Polishing powder is usually composed of cerium oxide, aluminum oxide, silicon oxide, iron oxide, zirconia, chromium oxide and other components. Different materials have different hardness and chemical properties in water, so they are used in different occasions. The Mohs hardness of alumina and chromium oxide is 9, that of ceria and zirconia is 7, and that of iron oxide is lower.

Cerium based rare earth polishing powder is one of the most important rare earth products. Because of its strong cutting ability, short polishing time, high polishing accuracy, clean operating environment and other advantages, it is better than other polishing powders (such as Fe2O3 red powder) in use. At present, this product has developed rapidly in China, has been widely used, and its output has soared, with a promising development prospect.

1.1 Development process of rare earth polishing powder

Red powder (iron oxide) is an earlier polishing material in history, but its polishing speed is slow, and the rust pollution cannot be eliminated. With the development of rare earth industry, in the 1930s, rare earth oxide was first used as polishing powder to polish glass in Europe. In the Second World War, an employee of WF and BarnesJ in Rockford, Illinois, proposed a rare earth oxide polishing powder called Barnesite in 1943. This polishing powder soon became successful in polishing precision optical instruments. The rare earth polishing powder has the advantages of high polishing efficiency, good quality and low pollution, which has aroused the research of the United States and other countries. In this way, rare earth polishing powder is rapidly developing to replace the traditional polishing powder.

Foreign countries began to produce rare earth polishing powder 60 years ago. In the 1990s, various standardized and serialized products reached more than 30 specifications and brands.

1.2 Composition and classification of rare earth polishing powder

1.2.1 Divided by CeO2 content in rare earth polishing powder:

The main component of rare earth polishing powder is CeO2. According to the amount of CeO2, the cerium polishing powder can be divided into two categories: one is the high cerium polishing powder with high CeO2 content and high price and high quality, generally CeO2/TREO ≥ 80%; the other is the cheap low cerium polishing powder with low CeO2 content, whose cerium content is about 50% or less, and the rest is composed of La2O3, Nd2O3, Pr6O11.

For high cerium polishing powder, the higher the grade of cerium oxide is, the greater the polishing capacity is, and the longer the service life is. Especially when hard glass is polished for a long time (quartz, optical lens, etc.), it is appropriate to use high grade cerium polishing powder. Low cerium polishing powder generally contains about 50% CeO2, and the remaining 50% is La2O3? SO3,Nd2O3? SO3,Pr6O11? Alkaline anhydrous sulfate such as SO3 or alkaline fluoride such as LaOF, NdOF, PrOF, etc. This kind of polishing powder is characterized by low cost and its initial polishing ability is almost the same as that of high cerium polishing powder. Therefore, it is widely used for polishing flat glass, picture tube glass, spectacle lenses, etc., but its service life is inevitably lower than that of high cerium polishing powder.

1.2.2 According to the size and particle size distribution of rare earth polishing powder:

The particle size and particle size distribution of rare earth polishing powder have important influence on the properties of polishing powder.

For polishing powder with certain components and processing technology, the larger the average particle size is, the greater the glass grinding speed and surface roughness are. In most cases, the particle size is about 4 μ M polishing powder grinding speed is higher. On the contrary, if the average particle size of polishing powder is small, the amount of grinding is reduced, the grinding speed is reduced, and the flatness of the glass surface is improved. The standard polishing powder generally has a narrow particle size distribution, and there are few too fine and too coarse particles. The polishing powder without large particles can polish a high-quality surface, while the polishing powder with small fine particles can improve the grinding speed. In addition, rare earth polishing powder can also be classified according to different types of additives. The production technology of rare earth polishing powder belongs to micro powder engineering technology, and the rare earth polishing powder belongs to ultra-fine powder. Generally, there are three kinds of ultra-fine powder in the world: nanometer (1nm~100nm); Submicron (100nm~1 μ m）； Micron (1 μ m~100 μ m) According to the classification method, the rare earth polishing powder can be divided into three categories: nanometer rare earth polishing powder, submicron rare earth polishing powder and micrometer rare earth polishing powder. Generally, the rare earth polishing powder we use is micrometer, and its particle size distribution is 1 μ m~10 μ M, rare earth polishing powder is generally used in the rear process of glass polishing according to its physical and chemical properties for fine grinding, so its particle size distribution is generally not greater than 10 μ m. Particle size greater than 10 μ M polishing powder (including rare earth polishing powder) is mostly used for rough grinding at the initial stage of glass processing. Less than 1 μ The submicron rare earth polishing powder of M has been paid more and more attention in the field of liquid crystal display and computer optical disk, and its output has increased year by year. Nanometer rare earth polishing powder has also come out. With the development of modern science and technology, its application prospect is unpredictable, but its market share is still small, which belongs to the research and development stage.

1.3 Raw materials for polishing powder production

At present, the raw materials for the production of cerium rare earth polishing powder in China are as follows: (1) cerium oxide (CeO2), obtained from the separation of mixed rare earth salts (w (CeO2)=99%); (2) Mixed rare earth hydroxide (RE (OH) 3) is the intermediate raw material after chemical treatment of rare earth concentrate (w (REO) ≥ 50%) (w (REO)=65%, w (CeO2) ≥ 48%); (3) Mixed rare earth chloride (RECl3), rare earth chloride with less europium extracted from the mixed rare earth chloride (mainly containing La, Ce, Pr and Nd, w (REO) ≥ 45%, w (CeO2) ≥ 50%); (4) High grade rare earth concentrates (w (REO) ≥ 60%, w (CeO2) ≥ 48%) include Baotou mixed rare earth concentrates in Inner Mongolia, and bastnaesite concentrates in Weishan, Shandong Province and Mianning, Sichuan Province.

Except for the first raw material, the second, third and fourth raw materials all contain light rare earth (w (REO) ≈ 98%), mainly CeO2, and w (CeO2) is 48%~50% China is rich in cerium resources. It is estimated that its industrial reserves are about 18 million tons (calculated by CeO2), which has laid a solid foundation for the sustainable development of rare earth polishing powder in China in the future. It is also a unique advantage of China, and can promote the continued rapid development of China's rare earth industry.

1.4 Main production process and equipment

1.4.1 Production of high cerium rare earth polishing powder

Using cerium oxide separated from rare earth mixture as raw material, the powder product with large hardness, uniform particle size, fine size and face centered cubic crystal is processed by physical and chemical methods. The main process is: raw materials → high temperature → calcination → water quenching → hydraulic classification → filtration → drying → good cerium rare earth polishing powder products.

The main equipment includes: calciner, water quenching tank, classifier, filter, drying box.

Main indicators: w (REO)=99%, w (CeO2)=99%; The recovery rate of rare earth is about 95%; Average grain size 1 μ m~6 μ M (or the particle size is 200~300 meshes), and the crystal shape is intact. This product is suitable for high-speed polishing. This cerium polishing powder has earlier replaced the classical polishing iron oxide powder (red powder).

1.4.2 Preparation of medium cerium rare earth polishing powder

The mixed rare earth hydroxide (w (REO)=65%, w (CeO2) ≥ 48%) is used as the raw material, the rare earth salt solution is obtained by chemical pretreatment, and the intermediate (precipitator) is added to convert it into intermediate cerium rare earth polishing powder with w (CeO2)=80%~85%. The main process is as follows:

Raw materials → oxidation → preferential dissolution → filtration → acid dissolution → precipitation → washing and filtration → high temperature calcination → fine grinding and screening → intermediate cerium rare earth polishing powder products. Main equipment: oxidation tank, superior dissolution tank, acid dissolution tank, sedimentation tank, filter, calciner, fine grinding screening machine and packaging machine.

Main indicators: w (REO)=90%, w (CeO2)=80%~85%; The recovery rate of rare earth is about 95%; Average particle size 0.4 μ m~1.3 μ m. This product is suitable for high-speed polishing and has better performance than good cerium rare earth polishing powder in high-speed polishing.

1.4.3 Preparation of low cerium rare earth polishing powder

Low grade cerium rare earth polishing powder can be prepared by using rare earth chloride with less europium (w (REO) ≥ 45%, w (CeO2) ≥ 48%) as raw material and using synthetic intermediate (precipitant) for double salt precipitation. The main process is as follows:

Raw materials → dissolution → double salt precipitation → filtration and washing → high-temperature calcination → grinding → fine grinding and screening → low-grade cerium rare earth polishing powder products.

Main equipment:

Dissolving tank, sedimentation tank, filter, calciner, pulverizer, fine grinding and screening machine. Main indicators: in the product, w (REO)=85%~90%, w (CeO2)=48%~50%; The recovery rate of rare earth is about 95%; Average particle size 0.5 μ m~1.5 μ M (or particle size 320~400 mesh). This product is suitable for high-speed polishing of optical glass, etc. The low-grade cerium rare earth polishing powder can be directly produced by using mixed bastnaesite high-grade rare earth concentrate (w (REO) ≥ 60%, w (CeO2) ≥ 48%) as raw materials and directly processing by chemical and physical methods, such as grinding, calcination and screening.

The main process is as follows:

Raw materials → dry fine grinding → ingredients → powder mixing → baking → grinding and screening → low-grade cerium rare earth polishing powder products. Main equipment: ball mill, mixer, calciner, screening machine, etc. Main indicators: w (REO) ≥ 95%, w (CeO2) ≥ 50%; Rare earth recovery ≥ 95%; The product granularity is 1.5 μ m~2.5 μ m. The product is suitable for high-speed polishing of eyeglasses and TV picture tubes. At present, there are many low-grade cerium rare earth polishing powders produced in China, accounting for more than 90% of the total output.

1.5 Application of rare earth polishing powder

Because of its superior chemical and physical properties, cerium based rare earth polishing powder has been widely used in polishing industrial products, such as polishing various optical glass devices, TV picture tubes, optical glasses, oscilloscope tubes, flat glass, semiconductor chips and metal precision products.

Polishing refers to a processing method that uses mechanical, chemical or electrochemical effects to reduce the surface roughness of the workpiece to obtain a bright and flat surface. It refers to the surface modification of the workpiece by using polishing tools, abrasive particles or other polishing media.

Polishing can not improve the dimensional accuracy or geometric shape accuracy of the workpiece, but aims to obtain a smooth surface or mirror luster, sometimes used to eliminate luster (matting). The polishing wheel is usually used as the polishing tool. The polishing wheel is generally made of multi-layer canvas, felt or leather, and its two sides are clamped with metal round plates. Its rim is coated with a polishing agent uniformly mixed with fine abrasive and grease.

During polishing, the high-speed rotating polishing wheel (with a circumferential speed of more than 20 m/s) presses against the workpiece, so that the abrasive can roll and slightly cut the workpiece surface, thus obtaining a bright machined surface, and the surface roughness can generally reach Ra0.63~0.01 μ m; When a non greasy matting polishing agent is used, the bright surface can be matt to improve the appearance. When large quantities of bearing steel balls are produced, the method of drum polishing is often used.

During rough polishing, a large number of steel balls, lime and abrasives are placed in an inclined tank cylinder. When the cylinder rotates, the steel balls and abrasives will roll and collide randomly in the cylinder to remove the surface protrusions and reduce the surface roughness. The allowance of about 0.01 mm can be removed.

During fine polishing, steel balls and fur fragments are loaded into the wooden barrel, and the dazzling and bright surface can be obtained by continuously rotating for several hours. The polishing of precision linear rule is carried out by immersing the machined surface in the polishing solution, which is composed of chrome oxide powder with particle size of W5~W0.5 and emulsion. The polishing wheel is made of wood with uniform material and degreased treatment or specially made fine felt. Its motion track is uniform and dense mesh. The surface roughness after polishing is not greater than Ra0.01 μ m. No surface defects can be observed under a microscope with a magnification of 40 times. In addition, there are electrolytic polishing and other methods.

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