Ceramic muffles

The main part of a conventional Verneuil apparatus is a vertical inverted oxyhydrogen burner. The burner consists of several concentric tubes [1, 2]. The inner tube supplies oxygen and crystal powder to the flame, which burns into a ceramic muffle. A seed crystal is positioned in such a manner that only a thin film of liquid phase is on the top surface of the crystal. If the powder supply, i.e. the feeding of molten droplets into the molten layer, and the crystal growth rate are weU balanced, the crystal will grow under constant conditions. The principal design and operation of a Verneuil flame fusion apparatus was described earlier [3]. 

When used for ceramic heating, furnaces are called Idlus. Operations include drying, oxidation, c cination, and vitrification. These Idlus employ horizontal space burners with gaseous, hquid, or solid fuels. If product quahty is not injured, ceramic ware may be exposed to flame and combustion gases otherwise, muffle Idlus are employed. Dutch ovens are used frequently for heat generation. 

A muffle is an impenetrable ceramic or metal barrier between the firing chamber and the interior of the furnace. It heats the process charge by radiation and furnace atmosphere convection. 

This study is concerned with four different mixtures, including kaolinite and calcite (kc) kaolinite and dolomite (kd) montmorillonite and calcite (me) and montmorillonite and dolomite (md). All the mixtures, by weight, were 95% clay and 5% carbonate mineral. The minerals were first ground to a fine powder and thoroughly mixed by hand before heating in a muffle furnace (temperature controlled to within 20 °C). Each mixture was heated for 1 h, air quenched at room temperature, and analyzed by X-ray diffraction. X-ray films were made in cameras of 11.46-cm diameter with filtered copper radiation and exposure times of 6 h. Several wet mixtures that simulated ceramic paste before firing were heated and studied in like manner, but they showed no differences from the dry mixtures. 

Silicon carbide refractories are employed in considerable quantities in kiln furniture and in muffles in the ceramic industry, where use is made of their thermal conductivity. The life depends on conditions it is impaired by access of air and water vapour. Oxidation results in volume expansion, embrittlement and cracking of the product. 

The thermal efficiency of ceramic kilns depends on a number of factors, in particular on the kiln type. The consumption of heat related to unit amount of ware is highest for periodic furnaces, and lower by about one half for tunnel or continuous multi-chamber kilns. The consumption of heat also depends on the kind of firing, a direct-fired kiln having a lower consumption than a muffle kiln where the ware is protected from direct contact with combustion gases. For example, a direct-fired tunnel kiln for glost firing of tiles has a fuel consumption 40% lower than a similar muffle tunnel kiln (Holmes, 1978),... 

Thermal conductivity. Thermal conductivity is another practically significant property of refractory ceramics. It is greatly responsible for thermal losses of kilns and furnaces and to a certain degree for thermal shock resistance. A high thermal conductivity is required in some instances to permit as much as possible of the heat to pass into the product (crucibles, muffles, etc.). 

Despite its comparatively high price, silicon carbide is a significant refractory product due to its exceptional properties, such as its high thermal conductivity, high hardness and mechanical strength. It is used in zinc distillation kilns and in the manufacture of muffles, capsules and kiln furniture for the clay ceramic industry (see Section 5.5.4.7.2). In recent years silicon carbide has also been used in the refractory linings of blast furnaces and utilization in other sectors of the steel producing industry is in evaluation. 

Allow a crucible that has been subjected to the full flame of a burner or to a muffle furnace to cool momentarily (on a wire gauze or ceramic plate) before transferring it to the desiccator. 

Ash Platinum boat inside Ceramic crucible In crucible over Muffle furnace... 

Porcelain items only should be placed in a muffle furnace. No glass items should be placed in a muffle furnace. The glass will melt and cause a real mess. If such an accident happens, the furnace must be shut off, and cooled completely, and the glass carefully chipped out. You will likely never get it all out, so a ceramic plate must be placed on the bottom to keep other items from sticking. 

The two most important applications of batch sintering are in the ceramic and powder metal industries. Muffle, bell, elevator, or pit type furnaces are used. As shown in Tab. 9.1, the processes occurring during sintering are somewhat different for ceramic and powder metal parts. 

In the ceramics industry, muffle furnaces are typically used for batch sintering [B.97]. 

Ceramic crucibles that contain the necessary amount of reagent for the expected analyses are placed in a muffle furnace at 750 C the prior day. Ceramic sample boats are similarly stored in the muffle furnace to keep them free of ambient mercury contaminatiorL The sample boats and reagents are allowed to cool before use in a covered pan that is over-pressured by dry, mercury-free, filtered air from the instrument. It is important to allow the surfaces that the sample contacts to reach ambient temperature to minimize loss of elemental mercury. 

Fig. 1.14. Muffle furnace. The muffle heavy black line) may be of high temperature alloy or ceramic. It is usually pumped full of an inert gas.

The insulation adjacent to the hot muffle will have to be either carbon or graphite and as the temperature is dropped, it can be replaced by a ceramic form, although many subscribe to the view that it is best to keep all insulation black in order to avoid any possible interaction with the water and Si02 that may be present in the ceramic insulation, or the Na evolved from the product. Three forms of carbon/graphite insulation can be used, namely, board, felt and powder. The use of powder is not recommended, since there are many handling problems, including possible contamination during use with NaCN. 

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