Fabricated Plastics Limited of Maple, Ont., had previously built zinc sulfate cooling towers of FRP for various zinc refineries in North America when it was awarded a contract to produce six of the giant cooling units for a refinery in Tasmania, Australia.
Each 5.9 m (19'4') in diameter and 18.3 m (60 ft.) high the Australian towers are major components of the plant which transforms raw zinc ore into the pure metal for use in foundries, separating out gypsum as a byproduct.
In the refinery, the concentrated ore is dissolved in sulfuric acid to form a zinc sulfate solution from which the metal is then recovered by electrode-position on the cathode of an electrolytic cell.
A stream of air fed in at the bottom of each tower is forced through to cool the hot sulfuric acid introduced at the top, causing the gypsum to drop out of the solution and collect at the bottom of the tower.
'Originally, we had planned to work with FRP molders in Australia to produce the towers,' says FPL's president Don Sablinskas.'But because of the dimensionally large size, special materials and fabricating technology needed to meet the extremely high chemical resistance requirements of the job, we were forced to mold them in our own plant in Canada.
'For one thing,' he explains 'we felt it was essential to use ECRGLAS along with a vinyl ester resin to produce a laminate that would withstand the attack of the highly corrosive sulfuric acid. And ECRGLAS was not available in Australia at the time.'
Also, Nexus veil was required in the layup of each tower to provide a mirror- like interior surface - one that would withstand the abrasive action of the falling gypsum.
We were commissioned to carry out the contract in two phases. It took approximately eight months to complete, over all, with very heavy input from our engineering department because of many difficult problems associated with the job.
'For one thing, we had to provide 18.3 m (60 ft.) high towers that would be elevated on 12 m (40 ft.) high support structures for a total height of more than 30.5 m (100 ft.).' And to complicate matters,' adds Lam Woo, Fabricated Plastics VP of engineering,' the whole plant was to be built on a hill.'
Much too large to be completed in Canada and shipped intact half way around the world, each cooling tower structure had to be produced in sections small enough to ship by road to a dock in Vancouver and by boat to Australia.
The wall of each cylindrical structure consists of five curved panels - each approximately 3.6 m (12 ft.) wide and 18.3 m (60 ft.) long- which could be stacked five-high in shipping crates. Flanges on the sides of the panels were provided for use in the final assembly.
The towers were produced by a combination of hand layup and filament winding (with integrally molded ribs to help withstand heavy wind loads) - initially as complete cylinders which were then cut into the separate panel segments. An FRP bottom was also molded for the base of each tower to collect the gypsum.
'Each bottom was lined with polypropylene,' says Mr.Sablinskas,' an exceptionally good abrasion-resistant material that would not be damaged when the gypsum was being shoveled out by the maintenance staff.' The towers were supplied complete with distribution piping systems, FRP fans and fan motors, pultruded grating for walkways and safety maintenance platforms directly over the top sections of the towers.
An access manway at the base of each tower allows a work crew to walk in to service it, and direct access is provided at the top for servicing the piping systems.
The slightly conical shape at the top of the cooling units helps to control the flow of the gas stream through the towers, funneling the stream through the mist eliminators which remove the corrosive acid components.' Close to 99.9 percent of the acid is removed in the process,' says Lam Woo.