The dehydration of kaolinite

the dehydration of kaolinite primary product, ranging from approximately 600 ° C, is closest in structure to the kaolinite product metakaolin. Given that the profitability of energy mullite formation is much greater than that of metakaolin. accompanied by a dehydration process kaolinite razrusheniemego complete structure to form a mixture of free oxides. It was found that the endothermic effect of dehydration of kaolinite and its exotherm phase prevrascheniyapo with increasing heating rate of 10 to Bo / min shift toward high temperatur.Prevrascheniya by heating kaolinite were for decades one of the issues subjected to particularly intensive study. There were differing views on the nature of the dehydration of kaolinite product (metakaolin or a mixture of silicon and aluminum), and on the process, which is celebrated on the thermograms ekzotermicheskimeffektom at 900-1000 ° (crystallization from A1., 0z, mullite or spinel).

The reactions during the sintering

In the sintering zone of the most fusible minerals ZSa0-A1203 and 4SaO> Al203 * Fe203 melted. The resulting liquid phase is partially dissolved 2Ca0-Si02 and saturate it with lime to 3Ca0-Si02.

Tricalcium silicate is significantly less soluble in the melt than dicalcium silicate. Therefore, once there its formation, the melt becomes supersaturated with respect to this and tricalcium silicate mineral melt drops in the form of tiny crystalline solid, which under these conditions is then able to increase in size.

Dissolution 2CaO> Si02 and absorption they know is not right in the entire mass of the mixture and its individual portions. Therefore, for a more complete assimilation dvuhkal- tsievym lime silicate materials need to withstand a certain period at the sintering temperature (1300-1450 ° C). The longer this delay is, the more complete will the binding of lime, and at the same time become larger crystals CJSIC - Si02.

However, long withstand the clinker at the sintering temperature or it is not recommended to cool it slowly; Portland cement, which has a Si02 CJSIC fine crystalline structure has higher strength.

The duration of exposure depends on the temperature of the clinker: the higher it is in the sintering zone, the faster formed clinker. However, excessively high, and most importantly a sharp rise in temperature quickly formed a lot of melt and burns the mixture may start komkovatsya. Formed with the large grains are heated and difficult transition process in the C2S C3S violated. As a result, the clinker is badly burned (it will be a little tricalcium silicate).

To accelerate the process of clinker as well as in those cases when it is necessary to obtain high clinker 3Ca0-Si02, apply some substances (calcium fluoride, CaF2, iron oxide, etc.), Having the ability to lower the melting point of the raw mixture. An earlier generation of the liquid phase shifts the clinker formation process to less high temperatures.

The sintering period is sometimes all the lime mixture has no time to fully assimilate silica; this assimilation process is slower due to the depletion of a mixture of lime and 2CaO • Si02. In the clinker with high saturation coefficient, which require maximum absorption in the form of lime CJSIC • Si02, will always be free lime.

1-2% of free lime is not reflected in the quality of Portland cement, but more of its high content of non-uniformity causes changes in the volume of portland cement hardening and therefore inadmissible.

The clinker falls out sintering zone to the cooling zone (VI), where the flows of cold air moving towards the clinker.

Reaction with cooling

An important process is the preparation of cement clinker it is cooled. From the rotary kiln clinker cooling zone out at a temperature of 1100... 1300 0C. Final cooling is carried out in its refrigerators.

Cooling the clinker has a significant effect on the structure, the mineralogical composition, grindability and thus the quality of the cement produced from it.

Primarily clinker cooling rate affects the ratio of crystalline and glassy phases. Slow cooling crystallization occurs, and with rapid - it slows down the formation of crystals and substantial portion of the melt solidifies in the form of clinker glass. The proportion of clinker melt in the rotary kiln is 20... 25%.

The rate of cooling affects the consistency of clinker cement volume change. Rapid cooling becomes large amounts of MgO in the glassy phase remain a microcrystalline state (grain size up to 5... 8 m). On slow cooling crystals of MgO increase in size, up to 30... 150 microns, which causes uneven changes in the volume of cement hardening. With a sharp firing and rapid cooling of the clinker formed small crystals alite, which increases the strength of the cement stone.

clinker cooling process also causes the chemical resistance of cement. Rapid cooling of the clinker increases cement sulphate. This is because C3A determining resistance clinker towards sulfate aggression by rapid cooling generally proceeds in a glassy form and becomes less sensitive to the effects against sulfates.

Grindability clinker depends on many factors, including the and the cooling rate. Comparison of grindability data quickly and slowly cooled clinker indicate that the clinker is cooled (slowly) in a drum cooler, requires higher energy consumption for grinding than clinker is cooled (fast) in a grate cooler. A higher content of the glassy phase and the small size of the crystals of clinker minerals grindability increases rapidly cooled clinker compared with cooled slowly. These data demonstrate the need for rapid cooling clinker.

Homogeneity of the raw meal

In the clinker burning process and fuel consumption affect the chemical composition, fineness of grinding, crystal size, nature of raw materials, the type and amount of impurities, the homogeneity of raw muki.Himichesky composition of the raw meal has a significant impact on the required firing time. It is defined as the time it takes to during firing the raw material mixture at a certain temperature the content of free CaO in the clinker should not exceed 2%. Oxides of alkali metals in larger amounts (about 2.5%) retard (inhibit) binding of lime during firing, while the content of MgO (about 2.0%) accelerates the firing.

The homogeneity of the composition of the raw meal is a necessary condition for the uniform clinker mineralogical structure and uniform operation of the kiln system. To do this, you must make sure that the raw material mixture in the smallest volume in the range (<1 mm3) is representative of a composition (thoroughly mixed, homogeneous). If the meal is poorly mixed, the clinker having "nest" of different phases, for example, free lime and belite clusters that formed in non-uniform material with valuable tricalcium silicate - alite. Using mineralizers (such as fluorspar, zinc oxide, manganese sulfide), the firing temperature can be lowered, and thus, can be achieved energy savings in the production of clinker.