Getting Ideas for Energy Management Opportunities
Ladles
Preheating of ladles consumes a lot of natural gas. We have already mentioned ways to minimize the amount of heat needed to bring them up to the temperature required, as well as methods to cut the heat loss from the ladles by using new types of low-mass, low thermal conductivity ceramic fibre panels or inserts. The ladle heater burners often get inadequate attention; the burner efficiency averaged 53% in iron foundries and was even lower in steel and non-ferrous foundries. With the proper air/gas balance, the combustion efficiency is in the 62–65% range. The appearance of blue flame (i.e., carbon monoxide burning) indicates efficiencies below 58%.
In addition, the radiation losses from a ladle holding molten metal are very substantial. Commonly, the foundry may compensate for the radiation heat loss by superheating the metal in the melting or holding furnace, so that when the ladle gets to the pouring station, the metal is at the right temperature. This is, of course, a wasteful practice. The full extent becomes apparent when one realizes the cumulative effect of the many ladle transfers in a year. Other costs may arise due to chills and increased scrap levels. Covering the ladle with a lightweight ceramic-fibre cover during transfers will enable dropping the tap temperatures of the furnace substantially (maybe 50°C), therefore also saving wear and tear on the furnace lining, and improving furnace productivity.
Filters
In non-ferrous foundries, energy and cost savings can be achieved by the use of filters for straining the molten metal prior to casting. Many foundries do not yet filter, perhaps because they are unaware of the likely cost benefits. Inserting a disposable ceramic filter just ahead of the mould can provide a cost-effective way of improving product quality and reducing operating costs. Studies were made in aluminum and copper-based alloy foundries, which proved conclusively significant cost savings for little expenditure – on an average about $800/year per tonne of good castings. Other non-energy related savings were almost as high as well.
The cost of adjusting patterns or dies to accommodate filters is minimal. The benefits of filtering molten metal prior to casting include:
- Higher casting quality (and hence customer satisfaction);
- Reduced scrap and reject rates;
- Higher overall yields; and
- Reduced energy, metal, sand and labour costs.
Use of ultrasound
In an effort to reduce porosity of castings and improve their quality, reduce energy usage and increase yields, a foundry in the U.K. employed ultrasound. The process is cheap and simple to install. To convert electrical energy to acoustic energy, a converter employing the precious metal niobium was chosen. While niobium has excellent corrosion resistance, it absorbs a lot of ultrasonic energy.
Research is being carried out to find a better metal composition for the ultrasound inductor. Nevertheless, the successful pilot application demonstrated the following:
- 95% reduction in traditional de-gassing time;
- Improved metal nucleation upon solidification, leading to a finer grain structure;
- Improved mechanical properties of the castings; and
- Possibility to reduce molten metal temperature by increasing its fluidity.
Energy savings ensued, as did availability of the holding furnaces and increase in the furnace yield.
Robotics may also be used for pouring operations. Installation of the “autoladler” achieves consistency in pouring, is faster and eliminates operator’s fatigue and ergonomic problems.
Other EMOs
Housekeeping
- When preheating the ladle, place an insulating blanket on top of it (e.g., using Fiberfrax™ ceramic fibre).
- Analyse the metal transfer and pouring operation with an eye for unnecessary delays, which contribute to the heat loss.
Low cost
- Control process air dew point to minimize re-gassing of metal.
- In permanent mould casting, consider alarming machines for water line restrictions.
- For ladle preheating, consider placing the ladle in a tipped position in front of a wall of refractory material, through which the preheating burner is inserted into the ladle. The small gap between the ladle and the wall, together with the heat reflection from the wall back into the ladle will reduce the burner gas consumption. While this is an Ontario foundry’s invention, commercial gas burner ladle preheaters, operating on the same principle using a heat shield-reflector, are available.
- For ladle preheating, consider the economics of using a dip-in, mechanized ladle heater using electricity instead of gas. The preheater modulates the energy input independently of the temperature reached.
Retrofit; high cost
- Optimize the molten metal transfer from the melting or holding furnace to the pouring stations in order to shorten the transfer times, thus minimizing the heat losses and enabling reduction of the tapped metal temperature.
- In batch operations, when superheating of the metal is required, apply it to the smaller quantities in a holding furnace rather than to the larger melting furnace.
- Investigate the possibility of continuous casting (since the process eliminates much of the heating and cooling and saves energy).
- For permanent moulds, rather than “pushing” the metal in, or using gravity fill, consider using vacuum as the medium to draw the molten metal into the mould (it has a positive impact on quality and thus on energy efficiency as well).
- When considering a major upgrade of casting operations for low-pressure permanent moulds and applicable metals, consider automation using a PC-based control system to monitor and control critical factors, such as water cooling, mould temperature, fill pressure and time, etc., for improvements in quality and thus in energy efficiency.
- Similarly, in such situations and for applicable metals, consider application of low pressure, bottom fill casting to improve casting yield and quality, all impacting on energy efficiency.