Modern Kilns

After the product has dried, it is heated or fired in a furnace or kiln. Modern ceramics generally require certain heating schedules that include the rate and duration of heating and conditions such as the presence or absence of air. This is similar to procedures used to produce carbon fibers where the heating schedule is critical to the end product s properties. 

A development of interest to the chemical industiy is the tubular precipitator of reinforced-plastic construction (Wanner, Gas Cleaning Plant after T1O2 Rotary Kilns, technical bulletin, Lurgi Corp., Frankfurt, Germany, 1971). Tubes made of polyvinyl chloride plastic are reinforced on the outside with polyester-fiber glass. The use of modern economical materials of construction to replace high-maintenance materials such as lead has been long awaited for corrosive applications. 

Modern lime kilns operate at approximately 1,200°C to 1,300°C. Limestone, which has been crushed and screened into pieces with diameters of several inches, is fed into the top of the kiln. Air fed into the kilns bottom fluidizes the limestone, allowing for greater reaction... 

There Is something strangely suggestive in the relies of ancient pottery-kilns that still linger on the scene of their former active operations. The modern Staffordshire potter has derived do heritage from his Roman predecessor. Thera had been no continuity of work. Centnries of disuse had obliterated the very memory... 

Except for very old mixed-feed vertical kilns, time kilns operate with countercurrent How of raw material anil heal. Modern lime kilns utilize coolers to preheat air by recuperating heal from the hot quicklime, Lime kilns may be tired directly with coal, oil, or gas. 

Some characteristics of a modern grate-kiln iron ore pelletizing machine [7]. 

Except for the rotary kiln itself, which now represents only a fraction of the total capital cost, today s modern coke calcining plants bear little resemblance to the original facilities (Figure 2). Even the kiln itself has undergone major changes, not the least of which is increased capacity. A typical modern kiln,... 

To improve energy efficiency, refractories with superior K factors are used in lining the kiln, thus reducing radiant heat losses. Moreover, kiln mounted blowers now inject combustion air into the kilns in the zone where the volatiles evolve from the coke, thus permitting utilization of the Btu content in these previously wasted gases. In modern calciners, most of the energy required is obtained by burning the coke volatiles and fine particulate matter in the kiln. In some instances, rotary kilns equipped with kiln mounted blowers actually operate without external fuel (except for start-up). When these units are also equipped with incinerators (to combust the unburned volatiles and emitted coke fines) and waste... 

Nodules of clinker, typically 3-20 mm in diameter, are formed in a semisolid state in the burning zone, and solidify completely on cooling, which begins in a short cooling zone within the kiln, and continues in a cooler. In modern plants, when the nodules leave the kiln, their internal temperatures are around 1350°C, but their surface temperatures are considerably lower. 

The burning temperature for production of Portland cement clinker can be decreased by about 150°C through the use of fluxes, but opinions have differed as to the energy saving thereby obtainable, Klemm and Skalny (K52), who reviewed the subject, estimated it at 630kJkg" . Christensen and Johansen (C56) considered that this figure, while possibly realistic for an inefficient, wet process kiln, was unlikely to be so for a modern, precalciner-preheater kiln, in which heat recovery is efficient. They considered a value of lOSkJkg" more realistic. 

SiC kiln furniture, the heat consumption was 31.400 kJ/kg of ware in fast firing the consumption can be reduced to 12.600 — 14.000 kJ/kg (Harms, 1978). In another tunnel kiln for the firing of porcelain flat ware, the heat consumption was 24.300 kJ/kg (firing cycle 21 hours, 1430 °C). For decorative firing at 960 C, the consumption was 3800—4600 kJ/kg. According to Schroder and Goerg (1978), a value lower than 2400 kJ kg of fireclay bricks has been achieved in a modern tunnel kiln. A similar heat consumption is reported for modern brickmaking tunnel kilns (Staff Report, 1977). 

Currently shaft kilns with the most modern technology and rotary kilns with preheaters are mainly used, depending upon the required product properties. 

Modern rotary kiln plants have capacities of up to ca. 500 t/d, continuous grate furnaces up to 1000 t/d. The residence times are less than 1 hour. In kettles throughputs up to 150 t/d with residence times of several hours are aimed for. Box shaft kilns are no longer viable due to the lengthy (several day) calcination times. 

Three aspects are common to all shaft kiln designs, namely charging, drawing of the ore, and combustion. A shaft kiln is essentially a vertical refractory lined cylinder or ellipse. The ore is charged in at the top of the furnace, along with, in some cases, a solid fuel such as coke or anthracite coal. Other fuels such as natural gas and oil can also be employed. There are a number of different variants of shaft kilns, such as the mixed feed, traditional type and modern basic design, annular, parallel-flow regenerative, double inclined, and multichamber. 

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