Al2O3-MgO system: magnesia and spinel Magnesia

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1 Al 2 O 3 -MgO system: magnesia and spinel Magnesia Magnesium oxide (MgO, magnesia) occurs naturally as the mineral periclase; a metamorphic mineral formed by the breakdown of dolomite, CaMg (CO 3 ) 2, and other magnesium minerals. Occurrences of periclase are rare and are of no commercial importance. The principal commercial sources of MgO are magnesite (MgCO 3 ) and magnesium hydroxide [Mg (OH) 2]. Alkaline-earth elements form oxides with the general formula MO with high melting temperature ( 2,800 C for MgO, 2,580 C for CaO, 2,430 C for SrO). Owing to reactivity with water and propensity to carbonation, the most common minerals that contain these elements are calcite CaCO 3 for calcium, magnesite MgCO 3 and its hydrated varieties for magnesium, and dolomite (CaMg) CO 3 which combines both cations. Major deposits of magnesite occur in many countries including China, Turkey, and Russia. The magnesite contains varying amounts of impurities including silica, iron, aluminum, manganese, and calcium, usually present in the form of various minerals, for example, quartz, talc, mica, and magnetite. After mining the ores must be beneficiated. The methods for beneficiation vary, for example, crushing, screening, washing, magnetic separation, and froth floatation. After the impurities have been separated the magnesium carbonate is calcined. Calcining at temperatures between 800 and 900 o C produces a very reactive fine grained MgO called caustic magnesia. Sintered, or dead burned, magnesia is obtained by calcining the magnesium carbonate at temperatures above 1700 o C. During this process the reactive crystals grow and lose their activated state. Magnesia can be produced from seawater and magnesium-rich brines. About 60% of the U.S. production of magnesium compounds is from these sources. Seawater contains about 1.28 g Mg 2+ /kg. The most important process for the production of MgO from seawater is precipitation of magnesium hydroxide [Mg(OH) 2 ] from solutions of magnesium salts by a strong base: 1

2 Mg 2+ (aq) + 2(OH) (aq) Mg (OH) 2 (s) The Mg (OH) 2 precipitate is washed, filtered, and calcined to produce MgO. Another means of obtaining magnesia is from brines. This process is based on the decomposition of MgCl 2 at C: MgCl 2 + 2H 2 O Mg (OH) 2 + 2HCl World magnesia production capacity is about 10 Mt/year: ~9.0 Mt from natural magnesite and ~1.5 Mt from seawater and brines. The major application for magnesia is as a refractory lining in furnaces. In lesser quantities, it is made into a well-known milky solution and ingested. It is also used to manufacture other ceramics such as chrome-free spinels. Nonchrome spinel is not available in nature on an industrial scale. At Asahi Glass, spinel is produced by electrofusing magnesia with alumina. The archetype of oxides with ionic bond, magnesia MgO crystallizes in a NaCl- like structure. The simplicity of this structure, the very low difference from stoichiometry and the possibility of having of very pure monocrystals and perfectly densified polycrystals make MgO a model material to understand the properties of oxides, such as the dislocation movement. The plasticity of MgO is remarkable, at least for a ceramic compound: if, below 350 C, slips occurs only according along (110) <110 >, at high temperature ( 1,700 C) five independent slip systems are activated and rupture is no longer brittle, because the Von Mises criterion is met. The majority of magnesia produced is used for refractories in the iron and steel industry, where the basic oxide properties of the material are necessary (steelworks use the BOF method = basic oxygen furnace). Magnesia withstands the very high temperatures of converters (1,700 C), where it can dissolve several times its weight of iron oxide without melting and it effectively resists sealing and the attack of slag. The high reactivity of magnesia with water is a disadvantage for its use, but this reactivity decreases if we decrease the specific surface area of Chamottes by a high temperature treatment, which also makes impurities, including silicate impurities, form a less reactive skin on the grain 2

3 surface. This is called dead-burnt magnesia. Calcined dolomite, which is an almost 50:50 mixture of CaO + MgO (in moles), is less sensitive to hydration, but it is carbon products that ensure, in most cases, the protection of magnesia and also less wettability with respect to the attacking agents, such as slag. Magnesia-carbon refractories (about 30% carbon, in various forms) are the reference refractories in steel-works. Spinel Spinel MgAl 2 O 4 has given its name to a crystalline structure adopted by several mineral phases. Based on the notation Mg 2+ Al 3+ 2 O 4, Mg 2+, Mg 2+ can be replaced by other divalent cations like Fe 2+, Mn 2+ or Zn 2+, and Al 3+ can be replaced by other trivalent cations like Fe 3+ or Cr 3+. The spinel structure AB 2 X 4 is made up of a face-centered cubic stacking of X 2- ions (O 2- in the case of MgAl 2 O 4 ) containing eight structural units per array, i.e. a total of 32 X 2- (eight units multiplied by four nodes per CFC array). The cations A and B are placed in the interstitial sites. There are 64 tetrahedral interstices of which eight are occupied and 32 octahedral interstices of which 16 are occupied. Based on the occupancy of the interstitial sites, we can distinguish two cases: In a normal spinel, like MgAl 2 O 4, the trivalent Al 3+ ions are in octahedral sites and the bivalent Mg 2+ in tetrahedral sites; In an inverse spinel, like Fe 3 O 4 (written as Fe 2+ Fe 3+ 2 O 4 ), the trivalent Fe 3+ ions are divided half/half between octahedral sites and tetrahedral sites, and the bivalent Fe 2+ are in octahedral sites. These two cases are borderline cases, but there can be various deviations from this distribution. Statistical spinel corresponds to complete disorder where trivalent and bivalent cations are distributed randomly in the A and B sites. Besides II-III spinels that we have just considered, the spinel structure can adapt to cations with other nominal oxidation states: II-IV spinels like Mg 2 SiO 4, I-VI spinels like Na 2 WO 4 and I-III spinels like K 2 Zn (CN) 4. Gamma alumina has a lacunar spinel structure: the aluminum ions are all trivalent, but electric balance 3

4 is respected by considering that a bivalent entity M 2+ corresponds to 2/3Al 3+ plus 1/3V Al x (by using the Kr ger notation of point defects, where V Al x indicates a constitutional vacancy, whose effective charge compared to the normal occupation of the site is zero). Figure 1.3 MgO-Al 2 O 3 Diagram The magnetic properties of magnetite Fe 3 O 4 (also written as FeOFe 2 O 3 ) have been known since antiquity. This iron ferrite (different from ferrite, the centered cubic form of iron) crystallizes into inverse spinel. There is an antiferromagnetic order between octahedral and tetrahedral sites, therefore the magnetic moments of Fe 3+ ions cancel each other out and overall magnetization is due only to the sole contribution of Fe 2+ ions: this is called ferrimagnetism. The magnetic properties of ceramics are the foundation for the tape recording industry. The balance diagram of Al 2 O 3 -MgO (see Figure 1-3) shows that, at high temperatures, spinel is not stoichiometric, but allows a broad solid solution 4

5 domain, in particular on the alumina-rich side. The fusion of MgAl 2 O 4 occurs at 2,105 C, i.e. more than 50 C above the fusion of alumina. In addition to its refractarity, the interest of spinel as refractory chamotte is due to its possible formation by in situ reaction, for example, in concretes made up of magnesia grains bound by aluminous cement. Dense sintered spinel is transparent in a broad domain of wavelengths, which explains its use in the manufacture of optical windows (military applications in Particular). 5