GB2251611A - Descaling composition - Google Patents

Abstract

A composition useful in removing limescale from domestic appliances and apparatus comprising at least one medium strength to weak protonic acid or partly deprotonated acid that yields at least one proton and a Lewis acid which consists at least in part of the salt of a strong acid with cations of a 2-valent main group element.

Description

DESCALING COMPOSITION This invention relates to descaling compositions, particularly to compositions which are non-toxic and environmentally friendly and which may be used to remove limescale deposits from domestic appliances and apparatus, etc.

Limescale deposits from water, which consist principally of magnesium and calcium carbonates, are not only unsightly but can also lead to inefficient functioning of domestic appliances and apparatus such as for example electric kettles and shower jets.

compositions based on medium-strength to weak protonic acids such as for example citric acid are known to be of use as descalers. US-A-4747975 thus describes a descaling composition comprising a medium strength to weak protonic acid, or a partly deprotonated acid that yields at least one proton, and a Lewis acid, the latter being a salt of strong acid with cations of 3- or 4valent main group or transitional elements or 3- or 4valent lanthanides or actinides, preferably cations which correspond to metals upon which the limescale is deposited. The ratio of protonic acid to Lewis acid in these compositions is between 1:1 and 6:1.

We have now surprisingly found that in compositions of the kind described in US-A-4747975, the Lewis acid as described therein can with advantage be replaced at least in part by a Lewis acid which is a salt of a strong acid with cations of a 2-valent main group element.

According to one aspect of the present invention, we therefore provide a descaling composition comprising at least one medium strength to weak protonic acid or partly deprotonated acid that yields at least one proton and a Lewis acid which consists at least in part of the salt of a strong acid with cations of a 2-valent main group element.

The protonic acids and partly deprotonated acids used in the composition according to the present invention can be as described in US-A-4747975. The protonic acids used are preferably non-toxic carboxylic acids more preferably di- and tri-carboxylic acids such as for example tartaric and citric acid, with citric acid being particularly preferred. It is in general advantageous to utilise protonic acids or partly deprotonated acids which are water-soluble at ambient temperatures as this facilitates the use of the compositions according to the invention for descaling purposes.

The strong acid anion of the Lewis acid used in the compositions according to the invention is preferably sulphate, chloride, nitrate or phosphate, with chloride being particularly preferred. Preferred 2-valent metal cations are calcium and magnesium, with magnesium being particularly preferred. Magnesium chloride is thus the most preferred Lewis acid for use in compositions according to the invention.

Limescale contains magnesium and calcium carbonates and it is considered surprising that a Lewis acid such as magnesium chloride or calcium chloride when present in the compositions according to the invention can promote a descaling action. Furthermore, the compositions according to the invention containing magnesium chloride have the advantage of being non-toxic and environmentally friendly, magnesium being an essential element to human metabolism and its chloride being generally accepted as safe and non-irritant to handle. Aluminium salts are favoured ingredients for the compositions of US-A-4747975 and, in contrast to the situation with magnesium, there is an increasing body of literature linking aluminium with diseases such as Alzheimers disease.Furthermore magnesium and caLcium are environmentally friendly elements, being commonly occurring in the earth's crust and, in the case of magnesium, in sea water. Compositions according to the invention also have the advantage of low corosivity which can be important for the treatment of metal vessels and domestic appliances and apparatus.

In the compositions of the present invention, the molar ratio of protonic or partly deprotonated acid to Lewis acid is preferably from 3:1 to 1:3, more preferably from 2:1 to 1:2. With compositions according to the invention containing tartaric or citric acid as the protonic acid, and magnesium or calcium chloride as the Lewis acids molar ratios of from 1:1 to 1:2, more especially from 1:1 to 1:1.5, are particularly preferred. A mixture comprising citric acid and magnesium chloride hexahydrate in a molar ratio of about 4:5 is one particularly referred composition according to the present invention.

If desired, the compositions according to the invention may additionally contain a Lewis acid which is the salt of a strong acid with cations of a 3- or 4valent main group element, or transitional or 3- or 4valent lanthanide or actinide, as described in US-A4747975. The compositions can also if desired contain other optional ingredients, e.g. preservatives such as Bronopol.

The compositions of the present invention are conveniently in the form of dry mixes or concentrated solutions. In the latter case, it is preferred to adjust the pH to at least 2 or higher, e.g. by incorporation of ammonia.

According to a further feature of the present invention there is provided a method of dissolving a limescale deposit which comprises contacting the deposit with a composition according to the invention as hereinbefore defined in the form of an aqueous solution.

The method according to the invention may be carried out at ambient or elevated temperatures. Preferably the aqueous solution used contains from 0.075 to 0.5 moles/litre of the protonic or deprotonated acid and from 0.075 to 1.00 moles/litre of the Lewis acid. Where the composition used contains tartaric or citric acid as the protonic acid and magnesium or calcium chloride as the Lewis acid, the aqueous solution more preferably contains from 0.075 to 0.5 moles/litre of protonic acid and from 0.075 to 1.0 moles/litre of Lewis acid. Where the composition used contains citric acid and magnesium chloride, contents of from 0.075 to 0.5 moles/litre of citric acid and from 0.075 to 0.75 moles/litre of magnesium chloride are especially preferred.

The method according to the invention is particularly suitable for use for the dissolution of limescale deposits on domestic appliances and apparatus including for example electric kettles, shower jets, water taps, etc.

The following Examples illustrate the invention.

Example 1 Dissolution of Limescale 40g of anhydrous citric acid and 49.2g of magnesium chloride hexahydrate were dissolved in water to produce a solution with a final weight of 1keg.

Boiler limescale was broken into pieces measuring approximately 3 x 3 x 2 mm and fines were removed by sieving.

Three aliquots of the above-described solution were heated to 95"C and 15g of the prepared pieces of boiler limescale were added to each. The solutions were allowed to cool under ambient conditions and descaling estimated at the end of fixed intervals which varied for each of the three test samples. At the end of each interval, the residual scale was removed by filtering through a fine metal mesh, of size lmm x lmm, and the residue allowed to dry at 105 C for one hour. The residue was then weighed and the difference between the residual weight and initial weight expressed as a percentage descale. The results obtained were as follows: Time/Mins percentage descale 5 44 15 55 30 66 These results compare favourably with results obtained using combinations of citric acid with aluminium salts in accordance with US-A-4747975. Furthermore, as discussed above, the formulation of this example has advantages as regards non-toxicity and environmental friendliness when compared with combinations of citric acid with aluminium salts.

Example 2 Corrosion Studies A 1 kg solution of citric acid and magnesium chloride hexahydrate was prepared according to Example 1, poured into an electric kettle (New Haden Autojug Model AJ2) and left for 25 days. The solution was removed and inspection of the element revealed that it was in a near perfect condition.

Claims (17)

Claims

1. A descaling composition comprising at least one medium strength to weak protonic acid or partly deprotonated acid that yields at least one proton and a Lewis acid which consists at least in part of the salt of a strong acid with cations of a 2-valent main group element.

2. A composition as claimed in claim 1 wherein the protonic acid is a carboxylic acid.

3. A composition as claimed in claim 2 wherein the carboxylic acid is tartaric or citric acid.

4. A composition as claimed in any one of claims 1 to 3 wherein the strong acid anion is sulphate, chloride, nitrate or phosphate.

5. A composition as claimed in claim 4 wherein the strong acid anion is chloride.

6. A composition as claimed in any one of claims 1 to 5 wherein the 2-valent main group element is calcium or magnesium.

7. A composition as claimed in claim 1 wherein the Lewis acid is magnesium chloride.

8. A composition as claimed in any of claims 1 to 7 wherein the molar ratio of protonic or partly deprotonated acid to Lewis acid is from 3:1 to 1:3.

9 A composition as claimed in claim 8 wherein the molar ratio is from 2:1 to 1:2.

10. A composition as claimed in claim 1 wherein the protonic acid is tartaric acid or citric acid, the Lewis acid is magnesium chloride or calcium chloride and the molar ratio of protonic acid to Lewis acid is 1:1 to 1:2.

11. A composition as claimed in claim 10 wherein the molar ratio is 1:1 to 1:1.5.

12. A composition as claimed in claim 1 comprising citric acid and magnesium chloride hexahydrate in a molar ratio of about 4:5.

13. A method of dissolving a limescale deposit said method comprising contacting the limescale deposit with an aqueous solution of a composition as claimed in any one of claims 1 to 12.

14. A method as claimed in claim 13 wherein the aqueous solution comprises from 0.075 to 0.5 moles/litre of protonic or deprotonated acid and from 0.075 to 1.00 moles/litre of Lewis acid.

15. A method as claimed in claim 13 wherein the aqueous solution comprises from 0.075 to 0.5 moles/litre of citric acid and from 0.075 to 0.75 moles/litre of magnesium chloride.

16. A descaling composition substantially as herein described.

17. A method of dissolving limescale deposit as herein described with reference to either of the Examples.