What Impurities Need to be Removed to Produce High-Purity Alumina?

July 30, 2024

In high-purity powder materials, purity is the foundation of their special properties and applications. The presence of impurity elements often has a fatal impact on their special properties and high added value, similar to the saying "one bad apple spoils the whole barrel."For example, high-purity alumina is widely used in high-tech industries such as phosphors, transparent ceramics, electronic devices, new energy, catalytic materials, and aerospace materials. It is one of the highest added value and most widely used high-end materials in modern chemical engineering. High-purity alumina has several advantages that make it an ideal medium for sandblasting and shot peening processes:

High Hardness: High-purity alumina has high hardness, which can effectively remove oxide layers, rust, and other impurities from the surface of the workpiece without breaking easily, thereby extending the lifespan of the medium.
High Wear Resistance: Due to its excellent wear resistance, high-purity alumina can maintain stable performance during sandblasting and shot peening processes, reducing wear and replacement frequency, thereby lowering costs.
Chemical Stability: High-purity alumina is chemically stable and does not easily react with the workpiece being treated, ensuring the quality and consistency of the workpiece surface.
High-Temperature Performance: High-purity alumina can withstand high temperatures, making it suitable for sandblasting and shot peening operations in high-temperature environments, avoiding decomposition or deterioration of the medium under high temperatures.

However, in actual production, there can be impurities such as iron, sodium, magnesium, silicon, and calcium. These impurities mostly exist in the form of oxides like Fe₂O₃, Na₂O, CaO, MgO, and CuO. Their varying content can directly affect the performance of the material. For example, Fe₂O₃ can reduce the luminescence efficiency of phosphor materials, and SiO₂ can deteriorate the sintering performance of alumina, thereby greatly limiting the application of high-purity alumina in various fields.

If the alumina used as a medium for sandblasting and shot peening is not of high purity, it can cause the following issues:

Reduced Hardness and Wear Resistance: Alumina with insufficient purity has lower hardness and wear resistance, making it more prone to breaking or wearing out during use, thus reducing the lifespan of the medium and increasing replacement frequency and costs.
Impacts on Treatment Effect: Low-purity alumina may contain impurities that can contaminate the workpiece surface during sandblasting or shot peening, affecting the treatment effect. This can result in uneven treatment marks or residues on the workpiece surface, impacting the final product's quality and appearance.
Chemical Reactions: Low-purity alumina may contain active chemical substances that can react with the workpiece during sandblasting or shot peening, causing corrosion or deterioration of the workpiece surface, thus affecting its performance and lifespan.
Poor Thermal Stability: Low-purity alumina is less stable at high temperatures, which may lead to decomposition or deterioration, affecting the stability and effectiveness of the sandblasting and shot peening process. In high-temperature operations, this can cause the medium to fail quickly.
Increased Dust and Pollution: Low-purity alumina is more likely to produce dust and debris during use, increasing the difficulty of cleaning and maintenance and potentially posing health hazards to operators and causing environmental pollution.

Therefore, using high-purity alumina media is crucial to ensure the effectiveness of the sandblasting and shot peening process and the quality of the workpiece.

The main methods for preparing high-purity alumina include the hydrolysis of aluminum alkoxide, thermal decomposition of ammonium aluminum sulfate, and thermal decomposition of ammonium aluminum carbonate. Currently, research on impurity removal for high-purity alumina focuses primarily on purifying the raw materials involved in these methods.

Iron Impurities

Iron impurities mostly exist in the form of divalent or trivalent oxides in the main product. There is considerable research on the removal of iron impurities from different substances, such as bauxite and isopropyl alcohol aluminum. Bauxite, formed in tropical or subtropical climates, is widely found in various rocks and shales, containing common impurities such as ferrite, goethite, quartz, kyanite, corundum, and titanium dioxide. High-quality bauxite contains at least 40% Al₂O₃ and at most 15% SiO₂. Low-quality bauxite impurities primarily include quartz, Fe₂O₃, and CaO. In the process of recovering alumina from bauxite using sulfuric acid leaching, A Aziz found that using 68% ethanol (C₂H₅OH) almost completely removed the Fe impurities, although ethanol is expensive. To make the process more economical, ethanol can be recovered and reused through condensation.Iron is also commonly found in the preparation of alumina using the aluminum alkoxide method. Isopropyl alcohol aluminum can be purified using extraction-complexation and complexation-crystallization methods, selecting appropriate complexing agents to separate impurities based on the differing solubilities of different substances. This method's advantages include obtaining high-purity isopropyl alcohol aluminum at a low cost, although the process is complex and time-consuming.

When preparing high-purity alumina from ammonium aluminum sulfate, common methods for Fe removal include precipitation, organic primary amine extraction, and recrystallization. Precipitation can be achieved by adding MnO₂, KMnO₄, K₃[Fe(CN)₆], or K₄Fe(CN)₆·3H₂O. This method's advantages include low cost and ease of operation, although it can lead to significant aluminum ion loss and introduce new impurities through the additives used.

Silicon Impurities

Silicon impurities are relatively inert and difficult to remove. When using the aluminum alkoxide hydrolysis method, silicon is one of the many impurities in isopropyl alcohol aluminum, and can be removed using a method that involves adding a lanthanum oxide additive during vacuum distillation. In this process, lanthanum oxide reacts with silicon to form a high-boiling substance, which remains in the reaction vessel after distilling the isopropyl alcohol aluminum, thereby purifying the isopropyl alcohol aluminum. This method is energy-efficient, simple to operate, and has a short reaction time compared to general extraction and crystallization methods.

Si and Fe impurities in roasted samples can be exposed and then removed through repeated ultrasonic acid washing and water washing, resulting in hydroxide alumina with Si and Fe contents below 0.001%. It is also suggested to use activated carbon columns or microporous titanium filtration membranes for raw material filtration to remove impurities. Lime can be used as a desiliconizing agent in sodium aluminate solution, reacting with water to form calcium hydroxide, which then reacts with sodium aluminate to form tricalcium aluminate. The silicate ions react with tricalcium aluminate to form insoluble hydrated garnet, precipitating out and achieving a silicon removal rate of up to 98%.

Some methods may introduce impurities such as Na and Ca, while others are suitable for removing trace amounts of Si but not for deep removal of trace Si. Thus, researching an effective method for removing trace amounts of Si is crucial.

Calcium Impurities

Calcium impurities can be removed through extraction agents, chemical precipitation, salting-out crystallization, ion exchange, and chelation. Initially, the low solubility of calcium sulfate can be utilized to precipitate Ca²⁺ as CaSO₄, followed by secondary removal using methods such as solvent extraction. The extraction agent P204, which is inexpensive and an acidic extractant, follows the extraction order of Fe³⁺ > Zn²⁺ > Cu²⁺ > Co²⁺ > Mg²⁺ > Mn²⁺ > Ca²⁺. H. Xie used P204 to extract calcium ions from a mixture of calcium, magnesium, and manganese, significantly reducing calcium ion concentration, although it did not achieve high purity. R. Zhao effectively removed calcium impurities from phosphogypsum using tri-n-butyl phosphate, obtaining calcium sulfate particles with a purity greater than 99%.

Sodium Impurities

Common methods for removing sodium impurities in alumina preparation include washing, hydrothermal treatment, and adding boric acid. L. Lu used a washing method to remove sodium from ammonium aluminum sulfate crystals, heating the crystals to 150-200°C to facilitate sodium removal. J. Li compared hydrothermal treatment and boric acid addition for sodium removal during alumina preparation, choosing hydrothermal treatment for its higher purity results. H. He added boric acid during aluminum hydroxide calcination to react with sodium impurities to form sodium metaborate, followed by acid washing and drying to obtain alumina with a sodium content below 0.001%.