Metal Melting
The term “melting” refers to the process of transforming solid materials, such as metals, into liquid form by applying temperatures that exceed their melting points. This process is typically conducted to purify metals from impurities, to mix them and create alloys with varying properties compared to the original metals, or to cast them into specific molds for manufacturing metal products and machinery. Specialized furnaces, made from heat-resistant materials, are utilized for melting. In this article, we will explore various types of furnaces used in the metal melting process.
Classification of Melting Furnaces Based on Energy Source
Historically, charcoal was the initial source of thermal energy for melting processes. Over time, bituminous coal and coke became increasingly common. In recent years, petroleum derivatives and electric energy have emerged as the predominant sources. All these energy sources are still utilized to varying degrees, depending on the size of the melting facility and the required production volumes. Melting furnaces can be classified based on their energy sources as follows:
Solid Fuel Furnaces:
- Dome Furnace: Primarily used for melting iron, this furnace consists of a long steel cylinder. Its lower end is supported by bases encased in firebrick, while the upper end connects to a flue and features an opening for charging the furnace with iron and coke. The heat generated from the combustion of coal melts the iron, which collects in the furnace’s bottom trough before being poured into casting molds.
- Flame Reversing Furnace: This furnace comprises two adjacent chambers made of firebrick, sharing a common roof. Its design efficiently directs the flame from the first chamber, known as the furnace or combustion chamber (where coal is burned), to the second chamber, called the melting bowl. While this type of furnace can be used for various metals, its usage has declined in recent years.
Liquid and Gaseous Fuel Furnaces:
- Flame Furnace: Characterized by a long trough made of firebrick and refractory earth, this furnace includes a flame channel at one end where gas mixes with air to ignite, subsequently guiding it into the melting bowl, while the other end features a chimney for exhaust gases.
- Crucible Furnace: This type consists of a hollow steel cylinder with an opening on the top connected to a flame channel on one side. The metals to be melted are placed inside a crucible, which is then positioned within the furnace. Flame is applied to the crucible without direct contact with the metals, making this furnace ideal for small-scale production facilities.
Electric Furnaces:
Electric furnaces utilize electrical energy for heating and melting metals through one of the following heating methods:
- Resistance Heating: This can be either direct or indirect. In direct heating, the furnace consists of a hollow cylinder installed horizontally on rollers, connected to a power supply via a graphite column extending along its axis, which leads to the heating and melting of metals. In indirect heating, a crucible similar to those used in liquid fuel furnaces is employed, but it is encapsulated with electrical resistors that provide the necessary thermal energy.
- Electric Arc Furnaces: This type features a cylinder with a concave base and a convex ceiling, from which metals are charged. Three steel electrodes hang from the ceiling, capable of vertical movement, generating the heat required to melt the metals.