Reduction Process Chemistry

Gaseous sulfur dioxide is a widely used reducing agent. The reduction occurs when sulfurous acid, produced through the reaction of sulfur dioxide and water, reacts with chromic acid as follows  

Reduction With Sodium Metabisulfite and Sodium Bisulfite 

Metabisulfite and bisulfite are used for reduction of chromium. Metabisulfite hydrolyzes to sodium bisulfite, and bisulfite in turn dissociates to sulfiirous acid, which reduces the chromium. The reaction with metabisulfite and bisulfite occurs as follows  

The reduction reaction is highly dependent on both pH and temperature. The dissociation of sodium bisulfite (NaHSOg + H2O H2SO3 + NaOH) produces sodium hydroxide (NaOH), thereby requiring acid addition for pH control during the reaction. 

A common batch system for chromium reduction with sodium bisulfite consists of a collection tank and a reaction tank with a 4-h retention time. Sodium bisulfite solution is metered into the reaction tank and the pH is controlled by sulfuric acid addition. 

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The gangue content of DRI is typically comprised of oxides such as Si02, AI2O2, CaO, MgO, Ti02, K2O, Na20, MnO, etc, and is dictated by the chemistry of the iron ore used. The phosphoms in DRI is normally in the form of P2 5- Sulfur content in the DRI depends on the sulfur level in the ore and reductant, and the amount of sulfur released or absorbed by the DRI during the reduction process. 

Addition Reactions. The addition of nucleophiles to quinones is often an acid-catalyzed, Michael-type reductive process (7,43,44). The addition of benzenethiol to 1,4-benzoquinone (2) was studied by A. Michael for a better understanding of valence in organic chemistry (45). The presence of the reduced product thiophenyUiydroquinone (52), the cross-oxidation product 2-thiophenyl-1,4-benzoquinone [18232-03-6] (53), and multiple-addition products such as 2,5-(bis(thiophenyl)-l,4-benzoquinone [17058-53-6] (54) and 2,6-bis(thiophenyl)-l,4-benzoquinone [121194-11-4] (55), is typical ofmany such transformations. 

The dramatic improvements in the physical and chemical properties of tantalum powder produced by the sodium reduction process are evident in the lessening of chemical impurities (see Table 5). The much-improved chemistry reflects the many modifications to the process put in place after 1990. 

Understanding the chemistry of the process also provides the greatest opportunity in applying the principles of inherent safety at the chemical synthesis stage. Process chemistry greatly determines the potential impact of the processing facility on people and the environment. It also determines such important safety variables as inventory, ancillary unit operations, by-product disposal, etc. Creative design and selection of process chemistry can result in the use of inherently safer chemicals, a reduction in the inventories of hazardous chemicals and/or a minimization of waste treatment requirements. 

An overview is presented of plutonium process chemistry at Rocky Flats and of research in progress to improve plutonium processing operations or to develop new processes. Both pyrochemical and aqueous methods are used to process plutonium metal scrap, oxide, and other residues. The pyrochemical processes currently in production include electrorefining, fluorination, hydriding, molten salt extraction, calcination, and reduction operations. Aqueous processing and waste treatment methods involve nitric acid dissolution, ion exchange, solvent extraction, and precipitation techniques. 

The following pages will describe several examples of pyrochemical processing as applied to the recycle of plutonium, and will briefly review the fundamental chemistry of these processes. We shall review the conversion of plutonium oxide to plutonium metal by the direct oxide reduction process (DOR),the removal of americium from metallic plutonium by molten salt extraction (MSE), and the purification of metallic... 

Applications of peroxide formation are underrepresented in chiral synthetic chemistry, most likely owing to the limited stability of such intermediates. Lipoxygenases, as prototype biocatalysts for such reactions, display rather limited substrate specificity. However, interesting functionalizations at allylic positions of unsaturated fatty acids can be realized in high regio- and stereoselectivity, when the enzymatic oxidation is coupled to a chemical or enzymatic reduction process. While early work focused on derivatives of arachidonic acid chemical modifications to the carboxylate moiety are possible, provided that a sufficiently hydrophilic functionality remained. By means of this strategy, chiral diendiols are accessible after hydroperoxide reduction (Scheme 9.12) [103,104]. 

Industrial electrochemical reduction processes exist for the conversion of 3-hydroxybenzoic acid to 3-hydroxybenzyl alcohol and 4-nitroben-zoic acid to 4-aminobenzoic acid. How may these processes be carried out Compare these processes in terms of the Principles of Green Chemistry with alternative non-electrochemical methods. 

In the broadest sense, coordination chemistry is involved in the majority of steps prior to the isolation of a pure metal because the physical properties and relative stabilities of metal compounds relate to the nature and disposition of ligands in the metal coordination spheres. This applies both to pyrometallurgy, which produces metals or intermediate products directly from the ore by use of high-temperature oxidative or reductive processes and to hydrometallurgy, which involves the processing of an ore by the dissolution, separation, purification, and precipitation of the dissolved metal by the use of aqueous solutions. 4... 

In the absence of radical traps, the radical R is converted immediately into the carbanion R by an ECE or a DISP mechanism, according to the distance from the electrode where it has been formed. B is a strong base (or nucleophile) that will react with any acid (or electrophile) present. Scheme 2.21 illustrates the case where a proton donor, BH, is present. The overall reduction process then amounts to a hydrogenolysis reaction with concomitant formation of a base. This is a typical example of how singleelectron-transfer electrochemistry may trigger an ionic chemistry rather than a radical chemistry. This is not always the case, and the conditions that drive the reaction in one direction or the other will be the object of a summarizing discussion at the end of this chapter (Section 2.7). 

Many issues in one management area are bound to affect performance in other areas. For example, an inherent safety review may propose a change in the process chemistry that will allow a definite reduction in chemical reactivity hazards, perhaps by eliminating a reactive intermediate. Such changes will have to fit with product quality requirements, and the customer may need to be included in the process of changing to the inherently safer alternative. Effective communication among all parts of the management team will avoid many problems and help identify what works best. 

Electroanalytical techniques are an extension of classical oxidation-reduction chemistry, and indeed oxidation and reduction processes occur at the surface of or within the two electrodes, oxidation at one and reduction at the other. Electrons are consumed by the reduction process at one electrode and generated by the oxidation process at the other. The electrode at which oxidation occurs is termed the anode. The electrode at which reduction occurs is termed the cathode. The complete system, with the anode connected to the cathode via an external conductor, is often called a cell. The individual oxidation and reduction reactions are called half-reactions. The individual electrodes with their half-reactions are called half-cells. As we shall see in this chapter, the half-cells are often in separate containers (mostly to prevent contamination) and are themselves often referred to as electrodes because they are housed in portable glass or plastic tubes. In any case, there must be contact between the half-cells to facilitate ionic diffusion. This contact is called the salt bridge and may take the form of an inverted U-shaped tube filled with an electrolyte solution, as shown in Figure 14.2, or, in most cases, a small fibrous plug at the tip of the portable unit, as we will see later in this chapter. 

Postma, D., C. Boesen, H. Kristiansen, and F. Larsen (1991), "Nitrate Reduction in an Unconfined Sandy Aquifer Water Chemistry, Reduction Processes, and Geochemical Modeling", Water Resources Research 27/8, 2027-2045. 

In contrast there are many examples for reduction processes on polymeric supports, because it is an especially useful transformation for aromatic nitro compounds in solid-phase chemistry. The reaction can be divided into two general classes polymer-bound substrates and polymer-bound oxidant- and reductant-reagents. 

Reduction and oxidation reactions in the subsurface environment lead to transformation of organic and inorganic contaminants. We consider chromium (Cr) as an example of an inorganic toxic chemical for which both oxidation and reduction processes may transform the valence of this element, in subsurface aqueous solutions, as a function of the local chemistry. 

FAD shares a lot of features with NAD+ and NADP+, but contains two new variants a sugar that is neither ribose nor deoxyribose, and a fairly complex heterocyclic base flavin. The new sugar is ribitol, non-cyclic because it contains no carbonyl group (see Section 12.3). The chemistry of FAD is concentrated in the flavin part, and features oxidation/reduction processes (see Box 11.14). 

Although at first sight, the Citrate Process may not appear to be in any way related to the traditional Claus, it is in fact an H2S/SO2 redox reaction in solution with the activating bauxite, carbon, or metal salt type catalyst replaced by a citrate complex with SO2. The chemistry of the process is clearly interesting and of some importance but for the purposes of this review it is sufficient to draw the analogy indicated above. The Citrate Process is yet another reduction process that may require the ancillary generation of H2S from natural gas and product sulphur if the effluent gas stream is solely SO2 as far as sulphur content is concerned. 

From the viewpoint of inorganic chemistry, the reaction of potassium nitrate with sulfur and charcoal can be described as an oxidation-reduction reaction in which electron transfer between reacting species involves a loss or gain of electrons resulting in an oxidation or reduction process respectively. 

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