Impurity levels

The coking process produces electrode quality coke from vacuum residues of good quality (low metal and sulfur contents) or coke for fuel in the case of heavy crude or vacuum residue conversion having high impurity levels. 

The conversion products, other than gas and hydrogen sulfide (H2S), are essentially a gasoline fraction that, after pretreatment, will be converted by catalytic reforming an average quality distillate fraction to be sent to the gas oil pool and an atmospheric residue or vacuum distillate and vacuum residue whose properties and impurity levels (S, N, Conr. 

The ionization energies and impurity levels are shown in the flat-band figure next to the configuration diagram. 

Figure 9.9 Impurity levels I in (a) an n-type and (b) a p-type semiconductor C is the conduction band and V the valence band...

Chemical Conversion. Except for control of nitrogen impurity levels, the same chemical conversion methods used for nitrogen purification at low flow rates can also be used for argon purification. Although used less commonly for argon purification than for nitrogen purification, these chemical conversion methods are appHed in point-of-use purifiers located close to where the gas is consumed. 

Cq ), where is the blended impurity concentration of impurity a Cq, the background impurity level and the multiplication constant. Possible sources of background response include instmment noise, sample system outgassing, or interference from other impurity response signals. Proper setup, purging, and operation of the instmment should reduce background levels weU below ippb. 

Type of process Operating temperature, °C Extraction Crystal conversion Acid concentration, % P3O3 Acid impurity level vs dihydrate acid P3O3 recovery, %... 

The 30% reagent-grade hydrogen peroxide is purer than the industrial grades, is covered by ACS reagent specification, and is used as a laboratory reagent and in some specialty uses (see Fine chemicals). Several grades are also marketed for electronics use and thus have exceptionally low impurity levels. Some of these latter contain very Httie or no stabilizers (see Electronic materials). 

Specifications and Standards, Shipping. Commercial iodine has a minimum purity of 99.8%. The Committee of Analytical reagents of the American Chemical Society (67) and the U.S. Pharmacopoeia XXII (68) specify an iodine content not less than 99.8%, a maximum nonvolatile residue of 0.01%, and chlorine—bromine (expressed as chlorine) of 0.005% (ACS) and 0.028% (USP), respectively. In the past these requirements were attained basicaHy only by sublimation, but with processing changes these specifications can be met by direct production of iodine. Previously the impurities of the Chilean product were chiefly water, sulfuric acid, and insoluble materials. Improvements in the production process, and especiaHy in the refining step, aHow the direct obtainment of ACS-type iodine. Also, because of its origin and production process, the Chilean iodine has a chlorine—bromine impurity level of no more than 0.002%. 

Physical Properties. An overview of the metallurgy (qv) and soUd-state physics of the rare earths is available (6). The rare earths form aUoys with most metals. They can be present interstitiaUy, in soUd solutions, or as intermetaUic compounds in a second phase. Alloying with other elements can make the rare earths either pyrophoric or corrosion resistant. It is extremely important, when determining physical constants, that the materials are very pure and weU characteri2ed. AU impurity levels in the sample should be known. Some properties of the lanthanides are Usted in Table 3. 

GaP N, is clearly evident. The addition of N shifts the peak to longer wavelengths and broadens the spectral emission. The curves for the AIGalnP LEDs represent devices of three different alloy compositions, all exhibiting recombination for the conduction band direct minimum. The emission spectmm of the blue InGaN LED exhibits uniquely broad emission, most likely as a result of recombination via deep Zn impurities levels (23). 

The impurity atoms used to form the p—n junction form well-defined energy levels within the band gap. These levels are shallow in the sense that the donor levels He close to the conduction band (Fig. lb) and the acceptor levels are close to the valence band (Fig. Ic). The thermal energy at room temperature is large enough for most of the dopant atoms contributing to the impurity levels to become ionized. Thus, in the -type region, some electrons in the valence band have sufficient thermal energy to be excited into the acceptor level and leave mobile holes in the valence band. Similar excitation occurs for electrons from the donor to conduction bands of the n-ty e material. The electrons in the conduction band of the n-ty e semiconductor and the holes in the valence band of the -type semiconductor are called majority carriers. Likewise, holes in the -type, and electrons in the -type semiconductor are called minority carriers. 

Fumaric acid is sold as resia-grade and food-grade. The general sales specification under which resia-grade fumaric acid is sold ia the United States specifies white, crystalline granules with a minimum assay of 99.6% and maximum ash content of 0.05%. The moisture specification is 0.3% maximum with < 10 ppm heavy metals. The color of a 5% solution ia methanol is to be less than 10 APHA. Food-grade fumaric acid calls for somewhat lower impurity levels. Particle size and particle size distribution are important ia many appHcations. 

The impurity levels in electrolytic manganese metal are as follows ... 

The purity of oxygen from chlorate candles before and after gas filtration is indicated in Table 2. A particulate filter is always used. Filter chemicals are HopcaUte, which oxidizes CO to CO2 molecular sieves (qv), which remove chlorine compounds and basic materials, eg, soda lime, which removes CO2 and chlorine compounds. Other than H2O and N2, impurity levels of <1 ppm can be attained. Moisture can be reduced by using a desiccant (see Desiccants). Gas purity is a function of candle packaging as well as composition. A hotter burning unit, eg, one in which steel wool is the binder, generates more impurities. 

Table 2. Impurity Levels in O2 from Chlorate Candles...

The responsivity and g-r noise may be analyzed to obtain background photon flux and temperature dependence of responsivity, noise, and detectivity. Typically, n > p, and both ate determined by shallow impurity levels. The minority carrier density is the sum of thermal and optical contributions. 

The ultraviolet cutoff or the absorption edge for pure vitreous siUca is 8.1 eV or 153 nm (171). This uv cutoff is influenced by the impurity level and stoichiometry of the material. Several impurities, such as the transition metals (Fe, Cu, Ti, etc) and alkaU metal ions (Na, Li, K), degrade the ultraviolet performance, shifting the uv cutoff to longer wavelengths. Ferric ions (Fe " ) cause absorption or result in network defects under reducing conditions. This contaminant at only a few ppm can be detected as an absorption at 230 nm and below (176). 

Table 5. Comparison of Impurity Levels in Tantalum Powders...

Residual traces of zinc are released during vacuum sintering of cemented carbides made with recovered powders. This can be troublesome when a buildup of zinc occurs in the furnace. Teledyne Advanced Materials further developed this process on a commercial basis by achieving zinc levels in the low ppm range (<30 ppm). The fact that the materials were vacuum-sintered in their original form where certain impurities are removed leads to lower impurity levels in the recovered powders. There is a slight oxidation or loss of carbon that must be compensated, otherwise the recycled powder is not in any way inferior to the original. 

Table 6. Typical Impurity Levels in Monomer Grade Vinyl Chloride ...

Production of ultra-pure barium metal has been investigated on a laboratory scale. Redistikation (23,24), zone recrystakization (25,26), and combinations of these techniques (27) have been studied. Impurity levels of less than 100 ppm have been attained. 

Product Specifications. Specifications for the maximum allowable impurity levels for borate products are given in Table 12. Where maximum levels are not set, typical values are given. Typical levels of impurities generally fall well below the maximum specification. Both borax decahydrate and pentahydrate are sometimes overdried in manufacture and may give higher than theoretical assays. 

Redistillation does not gready reduce the impurity level of volatile materials such as magnesium. Volatile alkaH metals can be separated from calcium by passing the vapors over refractory oxides such as or Cr202 to form the nonvolatile Na20 and K O (14). Purification techniques include... 

The carbon monoxide purity from the Cosorb process is very high because physically absorbed gases are removed from the solution prior to the low pressure stripping column. Furthermore, there is no potential for oxidation of absorbed carbon monoxide as ia the copper—Hquor process. These two factors lead to the production of very high purity carbon monoxide, 99+ %. Feed impurities exit with the hydrogen-rich tail gas therefore, the purity of this coproduct hydrogen stream depends on the impurity level ia the feed gas. 

Although copper catalysts were known to be highly active for this reaction for many years, it was not until the late 1960s that gas purification processes for synthesis gas were introduced that would allow the commercial use of these catalysts, which require very low sulfur, chlorine, and phosphoms feed impurity levels to maintain catalyst activity. 

Analysis and purities of the metal or compounds are determined by difference, subtracting the sum of the analyzed levels of all impurities from 100%. Analysis of impurity levels is carried out by the most appropriate technique, which may include spectroscopy, atomic absorption, and photometry. 

Table 9. Average Impurity Levels and Physical Properties of Wire-Bar Copper Samples ...

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