ZERO WASTE TECHNOLOGIES
Because of the temperature difference between the dry ice particles (-109°Fahrenheit) and the surface being treated, thermal shock occurs during the blasting process. This breaks down the bond between the substrate and the contaminant to be removed from it. The dry ice vaporizes on impact, so no secondary waste stream is created.
A number of industries worldwide have discovered the advantages of this precision cleaning process. Applications range from PCB removal to lead paint abatement and masonry scarification.
What is dry ice blasting?
Dry ice (carbon dioxide) blasting is similar to sand blasting, bead blasting, or soda blasting where a media is accelerated in a pressurized air stream (or other inert gas) to impact the surface to be cleaned.
Why would I use dry ice instead of a traditional blast media?
Most other blast media leaves secondary waste behind. Dry ice sublimates (vaporizes) upon impact with the surface. All that remains is the contaminate that you are removing. Also, since dry ice vaporizes on impact, the process can be used to clean complicated cavities where typical grit blast media will become trapped. Some typical traditional methods include: SOLVENT CLEANING, (ULTRA-)HIGH-PRESSURE WATER BLASTING and ABRASIVE BLASTING.
THE FUNDAMENTALS OF DRY ICEBLAST CLEANING
Today, CO2 blasting is being effectively used in a wide array of applications from heavy slag removal to delicate semiconductor and circuit board cleaning. Imagine a process that can be used on-line without damaging equipment or requiring a machine "teardown". Unlike conventional toxic chemicals, high pressure water blasting and abrasive grit blasting, CO2 blasting uses dry ice particles in a high velocity air flow to remove contaminates from surfaces without the added costs and inconvenience of secondary waste treatment and disposal.
In the early 1930's, the manufacture of solid phase carbon dioxide (CO2) became possible. During this time, the creation of "Dry Ice" was nothing more than a laboratory experiment. As the procedure for making Dry Ice became readily available, applications for this innovative substance grew. Obviously, the first use was in refrigeration. Today, Dry Ice is widely used in the Food Industry for packaging and protecting perishable foods.
In 1945, stories exist of the U.S. Navy experimenting with Dry Ice as a blast media for various degreasing applications. In May 1963, Reginald Lindall received a patent for a "method of removing meat from bone" using "jetted" Carbon dioxide particles. In November 1972, Edwin Rice received a patent for a "method for the removal of unwanted portions of an article by spraying with high velocity Dry Ice particles". Similarly, in August 1977, Calvin Fong received a patent on "Sandblasting with pellets of material capable of sublimation".
The work and success of these early pioneers led to the formation of several companies in the early 1980's that pursued the development of Dry Ice Blasting Technology.
Dry Ice pelletizers and blast machines entered the industrial markets in the late 1980's. At this time the blast machines were physically large, expensive, and required high air pressure for operation (pressures greater than 200 psi). As the CO2 blast technology advanced, the blast machine's size and cost dropped. The latest nozzle technology has made blasting effective at shop air pressures (80 psi).
What Is Dry Ice?
Dry Ice is the solid form of Carbon Dioxide (CO2), which is a colorless, tasteless, odorless gas found naturally in our atmosphere. Though it is present in relatively small quantities (about 0.03% by volume), it is one of the most important gases we know of.
CO2 is a natural media, which serves many life-sustaining purposes. It is a key element involved in the carbon cycle; it is the only source of carbon for the carbohydrates produced by agriculture; it stimulates plant growth; and it helps to moderate the temperature of the earth overall. Animal respiration is believed to add 28 million tons of Carbon Dioxide per day into the atmosphere. By contrast, the U.S. CO2 industry can supply only 25,000 tons per day and 95% of this amount is from by-product sources, or less than 0.04% of the other sources combined.
With a low temperature of -109° F, Dry Ice solid has an inherent thermal energy ready to be tapped. At atmospheric pressure, solid CO2 sublimates directly to vapor without a liquid phase. This unique property means that the blast media simply disappears, leaving only the original contaminant to be disposed of. In addition, cleaning in water sensitive areas is now practical.
The grade of carbon dioxide used in blasting is the same as that used in the food and beverage industry and has been specifically approved by the FDA, the EPA and the USDA. Carbon dioxide is a non-poisonous, liquefied gas which is both inexpensive and easily stored at work sites. Of equal importance, it is non-conductive and non-flammable.
CO2 is a natural by-product of several industrial manufacturing processes such as fermentation and petrol-chemical refining. The CO2 given off by the above production processes is captured and stored without losses until needed. When the CO2 is returned to the atmosphere during the blasting process, no new CO2 is produced. Instead, only the original CO2 by-product is released.
Listed in Table 1 are the physical properties and conversion factors for CO2 in its various forms: Table 1. Carbon Dioxide (CO2 ) properties.
97.5 LB/ft3 at -109° F
63.7 LB/ft3 at 0° F
0.123 LB/ft3 at 32° F
-69.9° F at 75.1 psia (triple point)
-109.3° F (sublimates)
Liquid-to-Gas Conversion Rate
8.726 SCF (gas)/LB (liquid at 0°F and 305 psia)
Liquid-to-Snow Conversion Rate
.46 LB snow / LB liquid at 0.0° F
.57 LB snow / LB liquid at -55.0° F
How Does Dry Ice Blasting Work?
The Basic Process
Dry Ice particle blasting is similar to sand blasting, plastic bead blasting, or soda blasting where a media is accelerated in a pressurized air stream (or other inert gas) to impact the surface to be cleaned or prepared. With Dry Ice blasting, the media that impacts the surface is solid carbon dioxide (CO2) particles. One unique aspect of using Dry Ice particles as a blast media is that the particles sublimate (vaporize) upon impact with the surface. The combined impact energy dissipation and extremely rapid heat transfer between the pellet and the surface cause instantaneous sublimation of the solid CO2 into gas. The gas expands to nearly eight hundred times the volume of the pellet in a few milliseconds in what is effectively a "micro-explosion" at the point of impact. Because of the CO2 vaporizing, the Dry Ice blasting process does not generate any secondary waste. All that remains to be collected is the contaminate being removed.
As with other blast media, the kinetic energy associated with Dry Ice blasting is a function of the particle mass density and impact velocity. Since CO2 particles have a relatively low hardness, the process relies on high particle velocities to achieve the needed impact energy. The high particle velocities are the result of supersonic propellant or airstream velocities.
Unlike other blast media, the CO2 particles have a very low temperature of -109° F. This inherent low temperature gives the Dry Ice blasting process unique thermodynamically induced surface mechanisms that affect the coating or contaminate in greater or lesser degrees, depending on coating type. Because of the temperature differential between the Dry Ice particles and the surface being treated, a phenomenon known as "fracking" or thermal shock can occur. As a material's temperature decreases, it becomes embrittled, enabling the particle impact to break-up the coating. Refer to Figures 2 and 3.
Also, the thermal gradient or differential between two dissimilar materials with different thermal expansion coefficients can serve to break the bond between the two materials. This thermal shock is most evident when blasting a non-metallic coating or contaminate bonded to a metallic substrate.
Quite often companies examining this process are concerned with the effect the thermal shock will have on the parent metal. Studies have shown that the temperature decrease occurs on the surface only, there is no chance of thermal stress occurring in the substrate metal. To illustrate this principle, an experiment was performed where thermocouples were imbedded into a steel substrate at varying depths (flush with the surface to 2 mm deep). Refer to Figure 4.
A CO2 blast jet was constantly traversed across the test specimen for 30 seconds (a relatively long dwell time for this process) and the thermal couples recorded the changing temperatures at the various depths. As shown in Figure 5, the surface mounted thermocouple shows a temperature drop each time the blast jet impinged directly upon the thermocouple (50° C in about 5 seconds). In contrast, the thermocouples imbedded at various depths in the substrate recorded a slow gradual drop in temperature corresponding to the overall test plate temperature drop. The thermocouple 2mm deep only dropped 10° C after 30 seconds. This curve illustrates that the "Thermal Shock" occurs only at the surface where the coating or contaminate is bonded to the substrate (Reference 1) and has no detrimental effect to the substrate.
Another approach to looking at thermal stress is by studying the use of Dry Ice blasting in the molded rubber industry. Here, hot steel molds operating at 300+ °F are blasted with -109 °F Dry Ice particles. The temperature difference between the hot mold and cold Dry Ice will not cause cracking. There are two reasons for this phenomenon. First, as seen above, the temperature gradient occurs at the surface. Secondly, the thermal stresses involved are much less than those encountered during normal heat treatment.
Even at high impact velocities and direct "head-on" impact angles, the kinetic effect of solid CO2 particles is minimal when compared to other media (grit, sand, PMB, etc.). This is due to the relative lack of hardness of the particles and the almost instantaneous phase change to a gas on impact which effectively provides an almost nonexistent coefficient of restitution in the impact equation. Because CO2 blasting is considered non-abrasive and relies on the thermal effects discussed above, the process may be applied to a wide range of materials without damage. Soft metals such as brass and aluminum cladding can be CO2 blasted for the removal of coatings or contaminates without creating surface stresses (pinging), pitting, or roughness.
Benefits of CO2 Blast technology
The natural sublimation of Dry Ice particles eliminates the cost of collecting the cleaning media for disposal. In addition, containment and collection costs associated with water/grit blasting procedures are also eliminated.
Because CO2 blast systems provide on-line maintenance capabilities for production equipment (cleaning "on-line"), timely and expensive detooling procedures are kept to a minimum. Dedicated cleaning cycles are no longer required as preventative maintenance schedules can be adopted which allow for equipment cleaning during production periods. As a result, throughput is increased without adding labor or production equipment.
A Dry Process
Unlike steam or water blasting, CO2 blast systems will not damage electrical wiring, controls, or switches. Also, any possible rust formation after cleaning is far less with Dry Ice blasting when compared to steam or water blasting. When used in the Food Industry, Dry Ice blasting reduces the potential for bacteria growth inherent to conventional water blasting.
Carbon dioxide is a non-toxic element that meets EPA, FDA, and USDA industry guidelines. By replacing toxic chemical processes with CO2 blast systems, employee exposure and corporate liability stemming from the use of dangerous chemical cleaning agents can be materially reduced or eliminated completely. Since CO2 gas is heavier than air (CO2 gas displaces oxygen), care must be taken if blasting in enclosed areas or down in a pit.
There are many names and types of production fixtures, but virtually any item that is part of the production process and is difficult to clean on-line or during production hours by traditional means may be an excellent Dry Ice application. Applications such as:
- Conveyor Components
- Material Handling
- Car Carriers
- Weld slag Removal From Robotics, Fixtures, Carriers
- Removal of Oils and Grease from chains, machinery, etc.
- Cleaning Packaging Equipment
- Removal of Adhesives
We can't predict the future, but we do know that CO2 particle blast cleaning is not a mature technology. Only eight years have passed since the process was first introduced and the technology is continuing to experience rapid growth, change and advancement.
Today's environmental issues are only the beginning. Legislation on Air and Water quality will continue to impose stricter regulations on general industry and Dry Ice blasting is continually moving forward to meet these new demands.
One thing is certain. The successful company of the future will have SCE incorporate the CO2 blast process into its operation.