Past and Future
The industry’s voluntary product stewardship initiatives to collect and recycle cadmium containing products would contribute to the sustainable and safe use of cadmium in modern society.
Cadmium is generally characterized as a soft, ductile, silver-white or bluish-white metal, and is listed as 64th in relative abundance amongst the naturally occurring elements. Cadmium is found principally in association with zinc sulfide based ores and, to a lesser degree, as an impurity in lead and copper ores. It is also found in sedimentary rocks at higher levels than in igneous or metamorphic rocks, with the exception of course of the nonferrous metallic ores of zinc, lead and copper.
It was first discovered in Germany in 1817 as a by-product of the zinc refining process. Its name is derived from the Latin word cadmia and the Greek word kadmeia that are ancient names for calamine or zinc oxide.
Naturally-occurring cadmium-sulfide based pigments were used as early as 1850 because of their brilliant red, orange and yellow colors, and appeared prominently in the paintings of Vincent Van Gogh in the late 1800s. Germany was the first and only commercial producer of cadmium metal for industrial applications up until World War I. Thomas A. Edison in the United States and Waldemar Junger in Sweden developed the first nickel-cadmium batteries early in the 20th Century. However, the most significant early use of cadmium was as a sacrificial corrosion protection coating on iron and steel.
Cadmium is often described as a “heavy metal”, but the term has no meaning in fact since it possesses an average density, atomic number and atomic weight compared to other metals. The density of cadmium is 8.64 g/cm3 at room temperature compared to 1.85 g/cm3 for beryllium, the lightest of the metallic elements, to 19.3 g/cm3 for gold or tungsten, amongst the heaviest of the metallic elements. The atomic number for cadmium is 48, again right in the middle of the range of 4 for beryllium and 92 for uranium amongst the naturally occurring metallic elements. The atomic weight of cadmium is 112.1, which once again is very near the average atomic weight of the metallic elements. Beryllium is the lowest at 9 while other metallic elements with high atomic weights include lead at 207, bismuth at 209, and Thorium and Uranium at 232.
The effect of cadmium exposure on human health has been studied extensively since the 1950s following the occurrence of the painfull Itai Itai bone disease in Japan as the result of post-menopausal women with low iron and zinc levels ingesting cadmium-contaminated rice.
Exposure to certain forms and concentrations of cadmium is known to produce toxic effects on humans. Long-term occupational exposure to cadmium at excess concentrations can cause adverse health effects on the kidneys and lungs. Adverse human health effects have generally not been encountered under normal exposure conditions for the general population except in areas of historically high cadmium contamination. The potential risks from cadmium exposure have been extensively studied, and are now tightly controlled by occupational exposure standards, regulations for cadmium in ambient air, water and soil, and legislation covering cadmium emissions, labelling and disposal of cadmium-containing products, and impurity levels in other products such as fossil fuels, fertilizers and cement.
the Present and the Future
Most cadmium metal today is produced as a by-product of the extraction, smelting and refining of the nonferrous metals – zinc, lead and copper. As an alternative to environmentally sound disposal as waste, engineers have been able to utilize its unique properties for specific industrial applications. Cadmium is also produced from the recycling of spent nickel-cadmium batteries, its largest use, and secondary or recycled cadmium now accounts for about 23% of total cadmium supply.
Cadmium will invariably be present in our society, either in useful products or in controlled wastes. Today, the use of cadmium in the EU is restricted by the REACH and RoHS regulation to professional and industrial use, a trend that gradually expands to other parts of the world.
This minimizes the risk of unsafe use and inappropriate end of life discharge. Its health effects are well understood and well regulated. Occupational exposure in the EU has drastically reduced over the past decades and the health risk is now virtually zero . A further restriction or ban on the use of cadmium will have insignificant effect on human health.
Nickel-cadmium batteries are essential and irreplaceable in many industrial applications, particularly those requiring highest reliability for high power and long cycle lives at both high and low temperature. As such, Ni-Cd batteries are making important contributions to the safety of electrified public transportation, thereby contributing to the improvement of urban air quality. The materials in recyclable Ni-Cd batteries can be more than 99% recovered for reuse in the production of new Ni-Cd batteries. World-wide initiatives have been undertaken in Japan, North America, Europe, Australia and the OECD to promote the collection of Ni-Cd batteries for recycling, thereby improving overall recycling rates.
Cadmium pigments are important additives in certain specialised plastics, glasses, ceramics and enamels to achieve bright colours along with long service lives, even in very demanding applications. From an ecological point of view, it is important to develop and maintain functional products with long service lives, once again to minimise the input into the world’s waste stream. Inferior substitutes which produce shortened service lives will ultimately only increase the volume of the world’s waste. It should also be emphasised that cadmium pigments are very stable chemicals, highly insoluble, and embedded in the product’s matrix.
In most regions, Cd based PVC stabilisers have been replaced by less hazardous alternatives and are disappearing from the recycling loops. This trend is likely to continue.
Cadmium coated components, likewise, provide outstanding corrosion resistance along with low friction coefficient, low electrical resistivity, good galvanic comparability, good plating coverage, ability to coat a wide variety of surfaces, and good braze ability and solderability. For these reasons, cadmium coated products are preferred for a wide range of critical and safety related applications in the aerospace, electrical, defence, mining, nuclear and offshore industries. In addition, cadmium coating wastes and products are easily recycled.
In newer applications, like Cadmium Telluride based photovoltaic cells, CdTe is embedded in a matrix with extreme low solubility. Dedicated recycling processes have been put in place to recover cadmium from production waste and end of life cells. This ensures that new applications of cadmium are sustainable.
The recovery of cadmium from cadmium products through recycling programs not only ensures that cadmium will be kept out of the waste stream and out of the environment, but it also conserves valuable natural resources as well. Attempts to ban or restrict cadmium products are considered unnecessary, taking into account the ever-decreasing human cadmium intake level which is already well below the WHO-standard, and the very small relative contribution of cadmium products in this respect. Such measures will only have the effects of undermining extensive efforts to collect and recycle cadmium products around the world. It will also have the marked effect of reducing the European Union’s competitiveness in international markets with its attendant delocalisation of plants and losses of jobs. These conclusions have already been reached in North America and Japan which have no intention of restricting cadmium use in the manner proposed by the European Union. A similar conclusion was also reached in the OECD Risk Reduction Programme on Cadmium.
Rather than restricting cadmium products, it is argued that the European Union should co-operate with and encourage the industry’s voluntary product stewardship initiatives to collect and recycle cadmium containing products which would contribute to the sustainable and safe use of cadmium in tomorrow’s society.