Pyrometallurgy is the extraction and purification of metals by heat-based methods. It entails the heat treatment of minerals, metallurgical ores, and concentrates in order to cause physical and chemical alterations in the materials, allowing for the recovery of valuable metals.
Pyrometallurgy is an extractive metallurgy field that uses high-temperature techniques to extract and purify metals, alloys, or mattes from primary ores and/or secondary resources. Because operating temperatures frequently near 1500°C, pyrometallurgical processes are difficult, high-risk, and energy-intensive. Pyrometallurgy may yield marketable products such as pure metals, intermediate compounds, or alloys suitable for further processing. The oxides of less reactive elements such as iron, copper, zinc, chromium, tin, and manganese are examples of elements recovered through pyrometallurgy.
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Processes Involved in the Pyrometallurgy
Different processes are involved in Pyrometallurgy. they are:
Drying
Drying is the heat removal of unbound liquid moisture from a substance. Typically, moist substances are dried by contacting them with hot combustion gases produced by the combustion of fossil fuels. Heat for drying can be delivered in some situations by hot air or inert gas that has been indirectly heated. The quantity of heat needed for a specific drying process includes the heat needed to vaporize the liquid moisture, the heat needed to increase the temperature of the products (dry solids and water vapor) to the final drying temperature, and the heat needed to balance radiant heat losses.
Several types of industrial dryers are used to dry moist solids, including rotary dryers, fluidized bed dryers, and flash dryers.
Rotatory drier
The rotary dryer is a type of industrial dryer that uses direct contact with heated gas in order to reduce or minimize the liquid moisture content of the material it is treating. A rotary dryer is used to dry metallic and non-metallic minerals, clay in the cement industry, and coal slime in coal mines, among other things. Rotary dryers are frequently used to dry a variety of materials and are simple to use.
Fluidized bed dryers
Fluidized bed dryers (also known as fluid bed dryers) are pieces of equipment that are widely used in the pharmaceutical industry to reduce the moisture content of medicinal powders and granules. The apparatus operates on the fluidization of feed materials.
It has high moisture removal rates because of its outstanding gas-particle constant, which results in high heat and mass transfer rates.
High thermal efficiency is frequently attained when some of the thermal energy is supplied by the internal heat exchanger for drying. It also offers cheaper capital and maintenance costs.
Flash dryers
Many sectors, including agrifood, chemical, and mineral, have employed flash dryers to dry products. Powders, cakes, granules, flakes, pastes, gels, and slurries are among the feed items that can be processed.
Back-mixing of the wet feed with a portion of the dry product is required to produce sufficient conditioned material for slurries, pastes, or sticky materials.
It improves product quality. It is adaptable to different dewatering methods and can be used with friable and non-friable wet feeds.
Spray drying
Spray drying is another type of drying that occurs when the substance to be dried is entirely dissolved in aqueous solution. The solution is sprayed (typically through a specially constructed nozzle) into a heated chamber, and solids crystallize as the water evaporates. The dryer’s water vapor is evacuated, and dry solids are collected, typically in a conical part of the dryer. The particle size and form of solid material produced by a spray dryer are frequently controlled by the concentration of dissolved material in the solution and the design of the atomizing spray nozzle.
Calcination
Calcination (also known as calcining) is a thermal treatment procedure used to cause a thermal breakdown, phase transition, or the elimination of a volatile portion in ores and other solid materials. The calcination process is typically carried out at temperatures lower than the melting point of the finished materials. Calcination differs from roasting in that roasting involves more complex gas-solid reactions between the furnace environment and the solids, whereas calcination occurs in the absence of air.
Roasting
Roasting is a metallurgical process that converts sulfide ores to oxides before smelting. It varies from calcination in that the chemical change in calcining limestone or other carbonate ores is accomplished solely through the application of heat, whereas roasting entails an interaction between gas and the solid ore.
The reaction is exothermic. Roasting is primarily a surface reaction in which the oxide layer forms first and remains as a porous layer through which oxygen can enter into the still unreacted inner sulfide portion of the particle and the SO2 gas generated is released. Sulfur dioxide (SO2) is thus produced and utilized to make sulphuric acid.
For example:
CuS + 1.5O2 → CuO + SO2
2ZnS + 3O2 → 2ZnO + 2SO2
The solid byproduct of roasting is sometimes referred to as “calcine.” If the temperature and gas conditions in sulfide roasting are such that the sulfide feed is entirely oxidized, the process is known as “dead roasting.”
When pre-treating reverberatory or electric smelting furnace feed, the roasting process is sometimes carried out with less than the required amount of oxygen to thoroughly oxidize the feed. In this situation, the technique is known as “partial roasting,” because the sulfur is only partially eliminated. Finally, if the temperature and gas conditions are such that the sulfides in the feed react to generate metal sulfates rather than metal oxides, the process is known as “sulfation roasting.”
When temperature and gas conditions are maintained in such a way that a mixed sulfide feed (for example, a feed containing both copper and iron sulfide) reacts in such a way that one metal forms sulfate and the other forms an oxide, the process is known as “selective roasting” or “selective sulfation.”
Smelting
Chemical reduction, sometimes known as smelting, is a type of extractive metallurgy. The primary purpose of smelting is to extract a metal from its ore. This comprises iron extraction (for steel manufacturing) from iron ore, as well as copper and other base metal extraction from their ores.
To change the oxidation state of the metal ore, it uses a chemical reducing agent, often a carbon-containing fuel such as coke or, in former times, charcoal; however, factories for the electrolytic reduction of aluminum are also commonly referred to as smelters. The carbon or carbon monoxide derived from it eliminates oxygen from the ore, allowing the metal to be extracted. Carbon dioxide and carbon monoxide are produced as a result of the oxidation of carbon.
Sintering
Sintering, also known as frittage, is the process of compacting and creating a solid mass of material using heat or pressure at temperatures below the melting point (in the sinter plant, a mixture of iron ore, fluxes, and coke). It is the agglomeration of small mineral particles into a porous and lumpy mass caused by incipient fusion caused by heat generated by solid fuel burning within the mass itself. The atoms in the substance diffuse across the particle borders, fusing the particles and forming one solid piece. Sintering, in other terms, is a heat treatment that is widely used to increase the strength and structural integrity of a specific material.
Refining
Refining (as in non-metallurgical applications) is the process of purifying an impure material, in this case, a metal. It differs from other processes such as smelting and calcining in that those two involve a chemical change to the raw material, whereas in refining, the end material is usually identical chemically to the original one, only purer. Processes employed include pyrometallurgical and hydrometallurgical procedures.
Advantages of Pyrometallurgy
- Since Pyrometallurgy is a high-temperature technique, the reaction is quick. As a result, more metal is produced.
- The cost of reducing agents and raw materials is cheap.
- The separation of liquid metal and slag aids in the metal extraction process.
- The pyrometallurgy process extracts metals such as Fe, Zn, Pb Al, and Mg.
- The pyrometallurgy technique uses less chemicals than other processes.
- it is compatible with ore cracking.
Disadvantages of Pyrometallurgy
- Pyrometallurgy demands a lot of energy.
- Pyrometallurgy generates toxic substances.
- High demand for construction materials.
- It might be challenging to handle when Pyrometallurgy is combined with ore cracking and metal reduction.
References
- https://www.britannica.com/technology/pyrometallurgy
- https://www.metallurgyfordummies.com/pyrometallurgy.html
- https://chem.libretexts.org/Courses/University_of_Missouri/MU%3A__1330H_(Keller)/23%3A_Metals_and_Metallurgy/23.2%3A_Pyrometallurgy.
- Khalafalla, S.E. et al. (1979). Pyrometallurgical Processes. In: Sohn, H.Y., Wadsworth, M.E. (eds) Rate Processes of Extractive Metallurgy. Springer, Boston, MA. https://doi.org/10.1007/978-1-4684-9117-3_5.
- https://www.slideshare.net/jeenus101/pyrometallurgy
