Argon Oxygen Decarburization

Argon Oxygen Decarburization (AOD) is a process primarily used in stainless steel making and other high grade alloys with oxidizable elements such as chromium, aluminum, etc. After initial melting the metal is then transferred to an AOD vessel where it will be subjected to three steps of refining; decarburization, reduction, and desulphurization. AOD was invented in 1954 by Praxair.

Decarburization

The decarburization step is controlled by ratios of oxygen to argon or nitrogen to remove the carbon from the metal bath. The ratios can be done in any number of phases to facilitate the reaction. The gases are usually blown through a top lance (oxygen only) and tuyeres in the sides/bottom (oxygen with an inert gas shroud). The stages of blowing remove carbon by the combination of oxygen and carbon forming CO gas.

:$mathrm\{3\; CO\; \_\{(gas)\}\; +\; 2\; Cr\; \_\{(bath)\}\; longrightarrow\; Cr\_2O\_\{3\; (slag)\}\; +\; 3\; C\; \_\{(bath)$

To drive the reaction to the forming of CO the partial pressure of CO is lowered using argon or nitrogen. Since the AOD vessel isn't externally heated, the blowing stages are also used for temperature control. The burning of oxygen increases the bath temperature.

Reduction

After a desired carbon and temperature level have been reached the process moves to reduction. Reduction recovers the oxidized elements such as Cr from the slag. To achieve this, alloy additions are made with elements that have a higher affinity for oxygen than Cr, using either a Silicon alloy or Aluminum. The reduction mix also includes lime (CaO) and fluorspar (CaF₂). The addition of lime and fluorspar help with driving the reduction of Cr₂O₃ and managing the slag, keeping the slag fluid and volume small.

Desulphurization

Desulphurization is achieved by having a high lime concentration in the slag and a low oxygen activity in the metal bath.

:$mathrm\{S\; \_\{(bath)\}\; +\; CaO\; \_\{(slag)\}\; longrightarrow\; CaS\_\{(slag)\}\; +\; O\; \_\{(bath)$

So, additions of lime are added to dilute sulfur in the metal bath. Also, aluminum or silicon maybe added to remove oxygen. Other trimming alloy additions might be added at the end of the step. After sulfur levels have been achieved the slag is removed from the AOD vessel and the metal bath is ready for tapping. The tapped bath is then either sent to a stir station for further chemistry trimming or to a caster for casting.