AfriSam’s Rooikraal Quarry is located near Brakpan, Gauteng and mines competent dolerite rock. The quarry is a big supplier to asphalt markets which it supplies with its sought-after products – especially 10 and 20 mm road stone. In the production of aggregate, sand and gravel used in roadbuilding, raw material is mined from a deposit of high-grade material that is intermixed with other undesirable materials and detritus. As asphalt cannot have aggregates with any particles, the aggregate must be washed to get rid of such materials. Modern Mining visited Rooikraal Quarry to learn about the closed circuit it has established to wash its products and which furthers AfriSam’s environmental credentials.
The Rooikraal Quarry has a Life of Mine of 240 years. “The property itself is large and only two portions are used for quarrying purposes. These consist of two adjacent pits – one for dolerite mining and one for dolomite mining. “Before AfriSam acquired the quarry in 2001, dolomite was being mined, but it is mining dolerite now,” says Zielas du Preez, AfriSam’s Regional Manager - Gauteng and Western Cape Operations. “The historical dolomite pit is used as tailings catchment facility for the quarry’s recycling process and the tailings produced from the washing process is pumped into it.”
Louis Sterley, Rooikraal’s Works Manager explains the reason for the change from dolomite to dolerite. “Although dolomite has excellent cementitious properties and a pozzolanic effect (the chemical reaction between reactive silica or alumina present and portlandite formed during the cement hydration in the presence of water) and therefore ideal for ready-mix and precast customers, it is limited to these markets. “Dolerite has a wider use with which to produce saleable product,” he says.
Rooikraal Quarry’s dolerite is crushed to produce an array of material sizes including G6 (base course); ballast; 7,1 mm, 22 mm and 28 mm concrete stone; 10 mm, 14 mm and 20 mm road stone, as well as crusher sand.
“Apart from major companies in Gauteng that are involved in the supply of asphalt for road building, Rooikraal supplies its aggregate to the Free State, from Heilbron to Deneysville to Villiers. It has even supplied its product – because of the competent rock that is mined – to some projects in KwaZulu-Natal,” says Sterley. In addition, the quarry supplies material to the construction companies and sister establishments within AfriSam.
A closed circuit for washing
“All the products produced by the quarry are washed. The plant was originally designed for the asphalt market. In road building the aggregate must be devoid of any particles,” says Du Preez. Dust on the aggregate surface adversely affects bituminous hot mix pavement performance and decreases the long-term performance of roads.
“In order to produce the various products for the road building industry, a washing process is needed to meet the requirements and specification of the saleable products. During this process those fines that are by-products of the washing process are discarded.”
The tailings produced as a result of the washing of aggregates is pumped to the unused dolomite pit that is used as a tailings dam and its rehabilitation facility. “We have a closed circuit regarding water within the operation itself,” explains Du Preez as environmental regulations prohibit the release of untreated contaminated water into the stream that runs alongside the site, and it is economically feasible to recover and reuse the same process water again.
Rooikraal Quarry does not make use of municipal water as this will be uneconomical given the volume of water needed for washing. Instead, the quarry is dependent on rainwater that is collected in both the dolerite and dolomite pits. “The last time the facility used municipal water was in 2014. Since then seasonal rain has provided us with enough water to run the closed circuit. On the dolerite side there is a sump too and we collect water there and pump the water to the dolomite side,” says Sterley.
There is an HDPE pipeline running underground from the unused pit (which functions as a tailings dam) to the primary, secondary and tertiary sites of the plant and a pipeline taking slime away from the plant to the pit.
“We supply water to all the screens and cyclone. Including the dewatering screen, there are six screens across the primary, secondary and tertiary plant,” says Du Preez. In the primary section rock is broken down from 600 mm to 85 mm fragmentation. “We have a secondary crusher in the primary section which crushes the stone down further before it goes onto the ISP. Our secondary crushing is the biggest process and it has the most conveyors and most screens. This where smaller products such as concrete stone and sand are produced.”
The tertiary plant makes a coarse and a fine sand. The latter is used by the likes of Much Asphalt and the former by ready-mix customers. At the tertiary plant smaller sizer of aggregate, 6, 7, 10, 14 mm are produced while it also produces 20 road stone aggregate.
Challenges
Even though the facility is using its own water, there is no getting away from using Eskom. “Because the water for the screens are pumped via the underground pipelines, the escalating electricity cost has been significant. Even with the lower demand for especially road building product resulting in us running at lower capacity, our electricity costs are still almost 50% more than before,” says Du Preez.
In addition to the steep increases in electricity cost, the ongoing load shedding has been challenging from a scheduling, planning and especially time perspective. “If the load shedding schedule is not adhered, resulting in a power outage, while under load and rock gets stuck in the jaw crusher, that machine can be non-functional for a day or two,” says Du Preez.
Optimising efficiency
Rooikraal Quarry is fortunate that the depth from the overburden to competent rock is mere 0,5 m. The result is that development costs are very low, and planned drilling and blasting only has to happen once a month. “The fairly square shape of the shallow pit allows for big blasts,” says Sterley. “This leads to cost savings as up to 120 000 tonnes of rock is blasted at the same time. The quarry also has wide benches which allow space for bigger blasts.”
In addition Villa Liza, the closest community, is 2 km from the quarry. The only structures that are relatively close are 88 kVA Eskom transformer which is located about 260 m from the pit and the quarry’ office which is about 500 m from the area where blasting takes place.
This potential risk has been mitigated by innovation in blast designs.
“The better you blast the less you have to crush,” explains Sterley. “Our blasting achieves increased fragmentation which leads to easier rock removal, increased truck capacity and less wear and tear on crushing equipment,” says Sterley.
“The biggest challenge is sales demand. It is a cut throat market. Road projects are scarce and there are many competitors. In addition, we still have certain customers needing certain products. We are making a basket of products and cannot close taps to make a single product,” says Du Preez. “Because of the demand for 10 mm stone in the asphalt market, the tertiary plant used to run 24 hours a day, the secondary plant operated in two shifts of 9 hours each per day and the primary plant has always been a single shift. The current climate has forced us to play around and be flexible with what plant is operational and when. While we need to maintain our agreed stock levels, we cannot just run the plant. This is a challenge,” concludes Du Preez.