|NDT.net||January 2004, Vol. 9 No.01|
Ecological and Economical solutions in Film RadiographyKris Marstboom (AGFA NDT)
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AbstractFilm radiography is often regarded as an environment unfriendly technology, involving the application of messy chemicals and polluting the drained water with silver. Ecological legislation concerning silver content of the wash water differs from country to country. Sometimes these laws are causing confusion. Agfa now is capable to offer the industrial radiographers ways to live up to installed legislation or specific directives on silver contents in the water.
First of all two brand new film processors, the Structurix NDT Si and Meco are introduced to the NDT market. Both machines are equipped with the Cascade Fixing Technology, allowing the silver freight in the wash water to be in accordance with the most stringent laws or directives.
Next the new Eco Film System is presented. This system is based on the new machines, specially adapted films and carefully developed new chemicals. Eco stands here not only for ecological performance. Also economic issues will be addressed. The customer will now be enabled to process films in a very short cycle, to save lots of money and still work in line with the most stringent ecological legislation.
This paper will also address other means to reduce the silver freight in the wash water. Opportunities but most of all threats involved in the application of these technologies will be highlighted. Thus the presenter of this paper will demonstrate how film radiography is not necessarily a threat to the environment without causing extra financial burdens for the user.
IntroductionMolecules carrying the silver ion are the basic sensitive materials used in today's radiographic films. During processing of these films, silver is released and possibly put to drain, sometimes even ending up in the sanitary sewer system. Local and/or National legislation or directives require careful monitoring of the silver output. Some laws prescribe an absolute amount of silver that can be put to drain per volume of film processed (silver freight) while most state a maximum silver concentration per litre of water coming to the sewer. The latter form is mostly expressed in amount of silver released with each litre of water (g/l or ppm). Where the freight norm clearly limits the amount of silver put into the environment, the concentration restriction theoretically allows to put as much silver as desired to drain, provided one adds a sufficient amount of water to reach the required concentration. This can hardly be considered an environmental friendly procedure.
A classical processor releases approximately 20 ppm of silver with the wash water. The silver is carried over into the wash water by the film coming from the fixer solution. If one wants to limit the amount of silver in the wash water, one may go for post treatment, such as electrolysis, ion exchange, metallic replacement, precipitation or distillation. Not only do these methods implement high start-up and exploitation costs, the flow of the wash water is too high to be handled by these technologies. (1) The better solution to keep the silver concentration in the water under critical limits is to reduce the amount of silver carried over from the fixer by limiting the silver concentration in the fixer itself.
Is silver a hazard?Silverware, sometimes even used to eat off, silver jewellery or silver filling in teeth are so commonly used it is hard to believe silver has to be considered as hazardous waste! Reason for this is the form in which the silver presents itself. Metallic silver, as used in eating utensils can be regarded as non-toxic. The cation of silver (Ag+) however can be very dangerous for aquatic organisms. In fact, silver nitrate is used in medicine as a biocide fighting bacterial inflammations. Also in wastewater treatment this biocide is used. When released however in aquatic environment, the biocide will be a hazard to all organisms living there.
Can closed loop silver recovery reduce the silver concentration in the fixer?Yes, it can. In-line or closed loop silver recovery, where the fixer is led through an electrolysis cell for silver recovery before being pumped back into the fixer tank, can even reduce the consumption of fixer by as much as 50%. There is however a downside which needs to be considered seriously. In order to recover sufficient silver, one has to increase the current for the electrolysis process gradually. Doing so one will consume all sulphite in the fixer. This will in the first place increase the acidity of the solution until sulphur precipitates. Also the thiosulphate will be conversed, not only poisoning the electrodes and thus destroying the electrolysis cell, but also lowering archivability by keeping non-soluble salts in the film. Forced regeneration of the fixer solution by nearly doubling the replenishment rate may help but is economically less desirable.
How to reduce silver concentration in the fixer solution?Obviously there are different parameters determining the actual silver content of the fixer and hereby the resulting ppm in the wash water. First and actually supplier of all silver: the optical density of the film processed. Fig. 1 shows a relation between the average optical density of film processed and the resulting silver concentration in the wash water.
Obviously, the density of the film processed cannot be chosen freely and is completely depending from the specifications of the radiographic application involved. Given a certain density, one may influence the silver concentration in the water by diluting the fixer with fresh fixer. This is feasible by increasing the regeneration rate of the fixer. Fig. 2 gives a relation between both. One can deduce that a low enough silver concentration requires a fixer replenishment which is more than twentyfold of the usual settings (1200 ml/m2)!
Since all silver is being introduced into the wash tank by the film carrying over the silver from the fixer tank, and one therefore is compelled to reduce the silver concentration in the fixer tank, an obvious solution is using a cascade fixing system. Such a system, as used by the AGFA NDT Si and Meco processors, consists out of two consecutive fixer tanks. The replenishment of the fixer is done counterflow-wise regenerating the second fixer tank, of which the overflow will regenerate the first fixer tank. The resulting silver concentration in the wash water is reduced spectacularly as shown in Fig. 3. The combined effects of the cascade fixing and the counterflow regeneration have a drastic lowering of the silver content in the wash water as a result. For this mathematical model, the volume of the fixer tanks were chosen in such a way that their total equals the volume of the non-cascade example. In reality these volumes are bigger and thus the silver concentration is even much lower.
The STRUCTURIX ECO Film SystemUsing the NDT Si machine as described above, the STRUCTURIX Films and the newly developed chemicals: ecoDEV and ecoFIX, the eco Film System is formed. Thanks to the technology of the Cascade Fixing the ecological restrictions are being dealt with. The boron free chemicals add to the ecological performance of this system. Most spectacular however is the economical advantage offered when using this system. Both ecoDEV and ecoFIX allow a significant reduction in replenishment when used in the short 5-minute cycle. At replenishment rates for developer and fixer equalling 550 and 700 ml/m2, the system performs in an equal way to the standardised 8-minute cycle with 900/1200 replenishment rate settings! Thereby the customer gets multiple benefits in one go. Thanks to the STRUCTURIX ECO Film System he will be able to meet the most stringent ecological requirements without having to consume more chemistry. The user is even offered a substantial reduction in the amount of chemistry he consumes. The best thing about this film system is that in spite of all these advantages and savings, the user does not have to sacrifice any quality: the ECO Film System is a fully certified film system complying to all international standards on film system classification: EN 584-1 ; ASTM E1815 ; ISO 11699. One can say therefore that the "eco" does not stand for ecology alone, also economical benefits are offered with this system, allowing the user to maintain the quality of a certified system with shorter cycle times, reduced chemistry consumption and a higher throughput.