Energy costs for aluminium casting

Like many energy intensive industries, the aluminium casting sector is under severe global competitive pressure and needs to reduce costs wherever possible. In the past when energy prices were relatively low, the underlying ‘energy policy’ in most firms was based on reducing the price per unit of energy, rather than reducing the quantity of units used. One reason may have been because so called ‘energy efficiency initiatives’ were often the responsibility of purchasing or accounts and were based purely on the benefits of cost saving rather than energy saving. Today, cost saving by ‘playing’ the energy markets is not so easy – if not impossible particularly for smaller foundries. So reducing actual energy used is the only option for minimising energy costs. Energy managers must now concentrate on their task of managing energy and not only buying it.

At this time when quite a few foundries may be experiencing a significant fall in demand or at least the prospect of it, energy costs per casting will rise as a proportion of overall costs as there are some energy costs that are more or less fixed, regardless of output. Therefore all possible cost saving measures must be taken for any enterprise to remain both competitive and profitable.

In the past the cost of energy may have been a relatively small proportion of the total production costs for many foundries, so that there was little financial incentive for energy saving measures. But as energy prices have risen sharply in the last year or so, the picture has changed dramatically; now there is a powerful financial incentive to improve energy efficiency – but that does not necessarily mean investing in new equipment. In many cases, reducing energy waste is the answer.

In aluminium foundries, on average only about 25 to 30% of total site energy is used in the actual casting process itself – for metal melting and holding, moulding and core making, and casting machines. The balance is used for site services – such as motive power for fettling, shot-blasting, sand preparation, materials handling, compressors, pumps and environmental ancillary plant such as fume extraction and filtering.

But there are other costs associated with energy use – such as the metal itself. Reducing metal losses saves energy as does reducing scrap. After all, it takes at least the same amount of energy to generate oxides as that for usable metal. It also takes as much resource to make a casting that is subsequently scrapped, as that to make a good one!

Energy saving in foundries is sometimes achievable by changing working shift arrangements and reducing the number of furnaces in operation. Furnaces in use then can be operated for longer periods nearer to their optimum output. Changing working practices and operating less shifts of longer duration, or in some cases working extra shifts, improves plant utilization and minimizes the effects of process variables caused by production breaks. When furnaces are used continuously, the overall energy efficiency improves by eliminating the energy expended to maintain furnaces at temperature between shifts when not melting, or frequently heating up furnaces from cold.

Better ‘melting management’ can significantly improve energy efficiency. Many aluminium foundries – particularly smaller ones, only cast metal for seven or eight hours each day; but furnaces may be run either continuously, or are started up three or four hours before the beginning of the working shift. Procedures usually depend upon other factors such as the economic advantage of holding surplus liquid metal at temperature rather than waste the energy stored in it by casting it into ingots that are re-melted again – but both use unnecessary energy. Melting only as much metal as needed clearly reduces energy costs; melting more than needed is entirely wasteful. Furthermore, the practice of holding for long periods in the liquid state at casting temperature is impractical for some alloys, the metallurgical quality of which may be impaired by loss of alloying elements. But the principal causes of energy waste in aluminium foundries are due to ineffective furnace operation, poor furnace maintenance, high metal loss due to oxidation, low mould yields and scrapped castings.

Scrap is an emotive subject and is rarely revealed even if it is actually measured in foundries - but it is likely that scrap is a major contributor to low energy efficiency. Past surveys have indicated that something like 10% of all castings poured are scrapped either in the foundry or subsequently rejected by customers. Although now this might have changed for the better, ‘quality control’ in some foundries often means just sorting for defective castings rather than reducing the number of defective ones produced by investigating and where possible eliminating the causes of defects.

Improving mould yield saves energy as less metal is melted per casting. Therefore tooling for new products should be designed to optimize yield and when possible reduce pouring temperatures, of course without compromising mould filling and feeding during solidification. In some cases this may also raise productivity by reducing cycle times.

There is nothing new in all of this, but it may be a timely reminder of some of the most basic aspects of energy efficiency in aluminium foundries. Below is a checklist of some ‘no-cost’ and ‘low-cost’ energy saving measures most of which are good practice, based simply on good ‘housekeeping’ and good foundry management.

Furnaces:

  • improve temperature control;
  • use lowest practical melt temperature;
  • repair or replace defective insulation;
  • check combustion conditions regularly (fuel-fired furnaces);
  • replace defective heating elements (electric resistance furnaces);
  • install lids on crucible furnaces;
  • close lids whenever access to the crucible is not required;
  • clean crucible walls frequently to maintain conductivity;
  • remove slag and dross regularly from reverberatory and ‘tower’ furnaces;
  • maintain door seals to avoid air ingress which increases energy losses and oxide generation;
  • reduce metal loss in furnaces;
  • change operating procedures where practical.

Management:

  • better planning – melt only sufficient for planned production;
  • raise yields from existing tooling where possible;
  • avoid holding liquid metal during extended non-productive breaks such as over-night and weekends;
  • check that all process procedures are followed;
  • review quality control procedures regularly.

In conclusion, we are constantly told that carbon dioxide emissions from energy use is the cause of global warming and that it in turn may lead to changes in the earth’s climate with potentially disastrous consequences for future generations. Industry must use energy to fulfil its contribution to the economy, but must avoid energy waste wherever possible. Today, all enterprises including foundries have a moral obligation to find a balance between environmental compatibility and economic feasibility. The aim must be to do more with less - that is an underlying principle of sustainability. After all – economy in any form, apart from energy use, is an essential basis for good business and a sound, sustainable ecosystem.