Alkaline advances

Frank McDonough and Jim Vandenberg of Quaker Chemical Corporation explore the benefits of using advanced alkaline cleaning technology for aluminium beverage cans

For nearly 50 years in the United States, two-piece aluminium beverage cans have been manufactured using the drawing and ironing process (D&I). Early innovators of this technology included Kaiser Aluminum, Reynolds Metals, and the Adolph Coors Company as early as 1963. David Reynolds and Ed Maeder, of Reynolds Metals, are considered the visionaries for the development of the aluminium can and the ‘professor of the drawing and ironing process,’ respectively.

Aluminium CoilsDuring the two-piece can manufacturing process, aluminium coils are converted into cups, which are then drawn and ironed to form elongated cans to achieve the desired length. Each can is then trimmed and sent to a multistage washer for cleaning prior to subsequent surface decoration. During early development of this two-stage process, the can cleanliness was targeted as critical to overall decorative quality and ink and varnish adhesion. The degree of cleanliness also heavily influenced the mobility of the cans as they moved through the decorating process. Today, these early discoveries remain critical to understanding can quality.

One industry method of early success was with a cleaning system based on a blend of sulfuric acid, hydrofluoric acid, and combinations of a variety of suitable surfactants. Through this cleaning method, the surfactants – delivered in a sulphuric acid medium – removed the organic soils (eg, cupping lubricants and bodymaking coolants), and the hydrofluoric acid removed the inorganic soils (surface oxides and aluminum fines), which resulted in etching of the can surface. Even today, this basic system represents the predominant global aluminium can cleaning process.

In the early 1980s, the Anheuser-Busch flavour laboratories conducted a number of experiments with aluminium cans to determine the impact of the specific manufacturing process on the flavour of the beer products. One study compared cans cleaned with acidic cleaners to those cleaned with alkaline cleaners.

The study determined that there was a slight flavour advantage to alkaline cleaned cans over acid cleaned cans. In an article published in the Journal of the American Society of Lubrication Engineers in 1984, Bill Hardwick of Anheuser-Busch proposed that certain unsaturated fatty acids used in the forming lubricants could be more completely removed during the cleaning process with the use of alkaline cleaning chemistry, thus reducing the incidence of an off-flavour in canned beer products [1].

As a result of this study, Anheuser-Busch requested that Ball Corporation’s Metal Container Division convert its cleaning systems to an alkaline-based cleaning method. Ball’s Saratoga Springs, New York plant was converted to the first generation of alkaline cleaners in 1984. In an effort to accommodate this change to alkaline-based cleaners, two suppliers of cleaners, Amchem and Parker Chemical, began development of alkaline-based cleaning systems. The parties merged and the can business was eventually acquired by PPG Industries in 1997.

In 1986, as a response to the shift to alkaline-based cleaning methods, Quaker Chemical Corporation introduced an alternative alkaline cleaning technology to Ball Corporation. The earliest versions of alkaline cleaning systems continued to use small quantities of hydrofluoric acid in the first rinse stage for aluminium fines removal; however, the newer generation of Quaker technology and subsequent industry versions have eliminated this need and are therefore classified as ‘fluoride free.’

Benefits:Anheuser-Busch

Safety – Alkaline cleaning technology allows the can plant to eliminate all hydrofluoric acid (HF) from the facility. HF is one of the more dangerous chemicals, and exposure to it requires immediate, specific and unique medical treatment. Adequate personal protective equipment must be provided to each employee who may be exposed to hydrofluoric acid. Currently, most can plants using an acid-based cleaning system store significant volumes – up to 20 per cent – of HF solutions on-site.

In addition, sulfuric acid mist was declared an IARC carcinogen in the US in 2003. This is particularly relevant because there is a considerable amount of misting of the sulphuric/HF solution in stage 2 of an acid washer, which are typically run at ~ 140ºF.

Productivity – Aluminium cans are cleaned faster with alkaline cleaning solutions versus acidic solutions, typically about 25 per cent faster than HF. Alkaline cleaners have a long history in a variety of industries as being used in high speed cleaning operations to quickly emulsify fatty acids and esters.

Higher productivity is achieved through faster can cleaning and faster mat speeds, especially in plants where the washer is considered a bottleneck to higher output.

Equipment – Acidic cleaning solutions often attack stainless steel washer tanks, plumbing, pumps and washer mats, which increase maintenance and repairs costs, thus limiting productivity because of increased downtime. In comparison, an alkaline environment in and around the washer is far less corrosive than an acidic environment, especially where HF is present, and at elevated temperatures.

Energy – Typically, stage 2 baths are maintained at 105-110ºF, compared to 135–145ºF with acidic systems. Thus, due to the significantly lower stage 2 operating temperature in an alkaline system, energy savings are inherent. In some cases, stage 2 solutions may require cooling rather than heating dependent upon the temperature in stage 1.

Waste treatment – Can plants utilising HF in stage 2 of the washers require lime (calcium hydroxide) to complex and precipitate fluoride ions from the effluent systems to meet government fluoride discharge regulations. In comparison, this is not required with effluent from an alkaline system since there is no fluoride present. As a result, the purchase, storage, and use of large amounts of lime are eliminated, which significantly reduces generated solid sludge realised savings by the plant(s). Reporting requirements are also streamlined due to the less hazardous and lower volume nature of the waste stream.

Limitations

The speed with which alkaline cleaners clean aluminium cans exposes fresh metal surfaces, creating greater susceptibility to oxide staining during line stoppages. However, can makers have addressed this limitation through improvements to line control techniques by minimising contact time of cans in stage 2.

A byproduct of alkaline aluminium can cleaning is the formation of insoluble aluminium complexes, which can plate out over time on the washer housing, which may occasionally result in a higher level of blocked nozzles. However, routine washer maintenance can reduce the risk of such blockages.

Water usage can be slightly higher with alkaline cleaners because the baths are maintained in a narrow reactive product (RP) range for optimum cleaning and costs. However, stage 2 discards are utilised to maintaining the RP target.

Summary

Alkaline aluminium can cleaning technology continues to be a successful alternative to hydrofluoric acid based cleaning. The benefits of alkaline cleaning include improved worker safety, reduced washer corrosion, lower operating costs, shorter cleaning times, lower freight costs and reduced total waste.

Furthermore, converting a system from hydrofluoric acid to alkaline requires a limited amount of down time and capital expense.

– Frank McDonough is senior product manager and Jim Vandenberg is can industry business manager, Americas, both at Quaker Chemical Corporation

References:

[1] Journal of the American Society of Lubrication Engineers, October 1984

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