Reductive precipitation processes

Reduction of soluble chromium(VI) to soluble chromium(III) as insoluble chromium hydroxide is a typical example of the chemical reduction precipitation process. 

Chemical reduction alone will only convert metals from a higher valence start to a lower one to either decrease toxicity or to encourage a given chemical reaction. Explain how a combination of chemical reduction and chemical precipitation together (i.e., chemical reduction precipitation process) can remove highly toxic hexavalent chromium ions from an industrial efQuent. 

The first equation is an example of hydrolysis and is commonly referred to as chemical precipitation. The separation is effective because of the differences in solubiUty products of the copper(II) and iron(III) hydroxides. The second equation is known as reductive precipitation and is an example of an electrochemical reaction. The use of more electropositive metals to effect reductive precipitation is known as cementation. Precipitation is used to separate impurities from a metal in solution such as iron from copper (eq. 1), or it can be used to remove the primary metal, copper, from solution (eq. 2). Precipitation is commonly practiced for the separation of small quantities of metals from large volumes of water, such as from industrial waste processes. 

At one stage in our project we were surprised to learn that some workers had found difficulties in preparing the tetroxide from the dioxide, until we experienced the same trouble. This problem has now been resolved (3). Ruthenium dioxide is available commercially in both anhydrous and hydrated forms, the former being obtained by direct oxidation of ruthenium metal and the latter by a precipitation process. Only the hydrated form is oxidizable under the mild conditions (2,3) that we use and this form must be specified when purchasing the dioxide. It is noteworthy that the dioxide recovered from carbohydrate oxidations is always easily re-oxidized to the tetroxide. The stoichiometry has been determined of both the oxidation of the dioxide by periodate and reduction of the tetroxide which results on oxidation of an alcohol. 

Equations (141) and (142) describe the equilibrium between the hydrolysis of complex fluoride acids (shift to the right) and the fluorination of hydroxides (shift to the left). Near complete precipitation of hydroxides can be achieved by applying an excessive amount of ammonia. Typically, precipitation is performed by adding ammonia solution up to pH = 8-9. However, the precipitate that separates from the mother solution can be contaminated with as much as 20% wt. fluorine [490]. Analysis of niobium hydroxides obtained under different precipitation conditions showed that the most important parameter affecting the fluorine content of the resultant hydroxide is the amount of ammonia added [490]. Sheka et al. [491] found that increasing the pH to 9.6 toward the end of the precipitation process leads to a significant reduction in fluorine content of the niobium hydroxide. 

The spent salt from MSE is currently sent to an aqueous dissolution/carbonate precipitation process to recover plutonium and americium. Efforts to recover plutonium and americium from spent NaCl-KCl-MgCl2 MSE salts using pyrochemistry have been partially successful (3). Metallothermic reductions using Al-Mg and Zn-Mg alloys have been used in the past to recover plutonium and americium, and produce salts which meet plant discard limits. Attempts at direct reductions of MSE salts using... 

Precipitation Processes. Plutonium peroxide precipitation is used at Rocky Flats to convert the purified plutonium nitrate solution to a solid (14) the plutonium peroxide is then calcined to Pu02 and sent to the reduction step. The chemistry of the plutonium peroxide precipitation process is being studied, as well as alternative precipitation processes such as oxalate, carbonate, fluoride, and thermal denitration. The latter method shows the most promise for cost and waste reduction. 

The penultimate categorization of the chemical precipitation processes detailed in this chapter is reduction. The process basically involves electron transfer from different ions. The reductive precipitation may be either homogeneous (which may be ionic or non-ionic), or heterogeneous (which may be electrochemical or electrolytic). Electrolytic processes are described in Chapter 6, and no account of these need be given here. 

It is seen from Figure 6.7 that this system is much simpler, more cost-effective, and easier to operate in comparison with all other process systems discussed earlier. The treatment efficiency of the new flotation-filtration system is expected to be higher than that of the conventional reduction-precipitation system. The new flotation-filtration system also requires much less land space.15... 

In addition to the heavy metals stated in Table 22.10, ferro- and ferricyanide are also part of the pollutants in the wastewater generated in a chrome pigment plant. These wastes are generally combined and treated through reduction, precipitation, equalization, and neutralization to be followed by clarification and filtration processes. Most of the heavy metals are precipitated using lime or caustic soda at specific pH. Chromium is reduced by S02 to a trivalent form, wherein it is precipitated as chromium hydroxide at specific pH. Sodium bisulfide is also employed to precipitate some of the metals at a low pH. The treated water is recycled for plant use while the sludge is sent to landfills (Figure 22.7). 

Cementation is the process of recovery of metals from dilute aqueous solution by reductive precipitation using another metal with a more negative electrode potential, e.g., Cu + Fe° Cu° + Fe. The product, in this case cement copper, is relatively impure because of iron contamination. However, cementation can be used in conjunction with a solvent extraction flow sheet to remove small amounts of a metallic impurity, for example, removal of copper from a nickel solution by cementation with nickel powder. Here the dissolved nickel conveniently augments the nickel already in solution. 

It is state of the art to deposit the platinum catalyst on the membrane surface by a diffusion process in which platinum salt solutions (from the cathode side of the membrane) and a reductant, like hydrazine, are counter-diffusing causing reductive precipitation of dispersed platinum close to and on the surface of the membrane. The BBC Membrel cell is reported to contain a catalyst load of only 0.2 mg Pt per square centimeter (97). The cathode is reported to exhibit only from 50 to 70 mV overpotential at current densities of 1 A/cm2 and 80°C. 

The recovery or removal of metals from solutions derived from the leaching of minerals is an important step in any hydrometallurgical process. Precipitation by reduction to the metallic state in electrochemical cells will be discussed in Section 63.3.5 this section will cover the use of chemical reagents to control the precipitation process. Therefore, although the production of metallic powders by the reduction of metal ions with hydrogen or less-noble metals (cementation) is electrochemical in nature, it will be discussed under this heading. 

The make-up of the reactant solutions is an important link in the preparation of nano copper by reduction-precipitation affecting the efficiency of the process. The procedures for the two solutions are as follows ... 

Recently, workers (2) have been examining the equilibrium and kinetic factors that are important at the oxic-anoxic interface. The kinetic behavior is difficult to characterize completely due to varying rates of oxidation and absomtion above the interface and varying rates of reduction, precipitation and dissolution below the interface (2.51. Bacterial catalysis may also complicate the system (1). Although one can question the importance of abiotic thermodynamic and kinetic processes at this interface, we feel it is useful to use simple inorganic models to approximate the real system. Recently, the thermodynamics and kinetics of the H2S system in natural waters has been reviewed (0. From this review it became apparent that large discrepancies existed in rates of oxidation of H2S and the thermodynamic data was limited to dilute solution. In the last few years we have made a number of thermodynamic (7.81 and kinetic (9 101 measurements on the H2S system in natural waters. In the present paper we will review these recent studies. The results will be summarized by equations valid for most natural waters. 

Although all potentiometric measurements (except those involving membrane electrodes) ultimately are based on a redox couple, the method can be applied to oxidation-reduction processes, acid-base processes, precipitation processes, and metal ion complexation processes. Measurements that involve a component of a redox couple require that either the oxidized or reduced conjugate of the species to be measured be maintained at a constant and known activity at the electrode. If the goal is to measure the activity of silver ion in a solution, then a silver wire coupled to the appropriate reference electrodes makes an ideal potentiometric system. Likewise, if the goal is to monitor iron(UI) concentrations with a platinum electrode, a known concentration of... 

Ruthenium Black from Ruthenium Hydroxide. The ruthenium hydroxide (1 g) prepared as described above is suspended in 100 ml of water in a hydrogenation bottle and reduced with atmospheric pressure of hydrogen at room temperature or 40-50°C until black precipitates of ruthenium are separated out. The supernatant liquid, which is not always clear but is often colored brown, is decanted and the precipitate is washed thoroughly with distilled water. To obtain a catalyst with lesser amounts of alkaline or acidic impurities, the reduction-washing process is repeated until the supernatant liquid becomes neutral. 

Reductive Precipitation. Reductive precipitation involves the production of reduced species with limited solubility. An example of reductive precipitation in the environment involves the reduction of S04 to H2S and the precipitation of metals as metal-sulfides. In nature, the process of reductive precipitation is mostly microbiologically controlled. Production of H2S is the rate-controlling reaction of metal-sulfide precipitate formation. 

Phase II. Additional crystallization studies revealed that the product could be crystallized from toluene-heptane and acetone-water. The product also exhibited limited solubility in toluene. With this information at hand, both reactions were conducted in toluene as before however, during the work-up, dichloromethane was not added. Instead, aqueous hydrochloric acid was added to neutralize excess triethylamine. Heptane was then added to precipitate the product directly from the reaction mixture. The crude wet product was crystallized from acetone-water with an overall yield of 80%. The color of the product was brown. The advantages of this modification are the elimination of several extraction and back-extraction steps and a reduction in processing time and in solvent consumption. 

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