Hardness

Hardness is defined as the concentrations of calcium and magnesium ions expressed in terms of calcium carbonate. Concentration is measured in degrees hardness. There are many different definitions around the globe, such as German hardness, French hardness, English Hardness and American Hardness.

Definition

1 ° dh equals

  • 10 mg/l CaO = 7.14 mg/l Ca = 17.8 mg/l CaCO3
  • 7.19 mg/l MgO = 4.34 mg/l Mg
  • 0.18 mmol/l Earthalkali-ions

The reaction of Water Hardness Compounds

Water hardness compounds react with fatty acids and some vegetable oils with a high free fatty acid content to form hard magnesium and calcium soaps. These soaps result in a white sticky layer or a white powdery residue both of which are difficult to remove.
Therefore seawater, as well as untreated freshwater, should not be used for cleaning these cargoes.

Freshwater hardness varies depending on the source of supply.

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For example:

Location ° dh
Gothenburg 2.5
Helsinki 6.3
Hamburg 16
Antwerp 9
Milan 21
Palermo 27

Seawater has a hardness of some 6600 mg/L

Comparison Table

CaO mg/l Ca mg/l Degree German Hardness ° dh Degree English Hardness ° e Degrees French Hardness ° f Degrees American Hardness ° US
10 7.14 1 1.25 1.78 17.8

Fresh Water and Sea Water Hardness

Often water is classified with common terms like soft, hard etc.

soft moderately hard hard very hard
° dh 0-7 8-12 13-30 30 +
mg/l 0-75 75-150 150-300 300+

 

Removal of Water Hardness

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To remove the water hardness the following chemicals can be used

  • Sodium Carbonate Na2 CO3 settles hardness compounds as carbonate powder
  • Sodium Triphosphate Na3 PO4 settles hardenss compounds as phosphate clouds/flakes

Preparation of the tank cleaning water:
If the tanks to be cleaned are coated the chemicals should not be applied in excess, because this causes alkalinity of the prepared solution whivh might be not compatible with the coating. It is recommended to use the formulöa which is based on known water hardness.
Determine the amount of chemicals required:
If there is no possibility to determine the water hardness of the fresh water the following recipie should be applied:
Fill the necessary amount of fresh water into a separate and clean tank and add slowly one of the chemicals as recommended above. Use either 15 kg Na2 CO3 or 15 kg Na3 PO4 per 10 m3 fresh water. Do not use a zinc coated tank, due to the alkalinity of the solution (ph 12-13).
If the hardness can be measured the following concentrations should be used for 10 m3 fresh water:
Use either 0.2 kg Na2CO3 or 0.2 kg Na3PO4 per 10 m3 water and per ° dh
Example: 10 m3 water with a hardness of 30 ° dh =“> 30*0.2″ = 6 kg of either chemical
Circulate the solution 0.5 hours
Wait another hour to allow settling of the sediments/flakes before using the water.
Removal of sediments/flakes after usage of the water from the service tank can be done by means of an acid like a standard Metal Brightener.