COUNTER CURRENT ATMOSPHERIC LEACH PROCESS

FIELD OF THE INVENTION

The invention relates to the hydrometallurgical processing of nickeliferous ores. In particular, although not exclusively, the invention relates to the extraction of nickel and/or cobalt from high magnesium, or saprolitic, laterite ores by atmospheric pressure acid leaching in combination with the extraction of nickel and/or cobalt from low magnesium, or limonitic, laterite ores by high pressure acid leaching. BACKGROUND TO THE INVENTION

A High Pressure Acid Leaching (HPAL) process is commonly used to extract nickel and/or cobalt from limonitic laterite ores. The process comprises adding concentrated sulphuric acid to ore slurry and heating to approximately 25O ⁰ C at a raised pressure. This enhances nickel and cobalt leaching kinetics and promotes the precipitation of iron as hematite.

It is beneficial to the extraction of nickel and/or cobalt to maintain an excess of acid, due to the consumption of acid by magnesium, aluminium, iron and the like which are present as constituents in the ore. The solution phase of the leach product, containing dissolved nickel and cobalt (the "pregnant solution"), typically also contains approximately 50 g/L of "free acid". This free acid is subsequently neutralized (totally or partially) by adding limestone or an equivalent alkali material prior to separation of the solid leach residue from the leach solution.

Increasing the pH of the slurry by neutralizing a portion of the free acid precipitates some of the impurities, such as iron and aluminium, from the pregnant solution and thus assists in downstream processing. However, the use of limestone or other alkaline neutralizing agents often results in voluminous gypsum and/or hydroxide precipitates, which are difficult to separate from the pregnant solution, and the release of significant quantities of carbon dioxide, which contributes to greenhouse gas emission. Additionally, this neutralization step can result in the co- precipitation, and therefore loss, of nickel and/or cobalt compounds. Saprolitic laterite ore, in place of all or part of the limestone, has been used to neutralise the free acid, with the added benefit of additional nickel and/or cobalt extraction from the saprolitic ore. US Patent 3,991 ,159 (Queneau et al) teaches neutralizing leach slurry from a high pressure and temperature acid leach of low magnesium laterite ore with a high magnesium ore. The neutralization process is carried out at a high temperature (200°C-300°C) and pressure (225psig-1750psig). Queneau alternatively teaches pre-treating the high magnesium ore with a leach solution from the high pressure and temperature leach, at a temperature not exceeding 15O ⁰ C. Thickened pulp from this low temperature leach is used in the high temperature neutralization described above, whilst metal values are recovered from the separated solution. However, the recovery of nickel from the high magnesium ore in this process is low compared to extraction by the HPAL process and high temperature and pressure are required for the neutralization step.

US Patent 4,548,794 (Lowenhaupt et al) teaches mixing the pregnant solution from an HPAL process with a high magnesium ore fraction, followed by a low pressure (90-300psig) leach. Whilst this process results in acid neutralization and the removal of impurities as sulphate precipitate which are easier to separate, the low pressure leach neutralization still requires the use of pressure vessels and the nickel extraction levels are relatively low.

More recently, US 6,379,636 (Arroyo et al) teaches a simple acid pre-leach of a saprolitic ore prior to its addition to an HPAL discharge in a neutralisation stage. Whilst this increases nickel extraction, a maximised nickel extraction requires maintaining an excess of acid throughout the leaching process which is contrary to the objective of acid neutralisation.

Similarly, WO 2006/084335 (Liu & Krebs) teaches a simple acid preleach of saprolitic ore prior to its addition to a primary HPAL slurry. This process also relies on the use of process water that has an ionic composition that substantially avoids the formation of jarosite. That is, the process water has low levels of sodium, potassium and ammonia.

As previously suggested by Arroyo et al, extra acid is added in the preleach which contradicts the objective of acid neutralization. Additionally, maintaining process water to contain only low ionic levels would adversely impact on the HPAL reaction kinetics, necessitating more reaction time with associated increase in equipment cost.

Thus the problem of achieving high extraction of nickel and cobalt, by employing high free acid levels in the atmospheric pressure leaching of

saprolitic ores without uneconomic consumption of sulphuric acid, has not yet been satisfactorily overcome.

OBJECT OF THE INVENTION

It is an object of the invention to overcome or at least alleviate one or more of the above problems and/or provide the consumer with a useful or commercial choice.

DISCLOSURE OF THE INVENTION

In one form, although it need not be the only or indeed the broadest form, the invention resides in a counter current atmospheric leach process comprising:

• subjecting a saprolite ore slurry to a first stage leach at atmospheric pressure;

• subjecting a first stage leach residue resulting from the first stage leach to a further leach at atmospheric pressure; • combining a high pressure acid leach (HPAL) discharge slurry with a further leach discharge resulting from the further leach;

• separating a further leach solution from the combined HPAL discharge slurry and the further leach discharge; and • recycling the further leach solution to the first stage leach at atmospheric pressure of the saprolite ore slurry. Suitably, the counter current atmospheric leach process further includes:

• presenting a first stage leach solution from the first stage

leach at atmospheric pressure, and a further leach residue from the combined HPAL discharge slurry and further leach discharge, either mixed or separately, to downstream processing for the recovery of dissolved nickel and cobalt. In a preferred embodiment the counter current atmospheric leach process further includes mixing together for a period of time the HPAL discharge slurry with the further leach discharge resulting from the further leach prior to the step of separating the further leach solution from the combined HPAL discharge slurry and the further leach discharge. In a further preferred embodiment of the counter current atmospheric leach process a portion of the further leach solution is recycled to the further leach at atmospheric pressure and a portion of the further leach solution is recycled to the first stage leach at atmospheric pressure. Further features of the present invention will become apparent from the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS To assist in understanding the invention and to enable a person skilled in the art to put the invention into practical effect preferred embodiments of the invention will be described by way of example only with reference to the accompanying drawings, wherein:

FIG. 1 shows a flow diagram of a first embodiment of the counter- current atmospheric leach process;

FIG. 2 shows a flow diagram of a second embodiment of the

counter-current atmospheric leach process; and

FIG. 3 shows a flow diagram of a further embodiment of the counter-current atmospheric leach process.

DETAILED DESCRIPTION OF THE INVENTION The present invention provides a counter current atmospheric leach process of high magnesium, or saprolitic, fraction of lateritic ore, with improved utilization of sulphuric acid, both freshly added and as byproduct of a parallel high pressure acid leach (HPAL) of low magnesium, or limonite, fraction of lateritic ore. This method maximizes the extraction of nickel and cobalt whilst minimizing the free acid level of the resultant pregnant leach solution.

Referring to FIG. 1 , in one embodiment of the invention a continuous counter current atmospheric leach process is described in which saprolite ore is mixed with an aqueous phase (e.g. fresh water, seawater, or process solution) to produce saprolite ore slurry 101.

The saprolite ore slurry 101 is mixed with a second stage leach solution 110 and the mixture is subjected to a first stage leach 102 at atmospheric pressure and a temperature close to the boiling point of the slurry, preferably 7O ⁰ C to 105 ⁰ C. In the first stage leach 102, nickel, cobalt and other metals are solubilized from the saprolite ore slurry 101 into a solution phase. Heating of the mixture of saprolite ore slurry 101 and second stage leach solution 110 may be achieved by the addition of steam from an HPAL process, or other conventional method as would be known to a person skilled in the field. A reductant 119 (e.g. solid sodium

metabisulfite (SMBS) or SO ₂ gas) may be added to improve extraction of valuable metals.

Retention in the first stage leach 102 is typically 1-6 hours. A first stage leach discharge 103 is directed to a first separation device 104 and separated into a first stage leach solution 105 and a first stage leach residue 106. The first separation device 104 may be a thickener or other conventional separation method either separately or in combination. The residual free acid present in the first stage leach solution 105 is typically less than about 50g/L, preferably less than 10g/L. The first stage leach residue 106 with the addition of sulfuric acid

113, preferably concentrated, is then subjected to a second stage leach 107 at atmospheric pressure and a temperature close to the boiling point, preferably 7O ⁰ C to 105 ⁰ C. The quantity of sulfuric acid 113 added is adjusted to maintain sufficient excess acid in the second stage leach 107 as required to maximize nickel and cobalt extraction, with excess acid levels typically 50g/L to 150g/L, preferably 100g/L of contained solution. As in the first stage leach 102, reductant 119 (e.g. solid sodium metabisulfite (SMBS) or SO ₂ gas) may be added to the second stage leach 107 to improve extraction of valuable metals. Retention in the second stage leach 107 is typically 1-6 hours. A second stage leach discharge 108 is directed to a second separation device 109. The second separation device 109 may be a thickener or other conventional separation method either separately or in combination.

Concurrently to the above processes, a limonite ore is subjected to

a conventional HPAL process (not shown) at elevated temperature and pressure. The HPAL leach residue and HPAL leach solution are discharged to atmospheric pressure and consequently release a quantity of steam and cool to boiling point, forming HPAL discharge slurry 112. The residual free acid in the HPAL discharge slurry is typically 25g/L to 75g/L, preferably 50g/L.

The HPAL discharge slurry 112 is combined with the second stage leach discharge 108 in the second separation device 109. Operation of the second separation device 109 results in a second stage leach solution 110 and a second stage leach residue 111. The residual free acid present in the second stage leach solution 110 is typically 40g/L to 100g/L, preferably 70 g/L to 80 g/L. The second stage leach solution 110 is then directed to the first stage leach 102.

The second stage leach residue 111 and the first stage leach solution 105 are processed, either in combination or separately, for the recovery of dissolved nickel and cobalt.

The first stage leach 102 and the second stage leach 107 are carried out as part of a continuous process, each in either one or a series of agitated tanks. FIG. 2 shows a second embodiment of the counter-current atmospheric leach process. Similarly to the process shown in FIG. 1 and described above, the saprolite ore slurry 101 is mixed with a second stage leach solution 110 and the mixture is subjected to a first stage leach 102 at atmospheric pressure. A first stage leach discharge 103 is directed to a

first separation device 104 and separated into a first stage leach solution 105 and a first stage leach residue 106.

The first stage leach residue 106 with the addition of sulfuric acid 113, preferably concentrated, is then subjected to a second stage leach 107 at atmospheric pressure. A second stage leach discharge 108 is then directed to a discharge slurry mix tank 114. The discharge slurry mix tank 114 may be one or a multiple of tanks equipped with mixing or agitating apparatus.

HPAL discharge slurry 112, as described in the process of FIG. 1 above, is directed to and combined with the second stage leach discharge 108 in the discharge slurry mix tank 114 and agitated for an extended time. A mix tank discharge 115 is then directed to a second separation device 109. Operation of the second separation device 109 results in a second stage leach solution 110 and a second stage leach residue 111 , as discussed in the process of FIG. 1 , and the second stage leach solution 110 is then directed to the first stage leach 102.

The second stage leach residue 111 and the first stage leach solution 105 are processed, either in combination or separately, for the recovery of dissolved nickel and cobalt. In a further embodiment of the invention, shown as an optional feature in FIG. 3, a portion of the second stage leach solution 110 may be diverted 116 to the second stage leach 107 to lower the leach solids concentration. A lower solids concentration in the second stage leach 107 may increase the leach kinetics. The diversion 116 of the second stage

leach solution 110 may occur in either of the embodiments shown in FIG. 1 or FIG. 2.

In another embodiment of the invention, the saprolite ore used in any embodiment of the invention may undergo a pre-treatment process. Pre-treatment of the saprolite ore prior to the production of the saprolite ore slurry 101 may increase the efficiency of the counter-current atmospheric leach process. Two examples of pre-treatments are shown as optional features in FIG. 3. In one pre-treatment process, the saprolite ore undergoes a calcination process 117. Calcination may increase the reactivity of the saprolite ore slurry 101 and thus increase its acid neutralization capacity. In an alternative pre-treatment process, the removal of an ultra-fine size fraction 118 from the saprolite ore slurry 101 may increase the efficiency of the first and second separation devices, thus increasing the efficiency of the process. The embodiments of the invention maximize nickel and/or cobalt extraction from the saprolite ore into a solubilised form in the liquid phase by retaining high acid strength leach conditions, yet simultaneously reduce concentrated sulfuric acid consumption. A further benefit is the reduction in the quantity of limestone required for downstream neutralization and consequently a reduction in carbon dioxide evolution.

The following example illustrates, but does not limit, an embodiment of the present invention.

The example shows laboratory simulated continuous processing of nickeliferous saprolite ore by an embodiment of the counter-current

atmospheric leach flowsheet as depicted in FIG. 1.

The first stage leaching had a retention time of 6 hours. Low magnesium saprolite ore slurry was mixed with a simulated second stage leach solution to give slurry at 10% solids and heated to 90-95 ⁰ C. Separation of the first stage leach discharge via a thickener gave a first stage leach solution with a free acid content of 35 g/L and a first stage leach residue.

The first stage leach residue was directed to a second stage leach, which had a retention time of 6 hours. Concentrated sulphuric acid was added and the temperature maintained at 90-95 ⁰ C. The second stage leach solution, contained 89 g/L free acid.

With additional utilization of excess acid in the HPAL discharge of about 40 kg/t limonite ore (HPAL feed), the total acid addition required was 550 kg/t saprolite ore processed. Thus a saving of acid used of 228 kg/t of saprolite ore was achieved.

EXAMPLE 1

(1) utilization of residual acid in HPAL discharge slurry

Throughout the specification the aim has been to describe the invention without limiting the invention to any one embodiment or specific collection of features. Persons skilled in the relevant art may realize variations from the specific embodiments that will nonetheless fall within the scope of the invention. For example, a. an HPAL discharge may be combined with the residue from an initial atmospheric pressure leach prior to a second atmospheric pressure leach, b. a leach solution from an initial atmospheric leach may be processed downstream either together with or separate from a combined HPAL and second atmospheric leach residues, and c. additional atmospheric leach stages may be added in similar counter current mode. It will be appreciated that various changes and modifications may be made to the embodiment described without departing from the spirit and scope of the invention.