Plant secretions promote the development of microorganisms around the root, in an area known as the rhizosphere.
Additionally, leaves and other material that fall from plants decompose and contribute to soil composition. Time is an important factor in soil formation because soils develop over long periods.
Soil formation is a dynamic process. Materials are deposited over time, decompose, and transform into other materials that can be used by living organisms or deposited onto the surface of the soil. Soils are named and classified based on their horizons. The soil profile has four distinct layers: 1 O horizon; 2 A horizon; 3 B horizon, or subsoil; and 4 C horizon, or soil base Figure The O horizon has freshly decomposing organic matter—humus—at its surface, with decomposed vegetation at its base.
Humus enriches the soil with nutrients and enhances soil moisture retention. Topsoil—the top layer of soil—is usually two to three inches deep, but this depth can vary considerably. For instance, river deltas like the Mississippi River delta have deep layers of topsoil. The A horizon consists of a mixture of organic material with inorganic products of weathering, and it is therefore the beginning of true mineral soil. This horizon is typically darkly colored because of the presence of organic matter.
In this area, rainwater percolates through the soil and carries materials from the surface. In some soils, the B horizon contains nodules or a layer of calcium carbonate. The C horizon , or soil base, includes the parent material, plus the organic and inorganic material that is broken down to form soil. The parent material may be either created in its natural place, or transported from elsewhere to its present location.
These larger plants are very often remote from large centers of population which is likely to mean there is a lower level of industrial activity and consequently limit some opportunities for the use of bauxite residue.
This makes the transport cost issue even more critical when considering uses. It is generally estimated that some 2—4. Roads see Fig. Construction materials bricks see Fig. Bauxite residue can provide valuable iron and alumina values in the production of Ordinary Portland cement clinker and it has the added benefit of substantially reducing the carbon dioxide emissions during production.
In addition, it can behave as a supplementary cementitious material which is used in mortar or concrete mixes, this can provide: better mechanical properties; more efficient use of resources; lower carbon dioxide emissions [ 19 — 21 ]. The Nikolayev refinery blends the residue produced to give the cement plant a consistent feed and the climate allows a reasonably low moisture product to be produced.
There has also been some historical usage of bauxite residue from the refinery in Tulcea in a local Romanian cement plant and also reportedly small-scale usage of bauxite residue from the Gandja alumina refinery in Azerbaijan in cement. The use of bauxite residue in cement in China was formerly several million tonnes a year [ 46 , 47 ] but this has fallen because of the changes in construction industry standards and also a reduction in the number of plants operating a sinter or Bayer-sinter extraction route.
It should be noted that the bauxite residue produced from the sinter or Bayer-sinter extraction route is very different chemically from that produced in a conventional Bayer alumina plant. It is important to consider these contents and realize the difficulty in even closely matching the economics against using virgin iron ore, especially at the current price of iron ore.
Notably, success has also been achieved using magnetic separation techniques as a first stage of processing to concentrate the iron fraction.
The bauxite residue material is also wet and has a high sodium content which is a disadvantage in steel production. Recent research in Russia using low alkaline bauxite residue and bentonite has been very effective in raising the iron content, improving agglomeration quality and thereby giving greater yields in the production of iron ore pellets [ 51 ].
The simultaneous recovery of other metals, for example, titanium and aluminum, would improve the economics of using bauxite residue for iron recovery in steel production. The only non-Chinese plant using bauxite residue for making steel is based in the Urals. The work on iron extraction from bauxite residue in the Southern Chinese alumina plants is discussed later.
China is worth discussing separately as it devoting a very considerable effort into searching for and implementing reuse of bauxite residue, much of it being driven by Chinese Government initiatives [ 41 — 43 , 46 , 47 ]. The alumina manufacturing routes have historically been very different because of the nature of the indigenous bauxite. Sinter routes or combined Bayer-sinter routes were widespread but are now declining sharply, except for the production of chemical grade aluminas, and the industry has become more dependent on imported bauxites.
This change in route has significantly changed the characteristics and composition of the bauxite residues being produced.
Hitherto much of the imported bauxite was from Indonesia and Australia but curtailment of bauxite exports from Indonesia is changing the nature of the bauxite residue yet again.
Traditionally, the alumina plants in the Northern part of China produced a residue very high in calcium and silicon oxides but low in iron oxide making them suitable for cement production while those in the south of China have a residue high in iron which makes the recovery of iron the most likely option for them to pursue.
In , a cement plant was built to consume residue from Shandong Alumina and by the late s over 6 million tonnes of residue were used to make OPC and oil well cements. However, new standards for cement were introduced which restricted sodium level content and the progressive move to imported bauxites led to a reduction in the amount going into cement.
A very strong driving force in China has been government imposed legislation requiring that bauxite residue is reused. Since very large-scale effort to utilize more bauxite residue in many areas with the main effort being devoted to high-intensity magnetic separation to concentrate iron; glass ceramics CaO—SiO 2 —Al 2 O 3 ; bricks with lime fly ash ; and polymer fillers.
The manufacture of bricks, tiles, and other building materials has been shown to be technically possible by many groups of workers from a wide variety of sources of bauxite residue using both fired and chemically bonded methods. Outside China, however, while plants have started up, production has not continued. Use of bauxite residue for capping municipal landfills is carried out in France and has been trialled in the USA.
The bauxite residue must be in a form that can safely and readily be carried in trucks on public roads. Possible concerns are dust from the bauxite residue when dry and heavy metal leaching characteristics. However, controversy over two decades has prevented its implementation until recently.
The target is to replace virgin sand and crushed limestone for sub-grade and top dressing: for each cubic meter of virgin sand replaced there are savings of 4. The benefits include lower carbon footprint; reduced loss of vegetation as it avoids use of quarries; cost savings; less water usage; and reduced eutrophication.
The global desire for the more efficient use of resources has led to a greatly increased effort looking at uses of bauxite residue over the past 3 years. Increased enthusiasm from industry, more university activities and the input of funds from organizations such as the EU has led to greater co-operation between industries and academia.
In , under the Horizon initiative, the EU agreed to fund 15 PhD students to work on the recovery of materials and the utilization of bauxite residue. Some Euro 3. The work is focussing on the extraction of iron, aluminum, titanium and rare earth elements including scandium as well as the production of new building materials [ 52 ].
This seeks to bring together industry with researchers and stakeholders to explore the best available technologies to recover critical raw materials. The strongly growing demand for rare earth elements REEs , and the concentration of production in China has led to a renewed interest in the extraction of REEs from bauxite residue [ 18 , 53 — 58 ].
The REEs, normally taken to mean the lanthanides plus scandium and yttrium, are essential constituents of modern permanent magnets, nickel metal hydride batteries, and lamp phosphors with dramatic growth driven by increasing demand for electric and hybrid cars, wind turbines, and compact fluorescent lights. The very strong demand for scandium is driven by its emerging use in aluminum alloys where its ability to control grain size leads to exceptionally high strength but light components.
During the Bayer process, the REEs remain undissolved so are concentrated up in the resulting residue. Much work was done during the s on Jamaican bauxite residues using dilute acid as the first stage leaving behind the iron and titanium oxides, the REEs were then recovered by selective precipitation [ 18 ], but many new projects have started in recent years. These include a joint venture between the Jamaican Bauxite Institute and Nippon Light Metals to construct a pilot plant for extraction from locally arising bauxite residues [ 53 ].
Examples include selective leaching after sulfation roasting at KU Leuven [ 54 ], and use of cation exchange chromatographic techniques at the National Technical University of Athens [ 55 ]. In global terms, over , t of REEs are sent to bauxite residue disposal ponds which compares to some , t of REEs produced annually. Orbite Technologies Inc. Using a modification of this route, they have developed and patented the recovery of alumina, titania, iron oxide, magnesia, rare earth elements, and other rare metals from bauxite residue.
The ability of bauxite residues to react with heavy metals, especially from mine and mineralogical processing sites, has been examined by several groups around the world [ 23 — 25 ] so far with only modest tonnages being sold but several new approaches are emerging. In Italy, using residues from the Eurallumina plant, good heavy metal absorption results were obtained by neutralizing the material with seawater. In some formulations, the bauxite residue was mixed with fly ash which improved the absorption of arsenic.
Work on bauxite residue from San Ciprian mixed with gypsum was found to have a good capability to remove copper, zinc, nickel, and cadmium from waste streams. Work at the Chinese Academy of Science has shown good benefits in the use of bauxite residue for the adsorption of lead, cadmium, and copper. A British group of entrepreneurs, Purgo Limited, has developed an innovative approach using bauxite residue in combination with different waste materials in an unconventional way to manufacture construction materials such as building panels, bricks, tiles, and aggregates.
The early history of bauxite residue disposal and storage involved using estuaries or land impoundment areas adjacent to the factory. Disposal into rivers, estuaries, or the sea became common for a number of years but this has now totally ceased.
In contrast to the swashbuckling adventures of Indiana Jones, archaeologists actually spend the vast majority of their fieldwork time doing something rather mundane: moving dirt. Although their traditional dig-and-sift method works well, it is labor-intensive and time-consuming--and therefore rather costly.
As a result, researchers have increasingly enlisted the aid of imaging techniques such as ground-penetrating radar GPR and seismic exploration technologies to help narrow their search. Now another tool is in the works. Impact - soft hit - soil on wood - throwing with particles, dust, powder, and debris - with detail - LR 4.
Soft Impact Hit Soil on W Impact - soft hit - soil on wood - throwing with particles, dust, powder, and debris - with detail - LR 3. Impact - soft hit - soil on wood - throwing with particles, dust, powder, and debris - with detail - LR 2. Impact - soft hit - soil on wood - throwing with particles, dust, powder, and debris - with detail - LR 1. It will be almost impossible to support the animal and human life without land.
Biodiversity relies on soil at all times. The soil is necessary for water supply. This is the magic of nature. The land is also necessary to ensure the quality of water we derive from our earth.Skip to 0 minutes and 12 seconds Have you ever thought about the soil beneath us?. Skip to 0 minutes and 22 seconds Did you know that the rise and fall of civilizations depends on soils, or that there are more living things in one teaspoon of soil than there are people on the planet? We rely on soil for many resources, not least food and water. But soils can take thousands of years.