Carbon dioxide (CO2) blasting is a relatively new process that has been developed primarily for the removal of contaminants from surfaces that are easily damaged by traditional shot or bead blasting, either because of the material of construction or complexity of design.

Replacing the abrasive media with pellets of solid CO2 or dry ice eliminates the physical impact of solid particles and delivers an impressive range of other benefits.

Characteristics of dry ice pellets
CO2 is available commercially as a liquefied gas delivered by many industrial gas suppliers, either in portable liquid containers or from a bulk tanker into a dedicated storage vessel at the customer’s premises.

In terms of physical composition, CO2 is a colourless, odourless, non-toxic gas, but can produce a sour taste in the mouth and a stinging sensation in the nose & throat when inhaled at concentrations much higher than usual atmospheric levels.

These effects result from the gas dissolving in the mucous membranes and saliva, forming a weak solution of carbonic acid.

Concentrations above 5,000 ppm are considered very unhealthy, and those above about 50,000 ppm (equal to 5% by volume) are considered dangerous to human and animal life.

CO2 solidifies at pressures lower than 5.1atm and the storage pressure must be maintained to prevent the formation of a CO2 ‘lollipop’.

Pellets meanwhile, are formed by expanding the liquid at ambient pressure so that it forms ‘snow’ which can be easily compressed through a die, to form solid rice-sized pellets.

Its boiling temperature at atmospheric pressure is -78.5°C but CO2 ice is referred to as dry-ice, because at this pressure it does not liquefy, but sublimes directly to the gas phase when exposed to heat. The temperature of dry-ice pellets cannot be called cryogenic, but with a latent heat of sublimation of 571.3 kJ/kg, can deliver rapid surface cooling in the blasting process.

The CO2 blasting process
CO2 pellets are propelled by compressed air through a nozzle to achieve supersonic speed, and directed at the contaminant-coated item or assembly to produce a clean, dry, undamaged surface.

The coating removal is achieved as the combined result of three distinct mechanisms, notably Kinetic Energy Transfer, Thermal Shock, and Velocity induced pressure differential.

Kinetic Energy Transfer occurs when the high velocity pellets impact with the contaminated surface. The kinetic energy transferred is sufficient to knock some of the contaminant coating loose, and this is the primary cleaning mechanism.

Thermal Shock occurs when the pellets change from solid to gaseous CO2 almost instantaneously and absorb the required latent heat, mainly from the top layer of coating still adhered to the substrate.

This creates a large temperature difference between successive layers of coating and the stress produced as these layers rapidly contract, is sufficient to cause de-lamination and results finally, in failure of the bond at the surface of the substrate.

Velocity induced pressure differential is caused by the rapid expansion of the CO2 molecules after sublimation, because this phase change involves an 800-times increase in volume. The gas expands explosively away from the point of impact and accelerates to high velocity parallel to the substrate surface.

The resulting low pressure zone exerts a lifting force that effectively peels away the fractured layers of coating.

Environmental impacts
Although the CO2 used in the blasting process is a greenhouse gas and extensive programmes are in place to reduce its accumulation in the atmosphere, this process is essentially Carbon Neutral because the CO2 supplied commercially is usually derived from flue gases of industries that otherwise would have released it into the air.

CO2 blasting does not contribute any additional carbon into the environment.

As a substitute for traditional cleaning methods that employed solvents, chlorinated compounds, corrosive or toxic chemical cleaning agents and other dangerous substances, the CO2 blasting process uses only Food Grade liquid CO2 and therefore reduces the risk of these harmful substances to the environment. It will also not compromise existing ISO 14001 programmes.

Conventional blasting processes using steel or iron media as the abrasive normally recycle the media up to 3000 times before it is discarded. In the case where the contaminant to be removed is a dangerous substance, the media becomes contaminated and not only requires replacement, but must also be safely disposed of as a dangerous material after use.

Productivity Factors
CO2 blasting delivers increased productivity in a wide range of industrial, commercial and domestic applications.

The aggressiveness of the process can easily be controlled by the operator, meaning that it can be used without any risk of damage on sensitive or even fragile items, such as books that have been contaminated with soot from a building fire.

Alternatively, by selecting the appropriate nozzle and operating parameters, the operator can easily remove tough deposits like bitumen sludge or polyurethane overspray, with practically no limit.

This means that the same cleaning process can be applied to virtually all cleaning applications, and there is no need to use multiple cleaning processes with the required variety of equipment and materials.

Using CO2, cleaning can usually be done in-situ without disconnecting, disassembling or moving machinery or plant equipment. Separate facilities for grit blasting rigs and hoods, fume and dust catchers, solvent sprayers, soaking tanks, pressure washers, steam cleaners and drying rooms are no longer required.

The storage of hazardous cleaning materials is eliminated and there is no need for specialised protective clothing, while the space previously required for storage and operating various pieces of cleaning equipment, becomes available for productive activities.

CO2 pellets are converted completely into gas that dissipates naturally into the air, leaving absolutely no residue or moisture. This makes the process particularly useful for electrical equipment like motors, transformers, controls and switchgear.

Dust and dirt continually plague electrical systems, causing sensors and controls to become unreliable and even causing dangerous energy releases in high voltage systems.

CO2 blasting however, allows regular cleaning to be performed with far less downtime, because disconnection is often unnecessary and no drying time is required before putting electrical systems back into service. No flammable solvents are involved and no fire hazard is introduced.

The inherent cleanliness of the CO2 blasting process implies that after the cleaning task is completed there is no other mess, contamination or spillage to deal with, apart from that of the contaminant material removed by cleaning. There is no secondary waste generated as in most other cleaning processes.

CO2 also boasts the added benefit of natural anti-bacterial properties that destroy mould, fungus and algae immediately on contact making any further disinfection with toxic chemicals unnecessary.

Complex and expensive machinery like gas turbine rotors can be cleaned with CO2 blasting in a fraction of the time that hand-cleaning requires. Internal boiler tubes, elevated silos or smoke stacks are tackled easily.

CO2 blasting equipment can be integrated into automated systems like assembly lines to enable continuous cleaning of critical components.

Conclusion
CO2 blasting is therefore emerging as an easy to use, practical example of the countless ways that industrial gases have contributed to industrial progress.