Sorry but it isn't that simple:
There's a total world demand on let's say Iron, and based on that demand, entities produce the demanded Iron amounts, plus they generally keep a reserve in case an entity wants to make a quick buy. Usually there's a whole department that deals with company reserves and the costs associated with having them, there's even mathematical theory about stockpile management costs. There's also studies that analyze if a certain resource is going to be in more demand in the future or less, this helps companies understand if they need to increase or decrease production.
So if I am Spain and I need to store a few tons of Iron, I can increase the world demand by placing an order of such resource in the market, usually small increases to the world demand just means that some company will get a contract and will start by selling their RESERVES to the buyer at the ratio required, X tons per day, giving them time to hire more workers, buy more machinery and increase production to the new levels now required, often the contracts have incremental supplying, on first year it delivers X amount, on the second more than X and so on until it stabilizes.
Now if we are talking about placing huge demands on the market, yeah it will take time for the mining industry to get all that Iron pumping out, but guess what, in all this process, the iron mining industry JUST DOESN'T CARE if you have industrial capacity or not.
