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, но также уже специальной формы и из специальных материалов [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 by 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. Комбинация всех этих факторов приводит к возрастанию температуры печи и созданию в печи более агрессивной атмосферы [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

Огнеупорная футеровка типичных плавильных (раздаточных) печей для алюминия должна выдерживать большое разнообразие физических и химических воздействий [1-3]. Various furnace zone, which have their own particular operation, shown in Figure 1.


Figure 1 – Зоны футеровки плавильно-раздаточной печи [4]

Generally, the furnaces have three distinct zones, которые различаются по условиям их эксплуатации [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.
  • «шлаковый пояс» (belly band – “belly”), which adjoins the bottom area.

The liner in contact with the metal, i.e. in the lower zone of the furnace, изнашивается в результате:

  • 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.

Промежуточная зона печи («шлаковый пояс») должна выдерживать:

  • 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, которые могут возникать при применении флюсов (например, эkzotermicheskih flux – look. here).

Education coating of corundum

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, а также с кремнием и алюминием [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.

On the image 2 diagram corundum formation on the furnace lining.

Figure 2 – Схема образования наростов корунда на футеровке печи [2]

Reaction formation of corundum

Aluminum has strong affinity with oxygen, i.e, in other words, high ability to actively connect with oxygen. Это приводит к различным механизмам окисления жидкого алюминия (рисунок 3) [1]:

  • На поверхности ванны жидкого алюминия происходит его прямое окисление:4 Al + 3 O2 → 2 Al2O3
  • of coating, начиная с поверхности жидкого контакта (ниже уровня расплава). Металлический алюминия за счет высокого сродства с кислородом диффундирует из расплава в футеровку и реагирует с ней согласно реакции:4 Al + 3 SiO2 → 2 Al2O3 (тв.) + 3 Si

Эти два механизма могут взаимодействовать друг с другом и катализировать друг друга [1].


Figure 3 – Реакция жидкого алюминия с футеровкой с образованием корунда [1]

What is a good lining

features linings, которые могут влиять на образование корунда:

  • Тип огнеупорного заполнителя (порошка)
  • Method assembly linings
  • Матрица огнеупора (очень важно)

refractory Matrix - a continuous phase of crystalline or vitreous refractory, содержащая изолированные включения других фаз [5].

Существует несколько путей модифицирования футеровки для повышения ее стойкости к воздействию жидкого алюминия:

  • Reduction of the chemical components of the refractory lining, which have a lower affinity for oxygen, than aluminum
  • Оптимизация пористости огнеупорной футеровки (рисунок 4)
  • Reduced wettability of surface of the molten aluminum of the refractory lining.


Figure 4 - Penetration of aluminum into the lining
в зависимости от диаметра пор [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, различные типы фторидов (AlF3, CaF2 and so on. п.) и другие. 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, что особенно важно при выполнении ремонта футеровки [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