|The myriad of technical resources used by metal sorting facilities continues to evolve.|
In a society which is increasingly affected by the seesaw of production and consumption, the recycling of all available secondary raw materials is gaining more importance.
However, this recycling is only possible if the available separation technology can produce a high-quality product for the different recycling industries.
For the metals recycling industry, this means that various metal products from different sources and of greatly differing quality have to be upgraded to refinable products to be used as input materials in steel mills and in nonferrous smelters.
Demand for these semi-finished metal products has increased significantly in recent years. Thereby, technical resources used by metal sorting facilities also are increasing.
Downstream system success depends on the right combination of a variety of separation technologies, ranging from traditional magnetic separation equipment to highly technical sensor sorters of various types.
On high-capacity car shredders, advanced ferrous and nonferrous downstream systems offer a good overview of the current state of metal sorting technology. Clearly, the days when zorba and zurik were final products are over.
Since between 70% and 75% 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. Even though these magnetic drums were introduced to the market decades ago, components inside the drums have significant differences because they need to generate a deep-draw field over a preferably high working gap to recover a clean steel product.
Steinert’s Hybrid MTE series is capable of operating with a 450 millimetre (18-inch) working gap, allowing for a high-purity steel product after the first pass, but at the same time offering high recovery rates and minimising steel loss to the nonferrous stream.
Recently XRF (X-ray fluorescence) sensor sorters found their way into the ferrous downstream. Used to remove copper contaminants from shredded steel, the XRF sensor sorter solves the age-old problem of how to reduce copper contents in steel scrap. Copper content has increased to alarming levels over the past years, thus drawing steel mills’ attention to shredded steel products.
XRF sorters perform an elementary scan of the feed material based on specific energy emitted from electrons which change their energy level in the atomic shale model when energized by the radiation, thereby detecting copper within the steel scrap. The copper can, for example, be found in copper armatures, the so-called “meatballs,” which will be detected and ejected by the Steinert XSS-F system despite their unfavorable shape and heavy weights.
After the steel has been removed from the shredded material, the remaining material is processed in the nonferrous downstream. Once neglected in car shredding because steel was thought to be the only sellable product, the nonferrous downstream has evolved to a highly complex array of separation technology. Advanced sensor sorting systems are being used to recover all metals from the auto shredder residue, or ASR.
Where once the main attraction at a shredder yard was the monstrous shredder itself, some massive nonferrous systems have recently stolen the show. If someone is lucky enough to visit a yard with this type of installation, they are usually wowed not by the size of the shredder but by the nonferrous system, clearly demonstrating the importance and future of metal separation.
Today’s nonferrous metals separation should always start with some effective screening processes to prepare the feed material into different fractions. This greatly enhances the efficiency of the following separation equipment. Largely accomplished by reliable trommel screens, today more and more screening technology is working in many yards.
Based upon the dual-vibration principle, many other types of screening equipment are hitting the yards because of their more efficient screening abilities, especially when it comes to the finer screen cuts and when processing wet material, an important factor because most yards still store and process material in the open.
With the material being screened and split into different size ranges, material continues to the first sets of separation equipment, usually being combinations of single- and dual-stage magnetic preseparators and eddy current separators (ECS). Magnetic preseparators prepare the material to be more effectively separated on the following eddy current separators which separate the nonferrous metals (aluminium, copper, brass and zinc) from the residual material. Different magnetic rotors can be used in each Steinert ECS to enhance its performance in the respective particle size range, thus reaching recovery rates of up to 99% by weight.
The mixed nonferrous product coming from the ECS, 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, leaving a high concentrate aluminium.
Aluminium shredders and smelters take this separation one step further, adding aluminium alloys containing copper and zinc to the list of metals being detected and ejected. Thereby they are able to produce high quality aluminium series products as input materials for the smelter.
Steinert’s XSS-T X-ray transmission system is used by several aluminium shredders to not only eliminate heavy nonferrous metals but also to distinguish 2000-series aluminium (major alloy copper) and 7000-series aluminium (main alloy zinc).
With today’s increasing demands on the infeed material of an aluminium smelter going down to less than 0.01% allowable zinc and copper in the aluminium melt, the XSS-T is designed to reliably achieve these high standards. Thus the XSS-T generates additional profit for the customer through premiums being paid on the post XSS-T aluminium product.
After the nonferrous metals have been separated on an ECS, what’s left in the drop is basically all of the nonmetallic material plus two types of metals which are not influenced by magnets nor ECS: stainless steel and insulated copper wire (ICW).
To recover these two metals and turn them into sellable products, Steinert has developed a three-step philosophy using its ISS and KSS sorting systems. The first of the three steps uses an ISS induction sorting system to remove the remaining metals from the ECS-waste stream, including stainless steel and ICW. Set to a high sensitivity, this machine has been known to leave less than 1% 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 to eject the stainless steel to produce a high quality zurik product.
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 detecting not only metal signals but also measuring shape and volume of each particle in the stream. This is 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 which are then used to separate the ICW.
A recently commissioned KSS-unit in the U.S. Midwest recovers more than 90% of the present ICW into the final ICW product.
Because copper prices remain high, many larger plants worldwide are pushing the envelope on recovering ICW. Today many large plants operate multiple machines in lines dedicated solely to this one purpose. These ICW lines are often found in the 20-50 millimetres (¾-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 can be used for other metals sorting applications, such as sorting airbags from zorba or zurik or coins from mixed nonferrous heavy metals.
The Shape of Things to Come
Digging deeper into the various alloys, for example of aluminium and stainless steel, is one of the trends for the future of metals separation. The new rule behind separating material appears to be 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. With these alloys finding their ways into the metals recycling industry, the road of metals sorting is paved with new challenges to obtain high-quality products from each separation step.
As a full-range supplier of sorting equipment, Steinert is prepared for these tasks. The combination of several sensors to define as many physical and chemical properties as possible is a key philosophy.
Stephen Krehla is an application specialist and regional manager with Steinert Elektromagnetbau in Cologne, Germany. He can be contacted at firstname.lastname@example.org.