Slag and/or Dross Formation Tendency

The dictionary meaning of slag and dross are the same that is residue left behind in the melting process of metals. This residue is a waste by product and processors would like to avoid this waste.

In foundry terminology, slag and dross do not hold exactly same meaning. Slag is said to be refuse produced as a result of melting non ferrous material which have a high melting point. These products are usually made up of liquid non metallic components and are an outcome of alloying and oxidation in air. Whereas dross is said to be related to non ferrous alloys, which have a low melting point. These are produces when molten metal comes in contact with air.

Some molten metals leave behind more slag / dross than others. Castings made from such alloys leave behind higher quantity of non metallic refuse. However, some casting techniques, quality checks and design factors which can help significantly reduce occurrence of non metallic components. Better design principles if used reduce occurrence of non metallic components in casting process which affect the superiority of the casting.

The best way to reduce the occurrence of non metallic components is to stop their formation through good casting procedures and practices. Alloys which have a high fluid life can use ceramic filters which increasingly help a foundry reduce non metallic inclusions. Alloys like titanium which have a high incidence of dross should use vacuum melting and pouring to avoid dross.

Aluminum oxide dross layer, which forms a layer on the top of molten metal, can be removed from the molten metal through drossing fluxes. Dross and metal (in liquid or solid form) are usually blended together in this layer of dross. When drossing fluxes react on aluminum oxide in slag or dross layer, metal can usually be separated and recovered. The fluorides present in drossing fluxes liquidate and melt the oxide layer.

This reaction is 6Na2SiF6 + 2Al2O3 ? 4Na3AlF6 + 3SiO2 + 3SiF4

If the reaction causes a sufficient shaking up of the oxides, the oxide layer is broken down and metal can be separated from the film. Aluminum industry vastly uses this technique of drossing fluxes to avoid wastage of metal in dross. Raw dross may have as much as 60 % - 85% metal content, which is wasted, if it is allowed to burn up.

Wall cleaning fluxes can be applied to walls with a gunning device. These fluxes help reduce the oxide layer that forms on the walls.

Molten zinc can also be recovered from dross where zinc coating machinery and zinc pot containing metal which is to be quoted is being used.

  • A pressurized motor is used to supply power. This motor has a piston joined to the cylinder.
  • A ladle with a long handle is used to manually skim the dross for metal. This skimmer is U shaped.
  • A firm support is provided to the ladle in the form of a C shaped frame. The lower portion of this frame has a spherical seat, which is similar to the U shape of the skimming apparatus. This C frame supports the ladle under the ram.
  • A drive shaft is attached to the upper portion of the C frame at one end and the motor supplying power at the other end. This helps to and fro movement of the ram.
  • The top portion of the ram is connected to the drive shaft. The ram top is shaped as an inverted U.
  • The u shaped punctured wall is similar in shape to the top of the ram.
  • The curved shapes of the ram and ladle make a concentric shape is powerfully inserted into pool of dross and zinc. The droplets of metal thus combine together and attach themselves into the ladle wall. The metal is forcibly separated from the dross.
  • The molten zinc extracted from the dross is collected in the zinc pot.
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