Changes to pulp mill ‘difficult’ for Gunns
The Examiner Newspaper
9 Oct 2010
IT WOULD be difficult to persuade Tasmanian timber company Gunns to build a totally chlorine-free pulp mill because of the lower-grade pulp produced, says industry analyst Robert Eastment.
Mr Eastment was commenting yesterday after the state’s environment groups said they might be willing to negotiate on a different kind of mill from that proposed for Bell Bay.
Environment Tasmania, The Wilderness Society and the Australian Conservation Foundation said that they had agreed that a pulp mill could be part of the state’s economic future but the existing Gunns’ proposal was unacceptable.
Mr Eastment said that it was unlikely that Gunns would go a step further with its design to build a totally chlorine-free pulp mill because of the quality of pulp produced.
He said that the pulp produced from a partly chlorine-free mill was of a higher grade with much stronger fibres that could be used for higher quality goods.
Mr Eastment said that Gunns’ proposed mill was “on a ball park with the best in the world”.
He said that the water from the mill would only need to be treated to make it look more presentable by removing the tannin but the quality would be acceptable for re-use.
The facts are these, says pulp mill expert, Dr Warwick Raverty:
1. TCF bleached eucalypt kraft pulp has exactly the same strength as ECF bleached pulp.
2. Strength (tensile strength or tearing strength) is not a criterion of prime consideration when buying bleached pulps to make printing and writing grades of paper (the overwhelming market for the proposed mill).
3. The only quality deficit in TCF bleached eucalypt kraft pulps compared to their ECF cousins is in brightness – TCF pulps tend to be very slightly yellower in colour (87% Brightness) to their ECF cousins (90 Brightness) WHEN PRODUCED USING THE SAME AMOUNT OF BLEACHING CHEMICAL ON AN OPERATING COST BASIS. It is quite likely that a few years ago the potential Chinese customers for the pulp would have discounted the price that they were willing to pay for 87 Brightness pulps compared to the 90 Brightness that ECF produced. Now that China seems to be leading the world in many areas of environmental consciousness and action it is arguable that the Chinese pulp buyers might pay the same for TCF and ECF pulps, or even a premium for TCF.
4. Sodra (the potential Swedish partners who seem to be somewhat reductant to come to the altar with Gunns – presumably because of her unpopularity in the community and the ‘mud’ on her wedding dress) do make 90 Brightness TCF pulps at some of their Swedish mills using larger amounts of TCF chemicals at a higher operating cost.
5. Mr Eastment is totally wrong about ‘tannins’ in either ECF, or TCF effluent from a pulp mill bleach plant. Any ‘tannins’ in pulp wood are completely removed by the kraft pulping process. The pulp is free of tannins before it is sent to the bleach plant. The brown colour that remains in the unbleached pulp is a chemically resistant form of the lignin biopolymer that nature uses to make wood waterproof. This residual lignin broken down into water-soluble fragments by the strong oxidising agents used in both ECF and TCF bleach plants. The solution of these fragments together with the spent bleaching chemicals is yellow in colour and has a high demand for oxygen (so-called Biological Oxygen Demand measured usually over a seven day period – referred to as BOD7). If this solution (effluent) is sent out of the mill without treatment, it causes anaerobic conditions to develop in the sea water into which it is discharged and fish and other marine organisms die. In addition to the oxygen deficit, their are also toxins in untreated effluent. The ‘Environmental emission guidelines for any new bleached eucalypt kraft mill in Tasmania’ developed by the RPDC therefore stipulate that both primary treatment (to remove suspended fibre) and secondary treatment (biological treatment that reduces the toxins and BOD7 to environmentally acceptable levels according to world’s best practice) be used to treat the effluent before discharge.
6. ECF effluent cannot be recycled back into the mill for re-use (closed loop technology) because the product of bleaching with chlorine dioxide is chloride ion (present as sodium chloride – table salt) which would build up in the mill’s process water circuit and cause severe corrosion of all the steel piping and tanks. This consequence was confirmed in Canada in the 1970s when a bleached kraft pulp mill tried to run in closed loop mode for 6 months. The trial had to be abandoned.
7. No one has found an economically feasible way of operating a TCF bleached kraft pulp mill in closed loop mode, because although the effluent contains no sodium chloride, wood contains many ‘non-process elements’: potassium ions, magnesium ions, iron ions and manganese ions being among the most problematic. These elements would gradually build up in the mill’s water circuit of run in closed loop mode and prevent the bleaching process from working as well as corroding the chemical recovery boiler. This build up has been modelled extensively by researchers at H A Simons in Vancouver using sophisticated computer programs and no-one has yet found a cost-effective way of removing the non-process elements from a closed loop mill water circuit. Discharge of fully (primary and secondary) treated effluent through a diffuser is the only way of addressing this problem.
8. Tertiary treatment (as normally practised) is essentially a back-up ‘buffer’ treatment to ‘polish’ the effluent using natural (wind-blown) aeration and biological treatment for periods of 14 – 21 days. In a well run pulp mill it achieves very little other than acting as a buffer in the event of the secondary treatment breaking down for a few days. Just as in the case of sewage treatment systems biological treatment, that relies on a huge population of microbes to do its job effectively, can fail if the microbes stop growing for some reason – usually a failure to keep the pH and temperature within control limits, or a mechanical failure of equipment. In such cases, the presence of a tertiary treatment lagoon containing between 14 and 21 days times the daily effluent discharge of the mill will dilute the few days of untreated effluent AND provide some additional reduction of the toxicity and BOD7 thereby reducing the environmental impact of discharge of what would have been untreated effluent in the absence of tertiary treatment. The Environmental Emission Limit Guidelines did not specify tertiary treatment because they were written on the assumption that any mill in Tasmanian would be operated by an experienced and professional company (well before it was known that Gunns would be a proponent).
Earlier on Tasmanian Times: Lennon: Peace push ‘breakthrough’; mill will be built in Tamar Valley. Biomass row.
Karl: Anatomy of the Tasmanian wood fibre industry: HERE