There are around 40 non-ferrous metals in production. Those produced in the greatest quantities are aluminium (37.6 million tonnes), copper (18.8 million tonnes), zinc (11.4 million tonnes), lead (8.2 million tonnes) and nickel (1.5 million tonnes). There are also a number of important alloys produced from these elements, such as brass (made up mainly of copper and zinc) and bronze (copper and tin). And then there are the precious metals, the most significant of which are silver, gold, platinum, rhodium, and palladium.
Non-ferrous metals are in demand as they are used in an incredibly diverse variety of environments, ranging from the catalysts in car exhaust systems to beer cans. Although prices are on the rise, the industry does face increased costs due to stricter environmental controls on production. So the non-ferrous metal producers may need to investigate new production processes which emit lower amounts of CO2 and pollutants.
Recycling of non-ferrous metals is another obvious way of reducing the environmental impact of the industry. The British Metals Recycling Association says that in the EU the use of recycled metals cuts emissions of carbon dioxide by 200 million tpy, which is equivalent to about one half of the total emissions of France. Recycling aluminium is most effective in terms of energy saving, as scrap aluminium melts at 660°C whereas extracting metal from the bauxite ore requires temperatures of around 900°C. Lead production uses the largest proportion of recycled material, with 74% of newly produced lead coming from recycled material.
So what are the prospects for those supplying gases to the industry? The downside is that the scale of production is much smaller than in the steel industry. While a steel plant may require up to 1300 tpd of oxygen, a typical non-ferrous metal plant requires supplies of between 5 and 100 tpd. However, on the positive side, demand is growing strongly. Joachim von Schéele, Marketing Manager for the Metals and Glass division at Linde, said prospects for business with the primary non-ferrous metals industry are good, “The high prices are changing the situation. What wasn’t classified as ore 2 years ago is suddenly ore, because the prices are high.”
Oxygen is the main gas used in the industry. Its primary role is to enhance the efficiency of smelting and furnace processes. It can either be used to enrich the combustion air stream or is input via burners. Using oxygen reduces the warm-up time needed for the smelter, reduces fuel consumption, improves the quality of the exhaust gas, and lowers the amount of dust emitted.
Dr Pravin Mathur, Director of Business Development at Praxair, pointed to the copper industry as an example of where there has been increased oxygen use, “Copper producers are generally in more of a mode to increase production and there’s been activity in putting industrial gases like O2 into copper production areas.”
Smelting is the chemical reduction of metal ores to extract the metal. This is carried out using one of the four main types of furnaces: blast, reverbatory, induction or electric arc.
Blast furnaces (also known as cupolas) consist of vertical cylinders set above a crucible. Ore and fuel is introduced at the top and air (sometimes oxygen enriched) is fed in via tuyeres at the bottom. Molten metal is tapped off at the bottom of the furnace. Limestone is also part of the charge and this decomposes to form calcium oxide, which reacts with acidic impurities in the ore to form a slag. Blast furnaces are used in the production of lead, copper and tin.
Reverbatory (or reverb) furnaces keep the fuel separate from the material being processed. The metal ore is heated through contact with the combustion gases. This is less efficient than a blast furnace and the output of these furnaces can be increased by using oxy-fuel burners. Air Products state that for a typical reverb furnace, using oxygen can reduce fuel use by 20-40%, increase production by 20-35% and reduce the flue gas volume by up to 60%. In copper and nickel flash smelters, the ore is burned using an oxygen-enriched burner set in a vertical shaft above a reverb furnace. This results in the sulphur in the ore burning off as sulphur dioxide and the remaining semi-molten metal particles falling into the hearth where they melt completely. The process requires oxygen supplies of between 200 and 800 tpd. In rotary reverbatory furnaces, the furnace chamber rotates horizontally. Rotary furnaces operate with less heat loss and lower fuel consumption than other reverb furnaces and so are economically viable for processing lower metal content scrap and residue. Some can be tilted to remove waste products.
Induction furnaces are heated using electrical power. Magnetic coils are wrapped around a crucible and when a high-frequency electric current is passed through them, an electrical charge is induced in the metal within the crucible. This results in rapid heating. Induction furnaces are used for melting copper, aluminium and precious metals.
Electric arc furnaces are also powered by an electric current. This is either fed through a graphite electrode placed above the ore and scrap input, or via submerged Soderberg electrodes made from carbon paste. Some copper is produced using these furnaces, as well as non-ferrous alloys. Some furnaces incorporate oxy-fuel burners to accelerate the melting rate.
Aluminium, the non-ferrous metal produced in the greatest quantities, is not produced by any of these pyrometallurgical methods, but instead by electrolysis in the Hall-Héroult process. This process involves dissolving aluminium oxide in a compound called cryolite and then passing an electric current through it. Aluminium is deposited at the anode and oxygen is liberated at the cathode; both electrodes are made of carbon.
After the metal is extracted by one of the processes above, it undergoes secondary processes such as refining, alloying, casting, and the application of surface finishes.
Burner technology is developing in the non-ferrous metals sector, with oxy-fuel burners becoming more widely used. Oxy-fuel burners combine a stream of up to 95% oxygen with recycled flue gases. Joachim von Schéele described Linde’s experiences in the aluminium industry, “In the past the aluminium producers were afraid of applying oxy-fuel because you got hot-spots and oxidation of the material but now we can show we can solve these problems, which means that they are prepared to change their processes.”
A lot of research effort is being dedicated throughout the industry to developing more effective oxy-fuel burners. Paul Grohmann explained the benefits of Messer’s new burners which are designed to prevent overheating, “The problem of excessive temperatures has been solved by diluted combustion. This enabled us to lower the flame temp by recycling the combustion atmosphere within the furnace. There
is a high velocity of O2 which has a large momentum that sucks the atmosphere from the furnace into the flame. NOx formation is reduced, which is a further advantage beyond the fuel savings and the increase in productivity.”
He went on to say, “We have brand new burners (Oxipyr Flex and Oxipyr Air) which are adjustable to the conditions, for which you can adjust the momentum of O2 and set the flame length. At first you need a very short fl ame as non-molten metal is in front of the burner and afterwards you need a longer flame. The market is very hungry for such burners.”
A number of other gases in addition to oxygen are used by non-ferrous metals producers. Nitrogen is used for cooling and stirring, as well as inerting, although argon often has to be used instead of nitrogen for this latter role. Argon is used to protect metals such as chromium, vanadium, and magnesium, which would react with nitrogen or incorporate it within
Nitrogen and argon are also required in aluminium processing. Molten aluminium absorbs hydrogen from atmospheric moisture, which creates porosity in the metal. Degassing is usually carried out using a purge gas, in a process that also removes particulates. The
gas forms bubbles, within which hydrogen atoms
come together to form molecules.
Nitrogen is the most commonly used purge gas; argon is used for higher performance aluminium alloys. The gas is sometimes supplemented by up to 10% of an active halogen gas such as freon, chlorine or sulphur hexafluoride, which makes the process more effective by changing the surface tension of the bubbles. One common process is called rotor impeller degassing, a patented version of which is called the Spinning Nozzle Inert Flotation, developed by the then Linde Division of Union Carbide in the 1970’s.
In this process, the purge gas is introduced via a rapidly spinning shaft or impeller. The kinetic energy of the shaft shears the bubbles of purge gas into extremely small bubbles, thus increasing their surface area and promoting the
collection and then removal of the hydrogen.
Less effective aluminium degassing processes use static lances to input the purge gas or tablets that create bubbles of aluminium chloride to absorb the hydrogen.
There are now research programmes underway that
work on using ultrasonic vibrations to break up the size of the bubbles of purge gas, reducing the quantity of gas needed.
Another process that makes use of argon is hot
isostatic pressing (HIP) which involves subjecting metal powders to high pressure (typically 200 Mpa)and high temperature (up to 2000°C). Isostatic pressure means pressure from all directions, which can be achieved when a gas is used as the pressurizing medium. Argon is chosen as it will not react with the molybdenum and graphite heating elements that surround the chamber. Key advantages of the HIP process are that near-net shape objects can be created, thus reducing the wastage of raw materials, with porosity of the finished article is very low. It can also be used to create alloys that cannot be made by combining the elements in their molten form.
Other gases used in the industry include hydrogen, which is used for annealing and hot-dip galvanizing, and carbon dioxide, which is used for stirring ladles and vessels. Nitrogen/helium mixtures can be used for
checking the tightness of cast parts.
Although production in the non-ferrous metals
industry is not on the same scale as steel
manufacturing, it is undergoing a period of strong growth. The environmental pressures on the metal producers will create opportunities for gas suppliers, as manufacturers work to develop cleaner, more efficient processes.