Methods and systems for transferring feed materials between zones having substantially different pressures, where the transfer can be continuous or semi-continuous. The methods and systems include a plurality of lock hoppers to receive feed material from a low pressure zone and pressurize it with fluid to a pressure of a high pressure zone. The pressurized material can be discharged to a circulation loop, which carries the pressurized material to one or more receiving unit(s) of a pressurized system. At least some feed material remains in the receiving unit(s) and at least a portion of the fluid exits to become part of the circulation loop. After discharge, the lock hoppers can be depressurized so the next pressurization cycle can begin with additional feed material. The lock hoppers can be operated in a time-staggered manner to provide continuous or semi-continuous transfer of material.