EP0234425B1 - Process for cleaning aluminium containers - Google Patents

Abstract

Aluminium containers are cleaned using alkaline cleaning solns., in which the alkaline cleaning is preceded by a precleaning with an aq. acidic soln. and followed by rinsing with an aq. soln. wherein at least a part of the rinsing soln. is employed in the precleaning step. A soln. of pH from 4 to less than 7, pref. 4.5-5.5 is employed in the precleaning step. The soln. contains 0.005-5 g/l, pref. 0.1-1 g/l, of a tensile, and has a temp. of less than 65.6 deg.C, esp. 32.2-54.4 deg.C.

Description

The invention relates to a method for cleaning aluminum containers with the aid of alkaline cleaning solutions, the alkaline cleaning being preceded by a preliminary cleaning with an acidic solution and a rinsing with an aqueous solution, and its use as a pretreatment of aluminum containers before the application of conversion coatings.

Aluminum containers have found widespread use and are used in particular for packaging food and beverages, such as soft drinks and beer. It is common practice in the manufacture of such containers to first punch round disks from aluminum sheet, to bring them into a preliminary cup shape with suitable devices and finally to deform them in one or more stages by pulling and smoothing them into the final can. The cans are then trimmed and subjected to various cleaning and treatment stages. The conclusion is usually the application of an inner varnish and a decorative outer cover.

To facilitate the aforementioned process with rolling, punching, drawing and smoothing, various lubricants and cooling lubricants are used which - together with fine metal particles that are formed during the shaping process - remain in certain quantities. It is essential that such lubricant residues and metal particles are removed before the containers are subjected to further treatment.

There are numerous acidic or alkaline cleaners for this purpose. Although these cleaners are quite effective in terms of residue removal, problems often arise with regard to the taste of the contents of the container. This problem is particularly annoying because it occurs only occasionally and in an unpredictable manner and then leads to rejection of entire batches of cans due to impairment of the contents of the can. The actual reasons for the sudden appearance of such taste impairments are not known. A somewhat speculative assumption is that residues from cleaning and any chemical treatment lead to the problem in question. In this respect, adjustments or changes in the concentration of the active components of the cleaning solution have no effect. In addition, there is a limit to the amount of change in the concentrations of the active ingredients since they lead to adverse effects on the aluminum surfaces, e.g. an undesirable matt appearance.

The type and thoroughness of the rinsing after the cleaning treatment is also of crucial importance for a possible taste impairment, but also the appearance of the surface. In particular, the detergent solution adhering to the aluminum container and added to the rinsing stage must be practically completely removed.

The state of the art includes a process for cleaning aluminum containers with the help of alkaline cleaning solutions and subsequent water rinsing, in which one works in the rinsing stage with circulating rinsing water, which is kept approximately neutral to acidic by pH control (EP - A2 - 181 673). Although this method has considerable advantages, particularly with regard to the consumption of rinsing water, the desired advantages with regard to the cleanliness of the containers and their mobility in the transport device of the treatment system and on the way from there to the further production stages, such as printing etc., cannot be achieved on a regular basis.

Another state-of-the-art method for cleaning aluminum containers by means of alkaline cleaning solutions provides for rinsing treatment with an aqueous solution, the pH of which is set to 2.5, to be followed (EP application 87 100 373.7). With the aid of this method, it is possible to obtain aluminum containers with great regularity, which are distinguished by high purity and, as a result of the mobility thereby achieved, permit high throughput rates, in particular also in a printing device which may be connected downstream. One embodiment of the aforementioned method consists in rinsing with an acidic aqueous solution before the treatment with the alkaline cleaning solution. As a result, a considerable part of the impurities present on the aluminum container and consisting of fine aluminum particles and organic residues can be removed, so that the subsequent step of alkaline cleaning is less stressed.

Although this method is associated with the above-mentioned special design with excellent results, it does require some effort in pre-rinsing and rinsing agents.

US-A-4 540 444 relates to a process for the alkaline cleaning of aluminum containers with the following process steps: After performing an acid prewash, an acid cleaning is carried out. Following the acidic cleaning, a rinsing step is carried out, followed by the alkaline cleaning. This is followed by a rinse step with tap water, which is circulated, followed by a further rinse with fresh tap water. A rinse with deionized water is then carried out.

The object of the invention is to reduce this effort and still maintain the excellent results with regard to the regular maintenance of aluminum containers of high purity and with great mobility.

The object is achieved by designing the method of the type mentioned at the outset in accordance with the invention in such a way that at least part of the rinsing solution is introduced into the pre-cleaning stage.

Depending on the pH setting, the rinse solution used can be entered in the pre-cleaning stage without any special measures or after adding acid, especially sulfuric acid.

According to the invention, a solution is used in the pre-cleaning stage, the pH of which is adjusted to 4 to less than 7, preferably to 4.5 to 5.5.

In contrast to the generally common cascade-like guidance of the cleaning solution in the cleaning stage, i.e. with alkaline pre-cleaning, the pre-cleaning with acidic solution has the advantage that the progressive increase in the concentration of emulsified oil and organic contaminants, which is accompanied by sludge formation, contamination of container walls and pipes and also leads to renewed settling on the aluminum containers, is avoided. Instead, oil and organic contaminants split off due to the breaking of the emulsion in the acidic medium, float and can be removed in the simplest way by stripping or decanting and the like.

The aforementioned effect can be enhanced if, according to a further advantageous embodiment of the invention, a solution is used in the pre-cleaning stage which contains a surfactant. Concentrations of 0.005 to 5 g / l, preferably 0.1 to 1 g / l, are particularly expedient ^.

These surfactants can be anionic, cationic or nonionic in nature.

The pre-cleaning stage solution can be used at a temperature ranging from 15.5 ° C to 93.3 ° C. According to a particularly expedient embodiment of the invention, its temperature is set to <65.6 ° C., in particular to a value from 32.2 to 54.4 ° C. The solution can be applied in the usual way, e.g. by flooding, dipping or spraying. Spray treatment is preferred because of the uniform distribution of the solution over the inner and outer surfaces of the aluminum container.

Part of the solution from the pre-cleaning stage is withdrawn continuously or at certain intervals and sent to the wastewater treatment. This part is usually replaced quantitatively by acidified rinsing water.

After pre-cleaning, alkaline cleaning takes place. It is preferably carried out using a cleaning solution which, in addition to alkaline components, contains a complexing agent and at least one surfactant with an HLB number of at least 12 and a pH of at least 10.

In this cleaning solution, the alkaline component can be alkali or alkaline earth borate, carbonate, hydroxide or phosphate and mixtures thereof. Alkali hydroxide and carbonate are the preferred ingredients. On the one hand, their concentration must be sufficient to remove the aluminum particles on the container surface, but on the other hand it must not cause excessive etching of the aluminum surface. The result must be a clean, shiny appearance of the surface. The pH value, which is capped at 10, can go up to around 13 depending on the nature of the container surface. It is preferably in the range from 11.5 to 12.5. The concentration is usually between 0.05 and 10 g / l, preferably between 0.4 and 3.5 g / l.

The complexing agent must be soluble in the cleaning solution and compatible with the other components. He must also be able to complexly bind the ions in the solution to the extent that the formation of troublesome precipitates is avoided. Sugar acids such as gluconic acid or glucoheptanoic acid, citric acid, ethylenediaminetetraacetic acid, tartaric acid or salts of these acids and sodium tripolyphosphate are particularly suitable as complexing agents. The concentration of the complexing agents is usually in the range from 0.01 to 3 g / l, preferably in the range from 0.05 to 1 g / l.

The HLB number of the surfactant is a measure of the size and strength of the hydrophilic surfactant component in relation to the lipophilic surfactant component (cf. Römpps Chemie-Lexikon, Franckh'sche Verlagsbuchhandlung, Stuttgart, 8th edition, page 1715). By choosing surfactants with an HLB number of at least 12 and preferably a maximum of 15, effective removal of lubricants and organic contaminants, which result in particular from the deformation and smoothing of the aluminum containers, is achieved. The formation of white rust is also inhibited. When using surfactants with an HLB number of over 15, higher concentrations are usually required if complete cleaning is achieved and impurities are re-deposited on the surface Container surface should be avoided.

Suitable surfactants are, for example, those whose hydrophobic group is formed by alkylphenol, straight-chain or branched-chain alcohols and propylene glycol / propylene oxide polymers. The hydrophilic constituent can originate from ethylene oxide groups, the end groups of which may be etherified or otherwise occupied. Surfactants based on alkoxylated hydrocarbons are particularly suitable.

The aforementioned surfactants can be used individually or in combination. Their concentration should usually be 0.003 to 5 g / l, preferably 0.02 to 1.0 g / l.

The application of the cleaning solution is carried out as usual by e.g. Flooding, dipping or spraying at temperatures of about 15 to 65 ° C, preferably 32 to 54 ° C, for a period of time that ensures the purity of the aluminum surface.

After the alkaline cleaning, a rinsing treatment with an aqueous solution is carried out, preferably in several stages. The pH of the rinsing solution should be below 7.5, preferably 6.5 to 7. At higher pH values, the aluminum containers can easily turn brown when the treatment system is at a standstill in the area of the rinsing zone. The final rinse is generally carried out with deionized water.

If no further wet chemical treatment is intended, the aluminum containers are dried after the rinsing treatment, usually also painted, printed or otherwise provided with decorative coatings.

The method according to the invention is particularly advantageously suitable for the treatment of aluminum containers before the formation of conversion coatings. Conversion coatings greatly improve the mobility of the containers (rotation, sliding against each other) as they pass through the treatment plant and through the subsequent production stages and also increase the corrosion resistance. Such conversion coatings can be produced, for example, from solutions based on heavy metal phosphate or titanium, zirconium and hafnium fluoride with or without additional tannin content. Examples of methods for forming such conversion coatings are given in U.S. Patents 4,017,334, 4,054,466 and 4,338,140.

Also in the case of the aforementioned production of conversion coatings, the aforementioned drying, painting, etc. usually follows, depending on the type of process, after a previous rinsing treatment.

With the help of the invention it is possible to save considerable amounts of washing-up liquid and thus also to considerably reduce the amount of waste water which requires processing. As a rule, it is possible to cover the entire requirement of the solution required for the pre-cleaning stage from the rinsing stage, so that the total amount of waste water to be treated is reduced accordingly.

Claims (8)

1. A process for cleaning aluminum containers, comprising the steps of

a) pre-cleaning using an aqueous acidic solution, the pH value of which has been adjusted to from 4 to less than 7,

b) alkaline cleaning, subsequently to step a), and

c) rinsing, subsequently to step b) with aqueous solution, the pH value of which is adjusted to less than 7.5 by introducing at least part of the rinsing solution into step a) and recirculating same.

2. The process according to claim 1, characterized in that in the pre-cleaning step a solution is used, the pH value of which has been adjusted to from 4.5 to 5.5.

3. The process according to claims 1 or 2, characterized in that in the pre-cleaning step a solution containing a surfactant is used.

4. The process according to claim 3, characterized in that in the pre-cleaning step a solution containing a surfactant in an amount of from 0.005 to 5 g/I is used.

5. The process according to claim 3, characterized in that in the pre-cleaning step a solution containing a surfactant in an amount of from 0.1 to 1 g/I is used.

6. The process according to any one or more of claims 1 to 5, characterized in that in the pre-cleaning step a solution is used, the temperature of which has been adjusted to <65.6 ° C.

7. The process according to any one or more of claims 1 to 5, characterized in that in the pre-cleaning step a solution is used, the temperature of which has been adjusted to a value within the range of from 32.2 ° C to 54.4 ° C.

8. The process according to any one or more of claims 1 to 7 as pre-treatment of aluminum containers prior to the application of conversion coatings.