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HYDROMETALLURGY PROCESS OVERVIEW

1. SYNOPSIS

Ore brought to the surface from mining operations is treated in extractive metallurgical processes to extract the valuable contained components. These processes involve the major divisions of mineral processing, pyrometallurgy and hydrometallurgy. In general upfront mineral processing is followed by either pyro or hydrometallurgical processing or a combination of both.

This article focuses on hydrometallurgy processes notable for their ability to successfully recover a diversity of valuable metal products from a wide variety of feed materials. By definition these processes are carried out in aqueous solution generated in an initial dissolution step and subsequently upgraded using chemical process steps with final recovery of metal or metal compound product from the purified solution. Several flow sheets based primarily on hydrometallurgical processing are discussed. These include the recovery of gold from ores, the pressure acid leach process for lateritic nickel ores and the treatment of oxidised copper ores.

2. EXTRACTIVE METALLURGY – THE TREATMENT OF MINED ORE

Mined ore is treated via extractive metallurgical processes to extract intermediate or high-grade metal products. Each process flow sheet depends on the size, nature and grade of the ore deposit, the mineralogy of the ore, local infrastructure, availability of reagents and services such as power and water and the nature of the products produced. Extractive metallurgy processes can be divided up into a series of so-called unit operations

which involve the major process sectors of mineral processing, pyrometallurgy and hydrometallurgy [1]. Figure 1 is a generic extractive metallurgical processing scheme.

Upfront mineral processing, a feature of all extractive metallurgical flow sheets, encompasses steps aimed at making the feed material more amenable for downstream processes. The steps include comminution (crushing, grinding and classification) and physical separation/concentration (gravity separation, magnetic and electrostatic separation and flotation). The mineral processes product is an upgraded material (concentrate) that in many cases is the saleable product (iron and manganese ore processing and the processing of base-metal sulfides like copper, nickel and zinc) but typically is the feed to further processing by pyro- or hydrometallurgical processes or a combination of the two.

Pyrometallurgical processing is conducted at elevated temperatures close to or above the melting point of the metallic components of the minerals present. Some of the best-known successful industrial extractive metallurgy flow sheets feature a key pyrometallurgical step such as roasting or smelting that precedes the hydrometallurgy processes.

• Roasting: The high-temperature roasting of ore or concentrate (performed under oxidising or reducing conditions) yields an intermediate product more amenable to hydrometallurgical processing. The conventional roast/leach/electrowin processing route for the extraction of zinc from sulfide concentrates employs an oxidative roast of concentrate followed by dissolution of the calcine using spent zinc electrolyte. The solution is advanced to zinc electrowinning after undergoing impurity removal (the Zincor Refinery was South Africa’s only such refinery and has recently been decommissioned). The reductive roast/leach/precipitation process for the recovery of nickel and

cobalt from lateritic ores (the Caron Process used at Queensland Nickel Industries Yabula Refinery in Australia).

• Smelting: High temperature smelting of concentrate generates an impure metal (blister copper) or matte (mixture of synthetic metal sulfides) intermediate that feeds a hydrometallurgical process to yield a final metal product. In the copper smelting/electrorefining process blister copper produced by smelting copper-sulfide concentrate become the impure anodes refined into pure metal cathodes by electrolysis (practiced at Palabora Mining Company in South Africa and Mopani Copper Mines in Zambia).

The nickel matte smelting/ammoniacal leach/reduction process recovers nickel metal from sulfide concentrate at the WMC and Kwinana refineries in Western Australia. The platinum group metal (PGM) copper-nickel matte smelting/acid leach/electrowinning process for the recovery of base metals from PGM-containing concentrate as practised by South African platinum producers Anglo Platinum and Lonmin. Figure 1: A generic extractive metallurgical processing scheme

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3. HYDROMETALLURGICAL PROCESSES FEED MATERIALS

The treatment of sulfide minerals often involves a pyrometallurgical step, as in the examples above, but this is not always the case as sulfide minerals are also treated by only hydrometallugical upgrading (pressure leaching of base-metal sulfide concentrate). Similarly oxidised minerals are typically treated in hydrometallurgy-based extractive metallurgical flow sheets sometimes without undergoing concentration by mineral processing techniques.

One of the major advantages of hydrometallurgical processes is the ability to successfully recover valuable metals from a wide variety of feed materials. Low and high-grade examples are given in Table 1 [2].

Table 1: Range of feed materials treated in hydrometallurgical processes

Feed Materials Industrial ExamplesLow-grade ores • Gold ore - 1 to 5 g/t gold

• Low-grade copper ore – 0.4% Cu and up to 10% Fe

• Lateritic nickel ore – 1.5% Ni, 0.2% Co, 70% Fe

Low-grade concentrates and calcines

• Vanadium ore after magnetic concentration – 3% V2O5, 8% Fe2O3

High-grade concentrates and calcines

• Calcine from roasting of zinc concentrate – 75% ZnO, 8% Fe2O3

• Bauxite ore – 50% Al2O3, 15% Fe2O3

High-grade mattes • Nickel concentrate smelter matte – 40% Ni, 30% Cu, 20% S, 1% Precious Metals

High-grade metals • Copper anode produced in smelting process and fed to electrorefining – 95% Cu, 2% Ni

A further demonstration of the versatility of hydrometallurgical processes is the diverse range of metals and metallic products produced (Table 2).

4. UNIT OPERATIONS IN HYDROMETALLURGY

The major unit operations in hydrometallurgy can be sub-divided into three main groups, namely; leaching, separation and metal recovery processes.

4.1 Leaching (or dissolution)

The selective dissolution of the desired mineral in an ore, concentrate or intermediate product can involve many different chemical reactants (leachants or lixiviants, and oxidising/reducing reagents) and can be carried out in many different ways. The degree of complexity can vary from simple relatively inexpensive heap leaching to expensive high pressure and temperature autoclave processes. Mineralogy is the primary consideration in choosing leaching conditions inclusive of the chemical regime

and parameters like temperature, pressure and degree of mixing of component phases. Table 2 lists industrial metal recovery leachants along with possible end products.

Table 2: Metal recovery leachants for various feed materials with possible end products

Feed Leachant system Products

Gold/Silver ore CN- plus oxidant (O2)Au and Ag metal

Copper ore (oxidised)

H2SO4 Cu metal and sulfate

Copper (sulfide ore or concentrate)

H2SO4 plus oxidant (Fe3+, O2)

Cu metal and sulfate

Zinc calcine from roast of sulfide concentrate

H2SO4 Zn metal

Nickel laterite ore H2SO4 Ni metal, or NiONi laterite post reducing roast

NH3/(NH4)2CO3 NiCO3, Ni or NiO rounds

Ni/Co matte (sulfide)

H2SO4 or NH3/(NH4)2CO3

Ni and Co metal and sulfate

Cobalt ore (oxidised)

H2SO4 plus reductant (SO2)

Co metal or salt (hydroxide or carbonate)

Vanadium concentrate or calcine

Na2SO4 V2O5 and ammonium vanadate (AMV)

Uranium ore (U6+)

H2SO4 or CO3/HCO3 Uranium peroxide and ammonium di-urinate (ADU)

Uranium ore (U4=)

H2SO4 or CO3/HCO3 plus oxidant (MnO2)

Uranium peroxide and ADU

Rutile HCl or H2SO4 Synthetic rutile and TiO2

Bauxite NaOH Alumina (Al2O3)Tantalum/Niobium ore or concentrate

HF/H2SO4 Ta2O5 and Nb2O5

Manganese dioxide post reducing roast

H2SO4 Mn metal, electrolytic manganese dioxide (EMD) and sulfate

PGM concentrate HCl/Cl2 PGM metal and saltsRare earth intermediate (95% TREO)

HNO3 Rare-earth compounds

4.2 Separation, Concentration and Purification

After the leaching operation, the resulting slurry (pulp) or solution after solid-liquid separation must be subjected to one or more chemical process step designed to remove the impurities and/or concentrate the solution so that the desired metal can be successfully recovered in a pure form. These processes can involve selective precipitation, crystallisation, cementation, solvent extraction (SX), adsorption or ion exchange (IX). In-pulp processing is only viable for adsorption and IX processes that are particulate media systems.

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4.3 Precipitation and Reduction

The final metal or metal compound is produced from the purified solution by a precipitation, crystallisation or reduction step depending on the desired product. The reduction step can involve electrons as in electrorefining or electrowinning but can also involve a reductant such as hydrogen gas.

5. SELECTED HYDROMETALLURGY-BASED PROCESSES

A selection of process flow sheets that showcase the versatility of hydrometallurgical processing are discussed in this section. Each is a unique example of how hydrometallurgy steps are applied for the economic processing of ore that has undergone little or no prior upgrading.

5.1 Processing of Gold Ore

The processing of gold ore is probably the most remarkable example of the application of hydrometallurgical processes as it has made possible the economic recovery of gold from ore with grades lower than 2 g/t, achieving >90% recovery into a metal product of 99.999% Au purity. The cyanide-based recovery of gold, first commercialised around 1900, revolutionised gold ore processing.

Early process technology limited economic recovery to ore grades of >10 g/t Au. It was only in the early 1980s with the introduction of activated-carbon technology that the processing of low-grade ores of <5 g/t Au became feasible, significantly increasing the amount of economically viable gold resource in the ground [3]. The inherently simple and forgiving activated-carbon process technology was rapidly adopted by industry largely due to the open exchange of information and the large amounts invested in R&D globally. Figure 2 is a modern gold recovery flow sheet

incorporating activated-carbon technology. The reader is referred to the excellent publication edited by Stanley [4] that thoroughly covers the extractive metallurgy of gold.

Gold is the most noble of all the metals and is the only metal that is generally found in nature in the metallic state. It is the only metal, for example, that is not attacked in air or water by either oxygen or sulfur, and its durability under the most corrosive conditions has led to its widespread use in coinage and jewellery. This distinctive feature permitted the refining of gold from less noble metal from ancient times simply by oxidising the metals that as oxides are then easily separated from molten gold. Despite its extraordinary chemical stability gold is readily dissolved under oxidising conditions in acidic halide or dilute basic cyanide solution.

The successful commercialisation of the cyanide system for industrial-scale recovery of gold is attributed to the particularly high stability of the aqueous aurocyanide species. In the early flow sheets soluble gold was recovered from solution by cementation with powdered zinc dust (Merrill Crowe Process). The process is not suitable for all ore types, is technically challenging and not robust to process upsets. The counter-current decantation (CCD) process used for the solid-liquid separation of slurry excludes treating clay-type ores and soluble gold losses impose limitations on ore feed grade.

In the carbon-in-pulp (CIP) process granular activated carbon (burnt coconut shells) found to be a good adsorbent for gold is contacted directly with the slurry or pulp phase. In-pulp processing eliminates the expensive solid-liquid separation equipment and with it the associated soluble gold losses. Increased gold recovery from the solid pulp phase can be achieved by introducing the carbon into the leach. This so-called carbon-in-leach (CIL) process is especially suited for ores with gold adsorbent (preg-

robbing) mineral components as associated gold loss to the residue is prevented due to the higher gold-adsorbent affinity of activated carbon.

Gold as dilute as <1 mg/L can be upgraded several 1000 times by adsorption onto the particulate activated carbon and further concentrated into solution generated by the carbon desorption step (elution) performed at around 100 °C. The gold is finally recovered onto high-surface area steel wool cathode in an electrowinning step. The cathode is smelted in a furnace to produce gold and silver bullion. The carbon is recycled to adsorption after thermal treatment to recover its activity (gold adsorption capacity).

Figure 2: Modern gold recovery flow sheet

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South Africa has played a pivotal role in the technical development of CIP technology both at the state metallurgical laboratory, Mintek and the Anglo American Research Laboratories. In South Africa the application of CIP technology has enabled the economic recovery of gold from old tailings or dumps. Anglo American’s Ergo tailings retreatment plant, once the world’s largest gold production facility, is still in operation today (by DRD Gold Ltd) albeit on a much reduced scale. The economic recovery of gold from feed material at a grade of 0.3 g/t at this plant is testimony to the success of CIP process technology.

Further advances in gold recovery are expected with the industrial-scale commercialisation of the use of physically robust gold-selective IX resins suited for in-pulp application. The resins feature better gold loading capability than activated carbon, can be chemically eluted at ambient temperatures and don’t require thermal regeneration [5].

5.2 Processing of Nickel Laterites

While high-grade nickel sulfides are processed by smelting, low-grade oxidised laterites (0.5 to 3% Ni) until recently could not be economically treated with available technologies. Since 1998 several new laterite flow sheets have been commissioned [6]. These processes all use pressure acid leaching (PAL) to solubilise the nickel, but the downstream hydrometallurgy flow sheets have significant differences. A comprehensive review of the flow sheets can be found in reference [7]. Many of the process plants were for various reasons closed and only a few remain operational today.

Figure 3: Goro nickel laterite flow sheet

The Goro process developed by INCO (now Vale) features perhaps the most interesting process flow sheet, shown schematically in Figure 3. The plant situated in New Caledonia is still in the commissioning phase and when nameplate capacity of 55 000 t/a nickel is reached the Goro plant will be the largest capacity of the new generation laterite circuits and promises to be the world’s lowest cost producer [8].

The ore is leached using sulfuric acid under pressure at 270 °C. After clarification the solution is partially neutralised to remove impurities like iron, aluminium and chrome. Bulk extraction of nickel and cobalt is achieved using Cyanex 301, the sulfur-substituted analogue of Cyanex 272 (both extractants from Cytec Industries) used widely in industry for cobalt-over-nickel extraction. At low pH Cyanex 301 is shown to be selective for nickel and cobalt

Figure 4: pH dependence of cation extraction from sulfate medium with Cyanex 301 [6].

over manganese, calcium and magnesium, the main impurity elements present in the ore (Figure 4).

Since Cyanex 301 is irreversibly poisoned by copper (Figure 4 illustrates the strong affinity for Cu) all traces need to be removed prior to SX. This is achieved to a level of < 0.04 mg/L copper in an IX step.

The SX extraction step is carried out in pulsed columns easily sealed to prevent the ingress of deleterious oxygen. Very slow reaction kinetics precludes the use of sulfuric acid for stripping. The loaded solvent is stripped instead with hydrochloric acid as the reaction is kinetically positive and conveniently places the nickel and cobalt in a chloride

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medium in which nickel-from-cobalt separation is well proven industrially.

In chloride solution nickel is present as a neutral species (NiCl2) and cobalt and zinc as the anionic species, MCl4

2-. Anionic zinc is removed in an IX step. Cobalt is separated from nickel using a tertiary amine extractant that extracts the anionic cobalt species while leaving the neutral nickel species in solution. Nickel is recovered as NiO in a pyro-hydrolysis process that also produces HCl for recycle to the SX. Cobalt is recovered as a carbonate salt.

5.3 Processing of Copper Oxide Ore

More than 20% of total copper production is today via hydrometallurgical processing using a combination of leaching in sulfuric acid, upgrading and purification of the copper by SX, and recovery of the high-purity copper metal by EW. The key enabling technology is the SX step that for the primary processing of copper has enjoyed spectacular growth over the past 35 years due primarily to developments in copper-specific extractants used for the extraction of copper from acidic sulfate liquors. These hydroxyoxime extractants exhibit excellent selectivity for copper other cations commonly found in sulfate liquors as is evident in the extraction versus pH-data shown for a modern ketoxime extractant in Figure 5.

It is other chemical properties of this type of acidic chelating extractant that enable unique acid balances to be achieved between the various hydrometallurgy process steps. The acid balance between leaching, SX and EW is shown in Figure 6, a simplified copper hydrometallurgy flow sheet showing the chemical reactions pertinent to each process step. The leaching reaction is for the copper mineral malachite and RH is the organic extractant used in the SX [10].

The extraction of each copper ion in the SX step releases two protons, thereby providing a useful source of acid for further leaching. Since the complexation of copper with the extractant molecules is an equilibrium reaction, copper can be stripped from the organic phase by applying the reverse reaction, contacting

Figure 5: pH-dependence of extraction of cations from sulfate solution with a ketoxime extractant [9]

the loaded organic phase with strong acid. This generates a purified, concentrated copper solution from which the metal can be recovered by EW. The EW reactions also generate acid and the spent electrolyte from the EW circuit is recycled as the strip liquor to the SX circuit, providing a closed loop for the acid requirements of the process.

Heap leaching technology is used for the low-grade copper deposits found in South and North America while agitation leaching is the predominant method used for the higher grade African copper-belt ores. A typical flow sheet for treating African Copper Belt ore is shown in Figure 7 [11].

The African Copper Belt deposits contain significant reserves of cobalt associated with the copper. Secondary supergene minerals dominate the oxide ore bodies–malachite (Cu2(CO3)(OH)2) for copper and heterogenite (CoO(OH)) for cobalt, which give way to sulfide phases such as chalcocite (Cu2S) and carrollite (CuCo2S4) at increasing depths. Grades vary between 0.7 and 6 % for copper and up to 1.03 % for cobalt. Oxide copper leaches readily in dilute sulfuric acid; secondary sulfides require oxidative conditions, and cobalt, present as Co3+, requires reductive conditions to ensure good recoveries.

Soluble losses of copper and acid in discarded leach tails result in lost copper revenues and acid credits for the leach. With copper

prices of greater than 8,000 US$/t and the high cost of acid, it is vital to minimise these losses. Minimisation of soluble copper and acid losses is ensured by including high (HG) and low-grade (LG) copper SX circuits in the the flow sheet. Leach slurry fed to a primary thickener produces a high-grade copper overflow stream and the underflow reports to the CCD wash circuit. Exiting washed residue contains reduced levels of soluble copper and acid and the overflow is a low-grade copper stream. Treating the high and low-grade copper streams in separate SX circuits allows high-grade raffinate to be returned to leach, maximising acid recycle, and reducing the quantity of acid and copper that reports to the wash residue. The efficacy of the split circuit concept depends on how well the leached ore responds to the primary S/L separation

Figure 6: Schematic copper hydrometallurgy flow sheet [10]

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step and enables the copper SX to be configured to produce a feed to the cobalt circuit with a low copper tenor.

It is crucial to minimise liquid discharges to tails and the input of fresh water, an important consideration in a high rainfall region like the DRC. Water balance maintenance is greatly eased by decreasing the volume of water that enters with the milled ore. Two ways to achieve this are by high-compression thickening of pre-leach solids and milling in SX raffinate. The latter practise, successfully used at the Sepon operation in Laos, requires expensive materials of construction in the pre-leach circuit.

Figure 7: Copper oxide treatment flow sheet

The relatively high levels of manganese in the DRC Copper Belt ores are potentially problematic. Manganese, transferred via entrainment into the advance electrolyte from the SX circuit can be oxidised at the anodes in the EW circuit. The oxidised species can cause degradation of the SX organic phase. Minimising manganese transfer to the EW circuit is done by selecting an extractant that exhibits good physical phase-separation and minimal crud generation properties or by including an organic wash stage in the SX configuration.

6. REFERENCES

1. Handbook of Extractive Metallurgy, Vols 1, 2, 3 and 4, F. Habashi, Ed., Wiley, Weinheim, Federal Republic of Germany, 1997.

2. M. J. Nicol, Hydrometallurgy Theory and Practice, Introduction to Hydrometallurgical Processes, M.Sc. Course Notes, University of Cape Town, 2003.

3. M. J. Nicol, Developments in Hydrometallurgy since Mintek 50, Mintek 75 Conference, Randburg, 2009.

4. The Extractive Metallurgy of Gold in South Africa, Volume 1, G. G. Stanley, Ed., South African Institute of Mining and Metallurgy

Monograph Series M7, 2001.5. M. Kotze, B. Green, M.

Mackenzie and M. Virnig, Resin-in-Pulp and Resin-in-Solution, Chapter 2.5.3, Submission for publication in: Advances in Gold Ore Processing

6. K. C. Sole, Solvent Extraction in the Processing and Purification of Metals: Process Design and Selected Applications, In Solvent Extraction and Liquid Membranes: Fundamentals and Applications in New Materials, M. Aguilar, J. L. Cortina, Eds, Taylor and Francis, New York, 2008, pp 158 - 166

7. K. C. Sole and P. M. Cole, Purification of Nickel by Solvent Extraction, In Ion Exchange and Solvent Extraction, Y. Marcus, A. K. SenGupta, Eds, Vol 15, Marcel Dekker, Switzerland, 2002, pp. 143 – 195

8. G. Bacon and I. Mihaylov, Solvent extraction as an enabling technology in the nickel industry, In Proceedings of the International Solvent Extraction Conference, Vol. 1, K. C. Sole, P. M. Cole, J. S. Preston, D. J. Robinson, Eds, South African Institute of Mining and Metallurgy, Johannesburg, South Africa, 2002, pp. 1 - 13

9. K.C. Sole, Solvent Extraction in the Processing and Purification of Metals: Process Design and Selected Applications, In Solvent Extraction and Liquid Membranes: Fundamentals and Applications in New Materials, M. Aguilar, J. L. Cortina, Eds, Taylor and Francis, New York, 2008, pp 148 – 155

10. M.E Schlesinger, M.J. King, K.C Sole and W.G. Davenport, Extractive Metallurgy of Copper, 5th Ed, Elsevier, Amsterdam, Netherlands, 2011.

11. P.M. Cole and A. M. Feather, Processing of African Copper-Belt Copper-Cobalt Ores: Flowsheet Alternatives and Options, In Proceedings of the ISEC 2008 International Solvent Extraction Conference, Vol. I, B. A. Moyer, Editor-in-Chief, Canadian Institute of Mining, Metallurgy and Petroleum, Montreal, Canada, 2008, pp 131 – 138.