Thermal processes for corrosive media – plants in special materials
Schulz+Partner Verfahrenstechnik in Teningen, Germany, designs and supplies evaporators, crystallisers, columns and heat exchangers for media where stainless steel fails: hydrochloric acid, sulphuric and phosphoric acid, HNO₃/HF mixed acids, chloride-bearing pickling liquors and metal salt solutions. We select the material for each component – from graphite and silicon carbide to tantalum, zirconium and titanium, through to Hastelloy, PFA linings and GRP (glass-fibre reinforced plastic, also known as FRP) with plastic liner. This keeps the plant resistant and still economical. We deliver worldwide and commission the plant on site.
Why stainless steel is often not enough
Standard stainless steels such as 316L (EN 1.4404) are adequate for many aqueous media. With chlorides, reducing acids, fluorides or high temperatures, however, pitting, stress corrosion cracking or uniform corrosion become a risk. Often it is not the main components that decide, but traces: even small amounts of copper or iron ions, fluoride or oxidising agents can rule out an otherwise suitable material. That is why we check the full composition of the medium – including the concentrated state at the end of evaporation.
Materials at a glance
| Material | Typical media | Typical components |
|---|---|---|
| Graphite (impregnated) | hydrochloric acid, dilute to medium sulphuric acid, phosphoric acid | heat exchangers, heaters, condensers |
| Silicon carbide (SiC) | highly corrosive acids, including HNO₃/HF mixed acid | shell-and-tube heat exchangers |
| Tantalum | hot hydrochloric and sulphuric acid, nitric acid (not for hydrofluoric acid) | heaters, heating surfaces |
| Zirconium | hydrochloric acid, nitric acid, alkalis | condensers, heat exchangers |
| Titanium | oxidising and chloride-bearing media, e.g. chromic acid | heat exchangers, vessels |
| Nickel-based alloys (Hastelloy C-276, C-2000 etc.) | chloride-bearing acids, mixed oxidising/reducing | vessels, plate heat exchangers, instrumentation |
| PFA lining | almost all acids and alkalis, also under vacuum | vessels, piping, valves |
| GRP with liner of PVDF, PFA or FEP (dual laminate) | acids and salt solutions, large tanks and columns | vessels, tanks, separators |
| Rubber lining, PP, PVDF, E-CTFE | acids and salt solutions at moderate temperature | tanks, piping, periphery |
How we select the material
- Analyse the medium completely: main components, trace substances, pH, temperature – in the feed and in the concentrate.
- Decide component by component: heater, separator, condenser and piping see different conditions. We use expensive materials only where they are needed.
- Combine experience and manufacturer data: our own resistance data from more than 30 years of projects plus recommendations from material and equipment manufacturers.
- Tests with the original medium: in our testing laboratory with pilot plants we check how your medium behaves before you invest.
- Fabrication by specialists: we design and specify the equipment; it is fabricated by manufacturers specialised in the respective material. We supervise fabrication, testing and commissioning.
Practical examples
Hydrochloric solution from the precious metals industry: heat pump vacuum evaporator with tantalum heater and zirconium condenser. The solution is concentrated to about one fifteenth of its volume – at around 0.2 kWh of electricity per litre of distillate.
Concentrating sulphuric acid from 41 to 85 %: vacuum evaporation with PFA-lined equipment.
Mixed acid from zirconium pickling: recovery of more than 98 % of the nitric acid and more than 45 % of the hydrofluoric acid with silicon carbide heat exchangers.
Sulphuric metal salt solution containing copper: the dissolved copper ions rule out an otherwise common nickel-molybdenum alloy. Hastelloy C-276 was selected.
Phosphoric acid up to 85 %: forced-circulation vacuum evaporator with a plate heat exchanger in Alloy C-2000 and plastics in the periphery.
Frequently asked questions
When is 316L (1.4404) stainless steel no longer sufficient?
With hydrochloric acid, chloride-bearing solutions at elevated temperature, hydrofluoric acid or reducing acids. What counts is the concentration at the end of evaporation, not in the feed.
Why not make everything from plastic or tantalum?
Because it is rarely necessary. Plastic linings have temperature limits and conduct heat poorly; tantalum is expensive. We combine materials component by component.
Why PFA instead of PTFE for vacuum equipment?
For vacuum equipment we use PFA linings or GRP vessels with a liner of PVDF, PFA or FEP. They are suitable for vacuum operation – unlike classic PTFE linings, which can collapse under vacuum; external support vacuum helps only to a limited extent. This keeps the plant permanently tight, even with aggressive acids.
Do your plants comply with EU regulations?
Yes. For the EU we supply plants with CE marking and an EU declaration of conformity in accordance with the Machinery Directive 2006/42/EC (from 20 January 2027: Machinery Regulation (EU) 2023/1230) and, for pressure equipment and assemblies, the Pressure Equipment Directive 2014/68/EU (PED). The electrical equipment complies with the Low Voltage Directive 2014/35/EU and the EMC Directive 2014/30/EU; where explosion protection is required, for example with solvents, we design to ATEX 2014/34/EU. For installations outside the EU we clarify the applicable requirements with you at the start of the project.
Can you check the resistance in advance?
Yes. In our testing laboratory we test your original medium in pilot plants. This determines the material selection, concentration limits and energy demand.
Which processes do you offer in special materials?
Evaporation (also with heat pump), crystallisation, rectification and liquid-liquid extraction – each including engineering, supply and commissioning.
Your medium – our material proposal
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