GB2186292A - Process for cleaning aluminium - Google Patents

Abstract

Aluminium containers are cleaned by contact with an aqueous alkaline cleaning solution to remove aluminium fines and organic residues and are then rinsed with an aqueous acidic rinse solution having a pH of not more than 6. This solution preferably contains fluoride and/or phosphate as accelerator for removal of oxide film remaining from the cleaning step and preferably has pH below 2.5. The containers may be subjected to at least two of the said acidic rinses. The aluminium containers may be rinsed with tap water after the alkaline cleaner and before the acidic rinse, the pH of the acidic rinse being preferably provided by hydrogen fluoride and/or sulphuric acid.

Description

SPECIFICATION Process for cleaning aluminium The present invention broadly relates to a process for cleaning aluminium surfaces employing a sequential cleaning cycle including a primary alkaline cleaning solution and a rinse. The process is designed to effect removal of organic contaminants and aluminium fines from the surfaces of the containers and to further enhance the mobility of the containers facilitating their transport in high-speed can lines having a capacity in excess of about 1,000 cans per minute. The process of the present invention is particularly adaptable for cleaning drawn and ironed aluminium container bodies of the types employed in the packaging of foodstuffs and beverages.The cupshaped and dished integral bottom of such container bodies, because of their configuration, are conducive to entrapment of the various cleaning and rinse solutions during the cleaning cycle which has in some instances resulted in an objectionable localised staining of the surfaces thereof during line stoppages during the cleaning process.

We have described in GB 2,166,757 (not published at the priority date hereof) various processes for cleaning aluminium containers, including a process in which they are cleaned with an aqueous alkaline cleaning solution that removes aluminium fines and organic soil and they are then rinsed with rinse water that initially has a pH below 7.5 but is used for rinsing sufficient of the containers that its pH would rise above 7.9 but is held below 7.5 by neutralisation of used rinse water. We state that the pH is preferably held at 7 or below but the lowest figure that is quoted for the pH at which the rinse water is maintained is 7.0. The processes described in that specification do give a significant improvement in mobility and other results, but there is room for further improvement.

In the invention a plurality of aluminium containers are cleaned by contact with an alkaline cleaning solution that can remove aluminium fines and organic residues and the containers are then rinsed with an aqueous acidic rinse solution having a pH of not more than 6.

In one aspect of the invention the rinse solution has a pH of 2.5 or below and optionally contains fluoride. In a second aspect of the invention the rinse solution can have a pH of not more than 6, preferably below 5, and contains fluoride and/or phosphate as an accelerator for removal of oxide film remaining from the cleaning step.

By the invention it is possible to obtain increased mobility in high speed can processing lines and less staining.

The containers are preferably drawn and ironed aluminium containers. The aluminium containers are transferred from the body-making and trimming apparatus to a multiple stage high-speed cleaning apparatus to effect a removal of the residual body-forming lubricants and aluminium fines or smut formed on the surfaces of the containers during the manufacturing operation. The aluminium fines comprise minute particles of aluminium on the container surfaces and adhered thereto in combination with various lubricants and metal working compounds employed during the forming operations of the container.

The first stage of the cleaning cycle preferably comprises contacting the surfaces of the containers with an aqueous pre-wash solution, preferably an aqueous acidic pre-wash solution, to effect a removal of at least a portion of the fines and/or organic soils from the container surfaces thereby reducing a buildup of such contaminants in the succeeding primary alkaline cleaning stage.

The pre-wash solution is applied to the aluminium container surfaces at temperatures ranging from ambient (i.e., from about 15"C) up to about 93"C, and preferably at a temperature below about 65"C, such as from about 32"C to about 55"C. The pre-wash solution is contacted with the aluminium surfaces to be cleaned such as by flooding, immersion or spraying of which the latter constitutes the preferred method to assure uniform distribution of the pre-wash solution on both the interior and exterior surfaces of the container.

Following the pre-wash stage, the preliminarily cleaned aluminium containers are transferred directly to a primary alkaline cleaner stage incorporating an aqueous alkaline cleaning solution of a composition preferably as described in GB 2,166,757. Typical of suitable aqueous alkaline cleaning compositions are those containing an alkalinity agent present in an amount to achieve satisfactory removal of residual aluminium fines on the surfaces of the containers without incurring undesirable etching of the surfaces. Generally, the pH of the alkaline cleaning solution ranges from at least about 11 up to about 13.The alkalinity agent may comprise an alkali metal hydroxide and/or alkali metal carbonate in further combination with a suitable complexing agent present in an amount effective to complex at least some of the metal ions present in the cleaning solution which otherwise tend to form solution insoluble precipitates. The alkaline cleaning solution can optionally further contain a foam-suppressant agent of any one of the types conventionally employed which is utilised in consideration of the types and concentration of the surfactants employed. The foam suppressant agent is used at a level sufficient to prevent undesirable foaming of the cleaning solution particularly when it is applied by spray application.

The alkaline cleaning solution is preferably controlled at a pH ranging from about 11 up to about 13 with a pH of from about 11.5 to about 12.5 being preferred. In order to achieve an alkalinity within the foregoing range, the alkalinity agent such as a mixture of sodium hydroxide and sodium carbonate is employed at concentrations of from about 0.05 up to about 10 g/l.

Complexing agents suitable for use in the alkaline cleaning solution comprise sugar acids as well as salts thereof such as, for example, sodium gluconate, sodium citrate, sodium tripolyphosphate, as well as other acids including glucoheptanoic acid, tartaric acid, EDTA and the like as well as the bath soluble and compatible salts and mixtures thereof.

The alkaline cleaning solution preferably contains at least one or a combination of surfactants which are usually selected from the group comprising hydrocarbon alkoxylated surfactants which are characterised as individually or in their blended combination as having a Hydrophile-Lipophile Balance (HLB ratio), i.e., the balance of the size and strength of the hydrophilic (water-loving or polar) and the lipophilic (oil-loving or non-polar) groups of the molecule within an HLB ratio of at least about 12, preferably at least about 12 to about 15.While other surfactants can be employed to provide for effective cleaning of the alkaline cleaning solution, it has been found that employing surfactants characterised by the foregoing HLB ratios substantially reduces or eliminates a tendency of white staining on the surfaces of the aluminium container occasioned by line stoppages which frequently occur in high-capacity, high-speed container washers. Such line stoppages may range from about one-half minute to as long as about one hour and the presence of residual cleaning solution on the container surfaces has occasioned such whitestaining problems. By employing surfactants of the appropriate HLB ratio, such white-staining has been substantially eliminated.

The composition may contain an antifoam agent.

The aqueous alkaline cleaning solution can also be applied, as in the case of the acidic prewash solution, by flooding, immersion and preferably by spray application for a period of time sufficient to effect a cleaning of the surfaces thereof. Generally, the aqueous alkaline cleaning solution is employed at moderate temperatures below about 65"C to about ambient temperature with temperatures of from about 32"C to about 55"C being preferred.

Following the aqueous alkaline cleaning stage, the cleaned containers are transferred to a water rinse stage, preferably a multiple stage water rinse section of the conveyorised washer.

As described in GB 2,166,757 it had previously been found that maintenance of the pH of the rinse water at below 7.5 reduced the tendency to form brown stains, especially when line stoppages occur.

It has now been further discovered that by increasing the acidity of the water rinse solution employed in the rinse stage following the primary alkaline cleaning step, with or without an intervening water rinse step, not only brown staining is prevented, but a further substantial improvement in the mobility of the cleaned aluminium containers is obtained. It is believed that this increased mobility is a result of the substantial reduction of or removal of an aluminium oxide film formed on the aluminium container surfaces during the alkaline cleaning step. The substantial reduction of the aluminium oxide film on the aluminium surfaces also effects a substantial or complete removal of objectional stains that may have formed on the aluminium container surfaces during and following the alkaline cleaning stage.After the alkaline cleaning stage and prior to the acidulated water rinse stage, the containers can be subjected to a conventional tap water rinse to remove a substantial portion of the residual alkaline cleaner from the surfaces thereof prior to the acidulated water rinse stage. Any staining occurring in the tap water rinse stage, which may become somewhat alkaline because of drag-in, as a result of line stoppages, can be substantially and/or completely removed in the following acidulated water rinse stage producing containers of acceptable commercial quality.

The acidulated water rinse solution should be at an acid pH low enough for the specific contact time and rinse solution temperature, method of application and concentration, if any, of accelerating agents to achieve removal of substantially all stains formed during any prior stages.

The rinse solution must have a pH of less than about 6, and preferably less than about 5. It preferably contains free fluoride and/or phosphate ions as an accelerator for the removal of oxide coating. The presence of this accelerator is especially important when the pH of the solution is above 2.5 but the accelerator can desirably be present even at lower pH values. If the solution does not contain such an accelerator it is preferred for it to have a pH of below about 2.5, often about 2 or less. It is generally unnecessary for the rinse solution to have a pH of below about 1.5. Best results are often obtain when the rinse solution has a pH in the range of 1.5 to 2.5, especially when it also contain fluoride.

When tertiary stage acidic water rinse section is employed, the second-recirculating stage of the three-stage water rinse should contain the rinse solution at the required maximum pH. The water rinse solution from the third or last rinse stage can conveniently be counterflowed to the preceding stages such as in a three-stage water rinse section. The acidulated water rinse solution is applied to the containers in the same manner and at the same temperatures employed for the pre-wash and alkaline cleaning solutions.

Acidulation of the water rinse solution can be achieved by employing any one or combinations of commonly available organic and inorganic acids of which sulphuric and/or hydrofluoric acids constitute the preferred materials. The presence of free-fluoride through the addition of hydrofluoric acid or simple or complex fluoride and soluble salts may also be employed in amounts sufficient to enhance mobility or improve stain suppression/removal.

The effectiveness of the reduction of oxide film to an acceptable level in the acidulated water rinse stage is related to the temperature employed and the duration of contact of the acidulated rinse solution with the container surface. The method of application of the acidulated rinse solution also constitutes a variable with spray application at high pressure (high volume) constituting the preferred practice. In commercial can washers of the general types presently employed, prewash contact times of about 20 seconds up to about one minute are usual while alkaline cleaner contact times generally range from about 10 seconds up to about one minute.

Acidulated water rinse stages have normal contact times ranging from about 10 up to about 30 seconds. Under some manufacturing conditions, the can washers will operate at one-half speed for certain time intervals under which conditions the aforementioned contact times are doubled.

While mineral acids such as sulphuric acid itself is effective to reduce the oxide film when employed at temperatures approaching 93"C, such elevated temperatures are energy intensive, and for this reason, temperatures below about 65"C are preferred. It has been found however, that an acidulated water rinse temperatures below about 71"C employing usual contact times, sulphuric acid itself is only marginally effective to remove the necessary quantity of oxide film and any prior staining that may have occurred on the container surfaces.Accordingly, when lower rinse temperatures are employed, such as from about 32"C to about 55"C, the addition of an effective amount of free fluoride and/or phosphate ions to accelerate the oxide film removal properties of the acidulated rinse solution has been found necessary and preferred practice under conventional commercial operations.

The preferred accelerator is free-fluoride due to the relatively small quantities required to achieve the necessary acceleration in comparison to phosphate ions which must be added in amounts substantially higher than fluoride to achieve equivalent results. Generally,= free-fluoride ions in amounts as low as 1 ppm provide for an increase in acceleration with amounts as high as 100 ppm or higher to as high as about 1,000 ppm being usual subject to environmental restrictions. Excessive free-fluoride concentrations, that is, above about 200 ppm under certain processing conditions have been observed to cause the undesirable etching of the aluminium container surface which detracts from the shiny metallic appearance of the surface. Because of this, the free-fluoride ion concentration is preferably controlled at a level below about 100 ppm.

Generally, free-fluoride in amounts of less than about 40 ppm are satisfactory and preferred from both an economic and environmental standpoint.

The free-fluoride ions can be added to the acidulated water rinse solution as simple or complex fluorides and/or soluble salts. Hydrofluoric acid is the preferred source.

The cleaned and acidulated water rinse containers may be given a substantially neutral rinse.

For instance, they may be subjected to a final third-stage-exit water flush-off rinse upon emerging from the acidulated rinse section, which serves to remove residual acidulated rinse solution therefrom, as well as supplying fresh make-up water to the rinse stage.

Following the water rinsing of the cleaned aluminium containers, if further mobility enhancement is desired and/or an improvement in the pasteurisation characteristics of the filled containers, one may optionally subject the containers to a conversion treatment such as by employing treating solutions based on chromium phosphate or titanium or zirconium with or without tannin.

Exemplary of such conversion coatings suitable for use in the present process are those described in U.S. Patent Nos. 4,017,334; 4,054,466 and 4,338,140.

The cleaned and treated containers can thereafter be dried and subjected to the application of one or a plurality of sanitary lacquer coatings, decorative coatings, inks, and the like in accordance with conventional practice prior to the filling and sealing thereof with appropriate food stuffs or beverages.

In order to further illustrate the improved process of the present invention the following specific examples are provided. It will be understood that the examples are provided for illustrative purposes and are not intended to be limiting of the scope of the present invention as herein described and as set forth in the subjoined claims.

Example 1 In order to demonstrate the effectiveness of the present process in improving can mobility in high-speed can processing lines, an experimental field test was conducted employing a multiple stage high-speed cleaning apparatus to effect a removal of residual body-making lubricants and aluminium fines from the surfaces of drawn and ironed aluminium containers. The multiple stage washer contained three primary stages in which Stage 1 comprises an aqueous prewash employing a solution at a pH of about 10.5; Stage 2 comprises an aqueous alkaline primary cleaning solution of the type previously described at a pH of about 11.9; Stage 3 comprises an aqueous acidulated water rinse stage controlled at a pH of about 2.5 which was followed by a tap water flush-off rinse stage prior to transfer to a conversion treatment stage operated at a pH of from about 2.8 to about 3.2.Following the conversion treatment, the containers were water rinsed followed by a deionised water rinse and were thereafter oven dried. Following drying, the cans were transferred by conveyorised transfer lines and chutes to a printer at a nominal rate of about 980 to about 1,000 cans per minute. Under the foregoing operating conditions, the mobility of the cans was acceptable providing for high-speed printing.

By reducing the acidity in Stage 3 of the acidulated rinse solution from an operating pH of about 2.5 to a pH of about 5, can mobility was significantly reduced whereby the capacity of the printer had to be reduced to a rate of 710 cans per minute to provide satisfactory operation. When the acidulated rinse solution was again further acidified to restore its pH at a level of about 2.5, the mobility of the cans improved and the rate of printing was also restored to a level of about 980 to about 1 ,000 cans per minute.

The benefits of applying a conversion treatment to the aluminium containers following the acidulated rinse step in accordance with a preferred practice was also demonstrated by omitting the conversion treatment for a 24 hour test period. Can mobility was reduced apparently due to an oxide build-up on the conveyorised rails and chutes such that the rate of printing had to be reduced to a level of about 820 cans per minute. When the conversion treatment was restored, can mobility was again restored and the printing rate returned to normal.

Example 2 In order to demonstrate the effectiveness of free-fluoride ions to accelerate the removal of oxide film from aluminium container surfaces, aluminium cans were cleaned in a laboratory can washer by spray application employing 19 litres of cleaning solution. The cleaning solution consisted of sodium hydroxide dissolved in water to provide a pH of 12. 1. The cleaning solution was heated to 49"C and spray applied to the aluminium containers for a period of one minute.

The containers were inverted so that the concave dome retained a residual quantity of cleaning solution which was allowed to stand on the container for a period of one-half hour. The containers were thereafter subjected to a tap water rinse. An objectionable brown stain was present in the dome of the container along the periphery of residual alkaline cleaning solution that was retained therein.

An acidulated water rinse solution totalling 19 litres was prepared containing one gram per litre of tartaric acid which was adjusted to a pH of 1.91 by sulphuric acid. The acidulated rinse solution at a temperature of 49"C was applied for one minute to the stained aluminium containers whereafter the containers were water rinsed and inspected for stain removal. A rating system was employed in which a rating of 1 indicates no stain removal while a rating of 5 indicates a complete stain removal. An inspection of the aluminium container revealed a stain rating of 1 indicating no stain removal.

Example 3 To the acidulated water rinse solution as described in Example 2, 0.5 millilitres of 50 percent hydrofluoric acid was added to the 19 litres of rinse solution. An acidulated water rinsing of stained aluminium containers was repeated at a temperature of 49"C for a period of one minute.

Following the acidulated water rinsing, the containers were water rinsed and inspected. The stain rating of the aluminium containers containing the hydrofluoric acid was increased to 4.8 indicating almost complete stain removal.

Example 4 The inter-relationship of acidity, free-fluoride concentration, temperature and time of the acidulated water rinse treatment relative to the effectiveness of stain removal was evaluated by forming an acidulated water rinse solution containing sulphuric acid in variable amounts to provide the desired pH. Stained aluminium containers prepared as previously described in connection with Example 2 were acid rinsed employing such solutions at variable temperatures of 32"C and 49"C for time periods of 1 5 seconds and one minute to determine the concentration of free-fluoride present to provide a stain rating of 5. The results obtained are set forth in the following table: ppm Free-Fluoride Required for Stain Rating of 5 pH3 pH2.14 pH 1.7 pHO.4 TEMPERATURE 32"C 49"C 49"C 49"C 32"C 49"C TIME, 15 sec 171 68 64 69 190 109 TIME, 60 sec 68 23 13 18 - The results as set forth in the foregoing table indicates that as temperature increases or time increases, the concentration of free-fluoride in ppm to effect a rating of 5 decreases. As the pH of the acidulted rinse solution decreases below a pH of about 2, as evidenced by the data provided for the acidulated rinse solution at a pH of 0.4, the concentration of free-fluoride increases in comparison to that required at a pH of from about 2 to about 3.The foregoing test data indicates that when moderate acidulated rinse solution temperatures are employed and are accelerated by employing free-fluoride ions, the optimum pH for removing brown stains ranges from about 1.5 to about 2.5 Example 5 The inter-relationship of acidity, solution temperature and time of contact at a constant freefluoride concentration was evaluated relative to the effectiveness of stain removal by forming an acidulated water-based rinse solution containing 1,000 ppm free-fluoride added as sodium fluoride and containing sulphuric acid in variable amounts to provide the desired pH.Separate groups of drawn and ironed aluminium containers were processed through a conveyorised laboratory pilot can washer including a first cleaning stage employing an alkaline cleaning solution containing sodium hydroxide, a chelating agent and a surfactant at a pH ranging from about 12.0 to about 12.2 at a temperature of 55"C for a period of 45 seconds by spray application. Following the alkaline cleaning stage, the containers were transferred into a second tap-water rinse stage with the rinse water turned off and the cleaned containers were permitted to stand for a period of 20 minutes containing alkaline cleaner in the dome sections thereof simulating a line stoppage.

Following the 20 minute dwell, one can was removed and the stain in the dome section was inspected. The remainder of the containers were retained in stage two whereafter the tap-water sprays were turned on for a period of 15 seconds at room temperature and the water rinsed containers were thereafter transferred to a third-stage containing an acidulated highly accelerated rinse solution containing 1 ,000 ppm free-fluoride. A series of tests were conducted at each pH level commencing at a pH of 7 at two different temperatures and at two different contact times.

The temperatures selected were 32"C and 49"C and the spray contact times were selected at 15 seconds and 60 seconds. Each series of tests started employing the longest contact time at the highest temperature to evaluate the effectiveness of stain removal compared to the container removed from the second-stage prior to tap-water rinsing. If only a marginal improvement was obtained under such conditions, the remaining tests of that series employing lower temperatures and/or lower contact times were not run in anticipation that the results would be less favourable than those obtained at the higher temperature and contact time condition.

Following the accelerated acidulated third rinse stage, the containers were transferred to a fourth stage containing a deionised water rinse whereafter the containers were dried and the stain remaining in the dome section thereof was compared to the control sample removed from the second tap-water rinse stage. In accordance with the rating system described in Example 2, a stain rating of 1 was assigned to those containers in which no perceptible stain removal was obtained while a stain rating of 5 was assigned to test samples exhibiting a substantially complete stain removal. In addition to an inspection of the degree of stain removal, the aluminium container surfaces were also inspected for undesirable etching which in some commercial operations would result in a rejection of such containers as unacceptable.The results of the five groups of separate test runs are summarised in the following table including an indication as signified by the letter "E" whether the surfaces were highly etched.

pH 7 pH 6 pH 5 pH 4.5 pH 4 Temp. 32"C 49'C 32"C 49"C 32"C 49"C 32"C 49"C 32"C 49"C Time, 15 sec - - - 2 5 5 5 5E" Time, 60 sec - 1 - 1 2 5 5 5 5E 5E = undesirable etching of surface It is apparent from the data as set forth in the foregoing table under the specific conditions employed, that the acidulated rinse solution at a pH of about 7 was ineffective to remove any stain from the dome of the container employing a temperature of 49"C and a contact time of 60 seconds. Accordingly, the remaining three tests at pH 7 were not run in anticipation that the results would even be poorer. Similarly, at a pH of 6, the highly accelerated acidulated rinse solution was ineffective to remove any appreciable stain employing a solution temperature of 49"C and a contact time of 60 seconds. Accordingly, the remaining three tests of that group were not conducted. At a pH of 5, however, at a temperature of 49"C and at a contact time of 60 seconds, the highly accelerated acidulated rinse solution was effective to remove substantially all of the stain on the containers.A stain rating of 2 was obtained when the contact time was reduced to 1 5 seconds or when the solution temperature was reduced to 32"C. In view of these results, the test at 32"C and 1 5 second contact time was omitted.

At a pH of 4.5, stain ratings of 5 were obtained under all of the temperature and contact conditions specified. As the pH was further reduced to 4, undesirable etching was observed except for the test conducted at 32"C and 15 second contact time.

It is apparent from the relationship of the results as set forth in the foregoing table, that optimum stain removal without experiencing undesirable etching can be achieved by an appropriate correlation of the pH, temperature, contact time, concentration of the accelerating agent, and manner of application of the acidulated rinse solution in accordance with routine testing for any specific commercial installation.

Claims (16)

1. A process in which a plurality of aluminium containers are cleaned by contact with an aqueous alkaline cleaning solution that can remove aluminium fines and organic residues and the containers are then rinsed with an aqueous acidic rinsing solution having a pH of not more than 6.

2. A process according to claim 1 in which the rinse solution has a pH of below 5.

3. A process according to claim 1 in which the rinse solution has a pH of below 2.5.

4. A process according to claim 3 in which the rinse solution contains fluoride.

5. A process according to any preceding claim in which the rinse solution contains fluoride and/or phosphate as an accelerator for removal of oxide film remaining from the cleaning step.

6. A process according to claim 5 in which the rinse solution contains fluoride and has pH 1.5 to 2.5.

7. A process according to any of claims 4 to 6 in which the rinse solution contains fluoride in an amount of from 1 to 200 ppm.

8. A process according to any preceding claim in which the containers are subjected to at least two of the said acidic rinses.

9. A process according to claim 8 in which the rinse solution in a latter downstream stage is counterflowed to the adjacent upstream stage.

10. A process according to any preceding claim in which the rinse solution is applied by spray.

11. A process according to any preceding claim in which the rinse solution is applied at a temperature of from 15 to 650C.

12. A process according to any preceding claim in which the aluminium containers are rinsed with tap water after the alkaline cleaner and before the acidic rinse.

13. A process according to any preceding claim in which the pH of the acidic rinse solution is provided by the addition of hydrogen fluoride and/or sulphuric acid.

14. A process according to any preceding claim in which the containers are given an aqueous prewash before the alkaline cleaner.

15. A process according to any preceding claim in which the alkaline cleaner has a pH of from 11 to 13 and contains complexing agent and surfactant.

16. A process according to any preceding claim in which a conversion coating is formed on the rinsed containers.