July/August 1989
What’s the best way to sort your scrap metal? Is visual testing satisfactory for your needs? Or should you invest in wet lab testing? This article presents a range of metal sorting options, including some new developments in
By Jerry Cohen
Jerry Cohen is president of Atlantic Stainless Co., Inc., North Attleboro, Massachusetts.
How much more simple could it be? First you take an unknown sample and clean a small area free of dirt, rust, grime, and other surface contaminants. Then you place a hand-held box (approximately 6 inches by 8 inches by 2 inches) with a self-contained trigger on the clean surface of the unknown material. You hold the trigger down for five seconds. When you release the trigger you see either a chemical analysis or a specific alloy on a screen enclosed in another larger box.
That's all you must do to determine the analysis of an unknown sample. Could it be any more simple? However, unless you can afford to spend approximately $40,000 or more to purchase the equipment, you will not find sorting material that easy. But there are many alternatives.
In choosing the equipment best suited for you, the overriding consideration is available cash. The more funds you have to spend on sorting techniques, the more easily and the more accurately you can sort scrap. From the simplest and least expensive testing, which starts with making sensory determinations, to the most elaborate and expensive wet laboratory, there are many types of equipment and procedures from which the scrap metal processor can choose.
To determine what is best for your situation, first evaluate the quantity of material that should be tested. This estimate should help you decide approximately how much money you want to spend on sorting methods and equipment. It is important to keep in mind that some of the equipment to be discussed has an ongoing cost. For instance, the X-ray and optical emission equipment have sources and electrodes that must be replaced regularly.
Before making a decision about equipment, make sure you speak at length with processors who currently own the equipment. Their testimonies, based on actual use, are essential in evaluating purchases.
Visual Inspection
Often, sorting can be easily accomplished or at least facilitated by visual inspection. Once taught, a good sorter can look at certain materials and invariably tell what they are just by the color or shape of the material. If nothing else, it is a start.
So start sorting with your eyes. For example, you may have received a large rectangular grayish tube 10 feet long, weighing hundreds of pounds—a muffler used in a heat treating machine. Most mufflers are Inconel 600 and are standard equipment in these machines. You can visually predict that this material is Inconel 600. It's important to further test this material since the replacement tubes, as well as some original equipment, consist of other alloys, for example, Incoloy 800. In addition, you'll find the ends are often iron, which must be considered in settling with the customer from whom this was purchased.
Magnet Testing
Magnet testing is an extremely useful tool for metal sorters. There are three ferromagnetic metals: iron, nickel, and cobalt. Because of the properties of these ferromagnetic metals, a magnet can be employed to differentiate between 300 and 400 series stainless, R & K Monels, commercial bronze and manganese, or silicon bronze. It is important to understand that some metals will pick up magnetism by cold working. Although 300 series stainless steel is nonmagnetic in its annealed state, once cold-worked the material easily can become highly magnetic. Also, tempered stainless can be magnetic.
Spark Testing
The most generally employed technique for sorting is spark testing. A portable die grinder or a stationary table grinder run by an experienced spark tester is a valuable tool at an extremely inexpensive price tag.
Different elements cause the sparks to have different characteristics. For example, nickel will cause a suppression of the spark stream, slight bursts, and the appearance of forked tongues. In contrast, tungsten suppresses the effect of all other elements and imparts a reddish orange color in greater amounts proportionate to the percentage of tungsten content.
You may frequently wish to differentiate between Type 316 and 18/8 Stainless Steel. By sparking the stainless sample, you can observe the difference by looking at the stream of sparks and determining whether there are star bursts at the ends of the stream, indicating the material is 18/8. However, if the stream is perfectly straight without evidence of star bursts, you might assume the presence of Moly, ergo Type 316 Stainless.
A trained eye also is required to spark test for Type 309 and Type 310 Stainless Steel. Both alloys initially spark red, Type 309 quickly turns yellow, and Type 310 eventually turns yellow. The difference is not distinctive to the novice.
The more experience the person doing the spark testing has, the more efficient and accurate he will be. Because of the development of optical emission, X-ray emission, and thermoelectric devices, the art of spark testing is becoming more and more a thing of the past. Unfortunately, we are no longer training sorters to spark test metals. With our increasing dependence on machinery, sorters are becoming a lost breed.
Chemical Spot Testing
Another widely used form of testing unknown metals, until recent years, has been chemicals, particularly acids. A sample can be tested by placing one drop of one or more chemicals on the clean surface of the metal. By observing the reaction or reactions between the chemicals and the metal you can make a relatively accurate deduction as to the identity of that metal. It often is helpful to use both magnet and spark testing before chemical spot testing to eliminate certain possibilities as to the type of metal. By eliminating certain metals, the tester can limit the number of chemicals necessary to make a determination.
Chemical spot testing is very inexpensive but requires a great deal of training. Even the trained eye can be easily fooled. One example might be in testing a sample thought to be K Monel by placing one drop of concentrated nitric acid on the clean surface of the metal. The nitric acid turns a greenish color on K Monel. On 70/30 Cupro Nickel the acid turns a bluish color. These two colors can be difficult to distinguish except to the well-trained eye.
For the most part, chemical spot testing is a reasonably accurate tool. It is rather time consuming, so it cannot be used for high-production sorting.
Wet Laboratory Testing
The most expensive testing procedure is wet laboratory testing. The cost of maintaining a complete wet chemical laboratory, including the cost of employing a full-time chemist, is prohibitive for most scrap metal processors. But wet analysis is certainly the most accurate as it gives very close quantitative results.
Thermoelectric Testing
Thermoelectric machines have limited usefulness in a scrap facility. These devices measure the "potential difference" between an unknown metal and a heated known surface. By comparing this "potential difference" through a direct-current voltmeter, a judgment can be made as to the chemical analysis of the unknown sample.
This method is extremely inaccurate because the testing conditions can be so dissimilar from test to test. For instance, the temperature and cleanliness of the surface, and the structure and temper of the material must be controlled. If you use this method, understand its limitations.
Optical Emission Devices
Other than wet laboratory chemical analysis, the procedures mentioned to this point are relatively inexpensive-but they all have their limitations. Over the past couple of decades, new and exciting equipment has been developed that tests unknown materials in a matter of seconds with minimal effort.
Optical emission devices are used for semiquantitative analysis (precise elemental quantities are not possible). Optical emission testing uses an electrical source to vaporize a sample. The vapor is measured for characteristic wavelengths. By programming a multitude of known responses into the device, the machine compares the sample tested to make fairly accurate quantitative responses. The machines employed today are extremely accurate for qualitative responses and accurate enough quantitatively to identify alloys.
Optical emission devices range from mobile units on carts to room-size machines. The cost of the less expensive machine once again carries with it limitations in testing. The more expensive machines are as good as they are expensive. There are continual upkeep costs since graphite probes are used.
It should be noted that chemical, spot, thermoelectric, and optical emission testing are destructive forms of testing, which means that some part of the unknown material is stained, scratched, or burned during the procedure. This is not terribly important for most scrap metal processors. However, if it is important not to mar the surface even slightly, do not use these methods.
X-Ray Emission Devices
One major breakthrough in recent years is a nondestructive form of testing equipment: X-ray emission devices. X-ray emission devices utilize X rays as the excitation source, as opposed to electricity in optical emission and thermoelectric testing. To achieve the necessary release of X rays, different radioisotopes are utilized. As stated earlier, X-ray emission testing is very simple. The results, as in optical emission testing, are semiquantitative, but for scrap processors they are quite acceptable. X-ray emission devices carry with them very large price tags as well as expensive upkeep costs. The source of radiation must be changed regularly according to how often the machine is used--every two years is average.
X-ray emission devices use extremely small quantities of radiation, but you should know that there are some places, particularly some government facilities, that prohibit these units from being carried onto the premises. These units are quite safe-however, they are not always allowed.
What's Best for You?
Choosing any equipment for a scrap processing facility is never simple, and choosing the best piece of testing equipment is no amateur's task. So many issues must be evaluated prior to making the final decision. The first question to ask is, Exactly what do you wish to accomplish with the equipment being purchased? The second (obvious) question is, How much can you afford to spend? Once these two questions are answered, the rest is relatively simple.
Whose equipment do you purchase? You must question the manufacturers, but equally important is to contact the individuals who have used this equipment for the same purposes. There are many avenues to pursue, but knowing your options helps make your final choice a wise one.•
What’s the best way to sort your scrap metal? Is visual testing satisfactory for your needs? Or should you invest in wet lab testing? This article presents a range of metal sorting options, including some new developments in