The Impact of Sodium Oxide on White Fused Alumina: Performance Changes and Control Measures

In the production and application of white fused alumina (WFA), the content of sodium oxide (Na₂O) significantly affects the material’s physical properties, chemical performance, and application outcomes. This report provides an in-depth analysis of the mechanisms by which Na₂O influences various aspects of WFA performance, as well as industrial strategies for controlling Na₂O content to enhance WFA quality.

Basic Characteristics of White Fused Alumina and Forms of Na₂O

White fused alumina is an artificial abrasive produced by melting and recrystallizing high-purity alumina at high temperatures. Its main crystal phase is α-Al₂O₃ (alpha-alumina), featuring high hardness, excellent toughness, and outstanding heat resistance. Although WFA can reach purities above 99%, it still contains minor impurities, among which Na₂O is a key factor affecting performance.

Forms of Na₂O in White Fused Alumina

Na₂O exists in WFA mainly in three forms:

1. Sodium silicate-bound alkali

2. Surface-attached sodium salts

3. Lattice alkali formed during the decomposition of sodium aluminate solution

Lattice alkali is the most abundant and the hardest to remove. During high-temperature smelting, Na₂O reacts with Al₂O₃ to form β-Al₂O₃ (Na₂O·11Al₂O₃). As the Na₂O content increases, the amount of β-Al₂O₃ formed also rises.

The Effect of Na₂O on the Crystal Structure of WFA

1. Formation and Characteristics of β-Al₂O₃

When the WFA melt cools and crystallizes, β-Al₂O₃, which has a lower melting point and density, tends to segregate in the upper and middle parts of the ingot. β-Al₂O₃ has a layered structure, with sodium atoms located in discontinuous layers separated by layers of aluminum and oxygen atoms, forming a spinel structure. Its ideal chemical formula is Na₂O·11Al₂O₃.

2. Inhibition of α-Al₂O₃ Phase Transformation

Studies show that even low Na₂O content (wt 0.14%) can inhibit the transformation of transitional phases to α-Al₂O₃. This is mainly because Na₂O reacts with alumina to form corresponding aluminates, creating spatial hindrance and reducing the transformation rate. This inhibition directly affects the proportion of α-Al₂O₃ in WFA, thereby impacting its overall performance.

The Impact of Na₂O on the Physical Properties of WFA

1. Hardness Reduction

The hardness of β-Al₂O₃ (1300–1600 kg/mm²) is significantly lower than that of α-Al₂O₃. Low-sodium WFA can reach a microhardness of 2300–2400 kg/mm², while regular WFA is about 2200 kg/mm². High Na₂O content leads to reduced WFA crystal hardness, directly affecting its cutting and grinding efficiency.

2. Decreased Density and Strength

Since β-Al₂O₃ has a lower density than α-Al₂O₃, high Na₂O content reduces both the bulk and particle density of WFA. Low-sodium WFA has higher bulk density and specific gravity than regular WFA, and a lower porosity. High Na₂O content also leads to decreased compressive and flexural strength.

3. Reduced Thermal Stability

The melting point of β-Al₂O₃ is lower than that of α-Al₂O₃, severely affecting the refractory performance of WFA. High Na₂O content reduces WFA’s thermal stability, making the material more prone to softening or melting at high temperatures.

The Impact of Na₂O on WFA Application Performance

1. Grinding and Cutting Performance

High Na₂O content reduces the crystal hardness of WFA, thus lowering its cutting ability. In abrasive applications, low-sodium WFA offers improved durability, higher particle strength, and is less prone to fragmentation and powdering. This increases the number of sandblasting cycles and, when used in grinding wheels, enhances grinding force and efficiency.

2. Refractory Performance

In the refractory industry, high-strength low-sodium WFA is used as aggregate and castable, improving the high-temperature resistance, creep resistance, and service life of refractory products. Low-sodium WFA’s low porosity enhances slag resistance and the high-temperature load softening point of refractory products.

3. Other Application Fields

In electronic ceramics, the Na₂O content significantly affects material performance. Since β-Al₂O₃ is somewhat conductive, electronic ceramic powders generally require Na₂O content below 0.1% to avoid affecting electrical properties. Control of Na₂O is also important in pharmaceutical, chemical, and catalyst applications.

Control Measures for Na₂O Content in WFA

1. Selection of Low-Sodium Raw Materials

Choosing low-sodium alumina powder as the raw material is a direct way to control Na₂O content in finished WFA. The industry has developed low-sodium alumina with Na₂O content below 0.2% and micro-sodium alumina below 0.1%, suitable for producing low-sodium WFA.

2. Control During the Smelting Process

During smelting, a small amount of aluminum fluoride can be added to promote the volatilization of Na₂O at high temperatures. This method effectively reduces Na₂O content in WFA, but also produces some harmful fluoride gases, which can impact the environment.

3. Post-Treatment Technologies

Industrial de-sodium methods such as acid-base treatment can remove some Na₂O, but often have low efficiency, high energy consumption, and cause pollution. New techniques like the "co-dissolution method" have achieved better results and can more effectively control Na₂O content.

Classification and Performance Indicators of Low-Sodium WFA

Classification by Na₂O Content

Based on Na₂O content, WFA can be classified as follows:

1. Regular WFA: Na₂O content 0.3–0.4%

2. Low-sodium WFA: Na₂O content ≤0.2%

3. Micro-sodium WFA: Na₂O content ≤0.1%

Comparative Performance Indicators

Compared to regular WFA, low-sodium and micro-sodium WFA offer the following advantages:

1. Higher hardness: Microhardness of 2300–2400 kg/mm² for low-sodium WFA vs. 2200 kg/mm² for regular WFA

2. Higher compressive and flexural strength: Improves high-temperature and creep resistance of refractory products

3. Higher refractory temperature: Provides better stability in high-temperature environments

4. Higher bulk density and specific gravity: Lower porosity, stronger slag resistance

5. Higher α-Al₂O₃ content: Improves overall product performance

Conclusion

Na₂O, as a major impurity in WFA, has a significant impact on its properties. Na₂O mainly forms β-Al₂O₃ with Al₂O₃, reducing the hardness, density, strength, and thermal stability of WFA. As industrial applications demand higher WFA quality, controlling Na₂O content is key to improving performance. Low-sodium and micro-sodium WFA, with their superior properties, have broader applications in refractories, abrasives, electronic ceramics, and other high-end fields.

In WFA production, selecting low-sodium raw materials, adding auxiliary agents to promote Na₂O volatilization, and adopting advanced post-treatment technologies are effective ways to control Na₂O content. With ongoing technological advancements, it is expected that WFA with even lower Na₂O content and better performance will be developed to meet the demands of more high-end applications.

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