Mary Frances Hill Vancouver Sun
A chlorine dioxide leak at a Mackenzie pulp mill recently taken over by the provincial government to avoid an environmental disaster forced 35 people out of their homes and workplaces on Sunday. Chlorine dioxide was leaking from a valve in one of three fibreglass tanks at the Worthington mill at the rate of about two gallons per minute, said Pat Bell, MLA for Prince George North. Used to bleach wood-pulp, liquid chlorine dioxide is one of the most dangerous chemicals stored at the pulp mill. When concentrated chlorine dioxide is exposed to air and light, it decomposes into chlorine gas. Read more, Comment here
What Warwick Raverty told Jeremy Ball …
Hi Jeremy,
I don’t know the background to the article in the ‘Vancouver Sun’, but based on what I read, I assume that the company that owns Mackenzie Pulp Mill in British Columbia must have had to file for bankruptcy as a result of the economic recession, leaving the safe shut down of the Mill in doubt. Hence the comment in the article ‘the province [i.e. the Provincial Government of British Columbia] is going to have to run the Mackenzie mill to solve the problem’.

My recollection from Paul Lennon’s statement to the House of Assembly (you will need to check Tasmanian Hansard for Aug – Sept 2007) is that Gunns agreed (reluctantly, following Sweco Pic’s agreement with my earlier advice!) to use the methanol process (one of two approved under the Environment Emission Guidelines) for generation of their chlorine dioxide. Which process is used, the methanol process, or the hydrogen peroxide process (both Accepted Modern Technology) would not change the situation in which Mackenzie Pulp Mill finds itself. Like elemental chlorine, chlorine dioxide is a gas under atmospheric pressures and temperatures, but it is soluble in water and it is this solution that is stored in tanks at Mackenzie Mill and all other ECF kraft pulp mills. This is the reason for the incorrect statement in the article that chlorine dioxide is a liquid. Mill workers, like the Rod Clark quoted, do not have tertiary degrees in chemistry and so often refer to solutions of chlorine dioxide in water as ‘chlorine dioxide’. The difference between elemental chlorine and chlorine dioxide is that, whereas elemental chlorine is stable and can be compressed to a liquid and stored and transported in reinforced metal cylinders, chlorine dioxide is unstable and detonates spontaneously when the concentration of dioxide in air, or water vapour rises above 9% – forming elemental chlorine gas and oxygen. So 99% of all chlorine dioxide made in the world is generated by reducing a solution of sodium chlorate in dilute sulfuric acid (‘chloric acid’) with methanol (or hydrogen peroxide), generating a solution of chlorine dioxide and sodium chloride (sea salt) in dilute sulfuric acid. In order to separate the chlorine dioxide generated in this way, steam is bubbled through the ‘chloric acid’ solution creating a mixture of chlorine dioxide in water vapour that must be kept below 9% concentration and which is then bubbled through water to create the chlorine dioxide ‘bleaching chemical’ that is stored in tanks and eventually mixed with the unbleached kraft pulp after it has been subjected to oxygen delignification. Oxygen is only capable of removing about half of the residual lignin remaining after kraft pulping, hence the need for chlorine dioxide (or the ‘totally chlorine free’ alternative, ozone that is used to make TCF bleached kraft pulp under a slightly more expensive process). Attempts to compress the water vapour-chlorine dioxide mixture into a liquid for transport always result in an explosion – hence the need for it always to be made on the mill site as no mixture of anything in water vapour at 7-8% concentration could ever be transported cost effectively. All AMT chlorine dioxide bleach plants incorporate bursting disks and other ‘relief equipment’ that cater for small detonations (called ‘puffs’ in the pulping industry!) that occur periodically during normal operation because of inadequate mixing of the chlorine dioxide with the air and build up of pockets of chlorine dioxide above the ‘chloric acid’ solution.

Under normal conditions, if the company running a pulp mill became insolvent, there would have to be provision in the Environmental Management Plan modules (the documents that Minister Garrett and his Independent Expert Group are approving for Gunns at present) to have a contingency plan in place (perhaps a sum of money held in trust that enabled the mill to continue running for long enough for the total storage of chlorine dioxide solution in water to be mixed with unbleached pulp and rendered harmless). So, although Gunns propose to store chlorine dioxide solution on site – as is normal practice, if Minister Garrett’s IEG does its job professionally, there should be no risk of exposure to chlorine dioxide in the event that Gunns goes bankrupt at some point during future operation of the mill. It would be well worthwhile Kim Booth writing officially to Minister Garrett, to alert him to what has happened at Mackenzie and to seek assurance that the EMP modules (I unfortunately don’t have time to read them!) prevent the same thing happening in the Tamar Valley. With Gunns’ share price having bottomed at 65 cents fairly recently, the contingencies of insolvency and bankruptcy should certainly be planned for by the Minister in my opinion.

What would happen in the event that a storage tank was breached (by accident, or by deliberate criminal act) is that the solution would be contained in a bund around the tanks and flow into drains to a ‘spill dam’ and covered with a suppressant foam to limit escape of chlorine dioxide – a normal part of any the pollution prevention measures in any modern mill (and part of the Tasmanian-Commonwealth Environmental Emission Guidelines) where it would have to be neutralised by adding a reducing agent (such as kraft pulping chemical (‘white liquor”), methanol or hydrogen peroxide). Nevertheless, emergency services personnel dealing with the spill would have to wear protective clothing and gas masks – a so called HAZOPS plan is a normal part of any EMP for a facility where dangerous goods are stored or handled and a spill of that sort is exactly the sort of event that HAZOPS is intended to deal with.

In the event of a deliberate act of terrorism, where explosives might be used to disperse the contents of the tanks for example, or perhaps in the event of an earthquake (the Tamar Valley is geologically active according to at least one experienced risk assessor and Beaconsfield, site of the mine tragedy that arguably had a strong geological instability component, is less than 10 km from the proposed mill site) the consequences for the local community and animals would be much more serious. If the chlorine dioxide were to be dispersed into the air by explosive force (presumably much of it being converted quickly to chlorine and oxygen) a heavier than air cloud of lethal gas would drift with the breeze similar to the chemical warfare practised in the trenches in France and Russia between 1914-1918 with tragic consequences for people living near the mill (how near would have to be modeled by computer!). Even a rupture of the tank and bund by earthquake would arguably allow many kilolitres of solution to flow down hill on the steeply sloping site, releasing considerable quantities of chlorine dioxide and chlorine into the atmosphere as it travelled. Again, how much would find its way into the Tamar Estuary would have to be subjected to computer modeling.

I trust that you find this information helpful.
regards,

Warwick Raverty
paperscience@bigpond.com