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ChemWiki: The Dynamic Chemistry E-textbook > Inorganic Chemistry > Descriptive Chemistry > d-Block Elements > Metallurgy

Metallurgy

This section describes the metallurgy of the transition metal elements. More importantly, it discusses the methods in which transition metal elements (Groups 3-12) are extracted from their metal ores. It concludes with a brief description of steel production using the concepts of equilibrium, thermodynamics, & kinetics.

Introduction

Transition metal elements, unlike metals of groups 1,2, & Al of group 3, are obtained from a process of extractive metallurgy.

Definitions

  • Metallurgy: science/study of metals & their properties.
  • Ores: compounds from which metals are separated
  • Extractive metallurgy: extraction of metals from their ores.

The table above shows some examples of transition metals & their corresponding ores in which they naturally occur.

 

Processes of Extraction

Concentration (removing the impurities)

Ores mined from the earth’s crust will contain impurities. In fact the desired metal contained in the mineral ore will constitute a small % of the mined material.  These impurities are known as gangue (i.e sand,& gravel)

Froth Flotation 

  •  used if ore density is less than density of impurities.                                                                                               

Magnetic Separation  

  • Another method of concentrating Ores.
  • Magnetic ores (i.e Fe3O4) are separated from their impurities by being passed through a magnetic field. The field attracts the magnetic ore & repels the nonmagnetic impurities.

Roasting

Ore is crushed and heated to a high temp using a strong blast of hot air. The process converts the ores to their oxides which can then be reduced. Eg: The natural occurring ores of zinc are ZnS (sphalerite) & ZnCO3 (smithsonite). When roasted, smithsonite decomposes to to ZnO(s) & CO2(g). The strong blast of hot air involved in roasting supplies O2(g) to the sphalerite, thus, becoming oxidized as a result. The oxidation produces ZnO(s) & SO2(g). 

ZnCO3(s) + (heat) → ZnO(s) + CO2(g)

2 ZnS(s) + 3 O2(g) + (heat) → 2 ZnO(s) + 2 SO2(g)

Reduction

C(s) & CO(g) are often used as reducing agents in simultaneous reactions. Oxides of Cr, V, & Mn are reduced using Al. During the reduction process the metal oxide is heated to a temperature above its boiling point in order to vaporize it & condense as a liquid. 

Eg: The ZnO(s) that is produced in the roasting process is combined with C(s)/CO(g) & heated @ a temperature of 1100 oC allowing for reduction & vaporization to occur. 

ZnO(s) +C(s)  + (heat)→  Zn(g) + CO(g)

ZnO(s) + CO(g)+ (heat) → Zn(g) + CO2(g)

Pyrometallurgical Process

Pyrometallurgy is the processes of roasting an ore @ a high temperature & then reducing its oxide product. Its characteristics include:

  1. large amount of waste as a product of concentration.
  2. high energy consumption to maintain high temps.
  3. gaseous emissions that must be controlled (i.e CO2 & SO2)

Refining

The impurities contained in the metal product of the roasting/reduction process are removed. 

Eg: The Zn(l) produced in the reduction process often contains impurities of Cd & Pb. Though fractional distillation of Zn(l) will work in the refining process, a more commonly used and efficient method is electrolysis. The ZnO(s) produced from roasting is dissolved in H2SO4(aq) & then Zn(s) powder is added allowing the impurities to be displaced. The solution is then electrolyzed & \(Zn^{2+}\) is reduced to its pure metallic Zn(s) as a result.

Hydrometallurgical Process
  • The method of refining zinc described above is an example of a hydrometallurgical process.
  • The hydrometallurgical process eliminates the need to control gaseous emissions that are often produced in roasting. 

Hydrometallurgy is the process of extraction & refining that involves the use of water & aqueous solutions. It is carreid out at moderate temps & is generally carried out in 3 steps:

  1. Leaching: metal ions are extracted from their ore by water/acids/bases/salt solutions. Redox reactions that occur are often essential.
  2. Purification & concentration: Impurities are separated either by absorption on the surface of activated charcoal, ion exchange, or water evaporation; leaving behind a more concentrated solution.
  3. Precipitation: the process of electrolysis is often used to precipitate the desired metal ions in an ionic solid or reduce them to their free metal.                                                                                                               

Zone Refining

A rod containing the desired pure metal & impurities is passed through a series of heating coils and cooled again. The process isolates the impurities that concentrate in the molten zones, leaving the portions behind them somewhat more pure. This process is repeated until impurities are moved to the end of the rod and cut off, resulting in an almost completely pure metal rod. 

Alternative Methods 

In commercial production sometimes it is important to leave metals in their oxide or naturally occurring ore form in order to produce a metal alloy.

 Eg 1: Iron Alloys of Cr,V, & Mn:

  1. Ferrochrome ( alloy of Fe & Cr) can be produced in the reduction of Cr in an Fe compound containing its chromite ore, Fe(CrO2)2.
  2. Ferrovanadium ( alloy of Fe & V) can be produced in the reduction of V that occurs when adding an Fe compound to a V oxide (V2O5).
  3. Ferromanganese (alloy of Fe & Mn) can be produced in the reduction of Mn that occurs when adding an Fe compound to a Mn oxide (MnO2).

 Eg 2: Titanium Production(The Kroll Process):                                                                                                              

  • Ti is used in the military & aircraft industry b/c of its low density & ability to maintain its strength @ high temps. It is produced using the following steps: 

          The Kroll process described above is slow  & proposes a health/safety risk due to the high- temp. vacuum distillation                                                      

Eg 3: Alternative Means of Titanium Production:

The electrolysis of TiO2 pellets:

  1. The pellets are placed @ the cathode end(usually Ti(s) or graphite) of an electrolytic cell.
  2. They are dissolved in a molten CaCl2(l) electrolyte.
  3. The O2- anions are discharged as O2(g) @ the anode(graphite).  
  4. The Ti4+ cation is reduced @ the cathode resulting in a tatanium sponge                

Eg 4: Copper Production:

  • Cu ores often contain iron sulfides. In order to achieve a pure(uncontaminated) Cu product, the ores undergo the following processes : 
  • Concentration of ore by flotation
  • Conversion of Fe sulfide ores to Fe oxides through roasting
  1. Smelting: mixing with coke/charcoal &  heated @ a high temp(i.e 800oC+ for Cu in order to keep Cu from remaining in sulfide form). Impurities are further removed by adding flux if not already present in ore( if gangue is acidic basic flux is added, if gangue is basic acidic flux(i.e SiO2) is used. Smelting separates the material to separate into 2 layers( copper matte: bottom layer containing molten sulfides of Fe & Cu. & silicate slag: top layer formed by reaction of oxides of  Fe, Ca, & Al w/ SiO2.)                                                                                                                                                                                    
  2. Reaction illustrated:    FeO(s) + SiO2(s) +(heat) → FeSiO3(l)                                                                                                                                              
  3. Conversion: air is blown through Cu matte in a separate furnace. This converts the remaining Fe sulfide to Fe oxide followed by the formation of FeSiO3(l) slag. The slag is poured off & the process is repeated resulting in blister copper with bubbles of SO2(g) still present.                                                                                                                                                                                                                                                                                                            
  4. Reaction illustrated:  2CuS(l) + 3O2(g) + (heat) → 2Cu2O(l) + 2SO2(g)                                                                                                                                                    2Cu2O(l) + Cu2S(l) + (heat) → 6Cu(l) +SO2(g)                                                                                                                              
  5. Blister Cu is further refined through electrolysis to achieve a high-purity Cu product.                                       

Relating Thermodynamics to Extractive Metallurgy

To establish the conditions under which a reducing agent will completely reduce an oxide of a transition metal to its desired pure metal we must look @ the tendencies of the metal & reducing agent to undergo oxidization. This is due to the fact that both the oxide & reducing agent are competing for Oxygen atoms. The ability of a metal & reducing agent to undergo oxidization is represented by the change in Gibbs Free Energy related to Temperature. For the reducing agent to be sufficient in the reduction of the transition metal, the reaction must be spontaneous. Many times, the ore must be heated to a high temperature make the overall reaction spontaneous.             

                               

Eg 2: For reaction that occurs with the reduction of Mg oxide using Carbon ( 2MgO(s) + 2 C(s) → 2 Mg(g) + 2 CO(g)), spontaneity only occurs for temps exceeding 1700 oC. Since the temperature is extremely high it is not used in the production of pure Mg metal.  

Iron Metallurgy & Steel Production 

  • Iron is the most commonly used & commercially produced transition metal especially in its alloy steel.  
  • Iron ore is reduced to pig iron(95% iron) in a blast furnace.  

The figure above illustrates the reactions that take place in the different regions of the blast furnace. Iron ore, coke, & limestone are added to the top of the blast furnace. The bottom of the furnace has a blast of hot air & is the region where the highest temperatures are attained. The varying temperature regions of the blast furnace allow for reduction, slag formation, formation of reducing agents, & extraction of pig iron.

  • The primary reducing agent used in a blast furnace is coke(CO). 

Reaction illustrated

\[Fe_2O_3(s) + 3CO(g) \rightarrow 2 Fe(l) + 3CO_2(g)\]

The primary components of the blast furnace are:

  1. Charge (solid reactants): includes Fe ore, coke, flux, & scrap iron.  
  2. Flux (maintains acid to base ratio allowing for liquid slag formation): generally limestone(CaCO3) or dolomite(CaCO3MgCO3) is used do to the fact that the acidic oxides (SiO2, Al2O3, & P4O10) are predominate over basic oxides.
  3. Slag(mixture of impurities that are separated/ poured off from molten Fe do to its less dense liquid state): silicate, aluminate, or phosphate.                          
  4. Pig Iron product( 95% Fe, 4% C, 1% other impurities): used in steel production & can be poured into molds creating hard & brittle cast iron.

Steel Production

  1. Steel is made in a basic oxygen furnace using the following methods & reactions:
  2. O2(g) kept @ pressure of 10atm & powdered limestone are fed through a water-cooled tube(lance) & exposed to molten pig iron
  3. The following reactions take place, which remove the pig iron of all its impurities [4%C,1% (Si,Mn,&P)]:                                                                                                                                                                                                                                                        
  4. The vessel is tilted to pour out the impure liquid slag on top of pure molten Fe.  
  5. The pure iron is alloyed with the desired alloying elements( such as Cr, Ni, Mn, V, Mo, & W).

References

  1. Geselbracht, Margaret J.; Ellis, Arthur B.; Penn, Rona L.; Lisensky, George C.; Stone, Donald S. "Mechanical Properties of Metals: Experiments with Steel, Copper, Tin, Zinc, and Soap Bubbles." J. Chem. Educ. 1994, 71, 254.
  2. Petrucci, Ralph H., William S. Harwood,F. Geoffrey Herring, Jeffry D. Madura. General Chemistry: Principles and Modern Applications-9th Edition. New Jersey: Pearson Education, 2007. Chapter 23: The Transition Elements (pp.967-978) 
  3. Department of Education, Government of Kerala. Kerala Reader.Chapter 8: Extraction of Metals (pp.91-102)

Problems

  1. Describe 2 methods of ore concentration and list the conditions under which they are used.
  2. Explain the conditions under which an ore might be roasted.
  3. Explain the conditions under which an are might be electrolyzed.
  4. What types of reactions occur at the cathode and anode in an electrolyzed solution?
  5. How might the transition metal, zinc, be refined?
  6. Explain the process of zone refining and how it may make an extracted metal more pure.
  7. Explain how a change in free energy is related to extractive metallurgy. What specific process or reaction does it pertain to?
  8. Give a brief explanation of why an ore might be heated to high temperatures using the principles of Thermodynamics. 
  9. What are the characteristics of a pyrometallurgical process & how do they compare with those of a hydrometallurgical process?
  10. What are the applications of pig iron and how might it be refined to make pure iron.
  11. What are the reactions that take place in the process of making steel? 
  12. How might an alloy of differing metals effect the properties of steel?
  13. Explain 2 alternative methods of extractive metallurgy and how they differ from traditional methods. 

Summary

Many methods of extractive metallurgy may be applied to get a pure metal from its naturally occurring ore. First ores must be concentrated and separated from their earthly impurities. Concentration of ores may be achieved by froth flotation in which an ore is contained in a froth that floats at the top of a water vat. After the ore has been concentrated it must be roasted to a high temperature allowing for the desired metal to be oxidized. Once the metal is oxidized it is reduced by a reducing agent. Reduction may require the metal oxide to be heated at a high temperature. The metal must be further refined to remove all of the impurities that it may contain. Refining a metal can be accomplished by electrolysis or zone refining. Certain ores such as those of copper and titanium require special treatment before they can be refined. The extraction of iron ore is particularly important for commercial uses because of the mass production of its alloy, steel. 

Contributors

  • Eric Knotts (UCD)

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Last Modified
09:47, 21 Dec 2013

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