Viability of the Copper-Alloy Scrap Stream
The restricted use of lead in public water systems starting in the U.S. in 1986 led to the development and use of lead-free alloys. While the new lead-free alloys meet federal and state requirements, they also will eventually affect the scrap stream for copper castings.
Scrap is vital to the business case and sustainability of all brass products. Brass rod and brass ingots for castings are made almost entirely from recycled content (Fig. 1). Maximizing the use of low-cost feedstock gives the most economic value and reduces carbon footprint. Brass turnings, for example, can be reclaimed for 75-90% of the original value, offsetting initial raw material cost and contributing to sustainable design practices.
While drinking water system components must comply with lead-free regulations, other copper markets exist without those standards. Brass parts used in construction, industrial machinery, transport equipment, and electronics may contain small amounts of lead.
Adding small amounts of lead to copper and brass alloys can greatly improve manufacturing properties. It helps break chips during machining, lubricates tools, increases corrosion resistance and fills microscopic voids in castings.
So, the brass scrap stream now contains leaded alloys and lead-free alloys that contain new elements not before present on a significant scale. It is no longer relatively homogeneous, and cross contamination can introduce harmful impurities.
New Elements Added
In response to the lead-free regulations, several types of wrought and cast alloys are now available, with different costs, performance and design properties. Some are binary alloys in which no lead or alloying elements are added. These alloys contain copper and zinc, only. Others are tertiary alloys, where lead is substituted for other elements, such as bismuth, silicon, sulfur and tin.
Certain elements can act as deleterious impurities, depending on the scenario. Trace amounts of impurities can cause serious issues:
- High residual stress leading to product failures (e.g. stress corrosion cracking).
- Production failures during hot-working.
- Altered mechanical properties that affect material performance.
Strict segregation of leaded and certain types of lead-free scrap is necessary. Scrap from lead-containing alloys must be kept separate from lead-free scrap, and different types of tertiary alloy scrap must be kept separate from each other and from any other alloy. Scrap from binary alloys can be recycled into any other brass alloy.
Cross-contamination of the brass scrap stream is a complex issue. Scrap is an internationally traded commodity, and there are also limitations of secondary refining technology. Segregation of primary scrap sourced directly from manufacturers can be managed with professional care, but end-of-life scrap and scrap sourced from dealers can be more difficult as alloys are indistinguishable visually.
Fortunately, impurities are not showing up in large quantities yet, and the long service lives of plumbing components is delaying impact. However, the issue could accelerate as more end-of-life lead-free scrap enters the stream.
The brass scrap stream is a finite and limited resource. Increasing contamination issues could hinder future access to clean charge material and potentially lead to the stockpiling of unusable scrap. An increased dependence on higher scrap grades or cathode as feedstock would increase the cost of the material, and degrade the competitive position and sustainability of brass products. The industry will need to enhance monitoring programs to scrutinize incoming scrap and suppliers, and upgrade laboratory testing equipment to detect impurities.
To mitigate impurities in important primary and secondary brass scrap streams, the Institute of Scrap Recycling Industries (ISRI) introduced new purchasing specifications in 2016. The goal was to implement harmonized industry specifications to control for bismuth, silicon and lead impurities in both leaded and lead-free brass scrap grades. The following ISRI specifications were approved and are now being put to use.
Leaded brass scrap turnings: NASCENT
Shall consist of scrap borings and turnings alloyed with copper, zinc and lead. Turnings shall be unmixed and have less than 0.01% alloyed bismuth and silicon each and other impurities as agreed between buyer and seller.
Leaded brass scrap rod ends and forgings: NICHE
Shall consist of scrap rod ends and forgings alloyed with copper, zinc and lead. Solids shall have less than 0.01% alloyed bismuth and silicon each and other impurities as agreed between buyer and seller.
Lead-free, bismuth brass solids: EBULLIENT
Shall consist of scrap castings alloyed with copper, tin, bismuth and zinc. Castings shall be free of leaded brass attachments and have less than 0.2% alloyed lead or as agreed between buyer and seller. Examples that meet this specification include, but are not limited to, CDA 89833/35/36/37/41/42 and 45.
Lead-free, bismuth brass turnings: ECSTATIC
Shall consist of scrap borings and turnings alloyed with copper, tin, bismuth and zinc. Turnings shall be unmixed and have less than 0.2% alloyed lead or as agreed between buyer and seller. Examples that meet this specification include, but are not limited to, CDA 89833/35/36/37/41/42 and 45.
Beyond new specifications, handheld analyzers such as X-ray fluorescent spectrometers are a common tool used to check and verify the chemistry of incoming scrap through representative sampling. While these devices are useful, there are practical limitations as it is not possible to catch all impurities in large shipments.
On a larger scale, advancements in automated sensor-based sortation equipment may offer new solutions to separate some forms of mixed copper alloy scrap. Cost, throughput, and preparation of input material are important factors that influence the value proposition for advanced sortation technology.
Copper alloy scrap provides about one-third of the copper consumed in the U.S. each year. Copper alloy cast products depend on this scrap stream for production of 100% of the ingot and other furnace charge metal. As the copper scrap stream becomes affected by new types of alloys being recycled, the industry should be aware of the challenges this can pose and collaborate on solutions.
Click here to see this story as it appears in the March 2019 issue of Modern Casting