Source: Chemical Processing
The protection of the Shenandoah River and the reclamation of valuable zinc hydroxide were both achieved by Avtex Fibers, the nation's largest producer of rayon fibers, through the use of a portable sediment remover.
For years, Avtex had pumped zinc hydroxide, a by-product of their manufacturing process, into settling ponds surrounding the plant. The settling ponds located at Avtex's Front Royal plant in Virginia suddenly started to fill up with sludge and sediment - valuable sediment - since it was determined that if the zinc hydroxide could be reclaimed it could be processed into useful zinc sulfate.
After investigating various methods to recover zinc from the ponds, Avtex decided on a portable sediment removal system that can clean industrial settling ponds while the ponds remain in service. With over 85 acres of zinc hydroxide in seven ponds, the machine's portability was a must.
The equipment, which moves by winching itself along a cable anchored on shore, can operate in water as shallow as 21 inches. A special mudshield surrounds the cutter head, entrapping suspended material and minimizing turbidity.
The principal controls of the sediment remover are hydraulically operated. A patented horizontal cutter, mounted on the head of a boom, is equipped with cutter knives that dislodge and cut up material with scissor-like action. A spiral auger located in the cutter head drives the material into a pump suction intake. It removes sediment in a nine foot wide cut down to a depth of fifteen feet. Sloping sides of lakes or settling ponds can be accommodated by tilting the auger head left and right up to a 45 degree angle.
Powered by a heavily muffled 175 horsepower diesel engine, the 19,000 pound sediment remover literally eats its way through solid materials.
In the first phase of the operation involving reclamation, the portable machines mine and transfer sludge at fairly high concentrations to the Zinc Recovery Plant. The machines pump this sludge through a 2000 foot pipeline at a rate of 1600 gpm to a large holding tank. From the tank the sludge is pumped through heat exchangers. The sludge is then filtered in a large filter press to remove water. The sludge cakes from the press are slurried with sulfuric acid to dissolve the zinc. Additional processing of the slurried cake removes iron and calcium to form the final product: zinc sulfate solution.
In the second or pollution control phase of the application, the units are used to help protect the Shenandoah River. Wastewater at a flow rate of 6000-8000 gallons per minute, or 8-12 million gallons per minute, or 8-12 million gallons per day, 365 days per year, is processed in an extensive treatment facility at the Avtex Plant. The wastewater is first neutralized and clarified. The water flows from the two clarifiers into two polishing basins to settle out any suspended solids not removed in the clarifiers.
The sludge which accumulates in the polishing basins is removed by the sediment removers. Machine operation is critical here, as they remove sludge from the polishing basins without stirring up the solids. If the solids were resuspended, they would overflow into the secondary treatment plant.
The excellent results obtained in the above applications have led Avtex to evaluate the sediment removal system for another use. The unit is being tried as a means of removing fly ash from settling ponds. Currently, the ponds are taken out of service in rotation and the fly ash cleaned out using a dragline and dump trucks. Appreciable savings would result from a successful application in this case.
Reprinted from: Chemical Processing