chapter20

= Chapter 20: Metallurgy and the Chemistry of Metals = toc

Extractive Metallurgy
is the process of separating metals from their ores and of compounding alloys. There are three general categories of extractive metallurgy:

**1. Preparation of ore:**
The mineral must first be separated from the surrounding materials in the ore before it can be used. //**mineral processing**// - a category of processing methods that rely on physical and chemical properties of metals to separate them from other materials in the ore // flotation -// hydrophobic properties of metals can be altered by chemicals. The finely grinded ore is submerged in solution and the hydrophobic mineral particles are allowed to be carried to the surface, where they form a froth. The froth is then removed and allowed to collapse, and the metal is recovered. // magnetism -// ferromagnetic metals can be collected by using a magnetic field to separate the metal from other nonmagnetic material in the ore // amalgams // - amalgams are alloys of mercury with other metals. Mercury can form alloys with metals in ore, and the resulting liquid is easily separated from the ore. The amalgam is then broken up by distilling off the mercury, leaving the pure mineral. //**pyrometallurgy**// - ore is treated at high temperatures to be converted to raw metal or intermediate compounds for further refining // roasting -// used to extract metals from sulfide ores. Ore is heated in the presence of oxygen, which oxidizes the sulfur to form sulfur dioxide gas, leaving pure metal. Example: the equation for the roasting of calcium sulfide is //smelting// - used to extract metals from oxide ores. The ore is heated with a form of carbon that combines with oxygen to form carbon dioxide, which enters the atmosphere and leaves pure metal behind. Combined metals are always in oxidized form. A reduction process is necessary to produce free metal. Example: the equation for the smelting of iron(III) oxide is source: []

**2. Reduction of metal:**
Metals typically have positive oxidation numbers when combined and need to be reduced before they can be used. There are two main types of reduction. //**chemical reduction**// - a more electropositive element can be used as a reducing agent Example: the equation for the reduction of titanium (IV) chloride with magnesium is

//**electrolytic reduction**// - used for highly electropositive elements such as alkali metals that are unable to be reduced chemically. The process is carried out on the molten oxide or hydride of the metal. An electric current is passed through the metallic ions, and the positive metallic ions are collected at the cathode while the waste ions are collected at the anode.

**3. Purification of metal:**
Impurities can be removed with further treatmentI. //**fractional distillation**// - uses differences in volatility to separate substances. When two liquid substances of different volatility are heated, the more volatile substance dominates the vapor that is produced. As a result, the liquid becomes richer in the less volatile substance. Distillation cannot be used to fully purify a metal. //**electrolysis**// - similar to electrolytic reduction; however, the impure substance acts as the anode while a sample of the pure substance acts as the cathode. At the anode, the substance is oxidized, and subsequently reduced at the cathode. Reactive contaminants present in the impure substance do not undergo reduction and fall to the bottom of the cathode. Non-reactive elements do not undergo oxidation and remain at the anode. The mass that is separated during an electrolysis reaction is quantified by Faraday's laws of electrolysis. where //m// is the mass of the substance that is electrolyzed (removed from the sample and separated to the anode and cathode), //Q// is the total electric charge transferred to the substance, //F// is the faraday constant, 96.485 C/mol, //M// is the molar mass of the substance, and //z// is the number of electrons transferred per atom. Example: How much time is required to deposit 54g of silver from a silver nitrate solution using a constant current of 4.5A? The equation for the reduction of silver is Thus 1 mole of electrons is required to deposit 1 mole of silver, so z = 1. 54g is 0.5 mol of Ag Use Faraday's equation in the form where //n// is the number of moles of Ag produced, 0.5. Q = (0.5)(96500) = 48250C. Q = I * t when the current is constant, so we find the time to be t = 48250 / 4.5 = 10,722 seconds.

Steelmaking:
Steel is an alloy of iron containing 0.03% to 1.4% carbon, as well as trace amounts of other elements. Conversion of iron to steel is an oxidation process; unwanted impurities are removed by reacting with oxygen gas. The main method used is called the //basic oxygen process//. Source: []

Molten iron is poured into an upright cylindrical vessel, and pressurized oxygen gas is injected through a water-cooled tube. Under these conditions, manganese, phosphorus, silicon, and carbon react with oxygen to form their respective oxides. A flux is then added to the mixture to react with the oxides to form a slag.

The three major fluxes used for manganese, phosphorus, and silicon oxides are shown below. The molten steel is sampled at certain intervals. When the desired blend of impurities has been reached, the vessel is rotated to a horizontal position and the molten steel is removed through the taphole.



** Band Theory: **
In theory, atoms can have an infinite number of bands. However, because most of these lie at such high energies, electrons rarely occupy more than a few of them. When multiple atoms are brought together, the number of molecular orbitals increases and is proportional to the number of atoms. However, no matter how many atoms are brought together, some areas remain unoccupied by orbitals. These areas are known as band gaps, where electrons are forbidden to be. A sufficiently excited electron can jump from one band to the next band through this area.

**Conductivity:**
The uppermost, occupied band is known as the valence band. The next highest energy level band is the conduction band. The conduction band is responsible for conductivity properties; the more electrons that occupy the conduction band, the greater the conductivity of the metal. The valence band is almost always full or nearly full, and thus its occupying electrons have very limited mobility and cannot conduct current. The conduction band is not nearly as full, and electrons in it are free to move and transmit electric current. //metals/conductors//: the band gap for metals (conductors) is 0; the valence band overlaps with the conduction band. Electrons are free to move from one band to the other. //semimetals/semiconductors//: the band gap is larger; however, with a little external energy, electrons can still jump the band gap. The main mechanism for exciting these electrons is thermal energy. //insulators//: the band gap is insurmountably large and electrons cannot travel to the conduction band in any sufficient quantity to cause an appreciable amount of conduction.

source:

The Fermi level, //ζ,// is the energy level where no electrons will have sufficient energy to pass without external influence. It is also sometimes known as chemical potential, denoted by //µ////.//

**Doping:**
Semiconductors can be "doped" with other elements to create impurities that modify the conducting properties of the semiconductor. An //n-type dopant// contains extra electron(s) than the element which it replaces, and thus, after forming bonds, the solid has free electron(s) to move around. A //p-type dopant// has fewer electron(s), and after forming bonds, the atom has positively charged //hole(s)// around it. Electrons and holes are known as //charge carriers,// or simply //carriers//. The presence of a hole makes it easier to excite an electron from a nearby energy level to the hole. The hole is then said to move to the location of the displaced electron. This process is known as carrier generation and recombination. Doping typically increases the conductivity of a semiconductor.

**Organic Semiconductors:**
Because the Earth's atmosphere is oxygen-rich (which has more electrons than carbon, thus creating an oxidizing environment), n-type doping is rare. An n-type conductor would react immediately with oxygen to "un-dope" itself. chemical doping: the material is exposed to an oxidant such as iodine or bromine in p-type doping. Typically an alkali metal would be used in n-type doping. electrochemical doping: the material is coated on an electrode and placed in an electrolyte (conductive) solution that does not dissolve the polymer. A potential difference is created between the two electrodes, causing charge to flow. Depending on the direction of the flow (which depends on the sign of the potential difference), the polymer can be oxidized or reduced.



source: []

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Metallic Properties:
Metals are lustrous in appearance, solid at room temperature (with the exception of mercury), malleable, ductile, and good conductors of heat and electricity. The strength of metal properties follows the opposite trend of periodic electronegativity, increasing from right to left and top to bottom.

**Alkali Metals**
Always have an oxidation number of +1. Have low melting points and are soft enough to be sliced with a knife. All alkali metals possess body-centered crystal structures, which have low density, thus accounting for the low density of alkali metals.

**Alkaline Earth Metals**
Have an oxidation number of +2 and possess similar chemical properties to those of alkali metals.

**Aluminum**
Aluminum is the most abundant metal found on this planet. It does not occur in its elemental form, the most common ore being bauxite Aluminum has high tensile strength, which means it can be stretched or drawn out. Its conductivity is about 65% of copper, but it is much cheaper and thus commonly used in high-voltage lines. Aluminum alloys created by adding trace amounts of other metals such as copper and magnesium can greatly improve its strength. Aluminum is not used by any living creature.

Review Questions:
Write the equation for the roasting of Iron (III) Sulfide.

How much time is required to deposit 446g of francium from a francium telluride solution using a constant current of 10A?

What flux is used to make a slag with silicon dioxide?

Replacing some carbon atoms in graphene with boron atoms is what type of doping?

How does doping affect the conductive properties of a semiconductor?

What purification method uses differences in volatility to separate substances?

Electroplating Lab (Galvanizing a Nail)
(adapted from wserver.scc.losrios.edu/~millerb/**lab**s/401_expt11_spr06.pdf)

Equipment needed: 50mL beaker cardboard zinc strip iron nail Lab-Volt HCl/sandpaper/etc. (to clean nail) 0.5 **M** ZnSO4 ammeter

Procedure:

1. Clean the rust and oil of an iron nail by placing the nail in HCl for a few minutes or by scraping it off using sandpaper. 2. Insert a strip of zinc and a cleaned, weighed nail into a cardboard square. 3. Obtain about 8 mL 0.5 ** M ** ZnSO4 in a 50 mL beaker. Place a piece of cardboard in the middle of the beaker to separate the two electrodes once you put them into the beaker. 4. Put your test tube clamp around the beaker to prevent it from falling over. Place the cardboard square with the 2 electrodes into the 10 mL beaker - this is your electrolysis cell. 5. Connect the power supply such that the electrons are going to the nail and the electrons are coming from the zinc strip. Connect an ammeter to the current. 6. Switch on the power supply. Turn the voltage knob until the ammeter reads approximately 0.1A. 7. Allow the electroplating to proceed for 10 minutes. Keep the current as constant as possible. Record the final mass of the nail after allowing it to dry. 8. Calculate the expected amount of zinc that should be on the nail. Compare this to the measured amount.

setup

the galvanized nail this text is just to take up space ignore it the oxidized zinc strip

~0.1A current passed for 10 minutes Total charge passed: 60C Total electrons passed: 60 / 1.602*10^-19 = 3.745 * 10^20 Expected number of zinc (+2) atoms reduced = 3.745 * 10^20 / 2 = 1.872 * 10^20 Expected number of moles: 1.872 * 10^20 / 6.022 * 10^23 = 0.000312 Expected mass of zinc accrued on nail: 0.000312 * 65.39 = 0.020g