Bauxite and Alumina Mining

Bauxite ore with high concentrations of aluminium oxides (65% or greater) can only be economically mined when located near tropical regions both sides of the Equator; gibbsite and boehmite deposits typically fall in these locations while diaspore is predominantly located in northern regions.

The Bayer process entails four steps: digestion, clarification, precipitation and calcination. Energy usage results for these processes have been displayed below using LCA simulations using gibbsite and boehmite-diaspore bauxite as examples.

Digestion

Aluminum is the third most abundant element on Earth’s crust and its main source for production is bauxite (Al2O3), mined from open pit mines and transported to refineries where it is processed into aluminum using the Bayer process. Energy consumption during refining includes digestion of bauxite and calcination of alumina – with combustion of natural gas accounting for 66% of GHG emissions from this process.

Digestion of bauxite is the initial step of Bayer process. Caustic soda concentration plays a significant role in determining its digestibility; efficiency increases with increasing concentration up to its maximum value of 260 g/L.

Wet grinding of bauxite ore in rod mills, ball mills or semiautogenous grinding mills produces fine slurry which is then transferred to digestion tanks where it is mixed with hot water at high pressure and temperature to form “green liquor.” Red mud residues are then separated and washed off using crude cyclones and raking thickeners and their residue returned for more digestion as “spent liquor.”

Bayer digestion of non-magnetic bauxite involves the separation of iron-bearing minerals from alumina through reduction roasting and magnetic separation processes, with diaoyudaoite and sodium aluminosilicate being main sources of alumina when processed at lower digestion temperatures.

The digestion stage consumes the highest energy requirements in refining alumina, due to its large consumption of thermal energy used for heating caustic soda. Furthermore, water and electricity consumption is substantial during this stage as well. When considering an average EU energy mix for analysis purposes, LCA results indicate 639 kg CO2-equivalent emissions directly produced per ton produced with Bayer process using natural gas as heat source in digestion process; emissions were substantially decreased when natural gas heat source used instead indicating its influence over environmental indicators in LCA calculations.

Precipitation

Aluminum is a soft silvery metal that occurs naturally bound with oxygen in minerals and rocks like bauxite. To produce aluminum, however, bauxite must first be processed – this requires adding water and various impurities such as iron oxide, silica and ferric oxide; only those bauxites with concentrations of 30-66% alumina make economical sense for processing. Only certain countries possess sufficient deposits with sufficient economic alumina concentrations to meet world aluminum demand.

To produce alumina, bauxite must first be dissolving in caustic soda at high temperatures through digestion – this process is known as digestion. Once complete, the solution must then be clarified in thickeners before being separated into red mud and sodium aluminate solutions – red mud being known as the insoluble residue from bauxite processing; while sodium aluminate solutions are taken through tall tanks called precipitators where aluminum hydroxide seeds (very small particles) are added so when cooling, these seeds promote solid aluminum hydroxide crystal formation that settle at the bottom of these tanks before removal from them allowing further processing or disposal.

At least two tons of waste is generated for every ton of aluminum produced, including mining wastes, production residues from alumina production and other scrap materials or solid wastes. Smelting also releases greenhouse gases.

Bauxite mining is an intensive labor-intensive process requiring heavy machinery on steep, unforgiving terrain. Furthermore, operating equipment like scrapers and drills may cause whole-body vibration that leads to musculoskeletal conditions and spinal disorders as well as hand-arm vibration from using vibrating tools like hammers and power tools – further adding to this risk.

Although acid and electrothermal processes exist for producing alumina, they are expensive. The Bayer process is the dominant refining method and consists of four steps – digestion, clarification, precipitation and calcination – used to refine it. Australia, China and Russia are currently the world’s top three producers; Australia having mines and refineries in Western Australia (Kwinana and Wagerup) and Queensland; Tasmania Queensland Yarwun aluminium smelters among them while China relies heavily on imports from Australia Brazil Guinea etc for its production.

Calcination

Bauxite, the primary ore of aluminium, often forms weathered blankets known as laterite or duricrust over various rock types containing alumina-bearing minerals, including iron oxides, silicon dioxide and titanium oxide. Alumina can be extracted by digesting and precipitating laterite/duricrust layers.

Calcination of alumina produces commercially pure aluminum oxide. Filtered and washed to remove entrained caustic and solution, then heated in either rotary kilns or stationary fluidized-bed flash calciners up to 960 degC (1,750 degF) to complete this process.

Gibbsite and boehmite undergo transformation during calcination to alpha alumina via release of OH groups with up to 30% water loss, providing superior transformation rates and product properties. For maximum effectiveness and optimal transformation rates, gibbsite should have larger mean particle sizes relative to boehmite for best transformation rates and product properties – supported by its surface area increasing nearly 280 times greater than its counterpart GB precursor.

These textural parameters are critical in creating custom alumina powders tailored for use as catalysts and absorbents, as well as determining its attrition resistance and low LOI value. Once manufactured, this alumina is then melted down to produce metallic aluminum.

Smelting requires heating alumina at high temperatures in an electric arc furnace to produce molten aluminium, while impurities such as iron and silicon are removed via electrochemical anodes, leaving only pure aluminium on cathodes for casting into ingots for further processing.

Alumina can then be processed further to produce bauxite, used to manufacture various products with its aluminium content ranging between 10%-30% by weight. Plastic is an exceptionally flexible material with multiple physical characteristics such as its strength, hardness and electrical conductivity. It can be found in building materials such as windows and doors as well as automotive applications like brake linings and transmission components. Bauxite is widely recognized as a nontoxic mineral that poses no significant threats to both people and the environment. However, its mining industry must strictly manage the environmental risks it encounters; older alumina refineries may contain asbestos-containing minerals which have been linked with mesothelioma as well as other lung disorders.

Refining

Alumina (also known as aluminium oxide) is the main raw material required to produce aluminium, obtained through refining bauxite and refined. Refined alumina can then be used to produce primary aluminium through an electrolytic process; roughly two tonnes are necessary per ton produced. Refined alumina also finds use as water purification media as well as an additive in steel production, refractories, and ceramic industries.

Alumina production follows the Bayer Process, a four-step procedure. Digestion begins by mixing bauxite with caustic soda at very high temperatures in special digester vessels; precipitation then recovers alumina crystals from this solution by mechanical stirring in open tanks; precipitated alumina is then classified by gravity or cyclone into various sizes – those larger in size will go on to calcination while those smaller are sent for clarification.

Bauxite mines produce significant waste products as a result of mining and refining processes, such as soil contamination, alumina fines, and other waste materials. Alumina refineries produce about two to 2.5 tons of solid waste for every ton of alumina produced – some of this waste may even contain toxic tenormine (TENORM).

Safety is of utmost importance during both bauxite mining and refining operations, with workers exposed to health and safety hazards associated with both activities. Workers are closely monitored for any insect bites at certain mine locations where mosquito netting or other preventive measures may be employed to lower this risk; malaria prophylaxis may be recommended while yellow fever vaccination may also be required depending on location.

Only 20 countries around the world operate alumina refineries, with most located in China, Brazil and Australia. Alumina produced from these refineries is typically sold to aluminium smelters as raw material for primary production with any remaining quantity sold to other manufacturers for various uses. Refineries require significant investments of time, money and human resources for optimal operations.