Metallurgy is a subject which deals with the science and technology applied for the extraction of metals economically on a large scale from their respective ores
Mineral: The compounds of a metal which are naturally available in the earth’s crust and can be obtained by mining.
Ore: The minerals from which a metal can be extracted economically and conveniently
Principal Steps Involved in the Recovery of a Metal from its Ore:
Concentration of Ore or Dressing of Ore: Two methods. (i) Physical: Gravity separation or levigation, magnetic separation and froth floatation. (ii) Chemical: Leaching-by using chemical reagent.
Conversion of Ore into its Oxide: Calcination (heating to high temp. in limited supply of air or in absence of air) and roasting (heating to high temp. in presence of excess air).
Production of Metals by Reduction: Carbon reduction, self reduction (PbS, Cu2S and HgS), thermite reduction (Goldschmidt-thermite process- for high m.p. metal oxides) , metal replacement method (hydrometallurgy), electrolytic reduction (for strong electropositive metals) and thermal decomposition method (e.g. Hg, Ag etc)
Purification and Refining of Metals: The metals obtained by reduction methods from the concentrated ores are usually impure. These impure metals may be associated with small amounts of: (i) Unchanged ore, (ii) Other metals produced by the simultaneous reduction of their compounds originally present in the ore, (iii) Non-metals like silicon, carbon, phosphorus, etc., (iv) Residual slag, flux, etc. (v) The impure metal is thus subjected to some purifying processes known as refining in order to remove the undesired impurities.
INORGANIC COMMODITY CHEMICALS
Manufacture of Sodium Hydroxide and Chlorine by the Chlor-alkali process
The term chlor-alkali refers to the two chemicals (chlorine and an alkali) which are simultaneously produced as a result of the electrolysis of brine. The most common chlor-alkali chemicals are chlorine and sodium hydroxide (caustic soda) but can include potassium hydroxide when a potassium brine is used.
Brine electrolysis produces chlorine at the anode and hydrogen along with the alkali hydroxide at the cathode. The two products are removed in separate streams. The overall chemical reaction of electrolysis of sodium chloride is:
If chlorine is not separated from the sodium hydroxide, side reactions such as the following would occur:
Solvay process is used to prepare sodium carbonate.
When carbon dioxide gas is bubbled through a brine solution saturated with ammonia, sodium hydrogen carbonate is formed. This sodium hydrogen carbonate is then converted to sodium carbonate.
Step 1: Brine solution is saturated with ammonia.
This ammoniated brine is filtered to remove any impurity.
Step 2: Carbon dioxide is reacted with this ammoniated brine to result in the formation of insoluble sodium hydrogen carbonate.
Step 3: The solution containing crystals of NaHCO3 is filtered to obtain NaHCO3.
Step 4: NaHCO3 is heated strongly to convert it into NaHCO3.
Step 5: To recover ammonia, the filtrate (after removing NaHCO3) is mixed with Ca(OH)2 and heated.
The overall reaction taking place in Solvay process is
SYNTHESIS GAS PROCESSES
Manufacture of Ammonia
The main uses of ammonia include the manufacture of:
• Fertilizers ((ammonium sulfate, diammonium phosphate, urea)
• Nitric acid
• Fibres, synthetic rubber, plastics such as nylon and other polyamides
• Refrigeration for making ice, large scale refrigeration plants, air-conditioning units in buildings and plants
• Pharmaceuticals (sulfonamide, vitamins, etc.)
• Pulp and paper
• Extractive metallurgy
• Cleaning solutions
For a long time, commercial development of nitrogen fixation ammonia process had proved elusive. Old methods used to produce ammonia included dry distillation of nitrogenous vegetable and animal waste products. Here, nitrous acid and nitrites were reduced with hydrogen according to the following equation:
Ammonia was also produced by the decomposition of ammonium salts using alkaline hydroxides such as quicklime as shown in the following equation.
Haber invented a large-scale catalytic synthesis of ammonia from elemental hydrogen and nitrogen gas, reactants which are abundant and inexpensive. By using high temperature (around 500 o C), high pressure (approximately 150-200 atm), and an iron catalyst, Haber could force relatively unreactive gaseous nitrogen and hydrogen to combine into ammonia.
Chemical Reaction and Equilibrium
Ammonia synthesis from nitrogen and hydrogen is an exothermic reversible reaction and can be described by the following overall reaction.
The reaction is accompanied by decrease in volume and by Le Chatelier’s principle, increasing the pressure causes the equilibrium to shift to the right resulting in a higher yield of ammonia. Since the reaction is exothermic, decreasing the temperature also causes the equilibrium position to move to the right again resulting in a higher yield of ammonia. We can conclude then that ammonia synthesis as per equation is an equilibrium reaction that is favoured by low temperature and high pressure. Thermodynamics gives us equilibrium conditions of the reaction but does not give us any idea about the rate of reaction. The reaction does not proceed at ambient temperature because nitrogen requires a lot of energy to dissociate. In the gas phase this dissociation occurs only at around 3000°C. Even the hydrogen molecule, which has a weaker molecular bond, only dissociates markedly at temperatures above 1000°C.
NITRIC ACID PRODUCTION
Ostwald Process for making Nitric Acid
Step 1 – Catalytic Oxidation Reaction
Primary Oxidation (Formation of Nitric Acid)
The main goal in this process is the conversion of ammonia into nitric acid. The process begins in a catalyst chamber where one ammonia part and eight oxygen parts are introduced by volume. The chamber temperature is around 600oC. This chamber uses a catalyst-like platinum gauze or copper and nickel can also be used.
The oxidation of ammonia takes place and it is converted into nitric oxide (NO). The process is reversible and exothermic. The change in temperature encourages a forward reaction.
4NH3 + 5O2 ↔ 4NO + 6H2O | H -24.8 Kcal/mol
Secondary Oxidation (Formation Of Nitrogen Dioxide)
The nitric oxide gas produced by oxidation of ammonia is in a very hot state. It is passed through a heat exchanger in which the temperature of nitric oxide is lowered to 150oC. After cooling, nitric oxide is transferred to another oxidizing tower where nitrogen dioxide (NO2) is oxidized at about 50oC.
2NO + O2 ↔2 NO2
Step 2 – Absorption of No2 (Formation of HNO3)
In a special absorption tower containing water, the nitrogen dioxide from the secondary oxidation chamber is introduced. NO2 gas is passed through a tower where it absorbs the water. Nitric acid is then obtained through this process.
3NO2 + H2O -> 2HNO3 + NO
Nitric acid that is obtained is very dilute. NO is recycled to increase the concentration level which is generally kept around 68 per cent. The acid-in-water solution can also be dehydrated by distillation with sulfuric acid. Here, the pressure is kept between 4–10 standard atmospheres and the temperature is set at 870–1,073 K.
This reaction is exothermic as there is a release of energy. However, since the water and oxygen are constantly added to this cycle as reactants, the increasing concentration will create optimal equilibrium conditions.
Sometimes when producing nitric acid by the Ostwald process can be dangerous due to unfavourable conditions that may arise. More significantly, the concentration and corrosive behaviour of nitric acid can be a safety hazard.
Methanol is among 10 most important organic chemical. Most of it is used in the manufacture of formaldehyde which in turn is used to make formaldehyde-phenol resin and urea formaldehyde resin.
- The molecular weight of methanol is .
- The boiling and melting point of methanol is 64.7 and - 97.6 respectively.
- The density of methanol is 0.792 , which is lower than that of water.
- Methanol has a hydroxyl group in its structure, making it both a polar molecule and soluble in water.
The term petroleum comes from the Latin stems petra, “rock,” and oleum, “oil.” It is used to describe a broad range of hydrocarbons that are found as gases, liquids, or solids beneath the surface of the earth. The two most common forms are natural gas and crude oil.
Natural gas: Natural gas which is a mixture of lightweight alkanes, accumulates in porous rocks. A typical sample of natural gas when it is collected at its source contains about 80% methane, 7% ethane, 6% propane, 4% butane and isobutane, and 3% pentanes. The C3 , C4 , and C5 hydrocarbons are removed before the gas is sold. The commercial natural gas delivered to the customer is therefore primarily a mixture of methane and ethane. The propane and butanes removed from natural gas are usually liquefied under pressure and sold as liquefied petroleum gases (LPG).
Crude oil is a composite mixture of hydrocarbons (50-95% by weight) occurring naturally. The first step in refining crude oil involves separating the oil into different hydrocarbon fractions by distillation. Each fraction is a complex mixture. For example, more than 500 different hydrocarbons can be found in the gasoline fraction
The oil is sometimes under high pressure and can flow to the surface on its own without pumping. However, most wells require induced pressure using water, carbon dioxide, natural gas or steam in order to bring the oil to the surface. Petroleum refining has evolved continuously in response to changing consumer demand for better and different products. The original requirement was to produce kerosene as a cheaper and better source of light than whale oil. The development of the internal combustion engine led to the production of gasoline and diesel fuels. The evolution of the airplane created an initial need for high-octane aviation gasoline and then for jet fuel, a sophisticated form of the original product, kerosene. Present-day refineries produce a variety of products including many required as feedstock for the petrochemical industry. Common petroleum products include gasoline, liquefied refinery gas, still gases, kerosene, aviation fuel, distillate fuel oil, residual fuel oil, lubricating oils, asphalt, coke and petrochemical feedstocks.
Composition of petroleum
Crude petroleum contain hydrocarbon and non-hydrocarbon compounds.
Paraffins - The paraffinic crude oil hydrocarbon compounds found in crude oil have the general formula Cn H2n+2 and can be either straight chains (normal) or branched chains (isomers) of carbon atoms. The lighter, straight chain paraffin molecules are found in gases and paraffin waxes. The branched-chain (isomer) paraffins such as isobutene are usually found in heavier fractions of crude oil and have higher octane numbers than normal paraffins.
Aromatics: The aromatic series include simple aromatic compounds such as benzene, naphthalenes and the most complex aromatics, the polynuclears which have three or more fused aromatic rings. They have high anti-knock value and good storage stability.
Naphthenes (Naphtha): These are saturated hydrocarbon groupings with the general formula Cn H2n, arranged in the form of closed rings (cyclic) and found in all fractions of crude oil except the very lightest. Single-ring naphthenes (monocycloparaffins) with five and six carbon atoms such as cyclohexane predominate. Two-ring naphthenes (dicycloparaffins) are found in the heavier ends of naphtha.
Alkenes (Olefins): Olefins such as ethylene, butene, isobutene are usually formed by thermal and catalytic cracking and rarely occur naturally in unprocessed crude oil. They are unstable and also improve the anti-knock tendencies of gasoline but not as much as the iso-alkanes. When stored, the olefins polymerise and oxidize. This tendency to react is employed in the production of petrochemicals.
Dienes and Alkynes: Examples of dienes or diolefins, are 1,2-butadiene and 1,3- butadiene. Acetylene is a typical alkyne. This category of hydrocarbons is obtained from lighter fractions through cracking.
Sulfur Compounds: Sulfur may be present in crude oil as hydrogen sulfide (H2 S), as mercaptans, sulfides, disulfides, thiophenes, etc. or as elemental sulfur.
Sulphur is an undesirable component because of its strong offensive odour, corrosion, air pollution by some of its compounds and its effect of reducing tetraethyl lead (anti-knock agent). Hydrogen sulfide is a primary contributor to corrosion in refinery processing units. Other corrosive substances are elemental sulfur and mercaptans. The corrosive sulfur compounds also have an obnoxious odor. The combustion of petroleum products containing sulfur compounds produces undesirables such as sulfuric acid and sulfur dioxide. Catalytic hydrotreating processes such as hydrodesulfurization remove sulfur compounds from refinery product streams. Sweetening processes either remove the obnoxious sulfur compounds or convert them to odorless disulfides, as in the case of mercaptans.
Oxygen Compounds: Oxygen compounds such as phenols, ketones, and carboxylic acids occur in crude oils in varying amounts.
Nitrogen Compounds: Nitrogen is found in lighter fractions of crude oil as basic compounds, and more often in heavier fractions of crude oil as nonbasic compounds. Nitrogen oxides can form in process furnaces. The decomposition of nitrogen compounds in catalytic cracking and hydrocracking processes forms ammonia and cyanides that can cause corrosion.
Trace Metals: Metals, including nickel, iron, and vanadium are often found in crude oils in small quantities and are removed during the refining process. Burning heavy fuel oils in refinery furnaces and boilers can leave deposits of vanadium oxide and nickel oxide in furnace boxes, ducts, and tubes. It is also desirable to remove trace amounts of arsenic, vanadium, and nickel prior to processing as they can poison certain catalysts.
Carbon Dioxide: Carbon dioxide may result from the decomposition of bicarbonates present in or added to crude, or from steam used in the distillation process.
Naphthenic Acids: Some crude oils contain naphthenic (organic) acids, which may become corrosive at temperatures above 230°C when the acid value of the crude is above certain level.
Refining crude oil involves two kinds of processes: First, there are physical processes which simply refine the crude oil (without altering its molecular structure) into useful products such as lubricating oil or fuel oil. Petroleum refining begins with distillation, or fractionation, which separates crude oil in atmospheric and vacuum distillation towers into groups of hydrocarbon compounds of differing boiling-point ranges called “fractions” or “cuts.”
Second, there are chemical conversion processes which alter the size and/or molecular structure of hydrocarbon molecules to produce a wide range of products, some of them known by the general term petrochemicals. Conversion processes include:
Decomposition (dividing) by thermal and catalytic cracking; • Unification (combining) through alkylation and polymerization; and • Alteration (rearranging) with isomerization and catalytic reforming.
Octane number and the development of cracking and reforming processes
The most commonly used measure of a gasoline's ability to burn without knocking is its octane number. Octane numbers compare a gasoline’s tendency to knock against the tendency to knock of a blend of two hydrocarbons heptane and 2,2,4-trimethylpentane, (isooctane). Heptane produces a great deal of knocking while isooctane is more resistant to knocking. Gasolines that match a blend of 87% isooctane and 13% heptane are given an octane number of 87.
By 1922 a number of compounds had been discovered that could increase the octane number of gasoline. Adding as little as 6 ml of tetraethyllead to a gallon of gasoline, for example, can increase the octane number by 15 to 20 units. This discovery gave rise to the first «ethyl» gasoline, and enabled the petroleum industry to produce aviation gasolines with octane numbers greater than 100.
Another way to increase the octane number is thermal reforming. At high temperatures (500-600o C) and high pressures (25-50 atm), straight-chain alkanes isomerize to form branched alkanes and cycloalkanes, thereby increasing the octane number of the gasoline. Running this reaction in the presence of hydrogen and a catalyst such as a mixture of silica and alumina results in catalytic reforming, which can produce a gasoline with even higher octane numbers.
The yield of gasoline is increased by “cracking” the long chain hydrocarbons into smaller pieces at high temperatures (500 o C) and high pressures (25 atm). A saturated C12 hydrocarbon in kerosene, for example, might break into two C6 fragments. Because the total number of carbon and hydrogen atoms remains constant, one of the products of this reaction must contain a C=C double bond.
The presence of alkenes in thermally cracked gasolines increases the octane number (70) relative to that of straight-run gasoline (60), but it also makes thermally-cracked gasoline less stable for long-term storage. Thermal cracking has therefore been replaced by catalytic cracking, which uses catalysts instead of high temperatures and pressures to crack long-chain hydrocarbons into smaller fragments for use in gasoline.
Ethylene and propylene are the most important organic chemical feedstocks accounting for over 50-60% of all organic chemicals. But because of their relatively high reactivities, very few olefins are found in natural gas or crude oil. Therefore, they must be manufactured by cracking processes.
The purpose of cracking is to break complex hydrocarbons into simpler molecules in order to increase the quality and quantity of lighter, more desirable products and decrease the amount of residuals. The heavy hydrocarbon feedstock is cracked into lighter fractions such as kerosene, gasoline, LPG, heating oil, and petrochemical feedstock. LPG gases are feedstock for olefins such as ethylene and propylene.
The decomposition takes place by catalytic action or heating in the absence of oxygen (pyrolysis). The catalysts used in refinery cracking units are typically zeolite, aluminum hydrosilicate, treated bentonite clay, fuller’s earth, bauxite, and silica-alumina all of which come in the form of powders, beads, or pellets. The formation of gasoline (with low molecular weight) from heavy gas oil of high molecular weight is shown in the following equation:
There are three basic functions in the catalytic racking process: • Reaction - Feedstock reacts with catalyst and cracks into different hydrocarbons • Regeneration - Catalyst is reactivated by burning off coke • Fractionation - Cracked hydrocarbon stream is separated into various products.
Catalytic reforming is an important process used to convert low-octane naphthas into high-octane gasoline blending components called reformates.
The conversion is illustrated by the following reaction in which a cycloalkane is converted to an aromatic compound, usually of higher octane number.