Aluminates

Aluminates 

the salts of aluminum acids: orthoaluminum acid H₃AlO₃, metaaluminum acid HAlO₂, and others. The most widely distributed aluminates are those of the general formula R[A1₂O₄], where R represents Mg, Ca, Be, Zn, and others. This general type can be subdivided into two groups: octahedronal varieties called spinels, such as Mg[Al₂O₄] (true spinel), Zn[Al₂O₄] (gahnite, or zinc spinel), and others, and rhombic varieties, such as Be[Al₂O₄] (chrysoberyl). (Note that in chemical formulas of minerals, the atoms forming the structural group are usually placed in square brackets.)

The aluminates of alkali metals result from the interaction of Al or Al(OH)₃ with caustic alkalis: Al(OH)₃ + KOH = KA1O₂ + 2H₂O. Of this group, sodium aluminate or NaAlO₂, a by-product of the alkali process of obtaining alumina, is used in the textile industry as a mordant. Aluminates of the alkali-earth metals are obtained by fusing their oxides with A1₂O₃; of that group, calcium aluminate (CaAl₂O₄) serves as the primary ingredient of quick-hardening alumina cement.

Aluminates of the rare-earth elements have come to have practical importance. They are obtained by dissolving the oxides of the rare-earth elements R₂O₃ and A1(NO₃)₃ in nitric acid, evaporating the solution until the salts crystallize and then calcining the crystallized salts at 1000–1100°C. Formation of the aluminates is controlled by X ray diffraction analysis and chemical phase analysis. The latter procedure is based on the difference in solubility of the initial oxides and the resultant compounds. For example, aluminates are stable in acetic acid, while oxides of the rare-earth elements dissolve well in acetic acid. Aluminates of rare-earth elements have great chemical stability, depending on the temperature of their preliminary calcination. They are stable in water at high temperatures (up to 350°C) under pressure.

Table 1. Calcination of aluminates
Compound	Color after calcination at over 1380°C	Melting point (°C)
La AIO3	cream	2100
Pr AIO3	yellow	2088
Nd AIO3	lilac	1950
Sm AIO3	cream	2020
Eu AIO3	pink	1940
Gd AIO3	“	1960
Dy AIO3	“	1880

The best solvent for aluminates of rare-earth elements is hydrochloric acid. The aluminates of rare-earth elements are distinguished by their high refractoriness and characteristic coloring. Their densities range from 6,500 to 7,500 kg/m.

The microhardness of fused aluminates of rare-earth elements is 16–17 giganewtons per m² (1,600–1,700 kilograms-force [kgf] per mm²); the microhardness of oxides of rare-earth elements is 4 - 4.7 giganewtons per m²(400 - 470 kgf/mm²).

Aluminates of the rare-earth elements are promising materials for the production of special ceramics and optical glass and in nuclear technology and other branches of the national economy, successfully substituting for the oxides of rare-earth elements.

REFERENCES

Portnoi, K. I., and N. I. Timofeeva. “Sintez i svoistva monoaliuminatov redkozemel’nykh elementov.” Izv. AN SSSR: Neorganicheskie materialy, 1965, vol. 1, no. 9.
Tresviatskii, S. G., V. I. Kushakovskii, and V. S. Belevantsev. “Izuchenie sistem Al₂O₃—Sm₂O₃ i A1₂O₃—Gd₂O₃.” Atomnaia energiia, 1960, vol. 9, issue 3.
Bondar’, I. A., and N. V. Vinogradova. “Fazovye ravnovesiia v sisteme okis’ lantana—glinozem.” Izv. AN SSSR: Ser. khimicheskaia, 1964, no. 5.

K. I. PORTNOI