Refractory manufacturers, facing problems.

The main refractory material in preheating machine is the castable fireclay of insulating brick and Liaoning magnesia chrome brick of the same grade. In the preheating machine, the refractory material should be light, fire resistance and fire resistance should be enough. In the drying process of rotary kiln, the powder entrained by air flow has abrasion on the inner wall of cyclone, especially at the turning or changing direction of air duct. The bottom of the hammer body of the cyclone also has the deacidification high-temperature raw meal in contact with the cylinder wall. There is coal injection in the rising pipe of kiln gas and the tertiary air pipe for carbonate decomposition in the preheater, and the temperature is also relatively high, so the refractory requirements here are more stringent.

Sources of hexavalent chromium in cement

Generally speaking, hexavalent chromium in cement mainly comes from raw materials, admixture, crushing and grinding equipment and refractory materials. In the raw materials of cement production, there is a high content of chromium in iron correction raw materials. Marl, limestone, clay and iron tailings often contain trace chromium, which will be brought into the clinker in the process of clinker calcination by using these raw materials. A large amount of industrial waste residue has become an important way for sustainable development of cement industry. As the waste with high chromium content is used as alternative fuel, raw material or cement admixture, chromium element will inevitably be brought into cement products. In the process of cement production, chromium is introduced into cement products by crushing and grinding equipment in raw material crushing, raw meal and cement grinding due to the crushing of chromium bearing grinding body and wear of grinding medium. Under the influence of high temperature of rotary kiln, high air pressure at outlet and high basicity of furnace charge, chromium is oxidized and mixed into clinker, resulting in water-soluble hexavalent chromium in cement clinker.

Effect of chromium on cement production and quality

In addition to environmental safety, magnesia chrome brick as raw material ingredient and admixture has a certain impact on cement clinker firing and cement performance. In the past 20 years from 1989 to 2008, many scholars in China proposed to use the waste slag produced from the production of chromium salt to cement production. The influence of chromium slag on cement production mainly includes magnesite (MgO) and chromium containing compounds in chromium slag. Part of the research results show that proper amount of chromium compound is beneficial to clinker burning, chromium slag can play a role of melting and mineralization, magnesium and chromium in chromium slag can shorten clinker burning time, reduce unit clinker coal consumption and cement grinding power consumption. Research on the influence of chromium on cement clinker sintering has been carried out earlier abroad. In 1990, Portland Cement Association of America pointed out in the report on the effect of trace elements on cement production and application that chromium in raw materials can reduce the viscosity of clinker liquid phase and improve the burnability of clinker due to its high ionic charge. Chromium in clinker exists in different oxidation states, Cr3 + and Cr6 + are the most stable, which is closely related to the oxygen content in cement kiln. Higher oxygen content tends to form Cr6 + compounds, such as chromate, which dissolve in water and affect the hydration behavior of pastes; reduction atmosphere helps to form Cr3 + compounds, which are almost insoluble in water. In oxidizing atmosphere, chromium may also exist as Cr4 + and Cr5 + in dicalcium silicate (C2S) minerals, and some of them are converted into more stable C