Alloys for Chemical Processing

The Chemical Processing Industry (CPI) encompasses a broad range of products, including petrochemical and inorganic chemicals, plastics, detergents, paints, pigments and more. Given the specific nature of individual processes, we focus here on the production of phosphoric acid - a key precursor in the large-scale production of fertilisers and agri-chemicals - but there is some commonality between the process conditions experienced in other acid-based processes.

Phosphorus is an essential plant nutrient (usually taken up by plant roots as H₂PO₄^- derived from phosphoric acid, H₃PO₄). Therefore, the manufacture of fertilisers depends on the availability of supplies of phosphoric acid.

Phosphoric acid is made by two processes, either the so-called 'wet' process or the 'thermal' process. Phosphate rock (fluorapatite) is the predominant source of phosphorus and is mined in countries such as Brazil, South Africa, Morocco, Jordan and Algeria.

Production processes

Wet process

Phosphoric acid is produced from phosphate rock by reacting it with concentrated sulphuric acid in a series of reactor vessels. This results in phosphoric acid and calcium sulphate (gypsum) together with smaller volumes of impurities from the mined rock.

There are a number of variants of the wet process, based on either di-hydrate or hemi-hydrate technologies; the main difference between the processes being the temperature of operation. The hemi-hydrate process runs at a higher temperature (90-110°C) than the di-hydrate process (70-80°C) and hence is more demanding of the materials employed.

Thermal process

Here, phosphorus is burnt in air at elevated temperatures and passed into a hydration tower where the phosphorus oxide gas is absorbed into a phosphoric acid solution. This is a more energy-intensive process, although it will produce a more concentrated and purer solution. Given the main application in fertilisers and cost-sensitivities, the wet process dominates.

The role of super duplex stainless steel

Stainless steels have been used by the chemical industry for many years in applications requiring corrosion resistance better than that of carbon steel. Super duplex stainless steels provide a cost-efficient alternative to nickel-based alloys for a wide range of acid processing applications due to their very high resistance to localised corrosion in chloride-containing environments, plus their high mechanical strength. Ferralium^® has been designed to maximise this combination of properties, making it the alloy of choice for a vast range of acid production processes.

Corrosion challenges at each stage

Uniform corrosion from impurities (HF, H₂SiF₆ etc.) - the 'attack step'

Digester tanks have been historically constructed from stainless steels such as Alloy 316L, with rubber- or brick-lining selectively applied at the bottom of the tank. However, super duplex steels, and specifically Ferralium^®, were specified in significant quantities from the 1990s onwards and have provided excellent service.

Erosion corrosion from solid particles

The ground phosphoric rock is mixed with water to form a slurry that can be easily pumped into the digester for reaction with sulphuric acid. Such a slurry will lead to erosion corrosion unless appropriate materials are specified.

Ferralium^® is widely specified in pumps and valves due to its combination of corrosion resistance and mechanical properties. In addition, it is commonly used in the construction of agitators, for both the shafts and blades.

Effects of temperature - di-hydrate vs hemi-hydrate process

As the hemi-hydrate process operates at higher temperatures (90-110°C) than the di-hydrate process (70-80°C), the rate of corrosion will be higher. Moving from austenitic stainless steels (Alloy 316L, Alloy 317L) to super duplex stainless steels is therefore a prudent move.

Ferralium^® has been used in evaporators as tank heads, where the parent metal provides equivalent or better performance than lined alternatives, with simpler construction, lower cost and less potential for maintenance.

Localised corrosion under deposits - filtration step

The filtration step can be relatively overlooked with respect to material selection in comparison with the digesters - as the perceived level of corrosion is deemed less compared with the steps involving concentrated acids. Localised crevice and pitting corrosion from scale formation during the digestion reaction and filtration stages of phosphoric acid manufacture can be a major concern for production facilities. Field tests in a Lamella decanter containing 40% P₂O₅ solution show that Ferralium^® has a corrosion resistance more than 5x better than Alloy 904L and 2x that of Alloy 316 stainless steel under these conditions.

Localised corrosion due to chlorides

Ferralium^® has a superior corrosion resistance to many other metals in chloride-containing media at most concentrations, including both Alloy 904L and 3xx series austenitic stainless steels. It also has a lower threshold chloride content for the initiation of localised corrosion; consequently, it outperforms these alloys in the hemihydrate process where operating conditions are harsher due to the higher temperature and lesser rock quality requirement.

Uniform corrosion from sulphuric acid (H₂SO₄)

Ferralium^® outperforms a large number of austenitic, duplex and super duplex stainless steels in sulphuric acid at all concentrations and temperatures. Its high copper content compared with other duplex and super duplex grades improves corrosion resistance in sulphuric acid, and it has been shown to outperform Alloy 904L (UNS N08904) in most areas of the phosphoric acid manufacturing process.

Proven applications

In comparison with UNS N08904, Ferralium^® provides:

• Improved corrosion resistance against impurities such as HF, H₂SiF₆ and others
• Better resistance to erosion-corrosion during the attack process
• Enhanced resistance to corrosion from chlorides
• Resistance to crevice corrosion i.e. corrosion under deposits
• More corrosion resistant at higher temperatures (hemi-process)
• Lower rate of corrosion in sulfuric acid

Components where Ferralium^® has been used successfully include storage tanks for concentrated super phosphoric acid, storage tanks for intermediate 54% phosphoric acid, slurry feed tanks, water circulation tanks, pre-neutraliser tanks, valves, filter pans, pipework (pipe, flanges, elbows, fittings) and fasteners.

For such tanks, the higher mechanical properties of Ferralium^® can result in material savings by reducing the wall thickness by up to 25% compared with lower grade stainless steels, which can also simplify and speed-up the welding operation.

Whilst 25% Cr super duplex stainless steel meets most material requirements for successful phosphoric acid production, there are occasional instances where more expensive materials such as nickel-based alloys have to be deployed in areas of plant where high concentrations of hydrofluorosilisic acid or hydrofluoric acid are experienced.