LIMONITE AND SAPROLITE HEAP LEACH PROCESS

This invention relates to the hydrometallurgical process for the recovery of nickel from oxide ores, in particular the heap leaching of lateritic ores that include both saprolitic and limonitic components.

Background of the Invention

Laterite ores are potentially the world's largest source of nickel and cobalt. In general, most deposits of nickel/cobalt laterites contain three major zones based on morphology, mineralogy and chemical composition. These three zones, from the base to the surface, atop weathered parent bedrock materials are the saprolite zone, the transition zone and the limonite zone. There is generally a large variation in total thickness of the laterite deposit, as well as individual zone thickness.

The saprolite zone consists predominantly of "saprolitic serpentine" minerals and a large variety of nickel/magnesium silicate minerals. The weathering process, or "serpentinization" of the parent bedrock material is characterised by a decrease in the magnesium content and an increase in the iron content of the top layer of ore body. The resulting saprolite zone contains between 0.5% and 4% nickel and a higher magnesium content, which is normally over 6%wt.

The not well defined transition zone is composed essentially of limonite and saprolite. It also commonly contains nickel in the range of from 1.0% to 3.0% with co-existing cobalt ranging from 0.08% up to 1 % (associated with asbolane, a hydrated manganese oxide).

The limonite zone, located on the top zone of lateritic ore body, contains nickel ranging from about 0.5% to 1.8% and consists of goethite-rich and/or hematite- rich ore, which is rich in iron and cobalt. It has a lower magnesium content than saprolitic type ore. Due to stronger weathering, limonitic ore contains dominantly fine and soft particles of goethite and/or hematite. Sometimes the weathering has not been fully completed and either the hematite or the

goethite rich sections are not present. Alternatively, depending upon the climatic condition, the limonite zone will still contain residual iron/aluminium silicates, such as nickel-containing smectite, nontronite and chlorite. At atmospheric pressure and ambient temperature, the acidic leach of limonite is slow. The whole-ore dissolution reaction using sulfuric acid is shown as follows: Limonite leach

(Fe,Ni)O.OH + H ₂ SO ₄ → NiSO ₄ + Fe ⁺³ + SO ₄ ⁺² + H ₂ O Eq. 1

Goethite

(Fe ₅ Ni) ₂ O ₃ + H ₂ SO ₄ → NiSO ₄ + Fe ⁺³ + SO ₄ ⁺² + H ₂ O Eq. 2

Hematite

The iron content of limonite ore is normally in the range of 25-45%wt which corresponds to 40-72%wt goethite (FeOOH) or 36-64%wt hematite (Fe ₂ O ₃ ). Consequently the dissolution of Ni-containing goethite or hematite of a limonitic heap causes the instability of a heap, such as severe volumic slumping or shrinkage, and poor irrigation permeability.

The less-weathered, coarse, siliceous and higher nickel content saprolites tend to be commercially treated by a pyro metallurgical process involving roasting and electrical smelting techniques to produce ferro nickel. The power requirements and high iron to nickel ore ratio for the lower nickel content limonite blends make this processing route too expensive. Limonite ores are normally commercially treated by a combination of pyrometallurgical and hydrometallurgical processes, such as the High Pressure Acid Leach (HPAL) process, or the reduction roast - ammonium carbonate leach process.

Acid leaching of saprolitic ore is not practised commercially for the reason that a process has not been developed for recovering the nickel from the leach solution in an economical and simple manner.

While heap leaching copper ores is well known as a commercial operation, there are several differences between heap leaching of copper containing ores

that also contain some clay components, and the lateritic ores that have substantial fine and/or clay components. In addition, the acid consumption of laterite ore is ten-fold that of heap leaching copper ores.

It has been found that the permeability of lateritic ore is largely controlled by the type of mineral occurrence, mineral morphology and particle size. Although the mineralogy of lateritic ore is rather complex and widely variable from deposit to deposit, there is some commonality or similarity of mineral morphology in the worldwide lateritic nickel deposits. These morphological structures enhance permeability of solution and preserve physical stability of individual minerals.

Heap leaching of nickeliferous oxidic ore has been proposed in recovery processes for nickel and cobalt and is described, for example in U.S. Patents 5,571 ,308 and 6,312,500, both in the name of BHP Minerals International Inc.

U.S. Patent 5,571 ,308 describes a process for heap leaching of high magnesium containing laterite ore such as saprolite. The patent points out that the fine saprolite exhibits poor permeability, and as a solution to this, pelletisation or agglomeration of the ore is necessary to ensure distribution of the leach solution through the heap.

U.S. Patent 6,312,500 also describes a process for heap leaching of laterites to recover nickel, which is particularly effective for ores that have a significant clay component (greater than 10% by weight). This process includes sizing of the ore where necessary, forming pellets by contacting the ore with a lixiviant, and agglomerating. The pellets are formed into a heap and leached with sulfuric acid to extract the metal values. Sulfuric acid fortified seawater may be used as the leach solution.

International application PCT/AU2006/000606 (in the name of BHP Billiton SSM Technology Pty Ltd) also describes a process where nickeliferous oxidic ore is heap leached using an acid supplemented hypersaline water as the

lixiviant with a total dissolved solids concentration greater than 30 g/L in order to leach the heap.

Heap leaching laterites offers the promise of a low capital cost process, eliminating the need for expensive and high maintenance, high pressure equipment required for conventional high pressure acid leach processes.

These patents and applications exclude the processing of limonitic laterite for heap leach because, in addition to the low reactivity, the reaction mechanism of whole-ore dissolution shown in Eq.1 and 2 may lead to the collapse and/or poor permeability of the heap due to the dissolution of nickel containing goethite or hematite as outlined above.

Each of these patents/applications does not claim to explore the whole ore body or the fraction of an ore body such as transition zone, which contain considerable limonite and saprolite. Most heap work testing to date has been conducted with large components of the heap comprising the coarser saprolitic component of a laterite ore.

One problem hindering the heap leaching of nickel and cobalt containing nickeliferous oxidic ores is the substantial clay or fine-particle component of such ores, particularly ores which contain significant quantity of limonitic type ores. The type of clay content is dependent on the parent rock and the physicochemical environment of the clay formation, but most clays have a detrimental effect on the percolation of the leach solution through the ore.

The present invention aims to develop a process where a heap comprising at least a significant proportion of both limonitic and saprolitic type ore components can be placed in a heap and subjected to heap leaching.

A reference herein to a patent document or other matter which is given as prior art is not to be taken as an admission that that document or matter was known or that the information it contains was part of the common general knowledge as at the priority date of any of the claims.

Brief Description of the Invention

The applicants have developed a process where a transition zone ore of a laterite ore body and/or the mixed blend of both limonite and saprolite ore may be heap leached without the associated difficulties, such as collapse and/or poor irrigation permeability of the heap, which may generally be expected when heap leaching a heap with a significant limonite content. The process of the invention includes the step of maintaining a sufficient proportion of both limonite and saprolite within the heap to maintain the integrity of the heap.

Accordingly, the present invention provides a process for the recovery of nickel and/or cobalt from a lateritic ore by heap leaching, the process including the steps of:

a) forming one or more heaps from a lateritic ore body wherein that lateritic ore body includes a blend of both limonitic and saprolitic type ores; b) leaching the ore heap with a leach solution; and c) recovering the nickel and/or cobalt from the resultant heap leachate.

The inventors have surprisingly found the outline shape of reacted saprolite or leaching residue during an acidic leach of a saprolite type ore was not changed due to the formation of SiO ₂ (Quartz). The reaction of acidic leach of saprolite showing the formation of quartz is shown in Eq, 3.

Saprolite Leach:

(Mg, NiJ ₂ - _S Si ₂ O ₅ (OH) ₄ + H ₂ SO ₄ → Mg ⁺² + Ni ⁺² + SO4 ^'2 + SiOp + HpO Eg.3

Serpentine

The mechanism could be described with "Shrinking Core Model" (SCM) with porous solid product (quartz) formation as shown in the photo of Figure 1. The formation of acid-resistant quartz can act as a "backbone" within the agglomerated pellets and can strengthen and stabilize a heap that includes limonite.

Consequently, the inventors have found that a heap charged with lateritic ore may be stable during acidic leach if the ratio of saprolite/limonite type ores is appropriately selected. In one embodiment, the inventors have found that a stable heap may be maintained if the limonite/saprolite blended heap includes 15%-85% wt of limonitic type ore and 15%-85% wt of saprolitic type ore. Preferably the blend will include 40%-60% wt of limonitic type ore and 40%- 60% wt of saprolitic type ore. The process of the invention is aimed at processing a blend of limonitic and saprolitic ore types, and includes transition zone ore where the limonite and saprolite components are already naturally blended. It also includes processing limonite and saprolite blends where the limonite and saprolite has been sourced individually from limonitic, saprolitic and/or transition zones.

Preferably the limonitic type ore to saprolitic type ore weight ratio in the blend within the heap will be anywhere from 5:1 to 1 :5, but most preferably about 1 :1. Further, the overall silicon content of the blended ore is preferably greater than 13% wt silicon, and more preferably within the range of 15% to 40% wt silicon. The blended limonite and saprolite preferably has a quartz content greater than 28% wt, more preferably, the blend will have a quartz content in the range of from 32% to 86% wt.

An advantage of the process of the present invention is that the run of mine ore may be used without post mining separation and/or classification, which includes directly processing transition type ore. This leads to exploration efficiencies as the whole of ore may be subjected to heap leaching.

In the process of the invention, the heap is subjected to a leach process by contacting a heap of the ore with a mineral acid selected from the group consisting of hydrochloric acid, sulfuric acid and/or nitric acid at an acid concentration sufficient to effect the dissolution of nickel, for example, at least about 0.1 molar (equals to 10 g/L H ₂ SO ₄ ). Preferably sulfuric acid is used. The aqueous leach solution may be acid supplemented fresh water, seawater or saline water.

The leaching is preferably carried out at a temperature of at least ambient temperature and ranging up to about 80 °C, if additional exothermic reaction such as bio-oxidation is introduced. The reaction time should be sufficient to dissolve substantial amounts of nickel and some iron and magnesium to provide a pregnant solution thereof.

The limonite ore generally consists of goethite and/or hematite type particles and is particularly fine and dissolved with acid as shown in Eq. 1 and 2. The saprolite ore is coarse silicates which volume was found unchanged during leach as shown in Eq.3 and the photo (Figure 1 ). Accordingly there needs to be a certain quantity of saprolitic type ore within the heap in order to give the heap some structure backbone during the leaching phase. The applicants have found that it is still possible to leach a blend of limonite and saprolite ore within a heap with as little as 15%wt of the heap comprising saprolitic type ores, and the blended material containing greater than 13% silicon, preferably in the range of from about 15% to about 40% silicon.

It is preferred to agglomerate or pelletize the particles prior to leaching in order to maintain high percolation flux to provide accelerated heap leaching kinetics. Further, pelletisation also enables for control of the iron inside the heap and decreases acid consumption by making larger particles from the fine particles.

In order to agglomerate the ore, a nickel containing lateritic ore may be crushed so that the particles are less than 2.5cm. The particles may then be agglomerated or pelletized by mixing the crushed lateritic ore particles with a concentrated acid, for example, in a rotary disk, drum or other suitable apparatus. Concentrated sulfuric acid is a preferred acid. The amount of acid used to agglomerate the pellets is generally that amount required to initially attack the nickel containing mineral matrix. In general, the amount of acid ranges from about 10kg to about 125kg of acid per tonne of ore, depending on the ore characteristics such as saprolite or limonite or smectite/nontronite/chlorite.

The pellets are then formed into a heap having a base and a top. A leach solution is applied to the top of the heap and allowed to percolate downward through the heap. The leach solution is collected at the bottom and may be recycled, or collected for nickel and/or cobalt recovery.

Preferably, a plurality of heaps is formed and arranged in at least a primary and secondary heap, the process including the steps of: a) adding the leach solution to the secondary heap to produce an intermediate pregnant leachate solution; and b) adding at least a part of the intermediate pregnant leachate solution to the primary heap to leach the primary heap in a counter current process, and producing a nickel and cobalt rich heap leachate for further nickel and/or cobalt recovery.

The intermediate pregnant leachate solution is rich in nickel and cobalt with low acidity, but also contains iron and a number of other impurities. The counter current heap leach process has the advantage of lowering acid consumption, and also achieves lower iron concentration and higher nickel concentration in the pregnant leachate solution and results in a cleaner product liquor of lower acidity than the single heap system.

The intermediate pregnant leachate solution may be divided into two fractions with the fraction having a pH of greater than 2 being removed and transferred downstream for nickel and/or cobalt recovery. The acidic fraction with a pH of less than 2 may be used for the leach solution in the counter current system. The nickel and cobalt rich heap leachate may be divided in a similar manner with the fraction having a pH greater than 2 being transferred for nickel and cobalt recovery, while the acidic heap leachate with a pH of less than 2 is used to leach a fresh heap in the counter current process.

The nickel and/or cobalt recovery may include ion exchange (IX), solvent extraction (SX), electrowinning (EW), multi-stage neutralisation to produce Ni/Co hydroxide, pyrohydrolysis to produce Ni/Co oxides and sulfidation to produce Ni/Co sulfides.

Brief Description of the Drawings

The figures of the accompanying drawings illustrate aspects of particular preferred embodiments of the present invention, wherein:

Figure 1 a porous solid product (quartz) formation used in an ore heap of one embodiment of the process according to the present invention.

Figure 2 shows progressive nickel extraction graphs of blended ores

Nos. 1 to 3 for example 5.

Examples

Example 1 : Mineralogy of the Blended Composite Ore

The blended ore sample taken from the same deposit for tests of Examples 2 to 4 was lightly ground using a mortar and pestle to separate aggregated grains, loaded into a stainless steel sample holders and analysed by XRD using the Scintag XTra diffractometer scanning between 2 and 80 degrees 2- theta with CuK _a ι _pha radiation. Polished sections of each sample were coated with around 40 nanometres of carbon and examined by scanning electron microscopy (SEM) and energy dispersive X-ray microanalysis (EDS) to obtain elemental compositions of individual minerals and determine speciation and distribution of nickel and cobalt. The results shown in Table 1 indicate that there were considerable existence of limonite and saprolite in the blended ore, classified with characteristic mineral goethite and serpentine respectively.

Table 1 : Mineralogy of the Blended Composite Ore

Example 2: Single-pass Column Leaching and Counter-current Column Leach without Acid Dose during Agglomeration

The laterite ore used for these tests were the blended composite ore with limonite/saprolite weight ratio of around 1 :1. Table 2 illustrates the chemical composition of the composite ore.

Table 2: Chemical Composition of Blended Laterite with Limonite/Saprolite Weight Ratio of 1 :1

No sulfuric acid was used during agglomeration as the ore contained clay marked by aluminium, which acted as a binding material for agglomeration. The agglomerated laterite was charged into two columns BC01 and BC02 with size of D x H = 160cm x 400 cm. In a single-pass style test, the blank sulfuric solution with constant acidity of around 100 g/L was fed into column BC01 to produce a PLS (pregnant leachate solution). The PLS fraction with pH>2 was directly transferred to downstream for nickel recovery. The acidic PLS fraction with pH<2 was transferred to column BC02 filled with freshly agglomerated ore

for counter-current leach. After the nickel extraction of BC01 approaching the targeted extraction and shut down, the feed solution to BC02 was switched to the blank sulfuric acid with acidity of around 100 g/L. The treatment of the PLS of Column BC02 was the same as Column BC01 : transfer the PLS fraction with pH>2 to nickel recovery section and transfer the acidic PLS fraction with pH<2 as feed solution for next column. The test conditions and results are listed in Table 3.

Table 3: Test Conditions and Results

Example 3: Single-pass Column Leaching and Counter-current Column Leach with Acid Dose during Agglomeration

The laterite ore used for these tests were the blended composite ore with limonite/saprolite weight ratio of 1 :1. Table 4 illustrated the chemical composition of the composite ore.

Table 4: Chemical Composition of Blended Laterite with Limonite/Sa rolite Wei ht Ratio of 1 :1

Concentrated sulfuric acid was used during agglomeration. The agglomerated laterite was charged into two columns BC08 and BC07 with size of D x H = 160cm x 400 cm. In a single-pass style test, the blank sulfuric solution with constant acidity of around 75 g/L was fed into column BC08 to produce a PLS (pregnant leachate solution). The PLS fraction with pH>2 was directly transferred to downstream for nickel recovery. The acidic PLS fraction with pH<2 was transferred to another column BC07 filled with freshly agglomerated ore for counter-current leach. After the nickel extraction of BC08 approaching

the targeted extraction and shut down, the feed solution to BC07 was switched to the blank sulfuric acid with acidity of around 75 g/L. The treatment of PLS of Column BC07 was the same as Column B08: transfer the PLS fraction with pH>2 to nickel recovery section and transfer the acidic PLS fraction with pH<2 5 as feed solution for next column. The test conditions and results are listed in Table 5.

Table 5: Test Conditions and Results

Example 4: Reproducibility Tests of Single-pass Column Leaching with Acid Dose during Agglomeration and Low Acid Feed Flow

The laterite ore used for these reproducibility tests was the blended composite ore with limonite/saprolite weight ratio of 1 :1. Table 6 illustrates the chemical composition of the composite ore.

Table 6: Chemical Composition of Blended Laterite with Limonite/Sa rolite Wei ht Ratio of 1 :1

Concentrated sulfuric acid was used during agglomeration. The agglomerated laterite was charged into two columns with size of D x H = 25cm x 300 cm. With a single-pass style test, the blank sulfuric solution with constant acidity of 50 g/L was fed into column SC30 and SC31 to produce a PLS (pregnant leachate solution). The test conditions and results are listed in Table 7.

Table 7: Test Conditions and Results

Example 5: Column Leach tests with blended limonite/saprolite composite ores Nos. 2 to 4

Three separate blended limonite/saprolite ore samples were subjected to column leach tests over a period of 10 months. The separate limonite/saprolite mineralogy, blended ore sample limonite/saprolite ratios, and 0 the blended ore sample chemistry are shown below in Table 8.

Table 8

The column leach independent variables and dependent variables are shown below. This data in Table 9 describes the test conditions at the completion of leaching.

The progressive nickel extraction graphs of blended ores Nos. 1 to 3 are shown in Figure 2.