DE2153294A1 - Process for the production of aluminum by electrolysis of aluminum oxide in a fluoride melt flow - Google Patents

Description

Process for the production of aluminum by electrolysis of Alumina in the fluoride melt flow

Priority: December 1, 1970, Switzerland, No. 17762/70

For the production of aluminum by electrolysis of aluminum oxide (A] O, alumina), this is dissolved in a fluoride melt. The electrolysis takes place in a temperature range of about 9-10 to 975 C. Anodes made of amorphous carbon are immersed in the melt from above . The cathodically deposited aluminum collects under the fluoride melt on the bottom of the cell. At the anodes, the electrolytic decomposition of the aluminum oxide produces oxygen, which combines with the carbon of the anodes to form CO and CO.

The principle of an aluminum electrolysis cell with prebaked (prebaked) anodes can be seen from the figure, which shows a section in the longitudinal direction. The Fluoridschrnelze 10 (the electrolyte) is located in a lined carbon 11 Stahhvanne 12, which is provided with a thermal insulation 13 from hitzebeständigeni, -.värmedänimendcm Au.skleidung.smatcrinl. The cathodically deposited aluminum 14 lies on the floor 15 of the cell. The IG surface of the

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_t ■ liquid aluminum, the cathode is. Li the carbon lining 11 iron cathode bars 17 are inserted, which lead outwards from the bottom of the cell according to the current. Anodes 18 made of amorphous carbon, which feed the direct current to the electrolyte, are immersed in the fluoride melt 10 from above. They are firmly connected to the anode bar 21 via conductor rods 19 and locks 20. The electrolyte 10 is covered with a crust 22 of solidified melt and an aluminum oxide layer 23 located above it. The distance d between the anode underside 24 and the aluminum surface 16, also called the interpolar distance, can be changed by raising or lowering the anode bar 21 with the aid of the lifting mechanisms 25 which are mounted on columns 26. As a result of the attack by the oxygen released during electrolysis, the anodes on their underside are used up daily

approx. 1.5 to 2 cm depending on the cell type.

The principle of an aluminum elec

A trolysis cell with a self-baking anode (Soederberg anode) is the same as that of an aluminum electrolysis cell with pre-burnt ψ anodes. Instead of a plurality of prefired anodes, there is a single anode which is surrounded by a steel jacket; this anode is supplemented by pouring green electrode mass into the steel jacket as required and the green electrode mass is gradually baked by the heat of the cell.

By hammering in the upper electrolyte crust 22 (the encrusted bath surface) the aluminum oxide 23 located above it is brought into the electrolyte 10. In the course of the 'electrolysis, this is depleted in aluminum

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umoxid. With a lower concentration of 1% aluminum oxide in the electrolyte, the anode effect occurs, which results in a sudden increase in voltage from normally 4 to 4.5 V to 30 V and above. At this point, at the latest, the crust must be smashed in and the Al ₉ O concentration increased by adding new aluminum oxide so that the duration of the high energy supply to the cell during the anode effect is limited as much as possible (this high energy supplied during the anode effect is hereinafter referred to as " Anode effect energy "). Although the crust is knocked in immediately after the anode effect has occurred, it cannot be prevented that its duration is between 2 and 5 minutes. Since a multiple of the normal electrical energy is supplied in this short period of time, the temperature of the fluoride melt rises very quickly to values of 980 to 1040 C. After adding the aluminum oxide, the high voltage of the anode effect breaks down, especially when the melt is mechanically operated such as wooden poles, hooks, etc. is stirred vigorously. It often takes up to an hour before the temperature * of the electrolyte has returned to a normal value between 940 and 975 C. However, increased temperature means poorer current yield, since the electrolyte dissolves a larger proportion of metallic aluminum, which is then reoxidized to aluminum oxide in contact with the anode gas CO. Electricity yield is understood to mean that. Ratio of the actually produced aluminum to the theoretically produced according to FARADAY.

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The anode effect usually occurs one to four times, depending on the type of operation every day in every cell.

From the circumstances described, it is understandable that the current yield drops as a result of frequent anode effects, since the temperature of the fluoride melt rises every time. Once the electrolyte temperature is substantially above 980 C, increase the heat losses of the cell, since the temperature difference between the fluoride electrolyte and the cell sur- rounding W is large the hall air. Heat losses are not useful energy, so

that all too frequent anode effects increase the specific electrical energy consumption (kWh / kgAl) in a noteworthy way. The part of. electrical energy, which causes the higher heat losses of the cell during the anode effect, is to be regarded as additional energy loss. · '"-

However, the remaining part of the anode effect energy cannot be regarded as loss energy, as it is required to maintain a normal working temperature of 940 to 975 C. This can be seen from the fact that after reducing the anode effect energy (by reducing the frequency of the anode effects and / or reducing their duration), the open-circuit voltage of the cell must be increased. Rest voltage is understood to mean that cell voltage which is preferably αίη ^ ακΙιΊΗ as the minimum voltage of the cell approximately half an hour after the cell has been operated by crushing and adding Al O to maintain the lower limit of the working temperature.

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^JÄ 'BAD ORKSfNAU

After reducing the anode effect energy, it is found that part of the energy saved by the cell by increasing the open-circuit voltage, i.e. in the periods between the anode effects. This portion of the anode effect energy is useful energy.

If you do not take countermeasures, for example by increasing the Rest voltage, initiates, the fluoride melt cools down in an impermissible manner and falls below its liquidus point, which causes disruptions in operation enter the cell. It is therefore necessary after reducing the Anode effect energy to supply the missing useful energy to the cell in a different way. So far, this has only been achieved by the rest voltage was increased by increasing the interpolar distance. Voltage, unlike current / but does not produce metal; by increasing the interpolar distance to increase the voltage, the current yield cannot be improved unless the Before the rest voltage is increased, the interpolar distance is at a lower value that can hardly be controlled, for example 4 cm.

The invention relates to a method for the supply of the after reduction The energy missing from the anode effect energy, which is usually supplied by increasing the open-circuit voltage.

According to the invention in a cell in which the frequency and / or Duration of the Anodenoffcktc is reduced, the amperage by one

such a permanent amount increases that in the cell that amount of heat generated by electricity, which makes up the missing share of useful energy corresponds to the reduced anode effect energy.

So you increase the electrolysis current. This is by electricity heat the missing Joule heat is supplied to the electrolyte and at the same time the production of the cell is increased, as the Electrolyte temperature does not reduce the current yield.

However, the current strength may only be increased to the extent necessary to cover the lack of useful energy due to a reduction in the duration and / or frequency of the anode effects. If too little energy is supplied for compensation (i.e. if the current strength is not increased too much), the electrolyte temperature drops and there is a risk that it will fall below the liquidus point, with individual components of the. Electrolyte precipitate in solid form, located at the bottom of the cell ansam- cause my ψ and interfering sludge deposits and Bödenverkrustungen. If, on the other hand, the current strength is increased more than is necessary to compensate for the lack of useful energy, the temperature of the fluoride electrolyte rises with the consequence that the electrolyte's dissolving power for aluminum also increases and the current yield decreases as a result.

When the method according to the invention is used, that current strength increase is calculates which is the missing useful energy in terms of heat

2 0 9 8 2 5 Jf® & $$ C BAD ORIGINAL

compensated so that the temperature of the fluoride electrolyte does not change if the interpolar distance remains the same. ₍

It does not need to be examined here in detail by which one Means the anode effect energy is reduced. This is done in practice, for example, by means of rapid automatic crust wrapping during Anode effect, by maintaining an Al O concentration between 5 and 7% over extended periods of time and by other known methods achieved.

The following shows how this can be used to maintain the electrolyte temperature necessary increase in current intensity can be calculated after reducing the anode effect energy.

Output data of a cell

Amperage.

Average cell voltage and current yield

Specific electrical energy consumption

Average cell voltage without partial anode effects

Voltage component due to anode effects

Voltage drop in all electrical conductors

outside the tub (entrance cell

to anodes in electrolyte level and end

Cathode bar to exit Zolle) U (V)

X.O

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Il (IcA) »10 (V) ■ (kWh / kgAl) ^u n (V), ^U 12 (V)

Gross anode consumption

Net anode consumption (gross anode consumption minus anode residues)

2 15329 k

(gC / kgAl) (gC / kgAl)

Data after the increase in amperage and decrease in the anode effect energy

Current strength (I = I + ΔD
Average cell voltage and current yield

(7? ₂ =? 2, since constant electrolyte temperature is assumed on average)

Specific electrical energy consumption

Average cell voltage without partial anode effects

Voltage component due to anode effects ^(U 20 ⁼ U ₂ I ⁺¹ V

Voltage drop in all conductors outside the tub

Gross anode consumption

(C = C because the other cell parameters have not changed) Net anode consumption
. ^(C 22 ^SC 12>

2 U

(not specified)
(V)

(kWli / kgAl)

(V)

(gC / kgAl)

(gC / kgAl)

Calculation of the saved energy

Measures not specified here reduce the voltage component from U ₉ to U due to anode effects. In the Zf;] lc -n \ vanno the following energy is introduced less in 24 hours: '

kcal

24.

(D

According to general experience, however, only 0.1 to 0.1 V of this can be used. For safety reasons, the lower value is included in the calculation. It turns out that of the total saved,

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> - ⁵ SAD ORtGINAi

■ Energy 4 A ¹ only the following proportion ΔΑ "can be used to increase the current:

Δ / A "= 0.1 · I- 24 - 8GO ~~ (2)

1 λ4 Il

^{(for U} 12- ^U 22> ° ' ^1V)

Δ A "= (U ₁₂ - U ₂₂ ) I ₁ · 24 · 860 Iff (3)

(for U ₁₂ -U ₂₂ * 0.1 V)

Equation (1) means that the total anode effect duration (i.e. number of anode effects per cell and day multiplied by the Duration of the individual anode effect around (U U) 100

Z ₁ =% (4)

12th

has been shortened. It doesn't matter here; whether you can get the frequency the anode effects whose duration or both has changed.

Equation (2) means that one can only benefit from the reduced anode effect energy; ^: ·

₇ P, l -100 Of

can use for an increase in amperage.

In the case of equation (3), 100% of the reduced energy is used, i.e. it becomes

Z ₀ = 100% '(6)

10 · Z 'Z indicates what percentage of the total, original anode effect energy is used to increase the current intensity can be.

Generated . " Amount of heat in the tub before reducing the A nod enc'ffck t energy

Jon] r - '. See before Wilrmr: in the Elok trolytr-n and in the Ko hl ο bod on

The chemical Gogen. ^c ; p.'umung is assumed to be 1.05 V.

The tub voltage (Zc]] on. ^F ; p «'guild minus that of the current conductors iiusHc-rhallj flor tub :) is

2 0 9 8 2 5jfeQ # 0.ic *, 3 ^BAD ORIGINAL

^U W1 ^{= U} 10 - ^U 12 - ^U 13 ^V '

The ohmic part of this is:

"Witt" ¹¹ XO - ⁰ H- ¹¹ IS- ¹ - ^{65 V (B)}

The amount of heat generated is thus

Amount of heat generated by the anode effect

^W 12 * ^U 12 · h · ²⁴ ■ ⁸⁶ ° fit ⁽¹⁰⁾

Amount of heat generated by the C + Q reaction ^W 1S " ¹ I ^ ₁ C ₁₂ -O ₁₈ IS- 94!) F | f (H) Amount of heat through reoxidation

) - 299 IFF (12)

Bound energy in Al Q decomposition

Warming up the aluminum oxide and dissolving it in an electrolyte

W ■ = I · 0 -47 25 ~ - (14)

^W 1G 1 / l '24 h ^l '

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Warming up the anodes

= 2.72 * I _{1 -Jr 1} . C _n - 10- ² Hf ⁽¹⁵⁾

summary

It is the sum for the formation of the heat losses

W to form J> JL

^W L1 * ^W ! L ^{+ W} ! 2 ^{+ W} 13 ^{+ W} 14 " ^W » " ^W 16" ^W ! 7

The proportions for heating cryolite and AlF are ver been neglected.

Heat generated in the tub after reducing the Anode effect energy

Joule's heat in the electrolyte and in the carbon base W ». = (UUU -L65) t- I * * 24 * 8C0 ^kcal

2i ^ ₁ O ¹ ViS ¹ - ^β8 »^ h ■ ^{24 8G0} IfiT

Amount of heat generated by the anode effect

= \ v -AA ^W 12 - ^Si

\ v AA 22 ^W 12 - ^Si 24 h

Amount of heat generated by the reaction C + O

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Amount of heat through reoxidation

299 If ^ -. (20)

Bound energy in the Al O decomposition

- 14, 40 - 10 ³ - I ₂ ^ JL (21)

Warm up the aluminum oxide and dissolve it in the electrolyte

"- ²⁵ ITS · ⁽²²⁾

Warming up the anodes
on ~> ο 'λ' 11 * (/ ο /

To collect _nf assu η g

»W = W + W + W + W - W - W - W (24)

L2 .21 22 23 24 25 26 2 7 ^K '

equal weight sbcding ung

It is by definition

W (equation IG) --- W, ₀ (equation 24) (25)

The is calculated from this . Current. Strength I ₉ after the increase by ΔI (see data after the current strength increase and decrease i ' Anodenel'fe cycleei-gie).

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Calculation of the new cell voltage

S ^ ⁺¹ - ^{ee Y (26)}

• Calculation of the new specific electrical energy consumption

0.3354 ■ ^ ₁ kgAl

Calculation example

In an electrolysis system with cells that are pre-burned (pre-baked) anodes are equipped, each cell had an average of 3.6 anode effects per day with a voltage increase of around 35 V and a duration of 2.3 minutes each 0.328 χ 10 "kcal were generated in the cell tray in 24 h. This value is calculated as follows:

³⁵ · ³ ' ⁶ ' ² ·

60

_{= 4} 8V

This is used to calculate the amount of current in each cell tray of 79.2 IcA by the anode effect energio on average daily amount of heat generated.

W = 4.8 x 79.2 x 860 = 0.328 ■ 10 ° kcal

Initial data of this electrolysis system (average of one year)

Amperage

Average cell voltage sir oinaus prey

Specific electrical energy consumption

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¹ I " 79 , 2 n / a ^u io ■ 4, 36 V Vi ' 93 , 3 % E ₁ - 13th , 93 kWh
kgAl

Average cell voltage without proportional ano'd en effects

U ₁₁ = 4, 16 V- "l2 - 0.20V "13 = 0.32V C _n = 513 gC / kgAl ^C 12 = 454-gC / kgAl

Voltage component due to anode effects

Voltage drop in all current conductors outside the tub. Gross anode consumption Net consumption of anode

By measures known per se (e.g. more frequent knocking in of the electrolyte crust and the associated higher average Al O concentration in the electrolyte and faster impact when an anode effect occurs) the number of anode effects will be 1, 3 per cell per day and their duration reduced to 1.6 minutes on average.

Data after the increase in amperage, some of which are to be calculated

Current I kA

Average cell voltage U _0n V

Current yield p »93.3%

Specific electrical energy u \\ rh

consumption E

2 kgAl

Average cell voltage

without proportional anode effects Up ₁ V

ψ Voltage component due to anode effects U ₉₉ = 0.05 V

Voltage drop in all current conductors outside the tub U ₀ "V

Gross anode consumption C ₉ = 513 gC / kgAl

Net anode consumption C _{? O} = 454 gC / kgAl

Calculation of the saved energy

There were

(G. 20 - 0.05) 100 "",
Ζ. = ^J - ^J Q- '- = (5% anode efficiency saved.

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Energy saved

ΔΑ "= 0.163 x 10 kcal / 24h From the decreased anode effect energy

_ze 0.10 100 _{= 66} 7% ^Z 2 0.20-0.05 ^bb) ' ^{/ 0}

used for an increase in amperage.

Be from the original anode effect energy

10 " ² Z ₁ * Z = 75 * 66, 7 * 10" ² = 50% used for an increase in current intensity.

Amount of heat generated in the tub before the anode effect energy was reduced

W _T1 = 3.467 - ΙΟ ⁶ ~

Ll '24 h

Amount of heat generated in the pan after reduction the anode effect is given

W ₁ = 570.7 I ² - 5569.7 I. + 0.164 x 10 ^{6 kcal}

24 hours

Equilibrium conditions

3.467 ΙΟ ⁶ = ^{570.7 I 2} - 5569.7 I + 0.164

Ct Ci

Solving the equation gives I = 81, 1]: A

ΔΙ = oil, 1 - 79, 2 = 1, 9 IcA

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it

Calculation of the new cell voltage

RII

Total cell voltage on average:

= 4.27V

No-load voltage:

Calculation of the new specific electrical energy consumption

, ₌ 4.27 x 100 '2 0.3354 x 93.3

= 13.65

kWh kgAl

Comparison of the values

Amperage

Cell voltage current efficiency

Specific electrical energy consumption

Resting tension

Stress component due to anode effect

Gross anode ο consumption Net anode consumption Heat losses from the aluminum production tub

before

n / a 79.2 V. 4.36 % 93.3 kWh / kgAl 13, 93 V 4, 16 V 0, 20 gC / kgAl 513 gC / kgAl 454 kcal / 24 h 3,4G7. 10 kgAl / Zclle. Day 594, 8

afterwards 81.1 4.27 93.3

13.65 4.22

0.05 513

454

3.467.

609, 1

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^Jf ^ ^J; 'BÄfr ORIGINAL

The inventive method can be used to reduce the anode effect energy increase the amperage by almost 2 kA in the example given. This means an increase in metal production by about 2.2%.

2 O 9 8 2 5 / Ojfep? Fe

Claims (1)

ie Claim Process for the production of aluminum by electrolysis of aluminum oxide in the fluoride melt flow, characterized in that in a Cell in which the frequency and / or duration of the anode effects is reduced the cell current is increased by such an amount that the amount of heat generated in the cell by electrical current which corresponds to the missing share of useful energy of the reduced anode effect energy. 825 / .0.§3 BÄPiS