Objectives General Principles and Gener Processes of ...

1 A few elements like carbon, sulphur, gold and noblegases, occur in free state while others in combined formsin the earth s crust. The extraction and isolation of anelement from its combined form involves variousprinciples of chemistry. A particular element may occurin a variety of compounds. The process of metallurgyand isolation should be such that it is chemically feasibleand commercially viable. Still, some General principlesare common to all the extraction Processes of obtaining a particular metal, first we look forminerals which are naturally occurring chemicalsubstances in the earth s crust obtainable by of many minerals in which a metal may be found,only a few are viable to be used as sources of thatmetal.

2 Such minerals are known as , an ore contains only a desired is usually contaminated with earthly or undesiredmaterials known as gangue. The extraction and isolationof metals from ores involve the following major steps: Concentration of the ore, Isolation of the metal from its concentrated ore, and Purification of the entire scientific and technological process usedfor isolation of the metal from its ores is known studying this Unit, you will beable to explain the terms minerals,ores, concentration, benefaction,calcination, roasting, refining, etc.; understand the Principles ofoxidation and reduction as appliedto the extraction procedures; apply the thermodynamicconcepts like that of Gibbs energyand entropy to the Principles ofextraction of Al, Cu, Zn and Fe; explain why reduction of certainoxides like Cu2O is much easierthan that of Fe2O3; explain why CO is a favourablereducing agent at certaintemperatures while coke is betterin some other cases.

3 Explain why specific reducingagents are used for the illustrates why only a certain reducing elementand a minimum specific temperature are suitable for reduction of ametal oxide to the metal in an Pal Pal Pal Pal Principles andrinciples andrinciples andrinciples andrinciples andPPPPP rrrrrocesses oocesses oocesses oocesses oocesses of Isolationf Isolationf Isolationf Isolationf Isolationooooof Elementf Elementf Elementf Elementf ElementsssssGenerGenerGenerGenerGeneral Pal Pal Pal Pal Principles andrinciples andrinciples andrinciples andrinciples andPPPPP rrrrrocesses oocesses oocesses oocesses oocesses of Isolationf Isolationf Isolationf Isolationf Isolationooooof Elementf Elementf Elementf Elementf Elementsssss6 UnitUnitUnitUnitUnit NCERTnot to be republished148 ChemistryIn the present Unit, first we shall describe various steps for effectiveconcentration of ores.

4 After that we shall discuss the Principles of someof the common metallurgical Processes . Those Principles shall includethe thermodynamic and electrochemical aspects involved in the effectivereduction of the concentrated ore to the vary in abundance. Among metals, aluminium is the mostabundant. It is the third most abundant element in earth s crust ( by weight). It is a major component of many igneous mineralsincluding mica and clays. Many gemstones are impure forms of Al2O3and the impurities range from Cr (in ruby ) to Co (in sapphire ). Ironis the second most abundant metal in the earth s crust. It forms avariety of compounds and their various uses make it a very importantelement. It is one of the essential elements in biological systems as principal ores of aluminium, iron, copper and zinc have beengiven in Table ofOccurrence ofOccurrence ofOccurrence ofOccurrence Oresof Oresof Oresof Oresof OresAluminiumBauxiteAlOx(OH)3-2x[where 0 < x < 1]Kaolinite (a form of clay)[Al2 (OH)4 Si2O5]IronHaematiteFe2O3 MagnetiteFe3O4 SideriteFeCO3 Iron pyritesFeS2 CopperCopper (OH)2 CupriteCu2 OCopper glanceCu2 SZincZinc blende or SphaleriteZnSCalamineZnCO3 ZinciteZnOMetalOresCompositionFor the purpose of extraction, bauxite is chosen for aluminium.

5 Foriron, usually the oxide ores which are abundant and do not producepolluting gases (like SO2 that is produced in case iron pyrites) are copper and zinc, any of the listed ores (Table ) may be useddepending upon availability and other relevant factors. Before proceedingfor concentration, ores are graded and crushed to reasonable of the unwanted materials ( , sand, clays, etc.) from the oreis known as concentration, dressing or benefaction. It involves severalsteps and selection of these steps depends upon the differences inphysical properties of the compound of the metal present and that ofthe gangue. The type of the metal, the available facilities and theenvironmental factors are also taken into consideration.

6 Some of theimportant procedures are described is based on the differences in gravities of the ore and the gangueparticles. It is therefore a type of gravity separation. In one such process,Table : Principal Ores of Some Important NCERTnot to be republished149 General Principles and Processes of Isolation of ElementsFig. : Magnetic separation (schematic)an upward stream of running water is used to wash the powdered lighter gangue particles are washed away and the heavier ores areleft is based on differences inmagnetic properties of the orecomponents. If either the oreor the gangue (one of thesetwo) is capable of beingattracted by a magneticfield, then such separationsare carried out ( , in caseof iron ores). The groundore is carried on a conveyerbelt which passes over amagnetic roller ( ).

7 This method has been in use for removing gangue from sulphide ores. Inthis process, a suspension of the powdered ore is made with water. To it,collectors and froth stabilisers are added. Collectors (e. g., pine oils, fattyacids, xanthates, etc.) enhance non-wettabilityof the mineral particles and froth stabilisers(e. g., cresols, aniline) stabilise the mineral particles become wet by oilswhile the gangue particles by water. A rotatingpaddle agitates the mixture and draws air init. As a result, froth is formed which carriesthe mineral particles. The froth is light and isskimmed off. It is then dried for recovery ofthe ore , it is possible to separate twosulphide ores by adjusting proportion of oilto water or by using depressants . Forexample, in case of an ore containing ZnSand PbS, the depressant used is NaCN.

8 Itselectively prevents ZnS from coming to thefroth but allows PbS to come with the Innovative WasherwomanThe Innovative WasherwomanThe Innovative WasherwomanThe Innovative WasherwomanThe Innovative WasherwomanOne can do wonders if he or she has a scientific temperament and is attentive toobservations. A washerwoman had an innovative mind too. While washing a miner soveralls, she noticed that sand and similar dirt fell to the bottom of the was peculiar, the copper bearing compounds that had come to the clothes fromthe mines, were caught in the soapsuds and so they came to the top. One of herclients was a chemist, Mrs. Carrie Everson. The washerwoman told her experience toMrs. Everson.

9 The latter thought that the idea could be used for separating coppercompounds from rocky and earth materials on large scale. This way an invention wasborn. At that time only those ores were used for extraction of copper, which containedlarge amounts of the metal. Invention of the Froth Floatation Method made coppermining profitable even from the low-grade ores. World production of copper soaredand the metal became : Froth floatation process (schematic) NCERTnot to be republished150 ChemistryIntext QuestionsIntext QuestionsIntext QuestionsIntext QuestionsIntext of the ores mentioned in Table can be concentrated bymagnetic separation method? is the significance of leaching in the extraction of aluminium?

10 Crudeof Crudeof Crudeof Crudeof CrudeMetal fromMetal fromMetal fromMetal fromMetal fromConcentratedConcentratedConcentrated ConcentratedConcentratedOreOreOreOreOreL eaching is often used if the ore is soluble in some suitable following examples illustrate the procedure:(a) Leaching of alumina from bauxiteThe principal ore of aluminium, bauxite, usually contains SiO2,iron oxides and titanium oxide (TiO2) as impurities. Concentrationis carried out by digesting the powdered ore with a concentratedsolution of NaOH at 473 523 K and 35 36 bar pressure. Thisway, Al2O3 is leached out as sodium aluminate (and SiO2 too assodium silicate) leaving the impurities behind:Al2O3(s) + 2 NaOH(aq) + 3H2O(l) 2Na[Al(OH) 4](aq)( )The aluminate in solution is neutralised by passing CO2 gas and hydratedAl2O3 is precipitated.