It is estimated that between 2.5 and 5 million tonnes is recycled annually as an input to cement production, road construction, refractories, soil amelioration, dyke construction and landfill covering.
Applications can broadly be broken down into five categories:
- use as a major component in the manufacture of another product because of its chemical composition, e.g. in cement (clinker production) for its iron and aluminium content;
- use as a component in a building or construction;
- use for specific properties, e.g. soil amelioration or landfill capping for its low permeability and phosphorus absorbing characteristics;
- recovery of specific components present, e.g. iron, aluminium, titanium, lanthanides, scandium, yttrium, and gallium;
- conversion of the bauxite residue to a useful material by modifying the compounds present in some way, e.g. acid mine drainage and heavy metal absorption.
Process changes at the refineries such as improved soda recovery and increased use of filter presses to generate a higher solids residue have become increasing common and resulted in the generation of more benign and useful materials. The increased development effort together with public, corporate and government attitudes have never presented such an encouraging environment for developing and implementing bauxite residue uses.
The prospects for using bauxite residue in Portland cement has been explored for over 80 years with a number of promising technical studies and several successful large scale commercial initiatives. The iron and aluminium constituents of the bauxite residue provide valuable additions in the production of Portland cement at a low cost.
The best estimates are that between 1,000,000 to 1,500,000 tonnes of bauxite residue are currently used annually in the production of clinker. The cement plants currently utilising the bauxite residue in clinker production are based in China, Ukraine, India, Russia, Georgia, Moldova and Greece.
Evidence from cement plants already using bauxite residue on an industrial scale demonstrate that bauxite residue can satisfactorily be used in cement clinker manufacture. With the appropriate bauxite residue, typically a usage rate of 3 to 5% can be accommodated. From the industrial experience, key aspects seem to be:
- a relatively low moisture content – approximately 30% moisture has been used in several plants and this can readily be achieved at many alumina refineries
- a relatively low moisture content – approximately 30% moisture has been used in several plants and this can readily be achieved.
- a moderately low sodium content – a value of <2.5% Na2O has been indicated as satisfactory but it will depend on the composition of the other raw materials
- the appropriate aluminium oxide to iron oxide ratio – an iron oxide to alumina ratio of 0.8:1.2 in the raw mix was found to give the best results in one study although some plants will use the bauxite residue to supplement the iron level whilst others use it for the alumina content
- the reasonable proximity of a cement plant – a distance of up to 1,200 km has been found to be acceptable in one case
Potentially the largest use of bauxite residue in cementious materials is in supplementary cementious materials. Considerable work has been undertaken on a laboratory/pilot scale by numerous groups. Opportunities for bauxite residue as a filler in blended cements have been explored, sometimes for their pozzolanic activity but also to improve the mechanical properties in blended cement by other mechanisms such as optimising packing density or rheological characteristics. In some instances the bauxite residues have been calcined and/or partially neutralised to improve its performance.
Some of the studies have shown that bauxite residue may successfully replace clinker in blended cements at dosages between 10 and 20 wt% whilst other work has shown that 25-30% replacement is possible. This would have a significant impact on CO2 emissions.
Studies on using bauxite residue in special cements such as calcium sulfo-aluminate cements are more recent than those on Portland cement. Despite some promising studies, there does not currently appear to be any industrial usage. During production, these belite cements generate less carbon dioxide than Portland cement so offer an environmentally attractive option. In addition, these cements possess high early strength characteristics, high ultimate strengths and good dimensional stability. The rapid strength development is due to the formation of ettringite. Calcium sulfo-aluminate type cements require a higher alumina content that Portland cement and bauxite is frequently used as the alumina source with the result that their raw material cost is markedly higher. The use of bauxite residue offers the opportunity for reducing this cost differential. Iron rich, special setting cements with improved strength when compared to Portland cement have been made with levels of up to 40% bauxite residue from Renukoot, India together with bauxite and gypsum.
When dewatered, compacted and mixed with a suitable binder, bauxite residue makes a good road building material and has been used to construct haul roads on bauxite residue areas for very many years. The good impermeability of bauxite residue when dewatered and compacted is an advantage in the construction of levees and dyke walls and is used in the construction of impoundment areas for containing bauxite residue. The material is sometimes mixed with other waste materials, e.g. fly ash, and then capped with clay to reduce water ingress and promote vegetative growth. This practice has been widely adopted across the world.
Mixtures with clay, shale, sand, and fly ash have been proposed and evaluated for brick manufacture by various teams of workers and has been undertaken using bauxite residue from Jamaica, Sardinia, Hungary, and Korea. Bricks have been made with a bauxite residue content of over 90% when using a firing temperature of approximately a 1000ºC.
Addition of bauxite residue to acidic and sandy soils can be beneficial in many ways and considerable work in this area has been undertaken in Western Australia. Bauxite residue was added to sandy soil together with 5% gypsum. The additions improved water retention and nutrient utilisation ability. Greatly increased ammonium and phosphorus retention behaviours were found showing how the usage of fertiliser could be reduced.
Another application that utilises the impermeability of bauxite residue when dewatered is the capping of landfills, especially municipal site; this has been extensively used in the Marseille area using Bauxaline® from Gardanne. In one site in the Gardanne area, the waste methane gas that evolves is collected under the Bauxaline® covering layer. Similar practices have been done in Louisiana where the pH reduced bauxite residue is mixed with clay.
Since the start of large scale manufacture of alumina using the Bayer Process in the 1890s, the industry has sought ways of using bauxite residue in a sustainable manner. Despite the enormous amount of work and countless studies over many years, the utilisation rate has historically been modest. However, recent efforts and initiatives within the industry and by universities and entrepreneurs has led to a greatly increased utilisation rate.