You're right to assume 'the...metal' in this sentence is iron because at the start of the sentence it is written, 'a hard type of iron,' and no other metal is mentioned. If another metal were mentioned, or if 'iron' hadn't been mentioned earlier, then of course, it would be vague and ambiguous. It's a fairly common stylistic choice to mention something in a specific way at first (e.g. iron) and then refer to it in a more general way later (metal). This keeps the language varied and avoids repetition. Of course, it can potentially make a sentence less clear. So it is a choice between style and clarity. You can make up your own mind about which is better! By the way, instead of saying 'hot liquid', when referring to metal you can say 'molten'. Molten metal, molten iron, molten gold, etc.

I belive you are correct. I think this is iron too.