Silica Literature

SILICA LITERATURE The wear of dinners refractories in the partition of coke batteries depends not only upon their physico-ceramic properties but also upon the necessary stable structure formed in all stages of production and especially in firing. Based on service condition in the partition of coke oven dinners parts must have low additional growth at 1450o C, a small quantity of residual quartz, and high Tridymite. The most stable form of silicon dioxide in dinners for coke oven is crystals of tridymite, which broadens the temperature boundaries of its heat resistance from 14700 C to 2000 C, In the fine grained portion of dinners tridymite is normally appear shaped twins forming a concretion. In tridymite cristobolite part concretion of tridymite crystals promote in the fine grained portion (<1 mm ) & Cristobality and residual quartz in the ground former quartzite (>1 mm). Such heterogeneous structure of the dinas is the result primarily of transformation of quartz in the fine grained portion under the action of the mineralizing additions and on the other hand in the quartzite grains in the absence of them each of the crystalline varieties of silicon dioxides determines the properties and behavior of the dinas refractories in service. An increase in the quality of dinas parts with a high degree of degeneration of quartz existing standards for addition of mineralizing additions to them must be reconsidered. The mineralizing additions added to the dinas, calcium and iron oxides interact in firing with silicon dioxide and form molten silicates. Iron oxide posses different properties and melting points. For example, FeO is a basic oxide and Fe2O3 an amorphous one, FeO melts at 13700 C and Fe2O3 and magnetite (Fe3O4) at 15600 C, and 15900 C, respectively. The most active form of iron oxides promoting early appearance of molten silicate, is the ferrous form. According to the date of in the FeO-SiO2 system molten material appears at 11780 C and in the Fe3O4-SiO2 system at 14250 C. The plan of the mechanism of the transformation of quartz into the high temperature modification (tridymite) is shown in fig. Form the plan it follows that in the absence of interaction of molten silicates α-quartz first transformations into meta-cristobolite, which then under the action of the molten material recrystalizes into α-Tridymite. 1

β- Quartz

5730C

α-Quartz

Silicon Dioxide Mineralizer

1050 - 12500C

Meta Cristobalite

1178-12000C

14500C 11780C

+

Molten silicate

1178 - 14700C

α-Tridymite

Fig 2.1 - plan of the mechanism of transformation of quartz into Tridymite The compositions of the mineralizing addition, the reducing medium and the temperature of appearance of the molten material have a significant influence on recrystalization. With addition to the dinas of mineralizing additions with a composition of 2-3 % CaO+0.6-1 % Fe2O3 molten silicate appears only at 1452o C while with addition of 2-3 % CaO+2% FeO the temperature of appearance of molten silicate drops to 1178o C – 1200o C. From this it follows that with the addition to dinas of Fe the temperature of the appearance of molten silicate is higher than the temperature reached in firing of dinas in 243m tunnel kiln, on the other hand, with the addition to dinas of Fe molten silicates are formed in the stages of the end of the heating zone and in the hold, determining the activity of

recrystallization of quartz and meta-cristobolite in to α-

Tridymite.(2) There is no generally accepted concept of the effect of mineralizing additives with different iron valences on the process of phase formation and the properties of dinas parts has been worked out. For example in order to improve the physico-ceramic properties and the structure of dinas it is desirable to introduce iron containing additives with bivalent iron .An investigation conducted by the authors has not shown any advantage of iron containing mineralizers with iron of different valences. This is explained by the fact that in heat treatment of dinas iron oxide can be oxidized and reduced in the following order: 2

Fe2O3

Fe3O4

FeO

Fe

The experiment has shown that the three employed types of iron containing additives (pyrite cinder, converter slim, scale) produce approximately the same effect. The properties and the phase composition of dinas specimens with different iron containing additives are virtually the same. This can be associated with oxidation of iron oxides in the course of heat treatment, which levels off difference in the effects of the additives. (3) Dinas parts with high iron oxide content may not be recommended for service in coke oven since an increased content of iron oxides promote reduction of silicon dioxide to its monoxide in the presence of carbon containing gases. (2) To increase the resistance of walls in the coking chamber it is desirable to use dinas without iron containing mineralizers. This idea is supported by positive experiences with the use of low iron dinas with increase in the mass proportions of silica. Dinas products made with the use of lime-iron mixtures (2.2-2.7 % CaO, 0.8-1.0 % Fe2O3) as mineralizers are used in Russia for building coke furnace. It is known however, that at high temperature the iron oxide acts as catalysts in reducing the silica and under the action of carbon and reducing gases accelerate the conversion of silicon dioxide to the monoxide at temperature above 1100o C, and especially at 1180o C – 1200o C; this occurs on the surface of the dinas in the walls of the coking chamber during prolonged holding of the coke discharge, without an appropriate reduction in the temperature in the above channels. The corrosion of the surface due to the reduction of silica has been noted in several coke batteries. 1. Ukrainian Scientific Research Institute for Refractory. “Formation of tridymite structure in Dinas

Refractories

“Vol–29,

No.11,

November-December,

1988.

Authors-

V.D.Tsigler,V.V.Martynenko, R.F.Rud,A.E. Fedovov & V.G. Barkar (Pg. no. : 679-685) 2. Ukanian Research Institute of Refractory.” Effect of mineralizing additives with different iron valences on phase formation & properties of dinas parts.“ Vol – 37, No.7–8, July-August 1996,March-1997.Authors-I.V.Khonchik,I. Drodz & L.E.Azarenkova (Pg. no : 227-230) 3. Ukainian Scientific Research Institute for Refractory “New types of calcium mineralizers for dinas production “ Vol – 32, No. 7-8, July-August 1991,March 1992. (Pg. no : 339-343). 3