What is melting? A detailed explanation of different metal melting techniques and melting furnaces.

Melting is a technique with a long history, dating back to ancient times, and is considered the first step in metal working. It is a field in which mankind has been constantly improving its technology since the Bronze Age. There are various methods of melting metals, and today efficient melting techniques and furnaces are used industrially. In this article we will look at the basics of metal melting and the types of furnaces used in the process.

What is melting?

In the context of metals, melting refers to the process of converting solid metal to liquid by heating it above its melting point, causing a phase change.

The term “melting” can have several meanings. For example, dissolving table salt in water to form a solution is also called melting. However, when referring to metals, it specifically means heating the metal to turn it into a liquid, and is sometimes referred to as fusion or liquefaction. Metals have different melting points, or the temperature at which they become liquid, depending on the material. For example, iron-based metals typically melt at temperatures of around 1500°C or higher. Lighter metals such as aluminium have a melting point of around 660°C, while copper-based metals melt at temperatures just above 1000°C. The exact melting point can vary depending on the specific alloy or composition of the metal.

Melting metal requires specialised equipment capable of reaching the necessary temperatures above the melting point. This is where melting furnaces come into play. The type of furnace used depends on the type of metal being melted and its intended application. In particular, industrial melting furnaces have evolved to melt large quantities of metal efficiently.

Throughout history, methods of handling metals have evolved. Melting techniques have undergone numerous innovations, resulting in furnaces that can operate at higher temperatures and with greater efficiency. As technology advances, metal melting furnaces continue to evolve to meet the demands of modern industrial processes.

Differences between combustion and electric furnaces in metal melting.

Melting furnaces can be broadly divided into two main types based on their heat source: combustion furnaces and electric furnaces.

Combustion furnaces generate the heat required for melting by burning various fuels.　There are several types of these furnaces: The direct-fired method, where the material is heated directly by the combustion gases from a burner; the indirect method, which involves combustion in radiant tubes or a muffle, where heat is transferred to the heated material through a partition; the gas circulation heating method, where heated gas is circulated through the furnace and directed at the material; and the jet heating method, where the heated gas is applied in a jet stream to heat the material.

While combustion furnaces offer the advantage of using a variety of fuels, they also produce CO₂ and other greenhouse gases that contribute to global warming.

Electric furnaces use electricity to generate the heat required for melting. There are several types of heat source in electric furnaces, the most common being the electric resistance type, which uses a heating element to generate heat through electrical resistance. An advantage of electric furnaces is their flexibility, as they can be powered by different methods of electricity generation. Depending on the method of power generation, it is possible to move towards carbon neutrality. In addition, unlike combustion furnaces, electric furnaces do not require waste gas treatment systems, making them more compact.

Both combustion and electric furnaces come in a variety of designs, so we will introduce the main types.

Types of combustion melting furnaces.

There are several types of combustion melting furnaces, of which the following are the most representative:

Blast furnace

A blast furnace is a furnace in which iron ore, coke and limestone are fed from the top by a device and hot air is blown in through the tuyères at the bottom to reduce and melt the iron ore to produce pig iron. It is also known as a “smelting furnace”. The fuel used in blast furnaces evolved from charcoal to coal and coke, allowing higher temperatures to be achieved. This development greatly increased the production of iron and laid the foundations for the Industrial Revolution.

Reverberatory furnace

A reverberatory furnace is a furnace in which heat rays and combustion gases generated by a heat source, such as a burner, are transferred to the melting furnace. These heat rays and gases are reflected off the ceiling and walls of the furnace, and by radiation the heat is transferred to the material being heated on the bottom of the furnace, causing it to melt. In practice, the heat absorbed by the furnace roof and walls is transferred to the furnace floor by radiative heat transfer. This process allows the efficient production of high quality pig iron, and as a result, reverberatory furnaces are widely used for melting and refining common metals and alloys. Due to their ability to handle a wide range of treatment temperatures and capacities, reverberatory furnaces are used in a variety of industries including metals, glass, ceramics and chemicals.

Cupola furnace

The cupola furnace is a tall, advanced version of the blast furnace and is also known as a “smelting furnace”. Like the blast furnace, it uses alternating layers of raw materials, such as iron ore and coke, to melt pig iron and scrap iron. The cupola furnace is designed so that raw materials can be added during operation, allowing it to melt large quantities of metal continuously. This makes it an efficient furnace for large-scale operations and essential for the mass production of castings. In addition, the coke used in the furnace replenishes the carbon content and the furnace also helps to refine impurities, which contributes to the production of high quality molten iron. Historically, the cupola has been the dominant melting furnace used in large scale operations. However, with growing concerns over CO₂ emissions and the push for environmental sustainability, many companies are now considering a switch to electric furnaces as a more environmentally friendly alternative.

Converter furnace

A converter furnace is a type of steelmaking furnace used primarily for refining and adjusting the composition of molten metal. Molten pig iron produced in furnaces such as the blast furnace or cupola is transferred to the converter furnace for further refining into steel. The furnace has a structure lined with refractory materials and contains a rotating mechanism mounted on a support shaft. Inside the furnace, oxidising gases are blown into the molten iron to remove carbon and impurities. Compared to other steelmaking furnaces, the converter furnace is known for its shorter refining time and higher productivity.

Types of electric melting furnaces

Electric furnaces all share the common feature of using electricity as their energy source, but they are classified based on the structure of heat generation.

Resistance furnace

A resistance heating furnace is designed to generate heat through the electrical resistance of the heating element itself, using two methods: direct heating and indirect heating. In the direct heating method, electrodes are attached directly to the material to be heated and electricity is passed through them, causing the material to melt due to its own electrical resistance. In the indirect heating method, heat is transferred from the heating element to the material, indirectly heating the entire furnace to melt the material. Indirect heating elements can be made of metals or non-metals. Resistance heating furnaces allow precise temperature control, a wide temperature range and can also be used in vacuum furnaces.

Induced melting furnace

An induction furnace is a furnace that uses electromagnetic induction for heating, where the material to be heated itself becomes the heating element. Electromagnetic induction occurs when a current flows through a coil, creating magnetic field lines. These magnetic field lines pass through the metal, creating eddy currents, which in turn generate Joule heat due to the electrical resistance of the material. As only the magnetic field lines affect the material, melting can take place even in a sealed furnace, allowing high quality molten metal to be produced. In addition, by adjusting the strength of the induced current, the amount of heat generated can be increased, allowing high temperatures to be achieved without relying on the thermal resistance of electrodes or other components.

Arc furnace

An arc furnace is a furnace in which material is melted by the arc discharge generated between the electrodes, or between an electrode and the material to be heated. The arc discharge provides a continuous source of heat that melts the metal instantly, and the temperature is easily controlled by adjusting the current. In addition, there is an indirect heating type of arc furnace where the heat generated by the arc discharge between the electrodes heats the entire furnace, indirectly melting the material.

Specialized melting furnaces

There are types of melting furnaces with specialised functions, and here we introduce four representative types of melting furnaces.

Vacuum melting furnace

A vacuum melting furnace operates in a vacuum environment, effectively blocking air and oxygen to prevent oxidation, which enables the production of molten metal without contamination. This furnace type typically uses induction heating, as the heating element must avoid direct contact with the material. It is ideal for processes like degassing and decarburization of stainless and high-alloy steels, degassing of aluminum and copper alloys, and melting of titanium alloys. Once a vacuum has been established within the furnace, inert gases can be introduced to maintain an inert atmosphere, facilitating melting without oxidation.

Plasma arc melting furnace

A plasma melting furnace uses plasma from an inert gas as a heat source. The plasma arc produces a highly stable and directional arc at temperatures of tens of thousands of degrees Celsius. The stability of the arc offers benefits such as reduced interference with electrical and radio systems, and less noise and dust. While atmospheric discharge arc furnaces can reach temperatures of around 3000°C, discharge in an inert gas such as argon, which has a higher insulation resistance, significantly increases the temperature. This furnace is used for melting metals with higher melting points than iron-based metals, as well as melting various alloys.

Electron beam melting furnace

An electron beam melting furnace melts material by direct irradiation with accelerated electron beams. The process takes place in a high vacuum chamber where the material is melted and then cooled using a water-cooled copper crucible placed inside the chamber, allowing the molten metal to be extracted as high purity ingots. This furnace can achieve even higher temperatures than a plasma melting furnace. The electron beam is created by accelerating electrons to high voltage, forming a beam. Due to the high energy of the electron beam itself, it is often used for high purity refining of high melting point metals, highly reactive metals and rare metals.

Frequently Asked Questions

Q1.Why is there a shift from combustion ovens to electric ovens?

The main reason is environmental. CO₂ emissions and smoke from the operation of blast furnaces and cupolas that use coke as a fuel contribute to environmental pollution. The type of furnace is selected based on the melting point of the material, and temperature control can also be used to produce different alloys.

Q2.What types of metals can be melted in a melting furnace?

Melting furnaces can be used to melt a wide range of metals including ferrous metals, low melting point light metals such as aluminium, heavy metals such as gold, silver and copper, and high melting point metals such as titanium. The type of furnace is selected based on the melting point of the material, and temperature control can also be used to produce different alloys.

Summary

A melting furnace is a specialised piece of equipment used to melt various metals and plays an essential role in the production and refining of metallic materials. The history of melting furnaces dates back to ancient times, when man first began to work with metals, and has evolved over the centuries to handle metals with higher melting points, starting with iron.

As a manufacturer of industrial furnaces, Sun Furnace can provide you with a customized atmosphere gas generation system for your application based on our experience in manufacturing more than 1,500 industrial furnaces. We also offer free technical consultations, so please feel free to contact us.

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