Chapter 7 - Metallurgical Processes

1 Introduction Metals occur in nature in two different forms. 1. Native state: The metals occur in native or free or uncombined state are non-reactive with the environment. They are noble metals, exist as such in the earth’s crust. Example: Au, Pt, Ag 2. Combined state: They exists in the form of stable compounds. These cpds are associated with impurities such as sand, clay, rocks etc. Example: Fe2O3, ZnO, PbS, CaCO3 etc. 2 7.1. Minerals and Ores ➢ Mineral is a naturally occurring materials present in the earth’s crust, which contains metal in the native or in the combined state. ➢ All minerals are not suitable for the extraction of metals commercially. ➢ Ore is a mineral from which a metal can be extracted profitably. Thus, “all ores are minerals, but all minerals are not ores”. 3 Cont’d…. Examples: clay (Al2O3, SiO2, 2H2O) and bauxite (Al2O3, 2H2O) are two minerals of aluminum. But Al can be extracted only from bauxite and not from clay. Therefore, —Bauxite is an ore of Al and clay is a mineral. Classification of ores: 1. Oxide ores (bauxite, cuprite, haematite) 2. Sulphide ores (cinnabar, zinc blende) 3. Carbonate ores (dolomite, limestone) 4. Halide ores (rock salt, cryolite) 5. sulphate ores (Epsom salt, gypsum) 6. silicate ores (feldspar, asbestos) 7. nitrate ores (chile salt peter) 8. phosphate ores (phosphorite) 4 Ores bright scarlet to brick-red form of mercury(II) sulphide (cinnabar) Cryolite, Na3AlF6 Cuprite Cu2O Zinc Blende, ZnS Dolomite is an anhydrous carbonate mineral composed of calcium magnesium carbonate, CaMg(CO₃)₂ Haematite, Fe (III) oxide Magnetite, FeO 5 7.2. Metallurgy Process of extracting metal from ores is called metallurgy which involves the following steps. 1. size reduction of ores 2. concentration of ores 3. extraction process 4. refining of impure metal 6 Cont’d… 7.2.1. size reduction of ores In order to process the ore to the metal, the ore must be reduced to the appropriate size first by crushing and then by grinding. 1. Jaw crushers 2. roll crushers 7 b) Grinding Screening The ground particles are to be screened for their size and any over size particles are to be recycled back to grinding. There are two types of equipments of commonly used for screening. 1. reels or trammels 2. flat screen 8 7.2.2. Concentration of Ores The concentration of ores involves removal of rock impurities (i.e. gangue or matrix) from the powdered ore. The concentration of ore can be brought by any of the following methods. 1. Gravity flotation process or hydraulic washing process 2. Froth flotation process 3. Electromagnetic separation process 4. Chemical methods 9 1. Gravity separation or hydraulic washing process This method is suitable especially for heavy “oxide” ores. The concentration of ores is based on the difference in the specific gravity (density) of the ore and the gangue particles. Principle: the powdered ore is treated with a stream of running water, during which the lighter gangue particles are washed away and heavier ore particles settle down. Gravity separation is carried out by two methods: i. wilfely table method ii. Hydraulic classifier method 10 2. Froth Flotation Process This process is based on the difference in wetting characteristics of the ore and gangue with oil and water. —Used for sulphide ores in which the impure ore (ore with gangue) is mixed with water and pine oil to make a suspension. The suspension is violently agitated by the rotating paddle/stirrer which draws in air, causing frothing. —Due to this the ore particles which are wetted by oil come to the surface with the froth, which can easily be separated to get pure ore. 11 Cont’d... —Sometimes it is possible to separate two sulphide ores by adjusting proportion of oil to water or by using 'depressants'. Example: In an ore containing ZnS and PbS, the depressant used is NaCN. It selectively prevents ZnS from coming to the froth but allows PbS to come with the froth. 12 Cont’d... • Foaming agents- pine oils • Collectors (floating agents)- ethyl xanthates, amyl xanthates • Activators or depressants ⇨ NaCN is used to create froth of sulphide ores to be concentrated 13 3. Electromagnetic separation process —Electromagnetic method can be used in which the impure ore is dropped over a conveyer belt moving around two rollers, one of which is an electromagnet. —If any one part of the impure ore is magnetic in nature (either ore or gangue), The magnetic parts will be attracted by the electromagnet and collect as a separate heap from the non-magnetic part. Examples; ✓Tin stone ore is non magnetic. It contains wolframite as impurity. The wolframite (FeWO4) is magnetic. ✓ Haematite (or) Magnetite ore is magnetic. It contains silica as impurity. Silica is non magnetic. 14 4. Chemical Methods Chemical properties are utilized in concentration or purification of the ore. Example: Crude bauxite ( Al2O3.2H2O), impurities are TiO2 and SiO2. Al2O3 + 2NaOH 2NaAlO2 + 3H2O NaAlO2 + 2NaOH Al(OH)3 + NaOH 2Al(OH)3 ∆ Al2O3 + 3H2O 15 7.3. Extraction Process ✓ It is an essential process of reduction ( it uses a Chemical Reduction method). ✓ For chemical reduction, the concentrated ore is converted into oxide by roasting or calcination followed by reduction. 7.3.1. Calcination Calcination is the process of heating the ore strongly in the absence of blast of air. Carbonates and oxides are generally calcined. This method removes moisture, CO2 and other volatile impurities. 16 Examples: 1. Calcination of limestone (CaCO3) to lime (CaO) CaCO3 CaO + CO2 2. When bauxite is calcined, water is removed and only anhydrous alumina is obtained. Al2O3. 2H2O Al2O3 + 2H2O The following changes occurs during calcinations. a) b) c) d) e) moisture is removed gases are expelled volatile impurities are removed The mass becomes porous thermal decomposition of the ore takes place 17 7.3.2. Roasting ✓This is the process of heating the ore strongly in excess of air. Sulfide ores are generally roasted. ✓ During roasting; sulfide ores are converted into oxide ores and volatile impurities are removed. Example: 2ZnS + 3O2 2ZnO + 2SO2 18 2ZnS +3O2 high temp. 2ZnO + 2SO2 FeS2+11O2 → 2Fe2O3 + 8SO2 2Cu2S + 3O2 → 2Cu2O + 2SO2 ― Some sulphide ores on roasting gives metal sulphate. This known as sulphatizing roasting. ZnS + 2O2 650oC ZnSO4 — Some sulphide ores on roasting gives metal and sulphur dioxide. This is known as auto-reduction (self-reduction). 2Cu2S + 3O2 → 2Cu2O + 2SO2 2Cu2O + Cu2S → 6Cu + SO2 19 7.3.3. Smelting Smelting is the process of heating the roasted or calcined ore along with a reducing agent and a flux to high temperature. Commonly used reducing agents in metallurgy: H2(g), CO(g), water gas, Al, Mg, Ca metals, coke(C) Example: Fe2O3 + 3C (reducing agent) 2Fe + 3CO 20 —Metal oxide ore on reduction with carbon or coke gives metal. This method is used for the extraction of metals like Zn, Sn etc. ZnO + C 900 oC Zn + CO SnO2 + 2C 1800oC Sn + 2CO — Haematite (Fe2O3) is reduced to iron mainly by carbon monoxide Fe2O3 + 3CO → 2Fe + 3CO2 — Metal oxide on reduction with hydrogen gas gives metal. This method is used for the extraction of metals like Cu, W, Mo etc. MoO3 + 3H2 I.R Mo + 3H2O WO3 + 3H2 850oC W + 3H2O Cu2O + H2 2Cu + H2O — NiO is reduced to Nickel by synthesis gas. 2NiO + (CO+H2) 300oC 2Ni + (CO2+H2O) — Metal oxide on reduction with Aluminum powder gives metal. This known as Gold-Schmidt alumino thermic process. Example Cr, Fe, Mn etc are reduced based on this method. Cr2O3 + 2Al 600oC 2Cr + Al2O3 Fe2O3 + 2Al 600oC 2Fe + Al2O3 21 Electrolytic reduction • Metals like Fe, Zn, Pb, Cr and Mn lie in the middle of electrochemical series. Their oxides can be reduced by C, CO or other reactive metals like Al, Na, or Ca. • However, the metals like Na, Al, Ca and Mg (very active metals) which are placed in the top of electrochemical series cannot be obtained by ordinary reducing agents. • So, they are obtained by passing an electric current through their purified molten ore. The metal get deposited at the cathode from where it is separated. 7.4. Refining of impure metal 23 Cont’d… — The pure metal from the electrolyte gets deposited on the cathode and the equivalent amount of metal from the anode pass in to the solution or are deposited down the anode forms an anode mud. Example: Cu is refined in this manner 24 7.4.2. Refining of metals by Liquation ― This method is used if the ore contains high melting gangue and easily fusible mineral particles 25 7.4.3. Poling • Impure metals extracted are refined by bringing them to molten state in a furnace and then treating with oxygen (air). • involves stirring the impure molten metal with green logs or bamboo sticks. The hydrocarbon contained in the pole reduce any metal oxide present as impurity. These oxidized impurities escape either as gas or form scum over molten metal. • Cu and Sn are refined by this method. 26 7.4.3. Fractional Distillation • The impure metal is taken in the furnace and is melted into liquid. The molten impure metal flows into the tower’s fractionating column containing a large number of refractory trays; • The vapours formed are condensed and collected on the top. The non-volatile fraction flows down the fractionating column through the holes and is collected in the bottom. 27 ➢ Purification is employed when the impurities are more soluble in the molten metal than the solid metal , and the molten metal on re-solidification crystallizes out as pure metal. Used to refine elements like Si, Ge, Ga etc. Used to produce ultra pure elements based on the difference in solubility of impurities in the molten and solid states of the metal It is applied by a movable heater around the rod of impure metal 28 N.B. • Gangue/matrix: the unwanted earthy impurities • Flux: is a chemical substance which combines with gangue at higher temperatures to form easily fusible material called slag. • Slag: is a fusible chemical compound formed by the reaction of flux with gangue. It’s not soluble in the molten metal and is thus separated. 29 Functions and properties of a slag • The majority of refining reactions required in liquid melts occur around the liquid slag - liquid metal interface and are commonly called slag-metal reactions. Then, its importance is to examine the functions, properties and structure of liquid slags in order to be able to select suitable metal-slag conditions for each refining operation. The functions of a slag used in liquid metal processing may be summarized as: — protection of the melt from contamination from the furnace atmosphere and combustion products of the fuel; — insulation of the melt; — accepting unwanted liquid and solid components; — controlling the supply of refining media to the melt through additions to the slag. 30 Cont’d… To be able to apply these functions, slags must have certain fundamental properties: — Lower melting point than the melt in order to maintain a liquid slag of high fluidity which covers the whole of the liquid metal surface, and allows good contact with the liquid metal. Fluxes such as lime, fluorspar, quartz or iron oxide may be added to lower the melting point of the slag and thereby increase its fluidity. — Lower specific gravity and immiscible in the melt, so that it rests above the melt as a separate liquid layer and can therefore accept impurities which are generally lighter than the liquid metal. — Correct composition so that it can accept and react with impurities from and be immiscible in the melt. 31 Diagram Showing Extraction of Aluminum from bauxite Ore Finely powdered bauxite containing Fe2O3, TiO2, SiO2 as impurities Crude bauxite is heated at 1100 ℃ with Na2CO3 Calcination FeO→ Fe2O3 Organic matters are removed Calcined ore (Digested with 45% NaOH at 150℃ under 80 lb per filtered) Residue (Fe2O3) Al2O3 + Na2CO3 →2NaAlO2+ CO2↑ Fe2O3 + Na2CO3 →2NaFeO2+ CO2↑ SiO2 + Na2CO3 →Na2SiO3+ CO2↑ CaO + SiO2 →CaSiO3 2NaAlO2 + CO2 + 2H2O→ 2Al(OH)3↓ + Na2CO3 Filtrate (NaAlO2+ Na2SiO3) Freshly prepared Al(OH)3 as seeding agent filtered Al(OH)3 precipitate Filtrate(NaOH, Na2SiO3) Heated at 1000℃ and pure Al2O3 is formed 32 Electrolytic reduction of Al uses carbon as a cathode and graphite rods as an anode which is immersed in 𝟔𝟎 𝐩𝐚𝐫𝐭𝐬 𝐜𝐫𝐲𝐨𝐥𝐢𝐭𝐞, 𝟐𝟎 𝐩𝐚𝐫𝐭𝐬 𝐟𝐥𝐮𝐨𝐫𝐬𝐩𝐚𝐫 𝐚𝐧𝐝 𝟐𝟎 𝐩𝐚𝐫𝐭𝐬 𝐩𝐮𝐫𝐞 𝐀𝐥𝟐 𝐎𝟑 as an electrolyte at 𝟗𝟎𝟎 ℃ temperature will produce a mineral containing sodium aluminium fluoride (𝐍𝐚𝟑 𝐀𝐥𝐅𝟔 ) Cryolite undergoes electrolytic dissociation first, and then 𝑨𝒍𝟑+ goes to the cathode, 𝑭𝟐 at the anode reacts with 𝑨𝒍𝟐 𝑶𝟑 produces 𝑨𝒍𝑭𝟑 . 𝑨𝒍𝑭𝟑 • 𝟑𝑵𝒂𝑭 → 𝑨𝒍𝟑+ + 𝟑𝑵𝒂+ + 𝟔𝑭− Reaction at cathode: 𝑨𝒍𝟑+ + 𝟑𝒆− → 𝑨𝒍 Reaction at anode: 𝟔𝑭− − 𝟔𝒆− → 𝟑𝑭𝟐 𝟐𝑨𝒍𝟐 𝑶𝟑 + 𝟔𝑭𝟐 → 𝟒𝑨𝒍𝑭𝟑 + 𝟑𝑶𝟐 ↑ As Cryolite has greater electrochemical stability and only increases the dissociation of 𝑨𝒍𝟐 𝑶𝟑 . 𝟑+ 𝐀𝐭 𝐜𝐚𝐭𝐡𝐨𝐝𝐞: 𝟐𝐀𝐥 − + 𝟔𝐞 → 𝐀𝐥; 𝐀𝐭 𝐀𝐧𝐨𝐝𝐞: 𝟑𝐎 𝟐− − − 𝟔𝐞 𝟑 → 𝐎𝟐 ↑ 𝟐 Reading Assignment Extraction of Fe, Cr and Cu !!! 33