MSE3541 Hydrometallurgy Course Outline PDF

MSE3541 Hydrometallurgy Online ASSOC. PROF. DR. METİN GENÇTEN Thursday [email protected] 15.00-16.45 [email protected] Week Subject 1. Introduction to hydrometallurgy. General information about the aim, subject and evaluation of the course. Historical developme...

MSE3541 Hydrometallurgy Online ASSOC. PROF. DR. METİN GENÇTEN Thursday [email protected] 15.00-16.45 [email protected] Week Subject 1. Introduction to hydrometallurgy. General information about the aim, subject and evaluation of the course. Historical development, importance, and advantages of hydrometallurgical methods. Comparison of the related method with 2. pyrometallurgical methods. Basic steps of hydrometallurgical methods. 3. Pre-preparation processes in hydrometallurgical methods (Crushing, Grinding, Enrichment, Roasting) 4. Purpose and fundamentals of leaching process in hydrometallurgical methods. Acidic, basic, and bacterial leaching processes. 5. Selection of leaching reagents and reactions 6. Solid-liquid separation processes 7. Recovery metals from solution I 8. Midterm 9. Recovery metals from solution II 10. Hydrometallurgy of gold and silver 11. Hydrometallurgy of copper and aluminum 12. Hydrometallurgy of arsenic and antimony 13. Hydrometallurgy of cobalt and nickel 14. Hydrometallurgy of uranium and thorium 15. Final exam ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 2  Evaluation I. Midterm: 30% Homework+presentation: 30% Final exam: 40% Attendance: at least 70% ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 3 Course Objectives To introduce the basic concepts and processes of production metallurgy. To provide training for evaluating our raw material resources and converting advanced technology into the products we need. To give examples from industrial applications. ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 4 Course Content Dissolution Techniques, Anodic and Cathodic Dissolution Reactions, Dissolution Thermodynamics and Kinetic / Factors Affecting Dissolution, Dissolution Techniques / Filtration, Physical and Chemical Precipitation Techniques / Problem Solving / Technical Trip / Hydrolytic and Ionic Precipitation / Homogeneous (Ionic and Non-ionic) and Heterogeneous (Electrochemical and Electrolytic) Reduction Deposition / Problem Solution Electrolytic Precipitation / Solvent Extraction (Ion Exchange) Deposition ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 5 What is metallurgy? Metallurgy is a domain of materials science and engineering that studies the physical and chemical behavior of metallic elements, their inter- metallic compounds, and their mixtures, which are called alloys. ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 6 Mineral and ore Ore is the rock from which the metal is extracted in a convenient and economical way. Ore has a composition that is definite. Metals that occur naturally in the earth’s crust are called minerals. Minerals that can profitably be used to get the metal are called ores. ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 7 ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 8  Extractive metallurgy Extractive metallurgy is the practice of removing valuable metals from an ore and refining the extracted raw metals into a purer form. ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 9 MSE3541 Hydrometallurgy Online ASSOC. PROF. DR. METİN GENÇTEN Thursday [email protected] 15.00-16.45 [email protected] What is metallurgy? Metallurgy is a domain of materials science and engineering that studies the physical and chemical behavior of metallic elements, their inter- metallic compounds, and their mixtures, which are called alloys. ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 2 Mineral and ore Ore is the rock from which the metal is extracted in a convenient and economical way. Ore has a composition that is definite. Metals that occur naturally in the earth’s crust are called minerals. Minerals that can profitably be used to get the metal are called ores. ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 3 ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 4  Extractive metallurgy Extractive metallurgy is the practice of removing valuable metals from an ore and refining the extracted raw metals into a purer form. ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 5 Extractive metallurgy 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. Electrometallurgy is a method in metallurgy that uses electrical energy to produce metals by electrolysis. It is usually the last stage in metal production and is therefore preceded by pyrometallurgical or hydrometallurgical operations. Hydrometallurgy is a technique within the field of extractive metallurgy, the obtaining of metals from their ores. Hydrometallurgy involve the use of aqueous solutions for the recovery of metals from ores, concentrates, and recycled or residual materials. ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 6 History of Hydrometallurgy Thousands of years ago people had learned how to build furnaces and use fire to melt rocks and produce metals but the use of aqueous solutions for ore processing came much later, mainly at the time of the alchemists when acids and alkalies became known and used. Modern hydrometallurgy, however, can be traced back to the end of the 19th century when two major operations were discovered: the cyanidation process for gold and silver extraction and the Bayer process for bauxite treatment. Later, in the 1940s, a breakthrough came during the Manhattan Project in USA in connection with uranium extraction. Since then, it has been advancing progressively and even replacing some pyrometallurgical processes. Canadian contribution is significant particularly in the recovery of uranium, nickel, cobalt, and zinc. ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 7  History of Hydrometallurgy The roots of hydrometallurgy may be traced back to the period of alchemists when the transmutation of base metals into gold was their prime occupation. For example, when an alchemist dipped a piece of iron into a solution of blue vitriol, i.e., copper sulfate, the iron was immediately covered by a layer of metallic copper. Cu2++Fe→Cu+Fe2+ The discovery of aqua regia by Jabir Ibn Hayyan (720–813 AD) (Fig. 1), the Arab alchemist, may be considered as a milestone marking the beginning of hydrometallurgy. Aqua regia, i.e., royal water, is a mixture of HCl and HNO3 that dissolves gold; neither of the acids alone has any dissolving action. Aqua regia is still used today for gold refining, and chlorine one of its active ingredients: 3HCl+HNO3→Cl2+NOCl+2H2O was utilized extensively for extracting gold from its ores till the 1890s. ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 8 History of Hydrometallurgy In the Middle Ages, certain soils containing putrefied organic matter were leached to extract saltpeter (=salt of stone, potassium nitrate), a necessary ingredient for the manufacture of gunpowder. The process was fully described by Vannoccio Biringuccio (1480–1539) in his Pirotechnia published in 1540. In the 16th century, the extraction of copper by wet methods received some attention. Heap leaching was practiced in the Harz mountains area in Germany and in Rı´o Tinto mines in Spain. In these operations, pyrite containing some copper sulfide minerals was piled in the open air and left for months to the action of rain and air whereby oxidation and dissolution of copper took place. A solution containing copper sulfate was drained from the heap and collected in a basin. Metallic copper was then precipitated from this solution by scrap iron, a process that became known as bcementation process, which is apparently derived from the Spanish bcementacio´nQ meaning precipitation. ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 9 History of Hydrometallurgy In the 18th century, one of the most important industries in Quebec was the production of potash for export to France to satisfy the needs of the soap and glass industries. Before the invention of the Leblanc Process for the manufacture of Na2CO3 from NaCl, the main source of Na2CO3 was from ashes of seashore vegetation, and that of potash was from ashes produced by burning wood in areas where the clearing of forests was in progress on a large scale. The importance of this process to hydrometallurgy lies in the fact that leaching was extensively practiced. During the period 1767–1867, wood ash was collected from domestic stoves and fireplaces, and from lime kilns, then agitated with water, filtered, then evaporated to dryness to yield potash. One ton potash required the burning of 400 tons of hardwood, which is equivalent to the cutting of about 10 acres of forest. ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 10 History of Hydrometallurgy The beginnings The birth of modern hydrometallurgy dates back to 1887 when two important processes were invented. The first, the cyanidation process for treating gold ores, and the second, the Bayer Process for the production of alumina. ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 11  History of Hydrometallurgy Cyanidation process The dissolving action of cyanide solution on metallic gold was known as early as 1783 by the Swedish chemist Carl Wilhelm Scheele (Habashi, 1987). L. Elsner in Germany in 1846 studied this reaction and noted that atmospheric oxygen played an important role during dissolution. The application of this knowledge to extract gold from its ores was proposed and patented much later in England by John Stewart MacArthur (1856–1920) (Fig. 2) in 1887 and became known as the cyanidation process. G. Bodla¨nder in 1896 made the important discovery that hydrogen peroxide was formed as an intermediate product during the dissolution of gold. ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 12 History of Hydrometallurg y Cyanidation process The cyanidation process had already been applied to each mining district in the world and still its chemistry was very obscure. Its impact on hydrometallurgy had been tremendous. As a result of introducing the cyanidation process worldwide, gold production increased greatly during the period 1900–1910 (Fig. 3). ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 13  History of Hydrometallurgy Bayer process The second major hydrometallurgical process of this era was the process invented by Karl Josef Bayer (1847–1904) (Fig. 4) for the preparation of pure Al2O3 and known as the Bayer Process. This process was concerned with leaching bauxite, discovered in 1821 in France in a small village called Les Baux, near Marseille, with sodium hydroxide solution above its boiling point in a pressure reactor. ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 14 History of Hydrometallurgy Bayer process After separating the insoluble material, the pure solution was then seeded to precipitate pure crystalline aluminum hydroxide which was filtered, washed, dried, and calcined to pure Al2O3 suitable for charging to the electrolytic reduction cell invented 2 years earlier. Bayer was an Austrian chemist working in Saint Petersburg, in Russia; his process is used at present in its original version with practically no change. It is interesting to point out the following: ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 15  The process was originally developed to satisfy the needs of the textile industry since aluminum hydroxide was used as a mordant in dyeing cotton. It was only after the invention of the electrolytic aluminum process in 1886 that the process gained importance in metallurgy. Bayer’s first contribution was in 1887 when he discovered History of that Al(OH)3 precipitated from alkaline solution was crystalline, easy to filter, and wash free from impurities Hydrometallurgy while that precipitated from acid medium by neutralization was gelatinous and difficult to filter and wash. Bayer process A few years earlier to Bayer’s invention, Louis Le Chatelier (1815–1873) in France described a method for making Al2O3 by heating bauxite with Na2CO3 at 1200 ̊C, leaching the sodium aluminate formed with water, then precipitating Al(OH)3 by CO2 which was then filtered, dried, and claimed to pure Al2O3. This process was abandoned in favor of the Bayer process (Fig. 5). ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 16  History of Hydrometallurgy Bayer process ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 17 History of Hydrometallurgy ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 18 History of Hydrometallurgy ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 19 History of Hydrometallurgy ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 20 History of Hydrometallurgy ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 21 Hydrometallurgy vs Pyrometallurgy ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 22 Hydrometallurgy vs Pyrometallurgy ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 23 Hydrometallurgy Hydrometallurgy is typically divided into three general areas: Leaching Solution concentration and purification Metal or metal compound recovery ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 24 MSE3541 Hydrometallurgy Online ASSOC. PROF. DR. METİN GENÇTEN Thursday [email protected] 15.00-16.45 [email protected] Hydrometallurgy Hydrometallurgy is typically divided into three general areas: Leaching Solution concentration and purification Metal or metal compound recovery ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 2 Leaching Leaching is a primary extractive operation in hydrometallurgical processing, by which a metal of interest is transferred from naturally occurring minerals into an aqueous solution. In essence, it involves the selective dissolution of valuable minerals, where the ore, concentrate or matte is brought into contact with an active chemical solution known as a leach solution. ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 3 Solution concentration and purification The solution from a leaching process invariably contains impurities that need to be removed or reduced to as low as possible levels. In some cases, the concentration of the metal of interest is too low, and some form of concentration needs to be applied before the metal can be extracted economically. The concentration and/or purifying process of a metal-bearing solution can be accomplished in one of three ways. ❖Evaporation ❖Precipitation process ❖Processes utilizing a carrier phase ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 4  Metal or metal compound recovery Metal recovery is the final step in a hydrometallurgical process. Metals suitable for sale as raw materials are often directly produced in the metal recovery step. Sometimes, however, further refining is required if ultra- high purity metals are to be produced. The primary types of metal recovery processes are electrolysis, gaseous reduction, and precipitation. For example, a major target of hydrometallurgy is copper, which is conveniently obtained by electrolysis. Cu2+ ions reduce at mild potentials, leaving behind other contaminating metals such as Fe2+ and Zn2+. ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 5 Pre-steps of Hydrometallurgical Process Some of the basic pre-steps of the hydrometallurgy can be given as Crushing Grinding Enrichment Roasting ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 6 Crushing Crushing is the largest process operation in minerals processing. The goal of this process is to produce rock or (more seldom) mineral fractions to be used as raw material for other industrial production. The quality parameters are normally strength, size and shape. There are two kinds of equipment used for crushing works. One is by using crushers and other one is by using impactors. Size reduction of ores is normally done in order to liberate the value minerals from the host rock. This means that we This diagram illustrates the stages of size must reach the liberation size, normally in the interval 100 – reduction from 1000 mm to 4 mm. 10 micron, see value curve 1 above. ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 7 Crushing of rock and minerals Crushing is the largest process operation in minerals processing. The goal is to produce rock or (more seldom) mineral fractions to be used as rock fill or ballast material for concrete and asphalt production. Quality parameters are normally strength, size and shape. The kinds of materials processed are Limestone, Granite, Gabbro, Basalt, River Stone, Coal Gangue, Quartz, Diabase, Iron Ore, Copper Ore, Zinc Ore and Manganese Ore. ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 8  Crushing of rock and minerals Crushing means different things for different operations and the production goals are not always equal. In Mineral dressing, these two approaches are adopted: 1. Crushing of rock and gravel 2. Crushing of ore and minerals. There are three stages in crushing as stage 1, 2 and 3. In each stage the reduction in size ranges as referred to as R1,R2 and R3. This diagram illustrates the stages with equipments and reduction ratios. From 1000mm to 100 micron levels the mass is crushed. ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 9  Crushing of rock and minerals A crusher is a machine designed to reduce large rocks into smaller rocks, gravel, or rock dust. Crushers may be used to reduce the size, or change the form, of waste materials so they can be more easily disposed of or recycled, or to reduce the size of a solid mix of raw materials (as in rock ore), so that pieces of different composition can be differentiated. Crushing is the process of transferring a force amplified by mechanical advantage through a material made of molecules that bond together more strongly, and resist deformation more, than those in the material being crushed do. Crushing devices hold material between two parallel or tangent solid surfaces, and apply sufficient force to bring the surfaces together to generate enough energy within the material being crushed so that its molecules separate from (fracturing), or change alignment in relation to (deformation), each other. ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 10  Types of Crushers: Jaw crusher A jaw crusher is generally used as a primary crusher in a crushing circuit. Product is fed into the top of the jaw crusher by an vibrating grizzly feeder. The eccentric rotating drive shaft causes the movable jaw to oscillate crushing the aggregate against a fixed jaw. Jaw crushers are run on belt drives driven by an electric motor or diesel engine. Jaw crushers are used extensively throughout the aggregate and mineral processing industry. ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 11 Types of Crushers: Gyratory crusher A gyratory crusher is similar in basic concept to a jaw crusher, consisting of a concave surface and a conical head; both surfaces are typically lined with manganese steel surfaces. The inner cone has a slight circular movement, but does not rotate; the movement is generated by an eccentric arrangement. As with the jaw crusher, material travels downward between the two surfaces being progressively crushed until it is small enough to fall out through the gap between the two surfaces. ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 12 Types of Crushers: Gyratory crusher The gyratory crushers are robust crushers with modern features. They are designed to give optimal capacity in primary crushing, increasing the total capacity in the mining crushing process. These crushers have a large feed opening and a grooved mantle, making them suitable for crushing large boulders. This diagram shows the setup of a gyratory crusher. ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 13  Types of Crushers: Cone crusher With the rapid development of mining technology, the cone crusher can be divided into four types: compound cone crusher, spring cone crusher, hydraulic cone crusher and gyratory crusher. According to different models, the cone crusher is divided into VSC series cone crusher(compound cone crusher), Symons cone crusher, PY cone crusher, single cylinder hydraulic cone crusher, multi-cylinder hydraulic cone crusher, gyratory crusher, etc. A cone crusher is similar in operation to a gyratory crusher, with less steepness in the crushing chamber and more of a parallel zone between crushing zones. This diagram illustrates the anatomy of a cone crusher. ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 14 Types of Crushers: Impact crusher Impact crushers involve the use of impact rather than pressure to crush material. The material is contained within a cage, with openings on the bottom, end, or side of the desired size to allow pulverized material to escape. There are two types of impact crushers: horizontal shaft impactor and vertical shaft impactor. ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 15 Grinding Process Size reduction by crushing has a size limitation for the final products. If we require further reduction, say below 5-20 mm, we have to use the processes of grinding. Grinding is a powdering or pulverizing process using the rock mechanical forces of impaction, compression, shearing and attrition. The two main purposes for a grinding process are: ❖To liberate individual minerals trapped in rock crystals (ores) and thereby open up for a subsequent enrichment in the form of separation. ❖To produce fines (or filler) from mineral fractions by increasing the specific surface. ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 16 Grinding methods The major grinding methods are carried out by tumbling, stirring and vibrations as shown in the following illustration: ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 17 Grinding methods The major grinding methods are carried out by tumbling, stirring and vibrations as shown in the following Fig. All crushers including impactors have limited reduction ratios. Due to the design there is a restricting in retention time for the material passing. In grinding as it takes place in more “open” space, the retention time is longer and can easily be adjusted during operation. ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 18 Size reduction by Grinding The grinding of solid matters occurs under exposure of mechanical forces that trench the structure by overcoming of the interior bonding forces. After the grinding the state of the solid is changed: the grain size, the grain size disposition and the grain shape. Milling also refers to the process of breaking down, separating, sizing, or classifying aggregate material. For instance rock crushing or grinding to produce uniform aggregate size for construction purposes, or separation of rock, soil or aggregate material for the purposes of structural fill or land reclamation activities. Aggregate milling processes are also used to remove or separate contamination or moisture from aggregate or soil and to produce "dry fills" prior to transport or structural filling. Grinding may serve the following purposes in engineering: increase of the surface area of a solid, manufacturing of a solid with a desired grain size & pulping of resources. ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 19 Grinding machines In materials processing, a grinder is a machine for producing fine particle size reduction through attrition and compressive forces at the grain size level. In general, grinding processes require a relatively large amount of energy; for this reason, an experimental method to measure the energy used locally during milling with different machines was recently proposed. Hammer- and hammer impact mills are suitable for crushing soft to medium hard materials (degrees of hardness according to F. Mohs 2 - 5). For example: agglomerates, coal, limestone, gypsum and slag. They are designed for large through-put volumes and trouble-free operation. ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 20  Grinding machines-Ball Mills A typical type of fine grinder is the ball mill. It consists of a slightly inclined or horizontal rotating cylinder is partially filled with balls, usually stone or metal, which grinds material to the necessary fineness by friction and impact with the tumbling balls. Ball mills normally operate with an approximate ball charge of 30%. Ball mills are characterized by their smaller (comparatively) diameter and longer length, and often have a length 1.5 to 2.5 times the diameter. Ball mills are used for wet grinding iron ore, gold/copper ore, nickel ore and other ores, as well as lime/limestone for flue gas desulphurisation systems, coal and other raw materials. ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 21  Grinding machines-Rod mill A rotating drum causes friction and attrition between steel rods and ore particles. But note that the term 'rod mill' is also used as a synonym for a slitting mill, which makes rods of iron or other metal. Rod mills are less common than ball mills for grinding minerals. The rods used in the mill, usually a high-carbon steel, can vary in both the length and the diameter. However, the smaller the rods, the larger is the total surface area and hence, the greater the grinding efficiency. A few of them are suitable under wet or dry conditions exclusively. This diagram illustrates the operations of rod mills. ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 22 Grinding machines-Rod mill ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 23 Other Grinding Machines ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 24 Enrichment of Ores Before extracting the metal from an ore, it is necessary to remove the impurities present in it. By removing these impurities we get a concentrated ore which contains a high percentage of metal in it. The process of removal of gangue particles from an ore to increase the percentage of metal in ore is called enrichment of ore. The processes used for removing the impurities from ores depend on the difference between physical or chemical properties of the ore and of the impurities ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 25 Enrichment of Ores Hydraulic Washing This method is used for the enrichment of those ores which are heavier than gangue particles present in them. In this method, a stream of water is passed through crushed and finely powered ore. The Lighter gangue particles are washed away with water while the heavier ore particles are left behind. Oxide ores of tin and lead are concentrated by this method. ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 26  Enrichment of Ores Froth Floatation Process This method is used for concentration of sulphide ores of copper, lead and zinc. In this method, powdered ore is put in a tank full of water. And then, some Pine oil is added to it. In the tank the particles of sulphide ore are wetted by pine oil whereas the gangue particles are wetted by water. Then air is passed through this mixture. This results in the agitation of water in tank, which cause the sulphide ore particles to stick with the oil and rise to the surface in the form of froth. The gangue particles being heavier remain behind at the bottom of water tank. The froth is separated and concentrated sulphide ore is obtained from it. ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 27  Enrichment of Ores Magnetic Separation This method is used for concentration of magnetic ores of iron (magnetite and chromite) and manganese (pyrolusite) by removing non- magnetic impurities present in them. This process involves the use of a magnetic separator. A magnetic separator consists of a leather belt which moves over two rollers. Out of two rollers one roller has a magnet in it. In this method, the finely powdered magnetic ore is dropped over the moving belt at one end. When the powdered ore falls down from the moving belt at the other end having a magnetic roller, the particles of ore are attracted by the magnet and form a separate heap from the non- magnetic impurities. ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 28 Enrichment of Ores Chemical Separation This method is based on the differences in some chemical properties of the gangue and the ore. For example, the impure ore of metal aluminium (bauxite or aluminium oxide) is concentrated by Baeyer’s process. ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 29 Roasting Roasting is a process of heating of sulphide ore to a high temperature in presence of air. It is a step of the processing of certain ores. More specifically, roasting is a metallurgical process involving gas–solid reactions at elevated temperatures with the goal of purifying the metal component(s). Often before roasting, the ore has already been partially purified, e.g. by froth flotation. The concentrate is mixed with other materials to facilitate the process. The technology is useful but is also a serious source of air pollution ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 30 Roasting Roasting consists of thermal gas–solid reactions, which can include oxidation, reduction, chlorination, sulfation, and pyrohydrolysis. In roasting, the ore or ore concentrate is treated with very hot air. This process is generally applied to sulfide minerals. During roasting, the sulfide is converted to an oxide, and sulfur is released as sulfur dioxide, a gas. For the ores Cu2S (chalcocite) and ZnS (sphalerite), balanced equations for the roasting are: ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 31 Roasting The gaseous product of Roasting operations sulfide roasting, sulfur 1.1 Oxidizing roasting dioxide (SO2) is often used 1.2 Volatilizing roasting to produce sulfuric acid. 1.3 Chloridizing roasting Many sulfide minerals 1.4 Sulfating roasting contain other components 1.5 Magnetic roasting such as arsenic that are 1.6 Reduction roasting released into the 1.7 Sinter roasting environment. ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 32 Types of Roasting Oxidizing roasting Oxidizing roasting, the most commonly practiced roasting process, involves heating the ore in excess of air or oxygen, to burn out or replace the impurity element, generally sulfur, partly or completely by oxygen. For sulfide roasting, the general reaction can be given by: 2MS (s) + 3O2 (g) -> 2MO (s) + 2SO2 (g) Roasting the sulfide ore, until almost complete removal of the sulfur from the ore, results in a dead roast. Volatilizing roasting Volatilizing roasting, involves careful oxidation at elevated temperatures of the ores, to eliminate impurity elements in the form of their volatile oxides. Examples of such volatile oxides include As2O3, Sb2O3, ZnO and sulfur oxides. Careful control of the oxygen content in the roaster is necessary, as excessive oxidation forms non-volatile oxides. In Roasting control on oxygen content is necessary ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 33 Types of Roasting Chloridizing roasting Chloridizing roasting transforms certain metal compounds to chlorides, through oxidation or reduction. Some metals such as uranium, titanium, beryllium and some rare earths are processed in their chloride form. Certain forms of chloridizing roasting may be represented by the overall reactions: 2NaCl + MS + 2O2 -> Na2SO4 + MCl, 4NaCl + 2MO + S2 + 3O2 -> 2Na2SO4 + 2MCl2 The first reaction represents the chlorination of a sulfide ore involving an exothermic reaction. The second reaction involving an oxide ore is facilitated by addition of elemental sulfur. Carbonate ores react in a similar manner as the oxide ore, after decomposing to their oxide form at high temperature. Sulfating roasting Sulfating roasting oxidizes certain sulfide ores to sulfates in a controlled supply of air to enable leaching of the sulfate for further processing ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 34 Types of Roasting Magnetic roasting Magnetic roasting involves controlled roasting of the ore to convert it into a magnetic form, thus enabling easy separation and processing in subsequent steps. For example, controlled reduction of haematite (non magnetic Fe2O3) to magnetite (magnetic Fe3O4). Reduction roasting Reduction roasting partially reduces an oxide ore before the actual smelting process. Sinter roasting Sinter roasting involves heating the fine ores at high temperatures, where simultaneous oxidation and agglomeration of the ores take place. For example, lead sulfide ores are subjected to sinter roasting in a continuous process after froth flotation to convert the fine ores to workable agglomerates for further smelting operations. ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 35 MSE3541 Hydrometallurgy Online ASSOC. PROF. DR. METİN GENÇTEN Thursday [email protected] 15.00-16.45 [email protected] Hydrometallurgy Hydrometallurgy is typically divided into three general areas: Leaching Solution concentration and purification Metal or metal compound recovery ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 2  Leaching Leaching is a primary extractive operation in hydrometallurgical processing, by which a metal of interest is transferred from naturally occurring minerals into an aqueous solution. In essence, it involves the selective dissolution of valuable minerals, where the ore, concentrate or matte is brought into contact with an active chemical solution known as a leach solution. ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 3  Leaching means dissolution of the metal value into a suitable reagent that may also dissolve many other metals present in the ore. Prior to leaching, the ore may be subjected to preliminary treatments like crushing, grinding, and Leaching concentration by mineral beneficiation methods, which will not be dealt here. Crushing and grinding of the ore to a particular size help to liberate the mineral particles that can be easily digested by the solvent. ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 4  Grinding is generally not necessary for porous ores. For leaching purposes products are divided into two groups, namely, slimes and sands. Leaching Slimes are finely ground products that tend to pack in a vat or tank and thus prevent the free circulation of liquid through the interstices of the ore bed. Sands are coarse products to permit circulation of solvents through the void space between the ore particles. ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 5  Leaching Hence, slimes and sands are treated by agitation and percolation leaching methods, respectively. The leaching of low-grade ore is done on a very large scale. The great bulk of the treated ore requires a large amount of solvent. In such cases, for a profitable process, the solvent must be cheap and should be regenerative, if necessary. However, the cost of reagents for leaching the mineral containing large amount of valuable metal concentrated in a small bulk is not an important factor. Due to the small amount of metal in the low-grade ores the leaching cost must be reduced by using cheap reagents in dilute solutions. Although most leaching process can be accelerated by heating the solvent, heating is seldom practical in large-scale leaching. ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 6 Some materials like oxidized copper and uranium ores and certain gold ores can be directly leached. Occasionally, ores are concentrated prior to leaching to reduce the bulk so as to economize the process by using less amount of solution. Leaching For example, gold ore is concentrated by flotation. The solvent to be used in leaching must be selective to the metal and not to the gangue, cheap and readily available in large quantities, regenerative and quick in carrying out dissolution for commercial production. ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 7  Leaching Acids, alkalis, and salts have been very effectively used in leaching, for example, ▪sulfuric acid in leaching of uranium and oxidized copper ores and roasted zinc concentrate, ▪sodium hydroxide in dissolution of bauxite, ▪sodium carbonate solution in leaching of scheelite and oxidized uranium ores and in The use of ferric salts in dissolution of CuS and Cu2S the presence of oxygen, depends on the fact that they are reduced to ferrous ▪sodium cyanide and potassium cyanide salts as evident by the following reaction: solution in leaching of gold and silver ores are being used on a commercial scale to bring metal values in the leach liquor according to the reactions (only a few selected ones): ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 8  Ferric sulfate also functions in a similar way. Oxide and carbonate minerals of copper can be easily dissolved in dilute sulfuric acid but leaching of sulfides Leaching requires an oxidant in addition to the acid. The rate of leaching is enhanced tremendously in the presence of certain bacteria. ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 9  The choice of a particular technique depends on factors such as type of ore deposits, desired leaching rates, composition of the ore, nature of the gangue associated with the ore, and the subsequent separation and precipitation or extraction technique to be adopted. A brief outline of each method is given below. Leaching ❖In-situ Leaching Methods ❖Heap Leaching ❖Percolation Leaching ❖Agitation Leaching ❖Pressure Leaching ❖Bacterial Leaching ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 10 In-situ Leaching This method of leaching also known as solution mining is used when very low-grade ore is left out in the worked-out mines and also for recovery of metals from low-grade deep- seated ore deposits. In this method, ore bodies are fractured at the surface for penetration of the lixiviant inside the mine and the resultant leach liquor is pumped out to recover the metal value. In-situ leaching has been successfully practiced for recovery of copper and uranium in the Western United States. ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 11  Heap Leaching It is practiced by spraying a solvent over the ore lumps of less than 200 mm diameter, stacked in open atmosphere with the facility for drainage for collection of the leach liquor. The process is slow with low recovery. ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 12  Percolation Leaching It is adopted for crushed ore of 6–10 mm size placed in large tanks by percolating a number of solutions in increasing concentration. For effective leaching the ore should be course enough so that the leaching solution can move freely through the voids. The method is also known as sand leaching because of the use of coarse particles. The tanks made of wood and concrete and lined with lead or asphalt are used. To facilitate addition and withdrawal of leach solution and wash water, tanks are fitted with filter at the bottom. ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 13  It is employed for ore fines ground to less than 0.4 mm diameter. This is also known as slime leaching. The slime and the leach solution are agitated in one or more agitators until the ore minerals have dissolved. Some agitators have mechanically driven paddles or elevators inside the agitation tank that facilitates continuous circulation of the pulp to achieve complete dissolution. Agitation For slime leaching, another type of tank known as pachuca Leaching in the form of a cylindrical vessel with a conical bottom fitted with a coaxial pipe, with both ends open, for introducing compressed air for agitation is more popular. In both types of tanks the particles remain suspended in leach solution and are stirred mechanically or with jets of the compressed air. Though expensive, this is a faster and more efficient method as compared to the percolation leaching. ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 14 Agitation Leaching ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 15 Pressure Leaching Leaching is carried out in autoclaves at high pressure and relatively at higher temperature than possible in open leaching tanks. High pressure leaching is advantageous when gaseous reagents, for example, oxygen and ammonia are involved. The amount of gas held in the solution increases with pressure. Equations 11.1 and 11.5 demonstrate that oxygen is necessary for dissolution of uranium oxide in dilute sulfuric acid and of gold and silver in cyanide solution. The rate of dissolution of gold in cyanide solution increases with increase of oxygen pressure, which optimizes at a certain value. It is possible to dissolve sulfides directly in acids or ammoniacal solution in the presence of oxygen at higher temperature and pressure. ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 16  Thus, pressure leaching is advantageous because it permits a much higher concentration of gaseous reagents and higher operating temperature, which hastens the dissolution. In some cases, both these factors are important, or at least one is significant. It also prevents dissolution of gangue minerals. Regarding concentration of the gas, it is interesting to record some facts about the solubility of substances in Pressure superheated water. Leaching At the critical point of water (i.e., at 374C and 218 atm) liquid water and steam become a single phase. Solubility of gases decreases with increase of temperature between the freezing point of water and the normal boiling point. Dissolved air and gas come out of the solution as water warms up and practically all gases are expelled at the boiling point of water. On the other hand, many salts show increased solubility in water with rise of temperature. ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 17  Pressure Leaching At higher temperature and pressure maintained in the autoclave, these effects are reversed. As water approaches the critical point, the solubility of salts and gases approaches zero and infinity, respectively. Presumably, this is because all the gases are mutually soluble, and as liquid water approaches the critical point it behaves more like a gas. Probably for the same reason it loses its solvent power for salts. Practically, this means that leaching cannot be carried out at excessively high temperature because materials will not stay in solutions. Also, excessively high temperature would require extremely high pressure; the pressure of superheated steam at 374C is 218 atm (~3200 psi). The chemical reactions taking place in pressure leaching are quite complex. ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 18  In the Bayer process, the crushed bauxite ore is ground in a fine grinding mill. Strong caustic soda solution (130–350 g Na2O per liter) is introduced in the mill to obtain a slurry. Pressure The resultant slurry is pumped into a horizontal leaching of mild steel digester tank (autoclave) heated by steam under pressure with constant agitation. bauxite Aluminum gets dissolved as AlO2− anion according to the reaction at 25 atm pressure and 200 oC. The leach liquor is separated from the insoluble residue containing oxides of iron, silicon, titanium, and other gangue materials. ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 19 Pressure leaching of bauxite The resulting sodium aluminate solution is cooled to 25–35 oC to precipitate Al(OH)3 and regenerate sodium hydroxide (NaAlO2(aq) + 2H2O = Al(OH)3 + NaOH). Crystals of hydrated alumina (Al2O3.3H2O) are precipitated by seeding the solution with freshly precipitated aluminum hydroxide. Sodium hydroxide is recirculated after concentration and evaporation and addition of some new sodium hydroxide to compensate the loss. Pure alumina is obtained by calcination of the hydrated alumina at 1100 oC. This is dissolved in cryolite for electrolytic extraction of aluminum. ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 20 Bacterial Leaching In previous process, while listing various leaching reactions it has been mentioned that oxide minerals of copper can be leached easily in dilute sulfuric acid but dissolution of sulfide requires oxygen in addition to H2SO4 as per the reaction: ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 21 Bacterial Leaching The Fe2+ ions produced by the above reaction are re- oxidized. Thus, reduction/oxidation reactions continue in a cyclic manner and the rate of leaching reactions (11.12–11.15) are enhanced extensively (up to million folds) by the bacteria enzyme catalysts. Thiobacillus ferroxidans, thiobacillus thiooxidans, and leptosprillum ferroxidans are the most commonly known bacteria for catalytic action. The role of ferric ions (present as FeCl3 or Fe2(SO4)3 in the leaching media) in dissolution of sulfide mineral of copper has already been stated in previous section. ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 22  Bacterial Leaching Geological weathering due to microbiological degradation processes has been known for a long time. Autotrophic bacteria have adapted themselves to live and grow in strongly acidic environments (pH 1.5–3) in the presence of many heavy metal ions. For the generation of cellular tissues these bacteria depend on atmospheric CO2 for the necessary carbon. Bacteria of interest in leaching of ores are given in Table 11.1. Thiobacillus ferroxidans and ferrobacillus sulfooxidans have been used successfully in bringing copper and uranium from chalcopyrite and bannerite, respectively into solution. The former bacterium prevents the dissolution of iron by oxidizing Fe2+ into Fe3+ and are thus helpful in increasing the dissolution rate of copper. ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 23 References Physical Chemistry Of Metallurgical Processes, First Edition. M. Shamsuddin. Hydrometallurgy Principles And Applications. Tomás Havlík Hydrometallurgy Fundamentals And Applications Michael L. Free ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 24 MSE3541 Hydrometallurgy Online ASSOC. PROF. DR. METİN GENÇTEN Thursday [email protected] 15.00-16.45 [email protected] SEPARATION OF DISSOLVED METALS Key Learning Objectives and Outcomes Understand basic metal concentration principles and terminology Understand how solvent extraction is performed Know how to evaluate solvent extraction processing Understand how ion exchange is performed Know how to evaluate ion-exchange processing Understand how carbon adsorption is performed Understand how precipitation is performed Understand how ultrafiltration is performed ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 2  Leaching Leaching is a primary extractive operation in hydrometallurgical processing, by which a metal of interest is transferred from naturally occurring minerals into an aqueous solution. In essence, it involves the selective dissolution of valuable minerals, where the ore, concentrate or matte is brought into contact with an active chemical solution known as a leach solution. ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 3 SEPARATION OF DISSOLVED METALS Metals dissolved by extraction or leaching need to be separated in order to obtain purified products. Separation of metal ions is based on differences in thermodynamic properties of each metal. Dissolved metals are commonly separated using solvent extraction, ion exchange, carbon adsorption, precipitation, and ultrafiltration. ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 4  LIQUID–LIQUID OR SOLVENT EXTRACTION Liquid–liquid or solvent extraction is a common process for selectively concentrating metals. Liquid–liquid extraction is performed using an organic extractant that is dissolved in an organic phase. The organic phase is allowed to contact an aqueous phase containing the dissolved metal or metal ion complex. Thus, two liquids are used, hence the term liquid–liquid extraction. The aqueous and organic phases are immiscible in each other. However, there is some loss of organic phase in the Wellens, S., Thijs, B., & Binnemans, K. (2012). An environmentally friendlier approach to aqueous phase that is often less than 15 ppm. hydrometallurgy: highly selective separation of cobalt from nickel by solvent extraction with undiluted phosphonium ionic liquids. Green Chemistry, 14(6), 1657-1665. ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 5  The organic phase contains an extractant and a diluent. The diluent effectively dilutes the extractant. LIQUID– The diluent commonly consists of paraffins, naphthenes, and alkyl LIQUID OR aromatics. SOLVENT Diluents are needed to facilitate pumping, processing, and settling of the extractant, which is often viscous and difficult to manage without a diluent. EXTRACTION The diluent also helps to distribute the extractant more effectively in organic phase droplets. The diluent effectively extends the presence of extractant at the droplet interface. Thus, the diluent is also referred to as the extender. ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 6  LIQUID–LIQUID OR SOLVENT EXTRACTION Solvent extraction is carried out using a mixer. The mixer disperses the organic phase in the aqueous phase as small droplets. Small droplets enhance the extraction kinetics. A schematic diagram of small-scale and industrial- scale solvent extraction systems are presented in Figure 6.1 and Figure 6.2. Solvent extraction loading involves intimate mixing of organic and aqueous phases during which metal ions are selectively absorbed into the organic medium. ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 7  The aqueous phase often requires some conditioning. Conditioning steps often include clarification to remove particulate matter. LIQUID– LIQUID OR Residual solvent is also removed from the aqueous phase when necessary. SOLVENT Crud, which typically consists of a mixture of EXTRACTION aqueous, organic, and solid matter, must also be removed. Crud removal can become an important maintenance issue. ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 8  Two main events are required for liquid–liquid metal extraction. One event is dehydration. The other event is charge neutralization. Because the extractants are organic, they do not Types of Liquid– have a high tolerance for hydrated ions. Liquid or Solvent The charge on the ions cannot be accommodated Extractants by the nonpolar nature of most organic molecules. As oil and water do not mix, it is evident that ions that are surrounded by water molecules do not mix in oil. The organic phase is essentially oil and does not easily accommodate water. Thus, dehydration is an important liquid–liquid extraction step. ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 9 Types of Liquid–Liquid or Solvent Extractants Three basic types of solvent extractants are used: ion-exchange, solvation, and coordination extractants. Each type facilitates dehydration and charge neutralization. Coordination extractants typically undergo ion exchange as part of the overall extraction process. Solvation extractants do not formally exchange ions. ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 10  Ion-exchange extractants include both basic and acidic compounds. The basic extractants often have a net excess of hydrogen ions. These ions are attracted to hydroxyl or other anions in solution. Some of these extractants are used in the basic or alkaline pH range. Ion- Acidic extractants have a net excess of negative charge that attracts cations. Exchange Acidic extractants are generally used in the acidic pH range. Extractants In acid, hydrogen ions occupy active sites until exchanged with a metal cation. Some common acidic extractants are carboxylates, sulfonates, and phosphates. The basic extractants are nearly always primary, secondary, tertiary, or quaternary amines. ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 11  Solvating extractants are effectively neutral. Consequently, they are sometimes referred to as neutral extractants. Solvating extractants remove metal ions and complexes by first replacing solvating water molecules. The replacement of water molecules with organic solvent molecules facilitates organic solubility. Next, ion associations (usually through protonation) are made. These associations effectively neutralize the overall charge. Solvating Most solvating extractants contain polar oxygen atoms that Extractants accept hydrogen ions. The accepted hydrogen ions associate with negatively charged metal complexes. Many metals form anionic complexes with chloride ions. The most common functional groups in solvating extractants include ketones, ethers, esters, and alcohols. Examples of solvating extractants are tri-n-butyl phosphate (TBP) and trioctyl phosphine oxide (TOPO). ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 12  Coordination extractants act by spatially coordinating metal- bearing ions. The most common type of coordination extractant is a chelating extractant. Chelating extractants utilize ion dissociation and association to form a coordinated complex. Coordination Extractants Coordinating extractants often have excess electron pairs on nitrogen or oxygen atoms. These pairs are separated from each other at the ends of heterocyclic organic rings. The special arrangement between nitrogen, neutral oxygen, and anionic oxygen is designed for specific metals. The resulting structure is very selective. ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 13  Coordination Extractants Typically, two of the extractant molecules coordinate and bond with one divalent metal ion in a very specific manner. Coordinating extractants for copper solvent extraction are often hydroxyoximes. Other general extractants such as carboxylates are also effective. Examples of coordination extractants are presented in Figure 6.3 and Figure 6.4. These figures illustrate the importance of structural or steric interactions. ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 14 ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 15  Ion exchange is a very common form of solution concentration and purification. It has been known for almost 150 years with respect to certain naturally occurring soils. ION Some soils containing alumino-silicates are known to have ion-exchange capabilities. EXCHANGE Ion exchange is commonly applied to separate and concentrate metals in addition to the well-known process of water ‘‘softening,’’ which removes calcium and magnesium from culinary water to reduce ‘‘hard water’’ deposits. ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 16 ION EXCHANGE Today, specific clays, which are layered silicate minerals, are used for ion exchange. Zeolites, which are porous silicate minerals, are also used. Porous resin beads are also used for ion exchange. Ion resin beads are usually made of a porous polymeric network. Functional groups with specific ion-exchange capability are placed inside pores. ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 17 ION EXCHANGE An example of resin beads is shown in Figure 6.15. A cross-sectional illustration of a typical resin particle is shown in Figure 6.16. ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 18  Ion-exchange resin was first developed in 1935. The resulting porous beads are capable of loading a large quantity of dissolved species. After adsorption occurs, the beads are stripped of the loaded species. ION Stripping is accomplished using a high EXCHANGE concentration of similarly charged species. This process is essentially the same as the solvent extraction process discussed previously. One difference between ion exchange and solvent extraction is the structure of the extraction medium. ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 19 ION EXCHANGE Ion exchange does not normally require solution clarification or filtration before extraction. Ion-exchange resins release negligible quantities of organic matter into the aqueous process streams. Thus, ion exchange has some advantages over liquid–liquid extraction. However, liquid–liquid extraction is generally much faster and easier to apply commercially. ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 20 ION EXCHANGE The terminology for ion exchange is different from solvent extraction. The aqueous solution from which the ions have been extracted is called the effluent. The stripping solution is called the eluant, and the solution into which the ions have been stripped is referred to as the eluate. ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 21  The types of resins used for ion exchange are similar to those used for liquid–liquid extraction. However, the resin beads contain a polymer backbone such as polystyrene–divinyl benzene copolymers. The backbone also contains functional groups that perform the extraction. ION Because the extractant is immobilized by the polymer EXCHANGE matrix, resins cannot participate in solvation mechanisms of extraction. Instead, the resin extracts by means of ion exchange and coordination through attached functional groups. The most common functional groups used in resin ion- exchange beads include amines, carboxylates, phosphonates, and sulfonates. ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 22  Ion-exchange resins are often used in packed columns. As solution enters the column, the ion exchange begins at the entrance. Resin near the entrance has first access to the solution. Consequently, resin near the entrance fills with adsorbed ions first. During initial stages, the exit region has little ION adsorption. EXCHANGE During later stages, the entry region is full, leaving more ions for the exit region. Eventually, the resin reaches its capacity to adsorb ions. Consequently, the concentration of desired ions leaving the resin column is low initially. However, as more of the resin is filled, the exit concentration rises. ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 23  ION EXCHANGE The concentration of adsorbing ions leaving the column changes as a function of volume as shown in Figure 6.17. The point at which the effluent (solution leaving the column) metal ion concentration surpasses the threshold concentration limit is known as breakthrough. The capacity of the resin to maintain the effluent concentration below the threshold limit is known as the breakthrough capacity. The breakthrough capacity is often reported on the basis of the number of bed volumes of the solution. ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 24 ION EXCHANGE Solution flow rate is an important parameter because of its relationship with mass transport. The effluent concentration is high with high flow rates. At high flow rates, ions often do not have time to adsorb before leaving. At low flow rates, ion diffusion into resin allows for greater adsorption and lower effluent concentrations. ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 25  ION EXCHANGE The same type of phenomenon occurs with stripping. However, as shown in Figure 6.18, the eluate (stripping solution leaving the column) concentration increases initially as metal is removed or stripped. The eluate concentration decreases as the resin becomes depleted. The concentration peaks at higher levels with lower flow rates. ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 26  In practice, ion-exchange resin beads are used in columns or in slurries. The most common method is in columns. However, in many instances, the resin is sufficiently durable to be placed in slurries. The capability of placing the resin in a ION EXCHANGE slurry or pulp (resin in pulp, RIP) allows for more rapid kinetics. Resin that is used in slurries is subsequently separated from the slurry by screening or flotation. The resin is subsequently stripped to remove the adsorbed ions. ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 27 ION EXCHANGE The same principles of selectivity, extraction coefficient, and fraction extracted that apply to solvent extraction also apply to ion exchange. However, with ion exchange, the area available for extraction remains constant, and the extraction can be a very predictable function of diffusion through the pores. In contrast, the extraction with solvents is dependent on interfacial tension, viscosities, velocities, and mixing within the solvent droplets. ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 28  Freundlich Model: The Freundlich adsorption Equilibrium model is derived by assuming a site distribution Ion- function that is based on the varying adsorption site free energy values. Exchange It reduces to the Langmuir equation for n = 1 at Adsorption low concentrations and for n=∞ at high concentrations. Models ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 29 Equilibrium Langmuir Model. The Langmuir adsorption model assumes that all surface adsorption Ion- sites are equivalent—regardless of whether Exchange neighboring sites are occupied (A + S ↔ AS). Adsorption Models ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 30 Equilibrium Tempkin Model: The Tempkin adsorption model is an empirical adsorption model Ion- that considers nonuniform site Exchange distribution. Adsorption Models ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 31 ACTIVATED CARBON ADSORPTION Carbon adsorption is a common method of species removal. Carbon is particularly adept at removing low levels of dissolved ions. The activated carbon can be made from a wide variety of organic starting materials. Items such as peach pits, coconut shells, and wood are used for activated carbon. ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 32  ACTIVATED CARBON ADSORPTION The material is converted into activated carbon by heating it in a low oxygen environment. Initially, the carbon source is carbonized by heating it to 500 ◦C with dehydrating agents to remove water and impurities. The carbon is then heated to 700–1000 ◦C with steam, carbon dioxide, and/or air to volatilize residues, develop pore structures, and form functional groups. Different types of carbon surfaces also provide active adsorption sites for dissolved ions. ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 33  ACTIVATED CARBON ADSORPTION A sample of coconut shell carbon is shown in Figure 6.19, Figure 6.20 and Figure 6.21 at three magnifications. The particles are approximately 2 mm in diameter. ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 34  ACTIVATED CARBON ADSORPTION A sample of coconut shell carbon is shown in Figure 6.19, Figure 6.20 and Figure 6.21 at three magnifications. The particles are approximately 2 mm in diameter. ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 35  ACTIVATED CARBON ADSORPTION A sample of coconut shell carbon is shown in Figure 6.19, Figure 6.20 and Figure 6.21 at three magnifications. The particles are approximately 2 mm in diameter. ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 36  Activated carbon is commonly used to extract aurocyanide from solution. The exact mechanisms are not well understood. Evidence suggests adsorption is an ion- exchange process. ACTIVATED However, for gold chloride complexes, it is CARBON believed that the adsorption process involves ADSORPTION reduction to metallic gold at the carbon surface. Extraction using carbon adsorption is most often diffusion controlled. The same theoretical treatment that applied to ion-exchange kinetics applies to carbon adsorption kinetics. ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 37 ACTIVATED CARBON ADSORPTION Activated carbon is often utilized in a countercurrent manner as depicted in Figure 6.22 to maximize adsorption. Countercurrent processing allows the carbon with the least adsorbed matter to contact the solution that is most depleted. Carbon that is nearly loaded contacts the solution that has the highest concentration of the desired species. ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 38  There are several processing options for activated carbon use. The process of loading carbon from leaching solution without ore or concentrate particles is known as carbon-in-column (CIC). ACTIVATED The carbon is commonly partially CARBON fluidized by the flow of leaching solution ADSORPTION upward through CIC columns. Locating columns at different elevations often creates the head pressure for solution flow. CIC utilization requires particle/liquid separation after leaching, before CIC loading. ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 39 ACTIVATED CARBON ADSORPTION Exposure of the carbon However, the particles to a gold leaching slurry must be ground to fine This process is often or pulp with particles particles to facilitate a operated similarly to present after leaching is screen separation of the CIC. known as the carbon-in- carbon from the pulp pulp (CIP) process. after adsorption. ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 40  Another way of adsorbing gold is to utilize carbon in a leaching slurry. This process offers an advantage of extracting gold at lower concentrations, thereby reducing gold losses from natural carbon sources in the ore. ACTIVATED The unwanted removal of gold from the gold- CARBON bearing “pregnant” solution is referred to as ADSORPTION pregrobbing. Therefore, carbon-in-leach (CIL) is sometimes used for gold loading from refractory ores that contain carbonaceous matter. The CIL process results in lower gold loading and higher carbon concentrations. ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 41  Factors that influence gold adsorption include pH, temperature, gold concentration, ionic strength, and system impurities. The lower the pH is, the more selective the carbon is for gold. However, the pH must generally be greater than 10 for safety reasons. ACTIVATED Temperature is a key parameter that is utilized for stripping because elevated temperatures favor CARBON desorption. ADSORPTION Adsorption is more rapid when the gold concentration in solution is higher. Increasing ionic strength improves loading. Cation impurities improve extraction, whereas anionic impurities decrease adsorption. The presence of silver, mercury, and copper can reduce gold loading capacity. ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 42  ACTIVATED CARBON ADSORPTION Figure 6.23 shows industrial gold adsorption data plotted in an inverse adsorption concentration versus inverse solution concentration format to identify the constants needed for the Langmuir adsorption isotherm. The associated constants can be used to show the model fit of the data presented in Figure 6.24. These data show that industrial gold adsorption on carbon can fit the Langmuir adsorption model. ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 43  Gold is often stripped or eluted from carbon at elevated temperatures (95–150 ◦C) in a caustic cyanide solution. Stripping is often performed using either the Zadra or the Anglo American Research ACTIVATED Laboratories (AARL) process. CARBON The stripping solution often contains 1% NaOH and may contain some sodium ADSORPTION cyanide. The elution or stripping process is strongly influenced by temperature. Consequently, most processes tend to strip carbon above 100 ◦C in pressurized vessels. ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 44 ACTIVATED CARBON ADSORPTION Activated carbon is exposed to a variety of chemicals during loading. Organic compounds and calcium and magnesium carbonate commonly deposit on the carbon surface and restrict gold adsorption access. ❖ Activated carbon often needs to be Consequently, activated carbon performance decreases with use thermally unless it is regenerated. regenerated. Regeneration is often performed by thermal activation in a rotary kiln at 600–900 ◦C (usually around 650 ◦C) in steam to remove organic debris. Thermal activation is often preceded or followed by hydrochloric acid washing to remove inorganic precipitates such as carbonates. ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 45 ULTRAFILTRATION OR REVERSE OSMOSIS Ultrafiltration is the process of filtering out solute from a solution through a membrane at high pressure. The process of ultrafiltration is illustrated in Figure 6.25. Ultrafiltration or membrane filtration is, in fact, the same as reverse osmosis. Ionic solutions have a significant osmotic driving force for acquiring additional solute molecules. ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 46 ULTRAFILTRATION OR REVERSE OSMOSIS Solute molecules must have a pathway for transfer to occur. The pathway for ion transfer can be a membrane. Membranes have small pores that selectively allow ion transport. Membranes with very small pores may allow only small ions or molecules to pass. Thus, if a solution is forced through such a membrane, solute molecules are retained. ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 47 ULTRAFILTRATION OR REVERSE OSMOSIS Ultrafiltration and membrane filtration systems utilize thin membranes with very small pores. These pores allow water molecules to pass. However, the small pores severely restrict the passage of hydrated ions and larger molecules. Ideally, the membranes are relatively thin and are supported by more porous material. The porous material and its support must withstand the high pressures. Membranes are made of materials such as cellulose acetate, but they exhibit tremendous resistance to flow (0.2 cm3/(s/atm m2) is a typical flow rate). High resistance makes it necessary to increase the surface areas dramatically to enhance the flow. ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 48 PRECIPITATION Precipitation is a common method for concentrating metal content and purifying solutions. Iron is commonly removed from solution by precipitation. Ferric ions have low solubility above pH 3. Many divalent metal ions such as ferrous ions are relatively soluble at low and intermediate pH levels. This difference in solubilities as a function of pH facilitates separations. ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 49 PRECIPITATION The differences in precipitation of metals at specific pH levels facilitate separations. As an example, ferric ions are often separated from other divalent metal ions at pH values of 3– 4 by selective precipitation. The ferric ions are generally precipitated as ferric hydroxide. One industrially important example of iron precipitation is found in zinc metal production. Precipitation can also be accomplished using gasses and/or water. Ferrous iron, for example, can be precipitated as hematite by the addition of oxygen and water. ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 50 References Physical Chemistry Of Metallurgical Processes, First Edition. M. Shamsuddin. Hydrometallurgy Principles And Applications. Tomás Havlík Hydrometallurgy Fundamentals And Applications Michael L. Free ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 51 MSE3541 Hydrometallurgy Online ASSOC. PROF. DR. METİN GENÇTEN Thursday [email protected] 15.00-16.45 [email protected] METAL RECOVERY PROCESSES Key Learning Objectives and Outcomes Key Chapter Learning Objectives and Outcomes Understand the principles and the practices of electrowinning in aqueous media Be able to calculate basic electrowinning parameters Understand the principles and practices of electrorefining Know how cementation is performed Know how metals can be recovered in solution as metals ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 2 METAL RECOVERY PROCESSES Metals are recovered from hydrometallurgical solutions in metallic form by electrochemical reduction. ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 3  ELECTROWINNING Electrowinning is the most common method of recovering metal from solution in its metallic form. Electrowinning is the electrolytic process of “winning” or recovering dissolved metal using an applied potential. An example of an electrowon copper plate is shown in Figure 7.1. This process is practiced extensively in the metals industry. Copper, zinc, and gold, as well as other metals, are produced by this process. ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 4 ELECTROWINNING Electrowinning utilizes an applied potential to drive electrochemical reactions in the desired direction. An external power source supplies the potential and the current. An inert anode is used to complete the circuit and the necessary counterreaction to metal recovery. Metal is recovered at the cathode. Ions or molecules are reduced at the cathode. Molecules or ions are oxidized at the anode. An electrolyte or ion-conducting medium must exist between the anode and cathode. Water containing dissolved ions is a common electrolyte. ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 5  ELECTROWINNING Commercial electrowinning in aqueous solutions often involves acid. The hydrogen ions in the acid as well as the counterions (usually sulfate) provide most of the solution conductivity. The process of electrowinning is depicted in Figure 7.2. ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 6  ELECTROWINNING The primary electrowinning parameters are potential and current. Current is often tracked through current density. Current density is a more practical term in an industrial setting. Potential and current density are related to thermodynamics and application parameters. Potential and current density are affected by solution and other resistances as well as the deposition area. The current density is a direct measure of the reaction kinetics. ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 7  ELECTROWINNING In the case of electrochemical reactions, the rate of reaction may be increased by several orders of magnitude by applying the proper voltage. Thus, in electrowinning, the voltage plays an important role in determining the overall reaction rate. There are, however, limitations imposed by the solution media. Solution limitations, such as mass transport, often prevent the practical application of high voltages. ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 8  ELECTROWINNING The effect of potential is shown in Figure 7.3 for copper electrowinning. Note that the anodic reaction occurs for the half-cell reaction with the highest equilibrium potential. The anodic reaction is often the decomposition of water to hydrogen ions and oxygen. The cathodic reaction is generally the reduction of metal to its metallic state. ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 9  ELECTROWINNING If the applied potential is greater than the difference between the two half-cell reactions, electrodeposition of the metal will occur. The rate of electrodeposition depends on the applied potential and the associated electrochemical reaction kinetics. A significant overvoltage, η, is needed to allow the water decomposition to occur at a reasonable rate. The overpotential for the metal deposition is generally not large. ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 10 ELECTROWINNING The applied voltage is related to the rate of the electrowinning as indicated in the diagram. If the voltage is not greater than the difference between the halfcell voltages plus the reaction overpotentials and solution and contact resistance voltage drops, no electrowinning occurs. The applied potential can be expressed mathematically as ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 11  Electrical resistance in an electrowinning cell is based on the electrolyte conductivity. The resistance in an electrowinning cell is inversely related to conductivity. Thus, in order to have low resistance, the conductivity of the solution must be high. ELECTROWINNING Consequently, salts and acids with high degrees of ionization and high mobility are desired. Sulfuric acid is a common acid in electrolyte solutions. Although sulfuric acid ionizes readily only to HSO4− and H+, these ions have good molar conductivity. ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 12  Other acids such as HCl and HNO3 ionize readily and have very good ion molar conductivities. Ions that form strong neutral complexes such as acetic acid have low molar conductivities. Strong acids are desirable from a ELECTROWINNING mobility perspective because H+ ions are much more mobile than most ions. As discussed in connection with ion mass transport, the fraction of current carried by individual ions is related to the transference number. The transference number is directly related to ion mobility. ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 13  Another factor that influences the resistance in the cell is the distance between the anode and the cathode. This distance is usually only a few centimeters. The combination of increasing deposit thickness, edge strips, shorting, and mechanical ELECTROWINNING harvesting requirements generally results in distances greater than 2 cm. However, shorter distances reduce solution resistance. Other resistances can form in association with electrode contacts. These resistances can be caused by corrosion of contact surfaces, dirt, salt, etc. ASSOC. PROF. DR. METIN GENÇTEN, HYDROMETALLURGY 14  In copper electrowinning, the difference between the cathodic (E◦ =0.34 V) and the anodic (E◦ =1.23 V) potentials is approximately 0.9 V. The anodic overpotential is usually 0.2–1.0 V. The cathodic overpotential is usually around 0.1 V. The solution voltage drop is on the order of 0.1 V. The other voltage drops across connectors and ELECTROWINNING wires are around