Iron in secondary aluminum

The most harmful impurities

Iron It is considered one of the most harmful impurities aluminum. This is especially noticeable in the manufacture of cast aluminum alloys for casting in a metal mold and a sand mold. Iron together with aluminum and other alloying elements, such as manganese, copper, magnesium and silicon form a phase intermediate iron, that significantly reduce the mechanical properties of the final product.

Iron, aluminum scrap

The main pollution iron aluminum It occurs during the melting of aluminum scrap, which is mixed with an iron bar. Moreover, Aluminum scrap may itself contain a high concentration of iron. It belongs, for example, to waste aluminum casting alloys under pressure. To minimize the problem of pollution of aluminum scrap iron, aluminum industry employs rigorous scrap sorting before loading it into a melting furnace. This involves manual sorting, magnetic sorting, electromagnetic sorting and so-called "air knife" for sorting crushed scrap particle density.

Dilute secondary aluminum with primary aluminum?

A simple way to reduce the iron content of aluminum is to dilute it with primary aluminum.. Often, however, it is not profitable.

Iron-containing particles in liquid and solid aluminum

In conventional solidification of aluminum-silicon casting alloys typically begins to harden the first aluminum phase. Other components of the alloy remains liquid accumulate in the areas between the grains of the primary phase. Iron contamination can lead to changes in the solidification order of the phases: particles of the iron-containing intermediate phase appear first, and only then crystallization occurs alumina grains. When these particles are formed before aluminum, they are free to grow and so grow up in the rough crystals, surrounded by a liquid phase.

Effect of manganese at an iron removal efficiency

In the casting of aluminum alloys typically used to reduce the aluminum silicon melt viscosity. Alloys of aluminum-iron-silicon has a relatively high solubility of iron in liquid phase even after removal of the primary crystals, containing iron. therefore, for reducing the iron content in the liquid phase in the alloy added manganese, which transforms the system aluminum-iron-Kremin in aluminum-iron-manganese-silicon.

The addition of manganese gives the change in the solidification mechanisms, facilitating release of intermediate phases, containing iron, as well as reduction of the residual concentration of iron in the liquid phase to acceptable limits for casting alloys. The molten aluminum is maintained at an intermediate temperature between the formation of the intermediate phase and the appearance of aluminum. Separating the solid phase from the liquid by filtration of the melt occurs at this temperature.

The process of removing iron from aluminum

The efficiency of manganese additives on the kinetics of formation of primary phases during solidification of the aluminum alloy casting provides a method for removing iron by precipitation of the intermediate phases, followed by filtration through a ceramic filter. However, the easier and cheaper it can not be called. Nonetheless, it makes it possible to use scrap aluminum from any source, regardless of the original content in the iron, for applications, which require low iron content, such as aluminum casting molds or sand molds.

Each composition of the secondary aluminum melt requires a certain amount of manganese and silicon additions (up to the composition of casting alloys) to achieve high iron removal efficiency. The amount of added manganese depends on the content of iron in the melt.

Removal of iron from aluminum is carried out in four stages (figure):
1) adding manganese to the melt and, if necessary, silicon;
2) controlled cooling of the melt to separate particles of the intermediate phase Al (FeMn) Si in the melt;
3) settling the melt for the deposition of iron-containing particles on the bottom of the furnace;
4) filtration of the melt through a ceramic filter.

Figure – Scheme of the process of removing iron from aluminum [1]

1. Removal of Iron from Molten Recycled Aluminum through Intermediate Phase Filtration // Materials Transactions, Vol. 47, No. 7 (2006)