ANALYSIS

Field of the Invention

The present invention relates to a method and a container for preparing a fused sample for analysis .

Background

Mineral samples containing noble or precious metals often need to be analysed in order to determine the quantity of the noble or precious metals within the mineral samples. One method of analysing mineral samples for precious metals involves the use of spectroscopic techniques, such as laser ablation or optical emission spectroscopy. In these methods, the mineral samples are first fused with a flux before being analysed. Sample preparation typically involves mixing the samples with a flux material, such as a litharge flux material, and positioning a resultant mixture in a crucible and into a furnace that is heated to form a melt. The litharge flux material is reduced to molten lead, which collects the precious metal and settles to the bottom of the crucible.

There are a number of ways to then separate out the lead and precious metals from the resulting slag to form a lead button. The lead button is then analysed to determine the amount of precious metal that is distributed within the lead. The amount of the precious metal can be determined by directly analysing the button using spectroscopic techniques. Alternatively, the lead button can be placed in a cupel and heated. The lead is absorbed into the cupel and the resultant prill can be weighed or analysed using wet chemistry techniques .

A problem with the above-described sample preparation relates to the often inhomogeneous distribution of the precious metal contained within the separated lead button. As the spectroscopic analysis techniques generally only detect the content of the precious metal within small areas of the sample, significant errors can occur if the precious metal is not homogenously distributed within the lead button. Spectroscopic techniques are essentially only able to analyse the outer surface of the sample. Further, often the precious metal is too diluted in the lead and is below a limit of detection.

Summary of the Invention

In a first aspect, there is provided a container for preparing a sample for analysis, the container comprising: a cavity for receiving a sample including a collector material and a precious metal, the cavity being suitable for melting the sample and oxidising the collector material, the cavity comprising:

a first region defined by a porous material that is capable of absorbing the collector material when oxidised and molten; and

a second region defined by a material that is largely incapable of absorbing the molten and oxidised collector material, the second region being capable of holding a volume of the molten sample; wherein the container is arranged such that, when the sample has melted and has reduced in volume due to absorption by the porous material of the first region, at least a portion of the remaining sample remains in the second region.

Throughout this specification, unless the context requires otherwise, the term "collector material" refers to any substance that can form an alloy with a precious metal at suitable temperatures, thus Collecting' it. Examples of collector materials include base metals such as lead and silver, or nickel sulphide. The terms may also be used to refer to the substance in its oxidised state, for example, lead and lead oxide.

In one specific embodiment of the present invention the first and second regions of the container are arranged such that collector material is absorbed by the porous material of the first region, which reduces the amount of the sample during cupellation until the sample

substantially only remains in the second region whereby then further absorption of the collector material is substantially avoided.

Embodiments of the present invention have the advantage that absorption of the collector material or "cupellation" ceases automatically once the material collector material in the first region is absorbed and concentrated sample in the second region is formed.

The second region may extend from a bottom portion of the first region. Further or alternatively, the second region may be positioned below the first region. The second region may have a smaller volume than the first region.

The container may be a cupel, and the second region may be wholly contained in the cupel and extending from the first region .

Throughout this specification, unless the context requires otherwise, the term "cupel" refers to a container comprising porous material capable of withstanding temperatures in the order of about 1000-1200°C. To provide context, cupels are commonly used in a technique known as fire assay to refine precious metals.

The porous material of the first region of the container may for example comprise bone ash or magnesium oxide. The second region may for example be defined by a suitable ceramic material that is largely incapable of absorbing the molten and oxidised collector material, which may be boron nitride, aluminium oxide or glassy carbon.

The container may comprise an insert that defines the second region and extends from the first region (typically from the bottom of the first region) . The insert may be cylindrically shaped having closed bottom portion and an open top portion for receiving the sample via the first region. The insert may be surrounded by the material that defines the first region.

In a second aspect, there is provided a method of

preparing a sample for analysis, the method comprising: providing the container in accordance with the first aspect of the present invention;

placing a fused sample including a collector material and a precious metal into the cavity of the container; and heating the sample in the container to a temperature sufficient to melt the sample and allowing a portion of the molten sample to be absorbed by the porous material; wherein the container is arranged and the method is conducted such that a portion of the molten sample that remains in the cavity retreats into the second region, preventing any further absorption by the porous material.

In one specific embodiment of the present invention the the method is conducted such that collector material is absorbed by the porous material of the first region, which reduces the amount of the sample until the sample

substantially only remains in the second region whereby further absorption of the collector material is avoided typically substantially automatically. The collector material may comprise silver as a co- collector material. The collector material may comprise a primary collector material that may comprise lead in the form of litharge . The step of heating the sample in the container typically involves oxidation of the collector material.

The method may comprise modifying a property of an environment surrounding the container during heating of the sample to increase or decrease a rate at which the container absorbs the collector material. Modifying the environment may comprise adding oxygen to increase oxidation of the collector material, and thus the rate at which the oxidised collector material is absorbed.

The method may comprise, after absorption of the collector material has ceased, pouring the remaining sample into a mould, such as a chilled mould. Further, the chilled mould may be a sample holder, such that the remaining sample can be subsequently analysed in the sample holder.

In a third aspect, there is provided a method of preparing a sample for analysis, the method comprising:

heating a fused sample including a collector material and a precious metal in a cupel to a temperature

sufficient to melt the fused sample and initiate

absorption of at least a portion of the collector material by the cupel; and

causing the absorption of the collector material by the cupel to cease automatically after a pre-determined period of time in a manner such that the remaining sample comprises a portion of the collector material.

The step of causing the absorption of the collector material by the cupel to cease may comprise reducing the temperature of the furnace after the pre-determined period of time or removing the cupel from the furnace after a predetermined time.

Alternatively, the hole may be a recess that is located in a bottom portion of the cupel without protruding through the bottom portion. The recess may have a diameter that is much smaller than that of the cupel. For example, the recess may have a diameter in the order of 5 - 10mm or about 5 - 10%, 10 - 20%, or 20 to 30% of the diameter of the cupel. The recess may have an internal volume that is less than 1%, 1-2%, 2-5%, 5 - 10%, or 10 - 20% of a total internal volume of the cupel. As the sample is absorbed in the cupel, an amount of the sample reduces until it is only located in the recess. As the surface area in the recess is significantly smaller than a total internal surface area of the cupel, the cupellation process

(absorption) is slowed making is easier to control the volume of sample remaining and the time for casting the sample .

In a fourth aspect of the present invention there is provided a system for automated preparation of a sample for analysis, the system comprising:

a furnace having at least one receiving station located within an interior of the furnace and an access port to facilitate access to the receiving station;

the container in accordance with the first aspect of the present invention, further configured to be received by the, or a respective, receiving station;

a loading mechanism for moving the container relative to the furnace; and

a controller for controlling the loading mechanism.

The controller may further be arranged to initiate a change in a furnace operation parameter after a pre- determined period of time . The change in the furnace operation parameter may be a reduction in the temperature inside the furnace, or may relate to opening or closing of the access port. The controller may be arranged such that the container is loaded or unloaded automatically after a pre-determined period of time.

The system may further be configured to decant the contents of the container into another container, such as a chilled mould.

The invention will be more fully understood from the following description of specific embodiments of the invention. The description is provided with reference to the accompanying drawings .

Brief Description of the Drawings

Figure 1 is a flow chart illustrating a method according to an embodiment of the present invention;

Figure 2 is a flow chart illustrating a method according to another embodiment of the present invention; Figure 3a is a plan view of a cupel according to an embodiment of the present invention;

Figure 3b is a cross-sectional view of the cupel shown in Figure 3a;

Figure 3c is a perspective sectional view of the cupel shown in Figures 3a and 3b; and Figures 4a and 4b are perspective views of a system according to an embodiment of the present invention.

Detailed Description of Specific Embodiments of the

Present Invention

The present invention relates to a method and a container for preparing a mineral sample for analysis, such as spectroscopic analysis, to determine a quantity of a precious metal in the sample.

Mineral samples for spectroscopic analysis are typically fused with a flux and collector material prior to

analysis. The flux may serve to lower the melting point and to impart a level of homogeneous fluidity in the sample. The flux may also include the collector material, such as litharge and silver. The mixture of the sample with flux is then placed in a furnace and heated to about 1000°C to form a melt. Molten slag floats to the top of the melt, and the collector material alloys with the precious metal in the sample and sinks to the bottom to form a pool of molten collector and precious metals, which is then separated from the molten slag and chilled rapidly to form a homogeneous button. Then, typically, spectroscopic analysis is performed directly on the button. Suitable spectroscopy techniques include laser ablation, optical emission spectrometry, or X-Ray fluorescence (XRF) . However, a problem with this is that the level of homogeneity in the button may be insufficient to produce accurate results, for example, if the button has not been produced by rapid chilling. This may in turn present a problem during spectroscopic analysis, which generally only measures small areas of the sample. As a result, significant errors can occur if the precious metal is not sufficiently homogeneous within the button. Also, impurities in the sample may cause

interference during analysis.

With reference to Figure 1 a method 100 in accordance with an embodiment of the present invention is now described. The method 100 comprises heating the button in a cupel to a temperature sufficient to melt the sample and oxidise a collector material to initiate absorption of at least a portion of the oxidised collector material by the cupel (step 102) .

In a specific embodiment of the present invention the sample is a fused sample containing lead and a precious metal. More specifically, the sample is derived from a fusion process wherein a mineral sample is combined with flux and a collector material (lead in the example) , and heated in a crucible to fuse the sample. During this process, the collector material (lead) melts and oxidises, collects the precious metals, and settles at the bottom of a crucible. Silver may also be added as a co-collector material. The lead and precious metals can then be separated from remaining slag to form fused sample or a "lead button". Examples of the precious metals include, but not limited to, gold, silver, platinum, palladium, ruthenium and rhodium.

The method 100 further comprises causing the absorption of the oxidised lead by the cupel to cease automatically after a pre-determined period of time in a manner such that a remaining sample comprises only a portion of the original lead (step 104) .

The cupel comprises a porous materials such as bone ash or magnesium oxide. In the step 102, the cupel and button are heated in a furnace to about 1000-1200°C. During this process, the lead oxidises due to a reaction with the flow of oxygen into the furnace. The lead oxide then melts and diffuses into the pores of the cupel by capillary action, thus separating from the precious metal. The precious metals are left behind unabsorbed. If the heating in the step 102 is continued for a

sufficiently long period of time, eventually all the lead in the sample will oxidise and be absorbed by the cupel, and a "prill" of precious metal having very high

percentage purity will remain. However, in accordance with the method 100, step 104 is carried out before this can occur i.e. oxidation of the lead is not continued to completion. Absorption of the collector material by the cupel is caused to cease after a pre-determined period, of time, such that a portion of the lead collector material still remains in the cupel. In other words, the method 100 only allows partial cupellation.

Thus, the method 100 increases both the concentration of precious metals, hich facilitates sample analysis.

Further, by performing a partial cupellation process, the impurities in the mixture of the sample and flux (which can cause interference during spectroscopic analysis) are removed also by absorption into the cupel material. Step 104 of causing the absorption of the collector material (lead oxide) by the cupel to cease may be carried out by reducing the temperature of the furnace at a predetermined time or the cupel may be removed from the furnace at a predetermined time .

In a further embodiment, the step 102 of the method 100 may comprise modifying an environment surrounding the cupel during heating of the sample to increase or decrease a rate in which the cupel absorbs the lead oxide. In one embodiment, modifying the environment comprises

introducing oxygen to the inside of the furnace in which the cupel is heated, to increase oxidation of the lead, and thus the rate at which the lead oxide is absorbed. The introduction of oxygen to the furnace may involve

injecting a pre-determined amount of oxygen gas into the furnace, or opening the furnace door for a pre-determined amount of time to allow inflow of additional air.

In one specific embodiment of the present invention the cupel has a recess that is located in a bottom portion of the cupel. The recess has a diameter that is much smaller than that of the cupel. For example, the recess may have a diameter in the order of 5 - 10 mm. As the collector material of the sample is absorbed by the cupel, an amount of the sample reduces until the sample is exclusively located in the recess . As the surface area in the recess is a lot smaller, the cupellation process (absorption) is slowed down significantly making is easier to control the volume of sample remaining and the time for casting the sample .

With reference to Figure 2, a method 200 of preparing a sample, specifically a fused sample in the form of a lead button, in accordance with a further embodiment of the present invention is now described.

The method 200 comprises providing a container having a cavity that has a first region defined by porous material that is capable of absorbing a collector material when oxidised and molten, and a second region defined by a material that is largely impermeable and capable of holding a volume of a molten sample (step 202) . The container will be described in more detail with reference to Figures 3 (a), (b) and (c) further below. Again, in one embodiment, the collector material is lead and the sample is a fused sample or a "lead button".

The method 200 also comprises placing the fused sample or lead button including the precious metals in the cavity (step 204) .

Further, the method 200 comprises heating the lead button in the cupel to a temperature sufficient to melt the lead button and allow the oxidised lead to be absorbed by the porous cupel material, thus reducing the volume of the lead button, wherein the container is arranged such that the remaining portion of the sample retreats wholly into - li the second region, preventing any further absorption by the porous material (step 206) .

The method 200 is similar to the method 100 in that both methods involve heating the sample in a container and, at a pre-determined time, prematurely causing the absorption of the lead collector material to cease. Thus, in both methods, the remaining sample to be analysed still comprises a portion of the lead collector material, and the precious metal is more concentrated and the sample has less impurities than if this partial cupellation step had not been carried out.

However, in the method 200, it is the configuration of the container that causes the lead collector material to be prevented from being absorbed any further.

In one embodiment and as mentioned above, silver may be added to the flux to act as a co-collector during fusion of the mineral sample with the flux which contains the principal collector material, usually lead. Silver may be added as a metal or in the form of silver salt. At the end of the fusion process the separated collector material, usually mainly composed of lead, will contain all the silver as well as all the other collected precious metals. For mineral samples which have been fused with a flux containing silver as a co-collector in addition to lead, the cupellation may be continued for a sufficient length of time to allow all the lead to be oxidized and absorbed into the porous body of the cupel leaving only a silver bead containing the collected precious metal remaining in the cupel. This silver bead is then used for analysis for example using optical emission spectroscopy. Further, when the collector material comprises lead and silver as a co- collector, the method 100 or 200 may further comprise allowing the entire lead to oxidise such that only the silver co-collector remains in the cupel with the

collected precious metals. The silver and other precious metals can then be analysed using spectroscopic

techniques, such as laser ablation or optical emission spectrometry . With reference to Figures 3a to 3c, in one embodiment, a cupel 300 that may be used for the method 200 is now described. Figures 3b and 3c show sectional views through the cut A-A shown in Figure 3a. The cupel 300 comprises a cavity 302 for receiving a material to be partially cupellated. The material to be received may be lead button including precious metal it has collected, as in the case of the method 100 and 200. The cavity 302 comprises a first region 304 and a second region 306.

The first region 304 is defined by porous material capable of absorbing the lead when oxidised and in melted form. The porous material can be the same as the material of the cupel suitable for use in the method 100, such as bone ash/magnesium oxide.

The second region 306 is defined by a material that is largely impermeable. In this embodiment, the second region comprises a surrounding wall 308 and a bottom portion. The second region 306 is capable of holding a volume of a molten sample.

The first and second regions 304 and 306 are arranged such that in use, when the mixture reduces in volume due to absorption of the collector material by the porous material, a remaining portion of the molten sample retreats wholly into the second region 306. As a result, any further absorption by the porous material of the first region 304 is prevented.

In this particular embodiment, when the cupel 300 is standing upright, the second region 306 is located below the first region 304 and extends from a bottom portion of the first region 304. In other words, an open bottom 310 of the first region 304 is adjacent to and in

communication with an open top 312 of the second region 306, thus allowing fluid communication between the first and second regions 304 and 306. In one embodiment, the first region 304 is configured with a concavely curved, inner surface 314, and the second region 304 is cylindrical, as shown in Figures 3a-3c. The second region 306 has a smaller volume than the first region 304.

Further, the surrounding wall 308 of the second region 306 may be in the form of an insert 316, particularly a cylindrical insert, comprising a base 318 and side wall(s) 320 extending upwardly therefrom. Thus, the insert 316 can be made separately to a main body of the cupel 300, and later combined.

The cupel 300 may be formed by firstly manufacturing the body of the cupel 300 entirely of bone ash or magnesium oxide, with a portion of the cavity 302 shaped to receive the insert 316. Then, the insert 316 can be placed in the associated portion of the cavity 302. The insert 316 can be made of any suitable material, such as ceramic, which can withstand temperatures of around 1000-1200°C and will not absorb or react with the collector material. In an alternative embodiment, a container for use in the method 200 may be formed from a combination of a cupel and crucible of impermeable material. For example, the cupel may have a longitudinal bore drilled through to a bottom end of the cupel . The crucible can then be coupled to the bottom end of the cupel in a manner which seals the opening created by the bore. The crucible may be formed to be fitted inside the bore, such that a bottom end of the crucible is flush with the bottom end of the cupel.

Alternatively, the crucible may be coupled to an outer surface of the cupel, thus forming an appendage to the cupel .

An advantage of using the above-described container arrangements for use in the method 200 is that a

consistent volume or desired amount of the sample can be accurately obtained for subsequent analysis . In other words, the method 200 can produce multiple samples having the same predetermined volume, thus enabling a more efficient method of precious metal assay.

In further embodiments, the methods 100 and 200 may comprise pouring the remaining sample into a chilled mould after the steps 104 and 206, respectively. Thus, after absorption of the collector material has ceased, or in other words the partial cupellation process is completed, the remaining sample is then allowed to cool and

resolidify in a chilled mould before being analysed for precious metal content. In one embodiment, the chilled mould also functions as a sample holder for subsequent spectroscopic or other analysis. The mould can be of any suitable shape.

With reference to Figures 4a and 4b a system 400 in accordance with an embodiment of the present invention is now described. The system 400 enables automated

preparation of a mineral sample for analysis to determine a quantity of a precious metal in the mineral sample. The system 400 may be used to carry out the methods 100 or 200. The system 400 comprises a furnace 402, a loading

mechanism 404 and a controller for controlling at least one component of the system, including the loading mechanism. The furnace 402 comprises receiving stations located inside the furnace 402. The receiving stations are capable of receiving suitable containers or cupels containing respective samples. For example, each container to be received by a respective receiving station may be in the form of the cupel 300 as described above and further configured to be received by the stations. The furnace 400 also comprises an access port to facilitate access to the receiving station. For example, the access port may be a door .

A plurality of receiving stations 404 may be provided, so that the automated process can assist in the preparation of one sample or a batch of samples. It will be

appreciated that the number of receiving stations 404 is exemplary only, and any number of receiving stations may be housed by the furnace 402. The controller is typically capable of controlling various components of the system 400, but is at least capable of controlling the loading mechanism 404 to perform handling tasks. Such tasks include loading, unloading and decanting contents of the container into another. As shown in

Figures 4a and 4b, the loading mechanism 404 is positioned relative to the furnace, but can also move relative to the furnace in order to perform these tasks.

In one example, the controller is arranged such that the container is loaded or unloaded automatically after a predetermined period of time. In other words, the controller may be programmed to execute predetermined motions relating to loading and unloading the container. For example, the mechanism 404 may be programmed to remove a container from the furnace 402 and decant its contents into a mould after the container and respective sample have been heated for a pre-determined period of time inside the furnace 402. Once the sample has cooled in the mould, the sample can then be analysed for precious metal content. The mechanism 404 can sequentially perform this task for each container in the furnace, thus automating the process .

In a further embodiment, the controller is also arranged to initiate a change in a furnace operation parameter after a pre-determined period of time or other condition/s being met. Furnace operation parameters may include a reduction in the temperature inside the furnace, and opening or closing of the access port. The furnace 402 may comprise control electronics configured to communicate with the controller, so that the controller can initiate these changes. For instance, the furnace may comprise electronic temperature sensors and/or a timer. Once the pre-selected conditions of temperature and/or duration have been met, the controller can automatically cause the furnace to reduce or cease heating of the container and sample. Then, the controller can control the loading mechanism to proceed with handling tasks such as unloading and decanting. The furnace 402 comprises a housing and a heater for heating an interior portion of the housing to a

temperature sufficient for causing melting of each sample. Further, in one embodiment, the receiving stations are in the form of an aperture or indentation located in a platform, being sized to receive respective containers .

In a further example, the furnace 402 comprises a rotating platform or carousel on or in which the receiving stations are located. The carousel may be configured to receive a plurality of (e.g. six) receiving stations radially disposed about a central longitudinal axis of the

carousel .

The carousel assists in moving the containers relative to the receiving stations, in particular, placing and removing the containers from the furnace. Thus, when a container is desired to be placed in or removed from a particular receiving station, the carousel is rotated so that the receiving station moves to a loading/unloading position relative to the furnace 402. Then, through the access port, the furnace 402 may allow the mechanism 404 to place or remove the container from the respective receiving station.

All such modifications and variations together with others that would be obvious to persons of ordinary skill in the art are deemed to be within the scope of the present invention, the nature of which is to be determined from the above description and the appended claims. For example, the first and second regions 304 and 306 may have an alternative shape and configuration to the above- described specific embodiment.