by David Phillips, Heyl & Patterson, USA
Bauxite is the world’s primary source of aluminum, one of the most useful and versatile metals known to man. Aluminum is used for cookware, baseball bats, chairs, aluminum foil, electronics, coins, cans, flagpoles, heat sinks and transportation.
One scholar of J.R.R. Tolkien even suggests that the fictional element mithril might actually be aluminum. The mineral was discovered in 1821 by the French geologist Pierre Berthier, near the village of Les Baux in Provence, southern France. French chemist Henri Sainte-Claire Deville later named the mineral after the town itself.
Bauxite is a mixture of hydrous aluminum oxides, aluminum hydroxides, clay minerals and insoluble materials such as quartz, hematite, magnetite, siderite and goethite. The aluminum minerals in bauxite include boehmite, gibbsite, diaspore, iron oxides haematite and goethite, kaolinite and anatase titanium dioxide. It is found as a secondary mineral that forms in weathered surface zones in limestone, clay deposits and low-silica igneous rocks.
Bauxite is plentiful all over the world, and Australia is the top producer by a wide margin. It is also mined in China, Brazil, Guinea, India, Jamaica, Kazakhstan and Russia from highest to lowest production, and several other nations maintain operations as well. In the United States, it can be found mainly in the southeast region — Alabama, Arkansas, Georgia, Mississippi, Tennessee and Virginia — and the town of Bauxite, Arkansas, is even named after it.
Aside from aluminum production, sintered bauxite can also be used as a sandblasting abrasive or in hydraulic fracturing as an oilfield proppant.
Because it is most often found near the surface, bauxite is usually strip mined, and processing and refining occur elsewhere.
To produce aluminum, bauxite first needs to be crushed. It is then purified using the Bayer process, which washes the resulting smaller particles in a hot solution of sodium hydroxide to leach the aluminum from the bauxite ore. This creates aluminum hydroxide, which is then calcined to form alumina. Next, aluminum is smelted from the alumina using the Hall-Heroult process. This dissolves the alumina in a molten bath of cryolite. Electrolysis then removes the molten aluminum from the solution. The Hall-Heroult process uses a great deal of electricity, which is why bauxite is often processed far from where it is mined.
A rotary dryer, or kiln, is used to dry bauxite. Drying is widely applied in industries such as metallurgy, fireproof materials, chemicals and energy. First the bauxite is crushed and cleaned, separating liquids from solids and removing clay and silica. The dried bauxite can then be used to smelt metal aluminum. The principal process for modern bauxite requires smelting in a rotary kiln under a salt cover. The salt slag is processed and reutilized later. Other methods of processing bauxite, such as smelting it in a hearth furnace or an induction furnace, need little or no salt and less energy, but are reserved for high-grade scrap.
Heyl & Patterson rotary dryers are some of the most versatile available, and are considered the workhorses of the bulk material processing industry. Able to effectively handle a broad range of material, from mineral powders to liquid sludges, Renneburg rotary dryers can be specified according to many different parameters in order to meet and exceed specific application requirements.
The heart of a rotary dryer is a rotating cylindrical shell that is slightly inclined from the horizontal. The heat transfer mechanism can be either direct or indirect, although direct-fired rotary dryers are more common than indirect. Direct-fired machines dry the wet material through direct contact with a hot gas, which can flow co-current or counter-current to the material stream. The rotating shell acts as both a stirrer and a conveyor, drying the material as it is transported through the interior. Rotary dryers are equipped with flights in the shell’s interior to lift and shower the material through the gas stream as it passes through the cylinder, while moving the particles from one end of the shell to the other.
Rotary bauxite dryers manufactured by Heyl & Patterson have several advantages over other types of drying equipment. They offer greater versatility in processing a broader range of materials, can handle frozen non-uniform sizes and feature a lower initial investment. They also tend to be more forgiving regarding feed material properties such as moisture content and particle size.
For more information about Heyl & Patterson rotary dryers for applications in bauxite processing, contact us .