Purification steps

These includes the following main steps separation of the undissolved digestion residue by filtration, and separation of the impurities dissolved in the sodium tungstate solution. Sometimes, the first step can be combined with the first precipitation step (silica precipitation). 

The first separation step is called silica precipitation but, besides silica, other ions like phosphate and fluoride are at least partially separated. 

The second step is a precipitation of sulfide-forming cations mainly applied to separate molybdenum, but others like arsenic, antimony, bismuth, cobalt, etc. are precipitated, too. 

The silica precipitation is performed in alkaline solution, while the sulfide precipitation is effective at pH 2.5-3.0. This change in pH is usually achieved by addition of sulfuric acid. An interruption at pH 6 offers the possibility to separate the above-mentioned [AIFg] ion (minimum solubility of Na3[AlF3]). The acidification transforms all excess Na into soluble sodium sulfate. 

Acidification can also be achieved by electrodialysis and offers in addition the possibility to recover NaOH solution, which can be used for digestion. This is a modem process until now used only in a pilot plant scale. However, economic as well as ecologic considerations will dictate such processes in the future. 

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Fine chemicals are produced by a wide spectmm of manufacturers, largely because the distinction between different kinds of chemicals is not sharp. There are specialty producers of fine chemicals. Many companies that manufacture dmgs also manufacture the chemical substances that are used in preparing the dosage forms. A number of companies manufacture dmg chemicals and food chemicals. Some fine chemicals are made by manufacturers of heavy chemicals, and either may be simply a segment of their regular production, or some of that production which has been subjected to additional purification steps. Many fine chemicals are imported into the United States from countries such as Japan, Germany, and the Netherlands. 

As a result of the development of electronic applications for NF, higher purities of NF have been required, and considerable work has been done to improve the existing manufacturing and purification processes (29). N2F2 is removed by pyrolysis over heated metal (30) or metal fluoride (31). This purification step is carried out at temperatures between 200—300°C which is below the temperature at which NF is converted to N2F4. Moisture, N2O, and CO2 are removed by adsorption on 2eohtes (29,32). The removal of CF from NF, a particularly difficult separation owing to the similar physical and chemical properties of these two compounds, has been described (33,34). 

If the hGH is exported to the culture medium the product can easily be collected by removal of the cells from the culture medium by centrifiigation. Purification of hGH from the culture medium is faciUtated by low amounts of contaminating proteins present. In fact, it has been shown that hGH can be purified on a laboratory scale by a single purification step on a reversed-phase hplc column (43). Mammalian cells growing in tissue culture have also been used as hosts to produce hGH, which is exported into the culture media (44). 

There has been an increasing interest in utilising off-gas technology to produce ammonia. A number of ammonia plants have been built that use methanol plant purge gas, which consists typically of 80% hydrogen. A 1250 t/d methanol plant can supply a sufficient amount of purge gas to produce 544 t/d of ammonia. The purge gas is first subjected to a number of purification steps prior to the ammonia synthesis. 

A wide range and a number of purification steps are required to make available hydrogen/synthesis gas having the desired purity that depends on use. Technology is available in many forms and combinations for specific hydrogen purification requirements. Methods include physical and chemical treatments (solvent scmbbing) low temperature (cryogenic) systems adsorption on soHds, such as active carbon, metal oxides, and molecular sieves, and various membrane systems. Composition of the raw gas and the amount of impurities that can be tolerated in the product determine the selection of the most suitable process. 

The decomposition of aqueous hydrogen peroxide is minimized by various purification steps during manufacture, use of clean passive equipment, control of contaminants, and the addition of stabilizers. The decomposition is zero-order with respect to hydrogen peroxide concentration. 

Many aminonaphthalenesulfonic acids are important in the manufacture of azo dyes (qv) or are used to make intermediates for azo acid dyes, direct, and fiber-reactive dyes (see Dyes, reactive). Usually, the aminonaphthalenesulfonic acids are made by either the sulfonation of naphthalenamines, the nitration—reduction of naphthalenesulfonic acids, the Bucherer-type amination of naphtholsulfonic acids, or the desulfonation of an aminonaphthalenedi-or ttisulfonic acid. Most of these processes produce by-products or mixtures which often are separated in subsequent purification steps. A summary of commercially important aminonaphthalenesulfonic acids is given in Table 4. 

Historically, ferrous sulfamate, Fe(NH2S02)2, was added to the HNO scmbbing solution in sufficient excess to ensure the destmction of nitrite ions and the resulting reduction of the Pu to the less extractable Pu . However, the sulfate ion is undesirable because sulfate complexes with the plutonium to compHcate the subsequent plutonium purification step, adds to corrosion problems, and as SO2 is an off-gas pollutant during any subsequent high temperature waste solidification operations. The associated ferric ion contributes significantly to the solidified waste volume. 

Improvements in the abiUty to control operating conditions and in contractor designs have allowed a steady reduction in the number of purification steps required. The THORP faciUty, commissioned as of 1994 in the U.K., uses only a single purification step. 

After the second extraction/stripping cycle, the plutonium is concentrated by evaporation or by preferential adsorption (qv) on ion-exchange resins. As in the case for uranium, the newer faciHties, such as THORP, use only a single purification step. 

The effluent from the reactor is a slurry of terephthaUc acid because it dissolves to a limited extent in almost all solvents, including the acetic acid—water solvent used here. This slurry passes through a surge vessel that operates at a lower pressure than the reactor. More terephthaUc acid crystallizes and the slurry is then ready to be processed at close to atmospheric conditions. The terephthaUc acid crystals are recovered by filtration, washed, dried, and conveyed to storage, from which they are in turn fed to the purification step. 

Some alkylphenol appHcations can tolerate "as is" reactor products, most significantly in the production of alkylphenol—formaldehyde resins. These resins can tolerate some of the reactant and by-product from the alkylphenol reactor because they undergo purification steps. This resin production route has both capital and operating cost advantages over using purer alkylphenol streams as feedstock. For these savings, the resin producer must operate the process in such a way as to tolerate a more widely varying feedstock and assume the burden of waste disposal of some unreactive materials from the alkylphenol process. 

After leaving the reactor, the reaction mixture consisting of aniline, water, and excess hydrogen is cooled and condensed prior to the purification steps. First, the excess hydrogen is removed and recycled back to the reactor. The rest of the mixture is sent to the decanter where the water and aniline are separated. The cmde aniline, which contains less than 0.5% of unreacted nitrobenzene and about 5% water, is distilled in the cmde aniline column. The aniline is further dehydrated in the finishing column to yield the purified aniline. Meanwhile, the aqueous layer from the decanter, which contains about 3.5% aniline, is extracted to recover the aniline and clean up the water before it is sent to the waste-water treatment plant. 

Carbon Dioxide Removal. The effluent gases from the shift converters contain about 17—19 vol % (dry) carbon dioxide (qv) which is ultimately reduced to a few ppm by bulk CO2 removal, followed by a final purification step. Commercial CO2 removal systems can be broadly classified as... 

Essentially no waste products are formed ia the USP process if hydriodic acid and either sodium hydroxide or sodium carbonate are used as reactants. Water results from use of the former a mole equivalent quantity of carbon dioxide is produced from the latter reagents. Higher quaUty grades may require some purification steps which may result ia wastes from the treatment. Disposal of these impurities must then be carried out. 

Development of conjugate and peptide vaccines requires the typical organic synthesis process and purification. This is a new area for vaccine technologists. Again, the main concern is to maintain the immunogenicity of the vaccine candidate during the chemical reaction and purification steps. 

A large number of patents describe various procedures for the mainly continuous hydrolysis and oxidation processes, as weU as for the purification steps requited to obtain high grade vanillin. Lignin is degraded either with sodium hydroxide or with calcium hydroxide solution and simultaneously... 

The cerium concentrate derived from bastnasite is an excellent polish base, and the oxide derived direcdy from the natural ratio rare-earth chloride, as long as the cerium oxide content is near or above 50 wt %, provides an adequate glass poHsh. The polishing activity of the latter is better than the Ce02 Ln0 ratio suggests. Materials prepared prior to any Ln purification steps are sources for the lowest cost poHshes available used to treat TV face plates, mirrors, and the like. For precision optical polishing the higher purity materials are preferred. 

Lime slurry is chlorinated in the presence of Ca(OCl)2 mother Hquor, NaOH, and NaOCl (185). After concentration, the resulting slurry of Ca(OCl)2 2H20 is filtered and the cake dried. A portion of the filtrate is treated with caustic, the recovered lime is recycled, and the mother Hquor used to prepare the requited NaOCl solution in an evaporator—chlorinator, which after separation of salt, is sent to the main reactor. In a slightly modified version, a lime purification step is added (186). 

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