Chromium removal

The sodium chlorate manufacturing process can be divided into six steps (/) brine treatment 2 electrolysis (J) crystallisation and salt recovery (4) chromium removal (5) hydrogen purification and collection and (6) electrical distribution. These steps are outlined in Figure 3. 

Chromium Removal System. Chlorate manufacturers must remove chromium from the chlorate solution as a result of environmental regulations. During crystallization of sodium chlorate, essentially all of the sodium dichromate is recycled back to the electrolyzer. Alternatively, hexavalent chromium, Cr, can be reduced and coprecipitated in an agitated reactor using a choice of reducing agents, eg, sodium sulfide, sulfite, thiosulfate, hydrosulfite, hydrazine, etc. The product is chromium(III) oxide [1333-82-0] (98—106). Ion exchange and solvent extraction techniques have also... 

Chemical precipitation is used in porcelain enameling to precipitate dissolved metals and phosphates. Chemical precipitation can be utilized to permit removal of metal ions such as iron, lead, tin, copper, zinc, cadmium, aluminum, mercury, manganese, cobalt, antimony, arsenic, beryllium, molybdenum, and trivalent chromium. Removal efficiency can approach 100% for the reduction of heavy metal ions. Porcelain enameling plants commonly use lime, caustic, and carbonate for chemical precipitation and pH adjustment. Coagulants used in the industry include alum, ferric chloride, ferric sulfate, and polymers.10-12... 

Other aquatic weeds such as reed mat, mangrove (leaves), and water lily (Nymphaceae family plants) have been found to be promising biosorbents for chromium removal. The highest Cr(III) adsorption capacity was exhibited by reed mat (7.18 mg/g), whereas for Cr(VI), mangrove leaves showed maximum removal capacity (8.87 mg/g) followed by water lily (8.44 mg/g). It is interesting to mention that Cr(VI) was reduced to Cr(III), with the help of tannin, phenolic compounds, and other functional groups on the biosorbent, and subsequently adsorbed. Unlike the results discussed previously for the use of acidic treatments, in this case, such treatments significantly increased the Cr(VI) removal capacity of the biosorbents, whereas the alkali treatment reduced it.118... 

Kleiman, I.D. and Cogliatti, D.H., Chromium removal from aqueous solutions by different plant species, Environmental Technology, 19 (11), 1127-1132, 1998. 

The ilmenite production from heavy mineral sands exclusively utilizes a physical separation method using magnetic separation, gravity concentration and electrostatic separation. Flotation is practiced mainly for beneficiation of fine mineral sands containing rutile, ilmenite and zircon. The ilmenite that is produced in a number of operations in Western Australia, India and the USA is high in chromium, which makes the ilmenite unusable. This section discusses a new process that was developed for chromium removal from ilmenite concentrates. 

The final flowsheet that was developed for chromium removal is shown in Figure 25.13. The concentrate was scrubbed with alkaline followed by desliming. The deslimed concentrate was subjected to chromium flotation followed by a single cleaning stage. 

Bulatovic, S., Chromium Removal from the Ilmenite Concentrate by Flotation from RZM Western Australia, Report of Investigation, 1993. 

Chemical precipitation with sedimentation or filtration Relies on proven technology limited installation for chromium removal 192,000 64,000 Effectiveness limited low removal efficiencies are reported in literature. 

The reverse reaction has also been studied (18). Treating the calcined Cr03/silica with dry HC1 strips off the chromium as chromyl chloride vapor leaving one OH for each former point of attachment. The silica base is otherwise unaffected. Hydroxyl measurements before and after the stripping give a direct indication of the bonding of Cr(VI), because chromate leaves two hydroxyls per chromium removed and dichromate leaves only one. 

Key words - adsorption from solution, activated carbon fibers, chromium removal. 

Usually the activated carbon granules are employed for chromium removal or for catalysts preparation. However in some previous reports the perspectives and advantages of activated carbon fibers (ACF) utilization for the same employment have been documented [2,4]. It seems from the analysis of the articles that the use of ACF or activated carbon cloth has a great potential. 

Figure 4 compares the amount of total chromium removed by ACF-1 at different temperatures for saturation time 48 h, pH=2 and initial concentration of Cr(Vl) in solution 200 mg/i. 

The inverse relationship between the amount of chromium ions removed and the temperature was found. This fact can be explained in the consideration that the amount of Cr(VI) reduced to Cr(lll) increases with the increase of the temperature and that the adsorption of Cr(lll) ions on ACF-1 is relatively low. So, more the Cr(lll) ions appears in the solution during ACF and Cr(Vl) species contact less the total amount of chromium removed. 

Thus, the factors which affect to Cr(VI) and Cr(lII) removal by ACF are not only temperature and initial concentration but also the oxii ion/reduction ability of fibers and theirs oxidation state before and during adsorption. The ACF used in the study can be useful to remove Cr(VI) and Cr(III) from aqueous solution. To ameliorate the rate of chromium removal it seems better to use at the same time the oxidized and non-oxidized ACF. The most usefid porous texture of ACF is when the total pore volume is more than... 

Ho, W.S., Supported liquid membrane process for chromium removal and recovery, US Patent No. 6,171,563, 2001. 

Moreno-Castilla and coworkers [139,140] did clarify the relationship between carbon surface chemistry and chromium removal. Table 3 summarizes some of the key results. Upon oxidation of carbon M in nitric acid (sample MO), the surface has become much more hydrophilic and more acidic, and the uptakes increased despite a decrease in total surface area. The enhancement in Cr(III) uptake was attributed to electrostatic attraction between the cations and the negatively charged surface. The enhancement in Cr(VI) uptake (at both levels of salt concentration) was attributed to its partial reduction on the surface of carbon MO (perhaps due to the presence of phenolic or hydroquinone groups), which is favored by the lower pH. The increase in uptake on carbon MO with increasing NaCl concentration is consistent with this explanation, from a straightforward analysis of the Debye-Hvickel and Nemst equations the decrease in uptake on carbon M was attributed to the competition of specifically adsorbed Cl and CrOj- ions on the positively charged surface. 

SchiffI H, Weidmann P, Weiss M, et al. Dialysis treatment of acute chromium intoxication and comparativeefficacy of peritoneal versus hemodialysis In chromium removal. Miner Electrolyte Metab. 1982 7 28-35... 

M. Pansini, C. Colella, and M. De Gennaro, Chromium removal from water by ion exchange using zeolite, Desalination 83, 145-157 (1991). 

In order to evaluate the utility of the removal model for additional metals, additional removal studies were conducted using Chromium and Cobalt in place of copper. Chromium removal was studied using solutions made by dissolving CrO, in dilute H2SO4 to produce solutions with chromium concentrations of 80 ppm. The oxidation state of the chromium in the resulting solutions was not determined. Intraparticle diffusivities of Cr, Co", and Cu were calculated with the removal model, and are compared in Figure 7. The removal model for Dowex XFS 4195.02 appears to be generally applicable to most metals. 

Dantas Neto, A.A., de Castro Dantas, T.N. and Alencar Moura, M.C.P. (2004) Evaluation and optimization of chromium removal from tannery effluent by microemulsion in the Morris extractor. /. Hazard. Mater., B114(l-3), 115-122. 

Ostertag s committee painted the picture of water pollution in shades quite different from those Dickey had used. Industrial wastes received equal billing with sewage. While the committee repeated the usual platitudes about industry s desire to reduce pollution, the facts it compiled made clear that real progress was slow. Numerous sites of industrial pollution were mapped, with chemical plants in Buffalo seen as a particular problem. Aside from the Long Island chromium removal systems, only three major industrial treatment facilities were built anywhere in the state in 1948. The total construction cost of needed industrial waste treatment... 

More SV, John S, Rao BS, Nair BU and Laxman RS (2001) Chromium removal and reduction in COD of tannery effiuents by actinomycetes. Indian J Environ Health 43108-113. 

Low pH favors rapid reduction of Cr(VI) to Cr(III). For example, James and Bartlett (1983) reported greater reduction of soluble Cr(VI) in unlimed (pH 5.3) soils than limed (pH 6.5) soils. Soil pH can also affect the form of Cr(VI) adsorbed on soil colloids. Below pH 6.4, HCrO is the dominant chromium form in aqueous solution. Above pH 6.4, HCrO dissociates to CrO. For then-adsorption studies, James and Bartlett (1983) reported that increasing soil pH from 5.4 to 7.0 by liming decreased the amount of chromium removed as phosphate exchangeable form from 7.2 to... 

In an effort to increase the reactivity of Fe° towards chromium removal, acid washed zero-valent iron [AW-Fe(O)] was evaluated under groundwater geochemistry conditions. It was found that AW-Fe(O) could remove Cr(VI) from synthetic ground-water in the absence of bicarbonate, magnesium, and/or calcium ions. The presence of bicarbonate alone had the mildest impact on adsorption while co-presence with calcium had a pronounced negative impact. In comparison with unwashed Fe(0), the AW-Fe(O) displayed a poorer Cr(VI) removal capability [ 57% to 77% drop in capacity (53)]. 

Wilkin RT, Su C, Ford RG, Paul CJ. (2005). Chromium removal processes during ground-water remediation by a zero valent iron permeable reactive barrier. Environmental Science and Technology 39(12) 4599-4605. 

The demonstration results indicate that approximately 200 g of hexavalent chromium was removed during about 700 h of system operation, and the average removal rate for the entire system was approximately 0.29 g/h. Nevertheless, a reasonable estimate of the chromium removal based on the pre- and posttreatment soil analysis was not possible in this case due to the nonhomogenous distribution of chromate concentration in pretreated soil and because the demonstration was not carried out until the completion. 

Chromium compounds are widely used in many industries metal finishing and electroplating, leather tanning, pigments manufacturing, photography and catalysts production [1]. The presence of chromium species in wastewater of all of these industries is a problem because of the affect onto die human physiolo. Chromium removal from wastewaters by adsorption onto activated charcoals is an important process in the environmental protection [2]. On the other hand the chromium species adsorption from aqueous solution is one of the processes for chromium catalysts supported on activated c ons production [3],... 

Campbell, H.J.. Jr N.C. Scrivner, K. Batzar, R.F. White, "Evaluation of Chromium Removal from a Highly Variable Wastewater Stream", Proceedings of the 32nd Purdue Industrial Waste Conference, Purdue University, Ann Arbor Science (1977)... 

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