It is important for scrap yard managers to track the separation performance of their facilities over time, making it easier to monitor losses and determine if problems exist in the processing circuit. Metal losses also can be used to justify adding new separation equipment.
The following calculations can help operators make better decisions regarding equipment purchases and scrap sales and can aid in communicating operating efficiency to employees.
Figure 1, page 94, shows a sample of material that has been fed to an eddy current separator. Although the ratio of metals to waste is above average, this hypothetical sample simplifies calculations.
This sample contains 12 blocks of equal weight. The four silver blocks are pure aluminum, the four red-orange blocks are pure copper, and the four blue blocks are plastic. When separated, the blocks on the yellow background go under the splitter in a negative sort and are sent to a landfill, while the blocks on the aqua background go over the splitter to the zorba product.
The most common measure of separation performance is yield. Yield, as shown in Figure 1, is the mass percentage of feed that ends up in a separated product. Most commonly, yield describes the fraction that is more valuable (i.e., zorba). In Figure 1, zorba yield is the area within the aqua background. Note that this area contains desirable metals and unwanted misplaced plastic. Because the blocks in the diagram are equal in weight, the yield for both the recovered and the rejected fractions is 50 percent. A yield describing the material to be sold as zorba is calculated by dividing the mass of the concentrated material (six blocks) by the mass of the feed materials (12 blocks) then multiplying the quotient (0.5) by 100, for a yield of 50 percent.
Yield changes can be caused by feed changes in feed composition or by equipment malfunction.
Recovery and rejection
Recovery and rejection calculations show the distribution of a specific component in the feed between the separated products.
Recovery and rejection measurements are better indicators of separation performance than yield or metals content in process waste. The valuable metal blocks that have been “recovered” are shown in the aqua area on Figure 2, below.
Zorba recovery—the percentage of recovered metals from the total metals in the feed—equals the mass of zorba metals in the concentrate (five blocks) divided by the mass of zorba metals in the feed (eight blocks), multiplied by 100. For every 100 tons of metals sent to the separator, 62 tons were recovered as zorba; the remaining 38 tons were lost as waste.
The 38 tons of lost metals can be described as rejected, also depicted in Figure 2. The blocks in the yellow area represent those in the waste stream. The zorba metals rejection percentage equals the mass of zorba metals in the tailings divided by the mass of zorba metals in the feed, multiplied by 100. The rejected metals are shown as the yellow area. Once again, the shaded area (plastic) is not used.
However, rejection also describes the amount of plastic in the zorba, or the degree of contamination in the product. Thus, a process with higher plastic rejection could mean higher grade zorba.
Recovery and rejection also are useful for determining metal losses per hour, which can be converted into revenue loss using current market prices.
Grade is the amount of a specific constituent present in a scrap sample, i.e., product purity. Grade is important for sales and processing calculations. The blocks in the unshaded region in Figure 3, above, are used to calculate the grade of the zorba.
Of the unshaded blocks, 50 percent are aluminum, making the zorba 50 percent aluminum. If indicated, the grade also can describe the overall metals content—the sum of aluminum and copper. Aluminum grade is calculated by dividing the mass of aluminum in the concentrate (three blocks) by the total mass of the concentrate (six blocks comprising aluminum, copper and plastic) and multiplying by 100.
Grade also can describe metal content in the waste stream, making waste grade a key parameter for monitoring metal loss.
Separation efficiency, which shows the effectiveness of the process, equals metals recovery in the zorba minus plastic recovery in the metals. Separation efficiency (Figure 4, below) is calculated by subtracting plastic recovery from zorba metals recovery—63 percent (five blocks out of eight) minus 25 percent (one block out of four)—resulting in a separation efficiency of 38 percent. Only the recovery of plastic and that of metals are used in the calculation.
Yield and recovery
In the example shown in Figure 5, above, the feed composition and the material splits change, yet yield stays the same, making it a poor indicator of performance.
While yield is still 50 percent, metals recovery has dropped from 62 percent to 57 percent (Figure 6,above), and overall separation efficiency has dropped from 38 percent to 17 percent. Copper was the only metal where recovery declined, being reduced from 50 percent to 33 percent. With copper being more valuable, this drop in recovery would be a problem.
If only yield is measured and not grade, the processor may not know of the drop in performance. This drop also can indicate a change in material shape, possibly indicating poor shredder performance or a problem with sizing or the separators.
Knowing these calculations and how to use them allows processors to get every penny from their scrap.
As a graduate research assistant at Virginia Tech, Scott Koermer, through an internship at Eriez, Erie, Pennsylvania, researched metals recycling.