De-icing salt
De-icing salt , colloquially road salt or Tausalz called, consists of at least 94% (ideally 98%) of conventional table salt and is used as Winter grit for melting snow and ice on roads used. It is therefore a winter service measure to counteract slippery snow and ice . However, the use of salt is prohibited in many communities, especially on sidewalks. In Germany, de-icing salt was not subject to the salt tax levied until 1993 , which is why it had to be offered denatured , i.e. no longer suitable for human consumption due to additives.
history
The systematic use of salt to remove snow and slipperiness began in the early 20th century. In Paris at this time salt was used on a large scale to combat black ice. The effects on the hooves of the draft animals and on the iron substructures and bridges were described as particularly disadvantageous .
With the increase in motorization, salt spreading began in Germany for reasons of road safety in the 1960s. At that time, the road maintenance depots also built salt halls to store the salt needed in winter. For use, the salt was loaded onto trucks, and then scattered from the open loading area by hand on the route. To make winter service more economical, mechanical spreaders were soon used.
However, sprinkling dry salt did not have the desired effect in every situation. For example, the salt was blown off the roadway relatively quickly by the wind on bridges that were prone to black ice. This problem was only solved with the development of pre-wetted salt in the early 1970s.
composition
The commercial de-icing salt consists largely of table or rock salt, i.e. sodium chloride (NaCl). It can also contain natural minor minerals such as anhydrite (calcium sulphate), magnesium sulphate or clay . Flow aids prevent clumping. Other de-icing agents used are calcium chloride (CaCl 2 ) and magnesium chloride (MgCl 2 ) and potassium chloride (KCl). Sometimes the mineral kainite is also used as a thawing agent.
Requirements for de-icing salt (here using the example of the Styrian state government in 2006):
- De-icing salt NaCl
- Minimum proportion of NaCl 98%
- Grain distribution steady
- Largest grain content over 1.6 mm maximum 10%
- Small grain content below 0.16 mm maximum 5%
- Products with a low sulphate content are preferred
- The proportion of insoluble substances must not exceed 0.25%.
- Moisture in hall storage maximum 1.2%
- Moisture in silo storage maximum 0.6%
effect
The effect of the de-icing salt is based on the physico-chemical effect of the lowering of the molar melting point , which causes the freezing point of a liquid to decrease the more particles are dissolved in it. In practice, these particles are the ions of the salt dissolved in the water. The de-icing salt is called brine , i.e. H. in aqueous solution, which therefore has a lower freezing point than ice, in which by nature significantly fewer particles are dissolved. The freezing point of a sodium chloride brine can be lowered to −21.1 ° C.
At the interface between the ice and the brine applied to it, water molecules change from the ice into the liquid state and become part of the brine. Since the brine continues to have a lower freezing point than the ice, no water molecules change from the brine to the solid state. As a result, the ice slowly dissolves, while the brine is increasingly diluted and its freezing point rises. Consequently, the brine used must have a sufficiently high concentration so that it does not reach the freezing point at a corresponding temperature.
De-icing salt cannot develop its effect if it is applied to the ice in its pure form (instead of as brine). It must always be dissolved in the water first in order to be able to develop its effect. This can happen when, e.g. B. due to the weather, liquid water is present in addition to the ice.
In addition to the shape, the ambient temperature in particular plays a role in the effectiveness of de-icing salt. Sodium chloride is only suitable for temperatures down to −10 ° C. Calcium chloride and magnesium chloride are more effective at lower temperatures. But at around −20 ° C the thawing effect of these two salts also fails.
application
The individual user in the small area sprinkles the salt in powder form on the icy area - as far as permitted in the respective municipality. In large-scale applications, especially in road traffic , it is slightly mixed with brine and then applied over a wide area using a spreader plate attached to the vehicle. Due to its damp condition, it is only carried away by the wind in small amounts (around 15%).
In order to achieve the full effect on the road, however, certain framework conditions must be in place. The road temperature may only fluctuate within a certain range. If it is too deep, no thawing effect can be achieved even with salt. In addition, the road must have a certain vehicle frequency. Moving cars cause the pressure of the tires to melt and crush the ice and to throw the mixture of salt solution and ice outwards. With the repeated whirling up, water also evaporates, which in turn concentrates the salt solution, which can then dissolve more ice. Tire-wide fairways soon form and the roads - starting from there - gradually become ice-free and dry.
De-icing salt can also be applied as wet salt . Here, the salt is moistened with a salt solution of sodium, calcium or magnesium chloride before it is applied to the road. The moistened de-icing salt enables larger spreading widths, adheres better to the road surface and keeps it free of ice for longer. Due to improved application technology and the predominant use of wet salt - also in combination with dry salt - the dosages could be reduced by approx. 70% compared to those in the 1970s with the same and longer thawing effect. According to information from the Federal Ministry of Transport from 1984, the use of de-icing salt in the Federal Republic of Germany could be reduced by 58% from 600,000 tons to 250,000 tons within three years.
Salt solutions and liquid mixtures of sodium, calcium and magnesium chloride are also used in winter services.
De-icing salt is used in many countries to combat ice. However, particularly snowy countries, such as Finland , Slovakia or Austria , use only little road salt. It is more likely that snow is removed there, followed by the scattering of dulling substances . In Switzerland, on the other hand, 300,000 to 350,000 tons of de-icing salt are used in severe winters, whereby the consumption can vary by more than three times between the years.
Environmental influences
The use of de-icing salt has numerous disadvantages for the environment . The salt gets into the ground with the meltwater . The excessive input of sodium and chloride ions into the soil caused by this has negative effects on the soil structure, it can lead to silting up and compaction.
Together with sea salt, de-icing salt contributes to an increase in the PM 10 content in the ambient air. Temperatures around freezing point and thawing weather enable vehicles to whirl up moisture, creating a salty aerosol .
The application of the salt in dissolved form (" spray salt ") is more effective, it reduces the amount of salt used , just like the replacement or admixture of sand or grit , etc. and thus reduces the environmental impact.
If the de-icing salt gets into the waters , which is more or less inevitably the case, at certain concentrations it can seriously damage flora and fauna . A study presented in the winter of 2014 by the American Cary Institute for Ecosystem Studies in Milbrook shows that the salt pollution from road salt from rivers and lakes in southeast New York state is almost the same and unexpectedly high in summer and winter. It assumes that de-icing salt loads that have entered the aquifers over decades are reflected in the increase in salt concentrations in surface waters and drinking water wells . In the cool and snowy Michigan , changes in the specific water densities were found in two of the lakes examined, with the resulting hindrance to the circulation between the water layers . In one of the lakes there was no longer any mixing at all: there was an oxygen-free and very salty, constant layer of water close to the ground with corresponding changes in flora and fauna.
De-icing salt also attacks the vegetation ; the linden trees , which are often planted on streets, are particularly sensitive . But maples , horse chestnut , red oaks and pines are sensitive to the increased ion concentrations in soil (see also: Osmoregulation ). Permanently high salt concentrations at roadsides can lead to the settlement of salt-loving coastal plants inland.
In domestic animals, de-icing salt in the paws damages the sensitive skin between the toes. The animals constantly lick the irritated areas clean and thus create inflammations that only heal slowly due to the salt. Further consequential damage is caused by the corrosive effect of the chloride ions contained in the de-icing salt on concrete components, steel girders and vehicles.
In Austria, the VKI has criticized the fact that the proportion of the much more aggressive calcium chloride in the grit is much higher than in Switzerland, for example. This criticism is also confirmed by numerous car importers, who are confronted with rust damage much more frequently than in other countries. Compared to Switzerland, these are significantly higher in Austria. Another point of criticism is above all the scattering of the municipal roads, as the individual communities, in contrast to the road and motorway maintenance depots, do not have the technical possibilities of a constant and economically correct distribution and thus more grit is applied than necessary.
Prohibition
Due to the disadvantages, the private use of road salt on public roads is prohibited in many municipalities and can be punished with a fine. The municipality itself usually reserves the right to use it for main roads and danger spots in the event of extreme weather conditions. The use of dulling grit as an alternative avoids the specific disadvantages of de-icing salt, but must be removed mechanically, i.e. swept up by cleaning vehicles.
In Germany, numerous municipalities prohibit the use of road salt on sidewalks or severely restrict its use. Three of the ten largest German cities, namely Berlin, Hamburg and Munich, completely ban road salt on sidewalks. In the remaining seven municipalities (Cologne, Frankfurt, Stuttgart, Düsseldorf, Dortmund, Essen and Leipzig) the statutes only allow the use of road salt in extreme situations ( e.g. freezing rain ), and then only very sparingly.
As in Germany, salt spreading is also prohibited in Austria by relevant regulations. In Vienna, Salzburg and Graz, for example, the spreading of de-icing salt is only permitted in exceptional cases.
Unlike in Germany and Austria, there is no general road salt ban in the Swiss municipalities.
Alternative substances
In response to these disadvantages, some manufacturers have developed salt-free litter materials based on urea ( ice flake point of a 30% by weight solution in water: −10 ° C). Since the use of urea can lead to over-fertilization of the soil, its use is prohibited in many cities, for example in Vienna. Another alternative de-icing salt is ammonium sulfate (freezing point of a 66% by weight solution in water −18.5 ° C). Like urea, this substance also acts as an undesirable nitrogen fertilizer .
In the USA and Canada , a by-product of sugar production is mixed with salt to form a dark, viscous mixture. In Switzerland and Austria, this substitute, known as Safecote , has been tested for different lengths of time. While this has been successful in Switzerland, the Austrian ASFINAG is not yet convinced. One of the disadvantages is the high conversion costs of the devices, while Safecote itself is only marginally more expensive. ASFINAG is testing brine made from potassium chloride on sections of the autobahn in eastern Austria, which can reach temperatures of up to 70 degrees due to process heat.
Various tests are also currently being carried out in Vienna, whereby the remains of grapes and granules from corn cobs are applied. A significant disadvantage of organic alternative substances is the released load of dissolved organic carbon ( DOC ), which can have a negative effect on the condition of adjacent waters. The water relevance of area-wide use on national roads and in residential areas was classified as low, provided the DOC is easily degradable.
See also
Web links
- Winter service guidelines (PDF; 773 kB), Provincial Government of Styria, Graz 2006.
- General information and statistics on de-icing salts (PDF file; 1.91 MB).
- Comprehensive information on the subject of de-icing salt (PDF file; 11.3 MB).
- Frequently asked questions about road salting ( eawag fact sheet ).
- Technical information from the Federal Office for the Environment
Individual evidence
- ^ Lueger, Otto: Lexicon of the entire technology and its auxiliary sciences. Volume 8, Stuttgart, Leipzig 1910, pp. 358-361.
- ↑ Josef Georg: The road warden earlier and today , Books on Demand GmbH, 2001, ISBN 3-8311-1999-6 , page 47.
- ↑ Trees are slowly dying - too much salt and too little care , article in DIE ZEIT, November 30, 1979, No. 49.
- ↑ Article The Invention of Moist Salt by the State Office for Mobility Rhineland-Palatinate ( Memento of the original from December 2, 2013 in the Internet Archive ) Info: The archive link has been inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. .
- ↑ a b Fact sheet road salt. In: eawag.ch . Retrieved December 24, 2018 .
- ↑ Bavarian State Office for Water Management: Salt scattering - Effects on waters (PDF file; 240 KB) .
- ↑ 3sat / nano: If salt is in the way, it bothers the ice crystal a lot .
- ^ Information on wet salt ( memento of November 9, 2010 in the Internet Archive ) from the Association of the Potash and Salt Industry.
- ^ Karl Moritz: Use of salt solutions from winter service to reduce pollutants. ( Page no longer available , search in web archives ) Info: The link was automatically marked as defective. Please check the link according to the instructions and then remove this notice. Lecture by the Federal Highway Research Institute 2008.
- ↑ a b Study on the effects of nitrogen-containing de-icing agents. ( Page no longer available , search in web archives ) Info: The link was automatically marked as defective. Please check the link according to the instructions and then remove this notice. (PDF; 1.6 MB) Institute for Forest Ecology (University of Natural Resources and Life Sciences), Vienna 2000.
- ↑ Frieder Monzer - The next winter is definitely coming! ( Memento of the original from December 20, 2015 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.
- ↑ ZEIT interview Snow in Finland "I've never seen road salt here" .
- ↑ Sweden - Motorists in Winter ( Memento of the original from October 17, 2015 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. .
- ↑ a b c d Fine dust source of road salt? Pros and cons in action against snow and black ice. (PDF; 85 kB) Helmholtz Center, Munich 2005.
- ↑ Dieter Gladtke, Anke Olschewski, Thomas Retny, Piet Risthaus: Contribution of sea and road salt aerosols to pollution from PM 10 in the Rhine-Ruhr area. In: Hazardous substances - cleanliness. Air . 73, No. 3, 2013, ISSN 0949-8036 , pp. 109-114.
- ^ A b Monika Seynsche : deutschlandfunk.de: Road salt and its consequences . Deutschlandfunk , Research News , January 6, 2015.
- ↑ Bavarian State Office for Water Management: Salt scattering - Effects on waters , pages 6 to 9. (PDF; 246 kB).
- ↑ Veitshöchheimer reports 84 (2005), page 9 ( Memento of the original from June 17, 2012 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. (PDF; 2.3 MB).
- ↑ Landratsamt Roth: Kreisfachberatung - Avoid road salt in private households, page 1 ( page no longer available , search in web archives ) Info: The link was automatically marked as defective. Please check the link according to the instructions and then remove this notice. (PDF; 79 kB).
- ↑ Consumer: The Austrian consumer magazine , issue 1/2013: Ätzende Auftraumittel , page 22f.
- ↑ Examples Munich , Hamburg ( Memento of the original from March 16, 2010 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. , Berlin ( Memento of the original from August 11, 2010 in the Internet Archive ) Info: The archive link was automatically inserted and not yet checked. Please check the original and archive link according to the instructions and then remove this notice. .
- ↑ List of federal German municipalities in which the use of road salt on sidewalks is prohibited (PDF file; 65 KB) ( Memento of the original from December 24, 2012 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. .
- ↑ BOKU, thawing agent study .
- ^ Salzburg - winter service .
- ↑ Graz Streummittelverordnung (PDF file; 261 KB) .
- ↑ Tages-Anzeiger - More salt than snow on Zurich's streets .
- ↑ City of Vienna: Away with the snow! How to clear it properly. (PDF; 1.2 MB).
- ↑ leaflet of the bayer. State Office for Water Management (PDF; 246 kB).
- ↑ a b Swiss satisfied with the sugar mixture on ORF from February 13, 2012, accessed on February 13, 2012.
- ↑ Fighting black ice with Safecote in winter service on national roads, pilot test report 2008 - 2010 in the area of the Spiez autobahn depot ( memento of the original from April 25, 2012 in the Internet Archive ) Info: The archive link was inserted automatically and not yet checked. Please check the original and archive link according to the instructions and then remove this notice. (PDF; 185 kB).
- ↑ ASTRA : Organic brine additives - assessment of the relevance of water bodies when using organic brine additives based on agricultural by-products , December 2014.
