New Approach for NPC

Natal Portland Cement’s new CDE plant set to change the face of aggregate washing in South Africa

Natal Portland Cement (NPC) have announced major productivity and efficiency improvements at their Sterkspuit Quarry in KwaZulu-Natal, South Africa, following the introduction of a new washing plant from CDE Global.

The loss of high-quality fines during the sand-processing phase of production has been eliminated following the introduction of the CDE EvoWash plant to replace the bucket-wheel dewaterer previously employed at the site.

NPC have also reported an industry first with the introduction of a CDE AquaCycle high-rate thickener, which permits the recycling of 90% of the water used in the washing of their crushed material. While thickener systems are used throughout the mining sector in South Africa, the CDE AquaCycle is believed to the first of its kind in a crushed rock or sand and gravel application in the country.

NPC are a subsidiary of global construction materials producer Cimpor of Portugal and operate 11 facilities in South Africa with total production of 1.7 million tonnes. These include seven ready-mixed concrete plants, three aggregate production plants (of which Sterkspuit is one), two grinding plants and one cement plant.

The washing process

Sterkspuit Quarry is located in the Cliffdale area, some 30km from Durban, on the eastern coast of South Africa. The CDE washing plant is used to process tillite from the existing blasting and crushing operations, and the washed material is sold by NPC for use in the production of various asphalt mixes and as a replacement for river sand in their own concrete plants.

From the crushing plant the –9.5mm material is first sent to a CDE ProGrade P2-75 double-deck rinsing screen fitted with modular Isenmann polyurethane panels on both decks. The ProGrade unit produces –9.5mm +6.7mm and –6.7mm +5mm aggregates, with the –5mm material being delivered to the sand-washing phase.

According to CDE, the design and construction of the ProGrade rinsing screen, which features a fully independent spray bar assembly, offers major advantages compared to many other screens on the market. ‘The spray bar assembly is not rigidly connected to the screenbox as is the case on many other rinsing screens,’ explained Des Crawford, CDE sales manager in South Africa. ‘This eliminates vibration and the associated risk of plant failure.

‘There is also a major focus on reducing the loss of both material and water during screening through the use of splash prevention over decks and specialist spray bar seals, which results in a safer, cleaner and more productive site.’

The advantages of polyurethane screen media on the ProGrade also became immediately clear at Sterkspuit. On the previous washing plant the wire meshes had been lasting only a few weeks owing to the abrasive nature of the material being processed, whereas the polyurethane screen decks on the ProGrade have shown enhanced performance in the first few months of operation, which has resulted in a reduction in the cost of maintenance both in terms of spare parts and man hours. An additional advantage from a health and safety perspective is the ease of fitting of the modules compared with side-tensioned wire meshes.

Increasing sand washing efficiency

Prior to the introduction of the new washing plant, the sand-washing process was the biggest area of concern for NPC at Sterkspuit Quarry. The bucket wheel was losing up to a third of the fine sand that NPC needed for the production of asphalt, and the very coarse sand from the wheel meant the company was having to buy in large quantities of natural fine sand to blend with it, leading to significantly increased production costs.

‘Sand production was very inefficient with the bucket wheel,’ explained Dave Kendall, general manager at the site. We had to buy in between 70% and 80% of the fine sand we needed as a result of these inefficiencies.’

The new CDE washing plant employs an EvoWash 71 system, which has been used in many similar applications throughout the world. ‘I visited Ireland and the UK and saw the EvoWash in action in many similar crushed rock applications,’ said Mr Kendall. ‘These visits backed up the manufacturer’s claims regarding the capability of the EvoWash to eliminate the loss of quality sand during the sand-washing phase. This offered us an opportunity to greatly increase the efficiency of our Sterkspuit operation and the new washing plant now installed is delivering on this.’

As a result of their increased capability to recover the fine-sand material, NPC have seen the quality of the washed sand improve and they have been able to dramatically reduce the need to purchase natural fine sand from external suppliers as the washed sand product now meets the specifications they require for asphalt mixes.

‘The move towards self-sufficiency that this new plant has given us makes a huge difference to our operations,’ explained Mr Kendall. ‘Higher-quality sand, increased volumes of sand and the effective elimination of the costs of buying in natural fine sand will help us deliver significant efficiency improvements in the months and years to come.’

Water recycling system

Following the sand-washing phase, the waste water from the EvoWash is delivered to an AquaCycle thickener, which allows for the recycling of 90% of the water used in the washing process.

The AquaCycle has been extensively used in recent years and provides operators with a means of reducing the environmental impact of their operations while also offering other advantages, such as improved health and safety on site and a significant reduction in the amount of space required to accommodate settling ponds.

‘We believe that the introduction of the AquaCycle to our operation is a first for sand and aggregate producers in South Africa,’ said Mr Kendall. ‘But with the environmental benefits and health and safety improvements that it offers, I’m sure it will be adopted by others in the not too distant future.’

Wash water containing –75 micron material from the EvoWash first enters the AquaCycle thickener via a de-aeration chamber located on the side of the thickener tank. The location of the waste discharge point at the top of the EvoWash allows for a gravity feed of this material, thereby eliminating the additional sumps and pumps that are often required with other similar systems.

In the de-aeration chamber the material is dosed with a small amount of flocculant, which is prepared in a FlocStation poly-dosing plant. Material is then delivered to the centre of the circular AquaCycle tank before being released. The flocculant that has been mixed with the waste material forces the –75 micron particles to bind together whereupon their cumulative weight causes them to descend to the bottom of the tank where a set of rotating rakes maintains an even consistency within the settled sludge. When the sludge reaches an appropriate density the rakes report back to the PLC control panel which activates the sludge pump. The settled sludge is then delivered to nearby settling ponds.

Meanwhile, the clean water overflows an integrated weir at the edge of the AquaCycle thickener and is sent to the AquaStore collection tank for recirculation to the washing plant.

‘The advantages of the AquaCycle thickener to our operation are two-fold,’ said Mr Kendall. ‘First, we have significantly reduced our use of natural resources through the water-recycling capability we now have, and secondly, the space required to accommodate our settling ponds has been massively reduced.’

The settling ponds at Sterkspuit previously required a substantial area due to a lack of water recycling coupled with the loss of large volumes of material to the ponds. The reduction in area that was possible following the introduction of the AquaCycle has given NPC additional security in relation to the safe storage of waste material from their operations.

‘Our settling ponds are located adjacent to a river so the introduction of the AquaCycle thickener is an example of our focus on environmental protection and sustainable production,’ said Mr Kendall.

South African success

This new washing plant represents the first installation for CDE in South Africa and the company hopes to secure further installations as a result of its success. ‘We have spent a lot of time in South Africa recently, and we know that we can offer operators greatly increased efficiencies based on the existing washing plants we have seen,’ commented Des Crawford. ‘The plant at Sterkspuit gives us a reference where our claims regarding plant performance are proven beyond question.’

For further information visit: www.cdeglobal.com