Collectors

General A collector is a surface-active chemical which has a polar and a non-polar group. The work of adhesion of a collector to a solid surface has been separated into three components by Leja:

where wd is the dispersive component, wh is the energy associated with polar adsorption of water at non-ionic sites and w' is the contribution through electrostatic interactions with the Stern layer associated with the solid surface (Figure 4).

The collector molecule must be firmly attached to the solid surface through its polar group and must be able to confer sufficient hydrophobicity to the surface through the non-polar component to facilitate bubble attachment. It appears that for effective attachment, multilayer adsorption is required. Fuer-stenau and Fuerstenau have suggested that the multilayer arise from the dispersive interactions between the non-polar parts of the molecules; they have referred to the resulting structure of the adsorbed layer as hemimicelles.

The attachment of a bubble to the solid ultimately involves an interaction between the polar groups adsorbed on the bubble surface and the multilayered collector on the particle's surface. As the science of the interactions is still imperfectly understood it is

Figure 4 The Stern layer. Reproduced with permission from Adamson AW (1982) Physical Chemistry of Surfaces, 4th edn. New York: John Wiley.

necessary briefly to review the nature of the collector in specific separations.

Thiol collectors The discovery of xanthates in the 1920s caused a rapid increase in the flotation process generally, but specifically with nearly all the sulfide ores. While they confer hydrophobicity on sulfide minerals, they do not affect the flotation of silicates, aluminosilicates, oxides or mineral salts. However, to achieve selective separation between sulfides within the same ore-body careful choice of reagent and control of operating conditions is required.

Although there appears to be a wide range of these sulfur-bearing compounds, they are derived from a restricted number of oxygen-bearing compounds through the substitution of sulfur for the oxygen. The great majority are derived from carbonic, carbamic and phosphoric acids, urea and the alcohols. They are usually surface-active with respect to the liquid-solid interface, but less so at the bubble-liquid surface. Some of the most important of these sulfhydryl collectors are discussed below, with their structures given in Figure 5(A). In all cases R denotes a non-polar group.

Alkyl d'th'ocarbonates or xanthates (R-O-CS"M#) Potassium ethyl xanthate (KEtX) is used for the selective flotation of Cu-Zn, Pb-Zn and Cu-Pb-Zn sulfides. The effective recovery of the sulfide increases with the length of the alkyl group R; n- and iso-propyl and butyl giving increased recovery over the ethyl homologue but with a reduced selectivity. These conflicting trends increase up to amyl and hexyl. All the xanthates mentioned are soluble in water and are restricted to a pH range of 8-13 as they undergo hydrolysis under more acid conditions.

Alkyl mercaptans, R-S-H Figure 5 Industrial flotation reagents

Dialkyl dithiocarbamate (R— R2-/>N-CS2~M+ These are selective collectors for Cu or Cu-activated Zn from FeS2. They are sparingly soluble and are used as emulsions in the pH range 4-9. The non-polar group R1 is usually CH3 or C2H5 while R2 is either propyl or butyl.

Dialkyl thiourea (R1NH-, R2NH-)C=S These are selective collectors for Cu, Pb and Ag ores and have been used in the flotation of complex Cu-Pb-Zn ores. They are not water soluble and have to be used as dispersions in water.

Dialkyl and diaryl thiophosphates (R1-O-, R2-O-) PS2M# The di-ethyl and di-sec-butyl derivatives are used as collectors for Au, Ag and Cu. The dicresyl derivative is used for Ag, Cu, Pb and Zn. Generally, the dithiophosphates (DTP) reagents are more selective than the xanthates particularly in preventing the flotation of iron sulfides. The alkyl derivatives are water-soluble and are used in 5-20% solutions over the pH range 4-12. The most common alkyl homologues are diethyl and dibutyl. The alkyl derivatives are essentially non-frothing.

The aryl derivatives are dicresyl and exhibit frothing behaviour. They are not water soluble and are used undiluted.

Alkyl mercaptans R-S-H Only homologues higher than C12 have been used. Dodecyl mercaptan is a powerful non-selective collector particularly for Cu. The mercaptans are not water soluble and are used either in an organic solution or as a liquid emulsion.

Xanthate derivatives R-O-S22M# These are used mainly in Cu flotation. They are water insoluble and are used either as an aqueous emulsion or as an organic solution. They are effective over a wide pH range.

Anionic collectors These are ionizable non-thio compounds derived from carbonic, sulfuric, phosphoric, phosphonic and arsonic acids (Figure 5(B)). They are anionic, depending on the pH of the flotation pulp and are used to collect a wide range of oxide, silicate and salt-type minerals. These minerals usually have positive (-potentials and consequently bind OH2 ions giving them a hydrophilic character. For the adsorbed surface to become hydrophobic the non-polar part of the anionic collector molecules must have longer carbon chains than those of the thiols. Their attachment is primarily through electrostatic interactions between the polar group of the collector and the particle surface while dispersive interactions between the non-polar groups of adjacent collector molecules increase the hydrophobicity of the surface by producing multilayered adsorbed films.

Fatty acid salts R-COO~ M# The cation M+ is either Na# or K#, with very few exceptions they operate at high pHs where they are highly dissociated.

The non-polar radical R most used lies in the C16-C18 range, typically oleic, linoleic and linolenic.

Unsaturated molecules are preferred. At alkaline pH the metal salts are soluble in water. They are used for the flotation of apatite, calcite, fluorspar and barite. Because of the electrostatic nature of their adsorption they are very powerful collectors for surfaces with positive (-potentials but selectivity is difficult to effect and requires careful control of the chemical environment which usually requires the use of depressants.

Alkyl sulfuric acid and salts R-O-SO33 M 3 These are stronger acids than the fatty acids and exist in an ionized water-soluble form above pH 1-2; the alkyl radical is usually C12 and saturated. Organic sulfates and sulfonates are used to float oxides and silicates such as iron ores, chromite, garnet, beryl and zircon. As the bonding with the solid surface is primarily electrostatic, pH control is critical, especially for the shorter chain length alkyl groups, as it affects their intermolecular dispersive bonding.

Alkyl and aryl sulfonic acid and salts R-SO3 M# A typical collector of this type is sodium dodecyl benzene sulfonate C12H25-C6H4-SO3-O3Na + which is a strong acid and will operate in pulps with pH values above 2.

Alkyl phosphoric acid and salts (R1O-, R2O-)PO3 M# R4 and R2 can be either aliphatic or aromatic. Dialkyls are more strongly frothing than the monoalkyls. They are used in separations from quartz gangue, in particular for the separation of heavy minerals from glass sands. They have also been used in flotation of tungsten, uranium and phosphate ores. They are not selective in the presence of apatite, calcite, fluorspar or barite.

Cationic collectors This group consist of amines which below ceratin pH values exist in the cationic form, RNH2, R1R2NH, R1(R2)2N. The amines are insoluble in the free base form but are treated by acids such as acetic or hydrochloric to solubilize them. They ionize in aqueous solutions as follows:

Typical collectors of this type are a primary aliphatic amine, n-amylamine (C5H11NH2), a primary aromatic amine, aniline (C6H5NH2), and pyridine (NC5H5), nitrogen in a benzene-like nucleus.

Particularly important are the quaternary ammonium salts, R1(R2)3N + X", which ionize strongly and are water soluble. The collecting action of this group is based on being strongly attached to surfaces with negative (-potentials and are consequently rarely selective but are effective over a wide pH

range. Important collectors of this type include alkyl propylenediamine, (RN^CHb^NHb)2 + 2X", and tetradecyltrimethylammonium bromide (TTAB), CH3(CH2)i3-(CH3)3-N + Br-.

Cationic collectors are used to separate sylvite (KCl) from halite (NaCl) in brine and are also used in the separation of silica from phosphate and in the flotation of zinc carbonates and silicates.

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