Matching Refractories to Molybdenum Electrodes in Electric Melting Furnaces

In an electric melting furnace the electrode is not an isolated component. It passes through the electrode block into the glass melt, surrounded by paving blocks, sidewall blocks and insulation. Whether the materials are chosen correctly often only becomes apparent months into operation: an enlarged electrode port, a cracked block, glass melt leakage. Trace any of these back and the cause is a matching problem.

The refractories around the electrode must satisfy three requirements at once: resistance to high temperature, resistance to glass melt corrosion, and compatibility with the thermal expansion behaviour of molybdenum. It is difficult for one material to carry all three, so duties are usually divided by location. The electrode block is in direct contact with the electrode and the hot glass melt and needs high density and low porosity; the sidewall blocks take the scouring of the melt flow and temperature fluctuations; the insulation layer is there for thermal insulation, keeping heat loss around the electrode port from becoming excessive. For specific furnace types, see the material combinations in the electric melting furnace solutions.

Thermal Expansion and Installation Clearance

The linear expansion coefficient of molybdenum is close to that of most oxide refractories, but differences remain. The electrode elongates as the furnace heats up; if the clearance allowed in the port is insufficient, the block may carry additional stress, while if the clearance is too large, glass melt can seep into the gap and form a hard shell on cooling that seizes the electrode. Design should calculate the elongation at the working temperature and set the clearance together with the sealing method. Dimensional and visual inspection of electrodes on receipt can follow the procedure described under quality control.

Setting the clearance also has to account for expansion of the block itself. The electrode block expands relatively quickly in the early stage of heat-up, while elongation of the electrode is more gradual, and the difference between the two peaks in a certain temperature interval. Calculating only at the working temperature can lead to compression at low temperature. The safer approach is to calculate several key temperature points and take the envelope value for the clearance.

Electrode Block and Electrode Port Design

The shape, taper and chamfer of the electrode port affect how the glass melt flows at the opening. If the opening is too sharp, the melt forms a vortex there and corrosion accelerates; if it flares too much, the heat-dissipating area increases, the temperature at the opening drops, and devitrification or condensation becomes likely. Practical designs usually draw on experience from similar furnaces and check the opening temperature by thermal calculation. The material of the electrode block also depends on the glass type: high-boron or fluorine-containing formulations corrode the block differently from soda-lime glass. For the material and dimensions of the electrode itself, see the product data.

The number and spacing of electrode ports affect the overall strength of the block. If the ports are too closely grouped, thin walls form between them and cracking under temperature fluctuation becomes likely; if the spacing is too wide, electrode power distribution becomes uneven. Design usually determines this together with the thermal calculation and the block structure, using shaped blocks to spread stress where necessary.

Checks During Operation

• Watch for glass melt seepage or white crystalline build-up around the electrode ports.
• Record electrode advance and current variation to judge whether the port is seizing.
• Check the surface temperature of the sidewall blocks; a local rise may indicate thinning of the block.
• After a furnace stop, measure electrode port diameter and block cracks, and update the corrosion record.

Matching problems are rarely corrected well after production starts; most have to be resolved at the design stage. Considering material properties, thermal expansion, port structure and installation practice together leaves far fewer anomalies in operation.

Once the blocks and electrodes of an electric melting furnace are mismatched, handling the problem on site usually costs more than the material selection work done up front.