Separation heavy metal

Chemical precipitation. Chemical precipitation followed by solids separation is particularly useful for separating heavy metals. The heavy metals of particular concern in the treatment of wastewaters include cadmium, chromium, copper, lead, mercury, nickel and zinc. This is a particular problem in the manufacture of dyes and textiles and in metal processes such as pickling, galvanizing and plating. 

In spite of their toxicity, alkyl phosphites have been used extensively as lubricant additives, corrosion inhibitors, and antioxidants. In addition to their use as intermediates in synthesis, organophosphorus compounds are useful for separating heavy metals by solvent extraction. Several insecticides that were formerly in widespread use are derivatives of organic phosphates. Two such compounds are malathion and parathion. 

Solvent extraction separates heavy metals (in particular, the actinides) from lighter metals and from each other and has been an important tool for nuclear chemistry over the last 60 years. As seen in the other chapters of this book, solvent-extraction chemistry remains of vital interest for nuclear fuel reprocessing and for the cleanup and segregation of nuclear waste. 

Separation of Cu(II) from an aqueous solution. Wang et al. [70] described a new method for separating heavy metal ions from dilute solution, and reported flotation of Cu(II) by CGAs. The effects of flow rate, amounts of CGAs introduced into the system, and surfactant concentration on the flotation efficiency have been investigated. The results show that the flotation efficiency at pH 5-6 has an optimum value for CGA flow rate and amount. When the pH is greater than 7, the flotation efficiency can be as high as 99% under optimum conditions. 

A patented hybrid process that combines NF with IX (or diffusion dialysis) for separating heavy metals from acids is shown in F ure 3.9. The process can treat both strong and weak adds with high or low metal concentrations. The waste feed is a spent 20 wt% sulphuric acid solution containing 9.6 g/1 aluminium. The NF permeate is purified acid. The NF membrane was Dow/Film-Tec NF45. The operating pressure was 34 bar g. Add preferentially permeated the membrane at 51% recovery. The NF concentrate... 

Hydroxyamine polymers have also been explored extmsively. Vernon and Etxles have prepared several hydroxyoxime-type polymers and recommended their use for copper separation from iron at low pH value. Amidoxime-type fonctional groups are found to chelate uranium at low concentrations in seawater and to offer a potential solution to the problem of separating heavy metals fnun acidic solutions. 

The analogous experiment in water allowed them to remove the 97 % of the initial 15 ppb of Pb ", validating the potentialities of this new type of magnetic biocompatible systems to detect and separate heavy metal toxins from different matrices. 

Due to their high efficiency, ELMs would appear to be attractive alternatives to SLMs. However ELM systems often suffer from problems of low solubility and difficulties with de-emulsification. To overcome these problems, a new liquid membrane process, the emulsion free liquid membrane (EI M), was developed by Kumar ef al. (60). Application of this novel technique to hydrometallurgical processes is quite promising (67). Recent studies of separating heavy metals, such as Cr(VI) and Cu(II) from electroplating effluents, with an EFLM system indicate that the technique is very efficient and superior to other types of LMs. 

Ozaki H, Sharma K, Saktaywin W (2002) Performance of an ultra-low-pressure reverse osmosis membrane (ULPROM) for separating heavy metal effects of interference parameters. 

The processes that can be used to separate heavy metals from industrial liquid waste streams are given in Table 3.4, processes that can treat various tars or solids are shown in Table 3.5, and processes suitable for treating slurries and sludges are listed in Table 3.6. 

TABLE 3.4 Processes That Separate Heavy Metals from Liquid Waste Streams... 

Heavy metals often can be removed effectively by chemical precipitation in the form of carbonates, hydroxides, or sulfides. Sodium carbonate, sodium bisulfite, sodium hydroxide, and calcium oxide are all used as precipitation agents. The solids precipitate as a floe containing a large amount of water in the structure. The precipitated solids need to be separated by thickening or filtration and recycled if possible. If recycling is not possible, then the solids are usually disposed of to a landfill. 

Reverse osmosis is a high-pressure membrane separation process (20 to 100 bar) which can be used to reject dissolved inorganic salt or heavy metals. The concentrated waste material produced by membrane process should be recycled if possible but might require further treatment or disposal. 

Finally, micellar systems are useful in separation methods. Micelles may bind heavy-metal ions, or, through solubilization, organic impurities. Ultrafiltration, chromatography, or solvent extraction may then be used to separate out such contaminants [220-222]. 

The ferrous ions that dissolve from the anode combine with the hydroxide ions produced at the cathode to give an iron hydroxide precipitate. The active surface of ferrous hydroxide can absorb a number of organic compounds as well as heavy metals from the wastewater passing through the cell. The iron hydroxide and adsorbed substances are then removed by flocculation and filtration. The separation process was enhanced by the addition of a small quantity of an anionic polymer. 

The bubble size in these cells tends to be the smallest (10 to 50 Im) as compared to the dissolved-air and dispersed-air flotation systems. Also, very httle turbulence is created by the bubble formation. Accordingly, this method is attractive for the separation of small particles and fragile floes. To date, electroflotation has been applied to effluent treatment and sludge thickening. However, because of their bubble generation capacity, these units are found to be economically attractive for small installations in the flow-rate range of 10 to 20 mVh. Electroflotation is not expected to be suitable for potable water treatment because of the possible heavy metal contamination that can arise due to the dissolution of the electrodes. 

By using an anionic collector and external reflux in a combined (enriching and stripping) column of 3.8-cm (1.5-in) diameter with a feed rate of 1.63 ni/n [40 gal/(h ft )] based on column cross section, D/F was reduced to 0.00027 with C JCp for Sr below 0.001 [Shou-feld and Kibbey, Nucl. AppL, 3, 353 (1967)]. Reports of the adsubble separation of 29 heavy metals, radioactive and otheiwise, have been tabulated [Lemlich, The Adsorptive Bubble Separation Techniques, in Sabadell (ed.), Froc. Conf. Traces Heavy Met. Water, 211-223, Princeton University, 1973, EPA 902/9-74-001, U.S. EPA, Reg. 11, 1974). Some separation of N from by foam fractionation has been reported [Hitchcock, Ph.D. dissertation. University of Missouri, RoUa, 1982]. 

SEPARATION OF HEAVY METALS IN AQUEOUS SOLUTIONS BY A NEW ION EXCHANGER BASED ON CELLULOSE... 

Sodium trimetaphosphate was used as an eluting agent for the removal of heavy metals such as Pb, Cd, Co, Cu, Fe, Ni, Zn and Cr from aqueous solutions. Distribution coefficients of these elements have been determined regarding five different concentrations of sodium trimeta phosphate (3T0 M 5T0 M 0.01 M 0.05 M 0.1 M) on this resin. By considering these distribution coefficients, the separation of heavy metals has been performed using a concentration gradient of 3T0 - 5T0 M sodium trimetaphosphate. Qualitative and quantitative determinations were realized by ICP-AES. 

Flocculation and sedimentation arc two processes used to separate waste streams that contain both a liquid and a solid phase. Both are well-developed, highly competitive processes, which arc oflcii used in the complete treatment of waste streams. They may also be used instead of, or in addition to, filtration. Some applications include the removal of suspended solid particles and soluble heavy metals from aqueous streams. Many industries use both processes in the rcmowal of pollutants from their wastewaters. These processes work best when the waste stream contains a low concentration of the contaminating solids. Although they are applicable to a wide variety of aqueous waste streams, these processes arc not generally used to treat nonaqueous or semisolid waste streams such as sludges and slurries. 

Residues containing high levels of heavy metals are not suitable for catalytic cracking units. These feedstocks may be subjected to a demetallization process to reduce their metal contents. For example, the metal content of vacuum residues could be substantially reduced by using a selective organic solvent such as pentane or hexane, which separates the residue into an oil (with a low metal and asphaltene content) and asphalt (with high metal content). Demetallized oils could be processed by direct hydrocatalysis. 

Deactivation of zeolite catalysts occurs due to coke formation and to poisoning by heavy metals. In general, there are two types of catalyst deactivation that occur in a FCC system, reversible and irreversible. Reversible deactivation occurs due to coke deposition. This is reversed by burning coke in the regenerator. Irreversible deactivation results as a combination of four separate but interrelated mechanisms zeolite dealu-mination, zeolite decomposition, matrix surface collapse, and contamination by metals such as vanadium and sodium. 

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