Pyrometallurgy III: Primary Aluminium Production and Refining

Primary Aluminium Production Overview

Aluminium ( $Al$ ) production is achieved through a two-stage process chain, starting from natural ore and resulting in pure metal. The transition from raw bauxite to finished product involves specialized chemical refining followed by electrolytic smelting.

The Bayer Process: This is a hydrometallurgical refining process used to extract aluminium oxide (alumina) from bauxite ore.
The Hall-Heroult Electrolytic Process: This is the smelting stage where the refined alumina is dissolved in a molten cryolite bath and electrolyzed to produce pure aluminium metal.

Industrial Process Flow

Mining: Primary sources of bauxite are located in Australia, Brazil, and China.
Refining (Bayer Process): Bauxite is processed into pure alumina ( $Al_2O_3$ ).
Reduction Cells (Hall-Heroult Process): Alumina undergoes electrolysis using carbon anodes and a cryolite ( $Na_3AlF_6$ ) flux.
Metal Handling: Liquid $Al$ is collected via a vacuum tapping ladle.
Holding and Alloying: The metal is transferred to a reverberatory furnace for alloy additions.
Casting: Final shapes are created in casting pits or through ingot casting.

Bauxite Ore Processing Methods

The choice of refining technology depends largely on the quality of the bauxite ore, specifically the mass ratio of alumina to silica ( $A/S$ ).

Bayer Process Suitability: Primarily used for high-grade bauxite ores where the $A/S$ ratio is above $9$ .
Sinter Process: Widely utilized in China and Russia for poor-grade diasporic bauxite ores with an $A/S$ ratio below $7$ . This involves sintering the ore with sodium carbonate ( $Na_2CO_3$ ) and limestone to produce sodium aluminate and calcium silicate.

The Bayer Process: Step-by-Step Refining

The Bayer process consists of five distinct stages to purify bauxite into alumina.

Step 1: Crushing, Mixing, and Desilication

Milling: Bauxite ore is pulverized, milled to reduce particle size, and screened.
Slurry Formation: Crushed ore is mixed with process liquor containing caustic soda ( $NaOH$ ). The resulting slurry contains 35\text{--}40 \text{%} solids.
Chemical Role: Sodium hydroxide is the primary chemical used to extract aluminium hydrate.
Desilication: Before digestion, soluble silica must be removed. This step occurs at lower temperatures ( $90\text{--}100 \text{ } ^\text{o}C$ ) to encourage silica to precipitate as a desilication product (DSP), which prevents excessive scaling in heat exchangers.

Step 2: Digestion

The slurry is pumped into a digester where the alumina compounds are solubilized as sodium aluminate ( $NaAlO_2$ ) using hot, concentrated $NaOH$ under pressure at temperatures ranging from $110\text{--}270 \text{ } ^\text{o}C$ . The chemical reactions depend on the mineralogy of the bauxite:

Gibbsite Reaction: $Al_2O_3.3H_2O (s) + 2NaOH (aq) \rightarrow 2NaAlO_2 (aq) + 4H_2O (l)$
Bohmite and Diaspore Reaction: $Al_2O_3.H_2O (s) + 2NaOH (aq) \rightarrow 2NaAlO_2 (aq) + 2H_2O (l)$

Step 3: Clarification

The slurry is moved to settling tanks. Insoluble oxide impurities (such as iron oxides, often called "red mud") do not dissolve in the caustic soda. These solids are separated from the "pregnant liquor" (the solution containing the dissolved sodium aluminate) via settling.

Step 4: Precipitation

Cooling: The filtered pregnant liquor is cooled from approximately $100 \text{ } ^\text{o}C$ to $75 \text{ } ^\text{o}C$ .
Seeding: The liquor is pumped into six-story tall tanks. Solid seed crystals of alumina hydrate are added from the top to trigger the precipitation of aluminium hydroxide crystals.

Step 5: Calcination

Processing: The precipitated aluminium hydroxide is washed, filtered, and then calcined in either a rotary kiln or a fluidized bed calciner.
Thermal Conditions: It is heated to $950\text{--}1000 \text{ } ^\text{o}C$ to remove chemically bonded water.
Energy Consumption: This step alone accounts for approximately 25 \text{%} of the total energy used in the Bayer process.
Chemical Change: $2Al(OH)_3 (s) \rightarrow Al_2O_3 (s) + 3H_2O (g)$
Product: The resulting anhydrous crystals are gamma crystals ( $\gamma\text{-}Al_2O_3$ ), known as smelter-grade-alumina (SGA).

Smelter-Grade-Alumina (SGA) Impurity Thresholds

$SiO_2$ : 0.01\text{--}0.02 \text{%}
$Fe_2O_3$ : 0.01\text{--}0.03 \text{%}
$NaO_2$ : 0.3\text{--}0.6 \text{%}

The Hall-Heroult Electrolytic Process

Principles of Aluminium Electrometallurgy

Aluminium is highly reactive and forms exceptionally strong bonds with oxygen. It cannot be produced via aqueous electrolysis (dissolved in water) because the metal reacts with water's protons to form hydrogen gas rather than depositing metal. Consequently, it must be extracted from a molten salt solution.

Electrolysis Configuration

Discovery: Independently discovered by Charles Martin Hall and Paul L. T. Heroult.
The Bath: Alumina is dissolved in molten cryolite ( $Na_3AlF_6$ ). A mixture of 89 \text{%} cryolite and 11 \text{%} alumina is used, which drastically reduces the melting point of alumina from $2050 \text{ } ^\text{o}C$ to $962.5 \text{ } ^\text{o}C$ .
Electrodes: * Anode: Carbon blocks suspended above and partially immersed in the electrolyte. * Cathode: A fixed carbon hearth lining the bottom of the cell.
Current: High direct current (DC) is passed through the bath.

Process Mechanics

Ion Movement: The current splits the alumina. Positive $Al^{3+}$ ions migrate to the negative cathode, where they receive electrons to form liquid aluminium metal ( $Al$ ).
Anode Consumption: Oxygen ions migrate to the carbon anode, where they react with the carbon to form $CO_2$ gas. This causes the anodes to be consumed at a rate of approximately $500 \text{ } kg$ of carbon per $1 \text{ } t$ (tonne) of aluminium produced.
Maintenance: Anodes are lowered continuously to maintain the optimal distance from the cathode until they are thin enough to require replacement.
Feeds: Alumina is added to the cells continuously to keep the concentration stable.

Metal Recovery and Casting

Temperature Maintenance: The heat generated by the electrical resistance of the bath maintains the molten state at approximately $960 \text{ } ^\text{o}C$ .
Siphoning: The dense molten aluminium settles at the bottom of the pot. It is siphoned off at regular intervals using a vacuum crucible (also called a vacuum ladle).
Final Processing: The molten metal is transported to a reverberatory furnace for holding. Here, alloying elements are added to meet specific material grades before the metal is cast into its final form.