1 Colloid Chemistry in Mineral Processing 1. 1. Work of adhesion and work of cohesion The work of adhesion between two phases (for example, between two immiscible liquids, or between solid and liquid) is equal to the reversible work required Wu=2YLv=-△G yur Yaw WSL=YSV+yLV-YSL=-△G yu ysr Fig. 1.1 to separate unit area of the liquid-liquid interface(for a two phase liquid-liquid system) and form two separate liquid air interfaces(see Fig. 1.1)and is given by the Dupre equation Wa=Ya+Yb-Yab (1.1) where ya and yb are the surface tensions of phases a and b, respectively, and Yab is the interfacial tension a-b. The work of cohesion for a single liquid corresponds to the work required to pull apart a column of liquid of unit cross-sectional area and is given by W2=2y。 (1.2) 1.2. Wettability In the case of a solid wetted by a liquid, the wettability is determined by the solid- liquid work of adhesion(WsL) and the work of cohesion of liquid (WLL). The classical boundary condition for the hydrophilic-hydrophobic transition is equality of Wst and WLL. According to Dupre's equation (Eq. 1.1)， for a solid-liquid system Wst is given by WSL=YSv+YLV-YSL (1.3) and
1 Colloid Colloid Colloid Colloid Chemistry Chemistry Chemistry Chemistry in Mineral Mineral Mineral Mineral Processing Processing Processing Processing 1. 1. Work of adhesion adhesion adhesion adhesion and work of cohesion cohesion cohesion cohesion The work of adhesion between two phases (for example, between two immiscible liquids, or between solid and liquid) is equal to the reversible work required Fig. 1.1 to separate unit area of the liquid-liquid interface (for a two phase liquid-liquid system) and form two separate liquid air interfaces (see Fig. 1.1) and is given by the Dupre equation Wa a b ab = γ + γ − γ (1.1) where γa and γb are the surface tensions of phases a and b, respectively, and γab is the interfacial tension a-b. The work of cohesion for a single liquid corresponds to the work required to pull apart a column of liquid of unit cross-sectional area and is given by Wc a = 2γ (1.2) 1.2. Wettability Wettability Wettability Wettability In the case of a solid wetted by a liquid, the wettability is determined by the solidliquid work of adhesion (WSL) and the work of cohesion of liquid (WLL). The classical boundary condition for the hydrophilic-hydrophobic transition is equality of WSL and WLL. According to Dupre’s equation (Eq. 1.1), for a solid-liquid system WSL is given by WSL SV LV SL = γ + γ − γ (1.3) and
2(1.4)Wu=2LvSince there are no experimental methods to determine the surface tension of asolid (sv and s) equation 1.3 can only be solved with the use of Young's equation(Fig, 1.2)(1.5)YSV=SL+YLVCOSOYtvYsvFig. 1.2. Sessile drop on a smooth solid surfaceBy combining Eqs. 1.3. and 1.5 one can get(1.6)Wst=Lr(cosO+I)Since both liquid surface tension (yv)and the contact angle ()can experimentally bedetermined, equation 1.6 is often used.Equations 1.4 and 1.6giveWa - r(I+cos @)(1.7)Wu2LVandWsL.(1.8)cOsO=2WuThe condition of hydrophobicity follows from Eq. 1.8; only for Ws.<Wu①+0.Fig. 1.3 depicts a sessile droplet resting on a flat solid surface; as this figureshows, the contact angle at the solid/liquid interface is determined by the values of theCOHESIONGasLiquidADHESIONSolid
2 WLL LV = 2γ (1.4) Since there are no experimental methods to determine the surface tension of a solid (γSV and γSL) equation 1.3 can only be solved with the use of Young’s equation (Fig. 1.2) γ SV = γ SL + γ LV cosΘ (1.5) Fig. 1.2. Sessile drop on a smooth solid surface. By combining Eqs. 1.3. and 1.5 one can get = (cosΘ + 1) WSL LV γ (1.6) Since both liquid surface tension (γLV) and the contact angle (Θ) can experimentally be determined, equation 1.6 is often used. Equations 1.4 and 1.6 give LV LV LL SL W W γ γ 2 (1+ cos Θ) = (1.7) and cosΘ = 2 −1 LL SL W W (1.8) The condition of hydrophobicity follows from Eq. 1.8; only for WSL <WLL Θ ≠ 0. Fig. 1.3 depicts a sessile droplet resting on a flat solid surface; as this figure shows, the contact angle at the solid/liquid interface is determined by the values of the
3work of adhesion of the liquid to the solid, and the work of cohesion of the liquidTheworkofadhesionofliquidtosolid(Wsi)canbesplitintoseveralcontributions (Fowkes). For water molecular interacting with a solid, the most importantare:van der Waals dispersion forces contribution (Ws),hydrogen bond contribution(Wsth)and, if the solid has an electrical charge,an electrical forces contribution (Ws)ThenWst = Ws + W +Ws(1.9)For the solid which does not have any polar groups and, therefore, does not interact withwater molecules through hydrogen bonding ( wst ~ O), and does not have an electricalcharge so that W ~0, substitution of Eq 1.9 to Eq. 1.8 giveswi(1.10)cOsO~21WuOMooSoCFig.1.4.Graphite and molybdenitecrystal latticesUsing this equation, Laskowski and Kitchener (1969) showed that because ofexceptionallyhighworkof cohesionofwater(Wu=145mJ/m2)forall solidsWdO)if they interacted withwater only through dispersion forces().This explains why some minerals (such asgraphite,molybdenite, sulfur, talc)are hydrophobic by nature since. This is perfectly in line with Gaudin who pointed out that all hydrophobic solids presentnon-polar molecular groups whereas hydrophilic solids have ionic or dipolar groupscapable of undergoing hydrationBy writing Eq. 1.8 in the formWs+Ws+Ws-1COsO=2(1.11)Wu
3 work of adhesion of the liquid to the solid, and the work of cohesion of the liquid. The work of adhesion of liquid to solid (WSL) can be split into several contributions (Fowkes). For water molecular interacting with a solid, the most important are: van der Waals dispersion forces contribution (WSL d ), hydrogen bond contribution (WSL h ) and, if the solid has an electrical charge, an electrical forces contribution (WSL e ). Then e SL h SL d WSL =WSL +W +W (1.9) For the solid which does not have any polar groups and, therefore, does not interact with water molecules through hydrogen bonding ( ≈ 0 h WSL ), and does not have an electrical charge so that ≈ 0 e WSL , substitution of Eq 1.9 to Eq. 1.8 gives cosΘ ≈ 2 −1 LL d SL W W (1.10) Fig. 1.4. Graphite and molybdenite crystal lattices. Using this equation, Laskowski and Kitchener (1969) showed that because of exceptionally high work of cohesion of water (WLL = 145 mJ/m2 ) for all solids LL d WSL 0) if they interacted with water only through dispersion forces( ). This explains why some minerals (such as graphite, molybdenite, sulfur, talc) are hydrophobic by nature since . This is perfectly in line with Gaudin who pointed out that all hydrophobic solids present non-polar molecular groups whereas hydrophilic solids have ionic or dipolar groups capable of undergoing hydration. By writing Eq. 1.8 in the form cos 2 −1 + + Θ = LL e SL h SL d SL W W W W (1.11)
4it becomes obvious that solid surface properties are strongly affected by the electricalsurface charge.1.3.Origin of electrical charge at solid/liquid interface(A). Ionic Solids. In the case of ionic solids such as Agl, BasO4, CaF2 etc. (soluble saltsalso belong to this group but will be discussed separately), the surface charge arises fromthetransportacrosstheinterfaceof ionsconstitutingthelattice.Thoseparticularionsthatare free to pass between both phases and therefore establish the electrical charge arecalledpotential-determiningions(PDI).ForAgthepotential-determiningionsareAgtand I,for a solid like calcite, CaCOs, the potential-determining ions are Ca2+ and CO,2but since their concentration also depends on pH, the potential-determining ions for thismineral are Ca 2+, CO, 2- and also Ht, OH and HCO3Agl/aqueous solutioninterphaseFig.1.5.Agl/aqueous solution interfaceThe surface charge of a solid is determined by theconcentration of potential-determiningionsonthesolidsurface(orinotherwordsisdeterminedbyadsorptiondensityofpotential-determining ions). In case of Agl surface, surface charge is given by(1.12)0, = F(T+-F-)where FAg+ is the adsorption density in mol/cm? of Agt ions, and Fi is that ofI ion, and Fis theFaraday constant.Agl is practically insoluble in water. The solubility product of AglK,sp = [Ag [/-] = 10-16(1.13)It was experimentally determined that in the system in which Agl is in equlibriumwith aqueous solution, the Agl/water interface does not have electrical charge at Agt ionsconcentration [Ag*] = 10-5.5 mol/dm3. In analogy with pH (pH = -log [H*D), suchconcentrations are expressed as pAg+=5.5. (pAg*=-log [Ag*1)
4 it becomes obvious that solid surface properties are strongly affected by the electrical surface charge. 1.3. Origin of electrical electrical electrical electrical charge at solid/liquid solid/liquid solid/liquid solid/liquid interface interface interface interface (A). Ionic Solids. In the case of ionic solids such as AgI, BaSO4, CaF2 etc. (soluble salts also belong to this group but will be discussed separately), the surface charge arises from the transport across the interface of ions constituting the lattice. Those particular ions that are free to pass between both phases and therefore establish the electrical charge are called potential-determining ions (PDI). For AgI, the potential-determining ions are Ag+ and I - , for a solid like calcite, CaCO3, the potential-determining ions are Ca 2+ and CO3 2- but since their concentration also depends on pH, the potential-determining ions for this mineral are Ca 2+ , CO3 2- and also H+ , OHand HCO3 - . Fig. 1.5. AgI/aqueous solution interface The surface charge of a solid is determined by the concentration of potential-determining ions on the solid surface (or in other words is determined by adsorption density of potential-determining ions). In case of AgI surface, surface charge is given by = (Γ + − Γ − ) Ag I σ s F (1.12) where ΓAg+ is the adsorption density in mol/cm2 of Ag+ ions, and ΓI- is that of I - ion, and F is the Faraday constant. AgI is practically insoluble in water. The solubility product of AgI 16 [ ][ ] 10 + − − KSP = Ag I = (1.13) It was experimentally determined that in the system in which AgI is in equlibrium with aqueous solution, the AgI/water interface does not have electrical charge at Ag+ ions concentration [Ag+ ] = 10-5.5 mol/dm3 . In analogy with pH (pH = -log [H+ ]), such concentrations are expressed as pAg+ = 5.5. (pAg+ = - log [Ag+ ])
5Theprevious equations is commonly written(1.14)pAgt + pI = 16(in the same way for H+ and OH ions in water, pH + pOH= 14)So, the point-of-zero-charge (p.z.c.) conditions for Agl crystals, which are inequilibrium with aqueous solution, are reached when the concentrations of potential-determining ions are: pAg*=5.5 (and pl = 10.5, since pAg+pl =16). In other words,at p.z.c.the concentration of [Ag*] =10-5.5.mol/dm3 is 10s times higher than theconcentration ofl ions (since at pzc pl = 10-10.5 mol/dm)For ionic solids the surfacepotential, o,can be calculated from the equationCRT(1.15)PZFCrFor the case of Agl this equation at room temperature givesY。= 0.059(pAg*o - pAgt) = 0.059(pl- pL) Volt(1.16)so to sum it up, particles of Agl in aqueous solutions are not electrically charged onlywhen [Ag'] = 10-5.5 (thus when []= 10-10.5). This shows that the concentration of silverionsinthesolutionmustbe1ostimeshigherthantheconcentrationof iodideionsfortheAgl particle not to have an electrical charge. This results from much higher hydrationenergy of Agions in comparison with that of I ions.+118459ASV42AA办814131218151067pl
5 The previous equations is commonly written pAg+ + pI - = 16 (1.14) (in the same way for H+ and OHions in water, pH + pOH = 14) So, the point-of-zero-charge (p.z.c.) conditions for AgI crystals, which are in equilibrium with aqueous solution, are reached when the concentrations of potentialdetermining ions are: pAg+ = 5.5 (and pI - = 10.5, since pAg+ + pI - = 16). In other words, at p.z.c. the concentration of [Ag+ ] = 10-5.5 . mol/dm3 is 105 times higher than the concentration of I - ions (since at pzc pI - = 10-10.5 mol/dm3 ). For ionic solids the surface potential, ψo , can be calculated from the equation o M M o C C zF RT + + Ψ = ln (1.15) For the case of AgI this equation at room temperature gives 0.059( ) 0.059( ) o o o pAg pAg pI pI + + − − Ψ = − = − Volt (1.16) so to sum it up, particles of AgI in aqueous solutions are not electrically charged only when [Ag+ ] = 10-5.5 (thus when [I - ] = 10-10.5 ). This shows that the concentration of silver ions in the solution must be 105 times higher than the concentration of iodide ions for the AgI particle not to have an electrical charge. This results from much higher hydration energy of Ag+ ions in comparison with that of I - ions
6Fig. 1.6. Surface potential at silver iodide/aqueous solution interfaceTable 1.1. The point-of-zero-charge of some ionic solids(after D.W. Fuerstenau, Principles of Flotation, Johannesburg, 1982, pp. 17-30)Materialpz.c.Barite,BaSOpBa 6.7Calcite, CaCO3pH9.5*pH 6*Fluoroapatite,Cas(PO)3(F,OH)pCa 3Fluorite, CaF2Hydroxyapatite, Cas(PO4):OHpH 7pCa4.8Scheelite, CaWO4Silver chloride, AgCIpAg 4pAg 5.6Silver iodide, AglpAg 10.2Silver sulfide, Ags* from the hydrolysis equilibria and solubility data.(B). Simple oxides (e.g. SiO2, SnO2, TiO2, Fe2O3, Al2O3, etc.). Quartz is probably themost researched mineral, and one of the least impure. Crystallographically, it is built bythree-dimensional network of alternating Si4+ and O2-ions. The Si-O bond is said to haveabout 50% covalent character and is certainly strong. Neither of the normal ions can existas such in water, quartz has an appreciable solubility amounting to about 10 mg/dm at25 °C, resulting from the reversible reaction SiO2 + 2H20= Si(OH)4aq. As the orthosilicicacid is a weak acid, rise of pH displaces the reaction to the right as silicate ions areformed and so increases the solubilityThe surface of quartz combines with water, which can be removed by heatingabove about 400 °C.Infra-red spectroscopy shows that siloxane groups, -SiOSi-areconverted by water into silanol groups, -SiOH, which are weakly acidic This is whyquartz in water shows a negative zeta potential, reaching-120 mV in very dilute alkaliand falling to nearly zero at about pH 2. The hydroxylated quartz is strongly hydrophilic,showing zero contact angle and a thick equilibrium film due to the electrical double layer.but when the surface is dehydroxylated by strong heating it becomes definitelyhydrophobic.Oxides are amphoteric and their surfaces after hydroxylation show a positivecharged at low pH and a negative at high pH, with a point-of-zero-charge (p.zc.)somewhere between.It is customary to ascribe the origin of the electrical charge at the oxidesurface/aqueous interfacetoprotonation/deprotonation of the surfaced hydroxyls-MOH + H+ = -MOH2*(1.17)-MOH + OH =-MO +H2Oand at p.zc
6 Fig. 1.6. Surface Surface Surface Surface potential potential potential potential at silver iodide/aqueous iodide/aqueous iodide/aqueous iodide/aqueous solution solution solution solution interface interface interface interface Table 1.1. The point-of-zero-charge point-of-zero-charge point-of-zero-charge point-of-zero-charge of some ionic solids (after D.W. Fuerstenau, Fuerstenau, Fuerstenau, Fuerstenau, Principles Principles Principles Principles of Flotation, Flotation, Flotation, Flotation, Johannesburg, Johannesburg, Johannesburg, Johannesburg, 1982, pp. 17-30) Material Material Material Material p.z.c. Barite, Barite, Barite, Barite, BaSO4 Calcite, Calcite, Calcite, Calcite, CaCO3 Fluoroapatite, Fluoroapatite, Fluoroapatite, Fluoroapatite, Ca5(PO4)3(F,OH) Fluorite, Fluorite, Fluorite, Fluorite, CaF2 Hydroxyapatite, Hydroxyapatite, Hydroxyapatite, Hydroxyapatite, Ca5(PO4)3OH Scheelite, Scheelite, Scheelite, Scheelite, CaWO4 Silver chloride, chloride, chloride, chloride, AgCl Silver iodide, iodide, iodide, iodide, AgI Silver sulfide, sulfide, sulfide, sulfide, AgS pBa 6.7 pH 9.5* pH 6* pCa 3 pH 7 pCa 4.8 pAg 4 pAg 5.6 pAg 10.2 * from the hydrolysis equilibria and solubility data. (B). Simple oxides (e.g. SiO2, SnO2, TiO2, Fe2O3, Al2O3, etc.). Quartz is probably the most researched mineral, and one of the least impure. Crystallographically, it is built by three-dimensional network of alternating Si4+ and O2- ions. The Si-O bond is said to have about 50% covalent character and is certainly strong. Neither of the normal ions can exist as such in water; quartz has an appreciable solubility amounting to about 10 mg/dm3 at 25 oC, resulting from the reversible reaction SiO2 + 2H2O = Si(OH) 4aq. As the orthosilicic acid is a weak acid, rise of pH displaces the reaction to the right as silicate ions are formed and so increases the solubility. The surface of quartz combines with water, which can be removed by heating above about 400 oC. Infra-red spectroscopy shows that siloxane groups, -SiOSi- are converted by water into silanol groups, -SiOH, which are weakly acidic This is why quartz in water shows a negative zeta potential, reaching –120 mV in very dilute alkali and falling to nearly zero at about pH 2. The hydroxylated quartz is strongly hydrophilic, showing zero contact angle and a thick equilibrium film due to the electrical double layer; but when the surface is dehydroxylated by strong heating it becomes definitely hydrophobic. Oxides are amphoteric and their surfaces after hydroxylation show a positive charged at low pH and a negative at high pH, with a point-of-zero-charge (p.z.c.) somewhere between. It is customary to ascribe the origin of the electrical charge at the oxide surface/aqueous interface to protonation/deprotonation of the surfaced hydroxyls: -MOH + H+ = -MOH2 + (1.17) -MOH + OH- = -MO- + H2O and at p.z.c
7[-MOH2+1 = [-MO-](1.18)As these reactions reveal, H* and OH ions are potential-determining ions foroxides. Concentration of these ions is measured as pH.In practice, it is common to characterize the electrical charge of solid particlesindirectly by measuring so-called electrokinetic potential (also know as zeta potential,since Greek letter, E (zeta), is internationally used as symbol for electrokinetic potential)Next figure shows the zeta potential-pH curves for various oxides. The pHvalues at which zeta potential is equal zero (=O) is called iso-electric-point (i.e.p.). It isusually identital (or close) to the p.z.c. As this figure shows why quartz particles in waterare practically always negatively charged, iron oxide particles are positively charged atpH6.5. For alumina the i.e.p. =9. This also indicatesthat while silica is more acidic alumina is more basic.It also explains, why for someminerals (such as oxides)pH affects so strongly their flotation properties.Al,OTi02FeOAETLLNELOENFig.1.7.Electrokinetic2PHpotential (zeta26potential)ofoxides vs. pH at40constantionicstrength.6sioC). Alumino-silicates (kaolinite, illite, montmorillonite,etc.)Tetrohedral SheeS14*(AI3*)O".OH"A/S+Octahedral ShO".OH"Si4(AIS*)Tetrohedrol SheFig. 1.8. Idealized sketch of the atomic structure of a three-layered clay mineralThenatureoftheclay-water interface is an illustrationof theinfluenceof crystalstructure on the surface properties of solids.The basic building units of clay minerals are
7 [-MOH2 + ] = [-MO- ] (1.18) As these reactions reveal, H+ and OHions are potential-determining ions for oxides. Concentration of these ions is measured as pH. In practice, it is common to characterize the electrical charge of solid particles indirectly by measuring so-called electrokinetic potential (also know as zeta potential, since Greek letter, ξ (zeta), is internationally used as symbol for electrokinetic potential). Next figure shows the zeta potential – pH curves for various oxides. The pH values at which zeta potential is equal zero (ξ=0) is called iso-electric-point (i.e.p.). It is usually identital (or close) to the p.z.c. As this figure shows why quartz particles in water are practically always negatively charged, iron oxide particles are positively charged at pH 6.5. For alumina the i.e.p. ≈9. This also indicates that while silica is more acidic alumina is more basic. It also explains, why for some minerals (such as oxides) pH affects so strongly their flotation properties. C). Alumino-silicates Alumino-silicates Alumino-silicates Alumino-silicates (kaolinite, (kaolinite, (kaolinite, (kaolinite, illite, illite, illite, illite, montmorillonite, montmorillonite, montmorillonite, montmorillonite, etc.). Fig. 1.8. Idealized sketch of the atomic structure of a three-layered clay mineral. The nature of the clay-water interface is an illustration of the influence of crystal structure on the surface properties of solids. The basic building units of clay minerals are Fig.1.7. Electrokinetic potential (zeta potential) of oxides vs. pH at constant ionic strength
8the sheets formed by linking together SiO tetrahedral and the two dimensional arrays ofoctahedral formed by the sixfold coordination of A/3+andMg2+ with oxygen and hydroxyl groups (see Fig.1.8).This group has been considerablystudied because of the importance of clays. Their special feature is their laminar crystalstructure, the exposed basal planes differ in topochemistry"from the edges of thecrystals. In case of montmorillonite the basal planes behave more-or-Fig.1.9.The edgeof montmorilloniteplatelet.Less as simple silicate(because the hydrated alumina layer is sandwiched between a pairof silica layers) while alumina is exposed at the edges. As a result of isomorphoussubstitution ofsomesilicon(Si4+)foraluminum(A/3+),thefacesurface carriesanegative electrical charge while =Al(OH)groups at the edges are either positively ornegativelychargeddependingonpHInneutralmediathefacesarenegativelychargedbut the alumina "sites" on the edges are positively charged and this is the origin ofcoagulation of clays in distilled water. In beneficiation of China clays,akaolin clay isfirstdispersedinalkalinepHlargerimpurityparticles(quartz,hematiteanataseetc.)are removed by classification, and the fine clay particles which constitute the finalproduct are thickened and filtered after adjusting pH down to acidic range.(D). Sulfide minerals. Minerals such as galena (PbS), sphalerite (ZnS), chalcocite (CuS),chalcopyrite (CuFeS2), etc. are important sources of metals and their surface chemistry isofvast interest in connection with the froth flotation process.In principle, such sulfides are in reversible equilibrium with aqueous metal andsulfide ions, according to a definite solubility products (Ksp)which are extremely low(for instance, for galena, Ksp = 10-28). But in the presence of dissolved oxygen galenaundergoessuperficialoxidationwithvariousoxidation products(leadhydroxide,leadthiosulfate, lead sulfate, ect) deposited on the surface. As sulfides are electronic semi-conductors,theiroxidation processes takeonthecharacter ofelectrolytic corrosionreactions, certain (anodic) areas oxidize preferentially while other are cathodic, as in therusting of iron.Anodic:MeS=Me2++So+2e(1.19)(1.20)Cathodic:1/202+H20+2e=20H
8 the sheets formed by linking together Si-O tetrahedral and the two dimensional arrays of octahedral formed by the sixfold coordination of Al3+ and Mg2+ with oxygen and hydroxyl groups (see Fig. 1.8). This group has been considerably studied because of the importance of clays. Their special feature is their laminar crystal structure; the exposed basal planes differ in “topochemistry” from the edges of the crystals. In case of montmorillonite the basal planes behave more-orFig. 1.9. The edge of montmorillonite platelet. Less as simple silicate (because the hydrated alumina layer is sandwiched between a pair of silica layers) while alumina is exposed at the edges. As a result of isomorphous substitution of some silicon (Si4+ ) for aluminum (Al3+ ), the face surface carries a negative electrical charge while =Al(OH) groups at the edges are either positively or negatively charged depending on pH. In neutral media the faces are negatively charged but the alumina “sites” on the edges are positively charged and this is the origin of coagulation of clays in distilled water. In beneficiation of China clays, a kaolin clay is first dispersed in alkaline pH, larger impurity particles (quartz, hematite, anatase, etc.) are removed by classification, and the fine clay particles which constitute the final product are thickened and filtered after adjusting pH down to acidic range. (D). Sulfide Sulfide Sulfide Sulfide minerals minerals minerals minerals. Minerals such as galena (PbS), sphalerite (ZnS), chalcocite (Cu2S), chalcopyrite (CuFeS2), etc. are important sources of metals and their surface chemistry is of vast interest in connection with the froth flotation process. In principle, such sulfides are in reversible equilibrium with aqueous metal and sulfide ions, according to a definite solubility products (KSP) which are extremely low (for instance, for galena, KSP = 10-28 ). But in the presence of dissolved oxygen galena undergoes superficial oxidation with various oxidation products (lead hydroxide, lead thiosulfate, lead sulfate, ect) deposited on the surface. As sulfides are electronic semiconductors, their oxidation processes take on the character of electrolytic corrosion reactions, certain (anodic) areas oxidize preferentially while other are cathodic, as in the rusting of iron. Anodic: MeS = Me 2+ + So + 2e (1.19) Cathodic: 1/2O2 + H2O +2e = 2OH- (1.20)
9MeS +1/202+H20=Me2++ So+20H(1.21Overall reaction:Presence ofvarious oxidation products (S°oxidizes further into S2O2, SO2-,forexample Fe2+ to Fe3+, etc.) on the surface makes identification of PDI ions very difficultOxidation of sulfide minerals can bemanipulated by controllingtheredoxpotential (Eh)ofthepulp duringprocessing (grinding and flotation).Me+ saAnodic oxidation9Cathodicreduction+1/202+H20=20HFig.1.1o.Schematiccorrosionreactionsonsulfidesurface(E). Soluble salts (e.g. KCl, NaC, etc.). The lattice ion hydration theory explains thesurface charge of the soluble minerals as in the case of Group A minerals. These mineralsare discussed here separately owing to their industrial importance. Because of theirsolubility in water there was no experimental method to measure experimentally theelectrical charge of such crystals in aqueous media, and the theoretical predictions couldonlyrecentlybeverifiedbyexperimentContrary to general belief that solid particles in concentrated electrolyte solutionscannotbeelectricallycharged,in1968,RJ.Roman,M.C.FuerstenauandD.C.Seidelpostulated that sylvite and halite particles carry opposite electrical charge in brine. It waspostulated that while KCl crystals were negatively charged NaCI crystals were chargedpositively.The hypothesis was based on the observation that while fine KCl suspensionsin KCI brine and fine NaCI particle suspensions in NaCI brine were very stable, the fineparticles of KCI and NaCl in KCI-NaCI brine strongly aggregated. Yalamanchili andMiller (1992) studied aggregation in various systems and confirmed these observations.Theyalsofound strong aggregationbetweenfine quartz and NaClparticles in NaCI brineThe observations suggested that these particles interacted in brine only because of theirelectrical charge. The electrical charge of the particles of water-soluble minerals inaqueous systems has been recently directly measured (J.D. Miller and M.RYalamanmchili, 1992).This was accomplished with the use of the Laser-DopplerElectrophoresis.In this method the movement of the salt particles (dissolvingveryquickly in water) is measured over a very short time (15 sec) in an applied electrical fieldVarying ionic strength does not allow the calculation of the zeta potential values, but themeasured electrophoretic mobility of the particles allows determination ofthe electricalcharge of such particles.Accompanyingfigure shows some results (Fig.1.22)
9 Overall reaction: MeS + 1/2O2 + H2O = Me 2+ + So + 2OH- (1.21 Presence of various oxidation products (So oxidizes further into S2O3 2- , SO4 2- , for example Fe2+ to Fe3+ , etc.) on the surface makes identification of PDI ions very difficult. Oxidation of sulfide minerals can be manipulated by controlling the redox potential (Eh) of the pulp during processing (grinding and flotation). Fig. 1.10. Schematic corrosion reactions on sulfide surface. (E). Soluble Soluble Soluble Soluble salts (e.g. KCl, NaCl, etc.). The lattice ion hydration theory explains the surface charge of the soluble minerals as in the case of Group A minerals. These minerals are discussed here separately owing to their industrial importance. Because of their solubility in water there was no experimental method to measure experimentally the electrical charge of such crystals in aqueous media, and the theoretical predictions could only recently be verified by experiment. Contrary to general belief that solid particles in concentrated electrolyte solutions cannot be electrically charged, in 1968, R.J.Roman, M.C. Fuerstenau and D.C. Seidel postulated that sylvite and halite particles carry opposite electrical charge in brine. It was postulated that while KCl crystals were negatively charged NaCl crystals were charged positively. The hypothesis was based on the observation that while fine KCl suspensions in KCl brine and fine NaCl particle suspensions in NaCl brine were very stable, the fine particles of KCl and NaCl in KCl-NaCl brine strongly aggregated. Yalamanchili and Miller (1992) studied aggregation in various systems and confirmed these observations. They also found strong aggregation between fine quartz and NaCl particles in NaCl brine. The observations suggested that these particles interacted in brine only because of their electrical charge. The electrical charge of the particles of water-soluble minerals in aqueous systems has been recently directly measured (J.D. Miller and M.R. Yalamanmchili, 1992). This was accomplished with the use of the Laser-Doppler Electrophoresis. In this method the movement of the salt particles (dissolving very quickly in water) is measured over a very short time (15 sec) in an applied electrical field. Varying ionic strength does not allow the calculation of the zeta potential values, but the measured electrophoretic mobility of the particles allows determination of the electrical charge of such particles. Accompanying figure shows some results (Fig. 1.22)
105040BanaSodiumChioridePotasskmCriorideg40.171±0.148s0.70±0.26310n1ELECTROPHORETICMOBILITY(um/sec/V/cm)Fig. 1.11. Electrophoretric measurements for sylvite and halite particles [Miller &Yalamnchili, Langmuir, 8, 1464(1992)]).Accordingtothelatticeionhydrationtheory,thesignofthesurfacechargeisdetermined by themagnitude of the hydration energy ofthe respective surface lattice ions.If the surface cation has a more negative hydration energy than the surface anion, that isif the escaping tendency of the cation is larger then that of the anion, the surface acquiresanegativecharge.The converseisalsotrue.in solution(N自图Water moleculecrystol of NaClFig.1.12.DissolutionofionicsolidAccordingtothelatticeionhydrationtheory,thesignofthesurfacechargeisdetermined by the magnitude of the hydration energyofthe respective surface lattice ions.If the surface cation has a more negative hydration energy than the surface anion, thecations will have a greater tendency to undergo hydration and be preferentially releasedto solution,thesurfacewill thus acquireanegative charge.The converse is also true[SaltsNegativefreeenergySign of SurfaceCharge
10 Fig. 1.11. Electrophoretric measurements for sylvite and halite particles [Miller & Yalamnchili, Langmuir, 8, 1464 (1992)]. According to the lattice ion hydration theory, the sign of the surface charge is determined by the magnitude of the hydration energy of the respective surface lattice ions. If the surface cation has a more negative hydration energy than the surface anion, that is if the escaping tendency of the cation is larger then that of the anion, the surface acquires a negative charge. The converse is also true. Fig. 1.12. Dissolution of ionic solid. According to the lattice ion hydration theory, the sign of the surface charge is determined by the magnitude of the hydration energy of the respective surface lattice ions. If the surface cation has a more negative hydration energy than the surface anion, the cations will have a greater tendency to undergo hydration and be preferentially released to solution; the surface will thus acquire a negative charge. The converse is also true. Salts Negative free energy Sign of Surface Charge