Crucible Sintering Process

High-Temperature Sintering of Magnesia Sand Crucibles

During the heating process of a magnesia sand crucible from low temperature to high temperature, various physical and chemical reactions occur. According to the reaction characteristics, the process can generally be divided into four stages.

Stage 1: Below 850°C

Below 850°C, the main reaction is dehydration of the sand material. In this stage, both adsorbed water and crystal water contained in the sand are completely removed. As moisture is eliminated, the bonding effect caused by hydroxides disappears, and the crucible strength drops to its lowest level.

In addition to dehydration, when the temperature rises above 800°C, a small amount of carbonate decomposition also occurs, such as:

MgCO3 = MgO + CO2 ↑
CaCO3 = CaO + CO2 ↑

This shows that a large amount of water vapor and carbon dioxide is released from the crucible during the first stage. Since the crucible strength is extremely low at this time, rapid gas release can easily cause cracks. Therefore, the heating rate must be slow and carefully controlled when designing the heating process.

Stage 2: 850-1500°C

In the range of 850-1500°C, low-melting-point compounds begin to melt, the sintering network starts to form, and the crucible body shows obvious shrinkage.

The compounds that begin to melt include:

MgO·Fe2O3
CaO·MgO·SiO2
2CaO·Fe2O3
3CaO·MgO·2SiO2
3CaO·Al2O3
4CaO·MgO·Fe2O3

As the purity of magnesia sand increases, the amount of impurity compounds decreases, and the volume change of the crucible becomes smaller. Therefore, the heating rate may be increased appropriately in this stage. Since the crucible strength gradually increases, a higher heating rate generally does not create a serious cracking risk.

Stage 3: 1500-1700°C

In the range of 1500-1700°C, the original complex compounds in magnesia sand, such as forsterite (2MgO·SiO2), dicalcium silicate (2CaO·SiO2) and magnesia-alumina spinel (MgO·Al2O3), begin to melt. New compounds start to form, MgO transforms into periclase, and fine periclase crystals fuse and grow.

The impurity compounds already present from the second stage, such as magnesium ferrite and calcium ferrite, begin to dissolve into periclase, reducing their harmful effect. At this point, the sintering network forms rapidly, the crucible shrinks sharply, and both density and strength increase significantly. Finally, a sintered layer mainly composed of periclase crystals is obtained. This sintered layer does not absorb moisture or deliquesce and has good chemical corrosion resistance.

Because the sand body shrinks greatly in this stage, the heating rate should be reduced appropriately to avoid cracking caused by excessively rapid temperature rise.

Stage 4: 1700-1850°C

In the range of 1700-1850°C, the purpose is to promote further growth of periclase crystals and obtain an ideal sintered layer thickness and cross-sectional structure.

The maximum sintering temperature has an important influence on porosity and bulk density. The temperature and holding time in this stage must meet the requirements for obtaining a sintered layer with the lowest porosity, highest bulk density and suitable thickness.

To obtain excellent sintering quality, the maximum sintering temperature should generally be 1800-1850°C. If a graphite core is used for sintering, the core center temperature should be controlled within 1850-1900°C, so that the sand material temperature can be maintained at 1800-1850°C.

Cross-Sectional Structure of the Sintered Crucible

From the cross-sectional structure of the crucible, the sintered layer is entirely composed of well-developed periclase. Its thickness accounts for about 30% of the total wall thickness and appears gray. In the semi-sintered layer, the periclase crystals gradually become smaller until they disappear.

A crucible with this structure has a long service life. Under intermittent use, a crucible with a capacity of 150 kg can usually be used about 50 times.

Low-Temperature Sintering of Magnesia Sand Crucibles

Magnesia sand materials containing additives such as boric acid, when rammed with a steel plate core, usually use a low-temperature sintering method.

The low-temperature sintering process is carried out in two steps. First, induction heating is used to heat the sand material to about 1300°C through the steel plate core for preliminary sintering. Then the core is removed, and low-carbon steel or industrial pure iron is charged for the second sintering step. At this stage, sintering is mainly carried out by using the temperature of molten steel.

The two-step sintering process can be divided into three stages.

Stage 1: Below 850°C

Below 850°C, the main reactions are moisture evaporation, dehydration and borate decomposition. Since sand containing boric acid is generally formed by wet ramming, a large amount of adsorbed water evaporates at the beginning of heating.

At the same time, crystal water introduced by boric acid begins to undergo dehydration reactions:

100°C: H3BO3 = HBO2 + H2O ↑
171°C: 2HBO2 = B2O3 + H2O ↑
302°C: Dehydration is complete

As the temperature rises, remaining carbonates begin to decompose and release carbon dioxide. This process ends at about 850°C. Therefore, below 850°C, because a large amount of gas is released from the sand material, the heating rate must be slowed down to prevent early cracking.

Stage 2: 850-1400°C

In the range of 850-1400°C, a large number of low-melting-point compounds containing B2O3 are formed, such as:

CaO·B2O3
CaO·B2O3·2SiO2
MgO·B2O3
SiO2·B2O3
2MgO·B2O3
3MgO·B2O3

The sintering network forms rapidly, the crucible strength increases, and the crucible body undergoes significant shrinkage at high temperature. In addition to the above compounds, low-melting-point compounds in the magnesia sand also participate in sintering.

Stage 3: 1500-1650°C

In the range of 1500-1650°C, the purpose is to further expand the thickness of the sintered layer after preliminary sintering and obtain an ideal sintered structure.

Fine cracks existing on the inner surface of the crucible after preliminary sintering can be welded together by the molten steel during further sintering, preventing them from extending outward. After this process, the crucible can be put into use and will continue to sinter during operation. This is the main feature of the low-temperature sintering method.

At present, large induction furnaces using basic crucibles mainly adopt the low-temperature sintering method, except for crucibles built with refractory bricks.

High-Temperature Sintering of Magnesia-Alumina Spinel Crucibles

Magnesia-alumina spinel crucibles are generally sintered at high temperature. Boric acid in the sand mainly promotes the formation of spinel and enables partial sintering in the semi-sintered layer. The sintering process can be divided into three stages.

Stage 1: Below 850°C

Below 850°C, the physical and chemical reactions are the same as those in the low-temperature sintering of magnesia sand crucibles.

Stage 2: 850-1400°C

In the range of 850-1400°C, spinel begins to form and a sintering network develops.

After industrial alumina is added to fused magnesia or other high-purity magnesia sand, all spinel can be formed within the temperature range of 1000-1300°C under the promoting effect of boric acid. However, Na2O and K2O in the sand have a harmful effect on spinel formation.

Stage 3: 1300-1700°C

In the range of 1300-1700°C, spinel aggregates and grows. Its connection with periclase becomes tighter, the porosity of the sand decreases, and the strength increases.

During high-temperature sintering, boric acid can improve compressive strength, but it does not affect thermal stability. At 1700°C, all Al2O3 forms spinel.

Because the thermal expansion coefficient of periclase is much smaller than that of spinel, obvious microcracks and voids remain between periclase and spinel crystals after firing, and they cannot be sintered into one fully integrated body. These microcracks and voids can buffer the stress caused by rapid heating and cooling. Therefore, spinel crucibles have excellent resistance to thermal shock.

Low-Temperature Sintering of Quartz Sand Crucibles

Acidic crucibles made from quartz sand and boric acid with a steel plate core are mostly sintered by the low-temperature method. The sintering process can generally be divided into three stages.

Stage 1: Below 850°C

Below 850°C, the main reactions are moisture evaporation, boric acid dehydration and polymorphic transformation of quartz.

During heating from room temperature to melting, quartz undergoes several polymorphic transformations accompanied by volume changes. This is very important when formulating the sintering process for acidic crucibles.

The volume expansion caused by quartz transformation below 850°C is very small and does not affect the sintering quality of the crucible. The heating rate below 850°C is similar to that used for low-temperature sintering of magnesia sand crucibles.