AU631413B2 – Recovery of gold, silver and the platinum group metals with various leachants at low pulp densities
– Google Patents
AU631413B2 – Recovery of gold, silver and the platinum group metals with various leachants at low pulp densities
– Google Patents
Recovery of gold, silver and the platinum group metals with various leachants at low pulp densities
Info
Publication number
AU631413B2
AU631413B2
AU52606/90A
AU5260690A
AU631413B2
AU 631413 B2
AU631413 B2
AU 631413B2
AU 52606/90 A
AU52606/90 A
AU 52606/90A
AU 5260690 A
AU5260690 A
AU 5260690A
AU 631413 B2
AU631413 B2
AU 631413B2
Authority
AU
Australia
Prior art keywords
gold
pulp
carbon
solution
solids
Prior art date
1989-03-07
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Ceased
Application number
AU52606/90A
Other versions
AU631413C
(en
AU5260690A
(en
Inventor
Dean Robert Butler
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Individual
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Individual
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1989-03-07
Filing date
1990-03-06
Publication date
1992-11-26
Family has litigation
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https://patents.darts-ip.com/?family=25643643&utm_source=google_patent&utm_medium=platform_link&utm_campaign=public_patent_search&patent=AU631413(B2)
“Global patent litigation dataset” by Darts-ip is licensed under a Creative Commons Attribution 4.0 International License.
1990-03-06
Application filed by Individual
filed
Critical
Individual
1990-03-06
Priority to AU52606/90A
priority
Critical
patent/AU631413C/en
1990-03-06
Priority claimed from AU52606/90A
external-priority
patent/AU631413C/en
1990-10-09
Publication of AU5260690A
publication
Critical
patent/AU5260690A/en
1992-11-26
Application granted
granted
Critical
1992-11-26
Publication of AU631413B2
publication
Critical
patent/AU631413B2/en
1994-02-24
Publication of AU631413C
publication
Critical
patent/AU631413C/en
2010-03-06
Anticipated expiration
legal-status
Critical
Status
Ceased
legal-status
Critical
Current
Links
Espacenet
Global Dossier
Discuss
Classifications
C—CHEMISTRY; METALLURGY
C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
C22B—PRODUCTION AND REFINING OF METALS; PRETREATMENT OF RAW MATERIALS
C22B11/00—Obtaining noble metals
C22B11/04—Obtaining noble metals by wet processes
C22B11/042—Recovery of noble metals from waste materials
C—CHEMISTRY; METALLURGY
C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
C22B—PRODUCTION AND REFINING OF METALS; PRETREATMENT OF RAW MATERIALS
C22B11/00—Obtaining noble metals
C22B11/04—Obtaining noble metals by wet processes
C—CHEMISTRY; METALLURGY
C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
C22B—PRODUCTION AND REFINING OF METALS; PRETREATMENT OF RAW MATERIALS
C22B3/00—Extraction of metal compounds from ores or concentrates by wet processes
C22B3/04—Extraction of metal compounds from ores or concentrates by wet processes by leaching
C—CHEMISTRY; METALLURGY
C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
C22B—PRODUCTION AND REFINING OF METALS; PRETREATMENT OF RAW MATERIALS
C22B3/00—Extraction of metal compounds from ores or concentrates by wet processes
C22B3/20—Treatment or purification of solutions, e.g. obtained by leaching
C—CHEMISTRY; METALLURGY
C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
C22B—PRODUCTION AND REFINING OF METALS; PRETREATMENT OF RAW MATERIALS
C22B3/00—Extraction of metal compounds from ores or concentrates by wet processes
C22B3/20—Treatment or purification of solutions, e.g. obtained by leaching
C22B3/22—Treatment or purification of solutions, e.g. obtained by leaching by physical processes, e.g. by filtration, by magnetic means, or by thermal decomposition
C22B3/24—Treatment or purification of solutions, e.g. obtained by leaching by physical processes, e.g. by filtration, by magnetic means, or by thermal decomposition by adsorption on solid substances, e.g. by extraction with solid resins
C—CHEMISTRY; METALLURGY
C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
C22B—PRODUCTION AND REFINING OF METALS; PRETREATMENT OF RAW MATERIALS
C22B3/00—Extraction of metal compounds from ores or concentrates by wet processes
C22B3/20—Treatment or purification of solutions, e.g. obtained by leaching
C22B3/42—Treatment or purification of solutions, e.g. obtained by leaching by ion-exchange extraction
Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
Y02P10/00—Technologies related to metal processing
Y02P10/20—Recycling
Description
RECOVERY OF GOLD, SILVER AND PLATINUM GROUP METALS
WITH VARIOUS LEACHANTS AT LOW PULP DENSITIES.
This invention relates to improvements in recovery of noble metals from ores and tailings. Throughout this specification noble metals are intended to include gold, silver and the platinum group.
This invention is partly predicated on the discovery that noble metals in extremely fine form are often present in higher concentrations than is revealed by normal assay techniques in common use.
For example platinum or gold ores can contain more metal than that recovered in conventional wet chemical or fire-assay methods. It is thought that where there are metal absorbing materials such as clay, carbon, or sulfides in the ore or other metals being analysed, some of the metal taken into solution becomes adsorbed onto these materials and is not detected. In the case of gold leached into solution by aqua regia in wet assays or by cyanide in the cyanide extraction process, the gold complex becomes absorbed by clay, carbon, sulfides, or other material and is thus undetected by solution assay. In the fire assay technique, because clays convert to a boro silicate glass under fire assay conditions, gold is also lost to detection in that technique. Conventionally, gold leached by the cyanide process, usually at pulp densities of 35 to 50%, may be recovered from the leach solution in a subsequent stage by contacting the solution or pulp with activated carbon, usually in a concentration range of 10 to 20 grams of carbon per litre of solution [carbon in pulp (CIP) process], but on occasions up to 40 grams per litre have been used. In some instances the carbon has been added to the leaching circuit as well (CIL process), in the same concentration ranges in order to improve gold leach rates so that the gold recorded equated with the assayed grade of the ore.
Particular treatments for clay or sulfide ores have been proposed. Australian Patent 569175 treats sulfide ores in a pressure oxidation step prior to cyanide
leaching. After leaching the pulp is diluted by washing to
improve the flocculation in the subsequent thickening stage, following which the liquor is separated from the pulp.
Gold is then extracted from solution and the concentrated pulp is then subjected to a carbon in leach circuit at 35 to 40% solids to extract further gold.
The conventional assay technique for gold is either by the wet method, which is leaching with aqua regia .
followed by measurement of the dissolved gold by atomic absorption spectroscopy or similar techniques, or by fire assay. In some instances when the recovery of gold by the CIP process was not up to the assayed grade, adoption of the CIL process, with addition of carbon to the leach circuit resulted in increased recovery. The amount of carbon was increased, in some cases to 40 grams per litre, until the head grade recovery was achieved. In other instances the Carbon in leach (CIL) process was adopted to improve gold leach rates and gold recovery rates and thereby decrease the required number of carbon contacting tanks, thus decreasing the capital cost of construction of the gold recovery plant. In some circumstances, however, increasing, the carbon loadings to the leach vessel, or the subsequent contacting stages, was found to be undesirable because of the formation of fine carbon particles caused by attrition during pulp agitation. The consequent loss of carbon with its attached gold reduced the effectiveness of the process. However prior to this invention it was not suspected that there were also undetected values of metal in some ore samples. These comments also apply to ore contentrate and tailings.
It is an object of this invention to improve recovery rates of noble metals including gold or other valuable metals from ores particularly clay containing ores.
In another aspect of the invention there is provided a method of recovering metal values by the leaching method in which the pulp density of the slurry is adjusted to below 15% either, prior to, during, or at the end of the leaching stage and subsequent to the leaching step
localized zones of high pulp density are avoided.
Pulp is defined as a mixture of one or more solids with one or more liquors. Pulp density is defined by Arthur F Taggart in his book “Handbook of Mineral Dressing” as “the decimal fraction of solids in pulp, by weight.” This is commonly referred to as a percentage figure, calculated as the decimal fraction of solids in pulp, by weight, multiplied by one hundred.”
By lowering the pulp density as compared to conventional processes, such as in the cyanide leaching process for gold ores where pulp densities in the range of 35 to 55% are normally used, more noble metal, particularly gold, is recovered than by conventional processes and in some cases gold recovery is higher than the assayed value obtained by using conventional assay technique.
Processes according to this invention utilizing collectors such as carbon or ion exchange resins generally have pulp densities in the collection stage less than 15%, preferably 0.1 to 10% more preferably 0.5% to 2%. The low pulp density may be used in the leaching stage or
alternatively conventional pulp densities may be employed in the leaching stage or the pulp may then diluted to densities of less than 15% prior to the stage in which noble metal is recovered from solution.
Once the dilution of the pulp has occurred it is an important aspect of this invention to minimize the
occurrence of localized zones in the slurry or in the recovery process in which pulp density is above 15% or higher than the preferred dilution.
Another alternative is to carry out the dilution in stages by separating the liquor from the solids as they leave the leaching circuit, sending the liquor to the recovery stage to recover some metal from solution and treating the separated solids with more leach liquors at a pulp density below 30% to recover further noble metal and again separating the solids from the liquor. The stages can be repeated until it is no longer economic to continue the process.
In this way the solids are contacted with a large volume Of liquor in total, even though most of the liquor may be a relatively small recirculating volume having multiple contacts with the solids. Separation of the liquor from the solids may be accomplished by any of the
conventional methods including centrifugation, filtration, decantation or similar. Leaching may also be performed at the higher pulp densities followed by dilution and recovery of noble metal by using a collector in an extended contactor circuit in which a tank/s containing carbon or other
collector is followed by a desorption tank/s containing little or no carbon. Thus the sequence of metal desorption from the solids and adsorption onto the collector is
maintained, but with a lesser inventory and attrition of the collector than if each tank in the extended circuit
contained normal levels of collector material. The
collector material may be carbon, ion exchange resins, chelating resins or polymeric adsorbent resins hereinafter referred to as resins.
In one preferred aspect of this invention it has been discovered that a higher recovery of the noble metal can be obtained if a lixiviant or leaching aid in addition to the leaching agent is used. A preferred lixiviant or leaching aid is chloride ions obtained, for example, from sea water. Chloride ion concentration is preferably in the range of 10 to 100 grams per litre.
Methods of recovering the leached material from solution need not be restricted to using collectors such as carbon, resins and other adsorbents, but may include methods such as zinc cementation, cementation, solvent extraction, electrowinning and precipitation. These alternative methods may be necessary to recover the gold or other metal from solution in an alternative procedure in which a
relatively short chain, polar, soluble organic material such as methanol or acetone is added to the pulp. The organic material, in the case of gold leaching, will alter the dielectric constant of the solvent and allow more gold to be
desorbed into the liquor than would normally be desorbed without the organic material present. When carbon or resins are used as collectors the organic material may interfere with the adsorption of gold by these materials and thus lower the recovery of gold from the liquor.
Testwork carried out has shown that the loading of gold onto carbon, both at high and low pulp densities and at high and low cyanide concentrations, is a reversible
reaction. There is a tendency for both the ore particles and the carbon to absorb gold from cyanide solution. The term “ore” can also mean ore concentrates, tailings, or other noble metal containing solids. A reversible equilibrium is formed between the ore particles, the liquor and the carbon. At high pulp densities [>30%] this equilibrium favours the ore particles. As the pulp density is lowered the
equilibrium moves toward the liquor and carbon. At low pulp densities [<5%] the equilibrium is also affected by both the cyanide concentration and pH. As the cyanide concentration is lowered the equilibrium moves back toward the ore particles. If the pH is too high the equilibrium will favour the ore particles and liquor rather than the carbon. The distribution of gold between the three phases is time dependent, in that for leaches where the conditions do not favour the carbon side of the equilibrium, the gold will unload from the carbon and load back onto the ore particles. It is for this reason that localized zones of high pulp density are to be avoided as gold recovery will be reduced.
Thus the dilution of the pulp density according to the present invention is for a different purpose to the dilution washing step disclosed in Patent 569175 where the dilution step assists in reducing the amount of flocculant required in the subsequent thickening and separation step. In contrast to the present invention Patent 569175 is not attempting to increase solution of gold and is not
attempting to recover gold above the conventional assay of value.
In another aspect, the present invention, provides a method of recovering noble metal values, particularly platinum, silver or gold, in which a noble metal bearing ore is leached with a leach solution to dissolve the metal in the presence of at least 65 grams per litre of a carbon source material or a resin having metal absorption
properties.
This process ensures increases in gold recovery.
When certain ore types such as those having clay, carbon, or sulfide materials in the ore are treated according to this invention, metal values recovered are greater than the assay grade of the ore when measured by conventional assay. For clay and silica containing ores, dilution below 10% shows immediate improvement in gold recoveries, but for sulphidic ores dilution below 2% appears to be necessary for
economically significant improvement in recovery.
Again these observations also apply to other noble metals. The recovery of the noble metal onto the collector may be achieved by adding the collector (e.g. carbon or resin) or the pulp on a recirculating tank or alternatively using a series of short columns packed with a collector through which the solution is passed a number of times so that the amount of collector exposed to each litre of solution is of the same order as that required in the recirculating tank.
The gold complexes are removed from solution and taken up by solids by several possible mechanisms. They include (1) ion exchange, (2) physical absorption, (3) chemical absorption (4) direct reduction, (5) method (1) or (2) followed with partial or complete reduction depending on the solid and the environment, (6) partial or complete complexation. The mechanism employed on any particular solid particle is dependent on both the solids type and the chemical and physical conditions in place at that time.
Accordingly, the present invention is able to achieve recoveries of noble metal values such as gold which are greater than conventional assay grades by simply
contacting the leach solution with gold adsorbent carbon source material or exchange resins. With conventional pulp densities of 35 to 55% ore by weight, carbon or resin
concentrations greater than 65 grams per litre are
preferred, with a more preferred range being above 300 grams per litre. Where lower pulp densities are used as taught by this invention, an even higher gold recovery is possible and carbon concentrations of from 100 to 300 grams per litre are practicable.
It is essential when considering increases in carbon or resin loadings to increase the carbon or resin concentrations in relation to the quantity of the ore in the pulp. In order to achieve improved recovery of gold above the conventional assay grade, at least 80 kilograms of carbon source material per tonne of ore should be used. It is preferred to use at least 90 kilograms per tonne and more preferably to use above 100 kilograms of carbon per tonne of ore. This increase in carbon loading per tonne of ore can be achieved either by adding more carbon, or by reducing the pulp density and maintaining the carbon concentration per litre. Where a recovery medium other than carbon is used e.g. ion exchange resin, the medium is to be used in
concentrations that will produce the same gold adsorbing capacity as activated carbon at the concentrations
recommended for this invention
In a further aspect of the leaching process, dispersants are added to the pulp to aid in the separation of the solids particles. It is thought that in the absence of very low (around 1%) pulp densities, a gold complex which is desorbed from a solids particle is likely to be adsorbed by another solids particle if these solids particles are in close proximity. Dilution increases the distance between these particles and the dispersant ensures that there is uniform separation. Any suitable dispersants may be used. Preferred dispersants are sodium silicate, sodium carbonate, tri-sodium polyphosphate and sodium hexametaphosphate. By leaching at a very low pulp density with moderate agitation,
the pulp solids may be naturally maintained in a condition of separation where by the Freundlich adsorption equation (C3=kCA n where C3 is the concentration of gold per unit area of the solids, CA is the concentration of gold per unit volume of the pulp liquor, k and n being constants)
describes the gold distribution in the pulp between the solids and the liquor. At higher pulp densities the solids particles proximity effect predominates and alters the gold distribution in the pulp in favour of the solids particles. This solids particles proximity effect may be lessened by the introduction of one or more dispersants into the pulp. By maintaining discrete solids particles rather than
agglomerates of floes, the dispersant lessens the likelihood of a gold complex being desorbed from one solids particle and then immediately being adsorbed by an adjacent solids particles.
The addition of the carbon material may require re-design of the subsequent carbon contacting circuit due to the increased carbon concentrations. By selection of a harder grade of carbon and control of agitation, the
attrition of the carbon particles expected at such high loadings can be reduced.
For gold extraction the preferred method of leaching, desorption and adsorption is as follows. The leaching is performed in a cyanide solution at a relatively high pulp density to minimise tankage requirements. At the end of the leach cycle the pulp is diluted to a low pulp density (less than 30%) and maintained for a minimum period of time in a holding tank. The pulp density and time will both vary according to the type and grade of the material being leached, the ore type, the liquor type and the
leachant. For most gold ores the economic optimum pulp density lies between 0.1 and 10% and the residence time in the holding tank is up to 30 minutes preferably 15 minutes. The pulp is then preferably centrifuged with a short
residence time in the centrifuge to separate solids from the liquor. Either liquor separated from the solids or
unseparated pulp is then passed through a column containing an absorbent material such as carbon or resin to absorb the gold which is present in the liquor. The pulp so treated will then desorb more gold into the liquor which is either barren or containing only low levels of gold, this
desorption occurs to maintain the constant ratio of gold per unit area of solids/gold per unit volume of liquor. If the centrifuge separation step has been carried out the solids may be recombined with gold depleted liquor to extract further gold from the solids. As the concentration of the gold per unit area of solids is lowered, so the amount of desorption of gold from the solids into the liquor is lowered. It has been observed that when the pH of the pulp is alkaline (>9) the collection efficiency of the carbon collector is inhibited. If the pulp was to be in contact with the carbon throughout this desorption stage until such time as the gold had desorbed to the equilibrium level for that pulp density, an extremely long and economically unviable contact time would be required. Not only would the operating costs be very high, but the capital costs would be immense.
This desorption effect, which occurs both in the presence and absence of carbon, may be utilised to aid the overall process. This is accomplished by using a short column of carbon to adsorb the majority of the gold from the liquor and then placing the pulp into an agitated tank containing little or no carbon. This allows the pulp enough residence time to desorb gold into the liquor to either the maximum level attainable in relation to the gold grade on the solids, or to such lower levels as may be desired by the operator. The pulp is then passed through a second carbon column and into a second desorption tank.
This cycle may be repeated until the value of the gold recovered per cycle is not greater than the cost of setting up and operating for that cycle. At any stage in these cycles the pulp density may be lowered one or more times to increase the gold recovery.
In all stages of the process, it is important that zones of high pulp density are not allowed to contact the pulp or partially clarified liquor in the interval between pulp dilution and the completion of adsorption of the metal onto the adsorbent. If a zone of high pulp density is allowed to form in this interval, then the metal values will be absorbed onto the solids in this high pulp density zone and will thus not be available for recovery by the process adsorbent. If the process adsorbent has surface
irregularities or internal pores where solids in the pulp may collect to form zones of localised high pulp density, then these solids will absorb the metal values in preference to the process adsorbent. With clay or fine ore particles present in the pulp, a slime layer may coat the surface of the carbon. Thus with process adsorbents such as carbon or ion exchange resins which have surface structures capable of hosting these localised high pulp density zones, the
preferred method of recovering the noble metal values is by contacting the process adsorbent with a clarified liquor. The method of production of this clarified liquor must be such that either no zones of high pulp density are formed, or if they are, that their area of contact with the pulp or resultant clarified liquor is minimal and such contact occurs for a minimum period of time.
If a solids removal step is not used, the collectors used such as zinc or ion exchange fibres must not cause pulp solids to form localized high pulp density zones.
Throughout this specification the preferred method is described in relation to gold recovery using cyanide leaching and carbon recovery. The process is equally
applicable to other noble metal values using the appropriate leaching solution and the appropriate recovery adsorbent such as a resin.
In another aspect, the present invention provides an improved wet assay technique for metal values,
particularly gold. If an aqua regia leach, which is being performed on an ore sample, has carbon material added to the
liquor, some gold will report to the liquor and some gold will report to the carbon. The leached ore is then
subjected to repeated cycles of releaching with fresh aqua regia and carbon until no significant gold is detected in either the carbon or the liquor for that cycle. This allows previously undetectable gold to be measured. The gold is measured in both the carbon and the leach solution.
In all examples in this patent the cyanide, chlorine and thiourea leaches were performed at ambient temperatures and pressures. Although results are not shown 1 in 5 examples were run as blanks. Abbreviations and symbols (shown in brackets) used in this patent include;
pulp density (PD), chlorine (Cl), cyanide (CN), thiourea (Tu), grams per tonne (g/t), parts per million (ppm), micrometres (um), micrograms (ug), grams (g), grams per litre (g/l), high volume aqua regia (HVAR), chloride ion (Cl-), greater than (>), less than (<) carbon in pulp process (CIP), carbon in leach process (CIL),
Di-iso-Butyl-Ketone (DIBK), Atomic Absorption Spectroscopy (AAS) and percentage (%). Eh millivolts are quoted with respect to the Standard Hydrogen Electrode.
Example 1
Test work with clay type ores indicates that more gold is maintained in cyanide solution if the pulp density of the slurry is lowered, all other conditions remaining constant. This is shown by the following results on <38 micron fractions, chosen to avoid any nugget effect. In this example, three (3) ore samples (<38 ppm) were leached for 6 hours at pH 10.5 and a cyanide concentration of 0.1% W>Download PDF in English
