We use plastic casings primarily due to cost, but also because of the static discharge problem with metal casings. Of course, metal creates an explosion hazard due to the risk of sparks when handling volatile solvents. On movable trolleys thus we use plastics, on presses where the pump is static, we use metal and ensure the pump is grounded. Always ground metal pumps.
The choice of the diaphragm will depend on what you are pumping, as per the solvent plus the chemical used to denature it (heptane?). Ratings change from manufacturer to manufacturer, so some will say Teflon (PTFE) is rated at 0C while others will say -20C. It depends on the blend they make the plastic with, etc… At cold temperatures, these diaphragms reach their glass transition temperatures and become brittle. Teflon works great under cold temperatures, but not when in motion, so Teflon is great for O-rings, balls, seals, and gaskets where there is no motion, but not in diaphragms where they move. I prefer Santoprene or Viton, but it depends on the chemicals to pump and the formulation of the plastic used by the manufacturer. PTFE as well as EDPM are options.
I read comments online where I understood the theory was that as diaphragms become brittle/rigid, the pump slows down. Also, the internal mechanisms of the pump are freezing and slowing the pump down. If the exhaust is clogged by ice, the pump will slow down because the air cannot exit and cycle normally. So the problem is airflow, not the ice or water in the compressed air. Remove the muffler or get a 3 in 1 air filter with dryer and regulator inline, a cheap and effective solution. Most importantly, most of these diaphragm pumps operate using a “Pneumatic Exchanger” which cycles the air to move the diaphragm piston. At around -20C the exchanger freezes and the pump slows considerably. Again, the pump slows down due mostly because of a frozen pneumatic exchanger, not a frozen exhaust or a brittle diaphragm. Of course, all this assumes you have enough supply of compressed air to begin with, which can be substantial!
I read about concerns about heat transfer where your solution might warm up. Heat transfer in my opinion is negligible especially on large batches, but when in doubt use a plastic pump.
If you are hating the fact that pressure oscillates on the pump discharge, you can purchase a “dampener” and smooth this out and eliminate this problem. This will create a steady stream of fluid instead of intermittent streams.
Some users complain about the life of the pump's diaphragms. Diaphragms are cheap and easily replaceable. So what if the longevity of a diaphragm goes from a year or two down to 6-12 months? IMO diaphragm life is not an issue.
Diaphragm pumps create suction (vacuum), they do not need to be primed as most centrifugal pumps do. A centrifugal pump will not work, usually, if the liquid does not wet the impeller. A positive displacement pump is required but will likely be electric and extremely expensive and require an explosion-proof motor!
If you have the money, invest in an FDA Food Grade pump with sanitary fittings that can easily be disassembled and cleaned. The crude oil left in these pumps will cause the diaphragms to seize. Clean your pumps with solvent after each use.
Diaphragm pumps are not perfect but are easy to use, safe, easy to service, and far cheaper than other alternatives with the same capabilities.
If you are making and selling medicine, please at least use food-grade equipment. Let’s avoid people needlessly getting sick due to poor practices.