The effect of biopretreatment on the flotation recovery of magnesite tailings

Pergamon

MineraL~Engineering,Vol,10, No. 8, pp. 813-824, 1997 © 1997ElsevierScienceLtd Printedin GreatBritain.Allrightsreserved 0892-6875(97)00059-9 0892--6875/97 $17.00+0.00

THE EFFECT OF BIOPRETREATMENT ON THE FI,OTATION RECOVERY OF MAGNESITE TAILINGS

J. GAWEL§, I. MALISZEWSKA'[" and Z. SADOWSKI§* § Dept. of Inorganic Chemistry and Metallurgy, Technical University of Wroclaw, 50-370 Wroclaw, Poland. E-mail: [email protected] i" Dept. of Biochemistry, Biotechnology & Organic Chemistry, Technical University of Wroclaw, Poland * Author for correspondence (Received 14 March 1997; accepted 28 April 1997)

ABSTRACT

The poten~:ial for using a heterotrophic microorganism (Aspergillus niger) to depress magnesite from magnesite tailing has been assessed. The influence of metabolic activities produced by AspergiUus niger on the surface properties of magnesite tailings was studied. In this research studies of the adsorption of sodium oleate microflotation tests and FTIR spectrascopy analysis were carried out. It has also been demonstrated that a depressing effect can be obtained using both selected amino acid ( Arginine, Asparagine and Methionine) and organic acid salts (potassium citrate and potassium oxalate). The mechanism of competition between a modifier reagent and sodium oleate was discussed. © 1997 Elsevier Science Lid

Keywords Mineral processing, Surface modification, Flotation froths, Bacteria, Biotechnology INTRODUCTION The separation of silica and silicate minerals from magnesite tailings is necessary for the use of these materials as refractory bricks. The selective flotation of magnesite from magnesite tailings using anionic collectors is inefficient, as both silica and silicate minerals are activated by Mg ÷2 ions [1,2]. Therefore application of depressant reagents is needed to achieve selective separation of carbonate minerals from gangue materials. Sodium silicate is one of the commonest used depressants [3]. The effect of sodium silicate on depression of salt-type minerals is dependent on the pH of the suspension. A major species responsible for the magnesite depression is SiO2(OH)2 -2. Sodium fluorosilicate as a magnesite depressant has been applied to magnesite-dolomite separation [4]. As with sodium fluorosilicate, sodium tripolyphosphate and hexamethaphosphate adsorbed at cationic sites on the mineral surface [5,6]. Hydrophilic organic polymers, such as gelatin, gum Arabic, starch and dextrin have been used as silicate depressants in magnesite flotation circuits [7]. These molecules can be adsorbed by hydrogen bonding with suitable sites on the solid surface and depress carbonate minerals. 813

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Recent developments in biotechnology have shown promise in solving some problems generated by mineral processing [8]. It is known [9] that certain microorganisms have a selective depressing effect in mineral froth flotation. Depression using microorganisms can be realized by: the adhesion of microorganism cells onto the mineral surface, the oxidation of the mineral surface by chemolithotrofic bacteria and attack of metabolic reagents produced by the microorganism cells. Biomodification of salt-type mineral surfaces by fungi Aspergillusniger has been used to enhance selectivity in separation of barite from barite - calcite mixed suspensions by application of a spherical agglomeration technique [10]. In this work, the effect of organic depressants produced by fungi Aspergillus niger and ordinary chemical reagents have been investigated in magnesite tailings flotation, with a view to selective separation of magnesite from the magnesite tailings. MATERIALS AND METHODS

Magnesite railings All biotreatment experiments were conducted using magnesite tailings from Wiry Mine, located in Lover Silesia (Poland). The material contains: MgO 45.9%, SiO2 10.6%, Fe203 4.7%. Chemicals Analytical grade chemicals and triple-distilled water were used. Pure sodium oleate powder was supplied by Riedel-de Haen (Germany) and used without further purification. The assay of fatty acids was 82%. A commercial grade potassium oxalate and potassium citrate were purchased from POCH in Gliwice (Poland). Chromatographically homogeneous amino acids (DL Methionine, L Asparagine anhydrous and DL Arginine monohydrochloride) were provided by Reagal Co., Budapest (Hungary).

Cultivation of the microorganism The microorganism used for all experiments was AspergiUusniger provided by the Department of Chemical and Food Industry, Academy of Economics, Wroclaw. Cells were transported with carbon black powder.

Aspergillus niger was cultivated in Erlenmeyer flasks at pH 6.8 at 270C in a growth medium. The nutrient medium had the following composition: 300 g molasses, 0.05 g Na2HPO 4, 0.01 g ZnSO 4, 2 10-5 g K4Fe(CN)6 in 1000 cm 3 of tap water.

Flotation experiments Flotation tests were carded out in a monobubble type Hallimond tube which was equipped with a calibrated receiver. This tube allows continuous measurement of recovery of solid as a function of time. The experiments were performed using 1.0g of magnesite tailings. A sample of magnesite tailings was agitated with 150 cc of aqueous collector solution for 10 minutes. The suspension was transferred to the Hallimond tube and floated with air. Air flow rate was 30 cc/minute.

Adsorption of sodium oleate The amount of sodium oleate adsorbed on the mineral surface was determined from the difference between the initial and final concentrations of sodium oleate in the solution. For all adsorption experiments, 1.0 g of magnesite tailings sample was used. The initial concentrations of sodium oleate were in the range of 5 10-5 - 3 10-3 kmole/m 3. The time of equilibrium for sodium oleate adsorption was 8h. After this , the magnesite tailings were separated from suspension centrifuge (Janetzki K26D 4000 r.p.m.). The

Biopretreatment on flotation recovery

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concentration of sodium oleate was analyzed using a two-phase extraction technique [11]. In the case of modifier agent application, the magnesite tailings were first conditioned with this agent then with the sodium oleate solution. Infrared spectroscopy study The spectra of the magnesite tailings were obtained using a Perkin-Elmer Model 1600 infrared spectrophotometer equipped with a multiple internal reflection (MIR) accessory. All the spectra were recorded between 4000 and 700 cm -1. A germanium internal reflecting crystal was used.

RESULTS AND DISCUSSION The first part of the experimental investigation was conducted to evaluate the effect of pH on the adsorption of sodium oleate o~a the magnesite tailing particles. Representative results are illustrated in Figure 1. It can be seen that the minimum uptake was at pH 8.5. With an increase in pH from 8.5 to 11.5, increasing amounts of sodium oleate were adsorbed on the mineral surface. A pH of 11.0 was chosen for further work. i

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Fig.l Adsorption of Sodium oleate as a function of pH The effect of nutrient, used for the microorganism growth, on sodium oleate adsorption was investigated. Figure 2 represents the adsorption isotherm for the magnesite tailings with and without nutrient. It is observed that the adsorption density of sodium oleate does not depend on the present of nutrient in the solution. The maximum adsorption of the surfactant was at 3 10-3 kmole/m 3 of sodium oleate. A similar trend was observed for both isotherms. In order to study th,: modification of the mineral surface by strains of Aspergillus niger, the influence of pretreatment time on sodium oleate adsorption has to be examined. Typical adsorption isotherms for sodium oleate on the miner~d particles at pH 11.0 are shown in Figure 3. As is seen in Figure 3 the adsorption of sodium oleate decreased with increasing pretreatment time, probably because the metabolic products produced by Aspergillus niger blocked the active surface sites, This speculation was confirmed by the results of the adsorption of sodium oleate in the presence of amino acids (Figure 4). Figure 4 shows the effect of amino acids on sodium oleate adsorption. These results indicate that, in the

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Gawel e t al.

investigated region of the surfactant concentration, the presence of amino acids caused a decrease in sodium oleate adsorption. Similar behavior is observed with the addition of both potassium citrate and potassium oxalate. The adsorption isotherms in the presence of these reagents are given in Figure 5.

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Fig.3 Effect of biopretreatment time on the sodium oleate adsorption The flotation tests provided kinetics in the form of recovery vs. time of flotation (Figure 6a). The floatability

Biopretreatmenton flotationrecovery

817

of magnesite tailings was investigated using sodium oleate as a collector. The recovery levels in the presence of an increasing amount of the collector are shown in Figure 6b. It should be noted that 100% recovery was obtained at a collector concentration of 5x10 -4 kmole/m 3, after 10 minutes of flotation.

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Fig.6a Kinetics of flotation of magnesite tailings in the presence of sodium oleate

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Biopretreatmenton flotationrecovery

819

The floatability of magnesite tailings, like sodium oleate adsorption, is expected to be a function of the suspension pH. Flotation tests as a function of pH were also carded out at a concentration of sodium oleate of 7x 10-5 kmole/m3. In Figures 7a and 7b it is shown that the recovery of magnesite tailings increases when the pH of the suspension increases from 9 to 11. A sharp increase of floatability was observed at pH 11.0. This supports our previous investigation of the adsorption of sodium oleate. l

60

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Fig.7a Kinetics of flotation of magnesite tailings at various pH values

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Gawel et al.

820

We were also interested in the relationship between the time of biopretreatment of a mineral suspension with

Aspergillus niger and the flotation activity of the mineral samples. Figure 8 shows the effect of conditioning time (1 day, 1 week and 2 weeks) on the recovery. It is worth noting that significant depression of flotation was obtained after 1 week of biopretreatment with Aspergillus niger.

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Fig.8 Kinetics of flotation of magnesite tailings after a biopretreatment The fungi Aspergillus niger is one of the major sources of production of metabolites such as citric acid, gluconic acid and other organic acids [13]. The effect of both potassium citrate and potassium oxalate on the flotation recovery of magnesite tailings is shown in Figure 9. In both cases, there was a decrease in the flotation recovery in the presence of 10--4 kmole/m 3 of organic acid salts. Figure 10 illustrates the results of further flotation experiments conducted with amino acids. It was found that a culture filtrate of heterotrophs contained a considerably high amount of amino acids [13]. Our studies demonstrated that the flotation recovery of magnesite tailings was decreased in the presence of selected amino acids (Figure 10). The results suggest that arginine is more effective than both methionine and asparaginase. The spectra for magnesite tailings without sodium oleate and with sodium oleate treated by Aspergillus niger for 24 h are shown in Figure 11. The carboxylate group has characteristic frequencies in the range 1800 1400 cm -1 [14]. The carbonyl stretching frequencies for both the unsaturated acids and their salts have double bonds occurring in the range 1670 - 1740 cm -l. The doublet at 2350 cm -! is caused by atmospheric CO 2. The stretching bonds of hydrocarbon chain are visible in the range 2800 - 3050 cm -1. Adsorbed oleate onto brucite, Mg(OH)2, surfaces has been evidenced by the band at 1572 cm -1 [15]. This position is quite different from that of sodium oleate 1562 cm -l and oleic acid 1710 cm -1 [16]. Figure 12 shows a part of the spectrum limited to the range 2800 - 3050 cm -1. The most important band is located at 2918 cm -1 ( the asymmetric - C H 2 stretching bond ) which was used for the adsorption density measurements [17].

Biopretreatmenton flotationrecovery 60

821

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Fig.9 Effect of organic acid salts on the flotation kinetics of magnesite tailings 60

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822

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FT-IR spectra of magnesite tailings with sodium oleate, biopretreatment time (1) = 24 h, (2) = 7 days, (3) = 14 days.

Quantification of spectroscopic data is based on the Beer-Lambart law. The shape of the spectra is basically the same for the three investigated cases ((1)- 24h., (2)-7 days and (3)-14 days biopreatreatment respectively) but the relative areas under the various absorption bands differ. The intensity of these bands corresponds to the total amount of all oleate species [18]. It is evident that, as the time of biopretreatment increases, the adsorbance at 2918 and 2850 cm -1 decreases. These results confirm that the metabolic products of Aspergillus niger act as a depressing reagent in the oleate flotation of magnesite tailings.

Biopretreatmenton flotationrecovery

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Figure 13 summarizes the result of the biotreatment of magnesite tailings. This Figure includes the results of the chemical analyses of the flotation concentrates. It can be seen that a significant reduction in carbonate (magnesite) content can be achieved after one or two weeks biopretreatment of the magnesite tailings.

70

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Fig.13 Recovery of magnesite as a function of biotreatment duration.

CONCLUSION The results of the initial work described here indicate that strains of Aspergillus niger are more efficient at producing a seriies of depressing agents for magnesite flotation. The complex formation between metabolites and surtace species can be considered as an adsorption mechanism for the investigated system. The competition between the anionic collector and metabolites produced by Aspergillus niger explains the decrease of magnesite tailings floatability after one or two weeks biotreatment. Results from this work provide a useful insight into the mineral-microbial-collector interactions.

ACKNOWLEDGMENTS This work was finarLced by a research grant from KBN No. 3P4 0300 506. Our sincere thanks are due to Mrs. W. Jagiello for the FTIR experiments

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