The monolithic lining of furnaces for aluminum

Refractory linings for aluminum

The aluminum industry used a wide variety of furnaces and units, which require refractory lining. At all stages of the treatment of primary and secondary aluminum basically currently used modern, so-called, monolitnaya coating. Only the smelting of primary aluminum in some furnaces and assemblies continue to use firebrick, but also already of a special form and made of special materials [1].

Below, the features of applying monolithic lining in melting and casting furnaces, which are used in the production process of primary and secondary aluminum.

Lining for molten aluminum

melting, and distributing, furnace for aluminum and aluminum alloys have an operating temperature of about 750 ºC. This is considerably lower than the operating temperature furnaces for the production of iron and steel - from 1200 to 1500 ºC. It could seem, that the operating conditions and requirements for the refractory lining of furnaces for melting aluminum must be significantly easier, than furnaces for iron and steel. However, lining, which works with liquid aluminum, It faces its own challenges and features.

In an increasingly competitive lining modern smelting furnaces is subjected to increasingly harsh operating conditions by improving their productivity and reducing the time for their maintenance. This is reflected in the use of more active flux and accelerated methods of cleaning ovens, replacing burners for more powerful, as well as increases in the proportion of the charge of aluminum scrap. The combination of all these factors leads to an increase in the temperature of the furnace and the creation of a more aggressive atmosphere in the furnace [2, 3].

The response to tightening operation conditions lining of furnaces for aluminum and aluminum alloys, it became widely used in these so-called monolithic lining.

lining zone

Refractory lining of typical melting (holding) furnaces for aluminum must withstand a wide variety of physical and chemical influences [1-3]. Various furnace zone, which have their own particular operation, shown in Figure 1.


Figure 1 - Lining zones of the melting and holding furnace [4]

In general aluminum melting furnace have three distinct zones, which differ in terms of their operation [1]:

  • the upper zone, wherein the burners are, and which comprises a hot furnace atmosphere.
  • the lower zone, which is in contact with liquid aluminum.
  • "Slag belt" (belly band – “belly”), which adjoins the bottom area.

The liner in contact with the metal, i.e. in the lower zone of the furnace, wears out as a result:

  • chemical reactions with aluminum lining material,
  • mechanical damage during cleaning oven to remove slag from the walls and hearth of the furnace and
  • thermal shock, that arise when loading charge.

The lining in the upper zone of the furnace is exposed

  • elevated operating temperature
  • alkali metals
  • thermal shock by opening the windows boot.

The intermediate zone of the furnace ("slag belt") must withstand:

  • all of the above effects for the lower, and upper zones of the furnace due to changes in the level of molten aluminum
  • high local temperature, which can occur when using fluxes (for example, эkzotermicheskih flux – look. here).

Сorundum layer formation

The reactions between the aluminosilicate and the molten aluminum cause the formation of corundum. Although corundum - a "mineral" name of aluminum oxide Al2O3, in the context of the furnace lining - a mixture of Al2O3 c particles liner, and also with silicon and aluminum [2]. This product is unstable composition is often formed below the level of the melt and grows up, violating the integrity of the lining elements. It is very difficult to remove, since it is firmly fixed to the lining due to diffusion in its pores.

The figure 2 diagram corundum formation on the furnace lining.

Figure 2 - Diagram of the formation of corundum build-up on the furnace lining [2]

Reaction formation of corundum

Aluminum has strong affinity with oxygen, i.e, in other words, high ability to actively connect with oxygen. This leads to different mechanisms of oxidation of liquid aluminum (figure 3) [1]:

  • On the surface of the bath of liquid aluminum, its direct oxidation occurs: 4 Al + 3 O2 → 2 Al2O3
  • of coating, starting from the liquid contact surface (below the melt level). Due to its high affinity for oxygen, metallic aluminum diffuses from the melt into the lining and reacts with it according to the reaction: 4 Al + 3 SiO2 → 2 Al2O3 (TV) + 3 Si

These two mechanisms can interact with each other and catalyze each other [1].


Figure 3 - Reaction of lined liquid aluminum to form corundum [1]

What is a good lining

features linings, which can influence the formation of corundum:

  • Refractory aggregate (powder) type
  • Method assembly linings
  • Refractory matrix (very important)

refractory Matrix - a continuous phase of crystalline or vitreous refractory, containing isolated inclusions of other phases [5].

There are several ways to modify the lining to increase its resistance to liquid aluminum:

  • Reduction of the chemical components of the refractory lining, which have a lower affinity for oxygen, than aluminum
  • Optimization of the porosity of the refractory lining (Figure 4)
  • Reduced wettability of surface of the molten aluminum of the refractory lining.


Figure 4 - Penetration of aluminum into the lining
depending on the pore diameter [1]

Increased resistance lining

To counter the ability to wet the aluminum lining, and then penetrate into it and respond to it, used a number of approaches. Their purpose is to inhibit or reduce the ability to recover aluminum linings components and, thereby, prevent formation reaction corundum.

The use in refractory additives, that increase their nesmachivaemost molten aluminum, a common approach, especially refractory concrete based on calcium aluminate. For this purpose, use a lot of different materials, and the mechanism of their action is often known. Such additives are barium sulfate, various types of fluorides (AlF3, CaF2 and so on. etc.) and others. Another approach is to add phosphate additives, which increase the non-wetting characteristics of refractories in contact with molten aluminum, and, Moreover, It does not decompose up to a temperature 1500 oC. Phosphate additives promote the formation of the connection between new and old refractory, which is especially important when repairing the lining [2].

Sources:

  1. Seven Refractories – Aluminium Indusry – 2019
  1. Refractory Considerations for Aluminum Melting and Holding Furnaces – Refractory Engineer, January 2015
  1. Improved Monolithic Materials for Lining Aluminum Holding & Melting Furnaces // Light Metals 2011 – ed. S.J. Lindsay – 2011
  1. Assessing Monolithic Refractories for Use in Aluminium Melt-Hold Furnace – Advanced Materials & Process – July 2011
  1. GOST P 52918-2008 Refractory. Terms and Definitions