Namakwa Diamonds

The mineral processing methods utilised by Namakwa for alluvial diamonds in its mining production areas of the Central Node, South Western Node, Southern Node and South Eastern Node in the North West Province are concentrated on traditional rotary pans (principally as a result of the acquisition of legacy equipment on mining sites), and DMS units. The final recovery of diamonds is obtained through flow sorting machines located at a single, centralised final recovery plant, which is utilised by each of the Namakwa sites in the North West Province and the Namaqualand District.

Mobile Rotary Pan Plant
The rotary pan plants are situated adjacent to the current open-pit pilot mining operations (Figure 36). The process for Rooikoppie gravels involves removing the +100mm material with static grizzlies from the ROM feedstock. The screened material is then fed to the rotary pan feed bins with a Front-end Loader (FEL). A conveyor belt transports the material into a trommel screen where the +32mm rocks are washed and removed. All -32mm and the wash water report to the pan where the fine material makes up the porrel. The density of the pan is controlled by adding or stopping make-up water to the pan.

The +32mm stones and the +2 – 32mm tailings from the pans are trucked back to the mining area and backfilled. This provides for a concurrent rehabilitation process with low cost compared to rehabilitation after mining has been completed. The -2mm slimes are pumped to specific slimes areas or evaporation ponds where rehabilitation is done after drying out. Water is recovered from the slimes areas for recirculating to the process.

The concentrate generate by the pans are tapped into bins of 2.5 ton capacity. These bins are transported under security supervision to the CFR where the diamonds are recovered.

The rotary pan plants are situated adjacent to the current open-pit mining operations. The process for Rooikoppie gravel involves removing rocks greater than 100mm with static grizzlies from the run of mine material. The screened material is then fed into the rotary pan feed bins using front-end loader trucks. A conveyor belt transports the material into a trommel screen where those rocks greater than 32mm are washed and removed. All material that is less than 32mm and the water used to wash the initial material is then inserted into the rotary pan. The density of the material within the pan is controlled by adding or reducing the flow of water into the pan that assists the centrifugal rotation of the pan and sifting of material.

Those stones that are greater than 32mm and those stones that are between 2mm and 32mm in size and which are rejected during the sifting process of the rotary pans are trucked back to the mining area and used to backfill the area. This provides for a concurrent, low cost, rehabilitation process compared to undertaking rehabilitation after mining has been completed. In addition, the water concentrated slime of less than 2mm that is produced as part of the rotary pan process, is pumped to specific dumping areas or evaporation ponds and the material is used in the rehabilitation process after drying out. Where possible, water is reused in the rotary pan process.

The diamondiferous concentrate generated by the rotary pans is tapped into secure, 2.5 tonne bins and transported under secure supervision to Namakwa's centralised final recovery plant, where the diamonds are recovered.

Those stones that are greater than 32mm and those stones that are between 2mm and 32mm in size and which are rejected during the sifting process of the rotary pans are trucked back to the mining area and used to backfill the area. This provides for a concurrent, low cost, rehabilitation process compared to undertaking rehabilitation after mining has been completed. In addition, the water concentrated slime of less than 2mm that is produced as part of the rotary pan process, is pumped to specific dumping areas or evaporation ponds and the material is used in the rehabilitation process after drying out. Where possible, water is reused in the rotary pan process.

The diamondiferous concentrate generated by the rotary pans is tapped into secure, 2.5 tonne bins and transported under secure supervision to Namakwa's centralised final recovery plant, where the diamonds are recovered.

DMS Plant Diamond Recovery
The Vleigruis DMS treatment plant is a classic alluvial diamond treatment plant. ROM ore is scalped with a vibrating grizzly and all +100mm rocks removed. The -100mm ore reports directly to a scrubber where clay is washed off. A further benefit of the scrubber is significant liberation of gravel from the softer calcretized ROM ore. The scrubber has a discharge trommel screen that scalps off +25mm gravels. All +25mm ore is stockpiled for future crushing and re-treatment. The scrubber screen removes the -2mm slime which is pumped to a dedicated slimes pit. This is part of the backfill operation. The cleaned gravel, +2mm -25mm, is conveyed to a surge bin that provides a constant feed to the DMS.

The DMS feed is from the surge bin via a feed conveyor. The conveyor discharges onto a preparation screen where the gravel is washed again prior to discharging into the mixing box. The mixture of Ferrosilicon and gravel is pumped to the separating cyclone where it is separated into sinks and floats fractions. The sink product is the concentrate. The floats are discarded and trucked back to the mining area and backfilled.

The sinks concentrate is discharged directly into a locked container. When full, the container is transported under security supervision to the CFR where the diamonds are recovered DMS units provide Namakwa with a greater probability of recovery of diamondiferous ore from harder calcrite gravel, but the units are less mobile than traditional rotary pans. The DMS process involves run of mine ore being scalped with a vibrating grizzly and all rocks greater than 100mm removed from the process. The resulting ore is then moved into a scrubber where clay is washed off and gravel is released from the softer calcretized material contained in the ore. The scrubber also contains a discharge trommel screen that removes all gravel that is greater than 25mm in size. This waste ore is then stockpiled for future crushing and retreatment. In addition, the water concentrated slime of less than 2mm that is produced as part of the scrubber process, is pumped to specific dumping areas or evaporation ponds and the material is used in the rehabilitation process after drying out. Where possible, water is reused in the DMS process.

The cleaned gravel of between 2mm and 25mm in size is then conveyed to a surge bin that provides a constant feed to the DMS unit itself via a further conveyor belt that discharges the ore onto a preparation screen where it is washed again prior to discharging into a mixing box containing ferrosilicon. This mixed material is then pumped into a separating cyclone where it is separated into material that sinks and material that floats.

The material that sinks is the diamondiferous concentrate and is discharged directly into locked containers, which are transported under secure supervision to Namakwa's centralised final recovery plant, where the diamonds are recovered. The material that floats is discarded and trucked back to the mining area and backfilled.

The significant volumes of pan concentrate being produced in the Wolmaranstad area have necessitated the construction of a centralized final recovery plant. All sorting of diamonds previously were done by the individual owners of the respective diggings. This is not possible in a high volume operation, and, while senior personnel attend the sorting process, a higher level of secure handling of concentrate had to be installed.

Concentrate are trucked to the CFR in locked concentrate bins. The bins are emptied into a dedicated receiving hopper. A vibrating screen below the hopper draws the concentrate out and provides a primary wash to remove some of the slimes adhering to the concentrate from the pan operations. The concentrate is then split into two fractions. The -19mm material is treated through a DMS for further concentration. The +19mm material reports directly to twin Flowsort units which have been specifically manufactured to treat the large particles. The concentrate from the DMS reports to twin Flowsort machines for sorting at a much lower rate.

The feed to the Flowsort machines are sized to enable all particles to present to the optical detection system. The fractions are based on the principle that the ratio between largest and smallest particle should not exceed three. The smallest particles are 2mm, and the first fraction is thus 2mm to 4mm. The next fractions for the fines Flowsort system is 4mm to 8mm, then 8mm to 12mm and then 12mm to 19mm. The larger concentrate is simply split into two fractions, being 19mm to 26mm and 26mm to 32mm.

The concentrate from the Flowsort machines is collected in self-locking containers. The containers are removed and transferred to the Sorthouse. The Sorthouse is adjacent to the Flowsort machine complex and within the high-security area. The concentrate is emptied into a receiving hopper with a reverse self-locking mechanism. All processes are hands-free and the concentrate does not come into contact with human hands after the Flowsorts.

Once the diamondiferous concentrate from the rotary pans and DMS units is transported to Namakwa's centralised final recovery plant, it is then passed through a further washing and DMS process before being distributed through an x-ray filtration system which seeks to identify diamonds from within the concentrate and which is constructed to minimise human contact with the concentrate. Once a stone is identified as a potential diamond, it is mechanically dropped into a locked box, together with the gravel matter around it. The contents of the locked box are then opened in a secure environment, their contents hand sorted and all diamonds recovered are recorded on site before being transported to a secure location for onward rough sale or polished production.