Ceramic Technology and Processing, King

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Gas Management System. The Figure 8.8 furnace has a poorly designed gas system replete with problems. In the figure, the vacuum system is in the back on the other side of the sight glass. An inlet is for introducing gases through a tube connected to the sight glass assembly. Without an outlet, one cannot sustain the flow. The furnace is pressurized. To sustain a flow of gas, valving is needed to both retain the pressure and to provide a gas flow across the sight glass. This may not be provided, and is something that has to be validated.

Solenoids designed to have the coil activated during the run result in a high amperage in the winding and the coils can run hot, which can quickly destroy them. Compression fittings are a good choice for the gas lines. Pay attention to the manufacturer's directions on tightening. Also, compression fittings are not designed to be dismantled and reassembled. Use a Union fitting where it has to be taken apart.

Solder joints are okay for larger diameter tubing. One needs a little skill to make good solder joints. Make sure that the tube and fitting are not damaged and be sure to clean the parts well. Once the solder flows into the joint, give it a quick wipe with a cloth and leave it alone.

Temperature controls. Figure 8.8 has a two-color optical pyrometer, which is a good choice if the sight glass is kept clean. Optical pyrometers do not work at low temperatures. If the manufacturer did not provide for this, one can make a fitting to insert a thermocouple into the furnace. Seal it with an "O" ring, measure the temperature up to where the optical pyrometer can take over, and then withdraw it. While not automatic, this procedure is tolerable although the temperature measured is on the OD of the heating element and not on the sample. It would be preferable to read the temperature on the sample, but this can be a little hard to do. Programmers can be simple or they can be complex. Operating manuals can be undecipherable, but procedures will work out. Again, a PC is a good choice, especially with the added complexity of gas handling and switching temperature measuring devices.

Operating Problems

Figure 8.8 and another similar carbon tube furnace can have many

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operating problems. Figure 8.11 is a sketch of the other carbon tube furnace.

Figure 8.11: Lab Furnace with a Ceramic Tube Accessory. Stresses on the tube during cooling can cause it to fracture.

Graphite is used for all the interior parts. Since graphite oxidizes in air, an alumina tube insert fires in air. "O" rings seal the tube at each end. As the furnace heats, the tube expands along its length placing the hot zone in compression. As the furnace cools, the tube contracts along its length, placing the hot zone in tension. The tube can break and trash the furnace. Friction between the "O" rings and the tube can be excessive. Refer to Figure 8.8 to look at another problem. At the top corner, there is an arrow with the caption "hazard." Voltage across the insulator is low and will not produce an electrical shock. However, if someone is wearing a ring, a metal banded wrist watch, or a metal bracelet, the metal will instantly melt,

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cutting off whatever appendage it is fastened to. Therefore, avoid wearing any metal object such as a ring or bracelet when operating this type of furnace. The circuit has low impedance, like a welder, so the amperage is high. This is irresponsible engineering. This area has to be at least wrapped with layers of duct tape for insulation, but this is a stop gap measure. Let us hope that anyone else using this equipment will understand the presence of the hazard.

Check List, Tube Furnaces, Reducing

•Materials quality

•ISO 9002 qualified

•Vacuum system

•Resistor design

•Electrical contacts

•Ceramic tube ends

•Gas plumbing and valving

•Sight glass cleaning

•Temperature sensors

•Controller/Programmer/PC

•Tube fracture

•Electrical hazards

Gas Fired Kilns

Ceramic production is predominantly fired in gas-air kilns. Costs are lower than the alternatives, unless they are operating in areas of low electrical rates. High performance ceramics are an exception. In the case where the product is ultimately to be fired in a gas kiln, it can be advantageous to do the lab work in a lab gas kiln. Transfer of a product to production is difficult enough without added complications. Figure 8.12 is of a laboratory gas fired kiln.

This kiln has four burners firing under the hearth. Burners are connected to a blower for the air supply and natural gas to make up the combustion mixture. Proportions of the two are adjustable for either

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oxidizing or reducing atmospheres. A zirconia oxygen gage measures and controls the oxygen partial pressure in the furnace.

Figure 8.12: Gas Fired Kiln. A wide variety of gas fired kilns are available according to size and temperature capability. (Courtesy of Bickley).

All manufacturers, as far as the author knows, have safety interlocks that turn off the gas supply when there is a malfunction. Flow gages are mounted on the shell for both gas and air. The door is double hinged so the hot face of the kiln will not be in one's face when the door to a hot kiln is open. A Pt-Pt/Rh thermocouple is used for temperature measurement and control. There is a plug in the door to accommodate an optical pyrometer. Automatic controls are available. With this kind of door, the front of the steel shell will overheat as the refractories become worn. This will result in warpage of the shell and a degenerating problem with heat loss around the sides of the door. The door closure refractories have

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to be maintained. While on vacation, someone failed to maintain the door, and the shell is now warped. What does one do to the kiln? With the door swung out of the way and the refractories covered, slots are cut with an oxyacetylene torch in the shell above the sides and top of the door. Then, the shell is hammered back in place and the refractories are repaired. In the figure, the manufacturer has thoughtfully stiffened the shell around the door with steel gussets. There, warpage has happened before.

Hot gases, called sting, exit from the kiln through ceramic tubes located in the top of the kiln. An experienced operator may be able to tell if the burn is going right by the appearance of the sting.

The atmosphere in a gas kiln is a mixture of gases: N2, H2O, CO2, O2, a little hydrocarbons, and CO. The atmospheric composition can be oxidizing, neutral, or reducing depending upon the ratio of the concentration of these gases. Ceramic parts can be sensitive to this atmosphere.

Several aspects of gas/air kilns include size, configuration, temperature, refractories, controls, and operation.

Size

The size has to be adequate for the parts, but a small kiln will have temperature gradients. It is a good idea to obtain a larger kiln to provide space around the ware and space in the free volume for uniform gas circulation and combustion. For general purposes, three to eight cubic feet is the range for a good lab size. Smaller kilns are apt to have temperature gradients and possibly direct flame impingements on the ware. There has to be space in the combustion chamber for combustion, convection, and heat transfer to occur. Burners should fire under the hearth, and this also takes up space. The top of the combustion chamber is an arch, again requiring space. It is best to not work with small kilns.

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Configuration

Dimensions are determined by the shape and the amount of firing. A lab kiln is usually a box with an arched roof. Hearths should always be raised with the burners firing under the hearth. Avoid direct impingement of the flame on the ware. Most lab kilns are front loading, but a bell design has advantages. There are more options in configuration if the firing temperature is not too high. Flatware is often fired on a broad hearth. When the temperature is high, the arch cannot be structurally stable over a broad span. Most lab kilns are shaped internally as a cube with all three dimensions being similar. When there is a requirement for firing long tubes, the kiln has to have a long vertical dimension, which may require additional tiers of burners.

Temperature

Maximum temperature is determined by the burners, combustion gas composition, and the refractories. For the lab, it is a good idea to over design, as the next project may require a higher temperature. Four burners, two on each side, with alternately firing from opposite sides under the hearth are a good design. Fiber blanket refractories are widely used in production kilns, are available in clever attachment schemes, and are usable to about 1600 °C. Lab kilns more commonly use fiber panels. High alumina fiber is available for use up to about 1700 °C. This fiber, available in blanket and formed shapes, is expensive. In the lab, expense is not as controlling a factor as capability. When the hot face is dense 99% alumina brick, the lining is usable to about 1800 °C. For even higher temperatures, the brick is a bonded calcia-stabilized, zirconia bubble.

When properly designed and tuned, a temperature up to 1800 °C is achievable with natural gas/air when large burners are used and the blast gate is open full bore. This is difficult to maintain, but one can either enrich the air with oxygen, preheat the incoming air, or use a gas such as MAP (a high-temperature-burning, welding gas). One can preheat the air by channeling the air through a gap between the hot face and the backup refractories. Design of this structure is critical as support for the hot face can be compromised causing it to sag inward. With sagging, one will need to reline the kiln; this will result in serious delay and considerable expense.

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Refractories

The kiln lining has to be engineered so that it can expand and contract. Usual construction is having a wall that holds up an arch through skew blocks with the top free to move up and down. Skew blocks transmit the lateral stresses from the arch through the brick work to the steel frame. Back up insulating brick should be tied to the hot face, and in turn tied to the kiln shell, usually with metal tabs or angled metal supports. These can oxidize and burn off, so it is necessary to make them from a refractory Ni/Cr alloy. Refractories can creep at high temperatures, changing the interior configuration of the wall. The tendency is for the hot face to sag inward, and, if this happens, the insulating brick will be exposed to the hot gases and will fail. Refractories need some maintenance to keep them in place. Refractory suppliers will have data on high temperature properties and hopefully ISO 9002 certification. Low alkali compositions and high density creep less. Burner blocks are castables, generally high alumina with a calcium aluminate bond. Burner blocks have to have a stable location to fire under the hearth without impinging on the hearth supports. Hearth plates are supported with high alumina refractory blocks on a stable base. The plates are a high alumina dense refractory or silicon carbide. High temperature gas kilns usually have high purity (99%+) alumina dense bricks.

Controls

Either thermocouples or optical pyrometers are satisfactory. One advantage of thermocouples is that one can locate them in various parts of the kiln to look for temperature gradients. Optical pyrometers are usually hand held and sight on the setting on the hearth. It helps to have a steady rest to hold the optic on the target. For best results, the optic should sight on a black body, which is a block with a hole having a depth twice the diameter. The burn is programmed with ramps and a soak. A programming problem is that the kiln cannot always keep up with the program, either on the upswing or cool-off. Naturally, the program has to fall inside the limits of the kiln's capability.

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Oxygen gages should be standard equipment on gas kilns. The ratio of gas/air is initially set with flow gages. Then, the oxygen sensor can maintain the specified level of oxygen fugacity. It is common to run a little on the oxidizing side when firing oxide ceramics. Pyrometric cones are still useful. They are inexpensive and easy to use. While not useful as controls, they easily reveal the amount of heat treatment adsorbed in various parts of the setting.

Operation

There are several steps to starting up a gas kiln.

Setting. Set the hearth and close the door. (Setting the ware is a sizable subject that will be discussed in a following section.)

Lighting Up. Unlike electric kilns, one starts the gas kiln manually. First, light the pilots. There will be a pilot for each burner, and they will be lit one at a time to establish a slow ramp for binder burnout. One can pull out these pilots to light them before returning them into the burner assembly. Electric spark ignition is also common. Gas kilns have safety features in that the main burners cannot be lit until the pilots are burning. Never bypass safety features; gas/air mixtures can be explosive. Depending on how steep a temperature ramp, the ware and kiln can stand, the burners can be lit one at a time, alternating from side to side and back to front.

Ramp Up. A butterfly valve (blastgate) is set to adjust the amount of air from the blower. The amount of natural gas is determined by the air volume. As the blastgate is opened another increment, the gas volume increases accordingly. The gas-to-air ratio can be fine tuned with information from an oxygen sensor. When the soak is reached, the blastgate is cut back to where the temperature is maintained. Whenever it becomes difficult to hold the soak temperature, the problem might be that the residence time of the combustion gases is too short. By placing a brick partially over the exit port on top, a little back pressure will increase the

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residence time. For higher temperatures, oxygen enrichment is another option. Some kilns have internal structures where the incoming air is preheated. This will also increase the maximum temperature. Be cautious as some of these structures are not structurally stable and can collapse as the inner lining sags.

Ramp Down. The temperature drop is much faster at high temperatures and it might be necessary to program this with the blastgate down for the initial ramp. At lower temperatures, the thermal conductivity of the insulation usually determines the cool down rate. If everything can stand the thermal shock, one can cool faster by blowing in cold air through the blastgate. A less appropriate option would be to open the door as this introduces cool air and thermal gradients. Whenever this does not damage the kiln or the ware, it is an option. It is a better idea to be patient and let the kiln do its own thing.

Check List, Gas Fired Kilns

•Size: not too small

•Configuration: to suit your needs; bell kilns have advantages.

•Temperature: Make a selection of refractories, burners, oxygen, or preheated air.

•Refractories: Check on quality and structure stability.

•Controls: Select a type. Check safety.

•Operation: binder burnout, ramp and soak, cool down

Other Furnace Types

There are many other furnace types, and it would be just too much to plod through them all. Shuttle kilns, roller hearths, and tunnel kilns are not included, as they are not usually seen in the laboratory. However, one usually fires samples in production kilns of these types. A discussion follows on rotary, induction, and vacuum furnaces.

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Rotary Furnaces

These furnaces are used for calcining or sintering granular materials. They are on a slant, with the material fed into the high end and exiting out of the low end as the kiln rotates. The burner is on the low end, firing up into the cylindrical kiln. Electrical heating is more common in lab rotaries. Rotary kilns are dusty and may need a dust collector on the effluent gases. Rotaries are available in lab sizes with metal or refractory tubes. Metals scale with time and will contaminate the batch. Contamination can also come from the seals at the feed end, where metal rubs between the stationary and rotating parts. Another source of metal contamination is from the bull ring and drive that transmit the rotary motion to the kiln. Some designs have chains hanging in the interior for breaking up the material as it sinters. However, this will furnish additional contamination. Just how serious contamination is depends on how much can be tolerated and how much can be removed by magnetic filters.

Material accumulates in the hot zone, making a ring that builds up with time. One uses a shotgun to blast this ring. Large production kilns are even better, as they use a machine gun.

Induction Furnaces

As this is about lab kilns, there are restrictions on size. Figure 8.13 is a sketch of a typical lab induction furnace.

Furnace construction is very simple consisting of the following: a shell, a coil, a base plate, a susceptor, and insulation. The susceptor is often graphite machined out of a billet. The coil is of copper tubing; it keeps the water out of the hot zone in case of a failure. Water is circulated in the coil to keep it cool. Thermal insulation can be graphite fiber, bubble alumina, or bubble zirconia, depending largely on the operating temperature. Tamped lamp black is also used but it is messy. Temperatures above about 1700 °C require graphite thermal insulation. For lab metal melting, the furnace is lined with refractories (often a crucible), and the metal charge can act as its own susceptor. Material for the shell has to be an electrical insulator and should be relatively impervious, especially if the other parts are graphite. Glass fiber-reinforced cement tubing can be used. The coil is