Topic 9 Mining and Industrial Chemistry is important in many areas of agricultural, mining and manufacturing industries. In this topic we will briefly examine how some of the principles we have examined apply to the winning of major metal and industrial chemicals. Extraction of metals from ores The Indispensable three : Copper Steel Aluminium Metal Ore Formula Important deposits Aluminium Bauxite Al 2 O 3.xH 2 O Australia, Brazil, Jamaica Chromium Chromite FeCr 2 O 4 Russia, South Africa Copper Chalcopyrite CuFeS 2 Canada, Chile, US Iron Hematite Fe 2 O 3 Australia, Ukraine, US Lead Galena PbS Australia, Canada, US Manganese Pyrolusite MnO 2 Gambon, Russia, South Africa Mercury Cinnabar HgS Algeria, Mexico, Spain Tin Cassiterite SnO 2 Bolivia, Malaysia Titanium Rutile Ilmanite TiO 2 FeTiO 3 Australia Australia, Canada, US Zinc Sphalerite ZnS Australia, Canada, US Chemical composition of ores More ionic More covalent s-block Group 3B p,d-block Noble metals 41
Metallurgy: (Extracting metal from ore) How do you extract metals from ores? Pretreatment Several methods of pretreating the ore to obtain the mineral: Flotation, Cyclone, Magnetism, Leaching Mineral to Compound The most common way to convert the mineral into a uniform chemical compound is to roast it in air, e.g. or CaCO 3 (s) + heat CaO(s) + CO 2 (g) 2ZnS(s) + 3O 2 (g) + heat 2ZnO(s) + 2SO 2 (g) Compound to Metal There are two main ways to form the pure metal from its oxide, depending on how reactive the metal is chemical or electrochemical: Blackman Figure 13.40 a) Reduction with carbon (smelting, known since Egyptian times) b) Reduction with hydrogen (when carbide formation is a problem c) Reduction with more active metal (when hydride formation is a problem) d) Electrochemical reduction (known for about 150 years) Reduction with carbon Consider the formation reactions: K p (300 K) (1) C(graphite) + 0.5O 2 (g) CO(g) + heat 4.7x10 23 (2) Zn(s) + 0.5 O 2 (g) ZnO(s) + more heat 2.3x10 54 (1)-(2) ZnO(s) + C(gr.) + heat Zn(s) + CO(g) 2.0x10-31 But K p is temperature dependant since ΔG o = -RTln K p so ln K p = -ΔG o /RT. This relationship can be used to construct Ellingham Diagrams as follows: 42
100 Ca - CaO Al - Al 2 O 3 80 ln K p 60 40 20 Zn - ZnO C - CO Fe - FeO Ni - NiO 0 400 600 800 1000 1200 1400 1600 1800 Temperature (K) Is there a chemical explanation for this trend? Metal Electronegativity Ca 1.0 Al 1.5 Zn 1.6 Fe 1.8 Ni 1.9 Oxygen is very electronegative. Which elements most readily give up their electrons to form stable oxides? Metal Least active Au, Pt Cu, Ag, Hg, Ni, Co Reduction Method None; found in nature as free metal Roast sulfide to oxide V, Cr, Mn, Fe, Ti, Sn Reduction using carbon, hydrogen, or more active metal Most active Al, Li, Na, Mg Electrolysis Example: Production of Copper Copper has been mined since ancient times, maybe 3500 BC Copper deposits form in magma pools and occur mostly as sulfides, CuS Near the surface, the minerals are altered by exposure to oxygen, into other minerals, e.g. cuprite (Cu 2 O), tenorite (CuO), malachite, Cu 2 (OH) 2 CO 3 43
Formation reaction for oxide: Cu(s) + 0.5 O 2 (g) CuO(s) + heat K p =4.3x10 22 ln(k) = 52.1 E.N. = 1.9 n Surface deposits of copper oxides soon ran out. n Larger deposits of sulfides were available. n Sulfides converted to oxides by roasting. CuS(s) + 1.5 O 2 (g) + heat CuO(s) + SO 2 (g) Largest deposits of copper are chalcopyrite, which contain iron Chalcopyrite is ground, then froth flotation is used to separate mineral from gangue The sulfide is smelted directly to Cu metal Blackman Figure 13.43 Overall reaction: 4 CuFeS 2 (s) + 10.5 O 2 (g) 4 Cu(s) + 2 FeO(l) + Fe 2 O 3 (s) + 8 SO 2 (g) Example: Steel Production Iron/steel is the most widely used of all metals, with some 750 million tonnes per year produced worldwide. Iron has been smelted since ~3000 BC In 1773, a cheap process to produce coke (carbon) from coal, and the advent of the blast furnace, led to large-scale iron production and the Industrial Revolution. Mineral type Mineral, formula Oxide Hematite, Fe 2 O 3 Magnetite, Fe 3 O 4 Ilmenite, FeTiO 3 Carbonate Siderite, FeCO 3 Sulfide Pyrite, FeS 2 Pyrrhotite, FeS The reaction (in a blast furnace) uses CO as reducing agent: Fe 2 O 3 (s) + 3CO(g) 2Fe(l) + 3CO 2 (g) But this is the overall reaction and hides a multitude of processes. 44
The process of converting iron ore into iron and steel involves a series of reductions of the ore, and acid-base steps. Although these seem simple enough, the detailed chemistry is very complex, and perhaps not fully understood even now. The iron from the furnace is impure and called pig iron. It contains typically 3-4% carbon Iron is purified and converted to steel by the basic oxygen process. High pressure O2 is blown over the molten iron. Impurities with very stable oxides form (e.g. C, Si, P, Mn, S). It is too hot for less stable oxides, including those of iron. CaO (lime) is added, which reacts with these oxides to form a variety of phosphates, silicates, etc, all termed slag, which again float on the surface of the molten steel. Now 1% C, and very low other impurities and ready for alloying, e.g. Cr. to make stainless steel. Blackman Figure 13.42 45
Top ten chemicals (US production) Rank Chemical Formula Amount Uses (million tonnes) 1 Sulfuric acid H 2 SO 4 41 fertilisers, chemicals, 2 Nitrogen N 2 36.3 inert atmosphere 3 Oxygen O 2 33.1 steel, medical, 4 Ethylene C 2 H 4 25.4 plastics, 5 Calcium oxide CaO 20.6 metallurgy (lime) 6 Ammonia NH 3 17.2 fertilisers, chemicals, 7 Propylene C 3 H 6 13.2 plastics, 8 Phosphoric acid H 3 PO 4 12.5 fertilisers, 9 Chlorine Cl 2 12.1 plastics, paper, 10 Sodium hydroxide NaOH 10.4 chemicals, soaps, Sulfuric acid Sulfuric acid was the first chemical to be produced on an industrial scale. Sulfuric acid is made in greater volume than any other chemical in the world. The main uses of sulfuric acid today are: Manufacture of sulfuric acid Three types of H 2 SO 4 plants: Rayon Chemicals and dyes Pigments Petrol refining Steel work Fertilizers 1% 2.5% 6% 61% 19% Uses of sulfuric acid 4% 6.5% 1. Sulfur burning A plant for the primary production of sulfuric acid. 2. Spent acid regeneration An SAR plant reconcentrates acid to usable strength. 1. Metallurgical plant A metallurgical plant uses toxic waste SO 2 gases from smelting of metal converts them into sulfuric acid and Step 1: Preparation of SO 2 : This is where the three methods of manufacture differ. Sulfur burning: S(l) + O 2 (g) SO 2 (g) + heat Spent acid regeneration: H 2 SO 4 (aq) + H 2 S(g) + O 2 (g) 2SO 2 (g) + 2H 2 O(l) 46
Metallurgical plant: MS(s) + 1.5O 2 (g) + heat MO(s) + SO 2 (g) (MS and MO are a metal sulfide and oxide respectively.) Step 2: Preparation of SO 3 This is the heart of modern manufacture. The original catalyst was platinum, now vanadium pentoxide is used. Catalytic conversion of SO 2 to SO 3 : SO 2 (g) + ½O 2 (g) SO 3 (g) + much heat Too much heat production is a problem (Why?) So this process is done in stages, where the heat is removed at every stage, and recycled, e.g. to produce heat for first stage of metallurgical plant. Step 3: Absorption of SO 3 This is where conc. or fuming sulfuric acid is made Reaction of SO 3 with water (usually with weaker H 2 SO 4 ) SO 2 (g) + H 2 O(l) H 2 SO 4 (l) + heat In practice, the sulfuric acid is recycled through the system, getting stronger and stronger each time, until reaching 98-99% purity. Again, the heat must be controlled. 47
Ammonia About 13% of all N-fixation is accomplished industrially, using the Haber process. Over 110 million tonnes are produced worldwide making it the largest produced chemical on a mole basis. The main uses of ammonia today are: Nitric acid Fertilizer Nylon Chemicals 79% 7.9% 5.9% 5% 2% Uses of ammonia Haber Synthesis of Ammonia Blackman Figure 14.15 N 2 (g) + 3 H 2 (g) 2 NH 3 (g) + 91.8kJ But N 2 is so stable how? Remove NH 3 (milking an equilibrium) Increase pressure Decrease temperature Too slow at low temperature Use a catalyst Increasing yield of NH 3 Increase pressure (within reason) Use a catalyst Remove NH 3 Intermediate temperature (compromise) Silberberg Fig B17.4 48
Ethylene / propylene Ethylene and propylene are both made from cracking hydrocarbons in the petroleum industry. By far their greatest use is in the manufacture of polymers (plastics). Chlorine / sodium hydroxide Styrene Ethylene oxide LDPE HDPE PVC Linear alkenes 32% 25% 14% 3% 7% 7% 12% Uses of ethylene (ethene) Chlorine (Cl 2 ) and sodium hydroxide are produced in the same process, involving electrolysis of common sea salt. We will look at electrolysis in the next topic. 2 Na + (aq) + 2 Cl (aq) + 2 H 2 O(l) Cl 2 (g) + H 2 (g) + 2 Na + (aq) + 2 OH (aq) 24% 17% 28% 4% 3% 22% 34% Uses of chlorine 8.1% 5.1% 7.1% 4% PVC Isocyanates/oxygenates Inorganic chemicals Chloromethanes Epichlorohydrin Solvents Source: www.eurochlor.org 26% 4% 13% Uses of sodium hydroxide Soaps and detergents Pulp and paper Inorganic chemicals Organic chemicals Water treatment Al manufacture Housecroft Source: and H&C, Constable p.597p597 49