Method and apparatus for extraction of amalgamatable metals from a slurry

A metal amalgamatable with mercury, gold in particular, is extracted from a slurry in which it is dispersed in low concentration, in particular from alluvial sands, by contact with mercury the surface of which is continuously freed of the sulfides which form thereon as a secondary process and is renewed continuously by electrolytic deposit of mercury.

The present invention relates to the extraction of metals and their salts 
which are amalgamatable with mercury, in particular gold, and which are 
present in dispersed form in slurries such as for example alluvial sands. 
In the following description specific reference will be made to gold and 
to alluvial sands it being understood that such reference is not intended 
as a limitation but only as an example. Indeed it is known that with the 
exception of iron almost all other metals amalgamate to a greater or 
lesser extend with mercury so that the method and the apparatus in 
accordance with the invention may be applied in a similar manner. It is 
also known that alluvial sands and deposits contain gold in very small 
proportions on the order of 1 gram approximately for each ton of river 
sand and that said gold is found in various physical forms including 
lamellae, particles occlusions in grains, e.g. quartz, or varying grain 
size. 
It is also know from the literature that gold combines very readily with 
mercury to form an alloy which takes the name amalgam and from which it 
can recoverdd equally readily. 
If an intimate contact is made between the sand and the mercury recovery of 
gold from the sand can reach 100% of the gold contained in the sand. 
Finally, it should not be forgotten that alluvial sands are used in the 
construction industry, for which they are extracted from quarrier, 
manipulated, and handled often in the form of slurries and acqueous 
solutions. 
Recovery of gold from alluvial sand with the amalgam formation method was 
well known in the rather distant past and is still in use today albeit 
sporadically, having been replaced by the cyanide process, and consists of 
passing the slurry or the acqueous solution of sand over a surface made of 
copper amalgamated with mercury preferably in the presence of silver. 
After a certain period of time on the order of at least several days, i.e. 
when the mercury's amalgamation capacity is believed to be exhausted, the 
copper and mercury amalgam support bearing the gold also amalgamated with 
the mercury is sent to treatment for recovery of the gold and optionally 
for restoration of the support. 
Although the preceding description is extremely concise it can readily be 
appreciated that the known method presents two serious disadvantages and 
problems, i.e. it is entirely empirical and allows no control of the 
process and in the second place but not of secondary importance it has the 
disadvantage of totally manual operations. It is clear that these problems 
and disadvantages preclude any modern industrial use of this process. 
On the other hand even the canide process presents not negligible drawbacks 
and risks. Finally it is worth noting that a quarry of not excessive size 
processes daily quantities of gravel and sand on the order of hundreds of 
tons sa that the real interest which the recovery of the gold of such sand 
could have, and indeed has, can be quickly understood. The main problem 
connected with reovery of the gold content of alluvial sands is however 
the formation of a surface film of mercury sulfides. Indeed often because 
of polluting substances contained in river water but also because of 
sulfur and/or sulfides present in alluvial sands, said sulfides combine 
with the mercury to form an impermeable film on the surface of the mercury 
which inhibits amalgamation with the gold. The main object of the present 
invention is is to solve said problems and drawbacks. 
One specific object of the present invention is to provide a method and 
equipment which would allow recovery in an acceptable and profitable 
manner of metals which are amalgamatable with mercury, more specifically 
noble metals and still more specifically silver and gold from alluvial 
sands and acqueous slurries. 
The method in accordance with the present invention of the type wherein a 
support preferably of copper amalgamated with mercury is put in contact 
with the slurry of the acqueous suspension to be processed in such a 
manner that the particles of metal to be recovered by amalgamation, in 
particular gold particles, come in contact with the mercury of said 
support, is characterized essentially by the operations: (a) continous 
removal from the mercury of the surface film which may contain sulfides 
and (b) continuous restoration of the surface on which the amalgam is 
formed with the metal particles to be recovered by electrolytic deposit of 
mercury thereon. 
One embodiment of this method cells for continous recovery of said film 
removed from the support and separation therefrom of the metal to be 
recovered. 
The apparatus in accordance with the present invention is characterized in 
that it comprises a cylindrical surface lined internally with a layer of 
copper amalgamated with mercury and in that is comprises first means held 
in contact with at least one portion of the surface of said copper layer 
amalgamated with mercury for the electrolytic deposit of mercury from an 
acqueous solution of mercury salts said first means receiving a positive 
electrical charge in such a manner as to act as an anode while the cathode 
consists of copper to be reamalgamated and second means for removing the 
surface film from the layer of copper amalgamated with mercury said second 
means being arranged upstream in the direction of movement of the linining 
of copper amalgamated with mercury with respect to the aforesaid first 
means so that the electrolytic deposit of mercury takes place on the 
surface free from sulfide surface encrustations. In conformity with a 
first embodiment of the apparatus according to the invention said 
cylindrical surface is described by a cylinder which turns about its axis 
preferably due to the action of external driving rolls said first means 
consisting of permeable buffers which are held in rubbing contact with the 
inner surface which bears a sleeve or lining of copper amalgamated with 
mercury said buffers being fed with a solution of mercury nitrate in water 
and there being positioned in said buffers electrodes passing through 
inner channels provided for inflow of the acqueous solution of mercury 
salt sand said electrdoes being connected to the positive pole of a 
voltage generator and siad copper sleeve or lining being connected to the 
negative pole. 
In accordace with the above first embodiment the voltage difference between 
the cathode and the anode is greater than that theoretically necessary for 
electrolysis of the acqueous solution of mercury salts and is such that it 
brings about electrolysis of the water so that there is a certain 
development of nascent hydrogen in the cathode area and the parachor of 
the cathodic mercury is attenuated. 
In a second embodiment of the apparatus according to the invention said 
cylindrical surface is formed of a cylinder with a horizontal axis bearing 
a lining of the type described above there being in said cylinder mounted 
in a turning manner a screw feeder fed upstreamt by a force pump for the 
slurry and communicating at the downstream end with a discharge cone 
having a vertical axis and turned upward so as to exploit change in 
velocity to hold back and then capture the gold particles not captured by 
amalgamation. 
Basically therefore in the first embodiment rotation of the cylinder fed 
with slurry causes intimate contact of the slurry and hence with the gold 
particles and the mercury while in the second embodiment it is the screw 
feeder which in its rotation projects the slurry against the inner surface 
of the horizontal cylinder and then against the surface of copper 
amalgamated with mercury. 
To recover the components of the amalgam recourse is made to a normal 
electrolytic cell in which the amalgam to be processed and a normal 
titanium basked coated with synthetic such as meracolon material 
constitute the anode while the cathode consists of copper to be 
reamalgamated. 
The electrolytic solution thanks to which mercury is redeposited on the 
cooper lining is for example an acqueous solution containing 20-30 grams 
per liter of KNO.sub.3 and 50-100 grams per liter of HgNO.sub.3. 
Preferably the solution contains 0.05% of AgNO.sub.3.

DETAILED DESCRIPTION OF THE INVENTION 
With reference first to FIGS. 1 and 2 the apparatus in accordance with the 
invention comprises a cylinder 10 arranged in an inclined manner and 
installed in such a manner as to rotate around its axis 11. For this 
purpose rolling bearings 16 are provided between the cylinder 10 and and 
the fixed casing 12 while rotation is provided by the motor 13 which 
drives the gear unit 14 in engagement with a ring gear 15 which is 
integral with the external surface of the cylinder 10. The casing 12 is in 
turn mounted on a bed 17. 
The internal surface of the cylinder 10 is coated with a layer 18 of copper 
amalgamated with mercury which turns together with the cylinder 10. One or 
more buffers 19 are mounted in such a manner as to be continuously in 
contact with the exposed surface of the layer 18. 
Said buffers are made of porous material in such a manner as to transfer to 
the adjacent surface the acqueous electrolytic solution which is fed from 
the tank 20 to the internal channel 21 of the buffers 19. 
In the channel 21 are also housed tubular electrodes 22 connected to the 
positive pole of a voltage generator of which the negative pole is 
connected to the layer of copper amalgamated with mercury. In this manner 
the solution which impinges on the surface of the layer 18 undergoes 
electrolysis, depositing metallic mercury on said surface. 
An the same time as already mentioned the voltage being higher than that 
necessary only for electrolysis of the mercury salts also brings about at 
least partial electrolysis of the water accompanied by development of 
nascent hydrogen at the cathode. 
For this purpose it has been found that the aforesaid voltage differential 
must be such as to cause the passage of the current in the contact points 
between the buffer 19 and the layer 18 of adequate intensity and voltage. 
From the practical tests conducted with the apparatus just described it was 
observed that the sulfide formation which when operating in accordance 
with the prior art would take place in a few seconds, at the most minutes, 
is completely eliminated. Moreover analysis of slurry issuing from the 
lower end shows that gold recovery is nerarly total. 
It should also be observed that the recovery process in accordance with the 
present invention does not interfere with normal sand processing and 
therefore does not affect normal operations in the quarry. 
Reference number 23 indicates schematically brush or knife means for 
scraping the surface of the layer 18 to remove the surface film formed by 
the amalgam of mercury, gold, and copper, as well as possible sulfide 
encrustations. 
This surface film is transferred, e.g. through channels or by suction, to 
the gold recovery means described above. 
It should be observed that by reason of the permeability of the amalgam of 
copper and mercury the buffers 19 may be positioned outside the layer 18, 
i.e. between the layer 18 and the cylinder 10 integrally with the 
cylinder. In the same manner the brush or knife means could be integrated 
with the buffers or shoes 19 to fulfill the same function. 
Now considering FIG. 3 an apparatus is shown comprising a horizontal 
cylinder 30 lined internally with a layer 31 of copper amalgamated with 
mercury which is renewed and treated in the same manner as that described 
for FIGS. 1 and 2 except that the treatment takes place externally by 
arranging on the surface of the layer of sleeve 31 buffers or shoes like 
those shown in FIGS. 1 and 2 with reference number 19 and similar in all 
ways to them (surface porosity, feeding with an electrolytic solution, 
arrangement of the electrodes) except that the buffers are caused to 
rotate around the external surface of the layer 31 which remains 
stationary. 
For the sake of clearer representation however these details are omitted 
from FIG. 3. 
Inside the cylinder 30 is mounted a screw feeder 32 made to rotate around 
its own axis 35 by drive means, not shown. 
At the upstream end of the cylinder 30 referring to the direction of flow 
of the slurry indicated by the arrows 33 is mounted a feed tank 34 to 
which a pump which is not shown feeds the slurry to be processed. At the 
downstream end of the cylinder 30 opens the mouth of a discharge duct or 
the lower one of a hopper-type separator 35 inside which is mounted an 
agitator 37 the shaft 38 of which is made to rotate by drive means not 
shown. In this manner the slurry undergoes slowing wich allows the gold 
particles which may have escaped formation of amalgam with the mercury to 
fall and to be optionally passed a second time through the cylinder 30. 
In this case the screw feeder projects the slurry violently against the 
sleeve 31 of copper amalgamated with mercury bringing about two 
concomitant effects to wit (a) intimate contact of the slurry with the 
mercury surface and (b) removal of the surface film or sulfide 
encrustations. 
In this case of course at periodic intervals the copper and mercury sleeve 
enriched with gold at withdrawn for recovery of the gold. 
The second embodiment of the apparatus according to the invention may be 
mounted on a mobile carriage allowing it to be transferred to and used in 
areas in which gold is more likely to be present. 
The essential characteristics of the invention have been described but it 
is understood that conceptually equivalent modifications and changes are 
possible and foreseeable without coming our its scope.