30. Chemical reactions during heat treatment of portland cement raw meal (main reaction clinker burning)

Firing - completing the process of clinker production operation. In the process of firing the raw mix specific chemical composition of the clinker, which consists of four main clinker mineralov.V composition of clinker minerals include each of the initial components of raw mixture. For example, tricalcium silicate 3Ca0-Si02, main clinker mineral is formed from three molecules of CaO - limestone mineral oxide and one molecule Si02- clay mineral oxide. Similarly obtained three clinker and other mineral - dicalcium silicate - 2Ca0-Si02, tricalcium aluminate - ZSa0-A1203 and four rehkaltsievy alumina ferrite - 4SaO A1203 • Fe203. Thus, for the formation of clinker minerals raw component - limestone and minerals second component - clay must chemically react with each other. Under normal conditions, the components of the raw mixture -.. Limestone, clay, 'and OE inert, ie, they do not react with one another. When heated, they become active and begin to be mutually reactivity. This is explained by the fact that with increasing temperature the energy of the moving molecules of solids becomes so large that between them is possible interchange of molecules and atoms to form a new compound. Formation of a substance resulting from reaction of two or more solids are called reaction in the solid phase.

However, the chemical reaction rate is further increased if the melt of the material, forming a liquid phase. Such partial melting of the sintering has been called, and the material - sintered. Portland cement clinker is calcined prior to sintering. Sintering, t. E. The formation of a liquid phase, it is necessary for a more complete chemical absorption of calcium oxide CaO, silica Si02 and receiving at the same time tricalcium silicate CJSIC • Si02.

Partial melting clinker raw materials starts with a temperature of 1300 ° C. In order to accelerate the reaction of formation of tricalcium silicate clinker burning temperature is increased to 1450 ° C. As various units in their structure may be used to produce clinker and principle of the thermal units. However, generally used for this purpose rotary kiln, they receive about 95% of the total output of clinker, 3.5% of clinker produced in shaft furnaces, and the remaining 1.5% - in other systems thermal units - Spekatelnye gratings reactors clinker burning in suspension or in fluidized bed. Rotary kilns are the main thermal unit both in the wet and the dry process clinker production. Calcining kiln is a rotating machine drum, lined inside with refractory materials. The drum is set obliquely on castors. With the raised end of the drum for receiving liquid or sludge granules. As a result of rotation of the drum sludge is moved to the lowered end. Fuel is supplied to the drum and is burned by the lowered end. Formed in this red-hot flue gases are moving towards the searing material and heat it. Calcined material as clinker exits the drum. The pulverized coal as a fuel used for the rotary kiln, oil or natural gas. Solid and liquid fuel is supplied into the furnace in atomized form. The air required for combustion is introduced into the furnace together with fuel and further is fed from the refrigerator furnace. In the refrigerator it is heated red-hot clinker heat, cooling the latter at the same time. The air that is introduced into the furnace together with fuel, is called the primary, and obtained from the refrigerator furnace - secondary. Formed during combustion hot gases moving towards searing material, it is heated and cooled by themselves. As a result, the temperature of material in the drum as they move all the time increases and the gas temperature - is reduced. material and the temperature distribution of the gas flow along the length of the rotating drum furnace. The abscissa (horizontal axis) is plotted length of the individual zones of the furnace as a percentage of the total length of the furnace drum and the ordinate axis - the temperature in each point of the furnace and the gas flow material. Character broken curve shows the temperature of the material that when heated feed mixture occurs therein various physico-chemical processes which inhibit heating in some cases (flat areas), and in other heat-promoting sharp (steep portions). The nature of these processes is as follows.

The feed slurry having a temperature of ambient air entering the furnace is subjected to high abrupt flue gas temperature and heated. The flue gas temperature at the same time reduced from about 800-1 Ltd. to 160-250 ° C.

Upon heating, the slurry thickens at first, and then thicken and lose a significant amount of water is converted into large lumps, which on further heating to become grains - granules. The evaporation of the sludge is mechanically admixed thereto water (drying sludge) lasts up to about the temperature of 200 ° C, as the moisture contained in the fine pores and capillaries material evaporates slowly.

By the nature of the processes occurring in the slurry at temperatures up to 200 ° C, this furnace zone is called the evaporation zone (/). As further promote the material falls into the region of higher temperatures and chemical processes begin to occur in the raw mix: at temperatures above 200-300 ° C burn organic matter is lost and the water contained in the clay minerals. Loss of clay minerals are chemically bound water (dehydration) leads to a complete loss of its bonding properties of clay and mud scattered pieces into a powder. This process takes up to temperatures of about 600-700 ° C. By essentially the processes occurring in the temperature range from 200 to 700 ° C, this furnace zone called the preheating zone (II).

As a result, the host feed mixture of calcium oxide is formed in such a temperature, so that the furnace zone (up to a temperature of 1200 °) called calcination zone (III).

The temperature of the material in this zone increases relatively slowly. This is because the heat of flue gas is consumed mainly in the decomposition of CaC03: for decomposition of 1 kg of CaO and CaCO of CO is required to expend 425 kcal of heat.

The appearance of the raw material mixture of calcium oxide and the presence of high temperature causes the beginning of the chemical interaction in the clay are oxides of silicon, aluminum and iron with calcium oxide. This interaction takes place between oxides in a solid state (solid phase). Reactions are developed in solid phase in the temperature range 1200-1300 ° C. These reactions are exothermic, t. E. Occurs with release of heat, why this furnace zone received .nazvanie exothermic reaction zone. As a result, the roasted blend stay in the exothermic region are formed: 2CaO • Si02; 4SaO • A1203 Fe203 and CJSIC • A12Oz.Obrazovanie tricalcium silicate (3Ca0-Si02) takes place at the next section of the furnace in the greatest heat, called sintering zone. 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 clinker tempera

Tours: 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. 2.1% of free lime is not reflected in the quality of Portland cement, but more of its high content of portland cement causes uneven volume changes during solidification and therefore nedopustimo.Klinker sintering zone falls from the cooling zone (VI), where the flows of cold air moving towards the clinker. From clinker leaves the cooling zone with a temperature of 1000-1100 ° C and final cooling to direct it in the refrigerator pechi.Oborudovanie in which these processes take place, it will be discussed in subsequent paragraphs.