Metal or ceramic ladles ? Pros and cons

The advantages of metal ladles are their relatively low price compared to ceramic, their robustness and therefore ability to withstand shocks and falls, and the fact that they are (or were?) easier to attach to robots or loaders. However, these ladles need a coating to prevent metallization, erosion and ferrous impurities and the coating must be redone several times during production.  They must also be pre-heated to reduce thermal shock and heat loss of the liquid alloy.

Thermal conductivity

The advantages of ceramic ladles are mainly these:

  •                 They are lighter
  •                 No need for pre-heating
  •                 They have excellent corrosion resistance
  •                 A very long operating life (up to 100,000 shots)
  •                 No ferrous impurities
  •                 Low wettability with aluminum alloys and therefore easy removal of residues                       (they are usually   coated with boron nitride)

However, advantages and disadvantages are not always quantifiable and the question about which is the best risks remaining a bit unanswered. But there is one point that is very important and that can be measured: the different thermal conductivity, i.e. how many degrees does the alloy lose while it is in the ladle?

The thermal conductivity of a preheated metal ladle can be as high as 30 W/mK (Watts per Kelvin meter). The ceramic ladles proposed by Mambretti Metalli are made of SiO2-CaO composite ceramic with high porosity (1600 kg/m3, the material is foamy/porous) and have a very low conductivity, 0.43 W/mK. But how does this translate into practice?

Let’s imagine a situation like this: a metal ladle contains 6 kg of aluminum, T=670 C° thermal conductivity: k=30 W/mK, ladle thickness: L=0.02 m, contact area: A=0.033 m2, time: t=20s

We feed the problem to the AI and in a few seconds, we have the solution: the temperature loss of the alloy (conduction only, no convection/radiation) varies from 18° to 86° depending on the temperature of the ladle after preheating, the ambient temperature and the distance of the liquid alloy from the wall of the ladle.

The same calculation with the ceramic ladle results in a temperature loss of 1–1.2 °C, i.e. there is virtually no temperature loss during the transport phase from the furnace to pouring.

The lower (or absent) loss of temperature has a double advantage: the temperature in the ladle remains homogeneous while with the metal ladle it can be very inhomogeneous between the areas near the walls and the central ones (moreover, the alloy begins to solidify along the walls and dendrites are formed from the outside to the inside); the holding furnace can be kept at a lower temperature and this is an easily quantifiable advantage.

The answer to the initial question is therefore to carefully evaluate the choice of ladle material. There are still cases in which the metal ladle can be convenient but for quality castings and sustainable productions the cost/benefit ratio could be reversed in favor of ceramic ladles.