Imagine a personal heating system that works indoors as well as outdoors, can be taken anywhere, requires little energy, and is independent of any infrastructure. It exists – and is hundreds of years old.
Iron oxide is not recyclable in a foundry. It would need to go back to a smelter to be smelt into iron again, which I assume is about the same efficiency as smelting iron ore. (Smelting hematite requires ~20 GJ / tonne [1] = ~ 4460 kJ / 0.223kg).
Spent iron oxide heat packs are probably best thrown in the compost.
I agree that an iron powder heat pack can be far more practical where a highly portable slow emission of heat are both important for the application. The heat output for something 10% the weight of a water bottle is pretty impressive:
1 litre water bottle cooling from 80 C to 40 C:
~4kJ/kg⋅K[2] * 40 K * 1 kg = 160 kJ
100g heat pack containing 50g iron:
50g * 1648kJ / 223g = 369 kJ
Iron powder heat packs need to be stored in suitable packaging (usually plastic) to protect them from moisture and oxygen until their single use.
Iron oxide is not recyclable in a foundry. It would need to go back to a smelter to be smelt into iron again, which I assume is about the same efficiency as smelting iron ore. (Smelting hematite requires ~20 GJ / tonne [1] = ~ 4460 kJ / 0.223kg). Spent iron oxide heat packs are probably best thrown in the compost.
I agree that an iron powder heat pack can be far more practical where a highly portable slow emission of heat are both important for the application. The heat output for something 10% the weight of a water bottle is pretty impressive:
Iron powder heat packs need to be stored in suitable packaging (usually plastic) to protect them from moisture and oxygen until their single use.
A hot water bottle needn’t be rubber or plastic.