Crystallization

Crystallization is the reverse process of melting. When a single substance is cooled down from liquid state, crystallization should initiate at the same temperature as fusion. A multi-component system will similar behave, read chapter 2.2.

If no crystallization is developed during cooling, the result is called “glassy state” or “glass”. Basic work about crystallization was done by Tammann [8]. In view of glasses, crystallization is defined as “devitrification”. Within this work, the term crystallization describes the formation of mineral particles in all kinds of investigated slags. Two important parameters represent crystallization [36]:

1. Nucleation, the formation of crystal seeds, and

2. crystallization velocity, the rate of forming and growing of crystals. Crystallization of melts occurs in three modes [114]:

1. Polymorphic crystallization, in which the glass transforms into a crystal of same composition.

2. Primary crystallization, in which glass and crystal have different compositions. 3. Eutectic crystallization, which is a cooperative transformation to two phases while

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5. Advanced Slag Viscosity Modelling 40 Water and oil are the best heat transfer media at room temperature [117]. Also, quenched samples can be examined by X-ray diffraction (XRD), chapter 6.3.2. 2. Hot-stage microscopy: A microscope with a heated sample stage takes

permanently pictures of a melt. The method is sufficient enough for small samples and moderately cooling rates [36].

3. Hot thermocouple microscopy: Two methods are available. First method is the single hot thermocouple technique (SHTT). There, the sample is pointed on the pearl of a single thermocouple and viewed by a microscope. An alternating or pulsed electric voltage is heating the TC. Between the pulsed heating cycles, the temperature at the tip is indicated by the measured thermoelectric voltage. The second method is double hot thermocouple technique (DHTT). In this case, the sample can be pointed between two thermocouples. Both thermocouples can be alternately used as heater or temperature indicator [113,118].

4. Differential thermal analysis (DTA) and differential scanning calorimetry (DSC): Detecting the heat of phase changes (melting/solidification/solid phase changes) is a quick method to observe the thermal behavior of a sample.