Covenant University MCE433 Extraction and Refining of Metals Lecture Notes PDF

COVENANT UNIVERSITY DEPARTMENT OF MECHANICAL ENGINEERING MCE433 Extraction and Refining of Metals LECTURE 4 Course Lecturers Engr. Prof. Inegbenebor A. O. Engr. Dr. Udoye N. E.  Outline  Re...

COVENANT UNIVERSITY DEPARTMENT OF MECHANICAL ENGINEERING MCE433 Extraction and Refining of Metals LECTURE 4 Course Lecturers Engr. Prof. Inegbenebor A. O. Engr. Dr. Udoye N. E.  Outline  Reactivity Series  Extraction of Metals  Electrolysis  Refining of Metals  Reactivity Series A reactivity series is an empirical, calculated, and structurally analytical progression of a series of metals, arranged by their "reactivity" from highest to lowest. It is used to summarize information about the reactions of metals with acids and water, double displacement reactions and the extraction of metals from their ores.  Extraction of Metals The Earth's crust contains metals and metal compounds such as gold, iron oxide and aluminium oxide. When metals or metal compounds are found in the Earth, they are often mixed with other substances. To get the desired useful metals, the metals have to be extracted from whatever they are mixed with. A metal ore is a rock containing a metal, or a metal compound, in a high enough concentration to make it economic to extract the metal. Major steps in extraction of metal are: 1. Ore concentration: Ore is purified and concentrated, unwanted rocks removed 2. Reduction to crude metal: Metal oxides to be reduced to metals, resulting in a mixture of metals collected 3. Refining to obtain pure metal: To obtain a specific metal, purify and remove unwanted metal impurities  Extraction of Metals The method used to Metals – in decreasing order of Extraction Method reactivity extract metals from Potassium Extracted by electrolysis the ore in which they Sodium are found depends Calcium Magnesium on their reactivity. Aluminum Thus the method of Carbon Zinc Extracted by reaction with carbon or extraction of a metal Iron carbon monoxide from its ore depends Tin Lead on the metal's Hydrogen position in the Copper Extracted by various chemical reactions reactivity series. Silver Gold Platinum  Electrolysis The highly reactive metals are the most difficult to extract from their ores. These metals are found in very stable compounds. Reduction by carbon won’t work. Carbon lies just under aluminum in the reactivity series. Therefore, carbon can’t displace the highly reactive metals from their ores. However, these metals can be gotten through electrolysis. Highly reactive metals are expensive to extract. In chemistry and manufacturing, electrolysis is a technique that uses a direct electric current (DC) to drive an otherwise non-spontaneous chemical reaction. Electrolysis is commercially important as a stage in the separation of elements from naturally occurring sources such as ores using an electrolytic cell. The voltage that is needed for electrolysis to occur is called the decomposition potential. Electrolysis is the passing of a direct electric current through an ionic substance that is either molten or dissolved in a suitable solvent, producing chemical reactions at the electrodes and separation of materials.  Electrolysis The main components required to achieve electrolysis are: An electrolyte: a substance, frequently an ion-conducting polymer that contains free ions, which carry electric current in the electrolyte. If the ions are not mobile, as in a solid salt then electrolysis cannot occur. A direct current (DC) electrical supply: provides the energy necessary to create or discharge the ions in the electrolyte. Electric current is carried by electrons in the external circuit. Two electrodes: electrical conductors that provide the physical interface between the electrolyte and the electrical circuit that provides the energy. Electrodes of metal, graphite and semiconductor material are widely used. Choice of suitable electrode depends on chemical reactivity between the electrode and electrolyte and manufacturing cost.  Electrolysis Case Study 1: Extraction of Aluminum from Bauxite Raw materials Bauxite: ore containing hydrated aluminum oxide Al2O3.2H2O Melting Point: ~2000 oC Molten Cryolite known as sodium aluminum fluoride Na3AlF6 Used to lower the melting point to ~900oC Carbon electrodes Cryolite is added to lower the melting point and to dissolve the ore, Then bauxite ore of aluminum oxide is continuously added When Potential Difference is applied, Al3+ is attracted to the negative cathode and O2- is attracted to the positive anode. At the cathode, Al3+ gains 3 electrons from the cathode to form molten aluminum, which is tapped off Al3+(l) + 3e- → Al (l) At the anode, O2- loses 2 electrons to the anode to form oxygen 2O2-(l) → O2(g) + 4e- Oxygen released attacks carbon anode, to form Carbon monoxide/dioxide. Carbon anode dissolved. Needs to be replaced regularly  Reduction with Carbon (Blast Furnace Method) Case Study 2: Extraction of Iron from Iron ore Iron is extracted from iron ore in a huge container called a blast furnace. Iron ores such as haematite contain iron oxide. The oxygen must be removed from the iron oxide to leave the iron behind. Reactions in which oxygen is removed are called reduction reactions. Raw materials of extraction of Iron Iron Ore e.g. haematite ore [iron(III) oxide, Fe 2O3] Coke (carbon, C) Hot air (for the O2 in it) Limestone (calcium carbonate, CaCO3)  Reduction with Carbon (Blast Furnace Method) Case Study 2: Extraction of Iron from Iron ore Stage 1 – Production of carbon dioxide The coke is ignited at the base and hot air blown in to burn the coke (carbon) to form carbon dioxide C(s) + O2(g) → CO2(g) The limestone is decomposed by heat to produce carbon dioxide & quicklime CaCO3(s)→CaO(s) + CO2(g)  Reduction with Carbon Case Study 2: Extraction of Iron from Iron ore Stage 2 – Production of carbon monoxide At high temperature, the carbon dioxide formed reacts with more coke (carbon) to form carbon monoxide CO2(g) + C(s) → 2CO(g) Stage 3 – Reduction of haematite The carbon monoxide removes the oxygen from the iron oxide ore. This frees the iron, which is molten at the high blast furnace temperature, and flows down to the base of the blast furnace.  Reduction with Carbon Case Study 2: Extraction of Iron from Iron ore Fe2O3(s) + 3CO(g) → 2Fe(l) + 3CO2(g) Other possible ore reduction reactions are... Fe2O3(s) + 3C(s) → 2Fe(l) + 3CO(g) 2Fe2O3 (s) + 3C(s) → 4Fe(l) + 3CO2 (g) Waste gases escape through the top of the furnace. Examples are: Carbon monoxide, carbon dioxide, nitrogen, etc.  Reduction with Carbon Case Study 2: Extraction of Iron from Iron ore Stage 4 – Removal of Impurities The original ore contains silica (SiO2, silicon dioxide). These react with limestone to form a molten slag of e.g. calcium silicate in 2 stages CaCO3 → CaO + CO2 CaO + SiO2 → CaSiO3 The molten slag forms a layer above the denser molten iron and can be separately, and regularly, drained away. The iron is cooled and cast into pig iron ingots / transferred directly to a steel producing furnace. Slag can be used for road surfacing  The effect of Carbon in Iron and Steels is that it is mainly responsible for the immense range of strengths and other useful properties that can be developed in Iron and steels. The main iron ores are: Haematite (Fe2O3)- about 70 % Fe Magnetite (Fe3O4) about 50-70 % Fe Limonite (2Fe2O3.H2O)-about 40-50 % Fe and sometimes containing 20-55 % Fe. Called bog iron Chydrates Haematite Siderite (FeCO3) about 30-40 % Fe  Note the following Melting point of iron is about 1535 oC Solid metals in loose granular (sponge-iron) is formed at about 800 oC Carbon in cast iron and other impurities causes brittle iron-carbide in it, thus making it brittle. Therefore, cast iron cannot be used for the same purpose as forged sponge iron.  Extraction of Low Reactive Metals The unreactive metals at the bottom of the reactive series can be found as the elements themselves. They are found native. They can be extracted by roasting ore by heating alone. Gold for instance is very unreactive, it is found as the native metal and not as a compound, so it does not need to be chemically separated. However, chemical reactions may be needed to remove other elements that might contaminate the metal.  Extraction of Low Reactivity Metals Case Study 3: Extraction of Gold The first step in this process is breaking down large chunks of rock into smaller pieces Grind to a fine slurry or powder Thicken the slurry with water to form pulp and run the pulp through a series of leaching tanks. Leaching dissolves the gold out of the ore using a chemical solvent. The most common solvent is cyanide, which must be combined with oxygen in a process known as carbon-in-pulp. As the cyanide and oxygen react chemically, gold in the pulp dissolves. When small carbon grains are introduced to the tank, the gold adheres to the carbon. Filtering the pulp through screens separates the gold-bearing carbon. The carbon moves to a stripping vessel where a hot caustic solution separates the gold from the carbon.  Extraction of Low Reactivity Metals Case Study 3: Extraction of Gold Another set of screens filters out the carbon grains, which can be recycled for future processing. Finally, the gold-bearing solution is ready for electrowinning, which recovers the gold from the leaching chemicals. In electrowinning, operators pour the gold-bearing solution into a special container known as a cell. Positive and negative terminals in the cell deliver a strong electric current to the solution. This causes gold to collect on the negative terminals.  Extraction of Low Reactivity Metals Case Study 3: Extraction of Gold Smelting, which results in nearly pure gold, involves melting the negative terminals in a furnace at about 2,100 degrees F (1,149 degrees C). Add a chemical mixture known as flux to the molten material, the gold separates from the metal used to make the terminals. Pour off the flux and then the gold. Molds are used to transform the liquid gold into solid bars called doré bars.  Refining of Metals Refining is the process of removing impurities or unwanted elements from metals. Most metals obtained by the reduction process are not very pure. These have to be further refined or purified. Purification of the metal is the last step in metallurgy. Refining is based on the difference between the properties of metals and their impurities.  Refining of Metals The following processes are used for refining. Liquation Distillation Oxidation Electrolytic refining The most widely used method for refining impure metals is electrolytic refining.  Liquation In this method the metals are melted and made to go into the liquid state. Metals that have low melting points such as lead, tin etc., can be purified by this method. A sloping hearth of a furnace is used on which the metal is placed and melted. The temperature of the furnace is maintained slightly above the melting point of the metal. Due to the heat the pure metal melts and flows down, leaving behind infusible impurities having higher melting point.  Distillation Refining of volatile metals like mercury, zinc etc. is done by distillation. The impure form of these metals can be distilled to get their vapours, which are then condensed to get the pure metal. The metal to be refined is heated above its boiling point when the impurities do not vaporize. Pure metal vapourizes and is condensed while the impurities are left behind.  Oxidation Impurities of sulphur, carbon, phosphorous etc. can be removed from the impure metals by passing calculated amount of oxygen or air through the molten metal. These impurities get oxidized to gaseous products like sulphur dioxide, carbon dioxide, phosphorous (V) oxide respectively. These then escape out from the metal. Electrolytic Refining Electrolysis can be used for both extractions of metal (which cannot be separated by chemical reduction process) as well as for further purification of metals obtained by any other method. In the electrolytic refining process, a block of impure metal is made the anode and a thin sheet of pure metal is made the cathode of an electrolytic cell containing an aqueous solution of the metal salt. When electric current of a suitable voltage is passed, impure metal at the anode gets dissolved to deposit the pure metal at the cathode. The impurities are left behind as anode mud near the anode. The anode finally disintegrates while the cathode gains in weight due to the collection of pure metal. This method is used for refining volatile metals like copper, silver, tin, nickel that have boiling points lower than their impurities. e.g., zinc, mercury. Thank You! 