Aqueous processing precipitation

An overview is presented of plutonium process chemistry at Rocky Flats and of research in progress to improve plutonium processing operations or to develop new processes. Both pyrochemical and aqueous methods are used to process plutonium metal scrap, oxide, and other residues. The pyrochemical processes currently in production include electrorefining, fluorination, hydriding, molten salt extraction, calcination, and reduction operations. Aqueous processing and waste treatment methods involve nitric acid dissolution, ion exchange, solvent extraction, and precipitation techniques. 

In the second stage of a representative aqueous process, an aqueous solution of polytyinyl alcohol) is heated with butyraldehyde and an acid catalyst. PVB precipitates from solution as il forms. Because PVB resin precipitates early in the reaction there is a tendency toward high levels of intennolecular acetalization. Cross-linking can be minimized by adding emulsifiers to control particle size or substances like ammonium thiocyanate or urea to improve the solubility of PVB in the aqueous phase. 

Hydrolysis in aqueous solution precipitates hydrated titanium dioxide which, after washing and drying, can be calcined at 800°C to remove water and residual Cl. This method has been the basis of producing titanium dioxide of 99.999% purity. If retaining a high specific surface area is important, it may be convenient to reduce the residual Cl content by Sohxlet extraction rather than calcination. For many electroceramic uses, the millability of the product is as important as the absolute purity. A number of process variants designed to modify particulate morphology have been described (67). 

Arsenic is a frequent contaminant in metal ores (especially sulfides) and therefore appears in hydrometallurgical processes applied to these ores. Because of its toxicity, it must be efficiently removed from aqueous process streams before discharge. Traditionally this has been done with lime, but a large excess is required and the product (calcium arsenite and arsenate) can re-release some arsenic under the influence of atmospheric carbon dioxide. A more reliable process is co-precipitation with iron salts under near-neutral conditions. This requires both element ores to be in their higher oxidation states and hydrogen... 

Lithium Recovery. The melt used in this process is relatively inexpensive except for the lithium carbonate which comprises approximately 84% of the salt cost. Therefore it is desirable to recover the lithium from the process filter cakes. An aqueous process has been developed for this purpose. The filter cakes are slurried with water and filtered to extract the very soluble sodium and potassium carbonates lithium carbonate remains with the ash because it is relatively insoluble under these conditions. The ash-lithium carbonate cake is slurried in water and the lithium is solubilized by conversion to the bicarbonate. The ash is removed by filtration and the soluble bicarbonate in the filtrate is precipitated as the carbonate. The lithium carbonate is separated by filtration and returned to the process stream the saturated lithium carbonate filtrate is recycled to conserve lithium. Laboratory tests have demonstrated that more than 90% of the lithium can be recovered by this technique. 

A prime responsibility of the Rocky Flats Plant since it was built by the Atomic Energy Commission some 25 years ago has been the recovery and purification of plutonium. This recovery and purification has been done using an aqueous process. One of the major steps in that process is the precipitation of plutonium peroxide. This step converts the plutonium from an aqueous to a solid form for further processing and conversion to metal. 

The production of catalyst particles of suitable configuration and hardness is an essential part of catalyst manufacture . Most heterogeneous catalysts are produced by processes that involve formation of solids from aqueous solutions. Precipitation is frequently employed in preparation of hydrous oxide catalysts. To avoid occluded or adsorbed impurities, ammonia or ammonium salts are often used as well as nitrates of the desired metal constituents. Calcination removes the nitrogen-containing components. Anions such as Cl or 864 or cations such as Na are avoided these often are poisons if they are present in the final catalyst. 

Although there are a number of potential methods for the manufacture of vinyl acetal polymers, in practice, only two basic processes with variants are used commercially today. By the solution process, formation of the acetal is as a solution in an organic solvent. By the aqueous process, the acetal is precipitated during the reaction of the aldehyde with an aqueous solution of poly(vinyl alcohol). These two reactions, hydrolysis and acetalization, can be conducted either sequentially or concurrently in the solvent process (1). 

PVB resin can be produced by either a solvent process (37,38) or an aqueous process (11,39,40). The flow sheets for these processes are shown in Figures 5 and 6. In the aqueous process, poly(vinyl alcohol) is dissolved in water or an aqueous solution that contains acetic acid, then acidifled with a mineral acid and reacted with butyraldehyde. During acetalization, PVB precipitates from the aqueous reaction mixture. 

Upon completion of the acetalization reaction in both solvent and aqueous processes, the acid catalyst is neutralized. PVB is precipitated into water during... 

Three major t)q)es of chemical processes occur in aqueous solution precipitation reactions, acid-base reactions, and oxidation-reduction reactions. 

In Sections 4.2-4.4 we will study three types of reactions in the aqueous medium (precipitation, acid-base, and oxidation-reduction) that are of great importance to industrial, environmental, and biological processes. They also play a role in our daily experience. 

Hydrothermal Process Precipitation from an aqueous solution of precursor salts under a high pressure and temperature condition in an autoclave Particle size control. By reaction temperature and duration, precursor concentration, pH, surfactant additives, and precursor materials General attributes High crystallinity often geometrical shapes mostly oxides... 

Freshly pressed yeast is extracted by boiling twice with one-and-one-half times the amount of water, containing 0.1 % acetic acid. Some neutral lead acetate is added to the filtrate and then a 10% solution of mercuric sulfate in 5 % aqueous sulfuric acid. After decomposition of the Hg salt with H,S and removal of the excess H,S by a stream of an inert gas the solution is made 0.5 N with sulfuric acid and reprecipitated with CujO. This process precipitates GSH as a colorless crystalline copper salt. The decomposition with H,S yields 1 to 1.2 gm. of GSH from 1 kg. yeast. A method of isolation using the S-benzyl derivative has been published in 1941 (9). A Japanese patent (10) claims a fourfold yield after incubation of the yeast with cysteine and malt extract for 8 hours. 

PLGA/HAp composites were also produced by colloidal non-aqueous chemical precipitation at room temperature by Petricca and coworkers [158]. The shape of the scaffolds with homogeneous distribution of HAp particles in PLGA matrix was easily engineered, and the mechanical properties of the PLGA/HAp composites indicated that the materials are suitable as bone substitution scaffolds. The decreased tensile strength was attributed to the presence of micropores within the scaffold, which are created upon removal of the ammonium nitrate, a by-product of the synthesis. The novelty of the process is the ability to incorporate up to 30 wt% HAp within the polymer matrix, thereby maximizing the material osteoconductivity, as evidenced by osteoblast-like cell adhesion experiments. 

About thirty years ago, van Dam and Cottier, et al. showed that an aqueous and non-aqueous process could produce 37% yield of 5-HMF. In the aqueous part of the system, 5-HMF takes up two molecules of water to form levulinic and formic acid while in the non-aqueous part, the hydrolysis of 5-HMF is suppressed. Nonetheless, cross-polymerization occurred under all circumstances leading to the formation of colored soluble polymers and insoluble brown precipitates. Thus, there is a need to come up with a suitable catalyst or a system for the selective formation of 5-HMF while not promoting the consecutive reactions... 

---