Category Archives: Smelting Process

Mar 06

Process Thermodynamic – Free Energy

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Gibbs Energy

To better explain the Gibbs energy concept applied to the alumina reduction, we will use first as an example the process of electrolysis of the water, depicted in the picture below, analyzed in condition of reversibility:

Electrolysis of water

The total energy necessary to electrolyze the water is equal to ΔH = 285.83 kJ/mol. This amount includes:

  • The change in the internal energy U linked with the change in the kinetic, potential and chemical bonds energy of the species involved in the electrochemical reaction
  • The energy required by the gas bubbles (O2 and H2) formed during electrolysis to expand under the pressure p:   work

being, as we know:

Change in enthalpy

Now arise a question: do we have, with the battery, to provide all the energy linked to the change in enthalpy? Let’s see…

The change in the internal energy U is equal to:

Change in internal energy

Where L is the work applied to the system while Q is the heat given to the system. In the case of a reversible reaction at constant temperature T the term Q is equal to TΔS. Furthermore, in our case the work L has to parts:

  • The electric work inputted by the battery: WEl
  • The work done by the gases expanding: -pΔV

Hence we have:

Change in internal energySubstituting the (8) into the (6):

Change in enthalpy

Now, in our example, the only kind of external energy we are deliberately providing to the water is the electrical energy of the battery, and this energy is equal to:

Change in Gibbs Energy - Free Energy

Since the reaction is characterized by an increase in entropy (ΔS > 0), the energy WEl that the battery has to provide from the external is less than the total energy required by the electrolysis reaction. The “missing” energy is taken as heat from the environment.

The term ΔH-TΔS is equal to the Gibbs energy, or “free” energy. So we can write:

Change in Gibbs Energy - Free Energy

Let’s take again in consideration the (1), the basic alumina reduction reaction, in reversible conditions (hence at 100% current efficiency), occurring in an environment at temperature T. For this reaction remain valid all the arguments we have developed for the previous example. Let’s calculate now the amount of electrical energy needed by the (1) to occur.

If we take again in considerations the reaction (4), with the same assumptions (electrolyte saturated with α alumina, PCO2 = 1 atm, temperature of 977°C), inserting the proper figures from the thermodynamic tables we have:

Change in Gibbs Energy - Free Energy

At 100% current efficiency we have:

Change in Gibbs Energy - Free EnergyThe difference:

Change in enthalpy

Is equal to the heat that is taken by the reaction (1) from the external environment. In the case of an alumina reduction cell the external environment is the bath itself, so the bath at its high temperatures (around 950°C) gives heat to the reaction (1). Obviously, the bath can not give indefinitely heat to the reaction, but this heat needs to be restored from the external. Practically, this means that we need to spend electrical energy also for the term TΔS.

Now we will calculate the minimum voltage to apply to a cell for the reaction (1) to occur in reversible conditions. In the (1) 12 moles of electrons are involved to produce 4 moles of aluminum.

In other terms, if F is the Faraday constant (the amount of electrical charge carried out by a mole of electrons) and E is the electrical potential we have:

Amount of electrical charge involvedInserting the values we get:

Decomposition potentialDepending on the temperature at which occur the reaction (940°C ÷ 960°C normal operating ranges in the industry). This voltage is equivalent to the minimum energy, in reversible conditions, to give from the outside for the reaction (1) to occur. The voltage needed to compensate for the term TΔS is equal to:

TDeltaS energy requirement as voltage

 

Mar 06

Process Thermodynamic – Enthalpy

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As we have seen, in a Hall-Héroult cell the molten aluminum is produced according to the following reaction:

Alumina Reduction Reaction

This reaction occurs at constant temperature T (the bath temperature) and constant pressure p (the atmospheric pressure).

Enthalpy

When we speak of enthalpy, we are talking of the 1st law of thermodynamic or, in other words, of the energy conservation principle. In the case of an alumina reduction cell, from the external the only energy we input is electrical energy (WEl). This energy is partially used by the  cell to produce aluminum while the rest is dissipated as heat. So we can write:

Electrical Energy Input

This equation tells us that the electrical energy we supply from the external is used to: 1) increment the internal energy content of the products of the reaction (1) compared to the internal energy of the reactants (term ΔU), 2) give the energy necessary for the CO2 bubbles to expand under the atmospheric pressure p (term pΔV), 3) the remaining part of WEl is lost in the external as heat (term QD).

We also know that the term (ΔU + pΔV ) is equal to the enthalpy change ΔH, so we can rewrite the (2) as:

Electrical energy input and change in enthalpy

Using thermodynamic tables and the basic reaction (1) we can calculate the enthalpy of the reaction, or, in other terms, the minimum energy required to produce aluminum.

But for a better estimation of the enthalpy we need to take in considerations the following practical aspects:

  • We need to consider the energy necessary to heat the reactants from the ambient temperature up to the bath temperature
  • The process is not 100% efficient, so we need to take into account the loss in current efficiency

With these considerations, if x is the current efficiency expressed as a fraction (CE = x ∙ 100%), (1-x) is the fraction of aluminum which reoxidizes and the (1) can be rewritten as:

Alumina Reduction Reaction with Current EfficiencyConsidering an electrolyte saturated with α alumina, with PCO2 = 1 atm, at a temperature of 977°C, inserting the proper figures from the thermodynamic tables we have:

Change in Enthalpy - Energy required

This equation takes into account:

  • The energy required to heat the reactants from ambient temperature to 977°C
  • A current efficiency lower than 100%

At 100% current efficiency:

Energy RequiredThe (5) should be further modified if we consider an electrolyte not saturated with alumina (as is the real case) and the fact that usually the alumina delivered to the cell is mostly γ alumina, but these changes introduce only slight modifications to the (5).

Mar 06

Detailed Description of a Cell and its Basic Functioning

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detailed industrial aluminum reduction cell drawing

Even though the basic elements in the aluminum production are still the same invented by Hall and Héroult more than 100 years ago, a modern industrial electrolysis cell is different from the first pots of the 19th century.

First of all, the size of the cells is changed to accommodate for the much larger current intensities used nowadays. Secondly, in a modern pot we find a series of elements used to reduce the energy consumption and the gas emissions.

The alumina reduction occurs in a vessel which is made of several parts designed altogether to:

  • Act as a container for the molten bath and aluminum
  • Resist to the high temperatures (around 950°C) of the molten liquids it contains
  • Resist to chemical attacks brought especially by the molten electrolyte constituents
  • Resist to wearing caused by alumina abrasive behavior
  • Reduce heat losses to a technical and economical optimal minimum
  • Be mechanically enough resistant, but also with sufficient elasticity in order to accommodate for the thermal and physical expansion of the materials it contains
  • Collect the electrical current coming from the anodes with a minimum voltage drop

To achieve all these properties the vessel is made with a combination of different materials. First, we find an outer steel container, usually referred as potshell, which contains all the other elements of the cell. On the potshell bottom are deposited some layers of thermally insulating bricks with the aim of reducing heat losses from the bath. Above this, we find a refractory bricks layer which are very resistant to prolonged periods of exposition to the cell high temperatures. The carbon blocks that physically constitute the container of the molten aluminum and electrolyte stay above these bricks layers. The bottom blocks are called cathodes, even though electrochemically speaking is the metal pool which acts as cathode. They also collect the current exiting from the metal pad. The container sides are made with other carbon blocks. Cathodes and side blocks are joined together with a mix of pitch and carbon dust that is pressed inside the joints between them. At the bottom of every cathode we find a slot that accommodates an iron bar, called collector bar, with the purpose to transport outside of the cell the current collected by the cathode. Cathode and collector bar are joined together filling the space between them with molten cast iron, which subsequently freezes bonding together the parts.

The vessel described above contains, as said before, the molten aluminum and electrolyte. Due to the different densities, the molten bath stays above the molten metal, as the oil stays on the top of the water to give a practical example. In the molten bath occur all the chemical reactions for the alumina reduction. This reactions are driven by the electrical current transported inside the bath by the carbon anodes partially immersed into the molten bath.

The term “anode” refers not only to the carbon block. More in general an anode is made by a rod, a yoke and a series of stubs (1 to 6 generally) partially housed in rounded cavities obtained in the carbon blocks. Anode rods are made with copper or aluminum while yoke and stubs are made with iron. The carbon part of the anode is joined together with the metallic part of the anode assembly pouring molten cast iron in the space between the anode cavity and the rod. The molten cast iron freezes joining together the carbon block and the metallic part. In a modern pot we can find up to 40 anodes and because the carbon of the anodes participates to the chemical reactions, hence being consumed, they need to be replaced on a regular basis.

All the anodes of a cell are fixed to an aluminum structure, called “bridge”. The bridge transports the electrical current to the anodes and is also equipped with an electrical motor and a series of levers in order to raise or lower all the anodes of a cell. In this way it is possible to control the voltage at which the pot is operating.

Because of the high working temperatures of the anodes, on the top of them is put a protective layer of material made of a mix of crushed bath and alumina, in order to avoid their burning to the air. This protective layer is called “crust”.

The CO2 formed by the reaction of the anodes with the alumina oxygen escapes from the bath as gas bubbles, while the aluminum, being no more bonded with the oxygen is deposited in the metal pad inventory, increasing its height as the production goes on.

As the aluminum production proceeds, the alumina dissolved into the bath is depleted, and needs to be restored on a regular basis. Each cell, hence, is equipped with an alumina bin with a feeding system which delivers alumina to the electrolyte.

This feeding system is made of a crust breaker, basically made with a steel rod operated with a pneumatic cylinder, which opens a hole on the crust, and a corresponding alumina feeder, which dumps a certain amount of alumina into the molten bath from the hole opened in the crust by the crust breaker. The bath is then restored in its alumina content. Depending on the pot size, each pot can have 1 to 6 crust breakers and alumina feeders. On some pot technologies, crust breaker and alumina feeder are integrated.

The feeding operations are usually controlled by a computer control system following some algorithms.

To collect the fumes escaping from the pot crust, due to the bath evaporation, the pots are completely closed with removable pot covers, which collect the gases coming from the bath and direct them towards the gas treatment center.

Mar 06

Prebake and Soderberg

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The alumina reduction cells, and in a broader sense, the aluminum smelters can be divided into two big categories depending on how is arranged their anodic system.

In the alumina reduction cells the anode is a block of carbon made of petroleum coke and pitch. What differentiate the two technologies is the way this carbon block is produced.

In a so-called pre-bake cell, the petroleum coke is mixed with pitch, which acts as a binder. Then, at this mixture, usually called green paste, it is given a parallelpiped shape with either a press or a vibrocompactor. The formed carbon block is then baked into furnaces in order to be transformed into a solid carbon block. The electric current arrives to the carbon block through a rod linked to it through nippels. A pre-bake pot contains several single anodes (usually 14 ÷ 40, mainly depending on the line current), which stay on the pots for a fixed amount of days (generally 26 to 30 days). Then, before being completely consumed, they are removed together with the rod, and the remaining carbon reused to produce new anodes.

In a Soderberg smelter the basic idea is to eliminate the sub-plants which form, bake and join the carbon block with the rod. A Soderberg cell has only one big anode, housed in a steel container, which gives to the anode its shape. From the upper part of this container it is introduced the green paste. During its movement from the top to the bottom of the container the green paste is baked. Unfortunately, the quality of the baked Soderberg anode is lower than the quality of the prebaked one, hence the Soderberg cells are always characterized by a lower current efficiency and a higher pot voltage, needed also to produce the extra heat necessary for the anode baking.

Presently all the new built smelters adopt the pre-bake technology, because of the higher current efficiency, lower specific energy consumption and lower emission (especially PAH). However, a good number of Soderberg plants are still in operations, sometimes retrofitted with additional technology aimed at increasing current efficiency and reduce emissions.

Throughout the rest of this website we will refer only to the pre-bake technology, even though most of the topics apply both at pre-bake and Soderberg technology.

Mar 06

Process Basics

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The aluminum is produced extracting it from the aluminum oxide (Al2O3), called also alumina, through an electrolysis process driven by electrical current. The process uses as electrolyte a molten salts called Cryolite (Na3AlF6) capable of dissolve the alumina. Carbon anodes are immersed into the electrolyte (usually referred as the “bath”) carrying electrical current which then flows into the molten cryolite containing dissolved alumina. As a result, the chemical bond between aluminum and oxygen in the alumina is broken, the aluminum is deposited in the bottom of the cell, where a molten aluminum deposit is found, while the oxygen reacts with the carbon of the anodes producing carbon dioxide (CO2) bubbles. The alumina reduction process is described by the following reaction:

 

Alumina Reduction Reaction

Once passed through the bath, the electrical current flows into the molten aluminum deposit and is then collected by the bottom of the pot, usually called “cathode”.

The following is a schematic picture of an aluminum electrolysis cell:

Schematic aluminum reduction cell