With discovery rates in decline, gold producers are turning to technology to make unattractive deposits more appealing. In particular, complex gold ores or
concentrates, which involve the presence of copper or other base metals that interfere with cyanidation, can be made more valuable through the novel and
ever-improving addition of SART technology. During cyanidation, copper and base metals dissolve and bind to cyanide, making it unusable. That process is
both costly, since cyanide consumption goes up, and environmentally dangerous, as cyanide bonded to copper is more difficult to break down.
SART (sulphidization, acidification, recycle, thickening) is a process designed to resolve these challenges, allowing the recovery and recycling of cyanide
with the added advantage of increased revenue from the sale of high-grade copper or zinc sulphide precipitate. But, so far at least, SART has not been as
widely adopted as might be expected, with only a few plants built worldwide. The technology was the subject of a number of papers at last month’s World
Gold conference in Brisbane, Australia, and real-world experience is building knowledge of how and when to use it.
In the SART process, leach solution containing copper cyanide complexes from cyanidation is treated with sulphuric acid and sulphide reagent to break the
copper-cyanide complex, precipitating copper as chalcocite (Cu2S), which is thickened and recovered as a value-added product. Acidification converts the
cyanide to hydrogen cyanide (HCN), which is recovered as free cyanide following the addition of lime and recycled back to the leach. Gypsum formed in the
lime addition, or neutralization, stage is also recovered.
Among the presenters at World Gold was Patrick Littlejohn, senior technology development specialist at the Vancouverbased BioteQ Environmental
Technologies, experts in industrial wastewater treatment. His company has experience in designing and operating metal recovery water treatment plants,
becoming experts in the precipitation, settling and dewatering of metal sulphides. This expertise, along with the company’s involvement with several
plants, has helped advance SART technology.
Littlejohn says SART has evolved from the initial plant designs, which “suffered from a lack of understanding of how sulphide chemistry behaves in a
continuous plant, what factors affect sulphide morphology and stability, and how sulphide chemistry affects reagent consumption.”
SART process control has improved through the more sophisticated application of pH and oxididation-reduction potential (ORP) monitoring and
instrumentation, Littlejohn says, adding, “BioteQ has successfully used these process controls to produce consistent copper and cyanide recovery while
avoiding problems with copper resolubilization, and achieved consistent plant performance and discharge levels results even when there are wide
fluctuations to the flow directed to the plant and to the plant feed composition.”
The company’s first plant was Lluvia de Oro in Mexico, for NWM Mining. Built in 2008, the SART plant lowered costs, enhanced gold recovery and purity, and
recovered a salable copper byproduct. Soon after the plant was commissioned, though, NWM closed the mine as it shifted its focus to developing new
deposits. In 2010, BioteQ designed and commissioned a SART plant at the Mastra mine near Gumushane, Turkey. The plant helped owner Koza Gold to reduce the
metallurgical interference of cyanide-soluble copper, improve gold purity, and maintain regulatory compliance for residual cyanide in tailings.
In 2011, BioteQ signed a contract with Kinross Gold to review and commission a SART plant being constructed at the Maricunga mine in Chile.
Actual success may vary
Paul Breuer, precious metals stream leader for Australia’s Minerals Down Under National Research Flagship at the Commonwealth Scientific and Industrial
Research Organisation (CSIRO), says implementation of SART technology is still constrained by the need to treat solution (not slurry), which with
counter-current decantation (CCD) separation creates a water balance issue, meaning “high capital costs and a more complex process compared to cyanide
Breuer says some operations amenable to SART use cyanide destruction instead because of capital cost constraints. “The real sticking point is that the
industry is at a place where companies will wear higher opex of cyanide destruction to have a simple and lower-capex process,” he explains.
The first commercial SART plant was commissioned in 2004 at Newcrest Mining’s Telfer mine in Australia, a past-producer that had previously closed in 2000
because of low gold prices. Breuer said Telfer is well-suited to SART because of exceptional levels of cyanide-soluble copper in the pyrite concentrate
stream. Its mill has flotation and cyanide circuits. High recoveries of copper and cyanide were achieved in SART, but the sulphide additions necessary to
achieve these recoveries were higher than expected.
“This problem has to do with retention time in the SART plant,” Breuer says. He and Andrew Simons, a PhD student at Curtin University, co-authored a paper
on the subject for the World Gold conference. “Over time, the chemistry of the acidified copper sulphide precipitation unravels and the [precipitated]
copper dissolves back into solution as copper cyanide. This process commences within minutes, triggering the need for the extra sulphide,” Breuer adds.
“Engineering solutions are out there, but there’s a cost attached.”
In any event, the Telfer SART plant’s iffy performance pales in comparison to chronic problems at the low-grade mine, notably production shortfalls,
reduced reserves from feasibility study estimates, and production costs that recently topped $1,570 per ounce. Newcrest is scaling back operations amid
rumours of a possible closure.
The SART process was also used at the profitable Yanacocha mine in Peru, which has produced 26 million ounces of gold for Newmont Mining and Buenaventura
since 2003. Built in 2008, the SART plant was effective in terms of recovering copper and cyanide, but the process was often difficult to control and
suffered from excessive sulphide consumption and low precipitate grades.
Yanacocha has lower cyanide soluble copper content in its ores than Telfer, resulting in lower precipitate grades, but its problems are common to most SART
plants treating cyanide solutions, Breuer says. “The process is robust in the lab and works efficiently at stoichiometric sulphide addition. The issue is
when you go to a [full-scale] plant and it’s not necessarily the case,” he explains.
Much has been learned from these early plants, however. The investigations by Simons have found that thickening time drastically affects SART performance
as sulphide [reaction] loss from the system causes re-dissolution of the copper sulphide precipitate. The sulphide precipitation circuit and its residence
time are considered critical to producing high-grade sulphide products.
Breuer says instrumentation is available to improve process control and optimization but can be difficult to maintain, resulting in poor reliability. “This
is a major challenge as most gold plants today operate on feedback control, through the measurement and analysis of samples, rather than
real-time/feed-forward control using online measurements.”
Monitoring excess sulphide is an option, Breuer says, but it is not easily achieved, with sulphide-selective electrodes ineffective due to HCN
interference. He adds that oxidation-reduction potential is an effective indicator of sulphide addition up to the stoichiometric amount but does not
provide a quantitative measure for controlling excess addition required by the process. Quantifying copper entering the process is the recommended control
parameter. While pH is used to control the acid addition, it does not provide any indication of excess sulphide.
“SART helps improve project economics but it’s not a cure-all,” Breuer warns.
Louie Diaz, Kinross communications manager, says the company’s Maricunga SART plant was completed in the second half of 2012. The location is a challenge
for the technology, given the high altitude and inherently harsh and windy winters in the Andes of Chile.
“The plant was enclosed in a building – a unique feature for the handful of SART plants in the world – to protect it from the weather,” Diaz says. “Proper
heating and ventilation were critical to ensure we meet our high safety and operating standards.”
The $80-million plant is performing to expectations, Diaz says. “As the SART process is a technologically complex endeavor, we introduced technologies that
were not previously used at Maricunga and built the plant with a high degree of automation. We continue to make small adjustments to the plant, including
fine-tuning the back end of the process where the copper product is produced.”
The Maricunga SART plant is one of the largest in the world, capable of treating 750 cubic metres of cyanide leachate per hour. Assuming its ongoing
success, this plant could kick-start broader application of the technology.