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Cooled finger technique uses the principle of fractional crystallization for the purification metals.

Origin
The concept is first mentioned in a 1982 Japanese patent for the production of high purity aluminum. But, further scientific investigations or publications were not found.

Recently, this concept is being researched and investigated by the pure metals team at the IME institute at RWTH Aachen university in Germany for the purification of various metals such as aluminum, magnesium and germanium among others.

Process
This technique can be used for the production of ultra high pure metals in relatively large quantities. Some highly pure metals need a significant amount of time and energy to produce by using conventional techniques such as vacuum distillation. The energy and time requirements can be significantly reduced when using the cooled finger apparatus to perform fractional crystallization.

Theory
This process makes use of the solubility difference of the impurity in the molten and solid phase of the base metal. The degree of purification due to solubility difference can be defined by the distribution coefficient k.

$$k = \tfrac{C_s}{C_l} $$

where Cs and Cl are the impurity concentrations in solid and liquid respectively. The value of the distribution coefficient for a certain impurity can be obtained from the binary phase diagram at a constant temperature if other impurities are ignored. However, this value gives only a theoretical assumption without considering the real scenarios like growth rate, diffusion of the impurity to the melt and the thickness of the boundary layer. The work done by Burton, Prim and Slichter considers the non ideal scenarios and gives an formula to find the effective distribution coefficient keff.

$$k_{eff} = \frac{k}{k+(1-k).\exp\left [ \frac{-V\delta}{D} \right ]} $$

where k is the distribution coefficient, 𝛿 is the thickness of the diffusion layer, D is the impurity diffusivity in the melt and V is the solid growth rate. Mechanical agitation, such as using the cooled graphite rotating rod, reduces the temperature gradient between the crystallization front and the melt, allowing the heat flow from the melt to the interface to be as small as possible. This rotation will also promote melt mass flow, resulting in not only a lower thermal gradient but also a stable boundary layer in front of the solid-liquid interface, allowing for uniform impurity segregation.

If the k value is lower than unity, the impurity gets removed from the solid phase and gets concentrated on the liquid phase and vice versa. Impurity elements having k value one order of magnitude less than unity (k < 0.1) can be more easily removed than those elements having k value slightly less than unity (0.1 < k < 1). By using this phenomenon, the impurity elements can be removed and a pure metal phase is obtained.

Device setup
The apparatus consists of long, thin graphite rod that is cooled by circulating a cooling gas, usually argon and can be rotated by the use of a motor. The graphite rod along with the motor is connected with a mechanism that can raise or lower its height.

A graphite crucible is used to contain the metal and it is placed inside a furnace. The furnace is connected to a control device, where the temperature can be set. A thermocouple is placed inside the crucible to measure the temperature and the temperature can be recorded for further evaluations.

Working
The crucible containing the metal is placed inside the furnace and the furnace is turned on. The furnace temperature is set just above the melting temperature of the metal inside the crucible to ensure complete melting.

The cooling gas supply and the motor for the graphite rod are turned on, and the flow rate and rpm are set to the optimum values. The graphite rod is lowered into the molten metal in the crucible. The molten metal starts to solidify on the graphite rod and grows laterally. After a period of time, the lateral growth stops and the graphite rod can then be raised away from the crucible and the rotation is stopped by the turning off the motor.

The furnace is turned off and everything is allowed to cool. Once the metal reaches room temperature, the graphite rod is disassembled and the metal can be sent for analysis.

Applications
Aluminum purification: Impurities such as iron and silicon can be removed effectively.

Magnesium purification: Impurities such as iron, aluminum, calcium, zinc can be reduced.

Selenium purification: Impurities such as iron, lead, mercury can be removed.

Germanium purification: Purification of the initial material from 98.8%. up to 99.9%.

Issues requiring future research

 * Hazardous metals require a relatively complicated setup to ensure concealment to prevent the evolution of harmful metal vapors.
 * Metals with a low self-ignition temperature such as magnesium necessitates an reaction atmosphere without oxygen.
 * It is harder to get rid of impurity elements having distribution coefficient greater than unity.