Pyrometallurgy
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Pyrometallurgy is a branch of extractive metallurgy. It consists of the thermal treatment of minerals and metallurgical ores and concentrates to bring about physical and chemical transformations in the materials to enable recovery of valuable metals.[1] Pyrometallurgical treatment may produce products able to be sold such as pure metals, or intermediate compounds or alloys, suitable as feed for further processing. Examples of elements extracted by pyrometallurgical processes include the oxides of less reactive elements like iron, copper, zinc, chromium, tin, and manganese.[2]
Pyrometallurgical processes are generally grouped into one or more of the following categories:[3]
- calcining,
- roasting,
- smelting,
- refining.
Most pyrometallurgical processes require energy input to sustain the temperature at which the process takes place. The energy is usually provided in the form of combustion or from electrical heat. When sufficient material is present in the feed to sustain the process temperature solely by exothermic reaction (i.e. without the addition of fuel or electrical heat), the process is said to be "autogenous". Processing of some sulfide ores exploit the exothermicity of their combustion.
Calcination
[edit]Calcination is thermal decomposition of a material. Examples include decomposition of hydrates such as ferric hydroxide to ferric oxide and water vapor, the decomposition of calcium carbonate to calcium oxide and carbon dioxide as well as iron carbonate to iron oxide:
- CaCO3 → CaO + CO2
Calcination processes are carried out in a variety of furnaces, including shaft furnaces, rotary kilns, and fluidized bed reactors.
Roasting
[edit]Roasting consists of thermal gas–solid reactions, which can include oxidation, reduction, chlorination, sulfation, and pyrohydrolysis.
The most common example of roasting is the oxidation of metal sulfide ores. The metal sulfide is heated in the presence of air to a temperature that allows the oxygen in the air to react with the sulfide to form sulfur dioxide gas and solid metal oxide. The solid product from roasting is often called "calcine". In oxidizing roasting, if the temperature and gas conditions are such that the sulfide feed is completely oxidized, the process is known as "dead roasting". Sometimes, as in the case of pre-treating reverberatory or electric smelting furnace feed, the roasting process is performed with less than the required amount of oxygen to fully oxidize the feed. In this case, the process is called "partial roasting" because the sulfur is only partially removed. Finally, if the temperature and gas conditions are controlled such that the sulfides in the feed react to form metal sulfates instead of metal oxides, the process is known as "sulfation roasting". Sometimes, temperature and gas conditions can be maintained such that a mixed sulfide feed (for instance a feed containing both copper sulfide and iron sulfide) reacts such that one metal forms a sulfate and the other forms an oxide, the process is known as "selective roasting" or "selective sulfation".
Smelting
[edit]Smelting involves thermal reactions in which at least one product is a molten phase. Metal oxides can then be smelted by heating with coke or charcoal (forms of carbon), a reducing agent that liberates the oxygen as carbon dioxide leaving a refined mineral. Concern about the production of carbon dioxide is only a recent worry, following the identification of the enhanced greenhouse effect.
Carbonate ores are also smelted with charcoal, but sometimes need to be calcined first.[citation needed] Other materials may need to be added as flux, aiding the melting of the oxide ores and assisting in the formation of a slag, as the flux reacts with impurities, such as silicon compounds.[citation needed] Smelting usually takes place at a temperature above the melting point of the metal, but processes vary considerably according to the ore involved and other matters.[citation needed]
Refining
[edit]Refining is the removal of impurities from materials by a thermal process. This covers a wide range of processes, involving different kinds of furnace or other plant.
The term "refining" can also refer to certain electrolytic processes. Accordingly, some kinds of pyrometallurgical refining are referred to as "fire refining".
See also
[edit]References
[edit]- ^ Brent Hiskey "Metallurgy, Survey" in Kirk-Othmer Encyclopedia of Chemical Technology, 2000, Wiley-VCH, Weinheim. doi:10.1002/0471238961.1921182208091911.a01.
- ^ Adalbert Lossin "Copper" in Ullmann's Encyclopedia of Industrial Chemistry 2005, Wiley-VCH, Weinheim. doi:10.1002/14356007.a07_471.
- ^ "Pyrometallurgy". Encyclopedia Britannica. Retrieved January 16, 2021.
External links
[edit]- U.S. Patent 5616168 Hydrometallurgical processing of impurity streams generated during the pyrometallurgy of copper
Pyrometallurgy
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Overview
Definition and Scope
Pyrometallurgy is a branch of extractive metallurgy that utilizes high-temperature thermal processes, typically above 500°C, to extract and refine metals from ores through chemical reactions involving gases, solids, and liquids.[5][6] These processes rely on heat to drive transformations such as decomposition, oxidation, and reduction, enabling the separation of valuable metals from gangue materials.[1] The scope of pyrometallurgy encompasses key unit operations including heating, melting, smelting, and gas-solid reactions, which facilitate the physical and chemical processing of ores and concentrates.[7] It contrasts with hydrometallurgy, which employs aqueous solutions for metal extraction and leaching, and electrometallurgy, which uses electrochemical methods for refining and deposition.[8] Core processes like roasting and smelting exemplify these operations within pyrometallurgy.[3] Key characteristics of pyrometallurgy include its high energy input to maintain elevated temperatures, often resulting in the production of byproducts such as slag—a mixture of metal oxides—and matte—a sulfide-rich phase.[7][9] It primarily targets oxide, sulfide, and carbonate ores for processing.[1] Common metals extracted via pyrometallurgical methods include iron, copper, lead, zinc, and nickel.Importance in Metallurgy
Pyrometallurgy constitutes a cornerstone of modern metallurgy, underpinning the bulk of primary metal production for essential industrial commodities. It accounts for approximately 80% of global primary copper output, where smelting and converting processes dominate the extraction from sulfide ores. For steel, pyrometallurgical methods via blast furnace-basic oxygen furnace routes represent about 71% of worldwide production in 2024, highlighting its enduring dominance in ferrous metallurgy. These contributions underscore pyrometallurgy's critical role in supplying metals vital for infrastructure, electronics, and energy sectors.[10][11] Beyond standalone application, pyrometallurgy is frequently integrated with hydrometallurgy in hybrid processing flowsheets to optimize recovery rates and resource efficiency, particularly for high-volume base metals like copper and nickel. This synergy allows for pretreatment via hydrometallurgical leaching followed by pyrometallurgical refinement, reducing waste and enhancing overall yield in complex mineral systems. Such combinations have become standard in operations targeting refractory ores, where pyrometallurgy's thermal intensity complements aqueous extraction.[12][13] The economic significance of pyrometallurgy is further amplified by its inherent advantages, including exceptional scalability for large-scale industrial operations and robust capacity to process complex, low-grade ores that challenge alternative methods. These attributes enable high-throughput facilities to meet surging global demand. In the context of iron and steel, smelting remains the predominant pathway, reinforcing pyrometallurgy's foundational impact on heavy industry.[14]Historical Development
Early Techniques
Pyrometallurgy's earliest techniques emerged in ancient civilizations, where empirical methods relying on charcoal fires and simple furnaces laid the foundation for metal extraction. The bloomery process, one of the oldest known pyrometallurgical methods, was used to produce wrought iron by reducing iron ore in a low-oxygen environment. Dating back to approximately 1200 BCE, this technique emerged in the Middle East, with early evidence from Anatolia, and spread to sub-Saharan Africa and across Eurasia by the first millennium BCE, involving the heating of ore mixed with charcoal in clay or stone furnaces to form a spongy mass of iron called a bloom, which was then hammered to remove slag.[15][16][17] These early furnaces, often shallow pits or small shafts, operated at temperatures around 1200°C, enabling the direct reduction of iron oxides without melting the metal.[18] In medieval Europe, advancements in pyrometallurgical refining focused on precious metals, particularly through cupellation, a technique that utilized oxidizing atmospheres to separate silver from lead. Dating back to the 4th millennium BCE in ancient Mesopotamia, cupellation involved melting argentiferous lead in shallow bone ash or clay vessels within a furnace, where the lead oxidized to form litharge, which was absorbed by the vessel, leaving a bead of pure silver.[19][20] This method, practiced widely in regions like England and France for silver production from lead ores, represented a key step in refining impure metals and was integral to medieval coinage and jewelry making.[21] Calcination, as an initial ore preparation step involving heating to remove volatiles, was occasionally employed prior to these processes to improve ore reactivity.[22] The 18th and 19th centuries marked significant empirical innovations in pyrometallurgy, driven by the need for larger-scale production during the Industrial Revolution. Reverberatory furnaces, developed in the late 17th century in Wales for copper smelting, allowed indirect heating of ores with coal or wood fires, preventing fuel contamination of the charge and enabling the processing of sulfide ores into matte.[23][24] These furnaces, refined through the 1800s in Swansea's copper districts, featured arched roofs that reflected heat onto the ore bed, achieving temperatures up to 1200°C for smelting without direct flame contact.[25] A pivotal development came in 1856 with the Bessemer process for steel production, patented by Henry Bessemer, which involved blowing air through molten pig iron in a pear-shaped converter to oxidize impurities and convert it to steel rapidly.[26] This air-blown method drastically reduced production time from days to minutes, revolutionizing steel output for railways and machinery. Key documentation of these early techniques was provided by Georgius Agricola in his seminal 1556 work De Re Metallica, which detailed roasting to prepare ores by heating them in air to convert sulfides to oxides, and smelting processes using furnaces for metal extraction.[22] Agricola's comprehensive descriptions, based on observations in Saxony's mining regions, bridged ancient practices with emerging industrial methods, influencing metallurgists for centuries.[27]Modern Advancements
One of the key 20th-century milestones in pyrometallurgy was the development of the Outokumpu flash smelting process, introduced commercially in 1949 at the Harjavalta copper smelter in Finland.[28] This autogenous process, which involves injecting finely ground sulfide concentrates and oxygen-enriched air into a reaction shaft for rapid combustion, significantly improved efficiency over traditional reverberatory furnaces by reducing energy consumption by approximately 30% through minimized fuel use and heat recovery from exothermic reactions.[28] By the 1950s, it had achieved anode copper production at around 1000 kWh per ton, compared to 3000 kWh for earlier electrical smelters, enabling scalable copper matte production with lower operational costs.[28] Post-1950s advancements included the widespread adoption of electric arc furnaces (EAFs) for steel recycling, marking a shift toward scrap-based production in mini-mills.[29] These furnaces, utilizing graphite electrodes to generate arcs up to 6000°C, enabled capacities of 150-200 tons per heat by the 1950s, competing effectively with fuel-based Martin furnaces for carbon steel from scrap.[29] The integration of continuous casting in the 1960s further boosted EAF viability, reducing defects and costs while promoting circular economy principles through high scrap utilization rates exceeding 90%.[29] Post-2000 developments have focused on submerged arc furnaces (SAFs) for ferroalloy production, incorporating design enhancements like DC operation and advanced cooling systems for greater stability and energy efficiency.[30] For instance, facilities such as Eramet's 99 MVA furnace in New Caledonia, commissioned around 2004, utilize self-baking electrodes and hollow electrode systems to process fines directly, reducing energy use to 12,000 kWh per ton for silicon metal while handling diverse ores like high-alumina manganese.[30] These upgrades, including finite element modeling for sidewall cooling, have extended furnace life and supported annual outputs exceeding 150,000 tons of high-carbon ferrochrome in projects like those in India.[30] Plasma smelting has emerged as a versatile method for waste recycling in pyrometallurgy since the 1990s, enabling high-temperature vitrification and metal recovery from hazardous materials.[31] This process uses electric arcs to generate plasma at 5000-10,000°C, treating electronic scrap and metallurgical dusts to recover zinc and alloys while producing inert slag, as demonstrated in facilities recovering over 90% of valuable metals from waste streams.[31] Complementing this, the ISASMELT technology, commercialized from the early 1990s, applies top-submerged lancing for efficient smelting of secondary materials like battery scrap, with plants in the UK and Malaysia processing up to 40,000 tons per year of lead waste into clean metal and slag.[32] As of 2025, recent trends emphasize sustainability through oxygen enrichment in blast furnaces, which reduces CO2 emissions by lowering coke rates via increased secondary fuel injection like pulverized coal.[33] This approach, achieving net CO2 savings of up to 3 million metric tons per year in large operations, maintains furnace temperatures while cutting cokemaking emissions, with oxygen production's footprint offset by a factor of 4-6.[33] Concurrently, automation and AI integration in smelter process control have optimized parameters such as fuel input and airflow, enhancing metal recovery by 5-10% and reducing waste through real-time predictive modeling in pyrometallurgical operations.[34] For example, AI systems at facilities like ERG's Kazchrome have improved smelting stability since 2025 implementations, minimizing downtime via machine learning-driven adjustments.[35]Fundamental Principles
Thermodynamics of Reactions
The thermodynamics of pyrometallurgical reactions governs the feasibility and direction of high-temperature processes used in metal extraction and refining, primarily through the analysis of Gibbs free energy changes (ΔG). The fundamental equation ΔG = ΔH - TΔS determines spontaneity, where ΔH is the enthalpy change, T is the absolute temperature, and ΔS is the entropy change; a negative ΔG indicates a spontaneous reaction under standard conditions. In pyrometallurgy, elevated temperatures (often exceeding 1000°C) amplify the -TΔS term, favoring reactions that increase gaseous products, such as the reduction of metal oxides by carbon monoxide or solid carbon, which release CO or CO₂. This principle underpins energy balances and equilibrium predictions, ensuring processes operate efficiently without excessive energy input.[1] Ellingham diagrams provide a graphical tool for assessing oxide stability and reduction potential, plotting the standard Gibbs free energy of formation (ΔG°) for the reaction 2M + O₂ → 2MO (normalized per mole of O₂) against temperature. These diagrams reveal that oxide stability decreases with temperature for most metals due to positive ΔS for dissociation, but reactions involving gas evolution, like 2C + O₂ → 2CO, exhibit negative slopes, enabling reductions at higher temperatures. For instance, the line for carbon oxidation to CO intersects the iron oxide (FeO) line at approximately 700°C, indicating that carbon can thermodynamically reduce FeO above this threshold, a key consideration in blast furnace operations. Such diagrams guide process design by identifying temperature thresholds where one oxide can reduce another, prioritizing more stable reductants like carbon over less effective ones like hydrogen.[36][37] Pyrometallurgical reactions encompass oxidation, reduction, and sulfation, each leveraging thermodynamic equilibria for selective metal recovery. Oxidation reactions, such as the roasting of sulfide ores (e.g., 2PbS + 3O₂ → 2PbO + 2SO₂), are spontaneous at moderate temperatures due to highly negative ΔG values driven by strong O₂ affinity for sulfur and metals. Reduction follows, as in the conversion of oxides using CO (e.g., PbO + CO → Pb + CO₂), where ΔG becomes negative above specific temperatures determined via Ellingham analysis. Sulfation reactions, employed for impurity separation, involve sulfate formation (e.g., ZnO + SO₃ → ZnSO₄), which is thermodynamically favorable at 500–700°C under controlled SO₂ partial pressures (10⁻³ to 10⁻⁶ atm), allowing selective volatilization of metals like zinc while retaining iron as oxide. Slag formation plays a crucial thermodynamic role in these processes by creating immiscible phases; fluxes like CaO react with silica impurities to form low-melting calcium silicates (e.g., CaO + SiO₂ → CaSiO₃), with negative ΔG promoting separation of oxides from molten metal and enhancing desulfurization through high partition coefficients (up to 70 for sulfur at low FeO levels).[1][38][39] A representative example is the reduction of wüstite (FeO) by CO: FeO + CO → Fe + CO₂. Thermodynamic calculations show ΔG° ≈ +10 kJ/mol at 1000°C under standard conditions. However, in smelting, CO/CO₂ ratios above the equilibrium value of approximately 2.5 make ΔG < 0, driving the reaction forward and confirming spontaneity under process conditions. These ΔG evaluations, derived from Ellingham data, optimize process conditions to minimize energy while maximizing yield.[36][1][40]Kinetics and Heat Transfer
In pyrometallurgical processes, reaction kinetics govern the rates at which thermal decomposition, reduction, and oxidation occur, often described by the Arrhenius equation, which relates the rate constant to temperature as $ k = A e^{-E_a / RT} $, where is the pre-exponential factor, is the activation energy, and is the gas constant.[41] This exponential dependence highlights how elevated temperatures, typical in pyrometallurgy (often exceeding 1000°C), exponentially accelerate reaction rates by providing the energy to overcome activation barriers.[42] In solid-state diffusion, a key mechanism for processes like sintering or phase transformations, activation energies range from 100 to 300 kJ/mol, reflecting the energy required for atomic jumps through lattices, which limits overall kinetics in dense materials.[43] Heat transfer in pyrometallurgical systems occurs via conduction through solid charge and furnace linings, convection driven by gas flows in the furnace atmosphere, and radiation, which becomes dominant above 1000°C due to high emissivity of molten slags and metals.[44] Conduction is prominent in static beds, while convection enhances mixing in fluidized or stirred reactors, and radiation accounts for up to 70-80% of heat flux in open-hearth or electric arc furnaces at operating temperatures.[45] Furnace efficiency, calculated as the ratio of useful heat absorbed by the charge to total heat input (typically 50-80% in smelters), is influenced by minimizing losses through refractory design and off-gas recovery, with examples showing thermal efficiencies around 75% in iron smelting operations.[46] Mass transfer plays a critical role in heterogeneous gas-solid reactions, such as roasting, where reactant gases must diffuse through boundary layers around particles and porous product layers to reach unreacted cores, often limiting the overall rate.[47] In sulfur removal during sulfide roasting, for instance, SO₂ evolution is controlled by diffusion across ash layers formed on pyrite (FeS₂) particles, with apparent activation energies of 80-100 kJ/mol indicating mixed chemical-diffusion control.[42] This diffusion-limited regime results in shrinking core models, where reaction fronts propagate inward, prolonging process times for larger particles.[48] Several factors modulate these kinetic and transfer processes: smaller particle sizes increase surface area, enhancing reaction speeds by up to 2-5 times in reduction steps, as seen in milled oxide ores where finer grains (<100 μm) accelerate zinc recovery.[49] Temperature gradients within furnaces, often 100-500°C from hearth to charge surface, create uneven heating that slows kinetics in cooler zones and risks thermal cracking, necessitating controlled gas injection for uniformity.[50] Stirring, via bottom gas bubbling or mechanical means, improves mass transfer by reducing boundary layer thickness and boosting convection, increasing slag-metal contact rates by 20-50% in converters and minimizing entrainment losses.[51]Core Processes
Calcination
Calcination is a key pyrometallurgical process involving the endothermic thermal decomposition of carbonate and hydrate minerals to remove volatile components such as CO₂ and H₂O, thereby converting the materials into more reactive oxides suitable for subsequent metal extraction steps.[1] This decomposition enhances the ore's porosity and reactivity, facilitating easier handling and reaction in later stages of metallurgical processing.[52] A classic example is the calcination of limestone (calcium carbonate), where CaCO₃ decomposes into CaO (lime) and CO₂ at temperatures typically ranging from 800°C to 1000°C:
This reaction requires careful temperature control to ensure complete decomposition without sintering the product.[1]
The process is usually carried out in specialized equipment such as rotary kilns, which provide continuous mixing and heat transfer, or fluidized bed reactors, which offer better gas-solid contact for efficient decomposition.[1][53] Energy input varies by equipment and material but generally falls in the range of 800–1200 kWh per ton for common applications, with fluidized beds achieving lower consumption (around 880 kWh/ton for lime production) compared to rotary kilns (up to 1290 kWh/ton).[1][54] These systems operate under controlled atmospheres to minimize unwanted side reactions, and the evolved gases are often vented or captured to manage emissions.
In industrial applications, calcination of limestone produces lime used as a flux in steelmaking and cement production, where it helps remove impurities like silica and phosphorus.[52] Similarly, the dehydration of bauxite (primarily aluminum hydroxides) via calcination yields alumina (Al₂O₃) for aluminum extraction, with the process consuming approximately 1150 kWh per ton in modern Bayer process facilities.[54] Another important reaction is the decomposition of magnesium carbonate to magnesium oxide:
at temperatures of 800–900°C, which is applied in refractory production and magnesium metal recovery.[55]
Given the significant CO₂ emissions from carbonate decompositions—accounting for up to 60% of process outputs in lime production—modern calcination incorporates CO₂ capture technologies, such as calcium looping or steam-assisted calcination, to mitigate environmental impact by separating and storing the gas stream.[56] Calcination often serves as a precursor step to roasting or smelting, preconditioning ores for these more intensive treatments.[1]
Roasting
Roasting in pyrometallurgy is a thermal process involving the controlled oxidation or sulfation of sulfide ores and concentrates at elevated temperatures, typically below 1000°C, to convert metal sulfides into oxides or sulfates while removing impurities.[57] This step prepares the material for subsequent smelting by enhancing reactivity and eliminating volatile elements such as arsenic and antimony.[58] The process generates sulfur dioxide (SO₂) as a byproduct, which is captured for sulfuric acid production in modern facilities.[57] Key types of roasting include oxidative, sulfating, and dead roasting, each tailored to specific ore compositions and objectives. Oxidative roasting involves partial or complete conversion of sulfides to oxides using oxygen, as exemplified by the reaction for iron sulfide:
conducted at 500–700°C to preferentially oxidize iron while preserving valuable metal sulfides.[59] For zinc sulfide, a representative reaction is:
which volatilizes sulfur and impurities like arsenic.[57] Sulfating roasting, often applied for chloride removal, forms metal sulfates (e.g., CuS + O₂ → CuSO₄) to facilitate leaching of impurities.[58] Dead roasting achieves near-complete sulfur elimination, producing an oxide-rich calcine with minimal residual sulfur (less than 1.5%), as in zinc processing where sulfides are fully oxidized.[60]
Equipment for roasting typically includes multiple hearth furnaces or flash roasters, which allow precise control of temperature and gas flow for efficient oxidation.[57] Multiple hearth roasters feature stacked levels for countercurrent gas-solid contact, while flash roasters inject preheated ore into a hot gas stream for rapid reaction. Off-gas treatment systems capture SO₂, with modern plants achieving up to 90% sulfur recovery through conversion to sulfuric acid.[61] In industrial applications, roasting is integral to copper and zinc production circuits; for instance, copper concentrates undergo partial oxidative roasting to adjust sulfur content before smelting, while zinc roasters employ dead roasting for high-purity oxide feed.[58] The roasted product directly feeds smelting processes to produce matte or metal.[57]
Smelting
Smelting represents a core pyrometallurgical process wherein roasted ores, charged with reductants such as coke and fluxes like limestone, undergo high-temperature melting and chemical reduction to separate molten metal from impurities in the form of slag. This operation typically occurs in furnaces maintained at 1500–2000°C, enabling the selective reduction of metal oxides while gangue materials form a silicate-rich slag that floats atop the denser metal phase. For iron production, the process yields pig iron, a crude form containing about 4–5% carbon, alongside slag primarily composed of calcium silicate (CaSiO₃).[1][62] Central to smelting are reduction reactions facilitated by carbon-based reductants. In the blast furnace for iron, coke first combusts with oxygen-enriched air to generate carbon monoxide:
This CO then reduces iron oxides stepwise:
where M denotes the metal, such as Fe from FeO.[1] In copper smelting, the process promotes the formation of matte, an immiscible liquid phase consisting of copper(I) sulfide (Cu₂S) and iron(II) sulfide (FeS), typically at compositions around 70% Cu₂S and 30% FeS, which settles below the slag.[1][63]
Various furnace designs support smelting depending on the metal and ore type. The blast furnace, a tall shaft reactor, dominates iron production, with hot air blasts injecting at the tuyeres to sustain the reducing atmosphere. Electric arc furnaces employ graphite electrodes to generate intense heat via electric arcs, suitable for ferroalloys or secondary smelting, while submerged arc furnaces immerse electrodes in a charge for high-efficiency reduction in processes like ferrosilicon production.[62] Energy requirements for iron smelting in a blast furnace range from 10 to 15 GJ per ton of pig iron, encompassing fuel combustion, heat losses, and endothermic reductions.[64]
Byproducts from smelting include slag, which is granulated and repurposed in construction materials such as cement and road aggregates due to its hydraulic properties, and off-gases rich in CO and CO₂ that are captured for energy recovery through combustion or chemical synthesis. The feed to smelting often derives from prior roasting to convert sulfides to oxides.[1][62]