Bauxite - Mine To Metal

Aluminum, the third most abundant element in the earth's crust (8.8%), is usually combined with silicon and oxygen in rock. Due to its chemical reactivity, it almost doesn’t exist in free form. Only a few minerals and rocks containing aluminum are suitable for industrial production. Aluminum is mainly produced from bauxite i.e., rock that contains high concentrations of aluminum hydroxide minerals.

Occurrence

Over 90% of the world’s bauxite resources are concentrated on the tropical and sub-tropical belt in Australia, Guinea, Jamaica, Surinam, Brazil, and India. The resources are distributed in the states of Odisha (52%), Andhra Pradesh (18%), Gujarat (7%), Chhattisgarh (5%), Maharashtra (5%), Madhya Pradesh (4%) and Jharkhand (4%). Major bauxite resources are concentrated in the East coast bauxite deposits in Odisha and Andhra Pradesh.

Bauxite usually contains certain principal ore forming minerals and is (i) Gibbsite Al2O3.3H2O; (ii) Boehmite Al2O3.H2O; and (iii) Diaspore Al2O3.H2O.

The presence and quantity of gibbsite mineral determine the value of bauxite ore in metal production since it is easily extractable in caustic soda at a lower concentration, temperature and pressure.

Aluminum Production

The production of primary aluminum is divided into (i) Bauxite mining, (ii) Production of Alumina (Al2O3) and (iii) Production of Aluminum (Al). Bauxite is the starting material for the production of aluminum through a common industrial production practice consisting of two distinct stages: one, the production of high grade metallurgical alumina (Al2O3) from bauxite that is performed according to the Bayer’s process and the other electrolytic reduction of alumina to aluminum, which is performed according to the Hall-Héroult process.

Bayer’s Process

Bayer’s process, in principle, involved the leaching of bauxite at high temperature and pressure with caustic soda and separating the resultant sodium aluminates solution which is, later on, precipitated as the hydrated aluminum oxide by seed crystallization. Bayer’s process is the standard and well-accepted process throughout the world for making alumina from bauxite.

Aluminum industry normally uses bauxite containing a minimum of 38% Al2O3. On an average the Bayer’s process requires 2.65 tons of bauxite ore to produce 1 ton of alumina, while one ton of aluminum is produced from every two tons of alumina.

In effect around 5 tons of bauxite must be mined to produce 1 ton of metal. It clearly implies that higher the grade of bauxite lower is the quantity required per ton of aluminum produced.

Consumption

The aluminum industry is the principal consumer (90%) of bauxite in the country, followed by cement (9%) and refractory (1%). Aluminum metal is quite an energy intensive and one of the most polluting industries.

Mining & Processing

Bauxite contains impurities of clay, silica and iron. Iron impurities does not pose any adverse impact on the metallurgical industry; however, it decreases effective digester capacity for alumina production and enhances red-mud generation. Red mud or bauxite residue is the denomination of the residual material obtained from the dissolution of the hydrated alumina minerals of bauxite in the caustic Bayer’s liquor. This red mud on accounts of its very fine nature pose problem for its disposal and environmental degradation. The presence of reactive silica (clay) in the Bayar’s process results in higher consumption of soda (NaOH), in addition formation of sodium alumino-silicate takes place and the same starts to dissolve in the spent liquor which is undesirable.

At present, no beneficiation technology is being adopted to produce high grade bauxite concentrate for industrial use in the country except selective mining followed by manual loading of coarse size lumps up to 50 mm size. However, in few cases clay is removed from lumps by deploying scrubber operation.

Need For Processing

Bauxite is basically a rock comprising of hydrated aluminum oxides (like gibbsite, boehmite and diaspore) and impurities in the form of silica, clay, silt, titanium minerals (like rutile, ilmenite and titaniferrous magnetite) and iron minerals (e.g. iron hydroxide, iron oxide, and pyrite). These impurities increase the cost of mining (in some cases up to 40% of bauxite is rejected as low/marginal grade), handling and transportation. More importantly, they increase costs of downstream products because of waste handling and disposal. The impurities also reduce the capacity utilization, productivity and efficiency of the plant.

The waste, in the form of ‘Red-Mud’ formed during alumina production, also contributes to the environment and groundwater pollution.

Bauxite is by far the most important raw material for the production of alumina. Its abundance, distribution, as well as mineralogical and chemical composition, greatly influences the mass flow involved with alumina production. The quality of the bauxite ore is highly variable between individual deposits and controlled by specific geologic factors.

Factors for Alumina Processing

Some important mineralogical chemical characteristic of bauxite ore which controls the efficacy of alumina processing are –

 a)   Recoverable Al2O3 which indicates the alumina content that can be made available by the Bayer’s process with respect of geochemistry and mineralogy of the ore- tri-hydrates are preferred over mono-hydrates.

b)   Bauxite/Al2O3 ratio, Red mud/Al2O3 ratio – this provides information on the amount of bauxite needed to produce the given amount of aluminum oxide.

c)   Reactive silica strictly influences negatively the consumption of energy in the Bayer’s process. The presence of reactive silica (clay) in the Bayar’s process results in higher consumption of soda (NaOH), in addition, the formation of sodium alumino-silicate takes place and the same starts to dissolve in the spent liquor which is undesirable.

d)   Reduction of iron content. Iron oxide content which results in larger consumption of bauxite and increased generation of red mud thus, hampering the productivity and necessitating more red handling facility per ton of alumina produced and increased environmental hazard.

e)   Another negative factor is the high content of TiO2. This has an indicative impact on alumina production. It causes incrustation of autoclave heating surfaces, impairs red mud thickening and washing, contaminates alumina and aluminum metal.

f)    Moisture content – this has a negative effect on processing and transportation.  These characteristics influence among others the amount of bauxite needed to produce one ton of alumina and the amount of energy and chemicals during the process. The requirement for high-quality bauxite has a sensitive effect, both on the amount and the regional distribution of available reserves. Different investigations have shown that it is possible to upgrade the quality of the run of mine (r.o.m.) Bauxite by proven Mineral Processing Technologies

The impurities present in the refractory grade bauxite are alkalis, lime, and magnesia which need to be removed to the lower level by various methods of processing. The process of beneficiation adopted for producing marketable bauxite products includes washing, screening, gravity separation, magnetic separation, floatation, acid leaching, etc.

Bauxite deposits in different place have different mineralogy. Indian ores are generally rich in goethite, hematite, ilmenite, rutile, anatase, clay and quartz as impurity phases. The main problems with the ores are high iron and titanium contents. So, the removal of iron and Titania is required to meet the refractory specifications. Besides, for the clay/silica rich ores, reactive silica needs to be reduced for the metallurgical specification.

The mining of bauxite is carried out by opencast method. Selective mining coupled with manual loading of up to 50 mm size lumps obviates the necessity of any further processing. At present, no beneficiation technology is being adapted to produce high-grade bauxite concentrate for industrial use.

Disclaimer: The above publication content is the excerpts of Aluminium Industry Interactions & Statistical Sources are from IMYB 2015-16