INDUSTRIAL FIRING PROCESSES
This section comprises a very brief overview of industrial ceramic firing methods. The information mostly relates to larger plants firing bricks and tiles. The most up to date information is available from your local industrial kiln manufacturer or technical ceramic sites on the web.
If you require faster turnaround times and high throughput's, the best approach is to fire with forced air burners. This system utilises sealed, usually high velocity, burners fed with clean gas and pressurised air from an air blower. There is a design in a well known ceramic paperback that uses a single forced air burner on a small batch type studio kiln but typically, for capital cost recovery, the kiln would need to be a minimum of 3-5 m3 or was fired every second day to consider this application.
FORCED AIR BURNERS PRODUCE:
Accurate control of the air and gas ratio through the firing. This is due to non reliance on atmospheric primary air for initial combustion and secondary air through flue draw.
High efficiencies through preheating combustion air, reuse of flue gasses for dryers and rapid firings.
High velocity convection forcing the water from the product and evenly heating all sections of a load.
Forced air burners, due to the capability of forcing relatively high volumes of combustion products into the kiln space in a short time, must have rapid shut down safety systems. It is this requirement that can increase the cost together with fully automatic start, programmable temperature controllers and accurate burner control valves that are usually specified for maximum benefit.
As expected ceramic compositions and development of ceramics is advancing apace. Covering of wear prone parts such as auger and mixer blades and press dies with ceramic materials is showing promise. Fully ceramic parts or more widespread use of PSZ should be commercially viable by now. Compositions of zircon and alumina silicates show promise in certain areas. Dies are either the usual high chrome steel or newer ceramic coated for longer life. Steel dies would be able to be machined 5 or 6 times prior to replacement, the ceramic dies usually have several pieces and can last much longer.
Thorough drying of the product is crucial for fast firing. Most industrial processes use waste heat from the kiln however a heat boost is required in the drying section. It would follow that the efficiency of the kiln process might be lower than is possible if a boost is not required as there would be excessive heat loss from the kiln section. If excessive water vapour is formed in the drying, increase the heat to overcome or increase the recirculating air volume.
The usual drying arrangement uses air fans and gas ducted burners to move the heated air (typically 110° C) through the product. Jet drying is favoured by some and this consists of higher pressures (typically 8 meter/sec.) forcing the heated air through a slotted side wall in the dryer. In some the slotted wall moves back and forth to distribute the air even more thoroughly. An ideal drying system would have the air delivery arranged to suit the product i.e. horizontal slots for thinner tiles.
Most modern high throughput tile or brick kilns are tunnel kilns. These have been around for a while now and replaced the earlier "Hoffman" kilns (continuous kilns relying on continuous draw of combustion products through the load) as these did not give the efficiencies or evenness of temperature required for modern processes.
The German Keller company uses tunnel kilns with multiple high velocity burners, special "cassettes" and cars designed to hold tiles and other products to allow efficient heat transfer and convection. Keller provides a total approach with savings on staff, work area taken into the total savings figure. Typical firings with multilayer stacks (typically 12 tiles high and wide) are approx. 8 hours versus the usual 20 or more hours in previous tunnel kiln plants although these vary from product to product.
The Italian Mori company pioneered the "roller kiln" using ceramic rollers to propel the product through the kiln in a single layer at fast speeds. This is a common concept in the steel industry but has developed in the heavy clay area as cheaper and stronger ceramics have become available. Typically 2-3 hour firings for tiles would be expected with 30 minute firings for domestic ware. The key is in the kiln design, the number of burners and placement for extremely even temperatures (the product must be heated at the same rate) and, most importantly, the composition of the body. This is formulated to withstand the stress and the rapid cool to hot to cool cycle. Mori would supply two kilns if more product was required but the newer Mori concept has several layers of shelves moving on rollers located on the kiln side in between three burner layers. To support the weight across the 1.5 or 2 meter width the tile cassette is made from exotic alloys. This idea may not be commercially viable yet due to the high alloy cost.
Both applications utilise many high velocity burners located above and below the stack for minimum temperature differential. Typically a kiln of this design would have 2-4° C difference in any part of the firing area at 1160° C. These tunnel kilns also have quite low stacks typically 600-750 mm high to achieve rapid transfer to the load. Low thermal mass insulation or interlocking refractory tiles are used and the impression of the kiln is quite small due to the smaller casing size and the reduced length of the tunnel. There is rapid forced cooling at the tunnel end with an air curtain to contain the heat loss. Flues are typically under the cars towards the drying end with facility for recirculating.
TOTAL PLANT CONCEPTS
Shorter firing cycles, better heat distribution, better insulation, better car and cassette design, better loading and unloading facility design etc.. all contribute to savings at the expense of higher initial capital cost. Throughput's of 20,000 to 45,000 pieces or more per day justify intense research on these concepts. Annual rates of 200,000 tonnes are noted.
Of course it is critical for economy of scale that these plants are run frequently without stoppages. The burners are typically turned down not off although rapid heat ups are possible from cold in the furnace zone but the extra heat shock and heat loss in the cars etc.. can be costly if the plant is stopped. Shifts of 7 day, 20 hours per day are common. Cars are typically good RI bricks with fibre covering wherever possible. In some countries, bricks would need to be redesigned due to the lack of surface area available to exhaust the water and the carbon. Maximum carbon content would be around 1.5 to 3% with free silica less than 25%. Handling of the product deserves attention to minimise breakage's and excessive handling. The loading and unloading from the extruded to the drying car is done by either suction pods or silicon type "fingers".
There is a "buffer" section between the manufacture, dryer and kiln and this provides a safety net to ensure the process is not stopped or all the parts are sufficiently dried. The buffer can be extra heating or a mechanism to off load a faulty cart. To ensure the products are not out of line, infrared detectors are used to line up the cars and reject loads before entering the dryer.
The modern plant and both roller hearth and tunnel kilns are very expensive and also call for a continuous expenditure program on the maintenance of the apparatus. High labour costs justify this investment in high wage countries. Minimum throughput's are required and many smaller plants are shutting, merging or committing to this upgrade as their competitors achieve greater economies of scale and higher efficiencies.
REMOTE AREA PLANTS
A plant in a remote area needs to have the following desirable features:
Minimal capital cost
Low maintenance equipment
Low thermal mass
Low transport costs
Greater flexibility than larger plants firing a variety of products, with shorter working cycles
As labour costs will be expected to be lower, it is possible to achieve similar economies with modern plants by utilising "older" technologies. Intermittent kiln designs such as the single or dual truck or portable cover would fulfil the above requirements. The burners would still need to be sealed higher velocity types to achieve consistent product and accurate temperature control and combustion product monitoring would be desirable. Recirculation of the exhaust heat to recuperators, regenerators and/or for drying must be installed. Gas firing should be considered as oils or other products would increase maintenance, produce inefficient firing and be more difficult to control. A typical batch type firing using high velocity burners could be to 950°C in 2-3 hours, to 1150°C in 3 hours, hold for 1 hour then down to 650°C. The efficiency of the plant is paramount and if a second car can be loaded and brought back to temperature as quickly as possible there will be less heat loss. Flue exits at 120° C with kiln exit at 40° C. Loading and unloading facilities and conveyor systems are critical for more consistent product and rapid kiln firing turnarounds. Rapid die changes for various product type should be considered.
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