Hydrolysis precipitation processes

Equations (141) and (142) describe the equilibrium between the hydrolysis of complex fluoride acids (shift to the right) and the fluorination of hydroxides (shift to the left). Near complete precipitation of hydroxides can be achieved by applying an excessive amount of ammonia. Typically, precipitation is performed by adding ammonia solution up to pH = 8-9. However, the precipitate that separates from the mother solution can be contaminated with as much as 20% wt. fluorine [490]. Analysis of niobium hydroxides obtained under different precipitation conditions showed that the most important parameter affecting the fluorine content of the resultant hydroxide is the amount of ammonia added [490]. Sheka et al. [491] found that increasing the pH to 9.6 toward the end of the precipitation process leads to a significant reduction in fluorine content of the niobium hydroxide. 

The most important synthetic routes to iron oxide pigments involve either thermal decomposition or aqueous precipitation processes. A method of major importance for the manufacture of a-Fe203, for example, involves the thermal decomposition in air of FeS04-7H20 (copperas) at temperatures between 500 °C and 750 °C. The principal method of manufacture of the yellow a-FeO(OH) involves the oxidative hydrolysis of Fe(n) solutions, for example in the process represented by reaction (1). 

With slight variations, the Stober silica precipitation process proceeds from the same chemicals. The starting material is TEOS, tetraethoxysilane, Si(OC2H5)4 the solvent is an alcohol (preferably ethanol) water is added and ammonia acts as the catalyst to initialiate the hydrolysis and condensation reaction. In a very schematic way the reaction could be described as follows ... 

Reactive crystallization or precipitation processes of industrial relevance include liquid-phase oxidation of pura-xylene to terephthalic acid, acidic hydrolysis of sodium salicylate to salicylic acid, and the absorption of ammonia in aqueous sulfuric acid to form ammonium sulfate. " A special type of reactive crystallization is the diastereomeric crystallization, widely applied in the pharmaceutical industry for the resolution of the enantiomers. Here, the racemate is reacted with a specific optically active material (resolving agent), to produce two diastereomeric derivatives (usually salts), which are separated by crystallization. Diastereomeric crystallization is commonly used in... 

Today, hydrometaUurgy is well established as the principal method for extraction of many important industrml metals. Hydrometallurgy for the direct treatment of base metal sulfide concentrates, as a widely used technology, must yet prove itself. The roast-leach electrowinning of zinc is a noteworthy exception and is evolving as standard practice hi the zinc industiy worldwide. Relatively recent developments by way of jarosite and iron oxide hydrolysis and precipitation processes have improved recovery and helped secure zinc hydromemllurgy as standard in the industiy.w... 

In this paper we have tried to present the chemical and mechanistic aspects of chemical bath deposition of chalcogenide compounds as they appear both in the recent literature and also in older studies dealing with hydrolysis of chalcogenide precursors. A better account of these aspects gives clues to understanding the properties of the films such as the dependence of composition on solution composition and competitive precipitation processes, and the dependence of structure on competition between atom-by-atom and colloidal growth deposition mechanisms. 

A luetic simulation of the microbial-facilitated calcite precipitation was conducted using geochemical conditions consistent with the SRPA, realistic concentrations of urea, and rates of microbial urea hydrolysis that are consistent with those measured on conqilex microbial communities. In the simulation, urea hydrolysis and then calcite precipitation proceeded at maximal rates. We expect that in a real field test of our process in which an electron donor (e.g., molasses) is added to the system to stimulate urea hydrolysis, the process would proceed even more rapidly. The modeling suggested that over tiie long periods that die process would occur in an aquifer, the prec itation rate would be controlled by the urea hydrolysis rates and would be independent of tiie concentration of calcium in the aquifer. 

They therefore took advantage of the iron hydrolysis - precipitation and acid regeneration process that occurs con-currently with leaching at elevated temperatures in pressure acid leaching (PAL). It was noted that PAL residues contained significant quantities of sulfate, consistent with precipitation of hydronium alunite, H30.Al3(S04)2(0H)6. 

Pseudospherical a-Fe203 nanoparticles obtained by the forced hydrolysis of 0.1 M Fe(CI04)3 solution for 24 hat 160°C(a) the addition of sodium polyanethol sulfonate at the beginning of the precipitation process caused the formation of hollow a- F6203 particles in the micometer range... 

The kinetics of the two competing reactions are controlled by the precursors used M and R) and the reaction conditions. For example, rapid addition of excess water to an alcoholic alkoxide solution generally leads to complete hydrolysis with little condensation. Because a hydroxide or hydrous oxide precipitate usually forms, this process resembles the sparingly soluble salt case. Polymeric species form when conditions are adjusted such that condensation occurs. It is the type and distribution of species in solution and how they react with water (hydrolysis) or among themselves (condensation) that determine the form and structure of the particulate or gel product. The ability to form large polymeric species is one of the primary differences between the sol-gel and salt precipitation processes. 

The hydrolysis of PVA-QA was measured in the presence of four metals, Co(II), Zn(II), Cu(II), and Ni(II). The first order rate constants (kobs) observed in the presence of a 5 1 or greater excess of each metal are listed in Table I. The rate of hydrolysis at pH 7.5 in the absence of metals was not measurable. The kinetics of hydrolysis in the presence of a 4 1 ratio of Cu(II) was biphasic, consisting of two simultaneous first order processes. The contribution of the faster component diminished as the Cu(II) PVA-QA ratio was decreased to 1 1. A single first order process was observed in the presence of Ni(II), Co(II), and Zn(II). A double reciprocal plot of k obs vs. [M]- for Ni(II) exhibited the expected linearity. Utilizing Equation 6, the value of k3 derived from the intercept of this plot was 0.013 min". Similar but less reproducible results were obtained for Co(II)-promoted hydrolysis. Precipitation of Zn(0H)2 prevented the use of this metal above a concentration of 1.7 X 10 M, and a maximum zinc(II) chelator ratio of 7 1. 

Clearly, Is much larger in the presence than in the absence of silica. From the values for experiments U30 and U31, it turns out that the size of the silica particles does not affect the overall kinetics of the precipitation process. The difference with respect to kj. between experiments U30 and U34 is caused by the higher rate of urea hydrolysis in experiment U30, which itself is brought about by the slightly higher temperature in that experiment. The four-fold difference in initial Mn concentration (experiment U30 versus U34) giving rise to comparable values for kj. - especially when the temperature difference is taken into account - further supports the overall precipitation being adequately described as a first-order process. 

Although the rate of hydrolysis of urea is larger than that of the removal of Mn, reaction (1) is the rate-determining step for the overall precipitation process. The selectivity of the utilization of the generated ammonium hydroxide is determined by equilibria (2) and (3). It is expected that hi er temperatures and/or a higher pH will favour (2) over (3). This Indeed explains that for experiment U33 the rate constant is lower than in the other cases, seeing that, as is shown in Fig. 2, the pH at which precipitation takes place is much higher. 

N-Benzylamides are recommended when the corresponding acid is liquid and/or water-soluble so that it cannot itself serve as a derivative. Phe benzylamides derived from the simple fatty acids or their esters are not altogether satisfactory (see Table below) those derived from most hydroxy-acids and from poly basic acids or their esters are formed in good yield and are easily purified. The esters of aromatic acids yield satisfactory derivatives but the method must compete with the equally simple process of hydrolysis and precipitation of the free acid, an obvious derivative when the acid is a solid. The procedure fails with esters of keto, sul phonic, inorganic and some halogenated aliphatic esters. 

The sweet water from continuous and batch autoclave processes for splitting fats contains tittle or no mineral acids and salts and requires very tittle in the way of purification, as compared to spent lye from kettle soapmaking (9). The sweet water should be processed promptly after splitting to avoid degradation and loss of glycerol by fermentation. Any fatty acids that rise to the top of the sweet water are skimmed. A small amount of alkali is added to precipitate the dissolved fatty acids and neutralize the liquor. The alkaline liquor is then filtered and evaporated to an 88% cmde glycerol. Sweet water from modem noncatalytic, continuous hydrolysis may be evaporated to ca 88% without chemical treatment. 

Separation Processes. The product of ore digestion contains the rare earths in the same ratio as that in which they were originally present in the ore, with few exceptions, because of the similarity in chemical properties. The various processes for separating individual rare earth from naturally occurring rare-earth mixtures essentially utilize small differences in acidity resulting from the decrease in ionic radius from lanthanum to lutetium. The acidity differences influence the solubiUties of salts, the hydrolysis of cations, and the formation of complex species so as to allow separation by fractional crystallization, fractional precipitation, ion exchange, and solvent extraction. In addition, the existence of tetravalent and divalent species for cerium and europium, respectively, is useful because the chemical behavior of these ions is markedly different from that of the trivalent species. 

Figure 17 summarizes the avadable sol—gel processes (56). The process on the right of the figure involves the hydrolysis of metal alkoxides in a water—alcohol solution. The hydrolyzed alkoxides are polymerized to form a chemical gel, which is dried and heat treated to form a rigid oxide network held together by chemical bonds. This process is difficult to carry out, because the hydrolysis and polymerization must be carefully controlled. If the hydrolysis reaction proceeds too far, precipitation of hydrous metal oxides from the solution starts to occur, causing agglomerations of particulates in the sol. 

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