"Iron Control in Leaching"

Field of the Invention

The present invention relates to a method for controlling iron concentrations in leaching processes. More particularly, the present invention relates to a process for inducing iron precipitation from an acidic ferric sulphate leach solution.

Background Art

Acidic ferric ion leaching is commonly practiced in the minerals industry. When acid contacts for example, an iron-containing sulphidic ore in an oxidising environment, acidic ferric sulphate leach solutions invariably form. The ferric ion plays an important role in the leaching process by oxidising the sulphidic ore and concomitantly being reduced to ferrous ion (Equation 1 ). Ferrous ion may then be re-oxidised to ferric ion by, for example, atmospheric oxygen (Equation 2). Such re-oxidation may be biologically-assisted by, for example, iron-oxidising bacteria. The equations below demonstrate such a process for chalcopyrite (CuFeS ₂ ) leaching.

Chalcopyrite leach: CuFeS ₂ +16Fe ³⁺ +8H ₂ O → Cu ²⁺ +17Fe ²⁺ +2SO ₄ ^2" +16H ⁺

Equation 1

Ferrous re-oxidation: 16Fe ²⁺ +4O ₂ +16H ⁺ -> 16Fe ³⁺ +8H ₂ O Equation 2

Overall reaction: CuFeS ₂ +4O ₂ -» Cu ²⁺ +Fe ²⁺ +2SO ₄ ^2" Equation 3

In acidic ferric sulphate leach processes, iron species may precipitate on the surface of the material being leached, resulting in hindered dissolution (reduced leaching rate) or "passivation". Hindered dissolution, or passivation, of for example chalcopyrite during acidic ferric sulphate leaching may be attributed to formation or precipitation of jarosite on the chalcopyrite surface (Equation 4).

Jarosite precipitation: 3Fe ₂ (SO ₄ ) ₃ + (K ₁ Na ₁ H ₃ O) ₂ SO ₄ + 12H ₂ O →

2(K,Na,H ₃ O)Fe ₃ (SO ₄ ) ₂ (OH) ₆ 4 + 6H ₂ SO ₄ Equation 4

In the context of the specification, the term jarosite refers to any iron-hydroxy- sulphate species of the general formula M _a Fe _b (SO ₄ ) _c (OH)d, where M is a metal ion such as Na ⁺ or K ⁺ , an ammonium ion (NH ₄ ⁺ ) or a hydronium ion (H ⁺ or H ₃ O ⁺ ).

The formation of jarosite is believed to be dependent on temperature (T) ₁ acid concentration (pH) and solution potential (Eh) where characteristically:

• the rate of jarosite formation increases with T;

• the rate of jarosite formation is slow below 60 ⁰ C;

• the rate of jarosite formation is at a maximum at about pH 2;

• the rate of jarosite formation is increasingly depressed at pH<1 , pH>3; and

• jarosite formation can be induced and accelerated by nucleation and / or seeding (e.g. with the provision of a high-surface-area, jarosite-like substrate).

Note that the pH region favouring jarosite formation is also the generally accepted "ideal" region for bio-leaching in terms of microbial activity. Illustrating this is the Eh-pH diagram for the Fe-S-K-O-H system at 25 ⁰ C shown in Figure 1 (Bigham, J. M., Schwertmann, U., Traina, S.J., Winland, R.L, WoIf ₁ M., 1996. Schwertmannite and the chemical modeling of iron in acid sulfate waters. Geochimica et Cosmochimica Acta, 60, 2111 -2121 ).

The rate of jarosite precipitation is enhanced by the addition of jarosite seed as shown in Figure 2 (Note that the activation energy of the process is unaltered.) (Dutrizac, J. E., 1996. The effect of seeding on the rate of precipitation of ammonium jarosite and sodium jarosite. Hydrometallurgy, 42, 293-312). This kinetic enhancement is important, as in the absence of seed, the conditions for

jarosite formation could exist without it apparently forming. Jarosite might thus appear a benign issue until massive precipitation occurred resulting in coated leaching surfaces.

Potentially, jarosite formation on leaching surfaces may be inhibited by providing conditions unfavourable to jarosite formation. However, preventing jarosite precipitation and still maintaining an ideal environment for oxidative dissolution is problematic. Although a low pH (~2) is required for jarosite formation, a very low pH (<1) is known to inhibit formation as shown in Figure 1. Possible alternatives would be to operate the process at either pH<1 or pH>3. In a biologically-assisted leach, this would necessitate the development of alternate microbial strains that are active away from the present ideal of just below a pH of 2. Further, an operating pH of <1 is unlikely to be economically attainable in practice in the presence of large amounts of acid-consuming gangue minerals, as for example in heap leach operations. Further still, controlling the leaching conditions in a heap is difficult compared to control in a reactor. Even if an average pH and temperature were targeted in a heap, local variations could occur that favoured jarosite formation.

There is a need to provide a method of controlling iron precipitation from acidic ferric sulphate leach solutions that provides a useful alternative to those already known.

Disclosure of the Invention

Those skilled in the art will appreciate that the invention described herein is susceptible to variations and modifications other than those specifically described. It is to be understood that the invention includes all such variations and modifications. The invention also includes all of the steps, features, compositions and compounds referred to or indicated in the specification, individually or collectively and any and all combinations or any two or more of the steps or features.

- A -

The present invention is not to be limited in scope by the specific embodiments described herein, which are intended for the purpose of exemplification only. Functionally equivalent products, compositions and methods are clearly within the scope of the invention as described herein.

The entire disclosures of all publications (including patents, patent applications, journal articles, laboratory manuals, books, or other documents) cited herein are hereby incorporated by reference.

In accordance with the present invention, there is provided a method for controlling iron precipitation from acidic ferric sulphate leach solutions in leaching of chalcopyrite and/or pyrite, the method comprising the steps of:

increasing the rate of formation of a ferric basic sulphate and/or promoting precipitation of a ferric basic sulphate; and

precipitating the ferric basic sulphate at a desired location.

The method of the present invention advantageously confers the ability to control the location in a leaching process where the ferric basic sulphate is precipitated. By controlling the location at which the ferric basic sulphate precipitates, the method of the present invention confers the ability to control iron levels in the leach solutions such that co-precipitation of the ferric basic sulphate with the chalcopyrite and/or pyrite is reduced.

The method of the present invention becomes particularly advantageous where the ore grade is low and/or the gangue contains considerable undesirable but reactive gangue material such as pyrite (FeS ₂ )

Advantageously, the method of the present invention enables higher valuable metal recovery and reduced reagent demand.

Preferably, the ferric basic sulphate is jarosite.

In one form of the invention, the method comprises the steps of:

increasing the rate of formation of a ferric basic sulphate; and

promoting precipitation of a ferric basic sulphate.

Preferably, the step of increasing the rate of formation of a ferric basic sulphate comprises the step of:

increasing the temperature of the leach solution.

Preferably, the step of increasing the temperature of the leach solution comprises the step of:

increasing the temperature of the leach solution at a first location.

Preferably, the temperature of the leach solution is increased at the location of precipitation of the ferric basic sulphate.

The step of increasing the temperature of the leach solution may be achieved by any method known in the art including the use of solar power, ultrasonic or microwave sources.

Preferably, the temperature at the location of precipitation of the ferric basic sulphate is between about 40 ⁰ C and about 80 ⁰ C.

Preferably, the pH at the location of precipitation of the ferric basic sulphate is about 2.

In one form of the invention, the step of promoting precipitation of a ferric basic sulphate comprises the step of:

adding seed to the leach solution.

It will be appreciated that seed with high surface area may be advantageous. Preferably, the seed comprises jarosite-like properties and in a preferred form of the invention, the seed is jarosite.

Without being limited by theory, it is believed that precipitation of a ferric basic sulphate may forego the need for further seeding.

Preferably, the step of adding seed to the leach solution comprises the step of:

adding seed to the leach solution at a second location.

Preferably, the seed is added to the leach solution at the location of precipitation of the iron-containing compound.

In one form of the invention, the step of promoting precipitation of a ferric basic sulphate comprises the step of:

in situ generation of seed in the leach solution.

It will be appreciated that the first location and the second location may be the same.

Preferably, the step of in situ generation of seed in the leach solution comprises the step of:

in situ generation of seed using ultrasonic or microwave sources.

Preferably, the step of precipitating the ferric basic sulphate at a desired location comprises the step of:

precipitating the ferric basic sulphate at a third location.

It will be appreciated that the third location may be the same as either the first location or the second location or the first location and the second location.

In one form of the invention, the method comprises the further step of:

removing the ferric basic sulphate from the leach solution.

The ferric basic sulphate may be removed from the leach solution by any means known in the art including the use of filters, screens, settlers and thickeners.

It will be appreciated that the method of the present invention may be applicable to any known method of leaching including heap leaching, dump leaching and vat leaching.

The method of the present invention may be performed as a batch process or a continuous process.

The method of the current invention may be performed in the same reactor volume as the leaching process, or as a separate process step in a separate reactor volume. In the context of the present invention, the term reactor volume shall be taken to include a specific vessel or a pond or an ore or concentrate heap or an in situ ore body.

In one form of the invention, the method comprises the further steps of:

separating at least a portion of the leaching solution into a separate reactor volume;

increasing the rate of formation of a ferric basic sulphate in the separate reactor volume and/or promoting precipitation of a ferric basic sulphate in the separate reactor volume; and

precipitating the ferric basic sulphate in the separate reactor volume.

Preferably, the method comprises the further steps of:

separating the ferric basic sulphate and the leach solution; and

returning the leach solution to the leaching process.

In one embodiment of the invention, the method comprises the further step of:

winning a desired material from the leach solution.

Where the ore material is chalcopyrite, the desired material is preferably copper.

The step of:

increasing the rate of formation of a ferric basic sulphate and/or promoting precipitation of a ferric basic sulphate;

may be conducted before, after or independent of the step of:

winning a desired material from the leach solution.

It will be appreciated that as the jarosite precipitation is an acid generation step, the rate of formation of jarosite and the rate of precipitation of jarosite may be enhanced by, but is not limited to, pH and sulphate adjustment.

Brief Description of the Drawings

The present invention will now be described, by way of example only, with reference to three embodiments thereof, and the accompanying drawings, in which:-

Figure 1 is a Eh-pH diagram for the Fe-S-K-O-H system at a temperature of

25 ⁰ C;

Figure 2 is an illustration of the significant enhancement of the rate of jarosite precipitation found in the presence of jarosite seed;

Figure 3 is a schematic illustration of the solar pond of the present invention;

Figure 4a is a flow diagram showing how a first embodiment of the present invention may be implemented in a typical heap circuit;

Figure 4b is a flow diagram showing how a second embodiment of the present invention may be implemented in a typical heap circuit;

Figure 4c is a flow diagram showing how a third embodiment of the present invention may be implemented in a typical heap circuit; and

Figure 5 is the predicted relative jarosite precipitation rate over a 40-60 ⁰ C solar pond range.

Best Mode(s) for Carrying Out the Invention

Those skilled in the art will appreciate that the invention described herein is amenable to variations and modifications other than those specifically described. It is to be understood that the invention includes all such variations and modifications. The invention also includes all of the steps, features, compositions and compounds referred to or indicated in the specification, individually or collectively and any and all combinations or any two or more of the steps or features.

By way of example, the method of the present invention is described in the context of the control of jarosite precipitation on chalcopyrite in a leaching process, although such should not be seen as limiting the generality of the foregoing description.

The invention describes the control of jarosite precipitation in a leaching process by careful control of the conditions amenable to jarosite formation and precipitation. By careful manipulation of the conditions throughout the leaching process through localised temperature changes and the addition of seed or the use of conditions amenable to seed formation, the fate and transport of jarosite in the leaching process may be controlled.

The process involves the combined or individual steps of increasing the leach solution temperature to accelerate jarosite formation, and/or the provision of seed material to promote jarosite formation away from the surface of the chalcopyrite.

According to a preferred embodiment of the invention, leach solution emanating from a chalcopyrite-containing heap is passed through a solar heated jarosite- precipitation pond for the purpose of lowering the iron content of the leach solution, thereby reducing iron precipitation and coating of the chalcopyrite in the heap.

This preferred embodiment has been found to be particularly useful in mitigating hindered dissolution (reduced leaching rates) or "passivation" attributable to jarosite coating of chalcopyrite surfaces in heap leaching.

The leachate is continually re-cycled and tapped off for copper recovery in a conventional fashion such as by solvent extraction, electrowinning or cementation. Advantageously, reducing background iron concentration can improve purity and recovery of the copper.

Before the leachate is re-cycled to the heap, it is directed to a solar pond and heated in the presence of jarosite seed. The solar-heated, jarosite precipitation circuit is provided in the form of a black plastic lined pond of a calculable area with a solar blanket and can be operated in batch or continuous mode. For continuous operation it would optionally be fitted with a staged weir design for concentration control. For batch mode the jarosite may remain in place and for the continuous weir design periodic jarosite removal may be required.

The pond would normally operate at a temperature elevated above the average core temperature of the heap and the leachate may only require a residence time of a few hours in the pond. Without being limited by theory, it is believed that residence time is governed by leachate chemistry and pond temperature. It will be appreciated that temperature, existing jarosite and/or other seed material surface area, mass flow rates compared to pond or reactor volume will all affect the residence time of the leachate in the pond. Part or all of the leachate emanating from the heap may be passed through the pond. It will be appreciated that both the heap and pond temperatures will be seasonally dependent. However, as the core temperature of the heap is also due to the sulphide

oxidation process it would be expected that the pond-to-heap temperature advantage would be at a maximum in summer.

Jarosite precipitation outside the heap provides advantages in addition to the ability to control jarosite precipitation. Jarosite precipitation generates substantial extra acid (see equation 4) which is particularly advantageous if the heap gangue is acid-consuming. Further, depending upon environmental rehabilitation requirements, the removal of the jarosite may present easier remediation/disposal options. If the heap were originally constructed with a view to capping, in accordance with current acid mine drainage (AMD) practice, there may be a significant reduction in future liability by acid control during the life of the heap; or indeed the leached heap may not actually require AMD remediation. Further still, some jarosites have commercial value as fertilizer additives. Still further, jarosite removal improves copper recovery and lowers iron levels in copper recovery circuits. It will be appreciated that jarosite precipitation prior to copper recovery may result in a small copper loss via jarosite incorporation.

Design of pond dimensions is dependent upon heap mineralogy/chemistry, tonnages, and copper recovery circuit capacity, but it should be designed to last the lifetime of the heap. Whilst periodic tap-off volume will set the pond leachate volume and the pond area, lifetime jarosite production will set the overall physical pond depth required. The intent of the pond design is that generally the jarosites remain in place once the heap is exhausted, i.e. although the jarosites can be recovered if desired (depending on purity), they can in fact be sealed at heap closure. It is expected that the volume of jarosite production would exceed the operating pond leachate volume by at least a factor of 10:1 or more over a typical heap leach operation lifetime.

Figure 3 schematically illustrates the solar pond cross-section for non-weir batch operation. The anticipated pond construction is below ground, for reasons of lower capital cost and reduced thermal losses, however the system can be constructed above ground. The primary elements of the pond are (i) carefully levelled base (for example, within 20 mm over entire pond bottom), (ii) black plastic lining (colours other than black will work but will reduce performance until

jarosite bed is consolidated), (iii) high R-value solar blanket. The vertical dimensions A, B, C and D are dictated by issues of heap leach operation (e.g. heap size, ore type and grade), solar radiation and ambient weather. In general, the overall height D is governed by the total expected jarosite recovery during the lifetime of the pond and is likely to be in the order of one to a few metres. It is believed that based on mineralogy and grade, the total maximum jarosite recovery is calculable. Initial operation can commence with seeding of the jarosite bed. Over heap and pond life the jarosite bed thickness C will continue to increase, eventually approaching D. The air gap A may range from zero through to a few 10's of millimetres. The air gap adds to the effective R-value of the blanket by reducing thermal conduction losses and prevents jarosite precipitation onto the blanket that would reduce solar transmission. The air-gap may be achieved by spaced plastic floats under the blanket (not shown). The optimum choice of leachate depth B is largely dependent upon available solar radiation, leachate composition and the preferred rate of jarosite and acid production. Lower values for B will result in higher leachate temperatures. For typical operations B would be expected to be, but not limited to, a value range of 100-200 mm. High R-value solar blankets are a commercial product with R-values ranging typically from 0.067 to 0.13. A secondary adjunct to the solar pond (not illustrated), advantageous in cooler climates, is the fitting of a light-weight clear UV-stabilized plastic film cover over the entire pond. This is conventional technology and can be fabricated, installed and supported in a number of ways (such as single skin, twin skin, fan inflated). Volumetric size and depth D are both operationally restrained and thus dictate the actual pond surface area. For a low grade (~1 % chalcopyrite) heap operation of 1 million tonnes, the pond might typically be in the order of 5000 m ² , with shape, aspect ratio and orientation to maximize solar performance. Side and bottom insulation would initially be advantageous, but with operation and jarosite deposition the jarosites' characteristics would dominate.

The extent to which the leachate's temperature can be elevated is decided essentially by the energy gain from solar radiation and the energy losses back to the environment. Characteristically, this is typically 10% to ground, 20% via radiation to sky and 70% via evaporation. Use of a solar blanket significantly

reduces the evaporative loss and converts that fraction to a smaller amount via conductive transfer. Both the solar transmission and the R-value of the blanket then become important. Energy loss as watts/m ² is determined by K/R where K is the gradient temperature difference ( ⁰ C or K) and R is the insulation value. Low K and high R minimizes this energy loss. Due to a drop in ambient temperature at night, the energy loss and leachate temperature drop can be substantial, so the preferred mode of operation is to substantively return pond leachate (and the associated low-grade heat) to the circuit in late afternoon. The actual temperature rise above ambient that is achievable is most critically dependent upon the leachate depth B. Maximizing the temperature rise increases the precipitation rate. Ignoring losses temperature rise inversely varies with B and the log of jarosite production rate varies linearly with the inverse of the temperature, so to a first approximation jarosite precipitation has an exponential dependence upon B; the smaller B the larger the precipitation rate. Adjustment of B gives the option for tailoring pond performance according to locality and environment. It will be appreciated that thermal considerations for pond design are within the knowledge of the skilled addressee.

Depending upon disposition of the pond with respect to the heap and other parts of the circuit, the most preferable configuration would have the pond gravity fed, but pump drained. Pump inlet for leachate removal, suitably guarded to not damage or entrain the pool blanket, is anticipated to be flexible, moored and fixed with floats to adjust to leachate level. End of batch pump operation would be dictated by the inlet resting on the jarosite bed.

Figures 4a to 4c schematically illustrate simplified flow diagrams showing how three embodiments of the present invention may be implemented in a typical heap circuit. The circuit incorporation could use any one of the three ways individually or in combination.

Figure 4a shows a schematic flow sheet comprising the steps of:

passing recycled low iron high acid leachate 12 through a heap 14;

passing the leachate 16 to a solar jarosite pond 18;

precipitating jarosite 19 and obtaining low iron high acid leachate 12;

returning the low iron high acid leachate 12 to the heap 14.

Figure 4b shows a schematic flow sheet comprising the steps of:

passing recycled low iron high acid leachate 22 through a heap 24;

removing pregnant leachate 26 from the heap 24;

passing the pregnant leachate 26 to a solar jarosite pond 28;

precipitating jarosite 29 and obtaining low iron high acid pregnant leachate 30;

directing the low iron high acid pregnant leachate 30; to copper recovery

32;

recovering copper and returning the low iron high acid leachate 22 to the heap 24.

Figure 4c shows a schematic flow sheet comprising the steps of:

passing recycled low iron high acid leachate 42 through a heap 44;

removing pregnant leachate 46 from the heap 44;

directing the high iron high acid pregnant leachate 46; to copper recovery 48;

recovering copper and obtaining high iron barren leachate 50;

passing the high iron barren leachate 50 to a solar jarosite pond 52;

precipitating jarosite 53 and returning the low iron high acid pregnant leachate 42 to the heap 44.

The invention may be operated in either continuous or batch mode, although batch mode may offer a number of advantages. In particular for the simpler pond design, without weirs, as illustrated in Figure 3, the intent is batch operation. The aim of the batch process is not to specifically maximize individual jarosite or acid production per batch period, but rather to increase the rate at which this is achieved as increasing rate lowers the general cost. Investigations of batch jarosite production specifically display an initial maximum rate that then tapers off; the rate being driven by the higher initial ferric and sulphate concentrations. The batch cycle may be any convenient time period, although the maximum solar benefit is gained by a leachate fill in early morning and then decant in late afternoon with the pond having low leachate levels at night. This reduces the nightly energy loss and allows hot acid to be returned to the heap during evening. Lack of leachate movement during the majority of the residence period assists the jarosite production. Residence time might typically be, but not restricted to, 8 hours. Leachate may be left overnight or for extended period without any subsequent performance issues. Moreover during period changeover the best performance will result from changing only part of the pond leachate; this would typically be but is not limited to, 50%. Partial changeover means not operating to excessive levels of iron removal, so maximizing rate, but it also achieves simpler operating conditions. This is also a practical acknowledgement on several points (i) excessive removal of ferric below 11.2 g/L can diminish immediate leaching activity in the heap; (ii) the requisite small values for B will restrict pump throat dimensions and (iii) full drainage is impractical.

A high surface area pond, without the clear plastic over-cover, is subject to rainfall issues. The optimum mode of operation is dependent not only upon the leachate chemistry and heap leach management but also the prevailing weather conditions. With a leachate depth of 100-200 mm, 24 hour rainfall periods with, for example, 20 mm of rain represents only a minor perturbation on return leachate chemistry, both in terms of dilution and depression of temperature.

Diagnostic choices for batch operation are relatively simple and require no sophisticated control methodology. Optimum depth B can be estimated by modelling and verified by practice; for a float defined air gap and known jarosite bed depth this is a simple periodic measurement. Monitoring pond leachate temperature with time gives a guide as to optimum return time for purposes of returning the low-grade heat. Monitoring pond leachate pH enables the progress of the precipitation reaction to be followed as it directly gives sulphuric acid production and indirectly both jarosite production and iron removal values. Simple T and pH indicators thus control residence time.

Based on ore grade mineralogy and expected heap lifetime, the gross design features of the solar pond can be determined in advance; the key parameters of B plus the T and pH for residence time can be optimised at operation commencement and changed as the heap changes behaviour, so the Invention represents a versatile and low capital cost solution to iron control. For a low- grade heap of 1 million tonnes with a project recovery time of 2 years, consisting of 1 weight % chalcopyrite, 2 weight % pyrite and the remainder unreactive gangue it is estimated that a 500 m ³ leachate volume pond could control the leachate iron problem. At a leachate depth of 100 mm the solar pond footprint of 5000 m ² would be only about 10% the footprint of the actual heap. With an overall depth of 1.2 m the pond envelope (in this example it would have a total volume of 6000 m ³ ) could store the entire jarosite output of the heap over the two years estimated lifetime based on 50% iron removal from the pond leachate for 8 hours batch operation and in the process returning over 16 tonnes of sulphuric acid per day.

Table 1 shows the sensitivity of iron removal both to temperature and to residence time for solar pond leachate conditions for iron levels at the lower end of the concentration range that would typically be expected for heap leachate. At higher iron levels the rate of iron removal would be expected to be higher. As noted, solar pond temperatures are environmentally dependent but can be manipulated by adjustment of B. Under most summer or dry season conditions an operational solar pond range of 50-60 ⁰ C would be expected and for temperate winter or wet season conditions an operational range of 40-50 ⁰ C. The lower end cool season

temperature may have an impact on performance so the B value may need to be reduced to maintain the leachate temperature toward 5O ⁰ C.

Table 1. Iron removal in relation to temperature and residence time Based on the known behaviour that Arrhenius activation energy for jarosite precipitation is preserved; though rate is dramatically enhanced in the presence of seed the typical results shown in Table 1 can be generalized with some confidence over other temperatures relative to a fixed value. This predicted relationship based on preserved activation energy is shown in Figure 5 and shows that over the 45-5O ⁰ C range the precipitation efficiency is 17-31 % compared to that at 6O ⁰ C. From Table 1 it can be seen that doubling residence time at the higher iron concentration end more than doubles the jarosite precipitation as a consequence of the lower turbulence. It is known that at sufficient seed concentration the initial behaviour is at least linear with time so that for an 8 hour residence time the worse case scenario for the cool season low end 45-5O ⁰ C range is at least 15-27% removal, though the reality is that it is likely to be much higher.