Heap leaching is widely used in the recovery of values from low-grade copper and gold ores, and is sometimes used for processing other ores including those containing uranium nickel and zinc.

Heap leaching has several advantages over the conventional processing routes of fine grinding followed by flotation or leaching.

Firstly, the cost of comminution required for heap leaching is much lower. The low-grade ore is usually crushed to a maximum size which may vary from 20mm to 0.5m. This crushing reduces the distance the leachant has to infiltrate through fractures and porosity in individual rocks to dissolve the valuable elements that are occluded in the rocks.

Such coarse crushing for heap leaching typically consumes around 10-20% of the amount of energy that is required to grind the rock to a p80 size less than 200 microns typically required for efficient flotation or leaching.

The second major advantage of heap leaching is the cost of the ‘reactor’. For heap leaching, the blasted or crushed ore is simply dumped from a truck or conveyor system onto a pad, and leachate is distributed across the surface of the resultant heap. Pregnant leachate is collected from the base of the heap, and metal of interest is recovered from the leachate.

This low cost ‘heap leach reactor’ avoids the capital and operating costs of contained a suspended slurry in a more intense processing step, whether by beneficiation techniques like flotation or agitation leaching. Heap leaching opens up the potential to leach the ore over periods of months or years, under ambient conditions, which is not economically possible in any form of an agitated reactor.

Furthermore, unlike leaching or flotation, the heap leach does not generate a fine residue which must ultimately be stored as a fine slurry in a special purpose tailings storage facility or undergo an expensive solid liquid separation required for subsequent storage as a dry stack.

There is however, one major downside to heap leaching, that restricts its application to lower grade ores. The recoveries of the valuable metals during conventional heap leaching is low. The extraction of the metals of value is typically less than 80% and often around 65%. And where passivation occurs during heap leaching, for example with primary copper ores, the extraction is even lower.

This low extraction in heap leaching is attributable to three factors:

1 . Diffusion of the leachate to access and dissolve values occluded in the middle of large rocks is very slow, and hence even where heap leaching time is a few years, a significant amount of the valuable metal remains un-leached. If the rocks were crushed finer to overcome the constraint, conventional heap leaching becomes restricted by heap permeability.

2. Secondly, the distribution of leachate as it flows down by gravity through the heap is influenced by variable permeability within the heap. The permeability differentials arise from uneven distribution of fines within the heap. The concentration of fines in specific zones of the heap occurs at both a macro scale, rain shadows formed below areas of low permeability, and at a micro scale where packing of fines around individual particles causes the leachate to channel and bypass particular rocks.

3. And thirdly, heap leaching often relies on utilization of the oxygen air as the oxidant during leaching. Again, areas of excessive fines can cause limited air circulation within parts of the heap, thus leading to ‘dead zones’ within the overall heap leach.

The magnitude of the impact of fines on permeability is sufficient for some heap leach operations to agglomerate the fines in the ore prior to heap leaching. Whilst agglomeration can enhance overall permeability of the heap and hence recoveries, it is a high cost unit operation and requires careful stacking to prevent destruction of the agglomerates.

Secondary downsides to heap leaching, relative to agitation leaching or flotation, include the time for leaching (slow cash flow and large land areas), the rehabilitation of large areas of mostly leached and partially drained ore, and the difficulty in maintaining a water balance over the extended leaching period (excessive evaporation in dry periods, and excessive dilution in wet periods).

The ideal heap leach structure would be easily stacked, have a particle size distribution which enables high extraction in a short leach duration, have a high and uniform permeability for the leachate, have a high air permeability if aeration is required, use a minimum amount of wash water, and leave a barren heap which is easily rehabilitated.

It is an object of this invention to provide for a heap leach structure to address the elements identified above. SUMMARY OF THE INVENTION

According to the present invention there is provided a method for recovering metal values from ore, including the steps of: depositing two or more processed ore fractions with different particle sizes in discrete layers to form a layered heap leach structure located on a heap leach pad; carrying out a leaching step in which the layered heap structure is leached with the leachate which permeates by gravity through the layers; and recovering a leachate containing dissolved values from a leachate collection point or points.

The processed ore fractions may be from an ore that has been crushed and classified to provide fractions of ore with different particle sizes, or the processed fractions may be waste streams from ore beneficiation processes, for example tailings from ore flotation processes.

The ore may be a sulphide ore, or an oxide ore, or a mixture of both. For example, the ore may be an oxidised copper ore, nickel laterite ore, primary copper ore, secondary copper ores, nickel sulphide ore, gold ore, zinc sulphide ore, or uranium ore.

The metal values may be copper, gold, nickel, uranium, or zinc.

Preferably, the discrete layers are deposited such as to have an even permeability in the vertical direction.

Typically, the discrete layers comprise alternating layers of a coarse ore fraction and a fine ore fraction. The coarse ore layers may have a particle size p80 between 0.1 and 10 mm, and preferably between 0.15 mm and 3 mm, and even more preferably between 0.2 mm and 2 mm.

The fine ore layers may have a particle size p80 size between 0.02 mm and 0.5 mm, and preferably between 0.05 mm and 0.3mm, and even more preferably between 0.1 mm and 0.2 mm.

Preferably, the layered heap leach structure is contained by a bund wall that preferably surrounds the layered heap leach structure.

The bund wall may be constructed using a portion of coarse ore sand fraction placed in a heap around a perimeter of the heap leach pad to form a permeable bund.

Alternatively, the bund wall may be constructed from permeable or impermeable materials such as waste rock or quarried material.

In an embodiment of the invention the bund wall is water impermeable, for example it may be lined with an impermeable material, to contain the layers of coarse ore and fine ore slurry, in effect creating a three-dimensional flooded reactor, from which leachant can be withdrawn from the permeable sand layers, in effect a very large-scale vat leaching reactor in combination with a solid liquid separation device.

Typically, the coarse ore and fine ore are deposited sequentially in alternating discrete layers composed of the coarse ore and fine ore fractions.

The coarse ore fractions can be deposited in thicker layers than the fine ore fractions depending on the ratio of the ore to be leached. For example, the coarse ore fractions may be deposited in layers of 0.2 m to 10 m thick or even more, possibly 2 to 10m or 5 to 8 m thick, and the fine ore fractions may be deposited in layers less than 5 m, and typically 0.2 to 2 m, preferably 1 m to 2 m thick. The fine ore fractions may be deposited as a single layer, or built up in a series of multiple thin layers to promote natural aeration during deposition.

Leachant may be mixed with the fine and coarse ore prior to deposition in the respective layers; and/or is subsequently irrigated through the sand layer on the top of the fine layers through the structure.

Leaching of the ore under ambient conditions may be between 1 day and 3 years, and preferably between 1 week and 1 year and even more preferably between 2 weeks and 6 months.

In one embodiment of the invention a fines layer at the top of the structure remains saturated through the leaching period to promote gravity flow of the leachate and subsequent wash water through the top layer into the underlying coarse layer; for transfer by draining to a leachate collection point.

In another embodiment of the invention, a partially leached fines layer is overlaid by another layer or channels of coarse ore and another layer of fine ore; and the stack is operated in flooded mode to enable leaching to continue in both the upper and underlying layers of the structure.

In a further embodiment of the invention, is accelerated aeration of the leaching may be promoted by forced aeration through the underlying and overlying coarse ore channels, or by mechanical turning of the ore in the exposed fines layer, or by progressive cycles of flooding and draining to create air channels within the fines.

Preferably, heap leaching takes place after addition of each layer or pair of layers, followed by deposition of another layer(s) and then heap leaching, and deposition of another layer(s), with heap leaching with the cycle continued to the desired height of the structure. In the case where the bund wall is water impermeable, a pump or multiple pumps may be provided to withdraw leachate from collection points located at coarse ore layers.

In the case where the bund wall is water permeable or includes water permeable layers, leachate may permeate through the bund and be collected a water collection point or points outside of the bund wall.

An interface between the coarse ore bund wall and a coarse ore layer may be sealed and flow of leachate or air in the layer controlled by an injection pipe through which leachate or air is injected or withdrawn. In this way, the layers can be either be flooded, or drained, or cycle between the two states.

When the ore is a sulphide ore, the sulphide ore may be classified with the finer fractions processed using coarse and conventional flotation of the sulphides, prior to deposition of the layers within the layered structure for heap leaching. In this way, the oxygen demand for dissolution of the sulphides in the fines fraction of the ore in particular, is decreased.

The heap may be structured and operated to achieve aeration from the surrounding air, including from the surface atmosphere, and forced air ingress through the coarse ore layers.

The heap leach method may be used in combination with other beneficiation techniques such as flotation and coarse particle flotation, to enhance metal recovery from the sulphide fractions of the ore, prior to deposition in the heap leach structure.

The coarse fraction of the ore may be subjected to a separate heap leach prior to the residue from this heap leach being added to or incorporated into the stack heap leach structure. The structure may be washed of entrained leach liquor by injecting wash liquor through the sand layer above the layer to be washed and recovered from the sand layer below.

The structure may be used to recover values from, and subsequently rehabilitate historical tailings dams

The invention also relates to a heap leach structure comprising: a heap leach pad; a multilayer heap leach structure contained by a bund wall, said multilayer structure comprising alternating layers of processed coarse and fine ore fractions.

The coarse ore fractions may have a particle size p80 between 0.1 mm and 10 mm, and preferably between 0.15 mm and 3 mm, and even more preferably between 0.2 mm and 2 mm.

The fine ore fractions may have a particle size p80 size 0.02 mm and 0.5 mm, and preferably between 0.05 mm and 0.3mm, and even more preferably between 0.1 mm and 0.2 mm.

A bund wall preferably surrounds the layered heap leach structure.

The bund wall may be constructed using a portion of coarse ore fraction which is placed in a heap around a perimeter of the heap leach pad to form a permeable bund.

Alternatively, the bund wall may be constructed from permeable or impermeable materials such as waste rock or quarried material.

In an embodiment of the invention the bund wall is water impermeable, for example it may be lined with an impermeable material, to contain the layers of coarse ore and fine ore slurry, in effect creating a three-dimensional flooded reactor, from which leachant can be withdrawn from the permeable sand layers, in effect a very large-scale vat leaching reactor in combination with a solid liquid separation device.

The coarse ore fraction layers may be 0.2 m to 10 m thick or even more, possibly 2 to 10 m or 5 to 8 m thick, and the fine ore fraction layers may be less than 5 m, and typically 0.2 to 2 m, preferably 1 m to 2 m thick.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a graph showing % copper extracted versus particle size in a column leaching of crushed ore from which fines have been removed;

Figure 2 is a cross-section of a heap layer structure of layered sand and tailings according to a first embodiment of the invention;

Figure 3 is a cross-section of a heap layer structure of layered sand and tailings according to a second embodiment of the invention;

Figure 4 is a cross-section of a heap layer structure of layered sand and tailings according to a third embodiment of the invention;

DESCRIPTION OF PREFERRED EMBODIMENTS

The current invention is a layered structure designed for effective heap leaching with high metal recoveries. In a method of the invention, ore is crushed to an ideal size for leaching, classified into size fractions which are placed in a layered structure achieving an even leachate permeability ensuring effective leachate distribution, with subsequent drainage and high recovery of the leachate. The layered structure in its various embodiments is suitable for treating a wide range of ore types, be they predominantly sulphides, or predominantly oxides, or a mix of both, or with a metallic fraction such as gold.

The structure can be used leach to sulphide ores which contain a significant oxide component. Since the oxide fraction of such ores is difficult to recover by flotation, and the sulphide fraction requires extended leaching times, one fraction or other of the ore is often lost. Using the current invention both fractions can be recovered. An example of this ore type is the oxidised cap of many copper resources.

The structure can also be used to recover predominantly oxide ore, which require extended leach duration under ambient temperature conditions. The structure allows reasonably fine grinding to increase the exposure of the oxide values to leachant, but through using the invention residence times of leaching can be extended to months or years at very low incremental cost of reactor volume. An example of such an ore is nickel laterites.

The current invention utilizes channels of coarse tailings to drain the liquid from the fine tailings, such as to leave a residue which has a low moisture content and is dimensionally stable. In effect, the structure is a large filter, which can transfer liquid from a dispersed state within the fine ore, and through the sand channels to a location from which it can be recovered.

The tailings disposal structure has a typical drainage time, from the time of deposition of tailings over unsaturated coarse ore underlayer, measured in terms of a few weeks to desaturate the tailings fraction of the structure. This drainage time can be further extended by retaining water saturation in the adjacent sand channels.

A similar layered sand and tailings structure can be utilized as both a scalable leach reactor and a filter. This layered structure provides the volume and residence time for leaching to occur, and also performs the solid liquid separation when leaching is complete. The dimensions of the tailings layer define the minimum residence time for a particular tailings to dewater, or be drained and washed, to recover the leachate that has been produced by dissolution. The maximum thickness is typically less than 5 metres, to enable effective leachate recovery. Whilst there is no minimum thickness of the tailings layers, costs of stack construction usually dictate a minimum height of around 0.5m.

The intervening channels of coarse ore must be such that leachant and wash water can be distributed into and recovered from the tailings. This demands a sand size which provides for suitable permeability and is such that the tailings will not mix with and block the permeable layer. This typically requires a sand size of between around 0.2 mm to 1 mm to be present in the coarse layer.

This sizing of tailings and sand for the structure enables comminution of most of the ore to the optimum size for subsequent extraction, similar to that which would be used in an agitated reactor, without concerns about heap permeability. The coarser sand fraction can also be leached in this way.

The stack leach structure enables ores that can be leached over periods of days, weeks, months or years to be extracted, whilst concurrently achieving the solid liquid separation to recover leachate from within the slurry.

As the stack leach structure can be extended over an extensive land area with minimal cost for external pad and bunding, the stack is scalable in away that agitators or vats are not. Hence the need for rapid leaching is reduced. The stack leach structure can be deposited in multiple lifts whilst still achieving high leachate extractions from each layer. The leaching duration can be adjusted by the rate of removal of the leachant from the underlying coarse ore channel, and hence extended to months or even years without incurring substantive reactor costs. The coarse ore fraction has a high leachate permeability. Whereas the fine ore fraction has a significantly lower permeability, that would normally cause issues in conventional heap leaching.

As is known to those skilled in the art, this maximum size of ore has a strong impact on both the leaching rate and overall recovery. An example of this impact is illustrated in Figure 1 , through column leaching of different maximum sizes of the coarse ore fraction. The copper ore was leached with acidic solution of copper chloride for 150 days.

An aspect of this invention is the formation of the coarse and fine fractions into a layered structure containing coarse and fine ores, having a reasonably even permeability in the vertical direction across each individual coarse and each fine layer within the structure.

In one embodiment of the invention, the stacked structure can be operated in flooded mode, in which the external bunds are impermeable, and progressive layers of sand and fines are laid down together with the leachant solution. After the required leach duration, the pregnant leachate can then be withdrawn from a selected sand layer.

With reference to Figure 2, a heap leach structure 10 comprises an impermeable leach pad 12 surrounded by an impermeable bund wall 14. The heap leach structure 10 comprises progressive layers of coarse sand 16 and fine ore 18. The coarse sand 10 and fine ore 18 may be from an ore containing at least part of its values as an oxide, that is crushed and classified to provide the coarse sand 16 as a fraction with a particle size p80 between 0.1 and 10 mm, and the fine ore 18 with a particle size p80 size between 0.02 and 0.5mm. The coarse layers 18 have a thickness of greater than 0.2m and typically around 1 m, and the fine ore layers 18 have a thickness of greater than 0.2 m and typically 1 -2m. One or more pumps 20 are provided to dewater selected sand layers. In this embodiment of the invention, the progressive layers of sand 16 and fines 18 are laid down together with a leachant solution. The leachant solution comprises a solution which will dissolve the values, and is typically an acid, or a basic solution containing a complexant such as ammonia or cyanide. After the required leach duration, the pregnant leachate can then be withdrawn from a selected coarse layer 16 using the pump 20.

With reference to Figure 3, a heap leach structure 10 comprises an impermeable leach pad 12 surrounded by an impermeable bund 14. The heap leach structure comprises progressive layers of coarse permeable sand 16 and fine ore 18. The coarse sand 10 and fine ore 18 may be from an ore containing at least part of its values as an oxide, that is crushed and classified to provide the coarse sand 16 as a fraction with a particle size p80 between 0.1 and 10 mm, and the fine ore 18 with a particle size p80 size between 0.02 and 0.5mm. The coarse layers 18 have a thickness of greater than 0.2m and typically around 1m, and the fine ore layers 18 have a thickness of greater than 0.2 m and typically 1-2m. A pump or pumps 20 is provided to dewater selected sand layers. In this embodiment of the invention, the heap leach structure 10 is operated in drained mode. The top tailings layer 18A comprises fines saturated with leachant, and the lower fines layers 18B are unsaturated. The leachant solution comprises solution which will dissolve the values, and is typically an acid, or a basic solution containing a complexant such as ammonia or cyanide. Leaching occurs in the freshly added upper layers of the structure, whilst the lower layer of the structure are unsaturated due to removal of leachant and subsequent wash water from the structure.

The solid liquid separation of leach liquor from the leached gangue occurs slowly utilising gravity, allowing most of the liquor to be removed through the coarse ore layers, with the leach residue remaining amenable to washing to remove any residual leachant once the leach is complete.

In effect, the structure enables the combination of the best features of both a heap leach and a vat leach reactor. The distribution of leachant to all parts of stack leach structure can be achieved by pre-mixing of the leachant and the ore prior to deposition, and/or working with a flooded system for part or all of the leaching duration. For example, leachate or wash water can be selectively injected into or recovered from selected sand layers within the stack leach structure

This pre-mixing can distribute the leachant in the solids, or include a period of agitated leaching and partial solid liquid extraction, prior to addition to the structure for further leaching residence time, and the requisite solid liquid separation to recover the leachate for metal recovery.

This hybrid reactor concept can be extended even further to leach the ore in an agitated reactor or vat, followed by what is a minimal soak time with leachate recovery, with washing and permanent disposal of the leached ore. Within the stack leaching structure, the washing of the pregnant leach liquor from the fine leach residues can be achieved by filling the sand layer above the fine leach residue with wash water, and recovering the displaced liquor from the sand layer below.

The presence of layers of fines in the stack leach structure limits air permeability through the structure. For oxidative leaching reactions which use oxygen as the oxidant, such as sulphide leaching, the requirement for aeration to achieve high extractions in the stack leach must be managed through the wider system design.

In one embodiment, this addition of oxidant can be through the use of a soluble oxidant which can be re-oxidised externally to the heap and reinjected into the stack leach structure for further dissolution. An example could be the addition of ferric or cupric ion to leach a modest quantity of base metal sulphides present in the ore.

Alternatively, some access of air can occur from the upper surface of the tailings that are exposed to the atmosphere immediately after deposition i.e. use of thin layers of fines, allowing the oxidation to occur, and then add to the layer thickness, and repeat the process.

Or in the circumstance of a shortage of available air to complete leaching, the availability can be augmented by channeling air through the unsaturated sand layers in the structure.

However, the most common embodiment for a mixed oxide and sulphide ore is to sequentially combine the flotation and stack leaching processes. The flotation can recover the sulphide metal fraction, with the flotation tailings being transferred to a stack heap leach to recover the oxide fraction. With most of the sulphide in the ore being successfully floated, the oxide fraction of the ore can be stack leached, and the required oxygen to achieve high extractions of any remaining sulphides is significantly reduced.

The stability of the final residue disposal structure makes the system applicable to leaching a wide variety of ores, whilst not incurring substantive rehabilitation costs. The system is well suited to extract metals not only for fresh ores, but also to combine further metal recovery from historic tailings dams, with their rehabilitation in a more stable landform.

As noted, the system can be utilized to recover the full complement of metal values in the ore either directly or in conjunction with other beneficiation systems.

For example, a copper sulphide ore which contains a significant oxide fraction is usually subjected to flotation for sulphide recovery. The oxide fraction is lost to the float residue. Using the current invention, the stack leach structure for recovery of the oxide fraction, simply through the addition of acid.

As a second example, a low-grade nickel sulphide ore may not sustain the cost of fine grinding required to recover all the floatable values. A tertiary crush will reduce the costs of comminution per tonne of ore, and liberate most of the sulphides in the size fraction suitable for coarse and conventional flotation. But some values will be occluded in the size fraction above around 0.5 mm, and hence report to coarse flotation residue, and the oxides in the ore will not be floatable. The stack leach structure provides a basis to separate this partially oxidised ore and stack leach both the fines and the sand.

Thus, the dual flotation and leaching recovery mechanism using the tailings structure is particularly relevant for both low grade ores and mixed oxide/sulphide ores that are inherently difficult to process by conventional means. The heap leach structure may for example be used in combination with a coarse particle flotation process such as that described in WO2016/1700437 (the content of which is incorporated herein by reference). In this example, coarse tailings from the process may be used as the coarse processed ore in the heap leach structure, and fine tailings used as the fine processed fraction in the structure.

With reference to Figure 4, in another embodiment of the invention, a heap leach structure 10 comprises an impermeable heap leach pad 12, and proportion of the coarse ore from classification placed in heaps around the perimeter of the leach pad 10 to form a permeable bund 14. The heap leach structure 10 comprises progressive layers of coarse permeable sand 16 and fine ore 18.

Whilst the layers of fines 18 have a low permeability, leachate will flow through the fine ore at a steady and even rate. For example, for a typical fines layer of around 1 -2 metres, a displacement volume of liquor from above will require less than a month, to permeate from top to bottom of the fines layer.

The heap is progressively built up from sequential layers of high permeability coarse ore 16 and low permeability fine ore 18, bunded on the outside by high permeability coarse ore. Pipes 22 may be provided in the bund 14 through which air or leachate may be injected or withdrawn. Once part or all of the structure is constructed, the fourth component of the current invention is the heap leaching.

Leachate 24 is either added to the ore fraction as it is being deposited in the structure or trickled down from the top of the structure to dissolve the values in the underlying layer.

The pregnant leachate 26 is recovered from the underlying coarse ore layers where it can mostly migrate laterally through the permeable coarse ore to the edge of the heap.

This leaching typically takes place following the deposition of each layer of the heap, to be followed by another raise of another layer and a repetition of the leaching cycle. However, leaching can also be delayed until after construction of multiple layers of the heap.

The leachant will distribute evenly through the coarse ore layers 16 just as it would in a highly permeable conventional heap.

And the leachate will slowly but evenly permeate the fine ore 18, dissolving the valuable metals in the fines fraction, until it reaches the layer of coarse ore below. Whilst this rate of permeation of the fine layer is slow, the fine nature of the ore implies that the rate of leaching is controlled by the chemical reaction rate, not diffusion of leachate through the particles. Thus, even distribution of leachate through all of the fine fraction is usually more important than the absolute rate of leachate permeation.

Some of the leachate flowing into the underlying coarse layer will permeate the fines layer below and ultimately to the coarse layer at the base of the heap, but some will also migrate laterally to the bund wall 14 and flow through this bund to a leachate collection point 26. The even distribution of leachate through the heap is controlled by the consistency of ore size in each of the layers, producing a uniform downward flow.

The ability to flood and drain individual sand layers of the layered structure, can be built into the design by sealing the interface between the coarse ore bund and the coarse ore layer with a sealant of low permeability, with flow controlled by an injection pipe through which leachant or air can be injected or withdrawn.

Thus, coarse ore layers 16 can potentially be flooded with leachant, to further enhance contact between the leachant and the values to be dissolved.

The even distribution of leachate and the modest particle size is sufficient to ensure high extractions in acceptable leach duration. Once overall extraction is sufficient, washing and draining of the ore can remove the remaining leachate and leave the structure in a form for easy rehabilitation.

Whilst the layered structure of coarse and fines allows for uniform gravity flow of leachant despite the presence of fines, it does not promote effective aeration.

So, in the case where oxidizing conditions are necessary for leaching, such as leaching a metal sulphide or gold ores, the heap design and operating procedures require specific adaptation to promote successful oxidative heap leaching.

This restricted aeration experienced, particularly by the fine ore layers within the layered structure, and in those parts of the coarse ore layer located far from the edge of the structure, the lack of air access can be overcome using one or more of the following methods.

The first method is to recover most of the sulphide mineralisation prior to heap formation. An ore containing substantial leachable sulphide values such as a copper sulphide ore, can be divided into multiple size fractions. The finer fractions, typically less than 1 mm, can be pretreated using sulphide beneficiation to recover most of the copper, and greatly reduce the oxygen demand in leaching of the layered structure. The coarsest ore fractions which are difficult to beneficiate can be assigned to forming the structural bunds, thus gaining greater access to aeration.

In an extreme case, these sulphide rich bunds can be heap leached before fines are inserted into the structure, thus ensuring excellent aeration. Then, each coarse ore layer in the structure can be mostly leached, before it is covered by the next fines layer which reduces the access of air to the underlying coarse layer.

As an example, for a copper ore of the approximate sizing for this sulphide beneficiation, the coarsest ore (> 0.6mm) is separated for bund construction. This bund construction material has the lowest copper sulphide grade of all the fractions. The bund material it is located where ongoing aeration and hence copper leaching is adequate. The ore < 0.6mm is further classified, with the ore between around 0.1 -0.6 mm being processed by coarse flotation to float the sulphide fractions into a separate concentrate. This coarse flotation concentrate will be ground further and conventionally floated. The residue from coarse flotation will be used to form the coarse ore layers in the heap leach structure. The finest fraction from the classification (< 0.15 mm) is beneficiated by conventional flotation, to form a concentrate which will not be heap leached. The sulphide depleted sand from coarse particle flotation and the sulphide depleted fines from conventional flotation can then be assigned as sequential coarse and fine layers within the bunds. If the sulphide containing ore has an oxidised component which cannot be floated, or any sulphide fractions which were not previously recovered, these flotation residues can then be recovered as previously described.

The second method to overcome any shortfall in oxidant, is to reduce the thickness of the leaching layers and the time available for their leaching to enable substantial dissolution prior to layer being ‘buried’ in the layered heap. As an example, dissolution of a gold containing ore may require some but not extensive aeration. The leachant can be added to the gold ore prior to deposition. Then by adjusting the deposition rate and the fines layer thickness, the amount of aeration of this fraction can be enhanced. In so doing most of the leaching can take place whilst the aeration from atmospheric contact from the layer surface remains sufficient for high extractions.

The third method to overcome any shortfall in aeration is to utilise the permeable coarse layers for forced air injection by pumping air into the layered structure. This ensures effective aeration of the coarse layer and provides for gradual permeation of air through the adjacent fines layers from both above and below.

In a fourth method, the uptake of air can be rendered less critical by use of an alternative oxidant in the leachant. One example might be the cuprous/cupric couple that has been considered for heap leaching in both chloride or ammine solutions. Other examples would be use the of acidic ferric sulphate to dissolve sulphides and the use of cupric thiosulphate to dissolve gold. The addition of this effective oxygen transfer agent makes access to oxygen less critical in the immediate vicinity of a sulphide grain to be dissolved; and also enables very efficient absorption of oxygen from the limited air available in the layered structure. The oxidation of the transfer agent can take place in part through air transfer within the heap, and in part when the leachate is recovered from the heap for recovery of the values and recycle for additional leaching.

There are also cases where heap leaching is best undertaken under mildly reducing conditions. A possible example is the heap leaching of nickel laterites, where conversion of goethite to magnetite is thought to enhance nickel dissolution. In such cases, the design and operation of the layered structure can be adapted to utilise the fines layers to reduce external air ingress during heap leaching. Once leaching is complete, the fifth component of the current invention is the drainage and washing of the leachate from the heap.

The layered design enables displacement washing of the layered structure with wash water injected above each layer of fines, to recover the last of the leachate, and to drain slowly to leave a ‘dry heap’ for rehabilitation.

Leaching in a stacked heap relies upon transfer of leachant through the fines within the fines layer within the structure, and subsequent drainage. Otherwise, the chemistry of stack leach is comparable to these other forms of leaching.

Measurement of the permeability of samples of flotation tailings has been undertaken in the laboratory and the hydrodynamics modelled to estimate the dewatering times for various thicknesses of copper and nickel tailings. Typical dewatering times were periods of weeks for tailings thicknesses of a few metres.

This modelling was confirmed through construction of a Perspex tank of dimensions 1 m ^* 1 m, which was filled with three consecutive layers of sand residue from coarse particle flotation to a settled height of around 0.1 m, and flotation tailings at 50% solids slurry to a settled height of 0.35 m. Each sand layer had a tap which enabled draining of the water from within the sand layer. The tank was instrumented with probes to enable the water content and pore pressures within the layers to be measured, and hence follow the migration of water through the structure.

When the taps were opened at each sand level, the phreatic surface of the water migrated steadily down through the layers, illustrating the flow of water is partially out through each sand layer and partially down through the tailings. The rate of drainage in each tailings layer could be controlled by opening or closing the valves in the underlying sand layer. The dewatering continued asymptotically from the lowest sand layer, towards a final water content of around 15% by weight water.

Whilst not intended as an exclusive list or limited to particular ore types or possible leachants of a chemical or biological nature, the following examples of the possible applications of the stack leach structure are listed below.

• Oxidised copper ores - leaching with sulphuric acid or ammonia

• Nickel laterites - leaching with sulphuric acid or ammonia

• Primary copper ores - leaching flotation tailings with ammonium chloride or acidic copper chloride

• Secondary copper ores - leaching floatation tailings with acidic ferric sulphate

• Nickel sulphides - leaching flotation tailings with ammonium chloride

• Gold ores - leaching of agitation leach or flotation tailings in cyanide or thiosulphate

• Zinc sulphides - leaching flotation tailings with acidic ferric sulphate

• Uranium - leaching with acidic ferric sulphate.

The stack leach method is potentially applicable to a wide variety of ore types; especially where natural leaching rates are slow, or ore grades are too low to justify fine grinding followed by agitation or vat leaching. It is also applicable to those ores where leaching rates are acceptable, but the filtration of the leach residue is slow.

Benefits of the Invention

The principle advantage of the current invention, relative to conventional heap leaching, is the increased extraction achievable from ores. This increased extraction arises from both the smaller particle size used in the layered structure, and the even distribution of leachate through the heaped ore. The optimum size of the crushing that can now be selected to enable high extraction in an acceptable leach period, without concern about fines reducing the permeability in parts of the heap. This enables the inherent benefits of heap leaching to be applied to processing higher grade ores, which otherwise would not be considered appropriate for heap leaching.

An advantage of the current invention relative to conventional flotation or agitation leaching is the ability to treat ores with both oxide and sulphide mineral values with high recovery from both mineral types. As examples, a copper ore containing mainly sulphide which has been ground fine enough to float most of its sulphide component, is too fine for subsequent heap leaching. Using the layered structure, the flotation residue can now be heap leached to recover the oxide fraction. And for the highly oxidised copper ore, agitation leaching will only dissolve a small proportion of the sulphidic copper. The layered structure enables simultaneous heap leaching of both the oxide and sulphide mineral types.

Another advantage of the current invention, relative to conventional flotation or agitation leaching, is the lower cost of comminution to a size where high extractions can be achieved. And the cost of containment and processing is also substantively reduced through use of heap leach in place of agitated vessels.

Where the heap leaching of the sulphide ore is problematic for reasons of ore passivation, for example with primary copper ores containing substantial proportion of chalcopyrite, the layered structure enables an efficient integration of coarse and conventional flotation. This dual approach recovers much of the chalcopyrite through coarse and conventional flotation, and the more effective drainage of heap leach reagents enables the use of expensive reagents such as copper chloride or ammonium chloride for leaching the remaining chalcopyrite from the oversize fraction.

In summary, the layered structure is a low-cost reactor, where most of the construction can be undertaken by hydraulic deposition. The reactor can either by operated in flooded mode, where ores are saturated with leachant, as would be achieved in vat leaching. Alternatively, the reactor can be operated in drained mode, as would be achieved in a heap leach. The layered structure enables high extractions from the ore. The reactor can be operated in multiple ‘lifts’ and hence requires a small area for its location. The structure allows for in-situ solid liquid separation and can be washed and dewatered to almost quantitatively recover the leachate, and to convert the leached heap into a useful landform.