Arc Control During the Start-Up Melting Stage of Molybdenum Electrodes

In the first hours after a cold start, a stable conductive path has not yet formed between the molybdenum electrode and the molten glass, and current can pass only through localized contact points. The contact area is small and the current density high, so the temperature near the contact point rises rapidly, the surrounding glass is vaporized and ionized, and visible arcing appears. This phenomenon is hard to eliminate completely during the start-up stage; what can be done is to keep its intensity and duration within an acceptable range.

Arcing damage usually appears in three places: the electrode end face, the side of the electrode at the melt line, and the gap where the electrode passes through the brick opening. Repeated arcing pits the end face and causes local melting erosion; once a crater has formed, the current tends to concentrate on its rim during normal operation. On the shop floor, whether intervention is needed can be judged by whether the arc changes from intermittent to continuous, whether the sound changes from short cracks to a sustained hum, and whether the swing of the current meter needle grows larger.

The Ramp Profile of the Start-Up Current

At the beginning of power-on it is not appropriate to set the target power directly. A more reliable approach is staged ramping, holding at each stage for a period so that a sufficiently large contact area is established between the electrode and the glass before moving to the next stage. If the ramp is too fast, the contact points cannot expand in time and arcing turns from occasional to continuous; if it is too slow, the glass melt is underheated and its viscosity stays high, convection around the electrode is difficult to establish, and conduction conditions likewise fail to form.

1. Preheat stage: pass a small current through the electrode and focus on whether there is abnormal arcing or smoke near the brick opening.
2. Transition stage: raise the current step by step while watching whether the melt surface begins to move slowly.
3. Stabilization stage: hold for a period after approaching the target power, and continue only once the range of current fluctuation has narrowed.

Melt Coverage and Observation Positions

The degree to which the molten glass covers the electrode directly determines how long arcing lasts. When the melt level is low, the upper part of the electrode is exposed to the furnace atmosphere, which makes it prone to oxidation and to repeated arcing near the melt line. During start-up, the melt level should where possible be kept a certain distance above the electrode end face, and prolonged operation at high current with a low melt level should be avoided. Observation positions are best fixed at two points, directly above the electrode and at a side observation port: the first shows the color and length of the arc, the second shows the movement near the melt line.

Judgment during the start-up stage is based mainly on trends: at the same current, a shorter arc, a more even sound and a narrower current swing indicate that conduction is improving; if the opposite is seen, the ramp should be slowed.

Order of Response for Abnormal Conditions

If continuous arcing appears and the current cannot be stabilized, the usual order is to reduce the current first, then check the melt level, and then check the condition of the electrode and the brick opening; it is not advisable to start with large power adjustments or frequent operations. Reducing the current lets the contact-point temperature fall back and gives the molten glass time to re-cover the electrode. Once the melt level is confirmed, the observation port can be used to judge whether the electrode has shifted and whether there is a gap at the brick opening. Service conditions vary considerably, and judgments involving electrode structure and furnace design can refer to the structural notes in the molybdenum electrode technical documentation, or involve the field service team in the assessment.

Records from the start-up stage matter as well, including the dwell time at each current stage, the location and duration of arcing, and changes in melt level. Reading these together with the data from subsequent normal operation helps to judge whether this start-up has damaged the electrodes.