A handheld metal detector specifies only "Alkaline" batteries and not "heavy duty" or "rechargeable". Since there are li-ion 9v batteries available not at low cost I was wondering what is different that requires these in a metal-detector specifically? Is there something about the metals or magnetism reacting within the device or is it an issue of power draw?
"Heavy Duty" batteries are what we engineers would call a zinc-carbon battery. This type of battery is cheap to make but has a lower capacity and a higher series resistance than an Alkaline battery.
Probably the metal detector draws more current than a zinc-carbon battery can handle properly and/or the battery's voltage will drop too much during extended use for the device to work properly. All of this relates to that series resistance.
So to prevent users from having issues, the manufacturer recommends using Alkaline batteries. That might save them from users complaining that the metal detector doesn't work well / for very long while that's actually caused by using an unsuitable battery.