Bauxite and Alumina Mining

Aluminum is one of the three most abundant elements on Earth, following oxygen and silicon. Bauxite ore is processed through Bayer refining for its extraction.

Bauxite ore is crushed, washed and dried before it is mixed with hot caustic soda to form sodium aluminate supersaturated solution, commonly referred to as red mud. This solution is then pumped to tall tanks called precipitators for collection and processing.

Bauxite

Bauxite is one of the world’s primary sources of aluminum, used in applications ranging from airplanes and automobiles to beverage cans and tin foil. Bauxite is an ore containing Al2O3, typically found near forested areas near surface terrain near tree cover. When mined strip-mining methods can be employed with soil replaced via forestation as part of rehabilitation procedures.

Bauxite ore is composed of aluminium-containing minerals such as gibbsite, bohmite and diaspore; their proportion varies depending on where you find it. Bauxite deposits can be found across all continents except Antarctica with rich reserves located in Guinea (Brazil), Australia, Indonesia and China. Bauxite formation occurs as a result of chemical weathering by water in tropical climates where rainwater leaches out silica from original igneous rocks that comprised its formation.

After mining bauxite, it must be transported to an alumina refinery where it will be transformed into alumina for use in manufacturing aluminum products. Bauxite can usually be transported by train or truck from its source mines directly to an alumina refinery; some shipments may even arrive by sea.

As part of the production process, bauxite must first be leached to separate out its alumina content, using a solution of 15-20% aqueous sodium hydroxide under pressure. Once this step has taken place, precipitated alumina can then be precipitated out and further processed through fluidized bed or rotary kiln calcination for 99.5% purity alumina production.

Bauxite mined from this mine is then exported for refinery use at Yarwun and Queensland Alumina Ltd in Gladstone, Australia, while some is also sent overseas (e.g. China).

Bauxite mining poses serious environmental impacts in countries like Guinea. A recent analysis of community impacts in Guinea’s largest mining area, Boke, revealed that its extraction was upending lives and livelihoods of rural communities through loss of access to land, depleted water resources, effects on health and violent attacks against project workers by local population members.

Red Mud

Production of aluminum comes with many environmental risks. Mining of bauxite decimates vast tracts of old-growth forest while its energy-intensive process necessitates massive dams that flood indigenous communities. Furthermore, toxic red mud produced during production poses serious health and safety threats when not contained properly.

Studies on how to make red mud less dangerous have been done extensively, with most efforts focused on alleviating rather than eliminating its dangers. A significant goal of much of this work has been encasing hazardous metals found in caustic waste into solid form called refractory minerals – these solids can then be utilized in many different ways.

One use for alumina manufacturers could be strengthening metal alloys they use in their products with rare earth metals like alumina. Researchers have even developed scandium-aluminum alloys which are 40% stronger than pure aluminum – this has aerospace manufacturers eager to use them, potentially helping reduce fuel consumption and emissions by making planes lighter and more fuel efficient; unfortunately though, its high price – $3500 per kilogram – prevents growth in this sector.

Refractory minerals have another application in building materials. While only about 3% of bauxite residue is recycled in this manner currently, this material could be utilized for walls and other construction projects; most is sent off to large waste ponds, dried mounts or landfills for disposal.

The primary challenge facing the industry today, however, is finding an environmentally sound way of disposing of vast amounts of bauxite residue that accumulate each year. Current methods — which involve dumping the waste directly in open nature — have proven both expensive and environmentally destructive.

Researchers from China have devised a technique to increase the stability and durability of caustic waste by mixing it with various binders. Biners also serve the additional benefit of binding harmful substances found in red mud such as hexavalent chromium, lead, selenium, fluoride, arsenic to reduce leaching or toxic effects; additionally they slow degradation while decreasing its permeability.

Pregnant Liquor

At this stage, hot caustic soda (NaOH) solution dissolves the aluminium-containing minerals found in bauxite such as gibbsite, boehmite and diaspore to produce sodium aluminate supersaturated solution known as pregnant liquor. Any insoluble residues left after digestion is known as red mud and may contain impurities like iron oxides, sodium silicate and titanium dioxide that must be removed to guarantee high-quality alumina products.

The aluminate solution is then sent through a series of vessels known as the reaction circuit for further processing, where it passes through security filters to separate solid alumina particles from caustic soda and prevent excessive losses of this expensive chemical. Finally, clear pregnant liquor rich in alumina trihydroxide will be routed back towards Bayer process’s precipitation section for further precipitation steps.

An approximate 10% to 25% by volume of the incoming pregnant liquor stream is directed into an agglomeration section, comprised of tanks containing fine aluminum hydroxide seed crystals with median particle sizes between 30-60 microns; then allowed to settle and agglomerate over an approximate six hour stay period in this agglomeration section.

Once the pregnant liquor has been transferred to the precipitation section, it is deposited in larger vessels known as growth section tanks containing coarse seed crystals of Alumina trihydroxide with median sizes ranging between 80-100 microns.

As the precipitation tanks cool down, alumina crystals begin to form and are gradually separated into various size ranges. Large crystals are classified as refractory grade products while smaller particles undergo calcination; during which process any trace of residual mud residue is washed away to ensure quality product.

After being thoroughly mixed and dried in a rotary dryer, alumina undergoes calcination – changing its composition and crystal structure without altering particle size – for further processing.

Precipitation

Bauxite ore is refined into aluminium oxide (Al2O3) using the Bayer process, producing white crystalline powder called alumina [Al2O3]. Smelted aluminium metal can then be made using this refined form. Four tons of bauxite yield two tons of alumina which has many applications such as abrasives and ceramics.

Bauxite ore is ground in rod or ball mills to form a fine slurry which is then digested with caustic soda at concentrations up to 170 g/L in pressure vessels under pressure and temperatures between 145-265 degC – this process is known as pregnant liquor digestion. Reaction proceeds in one direction via addition of caustic soda; supersaturated solutions form, which is then pumped into tall silolike precipitators before boehmite crystals precipitation occurs upon controlled cooling to form an alumina-bearing slag while any excess caustic soda returns for digestion.

Crude cyclones are used to separate coarse particles from slag containing alumina-bearing material, while finer particles are settled using raking thickeners with synthetic flocculants. Both thickeners operate under extreme temperature and pressure conditions – making them some of the most advanced pieces of processing equipment on the market today.

Like all mining operations, alumina plants face environmental and health hazards. Their refining process is energy intensive and produces significant waste materials, so plant owners must strive to minimize fuel and caustic soda usage while increasing production efficiency.

Health risks at alumina refineries can also be an important concern, particularly with regard to respiratory and gastrointestinal ailments. Furthermore, some locations are vulnerable to tropical diseases like malaria and dengue fever; workers therefore receive education on this matter and vaccinations against such illnesses.

Workers at bauxite mines and alumina refineries are at increased risk of asbestos and other toxic chemicals, though the aluminum industry typically has lower occupational asbestos exposure compared with some other industries. Mesothelioma has been linked with Australian bauxite mines and smelters; Positional and personal monitoring data collected at Pinjarra alumina refinery indicate urinary mercury levels for employees are well within current Australian guidelines of 20 mg/g creatinine.