Mixed-feed kilns

Another thermally efficient kiln is the modem mixed-feed vertical kiln ia which coke is admixed with 8.5—20 cm lump limestone and charged into the top of the vertical kiln by a mobile, overhead charging system. However, use of this kiln is waning since the quaUty of mixed-feed kiln lime does not equal that of the other three kilns described above, owing to ash contamination from the coke and poorer reactivity and to the higher cost of coke in most areas. 

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. 

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. 

Mixed feed kilns currently in operation generally use coke or anthracite [14.5, 14.10]. 

As coke starts to bum at 800 °C, relatively little of its heat is lost as low-grade heat. The net heat usage of coke-fired mixed feed kilns can be as low as 830 kcal/ kg [14.11]. The disadvantages of coke are its high cost per unit of heat and the relatively low availability of suitable grades. 

In the cooling zone, a less refractory lining may be used (e.g., 35 to 40 % alumina), although many operators prefer to use the same refractories as in the calcining zone. This is particularly true in the case of mixed-feed kilns, where the base of the calcining zone is not well defined. 

Intermittent mixed-feed kilns are similar in general design, but, instead of additional fuel being added on to the hearth, it is added in layers while the kiln is being charged with limestone. 

The earliest continuous kilns (also known as running kilns or draw kilns) were fired using the mixed-feed principle (Fig. 16.3). Alternate layers of limestone and fuel (wood or coal) were charged into the top of the kiln and lime was removed through the drawing door. 

Figure 16A Cross-section of a continuous, mixed-feed kiln...

The last mentioned design is used on some mixed-feed kilns [16.4]. 

Mixed-feed kilns are still widely used in the ammonia soda process [16.4] and elsewhere (e. g. [16.5]). 

Coke-fired mixed-feed kilns can have the lowest heat usage of all kilns. Net heat usages of about 850kcal/kg are reported [16.4,16.6], with 950 to 1,100 being more typical of routine operation. 

Another feature of mixed-feed kilns is that they can be operated to produce the consistently low reactivity lime favoured by some producers of aircrete. They can also produce higher reactivity lime, but the CaCOa levels are higher than can be obtained from more modem designs. 

Anthracite (see section 14.3.3.3) is widely used in mixed-feed kilns. As much of the volatile organic matter distils at below the dissociation temperature, part of the calorific value is lost and the total heat usage is increased (typically coke usage is in the range 130 to 150 kg/t lime). The volatiles also cause the emission of smoke, which is increasingly becoming environmentally unacceptable. 

Hard-burned dolomite is generally produced in mixed-feed shaft kilns, operating under reducing conditions. Hard-burned dolomite is generally produced in mixed-feed shaft kilns, operating under reducing conditions. 

C.L. Verma et al., Performance estimation vis-a-vis design of mixed-feed lime shaft kilns , Zement Kalk Gips 9,1988,471-477. 

Many aircrete producers specify low reactivity quicklime produced in mixed-feed kilns, fired with coke. The additional costs per tonne of lime, arising from the use of coke, and from operating at higher temperatures to produce hard-burned lime, are substantial, but are accepted as being necessary. 

Limestone is calcined (usually in a mixed feed kiln — see section 16.4.3 — using coke as fuel) to produce quicklime (used in stage 6) and an exhaust gas rich in carbon dioxide (used in stage 3). 

Anthracite is form of coal that has a volatiles content of between 3.5 and 10 %. It is suitable for use in mixed-feed kilns. 

Mixed-feed describes the process in which the limestone and fuel are both charged into the top of a shaft kiln. 

Large capacity, mixed feed, center burners Noncontact externally heated small capacity kilns used for niche applications Rotary hearth with traveling grate or calcimatic kilns... 

The ore is ordinarily ground to pass through a ca 1.2-mm (14-mesh) screen, mixed with 8—10 wt % NaCl and other reactants that may be needed, and roasted under oxidising conditions in a multiple-hearth furnace or rotary kiln at 800—850°C for 1—2 h. Temperature control is critical because conversion of vanadium to vanadates slows markedly at ca 800°C, and the formation of Hquid phases at ca 850°C interferes with access of air to the mineral particles. During roasting, a reaction of sodium chloride with hydrous siUcates, which often are present in the ore feed, yields HCl gas. This is scmbbed from the roaster off-gas and neutralized for pollution control, or used in acid-leaching processes at the mill site. 

For a coarse-grained (particle containing) graphite, the system depicted in Figure 1 is typical. The nin-of-kiln coke is brought in on railroad cars and emptied into pits where the coke is conveyed to an elevator. The elevator feeds a second conveyor that empties the coke into any one of a number of storage silos where the coke is kept dry. The manufacturer usually specifies a maximum moisture content in the incoming coke at about 0.1—0.2% to ensure that mix compositions are not altered by fluctuations in moisture content. 

Defluorinated Phosphate Rock. There is substantial production of defluorinated phosphate rock for fertilizer use in Japan (about 100,000 mt/year). Ground, high-grade rock is mixed with small proportions of sodium carbonate or sulfate and wet-process acid. The mixture is calcined at a temperature of 1350°C in an oil-fired rotary kiln 45.0 m in length and 2.7 m in diameter. The product contains 38-42 percent P205 of which more than 90 percent is soluble in neutral ammonium nitrate solution and is an effective fertilizer on acid soils. During the production of defluorinated phosphate rock, substantially all fluorine is driven off. Sodium bifluoride (NaHF2) is recovered as a byproduct. A similar product is made in the United States, but it is mainly used for animal feed supplement. 

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