Curvas IDA

The quality of lime is determined by the content of CaO in the calcined product.

Excess burning can reduce the availability of CaO, and low-temperature operation can result in an unreacted ore. If the calcination is carried out near the theoretical dissociation temperature, CaO will not be lost by reacting with impurities such as iron, silicon, and alumina, which are usually present in limestone. The first multi-stage-type fluidized bed commercial reactor^94,95 for calcination of limestone was built in 1949 for the New England Lime Company. The reactor was 4 m ID and 14 m long and had five compartments. The reactor was used for three functions: (1) preheating the air which was also the fluidizing agent and the oxygen source for burning the fuel oil, (2) carrying out the calcination reaction which could be accom-plished by the heat supplied by the combustion of sprayed fuel oil over the surface of the limestone, and (3) preheating the incoming limestone feed by the hot outgoing combustion product gas. The five-compartment concept was found to be useful for carrying out calcination economically, conserving the fuel oil. The calcination of limestone was accomplished at 1000°C with a heat input of 42.9 kcal/mol of lime-stone. Limestone conversion of 96.8% was achieved. A relatively large amount of unconverted fines, amounting to 14%, was entrained, and this was found to be responsible for lowering the overall conversion of limestone calcination. The particle size of the raw material fed into the top of the multicompartment calciner was in the range 6–65 mesh. In order to calcine calcium carbonate fines (<50 µm), a new technique known as pelletization⁹⁶ was developed. In this technique, soda ash or caustic soda is mixed along with the feed. The calcined material is coated over the fines as a sticky mass, thereby allowing the fines in the reactor to grow by adhesion or pelletization. The growth rate of the particles inside the reactor can be controlled by controlling the feed rate of the material into the reactor.

Limestone calcined in a fluidized bed reactor usually has improved properties;

hence, many commercial fluidized bed limestone calciners came into existence as early as three decades ago, and reports^97,98 on these are available. A multistage fluidized bed calciner that produced a very active quicklime was reported by Van Thoor.⁹⁹ Details and the kinetics of limestone calcination can be found in the literature.¹⁰⁰ The decomposition of limestone is assumed to start from the surface and then proceed toward the center. The reaction is believed to occur in a thin layer

between the unreacted limestone and the product. Although this type of theory is accepted, many conflicting issues crop up in terms of the controlling mechanism.

Calcination can proceed if the latent heat of decomposition is available. The heat flux and hence the rate of calcination are determined from knowledge of the sample geometry and external conditions using Fourier’s law of heat conduction.

2. Cement, Bauxite, and Phosphate Rock

A process for the production of cement clinker,¹⁰¹ named after the inventor, Pyzel, was developed in 1944. It incorporated a fluidized bed reactor with a 2.5-m ID and operated at 1300°C. The reactor, when operated in a single stage, lost heat through the exit gas, and the product obtained was of the order of 1050–1180 kcal/kg of clinker. Hence, improved heat recovery systems were deemed essential. Mitsubishi developed a new suspension¹⁰² preheating system for cement clinker production and incorporated a fluidized bed limestone calciner. Fine lime powders were produced by spraying lime sludge over the carrier bed of agglomerated lime particles. The slurry, after being sprayed on the bed, decomposed in a short time to fine lime particles, and the product was collected after cooling rapidly.

A fluidized bed calcination process for converting low-grade bauxite-contain-ing selenium oxide was reported¹⁰³ to be a viable technical route to produce aluminum sulfate after treating the calcined product with H₂SO₄. The calcination of low-grade bauxite with 1:1 soda ash at temperatures around 700–1400°C in a fluidized bed reactor can produce water-soluble sodium aluminate which, upon leaching and treating with sulfuric acid, can yield aluminum sulfate. Calcination of low-quality phosphate rock from the western United States was reported by Priestly.⁹⁷ The phosphate rock contained 3.5% hydrocarbon and could generate much of the heat required for the calcination carried out at 760°C in a 6-m-diameter three-compartment Dorr–Oliver fluosolid calciner. During the calcination of phos-phate rock, any limestone contained in it was also calcined, thereby yielding useful lime which was leached out easily. The calcined phosphate rock was cooled by direct water spray over a separate fluidized bed, thus bringing down the charge temperature from 538 to 121°C. A continuously operated pilot-scale fluidized bed reactor for the decomposition of ferrous sulfate at 700°C to produce a pigment containing 96% Fe₂O₃ with a mean pigment size of about 1 mm was reported by Fenyi.¹⁰³

3. Aluminum Trihydrate

a. Circulating Fluidized Bed

Vereinigte Aluminium Werke (VAW) and Lurgi developed a highly expanded fluidized bed for processing fine-grained solid particles. The reactor was operated at a high velocity, which increased the capacity significantly. Reh⁹² described highly expanded fluid beds for application in industrially important exothermic and endot-hermic processes and also gave an account of the developments carried out by VAW/Lurgi for the calcination of Al(OH)₃. In a highly expanded bed, the product

is collected mostly from the exit gas. VAW/Lurgi operated a pilot plant for several years to calcine aluminum trihydrate. The capacity of the pilot plant was 28 tpd, which was later scaled up to 560 tpd.

b. Operation

The circulating fluidized bed for calcining Al(OH)₃ was fluidized by air that was divided into primary and secondary supplies. The primary air was preheated by passing through suspended pipe coils immersed inside the four-stage fluidized bed of the hot product, alumina (200°C), and then was used to fluidize the Al(OH)₃ for calcination. The feed, Al(OH)₃, was dewatered and preheated using waste heat.

Venturi-type highly expanded fluid beds were employed for this duty. Venturi-type conical fluidized beds are grateless and offer uniform temperature distribution and relatively lower pressure drops than conventional fluidized beds. The heat for the calcination of Al(OH)₃ was supplied by direct oil burning inside the fluidized bed of high solid concentration. The secondary air supplied through the hot four-stage fluidized bed of hot alumina helped to effect complete combustion. The combustion was near stoichiometric without any soot formation or superheating. The CO content at the outlet of the reactor and the O₂ content in the flue gas were 0.5 and 1%, respectively. The operating gas velocity in the pilot plant was 3 m/s. The reactor was 100 mm ID and 800 mm in height. The mean solid concentration was 16 kg/m³ of the furnace volume. A schematic diagram of a typical circulating fluidized bed used for calcination is shown in Figure 2.16. Based on the experience with a 28-tpd pilot plant, a capacity calcination plant was commissioned. A 560-tpd-capacity circulating-type fluidized bed calciner is reported to require merely two-thirds the inside diameter and one-third the grate area of a conventional multistage fluidized bed calciner (6700 mm in diameter) with a capacity of 280 tpd.

4. Alumina

Schmidt et al.¹⁰⁴ gave a complete process description of the methods and means of producing alumina of different quality using a fluidized bed calciner. The fluidized bed process was reported to offer optimal performance to produce alumina of the desired quality. The two frequently used types of alumina are (1) fine-grained, high-calcined floury type and (2) large-grained, low-high-calcined sandy type. The VAW/Lurgi process for calcining alumina can optimally adopt the changeover from the production of floury-type to sandy-type alumina. Investigations carried out to minimize particle attrition and elutriation improved the process to meet pollution control requirements.

In the new Toth¹⁰⁵ process for the production of alumina, drying and calcination are accomplished in a fluidized bed. The Toth process adopts fluidized bed chlorination of clay followed by the reduction of aluminum chloride by manganese metal pellets.

The by-product, MnCl₂, is dechlorinated in a fluidized bed for recycling Cl₂ and Mn.

5. Waste Calcination

a. Chloride Waste

Chlorination of nonferrous ores and minerals usually ends up with the gener-ation of highly corrosive pollutants such as ferrous or ferric chloride. The loss of chlorine and the environment problem can be overcome by calcining the ferric chloride^106-108 over a bed of fluidized iron oxide. The calcined product is constituted of nonpolluting iron oxide and the chlorine obtained can be recycled for chlori-nation reactions. Recovery of chlorine and iron oxide by dechlorichlori-nation of FeCl₃

Figure 2.16 Typical schematic diagram of a circulating fluidized bed calciner incorporating waste heat and hot calcine heat recovery system.

using a fluidized bed of Fe₂O₃ was discussed by Paige et al.¹⁰⁹ For smooth operation and better dechlorination in a fluidized bed, a feed composition of 75% FeCl₃ and 25% Fe₂O₃ was recommended. An exit chlorine concentration >80 wt% (for feed FeCl₃ of bulk density <13 g/cc) and Fe₂O₃ (after washing) calcine of 70 wt%

suitable for blast furnace feed can be obtained from a fluidized calciner.

b. Radioactive Waste

Calcination in nuclear engineering, using fluidization, has been well accepted, especially for radioactive waste solutions. By this method of calcination, the radio-active solution is converted directly into granular solid in a fluidized bed. The solid form of the waste thus formed is safe and easy to store and dispose of. The voluminous radioactive solution after calcination is reduced to a volume that is tenfold less than the initial volume. Calcination of a radioactive solution in a fluidized bed is usually accomplished by spraying the solution over a hot bed of inert or reactive solids in the temperature range 400–600°C. The solidification of high-level radioactive waste solutions by calcination in a fluidized bed was described by Schneider.^110,111 The details of the fluidized bed as applied to waste disposal using fluidized electrode cells are described in Chapter 6, and its applications in nuclear engineering are described in Chapter 5.

c. Zirconium Fluoride Waste

An important promising application of fluidized bed calcination is to decompose aqueous zirconium–fluoride-bearing waste. The product of calcination is a safe solid waste, and this process has been proved to be reliable and viable. A fluidized bed 1200 mm in diameter was tested on a plant scale by the Idaho Nuclear Corporation, and the process which is claimed to be safe was described by Lohse et al.¹¹² There have been other developments in calcination of uranyl nitrate solution directly to arrive at useful oxides of uranium for use in nuclear fuel preparation. The details of this calcination (also known as denitrification) are described in Chapter 3 on uranium extraction.