WO2011130790A1

WO2011130790A1 - Smelting method using a fluxing agent

Info

Publication number: WO2011130790A1
Application number: PCT/AU2011/000454
Authority: WO
Inventors: Dean R Butler
Original assignee: Precious Metals Recovery Pty Ltd
Priority date: 2010-04-23
Filing date: 2011-04-21
Publication date: 2011-10-27

Abstract

A method of smelting a gold rich source material in a crucible comprising: i) adding a fluxing agent to the crucible; ii) heating the fluxing agent in the crucible to provide an initial molten fluid pool of fluxing agent in the crucible; iii) adding gold rich source material to the molten pool of fluxing agent; and iv) pouring out of the molten contents of the crucible with separation of slag to form a precious metal ingot.

Description

Smelting Method Using a Fluxing Agent

Field

[0001] The invention relates to the smelting of gold rich source materials derived from ore processing operations.

Background

[0002] The process of gold recovery frequently involves a leaching step and adsorption of gold and other precious metals onto an adsorbent such as carbon or a suitable synthetic resin. Improvements in the adsorption process such as the carbon in column (CIC), carbon in leach (CIL) and carbon in pulp (CIP) processes have led to efficient gold recovery which in some cases have even justified reprocessing of mine tailings. Precious metals are stripped from the adsorbent by elution using suitable solubilising liquors for precious metals to form a strip liquor, also referred to as pregnant liquor, containing the precious metals stripped from the absorbent.

[0003] Precious metals including gold and silver may be recovered from the strip liquor in an electrowinning process in which the precious metals are deposited from the strip liquor onto the cathode of an electrowinning cell.

[0004] The electrode-associated material includes materials such the direct cathode deposits and electrode-associated sludge which may collect on or below the cathode of a gold electrowinning cell.

[0005] The cathode in this process is usually a high surface area cathode, and may comprise steel wool. Both the material deposited on the cathode (often called wire-gold where the cathode is steel wool) and cathode slimes (deposits which collect beneath and in association with the cathode) are rich in precious metals, and the next step in the precious metal recovery process usually involves acid treatment to remove steel wool, followed by smelting and bullion formation. [0006] When copper is refined by electrolysis the anodes are frequently cast from processed blister copper placed into an aqueous solution of 3-4% copper sulfate and 10- 16% sulfuric acid. Cathodes are often thin rolled sheets of highly pure copper. At the anode, copper and less noble metals dissolve. More noble metals such as silver and gold as well as selenium and tellurium settle to the bottom of the cell as anode mud, which forms a saleable by-product. The anode mud therefore includes the anode associated gold.

[0007] Gravity gold, that is gold refined by a gravitational process such as tabling and/or spiral classification, is another gold rich source material.

[0008] A common method used to process gold-containing material, particularly gold concentrates (material at least partly refined to enrich precious metal content, involves smelting that material). Smelting involves placing the source material in a crucible, adding fluxing agents and heating to about 1250 ^QC. Some base metal contaminants are collected in the floating slag layer that forms over the molten precious metals. After cooling the slag can be physically separated from the dore metal bar and further processing can take place to obtain more highly purified gold.

[0009] The smelting of gold rich source material involves heating a combination of the source material and a flux to form a floating slag layer and submerged precious metal layer. The slag is physically removed after cooling and carries with it at least some base metal values that would otherwise contaminate the precious metal ingot. In order to provide efficient removal of base metals into the slag it is considered necessary by those skilled in the art that the source material and fluxing agent be well mixed. This is considered particularly important in the smelting of larger quantities of source material, for example quantities greater than 100g and particularly quantities greater than 300g.

[0010] The mixing of source material and fluxing agent prior to heating/smelting is generally considered to be a key feature of the smelting process. [0011] Examples of gold rich source materials include cathode associated material from a gold electrowinning cell, anode mud from a copper electrowinning cell, gravity gold concentrate and residues from recycling of electrical circuit components comprising gold.

[0012] 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.

Summary

[0013] There is provided a method of smelting a gold rich source material in a crucible comprising:

i) adding a fluxing agent to the crucible;

ii) heating the fluxing agent in the crucible to provide an initial molten fluid pool of fluxing agent in the crucible;

iii) adding gold rich source material to the molten pool of fluxing agent; and iv) pouring out the molten contents of the crucible with separation of slag to form a precious metal ingot.

[0014] Preferably at least 30% (such as at least 50%, at least 70%, at least 80% at least 90%, at least 95%) of the flux used in the smelting batch is present prior to addition of charges of source material.

[0015] The initial liquid phase of fluxing agent typically comprises little or no contamination from the source material. For example the fluxing agent charge preferably comprises less than 10% (preferably less than 5% such as less than 3%, less than 2% and less than 1 %) by weight of source material;

[0016] Smelting of the gold rich source material in a crucible may further comprise: forming a molten pool of collector metal, comprising a metal selected from the group consisting of gold, silver, copper and platinum group metals, beneath the molten pool of fluxing agent; and

adding the gold rich source material to the molten pool of fluxing agent and to the pool of molten metal through the molten pool of fluxing agent.

[0017] 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.

Detailed Description

[0018] Smelting involves heating the gold source material with a chemical substance called a fluxing agent also simply called flux. The flux bonds with the contaminants and floats on top of the melted gold. The gold is then cooled and allowed to harden in molds, and the flux-contaminant mixture (slag) is hauled away as a solid waste.

[0019] During the process of smelting gold the present inventor has found that a significant amount of gold, frequently of the order of from 1 to 3% or even more, is lost to slag. Even when the slag is ground and reintroduced into an earlier part of the gold recovery circuit this slag associated gold may be substantially unrecoverable. There is therefore a need for a process for liberating gold that the prior art has not recognised as recoverable. The inventor has also found that mixing of source material and fluxing agent may facilitate the loss of gold values to slag.

[0020] Despite the industry practice regarding premixing of fine mixture of flux with the source material to be smelted as a key feature of the smelting process we have found that the recovery of gold in the ingot is enhanced (probably due to reduced loss in the slag) when the source material is introduced to a fluid pool of molten fluxing agent. [0021] The smelting process is conducted in a crucible. A range of crucibles are known in the art. In one set of embodiments the crucible preferably contains no more than 10% carbon preferably no more than 5 % carbon, e.g. less than 1 %. The crucible preferably comprises clay.

[0022] Fluxing agents known for use in smelting gold rich material may be used. These include soda, borax, silica, sodium oxide and sodium carbonate and mixtures thereof. It is particularly preferred that the fluxing agent comprises borax such as at least 30% by weight, at least 50%, at least 70%, at least 80%, at least 90% and at least 95% by weight borax. Good results are generally obtained where the flux consists essentially of borax, however, in specific cases other fluxes or mixtures may perform acceptably. In one set of embodiments the initial liquid phase of flux comprises less than 10% (preferably less than 5% such as less than 3%, less than 2% and less than 1 %) by weight of gold rich source material;

[0023] Borax is also known as sodium borate, sodium tetraborate or disodium tetraborate.

[0024] Examples of source materials that are rich in precious metals include, but are not limited to:

a. cathode-associated material formed during electrolysis of a strip liquor. The strip liquor may arise when gold is stripped from an activated carbon,for example using a cyanide liquor;

b. anode-associated material formed during the electrolytic refining of copper or other base metal from a base metal cast anode; and

c. gravity gold.

[0025] The electrode-associated material includes materials such the direct cathode deposits and electrode-associated sludge which may collect on or below the cathode of a gold electrowinning cell. At least some of the cathode associated material generally has the morphology of a fine wire or fine wire coating. This material forms clumps comprising elongated high surface area components rather than solid independent particles.

[0026] The word clump and variation such as clumps is used herein to refer to a cluster or lump, particularly a bunch of filaments of elongated material such as obtained from cathode deposits of gold rich material derived from wire-type cathodes. Where used herein the term particle and variations such as particles and particulate are intended to include material in clump form.

[0027] The gold rich source material is preferably added to the molten flux in particulate form such as substantially sub 10 mm, preferably sub 5 mm and more preferably sub 1 mm and most preferably sub 0.5 mm.

[0028] It is generally preferred that the source material added to the molten flux contain little or no source material such as not more than 30%, preferably not more than 20% more preferably not more than 10% still more preferably not more than 5% such as no more than 2% and no more than 1 % by weight of the source material.

[0029] In one embodiment the source material which is added to the molten slag is in admixture with a collector metal. Examples of collector metals include copper silver, gold and platinum group metals. In this embodiment the source material charge is a mixture comprising particulate source material and particulate material comprising at least one metal selected from the group consisting of copper, silver gold and platinum group metals. In one set of embodiments the collector metal is present in an amount of up to 500 parts collector metal per 100 parts source metal such as 5 to 500 parts per 100 parts source material.

[0030] In one set of embodiments the gold rich source material is added to the molten pool of fluxing agent in divided form such as 90% by weight material passing through sub 10 mm aperture dry sieve, preferably sub 5 mm aperture dry sieve and more preferably sub 1 mm aperture dry sieve and most preferably sub 0.5 mm aperture dry sieve.

[0031] It is a feature of the method that the source material is added to a molten pool of flux. The flux is heated to provide a homogenous molten liquid at the time of addition of the source material. The flux will preferably contain little or no (less than 10% preferably less than 5% by weight) solid material such as gold rich source material. In one set of embodiments at least 30% (such as at least 50%, at least 80%, at least 90%, at least 95%) of the total flux used in the smelting is present prior to addition of charges of source material.

[0032] The method involves addition of source material to the molten flux so that the source material sinks through the molten pool of flux. The molten pool of fluxing agent is fluid and added charges of source material sink rapidly to be covered by the fluxing agent. The addition of source material may initially deposit source material on top of the fluxing agent particularly if the source material is added slowly or in small charges to the fluxing agent. If source material is added gradually the amount will build up on the fluxing agent until enough material is added to cause it to sink. In one set of embodiments the minimum quantity required for rapid sinking is at least 5 g, preferably at least 20g. The time taken for complete submersion depends on the size of the charge and viscosity of the fluxing agent which in turn is dependant on the temperature of the pool of fluxing agent. In one set of embodiments the temperature of the pool is such that the period of time for sinking of the source material to provide complete submersion of a charge of 20 g is less than 60 seconds preferably less than 30 seconds and more preferably less than 15 seconds. In the embodiment the temperature of the molten fluxing agent on commencement of addition of the source material is at least 1000^QC preferably 1 100^QC, more preferably 1 150^QC and even more preferably at least 1200^QC. In one set of embodiments the molten fluxing agent (preferably borax) is at a temperature in the range of from 1 150^QC to 1300^QC preferably 1 1 80^QC to 1250^QC.The source material is preferably added in charge sizes so as not to unduly cool the fluxing agent such as borax. Adding the source material all at once may unduly reduce borax temperature and the efficiency of the smelting process.

[0033] The source material may be added gradually to the molten pool of flux such as borax so as to maintain the fluidity of the pool with heating. In one set of embodiments the source material is added in a number of discrete sequential charges with time spacing during which the crucible is heated to maintain the desired fluidity of the molten pool.

[0034] When each charge of source material is added to the molten flux such as borax a reaction is observed to take place for about 10 minutes and dark coloured globules rise to the surface. When globules no longer appear, the reaction between newly added source material and molten flux is considered complete, and a further charge of source material may be added.

[0035] The weight of each charge of source material is at least 5 g preferably at least 20 g such as at least 25 g, at least 30 g at least 35 g, at least 40 g, at least 45 g at least 50 g, at least 100 g, at least 200g, at least 500 g.

[0036] The optimum rate of addition and weight of each addition will depend on the relative amounts of each charge, the total charge and the size of the molten pool at the commencement of a charge. The weight ratio of molten material in the crucible to source material in a charge at the commencement of addition of the charge is preferably in the range of from 20:1 to 2:1 (molten material:charge). It is preferred that there are a number of charges and the weight ratio in each of the charges may be in the range of from 20:1 to 2:1 (molten material:charge). Preferably the ratio (molten materiakcharge) is in the range of from 15:1 to 5:1

[0037] A preferred procedure for adding source material is to add charges of source material of 1 to 25 parts by weight of charge to 100 parts by weight of molten material in the crucible at the commencement addition of each charge. The addition may be repeated at time intervals of from 1 to 90 minutes and preferably about every 10 minutes to 20 minutes.

[0038] The period of delay between sequential charging may be chosen to ensure

(a) reversion to target temperature (for example at least 1000^QC preferably 1 100^QC, more preferably 1 150^QC and even more preferably at least 1220^QC; and

(b) completion of reaction by visual assessment.

[0039] In one set of embodiments the smelting of the gold rich source material in a crucible further comprises:

forming a molten pool of collector metal, such as comprising at least one metal selected from the group consisting of gold, silver, copper and platinum group metals, beneath the molten pool of fluxing agent; and

[0040] The molten pool of collector metal may be formed by addition of collector metal to the molten pool of fluxing agent and providing a temperature not less than the melting point of the collector metal.

[0041] The optimum rate of addition and weight of each addition of collector will depend on the relative amounts of each charge, the total charge and the size of the molten pool at the commencement of a charge. The weight ratio of molten material in the crucible to collector addition in a charge at the commencement of addition of the charge is preferably in the range of from 20:1 to 2:1 (molten materiakcharge). It is preferred that there are a number of charges and the weight ratio in each of the charges may be in the range of from 20:1 to 2:1 (molten materiakcharge). Preferably the ratio (molten material :charge) is in the range of from 15:1 to 5:1 [0042] The source material typically becomes incorporated in the molten pool of collector metal.

[0043] In embodiments in which a collector metal is used, particularly copper, it is generally preferred that the pool of molten metal cover the internal bottom wall of the crucible so that source material added to the crucible will contact the molten metal rather than the surface of the crucible. If the bottom of the crucible is concave or curved, sufficient metal collector is preferably used to ensure that the molten pool extends to the side wall of the crucible.

[0044] On completion of the addition of the collector metal it is preferred that the molten pool of fluxing agent cover the molten pool of collector metal and preferably the distance between the top of the molten pool of collector metal and the top of the liquid fluxing agent is at least 1 cm.

[0045] In order to optimize recovery it may be preferred to use coarsely divided collector metal preferably of particle size of at least 1 mm. Without being bound by theory we believe the use of coarser particles minimise oxidation of the collector particularly where the collector is copper.

[0046] The collector metal may be in particulate form and in one embodiment is in admixture with particulate source material.

[0047] The collector metal may, for example, be present in an amount of up to 5000 parts collector metal per 100 parts source metal such as 5 to 5000 parts per 100 parts source material.

[0048] The manner and rate of addition of a particulate mixture of source material and at least one collector metal selected from copper, silver, gold and platinum group metals may be optimised according to the specific nature of the materials used. In one set of embodiments, the mixture is gradually added to a preformed molten pool of fluxing agent. In one set of embodiments the collector metal comprises copper.

[0049] In one set of embodiments a molten pool of collector metal is formed from a solid particulate mixture comprising particles source material and particles of collector metal comprising at least one metal selected from copper, silver, gold and platinum group metals. In this set of embodiments a molten pool of metal is formed by addition of the mixture to the molten fluxing agent pool to form a pool of metal beneath the molten fluxing agent pool and further charges of mixture are added to the molten fluxing agent and molten metal pool formed from the collector metal and source material. The particulate mixture of the source material and at least one metal selected from copper, silver and gold is preferably added to a heated crucible such that a molten pool is formed during addition and further particulate mixture is added to and becomes part of the molten pool of metal.

[0050] In one set of embodiments the smelting method comprises adding the source material to a previously melted pool comprising at least one metal selected from copper, silver, gold and platinum group metals.

[0051] In one set of embodiments there is thus provided a method of smelting a gold rich source material in a crucible comprising: i) adding a fluxing agent to the crucible;

iii) providing a mixture of gold rich source material and at least one collector metal selected from the group consisting of copper, silver, gold and platinum group metals

iv) adding the mixture of gold rich source material and at least one collector metal selected from the group consisting of copper, silver, gold and platinum group metals to the molten pool of fluxing agent to form a molten pool of metal beneath the molten pool of fluxing agent ;and

v) adding further mixture of gold rich source material and at least one collector metal selected from the group consisting of copper, silver, gold and platinum group metals to the molten pool of metal through the molten pool of fluxing agent; and

vi) pouring out of the molten contents of the crucible with separation of slag to form a precious metal ingot.

[0052] 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

[0053] Cathode associated wire gold was taken from an electrowinning cell. The feed liquor in the electrowinning cell was derived from the following process sequence:

[0054] Gravity gold concentrate is treated by cyanide leaching and the leach liquor is contacted with activated carbon.

[0055] The loaded carbon is stripped with caustic cyanide to provide the electrowinning feed liquor.

Source Material

[0056] The cathode associated wire gold was submerged in 30% hydrochloric acid for 2 hours to dissolve the wire component. The residue was washed and dried and the complete washed/dried residue yield was homogenized with a blender comprising a rapidly moving blade. The resulting mixture comprised wire gold derived particulate material of size less than 5 mm and substantially less than 1 mm in size. The complete blended residue weighed 16.8 Kg. This was used as the source material in subsequent smelting examples. Comparative Example 1 - Standard Smelt Process

[0057] 10.8 kg of the source material referred to above was taken for standard smelting using cone and quartering methods. The smelting of the 10.8 kg of material took place in a graphite crucible in a gas fired kiln to provide an ingot containing 252.577 Troy ounces of gold (Perth Mint bullion assay).

[0058] The standard commercial process involved the use of a mixed flux formulation containing the following in the parts by weight specified:

borax (2 parts), sodium carbonate (1 part), silica flour (1 part) and sodium nitrate (0.25 part) as a fluxing agent.

[0059] 10.8 kg of the fluxing formulation was stirred together 1 0.8 kg of wire gold material. A graphite crucible was heated to approximately 1220^QC in a gas fired kiln and multiple charges of the mixed powder as described above were added. The weight of each charge was approx. 2 kg. The molten material was poured into a mold, cooled and the slag layer removed. The ingot contained 252.577 Troy ounces of gold (Perth Mint bullion assay). This corresponds to 2.3387 Troy ounces of gold per 100 g of wire gold source material.

Smelting Example 1

[0060] 500 g of source material was divided into 5 charges of 100 g.

1 kg of borax was added to a clay crucible in a digital electric furnace and the temperature was brought to 1220^QC. At this temperature the borax was a homogeneous and fluid liquid. The first charge of 100 g of source material was added to the crucible. A reaction took place and dark coloured globules were seen rising to the surface of the borax. After 10 minutes no more dark coloured globules were observed and the temperature was 1220^QC. Thereafter a second charge of 100 g source material was added to the crucible and further reaction took place. After 10 minutes a third charge of 100 g source material was added and the process repeated until the last of the five charges was consumed and any associated reaction was complete. The molten material was poured into a mould and allowed to cool. The slag component was removed and the ingot was found to contain 12.172 Troy ounces of gold (Perth Mint bullion assay). This corresponded to 2.4344 Troy ounces of gold per 100 g of wire gold source material. Relative to the standard smelt process described above, this corresponds to a gold increment of 4.09%.

Smelting Example 2

[0061] 600 g of source material was divided into 3 charges of 200 g by cone and quarter methods. 1 kg of borax was added to a clay crucible in a digital electric furnace and the temperature was brought to 1220^QC. At this temperature the borax was a homogeneous and fluid liquid. The first charge of 200 g of source material was added to the crucible. A reaction took place and dark coloured globules were seen rising to the surface of the borax. Globule formation was observed for longer than10 minutes (rather than 10 minutes as in Example 1 ), and the speed at which the dark coloured globules rose to the surface of the molten borax was slower than in Example 1 . After the addition of the charge, the contents of the crucible were at a reduced temperature, and the time necessary for the temperature to reach 1220^QC again was 15 minutes. Thereafter a second charge of 200 g source material was added to the crucible and further reaction took place. After 15 minutes a third and final charge of 200 g source material was added and the temperature maintained for a further 15 minutes. The molten material was poured into a mould and allowed to cool. The slag component was removed and the ingot was found to contain 14.563 Troy ounces of gold (Perth Mint bullion assay). This corresponded to a gold increment of 3.78% relative to the gold recovered in the standard smelting process.

[0062] In each case, the wire gold was present as finely divided particles (no greater than 5 mm and substantially less than 1 mm).

[0063] When larger clumps of wire gold source material (10 mm in diameter) were added to liquid borax in a heated clay crucible, the reaction characterised by the formation of dark coloured globules was found to persist for significantly longer than when the clumps were 5 mm or less.

Smelting Example 3

[0064] 400 g of source material was divided into 4 charges of 1 00 g by cone and quarter method.

1 kg of borax was added to a clay crucible in a digital electric furnace and the temperature was brought to 1220^QC. At this temperature the borax was a homogeneous and fluid liquid. 200 g of copper powder was added to the molten borax and the crucible contents were restored to 1220^QC. At this temperature the copper formed a molten pool underneath the molten borax. The first charge of 100 g of source material was added to the crucible and the source material descended through the molten borax into the molten pool of copper. After 10 minutes the temperature was restored to 1220^QC. Thereafter a second charge of 100 g source material was added to the crucible. After 10 minutes a third charge of 100 g source material was added and the process repeated once more so that the last of the four charges was added and any associated reaction was complete. The molten material was poured into a mould and allowed to cool. The slag component was removed and the ingot was found to contain 9.769 Troy ounces of gold (Perth Mint bullion assay). This corresponded to 2.4422 Troy ounces of gold per 100 g of wire gold source material. Relative to the standard smelt process described above, this corresponds to a gold increment of 4.43%.

Smelting Example 4

[0065] 400 g of source material was divided into 4 charges of 100 g.

1 kg of borax was added to a clay crucible in a digital electric furnace and the temperature was brought to 1220^QC. At this temperature the borax was a homogeneous and fluid liquid. Two charges of 200 g each of copper powder were added with a 10 minute interval to the molten borax and the crucible contents were restored to 1220^QC. At this temperature the copper formed a molten pool underneath the molten borax. The first charge of 100 g of source material was added to the crucible and the source material descended through the molten borax into the molten pool of copper. After 10 minutes the temperature was restored to 1220^QC. Thereafter a second charge of 100 g source material was added to the crucible. After 10 minutes a third charge of 100 g source material was added and the process repeated once more so that the last of the four charges was consumed and any associated reaction was complete. The molten material was poured into a mould and allowed to cool. The slag component was removed and the ingot was found to contain 9.788 Troy ounces of gold (Perth Mint bullion assay). This corresponded to 2.4470 Troy ounces of gold per 100 g of wire gold source material. Relative to the standard smelt process described above, this corresponds to a gold increment of 4.63%.

Examples 5 to 28

[0066] 2kg of borax was added to a crucible pot of type listed in Table 1 and heated to a temperature in the range of from 1 180°C to 1250°C in a tilting furnace to provide a molten pool of borax.

Where copper metal is indicated in Table 1 , the copper was gradually added as a powder to the molten borax pool at a rate so as to maintain the molten pool and allow the copper to quickly dissent into the pool.

Source material from a Carbon-ln-Pulp process (C-l-P) or Gravity Gold process (GG) was gradually added at a rate so as to maintain the temperature of the pool and globules of dark coloured material rose within the borax as reaction took place.

In examples indicated "#" the copper powder was blended with the particulate source material before the combination was added to the molten pool of borax.

The molten material was poured into a mould to form an ingot which was allowed to cool.

The slag component was removed from the ingot and the ingot was analysed by the

Perth Mint using a bullion assay. The uplift compared with the standard smelt process

(see Comparative Example 1 ) for the same source material is reported in Table 1 . Table 1

Gravity Gold source

Carbon in Pulp derived gold source

Copper was blended with source material as fine powder prior to addition to molten pool of borax Borax flux replaced by the flux mixture used in Comparative Example 1

Small silicon carbide crucible used

[0067] Graphite crucibles were found to be useful for relatively small amounts of source material but maintenance of graphite crucibles in contact with molten flux, particularly molten borax was found to result in significant loss of performance after 20 or 30 minutes. This can be seen by comparing Examples 5 to 8 which treated from about 1 .5kg source material with Examples 16, 22 and 23 in which the amount of source material was increased to over 2.0 kg.

[0068] As a result, the processing of commercial quantities of source material is preferably conducted using clay crucibles and in particular, clay crucibles containing no more than 10%, preferably no more than 5% and most preferably, no more than 1 % carbon.

[0069] The use of silicon carbide crucibles in the process provided an uplift in gold which was much less than when clay crucibles were used.

[0070] Borax is also corrosive to clay crucibles, however, clay crucibles provide the most consistent uplift of gold recovered and the corrosion issue can be addressed by using relatively thick or double walled clay crucibles.

Claims

1 . A method of smelting a gold rich source material in a crucible comprising:

i) adding a fluxing agent to the crucible;

iii) adding gold rich source material to the molten pool of fluxing agent; and iv) pouring out of the molten contents of the crucible with separation of slag to form a precious metal ingot.

2. A method according to any one of the previous claims wherein the crucible contains no more than 10% carbon.

3. A method according to any one of the previous claims wherein the crucible comprises clay.

4. A method according to any one of the previous claims wherein the fluxing agent comprises borax

5. A method according to any one of the previous claims wherein the fluxing agent comprises at least 70% borax.

6. A method according to any one of the previous claims wherein the temperature of the molten fluxing agent on commencement of addition of source material is at least 1 150^QC.

7. A method according to any one of the previous claims wherein the gold rich source material comprises material selected from the group consisting of:

a. cathode-associated material formed during electrolysis of a strip liquor comprising gold stripped from activated carbon; b. anode-associated material formed during the electrolytic refining of copper or other base metal from a base metal cast anode; and

c. gravity gold.

8. A method according to any one of the previous claims wherein the gold rich source material is added to the molten pool of fluxing agent in divided form wherein as 90% by weight material passes sub 5 mm aperture dry sieve.

9. A method according to claim 1 wherein the initial molten pool of fluxing agent present prior to addition of charges of source material comprises at least 30% of the total flux used in the smelting.

10. A method according to any one of the previous claims wherein the source material in the form added to the molten flux contains no more than 20% by weight of fluxing agent.

1 1 . A method according to any one of the previous claims wherein the initial liquid phase of flux contains less than 10% by weight of gold rich source material.

12. A method according to any one of the previous claims wherein the gold rich source material is added to the pool of molten fluxing agent in discrete charges of at least 5 g.

13. A method according to any one of the previous claims wherein the gold rich source material becomes submerged in the pool of molten fluxing agent.

14. A method according to any one of the previous claims wherein the pool of fluxing agent is sufficiently fluid for the time taken for complete submersion of a charge of 20 g of gold rich source material in the molten pool of fluxing agent to be less than 60 seconds from its addition.

15. A method according to any one of the previous claims wherein the weight ratio of molten material to a charge of source material at the commencement of addition of a charge is in the range of from 20:1 to 2:1 (molten material:charge).

16. A method according to any one of the previous claims wherein a plurality of charges of source material are added and each of the charges of source material are in the range of from 1 to 25 parts by weight of charge of source material to 100 parts by weight of molten molten material in the crucible at the commencement of addition of the charge.

17. A method according to any one of the previous claims wherein multiple charges of source material are added to the molten pool of fluxing agent at time intervals in the range of from 1 minute to 90 minutes.

18. A method according to any one of the previous claims wherein smelting of the gold rich source material in a crucible further comprises:

forming a molten pool of collector metal, beneath the molten pool of fluxing agent; and

adding the gold rich source material to the molten pool of fluxing agent.

19. A method according to claim 18 wherein the collector metal comprises at least one metal selected from the group consisting of gold, silver, copper and platinum group metals.

20. A method according to claim 18 or claim 19 wherein the collector metal comprises copper.

21 . A method according to any one of claims 18 to 20 wherein the source material becomes incorporated in the molten pool of collector metal.

22. A method according to any one of claims 18 to 21 wherein the molten pool of collector metal is formed by addition of collector metal to the molten pool of fluxing agent and providing a temperature not less than the melting point of the collector metal.

23. A method according to any one of claims 18 to 22 wherein the molten pool of collector metal is formed by the addition of collector metal of particle size of at least 1 mm.

24. A method according to any one of claims 18 to 23 wherein the collector metal is present in an amount of up to 5000 parts collector metal per 100 parts source material.