Project topics and materials
Get Complete Project Material File(s) Now! »
FROTH FLOTATION FOR THE BENEFICIATION OF PCB FINES
INTRODUCTION
As presented in Chapter 2, the material constituents of the comminution fines will be quite diverse. This will include copper from the traces; solder remains; precious metals – gold, palladium, platinum – relatively in trace proportions; chipped or torn particles of various alloys; synthetic or natural ceramics used in certain resistors, semiconductors, glazed components and chips; resin materials like epoxy; particles of plastics used in slots; and board reinforcement material such as glass fiber. It follows that flotation of native metals, plastics, and ceramics (oxides, conventional minerals) are all of relevance towards evolving logical flotation schemes for the sample. These and other relevant concepts are reviewed in this chapter. The concepts are used to advance probable schemes, and to make more specific objective statements for the investigation. As this application will traverse a broad spectrum of surfactants, a brief on surfactants are presented first.
SURFACE ACTIVE AGENTS
Surface active agents (surfactants) are organic compounds with a heteropolar molecular structure. The non-polar hydrocarbon chain group of the molecule prefers to attach to air, while the polar functional groups prefer aqueous phase. The surface activity derives from this property. They can therefore adsorb (accumulate) at air-water, air-mineral and/or water-mineral interfaces. The specific application of a surfactant is determined by the properties of the polar functional group, which can be ionic – cationic or anionic, or non-ionic. Ionic surfactants are electrolytes. They can adsorb at mineral-water interfaces, electrostatically and/or chemically. This makes them useful as collectors. Non ionic surfactants are non-electrolytes, and they adsorb majorly at air-water interface. Table 3.1 shows some common surfactants compounds. The adsorption at air-water interface lowers the surface tension of the solution. This makes thin films of the solution metastable and therefore supports frothing. It also creates wetting effects, lowering overall surface energy and tension.
where i1 refers to the surface excess (adsorption density) of specie i relative to the solvent at the interface, and ai is the activity of i, which approximates the concentration of i, Ci, in very dilute solutions.
However, Eq. 3.2 is not a continuous function. The surface tension decreases towards a limiting value which is that of the surfactant, and Ci is defined within the solubility range of the surfactant, beyond which micelle or droplet formation (phase separation) commences. With further increase in concentration such droplet will act as antifoamer in frothing application (Pugh, 2007). The selective wetting achievable by surface tension control is the basis for gamma flotation separation of plastics (see Section 3.4).
FLOTATION OF METALS AND ALLOYS FOR PCB FINES APPLICATION
Many studies have shown that metallic particles can be rendered hydrophobic and floated. With higher surface energies (than plastics), metallic particles can react with collector molecules (chemisorption) forming hydrophobic surfaces. Woods (1996) included the adsorption of xanthates on gold, platinum, silver and copper in a detailed review of chemisorption of thiols on metals and metal sulphides. For platinum and gold, voltamogramms of their electrodes in ethyl xanthate indicate anodic peaks at the region of the reversible potentials of the ethyl xanthate/dixanthogen couple:
2C2H5OCS2‾ (C2H5OCS2)2 + 2é
Peaks corresponding to the cathodic reduction of the anodic oxidation product can also be observed on the reverse potential scan. It was inferred that anodic oxidation of ethyl xanthate to dixanthogen and adsorption of the latter occurs on the metals. Tafel slope for the anodic oxidation, however, indicate that the oxidation to the dimer can proceed via an initial chemisorption step
2C2H5OCS2‾ 2(C2H5OCS2)ads + 2é = (C2H5OCS2)2 + 2é
Lins and Adamian (1993) used amyl xanthate as a collector to study the effects of some physical variables on gold flotation. Good recoveries were obtained for gold from a synthetic mixture of silica and gold particles over 0.16 mm – 0.71 mm size ranges. About 90 % recovery and 5 kg/ton gold grade in the float was achieved from a 167 g/ton feed with 0.16 mm dA sample at 18% pulp density (Figure 3.1). The work of Lins and Adamian is notable considering the good flotation of 0.71 mm gold particles that was claimed.
Dicresyl monothiophosphate (DCMTP) was used to achieve recovery of native gold against sulphides at a pH above 7 in some specific ores (Nagaraj et al., 1991). Basilio et al. (1992) suggested that DCMTP does not have effect on the floatability of pure gold as such, and infra red spectroscopic measurement confirming this. The improved gold recovery was linked to silver-DCMTP interaction in further studies (Nagaraj et al., 1992). It was shown by X-ray photoelectron spectroscopy (XPS) that more DCMTP was adsorbed, and higher recovery observed, when the percentage of silver alloyed with the native gold was higher. Hence, only silver-bearing native gold will respond to DCMTP. The interesting context here is that adsorption on the alloy surface is synergistic. It can therefore be projected that alloys in the PCB mixture will respond to a collector as much as one of its constituent elements interacts with that collector.
Forrest et al. (2001) using a range of collectors (Table 3.2) at varying pH on a free-gold bearing copper-pyrite ore, obtained free gold recovery from the sulphides at d90 of 106 µm. While chalcopyrite recovery was almost independent of pH in the range 8 – 13, and pyrite recovery drop with increasing pH over the range (see Figure 3.2), gold grade shot up above pH 11.5 (Figure 3.3) for aeroflot 7249 and aeroflot 208. Selectivity for free gold by these collectors was concluded for the gold grade increase at pH above 11.5, since pyrite was depressed and chalcopyrite recovery remained almost the same over the pH range.
Although reports on flotation of metals have concentrated more on those that occur in native forms which are the real situations in flotation operations, many metals and alloys in the PCB fines that do not naturally occur in native forms can also be expected to respond to collectors. Auger electron spectroscopy (AES) and XPS studies of the chemisorbed xanthate monolayer on chalcocite and galena showed the same chemical environment for the metal atoms in the substrate and the monolayer xanthate (Buckley and Woods, 1990, 1991; Shchukarev et al., 1994). This indicates direct interaction with the metal atoms in a substrate, and implies that under adequate conditions, the metals will interact with collectors and float. However, the potentials at which a collector compound will form on a pure metal and on its mineral compound are expectedly different (Woods, 1996).
As in conventional mineral processing, activation with CuSO4 should also apply for particles (such as of zinc) whose collector-metal compounds have fairly high Keq (dissociation constant) which makes their surface product fairly soluble and unstable. Possibility of unselective activation can be very important in a pulp with such diverse constituent. Mercaptans, such as sodium mercaptobenzothiazole (SMBT), are a selective collector of native copper (Wills, 1997) and it can be expected that this can be applied in obtaining a selective metal fraction.
In the event of certain alloy particles having tarnished or oxidised surface layer, sulphidation activation should be applicable as with conventional minerals to make such particle interact better with sulphydryl collectors. By addition of sodium sulphide (Na2S), hydrolysis to sodium hydroxide and hydrogen sulphide occur (Wills, 1997). The hydrogen sulphide dissociates giving hydrosulphide ions (HS–). These adsorb and dissociate at the surface of the particle, causing sulphur ions to pass into the crystal lattices:
Na2S + 2H2O2NaOH + H2S.
H2S H+ + HS–
HS– H+ + S2-
This gives a pseudo insoluble sulphide surface, allowing interaction with sulphydryl collectors. Careful dosage of this sulphidisers is very important because excess HS– ion at a surface gives the particle a high negative surface charge. This prevents adsorption of anionic collectors, causing depression, while too little dosage will not maximize the conditioning effect desired (Wills, 1997; Lee et al., 1998). The dosage required for a system will depend on the pulp pH. At high pH values, equation 3.6b and 3.6c proceed farther to the right, generating the depressant ions. Because of the possibility of altering (increasing) pulp alkanility due to NaOH dissociation from Equation 3.6a, sodium hydrogen sulphide (NaHS) is preferred to sodium sulphide for sulphidation.
In the event of comminution, it is possible that the particle surfaces will be tarnished or oxidised. In this regard, SMBT has been found effective collector for oxidized copper and tarnished ores (Wills, 1997). Also, from general flotation of oxide and silicate minerals, collectors that mainly physisorbs (interacts electrostatically), such as amines and sulphonates can be used.
Flotation of glass fiber can also be conceived, considering the flotation of quartz from heamatite. A reverse flotation of heamatite employs amine at pH 6-7. The amine adsorb on the negatively charged quartz particle to float, leaving the heamatite particles that are relatively neutral at that pH range (Fuerstanau and Healy, 1972).
Another approach floats quartz activated with calcium ion at pH 11-12 with a soap collector, while using starch to depress heamatite. Soap application which can also have the effect of gamma depression (see Section 3.5.2.4) is notable here, but isolating the activity or interactions may not be easy in this very diverse material mixture: depression is already involved, and influences on other particles exist. Interwoven and complicated reagent effects are not unexpected in a sample of this type.
As addressed above, different collectors used with native metal and alloys, and tarnished ores, in conventional minerals processing and in various investigations, can be expected to interact with the metals and alloys in the PCB mixture – xanthates, amines, SMBT are all candidate collectors. However, appropriate pH is critical to reagent-surface interaction.
Selectivity by pH Control
Pure water has a dissociation constant of 10–14 in which –log [OH–] = –log [H+] = 7, the pH of neutral water. The concentrations of OH– and H+ vary in proportion to maintain the equilibrium constant at other pH values. In aqueous medium, OH– and H+ are potential determining ions for particles of oxide minerals and those with bases in their dissociation products, such as carbonates (Kelly and Spottiswood, 1989). The pH of the medium therefore determines the surface charges or neutrality (point of zero charge, or PZC) of the suspended particles. Response to ionic collectors is a function of surface charge condition, and hence a function of pH. This is a basis of selectivity by pH control.
Hydration of the surface and the stable surface species similarly depends on the pH. With iron, for example, the Eh-pH diagram for Fe-O-H system indicates Fe(OH)3 as the stable specie for most of the positive potentials at alkaline pH (see Figure 3.4). This frustrates formation of the less stable metal-collector compound with xanthate collectors at high pH values. Pyrite, for example, will therefore float under xanthate only at pH around 4 or below (Wills, 1997).
Acknowledgement
Abstract
List of Figures
List of Tables
1.0 INTRODUCTION
2.0 BACKGROUND – PCB PHYSICAL PROCESSING
2.1 Introduction
2.2 PCB Characterization
2.2.1 Occurrence and Reserve
2.2.2 PCB Structure
2.2.3 PCB Material Make-Up
2.2.4 Physical Processing Implication
2.3 PCB Physical Processing Operations
2.4 Improving PCB Physical Processing: Fines Beneficiation
3.0 FROTH FLOTATION FOR BENEFICIATION OF PCB FINES
3.1 Introduction
3.2 Surface Active Agents
3.3 Flotation of Metals and Alloys for PCB Fines Application
3.3.1 Selectivity by pH Control
3.4 Selective Wetting of Plastics for PCB Fines Flotation Application: Gamma Flotation
3.5 Probable Flotation Schemes
3.6 Applicable Range of Kinetic Parameters and Sample Characterization
3.7 Investigation Objectives
4.0 MATERIALS AND METHODS
4.1 Introduction
4.2 PCB Comminution Fines Generation
4.3 Sample Characterization
4.4 Preliminary Microflotation Investigation
4.5 Applicable Kinetic Regime and the Natural Hydrophobic Response (NHR) Flotation Scheme
4.6 Chemical Conditioning Schemes
4.7 Follow Up Investigations from Chemical Conditioning Schemes
5.0 CHARACTERIZATION OF PCB COMMINUTION FINES FOR FROTH FLOTATION INVESTIGATION
5.1 Introduction
5.2 Density and Particle Size Distribution
5.3 Morphology and Liberation Assessment
5.4 Comparative Wet Spectroscopic Analysis
5.5 Thermogravimetric Analysis for Organic Constituents
5.6 Conclusion
6.0 NATURAL HYDROPHOBIC RESPONSE AND FAVORABLE KINETICS FOR PCB FROTH FLOTATION
6.1 Introduction
6.2 Preliminary Microflotation
6.3 The Natural Hydrophobic Response Scheme
6.4 Conclusion
7.0 INVESTIGATION OF CHEMICAL CONDITIONING SCHEMES FOR FROTH FLOTATION OF PCB CF
7.1 Introduction
7.2 Macromolecular Versus Gamma Depression
7.3 PAX Conditioning Schemes
7.4 SMBT Conditioning
7.5 TBAC Conditioning
7.6 Calcium dissemination in PCB CF and Presence in Process Water
7.7 SEM and AES Investigation of Particle Surfaces
8.0 CONCLUSIONS
REFERENCES
GET THE COMPLETE PROJECT
