LECTURE NOTES MetE 208 Chemical Principles of Materials Production Prof.Dr. Yavuz A. TOPKAYA
1. INTRODUCTION 1.1 Sources of Metals 1.2 Mineral Dressing 1.3 Preliminary Treatment 1.4 Extraction Processes 1.5 Fuels and Refractories 1.6 Ironmaking Flowsheet Blast Furnace Charges and Operation 1.7 Steelmaking Flowsheet Steelmaking Processes 1.8 Non-ferrous Metals 1.8.1 Copper Production (As flowsheets) 1.8.2 Zinc Production (As a flowsheet) 1.8.3 Lead Production (As a flowsheet) 1.8.4 Aluminum Production (As a flowsheet)
2. MATERIAL BALANCES (STOICHIOMETRY) (STOICHIOMETRY) 2.1 Stoichiometric Principles
2.1.1. 2.1.2. 2.1.3. 2.1.4. 2.1.5. 2.1.6.
Conservation of Mass Mass Relations in Metallurgical Reactions Volume Relations in Metallurgical Reactions The gram-atom, kg-atom, kg-atom, gram-mole, kg-mole Gas Laws Excess of Reactants
2.2 Analyses of Materials 2.3 Charge Calculations
1
2.4 Examples
2.4.1. 2.4.2. 2.4.3. 2.4.4. 2.4.5.
Combustion Excess Reactants Roasting Gas Treatment Problem Hours on Material Balances
3. HEAT BALANCE 3.1 Kinds of Energy and Conservation of Energy 3.2 Heat Balance 3.3 Procedure in Calculating a Heat Balance 3.4 Choice of Reactions 3.5 Examples of Heat Balance
4. REFERENCES 4.1
SCHUHMANN R., Metallurgical Engineering Vol I, “Engineering Principles”, Addison – Wesley, 1952
4.2
ROSENQVIST T., “Principles of Extractive Metallurgy”, McGraw-Hill, 1973
4.3
GILCHRIST J. D., “Extractive Metallurgy” Metallurgy” Pergamon, 1980
4.4
NEWTON J., “Extractive Metallurgy”, John Wiley, 1967
4.5
PEHLKE R.D., “Unit Processes of Extractive Metallurgy”, Elsevier, 1973
4.6
FINE H.A. and GEIGER G.H., “Handbook of Material and Energy Balance Calculations in Metallurgical Engineering”, AIME, 1979
4.7
SCHLESINGER
M.E.,
“Mass
and
Engineering”, Prentice-Hall, 1996
5. MARKING
1st Mid-Term
25 %
2nd Mid-Term
25 %
Final
40 %
Homeworks
10 %
6. COURSE ASSISTANT: PINAR BURCU KAYIN
2
Energy
Balance
in
Materials
2.4 Examples
2.4.1. 2.4.2. 2.4.3. 2.4.4. 2.4.5.
Combustion Excess Reactants Roasting Gas Treatment Problem Hours on Material Balances
3. HEAT BALANCE 3.1 Kinds of Energy and Conservation of Energy 3.2 Heat Balance 3.3 Procedure in Calculating a Heat Balance 3.4 Choice of Reactions 3.5 Examples of Heat Balance
4. REFERENCES 4.1
SCHUHMANN R., Metallurgical Engineering Vol I, “Engineering Principles”, Addison – Wesley, 1952
4.2
ROSENQVIST T., “Principles of Extractive Metallurgy”, McGraw-Hill, 1973
4.3
GILCHRIST J. D., “Extractive Metallurgy” Metallurgy” Pergamon, 1980
4.4
NEWTON J., “Extractive Metallurgy”, John Wiley, 1967
4.5
PEHLKE R.D., “Unit Processes of Extractive Metallurgy”, Elsevier, 1973
4.6
FINE H.A. and GEIGER G.H., “Handbook of Material and Energy Balance Calculations in Metallurgical Engineering”, AIME, 1979
4.7
SCHLESINGER
M.E.,
“Mass
and
Engineering”, Prentice-Hall, 1996
5. MARKING
1st Mid-Term
25 %
2nd Mid-Term
25 %
Final
40 %
Homeworks
10 %
6. COURSE ASSISTANT: PINAR BURCU KAYIN
2
Energy
Balance
in
Materials
CHAPTER 1 1.1. THE SOURCES OF METALS
Metallurgy is the science and art of extracting metals from their ores, refining them and preparing them for use. Ores are naturally occurring deposits in the Earth’s crust. The ores are mined and treated by various unit operations (mechanical processes) and unit processes (chemical metallurgical processes) to extract metals, and to convert them into the metallic (chemically uncombined) form. Earth’s crust refers to the outer siliceous shell of the Earth which is about 35 km thick. Metallic ores and other mineral products are produced from this crust. Therefore, the original source of all metals is the Earth’s crust. Average analysis of the Earth’s crust is given in Table 1. As it can be seen from the table that oxygen, silicon, aluminum and iron are the four most abundant elements. Since engineering metals Al, Fe, Mg and Ti are far more abundant than the other metals. There is never likely shortage of these due to exhaustion of the ore deposits. The commonly known metals, e.g. Au, Pb, Sn are so small in quantities in the Earth’s crust that their commercial recovery is impossible. But, by natural events, those are concentrated in certain parts of the Earth’s crust resulting in their economical recovery. Ore deposits are metal-bearing veins, beds, placer deposits and solutions, which are used to extract, metal commercially. Geological processes result in the concentration of metals in ore deposits. Table 1 Composition of the Earth’s Crust Element % Element
Element
%
O Si Al Fe Ca Na K Mg -
46.66 27.72 8.13 5.00 3.63 2.89 2.59 2.09 -
Ti H Mn P F S Cl C Rb Ba Zr Cr Sr V Ni Cu
0.44 0.13 0.10 0.08 0.08 0.052 0.048 0.032 0.031 0.025 0.022 0.020 0.015 0.015 0.010 0.010
Zn W Li Ce Sn Y Nb Nd Co La Pb Cf Th -
0.0080 0.0069 0.0065 0.0046 0.0040 0.0028 0.0024 0.0024 0.0023 0.0018 0.0016 0.0015 0.0011 -
TOTAL
98.65
TOTAL
1.110
TOTAL
0.0459
Less than 0.001 % (Cs, Ge, Be, As, U, Mo, Ta, Sb, etc.) Less than 0.0001 % (Hg, Tl, Bi, Cd, Ag, In, Se, Pd, Pt, Au, etc)
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%
1.1.1. Further Definitions
Ore is naturally occurring aggregate of minerals from which a metal or metals may be extracted at a profit. Mineral is a naturally occurring homogeneous inorganic substance of definite chemical composition and with certain characteristic physical properties; e.g. galena PbS, chalcopyrite CuFeS2. Ore minerals are those minerals that contain the valuable metals in an ore; e.g. PbS, CuFeS2 , ZnS…etc. Sulfide ores are ores that contain sulfides, e.g. non-ferrous ore minerals such as PbS, CuFeS2, and ZnS…etc. Oxide or Oxidized ores contain oxide, carbonate, sulfate, hydroxide or silicate ore minerals; e.g. Fe2O3, PbCO3, FeCO3, PbSO4, Zn2(OH)2.SiO3…etc. Gangue minerals are the valueless minerals found in ores, e.g. waste wall rock broken with ore. In a typical Pb – Zn ore, we have: galena (PbS-ore mineral), sphalerite (ZnS-ore mineral) and quartz (SiO2-gangue mineral). Tenor (grade) of an ore is the amount of valuable metal in the ore. This is given in percentage of metal or metallic oxide, except in precious metal ores; e.g. Au, Ag and Pt where the analysis is reported in gms / metri etric ton or troy ounces / short ton. (1 troy ounce = 31 grams, 1 short ton = 2000 lbs or 907 kg) Placer or placer deposits are ore deposits formed by the erosion of rocks by the action of wind and water. Rocks are broken down both chemically and mechanically, and the action of water tends to concentrate some of the minerals, e.g. native Au, Pt and cassiterite (SnO 2) Ferrous and non-ferrous ore: Ferrous ores are ores used in ferrous metallurgy; the metallurgy of iron and steel. Non-ferrous ores are used in the technologies of all metals other than iron. Alloys: An alloy is a substance, with metallic properties, that contains more than one element, e.g. brass is an alloy of copper and zinc; steel is an alloy of iron and carbon (C ≤2%). 1.1.2. Minerals and Ores of Common and Precious Metals Iron ores Ore minerals of iron are hematite ati te (Fe2O3), magnetite agnetite (Fe3O4), limonite (Fe2O3.xH2O) and siderite (FeCO3). Magnetite is a magnetic mineral. Hematite is the most important ore mineral of iron and it is weakly magnetic. Since siderite and limonite are ore minerals lower in grade than others, they are not so desirable in iron ores. Pyrite (FeS2) may be used after roasting as pyrite cinder (iron oxide ash) for the production of iron. Ores: Most iron ores contain large amounts of the iron ore mineral and relatively small amounts of gangue. Most of the direct-smelting ores contain about 50 % Fe, although ores with as low as 25-30 % Fe are smelted after concentration. Aluminum ores Ore minerals of aluminum are gibbsite (Al2O3.3H2O), diaspore (Al2O3.H2O), boehmite (Al2O3.H2O), corundum(Al2O3), kaolinite (Al2O3.2.SiO2.2H2O), etc. Ores: The only commercial ore of aluminum is bauxite. Bauxite is a rock containing the hydrated oxides of aluminum; gibbsite, diaspore and boehmite (Al 2O3.xH2O). Bauxites contain 55 – 61 % Al 2O3, 10 – 30 % combined water, 1 – 25 % Fe 2O3, 1 – 3 TiO 2 and 1 – 12 % SiO2. Bauxites with low SiO 2 are desirable.
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Copper ores Most important ore minerals of copper are chalcopyrite (CuFeS2), bornite (Cu5FeS4), chalcocite (Cu2S), covellite (CuS), enargite (CuAsS4), malachite (CuCO3.Cu(OH)2), cuprite (Cu2O) and chrysocolla (CuSiO3.2H2O). Ores: The primary ore mineral in copper ores is chalcopyrite. Others are secondary minerals formed by the alteration of primary chalcopyrite and chalcocite. Sulfide ores in nature are usually associated with pyrite (FeS2) or pyrrhotite (Fe1-xS) and other base metal sulfides such as ZnS, PbS, NiS, etc. The ore minerals are usually associated with siliceous and other gangue minerals. Therefore, the grade of ore is reduced to 1 – 2 % Cu. Lead ores The only important lead ore mineral is galena (PbS). Anglesite (PbSO4) and cerussite (PbCO3) are found in the upper portions of some lead ore deposits but they are not important commercially. Most lead ores are found as veins, so are not suited to bulk mining methods. As a result, the average grade of lead ore mined is higher than that of copper ore; in the range 3.0 to 8.0 % Pb and associated with zinc and silver. Zinc ores
Sphalerite(ZnS) is the only important zinc mineral and usually associated with PbS or CuFeS2 and cadmium. Smithsonite (ZnCO3) and calamine (Zn2(OH)2.SiO3) often occur in the oxidized portions of sphalerite ore bodies. Grade of zinc sulfide ores: 2 – 12 % Zn. The precious metal ores Gold, silver and platinum metals most commonly occur as native metals. They are in the metallic state but they are usually alloys rather than pure metals. In a few cases, gold occurs as calaverite (AuTe2). Besides native silver, other important ore minerals of silver are argentite (Ag2S), pyrargyrite (Ag3SbS3), etc. Gold, silver and platinum are usually associated with base-metal sulfides and pyrite. Chromium ores The economical mineral in chromium ore is chromite (FeO.Cr 2O3). Commercial chromium ores usually contain 40 % Cr 2O3 or more.
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1.2. MINERAL DRESSING
The first process most ores undergo after they leave the mine is mineral dressing (processing), also called ore preparation, milling, and ore dressing or ore beneficiation. Ore dressing is a process of mechanically separating the grains of ore minerals from the gangue minerals, to produce a concentrate (enriched portion) containing most of the ore minerals and a tailing (discard) containing the bulk of the gangue minerals. Since most ore minerals are usually finely disseminated and intimately associated with gangue minerals, the various minerals must be broken apart (freed) or “liberated” before they can be collected in separate products. Therefore, the first part in any ore dressing process will involve the crushing and grinding (which is also known by a common name called “comminution”) of the ore to a point where each mineral grain is practically free. 1.2.1. Comminution
Crushing and grinding are usually carried out in a sequence of operations by which the lump size is reduced step by step. There are 3 stages of crushing and 2 stages of grinding. i.
ii.
iii.
i.
ii.
Primary Crushing (coarse crushing): In primary crushing, ore or run-of-mine ore (up to 1 m in size) is crushed down to about 10 cm and it is done in a jaw or gyratory crusher. Secondary Crushing (intermediate crushing): In this case, ore is crushed from 10 cm to less than 1 – 2 cm size; for this purpose jaw, cone or roll crushers are used. These secondary crushers consume more power than primary crushers. Tertiary Crushing (fine crushing): By tertiary crushers ore is crushed from 1 – 2 cm to less than 0.5 cm. Short head cone crushers, roll crushers, hammer mills can be used for this purpose. The two stages of grinding are: Coarse Grinding: Rod mills are generally used as coarse grinding machines. They are capable of taking feed as large as 50 mm and making a product as fine as 300 microns. Fine Grinding: Fine grinding, which is the final stage of comminution, is performed in ball mills using steel balls as the grinding medium. The ball mill, after feeding 0.5 mm material may give a product that is less than 100 microns. Grinding is usually done wet.
The principle purposes of grinding are: i. To obtain the correct degree of liberation in mineral processing. ii. To increase the specific surface area of the valuable minerals for hydrometallurgical treatment; i.e. leaching. Mineral processing combines a series of distinct unit operations. The flowsheet shows diagrammatically the sequence of unit operations in the plant.
6
A simple flowsheet of a mineral processing plant Ore
Crushers (+) Oversize Screens (-) Undersize Grinding (+) Oversize Classification (-) Undersize Concentration Concentrate
Tailing
Comminution and concentration are two primary operations in mineral processing as it can be seen from the above flowsheet, but many other important steps are involved. e.g. Sizing – by screens and classifiers. Dewatering – by thickeners, filters and driers, etc. Auxiliary operations – conveying, sampling, etc. 1.2.2. Sizing Methods
There are two methods of industrial sizing. i. Screening ii. Classification Screening is generally carried out on relatively coarse material, as the efficiency decreases rapidly with fineness. Screening is generally limited to materials above about 250 microns in size, finer sizing normally being undertaken by classification. Industrial sizing is used in closed circuit with a crusher or a ball mill. For the large lump sizes coarse grizzlies made of rails or trommel (revolving) screens made of punched plate may be used. For finer material, screens are usually made of woven metal wire. The material that passes through the openings (apertures) of a particular screen is known as the undersize and material that remains on the screen is the oversize. Laboratory screening: The other use of screening is as a measuring technique with the purpose of determining the relative amounts of various particle sizes in a given material. The particle size distribution of crushed or ground ore is determined by means of a screen analysis. For this purpose standardized screening scales are developed. Most common is the American Tyler Screen Scale (Tyler Standard Series) where the screen number(mesh number) is given as the number of meshes (openings) or wires per linear inch (1 inch=2.54 cm): the
7
diagonal of each screen opening is equal to the edge of the previous screen. So, the linear dimension of each opening differs by a constant factor of √2 = 1.414. Tyler screen scale starts with 1.05 inch (26.67 mm), for smaller particle sizes the dimensions are usually given in microns (1 micron = 10 -3 mm). Thus, 200 mesh (#) is equal to 74 microns in the Tyler Screen Series, Table 2. The result of a screen analysis is given as the fraction of the sample which passes through one screen but which is stopped by the subsequent screen so we can say that a certain percentage is + 26.67 mm (coarser than the coarsest screen), another percentage is – 20 mesh + 28 mesh (dimensions between 0.833 mm and 0.589 mm) and finally that a certain percentage is – 200 mesh (finer than 0.074 mm or 74 microns). The mesh number does not directly indicate the size of the aperture, and the aperture can be calculated from the mesh number if the wire diameter is known. 1 inch (25.4 mm) = Number of wires * Wire diameter + Number of apertures * Aperture size Table 2 The Tyler Standard Series for Screen Analysis
s e i r e s
2
√ ←
Aperture Size Millimeters Microns 26.67 18.85 13.33 9.423 6.680 4.699 3.327 2.362 1.651 1.168 0.833 833 0.589 589 0.417 417 0.295 295 0.208 208 0.147 147 0.104 104 0.074 74 0.052 52 0.037 37
Tyler Mesh #
3 4 6 8 10 14 20 28 35 48 65 100 150 200 270 400
Classification: Classification is defined as a method of separating mixtures of mineral particles into two or more products according to their settling velocities in water, in air or in other fluids as given in below figure. Industrial classification may be carried out in different types of classifiers and these classifiers are; hydraulic classifiers, mechanical classifiers and cyclones. Basically they all work according to the principle that the particles are suspended in water which has a slight upward movement relative to the particles. Particles below a certain size and density are carried away with the water-flow, whereas the coarser and heavier particles will settle.
8
In classification mostly we use wet cyclones (hydrocyclones) in which rapid spinning of the pulp centrifuges the solid particles. Overflow (particles with terminal velocities < V)
Feed
Fluid Velocity, V
Spigot Product (particles with terminal velocities > V) 1.2.3. Concentration
The second fundamental (main) operation in mineral processing, after the release, or liberation, of the valuable minerals from the gangue minerals, is the separation of these values from the gangue, i.e. concentration. Concentration is usually accomplished by utilizing some specific difference in physical (or chemical) properties of the metal and gangue compound in the ore. In concentration the following terms are used: Head is the feed to a concentrating system. Concentrate is defined as the valuable mineral(s) separated from ore undergoing a specific treatment. Tailing is the fraction of ore rejected in a separating process. It is usually the valueless portion, i.e. discard or waste. Middlings are the particles of locked valuable mineral and gangue, i.e. liberation has not been attained. Further liberation can be achieved by further comminution. Recovery is the percentage of the total metal, contained in the ore that is recovered in the concentrate. Physical Concentration Methods
1. Separation dependent on optical and radioactive properties of minerals, i.e. hand pickling, optical sorting, radioactive sorting, etc. 2. Separation dependent on specific gravity (density) difference of minerals, i.e. heavy-media separation, gravity concentration by use of tables, jigs, cones, etc. 3. Separation utilizing the different surface properties (i.e. surface chemistry) of the minerals, i.e. froth flotation, etc. 4. Separation dependent on magnetic properties of the minerals, i.e. low and high, dry and wet magnetic separation, etc. 5. Separation dependent on electrical conductivity properties of the minerals, i.e. electrostatic separation, etc. So mineral processing is concerned mainly with the physical methods of separation of minerals. Pyrometallurgy and hydrometallurgy may also deal with raw materials but those processes change the character of some or all of the constituents of the raw materials.
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1.3. PRETREATMENT PROCESSES
Minerals recovered from ores are not always in the optimum chemical or physical state for conversion to metals. Oxides are more conveniently reduced to metals than sulfides, or the metal might be more readily leached from the ore if it were present as a sulfate, a chloride, or an oxide. Chemical conversion to the desired species often is an integral segment of the extractive process. Sulfide ores or concentrates, for example, usually are heated in an oxidized atmosphere (roasted) to convert them to an oxide or sulfate. The physical state of an ore may be too fine for charging to a process. Fine ores often are agglomerated by sintering prior to charging to a blast furnace, the principal smelting unit for lead and iron. In the case of iron ore, pelletizing is another very important agglomeration process that has achieved commercial adaptation in the iron and steel industry. In the sections below, the following pretreatment processes will be explained: i. Drying ii. Calcination iii. Roasting iv. Agglomeration 1.3.1. Drying Drying usually means the removal of mechanically held water or moisture from concentrate, or other solid materials by evaporation, i.e. expensive operation, usually done in a drying furnace (fixed or fluidized bed, or kiln) and usually accomplished by passing hot combustion gases through or above the substance. Drying may be accomplished either at atmospheric pressure by heating the substance above the normal boiling point of water, or, under reduced pressure where the atmospheric pressure is brought below the vapor pressure of water at the temperature in question.
H2O(l)
↔ H2O(g) ∆H298oK = +10.5 Kcal / gram-mole (Endothermic process)
Therefore, in addition to the heat needed to bring the substance to the drying temperature, the heat of evaporation must be supplied at that temperature. Moisture Determination Take a grab sample weighing 100 to 1000 grams or more (It should include representative portions from the top, bottom and center of the car of ore or concentrate) Sample Weigh the Sample Put into Drying Oven Dry @ 105°C Weigh again Determine the Loss in Weight Calculate % Moisture % Moisture = {(wet weight - dry weight) / wet weight} * 100
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1.3.2. Calcination Calcination is the thermal treatment of an ore or a concentrate to effect its decomposition and the elimination of a volatile product, usually CO 2, water vapor, or other gases. Therefore, by contrast with drying, calcination involves the removal of H 2O, CO2, etc., which are chemically bound as e.g. hydrates or carbonates.
Lost
Material
Free water
Drying
Lost
Chemically bound H2O,CO2
Completely dehydrated and calcined material
Calcination Increasing Temperature
Tcalcination >> Tdrying
Temperature necessary for the decomposition pressure to reach 1 atm varies. e.g. FeCO3 and Mg(OH)2 ; T ≥ 200°C MnCO3 , MgCO3 ; T ≥ 400°C CaCO3 ; T ≥ 900°C BaCO3 and Na2CO3; T ≥ 1000 °C Examples of calcination reactions: i. CaCO3 ↔ CaO + CO2 ∆H298oK = +42.5 kcal/gram-mole
T ≥ 900o C
This calcination reaction is endothermic. It is more endothermic than drying. ii.
MgCO3 ↔ MgO + CO2 @ about 1700 – 1800 °C MgO produced is called periclase. It is a stable crystalline structure (with no hydration or shrinkage), used as a refractory
iii.
2Al(OH)3 ↔ Al2O3 + 3H2O @ about 1000 °C chemically combined water is driven off. 11
• • •
1.3.2.1.Calcination Furnaces Shaft furnace – For the calcination of coarse limestone Rotary kiln – For the calcination of materials with mixed particle size or lumps which disintegrate during the process Fluidized bed – For materials of uniform, small particle size Fuels used in calcination furnaces are gas, oil, coke, pulverized coal, etc.
Temperature of gas and solids during calcination of limestone in a coke-fired shaft furnace, i.e. temperature profiles 1.3.3. Roasting General Definition: Roasting is the oxidation of metal sulfides to give metal oxides and sulfur dioxide. Typical examples are: 2ZnS + 3O 2 = 2ZnO + 2SO2, ∆H298oK = -220 Kcal/gm-mole Strongly Exothermic o 2FeS2 + 11/2O2 = Fe2O3 + 4SO2, ∆H298 K = -410 Kcal/gm-mole Reactions
Product ( calcine ) In addition other reactions may take place: SO2 + ½ O2 = SO3 Formation of SO 3 2PbS + 4O2 = 2PbSO4 Formation of metal sulfates or 2PbO + 2SO2 + O2 = 2PbSO4 Formation of metal sulfates ZnO + Fe2O3 = ZnFe2O4 Formation of ferrites (complex oxides) Typical ores or concentrates, which are roasted, are the sulfides of Cu, Zn and Pb. In these cases, the main purpose is to convert the metal sulfides, partly or completely, into oxides for subsequent treatment. SO 2 is then a by-product. But for pyrite roasting; 2FeS2 + 11/2O2 = Fe2O3 (pyrite cinder) + 4SO2 Fe2O3 is the by-product while SO 2 gas is the main product.
12
1.3.3.1.Temperature of Roasting Troast > 500 - 600 C: in order for the reactions to occur with sufficient velocity. Troast < 1000 C: roasting is usually carried out below the melting points of the sulfides and oxides involved and to avoid ferrite formation.
Dead roast or sweet roast: Roasting to completion with the elimination of most of the sulfur by overall reaction. Partial roasting: Removal of some of the sulfur by roasting. e.g. partial roasting of copper concentrates CuFeS2 + 4O2 = CuSO4 + FeSO4 2CuS + 7/2O 2 = CuO.CuSO4 + SO2 1.3.3.2.Types of Roasting 1. Oxidizing roast 2. Volatilizing roast 3. Chloridizing roast 4. Sulfating roast 5. Magnetizing roast 6. Carburizing roast 7. Sinter or Blast roasting 1.3.3.3.Types of Furnaces for Roasting Development sequence: Stationary heaps old days Hand rabbled furnaces later development Multiple hearth furnaces long time used for roasting Flash or Suspension roasting furnaces Fluidized bed roasting furnaces Others: Blast roasting or sinter roasting, roasting in a rotary kiln, etc. Fluidized bed roasting became the dominant technique for roasting FeS 2 (pyrite), sulfides of Cu, Zn, Co, Ni, etc. in recent years. It is very efficient and close control is possible. 1.3.4. Agglomeration When the particle size of an ore or concentrate is too small for use in a later stage of treatment, i.e. in the blast furnace, it must be reformed into lumps of appropriate size and strength that is agglomerated. Agglomeration is used particularly if the ore is to be smelted in a shaft furnace where fine-grained material would plug up the gas passage, i.e. decrease gas permeability.
Types of agglomeration i. Sintering ii. Pelletizing iii. Briquetting iv. Nodulizing Below sintering and pelletizing of iron ores or concentrates are explained.
13
1.3.4.1. Sintering Sintering may be defined as a process in which relatively coarse materials, e.g. for iron ore concentrate –8mm + 0.15mm (100 mesh), are converted into coarse agglomerates by partial melting and fusion. The sinter product has a porous structure. Sintering is generally done by the use of a Dwight – Lloyd sintering machine. Sectional-view of a Dwight – Lloyd sintering machine is given below.
1.3.4.2. Pelletizing Pellets are made by rolling critically moist finely divided material around in a drum or in a rotating inclined disc. Below pelletizing of iron ore concentrate is explained.
i.e. Iron ore concentrate: Particle size 70 % - 325 mesh (44 microns) with specific surface area, i.e. Blaine Number 1600 – 2200 cm 2/gram Bentonite (i.e. clay): Binder Flux : Calcium oxide powder (to control basicity)
14
After the production of green pellets, they are dried and fired at 1200 - 1375 °C, i.e. induration, in order to obtain fired pellets of sufficient strength. For hardening of green pellets, the following pellet firing processes or furnaces are available: i. Shaft furnace ii. Grate-Kiln machine iii. Travelling grate (similar to Dwight-Lloyd) iv. Grate-rotary hearth-shaft furnace Pelletizing is well suited for very fine grained concentrates which are not easily sintered on the grate. Pelletizing of chromium concentrates, copper concentrates, etc, are also being done.
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1.4. METAL EXTRACTION PROCESSES
Following the mining, mineral processing and pretreatment of minerals, their extraction is accomplished by application of chemical metallurgy in one of the three areas of extractive metallurgy: Pyrometallurgy, hydrometallurgy or electrometallurgy. 1.4.1.Pyrometallurgical Extraction Processes Pyrometallurgy deals with chemical reactions at high temperatures. These are reactions that take place at temperatures from 100 °C up to 3000 °C or more. Pyrometallurgy or fire metallurgy is the most important of the extraction processes and it is the oldest division of metallurgy. Practically all iron and steel, lead, and tin, most copper, and a small proportion of zinc, etc. are won from their ores and concentrates by pyrometallurgical methods. Pyrometallurgical reactions involve a number of different solids, liquids and gases and, take place in a variety of furnaces, converters, roasters and other devices. The most important pyrometallurgical extraction processes are as follows: i. Roasting – agglomeration – calcination and drying (these are pretreatment processes that are pyrometallurgical in nature.) ii. Smelting - Matte smelting (Cu and Ni, etc.) - Smelting for metal (Fe, Pb, etc.) iii. Converting (Cu, Ni, etc.) iv. Reduction of oxides, etc. (Direct reduction of iron ore, zinc ore, etc.) v. Refining (Cu, Pb, etc.) vi. Distillation (Zn, Hg, etc.) vii. Halide metallurgy 1.4.1.1. Smelting Smelting is mainly a process of melting and separation of the charge into two immiscible liquid layers, i.e. a liquid slag and a liquid matte or a liquid metal. There are two types of smelting: Smelting for matte and smelting for metal. Under Neutral or Reducing Conditions
Slag
e.g. SiO2, CaO, Al2O3, etc
Metal sulphides Matte Smelting e.g. Cu concentrate Matte e.g. Cu2S, FeS, etc 20-30% Cu 30-55% Cu Matte can be defined as molten mixture of sulphides of heavy metals. Liquid slag is the siliceous or oxidized part of the concentrate. Matte smelting is done in one of the following furnaces: (Diagrams given below) - Reverberatory furnace (for fine concentrates) - Flash furnace (for fine concentrates) - Blast furnace (for lumpy ore or agglomerated concentrates) - Electric furnace (for fine concentrates) Matte smelting is therefore a pyrometallurgical concentrating stage in the overall extraction of a metal from its sulphides. Fluxes (Under Reducing Conditions)
Metal oxides (ore or concentrates) e.g. Fe2O3, Fe3O4
Liquid slag e.g. SiO 2, CaO, Al2O3 Smelting for Metal etc Liquid metal e.g. Pig iron Reducing agent (coal, coke, etc.) Fe+4%C+1%Si, etc
16
17
Liquid slag contains the gangue minerals and fluxes. It removes the impurities. Liquid metal, such as pig iron, is an alloy of iron containing C, Si, Mn, etc. Smelting for metal is done in blast, reverberatory and electric furnaces. Metal oxide smelting is an important extraction process producing an impure metal which must be subsequently (fire) refined. Tmatte smelting < Tmetal oxide smelting 1.4.1.2. Converting
Flux
Matte Converting
air and/or oxygen blowing
Horizontal converter, e.g. Pierce-Smith Converter → Liquid matte → (sulphide matte) e.g. Cu 2S - FeS
→ Preferential oxidation of the more reactive impurity metal sulphides, e.g. FeS → FeO. Air blowing is controlled to convert the remaining more noble metal sulphide to the required metal, e.g. Cu2S → Cu (blister copper) Cu2S + 3/2O 2 → Cu2O + SO2 i. 2MeS + 3O2 → 2MeO + 2SO2 FeS + Cu2O → FeO + Cu2S FeS + 3/2O2 → FeO + SO2 Cu2S + 3/2O 2 → Cu2O + SO2 Cu2S + 2Cu2O → 6Cu + SO2 ii. MS + 2MO → 3M + SO2 Cu2S + O2 → 2Cu + SO2 Cutaway view of a horizontal side-blown Pierce-Smith converter is given below.
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1.4.1.3. Reduction of Metal Oxides Metal oxides → Reduction → Metal
↓
Reduction is by C, CO, H 2 or other metals (metallothermic reduction) e.g. 3MO + 2Al → 3M + Al 2O3 2MO + Si → 2M + SiO2 ∆G° - T diagram can be used to assess the various reducing agents: 2MO(s) + C(s) → 2M(s) + CO2(g) below 650°C MO(s) + C(s) → M(s) + CO(g) above 650°C e.g. Fe, Mn, Cr, Sn, Pb and Zn are the main metal oxides reduced with carbon. If Treduction>1800°C cost of reduction increases substantially due to; - Refractory problems - Reactivity of the produced metal with its environment. 1.4.1.4. Refining Fire Refining
Air and/or O2(g) blowing + Poling (Introduction of hydrocarbons CH4, wood…etc)
↓ Impure metal from sulphide matte smelting and converting e.g blister copper (98.5 % Cu, 0.02 – 0.1 % S) 0.5 – 0.8 % O
→
Fire refining preferential oxidation of impurity elements
→
slag
→
SO2(g)
↓ Fire refined metal e.g. fire refined copper (99.5 % Cu, 0.001 – 0.003 % S) 0.05 – 0.2 % O
Steelmaking is also a typical refining process; impurities in pig iron are lowered to acceptable levels. Flux Air and/or oxygen blowing ↓ Impure metal from metal oxide smelting Steelmaking Slag → → ↓ e.g. pig iron (Fe, 4%C, 1%Si, 1% Mn, etc.) Steel (Fe, 0.3%C, 0.001%Si, 0.3% Mn )
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1.4.1.5. Distillation
Low boiling point (Bpt) metals can be separated and refined from higher boiling point metals by distillation, i.e. evaporation and subsequent condensation to the pure metal due to their difference in vapour pressures. M(l) → M(g) The metals that may be refined by distillation are mainly limited to those with boiling point less than 1000°C. Use of vacuum distillation extends the range of metals that can be refined. Separate recovery of vapour Liquid mixture Distillation Residue Partial Vaporization e.g. Extractive metallurgy of zinc and mercury (Hg bpt : 357°C) Removal of As (arsenic) from (iron ores) liquid iron (fractional distillation) Vacuum dezincing (Zn bpt : 907°C) of desilverized lead bullion (Pb bpt :1740°C) 1.4.1.6. Halide Metallurgy
By using halogens (Cl, Br, F, I) metal halides (chlorides, iodides, fluorides, bromides) are formed.
→
Zircon ZrSiO4
Chloridizing roasting
→
ZrCl2
→
→ Reduced with Mg (Kroll Process @ 800 - 1000°C)
Zirconium metal
or some volatile metal halides are produced, e.g. SnCl 4, TiCl4, AlCl3 some metal halides have low decomposition temp. ZrI4 → Zr + 2I2 , @ 1400°C Therefore, it is possible to separate certain metal halides and to purify by distillation.
1.4.2. Hydrometallurgy (Water Metallurgy) st
1 definiton: Hydrometallurgy is concerned with the leaching of ores, concentrates and calcines with aqueous solutions to dissolve and recover the valuable metals. nd
2 definiton: The separation of a soluble substance from an insoluble by means of a solvent. The solvent is either water or an aqueous solution.
Essential steps
i. Leaching or lixivation (dissolving of metal) ii. Recovery of metal from pregnant leach solution
Auxiliary step
Separation of pregnant solution from the leach residue.
1.4.2.1. Advantages
-
High extraction of the value metal Requires very little fuel
20
-
-
Equipment needed is relatively simple and inexpensive Principle expense is the cost of the necessary chemical reagents. In some processes the solvent is regenerated Suited to the treatment of low-grade ores as well as concentrates e.g. gold ores, zinc concentrates, Al 2O3 extraction by the Bayer process, etc. Less environmental problems.
-
Many materials will not respond to treatment by leaching methods.
-
BUT
Water & Solvents
Crushing Ore → & Grinding solution
→ Sands (coarse) → Percolation leaching →
-6mm Slimes (v.fine)
→ → Agitation leach
Separation Metal of solution → recovery and residue from
Preliminary Treatment -
Sometimes unnecessary i.e. direct leaching Sometimes concentration and roasting
1.4.2.2. Solvents Requirements i. It must dissolve the ore minerals rapidly enough to make commercial extraction possible, and it should not attack the gangue minerals. ii. It must be cheap and readily available in large quantities. iii. If possible, it should be regenerated. Some solvents i. Water ii. Acids
iii.
Bases
iv.
Salts
v.
Bacterial leaching
e.g. CuSO4, ZnSO4 leaching (most sulfates are water soluble) e.g. Dilute H2SO4 for oxidized Cu and Zn ore → CuSO4, ZnSO4 ZnO + H2SO4 → ZnSO 4 + H2O e.g. NH4OH+oxygen, or NH 4CO3+NH4OH for leaching CuCO3 NaOH for leaching bauxite Al2O3 + 2NaOH→ 2NaAlO2 + H2O e.g. NaCN or KCN dissolves Au and Ag 4Au (or Ag) + 8NaCN + O 2 + 2H2O → 4NaAu(CN)2 + 4NaOH (Thio Bacillus Thiooxidans or Thio Bacillus Ferrooxidans) e.g. FeSO4 → Fe2(SO4)3 (using Thio Bacillus Ferrooxidans) FeS2 + Fe2(SO4)3 → 3FeSO4 + 2S
1.4.2.3. Leaching Methods (Continuous, batch) (Referring to diagrams below)
i. ii. iii. iv. v.
In place (in situ) e.g. mine water for leaching CuSO 4 Heap leaching (-8 inch or –20cm ore), Dump leaching Sand (Percolation) leaching (-1/4 inch or -6mm ore) in large vats or tanks Agitative (Slime) leaching is done by agitation of fine solids in steel tanks or Pachuca tanks (either mechanically or with compressed air) Pressure leaching in autoclaves
21
22
1.4.2.4. Solid-Liquid Separation
Separation of pregnant leach solution and leach residue (S/L) is by decantation or thickening followed by filtration. A continuous method of leaching and separation of the enriched solution from the gangue is by continuous counter–current decantation (CCD) where several thickeners are often used. CCD as used after cyanidation of gold and silver ores is shown in below flowsheet. With the use of CCD, the pregnant leach solution (L) is separated from the leach residue (S), at the same time the leach residue is washed with wash water so that no pregnant leach solution is lost with the leach residue.
1.4.2.5. Recovery of Metal from Solution
Recovery of metals from pregnant leach solution is done by precipitation (Ppt), electrowinning (EW), solvent extraction (SX), ion exchange (IX), cementation, gaseous reduction, etc. Cementation: Cu++ + Fe° → Cu° + Fe++ ↓ scrap iron
Cu is displaced from aqueous solution by metallic iron
sponge type finely divided cement copper
Cd++ + Zn° → Cd° + Zn++
↓
Cadmium cementation.
Cadmium sponge Electrochemical Series (25°C, 1 N solution) volts Li / Li+ + 3.095 Cr +0.744 Pb +0.126 2+ + K +2.925 Fe/Fe +0.440 H2/H 0.000 2+ Na +2.714 Cd +0.403 Cu / Cu - 0.337 Mg +2.363 Co +0.277 Hg -0.788 Al +1.662 Ni +0.250 Ag -0.799 Zn +0.763 Sn +0.136 Au - 1.680 23
The metal with the more positive (oxidation) potential will pass into the solution and displace a metal with a less positive potential.
valency=n
Gaseous Reduction: Mn+ + n/2 H2 → Mo + nH+ (or CO, SO , H S,etc.) 2 2
@ elevated T over 100 °C and P.
(Ni++ + H2 → Ni° + 2H+)
e.g. Co, Ni, etc.
Precipitation: Zn(zinc dust) + 2NaAu(CN)2 → Na2Zn(CN)4 + 2Au
↓
(without reduction)
precipitate NaAlO2 + 2H2O → Al(OH)3 + NaOH (Hydrolysis reaction) Solvent Extraction (SX): Liquid – liquid extraction (liquid – ion exchange) LIX (For solution concentration or purification purposes) Aqueous solution + organic mixing for SX → (insoluble in leaching solution, capable of sep arating (pregnant solution) from leach solution, must be highly selective)
Extraction in various organic Di(2-ethylhexyl)phosphoricacid (DEHPA)+kerosene types of mixing units (stirred tanks, columns, etc.) contact with leach solution containing uranium 1g/l U3 O8
Uranium ore
→ Crushing & grinding
→ Leaching → with H2SO4
→ pregnant solution ← organic ↓ ↓ DEHPA Leach residue mixer settler → raffinate CCD
(SX)
6 g/l U3O8 ↓ loaded organic HCl → stripping→ regenerated organic ↓100g/l U3O8 (after stripping) NH3 → ppt
↓ (NH4)2U2O7 (ADU)
Ion Exchange (IX): Generally used for low grade uranium ore processing, water softening, processing of rare-earth metals, etc.
e.g. Water Softening Water → Zeolite (sodium aluminate-silicate) → removes Ca/Mg from water → softened water IX resin by exchange with Na e.g. IX of Uranium pregnant solution + synthetic resin → IX (with uranium) (organic polymers, e.g. Amberlite IRA 425, Dowex 11, etc.)
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Reaction: Anionic exchange UO2(SO4)3-4 + 4 RX (complex anion)
→
R 4(UO)2(SO4)3 + 4X
-
↓ resin
negative ion Elution with NaCl, HCl eluent
Uranium ore
→
crushing → grinding
↓ H2SO4 leaching→ CCD → IX columns → loaded → solution→ metal ↓ ↓ resin (eluate) recovery leach effluent ↓ residue to waste regenerated resin
1.4.3. Electrometallurgy
Electrometallurgy deals with metallurgical processes that require electric current for chemical reduction. Electrolytic refining (e.g. Cu, Fe, Co, Pb, Zn, etc) i. Electrolysis Fused salt electrolysis (e.g. Al2O3, MgCl2 etc) ( production of Na, Zr, Ti, K, Ca, etc ) ii.
Electrowinning → Recovery of metals from pregnant leach solution (e.g. Cu, Au, Zn, Sb, etc)
Below electrowinning of copper, electrolytic refining of copper and fused salt electrolysis of alumina to obtain aluminum metal are explained as examples.
25
26
SUMMARY General Classification of Ores for Leaching
Barren solution (Solvent)
In-Place Leaching
Pregnant leach liquor
Mineral Deposit Transported for Leaching
Low Grade Ore Not suitable for Beneficiation
Dissolution
Direct Leaching Ore
Communition and Sizing
High Grade Ore Suitable for Beneficiation
Mineral Dressing
Pretreatment (optional)
Separation Dissolution Dump, Heap, Vat Leaching
Concentrate Pretreatment (optional)
Separation
Dissolution
e.g. Production of e.g. Cu dump leaching Leaching of waste products not economic to subject them to additional treatment. e.g. fine grinding, concentration, etc.
Uranium, Vanadium Gold, Silver Aluminum Nickel Beryllium Titanium Cobalt
27
Separation e.g. Production of Zn, Cu, Ni, Co, W Pt, Ti, Mn, Mo
Recovery of Metal from Solution (Separation of metal)
Leach solution (Solvent) Precipitation by contact reduction Cementation Gold, copper Cu2+ + Fe → Cu + Fe2+
Ore or concentrate
Dilute process stream
Concentrated liquor
Solvent Extraction Ion Exchange
Precipitation by reduction Hydrogen Ni, Co, Cu 2+ M + H2 → M + 2H+
Precipitation without reduction
Uranium, copper Electrowinning Cu, Zn
28
As salts, sulfides -Uranium (Na2U2O7) (as yellow cake) -Fe, Co, Cu, Ni as hydroxides -Aluminum as Al(OH)3 -Ppt of Mg, Be, etc.
1.5. FUELS and REFRACTORIES
Cost of energy is usually a high proportion of the total cost of the metal production; therefore, energy should be used efficiently. coal natural gas
Energy For
Smelting Reduction Handling Comminution Separation … etc
Ore
fuels (solid, liquid, gas) oil electricity (generated by fossil fuels, water or nuclear power) solar energy, etc
→ Metallurgical → Metal(s) Processing
Refractories 1.5.1. Fuels 1.5.1.1. Definitions A fuel is any substance (it may even be a metal) that may be burned rapidly enough so that the heat resulting from the oxidation is capable of being applied to industrial operations. Combustion is the term applied to the burning of fuel. Incomplete combustion is a term applied to combustion in which not all the fuel is burned (leaving unburned carbon in ashes for example). Imperfect combustion means that not all the fuel is oxidized to its highest degree (if CO is formed, for example instead of CO 2). Ignition temperature of fuel is the temperature at which combustion starts. This is not definite and it depends upon the physical condition of the fuel and atmospheric pressure. Ashes and cinders are the residue left after the fuel has burned. 1.5.1.2. Heat Producing Elements : These are given by the order of importance as:
C, H, S, Si, Mn, Al, and P. C is used in the elemental form (charcoal, coke), combined form (hydrocarbons) and partly free, partly combined form (lignite and bituminous coals). H occurs in its free state in some gaseous fuels, or combined as in hydrocarbons. S is important in roasting and in matte converting as the fuel but is undesirable as an impurity in the finished metals (for example in steel). Fe2O3+SO2 e.g. Used for Sulphide Concentrate→ Roasting → Heat Evolved → steam production e.g. FeS2 (exothermic) Sometimes enough heat evolved to sustain the process. e.g. Autogenous roasting of FeS 2, sintering of PbS, etc. i.e. No carbonaceous fuel added.
29
1.5.1.3. Classification of Fuels I-Solid Fuels a. Natural 1. Wood 2. Peat (accumulation of compacted and partially devolatilized vegetable matter) 3. Lignite Coal 4. Bituminous Coal 5. Anthracite Coal b. Prepared (Artificial) 1. Pulverized Coal 2. Briquetted (Compressed) Fuels 3. Carbonized (Distilled) Fuels a) Charcoal b) Coke II-Liquid Fuels a. Natural Petroleum of Crude Oil b. Prepared 1. Distilled Oils 2. Coal Tar 3. Residual Oils (Fuel Oil) III-Gaseous Fuels a. Natural Natural Gas (CH4, C2H6, C3H8, ...) b. Prepared 1. Coal (Coke-oven) Gas (H 2, CH4, ....) 2. Oils Gas (H2, CH4, CO, ...) 3. Blast Furnace Gas (N2, CO, H2, CO2, ...) 4. Producer Gas a) Water Gas (50%H2, 50%CO) b) Air Gas (30%CO, remainder N2) c) Mixture of the two 1.5.1.4. Choice of Fuels Factors to be considered when selecting the right kind of fuel are: 1. Cost: Cheapest of suitable fuels will be used for the purpose. 2. Availability: The metallurgical plant must be at a location with an access to fuel sources (coal mines or oil fields). 3. Suitibility for the process: For blast furnace for example only metallurgical coke can be used. It has sufficient strenght, porosity, calorific value and it is low in ash, S, etc. In hearth or reverberatory furnaces only pulverized coal, fuel oil or gas, all yielding long flames will be required. 4. Purity: This refers to ash content, P and S in the fuel which all are undesirable. Ash is non-combustible, decreases the calorific value and since its melting point is high, to make it fluid during smelting, it has to be fluxed along with the impurities in the ore or concentrate. 5. Calorific Value: Depends upon the carbon, hydrogen and ash contents of the fuel. 6. Calorific Intensity: This is the intensity attained by the combustion of the fuel.
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1.5.2. Refractories 1.5.2.1. Definitions Refractory materials or refractories are those materials that can withstand high temperatures, corrosion from liquids and abrasion of hot gases laden with dust. Acid refractory or acid oxide is one which will absorb oxygen ions when dissolved in a basic melt: SiO2+2O2 = SiO44Basic refractory or basic oxide is one which will provide oxygen ions when dissolved in a melt: MgO= Mg2+ +O2 Neutral refractory is a material that is attacked by neither acidic nor basic oxides and is used to replace basic refractories where the corrosive action is strong. 1.5.2.2. Refractory Materials The commercial refractories are based on either of the following substances: 1. Silica (SiO2) 2. Alumina (Al2O3) 3. Aluminosilicate (xAl2O3.ySiO2) 4. Lime (CaO) 5. Magnesia (MgO) 6. Forsterite(2MgO.SiO2) 7. Dolomite ( MgO.CaO) 8. Hematite (Fe2O3) or Magnetite (Fe3O4) 9. Chromite (FeO.Cr 2O3) 10. Carbon (Graphite) 11. Metals (Water-cooled) 12. Carbides (Silicon carbide and others) 1.5.2.3. Melting Point of Refractories Silica 1724°C Kaolin 1740 °C (High Al 2O3 clay) Bauxite Brick 1600-1820 °C (High in Al 2O3) Alumina 2050 °C (corundum) Magnesia Brick 2165 °C Chromite 2050-2200 °C Silicon Carbide (beyond 2700 °C) Carbon solid at 3600 °C The softening points of refractories, not their melting points should be considered when selecting a refractory for the lining of a furnace.
Temperatures Attained in some Metallurgical Process Copper Smelting 1000- 1100 °C Zinc Retorts 1400- 1600 °C Bessemer Converter 1600 °C Oxygen Converter 1850 °C Tuyeres in Fe Blast Furnaces 1900 °C Electric Arc Temperature 3600 °C Electric Arc Furnace for Steelmaking 1800 °C max
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1.5.2.4. Classification of Refractories
I-Acid Refractories a. Siliceous Materials Consist of SiO2 and are low in metallic oxides and alkalies (Na2O, K 2O) 1. Natural Rock (Sandstone, Quartzite) 2. Quartzite Sand (Ganister) 3. Silica Brick b. Aluminosilicates (xAl2O3.ySiO2) All silica should be in a chemically combined form with alumina. Since any free silica would lower the melting point. 1. Natural Rock 2. Prepared Mass (Fireclay) 3. Burned Brick (Firebrick) II- Basic Refractories a. Aluminum Oxides 1. Bauxite or Bauxite Brick 2. Alundum (Electrically fused bauxite) b. Calcium and Magnesium Oxide 1. Magnesia 2. Lime 3. Dolomite III- Neutral Refractories Aluminosilicates are sometimes classified as neutral refractories, but they exhibit an acid reaction when contact with basic slags. A. Carbonaceous Refractories (Graphite, carbon bricks) B. Chromite (chrome-magnesite) C. Artificial Refractories (Zirconium carbide ZrC, silicon carbide SiC) D. Metals (Fe, Cu, Mo, Ni, Pt, Os, Ta, Ti, W, V and Zr) E. Others (Forsterite, concrete, serpentine,etc) IV- Rarer Refractories: ThO2, TiO2, ZrO2, Y2O3, Ta2O3, CeO2, etc. 1.5.2.5. Some Properties of Refractories Thermal Conductivity: Must be low to minimize heat losses from furnace walls. Coefficient of Thermal Expansion: Must be low, since the refractories will expand when heated up to operating temperature. Allowance must be given to this during the construction of the furnace. Thermal Shock Resistance: Means the ability of a refractory to withstand repeated heating and cooling. The expansion and contraction, which accompanies heating and cooling, may cause the refractory to break or spall. To avoid this, all refractories are, in general heated and cooled very slowly. Porosity: Since most refractories are made by firing mixtures of solids, the formation of pores between the solid grains can not be prevented. The porosity should be kept at a minimum, since all the refractory properties (e.g. strength, thermal shock resistance) of the brick improve when the porosity is decreased, except in the case of insulating refractories.
32
Resistance to Chemical Attack: Most of the oxide or silicate refractories are already fully oxidized so that they will not be affected by oxygen. However, graphite and silicon carbide will oxidize at high temperature and actually will burn. Chemical attack usually results from the contact of acid and basic refractories, through slag or dust. With acid slag (i.e. slag high in silica) acidic refractories and with basic slag (i.e. high in CaO, MgO) basic refractories should be in contact. The product of the chemical reaction has a very low melting point (forms a eutectic). For the same reason acid and basic refractories should not be laid side by side in the heated portions of the furnace. Bricks may have to withstand abrasive damage or erosion by fast moving, dust-laden gases. Softening Point: It is the temperature at which, the refractory is plastically deformed under load. The selection of the refractory for a given service temperature should take the softening point into consideration not the melting point of the refractory. Ability to Stand High Temperatures: Refractories should stand high temperatures.
33
1.6. PRODUCTION of PIG IRON in an IRON BLAST FURNACE
Continuous Pyrometallurgical Smelting Operation Blast Furnace Gas (gaseous fuel (by-product)) CO 22-27% CO2 11-16% H2O 3-5% N2 56-57%
Slag CaO SiO2 Al2O3 MgO
Pig Iron Main product of blast furnace C 3.5 – 4.25 % Mn 0.5 – 2.5 % Si 1.0 – 3.0 % S, P, etc. 0.025% Fe Rest
Raw Materials 1. Coke: Fuel+Reducing agent ( -7.5 cm + 1.5 cm) 2. Limestone: For slag formation + Removal of impurities (-7.5 cm +2.5 cm) 3. Source of Iron Iron ore Fe>51% -75mm +8mm
For 1 ton of Pig Iron Production 1.50 tons of iron ore or sinter or pellets 0.50 ton of coke 0.25 ton of limestone 2.00 tons of air Total 4.25 tons
35-45% 30-35% 10-15% 2-10%
and/or
Sinter -50mm +8 mm and/or
Pellets 10-12 mm φ
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1.7. STEELMAKING
• Steelmaking is an intermittent process. • Steel is an alloy of iron and C (max 2%C) • Most steels contain< 0.6% C Steel Making Processes 1. Bessemer Process(Acidic)- Thomas Process (Basic) 2. Open-hearth Steelmaking (Siemens-Martin Furnaces) 3. Oxygen Steelmaking- L.D., LD-AC, Kaldo, Rotor, etc. (Ere ğli, İskenderun, Karabük) 4. Electric Steelmaking (Çukurova, Asil Çelik, Kroman, Çemta ş, etc.) Mpt
↓ o
1250-1450 C
BF
Flux: Lime (CaO)
↓
↓
Pig Iron
e.g. 4%C → Steelmaking 1%Mn (Pyrometallurgical 1%Si Refining Process) P,S vary
Acid Steelmaking (S,P-can’t be removed)
Basic Steelmaking (S,P can be removed)
Steel (Mpt: 1500-1600 oC) (Lower impurity contents) e.g. C : 0.04-0.80% Mn: 0.06-0.30% P : 0.01-0.05% S : 0.01-0.05% Si : Negligible During Steelmaking: • Si oxidized to SiO 2 - goes to slag • Mn oxidized to MnO - goes to slag • C oxidized to CO(g) – burns at the mouth of converter or collected • P oxidized to P2O5 - goes to slag S reacts with CaO→ CaS in slag (CaO+S=CaS+CO)
Example: Oxygen Steelmaking: L-D (Linz-Donawitz) Process BOF (Basic Oxygen Furnace) Refractories Steel Shell Magnesite Brick Magnesite or Dolomite (Working Lining)
At exothermic reactions, usually excess heat is produced; in order to cool it, scrap is added. So L-D steelmaking is autogenous.
35
1.8. EXTRACTIVE METALLURGY of NONFERROUS METALS 1.8.1. Extractive Metallurgy of Copper
Copper Ores → Sulphide Ores → Pyrometallurgical Extraction Oxidized Ores → Hydrometallurgical Extraction 1.8.1.1. Sulphide Ores of Copper e.g. CuFeS2 containing ore (0.5-2.0%Cu)
↓ Comminution
↓ Flotation →Tailing
↓ Cu Concentrate (20-30%Cu)
↓ or Drying
or Roasting
or Green Conc.
or Drying
↓ Electric Furnace
Ergani
Murgul
KBI
or Sintering or Pelletizing
↓ Reverberatory Furnace
Flash Furnace
↓
Cu Blast Furnace
↓
Matte (30-55% Cu) xCu2S.yFeS
Converting
Blister Cu (98.5% Cu)
Fire Refining (S, O removed)
Anodes (99.5% Cu)
Electrolytic Refining
Electrolytic Cu or Cathode Cu
→(99.99 %Cu)
36
or Continuous Process
1.8.1.2. Oxidized Ores of Copper
e.g. (oxides, carbonates, hydroxides, silicates, sulphates, etc. of copper) CuCO3.Cu(OH)2 Malahite, CuSiO3.2H2O Chrysocolla, Cu2O Cuprite, etc.
Lean Ore (Cu<1.0%)
Rich Ore (1-2 %Cu)
Heap Leaching with H2SO4
Vat Leaching with H2SO4
Pregnant Leach Solution (5 kg Cu/m 3)
Cementation by scrap
SX
Cement Cu
Electrolyte (40 kg Cu/m 3)
Electrolyte (35 kg Cu/m3)
Electrowinning
Cathode Copper (99.94% Cu)
37
Oxide Concentrate or Roasted Sulphide (20-30 %Cu)
Agitation Leaching
Electrolyte (≈40 kg Cu/m 3)
1.8.2. Extraction of Lead
Lead Sulphide Ore
↓
Comminution and Concentration
↓
Lead Concentrate (40-70% Pb) e.g. PbS Concentrate (S acts as fuel)
Fluxes -6 mm CaCO3 CaCO3→ CaO+CO2- Heat
Roasting and Sintering Moisture (8-10%) (Done together on a Dwight-Lloyd) sintering machine ↓ At 1100 °C Dust Laden Gases containing SO2 Sinter
SO2
Coke (fuel) Air Scrap → Lead Blast Furnace (For slag formation) 1400oC
Slag (SiO2, FeO, ZnO, CaO etc.)
Dust
H2SO4 BF gas
Lead Bullion (96-99%Pb)
Refining Waste Commercial Pb Reduction in Blast Furnace: PbO + C→Pb + CO PbO + CO → Pb + CO2
38
Ag, Sb, Zn, Cu, Sn. etc.
1.8.3. Extraction of Zinc
Zinc Sulphide Ore Comminution and Concentration Pyrometallurgical Route
Hydrometallurgical Route ZnS Concentrate (≈50%Zn)
Roasting
Pressure Leaching under O2
Sintering, Briquetting, etc. Horizontal Retort Furnace or Vertical Retort Furnace or Blast Furnace 1300oC or Other Furnaces
Roasting (ZnS+3/2O2 → ZnO+SO2+heat), T:900°C
Leaching with H2SO4 at 80oC ZnO+H2SO4→ZnSO4+H2O Solid/Liquid Separation
Solid (Leach Residue)
Liquid
Solution Purification Cd,Cu,Co,Ni,etc separated Electrolysis 0 2ZnSO4+2H2O+2e →2Zn + 2H2SO4+O2 Anodes: Pb or Pb-Ag Cathodes: Al
Slab Zn Reduction Reaction: ZnO + C → Zn +CO ZnO + CO → Zn +CO2
Electrolytic Zinc 99.99% Zn
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