SMELTING PROCESS

TECHNICAL FIELD

[0001] The present invention relates to a process for smelting a silver-rich ore or concentrate. In some embodiments, the present invention relates to a process for smelting a silver-rich ore or concentrate to produce a silver-rich matte.

BACKGROUND ART

[0002] It will be clearly understood that, if a prior art publication is referred to herein, this reference does not constitute an admission that the publication forms part of the common general knowledge in the art in Australia or in any other country.

[0003] In nature, silver generally occurs in minerals of copper, lead, or zinc. Silver mainly occurs as inclusion in the sulfide ores of copper (chalcopyrite - CuFeS ₂ ), lead (galena - PbS), and zinc (sphalerite - ZnS). These ores are mined for their main constituents, namely, copper from chalcopyrite, lead from galena, and zinc from sphalerite. Iri the process of extraction of these main constituents (copper, lead, and zinc) silver is produced as a by-product. Around 75% of the total silver is produced from these three sulphide ores. Some other less important and rare minerals of silver are argentite, pyrargyrite, proustite, stephanite, tetrahedrite, kerargyrite, embolite, polybasite, bromyrite, and iodyrite. About 25% of the total silver produced in the world is from the ores which are mined only for the silver values.

[0004] In the process of extraction of silver, the first step is typically to concentrate the silver bearing minerals obtained from mining. Sulfide minerals, the main sources of silver, are amenable to flotation for concentrating the mineral values. In general, silver is present in an amount of 1 to lOOOg/tonne (i.e. 0.0001% to 0.1%, by weight) in concentrates produced from such ores.

[0005] Silver may be recovered from ores or concentrates using hydrometallurgical processes or pyrometallurgical processes. Pyrometallurgical procees have been used to recover silver from lead-containing ores, copper containing ores and zinc-containing ores.

[0006] A recently discovered ore deposit can be used to form a concentrate that contains at least 2% by weight of silver (throughout this specification, all percentages are given as weight percentages). Testing has shown that a concentrate produced from this ore deposit can contain, for example, from 2% to 8% silver. This is an exceptionally high silver concentration in the concentrate.

[0007] The concentrate produced from this ore deposit can be described as a silver-rich poly-

*

metallic concentrate. The concentrate may also contain copper in an amount of from 3 to 15%, and lead in an amount of from 3 to 10%. The concentrate may also contain 20 to 40% silica and 5 to 15% alumina. The sulphur content may range from 3 to 10% and typically is in the range of around 5%. Thus, the concentrate has a somewhat unusual composition.

SUMMARY OF INVENTION

[0008] The present invention is directed to a process for smelting a silver-rich ore or concentrate. In some embodiments, the present invention is directed towards a process for smelting the silver-rich concentrate described in paragraphs [0006] and [0007] above.

[0009] Throughout the specification, the term "silver-rich ore or concentrate" means an ore or concentrate that contains at least 1% by weight silver, more preferably at least 2% by weight silver. The ore or concentrate may contain, for example, from 1% to 10% by weight silver, more preferably from 2% to 8% by weight silver, even more preferably from 3% to 8% by weight silver. The ore or concentrate may be a sulphide ore or concentrate.

[0010] In a first aspect, the present invention provides a process for smelting a silver-rich ore or concentrate comprising feeding the silver-rich ore or concentrate to a smelting furnace, adding a lead-containing flux material to the furnace and conducting smelting at elevated temperature and in the presence of oxygen to form a silver-rich matte and a slag.

[0011] The skilled person will, understand that a "matte" is a molten mixture of sulphide compounds. It may be a mixture of stoichiometric sulphide compounds, or it may diverge from stoichiometric ratios. Matte is a common intermediate product in copper smelting and nickel smelting.

[0012] In a second aspect, the present invention provides a process for smelting a silver-rich ore or concentrate containing at least 2% silver, the process comprising feeding the silver-rich ore or concentrate to a smelting furnace, adding a lead-containing flux material to the furnace and conducting smelting at elevated temperature and in the presence of oxygen to form a silver- rich matte and a slag. ¹

[0013] In a third aspect, the present invention provides a process for smelting a silver-rich ore or concentrate containing at least 2% silver, the process comprising feeding the silver-rich ore or concentrate to a smelting furnace, adding a lead-containing flux material to the furnace and conducting smelting at elevated temperature and in the presence of oxygen to form a silver- rich matte having a silver content of at least 10% and a slag.

[0014] In embodiments of the present invention, the silver-rich ore or concentrate comprises a silver-rich concentrate having at least 2% silver. The silver-rich concentrate may have a silver content of from 2% to 8%. The silver-rich concentrate may also contain copper in an amount of from 3 to 15%, and lead in an amount of from 3 to 10%. The concentrate may also contain from 20 to 40% silica, such as around 30% silica, and 5 to 15% alumina, such as around 10% alumina. The sulphur content of the concentrate may range from 3 to 10% and typically is in the range of around 5%.

[0015] The silver-rich concentrate may be produced by separating gangue from an ore and recovering the concentrate. The concentrate may be a concentrate produced by subjecting the ore to a flotation process. The concentrate may be a sulphide concentrate.

[0016] The silver-rich matte that is formed in the process of the present invention will have a higher silver concentration than the silver-rich ore or concentrate that is provided to the furnace. In one embodiment, the silver-rich matte has a silver content of at least 5%, more suitably from 5% to 60%, even more suitably from 10% to 40%. The silver-rich matte may also have a copper content of from 10% to 50%, a lead content of from 10% to 50%, a sulphur content of from 5% to 20% and an iron content of from 0 to 10%.

[0017] Desirably, the operating parameters of the smelting process are controlled such that a silver-rich matte having a composition as set out in paragraph [0016] above is formed. The operating parameters that can be controlled include composition of the concentrate, composition of flux material, relative amounts of the concentrate and flux material, amount of oxygen injected or supplied to the furnace, temperature and smelting time. The skilled person will readily be able to conduct simple experiments to determine the combination of operating parameters required to achieve the desired composition for the silver-rich matte.

[0018] The person skilled in the art will understand that the silver-rich matte will comprise a sulphide material. A small amount of metal may also report to the silver-rich matte.

[0019] The slag that is formed in the process of the present invention will comprise an oxide material. Desirably, most of the silica and alumina present in the silver-rich ore or concentrate that is fed to the furnace reports to the slag. Some of the lead added to the furnace as part of the lead-containing flux material also reports to the slag.

[0020] The slag that is formed in the process of the present invention will comprise an oxide material. Desirably, most of these silica and alumina present in the silver-rich ore or concentrate that is fed to the furnace reports to the slag. Some of the lead added to the furnace also reports to the slag.

[0021] The lead-containing flux material that is added to the smelting furnace may comprise a concentrate derived from a lead-sulphide ore, such as a galena ore. The lead-containing flux material may comprise a conventional lead concentrate. The lead-containing flux material may contain, for example, 50 to 60% lead, 5 to 10% iron, less than 1% copper, less than 1% silver and 15 to 20% sulphur. Other lead containing material may also be used, such as battery scrap.

[0022] The lead-containing flux material may be added in an amount such that the final slag contains 10-50 % lead oxide, more preferably 10-40% lead oxide.

[0023] The smelting process of the present invention may be operated at any suitable elevated temperature. In some embodiments, the smelting process is operated at a temperature of at least 1200°C. The temperature is preferably sufficiently high so that the slag is not overly viscous. For example, where the process is operated in a top entry submerged lance furnace, the slag should have a viscosity that allows the slag to splash and digest falling concentrate. The skilled person would understand that routine experiments could be conducted to determine an appropriate temperature for operating the process.

[0024] The smelting process of the present invention is operated with injection of oxygen. Oxygen may be provided in the form of essentially pure oxygen, air, or oxygen-enriched air.

[0025] The smelting process is preferably carried out in a furnace in which oxygen is injected through a lance. The smelting process may be carried out in a top entry submerged lance furnace, such as the furnace sold by the present applicant under the trademark ISASMELT™. Other furnaces that may be used in the process of the present invention include top blown rotary converter furnaces.

[0026] The process may also be operated in other furnaces known to be suitable for use in smelting processes. It is noted that embodiments of the present invention result in the production of significant quantities of slag and relatively small quantities of matte. Therefore, furnaces that are suitable for use in such environments are preferred for use in the present invention. [0027] The silver-rich matte that is formed in the process of the present invention may be recovered and sold as a product. In this embodiment, it is envisaged that the silver-rich matte will be tapped from the furnace and allowed to cool to form a solid matte. The solid matte can be sold as a product. The solid matte may be further treated to recover metals, including silver, therefrom.

[0028] Alternatively, the silver-rich matte may be further treated in the smelting furnace to convert the matte into metal. In this embodiment, the furnace may be sequentially operated in a smelting mode and then in a converting mode. As a further alternative, the silver-rich matte may be periodically tapped from the smelting furnace and sent to a converting furnace for production of metal. In this regard, the silver-rich matte and the slag are believed to be readily separable due to density differences. In a commercial process, it is expected that the matte and slag layers could be tapped separately.

[0029] Preferred embodiments of the present invention were developed following initial experimental work in which a silver-rich concentrate having a silver content of at least 2% (and having a composition generally as set out in paragraphs [0006] and [0007] above) was subject to smelting without having separate flux agents (it being assumed that the silica content of the concentrate plus oxides of the contained non-silver metals would be sufficient to form the slag during smelting). These tests were conducted in a top entry submerged lance furnace (laboratory scale). These tests showed that the slag foamed. In order to control slag foaming, addition of the lead-containing flux agent was tried in a subsequent series of tests. Although some foaming of the slag still occurred, foaming occurred to a markedly lesser degree than in the previous work. Indeed, the extent of foaming was sufficiently low to result in the process of the present invention being a commercially acceptable process.

[0030] In preferred embodiments of the present invention, a silver-rich concentrate (having a higher silver content when compared to conventional concentrates from which silver is recovered via smelting) is used as a feed material to a smelting process in which a silver-rich matte is formed. Again, the silver-rich matte has a significantly higher silver content than mattes that are formed in conventional smelting processes. The present inventor was faced with treating an ore or concentrate having an unusual composition. The present inventor was also faced with overcoming a number of difficulties before being able to achieve a smelting process that was likely to be operable on a commercial scale.

[0031] The reference to any prior art in this specification is not, and should not be taken as an acknowledgement or any form of suggestion that the prior art forms part of the common general knowledge.

[0032] In the present specification and claims (if any), the word "comprising" and its derivatives including "comprises" and "comprise" include each of the stated integers but does not exclude the inclusion of one or more further integers, unless the context of use indicates otherwise.

BRIEF DESCRIPTION OF DRAWINGS

[0033] Figure 1 shows a schematic view of the furnace used in the experimental work described below;

[0034] Figure 2 shows a graph of Fe content in matte vs Ag content in slag; and

[0035] Figure 3 shows a ternary projection of a phase diagram of a system containing AI2O3, CaC0 ₃ , Cu ₂ 0, Fe ₂ 0 ₃ , PbO and Si0 ₂ .

DESCRIPTION OF EMBODIMENTS

[0036] A series of seven experiments were conducted in which a silver-containing concentrate was smelted. Each test was performed using approximately 0.7 kg of material, contained in a crucible, with air bubbled through the material via a lance. Heat was applied via an electric induction loop. A 400 kHz induction furnace was used for heating. A schematic diagram of the 400 kHz induction furnace, heating station, off-gas duct and lance is shown in figure 1.

[0037] The crucibles used in this testwork were dense MgO crucible, with straight walls and a diameter of 60 mm and a height of 300 mm with a capacity of 600 cm ³ .

[0038] Lids of the same material as the crucible with a central hole for a flue were cemented to the top of the crucible, with a Pythagoras tube (30 mm O.D. and 60 mm long) cemented over the hole for the flue. The flue allowed a lance to be. passed down into the crucible and into the slag, and let exhaust gases exit and be collected in the gas off-take. Thermocouples sheathed in pockets made of alumina were located in the crucible or alongside the crucible. The crucible was heated by two susceptors, one inverted over the other, which were heated by the radio frequency induction. A Eurotherm programmable temperature controller was used to monitor temperature and control the furnace power. [0039] An alumina lance (5 mm O.D. and 3 mm I.D.) was used for injecting air into the bath, and for stirring the bath. The gas flow rates were delivered by an electronic mass flow controller.

[0040] A stainless steel hood over the crucible and steel ducting above the crucible mouth under negative pressure collected fumes and hot gases from the crucible. The fumes were drawn through the ducting and collected on large air sampling filter papers.

[0041] Smelting tests were conducted using a silver-containing concentrate obtained by subjecting a silver-containing ore to the flotation. The concentrate has a typical composition that is broadly as follows:

Ag: 2-8%

Cu: 3-15%

Pb: 3-10%

Si0 ₂ : about 30%

A1 ₂ 0 ₃ : about 10%.

A lead-containing flux material was also used in the smelting tests. The lead-containing flux material comprised a concentrate derived from flotation of a galena ore. The lead-containing flux material has a typical composition that is broadly as follows:

Pb: 50-60%

Fe: 5-10%

Cu: <1%

Ag: <1%

S: 15-20%

[0042] Both the silver-rich concentrate and the lead -containing flux material were analysed and the results of that analysis are set out in Table 1. In Table 1, the lead-containing flux material is referred to as "Galena Concentrate" because this material is a concentrate obtained from a galena ore: TABLE 1 : CONCENTRATE ELEMENTAL COMPOSITION

Silver- rich Galena

Concentrate Concentrate

wt % wt %

.. g_ 3.3 0.0635

Cu 4.1 0.09

Pb 3.9 53.2

A1 ₂ 0 ₃ 10.4 0.35

As 0.11 0.05

Ba 1.34 0.58

CaO 7.3 1.05

Fe 7.6 7.38

K 1.0

Na 0.18

Mg 0.6 0.14

Si0 ₂ 30 10.8

S 5.1 16.8

Ti 0.4

Mn 0.19

P 0.21

Zn 0.27 1.85

Co 0.02 0.026

Cd ^• 0.25 0.022

In 0.0015 0.0024

Total 76.27 92.40

[0043] The approximate mineralogicai analysis of the silver-rich concentrate and the lead - containing flux material are shown in Table 2:

TABLE 2: INERALOGICAL ANALYSIS

Silver-rich Galena

Concentrate Concentrate

Assay

Formula

Wt %

Ag-Sulphide A&S 2.8 -

Fe / Cu / As / 1.2 -

Ag sulphide

Fe / Cu / S / Ag 2.7

silicate

Fe / Cu / Ag 1.3 - silicate

Ag Silicate 1.3 -

Ag other 4.6 0.67

Chalcocite Cu ₂ S 1.7 -

Chalcopyrite CuFeS ₂ 9.5 5.62

Pyrite FeS ₂ 2.1 9.18

Galena PbS 4.9 71.70

Sphalerite ZnS 0.5 2.17

Quartz Si0 ₂ 5.6 4.13

Feldspars (K,Na)AlSi ₃ 0 ₈ 22.2 0.21

Muscovite KAl ₂ [AlSi ₃ O ₁₀ ](OH,F) ₂ 4.2 1.13

Biotite (Fe,Mg) ₃ [AlSi ₃ O ₁₀ ](OH,F) 1.2 -

2

Garnet (Fe,Mg,Ca) ₃ Al ₂ (Si0 ₄ ) ₃ 2.1 ^' 0.02

Amphibole (Fe,Mg) ₇ (Si ₈ 0 ₂₂ )(OH) ₂ 7.8 0.01

Wollastonite CaSi0 ₃ 0.5 0.13

Sphene CaTiSiOs 0.5 -

Kaolinite Al ₂ Si ₂ 0 ₅ (OH)4 0.5 0.06

Calcite CaC0 ₃ 4.0 1.00

Apatite Ca ₅ (P0 ₄ ) ₃ (F,Cl,OH) 1.1 0.01

Ilmenite FeTi0 ₃ 4.2 0.41

Limonite FeO(OH) 1.8 0.04

Barite BaS0 ₄ 0.6 0.21

Arsenopyrite FeAsS - 0.35

Others 1 1.1 2.95

Total 100.00 100.00

[0044] Experimental Method:

[0045] A 200 g bath of slag was prepared for tests 1 to 7. In the tests 1 & 2, this was achieved by pre-melting a mixture of lab reagent grade Al ₂ 0 ₃ , CaC0 ₃ , Cu ₂ 0, Fe ₂ 0 ₃ , PbO and Si0 ₂ in the desired proportions. For tests 3 to 7 it was decided that the most representative starting bath composition could be achieved by re-melting samples of the slag from previous tests.

[0046] In each test the crucible was slowly heated up to temperature. When the crucible was at the target temperature (i.e. 1260°C for tests 1 - 7) the lance was lowered to within 5 mm of the crucible base with a low flow of nitrogen to gently stir and homogenise the melt.

[0047] Concentrate, doping reagents and fluxes were added to the bath at a nominal rate of 300 g/h of dried silver-rich concentrate, plus a varying amount (approximately 180 g h) galena concentrate to act as flux. The addition rate of smelting air was initially determined from a mass balance, but was refined by empirical evidence as successive smelting tests were conducted. The air was injected into the crucible approximately 1 cm from the crucible base.

[0048] When the experimental temperature was attained, the lance was then lifted above the bath and a slag sample was collected on a steel dip rod. The gas was then changed to air at the required flow rate for the experiment and the lance lowered back to within 5 mm of the crucible base. At this point the timer clock was started and then concentrates and fluxes were added at the desired rate to the bath, while air was being injected into the bath. After 30 minutes of blowing time, the lance was raised from the bath, the clock stopped and the bath was allowed a short time to settle. Three to four grams of slag was quenched onto a steel dip rod.

[0049] The lance was lowered back into the bath and the timer restarted, with continuing air, concentrate and flux addition. Sampling occurred at 30 minute intervals over a period of up to 2 hours.

[0050] After the final samples were collected, the power to the furnace was shut off, the lance lifted and the crucible was allowed to cool under a blanket cover of nitrogen. Once the furnace was cold, the bag house was weighed to determine the mass of fume collected. The steel ducting was also swept clean and the fume collected. The crucible was then weighed and separated from the matte, slag and alloy with a hammer.

[0051] The alloy, matte and slag were weighed and then separated. Representative samples of each were collected for chemical analysis. The experimental conditions in experimental results for each of examples 1 to 7 are set out below. Example 1

Temperature 1260

Initial Bath PbO 77 g

composition Si0 ₂ 77 g

A1 ₂ 0 ₃ 15 g

Fe ₂ 0 ₃ 15 g

CaCO ₃ 20 g

Cu ₂ 0 6 g

Silver-rich 400 g

Concentrate Addition

Galena Concentrate 267g

Addition (flux)

Air addition 333 litres

Slag samples (wt%)

Ag A1 ₂ 0 ₃ CaO Cu20 Fe MgO PbO Si0 ₂ ZnO

0 min 0.05 7.0 4.7 3.25 5.2 1.17 42.8 ^■ N/A 0.14

60 min 1.25 7.3 5.0 3.68 7.5 0.74 23.4 31.3 0.84

Final 0.03 12.2 9.1 0.07 4.7 2.4 9.6 53.6 0.46

Matte sample (wt%)

Fume sample (wt%)

Example 2

Temperature 1260 Initial Bath PbO 77 g

Si0 ₂ 77 g

A1 ₂ 0 ₃ 15 g

Fe ₂ 0 ₃ 15 g

CaCO ₃ 20 g

Cu ₂ 0 6 g

Silver-rich , 540 g

Concentrate Addition

Galena Concentrate 360g

Addition

Air addition 540 litres

Slag samples

Ag A1 ₂ 0 ₃ CaO Cu20 Fe MgO PbO Si0 ₂ ZnO

0 min 0.02 6.9 5.0 2.83 5.1 0.8 39.1 37.9 0.11

60 min 0.21 9.5 6.4 0.64 9.8 2.3 22.3 40.9 1.0

Final 0.35 9.1 6.4 1.48 10.1 3.9 21.3 37.5 1.1

Matte sample

Metal sample

Fume sample

Example 3

Temperature 1260

Initial Bath 200 g

Final Slag from Test 2

Silver-rich 455 g

Concentrate Addition

Galena Concentrate 303g

Addition

Air addition 222 litres

Slag samples

Ag A1 ₂ 0 ₃ CaO Cu ₂ 0 Fe MgO PbO Si0 ₂ ZnO

0 min 0.23 9.5 6.3 1.45 12.5 4.6 19.1 40.1 1.0

60 min 3.27 8.8 5.6 5.84 1 1.7 3.2 18.3 35.8 1.1

Final 0.31 9.7 6.3 1.35 13.0 3.9 20.4 38.4 1.1

Metal sample

Fume sample

Example 4

Temperature 1260

Initial Bath 200 g

Final Slag from Test 2

Silver-rich 150 g

Concentrate Addition

Galena Concentrate 100g

Addition

Air addition 150 litres

Slag samples

Ag A1 ₂ 0 ₃ CaO Cu ₂ 0 Fe MgO PbO Si0 ₂ ZnO

0 min 0.09 9.1 6.3 1.44 9.8 2.4 19.0 39.0 1.0

60 min 0.08 10.0 6.8 0.47 10.3 2.1 16.0 37.9 1.1

Final

Matte sample

Fume sample

Test abandoned after 30 minutes due to loss of temperature control.

Example 5

Temperature 1260

Initial Bath 200 g

Final Slag 50/50 mix

from Test 3 & 4

Silver-rich 565 g

Concentrate Addition

Galena Concentrate 339g

Addition

Air addition 565 litres

Slag samples

Ag A1 ₂ 0 ₃ CaO Cu ₂ 0 Fe MgO PbO Si0 ₂ ZnO

0 min 0.41 9.1 6.0 2.06 9.4 3.8 22.1 40.3 1.1

60 min 1.51 9.0 6.2 2.79 10.1 3.6 Ϊ9Α 35.8 1.3

Final 0.07 7.8 6.0 1.31 10.2 3.4 21.8 36.6 1.4

Matte sample

Fume sample

Example 6

Temperature

Initial Bath

Final Slag 50/50 mix

from Test 3· & 4

Silver-rich

Concentrate Addition

Galena Concentrate

Addition

Air addition 575 litres

Slag samp es

Matte sample

Fume sample

Example 7 ¹

Temperature 1260 Initial Bath

Final Slag from Test 3 38.5 g

Final Slag from Test 4 38.5 g

Final Slag from Test 6 123 g

Silver-rich 410 g

Concentrate Addition

Galena Concentrate 220g

Addition

Flux addition CaC0 ₃ 50g

Air addition 410 litres

Slag samples

Ag AI2O3 CaO Cu ₂ 0 Fe MgO PbO Si0 ₂ ZnO

0 min 0.12 8.3 1.08 8.6 4.1 16.4 30.2 0.9

60 min 2.33 8.3 10.2 4.29 8.9 4.2 18.1 31.9 1.1

82 min 0.75 8.8 10.0 2.19 9.4 4.9 19.1 33.4 1.0

Final 0.07 9.6 10.5 1.92 9.6 5.4 18.3 33.6 1.1

Matte sample

[0052] Table 3 provides a summary of the composition of the complex silver mattes that were formed in examples 1 to 7. The silver mattes contained Ag-, Cu-, Pb- and Fe-sulphides. In examples 2, 3, 6 and 7, there were also small amount of metal formed. TABLE 3: SILVER MATTE COMPOSITION FROM SMELTING TESTS

Test #2 #5 U #J #6 #7

wt.% wt.% wt.% wt.% wt.% wt.% wt.%

5.16 23.2 30.6 14.1 32.0 25.7 21.9

Cu 7.93 39.2 47.4 23.5 38.2 43.9 35.8

Fe 17.6 0.83 0.11 4.78 0.08 0.5 1.22

Pb 45.9 15.1 3.4 38.5 10.0 8.9 17.0

S 12.1 14.7 16.8 16.3 15.7 16.2 14.2

[0053J The matte composition can be controlled by the degree of oxidation applied to the melt. Increasing oxidation drives the sulphide minerals to become oxides, and hence to leave the matte phase and join the slag phase. The same mechanism also oxidises the valuable metal, i.e. silver, and results in increased losses to slag. Therefore an inverse relationship exists between the level of iron in matte and the silver losses to slag. This relationship is illustrated in Figure 2.

[0054] For the purpose of flowsheet design, a matte composition must be chosen as the metallurgical target. For this smelting process a logical choice of matte composition is a point just prior to where the formation of silver bullion begins. This transition appears to occur for systems when the matte contains less than approximately 2 wt% Fe. So a matte containing 2 %Fe will be assumed, with no production of silver bullion. The other elements, (i.e. Cu, Pb & S) can be derived from this starting assumption. For example, using a concentrate composition matching the concentrate in these experiments, it would result in approximately 30 %Ag, 35 %Cu, 15 %Pb, 15 %S and 2 %Fe.

{0055] The expected slag composition, and smelting temperature of the concentrate mixture is shown (star marker) in the phase diagram shown in Figure 3. The commercially-available package FactSage™ was used to make the predictions.

[0056] According to the composition indicated in Figure 3, a wide range of potentially suitable slag chemistries can be obtained, providing that the amount of PbO in the slag is at least half as much as the amount of Si02.

[0057] Fluid slags were obtained in all tests. Slag temperatures in the range 1250-1270°C were used. It may be possible to obtain fluid slags at lower temperatures in accordance with the predictions from Figure 3 but, owing to the sensitivity of the experimental apparatus to slag properties, lower slag temperatures were not attempted.

[0058] The examples show that a smelting temperature of 1250°C is appropriate. [0059] A number of observations were made during this experimental work. The slag was observed to foam in smelting tests 1 to 7, but to a markedly lesser degree than in previous work conducted without the addition of the lead-containing flux material. Foaming slags are not uncommon in the ISASMELT™ process and, in commercial operations, care is taken to avoid vigorous foaming conditions. This work has demonstrated a slag system with foaming characteristics that are qualitatively similar to other slags that are already being used in successful commercial processes.

[0060] In general, the slag compositions and physical properties produced in the tests agreed well with theoretical predictions, and therefore there is a strong likelihood that this regime corresponds with a system in which equilibrium calculations are valid for a concentrate of a given elemental analysis.

[0061] The following conclusions can be made from the experimental work:

[0062] A fluid slag can be made at or above 1250°C in the Pb0-SiO ₂ slag system that is suitable for a silver smelting process.

[0063] The galena concentrate will be a suitable fluxing agent for the silver-rich concentrate when added in a ratio of approximately 2:5 to 2:3.

[0064] The propensity of the PbO-Si0 ₂ slag to form a stable foam is much reduced when flux is added to the feed mix.

[0065] A stable smelting process can be expected in an ISASMELT™ furnace by smelting the silver-rich concentrate, with galena concentrate, to form a matte. The target matte composition will depend on the initial composition of the silver-rich concentrate, but for the sample used in the experimental work for these tests it corresponds with approximately 30 %Ag, 35 %Cu, 15 %Pb, 15 %S and 2 %Fe.

[0066] At the target matte composition, a PbO-Si0 ₂ slag can be formed containing around 0.1 %Ag, 1 %Cu, 20 %Pb.

[0067] The silver-bearing matte and siliceous slag are readily separable, due to density difference, and on a commercial process it is expected that the matte and slag layers could be tapped separately.

[0068] Reference throughout this specification to "one embodiment" or "an embodiment" means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, the appearance of the phrases "in one embodiment" or "in an embodiment" in various places throughout this specification are not necessarily all referring to the same embodiment.

Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more combinations.

[0069] In compliance with the statute, the invention has been described in language more or less specific to structural or methodical features. It is to be understood that the invention is not limited to specific features shown or described since the means herein described comprises preferred forms of putting the invention into effect. The invention is, therefore, claimed in any of its forms or modifications within the proper scope of the appended claims (if any) appropriately interpreted by those skilled in the art.