Dry ice

Dry ice is the solid form of carbon dioxide (CO2), a white, ice-like and odorless solid.

At normal pressure, dry ice sublimates at approx. -79°C, i.e. it transitions directly to the gas phase without prior melting. As the triple point Pt of carbon dioxide is at Tt = -56.57°C and pt = 5.185 bar, carbon dioxide can only melt above 5.185 bar. The density is about 1.56 g/cm3, being higher than that of water. The critical point Pc of carbon dioxide lies at a temperature of Tc = 30.98°C and a pressure of pc = 73.75 bar (Fig. 1). 1m³ of dry ice weighs about 1,500kg depending on compacting pressure, while 1m³ of gaseous carbon dioxide only weighs 1.98kg. This means a 760-fold change in volume upon sublimation.

Production

Dry ice is produced from liquid carbon dioxide. In a dry ice pelletizer, liquid CO2 is expanded under controlled conditions. During this process, part of the carbon dioxide evaporates, cooling the remainder by evaporative cold until it freezes to form a so-called carbon dioxide or dry ice snow. This snow is then pressed though an extruder plate to obtain round, hard pellets. Depending on the extruder plate and the intended use,

  • oblong grains with a diameter of 3 or 1.7mm are produced for steel-related purposes (see Fig. 2) and
  • larger pellets, dry ice discs or dry ice blocks for cooling purposes (see Fig. 3).

    Small amounts of dry ice can be prepared by inversing a cylinder of CO2 and opening it. The escaping liquid CO2 largely evaporates on the spot, cooling so rapidly that part of it freezes in the form of CO2 snow. The same principle is used by CO2 fire extinguishers which extract heat from the fire in addition to displacing atmospheric oxygen and are to cool the burning materials to below the flash point.

    Application

    In the field of foundry technology, dry ice is used for a variety of applications, in particular for cleaning and degreasing molds, gravity dies, patterns and many more.

    In this respect, a distinction is made between the dry ice blasting (see also Dry ice blasting unit) and CO2 snow blasting process. In dry ice blasting, small dry ice pellets are blown onto the surfaces or tool surfaces to be cleaned at high speeds (compressed air) (Figs. 4 and 5). Soiling (grease, separating agents, oil, coatings, etc.) turn hard and brittle due to the cold and flake off.

    Literature references:

    Website of ASCO Kohlensäure AG
    Wikipedia, entry on dry ice

    • Fig. 1: Phase diagram of carbon dioxide and triple point (not so scale), source: Wikipedia Commons, author: Sponk
    • Fig. 2: Dry ice pellets with a diameter of 3mm, source: ASCO Kohlensäure AG, Romanshorn, CH
    • Fig. 3: Dry ice blocks, source: Wikipedia Commons, author: Mark S.
    • Fig. 4: Dry ice blasting of a low-pressure gravity when hot, source: ASCO Kohlensäure AG, Romanshorn, CH
    • Fig. 5: Cleaning of a core box using dry ice, source: ASCO Kohlensäure AG, Romanshorn, CH
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