Cerium-based rare earth polishing serbuk is the most popular rare earth polishing powder. It has excellent polishing performance, improving surface smoothness. It is known as the “king of polishing powders.” The glass processing and electronics industries are the main downstream applications. About 70% of polishing powder waste is from polishing failure each year. The waste mainly consists of rare earth powder residues, waste liquids, glass fragments, and polishing cloth residues. The rare earth content in the waste is around 50%. Handling the waste rare earth polishing powder has become a major challenge for downstream companies. Currently, the common methods for recycling waste rare earth polishing powder are physical separation and kimia separation.
Physical separation
Pengapungan
In recent years, flotation technology has been widely applied in solid waste treatment. The hydrophilic differences in the components of waste rare earth polishing powder are utilized. By selecting different flotation reagents, the affinity of components in aqueous solution is improved. Hydrophilic particles remain in the water, achieving separation. However, the saiz Zarah of polishing powder affects flotation recovery, and purity is insufficient.
When selecting different collectors, the impurity removal effect varies significantly. Research foundings that at pH 5 with styrene phosphonic acid, the recovery rate of cerium and lanthanum oxide reached 95%. The recovery rate of calcium fluoride and fluorapatite was only 20%. Particles smaller than 5 microns need further separation to remove impurities due to poor flotation.
Pemisahan magnet
Waste rare earth polishing powder is magnetic, which allows its recovery. Mishima and others designed a device with a vertical magnetic field to recover rare earth polishing slurry. When the slurry flow rate is 20 mm/s, cycle time is 30 minutes, slurry concentration is 5%, and pH is 3, the separation efficiency of cerium oxide and iron flocculant can reach 80%. If the magnetic field direction is changed to a horizontal gradient and MnCl2 solution is added, silicon dioxide and alumina with opposite magnetic properties can be separated from cerium oxide.
Other methods
The waste slurry, which is difficult for particles to settle, is frozen at -10°C. After being thawed in a 25°C environment, impurities and rare earth oxides form layers. This process makes it easier to collect and recover the useful substances from the waste.
Chemical separation
Chemical methods mainly use acid leaching and alkaline roasting for recovery, with reducing agents as auxiliaries. After impurity removal, extraction, and precipitation, rare earth polishing powder is obtained. This method has a high rare earth recovery rate, but the process is lengthy and costly. Excessive strong acids or strong alkalis produce large amounts of wastewater, and the purity of the polishing powder is low. As a result, the polishing performance is suboptimal.
Alkali treatment
Alumina (Al₂O₃) and silica (SiO₂) are the main impurities in waste rare earth polishing powder. At a temperature of 60°C, reacting the waste rare earth polishing powders with 4 mol/L NaOH solution for 1 hour removes the silica and alumina impurities.
The factors affecting the alkali leaching method for impurity removal are ranked as follows: The mass ratio of alkali to polishing powder waste> The leaching reaction time>The leaching reaction temperature>The concentration of the alkali.
Acid treatment
When recovering rare earth elements from polishing powder waste, nitric acid, sulfuric acid, and hydrochloric acid are often used for leaching. Cerium oxide, the main component in rare earth polishing powder waste, is slightly soluble in sulfuric acid. Increasing the reaction temperature, sulfuric acid concentration, and cerium oxide to sulfuric acid mass ratio speeds up the reaction. This can increase the dissolution rate of cerium oxide. If the cerium oxide particle size increases, it becomes more difficult to dissolve in sulfuric acid.
Reductant-assisted acid leaching
If acid is used directly to leach CeO₂, the effect is not ideal. However, adding a reducing agent to reduce Ce⁴⁺ to Ce³⁺ can improve the rare earth leaching rate. Using the reducing agent H₂O₂ to assist hydrochloric acid leaching of rare earth polishing powders waste significantly enhances the process. In addition to H₂O₂, iron powder, KMnO₄, and thiourea also have the same effect. However, using iron powder or KMnO₄ as reducing agents introduces new impurity elements.
When using thiourea to assist hydrochloric acid leaching, at 80°C, with 0.2g/g of thiourea, 3.5 mol/L hydrochloric acid, and a reaction time of 150 minutes, the cerium oxide recovery rate can reach 91.23%. However, thiourea as a reducing agent for acid leaching of rare earth polishing powder waste has drawbacks. In a strong acidic environment, thiourea easily decomposes, producing urea, H₂S, and waste gases.
Kesimpulan
In conclusion, rare earth polishing powder plays a crucial role in various industries, especially in glass and electronics, due to its superior polishing performance. However, the challenge of recycling and handling waste remains significant. Various methods, including flotation, chemical leaching, and the use of reducing agents, have been explored to improve recovery rates and minimize environmental impact. Continued innovation in recycling technologies is essential for improving the sustainability of rare earth polishing powders and reducing waste.
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