Flotation of oxidized lead minerals with derivatives of 2-mercaptobenzothiazole. Part 1: chemical equilibria in the system 6-methyl-2-mercaptobenzothiazole-lead salts

International Journal of Mineral Processing, 32 ( 1991 ) 23-43

23

Elsevier Science Publishers B.V., Amsterdam

Flotation of oxidized lead minerals with derivatives of 2-mercaptobenzothiazole. Part 1: chemical equilibria in the system 6-methyl-2mercaptobenzothiazole-lead salts Pawel N o w a k a, Maria Barbaro b and Anna M. Marabini b' apolish Academy of Sciences, Institute of Catalysis and Surface Chemistry, ul.Niezapominajek, 30239 Krakow, Poland bNational Research Council, Institute of Mineral Processing Rome, Via Bolognola 7, 00138 Rome, Italy (Received March 29, 1990; accepted after revision February 1, 1991 )

ABSTRACT Nowak, P., Barbaro, M. and Marabini, A., 1991. Flotation of oxidized lead minerals with derivatives of 2-mercaptobenzothiazole. Part l: chemical equilibria in the system 6-methyl-2-mercaptobenzothiazole-lead salts. Int. J. Miner. Process., 32: 23-43. A new flotation collector 6-methyl-2-mercaptobenzothiazole (MMBT) was investigated. The dissociation constant for the acidic form was measured. Two insoluble lead complexes of MMBT with lead were synthesized and characterized for the first time and their solubility products were measured. On the basis of these data and literature data for the equilibria between lead salts and aqueous solutions, the chemical equilibria for the systems: solution of MMBT-insoluble lead salt were calculated. Lead salts which may form lead minerals or may appear as oxidation products on the surface of galena were taken into account. The domains of existence for different species and equilibrium concentrations for MMBT at different conditions have been presented in the form of distribution diagrams. The results of calculations were compared with the Fourier Transform Infrared Internal Reflection Spectrometric (FT-IR IRS) measurements of the interaction of MMBT solution with the oxidized galena surface.

INTRODUCTION

There is a common believe that strong chemical adsorption o f a chemical compound at the surface of a mineral is a good prognostic for using this compound as a flotation collector. The condition for such bonding of a molecule to the mineral surface is the formation o f a covalent bond between one o f the atoms constituting a molecule o f the adsorbate and the metal forming the ~For all correspondence.

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© 1991 - - Elsevier Science Publishers B.V.

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mineral. The class of chemical compounds in which a covalent bond between metal and non-metallic constituents of a molecule is formed is that of metal complexes. A particular class of complexing reagents capable of forming stable, highly selective bonds with metal cations is chelating agents. These reagents have been investigated as possible flotation collectors for many years (Usoni et al., 1971; Rinelli and Marabini, 1973; Marabini et al., 1983 ). The published results of works in that field have been reviewed most recently by Somasundaran and Nagaraj (1984), Pradip (1988) and Marabini et al. (1989a). One of the compounds of that class is 2-mercaptobenzothiazole (MBT) (Leja, 1982). However, it has not found wide industrial application in flotation. According to the opinion of the present authors, the reason of this situation may be the lack ofa hydrophobic constituent in the molecule of MBT. Thus, a series of derivatives of MBT with the aliphatic chain connected to the aromatic ring of the MBT molecule either directly or through the etheric oxygen (see Fig. 1 ) was synthesized and investigated from the point of view of their flotation properties (Marabini et al., 1988). Good floatability for some oxidized lead minerals with those compounds was confirmed both in Hallimond tube tests as well as in plant experiments. The mechanism of action of one of the new collectors, namely 6-propoxy-2-mercaptobenzothiazole was studied and very strong adsorption was observed in the case of cerussite (Marabini et al., 1989b: Cozza et al., 1991 ). The lack of any physicochemical data for these series of compounds prevents any deeper insight in the mechanism of their action. So, one of these compounds, 6-methyl-2-mera

CH 3

SH

b m

c-3~.

_c \s ~ / - ~ s \ \s / \ i/"-~ N~ ' - C H 3

Fig. 1. Chemical formula o f 6-methyl-2-mercaptobenzothiazoleand some o f its derivatives. a - 6-methyl-mercaptobenzothiazole ( M M B T ) ; b - m o n o d e n t a t e complex o f lead with MMBT; c - bidentate complex o f lead with MMBT.

FLOTATION OF OXIDIZED LEAD MINERALS

25

captobenzothiazole (MMBT) was chosen for a more detailed study. The dissociation constants for the acidic form and solubility products for the lead complexes of mMBT were measured. On the basis of these data and literature data on the equilibria between insoluble lead salts and aqueous solutions the diagrams of chemical equilibria in the system aqueous solution of MMBTinsoluble lead salt were calculated. The lead salts which may either occur as lead minerals and/or may be the constituents of the oxidation products of the galena surface, have been considered. Due to the lack of data on redox potentials in the investigated system only precipitation-dissolution equilibria were taken into account in the calculations. EXPERIMENTAL

Materials The investigated compound (MMBT) was synthesized by the Montedison company (Bornengo et al., 1985 ), and purified by dissolution in 1 mol dm -3 NaOH, precipitation by 1 mol dm-3 HC1 and multiple crystallization from acetone. A pure compound in the form of white, long needles was obtained. All other reagents used were of analytical grade purity. Water for preparation of the solutions was obtained from the Millipore Q system. The aim of this work was to study the possibility of flotation of oxidized lead minerals with a new class of collectors. Therefore, heavily oxidized galena was used in the experiments: it was a Sardinian mineral which was left in the laboratory atmosphere for 2 years after grinding. Before experiments it was dry-classified. Two fractions were used in infrared measurements of interaction of MMBT solution with the galena surface: one fraction (sample I ), 30-38/~m (specific surface area 0.21 m 2 g-~) and the other (sample II) with mean particle diameter 5.6/tm (90% between 2 - 1 0 / t m ) and a specific surface area 0.67 m 2 g-l. Part of the experiments were performed also with the same galena, but additionally wet ground (sample III ). After grinding the galena was washed many times with water and kept under water all the time. The specific surface area of that sample was not measured.

Apparatus and procedure The infrared spectra were performed with a Perkin-Elmer 1760 FTIR spectrometer and ATR attachement produced by Zeiss using the Internal Reflection Spectroscopy technique (Harrick, 1967; Strojek et al., 1983). Germanium reflection elements of the refracting angle of 45 and 25 reflections were used. After treatment, the sample was clamped to the surface of the reflection element and the spectrum taken immediately. Both water and galena absorbed strongly infrared radiation. To extract the information about the sur-

thce species the substraction procedure was used. The spectrum of galena of higher granulometry treated in a similar manner was subtracted from the spectrum of the sample. To cancel the presence of the bands of oxidation products in the spectrum of the product of surface reaction the spectrum of the sample of the same granulometry but treated only with pure water was subtracted from the spectrum of the sample. All experiments were performed at room temperature. All calculations have been performed on an IBM computer. Statistical calculations were performed using the STATGRAF program. Specific surface area of the samples was measured using the BET method and Monosorb (Quantachrom) apparatus. The analysis of the solutions on lead were performed using the Perkin-Elmer ICP 6000 Plasma Emission Spectrometer. The concentration of MMBT in the solution was measured by UV spectrometry, using the Perkin-Elmer model 330 UV spectrometer. RESULTS AND DISCUSSION

Dissociation constant of M M B T H The acid-base properties of MBT are well known (Budesinsky and Svec, 1971 ). This molecule in aqueous solution may exist in ionic form, monoprotonated form (see Fig. 1 ) or biprotonated form (with the second H bound to nitrogen), however the biprotonated form exists only in extremely acidic solutions. A similar behaviour may be expected for MMBT, however no data may be found in the literature. The acid-base properties of a flotation reagent are very important because they determine the activity of the ionic and molecular forms in solution which influences all chemical equilibria in the system. Also the form of adsorbed species depends on the form of the adsorbate molecule in solution. So, the dissociation constant for the reaction of MMBTH dissociation (Table 2, reaction 2 ) was measured using the method described in the literature for MBT (Budesinsky, 1969). The series of solutions in phosphate buffer were prepared and the UV spectra measured. Figure 2 shows the UV spectrum of the solution of MMBT in ionic and acidic forms. The spectrum is very similar to the spectrum of MBT presented by Jones and Woodcock ( 1973 ). This is not surprising because the differences in structure between MMBT and MBT are very small. It can be seen from the figure that there is no wavelength at which only one form absorbs, but the differences in the spectra for ionic and monoprotonated forms are significant and the proportions of both forms may be calculated for any solution. Note, that there is the isosbestic point (point where the molar extinction coefficients for ionic and molecular form are the same) at 311 n m and this is the best analytical wavelength because the absorption of the solution does not depend on pH. The molar extinction coefficient for that wavelength was found to be

FLOTATIONOF OXIDIZEDLEADMINERALS

27

/ / /~"k

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220

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/ /

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36o

240

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3;,0

,/nm Fig. 2. UV spectrum of 0.0001 mol dm -3 M M B T solution in 0.1 mol dm -3 HCI solution (monoprotonated form) - dotted line, and 0.0001 mol dm -3 M M B T solution in 0.1 mol dm -3 N a O H solution (ionic form) - dashed line.

19180 _ 120 1 m o l - 1 c m - ~. So the proportions of ionic and acidic forms were calculated from UV spectra for each solution. The concentration of hydrogen ion was calculated from known pH values (the activity coefficient for hydrogen ion equal 0.83 for mean ionic strength of 0.1 mol dm -3 was calculated from the equation given in the monograph of Garrels and Christ ( 1965 ). The constant obtained (mean value from 12 measurements) was a practical constant for the ionic strength given above and was similar to the value reported for MBT in the literature (Budesinsky and Svec, 1971 ). In Fig. 3 the proportions of ionic and protonated forms in solutions of different pH are shown. The dissociation constant of MMBTH molecule is similar to the dissociation constant of water. Thus, in most of the pH range of flotation the MMBTH molecule is only partly dissociated. The other factor important from the point of view of practical application of a flotation collector is its solubility. Therefore, the solubility of the monoprotonated form of MMBT was determined. A series of suspensions of MMBTH in acidified water (pH between 2-6) was prepared and ultrasonicated for several hours and the suspensions were left for a week. Next they were filtered and the concentration ofmonoprotonated form was determined with UV measurements (for solutions with a higher pH, corrections for the presence of ionic form were made). The mean value obtained from 12 measurements was 1.45 × 10-4 mol dm-3. The solubility of the ionic form is very high. Thus, the total dissolved MMBT concentration is

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pH

7

e

9

i'o

pH

Fig. 3. (A) The dependence of proportions of molecular and acidic forms of MMBT in aqueous solution on pH. Solid lines - calculated from dissociation constant; points - measured by UV spectrometry for the 1 X 10-4 mol dm MMBT solution. (B) The theoretical solubility of MMBT (the sum of the concentration'of monoprotonated form and the concentration of MM BT ion) as a function ofpH.

the sum of the protonated form and dissociated form and may be calculated from the known MMBTH solubility and dissociation constant (see Fig. 3 ).

Insoluble complexes of lead ions and MMBT No information about the complexes formed by MMBT and lead ions can be found in the literature. Also for MBT there is no agreement in the literature about the complexes formed. Khullar and Agarwala (1975) and Shukla Banerji et al. ( 1982 ) reported the formation ofa bidentate complex, but Hopkala and Przyborowski ( 1971, 1975 ) reported the possibility that both monodentate and bidentate complexes (see Fig. 1 ) exist. The knowledge about the properties of insoluble complexes possibly formed between the collector molecule and metal ion constituting the mineral is important for two reasons. First, if the solubility product for the complex is low, the complex may precipitate in the solution. This may diminish the effective concentration of the reagent in solution and influence the adsorption equilibrium, leading at the same time to a high consumption of the collector. Further, it may be anticipated that the surface compounds responsible for rendering the surface hydrophobic should have a structure similar to that of the precipitated complex. The best examples of this are metal xanthates. In most cases the surface compound present at the sulphide surface treated with xanthate solution is similar to the insoluble xanthate precipitated from the solution of metal ions and xanthate (Allison et al., 1972; Mielczarski et al., 1981; Marabini and Cozza, 1983 ). The insoluble complexes of lead ions and MMBT were precipitated at

FLOTATIONOF OXIDIZEDLEADMINERALS

29

different conditions. The composition of solution was controlled by measuring pH, MMBT concentration (by UV) and lead ion concentration (by Plasma Emission Spectroscopy). The obtained precipitates were washed, dried and weighed. Next the infrared spectra of the obtained precipitates were measured using the KBr pellet technique (Figs. 4 and 5 ). From the mass balance for the synthesis at different conditions the existence of both monodentate and bidentate complexes was confirmed. The bidentate complex is pale-yellow and is formed in a wide pH range, from acidic (pH below 4) to moderately alkaline. The monodentate complex is white and is formed only in alkaline solutions. Note that based on the infrared spectra the possibility that

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1000

800

Fig. 4. Infrared spectra of bidentate (A) monodentate (B) complexes and lead hydroxide precipitated from neutral solution (C).

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4000

3000

2000

cm -1

Fig. 5. Infrared spectra of the monodentate lead-MMBT complex (C) and two samples of lead hydroxide precipitated from neutral solution and dried at the same conditions (A,B) in the range of-O-H stretching vibrations. Samples dried overnight in a vacuum oven at the temperature of 110°C.

the monodentate complex is a mixture of a bidentate complex and lead hydroxide may be excluded (see Fig. 4). The monodentate complex has a distinct spectrum: especially characteristic is the presence of a sharp band at 3469 cm-1 (see Fig. 5 ) which may be ascribed unequivocally to O - H stretching vibrations in the P b - O - H group. In fact a band of similar frequency (3535 cm -1 ) is observed in hydrocerussite but not in cerussite (Gasden, 1975). Lead hydroxide, precipitated and dried at the same conditions shows also a band at similar frequency (see Fig. 5 ), but that band disappears during drying under vacuum; on the other hand, the monodentate complex does not change during overnight heating in a vacuum oven at a temperature of 110 ° C. Moreover, the lack of any absorption band at 1640 c m - 1, characteristic for molec-

FLOTATION OF OXIDIZED LEAD MINERALS

31

ular water (see Fig. 4) excludes the possibility that the band at 3469 cm -1 belongs to crystallization water. The infrared spectra of both complexes are similar, but the spectrum of the bidentate complex shows doubling of some bands (see for example the band at approximately 1000 c m - 1 ). This means either the loss of symmetry of the molecule as a whole, or the non-equivalency of two MMBT ligands in the molecule of the complex. Consequently, it seems that the formula proposed by Shukla Banerij et al. (1982) for the bidentate complex is not correct, because it assumes a highly symmetric structure.

Solubility products of lead-MMB T complexes Two different series of experiments have been performed with the aim to measure the solubility product of the bidentate complex. In the first series a solution of lead nitrate was added to the solution of MMBT. Both the solutions were acidified previously to obtain a final pH in the range of 3-5, and the lead solution was taken in some excess. After precipitation of the complex the solution with the precipitate was ultrasonificated for 1 hour and left overnight. Next the precipitate was filtered out, the pH of the solution measured and lead in solution analyzed by plasma emission. Next a sample of solution TABLE 1 Infrared absorption bands of MMBT complexes (wavenumbers in c m - t ) Pb (MMBT)2

Pb ( O H ) M M B T

MMBTH

1598 1559

1596 1557

1600 1585

1465 1418 1390

1462 1422 1393

1495 1480 1449 1399

1307 1271 1243 1209

1305 1271 1248 1209

1325 1289 1255 1206

1128

1123

1144 1134 1111

1069

1062

1069

1014 1006

1006

1037 1037

892 867 808

890 858 808

895 850 805

32

P NO~,'kKEl 4l

1ABLE 2 Equations used to describe equilibria in the system MMBT solution-precipitated complex-lead salt and the values of equilibrium constants taken in calculations Nr I 2 3 4 5 6 7 8 9

10 11 12 13

Equation

Log constant

Reference

[ MMBTH ] = Ga, K ~ = [ M M B T H ] / [ M M B T ][H ÷1 Kb= [Pb2+ ] [ M M B T - ] A 2 Km=[pb2+][OH-][MMBT -] K,= [ p b 2 + ] [ O H - ] A2 K2 = [PbOH+ ]/[Pb-~+ ] [OH ] Ka= [Pb(OH)2]/[pb2+ ] [ O H - ] A 2 /G= [ P b ( O H ) s ]/[Pb2+ ] [ O H - ] A 3

-3.84_+0.05 7.07 _+0.03 - 19.0_+0.6 -16.3_+0.2 14.25 5.76 10.25 13.25 9.93 6.16 12.27

This work This w o r k This work This work Smith and Smith and Smith and Smith and Smith and Smith and Smith and Smith and Smith and

Ks=[HCO;]/[H+][C023

- ]

/G= [H2CO3]/[H+ ] [ H C O ; ] KT=[Pb2+][CO32-] Ks= [Pb2+] [SO 2- ] Kg=[H+][OH -]

-

6.93

13.81

Martell, Martell, Martell, Martell, Martell, MarteU, Martell, Martell, Martell,

1976 1976 1976 1976 1976 1976 1976 1976 1976

was analyzed by UV for MMBT species in solution. The UV spectra of the solutions containing a relatively high concentration of metal ions (in comparison with MMBT) show unequivocal evidence of the formation of soluble lead-MMBT complexes, which is not surprising because the parent compounds of the series, MBT, is known as a complexing agent. Therefore, the following reasoning was used to calculate the concentration of free MMBT ions in solution. From known spectral characteristics of MMBT species in solution two wavelengths were chosen at which absorption of one of the forms prevails. Next the concentration of the free acidic form (MMBTH) was determined and from the equation for dissociation of MMBTH (Table 1, eq. 2 ) the concentration of free MMBT ions in solution was calculated. The concentration o f H ÷ ions was known from pH measurements. The solubility product was next calculated according to formula 3 in Table 2. Another series of experiments was performed, in which the bidentate complex was partly dissolved in a 0.1 mol dm -3 HC1 solution with pH previously adjusted with NaOH. The suspension was ultrasonicated 1 hour, left overnight and filtered. The filtrate was analyzed for lead, MMBT and pH. A similar procedure was applied for the monodentate complex. In this case precipitation (or dissolution) was performed in a strongly alkaline solution. At such pH, lead in solution occurs mainly in the form of hydroxy complexes and analysis of the solution for free lead ions is impossible. So, the precipitation (dissolution) was performed at such pH that lead hydroxide should precipitate simultaneously and the concentration of free Pb 2÷ ions was calculated from eq. 5. The concentration of O H - ions was again calculated from pH readings and finally the solubility product from eq. 4. The obtained values

FLOTATION OF OXIDIZED LEAD MINERALS

33

of solubility products are presented in Table 2. Concentrations, not activities, were taken for calculations (the readings of pH were re-calculated to the concentration of H + ions using the formula [H ÷ ] = 10 ^ ( - p H ) / f w h e r e f i s the activity coefficient for hydrogen ion). Thus, the obtained solubility products were practical constants, measured for the ionic strength of 0.1 mol d m - 3.

Stability diagrams in the system MMBT solution-precipitated complexinsoluble lead salt The knowledge about chemical reactions which may occur between the mineral and the solution of the flotation reagent is of primary importance for designing the technological scheme of the flotation process. The stability diagrams for the several lead compounds and solution of MMBT were constructed on the basis of measured constants for this compound. All calculations were performed for 0.1 mol d m - 3 ionic strength. In the case of stability constants given in the literature for zero ionic strength, they were re-calculated for the 0.1 mol d m - 3 ionic strength using the following activity coefficient: H + - 0.83; O H - , HCO~- - 0.78; pb2+SO 2- , CO 2- - 0.37. The activity coefficients were calculated according to the formula given by Garrels and Christ ( 1965 ). The equations describing the considered equilibria are shown together in Table 2. The following procedure was used in the calculations. The lines on the stability diagrams are the lines describing the equilibrium between the two solid phases and the solution and all are described by the set of equations. Part of these equations are the equations describing the dependence of the equilibrium constant of a chemical reaction on concentrations of the species present in the system. Those equations are collected in Table 2 and the text will refer to this table. The others are the equations of mass or charge balance conservation and will be given for individual cases. Part of these sets of equations is a set of non-linear equations and they were solved by the modified Newton-Raphson method described by Daul and Goel (1977). The first system which is to be considered is the system precipitated lead hydroxide-precipitated complex-solid MMBTH-solution of MMBT. The stoichiometry of the precipitated Pb species was considered PbO.Pb (OH)2 (Smith and Martell, 1976). The diagram of stability for that system is presented in Fig. 6. The line for coexistence of Pb (MMBT)2 and MMBTH is described by an equation which is a combination of eqs. 1 and 2. The equilibrium between the precipitated complex and PbO.Pb (OH)2 is described by the combination of eqs. 3 and 5 or 4 and 5. It appears from the given equations that the system monodentate complex-lead hydroxide is unstable in comparison to the system bidentate complex-lead hydroxide and monodenrate complex should decompose to the bidentate complex and lead hydroxide. The conditions for the stability of the monodentate complex is that its solu-

3

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12

Fig. 6. Stability diagram for the system: hydrated lead oxide-insoluble lead MMBT complexprecipitated MMBTH-solution. Line 2 for the bidentate complex, line 1 for the monodentate complex.

bility product should fulfill the condition: log(Km)< 1/2 1og(Kb K~ ). However, the difference between these two values is small (log Km = - 16.3, 1/2 log (Kb K~ ) = - 16.6 ), lower than the uncertainty of the values of the solubility products due to the error of its calculation (see Table 2 ). Thus, it is very probable that the monodentate complex in equilibrium with precipitated lead hydroxide is stable, but the region of its stability is very narrow. It seems to be reasonable that the free energy change for the reaction: Pb(OH)2+Pb(MMBT)2,

, 2 P b ( O H ) (MMBT)

is close to zero. On the other hand, it is well known that lead ions in solution form very stable hydroxy complexes and it is very probable that the monodentate complex is simply the intermediate step in the formation of the bidentate one. The situation is similar to the case of metal xanthates. These compounds also form hydroxy complexes, as stated by Kakovsky and Arashkevich ( 1968 ), and some of them are considered bo be more stable than the metal xanthate itself (see the discussion of that problem in the work of Palsson and Forssberg, 1989). The problem of stability of the monodentate complex is important from the point of view of the possibility of the formation of an adsorptive bond between the MMBT molecule and the mineral surface. The formation of a surface complex of the structure similar to the bidentate complex is not very probable, due to steric reasons. Thus, the monodentate complex, rather than the bidentate, may serve as a model for a surface compound formed eventually in the process of adsorption. Note that, except strongly alkaline solutions, the equilibrium concentration of MMBT for the system hydrated lead oxide-precipitated complex is very low and any

35

FLOTATION OF OXIDIZED LEAD MINERALS

-3

MM lT'l-Soiutio/a / /

-4,

/ •

-

PkO. Pb{OH) 2 + $elontien . r,

Ph (MMBT)

,,

, +,.,.,,o,

iX/

5

-6. PhC O3,l.S| la t i o l r

i

i

T

8

9

,

10 PH

r

11

12

Fig. 7. Stability diagram for the system: lead carbonate-insoluble lead MMBT complexMMBTH-solution for the closed system. Line I for the monodentate complex, line 2 for the bidentate complex.

hydrated lead oxide should be converted into an insoluble complex in that solution. The other system of importance for flotation is the MMBT solution-lead carbonate. Lead carbonate occurs both as a mineral (cerussite) as well as an oxidation product at the galena surface. In Fig. 7 is presented the stability diagram for the system MMBT solution, lead carbonate, precipitated complex (bi- or monodentate). The diagram was constructed for a closed system solving simultaneously eqs. 2, 3 (or 4), 5-11 and 13 with two additional equations - one expressing the electroneutrality of the solution and the other expressing the fact that all the species in the solution proceeds from dissolution of the solids (no exchange with the environment, pH regulated by addition of hydroxide not reacting with other components of the system). Note that for such assumptions, the equilibrium COa pressure is almost never equal to the partial pressure of CO2 in the atmosphere. Note also that the bidentate complex starts to precipitate at very low concentrations at almost full pH range. However, the solutions in industrial processes always contain some dissolved carbonates. Because of this fact the stability diagrams for the MMBT solution at different fixed concentrations of total carbonates in solution are presented separately for the case of precipitation of mono- and bidentate complexes (Fig. 8 ). It can be seen that the presence of carbonates in solution prevents the precipitation efficiently. This is very important from the point of view of possible use of MMBT as a flotation collector for lead carbonate, because the precipitation of the complex in solution influences the process of adsorption in two ways. First, it leads to excessive consumption of the collector, second it lowers the concentration of the free collector to the equilibrium value calculated from the solubility product and prevents the adsorption (if

I' N(t'~,'\K I!1 k l .

36

(A)

-3 MMilTH+ Solution

"'

Selution

,,~., -4 !" //

/

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~,_5 j/

-6

PbC03+ Solutlen

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7

8

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10 pH

11

12

(B) - 3 MMilTH-I, $olut ion - 4- ~

Ph(MMliT)2-~ Solution

/

16-2

~-~

•

10-4

6

I

i

I

7

8

9

I

10 pH

I

11

12

Fig. 8. The influence of carbonate concentration on the equilibria in the system lead carbonateinsoluble lead complex-precipitated MMBTH-solution of fixed carbonate concentration. A monodentate complex, B - bidentate complex. Numbers near the curves show total carbonate concentration.

the final concentration of collector is lower than the value required to obtain sufficient coverage of the surface). Finally, Fig. 9 presents the stability diagram for the system in which the pressure of CO2 was kept constant and equal to the atmospheric pressure of CO2 (note that this assumption is unrealistic in alkaline solutions because it leads to very high concentrations of carbonates in solution). Also in that system the equilibrium concentration of MMBT for the precipitation of the insoluble complex is rather low (except highly alkaline solutions), but the precipitation may be prevented by tentative additions of carbonate to the solution as was stated before. Therefore the diagrams presented for the system PbCO3-MMBT solution cover all important cases from a practical point of view.

FLOTATION

OF OXIDIZED

37

LEAD MINERALS

-:':/

-3

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-4-

/

or Pfa(ON)(''")"~" 'oiHtJoe

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---s-

-6-

i

T

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8

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9

10 PN

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Fig. 9. Stability diagram for the system: lead carbonate-insolublelead MMBT complex-precipitated MMBTH-solution. Carbon dioxide pressure fixed at the value for carbon dioxide partial pressure in atmosphere. Line 1 - monodentate complex, line 2 - bidentate complex.

MM|TH÷ /

-

5

_

7

~ 7

Ph(lli)(MMiIT) {MMDT)2 -I- Selutien

8

9

PH 10

11

Fig. 10. Stability diagram for the system: lead sulphate-insolublelead MMBT complex - precipitated MMBTH-solution in closed system. Line 1 - monodentate complex, line 2 - bidentate complex.

The last lead compounds considered was lead sulphate. This compound appears as a mineral (anglesite) and may be present in the oxidation products on the surface of galena. The solubility product of lead sulphate is relatively high, in comparison with lead carbonate, so the precipitation of the complexes should occur in a wide range of conditions. Shown in Fig. 10 is the equilibrium concentration of MMBT for the precipitation of insoluble complexes in the closed system (every lead and sulphate species proceed from the

38

i, N{>~&,'~KUI M.

dissolution of mineral, pH regulated by addition of hydroxide not reacting with other components of the solution ). It can be seen that the equilibrium MMBT concentration is very tow and it is rather unlikely that MMBT might be used as a collector for anglesite,

Interaction of MMBT solution with oxidized galena surface The predictions from the solubility product relations were checked in the measurements of the sorption of MMBT at the surface of oxidized galena. In these experiments 1 g of galena was introduced to 100 ml of solution of MMBT of predetermined concentration and pH and stirred for some time. Usually the time of stirring was 15 min, but sometimes it was prolonged to 2 h or 20 h. Next the solution was decanted, filtered and the pH and MMBT concentra-

1500

13()0 CI.I,1-1

1;00

900

Fig. 11. Infrared spectra of the surface of oxidized galena. Upper spectrum - dry ground sample (II), lower spectrum-wet ground samplc III).

39

FLOTATION OF OXIDIZED LEAD MINERALS

J

/ ~so

1~5o

dso ~m

11~

16~o

-1

Fig. 12. Infrared spectra of oxidized galena surface after sorption in 1.10-s MMBT solution. Upper spectrum:initial pH 11.5,final pH 10.9,time ofsorption 15 rain. Lowerspectrum:initial pH 8.08, final pH 8.01, time of sorption 20 h. tion measured. At the same time the IR spectrum of the galena sample was taken. From the changes of solution concentration the surface coverage by MMBT was calculated. This coverage was expressed in MMBT molecules per one surface lead atom (the number of surface lead atoms for 1 g sample was simply calculated from known density of PbS and known specific surface area). The surface of galena oxidizes rather easily and the main oxidation products are lead oxide (or hydrated lead oxide), lead carbonate and sulphate-like species (Page and HazeU, 1989). In Fig. 1 1 are shown the spectra of dry ground (sample II ) and wet ground (sample III ) galena samples. Note the difference in the oxidation products present at the surface. The dry ground sample only shows sulphate-like species at the surface, sample III except sulphates shows also carbonates. Obviously both samples contain also oxide-like oxidation products, but lead oxide has no absorption bands in the accessible

P N( "~,,\K E1 M ,

40

/

i

/ /

I

I,--

/

/

/

/

/

/

!

I

I

I

i

I

I

I

I

I

1450

1350

1250

1150

1050

t~m -1 Fig. 13. Infrared spectrum of the oxidized galena surface after 15 min of sorption from I. 10- 3 mol dm MMBT solution, initial pH 8. Lower curve after substraction of the spectrum of oxidation products.

range of the spectrum. After 15 min of sorption in alkaline solution the absorption bands of both sulphate-like oxidation products as well as carbonatelike oxidation products disappear and, according to the predictions based on the solubility products, the monodentate lead MMBT complex appears at the surface (compare Figs. 12 and 4 ). Despite the fact that the absorption bands of the oxidation products are no longer visible in the spectra, sorption proceeds during the next 2 h, but much more slowly (about 50% of the sorption product is formed during the first 15 min ). This may be explained taking into account that the primary oxidation product of the galena surface is lead oxide (which has no absorption bands in the considered range of spectrum) - carbonates and sulphates are the secondary oxidation products, forming a layer on the underlying oxide. If the concentration of MMBT in solution is sufficiently high, the abstraction from the solution amounts for more than 20 MMBT molecules for one surface lead atom. Otherwise almost all MMBT is abstracted from the solution and the concentration drops to a value below

FLOTATION OF OXIDIZED LEAD MINERALS

41

1.10- 6 mol d m - 3. In alkaline solutions sorption is always accompanied by a drop in pH. All of the above mentioned observations lead to the conclusion that according to the calculations based on the solubility productrelations the products of galena oxidation reacts with the solution of MMBT giving rise to a monodentate insoluble complex. In neutral solutions the situation is quite different. Even after 20 h of sorption the oxidation products of the galena surface are still visible in the spectra. After 15 min of sorption (see Fig. 13 ) only a small part of oxidation products are converted into the form of lead MMBT complex (the amount of MMBT abstracted from 1.10-3 MMBT solution during the first 15 min is however higher than 1 monolayer), and the sulphate-like oxidation products disappear before carbonate-like oxidation products in accordance with the solubility products relation. No drop in pH is observed during the sorption in neutral solution. The final reaction product is the bidentate complex (compare Figs. 12 and 4) as predicted by calculations, however the reaction is kinetically strongly hindered. Furthermore, there is evidence from the IR spectra that at neutral pH adsorption at the surface occurs. The adsorption equilibria in the system lead minerals-MMBT solutions will be treated in the second part of this work. CONCLUSIONS

A new group of flotation collectors, the derivatives of 2-mercaptobenzothiazole with an alkyl chain linked to a phenyl ring, were investigated. However in order to use them in flotation, more physicochemical data are needed. Important physicochemical data have been measured for one of these compounds, 6-methyl-2 mercaptobenzothiazole. Due to the similarity in chemical structure of the particular members of that family the conclusions drawn for this compound will be representative for the whole group. The most important feature is the low solubility of the lead complexes of the MMBT. It is a known fact (Du Rietz, 1975) that a compounds forming with a metalconstituting mineral an insoluble complex of a solubility lower than the mineral itself may not be a good collector for that mineral. The solubilities of both mono- and bidentate complexes of lead with MMBT are of the same order of magnitude as lead xanthate and may cause significant problems in flotation practice because of the extensive precipitation of the insoluble complex. However, in the investigated system precipitation may be avoided when working with a solution containing an excess of carbonates. It is shown that at neutral pH the precipitation of the insoluble complexes is kinetically hindered, and in that region adsorption of MMBT at the surface may occur. The adsorption mechanism and flotation behaviour of these compounds will be treated in the second part of this study.

42

p N()WAK EI , \ 1

REFERENCES Allison, S.A., Goold, L.A., Nicol, M.J. and Granville, A., 1972. Determination of the products of reaction between various sulphide minerals and aqueous xanthate solution - and a correlation of the products with electrode rest potentials. Met. Trans., 3:2613-2618. Bornengo, G., Carlini, F.M., Marabini, A.M. and Alesse, V., 1985. Collettori per la flottazione selettiva di minerali di zinco e piombo. Patent no. 48019 A 85. Budesinsky, B.W. and Svec, J., 1971. Reactions of selenous acid with thiosalicyclic acid, 2mercapto-benzothiazole and 2-aminothiophenol. J. Inorg. Nucl. Chem., 33: 3795-3803. Budesinsky, B.W., 1969. Acidity of several chromotropic acid azo derivatives. Talanta, 16:12771288. Cozza, C., Di Castro, V., Polzonetti, G. and Marabini, A.M., 1991. An X-ray photoelectron spectroscopy study of the interaction of mercapto-benzo-thiazole with cerussite. Int. J. Miner. Proces., 34 (in press). Daul, C. and Goel, J.J., 1977. Numerical estimation of equilibrium concentrations. J. Chem. Soc. Faraday Trans., 73 985-990. Du Rietz, C., 1975. Chemisorption of collectors in flotation. Proc. XI I.M.P.C. (Cagliari, 377403. Garrels, R. and Christ, C., 1965. Solutions, Minerals and Equilibria. Freeman, San Francisco, CA, pp. 61-81. Gasden, J.A., 1975. Infrared Spectra of Minerals and Related Inorganic Compounds. Butterworths, London. Harrick, N.J., 1967. Internal Reflection Spectrocopy. Intersci. Publ., New York, NY. Hopkala, H. and Przyborowski, L., 1971. Reactions of 2-mercaptobenzothiazole and 2-mercaptobenzoimidazole with some metal ions and their use in gravimetric determination (in Polish). Ann. Univ. Mariae Curie-Sklodowska, Sect.D, 26: 57-72. Hopkala, H. and Przyborowski, L., 1975. Potentiometric titration of metal ions with 2-mercaptobenzothiazole. Chem. Anal. Warsaw, 20: 785-790. Jones, M.H. and Woodcok, J.T., 1973. UV Spectrophotometric determination of 2-mereaptobenzothiazole (MBT) in flotation liquors. Can. Metall. Q., 12: 497-505. Kakovsky, I.A. and Arashkevich, V.M., 1968. The study of the properties of organic disulphides. Proc. VIII I.M.P.C. Leningrad, Preprint $8. Khullar, I.P. and Agarwala, U., 1975. Complexes of 2-mercaptobenzothiazole with Cu(II), Ni(II), CO(II), Cd(II), Zn(II), Pb(II), Ag(I) and Fe(I). Can. J. Chem., 53:1165-71. Leja, J., 1982. Surface Chemistry of Froth Flotation. Plenum, New York, NY, 208 pp. Marabini, A.M. and Cozza, C., 1983. Determination of lead ethylxanthate on mineral surface by IR spectroscopy. Spectrochim. Acta, 388, 215. Marabini, A., Barbaro, M. and Ciriachi, M., 1983. A calculation method for selection of complexing collectors having selective action on a cation. Trans. IMM, Sec. C, 92: C20-C26. Marabini, A.M., Alesse, V. and Barbaro, M., 1988. New synthetic collectors for selective flotation of zinc and lead oxidized minerals. Proc. XVI I.M.P.C. Stockholm, pp. 1197-1208. Marabini, A., Cases, J. and Barbaro, M., 1989a. Chelating agents as collectors and their adsorption mechanism. In: K.V. Sastry and M.C. Fuerstenau (Editors), Challenges in Mineral Processing. AIME, Colorado, pp. 35-50. Marabini, A.M., Barbaro, M. and Passariello, B., 1989b. Flotation ofcerussite with a synthetic chelating collector. Int. J. Miner. Process., 25: 29-40. Mielczarski, J., Nowak, P., Strojek, J. and Pomianowski, A., 1981. Investigation of the products of ethylxanthate sorption on sulphides by IR-ATR spectroscopy. Proc. XIII I.M.P.C. Warszawa, Part A, 33. Page, P.W. and Hazell, L.B., 1989. X-ray photoelectron spectroscopy (XPS) studies of potas-

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sium amyl xanthate (KAX) adsorption on precipitated PbS related to galena flotation. Int. J. Miner. Process., 25: 87-100. Palsson, I. and Forssberg, K.S.E., 1989. Computer assisted calculations of thermodynamic equilibria in sphalerite-xanthate system. Int. J. Miner. Process., 26: 223-258. Pradip, 1988. Applications of chelating agents in mineral processing. Min. Met. Process., 25: 80-89. Rinelli, G. and Marabini, A., 1973. Flotation of zinc and lead oxide-sulphide ores with chelating reagents. Proc. X I.M.P.C. London, pp. 493-521. Shukla, Banerji, R.E., Byrne, S.E. and Livingstone, S.E., 1982. Metal complexes of 2-mercaptobenzothiazole. Transition Metal. Chem., 7: 5-10. Smith, R. and Martell, A., 1976. Critical Stability Constants, Vol. IV: Inorganic compounds. Plenum Press, New York, NY, p. 10. Somasundaran, P. and Nagaraj, D.R., 1984. Chemistry and applications of chelating agents in flotation and flocculation. In: M.J. Jones and R. Oblatt (Editors), Reagents in Mineral Industry. IMM-C.N.R., Rome, pp. 209-219. Strojek, J., Mielczarski, J. and Nowak, P., 1983. Spectrocopic investigations of the solid-liquid interface by the ATR technique. Adv. Coll. Interface Sci., 19: 309-327. Usoni, L., Rinelli, G. and Marabini, A., 1971. Chelating agents and fuel oil: a new way to flotation. AIME Centennial Annual Meeting, New York.