Engineering, equipment and service solutions supplier FLSmidth has developed an “innovative” skip discharging mechanism for vertical mine shafts – an alternative solution that can increase hoisting capacity and reduce hoisting duty cycles, as well as maintenance time and costs, FLSmidth commercial manager Terence Osborn tells Mining Weekly.

He notes that this solution forms part of the company’s dedication to innovation, and the continuous introduction of new products and solutions to the market, as well as increasing production and reliability at mine sites.

Owing to the downcycle in mining over the past four years, mining companies have increased their focus on productivity optimisation by evaluating the cost per ton of ore mined or processed, and reducing operating costs, indicates Osborn.

He highlights that a hoisting system allows for ore, waste products, and people to be brought to surface from underground shafts. Therefore, it is the “lifeblood” of an underground mine and needs to be efficient and optimised for overall productivity and cost effectiveness. As such, a hoisting system is designed in such a way that it often exceeds the life span of an underground mine, which is typically about 20 years.

However, owing to changing parameters at underground mines, there are instances where the hoisting system causes bottlenecks because the hoisting capacity is insufficient. Parameters include declining ore grades, which necessitates more ore to be brought to the surface for treatment. The hoist may, however, not be engineered to handle the increases in volume, as it is designed initially to handle a finite capacity.

Although a hoisting system is designed for a particular mine and duty, the discovery of an extended or adjacent orebody may change the requirements of the hoist when using the existing infrastructure to access this ore, Osborn states.

Mechanics

The skip discharging mechanism – conceptualised by FLSmidth mine shaft systems mechanical design engineer Francois Koekemoer in 2017 – for vertical mine shafts is the Crank Dump Type Mechanism (CDTM) – used for opening and closing the skip when discharging materials.

The mechanism, for which the provisional patent has been awarded, forms part of the hoisting system. It comprises a gearbox assembly and a set of crank arms.

Osborn explains that the mechanism’s notable feature is the crank arm. The crank motion naturally produces acceleration, constant speed and deceleration through the cycles, thereby eliminating the requirement for speed control. The mechanism uses power only when actuated.

This solution ensures that the ore hoisting capacity of the scroll discharge arrangement can be increased to accommodate changes, which is advantageous for mines that seek to increase capacity, as the hoist cycle time can be shortened, while output is increased.

Currently, mines use the conventional scroll discharge system for skips, which is the most widely used in underground mines, as well as the hydraulic or pneumatic discharge systems, which were developed as a more efficient, alternative solution to the scroll system and which are favoured by new mines.

However, the main disadvantage of the scroll discharge system is that the process of decelerating the skip to enter the scroll, followed by opening and closing the skip door, is slow, which subsequently “slows down production substantially”, says Osborn. The system is also costly because it requires considerable maintenance to the conveyance, owing to higher mechanical forces acting on the skip.

The hydraulic system uses a hydraulic ram to open and close the skip. While it is designed to be more efficient than the scroll system, the oil filters require ongoing maintenance to ensure optimal functioning.

The CDTM is more efficient than a scroll discharge system and, while the system’s efficiency level mirrors that of a hydraulic system, it requires less maintenance, owing to its simplistic design. Moreover, the gearbox, motor and its components can also “be bought off the shelf” and are, therefore, easy to replace or repair in emergencies, Osborn explains.

It offers the lowest operating costs and is, therefore, the most economic in terms of total cost of ownership, he adds. The mechanism can further be custom-designed to accommodate the type of skip used and shaft- specific parameters.

The detailed design for the manufacture of the mechanism is under way. Osborn enthuses that FLSmidth has already received a design order for the system from a European mine.

He notes that FLSmidth’s experience of more than 50 years and its involvement in the mining industry through its acquired subsidiaries, makes the company well equipped to offer innovative products and solutions in the mine shaft arena. The company also boasts a significant installed base of products for this industry in Southern Africa.

Osborn concludes that FLSmidth has a core group of hoisting mechanical experts committed to visiting clients regularly to conduct “critical” mechanical inspections on the hoist to ensure that it is safe, mechanically reliable and fulfils its operating requirements cementing its full life cycle support offering.