Typical Washing Process Sequence
Can Movement Stages 3A, B & C- First Multiple Rinses
The Can Washer Stage 0 - Initial Rinse
The liquid in Stage 2 is highly acidic; if a small quantity of this liquid from Stage 2 enters the Stage 4 tank, the acidity in this tank would plumet causing the chemical control system in stage 4 to close down. The chemical imbalance would cause exception failures as the cans pass through the quality control processes. The impact of these can failures would mean that large numbers of cans would be scrapped causing financial loss. Residual chemicals are removed by passing the cans through a number of separate rinse stages where treated water, supplied from a ’Reverse Osmosis’ process, is sprayed onto the cans. How can we tell if the rinsing is working? The effectiveness of the rinsing processes are managed using a technology called conductivity. Conductivity is how much electrical current will pass through the liquid. The small electric current is measured in micro Siemens. The principal is that acid content in the allows more current to flow therefore producing a higher number of micro Siemens, shown on the ‘Y Scale’. The graph below indicates how the conductivity is like to fall as the cans pass through the rinsing processes. Rinse 1
The purpose of Stage 0 is to re-wet and remove loosely bound organic “soils” i.e. soluble lubricants from the cupper and body making operation, rolling oils, tramp oil and general dirt and aluminum swarf from the can surfaces. An air blowdown nozzle is fitted across the full width of the washer. This is designed to move liquid from the inverted can dome because the rim of the cans sit on the conveyor. Can Washing Stage 1 - Pre Wash The purpose of stage 1 is to chemically remove the organic “soils” that are chemically attached to the can after the Stage 0-Initial Rise Stage. This stage is a heated stage designed to spray diluted Sulphuric Acid onto the can surfaces using spray nozzles at pressure (~ 2.5 bar). These nozzles produce high impact forces which would lift the cans and disturb the can pack; to prevent this a Hold Down conveyor is installed to hold the cans in place. The organic soils removed from the cans can cause a build- up of oil in the supply tank. To keep the liquid system clean, a coalescer (i.e. oil/water separator) is fitted to remove excess oil. Stage 2 - Wash The purpose of stage 2 is designed to chemically remove any remaining organic soils and surface inorganics (i.e. aluminum “fines” and oxides). The liquid sprayed onto the can surfaces is a ‘cocktail’ of diluted chemicals to clean and etch the surface, removing the Disturbed Layer producing a bright, chemically clean can ready for further treatment in stage 4. This stage is also a heated stage with high pressure nozzles, hold down conveyor and coalescer.
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FUNDAMETALS Newsletter
[ 12 ] Q1 2023
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