The green casting material of the 21 st century - rambling ceramic sand

Casting technology has been applied by human for more than 5000 years. For thousands of years, with the continuous development of casting technology, new materials and new processes have emerged. With the rapid development of science and technology, various new materials are constantly emerging. However, silica sand is still an important molding material in the casting industry all over the world.At present, the output of castings has reached about 100 million tons, of which China accounts for about half ( see Figure.1 ). The foundry industry consumes 45 to 50 million tons of raw sand every year, of which silica sand accounts for about 95 %. In addition to abundant reserves, easy exploitation and low price, silica sand has the characteristics of adapting to casting conditions, such as :

( 1 ) High refractoriness, suitable for pouring the vast majority of casting alloys, and non-wetting with molten metal. 

( 2 ) The particles are relatively hard and can withstand the impact and friction during molding and sand regeneration. 

( 3 ) At high temperature close to its melting point, there is still enough strength to maintain its shape.

However, silica sand also has many shortcomings, mainly in the following aspects : 

( 1 ) Poor thermal stability, phase transition at about 570 °C, and accompanied by a large volume expansion, is the root cause of various 'expansion defects ' in castings, and also the main factor affecting the surface roughness of castings. 

It is easy to react with FeO, MnO, etc., resulting in sand sticking on the surface of the casting. 

( 3 ) When it is used to prepare clay green sand, after repeated use, the surface of the sand accumulates a metamorphic sintered layer, which affects the quality of the casting. It has to be discarded and updated, and the regeneration of this waste sand is difficult. 

( 4 ) In the use of a variety of high-strength, low-dose binder conditions, the old sand regeneration processing, silica sand is more prone to fragmentation, and the fragmentation of the dust generated by the great harm to human health, which has gradually become a fatal weakness of silica sand.

In order to seek better performance, can replace the original sand of silica sand, since the 1840 s, foundry workers developed, developed and promoted dozens of kinds of special casting sand ( casting technology in the non quartz sand collectively referred to as special sand ). Special sand generally has the characteristics of high refractoriness, good thermal conductivity, small thermal expansion and strong resistance to slag erosion. Zircon sand, chromite sand, olivine sand, magnesia and so on are widely used. They are mainly used in the casting of alloy steel or carbon steel which is easy to adhere to sand. They are often used in coated sand, mold surface sand, coating, special core and so on. The whole production line is rarely used. These special sands are generally made by mineral processing, crushing, screening, and individual calcination without changes in composition and structure. They are generally called natural special sands. At present, it is faced with the gradual shortage of resources, limited mining and so on, and the price is more and more expensive.

Since the end of the last century, some enterprises and research institutions in Japan, China, Germany and other countries have used kaolin and bauxite as raw materials to produce ceramic spherical sand with different components and different phases by granulation, sintering, or melting, wind crushing and other processes. In addition to the above advantages of natural special sand, it also has the characteristics of low binder dosage, good fluidity, good permeability, high strength and crushing resistance, and adaptability to various molding and core making processes. It has been rapidly promoted and applied in the past ten years. These are the so-called ' artificial special sand '. According to the different production methods, it is divided into sintering method, melting wind crushing method, flame melting method and so on. The corresponding representative artificial special sand mainly includes ' Cerabeads ' of Itochu, Japan, ' Ceramic sand ' of Luoyang and Zhengzhou in Henan Province, ' Lunar sand ' of China Mining in Tianjin and so on. Among them, the current comprehensive performance is the best, and the most widely used is ceramic sand.

                                  Fig.1 Development trend of casting production in China