CBPC processes

In addition to the utility plant fly ash, one may also use volcanic fly ash, ash produced from burning municipal solid waste or any other combustion product that contains ash. The role of ash is also important in management of hazardous and radioactive waste because often such waste, if combustible, is incinerated to reduce its volume. The incinerated ash now is richer in inorganic hazardous components and needs to be stabilized. CBPC processes are ideal for stabilizing such ash because, phosphates are ideal materials to stabilize hazardous and radioactive contaminants, but as mentioned before, ash improves the physical and mechanical properties of the end products. Stabilization of such ashes is discussed in Chapters 16 and 17. 

When these acid phosphates are used as anion donors in the formation of CBPC, phosphates with lower solubility are formed. For example, H3PO4 readily dissolves in water, and the acid-base reaction in the CBPC process is too rapid. The resultant products are precipitates of acid phosphates that are soluble in water. On the other hand, for the acid phosphates formed by partial neutralization of H3PO4 that are listed in Table 4.1, the process is slightly slower, and hence one has better control over formation of a coordinated network of phosphate reaction products that are less soluble. For this reason, neutralization of phosphoric acid and formation of the acid phosphate was the common route followed by earlier workers as discussed in Chapter 2. 

The right conditions to form a CBPC are governed by the rate of reactions that control each of these three steps. Since acid phosphates selected for use in the CBPC process are soluble, their dissolution rate is comparatively high and, hence, uncontrollable. The phosphate reaction between dissolved cations and anions described in step 3 is also inherently fast and, again, cannot be controlled. Thus, the only reaction that can be controlled is the dissolution of oxides given in step 2. By selecting suitable oxides with appropriate reaction rates for forming CBPCs, one may allow sufficient time to mix the components in water and pour the slurry in molds, or spray the slurry, or apply it in any other suitable manner to form a ceramic. On the other hand, an oxide that dissolves fast... 

Reactions 5.5 and 5.6 represent the basic dissolution of the oxide or the hydroxide of a metal of valency x, and they form the basis for discussion of the CBPC processes. When n = 0 in reactions 5.5 and 5.6, we regain reaction 5.3 and 5.4, respectively these occur in highly acidic conditions. As the pH of the medium increases, reactions with increasing n occur. For example, for slightly higher pH, we put n = and have... 

Because the CBPC process is based on slow dissolution of the components, spontaneous dissolution of oxides is not desirable in the ceramic formation. This implies thatEq. 6.18 is a requirement for a dissolution reaction that is useful in forming a ceramic. Consider, for example, dissolution of MgO in a neutral medium given by Eq. 5.9c... 

As noted in Chapters 9, 11-13, CBPCs made with the room-temperature binders of Mg, Fe, Zn, and Ca exhibit physical properties similar to those of conventional cement, but suitable extenders can enhance them at very high loading. This improvement is not possible in conventional cement. Figure 14.1 illustrates the main difference between CBPCs and conventional cement. This difference is partly because the CBPC process is based on an acid-base reaction, while conventional cement is formed only in an alkaline region. Therefore, one can add acidic, neutral, or alkaline components in CBPCs at a high load factor. Cement, on the other hand, can accept only neutral and alkaline streams, even those at a modest load factor, as discussed below. 

The most common metal swarfs are iron-based [9,10] and produced by the machine tool and automobile industries. The resulting fine Fe particles oxidize in storage and form magnetite and hematite. Because they also contain flammable machine oils, this oxidation makes them pyrophoric and hence a liability. Because the particle surfaces are coated with oil, they cannot be incorporated in conventional cement. As demonstrated by Wagh and Jeong [3], the acid phosphate in the CBPC process acts like a detergent and exposes the surface of these particles to the acid-base reaction and binds them. 

The first deployment of CBPC for stabilization of Hg using Ceramicrete was reported by Singh et al. [58]. These authors used the CBPC process to stabilize crushed Hg light bulbs that were radioactively contaminated. Visual inspection of the waste revealed that 90 vol% of the waste was <60 mm in size. Typical size of the crushed glass ranged from 2 to 3 cm long by 1-2 cm wide down to fine particulates. Chemical analysis indicated... 

Unlike the case for hazardous waste streams using phosphates, the literature on stabilization of radioactive waste streams using CBPC processes is mainly limited to work... 

The survey, presented above, however, does not present the full picture of the recent research in the CBPC area. The Abstracts have not covered many modem CBPC applications such as those in radioactive and hazardous waste management. The purpose behind writing this monograph is to cover such areas in which CBPCs have made major inroads. In the process, we have built a discussion on the foundation of basic science and technology behind formation of these materials. We, therefore, hope that this monograph will be a comprehensive source for a wide readership interested in the science of CBPC materials and their applications. 

The various CBPC products discussed in the last chapter reveal that CBPC powder consists of one or more sparsely soluble oxides and an acid phosphate. When this mixture is stirred in water, the acid phosphate dissolves first and makes the solution acidic, in which the sparsely soluble alkaline oxides dissolve and an acid-base reaction is initiated. This reaction produces slurry that subsequently hardens and a ceramic hard product is formed. If the acid phosphate is phosphoric acid solution, the setting reaction is too rapid. Such a process becomes impractical for production of large ceramic objects because the rapid acid-base reaction is exothermic and that boils the reaction slurry. Therefore, less acidic acid phosphates (such as chhydrogen phosphates) are preferred for fabrication of practical ceramics. 

Aluminum is the second most abundant metal on earth s crust. It is a common metal in tropical soils called laterites (red soils). It is extracted from bauxite that is a rich laterite by Bayer process that involves dissolution and separation of the oxide in caustic soda solution between 150 and 250°C and 20 atm of pressure. Though abundant and inexpensive, alumina based CBPCs are difficult to form because even in an acid solution the solubility of alumina is very low. This solubility, however, can be enhanced by a mUd thermal treatment and suitable CBPCs can be formed. Alumina is available commercially as calcined alumina called corundum, or as its hydrated forms such as aluminum hydroxide (Al(OH)3), as bohmite, (A1203-3H20), gibbsite (AI2O3 H2O) or in impure forms as in kaolin clay. These mineral forms and their use in ceramic formation are discussed in Chapter 11. 

Wagh and Jeong [4] have reported that, once the metal ions are dissociated and screened in an acid solution that is rich with phosphate anions, the kinetics of transformation to a CBPC is very similar to that of the conventional sol-gel process of fabricating ceramics of nonsilicates [4] with the major difference here being that the acid-base reaction used in forming CBPCs carries the mixture all the way to the formation of ceramics, while in the sol-gel process the sols are ultimately sintered to form superior ceramics. Figure 5.1 illustrates the step-by-step kinetics of the formation of CBPCs. 

In most CBPC fabrication processes, the pressure is a constant, but the temperature of the system changes due to evolution or absorption of heat. In such cases, following Eq. 6.9, we obtain... 

This equation can be used to write the temperature dependence of the solubility product constant pA sp- Thus, AHq, ASq, and the specific heats from the data books can be used to calculate the thermodynamic parameters as well as the solubility product constant. Consequently, by manipulating the processing temperature of a CBPC, one can solubilize the starter oxides. 

The curves show that, before calcination of the powder, the pH rise is very steep but tapers off at pH 10. In contrast, the pH of the calcined powder increases very slowly at a constant rate. This constant rate of increase in the pH helps to produce Mg-based phosphate ceramics in large sizes and makes the process practical. Most commercially available MgO exhibits very high dissolution rates in acid solutions, and its calcination becomes a necessity for production of ceramics at a commercial scale. The titration test provides a good method for testing these powders for their suitability for CBPC formation. 

CBPC matrix composites can incorporate a high volume of industrial waste streams such as fly ash, mineral waste such as iron taUings and Bayer process residue from the aluminum industry (red mud), machining swarfs from the automobile industry, and forest product waste such as saw dust and wood chips. Table 14.1 lists some of these waste streams and potential products or applications. 

This incremental cost increase may be justified when stabilizing mixed waste streams or waste streams containing As, Cr, and Hg, because simple acid washing will not stabilize these waste streams. To stabilize Hg, in addition to the CBPC formation, a sulfide pretreatment is used [55]. The pretreatment converts the contaminants into their most insoluble sulfide forms, then the CBPC formation produces a waste form that is far superior to any other treatment. This dual treatment has the advantage of being performed at room temperature in a one-step mixing process. 

CBPCs may be placed as a paste or can be injected at the right place in a human body. It will harden rapidly after its delivery. It will attach itself to the adjacent surfaces and form a firm bond. The process is less intrusive as compared to hardened ceramics that need to be implanted surgically. 

Mineralization in body fluids. In vitro (outside the body) CBPC formation may not be the same as in vivo (inside the body). The body fluids will affect the mineralization process. Blood may wash away or dissolve the CBPC minerals prior to their setting. The components of blood may also become incorporated within the CBPC, modify its composition, and change the physical properties. 

The advancements in dental and biomaterials presented here demonstrate that CBPCs have distinct advantages over sintered ceramics. By means of room temperature processing, it has been possible to develop CBPCs that set under biological conditions, mimic bones with carbonate apatite, and are compatible with actual bones. Thus, chemical bonding to produce biomaterials may be a natural way to go. 

CBPCs may have an important role even in the production of artificial implants. Typically, one may exploit rapid-prototyping to produce exact shapes of the implants. From a practical standpoint, formation of a ceramic out of a paste would appear to be most suitable for rapid-prototyping processes [11]. Thus, coupling CBPC with rapidprototyping should lead to artificial body parts that not only match the namral bones in their composition, but in structure as well. The science of CBPCs paves the way for their use not only as dental cements and bioceramics for the 21st century, but as discussed in earlier chapters, many other applications as well. 

To be more specific. Chapter 2 provides an overview of Chemically Bonded Phosphate Ceramics. It is intended to streamline the earlier literature and present it in a suitable context. Since the many potential applications of CBPCs are likely to alfect the raw materials (such as phosphates) market, an overview of the raw materials, their general properties, and their manufacturing processes is given in the third chapter. Chapters 4-7 are devoted to the theoretical basis for formation of phosphate ceramics by chemical reactions, and much of the discussion in these chapters is based on thermodynamics. 

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