Challenge

With the high cost of handling, disposal and transportation of steel mill sludges, there is an increasing demand in the steel industry for waste products that can be recycled or landfilled without further processing.  U.S. Steel Gary Works in Gary, Indiana needed to find a way to generate less waste during sludge dewatering.

The existing dewatering system, which used centrifuges to dewater iron oxide sludge, produced solids only in the 44 to 50% range.  What’s more, the filter cake didn’t always pass the paint filter test and therefore could not be placed in the landfill.  Additionally, reuse of the filtration was desired.

The only solution was to add lime to the cake, which increased the volume of landfill material and shortened the landfill’s life.  Gary Works required a new dewatering system with a guarantee that the solids generated would consistently pass the paint filter test and produce the minimum amount of filter cake.  They also required a system that would operate 24 hours a day, 365 days per year.

Solution

Siemens Water Technologies met the challenge, providing two 75-chamber, 1200 mm overhead suspension filter presses.  By testing the sludge at the Gary Works’ site, and by applying experience gained from similar projects at other steel mills, Siemens Water Technologies was able to offer Gary Works the following performance guarantees:

• Filter cake with a minimum of 70% solids that passes the standard paint filter test.
• A throughput rate of 330,000 lbs of dry solids per filter per day if the underflow from the thickener met certain parameters.
• Total Suspended Solids (TSS) of less than 100 ppm in the filtrate.

Results

Started up in January 1998, the filters are meeting or exceeding these performance guarantees.  Siemen's experience with other systems enabled them to further improve the system’s design for even better performance:

• Operating pressure was increased from 100 to 135 psi filtration, increasing the solids range from 77 to 80%.
• Cleanouts were installed at strategic places to minimize system downtime should pluggage occur.
• Recirculation circuits were installed where appropriate to eliminate slurry settling.
• Long radius elbows and schedule 80 carbon steel pipelines were installed throughout.
• Variable speed drives were installed on the pumps.

By selecting a 40 mm cake in this particular application, Siemens Water Technologies was able to use fewer plates, reducing the cost and the cycle time to shift the larger number of plates.  In addition, Siemens Water Technologies helped Gary Works avoid a costly new building by suggesting a unique design that utilized an existing building.

Says John Cioffi, Process Engineer at U.S. Steel Gary Works, “The filters met all our requirements.  And, the good performance in this application was a consideration in buying three more filter presses for another application, the blast furnace recycle system.”