Alternative Technology for Sediment Remediation Demonstration Plant

Abstract

Duluth-Superior Harbor is a major port on Lake Superior located between the cities of Duluth, Minnesota, and Superior, Wisconsin. The harbor and the lower Saint Louis River that discharges into the harbor area have a history of water quality problems resulting primarily from municipal and industrial discharges in and upstream of the harbor. The port is a major debarking point for grain shipments overseas and for taconite pellets for the lower Great Lakes ports. To allow navigation, the shipping channels must be dredged annually. The dredged material has been stored in a confined disposal area developed at the Erie Pier location in Duluth. This facility is nearing its capacity and other methods for handling the dredged material must be found. The Coleraine Minerals Research Laboratory, a division of the Natural Resources Research Institute of the University of Minnesota - Duluth, has been studying the application of mineral processing techniques for treating contaminated soils. The laboratory sampled the Erie Pier site and designed a demonstration plant to treat about 50 tph of material from the site. Based on the previous work and the plant design, the U.S. Army Corps of Engineers awarded the laboratory a contract to construct and operate the demonstration plant. The plant consisted of a feeder followed by a grizzly screen to remove large rocks and miscellaneous junk. The grizzly undersize was conveyed to a double deck screen equipped with water sprays. The screen undersize flowed to a sump and pump. The slurry was then pumped to an agitated tank. Material from the tank was pumped to two cyclones to make a size separation. Cyclone overflows were collected and channeled to settling ponds to allow the solids to settle and to provide water for the plant. Cyclone underflow was stockpiled as a sand product. In addition to sending the cyclone overflow to the settling ponds, a belt filter press was tested for about two weeks to treat a portion of the overflow to produce a cake that could be easily handled and a clear filtrate that could be recycled. The objective of the program was to treat different types of materials found at the Erie Pier site to produce a coarse product (cyclone underflow) that contained less than 12 percent by weight particles finer than 200 mesh (75 microns). The underflow should be free draining so that it could be moved by loaders. The distribution of solids, water, inorganic compounds and organic compounds would be monitored. The settling characteristics of the cyclone overflow would be determined. A total of four separate samples were processed in the plant. Sample 1 was a sandy feed containing between 13 and 32 percent in the passing 200 mesh fraction. Sample 2 was a finer material that was removed from the site during construction of the settling ponds. Sample 2 contained between 30 and 52 percent in the passing 200 mesh fraction. Sample 3 was a fine sample dug from the north end of the site where the finest material should have been. Sample 3 was only run for one day due to a break down of the front-end loader used to transport the feed to the plant. The fourth sample was the drained cyclone underflow from the processing of samples 2 and 3. Maintaining a consistent feed to the plant was a continual problem. Clay material in the feed was difficult to disagglomerate and the material tended to form balls, which rolled down the screen decks. Additional water sprays and belting on the top screen deck improved the break up of the clay material but did not eliminate the problem. Another feed problem was the amount of vegetation in the feed. This material tended to bridge in the feeder and to plug the two screen decks, reducing screening capacity, at times significantly. Compounding the feed problem was the loss of the variable frequency drives on the two pumps. Loss of the drives effectively eliminated the ability to make any significant changes in the flowrate to the cyclones and, hence, the ability to affect the cyclone split. Attempts were made to control the cyclone feed by installing a by-pass line to return some of the cyclone feed back to the cyclone feed sump. These attempts were unsuccessful and on numerous occasions resulted in overloading the cyclone feed pump motor causing the motor to stop. Samples of the cyclone feed, overflow and underflow, as well as belt filter press cake and filtrate, when operating, were taken hourly. These samples were saved for future analysis. In addition to the saved hourly samples, a grab sample of each stream was taken hourly and made into a daily composite. The daily composites were filtered with a portion of the filtercake being used for size analysis and the remainder being air dried for chemical analysis. Sample 1 was processed at feed rates up to about 63 tph with no loss in performance. In all tests with Sample 1, the cyclone underflow contained less than 10 percent in the passing 200 mesh fraction. Weight recovery to the underflow ranged between 73.3 and 92.6 percent. In general, the heavy metals and organic material were concentrated in the cyclone overflow, but since the total weight recovery in the cyclone underflow was high, the majority of the heavy metals and organics in the feed remained with the cyclone underflow. The processing of Samples 2 and 3 were more difficult due to the large amount of vegetation contained in the feed. Plant feed rates were generally between 7 and 14 tph. The low feed rates were caused by the vegetation problem and by the need to feed the cyclone a low percent solids to try to make the desired size split. But even at the low percent solids in the feed, the cyclone underflow contained between 18 and 29 percent in the passing 200 mesh fraction. Weight recovery to the underflow ranged from 55 to 72 percent. Despite the high minus 200 content, the cyclone underflow was easy to dewater and formed into a steep sided conical pile. As with Sample 1, the heavy metals and organics were concentrated in overflow sample, which, due to the higher weight recovery, contained the majority of the heavy metals and organics from the feed. Since the cyclone underflows from Samples 2 and 3 still contained too many fines, the cyclone underflow pile was reprocessed through the plant. Resultant cyclone underflow contained between 10.9 and 14.7 percent in the minus 200 mesh fractions and recovered over 90 percent of the feed weight. Again the heavy metals and organics concentrated in the cyclone overflow. Performance of the belt filter press was very impressive. The resultant filtercake was very easy to handle by conveyor belts and would be very easy to haul by truck. The filtercake was almost dry to the touch. Filtrate from the belt filter press was very clean, with turbidity measurements less than 5 ntu. To produce these results required about 1.5 pounds of polymer flocculant for every 3900 gallons of cyclone overflow treated. Analysis of the filtrate indicated no residual polymer in the water.