The quality of raw diatomite is based on the comprehensive evaluation to three factors, its content of biological shell, the difference of its chemical composition and its physical characteristics.
（一） The relation between the quality and its content of diatomaceous shell
The higher the content of biological shell is, the higher the quality is. The content of diatomaceous shell in the first grade the raw soil is as high as 80 percent or higher. However, raw soil is inferior in quality once its content of diatomaceous shell is lower than 60 percent.
（二） The relation between the quality and its chemical composition
The content of SiO2 is one important parameter to judge the quality. The more noncrystalline SiO2 accounts for in raw diatomite, the higher the quality is. Generally, the raw diatomite whose SiO2 accounts for over 60 percent can be exploited and utilized. Besides, a low content of SiO2 in raw soil means that there are lots of impurity in it.
The content of Al2O3 is related to the clay mineral content in the original soil. Usually, that the content of Al2O3 is high in raw oil indicates that the quality is poor, but for the processing of catalyst carrier, it is necessary that the raw soil contains a moderate amount of Al2O3 to enhance the firmness of the carrier. When the raw soil is used to make light weight heat-preserved brick, a proper content of Al2O3 can increase the compression strength of brick.When used to make diatomite filter aid, the lower the raw soil contains Al2O3, the better it is.
The content of Fe2O3 depends on the content of iron minerals. However, its presence is harmful to any use of diatomite, especially the use of making catalyst carrier which requires that the Fe2O3 content in raw soil should be strictly less than 1%. Therefore, when the content of Fe2O3 in the raw soil exceeds a certain limit, it must be removed.
The content of CaO is related to the clay minerals and other impurities in the raw soil. The CaO in the raw soil is no benefit to promote the quality of diatomite, and the content of CaO in the most raw soil in China is about 1%.
The content of Mgo in most raw diatomite in China, is generally close to the content of CaO, which is also a kind of useless oxide. However, due to its low content, it has little effect on the quality of the products. As a result, there is no necessary to use special methods to remove it.
The loss on ignition is related to the content of organic compound in the raw soil. The loss on ignition of most of the diatomite raw mine in our country is less than 10%, but the loss on ignition in only a few areas is about 30%.
There are different amounts of quartz, feldspar and clay minerals and other impurities in different kind diatomite, and the chemical composition of all these impurities includes SiO2 crystalline silicon. When raw diatomite is tested through chemical analysis, there is no difference between the SiO2 in those impurities and the noncrystalline SiO2 in the biological silica shell. So, when those test reports are referred to, the evaluation to the quality of diatomite can not be only based on the level of SiO2 content in the reports. Other test methods (X ray diffraction and infrared spectroscopy) must be also adopted to make relative content determination of quartz, feldspar and clay minerals in the raw soil.
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