A raft of in-house plant improvements over the past 18 months – anchored by the automation of the secondary crushing circuit, installation of a new pan vibrating feeder and a new fines separator, among other upgrades – have increased production capacity by 30 – 40% at OMV Stilfontein. Munesu Shoko was recently on site and filed this report.
Having acquired a controlling interest (70%) in OMV (previously Oranje Mynbou en Vervoer) in 2014, JSE-listed Raubex Group, one of the largest construction and material supply companies in South Africa, has over the years continued to fund internal growth. The investments have helped turn the company from a small family concern into a formidable construction materials supplier.
Since the acquisition by the Raubex Group, OMV has over the years continued to optimise its operations, looking to maintain and increase value by implementing innovative measures to increase productivity and efficiency. During a recent visit to OMV Stilfontein, Modern Quarrying was afforded an exclusive plant tour to witness several plant improvements undertaken over the past 18 months.
Asaph Mathibeng, operations manager at OMV, explains that the several in-house improvements, including an extended conveyor belt and a feed box at the dump; a new pan vibrating feeder; an automated secondary crushing circuit; a new, larger fines separator; and a self-built asset management system, have addressed several bottlenecks previously experienced on site.
Process flow
Before we delve into the new developments, let’s go through the process flow at this particular site. OMV Stilfontein processes mine rock dumps in the Stilfontein area of the North West province. From the current rock dump, which is a heap of material tailed during shaft sinking of the Margaret Shaft, a national key point pumping station that pumps water to prevent underground flooding of mines located in the area, OMV Stilfontein processes quartzite material into road construction material and sand.
From the rock dump, front-end loaders feed anything up to 400 mm material into the feed box, which is then conveyed to the primary crusher via an overland conveyor belt. At the primary crusher, a 48 x 18 Hatfield jaw crusher, material is crushed to particle sizes of +/- 75 mm. At this stage, plant manager Richard Cawood, says that classification of material starts. “From here,” he says “we can already take out 55 mm ballast, 38 mm stone and fine sand.”
The 38 mm product is then classified at Sand Plant 2, where several material sizes, mostly the 19 mm, 13 mm builders mix and sand, are produced. “These are all the products we can take out at the primary crushing station and of note is that they are already washed,” explains Cawood.
The overrun from the primary section then goes over to the 6 x 16 triple-deck vibrating screen where anything between 24 and 55 mm is classified. Anything over that goes to the sage or intermediary stockpile (ISP), located on top of a tunnel, where 75 mm or less overrun product is stockpiled. This is where the new pan vibrating feeder has been installed.
The secondary crushing stage starts here. In the tunnel, a vibrating pan feeder feeds a conveyor belt that in turn feeds into a surge box. Material from the surge box is then fed into a 38” Osborn cone crusher (secondary crusher) where it is crushed to particle sizes of +/-25 to 32 mm. From here, material then goes up to a 6 x 16 double-deck screen where the first stage of classification in the secondary crushing plant starts.
“Here we classify -22 and -5mm aperture sizes,” explains Cawood, adding that the overrun from here is then returned to the surge box before being fed into a 2 x 36” Telsmith cone crusher (tertiary crusher) where material is crushed to between 10 and 13 mm closed side settings. From here, material goes over to the second screen, a 6 x 16, with the same classification of -22 mm and 5 mm capabilities. The overrun goes back to the hopper for re-crushing, and this process continues until material meets the required aperture sizes.
From here, material goes to the VSI (vertical shaft impactor) crusher, where flaky material is turned into a proper round shape. This process automatically produces sand as well. Material then goes over to a 6 x 16 triple-deck screen where -15 mm, -10 mm and -5 mm aperture sizes are classified. The saleable material from here are the 20 mm and 14 mm road stone.
From here, material is sent to another small screen, where -8 mm and -6 mm material is classified. “We also have got sand that goes through to a water well, where we take out -10 mm and -6 mm material as saleable product. Other saleable material obtained here are the 10 mm, 9 mm, 7 mm and 4,5 mm road products, and these are all washed,” explains Cawood.
At the sand section, a 6 x 6 Warman pump feeds material into a settling unit (another new installation) where material settles. From there a Warman WBH 100 bare shaft pump pumps into a CAVEX 500CVX hydrocyclone, which separates heavy material (sand) from light material (water). The final product then goes to a dewatering screen, and the result is sand, which is a saleable product.
The top selling materials on site are sand and the 20 mm road stone. In times of high demand for sand, the 10 mm and 4,5 mm products can be put through a crushing plant, the Sand Plant 1, to produce more sand to meet demand.
New installations
A number of plant improvements have been implemented over the past 18 months. To avoid front-end loaders (FELs) travelling long distances between the dump and the feed box, a 100-m conveyor extension was installed last year. This helps increase productivity, while increasing fuel efficiency.
Part of this improvement project was to install a new feed box, explains Philco Niemann, engineer at OMV. Because of the incessant breakdowns previously experienced on this part of the plant, “we came up with the idea to install a feed box with its own conveyor”. “Previously, we had to dump material directly onto the overland conveyor, which resulted in excessive wear and tear. We have addressed this problem by installing a new feed box with its own 6 m conveyor belt which then feeds the overland conveyor, which then takes material all the way to the primary crusher,” explains Niemann.
A second major new installation was the vibrating pan feeder on the secondary crushing plant. “Previously, we had a chute coming down on a conveyor with some chains and there was a person deployed to physically regulate the flow of material onto the conveyor belt. The human element here was a problem and resulted in production setbacks in some instances,” says Niemann.
The engineering team decided to install a vibrating pan feeder, manufactured and supplied by Weir Minerals, but installed and commissioned by the OMV team. With that came the automation of the secondary crushing process. “We also installed a PLC (programmable logic controller) and HMI (human machine interface) on the circuit to automatically regulate the feed box. It is always good practice to choke feed your crusher and previously it was difficult to do it manually, and this is now done automatically,” says Niemann.
Another major installation was the dewatering cone (fines separator). This was installed to recoup more sand. “The fines separator is quite bigger than the previous one, and allows us to produce more sand, given that this is a top selling material on site,” says Niemann.
Another major benefit of the new fines separator is that it allows for recouping of more sand by reducing the amount of saleable material discharged into the tailings pond. In the fines separator, the light material overflows into the tailings dam, and the sand settles at the bottom. “Basically, we want to eliminate the amount of certified microns and less out of our sand, as well the amount of saleable material into the tailings dam,” says Niemann.
“We did an analysis on the material at the tailings dam and established that we were losing substantial saleable material into the dam. With the installation of the new fines separator, we have managed to recover about 10% of saleable product previously discarded into the tailings pond. This allows us to increase the amount of saleable material on the floor, while reducing our environmental impact through less material into the tailings dam,” explains Niemann.
By implementing these changes, the operation has increased average production from about 700 t a day to 1 400 t per day. Previously, production could be pushed to 1 000 t a day, but with a lot of effort. Following these improvements, production capacity of 1 400 per day has become a norm, translating into a 30 – 40 % increase in capacity increase.
“In a nutshell, these improvements addressed a couple of bottlenecks. We had loading constraints at the dump, which were addressed by installing a new feeder box and extending the conveyor belt. We also had a bottleneck at the tunnel, where manual feeding was an impediment to the secondary crushing process. We addressed that by installing a new pan feeder and automating the process. We also had a problem at the sand section, which is a big selling item, and that has been addressed by the installation of a big fines separator. Previously, we could only produce 25 t of sand per hour, and with this new installation we can now do up 33 t per hour,” says Mathibeng.
Of note is that these improvements, says Niemann, were done in-house, except for the specialised items such as the vibrating pan feeder, which was manufactured and supplied by Weir Minerals, and the fines separator. However, the installation and commissioning of these items was done in-house, which is testimony to the in-house engineering prowess here.
Other innovations
Another major in-house innovation the engineering team is currently working on is the OMV asset management system. Niemann says the company struggled to get a system that could give comprehensive asset management capabilities in line with the company’s unique needs. “We looked at few systems in the market but we couldn’t find anything to cater for our specific needs,” he says.
Niemann and one of the engineers at OMV decided to build their own system to help with asset management across all OMV sites. This allows site managers to get information on their plants, including daily sales, daily production and availability of assets, all from a single dashboard. “This has given us more availability of our machinery, which increases productivity at the end of the day,” says Niemann.
When Modern Quarrying visited the site, the team was busy installing a PLC system to automate its water supply to the plant. At the time, this was done manually. “We are trying to remove the human element in some of our crucial processes, such as choke-feeding the crusher and water balancing,” explains Niemann.
In conclusion, Mathibeng says in future the ultimate goal is to automate the whole plant. “The vision is to eventually automate the whole system in future,” he says, adding that there is, however, a long way before that can be achieved.
Niemann says the installation of the Siemens S7-1200 on the secondary crushing circuit was to prove concept. “At this stage, the concept has been proven, and the next big step is to raise capital for a bigger automation project because bigger PLCs are costly,” concludes Niemann.
