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Fine coal cleaning for improved sulfur rejection – Part 3

World Coal,

Z. Ali, R. Bratton and G. Luttrell, Virginia Tech, M. Mohanty, Southern Illinois University Carbondale, A. Dynys and L. Watters, Taggart Global LLC, and C. Stanley, Knight Hawk Coal LLC, US, discuss the innovative design of fine coal cleaning circuits for improved sulfur rejection.

Classification testing
The use of a conventional “by-zero” froth flotation circuit was not considered to be a viable option for the Prairie Eagle coal preparation plant because of the extreme fineness (>85% minus 325 mesh) and poor quality (>60% ash) of the minus 100 mesh feed. The best alternative was considered to be a flotation circuit incorporating deslime classifying cyclones to discard the minus 325 mesh fraction followed by column flotation to upgrade the nominal 100 x 325 mesh cyclone underflow (Bethell, 2004). To examine the potential benefits of this alternative, a sample of minus 100 mesh overflow was acquired from the 15 in. dia. clean coal classifying cyclones currently operating in the plant. The sample was shipped to the laboratory and passed through a 6 in. dia. deslime cylone to determine the effects of classification on ash and sulfur partitioning.

Table 1 provides a summary of the deslime cyclone test results.

Table 1. Size-by-size ash and sulfur analysis of the 6-inch diameter feed, overflow and underflow.

The data was promising in that the overflow represented 63.4% of the feed weight and contained 71.8% ash. Very little (2.6%) material in the 100 x 325 mesh size fraction reported to the overflow. The data also indicated very little difference in sulfur contents of the overflow and underflow products for sizes larger than 325 mesh. This result was unexpected since it was assumed that sulfur levels in the underflow would be higher due to preferential partitioning of high-density pyrite to the underflow stream. Surprisingly, the plus 325 mesh sulfur contents ranged from 2.2 – 2.45% sulfur, which were well below the plant contract specification of 2.8 – 3.2% sulfur.

Upgrading of clean coal classifying cyclone overflow
The feed, overflow and underflow products from the raw coal deslime cyclone were subjected to laboratory froth flotation tests. The test work included both kinetic tests and release analysis tests to evaluate the practical and ultimate cleanability of each sample. The test results from the release analysis tests (Figures 2 and 3) showed that products containing less than 10% ash could be theoretically obtained from any of the three process streams.

Figure 2: Flotation release curves for deslime cyclone streams (ash data).

Figure 3. Flotation release curves for deslime cyclone streams (sulfur data).

However, high recoveries were difficult to obtain for either the feed or overflow samples due to the very high feed ash contents of these samples. In contrast, combustible recoveries approaching 90% were attainable for the deslimed underflow. The only apparent disadvantage of processing the underflow sample was that slightly higher sulfur values were obtained, perhaps due to the comparatively high sulfur content of the underflow feed sample.

Data from the kinetics tests (Figures 4 and 5) were found to be substantially inferior to those obtained from release analysis testing.

Figure 4. Flotation kinetic curves for deslime cyclone streams (ash data).

Figure 5. Flotation kinetic curves for deslime cyclone streams (sulfur data).

This large difference suggests that column flotation with froth washing to minimize entrainment would be the preferred option for treating any of these process streams. Based on this data, column flotation would be expected to produce a froth product containing about 7 – 8% ash, while conventional flotation would be expected to result in a froth product containing 17 – 18% ash. Both methods produced clean coal products containing about 2.5% sulfur, which as indicated previously was well below the contract specifications. The similar sulfur rejection levels achieved by kinetic and release testing indicates that the sulfur was recovered by flotation as a result of either poor liberation or pyrite floatability and not due to hydraulic entrainment in the froth water.

Upgrading of clean coal sieve underflow
The experimental results obtained from the initial round of classification and flotation tests indicated that deslime column flotation would be an ideal approach for upgrading the overflow from the clean coal classifying cyclones currently installed in the Prairie Eagle plant. Further investigations indicated that flotation performed well since most of the high-density pyritic sulfur in the minus 100 mesh feed slurry had already been captured in the underflow of the plant’s clean coal classifying cyclones. The cyclone underflow then reports to fine wire sieves where the sulfur-enriched fines eventual passed through as effluent. As shown in Table 2, sampling and size-by-size analysis of the sieve underflow showed that the effluent contained 39.8% ash and 5.6% sulfur, with the greatest proportions of the sulfur occuring in the sizes less than 100 mesh.

This created a delimina for plant designers since the sieve effluent contained too much valuable coal to discard despite being highly enriched in both ash and sulfur.

Several additional series of coal cleaning tests were conducted to determine how to best process the material present in the clean coal sieve underflow. In this round of tests, two different processes were considered: i.e., column flotation and fine spirals. In addition, a multi-property processing circuit consisting of density-based spirals followed by surface-based flotation was also evaluated for this unique application. This circuitry was considered to be a viable option since the upstream clean coal classifying cyclones and fine wire sieves had created a manageable low-volume high-impurity stream that could be easily handled by a multi-property processing circuit. The flotation test work was conducted using both laboratory and pilot-scale units, while the spiral tests were conducted using a full-scale single-start compound spiral rig. The spiral rig was equipped with a partitioned product launder that allowed six products to be collected across the spiral profile in addition to a primary reject. Although not shown, six separate sets of fine spiral tests were conducted to identify the optimum conditions (i.e. dry solids mass rate and slurry volume flow rate) for separating the nominal 100 x 325 mesh material from the sieve underflow.

Figures 6 and 7 show the combustible recovery versus ash and sulfur contents obtained from the flotation, spiral and flotation/spiral test runs.

Figure 6. Comparison of spiral, flotation and combined units for sieve underflow (ash data).

Figure 7. Comparison of spiral, flotation and combined units for sieve underflow (sulfur data).

As expected, the data plotted in Figure 6 effectively demonstrates that froth flotation was much superior to the fine spirals in removing ash-forming minerals. Flotation effectively reduced the ash of the feed from about 43% to about 15 – 18% ash, while the fine spirals never achieved ash levels lower than about 34% ash. While much of this problem can be attributed to the inability of the spiral to deal with misplaced ultrafines (minus 325 mesh solids), size-by-size analyses of the test data showed that froth flotation was superior to the fine spirals even for the coarsest size fraction of 100 x 200 mesh present in the feed (see Figure 8).

Figure 8. Comparison of spiral and flotation for sieve underflow (100x200 mesh size fraction only).

On the other hand, Figure 7 shows that fine spirals were much superior to flotation for reducing sulfur levels, often rejecting solids containing double-digit sulfur contents. The spiral unit lowered the feed sulfur from about 5.6% to about 4.2% sulfur. Flotation was not able to match this level of performance and, in fact, actually concentrated the sulfur-bearing components into the clean coal product. Consequently, most of the froth products from flotation testing had higher sulfur values than the original feed.

The most interesting and most promising results were obtained from the testing of the combined fine spiral and flotation circuit. As illustrated in Figure 6, the combined circuit was able to produce combustible recoveries in the range of 80 – 90% while reducing the total ash content into the 10 – 11% range. Moreover, the data provided in Figure 7 shows that the combined circuit also attained the best reductions in sulfur. In this particular case, sulfurs under 4% were obtained while keeping combustible recoveries in the 85 – 90% range. The improvement in performance achieved by the combined circuit is best illustrated by plotting the product ash and sulfur values obtained from the flotation, spiral and combined test runs on a single graph (Figure 9).

Figure 9. Impact of different processing methods on clean coal ash and sulfur values.

The format clearly illustrates the inherent capabilities of the different processes in dealing with ash- and sulfur-forming minerals and the synergistic effect of the two-stage combined in maximizing the purity of the final clean coal product.

The fourth part of the article about industrial demonstration and conclusion can be reached here.

This article was first presented at Coal Prep International 2013 and is presented here by permission of Penton Media. Coal Prep International 2014 will take place in Lexington, Kentucky between 18 April and 1 May 2014.

BETHELL, P.J. (2004): “Froth Flotation – To Deslime or Not to Deslime?” CPSA Journal, Vol. 13, No. 1, pp. 12-15.

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