Cement Energy and Environment

Raw materials: The main raw material for the manufacture of clinker is limestone. Approximately 1.65 tonnes of raw material input needed to make one tonne of clinker. Proportioning, mixing and raw milling: The proportions are governed by the chemical properties of the materials to be used and the desired properties of the clinker that is produced. After proportioning, the raw materials for a dry process kiln are mixed to produce an aerated dry raw meal. Burning of the raw mix in a kiln to form clinker: There are four stages to this process - smaller particle size than the less reactive low heat of hydration cements. Cement can absorb moisture from the environment leading to some degree of pre-hydration. 1.3 Co-processing in the Cement Industry Co-processing means the substitution of primary fuel and raw material by waste, recovering energy and material from waste. Waste materials used for co-processing are referred to as alternative fuels and raw materials (AFR). Different feed points utilizing to insert AFR into the cement production process. The most common ones are: evaporation and preheating, calcining, clinkering , • Vis feed into kiln through main burner and cooling. Evaporation and preheating remove moisture and raise the temperature of the raw mix preparatory to calcining. Calcining takes place at 800-900·c and breaks the calcium carbonate down into calcium oxide and carbon dioxide which is evolved in the process. Clinkering completes the calcination stage and fuses the calcined raw mix into hard nodules resembling small grey pebbles. Kiln temperatures in the burning zone range from 1350-14so·c . Clinkering is critical to the quality of cement and requires accurate control of the energy input. Insufficient heat will cause the clinker to be unburnt, containing unconverted lime, and reducing the hydration (setting and hardening) properties of the resulting cement. Excess heat will shorten the life of the refractory bricks lining the kiln, may damage the kiln shell and diminish product reactivity. The high temperatures required for burning of the raw mix mean that the process is energy intensive. Electrical energy is required for the raw milling and mixing, burning, and finish milling. The largest energy demand is for fuel for burning of the raw mix. This has been the focus of technological developments. Finish milling of clinker: Finish milling is the grinding of clinker along with gypsum to produce a fine grey powder called Cement. The gypsum controls the rate of hydration of the cement in the cement-setting process. The finer the grind, the more reactive is the finished cement. Rapid-setting cements have • via a pipeline or feeder to the precalciner Co-processing has the following characteristics during the production process: • The alkaline conditions and the intensive mixing favor the absorption of volatile components from the gas phase. This internal gas clean ing results in low emissions of components such as 802, HCI, • The clinker reactions at 1450°C allow incorporation of ashes and in particular the chemical binding of metals to the clinker. • The direct substitution of primary fuel by high calorific waste material causes a higher efficiency on energy recovery in comparison to other "waste to energy" technologies Co-processing and Waste Management Co-processing of waste in cement kilns offers advantages for the cement industry as well as for the authorities responsible for waste management. By this , fossil fuel and raw material consumption, can be reduced ,contributing to a more eco-efficient production. One of the advantages for authorities and communities is that this waste recovery method uses an existing facility, eliminating the need to invest in a new, purpose-built incinerator or secure landfill site. Co-processing should be considered in any approach to waste management. A detailed systems approach, comparing individual waste technologies and looking at the interface of combined processes (collection, storage, recycling and disposal) will help to optimize waste management from ecological, social and 5