Phosphate-silicates, condensed

The condensed compounds of this type so far investigated are mainly phosphate-silicate, sulfate-phosphate, vanadate-phosphate, and arsenate-phosphate. All the component 0x0 acids or the anions of these condensed compounds can form isopoly acids or their anions. As can be seen in silicate minerals, there are several structural types of isopoly silicates which are polymers of interconnected Si04 tetrahedra. For example, pyroxene has a chain structure 1 (13). In the structure of... 

From the experimental results, which will be described later in this chapter, it appears that the condensed compounds of phosphate-silicate, vanadate-phosphate, and arsenate-phosphate are hydrolyzed, when dissolved in water, by the scission of P-O-X linkages and, in some cases, X-O-X linkages. Monomer and polymer units of PO4 tetrahedra inserted between two atoms of X are released into the solution when the system is dissolved in water. Consequently, orthophosphate and/or chain phosphates are produced. As for condensed compounds of sulfate-phosphate, it has been found that copolymers of sulfate-phosphate persist in aqueous solutions. 

In the industrial production of phosphates as fertilizer or feed, silicates in raw materials remain as an impurity in the products. Therefore, it is important to know the role of the silicates in the process of condensation of calcium phosphates and the properties of the resulting phosphate-silicates. It is from this point of view that du Plessis (36) has investigated the Ca0-P205-Si02 system. 

Three Groups of Condensed Phosphates Derived from the Glasses of Calcium Phosphate-Silicates with a Ca0/P205 Ratio of 1 (36)... 

Average Chain Lengths of Polyphosphates Derived from Condensed Phosphate-Silicates (37) ... 

For structures 9, 10, 11, or 12, / = 4, 3, 2, or 1 and -2 = 0, 1, 2, or 3, respectively. Average chain lengths of polyphosphates derived from the condensed phosphate-silicates with various P/Si ratios, were calculated by equation 2 or 3. As shown in column I of Table XI, even the calculated values of the average chain length based on structure 9 are considerably larger than the experimental values, when z > 50. 

Other factors that shorten the average chain length of the polyphosphates produced from the condensed phosphate-silicates are the existence of branching points in phosphate chains and the presence of water of composition and other impurities. If all these factors are represented... 

The fate of silicic acid is of some interest. Silicic acid polymerizes, by condensation, and finally a silica gel is formed (Wilson Mesley, 1968). The insolubilization of silicic acid has been observed to parallel closely the precipitation of phosphate (Wilson Batchelor, 1967b) and is related to an increase of pH within the cement (Kent Wilson, 1969). A low concentration of silicic acid must remain in the matrix. All this is in accord with the known aqueous chemistry of silica. 

When magnesium sulphate was omitted from distilled water samples of phosphorus compounds, recovery was variable. Table 12.11 shows yields of a series of standards with and without the magnesium sulphate addition and with and without the final hydrolysis. The magnesium sulphate is used as an acidic solution (after addition to the seawater sample, the pH was about 3) to minimize silicate leaching from the glassware during evaporation. The acid and heating are necessary to hydrolyse any condensed phosphates in the final mixture. 

It was foimd that, in the sequence sulfates, phosphates, borates, silicates, aluminates, and simple oxides the probability of observing normal df emission decreases and the probabiUty of observing anomalous emission or no emission at all increases. Normal df emission is only observed in the sulfates MSO4 (M=Ca, Sr, Ba) and in the condensed borate SrB407. Anomalous emission... 

The first proposals for the employment of electrical heating in the production of phosphorus were made by Headman, Parker and Robinson.5 The simultaneous production of an alkali silicate by heating alkali phosphate, silica and carbon in a regenerative furnace was patented by Folie-Desjardins.6 In the Readman-Parker-Robinson process, as worked later, the phosphate, carbon and fluxes, previously heated to a high temperature, are introduced into the upper part of an electric furnace made of iron lined with refractory bricks and fitted with condensing pipes in its upper part. The gases pass through a series of copper condensers, the first of which contains hot water, the others cold water (or see p. 9). It has been found advisable to replace... 

Elementary phosphorus is now made by heating calcium phosphate with silica and carbon in an electric furnace. (Fig. 21-1). The silica forms calcium silicate, displacing phosphorus pentoxide, P4O1P, whkh is then reduced by the carbon. The phosphorus leaves the furnace as vapor, and is condensed under water to white phosphorus. 

Anions like silicate, phosphate, arsenate, etc. form heteropolytungstates. For information about their structure see Section 4.2.7.3.S. The heteropolytungstates are always higher condensed species in comparison to the isopolytungstates existing in solution under... 

Thilo (25) classified oxo acids of various elements into three groups on the basis of the stability of the oxo acid in aqueous solutions, as shown in Table I. Although Thilo placed the elements in order of their ionic radii, covalent radii of the elements are used in Table I. The most stable phosphate in aqueous solutions is the monomer, orthophosphate. However, isopoly acid anions of phosphorus, i.e., condensed phosphates, are generally stable in an approximately neutral aqueous solution at room temperature. The rate of hydrolysis of the condensed phosphates is very low under these conditions. On the other hand, polysulfates and polyarsenates are very rapidly hydrolyzed into their monomers in aqueous solutions. In an alkaline solution, vanadate anions are present as monomer, i.e., orthovanadate. When the pH of the solution is de- creased, the orthovanadate anions are successively polymerized to form polymers with medium degrees of polymerization. Although silicate anions behave similarly, a highly polymerized form of silicate anions,... 

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