1. Introduction

The complete range of processes undertaken in the minerals processing industry was outlined in CF 255. This combustion file gives more specific detail of the processes in use for the manufacture of [GLOSS]dead burnt[/GLOSS] magnesite.

2. Dead Burnt Magnesite

Magnesite ores are pure (>80%) magnesium carbonate.   These are calcined in two stages to produce magnesia MgO.   Gentle [GLOSS]calcination[/GLOSS] at 700^oC to 1000^oC, similar to the production of [GLOSS]lime[/GLOSS], produces caustic-calcined magnesia, which reacts with water to form Mg(OH)₂.   If this material is then heated to between 1450^oC and 1900^oC dead-burnt magnesia is produced, which is used extensively as a refractory.    Further heating at >3000^oC for six hours produces fused magnesia. An alternative source of magnesia is from seawater, whereby the magnesia is extracted as magnesium hydroxide by reaction with calcined dolomite ([GLOSS]dolime[/GLOSS]) or lime.

The calcination of caustic magnesia, dead-burnt magnesia and dolomite are all carried out in either [GLOSS]rotary kiln[/GLOSS]s or vertical shaft kilns.   Flame temperatures in the dead-burning process are sometimes enhanced by the use of oxygen.  Figure 2 shows the breakdown of magnesia production by principal countries.

Figure 2     World Production of Magnesia in 1993 (Industrial Minerals Handy Book)

The specific fuel consumption for this process is not readily available as it is considered by producers to be commercially sensitive, but it is estimated that the energy requirement to produce caustic magnesia is ~6.0 MJ/kg, whilst the requirement for dead-burnt magnesia is ~9.5 MJ/kg.   These two figures would be cumulative for many magnesite plants, as these processes are carried out in two stages, in separate units.  Approximately 80% of the annual world production is sold as dead-burnt magnesia.

Sources

[1] European Minerals Yearbook

[2] Industrial Minerals Handy Book