Performance of the Gladstone Raw Meal Silo

摘要

During the interval from start-up (late 1997) to mid-2000 Queensland Cement Limited has observed variations ('excursions') in the chemistry of the raw meal discharged from the raw meal silo of the new Gladstone Plant. The variations appear to be related to the flow of material within the silo and not to variations in the material fed to the silo (raw meal). The effect appears to be unrelated to the level of material in the silo.In late 1999 TUNRA was commissioned to determine the flow properties of the raw meal (1). According to the principles of JENIKE and others (2, 3) the angles and dimensions in the region of the silo outlet mitigate against mass flow discharge for the raw meal flow properties measured.A modification carried out in May 2000 has increased the diameter of the flow channels formed, with a resultant improvement in blending efficiency. Estimates of the 'dead volume' of material in the silo before and after the modification do not correlate well with the scale of the kiln feed excursions observed.In order to eliminate kiln teed excursions an expanded flow design is proposed. Expanded flow means that the body of the silo (above the cone) discharges by funnel flow while the outlet region (between the cone and the silo wall) discharges by mass flow. With expanded flow a sufficient cross-section of the silo ('flow channel') is mobile at any time such that the forrnation of dead zones is prevented. Mass flow in the outlet region (the cone) is the means by which sufficient cross-section is able to move. Mass flow in the outlet region also means that segregated material which enters the outlet region is re-mixed as it leaves the silo (4-8). It is inferred that a further improvement in blending efficiency will be found when expanded flow has been established. In order to achieve reliable mass flow at the silo outlets either the wall friction angle of the concrete inverted cone must be reduced (to something comparable with that of stainless steel, e.g. by changing the surface of the cone), or the geometry of the silo outlet region has to be changed (e.g., by increasing the cone angle from 60 deg to 70 deg).It is concluded that1. with the present outlet geometry, mass flow is not achieved or at best is achieved only intermittently2. the silo discharges by funnel flow through flow channels which form above the silo outlets3. feed to the silo segregates according to particle size and/or density in the course of flow into and through the silo4. the segregated material remains as an unstable 'dead volume' in the silo until some time when it is discharged as a slug of out-of-specification kiln feed, or 'excursion'5. given that segregation of raw meal is probably unavoidable, the resultant excursions in kiln feed chemistry must be expected from time to time with the present silo geometry6. no satisfactory explanation is offered for the apparently random occurrence of the excursions.The mixing bin below the silo has not been considered.