workshop hall heroult

7/27/2019 Workshop Hall Heroult

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Hall-Heroult Process

Electrochemical Process to Reduce Alumina to Aluminum Alumina is dissolved in a molten fluoride solvent called cryolite

2 Al2O3(cryolite) + 3 C(anode) 4 Al(liquid) + 3 CO2(g)12 e-

T = 960oC

Electrical work is needed:

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= -E 4 V

Electrolyte (cryolitic bath)

Na3AlF6 + (AlF3)excess + CaF2 + Al2O3

Hall-Heroult Process - 1


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Electrochemical Process to Reduce Alumina to Aluminum Alumina is dissolved in a molten fluoride solvent called cryolite

Electrolyte (cryolitic bath)

Na3AlF6 + (AlF3)excess + CaF2 + Al2O3

CR = NaF / AlF3 (molar)

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by CR and %Al2O3



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Examples:

1) Create a stream for a standard bath

2) Phase Diagrams (CR = 2.2; 5% CaF2)

3) Phase Diagrams (% Al2O3 vs xs-AlF3; 5%CaF2)

4) Adding reactants to the electrolyte (Al2O3)

5) Adding reactants to the electrolyte (Na2CO3)

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ng reac an s o e e ec ro y e 37) Carbide formation in the cathode blocks

8) Cathode / Collecting Bar / Refractory / Na(g)



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Example #1: Create a stream for a standard bath

Create a stream of the following electrolyte

Na3AlF6+ 12.2 wt.% AlF3+ 5.0% CaF2+ 3.0% Al O

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The stream will be constrained in

enthalpy at 960oC.

This stream will be used as reactant

in several following examples.



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FThall Database

79.8% Na3AlF6

+ 12.2 wt.% AlF3+ 5.0% CaF2+ 3.0% Al2O3


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Example #2: Phase Diagrams

The cryolite (Na3AlF6) liquidus and the alumina (Al2O3) solubilityare very important for the cell operation.

The formation of a side-ledge protects the cell

lining from the corrosive electrolyte.


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Alumina has to be transported in the bathto the reaction site under the anode.

So a high solubility is better.

A mapping of these phase diagram

features will be useful.



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Example #2: Phase Diagrams (CR = 2.2; 5% CaF2)

FThall database

CR = NaF / AlF3

CR = 2.2

(NaF)2.2(AlF3)1.0



File | Open HALLHEROULT_Isoplethal1


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X-axis

Y-axis T = 800 1050oC


( ) ( )100

111

100

3220.132.2

32=

++ OAlCaFAlFNaF

OAl

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Constants P = 1 atm


( ) ( )0.5

111

100

3220.132.2

2=

++ OAlCaFAlFNaF

CaF


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Example #3: Phase Diagrams (% Al2O3 vs xs-AlF3; 5%CaF2)

FThall database

Na3

AlF6

AlF3(excess)

CaF2Al2O3


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T = 960oC


File | Open HALLHEROULT_Isothermal2


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X-axis

Y-axis


5001111

100

322363

3=

+++ OAlCaFAlFAlFNa

AlF

1001111

100

322363

32=

+++ OAlCaFAlFAlFNa

OAl

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Constants T = 960oC

P = 1 atm


0.51111

100

322363

2=

+++ OAlCaFAlFAlFNaCaF


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Example #4: Adding reactants to the electrolyte

The thermal/heat balance on the electrolytic cell is very important

The following additions are frequent:

Al2O3 for the reduction reaction


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Na2CO3 to increase CR3 Na2CO3 + 2 AlF3 6 NaF + Al2O3 + 3 CO2

AlF3 to decrease CR

CR = NaF / AlF3 (molar)

All these reactants are much colder than the cell (10oC 200oC)



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Example #4: Adding reactants to the electrolyte (Al2O3)

File | Open HALLHEROULT_Add_Alumina



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1.0 ton of Bath

(stream @ 960oC)

Alumina @ 125o

C-Al2O3-Al2O3Al2O3(H2O)

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Na2CO3(s) (@ 20oC)

Al(liq.) (@ 960oC)

AlF3(s)

(@ 50oC)



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1.0 ton of Bath

(stream @ 960oC)

+ 5 kg Alumina-Al2O3-Al2O3Al2O3(H2O)

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Adiabatic Addition

Q = H = 0



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Example #5: Adding reactants to the electrolyte (Na2CO3)

File | Open HALLHEROULT_Add_Soda



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1.0 ton of Bath

(stream @ 960oC)

Na2CO3(s) (@ 20

o

C)

Al(liq.) (@ 960oC)


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2 3

added manually to thecell.

A bag can partially

touch the metal pad

below the bath and

react with it.



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1.0 ton of Bath

(stream @ 960oC)

Na2CO3(s) (@ 20

o

C)

Al(liq.) (@ 960oC)


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2 3

added manually to thecell = 10 kg

A bag can partially

touch the metal pad

below the bath and

react with it

= 0 or 2 kg



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Bags of Na2CO3 = 10 kg

metal pad

= 0 kg

Result


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T < 960oC

4.2 kg gas

(9.5 m3 gas)

986.7 kg bath

19.1 kg Na3AlF6



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Bags of Na2CO3 = 10 kg

metal pad

= 2 kg

Result


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T > 960oC

2.2 kg gas

(7.8 m3 gas)

1009.6 kg bath

0.2 kg C



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Example #6: Adding reactants to the electrolyte (AlF3)

File | Open HALLHEROULT_Add_AlF3


H ll H l P


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1.0 ton of Bath

(stream @ 960oC)

AlF3(s)

(@ 50oC)


H ll H lt P


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1.0 ton of Bath

(stream @ 960oC)

+ 5 kg AlF3(s)

(@ 50oC)

Adiabatic Addition

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H ll H lt P


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1.0 ton of Bath

(stream @ 960oC)

+ 5 kg AlF3(s)

(@ 50oC)

T = 956.00oC

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.


Hall Heroult Process


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Example #7: Carbide formation in the cathode blocks

The lifetime of the cell is very dependent on the cathode block(graphite) lifetime.

The cathode blocks can be degraded by mechanical erosion, but alsoby chemical reactions with the penetrating sodium vapors andliquid electrolyte.


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Sodium is generated at the metal

bath interface with

3 NaF + Al AlF3 + 3 Na

Residual N2 and CO can be

present in the carbon pores init.


Na



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Bath; CR=2.2

C with smallamounts of N & O


File | Open HALLHEROULT_Cathode1

Penetrating Na



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FACT53 Gas species

NaCN(liq.)

C

Fthall


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Metal




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X-axis

Y-axis


10..0100

32=

OAl

( ) ( )01.0..0

111

1

3220.132.2

=

++ OAlCaFAlFNaF

Na

00.01 kg Na / kg of bath

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Constants T = 960oC & P = 1 atm


( ) ( )5

111

100

3220.132.2

2=

++ OAlCaFAlFNaF

CaF

1113220.132.2

++ OACaFA FNaF

( ) ( )5

111

1

3220.132.2

=

++ OAlCaFAlFNaF

C

CaF2 is 5% of the mass of the bath

The carbon to bath mass ratio is fixed at 5.0(arbitrary)

0 10% Al2O3 in the bath



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Example #8: Cathode / Collecting Bar / Refractory / Na(g)

The current collecting bar (steel) entrapped inside the cathode block(C-graphite);

a SiO2/Al2O3 layer is beneath it;

Na(vapors) percolated through the

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Cathode block from the metal pad;

Unwanted reaction products can

increase the electrical resistivity

$$

T 900-960oC




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FACT53

Gas species FToxid

Refractory Mat.

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Steel


File | Open HALLHEROULT_Cathode_CollectingBar


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PNa is variedfrom 10-10 atm to10-1 atm




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Extract & Plot

Results

Na(g)




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X-axis : log10(PNa)

Na(g) is species #13

Y-axis : log10(grams)

Select all s ecies with


Mole (max) > 0



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At low log10(PNa)

C+ Austenite(fcc)

+ Refractory (SiO2 + mullite)

At intermediate log10(PNa)

C


+ Austenite(fcc)

+ Albite (NaAlSi3O8) + SiO2

Higher:+ Slag+ Nepheline (NaAlSiO4)

At high log10(PNa)

C+ Ferrite(bcc)+ Slag (Na2SiO3 + NaAlO2-NaAlSiO4(s.s.))

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