Choosing the right reagent to help process troublesome ores often can result in significant savings and higher recovery rates, without draining the capex budget
By Russell A. Carter, Managing Editor
When metal markets weaken and primary production costs come under closer scrutiny, producers can be forced to make some difficult decisions about allocating shrinking capex/opex funds to improve performance. As a recent article from Australia’s CSIRO research organization pointed out1, suppose a company had the flexibility to spend $1 million on a technology that would improve product recovery by 1% or spend the same amount on initiatives that would cut operating costs by 5%. Which is the better approach?
Given the cost and production parameters specified in the article—gold ore, 500 tons per hour (t/h) throughput, head grade of 3 g/t and $1,150/oz price—the numbers favor the improved recovery option by far, in terms of speed of payback. It’s a simplistic example, but it illustrates the point.
This is an area in which mining chemicals may figuratively be worth their weight in gold—or iron ore or phosphate, potentially enabling producers to tweak or otherwise adjust process-line inputs to obtain small, but valuable improvements in recovery without massive investment.
That isn’t always the case, however; as we’ll see later in the article, the world’s largest gold miner had to spend triple-digit millions to incorporate a new chemical treatment circuit into one of its Nevada operations—but plans to recoup that investment, and more, from recovery of gold in hard-to-process ores that would have required additional capital spending to accomplish by conventional methods.
A flurry of mining-chemical market studies emerged in early 2015, predicting a growth rate of anywhere from 4.8% to 6.5% CAGR through 2020. How those predictions stand up to subsequent economic developments affecting the mining industry during the current slump is anyone’s guess. What is for certain is that mining chemical suppliers often can provide solutions to processing problems that will undoubtedly proliferate as miners are forced to develop less-desirable, lower-grade deposits in increasingly remote locations.
TRACKING THE TRENDS
Against this backdrop of challenge and opportunity, E&MJ asked Kerim Sasioglu, global marketing manager–mining chemicals for AkzoNobel Surface Chemistry AB, a major specialty chemical supplier for the mining industry, to comment on industry trends.
E&MJ: How can mining chemical suppliers assist their mineral processing customers in improving recovery and cutting costs when faced with the challenges of lower commodity prices, less money for equipment upgrades, and lower-grade, more difficult ores to process?
Sasioglu: As ore grades are depleted, the utilization and optimization of flotation becomes even more important. In many cases, the traditional flotation chemicals need to be replaced completely or at least partly by more selective reagents. Most often, the solution is a compromise. It is challenging to both improve the selectivity of the collector to achieve optimal flotation of complex ores and at the same time improve flotation kinetics to deal with higher throughputs. One way to deal with higher throughputs can be to add a booster to the reagent to enhance flotation kinetics.
E&MJ: In what ways can mining chemical suppliers help their mineral processing customers reduce process water consumption and/or contribute to less environmental impact from flotation and other processing technologies?
Sasioglu: Process water circuits will become more and more closed. We anticipate the need for sustainable tailor-made reagents that can conform to such an environment.
E&MJ: What challenges or benefits do flotation chemical suppliers encounter as mineral processors move from smaller, more numerous flotation machines to fewer, larger-sized flotation cells? Is there just a simple linear relationship (i.e., bigger flotation cells, thus more chemical usage) or are there hidden factors to consider?
Sasioglu: The reagent dosage is measured by gram reagent/ton floated ore regardless of the size or number of the flotation cells. Pulp density, retention time and flotation kinetics all determine the size of the flotation cells.
E&MJ: What do you envisage as the next important near-term steps in the evolution of flotation technology?
Sasioglu: Flotation technology will continue to advance to meet the challenges of more complex ores/lower qualities. We anticipate the need for sustainable tailor-made reagents to increase. In order to improve the efficiency and the selectivity of the flotation reagent through the flotation circuit, more understanding of the mineralogy and flotation kinetics is needed. We anticipate a closer collaboration between engineering companies, flotation chemical suppliers and mineral processing companies for finding the optimal process.
AkzoNobel, along with other major chemical suppliers, provides both tailor-made and standard products, and The Netherlands-headquartered company prides itself on its level of customer collaboration and focus on a sustainable future. For example, the company said it has extensive knowledge and experience in assessing the impact of its flotation collector chemistry within the customer’s environment, and has developed unique analysis methods for detecting very low levels of its products in water and air.
“Choosing the best collector is based on three basic principles,” Sasioglu explained. “Each ore is unique; each ore is treated in a unique process; and the process performance is optimized with customized collectors.”
AkzoNobel flotation collectors offer optimized solutions for iron ore, phosphate, potash, calcite, and other industrial minerals. The products are recognized under the Armeen, Armac, Armoflote, Atrac, Berol, Ethomeen and Lilaflot trade names.
There has been a steady stream of notable developments in the search for methods to achieve better performance when dealing with hard-to-process ores and troublesome concentrates. For example, late last year, FLSmidth and BASF signed a joint development agreement to expedite the commercialization of FLSmidth’s Rapid Oxidative Leach process targeting difficult-to-process copper concentrates, such as primary sulphides and concentrates containing high levels of arsenic.
Earlier in 2015, FLSmidth reported a technological breakthrough with the Rapid Oxidative Leach process that it claimed to enable the copper industry to dissolve copper from low to mid-grade concentrates at 80°C. This capability, according to the company, will maximize the use of existing capacity when producers are faced with a transition from processing oxide to sulphide ores. The technology allows extraction of copper as an alternative to selling concentrate to smelters.
Under the joint development agreement, BASF will bring additional funding, resources and new novel chemical reagents and processes to more rapidly commercialize FLSmidth’s leaching technology. In particular, BASF said it will incorporate innovative solvent extraction reagents, which exhibit high resistance to degradation and a step change in copper selectivity.
As an example, the two companies pointed out that the decline of copper ore grades in Chile and Peru has driven companies toward new higher-copper-grade ore deposits containing arsenic. This requires costly cleaning, as arsenic is a health and safety risk for copper smelters, which limit arsenic content to a maximum of 0.5% arsenic in the concentrate.
According to BASF, the hydrometallurgical processing of primary copper sulphide concentrates and concentrates containing high levels of arsenic offers an exciting opportunity. “Several years ago, we focused on developing chemicals that would support the industry’ inevitable move towards mine more complex ores, chemicals that required a new leaching technology. We believe that the FLSmidth Rapid Oxidative Leach process combined with BASF’s novel reagents is the game changer that will enable the copper industry to process such ores,” said Christian Lach, vice president strategic marketing and innovation for water, oilfield and mining solutions at BASF.
In a recent paper2, researchers from FLSmidth described the background and details of the Rapid Oxidative Leach concept as developed by the company, explaining that “…the majority of efforts to improve primary copper sulphide leaching have focused on solution chemistry, temperature, O2 pressure, ultra-fine grinding, the use of catalysts, etc. Historically, very few studies have focused on the solid/solution interface.
“A new, patent-pending approach to catalyzed copper sulphide leaching has been discovered by FLSmidth that enables manipulation of the 2-D semi-conductor properties of chalcopyrite surfaces to the benefit of higher electrochemical reactivity. Copper dissolution rates are then further accelerated by incorporating a Stirred Media Reactor (SMRt) into the process. By using minute amounts of Cu2+ to ‘pre-activate’ chalcopyrite, leach times have been reduced from more than 20 hours with incomplete Cu dissolution to less than 2 hours with 98+% Cu dissolution at 80°C. The pre-activation process takes only minutes to complete at temperatures of 80°C or less.
“The total atmospheric leaching process, incorporating the pre-activation and Stirred Media Reactor, is compatible with existing SX/EW processes.”
MONEY WELL SPENT
Also in 2015, Barrick announced that a new thiosulphate process circuit at its Goldstrike mine in Nevada had reached commercial production status in the third quarter, and it expected full ramp-up of production from the facility in the first half of this year. The company developed the concept, which it labeled as the Total Carbonaceous Matter (TCM) project, over several years in collaboration with CSIRO and others to handle large volumes of double-refractory ore that were not amenable to Goldstrike’s conventional autoclave processing methods. Although the Goldstrike mill also has a roaster circuit for processing carbonaceous refractory ore, Barrick would have had to shut down its six autoclaves after the mine’s single-refractory ore was depleted, and then probably be forced to heavily invest in building another roaster to process additional refractory ore in order to reach its production targets.
The thiosulphate process now installed at Goldstrike gave Barrick—at a cost of $620 million—a cyanide-free alternative for recovering gold from stockpiled double-refractory ore that contains an estimated 4 million oz of gold. The company began stockpiling that ore in the early ‘90s.
Calcium thiosulphate is a relatively common chemical typically used in agricultural applications; however, it also has the ability to “hang on” to gold when used as a lixiviant in a leach process involving carbonaceous refractory ores, whereas cyanide will give up dissolved gold values to the carbon in those “preg-robbing” ores. For that reason, conventional CIL processing wasn’t a viable option for the stockpiled ore. The thiosulphate approach enabled Goldstrike to continue using its autoclaves by allowing the autoclaved ore slurry to be sent to a new resin-in-leach circuit where the slurry interacts with calcium thiosulphate and resin as it proceeds through the seven-tank circuit to maximize gold recovery.
The use of thiosulphate for this purpose had been a subject of study by Barrick and others for a number of years prior to 2009, at which time Barrick decided to move the concept out of the laboratory and into the demonstration plant stage. Although calcium thiosulphate offers a number of environmental advantages over cyanide in addition to its gold-recovery characteristics, without careful handling it can break down into ineffective chemicals during the resin-in-leach process. Solving that problem, along with developing a roadmap for smoothly transitioning from CIL to thiosulphate/resin leaching took some time, adding to the pressure the company faced in finding a way to keep its autoclave plant open after the amenable ore ran out.
Thiosulphate, under the trade name Thio Gold 300, is manufactured on-site at Goldstrike in a facility designed by Tessenderlo Kerley, a chemical supplier to the mining, agricultural, industrial and water treatment industries.
The capacity of Goldstrike’s TCM facility is just under 14,000 t/d, representing an annual gold recovery figure of 350,000–400,000 oz/y that possibly overshadows the other substantial benefit provided by the TCM project—avoiding a likely loss of money, jobs and production from shutting down its autoclave operation if a successful solution hadn’t been found.
MAKING MORE MOLY
In another example of how reagents can be adjusted to target specific objectives, researchers from Cytec point out in a recent issue of Cytec’s Solutions magazine3 that Cu-Mo separation has long been a challenging process in flotation of porphyry copper ore. The small amount of molybdenite (MoS2) in these ores is floated along with copper to a bulk Cu-Mo concentrate. The concentrate is then separated by depressing the copper minerals and floating MoS2. Sodium hydrosulphide (NaSH) (or sodium sulphide) is one of the traditional depressing reagents.
At the Wunugetu mine in China’s Inner Mongolia region, Cu and Mo grades are low and the proportion of secondary copper minerals is high. The porphyry copper ore body has 3 million tons of copper and 600,000 tons of molybdenum reserves, and is the fourth largest copper/molybdenum ore reserve in China. After commissioning in 2009, the mine implemented several technical modifications in the Cu-Mo circuit. In 2010, the mine completed a process optimization project that stabilized the Cu-Mo plant operation.
However, the operation still faced three major challenges: High NaSH consumption (cost), environmental and occupational health concerns associated with the reagent, and logistical arrangements for reagent supply. To solve these problems, the mine set a new target for improvement by searching for a reagent that could provide better dosage-performance, lower toxicity and easier handling.
In 2014, the mine conducted industrial scale trials with a high performance synthetic organic reagent, Cytec’s AERO 8371 PNR, as a copper depressant. The application of AERO 8371 PNR resulted in about 45% reduction in NaSH consumption, and delivered a number of benefits to the mine. NaSH dust and stench were reduced in the reagent warehouse and reagent preparation workshop. Reduction in NaSH consumption resulted in up to a 35% reagent cost saving.
Molybdenum product grade also increased from 45% to 47%, and the copper content in the molybdenum product has decreased from 1.2% to less than 1%, a threshold to allow upgrading the molybdenum product to Class 1 from Class 2. This is achieved with a shorter cleaner circuit, seven-stages vs. nine-stages before the trial.
Furthermore, according to the authors, the stability of the Cu-Mo circuit was improved, resulting in higher Mo recovery. The reduction in NaSH consumption also eased logistics matters and solved certain operational issues. The reduced need for NaSH flakes allowed the mine to purchase the raw material at a higher quality level, which not only benefitted Cu-Mo separation but also helped eliminate blockage of the NaSH solution delivery pipes.