Magnetic separation is separating components of mixtures by using different types of magnets to attract magnetic materials. The process that is used for magnetic separation detaches non-magnetic material from those that are magnetic.
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Magnetic separation can be used in different environments and markets, so are magnetic separators used primarily for recovering metal from waste or purifying secondary materials by removing metals. But magnetic separators are also used in electromagnetic cranes that separate magnetic material from scraps and unwanted substances. Magnetic separation is also used in the mining and mineral industry and plays a small role in the food and pharmaceutical industry. For example, to remove metal contaminants from product streams.
How is a magnetic separator used in recycling applications?
A magnetic separator consists of a powerful electromagnet placed or suspended from a ceiling or device. Materials can be passed over a tabletop magnetic separator, while suspended magnetic separators often hover over material to remove imperfections. Magnetic separators can also be cylinders that objects pass through.
In the recycling industry, magnets are commonly used to attract ferrous materials, such as tin, iron, steel, and many more. Magnets are found along assembly lines where they will be placed either above or below the conveyor belts to attract said materials or minerals.
The cost of biosolids storage, transportation, and end-use or disposal can significantly impact the choice of decanter centrifuges to perform thickening or dewatering functions. Generally speaking, sludge thickening before dewatering will reduce the tankage needed for storage by removing water; dewatering works to remove more water producing a drier cake material (source: EPA - Centrifuge Thickening and Dewatering of Biosolids).
According to ScienceDirect, Sludge thickening commonly produces sludge solids concentrations in the 3% to 7% range, which results in an 80% volume reduction. Several factors determine whether a sludge thickening system is sufficient in the treatment of biosolids. These include:
Types of Sludge
* Traditional decanter centrifuges used for sludge thickening can work using little to no polymer but require higher energy consumption or the capacity of the equipment is limited. Other thickening technologies (rotary drum, gravity belt, DAF) need polymer to produce 4-5% solids concentration
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**The Centrisys THK Sludge Thickener will achieve 4-5% solids concentration using no polymer, with 50% less energy consumption and increased capacity compared to traditional decanter centrifuges.
Sludge thickening using a decanter centrifuge before a digestion treatment process will reduce the size of the digester, and can be used before sludge storage and liquid land applications. Depending on the utilization of the sludge product (landfilled, land applied or dried) the drier the product the more cost effective it will be for storage, transportation and disposal.
Learn More >> Centrisys Sludge Thickener THK Series for more details and advantages of this type of a decanter centrifuge for sludge thickening.
Mechanical dewatering with a decanter centrifuge can result in a 95% reduction in volume and a concentration of between 15% to 35% dry solids, compared with 80% volume reduction and solids concentration of 3% to 7% with a sludge thickening-only operation. By removing more water and thereby producing a drier cake product, dewatering will offer significant savings in treatment, handling, and disposal costs.
Advantages of dewatering include the following (source: EPA - Biosolids Technology Fact Sheet on Centrifuge Thickening and Dewatering):
Learn More >> Centrisys Dewatering Centrifuge CS Series
In a Centrisys Dewatering Centrifuge, the more the equipment runs, the more fine-tuned the set-up becomes. Not only does the centrifuge controller automatically start the centrifuge system, the control package also allows the operator to preselect what mode the centrifuge should operate in. A Centrisys decanter centrifuge is typically 95% optimized within 45 minutes of start-up. The Rotodiff® hydraulic backdrive gives instant feedback (via a pressure correlation) of the cake solids level more accurately than can be determined by visual observation. Once the desired pressure is achieved, the Rotodiff hydraulic backdrive maintains the cake solids level in spite of changing feed concentration via an integral proportional controller. A plant operator will need to occasionally observe the centrate and adjust the polymer accordingly, but the system stays optimized with very little operator attention.
Process Performance. Municipal sludge and performance expectations are difficult to fully classify due to wide-ranging variations of different processes as well as domestic and industrial contributions to the biomass.
There is a need to:
Sludge definitions. To better quantify performance, limits need to be set and terms better qualified. All sludge types as defined below assume industrial contributions are less than 20% of the final dewatered solids concentration and that conventional wastewater treatment processes are employed. Chemical additives (such as potassium permanganate used in odor control) are assumed not to significantly affect sludge conditioning.
Raw Primary The feed solids are assumed to come off the bottom of a primary clarifier and therefore have a consistency of 2 &#; 7% ts. For thickening before anaerobic digestion or tanker hauling, cake at 5 &#; 10% ts is specified at +95% recovery and is easily obtainable. Waste Activated Sludge Most feed solids vary from 0.4 &#; 2.0% tss. Sludge thickening without polymer yields a 4 &#; 6% cake for recovery specs of 85 &#; 90%. Polymer is required at higher level of recoveries and/or while thickening at 7 &#; 10%. Dewatering and high solids dewatering typically occurs at 90 &#; 95% recovery levels. Raw Mixed Primary / Secondary Sludge Various combinations of mixed primary and secondary sludge are usually found at a concentration between 3 &#; 6% tss. This analysis assumes a 50:50 blend of sludge types. Recovery levels of +95% are typically achieved using polymer for all modes of separation. Mixed Anaerobically Digested Sludge Assuming a 50:50 blend of primary and secondary sludge to the digester, feed solids at 2 &#; 4% tss typically result in specifications at +95% recovery with polymer. Aerobically Digested Sludge For aerobically digested sludge, feed solids at 1 &#; 2.5% tss typically result. Polymer is used to effect separations at 90 &#; 95% recovery.
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