Bauxite

Slide on Bauxite

1. BAUXITE PRESENTED BY AZEEZ AKEEM ABIODUN HENRY K WEEFAR Lecturer: Dr. A.R Adetunji

2. Outlines Introduction History of bauxite Chemical composition and Physical appearance of Bauxite World reserve of bauxite Bauxite mining Refining bauxite Bayer’s process Alumina to Aluminium Uses of Aluminium References 2

3. Introduction Is an aluminium ore Is the main source of aluminium It is not a mineral rather it is rock that has been severely leached of silica and other soluble minerals. It is found majorly (90%) in wet tropical and subtropical climate. Bauxite are commonly classified in two genetic types according to mineralogy. Laterite, Karst-type deposits.

4. History of Bauxite It was first discovered in southern France in 1821. The village at which it was discovered is known as Les Baux . The name of the geologist that discovered it is known as Pierre Berthier .

5. History of Bauxite In 1847, Armand Dufrenoy named the ore “ beauxite ” In 1854, Henri Sainte-Claire Deville rename it as “bauxite” In Africa, the first country to mine Bauxite is Sierra Leone in the 1920s long before Diamond was discovered in the country

6. Chemical composition and Physical appearance of Bauxite Chemical formula: Al(OH)3 with possible additional Al and (OH ) Composition: Basic aluminum hydroxide Colour : yellow, white, gray, brown, reddish-brown, pink Streak: White Hardness: 1-3 HB Crystal system: Amorphous Transparency: Opaque Specific gravity: 2.0-2.6 Rock type: Igneous, Sedimentary

7. Chemical composition and Physical appearance of Bauxite cont’d Bulk Density: 3.1 (g/cm3) Tenacity: Brittle Luster: Dull

8. World reserve of Bauxite Source: mcgroup.co.uk

9. Bauxite Mining It is generally extracted by open cast mining because it is almost always found near the surface. The land needs to cleared of timbers and vegetation before the mining process can proceed. The top soil is usually stored for replacement during rehabilitation. Layer under the top soil is known as overburden, on average, the overburden thickness is around 2 meters. The bauxite layer beneath the overburden is broken up using methods such as blasting, drilling and ripping by large bulldozers.

10. Bauxite Mining cont’d Once the bauxite is loosened into manageable sizes, it is loaded into trucks or train. It is transported to crushing and washing plants or stockpiled before being moved to alumina refineries. Source: Bauxite.world-aluminium.org

11. Refining Bauxite Bauxite refining is done by a process known as Bayer. Two to three tones of bauxite are required to produce one tone of alumina. Alumina refineries are located near to the bauxite mines for efficient transport of raw materials and the final product.

12. Bayer’s Process Milling: The bauxite is washed and crushed, reducing the particle size and increasing the available surface area for the digestion stage. Lime are added at the mill to make a pumpable slurry. Desilication : Bauxite with high levels of silica (SiO2) go through this process to remove this impurity. Silica can affect the quality of the final product. Digestion: Hot caustic soda ( NaOH ) solution is used to dissolve the aluminium-bearing minerals in the bauxite (gibbsite, bohmite , diaspore ) to form a sodium aluminate supersaturated solution or pregnant liquor.

13. Bayer’s Process cont’d Gibbsite: Diaspore : The temperature varies between 140 C- 280 C depending on the nature of the ore. The pressure is approximately 3.5 Mpa . The slurry is then cooled in a series of flash tanks to around 106 C at atmospheric pressure and by flashing off steam. Clarification/Settling: The first stage of clarification is to separate the solid (bauxite residue) from the pregnant liquor through sedimentation. Chemical additives are added to assist the sedimentation process.

14. Bayer’s Process cont’d The bauxite residue sinks to the bottom of the settling tanks and then washed. Caustic soda is recovered during this stage. Precipitation: The alumina is recovered by crystallization from the pregnant liquor, resulting in formation of small crystals of aluminium trihydroxite commonly known as hydrate. Evaporation: The spent liquor is heated through a series of heated exchangers and subsequently cooled in a series of flash tanks. Classification: The gibbsite crystals formed in precipitation are classified into sizes ranges.This is normally done using cyclones or gravity classification tanks.

15. Bayer’s Process cont’d The coarse size crystals are destined for calcination while the fine crystals after being washed to remove organic impurities are returned to the precipitate stage as fine seed to be agglomerated. Calcination: The filter cake is fed into calciners where they are roasted at temperatures up to 1100 C to drive off free moisture and chemically-connected water, producing alumina solid. Alumina is the final product of Bayer’s process

16. Bayer’s process in picture Source: www.hydo.com

17. Alumina to Aluminium To produce pure aluminium, alumina is smelted using the Hall- Heroult electrolytic process. The raw materials needed for the smelting process are: Alumina, Carbon and electricity. Alumina has a high melting point and is converted through an electrolytic process. In the electrolytic cells, a very high direct current is run between a negative cathode and a positive anode, both made of carbon. The anode is consumed in the process, as it reacts with the oxygen in the alumina to form CO2 .

18. Alumina to Aluminium Cryolite is added to the alumina to lower its melting point. Liquid aluminium is drawn from the cells using specialized vehicles and is cast into extrusion ingots, sheet ingots or other ingots, depending on how it will be processed further .

19. Alumina to Aluminium in Picture Source: www.meritnation.com

20. Uses of Aluminium Source: nfalliance.org

21. References http :// bauxite.world-aluminium.org/mining/process.html http:// bauxite.world-aluminium.org/refining/process.html http://www.hydro.com/en/About-aluminium/Aluminium-life-cycle/Primary-production /

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