FLOTATION NATHANIEL ARBITER Columbia University, New York 27, N . Y
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LOTATIOS as a method for tlic separation of particulate golids was developed for and is used primarily in the mineral industry. Froth flot.ation contiiiuee to be the predominant method for mineral separation of particle sizes finer tha.u 35 niesh, with minor application of table, spiral, and belt flotation t o nonmetallics a t coarser sizes. The past year, as evidenced hy the literature, showed no evidence of dominant trends in this field. Theoretical investigatiom continue to emphasize the surface chemistry of flotation with increase in work on the dymtmics of the particle bubble system and on the separat,ion process. Laboratory studies of an applied nature are of interest because of the Ride variety of ore8 to which the operation is being applied. Descriptions of operating plants are few, and there is little evidence of extension of new applications out.side the mineral industry. The patent literature is noteworthy, however; mainly because of the large proportion issued dcaling with potash piirificittion.
Theory Particle-Bubble Adhesion. The basic phenomenon in flotation separations is tho potential adhesion of the mineral particle to an air bubble. Renewing the controversj- on inherent floatability of aiilfide minerals, Plaksin and Rcssonov ( 7 1 ) claim that fresh surfaces of galena, chalcopyrite, sphalerite, pyrite, pyrrhotite and arsenopyrite adhere to air hubbleu without reagent treatment, that oxidation decreases the adherence, and thc oxygenfree condition8 can preserve it. In contrast, the same authors ( 8 ) report that galena or chalcop>4teJground in a nonoxidizing atmosphere, do not, float without reagents: nor do they float with collector reagents unless some oxidation is permitted. Although the claim for inhcrent floatability of sulfide minerals has never been resolved (46, 61, 7 6 ) , similar claims for finite contact angles on pure nietalv by Bartell (3)were later shown by the same author to have been the result of contaminated surfuces (4).Reports of an effect of the gas phase on wettabilities of pyrite (63) and metal ( 6 ) surfaces were published, but thPir significance is obscure in view of Bartell's work. Evans published an important study of the mechanism of formation of the three-phase contact m g l e on an extended solid surf:tce (28). (See also reference f7.1 By treating pure silica with ethylchlor silane, an equilibriunl air contact angle of 66" pias obtained. I n the formation of the three-phase contact by pres.sirig a bubble against the surface, the intervening x-ater film rilptures around the periphery of the buhble contact when the filni is about 1500 A . thick for the particida,r system. Other investigat,ions dcaling with particle-bubble adhesion or with contact, angle reported: an apparatus (37) for measurcment of rate of adhesion of bubbles to mineral particles; a direct reading torsion wire tensiometer (43) for measuring either surface tenfiion where the slide is completely wet by the liquid or adhesion tension where thc slide shows an air-liquid contact angle; an analysis of forces involved in the adhesion of particles t o bubbles ( 6 4 ) ; t,he specific effects of time, pH, and tempera-
March 1956
tux, on the contact ang1t:s obtained by treating pyrolusite with oleic, stearic, and palmitic acid as collect~ors(9); a relation hetween wettability and magnctic susceptibility of pyrrhotite ( 4 5 ); and expcriment>alstudies of the relations between particle . size and floatability (65, 99). Electrokinetics. The study of electrokinetic effects continues to bc used with thc objective of e l u r i d a h g the fine structure of the solid-water interface. Gaudin and Fuerstenau (34) relate adsorption of activators and collectors by quartz to streaming potential studies. In the absence of cationic act)ivators, the qiiartz surface is charged negatively in contact with alkaline solutions. An anionic collector is not ad8orbed but affects the diffuse Gouy double layer. Cationic artivators such as Ba++ or A1+++ are in part specifically adsorbed into the Stern layer and in part act as counter ions in the Gouy layers. With polyvalent cations in the Stern layer, anionic collect,ors then can be adsorbed and affect wetting properties. Borivov (1I, 1 2 ) reports rneasurcments by electro-osmotic methods on apat'ite, barit,(., calcite, and dolomite in solutions of sodium carbonate, sodium silicate, and sodium oleate. IIo suggests as an electrokinetic iiidpx the change in zcta potential caused by a reagent,. A similar index byas proposed by Gaudin and Sun (56) in 1946. The significance of interfacial potentials i n flotation is nlso considered by Jo ( 4 7 ) .
Surface Chemistry Sulfide Minerals. The mechanism of depression of plrite by CS-, S--, SO4--, &InO4-,and Cr%O7-- is considered by hlajumdar (59, 60). I n the case of C Y , rcdiiction of Fei4 to F e + + and adsorption of CY- was postulated on thc basis of contact angle studies. The effect of NI€l+ salts an the floatability of +
pjrite and pyrrhotite was studicd by* Ono and others ( 6 7 ) . The salts &-ere used to overcome thP depressing effects of lime. In the casc of pyrite the optimiim pH was 7 while the floatability of p j rrhotite was relatively insensitivc to pII. Practical applications resulted in decreased rollectur consumption or increasrd recovrry. Galena depression by alkalics 13 discussed by Bessonov ( 7 ) . He claims the formation of surface lead s.ilts which prevent xanthate adsorption ii difference in degiw of activity betuecii potassium hydroxide and sodiiini hydroxide, pia8 found and an explanation for the differenre advanced. Hagihara ($ j , reports additional electron diffraction studies on the galena alkyl xaiithatr, and alkyl dithiophosphate SI stems For oxidized galena faces, the patterns are interpreted to indicate lead u n thatc crystallites in linear groups. For fresh or slightly oxidized faces, unimolecular patches of xanthic acid wcre postulated with a cross section area of 17.5 sq. X per moltwilr. \\'ith diethyldithiophosphate the patterns dependcd both on crystallographic face and on the reagent concentration. The adsorption site for the large dithiophosphate adsorbate involvrs several lead ions. Electron diffraction studies also are reported for the sphaleritexanthate sg.stem by Sato (85). After copper sulfate activation
INDUSTRIAL AND ENGINEERING CHEMISTRY
527
UNIT OPERATIONS REVIEW
Potash purification by flotation i s subject of new patents
COURTESY DENVER EQUIPMENT CO.
Flotation of fluorspar
of the sphalerite and treatincnt with xanthate, patterns attributed to cuprous xanthate were found in tn-o modjficntions; the first was interpreted as iormcd by randomly oriented crystallites and the second by crystallites arranged in a fibrous st,rrict,iue. Sat,o (85) also determined the thicknem of xanthate coatings 011 copper sulfate and silver nitrat,e-activated sphalerite and related the results to the electron diifraction findings. Plaksin and coxorkers studied the interaction of' santhates with chalcopyrite, sphalerite, and pyrite. Bl- using tracertagged reagents (;.$i they found that osygen accelerated t'he adsorpt,ion of collcctors by chalcopyiite and sphalerite, whereas it reduced the adsorption by pyrite. In another study on rraction rates for the same syst'em (YO),they reported a distinction between chemisorption which was unaffected by agitation and physical adsorption \I-hich depended on a,gitation. I t wit8 assumed that physical adsorption of an unoriented sant hate layer followed oriented chemisorption. Chemisorption was said to predominat#ein the flotation of the two minerah. Nonmetallic Minerals. I n interpreting studies on aluminum oxide flotation with oleic arid, Deshpande and Bhat ( 2 2 ) postulated molecule adsorption in neutral or acidic solutions, ion adsorption from alkaline solut,ions, and adsorpt.ion of a second layer of collector with rcverse orientation. DeHruyn ($0) studied the adsorption of ClzHZsNIIa by quartz using tracer techniques. For collector concentrations below 2 x 10-4 mole per liter, the adsorption density depended on t,he square root of collector concentration. Above this concentration, the density depended on a higher power of concentration. A complete monolayer was obtained at 10-3 mole per liter of collector, but adsorption continued beyond this region. The effect of plI variations on + rat>hcrthan tmhe free adsorption suggested that the C121'L25KH3 base \ m s the adsorbate. The adsorption of the samc reagcnt by
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hematite (E'e20.i) was studied by Gaudin and llorrow ( 3 6 ) . The isotherm closely xsembled that for quartz, alt,hough in the low concentration range the adsorption density was highcr and depended on the 0.6 povier of solution concentrations; the dependence of adsorption on pH was similar for both ininrralfi, and the adsorption was shown to be easily reversihle. Parallel measurements of conta,ct angles on both minerals indicated that for the same surface coverage, higher angles were obtained for hematite, and t,he dependence of contact angle on surface coverage differed significantly for the t v o minerals. The mechanism of' flotation of feldspar and quartz by C , ~ H ~ S NET2 \+-as studied by Buckenham ( I S ) . He postulated that fluoride activation is due to formation of a complex aluminun~ fluoride on feldspar surfaces. In dkaline solutions adsorption of both free amine and ion was inferred. Comparative studies indicated that very small contact angles were sufficient for the flotation of small particles. Miscellaneous. Sulfide minerals ground in argon were shown to t,ake up oxygen at different rates (72). The rates were dependent on p R and on the collector. Lime and sodium carbonate had strongest effects. Synergist,ic effects of a mixture of flot,at,ion collectors were reported involving xanthates and dithiophosphates ( Y S ) , or oleates with either of the foregoing collectors (38). The effect of substitution of active groups in starch on its depressing effect on iron oxide %vas studied by Chang (14) and by Schuls a,nd Cooke (84). The order of decreasing depression was aminoethyl starch, starch phosphate, pearl starch, starch sulfate, stmarchxanthate, and osidized starch. Khelan (98) measured the rffect of adsorbed flotation reagents on the polarizing angles obtained with pyrite, coal, and fluorspar surfaces. It was claimed t>hat with large molecules the detection of a single monolaj-er might be possible hy this method.
Application The following references largely describe flotation test results on specific ores. Only those details are reported here which are of general rather than limited significance. Sulfide Ores. Conventional flotation tests xere carried out on pyrite ore samples from the Alpha mine, A'ynaad, India (89). I n the flotation of a complex ore containing galena, sphalerite, and chalcopyrite n-ith a cresyl dithiophosphate collector, concentrat,es containing lead, zinc, copper, and iron wcre obtained in that order (96). Results mere presei~tedin the form of a block diagram. Recovery of sphalerite from a copper ore and of lead and zinc from tailings dump were reported (61). Laboratory studies led to the development of a flow sheet for a complex ore containing chalcopyrite, galena, sphalerik, and pyrite (95). A copper-nickel matte cont>aining 22.7% of nickel and 57.1% of copper was separat,ed by flotation into a nirkel concentrate conhining 54.470of nickel and 17.3% of copper and a copper concentrate containing i l . O % of copper, 6.5% of nickel. Nonsulfides. In the flotation of silica from iron ores with a cationic collector (Y5), starch was superior to tsn11i11, silicate, or dextrin for iron depression. The optirnum pII w3s 8 to 9 x i t h desliming not ncceseary. The flotation of I
