Materials 3F03

Module 6 Aluminum and Magnesium

Smelting

Aluminum

•  3rd most abundant element •  Production: ~1.5x106/annum •  Price:$1800/tonne •  Ore: Bauxite(hydrated aluminum

hydroxide) – Gibbsite (Al(OH)3)

– Boehmite (AlOOH) – Diaspore (Al2O3H2O)

Alumina Reduction

•  What is the minimum temperature required to reduce Al2O3 with carbon.

A/ 1000 C B/1500 C C/ 2000 C

Aluminum

•  Process Route – Pyrometallurgy?

•  No carbothermic reduction below 20000C

– Halide Metallurgy •  Al2O3+3Cl2+3C= 2AlCl3+3CO

•  AlCl3+ 3M=MCl + Al •  Only works for M=Kor Na •  Very Expensive!

Process Route

•  Electrometallurgy – Aqueous? – Won’t work – Why?

•  Fused Salt – Al2O3 dissolved in molten cryolite (Na3AlF6) – Hall Heroult process (1886)

Aluminum Electrolysis

•  Bauxite contains oxides of iron, silicon and titanium, can I charge bauxite directly to the electrolysis cell.

A/ Yes B/ No

Feed-Stock

•  Bauxite? – No –  Impurities; Ti, Fe, Si more easily electro-won – Must remove – Bayer Process – Pure Al2O3

Hall –Heroult Cell

•  Must contain corrosive cryolite – Graphite lined – Graphite anodes – Liquid Al cathodes – Al removed by suction

Electrolyte

•  87% cryolite, 8%CaF2, 5% Al2O3

•  Requirements: – Low Mpt---High Fluidity – Low density----Must float on Al – High Al2O3 solubility – Low Al Solubility – Will freeze on graphite to form protective skin

Possible Cell reactions

•  Al2O3+9/2C= 1/2Al4C3+3CO – ΔG0

1223= 137,224 cal/mol •  Al2O3+3/2C= 2Al+3/2CO2 – ΔG0

1223= 165,758 cal/mol •  Al2O3+3C= 2Al+3CO – ΔG0

1223= 150,530 cal/mol •  CO2 formed at anode for kinetic reasons! •  Some CO but this has higher heat

requirement

Energy Requirement

Al2O3+3/2C= 2Al+3/2CO2 – ΔG0

1223= 165,758 cal/mol – E0:~3.5KWh/Kg Heat : 6.3KWh/Kg

•  Al2O3+3C= 2Al+3CO – ΔG0

1223= 150,530 cal/mol – E0 :~3.2KWh/Kg Heat : 7.74KWh/Kg

•  Minimum Energy Requirement Dictated by Heat!

•  Reality Much Higher!

Optimize Energy

•  Electrolyte – High Conductivity – Low Al Solubility – High Al2O3 solubility

•  Electrodes – Prebaked---low resistance(7-10% of cost)

•  Anode Cathode Gap----Small

Minimise Anode/Cathode Gap

•  Problem: Reoxidation •  Solution: New Cell Design

Optimum Operation

•  Anode Overpotential – η=0.4+ 0.25logi –  Increased production –  Increased current –  Increased energy rquirement

•  Must balance cost of production with capital cost.

Aluminum Production

•  Hall Heroult- Only viable process •  Requires Bayer Alumina •  Energy Intensive

– Minimum dictated by heat requirement – Real dictated by resistance losses and

Current Efficiency

Energy Requirement

•  Minimum dictated by heat requirement – Do not achieve minimum – System resistances lead to greater V – Reoxidation reduces current efficiency

•  Main contribution to R – Bath resistance---want to decrease gap

•  Anode Overpotential----increase production –increase V.

Magnesium Smelting

•  Production; 31,000tpa •  Price; $1900/t •  Uses: Light Alloys , Reductant for Ti, Zr

and U •  Ores:

– Dolomite (CaOMgO) – Seawater (0.13%Mg) – Chloride brines (Salt lake; 0.45Mg)

Possible Production Routes

•  Carbothermic Reduction – ~20000C – Mg gas----reduced pressure ---decreased

reduction Temp •  Silicathermic

– >20000C – Reduce pressure and absorb SiO2 in slag to

increase driving force (Pigeon/Magnatherm process

Possible Production Routes

•  Fused Salt Electrolysis – MgCl2= Mg + Cl2

– E0= -2.55V – Heat = 3.66V – Reality>6V because of overpotentials, bath

resistance and low C.E. (electrolysis only 20% of total cost)

Electrolyte

•  Depends on MgCl2 source – Sea water 40%CaCl2, 30-40%NaCl, 5-20%

KCl, 10-15% MgCl2

– Anhydrous MgCl2 3%CaCl2 30-40% NaCl, 50%KCl, 10-15% MgCl2

•  More Dense than Mg!!!

Two Main Processes

•  IG Farben: – Uses anhydrous MgCl2

•  Dow: – Semi hydrated MgCl2

– Less preprocessing, lower productivity cells

Two Philosophies

•  Fully Anhydrous MgCl2 – Requires Carbochlorination – Easier cell operation

•  Semi- hydrated (~30%H2O) – Only operated by Dow – Produces 35% of Worlds Mg\ – Eliminates costly carbo-chlorination step – Causes problems with metal coalescence

Other Processes

•  Many have been attempted over the years •  Carbothemic reduction •  Hydrogen reduction •  Silicathermic reduction (only remaining

pyro process)

Summary

•  Energy Comparison – Electrolytic

•  325MJ/Kg(Dow, 50%electric)

– Carbothermic •  380MJ/Kg

– Pidgeon •  394MJ/Kg

– Magnatherm •  276 MJ/Kg(80%Electric)

Summary

•  Magnesium can be made from fused salt electrolysis or metalothermic reduction

•  Electrometallurgy – Salt more dense than Mg – Different options on degree of hydration – Electrolysis Energy only 20% of

The Future(Now)

•  Cell development –  diaphragm-less cell (Alcan, Norsk Hydro)

•  Rely on circulation patterns to affect separation •  Can use higher current densities with higher

efficiencies (close to the best aluminum cells 13-15MWh/t)

•  Larger anode cathode gap •  Greater voltage drop

•  New MgCl2 production (Magnola--Noranda) FAILED!