In a society that is increasingly affected by the seesaw of production and consumption, the recycling of all available secondary raw materials is gaining importance. However, recycling is only possible if the available separation technology can produce high-quality secondary commodities for the various consuming industries.
For the metals recycling industry, various metal products from different sources and of greatly differing quality have to be upgraded to refinable products suitable for use as input materials in steel mills and at nonferrous smelters.
In the past couple of years, the demand for these semi-finished metal products has increased significantly. To respond to the need for scrap metals to produce these products, scrap metal processing facilities are increasing their use of technical resources, such as shredding equipment and nonferrous processing plants. In fact, we have already seen how nonferrous downstream sorting systems have begun to proliferate following automobile shredders. Less than a handful of years ago, nobody would have ever thought these systems would have been employed to the degree that they are today.
The success of these downstream sorting systems depends heavily on the right combination of various separation technologies, ranging from traditional and proven magnetic separation equipment to highly technical sensor sorters of various types.
On high-capacity car shredders, advanced ferrous and nonferrous downstream sorting systems offer a good overview of the current state of metal sorting technology. Clearly, the time when zorba and zurik were final products are over.
Because from 70 percent to 75 percent of an average car shredder’s output is shredded steel, this metal is the first to be separated in the post-shredder downstream. Typically, underfed electromagnetic drums are used to separate the magnetizable steel from the remaining nonferrous material. These magnetic drums were introduced to the market decades ago. The components inside these drums can vary significantly because they need to generate a deep-draw field over a preferably high working gap to produce a clean steel product.
Drum magnets, such as Steinert’s Hybrid MTE series, can operate with a 450-millimeter, or 18-inch, working gap, allowing for a high-purity, first-pass steel product while also guaranteeing high recovery rates and minimizing steel lost to the nonferrous stream.
Recently XRF (X-ray fluorescence) sensor sorters found their way into ferrous downstream sorting systems. Used to remove copper contaminants from shredded steel, XRF sensor sorters are designed to solve the age-old problem of how to reduce copper content in scrap steel. Copper content has increased to alarming levels during the recent past, thus drawing the steel mills’ attention toward shredded steel products.
XRF sorters perform an elementary scan of the feed material based on specific energy emitted from electrons, which change their energy levels in the atomic shale model when energized by the X-ray’s radiation, thereby detecting copper within the steel scrap.
Copper can be found in copper armatures on electric motors, so-called “meatballs,” which will be detected and ejected by XRF sorters despite their unfavorable shape and heavy weights.
Shake it Out
After the steel has been removed from the shredded material, the remaining material is processed by the nonferrous downstream sorting system. Once neglected in automobile shredding because steel was thought to be the only sellable product, recovering nonferrous metals via downstream sorting equipment is now an area many auto shredders are pursuing by installing a highly complex array of separation technology. Advanced sensor sorting systems are being used to recover metals from auto shredder residue, or ASR.
Once the main attraction at a shredder yard was the monstrous shredder itself; however, some massive nonferrous systems have recently stolen the show. The scale and complexity of these nonferrous recovery and sorting systems clearly demonstrate where the importance and future of metal separation lies in the traditional shredder yard.
Nonferrous metals separation always should start with an effective screening process to sort the feed material into different fractions by size. This greatly enhances the efficiency of the separation equipment that follows. Largely accomplished using reliable trommel screens, today more and more screening technology is employed in many yards.
Based upon the dual-vibration principle, some screens may offer more efficient screening capabilities, especially in the finer screen cuts and with wet material, an important factor because most yards still store and process material in the open. After the material is screened and split by size, it continues to the first set of separation equipment.
Identifying & Recovering Nonferrous
Usually being combinations of single- and dual-stage magnetic preseparators and eddy current separators, this stage prepares the material to be more effectively separated on the following eddy current separators, which separate nonferrous metals (aluminum, copper, brass and zinc) from the residual material. Different magnetic rotors may be used to enhance an eddy current separator’s performance in the respective particle size range, thus reaching recovery rates of up to 98 to 99 percent by weight.
The mixed nonferrous product coming from the eddy current separator, the so-called zorba, can be further processed on an X-ray transmission system. Based on the differences in the density of the material, the X-ray transmission system can separate heavy metals, such as copper, brass and zinc, from high concentrate aluminum.
Aluminum shredders and smelters take this separation one step further, adding aluminum alloys containing copper and zinc to the list of metals being detected and ejected. Thereby they are able to produce high-quality aluminum series products as feedstock for the smelter. For instance, Steinert’s XSS-T X-ray transmission system is used by several aluminum shredders to not only eliminate heavy nonferrous metals but also to distinguish 2000-series aluminum (major alloy copper) and 7000-series aluminum (main alloy zinc).
By ensuring that the aluminum destined for a secondary smelter contains less than 0.01 percent allowable zinc and copper, processors can realize a premium for their material.
Down to the Wire
After the nonferrous metals have been separated with an eddy current separator, what’s left in the drop is basically all of the nonmetallic waste plus two types of metals that are not influenced by magnets nor by eddy current separators: stainless steel and insulated copper wire (ICW).
To get these two metals out of the waste stream and to turn them into sellable products, Steinert has developed a three-step process using an induction sorting system (ISS) and a KSS sorting system. First, an ISS takes out all of the remaining metals from the eddy current separator waste stream, including stainless steel and the ICW. Set to a high sensitivity, this machine is designed to leave less than 1 percent by weight of metals in the final waste, producing a mixed metals product.
This product then hits a second ISS, fine-tuned to a lower sensitivity, which only ejects the stainless steel, producing a zurik product consisting mainly of stainless steel.
The underflow of the second ISS machine then enters the third and final stage of this processing line: the so-called KSS. The KSS is a combined sorting system capable of detecting not only metal signals but also of measuring shape and volume of each particle in the stream. This can be achieved using the Steinert 3D laser technology in which a camera follows a projected laser line over the surface of each particle to determine its shape and volume. From this information, characteristics such as the shape of a copper wire can be derived and are then used to separate the ICW from the rest of the material.
A recently commissioned KSS unit in the U.S. Midwest recovers more than 90 percent of the present ICW into the final ICW product.
Because copper prices remain high, many larger plants worldwide are pushing the envelope on how to recover ICW. Today, many large plants operate multiple machines in lines dedicated solely to this one purpose. These ICW lines are often found in the three-quarter-inch to 2-inch fraction because the majority of the insulated copper wire present in the ASR ends up in this size range after screening.
The Steinert KSS also is suitable for other sorting applications, such as sorting air bags from zorba/zurik or coins from nonferrous heavy metal mixes.
Things to Come
Digging deeper into the various alloys of aluminum and stainless steel, for example, is only one of the trends visible for the future of metals separation. The new rule behind separating material appears to be rather simple: The cleaner your products, the higher the value.
The metal industry uses an increasing number of specific alloys to enable the manufacturing of components. These components have a value for strength and durability that in the past were thought not to be possible. With this increasing variety of alloys finding their way into the metals recycling industry, the road to metals sorting is paved with new challenges to obtain high-quality products from each step in the separation process.
The combination of several sensors to define as many physical and chemical properties as possible is already a reality. The use of metal signals; shape, volume and surface recognition; and color detection is boosting the capabilities of modern sensor sorters to new levels.
Stephen Krehla is an application specialist and regional manager with Steinert Elektromagnetbau in Cologne, Germany. He can be contacted at firstname.lastname@example.org.