Heat the 6,000 kg of water by how much?
The amount of heat you need is essentially set by the mass of water, its heat capacity and the temperature change. Divide that by 1 hour (your heat up time) and you know the rate you need to add duty to the system.
Condensing steam to do this releases about 970 BTU/lb at atmospheric pressure (as the condensing pressure increases, you get less heat out of a pound of steam, or kg or whatever mass units you want to use).
Divide the heat rate by the latent heat of steam and that's the flow rate of steam you need (actually, the average flow rate over the 1 hour period). You also need to determine if you have enough area in your jacketted tank to transfer that much heat in your desired period of time.