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Case Histories : Wastewater

V-SEP Filtration of Acid Mine Drainage
By Josh Miller
Apr 20, 2005
  E-mail article
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Overview: Since their development as lab filters in the early 1960s, polymeric membranes have grown in the number of uses at exponential rates. Membrane architecture and process design itself has undergone significant advancement. A unique membrane filtration system, known as VSEP (Vibratory Shear Enhanced Process), was developed by New Logic of Emeryville California. The technology employs vibrational oscillation of the membrane surface to improve the relative throughput per area of membrane used. This oscillation is used to prevent colloidal fouling of the membrane surface.

One unique benefit of the shear created by vibrational oscillation is the resiliency of the membrane system against fouling from crystallization of mineral salts. Studies recently conducted have shown that crystallization occurs out in the boundary layer of suspended solids as filtrate is removed and solubility limits are reached. Once precipitated, these insoluble mineral salts become just another suspended solid and can be easily washed from the membrane system with laminar crossflow of the process feed.

With conventional static or crossflow filtration subject to colloidal fouling, mineral scale formation would severely inhibit performance. As a result, these membranes have low tolerance for mineral hardness and would require elaborate pretreatment and chemical dosing to inhibit crystal formation using antiscalants. Even with pretreatment and chemical dosing, conventional membranes would be limited in the % recovery of filtrate that is possible.

It is because of this key limitation that membranes have not been used to a great degree in the processing of Acid Mine Drainage, until now. New Logic's V-SEP has the ability to perform membrane separations not possible using conventional membrane systems. Wastewater treatment systems that are compact, economical, and reliable are now possible for the mining industry.

Mining Regulation

One of the challenges of today's mining operations is that heavy metals which pose a potential environmental hazard are naturally occurring elements in the ore that is removed for processing. For a typical Copper mine, one ton of waste rock can contain several pounds of copper, five ounces of zinc, three ounces of lead, and two ounces of arsenic. On average, the earth's crust has background levels of about 2 ppm of arsenic.

Limits currently exist for heavy metals in industrial wastewater discharge. As a result of the Clean Water Act, the EPA is currently developing new tighter regulations on these metals. Since the average soil contains 2 ppm of arsenic, almost any water that has come in contact with soil and is then discharged to sewer could violate the new regulations.

The mining industry is one of the most heavily regulated by the EPA. Mining does nothing at all to increase the amounts of naturally occurring substances in the rock. Ore removed for processing can contain nearly all of the 650 elements and chemicals regulated as hazardous waste by the EPA. Unfortunately, the simple act of moving it from one place to another qualifies as a "release to the environment."

If you take the parts per million concentrations of controlled substances in the waste rock and multiply them by an average mine's daily tonnage of rock mined, the amounts increase dramatically. A copper mine's total "release" of all TRI reportable chemicals can be approximately 450 million pounds of hazardous materials per year. With the current laws, it makes no difference whether the materials are released to the environment or are stored in government permitted waste rock repositories and tailing impoundments. Either way the movement of the earth must be reported as a "release to the environment"

Many mining companies are forced to clean up historic wastes from mining in the 19th century. Again, the act of moving this material constitutes a "release" according to the EPA's method of reporting, even though the historic mining wastes are being placed in state or federally-approved impoundments that are safer for the environment than if the wastes remained in their current location.

The problem can be even more difficult if the mining involves rare earth metals where Uranium, Radium, or other radioactive elements can be found. As long as the radioactive elements are not disturbed, there is not classification as a hazardous material that needs superfund attention. But if the rock is moved from one place to another, a release of radioactive materials has occurred and must be reported.

Copper Mining Process

Rocks are blasted to break them into smaller pieces and loaded into large trucks for transport to the processing locations. The ore goes either to concentrating and smelting or to leaching and electrowinning. It depends on how much copper and the types of minerals it contains.

In one copper production process, rock that comes from the mine is crushed into smaller and smaller pieces by heavy steel balls in machinery called mills. Concentrating Ground up rock is mixed with water, air bubbles and small amounts of chemicals. The chemicals allow copper minerals to rise to the top and stick to floating air bubbles. The remaining mixture of crushed rock and water called tailing separates from the copper bearing bubbles. The copper minerals are skimmed off and dried to form copper concentrate, a powder-like material.

In the smelter, copper concentrate is melted and copper is separated from other substances in the concentrate. Molten copper is poured into molds called anodes. The unwanted material cools to a glass-like substance called slag. The natural metals that remain in slag are reported under EPCRA.

In an alternate copper production process, rock is taken from the mine directly to stockpiles. A solution of slightly acidic water is dripped on the stockpiles, percolating down through the rock and dissolving copper along the way. The solution containing the copper is collected and piped to holding ponds. In tanks, the copper-bearing solution is mixed with chemicals that transfer the copper to a more concentrated solution called electrolyte. The electrolyte is pumped to steel tanks. Starter sheets hang in the solution and, using an electric current, the copper is plated from the electrolyte on to the sheet, forming 99.99 percent pure copper plates. All solutions used during this process are recycled. Producing copper and other hard metals also takes a lot of water, which is why water management is such a crucial part of any mine's operations.

Vibratory Shear Process

VSEP's unique separation technology is based upon an oscillating movement of the membrane surface with respect to the liquid to be filtered. The result is that blinding of the membrane surface due to the build up of solids is eliminated and free access to the membrane pores is provided to the liquid fraction to be filtered. The shear created from the lateral displacement caused suspended solids and colloidal materials to be repelled and held in suspension above the membrane surface. This combined with laminar flow of the fluid across the membrane surface keeps the filtered liquid homogeneous and allows very high levels of recovery of filtrate from the feed material. In the case of Acid Mine Drainage, up to 97% of the water can be filtered in a single pass filtration using VSEP. Flux is inversely related to % recovery, so the optimum % recovery may vary for each application. Other methods like filter presses are done in batch mode with operators opening and cleaning the filter cake on a regular basis. VSEP is a continuous automated process requiring very little operator attendance.

The industrial VSEP machines contain many sheets of membrane, which are arrayed as parallel disks separated by gaskets. The disk stack is contained within a Fiberglass Reinforced Plastic (FRP) cylinder. This entire assembly is vibrated in torsional oscillation similar to the agitation of a washing machine. The resulting shear is 150,000 inverse seconds, which is ten times greater than the shear in crossflow systems. High shear has been shown to significantly reduce the fouling of many materials. The resistance to fouling can be enhanced with membrane selection where virtually any commercially available membrane materials such as polypropylene, polyester, polysulfone and Teflon can be used.

Each Series i system contains up to 2000 square feet of membrane filtration area. A single VSEP unit is capable of processing from 5 to 200 U.S. gallons per minute while producing crystal clear filtrate and a concentrated sludge in a single pass. This large throughput capability can be accomplished with a system, which occupies only 20 square feet of floor space and consumes 15 hp.

One other significant advantage is that the vibration and oscillation of the membrane surface itself inhibits crystal formation. Just as a stirred pot won't boil, lateral displacement of the membrane help to lower the available surface energy for nucleation. Free energy is available at perturbations and non-uniform features of liquid/solid interfaces. With the movement of the membrane back and forth at a speed of 50 times per second, any valleys. Peaks, ridges, or other micro imperfections become more uniform and less prominent. The smoother and more uniform a surface, the less free energy is available for crystallization. Crystals and scale also take time to form. The moving target of the membrane surface does not allow sufficient time for proper germination and development. Other stationary features within VSEP present a much more favorable nucleation site. Whereas, with conventional membranes that are static, scale formation on the membrane is possible and has plenty of time to develop and grow.

Results using VSEP

VSEP's Reverse Osmosis membrane module is capable of treating Acid Mine Drainage and providing a filtrate, which is free from suspended solids and low in Sulfates and Heavy Metals. The VSEP process does not involve any chemical addition, except for pH adjustment using Lime, and meets the process engineers' needs for automated PLC controlled production.

VSEP modules containing about 1300 SF (120m2) of filtration media are modular and can be run in parallel as needed to meet any process flow requirements. Each 84" VSEP module can produce 20 gpm of clean water from the leachate pond. Since the units are modular and can be used in parallel or in series, the number of VSEPs needed can be calculated based on the amount of material to be processed, (GPD or GPM). At 40C the membrane flux is about 20-30 GFD (Gallons per Square Foot per Day). System throughput is also a function of the extent to which the feed is concentrated.

Process Description

The mining leachate is collected and stored in holding tanks. Lime is added to raise the pH and to precipitate calcium sulfate and other slightly soluble mineral salts prior to filtration. After proper residence time, the Feed Liquor is pumped into the VSEP system for filtration. The viscosity of the material plays a big part in the rate of filtration. Heat will help to decrease the viscosity of the slurry and therefore improves the throughput of the VSEP system. Counter-current heat exchangers and recovery boilers are used to warm the feed material.

The heated leachate is pumped into the VSEP Filter Pack at about 450-psi. The contents of the feed tank are taken out of the side of a cone bottom tank so that settled solids are excluded. The resulting permeate is sent to a process water storage tank for reuse in the operations. The reject material, about 15% of the volume, is sent back to the leachate pond or on to evaporation ponds for disposal.

When the permeate rate drops off, the Filter Pack is cleaned using New Logic's formulated membrane cleaners out of a Clean in Place tank of about 260 gallons. Cleaning solution is recirculated with pressure and vibration to dissolve foulants that have found their way to the membrane. Actual site conditions at various mine locations have shown that the membrane can be cleaned easily and the results from week to week are predictable and stable.

System Components

The VSEP system is configurable for manual mode where the operator would initiate operating sequences, or for full automation including seamless cleaning operations with round robin cleaning or multiple units. The VSEP has a PLC (Programmable Logic Controller) which monitors pressure, flow rate, and frequency. It also provides the safety in operation by monitoring conditions and initiating an alarm shut down should some configurable parameters be reached. The control stand contains the PLC, Operator display and terminal strips for wiring connections to instrumentation.

The Filter Pack is mounted on the VSEP base unit and contains about 1300 SF, (120m2), of membrane area and is constructed out of high temperature materials. The VSEP drive system, which vibrates the Filter Pack, is engineered using space age alloys and materials to withstand the applied stress from a resonating frequency of about 50 Hz. Each base unit is fully stress tested and the factory prior to shipment. The VSEP drive system is made up of the Seismic Mass, Torsion Spring, Eccentric Bearing, and Lower Pressure Plate.

Project Economics

The table below shows the operating costs for the installation of one VSEP module as currently configured. The VSEP is uniquely energy efficient. It comes with a 20 HP dive motor and a 10 HP Pump Motor. Operators interface and maintenance is limited to starting and stopping the unit and a periodical cleaning of the membrane after an extended run. The membrane replacement is the largest operating cost and it is estimated that the life of each module is approximately 2 years. Operator care can improve the life and additional savings could be yielded if the Filter Pack lasts more than 2 years.

Company Profile

New Logic is a privately held corporation located in Emeryville, CA approximately 10 miles from San Francisco. New Logic markets, engineers, and manufactures a membrane dewatering and filtration systems used for chemical processing, waste streams, pulp & paper processing, mining operations, and drinking water applications.

For additional information visit: http://www.vsep.com/

Copyright 1998 - 2012 Water and Wastewater.com

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