- Email: [email protected]
Dextrin as a non-toxic depressant for pyrite in flotation with xanthates as collector
Minerals Engineering 17 (2004) 1001–1006 This article is also available online at: www.elsevier.com/locate/mineng
Dextrin as a non-toxic depressant for pyrite in flotation with xanthates as collector A. L opez Valdivieso a
a,*
, T. Celed on Cervantes a, S. Song a, A. Robledo Cabrera a, J.S. Laskowski b
Instituto de Metalurgia, Universidad Autonoma de San Luis Potosı, Avenue Sierra Leona 550, San Luis Potosı, S.L.P. 78210, Mexico b Department of Mining, University of British Columbia, Vancouver, British Columbia, Canada V6T 1Z4 Received 4 February 2004; accepted 6 April 2004
Abstract Depression of pyrite flotation by dextrin has been investigated through adsorption, electrokinetic and microflotation studies in systems open to the atmosphere. Adsorption of dextrin takes place through specific interaction with ferric oxyhydroxide species that result from the oxidation of pyrite surface. Dextrin shows an isoelectric point at pH 4 and pyrite does at pH 6.4. Within this pH range adsorption is suggested to be promoted by electrostatic interactions. Coadsorption of dextrin and isopropyl xanthate occurs on the surface of pyrite and is explained to happen through distinct mechanisms taking into account the heterogeneous nature of the surface. It is likely that dextrin depresses pyrite by enveloping the dixanthogen resulting from adsorption of xanthate ions. It is shown that dextrin is as effective depressant of pyrite as cyanide. 2004 Elsevier Ltd. All rights reserved. Keywords: Sulphide ores; Flotation reagents; Flotation depressants; Environmental; Mineral processing
1. Introduction Pyrite is the most widespread and abundant of naturally occurring metal sulfides. It is commonly present in base metal sulfides and frequently appears in coal as a major source of sulfur in coal. Pyrite lowers the quality of base metal concentrates and increases the amount of sulfur compounds produced in the base metal extraction processes. Combustion of high-sulfur coals poses serious environmental problems due to sulfur dioxide emissions. Therefore, depression of pyrite is desirable in the concentration of base metal sulfides by flotation. This can be achieved by floating in alkaline solutions (lime) using highly selective inorganic modifiers such as cyanides, sulfites, ferrocyanides and cyanides in combination with zinc sulfate (Fuerstenau et al., 1985). Of all depressants, cyanides have been the most common. Their use has raised concern on environmental grounds, however. Hence, natural, bio-degradable, non-toxic agents such as dextrin are gaining in importance and hold promise * Corresponding author. Tel.: +52-444-825-5004; fax: +52-444-8254326. E-mail address: [email protected] (A. L opez Valdivieso).
0892-6875/$ - see front matter 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.mineng.2004.04.003
to function as a selective depressant (Laskowski et al., 1993). Dextrin naturally degrades to carbon dioxide and water. Dextrin is derived from starch by partial thermal degradation under acidic conditions. The treatment causes the break down of starch macromolecules resulting in smaller molecules that are more branched (dextrin). The structure of dextrin is given in Fig. 1 and resembles amylopectin. All the three hydroxyl group (–OH) in the glucose monomeric unit may rotate in such a way that they face one side of the monomeric ring, making that side hydrophilic. The opposite side is consequently slightly hydrophobic due to the exposed –CH groups. Application of dextrin as a flotation depressant, both in the laboratory and in commercial processes, has been reported. In the flotation of Cu–Pb sulfide ores at Kidd Creek dextrin is utilized in alkaline environment whenever the content of galena in copper concentrate is too high. Its use along with sulfur dioxide in Brunswick Mines has also been reported (Schnar, 1978). In 1957, Lukkarinen discovered that dextrin could be used in the differential flotation of Cu–Pb concentrate; in this process pentlandite was depressed with dextrin while
1002
A. Lopez Valdivieso et al. / Minerals Engineering 17 (2004) 1001–1006 CH2OH
O H
HO HO
H
OH
H H
CH2OH
HO HO
H
O O H H OH
O CH2
H HO
O H O
HO
H
OH
involves formation of a complex between polysaccharide oxygen atoms at C-2 and C-3 and iron atoms in the flocculation of iron oxides with starch was confirmed by Weissenborn (1993). The aim of this study is to test dextrin as an alternative non-toxic depressant for pyrite. Adsorption studies have been carried out to delineate the mechanisms by which dextrin renders pyrite surface hydrophilic when using xanthates as collector. The tests have been carried out using natural crystals of pyrite and isopropyl xanthate collector.
H
H
Fig. 1. Schematic of the molecular structure of dextrin.
chalcopyrite was floated with xanthate under alkaline conditions. According to Laskowski and Namyekye (1994) this scheme was used in the Kotalahti mine in Finland until 1987 when the mine closed down. In the laboratory, Liu and Laskowski (1989a) showed that in acidic solutions chalcopyrite can be depressed by dextrine while galena can be floated with ethyl xanthate; whereas in alkaline solutions chalcopyrite is floated while galena is depressed. They have used dextrin to selectively depress heazlewoodite (Ni3 S2 ) in the flotation of chalcocite in the processing of INCO matte with amyl xanthate at a pH of 11.7. Reports on the use of dextrin as a depressant for pyrite in sulfide flotation are scarce. Kydros and Gallios (1994) have shown that dextrin effectively depressed pyrite at pH 4 and higher in the presence of ethyl xanthate. At pH 4, copper-activated sphalerite was floated using ethyl xanthate as a collector, while pyrite was depressed with dextrin in the flotation of the pyrite– sphalerite mixture. Xu and Aplan (1994) have proposed to use polysaccharides in the presence of iron ions to depress pyrite in coal cleaning. Adsorption of dextrin which renders metal sulfide surfaces hydrophilic has been proposed to be due to the interaction between the hydroxyl groups in the glucose unit and the metal hydroxides on the sulfide surface (Liu and Laskowski, 1989b). This mechanism is compatible with the findings by Angyal (1973), who showed, through Nuclear Magnetic Resonance studies, that the hydroxyl group of the C-2 y C-3 in D-Glucose interacts with multivalent metallic ions to form D-Glucose metal complex species. X-ray photoelectron spectroscopy and Auger electron spectroscopy studies performed by Liu et al. (1994) indicated chemical interaction of dextrin with metal hydroxide species on the surface of metal oxides. Through infrared studies, Khosla et al. (1984) have shown that amylopectin bonds chemically with Fe3 ions, giving evidence of the existence of chemical bonding of amylopectin on hematite surface. The mechanism similar to the one postulated by Liu and Laskowski (1989a,b, 1999; Liu et al., 2000) and which
2. Experimental The crystalline pyrite sample used in this study was from Zacatecas, Mexico. The crystals were hand ground using an agate mortar and pistil in order to obtain )150 + 70 lm and )37 lm size fractions, which were used for microflotation tests and adsorption and electrokinetic studies, respectively. These samples were kept under nitrogen atmosphere to prevent further surface oxidation and ensure reproducibility of the tests. The surface area of the )37 lm size fraction particles was determined to be 0.86 m2 /g by the BET method using krypton as the adsorbent and a Micromeritics model ASP 2010 equipment. The dextrin used in the work was supplied by A.E. Staley Manufacturing Co. Its molecular weight has been determined by Nyamekye (1992) to be 56,000. Dextrin solutions were prepared daily at 1 g/l concentration and 0.01 mol/l ionic strength, using NaNO3 as the supporting electrolyte. Sodium isopropyl xanthate obtained from Industrias Quimicas de Mexico was purified threefold through dissolution of the xanthate in acetone, followed by precipitation in ethylic ether. To prevent its decomposition, the purified xanthate sample was kept under nitrogen atmosphere in a fridge and was kept in a black glass bottle. Deionized water and analytical grade inorganic reagents were used to prepare the solutions used in this work. The deionized water was obtained by passing distilled water through ion exchange resins in a Nanopure Barnstead unit. The solutions used in all tests were prepared at a fixed ionic strength of 0.01 mol/l, using NaNO3 as the supporting electrolyte. Dilute solutions of NaOH and HNO3 were used to adjust pH. Microflotation tests were carried out using a modified Hallimond flotation tube. One gram of pyrite was conditioned in 150 ml solution at the desired xanthate concentration and pH for 5 min. When dextrin was used, the pyrite was conditioned first with this reagent for 30 min at a given concentration and pH, then with xanthate for an additional 5 min. The pH reported in this study is the pH value measured at the end of the conditioning time. Following the conditioning step, the pulp was transferred to the Hallimond tube for flota-
A. Lopez Valdivieso et al. / Minerals Engineering 17 (2004) 1001–1006
tion, which was carried out for 1 min using nitrogen at a flow rate of 30 ml/min. The conditioning procedure aforementioned was identical for both the adsorption and electrokinetic studies. The former was carried out using 1 g pyrite and 50 ml solution. After conditioning, the pulp was centrifuged and solutions were withdrawn from the supernatant for analysis of dextrin and xanthate residual concentrations. Dextrin concentration in solution was determined through the method described by Dubois et al. (1956), while xanthate concentration was determined with a UV spectrophotometer at a wave length of 301 nm. Electrokinetic studies were carried out with 100 mg pyrite and 50 ml solution. The zeta potential of pyrite was determined using a Riddick Zeta Meter model D and electrophoretic cell No S-2479. After conditioning, the pulp was transferred to the electrophoretic cell and the mobility of at least 10 pyrite particles was measured. The zeta potential was calculated from the electrophoretic mobility measurements using the Smoluchowski equation (Adamson and Gast, 1997).
3. Results and discussion Electrokinetic studies have been undertaken to delineate the adsorption mechanism of dextrin onto pyrite. Fig. 2 shows the zeta potential of pyrite as a function of pH in the absence and presence of various dextrin concentrations. This figure also shows the zeta potential of dextrin both from this study and after Liu (1988). As noted, dextrin exhibits point of zeta reversal at about pH 4. Fig. 2 shows that pyrite’s isoelectric point (iep) is at about pH of 6.4. Since the pHiep of non-oxidized pyrite has been reported to be about 2 (Fuerstenau et al., 1968; Fornasiero et al., 1992; Bebie et al., 1998) and that of
60 ZETA POTENTIAL, mV
DEXTRIN INITIAL CONC
40
None 10 mg/L 25 mg/L 100 mg/L 0.01 M NaNO3, 25°C
20 0 DEXTRIN
-20
1000 mg/L Dextrin (Liu et al., 1988) 1000 mg/L Dextrin (this work)
-40 -60 0
2
4
6
pH
8
10
12
14
Fig. 2. Zeta potential of pyrite as a function of pH in the absence and presence of various initial dextrin concentration. The zeta potential of dextrin as a function of pH is also presented.
1003
ferric oxides to range between 5.2 and 8.6 (Parks, 1965), this pHiep at 6.4 is an indicative that the surface of the pyrite used in this study underwent oxidation during the sample preparation and conditioning. Various techniques have been used to study the oxidized pyrite surface, such as cyclic voltammetry, surface enhanced Raman spectroscopy, X-ray photoelectron spectroscopy, to name a few (Buckley and Woods, 1987; Li et al., 1993; Zhu et al., 1993). They all have reported the formation of ferric oxyhydroxide species on the surface of pyrite, as dictated by thermodynamics. Todd et al. (2003) have suggested, based on X-ray absorption spectroscopy studies, that ferric oxyhydroxide, probably goethite, is the oxidation product. Nanopatches of ferric oxyhydroxide have been microphotographed on the surface of oxidized pyrite through atomic force microscopy studies by Miller et al. (2002) and Hochella (2003). These studies show that the surface of pyrite, when exposed to oxidation for a short period of time, is of heterogeneous nature with patches of ferric oxyhydroxide and FeS2 . The electric charge at the pyrite/aqueous solution interface can then be accounted for by FeOHþ 2 and FeO sites. The equilibrium between the oxidized surface species at interface with water may be viewed as a two step process (de Bruyn and Agar, 1962), FeOHðsurfÞ þ Hþ ¼ FeOHþ 2 ðsurfÞ
FeOHðsurfÞ þ OH ¼ FeO ðsurfÞ þ H2 O
ð1Þ ð2Þ
Eqs. (1) and (2) are equivalent to the following reactions at the non-oxidized pyrite/aqueous solution interface (Fornasiero et al., 1992): FeSHðsurfÞ ¼ FeS ðsurfÞ þ Hþ þ
FeSHðsurfÞ þ H ¼
FeSHþ 2 ðsurfÞ
ð3Þ ð4Þ
As noted in Fig. 2, dextrin shifts the pHiep of pyrite to a more lower value and decreases both the positive and negative zeta potential of pyrite. This is indicative that dextrin specifically adsorbs on the pyrite surface, decreasing the surface electric charge density. At a dextrin concentration of 100 mg/l, the zeta potential of pyrite is identical to that of dextrin. From the point of view of flotation, this means that pyrite completely loses its interfacial properties to acquire that of dextrin, which is highly hydrophilic as stated above. Adsorption of dextrin on pyrite as a function of pH is given in Fig. 3 at two distinct dextrin concentrations, namely at 50 and 500 mg/l. As noted, adsorption is strongly pH-dependent and rises sharply at pH 4. At the lowest dextrin concentration, a leveling off of the adsorption takes place above pH 4, while at the highest dextrin concentration, a maximum in the adsorption occurs around pH 6, close to the pHiep of pyrite due to ferric oxyhydroxide. Above this pH value adsorption decreases monotonically.
A. Lopez Valdivieso et al. / Minerals Engineering 17 (2004) 1001–1006
DEXTRIN ADSORPTION DENSITY, mg/m
2
18 DEXTRIN INITIAL CONC. 50 mg/L
16
400 mg/L
14
0.01 M NaNO3, 25°C
12 10 8 6 4 2 0 0
2
4
6
8
10
12
14
pH
Fig. 3. Adsorption density of dextrin on pyrite as a function of pH at 50 and 400 mg/l initial dextrin concentration.
The sharp increase in the adsorption takes place at a pH close to the pHiep of dextrin where the net surface charge of pyrite is positive. This seems to suggest that adsorption occurs not only by specific interaction contributions, but also by a contribution resulting from the electric charge at both the dextrin and the pyrite/aqueous solution interface. At the pH range between the pHiep of pyrite and the pHiep of dextrin, the electric charge for dextrin is negative and that of pyrite is positive. Under these conditions, electrostatic interaction should promote adsorption. On the other hand, the specific interaction contribution to adsorption can be accounted for by chemical bonding between dextrin and the ferric oxyhydroxide on the surface of pyrite. Liu et al. (1994) have shown that dextrin interacts with ferric hydroxide through chemisorption. Bogusz et al. (1997) have reported that pyrite adsorbed dextrin only after an oxidative treatment of its surface. At pH values above the pHiep of pyrite, both pyrite and dextrin carry a net negative electric charge. Under these conditions, the specific interaction contribution should be responsible for adsorption by surmounting that owing to electrostatic repulsion. The higher the pH, the larger is the negative electric charge at the pyrite/ aqueous solution interface. Accordingly, the electrostatic repulsion contribution increases causing adsorption to decrease as noted in Fig. 3. In addition, dextrin must compete with OH ions for the surface of ferric oxyhydroxide on pyrite. The dextrin adsorption did not occur at pH values below 4. This can be associated with the fact that under these conditions ferric hydrosulfate, Fe(OH)SO4 , is the main oxidation product on oxidized surfaces of pyrite, as shown by the X-ray absorption spectroscopy studies performed recently by Todd et al. (2003). They have
pointed out that ferric oxyhydroxide species predominantly form at pH ¼ 4 and higher. The electrokinetic and adsorption studies then predict that dextrin should be an effective depressant for pyrite with ferric oxyhydroxide on the surface. The adsorption studies show that optimal conditions for an effective depression of pyrite should be within the pH range between 4 and 12. Accordingly, microflotation tests were carried out to assess dextrin as a depressant for pyrite. Sodium isopropyl xanthate was used as the flotation collector. Fig. 4 shows the floatability of pyrite with 104 mol/l isopropyl xanthate in the absence and presence of various concentrations of dextrin as a function of pH. As can be observed, in the absence of dextrine, the flotation of pyrite is high up to about pH 12. With 10 mg/l dextrin, flotation of pyrite falls at pH 6. Increasing the concentration of dextrin up to 25 mg/l results in depression which begins at about pH 4. This pH of depression does not change further when the concentration of dextrin is increased up to 50 mg/l. These flotation results correlate well with the adsorption results in that pyrite is depressed within the pH range of dextrin adsorption. At pH values below 4, pyrite was not depressed due to the lack of adsorption of dextrin. Kydros and Gallios (1994) found a marked depression of pyrite by dextrin at pH 4 in flotation with ethyl xanthate, which is compatible with the results of this investigation. It is well known that the depressing effect of inorganic modifiers, such as cyanides, ferrocyanides and sulfites, is caused by lowering the pulp potential and making the surface of pyrite unsuitable for adsorption of sulfhydryl collectors. To understand further the mechanisms by which dextrin depresses pyrite, the adsorption of isopropyl xanthate at the pyrite/aqueous solution interface has been studied in the presence of dextrin at pH 8. In
100 FLOATABILITY, percent
1004
80 DEXTRIN INITIAL CONC. None
60
10 mg/L
40
25 mg/L 50 mg/L -4 1x10 M SIPX
20
0.01 M NaNO 3, 25°C
0 0
2
4
6
8
10
12
14
pH Fig. 4. Floatability of pyrite with 1 · 104 mol/l sodium isopropyl xanthate as a function of pH in the absence and presence of various initial dextrin concentration.
A. Lopez Valdivieso et al. / Minerals Engineering 17 (2004) 1001–1006
these tests, the xanthate concentration was fixed at 104 mol/l, while that of dextrin was varied. Fig. 5 shows the adsorption density of both isopropyl xanthate and dextrin on pyrite as a function of equilibrium dextrin concentration. As can be seen, the collector adsorption is not affected by dextrin. Dextrin does not adsorb until its concentration reaches about 8 mg/l, then adsorption increases monotonically until a plateau is obtained at 8 mg/m2 adsorption density. Accordingly, dextrin does not inhibit the adsorption of xanthate ions so that its depression effect can be attributed to the entrapment of adsorbed xanthate on the surface of pyrite. Flotation of pyrite with xanthate is due to dixanthogen formation on the surface of pyrite through the oxidation reaction of xanthate ions coupled with a reduction reaction of ferric oxyhydroxide species to ferrous species (Lopez Valdivieso et al., 2003; Bulut and Atak, 2002; Fuerstenau et al., 1968). Due to the heterogeneous nature of the surface of oxidized pyrite, adsorption of xanthate ions and dextrin may be viewed to take place at different sites on the surface. While dextrin adsorbs on ferric oxyhydroxide sites, xanthate ions do so on anodic sites located on the non-oxidized surface of pyrite. The adsorption of xanthate ions on the surface of pyrite has been proposed to be as follows: An odic oxidation reaction of xanthate ions to dixanthogen
X ¼ X2 ðlÞðsurfÞ þ 2e
ð5Þ
Cathodic reduction reaction of ferric oxyhydroxide sites to ferrous ions 2FeOOHðsurfÞ þ 6Hþ þ 2e ¼ 2Fe2þ þ 4H2 O
ð6Þ
where Eq. (6) should be in equilibrium with the hydrolysis and precipitation products of Fe2þ ions and the oxidation of Fe2þ ions to Fe3 species.
1005
The chemical interaction of dextrin with ferric oxyhydroxide sites on the surface of pyrite may be given as follows: FeOHðsurfÞ þ DOH ¼ FeODðsurfÞ þ H2 O
ð7Þ
Eq. (7) is a simplified version since a complete complex probably involves OH groups on C-2 and C-3 carbons interaction with surface metal sites as shown below (Liu and Laskowski, 1989b):
C O C O
C C C
O
Pb
Weissenborn (1993) showed the following complex for iron oxide interacting with starch: O
CH2OH
CH2OH
O
2
O
O
O HO
HO Fe O
Fe Fe
O
OH
3'
O
O
OH Fe
O
Fe
O
O
It is interesting to compare depression of pyrite by dextrin and sodium cyanide. Fig. 6 shows the floatability of pyrite with 104 mol/l sodium isopropyl xanthate at pH 8 as a function of the concentration of the two depressants. The depression of pyrite takes place at a lower dextrin concentration in comparison to that needed with sodium cyanide. It is to be pointed out however that these two, cyanide and dextrin, operate in quite a different way. While cyanide prevents xanthate
9
100
DEXTRIN
8
FLOATABILITY, percent
ADSORPTION DENSITY, mg/m
2
10
7 6
-4
1x10 M SIPX pH 8 0.01 M NaNO 3 , 25°C
5 4 3 2
ISOPROPYL
1
-4
1x10 M SIPX pH 8 0.01 M NaNO3, 25°C
80 60 40
NaCN
20 DEXTRINE
0
0 0
5
10
15
20
25
30
35
40
45
50
55
60
EQUILIBRIUM DEXTRIN CONCENTRATION, mg/L
Fig. 5. Adsorption density of dextrin on pyrite as a function of equilibrium dextrin concentration in the presence of 1 · 104 mol/l sodium isopropyl xanthate at pH 8.
0
10 20 30 40 DEPRESSANT CONCENTRATION, mg/L
50
Fig. 6. Floatability of pyrite with 1 · 104 mol/l sodium isopropyl xanthate as a function of sodium cyanide and dextrin concentration at pH 8.
1006
A. Lopez Valdivieso et al. / Minerals Engineering 17 (2004) 1001–1006
adsorption onto sulfide, dextrin imparts hydrophilicity to the surface on which it adsorbs.
4. Conclusions Dextrin is a good depressant for pyrite in flotation with xanthates at pH values higher than 4 owing to adsorption of dextrin onto the ferric oxyhydroxide, which results from oxidation of the surface of pyrite. This depressing effect of dextrin does not result from reduced xanthate adsorption, it results from enveloping the dixanthogen which is a hydrophobic entity on the surface of pyrite reacted with xanthate. Dextrin depresses pyrite as efficiently as cyanide.
Acknowledgements The authors acknowledge the financial support of CONACyT through Grant No. 2002-3953 and that of Fondo Mixto SLP-CONACyT and Minera Mexico through Grant No. FMSLP 2002-5505. T. Celedon Cervantes thanks the Fellowship by CONACYT for Postgraduate studies.
References Adamson, A.W., Gast, A.P., 1997. Physical Chemistry of Surfaces. John Wiley & Sons, New York. pp. 599–684. Angyal, S.J., 1973. Complex formation between sugars and metal ions. Pure Applied Chemistry 35, 131–146. Bebie, J., Schoonen, M.A., Fuhrmann, M., Strongin, D.R., 1998. Surface charge development on transition metal sulfides: an electrokinetic study. Geochimica et Cosmochimica Acta 62, 633– 642. Bogusz, E., Brienne, S.R., Butler, I., Rao, S.R., Finch, J.A., 1997. Metals ions and dextrin adsorption on pyrite. Minerals Engineering 10, 441–445. Buckley, A.N., Woods, R., 1987. The surface oxidation of pyrite. Applied Surface Science 27, 437–452. Bulut, G., Atak, S., 2002. Role of dixanthogen on pyrite flotation: solubility, adsorption studies and Eh, FTIR measurements. Minerals and Metallurgical Processing 19, 81–86. de Bruyn, P.L., Agar, G.E., 1962. Surface chemistry of flotation. In: Fuerstenau, D.W. (Ed.), Froth Flotation 50th Anniversary Volume, AIME, pp. 91–138. Dubois, M., Gilles, K.A., Hamilton, J.K, Rebers, P.A., Smith, F., 1956. Colorimetric method for determination of sugars and related substances. Analytical Chemistry 28, 350–356. Fornasiero, D., Eijt, V., Ralston, J., 1992. An electrokinetic study of pyrite oxidation. Colloids and Surfaces 62, 63–73. Fuerstenau, M.C., Kuhn, M.C., Elgillani, D.A., 1968. The role of dixanthogen in xanthate flotation of pyrite. Transactions of the AIME 241, 148–156. Fuerstenau, M.C., Miller, J.D., Kuhn, M.C., 1985. Chemistry of Flotation. AIME, New York. Hochella, M.F., 2003. Nanoscience and technology: the next revolution in the earth sciences. Earth and Planetary Science Letters 203, 593–605.
Khosla, N.K., Bhagat, R.P., Gandhi, K.S., Biswas, A.K., 1984. Calorimetric and other interactions studies on mineral-starch adsorption systems. Colloids and Surfaces 8, 321–336. Kydros, K.A., Gallios, G.P., Matis, K.A., 1994. Modification of pyrite and sphalerite flotation by dextrin. Separation Science and Technology 29, 2263–2275. Laskowski, J.S., Namyekye, G.A., 1994. Adsorption studies in flotation research: differential flotation of Cu–Ni sulfides using dextrin. In: Castro S. and Alvarez J. (Eds) Proceedings of the IV Meeting of the Southern Hemisphere on Mineral Technology and III Latin American Congress on Froth Flotation, Concepcion, Chile, Nov. 1994, 15–28. Laskowski, J.S., Subramanian, S., Nyamekye, G.A., 1993. Polysaccharides-emerging no-toxic modifiers for differential flotation of sulphides. XVIII International Mineral Processing Congress, Sidney, pp. 593–600. Li, J., Ximeng, Z., Wadsworth, M E., 1993. Raman spectroscopy of natural and ozidized metal sulfides. EPD Congress 1993. In: Hager, J.P. (Ed)., The Minerals, Metals and Materials Society, TMS, Warrendale PA., pp. 229–243. Liu, Q., 1988. Ph.D. Thesis, The University of British Columbia, Vancouver, Canada, 178 pp. Liu, Q., Laskowski, J.S., 1989a. The role of metal hydroxides at mineral surfaces in dextrin adsorption, II. Chalcopyrite-galena separations in the presence of dextrin. International Journal of Mineral Processing 27, 147–155. Liu, Q., Laskowski, J.S., 1989b. The interactions between dextrin and metal hydroxides in aqueous solutions. Journal of Colloid and Interface Science 130, 101–111. Liu, Q., Laskowski, J.S., 1999. Adsorption of polysaccharides onto sulfides and their use in sulfide flotation. In: Laskowski, J.S. (Ed), Proceedings of the 3rd UBC-McGill International Symposium on Polymers in Mineral Processing. Canadian Met. Soc., Quebec, Canada, pp. 71–88. Liu, Q., Laskowski, J.S., Li, Y., Wang, D.F., 1994. Synergetic effect of mineral surface constituents in dextrin adsorption. International Journal of Mineral Processing 42, 251–266. Liu, Q., Zhang, Y., Laskowski, J.S., 2000. The adsorption of polysaccharides onto mineral surfaces: an acid/base interaction. International Journal of Mineral Processing 60, 229–245. Lopez Valdivieso, A., Sanchez Lopez, A.A., Song, Sh., Robledo Cabrera, A., Celed on Cervantes, T., 2003. Flotation chemistry of pyrite with xanthates as collector and dxtrin as a depressant, In: Gomez, C.O., Barahona, C.A. (Eds.), Proceedings of Copper 2003Cobre 2003, The 5th International Conference. Met. Soc. Santiago, Chile, pp. 283–297. Miller, J.D., Du Plessis, R., Kotylar, D.G., Zhu, X., Simmons, G.L., 2002. The low-potential hydrophobic state of pyrite in amyl xanthate flotation with nitrogen. International Journal of Mineral Processing 67, 1–15. Nyamekye, G.A., 1992. Ph.D. Thesis, The University of British Columbia, Vancouver, Canada, 212 pp. Parks, G.A., 1965. The isoelectric point of solid oxides, solid hydroxides, and aqueous hydro complex systems. Chemical Reviews 65, 177–198. Schnar, J.R., 1978. Milling practice in Canada. CIM Bulletin 16, 158– 164. Todd, E.C., Sherman, D.M., Purton, J.A., 2003. Surface oxidation of pyrite under ambient atmospheric and aqueous (pH ¼ 2–10) conditions: electronic structure and mineralogy from X-ray absorption spectroscopy. Geochimica et Cosmochimica Acta 67, 881–893. Xu, D.D., Aplan, F., 1994. Use of polysaccharides and iron ions to depress coal pyrite. Minerals and Metallurgical Processing 11, 223–230. Weissenborn, P.K., 1993. Selective Flocculation of Ultrafine Iron Ore, Ph.D. Thesis, Curtin University of Technology. Zhu, X., Li, J., Bodily, D.M., Wadsworth, M.E., 1993. Transpassive oxidation of pyrite. Journal of Electrochemical Society 140, 1927– 1935.
Dextrin as a non-toxic depressant for pyrite in flotation with xanthates as collector A. L opez Valdivieso a
a,*
, T. Celed on Cervantes a, S. Song a, A. Robledo Cabrera a, J.S. Laskowski b
Instituto de Metalurgia, Universidad Autonoma de San Luis Potosı, Avenue Sierra Leona 550, San Luis Potosı, S.L.P. 78210, Mexico b Department of Mining, University of British Columbia, Vancouver, British Columbia, Canada V6T 1Z4 Received 4 February 2004; accepted 6 April 2004
Abstract Depression of pyrite flotation by dextrin has been investigated through adsorption, electrokinetic and microflotation studies in systems open to the atmosphere. Adsorption of dextrin takes place through specific interaction with ferric oxyhydroxide species that result from the oxidation of pyrite surface. Dextrin shows an isoelectric point at pH 4 and pyrite does at pH 6.4. Within this pH range adsorption is suggested to be promoted by electrostatic interactions. Coadsorption of dextrin and isopropyl xanthate occurs on the surface of pyrite and is explained to happen through distinct mechanisms taking into account the heterogeneous nature of the surface. It is likely that dextrin depresses pyrite by enveloping the dixanthogen resulting from adsorption of xanthate ions. It is shown that dextrin is as effective depressant of pyrite as cyanide. 2004 Elsevier Ltd. All rights reserved. Keywords: Sulphide ores; Flotation reagents; Flotation depressants; Environmental; Mineral processing
1. Introduction Pyrite is the most widespread and abundant of naturally occurring metal sulfides. It is commonly present in base metal sulfides and frequently appears in coal as a major source of sulfur in coal. Pyrite lowers the quality of base metal concentrates and increases the amount of sulfur compounds produced in the base metal extraction processes. Combustion of high-sulfur coals poses serious environmental problems due to sulfur dioxide emissions. Therefore, depression of pyrite is desirable in the concentration of base metal sulfides by flotation. This can be achieved by floating in alkaline solutions (lime) using highly selective inorganic modifiers such as cyanides, sulfites, ferrocyanides and cyanides in combination with zinc sulfate (Fuerstenau et al., 1985). Of all depressants, cyanides have been the most common. Their use has raised concern on environmental grounds, however. Hence, natural, bio-degradable, non-toxic agents such as dextrin are gaining in importance and hold promise * Corresponding author. Tel.: +52-444-825-5004; fax: +52-444-8254326. E-mail address: [email protected] (A. L opez Valdivieso).
0892-6875/$ - see front matter 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.mineng.2004.04.003
to function as a selective depressant (Laskowski et al., 1993). Dextrin naturally degrades to carbon dioxide and water. Dextrin is derived from starch by partial thermal degradation under acidic conditions. The treatment causes the break down of starch macromolecules resulting in smaller molecules that are more branched (dextrin). The structure of dextrin is given in Fig. 1 and resembles amylopectin. All the three hydroxyl group (–OH) in the glucose monomeric unit may rotate in such a way that they face one side of the monomeric ring, making that side hydrophilic. The opposite side is consequently slightly hydrophobic due to the exposed –CH groups. Application of dextrin as a flotation depressant, both in the laboratory and in commercial processes, has been reported. In the flotation of Cu–Pb sulfide ores at Kidd Creek dextrin is utilized in alkaline environment whenever the content of galena in copper concentrate is too high. Its use along with sulfur dioxide in Brunswick Mines has also been reported (Schnar, 1978). In 1957, Lukkarinen discovered that dextrin could be used in the differential flotation of Cu–Pb concentrate; in this process pentlandite was depressed with dextrin while
1002
A. Lopez Valdivieso et al. / Minerals Engineering 17 (2004) 1001–1006 CH2OH
O H
HO HO
H
OH
H H
CH2OH
HO HO
H
O O H H OH
O CH2
H HO
O H O
HO
H
OH
involves formation of a complex between polysaccharide oxygen atoms at C-2 and C-3 and iron atoms in the flocculation of iron oxides with starch was confirmed by Weissenborn (1993). The aim of this study is to test dextrin as an alternative non-toxic depressant for pyrite. Adsorption studies have been carried out to delineate the mechanisms by which dextrin renders pyrite surface hydrophilic when using xanthates as collector. The tests have been carried out using natural crystals of pyrite and isopropyl xanthate collector.
H
H
Fig. 1. Schematic of the molecular structure of dextrin.
chalcopyrite was floated with xanthate under alkaline conditions. According to Laskowski and Namyekye (1994) this scheme was used in the Kotalahti mine in Finland until 1987 when the mine closed down. In the laboratory, Liu and Laskowski (1989a) showed that in acidic solutions chalcopyrite can be depressed by dextrine while galena can be floated with ethyl xanthate; whereas in alkaline solutions chalcopyrite is floated while galena is depressed. They have used dextrin to selectively depress heazlewoodite (Ni3 S2 ) in the flotation of chalcocite in the processing of INCO matte with amyl xanthate at a pH of 11.7. Reports on the use of dextrin as a depressant for pyrite in sulfide flotation are scarce. Kydros and Gallios (1994) have shown that dextrin effectively depressed pyrite at pH 4 and higher in the presence of ethyl xanthate. At pH 4, copper-activated sphalerite was floated using ethyl xanthate as a collector, while pyrite was depressed with dextrin in the flotation of the pyrite– sphalerite mixture. Xu and Aplan (1994) have proposed to use polysaccharides in the presence of iron ions to depress pyrite in coal cleaning. Adsorption of dextrin which renders metal sulfide surfaces hydrophilic has been proposed to be due to the interaction between the hydroxyl groups in the glucose unit and the metal hydroxides on the sulfide surface (Liu and Laskowski, 1989b). This mechanism is compatible with the findings by Angyal (1973), who showed, through Nuclear Magnetic Resonance studies, that the hydroxyl group of the C-2 y C-3 in D-Glucose interacts with multivalent metallic ions to form D-Glucose metal complex species. X-ray photoelectron spectroscopy and Auger electron spectroscopy studies performed by Liu et al. (1994) indicated chemical interaction of dextrin with metal hydroxide species on the surface of metal oxides. Through infrared studies, Khosla et al. (1984) have shown that amylopectin bonds chemically with Fe3 ions, giving evidence of the existence of chemical bonding of amylopectin on hematite surface. The mechanism similar to the one postulated by Liu and Laskowski (1989a,b, 1999; Liu et al., 2000) and which
2. Experimental The crystalline pyrite sample used in this study was from Zacatecas, Mexico. The crystals were hand ground using an agate mortar and pistil in order to obtain )150 + 70 lm and )37 lm size fractions, which were used for microflotation tests and adsorption and electrokinetic studies, respectively. These samples were kept under nitrogen atmosphere to prevent further surface oxidation and ensure reproducibility of the tests. The surface area of the )37 lm size fraction particles was determined to be 0.86 m2 /g by the BET method using krypton as the adsorbent and a Micromeritics model ASP 2010 equipment. The dextrin used in the work was supplied by A.E. Staley Manufacturing Co. Its molecular weight has been determined by Nyamekye (1992) to be 56,000. Dextrin solutions were prepared daily at 1 g/l concentration and 0.01 mol/l ionic strength, using NaNO3 as the supporting electrolyte. Sodium isopropyl xanthate obtained from Industrias Quimicas de Mexico was purified threefold through dissolution of the xanthate in acetone, followed by precipitation in ethylic ether. To prevent its decomposition, the purified xanthate sample was kept under nitrogen atmosphere in a fridge and was kept in a black glass bottle. Deionized water and analytical grade inorganic reagents were used to prepare the solutions used in this work. The deionized water was obtained by passing distilled water through ion exchange resins in a Nanopure Barnstead unit. The solutions used in all tests were prepared at a fixed ionic strength of 0.01 mol/l, using NaNO3 as the supporting electrolyte. Dilute solutions of NaOH and HNO3 were used to adjust pH. Microflotation tests were carried out using a modified Hallimond flotation tube. One gram of pyrite was conditioned in 150 ml solution at the desired xanthate concentration and pH for 5 min. When dextrin was used, the pyrite was conditioned first with this reagent for 30 min at a given concentration and pH, then with xanthate for an additional 5 min. The pH reported in this study is the pH value measured at the end of the conditioning time. Following the conditioning step, the pulp was transferred to the Hallimond tube for flota-
A. Lopez Valdivieso et al. / Minerals Engineering 17 (2004) 1001–1006
tion, which was carried out for 1 min using nitrogen at a flow rate of 30 ml/min. The conditioning procedure aforementioned was identical for both the adsorption and electrokinetic studies. The former was carried out using 1 g pyrite and 50 ml solution. After conditioning, the pulp was centrifuged and solutions were withdrawn from the supernatant for analysis of dextrin and xanthate residual concentrations. Dextrin concentration in solution was determined through the method described by Dubois et al. (1956), while xanthate concentration was determined with a UV spectrophotometer at a wave length of 301 nm. Electrokinetic studies were carried out with 100 mg pyrite and 50 ml solution. The zeta potential of pyrite was determined using a Riddick Zeta Meter model D and electrophoretic cell No S-2479. After conditioning, the pulp was transferred to the electrophoretic cell and the mobility of at least 10 pyrite particles was measured. The zeta potential was calculated from the electrophoretic mobility measurements using the Smoluchowski equation (Adamson and Gast, 1997).
3. Results and discussion Electrokinetic studies have been undertaken to delineate the adsorption mechanism of dextrin onto pyrite. Fig. 2 shows the zeta potential of pyrite as a function of pH in the absence and presence of various dextrin concentrations. This figure also shows the zeta potential of dextrin both from this study and after Liu (1988). As noted, dextrin exhibits point of zeta reversal at about pH 4. Fig. 2 shows that pyrite’s isoelectric point (iep) is at about pH of 6.4. Since the pHiep of non-oxidized pyrite has been reported to be about 2 (Fuerstenau et al., 1968; Fornasiero et al., 1992; Bebie et al., 1998) and that of
60 ZETA POTENTIAL, mV
DEXTRIN INITIAL CONC
40
None 10 mg/L 25 mg/L 100 mg/L 0.01 M NaNO3, 25°C
20 0 DEXTRIN
-20
1000 mg/L Dextrin (Liu et al., 1988) 1000 mg/L Dextrin (this work)
-40 -60 0
2
4
6
pH
8
10
12
14
Fig. 2. Zeta potential of pyrite as a function of pH in the absence and presence of various initial dextrin concentration. The zeta potential of dextrin as a function of pH is also presented.
1003
ferric oxides to range between 5.2 and 8.6 (Parks, 1965), this pHiep at 6.4 is an indicative that the surface of the pyrite used in this study underwent oxidation during the sample preparation and conditioning. Various techniques have been used to study the oxidized pyrite surface, such as cyclic voltammetry, surface enhanced Raman spectroscopy, X-ray photoelectron spectroscopy, to name a few (Buckley and Woods, 1987; Li et al., 1993; Zhu et al., 1993). They all have reported the formation of ferric oxyhydroxide species on the surface of pyrite, as dictated by thermodynamics. Todd et al. (2003) have suggested, based on X-ray absorption spectroscopy studies, that ferric oxyhydroxide, probably goethite, is the oxidation product. Nanopatches of ferric oxyhydroxide have been microphotographed on the surface of oxidized pyrite through atomic force microscopy studies by Miller et al. (2002) and Hochella (2003). These studies show that the surface of pyrite, when exposed to oxidation for a short period of time, is of heterogeneous nature with patches of ferric oxyhydroxide and FeS2 . The electric charge at the pyrite/aqueous solution interface can then be accounted for by FeOHþ 2 and FeO sites. The equilibrium between the oxidized surface species at interface with water may be viewed as a two step process (de Bruyn and Agar, 1962), FeOHðsurfÞ þ Hþ ¼ FeOHþ 2 ðsurfÞ
FeOHðsurfÞ þ OH ¼ FeO ðsurfÞ þ H2 O
ð1Þ ð2Þ
Eqs. (1) and (2) are equivalent to the following reactions at the non-oxidized pyrite/aqueous solution interface (Fornasiero et al., 1992): FeSHðsurfÞ ¼ FeS ðsurfÞ þ Hþ þ
FeSHðsurfÞ þ H ¼
FeSHþ 2 ðsurfÞ
ð3Þ ð4Þ
As noted in Fig. 2, dextrin shifts the pHiep of pyrite to a more lower value and decreases both the positive and negative zeta potential of pyrite. This is indicative that dextrin specifically adsorbs on the pyrite surface, decreasing the surface electric charge density. At a dextrin concentration of 100 mg/l, the zeta potential of pyrite is identical to that of dextrin. From the point of view of flotation, this means that pyrite completely loses its interfacial properties to acquire that of dextrin, which is highly hydrophilic as stated above. Adsorption of dextrin on pyrite as a function of pH is given in Fig. 3 at two distinct dextrin concentrations, namely at 50 and 500 mg/l. As noted, adsorption is strongly pH-dependent and rises sharply at pH 4. At the lowest dextrin concentration, a leveling off of the adsorption takes place above pH 4, while at the highest dextrin concentration, a maximum in the adsorption occurs around pH 6, close to the pHiep of pyrite due to ferric oxyhydroxide. Above this pH value adsorption decreases monotonically.
A. Lopez Valdivieso et al. / Minerals Engineering 17 (2004) 1001–1006
DEXTRIN ADSORPTION DENSITY, mg/m
2
18 DEXTRIN INITIAL CONC. 50 mg/L
16
400 mg/L
14
0.01 M NaNO3, 25°C
12 10 8 6 4 2 0 0
2
4
6
8
10
12
14
pH
Fig. 3. Adsorption density of dextrin on pyrite as a function of pH at 50 and 400 mg/l initial dextrin concentration.
The sharp increase in the adsorption takes place at a pH close to the pHiep of dextrin where the net surface charge of pyrite is positive. This seems to suggest that adsorption occurs not only by specific interaction contributions, but also by a contribution resulting from the electric charge at both the dextrin and the pyrite/aqueous solution interface. At the pH range between the pHiep of pyrite and the pHiep of dextrin, the electric charge for dextrin is negative and that of pyrite is positive. Under these conditions, electrostatic interaction should promote adsorption. On the other hand, the specific interaction contribution to adsorption can be accounted for by chemical bonding between dextrin and the ferric oxyhydroxide on the surface of pyrite. Liu et al. (1994) have shown that dextrin interacts with ferric hydroxide through chemisorption. Bogusz et al. (1997) have reported that pyrite adsorbed dextrin only after an oxidative treatment of its surface. At pH values above the pHiep of pyrite, both pyrite and dextrin carry a net negative electric charge. Under these conditions, the specific interaction contribution should be responsible for adsorption by surmounting that owing to electrostatic repulsion. The higher the pH, the larger is the negative electric charge at the pyrite/ aqueous solution interface. Accordingly, the electrostatic repulsion contribution increases causing adsorption to decrease as noted in Fig. 3. In addition, dextrin must compete with OH ions for the surface of ferric oxyhydroxide on pyrite. The dextrin adsorption did not occur at pH values below 4. This can be associated with the fact that under these conditions ferric hydrosulfate, Fe(OH)SO4 , is the main oxidation product on oxidized surfaces of pyrite, as shown by the X-ray absorption spectroscopy studies performed recently by Todd et al. (2003). They have
pointed out that ferric oxyhydroxide species predominantly form at pH ¼ 4 and higher. The electrokinetic and adsorption studies then predict that dextrin should be an effective depressant for pyrite with ferric oxyhydroxide on the surface. The adsorption studies show that optimal conditions for an effective depression of pyrite should be within the pH range between 4 and 12. Accordingly, microflotation tests were carried out to assess dextrin as a depressant for pyrite. Sodium isopropyl xanthate was used as the flotation collector. Fig. 4 shows the floatability of pyrite with 104 mol/l isopropyl xanthate in the absence and presence of various concentrations of dextrin as a function of pH. As can be observed, in the absence of dextrine, the flotation of pyrite is high up to about pH 12. With 10 mg/l dextrin, flotation of pyrite falls at pH 6. Increasing the concentration of dextrin up to 25 mg/l results in depression which begins at about pH 4. This pH of depression does not change further when the concentration of dextrin is increased up to 50 mg/l. These flotation results correlate well with the adsorption results in that pyrite is depressed within the pH range of dextrin adsorption. At pH values below 4, pyrite was not depressed due to the lack of adsorption of dextrin. Kydros and Gallios (1994) found a marked depression of pyrite by dextrin at pH 4 in flotation with ethyl xanthate, which is compatible with the results of this investigation. It is well known that the depressing effect of inorganic modifiers, such as cyanides, ferrocyanides and sulfites, is caused by lowering the pulp potential and making the surface of pyrite unsuitable for adsorption of sulfhydryl collectors. To understand further the mechanisms by which dextrin depresses pyrite, the adsorption of isopropyl xanthate at the pyrite/aqueous solution interface has been studied in the presence of dextrin at pH 8. In
100 FLOATABILITY, percent
1004
80 DEXTRIN INITIAL CONC. None
60
10 mg/L
40
25 mg/L 50 mg/L -4 1x10 M SIPX
20
0.01 M NaNO 3, 25°C
0 0
2
4
6
8
10
12
14
pH Fig. 4. Floatability of pyrite with 1 · 104 mol/l sodium isopropyl xanthate as a function of pH in the absence and presence of various initial dextrin concentration.
A. Lopez Valdivieso et al. / Minerals Engineering 17 (2004) 1001–1006
these tests, the xanthate concentration was fixed at 104 mol/l, while that of dextrin was varied. Fig. 5 shows the adsorption density of both isopropyl xanthate and dextrin on pyrite as a function of equilibrium dextrin concentration. As can be seen, the collector adsorption is not affected by dextrin. Dextrin does not adsorb until its concentration reaches about 8 mg/l, then adsorption increases monotonically until a plateau is obtained at 8 mg/m2 adsorption density. Accordingly, dextrin does not inhibit the adsorption of xanthate ions so that its depression effect can be attributed to the entrapment of adsorbed xanthate on the surface of pyrite. Flotation of pyrite with xanthate is due to dixanthogen formation on the surface of pyrite through the oxidation reaction of xanthate ions coupled with a reduction reaction of ferric oxyhydroxide species to ferrous species (Lopez Valdivieso et al., 2003; Bulut and Atak, 2002; Fuerstenau et al., 1968). Due to the heterogeneous nature of the surface of oxidized pyrite, adsorption of xanthate ions and dextrin may be viewed to take place at different sites on the surface. While dextrin adsorbs on ferric oxyhydroxide sites, xanthate ions do so on anodic sites located on the non-oxidized surface of pyrite. The adsorption of xanthate ions on the surface of pyrite has been proposed to be as follows: An odic oxidation reaction of xanthate ions to dixanthogen
X ¼ X2 ðlÞðsurfÞ þ 2e
ð5Þ
Cathodic reduction reaction of ferric oxyhydroxide sites to ferrous ions 2FeOOHðsurfÞ þ 6Hþ þ 2e ¼ 2Fe2þ þ 4H2 O
ð6Þ
where Eq. (6) should be in equilibrium with the hydrolysis and precipitation products of Fe2þ ions and the oxidation of Fe2þ ions to Fe3 species.
1005
The chemical interaction of dextrin with ferric oxyhydroxide sites on the surface of pyrite may be given as follows: FeOHðsurfÞ þ DOH ¼ FeODðsurfÞ þ H2 O
ð7Þ
Eq. (7) is a simplified version since a complete complex probably involves OH groups on C-2 and C-3 carbons interaction with surface metal sites as shown below (Liu and Laskowski, 1989b):
C O C O
C C C
O
Pb
Weissenborn (1993) showed the following complex for iron oxide interacting with starch: O
CH2OH
CH2OH
O
2
O
O
O HO
HO Fe O
Fe Fe
O
OH
3'
O
O
OH Fe
O
Fe
O
O
It is interesting to compare depression of pyrite by dextrin and sodium cyanide. Fig. 6 shows the floatability of pyrite with 104 mol/l sodium isopropyl xanthate at pH 8 as a function of the concentration of the two depressants. The depression of pyrite takes place at a lower dextrin concentration in comparison to that needed with sodium cyanide. It is to be pointed out however that these two, cyanide and dextrin, operate in quite a different way. While cyanide prevents xanthate
9
100
DEXTRIN
8
FLOATABILITY, percent
ADSORPTION DENSITY, mg/m
2
10
7 6
-4
1x10 M SIPX pH 8 0.01 M NaNO 3 , 25°C
5 4 3 2
ISOPROPYL
1
-4
1x10 M SIPX pH 8 0.01 M NaNO3, 25°C
80 60 40
NaCN
20 DEXTRINE
0
0 0
5
10
15
20
25
30
35
40
45
50
55
60
EQUILIBRIUM DEXTRIN CONCENTRATION, mg/L
Fig. 5. Adsorption density of dextrin on pyrite as a function of equilibrium dextrin concentration in the presence of 1 · 104 mol/l sodium isopropyl xanthate at pH 8.
0
10 20 30 40 DEPRESSANT CONCENTRATION, mg/L
50
Fig. 6. Floatability of pyrite with 1 · 104 mol/l sodium isopropyl xanthate as a function of sodium cyanide and dextrin concentration at pH 8.
1006
A. Lopez Valdivieso et al. / Minerals Engineering 17 (2004) 1001–1006
adsorption onto sulfide, dextrin imparts hydrophilicity to the surface on which it adsorbs.
4. Conclusions Dextrin is a good depressant for pyrite in flotation with xanthates at pH values higher than 4 owing to adsorption of dextrin onto the ferric oxyhydroxide, which results from oxidation of the surface of pyrite. This depressing effect of dextrin does not result from reduced xanthate adsorption, it results from enveloping the dixanthogen which is a hydrophobic entity on the surface of pyrite reacted with xanthate. Dextrin depresses pyrite as efficiently as cyanide.
Acknowledgements The authors acknowledge the financial support of CONACyT through Grant No. 2002-3953 and that of Fondo Mixto SLP-CONACyT and Minera Mexico through Grant No. FMSLP 2002-5505. T. Celedon Cervantes thanks the Fellowship by CONACYT for Postgraduate studies.
References Adamson, A.W., Gast, A.P., 1997. Physical Chemistry of Surfaces. John Wiley & Sons, New York. pp. 599–684. Angyal, S.J., 1973. Complex formation between sugars and metal ions. Pure Applied Chemistry 35, 131–146. Bebie, J., Schoonen, M.A., Fuhrmann, M., Strongin, D.R., 1998. Surface charge development on transition metal sulfides: an electrokinetic study. Geochimica et Cosmochimica Acta 62, 633– 642. Bogusz, E., Brienne, S.R., Butler, I., Rao, S.R., Finch, J.A., 1997. Metals ions and dextrin adsorption on pyrite. Minerals Engineering 10, 441–445. Buckley, A.N., Woods, R., 1987. The surface oxidation of pyrite. Applied Surface Science 27, 437–452. Bulut, G., Atak, S., 2002. Role of dixanthogen on pyrite flotation: solubility, adsorption studies and Eh, FTIR measurements. Minerals and Metallurgical Processing 19, 81–86. de Bruyn, P.L., Agar, G.E., 1962. Surface chemistry of flotation. In: Fuerstenau, D.W. (Ed.), Froth Flotation 50th Anniversary Volume, AIME, pp. 91–138. Dubois, M., Gilles, K.A., Hamilton, J.K, Rebers, P.A., Smith, F., 1956. Colorimetric method for determination of sugars and related substances. Analytical Chemistry 28, 350–356. Fornasiero, D., Eijt, V., Ralston, J., 1992. An electrokinetic study of pyrite oxidation. Colloids and Surfaces 62, 63–73. Fuerstenau, M.C., Kuhn, M.C., Elgillani, D.A., 1968. The role of dixanthogen in xanthate flotation of pyrite. Transactions of the AIME 241, 148–156. Fuerstenau, M.C., Miller, J.D., Kuhn, M.C., 1985. Chemistry of Flotation. AIME, New York. Hochella, M.F., 2003. Nanoscience and technology: the next revolution in the earth sciences. Earth and Planetary Science Letters 203, 593–605.
Khosla, N.K., Bhagat, R.P., Gandhi, K.S., Biswas, A.K., 1984. Calorimetric and other interactions studies on mineral-starch adsorption systems. Colloids and Surfaces 8, 321–336. Kydros, K.A., Gallios, G.P., Matis, K.A., 1994. Modification of pyrite and sphalerite flotation by dextrin. Separation Science and Technology 29, 2263–2275. Laskowski, J.S., Namyekye, G.A., 1994. Adsorption studies in flotation research: differential flotation of Cu–Ni sulfides using dextrin. In: Castro S. and Alvarez J. (Eds) Proceedings of the IV Meeting of the Southern Hemisphere on Mineral Technology and III Latin American Congress on Froth Flotation, Concepcion, Chile, Nov. 1994, 15–28. Laskowski, J.S., Subramanian, S., Nyamekye, G.A., 1993. Polysaccharides-emerging no-toxic modifiers for differential flotation of sulphides. XVIII International Mineral Processing Congress, Sidney, pp. 593–600. Li, J., Ximeng, Z., Wadsworth, M E., 1993. Raman spectroscopy of natural and ozidized metal sulfides. EPD Congress 1993. In: Hager, J.P. (Ed)., The Minerals, Metals and Materials Society, TMS, Warrendale PA., pp. 229–243. Liu, Q., 1988. Ph.D. Thesis, The University of British Columbia, Vancouver, Canada, 178 pp. Liu, Q., Laskowski, J.S., 1989a. The role of metal hydroxides at mineral surfaces in dextrin adsorption, II. Chalcopyrite-galena separations in the presence of dextrin. International Journal of Mineral Processing 27, 147–155. Liu, Q., Laskowski, J.S., 1989b. The interactions between dextrin and metal hydroxides in aqueous solutions. Journal of Colloid and Interface Science 130, 101–111. Liu, Q., Laskowski, J.S., 1999. Adsorption of polysaccharides onto sulfides and their use in sulfide flotation. In: Laskowski, J.S. (Ed), Proceedings of the 3rd UBC-McGill International Symposium on Polymers in Mineral Processing. Canadian Met. Soc., Quebec, Canada, pp. 71–88. Liu, Q., Laskowski, J.S., Li, Y., Wang, D.F., 1994. Synergetic effect of mineral surface constituents in dextrin adsorption. International Journal of Mineral Processing 42, 251–266. Liu, Q., Zhang, Y., Laskowski, J.S., 2000. The adsorption of polysaccharides onto mineral surfaces: an acid/base interaction. International Journal of Mineral Processing 60, 229–245. Lopez Valdivieso, A., Sanchez Lopez, A.A., Song, Sh., Robledo Cabrera, A., Celed on Cervantes, T., 2003. Flotation chemistry of pyrite with xanthates as collector and dxtrin as a depressant, In: Gomez, C.O., Barahona, C.A. (Eds.), Proceedings of Copper 2003Cobre 2003, The 5th International Conference. Met. Soc. Santiago, Chile, pp. 283–297. Miller, J.D., Du Plessis, R., Kotylar, D.G., Zhu, X., Simmons, G.L., 2002. The low-potential hydrophobic state of pyrite in amyl xanthate flotation with nitrogen. International Journal of Mineral Processing 67, 1–15. Nyamekye, G.A., 1992. Ph.D. Thesis, The University of British Columbia, Vancouver, Canada, 212 pp. Parks, G.A., 1965. The isoelectric point of solid oxides, solid hydroxides, and aqueous hydro complex systems. Chemical Reviews 65, 177–198. Schnar, J.R., 1978. Milling practice in Canada. CIM Bulletin 16, 158– 164. Todd, E.C., Sherman, D.M., Purton, J.A., 2003. Surface oxidation of pyrite under ambient atmospheric and aqueous (pH ¼ 2–10) conditions: electronic structure and mineralogy from X-ray absorption spectroscopy. Geochimica et Cosmochimica Acta 67, 881–893. Xu, D.D., Aplan, F., 1994. Use of polysaccharides and iron ions to depress coal pyrite. Minerals and Metallurgical Processing 11, 223–230. Weissenborn, P.K., 1993. Selective Flocculation of Ultrafine Iron Ore, Ph.D. Thesis, Curtin University of Technology. Zhu, X., Li, J., Bodily, D.M., Wadsworth, M.E., 1993. Transpassive oxidation of pyrite. Journal of Electrochemical Society 140, 1927– 1935.