WO1989009837A1 - Noble metal recovery process - Google Patents

Abstract

A noble metal recovery process using counter current contact of a flow of slurry with a flow of mercury in droplet form which mercury is collected and filtered to remove amalgams for recovery of noble metals. An electric charge may be applied to the mercury droplets to enhance recovery. Other materials collected by the mercury may be removed in cleaning cycles designed for their removal. The process may be operated on a batch or continuous basis.

Description

TITLE: "NOBLE METAL RECOVERY PROCESS"

FIELD OF THE INVENTION This invention relates to noble metal recovery and in particular to a process and system which has particular utility in recovering gold from a slurry containing the same.

BACKGROUND OF THE INVENTION Mercury has been used for many years in the recovery of gold relying on its innate ability to amalgamate with gold contained in solution. Once an amalgam has been formed between the gold and mercury, it is relatively simple to separate the amalgam from the slurry and subsequently the gold from the mercury.

Previously, it has been known to pass the slurry feed over the mercury so that the gold is brought into contact with the mercury which is generally in the form of a pool at the bottom of a sluice in which the slurry is flowing.

A limitation with this technique is that it relies upon gold, being heavier than other materials withi the slurry, to gravitate towards the bottom of the sluice to come into contact with the mercury.

Although such a technique works satisf ctorily for relatively large particles of gold, for example down to 300 microns in size, the process is not as efficient for recovering finer sized particles of gold, for example down to 75 microns or less in size. The reason for this is that the finer sized gold particles behave in part according to their surface characteristics rather than their own mass resulting in a tendency to float rather than gravitate towards the pool of mercury at the bottom of the sluice.

For fine gold to settle in a sluice and come into contact with mercury, flow rate and turbulence must be reduced to a point where throughput is uneconomically low. To provide for adequate throughput under such circumstances, the plant size is excessive and even then, efficiency in collection of fine gold is reduced by other mercury surface phenomena so that previously used sluice and plate amalgam systems are not considered efficient for fine gold recovery.

OBJECT OF THE INVENTION

Accordingly, it is an object of the present invention to provide for the efficient and more economical recovery of relatively fine gold particles J from a slurry containing the same along with the more readily recoverable gold contained therein.

NATURE OF THE INVENTION In accordance with one aspect of the present invention there is provided a recovery process for noble metal entrained within a slurry, comprising:- generating a flow of slurry; dispersing mercury in counter current exchange with said slurry to substantially pervade said slurry during its passage therethrough so that said noble metal may amalgamate therewith; accumulating said mercury after its passage through said slurry; extracting the amalgam from the mercury; and extracting said noble metal from the resultant amalgam to recover said noble metal.

In accordance with another aspect of the present invention, there is provided a noble metal recovery apparatus for recovering noble metal from a slurry having said noble metal entrained therein by the above process comprising:- means to establish a flow of slurry; means to disperse a spray of mercury into the flow of slurry; a well for the accumulation of mercury; extraction means whereby accumulated mercury may be removed for collection of noble metals; a well disposed at said bottom for accumulating said dispersed mercury after passing through said slurry; first extracting means to remove said mercury from the well; a vessel to receive the accumulated mercury; second extracting means to remove said noble metal from the resultant amalgam; and means to create a flow of said slurry within said container whereby slurry containing said noble metal may be constantly supplied to said inlet region and treated slurry may be discharged from said outlet region.

Throughout the specification 'counter current exchange' is to be taken to encompass not only flows that are anti-parallel but also other configurations wherein a contacting of two flows is effected. The two flows which are contacted may include either of mercury flow through a slurry or mercury flow through another liquid such as a cleaning fluid. In the contacting of the two flows, the flow of the fluid to be contacted with mercury may be established in a vertical flow column, down a spiral surface or over a cone. In each case, mercury droplets are sprayed, dispersed, injected or otherwise created in the flow in a manner which enables mercury droplets to move through the slurry to a collection point or points. The intermixing of the two flows may be enhanced by the addition of paddles, baffles, etc. The creation of mercury droplets of particular size depends upon a plurality of factors such as the configuration of jets used, their size and orientation as well as pressures in the mercury circuit. Size of droplets can be a significant factor. Too small a drop and surface tension is too high for effective amalgamation. Too large a drop and the volumes of mercury that are to be pumped are high.

The recovery process of the present invention may be operated in either a batch or continuous mode. In a continuous plant, throughput can be increased by scaling up individual counter current devices and/or by connecting a plurality individual counter current devices in parallel. So as to increase residence time of a slurry within the apparatus, a series arrangement might be adopted. Residence time may be increased by other means as set out below. To increase residence time and to increase throughput clusters of series connected individual counter current devices may be connected in parallel. Of course, individual units may be scaled up or down to suit requirements within the clusters.

Residence time is a factor of things such as the number of columns the slurry is flowed through, the slurry velocity, the height of a particular column, etc. The velocity of the slurry is dependent upon factors such as feed pressure, column geometry etc.

To enable an electric charge to be applied to the mercury droplets,- an electrode may be contacted to the mercury in the mercury circuit such that droplets are charged on formation. If this is to be effected, the apparatus employed in performing the recovery process must be non-conducting and materials such as fibre glass and plastics are usefully employed. Carbon provides a useful electrode for applying electric power to the mercury circuit.

The mercury that is contacted with a slurry will pick up a variety of materials besides noble metals and depending upon the chemistry of a particular material, various techniques may be employed to remove it. Cleaning fluids may be applied to the mercury. Such fluids may be contacted with the mercury in a mercury circuit wherein contacting takes place in a counter flow arrangement. Individual cleaning units may be connected in series, clustered and connected in parallel as is done in the slurry circuit. 5 In both the slurry circuit and the mercury circuit, temperature may be a factor and temperature control may be effected.

Filters for collection of amalgams may be connected in parallel to increase capacity.

I^'D^' Arrangements may be made for back flushing to remove collected amalgams. Sintered metal filters may be used to withstand the substantial forward and back flushing pressures in the mercury circuit.

The process of this invention takes advantage

15 of the following properties of auriferous material and mercury and available equipment. a) Relative density.

There is a wide difference in specific gravity between mercury and gangue which enables counter 20 current flows with mercury falling with or without the force of the jet behind it through a flow of slurry. b) Gold/mercury affinity.

Affinity of non-contaminated mercury to amalgamate 5 gold and silver (small amounts of very fine gold retained within mercury, enhances this affinity) . c) Mercury impurities.

Certain impurities such as sulphides, and oxides, float to the surface of mercury, thus allowing 0 noncontaminated or less contaminated mercury to be pumped from the bottom of a reservoir pool, without contamination or with less contamination. d) Contact surface area.

There is an increase in mercury contact surface 5 available, when a given mass of mercury is pumped through jets to produce a "shower" of mercury droplets . Contact surf ace area affects amalgamation . e) Relative flow. Dependent on the size of the shower droplets produced, and/or pressure at the jet or the jet characteristics, it is possible to determine droplet size and to arrange the flow of the ore slurry so that its upward flow velocity is less than that due to gravitational free settling rate of the mercury shower through the slurry. f) Mercury filter.

Various sintered stainless steel filters are now available in useful aperture sizes. The significance of the availability of such a wide range of filter elements is that a major obstacle preventing automatic gold amalgam filtration has now been removed. That is, mercury containing gold can now be filtered down to 0.5 microns. An automatic circuit has been designed, which in one flow direction allows mercury to be filtered. In the opposite direction, the filter may be back- flushed with mercury (preferably not air) to recover the precious metal. In this situation it is important that the filter element itself will withstand high differential pressures in both directions, and that it will not react with mercur . g) Available pumps. A pump should have the following features:-

- ability to "stall" at a predetermined pressure and hold this fixed pressure without consuming excessive power or generating excessive heat.

- to have no heat, flame, or spark risk, and operates at ambient temperatures up to 50° celsius. - infinitely variable speed and output.

- capability of high output pressure with one stage.

- ability to interface with automatic controls. - ability to tolerate repetitive stop/start commands.

- if air driven, must not require an external line lubricator, thus preventing oil vapour contamination of the surrounding environment, and perhaps the ore or mercury itself.

- reliability.

- compactness.

- ease of maintenance (preferably mechanical only) .

- long life between overhauls. - "wetted" parts must not be materials capable of amalgamation, or reaction with mercury.

- hydraulic seals must prevent mercury escape into the atmosphere or air supply during operation over the pressure range used. DESCRIPTION OF THE DRAWINGS

The invention will be better understood in the light of the following description of several specific embodiments thereof. The description is made with reference to the accompanying drawings wherein:- FIG. 1 is a side elevation of a fine gold recovery plant in accordance with the first embodiment; FIG. 2 is a plan view of FIG. 1; FIG. 3 is a schematic view of the gold recovery system used in the plant described in the first embodiment;

FIG. 4 is a plan view of the plant in accordance with the second embodiment;

FIG. 5 is a plan view of an alternative gold recovery system for use in the second embodiment; FIG. 6 is a sectional elevation of the gold recovery system in accordance with the third embodiment; FIG. 7 is a plan view of FIG. 6; FIG. 8 is a schematic view of a column and associated mercury and slurry circuits to facilitate explanation of the concept of the gold recovery system described in any of the embodiments;

FIG. 9 is a plan view of a group of four columns connected in series which form part of the gold recovery system in accordance with the fourth embodiment; FIG. 10 is a schematic drawing of FIG. 9 showing the serial connection between the columns;

FIG. 11 is a plan view of a cluster of four groups of columns in accordance with the fourth embodiment; FIG. 12 is a sectional elevation of a cluster of groups of columns;

FIG. 13 is a plan view of a cluster of ten groups;

FIG. 14 is a plan view of a cluster of fifteen groups;

FIG. 15 is a plan view of a cluster of twenty groups; and

FIG-. 16 is a schematic elevation of the test tank used in the test plant described in the test embodiment.

PREFERRED EMBODIMENTS

The first embodiment is directed towards a pilot plant for fine gold recovery having a capacity of approximately one tonne per hour. This plant uses a fine gold recovery system comprising one cluster of columns.

As shown at FIGS. 1 and 2 of the drawings, the plant comprises two principal stages: a feed preparation stage; and a gold recovery stage. The feed preparation stage comprises a feed hopper 1, a variable speed belt feeder 2, a conveyor 3, an attritioner 4, .a conditioner 5, a pump 6, a vibratory sieve bend 7, a slurry hopper 8 and a pump 9. The feed hopper is adapted to receive fine auriferous material which is conveyed along the belt feeder 2 to be deposited upon the conveyor 3. The conveyor transports the material to the attritioner 4 where the material is scrubbed and disintegrated into -. its individual particulate components for the purposes of creating a slurry. From the attritioner 4, the material is passed into the conditioner 5 where suitable additives and water are combined with the particulate material to form the slurry of desired density. The subsequent slurry is pumped by the pump 6 to the sieve bend 7 where the gold containing fraction is separated from the rest of the slurry and reports to the hopper 8 and pump 9 to the gold recovery stage.

The gold recovery stage comprises a system 10 particularly adapted for fine gold recovery, i.e. gold of a particle down to a size of less than approximately 350 microns. The system 10 in the present embodiment comprises a single contactor column 13 constructed of plastic material which forms part of a cluster (not shown), a mercury circuit 15 and a slurry circuit 17, as shown at FIG. 3.

The column is formed with a circular side wall 19 and a bottom 21 into which is incorporated a well 23. The column is approximately 6 metres in height and is of a diameter of approximately 4000 millimetres. The top of the column is sealed with a removable lid 25 to which is mounted a coupling 27 for an inlet pipe of the slurry circuit 17 and from which is suspended a continuation of the feed tube which extends from the top of the column all the way down to a location spaced from the bottom 21. The tube is of a diameter of approximately 100 millimetres consequently defining an annular region between the outer surface of the feed tube and the inner " surface of the side wall 19 of the column.

The actual location of the draft, tube is better shown at FIG. 8 of the drawings and shall be described in more detail in the third embodiment of the description.

The side wall 19 of the column is provided with an outlet port 29 disposed proximate to the top of - the column and to which is connected an outlet pipe 47 of the slurry circuit.

The mercury circuit 15 of the system comprises a dispersing means 31 disposed within or around the annular region of the column, the well 23, a recirculating pipeline 33 and a mercury pump 35.

The dispersing means 31 is disposed towards the top of the column below the outlet 29 and comprises a circular manifold 37 concentrically disposed about the central axis of the column and to which is mounted a series of jets 39 disposed at equal angular locations around the manifold and each of which are directed axially downward relative to the column. The pump 35 is adapted to pump mercury accumulated within the well 23 along the pipeline 33 to the dispersing means 31 for dispersing within the annular region of the column. The mercury circuit 15 also includes cleaning means and extracting means, which shall be described in more detail in the third embodiment, for cleaning and extracting gold from a resultant gold amalgam containing mercury formed in the column to enable fine gold recovery.

The slurry circuit 17 comprises an inlet pipe 41 which is connected to the coupling 27, a slurry pump 43, a hopper tank 45 and an outlet pipe 47 connected to the port 29 of the column. The hopper tank includes a set of conditioner impellers 49 to circulate slurry within the tank and facilitate conditioning of the same as is necessary. The pump 43 is provided to pump the conditioned slurry from the tank 45 along the inlet pipe 41 and into the column 13 via the axial feed tube. Accordingly, the inflowing slurry is directed downwardly along the feed tube towards the bottom 21 whereupon the. slurry flow is inverted to enter the annular region of the column and flows upwardly in a circumfluous manner back, towards the top of the column. The outflowing slurry is subsequently discharged through the outlet port 29- σf the column where it flows through the outlet pipe 47 either to repeat the process in another column in the cluster and/or ultimately to tail.

Now describing the operation of the system 10 as shown at FIG. 1, conditioned slurry from the hopper tank 45 is pumped through the slurry feed line 17 in the manner previously described to pass vertically upwards via the annular region of the column at a velocity marginally greater than the settling velocity of the slurry. Conversely, mercury is pumped through the mercury circuit 15 in the manner previously described to be dispersed into the annular region of the column so as to pervade the rising slurry in a counter-current exchange. Thus an effective mercury shower is provided which gravitates downwardly through to slurry to eventually accumulate within the well 23.

In this arrangement, an extremely high probability of contact between gold entrained within the slurry and the mercury is provided to enable amalgamation of the two and thus a high percentage of gold recovery is attained from the slurry.

It should be noted that a particular advantage of the present embodiment is the cross sectional area of the annular region of the column. Accordingly this feature enhances the treatment of a relatively high volume of slurry in a given time and thereby increases throughput. To the extent that a number of such columns are used, throughput is increased to greater efficiency than in other systems. In order to increase residence time, the present invention presents a simple manner in which this can be achieved by simply adding additional columns, and/or by reducing the upward flow velocity within the annulus and/or varying the geometry, height, cross-sectional area of individual columns. Different embodiments of this arrangement are described in later embodiments of the description.

The second embodiment is substantially similar to the preceding embodiment, except that it is directed towards a plant which provides a high throughput and also higher residence time for the slurry to improve the amount of gold recovery.

As shown at FIG. 4 of the drawings, the feed preparation stage of the plant is much the same as that of the previous embodiment where a feed hopper 51 is provided with a variable speed belt feeder 53 to feed a conveyor 55. The conveyor 55 in turn feeds an attritioner 57 which is in turn connected via a wet screen 59 to feed a conditioner 61. The conditioner discharges slurry to a^" pump 63 for feeding to a sieve bend 65, and discharges oversized material via a chute 67 which feeds to a sluice 69 for conveying the oversized material to the tails hopper 71.

The sieve bend 65 discharges slurry material to a secondary sieve bend 73 via a pump 75. Both sieve bends are provided with discharge chutes for oversized material, the first sieve bend discharging oversized material to the sluice 69 and the second sieve bend discharging oversized material to another sluice 77 for conveyance to the tails hopper 71. The tails hopper 71 feeds oversized material via a pump 79 to tails. The gold recovery stage employs a system 80 comprising a group of four columns 81 which are connected serially so that the first column 81a has its inlet pipe 83 connected to the outlet of the secondary sieve bend 73 via a pump 85, the second column 81b has the inlet pipe thereof connected to the outlet pipe 86 5 of the first column 81a, and so on until the last column 81c has its outlet pipe 87 disposed to discharge treated slurry onto the sluice 77 for conveyance to the tails hopper 71.

By employing four columns, a typical

10. throughput of approximately 10 to 12 tonnes per hour and a typical residence time of about 90 seconds or more is obtainable with a slurry of around 20% solids by weight. It should be noted, however, that the residence time is influenced materially by the slurry particle size

15 derived from the underflow of sieve bend 73, and the dry specific gravity of the solids therein to be treated, since this determines the settling velocity of the slurry in the contactor columns and thereby the minimum pumping rate required from pump 85.

20 In an alternative arrangement of the gold recovery system, as shown at FIG. 5 of the drawings, the output chute 89 of the secondary sieve bend 73, instead of feeding oversized material directly to the sluice 77 and hence direct to the tails hopper 71, alternatively 5 feeds oversized material to a secondary conditioner 91 and distributor 93 to spirals 95. In this arrangement the spiral concentrate is reintroduced to the inlet of the slurry pump 85 for combining with the undersized material from the secondary sieve bend 73 and conveyance 0 to the inlet pipe 83 of the first column 81a. The distributor 93 directs material from the secondary conditioner 91 to spirals 95 either side for eventual discharge to tails. Consequently, the output line 87 of the last column 81c is also directed straight to the 5 tails. The alternative arrangement of the gold recovery system has the advantage that the oversized material from the secondary sieve bend 73 is treated with spirals 95 instead of a sluice 77 and thereby is possibly more efficient in certain operations.

It should be noted that in both the first and second embodiments, the plants are only adapted to process clays and fine slurry material but not boulders, which would normally be required to be separated from fine material or to be subjected to a crushing process before being subjected to treatment.

The third embodiment is directed towards an actual production plant which can provide a fine feed fraction throughput typically of up to 70 tonnes per hour or more, with a slurry of 20% solids by weight in a modular design to easily enable multiplies of this throughput to be attained as is desired.

In the present embodiment, the feed preparation stage is substantially identical to that described in the preceding embodiments and hence shall not be expanded upon. However, the gold recovery stage employs a system which is modularised and which can be clustered.

As shown at FIGS. 6 and 7 of the drawings, the system comprises a cluster of groups of columns 101 housed within a tank 103 of approximately 3 metres in diameter. The columns 101 are organised into serial groups comprising two or more columns. For example, as shown at FIG. 7 of the drawings, the columns are arranged into five groups of three each group comprising a first column 101a, a second column 101b and a third column 101c, serially disposed as in the previous embodiment.

Each group is disposed at equal angular locations about the central axis of the tank 103 along the inner circumferential wall of the tank. Furthermore, each group is connected to a common feed and discharge distributor 105 located centrally on the said tank through which slurry material is fed into and discharged from the various groups of columns via their inlet pipes 107 and outlet pipes 109 respectively.

As in the previous embodiment, the inlet pipe 107 is connected to the top of the first column 101a centrally and the outlet pipe 109 is connected- to the side of the last column 101c for discharge to tails. Accordingly, the outlet of the pipe of the first column is connected directly to the inlet pipe of the second column 101b and the outlet pipe of this column is connected to the inlet pipe of the next, which in the present embodiment is the third column 101c. To faciliate the discharge of slurry from one column and inlet to the next, the columns in each group effectively decrease in axial length to maintain a decreasing gradient for each of the feed pipes but a different arrangement may be used to ensure that the length of each column is constant.

The arrangement of the feed and discharge distributor 105 is better shown at FIG. 6 of the drawings, wherein a main discharge pipe 111 is connected to a central chamber 113 of the distributor 105 and a main feed supply pipe 115 with a check valve 115a is connected to an annular chamber 117 disposed about the outer circumference of the discharge pipe 111 proximate to the central discharge chamber 113. Futhermore, the annular chamber 117 is connected via an elbow to each inlet pipe 107 of each group of columns 101, and the central chamber 113 is connected to each of the outlet pipes 109 of each group. Thus, all of the columns are supplied with slurry from a common axial feed source and all columns discharge slurry eventually to a common axial discharge destination. By adopting this particular arrangement, the design of the system is modularised in that all inlet pipes are of a common size and shape and similarly all outlet pipes are of a common size and shape. Furthermore, each group of columns is idential thereby modularising the various columns _which constitute the groups.

Another important aspect of the present embodiment is that the feed and discharge distributor is located: towards the top of the tank 103 thereby providing a large central inspection/work space to all of the columns within the confines of the tank 103. Consequently, it is possible to locate the mercury circuit 119 within this work space and arrange it so that a common source of mercury is provided to each of the columns of the group thereby reducing duplication of the external components of the circuit and hence reducing costs in construction.

Now describing the construction of each column and the mercury circuit thereof in more detail, reference should be made in particular to FIG. 8 of the drawings. As can be seen from this drawing the contactor column 101 comprises a circumferential side wall 121 and an outwardly convex bottom 123. The top of the column is sealed with a lid 125 to which is attached the depending feed tube 127. As is shown, the feed tube 127 extends all the way from the top of column to terminate at a distal end 127a marginally spaced above the bottom 123 of the column. Thus, the feed tube 127 defines an annular region 129 between the outer surface thereof and the inner surface of the side wall 121 which extends along the full axial extent of the tube.

The top of the tube 127 communicates with the inlet pipe 107 via an appropriate coupling. An outlet port 131 is formed in the side wall 121 proximate to the top of the column so as to communicate with the annular region 129 of the column. The port 131 includes a coupling 133 which in turn is connected to the outlet pipe 109 associated therewith. In the drawings shown at FIG. 8, the outlet pipe connects directly to the central chamber 113 of the distributor 105 via a partitioned chamber 135. A pressure gauge 137 and slurry valve 139 are provided in the outlet pipe for control purposes.

The bottom 123 of the column incorporates the well for accumulating globules of mercury which gravitate down the column in the mercury circuit to form a pool 141. The surface of this pool is disposed marginally below the distal end 127a of the feed tube to be exposed to the slurry as it exits the tube. A drain is provided in the bottom of the well to drain the pool of mercury 141 and convey the mercury to a cleaning means and gold extracting means which form part of a mercury treatment means of the mercury circuit. Accordingly, a transfer pipe 143 connects the drain to a mercury pump reservoir 145 via a strainer 147 which incorporates a coarse filter of approximately 350 micron aperture to filter out extraneous complex precipitates from the mercury pool which could damage the mercury pump.

The pump reservoir 145 forms part of the cleaning means and is sealed to define a chamber which is completely filled by a residual volume of mercury 149 at the bottom thereof and a quantity of oxidant 151 at the top thereof.

The residual volume of mercury 149 is continually drained by way of the outlet pipe 153 which is connected to a mercury pump 155 for pumping the mercury up through a recirculating pipeline 157 to a charging reservoir 159 of a charging means disposed towards the top of the column, which also forms part of the mercury treatment means. The gold extracting means comprises a fine gauge extracting filter 161 which is connected in a feed back arrangement in the mercury circuit between the mercury pump 155 and the pump reservoir 145. Thus high pressure mercury in the circulating pipeline 157 is bled through the filter 161 to return to the low pressure pump reservoir, so that the filter can extract any gold amalgam residing within the mercury.

Describing the cleaning means in more detail, the:- oxidant 151 within the pump reservoir 145, being of a- lesser- specific gravity than the mercury 149, resides above the residual volume of mercury and is used to clean the mercury of base metal amalgams by reacting with the same so as to leave the comparatively inert gold amalgams within clean mercury. Accordingly, a supply of oxidant in the form of sulphuric acid or other proprietory products such as "MCC40" (trade mark) is contained within a sealed storage reservoir 163 equipped with a safety valve 165. The storage reservoir 163 is connected via a high pressure delivery pipe 167 and pump 169 to the mercury pump reservoir 145 and the latter is connected to the former by a return pipe 171 to complete the oxidant cleaning circuit. The delivery pipe 167 discharges acid into the residual mercury volume 149 to bubble up through the mercury cleaning the same.

The annulus 129 is connected via a tube 253 to a pressure transmitting device 254 which contains a rubber bladder 255 sealed to the inlet side of the device so as to apply pressure against a sealed air enclosure 250 and valve 251. The fluctuations of pressure within the annulus 129 are transmitted to the fluid contained behind the rubber bladder to the area 249 and thus via the tube 248 to the storage reservoir 163 thus maintaining a balance between the variable pressures applied to mercury both in the column pool 141 and the reservoir 149. In an enhancement of the cleaning circuit, a venturi suction pipe (not shown) may be connected between the transfer pipe 143 and the pumping reservoir 145 to communicate with the oxidant therein, whereby the 5 passage of mercury through the transfer pipe 143 creates a venturi which sucks oxidant through the venturi suction pipe from the reservoir into the mercury flow within the transfer pipe 143 so as to admix oxidant therewith and release the same into the residual volume

10 of mercury 149 upon entering the pump reservoir 145.

In a further enhancement of the cleaning circuit, a separate quantity of oxidant such as sulphuric acid, ferrous chloride or potassium di- chromate contained within a separate storage reservoir

15 173 can be introduced into the well directly via a discharge tube 175 and pump 177 so as to trickle down into the pool of mercury 141 at a point above the drain of the well. Accordingly, surface impurities on the mercury within the pool are cleansed directly where the

20 oxidant reacts chemically with extraneous base metal amalgams which disburse into the updraft slurry stream and leave behind a mercury pool 141 with a clean surface.

The fine grade extracting filter 161, as shown

25 in the drawing, is provided with an inlet valve and coupling* 179 and an outlet valve and coupling 181 to enable removal of the filter from the mercury circuit to facilitate recovery of gold amalgam retained therein. An air inlet port 183 is provided on the inlet side of

30 the filter to connect to a compressed air supply to facilitate discharge of residual mercury within the filter prior to its removal from the circuit.

The mercury pump 155 preferably employs a continuous pumping action, although the system can still

3.5 function quite adequately with a pulsating pumping action. A pump by-pass 185 and valve is provided to allow the mercury line 157 and tank 145 to be drained and also to allow the filter 161 to be positioned at the lowest point within the circuit to maximise the differential pressure across the filter 161. The mercury circulation stainless steel pipeline 157 introduces mercury into the bottom of the charging reservoir 159 via a heat exchanger 157a (where necessary) and a one way check valve 157b of approx. 50 psi to establish a residual volume of mercury 187 within -^: the; reservoir, within which an electrode connected to the negative side of a cell 189 via an electrical connection 191 is immersed. The remainder of the charging reservoir 159 is occupied by an electrolyte such as salt or the proprietory product "MCC50" (trade mark) into which is immersed a carbon electrode connected to the positive side of the cell 189 via an electrical connection 195.

By electrolysis, an effective negative charge is given to the mercury which is subsequently passed from the charging means to an inlet manifold 197 via a discharge pipe 199 at low pressure and at a controlled rate of flow so that mercury globules produced at jets 201 are not too small.

The mercury heating means 157a may be interposed between the gold extracting means and the charging means. The heating means comprises a heat exchanger to elevate the temperature of the mercury in the circulation pipeline 157 during its passage to the charging means to improve its amalgamation efficiency when low temperature slurries are treated, since heat enhances the ability of mercury to amalgamate with gold.

As described in the previous embodiments, the inlet manifold 197 is circular in shape and is disposed preferably within the annular region 129 of the column at a location approximate the top thereof. The manifold 197 is provided with a series of equally angularly spaced jets 201 which sprinke globules of mercury into the annular region of the column in a counter-current downward direction to the rising slurry within the region. The size of the jet aperture is quite critical 5 to the performance of the system as is the rate of flow of the mercury therefrom. Essentially, a delicate balance between a sufficiently small globule size to. pervade the slurry and a sufficiently large globule size to prevent flouring of the mercury is required to be

10. obtained. Accordingly, an aperture size of around 2 mm and a flow rate of about two feet per second for a feed size of typically minus 350 microns and a buoyancy velocity of about 75mm per second using riverbed sand of a specific gravity of about 2.65 has been found to be

15 generally effective. Accordingly, with departures from this, variations in parameters of the feed density, feed size and rate of flow and jet specification may require further varying to optimise recovery.

A pressure intensifier might be applied to the 0 mercury circuit. A large pneumatic cylinder is used to force oil from a small chamber at a pressure ratio consistant with the relevant sizes of the driving pistons to create a higher down stream hydraulic line pressure for example:- a differential in piston size of 5 10:1 would multiply downstream pressure by 10.

A reciprocating intensifier may be used and the Hasker air oil pump is an example of a satisfactory type for this purpose. It is air driven and can be stalled without damage at full operating pressure. 0 A function of the pumping circuit is that the mercury is to be filtered to collect the amalgamated particles which can be less than 2 micron. By nature, filters this fine have an extremely high pressure drop which increases as they are "blocked". As mercury is a :5 metal, it is not compressable and as reliefs cannot be used, it is necessary to stop pumping and hold that pressure. Air/oil intensifiers by nature will stand on load, if load (pressure) falls, then the pump will immediately react to deliver full pressure. The circuit can be provided with an auto change over which allows 5 the fluid to back flush the filter, passing this fluid into a second filter. The process can be a continuous operation. The flow can be divided into two pumps with one pump providing continuous flow at pressure through the contactor circuit. The second pump can be used more

10. specifically for mercury cleansing operations. When the filter is full (blocked) this condition is indicated by a higher brake pressure within the filter circuit and the system can automatically reverse to allow the mercury and the entrained mercury/gold amalgam to

15 reverse its flow within -the filter chamber, thereby back flushing the filter with contained mercury (not air) . The pneumatic control circuit is totally separate to the fluid circuit but as the pump is ratio relative it is possible to monitor drive pressure to effect control and

20 by regulating pressure it is possible to effect total control.

As the load increases so does the drive air pressure. By sensing this pressure, it is possible to control the operation. By using a pneumatic timer in

25 the circuit, the time used for this cleaning may be controlled. The system then becomes automatic. As the filter blocks, pressure increases, so does drive pressure. Once this reaches a set point, the system reverses so that the flow cleans the filter for a

30 selectable period of time. At the end it reverts to normal operation. Product from the cleaning process is collected in a separate filter from which it may be removed for final process.

With regard to the operation of the mercury

35 circuit, it is found that by charging the mercury and creating an effective shower of mercury to pervade the rising slurry, considerably higher percentages of gold recovery from the slurry can be obtained, upto 96% or more, within a far shorter time at a far greater throughput in comparison to prior art gold amalgamation recovery systems. In particular, the electrical charging of mercury causes a greater attraction of metals to the mercury to form amalgams than is the case in the absence of electrical charging but unfortunately, there is little differentiation between gold and certain other metals. Therefore, it is necessary to incorporate the cleaning circuit into the mercury circuit to reduce base metal amalgams so as to preferably leave only the gold amalgams for extraction by the extracting filter 161 from the mercury. In order to supplement the cleaning circuits which employ strong oxidants to react chemically with the unwanted amalgams, lime, plumbic or copper ions may be added into the slurry prior to entering the columns.

An additional aspect of the system comprises placing a charging plate near the distal end 127a of the draft tube 127 so as to impart a positive charge to the slurry upon it exiting the draft tube and being exposed to the mercury.

The fourth embodiment of the description is directed towards a plant using a cluster configuration for the columns in the system which is marginally different to the configuration described in the preceding embodiment. In this embodiment a greater residence time is achieved by using four columns in a group, whilst still maintaining the cluster type of configuration as previously described.

As shown at FIGS. 9 and 10 of the drawings, four columns 203 are arranged in a square formation in plan to form a group having the various inlet and outlet pipes 205 thereof interconnected so that the slurry flows serially through the columns as sourced from a main feeder pipe 207 connected to the first of the columns and a discharge pipe 209 connected to the last of the columns within the group.

As shown at FIG. 9 of the drawings, the

5 columns 203 are interconnected by an internal rigid framework 211 which also support the various slurry and mercury circuits, rather than a tank as described in the previous embodiment.

Using four columns arranged in this manner and

10. each: being of similar specification to those previously described would provide a throughput of approximately ten to twelve tonnes per hour. This would be generally satisfactory for most small plants, however, in larger plants it is necessary to increase the throughput well

15 beyond this capacity.

In order to achieve this increased throughput, each group of columns may be connected to a common distribution and discharge head 213 where the various groups _^ of columns 215 are arranged at equal angular

20- locations about the distributor and are at equal radial spacings. In this manner the feeder pipes 207 and discharge pipes 209 of each group 215 which interconnect a group of columns to the head 213 are of equal length providing a degree of modularity as well as maintaining

25 equal friction pumping losses. Furthermore, by adopting this particular configuration the throughput of the plant can be easily increased by increasing the number of groups of columns within the cluster. Thus, using four such groups as shown as FIG. 11 of the drawing, a

30 throughput of 40 to 50 tonnes per hour may be attained, using ten such groups in a cluster as shown at FIG. 13 of the drawings a throughput of 100 to 200 tonnes per hour may be attained, using 15 groups in a cluster as shown at FIG. 14 a throughput of 150 to 180 tonnes per

35 hour may be attained, and using 20 such groups in a cluster as shown at FIG. 15 of the drawings a throughput of 200 to 240 tonnes per hour may be achieved.

In order to handle these large size plants, the distributor 213 is required to be much larger than 5 that adopted in the previous embodiment, wherein the head is divided up into a slurry supply head 213a and a slurry discharge head 213b. The slurry supply head 213a is fed with slurry from a main supply line 217 and distributes the same to each feeder pipe 209,

Iff interconnecting the head and the respective groups 215. The supply head is disposed at an elevated location relative to each of the groups 215 of the plant. The common discharge head 213b is disposed below the supply head 213a to receive slurry from each of the discharge

15 tubes 207 and which directs the discharge slurry via a syphon breaker 207a to a main outlet pipe 219 for discharge to a tails dump. Another way to increase throughput is by altering individual units in the clusters. 0 The utility of the gold recovery system is demonstrated in the following description of the test embodiment which was performed using a tank and impellor unit as shown at FIG. 16 of the drawings. A programme was devised of different experiments each involving a 5 change in one or more parameters of the system considered to have an effect on the rate of gold recovery using the tank. In order to eliminate assay and sample errors, experiments were conducted on relatively coarse, barren, silica sand of minus 500 0 microns size to which pure gold of minus 20 microns was dosed.

As shown at FIG. 16 of the drawings, the tank impellor unit used in the test comprised an acrylic tank 221 which was constructed and mounted on a test frame 5 (not shown). A sub-frame (not shown) was adjustably mounted to the test frame and supported a variable speed drive motor 223 attached to an impellor 225 via a drive shaft 227. The impellor was permanently mounted inside a plastic draft tube 229 whereby the sub-frame assembly including a variable speed FHP controller 231 for the motor could be racked up and down axially relative to the tank upon the completion of each test for measurement and cleaning purposes.

The impellor 225 was constructed of stainless steel and the drive shaft and impellor were coated in polyurethene. . In each test the draft tube and impellor assembly was adjusted so that its vertical height from a mercury pool 233 disposed in the well 235 at the bottom of the tank was constant.

The bottom of the draft tube was fitted with a steel ring to which a 3mm open area stainless steel wire mesh screen 237 was fixed for later use as an electrode analogous to the charging plate described in the third embodiment. The impellor 225 was of conventional design so that known RPM/flow rate data could be translated into meaningful ongoing scaled up proportions pertinent to the test.

Into the top of the tank and located centrally in the annular region 239 defined between the outer wall of the draft tube and the side walls of the tank was disposed a circular plastic mercury outlet manifold 241 within which open, plastic jets were inserted. The palstic jets 243 were spaced at 30mm intervals around the circumference of the manifold and were oriented to spray directly downwards. The impellor 25 by controlling the electric motor could be varied in speed from zero to approximately 900 rpm. In practice, the speed used was approximately 475 rpm.

The mercury pool at the bottom of the tank was pumped by a 3 inch diaphragm dosing pump 245 the outlet of which was connected to the mercury manifold via a plastic pressure hose 247 and fittings. By operating the impellor 225 to pump slurry within the draft tube downwardly, as indicated by the arrows, towards the mercury pool an uprising or outflowing slurry was created within the annular region 239 between the tube and wall of the tank whereby the downwardly flowing slurry was inverted at the bottom of the tank to flow upwardly through the annular region 239 in a circumfluous manner to the inflowing slurry within the draft tube. Now describing the test technique in more detail, silica sand was hand screened with a 500 micron stainless steel sieve. The undersize was heaped and left for three weeks to dry on the laboratory floor. Each test sample was physically weighed on a Sartorius balance according to the following table:-

Water 60 litres (tap water) Sand 15% weight per weight 9.9 kilograms 20% weight per weight 13.7 kilograms 25% weight per weight 17.8 kilograms 30% weight per weight 22.1 kilograms

A beaker was tared on a weighing balance accurate to two decimal places and a mass of one gram of -20 micron gold was added to the beaker. This was then covered with approximately 20mls of methylated spirits. Prior to dosing considerable mechanical effort went into ensuring the gold particles were dissociated.

Mercury was measured in a measuring cylinder to the volume nominated in the table of test results provided at Table 1 herewith. Tap water was added to the tank to a predetermined measured 60 litre mark on the tank. Next, the desired volume of mercury was added to the tank and the mercury pump commenced. The impellor was then started and set to 475 rpm, whereupon the sand mass was added to the tank. The dissociated gold in alcohol was then added to the tank and the starting time recorded. The tank impellor was operated for a nominated residence time.

Upon the lapsing of the residence time, most of the mercury was drained from the well and recovered from the pipelines by blowing with compressed air through the mercury lines. The balance of mercury entrained in the slurry was recovered by panning the residual slurry to recover any mercury contained therein. The recovered mercury was then measured and prepared for acid digestion. This involved removal of all slurry particles with distilled water and then placing the mercury obtained in a container and heating it whilst adding concentrated nitric acid until only the gold remained. Alcohol was added to the resultant gold to depress any floating gold. Excessive alcohol was poured off and the resultant mixture heated until dry. After drying, the recovered gold was cooled off and weighed on a balance. This was then recorded and expressed as a percentage of the gold mass introduced in each test.

The various tests performed are summarised below and the recorded results were tabulated. The table is reproduced at Table 1. TEST 1 - 3 Initially, the mercury shower system was not used, and time intervals were reduced from 30 minutes to 5 minutes.

At this point the mercury volume was 500 mis and this resulted in a particular level (and given surface area) of mercury in the bottom of the tank.

The application of 2.2 volts, as anticipated, had no effect, but a very definite fall off in gold recovery was recorded as time intervals were reduced from 30 minutes (92%) to 5 minutes (29%). The impellor remained at 700 rpm. TEST 4 It was decided to conduct one test whereby the contents of the mercury sump was drained off after 15 minutes and mercury was added to this material outside the tank. The recovery recorded was 4% indicating that most fine gold was suspended in the slurry more or less uniformly. TEST 5 - 8

Four tests were conducted to test the application of an applied voltage. To do this, a voltage, was applied between the draft tube electrode and the mercury within the pump. Test conditions were recorded in Table 1: notwithstanding the reduction in mercury surface area by reducing the mercury volume to 400ml (20% reduction) gold recovery doubled for the same time .interval.

Notably, this occurred with or without the addition of salt as an electrolyte, but recovery fell off at higher voltages (2.5 volts) to 24%. TEST 9 - 13 In test 9 and 10 the mercury sprays were used with a 5 minute time interval and 500ml mercury. The recovery of 95% indicated the effect of providing greater opportunity for contact between gold particles and mercury. It was found, however, that a significant amount of mercury "flour" was produced, which was caused by excessively violent slurry flow rates. In subsequent tests, therefore impellor RPM was reduced to 450/475 rpm. This reduction virtually eliminated the problem. Recovery values in these two tests are corrected for the value of the amount of mercury not recovered in each test. This was done as a matter of interpretation, and it will be seen in tests 11 and 12 that excessive mercury was recovered (mistake made in blowing out mercury manifold) . Again recovery percentages are corrected for this factor. In any case, the application of a voltage (1.1 volts and 2.5 volts) appeared to have little effect on results, which were more sensitive to the mechanism of mercury contact than changes in charge potential. 5 However, the charge potential is influenced by the conductivity of the electrolyte (slurry) and this was not defined satisfactorily for interpretation.

Test 13 is particularly significant in that almost all (599/600) of the mercury was recovered. The 0^". value of 96% recovery was considered easily achievable for a five minute time interval. TEST 15 - 19

Starting with test 16, some attempt was made to explore the bottom end of the recovery/time curve. 5 In test 16 at 1.5 minutes a 95% was recorded so the time base was reduced to 1 minutes (test 17) which resulted in the first drop off in recovery (87%) recorded for the previous six tests.

Test 16 was repeated in test 18 and the result 0^' obtained (83%) when plotted with the 0.5 min recovery of 74% achieved in test 19 indicates that this is probably very close to "fitting" the recovery curve. TEST 19 - 23

It was then decided to test the effect of 25 changes in slurry density. Four tests were conducted with densities ranging from 15% w/w (solids by weight) in test 20 to 30% w/w in test 23.

Broadly, results indicate that recovery decreases at higher solid densities (73% at 30% w/w, 92% 20. at 15% w/w) .

The result of test 22 (72% at 0.5 min time interval) closely matched test 19 (74% at 0.5 min time interval) .

Since both tests were conducted with all other 35 variables the same this value is seen as being repeatably accurate within the constraints of the test equipment and conditions. TEST 24 - 25

These tests were designed to repeat 11 and 12. Test Mercury Gold Test Mercury Gold Number Volume Recovery% Number Volume Recovery

11 600ml 94.5% 24 500ml 84%

12 600ml 94.5% 25 500ml 90% However, in repeating the tests, a mistake was made in. using 500ml of mercury instead of 600ml. This resulted^', in a change of surface area of mercury in the bottom of the tank, and it is thought that this influence on "mercury contact" surface area also influenced recovery. TEST 26- 27

In test 26, 100% recovery was achieved at 1.5 minutes. A lot of time was spent endeavouring to work out whey the value exceeded test 16 (95% at 1.5 min) and test 18 (83% at 1.5 min). No explanation can be offered, except to say that perhaps some of the sample gold did not fully dissociate ^"and became therefore easier to recover. However, since all samples were prepared in the same manner this is unlikely.

The value of 81% recovered in one minute in test 27 is what one would expect when sighting results of test 17 (87% in one minute) and tests 19 and 22 ( 72- 74% at 0.5 minutes). TEST 28 BULK SAMPLE TEST This material was slimes from a recovery site, which was put through an attritioner and screened to minus 350 microns and adjusted in solids density to 20% w/w.

The tank unit is a batch unit, so that it became necessary to syphon off 20 litres of slurry from the top of the tank unit and then add a similar quantity of 20% w/w slurry each 30 seconds. The process circulation and mercury sprays continues operation.

The technique used was unavoidably inefficient, and some mercury was lost to tail via the syphon. However, results obtained were as follows:-

Sample mass 1.2 tonne

Mercury In 600ml

Mercury Out 545ml

Mercury Recovery 90.83% All Recovery 0.71 gms

Mercury Correction 0.78 gms

Allows 95% Recovery 0.82 gms

Correct for Tonnage 0.68 gms/t lead grade

Recovery 0.59 gms/t

Estimated Recovery = 87%

By experimenting with the disposition of the plastic jets 243, it was found that the jets had to spray mercury directly downward in opposition to the uprising path of slurry within the tank. It was found that if the jets were directed radially outwards, the mercury spray tended to produce excessive flouring over a period of time which was unsatisfactory for the purposes of gold recovery. This problem was exacerbated by the use of a diaphragm pump which produced pressure pulse peaks with resultant increases in the mercury stream velocity. By directing the jets downwardly, however, very little flour was produced since the sprayed mercury did not immediately encounter a container wall across the point of discharge.

The mercury jets used in most tests were 1.5mm in diameter and were screwed into the flexible manifold wall. However, the jet entry points projected marginally into the manifold itself and resulted, even with the appreciation of compressed air, with a certain amount of mercury being retained in the manifold. In addition a small amount of mercury was retained within the ball valve assembly of the diaphragm pump. It was considered that fine gold was more or less evenly distributed within the mercury charge since gold recovered was influenced by the percentage of recovered mercury.

In the testing, it was not possible to be absolutely accurate in timing of each test since most of the content of the tank was removed via a 1 inch diameter syphon introduced at the desired elapsed time interval. When the contents of the tank fell to the level of the top of the draft tube, circulation of the slurry ceased. When the level fell still further, to the vertical height of the impellor, the impellor rpm was reduced to zero. At this point, larger particles of the slurry then settled to the bottom of the tank and were removed by the dosing pumps suction line fitted into the bottom of the tank.

Bearing the above in mind, it can be seen from the table that very high recovery of fine gold was achieved at relatively low time intervals in slurries of 15-25% solids by weight. This rate of recovery was not possible without the use of the mercury pumping/jet syste . It was noted that recovery of very fine gold in the test unit was relatively easy to achieve notwithstanding that further improvements could be made with developments of a specialised mercury injection and delivery system and the beneficial use of known electro- capillary effects on mercury surface tension with applied voltages. Such improvements should mostly serve to reduce the physical size and cost of the contactor column and not greatly affect the percentage of gold recovery since this is already very high. Although the test unit utilised a tank rather than a column, so as to facilitate measurement, it was considered that the use of columns in actual plant operation is superior to the use of a tank for the following reasons:-

(a) the height of the mercury shower is simple to achieve within certain constraints due to pump pressures required; and

(b) the mercury shower within the column can be re¬ directed from side to side in such a way that the probability of slurry particles not coming into contact with mercury is very small indeed.

Additionally, in a container such as the tank it is much more difficult to get the mercury to return rapidly to the centre pumping well than it is in a column unit, due to the relatively large cross-sectional area of the tank compared with the column. Notwithstanding this, a greater pool surface area appears to increase recovery in the tank unit but this effect does not increase recovery to the same extent or at the same rate as the use of mercury showers employed in a small cross-sectional annular region as is the case in the column unit.

Accordingly, it should be appreciated that the tests performed using the tank were only demonstrative of conceptual aspects of the invention. It should be appreciated that the scope of the present invention is not limited to the particular emb¬ odiments herein described. Thus, it should be apprec¬ iated that many changes can be made to the plant and system described in the embodiments without departing from the scope of the invention. In particular, the annular region between the draft tube and wall of the column may have radially inwardly disposed baffles or the like to provide a tortuous path for the outflowing slurry, thereby increasing residence time and increasing the likelihood of the mercury shower coming into contact with gold entrained in the slurry flow.

Claims

CLAIMS :

1. A recovery process for noble metal entrained within a slurry comprising^*: generating a flow of slurry; dispersing mercury in counter current exchange with said slurry to substantially pervade said slurry during its passage therethrough so that said noble metal may amalgamate therewith; accumulating said mercury after its passage " through said slurry; extracting the amalgam from the mercury; extracting said noble metal from the result amalgam to recover said noble metal.

2. A recovery process as claimed in Claim 1 wherein: the accumulated mercury is interacted with one or more fluids before or after extraction of the amalgam to remove contaminates prior to the mercury being recycled through the process. 3. A recovery process as claimed in Claim 2 wherein: the fluid comprises an oxidant, sulphuric acid, hydrochloric acid, ferrous chloride, potassium dichromate, lime, plumbic or copper ions. 4. A recovery process as claimed in any one of Claims 1 to 3 wherein: the mercury is dispersed with an electric charge applied thereto.

5. A recovery process as claimed in Claim 4 wherein: the dispersing of mercury is performed by spraying or sprinkling said mercury into an uprising flow of slurry substantially throughout the cross- sectional extent of said flow to create relatively fine globules of mercury which gravitate through said flow in counter current exchange therewith towards the commencement of the flow where the said mercury is accumulated.

6. Noble metal recovery apparatus performing the process of any one of Claims 1 to 5 in either of a batch or continuous mode comprising: means to establish a flow of slurry; means to disperse a spray of mercury into the flow of slurry; a well for the accumulation of mercury; and ^"; extraction means whereby accumulated mercury amalgam may be removed for collection of noble metals.

7. Noble metal recovery apparatus as claimed in Claim 6 wherein: a vertical flow of slurry is established in a vertical counter flow slurry column and an injection means at the head of the column injects a dispersion of mercury droplets -to sink through slurry to the base of the column which forms a well for the accumulation of mercury. 8. Noble metal recovery apparatus as claimed in Claim 7 wherein: the column is a container having a circumferential side wall and a bottom for receiving said slurry at an inlet region and discharging the slurry from an outlet region after a prescribed residence period; the dispersing means for mercury or mercury containing noble metal and/or other amalgams is disposed within said container, spaced from said bottom, and being adapted to be connected to a supply of mercury for dispersing the same in counter current exchange with said slurry to pervade said slurry so that said noble metal may amalgamate with the dispersed mercury: the well is disposed at said bottom for accumulating said dispersed mercury after passing through said slurry, there being provided a first extracting means to remove said mercury from the well; a vessel to receive the accumulated mercury; second extracting means to remove said noble metal from the resultant amalgan; and means to create a flow of said slurry within said container whereby slurry containing said noble metal may be constantly supplied to said inlet region and treated slurry may be discharged from said outlet region.

9Lf. Noble metal recovery apparatus as claimed in Claim 8 wherein: said container is in the form of a column where said inlet region is disposed centrally at the top of the column and said outlet region is disposed annularly within the circumference of said side wall also at the top of said column, whereby inflowing slurry to the column is directed towards said botton, and outflowing slurry is directed oppositely towards said top, circumfluous to said inflowing slurry.

10. Noble metal recovery apparatus as claimed in Claim 8 wherein: said dispersing means comprises a plurality of jets arranged in an annular formation within said column in the path of the outflowing slurry, said jets being disposed to create a spray a particular size range of globules of said nercury in counter current exchange to the flow of said outflowing slurry, whereby said mercury may gravitate through said outflowing slurry to eventually accumulate within said well.

11. Noble metal recovery apparatus as claimed in Claim 8 wherein: an annular partition of smaller diameter than said side wall and concentric therewith is disposed within said column to extend from the top thereof to a position spaced from the bottom thereof to divide the column into two distinct regions: an inner region for conveying the inflowing slurry centrally of the column towards the bottom, and an outer region for conveying the outflowing slurry annularly within the column towards the top, at a lower velocity and circumfluous to said inflowing slurry there being a means to control the input to the column.

12. Noble metal recovery apparatus as claimed in Claim 8 wherein: ^' a cleaning means for cleaning said amalgam from impurities amalgamated therewith is connected in circuit between said well and said extracting means.

13. Noble metal recovery apparatus as claimed in any one of Claims 5 to 12 wherein: a cleaning column is provided to hold a body of fluid able to remove selected materials from mercury; a pump moves accumulated mercury in the well to the head of the cleaning column for dispersal in the fluid; and a pump exhausts mercury accumulated at the bottom of the cleaning column for reuse.

14. Noble metal recovery apparatus as claimed in Claim 7 wherein: a plurality of cleaning columns are provided in series with recovered mercury recycled into the slurry column.

15. Noble metal recovery apparatus as claimed in Claim 12 wherein: said cleaning means comprises a reservoir for containing a certain quantity of strong oxidant in apportion with mercury containing noble metal and other amalgams; a pipe valve from the reservoir to allow the said oxidant to be replaced after use; a pressure valve to allow escape of gas produced by the interaction of the said oxidant with certain impurities and amalgams; and a pipe to allow mercury containing amalgams to be continually recirculated through the said oxidant. 16. Noble metal recovery apparatus as claimed in Claim 8 wherein: said extracting means comprises a relatively small aperture filter connected in circuit to bleed relatively high pressure mercury from a recirculating ^' pipeline through which said mercury is pumped into the reservoir on the low pressure side of the filter to extract noble metal therefrom, said filter being of a sufficiently small size to extract relatively fine sized noble metal such as 10 microns or less from the amalgam. 17. Noble metal recovery apparatus as claimed in Claim 16 wherein: said recirculating pipeline is disposed to route said mercury from said extracting means to supply said dispersing means, thereby forming a continuous mercury cycle.

18. Noble metal recovery apparatus as claimed in Claim 16 wherein: said cycle includes charging means to electrically charge said mercury prior to re- introduction into said slurry.

19. Noble metal recovery* apparatus as claimed in any one of Claims 6 to 18 wherein: said system comprises a group of containers each having associated therewith a said dispersing means and well, whereby said containers are arranged in series so that the discharge of slurry from one container is fed into the next container in series to enable the residence time of said slurry within the said containers to be increased to improve the noble metal recovery individual groups of containers being connected in parallel. 20. Noble metal recovery apparatus as claimed in Claim 19 wherein: said cleaning means and extracting means combine with a common reservoir of mercury to provide a common mercury treatment means which is shared by all of said containers connected thereto.

21. Noble metal recovery apparatus as claimed in Claim 19 wherein: said system comprises a plurality of groups of containers arranged in a cluster about a common slurry feed and discharge distributor for all of the said groups, and all of the said containers share said common mercury treatment means.

22. Noble metal recovery apparatus as claimed in Claim 21 wherein: the common slurry distributor and mercury treatment means are coincident with a central axis about which said groups of containers are disposed to form said cluster. 23. Noble metal recovery apparatus as claimed in Claim 22 wherein: said cluster has all of said groups of containers therein disposed at equal angular and radial locations about said central axis.