Every paper or board mill has to ensure that its stock prep system is working at maximum efficiency
, particularly in mills that use OCC and mixed waste. Keeping the pulper clean has to be one of the main aims of the mill. The more reject that can be removed at the pulping stage results in less reject having to be removed further down the line. In a low consistency pulper, the first stage is to ensure that the ragger and heavy rejects dirt trap is working properly and the contents of the pulper are continuously cycled. One of the most common ways to remove rejects from the pulper is to pass the pulp through a contaminex, or similar machine, and up to a rotating Trommel or reject drum, with the large rejects being removed by the Trommel and passed to a reject compactor. At this stage it is important to ensure that the reject drum is correctly set up. Far too many mills use the wrong size perforations in the drum and do not allow the pulp enough time to be washed. The end result is that too much fibre is lost and ends up in the reject. The fibre must be retained at all costs. With a good pulper cleaning system the pulp is retained and the smaller rejects can then be removed using high density cleaners and a selection of pressure screens. Once the pulp reaches the stage in the cleaning process where it processed by the pressure screens, another very important part of the stock prep system. The pressure screens in a paper mill can be a combination of perforated screenbaskets and slotted screenbaskets. The finer the slots in the screenbaskets the smaller the reject that will be removed. These days there have been significant developments in pulp screening technology. Slotted Screenbaskets are commonly manufactured from a profiled wire and high consistency screening is far better understood and utilised by mills. Having the correct type of rotor is also important for the pressure screen as this will give higher throughputs with lower energy costs. Once the pulp has been screened, thin stock can go to the drum thickeners before going to the accepts tank.
Mills that use virgin fibre have a much easier job with their Stock Prep Machinery as the only contaminants in their pulp should be what the pulp has picked up in transit. So some low or High Density Cleaners will be required, and possibly some pressure screens. In general the stock prep of a virgin fibre mill is much simpler than that of a mill producing linerboard, fluting paper or corrugating medium.