WO2001022051A1

WO2001022051A1 - Standards for monitoring assaying of precious metals

Info

Publication number: WO2001022051A1
Application number: PCT/AU2000/001139
Authority: WO
Inventors: Alastair John Inglis
Original assignee: Alastair John Inglis
Priority date: 1999-09-21
Filing date: 2000-09-20
Publication date: 2001-03-29
Also published as: AUPQ299299A0

Abstract

Standards for use in assaying and analysis of precious metals such as gold and silver are provided, as well as a process for preparing such standards and the use of such standards in precious metal assays. Such standards have a composition including a precious metal in elemental form or a water insoluble salt thereof substantially homogeneously distributed in a matrix.

Description

STANDARDS FOR MONITORING ASSAYING OF PRECIOUS METALS

Technical Field

The present invention relates to standards for use in assaying and analysis of precious metals such as gold and silver, processes for preparing such standards, and the use of such standards in precious metal assays thereby enabling the assay or analytical performance to be monitored and the quality and consistency of assay results ensured.

Background of the Invention Extraction of precious metals

Precious metals including gold, silver and platinum, are extracted from their site of natural occurrence-generally ore deposits- and are subsequently refined to a purity suitable for commercial use. An ore is a metallic deposit from which one or sometimes more metals can be economically extracted. The precious metal is present in the ore in a form called the ore mineral which may be native metal or a compound of the metal. The ore generally also contains minerals without economic value, which are termed gangue minerals. The amount of valuable metal in the ore is termed the tenor or the ore grade, and can be given as the wt% of metal or oxide. For precious metals, the tenor is usually given in units of grams per metric ton or troy ounces per 2000 pounds.

The tenor of the ore is obviously very important to determine as this enables geologists to assess whether the concentration of the precious metal is high enough to warrant extraction-particularly in cases where extraction will be energy intensive and require a high degree of processing. Accordingly, once an ore has been located, representative samples are taken and analysis (assay) of its metal content is performed almost immediately. History of precious metal analysis Gold and silver have been assayed in one form or another since ancient times with the earliest recorded evidence being in the early Egyptian times. The techniques were fundamentally similar to those used in a modern day fire assay (discussed below), the main difference being in the sophistication and control afforded by modern instruments and techniques. The method used for quantitative determination of precious metals depends on the state of the sample to be analysed and on its expected precious metal content and interfering impurity levels. Until the early nineteen seventies, the commonly accepted technique to quantify gold and silver in mineral samples was the fire assay. However, with the introduction of modern instrumental techniques cheaper alternatives started to become popular. While the fire assay has continued to hold sway as the reference technique, overall it probably accounts for less than 40% of precious metal assays completed in Australia on an annual basis. Other techniques such as aqua regia and cyanide dissolutions have gained some acceptance over the past twenty years and probably now account for 50-60% of the precious metal assay market in Australia. Assay Laboratories and Techniques

As stated above, the assay technique used, depends on the state of the sample to be analysed, its expected precious metal content and impurity levels. The three main assay techniques-the fire assay, aqua regia leaching and cyanide leaching- are discussed below.

1. Fire Assay

This gravimetric technique is considered by many to be the ultimate reference and involves the complete destruction of the matrix or gangue of the sample. It remains the most reliable method for the accurate quantitative determination of precious metals in any mixture for concentrations from O.Olppm to 100% . In this technique, the finely ground ore sample is mixed with a flux usually containing sodium carbonate, silica, borax, flour and litharge and the mixture is placed in a fire clay crucible and heated to between 1000 and 1200°C for at least an hour to destroy the gangue and collect the noble metals. After cooling, a button of lead containing gold and other precious metals is isolated. The button is cupelled on a magnesia cupel which absorbs the lead oxides and a shiny, uniformly metallic coloured bead remains. The bead is treated with nitric acid to dissolve the silver leaving gold and any platinum metals present in the sample. The purity of silver in the solution is determined by instrumental techniques such as atomic absorption, emission spectroscopy and mass spectrometry. Similarly, the resulting gold is then analysed by either instrumental or gravimetric techniques. Determination of gold concentrations to about O. lppm in solution via atomic absorption spectrophotometry has become an increasingly popular technique and is available in most modern analytical laboratories. 2. Aqua Regia

This technique involves the leaching of the samples with aqua regia in order to dissolve the precious metal/s and thus obtain their soluble forms (such as [AuCl₄]^~). Aqua regia was discovered in the 8^th Century AD and is a mixture of hydrochloric and nitric acids. The mineral sample is first digested with aqua regia and then is evaporated to remove oxides of nitrogen and the residue is then dissolved in concentrated HCl. The gold or silver in solution is then quantified by a variety of spectrophotometric analytical techniques. Prior to analysis the precious metal in solution can be concentrated or separated from impurities by solvent extraction such as methyl isobutyl ketone, di isobutyl ketone, diethyl ether, ethyl acetate, or by chromatography on ion- exchange or complexing resins. The recoveries using this technique can be mineral specific and as a result, this technique is often used as a first pass exploration tool with fire assay checking of economic grades. The technique is often used where indicative results rather than absolute accuracy is required.

Cyanide Leach

This assay technique, based on the actual metallurgical extraction of gold or silver in a process stream using sodium or potassium cyanide, has gradually gained popularity over the past twenty years. Initially it was almost wholly used as a bulk leaching technique to quantify gold at very low levels in exploration samples but now is quickly gaining acceptance as a grade control and even metallurgical assay technique. Quality Control of assay techniques

It is generally recognised that quality control of assay techniques in respect of precious metals has become more important over the last few years for various reasons. a) The increasing competition in the analytical industry has forced prices and thus returns down. In an effort to maintain margins, laboratories have had to look at ways of becoming more efficient and this has usually meant reductions in the level of experience of staff and/or reductions in the level of quality control, b) The headgrade required to sustain the economic operation of a gold or silver mine has dropped substantially over the past twenty years due to improvements in mining, milling and recovery techniques. This has resulted in there being a heavy reliance placed upon the assay industry to produce the correct results. Small errors or bias in assaying can have profound effects on the viability of mines especially during periods of low commodity prices.

Part of the quality control is, or should be, provided by the assay laboratory as part of the normal assay technique. The sample provider also has a duty of care and should also ensure that the quality of the assays that are being used, be it for exploration, reserve definition or grade control, are of an adequate accuracy to allow quality decisions to be made. Commercial Assay Standards

Several companies are in the business of producing bulk standards for the use by the mining and exploration industries as well as assay laboratories to monitor the quality of the assay procedures in respect of precious metals. These standards are generally derived from large samples of ore which have been ground, classified and homogenised to provide a 'standard' sample. The assay assigned to each batch of standard is usually derived from round robin assaying. Mineral standards which are coarse in texture but which are somewhat homogenous in precious metal content have been produced but at relatively high cost. Several attempts have been made to produce gold standards by spiking samples with solutions containing gold. These attempts met with only minor success due to lack of homogeneity of the gold. Further, stability problems occur when the sample spiked with gold is heated during drying and fire assaying as the gold salt is volatile and low gold recovery results. As far as it can be ascertained, such spiking procedures have all but been abandoned. Further, problems exist with the mineral standards currently used in precious metal assays in respect of the differences in mineralogy between the standard and the actual samples to be assayed. Additionally, given that such standards have been already treated and do not need sample preparation, it is very obvious to the assay laboratories that they are standards. Maintenance of Assay Quality

Most analytical laboratories employ at least several techniques to monitor quality of precious metal assay performance. Most commonly, standard sample pulps are used to monitor the quality of the analytical work being produced by the laboratory. Such commercially available standard sample pulps are already prepared and packaged ready for assay. The results obtained by analysis of these standards are often used to " normalise" other analytical data. Such 'normalisation' involves using the analysis obtained from a standard (containing a known concentration of analyte) to compensate for any errors in the analytical procedure. If the analysis result obtained from the standard is for example, too low, then all the results for the 'unknown' sample/s, as well as the standard are factorised up so that the standard returns the correct value. Such 'normalisation' can lead to incorrect results if the error is only in the standard and not the sample/s.

Many laboratories reanalyse samples in a batch to confirm that the sample preparation has been adequate. The frequency of this repeat assaying varies from 1 : 10 to 1 :25 depending on the quality control regime of the particular laboratory.

Some laboratories confirm the data on anomalous assay results. Further, some laboratories screen selected pulps to ensure that the preparation is up to their clients' expectations. Finally, some laboratories have sought to maintain assay quality by joining professional organisations such as NAT A or by seeking ISO compliance. The sample provider-such as a geologist-also generally employs some form of quality control in an effort to validate the assay results received from the laboratory. Examples of techniques currently employed by the sample provider include: -Submission of standard samples in a pulp form with every batch of samples, -Resampling in the field to confirm anomalous results, -Insertion of duplicate field splits into each batch of samples, -Insertion of "blank" samples into each batch of samples.

It is common practice for geologists and other sample providers to use at least one of the above techniques, however some use none of the techniques, relying solely on the laboratory to provide quality data. Problems with Assay Quality Control Techniques

In theory, the quality control techniques listed above should provide the sample provider with reasonably good data but for one reason or another they fail to meet the demands placed upon them, sometimes providing inaccurate or inconsistent assay results. Reasons for such problems include: a) Laboratory Operations: i)In order to get work, laboratories have been known to make claims with respect to their proficiency, precision and accuracy with regards to assays that they have little chance of achieving. ii) Laboratories have also been caught short cutting analyses, especially sample preparation and this has led to a degree of mistrust of laboratories by the sample providers. iii)Laboratory quality control reports should be treated with a degree of scepticism as modern day Laboratory Information Management Systems have been written in such a way as to allow data manipulation. b) The use of standard samples by sample providers may not be effective as it is designed to be due to the following: i) the standard samples are usually pulps, are in different packaging, and are therefore easily identifiable by the laboratory; where a standard is readily identifiable, laboratory staff tend to take more care with its analysis; ii) in the laboratory, the standard samples are usually separated from the bulk of the samples as they seldom need preparation and thus do not follow the whole analysis route; iii) the standard samples are often analysed in duplicate by the laboratory to make sure that the results are correct, thus receiving a degree of attention and analysis by a laboratory which other non-standard samples are not necessarily given; iv) given that there is not an infinite number of prepared standards on the market and the fact that any one mining company will use only one or two of these standards for any one project, the analyst will, over time, become familiar with the standard values. As a result, the laboratory can not only spot the standard; the result can be spotted as well. v) The accepted value of these prepared standards is obtained by round robin assaying which is not foolproof and the values may alter as more data becomes available; and vi) the mineralogy of the standards seldom bears any resemblance to the mineralogy of the actual samples, which can lead to severe problems when assaying. c) The use of field splits suffers from the drawback that results can sometimes be spurious and the sample provider is then unsure whether s/he is dealing with a spurious gold problem or a sample preparation and/or an assay problem. d) The inclusion of blanks to check for contamination is a recommended procedure when low level analysis is being conducted, but it provides no information as to the authenticity of higher values obtained in the assay results. e) Internal standards in a laboratory are only useful if they are treated as unknown samples by the analyst. Too often special attention is given by the analyst to the standard to ensure that the "expected" result is obtained. Usually, all analysts know the "expected" result. f) Reanalysis of randomly selected samples in a batch is considered advisable and reanalysis of anomalous results is considered essential. Not all laboratories perform this reanalysis. g) The random screening of pulps as a quality control measure is worthwhile, but again it should be realised that a laboratory will not release data that could be incriminating. Objects of the Invention

Accordingly, it is the object of the present invention to overcome or substantially ameliorate at least one of the above disadvantages.

In particular, it is an object of the present invention to provide a standard containing a known quantity of precious metal in a matrix which is similar to the field samples, which has to follow the whole analytical process from sample preparation through to the final assay, without identifying the sample as a standard to the assay er. It is also an object of the present invention to provide a process for production of such a standard for use in precious metal assays.

It is also an object of the present invention to provide for the use of such standards in precious metal assays. Summary of the Invention A first aspect of the present invention provides a standard for use in precious metal assays, said standard having a composition including a precious metal in elemental form or a water insoluble salt thereof, said precious metal or salt thereof being substantially homogeneously distributed in a matrix.

A related aspect of the present invention provides a standard for use in precious metal assays, said standard having a composition including a precious metal in its water insoluble elemental form or a water insoluble salt thereof, said precious metal or salt thereof being substantially homogeneously distributed in a matrix, such that when added to a mineral sample, the standard composition is capable of being distributed substantially homogeneously throughout the sample.

The term 'substantially homogeneously' is used throughout the specification to mean evenly dispersed throughout, having a unity of character and composition. Advantageously, the precious metal in the standard is in the form of fine particles with a particle size of < lOμm, preferably < 7μm, more preferably < 5μm, even more preferably < 3μm and most preferably < lμm. Of course the smaller the particle size the better in order to achieve a homogeneous distribution of the standard in the sample. The invention advantageously provides a high proportion of particles (in the order of 80-90%) which are < l.Oμm, with many particles having a size of about 0.2μm.

Another related aspect of the invention provides a standard for use in precious metal assays, said standard having a composition including a precious metal in elemental form, or a water insoluble salt thereof, reduced to water insoluble fine particles, said precious metal or salt thereof being substantially homogeneously distributed in a matrix.

Advantageously, reduction to water insoluble fine particles of a precious metal can occur by way of chemical reduction of a solution of a salt of a precious metal. Chemical reduction is discussed further below. Advantageously, reduction of an insoluble salt of a precious metal to water insoluble fine particles can also occur by way of physical reduction or precipitation. Physical reduction to fine particles can occur by means of grinding, crushing or other mechanical means which renders a precious metal salt into particles.

A second aspect of the present invention provides a standard for use in precious metal assays, said standard having a composition including a precious metal in elemental form obtained by reduction of an aqueous solution of a salt of said precious metal, said precious metal being substantially homogeneously distributed in a matrix.

A related embodiment of the second aspect provides a standard for use in precious metal assays, said standard having a composition including a precious metal in elemental form or an insoluble salt thereof obtained by precipitation of said elemental precious metal or insoluble salt from an aqueous solution of a salt of said precious metal, said precious metal or salt thereof being substantially homogeneously distributed in a matrix. When added to a mineral sample and pulverised, the standard composition advantageously disintegrates readily and is capable of being distributed substantially homogeneously throughout the sample.

Advantageously, any precious metal or salt thereof which has been reduced or precipitated either chemically or physically to a fine particle size can be included in the standard product of the second aspect of this invention. Reduction is one way in which a fine particle size can be achieved which allows ease of sample preparation and results in substantially homogeneous distribution of the standard through a sample.

A third aspect of the present invention provides a process for preparing a standard for use in a precious metal assay, said process including the steps of (a) reducing a solution of a salt of a precious metal to the precious metal in elemental form, or precipitating an insoluble salt of a precious metal from a solution of a salt of said precious metal, and (b) distributing said precious metal or insoluble salt thereof substantially homogeneously in a matrix. In one embodiment, the process for preparing a standard for use in a precious metal assay which includes a step of precipitating an insoluble salt of a precious metal from a solution of a salt of said precious metal can include the step of reducing said solution.

Typically, step (b) includes the step of adding a matrix medium to the reduced or precipitated solution from step (a) to obtain a mixture in which particles of precious metal or a water insoluble salt thereof, are substantially homogeneously distributed in the medium.

More typically, the matrix medium can be added to the solution prior to its reduction. This addition of the matrix medium to the solution prior to its reduction helps the precious metal particles to distribute more homogenously, than when reduction occurs first.

Typically, the step of adding a matrix medium to the solution either before or after reduction occurs with mixing. It is also typical that the mixture of matrix medium and solution is then dried to obtain a dry mixture in which particles of precious metal or a water insoluble salt thereof, are substantially homogeneously distributed in the medium. The solution of the salt of a precious metal is typically an acidic solution containing precious metal in soluble form.

The reducing of the precious metal solution to obtain fine precious metal particles or salts thereof, can be effected by use of any reducing means-such as a chemical reducing agent/reductant.

While any reducing agent can be used in preparing the standard of the invention, in particular, the reducing agent can be selected from the group consisting of SnCl₂, Na₂SO₃, oxalic acid, hydrogen sulfide, phosphorus, ferrous sulphate, formaldehyde, tannin, quinol, catechol, resorcinol, reducing sugars, glycerol, carbon monoxide, nitric oxide, acetylene, hydroxylamines and hydrazines. More particularly the reducing agent is typically SnCl₂.

Further, any reducing agent in a concentration ranging from 0.001% to 100% wt/vol (ie undiluted reducing agent) can be used to produce fine metal for inclusion in a standard of the invention. Typically the concentration of SnCl₂ when used as the reducing agent is about 40%wt/vol.

The temperature of the reduction reaction affects the particle size of the precious metal in the resulting standard. It is therefore preferable that the reduction occurs at low temperatures such that the resulting precious metal particle size is small and consistent with no clumping occurring. In particular, a temperature of less than 10°C, more particularly less than 5°C is recommended.

An embodiment of the third aspect of the present invention provides a process for preparing a standard for use in precious metal assays, said process including the steps of adjusting the pH of a solution of precious metal, adding a matrix medium to form a thick paste-like mixture, cooling the mixture of the precious metal prior to its reduction, reducing the mixture to produce reduced precious metal particles, and mixing said reduced mixture to obtain a mixture in which reduced precious metal particles are substantially homogeneously distributed in the medium. Typically the mixture is then dried.

The pH neutralising agent typically used is lime which neutralises the excess acid in the standard and that resulting from the reduction reaction and provides an inbuilt protective alkalinity for use in cyanide extractions when the standard is used as an unidentifiable standard in assays. Typically excess lime is used. Other neutralising agents which could also be used are any other basic salts including sodium hydroxide, potassium hydroxide, sodium carbonate and potassium carbonate. If a neutralising agent is not added, then if any residual acid is left after precipitation of the precious metal, that acid will react with the precious metal again on heating resulting in the formation of a water soluble precious metal salt. The neutralising agent can also be added following reduction, if the reduced solution is acidic. The pH of the neutralised solution is between about 7 and 9, typically about 8.

Another embodiment of the third aspect of the present invention provides a process for preparing a standard for use in precious metal assays, said process including the steps of taking a solution of a precious metal, precipitating said precious metal or a salt thereof from said solution, adding a matrix medium to the solution containing said precipitated precious metal or insoluble salt thereof to obtain a mixture in which particles of precipitated precious metal or said salt thereof are substantially homogeneously distributed in the medium. Typically, the mixture is then dried.

Silica typically provides a chemically stable matrix medium for production of the standard. Silica is the preferred matrix as it is cheap, readily available and of course free of any precious metal. Alumina can also be used, as can beach sand, river sand, granite, basalt and hard minerals such as zircon, rutile, ilmenite. In fact any solid material which is cheap and can be readily obtained with no precious metal contamination is an appropriate medium. The silica matrix is preferred as silica is found in 99% of samples tested and thus will have little effect on the matrix of the sample. Preferably, any material which will bond with a bonding agent such as cement, as discussed below, is used.

The manufacturing process typically produces a standard containing fine precious metal such as gold, with a particle size of nominally < 1 μm, entrained in a silica/lime matrix. When added to a mineral sample and pulverised, the standard disintegrates readily and distributes homogeneously throughout the sample.

Advantageously, the standard can be formulated into the physical form of a capsule, tablet, pill or powder. In formulating a tablet or pill, a binding agent is simply added to the standard composition mixture which is then homogenised before being pressed. The standard composition, once placed in a sample which is to be assayed, follows the same process as the sample would if no addition of a standard had occurred. Accordingly, if the sample is pulverised, then the composition (powder, capsule, tablet or pill) will be pulverised as well. The precious metal used in a standard of the invention can be selected from the group of elements consisting of gold, silver, platinum, palladium, ruthenium, osmium, rhodium, iridium and rhenium. In particular, the precious metal is gold, silver, platinum or palladium. More particularly, the precious metal is gold. If silver is the precious metal of choice in a standard of the invention then it can be precipitated out of a soluble silver salt solution (such as silver nitrate) in the form of an insoluble salt such as AgCl or AgS . In such a case a precipitating agent such as zinc sulphide or chloride, sodium sulphide or chloride, or HC1 can be used. Gold, platinum and palladium will precipitate out of solution in the form of the elemental metal upon the addition of a reducing agent. For example, elemental gold can be readily obtained by reduction of dilute solutions of gold salts. Alternatively an insoluble gold salt (such as Aul) will precipitate out of an aqueous solution of a gold salt upon addition of an appropriate precipitating agent.

Up to about 20mg of precious metal, more particularly up to about lOmg of precious metal can be accommodated in a pill of lg in weight. Such a lg pill can contain as little as 0.005mg of precious metal, more particularly about O.OlOmg of precious metal which can be homogenised in such a lg pill. In another embodiment, about 0.025mg of precious metal can be homogenised in such a lg pill.

A sample containing such a standard of the invention can therefore contain an added amount of precious metal in any amount from about lppb to about 200ppm, more particularly in any amount from about lppb to about 20ppm, even more particularly from about lOppb to about 10 ppm. Generally, the standards of the invention are limited to precious metals which are present in low levels and which are economic at these low levels . A fourth aspect of the present invention provides for a method of using a standard of the invention in a precious metal assay as an unidentifiable means of monitoring the assay quality, said method including the steps of adding said standard to a sample to be assayed to produce a spiked sample and then subjecting said spiked sample to a precious metal assay . Given the financial stakes which depend upon determination of the existence of a viable precious metal ore, it is unfortunately a consideration that an unscrupulous person may (for example) seek to 'dope' a mineral sample with a quantity of the standards of this invention in order to obtain favourable assay results for samples from a particular site. Advantageously, the standard of the present invention can have its own " signature" so that its use can be confirmed if necessary and a suspect assay result can be checked in order to determine whether it is due to doping with a standard of the invention. For example, if the reductant which is used in preparing a standard of the invention is stannous chloride, then the resulting standard contains an amount of tin, which does not otherwise occur naturally with gold. The amount of tin in an analysed sample can then be ascertained in order to check as to whether a standard of the invention has been added to the sample. Other similar tracer elements can also be used. Optionally, the standard can also be colour coded to identify the grade ie the quantity of precious metal in the various standards. This colour coding can be achieved by simply including a dyeing agent such as a cement dye or other dye in the composition.

The standard is advantageously produced with a gold content better than ± 5% relative of that claimed and with a weight variability of better than 2% relative. In other words, the standards when formed into pills will typically be manufactured to a weight tolerance of +2% of the claimed weight, more typically ± 1 % of the claimed weight. Typically also, the amount of gold in each pill will be within +5 % of that claimed, more typically +2% of that claimed, with a built in allowance for analytical inaccuracy. The precious metal standard of the present invention is easy to use, it is reliable, it does not physically degrade under normal handling and is difficult to identify when placed in a sample. The standard of the invention can be hidden in a sample and the amount of the standard can be varied to hide its presence. Further, the standard is inexpensive and can be successfully used in cyanide leaching, aqua regia, and fire assays. The product is safe to handle as it does not contain dangerous chemicals.

The invention provides a standard which allows for assurance of consistency of precious metals assay results, assurance of quality of precious metal assay performance and facilitation of the analytical procedure. The standard is stable, easy to measure and is used for comparison and calibration in precious metal assay procedures, thus providing a basis for uniformity and accuracy of assay performance. Its use with standard, well characterised assay methods enables the accuracy and establishment of measurement traceability throughout the whole precious metal industry. In particular, a precious metal standard of the present invention in the form of an assay pill or capsule for example, can simply be inserted by a sample provider into a barren sample which then becomes a standard sample, thereby providing a means of matrix matching standards, which industry has long sought. The small amount of the standard added to the sample, makes no difference to the behaviour of the sample, as in effect the standards of the invention mimic the sample in which they are placed, with the sample provider being able to pick the matrix in which s/he is placing the standard. The precious metal contributed to a standard sample by each pill or capsule is accurately known, so it is possible, by accurately weighing the sample before submission, to predict the assay results of the standard sample. As stated above, this result can be compared with the result achieved by the laboratory to assess analytical precision and accuracy as the standard sample is subjected to all of the same analytical and preparation processes as those applied to the 'true' samples. The standards of the present invention can therefore be included with batches of drill and possibly even rock chip samples submitted to laboratories for determination of precious metal content. Data Interpretation

The data produced by use of the product of the invention can be used in several ways, the limitation being only in the data processing available. Some examples as to how the data may be used are included below: -a) Sample preparation can be remotely checked. If the preparation technique is good enough to thoroughly homogenise the standard with the sample, measured by the accuracy of the analytical result, one can reasonably assume that the preparation procedures are being followed. This does not mean that coarse gold in real samples will be broken up completely, but rather that the laboratory is making a reasonable effort to meet the preparation specification called upon in the method.

-b) If assay results vary considerably from the expected value, the sample preparation technique needs to be investigated. The decision as to when to act is obviously in the hands of those persons or entities submitting the samples but early disclosure will often result in any problems being addressed quickly . -c) It is commonplace for laboratories to split samples when they are too large to prepare. When done correctly, splitting utilises a specially designed splitter, however, as splitting is time consuming laboratories often take short cuts in splitting which invalidates the sample. Accordingly, varying the weight of the samples provided to a laboratory for analysis will often produce data which can indicate if samples are being split and at what weight they are being split.

Further, varying the weight of the sample will also "hide" the standard by varying the assay so in addition to the sample being "hidden" due to the fact that it looks and acts like a normal sample, it also produces different assays, which can further confuse the standard spotter. Standard spotting has been common and reasonably easy to perform prior to the present invention given that the standards in common use are i) different to the normal samples-already being pulverised and therefore already prepared for analysis, and/or ii) are of a small number consistently used by the sample providers. Accordingly, as the present invention involves inserting a known amount of precious metal (such as in the form of a weighed pill or powder) into a sample to be analysed, the actual sample weight governs the value of the assay. Thus, the sample weight can be varied, thus varying the assay. For example, if the sample weight is doubled, the assay is halved. The standard according to the present invention is not only invisible-in that it cannot be spotted visually as it looks like a normal sample-but also it is highly unlikely to be spotted from the assay results as the assay varies. The assays can be normalised to a standard weight to check overall assay quality .

-d) Contamination problems in the milling circuit may be identified. Poor analytical techniques can also be identified. Detailed Description of the Preferred Embodiments

Manufacturing Technique Preparation of dried powder

Proof gold with a minimum assay of 99.99% is dissolved in a minimum volume of aqua regia (3 parts hydrochloric acid and 1 part nitric acid) and evaporated down at a solution temperature of 90°C to eliminate all the free oxides of nitrogen from the solution. The solution is carefully made to volume in a tared A grade volumetric flask with distilled water. The flask and its contents are carefully weighed to the nearest O.Olg and the specific gravity of the solution calculated. A 50% wt/vol solution of stannous chloride (the reducer) is prepared in concentrated hydrochloric acid by dissolving lOg of stannous chloride in 15 ml of hot hydrochloric acid then bulking the solution to 20 ml with extra hydrochloric acid. A fixed volume of the gold solution is accurately pipetted into a preweighed Teflon or glass beaker. Alternatively, an approximate volume of the gold solution is transferred to a preweighed Teflon or glass beaker. The beaker and its contents are accurately weighed. The precise gold content of the addition being determined by calculation using the SG and the weight of solution transferred to the tared beaker. The solution is cooled to less than 5°C in a freezer. A block of ice (25ml) is added. Lime is added at a rate of lg per 1.3g of hydrochloride acid (from the gold solution, the stannous chloride and that generated in the reduction reaction).

2AuCl₃ + 3SnCl₂ + 6H₂O → 2Au + 3 SnO₂ + 12 HC1 A weighed amount of silica determined by the required grade of the pill is added and sufficient water added to obtain a smooth paste. The cold gold paste is vigorously stirred and the stannous chloride solution is added slowly to the gold paste at the rate calculated from the above reaction plus 1ml, with constant swirling and stirring. The amount which is added is therefore about lmL per 300mg of contained gold. The reaction produces sub-micron sized particles of gold and a very dark purple colour is imparted to the mixture. If required, a weight of cement dye equal to 5% of the weight of silica added, is added to the paste and the paste thoroughly mixed again to obtain a homogenous mixture. The cement dye is used to colour code the mixture to identify differing grades. The mixture is then dried at less than 120°C in a drying oven overnight. The beaker and the dried product is then weighed. The dried product is removed from the beaker, crushed by bottle rolling and screened through a 0.25mm aperture screen. The screened product is transferred to a tared glass bottle and lid and left to stand for a minimum of about 8 hours in an oven set at about 90° Celsius with the lid off. The lid is replaced on the bottle and the bottle is then transferred to a dessicator to cool. The bottle and its contents are then weighed. Pills

Immediately before the pills are to be pressed, 30% by weight of Portland cement is added to the dry powder and the mixture rehomogenised by roll mixing. Other binders which could be used include plaster of paris/gypsum, or other binding agents such as a polymeric binding agent. Any other binding agent could be used such that the material is amenable to milling, in which process it would be expected to behave just as a normal mineral and crack. The pills are generally pellet or tablet form weighing about 0.5-2g, conveniently about lg, and being about 1cm in diameter and about 0.3-0.8cm deep, more particularly 0.6cm deep. Safety.

The product is considered safe, containing only elemental gold or silver, precipitated silica, calcium chloride, calcium nitrate, tin chloride, Portland cement and water of hydration. Hydrochloric acid and nitric acid are used in the initial stages of manufacture but are neutralised with lime prior to the final homogenisation of the product. No other products generally considered to be toxic to flora or fauna are used in the manufacture of the product. Instructions for Use

Tablet/Pill (recommended when total sample preparation is required)

A sample of gold free (barren) material, preferably up to 50Kg, having a similar mineralogical content as the samples to be tested, is dried. The dried material is weighed into batches varying in size from 1.5 Kg to 3 Kg and the weight of each sample is recorded. Each sample is bagged and one tablet/pill is then dropped into each sample bag, and one 'standard' sample is sent with each batch of 100 'real' samples to the laboratory for analysis. In the laboratory, the samples are generally firstly dried at 100- 200°C and then pulverised prior to analysis. On receipt of the analysis results, the data is then normalised back to a nominated weight and the assay compared with the certified gold content of the tablet. If it is thought necessary, the pills/tablets can be crushed slightly as they are being placed into the barren samples. Powder (recommended when the sample is to be split using a Jones Riffle Splitter)

A sample of gold free material, preferably up to 50Kg, having a similar mineralogical content as the samples to be tested, is dried. The dried material is weighed into batches varying in size from 1.5 Kg to 3 Kg and the weight of each sample is recorded. Half of each sample is bagged and the contents of one preweighed portion of the powder is then sprinkled carefully onto the sample in the bag. The other half of the sample is then carefully added to the bag, the bag sealed and one 'standard' sample is sent with each batch of 100 'real' samples to the laboratory for analysis. On receipt of the analysis results the data is then normalised back to a nominated weight and the assay compared with the certified gold content of the tablet.

The tablet form of the product can also be used to generate standards data when samples are to be split. Chemistry

The chemistry of the production of the standard is such that the precious metal, for example gold, is mixed and stabilised having the following formulation

Ingredient Range

Assay Silica 0 <90%

Gold 0 < 10%

Stannous Chloride 0 < 10%

Lime 0 <5%

Portland Cement 0 < 30%

Cement Colouring 0 < 10% .

In one embodiment, lOOg of the final

Assay Silica 60-80g

Gold 0.2g

Lime 10-30g

Portland Cement 10-20g

Cement Colouring l-2g

In a preferred powder formulation of the composition of the invention, there is no requirement that a bonding agent (eg cement) be present, nor a colouring agent. Typically, silica (or other medium), the precious metal and the neutralising agent (eg lime) are present, as is the reducing agent (eg tin in the form of tin chloride) which is typically present at approximately between about 0.4-2.0%) wt/wt, more particularly at about 0.8% wt/wt-depending on the product. Serial dilutions can be performed on the above formulation so as to give a gold concentration in a sample in the low ppb levels- including as little as 4 or 5 parts per billion (ppb).

The form of precious metal in this product is chemically stable to the usual environmental influences such as heat and moisture, however it is readily soluble in a correctly conducted fire assay, an aqua regia digestion and in sodium or potassium cyanide solutions. The fact that it is not water-soluble is an important consideration and this feature sets it apart from other standard assay products, which have been tried in the past. When added to a mineral sample and pulverised, the product of the invention breaks down readily and distributes homogeneously throughout the sample. Other techniques of producing fine particles of precious metal such as fine gold are available using other chemicals, such as oxalic acid, and sodium sulphite. The use of stannous chloride as the reducing agent is however preferred in that the tin provides a tracer element not often found in high concentrations with gold. This tin tracer can, if necessary, be used to trace the fact that the product has been used in a sample, should the product be suspected of being used for unethical " salting" . Test Results Test 1

The following testwork was carried out to assess the solubility of gold in the various media commonly used to assay gold samples. The testwork was conducted on three different batches of the product of the invention.

One gram of each sample product was weighed into a 100ml volumetric flask. 50ml of nanopure water was added to the first flask, 10ml of hydrochloric acid and 3 ml of nitric acid was added to the second flask and 50ml of 0.05 % sodium cyanide solution was added to the third flask. The solutions were shaken intermittently over a period of six hours, bulked to volume with water and the quantity of gold in each solution was determined by ICPMS. The results, shown in Table 1 below were corrected from the volume of undigested silica left in the flask.

Table 1: Solubility of gold in water, aqua regia and cyanide

From the above table it can be concluded: The recovery of gold from water was on average < 0.05 % . The recovery of gold from aqua regia was on average 99.5% The recovery of gold from sodium cyanide was on average 101 %

The inventive product performed as expected for the water analysis, however the recoveries and the precision and accuracy achieved for the aqua regia and the cyanide leaches far exceeded expectations . Test 2 lg of each of the same powders used in test 1 was added to weighed 1kg, 1.5Kg and 2Kg samples of barren flush material. The samples were then milled to a nominal lOOμm, roll mixed then subsampled for assay. The assay portions were subjected to fire assay and aqua regia determinations. The gold in the sample was determined by A AS. The results are shown in Table 2 below.

l.OKg of Sample

1.5Kg of Sample

2.0Kg of Sample

From the above table it can be concluded:

The recovery of gold using fire assay was on average 99 % .

The recovery of gold using aqua regia was on average 99% .

These samples in Test 2 were slipped into the system anonymously, and no special care was taken with them. Accordingly, the results are as would be hoped for in a routine assay situation. The scatter of results is as would be expected given the precision and accuracy usually experienced from these techniques.

Test 3

The following testwork was carried out to assess the precision of the weight of the pill when produced at a commercial rate. The results are shown in Table 3 below.

Table 3: precision of production of gold standard pill

Test 4

Five samples of crushed rock were submitted for analysis by fire assay in duplicate, for gold. All five samples were weighed prior to submission and one pill from production batch 3 was added to each of four samples. The fifth sample was not spiked in any way. The samples were submitted anonymously with the laboratory being told only

1. The whole sample was to be prepared, and

2. The analysis was to be conducted in duplicate and all results were to be reported. At no stage was the laboratory made aware that the samples were actually standards. The spiked samples submitted for analysis had a predicted gold content of 1.94 g/T (due to mixing of a standard of the invention with lKg of barren material). The assay results are shown in Table 4 below.

Table 4: Fire Assay results showing gold content in sample material

From the above table showing the fire assay results, it can be concluded: 1. The milling of the samples has produced an evenly distributed homogeneous product based on the fact that all of the samples gave assay results within 10% of the expected value.

2. Recoveries in excess of 95 % are possible on a routine basis. 3. The average overall recovery is better than 98 % .

Test 5

Twenty-five Assay Pills from production batch 4 were sent to the Technical Services Superintendent at an Australian Gold Mine. The superintendent personally weighed out the barren ore and added twenty-one of the pills to the appropriate samples. The samples were submitted to the on site laboratory anonymously, and were subject to the company's routine assay procedure, which involves total sample preparation. The results of the testwork are tabulated below in Table 5.

Table 5: Assay results showing gold content in sample material Conclusions

The fire assay results produced by the gold mine Laboratory indicate that:

1. The milling of the samples has produced an evenly distributed homogeneous product based on the fact that all bar one of the samples gave assay results within 10% of the expected value.

The results prove that recoveries in excess of 95% are possible on a routine basis.

Test 6 Approximately 50Kg of -5mm screened aggregate of Mt Goyder syenite, appropriately dried and mixed was acquired as sample material. Individual samples were then prepared by shovelling the sample material into pre-numbered calico bags. Each sample weighed approximately either 2 or 3 Kg. The average weight of an empty sample bag was also determined. A total of five blank samples were first selected and were closed and accurately weighed on a Sartorius electronic balance. The weight of each sample and the absence of standard assay pills/capsules in each sample were recorded on the corresponding sample tickets. All blank samples weighed about 2Kg.

Standard samples were then selected with the contents of 1 , 2 or 3 Assay capsules emptied into a depression scooped by hand into the top of each sample. The depression was then covered with sample to keep the contents of the capsules away from the calico bag. Each bag was immediately closed and accurately weighed and the weight and number of assay capsules used in that sample were recorded on the corresponding sample ticket The sample provider was advised that each assay capsule should provide an assay of 0 9g/t Au in a one kilogram sample

Standard samples were also prepared where 1 , 2 or 3 Assay pills were inserted into a depression scooped by hand in the top of each sample The depression was then covered with sample Each bag was immediately closed and accurately weighed and the weight and number of assay pills used m that sample were recorded on the corresponding sample ticket The sample provider was advised that each assay pill should provide an assay of 1 OOg/t Au m a one kilogram sample

Each calico bag of sample was placed m a thick plastic bag which was sealed and then grouped into polywoven sacks which were closed with cable ties and delivered to a laboratory Assaying

Sample preparation was to crush each entire sample to -2000 microns and then pulverise each entire sample to -100 microns Duplicate gold determinations were then performed on 50 gram assay samples using fire assay /AAS gold determination All of the residues and pulps were retained by the laboratory and were then collected and weighed by the sample provider were and despatched to a second laboratory which assayed m duplicate four selected samples (which had returned questionable results from the first laboratory) by fire assay/ AAS gold determination on 50 gram assay samples Results

Analytical results from laboratory 1 are shown below on Table 6a and laboratory 2 on Table 6b Initial sample weights recorded by the sample provider as well as the weights of the recovered pulps and residues are listed in Table 6c along with the analytical results from both laboratoπes and resultant calculations

Table 6a: Lab 1 duplicate determinations on 50 g assay samples using fire assay

Table 6b: Laboratory 2 duplicate determinations on 50 g assay selected samples using fire assay

Conclusions: There is good agreement between the duplicate determinations by laboratory 1 and laboratory 2 as shown in Tables 6a and 6b, demonstrating excellent precision and accuracy and good sample comminution and even distribution of gold throughout the samples from both the pills and capsules. This indicates that the standard samples of the invention behave like any other experimental sample which is assayed. The close agreement between the determinations by laboratory 2 on the assay pulps and residue pulps returned from laboratory 1 support the conclusion that laboratory 1 has achieved good comminution of each sample and the gold from the pills/capsules was evenly distributed throughout the sample during sample preparation. The fine grain size of the gold in the pills and capsules would have contributed to this result.

Most importantly, the predictability of assay results which are obtained from the standard samples of the invention is evidenced by the data in Table 6c. From Table 6c, it is noted that results from laboratory 1 for three of the Assay pills samples (3, 18 and 23) and one of the capsule samples (sample 15) are significantly below the calculated grades of the samples. For example, from the column titled 'Calc PILLS' it is noted that the predicted assay result for sample 3 should have been 0.69g/t of Au, however, the assay result obtained was about 0.35g/t (see the column titled 'LABI Rec'). Due to the predictability of the assay results for the standard samples, the sample provider was immediately able to determine that the assay results for the three assay pills samples 3, 18 and 23 in fact differed from the calculated results by exactly one pill. The conclusion was that the pills had been lost/removed from these samples by laboratory 1. The almost identical results from laboratory 2 for both the assay pulps and residue pulps confirmed that the discrepancies did not arise because whole pills or parts of pills had been partitioned into the residue pulps during sample preparation stage, but that loss/removal of the pills had occurred prior to the samples being sent to laboratory 2. Upon consultation with staff of laboratory 1, it was confirmed that the laboratory had not followed instructions in its sample preparation procedure to crush and pulverise the entire sample, but had removed pills from some of the samples submitted.

Another feature to be noticed in Table 6c is the excellent gold 'recovery' rate (>95%) for the remainder of the assay pills samples. This % recovery is indicated in Table 6c in the column titled 'LAB 1 Rec %' with the recovery rates of >95% presented in boxes. Allowing for the loss of one assay pill from the three samples discussed above, the same high 'recovery' level applies to these samples as well and this can be seen from the column titled 'Adj Calc Pills g/t' in Table 6c. This column presents the predicted assay results for the standard samples containing the assay pills which have been recalculated assuming one less pill in each of samples 3, 18 and 23. The assay results obtained by laboratories 1 and 2 correlate closely with the recalculated predicted results for standard samples 3, 18 and 23. The consistently high gold 'recovery' from the assay pills samples contrasts with variable and generally lower gold 'recovery' in the capsules samples (72-95%). This implies some loss of the contents of the capsules either during preparation by the sample provider or during sample preparation in the laboratory. The result for sample 15 (72%) is consistent with this trend but at the extreme end of the scale. Again, the predictability of the assay results obtained from standard samples of the invention immediately alerts a sample provider to any anomaly that may arise in sample preparation or in laboratory analysis.

It will be understood by those skilled in the art that the invention has been described with reference to exemplary embodiments and that variations and modifications can be effected in the described embodiments without departing from the scope and spirit of the invention.

Claims

1 A standard for use in precious metal assays, said standard having a composition including a precious metal in elemental form or a water insoluble salt thereof, said precious metal or salt thereof being substantially homogeneously ι distributed in a matrix

2 A standard as claimed in claim 1, wherein said precious metal is m its water insoluble elemental form or a water insoluble salt thereof, said precious metal or salt thereof being substantially homogeneously distributed in a matπx, such that when added to a mineral sample, the standard composition is capable of being distπbuted substantially o homogeneously throughout the sample

3 A standard as claimed m claim 1 , wherein said precious metal or water insoluble salt thereof, is m the form of water insoluble fine particles, said precious metal or salt thereof being substantially homogeneously distnbuted in a matπx

4 A standard for use in precious metal assays as claimed in claim 1 wherein said s precious metal m elemental form or water insoluble salt thereof, is obtained by reduction of an aqueous solution of said precious metal or a salt thereof, said precious metal or water insoluble salt thereof being substantially homogeneously distπbuted m a matπx

5 A standard as claimed in claim 1, wherein said precious metal m elemental form or insoluble salt thereof is obtained by precipitation of said elemental precious metal 0 or insoluble salt from an aqueous solution of a salt of said precious metal, said precious metal or salt thereof being substantially homogeneously distπbuted m a matπx

6 A standard as claimed in claim 4 wherein said reduction is chemical reduction

7 A process for prepaπng a standard for use in a precious metal assay, said D process including the steps of (a) reducing a solution of a salt of a precious metal to the precious metal in elemental form, or precipitating an insoluble salt of a precious metal from a solution of a salt of said precious metal, and (b) distπbuting said precious metal or insoluble salt thereof substantially homogeneously in a matnx

8 The process as claimed in claim 7 wherein a matπx medium is added either to 0 the reduced or precipitated solution from step (a), or to the solution pπor to its reduction or precipitation, to obtain a mixture in which particles of precious metal or a water insoluble salt thereof, are substantially homogeneously distπbuted m the medium

9. The process as claimed in claim 7 wherein said process includes the steps of adjusting the pH of a solution of precious metal, adding a matrix medium to form a thick paste-like mixture, cooling the mixture of the precious metal prior to its reduction, reducing the mixture to produce reduced precious metal particles, and mixing said reduced mixture to obtain a mixture in which reduced precious metal particles are substantially homogeneously distributed in the medium.

10. The process as claimed in claim 9 wherein said pH is adjusted to be in the range of about 7 to about 9 by addition of a pH neutralising agent selected from the group consisting of lime, NaOH and KOH.

11. The process as claimed in claim 9 wherein said matrix medium is selected from the group consisting of silica, beach sand, alumina, granite, basalt, zircon, rutile and ilmenite.

12. A process for preparing a standard for use in precious metal assays as claimed in claim 7, said process including the steps of taking a solution of a precious metal or salt thereof, precipitating said precious metal or a water insoluble salt thereof from said solution, adding a matrix medium to the solution containing said precipitated precious metal or insoluble salt thereof and mixing to obtain a mixture in which particles of precipitated precious metal or said insoluble salt thereof are substantially homogeneously distributed in the medium.

13. A process as claimed in any one of claims 8 to 12 wherein the final mixture of matrix medium and solution containing particles of a precious metal or a water insoluble salt thereof is then dried to obtain a dry mixture in which said particles of precious metal or water insoluble salt thereof, are substantially homogeneously distributed in the medium.

14. A standard produced by the process as claimed in any one of claims 7 to 13.

15. A method of using a standard as claimed in any one of claims 1 to 6 or 14, in a precious metal assay as an unidentifiable means of monitoring the assay quality, said method including the steps of adding said standard to a sample to be assayed to produce a spiked sample and then subjecting said spiked sample to a precious metal assay.

16. The standard of claim 1 or the process of claim 7 wherein said precious metal is selected from the group consisting of gold, silver, palladium and platinum.