Clinker chemical processes

The production of Portland cement is essentially a chemical process in which the four main components, namely calcium oxide, silica, alumina and iron oxide are combined chemically to produce cement clinker. 

Both fossil fuels and hazardous waste fuels used in Southdown cement kilns contain metals. The raw materials (limestone, clay, sand) used to make cement clinker also contain metals. In fact, certain metals, such as iron and aluminum, are essential components of the final product. While metals cannot be destroyed, the Southdown cement kiln process effectively manages them in the following ways (a) cement kiln operators limit emissions by carefully restricting the metals content in wastes accepted for recycling (b) dust particles containing metals are returned to the kUn through closed-loop mechanisms, where metals are chemically bonded into the cement clinker (c) particles not returned to the kiln are captured in state-of-the-art pollution control devices and (d) small amounts are emitted from the stack in quantities strictly hmited by USEPA s BIF mle. 

The relative reactivity of the different mineral phases of cement with water is usually given as C A>C S>C S>C AF. Aluminate phases and their hydration products therefore play an important role in the early hydration process. Because of the high reactivity of calcium aluminate, the aluminate hydration reaction is carried out in the presence of sulfate ions. The latter provide control of the reaction rate through the formation of mixed aluminum sulfate products (ettringite and monosulfoaluminate) Calcium sulfate which is added to the cement clinker hence controls the properties of the aluminate hydration products. Sulfates thus play a crucial role in cement hydration and the influence of chemical admixtures on any process where sulfates are involved may be expected to be significant [127],... 

Nodulization, chemical reactions and evaporation of volatiles are interdependent. Alite formation and loss of volatiles affect nodulization, and the compaction that the latter entails must affect the kinetics of the former processes. Alite formation thus occurs in a changing environment. Initially, the clinker liquid is not a continuous and uniform medium enveloping the grains oflime, belite and other solid phases rather, it fills separate pores and capillaries, forming thin films on the particles, and there are large local variations in composition (Tl). As compaction proceeds, it becomes more continuous and more uniform. 

The development of the rotary kiln, now predominantly used for the production of Portland cement, started around the late 1870s but the technology was not patented until 1885 (Ransome [B.69]). A rotary kiln (Fig. 6.7-35) is a long refractory-lined steel cylinder that is inclined at about 3-6° to the horizontal. At the lower end is a burner (coal, oil, or gas fired) and the material to be calcined enters on the other end. While passing down the kiln, chemical and physical reactions take place in the oxidizing atmosphere. Hot clinker emerges at the burner end and must be suitably cooled before being milled to yield cement. The rotary kiln process may be carried out with wet (slurry), semi-dry, or dry feed. 

The clinker heat of formation only depends on the physical-chemical properties of the raw meal fed to the kiln. For such calculation, it is conventional to use the isothermal analytical method, which consists in quantifying step-by-step the heat required by each chemical and physical transformation of the material during the burning process. The reference temperature to which all heat quantities are referred is 273.16 K. The final heat of reaction is obtained subtracting from the total heat required for endothermic transformations the heat released by the exothermic transformations. 

The chemical composition of clinker is complex, however, it is easy to notice that the sum of four components, CaO, Si02, AI2O3 and Fe203 is as a rule higher than 95 %. The processes of Portland cement clinker phases ciystallization in the four components system can thus be presented. It is especially justified that MgO is not forming own compounds in the rich in calcium part of five-components system, but solid solutions with remaining clinker phases, or is present as periclase. However, the four-components system is seldom used, because the use of ternary systems is much more convenient. They are usually presented as the horizontal projection. 

Photograph 7-79 Free-lime nest colored by water etch in polished section. Chemical analysis gave clinker free-lime value of 2.69%. High maximum temperature, moderately long burning time, moderately slow heating rate, moderately slowly cooled, 31 MPa. Gas-fired, dry-process kiln, 3000 tons/day. (S A6697)... 

The intricate chemical and physical processes that take place in the burning of portiand cement raw feed are much too complicated to be dealt with here. But, as an introduction to these subjects, a brief survey of selected recent literature and a presentation of elementary steps in raw feed examination are offered. This is not to imply that all needed information has been published, for much systemafic research remains to be done. Our understanding of clinkering, however, demands an appreciation of exacfly whaf it is that we are burning. 

Unfortunately, some of the observations and interpretations presented in Chapter 7 ("Microscopical Interpretation of Clinkers") do not appear to be founded in systematic experimental design or statistical analysis. Statistical measures to determine the degrees of correlation and association of the observations, and their relationship to the various physical and chemical causal factors of the production process, are essential and urgently needed for several very important reasons ... 

The thermal decomposition of calcium carbonate is a highly endothermic reaction, and even though the formations of both dicalcium and tricalcium sihcate from CaO and Si02 are exothermic, the overall formation process of these compoimds from CaCOj and Si02 remains endothermic. A comparison of both reactions reveals that the overall chemical enthalpy of C3S formation is distinctly higher than that of C2S (1268 J/kg for C2S and 1770 J/kg for C3S). Thus the potential exists to reduce the energy consumption in the production of Portland clinker by increasing its behte content at the expense of alite. 

The thermal energy consumed in the production of Portland clinker may be subdivided into two categories the chemical enthalpy of clinker formation, and thermal losses. In addition, significant amounts of electrical energy are consiuned in the production process to power the equipment used. In contemporary cement plants the following values may be considered typical in the production of ordinary Portland cement ... 

The chemical enthalpy of clinker formation is the energy/enthalpy needed to convert the substances constituting the raw meal into Portland clinker in the absence of any heat losses and without changing the overall temperature of the system. Since the formation of clinker consists of a series of chemical and physical processes that are both endothermic and exothermic in nature, the overall chemical enthalpy will be derived from the difference between the energy liberated in the exothermic reactions and the energy consumed in the endothermic reactions taking place. 

By mixing Portland cement with water, the hydration process begins. The hydration process is the combination of physical and chemical changes during which, the cement previously mixed with water becomes a porous sohd. During this process a number of exothermic reactions give rise to cement matrix. In the case of Portland cement, the main reactions are based on interaction between two mineral components present in higher proportion clinker and water. These reactions can be written in simplified form thus [30, 65, 66] ... 

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