What is the Hall Heroult Process?
The Hall-Heroult process is an intricate electrolysis procedure used to produce aluminium. It is difficult to continuously measure certain important values due to the extreme operational conditions. Based on the available measurements, we hereby present a method for modelling and estimating the alumina concentration and anode-cathode distance. Initially, a state affine model is generated by combining physical-chemical relations and system identification. The desired signals are then recovered using a linear Kalman observer. On an industrial platform, the proposed approach is validated.
Hall Heroult Process
Aluminium is a non-corrosive silvery-white metal that is light in weight. It belongs to the periodic table's Boron group (group 13). Aluminium is the most abundant metallic element on the planet, accounting for approximately 8% of the earth's crust. Two scientists, Charles Martin Hall and Paul Heroult proposed the Hall-Heroult process. Despite not knowing each other, they both experimented with the extraction and refining of aluminium and applied for patents at the same time. As a result, this process is known by both their names. What is the Hall Heroult process? Electrometallurgy is the extraction of metals through the electrolysis of their fused salts. Electrons act as the reducing agent in this process. The Hall and Heroult process is used to obtain aluminium by electrolysis of a mixture of purified alumina and cryolite. This aluminium extraction process has three stages:
The concentration of bauxite
Extraction of metals from concentrated ore
Refining of metals
Bayer Process
The Bayer process extracts pure alumina from bauxite ore. Bauxite ore typically contains oxide, silicon, and titanium oxide impurities. At 473-523 K and 35-36 bar pressure, the powdered ore is heated with a concentrated (45 percent) NaOH. Under these conditions, alumina dissolves, yielding sodium meta-aluminate and silica (SiO2) as sodium silicate while leaving impurities behind.
$\underset{\text{bauxite}}{\mathrm{Al}_{2} \mathrm{O}_{3} \cdot 2 \mathrm{H}_{2} \mathrm{O}(\mathrm{s})}+2 \mathrm{NaOH}(\mathrm{aq})+2 \mathrm{H}_{2} \mathrm{O} {\xrightarrow{473 \mathrm{~K}-523 \mathrm{~K}}} \underset{\text{sod.meta - aluminate}}{2 \mathrm{Na}\left[\mathrm{Al}(\mathrm{OH}){ }_{4}\right](\mathrm{aq})}$
$\begin{array}{ll} \mathrm{SiO}_{2}+2 \mathrm{NaOH}(\mathrm{aq}){\xrightarrow{473 \mathrm{~K}-523 \mathrm{~K}}} & \mathrm{Na}_{2} \mathrm{SiO}_{3}(\mathrm{aq})+\mathrm{H}_{2} \mathrm{O} \\ \text { silica } & \text { sod.silicate } \end{array}$
When aluminium hydroxide precipitates, the solution is filtered (to remove undissolved impurities), cooled and its pH is adjusted downward either by dilution or neutralisation.
$2 \mathrm{Na}\left[\mathrm{Al}(\mathrm{OH})_{4}\right](\mathrm{aq})+2 \mathrm{CO}_{2} \longrightarrow \mathrm{Al}_{2} \mathrm{O}_{3} \cdot \mathrm{xH}_{2} \mathrm{O}(\mathrm{s}) \quad+2 \mathrm{NaHCO}_{3}(\mathrm{aq})$
To obtain pure alumina, hydrated alumina is filtered, washed, and finally heated to 1473 K.
$\mathrm{Al}_{2} \mathrm{O}_{3} \cdot \mathrm{xH}_{2} \mathrm{O} \stackrel{1473 \mathrm{~K}}{\rightarrow} \mathrm{Al}_{2} \mathrm{O}_{3}(\mathrm{~s})+\mathrm{xH}_{2} \mathrm{O}(\mathrm{g})$
Hall Heroult Process Reaction
The following steps summarise the aluminium extraction process:
At 1140K, pure alumina melts with cryolite and fluorspar, which are used in electrolysis (3:1 ratio). Electrolysis occurs in an iron tank with a heat insulator and sloping floor. The tank has an outlet where the molten aluminium can be tapped.
Hall Heroult Process Diagram
The cathode is made of graphite or gas carbon, while the anode is made of thick carbon rods. To prevent the anodic rods from burning, a coke powder coating is applied. This is to prevent heat loss from the electrolyte. The bath temperature is kept around 1173K. Hall Heroult process equation is:
$\begin{array}{ll} \text { cathode: } & \mathrm{Al}^{3+}(\text { melt })+3 \mathrm{e}^{-} \rightarrow \mathrm{Al}(\mathrm{l}) \\ \text { anode: } & \mathrm{C}(\mathrm{s})+\mathrm{O}^{2-}(\text { melt }) \rightarrow \mathrm{CO}(\mathrm{g})+2 \mathrm{e}^{-} \\ & \mathrm{C}(\mathrm{s})+2 \mathrm{O}^{2-}(\text { melt }) \rightarrow \mathrm{CO}_{2}(\mathrm{~g})+4 \mathrm{e}^{-} \end{array}$
Al ions reach the cathode quickly due to their lower position in the electrochemical series. As a result, at 950 degrees celsius electrolyte temperature, aluminium is deposited at the cathode and begins to melt in the tank. The anode generates nascent oxygen, which combines with the coke carbon to form carbon monoxide. Corbon dioxide is produced when carbon monoxide reacts with oxygen in the atmosphere. Carbon anodes must be replaced regularly because nascent oxygen reacts with them. This process yields metal that is 99.95 percent pure.
Hall Heroult Process Flow Chart
Conclusion
This overview of the industrial primary aluminium production process includes a brief description of electrolytic reduction technology, the history of aluminium and the importance of this metal and its manufacturing process in modern society. Aluminium's distinct properties have allowed for technological advancements as well as energy and cost savings.
The Hall Heroult process is used for the production of aluminium. It is difficult to model the current distribution and optimise the process during this complex electrolyte two-phase electrolysis.
FAQs on Hall Heroult Process with Reaction for JEE
1. What is the importance of cryolite in aluminium metallurgy?
The role of cryolite in aluminium metallurgy is two-fold:
A. Fused alumina (Al2O3) is a poor conductor of electricity, but the addition of cryolite (NaAlF6) improves it. As a result, electrolysis can be used to obtain aluminium.
B. Pure alumina has a very high melting point (2323 K), but the addition of cryolite lowers the melting point of the mixture to around 1140 K. As a result, electrolysis is performed at a much lower temperature (1173 K), resulting in significant energy savings.
2. What function does graphite perform in aluminium electrometallurgy?
In aluminium electrometallurgy, a fused mixture of alumina, cryolite, and fluorspar (CaF2) is electrolyzed with graphite as the anode and graphite-lined iron as the cathode. Al is liberated at the cathode during electrolysis, while CO2 is liberated at the anode.
at cathode: $\mathrm{Al}^{3+}($ melt $) \rightarrow \mathrm{Al}(\mathrm{l})$
at anode: $\begin{align} &\mathrm{C}(\mathrm{s})+\mathrm{O}^{2-} \rightarrow \mathrm{CO}(\mathrm{g})+2 \mathrm{e}^{-} \\ &\mathrm{C}(\mathrm{s})+2 \mathrm{O}^{2-} \rightarrow\mathrm{CO}_{2}(\mathrm{~g})+4 \mathrm{e}^{-} \end{align}$
If a metal is used as the anode instead of graphite, the liberated will not only oxidise the metal of the electrode, but will also convert some of the Al liberated at the cathode back to Al2O3. Because graphite is much cheaper than any metal, it is used as the anode.
Thus, graphite's role in Al electrometallurgy is to prevent O2 liberation at the anode, which would otherwise oxidise some of the liberated Al back to Al2O3.
3. What is the significance of aluminium in chemistry?
Aluminium fine dust is used in the production of aluminium paints and lacquers. Aluminium powder mixed with linseed oil, for example, shines like silver and is known as silver paint.
Due to its high reactivity, aluminium powder is used as a reducing agent in the aluminothermic process for extracting chromium and manganese from their oxides.
Since aluminium is light and a good conductor of electricity (on a weight-for-weight basis, Al conducts twice as much as Cu), it is used to make transmission cables and to wind the moving coils of dynamos. It is used in buildings for making angles for doors, windows etc.
Aluminium powder is used for flashlight bulbs in indoor photography.
