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An enclosed offloading, conveying and blending system replacing an open, double-handled lime process to control fugitive dust at Sappi Ngodwana Mill.
By replacing an open, double-handled lime process with direct truck-to-hopper offloading and enclosed mechanical conveying, RULA delivered Sappi Ngodwana Mill an integrated system for safer, cleaner and more reliable lime handling — from process and mechanical engineering through installation, commissioning and operator training.
The existing lime handling arrangement presented several operational and environmental challenges:
The upgraded system therefore needed to improve material flow while providing effective dust containment and reliable plant operation.
The project incorporated three primary areas of improvement:
A new enclosed offloading arrangement was developed to allow road side-tippers to discharge lime directly into a purpose-designed receiving hopper. The hopper was designed for approximately 20 tonnes / 18 m³ of material and incorporated a feed screw to provide controlled transfer of lime to the new vertical conveying system. The offloading facility was enclosed to contain airborne dust and incorporated a dedicated filtration system.
A new 50 tonnes-per-hour vertical bucket elevator was incorporated into the lime handling system, transferring fresh lime from the offloading hopper to the existing lime storage silos. At the top of the elevator, a distribution arrangement incorporating two diverting chutes allows the conveyed material to be directed to either of the two dedicated lime silos, providing controlled and reliable material transfer while minimising product spillage and dust generation.
The existing offloading shed was enlarged to accommodate road side-tippers and allow vehicles to enter, side-tip directly into the receiving hopper, and exit through the opposite side. The enclosed facility significantly reduces the opportunity for lime dust to escape into the surrounding plant environment. A stand-alone bag filtration system was incorporated into the facility, with collected material capable of being returned to the lime storage system.
The project also addressed dust emissions from the existing No. 3 lime conveying system. Sections of the existing conveying equipment were exposed to atmosphere, allowing wind to carry fine lime particles into surrounding areas of the mill. The bucket elevator was therefore enclosed using materials suitable for the operating environment while maintaining the access required for inspection and maintenance.
A lime blending system was incorporated to improve control of material discharged from the existing storage silos. The system was required to manage both bought-in fresh lime and kiln-produced lime, improving the controlled blending of the two material streams and reducing process instability.
The project required a multidisciplinary bulk materials handling solution incorporating:
The new equipment was designed for integration with the mill’s existing Honeywell Experion supervisory control system. The engineering scope included the required process descriptions, P&IDs, I/O requirements, control logic, interlocks, local control equipment, instrumentation and safety devices required for reliable plant operation.
Safety and maintainability formed an integral part of the design. The system incorporated appropriate guarding of moving equipment, emergency-stop facilities, safe access platforms and stairs, equipment identification and provision for inspection and maintenance. A HAZOP formed part of the engineering process prior to implementation.
RULA engineers and delivers bulk materials handling systems from initial design through commissioning.