Water environmental innovation at mining site

Sunday, 29 January, 2012

To meet Grange Resources’ stringent requirements to protect the Savage River from its nearby mining operations, pitt&sherry designed a unique hydrocarbon separator to ensure no hydrocarbons were released into the river.

Mining projects are increasingly being judged on their environmental credentials. It is clear that the world is moving rapidly to a position from which all initiatives will be determined based on their capacity to contribute to sustainability objectives, and companies including pitt&sherry are focused on exerting a positive influence on sustainability through their work.

In the Tarkine wilderness area in remote north-western Tasmania sits a tangible example - a cleverly designed full flow hydrocarbon separator, which led to pitt&sherry, Grange Resources and Shaw Contracting being highly commended at the 2011 Tasmanian Water Environmental Management Awards.

When Grange Resources (then Australian Bulk Minerals) began expanding its Savage River mine in 2007, it championed strict environmental standards. Grange Resources was committed to ensuring the health of the Savage River system, which runs through its mining operations, by exceeding environmental standards and ensuring no hydrocarbon was released to the river from the expanded mine.

The mine’s previous exit point offered no protection to the river, comprising a simple weir with a flow monitor but no hydrocarbon separation. pitt&sherry, a leading multispecialist infrastructure consultancy, was contracted to undertake the design work, which included developing a unique and cost-effective water treatment system that used a full flow hydrocarbon separator to remove harmful oils and impurities.

Facing challenges and delivering sustainable solutions

The development and design process at the Savage River site involved significant challenges - stringent environmental expectations, a remote location without power, a topography with wildly fluctuating annual rainfall and the need to design a cost-effective solution that did not require constant human intervention. These challenges led to pitt&sherry developing some unique solutions.

Addressing fluctuating flows

According to Dr Steve Edwards, Senior Consultant, pitt&sherry: “The major challenge was the need to address the mining operation’s flows as well as flows from the large natural catchment area that drains over and through the mine operations.”

While the local climate is dry in summer, rainfall in the wetter seasons constitutes a yearly average of about 2000 mm. Any design had to be capable of processing all of the flows. The design also had to be able to treat small oil or diesel leaks from machinery, as well as major contingencies such as fuel truck spills.

“Our design included two components, a hydrocarbon separator and an overflow chute, which work together as required. This design achieved what no commercial hydrocarbon separator could - complete trapping under all conceivable flow situations,” said Dr Edwards.

Containing and treating high flows

Early analysis found almost all of the water emanating from various parts of the mine ended up in the site’s artificial lake (South Lens). The exception was a drainage stream, which has since been diverted to flow into the lake. The lake became the integral focus of the design process and its capacity was increased to accommodate water rises experienced during a major storm.

“Lake water travels through the hydrocarbon separator before discharging to the river. A unique feature of our design is a flow limiter that works on the principle of limiting flow through multiple small holes. The number, size and orientation of these exit holes control the flow through the unit and ensure the separator is never exposed to flow levels beyond its design capacity to separate and store incoming hydrocarbon,” explained Dr Edwards.

Under extreme conditions, another tailored design element comes into effect, in the form of a separate but connected overflow chute with its own hydrocarbon barrier. By increasing the lake’s capacity and integrating the hydrocarbon separator and overflow chute, the system ensures all hydrocarbon is eventually trapped in the separator, where it is removed by vacuum truck.

Ensuring cost-effectiveness

It was theoretically possible to install many commercial separators to achieve Grange Resource’s desired outcomes; however, the costs were prohibitive. It was estimated 25 separators were needed to deal with a 1-in-100 year rainfall event - a $5m investment. Coupled with the expense were space limitations and the level of maintenance required to ensure redundant equipment (most of the separators would not have operated for much of the year) was always in operating condition.

pitt&sherry’s solution cost less than $0.5m completed, a significant cost reduction. Another key determinant was that none of the commercially available separators could deal with the site’s broad range of flows without requiring an untreated bypass during high flows, and such devices could not cope with a full tanker fuel spill under high rainfall conditions.

Using innovative materials and achieving minimal maintenance

To further protect the Savage River, Grange Resources imposed a design restriction on the use of any galvanised metals at or below the normal water line level. pitt&sherry’s design used some stainless steel and mostly HDPE (high density polyethylene), which created its own challenges.

The use of HDPE in such an exposed position meant there was a high level of thermal expansion (and contraction) of the material, so unique fasteners had to be designed and built. U bolts with blue HDPE rollers were used, with the inclusion of plastic rollers ensuring the U bolts do not grind against the metal frame and release extra metal. These two measures minimise the chance of any metal contamination of the water, and extend the unit’s life.

In such a remote, rugged site, a design requiring minimal maintenance was imperative. pitt&sherry’s open design enables anyone passing to observe the hydrocarbon separator’s performance. The absence of any moving parts or controls means the system is extremely robust under all weather conditions and immune to power failures. “The only human intervention needed is to empty trapped hydrocarbon using a suction truck, and future plans including the installation of a powered oil detector and skimmer will negate that,” said Dr Edwards.

Achieving outstanding results

pitt&sherry’s detailed analysis of the requirements and constraints resulted in an innovative design that has protected the environment and serviced all needs in a most cost-effective manner. The full flow hydrocarbon separator clearly protects the river ecosystem as since implementation there has been no recorded hydrocarbon release from the Savage River mine site despite a constant monitoring regime.

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