Manufacturing Example of a 150 kg Pure Magnesia Crucible

This section introduces practical examples of making crucibles with different refractory materials, providing a reference for actual production and operation.

1. Equipment Overview

The crucible has a capacity of 150 kg. The power supply is 100 kW, provided by a 2500 Hz medium-frequency generator set. It is mainly used for melting low-alloy steel and stainless steel.

2. Magnesia Sand and Particle Size Ratio

The magnesia sand used is first-grade brick-making magnesia with an MgO content of more than 92%. The particle size ratio is as follows:

No boric acid is added to the sand mixture. Only about 0.5% water is added to reduce dust during the ramming process.

The total amount of sand material is about 135 kg.

3. Crucible Ramming Method

The crucible is rammed manually and sintered at high temperature using a graphite core as the mold. The ramming process is divided into four parts: ramming the crucible bottom, placing the graphite core, ramming the crucible wall, and ramming and building the furnace mouth.

3.1 Ramming the Crucible Bottom

After laying glass fiber cloth and asbestos cloth inside the inductor, add two batches of sand material, totaling 30 kg. Use pointed steel rods with a diameter of 12-14 mm, and have two workers ram the material at the same time for 20-30 minutes.

Then add the third batch of sand material, about 15 kg, and continue ramming for 20-30 minutes. Add the fourth batch, also about 15 kg, and ram again for 20-30 minutes.

After ramming, check the position of the compacted sand layer and remove any loose surface sand. According to the shape of the end of the graphite core, dig a recess in the sand layer at the center of the inductor. The bottom of the recess should be 140-160 mm away from the furnace bottom brick, roughly corresponding to the second induction coil from the bottom. After this, prepare to place the graphite electrode core.

3.2 Placing the Graphite Core

Lift the graphite core wrapped with strawboard paper, align it with the center, and place it into the recess at the furnace bottom.

Measure the distance between the graphite core and the inductor on all sides, and adjust the position to keep it centered. Place a weight on the end of the graphite core to prevent it from shifting while the furnace wall is being rammed, which could affect the uniformity of the crucible wall thickness.

After the graphite core is positioned, check whether any foreign matter has fallen into the sand material. Then prepare to add more sand and ram the crucible wall.

3.3 Ramming the Crucible Wall

After the graphite core is placed, loosen the surface layer of sand at the furnace bottom with a steel rod. Add about 0.1-0.2 kg of fine magnesia powder with a particle size below 0.5 mm, mix it evenly, and then add the main sand material.

The crucible wall is rammed in 8-10 batches. Each batch contains 6-8 kg of sand material, and each ramming cycle lasts about 15 minutes. Two workers should alternate positions during ramming. Before each new batch is added, the same amount of fine magnesia powder should be added to help prevent layering.

The movement frequency of the steel ramming rod should be about 40 times per minute.

3.4 Ramming and Building the Furnace Mouth

When the crucible wall has been rammed up to the top induction coil, begin ramming the furnace mouth.

Because the furnace mouth area is above the slag line and has a lower sintering temperature, it cannot sinter as effectively as the lower part of the crucible. Therefore, the sand composition should be adjusted.

The furnace mouth sand mixture is:

Mix the materials evenly before use.

The furnace mouth sand is rammed in two batches with a flat-headed steel rod. Each batch should be rammed for about 10 minutes. After the furnace mouth is rammed, the pre-fired tapping spout and furnace mouth bricks can be laid. Once this is finished, the crucible forming process is basically complete.

4. Crucible Sintering

A dry-rammed crucible can be heated and sintered immediately. The sintering process curve for a 150 kg pure magnesia crucible is shown in Figure 3-21.

During sintering, the temperature is controlled by measuring the temperature inside the central measuring hole of the graphite core with an optical pyrometer. This measured temperature is used as the sintering temperature. The heating power and corresponding temperature during the sintering process follow the curve shown in Figure 3-21.

When the temperature reaches the planned level of about 1800°C, hold it for 10 minutes. Then loosen the graphite core, lift it slightly, and suspend it inside the crucible. Before loosening the graphite core, the power must be turned off. When the graphite core temperature drops to about 1400-1500°C, remove the core and inspect the inner surface of the crucible for cracks.

If no cracks are found, clean the crucible and begin furnace washing. If cracks are found on the inner surface, hot repair should be carried out. If hot repair is not possible, wait until the temperature drops before repairing.

Furnace washing is a very important step for a new crucible and must be done carefully to ensure smooth melting afterward. Industrial pure iron is the preferred washing material because it has a high melting point and poor fluidity. These properties help weld and seal microcracks on the working surface of the crucible, making the inner surface denser and improving the service life of the crucible.

After furnace washing, the crucible can be used for normal melting production. However, during the first few heats of a new crucible, it is better not to melt high-carbon steel or high-alloy steel. Low-carbon, low-alloy steel is more suitable for the initial heats.