Heat and energy recovery
Best Available Technique (BAT)
Energy recovery before or after abatement is applicable in the majority of cases but local circumstances are important, for example, where there is no outlet for the recovered energy.
The BAT conclusions for energy recovery are:
- Production of steam and electricity from the heat raised in waste heat boilers.
- The use of the heat of reaction to smelt or roast concentrates or melt scrap metals in a converter.
- The use of hot process gases to dry feed materials.
- Pre-heating of a furnace charge using the energy content of furnace gases or hot gases from another source.
- The use of recuperative burners or the pre-heating of combustion air.
- The use as a fuel gas of CO produced.
- The heating of leach liquors from hot process gases or liquors.
- The use of plastic contents in some raw materials as a fuel, provided that good quality plastic cannot be recovered and VOCs and dioxins are not emitted.
- The use of low-mass refractories where practicable.
Brief technical description
Energy and heat recovery is practised extensively during the production and casting of non-ferrous metals. Pyrometallurgical processes are highly heat intensive and the process gases contain a lot of heat energy. As a consequence recuperative burners, heat exchangers and boilers are used to recover this heat. Steam or electricity can be generated for use on or off site and process or fuel gases can be pre-heated. The technique used to recover heat varies from site to site. It is governed by a number of factors such as the potential uses for heat and energy on or near the site, the scale of operation and the potential for gases or their constituents to foul or coat heat exchangers.
The following examples are typical and constitute techniques to consider for use in the processes to produce non-ferrous metals The techniques described can be incorporated into many existing processes:
- The hot gases produced during the smelting or roasting of sulphide ores are almost passed through steam raising boilers. The steam produced can be used to produce electricity and/or for heating requirements. An example of this in where a copper smelter produces 25% of its electrical requirements (10.5 MVA) from the steam produced by the waste heat boiler of a flash furnace. In addition to electricity generation, steam is used as process steam, in the concentrate dryer and residual waste heat is used to pre-heat the combustion air.
- Other pyrometallurgical processes are also strongly exothermic, particularly when oxygen enrichment of combustion air is used. Many processes use the excess heat that is produced during the smelting or conversion stages to melt secondary materials without the use of additional fuel. For example the heat given off in the Pierce-Smith converter is used to melt anode scrap. In this case the scrap material is used for process cooling and the additions are carefully controlled, this avoids the need for cooling the converter by other means at various times of the cycle. Many other converters can use scrap additions for cooling and those that are not able are subject to process developments to allow it.
- The use of oxygen enriched air or oxygen in the burners reduces energy consumption by allowing autogenic smelting or the complete combustion of carbonaceous material. Waste gas volumes are significantly reduced allowing smaller fans etc to be used.
- Furnace lining material can also influence the energy balance of a melting operation. In this case Low Mass refractories are reported to have a beneficial effect by reducing the thermal conductivity and storage in an installation. his factor must be balanced with the durability of the furnace lining and metal infiltration into the lining and may not be applicable in all cases.
- Separate drying of concentrates at low temperatures reduces the energy requirements. This is due to the energy required to super heat the steam within a smelter and the significant increase in the overall gas volume, which increases fan size.
- The production of sulphuric acid from the sulphur dioxide emitted from roasting and smelting stages is an exothermic process and involves a number of gas cooling stages. The heat generated in the gases during conversion and the heat contained in the acid produced can be used to generate steam and /or hot water.
- Heat is recovered by using the hot gases from melting stages to pre-heat the furnace charge. In a similar way the fuel gas and combustion air can be pre-heated or a recuperative burner used in the furnace. Thermal efficiency is improved in these cases. For example, nearly all cathode/copper scrap melting shaft furnaces are natural gas fired, the design offers an thermal efficiency (fuel utilisation) of 58% to 60%, depending on diameter and height of the furnace. Gas consumption is approximately 330 kWh/tonne of metal. The efficiency of a shaft furnace is high, principally because of charge preheating within the furnace. There can be sufficient residual heat in the off-gas to be recovered and re-used to heat combustion air and gas. The heat recovery arrangement requires the diversion of the furnace stack gases through a suitably sized heat exchanger, transfer fan and ductwork. The heat recovered is approximately 4% to 6% of the furnace fuel consumption.
- Cooling prior to a bag filter installation is an important technique as it provides temperature protection for the filter and allows a wider choice of fabric. It is sometimes possible to recover heat at this stage. For example in a typical arrangement used by a shaft furnace to melt metal, gases from the top of the furnace are ducted to the first of two heat exchangers that produces preheated furnace combustion air. The temperature of the gases after this heat exchanger can be between 200 and 4500 The second heat exchanger reduces the gas temperature to 1300C before the bag filter. The heat exchangers are normally followed by a cyclone, which removes larger particles and acts as a spark arrester.
- Carbon monoxide produced in an electric or blast furnace is collected and burnt as a fuel for several different processes or to produce steam or other energy. Significant quantities of the gas can be produced and examples exist where a major proportion of the energy used by an installation is produced from the CO collected from an electric arc furnace installation. In other cases the CO formed in an electric furnace burns in the furnace and provides part of the heat required for the melting process.
- The re-circulation of contaminated waste gas back through an oxy-fuel burner has resulted in significant energy savings. The burner recovers the waste heat in the gas, uses the energy content of the contaminants and removes them. Such a process can also reduce nitrogen oxides.
- The use of the heat content of process gases or steam to raise the temperature of leaching liquors is practised frequently. In some cases a portion of the gas flow can be diverted to a scrubber to recover heat into the water, which is then used for leaching purposes. The cooled gas is then returned to the main flow for further abatement.
During the smelting of electronic scrap or battery scrap in metallurgical vessels the heat content of the plastic content is used to melt the metal content and other additional scrap and slag forming components.
The advantage of preheating the combustion air used in burners is well documented. If an air preheat of 400 °C is used there is an increase in flame temperature of 200 °C, while if the preheat is 500 °C the flame temperature increases by 300 °C. This increase in flame temperature results in a higher melting efficiency and a reduction in energy consumption.
The alternative to preheating the combustion air is to preheat the material charged to the furnace. Theory shows that 8% energy savings can be obtained for every 100°C preheat and in practice it is claimed that preheating to 400°C leads to 25% energy savings while a preheat of 500°C leads to a 30% energy savings. Pre-heating is practised in a variety processes for example the pre-heating of the furnace charge using the hot furnace off-gases during the production of ferro-chrome.
Heat and energy recovery is therefore an important factor in this industry and reflects the high proportion of costs that energy represents. Many techniques for energy recovery are relatively easy to retrofit but there are occasionally some problems of deposition of metal compounds in heat exchangers. Good design is based on a sound knowledge of the compounds released and their behaviour at various temperatures. Heat exchanger cleaning mechanisms are also used to maintain thermal efficiency.
Whilst these savings are examples of individual components of installations they are critically dependent upon the site and process specific conditions including economics.
Achieved environmental benefits
In a similar way the use of oxygen, the minimisation of the transfer of molten material in ladles and the collection and re-circulation or conversion of gases are significant factors in minimising cross media issues.
The choice of abatement process is also a significant factor.
The environmental cost of producing the energy required for processes and abatement is another important cross-media effect.
Non Ferrous Metals Industries