Air Quality Management
Several substances used to make castings in foundries can affect the air quality at your premises and also contribute to the larger problem of air pollution.
Mould and core production
The major pollutants emitted in mould and core production operations are
particulates from sand preparation, mould core forming and curing. In addition, volatile organic compounds (VOCs), carbon monoxide, and particulates may be emitted during core and mould curing or drying.
During the melting process scrap preparation and preheating may emit one or more of the following: fumes, organic compounds, carbon monoxide, or coarse particulates.
Induction and arc melting
The highest concentrations of furnace emissions occur during charging, backcharging, alloying, slag removal, and tapping operations. These emissions primarily include particulate (metal oxides) and possibly organics, depending on the scrap quality and pre-treatment.
Particulates are emitted during this stage of the process, although the quantity and composition of the particulates vary.
Other gases and organic compounds may be emitted as part of the melting process. These include carbon monoxide, sulfur oxides, lead and organic emissions. Sulfur and organic emissions may be affected by the amount of oil or grease on the scrap.
Pouring, casting, cooling and finishing
Particulate emissions can be generated during the treatment and inoculation of molten iron before pouring. Emissions from pouring consist of metal fumes, carbon monoxide, organic compounds and particulates evolved when the mold and core materials are contacted by the molten iron.
Finishing operations emit particulates during the removal of burrs, risers and gates and during shotblast cleaning. These particulates consist primarily of iron, iron oxide, and abrasive media. Painting castings can lead to a variety of VOC emissions.
Reduction and control of air emissions
When selecting collection devices for all processes you need to consider moisture, temperature and the presence of corrosive materials.
There are two ways of collecting foundry particulates: wet and dry. Dry collection methods include baghouses, mechanical collectors and electrostatic precipitators. Wet scrubbers include low and high energy types.
Baghouses are the most commonly used means of removing solid particulates from air. Fabric filters should have a fine weave or be finely felted. To maintain their efficiency make sure they are cleaned periodically by shaking them or using pulsed air.
Indications of bag failure in a fabric filter include:
- isolated instances of particulate fallout
- staining or corrosion around the exhaust stack
- pressure drop.
Fabric filter exhaust stacks can be fitted with a monitor to warn of bag failure. Make sure you use the most appropriate bags for your operations. When choosing bag type you should take into account:
- gas flow rate, temperature and dew point
- acid dew point
- particle size and distribution
- concentration of solids, and
- chemical and physical properties of solids.
Wet scrubbers may be used, particularly if there is also a need for acid gas cleaning. This is easily determined by checking the pH of the exhaust gas (a pH of less than 7 indicates a high acid content). The disadvantage of a scrubber is that a water treatment system is then needed.
Recycling certain metals can produce acid gases (e.g. recycling automotive batteries, aluminium products, used computers). Air pollution can be controlled using a caustic scrubber, perhaps followed by a fabric filter, or a dry scrubber.
Organic air toxics and odours
During casting, the resin bonding the sand is partially decomposed. This results in a
range of organic substances, some of which may be toxic. For example, phenol formaldehyde resins will produce formaldehyde and phenol derivatives.
These organic substances are the usual source of complaints about odour from foundries. Odours at the time of casting indicate that ventilation control around the casting floor needs to be improved. Odours at other times indicate problems with core baking or sand reclaiming ovens.
An afterburner is the most suitable control for organic substances. Afterburners typically operate at 0.5-second residence time and about 760 oC.
Chromium, cobalt, manganese, nickel, and vanadium may be associated with iron and steel foundries.
Antimony, arsenic, beryllium, cadmium, lead, nickel and tin may be associated with brass and bronze foundries. Arsenic may be a problem with some brasses and bronzes.
Fabric filters with effective fineness and temperature capabilities are effective controls. Scrubbers need to be high intensity to have the necessary fine spray.
Make sure the metal content of the waste is analysed to find out whether it needs to be treated as a hazardous waste.
Yellow pages: look under 'Air Pollution Measuring Equipment', 'Dust and Fume Control Equipment' and 'Environmental &/or Pollution Control Consultants'
WorkCover NSW, Tel: 131 050
DECC Environment Line, Tel: 131 555
Page last updated: 27 February 2011