OA21070A - Heap leaching. - Google Patents

Abstract

This invention relates to a method of recovering metal values such as gold, copper, nickel, zinc and uranium from ores containing said metal values. The method includes the steps of crushing an ore (10) to provide a sand containing metal values with a P80 of less than 5mm but greater than 1mm; classifying the sand (12) to remove a finer fraction to provide classified sand with a P10 of greater than 0.15mm, and a P90/P10 ratio of less than 25 and greater than 3, forming a heap (18) from the classified sand, and distributing leachant and air through the heap to leach the values from the sand in a pregnant leachate, from which the leached values may be recovered. The invention also relates to a heap formed from ore processed by this method.

Description

HEAPLEACHING

BACKGROUND OF THE INVENTION

Conventional heap leaching provides a low cost and water efficient method of métal recovery, but suffers from low extractions of the contai ned values due to • zones within the individual rocks with inadéquate conditions for leaching caused by micropermeability within the rock heap • zones within the heap with inadéquate conditions for leaching caused by variable macropermeability in the heap; and • reprecipitation of values caused by problematic gangue causing localized macropermeability

These low extractions mean heap leaching is only used for processing low grade ores, where the low cost is a more important factor than high recovery. For most of the worlds production, finer grinding and dotation or agitation leaching îs the preferred processing route.

Micro-permeability is term used to describe the ease with which leachant can access the contained values within the solid particles, allowing dissolution of the values, and then remigration of prégnant leachant out from the particle to ultimately be recovered through gravity at the base of the heap. This level of micro-permeability can be estimated using X-ray tomography. (Miller - Int. J. Miner. Process. 72 (2003) 33 l- 340), the content of which is incorporated herein by reference.

The greater the exposure of the mineraiised particles to leachant, whether it be through grain exposure on the surface of a gangue particle, or through a microcrack in the sunOtinding gangue, the greater the recoverable minerai of value.

The largest déterminant of micro-permeability is particle size. Smaller diameters increase the probability that the valuable minerai grain is located either on the surface of a particle, or at least accessible in a crack large enough for acceptable leachate access rates. For example, in the work of Miller, a copper ore showed exposures exceeding 90% at below 3mm.

-2 But the micro-permeability is also a function of the way the rocks are crushed. It is also dépendent on the mineralogical properties that affect rock fracture under stress.

The ultimate extension of this micro-permeability benefit is agitation leaching, where finely ground ore can be leachcd at rates and total extractions that are determined by the Chemical reaction rate, rather than through intra-particle diffusion. But agitation leaching cornes at a considérable capital and operating cost of the grinding and agitation leaching equipment; and becomes impractical for low grade ores or leach durations beyond around 24 hours.

For heap leaching, sol vin g the micro-permeabtlity constraint by crushing ftner, créâtes a different set of constraints in the macro-penneability of the heap. The term macro-penneability îs used to describe the permeability to fluid flow that exists through the bulk of the heap, i.e. over distances of centimeters or métrés in the various locations within the heap.

The macro-permeability of a heap decreases as the crush size is reduced, due to excessive proportion of fines impeding the flow of both leachant and air through the heap. Even at a reasonably coarse crush size, e.g. 100mm, ségrégation can occur during heap formation and compaction during operation, due to the wide particle size distribution.

Variable macro-permeability can impact both air and leachant flows within some sections of a heap, such that low leaching extractions are achieved in some zones either due to localized flooding or a defîciency of leachant within the zone of low permeability, or in the ‘rain shadow’ caused by this the low variability zone, or poor aération through the section of the heap.

This variability exists because of accumulation ofthe fines in the ore, resulting from either fracture and ségrégation during heap préparation; or by excessive comminution. They tend to further consolidate during stacking of the heap and leaching. The fines block ongoing leachant access to a zone within the heap.

Illustrating this factor is the Hazen équation (https://agupubs.onlinelibrary.wiley.com/doi/full/lO. 1002/2017WR.020888) which empirically relates the macro-permeability of any material to the 10^,h percentile of the particle size distribution în any zone within the heap.

-3The macro-permeability of an accumulation of particles is a function of absolute particle size. It îs also affected by the particle shape and the particle size distribution which défi ne the void ratio in the heap. Void ratio is important because mixtures of different particle sizes will naturally consolidate to a higher packing density, with the finer particles filling the interstices of the coarser particles.

Another expression of the macro-permeability is hydraulic conductivity. However, for heaps formed from ver y different particle sizes, this measure may be very different in the different zones within the heap. Thus, another effective measure is the time taken for the heap to drain.

So, in conventional heap leaching, the primary déterminant of macro-permeability of the heap is absolute size achieved during crushing. In effect, this crush size affects the proportion of fines generated during the crushing process. When the particle size becomes small, a layer of adhèrent liquid accumulâtes around the particles. Where this layer thickness is of a similar magnitude to the gap between sand grains, the flow of either liquid or gas phase is inhibited. A second déterminant is the relative particle size distribution, where unitbrmly sized particles hâve a higher conductivity than wide size ranges, as the latter can pack more tightly during heap consolidation.

For these reasons relating to macro-permeability, the top crush size of normal heap leaching is typically between 10mm and 500mm, thus avoiding the formation of excessive fines.

To reduce the impact of fines in conventional heap leaching, fines are sometimes agglomerated prior to heap construction. Agglomération causes the fines to adhéré strongly to the coarser rocks. With well controlled stacking to prevent excessive deagglomeration, it results in improved macropermeability between agglomérâtes, but has an adverse micro-penneability impact within each agglomerated entity. For this reason, the leachate is typically used as a binding agent for the agglomerate. This reduces the magnitude of micro-permeability issues caused by the coating of fines.

Whilst finer crushing and agglomération can increase extractions for some ores that are well suited to heap leaching, the balance of cost and benefits does not make it effective for ail ores. Nor does agglomération allow for varying the operating conditions of the heap, for example by utilising multiple leachants to treat different minerai species. Nor does agglomération fully overcome the issues of access of the leachant to valuable material locked in the coarser substrate pebbles, that form the centre of the agglomérâtes.

-4So, a balance is sought in conventional heap leaching involving either; coarser crushing with acceptance of a modest extraction in the heap leach (typically around 65%); or crushing to a finer size of around 12.7mm and agglomerating the fines prior to stacking to achieve a slightly higher extraction (typically around 80%).

Whilst not in commercial practice, physical removal of the fines prior to heap leaching has also been suggested. To optimise processing of fine component of ores by a beneficiation, both WO2016/170437 and US6146444 remove the fines for separate beneficiation, prior to heap leaching the remaining ore.

Both these patents are at a finer grind than has typically been used in conventional heap leaching. They are both directed towards novel processing routes from the finer fraction of the ore. Both nominate heap leaching of the residual coarser fraction of the ore, containing a modest proportion of the total values, following the size classification for the primary mode of values recovery.

The particle size claimed by WO2016/170437 is limited to an upper size of l mm, thus constraining the proportion of values recoverable b y heap leaching, rendering heap leaching a minor method of values production. Heap leaching of the ore above Imm is not considered.

And for US6l46444, the heap leach is directed to gold libération from pyrite, not direct gold extraction. Thus, quantitative extraction of the pyrite is not the key objective of the leach, in the same way that it would be if pyrite were the primary value.

Neither author considers the impact of the fines removal on the macro- and micro-permeability of the coarser fraction during its heap leaching, and the extraction efficiency and flexibility in heap operation.

The size séparation in US6146444 is by wet or dry screening of an ore crushed to between 6 and 20mm. The screening occurs at between 0.6 and 2mm, with the fine fraction being assigned to other beneficiation methods to recover pyrite and leach gold. The oversize fraction (>0.6-2mm) represents around half the weight of the ore, up to a top size of 25mm, is assigned to heap leach to dissolve pyrite. This heap leach is supplemented by adding back pyrite recovered during flotation or gravity séparation of the finer fraction. The additional pyrite not only libérâtes more contained gold, but also accélérât es bio leaching in the heap. These combined effects lead to higher gold extractions in a separate leaching process.

-5it is apparent to people ski lied in the art, that the removal of fines by U S6146444 will partially résolve issues of macro-permeability in the heap, particularly the desliming as was noted by US6146444. However, the quantitative impact ofthe removal ofthe ore smaller than 0.6-2mm on 5 heap macro-pemieability is unclear.

With respect to micro-permeability, LJS6146444’s upper crush size is only shghtly finer than the typical agglomération size in conventional heap leaching, and hence the issues of micropermeability remain.

1Û

This impact of micro-permeability on leaching rate of pyrite is clearly demonstrated in Figure 2 in US6146444, where the dissolution of 0.25-inch material, the fmest crush size claimed, îs slow. Only around 15% of the pyrite is bio-oxîdised in 300 days, compared to 55% extraction at 2mm. Whilst these extractions may be satisfactory for partially removing a problematic element such as 15 pyrite on a proportion of the total ore, they are inadéquate for recovery of the primary values during normal heap leaching.

WO2016/170437 follows a different comminution and beneficiation path, grinding the ore to a finer size, p80 less than 1mm and most preferably less than 0.6mm, then applying coarse particle 20 flotation in a teeter bed reactor. Coarse particle flotatîon recovery up to around 0.5mm is efficient, leaving a disposable residue. If the grind size is extended up to the 1mm limit of the claims, the coarse particle flotation process is split to generate a middlings residue stream. Recovery from this 0.5-1mm fraction of the ore is somewhat lower, due to the reduced libération of values during comminution. Hence the middlings residue is still contains significant values. WO2016/170437 25 notes that this residue is at a quite a low grade and suitable for storage or for heap leaching.

With these preferred and upper size dimensions in the claims, the middlings residue from coarse flotation will represent between 0-30% of the total weight of the ore being comminuted. And due to natural deportment during comminution and partial extraction ofthe values by coarse flotation, 30 it will typically contain less than 10-20% ofthe total métal values. As such, heap leaching is not a major component ofthe overall production.

No teaching is provided by WO2016/170437 on the impact ofthe middlings préparation on either the heap leaching conditions or heap préparation. Nor is guidance provided on methods b y which 35 the majority of the values could be recovered from this middlings fraction by heap leaching.

-6In a separate patent relating to heap leaching after removal of fines below around 0.5mm, WO2018/234880 utilises heap leaching a scavenging mechanism for the low-grade ore fractions rejected during bulk sorting, and coarse particle flotation, combining these streams from which the fines are removed into a heap for heap leaching, Optionally, further intermediate size classifications may be introduced, with the coarser ore fractions added to the heap leach feed.

Whist the removal of fines by WO2018/234880 will enhance the macro-permeability, the particle sizes from bulk sorting and screening are typical of conventional heap leaching and such that micro-permeability issues will remain,

The range of particle size distributions will be very wide and hence issues of macro-permeability will also occur due to consolidation in parts of the heap.

Retuming to conventional heap leaching, a further complication exists for the most abundant copper ores, which contain significant quantifies of chai copyrite. The chai copyrite reacts very slowly under normal heap leach conditions.

Other conditions hâve been identified for leaching primary copper ores containing significant chalcopyrite. Controlling the leach within a spécifie range of oxidation potential formed with the cupric-cuprous couple, in a high chloride acidic environment enables acceptable chalcopyrite leaching rates for considération in conventional heap leaching. (Muller - WO2007/134343A2).

Simîlarly, leaching with a ferrie sulphate solution more typical of that produced during biooxidation in a heap, at températures over around 60°C, enables acceptable chalcopyrite leaching rates for considération in conventional heap leaching. (Robertson - J. S. Afr. Inst. Min. Métall. vol.l 12 n.!2 Johannesburg Jan. 2012).

However, the macro- and micro-permeability of conventional heaps make these higher cost leachants problematic for conventional heap leaching of primary copper ores. For ex ample, utilising the acidic copper chloride solution over the extended heap leaching period consumes significant acid, and locks up substantial working capital, and results in excessive reagent dilution and losses over the full heap leaching cycle. In the case of high température heap leaching, initîating and maintaining the full heap at températures in excess of 60°C over the extended period of conventional heap leaching, requîtes significant extemal heat input.

-7 For ail these reasons, commercial heap leaching of primary copper ores has been limited to opportunisme leaching, with copper extractions up to around 20%. The chalcopyrite content of these ores goes largely unleached.

So, despite the many efforts to optimise conventional heap leaching, the overall extraction of metals using heap leaching technology remains lower than that achievable by dotation or agitation leaching of the same ore. Conventional heap leaching relies on lower costs for its applications and is primarily directed to treat low grade ore resources that can be readily dissolved.

In summary, the macro- and mîcro-penneability constraints resuit in conventional heap leaching being a second-tier method of metals production.

SUMMARY OF THE INVENTION

THIS invention relates to a method of recovering meta! values such as gold, copper, nickel, zinc and uranium from ores containing said métal values such as gold ore (including pyritic gold ore and copper gold ore), copper ore (including copper sulphide, primary copper, secondary, transition and oxidised copper ore), nickel ore (including nickel sulphide, mafic and uttramafic nickel ore), zinc ore, and uranium ore în a sand heap with high macro- and micro-permeabilîty.

The method includes the steps of:

• crushing an ore containing métal values to a size where at least 85% of the valuable minerai grains are exposed, to provide a sand containing métal values with a Pso of less than 5mm, and preferably less than 3mm, and even more preferably around 2mm, but greater than Imm;

• classifying the sand (i.e. passing the sand through a screen or screens) to remove a finer fraction (î.e. to remove particles less than 0.1mm, or less than 0.2mm, or less than 0.3mm or less than 0.4mm in size), to provide classified sand with a P to of greater than 0.15mm, or greater than 0.25mm or greater than 0.3mm or greater than 0.4mm, and a P90/P10 ratio of less than 25, less than 20, less than 18, or less than 15; and greater than 3, greater than 5, or greater than 8, and preferably a water permeability greater than 10' ⁵ m/s, more preferably greater than 5xl0^-4 m/s;

• forming a heap from the classified sand, where the heap preferably has a permeability greater than 10^-5 m/s, more preferably greater than 5x10^-4 m/s; and • distributing leachant and air through the heap to leach the values from the sand in a prégnant leachate, from which the leached values may be recovered.

Typically, sand heap leaching is used as the primary recovery method and more than 50% of the ore is recovered as sand, and processed by sand heap leach, and preferably more than 60%, and even more preferably around 70%.

Typically, there is no prior beneficiation step such as flotation, gravity séparation or magnetic séparation on the ore assigned to the leaching step.

The sand heap leach may be undertaken in a fixed or a dynamic heap with a résidence time of less than 2 years, and preferably less than 6 months and even more preferably less than 3 months.

The heap is preferably free draining, to achieve less than 15% contained water within 2 weeks of ceasing irrigation, and preferably within I week, and even more preferably around 3 days.

The heap may be subjected to more than one irrigation and drain cycles, to sequentially enhance aération and leaching.

Multiple leachants may be used sequentially to remove problematic gangue and then to recover the valuable components from the sand heap. For example, an ore containing both copper and gold could be heap leached initially to extract the copper, then washed, and subsequently leached with a different reagent to extract the gold.

Losses of leaching reagents, and management of water balance, may be reduced through efficient washing and draining ofthe leached sand heap.

The sand may be deposited on the heap by being flung from a discharge point using a hydraulic or mechanical device

The sand may be stacked in lifts of height of greater than 5 meters and preferably greater than 10 meters, even greater than 20 meters and up to 40 meters.

The sand may be leached in a dynamic heap, which is then removed from the dynamic pad by hydraulic mining techniques. The term “dynamic heap” means a heap which is constructed on a

-9fixed pad, leached, and then redaimed for storage elsewhere, leaving the pad available to leach further ore.

THIS invention also relates to a sand heap with high macro- and micro permeability, the sand in 5 the heap comprisîng a crushed ore containing métal values such gold, primary copper, secondary copper, nickel, zinc, and uranium, and the sand having a particle size P ιo of greater than 0.15mm, or greater than 0.25mm or greater than 0.3mm or greater than 0.4mm, and a PWPio particle size ratio of less than 25, less than 20, less than IS, or less than 15; and greater than 3, greater than 5, or greater than 8, and preferably a permeabilîty greater than 10'³ m/s, more preferably greater than 10 5xlÜ’⁴m/s.

The sand heap may be stacked in lifts of height of greater than 5 met ers and preferably greater than 10 meters, even greater than 20 meters and up to 40 meters.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1	is a block flowsheet of a method of heap leaching of the présent invention;
Figure 2	is illustrations of heap construction methods;
Z U Figure 3	is a graph showing the extraction of copper during the column leaching of a copper ore prepared into different particle sizes using acidic cupric chloride or ferrie sulphate. Lines represent solution-based extractions and data points represent mass balanced extractions;

Figure 4	is a graph showing the inventory-corrected total copper extractions as a function of particle size and time;
Figure 5 ο n	is a graph showing the minerai extractions as a function of particle size and time;
□ U Figure 6	is a graph showing hydraulic penneabîlity of several samples as a function of maximum particle size and the P90/P10 ratio, or sorting coefficient;
Figure 7	is a graph îllustratîng the degree of saturation as a function of application rate for
35	samples of a copper ore prepared into different size fractions;

-10Figure 8 is a graph showing the dependence of air conductivity on application rate for the copper ore samples reflected in Figure 7; and

Figure 9 is a graph showing the drain down profile of the copper ore samples reflected in Figure 7, on cessation of irrigation.

DESCRIPTION OF PREFERRED EMBODIMENTS

The current invention is a method in which sand is prepared and stacked such as to form a heap with suitable macro- and micro-permeability, to yield faster and higher extractions of the metals of interest.

The sélection of particle size is cri ti cal, to achieving both the micro-permeability and macropermeability required for rapid and complété heap leaching.

The création of macro- and micro-peirneability of the ore particles in the sand heap not only enables high extractions in sand heap leaching, but also créâtes heap properties which enable effective use of a wider range of lcachants. Examples of such leachants are those of higher cost, such as using cupric chloride as an oxidant, or glycine as a complexant, where the working capital and reagent losses are excessive in a conventîonal heap leach.

The macro-permeability is achîeved by preparing a sand with a high exposure of the minerais of value, at least 85%, and within a narrow size distribution, and with a lower size limit to allow free draining of leachant from the heap due.

This combination of properties enables sand déposition without excessive consolidation during the heap formation. The ratio of PWPio sizes ensures a satisfactory voîd ratio. With this narrow size distribution, the leachate and air can flow uniformly through the heap, whilst the leachant can access most of the available minerai species of value.

The lower size limit (Pio) is set to generate a free draining sand heap, i.e., the hydraulic conductivity will exceed 10^-5m/s, allowing the heap to drain within days to less than 15% contained moisture. The lower size limit is essential to achieve heap macro-permeability and is described by the Hazen équation. Résultant is the ability to drain the heap to achieve a high and uniform

-11recovery of leachate. The void ratio must be such that even during irrigation, air is able to flow between particles, to maîntain the oxîdation potential in the heap.

The optimum quantum of relatively fine sand is also dictatcd by the balance between gravitational and capillary forces, with sufficient fines present to enable latéral transfer of leachant through the heap. This is typically >5% by weight in the heap.

The upper size limit (Pw) of the sand is set to ensure effective micro-permeability enabling high extractions, and to ensure the void ratio within the heap is adéquate.

The present inventors hâve quite unexpcctedly found that with the micro-permeability and macropermeability characteristîcs attained by the process of the present invention, extractions using sand heap leaching can be increased exponentially to those achievable by fine grinding and extended agitation leaching, and even superior to that achievable by alternative recovery technologies, such as flotation.

As previously noted, the upper size at which the values will be suffi ci ently exposed to enable leaching of a particular ore, will be dépendent on grain sizes of the vahiable minerais, and the fracture properties of the minerais and gangue. And in practice, the acceptable extraction will be also dictated the head grade of the ore being leached to form a disposable residue.

For example, a coarse grained, low grade copper ore, which had been previously exposed to coarse benefidation; might hâve an upper size of 5mm with an exposure of around 85%, whereas a fine grained, high grade copper ore would require finer comminution to achieve a disposable residue after sand heap leaching. Above around 5mm, the differential fracture along grain boundaries is insufficient to generate the required microcracking.

Hence, according to the present invention, the preferred upper size of the comminuted ore is a Pso of less than 5mm, and preferably less than 3mm, and even more preferably around 2mm, but greater than 1mm.

To meet the macro-permeability requirements the ore must be of sufficient diameter that the heap is free draining with permeability exceeding 10'⁵ m/s, and preferably greater than 5x10’⁴ m/s. This requires a Pio of greater than 0.15mm, and preferably greater than 0.25mm. Achieving an effective void ratio requires a P90/P10 of less than 20, and preferably less than 15.

- 12To achieve these macro and micro-pemieability criteria, the comminuted ore must be classified to remove the fines prior to assignaient of the coarse fraction to sand heap leaching. Through efficient comminution and classification, up to around 70% of the ore can be assigned to sand heap leaching, within the specified size limits.

The remaining finer ore must be processed separately by flotation or agitation leaching. Like conventional heap leaching, but unlike US6146444, WO2016/170437 or WO2018/234880, sand heap leaching can be the primary method of values production; with supplemcntary production from the fines.

With reference to Figure 1, in an embodiment of the invention, ore is crushed 10 (in a crusher such as a HPGR (high pressure grinding rolls), S AG (serai autogenous grinding) mill, VSI (vertical shaft impactor), or Cône crusher), typically to less than 5mm in size, to provide a p80 of less than 5mm. Crushed ore is then classified (i.e. screened) 12 to remove fines 14 less than 0.4 mm in size and to provide a sand 16 with a particle size greater than 0.4 mm and a PWPio of about 12.5. The sand 16 is deposited in a heap 18 which has a typical hydraulic conductivity of greater than 5x10' ⁴ m/s. The heap 18 is subjected to a heap leach treatment with leachate 22 from which product 24 containing métal values is obtained, and which is recirculated to the heap leach 20. After the heap leach is completed a sand 26 depleted of métal values may be disposed of.

Leaching Flexibilîty Arising from Sand Heap Leach

The macro and micro-pemieability achievable in a sand heap with this particle size distribution create some additional characteristics that are quite different from a conventional heap. The distribution of leachant and air flows within the narrow particle size distribution of the sand heap is very uniform; the time required to achieve high extractions in sand heap leaching is short; and the heap drains uniformly and rapidly to a low moisture content.

These three unique characteristics of the sand heap provide a flexibilîty to adjust and control heap leach conditions, in ways that are not feasible in conventional heap leaching.

The uniform distribution of leachant and air within the sand heap enables ail zones of the heap to be exposed to effective leaching conditions. Indeed, through adjusting heap construction and operations, factors such as the oxidation potential and heap température can be controlied to a greater level of unifbrmity in the various zones within the heap.

-13 This improved control of oxidation potential within the heap is of particular relevance to leaching of primary copper ores, where tight control of potential in both sulphate and chloride solutions avoids the passivation of chalcopyrite. (Watling - Hydrometallurgy 140 (2013) 163-180), the content of which is incorporated herein by reference.

As a conséquence of the improved leachant and air distribution, higher extractions can be achieved.

The second source of flexibility arisîng from sand heap leaching is the high micro-permeability leading to a much shorter résidence time to achieve a high métal extraction.

For those minerais such as free milling gold, secondary and oxidised copper ores, in which the Chemical dissolution is fast, i.e. will leach completely within a couple of days of agitation leaching under ambient conditions, leaching times in a sand heap leach can typically be reduced to less than 3 months, and even less than l month.

This enables sand heap leaching of such ores to be undertaken on dynamic pads, at a rate and recovery commensurate with that achievable in vat or agitation leaching, and significantly higher than conventional heap leaching. These high extractions can be achieved without the infrastructure required for comminution to a fine size and materials movement during the vat or agitation leaching process.

The faster leaching rate achievable with sand heap leach also generates higher ténors of the prégnant leachate, enabling a réduction in the volume of leachate to be processed in subséquent métal recovery.

The leaching reaction for most ores is exothermic. Thus, the leaching créâtes a température increase in the heap, particularly where sulphide oxidation îs occurring at a rate greater than heat losses from the heap. As an example, when conventionally heap leaching of copper ores, températures of up to around 70°C hâve been rccorded in some zones within the heap. These température increases support more rapid bio-oxidation and increased diffusion rates to raise micro-permeability within the particles. The use of sand heap leaching provides a faster leaching rate of the readily oxidised secondary copper minerais, and hence a greater température increase in the heap.

This température increase contributes in part to the higher extractions that can be achieved using sand heap leaching of secondary copper ores.

-14The faster temperature increase also provides a method for initiating the oxidatîon of chai copyrite leaching, hence generating further heat. Furthermore, the relatively short period required to overcome micro-permeability constraints within particles to be leached, reduces the duration of that the heap must be maintained at the elevated températures to leach most of the chai copyrite. Thus, sand heap leaching, as indîcated in the current invention, enables the heap leaching of primary copper ores.

In yet another option for leaching primary copper ore at elevated températures, external heat înputs can be provided by techniques such as solar heating of the leachant. Nonually, the résidence time over which the heap must be retained at the elevated temperature is excessive, but with the shorter leach duration enabled by high micro-permeability, the potential for external heating is increased.

The third source of flexibility arising from sand heap leaching, is the free draining nature of the sand.

This efficient draining ensures a sharp tail of eluate on the completion of heap leaching. With a low residual lcachaie concentration in the heap, and with micro-permeability which enables quick release of the remaîning leachate. It also means the heap can be washed without major dilution of the leachate. The losses of reagents are lower, and the water balance of the sand heap leaching is more readily managed.

As such the sand heap provides opportunités for the use of expensive leachants which cannot be economically considered in conventional heap leaching, where fluid flows are much less consistent and leachate entrainment within the heap is higher. One example might be the use of acidic copper chloride to leach primary copper ores. Other examples are glycine to leach copper or nickel sulphide ores, and more concentrated cyanîde solutions to accelerate the leaching of gold.

The free draining and uniform nature of the heap also enables the intermittent application of leachate followed by a rest period during which most ofthe voidage in the heap is filled with air, without concerns about accessing zones which remain flooded or hâve been starved of leachant. This resting has been found to be bénéficiai in a number of conventional heap leaching operations.

The free draining nature of the heap also allows sequential use of different leachants without significant cross contamination between the leachants. This enables the use ofdual leachants in a single heap, to initially remove problematic gangue, before recovering the minerai of interest, such

-15 as with pyritic gold ores. It also provides for the opportunity to sequentially leach copper gold ores.

Recent developments in the conventional heap leaching of primary copper ores indicate that high extractions of chalcopyrite can be achieved over a period of several years using acidic copper chloride in strong brine solutions. However, the gangue éléments present in the ore consume significant acid, and since pyrite is not oxidised at the system’s oxidation potential, this acid represents a consumable cost. Beeause the heap is free draining, it is possible to undertake a conventional heap leach first, to use the acid generated from pyrite in the ore to neutralise the basic gangue, then couvert to the copper chloride system to leach the chalcopyrite content.

A similar pre-neutralisation can occur with nickel sulphide ores, with the use of pyrite and pyrrhotite generated during the flotation of the fines, to supplément the acid génération during sand heap leaching.

Extraction of gold, whether by agitation or conventional heap leaching, is usually limited to free milling gold ores. For those ores where the gold is locked in pyrite, either very fine grindîng or prior oxidation of the pyrite is required to liberate the gold.

Bio-oxidation of pyrite using is one well known as a method of liberating the gold, and heap leaching is a low-cost method to achieve the libération. But to recover the liberated gold using cyanide has complications. The bio-oxidation of pyrite takes place in an acidic environment and heap leaching is undertaken in a basic environment containing cyanide. The mixing of the two Systems is hazardous, and the reagent requirement to neutralize the heap prior to gold leaching is high. Hence processes similar to that of US6l 46444, use heap leaching to liberate the gold, and follow this with milling, neutralisation and agitation leaching to recover the liberated gold.

A well-drained sand heap enabled by the current invention enables such a dual leachant approach, initial bio-oxidation in an acidic environment, followed by draining and neutralizatîon, then cyanide heap leaching without hazard, with minimal additional reagent cost, and without the need for fine milling and agitation leaching.

Similar opportunities are created in gold ores with a high soluble copper content.

-16Ex amples of ore types that can be processed according to the présent invention, leachants and in some cases sequential leachants are provided in Table l below.

Table l

Ore type	Leachant Opportunities	Indicative p90 mm	Indicative plO mm	P90/P10 ratio	Sequential leachant required	Second leachant
Secondary and	Bioleach	4	0.2	20	No
oxidiscd Cu
	Acidic	4	0.2	20	No
	chloride
Transition and primary	Hot bioleach	4	0.4	10	No
copper				1
	Acidic chloride	4	0.4	10	No
	Glycine	3	0.4 _	1 7.5	No	—
	Ammonia	3	0.4	7.5	No
Primary	Hot bioleach	4	0.4	10	Yes	Cyanide
copper gold
Gold	Cyanide	3	0.2	15	No
Refractory gold in sulphide	Bioleach	3	0.2 I	15	Yes	Cyanide
Mafic	Acidic	3	0.3	10	No
nickel	bioleach
	Near neutral	3	I 0.3	10	No
	bioleach Glycine	3	I 0.3	10	No ”	I
	Ammonia	3	0.3	10	No
Ultramafic	Neutral	3	0.2	15	Yes	Ammonia
nickel	bioleach					or
			I			1 glycine
	Bioleach	3	0.2	15	No
	Glycine	3	0.3	1 10	No
	Ammonia	3	1 0.3	. io	No
Zinc	Bioleach	4	0.4	10	No
Uranium	Bioleach	4	0.3___	13.3	No

- 17Heap Construction Flexibility Created by Sand Heap Leach

The size range of sand in the current invention, as specified to meet the macro and micropermeability requirements of sand heap leaching, also créâtes opportunities for different methods 5 of heap construction and different heap designs.

Conventional heaps are generaily constructed by dump truck, but this causes issues with over compaction and fines génération caused b y the pressure from the heavy equipment travers! ng the heap prior to and after dumping. An alternative method of conventional heap formation is a 10 retreating conveyor Stacker. This technique is expensive, and the infrastructure is fixed in location relative to the heap it is creating. Whilst both these techniques can also be used for sand heap formation, the unifomily sized sand can also be ‘flung’ in multiple dimensions, either hydraulically or mechanically, from an easily relocated sand discharge point, (see Figure 2), which shows sand being deposited hydraulically using a high pressure water gun to carry the ore to be leached, and 15 then drain prior to commencing leaching.

In this way the sand heap can be formed without vehicle access and without equipment that constrains heap location and dimensions in the déposition cycle.

The uniform sand size also enabies use of hydraulic mining techniques to recover the spent heap 20 and pump the résultant residue slurry to a location for permanent disposai. As such, the potential for dynamic heap leaching on a permanent leach pad, is further enhanced, over and above the benefits of short résidence time noted previously.

Conventional heap leaching typically has a lift heîght of 5-10m, to retain effective vertical 25 irrigation through the heap. Because of the even particle size of sand and the consistent macropermeability, a reduced ability to consolidate, and the ability to drain and rest, this lift height for sand heap leaching can be increased significantly, particularly where under heap access is provided for forced air ingress.

The relatively small size of sand enabies placement of air pipes within the heap, and hence reducing the déplétion of oxygen in some zones, as the air flows through the heap. This placement of air pipes can be through drilling into the formed heap to inject the pipe; or as permanent fixtures in a dynamic heap which is formed around the fixed air pipe, with the sand later removed by hydraulic mining. Such an approach enabies further extension of the heap height.

- 18The uniform size of the sand in the sand heap leach créâtes an idéal distribution path for the flow of leachant and air through the heap. Ségrégation is limited during construction. Issues in conventional heaps associated with ‘ratholing’ and ‘dead zones’ are avoided in a sand heap of narrowly sized particles. This improved flow means that irrigation ofthe sides ofthe heap, and the aération of the centre of the heap are much less problematic in a sand heap leach.

Experimental

Various size fractions of a transition copper ore were prepared by crushing the ore to -2.4mm, 6.7mm and -25mm. The crushed fractions were then screened to yield relatively narrow particle size distributions as described in Table 2, which demonstrated excellent macro-permeability. Drainage of a 1m column of these sands to less than 8% moisture, on cessation of irrigation, occurred in a few hours as shown in Figure 9.

These fractions, containing 30-40% of the copper as clialcopyrite, were leached in 1m columns at 25°C using acidic cupric chloride at various pH, sait and cupric ion concentrations. Figure 3 présents copper extractions calculated by solution balance that are uncorrected for inventory changes as well as, where available, mass-balanced definitive extractions. These results show that high extractions of the transition copper ore could be achieved, with the finer ores dissolving more rapidly and completely. The chloride system results presented in Figure 3 represent identical experimental conditions apart from the use of intermittent irrigation for the -2.4mm fraction. In these tests, high extractions of the easily dissolved components in the ore were achieved in around 10 days, whilst the more refractory component, i.e., chalcopyrite, was largely extracted within 150 days. The decreasing rate and extent of copper extraction with increasing particle size highlights the effect of micro-permeability.

Table 2: Particle size characteristics of column leach samples

Sample Name	Pio (mm)	P50 (mm)	P90 (mm)	P90/P10
-l.25mm	0.17	0.38	0.79	4.5
-2.4mm	0.58	1.43	2.16	3.7
-6.7mm	0.51	3.30	5.97	11.7
-25mm	1.18	13.20	22.55	19.1

- 19To further demonstrate the impact of micro-permeability on achievable extractions, samples of the same ore were ground to less than l ,25mm before utilîzing coarse particle flotation to recover a concentrate and reject a lower grade sand, The résultant sand, which was towards the more difficult end of the size distribution for optimum macro-penneability, illustrated acceptable hydraulic conductivity for leaching but a significantly higher degree of saturation at équivalent flows as compared to the coarser sand fractions, as shown in Figure 7, which shows an exponentîal improveinent in degree of saturation for the fractions with a particle size with a Pso of greater than Imm. From Figure 7 it îs clear that in order to achieve a suitable degree of saturation, a particle size with a P80 greater than around Imm is required. In leaching Systems where reprecipitation of such species as oxides of iron, or sulphates of calcium and aluminium, or where the formation of elemental sulphur occurs, this higher degree of saturation may become increasingly problematic. The précipitation of such species within the heap, is common in many heap leach applications,

The -l ,25mm sand, was leached in both acidic cupric chloride and ferrie sulfate leachants at 25°C in identical conditions to the other fractions. Figure 3 shows that extraction in the chloride system was more than 85% in 100 days, with the chai copyrite proving the slowest minerai species to dissolve. In the sulfate system, 75% extraction was achieved in the same time, with high dissolution of each copper minerai species except chalcopyrite.

Figure 4 more clearly illustrâtes the increasing overall extraction with decreasing particle size, consistent with the previously quoted work of Miller et. al. on minerai exposure vs. particle size. Most surprisingly, there was also a marked increase in leaching rate associated with the particle size, suggestîng much improved access ofthe leachant to the surface ofthe valuable minerai grains. The idéal size range for rapid and complété extraction under relatîvely miid leaching conditions is less than around 6mm.

This effect of particle size highlighted further when considering the rate of dissolution of the different copper minerais in Figure 5.

The more easily leached minerais, in this case chalcocite and bomite, are less dépendent on particle size than the more refractory chalcopyrite for achieving extractions in excess of 85%. In the finest size fraction examined, in both leachant Systems, the extraction of the oxide and secondary sulphide fractions, consisting of predominantly delafossite, chalcocite and bornite, were higher than 98%. The impact of increasing rate and extent of extraction as a function of particle size below about 6mm is significant and unexpected. An additional 6m column was operated on the 21070

-20l .25mm fraction under identîcal experimental conditions and demonstrated an extraction of copper from chalcopyrite of almost 80% in about 190 days. For extension of effective heap leaehing to recover the more refractory minerais, such as chalcopyrite in the case of primary copper ores, size is a key parameter.

On the basîs of micro-permeabîlity alone, it would be advantageous to further reduce the particle size, the offsetting factor is macro-permeability. Figure 6 shows how macro-permeability reduces exponentially with decreasing particle size, even for extremely well sorted sands. The minimum particle size is thus set by a limît of ensuring effective macro-permeability for the practical application of heap leaehing as the primary method of values recovery from a particular ore and leachant System.

Hydrodynamic measurements of the fractions that were column leached are presented in Figures 7, 8 and 9. Given the narrow particle size distributions and the absence of signifîcant fines, the samples showed minimal compaction with dry bulk densities încreasing from about l .3 t/m³ to about l .4 t/m³ with an imposed compression équivalent to 40m of stack height. These results show thaï, while the fines t fraction examined demonstrated excellent mîcro-penneability, and corresponding high extractions, a signifîcant decrease in the macro-permeability was observed. Higher degrees of saturation at irrigation rates applicable to heap leaehing may prove more problematic for effective air permeability as shown in Figure 8. This may become even more problematic for cases where additional fine précipitâtes are formed within the heap. The marginally coarser fractions demonstrated excellent macro-permeability, having acceptable degrees of saturation and excellent air permeability at application rates applicable to heap leaehing, resisted consolidation and exhibit rapîd and extensive desaturation on cessation of irrigation.

In the context of heap leaehing the results illustrate that, with a sélection of particle size Pso of less than 5mm and a PWPio ratio of less than 20 and greater than 3, with appropriate adjustment of the particle size distribution of the ore, it is possible to achieve exponential micro-permeabîlity for rapid sand heap leaehing, whilst maintaining sufficient macro-permeability to create a free draining heap with excellent distribution of leachant and air.

With favorable mineraiogy, or sufficient time for dissolution of the slower reacting minerai species, extractions higher than 90% can be achieved from the sand in sand heap leach. These extractions are, quite unexpectedly, higher than the 80-85% extraction typically achieved b y flotation of the ore used in the expérimentation, îndicating that sand heap leaehing is equally

-21attractive for both low- and high-grade ores, and particularly attractive for leaching of highly oxidized ores. In addition, the comminution to a Pso less than 5mm is much easier than that required for flotatîon, the heap leaching enables direct production of cathode, and the overall environmentai footprint is lower.

References (the content of which is incorporated herein by reference)

Filmer and Alexander - WO2016/170437

Filmer and Alexander - WO2018/234880

Muller - WO2007/134343A2

Kohr-US6146444

Robertson - J. S. Afr. Inst. Min. Metall. vol.112 n.12 Johannesburg J an. 2012

Watlîng - Hydrometallurgy 140(2013) 163-180

Miller - Int. J. Miner. Process. 72 (2003) 33 1- 340 https://agupubs.onlinefibrary.wiley.com/doi/full/10.1002/2017WR020888

Beard and Weyl, 1973, Influence of texture on porosity and permeability of un Consolidated sand, The American Association of Petroleum Geologists Bulletin, Vol. 57, No. 2, 349-369

Guzman, 2013, Implications of hydrodynamic testing for heap leach design, Hydroprocess 2013, Conférence Paper

Claims (23)

1. l. A method of preparing and leaching of an orc containing métal values in a heap leach, the method including the steps of:

   • crushing an ore containing métal values to provide a sand containing métal values with a particle size Pso of less than 5mm and greater than 1mm;

   • classifying the sand to provide a classified sand with a particle size Pio of greater than 0.15mm, and a particle size P90/P10 ratio of less than 25 and greater than 3;

   • forming a heap from the classified sand; and • distributing leachant and air through the heap to leach métal values from the sand;

   • wherein the sand is stacked in lifts of height of greater than 5 meters.
2. 2. The method claimed in claim 1, wherein the ore is crushed a P80 of less 3mm, or a P80 of about 2mm.
3. 3. The method claimed in claim 1, wherein the classified sand has a Pio of greater than 0.15mm, a Pio of greater than 0.25mm, or a Pio of greater than 0.3mm, or a Pio of greater than 0.4mm,
4. 4. The method claimed in claim 1, wherein the classified sand has a P WP 10 ratio of less than 20, or a PWP10 ratio of less than 18, or a P90/P10 ratio of less than 15.
5. 5. The method claimed in claim 1, wherein the classified sand has a P90/P10 ratio of greater than 5, or a P90/P10 ratio of greater than 8.
6. 6. The method claimed in claim 1, wherein the classified sand and heap formed from the classified sand hâve a water permeability greater than 10’⁵ m/s, or a water permeability greater than 5xl0^-4 m/s.
7. 7. The method claimed in claim 1, wherein sand heap leaching îs the primary recovery method of values from the ore, and more than 50% of the ore is processed by sand heap leach, or more than 60% of the ore is processed by sand heap leach, or more than 70% of the ore is processed by sand heap leach.
8. 8. The method claimed m claim l, wherein the sand heap leach is undertaken in a fixed or a dynamic heap with a résidence time of less than 2 years, or less than 6 months, or less than 3 months.
9. 9. The method claimed in claim 1, wherein the heap comprises less than 1 5% contained water within 2 weeks of ceasing irrigation, or within 1 week, or around 3 days.
10. 10. The method claimed in claim 1, wherein the heap is subjected to more than one irrigation and drain cycles, to sequentially enhance aération and leaching.
11. 11. The method claimed in claim 1, wherein multiple leachants are used sequentially to remove gangue and then to recover the valuable components from the sand heap.
12. 12. The method claimed in claim 11, wherein an ore containing both copper and gold is heap leached initially to extract the copper, then washed with water, and subsequently leached with a different reagent to extract the gold.
13. 13. The method claimed in claim 1, wherein the classified sand is deposited on the heap by being flung from a discharge point using a hydraulic or mechanical device
14. 14. The method claimed in claim 1, wherein the sand is stacked in lifts of height of greater than 10 meters, or greater than 20 meters, or a height of up to 40 meters.
15. 15. The method claimed in claim 1, wherein the sand is leached in a dynamic heap, which is then removed from the dynamic pad by hydraulic mining techniques.
16. 16. The method claimed in claim 15, wherein the heap is constructed with air injection points to control the redox potential and température of zones across the heap.
17. 17. The method claimed in claim 1, wherein the métal values are selected from gold, copper, nickel, zinc and uranium, and the ores containing said métal values are selected from gold ore, copper ore, nickel ore, zinc ore, and uranium ore.
18. 18. A sand heap for heap leaching, comprising sand prepared from an ore containing métal values, the sand having a particle size Pso of less than 5mm, a Pm of greater than 0.15mm, and a PWPm particle size ratio of less than 25 and greater than 3, wherein the sand is stacked în lifts of height of greater than 5 meters.
19. 19. The sand heap claimed in claim 18, the sand having a particle size Pio of greater than 0.15mm, or a particle size Pm of greater than 0.25mm, a particle size Pm of greater 0.3mm, or a particle size Pm of greater 0.4mm.
20. 20. The sand heap claimed in claim 18, the sand having a PWPiû particle size ratio of less than 20 and greater than 5, a PWPio particle size ratio of less 15 and greater than 8.
21. 21. The sand heap claimed in claim 18, wherein the water permeability of the heap is greater than 10'⁵ m/s, or greater than 5x10’⁴ m/s.
22. 22. The sand heap claimed in claim 18, stacked in lifts of height of greater than 10 meters, or stacked in lifts of height of greater than 20 meters and up to 40 meters.
23. 23. The sand heap claimed in claim 18, wherein the métal values are selected from gold, copper, nickel, zinc and uranium, and the ores containing said métal values are selected from gold ore, copper ore, nickel ore, zinc ore, and uranium ore.