Purification chemical impurity removal

Solvent extraction—purification of wet-process phosphoric acid is based on preferential extraction of H PO by an organic solvent vs the cationic impurities present in the acid. Because selectivity of acid over anionic impurities is usually not sufficient, precipitation or evaporation steps are included in the purification process for removal. Cmde wet-process acid is typically concentrated and clarified prior to extraction to remove post-precipitated sludge and improve partition of the acid into the solvent. Concentration also partially eliminates fluoride by evaporation of HF and/or SiF. Chemical precipitation of sulfate (as Ba or Ca salts), fluorosiUcates (as Na salt), and arsenic (as sulfides) may also be used as a prepurification step preceding solvent extraction. 

Industrial examples of adsorbent separations shown above are examples of bulk separation into two products. The basic principles behind trace impurity removal or purification by liquid phase adsorption are similar to the principles of bulk liquid phase adsorption in that both systems involve the interaction between the adsorbate (removed species) and the adsorbent. However, the interaction for bulk liquid separation involves more physical adsorption, while the trace impurity removal often involves chemical adsorption. The formation and breakages of the bonds between the adsorbate and adsorbent in bulk liquid adsorption is weak and reversible. This is indicated by the heat of adsorption which is <2-3 times the latent heat of evaporahon. This allows desorption or recovery of the adsorbate from the adsorbent after the adsorption step. The adsorbent selectivity between the two adsorbates to be separated can be as low as 1.2 for bulk Uquid adsorptive separation. In contrast, with trace impurity removal, the formation and breakages of the bonds between the adsorbate and the adsorbent is strong and occasionally irreversible because the heat of adsorption is >2-3 times the latent heat of evaporation. The adsorbent selectivity between the impurities to be removed and the bulk components in the feed is usually several times higher than the adsorbent selectivity for bulk Uquid adsorptive separation. 

In the majority of impurity removal processes, the adsorbent functions both as a catalyst and as an adsorbent (catalyst/adsorbent). The impurity removal process often involves two steps. First, the impurities react with the catalyst/adsorbent under specified conditions. After the reaction, the reaction products are adsorbed by the catalyst/adsorbent. Because this is a chemical adsorption process, a severe regeneration condition, or desorption, of the adsorbed impurities from the catalyst/adsorbent is required. This can be done either by burning off the impurities at an elevated temperature or by using a very polar desorbent such as water to desorb the impurities from the catalyst/adsorbent. Applications to specific impurities are covered in the followings section. The majority of industrial applications involve the removal of species containing hetero atoms from bulk chemical products as purification steps. 

Concentrates are shipped from the uranium mill to a uranium refinery or conversion plant. Here chemical impurities are removed and the purified uranium is converted into the chemical form needed for the next step in the fuel cycle. Figure 1.14 shows concentrates being converted into uranium hexafluoride (UF ), the form used as process gas in the gaseous diffusion process for enriching U. Other possible products of a uranium refinery used in other fuel cycles are uranium metal, uranium dioxide, or uranium carbide. Uranium purification and conversion processes are also described in Chap. 5. 

Purification consists in removing from the impure uranium the rest of the nonuraniferous contaminants and producing a pure uranium compound. In most uranium reflneries purification is carried out before conversion of uranium to the chemical form finally wanted. This sequence of refining operations will be described in Secs. 9.2 through 9.6. However, in the process used by the Allied Chemical Company for producing UF from uranium concentrates, the sequence is reversed, with conversion to UF preceding purification of the impure UF by fractional distillation. This process will be described in Sec. 9.7. 

Special Chemical Methods are particularly important in cases in which impurities are difficult to remove using physical methods. In these cases, it is sometimes possible to resynthesise the substance to be purified into a material which is more readily purified. After the successful purification, the original substance must then be reconstituted. Chemical impurities can also be produced by oxygen, water, other solvents or by light, i.e. photochemically. Many organic compounds react sensitively to these influences. It is therefore often necessary to work under vacuum, in ultrahigh vacuum (UHV) or under inert gas, and to exclude light. 

Product purification. As mentioned in the introduction, this is a significant use for hydrogen peroxide, which is very diverse and cannot be covered exhaustively in this chapter. Purification may be required to remove odour, colour or chemical impurities to obtain a product of acceptable quality for sale. It may also be useful in recovery and recycle of reagents within an overall process. Some examples illustrating the breadth of current applications follow. 

The helium purification system purifies the helium primary coolant, removing fission products released from defective fuel particles, tritium and chemical impurities (HjO, CO, COj, N2, CHJ. Additionally, the helium purification system provided for helium supply (to the power module) and for helium pressure control. Two-fold redundant helium purification systems are provided for each power module. 

The helium purification system is installed for the purification of the primary coolant to specified values by removing chemical impurities (H2O, CO, CO2, N2, CH4), the supply... 

As water enters the plant, it is stored in large holding basins and allowed to settle out (Figure 10-16). A series of large pumps take suction off the basin and send water to a series of filters for additional purification. Chemicals are added to control pH and remove suspended or dissolved solids. Some filtered water is sent to demineralizers for additional treatment to remove dissolved impurities. 

Fig. 3.11 are measurements of the sequence of startup, rated operation, and shutdown of the HTTR operating test, which began on March 31,2004. The reactor power was increased in steps with monitoring aU of the parameters, ie, thermal parameters and coolant impurities. To minimize thermal stress in high-temperature components, the temperature was raised within the rate of 35°C/h when the outlet coolant temperature is less than below 650°C and 15°C/h when the coolant temperature is above 650°C. The reactor power was kept at 50% (15 MWth), 67% (20 MW,h), and 100% (30 MWth), each step for more than 2 days in a steady temperature condition in order to measure the power coefficients of the reactivity. The reactor power was also kept at 82%, at which the reactor outlet coolant temperature is slightly below 800°C, in order to remove the chemical impurity in the coolant by helium purification system. The calibration of the neutron instmmentation system with the reactor thermal power was performed at the 97% power level. 

Typical concentrations of major components and impurities in gases derived from coal are given in Table 4-4. The principal impurities removed by coal-gas purification processes are hydrogen sulfide and ammonia. These compounds are undesirable because they can cause corrosion, plugging, and ultimately, air pollution. In addition, both H2S and NH3 are relatively valuable chemicals, and their recovery and conversion to useful products, such as elemental sulfur and ammonia, can be of significant economic value. This was particularly true... 

Condensation is frequently used in the chemical industry to recover valuable products from gas streams. In many cases, it serves as the initial gas purification step by removing the bulk of the organic compounds from a gas stream before it undergoes final cleanup. As indicated in Figure 1-1, a high percentage removal of impurities by condensation can only be achieved when the inlet concentration is high (over about 5,000 ppmv). Removal efficiencies of more than about 95% are seldom attainable with condensation because the VOC content... 

Means should be provided to clean (i.e. demineralize) and purify (i.e. remove chemical impurities and fission and activation products from) the reactor coolant in all operational modes. The cleaning and purification systems for the reactor coolant should be capable of removing chemical impurities and fission and activation products from the reactor so as ... 

A.13. The pressure and inventory control system should include a purification system designed to control the chemical characteristics and activity of the coolant within specified limits by the removal of dissolved chemical impurities, radioactive substances including fission products, and suspended sohds. 

The cmde phthaUc anhydride is subjected to a thermal pretreatment or heat soak at atmospheric pressure to complete dehydration of traces of phthahc acid and to convert color bodies to higher boiling compounds that can be removed by distillation. The addition of chemicals during the heat soak promotes condensation reactions and shortens the time required for them. Use of potassium hydroxide and sodium nitrate, carbonate, bicarbonate, sulfate, or borate has been patented (30). Purification is by continuous vacuum distillation, as shown by two columns in Figure 1. The most troublesome impurity is phthahde (l(3)-isobenzofuranone), which is stmcturaHy similar to phthahc anhydride. Reactor and recovery conditions must be carefully chosen to minimize phthahde contamination (31). Phthahde [87-41-2] is also reduced by adding potassium hydroxide during the heat soak (30). 

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