FINAL
TECHNICAL REPORT
November 1, 1998, through April 30, 2000
Project Title: DEVELOPMENT AND DEMONSTRATION OF AN ENHANCED GRAVITY SEPARATOR FOR COAL CLEANING
Principal Investigator: Dr. Bradley Paul, Southern Illinois University
Other Investigators: Dr. D.P. Patil, Southern Illinois University
Project Manager: Dr. Ken Ho, Illinois Clean Coal Institute
ABSTRACT
The main objective of this project is to demonstrate the ability of an enhanced gravity technique combined with an advanced flotation technique to improve the separation efficiency of an existing fine coal circuit at a coal preparation plant.
Initial tests with the Falcon concentrator were conducted with feeds (-16mesh) having different characteristics using Jader refuse and feed to cyclone to identify the important variables. The results with Jader refuse show that a clean coal having 8.72% ash with 39.65% yield can be obtained from a feed coal having 47.23% ash. The total sulfur content was reduced from 7.21% to 2.64% with a combustible recovery of 60.32%. The results from composite feed to cyclone show that a clean coal having 9.41% ash and 2.26% total sulfur with 71.5% yield can be obtained from a feed coal having 24.50% ash and 5.54% total sulfur with a combustible recovery of 85.79%. Plant trials of the C10 Falcon concentrator were conducted at Turris coal preparation plant. The spiral feed (-8 mesh) was fed to the Falcon and long term runs were conducted after adjusting the Falcon parameters based on the study conducted on the Jader samples. Samples were collected from the Falcon and spirals for a period of 12 hours. The particle size analysis of the Falcon and spiral products showed that at average sizes of 82 and 56 mesh about 5% and 38%; and 9% and 24% of feed was reported to the tailings. The Falcon concentrator provided excellent separation at finer sizes than the spiral. However, at an average size of 12 mesh about 67% and 12% of the feed was reported to tailings for the Falcon and spiral respectively. Hence, the coarser particles will be lost to the tailings while treating a very coarse feed. Therefore, the Falcon concentrator operates better at finer sizes and it is necessary to control the feed size distribution. The Falcon concentrator provided lower product total sulfur (3.07%-4.14%) than the spiral concentrator (3.57%-4.66%). The maximum mass yield value obtained from the Jameson Cell treatment of Turris refuse was 41.83% at a product ash content of 13.33% from a feed containing 52% ash. This corresponds to a combustible recovery of 73.38%. The plant could produce approximately 54 tons/hr of clean coal using the Jameson cell technology.
Economic analysis indicates the potential for significant cost savings by plant optimization and savings as high as 72 cents/ton if enhanced gravity and flotation were both implemented. Flotation alone may not save money due to the inability to reject sulfur from the fines.
EXECUTIVE SUMMARY
Advanced fine coal cleaning technologies have been found to provide separation performance that are near or approaching the theoretical maximum based on either the washability curves for gravity separations or the release curve for froth flotation processes. Since the coal particles in the fine fraction represent the cleanest particles in the run-of-mine coal, the application of the advanced fine coal technologies should provide the highest possible product grades and mass yields in an operating preparation plant. This fine product can be blended with the coarse and intermediate fractions to produce an improved plant product at possibly higher plant yields, depending on the proportion of the fine fraction in the original feed. If a given overall plant product quality is to be maintained, higher gravity cut points on the coarse and intermediate fractions could be realized. In both cases, the result would mean an increase in the overall plant yield and separation performance.
Enhanced gravity concentration is a product of the research conducted aimed
at improving the efficiency of fine particle
processing. Research funded,
in a large part by the State of Illinois, has found that enhanced gravity
concentration has the ability to improve the rejection of both ash-bearing
material and coal pyrite while maximizing the recovery of fine
coal. The process effectively
treats particle sizes from 16 mesh to as small as 325
mesh. Based on successful research
programs conducted on bench, pilot plant and full-scale units, the present
project was funded to evaluate the application of a full-scale enhanced gravity
concentrator, commercially known as the Falcon, Concentrator in an operating
preparation plant.
The unit was installed and tested in conjunction
with an advanced flotation system for the treatment of –16 mesh coal
at the Turris preparation plant located in southern Illinois and the resulting
techno-economical benefits were
quantified.
The goal of this research project was to conduct plant trials of the Falcon concentrator in an operating coal preparation plant to identify the important parameters affecting its performance and to test the application of Jameson flotation to recover coal from refuse sample. The feed coal characteristics in a plant vary considerably. Hence, it was necessary to find out the effect of feed characteristics on the Falcon performance and identify its important parameters. To achieve this goal, the work in this research project initially involved the treatment of nominally -16-mesh coal samples having different feed characteristics using the Falcon concentrator. The refuse and feed to the cyclone samples from the Jader fuel preparation plant which treats the Illinois No. 2+3 seams were used for this study. The refuse coal contains about 45.33% ash having 5.52% total sulfur. Whereas the composite feed to cyclone contains 30.39% ash content with 3.91% total sulfur. The heat values for refuse and composite samples are 7789 and 9647 BTU/lb respectively.
Statistically designed experiments were carried using the Box-Benkhen design. The following parameters were used for the experimental campaign.
· Bowl speed (20,30,40 Hz)
· Percent solids (10,20,30 by wt.)
· Underflow rate (1, 3,6 lpm)
· Feed flow rate (10,20,30 gpm)
Using the Design Expert software the following coded empirical models describing the product ash content and combustible recovery as a function of the operating parameter values were derived in quadratic form for refuse coal and composite feed to cyclone.
Jader Refuse Coal
Product Ash (%) = 25.48 +
2.45´A +
3.65´B -
10.34´C +
4.15´D -
4.94´B2
+
2.67´C2
2.96´D
2
–
4.33´A´D -
4.49´B´C
[1]
Combustible recovery (%) = 75.06 +
16.13´B -
18.57´C +
13.28´D -
5.08´A2
-
7.70´B2 -
11.71´B´D
[2]
Jader Composite Feed to Cyclone
Product Ash (%) = 11.63 -
0.35´A +
1.06´B -
1.14´C +
0.75´D2
-1.65´A´D
+
0.94´B´D
[4]
-
9.71´B2
-
6.85´D2
+
7.95´A´B -
7.67´A´D
[5]
in which A is the bowl speed, B the feed solids content, C the underflow rate, and D the feed volumetric flow rate. (The values of the variables A, B, C, and D are scaled from 0 to 1 over the experimental range).
The effect of parameters on ash and combustible recovery were analyzed using perturbation plots. The underflow rate has a pronounced effect on both product ash content and combustible recovery, a small change in the underflow rate greatly affects product ash content and combustible recovery. The feed solids content is also quite important as indicated by its curvilinear nature. Bowl speed had the least effect both on product ash and combustible recovery. The factors according to their importance for product ash and combustible recovery for refuse sample can be written as:
Where as for the composite feed to the cyclone;
The simulations revealed that it was necessary to maintain a relatively high percentage of solids with low volumetric feed rates to achieve the ultimate separation performance while treating the refuse material. The studies with composite feed to cyclone showed that it was necessary to maintain low percent solids with high feed flow rate. Both conditions result from the need to build and maintain a stable bed of particles within the separator. Experiments were conducted near optimum regions as suggested by the simulation study.
Optimization results with Jader refuse show that it is possible to obtain a clean coal having 8.72% ash with 39.65% yield from a feed coal having 47.23% ash. At 39.65% mass yield, the Falcon concentrator is able to reduce the total sulfur content from 7.21% to 2.64% in a single stage with a combustible recovery of 60.32%. The washability data shows that at the same ash content of 8.72%, the yield is about 44% having 2.58% total sulfur. The pyritic sulfur content was reduced to 0.93% from 5.26%. The probable error (Ep) and specific gravity of separation (d50) were calculated by washability data of the product and tailings samples. The Ep and the d50 are 0.12 and 1.45 respectively.
The results with Jader composite feed to cyclone show that it is possible to obtain a clean coal having 9.41% ash and 2.26% total sulfur with 71.51% yield, from a feed coal having 24.50% ash and 5.54% total sulfur. The pyritic sulfur was reduced from 2.54% to 0.82%. The combustible recovery is about 86%. The washability data shows that at the same ash content of 9.41%, the yield is about 79% having 2.48% total sulfur. The Falcon results with Jader refuse and feed to cyclone show that it can effectively treat feeds containing varying amount of ash contents.
The Falcon concentrator is inefficient in rejecting ash from the –325 mesh coal fraction. The thickener underflow (130-tons/hr) of the Turris coal preparation plant contains about 65% of –325-mesh fraction having about 60% ash. Even though the flotation process is not as effective at reducing the pyritic sulfur content as the Falcon unit, it is an excellent desliming unit. The maximum mass yield value obtained from the Jameson Cell treatment was 41.83% at a product ash content of 13.33% from a feed containing 52% ash. This corresponds to a combustible recovery of 73.38%. The plant could produce approximately 54 tons/hr of clean coal using the Jameson cell.
An extensive separation performance comparison between the pilot-scale C10 Falcon concentrator and the existing spiral circuit was conducted at the Turris Coal Preparation plant for the treatment of coal from fine coal circuit. The fine coal circuit should contain particles nominally –16 mesh size. However, the feed coal contained about 9% of +16 mesh particles, the top size being 8-mesh. The percent solids in the feed to the Falcon was varying between 35% to 45% with a flow rate of 30 to 40 gallons per minute. The high percent solids combined with the presence of +16 mesh particles hindered the operation of the Falcon concentrator. Hence, the feed flow rate to the Falcon was reduced and water was added to the Falcon feed to reduce the percent solids. Finally the Falcon was operated at about 15 to 22 % solids with a flow rate varying between 15 to 20 gallons per minute.
The higher centrifugal force provided by the Falcon concentrator helps to separate fine particles by accelerating their settling characteristics. However, because of the higher centrifugal force the coarser particles will settle pretty quickly towards the wall of rotating bowl, and thus will report to tailings. The Falcon unit was found to be capable of providing reasonable metallurgical results from a feed containing –8-mesh coal particles. The results indicate that weight yields ranging from approximately 66-77% can be obtained at corresponding product ash contents of 10%-14% for the Falcon concentrator. Where as the spiral concentrator yields 70%-81% coal at about 10-14% ash, providing slightly better separation than the Falcon concentrator. However, the Falcon concentrator provides lower product total sulfur (3.07%-4.14%) than the spiral concentrator (3.57% - 4.66%). The Falcon concentrator was able to produce a clean coal with a reasonable variation in the ash content with varying feed characteristics while treating –8 mesh coal. However, the Falcon being an enhanced gravity separator performs better at finer sizes (-16 mesh) than on a feed containing –8 mesh. These results indicate both product quality and economic benefits could be obtained in using the units tested in this investigation in the fine coal cleaning circuits.
Economic analysis indicates the potential for significant cost savings by plant optimization and savings as high as 72 cents/ton if enhanced gravity and flotation circuit were implemented. The overall impact of this added revenue/savings for a 750-tph plant operating 4000 hours/year will be around $2.13 million/year. Because of reduction in pyritic sulfur, the increase in SO2 allowance cost from $150 to $250/ton would improve the economic benefits from $2.13 million to $3.48 million. Flotation alone may not save money due to its inability to reject sulfur from the fines.