Refractories for Channel Induction Furnaces: Applications and Development

Refractories are used very widely—in everyday life, high-temperature industries, aerospace and more. Here we introduce the application and development of refractories in channel (cored) induction furnaces.

A channel induction furnace has an iron core passing through the inductor and is powered at mains frequency, using electromagnetic induction to heat and melt the metal charge. It offers fast heating, high productivity, low metal loss, easy operation, low investment and significant economic benefits, and is widely used for melting iron, copper, zinc and aluminum—particularly advantageous for large-volume melting of a single product.

The choice of refractories for a channel induction furnace is critical and directly determines furnace performance. They are divided into backing refractories and lining refractories. Backing materials are generally ceramic insulating paper, insulating bricks and dense bricks; lining materials for the furnace body, throat and inductor are selected according to the metal being melted, the melting temperature and other factors.

1. Structure and working principle of channel induction furnaces

The furnace consists of an upper and a lower body. The upper body includes the cover and hearth; the lower body is the inductor, which contains the melting channel. The furnace acts like a transformer with an iron core: the induction coil is the primary winding and the metal in the channel is the secondary. When mains-frequency AC is applied to the primary, a large current is induced in the channel metal. Through thermal convection and electromagnetic forces, heat is continuously transferred from the channel to the metal in the bath above, melting the whole bath. Because the furnace can only operate with the channel closed (filled), it is suited to continuous operation.

2. Refractory requirements for each part of a channel induction furnace

By location, the furnace is divided into the cover, hearth, throat and channel. The throat and channel are the key areas that directly determine furnace life and operating cost.

2.1 Cover

Since it does not contact molten metal, the cover has relatively relaxed requirements: it must resist high-temperature radiation, insulate well, have high bonding strength and not fall away. It is usually a composite of an insulating layer and a hot-face layer, i.e., lightweight insulating material plus dense refractory.

2.2 Hearth

Hearth lining life depends mainly on reactions between the refractory and corrosive components in the slag. The refractory must therefore be chemically stable under prolonged exposure to high-temperature molten metal and resist attack by metal and slag. It needs sufficient cold and hot strength to withstand strong erosion by molten metal and the mechanical loads on the furnace body, as well as good volume stability—especially at the metal inlet and tap hole—with no excessive expansion or shrinkage at high temperature or after cooling.

2.3 Inductor

The inductor includes the channel, sealing mix, patching (leveling) mix and parting compound.

Channel: In a channel furnace the molten metal is heated by induction in the channel and raised in temperature or held as it circulates. The dry vibratable mix must therefore withstand high temperatures for long periods without softening, resist erosion and corrosion, and have good chemical stability and high strength.

Sealing mix: should be the same as or similar to the channel material, with high refractoriness and appropriate low- and high-temperature strength.

Patching mix: requires a small critical particle size, high refractoriness and good trowelability.

Parting compound: requires very high refractoriness to prevent the mating faces from sintering together and to ensure no metal leaks through the joint.

3. Selecting refractories for channel induction furnaces

3.1 Iron melting furnaces

Mains-frequency channel furnaces have higher efficiency and power factor than coreless furnaces and lose less heat, making them more economical. In large furnaces the iron temperature is stable and long holding causes little loss of alloying elements, so they are used to superheat iron and raise pouring temperature, markedly improving casting quality—ideal for high-volume continuous production. Many large enterprises now use channel induction furnaces of 30 t or more. Once started, these furnaces cannot be stopped at will—the longer the continuous campaign, the better—and forced shutdowns are often caused by lining damage. Analyzing the causes of lining failure and extending refractory life is therefore key to realizing the equipment's advantages.

Channel furnaces for iron are divided into grey iron and ductile iron furnaces. Grey iron is tapped at 1420–1470°C and the slag in grey iron melting is acidic; ductile iron is tapped at 1530°C and the slag alternates between acidic and basic as the metallurgical process proceeds, so a neutral lining refractory is required. After tapping, temperatures change sharply—even with a heel of iron retained, they drop by 300–400°C, and further still when the inductor is changed—so the hearth lining needs good thermal shock resistance. It must also withstand attack by iron, slag and nodularizer, as well as mechanical erosion from the churning iron and fluctuating bath level, requiring adequate hot strength and erosion resistance.

Hearth damage mainly takes the form of lining cracks after tapping: irregular, fairly wide cracks that cause large spalls where longitudinal and transverse cracks meet. Iron penetration accelerates cracking, with the metal inlet suffering the most damage. Based on analysis of inlet erosion, wall spalling and wear, and inductor build-up and channel clogging, suitable refractories are selected:

Cover: mostly lined with aluminum silicate fiber; alternatively, the insulating layer uses calcium-aluminate-cement-bonded castable based on lightweight high-alumina bauxite brick powder and hollow-sphere grains, with anchors or anchor bricks—usually heat-resistant steel studs and mesh—securing it to the cover shell.

Hearth: low-cement castable using fused corundum as aggregate and fines, bonded with pure calcium aluminate cement and SiO2 and Al2O3 ultrafine powders. To improve thermal shock resistance, a suitable amount of low-iron SiC powder can be added. Hearth lining life depends largely on the life and replacement frequency of the inductor refractories, typically 2–3 years.

Inductor: includes the channel, sealing mix, patching mix and parting compound. Channel: MgO–Al2O3 dry vibratable mixes or castables with MgO > 79% are widely used. The sealing mix is generally a dry ramming mix or wet castable of the same material as the channel; for Al2O3-based sealing mixes, pure calcium aluminate cement or phosphoric acid can be used as binder—phosphoric acid binder in particular prevents bursting at the joint during quick inductor changes and improves joining speed and efficiency. The patching mix is generally an Al2O3–SiO2 wet trowelling mix with a critical particle size of 2 mm. The parting compound is generally Cr2O3 powder, whose sintering temperature is higher than the Al2O3 hearth material or the MgO–Al2O3 inductor material, so essentially no ceramic sintered layer forms at service temperature, making it easier to separate the inductor from the throat refractory.

Practice has shown that low-cement corundum castables in the body of mains-frequency channel furnaces, exposed for long periods to erosion and corrosion by melts at 1500°C, show no cracking or spalling, with service life exceeding three years. MgO–Al2O3 dry vibratable mixes and their matching sealing, patching and parting materials last more than six months in 700 kW inductors and 1–2 years in other inductors.

3.2 Copper melting furnaces

Mains-frequency channel induction furnaces offer high electrical efficiency, fast heating and low metal loss, and more than 90% of the world's brass and copper alloys are melted in them. Common copper melting furnace capacities in China are 1.5 t, 2.0 t and 5.0 t, with some at 30 t. Copper is melted at 1200–1350°C, with channel temperatures reaching 1350–1450°C.

Molten copper has low viscosity, high density and strong penetrating power; penetrated copper oxidizes easily with volume expansion, causing the lining to craze and spall and shortening lining life.

Lining life depends on many factors: refractory selection, installation method, heat-up schedule, channel maintenance and monitoring, channel shape and furnace design. In Chinese mains-frequency channel furnaces for copper, quartz sand ramming mixes are cheap and quick to dry out but short-lived; with higher-grade quartz sand and careful installation, upkeep and maintenance, life generally reaches one year. Phosphate-bonded ramming or dry vibratable mixes of high-alumina bauxite with steel fibers last 4–7 years.

A correct installation method is the basic prerequisite for a successful start-up of a copper melting furnace. Choose a suitable ramming mix with a sound particle size distribution; adding a proportion of andalusite and silicon carbide is an effective way to increase sintered strength. During installation, feed material in layers of decreasing thickness, roughen the surface of each rammed layer and remove fines to improve bonding between layers. Ramming outward ring by ring from the center in a radial pattern, using two-stage compaction and a three-stage heat-up and start-up, effectively extends furnace life.

Upper body lining life and inductor life are generally not synchronized; the inductor's life is much shorter. One solution is to ram different materials according to the copper alloy being melted—for example, quartz sand ramming mix for brass and silicon carbide ramming mix for bronze and pure copper. Most enterprises use well-sintered high-alumina bauxite with mullite and corundum as the main phases for the inductor channel, adding a suitable amount of fine SiC to improve wetting behavior and resist strong copper penetration. Installation by phosphate-bonded ramming, or dry vibration forming with a sintering aid, has given good results.

3.3 Zinc melting furnaces

Mains-frequency channel induction furnaces are widely used in zinc processing, including melting and casting electrolytic cathode zinc, hot-dip galvanizing and zinc alloys. Zinc melts at 419.5°C; during normal galvanizing the bath is at about 460–485°C and the channel at about 600–670°C. Molten zinc flows very easily and penetrates strongly; when the lining is attacked and cracks, zinc enters the cracks and damages the furnace, clogging the channel mouth and causing zinc leakage from the inductor.

Zinc furnace linings currently use high-alumina bauxite castables or ramming mixes with about 85% Al2O3, with service life of 4–8 years or more. Inductor life depends not only on the coil, cooling and insulation technology but also closely on the life of the channel refractory. Large twin-channel inductors currently use wet castables, while single-channel inductors mainly use dry ramming mixes and wet castables, mostly mullite or high-alumina bauxite, with a life of about six months.

3.4 Aluminum melting furnaces

Because channel furnaces allow continuous operation with low energy use, many large enterprises use them to melt aluminum ingots, scrap and aluminum alloys. Aluminum melts at 660°C; the melt has low viscosity, strong penetrating power and high chemical activity, acting as a strong reducing agent at high temperature and readily reacting with refractories. Especially at the refractory–melt–gas triple interface, Al and its alloying elements such as Mg, Si and Mn readily penetrate the refractory and undergo redox reactions, degrading it and loosening its structure.

The lining of a channel furnace comprises four refractory areas: the cover (essentially the same as for iron furnaces), hearth, throat and inductor channel.

In recent years hearths have generally used high-alumina bauxite castables with corundum and mullite as main phases, bonded with pure calcium aluminate cement. With an added expansion agent they show high medium-temperature strength, an expansive state, and resistance to penetration and corrosion, with service life of 4–5 years.

The throat and channel are structurally complex and mainly use unshaped refractories—chiefly high-alumina castables with corundum and mullite as main phases—which show no signs of reaction in service and resist erosion and wear. For aluminum furnaces, particular attention must be paid to melt penetration: when aluminum enters pores and cracks, it reacts with the refractory, causing volume effects that damage the lining and shorten furnace life. Current non-wetting castables for the aluminum industry achieve good resistance to aluminum wetting by adding additives such as BaSO4; chrome corundum and boron nitride also work well but are costly. Adding BaSO4 or CaF2 together with Na3AlF6 and homogenizing the castable matrix gives even better resistance to aluminum penetration.

4. Maintenance of channel induction furnaces

Proper maintenance and monitoring, dealing with problems promptly—especially repairing lining damage as soon as it is found—are key to keeping the furnace running continuously and extending its life.

Maintenance and monitoring

During normal operation, monitoring and control of the lining—especially the channel—should be strengthened, and routine maintenance carried out. During start-up, process factors must be strictly controlled to prevent channel breakage, including casting quality of the channel former, density of the rammed material around the channel, water-jacket cooling flow, hearth temperature, channel temperature and input power.

Channel failure has long been the main problem with channel induction furnaces; mishandling can easily scrap the furnace and halt the entire production line. For routine lining maintenance, the following should be observed:

(1) Never cut off water or power arbitrarily

(2) Take special care when charging to avoid damaging the furnace bottom, and promptly clear slag above the channel

(3) Regularly monitor current changes in the melting and holding furnaces

(4) Observe the shape of the channel

(5) Minimize shutdowns; improper operation often causes channel breakage

(6) Regularly remove slag from the hearth and check for lumps of charge

(7) Monitor the temperature difference between cooling water inlet and outlet

(8) Regularly measure the surface temperature of the furnace body and inductor and compare the results. Alternate melting and holding cycles to prevent channel breakage during shutdowns.