Minerals Engineering, Vol. 9, No. 5, pp. 519-525, 1996 Copyright O 1996 Elsevier Seieace Ltd Printed in Cn'eatBritain. All fights raerved P l l : 80892--6875(96)00040-4 0892-6875/96 $15.00+0.00
THE EFFECT OF SURFACE POTENTIAL ON T H E F L O T A T I O N O F C H R O M I T E
S. SYSILA,§, H. L A A P A S t , K. H E I S K A N E N ~ a n d E. R U O K O N E N ¶ § Partek Industrial Minerals Oy, Finland t CUTEC-Institut GmbH, Germany :~ Helsinki University of Technology, Finland I Outoknmpu Chrome Oy, Finland (Received 13 October 1995; accepted 23 January 1996) ABSTRACT Theflotation of chromite from "slimefraction " of the gravity circuit of Kemi concentrator, Finland was studied. The main gangue minerals are chlorite, tremolite and talc. Amines or fatty acid collectors are normally used for this sort of mineral composition. Theoretically, a sufficient selectivity should be expected in the region where the surface charge of chromite is opposite to that of the gangue minerals. The electrokinetic potential of the main components was determined as a function of pH. The results showed that the region of selectivity lies at a pH below three, where chromite is positively charged and the other components negatively. This suggest the use of an anionic collector. No selective region was found for chromite with negative surface charge, excluding the use of amine collectors. Two conu~ercial fatty acid collectors, F2874 from Hoechst and AC825 from Cytec, were used in the actual flotation tests. Both are designed for low pH values. In an open circuit F2874 gave a concentrate around 95 % of chromite with a recovery of 70%. The flotation was also found to favour the recovery of chromite particles with high chrome to iron ratio. The experiments showed a distinct abundance between the chromite surface charge and the selectivity as well as reversible adsorption-desorption behaviour. This confirms the hypothesis of physical adsorption of fatty acids on chromite at low pH.
Keywords Oxide ores; froth flotation; flotation collectors; mineral processing; surface modification INTRODUCTION
Due to itshigh specificgravity, chromite is usually separated from gangue by gravity methods. These are, however, inefficientin recovering the finestfractionsbelow 500/~m. In some occasions the relativelyhigh susceptibility of chromite may offer the possibility of treating the fines with HGMS. The Kemi chromite in northern Finland is perhaps the most well-known deposit of this sort.
The feed in the Kemi plant is first crushed down to - 120 turn and then screened at 12 mm. The 12-120 519
mm fraction is treated in a two-stage sink-float circuit producing a lump concentrate and middlings. The float is discarded as tailings and the middlings are further crushed and combined with the - 12 mm fraction from screening. This represents the feed to the gravity plant. It is ground in a conventional rod-ball mill circuit and cycloned. The cyclone underflow goes to Reichert cones and the overflow is deslimed in a second cyclone circuit. The deslimed slurry, called "slime feed" is treated in a SALA HGMS and the concentrate is combined with Reicbert concentrate to so-called fine concentrate. In 1992 the concentrator produced 160 000 and 330 000 t of lump and fine concentrate assaying 35.9 and 47% of Cr203, respectively. Due to ore type variations the grinding fineness tends to fluctuate. Therefore, the amount of "slime feed" varies between 11 and 20 t/h. This, combined with fluctuations in magnetic properties of chromite, decreases the recovery in magnetic separation. An alternative method for recovering fine chromite is flotation. Numerous references are listed in the literature to float chromite with cationic and anionic collectors [1,2,3,4,5,6,7,8]. Earlier studies with Kemi ore have shown amines to be unsatisfactory . The stable surface structure of chromite should theoretically favour the physical adsorption of the collector. Depending on the ionic nature of the collector (anionic or cationic) and on the surface charge of individual minerals, the collector is attracted or repelled from the surfaces. The potential determining ions of chromite and associated gangue are H + and O H - . Their ratio, or pH, determines the potential at the Stern-layer (zeta-potential). The collector ions act as counter ions and are attracted to the surface if the surface is oppositely charged or near to the isoelectric point (zero Stern-potential). Good selectivity is achieved in the region where collector is adsorbed only on chromite surfaces. In the above conditions, the electrokinetic (zeta) potential is an indication of flotation or non-flotation.
EXPERIMENTAL Test materials The test samples were collected from the underflow of the slime circuit cyclones at the Kemi plant. Partial samples of each ore types were joined, dried at 105°C and riffled to 1 kg batches to be used in flotation tests. Four different ore types were sampled later indicated by E36, E182, NJ33 and NJ184. The Mohs hardnesses and Cr203-analyses of these materials were: Sample Mobs hardness % Cr203 E36 E182 NJ33 NJ184
4 2 3 1
26.0 25.6 27.5 31.1
The Mobs hardness and Cr203-value reflect the changes in gangue composition, and their strongest influence is on grinding fineness, demonstrated in Figure I.
Equipment and test procedures The electrokinetic (zeta) potentials were determined with Coulter DELSA 440. In this instrument a colloidal sample is suspended in an aqueous 0.01 M sodium chloride solution and syringed into the sample cell, which is placed in the ssmple chamber. The particle velOcities at appropriate potential fields are measured using laser Doppler-shift. The polarity of the cell electrodes is continuously reversed to prevent polarization. The minerals analysed were manually picked from core samples and ground to colloidal particle size using a ring mill. The concentration of ground powder in each test suspension varied between 12.5-100 ppm. The pH was controlled by sulphuric acid or by sodium hydroxide (both of p.a.-quality). The flotation tests were carried out in a Denver LAB D 12 laboratory unit. Both 2.6 and 1.4 liter cells
Effect of surface potential on chromite flotation
were used; the larger for roughing and the smaller for cleaning. The pulp pH was controlled with a Radiometer PHM 82 pH-meter connected to a Radiometer TTT 80 titrator. The pulp density in flotation was 35-50% solids by weight. The conditioning prior to flotation was done in the rougher cell at a pulp density of 60 % solids by weight.
E182 •" 0 -
o "0 e,
Particle size, pm
Fig. 1 Particle size distributions of test samples by sieving Petroleum sulphonate AC 825 (Cytec) and F2874 (Hoechst) were used as chromite collectors. The reagent F2874 is a sodium salt of a carboxylic acid containing sulphonate groups. Both were added undiluted. Dextrin was also used to improve dispersion and to avoid entrainment. It was added as a 100 g/l solution. Talc was floated with Montanol 300 (Hoechst), a mixture of branched alcohols. Flotation product.~ were dried, weighed and riffled for analysis. They were assayed for Cr203, SiO2 and Fe at the plant laboratory in Kemi by using neutron activation. Some comparative analyses were also done at the Helsinki University of Technology by XRF-method.
RESULTS AND DISCUSSION Figure 2 demonstrates the effect of pH on the electrokinetic (zeta) potential of chromite, chlorite, tremolite and talc. The given potentials represent median values of the potential distributions obtained from Coulter DELSA. The isolectric point of Kemi's chromite seems to be at a pH of 3.2. At more basic environment all the minerals are negatively charged and the charge is of the same order of magnitude. The chance of doing a selective separation above pH of 3.2 seems to be low with anionic or cationic collectors when we accept the theory of physical adsorption. But, at pH below 3.2 the possibilities for selective separation in terms of surface charge are enhanced suggesting the use of an anionic collector. Sulphonates and modified fatty acids are known to function in these very acid conditions. Talc, although negatively charged, has a very low surface tension and is natively hydrophobic. Therefore, it must be removed by frother addition prior to actual chromite flotation.
S. Sysila et al.
Fig.2 Zeta-potential of chromite and gangue minerals vs. pH Figure 3 shows the Cr203-content of the rougher concentrate as a function of pH. Sample NJ33 was used as a test material. The collector (F2874) addition was 4 kg/t and the conditioning time 15 minutes. Talc 33-
Fig.3 Cr203-content of the rougher concentrate vs. pH.
Sample--N J33, collator--F2874, Collector amount--4 kg/t
Effectof surfacepotentialon chromiteflotation
was removed by floating the pulp with Montanol 300 for 1 minute prior to conditioning. The flotation time in chromite roughing was 3 minutes. Figure 3 correlates well with the electrokinetic data shown in Figure 2, and the best selectivity is achieved at pH below 3.2. In this region chromite has a positive charge, whereas the gangue minerals remain negative. The collector adsorption seems to be controlled by the electrostatic forces on mineral surfaces, and only the positively charged chromite attracts collector molecules. This :mggests, that the dominating mechanism of collector adsorption on chromite is due to physical forces. Another observation, was that the pH-dependency of selectivity is reversible, typical to physical adsorption phenomena. Tables 1 and 2 summarise the results obtained with all four test materials. The experimental procedure of test series 1 (Table 1) was: 1. .
Talc flotation with Montanol 300 at natural pH Chromite roughing at pH of 3.2 with F2874 (dosages: 2 kg/t for E182 and NJ184; 4 kg/t for E36 and NJ33 Three-stage cleaning of rougher concentrate at pH of 2.5, 2, and 1.8
In all chromite flotation stages 100 g/t of dextrin was additionally used to improve the dispersion of the pulp. TABLE 1
Recovery of Cr203
In the test series 2 (Table 2) the collector was AC 825. The experimental procedure was equivalent to the series 1, apart frora the addition of a fourth cleaning stage for chromite. The pH-values from first to fourth cleaner flotation were 3, 2.5, 2 and 1.8. Dextrin was also added to the fourth cleaner stage (100 g/t).
Recovery of Cr203 %Fe
s. Sysilaet al.
The recovery of chromite with F2874 (Table 1) was reasonable with all materials in spite of open circuit flotation. In a continuous closed circuit the circulating loads will improve the final recovery. The concentrate grades were all high and the best selectivity was achieved with the finest feed. On the other hand, increasing fineness leads to higher chromite losses into the talc froth, due to entrainment. The selectivity of AC 825 was lower being more sensitive to feed fineness and to colloidal slime (Table 2). The sulphonate froth was excessive and thick containing heavily flocculated-partly heterocoagulatedhydrophobic chromite particles. This heavy froth remained undispersed in all four successive cleaner flotations.
SUMMARY AND CONCLUSIONS Electrokinetic measurements showed that chromite has a surplus of positive charges at low pH, whereas the gangue minerals are all negatively charged. This suggest the selective separation of chromite at pH below 3.2 by using fatty acid based anionic collectors. The flotation experiments confirmed that increasing pH is harmful for the selectivity. The best results in terms of recovery and selectivity were obtained by using F2874 as a collector. It proved to be insensitive against changes in feed composition and fineness. Petroleum sulphonate was less promising. Its hydrophobising effect was too strong leading to heterocoagulation. It was also more sensitive against colloidal slime and ore type. By floating the Kemi "slime feed" with F2874 in an acid circuit (pH below 3.2) it seems possible to obtain a concentrate containing 45 % of Cr203, equivalent to about 95 % of chromite. An open circuit gave a recovery of 70 %, but continuous closed circuit will definitely improve the final recovery.
ACKNOWLEDGEMENT The authors wish to express their gratitude to Outokumpu Chrome Oy for their assistance.
McDonald, Wm.T., A Useful New Selectivity Modifier in non-Sulphide Flotation. Mining and
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Effect of surfacepotentialon chromiteflotation 1
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