WO2020010400A1 - Mineral processing - Google Patents

Abstract

The disclosure provides a process of treating an ore comprising a sulphide material. The process comprises providing a slurry of the ore comprising the sulphide material and adding a first oxidising agent and a second oxidising agent to the slurry. The process also comprises milling the slurry in the presence of the first and second oxidising agents to at least partially oxidise the sulphide material. The second oxidising agent regenerates the first oxidising agent.

Description

Mineral processing

Technical Field

This disclosure relates generally to processes and systems for liberating metal values from sulphide-containing minerals.

Background

Metal values are found in a wide variety of ore deposits. To date, ore bodies that are easy (i.e. economical) to process to recover metal values therefrom have generally received the most attention in the production of these metal values. However, as the amount of these “easy deposits” begins to decrease, the focus has now turned towards ore bodies that are more difficult to process.

One type of ore that can be difficult to process to make economic recoveries of the value metal are those that contain sulphide-based minerals which make the ore refractory in nature (i.e. non-responsive to simple recovery methods). Sulphide minerals encountered in the recovery of value metals can be a hinderance to the cost-effective treatment of the metals concerned. The sulphide minerals can hinder the process of recovery by rendering the value metal unrecoverable by standard methods, as in the case of pyrite and arsenopyrite in gold ores. This is generally due to the gold particle being encapsulated by the mineral or in solid solution with the mineral. Having the value metal contained within the solid solution of the mineral, as is often the case with gold in arsenopyrite, means that the sulphide mineral needs to be chemically“destroyed” to release the contained gold. The processes and conditions required to chemically break down the sulphide mineral are generally harsh and may even produce a material that competes with the value metal of interest for the available process reagents in the down-stream processing stages. For example, copper minerals may consume the cyanide in conventional gold leaching which causes there to be inadequate cyanide remaining for gold recovery.

Despite the issues of recovering metal values from sulphide minerals, there are numerous processes available to enable the liberation and recovery of the metal value. However, these processes generally require high capital and operating costs, for example the use of autoclaves, expensive reagents and/or produce a detrimental by-product stream, which limits these processes to larger, higher grade ore, such as high grade refractory ore bodies. The low recovery of the metal values from low-grade ore bodies using current techniques makes these low-grade ore bodies uneconomical to process. It is to be understood that, if any prior art is referred to herein, such reference does not constitute an admission that the prior art forms a part of the common general knowledge in the art, in Australia or any other country.

Summary

The disclosure provides a process of treating an ore comprising a sulphide material, comprising:

providing a slurry of the ore comprising a sulphide material;

adding one or more oxidising agents to the slurry;

milling the slurry in the presence of the one or more oxidising agents to break down a matrix of the sulphide material.

The disclosure provides a process of liberating metal values from ore comprising a sulphide material, the sulphide material being a sulphide-containing mineral, the process comprising:

providing a slurry of the ore comprising the sulphide-containing mineral;

adding one or more oxidising agents to the slurry;

milling the slurry in the presence of the one or more oxidising agents to break down a matrix of the sulphide-containing mineral to liberate the metal value retained therein.

The disclosure also provides a process of treating an ore comprising a sulphide material, comprising:

providing a slurry of the ore comprising the sulphide material;

adding a first oxidising agent and a second oxidising agent to the slurry;

milling the slurry in the presence of the first and second oxidising agents to at least partially oxidise the sulphide material,

wherein the second oxidising agent regenerates the first oxidising agent.

The disclosure also provides a process of liberating a metal value from ore comprising a sulphide material, the method comprising:

providing a slurry of the ore comprising the sulphide material;

adding a first oxidising agent and a second oxidising agent to the slurry;

milling the slurry in the presence of the first and second oxidising agents to at least partially oxidise the sulphide material to break down a matrix of the sulphide material to liberate the metal value retained therein,

wherein the second oxidising agent regenerates the first oxidising agent.

The term“ore” is to be interpreted broadly to include native ore recovered during mining, ore that has been pre-treated, such as concentrated to form ore concentrate, prior to the process, and ore that has already been treated such as ore found in tailings. The term“adding” in the step of adding the one or more or the first and second oxidising agents to the slurry means adding one or more oxidising agents to the slurry and not relying on passive diffusion of oxidising agents from atmosphere or the surrounds into the slurry. As an example, when oxygen gas is an oxidising agent, the oxygen gas can be added to the slurry via pumping and injection rather than relying on passive diffusion of oxygen from the atmosphere into the slurry.

The sulphide material may be a sulphide contaminate. The sulphide material may be a sulphide contaminate within a value mineral ore. The sulphide material may be a sulphide- containing mineral. The sulphide contaminate may be present in a sulphide-containing mineral.

The first oxidising agent may oxidise the sulphide material so that the first oxidising agent is converted to a first reduced product. The second oxidising agent may regenerate the first oxidising agent by oxidising the first reduced product back to the first oxidising agent. The first oxidising agent may act as an oxidation catalyst that is regenerated by the second oxidising agent. The second oxidising agent may continually regenerate the first oxidising agent. The second oxidising agent may also at least partially oxidise the sulphide material.

The“matrix” of the sulphide material is the structure that surrounds and/or encapsulates the metal value e.g. particles retained therein. The matrix may also include the value metal of interest as part of its structure. Breaking down the matrix may include milling, grinding and abrading the sulphide material to physically break down apart the matrix to expose fresh interior structure of the sulphide material, and chemically breaking down the matrix, such as through oxidation and dissolution to chemically degrade the structure of the sulphide material.

Adding the one or more, such as first and second, oxidising agents to the slurry may occur before and/or during milling.

The process may further comprise cooling the slurry during milling by adding a cooling fluid to the slurry, for example to maintain integrity of the milling wear components. The process may further comprise adding additional reagents during the process. The additional reagents may be added before and/or during milling. During milling an average temperature of the slurry may be maintained at or below a predetermined temperature, such as approximately 75 °C to suit the milling equipment wear linings if they are polymer-based materials.

The process may further comprise additional oxidising of the slurry in an agitation tank after milling. This additional step may take advantage of the residual heat and oxidising conditions formed within the milling process. The agitation tank may have a heated oxygen- rich environment for oxidising the sulphide material, such as by breaking down the matrix. The one or more oxidising agents, such as the first oxidising agent, may include a nitrate salt. The one or more oxidising agents, such as the second oxidising agent, may include an oxygen source. Adding the first and/or second oxidising agents to the slurry may comprise adding the oxidising agent(s) to the slurry during milling. The oxygen source may comprise oxygen gas. Adding the oxygen source to the slurry may comprise bubbling oxygen gas through the slurry. In an embodiment, ammonium nitrate can be added to the slurry prior to milling and oxygen gas can be bubbled through the slurry during milling. The concentration of the nitrate salt may be approximately 7.5 wt.% based on the weight and composition of the ore. One of the one or more oxidising agents may be added in catalytic amounts, such as less than stoichiometric amounts of oxidising agent. For example, one or more oxidising agents, such as the first oxidising agent, may be added in amount that is <50 mol% based on a stoichiometric amount of sulphide.

The process may further comprise concentrating the ore comprising the sulphide material prior to milling the slurry. A value metal concentration of the ore may vary greatly after concentration and is dependent on the original nature of the originally mined ore and the concentrating process utilised. During milling the slurry sulphide material may be milled down to an average size of about 10 pm.

The sulphide-containing mineral may be pyrite and/or arsenopyrite. The metal value may be gold. The ore may be a low grade refractory deposit.

In some embodiments milling may be carried out at atmospheric pressures. In some embodiments milling may be carried out at elevated pressures, such as pressures above atmospheric pressure.

In an embodiment the slurry may be milled in a first mill. After the milling the slurry in the presence of the first and second oxidising agent in the first mill, the slurry may be separated into a fine slurry fraction and a coarse slurry fraction. The coarse slurry fraction may be further milled in a second mill positioned downstream of the first mill. The process may further comprise combining the fine slurry fraction and coarse slurry fraction back together. In some embodiments, the coarse slurry fraction may be injected back into the first mill. The latter such embodiment may obviate the need to have a second mill.

It is also noted that in at least some embodiments the disclosed process does not rely upon the sulphur in the sulphide material becoming a fuel source to provide heat for the process (as in the case of roasting or pressure oxidation) or“food source” to the active agents as in the case of bacterial oxidation and as such is more flexible in the quality or sulphur content of the material being processed.

The disclosure also provides a metal value liberated from an ore comprising a sulphide material, such as a sulphide-containing mineral, using the process as set forth above. By“liberated”, it is meant release of the metal value from the sulphide material or the metal value being in a located that is capable of being extracted and/or leached from the sulphide material.

The disclosure also provides a process of extracting gold from an ore using cyanide leaching, comprising:

treating the ore using the process as set forth above to make the gold available for cyanide leaching, such as breaking down a sulphide material so that any encapsulated or retained gold can be dissolved by cyanide leaching processes, and

cyanide leaching the gold.

The disclosure also provides a system for treating ore comprising a sulphide material, the system comprising:

a grinding mill for milling a slurry formed with the ore comprising the sulphide- containing mineral; and

a first inlet for adding one or more oxidising agents into the grinding mill such that the one or more oxidising agents contact the slurry when the slurry is being milled.

The system may further comprise a second inlet for adding a cooling fluid into the grinding mill so that in use of the mill the slurry is maintained at a temperature at or below a predetermined temperature. The system may add additional reagents to the slurry through the second inlet. The second inlet may be in communication with a reservoir for adding one of the one or more oxidising agents to the slurry.

The disclosure also provides a system for treating ore comprising a sulphide material, the system comprising:

a grinding mill for milling a slurry formed from the ore comprising the sulphide material; and

a first mill inlet for adding the slurry into the mill and a mill outlet for removing treated slurry from the mill;

a slurry flow path in communication with the mill inlet;

a first oxidising agent inlet for adding a first and/or a second oxidising agent into the slurry flow path prior to the slurry being added to the mill.

The mill inlet may be positioned towards a bottom of the mill and the mill outlet may be positioned towards a top of the mill. In an embodiment, a source of the first and/or second oxidising agents may be in communication with the second mill inlet for the cooling fluid. The system may be arranged so that the additional amounts of the first and/or second oxidising agent can be provided in the cooling fluid so that the additional amounts of the first and/or second oxidising agents can be added to the mill at the same time as the cooling fluid is added to the mill.

In an embodiment, the system may further comprise a second mill inlet for adding: (i) a cooling fluid into the grinding mill so that in use of the mill the slurry is maintained at a temperature that is at or below a predetermined temperature; and/or (ii) adding additional amounts of the first and/or second oxidising agents to the slurry in the mill.

The predetermined temperature may be approximately 75 °C.

The system may comprise a second oxidising inlet for adding the first and/or second oxidising agent to the grinding mill. The second oxidising inlet may be in communication with a reservoir for adding one of the one or more oxidising agents to the slurry.

The grinding mill may be vented to the environment at atmospheric pressure. The grinding mill may be a pressure grinding mill that is capable of being pressured up to about 10 atm (i.e. 10 bar).

The system may further comprise an agitation oxidation tank downstream of the mill outlet.

The grinding mill may comprise a first grinding mill. The system may further comprise a slurry separator downstream of the first grinding mill. The slurry separator may separate treated slurry into a coarse slurry fraction and a fine slurry fraction. The slurry separator may be positioned downstream of the mill, such as the first grinding mill, and in communication with the mill outlet. The system may include a second grinding mill configured to receive the coarse slurry fraction. The system may be configured such that the fine slurry fraction can bypass the second grinding mill. In some embodiments the coarse slurry fraction may be in fluid communication with the first mill. For example, the coarse slurry fraction may be recycled into the first grinding mill.

The system may be configured to perform the process as set forth above.

Brief Description of Figures

Embodiments will now be described by way of example only with reference to the accompanying non-limiting Figures.

Figure 1 shows an embodiment of a system for performing an embodiment of the disclosed process.

Figure 2 shows an embodiment of reaction conditions of Figure 1.

Figure 3 shows theoretical gold recovery percentage for a given dissolution percentage of As and S.

Figure 4 shows an embodiment of a system for producing gold that uses an embodiment of the disclosed process.

Figure 5 shows another embodiment of a system for performing an embodiment of the disclosed process.

Figure 6 shows another embodiment of a system for performing an embodiment of the disclosed process. Detailed Description

An embodiment of the disclosure provides a process of treating a sulphide material, such as an ore comprising a sulphide material. For example, sulphide contaminates in an ore may be treated by the process prior to extraction of metal values. In some embodiments treating an ore comprising a sulphide material includes liberating metal values from ore comprising a sulphide-containing mineral. For example, the ore may be a refractory ore. The ore may include pyrite (FeS₂) and/or arsenopyrite (FeAsS), and their analogues such as cattierite (CoS₂), vaesite (NiS₂), clinosafflorite ((Co,Fe,Ni)AsS), gudmundite (FeSbS), glaucodot or alloclasite ((Fe,Co)AsS) or ((Co,Fe)AsS), iridarsenite ((lr,Ru)AsS), osarsite or ruarsite ((Os,Ru)AsS) and ((Ru,Os)AsS). The ore may include chalcopyrite (CuFeS₂), stannite (Cu₂FeSnS₄), kesterite (Cu₂ZnSnS₄), talnakhite (Cu₉Fe₈Si₆), mooihoekite

(Cu₉Fe₉Si₆), haycockite (Cu₄Fe₅S₈), cubanite (CuFe₂Ss), argentopyrite (AgFe₂S₈), enargite (CU3ASS₄), proustite (AgsAsSs), calaverite (AuTe₂), stibnite (Sb₂Ss), sphalerite (ZnS), hawleyite (CdS), wurtzite (a-ZnS), greenockite (CdS), linnaeite (Co3S₄), violarite (FeNi₂S₄), carrollite (CuCo₂S₄), greigite (Fe3S₄), molybdenite (MoS₂), tungstenite (WS₂), and/or pentlandite ((Ni,Fe)₉Ss).

An embodiment of the process may allow for the removal of deleterious minerals from an ore which inhibit the usual processing of the metal value by way of interfering with process reactions. An example of this is the interference of cyanide consuming minerals (e.g. stibnite or various cyanide soluble copper minerals) in the conventional Carbon in Leach or Carbon in Pulp process. The metal value may be precious metals such as gold or value base metals such as copper. Alternatively, the process could be utilised in some embodiments in the cost-effective removal of contaminant sulphide species from other value mineral products (e.g. residual sulphide minerals from a zircon concentrate).

The process comprises providing a slurry of the ore comprising the sulphide material. Providing the slurry may comprise forming a slurry of the ore comprising the sulphide material. The slurry may be formed using water. Additives in addition to the oxidising agents may be added to the slurry to promote oxidation of the sulphide material. The slurry is contacted with one or more oxidising agents, such as a first oxidising agent and a second oxidising agent. The slurry is milled in the presence of the oxidising agents to oxidise the sulphide material which helps to break down a matrix of the sulphide material, for example to liberate metal values, such as gold, retained within the matrix. Breaking down the matrix of the sulphide material may include oxidation and dissolution. For example, sulphides can be converted to sulphates/sulphites, and arsenic compounds can be converted to various oxide forms depending on the reaction conditions. In this way, an embodiment of the process allows metal values that are present within a mineral matrix as a solid solution, and that would otherwise be unextractable using conventional processes, to be released from the matrix and extracted. For example, sulphide minerals often contain gold and can be impartial to cyanide attack, making it difficult for a cyanide leach solution to form a complex with the gold and extract it. Extraction of the metal value once it has been released and/or made accessible (i.e. liberated) for leaching from the matrix, through oxidation and dissolution of the sulphide material and matrix, may be carried out using traditional processes, such as cyanide leaching in the case of gold.

The terms“sulphide-containing mineral”,“sulphide mineral”,“sulphide material”, “sulphide” and the like are used interchangeably throughout this disclosure to refer to a material that is formed from a sulphide and can include a mixture of sulphides and/or materials.

One or more of the oxidising agents may be contacted with the slurry before milling.

In these embodiments, an amount of one or more of the oxidising agents is added to the slurry then the slurry is milled. Optionally, one or more of the oxidising agents may be contacted with the slurry during milling. For example, one or more of the oxidising agents may first contact the slurry in the mill. However, in some embodiments one of oxidising agents, such as the first oxidising agent, contacts the slurry prior milling and another of the oxidising agents, such as the second oxidising agent, contacts the slurry during milling. In some embodiments a first portion of e.g. the first oxidising agent is contacted with the slurry prior to milling, and a second portion of the first oxidising agent is contacted with the slurry during and/or after milling. Similarly, in some embodiments a first portion of e.g. the second oxidising agent is contacted with the slurry prior to milling and a second portion of the second oxidising agent is contacted with the slurry during and/or after milling. In some embodiments only one oxidising agent is used but a first portion of this oxidising agent first contacts the slurry prior to milling and a second portion contacts the oxidising agent during milling. The term“oxidising agent” is to be interpreted broadly to include a single species of oxidising agent and to a mixture that comprises two or more oxidising agents. In this interpretation“one or more oxidising agents” or the“first oxidising agent” and“second oxidising agent” may include specific mixtures, where each mixture comprises different oxidising agents and/or the same oxidising agents in different ratios.

In some embodiments one of the one or more oxidising agents, such as the first oxidising agent, includes a nitrate salt. The nitrate salt may be lead nitrate (Pb(NC>₃)₂), potassium nitrate (KNO₃), nitric acid (HNO₃), and/or ammonium nitrate (NH₄NO₃). The first oxidising agent may include permanganate salts. One of the oxidising agents may comprise a mixture of different oxidising agents. For example, a mixture of nitrate and/or

permanganate salts may be used as one of the oxidising agents. An advantage of ammonium nitrate over other oxidising agents is that it is relatively cheap and is commonly found on most mine sites since it is used as an explosive (e.g. ammonium nitrate fuel oil explosives). Ammonium nitrate can also be decomposed to nitrogen and nitrogen dioxide gas and water, which makes its removal post-process easier when compared to metal-based oxidising agents as there is a reduced likelihood of forming undesirable by-products with components of the ore. The first oxidising agent, such as ammonium nitrate, may be contacted with the slurry as a solid or a liquid. However, for more efficient mixing the first oxidising agent is generally added to the slurry as a solution. The addition of solution of the first oxidising agent may be regulated by valves and/or pumps. The first oxidising agent may be added to the slurry prior to milling. In some embodiments, the first oxidising agent is added to the slurry before milling and during milling. The amount of the first oxidising agent added in the process may range from about 1 wt.% to about 20 wt.%, such as about 5 wt.% to about 15 wt.%, based on a weight of the ore. The amount of the first oxidising agent added in the process may be dependent on the ore composition, desired level of oxidation, process efficiency and downstream processing configuration.

In some embodiments one of the oxidising agents, such as the second oxidising agent, includes an oxygen source. By oxygen source, it is meant a species capable of providing molecular and/or elemental oxygen, such as hydrogen peroxide, oxygen gas or other sources of available oxygen. Oxygen gas may be provided as a mixture of other gases and need not be pure. For example, oxygen gas may be provided from the atmosphere e.g. in the form of compressed air. In some embodiments the oxygen gas is purified to have a purity greater than 50%, 60%, 70%, 80%, 90%, >95% or >99% oxygen.

In an embodiment ammonium nitrate is contacted with the slurry prior to milling and oxygen gas is contacted with the slurry during milling. The oxygen gas may be injected into the slurry so as to be bubbled therethrough. The specific injection point of the oxygen gas in the mill will be dependent on the mill design e.g. horizontal or vertical mill, and size. Some embodiments use multiple injection points to contact the oxygen gas with the slurry. Some embodiments add additional first oxidising agent, such as ammonium nitrate, or other suitable oxidant(s) during the milling step.

Depending on the design of the mill, it may be desirable in some embodiments for the mill to be sealed in such a manner that the interior of the mill is operated at above ambient pressure. This may allow for improved oxidant reactivity (i.e. improved oxidation kinetics), especially in the case where one of the oxidants is in a gaseous form. The mill may be operated at a pressure above approximately 2 atm (i.e. 2 bar) with the upper limit of the mill operating pressure being limited by the structure and system of the mill and, for example the ability to seal any openings in the mill shell and the ability of the pumps feeding the mill to overcome the operational pressure. In some embodiments the mill is operated at about 2-10 atm of pressure, such as 5 atm. Some embodiments may use a pressure greater than 10 atm, for example 40 atm. The reference to pressure in units of atm does not preclude the use of other units to describe pressure, and the skilled person would readily understand that one unit of pressure can be converted into another unit of pressure, for example the conversion of atm to bar. In some embodiments, the mill is operated at a pressure that is determined by a maximum operational pressure of a compressed air system. In some embodiments an upper pressure limit of the mill may be about 10 atm. In some embodiments, the mill is run at ambient pressure, for example with an unrestricted discharge to maintain ambient pressures. In the case where elevated pressures are beneficial to the process, mill feed pumps, cooling pumps and oxidant injection systems are rated to provide adequate pressure to overcome the desired pressure in the mill reaction / comminution chamber. When elevated pressures are used for milling, the mill may comprise flash-off vessels and heat recovery systems.

When the sulphide-containing mineral is pyrite and/or arsenopyrite and a nitrate salt and oxygen gas are used as the oxidising agents, there are a number of reactions that can take place to break down the sulphide matrix to liberate the metal value retained therein. Pyrite will react with nitrate ions to form ferrous, sulphate and nitrite ions and excess acid according to eq. 1. Molecular oxygen can also react with pyrite to form ferrous and sulphate ions and excess acid according to eq. 2. When arsenopyrite is the sulphide-containing mineral it can react with nitrate ions according to eq. 3 to produce ferrous/ferric, sulphate, nitrite and various arsenic ions, along with acid. Oxygen can also react with arsenopyrite to form ferrous/ferric, sulphate and arsenous acid, as shown in eq. 4. Due to the generation of acid in eq. 1 to eq. 3, the oxidation of the sulphide results in a slurry having a pH ranging from about 1-2 after the milling and oxidising step.

With pyrite as the sulphide material during gold processing as an example, there is generally little need to chemically attack the matrix of pyrite with the oxidising agents as mechanical grinding may be all that is required to liberate the entrained gold particles, and there is generally only small quantities of gold in solid solution as the majority of the gold is generally present as encapsulated particulates. However, an advantage of an embodiment of the process is that oxidising the sulphide during milling helps to reduce the grinding energy required to access the gold as the matrix is both chemically and mechanically deteriorated. Therefore, an embodiment of the process may help to reduce the costs of grinding.

The presence of molecular oxygen in an acidic environment converts the nitrite ion to the nitrate ion, as shown in eq. 5. If oxygen gas is supplied in excess, then nitrite ions formed in eq. 1 and 3 should be continually converted to nitrate ions, which allows the nitrate ion (e.g. from ammonium nitrate) to act as a catalyst in some embodiments. This means that the amount of nitrate salt required to oxidise the sulphide-containing mineral can be reduced by the presence of oxygen as the nitrate ion is continually regenerated. This reaction may be enhanced at elevated pressures. Further, because the nitrate ion is continually regenerated, it helps to shift eq. 1 and eq. 3 to the right, which helps to favour the oxidation, breakdown and dissolution of the pyrite/arsenopyrite matrix and related analogue/substituted minerals to release any metal values retained therein.

7H2O + FeS₂ + I\I0₃- ®· Fe²⁺ + 2S0₄ ²- + N0₂- + 14H⁺ eq. 1 FeS₂ + 3.50₂

eq. 2

FeAsS + I\I0₃- + 3H₂0 ®· Fe^x+ + S0₄ ²- + As^x+ + N0₂- + 4H⁺ eq. 3

4FeAsS + 110₂ + 6H₂0 ®· 2Fe^x+ + H₃As0₃ + 4S0₄ ²- eq. 4

2N0₂- + 0₂ ®· 2N0₃- eq. 5

Eq. 1 to eq. 4 are exemplary only and there will also be competing side reactions, such as oxidation of ferrous to ferric ions, and oxidation of pyrite with ferric ions, that may take place at the same time as eq. 1 to eq. 4. The type(s) and kinetic(s) of these side reactions depends on the specific composition of the ore and the reaction conditions. It should be noted that any nitrites present may be reacted further to form various nitric oxides (NO_x), although these nitric oxides species would generally be short-lived and be converted back to the nitrite form. Depending on the composition of the sulphide-containing mineral, various combinations of eq. 1 to eq. 4 can occur simultaneously in some embodiments.

In some embodiments minimising or preventing side-products may determine the specific reaction conditions for a given ore composition. For example, if calcium peroxide is used as an oxygen source, under specific conditions the calcium peroxide decomposes into calcium hydroxide (hydrated lime) which can precipitate dissolved metal ions such as As^x+ and Fe^x+. Precipitated metals salts may form a surface coating on the particles of sulphide- containing mineral which may prevent further oxidation by the one or more oxidising agents. In these circumstances, further grinding and milling of the particles is required to expose fresh sulphide material and/or to break off the surface coating. The surface coating may also prevent leaching solutions from leaching the metal value retained in the sulphide-mineral matrix.

Oxygen gas is often used as an oxidising agent as it is cheap and readily available. However, an issue with using only oxygen gas as an oxidising agent is that the amount of oxygen available to perform oxidation in the slurry is determined by the temperature of slurry and the partial pressure of oxygen. Increasing the temperature of the slurry decreases the amount of dissolved oxygen, which lowers the kinetics of oxidation. This can be combated by increasing a partial pressure of oxygen, but the use of pressure can place restrictions on the type of mill used for milling as pressure vessels are required. If only nitrate salts were used as the oxidising agent, the amount of nitrate salts required to perform oxidation of the sulphide material will be dependent on the stoichiometric requirements. However, stoichiometric requirements may mean a significant amount of nitrate salt is required. Using an oxygen source as an oxidising agent to regenerate nitrate ions helps to reduce the amount of nitrate salt required to perform oxidation of the sulphide material to catalytic amounts whilst helping to minimise or eliminate the issues of using only oxygen gas as an oxidising agent. The use of a nitrate salt, or any other oxidising agent capable of acting as the first oxidising agent, helps to increase the relative concentration of oxygen present in the reaction media. For example, under standard conditions, oxygen gas has a solubility of about 20 ppm in aqueous systems, but the addition of a first oxidising agent helps to increase the availability of oxygen beyond the 20 ppm limit to oxidise a sulphide material.

Put another way, the first oxidising agent e.g. a nitrate salt acts as a transfer medium to increase the amount of oxygen present in the reaction mixture.

Oxidation and degradation of the matrix of the sulphide material occurs at the surface. Increasing the surface area of the mineral helps to increase the rate of reaction. Grinding in the presence of oxidising agents helps to mechanically break down the sulphide material to create fresh surfaces that can then react with oxidising agents. Additionally, the heat generated in the comminution process (localised at the mineral surface and in general in the grinding chamber) contributes to the accelerated oxidation rate of the mineral surface. This means that some embodiments of the process can break down the sulphide material using both mechanical and chemical processes at the same time. The sulphide material is generally ground down to a particle size determined by the end use requirement. In some embodiments the sulphide material is ground down to a particle size of around 10 microns. However, metal values that have a size less than 10 pm could still be encapsulated in the 10 pm mineral particle. Therefore, in the absence of chemically breaking down the mineral, some of the encapsulated metal value could not be extracted during leaching. Chemically breaking down the sulphide material with one or more oxidising agents in addition to mechanically breaking down the sulphide material may help to liberate more metal values compared to chemical or mechanical breaking alone.

Milling the sulphide material continually exposes fresh surfaces at which the oxidising agents can react with the sulphide material. This means that there may be no need for very high pressure and/or high temperature conditions typically used to break down the matrix. In some embodiments the milling step is carried out at a pressure close to atmospheric pressures. In some embodiments the mill used for the milling step is open to the atmosphere i.e. not a closed system. This means that an embodiment of the process may provide a more benign process for liberating metal values from the matrix of the sulphide-containing mineral compared to typical processes. This may also help to reduce the capital and operational costs of the equipment needed to perform the process, for example when compared to the use of autoclave systems.

Some embodiments may allow for elevated pressure milling which would provide more aggressive oxidising conditions and accelerate the comminution and oxidation processes and minimise or eliminate the need for post milling oxidation stages. This may help to minimising plant and equipment capital and operating expenses. For example, milling may be performed at pressures of greater than 1 atm, such as greater than 1.5 atm or greater than 2.0 atm.

Oxidation of the sulphide matrix to liberate the metal value retained therein generally is exothermic. Depending on the reaction conditions and ore type, oxidation may generate excess heat that needs to be removed. In these embodiments the process may further comprise cooling the slurry during milling. Cooling may be achieved by adding a cooling fluid to the slurry. The cooling fluid may be water. The water may comprise one or more reactants. The reactants may be one or more oxidising agents. The reagents may be separate from the oxidising agent(s). The water may comprise a nitrate salt.

The maximum temperature the process can reach during milling may be determined by the materials used to construction the mill. A lining of the mill that protects the mill from abrasion and wear may determine the maximum temperature. For example, a mill having a ceramic coating may allow a higher milling temperature compared to a mill not having a ceramic coating. In some embodiments an average temperature of the slurry is maintained at or below approximately 75 °C. It should be appreciated that localised heating where minerals are crushed and ground during milling will be significantly higher than the bulk of the slurry. Therefore, the term“average temperature of the slurry” means the average temperature of a bulk of the slurry and does not include areas of localised heating. The areas of localised heating may allow the oxidation and degradation of the sulphide matrix with the one or more oxidising agents to proceed at a higher rate of reaction compared to the rate of the reaction for the bulk of the slurry. Generally, a solid-solid reaction between a nitrate salt and the sulphide matrix will occur at a temperature of about 55 °C, whereas a reaction between a solution of nitrate ions and the sulphide matrix may occur at lower temperatures.

As oxidation of the sulphide matrix is exothermic, some embodiments of the process do not require an external heat source to heat the slurry during milling to allow the oxidation reaction of the sulphide matrix with the oxidising agents to occur. However, when the process is carried out in cold climates and/or during times of sub-optimal reaction kinetics, an external heat source may be used to at least initially heat the slurry and/or during milling to allow the slurry to react with the oxidising agents. A heater may be provided to heat the slurry prior to milling. Once oxidation begins, the excess heat generated may mean that the external heat source is not required.

In some embodiments milling (i.e. the oxidation step) is performed under pressure (e.g. 2-10 atm) and at elevated temperatures (e.g. >50°C). When the process is performed under pressure and at elevated temperatures, and ammonium nitrate and oxygen gas are used as the oxidising agents, the increased pressure helps to increase the concentration of oxygen dissolved in the slurry. This does two things: increases the rate of sulphide oxidation by the action of oxygen (e.g. eq. 2 and 4) by shifting the oxidation reaction equilibrium to the right; and increases the conversion of nitrite to nitrate (e.g. eq. 5) due to the increased oxygen concentration which results in an increase in the nitrate concentration thus increasing the rate of sulphide oxidation by action of nitrate (e.g. eq. 1 and 3). Increasing the temperature in addition to increasing the pressure also helps to increase the rate and kinetics of sulphide oxidation in some embodiments. Generally, heat is generated during mechanical grinding of the slurry in the comminution chamber which can heat the slurry during sulphide oxidation. If this heat is sufficient enough, and if the milling/oxidation is performed under pressure, then the process can be considered as operating under “autoclave” conditions. The mill used in the“autoclave” process can be considered an autoclave mill in that the mill uses both heat and pressure to oxidise the sulphide material. However, these conditions are generally more benign compared to autoclave reactors typically used for sulphide material oxidation.

Increasing the pressure and temperature helps to increase the efficiency of the process, but this is offset by additional capital and running costs. However, a more efficient process may require the use of a smaller mill, which helps to reduce the footprint of a plant or system used to perform the process. The“autoclave” conditions, e.g. maximum mill pressure and temperature reached during the process, will be determined by the mill design, such as maximum operating temperatures of wear linings, temperature-pressure

relationships, and so on.

Some embodiments of the process further comprise oxidising the slurry in an agitation tank or pipe reactor after milling. Oxidising the slurry in the agitation tank or pipe reactor may occur after milling the slurry. The agitation tank or pipe reactor may help to further break down the sulphide matrix by oxidising the matrix of the sulphide material to liberate further metal values encapsulated therein. Additionally, post milling oxidation with the selected oxidants may provide an opportunity for reagent optimisation and saving by incorporating an interstage solid/liquid separation and solution recycle system. Oxidising agents in addition to those contacted with the slurry during milling may be added to the agitation tank or pipe reactor. These oxidising agents may be different to the oxidising agents contacted with the slurry before and/or during milling. In some embodiments the same oxidising agents contacted with the slurry during milling are added to the slurry in the agitation tank or pipe reactor. For example, when ammonium nitrate and oxygen are used as the oxidising agents, additional ammonium nitrate and/or oxygen may be added to the agitation tank or pipe reactor. The agitation tank or pipe reactor may be heated. The agitation tank or pipe reactor may have an oxygen rich environment. In some embodiments the first oxidising agent, such as a nitrate salt, is contacted with the slurry before and/or during milling is carried through to the agitation tank or pipe reactor and additional second oxidising agent, such as oxygen, is contacted with the slurry in the agitation tank or pipe reactor, for example by bubbling oxygen through the slurry in the agitation tank or pipe reactor.

If a residence time of the slurry during milling is high enough then the matrix of the sulphur material may be broken down both physically and chemically to the desired level sufficient to liberate the required metal value retained therein. In these embodiments the step of oxidising the slurry in an agitation tank or pipe reactor may not be required. However, it may be cost prohibitive to employ a mill that can achieve such residence times, in which case a smaller mill in combination with the agitation tank or pipe reactor may be used.

The process may further comprise concentrating the ore comprising the sulphide- material prior to forming the slurry in some embodiments. However, the chemistry of the ore may determine the required sulphide concentrations, for example if the ore has species that could minimise and/or prevent oxidation of the matrix and/or subsequent leaching of the metal values. Concentration of the sulphide into a concentrate may allow improved efficiency and reduce relative reagent consumption and milling costs.

The ore may be a low grade refractory deposit. For example, the ore may be a low grade refractory deposit mined from the ground and/or may be tailings from other extraction processes. In some embodiment the process may be used as a pre-treatment step prior to leaching e.g. with cyanide. In some embodiments the process may be used as a post treatment step for example liberating residual gold from tailings that have already been subjected to processing and/or leaching. Therefore, the disclosed process may be used in an embodiment as a front-end or back-end process for liberating metal values from ore. The metal value released and/or leached from the matrix of the sulphide-containing material may be collected in its solid form. The solid form of the metal value may be further purified e.g. by leaching.

The specific ratio of one of the oxidising agents e.g. [nitrate salt]:[ore/sulphide material] depends on the properties of the ore, the concentration of sulphide, a reaction temperature used during oxidation and residence time of the ore during milling and oxidation, and the degree of oxidation sought to produce the desired value metal recovery. Generally, the shorter the residence time the greater the amount of oxidising agent(s) required per unit of ore, the higher the temperature the shorter the residence time, and so on. The specific parameters of embodiments of the process may also be determined by reducing operational running costs of a plant and/or system that uses the process. For example, the concentration of metal value in the ore and the ore composition may determine whether milling is performed at ambient pressure or under pressure to give a desired recovery rate. In an embodiment, a residence time of the slurry in the grinding mill may range from about 10 minutes to about 60 minutes, such as less than 60 minutes, less than 50 minutes, less than, 40 minutes, less than 30 minutes, or less than 20 minutes. In some embodiments the residence time is greater than about 60 minutes. In some embodiments the residence time is less than about 10 minutes. A ratio of the first oxidising agent to the second oxidising agent may range from about 10:1 to 1 :10. In some embodiments a ratio of the first oxidising agent to the second oxidising agent may range from about 1 : 1 to about 1 :10, such as 1 :1 to 1 :5. In some embodiments the ratio of the first and second oxidising agents may be based on weight of reagent. In some embodiments the ratio of the first and second oxidising agents may be based on moles of reagent, for example between relative amounts of oxidant species, such as [O].

The disclosure also extends to a metal value liberated from an ore comprising a sulphide-containing mineral according to the process as set forth above. The metal value may be a precious metal such as gold or a base metal such as copper. The metal value may be present in a solid form, i.e. not as a dissolved salt in solution. The metal value may be in its metallic form, or as a precipitated salt(s).

The disclosure also extends to a process of extracting gold from an ore using cyanide leaching. The process comprises treating ore comprising a sulphide-containing mineral using the process as set forth above prior to cyanide leaching.

An embodiment of a system 10 for treating ore comprising a sulphide material, such as for liberating metal values from ore comprising a sulphide-containing mineral, will now be described with reference to Figure 1. System 10 has a device 12 for forming a slurry. Device 12 is connected to grinding mill 16 via path 13. The specific type of grinding mill 16 will depend on the process requirements to achieve a desired particle size of the ore, mill configuration, and efficiency of the process and reagent usage. In some embodiments the mill 16 is a comminution chamber. In some embodiments, the mill 16 may act as a reactor. An embodiment of the grinding mill 16 uses a vertical style mill. In some embodiments the path 13 is a conduit that is equipped with a pump for pumping the slurry from device 12 to grinding mill 16. In some embodiments a heater is associated with path 13 for heating the slurry prior to entry into the grinding mill 16 (not shown). A reservoir 14 of the first oxidising agent, such as a nitrate salt including a ammonium nitrate solution, is injected into the slurry between the device 12 and grinding mill 16 along path 13, and this allows the slurry to be contacted with an oxidising agent prior to milling in the grinding mill 16. In some

embodiments reservoir 14 is connected directly to grinding mill 16 in addition to or in place of path 13. A source of oxygen 18 is connected to the grinding mill 16 via an inlet in the form of conduit 17 for injecting oxygen into the slurry in the grinding mill 16, for example prior to and during milling, so that the oxygen contacts the slurry. In some embodiments the oxygen source 18 is connect to the path in place of or in addition to the grinding mill 16. In an embodiment, oxygen gas in oxygen source 18 has a purity of about 90%. In the embodiment of Figure 1 the oxygen is injected towards a bottom of the grinding mill 16. Grinding mill 16 grinds and mills solids in the slurry down to a predetermined particle size such as about 10 pm. The predetermined particle size will be determined by a power of the mill, the mill type, ore composition, the amount and type of metal value retained therein and the impact of the downstream processes on value metal recovery.

Bubbling oxygen into the slurry in the grinding mill 16 provides a second oxidising agent to help oxidise a matrix of a sulphide-containing mineral that is present in the slurry. The oxygen also helps to regenerate the nitrate ion as best seen in Figure 2. The sulphide material 12a, such as a sulphide-containing mineral or untreated ore, in slurry reacts with nitrate ions (and/or oxygen, not shown in Figure 2) to form oxidised mineral 20a (also referred to as treated ore 20a) and nitrite ions. The nitrite ions can react with excess or unreacted oxygen from oxygen source 18 to be converted back to nitrate ions. Continually regenerating nitrate ions helps to shift the equilibrium of the oxidation of the matrix of the sulphide material (e.g. sulphide-containing mineral) to the right (i.e. towards oxidised mineral 20a) which helps to increase the rate of oxidation of slurry. This also helps to reduce the amount of ammonium nitrate required to promote oxidation and chemical degradation of the sulphide mineral. Oxidation of the sulphide-material 12a to form oxide 20a generally will occur on the surface of the sulphide material 12a. However, since oxidation occurs in the grinding mill 16, grinding oxidised material 20a in the slurry will expose fresh internal surfaces thereby exposing native sulphide material 12a which can then be further oxidised and degraded which is shown as step 16a. It should be appreciated that the process performed in system 10 in some embodiments does not need to achieve 100% conversion of sulphide to its respective oxide to liberate the metal values retained therein. Although nitrate ions are referred to specifically in Figure 2, other oxidising agents capable of being regenerated by another oxidising agent, such as oxygen, can be used. For example, the nitrate ions may be replaced by permanganate ions.

In an embodiment, slurry containing the sulphide material 12a is pumped into a bottom of the grinding mill 16 and pathway 19 collects milled slurry from a top region of the grinding mill 16. The flow of slurry from the bottom to the top of the grinding mill 16 is caused by the pumping of new slurry into the bottom of the grinding mill 16. In some embodiments the slurry in path 13 is oxygenated prior to injection into the grinding mill 16. Having an opening of the conduit 17 being positioned near the bottom of the grinding mill 16 means that oxygen can be bubbled through a majority of the slurry present in the grinding mill 16 so that oxygen present in the slurry is at or near saturation point for the given reaction conditions.

The treated ore 20a is then collected and washed in collection tank 21. For example, the collection tank 21 has, in some embodiments, a three stage counterwash set up. The collection tank 21 may be equipped with a water recovery system. The collection tank 21 is not required in all embodiments. In some embodiments the treated ore 20a in collection tank 21 is used as a direct feedstock into a leaching system 26. For example, when the metal value is gold, the treated ore 20a is pumped from grinding mill 16 into collection tank 21 where the metal values are isolated and then pumped to a cyanide leaching step (i.e.

processed by system 26). The metal values are isolated in their solid metallic form prior to further purification.

In some embodiments the treated ore 20a from grinding mill 16 is transported via pathway 19 to agitation tank 22 prior to collection tank 21. Agitation tank 22 is heated and oxygen from oxygen source 24 is delivered to the agitation tank 22 via a conduit. In some embodiments agitation tank 22 is in a heat exchange relationship with grinding mill 16. In some embodiments the agitation tank 22 is provided with its own heat source. The agitation tank 22 helps to further oxidise and break down the solids in the slurry prior to forming treated ore 22. A plurality of agitation tanks may be connected in series, for example to provide a multistage post-mill oxidation step. Agitation tank 22 is not required in all embodiments. The agitation tank 22 will generally be required where a residence time of the slurry in the grinding mill 16 is not sufficient to oxidise the sulphide-matrix to liberate an adequate amount of metal value retained therein. Agitation in the tank 22 can be achieved by way of a conventional mechanical turbine style agitator or alternatively through conventional slurry pumps and air/oxygen as used in Pachuca tanks. Additional amounts of the first oxidising agent may be added to the agitation tank 22.

Oxidation and grinding of slurry 12 to form oxidised material 20b generates heat. To help maintain the temperature of the grinding mill 16 below a predetermined threshold temperature, in some embodiments a cooling system 15 is connected to grinding mill 16 via inlet 21. Cooling system 15 is not required in all embodiments. In one embodiment the cooling system is water that is injected into the grinding mill 16 through conduit 21. Injecting water into the slurry dilutes the slurry. In some embodiments a recirculated style cooling system is used as cooling system 15. In some embodiments the cooling system 15 is also configured to deliver nitrate salts or other oxidising agents to grinding mill 16, in addition to the nitrate salts added via 14. In these embodiments, cooling system can be connected to reservoir 14. In some embodiments the predetermined threshold temperature of grinding mill 16 is approximately 75 °C.

Approximately 10-15 tonnes/hour of solids are milled and oxidised in grinding mill 16. Depending on the solids content of the slurry, the amount of solids in formed slurry and that is delivered to the grinding mill 16 is about 15-25 tonnes/hour. In an embodiment a solids content of the slurry is approximately 55% by mass, with a specific gravity of about 4, so approximately 16,000L/hour of slurry is pumped into grinding mill 16. However, the rate that slurry is pumped into grinding mill 16 is dependent on the mineral e.g. it’s specific gravity the solids content of the slurry, and a residence time required for oxidation.

The grinding mill 16 is vented to an atmosphere surrounding the grinding mill 16. This means that the grinding mill 16 operates at atmospheric conditions. Compared with prior art processes that require high pressures and/or temperatures to oxidise and degrade sulphide minerals post milling, such as autoclave systems, the system 10 and associated process may reduce both capital and operational costs of systems and processes for liberating metal values from sulphide-containing ore. However, in some embodiments grinding mill 16 is operated at pressures above ambient such as at 1.5 or 2.0 atm or greater such as 5 atm, 10 atm or 40 atm. The specific pressure(s) the grinding mill 16 is operated at will depend on the mill design, mill sealing system, system pump specifications, ore type, the require sulphide oxidation and associated kinetics, and any downstream requirements. When the mill 16 is operated at pressure, the system 10 may further comprise pumps for pumping the slurry into the mill, and flash off vessels and heat recovery systems on a downstream side of the mill 16.

In some embodiments the system further comprises a concentrator 28 for

concentrating the ore prior to milling and oxidation in the grinding mill 16. In the embodiment of Figure 1 , the concentrator 28 is positioned upstream of the device 12 for forming the slurry. In some embodiments the concentrator 28 and device 12 form part of the same device where concentration occurs at the same time as forming the slurry. The concentrator concentrates the ore so that it has a sulphide concentration ranging from about 10-50% ore after concentration. However, concentrator 28 is not required in all embodiments and the system 10 can be used with ore having a sulphide concentration outside of 10-50%.

Generally, an ore with a higher sulphide concentration helps to make the disclosed process more economical.

An embodiment of a flow diagram 100 using the system 10 is shown in Figure 4. Ore is processed using standard Carbon in Pulp cyanide leaching at leaching system 26. Gold recovered from the leaching system 26 is collected at collector 52. Tailings from the leaching system 26 are sent to the concentrator 28 to form concentrated tailings. Material that is discarded by the concentrator 28 is sent to a residue holding area 54 such as a tailings deposit. As the concentrated tailings are in the form of a slurry, the concentrated tailings are injected into the mill 16 for processing as described above. If the tailings are not in the form of a slurry, a device for forming a slurry, e.g. device 12, is used to form a slurry prior to injection of the slurry into the mill 16. Such circumstances may be used when the tailings are generated from a dried tailings deposit. In some embodiments of system 100 the

concentrator 28 is omitted and tailings from the leaching system 26 are sent directly to the mill 16. Oxidised slurry (i.e. treated ore 20a) is then removed from the mill 16 and optionally processed further in the agitation tank 22 (not shown in Figure 4) prior to collection in the collection tank 21. Material in the collection tank 21 is then sent to the leaching system 26 where the gold in the treated ore 20a is recovered by collector 52.

Some embodiments use two or more mills 16. The two or more mills 16 may be arranged in parallel or series relative one another. A slurry separator may be used to separate the slurry into different streams, for example, a fine fraction slurry stream or coarse fraction slurry stream, for distribution between the different mills 16. Similarly, some embodiments use two or more agitation tanks 22. The two or more agitation tanks 22 may be arranged in parallel or series relative one another. A further slurry separator may be used to separate the slurry into different streams, for example, a fine fraction slurry stream or coarse fraction slurry stream, for distribution between the different mills 16 and/or agitation tanks 22.

In some embodiments, the system 100 is provided with two or more mills 16. An example of a system using two mills is best seen in Figure 5. Figure 5 depicts system 200 having two mills 16a and 16b arranged in series. Certain features of the system 200 have been omitted for clarity, for example the device 12 for forming a slurry, oxygen source 18, and so on.

In system 200, a slurry separator 210 is positioned downstream of a first mill 16a and separates the slurry of the treated ore (e.g. 20a) into a coarse slurry fraction and a fine slurry fraction. The fine slurry fraction is diverted into fine fraction stream 212 (also referred as a bypass stream) to bypass the second mill 16b. The treated ore 22a in the fine slurry fraction typically has a higher degree of sulphide oxidation compared to the treated ore in the coarse slurry fraction and generally does not require any further oxidation to break down the sulphide matrix to liberate metal values retained therein. The matrix of the treated ore in the coarse slurry fraction is not as degraded as that in the treated ore in the fine slurry fraction.

In some embodiments the treated ore in the coarse slurry fraction requires further oxidation to liberate metal values retained therein. To help break down the treated ore in the coarse slurry fraction further, the coarse slurry fraction is delivered from the slurry separator 210 to the second mill 16b via coarse fraction pathway 214. The coarse slurry fraction is treated in the second mill 16b. In some embodiments, the second mill 16b may be operated at elevated pressures (e.g. as a pressure mill with an operating pressure > 1 atm), for example to degrade and oxidise the more oxidation resistant material. However, in some embodiments the second mill 16b is operated at atmospheric pressures and is open to the atmosphere. Separating the slurry into coarse and fine fractions may help to reduce the size of the second mill 16b, which may help to reduce capital costs such as when the second mill 16b is a pressure oxidation mill.

In some embodiments, additional oxidising agent(s), such as the first oxidising agent including nitrate salt(s), are added to the coarse fraction pathway 214 prior to the coarse fraction being treated in the second mill 16b. The by-product of the second mill 16b (e.g. treated coarse fraction) is delivered to the collection tank 21 via treated coarse stream 216. The treated coarse stream 216 and fine fraction stream 212 are combined in the collection tank 21. In some embodiments, the fine fraction stream 212 and/or coarse fraction stream 216 bypass the collection tank 21 and are delivered directly to the leaching system 26.

Another embodiment of a system 300 used to treat a sulphide material such as ore is best seen in Figure 6. Certain features of the system 300 have been omitted for clarity, for example, the device 12 for forming a slurry, oxygen source 18, and so on. System 300 has the slurry separator 210 positioned downstream of mill 16. Unlike system 200, in system 300 the coarse fraction pathway 214 delivers the coarse slurry fraction back to the mill 16 where the coarse fraction is further treated. Only the fine slurry fraction is delivered to the collection tank 21 via fine fraction stream 212. The delivery of the coarse fraction pathway 214 to mill 16a may eliminate the need for a second mill e.g. 16b.

Examples

Example 1

Exemplary laboratory-scale embodiments of the process were carried out according to Table 1. The composition of the samples used in Table 1 is listed in Table 2.

Sample 1 was ore concentrate with 41% of pyrite/arsenopyrite that was milled down to a particle size of approximately 10 pm then subjected to cyanide leaching to recover gold. Table 2 details a composition of the ore used in Table 1. Of the total gold retained in the pyrite mineral matrix only 25.2% was extracted. The sulphur content of the ore remained the same since the ore was not subjected to oxidation.

In Sample 2 calcium peroxide (Ca0₂) was used as an oxygen source during milling and the sulphur in the ore was oxidised as evidenced by the 22.0% sulphur reduction, but the gold extracted from the pyrite was 25.8% which is approximately the same as when no oxidation occurred (Sample 1). It is thought that upon releasing oxygen, the Ca0₂ oxidant decomposes to hydrated lime (CaOH) which acts to precipitate the Fe ions (and As ions when the ore comprises arsenopyrite) that have been dissolved during the oxidation process, onto a surface of the mineral forming a surface layer that prevents extraction of gold via cyanide leaching. When Ca0₂ was replaced with ammonium nitrate during milling (see Sample 3) the percentage sulphur reduction reduced, but the percentage of extracted gold increased to 54.6%. The use of ammonium nitrate does not result in precipitation of Fe and/or As on the surface of the mineral, so a greater percentage of the gold is available for cyanide leaching.

When the ore was only conditioned with either Ca0₂ or ammonium nitrate and not subjected to milling (Samples 4 and 5) the amount of gold extracted was about 33%.

In Samples 6 and 7 ammonium nitrate was used as an oxidising agent along with an oxygen source. In Sample 6 the oxygen source was Ca0₂ and in Sample 7 the oxygen source was H₂0₂. H₂0₂ was used in Sample 7 to simulate oxygen gas being injected into the mill as oxygen gas injection at laboratory scales is not efficient and does not give a representative approximation for large scale oxygenation with oxygen gas. Sample 7 was also oxidised in an agitation tank post milling. The inclusion of ammonium nitrate as an oxidising agent did not result in a noticeable increase in the percentage sulphur reduction when Ca0₂ was used as an oxygen source but the percentage gold extraction increased to 33.6% from 25.8% compared to use of Ca0₂ alone (c.f. Sample 2). When Ca0₂ was replaced with H₂0₂ as the oxygen source (Sample 7) the percentage sulphur reduction remained approximately the same, but the percentage gold extraction rose dramatically to about 63%. Again, H₂0₂ (as a provider of oxygen to the oxidising reaction) would not result in the formation of CaOH nor the formation of precipitates that would prevent gold from being extracted using cyanide leaching. The practical limit of sulphur reduction is driven by circuit economics. Figure 3 gives an indication of the As and S oxidation levels required to achieve gold recovery on a particular sample, and this relationship will vary with the different concentrates and pyrite to arsenopyrite ratios in any given ore.

Table 2

Mineral or mineral

Mass %

Arsenopyrite *14

Pyrite *27

Magnetite 0

Clay mineral 1

Chlorite 13

Kaolinite - serpentine 1

Annite - biotite -

3

phlogopite

Muscovite 5

Sodic and calcic

12

plagioclase

K-feldspar and/or rutile 0

Dolomite - ankerite 4

Siderite type carbonate 1

Alpha quartz 18

Both arsenopyrite and pyrite patterns show extremely strong preferred orientation, and the results carry larger than usual uncertainty. Some amorphous material is most likely present.

Example 2

Milling conditions using ammonium nitrate and hydrogen peroxide as oxidising agents at a concentration of [NH4Nq3]:[H2q2] at [75kg/t]:[25kg/t] were used to examine the effects of the disclosed process on a variety of ores with different compositions. The gold recovery for the various ores is shown in Table 3. Hydrogen peroxide was used to simulate oxygen gas being injected into the mill. However, oxygen gas can be used in place of hydrogen peroxide. As the ratio of pyrite to arsenopyrite increases, the amount of gold recovered in the head gold recovery (i.e. initial extraction process) using carbon in pulp leaching generally increases due to the increased ability of cyanide to leach gold from pyrite as pyrite tends to release gold particulates and gold solutions entrapped therein upon milling. However, even in samples that had high initial gold recoveries, such as Sample 18 having an initial gold recovery of about 93%, milling the tailings of the initial gold recovery in the presence of ammonium nitrate and hydrogen peroxide still recovered additional gold that was present in the tailings. Thus, even for ore containing a high percentage of pyrite, an embodiment of the disclosed process still helped to increase the amount of gold recovered from the ore. An increase in arsenopyrite in the ore tended to reduce the amount of gold in the head gold recovery. However, in all samples gold was recovered from the tailings by milling in the presence of a first and second oxidising agent, and in some samples the amount of gold recovered in the tailings was greater than that in the head gold recovery (e.g. Sample 24). In Table 4 the [NH4Nq3]:[H2q2] concentrations were adjusted to

[ 150kg/t] : [ 10Okg/t] and milling was performed on the same samples as in Table 3, which for most samples caused a significant increase in the recovery of gold from the tailings.

Table 3

Leach recovery of Au

Au leach recovery Total Au recovery

Sample Py/ASP ratio^# from tailings

on ore (%)

concentrate (%)*

8 0.07 39.32 24.1 1 63.43

9 0.10 45.48 21 .43 66.91

10 0.55 56.17 13.85 70.02

1 1 0.14 51 .59 25.51 77.10

12 0.08 30.41 13.62 44.03

13 0.87 72.56 8.53 81 .09

14 0.17 60.81 18.28 79.09

15 0.21 63.56 1 1 .98 75.54

16 9.67 76.51 10.63 87.14

17 1 1 .60 84.04 7.50 91 .54

18 10.51 92.99 1 .09 94.08

19 16.00 87.04 5.09 92.13

20 14.00 82.86 8.33 91 .19

21 16.00 85.51 7.32 92.83

22 0.22 31 .95 30.47 62.42

23 13.50 84.82 6.05 90.87

24 0.13 26.97 46.43 73.40

25 0.75 60.83 14.43 75..26

26 0.22 52.59 13.21 65.80

27 16.04 73.37 14.86 88.23

28 8.08 69.86 17.13 86.99

^#Py = pyrite, ASP = arsenopyrite; ^Treatment conditions: milling in the presence of

[NH₄N0₃]:[H₂0₂] at [75kg/t]:[25kg/t]

Table 4

Sample Py/ASP ratio Leach recovery of Au Leach recovery of Au from tailings from tailings

concentrate (%)* concentrate (%)^L

Low H2O2 High H2O2

8 0.07 24 76

9 0.10 21 77

10 0.55 14 81

1 1 0.14 26 81

12 0.08 14 63

13 0.87 9 81

14 0.17 18 80

15 0.21 12 81

16 9.67 1 1 80

17 1 1 .6 8 80

18 10.5 1 79

19 16.0 5 82

20 14.0 8 89

21 16.0 7 81

22 0.22 30 81

23 13.5 6 88

24 0.13 46 81

25 0.75 14 81

26 0.22 13 80

27 16.0 15 82

28 8.08 17 79

^#Py = pyrite, ASP = arsenopyrite; * Treatment conditions: milling in the presence of

[NH₄Nq3]:[H2q2] at [75 kg/t] : [25 kg/t] ; ^ATreatment conditions: milling in the presence of

[NH₄N0₃]:[H₂0₂] at [ 150kg/t] : [ 10Okg/t] .

Example 3

The effects of pressure on an embodiment of the disclosed process is shown in Table 5. Sample 29 was processed using the conditions described in Example 1 and Example 2, except a post-milling oxidation step was performed for Sample 29. Sample 30 was treated in the same was as Sample 29, except no post-milling oxidation step was performed for

Sample 30. These results show that the post-milling oxidation step helped to recover about 8% from gold that is present in the tailings. However, when milling was performed at a pressure of 6 atm, the amount of gold recovered from the tailings increased to about 58% from about 41 %. Increasing the pressure further to 10 atm (Sample 32) had little effect on the amount of gold recovered from the tailings. Increasing the concentration of ammonium nitrate and hydrogen peroxide (Sample 33) tended to decrease the amount of gold recovered from the tailings when compared to lower oxidant concentrations (Sample 32). The lower recovery of gold for Sample 33 may be attributed to sub-optimal reaction conditions for the give ore composition.

Table 5

* Treatment conditions: milling in the presence of [NH₄N0₃]:[H₂0₂] at [75 kg/t] : [25 kg/t] ;

treatment conditions: milling in the presence of [NH₄N0₃]:[H₂0₂] at [150kg/t]:[100kg/t].

It should be understood that the use of the disclosed process and systems to extract gold from ore is only one example of a metal value that can be extracted from a sulphide material using the disclosed process, and the disclosed process and system is not limited to the extraction of gold from sulphide materials and sulphide minerals. Accordingly, the disclosure includes the extraction of other metal values, such as copper, from sulphide materials.

It will be understood to persons skilled in the art of the disclosure that many

modifications may be made without departing from the spirit and scope of the disclosure.

In the claims which follow and in the preceding description, except where the context requires otherwise due to express language or necessary implication, the word“comprise” or variations such as“comprises” or“comprising” is used in an inclusive sense, i.e. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments.

Claims

Claims

1. A process of treating an ore comprising a sulphide material, comprising:

providing a slurry of the ore comprising the sulphide material;

adding a first oxidising agent and a second oxidising agent to the slurry;

milling the slurry in the presence of the first and second oxidising agents to at least partially oxidise the sulphide material,

wherein the second oxidising agent regenerates the first oxidising agent.

2. A process of liberating a metal value from ore comprising a sulphide material, the process comprising:

providing a slurry of the ore comprising the sulphide material;

adding a first oxidising agent and a second oxidising agent to the slurry;

milling the slurry in the presence of the first and second oxidising agents to at least partially oxidise the sulphide material to break down a matrix of the sulphide material to liberate the metal value retained therein,

wherein the second oxidising agent regenerates the first oxidising agent.

3. The process of claim 2, wherein the sulphide material is a sulphide-containing mineral.

4. The process of claim 2 or 3, wherein the metal value is gold.

5. The process of any one of claims 1 to 4, wherein the first oxidising agent oxidises the sulphide material so that the first oxidising agent is converted to a first reduced product, and wherein the second oxidising agent regenerates the first oxidising agent by oxidising the first reduced product back to the first oxidising agent.

6. The press of claim 5, wherein the first oxidising agent acts as an oxidation catalyst that is regenerated by the second oxidising agent.

7. The process of any one of claims 1 to 6, wherein adding the first and/or second oxidising agents to the slurry occurs before and/or during milling.

8. The process of any one of claim 1 to 7, further comprising cooling the slurry during milling by adding a cooling fluid to the slurry.

9. The process of any one of claims 1 to 8, wherein during milling an average temperature of the slurry is maintained at or below approximately 75 °C.

10. The process of any one of claims 1 to 9, further comprising oxidising the milled slurry in an agitation tank after milling, the agitation tank having a heated oxygen-rich environment for oxidising the sulphide material.

11. The process of any one of claims 1 to 10, wherein the first oxidising agents includes a nitrate salt.

12. The process of any one of claims 1 to 11 , wherein the second oxidising agent includes an oxygen source.

13. The process of claim 12, wherein adding the second oxidising agent to the slurry comprises adding the oxygen source to the slurry during milling.

14 The process of claim 12 or 13, wherein the oxygen source comprises oxygen gas, and wherein adding the oxygen source to the slurry comprises bubbling oxygen gas through the slurry.

15. The process of any one of claims 1 to 14, further comprising concentrating the ore comprising the sulphide material prior to milling the slurry, wherein a sulphide concentration of the ore is approximately 10-50 wt.% after concentration.

16. The process of any one of claims 1 to 15, wherein the sulphide material is milled down to an average size of about 10 pm during milling.

17. The process of any one of claims 1 to 16, wherein the sulphide material includes pyrite and/or arsenopyrite.

18. The process of any one of claims 1 to 17, wherein the ore is a low grade refractory deposit.

19. The process of any one of claims 1 to 18, wherein milling is carried out at a pressure that is greater than atmospheric pressure.

20. The process of any one of claims 1 to 19, wherein the second oxidising agent at least partially oxidises the sulphide material.

21. A metal value liberated from an ore comprising a sulphide material using the process according to any one of claims 1 to 20.

22. A system for treating ore comprising a sulphide material, the system comprising: a grinding mill for milling a slurry formed from the ore comprising the sulphide material; and

a first mill inlet for adding the slurry into the mill and a mill outlet for removing treated slurry from the mill;

a slurry flow path in communication with the mill inlet;

a first oxidising agent inlet for adding a first and/or a second oxidising agent into the slurry flow path prior to the slurry being added to the mill.

23. The system of claim 21 , wherein the mill inlet is positioned towards a bottom of the mill and the mill outlet is positioned towards a top of the mill.

24. The system of claim 22 or 23, further comprising a second mill inlet for adding:

(i) a cooling fluid into the grinding mill so that in use of the mill the slurry is maintained at a temperature that is at or below a predetermined temperature; and/or

(ii) additional amounts of the first and/or second oxidising agents to the slurry in the mill.

25. The system of claim 24, wherein a source of the first and/or second oxidising agents is in communication with the second mill inlet for the cooling fluid, and wherein the system is arranged so that the additional amounts of the first and/or second oxidising agent are provided in the cooling fluid so that the additional amounts of the first and/or second oxidising agents are added to the mill at the same time as the cooling fluid is added to the mill.

26. The system of claim 23 or 24, wherein the predetermined temperature is

approximately 75 °C.

27. The system of any one of claims 22 to 26, wherein the grinding mill is vented to an environment at atmospheric pressure.

28. The system of any one of claims 22 to 26, wherein the grinding mill is a pressure grinding mill that is capable of being pressured up to about 10 atm.

29. The system of any one of claims 22 to 28, further comprising an agitation oxidation tank downstream of the mill outlet.

30. The system of any one of claims 22 to 29, further comprising a slurry separator downstream of the mill and in communication with the mill outlet, the slurry separator separating the slurry into a coarse slurry fraction and a fine slurry fraction.

31. The system of claim 30, wherein the coarse slurry fraction is in communication with the first mill inlet.

32. The system of claim 30, further comprising a second mill that is configured to receive and treat the coarse slurry fraction, wherein the fine slurry fraction bypasses the second mill.

33. A process of extracting gold from an ore using cyanide leaching, comprising:

treating the ore using the process of any one of claims 1 to 20 to make the gold available for cyanide leaching, and

cyanide leaching the gold.