Production of magnesia - LEHMANN & VOSS & Co.
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Production of magnesia Magnesia / Infos / Production of magnesia CONVERSION OF CRUDE MAGNESITE INTO BURNT MAGNESITE Most of the mined magnesite is converted directly into magnesium oxide by burning (calcining). This is done by burning the lumps in horizontal rotary kilns, normally by direct firing with oil or gas. Grades with a very low sulphate content are obtained by burning with wood. The temperature and duration of the calcination procedure determines the respective reactive properties (grades) of the magnesium oxide. Decomposition of magnesium carbonate to form magnesium oxide and carbon dioxide begins at a temperature slightly above 400 °C. >400 °C MgCO3
MgO+CO2
Calcination temperatures of between 500 and 1,000 °C produce magnesium oxide with a relatively high specific surface area and remarkable reactivity. reactivity. These react readily with water and ev en fairly vigorously vigorously with diluted acid solutions. solutions. Grades produced at relatively low temperatures (up to approx. 1,000 °C) are called caustic calcined magnesite, also known commonly as causter. Caustic calcined magnesite is produced for a wide variety of applications. The majority of these require the natural magnesium oxide to be ground. Grinding is performed in hammer, ball or pendulum mills, which are made of abrasive-proof materials or coated with rubber to prevent contamination of the magnesia. This stage in the processing allows particles of different sizes to be obtained, the distinct grade required being dependent on the specific application. Jet mills are also used. Special applications may call for wet grinding followed by recalcination. In contrast to the above mentioned procedure, burning at temperatures above 1,600 °C produces dead burnt magnesite, a magnesium oxide with extremely low reactive properties. These grades are also called sinter or sinter m agnesite agnesite and are principally used in iron foundries as a r efractory material. CHEMICAL PRODUCTION OF MAGNESIA Due to its natural origin, magnesium oxide produced by calcination of magnesite may not satisfy all requirements concerning purity and activity. Processes have, therefore, been developed to produce either magnesium hydroxide or magnesium hydroxide carbonate chemically, which are then calcined to give magnesium oxide. Magnesium oxide produced this way is called precipitated, sometimes also chemical or synthetic magnesia. The following chemical processes are most common: common: Precipitation of magnesium hydroxide from seawater Lime milk is used to precipitate the magnesium content of seawater (approx. 1,300 ppm) in hydroxide form. Precipitation follows the -simplified- reaction equation: MgCl2 + Ca(OH)2
Mg(OH)2 + CaCl2
The precipitate is washed and calcined
Mg(OH)2
MgO + H2O
Depending on burning temperature and duration this produces caustic calcined or dead burnt magnesium oxide. Using milk of dolomite instead of lime milk for precipitation precipitation improves the magnesia yield relative to the ma gnesia gnesia content in the dolomite. Temp. MgCO3 + CaCO3 MgO + CaO + 2CO2
MgO + CaO + 2H 2O
1 f2
Mg(OH)2 + Ca(OH)2
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Production of magnesia - LEHMANN & VOSS & Co.
MgCl2 + Mg(OH) 2 + Ca(OH)2
http://nl.lehvoss.de/143.htm
2Mg(OH)2 + CaCl2
It is clear that magnesium hydroxide for dried use can be taken from these precipitation processes, instead of being calcined. Precipitation processes based on magnesium salt brines instead of seawater follow in principle the same reaction path PRECIPITATION OF MAGNESIUM HYDROXIDE CARBONATE The precipitation of magnesium hydroxide carbonate employs very specific processes. One of the most well known is the Pattinson process described here. Starting with dolomite or magnesia limestone, first of all a mixture of magnesium oxide and calcium oxide is produced by burning:
1.
>1000 °C CaCO3 + MgCO3 MgO + CaO + 2CO2
These light burnt oxides are hydrated to form the hydroxides 2.
Ca(OH)2 ¯ + Mg(OH)2 ¯
CaO + MgO + 2H2O
The carbon dioxide produced by 1 is now used to carbonatisate the hydroxide mixture. By selecting appropriate reaction conditions it is possible to precipitate insoluble calcium carbonate in a first step:
3. Ca(OH)2 + CO2
CaCO3 ¯ + H2O
In a second step with higher carbon dioxide pressure the soluble magnesium hydrogen carbonate is formed:
4. Mg(OH) 2 + 2CO2
Mg(HCO3)2
After separation of the calcium carbonate from the magnesium hydrogen carbonate solution, carbon dioxide is selectively released by raising the temperature of the solution. Insoluble magnesium hydroxide carbonate is thus precipitated and may be extracted: ~ 100 °C 5. 5 Mg(HCO 3)2 4MgCO3 • Mg(OH)2 • 4H2O ¯ + 6CO2 Carbon dioxide is fed back to the reactions 3 and 4. The magnesium hydroxide carbonate extracted in this way, if it is not be dried and used directly, may be calcined to magnesium oxide. The formula for magnesium hydroxide carbonate given above represents only one of a variety of possible compositions. The quality of magnesium hydroxide carbonate formed depends on the procedure followed. This process typically produces grades with a low apparent density. PYROHYDROLYSIS OF MAGNESIUM CHLORIDE BRINES In recent years a number of facilities have been built which apply pyrohydrolysis to magnesium Chloridee brine, as it occurs, e.g. as a by-product in the manufacture of potash. The process based on leaching magnesium minerals by hydrochloric acid followed by pyrohydrolysis of the resulting brine is also in use. The first method eventually gives caustic or dead burnt magnesia and hydrochloric acid as the by-product, whilst in the second method hydrochloric acid is fed back to the process in order to leach out further magnesia mineral Pyrohydrolysis MgCl2 + H2O MgO + 2HCl 450 - 1000° C In practice, this process also gives an intermediate magnesium hydroxide, since the oxide resulting from pyrohydrolysis still contains stable alkali and calcium Chloridees and has to be washed thoroughly. The magnesium hydroxide is rarely withdrawn for direct use, so it is normally calcined to caustic magnesia. Depending on the requirements, further compacting and sintering steps to form refractory magnesia will follow. OTHER PROCESSES Other processes based on magnesium sulphate, e.g. epsom salts or kieserite, are also known. A Japanese process uses magnesium metal as a base material to produce an extremely pure magnesium hydroxide which may be calcined to form magnesium oxide.
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