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Cement Mill

Cement milling in ball mills is a process which consumes large amounts of energy. For this reason, a system such as ADEX in Cementos La Robla is of interest as it not only drives the mills automatically but also minimises the specific energy consumed by ton milled and at the same time improves the quality of the milled product.

Traditional control based on PID’s is not suitable for carrying out the automatic operation of a ball mill in closed circuit for the following reasons;

  • In the first place, the considerable delays in transport and in obtaining precise variable measurements in laboratory.
  • Secondly, the strong interactions between various control actions such as fresh feed, separator speed, air to the separator and venting of the mill during the control of the mill load and the fineness of the milled product.
  • Thirdly, the relation between these control actions and the process variables are strongly non linear with respect to the mill load both in the first and second chambers and very variable with the grindability of the raw materials, particularly the clinker.

 

ADEX optimised adaptive control Cement Mill Optimised System (CMOS) for mill balls acts in a coordinated manner on the fresh feed, the speed of the separator, the air in the separator, the venting of the mill to automatically drive the mill toward the points of optimal performance where quality and production throughput is maximised and specific energy consumption is minimised.

In addition, it acts on the dosification of elements such as gypsum, ash and slag to adjust the sulphate content and the lost on ignition (LOI).

At Cementos La Robla, there are three horizontal cement mills in closed circuit, each one with the capability of milling up to six types of cement. The CMOS system works with individualised cement recipes by mill and type of cement which enables memorizing composition, set points and optimised values separately for each type of cement. In this way, the transition between different types of cement is carried out with the maximum efficiency obtaining the optimal point in the least time possible.

The CMOS cement mill control application at Cementos La Robla has enabled the automated operation of the three mills continuously including start ups, changes in quality, and has achieved improvements of between 2% and 10% with respect to manual control. It has also reduced the dispersal of fineness by more than 20%. 


Comparison between manual milling and ADEX milling



The graph above shows an example of manual milling. Normally, an estimated feed and separator are established and, according to experience, modified gradually leaving the load (expressed as elevator power, material return and mill power) to vary over a wide range of values. 




The graph below shows an example of automated milling using ADEX. It is the mill load (expressed as elevator power, material return and mill power) which stays constant at the optimum load for each situation and type of cement, changing the fresh feed as necessary. The purpose of the changes in the separator are to adjust the fineness within an acceptable band.


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