Aluminosilicate minerals, weathering process

Aluminum, a silver-white, malleable, and ductile metal, is the most abundant metallic element in the lithosphere, comprising about 8% of the earth s crust. It is never found free in nature, but occurs combined with other elements, most commonly as aluminosilicates, oxides, and hydroxides in rock, minerals, clays, and soil. It is also present in air, water, and many foods. Bauxite, a weathered rock consisting primarily of aluminum hydroxide minerals, is the primary ore used in aluminum production. Aluminum enters environmental media naturally through the weathering of rocks and minerals. Anthropogenic releases are in the form of air emissions, waste water effluents, and solid waste primarily associated with industrial processes, such as aluminum production. Because of its prominence as a major constituent of the earth s crust, natural weathering processes far exceed the contribution of releases to air, water, and land associated with human activities. 

Soil is the product formed when the rocks of the earth s crust are exposed at the surface and are subjected to various physical, chemical, and, eventually, biological weathering processes. The minerals in these rocks are predominantly silicates, which dominate the characteristics of most soils. Table 1 shows those elements that are found in the crust above an average concentration of 1% and their corresponding soil content. The importance of aluminosilicates in soil is clear from the enrichment factors of approximately 1 for O, Si, and Al. Some loss occurs of K, Fe, Ca, Na, and Mg as a result of soil processes. But two elements, C and N, show considerable enrichment in soil because of the crucial role played by organic matter. 

The Isolation of Aluminum Aluminum, the most abundant metal in Earth s crust by mass, is found in numerous aluminosilicate minerals. Through eons of weathering, certain of these became bauxite, a mixed oxide-hydroxide that is the major ore of aluminum. In general terms, the isolation of aluminum is a two-step process that combines several physical and chemical separations. In the first, the mineral oxide, AI2O3, is separated from bauxite in the second, which we focus on here, the oxide is converted to the metal. 

The product, Al2Si207-2H20, is known as kaolinite, and it is one of the aluminosilicates that constitutes clays used in making pottery and china. This example also shows how one mineral can be converted into another by the natural process of weathering. 

Of course, once the ore is obtained from its deposit, the actual work of extracting the desired metal has yet to be accomplished. In addition to metals, a variety of other substances comprise natural minerals. Since aluminum and silicon are the most prevalent elements in the Earth s crust, most of the metals exist naturally as aluminates, silicates, or aluminosilicates. The most common minerals are feldspars and clays. These materials have been used since ancient times for the production of materials such as pottery, brick, and china. An example of a feldspar is K2Al2Si60i6, which corresponds to a mixture of potassium superoxide, alumina, and silica (K20-Al203 6Si02). Upon contact with water and carbon dioxide, a weathering reaction results in kaolinite, an aluminosilicate clay (Eq. 1). However, in addition to these oxidized sources of metals, there are substances such as alkaline carbonates, sulfates, phosphates, as well as organic matter that need to be removed to yield the desired metal. As you would expect, the yield for this process is quite low ores typically possess less than 1 % of the desired metal ... 

---