Metallurgy Flashcards
metals are oxidising/reducing agent
reducing agents as they lose and donate electrons
define mineral
the compounds of various metals found in nature- associated with their earthly impurities are called minerals
define ore
naturally occurring minerals from which metals can be extracted profitably and conveniently
define metallurgy
large scale processes involved in the extraction of pure metals from their respective ores
define matrix/ gangue
rocky impurities including silica, mud, etc. associated with the ore
define flux
substance added to the ore to get rid of the matrix- forms a fusible compound SLAG
ores of aluminium
- bauxite (main)- hydrated aluminium oxide
- cryolite- sodium aluminium fluoride
- corundom- aluminium oxide
ores of zinc
- zinc blende (main)- zinc sulphide
- calamine- zin carbonate
- zincite- zinc oxide
ores of iron
- haematite (main)- iron 3 oxide
- magnetite- tri iron tetroxide
- iron pyrites- iron disulphide
- spathic iron ore- iron 2 carbonate
stages of extraction of metals
- dressing of ore/ concentration
- conversion of concentrated ore to oxide
- reduction of metallic oxides to its metal (impure)
- electro-refining of impure metal
dressing of ore
separation of ore from gangue
reason: to convert impure ore to pure concentrated ore
conversion of concentrated ore to its oxide
reason: to convert the pure concentrated ore to its oxide since oxides are easy to reduce
reduction of metallic oxide to its metal
reason: to obtain the metal, from the metallic oxide by reduction
method of reduction depends on position of metal in activity series of metals
electro-refining of impure metal
reason: to obtain the metal in its very pure form
methods of concentration
- hydrolytic
- froth floatation
- magnetic method
- chemical method
hydrolytic method/ gravity separation
separation of ore and gangue due to difference in density of particles
magnetic separation
separation of magnetic ore from non magnetic gangue, or non magnetic ore from magnetic gangue
froth flotation
separation of ore and gangue by preferential wetting
generally applied for sulphide ores
chemical method of concentration
separation of ore and gangue by use of a chemical NaOH in Baeyer’s process during extraction of aluminium
conversion of concentrated ore to oxide
done by
1. roasting (generally sulphide ores)
2. calcination (generally carbonate ores)
define roasting
process of heating the concentrated ore in presence of excess air to a high temperature before it is reduced to a metal
define calcination
process of heating the concentrated ore in a limited supply of air or in absence at a temperature just below its melting point or not sufficient to melt the ore
metal at the top of activity series is..
- most easily oxidised
- most electropositive and most reactive
- can displace the metal below from its salt solution
reduction of metallic ores to metal, processes
- electrolysis (K to Al)
- reducing agents (Zn to Cu)
- thermal decomposition (Hg and Ag)
types of electrorefining
- distillation refining
- oxidation refining
- electrolytic refining
dressing of ore- aluminium
bauxite is used
CHEMICAL METHOD
impure bauxite to pure aluminium oxide (alumina)
reduction of oxide to metal- aluminium
- method- electrolysis
- process- Hall Heroult’s process
- conversion- pure alumina to pure aluminium
- reaction: electrolytic reduction
- electrolyte: mixture of fused alumina, cryolite, fluorspar (1:3:1)
- electrodes: inert graphite
reason for use of cryolite and fluorspar
acts as a solvent for fused alumina, and also lower the melting point of the mixture, and increase conductivity of electrolytic mixture
why doesn’t aluminium occur in free state
reactive metal
function of sodium hydroxide
- bauxite is reacted with NaOH under pressure for 2 hrs
- impurities Fe2O3 and SiO2 remain unaffected since they are not amphoteric
- bauxite being amphoteric reacts with the base forming sodium aluminate and water, impurities separate out
hence NaOH is used during purification of bauxite
define seeding
sodium aluminate salt is diluted in water, and extra aluminium hydroxide is added to it to further precipitate out the aluminium hydroxide
electrolytic cell used during extraction of aluminium
rectangular steel tank with carbon lining
cathode: carbon lining of the cell
anode: thick graphite rods attached to copper clamps dipping into fused electrolyte
temp: 950 C
current: 100 A at 6-7 V
GR: fused alumina is electrolytically reduced to aluminium
it is a highly stable oxide
aluminium has a high affinity for oxygen
difficulties faced during electrolytic reduction of alumina
- fused alumina is a non electrolyte and has a high melting point
- large amount of electrical energy is required
- liberated Al can also volatize out and get wasted, so fusion temperature of the mixture is lowered
why is a layer of powdered coke sprinkled over the electrolytic mixture
- prevents burning of carbon electrodes in the air
- minimizes heat loss by radiation
why are the anode graphite rods continuously replaced
- oxygen released reacts with anode
- anode gets oxidised to CO which burns to give CO2 or escapes
- the anode is hence consumed and renewed periodically
processes of electrolytic reduction and electrorefining , and concentration of ore
reduction: Hall Heroult’s process
refining: Hoope’s process
Concentration: baeyers process
define alloy
a substance prepared by adding other metals to a base metal in appropriate proportions to obtain certain desirable properties
define amalgam
an alloy in which the base metal is mercury
