Flotation Chemistry

The search for new flotation reagents for various mineral separation systems has been one of the major aims in flotation chemistry development. Although xanthate, first used more than 70 years ago, remains the principal collector for sulfide mineral flotation, long chain surfactant has been introduced as the collector in oxide, silicate and sparingly soluble salt mineral flotation systems. The early trial-and-error approach in screening and searching for a new flotation collector has evolved into today's scientific design. Using quantum chemistry and electron density calculations, the structures of highly selective collectors have been proposed. A surfactant, with oxygen and nitrogen as the binding elements in its functional group (e.g. hydroxyoximes), was found to be a powerful and more selective collector for oxide minerals, while those with sulfur and nitrogen as the binding elements (e.g. thionocarbamate) is particularly selective for sulfide minerals. A common feature of these new collectors is their electron donor character, forming a five- to six-member closed ring structure with surface metallic elements. Many five-membered heterocyclic compounds (e.g. oxazole- or thiazole-based collectors) have recently been found to be of special selectivity in base metal ore flotation. A general correlation between flotation performance and collector chain structure (e.g. short versus long, single versus double, straight versus branched, single bond versus double bond, parafRnic versus aromatic, etc.) has also been established and a detailed list of newly developed collectors was compiled by Nagaraj in 1988. The use of a collector mixture has shown improved collecting power and selectivity, and warrants further development.

The invention of a water-soluble frother by Tveter (a polypropylene glycol ether, known as Dowfroth) was considered to be one of the major advances in frother development. Following the advent of various types of synthetic, propylene-based frothers, the effort in frother development has been directed to establishing a correlation between frother structures, frothing characteristics and their effect on recovery and selectivity. To this end, increasing branching in frother molecules has been identified as increasing flotation selectivity, often at the cost of reduced recovery. The use of mixed frothers in a flotation system to generate air bubbles with a wide range of sizes, each suitable for particles of a given size range, has also drawn considerable attention. An increased overall recovery has been demonstrated by using a mixture of 1:1 polyglycol: methyl isobutyl carbinol (MIBC) as compared to a single frother at the same total concentration level. The synergistic effect of a collector and a frother on bubble-particle collection has also been recognized, although the practical application has not gained its fair share of attention. The stabilization of air bubbles by simple inorganic electrolyte should not be overlooked. Developed in the early 1930s for natural hydrophobic coal flotation without using a frother, salt flotation provides a different avenue for recovering natural hydrophobic minerals, as the surface active frother tends to adsorb on natural hydrophobic minerals with unfavourable orientations for flotation, consuming added chemicals and reducing their floatability. In 1995 Weissenborn and Pugh confirmed that the hydration shell around the added inorganic (ionic) species or frother's polar groups is responsible for froth stabilization.

Development in the activator appears rather limited, although most of the positively charged metal hydroxy species have been found suitable for activation in silicate flotation. In sulfide flotation, copper sulfate remains the only activator extensively used today. In contrast, development in depressants has taken on a different pace. Shortly after the introduction of sodium dichromate (for PbS) and SO2 (for ZnS) in 1913, sodium cyanide (1922) and alkali sulfites (1923) appeared to be the popular depressants to use and remain the major depressants in modern sulfide flotation plants. Meanwhile, sodium silicate (1928) and macromolecular starch (1931) have become important depressants/dispersantsin oxide, silicate and sulfide flotation systems. In addition to nonionic dextrins, cationic polysaccharides and an-ionic carboxymethyl cellulose have been found to be effective depressants because of their multi-anchoring nature with mineral surfaces. Recent efforts have been directed to the search for polyamines, which are effective in iron sulfide depression, driven by the environmental pressure of reducing SO2 emission from smelters. Combined with SO2, di-ethylenetriamine (DETA) has been found effectively to depress the pyrrhotite in pentlandite flotation, although the depression mechanism remains to be identified.

The control of the mineral surface property by biotreatment is an emerging area and represents a special branch in flotation reagent development.

This approach is of special importance for desulfuri-zation in coal flotation, selective depression in base metal sulfide flotation and hydrophobization of nonsulfide minerals. The success of biotreatment in these systems lies in the extremely high selectivity of bacteria, such as Thiobacillus(T-)ferrooxidans, towards the oxidation of pyrite, without any adverse effect on the flotability of coal, resulting in a high desulfurization efficiency in coal flotation. Also reported is an improved floatability of sphalerite by pretreatment of T-ferrooxidans in an acidic medium. However, a high dose of T-ferrooxidans has been found to be detrimental to sphalerite and galena flotation. Although sulfate-reducing bacteria have a minimal effect on the floatability of molybdenite and galena, they have been found to depress the floatabil-ity of chalocopyrite and sphalerite, resulting in highly selective flotation. Brunet et al. (1998) reported that the combination of T-ferrooxidans, T-thiooxidans and Leptospirillum accelerated pyrite oxidation. The high selectivity of a bioprocess warrants the rapid growth of biotreatment in mineral flotation.

Accompanying the development of various flotation reagents is the recognition of surface reactions/adsorption and the understanding of collector/mineral interactions in selective flotation. The theory of sulfide flotation with xanthate family collectors has advanced from simple surface chemical reactions to a generalized electrochemical-chemical process. Recognizing the electrochemical nature of collector adsorption on sulfide surfaces was a quantum leap in sulfide flotation chemistry. The application of the mixed potential theory to a sulfide flotation system provides a scientific explanation for a required oxygen level to induce the floatability, and accounts for the role of pulp electrochemical potential (Eh) in sulfide flotation for a given collector chemistry. An important consequence of electrochemical involvement in sulfide flotation is the development of self-induced (also known as 'collec-torless') flotation by either controlled oxidation or sulfidization of pre-oxidized sulfide minerals. The use of cyclic voltammetry allows a direct correlation between collector adsorption (determined by charge integration), under a given applied electrode potential, and contact angle, which in turn determines the floatability of sulfide minerals. An important outcome from electrochemical studies is a new mechanism for differential flotation of complex sulfides by pulp potential control. However, the controversy regarding the collector reaction product on sulfide minerals is yet to be resolved. To this end, modern spectroscopic methods are useful. Surface reactions have been studied extensively using various surface analytical techniques, including: (i) Fourier transform infrared spectroscopy (FTIR in both in situ and ex situ modes); (ii) Raman spectroscopy; (iii) Auger and X-ray photoelectron spectroscopy (AES and XPS); (iv) fluorescence spectroscopy; (v) electron spin resonance spectroetry; (vi) laser ioniz-ation mass spectrometry (LIMS); and (vii) time-of-fight-secondary ion mass spectrometry (TOF-SIMS). For example, the monolayer formation of polysulfide as a sulfur oxidation product on PbS has been confirmed from synchrotron XPS characterization. However, whether or not polysulfide is responsible for collectorless flotation remains to be established. Following the pioneer work on in situ spectroelec-trochemical characterization of sulfide flotation chemistry by Leppinen et al. in 1988, the development of a spectroelectrochemical cell (Figure 2), combined with polarized FTIR spectroscopy, sets up an entirely new direction for sulfide flotation chemistry research. Using polarized infrared radiation, the orientation of the adsorbed molecular species can be derived, as shown in Figure 3. However, further research efforts are required to quantify the mo lecular orientation and to derive its practical implications in sulfide flotation practice.

In oxide and silicate mineral flotation, the interaction (i.e. adsorption of the collector, mostly surfactant) has been generally considered to be electrostatic rather than chemical in nature. An electrostatic interaction model has proven satisfactory when applied to silica and alumina flotation with ionic collectors of opposite charges from the surfaces. Progress has been made in predicting the point of zero surface charge, based on the minimum solubility theory and the sign of surface charge from the hydration energy of lattice ions. A more quantitative description of surface charge distribution has been made possible following the development of the surface triple-layer model in combination with the surface site-binding theory. Early adsorption studies have revealed the formation of surfactant hemimicelles on mineral surfaces at a bulk surfactant concentration of approximately one-hundredth of its critical micelle concentration (cmc). The formation of ionomolecular complexes has been found to enhance the floatability of oxides.

Figure 2 Schematic diagram of an in situ spectroelectrochemical cell suitable for studying sulfide flotation chemistry. Pushing a movable sulfide mineral working electrode against the CaF2 window with a screw type of mechanics ensures not only elimination of bulk water films to increase the sensitivity of infrared spectroscopy, but also reproducible positioning of the electrode (after each electrode polarization) for quantitative analysis. The use of polarized infrared radiation in external reflectance mode allows identification of molecular orientation.

Figure 2 Schematic diagram of an in situ spectroelectrochemical cell suitable for studying sulfide flotation chemistry. Pushing a movable sulfide mineral working electrode against the CaF2 window with a screw type of mechanics ensures not only elimination of bulk water films to increase the sensitivity of infrared spectroscopy, but also reproducible positioning of the electrode (after each electrode polarization) for quantitative analysis. The use of polarized infrared radiation in external reflectance mode allows identification of molecular orientation.

350 mV s-polarization

350 mV s-polarization

1400 1300 1200 1100 1000 Wave number (cm-1)

Figure 3 In situ infrared spectra obtained with a copper electrode polarized under electrode potentials of 150 (dotted lines) and 350 (continuous lines) mV/SHE (standard hydrogen electrode) in 2 x 10~3 mol L~1 potassium ethylxanthate solutions. By comparing the spectra obtained with s- and p-polarized infrared beams, a near perpendicular orientation of adsorbed copper xan-thate on copper electrode (inset) was derived to account for the absence of the band at 1050 cm-1, associated with COC molecular vibrations, with the s-polarized infrared beam. In contrast, a random orientation of dixanthogen, formed under a higher applied electrode potential, was ascertained by a similar spectral feature of characteristic dixanthogen bands obtained with both polarization modes.

This is consistent with recent observations on enhanced hydrophobicity of mica surfaces in a mixed cationic amine and neutral alcohol surfactant solution. The increased overall surfactant adsorption density at the solid-liquid interface is accounted for by screening elecrostatic repulsion between adjacent surfactant head groups.

Recently, a detailed study using a well-defined basal plane of crystalline sapphire in a surface forces apparatus showed that the formation of monolayer hemimicelles requires a near-cmc surfactant concentration in the surface region, while its bulk concentration has to be well below the cmc. Based on the well-known Stern-Grahame equation, this condition cannot be satisfied in the absence of any attractive driving force of electrostatic and/or chemical nature required to preconcentrate the surfactant in the surface region to the cmc level. As a result, the lack of hydrophobic monolayer formation and hence effective flotation is anticipated.

Clearly, effective oxide flotation requires creation of a chemical environment that maximizes the surface concentration of the surfactant at as low a bulk concentration as possible. Changing suspension pH to control surface charge density in oxide flotation serves as an excellent example. Under certain circumstances, activation by hydrolysed metal ions, which provide the linkage between an anionic collector and a negatively charged mineral, is necessary to induce floatability. It should be noted that the selectivity of separation in oxide flotation is relatively poor if the electrostatic force is the only driving force for collector adsorption. This is particularly true in fine particle flotation, as heterocoagulation between different minerals often induces a secondary locking which destroys the selectivity. To this end, searching for collectors which chemically anchor on to targets remains the focus in oxide flotation systems. For sparingly soluble mineral flotation, solution chemistry calculation has been proven to be one of the most valuable tools in searching for separation windows. Since bulk solution chemistry controls the flotation response, a bulk precipitation followed by surface deposition, with a switch-on type of adsorption characteristics, has been considered to be the most favourable mechanism in flotation of soluble-type minerals, where the monolayer adsorption is hardly recognizable.

A recent trend in laboratory studies of sulfide flotation chemistry is to use a mixed mineral system. With this approach, an enhanced xanthate adsorption on anodic minerals by galvanic contact of dissimilar minerals was revealed. Also derived from this type of research is the depression of pyrite by copper sulfate addition in a sphalerite/pyrite mixed mineral system, shown in Figure 4, as opposed to pyrite activation in a single mineral system. A similar approach has been used in oxide and salt-type flotation systems.

In summary, our understanding of the interaction mechanism of collectors with minerals in a flotation system has evolved significantly following the development of modern instrumentation. Future advances in the fundamental understanding of flotation systems are anticipated with the introduction of the atomic force microscope in mineral flotation research. A combination of electrochemistry, in situ spectroscopy and surface imaging at a molecular level will enable us to pinpoint the mechanism and roles of collector-mineral interactions in flotation.

Figure 4 Flotation recovery of pyrite in the presence of 10~5mol L~1 iso-propylxanthate alone (squares) or with cupric ions (triangles), sphalerite (inverted triangles) or both (circles). The flotation of pyrite was depressed by a combination of cupric ions and sphalerite, although cupric ions alone activated pyrite flotation, illustrating the importance of studying flotation chemistry with mixed mineral systems in the context of the separation practice.

Figure 4 Flotation recovery of pyrite in the presence of 10~5mol L~1 iso-propylxanthate alone (squares) or with cupric ions (triangles), sphalerite (inverted triangles) or both (circles). The flotation of pyrite was depressed by a combination of cupric ions and sphalerite, although cupric ions alone activated pyrite flotation, illustrating the importance of studying flotation chemistry with mixed mineral systems in the context of the separation practice.

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