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[3]:
50g * 1648kJ / 223g = 369 kJ [4]
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.