WO1992018654A1

WO1992018654A1 - Process for the treatment of ion flotation froths

Info

Publication number: WO1992018654A1
Application number: PCT/AU1992/000170
Authority: WO
Inventors: Malcolm David Engel; Neville Thomas Moxon; Stuart Kenneth Nicol
Original assignee: Broken Hill Proprietary Company Pty Ltd
Current assignee: Broken Hill Proprietary Company Pty Ltd
Priority date: 1991-04-19
Filing date: 1992-04-16
Publication date: 1992-10-29
Anticipated expiration: 1993-10-19
Also published as: ZW6192A1

Abstract

An ion flotation process for the recovery of cyanides of metals including gold, platinum and palladium from a feed solution (10) is performed in an ion flotation vessel (12) using a selective surfactant (14) such as cetyltrimethylammonium bromide which is at least partly derived from the collapsed froth product of a previous ion flotation process for the recovery of the metal cyanide from which collapsed froth product the metal value has been separated at (18) by electrowinning. The residual solution containing the surfactant may be supplied direct to the ion flotation vessel (12) or the surfactant may be separated from the solution at (32) and then passed to the vessel (12). Electrowinning of the metal value is advantageously performed at the pH of the solution in the vessel (12), preferably 10-11.

Description

PROCESS FOR THE TREATMENT OF ION FLOTATION FROTHS

This invention relates to a process for the recovery of metal ions from ion flotation froth concentrates. In particular, the invention is concerned with a process which involves both separating metal values from the froth concentrates as well as flotation treatments.

Particulate flotation is a physicochemical method of concentrating valuable minerals from finely-ground ore. The process involves a selective treatment of the valuable components to facilitate their attachment to air bubbles, which form a froth concentrate. Ideally, ion flotation is a procedure whereby valuable ions in a mixture of charged species are selectively removed by rising air bubbles. It resembles conventional froth flotation in that it employs a collector and similar equipment. It differs in that the substance to be separated is not usually present initially as a solid. The collectors are ionizable, surface-active organic compounds, cationic for the flotation of anions, anionic for the flotation of cations. These additives perform the dual function of complexing with the ions in solution and transporting these previously surface-inactive components to the foam phase. Such separation of ions is usually accomplished at low gas flow rates, producing a small volume of foam without tall columns or violent agitation of the liquid phase. Ion flotation is of enormous practical significance since ions are often successfully floated and concentrated from 10^~7 to 10~⁴ solutions.

The first of the low gas-flow rate foam separation techniques was introduced by Sebba who described his process in 1959 in Nature, 184, 1062. A surfactant ion of opposite charge to the ion to be removed was added in stoichiometric amounts. Sebba concluded that the collector must be introduced in such a way that its exists as simple ions and not micelles. The foam produced after subjecting this mixture to air bubbles then collapsed, thereby concentrating,the inorganic ion. Rubin et al. investigated other variables associated with the technique, including the effect of metal ion concentration, pH and temperature, using a soluble copper (II) ions recovered by a sodium lauryl sulphate (anionic) collector, as described in 1966 in I. & E.C. Process Design & Development, 5, 368. Berg and Downey studied the use of quaternary ammonium surfactants of the type R^N(R2)3Br as collectors in the flotation of anionic chlorocomplexes of platinum group metals, as described in 1980 in Analytica Chimica Acta, 120, 237.

We have now established that ion flotation can remove metal cyanides with varying degrees of selectivity from alkaline solutions. Specific reagent types have been proposed in our International Patent Applications

PCT/AU90/00124, PCT/AU90/00237 and PCT/AU90/00481 for the purpose of removing gold, and in our International Patent Application filed 7 April, 1992 for the purpose of removing copper, when present as metal cyanides. The present invention is directed to recovering metal values from froth concentrates, which are produced in flotation processes such as are described in the aforementioned International Patent Applications, and to improving the economics of flotation processes used for the recovery of gold, copper and other rare metal values such as silver, platinum, palladium and the like.

According to a first aspect of the present invention there is provided an ion flotation process for the recovery of metal cyanide from solution by subjecting the solution to rising gas bubbles and treating the solution with reagents whereby metal cyanide is selectively collected by the gas bubbles and carried to the surface of the solution to form a recoverable froth product, characterised in that said reagents include a surfactant which is derived from the collapsed froth product of a previous ion flotation process for the recovery of metal cyanide from which collapsed froth product the metal value has been separated by electrowinning.

According to a second aspect of the present invention there is provided a metal value recovery process which comprises recovering a metal cyanide from solution in an ion flotation process by subjecting the solution to rising gas bubbles and treating the solution with reagents including a surfactant whereby the metal cyanide is selectively collected by the gas bubbles and carried to the surface of the solution to form a froth product, collecting the froth product, collapsing the froth product, recovering the metal value from the collapsed froth product by electrolysing the collapsed froth product in an electrowinning cell and collecting the metal value as an electrodeposit to leave residual surfactant, and recovering further metal cyanide from solution in an ion flotation process by subjecting the solution to rising gas bubbles and treating the solution with reagents including said residual surfactant whereby the further metal cyanide is selectively collected by the gas bubbles and carried to the surface of the solution to for further froth product.

By the present invention recycled surfactant, left over after the metal value has been stripped from the collapsed froth product of a previous ion flotation process, is used in a subsequent ion flotation process. In contrast to common processes for the recovery of gold, copper and other rare metals, which are largely concerned with the recovery of conventional carbon strip solutions of those metals, we have discovered that it is possible to electrowin froth product solutions containing surfactant obtained from ion flotation operations. In carrying out the electrowinning, metal ions are removed from solution and a residue solution remains containing concentrated surfactant. Using recycled surfactant in subsequent ion flotation processes has very considerable economic advantages and is possible because we have found that the surfactant remains active even after the collapsed froth product is subjected to electrolysis to recover the metal value.

All or part of the residual surfactant may be recycled to an ion flotation feed solution. Some fresh surfactant may also be introduced to the feed solution or the recycled surfactant may be utilized alone. The quantity recycled will depend on the desired foaming activity in the subsequent flotation process. The surfactant may be recycled as a residual solution direct from the electrowinning step following the electrodepositing of the metal value. Alternatively, the surfactant may be stripped from the residual solution prior to recycling, for example by a recrystallisation or solvent extraction procedure.

Advantageously, the collapsed froth product is subjected to at least substantially no dilution, for example by water or alkali, prior to electrowinning of the metal value and recovery of the surfactant for recycling since this merely reduces the concentration of the metal value and surfactant in the collapsed froth product. However, other reagents may be added to the electrowinning cell to improve the separation of the ion-paired metal or metal cyanides from the surfactant.

The electrowinning of the metal value may thus be performed at the pH of the ion flotation process, which for the processes described with reference to our aforementioned International Patent Applications is generally in the range of 8.5 upwards, preferably from 10 to 11.

Generally, the electrowinning procedure may be conducted using an electrowinning cell of known design although, as noted, some modification of existing cell design may be advantageous.

Electrowinning of gold and silver from cyanide solutions has been practised for many years and industry experience in the use of electrowinning cells has led to a wide array of these devices being developed. Currently, one cell design is usually selected for all new operations, this being the "modified" or "rectangular" Zadra cell. It consists of a rectangular cell tank accommodating multiple anode-cathode pairs suspended vertically from busbars into the electrolyte. There are several variations on this design, among them the improved mass transfer (IMT) cell described in 1986 by Elges et al in "Staged Heap Leaching - Direct Electrowinning", Inf. Circ. U.S. Bur. Mines, IC 9059.

Collapse of the froth product from the ion flotation procedure is conveniently achieved naturally over a period of time. Alternatively any suitable technique known per se, e.g. by spraying water over the froth or by adding an appropriate reagent to the froth either before or during the electrowinning process. The pH of the froth may be adjusted by addition of acid or alkali.

The electrowinning cell contents will generally need to be agitated and cell conditions such as current, electrode packing density, pH and temperature may be selected for optimum performance. If the pH of the electrowinning cell solution is at or above about 12.5, an anode of, for example, stainless steel, may be used in the cell. However, for a solution which is at the preferred pH of the collapsed froth product of 10-11 this requires the addition of alkali, possibly in substantial quantities. Alternatively, therefore, we have found that electrowinning may be performed at a pH in the range of 8.5 to 12.5, preferably 10-11, without damaging the surfactant. Suitable anodes at these pHs may include: platinum coated reticulated vitreous carbon, platinum coated carbon felt, ruthenium dioxide coated titanium mesh, platinum coated niobium mesh, palladium coated titanium mesh, iridium oxide coated titanium mesh, platinum coated titanium mesh, platinum coated tantalum mesh and iridium oxide coated niobium mesh.

Various embodiments of process in accordance with the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 is a simplified flowsheet of one embodiment of the process; Figure 2 is a sectional view of experimental electrowinning apparatus for use in the process;

Figure 3 is a schematic view of an experimental flotation cell for use in the process; and

Figure 4 is a summary diagram of the pilot scale ion flotation/electrowinning process data given in Examples 3, 4 and 5. Referring initially to Figure 1, the simplified flowsheet illustrates schematically the various steps of a preferred process in accordance with the invention. Various of the steps are described in detail hereinafter. The process is described for convenience in relation to the recovery of gold from gold cyanide leach liquor but is equally applicable to the recovery of other metal values such as copper, silver, platinum and palladium using appropriate reagents.

In Figure 1, a leach liquor 10 containing gold cyanide is introduced to an ion flotation vessel 12 into which a gold cyanide selective surfactant 14 is also added. Ion flotation is performed by introducing air bubbles to the vessel which rise through the liquor and collect the gold cyanide/surfactant complexes to form a froth on the surface of the liquor. The ion flotation process is described in detail in our copending International Patent Application PCT/AU90/00124.

The froth product is collected and collapsed at 16. The collapse of the froth product preferably occurs naturally over a period of time and the collapsed froth product containing the gold cyanide/surfactant complexes is introduced to an electrolytic cell 18.

In the cell 18, gold is deposited on the anode and is recovered at 20 and subjected to further processing as necessary at 22. The residual solution following electrowinning of the gold contains surfactant and is recovered at 24 and recycled at 26 to the ion flotation vessel 10. The recycling may be direct as shown at 28 or indirect as shown at 30, or a mixture of both. In the indirect route 30, the surfactant is separated at 32 from the residual solution by solvent extraction, recrystallisation or, for example, re-flotation. At 26, only recycled surfactant may be introduced to the ion flotation vessel 10 or it may be combined with fresh surfactant 14.

If no or insufficient fresh surfactant 14 is introduced to the ion flotation vessel 10 with the recycled surfactant, the residual solution from the electrowinning cell 18 may after several cycles contain insufficient usable surfactant and be drained from the cell at 34 for recycling to the leach process from which the gold cyanide feed solution is derived.

ELECTROWINNING

The electrowinning experiments described hereinafter used an IMT cell, based on the aforementioned cell described by Elges et al, which in turn was based on the Zadra cell.

The electrolytic cell 40 shown in Figure 2 comprises three concentric cylindrical containers. An inner container 42, which defines the cathode compartment, is a perforated cylindrical insulating cup, made of polypropylene, which houses a perforated stainless steel central feed tube 44, surrounded by a perforated insulating sheath 46 made of polyproplyene.

Around the sheath 46 is a helical cathode 48 which consists of a helix 50 of stainless steel wool supported by a stainless steel wire helix 52. Between the inner container 42 and an outer container 54 is the anode compartment, which contains a cylindrical anode 56 of stainless steel mesh or other suitable material depending on the cell contents. The anode 56 is electrically connected to the feed tube 44, which thus serves as a secondary anode.

A recirculating pump 58 draws in the solution from the anode compartment, through inlet tubes 60 located on opposite sides of the anode compartment, and pumps it out through the feed tube 44 in the cathode compartment.

The solution comprises the collapsed froth product of the ion flotation process which consists primarily of gold cyanide/surfactant complexes. The collapsed froth product will usually have a pH of between 10 and 11 and alkali, or acid, may be added to it to control the pH.

In the gold industry, rectangular cells are preferred for electrowinning. However, the circular cell described above is suitable for laboratory experiments, as it is simpler to construct and it possesses similar operating characteristics to the rectangular IMT cell. The circular cell is also preferred for this experiment as less gold solution is needed due to the smaller cell volume. However the described design will be understood to be illustrative of all types of electrowinning cells.

Within the cell, the following reactions occur:

At the cathode:

Au(CN)₂ ^_ + e^" - ^{Au + 2CN"} and 2H₂0 + 2e" H₂ + 20H^" At the anode:

2H 0 = 4H⁺ + 0₂ + 4e^~

As gold is plated onto the stainless steel wool cathode 48, hydrogen is produced at the cathode and oxygen is produced at the anode. Some of the CN" ions are consumed by oxidation at the anodes in the electrowinning cell to form ammonia. HCN would be produced at a pH below about 8.5 so a pH below this level should be avoided.

Surfactant molecules will remain in solution as electrodeposition occurs. The residual solution from electrowinning can then be re-used as an ion flotation reagent feed in accordance with the invention, as described hereinafter.

The gold plating is removed from the cathodic stainless steel wool 50 by treating with acid to dissolve the iron in the cathode. The gold-containing sludge is then smelted. Alternative treatments for removing gold from steel wool electrodes are well known but are not described as they are outside the scope of this invention.

Some of the variables associated with electrowinning tests are cell current and voltage, liquid flowthrough rate, steel wool diameter and packing density, pH, anode material and temperature. The following values are non- limiting examples of some of these variables. The steel wool diameter was chosen as 0.11 mm, typical of that used in industrial practice, with a wool packing density of 3- 20 g/1. The pH was typically 9 to 13.5 and the temperature varied from ambient (about 20°C) to 95°C. Cell currents may vary up to 3 amps, voltages up to 25 volts, and liquid throughputs up to 2 litres/minute, for the cell 40.

EXAMPLE 1

In most experiments a solution of approximately one litre was prepared and poured into the cell 40, so that it just covered the stainless steel wool cathode 48 supported on the wire helix 52. Five millilitre samples were collected, using a pipette, at various time intervals. These samples were analysed for gold concentration. Most tests were run for a period not less than 15 minutes and not more than 10 hours.

The solution was prepared by generating a froth product from a gold ion flotation operation using a quantity of 0.08 g/1 of fresh long chain quaternary ammonium surfactant molecule cetyltrimethylammonium bromide and a gold cyanide feed solution obtained from a gold mine CIL operation (composition given in Table 1) following the procedure described in our International Patent Application PCT/AU90/00124.

Table 1. Ion Flotation Feed Solution Composition

The froth product from the ion flotation was found to contain 6.7 ppm of gold. This solution concentrate was then collapsed naturally over a period of time and passed directly to electrowinning for treatment in the manner described.

The electrowinning conditions were as follows: Steel Wool Diameter = 0.11 mm

Wool Packing Density - 3g/l pH - 13.2

Temperature = 22°C

Cell Current » 0.7 Amps Cell Voltage - 11 Volts

Liquid Throughput Rate = 1.5 1/min

Operating Time = 3 hours

Anode material = Stainless steel mesh

The pH of the collapsed froth product is generally between 10 and 11 so alkali in the form of sodium hydroxide was added to the solution prior to electrowinning to increase the pH to 13.2. Stainless steel anodes have been found to be ineffective below a pH of about 12.5 in recovering gold from the gold cyanide collapsed froth product.

Gold recovery (material reporting to the cathode surface as a deposit) as a function of time is calculated by the formula

R% = (1-C_t/C_Q) x 100

where G_j- is the liquid subsample gold concentration at time t, and C₀ is the concentration in the initial feed. The ratio C_t/C₀ represents the fraction of gold from the feed left in the cell at time t.

The gold concentration data for the electrowinning experiment is given in Table 2.

Table 2. Electrowinning Results

Initial Gold Final Gold Electrowinning

Concentration Concentration Recovery

(C₀, ppm) (C_t,ppm) ( )

6.7 0.28 96

Clearly electrowinning of gold-containing froth products from ion flotation experiments is possible with up to maximum efficiency. The use of cetyltrimethylammonium bromide would be illustrative for all surfactants.

The residue surfactant solution from this experiment was subsequently re-used in gold ion flotation experiments in the manner described below. SURFACTANT RE-USE BY ION FLOTATION

Reference is now made to Figure 3 which is a diagram of the experimental flotation apparatus 70 used in bench- scale laboratory experiments. The flotation apparatus 70 consisted of a modified Hallimond tube cell or column 72 of volume approximately 1L. A sintered glass frit 73 in the base of the column allows air to pass through the cell from an inlet 74, metered by appropriate flowmeters and regulators (not shown). A side port 76 fitted to the column 72 allows continuous monitoring of pH and/or temperature, while a side port 78 permits removal of small subsamples of the liquid contents of the cell. The liquid feed to the column 72 enters through a port 80 and the exit air stream flows out through port 82. The froth formed during flotation is discharged from an overflow lip 84 at the top of the column and is collected in another container (not shown). The column 72 may be completely drained at the end of a batch experiment by means of a tailings outlet port 86.

EXAMPLE 2

A solution containing a known concentration of metal ion (in this instance gold as aurocyanide ion) and no surfactant was mixed with a known quantity of electrowinning residue containing the surfactant cetyltrimethylammonium bromide from the Example 1. The two liquids were mixed thoroughly and this feed was injected into the flotation column 72 through port 80 and the air supply connected to inlet 74. Air was then immediately bubbled into the column and froth began to form at the top of the column. When the first drop of froth spilled over the lip 84 a timer was started and at known intervals after this point, sub-samples of the liquid contents of the cell were removed via the side port 78 and analyzed for their gold content by atomic 14 absorption spectrophotometry. At the completion of the experiment (when either the surfactant was exhausted or the elapsed time reached a certain value) the air supply was disconnected and the collected froth and a sub-sample of the final cell contents were analyzed for gold. During the test, pH remained at a resonably constant level between about 10 and 11, and was not regulated.

The ion flotation process was performed at room temperature for optimum economics. However, ion flotation at elevated temperature is possible, although usually with some change in surfactant selectivity characteristics.

Varying the molar ratio of surfactant to gold affects the recovery in any experiment. In the present case almost no gold was introduced to the flotation column 72 via the recycled surfactant solution since the prior electrowinning step (Example 1) had been almost entirely successful. The feed leach liquor contained approximately 1 ppm of gold which represented the major source of this metal for the flotation step. The quantity of recycled surfactant solution from the electrowinning stage which was mixed with new feed leach liquor is given in Table 3 for each of the experiments.

Table 3. Ion Flotation Conditions and Results

*Frothed until completion, air 260 cm3/_{min ce}ιχ capacity 800 g.

Gold recovery (material reporting to the froth) as a function of time is calculated by the formula R% = (1-C_t/C₀) x 100 where C_t is the liquid subsample gold concentration at time t, and C₀ is the concentration in the initial feed. The ratio C_t/C₀ represents the fraction of gold from the feed left in the cell at time t.

As shown in the right hand column of Table 3, the effect of increasing the proportion of recycled surfactant solution was to increase gold recovery. This example would be illustrative for all surfactants. Clearly it is possible to reuse and/or recycle ion flotation surfactants after electrowinning or metal precipitation stages to recover further amounts of solution gold and to enhance process economics overall.

EXAMPLE 3

ELECTROWINNING

Using a platinum coated titanium mesh anode an identical procedure was used as described in Example 1 to electrowin gold. To facilitate these experiments we generated a froth product from a gold ion flotation pilot-scale operation (described in Example 4) using a quantity of 0.055g/l of the long-chain quaternary ammonium surfactant molecule cetyltrimethylammonium bromide and 300 litres of feed solution obtained from a gold mine heap leach operation (composition give in Table 4) following the procedure described by Engel et al in 1991 in "Selective Ion Flotation of Gold from Alkaline Cyanide Solutions" Proc. World Gold 91, pp 121-131. Table 4 Ion Flotation Feed Solution Composition

The froth product from the ion flotation was found to contain 9.0 ppm of gold with 70% recovery of gold to product. The solution concentrate was then collapsed and passed directly to electrowinning and treated in the manner described in Example 1, using the following conditions:

Steel Wool Diameter = 0.11 mm

Wool Packing Density = 6 g/1 pH - 10.2

Temperature = 22°C

Cell Current = 1.7A

Cell Voltage = 7V

Liquid Throughput Rate * 2 1/min Operating Time = 3 hours

Anode Material = Platinum-coated titanium mesh

The gold concentration data for the electrowinning experiment is given in Table 5

Table 5 Electrowinning Results

The residue surfactant solution from this experiment was subsequently re-used in gold ion flotation pilot scale experiments in the manner desribed in Example 4. EXAMPLE 4

SURFACTANT RE-USE BY ION FLOTATION

The flotation equipment used in the pilot-scale laboratory experiments in this example was essentially the same as that described with reference to Figure 3 and Example 2, except that the flotation column width was 300mm and the column height overall was 4m. The air was sparged into the column using a sintered metal frit system.

The procedure for the pilot-scale experiment of this Example was similar to that of the bench-scale Hallimond tube work, described in Example 2.

Experimental Results

The surfactant residue solution was combined with 300 litres of feed solution with a composition given in Table 4. The froth products from the ion flotation was found to contain 5.6ppm of gold with a 58% recovery of gold to product. No additional fresh surfactant had been added at this stage. The solution concentrate was then collapsed and passed directly to electrowinning and treated in the manner described in Example 5.

EXAMPLE 5

ELECTROWINNING

The electrowinning conditions were identical to those described in Example 3 except that the electrowinning time used was 7 hours.

The gold concentration data for the electrowinning experiment is given in Table 6. Table 6 Electrowinning Results

The process of recycling surfactant in flotation experiments may continue for many stages in practice. A summary diagram of the pilot-scale flotation/electrowinning data given in Examples 3, 4 and 5 is shown in Figure 4. In all stages the pH was maintained at around 10 and the electrowinning anodes used were platinum coated titanium.

Clearly it is possible to reuse and/or recycle ion flotation surfactants after electrowinning or metal precipitation stages to recover further amounts of solution gold and to enhance process economics overall.

Claims

1. An ion flotation process for the recovery of metal cyanide from solution by subjecting the solution to rising gas bubbles and treating the solution with reagents whereby metal cyanide is selectively collected by the gas bubbles and carried to the surface of the solution to form a recoverable froth product, characterised in that said reagents include a surfactant which is derived from the collapsed froth product of a previous ion flotation process for the recovery of metal cyanide from which collapsed froth product the metal value has been separated by electrowinning.

2. A process according to Claim 1 wherein the metal value is selected from the group consisting of gold, platinum and palladium.

3. A process according to Claim 1 wherein the metal value is gold.

4. A process according to Claim 1 wherein no fresh surfactant is included with said reagents.

5. A metal value recovery process which comprises recovering a metal cyanide from solution in an ion flotation process by subjecting the solution to rising gas bubbles and treating the solution with reagents including a surfactant whereby the metal cyanide is selectively collected by the gas bubbles and carried to the surface of the solution to form a froth product, collecting the froth product, collapsing the froth product, recovering the metal value from the collapsed froth product by electrolysing the collapsed froth product in an electrowinning cell and collecting the metal value as an electrodeposit to leave residual surfactant, and recovering further metal cyanide from solution in an ion flotation process by subjecting the solution to rising gas bubbles and treating the solution with reagents including said residual surfactant whereby the further metal cyanide is selectively collected by the gas bubbles and carried to the surface of the solution to form further froth product.

6. A process according to Claim 5 wherein the metal value is selected from the group consisting of gold, platinum and palladium.

7. A process according to Claim 5 wherein the metal value is gold.

8. A process according to Claim 5 wherein no fresh surfactant is included with said reagents.

9. A process according to Claim 5 wherein the electrowinning anode material is selected from the group consisting of stainless steel mesh, platinum coated reticulated vitreous carbon, platinum coated carbon felt, ruthenium dioxide coated titanium mesh, platinum coated niobium mesh, palladium coated titanium mesh, iridium oxide coated titanium mesh, platinum coated titanium mesh, platinum coated tantalum mesh and iridium oxide coated niobium mesh.

10. A process according to Claim 5 wherein the pH of the collapsed froth product from which the metal value is electrowon is in the range 8.5 to 12.5.

11. A process according to Claim 10 wherein the collapsed froth product from which the metal value is electrowon is substantially at the pH of the ion flotation process.

12. A process according to Claim 10 wherein the electrowinning anode material is selected from the group consisting of platinum coated reticulated vitreous carbon, platinum coated carbon felt, ruthenium dioxide coated titanium mesh, platinum coated niobium mesh, palladium coated titanium mesh, iridium oxide coated titanium mesh, platinum coated titanium mesh, platinum coated tantalum mesh and iridium oxide coated niobium mesh.