chamber are preferably provided with crossmembers or lips (not 
shown) which extend across the fabric member of the belt assembly and fit 
closely beneath the lower surface of the fabric member, so that this rides 
over the lips at the ends of the inclined run of belt. These crossmembers 
are provided with another low-friction sleeve which, in this case, engages 
the bottom of the fabric member of the belt assembly so as to form a 
transversely extending seal across this; it has been found particularly 
effective to form this seal member (not shown) from Teflon.TM. tubing 
which is split in two longitudinally and fastened, one-half each, to the 
two crossmembers at the end of the inclined run of the belt, with the 
Teflon.TM. tubing being secured to these members by both fasteners (e.g., 
bolts) and adhesives (e.g., epoxy) to provide a durable installation 
which, amongst other things, resists the contraction and expansion of 
these components which occurs in a field environment due to climatic 
variations. 
The upper rim 155 of the plenum chamber also supports an outwardly 
extending angle bracket 160, the upper, generally horizontal surface 162 
of which forms a track for supporting the roller members 146 of the drive 
chain 154 as this is pulled across the top of the plenum by the drive 
system. In this regard, it will be observed that the diameters of the 
roller members 146 of the chain are slightly greater than the widths of 
the side plates 142, 144 to which they are mounted, so that the rollers 
rollingly engage the track 162 without the side plates rubbing thereon. 
This also shows the manner in which the core rod 112 of each riffle member 
bypasses through the roller member and side plates of the drive chain, so 
that longitudinal forces applied to the drive chain by the drive system 18 
are transmitted thereby to the riffle members, and from the riffle members 
to the fabric member and edge flights of the belt assembly, so that the 
assembly moves together over the frame of the machine. 
A second angle bracket 164 extends upwardly from the outer edge of the 
first angle bracket 160, and then inwardly over the drive chain so that 
its inner lip 166 is positioned relatively closely adjacent the outer 
surfaces of the edge flights of the belt assembly. This bracket 164 thus 
forms a cover or "cap" over the top of the drive chain, stabilizing this 
and preventing it from Jumping away from the top of the plenum chamber 
during operation, as, for example, when charges of gravel are dumped onto 
the belt assembly. 
FIG. 5 also provides a further illustration of the relationship between the 
ends of the riffle members 80 and the edge flights 32a, 32b. In 
particular, this shows the manner in which the extending boss portion 138 
at the end of the riffle member is sized to fit within the bore 134 so 
that the first flight 32b slips over this and closely abuts the outer 
polyurethane tube 120 at its inner surface. AS was noted above, the length 
of boss 138 corresponds to the thickness of the flight, so that when the 
central rod 112 of the riffle member is slipped through the smaller bore 
136 in the next adjoining flight 32a, this second flight fits flat against 
the first where these two are overlapped. The side plates of the drive 
chain 54, in turn, abut the outer surface of the second flight 32a so as 
to press the overlapped portions of the two flights together, ensuring 
that a fairly tight seal is maintained between these so as to prevent the 
escape of material along the edges of the belt assembly. Also, it is 
desirable that the flights be overlapped as shown in FIG. 1, so that the 
outwardly disposed edges of the flights are positioned toward the 
direction of travel of the belt and the inwardly disposed edges are 
positioned rearwardly, so as to obviate any tendency of material to 
accumulate against the edges of the flights and work out through the gaps 
between them. 
f. Removal of Excess Dust From Belt 
The foregoing discussion has centered largely on the structure of the belt 
assembly 14 and its associated fittings. Another significant feature of 
the present invention relates to the need to remove excess dust and other 
accumulations of undesirable (usually non-metallic) particulate material 
from the belt assembly prior to collection of the valuable metallic 
constituent. 
As is perhaps best shown in FIGS. 7 and 8, this is achieved in accordance 
with the present invention by means of an assembly which directs a flow of 
air towards the upper surface of the belt near the upper end of its 
inclined segment, so as to dislodge and remove the accumulations of 
unwanted particulate matter. In particular, FIG. 7 shows a hollow bar 
member or manifold 170 which extends transversely across the belt member 
14 near the upper end of the inclined segment 20. As is also shown in FIG. 
8, this is provided with a hollow interior 172 (this having a square cross 
section in the embodiment illustrated) which communicates with a series of 
orifices or nozzles 174 which extend across the width of the belt. As is 
best shown in FIG. 4, these nozzles 174 are angled to direct the blast of 
air which exits through them toward the underlying belt, with this angle 
being selected relative to the velocity and volume of the air flow to 
provide a blast which is sufficient to dislodge the unwanted particulate 
matter, but without removing the valuable metallic constituent from the 
belt. 
Manifold 170 is supported above the belt by a pair of stanchions 176, 178, 
these being mounted to the upper frame 24 of the machine. As is shown in 
FIG. 3, at least one of these stanchions (stanchion 176 in the embodiment 
illustrated) has a hollow construction and is in fluid communication with 
the interior of air plenum chamber 16 through a port 180. As was noted 
above, the interior of plenum chamber 16 is charged with compressed air; 
thus, air under pressure enters the stanchion from plenum 16, and travels 
through it to the interior of the hollow cross bar 170, which it enters 
through a second port 182. The air then exits the bar through the nozzles 
in the manner previously described, blowing the unwanted particulate 
material away from the surface of belt 14 prior to the collection of the 
metallic constituents. Preferably, this dislodged dust is collected in a 
vacuum hood or chamber, or like structure (not shown), so that this can be 
collected and disposed of with minimum impact to the surrounding 
environment. 
g. Build-Up of Electrostatic Charge on Airflow 
As was noted above, the plenum chamber 16 of machine 10 is charged with 
air; in the embodiment which is illustrated, this is done by means of fan 
assembly 76, which comprises (as is shown in FIG. 7) a blower 182 Which is 
driven off of a motor 184 by a belt 186 and pulley 188, with the discharge 
from this being directed into the plenum chamber 16 by the relatively 
large duct 34, so as to pressurize the chamber with the fluidization air. 
As the air enters the plenum chamber, it is "broken up" and directed 
therein by the baffle plates 38 which were described with reference to 
FIG. 1. Accordingly, it will be understood that the air flow comes into 
contact with several interior components of this assembly during the 
course of its flow through the machine, including, for example, the vanes 
of the blower, the interior of duct work 34, the baffle plates 38, the 
interior of plenum chamber 16, and so forth. It is a feature of the 
present invention that one or more of these components is coated with a 
suitable insulating material, such as a polyurethane or rubber coating. 
The function of this coating is two-fold: firstly, the coating serves to 
enhance the buildup of an electrostatic charge (preferably positive) on 
the air as this flows through the assembly, due to the friction which 
occurs as the air comes into contact with and passes through these 
components, thus complementing the buildup of the electrostatic charge on 
the fabric member of the belt assembly through which the air subsequently 
passes; secondly, the insulating qualities of the coating help prevent the 
dissipation of the electrostatic charge as this passes through these 
assemblies enroute to the belt. This feature has been found to 
significantly enhance the ability of the machine to build up the desired 
electrostatic charge on the inclined segment of the belt, to the extent 
that, in conjunction with the belt assembly constructed in accordance with 
the present invention, it has been found possible to dispense with the 
need for any sort of auxiliary electrostatic charge generator under the 
vast majority of operating conditions. 
As noted above, the system described herein may be used particularly for 
the separation of gold or silver from gravel particulate. It has been 
found that the apparatus incorporating the present invention requires only 
a relatively small amount of electrostatic charge to create an effective 
influence with respect to fine particle-sized gold. The charge is created 
by breaking up the air flow and creating turbulence in the manner 
described above. First, the air flow is broken up as it passes over the 
polyurethane, rubber, or other static-building material coating of the 
blower assembly and plenum chamber, so that this creates a relatively 
large electrostatic charge on air within the air box. This heavily charged 
air, in turn, charges the filter belt, by passing through this and 
imparting its charge to the belt assembly, and also due to the turbulence 
which is thus created, as was also described above. For separation of 
gold, it has been found effective to adjust the continuous flow of air 
through the belt to build a constant charge of roughly 20,000 to 25,000 
volts. A triboelectric positive charge is developed on the particles of 
gold, which attracts them to the moving belt assembly for concentration 
and subsequent recovery, as described above. 
While the invention has now been described with reference to its preferred 
embodiments, those skilled in the art will appreciate that various 
substitutions, changes, modifications, and omissions may be made without 
departing from the spirit thereof. Accordingly, it is intended that the 
scope of the present invention be limited solely by that of the claims 
granted herein.