Mixed cationic/anionic collectors in the flotation ...

Mixed cationic/anionic collectors in the flotation separation of albite from Greek stefania feldspar ore A. Vidyadhar National Metallurgical

Laboratory, Mineral Processing Division, Jamshedpur,

India

K. H. Rao, K. S. E. Forssberg Division

0/ Mineral

Processing, Lulea University of Technology, Sweden

ABSTRACT: The mixed alkyl diamine/sulfonate reagent scheme for the separation of albite from quartz is assessed through Hallimond flotation, zeta-potential studies using pure albite and quartz minerals. Furthermore, the reagent scheme is tested in bench scale flotation for the separation of albite from Gr~ek Stefania feldspar ore. The feasibility of selective albite flotation from quartz is indicated from single mineral flotation tests. The partial diamine flotation of albite at pH 2 is increased in the presence of sulfonate without affecting quartz flotation. The flotation performance is further enhanced when the hydrocarbons of diamine and sulfonate are compatibile with equal chain lengths. The eo-adsorption of sulfonate in the form of diamine-sulfonate (1: 1) complex is reasoned for the increased floatability of albite in mixed collector composition. The difference in the flotation and adsorption behaviour at pH 2 is attributed to the coarse and fme size particles employed in these studies respectively. Besides electrostatic interactions, the adsorption of alkyl ammonium ions through hydrogen bonding on surface silanol groups is explained for similar adsorption behaviour on albite and quartz where the surfaces are slightly negative and uncharged respectively at pH 2. Selective flotation of albite from feldspar ore is accomplished when the material is deslimed prior to flotation.

I INTRODUCTION The adsorption behaviour of mixtures of soluble surfactants at interfaces and their effect on interfacial properties varies significantly over a wide range of relative concentrations. There are a number of processes where mixtures of dissimilar surfactants have shown better properties than the individual components involved and thus a strong interest has appeared in the behaviour of mixed surfactant systems in recent years (Rosen, 1984; Scamehom, 1986). The interactions between two surfactants in solution and at the air/liquid interfaces have been studied intensively. In general, the physico-chemical properties of mixed mono layers do not obey the additive rule (Holland and Rubingh, 1992), i.e., the layer performance is either enhanced or suppressed as compared to the sum of the proportional contributions of the individual components. Such a non-additive behaviour is designated as synergism and negative synergism in the cases of property enhancement and suppression, respectively. Synergism in binary mixtures of surfactants is due to attractive interactions between the two types of surfactant being stronger than attraction between the molecules of one surfactant. Negative synergism occurs when attractive surfactant interactions in the 955

mixed system are weaker (e.g., due to electrostatic repulsion) than the individual surfactant systems. Although the adsorption of single surfactants at the solid-liquid interface has been studied thoroughly, far few studies exist for the case of adsorption from mixed surfactant solutions. Synergy in eo-adsorption of collectors is very promising direction in flotation research since it may permit achieving the same surface property at a lower consumption of the reagents. This is important in the flotation processes where the selectivity increases with decreasing collector concentration. The literature on mixed collector systems in flotation has been reviewed recently (Hanumantha Rao and Forssberg, 1997). Feldspars are conventionally separated from quartz using cationic long-chain alkyl amine collectors in highly acidic conditions generated by the use of hydrofluoric acid (HF). The use of HF is no longer acceptable due to environmental considerations and new reagent schemes, free from HF, have been reported from time to time (Mathieu and Sirois, 1984; Jiaying et al., 1993; Hanumantha Rao and Forssberg, 1994; Salmawy et al., 2000). Of particular interest is the mixed cationic/anionic collector system using cationic alkyltrimethylene

g/mol (specified by the manufacturer) and 400 g/rnol (an average molecular weight). The two cationic and anionic collectors are here into referred as diamine and sulfonate. High purity sodium salts of Cs and C]o-sulfonic acids (Sigrna Chemical Co., USA), C12sulfonic acid (Fluka Chemika, Switzerland) and, C]4 and Cwsulfonic acids (Aldrich Chem. Co.) were purchased. Analar grade NaOH and H2S04 were used for pH adjustment and de-ionised water (specific conductance, 004-0.7 IlS ern") was used in all the experiments except in the bench scale flotation tests.

diamine and anionic petroleum sulfonate (Katayanagi, 1973) or a combined cationic-anionic collector of tallow 1,3-propylene diamine dioleate (Malghan, 1976) for feldspar flotation. The latter process is reported to have been applied successfully in industrial practice. There are only a few attempts to understand the mechanism of mixed alkyl diarnine/sulfonate collectors in selective feldspar flotation (Shimoiizaka et aI., 1976; Hanumantha Rao and Forssberg, 1993). The aim of the present work is to apply the mixed cationic/anionic collector system for the selective separation of albite from Stefania feldspar deposit (Mevior S.A., Greece) and to understand the function of mixed collectors in feldspar flotation. In this paper, the results of zeta-potential measurements, as weJl as HaIlimond and bench scale flotation tests are presented and discussed.

2.3 Zeta-potential

Measurements

Zeta-potentials were determined with a Laser Zee Meter (Pen Kern Inc., model SOI) equipped with video ~rstem ~pparatus em~loying a flat cell. About 1.0 g I of mmeral suspension was prepared in 10-) KN03 supporting electrolyte solutions and conditioned for I hour at room temperature (22°C) at a predetermined collector concentration and pH.

2 EXPERIMENTAL 2.1 Materials Pure crystalline quartz (Si02) and albite (NaAIShOs) mineral samples were obtained from Mevior S.A., Greece. The chemical analysis of these samples showed that the purity of-the quartz is more than 99% and that of albite is 98.S% (pure with oxides content of 67.9 wt% Si02, 19.2 wt% AI203 and 11.7 wt% Na20). The samples were crushed and ground in an agate mortar. The products were wetsieved to obtain a particle size fractions of -ISO +38 urn and -38 urn. A portion of -38 urn was further ground and rnicro-sieved in an ultrasonic bath to obtain a -S urn size fraction. The coarse size fraction, -ISO +38 urn, was employed for Hallimond flotation tests while the fines (-S urn) were used in the zeta-potential measurements. The BET specific surface areas for the albite coarse and fine fractions were determined to be O.IS and 2.78 m2 g-I respectively where as those for quartz size were found to be 0.09 and 1.30 m2 respectively. The albite ore from the Stefania deposit near Thessaloniki, Greece was also received from Mevior S.A. The ore was crushed to -3 mm and then ground in a stainless steel rod mill to 80% passing 90 urn which is suitable for bench-scale flotation tests. The material contained about 7S.8 wt% Si02, 13.7 wt% AhO), 6.8S wt% Na20, 0.S4 wt% Fe203, 0.8S wt% K20 and 0043 wt% CaO.

204 Hallimond Flotation Tests Single mineral flotation tests were made using a glass frit Hallimond flotation cell of 100-ml capacity. Exactly I g of the mineral was conditioned for S min at the desired pH and predetermined collector concentration solution in a 100-ml volumetric flask. The suspension was then transferred to the Hallimond tube. Flotation was conducted for I min at constant agitation and aeration rate of8 ml min-I. 2.S Bench Scale Flotation Tests Flotation tests were performed with I Kg material in a WEMCO laboratory cell of the Fagergren type with a ceJl volume of2.7Iitres. Initially, I Kg of the material was ground in a stainless steel rod mill with 17 Kg charge and 800 ml water for a specified time. After grinding, the material was deslirned at 20 urn and transferred to the flotation cell. The pulp density was adjusted to 28 per cent and the sequential flotation test involving the diamine and sulfonate collectors was carried out as follows: The pulp was conditioned for 10 min at pH 7.5 was maintained during the course of flotation. Flotation was then conducted using sulfonate collector dosages of 300 g/t and 200 g/t, each for 4 min conditioning periods of S min. Afterwards, the pH was reduced to pH 2 and conditioned for S min. Three-stage albite flotation was then carried out with diamine collector additions of 100 g/t, 100 g/t and SO g/t, are for 2 min with S min conditioning periods.

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2.2 Reagents The cationic collector of tallow-I ,3-diaminopropane (Duomeen T) and anionic alkyl aryl sulfonate (Morwet 3008) collector were obtained from Akzo Nobel AB, Sweden and Witco SA, France, respectively. Respective molecular weights are 330

956

3 RESULTS AND DISCUSSION

flotation recoveries of albite increasing collector concentration results in Figs. 1 and 2 illustrate floated from quartz at pH 2 but only 30-40%.

3.1 Hallimond Flotation Studies Since albite is floated selectively from the present Greek feldspar ore with a combined cationic-anionic collector (N-tallow-I,3-diaminopropane dioleate, Duomeen TDO) where the ratio of cationic/anionic reagents cannot be changed, the effect of different ratios of cationic and anionic reagents was investigated using alkyl diamine (Duomeen T) and anionic alkyl aryl sulfonate collectors and pure minerals handpicked from the same feldspar ore deposit (Vidyadhar et al., 2002).

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The flotation responses of albite and quartz as a function of pH at two initial diamine concentrations are given in Figure I. Results show that albite recovery at pH 2 is about 40% at diamine collector concentration of 1 x 10-5 M, whereas quartz is nonfloatable. The albite and quartz recoveries are dependent on the diamine concentration in the pH region 3 to 5. Above pH 5, the flotation responses of both minerals are identical with 90% recovery, irrespective of the diamine concentration used. Thus, the diamine collector floats albite partially at pH 2 without influencing quartz flotation.

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Figure I. Flotation results as a function of pH at two initial diamine collector concentrations. The effect of diamine concentration on albite and quartz flotation at three pH values of 1.75, 2.0 and 6-7 is shown in Figure 2. At lower pH values of 1.75 and 2, the flotation response of albite is similar with increasing concentration with a marginal higher recovery at pH 2. The on-set diamine concentration for albite flotation is about 1 x 10-6 M and at 1 x 10-5 M concentration, the recovery is about 45%. At these pH values, there is no response of quartz flotation until about 2 x 10-5 M concentration where quartz begins to float. At the higher pHs of 6-7, the

957

The effect of sulfonate concentration on the diamine flotation of albite and quartz at pH 2 is presented in Figure 3. The results show that the presence of sulfonate increased the albite flotation without influencing quartz flotation at 2 x 10-6 M diamine concentration. At a higher level of diamine (5 x 10-6 M), the presence of sulfonate enhanced the flotation of albite significantly, where the recovery is increased to about 80% from 30%. Thus, the partial diamine flotation of albite at pH 2 can be enhanced with the presence of anionic sulfonate collector, but not quartz flotation.

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