Published on January 4, 2008
Chemistry of Materials:Bronze Age to Space Age: Chemistry of Materials: Bronze Age to Space Age Chapter Twenty-Four Metallurgy: From Natural Sourcesto Pure Metals: A mineral is a crystalline inorganic material in the Earth’s crust. An ore is a solid deposit containing a sufficiently high percentage of a mineral to make extraction of a metal economically feasible. Native ores contain free metals and include gold and silver. Oxides include iron, manganese, aluminum, and tin. Sulfides include copper, nickel, zinc, lead, and mercury. Carbonates include sodium, potassium, and calcium. Chlorides (often in aqueous solution) include sodium, potassium, magnesium, and calcium. Metallurgy: From Natural Sources to Pure Metals Extractive Metallurgy: Metallurgy is the general study of metals Extractive metallurgy focuses on the activities required to obtain a pure metal from one of its ores: Mining from deep mines or open-pit mines. Concentration by physical separation from waste rock. Roasting is often used to convert metal compounds to the corresponding oxides. Reduction may be performed by simple heating to decompose an oxide, or with a reducing agent such as coke, or by electrolysis. Slag formation removes high-melting impurities. One or more final steps of refining may be required. Extractive Metallurgy Concentration of an Oreby Flotation: Concentration of an Ore by Flotation In the flotation method, the ore is ground into a powder and mixed with water and additives. Particles of ore are attached to air bubbles and rise to the top. Undesired waste rock, called gangue, falls to the bottom. Hydrometallurgy: Metallurgical methods that use ore concentration, roasting, chemical or electrolytic reduction, and slag formation are often called pyrometallurgy. In some cases, these methods are being replaced by hydrometallurgy—methods that involve processing of aqueous solutions of metallic compounds. Operations include: Leaching the metal ions from the ores with water, acids, bases, or salt solutions. Purification and/or concentration to remove impurities. Precipitation and reduction to the desired metal. Hydrometallurgy Alloys: In many metallurgical procedures the desired product is an alloy—a mixture of two or more metals, or of a metal with a nonmetal. Some alloys are heterogeneous mixtures, like the familiar lead–tin alloy solder. Two other alloy types are homogeneous solid solutions: In substitutional alloys, atoms of one metal substitute for another in the crystal lattice. In interstitial alloys, atoms of one substance occupy voids in the crystal lattice. A few alloys are actually intermetallic compounds, such as the amalgam NaHg2. Alloys Iron and Steel: The iron formed in a blast furnace is an impure form called pig iron. It generally contains 3–4% C, 0.5–3.5% Si, 0.5–1% Mn, 0.05–2% P, and 0.05–0.15% S. The solid metal obtained from liquid pig iron is called cast iron and is used in automobile engine blocks, boilers, stoves, and cookware. Cast iron is brittle when cold but can be wrought by hammering at 800–900 °C. Most iron is converted to alloys known collectively as steel. Iron and Steel A Modern Blast Furnace: A Modern Blast Furnace Steel: Steel has more desirable properties (strength, malleability, corrosion resistance) for most purposes than does iron. Converting pig iron to steel requires: reduction of the carbon content to less than 1.5%. removal of major impurities (Si, Mn, P, S) and some minor impurities. Low-carbon steel (about 0.25% C) is used for construction beams and girders and for reinforcing rods in concrete. Harder, high-carbon steel (more than 0.7% C) finds use in cutting tools and railroad rails. The remainder of steel production is in the form of alloy steel, ordinarily containing Cr, Ni, Mn, V, Mo, Co, and/or W as a major component. Steel A Basic Oxygen Furnace: A Basic Oxygen Furnace The basic oxygen process has replaced most older methods of steel production in the U.S. Limestone, pig iron, and scrap steel are treated with high-pressure oxygen. The oxygen removes impurities as their corresponding oxides (CO2, SO2) or as slags [MnSiO3, Ca3(PO4)2]. Tin and Lead: Tin occurs in nature mainly as the ore cassiterite, SnO2, which can be concentrated by flotation, then reduced to the metal with coke. Recycling is an important source of tin. Treatment of tin plate with Cl2 converts the tin to SnCl4, which is volatile. SnCl4 is converted to SnO2, which is then reduced to tin metal. Lead is found chiefly as galena, PbS. The ore is concentrated by flotation, roasted to the oxide, then reduced with coke. The recycling of used lead is an important alternative to the production of new lead. Currently, about 70% of manufactured lead is recycled lead. Tin and Lead Copper, Zinc, Silver, and Gold: Copper ores commonly contain iron compounds, which complicates copper production. These ores undergo a four-step process to produce blister copper which is 97–99% pure Cu with entrapped bubbles of SO2. The recycling of used copper is now an important alternative to the production of new copper. Currently, nearly half of manufactured copper is recycled copper. Zinc occurs mainly as ZnS (sphalerite) and ZnCO3 (smithsonite). Zinc ore is roasted to produce ZnO, followed by reduction with coke at high temperature, and distillation of zinc vapor. Alternatively, zinc ore can be processed by hydrometallurgy, with electrolysis as the last step. Copper, Zinc, Silver, and Gold Copper, Zinc, Silver, and Gold (cont’d): Silver and gold are both found free in nature, but all easily accessible known deposits have been mined. A typical gold ore today contains only about 10 g Au per ton. A modern method of obtaining gold is by cyanidation. The ore is treated with cyanide, forming [Au(CN)2]–. Gold is then displaced from the complex, using an active metal such as zinc. A similar method is used for obtaining silver. Air is blown through an aqueous solution of cyanide ions in which highly insoluble Ag2S is suspended. Sulfide ion is oxidized to sulfate ion, and the silver appears in the complex [Ag(CN)2]–. Copper, Zinc, Silver, and Gold (cont’d) Slide15: Example 24.1 A Conceptual Example Consider the electrolytic method of zinc metallurgy previously described. Why must the ions of metals less active than zinc (for example, Cd2+) be removed before the electrolytic reduction of ZnSO4(aq) is carried out? The Free-Electron Modelof Metallic Bonding: In the free-electron model, a metal consists of more-or-less immobile metal ions in a crystal lattice, surrounded by a “gas” of the valence electrons. The Free-Electron Model of Metallic Bonding An applied electric potential causes the free-moving electrons to travel from (–) to (+). Deformation of a MetalCompared to an Ionic Solid: Deformation of a Metal Compared to an Ionic Solid In the free-electron model, deformation merely moves the positive ions relative to one another. Metals are therefore malleable and ductile. In contrast, deformation of an ionic solid brings like-charged ions into proximity; the crystal is brittle and shatters or cleaves. Band Theory: The free-electron model is a classical theory, which is less satisfactory in many ways than a quantum-mechanical treatment of bonding in metals. Band theory is a quantum-mechanical model. Band Theory The spacing between electron energy levels is so minute in metals that the levels essentially merge into a band. Band Theory: When the band is occupied by valence electrons, it is called a valence band. In band theory, the presence of a conduction band—a partially filled band of energy levels—is required for conductivity. Because the energy levels in bands are so closely spaced, there are electronic transitions in a partially filled band that match in energy every component of visible light. Metals therefore absorb the light that falls on them and are opaque. At the same time electrons that have absorbed energy from incident light are very effective in radiating light of the same frequency—metals are highly reflective. Band Theory Band Overlap in Magnesium: Band Overlap in Magnesium The 3s band is only partially filled because of overlap with the 3p band. The partially-filled band fulfills the requirement for electrical conductivity. Semiconductors: Semiconductors In an insulator, the energy gap between conduction and valence band is large. When the energy gap is small, some electrons can jump the gap; we have a semiconductor. Electrical Conductivityin Semiconductors: Electrical Conductivity in Semiconductors n-Type and p-Type Semiconductors: An n-type semiconductor is produced when a crystal (such as Si) is “doped” with an element (such as As) with more valence electrons. The energy levels of these donor atoms lie quite close to the conduction band and the “extra” electron(s) are lost to the conduction band. A p-type semiconductor is produced when a crystal is “doped” with another substance with fewer valence electrons. The energy levels of these acceptor atoms lie quite close to the valence band and electrons are easily promoted from the valence band into the acceptor level. n-Type and p-Type Semiconductors n-Type and p-Type Semiconductors: n-Type and p-Type Semiconductors A Semiconductor Device:The Photovoltaic Cell: A photovoltaic cell is a semiconductor device that converts light to electricity. The cell consists of a thin (1 x 10–4 cm) layer of p-type semiconductor, in contact with a piece of n-type semiconductor. A Semiconductor Device: The Photovoltaic Cell Some of the electrons in the p-type semiconductor absorb energy from the sunlight and are promoted to the conduction band. These electrons can cross the p–n junction and leave the cell as an electric current. Polymers: Polymers, also known as macromolecules, are made from smaller molecules, much as a brick wall is constructed from individual bricks. The small building-block molecules are called monomers. Synthetic polymers are a mainstay of modern life, but nature also makes polymers; they are found in all living matter. Polymers Natural Polymers: Three types of natural polymers are polysaccharides, proteins, and nucleic acids. Modifications to cellulose (a polysaccharide) are of economic importance. Modifications of cellulose take place at the many hydroxyl (OH) groups using acid–base reactions. Natural Polymers Addition Polymerization: In addition polymerization, monomers add to one another in such a way that the polymeric product contains all the atoms of the starting monomers. The steps for addition polymerization include: Initiation - often through the use of free radicals. Propagation - radicals join to form larger radicals. Termination - occurs when a molecule is formed that no longer has an unpaired electron. Addition Polymerization Molecular Models of a Segment of a Polyethylene Molecule: Molecular Models of a Segment of a Polyethylene Molecule Condensation Polymerization: In condensation polymerization, a small portion of the monomer molecule is not incorporated in the final polymer. Each monomer molecule contains at least two functional groups. The monomers are linked through the functional groups. Small molecules are formed as by-products as the monomers are linked. Condensation Polymerization Slide31: Example 24.2 Write a condensed structural formula for polypropylene, made by the polymerization of propylene (CH2=CHCH3). Physical Properties of Polymers: A thermoplastic polymer is one that can be softened by heating and then formed into desired shapes by applying pressure. Thermosetting polymers become permanently hard at elevated temperatures and pressures. High-density polyethylene (HDPE) consists primarily of linear molecules and has a higher density, greater rigidity, greater strength, and a higher melting point. Low-density polyethylene (LDPE) has branched chains and is a waxy, semi-rigid, translucent material with a low melting point. Physical Properties of Polymers Organization ofPolymer Molecules: Organization of Polymer Molecules HDPE molecules are linear and can pack closely together for increased strength. LDPE molecules have branches that keep the molecules from packing closely. A Small Segment of Bakelite®: A Small Segment of Bakelite® Numerous cross-links between chains produce an extensive three-dimensional structure that is highly rigid and strong. Flexibility and Elasticity: A polymer is flexible if it can yield to force without breaking. A polymer is elastic if it regains its original shape after a distorting force is removed. Elastomers are flexible, elastic materials. The natural polymer rubber is the prototype for this kind of material. Natural rubber is soft and tacky when hot. It can be made harder in a reaction with sulfur, called vulcanization. Flexibility and Elasticity Elasticity in Naturaland Vulcanized Rubber: Elasticity in Natural and Vulcanized Rubber In natural rubber the chains are separate and can slide over one another; when stretched, the product may not return to its original shape. Heating with sulfur cross-links the carbon chains to one another. When deformed as shown here, the cross-links tend to return the product to its original shape. Slide37: A larger distance between cross-links corresponds to a softer, more elastic product. Synthetic Rubber: Several kinds of synthetic rubber were developed during and after World War II. Neoprene (polychloroprene) is one example of this. Copolymerization is a process in which a mixture of two different monomers form a product in which the chain contains both monomers as building blocks. Synthetic Rubber SBR is a copolymer of styrene and butadiene. Fibers and Fabrics: A fiber is a natural or synthetic material obtained in long, threadlike structures that can be woven into fabrics. Cotton, wool, and silk are natural fibers of great tensile strength that have long been spun and woven into cloth. Synthetic polymers have revolutionized the clothing industry. Polyacrylonitrile (Acrilan®) is a synthetic addition polymer used for fibers. Polyesters (Dacron®) are synthetic condensation polymers. Polyamides (nylon) are synthetic analogs to proteins, and have properties similar to silk. Fibers and Fabrics Biomedical Polymers: One of the most interesting uses of polymers has been in replacements for diseased, worn out, or missing parts of the human body. Pyrolytic carbon heart valves are widely used. Knitted Dacron® tubes can replace arteries blocked or damaged by atherosclerosis. Polymers of glycolic acid and lactic acid have been used in synthetic films for covering burn wounds, and are less likely to be rejected by the body’s immune system. The development of biomedical polymers has barely begun. Biomedical Polymers Space Age Materials: Physical properties of polymers can be designed through control of their composition and molecular structure. In a similar way, other new materials are being designed and developed to meet the needs of advancing technologies. Examples include new alloys, composite materials, and materials structured on the nanometer scale (nanomaterials). In each of these cases, we will see that the introduction of a specific chemical composition or structure produces materials with unique and novel properties. Space Age Materials Slide42: Beryllium enhances the elasticity of copper. Waspaloy is usable to 600 °C, well above the useful temperature of most other alloys. Shape-memory alloys, when deformed, return to their original shape when heated. Composites: Composites are made of two or more physically distinct materials that, when combined, exploit the desired structural and mechanical properties of the individual components. One type of composite material widely used today is fiber-reinforced polymer (FRP), in which fiberglass is impregnated either with epoxy resin or polyester resin. FRP composites containing either graphite fiber or Kevlar in lieu of fiberglass are known for their extreme strength and rigidity. For high-temperature applications, a phenol–formaldehyde resin may replace the epoxy resin. Composites A Metal-Ceramic Composite: 3M Company’s Composite Conductor: A Metal-Ceramic Composite: 3M Company’s Composite Conductor Aluminum provides electrical conductivity. Aluminum oxide fibers provide strength needed for long electrical cables. Nanomaterials: A nanomaterial develops unique physical or chemical properties when the sample size is reduced to a nanometer scale. In order for the term nanomaterial to apply, there must be a change in some property or properties as the scale is reduced to the nanometer level. Coatings that incorporate nanometer-sized grains of one metal oxide in a matrix of a second metal oxide have been prepared. This inclusion has resulted in significantly increased hardness, an important property of protective coatings, by altering the mechanisms of mechanical deformation within the material. Nanomaterials Nanomaterials (cont’d): In carbon nanotubes (page 464), the electrical conductivity depends on the way the sheets have been rolled, because of the confinement of electrons within the nanometer-sized structure. Nanometer-sized particles of semiconducting materials exhibit optical properties that are related to the perturbation of electronic states within a very small sample of a material. The rate for reactions involving extremely fine particles of aluminum, just a few nanometers in diameter, is greater than the predicted rate based solely on increased surface area. This “nano” form of aluminum is being investigated for use in high-performance rocket propellants. Nanomaterials (cont’d) Slide47: Cumulative Example A polyurethane is a polymer involving the reaction of hydroxyl (—OH) groups and isocyanate (—N=C=O) groups: —OH + —N=C=O —NH(C=O)O— Suppose a polyurethane is to be prepared using pure 1,6-diisocyanatohexane (hexane diisocyanate, HDI) for the isocyanate functional groups and a 1:2 molar mixture of glycerol (1,2,3-trihydroxypropane) and 1,4-dihydroxybutane for the hydroxyl functional groups. (a) How many grams of the hydroxyl mixture must be used for every 100.0 grams of HDI? (b) Based on Table 24.2, what general physical properties might be expected for the product?