Chromium , reduction

C02 Chemical Precipitation - Sulfide C09 Chemical Precipitation - Other Cl 1 Neutralization C21 Chromium Reduction C31 Complexed Metals Treatment (other than pH Adjustment)... 

For metal compounds, the calculation of the reportable concentration and treatment efficiency is based on the weight ot the parent metal, not on the weight of the metal compounds Metals are not destroyed, only physically removed or chemically converted from one form into another. The treatment efficiency reported represents only physical removal of the parent metal from the wastestream, not the percent chemical conversion of the metal compound. If a listed treatment method converts but does not remove a metal (e.g., chromium reduction), the method must be reported, but the treatment efficiency must be reported as zero. 

C21 Chromium Reduction R02 Reuse as Fuel - Industrial Furnace... 

A 5% solution of chromium trioxide-pyridine complex in dry methylene chloride is prepared. The alcohol (0.01 mole) is dissolved in dry methylene chloride and is added in one portion to the magnetically stirred oxidizing solution (310 ml, a 6 1 mole ratio) at room temperature. The oxidation is complete in 5-15 minutes as indicated by the precipitation of the brownish black chromium reduction products. The mixture is filtered and the solvent is removed (rotary evaporator) leaving the crude product, which may be purified by distillation or recrystallization. Examples are given in Table 1.1. 

Appendix D presents the U.S. EPA data on a full-scale treatment of aluminum forming waste-water by chromium reduction, chemical precipitation, and sedimentation clarification. Chromium reduction, as described in detail by Wang, Hung and Shammas,9 is an important step prior to... 

It should be noted that dissolved air flotation (DAF) is a more effective process for clarification.8-10 As shown in Appendix D, with an additional step of chromium reduction, the secondary treatment system effectively removed chromium (over 99%), copper (89%), cadmium (64%), lead (67%), and zinc (77%). 

Full Scale Treatment of Aluminum Forming Wastewater by Chromium Reduction, Chemical Precipitation, and Sedimentation Clarification... 

Treatment for the removal of chromium and nickel from electroplating wastewater involves neutralization, hexavalent chromium reduction, pH adjustment, hydroxide precipitation, and final solid-liquid separation.15 37 48... 

Chromium hydroxide is an amphoteric compound and exhibits minimum solubility in the pH range of 7.5 to 10.0. Effluents from chromium reduction processes should be neutralized to the range of zero solubility (pH 8.5 to 9.0) to minimize the amount of soluble chromium remaining in solution. 

The first system, shown in Figure 6.6, is identical to the conventional reduction-precipitation in chemistry (i.e., neutralization, chromium reduction, pH adjustment, metal hydroxide precipitation, and so on). However, a flotation-filtration clarifier (Tank T101SF, as shown in Figure 6.6) is used. The unit consists of rapid mixing, flocculation, high-rate DAF, and sand filtration.1557... 

The process flow scheme consists of chromium reduction, lime precipitation, and clarification. 

The sources of theories and principles for chromium reduction using an acid, chemical precipitation using a base, and clarification are detailed in Refs. 8 to 10. 

The acid used for chromium reduction is sulfuric acid. 

Hexavalent chromium reduction through the use of sulfur dioxide and sodium metabisulfite has found the widest application in the metal finishing industry. It is not truly a treatment step, but a conversion process in which the hexavalent chromium is converted to trivalent chromium. The hexavalent chromium is reduced through the addition of the reductant at a pH in the range of 2.5-3 with a retention time of approximately 30-40 min (Figure 9.7). 

FIGURE 9.7 Hexavalent chromium reduction. (Adapted from U.S. EPA, Meeting Hazardous Waste Requirements for Metal Finishers, Report EPA/625/4-87/018, U.S. Environmental Protection Agency, Cincinnati, OH, 1987.)... 

Option 1 Emulsion breaking and oil separation by skimming, cyanide oxidation, chromium reduction, chemical precipitation and sedimentation, and sludge drying beds. 

System component Reaction tanks, a reagent storage and feed system, mixers, sensors, and controls for continuous chromium reduction. A single above-ground concrete tank with retention time of 45 min is provided. For batch operation, dual above-ground concrete tanks with 4h retention time are provided. 

Figure 10. Photocatalytic activity of Au/titania nanocomposites containing 0 to 5 % Au in phenol-oxidation and chromium-reduction reactions [57].

Ishibashi, Y., Cervantes, C. Silver, S. (1990). Chromium reduction in Pseudomonasputida. Applied and Environmental Microbiology, 56, 2268-70. 

Sodium metabisulfite or bisulfite is a commonly used chemical for chromium reduction. The metabisulfite hydrolyzes to sodium bisulfite, and the bisulfite in turn dissociates to sulfurous acid, which reduces the hexavalent chromium at a pH of 2-3. 

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