Ovens, Furnaces & Heating Equipment
The use of furnaces in recycling precious metals Robert Prior, Carbolite Gero
By the end of 2019 it was estimated that as many as 350,000 mobile phones were disposed of every weekday - with a gold content worth $200 million according to Carol Jegou, the CEO of All green Metal Recycling, when she spoke at the E-Waste World Expo in Frankfurt. Carol went on to say that for every million phones that are recycled 16 tonnes of copper and signifi cant quantities of silver, gold and palladium can be retrieved.
However, it isn’t simply mobile phones that are being recycled today. Global demand for precious metals continues to soar, whether for jewellery or for use in industrial applications. While gold and silver have been used for thousands of years in jewellery, more recently precious metals are performing useful and specialist functions in chemical processes, electronics manufacturing, aerospace and automotive systems, (eg catalytic converters).
More than a decade ago, in excess of 1,650 tonnes of fi ne gold was fed back into the global gold supply chain according to fi gures released by the Birmingham Assay Offi ce. This is hardly surprising as at the time it was estimated that the cost to extract the precious metals was $0.18 while the value of those same metals once extracted was over $0.75. Today, an even greater quantity of precious metals are recycled thanks to cost- effective techniques and the fact that precious metals can be recycled time and again without their properties degrading. At the time of writing in June 2020, the cost of palladium stands at over £50 per gram (a rise of over 370% in the last decade) and gold currently trades at over £45 per gram (up 70% over the last decade) so it comes as little surprise that recycling volumes are at an all-time high and continue to rise with the growing demand from high-tech manufacturers.
Specialist furnace equipment is used
throughout the world both for recycling these relatively rare commodities and for assessing their purity. There are two key processes in the precious metals recycling market - smelting and cupellation.
Smelting is the separation of precious metal from non-metallic impurities. When bulk ores are involved, very large installations are required, using heat and chemical reducing agents to decompose the ore, drive off other elements such as gases or slag and leave only the metal behind. When the process involves pre-used materials – for example, catalytic converters or jewellery - the equipment is smaller in scale but involves the same principles in order to ensure a high- quality end material.
Precious metals recovery is a typical use of smelting. Scrap material is heated to above its melting temperature (1100ºC for gold, 900 to 1000ºC for silver, 1500ºC for palladium and 1550 to 1800ºC for platinum). A great example of smelting to recycle can be seen by those recycling automotive catalytic converters which are designed for exhaust gases to pass through a ceramic honeycomb coated with a fi ne layer of palladium group metals (PGMs). This fi ne coating contains platinum ore, typically two to three orders of magnitude richer than the mined ore. During the recycling process which might recover as much as 98% of PGMs, the honeycomb ceramic is crushed into a powder and placed in crucibles where it is melted, perhaps even a number of times, in the smelting furnace with a low value material such as lead and a suitable fl ux to recover as much as 98% of PGMs. In its molten state the low value material combines with the precious metal and the impurities form a slag, which can be easily removed when the sample cools. The cooled can then be placed in a cupellation furnace to remove the low value material and to recover the precious metal.
High quality smelting furnaces with a single or twin chamber confi guration used by laboratories and assaying companies are specifi cally designed to provide the temperatures required for smelting, while withstanding the gases given off by the process. Those considering investing in such a smelting furnace will be looking for models with high- performance silicon carbide elements to allow continuous operation up to 1400oC. In addition, quality solutions will feature furnace chambers that are lined with silicon carbide
tiles to protect the elements and the thermal insulation, with hard-wearing refractory brick providing a solid hearth base.
The ground/powdered precious metal for recycling and the low value material is typically processed in ceramic crucibles of various sizes, and hinged chamber lids keep the heat of the insulation away from the operator when opened and these lids also typically incorporate chimneys to vent fumes. Furnace temperature is regulated by a microprocessor-based controller with a timer function allowing the equipment to be switched on automatically without an operator being present.
Accurate analysis is routinely carried out to determine the precious metal content in a wide variety of different materials. This is particularly important for fi neness determination for the hallmarking of precious metals for jewellery use. The precious metal concentration in different materials can vary widely from amounts measured in parts per million (ppm) for
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40
