Scale Up of the ISGS Froth Washer for Testing in a Commercial Plant

INTERIM FINAL TECHNICAL REPORT

November 1, 1998, through October 31, 1999

Project Title: SCALE UP OF THE ISGS FROTH WASHER FOR TESTING IN A COMMERCIAL PLANT

ICCI Project Number: 98-1/4.1A-1

Principal Investigator: Latif A Khan, ISGS

Other Investigators: John Lytle, William Roy, ISGS

Project Manager: Ken Ho, ICCI

ABSTRACT

The purpose of this project was to scale up the ISGS froth washer for testing at a commercial plant. Specifically, the approach taken was to scale-up the ISGS washer by more than a factor of ten from a 0.11-ft³ miniature cell to a 1.3-ft³ cell. The washer has been developed at the ISGS to increase the throughput and grade of clean coal from a froth flotation circuit.

The ISGS washer is in its third year of development. The washer was adapted to a subaeration cell, and tested on cleaning finely ground IBC-112 from the Illinois Basin Coal Sample Program. The modified cell was able to produce cleaner coal at a greater throughput than a packed column. The modified subaeration cell was especially effective in removal of pyritic sulfur, and under some conditions, it was able to produce a cleaner product than that predicted by the Advanced Flotation Washability analysis curve.

Our goal for this phase has been to determine the dimensions of the washer with especially emphasis on its height and width that are capable of processing effectively the concentrate generated in table top-scale subaeration cell, and to construct a similar washer for a pilot plant cell. The performance of pilot-plant cell was to be tested to ensure that its performance was similar to that of the table top cell and from that data to design, construct and test a plant-size unit

The ISGS froth washer is being scaled up by at least a factor of ten. One of the most important tasks in scaling up the ISGS froth washer has been the design of its shape and size to maximize throughput and cleaning. This task has been completed successfully. A rectangular tube-shaped washer that increases the cross-sectional area, but maintains a shallow froth to maintain a high rate of rejection of fine pyrite and clay particles was selected. Also, to limit the contact of the stream of contaminants with the stream of froth, grooves were added to the bottom of the washer to enhance the flow of the streams of contaminants thereby minimizing the contact of the froth with the streams of contaminants. Tests on the 1.3-ft³ subaeration cell equipped with the washer are being carried out. Even at 1/3 of its capacity it can handle, irrespective of the ash content of the feed, all the froth that is generated in this cell.

Pages 1-26 contain propriety information

EXECUTIVE SUMMARY

This goal of this project has been to scale up the inclined washer by more than a factor of ten from our 0.11-ft³ cell to a 1.3-ft³. This washer, when attached to a subaeration cell, has produced consistently, in single-stage flotation tests, a product that is cleaner than that produced with a packed column using the same feed material. Also, the rate of production of concentrate by the cell with the washer is greater than that of ordinary flotation columns. Commercial development and installation of such a device could have a significant impact on the future of the coal industry in Illinois by increasing recovery of fine coal and decreasing coal washing costs by more than a factor of ten.

During the intensive washing and enhanced drainage that occur in the inclined washer, both the particles that are mechanically carried over, unselectively attached to the froth and those that are trapped between the air bubbles are flushed out. To make sure that flushed particles do not get re-entrained in the lower layers of the froth, it is better to limit the vertical height of the froth that is being washed. In the inclined washer, the washed-out minerals are carried a short vertical distance to a separate stream along the lower part of the washer, and not allowed to become entrapped again.

Without the usual multiple stages of cleaning and re-cleaning, this inclined washer allows a single flotation cell to produce a product cleaner than that produced by a flotation column at the throughput rate of a common subaeration cell. In other words, this device is an improvement on the old system of subaeration cell batteries in which the froth or tails were recleaned to generate a product of desired quality. Equipped with this device each cell can produce the final product in a single run leading to greatly increased capacity of the plant or a decrease in the number of the cells required to process a given quantity of material. The result may be considerable savings in installation and/or operational cost. Costs also will be saved because of the better cleaning efficiency of fines and because of the associated saving on disposal of fines.

The inclined washer has been tested on both a subaeration cell and a column using both IBC-112 ground to ~90% -200 (74 m) mesh and preparation plant fines (~90% -100 mesh)(149 m). A froth flotation system retrofitted with the inclined washer produces better grades and recoveries than the most advanced systems being developed - sometimes even better grades and recoveries than those predicted by the advanced flotation washability curve. There are several reasons why the inclined washer can operate so effectively:

the inclined washer provides multi-stage washing along the length of the tube, with clean wash water at each stage;
because of the small froth depth, lower levels of the froth do not tend to become crushed and contaminants need only travel a short distance to a stream at the bottom of the tube to return to the pulp which is especially effective for rejecting fine clay and pyrite particles that have a very slow settling velocities;
to produce cleaner coal, the amount of wash water can be increased to levels that would flood a normal column without causing any difficulty in the operation of the inclined column;
to provide a cleaner coal product, the turbulence of washing can be increased by spraying the water onto the froth to the point that would destroy the froth in a vertical washer on a column, but because of the wet, flowing conditions, the froth in the inclined washer just becomes more fluid and cleaner under these conditions.

The inclined washer can be operated successfully to produce a well-drained dry froth or a very wet, water-laden froth depending on the needs of the system and the capacity of the filtration system. In either case, the coal produced is cleaner than that produced by advanced columns.

In the subaeration flotation aeration rate was used to increase the recovery. Unlike the results from the 4-inch inclined washer, it appears that the capacity of this 2-inch inclined washer was pushed to its limit as the throughput was increased to 200 lb/hr/ft³. At a smaller throughput however, a much smaller ash content of 5.9% was produced. The most likely reason that the 2-inch ID inclined washer could not be pushed to the same throughput capacity as the 4-inch ID washer was because of the velocity of the froth. The faster the froth moves through the washer, the greater the tendency for it to push some of the stream of water trying to carry contaminants down to the bottom of the washer back up instead. However, it appears that increasing the diameter of the washer can have a negative effect on rejection of the fine pyrite particles. Our best pyritic sulfur rejection (0.5% PS) was attained in the 2-inch ID washer at low throughput. The ability to reject fine pyrite particles appears to decrease with increased diameter and increased throughput. One of the most important tasks for scaling up the inclined washer was to design its shape and size to overcome that problem by designing a rectangular-shaped washer that increases the cross-sectional area, but maintains a shallow froth to maintain a high rate of rejection of fine pyrite and clay particles. Also, to limit the contact of the stream of contaminants with the froth, a grooved-bottom Washer was used for the streams of contaminants to flow along, minimizing the contact of the froth with the streams of returning contaminants.

One of the most important tasks for scaling up the ISGS froth washer has been the design of its shape and size to maximize throughput and cleaning. This task has been completed successfully. A rectangular tube-shaped washer that increases the cross-sectional area but maintains a shallow froth to maintain a high rate of rejection of fine pyrite and clay particles was selected. Also, to limit the contact of the stream of contaminants with the stream of froth, grooves were added to the bottom of the washer for the streams of contaminants to flow along, minimizing the contact of the froth with the streams of contaminants.

The goal of our proposed work for 1998-1999 has been to determine a variable approach to scale-up the ISGS inclined washer to a large flotation cell that can be tested at a commercial plant. The goal has been to find also the conditions under which inclined washers of various dimensions and design could process a stream of preparation plant fines effectively. The scale-up data generated from our 1.3-ft³ Denver subaeration cell (more than a factor of ten larger than the miniature subaeration cell) provided the needed information for the design of a much larger unit to be used in a commercial plant. Fine coal and rejects were used in these tests. The preparation plant rejects contained large amounts of clay-sized minerals, pyrite and a broad particle size distribution. As such, these fines required a carefully designed inclined washer. The impact of variables such as the angle of inclination of the washer, the amount of wash water, stages of washing/length of washer, aeration rate and feed rate on the quality and quantity of material produced in flotation machines equipped with different shapes and sizes of inclined washer were determined on the 0.11-ft³ cell in the laboratory. The results of these tests were compared with the results of 1.3-ft³ cell. Tests on the smaller scale version of the 1.3-ft³ subaeration cell are being carried out when the washer was used only at 1/3 of its capacity.

For a full copy of this report, e-mail ICCI (lisar@icci.org). You must specify the report name, the PI's name and your full address on your request. Report requests may also be sent by snail mail to ICCI, 5776 Coal Drive, Suite 200, Carterville, Illinois 62918, or by fax to 618-985-6166.