Calcining magnesium hydroxide

In Finland, investigations into stepwise carbonation processes initially involved the use of Finnish serpentine and (calcined) magnesium hydroxide powder ( 20gm, 99% pure) in a PTGA with humid C02, at 0.1 to 3.5 MPa pressure. The materials were first heated to 1273 K, and then cooled in (mainly) C02/N2 mixtures. Tests conducted at elevated pressures did not produce the expected increase in MgO carbonation rate, as the final carbonation conversions fell from 5.6% at 0.1 MPa to 2.9% at 3.5 MPa in 99%/l% C02/H20. Hence, the decision was taken to proceed with Mg(OH)2 under test conditions where MgO formation was thermodynamically unfavorable [85, 96]. 

Calcining magnesium hydroxide, such as that produced in either a brine or seawater process, involves heating a filter cake that contains between 50 and 72% magnesium hydroxide solids, the balance being water. The thermal decomposition involves the following reaction ... 

Anhydrous magnesium chloride in the molten state has been produced extensively from MgO obtained either from calcined magnesium hydroxide from seawater or from calcined magnesite, as shown in reaction (14.7) ... 

Magnesium Oxide n (ca. 1909) (magnesia, periclase) A white powder used as filler and as a thickening agent in polyester resins. It occurs naturally as the mineral periclase, but it is usually made in purer form by calcining magnesium hydroxide or carbonate. 

Dead Seas Periclase Ltd., on the Dead Sea in Israel, uses yet another process to produce magnesium oxide. A concentrated magnesium chloride brine processed from the Dead Sea is sprayed into a reactor at about 1700°C (127,128). The brine is thermally decomposed into magnesium oxide and hydrochloric acid. To further process the magnesia, the product is slaked to form magnesium hydroxide which is then washed, filtered, and calcined under controlled conditions to produce a variety of MgO reactivity grades. A summary of MgO purities, for the various processes is given in Table 20. 

Dow seawater A process for extracting magnesium from seawater. Calcined dolomite (CaO-MgO, dololime), or calcined oyster shell, is added to seawater, precipitating magnesium hydroxide. This is flocculated, sedimented, and filtered off. For use as a refractory it is calcined for the manufacture of magnesium chloride for the manufacture of magnesium metal, it is dissolved in hydrochloric acid. Developed by the Dow Chemical Company and later operated by the Steetly Company in West Hartlepool, UK, and Sardinia. 

There is no clear evidence to identify the active material for SO2 removal in a MgAl20 stoichiometric system. Figure 13 shows results for a 50-50 mole% magnesia-alumina material prepared from magnesium hydroxide and alumina sol and calcined at various temperatures. An attempt was made to correlate SO2 removal with compound formation, as measured by X-ray diffraction, and surface area. As indicated in the figure, SO2 removal ability decreased with Increasing calcination temperature as did surface area. X-ray diffraction analysis showed spinel formation increases as... 

Recently, the influence of the preparation method of various MgO samples on their catalytic activity in the MPV reaction of cyclohexanone with 2-propanol has been reported 202). The oxides were prepared by various synthetic procedures including calcination of commercially available magnesium hydroxide and magnesium carbonate calcination of magnesium hydroxides obtained from magnesium nitrate and magnesium sulfate sol-gel synthesis and precipitation by decomposition of urea. It was concluded that the efficiency of the catalytic hydrogen transfer process was directly related to the number of basic sites in the solid. Thus, the MgO (MgO-2 sample in Table IV) prepared by hydration and subsequent calcination of a MgO sample that had been obtained from commercially available Mg(OH)2 was the most basic and the most active for the MPV process, and the MgO samples with similar populations of basic sites exhibited similar activities (Table IV). 

In most commercial processes, the compound is either derived from the sea water or from the natural brines, both of which are rich sources of magnesium chloride. In the sea water process, the water is treated with lime or calcined dolomite (dolime), CaO MgO or caustic soda to precipitate magnesium hydroxide. The latter is then neutralized with hydrochloric acid. Excess calcium is separated by treatment with sulfuric acid to yield insoluble calcium sulfate. When produced from underground brine, brine is first filtered to remove insoluble materials. The filtrate is then partially evaporated by solar radiation to enhance the concentration of MgCb. Sodium chloride and other salts in the brine concentrate are removed by fractional crystallization. 

Lime is obtained by calcination of dolomite, CaCOs MgCOs, or limestone, CaCOs, under controlled conditions to remove all CO2. After bicarbonate removal, the seawater is then treated with calcium hydroxide, slaked dobme or sodium hydroxide to precipitate magnesium hydroxide ... 

While organo-silane treatments are extensively used in both thermoset and elastomer applications, their use in thermoplastics has so far been somewhat restricted. This is because they do not react with the surface of calcium carbonate, one of the principal fillers used in this type of polymer and because of the lack of a suitable reactive functionality for most of the thermoplastic polymers. Today they are principally used in conjunction with glass fibres, calcined clays, aluminium and magnesium hydroxides, micas and wollastonite. The main thermo-... 

Aluminium oxide is the oldest ceramic material used in medicine. Bauxite and corundum are the main natural sources of aluminium oxide. Bauxite is a mixture of diaspore, gibbsite, iron hydroxides, clay minerals and quartz. It is formed by the tropical weathering of silicate rocks during which quartz and the elements sodium, calcium, magnesium and potassium are largely washed away. This is the reason why the remaining material becomes richer in the resistant elements titanium, iron and aluminium. The latter is extracted from this mixture in the form of aluminium hydroxide. In its turn this compound is converted into aluminium oxide by heating the mixture to 1200-1300 °C, this is called calcination. The hydroxide is thus made anhydrous. 

Choudhary V. R. and Pandit, M. Y. Surface properties of magnesium oxide obtained from magnesium hydroxide - influence on preparation and calcination conditions of magnesium-hydroxide. Appl. Catal., 1991, 71, 265-274. 

In the process used by Norsk Hydro, magnesium hydroxide extracted from seawater with the aid of calcined dolomite is mixed with charcoal and magnesium chloride brine and is heated to 1000-1200°C in the presence of chlorine produced during subsequent electrolysis of magnesium chloride. The main reactions are [266]... 

Magnesium hydroxide is produced by precipitation of the magnesium ions with Ca(OH)2 obtained from dolomitic limestone. The limestone is calcined in a rotary kiln to produce lime the lime is reacted with water to give Ca(OH)2, which reacts with the Mg ions in the brine to give Mg(OH)2. One-half of the Mg(OH)2 comes from the dolomitic lime. The slurry of Mg(OH)2 in the CaCl2 brine is thickened in settling tanks and filtered, and the CaCl2 is washed out of the cake with water. 

A classic example of a solid—fluid ceramic powder synthesis reaction is that of calcination and dehydration of natural or synthetic raw materials. Calcination reactions are common for the production of many oxides from carbonates, hydrates, sulfates, nitrates, acetates, oxalates, citrates, and so forth. In general, the reactions produce an oxide and a volatile gaseous reaction product, such as CO2, SOg, or HgO. The most extensively studied reactions of this type are the decompositions of magnesium hydroxide, magnesium carbonate, and calcium carbonate. Depending on the particular conditions of time, temperature, ambient pressure of CO2, relative humidity, particle size, and so on, the process may be controlled by a surface reaction, gas diffusion to the reacting... 

The thereby formed sulfuric acid is neutralized by adding magnesium hydroxide. Independently of the production method, the precipitated uranium concentrate is washed to remove adhering salt solution and then dried. The precipitates produced with ammonia are subsequently calcined in a multiple hearth kiln at 750°C, ammonia, sulfite and chloride being driven off and U3O8 being formed ... 

CaCOa) are calcined to give lime (CaO), which is added to the seawater to yield magnesium hydroxide. The Mg(OH)2 slurry (a suspension in water) is washed and filtered in huge nylon filters. 

Derivation (1) By calcining magnesium carbonate or magnesium hydroxide, (2) by treating magnesium chloride with lime and heating or by heating it in air, (3) from seawater via the hydroxide. 

Typical fillers calcium carbonate, calcinated clay, aluminum hydroxide, magnesium carbonate, magnesium hydroxide, antimony trioxide, calcium borate, huntite, hydromagnesite, zinc oxide, talc, silica... 

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