WO2000034551A1

WO2000034551A1 - Cleaning solutions containing citric acid and uses thereof

Info

Publication number: WO2000034551A1
Application number: PCT/AU1999/001094
Authority: WO
Inventors: Gerald Joseph Ward Cairns
Original assignee: Cairnscorp Technology Pty Limited
Priority date: 1998-12-09
Filing date: 1999-12-08
Publication date: 2000-06-15
Also published as: AUPP762398A0

Abstract

An aqueous ammoniated citric acid cleaning solution effectively cleans metal parts at room temperature. The solution is prepared by diluting a concentrated aqueous citric acid solution containing between 1-10 % ammoniation, the concentrated solution being prepared in a special way. Dilution of this specially prepared concentrated solution provides a working solution which is much more effective in cleaning metal parts than conventionally prepared citric acid solutions which require hot temperature applications to work effectively.

Description

CLEANING SOLUTIONS CONTAINING CITRIC ACID AND USES

THEREOF

FIELD OF THE INVENTION This invention relates to cleaning solutions containing citric acid and various uses thereof and particularly relates to novel cleaning solutions which can have an improved cleaning ability especially at lower temperatures.

BACKGROUND ART Aqueous solutions of citric acid are well-known and are widely used as a chemical cleaner in the removal of ion oxides from ferrous metals. Aqueous citric acid solutions are widely used for cleaning steam generating plant and chemical processing equipment and is a safe and effective acid cleaner in a variety of metal finishing applications. It is used extensively for cleaning stainless steel parts which are liable to stress corrosion cracking in the presence of chloride ions and therefore cannot be cleaned with hydrochloric acid.

Citric acid reacts with ferric oxide to form a water soluble chelate. The most stable chelate is that containing one molecule of ferric ion and one molecule of citrate ion. This means that for each part by weight of ferric ion present, four parts by weight of citric acid is required. Exhaustion of the citric acid cleaning solution therefore sometimes results in the precipitation of ferric salts but it has been found that if an excess of 0.5% citric acid is maintained, precipitation of the ferric salts can be reduced or avoided.

The dissolution of mill scale, operational scale, and bare metal produces ferrous ions in solution. For example, soluble ferric ion in contact with steel is reduced to ferrous ion, and most cleaning operations are conducted in the absence of dissolved oxygen so that the ferrous ion is usually present to a much larger extent than the ferric ion.

Steel and ferrous oxide react with aqueous citric acid to provide a ferrous hydrogen citrate. The ferrous hydrogen citrate increases in concentration during the citric acid cleaning process and eventually precipitates as its solubility limit is reached.

To reduce the risk of precipitation, it is known to add ammonia to citric acid solutions. It is known to add sufficient ammonia to a 3 to 5% citric acid solution to increase the pH to 3.5. At this pH, one of the three carboxylic acid hydrogens of the citric acid is neutralized. A relatively large amount of ammonia is added to a very dilute (3 to 5%) citric acid solution, and it is found that this solution at pH 3.5 will dissolve rust more rapidly than citric acid. The reaction product is ferrous ammonium citrate (instead of ferrous hydrogen citrate), and the ferrous ammonium citrate is highly soluble and will not precipitate even when the pH of the solution is raised to about 10.

It is known to add 3 to 5% aqueous citric acid solutions to give a pH of 5.6 to form a diammonium citrate.

Current conventional literature to the use of citric acid solutions and ammoniated citric acid solutions are to high temperature solutions in order to clean the metal parts within an acceptable period of time. Thus, the solutions are applied at temperatures in the range of 60°C to 95°C. Lowering the temperature to room temperature or below does not provide a useful cleaning effect and the cleaning process takes far too long.

Several attempts have been made in order to speed up the cleaning process at lower temperatures. For instance, it is known to add small quantities (0.5 to 1.5%) of ammonium bifluoride as a catalyst which allows the dilute citric acid cleaning solutions to be used at room temperature.

However, the catalyst introduces fluoride into the solution which requires safe handling precautions when using this technique.

When cleaning large pieces of equipment, it has been difficult to date to properly clean the equipment by using hot dilute citric acid solutions as the solution must be kept hot for a period of time and this requires large energy and capital inputs. Heated soak solutions, heated foams, hot steam or sprays have also been used but suffer from the same disadvantages. The use of fluoride catalysts require safe handling procedures and techniques and is generally not considered desirable. Another disadvantage with dilute citric acid solutions is that the solution itself does not have a very long shelf life. In a fairly short period of time, the solution becomes contaminated with bacterial and fungal growth. Another disadvantage with known ammoniated dilute citric acid solutions is the difficulty in forming the solution. To make up large volumes of dilute solution requires the addition of fairly large amounts of ammonium hydroxide and ammonium hydroxide is a toxic chemical and requires safe handling procedures. The resultant solution can easily liberate ammonia and cause poisoning and toxic effects to workers.

After much research and experimentation, an improved cleaning composition containing citric acid has now been developed and which may overcome at least some of the abovementioned disadvantages or provide the consumer with a useful or commercial choice.

OBJECT OF THE INVENTION One form of the present invention is based on the realization that an ammoniated citric acid solution can be made more readily by adding ammonia to a saturated or near saturated aqueous citric acid solution as opposed to addition of ammonia to a dilute solution of citric acid.

Therefore, in one form the invention resides in a method for manufacturing an ammoniated concentrated citric acid solution which comprises gradual addition of ammonium hydroxide to a saturated or substantially saturated aqueous citric acid solution until a desired ammoniation level is obtained.

Citric acid has a solubility in water of 54% w/w at 10°C, 59.2% at 20°C, 64.3% at 30°C, 68.6% at 40°C, 70.9% at 50°C, 73.5% at 60°C, 76.2% at 70°C, 78.8% at 80°C, 81.4% at 90°C, and 84% at 100°C. The term "saturated aqueous citric acid solution" would be a term clearly evident to a person skilled in the art. The method need not be limited to a fully saturated aqueous citric acid solution but can include a substantially saturated or near saturated aqueous citric acid solution or concentrated citric acid solutions, it being appreciated that the citric acid solution is to be much more concentrated that the known method of ammoniating dilute citric acid solutions.

Although the concentrated citric acid solution can be ammoniated by ammonium hydroxide, it is envisaged that other types of amines may be used to form an ammoniated concentrated citric acid solution. In one form, the amount of ammonium hydroxide added is sufficient to provide an ammoniation of between 1 to 10%, and preferably an ammoniation of between 2 to 6% and most preferably an ammoniation of about 3.1%.

In another form, the invention resides in a concentrated cleaning composition comprising a saturated or substantially saturated aqueous acid solution which contains between 1 to 10% ammoniation. By saturated or substantially saturated is meant that the citric acid solution contains substantially as much citric acid as possible without appreciable precipitation occurring at the temperature of the solution. It is preferred that the solution contains between 2 to 6% ammoniation and most preferably about 3.1 % ammoniation. It is preferred that ammonium hydroxide is used to ammoniate the citric acid solution. The concentrated cleaning composition has several advantages. Firstly, the composition has an extremely long shelf life and has a reduced susceptibility to bacterial or fungal growth. The ammonia appears to stay stabile within the citric acid solution and does not appear to be appreciably liberated over time. This minimizes the requirement for safe handling procedure and possible poisoning by liberated ammonia or amines.

In one form, the concentrated cleaning composition includes one or more additives which can function to improve the cleaning capability of the composition. The additives may include surfactants which can increase the rate of penetration of the scale or other impurities which are to be cleaned.

The surfactant can be based on salts of fatty acids by reacting mono and/or polyunsaturated fatty acids such as canola oil firstly with a minimum of potassium hydroxide followed by urea, sodium sulphate and sucrose in the cold, then with the addition of a triazole corrosion inhibitor compound such as benzotriazole or tolytriazole results in a corrosion inhibitor surfactant which not only reduces smutting of cleaned surfaces but also in acceleration of the reactivity of the solution at low ambient temperatures. The concentrated cleaning composition, as described above, can be diluted with water to form a working solution. The dilution may be about 1 part of the concentrated composition to 8 parts of water. Working solutions may be between 1-20% w/w, and more preferably between 1-15% w/w.

While not wishing to be bound by theory, it appears that the diluted cleaning composition described above is superior to the known dilute 3 to 5% citric acid solutions which have been ammoniated and which cannot be stored because of susceptibility to bacterial and fungal attack. The cleaning composition according to the invention has a much higher free acid present in the ammoniated saturated solution which allows much of the ions to exist in a ferrous state without precipitating. It appears that these ferrous ions take part in auto oxidation reactions with ferric ion in the scale which functions synergistically and improves the reaction capacity to the cleaning solution. This would not be available if all the ion was in the form of a chelate which would be the situation with the known citric acid solutions.

It is also found that the cleaning compositions according to the invention when diluted allow metal products to be cleaned at ambient temperature and within a suitable period of time and therefore heating of the solution is not essential. This in turn reduces or avoids the need to provide heaters and other high energy dependents and high cost equipment to heat and maintain the solution in a hot condition during the cleaning process. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 illustrates a method of manufacturing an ammoniated concentrated citric acid solution in bulk.

Figures 2 and 3 illustrate the cleaning efficiency of the solutions

BEST MODE

Referring to Fgure 1 , a concentrated citric acid solution containing 600g of citric acid per litre of water is added to a stainless steel paddle mixer 10. Aqueous solution of ammonium hydroxide (225g per litre water) is metered into a large graduated polyethylene container 11 from a storage container 12 and via a pump 13. Container 11 has a gas overflow vent pipe 14, which extends into mixer 10 and below the level of water in the mixer so that no gas escapes to the atmosphere during charging container 11 with the appropriate volume of ammonium hydroxide.

Once the required amount of ammonium hydroxide has been added to container 11 , it passes through lower conduit 15 and via a valve to a pressure chamber 16. The pressure chamber forms a venturi junction with the main delivery pipe 17 on the output side of a circulating pump 18 which circulates the citric acid solution from mixer 10 in a closed loop through conduit 17 and back into the mixer. Ammonium hydroxide is gradually added to the circulating concentrated citric acid. The pressure chamber 16 helps to smooth out the gas or steam explosions which occur as the ammonium hydroxide comes into contact with the concentrated citric acid as the reaction is vigorously exothermic.

The method of manufacture results in an extremely low cost system which produces a highly concentrated solution of high specific gravity and which is ideally suited for packaging for commercial use. The method uses lower amine concentrations which have been shown to be effective and make the method of manufacture simple, safe and economical in large to small batch sizes. The system permits batches as large as 5,000 litres to be produced without the need for special heat exchange or other cooling devices.

Figures 2 and 3 illustrate the cleaning efficiency of the solutions. The figures show photographs of rusted sample tags and illustrated the ammoniated citric acid solution effects on the removal of the rust from the tags.

Figure 2 shows, in the Y axis, the times that the tags were immersed in the solution while the X axis shows 4 different solutions. The first and second column in Figure 2 are known conventional solutions. The first column is a dilute (3 to 5%) ammoniated citric acid solution at pH 5.6 to form the diamine and at 24°C. The second column is a similar citric acid solution with less added ammonia to form the monoamine solution of pH 3.5. The third column shows a concentrated citric acid solution made by the method described above and diluted to 2.5% w/w. The fourth column shows the same solution but diluted to 1.88% w/w.

In Figure 2, the solutions in columns 3 and 4 had 1 % surfactant additive the additive being described above. In Figure 3, columns 1 and 2 are the same as with Figure 2 and columns 3 and 4 illustrate that the diluted solution according to the invention provides good cleaning properties at ambient temperate (24°C) but as well at a greatly reduced temperature of 10°C (see column 4). The fifth column shows a dilute monoamine at 1.88% w/w. The solutions according to the embodiment and illustrated in columns 3 and 4 have 5% surfactant additive the additive being described above.

Careful observations of the sample tags illustrated in Figures 2 and 3 showed that the solutions according to the invention work faster than the traditional monoamine and diamine citric acid solutions. The surfactant enhances protection of the parent metal from any corrosive effects of the solution. The surfactant according to the invention has a particular affinity for oxides and while components of the surfactant are hydrophobic, once in contact with the rust they react to form aqueous soluble species which assists in the penetration and removal of the scale. The solutions according to the invention are distinguished from known citric acid solutions by the lesser amount of ammonia and therefore the higher free acid found in the solutions according to the invention.

It is found that approximately 3.1% ammoniation of the concentrated citric acid provides a rapid and effective method of producing a concentrated solution which is stable and economic for commercial storage and for deployment as a diluted 12% working solution with water. This is superior to the conventional method of ammoniating 3 to 5% citric acid solutions which cannot be stored due to bacterial and fungal attack and therefore must be used when prepared. The much higher free acid present in the solutions according to the invention allows much of the ion to exist in a ferrous state. The solution according to the invention is much more active at ambient temperatures and down to about 15°C or less thereby saving energy and time.

The invention therefore provides a practical one-step cleaning procedure for metal parts or other parts requiring cleaning.

Previously known citric acid solutions react very slowly and obtaining very clean metal surfaces suitable to accept various types of coatings such as paint, and galvanized zinc was very difficult as the slow rates of reaction permitted build-up of electrochemical polarization charges on the cleaned surfaces such that by the time the whole surface scale was removed, ionized solvated scale products were progressively attracted to the cleaned surfaces thereby creating a heavily smutted surface which was unsuitable for the application of coatings.

With respect to the surfactants according to an embodiment of the invention, although it is well-known that polyunsaturated fats react corrosively with rust and iron metal, by modifying the polyunsaturated fats as described above, the corrosion effect is reduced remarkably without limiting the reactivity with the oxides of ion.

The surfactant possesses the following properties: (a) Miscibility with water and wetting capacity in oily and dirty conditions. (b) Reactivity with metal oxides rendering them either directly soluble in water or making them much more reactive with the amine citrate components of the solution.

(c) Improving the cleanliness of the cleaned metal surface.

(d) Inhibition of corrosivity to the parent metal cleaned surfaces.

(e) Acts synergistically with a working solution based on the 3.1% ammoniated concentrated citric acid solution to improve low ambient temperature reactivity.

(f) Greatly improves the penetrating capacity of the solution as a whole.

(g) Has a citrate sparing effect reducing the rate of exhaustion of the working solution and extending the working life of any given solution. This significantly improves the economics by extending the capacity of the most expensive component of the solution namely citric acid. (h) Surfactant is workplace safe in keeping with the other components of the working solution, with a very low order of toxicity. (i) Complete biodegradability.

(j) Reduces flash rusting on removal from the solution by providing a degree of passivation of the cleaned metal surfaces, (k) Ambient temperature operation saving in time and energy. (I) Single step cleaning procedure eliminating the traditional multi-stage cleaning operations again saving energy and time. It should be appreciated that various other changes and modifications can be made to the embodiment described without departing from the spirit and scope of the invention.

Claims

CLAIMS:

1. A concentrated cleaning composition comprising a saturated or substantially saturated aqueous acid solution which contains between 1 to 10% ammoniation.

2. The composition of claim 1 wherein the solution contains between 2-6% ammoniation.

3. The composition of claim 2, wherein ammonium hydroxide is used as the ammoniation agent.

4. A cleaning solution for cleaning metal parts at ambient temperatures, the solution being the solution of claim 1 diluted to between 1.5

-15% w/w with water prior to use.

5. The solution of claim 4, containing between 1-5%w/w of additives which is a salt of a fatty acid and a triazole.

6. A method for forming the cleaning solution of claim 1 , the method comprising the steps of gradual addition of ammonium hydroxide to a saturated or substantially saturated aqueous citric acid solution.

7. The method of claim 6, wherein a concentrated citric acid solution containing 600g of citric acid per litre of water is added to a mixer, an aqueous solution of ammonium hydroxide (225g per litre water) is metered into a container from a storage container which has a gas overflow vent pipe which extends into the mixer and below the level of water in the mixer so that no gas escapes to the atmosphere during charging of the container with the appropriate volume of ammonium hydroxide, once the required amount of ammonium hydroxide has been added to the container , it passes through a conduit and via a valve to a pressure chamber which forms a venturi junction with a main delivery pipe on the output side of a circulating pump which circulates the citric acid solution from the mixer in a closed loop and back into the mixer, the ammonium hydroxide being gradually added to the circulating concentrated citric acid, the pressure chamber functioning to smooth out the gas or steam explosions which occur as the ammonium hydroxide comes into contact with the concentrated citric acid as the reaction is vigorously exothermic.