The feed to VISY 8 Paper Mill comprises 450 tonne/day of waste paper (newsprint, brown boxes, ..). The paper is then pulped by mixing with clarified process water. The pulp (4 % paper fibre) then goes through stock preparation, screening and cleaning using process water prior to entering the paper machine. A variety of screens and hydrocyclones are used. The rejects from this stage go to either landfill (20 wet tonnes/d at 50% water) or are used as fuel for the boiler (60 wet tonnes/d). The pulp then enters the paper machine which consists of two major sections – wet end and dry end. In the wet end, water is removed by low pressure suction followed by pressing. A number of chemicals are added to the pulp just prior to entering the paper machine – these assist with the paper quality but their composition can be considered negligible for the purposes of the material balances.
After the wet end, the solids fraction in the paper is approx 20%. Following the wet end, the paper passes through the press section, where it is essentially pressed between large rollers which press the water out of the paper and onto the press ‘felt’ material. After the press, the solids content is between 35-50%. The dry end section consists of a number of steam-heated rollers which the paper passes over. After the dry end, the water content of the paper is 5-10%.
Water is critical to the paper production process, and it is used in most stages of the process.
Fresh water enters the process in 3 streams – approx. 0.5 ML/d enters with chemicals at the dry end of the machine, approx. 1 ML/d enters at the wet end of the machine and the boiler consumes approx. 0.2 ML/d.
There is a major water recycle system:
? The ‘clarified process water’ is used for pulping and the stock preparation, and it then enters the wet end of the paper machine with the pulp. It is separated in the wet end (suction and squeezing) as ‘process water’ and is treated in the clarification stage, which includes dissolved air flotation and sand air flotation. The flow rate entering the pulper is estimated as 9.7 ML/d. Is this correct?
? The ‘clarified process water’ is used for showers in the cleaning process of the stock preparation. The clarified water flow rate leaving the clarification section and returning to the stock preparation is approximately 25 ML/d.
The effluent (0.9 ML/d) from the clarification stage is treated in a UASB anaerobic wastewater treatment system on-site prior to discharge to the sewer for additional treatment at the Gibson Island sewage treatment plant. The UASB removes 80% of the solids that are present in the effluent from the clarification stage. The UASB produces biogas as a result of the anaerobic degredation of the solids and other compounds in the wastewater. The biogas (approx (150 Nm3/hr) is used as fuel in the boiler. The boiler also uses approximately 100 tonne/d of coal. The boiler produces 1000 tonnes/d of steam which is used for drying the paper (via heating the rollers). The condensate is returned to the boiler. The boiler produces 20 tonnes/d of ash.
Data: assume Biogas 40% CO2, 60% CH4; assume density of process water 1000 kg m-3