Frothing

F. F. Apian and D. W. Fuerstenau, in Froth Flotation, D. W. Fuerstenau, ed., American Institute of Mining and Metallurgical Engineering, New York, 1962. 

Prior to about 1920, flotation procedures were rather crude and rested primarily on the observation that copper and lead-zinc ore pulps (crushed ore mixed with water) could be benefacted (improved in mineral content) by treatment with large amounts of fatty and oily materials. The mineral particles collected in the oily layer and thus could be separated from the gangue and the water. Since then, oil flotation has been largely replaced by froth or foam flotation. Here, only minor amounts of oil or surfactant are used and a froth is formed by agitating or bubbling air through the suspension. The oily froth or foam is concentrated in mineral particles and can be skimmed off as shown schematically in Fig. XIII-4. 

Fig. XIII-4. Schematic diagram of a froth flotation cell. Note the mineralized bubble shown in the inset. [Reprinted with permission from P. Somasumdaran, Interfacial ChemisUy of Particulate Flotation. AIChE Symp. Sen, 71(150), 2 (1975) (Ref. 58). Reproduced by permission of the American Institute of Chemical Engineers.]...

The basic phenomenon involved is that particles of ore are carried upward and held in the froth by virtue of their being attached to an air bubble, as illustrated in the inset to Fig. XIII-4. Consider, for example, the gravity-free situation indicated in Fig. XIII-5 for the case of a spherical particle. The particle may be entirely in phase A or entirely in phase B. Alternatively, it may be located in the interface, in which case both 7sa nnd 7sb contribute to the total surface free energy of the system. Also, however, some liquid-liquid interface has been eliminated. It may be shown (see Problem XIII-12) that if there is a finite contact angle, 0sab> the stable position of the particle is at the interface, as shown in Fig. XIII-5Z>. Actual measured detachment forces are in the range of 5 to 20 dyn [60]. 

The importance of the thin film between the mineral particle and the air bubble has been discussed in a review by Pugh and Manev [74]. In this paper, modem studies of thin films via SFA and interferometry are discussed. These film effects come into play in the stability of foams and froths. Johansson and Pugh have studied the stability of a froth with particles. Small (30-/ m), moderately hydrophobic 6c = 65°) quartz particles stabilized a froth, while more hydrophobic particles destabilized it and larger particles had less influence [75]. 

R. J. Pugh and E. Manev, The Study of Thin Aqueous Films as Models for Froths and Flotation," in Innovations in Flotation Technology, P. Mavros and K. A. Matis, eds., Kluwer, 1992, pp. 1-24. 

Ethyl bromide soon distils over, and collects as heavy oily drops under the water in the receiving flask, evaporation of the very volatile distillate being thus prevented. If the mixture in the flask A froths badly, moderate the heating of the sand-bath. When no more oily drops of ethyl bromide come over, pour the contents of the receiving flask into a separating-funnel, and carefully run oflF the heavy lower layer of ethyl bromide. Discard the upper aqueous layer, and return the ethyl bromide to the funnel. Add an equal volume of 10% sodium carbonate solution, cork the funnel securely and shake cautiously. Owing to the presence of hydrobromic and sulphurous acids in the crude ethyl bromide, a brisk evolution of carbon dioxide occurs therefore release the... 

Now add the diazonium solution slowly from a dropping-funnel to the vigorously-stirred arsenite solution, keeping the temperature of the latter at 5 7°. The frothing caused by the evolution of nitrogen will probably be dispersed by the stirrer if not, the addition of 1-2 ml. of ether, preferably in a fine jet from a wash-bottle, will cause it to subside. 

Attention is directed to the great advantage of continuous extraction over manual shaking in a separatory funnel for liquids or for solutions which tend to froth or which lead to emulsification comparatively little difficulty is experienced in the continuous extraction process. 

Fit a 1500 ml. bolt-head flask with a reflux condenser and a thermometer. Place a solution of 125 g. of chloral hydrate in 225 ml. of warm water (50-60°) in the flask, add successively 77 g. of precipitated calcium carbonate, 1 ml. of amyl alcohol (to decrease the amount of frothing), and a solution of 5 g. of commercial sodium cyanide in 12 ml. of water. An exothermic reaction occurs. Heat the warm reaction mixture with a small flame so that it reaches 75° in about 10 minutes and then remove the flame. The temperature will continue to rise to 80-85° during 5-10 minutes and then falls at this point heat the mixture to boiling and reflux for 20 minutes. Cool the mixture in ice to 0-5°, acidify with 107-5 ml. of concentrated hydrochloric acid. Extract the acid with five 50 ml. portions of ether. Dry the combined ethereal extracts with 10 g. of anhydrous sodium or magnesium sulphate, remove the ether on a water bath, and distil the residue under reduced pressure using a Claiseii flask with fractionating side arm. Collect the dichloroacetic acid at 105-107°/26 mm. The yield is 85 g. 

Place 84 g. of iron filings and 340 ml. of water in a 1 - 5 or 2-litre bolt-head flask equipped with a mechanical stirrer. Heat the mixture to boiling, stir mechanically, and add the sodium m-nitrobenzenesulphonate in small portions during 1 hour. After each addition the mixture foams extensively a wet cloth should be applied to the neck of the flask if the mixture tends to froth over the sides. Replace from time to time the water which has evaporated so that the volume is approximately constant. When all the sodium salt has been introduced, boU the mixture for 20 minutes. Place a small drop of the suspension upon filter paper and observe the colour of the spot it should be a pale brown but not deep brown or deep yellow. If it is not appreciably coloured, add anhydrous sodium carbonate cautiously, stirring the mixture, until red litmus paper is turned blue and a test drop upon filter paper is not blackened by sodium sulphide solution. Filter at the pump and wash well with hot water. Concentrate the filtrate to about 200 ml., acidify with concentrated hydrochloric acid to Congo red, and allow to cool. Filter off the metanilic acid and dry upon filter paper. A further small quantity may be obtained by concentrating the mother liquid. The yield is 55 g. 

Dissolve 200 g. of sodium nitrite in 400 ml. of water in a 2-litre beaker provided with an efficient mechanical stirrer, and add 40 g. of copper powder (either the precipitated powder or copper bronze which has been washed with a little ether). Suspend the fluoborate in about 200 ml. of water and add it slowly to the well-stirred mixture. Add 4-5 ml. of ether from time to time to break the froth. The reaction is complete when all the diazonium compound has been added. Transfer the mixture to a large flask and steam distil until no more solid passes over (about 5 litres of distillate). Filter off" the crystalline solid in the steam distillate and dry upon filter paper in the air this o-dinitrobenzene (very pale yellow crystals) has m.p. 116° (t.c., is practically pure) and weighs 29 g. It may be recrystallised from alcohol the recrystallised solid melts at 116-5°. 

While the diazotisation is in progress, cautiously add 165 ml. of concentrated sulphuric acid to 150 ml. of water in a 1-litre round-bottomed flask. Heat the mixture just to boiling. Add the supernatant Uquid (diazonium solution) from a separatory funnel supported over the flask at such a rate that the mixture boils very vigorously (about 30 minutes). Then add the residual damp soUd (or suspension) in small portions avoid excessive frothing. When aU the diazonium salt has been introduced, boil for a further 5 minutes and pour the mixture into a 1-Utre beaker... 

Place 20 g. of Orange II (Section IV,79) in a 600 ml. beaker and dissolve it in 250 ml. of water at 40-50°. Add, with stirring, 24-25 g. of sodium hyposulphite (Na SjO ) this discharges the colour and yields a pink or cream-coloured, finely-divided precipitate of a-amino-p-naphthol (compare Section IV,76). Heat the mixture nearly to boiling until it commences to froth considerably, then cool to 25° in ice, filter on a... 

Method 2. Intimately mix 99 g. of pure phthahc anhydride and 20 g. of urea, and place the mixture in a 1 litre long-necked, round-bottomed flask. Heat the flask in an oil bath at 130-135°, When the contents have melted, eflfervescence commences and graduaUy increases in vigour after 10-20 minutes, the mixture suddenly froths up to about three times the original volume (this is accompanied by a rise in temperature to 150-160°) and becomes almost sohd. Remove the flame from beneath the bath and allow to cool. Add about 80 ml. of water to disintegrate the sohd in the flask. Alter at the pump, wash with a httle water, and then dry at 100°. The yield of phthahmide, m.p. 233° (i.e., it is practically pure) is 86 g. If desired, the phthahmide may be recrystalhsed from 1200 ml. of methj lated spirit the first crop consists of 34 g. of m.p. 234°, but further quantities may be recovered from the mother hquor. 

The heating should not bo too rapid initially or the solution may froth over. 

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