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Ball Mill Assistant
 

The Mill Assistant is a computer control system carrying out the daily task of running a Ball Cement Mill.

The Mill Assistant will be especially helpful if the mill is equipped with a third generation separator. With a high efficiency separator many mills have stability problems running with the optimum circulation factor. They will have to run with lower circulation to stay stable (see later below). Often the traditional fixed total feed control will lead to an oscillation in fineness. This means that the reject will vary even though the total feed is kept constant. The Mill Assistant maintains the reject flow at the desired level and thereby also maintains the fineness of the material from the mill. This again leads to the fineness of the finished product being very constant.

The Mill Assistant uses several signals to maintain the mass flow stability. If taken in material direction the Folaphone (electronic ear), the mill motor power, the elevator power and the reject flow rate. Furthermore the gas flow through the separator is controlled at the desired level. The system can automatically adjust the reject flow to achieve the desired circulation factor, so we ensure the grinding is done as economical as possible. The rejects can also be kept constant, leading to an increased circulation factor if the grindability decreases. Taken short term the Mill Assistant will always keep the rejects constant.

The Mill Assistant is taking care of the fineness, measured as residue, Blaine or Particle Size Distribution by changing the separator speed. Also SO3 and LOI are controlled. The SO3 by changing the Gypsum feed percent and the LOI by changing the additive feeder percent.

The Mill Assistant can handle as many recipes as necessary. A recipe should carry information of the different mixing percents of the materials fed to the mill as well as the normal operating levels of auxiliary equipment.

Below is shown the main operator picture.


The buttons in the upper right corner leads to the recipe windows, where you define the different cements. Below an example of the Chemical Recipe is shown:

The mill shown here runs these 5 cement types, but as can be seen, with several different recipes for each. The reason to distinguish between the different recipes for the same cement is to enable statistics as help in finding the optimum grinding conditions e.g. the best reject flow rate (recirculation). Different recipes giving the same cement can be quite different to grind. Below is shown a plot of the mill performance as a function of reject setpoint:


 

Stability of the mass flow in the mill circuit is, as can also be seen from above plot, no problem if the mill is operating with rejects below the optimum. If the feed is a little too high, the rejects will increase and the mill will be able to grind more at this circulation factor. However, if the mill is operating at or above the optimum reject flow, the mill will be able to grind less if the feed is a little too high. This is direct instability, leading to a near exponential reject flow increase. Having a strong total feed control on the mill will just transform this into an oscillation in fineness. The reject control, used by the Mill Assistant, handles this gracefully.

The fineness is controlled using a EWMA filter on the measured fineness and adjusting the separator speed to get the required fineness. If two or more fineness measures should be obeyed simultaneously, like Blaine and residue, the system will chose the most needed action.

Producing blended cements, we strongly recommend to use residue or PSD for fineness, since the Blaine measurement will be unreliable due to the additives.

When controlling fineness we find it imperative to control the airflow through the separator just as accurately as the separator speed. The influence on a cement particle is airspeed squared, just as separator speed squared. We use the separator fan to measure the airflow because it is the most reliable way. The control is done by the fan speed or a damper.

Temperature control is traditional: Proportional with start at 115C - fully open water valve at 130C (or PI dynamics if requested).

Run factor is very close to 100% - the control is operational 10 minutes after the mill is started.

After commissioning the operation is remote monitored by Simeq for the first 6 months with near daily checking.

The above snapshots are from a 3800 kW FLS 2-compartment mill with SEPAX separator.

 

Should you like more information, please contact us.

 

                                                                

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