A treatment plant (1) transforms a material containing PET in flakes (2) into a depolymerized material (3). The plant includes a first treatment unit (10) having a first material inlet (11) for feeding the material containing PET in flakes (2), a first unit outlet (12) for outputting a semi-finished material of molten PET (4). A first unit path (14) extends between the first unit inlet (11) and the first unit outlet (12). A compression and mixing device (20) is arranged along the first unit path (14). An injection device (30) for injecting a treatment liquid (31) leads to an injection point (32) along the first unit path (14).