Studies of size segregation in filling and emptying a hopper

Abstract

Studies are described of in-bin segregation, flow pattern and discharge segregation of ternary mixtures of ore and coke (−8 +6.15; −5 +4 and −2 +1 mm) in a model Paul Wurth hopper. Measuring techniques used include heap freezing, tracer addition, contour mapping and screen analyses. The results show that on filling the hopper small particles segregate in the centre, large particles segregate towards the wall and medium-size particles remain essentially unsegregated, i.e. the concentration of these particles in the hopper is essentially uniform throughout and equal to that of the feed. The concentration of the other two sizes in the hopper (C) was found to depend on feed composition (C o), the nature of the material employed and the position in the hopper. It is shown that in-bin concentrations may be expressed by a relationship C = C o k, where k is an experimentally obtained segregation constant. It is also shown that material discharges from the hopper according to a sequence core flow-wall flow-scaffold flow, where the latter represents stagnant material at the hopper wall that discharges last. Analysis of the variances of tracer response curves indicates some material intermixing in the hopper during discharge. The extent of this intermixing in the core region follows a dispersed plug flow model exactly, whilst in the wall region intermixing is more enhanced with variance dependency there being related to the square of the material height in the hopper. In-bin segregation and flow pattern in the hopper influence size segregation in the discharging material. Concentration of the small particles in the discharge stream is high in the initial stages of discharge and low in the final stages of discharge. For large particles this pattern is reversed, whilst for medium-size particles the concentration in the discharge stream remains essentially constant throughout the discharge cycle. Experimental discharge segregation results show satisfactory agreement with industrial data. A method of predicting discharge segregation from knowledge of in-bin segregation and hopper flow pattern is suggested. This should interest a wide section of the bulk solids handling and processing industries.