So first fact's first. Almost (almost) everyone knows you don't put metal cutlery and foil in the microwave. Why? "IT'LL EXPLODE!" well. No. What happens is... Well, I'm not an engineer or particle physicist but it basically polarizes the particles in the metal, I researched it one time for a short story I was writing. I can't remember which but I feel strongly that the correct word is ions... The point however is that this is a problem on things with ridges. Foil is a problem because of ridges. Forks are a problem because forks. Basically the sparking you're seeing is electricity formed by that polarized flow of particles arcing between ridges. If the disk inside the hand warmer is flat metal, you're fine, if it, like mine, has ridges in it, you might want to think twice. That said, you should probably drop the thing in a bowl of water first, nuke for 1 minute, check, then repeat until the fluid is clear, any arcing that is caused will damage the product, not the device... Though you may potentially burn your self or make a mess if you aren't careful. Discretion is always the better part of valor but the truth is you stand a better chance of overheating the metal disk and melting the container or overheating the liquid and causing the sealed container to pop than you do of blowing up the microwave.
Special note. Only ridged metal "arcs" but the polarization of...again, the correct term eludes me but ions sounds right... Is present either way, metal does get very, very hot when exposed to microwaves. Even if it doesn't cause arcing. Which, unless you like third degree burns, is why you should not put plates with metal accents in the microwave.
There is however a better way than most listed. You may have to go through amazon but there is a device intended for use with wax scented candles. It's a heating plate. Idea is you place the jar on the heating plate to release the scented oils. They've fallen out of use hence why I suggest Amazon rather then wasting time searching stores. I've tested this my self. Lay the crystallized hand warmer on the heating plate for about an hour, you have plenty of time to activate multiple hand warmers over a standard eight hour work day, after about an hour the fluid will be clear and ready for use. Better, it will be hot. Not instantly burning your skin hot, more like hot wax hot. And it retains that heat for around 30 to 45 minutes. Meaning you can use one hand warmer for almost twice the time by taking it fresh off the hot plate, dropping it in a pocket don't do this if it's a pocket against bare skin, prolonged exposure may cause minor burns and you're good to go, after it cools down, a certain level of cooling may or may not be needed for reactivation... Just snap the plate and it's good for another half hour. Obviously this only works if you have private access to a power plug.