Method and apparatus for recovering small particles of heavy precious metals

There is disclosed a method and apparatus for recovering small particles of gold or platinum from a slurry by feeding the slurry onto a layer of Mercury which is flowing downwardly along a generally conical, downwardly converging inner wall of a housing. The housing is rotated about the vertical axis of the conical wall at a speed which induces particles to settle out or a malgamate with the layer of Mercury and causes the remainder of the slurry to move upwardly over the Mercury layer and out of the housing. Upon collection and separation of the particles therefrom, the Mercury is returned to the housing for resupply to the inner wall thereof so as to maintain a continuous layer of Mercury.

This invention relates to an improved method and apparatus for recovering 
small particles of heavy precious metals such as gold or platinum from the 
extraneous material with which they are mined. 
Large deposits of small, often microscopic particles of gold are known to 
exist in the mud and silt along and off the shoreline of Alaska. These 
particles, which are known as placers, have broken off from the veins of 
lode formations in the mountainous areas adjacent the shoreline, and, over 
the ages, have been washed down to the shoreline by the mountain streams. 
However, these gold particles are present in the mud and silt in very 
small concentrations, such as 0.005 ounces per ton of mud and silt. Hence, 
known chemical processes for removing large volumes of gold particles or 
similar small particles of platinum from the extraneous material with 
which they are mined are not economical or otherwise practical. 
It has long been the practice to recover gold particles from stream beds 
causing a slurry of the stream bed materials to flow through a sluice 
having Mercury traps along its length. The gold particles would separate 
out from the lighter elements of the extraneous material, and, being 
heavier than the Mercury, would either settle into or amalgamate with the 
Mercury. Periodically, the Mercury with the gold particles therein would 
be heated to its evaporation temperature, which is below that of the gold 
particles, to permit the latter to be separated out. However, this 
process, which relies solely upon the force of gravity to cause the gold 
particles to enter the Mercury, is obviously not practical on the mass 
production basis which would be required to economically recover gold 
particles in the small concentrations above noted. 
It is therefore an object of this invention to provide an economical and 
otherwise practical method and apparatus for removing small particles of 
gold or platinum from the formation materials with which they are mined. 
This and other objects are accomplished, in accordance with the illustrated 
embodiment of the invention, by a method and apparatus in which Mercury is 
supplied to a generally conical, downwardly converging inner wall of the 
housing to cause a layer of the Mercury to flow downwardly therealong, and 
a slurry containing the small particles and other extraneous material is 
fed onto the layer of Mercury. The housing is rotated about the vertical 
axis of the generally conical wall at a speed which induces the particles 
to settle in or amalgamate with the layer of Mercury and causes the 
remainder of the slurry to move upwardly over the Mercury layer and out of 
the housing, while permitting the Mercury layer to flow downwardly along 
the housing wall at a relatively slow rate. 
The Mercury layer and the particles which flow downwardly therewith over 
the conical wall of the housing are collected, and the particles are then 
separated from the Mercury. More particularly, the Mercury from which the 
particles have been separated is returned for resupply to the inner wall 
of the housing so as to maintain a continuous layer of Mercury thereover. 
In this way, the recovery process may be on a continuous basis, thus 
providing a practical way to recover the particles despite their small 
concentration in the extraneous material of the slurry.

With reference to the details of the drawings, the illustrated apparatus 
includes a housing, indicated in its entirety by reference character 10 
and comprising a hollow body 11 mounted on a vertically disposed shaft 12. 
The shaft is in turn mounted on a fixed horizontal support 13 by means of 
bearings 14 so as to permit it, and thus hollow body 11, to be rotated 
about its vertical axis. The shaft is rotated by means of a belt 15 
disposed about the lower end of the shaft beneath the support 13, and a 
sheave 16 mounted on a vertical shaft 17. A motor (not shown) is connected 
to shaft 17 in order to rotate it, and thus shaft 12, at a desired speed. 
As previously described, the body 11 of the housing has a generally 
conical, downwardly converging, inner wall 18, and Mercury is supplied to 
this wall to cause a layer thereof to flow downwardly therealong. More 
particularly, Mercury is supplied thereto from a header 19 comprising a 
tubular ring mounted above the upper end of wall 18 and having outlet 
ports 20 spaced circumferentially thereabout. Thus, as indicated by the 
arrow in FIG. 1, the Mercury is supplied in substantially equally spaced 
apart streams so as to form a layer M which quickly spreads about the 
entire circumference of the wall. 
Although centrifugal force due to rotation of the housing will force the 
Mercury layer against the wall 18, the angle of wall 18 with respect to 
the vertical and the speed of rotation of the housing are such that the 
layer nevertheless is free to flow downwardly therealong, although at a 
relatively slow rate. As it flows downwardly along wall 18, it passes 
through orifices 21 in the lower end of body 11 and thus into an annular 
collection container 22 mounted on support 13 and disposed about shaft 13. 
These orifices are preferably variable in size so that they may be 
adjusted depending on the flow rate of the Mercury layer. More 
particularly, the container is fixed to the stationary support 13 and 
carries a seal ring 23 to form a sliding seal about the shaft 12. 
A slurry containing gold particles, as well as water and the mud and silt 
in which the gold particles are contained, is fed to the Mercury layer M 
through a conduit 24 which passes upwardly and axially through shaft 12. 
Alternatively, of course, the particles may be platinum which, similarly 
to gold, will amalgamate with the Mercury. More particularly, the slurry 
is fed to the Mercury layer by means of a head 25 on the upper end of 
conduit 24 which is supported on the upper end of shaft 12 by means of a 
bearing 26. The head 25 has ports 27 spaced circumferentially thereabout 
so as to feed the slurry radially outwardly against the Mercury layer, as 
indicated by the arrows in FIG. 1. The slurry is pumped upwardly through 
conduit 24 and out of the head 25 by means of a pump 28, and slurry is 
supplied to the pump through another conduit 29 extending from the source 
of the slurry (not shown). 
As illustrated, conduit 24 as well as head 25 are not rotatable with the 
body, but instead, and as illustrated in the preferred embodiment of the 
invention, are stationary. This relative rotation between the Mercury 
layer and slurry as the latter is fed outwardly onto the Mercury layer, 
causes agitation at the inner face of the Mercury layer which is believed 
to promote settling and amalgamation of the gold particles in the slurry. 
In fact, the means for feeding the slurry may be rotated in a direction 
opposite to that of the rotation of the housing in order to promote even 
additional agitation. 
In any event, as the slurry is fed to the Mercury layer, it is caused to 
settle therein or amalgamate therewith not only by virtue of the gravity 
of the gold particles, but by the virtue of the centrifugal force on the 
slurry and the Mercury layer, which of course, induces movement of the 
particles into the Mercury. Furthermore, the angle of the wall 18 with 
respect to the vertical and speed of rotation of the housing are such that 
a layer S of the slurry is caused to move upwardly along the layer of 
Mercury M and over the upper edge of wall 18 for removal from the housing. 
This is made possible, of course, by virtue of the fact that the water and 
the extraneous solid particles in the slurry are sufficiently lighter than 
the Mercury that the centrifugal force overcomes the effect of gravity 
thereon to cause them to move upwardly depsite downward movement of the 
Mercury layer. 
As shown in the drawings, an annular trough 30 is mounted about the upper 
edge of the wall 18 so as to receive the upwardly moving layer of slurry S 
as it spills over the edge. A conduit 31 connects with the lower end of 
the trough for conducting the slurry either to a suitable place of 
disposal or return to the supply conduit 29 in the event a sufficient 
quantity of small particles have not settled out or amalgamated with the 
Mercury to make it economically feasible to recirculate the slurry through 
the housing. 
A conduit 32 connects the lower end of container 22 so as to conduct the 
Mercury and gold particles collected therein to a boiler 33. In the 
boiler, which may be of conventional construction, the Mercury and gold 
particles therein are heated to the vaporization temperature of the 
Mercury, which is below that of the gold so as to vaporize the Mercury and 
thus permit the gold to settle out therefrom. As the Mercury vaporizes, 
the gold particles thus fall by gravity into a hopper 34 beneath the 
boiler from which they may be removed through a conduit 35 connected with 
the lower end of the hopper. 
A conduit 36 connects the boiler with the header 19 through which Mercury 
is supplied, and a pump 37 is disposed in conduit 36 for returning the 
Mercury from which the gold particles have been separated back to the 
header, whereby it may again be supplied to the inner wall 18 of the 
housing. The Mercury vapor from boiler 33 is condensed within a condenser 
38 disposed in line 36 downstream of pump 37 so that it is in liquid form 
as it reaches the header 19. 
As shown in FIG. 1, a suitable means 39 is provided for applying a positive 
charge to each of slurry supply conduit 29 and hollow body 11. In this 
manner, each of the slurry, including the gold particles therein, and the 
layer of Mercury flowing downwardly over the inner wall 18 of the hollow 
body are also positively charged. Since the Mercury layer M is a 
conductor, its inner face which the slurry is fed is negatively charged. 
As will be understood, this negative charge attracts the positive charge 
applied to the gold particles in the slurry, and thus promotes movement of 
such particles into the Mercury layer. 
From the foregoing it will be seen that this invention is one well adapted 
to attain all of the ends and objects hereinabove set forth, together with 
other advantages which are inherent to the method and apparatus. 
It will be understood that certain features and subcombinations are of 
utility and may be employed without reference to other features and 
subcombinations. This is contemplated by and is within the scope of the 
claims. 
As many possible embodiments may be made of the invention without departing 
from the scope thereof, it is to be understood that all matter herein set 
forth or shown in the accompanying drawings is to be interpreted as 
illustrative and not in a limiting sense.