Yang Fluidization, Solids Handling, and Processing

Three-Phase Fluidization Systems 595

transfer could be explained by radiation and convection, while the rest was attributed to circulation of superheated carbonaceous materials within the slag. Ibaraki (1994) considered the heat transfer to be caused by two mechanisms; (i) radiation heat transfer from the gasses in the upper portion of the reactor to the slag layer and (ii) heat transfer from the flame of the oxygen jet through the surface of the slag cavity created by the oxygen jet. For the first mechanism, the radiation increased the temperature of the slag surface, and the slag motion made the temperature homogenous throughout the slag layer. Ibaraki (1994) concluded, based on his prediction, that the radiation heat transfer is not significant, accounting for 20% of the heat transfer. Based on another calculation using a flame temperature of 2600 K and an estimation of the cavity surface area, Ibaraki (1994) stated that the second mechanism accounted for only 20% of the heat transfer. Since these two mechanisms accounted for only 40% of the total heat transfer, Ibaraki (1994) proposed a third mechanism for heat transfer that accounted for the final 60% of the heat transfer. The third heat transfer mechanism is the transfer of heat from the combustion process in the slag to the reduction sites. The oxygen from the jet is not completely reacted upon reaching the slag layer, therefore, a portion of the oxygen reacted with gas, iron droplets, and char in the slag.

In order to provide further insight into the post-combustion ratio and the heat transfer efficiency, the factors that affect the PCR and HTE will be delineated. The factors that affect the PCR and HTE will be discussed separately with the understanding that a complex relationship may exist between the two parameters. The factors that affect the PCR are shown in Table 4, and Fig. 3 demonstrates the primary conditions for postcombustion. The PCR should be kept relatively high, since the fuel consumption decreases with an increase in the PCR at the same HTE (Aukrust, 1993). However, as mentioned, high PCR may lead to problems due to increases in:

-Off-gas temperature, with detrimental effects on the refractories and gas handling systems (Fruehan et al., 1989)

-Oxidation potential of the off-gas (Brotzmann, 1989)

-Volume of the off-gas, which may cause difficulties with the gas and dust handling (Hoffman, 1991)

596 Fluidization, Solids Handling, and Processing

Since a detailed discussion of the factors listed in Table 4 is beyond the scope of this review, the reader is referred to the listed references for further information. Several of the factors affecting the PCR also affect the slag layer and these are discussed in more detail in Sec. 2.4.

The heat transfer efficiency is significantly affected by the slag layer properties and behavior; therefore, those factors other than slag phenomena that affect HTE are presented in Table 5. As for the PCR, it is desirable to keep the HTE as high as possible. An increase in HTE at the same PCR decreases fuel consumption (Fruehan et al., 1989; Keogh et al., 1991) and the off-gas temperature (Fruehan et al., 1989; Takahashi et al., 1992).

Table 4. Factors That Affect the PCR

Three-Phase Fluidization Systems 597

Figure 3. Key factors to achieve efficient post-combustion in an iron-bath smelting reduction furnace. (From Takahashi et al., 1992.)

Table 5. Factors Other Than Slag Phenomena That Affect the HTE

598 Fluidization, Solids Handling, and Processing

Several authors have attempted theoretical modeling of post-com- bustion (Fruehan et al., 1989; Zhang and Oeters, 1991a, 1991b, 1993a, 1993b; Hardie et al., 1992; Gou et al., 1993; Gudenau et al., 1993). The models consider the convective and radiative nature of the heat transfer under steady-state conditions for various gas types including oxygen and air. The model equations consist of heat and mass balances of the various components in the furnace (i.e., iron and carbon dioxide) and, in some cases, momentum balances for the flow behavior. Gou et al. (1993) provided one of the more comprehensive models for heat transfer. The model, by incorporating the momentum balances, allowed not only for the calculation of the composition and temperatures but also the steady-state gaseous flow patterns. The model determined the location, shape, and temperature of the combustion flame front and coupled equations for the turbulent convective transfer to those for the radiative transfer. They use the DeMarco-Lockwood flux model to calculate the radiation transfer for a turbulent, high-temperature reacting flow system. In the DeMarcoLockwood flux model, the radiation fluxes for axisymmetric flow are expressed in the following form:

where Fz and Fr are the radiation heat fluxes in the z and r directions, respectively. The radiation heat fluxes are coupled to the convective heat transfer via a source term in the transport equation for the stagnation enthalpy of the form:

where SH is the source term from the enthalpy equation.

Three-Phase Fluidization Systems 599

Future requirements for the modeling of smelting furnaces will be for a detailed account of the heat transfer mechanisms as developed by Zhang and Oeters (1991a, 1991b, 1993a, 1993b) and a consideration of the fluid dynamics of the system such as that described above by Gou et al. (1993) while considering the effects of the raw materials to maintain model flexibility. The slag layer has tremendous effects on the overall behavior of the furnace, but the complex multiphase nature of the system is still in the early development stages of modeling. Another important factor that needs to be researched further and included in models is the reduction kinetics of iron oxide from liquid slags (Fine et al., 1989; Oeters, 1989; Oeters and Xie, 1995).

2.4Slag Layer Behavior

The slag layer is composed of molten slag, solid materials such as carbonaceous materials and undissolved ore, gas bubbles, and metal droplets. The operational characteristics, including post-combustion, heat efficiency, dust generation, and carbon content of the bath, of a SRF are highly influenced by the behavior of the slag layer (Katayama et al., 1993a). The slag added to a SRF consists primarily of lime (CaO) and silica (SiO2) with significant amounts of iron oxide (FeO) becoming present when the smelting reduction process commences. Small amounts of additives, such as P2O5, S, MgO, or CaF2, may also be present in the slag layer. The slag layer is present primarily to serve as a heat flux between the combustion space and the metal bath and to protect the metal bath from the oxidizing atmosphere above the slag. The two most researched topics concerning the slag layer behavior are the optimal height of the slag layer and the mechanisms and control of foaming of the slag layer. Smelting reduction processes are conducted with both a relatively thin slag layer, as in the HIsmelt process developed in Australia (Hardie et al., 1992), and with a thick slag layer, as in the DIOS process developed in Japan (Inatani, 1991).

The main reactions in smelting reduction processes are the gasification of coal and iron oxide reduction in the slag. These reactions produce large quantities of gaseous carbon monoxide and hydrogen on the slag/ iron or slag/carbon surface. The slag foam is created by the gas bubbles formed as a result of these reactions becoming entrained in the slag, and in not being able to readily coalesce, forming a system of tightly packed bubbles separated by one another by thin films of liquid slag (Gaskell,

600 Fluidization, Solids Handling, and Processing

1989). In studies conducted on the foaming behavior of metallurgical slags, it is generally agreed that the principle factors in controlling the foaming behavior are surface tension, viscosity, viscoelastic characteristics of the liquid films, basicity, solid particles, and bubble size. There is, however, considerable disagreement in the relative importance of each factor (Gaskell, 1989; Utigard and Zamalloa, 1993). Obviously, in a complicated three-phase system, it becomes extremely difficult to ascertain the individual effects of the stated factors, which is why many studies on the foaming behavior are conducted in non-reacting systems. The stabilization and intensification of foams are favored for low surface tension and high viscosity of the slag (Fine et al., 1989). Gaskell (1989) provided an example of how the presence of a surface active solute, in this case SiO2, in a nominally-basic slag led to stabilization of the foam. The surface active solute decreased the surface tension of the slag by preferential segregation of SiO2 to the slag surface and increased the viscosity as a result of the surface being more siliceous than the bulk melt (i.e., the viscosity of the surface is higher than that of the bulk melt). Aukrust and Dowling (1991) reported that a foaming slag was stable when the basicity was between 1.2 and 1.5. Good summaries of the more fundamental behavior of slag properties can be found in Gaskell (1989), Gudenau et al. (1992), Hara and Ogino (1992), Ogawa et al. (1993), Utigard and Zamalloa (1993), and Gou et al. (1994).

Other factors that are more engineering or design oriented are also found to affect the foaming behavior of the slag layer. Gudenau et al. (1992) reported that the top oxygen blowing parameters, such as lance penetration distance, gas velocity, and expansion angle of the gas stream, affected the foam generation and proper selection of these parameters can assist in controlling excess foaming. They also found that the foaming decreased with increased pre-reduction degree. Ibaraki et al. (1990) found that weak bath agitation and a low bath temperature promoted foaming. They also reported that bath agitation affected the iron droplet mixing in the slag and the slag circulation. An increase in bath agitation lifted more iron droplets into the slag resulting in lower PCR and increased the ferrous dust in the off-gas. They suggested a bath agitation of 2 to 4 kW/ton of metal. Ogawa et al. (1992) reported that an increase in iron ore feed rate promoted foaming.

Three-Phase Fluidization Systems 601

Foaming of the slag layer is inevitably going to occur during the smelting reduction process, therefore, ways of controlling the foaming to prevent overflow of the system have been studied. The most widely reported and possibly easiest way to control foaming is the addition of carbonaceous material to the slag. The DIOS smelting reduction furnace, under development in Japan, is chosen to demonstrate the control of the slag layer foaming because the process has been described in the literature in more detail than other processes and it operates with a thick layer of slag. The thick layer of slag is required to shield the stirred metal bath from the top blown oxygen jet. The thick slag layer consists of molten slag, carbonaceous materials, and gas bubbles. The carbonaceous material, consisting of char or coke, is suspended in the slag to control foaming. Ogawa et al. (1992) reported that the carbonaceous material promoted the coalescence of small bubbles into larger bubbles that are able to rise through the slag layer. With the use of x-ray fluoroscopic observation of the slag layer in a 1-ton furnace, Ogawa et al. (1992) concluded that the carbonaceous material controlled the slag foaming by increasing the coalescence of small bubbles on the surface of the carbonaceous material because the carbonaceous material was not easily wettable with slag and, therefore, the CO gas was likely to spread along its surface. The amount of carbonaceous material required for stable operation is between 10–20% of the slag’s weight (Ibaraki et al., 1990; Ogawa et al., 1992). Katayama et al. (1993a) measured the slag height and by assuming that all the carbonaceous materials were entrapped in the slag layer, calculated the volume proportions of the slag layer to be as follows: molten slag: 22–28 vol%, carbonaceous materials: 22–28 vol%, and gas bubbles: 44–55 vol%. Lump carbonaceous material is preferred over fine material, because the turbulence created by the larger particles lead to more frequent contact with the small bubbles, and the fine material is only sparsely scattered when charged into the furnace (Ogawa et al., 1992).

The slag layer plays important roles in heat transfer and the reduction of iron oxide. The role of the slag layer in heat transfer was described in Sec. 2.3. Katayama et al. (1992) proposed that the reduction of iron oxide occurred at the following three locations

(i)Interface between slag and metal bath

(ii)Interface between slag and metal droplets

(iii)Interface between slag and carbonaceous materials.

602 Fluidization, Solids Handling, and Processing

They estimated the overall rate of reduction to be equal to the rate of addition of ore at stable operation. By determining the overall rate as a function of the amount of slag in both 5-ton and 100-ton furnaces, they calculated the amount of reduction at each site. Katayama et al. (1992) found that for the 5-ton furnace operation, a high proportion (70%) of the reduction occurred at the bulk metal slag interface. The proportions of reduction at locations 2 and 3 were found to increase with increasing experimental scale and the amount of slag. For the 100-ton furnace containing a large amount of carbonaceous material, the amount of reduction at each site was almost equal (34% for site 1, 33% for sites 2 and 3).

Ibaraki et al. (1990) reported that the reduction reaction took place in the slag bulk as well as in the condensed iron zone around the static slag/ metal interface. They also reported that the reduction kinetics decreased as the oxygen jet penetrated the iron bath and concluded that the oxygen jet in the slag not only interfered with the reduction reaction, but also reduced the amount of post-combustion and increased the ferrous dust content in the off-gas. They recommended that the blown oxygen should not reach the condensed iron zone, located in the lower one-third of the slag. Katayama et al. (1993a) reported that the temperature of the lower 75% of the slag layer was almost equal to that of the metal bath, while the upper 25% could have large fluctuations in the temperature caused by the interaction with the gas phase in the upper portion of the furnace where temperatures may be 200–300°C higher than in the metal bath.

As mentioned previously, the modeling of the slag behavior is an area that requires further development. Ogawa et al. (1993) modeled the physical behavior of foaming slags by a mechanistic model that considered the effects of the physical properties of the slag and metal on the foam height. The model demonstrated that not only do the surface tension and viscosity of the slag affect the foam height, but the slag/metal interfacial tension and the surface tension of the metal affect the foam height as well. Gou et al. (1994) applied a one-dimensional, fluid mechanic model for gas and liquid two-phase flow to calculate the foam height and void fraction variation with gas velocity. The model requires an experimentally determined parameter of the ratio of the drag coefficient to the effective bubble diameter. The model was able to predict the slag level of those experiments that were used in determination of the above stated parameter. The model does not directly account for the raw materials.

Three-Phase Fluidization Systems 603

2.5Future of Smelting Reduction of Iron Ore

Table 6 provides areas of smelting reduction that require further research. The economics of the current smelting reduction processes under development need to be more accurately determined as the development stages become larger. One estimation of the economics of smelting reduction and a comparison between blast furnace, direct reduction, and smelting reduction was provided by Steffen (1989). As seen from Table 3, the development of smelting reduction processes is occurring worldwide. Ibaraki et al. (1995a, 1995b) summarized research performed in Japan between 1988 to 1991 using a 100 metric ton smelter, indicating the highest production rate achieved was 36.4 metric tons of iron/hour. Fine et al. (1989) estimated that based on research and development efforts in Japan, the USA, and Europe, the implementation of a smelting reduction process to produce steel is possible by the year 2000.

Table 6. Areas of Smelting Reduction Requiring Further Research (Fine et al., 1989; Oeters, 1989)

·Melting and reduction mechanisms of pre-reduced ore in iron oxide containing slag and in carbon containing iron

·Sticking behavior of ore fines

·Kinetics of high temperature oxidation of liquid iron

·Mechanism of carbon monoxide formation under the conditions of smelting reduction

·Flow processes in thoroughly mixed metal-slag-gas systems

·Mass and heat transfer during post-combustion in thoroughly mixed metal- slag-gas systems

·Reactions of FeO-bearing melts with refractory materials

·Optimization of reactor geometry in terms of charging and removal of materials

·Optimization of slag height and metal bath agitation

·Alternative heat sources such as electrical energy

·Use of increased amounts of scrap metal and lower grades of iron ore

·Removal of sulfur and other trace elements from the off-gas and liquid iron

604 Fluidization, Solids Handling, and Processing

3.0PAPER PROCESSING

3.1Introduction

The pulp and paper industry is characterized by many operations that are performed under multiphase flow conditions. Typical examples of the major processes that involve multiphase phenomena are the cooking of wood chips in a chemical solution to remove unwanted cross-linking components (lignin) leaving behind a fibrous wood pulp; bleaching of pulp in three-phase mixtures of pulp slurry and bleaching gas (chlorine, chlorine dioxide, oxygen, or ozone); and de-inking of recycled paper in flotation-type devices. Each of these processes and the nature of their three-phase behavior will be discussed in this section. The operational characteristics of these processes may differ somewhat from traditional three-phase fluidized beds, nevertheless, the processes still consist of three-phase mixtures that require intimate contacting of the phases. Since the pulp and paper mills of the future may only consist of improving on existing technologies (Rickard, 1994; Meadows, 1995), a further understanding of the three-phase nature of these processes will assist in attaining these necessary process improvements.

Three-phase mixtures in pulp and paper are unique because of the complex behavior of the solid phase. The solid phase (wood chips or paper pulp) in the pulp and paper industry is a low-density fibrous material (primary cellulose) of varying size, shape, and thickness. The nature of the solid phase is complicated because of the ability of the pulp fibers to absorb water and swell to several times the original volume of the oven dried (O. D.) pulp. This absorbed water becomes part of the fibers, and is not considered part of the suspending medium (Stenuf and Unbehend, 1986). This type of solid phase is in contrast to the high density, primary spherical solid phase seen previously in the smelting operation and the catalyst material used in chemical or hydrocarbon processing. While twophase pulp slurry pipe flow has been extensively studied (Stenuf and Unbehend, 1986), highlighted by the demonstration of the non-Newtonian and drag reduction characteristics, the three-phase flow behavior requires further study (Lindsay et al., 1995; Reese et al., 1996).