CALCINATION | Unit of Pyrometallurgy


● The temperature required 1000 - 1200℃ and this is the thermal treatment of the ore to bring,  about its decomposition and determined the volatile product usually carbon dioxide and water. The furnace use for calcination are
Lime calcination through shaft kiln
Lime calcination Shaft klin 

1. Shaft furnace.
2. Rotary furnace.
3. Fluidised bed furnace.

● The calcination resembles drying in that the material must be heated enough, so that the associate pressure of the decomposition compound is greater that pressure of the gas being evolved. The temperature required are generally much higher than these needed for drying.

Example :
                 * The burning limestone to make quick llme
                  CaCO3 =  CaO + CO2  ∆H  =  39900 cal.
                 * Periodically MgO is formed by calcination of MgCO3 at about 1000℃ to drying of carbon dioxide and convert MgO to a stable crystalline structure.
                 * In the refining of bauxite a precipitated of Al tri-hydrate is filled and then calcined at about 1000℃ to drying of common water.
                  2Al(OH)3 = Al2O3 + 3H2O.

Shaft Furnace :

A vertical, refractory-lined cylinder in which a fixed bed (or descending column) of solids is maintained, and through which an ascending stream of hot gas is forced.
For example : The pig-iron blast furnace and the phosphors-from-phosphate-rock furnace.

Working Principle | Operational Overview

● A furnace that has an upright working chamber of circular, elliptical, or rectangular cross section and is used to smelt or roast lumped materials. 
● The heat required for the smelting or roasting process is produced by the combustion of a fuel either directly in the furnace or in an external firebox from which hot combustion products are supplied to the furnace.
● Moderate velocities of the gaseous combustion products are characteristic of shaft furnaces. At such velocities, the bulk of the lumped materials is not entrained by the ascending gas stream and, in contrast to the case of a fluidized-bed furnace, maintains aerodynamic stability. 
● The countercurrent motion of the charge (from the top to the bottom) and of the gases forced through the charge (from the bottom to the top) and the direct contact between the charge and the hot gases result in good heat exchange and the generation of low temperature exhaust gases. 
● Consequently, shaft furnaces are characterized by a high thermal efficiency and a relatively high output. Such furnaces are widely used to smelt iron ores, pig iron, and the raw materials employed in non-ferrous metallurgy, as well as to roast, for example, iron ore (in the direct reduction of iron ore) and limestone.
Gas based shaft furnace
Gas based shaft f/c

● The shaft furnaces used in non ferrous metallurgy are designed for continuous operation. They are low (<8 m high) because of the need to carry out smelting without the reduction of substantial amounts of iron oxides and are narrow <2 m wide in the plane of the tuyeres, the length of the furnaces is 8–15 m. 
● The main components of such a furnace are as follows: a top, through which the charge is loaded and the gaseous combustion products are discharged; a shaft equipped with tuyeres, through which either a blast for fuel combustion or hot gases are supplied; and an inside crucible with a refractory lining, where the molten products collect. 
● The smelts are tapped through an outside crucible or directly from the inside crucible to a forehearth for the stripping of the slags. The forehearth is often equipped for electric heating.
● Formerly, shaft furnaces were constructed from metal jackets, through which water for cooling circulated, hence the obsolete name for shaft furnaces used in nonferrous metallurgy: the water-jacket furnace. 
● Later, evaporative cooling came to be used instead of water cooling. The shaft is constructed of thick-walled tubing welded into units. Shaft furnaces are usually not lined since iron slags in nonferrous metallurgy readily dissolve refractory materials.

Rotary Kiln

rotary kiln is a pyroprocessing device used to raise materials to a high temperature (calcination) in a continuous process. Materials produced using rotary kilns

Working Principle | Operational Overview

● The kiln is a cylindrical vessel, inclined slightly to the horizontal, which is rotated slowly about its axis. The material to be processed is fed into the upper end of the cylinder. As the kiln rotates, material gradually moves down towards the lower end, and may undergo a certain amount of stirring and mixing. 

● Hot gases pass along the kiln, sometimes in the same direction as the process material, but usually in the opposite direction. The hot gases may be generated in an external furnace, or may be generated by a flame inside the kiln. Such a flame is projected from a burner-pipe or "firing pipe" which acts like a large bunsen burner. The fuel for this may be gas, oil, pulverized petroleum coke or pulverized coal.
Rotary kiln
Cement rotary kiln

Rotary calciners, also commonly called indirect kilns, are used in thermal processing operations where exhaust gases must be minimized, when processing finely divided solids, or where temperature must be tightly controlled along the length of the kiln.

● A calciner is comprised of a rotating drum inside a furnace, which is externally heated. Unlike direct-fired kilns, which utilize direct contact between the material and process gas to carry out processing, in a calciner, heat is transferred from the shell of the externally heated kiln to the bed of material through radiation.

Fluidised Bed Furnace

Fluid bed furnaces are uniquely manufactured to meet the specific requirements of each customer. Our systems are designed for continuous processing of your advanced material, providing more effective chemical reactions and heat transfer.

Working Principle | Operational Overview

● Fluidized bed equipment is extensively used in coating of particles to obtain products with modified superficial properties at different industrial sectors such as chemical, agricultural, pharmaceutical and food. 
● The kilns are designed to send a powerful stream of fluid (gas or liquid) up through your particulate material, suspending and circulating the gas and solid particles. This fluidization produces an even distribution of air among the particles, making the fluid bed perfect for heat transfer and diffusion reactions. 
Fluidised bed principle
Fluidised bed F/c

● In which consider the temperature uniformity, fluidization gas velocity, reaction kinetics, gas/reactant concentrations, and the effects of bed height and diameter to create the exact design for your needs.
● Applications of fluid bed technology are justified with advantages such as the ease of operation and excellent conditions of heat and mass transfer within the bed, uniform particle mixing and uniform temperature gradients. 
● The ability to operate at a continuous state allows manufacturers to produce their materials more efficiently due to the removal of startup conditions in batch processes.

● Fluid Bed Capabilities

  • Preheated or indirect heating using natural gas or microwave technology
  • Feed materials including powder, spray dried granules, or other types of granules/conglomerates

● Typical Fluid Bed Applications

  • Chemical Reactions – carbonates to oxides, oxides to metals, and other chemical reactions
  • Decomposition
  • Calcination

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