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 115°C - fully
open water valve at 130°C (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.