Hi Renato thanks for your help.
The problem is with container resources!
I checked the PDS and there is no relationship there either. But I'm using a product storage definition which stores the HALB during the order to produce the HALB and then the order to produce the FERT consumes the material from the tank. I didn't mention this to simplify the question but after you suggested I should check the PDS it occurred to me that the storage defintion implictly realtes both PDSs. I attached an image of the result of the optimization showing the tanks. The product flow is set to fill the tank at the start of the operation. 
So I decided to test what happens if I remove the container resources form the scenario and after I did the optimization ran smoothly and there was no gap between the orders. I also tried removing the secondary resource and leaving the container resource (CARBOC) and again the optimizer ran smoothly.
But I can't understand how the container resource relates to the secondary resource. The container is assigned in the materials tab of the recipe while the secondary resource is assigned in the secondary resources tab of a phase in an operation. Any ideas of the realtion between container resources, product storage definitions and secondary reosurces?