Precipitation step

Both thermodynamic and kinetic aspects of mixed systems (e.g., the precipitation step in wet spinning) involve the properties of the other components (solvent and nonsolvent in wet spinning) as well as the polymer. 

The precipitated cellulose acetate is filtered from the dilute (25—36%) acetic acid. The acetic acid and salts remaining from the sulfuric acid neutrali2ation are removed by washing. The wet polymer is typically dried to a moisture content of 1—5%. The dilute acetic acid obtained from the washing and precipitation steps caimot be used in other stages of the process. Its efficient recovery and recycle are an economic necessity. 

Nickel and cobalt are recovered by processes that employ both pressure leaching and precipitation steps. The raw materials for these processes can be sulfide concentrates, matte, arsenide concentrates, and precipitated sulfides. Typically, acidic conditions are used for leaching however, ammonia is also effective in leach solutions because of the tendency for soluble cobalt and nickel ammines to form under the leach conditions. 

The key difference between the brine process and seawater process is the precipitation step. In the latter process (Fig. 6) the seawater is first softened by a dding small amounts of lime to remove bicarbonate and sulfates, present as MgSO. Bicarbonate must be removed prior to the precipitation step to prevent formation of insoluble calcium carbonate. Removal of sulfates prevents formation of gypsum, CaS02 2H20. Once formed, calcium carbonate and gypsum cannot be separated from the product. 

An alternative pretreatment for seawater is acidification of the bicarbonate followed by degasification to remove the carbon dioxide generated. The precipitation step for the seawater process is given by (76) ... 

Mixing. In method 1, a suspension of colloidal powders, or sol, is formed by mechanical mixing of colloidal particles in water at a pH that prevents precipitation (step A in Fig. 1) (8). In method 2 or 3, a Hquid alkoxide precursor such as Si(OR)4, where R is CH (TMOS), C2H (TEOS), or C Hy, is hydrolyzed by mixing with water (eq. 2). 

Wlien solid soda ash is used to supply all the carbonate v alues in the precipitation step (eq. 9), a ca 10% Na2S solution results from the primary filtration step wliich can be concentrated to 40% Na2S in a three-effect evaporator train. Final concentration to 60%Na2S occurs in a high v acuurn single-effect evaporator. Hiis concentrated solution can then be fed to a Baker to produce a 60% sodium sulfide flake wliich is sold as co-product. 

Two cocrystallization processes employ dibasic crystals as intermediates. The PPG process (199—202) is discussed under commercial processes. The PPC process (203) forms dibasic crystals from lime and recovered filtrates. The dibasic crystals are separated from thek mother liquor by decantation, slurried in caustic solution and chlorinated to produce a cocrystalline slurry of Ca(OCl)2 and NaCl. The slurry is sent to a flotation cell where the larger salt crystals settle out and the smaller hypochlorite crystals float to the top with the aid of ak and flotation agent. The hypochlorite slurry is centrifuged the cake going to a dryer and the centrate to the flotation cell. The salt-rich bottoms from the flotation cell are centrifuged and washed with dibasic mother Hquor. The centrates are recycled to the precipitation step. 

Commercial Processes. Olin s earlier triple salt process, originally commercialized in 1928, was modified in 1983. In the patented process, a slurry of dibasic calcium hypochlorite is mixed with a strong, low salt sodium hypochlorite solution and hypochlorite Hquors and chlorinated. The resultant Ca(OCl)2 2H20 slurry is filtered, the cake going to the dry-end and the filtrate to the dibasic precipitation step where it reacts with lime. 

Many crystalline products, including fine chemicals, foodstuffs and pharmaceuticals, require a final particle size that is significantly smaller than that produced during the crystallization or precipitation step. One way of achieving the required particle size is to employ a subsequent size-reduction step using some form of comminution device, frequently a mill. 

The precipitation step may be replaced by spray drying of a homogeneously stirred solution of, e.g., nitrates [31], or by spraying the precursors into a very hot flame at about 1500-2200 °C. This method is called flame pyrolysis. 

Cranston and Murray [35,36] took samples in polyethylene bottles that had been pre-cleaned at 20 °C for four days with 1% distilled hydrochloric acid. Total chromium Cr(VI) + Cr(III) + Crp (Crp particulate chromium) was coprecipitated with iron (II) hydroxide, and reduced chromium Cr(III) + Crp was co-precipitated with iron (III) hydroxide. These co-precipitation steps were completed within minutes of the sample collection to minimise storage problems. The iron hydroxide precipitates were filtered through 0.4 pm Nu-cleopore filters and stored in polyethylene vials for later analysis in the laboratory. Particulate chromium was also obtained by filtering unaltered samples through 0.4 pm filters. In the laboratory the iron hydroxide co-precipitates were dissolved in 6 N distilled hydrochloric acid and analysed by flameless atomic absorption. The limit of detection of this method is about 0.1 to 0.2 nM. Precision is about 5%. 

Typical protein precipitation procedures use one volume of plasma plus three to six volumes of acetonitrile or methanol (or a mixture) with the internal standard at an appropriate concentration for the assay. Poison et al.102 reported that protein precipitation using acetonitrile eliminates at least 95% of the proteins after filtration or centrifugation, the supernatant can often be directly injected into the HPLC/MS/MS system. Usually this step is performed using 96-well plates that are ideal for semi-automation of sample preparation. Briem et al.103 reported on a robotic sample preparation system for plasma based on a protein precipitation step and a robotic liquid handling system that increased throughput by a factor of four compared to a manual system. 

In addition to online filters and precolumns, a simple protein precipitation step often precedes online SPE LC/MS/MS to prolong cartridge life. Protein precipitation can also reduce analytical interference and shorten chromatographic separation time. Since an internal standard (IS) solution is often added to plasma samples and centrifugation is used to remove possible particles before loading into the autosampler, protein precipitation does not add labor to the process. 

Koal et al. (2004) measured four immunosuppressants (cyclosporine A, tacrolimus, sirolimus, and everolimus) in whole blood samples from transplant recipients. The samples were treated first with a protein precipitation step. The supernatant was extracted with a Poros Rl/20 perfusion column (30 x 2.1 mm, 20 tm, Applied Biosystems, Darmstadt, Germany) online. A Luna phenyl hexyl column (2 x 50 mm, Phenomenex, Schaffenburg, Germany) was used for separation. The total run time was 2.5 min. The lower limit of quantitation was 10 ng/mL for cyclosporine A and 1 ng/mL for the other three analytes. 

Hopfgartner et al. (2002) compared ternary column online SPE LC/MS and TFC with offline 96-well plate SPE LC/MS to quantitate three drug candidates in human plasma. A protein precipitation step was performed before the SPE LC/MS. Dual trapping columns (YMS AQ, 10 x 2.0 mm, 5 /tm) were used with an analytical column (Intertsil Phenyl, 50 x 2.1 mm, 5 /tm). The run cycle was 3 min calibration range was 0.2 to 250 ng/mL. The run cycle was 2 min with a calibration range of 5 to 1000 ng/mL for TFC. Offline SPE LC/MS achieved the same calibration range with a run time of 2 min. 

Application In Analytical And Inorganic Chemistry Knowledge about distribution coefficients is used in analytical chemistry to determine the feasibility of quantitative separation by precipitation. Therefore, D and X are also called separation factors. In order to precipitate 99.8% or more of the primary substance, X must be 3.2 x 10 or smaller. For larger values of X more than one precipitation step is necessary, and the number of steps can be calculated when X is known. 

A third plant uses a chemical precipitation step for removing arsenic and zinc from contaminated surface water runoff. Ferric sulfate and hme are alternately added while the wastewater is vacuum-filtered and sludge is contract-hauled. The entire treatment system consists of dual-media filtration, carbon adsorption, ion exchange, chemical precipitation, and vacuum hltration. Sampling results across the entire treatment system indicated that arsenic was reduced from 6.9 to 0.2 mg/L and zinc from 0.34 to 0.11 mg/L. 

Indium may be recovered from zinc ores by several patented processes. Usually it is recovered from residues obtained from zinc extraction. The residues, slags, fume, or dusts from zinc smelting or lead-zinc smelting are treated with a mineral acid. Other steps involved in recovery often vary, but mostly use solvent extraction and precipitation steps. In some processes, treatment with caustic soda yields indium hydroxide. The hydroxide is calcined to obtain oxide, which then is reduced with hydrogen at elevated temperatures to obtain the metal. Distillation or electrolysis are the final steps to... 

Insulin preparations used initially were little more than crude pancreatic extracts. The therapeutic value of such products was marginal, as severe adverse reactions were commonplace (due to the presence of impurities). This was made worse by the frequency of injections required. The introduction of an acid-alcohol precipitation step yielded insulin preparations of moderate purity, thus partially overcoming the range and severity of side effects noted. 

There are no available data on the formation of hydroperoxides derived from DNA within cells. This is likely explained, at least partly, by the fact that DNA is a poorer target than proteins for OH radical as observed upon exposure of mouse myeloma cells to ionizing radiation . However, indirect evidence for DNA peroxidation within cells may be inferred from the measurement of final degradation products that may derive from thymine and guanine hydroperoxidation as the result of oxidation reactions mediated by OH radical and O2, respectively (Sections n.A.2 and n.E.2). It may be pointed out that the measurement of oxidized bases and nucleosides within DNA has been the subject of intense research during the last decade and accurate methods are now available . This includes DNA extraction that involves the chaotropic Nal precipitation step and the use of desferrioxamine to chelate transition metals in order to prevent spurious oxidation of overwhelming nucleobases to occur . HPLC coupled to electrospray ionization... 

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