The workhorse: why gravity separation still wins on operational efficiency

The efficiency of simplicity

Gravity separation, such as heavy media separation (HMS), exploits the differences in density between materials. Because the process primarily uses water (or a media like ferrosilicon) and mechanical motion rather than high-energy sensors or expensive reagents, the operational efficiency profile is remarkably lean.

• Throughput rates: moving mountains: One of the primary reasons gravity separation dominates in industrial settings is its ability to handle massive volumes. High-tech sorting methods often struggle with "presentation", all materials must be spread thin in a single monolayer on a belt, so sensors or eddy currents can "see" them. In contrast, gravity systems are bulk-processing masters. For small materials, sensors see throughput rates of 1 to 5 tph at best, while 10+ tph are possible with larger material, provided the material can be presented as a single monolayer. HMS systems can easily handle 40 to 50 or even hundreds of tons per hour (tph), and they are designed for 24/7 continuous operation with minimal downtime and minimal user interaction. If there is a significant delta in specific gravity (SG), the HMS separation happens almost instantaneously within the medium, similar to what you see when you dump a bucket of Styrofoam and rocks into a tub of water.

• Effective across broad size range: HMS effectively separates across a broad range of sizes in bulk, from 4mm (5/32^nd-Inch) to 150mm (6-inches). Sensor separators require narrow size ranges to optimize effectiveness: the smaller the range, the better. Additionally, machines are optimized for specific size ranges, and one machine cannot separate across broad size ranges. A sensor separator designed for large material will not separate small material because (i) the piece may not be large enough for the sensor to detect it, and (ii) the air ejection nozzles near the head pulley would likely impact multiple pieces on the belt, causing product contamination. Conversely, a sensor separator designed for small material may not have the ejection force necessary to separate larger pieces. Sensor separators also see deterioration in recovery efficiency as particle sizes go below 25mm, because of detector/ejection resolution limitations. No technology matches HMS for separating broad size ranges at once.

• Separation/recovery efficiency: The separation efficiency of HMS is nearly perfect. Using the example above, if you dump a bucket of Styrofoam and rocks into a tub of water, all the rocks sink and all the Styrofoam floats. The separation is a function of the density of each piece, nothing else.
  
  Eddy currents also separate based on physical properties of each piece (conductivity and density), provided that feed material is presented as a single monolayer on the belt. Additional complications occur because all materials respond differently to eddy currents based on (a) density, (b) chemical composition of the piece (brass responds less than aluminum), (c) the shape of each piece, and (d) the moisture of the bulk material (no shredder residue operator likes rainy days). This means that the ejection trajectories for each component in these complex mixtures overlap with the ejection trajectories of all other components in the mixture. The operator chooses the specific positioning of the eddy current’s splitter plate, trying to ensure the product is sellable, while maximizing metal recovery and minimizing metal loss to waste. Invariable recovery is less than perfect, particularly for less eddy current responsive metals (i.e., copper and brass). For large sized aluminum, 95-99% recovery is achievable; but for smaller sized materials recovery rates can fall below 50%, especially for less reactive metals like copper or brass. Like eddy currents, sensor separators require monolayer presentation, but also require that each piece is a match for the sensor and air jet resolutions. Across materials >25mm or 1-inch, recovery efficiencies can approach 80% or more; but recovery efficiencies decrease rapidly with piece size. No comparison to the near perfect separation seen with HMS.

• Product purity: HMS is perfect on density, but product purity depends on the infeed mix. If the infeed to an HMS is magnesium and aluminum, the separation will be nearly perfect. If the mixture is rocks, glass, aluminum and magnesium, the magnesium will be separated perfectly, but the other three materials will be mixed. Eddy currents suffer a similar fate. If the infeed material includes magnesium, aluminum, rocks and glass, the magnesium would be mixed with the aluminum, separate from the rocks and glass. When attempting to capture several different metals from a complex stream like ASR, it is a well-known fact that when you make eddy current adjustments to improve the metal content of your product (purity), you lose metal recovery. Similarly, when you make adjustments to improve metal recovery, you lose purity. The trick is to take advantage of HMS as a process enhancer; combining HMS with eddy currents can produce results that are almost magical.

• Automation & ease of operation: Today’s modern HMS systems are fully automated: sensors and PLC controls assure routine, optimal performance from when you turn the system on Monday morning, till you turn it off on Friday. You can expect nearly 100% separation, 95% plus uptime, and all of this without manual adjustments. This is a break from HMS systems of the past…and it’s even a contrast to modern eddy current and sensor systems of today. When processing complex mixtures like shredder residue, eddy currents need regular inspection. Sometimes splitter plates need cleaning, and when materials change or get wet from rain, eddy current settings and splitter plates often need adjustments. HMS systems are automated.