As tin has been one of the higher valued base metals and as the infrastructure to reclaim tin has been well-developed for many years, the metal has significant recycling rates, though lower than some other metals. The metal is a relatively scarce element with an average abundance in the Earth’s crust of about 2 parts per million (ppm) compared with 63 ppm for copper, 94 ppm for zinc, and 12 ppm for lead. The metal is produced from hard rock deposits and placer deposits derived from the lodes.
Most lode tin deposits occur as greisen, replacement, skarn, or vein-type deposits related with granitic rocks or their extrusive equivalents where tin has been concentrated by magmatic differentiation techniques. Placer deposits of various types have been derived from lode tin deposits by weathering and erosion. As cassiterite is heavy and chemically resistant, it is concentrated by weathering in place that removes lighter minerals, or by erosional processes on a slope or in streams.
Most of the world’s past tin production has been from placer deposits that usually can be mined more cheaply than lode deposits. From 1934 through 1987, Malaysia, Indonesia, and Thailand, contributed more than half of the world’s 10 million metric tons of tin production, by using major dredge production from marine placer deposits.
Market Prices and Application Patterns
Sources of Magnesium Scrap The price of tin is the most important reason, which influences its recycling rate. In recent decades, tin usually has been the highest priced base metal. Though almost 20 years have passed since tin reached its historical peak price of more than $8 per pound, its recent pricing still offers an adequate incentive to recycle.
The important uses of tin include cans and containers, electrical equipment, construction, transportation, and other. These tin usage patterns are similar to most parts of the world.
Old scrap consists of tin-containing products, such as tin cans and electronic equipment, which have been discarded after use. Old scrap is sometimes also referred as „post consumer scrap.“
New scrap is tin waste, which is generated during the production processes and, for the most part, remains the property of the manufacturers and is is continually being recirculated. Home scrap is new scrap, which is immediately redirected back into the production process without leaving the plant. Sometimes new scrap is collected and later sold to scrap dealers who again sell it to similar manufacturers. An example of new tin scrap would be the side trimmings built on a coil of tinplate in a steel mill to satisfy a customer’s width specification.
Due to the relatively high value of tin compared with other base metals, such as copper, lead, zinc, etc.; tin recycling industries have become more efficient in recapturing discarded tin products. Also due to its value, the metal has been the target of smuggling attacks in areas, which range from tin smelter sites in developing countries to scrap yard sites in the developed countries.
Tin is normally considered as a nontoxic material and hence its environmental considerations are not major factors in recycling.
Recycling Efficiency of Old Scrap
Sources of Magnesium ScrapThe recycling efficiency of old tin scrap is significant despite the fact that its recycling rate is relatively low. Old scrap recycling efficiency that is the amount recovered and reused compared to the total amount of old scrap generated and excludes dissipative uses, was 75% in 1998, while the recycling rate was 22%. One of the main reason for lower recycling rate was due to the fact that the third leading use for tin has end uses, which are nearly all dissipative.
Detinning facilities are important to the tin scrap industry because no other major metal industry has numerous large-scale plants designed for the removal of metal plating. Detinning works are performed on new tinplate scrap from tin mills and can manufacturing plants and from old scrap in the form of used tin cans.
The metal recycling industry, as a whole, has been undergoing significant consolidation during the past 3 decades. Before consolidation, the industry was characterized by numerous small, family-run firms, many of which dated back to the 19th century. Generally, the firms operated only locally or perhaps regionally. However, from 1970 onward, the industry has been characterized by fewer, large size units, which serve national or international markets.
The Steel Recycling Institute (SRI), which is funded by the major steel producers of United States, has encouraged the recycling of used tin cans for more than a decade. These used tin cans have become a crucial raw material for the US steel industry during the past 2 decades. The increased recycling is essential to tin recovery as most of the steel cans are made from tinplate.
Processing of Scrap Metals
Typically, the tin alloys, such as brass and bronze, jeweler’s metal, or babbitt, contain from 2-11 % of tin as a minor component of the alloy. The prime exception is solder. Solders for plumbing and construction-related applications have conventionally been 50 % tin-50 % lead, and those for electronics have usually been 70 % tin-30 % lead. The bulk of the secondary tin industry processes several forms of tin alloy (brass, bronze, solder, etc.); the recovered tin is recycled within its own product-line industries and, hence, is used again in alloys.
The scrap materials of all tin alloys are collected via long-established commercial routes and transported back to the original alloy producers. Usually, these alloy producers then use a mix of primary tin along with the primary elements of the other components of the alloy and combine them in a batch heating technique with the collected scrap materials to manufacture new units of the same alloys. Thus, the secondary tin industry is integral to the process of successive regeneration with old and new scrap alloys being mixed with some primary material added for each new production of alloys.
For most of this century, detinning has been the only method in the secondary tin industry through which tin alone, not in alloys, has reached the industry. Accordingly, detinning of tinplate is one of the most important sources of tin scrap in many countries. Tinplate scrap suitable for detinning is collected at different stages – off-specification tinplate generated in the tin mills at steel plants, reject tinplate and tin cans generated at can making plants, reject cans at can-filling operations, and old scrap tin cans accumulated by municipalities via curbside collection programs.
Conventionally, tin scrap is not used to make tinplate due to purity requirements. The tin coating thickness on steel in modern tinplates is on average, about, one-half to one-third of what it was in the early 1960’s. Presently, the nominal tin coating thickness on each side of tinplate is only 0.000381 mm. Hence, the economics of detinning is not nearly as profitable as it once was. However, for maximum steel cleanliness, some steel manufacturers prefer to pay more to have tinplate scrap detinned before using it, the presence of tin on the surface of scrap steel often is believed to be a major detriment to good quality steel.
Types / Grades
Different grades and varieties of tin scrap scrap is used for recycling purposes and to recover pure tin from the scrap. Given in the table below are some of the important grades / types of tin used in recycling.
The stable consumption pattern of tin and the comparatively high prices of tin compared with other base metals indicates that plentiful amounts of tin will be available for recycling in coming years. In addition, the expected technological advances will tend to encourage more recycling of tin in the next few decades. Except for dissipative uses, tin scrap for recycling will continue to stem from all the consumption sectors. The prices of tin will probably remain as the most important influence on further recycling rates.
Trends toward consolidation in the recycling industry are anticipated to continue, at least in the coming future. The resulting bigger enterprises presumably will be able to attract easy funds for modernization of equipment and technological improvement, which often result in lower unit processing costs. Hence, consolidation trends augur well for increased tin recycling. The recycling of tin raises the sustainability of production of tin by reducing the need for primary production, thus saving energy and extending the longevity of natural resources.
The prospects for increasing the recycling rate for tin chemicals are not especially good, but, since tin chemicals are mostly a dissipative end use. Also, some of the more assuring new applications for tin, such as ammunition, are essentially dissipative.
Zinc brings a multitude of social and economic benefits to society. Man has discovered a wide range of uses for this versatile natural element with valuable characteristics and properties for various industrial applications.
The most important use of zinc is in protecting steel from corrosion using hot-dip galvanizing technique. The metal is used to prolong the durability of steel, which is one of the most used materials on the planet. Both zinc and steel are 100% recyclable. The zinc-steel combination has considerable economic benefits in terms of life-cycle costs. Improved air quality in several industrialized countries, with diminishing levels of sulfur dioxide (SO2), means that today zinc coatings facilitate even longer protection for steel.
Increased attention to life-cycle costing is prompting the designers, specifiers and investors to prefer zinc-coated steel in many traditional and new applications, from electricity distribution poles to safety barriers, from construction to automobiles and from farm gates to ski-lifts.
The 27th most common element in the Earth’s crust, Zinc is completely recyclable. According to estimates, at present, 70 % of the zinc produced originates from mined ores while 30 % comes from recycled or secondary zinc. The level of recycling is increasing in step with progress in zinc production technology and zinc recycling technology.
Today, more than 80 % of the zinc available for recycling is recycled. The metal is recycled at all the stages of production and use, for example, from scrap, which arises during the manufacturing of galvanized steel sheet, from scrap generated during production and installation processes, and from end-of-life products. Given below is the pie chart that describes the percentage of recovered zinc from different products manufactured with zinc.
The life of zinc-containing products varies and may range from 10 to 15 years for household appliances or cars, to over 100 years for zinc sheet used for roofing. Street lighting columns constructed of zinc-coated steel can remain in use for 40 years or more, and transmission towers for over 70 years. All these products tend to be replaced because of obsolescence, and not because the zinc has ceased to protect the underlying steel.
The presence of zinc coating on steel does not restrict its recyclability and all types of zinc-coated products can be recycled. Zinc coated steel is recycled along with other steel scrap during the processes of steel production – the zinc volatilizes and is then recovered. According to estimates, the supply of zinc-coated steel scrap is expected to double over the coming years, as more zinc-coated vehicles enter the recycling stream.
Types / Grades of Zinc Scrap
Different grades and varieties of zinc scrap is used for recycling purposes and to recover pure zinc from the scrap. Given in the table below are some of the important grades / types of zinc scrap used in recycling –
The secondary zinc industry processes metal scrap for the recovery of zinc in the form of zinc oxide, zinc slabs, or zinc dust. The process of zinc recovery involves three general operations, viz. –
Secondary recovery starts with the separation of zinc-containing metals from other materials, typically by magnetics, sink-float, or hand sorting.
When non-ferrous metals have been mixed in shredder scrap, zinc metal can be separated from higher-melting metals, such as such copper and aluminum, through selective melting in a sweating furnace. A sweating furnace (rotary, reverberatory, or muffle furnace) slowly heats the scrap containing zinc and other metals to approximately 419.4°C; this temperature is adequate to melt zinc, however it is still below the melting point of the remaining metals.
In the case of zinc-galvanized steel, the zinc can be recovered largely in furnace dust after the scrap is charged into a steel making furnace and melted. Although it is expected that recycling will continue to be encouraged on the grounds of environmental sustainability.
Almost all of the zinc in electric arc furnace dust is first retrieved in an upgraded, impure zinc oxide product, and is then shipped to primary pyro-metallurgical zinc smelter for refinement to metal.
According to International Zinc Association, there are 50 secondary zinc operators in the world with varying recycling processes. New scrap is usually just remelted. In the case of mixed non-ferrous shredded metal scrap, zinc is separated by hand or magnetically, before it is re-refined by retorting. An important source of secondary zinc is flue dust from secondary electric arc furnaces for production of iron, using galvanized iron scrap.
Consumption of refined zinc metal in recent years has been slightly more than total metal production, perhaps due to the considerable amount of refined zinc includes zinc used directly in alloys irrespective of source material, whether concentrates, slags, residues or scrap.
The recycling picture for old scrap is quite confused. Based on the physical characteristics, most metallic zinc is theoretically recoverable, even after very long periods of use. According to International Lead and Zinc Study Group, the recovery efficiency for zinc, meaning the fraction of old scrap available for recycling, which is actually recycled, is 80 %; this is consistent with a very long lifetime of zinc products in use, as much as 100 years.
Zinc recovery from recycled galvanized iron and steel is becoming more and more important, but is still small; the remainder was primarily from brass products, flue dust, old die casting and old rolled zinc products.
A major problem for recyclers is that secondary zinc is almost invariably associated with other metals, including lead, copper, cadmium, arsenic and chromium.
Depending on the type of steel and the galvanizing process, between 10 % and 40 % of the total consumed zinc ends up in residues. Zinc that is metallurgically combined with steel during galvanizing process, cannot be easily separated from galvanized steel scrap. It is generally recovered from flue dust generated during the reprocessing of steel scrap.
Zinc containing materials are highly variable and include automobile scrap, brass scrap, flux skimming dross from the wet galvanizing process, zinc ash from the dry process, arc furnace dust, and so on. Composition may vary widely, and the recovery processes would have to be equally variable.
Scrap produced only at the end of the useful life of the product into which it is incorporated is more difficult to recover; and hence, only a small portion is recycled. Until 1993, about, all zinc used in galvanized steel products was lost either in landfills or in slag at steel scrap processing plants.
In recent years, zinc has been recovered from flue dust, and advanced technology to de-zinc galvanized steel has been developed. This new de-zincing technique should increase the relatively low recycling rate of old zinc scrap, which enhances the sustainability of primary zinc production.
Zinc score consists of clean, dry and unalloyed zinc sheet scrap. Zinc Score is being used for production of zinc sheets also meant for printing industry, for common castable alloys and lead copper-zinc alloys; for hot zinc plating of ready-made products as well as materials; for producing zinc powder used in metallurgical industry and also for production of zinc oxide used in chemico-pharmaceutical industry; for fine chemicals; for acquiring zinc powder used in accumulators.