Neutral refractories

Neutral refractories do not react with either acidic or basic slags. Hence, they are useful in acidic and basic conditions. Examples are carbon, chromite (Fe0.Cr203), and fosterite (2Mg0.Si02). 

Certain refractories are grouped imder special refractories. Examples are zir-conia, thoria, and beryllia. They possess special properties that make them useful in special applications. For example, thoria is a nuclear fuel that can sustain radiation, damage, and high temperatures. 

Bricks are commonly produced by refractories and come in standard and nonstandard shapes. Nonstandard shapes are costlier. Standard shapes include rectangular prisms, tapered bricks, and tubular sleeves. Some refractory materials are supplied in granular form. These are called pea-sized refractories and are used in furnaces. To make the top layer of a furnace hearth, these pea-sized refractories are thrown onto the hearth while hot. They also are mixed with hot tar as a binder and are used as a lining for furnace hearths. Running repairs are made from castables. These are plastic preparations that can be made in any shape they are dried and fired in situ. 

Another class of refractories is called insulating refractories. These are designed to have very low thermal conductivity. This is achieved mostly by incorporating a high proportion of air into the structure. Bricks made in this way are called porous bricks. Another example of an insulating refractory is the mineral wool. This is not self-supporting. Hence, it should be contained for use. Asbestos is a natural insulator but is not useful as a refractory. 

The widely used representatives from all these five classes will be discussed in Section III. 

---

Neutral refractory materials include graphite, charcoal, coke, chromite and various carbides. 

Refractory Linings. The refractory linings (2,3) for the hearth and lower wads of furnaces designed for melting ferrous materials may be acidic, basic, or neutral (see Refractories). Sdica has been widely used in the past, and is stid being used in a number of iron and steel foundries. Alumina, a neutral refractory, is normally used for furnace roofs and in the wads for iron foundries, but basic brick can also be used in roofs (4). 

Acid refractories Basic refractories Neutral refractories Rarer... 

In technological practice, refractories are usually classified according to content of SLO2 and divalent oxides, as acid, neutral or basic types. The acid types comprise silica refractories and siliceous fireclay neutral refractories are alumina, mullite and chromite refractories magnesite, chrome-magnesite and dolomite refractories are... 

Neutral refractories (e.g. bauxite, lime. These are used on areas where slags and atmosphere are basic they are stable to alkaline Artificially used refractories Silicon carbide, zirconium carbide etc. 

Neutral Refractory. A refractory material such as chrome ore that is chemically neutral at high temperatures and so does not react with either silica or basic refractories. 

Many refractories are derived directly from natural minerals, but synthetic materials are also widely use. Clays, espeeially those having inherent temperature resistance, are the oldest and most eommon of the naturally occurring refractory minerals. Major natural refi actoiy materials are kaolin, chromite, bauxite, zirconia, and magnesite. These are often marketed under specific trade names. Refractory materials may be acid, such as silica, or basic, such as magnesite or bauxite, for use in aeid- or basic-process steel furnaces. Graphite and chromite are generally considered neutral refractories. 

Refractories can be classified in two ways. One way is based on chemical composition. The various refractories are silica (Si02), alumina (AI2O3), magnesia (MgO), chromia (Cr203), alumino-silicate, and magnesia-chromia. In the second method, the refractories are classified as acidic, basic, and neutral refractories. This classification is based on the behavior of refractories toward slags. Following is a brief discussion of each of these classes. 

There are a number of interferences that can occur in atomic absorption and other flame spectroscopic methods. Anything that decreases the number of neutral atoms in the flame will decrease the absorption signal. Chemical interference is the most commonly encountered example of depression of the absorption signal. Here, the element of interest reacts with an anion in solution or with a gas in the flame to produce a stable compound in the flame. For example, calcium, in the presence of phosphate, will form the stable pyrophosphate molecule. Refractory elements will combine with 0 or OH radicals in the flame to produce stable monoxides and hydroxides. Fortunately, most of these chemical interferences can be avoided by adding an appropriate reagent or by using a hotter flame. The phosphate interferences, for example, can be eliminated by adding 1 % strontium chloride or lanthanum chloride to the solution. The strontium or lanthanum preferentially combines with the phosphate to prevent its reaction with the calcium. Or, EDTA can be added to complex the calcium and prevent its combination with the phosphate. 

The common industrial refractories are classified into acid, SiO and Z1O2 basic, CaO and MgO and neutral, AI2O3 and G Ch. Oxides within... 

Bevacizumab, a humanized IgG and cetuximab, a chimeric IgGx, are currently marketed in the US for treatment of metastatic colorectal cancer [92, 93]. Bevacizumab neutralizes the biological activity of vascular endothelial growth factor (VEGF), while cetuximab binds specifically to the extracellular domain of the human epidermal growth factor receptor (EGFR). Bevacizumab, in combination with IV 5-fluorouracil (5-FU) -based chemotherapy, is indicated for first-line treatment of metastatic colorectal cancer, whereas cetuximab is used in patients refractory to or intolerant to irinotecan-based chemotherapy. The clinical pharmacokinetics of cetuximab are discussed in detail in Chapter 14. 

In concluding this chapter, we point out that there are far more research opportunities than hard answers in this field of ion emitters. This field is dominated by systems in which the element from which ions are emitted is embedded in a matrix that enhances ion emission. Indeed, with the exception of the small number of emitters in which ions are emitted from pure refractory metals at the temperature limits of the material, pure materials predominantly volatilize neutral atoms and/or molecules when heated to temperatures sufficiently high to force volatilization to the gas phase. Thus, the key to the development of superior ion emitters seems to be to develop better understanding of the processes that cause the matrices to force the element to volatilize as ions rather than neutrals. With this better understanding perhaps new and better ion emitters can be developed. 

Sometimes one has to live with this problem, and some of the finest work, particularly on transient molecular species, has been achieved using source modulation. This includes some of the earliest work on neutral free radicals, by Kewley, Sastry, Winnewisser and Gordy [5], who studied the SO and CS species. With the benefit of hindsight, we know these radicals to be very long-lived. We shall illustrate the details of source modulation by describing two other experiments. The first is the classic work of Woods [6] and Dixon and Woods [7] who obtained the first microwave spectrum of a molecular ion, namely CO+. The second is an example of a high-temperature microwave cell for the study of refractory materials. 

---