dirdeppe.pdf [3.78 mb] |Mechanismus und Beeinflussung von Trockenausblühungen aus Kalziumsulfat bei der Konvektionstrocknung von Ziegelrohlingen|
|Drying dryer scumming brick calcium sulphate crystallisation diffusion convection|
In the production of brickworks, clay roofing tile works, facing brickworks, clinker and paving tiles, flowerpots and other products, of which the surfaces are in the visible state of use, suffer from the fact that in the drying of the green ware, efflorescence of calcium sulphate occurs. After firing this is clearly recognizable as a greyish-white coating which cannot be removed and results in such bricks and tiles being unsaleable. Until now the problem has been effectively dealt with by the addition of barium carbonate. This is expensive and the usual additive amounts can have a considerable effect on the costs of production. Experience shows that despite higher cost expenditure, dryer scumming does occasionally occur. The suspicion arises that different drying conditions have great influence on the origin of dryer scumming. A systematic explanation however has not yet been found so far. It is therefore the aim of the research measures introduced here to explain the causes of dryer scumming and the interrelationship between the convection of the mixing water to the green brick surface – the actual drying process – and to analyze the diffusion of calcium sulphate deposits directed in opposition to this convection flow, to form them into a mathematical model and to draw up a computer programme for the simulation of the dryer scumming processes. With the help of the mathematical model it was possible to present the influence of the drying conditions in concrete form on the origin of the dryer scumming and to make the anticipated amounts of dryer scumming capable of calculation. For testing the mathematical model developed, use was made of brickmaking raw materials at the works with reference to which good practical suitability is found. The results of laboratory measurements and computer simulation show that only a comparatively small amount of the calcium sulphate contained in the green product proves to be mobile and can therefore contribute to the dryer scumming. For this amount a distinctly marked dependence on the tendency to dryer scumming exists in the process engineering drying conditions, which can be effectively counteracted with comparable conditions in space and time and drying conditions adapted to the raw material and the unfired product geometry.