New Start-Up Processes for Submerged Arc Furnaces

New Start-Up Processes for Submerged Arc Furnaces

Submerged arc furnaces must be restarted after major or medium-scale repairs. Before the 1980s and 1990s, the start-up methods used for these furnaces largely followed traditional practices, which were characterized by long start-up cycles, high consumption, and heavy manual labor. In recent years, in order to reduce start-up costs, ferroalloy researchers and engineers in China and abroad have conducted extensive research and practical innovation to develop new furnace start-up processes based on traditional methods.

The main objectives of starting up a ferroalloy electric furnace are to bake the furnace lining, remove gases, and sinter and consolidate the lining; additionally, the electrodes must be baked to ensure that both the furnace and electrodes meet the required metallurgical conditions before charging materials.

Traditional Start-Up Methods

1. Start-Up Process for Newly Built Furnaces or Furnaces After Major Overhauls

Build a checker wall (or iron barrier) → wood drying → coke baking → electric baking → charging.

2. Start-Up Process After Long-Term Shutdown

Dig out the old burden, repair the furnace lining → build a checker wall (or iron barrier) → wood drying, coke baking → electric baking → charging.
These traditional methods require a long time, consume large amounts of materials, incur high costs, and impose high labor intensity. Based on practical experience, several improved “new start-up processes” have been summarized.

New Start-Up Processes

1. Direct Electric Baking of Electrodes

The electrodes are placed directly on the furnace bottom, and conductive materials such as coke are packed around them to a height of 0.5–1 times the electrode diameter, slightly higher between the electrodes. After power is supplied, a circuit (mainly a delta connection) is formed between the electrodes, with coke serving as the conductive medium. In practice, several scenarios have been encountered:

1. Pre-baking with Coke:
   Approximately one-third of the baking length is achieved using coke, followed by continued electric baking.

2. Retaining hard electrode tips after major furnace overhaul:
   Each electrode retains about 0.5 m of hard tip.

3. No hard electrode tips remaining:
   The electrode ends are completely sealed with welded steel plates, refilled entirely with electrode paste, and then directly baked electrically.

After power is supplied, an appropriate power-supply regime is adopted to bake the electrodes while gradually heating the furnace lining.

2. Direct Electric Heating of the Furnace

For furnaces stopped without burden removal (or after burden removal but keeping the old electrode tips intact during medium/major repairs), low-load power can be supplied initially to directly heat the furnace for start-up.

Theoretical Basis of the New Start-Up Processes

When retaining old electrode tips and starting the furnace through direct electric heating, the primary concern is preventing hard electrode breakage after power application. Smooth temperature transition—from room temperature to the high temperatures required for smelting—is critical to start-up success.

In self-baking electrodes, internal stresses arise from uneven temperature distribution, load conditions, and microstructural differences. When internal stresses exceed the ultimate strength, cracks develop. Frequent or sudden temperature fluctuations cause cracks to merge and grow, leading to catastrophic electrode breakage.

Studies on the effects of electrode current fluctuations on thermal stress (see Fig. 1) show that thermal stress increases with the number of cyclic current variations. Surface stresses are 1.6 times higher than stresses at the electrode center. According to the mechanisms of thermal stress generation and distribution, reducing the rate of change in electrode current can prevent crack initiation and propagation within the electrode.

Measures to prevent electrode breakage include:

• Reduce the current gradually before shutdown; never shut down directly from full load.

• Increasing furnace load slowly when re-energizing;

• Ensuring that load increases as continuously as possible within the transformer’s tap-changing limits to minimize the current variation rate.

These measures help prevent thermal shock and hard electrode breakage during start-up.