Precipitation experiments

Guichardon etal. (1994) studied the energy dissipation in liquid-solid suspensions and did not observe any effect of the particles on micromixing for solids concentrations up to 5 per cent. Precipitation experiments in research are often carried out at solids concentrations in the range from 0.1 to 5 per cent. Therefore, the stirred tank can then be modelled as a single-phase isothermal system, i.e. only the hydrodynamics of the reactor are simulated. At higher slurry densities, however, the interaction of the solids with the flow must be taken into account. 

Figure 8. Regression of batch oxidation and precipitation experiment of Bullen et al. (2001), where Fe(II),q was oxidized, followed by precipitation of ferrihydrite, over a 24 h period. The reaction progress (F) is well-fit by a first-order rate law, where the rate constant is 0.0827 F/h, with an of 0.96. In the model illustrated by Equations (5)-(8) in the text, this rate constant would be set to k,.

Figure 10. Fe isotope compositions for total aqueous Fe (Fe,(,T) and ferrihydrite (FH) precipitate and aqueous Fe-ferrihydrite fractionations from the batch oxidation and precipitation experiment of Bullen et al. (2001). (A) Measured S Fe values from Bullen et al. (2001), compared to simple Rayleigh fractionation (short-dashed lines, noted with R ) using 10 1naFe.,-FH = 0.9%o, as well as the two-step re-equilibration model discussed in the text (i.e., Eqn. 12), shown in solid gray lines for the aqueous Fe and ferrihydrite components the predicted 5 Fe value for Fe(III), is shown in the heavy dashed line, which reflects continual isotopic equilibrium between Fe(II), and Fe(III),(. Note that in the experiment of Bullen et al. (2001), aqueous Fe existed almost entirely as Fe(II),(,. (B) Measured fractionation between total aqueous Fe and ferrihydrite precipitate, as measured, and as predicted from simple Rayleigh fractionation (black dashed line) and the two-step model where isotopic equilibrium is maintained between aqueous Fe(II),q and Fe(III),q (solid gray line).

Liquefaction to produce filtered extract solution was carried out in a 2-1 autoclave under the conditions described previously and elsewhere (9). Solvent precipitation experiments using the filtered extract solution were carried out in a 0.2-1 autoclave. The extract and solvent were mixed together cold, in the autoclave, and heated to the extraction temperature in a fluidised sand bath. The mixture was agitated at the required temperature for 20 minutes and then filtered through a pressure filter containing a nomex cloth, at the extraction temperature, using the pressure in the autoclave or an iq lied nitrogen pressure. In some eiq)eriments, the solvent was injected, under pressure, into extract solution already at the required temperature. 

Sanyal A, Nugent M, Reeder RJ, Bijma J (2000) Seawater pH control on the boron isotopic composition of calcite evidence from inorganic caldte precipitation experiments. Geochim Cosmochim Acta 64 1551-1555... 

Figure 3.2. Schematic diagram of apparatus used in precipitation experiments of Phelan and Mattigod (1987). (Reprinted with permission of the publisher.)...

Precipitation experiments Mucci and Morse (1984) reported solubility data for several magnesian calcites produced from precipitation experiments (see Figures 3.7 and 3.14). In their experiments overgrowths of magnesian calcite were precipitated on calcite seeds in a pH-stat which maintained constant solution composition. The overgrowth compositions were determined by atomic absorption spectrophotometry and X-ray diffraction. The Mg contents of the overgrowths varied only with the Mg Ca ratio in solution, and the overgrowths were shown to be... 

Figure 3.14. Stabilities of calcite, and synthetic (closed squares) and biogenic (closed circles) magnesian calcites as a function of composition. Stabilities are expressed as -log IAPMg-Calcite- The curve is a hand-drawn "best" fit to the synthetic data. Also plotted are the results of precipitation experiments by Mucci and Morse (1984, open squares) and biogenic dissolution experiments by Walter and Morse (1984a, open circles). (After Bischoff et al., 1987.)...

In discussions of the precipitation of calciinn phosphate, the phase which is usually emphasized is the thermodynamically most stable, HAP. However, most calcium phosphate solutions in which precipitation experiments are made, are initially supersaturated with respect to four additional phases. The solubility isotherms are shown in Figure 1 as a function of pH. Thus, at a pH of 7.0, in order of increasing solubilities, it is necessary to take into account tricalcium phosphate (Ca3(PO )2 hereafter TCP), octacalcium phosphate (Ca H(PO ). 2 1/2 H2O, hereafter OCP), anhydrous dicalcium phosphate (CaHPO, hereafter DCPA), and dicalciiam phosphate dihydrate (CaHPO. 2H2O, hereafter DCPD). The corresponding thermodynamic solubility products. 

The results confirm previous suggestions based upon approximate stoichiometry determinations using conventional precipitation experiments that at sufficiently low supersaturation HAP can precipitate without the need for precursor formation both at the surface seed crystals (6)and spontaneously from solution (56). 

These results suggest that even if the model of Dickie and Yen (21) might explain some of the phenomena, the asphaltenes are a system with dynamic behavior depending on all system components these experiments and the precipitation experiments of Boduszynski (6) are suggesting the same thing, that we do not yet understand the actual nature of asphaltenes. 

Precipitation of Coal-Derived Bases from Toluene. A solution of coal-derived material in toluene was prepared by stirring 50 g of sample per liter of solvent at reflux temperature for 6-8 h. After cooling, suction filtration through a sintered glass funnel was used to remove the undissolved solids, which were dried in vacuo at 80° C and weighed the quantity of material dissolved was determined by difference. The filtrate was placed in a stoppered bottle and used as a stock solution for several precipitation experiments. Immediately before the use of the toluene solution, an accurately pipetted volume was evaporated to dryness in vacuo and the remaining solid was weighed to calculate the concentration. 

Preparation of -Glycerophosphoric Acid. The 0-glycerophosphoric acid used in the precipitation experiments was freshly prepared before each use. The acid, which decomposes slowly at room temperature, was made from its disodium salt (Reliable Chemical Co.) by ionic exchange in water using Dowex 50W-X8 cation exchange resin. After exchange, the water was removed in vacuo to give the acid as a viscous liquid. 

In Table III are reported elemental analyses of the original sample (toluene-soluble and toluene-insoluble fractions), of the bases obtained from them, and of the unprecipitated remainder. The preasphaltenes, though defined as tetrahydrofuran-soluble, toluene-insoluble materials, are sufficiently soluble in toluene (approximately 1 g/100 mL) to be used in these precipitation experiments. The elemental analysis data suggest that no significant fractionation has taken place in this dissolution step. 

In addition to concerns over the incomplete solubility of some of the H-coal vacuum-still bottom materials in toluene, the precipitation experiments just discussed are subject to problems associated with two-phase reactions in general. Incomplete contact between the organic compounds in toluene solution and the aqueous acids can lead to inconsistent precipitation results. With this in mind, a different approach was developed in which various H-coal vacuum-still bottoms were treated with phosphotungstic acid, which is soluble in tetrahydrofuran. This reagent allows precipitation to be accomplished from an initially one-phase system, and gives rise to crisp, easily collected salts. 

Buck MJ, Lieb ID. ChIP-chip considerations for the design, analysis, and application of genome-wide chromatin immuno-precipitation experiments. Curr. Opin. Genet. Dev. 2006 16 157-164. 

The results of precipitation experiments are shown in Figure 3. The amounts of... 

Both inorganic precipitation experiments (Sanyal et al., 2000) and culture experiments (Sanyal et al., 1996, 2001) have shown that the isotopic composition of boron incorporated into calcite is a function of pH (Figure 13). The measured of calcite precipitated over a... 

Van Cappellen and Berner (1991) smdied the growth kinetics of FAP in seeded precipitation experiments using carbonate-free solutions. The growth of FAP was inhibited by Mg " " at concentrations typical of marine pore waters and enhanced by H" " ions in the pH range of 7-8.5. At moderate degrees of supersaturation, a precursor phase similar in composition to octacalcium phosphate formed on the FAP... 

A variety of methods have been used to characterize the solubility-limiting radionuclide solids and the nature of sorbed species at the solid/water interface in experimental studies. Electron microscopy and standard X-ray diffraction techniques can be used to identify some of the solids from precipitation experiments. X-ray absorption spectroscopy (XAS) can be used to obtain structural information on solids and is particularly useful for investigating noncrystalline and polymeric actinide compounds that cannot be characterized by X-ray diffraction analysis (Silva and Nitsche, 1995). X-ray absorption near edge spectroscopy (XANES) can provide information about the oxidation state and local structure of actinides in solution, solids, or at the solution/ solid interface. For example, Bertsch et al. (1994) used this technique to investigate uranium speciation in soils and sediments at uranium processing facilities. Many of the surface spectroscopic techniques have been reviewed recently by Bertsch and Hunter (2001) and Brown et al. (1999). Specihc recent applications of the spectroscopic techniques to radionuclides are described by Runde et al. (2002b). Rai and co-workers have carried out a number of experimental studies of the solubility and speciation of plutonium, neptunium, americium, and uranium that illustrate combinations of various solution and spectroscopic techniques (Rai et al, 1980, 1997, 1998 Felmy et al, 1989, 1990 Xia et al., 2001). 

Mixed calcium cerium oxide samples were also prepared by the incipient wetness method. Experimental data from both nitrogen isotherm analysis and FTIR spectra indicate the presence of calcium cerium mbced oxide and pure calcium and cerium oxide phases, thus reinforcing the idea that for the precipitation experiments, mixed phases are indeed prepared. 

Oxalate precipitation experiments. Simulated and fully active HAW solutions have been utilized to carry out experimental tests on the separation of actinides by oxalates precipitation and on the actinide/RE separation steps. 

To prevent during the denitration step the formation of precipitates on which Pu and Am were partially and irreversibly adsorbed, denitration and oxalate precipitation were carried out in a single step by addition of the waste solution to the formic and oxalic acid mixture, the latter acid acting as a metal complexant during the denitration step. By experimental tests performed on simulated HAW according to this modified process scheme, separation yields of about 99.5% for Pu and 99.8% for Am were measured. A further reduction of the actinide content was reached by flowing the clarified HAW solution through a Dowex 50 resin column. The oxalate precipitation experiments on fully active HAW solutions have practically been completed. The results obtained from five runs (Table IV) confirmed the previous results obtained on simulated solutions. 

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