silicon for the chemical and solar industry XIII

coarse quartz lumps that do not have time to heat properly all the way to the center

before entering the hot zone. A third reason is if the set point for the current is too

low, corresponding to a low value of the Westly constant, also known as C

(see Eq.

8, where I is the current in kA and P is the furnace load in MW).

= I / P

2/3

(8)

Typical values of C

for medium sized furnaces are in the range 8,5 – 9,5. Lower

values of C

means that a higher fraction of the furnace load is delivered to the upper

part of the furnace. This increases the melting of quartz, which in turn increases the

amount of charge that is supplied to the hot zone. At the same time, there is less

energy available in the hot zone and the chemical conversion rate there decreases. A

too low C

value therefore results in more raw materials being supplied to the hot

zone than can be consumed by the endothermic reactions. Raw materials can then

build up below the electrode. Three main problems then occur:

1. The molten quartz does not conduct electricity. Too large amounts of it below

the electrode can thus prevent the electric arc from burning down towards the

metal pool. Instead it may be forced to burn from the electrode flank to the

side of the cavity wall. This means that even more of the energy will be

delivered to the upper part of the furnace and the problem may escalate.

2. A surplus of molten quartz may gather in the tapping channel or in its inlet and

cause problems in tapping the furnace.

3. The temperature may decrease in the hot zone because more energy is

delivered high up in the gas-filled cavity and less further down. This will

increase the SiO/CO ratio of the gas leaving the hot zone, which is

unfavorable as described earlier.

Good furnace operation requires that the C

value is chosen such that the material

transport to the hot zone matches the chemical consumption there. The energy balance

is then correct.

Situation 1 above normally changes the dynamic behavior of the electrode. An

electric arc that burns downwards against the metal pool with no major obstructions

will usually move only short distances up and down, typically less than 15 cm, in

response to relatively small dynamic changes in the furnace resistance. However, a

major change in resistance occurs once insulating quartz blocks the electric arc so that

it suddenly jumps over to a highly conducting area of the cavity wall. The current then

increases abruptly and the electrode regulator responds by lifting the electrode to

lower the current as it expects to increase the length of the electric arc by this.

However, since the electric arc now all of a sudden burns high up on the electrode

flank, there will be little, if any, increase in the length of the electric arc as the

electrode moves up. Instead the root of the electric arc just slides along the electrode

flank until the tip of the electrode reaches up to where the electric arc burns. Then the

resistance starts to increase and the electrode lifting stops. We typically see an

electrode that moves up 30-50 cm in a few minutes as it connects to the side of the

cavity wall in this way.

Once the electrode tip is lifted this far up, there is no easy way for it down again, at

least if the cavity wall has not become a narrow cylinder around the electrode. Instead

it must work its way gradually down again by reacting or melting away parts of the

cavity wall so that the electric arc length increases or the electric conductivity of the