Aluminium salts, hydrolysis

NANOPARTICLES OF ALUMINIUM SALTS HYDROLYSIS PRODUCTS IN WATER TREATMENT AND DISINFECTION... 

Keywords aluminium salts, hydrolysis products, size, electrokinetic potential, hydration, aggregation rate, salt dosage, pH, alkalinity, ionic strength, water treatment, disinfection, electric sparks... 

In the last decade the chemistry of aluminium salts hydrolysis related to the application of the products of their partial hydrolysis in coagulation has been studied in detail [5]. It has been found that as well as the pH value, temperature, precipitate ageing and solution, the composition of the hydrolysis product also depends on the initial ratio of the concentration of OH ions and aluminium, as well as on the rate of the base addition. The hydrolysis of aluminium salts influences a number of side reactions depending on the overall composition of water, since aluminium forms complexes with other ligands, such as sulphate, phosphate and fluoride. 

Therefore, when an anhydrous aluminium salt is dissolved in water initially, the octahedral ion [Al(H20)j,] " is formed by hydration of the A1 ion. However, since some hydrolysis occurs, the solution will contain and be acidic. Addition of any molecule or ion... 

The addition of an acid to the wash solution will prevent the hydrolysis of iron(III) or similar salts thus dilute hydrochloric acid will serve to remove iron(III) and aluminium salts from precipitates that are insoluble in this acid. 

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... 

With ASA, good first pass retention of ASA is paramount to good sizing efficiency, as any ASA not retained in the first pass can be easily hydrolysed in the white-water system. This is because the ASA dispersion is not as stable in the wet-end as that of rosin, or AKD, as these products are designed to be stable for a period of months. This hydrolysis product can react with calcium and/or magnesium to form sticky salts, which can deposit and cause many problems. If aluminium is not added to the ASA dispersion, then it should be added to the white-water system to react with any unretained, hydrolysed, ASA to prevent the formation of these salts, by formation of the less/non-sticky aluminium salt. There are appUcations of ASA that do not use aluminium species, where first pass ASA retention is optimised, but these tend to be in the minority. 

Additional examples of salt hydrolysis occur with salts whose metal cation is small and highly charged. Examples include copper(ii) sulfate, CUSO4, aluminium sulfate, Al2(S04)3 and iron(iii) chloride, FeCl3 (Figure 18.13). 

The coagulation of hydrolysed aluminium salts has been studied using A1 NMR spectroscopy. The hydrolysis of mixed solutions of Al , Ga ", Ln , and Zr" has been studied by Al and Ga NMR spectroscopy. A C and A1 NMR study of the interaction between Al and tiron, salicylic acid, and phthaUc add has been reported. B, A1, and Si NMR studies of the temperature dependent structural changes in borate, borosilicate, and boro-aluminate Uquids have been reported. The interaction of aluminosilicates by HF has been investigated. Aluminium in polysilicic acid has been speciated using A1 NMR spectroscopy. C, A1 and P NMR spectroscopy has been used to study the formation of Al-NTA-phosphate complexes. The selfdiffusion coefficients of the ATP complex of Al have been studied by H and P pulsed-field gradient NMR spectroscopy. H NMR spectroscopy has been used to study the interaction of Al with phosphatidylcholine vesicles. ... 

Aluminium - CIDNP is observed in the reaction of [EtsAl] with CCI4, catalysed by [Ni(acac)2]. Al NMR spectroscopy has been used to investigate the hydrolysis polymerization of aluminium salts. The reductive coupling of 2-cyanopyrroles when treated with LiAlH4 has been investigated with Al NMR spectroscopy. The reaction of HF and H2Sip6 with aluminosilicates has been followed by NMR spectroscopy. 

Schofield, R.K. and Taylor, A.W. (1954) The hydrolysis of aluminium salt solutions. 

For this purpose a metallic salt is reduced at room temperature by an aluminium alkyl in homogeneous phase, and the slurry is formed by heating this solution up to 200 C under a stream of synthesis gas the water which forms, in situ, by the reduction of the carbon monoxide is used for the hydrolysis of the aluminium salt into alumina (3). 

Urea possesses negligible basic properties (Kb = 1.5 x 10 l4), is soluble in water and its hydrolysis rate can be easily controlled. It hydrolyses rapidly at 90-100 °C, and hydrolysis can be quickly terminated at a desired pH by cooling the reaction mixture to room temperature. The use of a hydrolytic reagent alone does not result in the formation of a compact precipitate the physical character of the precipitate will be very much affected by the presence of certain anions. Thus in the precipitation of aluminium by the urea process, a dense precipitate is obtained in the presence of succinate, sulphate, formate, oxalate, and benzoate ions, but not in the presence of chloride, chlorate, perchlorate, nitrate, sulphate, chromate, and acetate ions. The preferred anion for the precipitation of aluminium is succinate. It would appear that the main function of the suitable anion is the formation of a basic salt which seems responsible for the production of a compact precipitate. The pH of the initial solution must be appropriately adjusted. 

Solutions which prevent the hydrolysis of salts of weak acids and bases. If the precipitate is a salt of weak acid and is slightly soluble it may exhibit a tendency to hydrolyse, and the soluble product of hydrolysis will be a base the wash liquid must therefore be basic. Thus Mg(NH4)P04 may hydrolyse appreciably to give the hydrogenphosphate ion HPO and hydroxide ion, and should accordingly be washed with dilute aqueous ammonia. If salts of weak bases, such as hydrated iron(III), chromium(III), or aluminium ion, are to be separated from a precipitate, e.g. silica, by washing with water, the salts may be hydrolysed and their insoluble basic salts or hydroxides may be produced together with an acid ... 

The review of Martynova (18) covers solubilities of a variety of salts and oxides up to 10 kbar and 700 C and also available steam-water distribution coefficients. That of Lietzke (19) reviews measurements of standard electrode potentials and ionic activity coefficients using Harned cells up to 175-200 C. The review of Mesmer, Sweeton, Hitch and Baes (20) covers a range of protolytic dissociation reactions up to 300°C at SVP. Apart from the work on Fe304 solubility by Sweeton and Baes (23), the only references to hydrolysis and complexing reactions by transition metals above 100 C were to aluminium hydrolysis (20) and nickel hydrolysis (24) both to 150 C. Nikolaeva (24) was one of several at the conference who discussed the problems arising when hydrolysis and complexing occur simultaneously. There appear to be no experimental studies of solution phase redox equilibria above 100°C. 

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