During the past decade, several enhanced gravity concentrators have evolved to commercial scale as a result of the need to achieve efficient gravity-based separations for the treatment of ultrafine particles. The initial interest in enhanced gravity separators (EGS) was for the recovery of fine gold, which represents a low product flow rate application whereby the concentrated gold stream represents 1% or less of the total feed. As a result, semi-batch units are the most common EGS used in gold applications, in which, the feed and overflow streams are continuous while the heavy gold particles are collected against the wall of a spinning bowl and periodically flushed. However, due to the need to treat materials having a high-density particle content greater than 1%, fully continuous units have been developed in which the high-density particles are continuously removed from the units (e.g., Falcon C-series Concentrator, Knelson CVC-series concentrator, Multi-Gravity Concentrator, and Kelsey Jig).
Fine coal cleaning represents an application whereby the heavy reject fraction typically accounts for more than 1% of the feed. Studies to date have shown that continuous EGS units achieve relatively efficient separations for particle size fractions in the range of 1 mm by 44 µm (Luttrell et al., 1995). Excellent ash and total sulfur reductions have been obtained with the latter being the most significant. Froth flotation, which is the most commonly used fine coal cleaning process, tends to concentrate the sulfur in the clean coal product due to the natural hydrophobicity of the coal pyrite. Due to the large density difference between coal and pyrite, the EGS units reportedly achieve sulfur reductions on the entire -1 mm particle size fraction. Compared to froth flotation, each of the enhanced gravity separators was also found to provide a superior ash rejection for the particle size fractions greater than 75 µm.
The Altair Centrifugal Jig is one such enhanced gravity concentrator, which is simpler in design with a lower number of moving parts than its competing technology, i.e., Kelsey Jig. Originally invented in 1971 by Thomas Campbell, the Altair Jig technology has been modified over the years to be able to achieve an enhanced gravity field of nearly 100 g’s with its current 12-8 model. Some of the past coal cleaning feasibility studies (Parkinson, 1989) using the Altair Jig reported efficient sulfur rejection results on the entire -600 µm particle size fraction, which includes the finest fraction (i.e., -75 µm). However, the ash rejection performances obtained from the same study were not as encouraging.
Since this study evaluated the effectiveness of the jig on cleaning -600 µm, -150 µm and -75 µm coal samples, the inability to effectively reduce the ash content was most likely due to the reporting of the slimes to the clean coal product. The slime fraction in coal (i.e., -25 µm) typically represents a significant quantity and characteristically contains a large amount of submicron clay, which reports to the product through hydraulic entrainment. All of the EGS technologies have been found to have this problem despite the use of centrifugal fields up to 300 g’s (Luttrell et al., 1995). Hazen Research also conducted a study in 1984 testing the Altair Jig technology on sand, gravel and coal. Although the coal results were encouraging, the recommendation of Hazen was to modify the equipment so that the jigging compartment g-force and the water pulse frequency can be independently controlled. Such modification may improve the metallurgical performance achieved by the technology; however, it will add complexity to the system.
This publication will present the results obtained from a study, which was conducted to evaluate the Altair Centrifugal Jig’s current 12-8 model essentially for cleaning the 600 x 25 µm fraction of a coal sample. The key operating parameters were identified and the individual parameter and their interaction effects on the important separation performances were studied. The potential of operating the Altair Jig without the use of any ragging material as a means of enhancing the throughput capacity of the equipment has been investigated.
Altair Centrifugal Jig
The Altair Jig, as shown in Figure 1, essentially consists of a rotating bowl, which is placed inside a static casing having separate launders for collecting the concentrate and tailing samples. The rotating bowl contains a cylindrical screen with a lip, whose height can be adjusted to vary the natural depth of the ragging bed. The ragging material remains in a vertical position on the screen due to the rotation of the bowl. The feed slurry, which is introduced from the top at the center of the rotating bowl, is distributed into the ragging bed on the screen by the diffuser plate placed under the feed inlet. Pressurized water is injected under the bed periodically through the four pulse-blocks to cause alternating dilation and contraction of the ragging and feed bed. This action, coupled with the high centrifugal force generated from the rotation of the bowl, provides the jigging mechanisms and kinetics required to achieve gravity-based separations for fine and ultrafine particles. The tailings material, which contains particles having a relatively high density (e.g., 2.4 gm/cc), settles through the ragging bed and screen into the hutch and reports to the tailings launder through the discharge ports. On the other hand, the clean coal particles, which have a relatively low density (e.g., 1.3 gm/cc), do not have sufficient retention time to settle through the screen and, thus, report to the concentrate launder.
Conclusions
- For the 600 x 44 µm particle size fraction of a Pittsburgh No. 8 seam coal, the Altair Centrifugal Jig was found to provide a reduction in ash content from an average of 30% to 10% while recovering 86% of the combustible material. The corresponding reduction in total sulfur content was from 2.50% to 1.40%. This separation performance resulted in a 78% rejection of ash-bearing material and 62.5% rejection of total sulfur. Considering that the feed material contained a significant amount of near-gravity material due to its origination from the middlings stream of a jig, the high separation efficiency value of around 55% is an excellent achievement.
- Partition curves derived from Altair Jig washability data indicate the ability to achieve a relatively low specific gravity cut point (D50) of nearly 1.50 at a probable error value (Ep) of 0.08. These process efficiency values represent an improvement compared to that obtainable from current conventional fine coal cleaning technologies.
- The separation performance achieved on the 600 x 44 µm particle size fraction by the Altair Jig was found to be superior to the results obtained from the advanced washability (release) analysis, which represents the ultimate performance achievable by any flotation technology.
- The separation performance results obtained as a function of particle size indicates that the optimum separation was achieved on the 150 x 75 µm particle size fraction. For the coarsest particle size fractions, the ash-forming material was found to have difficulty in passing through the screen. Thus, ash recovery to the product is sufficiently higher than the finer material. However, the excellent separation performances achieved on the particle size fractions below 150 µm are superior to other commercially available enhanced gravity technologies. This finding is especially unique due to the use of a relatively low centrifugal force of 45 g’s. As discussed, higher throughput capacities necessitate the use of increased g-forces to maintain high process efficiencies.
