Thermal cleaning is a combined process involving

pyrolysis The pyrolysis (or devolatilization) process is the thermal decomposition of materials at elevated temperatures, often in an inert atmosphere. It involves a change of chemical composition. The word is coined from the Greek-derived elements ''py ...

and

oxidation Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a d ...

. As an industrial application, thermal cleaning is used to remove

organic Organic may refer to: * Organic, of or relating to an organism, a living entity * Organic, of or relating to an anatomical organ Chemistry * Organic matter, matter that has come from a once-living organism, is capable of decay or is the product ...

substances such as

polymer A polymer (; Greek '' poly-'', "many" + ''-mer'', "part") is a substance or material consisting of very large molecules called macromolecules, composed of many repeating subunits. Due to their broad spectrum of properties, both synthetic a ...

plastic Plastics are a wide range of synthetic or semi-synthetic materials that use polymers as a main ingredient. Their plasticity makes it possible for plastics to be moulded, extruded or pressed into solid objects of various shapes. This adaptab ...

s and

coating A coating is a covering that is applied to the surface of an object, usually referred to as the Substrate (materials science), substrate. The purpose of applying the coating may be decorative, functional, or both. Coatings may be applied as liquid ...

s from parts, products or production components like extruder screws,

spinnerets A spinneret is a silk-spinning organ of a spider or the larva of an insect. Some adult insects also have spinnerets, such as those borne on the forelegs of Embioptera. Spinnerets are usually on the underside of a spider's opisthosoma, and are ...

and

static mixer A static mixer is a precision engineered device for the continuous mixing of fluid materials, without moving components. Normally the fluids to be mixed are liquid, but static mixers can also be used to mix gas streams, disperse gas into liquid o ...

s. Thermal cleaning is the most common cleaning method in industrial environment. A variety of different methods have been developed so far for a wide range of applications.

Process

Heat is supplied for pyrolysis and air is supplied for oxidation. Depending on the procedure, pyrolysis and oxidation can be applied consecutively or simultaneously. During thermal cleaning, organic material is converted into

volatile organic compounds Volatile organic compounds (VOCs) are organic compounds that have a high vapour pressure at room temperature Colloquially, "room temperature" is a range of air temperatures that most people prefer for indoor settings. It feels comfortable to a ...

hydrocarbon In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons are examples of group 14 hydrides. Hydrocarbons are generally colourless and hydrophobic, and their odors are usually weak or ex ...

s and

carbonized Carbonized were a Sweden, Swedish avant-garde metal band formed 1988 in Saltsjöbaden. The band was formed by Lars Rosenberg in 1988, with Dismember (band), Dismember vocalist Matti Kärki. Joined by drummer Piotr Wawrzeniuk, the trio was compl ...

gas.

Inorganic In chemistry, an inorganic compound is typically a chemical compound that lacks carbon–hydrogen bonds, that is, a compound that is not an organic compound. The study of inorganic compounds is a subfield of chemistry known as '' inorganic chemist ...

elements remain. Typical process temperatures range between 400 °C to 540 °C (750 °F to 1000 °F). Several types of industrial thermal cleaning systems are available:

Fluidized bed systems

Fluidized bed systems use

sand Sand is a granular material composed of finely divided mineral particles. Sand has various compositions but is defined by its grain size. Sand grains are smaller than gravel and coarser than silt. Sand can also refer to a textural class of s ...

or aluminium oxide (alumina) as heating medium. They apply pyrolysis and oxidation simultaneously. These systems clean fast, from 30 minutes process time up to two hours. The medium does not melt or boil, nor emit any vapors or odors. Thermal shock can be a problem with some parts. Pollution control devices may be needed to protect the environment.

Vacuum ovens

Vacuum ovens use pyrolysis in a

vacuum A vacuum is a space devoid of matter. The word is derived from the Latin adjective ''vacuus'' for "vacant" or "void". An approximation to such vacuum is a region with a gaseous pressure much less than atmospheric pressure. Physicists often dis ...

. This method is very safe because uncontrolled combustion inside the cleaning chamber is avoided. The cleaning process in this relatively new approach takes 8 to 30 hours. Vacuum pyrolysis is the only method that applies pyrolysis and oxidation consecutively. In two-chamber versions, molten plastic drains into an unheated chamber to capture the bulk of the polymer to reduce the fumes. Vacuum ovens are also electrically powered.

Burn-off ovens

Burn-off ovens, also known as heat-cleaning ovens, are gas-fired and used for removing organics from heavy and large metal parts. The process time is moderate, approximately 4 hours. Fires can occur from the fumes created during cleaning. The design is simple and inexpensive. Different types are available. Modern types contain an additional afterburner that operates at a minimum of 1,500°F (816°C) and consumes any smoke created by the process.

Molten salt baths

Molten salt baths belong to the oldest thermal cleaning systems. Cleaning with

molten salt Molten salt is salt which is solid at standard temperature and pressure but enters the liquid phase due to elevated temperature. Regular table salt has a melting point of 801 °C (1474°F) and a heat of fusion of 520 J/g.< ...

is fast: 15 to 30 minutes process time. The process has the risk of dangerous splatters due to chemical reactivity, or other potential hazards, like explosions or toxic

hydrogen cyanide Hydrogen cyanide, sometimes called prussic acid, is a chemical compound with the formula HCN and structure . It is a colorless, extremely poisonous, and flammable liquid that boils slightly above room temperature, at . HCN is produced on an ...

gas. Another disadvantage is that parts can be warped or altered in design tolerances. Molten salt baths can be environmentally unfriendly. Due to their disadvantages, they are rarely used today.

Areas of application

Plastics industry The plastics industry manufactures polymer materials—commonly called plastics—and offers services in plastics important to a range of industries, including packaging, building and construction, electronics, aerospace, and transportation. It is ...

: Cleaning of production components like extruder screws, spinnerets, or static mixers, of plastic residues polymers. * Remanufacturers: The rebuilding industry uses thermal cleaning to remove oils and greases from used parts such as engine blocks or starter housings. * Rewinding industry: In the rewinding industry there are applications to clean motor components of resins, oils and greases. * Industrial laboratories: Thermal cleaning is used to clean ceramics and glassware. * General industrial applications: Thermal cleaning is used to remove paint,

varnish Varnish is a clear transparent hard protective coating or film. It is not a stain. It usually has a yellowish shade from the manufacturing process and materials used, but it may also be pigmented as desired, and is sold commercially in various ...

es and coatings from industrial parts.

References

{{Reflist Pyrolysis Chemical processes Industrial processes Cleaning methods