The grinding stage in a cement plant is extremely energy-intensive, consuming almost half the plant’s total electric power requirement. An expert system, designed especially for cement production and based on components from the advanced process control (APC) library of the Simatic PCS 7 control system, uses real-time plant data to make precise statements about the plant’s quality parameters, and can be integrated into the existing control system with no additional hardware requirements.
At the Südbayerisches Portland-Zementwerk in Rohrdorf/Germany, raw materials extracted from nearby quarries are stored in a blending bed hall and then ground into “raw meal” in a raw mill. In the rotary kiln the raw meal is then burned to create Portland cement clinker, the basic material for all types of cement. The next step is to grind the clinker in a ball mill equipped with steel balls of varying sizes. Depending on the type of cement, other materials such as granulated blast furnace slag can be added at the grinding stage. The Rohrdorf plant has four ball mills with a total grinding capacity of around 270 t/h.
The grinding medium moves in different ways depending on the rotational speed of the mill. Low speeds create a cascade motion in which the balls merely roll around. As the speed increases, the balls are lifted and fall onto the milled material (“cataracting” motion). Above the so-called critical speed, the balls are held against the drum wall by centrifugal force, so scarcely any grinding takes place. The optimum operating speed is above the point at which cataracting begins, and below the critical speed.
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