Muffling

Alundum is used for highly refractory bricks (m.p. 2000-2100 C), crucibles, ref ractory cement and muffles also for small laboratory apparatus used at high temperatures (combustion tubes, pyrometer tubes, etc.). 

Example of a muffle furnace used for heating samples to maximum temperatures of 1100-1700 °C. 

Muds, dnlling Muffle furnace Muffle furnaces... 

A furnace may be dkect-fired or indkect-fired. The indkect-fired is known as a muffle furnace, and in such furnaces the combustion gases are separated from the stock being heated to prevent contamination. 

The first of these reactions takes place at temperatures of about 150°C, the second reaction proceeds at about 550—660°C. Typical furnaces used to carry out the reaction include cast-iron retorts the Mannheim mechanical furnace, which consists of an enclosed stationary circular muffle having a concave bottom pan and a domed cover and the Laury furnace, which employs a horizontal two-chambered rotating cylinder for the reaction vessel. The most recent design is the Cannon fluid-bed reactor in which the sulfuric acid vapor is injected with the combustion gases into a fluidized bed of salts. The Mannaheim furnace has also been used with potassium chloride as the feed. 

Different types of furnaces (rotary, muffle) can be used for produciag 2iac vapors. In most cases these furnaces are heated with gas. The oxidation of 2iac vapors is carried out with preheated air ia specially designed chambers. 

The market for fused siUca started ia 1906 with the sale of siUca muffles and pipes. That same year resulted ia the iacorporatioa of the Thermal Syadicate Ltd. Siace that time, worldwide sale of vitreous siUca material and fabricated products has continued to grow. The sales of vitreous siUca iagots, tubes, rods, plates, fabricated products, photomask blanks, cmcibles, and optics was estimated to be between 800 million to 1 biUion ia 1995. These figures do aot, however, take iato accouat the optical waveguide market based oa fused siUca technology. 

Vitreous silica is used for gas-heated or electrically heated devices ia various shapes, eg, as a tube or muffle because of its electrical resistivity, impermeabihty, and low expansion. In its simplest form, an electric-resistance furnace consists of a vitreous siUca tube or pipe on which the resistance element is wound (see Furnaces, ELECTRIC). Because of its iadifference to temperature gradients, a tubular furnace of vitreous siUca maybe made to operate at different temperatures at various portions of the tube, either by arrangement of the heating elements or by cooling sections of the tube with water. Vitreous siUca pipes may be employed ia vacuum-iaduction and gas-fired furnaces (see Vacuum technology) (221). 

At red heat, a low carbon ferrous metal, in contact with carbonaceous material such as charcoal, absorbed carbon that, up to the saturation point of about 1.70%, varied in amount according to the time the metal was in contact with the carbon and the temperature at which the process was conducted. A type of muffle furnace or pot furnace was used and the kon and charcoal were packed in alternate layers. 

In a fire-assay method used at the smelters, a weighed quantity of concentrate is mixed with sodium cyanide in a clay or porcelain cmcible and heated in a muffle furnace at red heat for 20—25 min. The tin oxide is reduced to metal, which is cleaned and weighed. Preliminary digestion of the concentrate with hydrochloric and nitric acids to remove impurities normally precedes the sodium cyanide fusion. 

It is prepared by pasting potassium or sodium dichromate with three times its weight of boric acid, roasting the mixture at 500°C in a muffle furnace in an oxidizing atmosphere, then hydrolyzing the melt with water and superheated steam. The product is then dried and ground. 

Ash. After the sample is heated ia a cmcible over a hot plate to drive off volatile solvents and moisture, it is charred over a Bunsen burner and then transferred to a muffle furnace where final ignition is completed. The weight of the ash is determined and reported as a percentage of the weight of the original sample. 

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. 

Continuous Furnaces Continuous furnaces are employed for the same general duties cited for batch furnaces. Units are gas, oil, or electrically heated and utihze direct circulation of combustion gases or muffles for heat transfer. Continuous furnaces frequently have an extension added for cooling the charge before exposure to atmospheric air. 

Indirect Heating If the process material cannot tolerate exposure to the combustion gas or if a vacuum or an atmosphere other than air is needed in the furnace chamber, indirect firing must be employed. This is accomplished in a muffle furnace or a radiant-tube furnace (tubes carrying the hot combustion gas run through the furnace). 

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. 

Bricks of silicon carbide, either recrystaUized or clay-bonded, have a high thermal conductivity and find use in muffle walls and as a slag-resisting material. 

Figure 7.14 Motor with muffling cone and lining of round felt wool to absorb the exhaust noise of air...

Charcoal is generally satisfactorily activated by heating gently to red heat in a crucible or quartz beaker in a muffle furnace, finally allowing to cool under an inert atmosphere in a desiccator. Good commercial activated charcoal is made from wood, e.g. Norit (from Birch wood), Darco and Nuchar. If the cost is important then the cheaper animal charcoal (bone charcoal) can be used. However, this charcoal contains calcium phosphate and other calcium salts and cannot be used with acidic materials. In this case the charcoal is boiled with dilute hydrochloric acid (1 1 by volume) for 2-3h, diluted with distilled water and filtered through a fine grade paper on a Buchner flask, washed with distilled water until the filtrate is almost neutral, and dried first in air then in a vacuum, and activated as above. To improve the porosity, charcoal columns are usually prepared in admixture with diatomaceous earth. 

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