Potash alum

Confusion arises ia the nomenclature of alum because double salt compounds, M(I)Al(SOwhere M is ia the +1 oxidation state, have also traditionally been called alums. In particular, potassium aluminum sulfate [15007-61 -1] KAl(SOreferred to as ordinary alum or potash alum. 

The word alum is derived from the Latin alumen, which was appHed to several astringent substances, most of which contained aluminum sulfate (20). Unfortunately, the term alum is now used for several different materials. Papermakers alum or simply alum refers to commercial aluminum sulfate. Common alum or ordinary alum usually refers to potash alum which can be written in the form K SO Al2(SO 24H20, or it can refer to ammonium alum, ammonium aluminum sulfate. The term is also appHed to a whole series of crystallised double sulfates [M(l)M (lII)(SO 2 12H20] having the same crystal stmcture as the common alums, in which sodium and other univalent metals may replace the potassium or ammonium, and other metals may replace the alurninum. Even the sulfate radical may be replaced, by selenate, for example. Some examples of alums are cesium alum [7784-17-OJ,... 

Potassium Aluminum Sulfate. Potassium aluminum sulfate [7784-24-9]. KAl(SO 12H20, is a white, astringent crystal known as potassium alum, ordinary alum, or potash alum. Its formula weight is 474.39 mp 92.5 °C sp gr 1.75 and solubiUty 11.4 g per 100 mL H2O at 20°C (8). It is soluble in dilute acid and insoluble in alcohol. It dehydrates at about 200 °C to porous desiccated potassium alum [10043-67-1], KAl(SO dried or burnt alum, which has a formula weight of 258.20. 

Other solvents or impurities ean have unpredietable results on solubility. For example, addition of a miseible seeond solvent, sometimes ealled a eo-solvent or diluent, often reduees solubility and is a eommon means of indueing erystal-lization (see Figure 3.1). For example, a signifieant deerease in solubility of potash-alum in aqueous solutions was seen upon the addition of aeetone (Mydlarz and Jones, 1989). Again this effeet ean be eorrelated by an empirieal expression of the form... 

A pilot-scale continuous MSMPR crystallizer of 10 litre capacity is used to crystallize potash alum from aqueous solution, supersaturation. This is being achieved using a 15-min residence time, a 100-ml slurry sample was taken and the crystals contained in this sample subjected to a size analysis. The results of this analysis are given below... 

Garside elal. (1979) measured size distributions of seeondary nuelei and reported their variation with supersaturation. Signifieant inerease of nuelei with supersaturation is observed. Thus the proeess is not simply an attrition event alone, but is also related to the level supersaturation at whieh parent erystal is growing. Jones elal. (1986) also observed anomalous growth of seeondary nuelei in a study of the eontinuous MSMPR erystallization of potassium sulphate with eonsequenees inferred for seeondary nueleation rates. Girolami and Rousseau (1986) demonstrate the importanee of initial breeding meehanism in seeded potash alum bateh erystallization. The number of erystals... 

Mydlarz and Jones (1990a,b) determined agglomeration of potash alum in eontinuous MSMPR erystallization from aqueous solution and its effeet on slurry filterability (see Chapter 9). 

Calculate the residence time and volume of an MSMPR crystallizer required to produce 1000 kg/li of potash alum having a dominant crystal size of 600 pm using a slurry density of 250 kg crystals/m slurry. 

Garside, J. and Jancic, S.J., 1976. Growth and dissolution of potash alum crystals in the sub-sieve size range. American Institution of Chemical Engineers Journal, 22, 887. 

Jones, A.G. and Mydlarz, J., 1990a. Continuous crystallization of potash alum. MSMPR kinetics. Canadian Journal of Chemical Engineering, 68, 250-259. 

Ristic, R.I., Sherwood, J.N. and Shripathi, T., 1991. The role of dislocations and mechanical defonuation in growth rate dispersion in potash alum crystals. In Advances in Industrial Crystallization. Eds. J. Garside, R.J. Davey, and A.G. Jones. Oxford Butterworth-Heinemann, pp. 77-91. 

Note. Prepare the precipitating reagent by dissolving 5.0 g aluminium potassium sulphate (potash alum) in 90 mL warm water. Cool and add dropwise with stirring, while cooling in ice, a solution of 2.0 g sodium hydroxide in 5.0 mL water until the initially formed precipitate re-dissolves. After standing for 12 hours, filter, adjust the pH to 12.6, and dilute to 100 mL with water. 

Amara N, Ratsimba B, Wilhelm A, Delmas H (2001) Crystallization of potash alum effect of power ultrasound. Ultrason Sonochem 8(3) 265-270... 

Table 9.15 Precipitation of Arsenic (as sulphide) using Potash alum at different concentrations and pH for 1-h sonication...

As present in parent solution (g/l) Potash Alum (g/l) pH Precipitate of Arsenic (g/l) Removal of Arsenic (%)... 

Garside, J. and Jancic, S. J. AIChE J. 25 (1979) 948-958. Measurement and scale-up of secondary nucleation kinetics for the potash alum-water system. 

The rate Oi diffusion.—In order to find if the components of a double salt are dissociated in soln.—say, alum K SO. A SO saq.v K SOa+A SO g+Aq.— T. Graham 3 assumed that the dissociated parts would diffuse with different velocities, and he found that potassium and aluminium sulphates diffused at different rates from an aq. soln. of potash alum into the pure solvent. Hence, he assumed that alum is partially dissociated into its constituents when in aq. soln. He likewise inferred that the components of the double sulphates of potassium and copper, and of potassium and magnesium, are not dissociated in aq. soln. since under the same conditions there is no sign of any difference in the rates of diffusion of the components. E. Fischer and E. Schmidmer 4 determined the relative quantities of the components of double salts which were drawn up by capillary attraction into rolls of filter paper. If a double salt is dissociated a larger proportion of the more diffusible component ascends the paper. Dissociation is far more pronounced in aq. than in alcoholic soln. 

Sulfates. Potassium sulfate. [CAS 7778-80-5], sulfate of potash, K2.SO4, white solid, soluble. Common constituent of potassium salt minerals. Used (l)as an important potassium fertilizer, (2) in the preparation of potassium or potash alums potassium hydrogen sulfate, KHSO4, white solid, soluble potassium pyrosulfate. K2S2O7, white solid, soluble, formed by heating potassium hydrogen sulfate to complete loss ot H2O. See also Fertilizer. 

Chrome Potash Alum. See Potassium-Chrome Alum, Vol 1, pl56-R... 

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