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Magnesia / Infos / Production of magnesiaCONVERSION 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 thelumps in horizontal rotary kilns, normally by direct firing with oil or gas. Grades with a very low sulphate content are obtainedby 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 above400 C.

>400 CMgCO3 MgO+CO2

Calcination temperatures of between 500 and 1,000 C produce magnesium oxide with a relatively high specific surface area andremarkable reactivity. These react readily with water and even fairly vigorously with diluted acid solutions.

Grades produced at relatively low temperatures (up to approx. 1,000 C) are called caustic calcined magnesite, also knowncommonly as causter.

Caustic calcined magnesite is produced for a wide variety of applications. The majority of these require the natural magnesiumoxide to be ground. Grinding is performed in hammer, ball or pendulum mills, which are made of abrasive-proof materials orcoated with rubber to prevent contamination of the magnesia. This stage in the processing allows particles of different sizes to beobtained, the distinct grade required being dependent on the specific application. Jet mills are also used. Special applicationsmay call for wet grinding followed by recalcination. In contrast to the above mentioned procedure, burning at temperaturesabove 1,600 C produces dead burnt magnesite, a magnesium oxide with extremely low reactive properties. These grades arealso called sinter or sinter magnesite and are principally used in iron foundries as a refractory material.

CHEMICAL PRODUCTION OF MAGNESIA

Due to its natural origin, magnesium oxide produced by calcination of magnesite may not satisfy all requirements concerningpurity and activity. Processes have, therefore, been developed to produce either magnesium hydroxide or magnesium hydroxidecarbonate chemically, which are then calcined to give magnesium oxide. Magnesium oxide produced this way is calledprecipitated, sometimes also chemical or synthetic magnesia.

The following chemical processes are most 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. Precipitationfollows 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 ofdolomite instead of lime milk for precipitation improves the magnesia yield relative to the magnesia content in the dolomite.

Temp.MgCO3 + CaCO3 MgO + CaO + 2CO2

MgO + CaO + 2H2O Mg(OH)2 + Ca(OH)2

Production of magnesia

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MgCl2 + Mg(OH)2 + Ca(OH)2 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 thePattinson process described here. Starting with dolomite or magnesia limestone, first of all a mixture of magnesium oxide andcalcium oxide is produced by burning:

>1000 C1. CaCO3 + MgCO3 MgO + CaO + 2CO2

These light burnt oxides are hydrated to form the hydroxides

2. CaO + MgO + 2H2O Ca(OH)2 + Mg(OH)2

The carbon dioxide produced by 1 is now used to carbonatisate the hydroxide mixture. By selecting appropriate reactionconditions 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 selectivelyreleased by raising the temperature of the solution. Insoluble magnesium hydroxide carbonate is thus precipitated and may beextracted:

~ 100 C5. 5 Mg(HCO3)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 bedried and used directly, may be calcined to magnesium oxide. The formula for magnesium hydroxide carbonate given aboverepresents only one of a variety of possible compositions. The quality of magnesium hydroxide carbonate formed depends on theprocedure 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 acidfollowed by pyrohydrolysis of the resulting brine is also in use. The first method eventually gives caustic or dead burnt magnesiaand hydrochloric acid as the by-product, whilst in the second method hydrochloric acid is fed back to the process in order to leachout further magnesia mineral

PyrohydrolysisMgCl2 + H2O MgO + 2HCl 450 - 1000 C

In practice, this process also gives an intermediate magnesium hydroxide, since the oxide resulting from pyrohydrolysis stillcontains stable alkali and calcium Chloridees and has to be washed thoroughly. The magnesium hydroxide is rarely withdrawnfor direct use, so it is normally calcined to caustic magnesia. Depending on the requirements, further compacting and sinteringsteps 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 usesmagnesium metal as a base material to produce an extremely pure magnesium hydroxide which may be calcined to formmagnesium oxide.

Email: info@lehvoss.nl Tel.: +31 (0)10 236 27 06

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