09/15/2011 | Autor / Editor: Albert E. Maton / Marcel Dröttboom
Due to bad transfer chute design the reliability of belt conveyors not always is reflected in an increase in reliability for the overall plant. Improving the design of transfer stations requires observation and experience but increasingly it requires an improvement in the application of available technologies such as DEM.
The present requirement for higher annual throughput capacity in the iron ore industry generally demands higher design capacity and speeds for belt conveyors. Conveyor belt technology has advance to meet the demand. However the reliability of the belt conveyor at high capacity is not reflected in an increase in reliability for the overall plant.
In part this is due to the transfer chute, an increase in unscheduled downtime such as blockages and schedule downtime such as increase in maintenance requirements.
Improving the design of transfer stations requires observation and experience but increasingly it requires an improvement in the application of available technologies such as flowability technology and discrete element methodology. These technologies require test work on representative samples to determine the input data. Reliable data is required to predict outcomes such as the likely hood of blockage and predictable flow trajectory.
This article illustrates present knowledge and future potential for transfer chute design.
The design of a conveyor transfer chute needs to address two major concerns:
To minimise blockages it is necessary to consider the flowability of the material and to minimise wear it is necessary to determine the trajectory of the material throughout the chute. To design for both these problems it is necessary to determine the variability of the material properties which the transfer chute is required to handle.
Design guides such as Taylor  recommend flowability tests should be undertaken in accordance with Arnold . The material tested must represent the range of material properties handled by the chute particularly a sample which represents the fine fraction and worst case moisture content, surface cohesion and stickiness.
To determine the trajectory through the chute it is recommended that a DEM model be used particularly for the larger size fractions (> 30 mm). Testwork needs to be undertaken to determine the particle interaction properties on the lines suggested in recent publications (Chandramohan , Grima  and Donohue ).
It must be noted that DEM models are very limited in their application and considerable experience and observation of operating chutes is required to understand these limitations. Reservations include assuming particles are spheroidal not irregular, jagged shapes and also difficult to model fine, cohesive and sticky material.
In this article the material is limited to iron ore mining where material properties are very variable being so called hard or soft rocks and sticky or free flowing fines. The fact that lumps increase maintenance problems while sticky cohesive fines cause operating problems results in a design dilemma.
Conveyor belts may be 1800 millimetres wide and travelling at 5.5 metres per send which means the chute needs to be designed for a capacity of 14 800 tonnes per hour with surges up to 17 000 tonnes per hour if the ore is delivered to the conveyor by a bucket wheel reclaimer.
A typical transfer chute is illustrated in Fig. 2. The overall height (belt line to belt line) is between 6.5 to 7.5 metres. The chute may be divided into three sections: a collecting section, a transferring section and a discharge section.
The collecting section can be either a rock box, flat impact plate or a curved hood. This section receives the material from the conveyor pulley discharge trajectory and directs the flow through the transfer section to the discharge section. It must be designed to contain resulting sideways impact path of large particles which under certain conditions cause serious damage to the walls of the chute.
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