Magnesium-aluminum/magnesium-chromium/iron-aluminum spinel sand structure crystal phase and index

1. Magnesia-alumina spinel sand.

Magnesium-aluminum spinel chemical formula MgAl2O4 or MgO-Al2O3, theoretical content ω(MgO) = 28.3%, ω(Al2O3) = 71.7%. Natural magnesia-alumina spinel is rarely found, and all of the industrial applications are synthetic products. Magnesia alumina spinel has good resistance to erosion, abrasion and good thermal shock stability. According to the synthesis method is divided into sintering method and electrofusion method.
The melting point of MgAl spinel solid solution is 2135℃. Due to the reaction of MgO and Al2O3 to form spinel, there is about 5% to 8% volume expansion, which poses some difficulties to the densification of Mg-Al spinel synthesis process. The synthesis of Mg-Al spinel belongs to the solid phase reaction, which can be seen as the oxygen ions of larger radius do compact accumulation, while the smaller radius Mg and Al ions diffuse each other in the framework of fixed oxygen ion compact accumulation.
(1) Sintered magnesia-alumina spinel sand.
Sintered synthetic spinel is usually high purity lightly burned MgO (impurity mass fraction less than 3%) and industrial alumina or alumina mixed grinding, balling, into high temperature rotary kiln or inverted flame kiln calcination. Mostly used in the preparation of cement kiln with magnesite a spinel brick.

The following table shows the typical technical index of magnesia-rich spinel sand.

SiO2	CaO	MgO	Al2O3	Fe2O3	Bulk density/g-cm-3
0.23	0.64	47.53	50.78	0.31	3.31

The microstructural characteristics of sintered magnesia-alumina spinel depend on the raw material type and the synthesis process parameters. The experiments chose industrial alumina powder and lightly burned magnesium oxide powder produced by a company as raw materials for the synthesis of magnesium-rich spinel. The main crystalline phase of magnesium-rich spinel is magnesia-alumina spinel and magnesite, magnesia-alumina spinel grain size is 10-30μm, intergranular distribution of granular magnesite crystals, the size of smaller than magnesia-alumina spinel grains.
(2) Electrofused magnesia-alumina spinel sand.
Electrofused magnesia-alumina spinel is made of magnesite and industrial alumina as raw materials, using electric arc furnace to melt and synthesize spinel, which has the characteristics of simple process and large crystals with high density of the synthesized spinel. Electric fusion synthetic spinel is often used as raw materials for refining furnaces, sliding plates and other spinel products.

The following table shows the typical electric fused magnesium aluminum spinel sand technical index.

SiO2	CaO	MgO	Al2O3	Fe2O3	TiO2	Bulk density/g-cm-3
2.51	0.60	30.53	61.54	0.73	2.79	3.40

The microstructure of electrofused magnesia-alumina spinel is characterized by the main crystalline phase magnesia-alumina spinel with a small amount of magnesia-olivine. The spinel crystals are complete and coarse, above several hundred microns, with high density.
2. Magnesium-chromium spinel sand.
Magnesia-chromium spinel sand is a magnesia complex refractory raw material obtained by artificial synthesis with magnesia base crystal and secondary spinel as the main mineral. the Mg0-Cr2O3 system is very similar to MgO-Al2O3, and there is a compound magnesia-chromium spinel MgO-Cr2O3 in the system, which is the theoretical basis of magnesia-chromium spinel sand synthesis. Usually magnesia-chromium spinel and magnesia sand composite manufacturing of magnesia-chromium bricks, especially directly combined magnesia-chromium bricks, widely used in metallurgy, building materials and other industrial fields.

At present, the market is widely used mainly 20 magnesium-chromium sand and 36 magnesium-chromium sand. Usually the grade of magnesium-chromium sand is related to the content of chromium oxide in magnesium-chromium sand. The following table shows the typical magnesium-chromium sand technical index.

Title	SiO2	CaO	MgO	Al2O3	Fe2O3	Cr2O3	Bulk density/g-cm-3
20 Magnesium-chromium sand	0.99	0.98	67.76	3.46	5.81	20.94	3.65
36 Magnesium-chromium sand	0.78	0.53	48.72	5.74	8.43	36.12	3.82

The microstructure of magnesia-chromium sand is characterized by the main crystalline phase magnesite and magnesia-chromium spinel. From its light sheet morphology diagram, it can be seen that the gray is the main crystalline phase of magnesite, the white is the secondary spinel of desolvation, the gray-white is the silicate phase, and the magnesite grains are combined by the spinel phase and silicate between them. Compared with the electrofused 20 chromium magnesium-chromium sand, 36 chromium magnesium-chromium sand spinel crystalline size is large, and there are spinel aggregation phenomenon.
3. Iron-aluminum spinel sand：

Iron-aluminum spinel is a rare mineral in nature. The chemical formula is FeAl2O4. It is an orthoclinic, isometric crystal system, mostly octahedral crystallization. Its melting point is 1780℃. It is the only stable compound in the FeO-Al2O3 system. FeO-Al spinel has good properties, with high melting point (1780℃) and low coefficient of thermal expansion. When added to refractory bricks, iron-aluminum spinel imparts excellent physicochemical properties to the bricks. However, it rarely exists in nature and must be synthesized artificially, generally by electrofusion and sintering methods to synthesize iron-aluminum spinel.
Observing the phase diagram of the FeO-Al2O3 system, it is found that there exists a molten compound with a melting point of 1780°C below 1750°C is a stable compound only in the region where ferrous oxide FeO can exist stably to ensure that the compound formed with Al2O3 is FeO-Al2O3 spinel. And in the conditions outside the region of stable presence of FeO, the product obtained by the action of iron oxide and Al23 are difficult to say that it is FeO-Al2O3 spinel, but may be a solid solution containing a large amount or mainly Fe2O3-Al2O3.
From the microstructure morphology of electrofused iron-aluminum spinel and sintered iron-aluminum spinel, it seems that the main crystalline phase of both is iron-aluminum spinel. The difference is that the crystal size of fused iron-aluminum spinel is large, reaching several hundred microns, and the density of raw materials is high, but the uniformity is not high, and the impurity phase is concentrated. Sintered iron-aluminum spinel crystal size between 20 ~ 40m, crystal size is more uniform raw material denseness is not high, containing more open pores, and pore size is larger.