Zee-Man is dead on: those handwarmers are just iron filing sachets. The handwarmers are MUCH cheaper than O2 absorbers, but do the same thing.
For the scientifically inclined, 4Fe+3O2+6H2O-->4Fe (OH)3 is the basic formula for rust. There's 4 Iron, 3 oxygen pairs, and 6 water per molecule. That means there MUST be moisture for your O2 absorbers to work. The ambient moisture in the air is plenty for the job. I only say this so you don't use O2 absorbers AND desiccant (like silica) at the same time. They defeat each other. The hand warmers are extremely fine iron filings so they react quickly and the rusting process gives off heat. In storage, I have a few 5 gal pails with sealing lids that contain rice, rolled oats, beans, and a few other sundries. After the pails were filled, I opened two handwarmer packets, shook the sachets so they started heating, put on top of the stuff in the pail, and sealed the lid.
For seeds: a desiccant like silica to keep them dry is great. Zee-man is, again, correct. Seeds will not survive in a vacuum; they require o2.
Hand warmers cannot be used after they stop reacting. All you can do is toss them.
To figure out how many to use in a bucket, I did some experimenting. I put one pair in the bucket of rice and then opened the bucket two days later. The handwarmers were spent. I stirred the rice to re-aerate and put two pairs in. After two days, I opened the bucket and the handwarmers started heating up again. That tells me they stopped reacting because they ran out of either water or oxygen in the bucket.
I am going to do the same with pelletized feed, but right now I'm using a rotational system where I buy feed, move what I have in storage to the barn, and put my new purchase in storage. Buying from a feed store instead of a farm & ranch retailer has resulted in much fresher pellets that are green instead of brown. I keep my feed in a dead chest freezer that provides dry, rat/mouse resistant storage. I just hate sending those boxy appliances to the landfill.
We also feed branches from willow, mulberry, and cottonwood (poplar), along with ditch hay and lawn mower clippings, which is easy since we have a colony set up.