This case study tells the story of a mine’s water treatment plant upgrade. How upgrading an existing water treatment plant optimised the use of abundant and free fissure water, minimising its reliance on municipal water – and limiting the associated reticulated water costs.

The mining operation, located on the West Rand of Gauteng in South Africa, is water-positive. Underground water is pumped from the mine to ensure the mine’s safety. The underground water is entrapped within the rock strata and is referred to as fissure water. Fissure water is not related to surface groundwater which frequently decants as acid mine drainage.

The quality of the pumped and filtered fissure water is good, and compliant with the SANS241-2015 potable water standard. The rock strata containing the fissure water is dolomitic causing the water to be saturated with calcium carbonate. The high scaling tendency of the fissure water poses serious scaling risks when used in cooling, process and domestic water systems.

In order to save on reticulated water costs and provide water with acceptable scale forming potential, the mine blended municipal water with filtered fissure water for many years. The water demand for the mine operation and the local community was 24Ml/day, made up of 16,8Ml/day filtered fissure water plus 7,2Ml/day municipal water.

Soaring costs lead to water optimisation projects

In 2009, an ion exchange water softening plant was constructed to reduce the mine’s dependence on municipal water. The plant saved the targeted 7,2Ml/day of municipal water. However, it was abruptly shut down due to the negative impact of the waste brine from the resin regeneration process on the mine’s effluent. As a result, the mine reverted to blending fissure water with municipal water.

By 2013 the cost of municipal water escalated to R2 500 000 per month (R11,20 – R11,57/m3), spurring the mine to initiate water optimisation projects to reduce consumption of the municipal water.

It was recognised that the plant could be recommissioned with modifications that would allow for the resumption of municipal water savings. The modifications were done at minimal capital cost utilising as much salvaged equipment as possible.

The recommissioned plant produced treated water complying with the environmental standards required of the waste. In order to generate 24Ml/day of potable water with acceptable scaling tendency, 12Ml/day of softened water was required. Actual production averaged 0 – 0.5Ml/day representing availability of between 3% and 11%. In essence, no real municipal water savings were realised.

The removal of unwanted ions from water using ion exchange resins is referred to as resin technology. The ion exchange process removes soluble ionised dissolved solids from the water via a reversible ionic exchange between a solid phase (resin bead) and a liquid phase (water). Ion exchange has been used, extensively and effectively, for decades as a form of water treatment. The most common application is for water softening to minimise the threat of scale by removing the hardness and alkalinity ions. Advances in resin technology have opened opportunities for contaminant ion removal from more complex waters. Hazardous ions such as nickel, uranium, arsenic and lead are now successfully removed using resin technology to produce safer water. Nitrates present a unique challenge in underground mine service water since it reacts with chlorine-based microbicides used for underground water disinfection forming volatile and toxic chloramines.

Drawing on experience and technical capabilities to maintain and operate a mine water plant

In June 2014, Watercare Mining was contracted to:

● Identify and evaluate operational and process-related issues
● Develop an operational mass balance
● Assist with the optimisation of the process by completing resin tests, vessel evaluations and inspections

Watercare Mining was the company of choice for its experience and capabilities in the following fields:

● Operation and maintenance of water treatment plants in the mining environment
● Ion exchange technology – Watercare has extensive experience in the development of unique and innovative ion exchange processes
● Proven capabilities in plant design, engineering, fabrication and construction
● Expertise and knowledge of the site – Watercare had operated at this plant for decades and has an in-depth understanding of the mine’s water balance and water reticulation systems

The ideal requirement of an efficient and effective water plant was to:

1. Produce water compliant with SANS 241:2015 potable water standards
2. Produce water with an LSI of 0.18 at 65°C, comparable to municipal water LSI of 0.09
3. Produce the required water volume of 25 Ml/day on average and 28 Ml/day at peak times
4. The upgraded plant must be able to tolerate changes in the feedwater quality
5. Produce waste that is environmentally compliant
6. Be economically viable for the mine

Phased-in approach to achieve water optimisation objectives

Extensive research and technical analysis highlighted the need for several interventions resulting from ageing equipment and a deterioration of the feedwater quality.

Watercare Mining signed an “Operate and Maintain” agreement with the mine, phasing in the implementation of proposed interventions.

Some of the actions taken included:

1. The placement of full-time operators in the plant
2. The phased-in refurbishment of the filter plant eliminated operational risks associated with suspended solid overflow into the softener plant
3. The ion exchange control system was reprogrammed. Historically the control programme limited the operation of the ion exchange plant to 30% of its capacity. It also contributed to approximately 40% downtime. The upgrading enabled full utilisation of the ion exchange plant’s capacity
4. The centrifuge system in the salt store was decommissioned and replaced with a brine recovery and precipitation process enabling the brine to be recycled and reused for resin regeneration. This recycling process was key to the compliance required of the waste generated
5. The lime dosing system was automated
6. Instrumentation was improved to fully automate operations such as the brine recovery cycle
7. Actuated valves on the softening plant were replaced over a period of time. This reduced downtime resulting from faulty valves

An overview of the water treatment plant which now consistently and reliably produces 18Ml/day of potable water, 9Ml/day of softened fissure water blended with 9Ml/day of filtered water, pH adjusted and disinfected before reticulated to the various points of use.

The table below highlights the relative cost associated with the production of potable water through the mine’s water treatment plant.

The mine’s water plant is able to produce SANS241:2015-compliant water 68% cheaper than the unit cost of municipal water. Reliance on municipal water has reduced from 7,2Ml/day to 6,0Ml/day (16.7% reduction) with Watercare Mining operating and maintaining the water plant.

This relates to a cost saving of approximately R760 000 per month at the current municipal water rate.

Moreover, the mine is assured of a consistent, reliable supply of potable water, without the headache of managing the water plant. It can focus on its core activity of mining.