“Once upon a time….” The early life and modern developments of the Hammer Mill.
Hammer mills were created to shred a variety of scrap metal including collected scrap, ELV cars as well as large household appliances (white ware and WEEE). From the middle of last century, we drew our inspiration to create the first hammer mills from the application of well-established technologies such as those found in rock and mineral crushers.
Only from the late nineteenth and early twentieth century the first applications of articulated hammer mill appeared. The percussion technique used in hammer mills exploits the effects of violent and continuous impacts of the rotating hammers on the materials as well as the materials themselves spinning and impacting within the mill chamber. The continuous impact on the introduced material causes a progressive reduction in size.
These machines use movable percussion bodies (hammers) and fixed impact plates, among which materials are beaten and bounced a number of times within the drum until the material is reduced in size sufficiently small enough to fall through the mills grid.
The term “hammer mill” and the use of such technologies was first introduced in the United States around 1958.
At that time, the first attempts were made to apply the shredding technique to recover the iron from scrap cars destined for demolition. This process allowed recyclers to obtain a supply of iron with an average density of 1 t / mc.
Steel mills at the time were demanding a high quality of scrap metal and the market had thousands of tons of cars to be demolished. The cars were transferred to the mills, either whole or flattened, to reduce storage and transport costs.
Materials such as glass, synthetic materials and chrome were constantly present in the composition of the car; and such materials do not conform to the scrap requirements of the still mills which classified them as contaminants.
In addition, the cars to be processed contained considerable amounts of chrome which, at the time, was used to fill dents and in repairs. Tin is particularly damaging when introduced to the steel production process, especially when producing thin metal sheets.
The negative effects of adding contaminants to the melting mix only got worse after the old style Martin-Siemens furnaces were replaced with the most advanced BOF (Basic Oxygen Furnace). BOF furnaces use less time during the casting process which means there is not enough time to melt an entire car bale. Steel mills also demanded scrap that was devoid of impurities which are found in scrap bales.
Contaminants in the iron contribute to the production of slag. Slag is a term used to describe the granulated impurities left at the bottom of the furnace after the melting process. Slag has very little value, is costly and time consuming to remove so reducing the build up of slag by introducing a purer iron scrap to the furnace is desirable.
During 1955, in the United States, at least 14 million cars were abandoned at collection centers because the scrap value of the cars was not enough to justify their collection, storage and transfer to the foundries.
Collection centers throughout the US quickly started to overflow and the federal government of the United States was forced to approve a national campaign aimed at eliminating the problem.
Around this time, Mr Sam Proler, inspired by the requests of the steel mills to supply them with purer high quality scrap supplies, builds a large HAMMER MILL which he uses to process used cars. The end result was a clean, pure iron fragments which was known in the industry as “Proler”.
At the same time, the LURIA BROTHERS, then the largest scrap company in the United States, builds its own mill and called it LURMET. The LURMET mill uses a vertical feeding shoot to feed the cars to the mill. The car body is placed directly on top of the rotor from above which requires a disproportionately large amount of effort to process effectively. Another shortcoming of the LURMET mill is that continuous feeding of the mill was not possible which caused large fluctuations of power requirements and big inefficiencies of then 4000 HP engines.
The first generation of hammer mills were of the Bottom Discharge type where by the crushed material is expelled from a grid mounted under the rotor.
Newell Industries, a well known Shredder manufacturer also built its mill in 1959 and which steel mills to process entire cars. The Newell shredder was the first to use the TD or Top Discharge system, which ejected shredded material from the top of the shredder. Top discharge was possible due to the introduction of horizontal feeding. Horizontal feeding of the mill from the side placed much less stress on the mill and allowed for a more even shredding process with less power peaks.
Not long after inventing Horizontal feeding Newell introduced the TBD system also known as “Top and Bottom Discharge”. TBD systems allowed shredded material to be discharged both from the top and the bottom of the mill. The first versions of the TBD hammer mills demonstrated that is was possible to process also the difficult to shred parts of cars such as engines, gearboxes, semi-axles, springs and whole gearboxes, with the added advantage of separating ferrous from non-ferrous metals.
Further evolutions of the Hammer mills allowed shredders to process a much wider variety of scrap due mainly to the ability to build larger rotors with larger and thickener discs combined with heavier hammers. The integral parts including the grinding grids and internal ware plates of the shredding chamber are made from ware resistant steel.
The shredders evolutionary process reached its current state when, with the adoption of heavy rotating masses and high power engines, entire car bales were able to be shredded.
The bales however should not exceed certain dimensions and should not be too compressed. Finally, with the adoption of modern hammer mills, metal recyclers were able to meet the demands of the steel mills at a price that was not only cost effective for the mills but also returned a healthy profit to the operators.
Hammer mills continued to evolve, getting even more powerful with the goal of minimizing “per ton” production costs. Experimenting with new rotors and different loading systems to reduce the power while also increasing the performance and efficiency of the shredders also took place.
After optimizing the performance of the hammer mills, manufacturers turned their attention to introducing post shredder technologies and full automation of the entire process.
These capabilities were achieved through the use of feed and transfer system, scrap transportation units, dust removal, metal recovery and post shredder separation systems.
The entire complex line of machines and systems is controlled and managed by a sophisticated computer program to be as automated and simple to use as possible.
Today and in the future, recycling machine manufacturers efforts are aimed at seeking economic solutions even for the low value post sortation shredder waste also known as shredder fluff. Shredder fluff can be as much as 30% of the total output. 30% is a significant figure which, if not reduced through new technologies needs to be disposed in landfills at a high cost both to the recycler and the environment.
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