US20120090433A1

US20120090433A1 - Enhanced recovery of gold

Info

Publication number: US20120090433A1
Application number: US13/266,037
Authority: US
Inventors: Dean R. Butler
Original assignee: Precious Metals Recovery Pty Ltd
Current assignee: Precious Metals Recovery Pty Ltd
Priority date: 2009-04-24
Filing date: 2010-04-23
Publication date: 2012-04-19
Also published as: AU2010239156A1; ZA201107781B; CN102459660A; AU2010239157A1; CA2759804A1; WO2010121318A1; AU2010239083A1; WO2010121329A1; US20120067170A1; CA2760345A1; CN102459661A; CA2760354A1; CN102459659A; ZA201107780B; US20120067169A1; ZA201107782B; WO2010121317A1

Abstract

An improved method for recovering refractory gold from a material comprising treating the material to at least partially remove nitric acid-insoluble lead moieties.

Description

FIELD

This invention relates to methods of enhancing the recovery of gold from a material. In particular the invention relates to the recovery of refractory gold not normally amenable to cyanidation. Indeed as the presence of refractory gold is not always evident the invention provides a method for the identification and recovery of refractory gold.

BACKGROUND

Cyanidation is frequently used to recover gold from source materials; however in some materials a proportion of the gold or even a majority of the gold content may not be amenable to direct cyanidation. The ability to properly identify and recover refractory gold can represent a significant economic benefit. A “refractory” gold containing material is a gold containing material such as an ore or a processed ore fraction wherein at least some of the gold cannot be liberated by standard cyanide processing. These refractory materials often require pre-treatment in order for cyanidation or other treatments to be effective in recovery of the gold. Examples of known refractory ores include sulphide containing minerals, carbon containing ores, or materials containing both sulphide minerals and carbon. Sulfide minerals often trap or occlude gold particles, making it difficult for the leach solution to complex with the gold. Carbon present in gold ore may adsorb dissolved gold-cyanide complexes in much the same way as activated carbon, thereby reducing the amount of gold that can be recovered from leach liquor. Gold that is present but is not amenable to direct cyanidation is referred to as refractory gold.
Refractory gold materials may include raw ores, concentrates, flotation concentrates, mine and process tailings and cyanide tailings and residues. These materials may be refractory because precious metals are bonded to minerals in the ore, for example in a silver-manganese dioxide ore, silver may be bonded to manganese and direct cyanidation does not substantially break this bond. Source materials may also be refractory because precious metals are bound or encapsulated in other minerals. One example of this is the instance where precious metals are finely dispersed in sulphide ore so that there is little surface area of precious metal exposed to cyanide leach liquor. The recovery of precious metals from such ores may involve hydrometallurgical or pyro-metallurgical techniques or fine grinding prior to the cyanidation step. Such pre-treatments are designed to render refractory gold amenable to dissolution in a subsequent (frequently cyanide) leach step.
Hydrometallurgical techniques of improving gold recovery include leaching with sulphuric acid, hydrochloric acid or nitric acid. Treatment of source material with bacteria that oxidise sulfidic ores is also possible. Pyrometallurgical techniques for improving gold recovery include roasting, pressure oxidation and chlorination. The great majority of such techniques involve matrix erosion (and frequently matrix oxidation) of the source material. In other words, a large proportion of the gold containing material is eroded in the effort to recover further gold. Matrix erosion, matrix oxidation and/or chemical transformation of the matrix material adds significantly to the cost and waste associated with the process and may render such processes economically or environmentally unacceptable. Typically over 20% by weight of the matrix material in a process stream is eroded or transformed by treatments to liberate refractory gold from that stream.
Gold in an ore may be present in parts per million, so that sampling errors can be multiplied dramatically. It is well known that even small inconsistencies in source materials (e.g. regions of high local concentration of gold) can lead to dramatic errors when multiple samples are taken and analysed for gold (this is sometimes called the nugget effect). A well-developed and practical method for identifying the presence of refractory gold is the method of diagnostic leaching. Diagnostic leaching has been discussed by Lorenzen and Deventer in Minerals Engineering, Vol 6, Nos 8-10, 1013-1023 (1993), “The Identification of Refractoriness in Gold Ores by the Selective Destruction of Minerals”, the contents of which are incorporated by reference. In the diagnostic leach method, specific components in a sample are first eliminated using a selective (e.g. oxidative) leach, and cyanidation is then used to extract the precious metal liberated by destruction of this component. The precious metal extracted can be measured in solution to give a fairly accurate record of the amount of the precious metal associated with that component. Furthermore, the entire residue from this first leaching stage can be subjected to one or more further selective leach stages, leading to the destruction of further components that may prevent the extraction of gold. Washes with at least one of dilute acid and cyanide between these stages can also be used to destroy surface deposits, and to quantify the amount of refractory gold held up in the components that have been destroyed by selective leaching. The procedure can be varied to suit the mineralogy of the matrix material (i.e. the gold containing material). It is customary to quote assay results obtained from leach liquors as a fraction (e.g. percent, or parts per million, or other fraction) of water-washed and dried starting sample. In this way all assay results are referred to the starting sample and can be directly compared.
Diagnostic leaching is not limited to ores and residues, but can deal with any type of intermediary product that occurs on a plant.
Lorenzen and Deventer note that one leach in a diagnostic leach sequence can be a nitric acid leach, followed by a cyanidation step. The nitric acid is said to decompose all ore matrix materials except silicates, and some gold may remain locked in these silicates. This silicate residue is described as “the ultimate residue”, because of the belief that all further gold in the sample is held up inside a silicate matrix. A typical diagnostic leach procedure is:

- (a) cyanide washing to liberate precipitated gold;
- (b) cyanide leaching to liberate free gold;
- (c) HCl/cyanidation to liberate gold associated with pyrrhotite, calcite, ferrites etc;
- (d) H2SO4/cyanidation to liberate gold associated with labile copper sulfides, labile pyrite, base metal sulfides, uraninite etc;
- (e) HNO3/cyanidation to liberate gold associated with pyrite, arsenopyrite, marcasite;
- (f) inter-stage acid washes to liberate surface coatings; and
- (g) acetonitrile elution to liberate gold absorbed on carbon, kerogen, coal.

Henley et al. in Minerals Engineering Vol 14, Issue 1, January 2001, pages 1-12, “Evaluation of a diagnostic leaching technique for gold in native gold and gold/silver tellurides” describe a stage-1 leach in 0.1% cyanide at pH 9.5 for 24 hours, and a stage-2 leach in 2% cyanide at pH 12.5 for 96 hours.
Goodall et al. in Minerals Engineering Vol 18, Issue 10, August 2005, pages 1010-1019, “Applications of PIXE and diagnostic leaching in the characterisation of complex gold ores” describe the use of various diagnostic leach sequences on source materials to provide samples for analysis by Proton-Induced X-Ray Emission (PIXE). From these studies it was concluded that refractory gold may be locked within a sulphide matrix.
Resource Development Incorporated offers customised diagnostic leaching which involves progressively more aggressive leaches and interstage cyanidation to determine deportation of gold in various mineral types. Celep et al. in DPU Fen Bilimleri Enstitusu 16 Sayi Eylul 2008 “Application of Diagnostic Leaching Technique for Refractory Gold Ores” note that diagnostic leaching can attribute refractoriness in an ore to fine gold particles locked up within carbonates, oxides, sulphides and silicates.
The key feature of the diagnostic leach technique is that the characterisation of a source material is achieved by specifying a leach sequence in conjunction with associated analysis protocol for successive leach liquors. Since gold is generally present at rather low weight fractions in a source material, the removal of “easy” gold and “easy” matrix materials prior to the performance of more invasive leaching steps designed to liberate more refractory gold is a logical approach to characterising gold-containing materials, an approach that minimises analytical ambiguity (and in particular minimises analytical difficulties that may arise when one component in the material interferes in the analysis of another component).
Diagnostic leaching provides an efficient framework for defining novel process steps that are effective in releasing refractory gold. Diagnostic leaching furthermore provides a framework for characterising components of source materials that interfere with gold recovery. In prior art, such interfering components have been considered to be present in significant excess (as weight fraction in source material) over gold. There has been little attempt to address interfering components that are present at low levels such as less than 1000 parts per million and especially less than 100 parts per million (by weight).
In some instances the presence of refractory gold in a material may remain undiscovered until a means is found of exposing the presence of interferent and of removing the interferent. In these cases the gold content of the material may be substantially masked or undervalued.
In summary, the reasons for the routine use of diagnostic leaching the gold industry include:

- Avoidance of the nugget effect;
- Minimisation of interferences in analytical procedures;
- Capacity to characterise material responsible for inhibiting gold recovery; and

Capacity to streamline process development by the provision of clear signals on the impact of individual unit operations, and combinations of unit operations on components of samples that contain gold.
The discussion of documents, acts, materials, devices, articles and the like is included in this specification solely for the purpose of providing a context for the present invention. It is not suggested or represented that any or all of these matters formed part of the prior art base or were common general knowledge in the field relevant to the present invention as it existed before the priority date of each claim of this application.
There is an ongoing need to develop pre-treatments that liberate refractory gold, both for use in gold analysis and in gold recovery.

SUMMARY

The present inventor has found that in many cases it is possible to recover refractory gold by removal of interferents comprising lead moieties which are nitric acid insoluble. The removal of such interferents during processing of the gold containing material can lead to gold being recovered in yield greater than would otherwise be expected.
There is thus provided an improved method for recovering refractory gold from a material comprising at least partially removing nitric acid-insoluble lead moieties from the material.
In a preferred set of embodiments the method comprises treating the material to transform at least a portion of the nitric acid insoluble lead moieties into leachable lead moieties and leaching the material to remove at least some of the transformed lead moieties.
Examples of processes adapted to at least partially transform nitric acid-insoluble lead moieties from the gold containing material into leachable lead moieties include contacting the gold containing material with an agent selected from the group consisting of reducing agents, lead solubilising agents and lead complexing agents. It may be the case that that the effectiveness of an agent for removing nitric acid-insoluble lead moieties or for transforming nitric acid-insoluble lead moieties into leachable lead moieties can be enhanced by (a) providing intensive agitation or cavitating agitation to the gold-containing source material, or (b) adopting a process to remove or partially dislodge surface-residing scale. Said process may include the use of heat treatments to evaporatively remove liquor from the surface of the source materials and/or contacting the source material with organic solvents.
The present inventor has found that the use of ultrasonic agitation is particularly effective for enhancing the capacity of an agent to remove nitric acid-insoluble lead moieties or to transform nitric acid-insoluble lead moieties into leachable lead moieties.
The present inventor has also found that the use of a heated agent is effective for enhancing the capacity of said agent to remove nitric acid-insoluble lead moieties or to transform nitric acid-insoluble lead moieties into leachable lead moieties.
The above agents may be gaseous or liquid agents. The gaseous agent may be dissolved in a liquid carrier, and may be stabilised by keeping said carrier under pressure. The gaseous agent may comprise ammonia or other amine functionality.
If the above-mentioned agent is a liquid, it may be an aqueous liquid. The aqueous liquid may provide a reducing environment or an oxidising environment, or an environment which is neither reducing nor oxidising. The oxidation potential of the aqueous liquor may vary from a negative (reducing) value to a less negative or even positive (oxidising) value during the course of the treatment. Alternatively the oxidation potential of the liquid may vary from a positive (oxidising) value to a less positive or even negative (reducing) value during the course of the treatment. Independently of the oxidation potential of the liquid, the pH of the liquid may be acidic, neutral or alkaline.
In one preference the aqueous liquid may comprise ammonium ions or other ions that comprise amine functionality.
One or a plurality of sequential processes may be used to at least partially remove nitric acid-insoluble moieties from the source material.
In one preference, fine particles are at least partially removed from the source material prior to treatment to at least partially remove nitric acid-insoluble moieties from the source material. In one preference, fine particles are at least partially removed by passing a 10% slurry of source material through a 2-inch Mozley hydrocylone at 350 kPa input pressure, rejecting the overflow stream and collecting that fraction of source material in the underflow stream for further treatment.
Many processes that involve contacting gold containing material with reducing agents, lead solubilising agents, lead complexing agents are ineffective in removing nitric acid-insoluble lead moieties. This is because nitric acid is a powerful reagent for extracting lead moieties from a sample.
The invention provides a method for identifying suitable agents and conditions for at least partially removing nitric acid-insoluble lead moieties from a sample. Suitable agents and conditions can be established using the “nitric acid-insoluble lead detection test” as herein described. This test involves (a) a first step of providing multiple nitric acid leaching steps to a sample until the level of lead solubilised in the final nitric acid leaching liquor is reliably less than 5 ppm, preferably less than 1 ppm, (as a weight fraction of the initial sample weight)—this leaves an ultimate residue devoid of nitric acid-soluble lead moieties and (b) a second step of applying candidate agents and conditions to the ultimate residue—those agents and conditions that facilitate the liberation of more than 1.5 ppm lead from the ultimate residue are recognised as having enabled the at least partial removal of nitric acid-insoluble lead moieties from the sample.
An important aspect of the invention is that agents and conditions identified in the above test may be applied directly to source material as a treatment to enable the recovery of refractory gold from the source material.
Examples of suitable reducing conditions include reducing conditions provided by an electrode in contact with a liquor, and/or reducing conditions provided by the use of reducing agents, and/or reducing conditions provided by the use of reducing micro-organisms.
Reducing agents are preferably chosen from the set of reducing agents compatible with water. Preferably these reducing agents are chosen from the set comprising chromium(II), tin(II), copper(I), titanium(II) and titanium(III) moieties, and also comprising sulfites, sulphur-containing reducing agents oxalic acid and other organic reducing agents.
In one preference, the a suitable aqueous reducing liquor will comprise tin(II) moities, more preferably in the form of stannous chloride.
In one preference, the aqueous reducing liquor acidic, and preferably the pH is less than 1.
Examples of lead complexing or lead solubilising agents may be aqueous liquors comprising one or more selected from the group consisting of hydrochloric acid, nitric acid, alkaline material such as sodium hydroxide or other hydroxide moities or other water-compatible alkalis, lead acetate, ammonium chloride, chlorides, carboxylic acids and their salts, chelating agents, fluoro silicate, phenol sulfonate, peroxy-disulfate and any other agent that enhances the solubility of lead oxide moieties in water, When the lead complexing or solubilising agent is selected from carboxylic acids and their salts or chlorides it is preferred that (a) the carboxylic acids are selected from the group consisting of citric acid, lactic acid, acetic acid, formic acid, iso-butyric acid, acetyl salicylic acid and their salts such as the alkali and alkaline earth metal salts and (b) the chlorides are selected from the group consisting of ammonium chloride, sodium chloride, potassium chloride, calcium chloride and strontium chloride.
In a further set of embodiments there is provided a method of determining the presence of refractory gold in a material suspected of containing refractory gold comprising separating a sample from the material; subjecting the sample to a treatment adapted to at least partially remove nitric acid-insoluble lead moieties from the sample, and analysing the treated sample for gold.
In this set of embodiments we have found that when the sample, after multiple nitric acid leaching steps releases less than 1 ppm of lead in the last nitric acid leach liquor, and when the treatment adapted to at least partially remove nitric acid-insoluble lead moieties liberates at least 1.5 ppm lead from the residue formed after the nitric acid leach sequence, this is a good indication that the said treatment will be effective in enabling refractory gold to be analysed in the sample.
Accordingly, the method may comprise determining the presence of refractory gold and recovering the refractory gold by treating the material with the method adapted to solubilise nitric acid insoluble lead moieties.

DETAILED DESCRIPTION

The present inventor has investigated a variety of gold-containing source materials using diagnostic leach methodology. They have particularly investigated source materials using a diagnostic leach protocol comprising a sequence of nitric acid leaches. Residues from such multiple nitric acid leach treatments will be denoted “ultimate residues” in accordance with the prior art nomenclature. The prior art states that any gold remaining in ultimate residues will be locked in a silica matrix, and can only be un-locked by methods that provide matrix erosion of the silica matrix, e.g. treatment with hydrofluoric acid.
The inventors have made the surprising discovery that refractory gold may exist in ultimate residues in association with lead moieties. Such lead moieties are not locked in a silica matrix and are noteworthy in that they are insoluble in a succession of nitric acid leaching steps—henceforth they will be referred to as “nitric acid-insoluble lead moieties”. The association between refractory gold and nitric acid-insoluble lead moieties has not previously been identified.
Notwithstanding that the at least partial removal of nitric acid insoluble lead moieties may be important to the recovery of refractory gold there is no requirement to conduct nitric acid leach steps in the method of the invention. Diagnostic leaching may, if desired, be used to identify the presence of nitric acid insoluble lead and may also be useful in conducting trials to examine agents and processes for the at least partial removal of nitric acid insoluble lead moieties.
Nitric acid insoluble lead moieties have a significant effect on the ability to recover gold even when they are present at low levels relative to nitric acid soluble lead moieties. For example we have found the nitric acid insoluble lead moieties, when transformed and solubilised in a leach liquor are frequently present at less than 10% such as less than 5%, less than 2% and less than 1% of the cumulative amount of nitric acid soluble lead moieties that can be recovered in successive nitric acid leaching steps.

Definitions

“refractory-gold materials” refers to source materials containing gold which can not be recovered by direct cyanide treatment.
“refractory gold” refers to gold that is present in refractory source materials but which is not amenable to removal by direct cyanidation A discussion of refractory source materials is provided in U.S. Pat. No. 5,232,490 (Bender et al., filed 1992), the contents of which are incorporated by reference.
“matrix erosion pre-treatments” refers to pre-treatments such as hydrometallurical and pyrometallurgical pretreatments which lead to the dissolution/erosion of a significant fraction of the source material (more than 15% by weight, frequently more than 20% by weight, calculated on the weight of water-washed and dried source material).
The term “nitric acid-insoluble lead moieties” refers to lead moieties which are insoluble in a succession of nitric acid leaching steps. These moieties may act as interferents in conventional gold recovery processes.
The terms “ultimate nitric acid leach residues” and “ultimate leach residues” refer to residues from such multiple leach treatments with nitric acid.
Throughout the description and the claims of this specification the word “comprise” and variations of the word, such as “comprising” and “comprises” is not intended to exclude other additives, components, integers or steps.
The inventors have found that treatments of ultimate residue that at least partially remove nitric acid-insoluble lead moieties may also be applied directly to material believed to contain refractory gold as treatments or pre-treatments for recovering refractory gold.
Without wishing to be bound by theory, the chemical nature of the nitric acid-insoluble lead moieties that have been discovered by the present inventor to interfere with gold recovery are believed to include moieties encompassed by the term “sulphur-deficient lead sulfate”. One such moiety is the mineral lanarkite, however, it is believed that gold may also be in association with other sulphur-deficient lead sulfate moieties, and that said moieties may be present as a coating that resists dissolution and that surrounds microfine dispersed gold in the source materials.
Agents and conditions for at least partially removing nitric acid insoluble lead from a given material may be chosen without undue experimentation by examination of the material using the Nitric insoluble lead detection test described below. Once suitable conditions for at least partially removing nitric acid insoluble lead moieties have been determined the given material may be processed using the chosen conditions (i.e. without the use of nitric acid leaches).

Nitric Insoluble Lead Detection Test

Part 1—Diagnostic Leach Procedure For Generation of Standard Ultimate Residue

Particulate source material, generally less than 500 microns in size, (CHECK) is water washed and dried. 10 g of this material is placed in a 600 ml beaker containing 200 mls of liquor comprising 1 part by volume water and 1 part by volume of conc. nitric acid. The beaker is placed in thermostatted ultrasonic bath at 60° C. (bath frequency 42 kHz) with the power set at 150 Watts (i.e. 60% of maximum 250 Watt power). Ultrasonic agitation was applied for one hour followed by one hour with the power off, then one hour with power on etc. for a total of eight hours. The residue is then filtered and the filtrate tested for lead using flame AA. The entire residue is again leached in 50% conc. nitric acid according to the above protocol, and the filtrate again assayed for lead. Further leaches are carried out until the lead in filtrate (expressed as a fraction of the weight of initial water-washed and dried source material) is less than 1 ppm in 2 successive leach steps. Up to nine leaches or more may be applied, until the lead found in filtrate is less than 1 ppm on 2 successive leach steps.

Part 2—Evaluation of Candidate Treatment For At Least Partially Removing Nitric Acid Insoluble Lead Moieties

Standard ultimate residue is generated from the source material as outlined in Part 1 above and the candidate treatment is applied to the resultant ultimate residue. The last step in the candidate treatment is an aqueous leaching step. Liquor for this aqueous leaching step is filtered off and assayed for lead using flame AA. If lead in filtrate (expressed as a fraction of the weight of the initial water-washed and dried source material) exceeds 1.5 ppm this is taken as confirmation that the candidate treatment at least partially removes nitric acid-insoluble lead moieties from the source material. The power of the diagnostic leach process is that sequential leaches of the same sample of source material are used to establish all aspects of the characterisation—in particular there is no need to prepare two “identical” samples of source material for comparison. This overcomes the well-known problem designated the nugget effect—it is that when gold is present at low levels, a chance occurrence of a gold-rich local region in one sample cannot be replicated in another sample. An analogue of the nugget effect also applies to other materials that may be present at low levels, and may associate with gold in such a way that recovery of the gold is prevented. Such low-level interfering materials (e.g. nitric acid-insoluble lead moieties) will also be subject to the nugget effect, so that attempts at direct analytical determination lead to ambiguous results. The use of diagnostic leaching also overcomes problems associated with the analogue nugget effect for interfering materials. An important preference for performance of the invention includes the use of diagnostic leach methodology to characterise gold recovery treatments.

Criterion For Establishing That A Candidate Treatment At Least Partially Transforms Nitric Acid-Insoluble Lead Moieties To Nitric Acid-Soluble Lead Moieties In A Source Material

Standard ultimate residue is generated from the source material as outlined above and the candidate treatment is applied to the ultimate residue. Thereafter the resulting residue is leached in a liquor capable of dissolving lead (e.g. 50% nitric acid in water according to the protocol provided in the above section on diagnostic leach procedure), and the filtrate is assayed for lead using flame AA or an equivalent accredited method. If lead in filtrate after this final leach (expressed as a fraction of the weight of the initial water-washed and dried source material) exceeds 1.5 ppm, this is taken as confirmation that the treatment at least partially converts nitric acid-insoluble lead moieties to nitric acid-soluble lead moieties in a source material.

Definition of “Indicative Level of Nitric Acid-Insoluble Lead Moieties”

The total quantity of nitric acid-insoluble lead moieties present in a sample may be difficult to determine in an absolute and unambiguous manner, in the same way as the total level of refractory gold in a sample is difficult to determine. This is because whatever the quantity of nitric acid-insoluble lead moieties recovered from treated ultimate residue in a leach liquor, further quantities may still reside in the residue. However the inventors have found that if a standard ultimate residue is generated from a sample of source material, a leach (under ultrasonic agitation at 60 deg C as described above in the section on Diagnostic Leach Procedure for generation of Standard Ultimate Residue) of the standard ultimate residue with a liquor made by adding 100 ml conc HCl to 100 ml water, and dissolving 8 g stannous chloride dihydrate in the resultant 50% HCl frequently liberates significantly more lead into the leach liquor than was liberated in the previous nitric acid leach (used to generate the standard ultimate residue).
The excess of lead from the acid/stannous chloride leach over the preceding nitric acid leach (expressed as a fraction of original water-washed and dried source material in ppm) will, here and henceforth be designated as the indicative level of acid-insoluble lead moieties in the source material. The indicative level of nitric acid-insoluble lead moieties in a sample is a quantity that is well-defined in terms of diagnostic leach procedures, although clearly the content of acid-insoluble lead moieties in a sample may be greater than the indicative level.
The fire assay is a common technique used to estimate the gold content of a source material and is the subject of Australian Standard 2917.2 (1994) “Fire assay gravimetric and atomic absorption spectrometric method” and coresponding international standards. The presence of refractory gold is generally demonstrated by noting that the gold yield from cyanidation is significantly less, in percentage terms, than the gold yield by fire assay. Fire assays are usually conducted by specialist laboratories such as Amdel Laboratories in Australia.
In some instances the present inventor has found that pre-treatments which at least partially remove nitric acid-insoluble lead moieties from the material can lead to recovery higher than indicated by fire assay of the material.
Thus, pretreatment can lead to the recovery of more gold (in a subsequent leach or leach sequence) than the gold yield in the fire assay of the original material.
In a preferred set of embodiments the method comprises treating the material to transform at least a portion of the nitric acid insoluble lead moieties into leachable lead moieties and leaching the material to remove at least some of the transformed lead moieties.
In one preference, there is provided an improved method for recovering refractory gold from a material, wherein the source material comprises both nitric acid-soluble lead moieties and an indicative level of nitric acid-insoluble lead moieties, and wherein the ratio of nitric acid-soluble lead moieties to the indicative level of nitric acid-insoluble lead moieties is greater than 10:1, more preferably greater than 100:1.
In one preference, the method involves relatively minor erosion of the matrix of the material treated to remove nitric acid insoluble lead moieties (based on weight of water washed and dried matrix) and preferably less than 10%, such as less than 5%, less than 2% and less than 1%. Less preferred matrices involve those that undergo significant erosion. For example, a mineral acid used on a carbonate matrix, would cause significant corrosion. Specifically, the preferred method involves relatively minor erosion of the matrix as compared to the same leaching solution in the absence of reducing components (eg. HCl/stannous leach as compared with an HCl leach alone)
In one preference, the method of the invention involves treating the source material with a reducing aqueous liquor or a reducing gas. The reducing leach may produce at least partial removal of a base metal from the source material. Without being bound by theory it is believed likely that the use of a reducing leach may facilitate the dissolution of moieties comprising Iron (III), and that these moieties are responsible or partially responsible for immobilizing gold. Evidence for the dissolution of moieties comprising iron III includes decoloration of material after leaching. Leaching may be carried out in liquors comprising 1% HCl and one or more reducing agents such as tin (II) chloride, chromium (II) chloride and oxalic acid. Based on the observed degree of decoloration the effectiveness of reducing agents decreases according to the ranking tin (II) chloride, ≧chromium (II) chloride>oxalic acid
The invention will now be described with reference to the following examples. It is to be understood that the examples are provided by way of illustration of the invention and that they are in no way limiting to the scope of the invention.

EXAMPLES

Example 1

This example shows how ultimate residue is generated from a source material.
1 part by weight of fine playa source material (sub 100 microns) was taken from near Silver City in Mew Mexico USA and was slurried in water (10 parts by weight), then the slurry was passed through a 2-inch Mozley hydrocyclone at 350 kPa pressure. The overflow stream was rejected, and the underflow stream was filtered under air pressure to form a moist residue which was placed in an oven at 80 deg C overnight to dry, giving a de-slimed dried, water-washed material for further treatment,
This material was subject to a succession of nitric acid leaching steps with ultrasonic agitation, according to the following protocol:
L1: 10 g of de-slimed washed and dried playa material was placed in a 600 ml beaker containing a liquor made by adding 100 ml of concentrated nitric acid to 100 ml water. The beaker was placed in a Soniclean thermostatted ultrasonic bath (250 W max power setting, bath frequency 42 kHz). The ultrasonic bath temperature was set at 60 deg C, and the power setting was 60%, corresponding to 150 W power. Ultrasonic power was applied for one hour, followed by one hour power-off, then one hour power on and so forth, for a total of 8 hours bath time. The slurry in the beaker was filtered using Whatman no 1 filter paper, and the filtrate was taken for lead analysis by AA. The lead content of filtrate (expressed as a fraction of the 10 g of starting material) was 37.1 ppm.
L2: The moist residue on the filter paper from L1 was quantitatively transferred to a second 600 ml beaker and to it was added of 100 ml of water and 100 ml of concentrated nitric acid. Thereafter the protocol was as for L1. The filtrate was taken for lead analysis (by AA) and found to contain 3.12 ppm lead (expressed as a fraction of the 10 g of original starting material).
L3: Moist residue from L2 was quantitatively transferred to a third 600 ml beaker and to it was added 100 ml of water and 100 ml of concentrated nitric acid. Thereafter the protocol was as for L1. The filtrate was taken for lead analysis (by AA) and found to contain 1.01 ppm lead (expressed as a fraction of the 10 g of original starting material).
L4: Moist residue from L3 was quantitatively transferred to a fourth 600 ml beaker and to it was added 100 ml of water and 100 ml of concentrated nitric acid. Thereafter the protocol was as for L1. The filtrate was taken for lead analysis (by AA) and found to contain 0.65 ppm lead (expressed as a fraction of the 10 g of original starting material).
L5: Moist residue from L4 was quantitatively transferred to a fifth 600 ml beaker and to it was added 100 ml of water and 100 ml of concentrated nitric acid. Thereafter the protocol was as for L1. The filtrate was taken for lead analysis (by AA) and found to contain 0.0 ppm lead (expressed as a fraction of the 10 g of original starting material).
The resultant residue was designated ultimate residue derived from the starting material, since nitric acid leaching with ultrasonic agitation was found on 2 successive occasions to liberate less than 1 ppm lead into the leach liquor. This ultimate residue was taken to be substantially free of nitric acid-soluble lead moieties.
In repeat experiments with samples of the same de-slimed washed and dried playa material, it was found that 5 successive nitric acid leaching steps with ultrasonic agitation was sufficient to generate ultimate residue—in each experiment the last 2 sequential leach steps liberated less than 1 ppm lead into the leach liquor.

Example 2

This example illustrates a procedure, according to the current invention, for identifying agents and conditions that enable the at least partial removal of nitric acid-insoluble moieties from a sample. The sample is the playa material described in example 1. The basis of the procedure is to first rigorously remove nitric acid-soluble lead moieties from the sample, and then to search for the appearance of lead in subsequent trial leaching steps. In this example, the trial leaching steps involve the use of stannous chloride in aqueous acid conditions.
Ultimate residue was generated from 10 g de-slimed washed and dried playa material as described in example 1. The ultimate residue was transferred to a 600 ml beaker and to it was added an aliquot of liquor consisting of (a) 100 ml water; (b) 100 ml concentrated hydrochloric acid; and (c) 8 g stannous chloride dihydrate (dissolved). Stannous chloride is a known reducing agent. The beaker was placed in a Soniclean thermostatted ultrasonic bath (250 W max power setting, bath frequency 42 kHz). The ultrasonic bath temperature was set at 60 deg C, and the power setting was 60%, corresponding to 150 W power. Ultrasonic power was applied for one hour, followed by one hour power-off, then one hour power on and so forth, for a total of 8 hours bath time. The slurry in the beaker was filtered using Whatman no 1 filter paper, and the filtrate was taken for lead analysis by ICP (samples were sent to Veolia Laboratories in Adelaide, Australia). The lead content of filtrate (expressed as a fraction of the 10 g of original washed and dried playa source material) was 21 ppm. This quantity of lead was significantly greater than the lead (less than 1 ppm) found in the ultimate and penultimate nitric acid/ultrasonic leach steps used to generate ultimate residue. In other words, the above-described reducing acid leach with hydrochloric acid and stannous chloride led to the at least partial removal of nitric acid-insoluble lead moieties from the ultimate residue. In this case, the indicative level of nitric acid-insoluble lead (as previously defined) in the de-slimed washed and dried playa source material was 21 ppm.
The residue from the filter paper obtained after the above-described acid reducing leach was transferred to a 600 ml beaker, and to it was added 200 mls of 4% aqueous sodium hydroxide solution. The beaker was placed in the ultrasonic bath and subjected to the leaching protocol in the bath for 8 hours as described previously. After the leaching/ultrasonic treatment was finished, the slurry was filtered through a Whatman 1 filter paper, and the filtrate analysed for lead by flame AA. The lead content of filtrate (expressed as a fraction of the 10 g of original washed and dried playa source material) was 6.5 ppm. From this result it is clear that the above-described sequence of (a) a leaching step under acid reducing conditions, and (b) a leaching step under alkaline conditions is particularly effective in removing nitric acid-insoluble lead moieties from a sample.

Caustic Liquor Leaching Does Not Remove Nitric Acid-Insoluble Lead Moieties From Ultimate Residue Generated From This Source Material

Ultimate residue from a 10 g sample of de-slimed washed and dried playa material (prepared as described in example 1) was directly leached in 4% aqueous sodium hydroxide solution as described above. After the leaching/ultrasonic treatment was finished, the slurry was filtered through a Whatman 1 filter paper, and the filtrate analysed for lead by flame AA. The lead content of filtrate (expressed as a fraction of the 10 g of original washed and dried playa source material) was less than 1 ppm. This experiment shows that leaching with ultrasonic agitation using aqueous sodium hydroxide liquor does NOT lead to the at least partial removal of nitric acid-insoluble lead moieties. The above-described appearance of lead in the alkaline leach liquor has occurred after a prior leaching step in acid reducing conditions (see above). This can be rationalised as follows: under acid/reducing leach conditions, some of the nitric acid-insoluble lead moieties are transformed into (leachable) lead moieties that can be leached into caustic liquor.
According to the teaching of the present invention, the above-described sequence of (a) a leaching step under acid reducing conditions, and (b) a leaching step under alkaline conditions is adapted to provide a pre-treatment of source material that will enable the subsequent recovery of refractory gold (see example 3).

Example 3

This example shows how the use of agents and conditions that at least partially remove nitric acid-insoluble lead moieties (see example 2) can enable the recovery of refractory gold. The agents and conditions comprise a) a leaching step under acid reducing conditions, and (b) a leaching step under alkaline conditions. Both steps include the use of ultrasonic agitation.
10 g of de-slimed washed and dried playa material (described in example 1) was sent to a specialist gold assay laboratory (Amdel) for the fire assay determination of gold. The gold assay result was 0.3 ppm.
Another sample consisting of 10 g of de-slimed washed and dried playa material was given the following pre-treatment (see example 2): the sample was transferred to a 600 ml beaker and to it was added liquor comprising (a) 100 ml water; (b) 100 ml concentrated hydrochloric acid; and (c) 8 g stannous chloride dihydrate (dissolved). The beaker was placed in a Soniclean thermostatted ultrasonic bath (250 W max power setting, bath frequency 42 kHz). The ultrasonic bath temperature was set at 60 deg C, and the power setting was 60%, corresponding to 150 W power. Ultrasonic power was applied for one hour, followed by one hour power-off, then one hour power on and so forth, for a total of 8 hours bath time. The resultant slurry in the beaker was filtered using Whatman no 1 filter paper, and the residue from the filter paper was transferred to a 600 ml beaker, and to it was added 200 mls of 4% aqueous sodium hydroxide liquor. The beaker was placed in the Soniclean ultrasonic bath and treated according to the ultrasonic bath protocol described above. As noted in example 2, the above combination of steps accomplished the at least partial removal of nitric acid-insoluble moieties from the source material.

Gold Recovery

The resultant slurry was filtered using a Whatman 1 filter paper, and the residue was assayed for gold content using the following aqua regia protocol: 200 ml concentrated hydrochloric acid and 50 ml concentrated nitric acid were added to a 600 ml beaker, and the residue was promptly added. The beaker was added to the ultrasonic bath (60 deg C) and agitated for 2 minutes at 250 W (full power). The beaker was placed on a hot plate and heated to incipient boiling for 5 minutes, then added to the ultrasonic bath again with agitation at 250 W for another 2 minutes. The beaker was placed on a hot plate and heated to incipient boiling yet again for 5 minutes, then added yet again (a third time) to the ultrasonic bath with agitation at 250 W power for 2 minutes. The beaker was then returned to the hot plate and boiled until (a) evolution of nitrous fumes ceased, and (b) the liquor volume in the beaker was down to 80 ml. The slurry in the beaker at this point was hot-filtered through a Whatman 1 filter paper, and the filtrate was retained for AA analysis for gold. A gold content of 7.0 ppm was found (expressed as a fraction of the 10 g of original washed and dried playa source material). Compared with the gold assay value discovered by standard fire assay (0.3 ppm), it can be seen that the method of the invention provides a pre-treatment that enables recovery of refractory gold that is significantly in excess of the fire assay content.

Example 4

This example shows how agents and conditions quite distinct from those described in example 2 can be used to at least partially remove nitric acid-insoluble lead moieties from a particular sample. As in example 2, the appropriate agents and conditions are explored by first rigorously removing nitric acid-soluble lead moieties from the sample, and then searching for the appearance of lead in subsequent trial leaching steps. The example shows a 2-step trial process wherein (a) a first leach with aqueous ammonia transforms nitric acid-insoluble lead moieties to leachable form (however lead moieties are not liberated into the ammonia liquor); and (b) a second leach with aqua regia liberates the leachable lead moieties formed in the first leach. Both leaches were performed with ultrasonic agitation.
Ultimate residue was generated from 10 g de-slimed washed and dried playa material as described in example 1. The ultimate residue was transferred to a 600 ml beaker and to it was added liquor consisting of 200 mls 7.5% aqueous ammonia. The beaker was placed in a Soniclean thermostatted ultrasonic bath (250 W max power setting, bath frequency 42 kHz). The ultrasonic bath temperature was set at 60 deg C, and the power setting was 60%, corresponding to 150 W power. Ultrasonic power was applied for 10 minutes, followed by 2 hours power-off, then ten minutes power on and so forth, for a total of 8.5 hours bath time. The slurry in the beaker was filtered using Whatman no 1 filter paper, and the filtrate was taken for lead analysis by AA. The lead content of filtrate (expressed as a fraction of the 10 g of original washed and dried playa source material) was 0.92 ppm. This quantity of lead was NOT significantly greater than the lead (less than 1 ppm) found in the ultimate and penultimate nitric acid/ultrasonic leach steps used to generate ultimate residue.
The residue from the above ammonia leach was treated using the aqua regia protocol described in experiment 3 above, except that the final liquor was assayed for lead rather than gold (aqua regia has a considerable capacity for solubilising lead, albeit less than nitric acid). A lead content of 10.23 ppm was found (expressed as a fraction of the 10 g of original washed and dried playa source material)—this is significantly greater than the lead found (less than 1 ppm) in the penultimate and ultimate nitric acid/ultrasonic leach steps used to generate ultimate residue.
This result shows that the ammonia leaching step described above converts nitric acid-insoluble lead moieties in the residue to leachable lead moieties (that were at least partially removed by the aqua regia leach).
According to the teaching of the present invention, the above-described ammonia leach, in conjunction with the use of a leach that removes leachable lead moieties (formed in the ammonia leach), is adapted to provide a pre-treatment of source material that will enable the subsequent recovery of refractory gold (see example 5).

Example 5

This example shows how washed and dried playa source material can be treated with an ammonia leach followed by a further aqueous leaching step that removes (leachable) lead formed in the ammonia leach. This treatment at least partially removes nitric acid-insoluble lead moieties (see example 4) and is shown to enable the recovery of refractory gold.
10 g of de-slimed washed and dried fine playa source material was sent to a specialist gold assay laboratory (Amdel) for the fire assay determination of gold. The gold assay result was 0.3 ppm (average of duplicate samples).
Another sample consisting of 10 g of de-slimed washed and dried playa material was given the following pre-treatment (see example 4): The sample was transferred to a 600 ml beaker and to it was added liquor consisting of 200 mls 7.5% aqueous ammonia. The beaker was placed in a Soniclean thermostatted ultrasonic bath (250 W max power setting, bath frequency 42 kHz). The ultrasonic bath temperature was set at 60 deg C, and the power setting was 60%, corresponding to 150 W power. Ultrasonic power was applied for 10 minutes, followed by 2 hours power-off, then ten minutes power on and so forth, for a total of 8.5 hours bath time. The slurry in the beaker was filtered using Whatman no 1 filter paper, and the residue was analysed for gold using the aqua regia protocol described in experiment 3. A gold content of 3.5 ppm was found (expressed as a fraction of the 10 g of original washed and dried playa source material). Compared with the gold assay value discovered by standard fire assay (0.3 ppm), it can be seen that the method of the invention provides a pre-treatment that enables recovery of refractory gold that is significantly in excess of the fire assay content.

Example 6

This example illustrates the use of the present invention on a different starting material. The starting material was designated CH-4 by CANMET Mining and Mineral Sciences Laboratories, 555 Booth St, Ottawa, Ontario, Canada. The sample was characterised as part of the Canadian Certified Reference Materials Project. The source material for CH-4 was donated by Corporation Miniere Inmet, Division Troilus, Chibougama, Quebec in 2000. After crushing, milling, sieving and blending, the yield was 37%. The material is a fine powder with a mesh size of less than 45 microns (325 mesh). The host rock of the raw material is meta-anorthosite. The mineralogy includes pyrrhotite, pyrite and chalcopyrite, and small amounts of spalerite, galena and molybdenite. The mean gold content of the CH-4 CANMET standard is given as 0.88 ppm, with a within-lab standard deviation of 0.04 ppm, and a between-labs standard deviation of 0.04 ppm.
The procedure was to first rigorously remove nitric acid-soluble lead moieties from CANMET sample CH-4, and then to search for the appearance of lead in subsequent trial leaching steps. In this example, the trial leaching steps involve the use of stannous chloride in aqueous acid conditions.
Ultimate residue was generated from 10 g CANMET CH-4 material as described in example 1. No detectable lead was found in the 4^thand 5th nitric acid leach liquors. The ultimate residue was transferred to a 600 ml beaker and to it was added an aliquot of liquor consisting of (a) 100 ml water; (b) 100 ml concentrated hydrochloric acid; and (c) 8 g stannous chloride dihydrate (dissolved). Stannous chloride is a known reducing agent. The beaker was placed in a Soniclean thermostatted ultrasonic bath (250 W max power setting, bath frequency 42 kHz). The ultrasonic bath temperature was set at 60 deg C, and the power setting was 60%, corresponding to 150 W power. Ultrasonic power was applied for one hour, followed by one hour power-off, then one hour power on and so forth, for a total of 8 hours bath time. The slurry in the beaker was filtered using Whatman no 1 filter paper, and the filtrate was taken for lead analysis by ICP (samples were sent to Veolia Laboratories in Adelaide, Australia). The lead content of filtrate (expressed as a fraction of the 10 g of CH-4 starting material) was 14 ppm. This quantity of lead was significantly greater than the lead (less than 1 ppm) found in the ultimate and penultimate nitric acid/ultrasonic leach steps used to generate ultimate residue. In other words, the above-described reducing acid leach with hydrochloric acid and stannous chloride led to the at least partial removal of nitric acid-insoluble lead moieties from the ultimate residue. In this case, the indicative level of nitric acid-insoluble lead (as previously defined) in the de-slimed washed and dried playa source material was 14 ppm.
The residue from the filter paper obtained after the above-described acid reducing leach was transferred to a 600 ml beaker, and to it was added 200 mls of 4% aqueous sodium hydroxide solution. The beaker was placed in the ultrasonic bath and subjected to the leaching protocol in the bath for 8 hours as described previously. After the leaching/ultrasonic treatment was finished, the slurry was filtered through a Whatman 1 filter paper, and the filtrate analysed for lead by flame AA. The lead content of filtrate (expressed as a fraction of the 10 g of original washed and dried playa source material) was 2.8 ppm. From this result it is clear that the above-described sequence of (a) a leaching step under acid reducing conditions, and (b) a leaching step under alkaline conditions is particularly effective in removing nitric acid-insoluble lead moieties from a sample.

Example 7

This example is based on an investigation of CANMET sample CH-4. The example shows how the use of agents that at least partially remove nitric acid-insoluble lead moieties (see example 6) can enable the recovery of refractory gold. An investigation is made of the use of acid reducing conditions to liberate refractory gold from starting material.
CH-4 material contained 0.88 ppm gold on the basis of conventional gold assays (see example 6).
A sample consisting of 10 g of CANMET CH-4 material was given the following pre-treatment: the sample was transferred to a 600 ml beaker and to it was added liquor comprising (a) 100 ml water; (b) 100 ml concentrated hydrochloric acid; and (c) 8 g stannous chloride dihydrate (dissolved). The beaker was placed in a Soniclean thermostatted ultrasonic bath (250 W max power setting, bath frequency 42 kHz). The ultrasonic bath temperature was set at 60 deg C, and the power setting was 60%, corresponding to 150 W power. Ultrasonic power was applied for one hour, followed by one hour power-off, then one hour power on and so forth, for a total of 8 hours bath time. The resultant slurry in the beaker was filtered using Whatman no 1 filter paper, and the residue from the filter paper was transferred to a 600 ml beaker, and to it was added 200 mls of 4% aqueous sodium hydroxide liquor. The beaker was placed in the Soniclean ultrasonic bath and treated according to the ultrasonic bath protocol described above. As noted in example 2, the above combination of steps accomplished the at least partial removal of nitric acid-insoluble moieties from the source material

Gold Recovery

The resultant slurry was filtered using a Whatman 1 filter paper, and the residue was assayed for gold content using the following aqua regia protocol: 200 ml concentrated hydrochloric acid and 50 ml concentrated nitric acid were added to a 600 ml beaker, and the residue was promptly added. The beaker was added to the ultrasonic bath (60 deg C) and agitated for 2 minutes at 250 W (full power). The beaker was placed on a hot plate and heated to incipient boiling for 5 minutes, then added to the ultrasonic bath again with agitation at 250 W for another 2 minutes. The beaker was placed on a hot plate and heated to incipient boiling yet again for 5 minutes, then added yet again (a third time) to the ultrasonic bath with agitation at 250 W power for 2 minutes. The beaker was then returned to the hot plate and boiled until (a) evolution of nitrous fumes ceased, and (b) the liquor volume in the beaker was down to 80 ml. The slurry in the beaker at this point was hot-filtered through a Whatman 1 filter paper, and the filtrate was retained for AA analysis for gold. A gold content of 3.0 ppm was found (expressed as a fraction of the 10 g of starting material). Compared with the gold assay value discovered by standard fire assay (0.88 ppm), it can be seen that the method of the invention (using acid reducing leach conditions) provides a pre-treatment that enables recovery of refractory gold that is significantly in excess of the fire assay content.

Claims

1. An improved method for recovering refractory gold from a material comprising treating the material to at least partially remove nitric acid-insoluble lead moieties.

2. A method according to claim 1 comprising treating the material to transform at least a portion of the nitric acid insoluble lead moieties into leachable lead moieties and leaching the material to remove the leachable lead moieties.

3. A method according to claim 1 wherein the process of at least partially removing nitric acid-insoluble lead moieties from the gold containing material comprises contacting the gold containing material with an agent selected from the group consisting of reducing agents, lead complexing agents and lead solubilising agents.

4. A method according to claim 1 comprising contacting the material with a reducing agent.

5. A method according to claim 4 wherein the reducing agent is selected from the group consisting of chromium(II), tin(II), copper(I), titanium(II) and titanium(III) moieties, and also comprises sulfites, sulphur-containing reducing agents, oxalic acid, and other organic reducing agents.

6. A method according to claim 1 comprising contacting the gold containing material with a lead complexing agent.

7. A method according to claim 6 wherein the lead solubilising and complexing agents are selected from the group consisting of carboxylic acids and their salts, chlorates, perchlorates, alkalis, chlorides, fluorosilicate, phenol sulfonate and peroxy-disulfonate.

8. A method according to claim 7 wherein (a) the carboxylic acids are selected from the group consisting of citric acid, lactic acid, acetic acid, formic acid, iso-butyric acid, acetyl salicylic acid and their salts and (b) the chlorides are selected from the group consisting of ammonium chloride, sodium chloride, potassium chloride, calcium chloride and strontium chloride.

9. A method according to claim 4 wherein the method of at least partially removing nitric acid insoluble lead moieties further comprises at least one step selected from the group consisting of (a) providing intensive agitation or cavitating agitation to the gold-containing source material preferably ultrasonic agitation, (b) adopting a process to at least partially remove surface scale and (c) conducting the at least partial removal of nitric acid insoluble moieties at a temperature of at least 40° C.

10. A method according to claim 9 wherein the method of at least partially removing nitric acid insoluble lead moieties further comprises ultrasonic agitation.

11. A method of recovering refractory gold from a material suspected of containing refractory gold comprising separating a sample from the material; subjecting the sample to diagnostic leaching; determining the lead content of the filtrate from the last 2 nitric acid leaches in preparation of the ultimate leach residue and ensuring that in both cases the lead content is less than 5 ppm;

subjecting the ultimate leach residue to a treatment adapted to solubilise nitric acid insoluble lead moieties; analysing the filtrate from the treated ultimate leach residue for lead; wherein lead in the filtrate of the treated ultimate residue exceeds 1.5 ppm and the method further comprises treating the material with the method adapted to solubilise nitric acid insoluble lead moieties to produce a residue depleted in nitric acid insoluble lead moieties and recovering gold from the residue depleted in nitric acid insoluble lead moieties.

12. A method according to claim 11 wherein the method of at least partially removing nitric acid insoluble lead is a hydrometallurgical treatment which when applied to the ultimate residue produces a lead in filtrate which exceeds 1.5 ppm.

13. A method according to claim 1 wherein at least partially removing nitric acid insoluble provides gold recovery higher than indicated by fire assay of the material.

14. A method according to claim 11 wherein the material comprises a non-silica interferent which remains in the ultimate residue and comprises the nitric acid insoluble lead moieties.

15. A method according to claim 11 wherein the nitric acid insoluble lead moieties, when transformed and solubilised in a leach liquor are present at less than 10% of the cumulative amount of nitric acid soluble lead moieties that can be recovered in successive nitric acid leaching of the material.

16. A method according to claim 1 wherein the nitric acid insoluble lead moieties comprise sulfur-deficient lead sulfate moieties.

17. A method according to claim 1 wherein material is selected from the group consisting of playa material and copper ores.

18-21. (canceled)

22. A method according to claim 1, wherein the material is leached with an aqueous liquor comprising a reducing agent whereby the aqueous liquor has a negative oxidation potential.

23. A method according to claim 22 wherein the liquor has an acid pH.

24. A method according to claim 23 wherein the liquor has a pH less than 1.