S.K. Kawatra

A review of binders in iron ore pelletization

The majority of iron ores must be ground to a fine particle size to allow the iron oxides they contain to be concentrated, and the concentrate must then be agglomerated back into large enough particles that they can be processed in blast furnaces. The most common agglomeration technique is pelletization, which requires the use of binders to hold the iron oxide grains together so that the agglomerates can be sintered into high-strength pellets. Although bentonite clay is the most commonly used binder, there are many other possibilities that could be competitive in a number of situations. This article reviews the numerous types of binders (both organic and inorganic) that have been considered for iron ore pelletization, including discussion of the binding mechanisms, advantages and limitations of each type, and presentation of actual pelletization results, so that the performance of the various types of binders can be compared and evaluated.

Developing and understanding the bentonite fiber bonding mechanism

Abstract The classically accepted mechanism for binding fine iron ore concentrates by bentonite clay is that the clay becomes hydrated, expands, disperses, and then dries into a bonding film that holds the iron ore particles together. However, bentonite has been observed to form a fibrous structure on the surface of foundry sands, which is relevant because foundry sands are produced under similar conditions to iron ore pellets. The classical bentonite binding mechanism cannot explain how bentonite fibers are formed. Is a more complete understanding of bentonite binding mechanisms important? How can the iron ore pellet industry benefit? In this paper, scanning electron microscopy was used to attempt to image bentonite fibers formed on the iron ore concentrate that composes the pellets. It was determined that bentonite fibers were not inherently formed during iron ore pellet production. Experimental methods were designed that allowed bentonite fibers to be developed and studied. The bentonite fiber mechanism has been developed into a new theory. Based on this theory, experiments conducted under conditions that would promote formation of bentonite fibers were found to be associated with an increase in bonding. Once the fundamentals of bentonite binding mechanisms are completely understood, what can be changed to improve the quality of iron ore pellets? Implications to improving iron ore pellets are discussed. How can this knowledge be used to benefit other industries that use bentonite as a binder?

Factors Affecting Zeta Potential of Iron Oxides

This review examines the factors that affect the isoelectric point (IEP) and the magnitude of the Zeta potential for hematite. It addresses the various values that have been reported in the literature, with natural hematite typically having a more acidic IEP than does high-purity synthetic hematite. The key finding is that the IEP for natural hematite is very similar to that for silica. This is consistent with a coating of silica slimes forming on natural hematite, causing the hematite surfaces to behave as if they were silica surfaces. Adsorption of chemical reagents on the hematite surfaces causes changes in the magnitude of the Zeta potential, generally without changing the IEP.

Effect of viscosity on the cut (d50) size of hydrocyclone classifiers

Abstract On-line measurement of slurry viscosity was carried out to study the effect of viscosity on the cut (d 50 ) size of hydrocyclone classifiers. As slurry viscosity increases, the settling rate of particle decreases, causing the d 50 size to become coarser. The viscometer set-up used a vibrating sphere viscometer and a specially designed slurry presentation device to avoid settling of solids during viscosity measurement. This set-up was mounted on a test rig for a 10.2 cm diameter hydrocyclone. Test samples were prepared from ground silica (80% passing 65 microns) and water. Both the solids content and temperature of the samples were varied to change their viscosities. Samples from overflow and underftow streams were collected at regular intervals, and data from these samples were used to calculate the d 50 (c) size. Temperature and viscosity were recorded simultaneous during these tests. From these data it was observed that d 50 (c) was proportional to the 0.35th power of the slurry viscosity. This relationship was then introduced in the existing Lynch and Rao model for siliceous material, to develop a modified model for hydrocyclone classification. This modified Lynch and Rao model incorporated a viscosity parameter from direct measurement, and predicted the cut size precisely when the viscosities of the slurries were altered by factors other than changing percent solids, such as temperature variation. This was not possible with the original Lynch and Rao model, which did not include any viscosity term. It was also determined that increasing slurry viscosity produced an increase in the bypass fraction, R f This effect was due to increased fluid drag in the hydrocyclone as the viscosity increased.

Pyrite recovery mechanisms in coal flotation

Abstract In most operating coal-cleaning plants, a significant amount of pyrite is recovered in the froth during flotation of high-sulfur coal. This pyrite recovery is commonly believed to be a result of pyrite particles floating due to hydrophobicity. However, even though a wide range of pyrite depressants have been reported over the years, there is no use of these depressants for industrial coal flotation, which suggests that the mechanism of pyrite flotation is not properly understood. In order to reduce the pyrite recovery during flotation, it is first necessary to identify the primary recovery mechanism, so that the appropriate method for correcting the problem can be selected. In this paper, it is shown that flotation of liberated pyrite due to its intrinsic hydrophobicity is not an important mechanism for recovery of pyrite from freshly ground coal, and that the bulk of the floated pyrite reaches the froth either as a result of simple entrainment or by mechanical locking with floatable coal particles. The experiments were carried out using both a conventional flotation cell, and a horizontally baffled flotation column.

Rheological effects in grinding circuits

Abstract The grinding efficiency of circuits using tumbling media mills in closed circuit with hydrocyclone classifiers may be improved by control of the mill slurry rheology. The rheology affects both the rate of fine material production by the grinding mill, and the separation performance of the hydrocyclone. In addition, the manner in which rheology changes are produced will affect hydrocyclone performance. This parameter is neglected in process control situations due to the difficulty of making rheological measurements of dense slurries on-line. In recent years, a number of new viscometers has been developed which may be useful for this purpose.

On-line measurement of viscosity and determination of flow types for mineral suspensions

A viscometry system involving a vibrating sphere viscometer and a rotational viscometer has been developed for on-line measurement of viscosity, and for rheological characterization of mineral slurries into either Newtonian or non-Newtonian flows. Special precautions were taken to allow measurements of viscosity of rapidly settling mineral suspensions. Both the viscometers were able to measure viscosity as low as one centipoise, which is the approximate room temperature viscosity of water. Because the vibrating sphere viscometer operated at a much higher shear rate than the rotating viscometer, the two instruments together could determine the shear-rate dependency of the viscosity. Ground silica of 80% passing 65 μm size was suspended in water, and was used to prepare slurries at different percent solids. Viscosity of each slurry sample was measured simultaneously by both the viscometers, and the results were compared to determine the rheological characters of the slurries. With this technique, it was found that all the silica slurry samples (up to 70 wt% solids) at the given size distribution were in the Newtonian flow regime.

WATER CHEMISTRY EFFECTS IN IRON ORE CONCENTRATE AGGLOMERATION FEED

ABSTRACT In two different iron ore processing plants, it was demonstrated that when magnetite concentrate was filtered to prepare it for use as pelletization feed, the moisture that remained in the filter cake had greatly elevated levels of cations compared to the water removed by the filtration process. In particular, calcium concentrations in the filter cake were 565 times higher than in the filtrate water in Plant 1 and 19.4 times higher than the filtrate water in Plant 2. This effect is due to the adsorption of divalent cations on the magnetite surfaces resulting from electrostatic attraction when the pH of the solution is in the range where magnetite has a negative Zeta potential. Measurements of the Zeta potential as a function of pH for magnetite from Plant 2 determined that its point of zero charge (PZC) occurred at pH 3.0 to 3.5, which was much lower than is commonly reported in the literature and demonstrates that the PZC for magnetite is much more variable than is commonly thought. Plant experiments were carried out using carbon dioxide to reduce the pH of filter slurry so that the PZC could be approached. This was intended to allow adsorbed cations to be released from the magnetite surfaces during filtration. The pH adjustment was demonstrated to improve filtration rates by as much as 23%.

The effect of slurry viscosity on hydrocyclone classification

Abstract The effect of slurry viscosity on hydrocyclone classification was studied by installing an on-line viscometer set-up on a hydrocyclone test rig. This system used a vibrating sphere viscometer and a specially designed slurry presentation device to avoid settling of solids during viscosity measurement. Test samples were prepared from ground silica (80% passing 65 microns) and water. Both the solids content and temperature of the samples were varied to change the slurry viscosities. A modified version of Plitts model was then used to predict the d50(c) size. Unlike the original model where slurry viscosity was inferred from solids concentration, the modified model incorporates a viscosity parameter from direct measurement. Therefore, the new modified model predicted the cut size precisely when the viscosities of the slurries were altered by means other than changes in the percent solids. In other work, it was found that the slurries in our experiments were in the Newtonian flow regime, and their viscosities were independent of shear rate. As a result the measured apparent viscosity in this work was identical to the true viscosity. From this study it was found that d50(c) is proportional to the 0.35th power of the slurry viscosity. The water-to-underflow ratio (Rf), also tended to increase with increasing viscosity, while the reduced efficiency curve was not affected.

Rheological characterization of mineral suspensions using a vibrating sphere and a rotational viscometer

Abstract A new technique has been developed for the characterization of the rheology of mineral slurries into Newtonian and non-Newtonian flows. It utilizes a rotating type viscometer to measure apparent viscosity at a low shear rate, and a vibrating sphere type viscometer to measure the apparent viscosity at a high shear rate. Special precautions were taken to allow measurements of apparent viscosity of rapidly settling mineral suspensions. Both the viscometers are able to measure apparent viscosity as low as one mPa · s (millipascal-seconds) (1 mPa · s = 1 centipoise), which is the approximate room temperature viscosity of water. Because the vibrating sphere viscometer operates at a much high shear rate than the rotating viscometer, the two instruments together can determine the shear-rate dependency of the viscosity. Ground silica of −65 mesh size was used to prepare slurries in water at different percent solids. The apparent viscosity of each slurry sample was measured simultaneously by both viscometers, and the results were compared with each other. In this way, it was determined that silica slurries, for a given particle size distribution, between 0–70% solids by weight in distilled water have Newtonian flow behavior.