SYMPOSIUM ON MATERIALS OF CONSTRUCTION IN FERTILISER PLANTS.
1. PROTECTING FERTILISER MANUFACTURING PLANT: THE ROLE OF SURFACE COATINGS.
Although employed by a surface coatings manufacturer, the author believes sincerely the question ‘Why protect’ should be more carefully considered at the drawing board stage and sensible efforts made to design out materials which will dissolve or corrode in a particular environment.
Given the use of mild steel is inevitable, so is the reversion of the original iron ore extraction process! 90% of industrial and marine corrosion is an electrochemical process; surface coatings defeat corrosion cells by keeping water out, restricting the rate at which oxygen can reach the steel, and excluding corrosive media. They can be applied on or off site.
The choice of protective coatings is complicated by the sheer number of compositions available, but practical experience in the U.K. proves the bulk of steel in fertiliser plants can be protected by relatively simple chlorinated rubber paint processes; even the traditional oil-based micaceous iron oxide paints give excellent service in situations where the atmosphere does not contain chemicals destructive of the coating. While there is no blanket specification to suit all sites, the two basic paints can be tailor-made into processes to meet specific situations by choosing the correct primer, pre-protecting the steel with aluminium or zinc etc. This minimises the need to resort to exotic coatings.
Wherever possible, the bulk of a surface coatings process should be applied off site, ensuring the steelwork is never exposed to contamination. This approach has resulted in a marked decrease in the requirements for maintenance painting. There is a divergence of opinion regarding whether this method can realistically include pipework and vessels. The answer may well be ‘No’, but the author would welcome opinions from members of the Fertiliser Society.
Maintenance painting is simplified where the initial preparation and the bulk of paint application can be executed off site. Above all, the initial removal of millscale and rust pays long term dividends. The results of inadequate surface preparation may never be overcome.
Ideally, corroded, pitted steel needs blasting before repainting. Where surfaces are chemically contaminated, washing as part of the surface preparation will increase the life of the maintenance coatings. Even where preparation is limited to mechanical chipping, scraping and wire-brushing this remains true.
There is much to be said for the use of quick-drying oil-based primers in maintenance. Irrespective of whether they contain red lead, zinc chromate or zinc phosphate as the inhibitive pigment, such primers wet the surface more effectively and provide better corrosion resistance than analogues wholly based on chemically resistant film formers.
Truly hot surfaces should not be painted! Where surface coatings are used, cheap and cheerful aluminium paints are likely to perform as well as complex heat resisting formulations when applied to manually or mechanically prepared steel. Where blasting is possible, inorganic zinc silicates can give excellent results.
R S Hullcoop, ICI Paints Division, Billingham, UK
2. THE CORROSION AND PROTECTION OF CONCRETE IN AN AMMONIUM NITRATE ENVIRONMENT.
Ammonium Nitrate (AN) corrodes concrete; fresh concrete even more rapidly. Once it has penetrated or been absorbed by concrete, it can remain dormant until suitable environmental conditions are established for it to start a chemical attack. Contaminated concrete cannot be assumed to have stopped deteriorating, nor can it be protected or delayed by coverings of paint or render.
It is essential to detect AN attack in its early stages, by establishing routine inspections of foundations, process drainage, and AN dust-laden overhead structures, specifically to search out and identify contamination and monitor its significance.
Recognition of the symptoms, therefore, is important, but also it is essential to properly prepare contaminated concrete before any remedial work is attempted.
Coatings, patch mortars, overlays, admixes and other protective applications have their uses, but great care must be exercised in their application, as very often, if wrongly applied, or even correctly applied in the wrong circumstances, they can do much more harm than good.
It is not intended to specify a guaranteed remedy or repair procedure, (as this largely depends on the repair location or environmental conditions) but rather to focus on the effectiveness of completed repairs and their associated problems, as a guide to those who may encounter similar problems in the future, and to promote a wider exchange of information, and by this exchange avoid wasteful duplication of effort or expense by those presented with the problem of repair.
D W May, UKF Fertilisers Ltd., Ince, Chester, UK.
15 pages, 5 figures.
3. SOLVING WEAR PROBLEMS IN A COMPOUND FERTILISER PLANT.
A brief description of the compound fertiliser plant and its recycle loop is given. Several items of equipment suffer significant wear. These are the blunger, the drier feed screw, the drier exhaust fan and the oversize pulveriser.
Several attempts have been made to solve the problems of rapid wear on these items. Hard surfacing by welding has been found to be most effective in our case. Weld surfacing has the advantage that worn components maybe reclaimed by further weld surfacing. The lives of the various items have been increased significantly, in some cases components last from one annual shut-down to the next.
The problem can never be considered fully solved for two reasons. Firstly, new surfacing materials are being introduced which may offer advantages over those being employed; and secondly, changes in fertiliser formulation can significantly alter the type of wear taking place.
P Furnival, ICI Agricultural Division, Billingham, UK.
17 pages, 10 figures, 3 tables.