Coprecipitation process

In the oxalate coprecipitation method, oxalate acid reacts with metal cations to form the precipitate, which is subsequently calcined to obtain the products. The advantage of oxalate acid as a precipitant is that, unlike hydroxides, oxalates are less sensitive to the treatment conditions, such as washing and drying. In addition, the 

Nevertheless, the oxalate coprecipitation method has some problems. For example, this method usually results in rodlike doped ceria particles, which are agglomerations of smaller particles with irregular shapes. Hence, the green density of the compact body is relatively low, so it is difficult to fabricate a dense electrolyte film or membrane. In addition, the poor flow of the rodlike powder makes forming difficult. 

Most of the literature focuses on the aspects of sinterability and microstructure, but limited data on the electrical properties is available. Tok [152] reported a conductivity of 18.3 x 10-3 Scm-1 at 600°C for Gd0 jCeo.gOj 95, and we measured a high conductivity of 22 x 10-3 scm-1 for Sm0 2Cc08O 9 at the same temperature. Their activation energies are relatively low—less than 0.7 eV. Although conductivity data reported for doped ceria prepared with carbonate precipitation is varied from different authors [153-155], the conductivity is generally high and the activation energy is usually low for ceria electrolytes fabricated with this method. 

Solid Oxide Fuel Cells Materials Properties and Performance 

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As the first step in the coprecipitation process, ferric hydroxide precipitates either from the effect of the changing pH on the solubility of ferric iron,... 

Sol id Sol utions. The aqueous concentrations of trace elements in natural waters are frequently much lower than would be expected on the basis of equilibrium solubility calculations or of supply to the water from various sources. It is often assumed that adsorption of the element on mineral surfaces is the cause for the depleted aqueous concentration of the trace element (97). However, Sposito (Chapter 11) shows that the methods commonly used to distinguish between solubility or adsorption controls are conceptually flawed. One of the important problems illustrated in Chapter 11 is the evaluation of the state of saturation of natural waters with respect to solid phases. Generally, the conclusion that a trace element is undersaturated is based on a comparison of ion activity products with known pure solid phases that contain the trace element. If a solid phase is pure, then its activity is equal to one by thermodynamic convention. However, when a trace cation is coprecipitated with another cation, the activity of the solid phase end member containing the trace cation in the coprecipitate wil 1 be less than one. If the aqueous phase is at equil ibrium with the coprecipitate, then the ion activity product wi 1 1 be 1 ess than the sol ubi 1 ity constant of the pure sol id phase containing the trace element. This condition could then lead to the conclusion that a natural water was undersaturated with respect to the pure solid phase and that the aqueous concentration of the trace cation was controlled by adsorption on mineral surfaces. While this might be true, Sposito points out that the ion activity product comparison with the solubility product does not provide any conclusive evidence as to whether an adsorption or coprecipitation process controls the aqueous concentration. 

If catalysts are prepared by coprecipitation, the composition of the solutions determine the composition of the final product. Often the composition of the precipitate will reflect the solution concentrations, as was shown for CuO/ZnO catalysts for methanol synthesis [18], but this is not necessarily the case. For al-minum phosphates it was found that at low P A1 ratios the precipitate composition is identical to the solution composition, but if the P A1 ratio in the solution comes close to and exceeds unity, the precipitate composition asymptotically approaches a P A1 ratio of 1 [19]. Deviations from solution composition in coprecipitation processes will generally occur if solubilities of the different compounds differ strongly and precipitation is not complete or, if in addition to stoichiometric compounds, only one component forms an insoluble precipitate this the case for the aluminum phosphate. 

This section shows, for four examples of increasing complexity, how precipitates are formed and how the properties of the precipitates are controlled to produce a material suitable for catalytic applications. The first two examples comprise silica, which is primarily used as support material and is usually formed as an amorphous solid, and alumina, which is also used as a catalytically active material, and which can be formed in various modifications with widely varying properties as pure precipitated compounds. The other examples are the results of coprecipitation processes, namely Ni/ AI2O3 which can be prepared by several pathways and for which the precipitation of a certain phase determines the reduction behavior and the later catalytic properties, and the precipitation of (VOjHPCU 0.5 H2O which is the precursor of the V/P/O catalyst for butane oxidation to maleic anhydride, where even the formation of a specific crystallographic face with high catalytic activity has to be controlled. 

Alumina-supported nickel catalysts are an excellent example for the advantages of and the problems associated with coprecipitation processes for the manufacture of catalysts. Such catalysts are accessible via several pathways, as impregnation, deposition/precip-itation, coprecipitation from alumina gels, and more conventional coprecipitation routes. Also, for coprecipitation, different routes are possible, the first examples originating from the 1920s [48]. Starting from the nitrate solutions of nickel and aluminum, there are at least three different routes ... 

In the coprecipitation process, the multi-element product can be of 3 types ... 

Chitin metal silicates coprecipitates [53] Coprecipitation of a metal silicate on chitin particles offers industrial potential for use as a single filler which has binding as well as super-disintegration properties and can be used in directly compressed tablets or in wet granulation methodologies. The coprecipitation process causes physical adsorption of the metal silicates onto chitin particles (Fig. 2.39) without any chemical interaction which has been proved by IR and XRPD analysis. 

In comparison, the stationary phases prepared using the coprecipitation process of Zhang, Feng, and Da with... 

Although formation of arsenic oxoanion minerals may be uncommon in the subsurface, many other minerals in which oxoanions are the fundamental structural unit (most commonly sulfates, phosphates, and carbonates) can assimilate trace amounts of arsenic via adsorption and coprecipitation processes. By virtue of their ubiquitous nature in sediments, these phases... 

K. Yang, M. Misra, and R. K. Mehta, Removal of Heavy Metal Ions from Acid Mine Water by Feirite Coprecipitation Process, in Separation Processes Heavy Metals, Ions and Minerals, M. Misra (ed.), TMS, Warrendale, PA 1995, pp. 37—48. 

The microstructures of the two samples (n=0.4), NO3-I and Cl-Br-2, are compared in Fig. 5. NO3-I was mainly composed of submicron particles, but large a lomerates were also formed. By contrast, Cl-Br-1 was composed of highly dispersed fine particles with uniform size less than 0.1 mm. These are noncrystalline mixtures as evident from Fig.4. The amorphous-like nature of MnOx-Zr02 appears to be originated from the growth process of hydroxides in the coprecipitation process. With an addition of base (OIT) to the aqueous solution of Mn(N03)2, hydroxide [Mn(OH)6] clusters formed would be bound together to grow into Mn(OH)2 crystallites. 

In the present coprecipitation process, the interaction between hydroxide clusters of Mn and Zr would suppress each other to grow into large crystallites. This is a possible reason for the small particle size and thus the large surface area of the MnOx-ZrOa system. 

The diagram in Figure 4.15 emphasizes the continuity of precipitation and coprecipitation processes with chemisorption, both in time and space. Low levels of adsorbate (whether metal cations or anions) are usually bound by chemisorption, higher levels by the formation of sohd solutions or by the nucleation of small adsorbate clusters at surfaces. The highest levels of adsorbate lead to precipitation of separate mineral phases, a process that can be viewed as an extension of cluster growth that allows a new solid phase to become detectable. 

It has been shown that the predominate state of the selenium in these SSW streams is as the selenocyanate (SeChT) [1]. The selenocyanate in SSW is oxidized by refinery biotreaters to selenite (SeOs ), which can then be removed via an iron based treatment process. One of the most effective of these iron-based processes is iron co-precipitation process. However, the selenium can also occur as selenate (Se04 )> which is not removed during the iron coprecipitation process [2]. In order to design effective treatment protocols to remove the selenium, more than just the concentration of the metal must be determined. Without knowledge of the types of selenium species present in the water, design of an effective removal treatment protocol... 

Can FIA provide a solution to the automation of precipitation-dissolution and revitalize its function in modem analytical chemistiy The answer to this question came quite late, well after the application of FI techniques to solvent extraction and column separations. The reason for this delay obviously comes from the difficulties in on-line continuous manipulation of a heterogeneous system which could potentially create serious blockage problems in a standard FIA system. However, recent research efforts in this direction have been quite rewarding, and from the achievements described in this chapter the reader will see that the major difficulties in the automation of precipitation and even coprecipitation processes using FI techniques have been overcome. 

An important difference between the batch and the continuous mode of precipitation is the available reaction time. Different standing times are normally used in batch procedures to ensure the completeness of the precipitation reaction or/and the form of the precipitate to facilitate filtration and minimize contamination. Standing times of IS min to a few hours are typical, occasionally with elevated temperatures. Such procedures are obviously not feasible in continuous on-line precipitation systems where reaction times are typically in the range of a few seconds to a few tens of seconds. Quantitative recovery of analyte through precipitate collection is therefore not likely unless the precipitation (or coprecipitation) process is extremely fast. 

The form and amount of the precipitate formed. Crystalline precipitates, being more compact, produce more impedance in the filters. Large amounts of precipitate formed in coprecipitation processes also create impedance which limits the upper range of the flow-rate. 

This process has been successfully applied to obtain nano- and microparticles of different substances, with narrow PSDs. It is also suitable for the production of films. More recently, it has also been used for coprecipitation processes. Usually, the process parameters with a larger influence on product characteristics are the nozzle design, and the preexpansion pressure and temperature. The main limitation of this process is that it can only be applied which substances which have a relatively large solubility in SC-CO2, since its application to substances with low solubilities requires large amounts of CO2, which makes the process uneconomical. This limitation precludes the application of the RESS process to high-molecular-weight or to polar substances, since these materials typically have very low solubilities in SC-COj. 

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