Nearly all railroad tank cars are covered hopper cars, nearly all tanks on trucks, and many storage containers, are lined or coated on the inner surface. These protective linings are made of either latex or rubber (including synthetic rubber). Protective linings made of latex mixtures are used in tank cars and storage containers primarily in the food processing industry or with non-caustic, non-volatile, non-corrosive applications. Protective linings made of rubber are used in tanks cars and storage containers for transportation or storage of chemicals, caustic materials, corrosive materials, abrasives, acids, petroleum and other volatile substances. During heavy use all linings lose their protective qualities and high resistance to the substance being transported or stored, and eventually must be replaced. The useful life of each lining varies, but eventually all such linings must be replaced, the frequency depending on the nature of the lining and the severity of service. Should a vessel with a protective lining be kept in service beyond the useful life of the lining, the substance being transported or stored will invade the area between the lining and the interior (usually steel) of the tank car or storage container, and interfere with the structrual integrity of the tank. If such invasion occurs, costly and often irreparable damage can result.
Current processes for removing unwanted protective linings involve extensive man-hours, energy use and material waste. In addition, some processes require the use of materials, such as finely divided abrasives or solvents, which may present hazards to the workers.
Latex linings are currently removed as follows: First, the inside of the lined vessel is washed to remove any traces of the material which was stored or transported in the vessel. Then, the lining is removed by sand blasting until "white metal" (bare metal) shows. This requires at least two men to be inside the vessel, using high pressure air hoses and wearing protective clothing. masks and breathing filters. The vessel interior is then swept clean of sand, steel shot and latex fragments. Finally, the vessel interior is cleaned, e.g. by sweeping or vacuuming, to remove any debris. After cleaning and inspection, the vessel is ready for relining.
To reline the vessel, the clean metal surface is primed, the primer coat is allowed to cure, and then the new latex lining is applied, as for example by spraying.
A representative process for removing a rubber lining is as follows: After the interior of the vessel has been washed, it is heated to about 600.degree. C., causing the rubber to char. The charred rubber lining is then scraped away with hand tools by men inside the vessel. Any lining which remains fixed to the walls of the tank is removed by means of blow torches. The interior of the vessel is then again washed to remove debris. Immediately after washing, the interior walls of the vessel are dried, with propane gas dryers, for example, to remove any remaining moisture and thereby prevent oxidation. The dry interior wall of the vessel may be sand blasted if required. After sand blasting is completed, the vessel interior is again swept free of debris and inspected.
A new lining is applied to the clean metal surface by means similar to those described above for applying latex linings. That is, the metal surface is first primed, and then a rubber lining is applied by known means.
Cryogenic processes for removing adherent coatings or linings from metal substrates are also known. As far as applicant is aware, none of these is in commercial use.
U.S. Pat. No. 4,409,034 describes a cryogenic process for removing a contaminant material from the surface of a metallic substrate (which may be the interior of a vessel such as a railroad or truck tank car), which comprises spraying a cryogenic liquid, either carbon dioxide or nitrogen, against the surface of the contaminant, vibrating the surface to loosen and remove the contaminant material, and then removing the contaminant material, e.g. by a conveyor, shovel or hydraulic scoop. Cryogenic temperatures in the range of about -100.degree. to about -120.degree. C. are required for injection of the cryogenic liquid into the contaminant substance.
U.S. Pat. No. 4,409,034 does not suggest any good way for removing contaminant material debris once it has been separated from the metal surface. Instead, the debris is simply collected in a pile and removed by a conveyor, for example. The method of this patent requires the presence of both workmen and mechanical devices at the location where such debris collects, which would be inside the tank car or vessel when the cleaning method is applied thereto.
U.S. Pat. No. 4,020,992 describes another cryogenic process for stripping plastic from metal. According to the process described therein, a scrap material, such as a plastic lined metal bottle closure, is treated with liquid nitrogen at about -150.degree. F. to about -200.degree. F. to cause the plastic liner to become brittle. The chilled metal closures and liners are agitated in a hammer mill to break the bond between the closures and the liners. The plastic particles are then separated from metal pieces, (e.g., by a tumble barrel to separate out large metal pieces) followed by an electrical separator.
U.S. Pat. No. 3,527,414 describes a cryogenic process for removing plastic insulation from metal wire. The wire, chopped into short length, is cooled to -20.degree. to -195.degree. C. to embrittle the insulation.
None of the above processes describes the use of pressure, except to keep the cryogenic liquid in the liquid phase, suggestion that actual contact between the cryogenic liquid and the coated substrate is carried out at atmospheric pressure.
Although processes for separating adherent coatings from metallic substrates by use of cryogenic liquids are know, none has overcome one of the major problems which attends the older methods described above, and that is the necessity for manpower inside the vessel to be cleaned.
Quite a different type of metal article having an unwanted adherent coating or lining is a water pipe or main. Mineral deposits gradually build up on the inner walls of water pipes and mains, particularly in hard water areas as is well known. Home and industrial users may use water softeners, which greatly reduce the rate at which such buildup occurs. Municipalities seldom if ever use water softening techniques, because of the large volumes of water which must be treated and the prohibitive costs of such treatment. Instead, mineral deposit or scale gradually builds up and eventually it is so thick that the carrying capacity of the pipe or main is materially reduced. At present there is no practical way to remove this deposit or scale. Instead, it is necessary to replace the pipe or main once the scale has reached such a thickness that water flow is materially impeded.