Energy efficiency and metal purity
Energy efficiency can be described using a simple metric: the company’s total energy consumption per unit of turnover. The key factors influencing energy efficiency are:
- net process yield.
- plant utilisation and melting capacity.
- minimisation of energy losses.
- utilisation of waste energy.
Net process yield has a significant impact on energy efficiency, for the simple reason that a company is only paid for what it sells, not for what is melted. By increasing the net yield, a company can sell more for a standard unit of melt.
Example: A foundry sells 32,000 tonnes of good-quality castings per year; with a total melt of 67,000 tonnes, the net process yield is 47.8 per cent. The foundry has efficient melting capacity with an average consumption of 585 kWh/tonne. The total amount of energy consumed is 39,195,000 kWh. By increasing the net yield by 10 per cent, the foundry will produce the same 32,000 tonnes of castings with a total melt of 55,363.3 tonnes at 585 kWh/t, which corresponds to 32,387.531 units – a saving of 6,807,469 units. If we use 0.20 pounds as the average unit cost, this equates to a saving of 1,361,493.00 pounds per year with an increased capacity of 6,726 tonnes.
The net process yield is influenced by four main factors: melt yield, overflow and pigging, box yield, and scrap. Mould yield is a key factor, and significant improvements can be achieved through the use of state-of-the-art gating techniques – such as the John Winter MT/MTV High Density Exothermic Sleeves, which are far more efficient than conventional sleeves.
Furthermore, these sleeves offer high compressive strength and small contact areas, meaning they can be precisely positioned in areas of the mould where conventional sleeves cannot be used, thereby offering the possibility of increasing the number of cavities per mould and thus further improving yield. Another option, which is generally more suitable for contract foundries, is the use of direct casting processes. This method has a significant impact on increasing the box yield. John Winter supplies exothermic TXP casting units in conjunction with a range of filters, offering foundries a further means of increasing yield.
Maximising the increase in melt yield – or, conversely, minimising melt losses – is essential. Put simply: if you add a lot of non-metallic materials to the furnace, your melt yield will fall due to increased slag production. In non-ferrous applications, the more oxidation losses there are, the less metal is produced. Therefore, the use of clean scrap and foundry return, as well as the use of a protective covering flux, can play an important role in improving the smelting yield. Every tonne of slag removed means one tonne less metal produced using the same amount of energy. John Winter offers a range of cover fluxes and metal treatments that improve metal purity and melt yield.
Overflow and pigging depend on casting problems, metal changes and plant operating time. The following points should be subject to continuous improvement:
- Casting techniques.
- Ladle lids and insulation.
- Reducing the number of metal changes.
- Reducing plant downtime, which can lead to an increased volume of pigging. This is particularly important when unheated automatic casting lines are used.
Reducing scrap and waste is particularly important, as high scrap rates, in addition to the associated negative costs, have a devastating impact on net process yield.
Plant utilisation and melting capacity: Foundries are energy-intensive industries, and maximising plant efficiency to increase hourly output plays a key role in energy efficiency. The foundry must produce the castings we sell in the shortest possible time or increase total output within a given timeframe.
During melting, it is essential to maximise the furnaces’ melting capacity in order to reduce metal consumption and lower the kWh per tonne of metal produced. Another factor to consider is the cleanliness of the melt, which affects slag formation on the linings; this reduces the actual furnace capacity and increases melting cycle times due to difficult slag removal.
Slag formation shortens the service life of the lining and, in heated automatic casting furnaces, adversely affects the service life of the inductor and causes blocked filling nozzles, leading to downtime. John Winters’ SlagCure K, a fluorine-free cleaning flux, can and is used in melting furnaces, treatment and casting ladles, and casting furnaces to reduce deposits on filling spouts and inductors.
This applies to the entire spectrum of the foundry process, from melting to finishing. During melting, in addition to what we have discussed in the previous sections, a reduction in energy losses can be achieved through simple measures such as:
- Keep furnace lids and covers in place.
- Using insulating refractory materials with a lower density (where applicable) to reduce energy losses and avoid the need to preheat the ladles.
- Use of enclosed, concentrated extraction systems to reduce the total volume of air extracted. Put simply, for every cubic metre of air extracted from a system, an equal volume must enter it. This is particularly important in colder climates.
- Use of air supply systems to draw in outside air into enclosed extraction systems, thereby reducing the energy required to heat the system.
- Ensure that the extraction system operates efficiently and is kept clean. A blocked extraction system places greater demands on the system’s motors and thus increases electricity consumption, whilst dust is not being extracted.
- Shortening and monitoring blasting times, improving sand release and adhesion to castings through better sand control and the use of coatings such as John Winter Cleancast mould and core coatings.
- Use of power factor control throughout the plant to keep the COSø as close to 1.0 as possible.
- Zoning the compressed air supply to allow it to be isolated when not in use.
- Using fewer centralised compressed air systems and focusing on reducing leaks. Compressed air is one of the most energy-intensive resources used in foundries.
- Where possible, switch to electric drives and motors.
- Invest in new machinery or modernise existing equipment. Modern technologies are generally more energy-efficient.
- Switching off compressors and lighting systems.
- Using energy-saving lighting systems.
- Insulating buildings.
- Installing solar panels: foundries have large roof areas, and solar panels can be used to offset electricity costs. John Winter has recently started using solar energy, and it is paying off.
Utilising waste energy: As mentioned earlier, the foundry process is energy-intensive, and in most cases the energy is lost to the environment as heat. Exploring ways to utilise waste heat can be highly beneficial, for example:
- Using heat pumps to generate hot water from exhaust air, which can be used to heat offices or supplied to other facilities as a heat source.
- Using hot exhaust air from foundry plants for core and paint drying systems,
- Using hot water from cooling systems to cool moulds (permanent moulds).
In summary, it can be said that, in view of rising energy costs, it is becoming increasingly important for energy use and recovery to form part of future planning for energy-intensive industries such as foundries. Company management should give the highest priority to energy efficiency in day-to-day operations and future planning in order to reduce the financial burden.
Andrew Tagg
Technical Director