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What Is Dry Ice?
Dry ice pellets are made by taking liquid carbon dioxide (CO2) from a pressurized storage tank and expanding it at ambient pressure to produce snow. The snow is then compressed through a die to make hard pellets. The pellets are readily available from most dry ice suppliers nationwide. For dry ice blasting, the standard size is 1/8-inch high-density dry ice pellets.
What Is Dry Ice Blasting?
It is a process in which a blasting gun fires dry ice particles (rice-sized) at supersonic speed to impact and clean a surface. The particles are accelerated by compressed air, just as with other blasting systems. Upon impact the dry ice sublimates (goes from a solid to a gas without passing through a liquid phase). The substrate (surface) is left free of mold spores.
There are three phases in the dry ice blasting process. Energy transfer works when dry ice pellets are propelled out of the blasting gun at supersonic speed and impact the surface. The energy transfer helps to knock off the contaminant with little or no damage to the surface.
Micro-thermal shock occurs when the freezing effect of the dry ice pellets hitting the contaminant creates a micro-thermal shock (caused by the dry ice temperature of -79º C) between the surface contaminant and the substrate. This phase isn't as much a factor for removal of mold as it is with resins, oils, waxes, food particles and other contaminants. For substances such as these, the thermal shock causes cracking and delamination of the contaminant, furthering the elimination process.
The final phase, gas pressure, has the dry ice pellet explode on impact and, as the pellet warms, it converts to a CO2 gas, generating a volume expansion of 400 to 800 times. The rapid expansion underneath the contaminant on the substrate forces off the contaminant from behind. The energy transfer and gas pressure dynamics cause the contaminant to be relocated, becoming airborne (as with mold spores) or falls to the ground. The mold spores then need to be removed by HEPA filters. Since the dry ice sublimates into a gas, no media remains to be cleaned up.
As for air system requirements, a large number of applications using dry-ice blasting equipment only require between 80 to 100 psi and 120 to 150 CFM. An evaluation of system air is usually recommended to determine if the facility has sufficient capabilities to run dry ice blasting equipment at the levels desired for each specific application.
Mold Remediation Process
Utilizing dry-ice blasting technology does not alter the mold-cleaning process very much. Take the example of a second floor residence with no attic and having drywall on walls and ceiling being infested with mold. First, the second floor needs to be isolated from the first floor. Next, negative pressure needs to be created using a HEPA-filtered air scrubber. If there is any severely damaged carpeting and drywall, it should be double bagged and discarded. For the sake of this example, we will say that one half of the ceiling and all exterior wall drywall needed to be stripped and removed.
At this point, dry-ice blasting can be utilized to clean the plywood and support beams. The blasting gun can easily be managed to target the desired mark. Specific nozzle types best suited for cleaning wood can be utilized, e.g. a fan-shaped nozzle, creating a pattern several inches wide, can be used to "sweep" up and down boards and beams. The dry ice being fired on the wood can typically remove mold in a way that is clearly visible and in a methodical manner.
Once the blasting phase is complete and all surfaces have been vacuumed and cleared of sawdust and other debris, mold remediators should follow up by applying a micro-biocide spray to remediated areas to inhibit future growth.
A few details need to be considered to operate dry-ice blasting equipment. Blasting in an enclosed area is generally safe with proper ventilation. However, because CO2 is 50 percent heavier than air, and containments may limit ventilation enough where excessive levels of CO2 may accumulate, to maintain negative pressure differentials exhaust air volume may need to be greatly increased. In small areas or ones such as crawl spaces, great care should be given to sustaining proper air levels. If the OSHA Permissible Exposure Limit (PEL) for CO2 (5,000 ppm or 0.5 percent for an eight-hour time-weighted average) is exceeded, supplied-air respirators must be used.
Ear protection is necessary, as the process can get very noisy. Second, because the temperature of dry ice can be as low as -79oC (-109 oF), insulated gloves should always be worn when working with it. It is also very important that full personal protective equipment (PPE) is worn.
With dry ice blasting as it pertains to mold removal, it is common to see a 60 percent time savings over other methods. Blasting can also effectively and easily clean in tight spaces that would be difficult for hands or tools to reach.
When the dry ice changes from a solid to a gas, the volume expansion over surfaces such as wood, concrete or stone efficiently results in a stripping effect removing the mold from the surfaces. Typical results show less than 1 percent of toxic mold spores remain.
Dry ice sublimates and leaves no media for cleanup. The blasting process will generate saw dust, just as sanding would; however, when compared to other blasting methods, the time and cost to address the secondary waste generated by the media is eliminated.