Bellsbank Processing Plant
Diamonds produced at the Bellsbank operation are derived from tailing material. This feed stock is the coarse by-product of the historical mining and processing of diamondiferous kimberlite fissures from underground workings over a period of 60 years.
This primary processing of kimberlite ore required kimberlite to be put through various stages of crushing to liberate diamonds followed by gravity separation using diamond rotary washing pans. The preferred size of feed produced by crushing and sent to the rotary pans is a function of the optimum size fraction, determined by the operator, required to liberate the maximum number of diamonds without excessive damage to individual stones, whilst at the same time minimising the amount of costly crushing required. The by-products of rotary pan washing broadly fall into two categories, firstly slimes (ultra fine rock and clays) and secondly coarse tailing material. The majority of diamonds that would have been lost during processing will report to the coarse tailing fraction.
Diamonds recovered by KimCor are all found in coarse tailing material. These diamonds occur in the tailing material in two forms. Firstly, as free diamonds present in the coarse matrix. These stones were liberated by primary crushing but not recovered by the rotary pans. Secondly, as diamonds that remain locked up in kimberlite fragments that were not reduced in size by primary crushing.
The tailing by-product produced during primary processing was stockpiled in dumps. The dumps were constructed by means of mobile conveyors transporting material directly from the processing plant to the dump site or by haul trucks end tipping material.
The principles applied by Bellsbank to recover diamonds differs depending on whether the diamonds are already liberated or remain locked up in kimberlite.
Those stones that are still contained within kimberlite are subject to secondary crushing which reduces the coarse tailing material down to -6mm in size. Those stones that are already present in the tailing matrix (together with the stones liberated by crushing) are passed through a rod mill. The rod mill action provides a scrubbing effect which removes any coarse fragments of kimberlite still attached to the stones subject to crushing and at the same time removes any film or secondary deposition that may have covered the diamonds in the matrix, and accumulated over time whilst in the dumps.
Diamonds can be damaged or destroyed in the mill if the mill operating parameters are not optimised for the particular feed source being processed. In its simplest form, the key components of mill operation include the number of rods used, the speed of rotation and feed rate of the mill and the addition of water to generate a density that helps to protect the stones.
Rotary Washing Pan technology is based on a gravity separation process. In essence a "puddle" of fine kimberlite, clay and sand mixed with water is agitated by a rotating rake and the light fraction of the crushed kimberlite is floated off towards the middle of the pan whilst the concentrate (coarse heavy component) is pushed by the rake towards the outside of the pan and collected from the bottom of the unit on either a continuous or intermittent basis. Pan maintenance, especially the monitoring, replacement and realignment of the teeth that make up the rake is vital if recovery rates are to be optimised.
The concentrate fraction produced by the rotary pans is collected in concentrate bins and fed onto screens in the Final Recovery Section. These screens remove fine material and clays and produce a clean concentrate that can be passed across vibrating grease tables that recover diamonds.
Project Image Gallery
1. Kimberlite fissure exposed in sidewall of old excavation
2. Example of typical coarse tailing found at Bellsbank
3. Typical Bellsbank tailing dump
4. Bellsbank milling section
5. Bellsbank conveyor section feeding the mill section
6. Bellsbank rotary pan in action
7. Final Recovery Section - Grease tables in operation