Xanthate with Sulfides

The interfacial behavior of alkyl xanthates (a typical member of the O-alkyldithio-carbonate collector family) (Fig. 7.22) is of great interest in flotation studies, where these reagents have been used for the selective hydrophobization of sulfides since 1925. A variety of hypotheses have been put forward to explain this phenomenon (see Refs. [365, 369, 464-466] for review) that can be classified as either chemical or electrochemical. The former category includes adsorption [467-469], coordination [470], and replacement of surface oxidation products [471-473] or lattice ions [474, 475]. However, this hypothesis neglects 

The elecirochemical mechanism, also called the mixed-potential mechanism [477], assumes charge transfer within a particle from the cathodic patch, at which oxygen is reduced, to the anodic one, at which the sulfide itself and/or xanthate anion are oxidized. This mechanism describes a broad spectrum of interfacial phenomena involving, as an intermediate step, a redox reaction in which the anodic and cathodic processes are spatially separated. Some examples of this include electrocatalytic chemisorption of xanthate and synthesis of dixanthogen and precipitation of xanthate-metal complexes (nncleation of a microphase of metal xanthate). In the latter reaction, the anodic sulfide dissolution is initiated with the ionization of surface metal atoms, and the metal ions thus produced on the surface are transferred into aqueous solution to form hydrated metal ions or metal-ion complexes associated with anions [478]. The ionization of surface metal atoms is an electrochemical oxidation, whereas the hydration or complex-ation of metal ions is a chemical process (an acid-based reaction). 

The analysis of IR spectra regarding the different adsorption forms of xanthate is considerably simplified by the sensitivity of IR spectra to the coordination of the xanthate group (Fig. 7.23) [369, 479]. As the degree of covalency of the bond between the sulfur atom of xanthate and the heavy metal cation 

The ex situ studies of xanthate adsorption under chemically controlled conditions have been conducted by transmission [487-489] and DRIFTS [375-377, 385, 386, 480, 484, 490-495] on powdered sulfides, by ATR on thin polycrystalline synthetic films of PbS [496-498] and single-crystal sphalerite [499], and by IRRAS on sulfide and metal plates [327, 329, 330, 481, 482, 500], 

Persson et al. [385, 386, 484, 493] performed comparative DRIFTS studies of ethyl xanthate (EX) adsorption on fresh and oxidized natural minerals and sulfur-and metal-rich synthetic CU2S, ZnS, CdS, and PbS following different pretreatments of the absorbents. Only small amounts of Cu(I)EX were found on the 

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The type of the oxidation product on galena is independent of the chemical environment during preparation. Rao152) measured the adsorption heat of K amyl xanthate (KAX) on unactivated and Cu2+-activated pyrrhotite (FeS) and compared his results with heats of the reaction between KAX and Fe2+ or Cu2+ salts. With the unactivated mineral, the interaction involves a chemical reaction of xanthate with Fe2+ salts present at the interface (i.e. not bound to the crystal surface). The adsorption enthalpy is identical with the formation of Fe2+ amyl xanthate FeS04 + 2 KAX —> FeX2 + K2S04, and -AH = 97.45 kJ/mol Fe2+). As revealed from the enthalpy values and the analysis of anions released into the solution, the interaction of xanthate with Cu2+-activated pyrrhotite consists of xanthate adsorption by exchange for sulfate ions (formed by an oxidation of sulfides) at isolated patches (active spots), and by further multilayer formation of xanthate. The adsorption heat of KAX on pyrrhotite at the initial pH 4.5 was - AH (FeS unactivated) = 93.55 kJ/mol Fe2+ and - AH (FeS activated) = 70.03 kJ/mol Cu2+. 

This includes reactions of the polymer groups with metallic sites on the particle surface that may result in the formation of stable or insoluble compounds through covalent, ionic or coordination bonding. Carboxyl flocculants such as polyacrylic acid and carboxyl-methyl cellulose can chemisorb on the surface of calcite and sphalerite which have calcium or zinc sites on them. Certain flocculants, such as cellulose and starch with xanthate and polyacrylamide with dithiocarbamate with high chemically active groups, have been found to exhibit selective reaction with sulfide minerals. Such complexing polymers have been investigated for their use in selective flocculation processes. 

The determination of flotation recovery using particulate sulfide mineral bed electrodes under potential control was refined by Richardson and co-workers and applied to the study of the interaction of ethyl xanthate with a range of sulfide minerals. " " The approach introduced by these authors was to employ relatively large particles (590-840 //m) and to keep the bed under positive pressure during potential conditioning. With this technique, they were able to combine flotation recovery determinations with voltammetry and with UV-vis spectroscopy of the solution phase. 

Carbon disulfide [75-15-0] is a clear colorless liquid that boils at 46°C, and should ideally be free of hydrogen sulfide and carbonyl sulfide. The reaction with alkaU cellulose is carried out either in a few large cylindrical vessels known as wet chums, or in many smaller hexagonal vessels known as dry chums. In the fully continuous viscose process, a Continuous Belt Xanthator, first developed by Du Pont, is used (15). 

AEROPHINE 3418A promoter is widely used ia North and South America, AustraHa, Europe, and Asia for the recovery of copper, lead, and ziac sulfide minerals (see Elotatton). Advantages ia comparison to other collectors (15) are said to be improved selectivity and recoveries ia the treatment of complex ores, higher recoveries of associated precious metals, and a stable grade—recovery relationship which is particularly important to the efficient operation of automated circuits. Additionally, AEROPHINE 3418A is stable and, unlike xanthates (qv), does not form hazardous decomposition products such as carbon disulfide. It is also available blended with other collectors to enhance performance characteristics. 

Zinc ores are generally floated at the mine (18). In the case of simple zinc sulfide ores, flotation is carried out by treatment with copper sulfate to activate the sphalerite causing it to be wet by the organic collector (eg, xanthate). The now-hydrophobic zinc ore particles attach themselves to the rising bubbles. Oxidized ore particles present must be sulftdized with sodium sulfide to be floated (19). Flotation produces concentrates which are ca 50—60% zinc. In mixed ore, the lead and copper are usually floated after depressing the sphalerite with cyanide or zinc sulfate. The sphalerite is then activated and floated. 

Sodium azide does not react with carbonyl sulfide to form 5-hydroxy-1,2,3,4-thiatriazole, nor with carboxymethyl xanthates, RO-CS SCH2COOH, to form 5-alkoxy-l,2,3,4-thiatriazoles. The latter, however, could be prepared from xanthogenhydrazides (RO-CS NHNH2) and nitrous acid. They are very unstable and may decompose explosively at room temperature only the ethoxy compound (6) has been examined in detail. This is a solid which decomposes rapidly at room temperature and even at 0°C is transformed after some months into a mixture of sulfur and triethyl isocyanurate. In ethereal solution at 20° C the decomposition takes place according to Eq. (16)... 

The adsorption of collectors on sulfide mineral occurs by two separate mechanisms chemical and electrochemical. The former results in the presence of chemisorbed metal xanthate (or other thiol collector ion) onto the mineral surface. The latter yields an oxidation product (dixanthogen if collector added is xanthate) that is the hydrophobic species adsorbed onto the mineral surface. The chemisorption mechanism is reported to occur with galena, chalcocite and sphalerite minerals, whereas electrochemical oxidation is reportedly the primary mechanism for pyrite, arsenopyrite, and pyrrhotite minerals. The mineral, chalcopyrite, is an example where both the mechanisms are known to be operative. Besides these mechanisms, the adsorption of collectors can be explained from the point of interfacial energies involved between air, mineral, and solution. 

The copper sulfide formed on the surface of the sphalerite mineral reacts readily with the xanthate, and forms insoluble copper xanthate, which makes the sphalerite surface hydro-phobic. Such a reaction for activating sphalerite occurs whenever the activating ions are present in the solution. It is thus necessary to deactivate sphalerite (to prevent the occurrence of natural activation) in the case of some ores. With lead-zinc ores, for example, natural activation occurs due to Pb2+ in solution... 

The products of hydrolysis and dissociation depend on the pH. In an acid medium, hydrogen sulfide, which has no depressing action, evolves. It is, therefore, necessary to use alkaline circuits in which HS, predominates. These sulfide ions are adsorbed on the copper sulfide mineral surface and react with the surface previously coated with cuprous xanthate. The reaction causes desorption of the collector, and as a result of this desorption the copper sulfide minerals generally become hydrophilic. There is, however, no action of the sulfide ions on molybdenite, and so molybdenite retains its hydrophobic character. 

Suitable collectors can render hydrophilic minerals such as silicas or hydroxides hydrophobic. An ideal collector is a substance that attaches with the help of a functional group to the solid (mineral) surface often by ligand exchange or electrostatic interaction, and exposes hydrophobic groups toward the water. Thus, amphi-patic substances (see Chapter 4.5), such as alkyl compounds with C to C18 chains are widely used with carboxylates, or amine polar heads. Surfactants that form hemicelles on the surface are also suitable. For sulfide minerals mercaptanes, monothiocarbonates and dithiophosphates are used as collectors. Xanthates or their oxidation products, dixanthogen (R - O - C - S -)2 are used as collectors for... 

The synthesis and some reactions of meso-ionic 1,2-dithioM-ones (388) have been recently reported. The brown compound 388, R = R = Ph, has been prepared by several methods (i) the reaction betw i l,l,3,3-tetrabromo-l,3-diphenylacetone (PhCBrjCOCBrjPh) and potassium ethyl xanthate, (ii) the reaction between 1,3-diphenyl-propanetrione hydrate and tetraphosphorus decasulfide, (iii) 1,3-diphenylpropanetrione with hydrogen sulfide-hydrogen chloride in ethanol-chloroform yields the salt 389, R = R = Ph, X = Cl, which gives the meso-ionic I,2-dithiol-4-one with triethylamine, pyridine, or aqueous sodium bicarbonate. ... 

Amphipathic substances such as we have discussed throughout this chapter are used as collectors. Alkyl compounds with C8 to C18 chains are widely used with carboxylate, sulfate, or amine polar heads. For sulfide minerals, sulfur-containing compounds such as mercaptans, monothiocarbonates, and dithiophosphates are used as collectors. The most important collectors for sulfides are xanthates, the general formula for which is... 

Ethylenimine reacts with xanthates to give a thi eolidine derivative ao (Eq. 67) and with hydrogen sulfide plus various ketones to give dialkyl thiazolidines 6 (Eq. 58). 

Interaction of 2-, 3- or 4-chlorobenzenediazonium salts with (9-alkyldithiocarbon-ate ( xanthate ) solutions [8] or thiophenoxide solutions [9] produces explosive products, possibly arenediazo aryl sulfides. The intermediate diazonium xanthate produced during the preparation of m-thiocresol can be dangerously explosive under the wrong conditions [8], while the reaction of 3-nitrobenzenediazonium chloride with xanthate solution at 70—75°C proceeds with near-explosive evolution of nitrogen [4]. The product of interaction of 2-chlorobenzenediazonium chloride and sodium 2-chlorothiophenoxide exploded violently on heating to 100°C, and the oil... 

Depressants (or deactivators) are chemicals that ensure that undesired particles remain hydrophilic and therefore do not get floated. Conversely to the activation of zinc sulfide by copper ions above, zinc ions from zinc sulfate act as a depressant for zinc sulfide. Another example is the use of cyanide to complex with copper and prevent adsorption of collectors in the flotation of base-metal sulfides with xanthates. There are many other depressants but they tend to be quite specific to one of a few types of minerals. In some cases, such as some uses of cyanide as a depressant, the mechanism of depressant action remains unclear. 

The rate constant of R with (Me3Si)3SiH is about 105 M-1 s-1. The rate constants of (Me3Si)3Sf with alkyl halides and selenides are as follows 109M-1 s-1 for alkyl iodides 108 107 M-1 s-1 for alkyl bromides and alkyl methyl xanthates 107 M 1 s 1 for alkyl selenides 106 M 1 s-1 for alkyl sulfides. 

The rest of the chapter has been devoted to special topics and in materials science there are many possibilities. Those selected include the mechanism of the flotation of minerals in which the addition of a certain organic to the solution causes a specific mineral to become hydrophobic so that it is exposed to air bubbles, the bubbles stick to it and buoy the mineral up to the surface, leaving unwanted minerals on the bottom of the tank. It turns out that the mechanism of this phenomenon involves a mixed-potential concept in which the anodic oxidation of the organic collector, often a xanthate, allows it to form a hydrophobic film upon a semiconducting sulfide or oxide, but only if there is a partner reaction of oxygen reduction. This continues until there is almost full coverage with the dixanthate, and the surface is thereby made water-repelling. 

Minute traces of suspended sulfur resulting from the chemical decomposition of cellulose xanthate must be removed by washing with a solution of sodium sulfide. It is expedient to bleach the newly formed fibers with hypochlorite to improve their whiteness an antichlor follows. The chemicals originally present and those used to purify the fibers must be removed by washing. As a final step, a small amount of lubricant is placed on the filaments to reduce friction and improve processibility in subsequent operations. 

Metal-Organics. Many organic materials also form low-solubility species with certain metals. Among these are humic acids. The most widely publicized insoluble substrate for heavy-metal immobilization has been insoluble starch xanthate (ISX). In contact with metal ions, the metal links to the sulfur group much as it would with the S-2 in inorganic sulfides ... 

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