BACKGROUND OF THE INVENTION

C02 sequestration with high MgO rocks

The use of naturally occurring materials containing high levels of MgO such as ultramafic rocks as a method of sequestering carbon dioxide, has been the subject of extensive laboratory research.

Typically, the MgO content of the common rocks suitable for such sequestration exceeds around 30%, as tabled by Walder US 9,194,021 B2.

A summary of the many different approaches to achieving C02 sequestration is provided by Kelemen et. al. in “An Overview of the Status and Challenges of C02 Storage in Mineral and Geological Formations, Front. Cli. November

2015,

These possible approaches to sequestration range from in-situ disposal to ex-situ, and from injection of preconcentrated C02 to natural atmospheric sequestration, and from chemically preparing the rock to rapidly react with C02, to utilising the rock in its natural state.

Despite the importance and urgency of controlling C02 content in the earth’s atmosphere, and the commonplace mining of these ores for their contained metals, none of these sequestration techniques has found widespread commercial application.

The closest to common commercial practice is the storage of C02 using in- situ disposal of high-grade C02 gas streams in porous rock reservoirs. The rock types with adequate porosity are not typically those which will sequester the C02. The gas is pumped into this voidage purely for storage, rather than seeking to have the C02 absorbed by the rock over an extended time. Given the inherently low in-situ porosity of most rocks, this storage disposal requires the transport of the C02 enriched gas, from its source of production to areas where there is an abundance of suitable rock, Examples include storage of C02 in old oil and gas reservoirs.

The gas destined for such disposal is usually enriched to near 100% C02 using well established technology, then transported by pipeline.

An alternative is to concentrate the C02 from the natural atmosphere at the point of disposal. This direct air capture concept is relatively new and still faces issues with the high cost of concentrating C02.

It is obviously desirable to have a complete and permanent disposal of C02, without any potential for future leakage of stored C02 from its storage location.

Some small-scale efforts to store and subsequently sequester C02 in areas of recent geothermal activity https://www.miningweekly.com/article/rio-tinto- assembles-team-to-explore-new-approaches-to-carbon-mineralis ation- 2022-02-14/rep_id:3650

There have also been suggestions to mine, comminute and chemically prepare suitable rock to create a material which reacts rapidly with flue gas to sequester the contained C02.

This preparation of the rock can be through acid dissolution of the rock to dissolve the magnesium, then adding an alkali and C02 to precipitate magnesium carbonate. Alternatively, the rock can be calcined to generate MgO, prior to C02 addition.

Residues from processing some resources that were mined for their metal values, can naturally absorb significant C02 from the atmosphere.

In one example of a mining residue, the waste rock from a historical asbestos mine at Thetford, demonstrated that natural sequestration of C02 can occur when ultramafic rocks are exposed to the normal atmosphere, Lechat et.al. “Field evidence of C02 sequestration by mineral carbonation in ultrafamic milling wastes, Thetford Mines, Canada” Internartional Journal of Greenhouse Gas Control, Volume 47, April 2016, Pages 110-121 https://www.sciencedirect.com/science/articie/pii/S175058361 S300366

Low C02 concentrations were measured in a 9mx9mx2m heap of heterogeneously sized waste rock with a p70 of around 20mm, illustrating that the waste rock material was reactive, but the rate of sequestration was ultimately controlled by the flow rate of air through the heap. The waste heap, despite a high porosity (35%) had a low hydraulic conductivity (2x10 ^-6 m/s). Also, mineralogy studies, after a few years of sequestration, indicated that the carbonation was confined to the surface of the rock particles, with a coating thickness of some 50 microns.

The estimated C02 absorption rate at Thetford, being constrained by both atmospheric diffusion into the low-profile heap and the rock size, was around 4kg C02/m ³ /year.

In a second example, natural sequestration a flotation tailings of an ultramafic nickel ore has been demonstrated at Mount Keith. Wilson et.al. Offsetting of C02 emissions by air capture in mine tailings at the Mount Keith Nickel Mine, Western Australia: Rates, controls and prospects for carbon neutral mining, Journal of Greenhouse Gas Control, Volume 25, June 2014, Pages 121-140. https://www.sciencedirect.com/science/article/pii/S175058361 4000851

In this case, the ultramafic ore has been ground to less than 0.15mm, and processed to form a concentrate, and a fine flotation tailings which is stored in a tailings storage facility. The surface area of rock in the rock particles in slurry is very high, but the observed sequestration was confined to the top few centimeters of the tailings even over a period of almost 10 years. The air permeability through the fine saturated tailings is slow, thus limiting sequestration rate. A third concept by which C02 can be sequestered by a waste from mining has been claimed by Walder US 9,194,021 B2. Walder teaches a method in which waste rock from mining is stacked in a normal heap leaching arrangement, with leachant trickled down through the heap, and C02 containing gas passed counter currently up the heap. No experimental data is supplied, and no mention is made of prior comminution of the waste rock. So, by comparison with the Thetford studies, the Walder approach using a waste rock heap would remove C02 initially, but then reaction would slow as the surface area of exposed rocks was converted to carbonate, requiring C02 to then diffuse very slowly into the underlying rock volume.

Walder also teaches leaching the magnesium content of the ore with acid, and then precipitating this magnesium rich solution lower in the heap or externally. Such precipitation would require the addition of separately generated alkali such as caustic soda to drive the precipitation equilibrium to completion.

Mg2+ + C02 + 20H- » MgC03 + H20

In summary, Walder has partially addressed the permeability issue allowing air flow through a rock heap but did not address the surface area required to achieve an acceptable level of sequestration of MgO.

SUMMARY OF THE INVENTION

THIS invention relates to a method of sequestering C02 in a 3 dimensional heap structure of MgO containing crushed rock by:

• crushing the rock to a particulate size suited to sequestration and having a particle size of less than 5 mm, typically in the range between 0.2-5 mm;

• reducing the proportion of fines generated in the crushed rock, for example by sieving, such that the proportion of <75 micron particles is less than 20% by weight; • stacking the crushed rock in a heap such that the stacked crushed rock exceeds a hydraulic conductivity of 10 ^_5 m/s; and gas can flow through the crushed rock at a rate of greater than 20 l/m2/h

• maintaining the stacked crushed rock in an unsaturated state with internal moisture between 10-25% by weight, if necessary by irrigating the heap with water, and

• and passing a C02 containing gas through the heap, such that the residual particulate matter is accessed by and reacts with C02 in the gas; and the C02 in the gas is converted to a solid carbonate material.

The particulate rock material allowing bulk flow of gas through the heap is typically prepared by classification to reduce the quantity of fines.

The particulate rock material containing MgO typically comprises > 20% MgO by mass, preferably > 30% MgO.

The rock is typically a sulphide ore, and contains greater than 30% MgO, including naturally occurring nickel, copper and cobalt containing sulphide ore resources. Suitable rock, typically containing greater than 30% MgO but without valuable quantities of nickel, copper and cobalt, can also be carbonated.

The rock may be processed by one or more of heap leaching, coarse particle flotation, or conventional flotation, to beneficiate the ore and recover the metal values from said ore, and the crushed rock containing MgO for sequestration is a residue from said process/es. In this case, the proportion of <75 micron particles in the crushed rock is typically less than 20% by weight.

The particulate rock material containing MgO residue may be stored in heaps with one or more of under-heap aeration and air access pipes to promote gas flow through all fractions of the comminuted ore. Typically, the particulate rock material containing MgO is irrigated from an irrigation system located near the top of the heap.

The gas added to the heap is typically enriched in C02 content. By enriched C02 content is meant that typical of a combustion process and with C02 content greater than 10% and preferably greater than 20%, and up to 100% C02.

Preferably, the particulate rock material containing MgO is stacked in a free draining heap, and air is externally heated to promote airflow and sequestration.

Bulk sorting may be used to segregate run of mine ore into fractions according to their MgO content prior to sequestration.

Historical tailings may be reclaimed and prepared for sequestration.

Rock to be processed and stored for sequestration may be the output of another industrial process, such as slag from steel manufacture, or fly-ash from power generation.

The proportion of <75 micron material in the stacked heap typically is less than 20%, and preferably less than 15% and even more preferably around 10%.

The top size from crushing may be less than 5mm, and preferably less 3mm, and even more preferably 2mm or less.

Bulk sorting can be used to segregate run of mine ore into fractions according to their metal and MgO content, one fraction of which contains MgO > 30%, but with metal values which are below the cut-off-for immediate metal recovery. A fraction which contains MgO > 30% but is low in metal values, may be further crushed and classified to remove fines, prior to stacking in a form suited for carbon sequestration.

Heap leaching (SHL) may be applied to a size fraction between p20 of 0.2mm and p80 of 10mm, and preferably less than p80 of 5mm and even more preferably less than 3mm.

Leachants for SHL may be selected from those that operate in mildly acidic solution (pH>4) to basic conditions acidic solution (pH>4) to basic conditions (pH up to 10.5) such as ammonia or glycine, to ensure that the majority MgO content remans unleached and hence in the residue in a form suited to subsequent carbon sequestration.

Coarse particle flotation (CPF) may be applied to a size fraction between p20 of 0.1 mm and p80 of 0.5mm, and preferably between 0.1 mm and 0.35mm, and even more preferably between 0.1 mm and 0.25mm.

CPF or SHL residues may be utilised to provide the particulate material containing MgO for the permeable layers in a hydraulic dry stack (HDS) type configuration, enabling additional quantities of fine flotation tailings (<75 micron) to be stored and sequestered within the permeable heap.

Flotation residues may be pelletised or agglomerated to reduce the total quantum of material <75 microns contained in the heap.

Flotation residues including historical tailings may be reclaimed and stored in an HDS structure, with sand from either the tailings or external sources used to dewater thin layers of the tailings and increase air permeability to increase the rate of carbon sequestration in these layers.

The heap may be used to sequester gases in which concentration of C02 in the heap is enhanced beyond that in the surrounding atmosphere. For example, the gas may have a C02 content greater than 10% and preferably greater than 20%, and up to 100% C02.

Fines may be agglomerated or pelletised to reduces the amount of material less than 75 micron.

C02 may be introduced into the heap by forced air flow.

C02 flow into the heap may bepromoted by using the natural characteristics of air flow and temperature variation, through provision of air corridors such as air pipes to various points in the base and towards the centre of the heaps, the temperature differential and hence density difference of air in the interior of the permeable heap and that of the external air, to promote air circulation through the pipes and within the heap.

Heat may be generated in the heap utilizing diurnal temperature differences on the heap surface as a driving force for air movement in the heap by the addition of relatively impermeable slopes on the sides of the heap and aeration piping and/or a coarse, highly permeable base to the heap, diurnal temperature variation can be optimally utilized so that during the day, the top of the heap heats substantially more than lower levels, establishing a pressure gradient over the heap and draws air into the base of the heap, exposing the equisized sand to ambient concentrations of C02, during the night, the top of the heap will cool substantially faster than the rest of the heap.

Air from the aeration pipes may be heated by solar heating solar heating.

In an embodiment of the invention C02 is sequestered in a 3 dimensional heap structure comprising layers or channels of:

MgO containing rock crushed to a coarse particulate material with a size suited to sequestration and having a particle size of less than 5 mm and a proportion of <75 micron particles is less than 20% by weight; and fine flotation tailings; the crushed rock is in an unsaturated state with internal moisture between 10-25% by weight, and and C02 containing gas is passed through the heap, such that the residual particulate matter is accessed by and reacts with C02 in the gas; and the C02 in the gas is converted to a solid carbonate material.

Typically, the fine tailings layers of channels have a thickness of less than 5 metres, and preferably less than 3m, and even more preferably less than 2m.

The invention also relates to a 3 dimensional heap structure for use in a method of sequestering C02 including: crushed MgO containing rock with a particle size of less than 5 mm and a proportion of <75 micron particles less than 20% by weight; wherein the crushed rock is an unsaturated state with internal moisture between 10-25% by weight.

BRIEF DESCRIPTION OF THE DRAWINGS

Figures 1 A-F are graphs showing the modelled reaction rates of olivine sequestration expressed as the % sequestration for different temperatures and particle sizes.

Figure 2 is a flow diagram of a method according to an embodiment of the invention;

Figure 3 is a flow diagram of a method according to an embodiment of the invention; and

Figure 4 is schematic illustrations of stacked heaps that can be used in the method of the present invention. DESCRIPTION OF PREFERRED EMBODIMENTS

Whilst the experience of C02 absorption into pre-existing mining residues has been disappointing in terms of rate and extent of C02 uptake, it has been complemented by a large body of laboratory studies.

These laboratory studies have shown that the rate of C02 absorption into various rocks is dependent on four main variables.

The first variable is rock type, with extreme ultramafic rocks such as those containing brucite and olivine and some calcium rich smelter slags; reacting much faster than less basic resources such as the basalts or peridotites.

The second variable is the available surface area of the solid reactive material. Finely ground rocks, with high surface area exposed to the C02 containing gas, react much more rapidly than boulders.

The third variable is the C02 partial pressure at the reaction surface. 100% C02 reacts much more rapidly than the natural atmosphere (around 400ppm C02). And wherever the C02 activity becomes depleted at the reactive surface, the sequestration reaction is retarded.

The fourth variable is temperature, where sequestration reactions which are slow at room temperature, increase significantly with temperature.

An example of these factors is shown in Figures 1 A-F. This graphic models data to show the extent of the sequestration reaction plotted against temperature for various particle sizes, of olivine particles when exposed to the C02 levels present in the atmosphere.

The influence of temperature and particle size are clearly illustrated in Figures 1 A-F. If the modelling were undertaken at higher C02 partial pressures were increased, reaction rates would increase proportionally. And if a peridotite material was substituted for olivine, the reaction rates would decrease substantially.

The reaction durations are measured in months or years. This duration illustrates that a passive reactor like a heap is required, not an agitated reactor.

So, an ideal system for sequestration is a one in which ultramafic rock containing mineral species such as olivine and brucite is ground to a fine size to increase surface area, and then all of this surface area is presented in a permeable structure such that high concentrations of C02 can be maintained throughout the solid, preferably operated at an elevated temperature.

It is clear that the prior art cannot achieve all these factors simultaneously.

The problem to be solved is how to present a high surface area of rock, whilst maintaining high permeability for gas flow, in a reactor which has residence times of months or years.

Achieving this is the object of the present invention.

The current invention utilises methods for the crushing and storage of ores; including the potential to combine sequestration with the recovery of other contained values, in permeable 3-dimensional structures which supports rapid and extensive carbon sequestration.

The invention can also be used to reclaim and prepare historical mineral residues, into a permeable structures suitable for sequestration.

A schematic of a first embodiment of the invention is illustrated in Figure 2.

In this first embodiment, ultramafic ore 10 containing > 20% MgO and preferably > 30% MgO and even more preferably around 40% MgO is crushed 12 to a p80 <5mm, and classified 14 to remove the fines 16 (<0.1 mm) to prepare a coarse sand 18 containing less than 20% fines <75 microns, and preferably less than 15% is stacked 20 for subsequent beneficiation.

Optionally, the fines 16 may be agglomerated or pelletised to form coarser agglomerates which are then reintroduced to the stacking 20.

Optionally, as illustrated at 2A in Figure 2, the coarse sand 18 is stacked in a permeable heap 20 suited for heap leaching to recover metal values 22 such as nickel prior to C02 sequestration at 2B. The heap 20 has provision for air or oxygen gas injection 24 at the base 26 through an aeration system 28, and an irrigation system 30 is installed near the upper surface of the heap 20. The heap 20 is supplied with leachant 32 through the irrigation system 30 and an oxygen containing gas 24 through the aeration system 28, to carry out a heap leach process and recover metal values 23.

After a heap leach process 2A has been completed, a C02 sequestration process takes place on the heap 20 at 2B, utilising the existing or modified aeration and irrigation systems.

The heap 20 at 2B may, alternatively, be a purpose-built heap for C02 sequestration. The heap 20 has provision for C02 gas injection 25 at the base 26 through an aeration system 28, and an irrigation system 30 is installed near the upper surface of the heap 20. C02 containing gas 25 is fed through an aeration system 28 into the base 26 of the heap 20, and permeates upwards through the heap. Optionally, this gas 25 can be preheated prior to introduction to the heap 20. The moisture level in the heap is maintained at between 10-25% by weight naturally or by use of the irrigation system 30 at the top of the heap 20. As the gas passes through the heap, sequestration of the C02 occurs, leaving a lower concentration of C02 in the gas flow exiting the heap. The use of comparatively equisized sand or agglomerates not only increases the surface area of reactive rock for sequestration by more than an order of magnitude relative to normal heap leaching, but also ensures more uniform distribution of gas and liquid through the rock. It also creates a high porosity within the heap, thus enabling the sequestration to proceed without adversely affecting bulk gas flow.

Removal of the fines enhances the permeability of the stacked heap 20, thus allowing both liquid 32 and gas 24/25 to flow through the heap, whilst still presenting a much higher surface area than typically found in the coarsely crushed rock typically used in heap or dump leaching as taught by Walder. This finer crushing and classification enhances the surface area of the mineral species available to be sequestered. Preferably the hydraulic conductivity of the heap 20 is greater than 10 ^_5 m/s and even more preferably > 2 ^* 10 ^A -5m/s.

Air flow through the heap 20 is greater than 20l/m2/h and even more preferably greater than 40l/m2/h.

Preferably the moisture content in the heap is controlled to provide sufficient water for the sequestration reaction, but not excess water that inhibits the gas flow through the heap. Preferably the water content is between 10 and 25% by weight, and even more preferably between 15 and 20%.

Preferably the gas to be sequestered is air, and even more preferably the gas is enriched in C02, such as that generated during combustion processes such as energy generation or cement and steelmaking, or fermentation, or from natural gas production.

C02 is stripped from the gas as it passes through the permeable heap 20.

In the option where heap leaching precedes sequestration, the residual coarse sand heap retains its high surface area and permeability to both liquid and gas flow, and hence is ideally suited to sequestration.

The sequestration reaction is exothermic and generates heat but retaining this heat for extended periods within the well aerated heap is problematic. To further raise the temperature in the 3-dimensional structure, the gas or liquid entering the heap can be preheated. For example, the sources of such heat could be from locally available waste heat, or by solar collection.

The first key to accelerating sequestration is to select preferred particulate matter containing >20% MgO, and preferably >30% MgO and even more preferably around 40% MgO or greater. To pre-select only the most suitable rocks for sequestration, techniques such as bulk sorting can be utilized.

In bulk ore sorting (BOS) ore that has been fragmented by blasting, is transported by truck or conveyor to a primary crusher, and by conveyor to grinding. On the conveyor either before or after the primary crusher, the grade of the ore (or deleterious contaminants) can be analysed, using techniques such as magnetic resonance, or neutron activation. For example a cross belt analyser available from SODERN which makes use of a CNA (Controlled Neutron Analyser) using an electrical neutron source with stabilised emission htp://www.sodern.com/sitea/en/ref/Cross-beit- Anaiyser 7t .html, allowing a decision to divert the stream of rock to ore or to waste.

The second key to accelerating sequestration is provide a high surface area of solids for sequestration. The current invention achieves this high surface area through crushing to finer sizes and stacking the prepared ore in a 3- dimensional space, but without allowing the fines generated during crushing to reduce the heap permeability to the detriment of C02 access.

The third key to accelerating sequestration is to maintain a high C02 partial pressure within the heap. High heap permeability is required when seeking to sequester C02 from natural atmosphere, along with a high gas flow to maintain the C02 levels within the heap. Preferably, an enriched source of C02 is utilised to sequester C02 more rapidly and at a slower gas flow rate, either through locating the heap close to the C02 source or bringing the C02 source to the heap. The gas which is depleted in C02 content can then naturally vent to atmosphere. The fourth key to sequestration is to maintain moisture levels within the heap such that moisture is provided for the sequestration reaction but not such that it impedes heap permeability. The invention provides for irrigation from the top of the heap to control the moisture level and compensate for water which is evaporated or consumed in the sequestration.

The fifth key to accelerating sequestration is temperature. Whilst the sequestration reaction is exothermic, and a 3-dimensional reactor can transfer this heat internally and retain heat through balancing gas and liquid flows, some heat is inevitably lost in the off-gas. This internal heat generation can efficiently be complemented by adding passive heat to the heap by preheating the irrigant or gas source. Such indirect heating could for example be supplied using solar energy or waste heat from another process.

An embodiment of the invention which utilises all the processes of pre- beneficiation and residue storage is illustrated in Figure 3.

Depending on the characteristics of the ore, embodiments of the invention will include one or more of the steps of bulk ore sorting (BOS) described above, sand heap leach (SHL), coarse particle flotation (CPF), and hydraulic dry stacking (FIDS), used in combination with conventional flotation.

The third stage of the multistage beneficiation is coarse particle flotation. This process utilises the heterogeneity at the sand (sub 1 mm) size level, for a chemically assisted gravity separation. The partially ground ore is classified to produce a sand fraction, which is beneficiated using a fit for purpose flotation machine such as the Eriez™ Hydrofloat. The Eriez Hydrofloat™, carries out the concentration process based on a combination of fluidization and flotation using fluidization water which has been aerated with micro bubbles of air. The flotation is carried out using a suitable activator and collector concentrations and residence time, for the particular mineral to be floated. At this size, the ore is sufficiently ground to liberate most of the gangue and expose but not necessarily fully liberate the valuable mineral grains. The coarse flotation recoveries of partially exposed mineralisation is high, and the residual gangue forms a sand which does not warrant further comminution and conventional flotation. The reject sand from coarse flotation can be stacked and drained to recover water.

In the normal configuration, the coarse flotation size range will be bounded by the maximum size where the valuable minerals are sufficiently exposed to be floated, with sufficient recoveries such as to produce a sand residue suitable to discard. The minimum size is set by the particle size at which the coarse flotation machine can operate efficiently to produce a free draining sand for disposal. Depending on the mineralogy, the fracture characteristics of the ore, and the design of the classification circuit; this lower size range is typically around 100-200 microns, and the upper size is typically between 350 and 600 micron. Depending on the size range for coarse flotation, and the classification efficiency, this scalping captures for coarse flotation between 40-60% of the total feed to comminution, with the remainder reporting to conventional flotation.

In a conventional froth flotation process, particle sizes are typically less than 0.1 mm (100 pm). The ore particles is mixed with water to form a slurry and the desired mineral is rendered hydrophobic by the addition of a surfactant or collector chemical. The particular chemical depends on the nature of the mineral to be recovered. This slurry of hydrophobic particles and hydrophilic particles is then introduced to tanks known as flotation cells that are aerated to produce bubbles. The hydrophobic particles attach to the air bubbles, which rise to the surface, forming a froth. The froth is removed from the cell, producing a concentrate of the target mineral. Frothing agents, known as frothers, may be introduced to the slurry to promote the formation of a stable froth on top of the flotation cell. The minerals that do not float into the froth are referred to as the flotation tailings or flotation tails. These tailings may also be subjected to further stages of flotation to recover the valuable particles that did not float the first time. This is known as scavenging. The first step, in the embodiment which includes all pre-beneficiation techniques and tailings storage zones within the heap is BOS, in which the run-of-mine ore is analysed whilst being transferred from mine to processing facilities, and on this basis segregated into multiple streams.

With reference to Figure 3, run of mine (ROM) ore 40 is subjected to a primary crush 42, and crushed ore 44 is sorted in a bulk ore sorter (BOS) 46. The BOS 46 can be configured to produce at least two fractions selected from;

48 a. a high grade metals values stream of sulphide ore suited to immediate further beneficiation for recovery of metal values and storage of residues for carbon sequestration 48 b. a lower grade metals values stream for beneficiation suited to removal of fines for processing with the high-grade ore for values recovery, and storage of the coarser or lower grade rock for immediate carbon sequestration and potentially awaiting later beneficiation for recoveries of metal values,

48 c. a stream which is below cut-off-grade for values recovery but with MgO > 30% content suited for carbon sequestration 48 d. and a waste stream which has no value in terms of enhanced sequestration rates or values recovery.

Streams 48 a b and c will be further comminuted 50 a b and c, respectively, to a size suited for the next stage of beneficiation that has been selected for values recovery.

Stream 48 b can be further comminuted (50 b) to a size suited for more rapid sequestration; and classified to remove the fines, which are typically at a higher average grade than the coarse ore, to create a particle size distribution which enhances airflow through the storage heaps. The fines (52 b) from this classification can be further processed along with stream 56 a.

Stream 54 b will be stockpiled to enable high air permeability and to be suitable for carbonation and perhaps later reclamation and beneficiation, whilst stream 54 c will be stacked to enable high air permeability to promote carbonation.

Stream 48 d will be assigned to a normal waste rock storage facility.

This configuration of bulk sorting and size classification enables the benefits from both recovery of metal values and sequestration to be optimised for individual ores and individual locations.

A fines fraction 56 a may be subjected to conventional flotation 58 a to provide a Ni concentrate product 60 and tailings 62 a.

A coarse fraction 64 a may be stacked in a heap 66 a and heap leached, and a mid-size fraction 68 a may be subjected to coarse particle flotation 70 a. Pregnant leachant from the heap leach 66 a containing metal values is processed 74 a to recover a nickel product 76 a, and leachant is recirculated to the heap 76 a. Spent sand 72 a may be stacked suitable for carbon sequestration or used to supplement sand generated in 70 a, as may be required for HDS construction.

Since, the grain size of the valuable sulphides in ultramafic rock is typically less than 100 micron, and hence very fine grinding is required to fully liberate the valuable grains suitable for further recovery, additional beneficiation techniques are desirable prior to fine grinding.

The second step in the embodiment using all forms of pre-beneficiation is SHL 66 a, initially utilised with stream a, and then later with stream b.

The stream 48 a ore is crushed 50 a to less than 10mm, and preferably less than 5mm, and even more preferably around 2-3mm, and then classified to remove the fines that are more suited to recovery using the other beneficiation processes, CPF 70 a and flotation 78 a. By crushing the ore in stream 48 a to a finer size, the exposure of values and hence percentage extraction achievable by heap leaching is increased, but proportion of the ore assigned to SHL is decreased.

Classified coarse ore 64 a is stacked in a typical heap leaching structure provided from the base of the heap and leachant irrigated from the top. (similar to that described by US 9,194,021 ), but containing a much-enhanced particle area and size distribution that enables high air permeability through the heap. This makes subsequent sequestration much faster and with a higher conversion of the rock to carbonate.

The use of equisized sand, vs. the rock pile used by Walder, increases the surface area of reactive rock for sequestration by more than an order of magnitude and ensures more uniform distribution of leachant, air and water through the rock. This enables sequestration from the natural atmosphere over an extended period of time, rather than from an emission gas. The removal of fines from the sand heaps 54 c, 54 b and 52 a, imply much greater heap permeability and hence increases the natural airflows substantially. The sequestration process can occur not only during leaching, but continue on over subsequent years. The pre-beneficiation steps in the current invention also enable recovery of valuable metals, such as nickel, copper and cobalt, with a much greater extraction than that which can be achieved in the conventional crushed rock heap described by Walder. And finally, the fines, with or without prior agglomeration, can be contained in a HDS 62a in the form where air flow can occur at acceptable rates through the unsaturated fines.

Leachants for SHL 66 a can be selected from those that operate in mildly acidic solution (pH>4) to basic conditions to ensure that the majority MgO content remains unleached and hence in the residue in a form suited to carbon sequestration.

As one example, if using the bioleach technology operating at around pH 5, as described by Cameron et. al “Bioleaching of a low-grade ultramafic nickel sulphide ore in stirred-tank reactors at elevated pH” Hydrometallurgy, Volume 97, Issues 3-4, July 2009, Pages 213-220 https://www.sciencedirect.com/science/article/pii/S0304386X0 9000589, the nickel can be heap leached at sizes up to around 5mm, with high nickel recoveries. The acid consumed at pH 5 in Cameron’s agitation leach is around 10Okg/tonne which may be higher than desirable for agitation or heap leaching nickel ores. But at a slightly higher pH, it is possible to rely mostly on the acid generation through bio-oxidation of the sulphides present, with magnesium being the main acid consuming gangue element. The nickel in the pregnant liquor from this leach can be recovered by many techniques, the simplest being precipitation as a mixed hydroxide product, suitable as a precursor to battery manufacture.

As a second example, leaching with an ammonium salt solution at around pH 8.5 will not dissolve the MgO, but rather be buffered at around this pH by the rock. Ammonia activity in the leachant will be sufficient to dissolve the nickel and cobalt present, but not so high that excessive losses to atmosphere will occur.

As a third example, a similar dissolution of nickel can occur in glycine, operating at an alkaline pH.

Once the values are leached with any suitable leachant system, the sand residue from SHL has a low fines content and high surface area due to the prior comminution and classification, and is suited to ongoing airflow through the sand heap that is required for continuing carbon sequestration.

The third step in the embodiment of the invention that includes all pre- beneficiation techniques, is CPF. As an example of CPF, coarse particle flotation (CPF) enables the efficient flotation of values such as nickel and cobalt, present in the ore as sulphides or alloys, at a p80 grind size of around 150-200 microns coarser than is achievable with conventional flotation. This implies typical upper size p80 constraints for CPF feed of around 250 to 400 microns. The intermediate concentrate from CPF can either be directed to SHL for leaching, or ground more finely to recover the metal in conventional flotation

The residue from CPF is ideally suited for stacking as a free draining sand, or for use as sand layers in the FIDS.

Where stacked as a free draining sand, the use of equisized sand, vs. the rock pile used by Walder, increases the surface area of reactive rock for sequestration by more than an order of magnitude and ensures more uniform distribution of air through the rock.

The finest fraction of the ore from crushing and classification, typically less than 150 micron, is beneficiated using conventional flotation, to produce a saleable concentrate.

An alternative storage that enables good air permeation must be considered to achieve satisfactory sequestration on these flotation tailings. They may be pelletised or agglomerated to reduce the fines content prior to stacking for sequestration.

The final step illustrated in the embodiment shown in Figure 4, to render all the residue from beneficiation suited to carbon sequestration is the FIDS structure, in which the mix of unsaturated layers of flotation tailings and sand are placed in a heap, enabling water removal, greatly increasing air permeability and hence access of C02 to the finely ground flotation tailings from ultramafic ores. The layers unsaturated fine tailings form less permeable zones within the heap structure, but remain capable of transferring C02 into these layers from the more permeable heap.

Depending on the crush size selected for the various pre-beneficiation steps, this conventional flotation residue can represent between 20-80% of the selected ore stream that is sent from crushing to classification for nickel metal recovery. The conventional flotation tailings are saturated and hence impermeable to aeration if not stored in the thin unsaturated layers

If the flotation tailings were filtered and stacked, the fines would be converted to an unsaturated state but the small pore size between solid particles will inhibit air penetration through the stack and hence result in very slow carbonation below the first metre or two. And spreading filtered tailings in a 1 -2m high stack over an extended area to enable aeration, is untenable environmentally.

But when the flotation tailings are placed in a HDS structure, it enables the dewatering of the tailings through layers of porous sand generated by CPF. Once the system is dewatered, the sand layers are permeable to air flow, and hence can form channels through which both the top and the bottom of the tailings layers can be exposed to air containing high levels of C02, thus accelerating the air penetration into the fine tailings and enhancing carbonation in all the ore present in stream a.

The quantity of flotation tailings that are incorporated in the heap can be further increased by agglomerating or pelletising the fines, such that the fraction of material with particle size less than 75 micron does not exceed 20%.

The invention has teachings to describe how to both achieve high recovery of values and create the maximum surface area of residues in a form that is permeable to air, such that the faster reaction of the residue with the carbon dioxide is enabled.

Heap design for the storage of such residues is also important to achieve effective ongoing aeration at the exposed residue surfaces. Since this residue storage must balance land area available for residue, and the flow of air through the storage structure. A further addition to the inventive step is to ensure that when the residues are stored on heaps, the natural air flow of air through the heap is promoted, such that the C02 concentration in the air present in the residue at any time is maintained at a sufficient level to promote high reactivity.

This can be achieved by forced air flow through the various heaps by use of blowers; but can also be promoted by using the natural characteristics of air flow and temperature variation.

The carbonation of high magnesia rock is an exothermic reaction, as is the oxidation of sulphides contained in the rock. These reactions create a natural temperature differential between the heap and the surrounding environment. The daily and seasonal temperature variation of the heap is also lower than that of the surrounding air. Through provision of air corridors such as air pipes to various points in the base and towards the centre of the heaps, the temperature differential and hence density difference of air in the interior of the permeable heap and that of the external air, will promote air circulation through the pipes and within the heap.

This is illustrated schematically in Figure 4, for both for the Hydraulic Dry Stack (HDS) embodiment and a dry stacked sand.

The dry stacked sand 80 includes aeration pipes 84 at the base thereof for supplying air to the stack 80. The air in the aeration pipe is optionally heated by a solar heater 86 to promote air circulation and C02 sequestration.

The hydraulic dry stack (HDS) structure 82 comprises sequential layers or channels of coarse particles (crushed rock) 90 with a particle size of greater than 150 micron, and typically a p80 of around 0.3mm and fine flotation tailings layers or channels 88 with a particle size less than 150pm, typically less than 100pm. Drains 94 are provided for draining water from the coarse particles and hence desaturating the tailings layers or channels 88. Optionally then drains can be reversed in an unsaturated heap to inject C02 containing gas into the heap. Aeration pipes 92 with the C02 containing air in the aeration pipes are provided at the base of the structure 82 optionally being heated by a solar heater 86 to promote air circulation and C02 sequestration. For the layers or channels of fines tailings 88 in the HDS, even though they are unsaturated, permeability to air will be slow. The sequestration can be optimised by restricting the thickness of the tailings layers or channels 88, typically less than 5 metres, and preferably less than 3m, and even more preferably less than 1 m. The coarse particle layers or channels 90 have a thickness sufficient for the C02 containing gas to migrate through the structure, typically greater than 0.1 m and possibly up to 5mm. In this way, the diffusion length is through the tailings is limited, either from the sand layer above or below to the centre of the tailings layer. Although the airflow is slow through the unsaturated tailings layers 90, storage in this form is far superior to conventional tailings storage facilities where the ore is saturated.

Optionally, the air flows through the heap can be controlled by inserting a vent pipe or chimney 96 into the structure such that gas which has flowed through the heap and has been largely denuded of its C02, can be vented. This is illustrated schematically in Figure 4.

A further option is to generate a heap design that fully utilizes diurnal temperature differences on the heap surface as a driving force for air movement in the heap. By the addition of relatively impermeable slopes on the sides of the heap and aeration piping and/or a coarse, highly permeable base to the heap, diurnal temperature variation can be optimally utilized. During the day, the top of the heap heats substantially more than lower levels. This establishes a pressure gradient over the heap and draws air into the base of the heap, exposing the equisized sand to ambient concentrations of C02. During the night, the top of the heap will cool substantially faster than the rest of the heap. The addition of a grid of breathing pipes which end below 0.3-2m of the heap surface will allow the top of the heap to act as an insulating layer against substantial convective and especially radiative heat losses. The “stack effect” that was readily established during the day can still be achieved, albeit as a possibly slower rate. Breathing pipes may alternatively be capped during winter to reduce the intake of colder air to the base of the heap.

A further option is to utilise solar heating of the air being assigned to the aeration pipes, for example using solar collectors. This not only accelerates air flow through the heap due to the differential air density, but also accelerates the carbonation reaction.

With the residues from pre-beneficiation from which fines have been removed from all the fractions of the ore except flotation tailings, this natural convection will provide adequate aeration. And the size of the stored residues is such that maintaining water levels throughout the heap without causing saturation of parts of the heap, is possible.

Whilst the process and structures of residue are designed for natural aeration, it is also possible to utilise the invention for sequestering gasses in which the C02 concentration is enhanced. This may be through colocation of the facilities, or through localized concentration of atmospheric C02 to deliver an enriched air source to the heap.

The concepts inherent in the storage of the sand and fines, is equally applicable to other materials which naturally absorb C02.

If the C02 absorbing materials are present as fines, such as with fly-ash from thermal power stations or historical tailings from mineral processing, they can be pelletised or agglomerated and then stacked. Alternatively, they can be stacked with sand from a reactive or non-reactive source to create semi- permeable layers through the heap in a HDS structure containing the zones of fines

If the C02 absorbing materials are coarser, such as slag from pyrometallurgical production of metal, they can be crushed, optionally treated for metal recovery, and stored in the HDS to promote effective air permeability.

Through the selection and operation of pre-beneficiation and removal of fines into a separate processing stream, and through the adaptation of residue storage facilities to promote aeration, the current invention enables both high recoveries of metal values from suitable ores, and high rates of carbon sequestration in the resulting residues.