Factors Affecting the Quality of Calcium oxide

　　Calcium oxide (CaO), commonly known as quicklime or burnt lime, is a widely used chemical compound.it's CAS Registry Number is 1305-78-8.It is a white, caustic,alkaline crystalline solid at room temperature. The broadly used term "lime" connotes calcium-containing inorganic materials, which include carbonates, oxides and hydroxides of calcium, silicon, magnesium, aluminium, and iron predominate, such as calcium oxide. By contrast, "quicklime" specifically applies to a single chemical compound.

Calcium oxide，1305-78-8

　　Next are several areas of calcium oxide calcination process which should requires attention, those espect are able to control the quality of Calcium oxide if it is cntrol well.
　　1.Chemical composition of calcium oxide
　　The chemical composition of the calcium oxide cannot be controlled without a major cost impact on manufacturing, therefore, variation is generally accepted. The calcination temperature should be controlled very closely. To heat the calcium oxide in the kiln uniformly, the particle size of the feed must be relatively uniform. In addition, to avoid long residence time in the kiln, the particle size of the calcium oxide must be small, typically about 1.5 inches. However, due to the nature of the crushing operation, there is a range in size from 0.5 to 2 inches. Since the residence time and temperature in the kiln is constant, theheat penetration in the particles of calcium oxide is different due to variation of the size of the calcium oxide.  A larger size of stone heat does not quite penetrate to the core; therefore the center of these pieces remains as calcium carbonate while the outside is converted to CaO. These center cores are referred to as grit. For medium size stones, the heat penetration is complete and the entire stone is converted to CaO.  For the smaller stones, the heat reaches the core rapidly and the outside layer is overheated forming a hard outer shell where water cannot penetrate, therefore, the slaking process is greatly retarded or prevented.  Here, the large and medium size particles are highly reactive, soft burned, quicklime and the smaller particles are called hard-burned quicklime.
　　2.Kiln temperature
　　The theoretical temperature required for calcination is about 900°C; however, in practice, we find this temperature to be much higher about, at 1,350°C. To determine the correct temperature in the kiln is an art rather than a science, and it depends on the calcium oxide size as well as type of kiln and type of fuel used. The kiln operator mustexperiment to determine the exact temperature for the particular size calcium oxide that is being used. In general, it is best to use the lowest temperature with the shortest possible residence time to achieve full calcination. Higher calcination temperature will cause increased shrinkage and reduction in volume. Higher temperature also will cause recarbonation of the surface of CaO pebbles with the presence of CO2, which makes the lime non-porous, and thus unsuitable for hydration.
　　Kiln temperature affects the quality of CaO produced and the resultant hydroxide produced is from slaking this CaO. Very small particle sizes with large specific surfaces are the most desirable end product from calcium oxide.  A soft-burned lime pebble is full of small hair-like cracks where CO2 has escaped from the calcium oxide during the calcination process. When this lime is exposed to water the water penetrates the cracks in the quicklime pebbles and fills these cavities. The hydration takes place quickly, releasing a lot of heat energy. This heat will boil off the water and generate steam, which makes the particles burst, exposing the inner surfaces to water for further slaking. This process will continue until hydration is complete.
　　3.Rate of temperature rise
　　The temperature rise must be gradual and even. It is particularly important when using larger size calcium oxide (10 to 15 cm or 4 to 6 in.).  When calcining this size calcium oxide, the calcium oxide must remain porous during the process. As the temperature rises, the outer layer of calcium oxide is heated to disassociation temperature, where CO2 escapes the stone, leaving capillary passages making the lime porous. As the gas escapes, the calcium oxide shrinks in volume by as much as 40%.  This shrinkage in volume restricts the passage of gas from the center of the calcium oxide, preventing any additional CO2 gas from escaping.  Too long of a residence time will combine the CaO and CO2 back to CaCO3 (recarbonation) at temperatures above 1350°C. A good practical size for calcium oxide in VSK kilns is 5 to 10 cm (2 to 4 in.).  This size will allow for quick heating, short residence time and a minimum amount of cores that create grit.  In conclusion, the smaller size calcium oxide (4 to 5 cm or 1.5 to 2 in.) is most suitable for calcination in rotary kilns and will allow optimum residence time. This lower calcining temperature will also allow less fuel consumption. However, larger size calcium oxide and low calcining temperature are needed for vertical single-shaft and multi-shaft kilns. If the temperature rise is too rapid, the outer layer of the calcium oxide pieces is calcined very fast. As the temperature rises, the surface of pebbles will shrink, closing the pores created by the escape of CO2.  This produces increased internal pressure within the calcium oxide. Since the gas cannot escape, the calcium oxide will explode and disintegrate, producing unwanted fines, reducing the quality of the resultant calcium oxide.
　　4.summary 
　　There are many other factors that will affect the quality of calcium oxide, mainly in its firing process that need attention, such as its temperature, heat, density and so on, so in order to get a good calcium oxide, requiring careful clinkering process.