Differences in Smelting Process And Raw Materials Of 5 Common Fused Alumina Types

Fused alumina is a product formed by melting alumina in an electric furnace, followed by cooling and solidification. It has a crystal structure of α-Al₂O₃.

Classified by grade, fused alumina includes fused brown alumina, fused sub-white alumina, fused white alumina, fused dense alumina, and fused tabular alumina. Their properties vary depending on the processing principles used.

Fused brown alumina is a brown fused material produced by proportioning and mixing three raw materials-high-alumina bauxite clinker, carbon materials, and iron filings. The mixture is then fed into an electric arc furnace, where it undergoes high-temperature melting and impurity reduction, and is finally cooled to form crystals. Its main chemical component is Al₂O₃, with a content ranging from 94.5% to 97%.

1.1 Formation Principle of Brown Fused Alumina

The smelting of brown fused alumina is based on the fundamental principle that aluminum has a stronger affinity for oxygen than iron, silicon, titanium, and other elements. By controlling the dosage of reducing agents, the main impurities in bauxite can be removed through reduction smelting. The reduced impurities form ferrosilicon alloys and separate from the molten alumina, yielding brown fused alumina with qualified crystal quality and an alumina content of over 94.5%.

Theoretically, when producing fused alumina directly from high-alumina bauxite, adding sufficient carbon and controlling the temperature within the range of 1800–1900°C can reduce most oxides in bauxite (except Al₂O₃ and CaO) into metallic substances for separation, while keeping Al₂O₃ stable.

Among the reduced metallic elements, potassium, sodium, and magnesium volatilize in gaseous form, while most metals such as iron, silicon, and titanium sink in the form of alloys. This process achieves the separation of Al₂O₃ from other oxides.

The relevant reduction reactions are as follows:

3Fe₂O₃+C→2Fe₃O₄+CO↑

Fe₃O₄+C→3FeO+CO↑

FeO+C→Fe+CO↑

SiO₂+Fe+2C→FeSi+2CO↑

TiO₂+3C→TiC+2CO↑

TiO₂+2C+3Fe→Fe₃Ti+2CO↑

1.2 Raw Materials for Smelting Brown Fused Alumina

(1) High-Alumina Bauxite

High-alumina bauxite is generally required to contain more than 76% Al₂O₃, less than 7.0% Fe₂O₃, less than 5.5% TiO₂, and less than 1.2% CaO + MgO.

The particle size of high-alumina bauxite should be neither too coarse nor too fine and is normally controlled within the range of 10–25 mm.

(2) Carbon Materials

Carbon materials act as reducing agents in fused alumina smelting, reducing impurities contained in high-alumina bauxite. Commonly used carbon materials include petroleum coke, coke, and anthracite.

The effective component of carbon materials for reduction is fixed carbon, and a higher content is preferable. Volatile matter and ash are detrimental to the smelting process and product quality, so their contents must be strictly controlled.

Excessive volatile matter and moisture generate a large amount of gas during smelting, which adversely affects the stability of furnace operation.

(3) Iron Filings

Iron filings are also used as diluents. Since the iron oxide content in high-alumina bauxite in China is relatively low, a certain proportion of iron filings is usually added to the raw material mixture for alumina smelting. Cast iron filings are most commonly used.

Quality requirements for iron filings include chemical composition and particle size. Small-sized iron filings are easy to convey and are suitable for fused alumina production. The aluminum content in iron filings must be strictly controlled.

The amount of iron filings added is determined by the SiO₂ content in high-alumina bauxite. The iron oxide content in bauxite also affects the required dosage of iron filings; iron generated from the reduction of iron oxide can partially reduce the amount of iron filings that needs to be added.

1.3 Smelting Process

The smelting process of brown fused alumina is mainly divided into four stages: furnace startup, melting, regulation, and refining.

The melting stage accounts for over 80% of the total smelting time. During this stage, the furnace charge melts into a liquid state, and impurities are reduced to form ferrosilicon alloys, which then separate from the molten alumina.

Refining is the final stage. Its purpose is to fully reduce residual impurities, allow the ferrosilicon alloy to settle and aggregate sufficiently, and ensure the smooth discharge of furnace gases.

The main smelting methods include the pouring method, the ingot method, and the lapping method.

In recent years, institutions such as the Sinosteel Luoyang Institute of Refractories Research have adopted a reduction smelting method to utilize China's abundant high-alumina bauxite resources to produce off-white alumina with an Al₂O₃ content of no less than 98.5%, commonly known as sub-white alumina.

2.1 Formation Principle of Fused Sub-White Alumina

The smelting mechanism of sub-white alumina differs from that of brown fused alumina. The production of brown fused alumina mainly reduces SiO₂ and Fe₂O₃ in bauxite into elemental silicon and iron for separation.

In contrast, sub-white alumina production not only reduces SiO₂ and Fe₂O₃ into silicon and iron for removal but also minimizes other oxides aside from Al₂O₃. In particular, TiO₂ in bauxite must be reduced to metallic titanium and separated.

To reduce TiO₂, a relatively stable impurity with a high reduction temperature, a large amount of carbon must be added. This inevitably leads to the formation of aluminum carbide (Al₄C₃), which is a harmful impurity in fused alumina ingots.

Al₄C₃ will hydrolyze when exposed to water or stored in humid air, releasing methane (CH₄) and causing the alumina grains to pulverize.

Therefore, decarburization treatment must be carried out in the later stage of smelting to suppress the formation of Al₄C₃. Common practices include adding decarburizing agents such as mill scale or introducing oxygen during the later smelting period. The carbon in the melt reacts with the oxidizing agent to generate CO gas, which escapes, thereby preventing the formation of Al₄C₃.

Oxygen blowing into the melt is also used, allowing carbon in the melt to convert into CO or CO₂ and escape, thereby preventing carbide formation.

In addition, adding high-purity silica powder in the later melting stage can further reduce and limit carbide formation as well as the free carbon content in alumina. The relevant reaction equations are shown below:

Al₄C₃+3Fe₂O₃→2Al₂O₃+6Fe+3CO↑

Al₄C₃+9FeO→2Al₂O₃+9Fe+3CO↑

2Al₄C₃+9SiO₂→4Al₂O₃+9Si+6CO↑

The elemental iron and silicon produced by the reaction form a ferrosilicon alloy, which sinks to the lower part of the molten liquid and separates from the fused alumina.

2.2 Raw Material Requirements for Sub-White Fused Alumina Smelting

The raw material requirements for smelting sub-white fused alumina are stricter than those for brown fused alumina. In particular, higher standards are set for high-alumina bauxite clinker. It not only requires a high Al₂O₃ content but also requires the content of difficult-to-reduce oxides to be as low as possible; otherwise, the product quality requirements cannot be met.

2.3 Smelting Process of Sub-White Fused Alumina

The smelting process of sub-white fused alumina (Al₂O₃ ≥ 98% and Al₂O₃ ≥ 98.5%) differs from that of brown fused alumina. It is characterized by the addition of excess carbon for reduction in the middle stage, followed by the addition of decarburizing agents for treatment during the later refining and clarification stage. In other words, the early and middle stages are reduction smelting periods, while the later stage is an oxidative refining period.

Oxygen blowing for decarburization is an effective method that does not introduce additional impurities; however, it requires dedicated oxygen blowing equipment and oxygen lances.

The dosage of the decarburizing agent is determined by the color of the sampling rod after the completion of the reduction smelting stage, and the feeding rate of the decarburizing agent (mill scale) is controlled according to the reaction intensity. Generally, mill scale is added in several batches.

Pickling treatment can remove residual substances after mechanical processing, thereby increasing the Al₂O₃ content and reducing the Fe₂O₃ content. Heat treatment by high-temperature oxidation can also separate free carbon from alumina and oxidize carbides.

Fused white alumina is a white fused material produced using industrial alumina powder as the raw material. It is melted in an electric arc furnace, followed by cooling and recrystallization. Its main chemical component is Al₂O₃, with a content of over 99% and extremely low impurity levels.

3.1 Formation Principle of White Fused Alumina

The smelting process of white fused alumina is essentially a process of melting and recrystallization of industrial alumina powder, with no reduction reactions involved.

Industrial alumina contains more than 98.5% Al₂O₃, along with small amounts of impurities such as Na₂O, SiO₂, and trace Fe₂O₃. Electric melting provides a certain degree of purification; however, it cannot completely remove these impurities.

In the molten state, Na₂O reacts with Al₂O₃ to form β-Al₂O₃ (Na₂O·11Al₂O₃), and the amount formed increases with higher Na₂O content.

Due to the low melting point and low density of β-Al₂O₃, it segregates in the upper and middle portions of the ingot during cooling and crystallization. Although it can be partially removed by crushing and sorting, a small portion still remains in the alumina matrix, significantly degrading the refractory performance of white fused alumina ingots. Therefore, the Na₂O content in industrial alumina must be strictly controlled.

To eliminate or reduce the harmful effects of Na₂O, quartz sand or aluminum fluoride (AlF₃) is added. Quartz sand converts β-Al₂O₃ into nepheline, while AlF₃ promotes the volatilization of Na₂O.

3.2 Smelting Process

The smelting procedure for white fused alumina is basically the same as that of brown fused alumina and is also divided into four stages: furnace startup, melting, regulation, and refining.

Its smelting methods are similar to those of brown fused alumina, including the pouring method, ingot method, and lapping method. In general, the smelting process of brown fused alumina is relatively simpler.

To meet the requirements of metallurgical technological development for the quality and variety of refractory materials, dense alumina is produced by electric melting using industrial alumina as the raw material.

This type of alumina ingot is light gray in appearance and features high purity, low porosity, and high bulk density. It has been used to replace imported products in the production of iron trough castables, achieving remarkable application results in large blast furnaces at Baosteel Shanghai.

Industrial alumina powder is proportionally mixed with additives and charged into a tilting electric arc furnace for complete melting under oxidizing conditions.

In the presence of carbon, and based on the relationship between the standard Gibbs free energy of oxides and temperature, sodium oxide and potassium oxide above 1000°C can be reduced to alkali metal vapors (Na, K) and removed. The reaction equations are as follows:

Na₂O+C→2Na+CO↑

K₂O+C→2K+CO↑

The oxidizing process helps remove residual gases and carbonaceous substances from the molten material.

Unreasonable melting parameters and insufficient refining time may lead to excessive reduction of alumina and the formation of aluminum–carbon compounds. Alumina containing aluminum–carbon compounds will pulverize when exposed to moisture or water, undergo acidolysis when reacting with acids, and disintegrate during calcination. Castables and ramming materials made from such alumina may crack during baking.

To reduce or avoid excessive reduction of Al₂O₃ during melting, a small amount of SiO₂ is added during the smelting of industrial alumina. It promotes the volatilization of Na₂O and simultaneously helps remove excess carbon. The relevant reaction equations are:

SiO₂+C→SiO↑+CO↑SiO+C→Si↑+CO↑

Therefore, dense fused alumina can only be obtained by mastering proper melting operation techniques, controlling the refining process, and ensuring the complete removal of potassium and sodium vapors.

The fully melted liquid is poured into a ladle and cooled at a controlled rate to produce qualified dense alumina ingots.

Tabular alumina was originally developed by Alcoa (Aluminum Company of America). Sintered alumina produced by the sintering method is a polycrystalline aggregate of corundum composed of tabular crystals. Based on the process parameters for preparing tabular alumina via the sintering method, Tianjin University successfully developed tabular alumina using an electric melting process.

The ultra-high-temperature conditions of the electric melting method, along with the transition of alumina raw materials from unmelted to semi-molten states, promote the volatilization of sodium oxide. Accordingly, the formation process of tabular alumina is also a self-purification and refining process accompanied by the volatilization of impurity oxides such as Na₂O.

Fused tabular alumina forms during the rapid growth of alumina grains. The formation of a large number of closed pores inside the crystals is closely related to the rapid grain growth rate of tabular alumina. The migration rate of corundum grain boundaries is higher than the movement rate of closed pores toward grain boundaries and their escape through them. As a result, closed pores inside the crystals merge and grow larger, making them difficult to fully remove. In addition, the volatilization of Na₂O in alumina raw materials in the presence of liquid phases is also one of the causes of internal closed pore formation in fused tabular alumina.

Industrial alumina raw materials with suitable chemical composition and particle size for tabular alumina production are placed into a dish-shaped layer of a certain thickness inside an electric arc furnace. After arc ignition, feeding and melting are carried out simultaneously. The melting regime is controlled to maintain a stable temperature zone of 1900–2000°C within the furnace charge during smelting.

Practice has shown that by reasonably controlling voltage and current parameters according to the model and capacity of the electric arc furnace, and maintaining an appropriate melting rate, it is feasible to stably maintain a temperature zone of around 1900–2000°C. In this ultra-high-temperature zone, alumina crystals rapidly grow in two-dimensional directions, achieving the tabular structure of alumina grains.

As a high-grade refractory material, fused alumina exhibits excellent refractoriness, corrosion resistance, slag resistance, erosion resistance, and high-temperature strength. It is widely used in various metallurgical equipment and high-temperature furnaces. In addition, fused alumina is also used as a raw material in various coating materials and ramming mixes to improve their overall properties.

Fused cast alumina products, especially fused cast alumina bricks, have an even wider range of practical applications. The production process of fused cast alumina bricks consists of melting, casting, cooling and annealing, and mechanical processing. Each stage has a significant impact on product quality and directly affects the service life of high-temperature thermal equipment.

Therefore, the production technologies of fused alumina and fused cast alumina bricks still require further development and improvement, with a direction toward mechanized production.