Phosphate matrix

Although an acid phosphate matrix cannot be excluded it is not essential for cement formation. In fact, it must be remembered that when these cements are prepared the oxide or silicate powder is normally in excess of that required for the reaction. Under these conditions most oxides (MgO... 

It is interesting that this cement has been known for over 100 years and yet certain features of its chemistry remain obscure. The exact nature of the matrix is still a matter for conjecture. It is known that the principal phase is amorphous, as a result of the presence of aluminium in the liquid. It is also known that after a lapse of time, crystallites sometimes form on the surface of the cement. A cement gel may be likened to a glass and this process of crystallization could be likened to the devitrification of a glass. Therefore, it is reasonable to suppose that the gel matrix is a zinc aluminophosphate and that entry of aluminium into the zinc phosphate matrix causes disorder and prevents crystallization. It is not so easy to accept the alternative explanation that there are two amorphous phases, one of aluminium phosphate and the other of zinc phosphate. This is because it is difficult to see how aluminium could act in this case to prevent zinc phosphate from crystallizing. 

All commercial examples of phosphoric add solutions used in these cements contain metal ions, whose role has been discussed in Section 6.1.2. In the case of the dental silicate cement, aluminium and zinc are the metals added to liquids of normal commerdal cements and have a significant effect on cement properties (Table 6.8) (Wilson, Kent Batchelor, 1968 Kent, Lewis Wilson, 1971a,b). Aluminium accelerates setting for it forms phosphate complexes and is the prindpal cation of the phosphatic matrix. Zinc retards setting for it serves to neutralize the addic liquid - it... 

The role of organic sulfur species (other than those in thiophosphate form) in the tribochemical process is to react immediately with the nascent iron surfaces and ZnO to form metal sulfides (FeS, ZnS) embedded in the short chain phosphate matrix. 

Figure 8.3. ID analysis of B1O3" in drinking water by ion chromatography with ICP-MS detection study of polyatomic ions produced at mass 79 and 81 from a 300 p,g g-1 sulfate matrix a) and a 100 p,g g-1 phosphate matrix (b). Std denotes the peak produced by a 100 p.L post-column standard injection of BrOj". Reprinted from [395] with permission Copyright 1999 American Chemical Society.

Development of superior CBPC products for the wide-ranging applications shown in Fig. 2.1 requires a fundamental understanding of their kinetics of formation and their properties. This topic is extensively addressed in Chapters 4-6. The dissolution model described in these chapters also helps in understanding the role of individual components in formation of ceramics and the end performance of the ceramics. In addition, the dissolution model explains how hazardous and radioactive components are stabilized in a phosphate matrix. The stabilization mechanisms are discussed in Chapters 16 and 17. 

Arsenates have very low solubility, but AS2O3 is sparsely soluble. Like HgO, its solubility in the entire acidic range is almost constant and relatively high. Because of its higher solubility, phosphate treatment should be able to stabilize this compound. However, because other arsenates are nearly insoluble, they can be encapsulated in the phosphate matrix and will not be available for leaching. 

Most waste streams contain more than one contaminant. Some may have contaminants such as Hg, whose sulfide has a higher pAQp than its phosphate, and Ba, whose sulfide is soluble, but phosphate is insoluble. Even in these cases, sulfide treatment followed by phosphate ceramic formation is very effective. The sulfide treatment will produce insoluble HgS that will be microencapsulated in the phosphate matrix, but Ba will be converted partially into soluble BaS, which subsequently will dissolve and will be converted to insoluble phosphate in the CBPC matrix. Thus, the dual treatment is very effective even when several contaminants with varying solubility of sulfides and phosphates are found in the same waste. 

As discussed in Chapter 16, chemical stabilization is a result of conversion of contaminants in a radioactive waste into their insoluble phosphate forms. This conversion is solely dependent on the dissolution kinetics of these components. In general, if these components are in a soluble or even in a sparsely soluble form, they will dissolve in the initially acidic CBPC slurry and react with the phosphate anions. The resultant product will be an insoluble phosphate that will not leach into the groundwater. On the other hand, if a certain radioactive component is not soluble in the acid slurry, it will not be soluble in more neutral groundwater, because the solubility of such components is lower in neutral than in acidic solutions. Such a component will be simply microencapsulated in the phosphate matrix of the CBPC. Thus, the solubility of hazardous and radioactive components is key to chemical immobilization. 

Again, as in the case of hazardous contaminants discussed in Chapter 16, the solubility of a radioactive contaminant plays a major role in its stabilization in a phosphate matrix. Therefore, one needs to understand the aqueous behavior of a radioactive contaminant prior to selecting the acid-base reaction that will form the CBPC used for fabricating the waste form matrix. In this respect, actinides, fission products, and salts have unique solubility behavior. This behavior is discussed below. 

As mentioned in Section 17.3.1, retention of quadrivalent actinide oxides within the phosphate matrix is not a major issue because these oxides are insoluble in water, and all that is needed is their microencapsulation by the phosphate components of the matrix. This was demonstrated in a number of studies on UO2 and PUO2 and their surrogate Ce02. If the actinides are found in a trace amount in the waste, their chemical form is not so important because the phosphate matrix immobilizes them very efiectively. For example, the wastewater in the case study given in Section 16.3.2.2 contained 32 pCi/ml of and 0.6 pCi/ml of The ANS 16.1 tests conducted on the waste forms with 18.6pCi/g loading of combined U in the waste form showed that the leaching index was 14.52. XCLP tests also showed that levels in the leachate were below the detection limit of 0.2 pCi/ml. This implies that microencapsulation of trace-level U is very efiective in the Ceramicrete matrix. 

This conversion of actinide oxides of lower oxidation states into fully oxidized forms has a great advantage. The actinides of lower oxidation states are pyrophoric. Once converted into their fuUy oxidized form and encapsulated in the phosphate matrix, they are not spontaneously combustible and, hence, are safe for transportation and storage. Thus, because of this oxidation, the phosphate matrix removes the pyrophoricity during the stabihzation process. 

The most widely used industrial process for the manufacture of hexagonal boron(III) nitride is the reaction of boron(III) oxide with ammonia at 800 to 1200°C in a calcium phosphate matrix ... 

O2 transferring O to the phosphate matrix. This slow step determines the... 

Silsbee et al. (1991) studied materials that had been prodneed by eombining ealeined gibbsite or an X-ray amorphous form of alumina with phosphorie acid and by subsequent heat treatment at temperatures between 60 and 300 °C. The stmctuie of the resultant material was characterized by the presence of residual unreacted material (gibbsite crystals) embedded in an amorphous aluminum phosphate matrix The amorphous fraction exhibited some heterogeneity on a very fine scale. Splitting tensile strengths between about 10 and 20 MPa were attained. 

In water and beverages strontium can be measured directly, but food and biological materials require a pretreatment with hydrochloric acid (3 M) and lanthanum chloride [91]. In urine strontium can be determined after dry-ashing and addition of lanthanum [91] or directly after 1 2 dilution with an acidic lanthanum chloride solution [92], The determination of strontium in bone requires special attention because the bone matrix contains high amounts of calcium and phosphate, which can easily interfere with the determination of strontium. Razmilic described a method to isolate strontium from the calcium phosphate matrix by ion exchange chromatography. The pretreated samples then can be analysed by both emission and absorption spectrophotometry measurements without chemical, ionization, or bulk interferences [93,94]. 

Some effort has also been focused on the determination of other isotope ratios, particularly St/ St with TOF-ICP-MS and LA-TOF-ICP-MS [104].These efforts have not, however, yielded precision and accuracy useful for the characterization of human tooth enamel for population movement studies. Part of the problem probably arises from interferences produced by the calcium phosphate matrix [106], but there also appears to be TOF detector-related fractionation akin to but more extreme than that observed for Pb/ Pb. 

Horstwood et al. [Ill] reported results of experiments to determine strontium isotopes in archaeological tooth enamel using LA-MC-ICP-MS and pointed out the need for careful attention to interferences from the calcium phosphate matrix. In another study, Richards et al. [112] detected evidence of Neanderthal mobility via LA-MC-ICP-MS characterization of strontium isotope ratios in tooth enamel. Enamel values from a Neanderthal molar recovered from a coastal limestone environment in Greece were found to be consistent with... 

Yaszemski et al. (1995) incorporated sodium chloride as a pore forming component into a composite formulation also including /3-tricalcium phosphate (yS-TCP) or a calcium phosphate matrix. The mixture was crosslinked with VP to form a biodegradable bone cement. The mechanical properties of the composite were studied over several varying factors in a fractional factorial design. These mixtures... 

The main task of ReLIBS is to use this analyzer for horizon control in open mining. Phosphate rock is usually found 15-50 ft beneath the ground in a mixture of phosphate pebbles, sand and clay known as phosphate matrix. The sandy layer... 

White iron should not he confused with steel. Certain grades of steels are used in slurry, dredging, and phosphate matrix pumps. They are cast at a lower hardness than white iron and hy being more ductile can withstand higher disk stresses. Impellers cast in steel can he used in slurry pumps up to a tip speed of 45 m/s (8858 ft/min). 

A particular type of pump is the phosphate-matrix-handling pump. It does resemble in many aspects a sort of dredge pump, but is built of materials to handle both corrosion and... 

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