Hydrophobic flocculation characteristics of calcite and effects of some inorganic dispersants

Size: px

Start display at page:

Download "Hydrophobic flocculation characteristics of calcite and effects of some inorganic dispersants"

Jerome Watson
5 years ago
Views:

1 Indian Journal of Chemical Technology Vol. 13, September 2006, pp Hydrophobic flocculation characteristics of calcite and effects of some inorganic dispersants H Ucbeyiay & A Ozkan * Selcuk University, Department of Mining Engineering, Konya, Turkey * alpozkan@selcuk.edu.tr Received 30 November 2005; revised received 23 May 2006; accepted 20 June 2006 Hydrophobic flocculation characteristics of calcite mineral were investigated using sodium dodecyl sulphate (SDS) and Aero 845. Calcite suspensions could be flocculated by these anionic surfactants in the ph range of Especially SDS was more effective on the calcite suspensions at ph values lower than The flocculation of calcite with SDS and Aero 845 increased rapidly with increasing surfactant concentration. However, it started to decrease at concentrations higher than 30 mg/l for Aero 845 and 50 mg/l for SDS. It was found that the maximum aggregation degree of calcite particles could be achieved with a stirring speed of 500 rpm and a flocculation time of 3 min. The effects of sodium silicate, sodium polyphosphate and sodium phosphate, used as inorganic dispersants, on the flocculation of calcite were also studied. In the presence of sodium silicate and sodium polyphosphate, the flocculation power values obtained with both surfactants decreased with increasing dispersant concentration till approximately 0.5 and 1 g/l concentrations, respectively. Also, the flocculation of calcite with Aero 845 and SDS decreased towards 0.5 and 1 g/l concentrations of sodium phosphate, respectively. At higher dispersant concentrations, these power values remained relatively constant for sodium silicate, and different effects were also observed for other dispersants depending on surfactant type used in the experiments. In addition, it was found that sodium silicate was more effective to disperse the calcite suspensions with SDS and Aero 845. Keywords: Calcite, Hydrophobic flocculation, Sodium dodecyl sulphate, Aero 845, Dispersant IPC Code: C02F1/52 Size reduction by grinding of ores is usually required to liberate the valuable constituents, and separation results in the concentration operations strongly depend on the degree of liberation. However, efficiency of the separation decreases when the treated particles are very fine 1,2. The ultrafines, due to their hydrodynamic limitation, respond poorly in separation processes. Hence, selective size enlargements processes may be beneficial for mineral processing operations 3-6. One way to recover fine valuable minerals from slimes is to increase their size by selective flocculation and then to float the flocs. By this way the recovery in the concentration operations improves. Secondly, either the selective polymeric flocculation or selective hydrophobic flocculation methods is used to separate valuable minerals from fine particle mixtures, with the aggregation of the desired mineral 4,5. The advantage of hydrophobic flocculation is the production of stable hydrophobic flocs which should be tough enough to withstand the turbulence in subsequent mineral processing operations. The hydrophobic flocs can be separated from remaining hydrophilic dispersed particles by flotation 6. Hydrophobic flocculation is the aggregation of fine particles at a convenient stirring regime after hydrophobization by the adsorption of surfactants. Hydrophobic electrically charged fine particles can also form stable suspensions. Therefore, the hydrophobic flocculation process requires mechanical energy to overcome energy barriers, arising from the electrical charge on the particle surfaces 4. The aggregation is due to hydrophobic attraction between particles, and hydrocarbon chain association when the surfactant layers adsorbed on particle surfaces contact each other 6-8. Long-chain surfactants are more effective for obtaining a good flocculation, because the hydrocarbon chain association depends on the carbon number of adsorbed surfactant chain 9. However, selectivity in separation of minerals by hydrophobic flocculation process is decreased in the presence of long-chain surfactants. On the other hand, dispersants are used in the selective flocculation applications to prevent aggregation of particles of unwanted mineral in the suspension. In this way, selectivity in the separation of minerals by selective flocculation process is improved 10. Especially when the treated minerals are very similar to each other

2 UCBEYIAY & OZKAN: HYDROPHOBIC FLOCCULATION CHARACTERISTICS OF CALCITE 449 from the physicochemical properties, the selection of dispersant type appears to be more important. Therefore, determination of individual flocculation behaviours of a mineral in the presence of various dispersants will help in predicting the flocculation behaviours of that mineral in ore suspension systems. Calcite (CaCO 3 ) is one of the most common minerals in the world. It is widely used in industries such as paper, paint, plastic, adhesive, construction (plaster, cement etc.), food and ceramic. However, calcite is also known to be a gangue mineral and it is a salt-type mineral in regard to its physicochemical properties This paper aims to determine the flocculation characteristics of calcite with SDS and Aero 845 in the absence and presence of sodium silicate, sodium polyphosphate and sodium phosphate. Determination of such characteristics experimentally regarding to calcite mineral will help fine particle processing of calcite ores and ores containing calcite as gangue mineral. Experimental Procedure Materials Pure calcite mineral obtained from Nigde, Turkey, was used in this study. The calcite sample had the following chemical composition: 99.14% CaCO 3, 0.30% SiO 2 and 0.40% others (Fe 2 O 3 +Al 2 O 3 +MgO). The sample was dry ground to below 38 µm particle size by a ceramic ball mill. Sodium dodecyl sulphate (C 12 H 25 SO 4 Na) and Aero 845 (an alkyl succinamate surfactant 14 ) were purchased from Merck and Cyanamid Company, respectively. Sodium hydroxide (Merck) was prepared as 1 and 5% solutions for modification of ph values. Sodium silicate and sodium polyphosphate were purchased from Merck, and sodium phosphate was obtained from Sigma. All of these chemicals were of analytical grade. The ph of the suspension was measured by a digital ph meter. Distilled water was used for all experimental work. Flocculation experiments The flocculation experiments were carried out in a 300 ml cylindrical cell with four baffles using 0.5 g calcite and 300 ml water. The dispersed suspension, at desired ph, was first conditioned at 500 rpm for 5 min and the surfactant was added to the suspension at an impeller speed of 500 rpm. After 3 min, the stirring speed was reduced to 160 rpm and continued for 2 min to allow floc growth. After a settling time of 2 min, 30 ml of supernatant was taken out, from a Fig. 1 Diagram of hydrophobic flocculation of calcite suspension. depth of 4.5 cm below the liquid surface, for turbidity measurements. The turbidity of the supernatant was measured by a WTW Turb 550 turbidimeter. In the experiments performed to determine the effects of dispersants, the dispersant was added into the calcite suspension at the third minute of ph conditioning time. The experiments were carried out at 20±1 C. A schematic representation of the experimental set-up of the hydrophobic flocculation experiments for calcite is shown in Fig. 1. The performance of the flocculation process was assessed using the formula given below 15,16, Hydrophobic flocculation power = [(T 0 T hf )/T 0 ] 100 (1) where T 0 is the initial turbidity (nephelometric turbidity unit) of well-dispersed suspension of calcite at 0.5 g/300 ml of water and T hf is the turbidity of supernatant when sedimentation is assisted by a surfactant. Results and Discussion Flocculation characteristics of calcite with surfactants Figure 2 shows the flocculation of calcite with sodium dodecyl sulphate (SDS) and Aero 845 as a function of ph ( ). The isoelectric point (i.e.p.) of calcite has been reported 12,17,18 to be in the ph range of The adsorption of sodium dodecyl sulphate and sodium dodecyl sulphonate onto negatively charged calcite surface at ph values above its i.e.p. is attributed to chemical adsorption mechanism 19,20. The studies indicate that marginal

3 450 INDIAN J. CHEM. TECHNOL., SEPTEMBER 2006 Fig. 2 Hydrophobic flocculation of calcite with SDS and Aero 845 as a function of ph. Fig. 3 Effect of the concentration of SDS and Aero 845 on the hydrophobic flocculation of calcite at ph 9.3. adsorption occurs to a certain surfactant concentration and precipitation of the salt (calcium dodecyl sulphate/dodecyl sulphonate) takes place on the mineral surface beyond such a concentration. As seen from Fig. 2, the hydrophobic flocculation power values obtained with SDS for calcite suspension below ph 10.5 were higher than those obtained above ph Hence, it may be said that the flocculation of calcite suspension below ph 10.5 was enhanced by electrostatic adsorption of SDS onto positively charged calcite surfaces prior to the chemical adsorption of anionic surfactant. The similar result regarding to this occurrence was also reported for the study performed on the floatability of calcite with sodium dodecyl sulphonate. The increased floatability at ph values below the i.e.p. of calcite has been explained with increased adsorption density on the mineral surfaces 19. For Aero 845, the flocculation power was marginally higher below ph 10.5 than that above ph The effect of the concentration of sodium dodecyl sulphate (SDS) and Aero 845 on the flocculation of calcite suspension at ph 9.3 is given in Fig. 3. As seen in Fig. 3, below a concentration level of 30 mg/l, Aero 845 was more effective on the calcite suspensions than SDS. In addition, the flocculation of calcite increased rapidly with increasing surfactant concentration. However, the hydrophobic flocculation power values of calcite decreased at concentration values higher than 30 mg/l for Aero 845 and 50 mg/l for SDS. Therefore, the optimum concentration values of SDS and Aero 845 under the studied experimental conditions were determined as 50 and 30 mg/l, respectively and other flocculation experiments were carried out with these surfactant concentrations. It is well known that the hydrophobic flocculation is closely correlated with the particle hydrophobicity or wettability. That is, the hydrophobic flocculation increases with increasing hydrophobicity or decreasing wettability 6,8,16,21. Hence, the hydrophobic flocculation of calcite increased with increasing surface hydrophobicity depending on increasing surfactant concentration. However, the flocculation of calcite decreased above a particular point of surfactant concentration, as seen from Fig. 3. Positively charged calcite surfaces at ph values below its i.e.p. get a negative charge in the presence of anionic surfactants, and this negative charge increases with increasing anionic surfactant concentration 19,22,23. Therefore, the decreasing trend of the hydrophobic flocculation power curves obtained at ph 9.3 was attributed to the increasing negative surface charge of calcite particles. That is, repulsive interactions between the electricaldouble layer of particles resulting from particle

4 UCBEYIAY & OZKAN: HYDROPHOBIC FLOCCULATION CHARACTERISTICS OF CALCITE 451 Fig. 4 Hydrophobic flocculation of calcite with SDS and Aero 845 as a function of stirring speed at ph 9.3. Fig. 5 Effect of flocculation time on the hydrophobic flocculation of calcite with SDS and Aero 845 at ph 9.3. surface charge decreased the aggregation of calcite particles. The hydrophobic flocculation behaviour of calcite suspensions performed at various stirring speeds is shown in Fig. 4. It can be seen that a value of 500 rpm of stirring speed was required to achieve the maximum degree of flocculation with SDS and Aero 845 under the studied experimental conditions. Below 500 rpm the collision efficiency of particles is reduced in the suspension, thereby decreasing the flocculation of the calcite suspension. On the other hand, it can be stated that the higher stirring speeds rupture the flocs. Essentially, the kinetic energy required to force the hydrophobic particles into adhesive contact depends on the surface charge of particle and the particle size. Hydrophobic flocculation requires a sufficient stirring strength to provide the particles enough kinetic energy to overcome the energy barrier, arising from the electrical charge on the particle surfaces. Hence, the critical shear rate required for hydrophobic flocculation of mineral suspensions should decrease with decreasing surface charge. On the other hand, the finer particles require higher hydrophobic rates than larger particles 6,8. Figure 5 shows the effect of flocculation time (surfactant conditioning time) on the hydrophobic flocculation of calcite. As can be seen, a flocculation time of 3 min was sufficient to obtain the highest degree of flocculation of calcite with SDS and Aero 845 for the studied experimental conditions. Although the hydrophobic flocculation is stated to be a slow process 6, the high hydrophobic flocculation degrees obtained at low flocculation times have been reported for some minerals 8,16,21,24. The flocculation time required for the hydrophobic flocculation of minerals depends on the surface charge, the particle size and the particle concentration 6. In general, while a high surface charge requires long flocculation time, a low charge may cause rapid aggregation. In addition, low shear rates may require longer conditioning times 6. Effects of dispersants on flocculation of calcite The effect of the concentration of sodium silicate on the flocculation of calcite suspension at ph 9.3 and 10.5 is given in Fig. 6. As seen from Fig. 6, the flocculation of calcite with SDS and Aero 845 decreased sharply with increasing sodium silicate concentration and approximately a constant value was reached at about 0.5 mg/l. Also, the effect of sodium silicate was more on the calcite suspensions when SDS as surfactant was used compared to that when Aero 845 was used. Various researchers have carried out floatability studies of calcite with sodium oleate, sodium dodecyl sulphate and sodium dodecyl

5 452 INDIAN J. CHEM. TECHNOL., SEPTEMBER 2006 Fig. 6 Effect of the concentration of sodium silicate on the hydrophobic flocculation of calcite suspension at ph 9.3 and sulphonate using sodium silicate as depressant 19,20,25. They reported that this depressant was adsorbed on calcite surfaces. That is, Ca 2+ ions can react with silicate species [Si(OH) 4(aq), SiO(OH) 3 ] to form CaSiO 3 compound. This compound can render the surfaces of calcite more hydrophilic, therefore decreasing its floatability. Similar to the calcite flotation, it can be clearly said that the hydrophobic flocculation of calcite suspension also decreased due to the similar reason in the presence of sodium silicate. Because, it is well known that hydrophobic flocculation strongly depends on the surface hydrophobicity of particles as mentioned above. Figure 7 shows the effect of the concentration of sodium polyphosphate on the flocculation of calcite suspension at ph 9.3 and As seen in Fig. 7, the hydrophobic flocculation power values of calcite for both surfactants in the presence of sodium polyphosphate decreased with increasing dispersant concentration till 1 g/l. At concentrations higher than 1 g/l, these power values increased slightly for SDS and remained relatively constant for Aero 845. The effect of the concentration of sodium phosphate on the flocculation of calcite suspension at ph 9.3 and 10.5 is shown in Fig. 8. As can be seen, the flocculation of calcite with Aero 845 decreased towards 0.5 g/l concentration of sodium phosphate. However, an increase in the flocculation power values Fig. 7 Effect of the concentration of sodium polyphosphate on the hydrophobic flocculation of calcite suspension at ph 9.3 and Fig. 8 Effect of the concentration of sodium phosphate on the hydrophobic flocculation of calcite suspension at ph 9.3 and obtained with Aero 845 was observed at concentrations higher than this value. On the other hand, the flocculation of calcite with SDS also decreased towards 1 g/l, and thereafter remained

6 UCBEYIAY & OZKAN: HYDROPHOBIC FLOCCULATION CHARACTERISTICS OF CALCITE 453 relatively constant. In addition, the trends of the hydrophobic flocculation power curves obtained at ph 9.3 and 10.5 for each dispersant used in the experiments are very similar to each other, as clearly seen in Figs 6-8. That is, the flocculation of calcite suspensions with SDS and Aero 845 in the presence of these inorganic dispersants was not much affected by the ph changes of the suspension. On the other hand, Parsonage et al. 26 studied the effects of sodium triphosphate on the flotation of calcite, dolomite and apatite. They reported that the recovery of calcite and dolomite increased at sodium triphosphate concentrations higher than 1 g/l. However, they could not explain this phenomenon. Polyphosphates can adsorb onto solid surfaces either electrostatically or by hydrogen bonding, or by forming strong covalent bonds with a number of metallic cations 10. The action of phosphates and polyphosphates takes place via blocking mechanism or cleaning mechanism. The blocking mechanism occurs when metal ion-polyphosphate complex forms on the mineral surface, preventing reaction with surfactant. In the cleaning mechanism, phosphates or polyphosphates form complexes with metal ions and selectively dissolve metal ions from the mineral surface. Thus, this mechanism imparts a negative charge to the particles via decreasing the number of positively charged active centers on the mineral surface 10, Various studies were carried out to determine the effects of phosphates and polyphosphates on the flotation of calcium minerals 26,27. The results obtained from these studies indicated that the depression of the calcium minerals by these inorganic reagents was closely related to the selective complexation and dissolution of the calcium ion of the minerals. Conclusions Calcite fines in aqueous suspensions could be flocculated using sodium dodecyl sulphate (SDS) and Aero 845 as anionic surfactants in the ph range of The flocculation power values obtained with especially SDS below ph 10.5 were higher than those obtained at ph values above Aero 845 strongly flocculated the calcite suspension than SDS below a surfactant concentration level of 30 mg/l. The flocculation of the calcite suspension increased rapidly with increasing surfactant concentration, rendering the particle surfaces more hydrophobic. However, it started to decrease above a particular addition of the surfactant. The increased surface charge on the calcite surfaces is one possible reason for the reduced flocculation. The flocculation of calcite suspension with SDS and Aero 845 reached the maximum value at a stirring speed of 500 rpm within 3 min. The stirring speeds higher than 500 rpm caused the flocs rupture. The prolonged flocculation time also ruptured the flocs, especially in the presence of SDS. The dispersive capability of sodium silicate, sodium polyphosphate and sodium phosphate on the flocculation of calcite suspension varied depending on the surfactant type. Sodium silicate was most effective among the three dispersants used. The hydrophobic flocculation power values decreased beyond a particular dispersant concentration due to the decreasing surface hydrophobicity. Hence, it is concluded that calcite mineral in the selective flocculation applications of ores containing calcite as gangue mineral can be dispersed by these inorganic dispersants, especially with sodium silicate. Also, dispersant concentration required for dispersing calcite mineral in the suspension is important. Acknowledgements The authors acknowledge the financial support of this study provided by Selcuk University Scientific Research Project Fund for project no References 1 Somasundaran P, in Fine Particle Processing, edited by Somasundaran P (AIME, New York), 1980, Sivamohan R, Int J Miner Process, 28 (1990) Patil D P, Andrews J R G & Uhlherr P H T, Int J Miner Process, 61 (2001) Warren L J, J Colloid Interface Sci, 50 (1975) Laskowski J S, in Proc 8th Int Miner Process Symp, edited by Ozbayoglu G, Hosten C, Atalay M U, Hicyilmaz C & Arol A I (Balkema, Rotterdam), 2000, Warren L J, in Colloid Chemistry in Mineral Processing, edited by Laskowski J S & Ralston J (Elsevier, New York), 1992, Warren L J, Chem Technol, 11 (1981) Song S, Lopez-Valdivieso A, Reyes-Bahena J L, Bermejo- Perez H I & Trass O, J Colloid Interface Sci, 227 (2000) Zollars R I & Ali S I, J Colloid Interface Sci, 114 (1986) Laskowski J S & Pugh R J, in Colloid Chemistry in Mineral Processing, edited by Laskowski J S & Ralston J (Elsevier, New York), 1992, Wills B A, Mineral Processing Technology (Pergamon Press, New York), Hanna H S & Somasundaran P, in A.M. Gaudin Memorial Volume, edited by Fuerstenau M C (AIME, New York), 1976, 197.

7 454 INDIAN J. CHEM. TECHNOL., SEPTEMBER Ozkan A, Indian J Chem Technol, 11 (2004) Cyanamid catalogue, Mining Chemicals Handbook, Mineral Dressing Notes No. 26-1, (USA), Osborne D G, Trans Inst Miner Metall Sec C, (1978) C Ozkan A & Yekeler M, J Colloid Interface Sci, 273 (2004) Sadowski Z & Laskowski J S, Colloids Surf A Physicochem Eng Aspects, 1 (1980) El-Shall H & Somasundaran P, Powder Technol, 38 (1984) Martinez-Luevanos A, Uribe-Salas A & Lopez-Valdivieso A, Miner Eng, 12 (1999) Martinez-Luevanos A, Uribe-Salas A, Carrillo F R, Coreno J & Ortiz J C, Int J Miner Process, 70 (2003) Ozkan A, J Colloid Interface Sci, 277 (2004) Cicerone D S, Regazzoni A E & Blesa M A, J Colloid Interface Sci, 154 (1992) Sonmez I & Cebeci Y, Colloids Surf A Physicochem Eng Aspects, 225 (2003) Dawei W, Kewu W & Jicun Q, Int J Miner Process, 20 (1987) Hernainz Bermudez de Castro F & Calero de Hoces M, Int J Miner Process, 37 (1993) Parsonage P, Melven D, Healy A F & Watson D, in Reagents in the Minerals Industry, edited by Jones M J & Oblatts R (Inst Min Metall, London), 1984, Yongxin L & Changgen L, Int J Miner Process, 10 (1983) Changgen L & Yongxin L, Int J Miner Process, 10 (1983) Rashchi F & Finch J A, Miner Eng, 13 (2000) 1019.

FLOTATION OF QUARTZ AND HEMATITE: ADSORPTION MECHANISM OF MIXED CATIONIC/ANIONIC COLLECTOR SYSTEMS

Paper No. 548 FLOTATION OF QUARTZ AND HEMATITE: ADSORPTION MECHANISM OF MIXED CATIONIC/ANIONIC COLLECTOR SYSTEMS A Vidyadhar 1, *, Neha Kumari 2 and R P Bhagat 3 ABSTRACT Using pure quartz and hematite