Pretreatment

Straw bales or chopped straw are feed in a pulper designed for a continuous operation.

The heavy contaminants (metal, stones, etc.) are continuously discharged from the pulper by a special "junk remover" device; the light contaminants, plastic films as well as steel or plastic wires fastening the bales are collected via a specially designed ragger unit.

Alkali (1-2% on the incoming straw) as NaOH and/or as Na₂CO₃ from the recovery is added. A 15-20 minutes average retention time at 50°C in presence of a high turbolence of the 6-7% straw suspension guarantees a pretreated straw well sliced and shortened at 40-50 mm. length.

The straw suspension from the turbopulper is continuously pumped via a specially designed pump to a fluidification device to complete the separation of small contaminants and sand. The defibrated straw is then washed and dewatered at 10% via a rotating drum. The pretreated straw is finally pressed in a DKP press up to 25-30% dryness.

The pretreatment assures:

• a reduction to about 50% of the silica content of the incoming straw
• a 30% reduction of the parenchyma and epitelial cells which are difficult to be bleached and have no papermaking value.
• material practically free from waxes and ready for an easy chemical impregnation.
• a reduction of the NaCl content that could negatively influence the incineration and chemical recovery when the Cl- content in the straw is higher than normal, due to the soil composition.

The yield of the pretreatment ranges between 80-90% depending from the straw quality and its chemical composition.  The raw material with high moisture content and/or partially rottened by an improper collection and/or storing can give pulp with standard properties but the yield will be of course lower.

After separation of fines and most of the sand in a screening drum, the effluent from the pretreatment is processed in an anaerobic treatment for BOD and COD  reduction.