Rheology of ore suspensions with fibrous minerals and its impact on flotation performance
- 65 Downloads
Flotation separation and recovery of value minerals from ores containing fibrous silicate minerals are known to be quite challenging. Earlier studies have established that the metallurgical challenges (poor selectivity and recovery, high energy consumption, etc.) arise from the impact of the fibrous minerals (with high aspect ratio) on pulp rheology, which was found be rather complex. The complex rheological behavior of ore suspensions containing particles with high aspect ratio and its influence on flotation outcome are investigated in this study. Flotation tests and rheological measurements were carried out with both ores (containing fibrous minerals) and a model system comprising nylon fibers (chemically inert) added to a copper ore that had no fibrous mineral. These studies allowed the determination of contributions of physical properties of the fiber to pulp rheological behavior. Optical microscopy images of the fiber pulp indicated that the fibers entangled to form two levels of microstructures: two-dimensional (2D) aggregates under semi-dilute (< 20% solids) conditions and three-dimensional (3D) microstructures at concentrations > 30% solids. The yield stress (σ) values determined for the fiber pulp were logarithmically related to the volume fraction of the 2D/3D structures. The relationships developed here for different ARs showed that the yield stress of a flotation pulp changes with respect to shape, size, and the concentrations of the microstructures. Implications to the effects of such entangle networks on selectivity of flotation separation are discussed.
KeywordsMicrostructure Planar aggregates Aspect ratio Flotation Froth phase
Rheological behavior of suspensions containing fibrous particulates is of importance in governing processes in the pulp and paper industries and in froth flotation beneficiation of ores containing fibrous minerals. For suspensions containing long and flexible fibers, such as chrysotile, experimental and theoretical studies of the rheological behavior are often complex due to the interdependence of the factors that contribute to the rheological behavior, such as fiber-wall interactions, fiber-solvent interactions, and most importantly inter-fiber interactions [1, 2]. Inter-fiber interactions are mostly the result of the suspension hydrodynamics that leads to the formation of regions of high concentrations of fibers, otherwise referred to as fiber flocs with entangled structures [3, 4]. With the increase in the fiber concentrations of suspensions, the probability of fiber-fiber contacts increases, leading to the formation of network structures that spread throughout the suspension . Such network structures are inhomogeneous in terms of the shapes and sizes of the fiber flocs. The associative strengths of such flocs differ depending on the aspect ratios (ARs) of the fibers, where larger fibers make more contact points than the smaller fibers, and the net cohesive strengths become higher owing to the frictional forces at the contact points. Here, we emphasize development of a correlation between the aspect ratio (AR—typically represented as width (thickness)/length) of fibers and types of entangled structures that are formed with an increase in fiber concentrations.
In this investigation, froth flotation studies conducted with pulp containing fibrous chrysotile minerals and nylon fibers that were chemically inert. The fibrous chrysotile minerals pose challenges in the selective flotation separation of minerals , which has been attributed to poor collector adsorption due to slime coating (electrostatic attraction between positively charged fibrous chrysotile and negatively charged valuable minerals), a widely proposed cause for reduced mineral separation . In this investigation, the focus was on assessing the contributions of morphological aspects of chrysotile fibers. Chrysotile belongs to the serpentine group of minerals with fibrous morphologies . Our earlier work had indicated that suspensions containing fibrous chrysotile minerals are viscous and, therefore, selective floatability is significantly affected; a decrease in value recovery occurs with an increase in suspension viscosity due to an increase in pulp fiber content [9, 10]. However, the impact of fiber ARs and characteristics of the microstructures on the rheological behavior and the forth phase stability has not been fully explored. In this study, the rheological and flotation studies were carried out with three different types (different ARs) of fibrous ores in order to determine the morphological effects of fibrous particles on the flotation performance and the rheological behavior. In order to isolate physical effects from surface chemical effects, experiments were conducted with a model ore system in which nylon fibers with different ARs were added to a Cu ore that had no fibrous minerals. Nylon is chemically inert under conditions prevailing in flotation pulps; therefore, the surface chemical contributions from nylon fibers to flotation performance, such as slime coating, were deemed negligible. The emphasis of this work was to understand the type of the microstructures that are formed with an increase in both content and ARs of fibers and how such microstructures affect pulp rheological behavior.
2 Materials and Methods
Different Ni ore samples, designated as 1, 2, and 3, were obtained from Vale, Inco Corporation, and contained approximately 1 wt.% Ni sulfides and 60–65% of serpentine minerals. The non-fibrous copper ore from a North American mine contained 0.7% copper and had no fibrous minerals based on to SEM (scanning electron microscopy) analysis. Nylon fibers (AR of 1000) of 1 cm long and approximately 10 μm in thickness were obtained from Nyconn Industries. The fibers were subjected to size reduction to obtain an aspect ratio of value around 100.
Flotation tests were carried out at 25% solids in a 2.5-L Denver cell for 12 min at pH 4–5 with the three fibrous serpentine ore types. Ore was ground at 50% solids in a rod mill and flotation was conducted using a dithiophosphate-based collector (40 g/t) and MIBC (20 g/t) frother. The pH was adjusted using 50% H2SO4. Flotation tests in another set of experiments included a set of model ore systems where copper ore was spiked with 1 and 2% nylon fibers. The model ore mixture was ground at 60% solids in a rod mill and tests were done at 25% solids, at pH 10, for 7 min.
Rheological measurements were carried out using an Anton Paar DSR 301 rheometer calibrated with standard Cannon viscosity standards using a vane type probe. Rheological characterization was carried out on three types of suspension samples: (1) Ni ore suspension, (2) copper ore suspension to which nylon fibers in desired amounts were added before grinding, and (3) nylon fiber suspensions. The ore suspension obtained from the rod mill was transferred into a 100-mL sample holder, and a vane geometry sensor was used to measure the yield stress values. Solid concentrations of the ore suspensions were increased in steps by removing 10 ml of water from the suspension in each step using a syringe having a 0.2-μm filter. Yield stress values of the pulp were measured after each step. In a separate set of experiments, nylon fiber suspensions were prepared by adding the fibers to water at a concentration in the range of 1–5 wt.%. Details of the rheological characterization are similar to that described elsewhere (Bennington, 1990). In brief, the rheological measurement procedure using an Anton Paar DSR 301 rheometer involved three sequential steps: (1) homogenization of the suspension, (2) suspension stabilization for 5 s, and (3) measurement of yield stress. Homogenization of samples was carried out to uniformly disperse the particles in suspension and avoid settling of particles during rheological measurements in step 3. In step 1, homogenization shear rate and time were standardized by trial and error where the criteria were to prevent particle settling and repeatability of yield stress values. Step 2 was incorporated to eliminate stress inertia developed in the homogenization step. Thus, the parameters (shear rate, ramp values, and ramp speed) that were set in the three steps were standardized. A stress ramp (Δσ/Δt = 1 Pa/s) was applied until the deformation γ diverged and flow occurred which marked the end of the experiment. The speed of the stress ramp Δσ/Δt) did not affect the trend observed with the variations in yield values at different percentages of solids in suspension but only the shape of the γ divergence.
Optical images of the nylon fiber suspension were taken using an Olympus Optical microscope. For SEM studies, the ore suspension samples were acquired from the ball mill and were tumbled in 10-mL glass vials. Drops of the ore suspensions taken from the vials were mounted on an SEM stub. A technique known as acetone-replacement drying  (Fitzpatrick, 1993) was used instead of conventional drying to maintain the structural integrity of the sample. The samples after drying were carbon-coated and SEM micrographs were obtained using the Zeiss DSH 982 Gemini SEM system and a low electron beam voltage (3 KeV).
3 Results and Discussions
Relationships and correspondence between results obtained from flotation and rheological studies were analyzed to determine the role of the suspension microstructures in flotation outcome (both recovery and grade).
3.1 Flotation and Rheological Studies with Different Ore Types
Flotation recovery of Ni was higher (~ 90%) for ore type 1 than that for ore type 2 (~ 81% Ni recovery). The yield stress value for the pulp of ore type 2 was ~ 50 Pa, whereas that for ore type 1 was < 5 Pa. Thus, the decrease in the Ni recovery for ore type 2 could be attributed to the increase in the yield stress of the ore pulp.
3.2 Impact of aspect ratio
As reported earlier , chemical factors such as slime coating or other factors owing to mineral surface chemistry could play a role in the reduced recovery of Ni minerals in froth flotation processes. In order to isolate such contributions due to chemical factors leading to poor recovery of Ni, flotation and rheological studies were carried out with a model ore system containing chemically inert nylon fibers of different ARs (described in the experimental section).
S (S = Δσ/Δc) for ore containing nylon fibers of varying ARs and concentrations
Aspect ratio and concentration
3.3 Conceptual relation between entangled fiber structures and rheological behavior
3.4 Effect of suspension microstructures on the behavior of flotation froth
4 Summary and Conclusions
Ore suspensions become more viscous and have higher yield stress when high aspect ratio (AR) fibrous minerals (or fibers of other solids such as nylon) are present. There is also a concomitant adverse impact on the flotation performance from such ore suspensions. With an increase in fiber concentrations in suspensions, two types of macro-structures of sizes as large as 1–2 cm are formed: planar (2D) aggregates and three-dimensional flocs. Contributions from these 2D and 3D aggregates to the rheological behavior is more than the contribution expected from simply accounting for the volume fractions of the fibrous particles in a suspension. This study reveals that reducing the ARs of the fibrous minerals in froth flotation pulp is a plausible approach to improve froth flotation recovery of valuable minerals from ores containing fibrous minerals.
The authors acknowledge the support of Manqiu Xu, Zongfu Dai, Andrew Lee, and Ken Scholey at the Vale Technical Services in Mississauga, Canada. The authors acknowledge the support received for this research work from the National Science Foundation Industry/University Collaborative Research Center of Particulate and Surfactant Systems (IIP 1362078).
Compliance with ethical standards
Conflict of Interest
The authors declare that they have no conflict of interest.
- 2.Bennington CPJ, Azevedo G, John DA, Birt SM, Wolgast BH (1995) The yield stress of medium- and high-consistency mechanical pulp suspensions at high gas contents. J Pulp Paper Sci 21(4):111–118Google Scholar
- 3.Mason SG (1950) The flocculation of pulp suspensions and the formation of paper. TAPPI J 33:440–444Google Scholar
- 5.Kerekes RJ, Schell CJ (1992) Characterization of fiber flocculation regimes by a crowding factor. J Pulp Paper Sci 18:J32–J38Google Scholar
- 6.Eltham JA, Tilyard PA, (1973) An approach to the flotation of Western Australian Ni ores, Proceedings, Australasian Institute of Minerals and Metallurgy Annual Conference, Western Australia, 417–429Google Scholar
- 10.Patra P, Nagaraj DR, Somasundaran P, (2010) Impact of pulp rheology on selective recovery of value minerals from ores, XI international seminar on mineral processing technology, 15–17 December, NML, Jamshedpur, IndiaGoogle Scholar
- 11.Fitzpatrick EA (1993) Soil microscopy and micromorphology. John Wiley and Sons, New York ISBN-10: 0471938599Google Scholar
- 12.Doi M, Edwards SF (1986) The theory of polymer dynamics. Clarendon Press, Oxford ISBN: 9780198520337Google Scholar
- 13.Doi M, Edwards SF (1978) Dynamics of rod-like macromolecules in concentrated solution: part 1, J Chem Soc, Faraday Trans 2, (74), 560–570. https://doi.org/10.1039/F29787400560