WO1988006193A1

WO1988006193A1 - Process and apparatus for recovering precious metal particles from soil materials

Info

Publication number: WO1988006193A1
Application number: PCT/US1988/000375
Authority: WO
Inventors: John D. Green
Original assignee: Individual
Current assignee: Individual
Priority date: 1987-02-10
Filing date: 1988-02-08
Publication date: 1988-08-25
Anticipated expiration: 1989-08-10
Also published as: ZA881171B; CA1318641C; AU1294488A

Abstract

A system for recovering precious metal particles from a slurried soiled material by amalgamation comprises an amalgamating apparatus (3) through which a soil material is circulated with the soil material in slurry form and submersed in a body of amalgamating mercury. The system includes a soil material conditioner (1) that provides a slurried soil material having a predetermined small particle size to the amalgamation apparatus (3). The system also includes a mercury conditioner (4) to purify and charge the mercury for use in the amalgamating mercury body, and includes a mercury amalgam recovery separator (5).

Description

PROCESS AND APPARATUS FOR RECOVERING PRECIOUS METAL PARTICLES FROM SOIL MATERIALS FIELD OF THE INVENTION

This invention relates to process and apparatus for recovering gold and other precious metal from soil materials by amalgamation. In particular it relates to the recovery of micro sized particles of precious metals that typically are left in the tailings of conventional recovery processes. BACKGOUND OF THE INVENTION The process and apparatus of this invention are specificall directed to the recovery of what is known as "micron gold" , although the system also recovers other precious metals that are often present with gold in the processed soil materials. "Micron gold" is gold having a particle size in range of a few microns down to about 0.25 microns. It is very fine particled and has proven extremely difficult to recovery in a commercially-feasible recovery system. The tailings from almost all gold mining operations contain a substantial, measurable portion of this micron gold that is often simply washed away with the mining residue in the form of a slurry.

Amalgamation processes have been in existence to recover gold for a very long time. These use amalgamation techniques, however, are not" suitable for efficient recovery of the very fine gold particles for which the present invention is designed. These prior amalgamation processes do not appear to be able to recover gold finer that about 30 microns in particle size. SUMMARY OF THE INVENTION

The process and apparatus of the present invention utilize an amalgamation technique quite unlike any other to recover gold and other precious metal particles as fine as 0.25 microns. The gold bearing material to which the process of this invention is applied is reduced in particle size to about 1/4 inch or less and provided as a slurry. The slurry is fed to a stationary amalgamating apparatus where it is submersed in mercury. The precious metal particles are amalgamated while the slurry is passed though the mercury, and the amalgamated particles sink to the bottom of the mercury for withdrawal. The residual slurry passes through the mercury and exits the apparatus for discharge, The amalgamating apparatus is designed so that the pressure of the slurry stream is sufficient to cause the submersion and passage of the slurry though the mercury. No large water volume is required; only sufficient water to liquify the slurry to the point where it can be fed into the amalgamating apparatus through an inlet conduit. The mercury within the amalgamating apparatus is conditioned so that it will "wet" the amalgamating apparatus surfaces that come in mercury contact; thus insuring that the slurry material must all pass through the mercury and that no material can reach the exit without being submerged in the mercury. The laden mercury is withdrawn from the amalgamating apparatus and filtered to separate the precious metal amalgam from free mercury. The amalgam is removed from the filter and precessed to recover the precious metals. The free mercury is passed through a conditioner and then returned to the amalgamating apparatus.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a top view, partially cut away, of the amalgamating apparatus of this invention;

Figure 2 is cross section view taken along the line 2 - 2 in Figure 1; and

Figure 3 is a diagrammatic view of the process of this invention. DETAILED DESCRIPTION OF THE INVENTION

The amalgamating apparatus 10 depicted in Figures 1 and 2 comprises a circular outer housing 12 having a tangential slurry inlet 16 and an axial slurry outlet 20. A circular inner housing 14 defines, with outer housing 12, an annulus within the outer housing within which the slurry from inlet 16 is fed. A circular slurry collector 45 is located within the inner housing and positioned axially around the slurry outlet 20. The bottom 26 o outer housing 12 is designed to hold a shallow body of mercury and is provided with a depressed center section 28, located beneath the inner housing 14, to provide a deeper mercury bed from which laden mercury may be withdrawn through passage 52. The bottom 35 of inner housing 14 is spaced above the outer housing bottom 26 by a small amount, on the order of 1/4 inch, and is designed to be in contact with the body of mercury. The slurry collector 45 has no bottom and its interior is open to th body of mercury overlying the outer housing depressed center section 28. The slurry outlet 20 extends axially through the depressed center section 28 and up through the body of mercury into the interior of slurry collector 45 to an elevation substantially above the surface of the mercury body. A mercury make-up inlet conduit 18 provides for addition of mercury to the mercury body as needed to maintain the mercury body at a level sufficient to keep its upper surface in contact with the inner housing bottom 35. The mercury - contacting surfaces are preferably constructed of copper so that these surfaces will be "wetted" or "amalgamated" with mercury from the mercury body to insure that there will occur no gaps through which soil material in the slurry could escape to the slurry outlet except through submersion in the body of mercury.

In operation, the body of mercury present would have a sufficient depth to at least completely contact the undersurface of the inner housing bottom 35, and would preferable be slightly greater than the minimum. Slurry would be introduced through inlet conduit 16 at whatever line water pressure existed, on the order of 50 psig or greater, and would circulate in the annulus. Because the slurry exit would be maintained at atmospheric pressure, the pressure differential between the inlet and outlet would drive the circulating slurry down into the mercury body over the surface area exposed on the bottom of the annulus. This will cause the mercury body to circulate also. Because of the pressure differential and the tangential input of the slurry, the circulating slurry will penetrate into the mercury body and begin a spiral circulation inward and through the mercury body, underneath the inner housing bottom 35, and eventually will break free from the mercury body into the slurry collector 45. The slurry within the slurry collector 45 will continue circulating and rise high enough to overflow into the outlet conduit 20 for exit from the apparatus. Because of the spiraling flow of the slurry through the mercury body, the contact time of the soil material particles is greatly extended. Consequently, there is a substantial opportunity for the precious metal particles to be attracted to the mercury and become amalgamated therewith. As the mercury becomes laden with amalgamated precious metal particles, the laden mercury will settle out onto the depressed center section 28 for withdrawal through conduit 52.

Four narrow metal plates or rods 39-42 extend horizontally across the top of housing 14 at 90 degree intervals, connecting the top edges, respectively, of the outer and inner peripheral walls 43 and 45. Housing 14 is supported within the outer housing 12 by a central vertical rod 36, which extends down into the outer housing through lid 30. At the internal end 47 of rod 36, a circular flange 40 is mounted, the peripheral edge of which is secured to the top edge of the inner peripheral wall 45.

The rod 36 and hence the housing 14 may be biased downward into housing 12 by a spring 42 positioned in a U-shaped cup 49 that surrounds rod 36. The bottom of cup 49 is secured to the lid 30 of housing 12. The upper end of spring 42 abuts a handle 51 which is attached to the rod 36. Through this mechanism, the spacing between the underside 35 of housing 14 and the bottom 26 of housing 12 may be adjusted.

The outer housing 12 is airtight so that the pressure differential between the slurry inlet and outlet will drive this part of the process. If desired, an air valve 44 and a pressure guage 46 may be mounted to lid 30. - D -

An amalgamator apparatus of the type described above, is part of the system and process depicted in Figure 3. That syste comprises a soil material screening plant 1, a soil material scrubber 2, an amalgamator apparatus 3, a mercury conditioner or charger 4, and a laden mercury filter 5. The screening plant sorts out and reduces the soil material to a particle size of 1/ inch minus or less to insure that micron-sized precious metals will become exposed for subsequent amalgamation. From the screening plant, the sized soil material is passed to the scrubber for further conditioning. In the scrubber, the particles of the soil material are literally scrubbed together t break the oxidized coating off the precious metal particles. Other means such as ultrasonic scrubbers could be employed to break the oxidized coatings and to expose precious metal particl surfaces for amalgamation. The screened and scrubbed soil material is then mixed with a quantity of water and transferred by inlet conduit 16 into the amalgamator apparatus for processin in the manner described above.

In this process, the mercury is conditioned by charging in charger 4. Mercury from filter 5 or from an external source, as for make-up, is fed into the D.C. charger 4 and withdrawn in a charged state with a negative charge and transferred in inlet conduit 18 to the amalgamator apparatus. Charger 4 may be a three cell device with one or more mercury inlets located at the sides and a mercury outlet located in the bottom center of the device. The mercury level within the device is kept below the electrodes 75 and an electrolyte solution of water and soda (bicarbonate of soda) is added over the mercury body. The electrodes may be supported in pans 76 to ride on the electrolyt solution if desired. The electrodes are positively charged and the casing of the charger is grounded. In this configuration, the outer two cells are acidic and the middle cell is alkaline. Mercury withdrawal from the charger should be from the vicinity of the alkaline portion of the device. The mercury from the filter 5 that is transferred to the charger in conduit 77 is dar and dirty looking. The conditioned mercury leaving the charger is bright and active and is itself an amalgam with sodium. The conditioned mercury/sodium amalgam easily coats the amalgamator surfaces with which it comes in contact and is highly attractive to the precious metal particles that will pass through it within the amalgamator. This conditioned mercury is so active that it will wet other metals in addition to copper. Consequently, it i possible to construct the mercury-holding or mercury-contacting surfaces within the amalgamator out of non-copper material such as steel, if desired, to reduce the cost of the amalgamator.

The laden mercury within the amalgamator is withdrawn through exit conduit 52 and transferred to a pressurized filter press 5. Within the press, the amalgam is forced under elevated pressure onto a filter screen 79 and the non-amalgamated mercury is collected in the bottom of the filter for transfer to the charger. An air conpressor 6 may be provided to provide compressed air to drive the filtering procedure. An air pressur of about 40 psig is suitable. When one filter screen is saturated with amalgam, transfer valve 80 can direct the laden mercury stream from conduit 52 into a companion filter device fo processing while the saturated filter screen is removed for amalgam processing. The amalgam may be processed by one of the conventional methods to recover the precious metals, such as wit nitric acid or by retorting.

An amalgamating apparatus of the type shown in Figures 1 an 2, with a size of about 25 inches outside diameter by 3 1/2 inches high, a depressed bottom 28 of about 6 inches diameter an 1 inch deep, and an inner housing 14 with an outside diameter of about 18 inches, with a gap of about 3/8 inch between the bottom surfaces 26 and 35, can process approximately 2 1/2 tons of soil material per hour. This is equivalent to processing about 3 gallons of slurry per minute. The particulate slurry may be continuously added to the amalgamator at a defined rate, and the mercury may be continuously recirculated, so that the process is truly continuous. The continuous discharge from outlet 20 may directed to a holding pond, where the particulates therein settle, permitting the liquid to be reused to make up the slurry. Hence, once in operation, the process requires relatively little water to operate effectively. Also, little power is required to operate the system inasmuch as a small pump may be used to circulate the mercury and a battery charger type of apparatus may be employed to charge the mercury conditioner. The above-described apparatus is thus capable of being conveniently and practically used on-site in remote mining operations, thus eliminating high material transportation costs. Although a preferred embodiment of the invention has been disclosed herein for illustration, it should be understood that various changes, modification and substitutions may be incorporated in such embodiment without departing from the spirit of the invention as defined by the claims appended hereto

Claims

IN THE CLAIMS

1. A system for recovering fine particles of precious metals by amalgamation from soil materials which comprises an amalgamation apparatus comprised of housing means for containing a body of amalgamating mercury with an upper surface exposed to the interior of said housing means; soil material inlet means for introducing slurried soil material to the interior of said housing means into contact w^τith the exposed surface of said mercury body; inner housing means positioned within said housing means to divide said interior into a first outer annular zone for receipt of said slurried soil material from said inlet means and circulation of said slurried soil material over an exposed annular surface of said mercury body, a second intermediate annular zone completely filled with said mercury body positioned radially inward of said first zone and confined so as to be conpletely filled with said mercury body, and a third axial zone positioned radially inward of said second zone and partially filled with said mercury body; soil material outlet means for discharging slurried soil material from the interior of said housing means and positioned to communicate with a space above the mercury body in said third zone; said inlet and outlet means and said inner housing means being so constructed and arranged that slurried soil material introduced into said housing means will circulate around said first zone until it penetrates said annular exposed mercury body surface, submerge in and travel circuitously through the confined mercury body in said second zone and exit said mercury body in said third zone and be discharged through said outlet means whereby precious metal particles contained within said slurried soil material will be amalgamated with mercury and settle to the bottom of said mercury body for removal therefrom as an amalgam.

2. The system of claim 1 wherein the mercury - contacting surfaces within said housing means are provided of a metal material that is wettable by said mercury body whereby a mercury barrier is created between said inlet and outlet means through which said slurried soil material must pass in submersion befor it is discharged from said apparatus.

3. The system of claim 1 including soil material conditioning means for providing a slurried soil material havin a predetermined small particle size to said inlet means; a mercury conditioning means for providing a supply of purified a negatively-charged mercury for said mercury body; and amalgam separating means communicating with said mercury body for removing laden mercury from said housing means and separating t amalgam therefrom and for transferring recovered mercury therefrom to said mercury conditioning means for purification before return to said mercury body.

4. The system of claim 1 wherein said housing means is provided with a depressed bottom wall beneath said second zone t provided an amalgam settling region where amalgamated precious metal particles may settle out from said slurried soil material as slurried soil material circulates through said mercury body.

5. The system of claim 1 wherein the mercury - contacting surfaces within said housing means are provided of a metal material that is wettable by said mercury body whereby a mercury barrier is created between said inlet and outlet means through which said slurried soil material must pass in submersion before it is discharged from said apparatus; wherein said housing means is provided with a depressed bottom wall beneath said second zon to provided an amalgam settling region where amalgamated preciou metal particles may settle out from said slurried soil material as slurried soil material circulates through said mercury body; and including soil material conditioning means for providing a slurried soil material having a predetermined small particle siz to said inlet means; a mercury conditioning means for providing supply of purified and negatively-charged mercury for said mercury body; and amalgam separating means communicating with said mercury body for removing laden mercury from said housing means and separating the amalgam therefrom and for transferring recovered mercury therefrom to said mercury conditioning means for purification before return to said mercury body.

6. A system for recovering fine particles of precious metals by amalgamation from soil materials w^τhich comprises an amalgamation apparatus comprised of housing means for containing a body of amalgamating mercury, said housing means being so constructed and arranged that a pressure differential between its inlet and outlet will cause a slurried soil material to be submerged in and circulated through the amalgamating mercury body; the mercury - contacting surfaces within said housing means being provided of a metal material that is wettable by said mercury body whereby a mercury barrier Is created between said inlet and outlet through which said slurried soil material must pass in submersion before it is discharged from said apparatus; soil material conditioning means for providing a slurried soil material having a predetermined small particle size to said housing inlet; a mercury conditioning means for providing a supply of purified and negatively-charged mercury for said mercury body; and amalgam separating means communicating with said mercury body for removing laden mercury from said housing means and separating the amalgam therefrom and for transferring recovered mercury therefrom to said mercury conditioning means for purification before return to said mercury body.