Coagulation with aluminum salts

Ion exchange, in contrast, creates an effluent that contains between two and five times the mass of inorganic material removed from the product water. Coagulation with aluminum or iron salts creates a sludge, which creates a disposal problem. Green pressure, especially in Switzerland and mid-west USA, which lie in the middle of large land masses, has started to force industrialists to install alternative membrane processes to avoid these discharges. 

The basic pattern of the coagulation of polymer latices with aluminum salts has been clearly demonstrated by the work of Matijevic and his collaborators (1968) using styrene-butadiene. poly(vinyl chloride) (1977). and PTFE (1976) latices. The results obtained by Matijevic and Force (1968b) for the coagulation of styrene-butadiene latices using alumi num nitrate are shown in Fig. 13. From these it can be seen that up to a pH of 3.4 the ccc remains constant at 5 x 10 mol dm" and then decreases between pH 3.4 and 4.8 to reach a constant value of... 

Effluents from pulp, paper and board mills contain wood materials in solid, colloidal and dissolved form. The effluents also contain some chemicals used in the production process, either in their original or modified form. The principal mechanical methods used to remove these compounds are filtration (including membrane and nanotechnology), sedimentation and/or flotation. All these methods require very good flocculation and coagulation of these undesirable substances in the effluent. Only then can they be separated from the clear water. The main chemicals used for coagulation are aluminum salts (e. g. Al2(S04)s), iron salts (e.g. FeCU or Fe2(S04)s or FeS04) and hme with addition rates of 100-500 g m effluent To achieve optimum flocculation results, it is often necessary to feed a suitable... 

Alum has two prime uses. About two thirds of it is bought by the pulp and paper industry for coagulating and coating pulp fibers into a hard paper surface by reacting with small amounts of sodium carboxylates (soap) present. Aluminum salts of carboxylic acids are very gelatinous. 

It has been observed for more than 30 years that preozonation ahead of particle removal units can improve the efficiency significantly, can induce a lower coagulant demand or allow higher flow rates, e. g. in deep-bed filtration. Ozone gas is added either before or together with the coagulant (ferric or aluminum salts or cationic polymers) at rather low dosages of 0.5-2 mg L l. The terms microflocculation or ozone-induced particle destabilization are used in practice (Jekel, 1998). 

It was found in this way that in the presence of an excess of silica sol, no free aluminum hydroxide is precipitated by the addition of a base to aluminum salt solutions. Instead, at an acidity level higher (ca. 1 pH unit lower) than corresponds to the precipitation of aluminum hydroxide, the primary products of hydrolysis of aluminum ions are consumed by the interaction with silica sol particles. One equivalent of acid is produced for each atom of aluminum consumed in this reaction, indicating the formation of an aluminosilicic acid, so that ultimately four equivalents of a base are required to reach neutrality for each aluminum atom present. AH aluminum ions are consumed by this reaction if an excess of silica sol is present (see Fig. 14). No visible precipitate is formed and the insoluble alkali salt remains in the sol form it can be converted to gel or coagulated... 

The removal of negatively charged particles is commonly accomplished via coagulation with ferric or aluminum salts (Stumm and O Melia, 1968). Common ferric salts include ferric sulfate and ferric chloride, both of which hydrolyze in water to form the aqueous species Fe, Fe(OH) ", Fe(OH)2, Fe(OH)3, and Fe(OH)4, as well as the solid precipitant Fe(OH)3(s). Common aluminum salts include alum, PACl, and AICI3, which hydrolyze to Al +, Al(OH) +, Al(OH)2+, Al(OH)3°, and Al(OH)4, as well as the solid precipitant Al(OH)3(s). In addition to the common hydrolysis species, aluminum is prone to the formation of polymeric aluminum species. Alum is the aluminum sulfate (Al2(S04)3 18H20) and contains 0.081 (g Al/g alum). 

Coagulation of silver halide by hydrolyzed aluminum salts was shown by Matijevi6, Kratohvil, and Sticles (267) to be inhibited by fluoride ion which formed complex ions with aluminum. Similar reduction in coagulation effectiveness of other metal cations such as Fe and Th on colloidal silica may be anticipated. 

Precipitation occurs simultaneously with the use of inorganic coagulants (iron and aluminum salts) during clarification of surface water and physicochemical treatment of oily water. 

This polycondensation reaction may also involve the participation of aluminum salts. In this reaction, first, a sol of silicic acid or aluminosilicic acid is formed. This sol then undergoes coagulation and forms a gel, which serves as the primary coagulation structure. In the presence of supersaturation, the gel particles undergo bridging with the formation of phase contacts, and the condensation structure develops. This process is the basis for the synthesis of numerous catalysts and adsorbents. 

The coagulant sodium aluminate (NaA102) is strongly caustic (contributing hydroxide alkalinity to the BW) and reacts with calcium and magnesium salts and any silica present to form a zeolite sludge of calcium-magnesium-aluminum silcate. 

The exact nature of the polymeric spedes in solutions of aluminium salts at the pH conditions for charge reversal is not known with certainty. It is possible that several species coexist, depending on their staUlity constants, and that these also change with time with the ultimate product of hydrolysis being aluminum hydroxide particles. A number of species have been proposed in the literature and Matijevic et a . (1964) have provided evidence for the existence of AIg(OH)fo from coagulation studies. The higher valence of this type of species again reduces the concentration of ions... 

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