childrens_club_seminar_-2009

Specific Industrial Waste Treatments Chemical Industry: 

Specific Industrial Waste Treatments Chemical Industry P. RAMANA MURTHY

Introduction: 

Introduction The chemical industry comprises the companies that produce industrial chemicals. It is central to modern world economy, converting raw materials (oil, natural gas, air, water, metals, minerals) into more than 70,000 different products. The chemical industry involves the use of chemical processes such as chemical reaction and refining methods to produce a wide variety of solid, liquid, and gaseous materials. Most of these products are used in manufacture of other items, although a smaller number are used directly by consumers. Solvents, pesticides, dye, washing soda, and portland cement are a few examples of product used by consumers. The industry includes manufacturers of inorganic and organic industrial chemicals, ceramic products, petrochemicals, agrochemicals, polymers and rubber, oleochemicals (oils, fats, and waxes), explosives, fragrances flavors and pharmaceutical industry.Other closely related industries include petroleum, glass, paint, ink, sealant, adhesive, and food processing manufacturers. The chemical industry comprises the companies that produce industrial chemicals. It is central to modern world economy, converting raw materials (oil, natural gas, air, water, metals, minerals) into more than 70,000 different products. The chemical industry involves the use of chemical processes such as chemical reaction and refining methods to produce a wide variety of solid, liquid, and gaseous materials. Most of these products are used in manufacture of other items, although a smaller number are used directly by consumers. Solvents, pesticides, dye, washing soda, and portland cement are a few examples of product used by consumers. The industry includes manufacturers of inorganic and organic industrial chemicals, ceramic products, petrochemicals, agrochemicals, polymers and rubber, oleochemicals (oils, fats, and waxes), explosives, fragrances flavors and pharmaceutical industry.Other closely related industries include petroleum, glass, paint, ink, sealant, adhesive, and food processing manufacturers.

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In the chemical industry, waste is either considered inert or contaminated.  Inert waste can be recycled or released into the environment.  Malformed polymer or leaking steam may be considered inert wastes, although they are not chemically inert. Treating wastewater is certainly an expensive endeavor.  In short, there is much motivation to minimize waste in the chemical and other industries. The first priority is to reduce the amount of waste generated. Again manufacturing processes can be modified to yield substantial waste reduction. For example changes in the chemical reaction conditions can minimize production of by-product hazardous substances. In some instances potentially hazardous catalysts, can be replaced by catalysts that are non hazardous or that can be recycled rather than discarded. Many industrial processes that use solvents are equipped for solvent recycle for reasons of both economics and pollution control. A number of operations are used in solvent purification. Entrained solids are removed by settling, filtration, or centrifugation. Even with after vigorous hazardous waste reduction program, there will still be large quantities of hazardous wastes that will require treatment and disposal. The treatment technologies have been categorized as physical, chemical, biological, thermal, or stabilization.

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Physical treatment processes include gravity separation, phase change systems, such as air and steam stripping of volatiles from liquid wastes, and various filtering operations, including carbon adsorption. Chemical treatment transforms waste into less hazardous substances using such techniques as pH neutralization, oxidation or reduction, and precipitation. Biological treatment uses microorganisms to degrade organic compounds in the waste stream. Thermal destruction processes include incineration, which is increasingly becoming a preferred option for the treatment of hazardous wastes, and pyrolysis, which is the chemical decomposition of waste is brought about by heating the material in the absence of oxygen Stabilization techniques involve removal of excess of water from a waste and solidifying the remainder either by mixing it with a stabilizing agent such as Portland cement, or vitrifying it to a glassy substance

Physical methods of treatment of hazardous wastes: 

Physical methods of treatment of hazardous wastes Electrodialysis Reverse osmosis Adsorption Sedimentation Solvent extraction Distillation Evaporation Filtration Flocculation

Electrodialysis (ED) : 

Electrodialysis (ED) What is Electrodialysis (ED)? Electrodialysis is an electro membrane process in which ions are transported through ion permeable membranes from one solution to another under the influence of a potential gradient. The electrical charges on the ions allow them to be driven through the membranes fabricated from ion exchange polymers. Applying a voltage between two end electrodes generates the potential field required for this. Since the membranes used in electro dialysis have the ability to selectively transport ions having positive or negative charge and reject ions of the opposite charge, useful concentration, removal, or separation of electrolytes can be achieved by electro dialysis.

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The ion permeable membranes used in electro dialysis are essentially sheets of ion-exchange resins. They usually also contain other polymers to improve mechanical strength and flexibility. The resin component of a cation-exchange membrane would have negatively charged groups (e.g., -SO3-) chemically attached to the polymer chains (e.g. styrene/divinylbenzene copolymers). Ions with a charge opposite to the fixed charge (counter ions) are freely exchanged at these sites. The concentration of counter ions (e.g., Na+) is relatively high; therefore, counter ions carry most of the electric current through the membrane. The fixed charges attached to the polymer chains repel ions of the same charge (co-ions), in this case the anions. Since their concentration in the membrane is relatively low, anions carry only a small fraction of the electric current through a cation permeable membrane. Attachment of positive fixed charges (e.g., -NR3+ or C5H5N+R where commonly R = CH3) to the polymer chains forms anion permeable membranes , which are selective to transport of negative ions, because the fixed -NR3+ groups repel positive ions. This exclusion, as a result of electrostatic repulsion, is called Donnan exclusion . Ion Permeable Membranes

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The process depends on special synthetic membranes, usually based on ion exchange resins which are permeable only to a single type of ion. Cation exchange membranes permit passage only of positive ions, under the influence of electric field, while anion exchange membranes permit passage only of negatively charged ions. A dc potential applied across the stack causes the positive and negative ions to migrate in opposite directions. This technique has been used for desalination to produce potable water from brackish well water. In food industry electrodialysis is used for desalting whey and deashing sugars. The chemical industry uses this technique for enriching or depleting solutions, and for removing mineral constituents from product streams.

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Reverse osmosis Reverse osmosis process Reverse osmosis is a filtration process that is often used for water. It works by using pressure to force a solution through a membrane , retaining the solute on one side and allowing the pure solvent to pass to the other side. This is the reverse of the normal osmosis process, which is the natural movement of solvent from an area of low solute concentration, through a membrane, to an area of high solute concentration when no external pressure is applied

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This technique which is most widely used consists of a membrane permeable to solvent but impermeable to most dissolved species, both organic and inorganic. These devices use pressure to force the contaminated water against the semi permeable membrane. The membrane acts as a filter, allowing the water to be pushed through the pores, but restricting the passage of larger molecules that are to be removed. The reverse osmosis technique has been widely used for desalination of sea or brackish water It has also been successfully used in the treatment of electroplating rinse waters, not only to meet effluent discharge standards, but also to recover concentrated metal salt solutions for reuse. It has also been used for treatment of waste stream from paper and food processing industries.

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Adsorption Adsorption is a process that occurs when a gas or liquid solute accumulates on the surface of a solid or a liquid (adsorbent), forming a film of molecules or atoms (the adsorbate ). The factors that relate to adsorption capacity are: • Greater surface area produces greater adsorption capacity [e.g.: activated carbon has large surface area (500-1500 m 2 /g)] • Adsorptivity increases as the solubility of the solute (in solvent) decreases. thus, for hydrocarbons, adsorption increases with molecular weight. • For solutes with ionisible groups, maximum adsorption will be achieved at a pH corresponding to minimum ionization. • Adsorption capacity decreases with increasing temperature

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Carbon adsorption is applicable only to single phase fluid waste streams, e.g., liquid solutions or gas mixtures. In actual applications, both aqueous and nonaqueous liquids are treated with carbon. the nonaqueous streams include petroleum fractions, syrups, animal and vegetable oils, and pharmaceutical preparations, color removal is the most common objective in such cases.

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Distillation Distillation is expensive and energy intensive and can probably be justified only in cases where valuable product recovery is feasible (e.g., solvent recovery). This technique has only limited application in the treatment of dilute aqueous hazardous wastes . Distillation is a method of separating mixtures based on differences in their volatilities in a boiling liquid mixture. Distillation is a unit operation, or a physical separation process, and not a chemical reaction. 1: Heat source 2: Still pot 3: Still head 4: Thermometer/Boiling point temperature 5: Condenser 6: Cooling water in 7: Cooling water out 8: Distillate/receiving flask 9: Vacuum/gas inlet 10: Still receier 11: Heat control 12: Stirrer speed control 13: Stirrer/heat plate 14: Heating (Oil/sand) bath 15: Stirrer bar/anti-bumping granules 16: Cooling bath.

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Evaporation Evaporation process is used for the treatment of hazardous waste such as radioactive liquids and sludge's and concentrating of plating and paint solvent waste among many other applications. It is capable of handling liquids, slurries and sometimes sludge's, both organic and inorganic, containing suspended or dissolved solids or dissolved liquids, where one of the components is essentially non volatile. It can be used to reduce waste volume prior to land fill disposal or incineration. The major disadvantages of evaporation are high capital and operating costs and high energy requirements. This process is more adaptable to waste waters with high concentrations of pollutants

Chemical methods of treatment of hazardous wastes: 

Chemical methods of treatment of hazardous wastes Chemical Oxidation and reduction Acid-base neutralization Chemical precipitation Hydrolysis Ion exchange Thermal treatment methods Photolysis Biological treatment of hazardous wastes

Chemical Oxidation and reduction : 

Chemical Oxidation and reduction Oxidation reduction methods provide another important chemical treatment alternative for hazardous wastes. One important chemical redox treatment involves the oxidation of cyanide wastes from metal finishing industry, using chlorine in alkali solution. In this reaction CN - is first converted to a less toxic cyanate. Further chlorination oxidizes the cyanate to simple carbon dioxide and nitrogen gas. NaCN + Cl 2 + 2NaOH →NaCNO + 2NaCl +H2O 2NaCNO + 3Cl 2 + 4NaOH → 2CO 2 + N 2 + 6NaCl +2H 2 O Another important redox treatment process is the reduction of hexavalent chromium Cr (VI) to trivalent chromium Cr (III) in large electroplating operations Sulphur is used as the reducing agent and the reactions are as follows. 3 SO 2 + 3 H 2 O → 3 H 2 SO 4 2 CrO 3 + 3 H 2 SO 4 → Cr 2 SO 4 + 3 H 2 O

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Iron (II) in solution can be precipitated as ferric hydroxide by oxidation. 4 Fe 2+ ­- O 2 + 10 H 2 O → 4 Fe (OH) 3 + 8 H + In a similar way sulphurdioxide is oxidised to sulphuric acid 2SO 2­ +O 2 + 2H 2 O → 2H 2 SO 4 A large variety of oxidisable contaminants in waste water and sludges are oxidised by ozone which can be generated on site by an electrical discharge through dry air or oxygen. (CH 2 O) + 2[O]→CO 2 +H 2 O CH 3 CHO+ [O] →CH 3 COOH

Acid-base neutralization : 

Acid-base neutralization Hazardous wastes are categorized as corrosive when their solution pH is less than 2 or more than 12.5. Such wastes can be chemically neutralised. Generally acidic wastes are neutralised with slaked lime [Ca(OH)2] in a continuously stirred chemical reactor. The rate of addition of lime is controlled by feed back control system which monitors pH during addition. Lime is least expensive and is widely used for treating acidic wastes. Since the solubility of lime in water is limited, solution of excess lime do not reach extremely high pH values. Alkaline wastes may be neutralised by adding sulphuric acid. It is a relatively inexpensive acid. For some applications acetic acid is preferable since it is non toxic and biodegradable. Alkaline wastes can also be neutralised by bubbling gaseous carbondioxide forming carbonic acid. The advantage of CO 2 is that it is often readily available in the exhaust gas from any combustion process at the treatment site.

Chemical precipitation : 

Chemical precipitation This technique can be applied to almost any liquid waste stream containing a perceptible hazardous constituent By properly adjusting pH, the solubility of toxic metals can be decreased, leading to the formation of a precipitate that can be removed by settling and filtration. Lime [Ca (OH) 2 ] or caustic soda is used for precipitation of the metal ions as metal hydroxides M +2 + Ca (OH) 2  M (OH) 2 + Ca +2 Chromium is precipitated as hydroxide. Cr 3+ + (OH) - → Cr (OH) 3 . Sodium carbonate also has been used to precipitate metals as hydroxides (Fe(OH) 3 .XH 2 O), carbonates (CdCO 3 ), basic carbonate salts (2PbCO 3 .Pb(OH) 2 ). Carbonate ion hydrolyses in water to give hydroxide ion. CO3 2- + H 2 O → HCO 3- + OH − Reducing agents such as sodium borohydride can be used to precipitate the metal ions from solution in the elemental form. 4Cu 2+ + NaBH 4 + 2H 2 O → 4Cu + NaBO 2 + 8H+

Hydrolysis : 

Hydrolysis Hydrolysis treatment can be given to those hazardous waste constituents which are very reactive with water. Examples of those substances are halides, carbide, hydride, alkoxide, and active metal. SiCl 4 + 2H 2 O → SiO 2 + 4HCl CaC 2 + 2H 2 O → Ca(OH) 2 + C 2 H 2 NaAlH 4 + 4H 2 O → 4H 2 +NaOH+ Al(OH) 3 NaOC 2 H 5 +H 2 O → NaOH+ C 2 H 5 OH Ca + 2H 2 O → Ca(OH) 2 + H 2

Ion exchange : 

Ion exchange Ion exchange is judged to have some potential for the application of interest in situations where it is necessary to remove dissolved inorganic species. This is the use of anion exchanges for the removal of anionic nickel cyanide complex and chromate ions from waste solutions. 2Res + OH - + [Ni (CN) 4] 2- → (Res + ) 2 [Ni (CN) 4] 2- + 2OH - 2Res + OH - + CrO4 2- → (Res + ) 2 (CrO 4 2- ) + 2OH - Ion -exchange resins have also been used in the removal of radionuclide from radioactive wastes.

Thermal treatment methods : 

Thermal treatment methods Thermal incineration is a process that uses high-temperature thermal oxidation to convert a waste to a less bulky, less toxic or less noxious material It can be considered as a volume-reduction process in that many of the component elements of organic materials, including the most common ones (carbon, hydrogen and oxygen) are converted wholly or partially to gaseous form, leaving only the non combustible inorganic volume. The principal products of incineration are carbon dioxide, water vapour and ash C (organic) + O 2 → CO 2 + heat 4H (organic) + O 2 → 2 H 2 O + heat

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The solid and liquid effluents may require treatment prior to ultimate disposal or discharge. The most critical factors that determine complete combustion of hazardous wastes are High combustion temperatures above about 900 o C to ensure that the thermally resistant compounds react. (ii) Availability of sufficient amount of oxygen for combustion. (iii) Sufficient residence time to allow the reactions to occur.

Photolysis : 

Photolysis In photolysis, chemical bonds are broken under the influence of light. In primary photochemical process, the target species is converted to an electronically excited state, usually a diradical, which is sufficiently energetic to undergo chemical reaction The fate of the excited molecule and therefore the effectiveness of a photolysis treatment process, depends on its chemical structure and on the medium in which it is carried out For the photolysis process to be effective in the treatment of hazardous wastes stream, the radiation source must be sufficiently energetic, must be absorbed by the target species and the final photochemical products must be less toxic

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To date much of the research work on the treatment of highly toxic wastes has centered on two types of constituents Polychlorinated biphenyls (PCBs) and chlorinateddibenzo-p-dioxins (CDDs) eg: tetrachloro dibenzo-p-dioxin (TCDD). The three requirements of photolysis of TCDD are 1) Dissolution in a light transmitting film 2) Presence of organic hydrogen odour 3) Ultraviolet light. In photolysis Reactions initially a reactive intermediate such as HO• is formed which participitate in chain reactions that lead to the destruction of the compound

Biological treatment of hazardous wastes : 

Biological treatment of hazardous wastes Biological processes are in general, the most cost effective techniques for treating aqueous waste streams containing organic constituents. The physical and chemical properties of the compound influence its biodegradability. With appropriate organisms and under right conditions, even phenol which is considered to be biocidal can be degraded The microorganism must be allowed to acclimate to the waste to be treated prior to routine operation of the process Even some compounds which were considered as biorefractory may be degraded by microorganisms adapted to their biodegradation. DDT for example is degraded by properly acclimated pseudomonas The relatively highly Chlorinated PCBs are degraded by anaerobic bacteria under less anaerobic condition. These products can be further decomposed

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To increase the biodegradability of the hazardous wastes, the pH of the medium should be adjusted to an optimum value of 6-9 and the oxygen level should be high Concentrations of soluble inorganic in the hazardous wastes should be kept low so that enzymatic activity is not inhibited. Trace concentrations of inorganic may be partially removed from the liquid waste stream during the biological treatment, because of adsorption onto the microbial cell coating In aerobic waste treatment, hazardous wastes such as chemical processes wastes and land fill leachates can be degraded by aerobic microorganisms such as bacteria and fungi in the presence of oxygen. In anaerobic waste treatment, microorganisms degrade different organic wastes in the absence of oxygen. During the process H2S is generated which precipitates toxic heavy metal ions as their sulphides. The overall degradation of the hypothetical organic compound (CH 2 O) can be written as follows. 2(CH 2 O) →CO 2 +H 2 O

References : 

References Yen-Hsiung kiang, Amir A.Metry, Hazardous Waste processing technology,Ann Arbor Science , 1982. p. 351-543. Y.M.Chao, T.M. Liang , Desalination 2008,221, 433–439. Seoung-Hyun Kim et.al.,Korean J. Chem. Eng ., 2002, 19 (5), 895-902. F. A. Nasr, H. S. Doma, H.S. A.Halim, S.A.E.Shafai, Environmentalist 2007, 27,275–286. www.wikipedia.org