Alumina Feeding Diagrams

A graphical representation is more instructive than a listing of numbers. The next figure shows a graphical representation of the five feeding phases discussed on the Alumina Feeding Page. The alumina concentration is plotted against time. AlPrg represents the phases as differently shaded areas and the start respectively the end alumina concentration by circles. The upper dark region is limited by the maximal alumina solubility i.e. the alumina concentration (cAl2O3) is greater than the maximal alumina concentration (cAl2O3(max)). The lower dark region represents the area where cAl2O3 is smaller than the anode effect concentration (cAl2O3(AE)).  

When you move the mouse pointer over a feeding phase region AlPrg shows informations about the phase: phase name (Phase 1), alumina addition (Add. = 162.8 %), start (2 %)  and end alumina concentration (4.0 %), time period (130 min) and finally the feeding frequency (f  = 56.2 1/h).

Change the Feeding Phase Parameters

When you move the mouse pointer over an alumina concentration circle the mouse pointer changes to a hand and AlPrg draws the circle in red color:

You may drag the activated alumina concentration circle to another position on the diagram by pressing the following left mouse button (LMB) and key combination:

LMB:	change the alumina concentration (vertical dragging),
LMB+Shift-Key:	change the time period (horizontal dragging),
LMB+Ctrl-Key:	change simultaneaously the alumina concentration and time period.

AlPrg calculates a new alumina concentration and time period corresponding to the modified position of the data circle and updates accordingly the phase parameters.
In the next figure the user dragged the alumina end concentration of Phase 1 (respectively the start concentration of Phase 2) to the new value of 3.0 %.

When you drag the an alumina concentration circle with the Shift-key down you move horizontally the mouse pointer with the alumina concentration circle. In this way you modify the time period value of the phase while the alumina concentration stays constant.

Finally when you drag the an alumina concentration circle with the Ctrl-key down you change simultaneously the alumina concentration and time period:

After you have finished your dragging action AlPrg recalculates the values of the phases and draws a new diagram.  

Select Items to be Drawn

Beside the alumina concentration AlPrg draws other values in function of the time. You click on the select button to open the Select Item Dialog Box and to select such an item. The next figure shows the Select Items Dialog Box with the checked cell voltage and the corresponding Alumina Feeding Diagram:

The typical shape if the cell voltage curve in function of time i.e. in function of the alumina concentration is the basis of the indirect determination of alumina concentration by the electrolysis process control. The slope and the position of the minimum of this curve indicate the momentary alumina content in the electrolyte.
The next figure shows same diagram representing all items: alumina concentration, cell voltage, electrolyte temperature and the electrochemical bath voltage and ohmic bath voltage drawn versus time. The cell resistance is not shown because it contains the same information as the cell voltage:

Change Anode Type

The next figure shows what happens when you change the alumina between the 1  and 3 % in a time period of 90 minutes: the cell voltage fluctuates between 3.7 and 3.8 V while the cell temperature changes by 15°C because the superheat is constant. This voltage behavior is due to the different dependence of the electrochemical and ohmic bath voltage on the alumina concentration:

If you change the anode type from carbon to inert anode this voltage change is much smaller namely by about 0.03V:

Consequently it is more difficult to use the cell voltage as indicator for the alumina concentration for electrolytic cells with inert anodes .