Award Winning Large Diameter FRP Stacks For Magnesium Extraction Plant

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Courtesy of Fabricated Plastics Limited

When Fabricated Plastics Limited of Maple (Toronto), Ontario, Canada was awarded a contract by a world renowned Mining/Metallurgical Company, it proved to be a chance to exhibit their engineering excellence in several ways. First they had to demonstrate that their proposed use of Fibreglass Reinforced Plastic (FRP) was clearly the material of choice in competition with other materials. Then they needed to produce an engineering design to satisfy a demanding specification requirement. Also, given the large diameter and height, it was necessary to create an optimum relationship between design, engineering and manufacturing that would facilitate the least costly shipment and installation of the finished product.

The requirement was for two self-supporting FRP stacks to be installed at a new magnesium extraction facility, in Quebec. In this area, there is roughly 300 years of magnesium contained in the serpentine tailings of the old asbestos mines. The two FRP stacks were identical and were 64.5 m (211.6 ft) high with a diameter of 2.38 m (94 in). They had to meet stringent wind and seismic loads and include a lightning arrest system to comply with The National Building Code of Canada. The towering height and unusually large diameter of the stacks presented a considerable challenge to the design engineers, particularly as they were to be self-supporting and used in the extraction of highly corrosive fumes.

Serpentine tailings contain about 24% Mg. The extraction process starts with feed preparation, which involves drying in a gas fired rotary kiln. The serpentine is then screened and crushed before being leached with hydrochloric acid in reactors. It is then neutralized with magnesium oxide (MgO) in three tanks. A pair of buffer tanks completes the gravity flow circuit.

The resulting brine, with its magnesium content, is then separated from the FeS residue with belt filters and subsequently pumped to a storage pond. Caustic soda (NaOH) is used to purify the brine and any metal impurities are removed at this stage. Purification residue and brine are then separated in fillter presses and the residue is packed in containers for storage at the magnesium site. The final stage is creating a pure feed for electrolysis in chlorination. By adding highly concentrated hydrochloric acid, the remaining water is driven out of the prills and magnesium oxide is converted to magnesium chloride. In its molten state, this becomes the high purity feed for electrolysis.

By incorporating three different design modalities, Fabricated Plastics engineers produced an integral unit that separated the FRP stacks into three parts: the lower 3.20 m (10 ft - 6 in) diameter section to have a 2.44 m (8 ft) side inlet breaching, the middle section, representing the highest stress area, to have the proven Fabricated Plastics designed steel cage concept, and the upper section to have 3 helix vortex breaker strakes to eliminate oscillations caused by vortex shedding. The three sections used bolted connections in order to minimize installation time and its attendant cost.

The lower section had a height of 18.9 m (62 ft), the middle at 20 m (65.6 ft) and the upper at 25.6 m (83.9 ft).

All of the necessary ducting was designed, engineered and manufactured by Fabricated Plastics as well. This was an additional challenge as the duct diameters were closer to those of a tank. The resultant vacuum created a need for ribbing reinforcement to ensure stability and efficient operation.

Although not part of the contract, to ensure proper fit and alignment, Fabricated Plastics trial assembled both frp stacks horizontally. Laser transits were used to ensure the stacks were true and plumb. Well within the allowable tolerance of 25 mm /30 m(1 in /100 ft).

Fabricated Plastics personnel provided technical assistance at the job-site during the installation phase.

The Fabricated Plastics' team, including engineers, designers and manufacturers, is dedicated to providing leading edge solutions to the challenges of their customers' requirements. Their satisfaction is derived from knowing they have given their very best and have fully satisfied the customer's needs. Nevertheless, it is a great source of pride when they are given accolades from within the industry. This particular project won the coveted 'Fabricator Excellence Award' for DERAKANE Resins, presented by Dow Chemicals.

As Don Sablinskas, President of Fabricated Plastics says, 'we are satisfied in knowing that we have provided the best possible solution. But, as with all creative work, we admit that recognition of our efforts is most gratifying and an inspiration to continue in our role as one of the top players in our industry.'

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