Fused chrome sand, also known as magnesia chrome ore or fused spinel, is a light burnt magnesia powder made from magnesia raw materials (sintered magnesia, natural magnesite or sea (brine) water magnesium hydroxide) and ferrochromium. Alkaline refractory raw materials with periclase and chrome spinel as the main constituent minerals obtained by artificial synthesis (sintering or electric sintering). Fused aluminum chrome sand
1. Raw materials for making magnesia-chrome sand
① The raw materials with MgO content include magnesite, flotation magnesium concentrate, magnesium hydroxide extracted from sea (brine) water, light-burned magnesium powder, etc.
The basic requirement for it is that the content of SiO2, Al2O3 and Fe2O3 is as low as possible. Generally, purer raw materials such as light-burned magnesium powder extracted from seawater or light-burned magnesium powder obtained by flotation of magnesium concentrate are used. The latter is mostly used in my country, requiring SiO2 content to be lower than 0.5% to 1.5% and loss on ignition <2%.
Fused aluminum chrome sand
②Chromite can choose better quality chromite or chrome concentrate. Requires low SiO2 content in chromite: no special restrictions on F2O3 content. The quality of chromite in my country is poor (low content of Cr2O3 and high content of SiO2), so it is not suitable to be directly used for synthesizing magnesia-chromium sand, and chromium concentrate should be selected. Depending on the application requirements, chromite ore sections can also be used in conjunction with chrome concentrates to control the overall SiO2 content.
2. Content of Cr2O3 in magnesia-chromium sand
The content of Cr2O3 in magnesia-chromium sand depends on the use of magnesia and its performance requirements. Usually, the more chromite is added, the higher the total amount of spinel in magnesia-chromite sand, and the secondary spinel (the spinel in chromite is primary spinel, the The spinel diffused in the periclase and formed on the edge of the periclase is called secondary spinel. The content is 5%~15%, fused magnesia is 15%~25%, and some are as high as 30%~35%.
According to our country's experience in manufacturing sintered magnesia-chromium sand with light calcined powder of flotation magnesia concentrate and chrome concentrate, the content of Cr2O3 is preferably 5% to 10%. When it is lower than or beyond this range, the apparent porosity of synthetic sand is high and the particles are loose; when calcined in a high-temperature shaft kiln, it is easy to stick to the kiln and agglomerate, which often hinders the calcination process.
The technical process of producing sintered magnesia-chromium sand by using light-burned yon powder and chromite (chromium medium ore powder and chromium concentrate are used together) as raw materials. Various raw materials are weighed, mixed in proportion, and then sent into a briquetting machine to be pressed into a ball blank (dry method), the diameter of the material ball is 16～32mm, and the density of the ball blank is 2.1～2.2g/Cm3. The ball billets are calcined in an oil-fired shaft kiln at a firing temperature of 2000 °C. The atmosphere in the kiln should be well controlled, and the quality of the synthetic sand calcined in an oxidative atmosphere is better. The kiln temperature of sintered sand is 100~150℃. When light-burned magnesia powder and chromite are used as raw materials, the heat consumption of magnesia-chromium sand is 600~700 kcal/kg. Magnesia-chrome sand can also be calcined at high temperature in tunnel kilns and back brick kilns.
Electrofusion method is also a common method for producing synthetic magnesia-chromium sand, especially for synthetic sand with high Cr2O3 content. The difference between it and sintered sand is that periclase and spinel have a direct bonding structure, the intergranular spinel has a high degree of self-shape, the structure is dense, the particle porosity is low, and the silicate phase content is relatively low. Therefore, the fused magnesia-chromium sand has better corrosion resistance. Using it to make large particles combined with magnesia-chrome bricks can even achieve the performance of fused magnesia-chrome bricks, which is obviously better than that of sintered sand. The sintered sand has good thermal shock stability and low cost.
Magnesium-chromium spinel is one of the main constituent minerals of magnesia-chromium sand. Its theoretical composition is MgO 21.0%, Cr2O3 79.0%. The coefficient of linear expansion is 5.70~8.55×10-6℃-1.
Because it often contains a variety of spinels composed of FeO, MgO and Fe2O3, Al2O3, Cr2O3. The Cr2O3 content of sintered magnesia-chromium sand is generally 5%~15%, and the bulk density is 3.30~3.40g/cm3; fused magnesia-chromium sand is divided into medium chromium and high chromium, and the Cr2O3 of medium chromium sand is 15%~30% , Cr2O3 content greater than 30% can be counted as high chromium sand. The bulk density of fused magnesia-chrome sand is 3.6~4.2g/cm3. Table 1 shows the physical and chemical properties of various magnesia-chromium sands.
In the structure of sintered magnesia-chromium sand, periclase has been obviously spinelized, that is, there are star-shaped magnesia-chromium spinel inside the periclase crystal, the size is generally 15~30μm, the periclase crystal is larger, generally It is 150~180μm periclase crystals with fully developed magnesia-chromium spinel, generally 0~40μm. The silicate phase is rare, the pores are few, and the size is basically the same, and the distribution is uniform. In the sintered magnesia-chromium sand with high Cr2O3 content, there are sometimes chromite particles, which are surrounded by magnesia-chromium spinel, and the thermal shock stability of bricks made of this kind of sand is better.
The mineral composition of synthetic magnesia-chromium sand is very complex. In addition to periclase and chrome spinel, there are calcium forsterite (CMS), forsterite (M2S) and tetracalcium ferric aluminate (C4AF).