Abstract

LOTZ, M.. Fluorinated melt-extruded thermoplastic linings in off-gas pipelines of sulphuric acid production plants and in tailor-made equipment for sulphur oxide and sulphuric acid handling. J. S. Afr. Inst. Min. Metall. [online]. 2013, vol.113, n.8, pp.611-617. ISSN 2411-9717.

Gases containing sulphuric acid and sulphur oxide (SO₂ , SO₃ ) can lead to severe corrosion problems, in particular in high concentrations combined with water vapour. If acid condensation takes place, this can lead to the extremely destructive dew-point corrosion. In these cases, even materials like titanium and stainless steels can be attacked in the aggressive environments, or suffer variable but significant degrees of material loss by surface degradation. An excellent option for solving the corrosion problems induced by sulphuric acid and sulphur oxide is linings made from fluorinated melt-extruded thermoplastics. Those can be divided into partially fluorinated polymers like polyvinylidenefluoride (PVDF) and ethylene-chlorotrifluoroethylene (ECTFE), and fully fluorinated materials like tetrafluoroethylene-hexafluoropropylene (FEP) and tetrafluoroethylene-perfluoroalkylvinylether (PFA). The latter have a much higher temperature and chemical resistance than partially fluorinated thermoplastics, but share their outstanding processing properties. Fully fluorinated melt-extruded thermoplastics extend the application range of the lining solutions remarkably. The linings can be installed in sheets, tanks and reactors for liquid sulphuric acid, as well as in scrubbers and pipelines for the conversion and transport of sulphur oxide and sulphuric acid gases and fumes, including the shut-off valves for flow regulation. An introduction to and comparison of the lining materials and installation methods will be presented, illustrated by means of selected relevant applications. Application ranges for partially and fully fluorinated melt-extruded thermoplastics will be discussed against the background of the chemical compounds and temperatures involved. Case studies of two recently installed and successfully running pipelines in sulphuric acid producing metallurgical plants will be shown in detail. One is a fibre-reinforced plastic (FRP) reinforced FEP liner pipe for off-gases containing droplets of sulphuric acid. The other is FEP-lined blast spools made from stainless steel, also installed in gas streams containing sulphuric acid. Fabrication methods for transition forms, over-dimensional pipes, and other tailor-made parts will be explained, and relevant quality standards and best practices will be discussed.

Keywords : extrusion; fabric backing; ECTFE; PFA; PTFE; PVDF; FEP; fluoropolymer; fluoroplastic; fully fluorinated; partially fluorinated; dual laminate; FRP; bonded steel lining; liner pipe; loose lining; fix point lining; sulphuric acid; sulphur oxide; corrosion; melt-extrusion.

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