26.08.2010 | Author / Editor: D. Goodwill, B. Pittenger / Marcel Dröttboom
Stockpile Recommendations
Above the tunnel roof, steep-sided tent shaped inserts – often referred to as a ‘hogs-back’ – run between adjacent hoppers across the width of the bin floor. Their purpose is to extend the sloping sides of the mass flow hoppers upwards so that adjacent flow channels intersect, thus preventing a stable rathole from forming. This design approach is a good example of an expanded flow pattern: a series of mass flow hoppers forming a slotted outlet beneath a flat-bottomed funnel flow bin.
Glensanda quarry operators specified that the moisture content of the incoming dust would not exceed 1.5 percent. This was an important stipulation to ensure reliable flow, as flow tests on dust samples at higher moisture levels showed it to be cohesive, with a strong tendency to both arch and rathole in funnel flow bins. A key goal in this project was to achieve flow rate uniformity as a function of conveyor belt speed (or motor rpm), as this would then permit accurate blending with other stone sizes to produce an ‘end-product’ with the required granulometry.
All too often, a feeder or control gate is selected and sized solely on its ability to move material with no regard to the capability of the material to flow through the hopper outlet onto the feeder. To meet the design requirements for a feeder or control gate, it is crucially important that the minimum achievable gravity flow rate – i.e. the limiting discharge rate – through a hopper outlet exceeds the specified mechanical operating capacity of the feeder or gate.
Based on the measured flow properties – i.e. bulk density, cohesive strength, permeability and compressibility – of a ‘worst-case’ granite dust sample, it was predicted that the flow rate from a single flow-control gate would increase linearly up to a maximum rate of 3000 tonnes per hour. However, calculations also showed that doubling the belt speed from 1.5 meters per second to 3 meters per second would only increase the limiting discharge rate from 3000 tonnes per hour to 3200 tonnes per hour. This is a typical example of a limiting discharge rate condition. To increase the flow rate linearity up to the 6000 tonnes per hour required, it was recommended to operate a pair of adjacent gates in tandem.
The project, built in accordance with the recommendations by Jenike & Johanson, has been operating successfully for three years. Actual measurements of flow rate versus speed for two gates operating in tandem are shown in Fig. 3. The flow rate linearity is excellent, especially in the range from 500 tonnes per hour to 3500 tonnes per hour, thus ensuring blended products to meet their product grading specification.
Live recovery from the bin is exactly 60 000 tonnes, as predicted, and the flow rate of 6000 tonnes per hour is easily met. In addition, the need for excavators to move approximately 400000 tons per year of ‘dead material’ to the feeder openings of the old storage bin has been totally eliminated together with the need to supercharge the bin with 170 000 tons per year. As a further advantage, the improvement of the facility allows shipping planners the flexibility to store sufficiently large quantities of the product and ship to suitable market locations, which alone has realised significant savings. Again, the involvement of a qualified and experienced bulk solids handling specialist at the planning stage made a significant contribution to the return on investment. Only with sound research data and engineering input can a project of this importance proceed with confidence to the eventual outcome.
Jenike & Johanson, Inc.
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